Reduced ability of old male rats to release ACTH and corticosterone in response to CRF administration.

A single injection of oCRF at either 2 or 15 micrograms/kg BW was given to old (19-21 mo) and young (4 mo) male rats, and plasma ACTH and corticosterone (CORT) were measured. Both doses of CRF caused a significant increase in plasma ACTH by 5 min in both age groups (p less than 0.01), but young rats had significantly greater ACTH levels by 25 min after injection (p less than 0.05) compared with old. Plasma CORT in both age groups was significantly elevated (p less than 0.01) above preinjection values by 5 min after either dose of CRF. However at 5, 15, and 25 min, young animals had significantly greater levels of CORT (p less than 0.05) compared with old. These results indicate that old male rats have a diminished capacity to secrete ACTH in response to oCRF; this decreased sensitivity may be responsible for decreased release of ACTH and CORT observed in old rats in response to stress.