Endocrine Action on Midbrain
Writer and Curator: Larry H. Bernstein, MD, FCAP
- Brain’s Role in Browning White Fat
- Insulin and leptin act on specialized neurons in the mouse hypothalamus to promote conversion of white to beige fat.
By Anna Azvolinsky | January 15, 2015
JUSTIN HEWLETT, MNHS MULTIMEDIA, MONASH UNIVERSITY
Ever since energy-storing white fat has been shown to convert to metabolically active beige fat, through a process called browning, scientists have been trying to understand how this switch occurs. The immune system has been shown to contribute to activation of brown fat cells. Now, researchers from Monash University in Australia and their colleagues have shown that insulin and leptin—two hormones that regulate glucose metabolism and satiety and hunger cues—activate “satiety” neurons in the mouse hypothalamus to promote the conversion of white fat to beige. The results are published today (January 15) in Cell.
Hypothalamic appetite-suppressing proopiomelanocortin (POMC) neurons are known to relay the satiety signals in the bloodstream to other parts of the brain and other tissues to promote energy balance. “What is new here is that one way that these neurons promote calorie-burning is to stimulate the browning of white fat,” said Xiaoyong Yang, who studies the molecular mechanisms of metabolism at the Yale University School of Medicine, but was not involved in the work. “The study identifies how the brain communicates to fat tissue to promote energy dissipation.”
“The authors show that [insulin and leptin] directly interact in the brain to produce nervous-system signaling both to white and brown adipose tissue,” said Jan Nedergaard, a professor of physiology at Stockholm University who also was not involved in the study. “This is a nice demonstration of how the acute and chronic energy status talks to the thermogenic tissues.”
Although the differences between beige and brown fat are still being defined, the former is currently considered a metabolically active fat—which converts the energy of triglycerides into heat—nestled within white fat tissue. Because of their energy-burning properties, brown and beige fat are considered superior to white fat, so understanding how white fat can be browned is a key research question. Exposure to cold can promote the browning of white fat, but the ability of insulin and leptin to act in synergy to signal to the brain to promote browning was not known before this study, according the author Tony Tiganis, a biochemist at Monash.
White fat cells steadily produce leptin, while insulin is produced by cells of the pancreas in response to a surge of glucose into the blood. Both hormones are known to signal to the brain to regulate satiety and body weight. To explore the connection between this energy expenditure control system and fat tissue, Garron Dodd, a postdoctoral fellow in Tiganis’s laboratory, and his colleagues deleted one or both of two phosphatase enzymes in murine POMC neurons. These phosphatase enzymes were previously known to act in the hypothalamus to regulate both glucose metabolism and body weight, each regulating either leptin or insulin signaling. When both phosphatases were deleted, mice had less white fat tissue and increased insulin and leptin signaling.
“These [phosphatase enzymes] work in POMC neurons by acting as ‘dimmer switches,’ controlling the sensitivity of leptin and insulin receptors to their endogenous ligands,” Dodd told The Scientist in an e-mail. The double knockout mice also had an increase in beige fat and more active heat-generating brown fat. When fed a high-fat diet, unlike either the single knockout or wild-type mice, the double knockout mice did not gain weight, suggesting that leptin and insulin signaling to POMC neurons is important for controlling body weight and fat metabolism.
The researchers also infused leptin and insulin directly into the hypothalami of wild-type mice, which promoted the browning of white fat. But when these hormones were infused but the neuronal connections between the white fat and the brain were physically severed, browning was prevented. Moreover, hormone infusion and cutting the neuronal connection to only a single fat pad resulted in browning only in the fat pad that maintained signaling ties to the brain. “This really told us that direct innervation from the brain is necessary and that these hormones are acting together to regulate energy expenditure,” said Tiganis.
These results are “really exciting as, perhaps, resistance to the actions of leptin and insulin in POMC neurons is a key feature underlying obesity in people,” said Dodd.
Another set of neurons in the hypothalamus, the agouti-related protein expressing (AgRP) or “hunger” neurons, are activated by hunger signals and promote energy storage. Along with Tamas Horvath, Yale’s Yang recently showed that fasting activates AgRP neurons that then suppress the browning of white fat. “These two stories are complimentary, providing a bigger picture: that the hunger and satiety neurons control browning of fat depending on the body’s energy state,” said Yang. Activation of POMC neurons during caloric intake protects against diet-induced obesity while activation of AgRP neurons tells the body to store energy during fasting.
Whether these results hold up in humans has yet to be explored. Expression of the two phosphatases in the hypothalamus is known to be higher in obese people, but it is not clear whether this suppresses the browning of white fat.
“One of the next big questions is whether this increased expression and prevention of insulin plus leptin signaling, and conversion of white to brown fat perturbs energy balance and promotes obesity,” said Tiganis. Another, said Dodd, is whether other parts of the brain are involved in signaling to and from adipose tissue.
- Dodd et al., “Leptin and insulin act on POMC neurons to promote the browning of white fat,”
Cell, 2015. http://dx.doi.org:/10.1016/j.cell.2014.12.022 http://medicine.yale.edu/lab/horvath/index.aspx
Our main interest is the neuroendocrine regulation of homeostasis with particular emphasis on metabolic disorders, such as obesity and diabetes, and the effect of metabolic signals on higher brain functions and neurodegeneration. We have active research programs to pursue the role of synaptic plasticity in the mediation of peripheral hormones’ effects on the central nervous system.
We also study the role of mitochondrial membrane potential in normal and pathological brain functions with particular emphasis on the acute effect of mitochondria in neuronal transmission and neuroprotection. We combine classical neurobiological approaches, including electrophysiology and neuroanatomy, with endocrine and genetic techniques to better understand biological events at the level of the organism.
Leptin and Insulin Act on POMC Neurons to Promote the Browning of White Fat
Garron T. Dodd, Stephanie Decherf, Kim Loh, Stephanie E. Simonds, Florian Wiede, Eglantine Balland, Troy L. Merry, et al.
http://dx.doi.org/10.1016/j.cell.2014.12.022
Highlights
- Insulin and leptin act synergistically on POMC neurons to promote WAT browning
- Increased POMC-mediated WAT browning prevents diet-induced obesity
- PTP1B and TCPTP attenuate leptin and insulin signaling in POMC neurons
- Combined PTP1B and TCPTP deficiency in POMC neurons promotes white fat browning
The primary task of white adipose tissue (WAT) is the storage of lipids. However, “beige” adipocytes also exist in WAT. Beige adipocytes burn fat and dissipate the energy as heat, but their abundance is diminished in obesity. Stimulating beige adipocyte development, or WAT browning, increases energy expenditure and holds potential for combating metabolic disease and obesity. Here, we report that insulin and leptin act together on hypothalamic neurons to promote WAT browning and weight loss. Deletion of the phosphatases PTP1B and TCPTP enhanced insulin and leptin signaling in proopiomelanocortin neurons and prevented diet-induced obesity by increasing WAT browning and energy expenditure. The coinfusion of insulin plus leptin into the CNS or the activation of proopiomelanocortin neurons also increased WAT browning and decreased adiposity. Our findings identify a homeostatic mechanism for coordinating the status of energy stores, as relayed by insulin and leptin, with the central control of WAT browning. http://www.cell.com/cms/attachment/2023992410/2043906325/fx1.jpg
Light on the Brain
Researchers find that photoreceptors expressed in zebrafish hypothalamus contribute to light-dependent behavior.
By Sabrina Richards | September 20, 2012
A 21 day old zebrafish. Their optical clarity and relatively easy maintenance make them a favorite for geneticists and developmental biologists. In this fish, the muscles can be seen as chevron shapes in the tail, the swim bladder as a “bubble” just behind the head, and the food that the fish has been eating as a brown patch just below the swim bladder.
Juvenile zebrafish. Shawn Burgess, NHGRI
Zebrafish larvae without eyes or pineal glands can still respond to light using photopigments located deep within their brains. Published today (September 20) in Current Biology, the findings are the first to link opsins, photoreceptors in the hypothalamus and other brain areas, to increased swimming in response to darkness, a behavior researchers hypothesize may help the fish move toward better-lit environments.
“[It’s a] strong demonstration that opsin-dependent photoreceptors in deep brain areas affect behaviors,” said Samer Hattar, who studies light reception in mammals at Johns Hopkins University but did not participate in the research.
Photoreceptors in eyes enable vision, and photoreceptors in the pineal gland, a small endocrine gland located in the center of the vertebrate brain, regulate circadian rhythms. But photoreceptors are also found in other brain areas of both invertebrates and vertebrate lineages. The function of these extraocular photoreceptors has been best studied in birds, where they regulate seasonal reproduction, explained Harold Burgess, a behavioral neurogeneticist at the Eunice Kennedy Shriver National Institute for Child Health and Human Development. Many opsins have been reported in the brains of tiny and transparent larval zebrafish, raising the possibility that light could be stimulating the photoreceptors even deep in the brain. To test for behaviors that may be regulated by deep brain photoreceptors, Burgess and his colleagues in Wolfgang Driever’s lab at the University of Freiburg removed the eyes of zebrafish larvae, and compared their behavior to larvae that retained their eyes. Although most light-dependent behavior required eyes, the eyeless larvae did respond when the lights were turned off, increasing their activity for a several minutes, though to a somewhat lesser extent than control larvae. But the fact that they responded at all suggests that non-retinal photoreceptors contributed to the behavior.
To confirm the role of the deep brain photoreceptors, the researchers also tested eyeless larvae that had been genetically modified to block expression of photoreceptors in the pineal gland. This fish still showed this jump in activity for several minutes after entering darkness.
Two different types of opsins—melanopsin and multiple tissue opsin—are expressed in the same type of neuron in zebrafish hypothalamus. Burgess and his colleagues looked at zebrafish missing the transcription factor Orthopedia, which is unique to these neurons, and found that the darkness-induced activity boost is nearly absent in these fish. To further narrow the search for the responsible photoreceptors, the researchers overexpressed melanopsin in hypothalamus neurons that co-express Orthopedia and melanopsin, and found that it increased the sensitivity of eyeless zebrafish to reductions in light. The results point to both melanopsin and Orthopedia as key players in modulating this behavior and pinpoint the location to neurons that coexpress these factors in the zebrafish hypothalamus.
Interestingly, the hypothalamus is one of the oldest parts of the vertebrate brain, said Detlev Arendt, a developmental biologist at the European Molecular Biology Laboratory in Heidelberg. “It’s very possible that this is one of the oldest functions”—one that evolved in “non-visual organisms” that had no eyes but still needed to sense light.
Although not as directed and efficient as eye-dependent behaviors that help fish swim toward light, Burgess speculates that deep brain opsins can still benefit zebrafish larvae. “You could imagine situation where it can’t see light, if a leaf falls on it and it doesn’t know where to swim. I think this behavior puts it in a hyperactive state where it swims wildly for several minutes until it reaches enough light for eyes to take over,” he explained, noting that such behavior is common in invertebrates.
It remains to be seen whether these deep brain opsins regulate other behaviors, perhaps in similar fashion to seasonal hormonal regulation in birds, but Hattar believes it is likely. “It’s beyond reasonable doubt there are many functions for these deep brain photoreceptors.”
Fernandes et al., “Deep brain photoreceptors control light-seeking behavior in zebrafish larvae,” Current Biology, 22:1-6, 2012.
Neuroendocrine basis of sexuality, mood, anxiety, social consciousness
Physiology, signaling, and pharmacology of galanin peptides and receptors: Three decades of emerging diversity
Lang, R., Gundlach, A.L., Holmes, F.E., (…), Hökfelt, T., Kofler, B.
Pharmacological Reviews 2015: 67 (1), pp. 118-175
http://dx.doi.org:/10.1124/pr.112.006536
Galanin was first identified 30 years ago as a “classic neuropeptide,” with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides—galanin-like peptide and alarin—with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein–coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor–specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.
Estradiol regulates responsiveness of the dorsal premammillary nucleus of the hypothalamus and affects fear- and anxiety-like behaviors in female rats
Litvin, Y., Cataldo, G., Pfaff, D.W., Kow, L.-M.
European Journal of Neuroscience 2014; 40 (2), pp. 2344-2351
10.1111/ejn.12608
Research suggests a causal link between estrogens and mood. Here, we began by examining the effects of estradiol (E2) on rat innate and conditioned defensive behaviors in response to cat odor. Second, we utilized whole-cell patch clamp electrophysiological techniques to assess noradrenergic effects on neurons within the dorsal premammillary nucleus of the hypothalamus (PMd), a nucleus implicated in fear reactivity, and their regulation by E2. Our results show that E2 increased general arousal and modified innate defensive reactivity to cat odor. When ovariectomized females treated with E2 as opposed to oil were exposed to cat odor, they showed elevations in risk assessment and reductions in freezing, indicating a shift from passive to active coping. In addition, animals previously exposed to cat odor showed clear cue + context conditioning 24 h later. However, although E2 persisted in its effects on general arousal in the conditioning task, its effects on fear disappeared. In the patch clamp experiments noradrenergic compounds that typically induce fear clearly excited PMd neurons, producing depolarizations and action potentials. E2 treatment shifted some excitatory effects of noradrenergic agonists to inhibitory, possibly by differentially affecting α- and β-adrenoreceptors. In summary, our results implicate E2 in general arousal and fear reactivity, and suggest these may be governed by changes in noradrenergic responsivity in the PMd. These effects of E2 may have ethological relevance, serving to promote mate seeking even in contexts of ambiguous threat and shed light on the involvement of estrogen in mood and its associated disorders.
Endogenous opiates and behavior: 2013
Richard J. Bodnar
Peptides 62 (2014) 67–136
http://dx.doi.org/10.1016/j.peptides.2014.09.013
This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.
Brain aromatase (cyp19a1b) and gonadotropin releasing hormone (gnrh2 and gnrh3) expression during reproductive development and sex change in black sea bass (Centropristis striata)
Timothy S Breton, Matthew A DiMaggio, Stacia A Sowe, David L Berlinsky, et al.
