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At the large excitatory calyx of Held synapse, the quantal size during an evoked EPSC and the number of active zones contributing to transmission are not known. We developed a nonstationary variant of EPSC fluctuation analysis to determine these quantal parameters. AMPA receptor-mediated EPSCs were recorded in slices of young (postnatal 8–10 d) rats after afferent fiber stimulation, delivered in trains to induce synaptic depression. The means and the variances of EPSC amplitudes were calculated across trains for each stimulus number. During 10 Hz trains at 2 mm Ca2+concentration ([Ca2+]), we found linear EPSC variance–mean relationships, with a slope that was in good agreement with the quantal size obtained from amplitude distributions of spontaneous miniature EPSCs. At high release probability with 10 or 15 mm [Ca2+], competitive antagonists were used to partially block EPSCs. Under these conditions, the EPSC variance–mean plots could be fitted with parabolas, giving estimates of quantal size and of the binomial parameter N. With the rapidly dissociating antagonist kynurenic acid, quantal sizes were larger than with a slowly dissociating antagonist, suggesting that the effective glutamate concentration was increased at high release probability. Considering the possibility of multivesicular release and moderate saturation of postsynaptic AMPA receptors, we conclude that the binomial parameter N (637 ± 117; mean ± SEM) represents an upper limit estimate of the number of functional active zones. We estimate that during normal synaptic transmission, the probability of vesicle fusion at single active zones is in the range of 0.25–0.4.

Estimation of Quantal Size and Number of Functional Active Zones at the Calyx of Held Synapse by Nonstationary EPSC Variance Analysis