THE SALMON GNRH ENCODING GENE IN TELEOST FISH

This chapter describes the salmon gonadotropin-releasing hormone (GnRH) encoding gene in teleost fish. Following the identification of the mammalian GnRH variant, designated “mGnRH,” two GnRH forms have been isolated from chicken hypothalamus—cGnRH-I and cGnRH-II. Sherwood et al. characterized two novel GnRH variants from salmon and lamprey, designated “sGnRH” and “lGnRH,” respectively. The GnRH variants are distributed in the different vertebrate groups. Mammalian studies have demonstrated that the GnRH neurons originate outside the brain in the olfactory placode. During embryogenesis, these neurons migrate into the forebrain along with the terminal nerve. The spatial and temporal appearance of GnRH neurons in the brain of chicken and the newt Cynops pyrrhogaster during embryogenesis indicates a similar migration route in these vertebrate groups. sGnRH neurons are widely distributed in the teleost brain. Immunoreactive sGnRH cell bodies have been localized in the olfactory nerve and the bulbs along the ventral part of telencephalon and the preoptic area and in the lateral-basal hypothalamus of several species. The amount and distribution of sGnRH and cGnRH-II are the two most common GnRH variants in teleosts, which have been studied in the brain of several species using specific antisera. sGnRH fibers innervate the pituitary directly, whereas cGnRH-II fibers are absent from the pituitary. © 1993, Elsevier Science Publishers, B.V.