ONTOGENIC DEVELOPMENT OF SALMON GNRH AND CHICKEN GNRH-II SYSTEMS IN THE BRAIN OF MASU SALMON (ONCORHYNCHUS-MASOU)

Ontogenic development of salmon gonadotropin-releasing hormone (GnRH) and chicken GnRH-II systems in masu salmon (Oncorhynchus masou) was ex amined. Salmon GnRH was first detected by radioimmunoassay in the embr yo on day 36 after fertilization. Salmon GnRH-immunoreactive fibers we re detected initially by immunocytochemistry in the vicinity of the ol factory placode of the embryo (day 36) and were distributed widely in the brain as well as in the pituitary gland of fish just after hatchin g (day 80). Salmon GnRH-immunoreactive neuronal somata were first dete cted about 6 months after fertilization in the rostroventral brain are a, ranging from the olfactory nerve to the preoptic area. Salmon GnRH neuronal somata were detected earlier by in situ hybridization than by immunocytochemistry. Neuronal somata expressing salmon GnRH mRNA were first detected in the vicinity of the olfactory epithelium on day 40 and then were seen to be migrating from the olfactory epithelium, alon g the olfactory nerve, on day 60 and in the transitional area between olfactory nerve and olfactory bulb on day 80. In the brain, these neur ons were first detected in the ventral olfactory bulb on day 80, and t hereafter they were also detected in the caudal brain regions. The chi cken GnRH-II system was detected later than the salmon GnRH system; ch icken GnRH-II was first detected by radioimmunoassay on day 57, and ch icken GnRH-II-immunoreactive fibers were first detected on day 67. Chi cken GnRH-II-immunoreactive neuronal somata were not detected during t he observation period. These results suggest that salmon GnRH neurons derive from the olfactory placode and then migrate into the brain and that salmon GnRH is synthesized before chicken GnRH-II.