CHRONIC GLUCOCORTICOID ADMINISTRATION AS WELL AS REPEATED STRESS AFFECTS THE SUBSEQUENT ACUTE IMMOBILIZATION STRESS-INDUCED EXPRESSION OF IMMEDIATE-EARLY GENES BUT NOT THAT OF NGFI-A

We reported that repeated immobilization for six days attenuates the s ubsequent acute immobilization stress-induced expression of the immedi ate early genes c-fos, fos B, jun B and nerve growth factor-induced ge ne-B (NGFI-B), but not of NGFI-A, in the rat paraventricular hypothala mic nucleus. In this study, we confirmed these findings by means of a time-course study, and further investigated whether the elevated plasm a basal glucocorticoid level induced by repeated stress underlies the attenuated response of immediate early genes and the preserved reactiv ity of NGFI-A. Rats implanted with 100, 200 or 400 mg corticosterone o r placebo pellets (control), were immobilized for Ih and decapitated s even days later. In control rats acute immobilization induced c-fos, f os B, jun B, NGFI-A and NGFI-B messenger RNA in the paraventricular hy pothalamic nucleus, whereas all of them except NGFI-A, were significan tly reduced in rats given 200 and 400 mg corticosterone implants. The similarity of the results from the two procedures suggests that glucoc orticoid is involved in regulating immediate early genes in the parave ntricular hypothalamic nucleus under repeated stress and that the NGFI -A gene is not regulated by this mechanism. However, the plasma basal corticosterone level in repeatedly stressed rats was lower than that o f rats implanted with 100 mg corticosterone, suggesting that a repetit ive stress-induced corticosterone surge also contributes to this mecha nism. (C) 1997 IBRO. Published by Elsevier Science Ltd.