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 neuro-muscular 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 μM) 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 μM 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 μM 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.