Ultrastructural demonstration of salmon-type gonadotropin-releasing hormone (sGnRH) in the olfactory system of masu salmon (Oncorhynchus masou).

Immunocytochemical and immunoelectron microscopic localization of salmon-type gonadotropin-releasing hormone (sGnRH) were observed in the olfactory system (olfactory epithelium, olfactory nerve and olfactory bulb) of masu salmon (Oncorhynchus masou) to investigate its possible involvement in the olfactory functions. sGnRH-immunoreactive (ir) bipolar neuron, which might be related to the terminal nerve, was located in dorsal portion of the olfactory nerve. sGnRH-ir gold particles were concentrated on electron-dense granule-like structures of 50 nm in diameter in fibers of the olfactory nerve close both to the olfactory epithelium and to the olfactory bulb. These findings suggest that sGnRH might participate in neurotransmission and/or neuromodulation in the olfactory system of masu salmon. sGnRH would become a useful molecular marker for studying the olfactory imprinting and homing mechanisms in salmonids.