Evidence for gonadotropin-releasing hormone receptor mRNA expression by estrogen in rat granulosa cells

The hormonal regulation of ovarian gonadotropin-releasing hormone (GnRH) receptor mRNA expression has been examined by in situ hybridization in hypophysectomized immature rats. In hypophysectomized rats, GnRH receptor mRNA expression is localized in the interstitial cells. After diethylstilbestrol treatment, most follicles grow to form early antral follicles and express GnRH receptor mRNA in the peripheral part of the granulosa layer, indicating that the expression in the growing follicles is estrogen-dependent. Only weak or no expression of the receptor mRNA is detectable in the atretic follicles of hypophysectomized rats, whereas very strong expression has been observed in the granulosa cells of atretic follicles of intact immature rats. Administration of testosterone or a GnRH agonist, both of which are atretic agents for ovarian follicles, to hypophysectomized rats markedly increases the apoptotic cell death of the granulosa cells but fails to induce GnRH receptor mRNA expression. The co-administration of these agents with diethylstilbestrol causes the granulosa cells of atretic follicles to express the receptor mRNA very strongly, suggesting that this mRNA expression in the atretic follicles is also estrogen-dependent. On the other hand, expression of the receptor mRNA in the ovarian interstitial cells is not affected by hypophysectomy or hormone treatments. All of these results clearly indicate that estrogen is essential for the expression of ovarian GnRH receptor mRNA in the granulosa cells of atretic follicles and growing follicles, whereas the expression in the interstitial cells is estrogen-independent.