CHARACTERIZATION OF GONADOTROPIN-RELEASING-HORMONE (GNRH) RECEPTORS IN THE OVARY OF COMMON CARP (CYPRINUS-CARPIO)

Gonadotropin-releasing hormone (GnRH) binding sites have been characte rized in the fully mature common carp ovary, using an analog of salmon GnRH ([D-Arg6,Trp7,Leu8,Pro9-NEt]-GnRH; sGnRH-A) as a labeled ligand. Binding of sGnRH-A to carp follicular membrane preparation was found to be time-, temperature-, and pH-dependent. Optimal binding was achie ved after 40 min of incubation at 4-degrees-C at pH 7.6; binding was f ound to be unstable at room temperature. Binding of radioligand was a function of tissue concentration, with a linear correlation over the r ange of 8.0-40.0-mu-g membrane protein per tube. Incubation of membran e preparations with increasing levels of [I-125]sGnRH-A revealed satur able binding at radioligand concentrations greater than 400 nM. The bi nding of [I-125]sGnRH-A to the carp ovary was also found to be reversi ble; addition of unlabeled sGnRH-A (10(-6) M) after reaching equilibri um resulted in complete dissociation of [I-125]sGnRH-A within 30 min, and the log dissociation plot indicated the existence of a single clas s of binding sites. Addition of unlabeled sGnRH-A displaced the bound [I-125]sGnRH-A in a dose-related manner. Hill plot as well as Scatchar d analysis suggested the presence of one class of high affinity GnRH b inding sites. Bound [I-125]sGnRH-A was also found to be displaceable b y other GnRH peptides, including sGnRH ([Trp7,Leu8]-GnRH), cGnRH-II ([ His5,Trp7,Tyr8]-GnRH) and a GnRH antagonist ([D-pGlu1,D-Phe2,D-Trp3,6] -GnRH; GnRH-ANT) in a parallel fashion, indicating that these peptides bind to the same class of binding sites. sGnRH-A and cGnRH-II were fo und to bind with greater affinities than sGnRH and GnRH-ANT to the car p ovarian binding sites. These results provide for the first time char acterization of GnRH binding sites in the ovary of a teleost species, Cyprinus carpio.