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16-September-2008 16:15:08 - Hypothalamic-pituitary-adrenal axis Redirected from HPA axis The hypothalamic-pituitary-adrenal axis HPA or HTPA axis, also known as the limbic-hypothalamic-pituitary-adrenal axis LHPA axis, is a complex set of direct influences and feedback interactions among the hypothalamus a hollow, funnel-shaped part of the brain, the pituitary gland a pea-shaped structure located below the hypothalamus, and the adrenal or suprarenal glands small, conical organs on top of the kidneys. The interactions among these organs constitute the HPA axis, a major part of the neuroendocrine system that controls reactions to stress and regulates many body processes, including digestion, the immune system, mood and emotions, sexuality, and energy storage and expenditure. A wide variety of species, from the most ancient organisms to humans, share components of the HPA axis. It is the common mechanism for interactions among glands, hormones, and parts of the midbrain that mediate the general adaptation syndrome GAS. Contents 1 Anatomy 2 Function 3 Research 4 See also 5 References 5.1 General 5.2 Relation to illnesses 6 External links Anatomy The key elements of the HPA axis are: The paraventricular nucleus of the hypothalamus, which contains neuroendocrine neurons that synthesize and secrete vasopressin and corticotropin-releasing hormone CRH. These two peptides regulate: The anterior lobe of the pituitary gland. In particular, CRH and vasopressin stimulate the secretion of adrenocorticotropic hormone ACTH, once known as corticotropin. ACTH in turn acts on: the adrenal cortices, which produce glucocorticoid hormones mainly cortisol in humans in response to stimulation by ACTH. Glucocorticoids in turn act back on the hypothalamus and pituitary to suppress CRH and ACTH production in a negative feedback cycle. CRH and vasopressin are released from neurosecretory nerve terminals at the median eminence. They are transported to the anterior pituitary through the portal blood vessel system of the hypophyseal stalk. There, CRH and vasopressin act synergistically to stimulate the secretion of stored ACTH from corticotrope cells. ACTH is transported by the blood to the adrenal cortex of the adrenal gland, where it rapidly stimulates biosynthesis of corticosteroids such as cortisol from cholesterol. Cortisol is a major stress hormone and has effects on many tissues in the body, including on the brain. In the brain, cortisol acts at two types of receptor - mineralocorticoid receptors and glucocorticoid receptors, and these are expressed by many different types of neuron. One important target of glucocorticoids is the hippocampus, which is a major controlling centre of the HPA axis. Vasopressin can be thought of as water conservation hormone and is also known as antidiuretic hormone. It is released when the body is dehydrated and has potent water-conserving effects on the kidney. It is also a potent vasoconstrictor. Important to the function of the HPA axis are some of the feedback loops: Cortisol produced in the adrenal cortex will negatively feedback to inhibit both the hypothalamus and the pituitary gland. This reduces the secretion of CRH and vasopressin, and also directly reduces the cleavage of proopiomelanocortin into ACTH and β-endorphins. Epinephrine and norepinephrine are produced by the adrenal medulla through sympathetic stimulation and the local effects of cortisol upregulation enzymes to make E/NE. E/NE will positively feedback to the pituitary and increase the breakdown of POMCs into ACTH and β-endorphins. Function Release of CRH from the hypothalamus is influenced by stress, by blood levels of cortisol and by the sleep/wake cycle. In healthy individuals, cortisol rises rapidly after wakening, reaching a peak within 30-45 minutes. It then gradually falls over the day, rising again in late afternoon. Cortisol levels then fall in late evening, reaching a trough during the middle of the night. An abnormally flattened circadian cortisol cycle has been linked with chronic fatigue syndrome MacHale, 1998, insomnia Backhaus, 2004 and burnout Pruessner, 1999. Anatomical connections between brain areas such as the amygdala, hippocampus, and hypothalamus facilitate activation of the HPA axis. Sensory information arriving at the lateral aspect of the amygdala is processed and conveyed to the central nucleus, which projects to several parts of the brain involved in responses to fear. At the hypothalamus, fear-signaling impulses activate both the sympathetic nervous system and the modulating systems of the HPA axis. Increased production of cortisol mediates alarm reactions to stress, facilitating an adaptive phase of a general adaptation syndrome in which alarm reactions including the immune response are suppressed, allowing the body to attempt countermeasures. Glucocorticoids have many important functions, including modulation of stress reactions, but in excess they can be damaging. Atrophy of