HPA axis

Hypothalamus-pituitary-adrenal (HPA) Axis Circadian Rhythms Appetite Control Orexins... 

Figure 20.7 Possible interactions between hormones of the HPA axis, extrahypothalamic CRF and the central monoamines, noradrenaline and 5-hydrox5hryptamine. See text for a full explanation...

The HPA axis provides a critical mechanism for regulation of the stress response and its effects on the brain and other organ... 

Fhtients may require a lower dose of glucocorticoid than those with primary adrenal insufficiency. Some patients will only require glucocorticoid replacement temporarily, which can be discontinued after recovery of the HPA axis (e.g., drug-induced adrenal insufficiency, adrenal insufficiency following treatment for Cushing s syndrome). 

Understand that recovery of the HPA axis may take up to a year after glucocorticoid discontinuation during which the patient may require supplementation therapy during periods of physiologic stress. 

Monitor morning cortisol or response to ACTH stimulation every 3 to 6 months to assess for HPA axis recovery. Discontinue glucocorticoid replacement therapy when cortisol concentrations are greater than 19 mcg/dL (524 nmol/L) on either test. 

Hypothalamic-pituitary-adrenal (HPA) axis A neuroendocrine feedback loop that controls response to stress. 

In animal studies, high levels of cortisol have been shown to induce (increase) the activity of the enzyme tryptophan 2,3-dioxygenase in the liver, thereby decreasing the bioavailability of tryptophan to the brain. It is interesting to note that low acute doses of a number of different antidepressants inhibit the activity of this enzyme and, as a result, increase brain tryptophan concentrations, thus stimulating 5-HT synthesis (Badawy and Evans, 1982). In this way a link between the two key monoamine neurotransmitters and the hormone may be seen namely, reduced brain NA activity leads to decreased inhibition of the HPA axis, while increased levels of cortisol reduce 5-HT activity in the brain. Activation of the HPA axis has also been shown to result in tissue atrophy, in particular of the limbic system s hippocampus, and a reduction in the levels of neurotrophic factors responsible for the maintenance and optimal function of brain neurons (Manji et al., 2001). In conclusion, manipulation of the HPA axis (Nemeroff, 2002) and stimulation of neurotrophic factor activity (Manji et al., 2001) might open up new avenues for the treatment of affective disorders. 

Hypothalamic-Pituitary-Adrenal (HPA) axis An integrated neuroendocrine system linking the hypothalamus (noradrenaline and corticotropin-releasing factor CRF) with the anterior pituitary (corticotropin or ACTH) and the adrenal cortex (cortisol). 

Activation of the HPA axis appears to play a critical role in mediating hippocampal atrophy 898... 

The potential hyperactivation of the HPA axis in mood disorders has been revisited in recent years, in large part due to the growing recognition of the specific brain areas in which atrophy (loss), a neuroplastic event, may be present in many patients. To what extent these findings of atrophy represent the sequelae of the biochemical changes (for example, in glucocorticoid levels) accompanying repeated affective episodes, remains to be fully elucidated. 

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