Figure 3. Prominent decrease in quantal size during fibre-stimulation evoked EPSC trains in immature calyx of Held synapses.

A, average eEPSC waveform (A1, stimulus artifacts blanked for clarity) and peak amplitude fluctuations of first (EPSC₁, black trace) and last (EPSC₁₀, grey trace) synaptic response in the train (A2), from a representative recording in a P6 synapse (30 Hz, 15 s inter-stimulus interval, V_h=−70 mV, 2 mm Ca²⁺, 1 mm Mg²⁺). In this cell, q_i decreased from 58 to 6 pA for EPSC₁ and EPSC₁₀, respectively. A3, average waveforms for eEPSC₁ and eEPSC₁₀ shown superimposed on an expanded time scale and after scaling to the same peak amplitude. Before averaging, eEPSCs were aligned at their rising phase to account for fluctuations in the timing of presynaptic APs. Rise and decay of eEPSC₁₀ are slightly slower when compared with those of eEPSC₁, presumably attributable to a slight broadening of presynaptic APs during trains. B, normalized average values for eEPSC peak amplitudes during trains evoked at various stimulus frequencies (white symbols: 10 Hz, n = 17; grey symbols: 30 Hz, n = 18; black symbols: 100 Hz, n = 13). C, CV⁻² analysis is consistent with a reduction in q_i during eEPSC trains as indicated by data points above the identity line (dashed line). D and E, normalized average q values (D) and M values (E) from σ²/I analysis of eEPSC trains shown in B. Note the rapid and pronounced reduction in q_i to ∼40% of control even at the lowest stimulus frequency (D, white symbols). Inset in E, a lower limit of the size of the readily releasable pool was obtained from the 100 Hz depression curve. The extrapolated line fit to the first 5 eEPSCs in the M_iversus cumulative M_i plot (Elmqvist & Quastel, 1965) yielded an estimate of 707 ± 60 quanta.