MINIATURE EXCITATORY SYNAPTIC CURRENTS CORRECTED FOR DENDRITIC CABLE PROPERTIES REVEAL QUANTAL SIZE AND VARIANCE

1. Non-NMDA (N-Methyl-D-aspartate) receptor mediated miniature excitat ory synaptic currents (mEPSCs) were recorded from motoneurons in organ otypic cultures of embryonic rat spinal cord. 2. Amplitude histograms of mEPSCs were unimodal and skewed toward larger events. The mean of t he modes of the amplitude histograms was -18 pA with a maximal amplitu de range of -4 to -160 pA for individual mEPSCs. 3. Current transients to a short voltage pulse were used to estimate the passive cable para meters of the motoneurons. The mean membrane time constant (tau) and t he mean electrotonic length (L) were 20 and 0.96 ms, respectively. 4. The amplitudes of the mEPSCs were corrected for imperfect space and vo ltage clamp. The resulting amplitude histograms could be fitted by the sum of two Gaussian curves, revealing a mean quantal size of -48 pA w ith 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 th e neurons and that simultaneous release of elementary quanta occurs oc casionally.