HOMOLOGOUS DESENSITIZATION OF GONADOTROPIN-RELEASING-HORMONE (GNRH) RECEPTORS IN THE GOLDFISH PITUITARY - EFFECTS OF NATIVE GNRH PEPTIDES AND A SYNTHETIC GNRH ANTAGONIST

Gonadotropin-releasing hormone (GnRH) stimulates release of gonadotropin hormone (GTH) through interaction with high affinity receptors in the goldfish pituitary. In the present study, we investigated desensitization of two native GnRH peptides, [Trp 7, Leu 8]-GnRH (sGnRH) and [His 5, Trp 7, Tyr 8]-GnRH (cGnRH-II), using superfused fragments of goldfish pituitary in vitro. Pulsatile treatment with either sGnRH or cGnRH-II (2-min pulses given every 60 min) resulted in dose-dependent secretion of GTH from the goldfish pituitary; cGnRH-II had a greater GTH release potency and displayed a greater receptor binding affinity than sGnRH. Both sGnRH and cGnRH-II-induced GTH release were partially inhibited by concomitant treatment with either [D-Phe 2, Pro 3, D-Phe 6]-GnRH or [D-pGlu 1, D-Phe 2, D-Trp 3,6]-GnRH. These antagonists had greater receptor binding affinities than the native peptides, with no stimulatory action on GTH release in the absence of the GnRH agonists. Continuous treatment with either sGnRH or cGnRH-II (10(-7) M), rapidly desensitized pituitary GTH release in a biphasic fashion; initially there was a rapid increase in GTH release of approximately 10-20-fold (phase 1), followed by a sharp decline in GTH release, reaching a stable concentration 2-3-fold above the basal level (phase 2). Further stimulation of the pituitaries with sGnRH or cGnRH-II (10(-7) M) (second treatment) after 60 min recovery resulted in a significantly lower sGnRH or cGnRH-II-induced GTH release compared to that observed during the initial treatment period. The desensitized GTH response observed during the second treatment period was agonist-induced, since initial treatment with a GnRH antagonist ([D-pGlu 1, Phe 2, D-Trp 3,6]-GnRH; GnRH-ant), did not reduce sGnRH- or cGnRH-II-induced GTH release during the second treatment period. Both sGnRH and cGnRH-II treatments resulted in significant reduction in the pituitary GnRH receptor content of the high affinity sites with no effect on the binding affinities; treatment with GnRH-ant was without effect. In summary, the present study demonstrates for the first time GnRH desensitization in a teleost species and provides information on relative desensitizing effects of two native GnRH molecules in the goldfish pituitary. It is evident from the results that the GnRH-induced desensitization is an agonist-induced process and may be in part the result of a change in GnRH receptor content in the goldfish pituitary.