Sub-MEPPs, Skew-MEPPs and the Subunit Hypothesis of Quantal Transmitter Release at the Neuromuscular Junction

Abstract

sub-MEPPs, skew-MEPPs and bell-MEPPs. In 1973 we reported that there is a class of small MEPPs (sub-MEPPs) at the normal adult frog neuromuscular junction and that the normally low percentage of these small MEPPs is temporarily increased after a heat challenge or nerve stimulation (Gross and Kriebel, 1973; Kriebel and Gross, 1974; Bevan, 1976) (Fig. 1) or with focal depolarization (Cooke and Quastel, 1973). Sub-MEPPs are absent with curare (Cooke and Quastel, 1973) and potentiated by eserine similarly to the larger MEPPs that were initially described by Fatt and Katz (1952). The larger MEPPs are termed bell-MEPPs because the amplitude distribution is bell-shaped (Fig. 2). Since both sub-MEPPs and the larger MEPPs are found in the normal preparation, the term normal-MEPP or classical-MEPP is inaccurate. The sub-MEPPs form a distinct class separate from the bell-MEPPs as indicated by the discontinuity in the profile of the MEPP amplitude histogram. In the frog neuromuscular junction, the small MEPP amplitudes usually form a bell distribution and are termed sub-MEPPs or s-MEPPs. However, in the mouse neuromuscular junction, the class of small MEPPs shows an overall skew distribution with a mode near the noise level (hence termed skew-MEPPs ; term from Harris and Miledi, 1971). The skew class of MEPPs may show integral peaks providing that enough MEPPs are present in each histobar and if adequate resolving power of the histogram is used (i.e., bins between peaks; Fig. 2; Matteson et al., 1979). When the skew-class shows multiple peaks, the first peak is the sub-MEPP (s-MEPP) and its modal value is the same as that of the overall skew-class. The skew-class thus includes the sub-MEPPS (s-MEPPs). Doublet MEPPs (two beli-MEPPs, Kriebel and Stolper, 1975), are not discussed here.