Ultrastructural demonstration of nonsynaptic release sites in the central nervous system of the snail Lymnaea stagnalis, the insect Periplaneta americana, and the rat.

Release of neuronal secretory products by exocytosis was studied ultrastructurally in the central nervous systems of three different species (the snail Lymnaea stagnalis, the cockroach Periplaneta americana and the rat). Tissues were fixed with: (1) a mixture of glutaraldehyde and osmium tetroxide, (2) the tannic acid-glutaraldehyde-osmium tetroxide (TAGO) method, and (3) the tannic acid-Ringer incubation (TARI) method. Especially after TARI-treatment, release of the contents of the secretory vesicles by exocytosis could be clearly demonstrated in: (1) synapses, (2) neurohaemal axon terminals (L. stagnalis), and (3) neuronal processes without morphological synaptic specializations (nonsynaptic release sites). Release from nonsynaptic release sites occurs in most cases over a large area of the plasma membrane of a neuronal process facing several neural elements. On the basis of the differences in morphology of the secretory vesicles at nonsynaptic release sites, it is proposed that various types of (peptidic) messenger are released from such sites. In some neurones of L. stagnalis nonsynaptic release sites have been found together with synapses, or with neurohaemal axon terminals (caudodorsal cells, light green cells and light yellow cells). The possibility that nonsynaptic release sites represent the morphological correlates of nonsynaptic communication in the central nervous system has been discussed.