The effect of diamide on transmitter release and on synaptic vesicle population at vertebrate synapses

Diamide, a sulfhydryl-oxidizing agent, has previously been shown to cause acetylcholine release in two preparations in the absence of added Ca2+. Similarities in action between diamide and alpha-latrotoxin, a component of black widow spider venom which causes transmitter release with no added Ca2+, and which seems to require a disulfide bond for its action, led us to study further the transmitter-releasing properties of diamide. In rat cerebral cortical slices we show that diamide, like alpha-latrotoxin, released all transmitters studied; GABA, acetylcholine, norepinephrine and dopamine. The response reached a peak after a delay (5-15 min), in contrast to the much faster release evoked by high K+ (within 3 min). Diamide-induced GABA release was found to occur equally well in the absence of added Ca2+, and was blocked when diamide was reduced prior to addition. Our ultrastructural studies of the frog neuromuscular junction showed that whereas alpha-latrotoxin caused the elimination of synaptic vesicles, diamide did not. Dithiothreitol, a disulfide-reducing agent, also caused GABA release, but this effect was Ca2+-dependent, blocked by high Mg2+, and occurred without delay. These observations comparing the 3 transmitter-releasing agents have further delineated the sulfhydryl/disulfide-group involvement in transmitter release and have demonstrated that dithiothreitol is operating at a different site from either alpha-latrotoxin or diamide.