Short-Latency Suppression of Pituitary-Adrenal Function with Physiological Plasma Levels of Corticosterone in the Female Rat

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Abstract
Previous studies showed suppression of non-stress pituitary-adrenal function 2 h after exposure to physiological levels of plasma corticosterone. To study the latency of such suppression, adult female rats were killed at intervals after receiving 1 mg/kg corticosterone or vehicle s.c. at 14:30. Plasma and adrenal concentrations of corticosterone, measured fluorometrically, were used as indices of magnitude of feedback signal and level of ACTH secretion, respectively. Plasma steroid levels reached a peak of approximately 150 µ g/100 ml 5 min after corticosterone injection and by 30 min were declining and approaching those of vehicle-injected controls. Concurrently, corticosterone levels in adrenals fell to basal levels by 30 min after corticosterone injection. In attempts to find the minimum feedback-active dose of corticosterone, interference associated with the estrous cycle was encountered. At 30 min after injection of 0.5 mg/kg corticosterone, adrenal steroid levels in diestrous rats were lower than those of rats in estrus. Injection of 0.5 mg/kg corticosterone in diestrous rats resulted in peak plasma levels of 90 µ g/100 ml at 5 min and a return to control levels by 30 min. At 30 min after corticosterone, adrenal levels were totally suppressed. These findings suggest that transient feedback signals within the physiological range may cause rapid and complete suppression of non-stress pituitary-adrenal function in the rat.

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