Pituitary Gonadotropin-Releasing Hormone Receptors on Proestrus: Effect of Pentobarbital Blockade of Ovulation in the Rat*

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Abstract
Pituitary GnRH receptors vary markedly during the estrous cycle in rats. Concentrations are maximal on diestrus and early proestrus before falling rapidly on the afternoon of proestrus. The present studies were performed to delineate the temporal relationship between pituitary GnRH receptors and the changing hormonal milieu on proestrus. Rats demonstrating 4-day cycles were studied at 2-h intervals beginning at 0800 h on proestrus and every 30 min between 1300-1700 h. The gonadotropin-releasing hormone-binding capacity (GnRH-BC) was measured using the analog [D-Ala6-des-Gly10]GnRH ethylamide as ligand. GnRH-BC was elevated between 0800–1330 h on proestrus [range, 473 ± 109 to 646 ± 162 fmol/mg protein (mean ± SE)] before falling abruptly to 258 ± 50 fmol/mg at 1400 h. Thereafter, GnRH-BC increased rapidly to 497 ± 76 at 1500 h and 618 ± 57 at 1700 h before declining gradually to 302 ± 35 at midnight. The abrupt fall in GnRH-BC between 1330 and 1500 h occurred while serum estradiol was still elevated and preceded both the main LH and FSH surges and the rise in progesterone. Pentobarbital blockade of ovulation abolished the abrupt fall in GnRH-BC, but BC fell gradually through the afternoon and evening of proestrus. At midnight, receptor concentrations (297 ± 40 fmol/mg) were similar to those in untreated animals. The intravenous injection of GnRH at 1400 h on proestrus in pentobarbital-treated animals increased serum LH, FSH, and progesterone, but GnRH-BC was not reduced when LH release was maximal, indicating that the abrupt fall in measured GnRH binding is not due to receptor occupancy by GnRH. We conclude that pituitary GnRH receptors on proestrous afternoon are in dynamic equilibrium, capable of rapid changes that may reflect receptor internalization or altered cellular processing of hormone-receptor complex.