Regulation of amounts of mRNA for GnRH receptors by estradiol and progesterone in sheep.

Expression of GnRH receptors increases prior to the onset of the preovulatory surge of LH in sheep. Two experiments were conducted to investigate the interactions of progesterone (P) and estradiol (E) on amounts of mRNA for GnRH receptors and the number of receptors for GnRH. The first study was designed as a 2 x 2 factorial arrangement of treatments to investigate effects of removal of P and the presence of E. Ewes that had been ovariectomized (OVX) for at least 4 wk received one silastic implant containing E and two silastic implants containing P for 6 d to mimic concentrations of these steroids during the luteal phase of the estrous cycle. Anterior pituitary glands were collected (n = 4 animals/group): 1. Prior to implant removal and 12 h after removal of: 2. P only. 3. E only. 4. P and E. Regardless of whether or not E was present, amounts of mRNA for GnRH receptors (P = 0.87) and number of GnRH receptors (P = 0.43) were not different within 12 h after removal of P. In the second experiment, ewes were OVX on d10-12 of the estrous cycle (d0 = estrus), and immediately received silastic implants containing E and P as described above. Anterior pituitary glands were collected on d12 of the estrous cycle (n = 5), prior to implant removal (n = 5), and from the remaining ewes 24 h after removal of P only (n = 7) or removal of P and E (n = 6). Relative amounts of mRNA for GnRH receptors and the number of GnRH receptors were similar (P > 0.05) on d12 of the estrous cycle and prior to implant removal. Removal of both P and E did not affect (P > 0.05) amounts of GnRH receptor mRNA or number of GnRH receptors. However, the removal of P and the presence of E increased (P < 0.05) amounts of mRNA for GnRH receptors, but did not affect (P > 0.05) the number of GnRH receptors. We conclude that increased amounts of GnRH receptor mRNA require the removal of P and the presence of E.