View source for Hypothalamic–pituitary–adrenal axis

{{Disputed|date=December 2009}}

The '''hypothalamic-pituitary-adrenal axis''' ('''HPA or HTPA axis'''), also known as the 
'''[[limbic]]-hypothalamic-pituitary-adrenal axis''' (LHPA axis) and, occasionally, as the '''hypothalamic-pituitary-adrenal-[[gonadotropic]] axis''', is a complex set of direct influences and [[feedback]] interactions among the [[hypothalamus]], 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 [[neuroendocrinology|neuroendocrine system]] that controls reactions to [[stress (medicine)|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).

== 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 cortex|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 [[corticosteroid]]s 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 receptor]]s and glucocorticoid receptors, and these are expressed by many different types of neurons. One important target of glucocorticoids is the [[hypothalamus]], 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]] (POMC) 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 (medicine)|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 (psychology)|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 [[glucocorticoid]]s. 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 disorder]]s and functional illnesses, including [[anxiety disorder]], [[bipolar disorder]], [[insomnia]], [[post-traumatic stress disorder]], [[borderline personality disorder]], [[ADHD]], [[major depressive disorder]], [[Burnout (psychology)|burnout]], [[chronic fatigue syndrome]], [[fibromyalgia]], [[irritable bowel syndrome]], and [[alcoholism]].<ref>Robert L. Spencer, Kent E. Hutchinson, [http://findarticles.com/p/articles/mi_m0CXH/is_4_23/ai_63506636/pg_1 Alcohol, Aging, and the Stress Response], Alcohol Research and Health, Winter 1999.</ref> [[Antidepressants]], which are routinely prescribed for many of these illnesses, serve to regulate HPA axis function.<ref>Carmine M. Pariante, Institute of Psychiatry, King’s College London [http://www.neuroendo.org.uk/content/view/31/11/ Depression, stress and the adrenal axis.] The British Society for Neuroendocrinology, 2003.</ref>

==Research==

Experimental studies have investigated many different types of stress, and their effects on the HPA axis in many different circumstances.<ref>{{cite journal | author=Douglas A | title=Central noradrenergic mechanisms underlying acute stress responses of the Hypothalamo-pituitary-adrenal axis: adaptations through pregnancy and lactation. | journal=Stress | volume=8 | issue=1 | pages=5–18 | year=2005 | pmid=16019594 | doi=10.1080/10253890500044380}}</ref> 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.<ref>{{cite journal | author=Engelmann M, Landgraf R, Wotjak C | title=The hypothalamic-neurohypophysial system regulates the hypothalamic-pituitary-adrenal axis under stress: an old concept revisited. | journal=Front Neuroendocrinol | volume=25 | issue=3-4 | pages=132–49 | pmid=15589266 | doi=10.1016/j.yfrne.2004.09.001 | year=2004}}</ref> Several [[monoamine]] neurotransmitters are important in regulating the HPA axis, especially [[dopamine]], [[serotonin]] and [[norepinephrine]] (noradrenaline). In [[herbal medicine]], [[adaptogen]]s work by reregulating the HPA axis.{{Citation needed|date=February 2010}}

The HPA axis is a feature of mammals as well as other vertebrates. For example, biologists studying stress in fish showed that [[subordinate|social subordination]] leads to chronic stress, related to reduced [[Aggression|aggressive]] [[interact]]ions, to [[Impulsivity|lack of control]] and to the constant threat imposed by [[Dominant species (ecology)|dominant]] fish. Serotonin (5HT) appeared to be the active neurotransmitter involved in mediating stress responses, and increases in serotonin are related to increased [[blood|plasma]] [[α-MSH]] levels, which causes skin darkening (a social signal in [[salmon]]oid 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.<ref>({{cite journal | author=Winberg S, Øverli Ø, Lepage O | title=Suppression of aggression in rainbow trout (Oncorhynchus mykiss) by dietary L-tryptophan. | journal=J Exp Biol | volume=204 | issue=Pt 22 | pages=3867–76 | year=2001 | pmid=11807104}}</ref> However, the study mentions that plasma cortisol was not affected by dietary L-tryptophan.

