Gonadotropin releasing hormone receptor interaction 5. down regulation of pituitary receptors for gonadotropin releasing hormone in ovariectomized ewes by infusion of homologous hormone
Nett, T.M.; Crowder, M.E.; Moss, G.E.; Duello, T.M.
Biology of Reproduction 24(5): 1145-1156
Concentrations of pituitary receptors for gonadotropin-releasing hormone (GnRH) were examined after infusion of GnRH into ewes. Twenty-four ovariectomized ewes were given i.v. infusions of GnRH (2.5 .mu.g/h) for 0, 1, 2, 4, 12, or 24 h. Jugular blood samples were collected at 30 min intervals beginning 2 h prior to and continuing throughout the infusions. At the end of treatment, pituitaries were collected and concentrations of receptors and gonadotropins in the anterior pituitary gland were quantified. Number of receptors for GnRH in a standard pool of pituitary membranes were determined by Scatchard analysis using [125I]-D-Ala6-desGlyNH210-GnRH-ethylamide ([125I]-D-Ala6-GnRH-EA) as labeled ligand. Increasing quantities of the membrane pool were used to construct a standard curve. To quantify receptors in each pituitary, binding of [125I]-D-Ala6-GnRH-EA to an enriched membrane fraction was compared with the standard curve. Concentrations of GnRH receptors (moles receptor/milligram pituitary .times. 10-16) were 1.96 .+-. 0.38, 2.72 .+-. 0.37, 2.50 .+-. 0.58, 3.85 .+-. 0.71, 1.41 .+-. 0.14, and 1.00 .+-. 0.10 after 0, 1, 2, 4, 12, and 24 h of infusion of GnRH, respectively. Serum concentrations of both LH [luteinizing hormone] and FSH [follicle stimulating hormone] were maximal after 2 h of infusion. Concentratons of both gonadotropins then began to decrease and had returned to baseline within 5 h (FSH) or 20 h (LH) even though infusion of GnRH continued. Analysis of variance indicated a significant effect of time treatment (P < 0.05) on pituitary receptors for GnRH and serum concentrations of gonadotropins. Pituitary concentrations of gonadotropins did not change (P > 0.05) during the infusion as demonstrated quantitatively by radioimmunoassay and qualitatively by immunocytochemical staining of LH. Since there was a decrease in the number of receptors for GnRH at 12 and 24 h after infusion of GnRH, a 2nd experiment was performed to determine if down-regulation of receptors was responsible for refractoriness of the pituitary to infused GnRH. After i.v. infusion of GnRH (2.5 .mu.g/h) into 4 ovariectomized ewes for 16 h, the ewes were given a bolus injection of 100 .mu.g GnRH in an attempt to saturate the remaining receptors. This treatment resulted in a 2nd peak of LH release, but of lesser magnitude (.apprx. 15%) than the initial peak. GnRH can participate in the regulation of its own receptor and down-regulation of receptors decreases the magnitude of LH release in response to a bolus injection of GnRH.