ZINC BLOCKS ACETYLCHOLINE-RELEASE BUT NOT VESICLE FUSION AT THE TORPEDO NERVE-ELECTROPLATE JUNCTION

The combined effects of Zn2+ treatment and nerve stimulation were stud ied on cholinergic synapses of the Torpedo marmorata electric organ, I ncubation of small pieces of electric tissue in 250 mu M ZnCl2 for 2 h irreversibly blocked synaptic transmission by inhibiting the release of acetylcholine. This treatment, however, did not cause any significa nt fine structural alteration in the nerve-electroplate junctions. Pre parations treated with Zn2+ were submitted to electrical stimulation. In spite of the fact that no transmitter was released, stimulation res ulted in the accumulation of calcium in the tissue, and in marked ultr astructural changes, The density of synaptic vesicles was significantl y reduced and many of the remaining vesicles were found in close proxi mity to the presynaptic membrane, Images of vesicles fused with the pl asmalemma were abundant, indicating that numerous vesicles were caught in different phases of exocytosis or endocytosis. Freeze-fracture rep licas made from quick-frozen or chemically fixed material showed a hig h number of vesicle openings (pits) in the presynaptic plasmalemma. No recovery occurred even after a prolonged period of rest, indicating t hat retrieval was impaired by zinc treatment. In conclusion, the prese nt experimental paradigm created an unusual situation where fusion of synaptic vesicles to the plasma membrane could be activated independen tly from the release of transmitter.