The effects of L-vesamicol, an inhibitor of vesicular acetylcholine uptake, on two populations of miniature endplate currents at the snake neuromuscular junction

Abstract

The actions of the active L-isomer of vesamicol, an inhibitor of the vesicular storage of acetylcholine, has been studied on spontaneous and evoked acetylcholine release at the snake neuromuscular junction. Miniature endplate currents and endplate currents were recorded from cut muscle fibres of the garter snake, Thamnophis sirtalis. In controls, prolonged periods of high frequency nerve stimulation produced a bimodal distribution of miniature endplate current amplitudes. The stimulation induced "small-mode" miniature endplate currents had a mean amplitude of around 40-55% of the pre-stimulation miniature endplate current. Relative to the normal-sized post-stimulation miniature endplate current, the proportion and, to a lesser extent, amplitude of the small-mode miniature endplate currents was related to both the frequency and duration of nerve stimulation and to the extracellular calcium ion concentration. In unstimulated preparations, L-vesamicol (2-5 microM) did not affect either endplate current quantal content or miniature endplate current amplitude or frequency. However, at these doses, the mean amplitude of the stimulation-induced, small-mode miniature endplate current was reduced by L-vesamicol in a concentration-dependent manner such that they were not visible at the highest dose. L-Vesamicol had no affect on the mean or coefficient of variance of amplitude of the larger, normal-sized miniature endplate current. Additionally, the stimulation-induced increase in overall miniature endplate current frequency seen in controls was abolished by 5 microM L-vesamicol. After prolonged 10 Hz nerve stimulation endplate current amplitude was markedly reduced in both controls (by 94%) and in the presence of 5 microM L-vesamicol (by 98%). Analysis of endplate current amplitude variance showed that in control the decrease was due to reductions in both quantal content and quantal size while in L-vesamicol the decrease was due entirely to a change in quantal content with no change in quantal size. Thus, we have observed that L-vesamicol selectively reduces the amplitude of a population of stimulation-induced small-mode quanta both as miniature endplate currents and as constituents of endplate currents. We suggest that these quanta are derived from a highly active, readily releasable pool. An action of L-vesamicol on this labile pool is consistent with previous observations on its ability to inhibit the vesicular storage of acetylcholine.