the hippocampus in humans and animals exposed to severe stress is believed to be caused by prolonged exposure to high concentrations of glucocorticoids. Deficiencies of the hippocampus may reduce the memory resources available to help a body formulate appropriate reactions to stress. The HPA axis is involved in the neurobiology of mood disorders and functional illnesses, including anxiety disorder, bipolar disorder, insomnia, post-traumatic stress disorder, ADHD, major depressive disorder, burnout, chronic fatigue syndrome, fibromyalgia, irritable bowel syndrome, and alcoholism.1 Antidepressants, which are routinely prescribed for many of these illnesses, serve to regulate HPA axis function.2 Research Experimental studies have investigated many different types of stress, and their effects on the HPA axis in many different circumstances.3 Stressors can be of many different types - in experimental studies in rats, a distinction is often made between social stress and physical stress, but both types activate the HPA axis, though via different pathways.4Several monoamine neurotransmitters are important in regulating the HPA axis, especially dopamine, serotonin and norepinephrine noradrenaline. In herbal medicine, adaptogens work by reregulating the HPA axis. The HPA axis is a feature of other vertebrates as well as of mammals. For example, biologists studying stress in fish showed that social subordination leads to chronic stress, related to reduced aggressive interactions, to lack of control and to the constant threat imposed by dominant fish. Serotonin 5HT appeared to be the active neurotransmitter involved in mediating stress responses, and increases in serotonin are related to increased plasma α-MSH levels, which causes skin darkening a social signal in salmonoid fish, activation of the HPA axis, and inhibition of aggression. Inclusion of the amino acid L-tryptophan, a precursor of 5HT, in the feed of rainbow trout made the trout less aggressive and less responsive to stress 5However, the study mentions that plasma cortisol was not affected by dietary L-tryptophan. See also Hypothalamic-pituitary-gonadal axis Neuroendocrinology Hypothalamus Stress Cushing's syndrome Adrenal insufficiency Addison's disease ACTH stimulation test Dexamethasone suppression test Glucocorticoids Cortisol Dexamethasone Hydrocortisone References ^ Robert L. Spencer, Kent E. Hutchinson, Alcohol, Aging, and the Stress Response, Alcohol Research and Health, Winter 1999. ^ Carmine M. Pariante, Institute of Psychiatry, King's College London Depression, stress and the adrenal axis. The British Society for Neuroendocrinology, 2003. ^ Douglas A 2005. Central noradrenergic mechanisms underlying acute stress responses of the Hypothalamo-pituitary-adrenal axis: adaptations through pregnancy and lactation.. Stress 8 1: 5-18. doi:10.1080/10253890500044380. PMID 16019594. ^ Engelmann M, Landgraf R, Wotjak C 2004. The hypothalamic-neurohypophysial system regulates the hypothalamic-pituitary-adrenal axis under stress: an old concept revisited.. Front Neuroendocrinol 25 3-4: 132-49. doi:10.1016/j.yfrne.2004.09.001. PMID 15589266. ^ Winberg S, Ã~verli Ã~, Lepage O 2001. Suppression of aggression in rainbow trout Oncorhynchus mykiss by dietary L-tryptophan.. J Exp Biol 204 Pt 22: 3867-76. PMID 11807104. General Merali Z. et al. 1998 Aversive and appetitive events evoke the release of corticotropin-releasing hormone and bombesin-like peptides at the central nucleus of the amygdala J Neurosci 18:4758-99 Relation to illnesses Backhaus J et al. 2004 Sleep disturbances are correlated with decreased morning awakening salivary cortisol Psychoneuroendocrinology 29:1184-91 Pruessner JC et al. 1999 Burnout, perceived stress, and cortisol responses to awakening Psychosom Med 61:197-204 MacHale SM et al.1998 Diurnal variation of adrenocortical activity in chronic fatigue syndrome Neuropsychobiology 38:213-7 Patacchioli FR et al. 2001 Actual stress, psychopathology and salivary cortisol levels in the irritable bowel syndrome IBS J Endocrinol Invest 24:173-7 Winston, David Maimes, Steven. Adaptogens: Herbs for Strength, Stamina, and Stress Relief, Healing Arts Press, 2007 External links NeuroWiki page on HPA axis Mind-Body-Health.net page on HPA axis a_76/12172218 at Dorland's Medical Dictionary v d e Human anatomy, endocrine system: endocrine glands Hypothalamic/ pituitary axes Thyroid axis Thyroid gland Parafollicular cell, Thyroid epithelial cell, Thyroid isthmus, Lobes of thyroid gland, Pyramid of thyroid Parathyroid gland Oxyphil cell, Chief cell Adrenal axis Adrenal gland Gonadal axis Testes - Ovaries - Corpus luteum Pineal gland Pinealocyte Islets of pancreas Alpha cell - Beta cell - Delta cell - PP cell - Epsilon cell Retrieved from http://en..org/wiki/Hypothalamic-pituitary-adrenal_axis Categories: Neuroendocrinology Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Español Polski This page was last modified on 31 July 2008, at 17:24
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