A Gonadotropin-Releasing Hormone (GnRH) Antagonist Decreases Androgen Production and Spermatogonial Multiplication in Frog (Rana esculenta): Indirect Evidence for the Existence of GnRH or GnRH-Like Material Receptors in the Hypophysis and Testis*

The effects of a GnRH antagonist (GnRHA) on GnRH agonist (GnRH*)-induced androgen production and spermatogonial multiplication were studied in the frog, Rana esculenta, in vivo and in vitro. Intact and hypophysectomized (PDX) animals were kept at 22 +/- 2 C and treated with GnRH (45 ng/g BW) and GnRH* plus 1X and 10X concentrations of GnRHA on alternate days for 2 weeks. Androgen concentration in GnRH* plus GnRHA-treated animals decreased in the testis by about 50% with the 10X dose whereas the increase obtained in GnRH*-treated PDX group was completely abolished with the 1X dose. Histological sections were evaluated with respect of the mitotic index (MI) of the primary spermatogonia. Both GnRHA-treated intact and PDX frogs showed a dose-dependent MI decrease which reached 59% and 57% of control, respectively. In vitro incubations were carried out on testis halves at 15 C for 0, 2, 4, 6, and 8 h with the addition of 1 microgram GnRH* and 1 microgram GnRH* plus 1 or 10 micrograms GnRHA. The stimulatory effect of GnRH* and the inhibitory effect of GnRHA were apparent within 2 h. The basal mitogenic activity was affected by antagonist treatment and the inhibitory effect on the MI was evident within 2-4 h in the 10X-treated groups or within 6-8 h in the 1X treated groups. Since GnRH* and GnRHA bind to the same receptor these data strongly indicate that the effects of putative GnRH-like materials in the frog, Rana esculenta, are mediated throughout stereospecific recognition sites in both pituitary and testis.