RECEPTOR-BINDING OF GONADOTROPIN-RELEASING-HORMONE ANTAGONISTS THAT INHIBIT RELEASE OF GONADOTROPIN-II AND GROWTH-HORMONE IN GOLDFISH, CARASSIUS-AURATUS

In goldfish, GnRH stimulates gonadotropin-II (GTH-II) and growth hormo ne (GH) release. The two native forms of GnRH, salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), bind to two classes of GnRH binding sites : high-affinity/low-capacity sites and low-affinity/high-capacity site s. Our previous in vitro perifusion studies of goldfish pituitary frag ments showed that [Ac-Delta(3)-Pro(1), 4FD-Phe(2), D-Trp(3,6)]-mGnRH ( analog E), [Ac-Delta(3)-Pro(1), 4FD-Phe(2), D-Trp(3,6)]-sGnRH (analog C), and [Ac-D(2)Nal(1), 4Cl-D-Phe(2), D-(3)Pal(3,6)]-cGnRH-II (analog N) inhibited both sGnRH- and cGnRH-II-stimulated GTH-II and GH release . Interestingly, analog C stimulated GH release but not GTH-II release . The objectives of the present study were 1) to test the site of acti on of GnRH antagonists in goldfish, 2) to test the relationship betwee n receptor binding affinity of antagonists and their in vitro inhibito ry potencies and apparent duration of action, and 3) to compare the bi nding characteristics of analog C with its differential action on GTH- LI and GH release. As in previous studies, analog E suppressed sGnRH-s timulated GTH-II and GH release from perifused pituitary fragments. Si milarly, analog E suppressed both sGnRH- and cGnRH-II-stimulated GTH-I I and GH release from perifused dispersed goldfish pituitary cells, in dicating the direct action of GnRH antagonists at the pituitary cell l evel. In the receptor binding studies, analog E displaced I-125-[D-Arg (6), Pro(9)NHEt]-sGnRH (sGnRH-A) from crude goldfish pituitary membran e preparations in a dose-dependent manner. The binding affinities to h igh-affinity GnRH binding sites of analog E, analog C, and analog N we re significantly higher than those of native sGnRH and sGnRH-A. The ra nk order of high affinity was analog N greater than or equal to C > E, and binding affinity had no positive relation with the inhibitory eff ects of these analogs on GTH-II or GH release in vitro. In conclusion, GnRH antagonists inhibit native GnRH-stimulated GTH-II and GH release by competitively binding to binding sites at the pituitary cells, alt hough there was no positive relation between receptor binding affinity and in vitro GTH-II or GH release-inhibiting potency of the analogs t ested.