Comparative Biochemistry and Physiology, Part A 181 (2015) 45–53
http://dx.doi.org/10.1016/j.cbpa.2014.11.020
Teleost fish exhibit diverse reproductive strategies, and some species are capable of changing sex. The influence of many endocrine factors, such as gonadal steroids and neuropeptides, has been studied in relation to sex change, but comparatively less research has focused on gene expression changes within the brain in temperate grouper species with non-haremic social structures. The purpose of the present study was to investigate gonadotropin releasing hormone (GnRH) and brain aromatase (cyp19a1b) gene expression patterns during reproductive development and sex change in protogynous (female to male) black sea bass (Centropristis striata). Partial cDNA fragments for cyp19a1b and eef1a (a reference gene) were identified, and included with known gnrh2 and gnrh3 sequences in real time quantitative PCR. Elevated cyp19a1b expression was evident in the olfactory bulbs, telencephalon, optic tectum, and hypothalamus/
midbrain region during vitellogenic growth, which may indicate changes in the brain related to neurogenesis or sexual behavior. In contrast, gnrh2 and gnrh3 expression levels were largely similar among gonadal states, and all three genes exhibited stable expression during sex change. Although sex change in black sea bass is not associated with dramatic changes in GnRH or cyp19a1b gene expression among brain regions, these genes may mediate processes at other levels, such as within individual hypothalamic nuclei, or through changes in neuron size, that warrant further research.
Evaluation for roles of neurosteroids in modulating forebrain mechanisms controlling vasopressin secretion and related phenomena in conscious rats
Ken’ichi Yamaguchi
Neuroscience Research xxx (2015) xxx–xxx
http://dx.doi.org/10.1016/j.neures.2015.01.002
Anteroventral third ventricular region (AV3V) regulates autonomic functions through a GABAergic mechanism that possesses neuroactive steroid (NS)-synthesizing ability. Although NS can exert effects by acting on a certain type of GABAA-receptor (R), it is not clear whether NS may operate to modulateAV3V GABAergic activity for controlling autonomic functions. This study aimed to investigate the issue.AV3V infusion with a GABAA antagonist bicuculline increased plasma vasopressin (AVP), glucose, blood pressure (BP), and heart rate in rats. These events were abolished by preinjecting its agonist muscimol, whereas the infusion with allopregnanolone, a NS capable of potentiating GABAA-R function, affectednone of the variables in the absence or presence of such bicuculline actions. Similarly, AV3V infusion with pregnanolone sulfate, a NS capable of antagonizing GABAA-R, produced no effect on those variables.AV3V infusion with muscimol was effective in inhibiting the responses of plasma AVP or glucose, orBP to an osmotic loading or bleeding. However, AV3V infusion with aminoglutethimide, a NS synthesis inhibitor, did not affect any of the variables in the absence or presence of those stimuli. These results suggest that NS may not cause acute effects on the AV3V GABAergic mechanism involved in regulating AVP release and other autonomic function.
Novel receptor targets for production and action of allopregnanolone in the central nervous system: a focus on pregnane xenobiotic receptor
Cheryl A. Frye, Carolyn J. Koonce, and Alicia A. Walf
Front in Cell Neurosci Apr 2014; 8(106)
http://dx.doi.org:/10.3389/fncel.2014.00106
Neurosteroids are cholesterol-based hormones that can be produced in the brain, independent of secretion from peripheral endocrine glands, such as the gonads and adrenals. A focus in our laboratory for over 25 years has been how production of the pregnane neurosteroid, allopregnanolone, is regulated and the novel (i.e., non steroid receptor) targets for steroid action for behavior. One endpoint of interest has been lordosis, the mating posture of female rodents. Allopregnanolone is necessary and sufficient for lordosis, and the brain circuitry underlying it, such as actions in the midbrain ventral tegmental area (VTA), has been well-characterized. Published and recent findings supporting a dynamic role of allopregnanolone are included in this review. First, contributions of ovarian and adrenal sources of precursors of allopregnanolone, and the requisite enzymatic actions for de novo production in the central nervous system will be discussed.
Second, how allopregnanolone produced in the brain has actions on behavioral processes that are independent of binding to steroid receptors, but instead involve rapid modulatory actions via neurotransmitter targets (e.g., g-amino butyric acid-GABA, Nmethyl-D-aspartate- NMDA) will be reviewed.
Third, a recent focus on characterizing the role of a promiscuous nuclear receptor, pregnane xenobiotic receptor (PXR), involved in cholesterol metabolism and expressed in the VTA, as a target for allopregnanolone and how this relates to both actions and production of allopregnanolone will be addressed. For example, allopregnanolone can bind PXR and knocking down expression of PXR in the midbrain VTA attenuates actions of allopregnanolone via NMDA and/or GABAA for lordosis. Our understanding of allopregnanolone’s actions in the VTA for lordosis has been extended to reveal the role of allopregnanolone for broader, clinically-relevant questions, such as neurodevelopmental processes, neuropsychiatric disorders, epilepsy, and aging.
Long-term dysregulation of brain corticotrophin and glucocorticoid receptors and stress reactivity by single early-life pain experience in male and female rats
Nicole C. Victoria, Kiyoshi Inoue, Larry J. Young, Anne Z. Murphy
Psychoneuroendocrinology (2013) 38, 3015—3028
http://dx.doi.org/10.1016/j.psyneuen.2013.08.013
Inflammatory pain experienced on the day of birth (postnatal day 0: PD0) significantly dampens behavioral responses to stress- and anxiety-provoking stimuli in adult rats. However, to date, the mechanisms by which early life pain permanently alters adult stress responses remain unknown. The present studies examined the impact of inflammatory pain, experienced on the day of birth, on adult expression of receptors or proteins implicated in the activation and termination of the stress response, including corticotrophin releasing factor receptors (CRFR1 and CRFR2) and glucocorticoid receptor (GR). Using competitive receptor autoradiography, we show that Sprague Dawley male and female rat pups administered 1% carrageenan into the intraplantar surface of the hindpaw on the day of birth have significantly decreased CRFR1 binding in the basolateral amygdala and midbrain periaqueductal gray in adulthood. In contrast, CRFR2 binding, which is associated with stress termination, was significantly increased in the lateral septum and cortical amygdala. GR expression, measured with in situ hybridization and immunohistochemistry, was significantly increased in the paraventricular nucleus of the hypothalamus and significantly decreased in the hippocampus of neonatally injured adults. In parallel, acute stress-induced corticosterone release was significantly attenuated and returned to baseline more rapidly in adults injured on PD0 in comparison to controls. Collectively, these data show that early life pain alters neural circuits that regulate responses to and neuroendocrine recovery from stress, and suggest that pain experienced by infants in the Neonatal Intensive Care Unit may permanently alter future responses to anxiety- and stress provoking stimuli.
Dysruption of Corticotropin Releasing Factor in hypocampal region
Stress and trauma: BDNF control of dendritic-spine formation and regression
M.R. Bennett, J. Lagopoulos
Progress in Neurobiology 112 (2014) 80–99
http://dx.doi.org/10.1016/j.pneurobio.2013.10.005
Chronic restraint stress leads to increases in brain derived neurotrophic factor (BDNF) mRNA and protein in some regions of the brain, e.g. the basal lateral amygdala (BLA) but decreases in other regions such as the CA3 region of the hippocampus and dendritic spine density increases or decreases in line with these changes in BDNF. Given the powerful influence that BDNF has on dendritic spine growth, these observations suggest that the fundamental reason for the direction and extent of changes in dendritic spine density in a particular region of the brain under stress is due to the changes in BDNF there.
The most likely cause of these changes is provided by the stress initiated release of steroids, which readily enter neurons and alter gene expression, for example that of BDNF. Of particular interest is how glucocorticoids and mineralocorticoids tend to have opposite effects on BDNF gene expression offering the possibility that differences in the distribution of their receptors and of their downstream effects might provide a basis for the differential transcription of the BDNF genes. Alternatively, differences in the extent of methylation and acetylation in the epigenetic control of BDNF transcription are possible in different parts of the brain following stress.
Although present evidence points to changes in BDNF transcription being the major causal agent for the changes in spine density in different parts of the brain following stress, steroids have significant effects on downstream pathways from the TrkB receptor once it is acted upon by BDNF, including those that modulate the density of dendritic spines.
Finally, although glucocorticoids play a canonical role in determining BDNF modulation of dendritic spines, recent studies have shown a role for corticotrophin releasing factor (CRF) in this regard. There is considerable improvement in the extent of changes in spine size and density in rodents with forebrain specific knockout of CRF receptor 1 (CRFR1) even when the glucocorticoid pathways are left intact. It seems then that CRF does have a role to play in determining BDNF control of dendritic spines.
Central CRF system perturbation in an Alzheimer’s disease knockin mouse model
Qinxi Guo, Hui Zheng, Nicholas John Justice
Neurobiology of Aging 33 (2012) 2678–2691
http://dx.doi.org:/10.1016/j.neurobiolaging.2012.01.002
Alzheimer’s disease (AD) is often accompanied by changes in mood as well as increases in circulating cortisol levels, suggesting that regulation of the stress responsive hypothalamic-pituitary-adrenal (HPA) axis is disturbed. Here, we show that amyloid precursor protein (APP) is endogenously expressed in important limbic, hypothalamic, and midbrain nuclei that regulate hypothalamic-pituitary-adrenal axis activity. Furthermore, in a knockin mouse model of AD that expresses familial AD (FAD) mutations of both APP with humanized amyloid beta (hA), and presenilin 1 (PS1), in their endogenous patterns (APP/hA/PS1 animals), corticotropin releasing factor (CRF) levels are increased in key stress-related nuclei, resting corticosteroid levels are elevated, and animals display increased anxiety-related behavior. Endocrine and behavioral phenotypes can be normalized by loss of 1 copy of CRF receptor type-1 (Crfr1), consistent with a perturbation of central CRF signaling in APP/hA/PS1 animals. However, reductions in anxiety and corticosteroid levels conferred by heterozygosity of CRF receptor type-1 do not improve a deficit in working memory observed in APP/hA/PS1 mice, suggesting that perturbations of the CRF system are not the primary cause of decreased cognitive performance.
Alzheimer’s disease-like neuropathology of gene-targeted APP-SLxPS1mut mice expressing the amyloid precursor protein at endogenous levels
Christoph Kohler, Ulrich Ebert, Karlheinz Baumann, and Hannsjorg Schroeder
Neurobiology of Disease 20 (2005) 528 – 540
http://dx.doi.org:/10.1016/j.nbd.2005.04.009
Most transgenic mice used for preclinical evaluation of potential disease-modifying treatments of Alzheimer’s disease develop major histopathological features of this disease by several-fold overexpression of the human amyloid precursor protein. We studied the phenotype of three different strains of gene-targeted mice which express the amyloid precursor protein at endogenous levels. Only further crossing with transgenic mice overexpressing mutant human presenilin1 led to the deposition of extracellular amyloid, accompanied by the deposition of apolipoprotein E, an astrocyte and microglia reaction, and the occurrence of dilated cholinergic terminals in the cortex. Features of neurodegeneration, however, were absent. The pattern of plaque development and deposition in these mice was similar to that of amyloid precursor protein overproducing strains if crossed to presenilin1-transgenics. However, plaque development started much later and developed slowly until the age of 18 months but then increased more rapidly.
Central Cholinergic Functions In Human Amyloid Precursor Protein Knock-In/Presenilin-1 Transgenic Mice
Hartmann, C. Erb, U. Ebert, K. H. Baumann, A. Popp, G. Koenig, J. Klein
Neuroscience 125 (2004) 1009–1017
http://dx.doi.org:/10.1016/j.neuroscience.2004.02.038
Alzheimer’s disease is characterized by amyloid peptide formation and deposition, neurofibrillary tangles, central cholinergic dysfunction, and dementia; however, the relationship between these parameters is not well understood. We studied the effect of amyloid peptide formation and deposition on central cholinergic function in knock-in mice carrying the human amyloid precursor protein (APP) gene with the Swedish/London double mutation (APP-SL mice) which were crossbred with transgenic mice overexpressing normal (PS1wt) or mutated (M146L; PS1mut) human presenilin-1. APP-SLxPS1mut mice had increased levels of Aβ peptides at 10 months of age and amyloid plaques at 14 months of age while APP-SLPS1wt mice did not have increased peptide levels and did not develop amyloid plaques. We used microdialysis in 15–27 months old mice to compare hippocampal acetylcholine (ACh) levels in the two mouse lines and found that extracellular ACh levels were slightly but significantly reduced in the APP-SLPS1mut mice (-26%; P=0.044). Exploratory activity in the open field increased hippocampal ACh release by two-fold in both mouse lines; total and relative increases were not significantly different for the two strains under study. Similarly, infusion of scopolamine (1 µM) increased hippocampal ACh release to a similar extent (3–5-fold) in both groups. High-affinity choline uptake, a measure of the ACh turnover rate, was identical in both mouse lines. Neurons expressing choline acetyltransferase were increased in the septum of APP-SLPS1mut mice (26%; P =0.046). We conclude that amyloid peptide production causes a small decrease of extracellular ACh levels. The deposition of amyloid plaques, however, does not impair stimulated ACh release and proceeds without major changes of central cholinergic function.
Glutamate Neurotoxicity
Glutamate Neurotoxicity and Diseases of the Nervous System
Dennis W. Choi
Neuron. Oct, 1988; 1: 623-634
A growing number of studies now suggest that the cellular mechanisms which normally participate in signaling in the central nervous system (CNS) can be transformed by disease into instruments of neuronal cell destruction. Excitatory synaptic transmission in the mammalian CNS is principally mediated by L-glutamate. In fact, glutamate excites virtually all central neurons and is present in nerve terminals at millimolar levels (Curtis and Johnston, 1974). Normally, the extracellular levels of glutamate rise to high levels only in the brief and spatially localized fashion appropriate to synaptic transmission. This is fortunate, because as Lucas and Newhouse first showed in 1957, sustained exposure to glutamate can destroy retinal neurons. In a subsequent set of pioneering experiments, Olney (Olney and Sharpe, 1969; Olney et al., 1971) established that this toxicity, which he later called excitotoxicity, was not unique to glutamate or to retinal neurons, but was a feature common to the actions of all excitatory amino acids on central neurons. He postulated therefore that glutamate, or related compounds, might be the cause of the neuronal cell loss found in certain neurological diseases. In recent years, this hypothesis has gathered considerable support, fueled by new insights into glutamate receptor function and the development of effective glutamate antagonist drugs. The evidence is most convincing in diseases involving an acute insult to the brain, as occurs in a stroke, with abrupt deprivation of blood supply. But neurotoxicity due to excitatory amino acids may also be involved in slowly progressive degenerative diseases such as Huntington’s disease. Although the detailed molecular basis of glutamate neurotoxicity is not known, it appears that Ca2+ influx may play a critical role.