==See also==
* [[Hypothalamic-pituitary-gonadal axis]]
* [[Neuroendocrinology]]
* [[Hypothalamus]]
* [[Stress (medicine)|Stress]]
* [[Cushing's syndrome]]
* [[Adrenal insufficiency]]
* [[Addison's disease]]
* [[ACTH stimulation test]]
* [[Dexamethasone suppression test]]
* [[Glucocorticoid]]s
* [[Cortisol]]
* [[Dexamethasone]]
* [[Hydrocortisone]]
* [[Cortisol awakening response]]
* [[Sickness behavior]]

==References==
{{reflist}}

===General===

*{{cite journal |author=Merali Z, McIntosh J, Kent P, Michaud D, Anisman H |title=Aversive and appetitive events evoke the release of corticotropin-releasing hormone and bombesin-like peptides at the central nucleus of the amygdala |journal=J. Neurosci. |volume=18 |issue=12 |pages=4758–66 |year=1998 |month=June |pmid=9614249 |url=http://www.jneurosci.org/cgi/pmidlookup?view=long&pmid=9614249}}

===Relation to illnesses===

*{{cite journal |author=Backhaus J, Junghanns K, Hohagen F |title=Sleep disturbances are correlated with decreased morning awakening salivary cortisol |journal=Psychoneuroendocrinology |volume=29 |issue=9 |pages=1184–91 |year=2004 |month=October |pmid=15219642 |doi=10.1016/j.psyneuen.2004.01.010 |url=http://linkinghub.elsevier.com/retrieve/pii/S0306453004000125}}
*{{cite journal |author=Pruessner JC, Hellhammer DH, Kirschbaum C |title=Burnout, perceived stress, and cortisol responses to awakening |journal=Psychosom Med |volume=61 |issue=2 |pages=197–204 |year=1999 |pmid=10204973 |url=http://www.psychosomaticmedicine.org/cgi/pmidlookup?view=long&pmid=10204973}}
*{{cite journal |author=MacHale SM, Cavanagh JT, Bennie J, Carroll S, Goodwin GM, Lawrie SM |title=Diurnal variation of adrenocortical activity in chronic fatigue syndrome |journal=Neuropsychobiology |volume=38 |issue=4 |pages=213–7 |year=1998 |month=November |pmid=9813459 |url=http://content.karger.com/produktedb/produkte.asp?typ=fulltext&file=nps38213}}
*{{cite journal |author=Patacchioli FR, Angelucci L, Dellerba G, Monnazzi P, Leri O |title=Actual stress, psychopathology and salivary cortisol levels in the irritable bowel syndrome (IBS) |journal=J. Endocrinol. Invest. |volume=24 |issue=3 |pages=173–7 |year=2001 |month=March |pmid=11314746 }}
*Winston, David & Maimes, Steven. “'''Adaptogens''': Herbs for Strength, Stamina, and Stress Relief,” Healing Arts Press, (2007)

==External links==
*{{commons-inline|Category:Hypothalamic-pituitary-adrenal axis|HPA axis}}
* [http://www.ifi.unizh.ch/~andel/neurowiki/nw.cgi?Hypothalamic-Pituitary-Adrenal_Axis NeuroWiki page on HPA axis]
* [http://www.mind-body-health.net/hpa.html Mind-Body-Health.net page on HPA axis]

{{Endocrine system}}

[[Category:Neuroendocrinology]]
[[Category:Attention]]
[[Category:Attention disorders]]
[[Category:Mood disorders]]
[[Category:Stress]]
[[Category:Anxiety]]
[[Category:Attention-deficit hyperactivity disorder]]
[[Category:Treatment of bipolar disorder]]

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