Reporter: Howard Donohue, PhD (EAW)
The hypothalamic-pituitary-adrenal (HPA) axis – which can be thought of as a series of closely linked endocrine structures in the brain – has a key role in triggering the body’s stress response through the secretion of cortisol. In explaining how the HPA axis is itself regulated, for example how its activity is increased in response to a perceived environmental threat, we can infer that the diverse brain areas with which it shares neural interconnections have a crucial role (for a review, see [1]). An equally important question relates to how the activity of the HPA axis is returned to normal when the stress response is no longer needed. To answer this, it is well known that the same “neurosteroid” hormones released by the HPA axis that trigger stress-related biological adaptations also serve to dampen its activity through a “negative feedback” mechanism. In re-defining the biological model of how neurosteroids control the HPA axis, a study led by Jamie Maguire, PhD at Tufts University (Boston, MA) provides some fascinating insights [2]. Moreover, this study has some extremely interesting and counter-intuitive implications for understanding the functions of the “inhibitory” brain chemical gamma-aminobutyric acid (GABA), which is best known for opposing the effects of “excitatory” brain chemicals in order to balance the flow of electrical activity in the brain.
To study how the HPA axis is regulated by neurosteroids, Maguire’s team performed investigations in mice using the neurosteroid tetrahydrodeoxycorticosterone (THDOC). The investigators found that THDOC, when applied to a discrete population of cells in the thalamus called the paraventricular nucleus (PVN), resulted in a decrease in blood levels of corticosterone (the mouse equivalent of the human stress hormone, cortisol). This finding highlights the importance of the PVN as a key anatomical locus in the brain where neurosteroids act, and is consistent with the traditional view of neurosteroids as “negative regulators” of the HPA axis. However, in mice that underwent a stressful “restraint” procedure, it was found that a prior treatment with THDOC (thirty minutes before the stressful experience) resulted in augmentation of corticosterone levels (i.e. relative to mice that underwent the stressful experience but did not receive prior THDOC treatment). In parallel, it was shown that while application of THDOC normally decreased the electrical activity of PVN cells, it actually led to increases in mice that had undergone restraint. Taken together, these findings provide evidence that neurosteroids can have opposite effects on the HPA axis depending on the “stressed” state of the organism.
Thinking about how a neurosteroid hormone can exert opposite effects on PVN cells in the thalamus may be confusing, but what may be more confusing is that these different actions depend on the same “inhibitory” brain chemical, GABA (a neurotransmitter), as well as the same molecular “machinery” (or receptors) with which GABA interacts. This was demonstrated by using mice in which a particular sub-component (or subunit) of the GABA receptor, the gamma subunit, had been genetically deleted; neurosteroids had absolutely no effect on the activity of the HPA axis (neither positive nor negative) in these gamma subunit-deficient mice.
How is it possible to explain the seemingly paradoxical finding that neurosteroids can exert opposite effects on the HPA axis through the same neurotransmitter system? In addressing this question, it is important to remember that although neurotransmitters may be thought of as excitatory or inhibitory, their ability to trigger these effects depends solely on the molecular and cellular apparatus with which they interact. Normally, the inhibitory actions of GABA upon the electrical activity of nerve cells depend on the maintenance of an “electrochemical” gradient by a “transporter” molecule called KCC2 (which transports chloride ions out of cells). Maguire’s team showed that “dephosphorylation” (i.e. the removal of a small chemical moiety – the phosphate group – which is covalently bound at a specific site on the molecule) of KCC2 resulted in lower detectable levels of this transporter in the PVN. Similarly to innumerable other examples in biology where dephosphorylation (or the reverse, phosphorylation) serves as an exquisite regulatory mechanism for controlling the activity of molecular networks, removal of the phosphate group from KCC2 acts as a molecular “switch” that causes the breakdown of the electrochemical gradient. The outcome is that GABA has an excitatory influence on neural activity instead of the inhibitory influence with which it is usually associated.
In common with many important contributions to scientific understanding, these findings should serve as a reminder that it is often necessary to challenge and question what is already “accepted” in our theoretical models, in the light of unexpected and sometimes counter-intuitive experimental results. Whatever the line of scientific inquiry may be, the reward for doing so will be a deeper and more comprehensive understanding of the natural phenomena being studied. The findings of Maguire and colleagues, published in the Journal of Neuroscience, have possible therapeutic implications for disorders associated with disrupted function of the HPA axis, including epilepsy and depression.
