New synapses associated with the granule-containing cells of rat sympathetic ganglia.

The synapses of the rat superior cervical sympathetic ganglion were studied with both conventional and ultrastructural histochemical methods. Besides the cholinergic synapses polarized from preganglionic fibers to sympathetic ganglion neurons, two morphologically and functionally different types of synapses were observed in relation to the small granule-containing (catecholamine-containing) cells of the rat superior cervical ganglion. The first type is an efferent adrenergic synapse polarized from granule-containing cells to the dendrites of the sympathetic ganglion neurons. This type of synapse might mediate the inhibitory effects (slow inhibitory postsynaptic potentials) induced by catecholamines on the sympathetic neurons. The second type is a reciprocal type of synapse between the granule-containing cells and the cholinergic preganglionic fibers. Through such synapses, these cells could exert a modulating effect on the excitatory preganglionic fibers. Therefore, we propose that these cells, through their multiple synaptic connections, exhibit a local modulatory feedback system in the rat sympathetic ganglia and may serve as interneurons between the preganglionic and postganglionic sympathetic neurons.