Chemical modification of vesamicol binding sites and frequency augmentation-potentiation in neuromuscular...

Vesamicol (AH5183), a potent and specific inhibitor of the vesicular acetylcholine uptake system, reduces at relatively high doses the slope of the log-linear frequency augmentation-potentiation (FAP) relation observed between the mean quantal content of endplate potential and the stimulation frequency. Recent biochemical studies identified two binding sites for vesamicol; one is the vesicular acetylcholine uptake system and the other is the vesamicol binding protein localized in the nerve terminal membrane. The present experiments were designed to determine the exact site of action of vesamicol on the FAP relation. Studies using epsilon-maleimidocaproic acid and picrylsulfonic acid, both membrane-impermeant protein-modifying reagents, revealed that these reagents diminished the vesamicol effect concomitant with an impairment of the functioning site for FAP. Because of the membrane-impermeant nature of the reagents, the present results were interpreted to mean that a vesamicol binding protein or a related component, localized in the nerve terminal membrane, is the target of action by vesamicol. This vesamicol binding component seems to play an important role in the stimulation frequency-dependent modulation of the evoked release of transmitter quanta observable as FAP.