Miniature excitatory synaptic currents corrected for dendritic cable properties reveal quantal size and variance.

1. Non-NMDA (N-Methyl-D-aspartate) receptor mediated miniature excitatory synaptic currents (mEPSCs) were recorded from motoneurons in organotypic cultures of embryonic rat spinal cord. 2. Amplitude histograms of mEPSCs were unimodal and skewed toward larger events. The mean of the modes of the amplitude histograms was -18 pA with a maximal amplitude range of -4 to -160 pA for individual mEPSCs. 3. Current transients to a short voltage pulse were used to estimate the passive cable parameters of the motoneurons. The mean membrane time constant (tau) and the mean electrotonic length (L) were 20 and 0.96 ms, respectively. 4. The amplitudes of the mEPSCs were corrected for imperfect space and voltage clamp. The resulting amplitude histograms could be fitted by the sum of two Gaussian curves, revealing a mean quantal size of -48 pA with a coefficient of variation (cv) of 0.28. 5. Our data suggest that quantal size and its variance are masked by the cable properties of the neurons and that simultaneous release of elementary quanta occurs occasionally.