Figure 2. The strength of transmission at the RB–AII synapse is reduced by tonic presynaptic depolarization. A, The presynaptic RB is stepped to prepulse potentials between −60 and −42 mV for 750 ms before a test pulse: a 10 ms depolarization of +10 mV relative to the prepulse potential. Averaged (n = 4 responses at each potential) presynaptic Ca currents (middle) and evoked EPSCs (bottom) are illustrated. Some inactivation of the presynaptic Ca current during the prepulse is evident at −48 and −42 mV. EPSCs have been separated by a vertical offset. B, Ca currents and EPSCs are illustrated at higher temporal resolution. EPSCs are not offset. C, Summary of n = 7 experiments. Ca currents (integrated) and EPSCs (quantal contents) recorded during the test pulse from each RB–AII pair were normalized to the largest currents recorded in that pair. The magnitude of Ca influx was relatively constant following each prepulse, but evoked exocytosis was inhibited significantly by the more depolarizing prepulses. Error bars indicate SEM. D, Examples of EPSCs recorded in the 100 ms before the test pulse demonstrate both the increased exocytosis evoked by tonic depolarization (colors as in A) and our ability to resolve and evaluate individual synaptic events (denoted as vertical lines in the traces above the EPSCs). E, The EPSC amplitudes do not vary with interevent interval: amplitudes are normalized to the mean quantal EPSC amplitude (recorded in the absence of presynaptic stimulation), and data are binned in 3 ms bins. The quantal content of the EPSCs is larger than one (average mEPSC, 27.6 ± 3.1 pA), reflecting the multivesicular release that is known to occur at this synapse. The average interevent intervals at −54, −48, and −42 mV were 37.2 ± 33.8, 10.3 ± 8.7, and 4.9 ± 1.8 ms, respectively. Events were exponentially distributed, indicating that they arose from a Poisson process. The average interevent interval in the absence of stimulation was 208.9 ± 38.0 ms (n = 11 paired recordings).