Figure 4.

Large recycling pool. A, The accumulated QEPSC (ΣQEPSC, circles) induced by Train_1Hz applied every 30 s is plotted versus the Train_1Hz number in the absence of glutamate (analyzed from Fig. 1C). ΣQEPSC is normalized to the QEPSC induced by the first 20 ms depolarization of the first Train_1Hz, i.e., the RRP size. Black curve is a monoexponential fit with a plateau value of 46 times the RRP size. The red curve is the predicted change assuming random mixture of recycling vesicles (see Results). B, Sampled EPSCs induced by a presynaptic AP-e train at 5 Hz for 180 s (bar) with no glutamate in the pipette (applies to B–E). C, The mEPSC amplitude (n = 8 synapses) in the first and the last 30 s during the AP-e train at 5 Hz for 180 s. Sampled mean mEPSCs at the same time period from a synapse shown in B are also plotted above the bar graph (left: averaged of 97 mEPSCs; right: 93 mEPSCs). D, The cumulative EPSC amplitude (ΣEPSC Amp) and the number of vesicles (N_vesicle) released by an AP-e train at 5 Hz (n = 8 synapses). Data are shown as the mean (solid curve) ± SEM (gray). E, The amplitude of EPSCs (bottom) and sampled EPSCs (top, from one synapse) induced by an AP-e at ∼30 s before (left) and after (right) an AP-e train at 5 Hz for 180 s (n = 8 synapses). F–H, Same arrangements as in C–E, respectively, except that the pipette contained 10 mm glutamate and the bath contained 300 μm CPPG (n = 5 synapses). mEPSC traces in F: left, averaged of 47 mEPSCs; right, 56 mEPSCs. I–K, Same arrangements as in C–E, respectively, except that the pipette contained 10 mm glutamate (no CPPG in bath, n = 5 synapses). mEPSC traces in I: left, averaged of 136 mEPSCs; right, 58 mEPSCs. L–N, Same arrangements as in C–E, respectively, except that whole-cell AP-e was replaced with an extracellular stimulation that induced axonal action potentials (see Results), and the bath contained 6 μm bafilomycin A1 (n = 4, synapses). mEPSC traces in M: left, 44 mEPSCs; right, 51 mEPSCs.