Quantitative ultrastructural tannic acid study of the relationship between electrical activity and peptide secretion by the bag cell neurons of Aplysia californica.

Release of neurosecretory material by the neuroendocrine bag cells (BC) of the gastropod Aplysia californica was studied, using quantitative electron microscopy and the tannic acid method for the demonstration of exocytosis of neuropeptides. Axon terminals of electrically inactive BC located in the periphery of the pleurovisceral connectives are filled with secretory granules and show low exocytosis activity (one exocytosis figure per 8 terminal profiles). In terminals of BC stimulated to an electrical discharge, in contrast, granules are scarce or absent and exocytosis activity has increased 24-fold. During rest and, particularly, during electrical discharge, BC apparently release secretory material into the hemolymph by exocytosis from axon terminals. Release furthermore takes place from axons running in the connective tissue surrounding the connectives.