Nitric oxide synthase containing nerves in the atrioventricular node of the guinea pig heart

Immunoelectron microscopy was performed to localize immunoreactivity for nitric oxide synthase (NOS) in the guinea-pig atrioventricular node. Many small ganglia were found in the interatrial septum near the atrioventricular node. A small number of neurons in these ganglia were immunoreactive. NOS immunoreactive axons were observed in nerve bundles near or within the node. Very thick immunoreactive axons, 8-10 microns 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. Intranodal ganglia were observed among the specialized cardiac muscles. They were surrounded by numerous axons, some of them were immunoreactive. Direct axo-somatic synapses from NOS-immunoreactive terminals to the intranodal ganglion cells were observed. The present 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.