Gonadotropin-releasing hormone-immunoreactive neurons and associated nicotinamide adenine dinucleotide phosphate-diaphorase-positive neurons in the brain of a teleost, Rhodeus amarus

Using combined nicotinamide adenine dinucleotide phosphate-diaphorase (NADP Hd) histochemistry and salmon gonadotropin-releasing hormone (sGnRH) immuno cytochemistry, it is reported for the first time that possible potential co ntacts occur between the nitric oxide (NO) and the GnRH-containing neurons in the brain of a freshwater teleost, Rhodeus amarus. GnRH-immunoreactive ( Ir) neurons were observed in the olfactory nerve (OLN), olfactory bulb (OB) , medial olfactory tract (MOT), ventral telencephalon (VT), nucleus preopti cus periventricularis (NPP), nucleus lateralis tuberis (NLT), and midbrain tegmentum (MT). Although NADPHd neurons were widely distributed in the brai n, only those having an association with GnRH-ir neurons are described. Bas ed on the nature of the association between the GnRH and the NADPHd neurons , the former were classified into three types. The Type I GnRH neurons were characterized by the presence of NADPHd-positive granules in the perikarya and processes and occurred in the OLN, OB, MOT, and VT. The Type II GnRH n eurons, having soma-soma or soma-process contacts with the NADPHd neurons, were restricted to the MT; the long processes of NADPHd cells crossed over either the perikarya or the thick processes of GnRH cells. However, the Typ e III GnRH neurons, found in the NPP and NLT, did not show direct contact, but a few NADPHd fibers were present in the vicinity. The terminal-soma con tacts in the olfactory system and the VT and the soma-soma contacts in the MT represent the sites of possible potential contacts indicating a direct N O involvement in GnRH function, although NO action by diffusion remains pos sible. NO may influence the NPP and NIT GnRH cells by diffusion only, since a direct contact was not observed. (C) 2000 Academic Press.