Glutamate interacts with at least three classes of membrane receptors, each commonly referred to by preferred pharmacological agonists: N-methyl-o-aspartate (NMDA), quisqualate, and kainate (Watkins and Olverman, 1987) (Figure I). These three classes are linked to membrane cation channels. A second type of quisqualate receptor has been additionally linked to a second messenger system (see below). It has been suggested that all three classes might actually be substates of a single molecular complex, but binding studies and newer physiological studies favor separate structures.
Quisqualate NMDA Kainate
Three Classes of Glutamate Receptors
Three Classes of Glutamate Receptors
One type of quisqualate receptor stimulates the formation of inositol 1,4,5-trisphosphate UPS) and diacylglycerol (DAG) from phosphatidylinositol-4,5-biphosphate (PIP,); the other is linked directly to a Na+ ionophore. Activation of the quisqualate receptor-ionophore complex can be potentiated by Zn2+. The NMDA receptor opens a channel permeable to Ca2+ as well as Na+; this receptor-channel complex has several modulatory sites discussed in the text. The kainate receptor opens an ionophore permeable to Na+.
Best defined is the NMDA receptor. This receptor opens a distinctive membrane channel characterized by high conductance (main state about 50 pS), voltage dependent Mgz+ blockade and permeability to both Ca2+ and Na+. The NMDA receptor can be selectively activated by several endogenous compounds, including L-aspartate, homocysteate, and quinolinate. Activation requires the coavailability of glycine in near micromolar concentrations. The action of glutamate at the NMDA receptor can be selectively antagonized: competitively by 2-amino-5-phosphonovalerate (APV) and 2-amino-5-phosphonoheptanoate (APH), or noncompetitively by drugs that bind to the phencyclidine site within the open channel (such as phencyclidine, MK-801, dextrorphan, or ketamine. The NMDA receptor-activated channel can also be blocked noncompetitively by Znz+, most likely at a site different from that which binds Mg2.
Although glutamate has high affinity for all three classes of postsynaptic receptors, it is not easy to demonstrate its neurotoxicity in vivo. Even when directly injected into brain, bypassing the blood-brain barrier, extremely high doses of glutamate are required to create lesions. Mangano & Schwartz found that they could infuse 0.5 crl/hr of a 300 mM glutamate solution into the hippocampus of a rat for 2 weeks without producing neuronal injury. This apparent low in vivo neurotoxic potency of glutamate may represent one reason why Olney’s “glutamate hypothesis” of neurological disease did not initially achieve a more widespread following. However, in fact, glutamate is a potent and rapidly acting neurotoxin; its neurotoxicity in vivo is likely masked by the efficiency of normal cellular uptake mechanisms in removing glutamate from the extracellular space. Glutamate neurotoxicity can be most directly studied in cell culture where bath exposure is not limited by cellular uptake.
The toxic changes produced by glutamate or related excitatory amino acids in vivo are of two sorts:
- acute swelling of neuronal dendrites and cell bodies and a
- more slowly evolving neuronal degeneration (Olney, 1986).
Axons and glia are relatively spared, although high levels of excitatory amino acids can produce some swelling of glia. A hallmark of excitatory amino acid neurotoxicity is its cellular selectivity, with distinctive patterns of neuronal loss produced by different excitatory amino acids and different routes of administration. For example, Nadler and co-workers (1978) found that intraventricular kainate preferentially destroys hippocampal CA3 neurons but spares dentate granule neurons. Different neuronal subpopulations
may differ in their intrinsic vulnerability to damage.
Possible Mechanisms Involved in Glutamate Neurotoxicity
How Ca*+ may mediate glutamate-induced neuronal degeneration. Glutamate acts on NMDA, non-NMDA, and “metabotropic” receptors (the quisqualate receptor linked to a second messenger system) to produce an increase in cytosolic free Ca*+. This cytosolic Ca *+, in concert with diacylglycerol liberated by the quisqualate-triggered second messenger system, activates protein kinase C, which acts via a number of mechanisms (primarily by altering membrane ion channels) to increase neuronal excitability and further increase cytosolic Ca*+. Elevated cytosolic Ca2+ then activates several enzymes capable of either directly or indirectly (through free radical formation) destroying cellular structure. Glutamate released from synaptic terminals or leaking nonspecifically from ruptured neurons contributes to additional injury propagation.
Glutamate Neurotoxicity in Perspective
The hypothesis that excitatory amino acids may specifically mediate pathological neuronal injury gives new form to this age-old enemy and raises the tantalizing possibility that current molecular and cellular insights into excitatory amino acid transmitter systems might be harnessed to develop an efficacious clinical therapy. Some points of attack are already apparent; others will likely be defined as the biology of excitatory amino acids continues to be unraveled. An intriguing area for investigation is the relationship between excitatory amino acid neurotoxicity and normal neuronal processes such as maturation, neurite outgrowth, and synaptic plasticity.
Glutamate Toxicity in a Neuronal Cell line Involves Inhibition of Cystine Transport Leading to Oxidative Stress
Timothy H. Murphy, M Miyamoto, A Sastre, R Schnaar and JT Coyle
Neuron 1989: 2: 1547-88.
Glutamate binds to both excitatory neurotransmitter binding sites and a W-dependent, quisqualate- and cystine-inhibited transport site on brain neurons. The neuroblastoma-primary retina hybrid cells (NWRE-105) are susceptible to glutamate-induced cytotoxicity. The Cl–dependent transport site to which glutamate and quisqualate (but not kainate or NMDA) bind has a higher affinity for cystine than for glutamate. Towering cystine concentrations in the cell culture medium results in cytotoxicity similar to that induced by glutamate addition in its morphology, kinetics, and CaZ+ dependence. Glutamate-induced cytotoxicity is directly proportional to its ability to inhibit cystine uptake. Exposure to glutamate (or lowered cystine) causes a decrease in glutathione levels and an accumulation of intracellular peroxides. Like NW-RE-105 cells, primary rat hippocampal neurons (but not glia) in culture degenerate in medium with lowered cystine concentration. Thus, glutamate-induced cytotoxicity in N18-RE-105 cells is due to inhibition of cystine uptake, resulting in lowered glutathione levels leading to oxidative stress and cell death.
Mechanism of glutamate-induced neurotoxicity in HT22 mouse hippocampal cells
Masayuki Fukui, Ji-Hoon Song, Jinyoung Choi, Hye Joung Choi, Bao Ting Zhu
European Journal of Pharmacology 617 (2009) 1–11
http://dx.doi.org:/10.1016/j.ejphar.2009.06.059
Glutamate is an endogenous excitatory neurotransmitter. At high concentrations, it is neurotoxic and contributes to the development of certain neurodegenerative diseases. There is considerable controversy in the literature with regard to whether glutamate-induced cell death in cultured HT22 cells (an immortalized mouse hippocampal cell line) is apoptosis, necrosis, or a new form of cell death. The present study focused on investigating the mechanism of glutamate-induced cell death. We found that glutamate induced, in a time dependent manner, both necrosis and apoptosis in HT22 cells. At relatively early time points (8–12 h), glutamate induced mostly necrosis, whereas at late time points (16–24 h), it induced mainly apoptosis. Glutamate-induced mitochondrial oxidative stress and dysfunction were crucial early events required for the induction of apoptosis through the release of the mitochondrial apoptosis-inducing factor (AIF), which catalyzed DNA fragmentation (an ATP-independent process). Glutamate-induced cell death proceeded independently of the Bcl-2 family proteins and caspase activation. The lack of caspase activation likely resulted from the lack of intracellular ATP when the mitochondrial functions were rapidly disrupted by the mitochondrial oxidative stress. In addition, it was observed that activation of JNK, p38, and ERK signaling molecules was also involved in the induction of apoptosis by glutamate. In conclusion, glutamate-induced apoptosis is AIF-dependent but caspase-independent, and is accompanied by DNA ladder formation but not chromatin condensation.
Understanding Low Reliability of Memories for Neutral Information Encoded under Stress: Alterations in Memory-Related Activation in the Hippocampus and Midbrain
Shaozheng Qin, EJ Hermans, HJF van Marle, and G Fernandez, et al.
The Journal of Neuroscience, Mar 21, 2012; 32(12): 4032–4041
http://dx.doi.org:/10.1523/JNEUROSCI.3101-11.2012
Exposure to an acute stressor can lead to unreliable remembrance of intrinsically neutral information, as exemplified by low reliability of eyewitness memories, which stands in contrast with enhanced memory for the stressful incident itself. Stress-sensitive neuromodulators (e.g., catecholamines) are believed to cause this low reliability by altering neurocognitive processes underlying memory formation. Using event-related functional magnetic resonance imaging, we investigated neural activity during memory formation in 44 young, healthy human participants while incidentally encoding emotionally neutral, complex scenes embedded in either a stressful or neutral context.
We recorded event-related pupil dilation responses as an indirect index of phasic noradrenergic activity. Autonomic, endocrine, and psychological measures were acquired to validate stress manipulation. Acute stress during encoding led to a more liberal response bias (more hits and false alarms) when testing memory for the scenes 24 h later. The strength of this bias correlated negatively with pupil dilation responses and positively with stress-induced heart rate increases at encoding. Acute stress, moreover, reduced subsequent memory effects (SMEs; items later remembered vs forgotten) in hippocampus and midbrain, and in pupil dilation responses.
The diminished SMEs indicate reduced selectivity and specificity in mnemonic processing during memory formation. This is in line with a model in which stress-induced catecholaminergic hyperactivation alters phasic neuromodulatory signaling in memory-related circuits, resulting in generalized (gist-based) processing at the cost of specificity. Thus, one may speculate that loss of specificity may yield less discrete memory representations at time of encoding, thereby causing a more liberal response bias when probing these memories.
Neuroendocrinology – Signaling, neuron plasticity and memory
Leptin Signaling Modulates the Activity of Urocortin 1 Neurons in the Mouse Nonpreganglionic Edinger-Westphal Nucleus
Lu Xu, Wim J. J. M. Scheenen, Rebecca L. Leshan, Christa M. Patterson, et al.
Endocrinology 152(3): 979–988, 2011
http://dx.doi.org:/10.1210/en.2010-1143
A recent study systematically characterized the distribution of the long form of the leptin receptor (LepRb) in the mouse brain and showed substantial LepRb mRNA expression in the nonpreganglionic Edinger-Westphal nucleus (npEW) in the rostroventral part of the midbrain. This nucleus hosts the majority of urocortin 1 (Ucn1) neurons in the rodent brain, and because Ucn1 is a potent satiety hormone and electrical lesioning of the npEW strongly decreases food intake, we have hypothesized a role of npEW-Ucn1 neurons in leptin-controlled food intake. Here, we show by immunohistochemistry that npEW-Ucn1 neurons in the mouse contain LepRb and respond to leptin administration with induction of the Janus kinase 2-signal transducer and activator of transcription 3 pathway, both in vivo and in vitro. Furthermore, systemic leptin administration increases the Ucn1 content of then pEW significantly, whereas in mice that lack LepRb (db/db mice), then pEW contains considerably reduced amount of Ucn1. Finally, we reveal by patch clamping of midbrain Ucn1 neurons that leptin administration reduces the electrical firing activity of the Ucn1 neurons. In conclusion, we provide ample evidence for leptin actions that go beyond leptin’s well-known targets in the hypothalamus and propose that leptin can directly influence the activity of the midbrain Ucn1 neurons.
Leptin regulation of hippocampal synaptic function in health and disease
Andrew J. Irving and Jenni Harvey
Trans. R. Soc. B 369: 20130155 http://dx.doi.org/10.1098/rstb.2013.0155
The endocrine hormone leptin plays a key role in regulating food intake and body weight via its actions in the hypothalamus. However, leptin receptors are highly expressed in many extra-hypothalamic brain regions and evidence is growing that leptin influences many central processes including cognition. Indeed, recent studies indicate that leptin is a potential cognitive enhancer as it markedly facilitates the cellular events underlying hippocampal-dependent learning and memory, including effects on glutamate receptor trafficking, neuronal morphology and activity-dependent synaptic plasticity. However, the ability of leptin to regulate hippocampal synaptic function markedly declines with age and aberrant leptin function has been linked to neurodegenerative disorders such as Alzheimer’s disease (AD). Here, we review the evidence supporting a cognitive enhancing role for the hormone leptin and discuss the therapeutic potential of using leptin-based agents to treat AD.
The Y2 receptor agonist PYY3–36 increases the behavioral response to novelty and acute dopaminergic drug challenge in mice
Ulrike Stadlbauer, Elisabeth Weber, Wolfgang Langhans and Urs Meyer
International Journal of Neuropsychopharmacology (2014), 17, 407–419
http://dx.doi.org:/10.1017/S1461145713001223
The gastrointestinal hormone PYY3–36 is a preferential Y2 neuropeptide Y (NPY) receptor agonist. Recent evidence indicates that PYY3–36 acts on central dopaminergic pathways, but its influence on dopamine-dependent behaviors remains largely unknown. We therefore explored the effects of peripheral PYY3–36 treatment on the behavioral responses to novelty and to dopamine-activating drugs in mice. In addition, we examined whether PYY3–36 administration may activate distinct dopamine and γ-aminobutyric acid (GABA) cell populations in the mesoaccumbal and nigrostriatal pathways. We found that i.p. PYY3–36 injection led to a dose-dependent increase in novel object exploration. The effective dose of PYY3–36 (1 μg/100 g body weight) also potentiated the locomotor reaction to the indirect dopamine receptor agonist amphetamine and increased stereotyped climbing/leaning responses following administration of the direct dopamine receptor agonist apomorphine. PYY3–36 administration did not affect activity of midbrain dopaminergic cells as evaluated by double immuno-enzyme staining of the neuronal early gene product c-Fos with tyrosine hydroxylase. PYY3–36 did, however, lead to a marked increase in the number of cells co-expressing c-Fos with glutamic acid decarboxylase in the nucleus accumbens and caudate putamen, indicating activation of GABAergic cells in dorsal and ventral striatal areas. Our results support the hypothesis that acute administration of the preferential Y2 receptor agonist PYY3–36 modulates dopamine-dependent behaviours. These effects do not seem to involve direct activation of midbrain dopamine cells but instead are associated with neuronal activation in the major input areas of the mesoaccumbal and nigrostriatal pathways.
