Estradiol increases amounts of messenger ribonucleic acid for gonadotropin-releasing hormone receptors in sheep

Two experiments were conducted simultaneously to investigate regulation of amounts of mRNA for GnRH receptors during the periovulatory period in sheep. In the first experiment, amounts of mRNA for GnRH receptors were measured before and after the preovulatory surge of LH following regression of the CL by prostaglandin F(2α) (PGF(2α)). So that the time of the preovulatory surge of LH could be accurately predicted, ewes received two injections of PGF(2α) on Day 14 of the estrous cycle. Anterior pituitary glands were collected from 5 control ewes on Day 14 of the estrous cycle (0 h after PGF(2α)) and at 48, 72, and 96 h after PGF(2α) (5 ewes per group). The second experiment was conducted to investigate the effects of 17β-estradiol on amounts of mRNA for GnRH receptors. On Day 14 of the estrous cycle, 20 ewes were ovariectomized (OVX); 15 of these ewes received estradiol implants when they were OVX (OVXEI). Sixteen hours after OVX, anterior pituitary glands were collected from 5 OVX and 5 OVXEI ewes, and the remaining OVXEI ewes received an i.m. injection of estradiol (25 μg in corn oil; OVXEI + E) to induce a preovulatory-like surge of LH. Anterior pituitary glands were collected from OVXEI + E ewes 18 or 54 h after injection of estradiol (n = 5 per group). Half of each anterior pituitary gland was used to measure the number of GnRH receptors. Poly(A)⁺ RNA was isolated from the remaining half of each anterior pituitary gland, applied to slot blots, and hybridized with a radioactive cDNA probe encoding the ovine GnRH receptor. Amounts of GnRH receptor mRNA were elevated (p < 0.05) approximately 2-fold 48 h after PGF(2α) (i.e., prior to the onset of the preovulatory surge of LH) compared to 0, 72, or 96 h after PGF(2α). Acute (i.e., 16 h) removal of ovarian hormones did not influence amounts of mRNA for GnRH receptors (p > 0.05) compared to the amount on Day 14 of the estrous cycle. Administration of estradiol implants to OVX ewes for 16 h increased (p < 0.05) amounts of mRNA for GnRH receptors approximately 1.6-fold as compared to the amount in ewes OVX for 16 h or that on Day 14 of the estrous cycle. There were, however, no further increases (p > 0.05) in GnRH receptor mRNA after injection of estradiol to induce a preovulatory-like surge of LH. We conclude that removal of ovarian hormones was not sufficient to increase amounts of mRNA for GnRH receptors. In contrast, estradiol increased amounts of mRNA for GnRH receptors in OVXEI ewes. Ovarian hormones other than estradiol do not appear to be required for the increase in GnRH receptor mRNA that occurs prior to the preovulatory surge of LH in ewes.