Nitric oxide synthase containing neurons in the cardiac ganglia of the guinea pig.

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Immunoelectron microscopy was performed to localize immunoreactivity for nitric oxide synthase (NOS) in guinea pig heart. Many round intracardiac ganglia, close to the openings of the pulmonary veins, contained NOS-immunoreactive neurons. A small number of neurons were immunoreactive, but most of the non-immunoreactive neurons were surrounded by immunoreactive axons. Direct axo-somatic or-spinic synapses from NOS-immunoreactive axon varicosities to intracardiac ganglionic neurons were observed. Many small ganglia were found in the interatrial septum near the atrioventricular node. A small number of neurons in these ganglia were also immunoreactive. NOS immunoreactive axons were observed in nerve bundles near or within the node. Very thick immunoreactive axons, 8-10 mum in diameter, were found between the conductive tissue and the ordinary cardiac muscles. Electron-microscopic examination confirmed that these axons were unmyelinated. Nerve bundles in this region contained both myelinated and unmyelinated axons ; however, immunoreactivity was detected exclusively in unmyelinated axons. Fine NOS-immunoreactive nerve fibers with terminal varicosities were often seen in the atrioventricular node. Close contact between NOS-immunoreactive axon varicosity and specialized cardiac muscle cell was observed. These results indicate a possibility that nitric oxide plays a role in the neural control of the conductive tissue in the heart through direct neuromuscular contact.