Age-related adaptation of pituitary-adrenocortical responses to stress.

It has been reported that aged rats show impaired feedback regulatory control of pituitary adrenocorticotropic hormone release by adrenal glucocorticoids, yet, show no age-related deficit in eliciting an adrenocortical stress response when compared to younger animals. However, the effects of age on the capacity of the pituitary-adrenocortical system to adapt from an acute to a chronic stress situation have not been fully resolved. In the present study, groups of 6-month-old (young) and 22-month-old (old) F-344 rats were sacrificed at various times during the 1st (day 1) and 3rd (day 3) acute exposure to a two-way electric shock-escape stress procedure and subsequently during the 28th (day 28) and 56th (day 56) chronic exposure. Determinations of stress-induced corticosterone and adrenocorticotropic hormone responses indicated that: (1) adrenocortical responses were similar between young and old rats on the first stress exposure, whereas by the third stress session corticosterone responses were higher in young than in old rats; (2) attenuation of pituitary-adrenal responses to chronic stress was less in old compared to young rats, and (3) environmental factors may delay the development of age-related physiological alterations in the pituitary-adrenocortical system.