REGULATION OF GONADOTROPIN-RELEASING-HORMONE (GNRH) RECEPTOR GENE-EXPRESSION IN SHEEP - INTERACTION OF GNRH AND ESTRADIOL

GnRH and estradiol are important regulators of GnRH receptors. When de livered to the anterior pituitary gland continuously, GnRH decreases n umbers of GnRH receptors on gonadotropes. Treatment with estradiol con sistently increases numbers of GnRH receptors. Because estradiol acts via intracellular receptors while GnRH exerts its effects through a me mbrane receptor, it is likely that these hormones influence GnRH recep tor expression via different mechanisms. In this experiment, we tested two hypotheses: 1) continuous infusion of GnRH will decrease expressi on of the GnRH receptor gene; and 2) estradiol will override the negat ive effects of continuous infusion of GnRH on GnRH receptor expression . Ovariectomized ewes were administered either GnRH (10 mu g/h, n = 10 ) or saline (n = 10) continuously for 136 h. At 124 h, 5 ewes in each group were administered estradiol (25 mu g im) and anterior pituitary glands were collected 12 h later. Treatment with GnRH caused an abrupt increase in circulating concentrations of LH, and the maximal mean co ncentration was observed 4 h after the start of GnRH infusion. Followi ng this increase, concentrations of LH in GnRH-treated ewes declined a nd were similar to those in saline-treated ewes from 8 h to 124 h. Aft er injection of estradiol at 124 h, circulating concentrations of LH i ncreased in both GnRH- and saline-treated ewes. However, this response occurred within 6 h in ewes treated with GnRH compared with 9 h in ew es treated with saline (P < 0.05). Compared with saline-treated contro ls, treatment with GnRH decreased mean steady-state amount of GnRH rec eptor messenger RNA (mRNA) (P < 0.01) and concentration of GnRH recept ors (P < 0.05). Treatment with estradiol caused an increase in concent rations of GnRH receptor mRNA (P < 0.05) and GnRH receptors (P < 0.01) . Amounts of GnRH receptor mRNA and numbers of GnRH receptors in ewes treated with both GnRH and estradiol were not different from those in the control group brit were higher (P < 0.002) relative to ewes treate d with GnRH alone. Treatment with GnRH and estradiol also influenced t he expression of genes encoding the LH beta and FSH beta subunits. Com pared with saline-treated controls, treatment with GnRH reduced steady -state amounts of mRNA encoding LH beta subunit (P < 0.005) and FSH be ta subunit (P < 0.05). Treatment with estradiol caused a decrease in c oncentrations of FSH beta subunit mRNA (P < 0.01) but did not affect a mounts of LH beta subunit mRNA. The combined treatment of GnRH and est radiol reduced concentrations of mRNA encoding LH beta subunit (P < 0. 01) and FSH beta subunit (P < 0.005). From these data we conclude that 1) reduced numbers of GnRH receptors during continuous infusion of Gn RH are mediated in part by decreased expression of the GnRH receptor g ene; and 2) estradiol is able to override the negative effect of GnRH by stimulating an increase in GnRH receptor gene expression and GnRH r eceptor concentrations. Therefore, although the gonadotrope becomes re fractory to GnRH during homologous desensitization, this desensitizati on does not affect the cell's ability to respond to estradiol.