LOCALIZATION OF SALMON GONADOTROPIN-RELEASING-HORMONE MESSENGER-RNA AND PEPTIDE IN THE BRAIN OF ATLANTIC SALMON AND RAINBOW-TROUT

The decapeptide gonadotropin-releasing hormone (GnRH) is a key hormone for the central regulation of reproduction. The distribution of salmo n GnRH (sGnRH), which is the major form in salmonids, has been studied in different fish species by immunocytochemistry. Discrepancies in da ta concerning the distribution of sGnRH perikarya led us to investigat e this problem in two species, the Atlantic salmon and the rainbow tro ut, with in situ hybridization of sGnRH messenger, a highly specific m olecular tool. By Northern blot analysis, the rainbow trout sGnRH mess enger appears to be about 500 bases in length, which is close to those isolated from Atlantic salmon or masu salmon and characterized previo usly. In situ hybridization with riboprobes generated with Atlantic sa lmon sGnRH cDNA demonstrated that sGnRH perikarya are restricted to th e ventral part of olfactory bulbs, telencephalon, and preoptic area. T hey are distributed on a nearly continuous line extending from the olf actory bulbs to the preoptic area in both salmonid species studied. De spite the presence of GnRH-like immunoreactivity in the preoptic magno cellular nucleus (NPOm) and in the tegmentum of the midbrain (MT), the sGnRH mRNA is not present in these two structures. Stained cells in N POm could be target cells for GnRH and immunoreactive neurons in MT ar e likely to be chicken GnRH-II containing cells. Our study not only gi ves a precise distribution of the sGnRH system in two salmonids, Atlan tic salmon and rainbow trout, but also clarifies the ambiguous data pu blished up to now in rainbow trout. (C) 1994 Wiley-Liss, Inc.