Action of Phospholipase Neurotoxins on Torpedo Synaptosomes: Changes in Membrane Potential and Phosphoglyceride Composition

A number of snake neurotoxins with intrinsic phospholipase A2 activity are potent neuromuscular blocking agents which inhibit transmitter release by a complex, but as yet poorly characterized, sequence of events (14, 15). With β-bungarotoxin (β-BtX), the best-known toxin of this group, there is a well documented triphasic effect on evoked transmitter release from motor nerve terminals (see 14 for literature citations) in which both phospholipase-independent and phospholipase-dependent actions are involved (3, 14). Following an early rapid inhibitory phase which is independent of phospholipase activity, transmitter release from β-BtX-poisonedjunctions transiently recovers before entering a third and final stage in which release gradually decreases to zero. Since the last two phases involve phospholipase activity they presumably reflect changes in the phospholipid composition of presynaptic membrane systems.