A transient increase in the FRP size can be induced by a single conditioning pulse. (A) Experimental protocol is the same as in Fig. 2 except that a train of AP-like pulses were replaced with a single depolarization pulse to 0 mV for 100 ms duration (conditioning pulse, second column). Presynaptic Ca²⁺ current (I_Ca,pre, top), EPSC (middle), and cumulative release (bottom) are shown in a sequential order. (B and C) Presynaptic Ca²⁺ current (B, top), EPSCs (B, bottom), and cumulative quantal release (C). Each panel shows three superimposed traces recorded at the same synapse as in A at the 40 s before (blue solid line) and 40 s (red solid line) and 120 s (broken line) after the conditioning pulse. (C, inset) A trace for the cumulative release on an expanded time scale. (D–I) Statistical mean of relative values before (ctrl) and after HFS (40 s and 120 s) normalized to the control value measured before HFS. (D) EPSC amplitude; (E) decaying time constant of EPSC; (F) size of the FRP; (G) size of the SRP; (H) the amplitude of I_Ca,pre; (I) size of the total RRP. Data of individual synapses are shown as circles connected with a line (right ordinate). Error bar, SEM; *, P < 0.05; **, P < 0.01; n.s., not significant (P > 0.05, paired t test). (J) Relative FRP sizes as a function of relative amplitudes of I_Ca,pre at 40 s after conditioning (n = 6). Each point of data was normalized to the value measured before applying a conditioning pulse.