FUNCTIONAL EVIDENCE REGARDING RECEPTOR SUBTYPES MEDIATING THE ACTIONS OF NATIVE GONADOTROPIN-RELEASING HORMONES (GNRH) IN GOLDFISH, CARASSIUS-AURATUS

In goldfish, two native gonadotropin-releasing hormone (GnRH) peptides, salmon GnRH (sGnRH) and chicken-II GnRH (cGnRH-II), stimulate gonadotropin (GTH-II) and growth hormone (GH) secretion. In the present study we tested whether sGnRH and cGnRH-II act through different or the same population of GnRH receptors on GTH and GH cells, using various approaches. A GnRH antagonist [Ac-Delta(3)-Pro(1),4FD-Phe(2),D-Trp(3,6)]sGnRH (analog C) equally inhibited GTH-II release by sGnRH and cGnRH-II. Similarly, [Ac-D(2)Nal(1),4Cl-D-Phe(2),D-(3)Pal(3),(6)]cGnRH-II (analog N) suppressed GTH-II stimulation by the two GnRH peptides equally. Further, in the continued presence of 100 nM sGnRH, a 100 nM pulse of cGnRH-II failed to cause additional GTH-II release and vice versa. Prolonged exposure of pituitary fragments to 100 nM sGnRH or cGnRH-II resulted in desensitization and reduced the GTH-II response to a subsequent pulse of either sGnRH or cGnRH-II equally. All these results indicate that both sGnRH and cGnRH-II act through the same population of GnRH receptors on gonadotrophs. Similarly, the GH responses obtained with (i) analog N treatment, (ii) pulse challenge to cGnRH-II during prolonged sGnRH treatment and vice versa, and (iii) desensitization studies suggest that both sGnRH and cGnRH-II act through the same population of GnRH receptors on somatotrophs. However, the properties of GnRH receptors on GTH-II and GH cells are different. Analog C stimulated GH release but not GTH-II release. In contrast, [Ac-Delta(3)-Pro(1),4FD-Phe(2),D-Trp(3),D-Arg(6)]mGnRH stimulated GTH-II release but weakly suppressed GH release. These results indicate that sGnRH and cGnRH-II act on the same population of receptors on GTH cells and on a functionally distinct population of receptors on GH cells. (C) 1994 Academic Press, Inc.