References
1. http://www.nature.com/nrn/journal/v10/n6/full/nrn2647.html
2. http://www.jneurosci.org/content/31/50/18198.long
This is interesting, but the lifespan, body surface area, and life activities are different in mice than in man. Not that this won’t hold up, but there is another layer, and a greater variability of conditions in humans.
Yves Ingenbleek and a coworker in Japan will soon have a lead article in Nutrition Review on the essentiality of S. To be brief:
1. dietary S is essential to plants and to animals. S availability is highest where S04*-2 is washed down from lava flows, and it is poorest in the region from Northern Pakistan through Northern India, to Bangladesh. But the sulfur in plants is considerably lower than in animals, which would have an effect on people with vegan diets because they are unable to maintain Met necessary for generation H2S.
2. This results in a severe hyperhomocysteine inducuced oxidative burden on mitochondria with intense uptake by mitochondria. If there is stress induced hypermetabolism, subclinical malnutrition, frail old age, then there is an associated downsizing of lean body mass and loss of total body sulfur.
3. Studies carried out about 35 years ago on Kwashiror in Senegalese showed that the protein transthyretin is decreased and is a measure of LBM.
A study in 1986 showed that in Senegalese with a Met shortage there is downsizing of LBM and TTR reflects fluctuations in LBM. Homocysteine levels rise, cystathionine-b-synthase declines, and the transsulfuration pathway is impaired. I need not comment on the effect of vasorelaxation by both H2S and NO here, except that there is also a paradoxical effect, and the effect on endothelium is not the same on large vessels.
4. The concern here is the pit-endocrine axis. The axis is the main sulfonation target modulating the endocrine organs. Met is critical for initiating protein synthesis through formyl-methionyl-tRNA. In oxidative stress Met captures ROS and undergoes met-sulfoxide conversion in signaling proteins.
There are several “biological features” that can be identified to get the full picture. The severely malnourished patients have goitrous enlargement, and the stressed state imposes a demand for corticosteroid, thyroid, and sex hormones.
“dephosphorylation” (i.e. the removal of a small chemical moiety – the phosphate group – which is covalently bound at a specific site on the molecule) of KCC2 resulted in lower detectable levels of this transporter in the PVN
This is perfectly compatible with the points I comment on.
Dr. Donohue,
Thank you for your first post. I selected few key words from the computer suggestion. After I’ll read I may find more research categories appropriate and will add a new comments.
Please connect to OUR Facebook, Twitter, Your Groups on LinkedIn
This will bring readers to your posts.
Dr Donahue, Thank you for this post which serves as an important reminder that science and medicine are dynamic disciplines. Acceptance of current therapies and practices with complacency in the status-quo of “usual and customary” stifles advancement and growth.
Dr. Donahue, Thank you for your post which is an excellent reminder that science is always dynamic and requires challenging of accepted facts rather than unquestioning continuation of accepted ideas and practices.
Dr. Donohue,
Thank you for your post challenging the state of science regaled to the inhibitory/excitatory function of GABA.
I believe that this post will be followed up with a full description ofthe drug therapies developed to increase and to decrease GABA availability at the synapse level.
Tufts University have another very renown Professor and Resercher on psychotropic therapeutics, D. Greenblatt. I’ll research and e-mail you who and what of his research, we would like you to digest for us.
I am thrilled to have our Research Category on Neurophysilogy, Neuropsychology and Psychotropic Therapy in your hands.
I would like to send your way some paper which I believe will add to the wealth of discussions that you will publish for us.
Welcome to our Scientific Web Site assuming an important role in a very important field of Medicine and Science.
Please review for us the article in NEJM, 200 years of Psychiatry. It might have been posted here by myself, following the Post on 200 years of medicine and of Surgery, search, by mt name or by NEJM on this site,
I mentioned Prof. David J. Greenblatt, MD at Tufts School of Medicine
He is one of the top five in the US in the following areas:
Pharmacokinetics, pharmacodynamics, and neuroreceptor properties of the benzodiazepine derivatives
Drug disposition and response in old age
Molecular mechanisms and consequences of drug interactions
Modulation of drug metabolism by nutrients and natural medicines, and
Regulation of expression and function of Cytochrome P450 enzymes and energy-dependent transport proteins
I would appreciated in one post of yours each month will present a paper based on the research of his Team. THIS IS THE FRONTIER of Science in this are, you are our Research Category OWNER of this area. Please review the list below:
Greenblatt DJ, Peters DE, Oleson LE, Harmatz JS, MacNab MW, Berkowitz N, Zinny MA, Court MH. Inhibition of oral midazolam clearance by boosting doses of ritonavir, and by 4,4-dimethyl-benziso-(2H)-selenazine (ALT-2074), an experimental catalytic mimic of glutathione oxidase. Br J Clin Pharmacol. 2009 Dec;68(6):920-7.
