Endo-exocytotic images and changes in synaptic transmission induced by diamide at a cholinergic junction

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 marmorata electric organ. Incubation of small pieces of electric tissue in 250 microM ZnCl2 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.

Modification of ionic currents underlying action potentials in mouse nerve terminals by the thiol-oxidizing agent diamide

The effect of diamide, a thiol-oxidizing agent, was tested using electrophysiological techniques to determine whether its ability to alter neuromuscular transmission in vitro could be attributed to alterations of ion channels controlling neuronal excitability and/or acetylcholine release. In mouse triangularis sterni preparations, diamide transiently increased the evoked release of acetylcholine and then blocked release. Extracellular recording of perineural waveforms associated with neuronal action potentials at motor nerve terminals showed that diamide reduced the waveforms associated with the delayed rectifier K+ current, a Ca2+ current and a Ca(2+)-activated K+ current (IK,Ca). Inhibition of quantal transmitter release was not associated with failure of action potentials to invade nerve terminals. Thus, diamide modifies the ionic currents underlying the nerve terminal action potential, some of these changes probably account for the complex effects of diamide on quantal transmission.

Role of glutathione in repair of free radical damage in hippocampus in vitro

Depletion of glutathione (GSH), an intrinsic antioxidant, increases vulnerability to free radical damage in a number of cell systems. This study investigates the role of GSH in limiting electrophysiological damage and/or recovery from free radical exposure in slices of guinea pig hippocampus. Synaptic potentials (PSPs) and population spikes (PSs) were recorded from field CA1. Free radicals were generated from 0.006% peroxide through the Fenton reaction. Analysis of the input-output curves showed that peroxide treatment decreased PSPs and impaired ability of the PSPs to generate PSs as previously reported. Recovery was nearly total within a half hour. Treatment with 5 mM buthionine sulfoximine (BSO) for 2 h depleted hippocampal GSH to 79.2% of control values. The extent of free radical damage was not increased. Recovery, however, was only partial. GSH was further depleted by oxidation with diamide or covalent bonding with dimethyl fumarate (DMF) immediately before and during the peroxide treatment. Neither diamide nor DMF treatment in BSO-incubated tissue enhanced peroxide-induced electrophysiological deficits. Following these treatments, however, tissue showed little recovery from free radical damage. We conclude that glutathione is essential for repair processes in hippocampal neurons exposed to oxidative damage.