ESTRADIOL INCREASES AMOUNTS OF MESSENGER-RIBONUCLEIC-ACID FOR GONADOTROPIN-RELEASING-HORMONE RECEPTORS IN SHEEP

Two experiments were conducted simultaneously to investigate regulatio n of amounts of mRNA for GnRH receptors during the periovulatory perio d in sheep. In the first experiment, amounts of mRNA for GnRH receptor s were measured before and after the preovulatory surge of LH followin g regression of the CL by prostaglandin F-2 alpha (PGF(2 alpha)). So t hat the time of the preovulatory surge of LH could be accurately predi cted, ewes received two injections of PGF(2 alpha) on Day 14 of the es trous cycle. Anterior pituitary glands were collected from 5 control e wes on Day 14 of the estrous cycle (0 h after PGF(2 alpha)) and at 48, 72, and 96 h after PGF(2 alpha) (5 ewes per group). The second experi ment was conducted to investigate the effects of 17 beta-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 i mplants when they were OVX (OVXEI). Sixteen hours after OVX, anterior pituitary glands were collected from 5 OVX and 5 OVXEI ewes, and the r emaining OVXEI ewes received an i.m. injection of estradiol (25 mu g i n corn oil; OVXEI + E) to induce a preovulatory-like surge of LH. Ante rior pituitary glands were collected from OVXEI + E ewes 18 or 54 h af ter injection of estradiol (n = 5 per group). Half of each anterior pi tuitary gland was used to measure the number of GnRH receptors. Poly(A )(+) RNA was isolated from the remaining half of each anterior pituita ry gland, applied to slot blots, and hybridized with a radioactive cDN A probe encoding the ovine GnRH receptor. Amounts of GnRH receptor mRN A were elevated (p < 0.05) approximately 5-fold 48 h after PGF(2 alpha ) (i.e., prior to the onset of the preovulatory surge of LH) compared to 0, 72, or 96 h after PGF(2 alpha). Acute (i.e., 16 h) removal of ov arian hormones did not influence amounts of mRNA for GnRH receptors (p > 0.05) compared to the amount on Day 14 of the estrous cycle. Admini stration of estradiol implants to OVX ewes for 16 h increased (p < 0.0 5) amounts of mRNA for GnRH receptors approximately 1.6-fold as compar ed 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 r eceptor mRNA after injection of estradiol to induce a preovulatory-lik e surge of LH. We conclude that removal of ovarian hormones was not su fficient 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 preovulat ory surge of LH in ewes.