Somatostatin and nociceptin inhibit neurons in the central nucleus of amygdala that project to the periaqueductal grey
Billy Chieng, MacDonald J. Christie
Neuropharmacology 59 (2010) 425e430
http://dx.doi.org:/10.1016/j.neuropharm.2010.06.001
The central nucleus of amygdala (CeA) plays an important role in modulation of the descending antinociceptive pathways. Using whole-cell patch clamp recordings from brain slices, we found that CeA neurons responded to the endogenous ligands somatostatin (SST) and nociceptin/orphanin FQ (OFQ) via an increased K-conductance. Co-application with selective antagonists suggested that SST and OFQ act on SSTR2 and ORL1 receptors, respectively. Taking account of anatomical localisation of recorded neurons, the present study showed that many responsive neurons were located within the medial subdivision of CeA and all CeA projection neurons to the midbrain periaqueductal grey invariably responded to these peptides. Randomly selected agonist-responsive neurons in CeA predominantly classified physiologically as low-threshold spiking neurons. The similarity of SST, OFQ and, as previously reported, opioid responsiveness in a sub-population of CeA neurons suggests converging roles of these peptides to inhibit the activity of projections from CeA to vlPAG, and potentially similar antinociceptive actions in this pathway.
In vitro identification and electrophysiological characterization of dopamine neurons in the ventral tegmental area
Tao A. Zhang, Andon N. Placzek, John A. Dani
Neuropharmacology 59 (2010) 431e436
http://dx.doi.org:/10.1016/j.neuropharm.2010.06.004
Dopamine (DA) neurons in the ventral tegmental area (VTA) have been implicated in brain mechanisms related to motivation, reward, and drug addiction. Successful identification of these neurons in vitro has historically depended upon the expression of a hyperpolarization-activated current (Ih) and immunohistochemical demonstration of the presence of tyrosine hydroxylase (TH), the rate-limiting enzyme for DA synthesis. Recent findings suggest that electrophysiological criteria may be insufficient for distinguishing DA neurons from non-DA neurons in the VTA. In this study, we sought to determine factors that could potentially account for the apparent discrepancies in the literature regarding DA neuron identification in the rodent brain slice preparation. We found that confirmed DA neurons from the lateral VTA generally displayed a larger amplitude Ih relative to DA neurons located in the medial VTA. Measurement of a large amplitude Ih (>100 pA) consistently indicated a dopaminergic phenotype, but non-dopamine neurons also can have Ih current. The data also showed that immunohistochemical TH labeling of DA neurons can render false negative results after relatively long duration (>15 min) wholecell patch clamp recordings. We conclude that whole-cell patch clamp recording in combination with immunohistochemical detection of TH expression can guarantee positive but not negative DA identification in the VTA.
Dopamine Enables In Vivo Synaptic Plasticity Associated with the Addictive Drug Nicotine
Jianrong Tang and John A. Dani
Neuron, Sept 10, 2009; 63, 673–682
http://dx.doi.org:/10.1016/j.neuron.2009.07.025
Addictive drugs induce a dopamine signal that contributes to the initiation of addiction, and the dopamine signal influences drug-associated memories that perpetuate drug use. The addiction process shares many commonalities with the synaptic plasticity mechanisms normally attributed to learning and memory. Environmental stimuli repeatedly linked to addictive drugs become learned associations, and those stimuli come to elicit memories or sensations that motivate continued drug use. Applying in vivo recording techniques to freely moving mice, we show that physiologically relevant concentrations of the addictive drug nicotine directly cause in vivo hippocampal synaptic potentiation of the kind that underlies learning and memory. The drug-induced long-term synaptic plasticity required a local hippocampal dopamine signal. Disrupting general dopamine signaling prevented the nicotine-induced synaptic plasticity and conditioned place preference. These results suggest that dopaminergic signaling serves as a functional label of salient events by enabling and scaling synaptic plasticity that underlies drug-induced associative memory.
NCS-1 in the Dentate Gyrus Promotes Exploration, Synaptic Plasticity, and Rapid Acquisition of Spatial Memory
Bechara J. Saab, John Georgiou, Arup Nath, Frank J.S. Lee, et al.
Neuron, Sept 10, 2009; 63, 643–656
http://dx.doi.org:/10.1016/j.neuron.2009.08.014
The molecular underpinnings of exploration and its link to learning and memory remain poorly understood. Here we show that inducible, modest overexpression of neuronal calcium sensor 1 (Ncs1) selectively in the adult murine dentate gyrus (DG) promotes a specific form of exploratory behavior. The mice also display a selective facilitation of longterm potentiation (LTP) in the medial perforant path and a selective enhancement in rapid-acquisition spatial memory, phenotypes that are reversed by direct application of a cell-permeant peptide (DNIP) designed to interfere with NCS-1 binding to the dopamine type-2 receptor (D2R). Moreover, the DNIP and the D2R-selective antagonist L-741,626 attenuated exploratory behavior, DG LTP, and spatial memory in control mice. These data demonstrate a role for NCS-1 and D2R in DG plasticity and provide insight for understanding how the DG contributes to the origin of exploration and spatial memory acquisition.
Neuroligin 2 Drives Postsynaptic Assembly at Perisomatic Inhibitory Synapses through Gephyrin and Collybistin
Alexandros Poulopoulos, Gayane Aramuni, Guido Meyer, Tolga Soykan, et al.
Neuron 63, 628–642, Sept 10, 2009
http://dx.doi.org:/10.1016/j.neuron.2009.08.023
In the mammalian CNS, each neuron typically receives thousands of synaptic inputs from diverse classes of neurons. Synaptic transmission to the postsynaptic neuron relies on localized and transmitter-specific differentiation of the plasma membrane with postsynaptic receptor, scaffolding, and adhesion proteins accumulating in precise apposition to presynaptic sites of transmitter release. We identified protein interactions of the synaptic adhesion molecule neuroligin 2 that drive postsynaptic differentiation at inhibitory synapses. Neuroligin 2 binds the scaffolding protein gephyrin through a conserved cytoplasmic motif and functions as a specific activator of collybistin, thus guiding membrane tethering of the inhibitory postsynaptic scaffold. Complexes of neuroligin 2, gephyrin and collybistin are sufficient for cell-autonomous clustering of inhibitory neurotransmitter receptors. Deletion of neuroligin 2 in mice perturbs GABAergic and glycinergic synaptic transmission and leads to a loss of postsynaptic specializations specifically at perisomatic inhibitory synapses.
A Subset of Ventral Tegmental Area Neurons is Inhibited by Dopamine, 5-Hydroxytryptamine and Opioids
L. Cameron, M. W. Wessendorf and J. T. Williams
Neuroscience 1997; 77(1), pp. 155–166 PII: S0306-4522(96)00444-7
Neurons originating in the ventral tegmental area are thought to play a key role in the formation of addictive behaviors, particularly in response to drugs such as cocaine and opioids. In this study we identified different populations of ventral tegmental area neurons by the pharmacology of their evoked synaptic potentials and their response to dopamine, 5-hydroxytryptamine and opioids. Intracellular recordings were made from ventral tegmental area neurons in horizontal slices of guinea-pig brain and electrical stimulation was used to evoke synaptic potentials. The majority of cells (61.3%) hyperpolarized in response to dopamine, depolarized to 5-hydroxytryptamine, failed to respond to [Met]5enkephalin and exhibited a slow GABAB-mediated inhibitory postsynaptic potential. A smaller proportion of cells (11.3%) hyperpolarized in response to [Met]5enkephalin, depolarized to 5-hydroxytryptamine, failed to respond to dopamine and did not exhibit a slow inhibitory postsynaptic potential. These two groups of cells corresponded to previously described ‘‘principal’’ and ‘‘secondary’’ cells, respectively. A further group of cells (27.4%) was identified that, like the principal cells, hyperpolarized to dopamine.
However, these ‘‘tertiary cells’’ also hyperpolarized to both 5-hydroxytryptamine and [Met]5enkephalin and exhibited a slow, cocaine-sensitive 5-hydroxytryptamine1A-mediated inhibitory postsynaptic potential. When principal and tertiary cells were investigated immuno-histochemically, 82% of the principal cells were positive for tyrosine hydroxylase compared
with only 29% of the tertiary cells. The 5-hydroxytryptamine innervation of both these cell types was investigated and a similar density of putative contacts was observed near the somata and dendrites in both groups. This latter finding suggests that the existence of a 5-hydroxytryptamine-mediated inhibitory postsynaptic potential in the tertiary cells may be determined by the selective expression of 5-hydroxytryptamine receptors, rather than the distribution or density of the 5-hydroxytryptamine innervation.
We conclude that tertiary cells are a distinct subset of ventral tegmental area neurons where cocaine and μ-opioids both mediate inhibition.
Dopamine reward circuitry: Two projection systems from the ventral midbrain to the nucleus accumbens–olfactory tubercle complex
Satoshi Ikemoto
Brain Research Reviews 56 (2007) 27–78
http://:dx.doi.org:/10.1016/j.brainresrev.2007.05.004
Anatomical and functional refinements of the meso-limbic dopamine system
of the rat are discussed. Present experiments suggest that dopaminergic neurons localized in the posteromedial ventral tegmental area (VTA) and central linear nucleus raphe selectively project to the ventromedial striatum (medial olfactory tubercle and medial nucleus accumbens shell), whereas
the anteromedial VTA has few if any projections to the ventral striatum,
and the lateral VTA largely projects to the ventrolateral striatum (accumbens
core, lateral shell and lateral tubercle). These findings complement the recent behavioral findings that cocaine and amphetamine are more rewarding when administered into the ventromedial striatum than into the ventrolateral striatum. Drugs such as nicotine and opiates are more rewarding when administered into the posterior VTA or the central linear nucleus than into
the anterior VTA. A review of the literature suggests that
(1) the midbrain has corresponding zones for the accumbens core and medial shell;
(2) the striatal portion of the olfactory tubercle is a ventral extension of the nucleus accumbens shell; and
(3) a model of two dopamine projection systems from the ventral midbrain to the ventral striatum is useful for understanding reward function.
The medial projection system is important in the regulation of arousal characterized by affect and drive and plays a different role in goal directed learning than the lateral projection system, as described in the variation–selection hypothesis of striatal functional organization.
Metabolic hormones, dopamine circuits, and feeding
Nandakumar S. Narayanan, Douglas J. Guarnieri, Ralph J. DiLeone
Frontiers in Neuroendocrinology 31 (2010) 104–112
http://dx.doi.org:/10.1016/j.yfrne.2009.10.004
Recent evidence has emerged demonstrating that metabolic hormones such as ghrelin and leptin can act on ventral tegmental area (VTA) midbrain dopamine neurons to influence feeding. The VTA is the origin of mesolimbic dopamine neurons that project to the nucleus accumbens (NAc) to influence behavior. While blockade of dopamine via systemic antagonists or targeted gene delete can impair food intake, local NAc dopamine manipulations have little effect on food intake. Notably, non-dopaminergic manipulations in the VTA and NAc produce more consistent effects on feeding and food choice. More recent genetic evidence supports a role for the substantia nigra-striatal dopamine pathways in food intake, while the VTA–NAc circuit is more likely involved in higher-order aspects of food acquisition, such as motivation and cue associations. This rich and complex literature should be considered in models of how peripheral hormones influence feeding behavior via action on the midbrain circuits.
Control of brain development and homeostasis by local and systemic insulin signaling
Liu, P. Speder & A. H. Brand
Diabetes, Obesity and Metabolism 16 (Suppl. 1): 16–20, 2014
Insulin and insulin-like growth factors (IGFs) are important regulators of growth and metabolism. In both vertebrates and invertebrates, insulin/IGFs are made available to various organs, including the brain, through two routes: the circulating systemic insulin/IGFs act on distant organs via endocrine signaling, whereas insulin/IGF ligands released by local tissues act in a paracrine or autocrine fashion. Although the mechanisms governing the secretion and action of systemic insulin/IGF have been the focus of extensive investigation, the significance of locally derived insulin/IGF has only more recently come to the fore. Local insulin/IGF signaling is particularly important for the development and homeostasis of the central nervous system, which is insulated from the systemic environment by the blood–brain barrier. Local insulin/IGF signaling from glial cells, the blood–brain barrier and the cerebrospinal fluid has emerged as a potent regulator of neurogenesis. This review will address the main sources of local insulin/IGF and how they affect neurogenesis during development. In addition, we describe how local insulin/IGF signaling couples neural stem cell proliferation with systemic energy state in Drosophila and in mammals.
Pharmacology, Physiology, and Mechanisms of Action of Dipeptidyl Peptidase-4 Inhibitors
Erin E. Mulvihill and Daniel J. Drucker
Endocrine Reviews 35: 992–1019, 2014
http://dx.doi.org/10.1210/er.2014-1035
Dipeptidyl peptidase-4 (DPP4) is a widely expressed enzyme transducing actions through an anchored transmembrane molecule and a soluble circulating protein. Both membrane-associated and soluble DPP4 exert
catalytic activity, cleaving proteins containing a position 2 alanine or proline. DPP4-mediated enzymatic cleavage alternatively inactivates peptides or generates new bioactive moieties that may exert competing or novel activities. The widespread use of selective DPP4 inhibitors for the treatment of type 2 diabetes has heightened interest in the molecular mechanisms through which DPP4 inhibitors exert their pleiotropic actions. Here we review the biology ofDPP4with a focus on:
1) identification of pharmacological vs physiological DPP4 substrates; and
2) elucidation of mechanisms of actions of DPP4 in studies employing genetic elimination or chemical reduction of DPP4 activity.
We review data identifying the roles of key DPP4 substrates in transducing the glucoregulatory, anti-inflammatory, and cardiometabolic actions of DPP4 inhibitors in both preclinical and clinical studies. Finally, we highlight experimental pitfalls and technical challenges encountered in studies designed to understand the mechanisms of action and downstream targets activated by inhibition of DPP4.
Dipeptidyl peptidase-4 (DPP4) is a multifunctional protein that exerts biological activity through pleiotropic actions including:
- protease activity (1),
- association with adenosine deaminase (ADA) (2),
- interaction with the extracellular matrix (3),
- cell surface coreceptor activity mediating viral entry (4), and
- regulation of intracellular signal transduction coupled to control of cell migration and proliferation (5).
The complexity of DPP4 action is amplified by the panoply of bioactive DPP4 substrates, which in turn act as elegant biochemical messengers in multiple tissues, including the immune and neuroendocrine systems.
DPP4 transmits signals across cell membranes and interacts with other membrane proteins (Figure). Remarkably, most of the protein is extracellular, including the C-terminal catalytic domain, a cysteine-rich area, and a large glycosylated region linked by a flexible stalk to the transmembrane segment. Only six N-terminal amino acids are predicted to extend into the cytoplasm. The active site, Ser 630, is flanked by the classic serine peptidase motif Gly-Trp-Ser630-Tyr-Gly-Gly-Tyr-Val.
