CYTOPHYSIOLOGY OF GONADOTROPIN-RELEASING-HORMONE NEURONS IN CHUM SALMON (ONCORHYNCHUS-KETA) FOREBRAIN BEFORE AND AFTER UPSTREAM MIGRATION

Cytophysiology of gonadotropin-releasing-hormone neurons in chum salmo n (Oncorhynchus keta) was examined before and after upstream migration by an immunocytochemical technique with a specific antiserum to salmo n gonadotropin-releasing hormone and an in situ hybridization techniqu e with an Oligonucleotide encoding salmon gonadotropin-releasing-hormo ne precursor (pro-salmon gonadotropin-releasing hormone). In the foreb rain (olfactory nerve, olfactory bulb, telencephalon, and preoptic are a), salmon gonadotropin-releasing hormone-immunoreactive neurons and n eurons showing signals for pro-salmon gonadotropin-releasing-hormone m RNA were compared between fish from the coastal sea and those from the spawning ground, Neurons in the dorsal region of the olfactory nerve and in the ventral region of the transitional area between olfactory n erve and olfactory bulb showed strong salmon gonadotropin-re leasing-h ormone immunoreactivity and strong hybridization signals in fish from the coastal sea, but these activities and signals were not observed or were decreased in number in fish from the spawning ground. The neuron s in the olfactory bulb, telencephalon, and preoptic area consistently revealed salmon gonadotropin-releasing-hormone immunoreactivity and h ybridization signals, and the hybridization signals of salmon gonadotr opin-releasing hormone in the telencephalon and the preoptic area were stronger in fish from the spawning ground than in those from the coas tal sea. These findings suggest that salmon gonadotropin-releasing-hor mone neurons in the olfactory nerve and the transitional area between olfactory nerve and olfactory bulb have different patterns of hormone production than those in the telencephalon and the preoptic area.