The pituitary gonadotropin-releasing hormone (GnRH) receptor of the female rabbit: characterization and developmental aspects

The aim of the present study was to characterize the pituitary gonadotropin-releasing hormone (GnRH) binding site in the rabbit and investigate its possible role in sexual maturation of the female rabbit. A radioligand binding assay was established, and the presence of specific 125I-labelled D-Ala6-des-Gly10-GnRH ethylamide (125I-DAl6EA) binding sites in the anterior pituitary gland of the rabbit was demonstrated. 125I-DAla6EA binding was saturable, specific, displaceable, reversible, correlated with increasing tissue concentrations, and susceptible to physiological manipulation. 125I-DAla6EA binding indicated the presence of two binding sites in the female adult rabbit pituitary: a high affinity, low capacity site (KD = 0.3-0.4 nM; Bmax = 100-200 fmol/mg protein) and a lower affinity, high capacity site (KD = 30 nM; Bmax = 5-8000 fmol/mg protein). Ontogeny of 125I-DAl6EA binding in the female rabbit (40-120 days of age) did not show a correlation between binding site number and serum luteinizing hormone (LH). In addition, the net serum LH response in female rabbits to a subcutaneous injection of DAla6EA (10 ng, 100 ng, and 1 microgram per kilogram body weight) was not significantly different between animals 40, 75, and 120 days of age. This suggests that a decrease in pituitary responsiveness to GnRH is not associated with sexual maturation in the female rabbit. Results indicate that factors other than and (or) in addition to GnRH binding site number, such as postreceptor events, play a role in gonadotropin secretion in the female rabbit.