3 FORMS OF GONADOTROPIN-RELEASING-HORMONE IN A PERCIFORM FISH (SPARUS-AURATA) - COMPLEMENTARY DEOXYRIBONUCLEIC-ACID CHARACTERIZATION AND BRAIN LOCALIZATION

Three forms of GnRH-salmon (sGnRH), seabream (sbGnRH), and chicken (cG nRH-II)-have been described in the gilthead seabream (Sparus aurata) b rain, and the cDNA encoding the sbGnRH precursor was recently isolated . In the present study, the cDNAs encoding the sGnRH and cGnRH-II were isolated and characterized, and the neurons producing the three GnRHs were localized in the seabream brain. Fragments of sGnRH and cGnRH-II cDNAs were amplified by polymerase chain reaction and used as probes to isolate the full-length cDNAs from a brain cDNA library. The cDNA e ncoding the cGnRH-II precursor is 573 nucleotides (nt) long, and the c DNA encoding the sGnRH precursor is 1971 nt in length with an unusuall y long 5' untranslated region. Specific single-strand DNA probes for i n situ detection of mRNA were designed according to nonconserved regio ns among the three GnRH cDNAs. Localization of GnRH mRNA-producing cel ls in the brain revealed five distinct populations of cells: sGnRH-pro ducing cells in the ventromedial olfactory bulbs and the terminal nerv e, sbGnRH-producing cells in the preoptic area and the ventral thalamu s, and cGnRH-II-producing cells in the midbrain tegmentum. The discret e sites of expression of the three forms of GnRH indicate that only sb GnRH is directly involved in the control of gonadotropin secretion.