Membrane-bound DPP4
Membrane-bound DPP4 contains residues 1–766, whereas sDPP4 contains residues 39–766. sDPP4 is lacking the cytoplasmic domain [residues 1–6], transmembrane domain [residues 7–28], and the flexible stalk [residues 29–39]. Both membrane-bound and circulating sDPP4 share many domains including the glycosylated region [residues 101–535, specific residues 85,92, 150], ADA binding domain [340–343], fibronectin binding domain [468–479], cysteine-rich domain [351–506, disulfide bonds are formed from 385–394, 444–472, and 649–762], and the catalytic domain [507–766 including residues composing the catalytic active site 630, 708, and 740].
DPP4 activity is subject to regulation at many levels, including control of gene and protein expression, interaction with binding partners, and modulation of enzyme activity. The DPP4 gene does not contain conventional TATAA or CCAAT promoter sequences but is characterized by a cytosine/guanine-rich promoter region.
DPP4 contains eight to 11 potential N-glycosylation sites, which can contribute to its folding and stability. Although glycosylation may contribute approximately 18–25% of the total molecular weight, mutational analysis has determined that the glycosylation sites are not required for dimerization, catalytic activity, or ADA binding. However, N-terminal sialylation facilitates trafficking of DPP4 to the apical membrane. Interestingly, molecular analysis of DPP4 isoforms isolated from the rat kidney brush border membrane reveals extensive heterogeneity in the oligosaccharides of DPP4.DPP4 was first investigated for its role in hydrolysis of dietary prolyl peptides (58); subsequent studies using DPP4 isolated using immunoaffinity chromatography and ADA binding identified DPP4 as the primary enzyme responsible for the generation of Gly-Prop-nitroanilide substrates in human serum. It is now known that DPP4 can cleave dozens of peptides, including chemokines, neuropeptides, and regulatory peptides, most containing a proline or alanine residue at position 2 of the amino-terminal region. Despite the preference for a position 2 proline, alternate residues (hydroxyproline, dehydroproline > alanine >, glycine, threonine, valine, or leucine) at the penultimate position are also cleaved by DPP4, suggesting a required stereochemistry. The DPP4 cleavage at postproline peptide bonds inactivates peptides and/or generates new bioactive peptides (see Figure), thereby regulating diverse biological processes.
DPP4 cleavage regulates substrate-receptor interactions
DPP4 cleavage regulates substrate/receptor interactions. A, DPP4 cleaves NPY [1–36] and PYY [1–36]. The intact forms of these peptides signal through Y1R-Y5R. After DPP4 cleavage, NPY [3–36] and PYY [3–36] are generated and preferentially signal through the Y2R and Y5R. B, DPP4 cleaves SP [1–11], which signals through the NK1R receptor to generate SP [5–11], which can signal through (NK1R, -2R, -3R).
GHRH and IGF-1
GHRH [1–44] and [1–40] are produced in the arcuate nucleus of the hypothalamus and bind its receptor on the anterior pituitary to stimulate the release of GH, and in turn, GH stimulates hepatic IGF-1 release. GHRH was among the first peptides to be identified as a DPP4 substrate; it is rapidly degraded in rodent and human plasma to GHRH [3–44]/GHRH [3–40], and this cleavage was blocked upon incubation of human plasma with the DPP4 inhibitor, diprotin A (99).GHRH[1–44] or [1–40] exhibits a very short half-life (6 min) andDPP4 cleavage was initially perceived to be a critical regulator of GHRH bioactivity and, in turn, the GH-IGF-1 axis. IGF-1, the downstream effector of GHRH and GH, is a 105-amino acid protein produced mainly by the liver.
IGF-1 was identified as a pharmacological DPP4 substrate by matrix-assisted laser desorption/ionization-time of flight analysis of molecular forms of IGF-1 generated after incubation with DPP4 purified from baculovirus-infected insect cells. However, studies in pigs treated with sitagliptin at doses inhibiting 90% of DPP4 activity failed to demonstrate an increase in active intact IGF-1.
Clinically, treatment of healthy human male subjects with sitagliptin (25–600 mg) for 10 days did not produce increased concentrations of serum IGF-1 or IGF-binding protein 3 as measured by ELISA. Furthermore, Dpp4/ mice or rats do not exhibit increased organ growth or body size. Hence, the available data suggest that although DPP4 cleaves and inactivates both GHRH and IGF-1, enzymatic inactivation by DPP4 is not the major mechanism regulating the bioactivity of the GHRH-IGF-1 axis.
The role of acute cortisol and DHEAS in predicting acute and chronic PTSD symptoms
Joanne Mouthaan, Marit Sijbrandij, Jan S.K. Luitse
Psychoneuroendocrinology (2014) 45, 179—186
http://dx.doi.org/10.1016/j.psyneuen.2014.04.001
Background: Decreased activation of the hypothalamus—pituitary—adrenal (HPA) axis in response to stress is suspected to be a vulnerability factor for posttraumatic stress disorder (PTSD). Previous studies showed inconsistent findings regarding the role of cortisol in predicting PTSD. In addition, no prospective studies have examined the role of dehydroepiandrosterone (DHEA), or its sulfate form DHEAS, and the cortisol-to-DHEA(S) ratio in predicting PTSD. In this study, we tested whether acute plasma cortisol, DHEAS and the cortisol-to-DHEAS ratio predicted PTSD symptoms at 6 weeks and 6 months post-trauma. Methods: Blood samples of 397 adult level-1 trauma center patients, taken at the trauma resuscitation room within hours after the injury, were analyzed for cortisol and DHEAS levels. PTSD symptoms were assessed at 6 weeks and 6 months post-trauma with the Clinician Administered PTSD Scale. Results: Multivariate linear regression analyses showed that lower cortisol predicted PTSD symptoms at both 6 weeks and 6 months, controlling for age, gender, time of blood sampling, injury, trauma history, and admission to intensive care. Higher DHEAS and a smaller cortisol-to-DHEAS ratio predicted PTSD symptoms at 6 weeks, but not after controlling for the same variables, and not at 6 months. Conclusions: Our study provides important new evidence on the crucial role of the HPA-axis in response to trauma by showing that acute cortisol and DHEAS levels predict PTSD symptoms in survivors of recent trauma.
Neurobiology of DHEA and effects on sexuality, mood and cognition
- Pluchino, P.Drakopoulos, F.Bianchi-Demicheli, J.M.Wenger
J Steroid Biochem & Molec Biol 145 (2015) 273–280
http://dx.doi.org/10.1016/j.jsbmb.2014.04.012
Dehydroepiandrosterone (DHEA) and its sulfate ester, DHEAS, are the most abundant steroid hormones in the humans. However, their physiological significance, their mechanisms of action and their possible roles as treatment are not fully clarified. Biological actions of DHEA(S) in the brain involve neuroprotection, neurite growth, neurogenesis and neuronal survival, apoptosis, catecholamine synthesis and secretion, as well as anti-oxidant, anti- inflammatory and antiglucocorticoid effects. In addition, DHEA affects neurosteroidogen is and endorphin synthesis/release. We also demonstrated in a model of ovariectomized rats that DHEA therapy increases proceptive behaviors, already after 1 week of treatment, affecting central function of sexual drive. In women, the analyses of clinical outcomes are far from being conclusive and many issues should still be addressed. Although DHEA preparations have been available in the market since the 1990s, there are very few definitive reports on the biological functions of this steroid. We demonstrate that 1 year DHEA administration at the dose of 10mg provided a significant improvement in comparison with vitamin D in sexual function
and in frequency of sexual intercourse in early postmenopausal women. Among symptomatic women, the spectrum of symptoms responding to DHEA requires further investigation, to define the type of sexual symptoms (e.g. decreased sexual function or hypoactive sexual desire disorder) and the degree of mood/cognitive symptoms that could be responsive to hormonal treatment.
In this regard, our findings are promising, although they need further exploration with a larger and more representative sample size.
Although adrenal cortex is considered to be the primary source of DHEAS in the brain, it was reported that DHEAS did not dis- appear or decrease in the brain 15 days neither after orchiectomy, adrenalectomy, or both, nor after the inhibition of adrenal secretion by dexamethasone. DHEA and DHEAS were among the first neurosteroids identified in rat brains. Cytochrome P450c17 was found in a subset of neurons of embryonic rodent brains. While P450c17 protein was readily detected in the brain, the abundance of P450c17 mRNA transcripts in the embryonic mouse brain or hippocampus of adult male rats was low, and was approximated to be 1/200th of the expression in testis.
DHEAS may be synthesized in the brain from DHEA. Sulfation of DHEA has been observed in the brains of rhesus monkeys in vivo and in human fetal brain slices in vitro. DHEA sulfotransferase (HSTor SULT2A1) is an enzyme that sulfonates DHEA (in addition to pregnenolone).Western blotting and immune-histochemistry showed protein expression of an HST in adult Wistar rat brain. In addition SULT2A1 mRNA expression has been shown in rat brains. DHEAS is predominately transported out of the brain across the blood–brain barrier and DHEAS found in the brain is most likely due to local synthesis . DHEA(S) may mediate some of its actions through conversion into more potent sex steroids and activation of androgen or estrogen receptors in tissue.
According to existing assumption of the biology of depression, DHEA(S) ability to modulate many neurobiological actions could underlie relationships between endogenous and/or exogenously- supplemented DHEA(S) concentrations and depression. There is evidence that DHEAS concentrations are negatively correlated with ratings of depressed mood. However, the remaining literature examining plasma and serum DHEA(S) concentrations in depression is still inconsistent and other plasma indexes were studied in order to more accurately discriminate depressed from nondepressed individuals. Hypothalamic–pituitary–adrenal axis (HPA) hyperactivity has
been demonstrated in chronic diseases affecting nervous system disorders like depression. The end products of HPA axis, glucocorticoids (GCs), regulate many physiological functions and play an important role in affective regulation and dysregulation. Despite DHEAS levels which markedly decrease throughout adulthood, an increase in circulating cortisol with advanced age has been observed in human and nonhuman primates.
The most relevant aspect meriting attention is certainly the controversial finding among the studies that investigate the correlation of the endogenous DHEA sulfate (DHEAS) level, the aging process or organ illness with the results coming from studies focusing on the effects of exogenous DHEAS administration on brain function, sexuality, cardiovascular health and metabolic syndrome. Indeed, the marked age-related decline in serum DHEA and DHEAS has suggested that a deficiency of these steroids may be causally related to the development of a series of diseases that are generally associ- ated with aging. The postulated consequences of low DHEA levels include insulin resistance, obesity, cardiovascular disease, cancer, reduction of the immune defense system as well as psychosocial problems such as depression and a general deterioration in the sensation of well-being and cognitive function, DHEA replacement may seem an attractive treatment opportunity. Nevertheless, the analyses of clinical outcomes are far from being conclusive.
Dehydroepiandrosterone, its metabolites and ion channels
Hill, M. Dusková, L. Stárka
J Steroid Biochem & Molec Biology 145(2015)293–314
http://dx.doi.org/10.1016/j.jsbmb.2014.05.006
This review is focused on the physiological and pathophysiological relevance of steroids influencing the activities of the central and peripheral nervous systems with regard to their concentrations in body fluids and tissues in various stages of human life like the fetal development or pregnancy. The data summarized in this review shows that DHEA and its unconjugated and sulfated metabolites are physiologically and pathophysiologically relevant in modulating numerous ion channels and participate in vital functions of the human organism. DHEA and its unconjugated and sulfated metabolites including 5 _/ _-reduced androstane steroids participate in various physiological and pathophysiological processes like the management of GnRH cyclic release, regulation of glandular and neurotransmitter secretions, maintenance of glucose homeostasis on one hand and insulin insensitivity on the other hand, control of skeletalmuscle and smooth muscle activities including vasoregulation, promotion of tolerance to ischemia and other neuroprotective effects. In respect of prevalence of steroid sulfates over unconjugated steroids in the periphery and the opposite situation in the CNS, the sulfated androgens and androgen metabolites reach relevance in peripheral organs. The unconjugated androgens and estrogens are relevant in periphery and so much the more in the CNS due to higher concentrations of most unconjugated steroids in the CNS tissues than in circulation and peripheral organs.
Neurotrophins are proteins found within a broad range of cell types in the brain and periphery that facilitate neuronal growth, survival, and plasticity. The neurotrophin ‘‘superfamily’’ includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3), neurotrophin-4/5 (NT4/5), and neurotrophin-6. Target tissues are hypothesized to regulate neuron survival by making neurotrophins available in limited amounts, resulting in selection of neurons with the best connectivity to the target tissue. NGF, in particular, is released by the target tissue and taken up in responsive neurons by receptor-mediated endocytosis. It is then transported retrogradedly into the cell where it exerts trophic effects. Lu et al. proposed a ‘‘Yin and Yang model,’’ whereby neurotrophic action is mediated by two principal classes of transmembrane receptor systems: the tyrosine kinase (Trk) receptors (including TrkA [selective for NGF], TrkB [selective for BDNF and NT4/5], and TrkC [selective for NT3]) and the neurotrophin receptor p75NTR. Each receptor type binds mature neurotrophins and/or neurotrophin precursors (proneurotrophins), creating a complex ‘‘balance’’ that then causes neuronal survival or death.
DHEA has been shown to evoke NGF mRNA expression in target cells. In a study of pregnant women, Schulte-Herbrüggen et al. showed no relationships between serum DHEAS and NGF. In contrast, we showed that DHEAS independently associated with salivary NGF (sNGF) in military men under baseline conditions, while DHEA did not. We now know that both DHEA(S) and NGF respond affirmatively to stressful insult, yet the association between these analytes during stress exposure is not understood. Characterization of this relationship has implications for prevention and treatment of traumatic stress and injury, degenerative disease management, and nerve repair. In this report, we extended our prior study of neuroprotective properties of DHEAS in men under baseline conditions to a prospective paradigm involving intense stress exposure in both men and women. We hypothesized that
(a) robust associations would prevail between total output of DHEAS and sNGF across the stress trajectory and at each time point,
(b) changes in DHEAS would predict corresponding changes in sNGF, and
(c) baseline DHEAS would positively predict total sNGF output across the stress trajectory.
We also explored the roles of testosterone and cortisol. In light of less definitive prior literature, directional hypotheses were not stated regarding these analytes.
