Histochemical and immunocytochemical localization of nitric oxide synthase in the supramedullary neurons of the pufferfish Tetraodon fluviatilis

The presence of the nitric oxide (NO) converting enzyme, constitutive neuronal NO synthase (nNOS), was investigated in the supramedullary neurons (SN) cluster of the pufferfish Tetraodon fluviatilis. The identification of NADPH diaphorase- (NADPHd-) positivity and the demonstration of nNOS with the BAS technique and with immunofluorescence together, strongly indicate the presence of a constitutive NO converting enzyme in SN cellular bodies and axons, and provides evidence that the SN cluster represents a distinct nitrergic neuronal system in the vertebrate CNS. The possible roles of NO in the cluster are discussed, including an involvement in communication among neurons and between neurons and the glial cells in the cluster.