Synaptic junctions between the adrenergic axon varicosity and the pinealocyte in the rat.

Intercellular relationships of sympathetic nerve fibers and cells in pineal glands of Long-Evans rats and yellow-bellied country rats (Rattus losea Swinhoe) were studied with conventional electron microscopy. Typical synapses were found between adrenergic axon varicosities and pinealocytes. The synaptic cleft was 20-30 nm thick and contained granular or filamentous material of moderate electron density. Some small granulated vesicles attached to, or fused with, the presynaptic axolemma, a feature suggesting an exocytotic release of the vesicle content. The occurrence of membranous cisternae or tubules in the synaptic nerve ending may connote the phenomenon of synaptic vesicle recycling. Beneath the postsynaptic pinealocyte plasmalemma there was a dense plaque corresponding to the postsynaptic density of neurons. Possible origins and functional significance of the synapse-making nerve fibers are also discussed.