Involvement and role of antidepressant drugs of the hypothalamic-pituitary-adrenal axis and glucocorticoid receptor function.
Changes in the hypothalamic-pituitary-adrenal (HPA) axis are characteristic of major depression. Because the effects of glucocorticoids are mediated by intracellular receptors including, most notably, the glucocorticoid receptor (GR), several studies have examined the number and/or function of GR's in depressed patients. Review scientific evidences have consistently demonstrated that GR function is impaired in major depression, resulting in reduced GR-mediated negative feedback on the HPA axis and increased production and secretion of corticotropin-releasing hormone (CRH) in various brain regions postulated to be involved in the causality of major depression. Hyperactivity of HPA axis is the main biochemical change, besides disturbed monoaminergic neurotransmission, observed in the patients suffering from a major depression. High incidence of depression in Cushing's syndrome as well as antidepressant effects of adrenocortical enzyme inhibitors in major depression support hypothesis that hyperactivity of HPA axis may be involved in pathogenesis of depression. Major alterations of the HPA axis that can be reversed by successful antidepressant therapy are often seen in depressed patients. A possible explanation for this is that the antidepressant-induced increase in GR's renders the HPA axis more sensitive to glucocorticoid feedback. This new insight into antidepressant drug action suggests a novel approach to the development of antidepressant drugs.