GONADOTROPIN-RELEASING-HORMONE (GNRH) BINDING CHARACTERISTICS IN THE TESTIS OF GOLDFISH (CARASSIUS-AURATUS)

Gonadotropin-releasing hormone (GnRH) binding sites were characterized in the goldfish testis using an analog of salmon GnRH ([D-Arg6, Trp7, Leu8, Pro9-NEt]-GnRH; sGnRH-A) as a labeled ligand. Binding of sGnRH-A to goldfish testicular membrane homogenate was found to be saturable, reversible, and dependent on time, temperature, and tissue concentration. Addition of unlabeled sGnRH-A displaced the bound I-125-sGnRH-A in a dose-related manner. Hill plot as well as Scatchard analysis indicated the presence of two classes of binding sites, a high affinity/low capacity site and a low affinity/high capacity site in the goldfish testis. Bound I-125-sGnRH-A was also found to be displaceable by sGnRH ([Trp7, Leu8]-GnRH) and cGnRH-II ([His4, Trp7, Tyr8]-GnRH) which occur naturally in the goldfish brain. sGnRH-A and cGnRH-II were found to bind with greater affinity than sGnRH to the goldfish testicular GnRH binding sites. The findings provide the first characterization of testicular GnRH binding sites in a teleost species. (C) 1993 Wiley-Liss, Inc.