Ansell J, McDonough M, Zhao Y, Harmatz JS and Greenblatt DJ. The absence of an interaction between warfarin and cranberry juice: A randomized, double-blind trial. Journal of Clinical Pharmacology 2009;49:824-830.
Knox TA, Oleson L, von Moltke LL, Kaufman RC, Wanke CA and Greenblatt DJ. Ritonavir greatly impairs CYP3A activity in HIV infection with chronic viral hepatitis. JAIDS 2008; 49:358-368.
Perloff MD, von Moltke LL, Fahey JM and Greenblatt DJ. Induction of P-glycoprotein expression and activity by ritonavir in bovine brain microvessel endothelial cells. Journal of Pharmacy and Pharmacology. 2007;59:947-953.
Cysneiros RM, Farkas D, Harmatz JS, von Moltke LL, Greenblatt DJ. Pharmacokinetic and pharmacodynamic interactions between zolpidem and caffeine. Clin Pharmacol Ther. 2007 Jul;82(1):54-62. Epub 2007 Apr 18.
Farkas D, Oleson LE, Zhao Y, Harmatz JS, Zinny MA, Court MH, Greenblatt DJ. Pomegranate juice does not impair clearance of oral or intravenous midazolam, a probe for cytochrome P450-3A activity: comparison with grapefruit juice. J Clin Pharmacol. 2007 Mar;47(3):286-94.
Greenblatt DJ, Legangneux E, Harmatz JS, Weinling E, Freeman J, Rice K, Zammit GK. Dynamics and kinetics of a modified-release formulation of zolpidem: comparison with immediate-release standard zolpidem and placebo. J Clin Pharmacol. 2006 Dec;46(12):1469-80.
Fahey JM, Pritchard GA, Reddi JM, Pratt JS, Grassi JM, Shader RI, Greenblatt DJ. The effect of chronic lorazepam administration in aging mice. Brain Res. 2006 Nov 6;1118(1):13-24.
Culm-Merdek KE, von Moltke LL, Gan L, Horan KA, Reynolds R, Harmatz JS, Court MH and Greenblatt DJ. Effect of extended exposure to grapefruit juice on cytochrome P450 3A activity in humans: Comparison with ritonavir.: Clin Pharmacol Ther 2006;79:243-254.
He P, Court MH, Greenblatt DJ, Von Moltke LL. Genotype-phenotype associations of cytochrome P450 3A4 and 3A5 polymorphism with midazolam clearance in vivo. Clin Pharmacol Ther. 2005 May;77(5):373-87.
Greenblatt DJ, Harmatz JS, von Moltke LL, Wright CE, Shader RI. Age and gender effects on the pharmacokinetics and pharmacodynamics of triazolam, a cytochrome P450 3A substrate. Clin Pharmacol Ther. 2004 Nov;76(5):467-79.
von Moltke LL, Granda BW, Grassi JM, Perloff MD, Vishnuvardhan D, Greenblatt DJ. Interaction of triazolam and ketoconazole in P-glycoprotein-deficient mice. Drug Metabolism and Disposition. 2004 Aug;32(8):800-4.
Mitin T, Von Moltke LL, Court MH, Greenblatt DJ. Levothyroxine up-regulates P-glycoprotein independent of the pregnane X receptor. Drug Metabolism Disposition. 2004 Aug;32(8):779-82.
Greenblatt DJ, von Moltke LL, Harmatz JS, Chen G, Weemhoff JL, Jen C, Kelley CJ, LeDuc BW, Zinny MA. Time course of recovery of cytochrome p450 3A function after single doses of grapefruit juice. Clin Pharmacol Ther. 2003 Aug;74(2):121-9.
Patki KC, Von Moltke LL, Greenblatt DJ. In vitro metabolism of midazolam, triazolam, nifedipine, and testosterone by human liver microsomes and recombinant cytochromes p450: role of cyp3a4 and cyp3a5. Drug Metabolism and Disposition. 2003 Jul;31(7):938-44.