IMMUNOCYTOCHEMICAL LOCALIZATION OF MAMMALIAN GNRH (GONADOTROPIN-RELEASING-HORMONE) AND CHICKEN GNRH-II IN THE BRAIN OF THE EUROPEAN SILVER EEL (ANGUILLA-ANGUILLA L)

Using specific antibodies for the two molecular forms of gonadotropin- releasing hormone (GnRH) present in the European eel, Anguilla anguill a, (mammalian GnRH, mGnRH, and chicken GnRH II, cGnRH-II), we employed immunocytochemistry to determine the distribution of these two peptid es in the brain and in the pituitary. The results indicate that mGnRH and cGnRH-II are localized in different neurons: nGnRH-immunoreactive (ir) perikaria were observed in the olfactory bulbs, the junction betw een olfactory bulbs and telencephalon (nucleus olfactoretinalis), the telencephalon, the preoptic region adn the mediobasal hypothalamus. Th ese cell bodies are located along a continuum of ir-fibers that could be traced from the olfactory nerve to the pituitary. Mammalian GnRH-ir fibers were detected in many parts of the brain (olfactory bulbs, ven tral telencephalon, hypothalamus, optic tectum, mesencephalon) and in the pituitary. Chicken GnRH-II-ir cell bodies were detected in differe nt parts of the brain. The pituitary exhibited very few cGnRH-II-ir fi bers, contrasting with an extensive mGnRH innervation. These results a re in agreement with our previous data obtained in the same species us ing specific radioimmunoassays for mGnRH and cGnRH-II. They demonstrat e a differential distribution of the two forms of GnRH in the brain of the eel, as in the brain of some other vertebrae species, and suggest differential physiological roles for the two GnRH forms in the eel. T hey also provide information concerning the evolution of the GnRH syst ems in vertebrates.