Gonadotropin-releasing hormone (GnRH) binding sites have been characterized 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 achieved after 40 min of incubation at 4 °C at pH 7.6; binding was found to be unstable at room temperature. Binding of radioligand was a function of tissue concentration, with a linear correlation over the range of 8.0–40.0 μg membrane protein per tube. Incubation of membrane preparations with increasing levels of [125I]sGnRH-A revealed saturable binding at radioligand concentrations greater than 400 nM. The binding of [125I]sGnRH-A to the carp ovary was also found to be reversible; addition of unlabeled sGnRH-A (10−6 M) after reaching equilibrium resulted in complete dissociation of [125I]sGnRH-A within 30 min, and the log dissociation plot indicated the existence of a single class of binding sites. Addition of unlabeled sGnRH-A displaced the bound [125I]sGnRH-A in a dose-related manner. Hill plot as well as Scatchard analysis suggested the presence of one class of high affinity GnRH binding sites. Bound [125I]sGnRH-A was also found to be displaceable by 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 found to bind with greater affinities than sGnRH and GnRH-ANT to the carp ovarian binding sites. These results provide for the first time characterization of GnRH binding sites in the ovary of a teleost species, Cyprinus carpio.Key words: gonadotropin-releasing hormone, luteinizing hormone releasing hormone, receptor, ovary, carp, Cyprinus carpio.
Gonadotropin-Releasing Hormone Stimulation of Pituitary Gonadotrope Cells Produces an Increase in Intracellular Calcium