The present study was designed to characterize the regulation of the type II corticosteroid receptor (GR) mRNA in two tissues involved in the control of the hypothalamic- pituitary-adrenal axis. We have used a solution hybridization/Si nuclease protection assay to quantitate GR mRNA levels in the rat hippocampus and anterior pituitary after CRF, dexa- methasone (DEX), or corticosterone (CORT) treatment. In general, hippocampal GR mRNA levels increased after removal of endogenous corticosteroids by surgical adrenalectomy and de-creased in response to glucocorticoid treatment. More specifically, in the hippocampus 1) GR mRNA expression was decreased when adrenalectomized (ADX) animals were replaced with a relatively low dose of CORT, but not with a low dose of DEX; 2) acutely, CRF was more effective than DEX in decreasing the levels of GR mRNA in intact animals; however, under the same paradigm in ADX animals, DEX decreased the level of GR mRNA, whereas CRF was ineffective; and 3) in contrast to the decrease in GR mRNA levels observed after acute and low doses of glucocorticoid treatment, chronic treatment with either DEX or CORT did not change the level of hippocampal GR mRNA. These results suggest that in the hippocampus the decrease in GR mRNA expression after CRF treatment is probably via the release of glucocorticoids, and that this tissue is more sensitive to endogenous glucocorticoids than DEX. Anterior pituitary GR mRNA was differentially regulated compared with that in the hippocampus. In marked contrast to GR mRNA in the hippocampus, ADX did not alter anterior pituitary GR mRNA expression, and glucocorticoid treatment led to an increase in GR mRNA levels. In the anterior pituitary 1) glucocorticoid treatment led to an increase in GR mRNA expression, when replaced with a relatively low dose of DEX, but not when replaced with a low dose of CORT; 2) acutely, neither CRF nor DEX altered levels of GR mRNA in intact animals; however, under the same paradigm DEX increased levels in ADX animals; and 3) chronic DEX or CORT treatment of intact animals elevated levels of anterior pituitary GR mRNA. In summary, these data have demonstrated tissue-specific regulation of GR mRNA in the hippocampus and anterior pituitary, which is dependent on both the dose and length of treatment and, in addition, on the glucocorticoid itself.
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