Dynamics of ethanol-induced transmitter packet release in the frog neuromuscular junction

Intervals of spontaneous miniature endplate potentials (MEPPs) are usually thought to follow random kinetics because number vs. interval size plots can fit a Poisson distribution such that the largest number of intervals are found in the smallest interval bin. Even though several studies report that MEPPs have a tendency to clump together, an alternative dynamic has not gained acceptance. We found that spontaneous MEPP interval plots with commonly used bin sizes (10 ms or larger) from muscle fibers of diameters usually studied (20 microns) also skew to the smallest interval bin. In order to study very small junctions which do not generate enough intervals for a detailed analysis of high resolving power (small bin width) at normal frequencies of 0.05 MEPPs/s we increased MEPP frequencies with ethanol to obtain large samples. Ethanol accentuated the normal clustering of MEPPs and number vs. interval plots of 0.5 ms show periodic peaks at 400 Hz. A periodogram of power vs. frequency also showed a preferred frequency of 400 Hz. The first peak indicates a refractory period and the 400 Hz oscillation demonstrates that MEPPs are not independent. A release channel with a rhythmical nature is discussed as a possible molecular process which generates clusters of MEPPs.