Regulation of gonadotropin-releasing hormone (GnRH) receptor messenger ribonucleic acid and GnRH receptors during the early preovulatory period in the ewe

Estradiol increases the number of GnRH receptors in the ewe. Although results from studies conducted in vitro indicate that progesterone may have a negative influence on the number of ovine GnRH receptors, this effect of progesterone has not been documented in vivo. To explore the regulation of GnRH receptors at the level of gene expression, a partial complementary DNA (cDNA) encoding ovine GnRH receptor was isolated using reverse transcription and polymerase chain reaction methodology. This partial cDNA (701 basepairs) was used to isolate a full-length cDNA encoding GnRH receptor from an ovine pituitary cDNA library. Northern blot analysis of RNA from ovine pituitary glands using the partial cDNA as a molecular probe revealed four messenger RNA (mRNA) transcripts at 5.6, 3.8, 2.1, and 1.3 kilobases. In some samples, a fifth transcript at 0.8 kilobases was also evident. GnRH receptor mRNA was not detected in ovine brain, heart, kidney, adrenal, or liver tissues. To examine the regulation of GnRH receptor mRNA and GnRH receptors during the early preovulatory period, relationships among steady state concentrations of GnRH receptor mRNA, numbers of GnRH receptors, and circulating concentrations of progesterone and estradiol during luteolysis were characterized. We hypothesized that during luteolysis, decreased concentrations of progesterone would be associated with increased concentrations of GnRH receptor mRNA and increased numbers of GnRH receptors. On day 11 or 12 of the estrous cycle, luteolysis was induced in 14 ewes by treatment with prostaglandin F(2α) (PGF(2α)). Four ewes were treated with saline (saline controls). Anterior pituitary tissue was collected 4 h (n = 4), 12 h (n = 5), and 24 h (n = 5) after treatment with PGF(2α) or 24 h after treatment with saline and from four untreated ewes on day 11 or 12 of the estrous cycle (untreated controls). Twelve hours after treatment with PGF(2α), circulating concentrations of progesterone had decreased (P < 0.05) to 46% of the control values; however, concentrations of estradiol were not different from those in control ewes. Concentrations of GnRH receptor mRNA increased 2-fold during luteolysis and were higher than control values 12 h after PGF(2α) treatment (P < 0.05). This increase in GnRH receptor mRNA was not accompanied by an increase in the number of GnRH receptors. Twenty-four hours after treatment with PGF(2α), concentrations of progesterone in PGF(2α)-treated ewes had decreased (P < 0.05) to 15% of control values, whereas concentrations of estradiol had increased (P < 0.05) to 321% of control values. Concentrations of GnRH receptor mRNA remained elevated 24 h after treatment with PGF(2α), but did not differ from those at 12 h. Increased concentrations of GnRH receptor mRNA 24 h after treatment with PGF(2α) were accompanied by greater numbers of GnRH receptors (P < 0.05) relative to those in controls. Concentrations of hypophyseal receptors for estradiol in PGF(2α)-treated ewes were not different from those in controls at any time; therefore, it is unlikely that increases in concentrations of GnRH receptor mRNA were due to increased sensitivity of the pituitary gland to estradiol. In summary, a dramatic increase in concentrations of GnRH receptor mRNA preceded a rise in the number of GnRH receptors during luteolysis and occurred when mean concentrations of progesterone in serum had decreased, but before circulating concentrations of estradiol increased. The temporal relationship between declining serum concentrations of progesterone and increased concentrations of GnRH receptor mRNA is consistent with the hypothesis that progesterone has a negative influence on the expression of GnRH receptors in the ewe. Increased amounts of GnRH receptor mRNA during the follicular phase may represent an early step in the series of events that enhances the sensitivity of the anterior pituitary to GnRH in preparation for the preovulatory gonadotropin surge.