Aging and the hypothalamus-pituitary-adrenocortical axis, with special reference to the dog

Aging affects the hypothalamus-pituitary-adrenocortical (HPA) system in various ways. It affects the receptors for glucocorticosteroids in the limbic system, the hypothalamus and the pituitary; the basal and stress-induced secretion of proopiomelanocortin-derived peptides and glucocorticoids; and the neuronal integrity, especially in the hippocampus. The homeostatic actions of glucocorticoids occur through the glucocorticoid and the mineralocorticoid receptors. It has been hypothesized that the balance between these two receptors, which are co-localized in the hippocampus, determines the basal HPA activity and the magnitude of the response to challenges. Feedback actions of glucocorticoids are mediated via glucocorticoid receptors in the hypothalamus and the pituitary. In aged rats many changes in the binding capacity of the mineralocorticoid receptor and glucocorticoid receptor and in the regulation of the HPA activity have been reported, but the findings often seem contradictory. The only consistent finding has been that the binding capacity of mineralocorticoid receptor in the hippocampus is reduced. The number of glucocorticoid receptors may be increased, reduced or unchanged in senescent rats. In old dogs the receptor changes were largely confined to mineralocorticoid receptor, there being a 60% reduction in the binding capacity in the limbic system, but glucocorticoid receptor was unchanged in all brain regions. Senescent dogs also had an increased basal secretion of ACTH, and of cortisol. The old dogs had exaggerated responses to stress and to administered corticotropin-releasing hormone, but the termination of the response by the feedback mechanism was unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)