Defective ACTH response to stress in previously stressed rats: dependence on glucocorticoid status

The effect of previous exposure to stress on the pituitary-adrenal response to a further stress was characterized in rats with different glucocorticoi d status: sham-operated rats (Sham), adrenalectomized (ADX) rats, and ADX r ats supplemented with a low corticosterone (B) dose in the drinking saline (ADX + B). Previous exposure of Sham rats to 1 h of immobilization (Imo) re duced, 2 h later, the ACTH response to a second severe stressor (Imo) but n ot to a less severe stressor (tail shock). In ADX rats, previous Imo totall y suppressed the ACTH response to Imo or to shock. In ADX + B rats the resp onse to shock was blocked and that to Imo tended to be lower. These changes were not explained by depletion of adenohypophysial ACTH stores. After pre vious Imo, reduced response to corticotropin-releasing factor was observed in Sham and ADX + B, but not in ADX, rats. Taken together, the present resu lts suggest that the reduced ACTH response of previously stressed rats to a second severe stress is observed in the presence and absence of glucocorti coids, but the main site at which such inhibition occurs might be criticall y dependent on the glucocorticoid status.