In the first regression model, total hormone output (AUCG) of the independent variables (DHEAS, testosterone, and cortisol) combined to explain 63.7% of variance in sNGF output (F = 65.4, p < 0.001). Standardized beta coefficients revealed that testosterone exerted an independent effect (b = 0.80, p < 0.001), while the other predictors were not significant. In light of this unexpected finding, we then used regression-based causal steps modeling to evaluate whether testosterone mediated a hypothesized direct effect of DHEAS on sNGF. Following this approach, DHEAS predicted sNGF in an initial regression model (b = 0.45, p < 0.001). When testosterone was added, the direct effect of DHEAS (path c0) on sNGF was nearly eradicated and no longer significant (b = .04, p = .57), thus suggesting a mediated effect. An alternate statistical test (Sobel Test; 34) evaluating the hypothesized difference between the total effect (path c) and the direct effect (path c0) of DHEAS on sNGF produced a similar result (test statistic = 4.0, p < 0.001). Fig. 1 depicts positive association of DHEAS to sNGF, while Fig. 2 depicts Positive association of testosterone to sNGF.
Positive association of DHEAS total output and sNGF total output
Positive association of DHEAS total output and sNGF total output
Positive association of testosterone total output and sNGF total output
Positive association of testosterone total output and sNGF total output.
The models were then decomposed at each time point. At baseline, the independent variables (DHEAS, testosterone, and cortisol) combined to account for 10.2% of variance in sNGF (F = 5.3, p < 0.01). Standardized beta coefficients showed that DHEAS exerted an independent effect on sNGF (b = 0.39, p < 0.001), while the other predictors were not significant. During stress exposure, the independent variables combined to account for 28.0% of variance in NGF (F = 15.8, p < 0.001). Again, DHEAS exerted an independent effect (b = 0.56, p < 0.001) while the other predictors were not significant. During recovery, the predictor set accounted for 18.0% of variance in sNGF (F = 9.2, p < 0.001), and DHEAS exerted an independent effect (b = 0.47, p < 0.001) while the other predictors did not.
The models were then decomposed relative to each change index. In terms of reactivity, the independent variables (DHEAS, testosterone, and cortisol reactivity) and covariate (sex) combined to account for 20.3% of variance in sNGF reactivity (F = 8.2, p < 0.001). Standardized beta coefficients revealed that DHEAS reactivity exerted an independent effect (b = 0.39, p < 0.001), while the other predictors were not significant. In terms of recovery, the predictors combined to account for 28.2% of variance in sNGF recovery (F = 15.5, p < 0.001); DHEAS recovery exerted an independent effect (b = 0.52, p < 0.001), as did testosterone recovery (b = [1]0.27, p < 0.01). In terms of residual elevation/depression, the independent variables explained 12.4% of variance in sNGF residual elevation (F = 6.2, p < 0.001). DHEAS residual elevation exerted an independent effect (b = 0.35, p < 0.001), while the other predictors did not.
Endocrine-Disrupting Chemicals and Human Growth and Maturation: A Focus on Early Critical Windows of Exposure
Julie Fudvoye, Jean-Pierre Bourguignon, Anne-Simone Parent
Vitamins and Hormones, 2014; 94: Chapt 1. 1-25.
http://dx.doi.org/10.1016/B978-0-12-800095-3.00001-8
Endocrine-disrupting chemicals (EDCs) are exogenous substances that interfere with hormone synthesis, metabolism, or action. In addition, some of them could cause epigenetic alterations of DNA that can be transmitted to the following generations. Because the developing organism is highly dependent on sex steroids and thyroid hormones for its maturation, the fetus and the child are very sensitive to any alteration of their hormonal environment. An additional concern about that early period of life comes from the shaping of the homeostatic mechanisms that takes place also at that time with involvement of epigenetic mechanisms along with the concept of fetal origin of health and disease. In this chapter, we will review the studies reporting effects of EDCs on human development. Using a translational approach, we will review animal studies that can shed light on some mechanisms of action of EDCs on the developing organism. We will focus on the major hormone-dependent stages of development: fetal growth, sexual differentiation, puberty, brain development, and energy balance. We will also discuss the possible epigenetic effects of EDCs on human development.
Several studies have reported that prenatal or early postnatal exposure to some EDCs is associated with alterations of cognitive or motor functions in children. Knowing the fundamental role played by thyroid hormones and sex steroids in cortex development, one can hypothesize that disruption of those hormones could cause alteration of the development of the cerebral cortex and of its functions later in life. We will review here the human data suggesting a causal effect for endocrine disrupters on impairment of cortical functions and approach some EDC mechanisms of action using animal models.
Thyroid hormones are known to be essential for brain development. They regulate progenitor proliferation and differentiation, neuron migration, and dendrite outgrowth (Parent, Naveau, Gerard, Bourguignon, & Westbrook, 2011). Even mild thyroid hormone insufficiency in humans can produce measurable deficits in cognitive functions (Zoeller & Rovet, 2004). Thyroid hormone action is mediated by two classes of nuclear receptors (Forrest & Vennstro¨m, 2000) that exhibit differential spatial and temporal expressions in the brain, suggesting that thyroid hormones have variable functions during brain development. This differential expression of thyroid hormone receptors explains the critical period of thyroid hormone action on brain development as suggested by models of maternal hypothyroidism or congenital hypothyroidism.
Depending on the timing of onset of hypothyroidism, the offspring will display problems of visual attention, gross or fine motor skills, or language and memory skills. Similarly, one can hypothesize that disruption of thyroid function by EDCs will have different effects based on the timing of exposure. However, few studies focused on that aspect. Polychlorinated biphenyls (PCBs) form a group of widespread environmental contaminants composed of 209 different congeners used in a wide variety of applications. Their production was banned in the 1970s but PCBs are still present in the environment due to their high stability. PCBs were among the first EDCs identified as responsible for alterations of cognitive functions. Indeed, impaired memory and altered learning abilities have been associated with prenatal exposure to EDCs in humans and In animal models, perinatal exposure to PCBs has been consistently associated with a decrease of thyroid hormone concentration in maternal serum as well as pup serum. Some but not all epidemiological studies in human have found an association between PCB body burden and thyroid hormone levels. This disruption of thyroid function could explain some of the effects of PCBs on the developing brain. Indeed, animal models have shown that the ototoxic effects of PCBs could be partially ameliorated by thyroxin replacement and PCBs seem to alter some of the developmental processes in the cortex and the cerebellum that are dependent on thyroid hormones. However, recent publications raise important issues.
As it is the case for other EDCs, some windows of susceptibility have been identified during pre- and postnatal brain development. Recent studies have shown that exposure to PBDEs causes alteration of thyroid hormone levels in pregnant women and infants as it is the case in rodents. Only very few studies, however, have focused on the molecular or cellular effects of perinatal exposure to PBDEs in vivo. Viberg et al. have reported a decrease of cholinergic nicotinic receptors in the hippocampus after exposure to BDE-99 and BDE-153. However, the link between such a decrease and the behavioral effects of PBDEs is still unclear. Other teams have reported that exposure to PBDEs reduced hippocampal long term potentiation and decreased brain-derived neurotrophic factor expression in the brain. While several studies have reported negative effect of PBDEs on brain development and cognitive function in animals, there is relatively little information about adverse health effects of PBDEs in humans. Some very recent studies have identified a correlation between prenatal exposure to PBDEs and alteration of cognitive functions.
Endocrine-Disrupting Chemicals: Elucidating Our Understanding of Their Role in Sex and Gender-Relevant End Points
Cheryl A. Frye
Vitamins and Hormones, 2014; 94: 41-98
http://dx.doi.org/10.1016/B978-0-12-800095-3.00003-1
Endocrine-disrupting chemicals (EDCs) are diverse and pervasive and may have significant consequence for health, including reproductive development and expression of sex-/gender-sensitive parameters. This review chapter discusses what is known about common EDCs and their effects on reproductively relevant end points. It is proposed that one way that EDCs may exert such effects is by altering steroid levels (androgens or 17-estradiol, E2) and/or intracellular E2 receptors (ERs) in the hypothalamus and/or hippocampus. Basic research findings that demonstrate developmentally sensitive end points to androgens and E2 are provided. Furthermore, an approach is suggested to examine differences in EDCs that diverge in their actions at ERs to elucidate their role in sex-/gender-sensitive parameters.
Reproductive dysfunction among adults and emotional, attentional, and behavioral disorders among children are on the rise. Sperm counts and fertility have declined in the last 50 years . Incidence of attention-deficit hyperactivity disorder (ADHD) and autism has increased in the last 30 years. These increases in reproductive dysfunction and developmental disorders may be due to increased exposure to environmental contaminants, although there is controversy about the relationship between exposure and these effects.
Many contaminants in the environment, including polychlorinated biphenyls (PCBs), dioxins, and metals, accumulate in exposed individuals and may have adverse consequences due to effects as endocrine-disrupting chemicals (EDCs). EDCs may have effects by altering steroid levels (androgens or 17β-estradiol, E2) and/or intracellular E2 receptors (ERs) in the hypothalamus and/or hippocampus.
Steroid hormones, during critical periods of development, organize sexual dimorphisms in brain and behavior and give rise to sex differences in later responses to steroid hormones. EDCs can profoundly disrupt reproductive responses following adult exposure and result in pervasive effects that extend throughout the life of their offspring. Many nonreproductive behaviors, such
as spatial performance, activity, and arousal, are also sexually dimorphic and organized and activated by steroid hormones. Thus, EDCs may affect reproductive and the aforementioned nonreproductive parameters by altering E2 levels and/or ER binding in the hypothalamus and/or hippocampus.
Results from the literature and preliminary data will be presented that demonstrate our use of a whole-animal model to begin to investigate effects of exposure (in adulthood and/or development) to EDCs on steroid levels (androgens and E2), actions at ERs (in hypothalamus and hippocampus), and reproductive-sensitive measures (anogenital distance, accessory structure weight, onset of puberty and sexual maturity, and reproductive behavior) and nonreproductive behaviors (spatial performance, play behavior, and arousal) throughout development.
A common feature of many environmental contaminants is their estrogenic effects. Some contaminants can alter production of E2 and/or androgens or act as agonists or antagonists for intracellular or membrane ERs. Thus, the term “endocrine-disrupting chemicals” (EDCs) in this chapter is used to refer to contaminants with these effects. An important question considered here is the extent to which EDCs’ actions to alter E2 levels and/or ER binding in the hypothalamus or hippocampus mitigates effects on reproductive or nonreproductive processes. There are potential pervasive, negative effects of endocrine disrupters on steroid sensitive tissues, which may confer risk to disease states, such as cancer, heart disease, and neurodegenerative disorders. The following discussion provides evidence that exposure to EDCs during development may result in permanent, lifelong differences in sexual function and reproductive ability, as well as cognitive function and/or emotional reactivity/arousal. Gonad development, sex determination, and reproductive success of offspring are highly dependent on sex hormone systems. The developing organism is exquisitely sensitive to alterations in hormone function. In the early embryonic state, the gonads of human males and females are morphologically identical. Sexual differentiation begins under hormonal influence during the fifth and sixth weeks of fetal development, and thus, alterations in hormones during this highly sensitive period can have profound consequences. Disruption of the sex steroid system during fetal stages of life results in profound adverse developmental reproductive effects, as is well known from the effects of DES. The balance of estrogens and androgens is critical for normal development, growth, and functioning of the reproductive system. Although especially important during development, this balance is important throughout life for the preservation of normal feminine or masculine traits, as well as the expression of some sexually dimorphic behaviors (sex, spatial performance, and arousal).
| Proposed negative effects of exposure to endocrine disrupters during development in people and in animals. The focus here is on vulnerability to sexually dimorphic processes that are estrogen-sensitive, such as reproductive, cognitive, and emotional development and associated behavioral processes |
The existing data clearly indicate that developmental exposure to EDCs can adversely affect sexual development of people and animals; however, there are different effects depending upon the EDCs and when in development exposure occurs. Therefore, we consider the next effects of EDCs exposure at different point in development and the consequences for reproductive development and behavior, as well as E2 levels and hypothalamic ER binding.
Steroid hormones also play a critical role in neurodevelopment that influences not only reproductive but also nonreproductive behaviors that show sex differences. Specific behavioral differences in nonreproductive behaviors between males and females include differences in spatial learning, play, exploration, activity levels, novelty-seeking behavior, and emotional reactivity. These sex dimorphisms are thought to reflect adaptive differences for behavioral strategies in coping as a result of sexual selection. Moreover, these sexually dimorphic behaviors may be relevant for concerns regarding increased developmental, cognitive, or emotional disabilities over the past 30 years. Also, behaviors are particularly sensitive measures of effects of EDCs.
EDCs can alter cognitive development. Some, but not all, studies have shown a predictive relationship between prenatal PCB exposure and cognitive development in infancy through preschool years. EDCs have direct effects on nervous system function. Long-term potentiation (LTP), a form of synaptic plasticity used as a model system for study of cognitive potential, is altered by PCBs and lead. The protein kinase C (PKC)-signaling pathway is involved in the modulation of learning, memory, and motor behavior and may be a target of E2’s actions. PCBs also alter PKC signaling. Although findings provide evidence that EDCs can alter cognitive performance, these measures of cognition are neither sexually dimorphic nor E2- or ER-dependent.
There are sex-specific effects of perinatal PCB and dioxin exposure on spatial learning. Yu-Cheng boys that were prenatally exposed to high levels of PCBs and PCDFs when their mothers were accidentally exposed to these contaminants in rice oil show more disrupted cognitive development, mainly spatial function, than did exposed girls. In animal studies, spatial learning that favors males is mediated by perinatal exposure to androgens. Gestational and lactational exposure to ortho-substituted PCBs produces spatial deficits at adolescence in male mice or adulthood in male rats. The sparse data suggest that developmental exposure to EDCs disrupts spatial memory. Furthermore, Exposure during adulthood to EDCs can also have activational effects on spatial memory. Females exposed to a phytoestrogen-rich diet exhibit “masculinized” spatial performance in a radial arm maze, while males fed with a phytoestrogen-free diet show “feminized” performance.
An important question is what are the mechanisms by which developmental and/or adult exposure to EDCs alters spatial performance? There is evidence for sex differences in spatial performance and activational effects of E2 in adulthood to alter spatial performance of rats. Systemic or intrahippocampal administration of E2 improves spatial performance of female rats. Further, E2’s actions at intracellular ERs in the hippocampus of adults do not seem to be required to mediate these effects on spatial performance.
