Zinc Blocks Acetylcholine Release but not Vesicle Fusion at the Torpedo Nerve-Electroplate Junction

The combined effects of Zn2+ treatment and nerve stimulation were studied on cholinergic synapses of the Torpedo rnarmorafa electric organ. Incubation of small pieces of electric tissue in 250 μM ZnCI2 for 2 h irreversibly blocked synaptic transmission by inhibiting the release of acetylcholine. This treatment, however, did not cause any significant fine structural alteration in the nerve‐electroplate junctions. Preparations treated with Zn2+ were submitted to electrical stimulation. In spite of the fact that no transmitter was released, stimulation resulted in the accumulation of calcium in the tissue, and in marked ultrastructural changes. The density of synaptic vesicles was significantly reduced and many of the remaining vesicles were found in close proximity to the presynaptic membrane. Images of vesicles fused with the plasmalemma were abundant, indicating that numerous vesicles were caught in different phases of exocytosis or endocytosis. Freeze‐fracture replicas made from quick‐frozen or chemically fixed material showed a high number of vesicle openings (pits) in the presynaptic plasmalemma. No recovery occurred even after a prolonged period of rest, indicating that retrieval was impaired by zinc treatment. In conclusion, the present experimental paradigm created an unusual situation where fusion of synaptic vesicles to the plasma membrane could be activated independently from the release of transmitter.