Pituitary and Gonadal Gonadotropin-Releasing Hormone Receptors during Sexual Maturation in the Rat*

The gonadotropin-releasing hormone (GnRH) agonist D-Ala6-des-Gly10-GnRH ethylamide (D-Ala analog) was used as ligand to study GnRH receptors in the pituitary and gonads during sexual maturation in the rat. The affinities of the GnRH receptor (Ka = 6.5 × 109 M-1) were similar in all three tissues and did not change during maturation. Pituitary GnRH receptor concentration increased 2-fold in both sexes. Peak values occurred earlier (20 days) and were higher (720 ± 52 fmol/mg protein) in females than in males (30 days; 594 ± 54 fmol/mg). The changes in GnRH receptors followed a pattern similar to that of plasma FSH, though GnRH receptors were maximal 10 days after the FSH peak. The binding capacity of the gonadal GnRH receptors also changed with age, and was maximal on day 20 in females (271 ± 25 fmol/mg) before declining to a constant level by day 60 (139 ± 20 fmol/mg). In males, binding to testicular interstitial tissue was not detectable at 30 days of age but increased rapidly to 256 ± 13 fmol/mg by 40 days, before declining to a stable level at 60 days (160 ± 14 fm/mg). The parallel rise in plasma FSH and pituitary GnRH receptor concentrations suggest that both are mediated by increased hypothalamic secretion of GnRH. The highest concentrations of GnRH receptors in the pituitary was found at an age when pituitary responsiveness to GnRH is known to be maximal. This suggests that an increase in GnRH receptor numbers form an important part of the mechanisms involved in increasing pituitary responses to GnRH. While the factors involved in determining gonadal GnRH receptors are unknown, the observed changes in receptor concentration suggest that GnRH or a GnRH-like peptide may be involved in controlling steroidogenesis during sexual maturation. (Endocrinology108: 1658, 1981)