EDCs may have effects on E2 metabolism in a number of ways. First, some EDCs can alter serum lipid concentrations. Cholesterol is the precursor for the production of E2 and other steroid hormones (see Fig. 3.3). Second, there is also evidence that some EDCs can alter metabolism enzymes that are necessary for converting cholesterol to steroid hormones. Induction of CYP occurs when EDCs, such as TCDD, bind the aromatic hydrocarbon receptor (AhR). There is a firm link between PCBs, enzyme induction, and AhR. The binding of EDCs with AhR can result in antiestrogenic activity through increased metabolism and depletion of endogenous E2. Elevated levels of CYP enzymes, primarily expressed not only in the liver but also in the brain and other tissues, result in increased E2 metabolism and excretion. Alternatively, compounds that are metabolized by P450s may result in a net estrogenic effect if they inhibit endogenous estrogens from being metabolized.
Steroid hormones are lipid molecules with limited solubility in plasma and are accordingly carried through the plasma compartment to target cells by specific plasma transporter proteins. Each transporter protein has a specific ligand-binding domain for its associated hormone. It is generally accepted that the “free” formof the steroid hormone, and not the conjugate of the hormone with its plasma transport protein, enters target cells and binds with the appropriate receptor. Receptors for the steroid hormones are proteins located primarily in the cell nucleus or partitioned between the cytoplasm and the nucleus. The unoccupied steroid receptors may reside in the cell as heterodimeric complexes with the 90 kDa heat-shock protein, which prevents the receptor from binding with the DNA until the receptor has first bound with its steroid hormone. Once the hormone binds to the receptor, the hormone receptor complexes with the heterodimeric heat-shock protein and undergoes a conformational change and is activated. The activated receptor binds with DNA at a specific site, initiating gene transcription.
Traditional effects of steroid hormones at their cognate steroid receptors
Traditional effects of steroid hormones at their cognate steroid receptors, which act as transcription factors. In this example, effects of steroid hormones, such as estradiol, to bind estrogen receptor (ER) subtypes, referred to as ERa and ERb, are shown.
Beyond traditional actions solely through intracellular cognate estrogen receptors (ERs; ERa and ERb), steroids, such as estradiol, and estradiol-mimetics (endocrine disrupters) may have novel actions involving membrane bound ERs, other neurotransmitter systems (e.g., NMDA receptor), and signal transduction cascades (e.g., growth factors, MAPK).
To date, there has been little investigation in a whole-animal model of the effects of EDCs on E2 levels and/or activity at intracellular ERs in the brain. Thus, changes in E2 levels and ER activity in the hypothalamus and hippocampus, concomitant with alterations in endocrine parameters and reproductive behavior and nonreproductive behavior, respectively, are
needed to elucidate tissue specificity of EDCs’ functions and mechanisms.
Low-Dose Effects of Hormones and Endocrine Disruptors
Laura N. Vandenberg
Vitamins and Hormones, 2014; 94: 129-165
http://dx.doi.org/10.1016/B978-0-12-800095-3.00005-5
Endogenous hormones have effects on tissue morphology, cell physiology, and behaviors at low doses. In fact, hormones are known to circulate in the part-per-trillion and part-per-billion concentrations, making them highly effective and potent signaling molecules.
Many endocrine-disrupting chemicals (EDCs) mimic hormones, yet there is strong debate over whether these chemicals can also have effects at low doses. In the 1990s, scientists proposed the “low-dose hypothesis,” which postulated that EDCs affect humans and animals at environmentally relevant doses. This chapter focuses on data that support and refute the low-dose hypothesis. A case study examining the highly controversial example of bisphenol A and its low-dose effects on the prostate is examined through the lens of endocrinology. Finally, the chapter concludes with a discussion of factors that can influence the ability of a study to detect and interpret low-dose effects appropriately.
Since EDCs began to be studied in depth in the 1990s, there has been intense debate over whether the public should be concerned about low level exposures to these chemicals. The low-dose hypothesis, proposed at that time, has steadily accumulated evidence that EDCs have actions at low doses, and these effects are not necessarily predicted from high-dose toxicology testing. In 2002, the NTP expert panel reported evidence for low-dose effects for a small number of EDCs and estradiol. In 2012, an updated approach identified several dozen additional EDCs with evidence for low-dose effects. Further, epidemiology studies continue to find relationships between EDC exposure levels and diseases in the general public, which has raised concerns because the general public is exposed to a large number of environmental chemicals at low doses. For decades, hormones have been known to produce striking changes in tissue morphology, physiology, and behaviors at exceedingly low doses.
A relatively large body of evidence suggests that EDCs, and in particular those environmental chemicals that mimic endogenous hormones, have similar effects at low doses. Although there is still no consensus about the universality of “low-dose effects” in the toxicology community, the Endocrine Society (Diamanti-Kandarakis et al., 2009; Zoeller et al., 2012) believes not only that there is sufficient evidence in support of this phenomenon but also that it is time for public health agencies to make changes to risk assessment paradigms and give greater consideration to studies that specifically identify low-dose effects when considering risks from chemical exposures.
Bisphenol A interferes with synaptic remodeling
Tibor Hajszan, Csaba Leranth
Frontiers in Neuroendocrinology 31 (2010) 519–530
http://dx.doi.org:/10.1016/j.yfrne.2010.06.004
The potential adverse effects of Bisphenol A (BPA), a synthetic xenoestrogen, have long been debated. Although standard toxicology tests have revealed no harmful effects, recent research highlighted what was missed so far: BPA-induced alterations in the nervous system. Since 2004, our laboratory has been investigating one of the central effects of BPA, which is interference with gonadal steroid-induced synaptogenesis and the resulting loss of spine synapses. We have shown in both rats and nonhuman primates that BPA completely negates the ~70–100% increase in the number of hippocampal and prefrontal spine synapses induced by both estrogens and androgens. Synaptic loss of this magnitude may have significant consequences, potentially causing cognitive decline, depression, and schizophrenia, to mention those that our laboratory has shown to be associated with synaptic loss. Finally, we discuss why children may particularly be vulnerable to BPA, which represents future direction of research in our laboratory.
Bisphenol-A rapidly promotes dynamic changes in hippocampal dendritic morphology through estrogen receptor-mediated pathway by concomitant phosphorylation of NMDA receptor subunit NR2B
Xiaohong Xu ⁎, Yinping Ye, Tao Li, Lei Chen, Dong Tian, Qingqing Luo, Mei Lu
Toxicology and Applied Pharmacology 249 (2010) 188–196
http://dx.doi.org:/10.1016/j.taap.2010.09.007
Bisphenol-A (BPA) is known to be a potent endocrine disrupter. Evidence is emerging that estrogen exerts a rapid influence on hippocampal synaptic plasticity and the dendritic spine density, which requires activation of NMDA receptors. In the present study, we investigated the effects of BPA (ranging from 1 to 1000 nM), focusing on the rapid dynamic changes in dendritic filopodia and the expressions of estrogen receptor (ER) β and NMDA receptor, as well as the phosphorylation of NMDA receptor subunit NR2B in the cultured hippocampal neurons. A specific ER antagonist ICI 182,780 was used to examine the potential involvement of ERs. The results demonstrated that exposure to BPA (ranging from 10 to 1000 nM) for 30 min rapidly enhanced the motility and the density of dendritic filopodia in the cultured hippocampal neurons, as well as the phosphorylation of NR2B (pNR2B), though the expressions of NMDA receptor subunits NR1, NR2B, and ERβ were not changed. The antagonist of ERs completely inhibited the BPA-induced increases in the filopodial motility and the number of filopodia extending from dendrites. The increased pNR2B induced by BPA (100 nM) was also completely eliminated. Furthermore, BPA attenuated the effects of 17β-estradiol (17β-E2) on the dendritic filopodia outgrowth and the expression of pNR2B when BPA was co-treated with 17β-E2. The present results suggest that BPA, like 17β-E2, rapidly results in the enhanced motility and density of dendritic filopodia in the cultured hippocampal neurons with the concomitant activation of NMDA receptor subunit NR2B via an ER-mediated signaling pathway. Meanwhile, BPA suppressed the enhancement effects of 17β-E2 when it coexists with 17β-E2. These results provided important evidence suggesting the neurotoxicity of the low levels of BPA during the early postnatal development of the brain.
Bisphenol-A rapidly enhanced passive avoidance memory and phosphorylation of NMDA receptor subunits in hippocampus of young rats
Xiaohong Xu⁎, Tao Li, Qingqing Luo, Xing Hong, Lingdan Xie, Dong Tian
Toxicology and Applied Pharmacology 255 (2011) 221–228
http://dx.doi.org:/10.1016/j.taap.2011.06.022
Bisphenol-A (BPA), an endocrine disruptor, is found to influence development of brain and behaviors in rodents. The previous study indicated that perinatal exposure to BPA impaired learning-memory and inhibited N-methyl-D-aspartate receptor (NMDAR) subunits expressions in hippocampus during the postnatal development in rats; and in cultured hippocampal neurons, BPA rapidly promotes dynamic changes in dendritic morphology through estrogen receptor-mediated pathway by concomitant phosphorylation of NMDAR subunit NR2B. In the present study, we examined the rapid effect of BPA on passive avoidance memory and NMDAR in the developing hippocampus of Sprague–Dawley rats at the age of postnatal day 18. The results showed that BPA or estradiol benzoate (EB) rapidly extended the latency to step down from the platform 1 h after foot shock and increased the phosphorylation levels of NR1, NR2B, and mitogen-activated extracellular signal-regulated kinase (ERK) in hippocampus within 1 h. While 24 h after BPA or EB treatment, the improved memory and the increased phosphorylation levels of NR1, NR2B, ERK disappeared. Furthermore, pre-treatment with an estrogen receptors (ERs) antagonist, ICI182, 780, or an ERK-activating kinase inhibitor, U0126, significantly attenuated EB- or BPA-induced phosphorylations of NR1, NR2B, and ERK within 1 h. These data suggest that BPA rapidly enhanced short-term passive avoidance memory in the developing rats. A non-genomic effect via ERs may mediate the modulation of the phosphorylation of NMDAR subunits NR1 and NR2B through ERK signaling pathway.
Bisphenol A promotes dendritic morphogenesis of hippocampal neurons through estrogen receptor-mediated ERK1/2 signal pathway
Xiaohong Xu, Yang Lu, Guangxia Zhang, Lei Chen, Dong Tian, et al.
Chemosphere 96 (2014) 129–137
http://dx.doi.org/10.1016/j.chemosphere.2013.09.063
Bisphenol A (BPA), an environmental endocrine disruptor, has attracted increasing attention to its adverse effects on brain developmental process. The previous study indicated that BPA rapidly increased motility and density of dendritic filopodia and enhanced the phosphorylation of N-methyl-D-aspartate (NMDA) receptor subunit NR2B in cultured hippocampal neurons within 30 min. The purpose of the present study was further to investigate the effects of BPA for 24 h on dendritic morphogenesis and the underlying mechanisms. After cultured for 5 d in vitro, the hippocampal neurons from 24 h-old rat were infected by AdV-EGFP to indicate time-lapse imaging of living neurons. The results demonstrated that the exposure of the cultured hippocampal neurons to BPA (10, 100 nM) or 17β-estradiol (17β-E2, 10 nM) for 24 h significantly promoted dendritic development, as evidenced by the increased total length of dendrite and the enhanced motility and density of dendritic filopodia. However, these changes were suppressed by an ERs antagonist, ICI182,780, a non-competitive NMDA receptor antagonist, MK-801, and a mitogen activated ERK1/2-activating kinase (MEK1/2) inhibitor, U0126. Meanwhile, the increased F-actin (filamentous actin) induced by BPA (100 nM) was also completely eliminated by these blockers. Furthermore, the result of western blot analyses showed that, the exposure of the cultures to BPA or 17β-E2 for 24 h promoted the expression of Rac1/Cdc42 but inhibited that of RhoA, suggesting Rac1 (Ras related C3 botulinum toxinsubstrate 1)/Cdc42 (cell divisioncycle 42) and RhoA (Ras homologous A), the Rho family of small GTPases, were involved in BPA- or 17β-E2-induced changes in the dendritic morphogenesis of neurons. These BPA- or 17b-E2-induced effects were completely blocked by ICI182,780, and were partially suppressed by U0126. These results reveal that, similar to 17β-E2, BPA exerts its effects on dendritic morphogenesis by eliciting both nuclear actions and extranuclear-initiated actions that are integrated to influence the development of dendrite in hippocampal neurons.
Tyreoliberin (Trh) – The Regulatory Neuropeptide Of Cns Homeostasis
Danuta Jantas
Advances In Cell Biology 2;(4)/2010 (139–154)
http://dx.doi.org:/10.2478/v10052-010-0008-4
The physiological role of thyreoliberin (TRH) is the preservation of homeostasis within four systems
(i) the hypothalamic-hypophsysiotropic neuroendocrine system,
(ii) the brain stem/midbrain/spinal cord system,
(iii) the limbic/cortical system, and
(iv) the chronobiological system.
Thus TRH, via various cellular mechanisms, regulates a wide range of biological processes (arousal, sleep, learning, locomotive activity, mood) and possesses the potential for unique and widespread applications for treatment of human illnesses. Since the therapeutic potential of TRH is limited by its pharmacological profile (enzymatic instability, short half-life, undesirable effects), several synthetic analogues of TRH were constructed and studied in mono- or adjunct therapy of central nervous system (CNS) disturbances. The present article summarizes the current state of understanding of the physiological role of TRH and describes its putative role in clinical indications in CNS maladies with a focus on the action of TRH analogues.
Breakthrough in neuroendocrinology by discovering novel neuropeptides and neurosteroids: 2. Discovery of neurosteroids and pineal neurosteroids
Kazuyoshi Tsutsui, Shogo Haraguchi
General and Comparative Endocrinology 205 (2014) 11–22
http://dx.doi.org/10.1016/j.ygcen.2014.03.008
Bargmann–Scharrer’s discovery of ‘‘neurosecretion’’ in the first half of the 20th century has since matured into the scientific discipline of neuroendocrinology. Identification of novel neurohormones, such as neuropeptides and neurosteroids, is essential for the progress of neuroendocrinology. Our studies over the past two decades have significantly broadened the horizons of this field of research by identifying novel neuropeptides and neurosteroids in vertebrates that have opened new lines of scientific investigation in neuroendocrinology. We have established de novo synthesis and functions of neurosteroids in the brain of various vertebrates. Recently, we discovered 7α-hydroxypregnenolone (7α-OH PREG), a novel bioactive neurosteroid that acts as a key regulator for inducing locomotor behavior by means of the dopaminergic system. We further discovered that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol (CHOL). The pineal gland secretes 7α-OH PREG and 3α,5α-tetrahydroprogesterone (3α,5α-THP; allopregnanolone) that are involved in locomotor rhythms and neuronal survival, respectively. Subsequently, we have demonstrated their mode of action and functional significance. This review summarizes the discovery of these novel neurosteroids and its contribution to the progress of neuroendocrinology.
