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.

Degree of inhibition of ACTH release by glucocorticoids in adrenalectomized rats.

The inhibitory effects of corticosterone, dexamethasone and prednisolone on activity of the hypothalamus-pituitary-adrenal axis were investigated in adrenalectomized rats infused with glucocorticoids for 6 h. Infusion of 202 micrograms corticosterone did not inhibit the plasma ACTH concentration, but 504 micrograms corticosterone significantly suppressed plasma ACTH levels. Infusion of 20 micrograms dexamethasone suppressed markedly the plasma ACTH concentration. These data suggest that the degree of inhibition of dexamethasone on ACTH release is about 25 times greater than that of corticosterone. The CRF content of the hypothalamus was not decreased by the administration of 202 micrograms corticosterone over a 6-hour period, but it was significantly diminished by 504 micrograms corticosterone. Infusion of 504 micrograms dexamethasone did not decrease the hypothalamic CRF content; however, infusion of 5 mg dexamethasone effectively suppressed the hypothalamic CRF content. Infusion of 2.5 mg prednisolone did not either decrease the CRF content. These data suggest that the degree of inhibition of natural steroid at the hypothalamus level is stronger than that of synthetic steroids. In rats pretreated with a single injection of dexamethasone (25 micrograms/200 g body weight) 22 h prior to the experiments, continuous infusion of 318 micrograms of dexamethasone significantly suppressed the hypothalamic CRF content, whereas infusion of 504 micrograms of dexamethasone failed to decrease the hypothalamic CRF content in the rats not pretreated with dexamethasone. This finding suggests that a latent period after the injection of dexamethasone is needed for the appearance of the inhibitory action of synthetic steroids at the level of hypothalamus.

Time course of hypothalamic CRF activity after the administration of two different stresses.

The time course of hypothalamic corticotropin-releasing factor (CRF) activity after the administration of ether stress was different from that after immobilization stress. The maximal response CRF activity was observed 2 min after ether stress, followed by a precipitous decrease 5 min after the stress. A gradual increase of CRF activity was subsequently observed for several minutes with fluctuated changes. Thus, response pattern was vibratory. But under immobilization stress, markedly fluctuant changes of CRF activity seen in the case of ether stress did not appear after the maximal response observed at 2 min, indicating that the response pattern was not vibratory. On the other hand, the concentration of plasma corticosterone increased significantly 5 min after the ether or immobilization stress with the peak value around 17 min.

Studies on fast feedback mechanisms by endogenous glucocorticoids.

Negative feedback in the hypothalamic-pituitary-adrenal axis by exogenous glucocorticoids has been well established, but it is still in doubt whether ACTH-releasing factor (CRF) activity in the hypothalamus is inhibited by endogenous glucocorticoids under conditions of stress. In our first experiment, CRF activity in the hypothalamus of rats was significantly increased, 2 min after onset of immobilization, for 2 min; a rapid drop of CRF activity followed immediately. On the other hand, the concentration of plasma corticosterone changed more slowly; it was increased at 5 min, and showed a progressive further increase until 17 min after the administration of the immobilization stress. A high plateau level persisted for the next 8 min, after which the concentration of plasma corticosterone fell, reaching the control level at 40 min. In order to test for negative feedback inhibition by endogenous glucocorticoids in the hypothalamic-pituitary-adrenal axis, a 2nd immunobilization stress was administered to rats 5 or 23 min after the 1st stress, and hypothalamic CRF activity and plasma corticosterone were examined after the 2nd stress. When the 2nd stress was applied 5 min after the start of the 1st stress, during the phase of a gradual increase of endogenous glucocorticoids, CRF activity was not increased 2 min after the 2nd stress. In contrast, an increase of CRF activity and plasma corticosterone was observed when the 2nd stress was administered 23 min after the onset of the 1st stress, during the phase of a high plateau level of endogenous plasma corticosterone. These data suggest that CRF activity in the hypothalamus is governed by a fast feedback, rate-sensitive mechanism.