PRESENCE OF GONADOTROPIN-RELEASING-HORMONE (GNRH) BINDING-SITES AND COMPOUNDS WITH GNRH-LIKE ACTIVITY IN THE OVARY OF AFRICAN CATFISH, CLARIAS-GARIEPINUS

GnRH binding was characterized in the African catfish ovary by use of an analog of salmon GnRH (sGnRH-A; [D-Arg(6), Trp(7), Leu(8), Pro(9)-N Et]GnRH) as a labeled ligand. Binding of sGnRH-A to catfish ovarian me mbrane preparation was found to be saturable, displaceable, reversible , and dependent on time, temperature, and tissue concentration. Optima l binding was achieved after 70 min of incubation at room temperature (similar to 22 degrees C) at pH 7.6. Addition of unlabeled sGnRH-A dis placed the bound I-125-sGnRH-A in a dose-related manner. Hill plot as well as Scatchard analysis indicated the presence of one class of high -affinity binding sites with a equilibrium dissociation constant (K-d) of 0.27 +/- 0.036 nM. Bound I-125-sGnRH-A was also found to be displa ceable by catfish GnRH (cfGnRH; [His(5), Leu(7), Asn(8)]-GnRH), chicke n GnRH-II (cGnRH-II; [His(5), Trp(7), Tyr(8)]-GnRH), and salmon GnRH ( sGnRH; [Trp(7), Leu(8)]-GnRH); all the peptides were found to bind wit h lower affinities than sGnRH-A to the catfish ovarian GnRH binding si tes. Further experiments using ovarian extracts indicated the presence of compounds with GnRH-like activity in the ovary of African catfish. The crude ovarian extract was found to stimulate pituitary gonadotrop in release h om goldfish pituitary, as well as displacing I-125-sGnRN- A binding in the catfish ovary. HPLC analysis of the catfish ovarian e xtract revealed the presence of two fractions that bind specifically t o the catfish ovary and release gonadotropin from cultured goldfish pi tuitary. These fractions include an early eluting peak that does not c orrespond with the retention time of known GnRH forms in addition to a fraction that co-elutes with the mammalian GnRH. Overall, the study p rovided characterization of GnRH binding sites in the catfish ovary, a nd evidence for the presence of compounds with GnRH-like activity in t he catfish ovary.