FUNCTIONAL SPECIFICITY FOR SALMON GONADOTROPIN-RELEASING-HORMONE (GNRH) AND CHICKEN GNRH-II COUPLED TO THE GONADOTROPIN-RELEASE AND SUBUNITMESSENGER-RIBONUCLEIC-ACID LEVEL IN THE GOLDFISH PITUITARY

GnRH is the key regulator of reproduction in the vertebrates. In this study, we investigated the release and synthesis of maturational gonad otropin hormone (GTH-II) stimulated by native GnRH forms, salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), in the goldfish pituitary. The experimental approach was to study the differences between desensitiz ation induced by sGnRH and cGnRH-II administered in homologous and het erologous fashion. Pulsatile alternate treatments with sGnRH and cGnRH -II (i.e. sGnRH/cGnRH-II or cGnRH-II/sGnRH) at 10(-8) M (every 30 min) resulted in a lower degree of desensitization compared with homologou s treatments with either sGnRH or cGnRH-II (sGnRH/sGnRH or cGnRH-II/cG nRH-II), or when combined together (sGnRH + cGnRH-II). We also investi gated the effects of continuous treatments with sGnRH and cGnRH-II, ad ministered in a homologous or heterologous fashion. Increasing concent rations of either sGnRH or cGnRH-II (10(-8)-10(-6) M) administered con tinuously (60 min) in a homologous fashion resulted in significant des ensitization of the pituitary GTH-II release. Alternate continuous tre atments with sGnRH and cGnRH-II (i.e. sGnRH/ cGnRH-II/sGnRH or cGnRH-I I/sGnRH/cGnRH-II) resulted in lower degree of desensitization compared to homologous treatments, particularly at lower doses. We further inv estigated the effects of sGnRH and cGnRH-II on GTH-II beta and GTH-II alpha subunit messenger RNA (mRNA) levels in the goldfish pituitary. I n sexually regressed animals, sGnRH treatment (4 mu g/fish) increased the accumulation of GTH-II beta and GTH-II alpha mRNA, whereas cGnRH-I I treatment was without effect. In sexually mature animals, however, b oth cGnRH-II and sGnRH stimulated accumulation of GTH-II beta and GTH- II alpha mRNA, with cGnRH-II exerting a greater effect on GTH-II subun it mRNA production. These results suggest a differential control of GT H-II subunit gene expression or mRNA stabilization by sGnRH and cGnRH- II in the goldfish pituitary based on the stage of gonadal recrudescen ce. In general, the present findings support the hypothesis that sGnRH and cGnRH-II regulate the release and synthesis of GTH-II through dif ferent receptor-effector mechanisms in the goldfish pituitary.