Post-tetanic increase in the FRP size at the expense of the SRP size. (A) Simultaneous recordings of presynaptic Ca²⁺ current (I_Ca,pre; top), EPSC (middle), and cumulative quantal release (bottom) are shown in a chronological order: 40 s before HFS (a train of AP-like pulses at 100 Hz for 4 s) and 40 s and 120 s after HFS. I_Ca,pre was evoked by a long depolarization pulse to 0 mV for 50 ms preceded by a predepolarization to +70 mV for 2 ms to ensure fast activation of presynaptic Ca²⁺ current. The second column shows EPSC (bottom) in response to HFS, a train of AP-like ramp pulses for 4 s at 100 Hz applied to presynaptic terminal (top). B and C show presynaptic Ca²⁺ currents (B, top), EPSCs (B, bottom), and cumulative quantal release (C). Each panel illustrates three superimposed traces recorded before HFS (blue solid line) and 40 s (red solid line) and 120 s (broken line) after HFS in the same synapse as in A. (C, inset) Cumulative quantal release on an expanded time scale. (D–K) Bar graphs illustrate 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) amplitude of I_Ca,pre; (G) size of the FRP; (H) size of the SRP; (I) size of total RRP; (J) release time constant (τ) of the FRP; (K) release τ of the SRP. Data of individual synapses are shown as circles connected with a line superimposed on the bar graph (right ordinate). Error bar, SEM; *, P < 0.05; **, P < 0.01; n.s., not significant (P > 0.05, paired t test). (L) Plot of the relative post-tetanic FRP size as a function of the relative post-tetanic amplitude of presynaptic Ca²⁺ current (I_Ca,pre) normalized to control values measured before HFS (n = 9).