Pituitary gonadotropin-releasing hormone receptors. Effects of castration, steroid replacement, and the role of gonadotropin-releasing hormone in modulating receptors in the rat.

Abstract

To study the role of gonadotropin-releasing hormone (GnRH) receptors in the regulation of gonadotropin secretion, we used D-125I-alanine6 des glycyl10 GnRH ethylamide (D-125I-Ala analog), a nondegradable, superagonist GnRH analog to assess GnRH receptors on rat pituitary membranes. Receptor affinity in intact adult rats was 5.0 X 10(9) M-1 and was unchanged after castration in both sexes. Castration of adult male and female rats produced a twofold increase in GnRH binding capacity by 7 d and binding capacity remained elevated for the subsequent 14 d. GnRH receptor number rose more rapidly after castration in males than females, and the time-course of receptor rise was similar to the increase in serum gonadotropin levels. The increase in GnRH binding capacity was prevented by gonadal steroid replacement at the time of castration in both sexes. Injections of the GnRH analog, D-Ser6 (TBu) des Gly10 GnRH ethylamide for 4 d produced a 70% increase in GnRH receptor number in intact male rats and testosterone-replaced castrates. The same regimen, however, failed to increase the elevated receptor numbers present after castration. Administration of rabbit anti-GnRH serum concomitant with castration inhibited the rise in both GnRH receptor number and luteinizing hormone. The changes in pituitary GnRH receptors parallel previously demonstrated changes in hypothalamic secretion of GnRH. Thus, GnRH probably regulates its own receptor in vivo and gonadal steroids may influence pituitary GnRH receptors by changing hypothalamic GnRH secretion.