Subunit composition of the spontaneous miniature end-plate currents at the mouse neuromuscular junction.

1. Adult, neonate and young mouse diaphragm muscle fibres were voltage clamped with a two-electrode clamp. Miniature end-plate currents (MEPCs) were recorded on magnetic tape and analysed with a computer. The MEPC amplitude, charge, rise time, time-to-peak, decay time constant and root mean square (r.m.s.) noise level were determined for each MEPC. 2. The MEPC amplitude and charge distributions showed integral peaks starting from zero. Peaks were enhanced by selecting MEPCs with uniform time characteristics, with low noise, with increased sample size, with a curve smoothing routine and/or with a selected bin size. 3. Integral peaks were found in histograms from neonate, young and old mice. The ratio of sub-MEPCs to bell MEPCs decreased during neonatal development. 4. The size of the peak intervals was the same in all preparations of the same developmental stage. The adult modal peak varied between 8 and 12 times the subunit value, but peak intervals were similar (0.44 +/- 0.04 nA). 5. Changes in the holding potential or the bath temperature, or addition of an anticholinesterase agent, changed the peak interval. 6. The number of peaks in the overall MEPC amplitude and area-to-peak (charge) histogram profiles were usually the same. 7. Integral peaks on MEPC amplitude profiles, notches and steps on the MEPC rising phase and changes in the overall MEPC profiles are explained by a subunit composition of the quantum of transmitter release.