The Pituitary Gonadotropin-Releasing Hormone (GnRH) Receptor of the Female Rabbit: Characterization and Developmental Aspects

Abstract

<p>The aim of the present study was to characterize the pituitary 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-DAla⁶EA binding sites in the anterior pituitary gland of the rabbit was demonstrated. ¹²⁵I-DA1a⁶EA binding was saturable, specific, displaceable, reversible, correlated with increasing tissue concentrations and susceptible to physiological manipulation. Significant ¹²⁵I-DA1a⁶EA binding was not present in the rabbit ovary suggesting that GnRH or GnRH-related peptides are not directly involved in the control of luteal function of the rabbit. 12S1_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-DA1a⁶EA binding in the female rabbit (40 to 120 days of age) did not show a correlation between binding site number and serum LH. In addition, the net serum LH response in female rabbits to a subcutaneous injection of DA1a⁶EA (10ng, 100ng, 1μg per kg 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 post-receptor events play a role in gonadotropin secretion in the female rabbit.</p>