The adrenocortical stress-response in the aged male rat: impairment of recovery from stress.

To test if the adrenocortical axis of the rat loses sensitivity to negative feedback control during aging, we examined corticosterone secretion under basal, stressed and post-stress conditions in young and in aged Fisher male rats. We find no age-related change in the speed or magnitude of the stress response or in the capacity to manifest a corticosterone response following chronic stress. However, we do observe in aging rats an elevation of basal corticosterone and an impaired capacity to adapt to and recover from stress. This latter finding is illustrated by longer latencies relative to young rats, in the return of serum corticosterone concentrations to basal values during sustained exposure to mild cold or following the end of immobilization stress. All of these deficits reflect an increased rate of corticosterone secretion during physiologically inappropriate circumstances. Such observations support the concept that there is an age-related loss of sensitivity of the brain and pituitary to the inhibitory effects of high circulating levels of corticosterone on ACTH release.