Direct synaptic contacts of nitric oxide synthase-immunoreactive nerve terminals on the neurons of the intracardiac ganglia of the guinea pig.

Immunoelectron microscopy was performed to localize immunoreactivity for nitric oxide synthase (NOS) in guinea pig atria. 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. The present results raise the possibility that nitric oxide plays a role in the neural control of the heart through synapses in the cardiac ganglia.