Postganglionic, direct axo-axonal contacts on the splenic nerve

The splenic nerve is made up almost exclusively of adrenergic fibers. Consequently it was used as a model system in the study of autonomic sympathetic neurotransmission. The splenic nerve regulates the vasoconstriction and volume reduction of the spleen. Brain-immune interactions via modulation of the splenic nerve activity may regulate peripheral cellular immunity. An inhibition of noradrenaline release by alpha(2)-adrenoceptor activation has been reported. As we were interested in a structurally detailed distribution of synaptophysin, immunocytochemical methods were applied to splenic nerve axons. In 1 microm plastic sections a network of synaptophysin-positive varicosities could be observed all along the splenic nerve. They were also positive for dopamine-beta-hydroxylase and cytochrome b561. At the ultrastructural level the varicosities were seen to establish direct contact with the splenic axons. In normal morphology the varicosities revealed small synaptic vesicles and several dense granules. It is demonstrated that a network of direct symmetric contacts of adrenergic nature is present all along the nerve. These terminals may have an inhibitory effect on the splenic nerve activity via axonal receptors. This finding opens new perspectives for the study of the splenic nerve in general and more particularly for its role in the regulation of peripheral cellular immunity.