Changes in MEPP and EPP amplitude distributions in the mouse diaphragm during synapse formation and degeneration

Miniature end-plate potential (MEPP) and end-plate potential (EPP) amplitude histograms were examined in the mouse diaphragm during degeneration, deterioration, re-innervation and neonatal development. MEPPs and EPPs were recorded with conventional electrophysiological techniques. Control MEPP amplitude distributions from mice 21-30 days old showed two classes of MEPPs. The larger class composed 80-90% of the MEPPs and formed a bell-shaped distribution (bell-MEPPs). The smaller class (skew-MEPPs) formed a skewed distribution with a peak 1/7 to 1/15 that of bell-MEPPs. Usually, MEPP amplitude distributions did not change during the course of nerve degeneration or during deterioration in the bath. MEPP amplitude distributions from newly re-innervated fibers were composed mainly of skew-MEPPs. At later stages of re-innervation the relative numbers of skew-MEPPs decreased. Many fibers from neonatal mice (2-3 days old) also showed mainly skew-MEPPs. Rise time vs amplitude plots were constructed from neonatal and re-innervating preparations. The skew-MEPP time-to-peak measurements fell on or below the regression line calculated from the time-to-peak data of the bell-MEPPs. This indicates that the skew-MEPPs originated from the same site as the bell-MEPPs. Unitary EPPs were recorded from neonatal and re-innervating preparations by reducing the evoked response with cobalt ions (4 mM). Distributions of unitary EPPs were similar to those of bell-MEPPs. It is concluded that there are two classes of spontaneous quanta. The skew-MEPP class dominates MEPP amplitude distributions during the early stages of re-innervation and early neonatal preparations. In all stages of development the unitary evoked EPPs have the same mean amplitude and time-to-peak as the bell-MEPPs. The data suggest that the skew class is not available for evoked release.