Mechanisms of crosstalk between endocrine systems: Regulation of sex steroid hormone synthesis and action by thyroid hormones
Paula Duarte-Guterman, Laia Navarro-Martín, Vance L. Trudeau
General and Comparative Endocrinology 203 (2014) 69–85
http://dx.doi.org/10.1016/j.ygcen.2014.03.015
Thyroid hormones (THs) are well-known regulators of development and metabolism in vertebrates. There is increasing evidence that THs are also involved in gonadal differentiation and reproductive function. Changes in TH status affect sex ratios in developing fish and frogs and reproduction (e.g., fertility), hormone levels, and gonad morphology in adults of species of different vertebrates. In this review, we have summarized and compared the evidence for cross-talk between the steroid hormone and thyroid axes and present a comparative model. We gave special attention to TH regulation of sex steroid synthesis and action in both the brain and gonad, since these are important for gonad development and brain sexual differentiation and have been studied in many species. We also reviewed research showing that there is a TH system, including receptors and enzymes, in the brains and gonads in developing and adult vertebrates. Our analysis shows that THs influences sex steroid hormone synthesis in vertebrates, ranging from fish to pigs. This concept of crosstalk and conserved hormone interaction has implications for our understanding of the role of THs in reproduction, and how these processes may be dysregulated by environmental endocrine disruptors.
Insights into the structure of class B GPCRs
Kaspar Hollenstein, Chris de Graaf, Andrea Bortolato, Ming-Wei Wang, et al.
Trends in Pharmacological Sciences, Jan 2014; 35(1)
http://dx.doi.org/10.1016/j.tips.2013.11.001
The secretin-like (class B) family of G protein-coupled receptors (GPCRs) are key players in hormonal homeostasis and are interesting drug targets for the treatment of several metabolic disorders (such as type 2 diabetes, osteoporosis, and obesity) and nervous system diseases (such as migraine, anxiety, and depression). The recently solved crystal structures of the transmembrane domains of the human glucagon receptor and human corticotropin-releasing factor receptor 1 have opened up new opportunities to study the structure and function of class B GPCRs. The current review shows how these structures offer more detailed explanations to previous biochemical and pharmacological studies of class B GPCRs, and provides new insights into their interactions with ligands.
Class B G protein-coupled receptors (GPCRs), also referred to as the secretin family of GPCRs, include receptors for 15 peptide hormones, which can be grouped into five subfamilies based on their physiological role (see Table 1 for an overview) [1]. These receptors are important drug targets in many human diseases, including diabetes, osteoporosis, obesity, cancer, neurodegeneration, cardiovascular disease, headache, and psychiatric disorders. However, the identification of small-molecule oral drugs for this family has proved extremely challenging.
(A,B) Crystal structures of the class B G protein-coupled receptors corticotropin-releasing factor receptor 1 (CRF1) [Protein Data Bank (PDB) identifier: 4K5Y] and glucagon receptor (PDB identifier: 4L6R) are shown in blue and orange ribbons, respectively, in two different views from within the membrane. Transmembrane (TM) helices and helix 8 are labelled. The disulfide bond tethering extracellular loop 2 (ECL2) to the tip of TM3 is shown as purple sticks. In CRF1 the small-molecule antagonist CP-376395 is shown in stick representation with carbon, nitrogen, and oxygen atoms colored magenta, blue, and red, respectively, and as skeletal formula in an inset. (C) Superposition of the two structures, with insets highlighting regions of particular interest. To highlight the structural differences in the extracellular halves of CRF1 and glucagon receptor, the distance of approximately 11 A° between the Ca-atoms of residues 7.33b at the N-terminal end of TM7 is indicated with a red arrow. The small molecule binding pocket is shown as a superposition of the two receptors around CP-376395, illustrating the antagonist binding mode and the substantial structural differences observed for TM6 of the two receptors.
- Overview of NMR and crystal structures of class B G protein-coupled
receptor (GPCR) extracellular domains (ECDs; magenta) and their complexes with peptide ligands (different colors). A complete overview of corresponding Protein Data Bank identifiers is presented in Table 1 (not shown). (B) Structure-based sequence alignment of representative peptide ligands of class B GPCR, adopted from Parthier et al. [6]. The residues of the peptide ligands solved in ECD–ligand complex crystal structures are marked using the same colour as in Figure 2A. The residues that are boxed black are found in an α-helical conformation in the complex. Peptide ligand residues that covalently bind receptors in photo-crosslinking or cysteine-trapping studies are colored and boxed green, whereas peptide ligand residues that have been mutated and studied in combination with receptor mutants are colored and boxed red. Note that the first residue of glucagon-like peptide-1 (GLP-1) is His7. A complete overview of all ECD structures and important peptide ligands for all class B GPCRs is presented in Table 1. Putative helix-capping residues [6] are coloured blue and cysteines involved in a disulfide-bridge (calcitonin) are coloured orange. D-phenylalanine (f), and norleucine (m) residues are indicated in stressin and astressin. The last 41 and 99 residues of parathyroid hormone (PTH) and PTH-related protein. (PTHrP), respectively, are not displayed. Abbreviations: CGRP, calcitonin gene-related peptide; CLR, calcitonin receptor-like receptor; CRF, corticotropin-releasing factor; CT, calcitonin; Ext-4, exendin-4; GHRHR, growth hormone releasing hormone receptor; GIP, glucose-dependent insulinotropic peptide; PAC, pituitary adenylate cyclase; PACAP, pituitary adenylate cyclase activating polypeptide; RAMP, receptor-activity modifying proteins; SCTR, secretin receptor; Ucn, urocortin; VPAC, vasoactive pituitary adenylate cyclase.
Figure 3. (not shown) (A) The spatial correspondence between residues in transmembrane (TM) helices of class A and class B G protein-coupled receptors (GPCRs) makes it possible to align the most conserved residues in class A (designated X.50, Ballesteros–Weinstein numbering) and class B (designated X.50b, Wootten numbering) for comparisons between GPCR classes (Box 1). (B) Structural alignment of corticotropin-releasing factor receptor 1 (CRF1; blue) and glucagon receptor (GCGR; orange) to two representative class A GPCRs, histamine H1 receptor (H1R; Protein Data Bank identifier: 3RZE) and CXC-chemokine receptor 4 (CXCR4; Protein Data Bank identifier: 3ODU/3OE0) (in grey). Helices are depicted as cylinders, and the ligands glucagon (for GCGR), CP-376395 (for CRF1), doxepin (for H1R), and IT1t and CVX15 (for CXCR4) are shown as sticks. The
location of the Ca-atoms of the most conserved residues of TM1–3 and TM5 among class A and class B GPCRs (Box 1) are indicated by spheres (TM4 is not depicted for clarity).
The GCGR and CRF1 crystal structures show distinct structural features and different binding pockets compared to class A GPCRs, and give new insights into the molecular details of peptide and small-molecule binding to class B GPCRs. The first two crystal structures of the TM domains of class B GPCRs provide a structural framework that will enable the design of biochemical and biophysical experiments detailing the complex structure of this class of receptors, and facilitate the design of stabilized constructs that might lead to the solution of full-length class B GPCR–ligand complexes. The structures furthermore present more reliable structural templates for the design of specific and potent small molecules for the treatment of type 2 diabetes (GCGR) and depression (CRF1) in particular, and open new avenues for structure-based small-molecule drug discovery for class B GPCRs as a whole.
Novel receptor targets for production and action of allopregnanolone in the central nervous system: a focus on pregnane xenobiotic receptor
Cheryl A. Frye, Carolyn J. Koonce and Alicia A. Walf
Front in Cell Neurosci Apr 2014; 8(106): 1-13.
http://dx.doi.org:/10.3389/fncel.2014.00106
Neurosteroids are cholesterol-based hormones that can be produced in the brain,
independent of secretion from peripheral endocrine glands, such as the gonads and
adrenals. A focus in our laboratory for over 25 years has been how production of the
pregnane neurosteroid, allopregnanolone, is regulated and the novel (i.e., non steroid
receptor) targets for steroid action for behavior. One endpoint of interest has been lordosis, the mating posture of female rodents. Allopregnanolone is necessary and sufficient for lordosis, and the brain circuitry underlying it, such as actions in the midbrain ventral tegmental area (VTA), has been well-characterized. Published and recent findings supporting a dynamic role of allopregnanolone are included in this review.
First, contributions of ovarian and adrenal sources of precursors of allopregnanolone, and the requisite enzymatic actions for de novo production in the central nervous system will be discussed.
Second, how allopregnanolone produced in the brain has actions on behavioral processes that are independent of binding to steroid receptors, but instead involve rapid modulatory actions via neurotransmitter targets (e.g., g-amino butyric acid-GABA, N methyl-D-aspartate- NMDA) will be reviewed.
Third, a recent focus on characterizing the role of a promiscuous nuclear receptor, pregnane xenobiotic receptor (PXR), involved in cholesterol metabolism and expressed in the VTA, as a target for allopregnanolone and how this relates to both actions and production of allopregnanolone will be addressed. For example, allopregnanolone can bind PXR and knocking down expression of PXR in the midbrain VTA attenuates actions of allopregnanolone via NMDA and/or GABAA for lordosis. Our understanding of allopregnanolone’s actions in the VTA for lordosis has been extended to reveal the role of allopregnanolone for broader, clinically-relevant questions, such as neurodevelopmental processes, neuropsychiatric disorders, epilepsy, and aging.
Genetically Encoded Chemical Probes in Cells Reveal the Binding Path of Urocortin-I to CRF Class B GPCR
Irene Coin, Vsevolod Katritch, Tingting Sun, Zheng Xiang, Fai Yiu Siu
Cell Dec 2013; 155, 1258–1269
http://dx.doi.org/10.1016/j.cell.2013.11.008
Molecular determinants regulating the activation of class B G-protein-coupled receptors (GPCRs) by native peptide agonists are largely unknown. We have investigated here the interaction between the corticotropin releasing factor receptor type 1 (CRF1R) and its native 40-mer peptide ligand Urocortin- I directly in mammalian cells. By incorporating unnatural amino acid photochemical and new click chemical probes into the intact receptor expressed in the native membrane of live cells, 44 intermolecular spatial constraints have been derived for the ligand-receptor interaction. The data were analyzed in the context of the recently resolved crystal structure of
CRF1R transmembrane domain and existing extracellular domain structures, yielding a complete conformational model for the peptide-receptor complex. Structural features of the receptor-ligand complex yield molecular insights
on the mechanism of receptor activation and the basis for discrimination between agonist and antagonist function.
Investigation of GPCR-Ligand Interactions under Native Conditions Using Genetically Encoded Chemical Probes GPCRs are integral membrane proteins containing multiple domains and various posttranslational modifications. To understand GPCR-ligand interactions by crystallography, receptors have to be extracted from the cell membrane and modified with a series of expedients such as deglycosylation, therm-stabilizing mutations, fusions with soluble proteins, or complexes with stabilizing nanobodies. We present here a method to investigate GPCR-ligand interactions at the intact fully posttranslationally modified receptor bound to its WT ligand on the membrane of the live cell, which mimics the native conditions for GPCR function. We first genetically incorporated into the receptor the photocrosslinking Uaa Azi, which served as
a proximity probe to provide an overall map of the ligand binding sites on the receptor. We then determined the relative position of the ligand in the binding pocket using a residue-specific chemical crosslinking reaction between Ffact genetically incorporated into the receptor and Cys introduced into the ligand. The derived intermolecular spatial constraints served eventually to build a detailed conformational model for the receptor-ligand complex.
Glutamate Neurons within the Midbrain Dopamine Regions
- Morales and D. H. Root
Neuroscience 282 (2014) 60–68
http://dx.doi.org/10.1016/j.neuroscience.2014.05.032
Midbrain dopamine (DA) neurons are hypothesized to play roles in reward-based behavior and addiction, reward prediction and learning by error detection, effort-based decision making, flexible reward-directed behaviors,
incentive salience, stimulus salience (e.g., prediction of rewarding and aversive events), aversion, depression, and fear. The extensive, divergent behavioral roles of midbrain dopamine neurons, predominantly from the ventral tegmental area (VTA), indicate that this system is highly heterogeneous.
This heterogeneity may be reflected in part by the diverse phenotypic characteristics among DAergic neurons and their interactive brain structures.
Midbrain dopamine systems play important roles in Parkinson’s disease, schizophrenia, addiction, and depression. The participation of midbrain dopamine systems in diverse clinical contexts suggests these systems are highly complex. Midbrain dopamine regions contain at least three neuronal phenotypes: dopaminergic, GABAergic, and glutamatergic. Here, we review the locations, subtypes, and functions of glutamatergic neurons within midbrain dopamine regions. Vesicular glutamate transporter 2 (VGluT2) mRNA-expressing neurons are observed within each midbrain dopamine system. Within rat retrorubral field (RRF), large populations of VGluT2 neurons are observed throughout its anteroposterior extent. Within rat substantia nigra pars compacta (SNC), VGluT2 neurons are observed centrally and caudally, and are most dense within the laterodorsal subdivision. RRF and SNC rat VGluT2 neurons lack tyrosine hydroxylase (TH), making them an entirely distinct population of neurons from dopaminergic neurons. The rat ventral tegmental area (VTA) contains the most heterogeneous populations of VGluT2 neurons. VGluT2 neurons are found in each VTA subnucleus but are most dense within the anterior midline subnuclei. Some subpopulations of rat VGluT2 neurons co-express TH or glutamic acid decarboxylase (GAD), but most of the VGluT2 neurons lack TH or GAD. Different subsets of rat VGluT2-TH neurons exist based on the presence or absence of vesicular monoamine transporter 2, dopamine transporter, or D2 dopamine receptor. Thus, the capacity by which VGluT2-TH neurons may release dopamine will differ based on their capacity to accumulate vesicular dopamine, uptake extracellular dopamine, or be autoregulated by dopamine. Rat VTA VGluT2 neurons exhibit intrinsic VTA projections and extrinsic projections to the accumbens and to the prefrontal cortex. Mouse VTA VGluT2 neurons project to accumbens shell, prefrontal cortex, ventral pallidum, amygdala, and lateral habenula. Given their molecular diversity and participation in circuits involved in addiction, we hypothesize that individual VGluT2 subpopulations of neurons play unique roles in addiction and other disorders. This article is part of a Special Issue entitled: Ventral Tegmentum & Dopamine. Published by Elsevier Ltd. On behalf of IBRO.
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