Dexamethasone Suppresses Gonadotropin-Releasing Hormone (GnRH) Secretion and Has Direct Pituitary Effects in Male Rats: Differential Regulation of GnRH Receptor and Gonadotropin Responses to GnRH*

Endogenous or exogenous glucocorticoid excess leads to the development of hypogonadotropic hypogonadism, but the site(s) and mechanisms of glucocorticoid action are uncertain. We studied the effects of various doses of dexamethasone (Dex) on the hypothalamic-pituitary-gonadal axis in intact and castrate testosterone-replaced (cast + T) male rats and attempted to determine possible sites of Dex effects. A dose-dependent suppression of basal gonadotropin secretion was induced by 5 days of Dex treatment (20, 100, 500, or 2,500 micrograms/kg.day), and the highest dose completely abolished the postcastration rise in pituitary GnRH receptor number (GnRH-R) and serum gonadotropin levels. Administration of exogenous GnRH (0.02-200 micrograms/day over 2 days) resulted in a dose-dependent induction in GnRH-R in both intact and cast + T rats, but the effect was significantly (P less than 0.01) augmented in Dex-treated animals. In contrast, acute LH and FSH responses to GnRH (10, 25, 50, 100, or 250 ng, iv) were significantly blunted in Dex-treated rats. The data suggest that 1) Dex suppresses hypothalamic GnRH secretion, thereby preventing the postcastration rises in GnRH-R and gonadotropins; 2) at the pituitary level, Dex dissociates GnRH-R and gonadotropin responses to GnRH, augmenting GnRH-R induction by GnRH and suppressing gonadotropin responses to GnRH at a postreceptor site; and 3) the model of Dex-treated rats may be useful to study differential GnRH regulation of GnRH-R and gonadotropin secretion.