NITRIC-OXIDE SYNTHASE-CONTAINING NEURONS AND NERVE-FIBERS WITHIN CARDIAC GANGLIA OF RAT AND GUINEA-PIG - AN ELECTRON-MICROSCOPIC IMMUNOCYTOCHEMICAL STUDY

The nitric oxide synthase-immunoreactivity and NADPH-diaphorase activi ty of intracardiac neurones in the rat and guinea-pig was studied at t he ultrastructural level. While some nitric oxide synthase-containing intracardiac neurones were very heavily labelled, with electron-dense immunoprecipitate distributed throughout the neuronal cell bodies and their processes, most of the labelled neurones exhibited a lighter and more patchy distribution of nitric oxide synthase-immunoreactive mate rial. Synapses made by nitric oxide synthase-negative nerve fibres wit h labelled intracardiac neurones were seen. Conversely, many nitric ox ide syn thase-containing nerve fibres that made synaptic contacts with unlabelled intracardiac neurones were also observed. Some small granu le-containing cells were nitric oxide synthase-immunoreactive and were associated with unlabelled nerve terminals, while non-immunoreactive small granule-containing cells that were innervated by nitric oxide sy nthase-immunoreactive nerves were also seen. Small patches of osmiophi lic electron-dense material were observed in the cytoplasm of NADPH-di aphorase-positive intracardiac neurones. This is the first description of the ultrastructural distribution of nitric oxide synthase-immunore activity and NADPH-diaphorase activity in a subpopulation of intracard iac neurones of rat and guinea-pig heart and provides further evidence in support of a role for nitric oxide in the local control of the hea rt by intrinsic neurones.