ACTH content in the anterior pituitary after the infusion of various doses of corticosterone and dexamethasone in normal and adrenalectomized rats.

It is well known that the administration of glucocorticoids inhibits the activities of hypothalamus-pituitary-adrenal axis. The present studies were undertaken to define the site of inhibition of both corticosterone and dexamethasone on this axis in rats. Infusion of 20 or 202 micrograms corticosterone over 6 hr did not change the ACTH content in the anterior pituitary and plasma ACTH levels even through the infusion markedly inhibited the hypothalamic CRF content in normal rats. The constant infusion of a relatively small dose of 20 micrograms dexamethasone inhibited the plasma ACTH level, but not the ACTH content in the anterior pituitary and the hypothalamic CRF content. In contrast to the above results, the infusion of 202 and 504 micrograms dexamethasone over 6 hr increased the ACTH contents in the anterior pituitary, but significantly reduced the hypothalamic CRF content and plasma ACTH levels. These results suggest that the mechanism of dexamethasone inhibition in the hypothalamus-pituitary-adrenal axis is the reduction of ACTH release rather than that of ACTH production in the anterior pituitary.