(A) Ca²⁺ transients (CaTs) evoked by a train of AP-like ramp pulses at 100 Hz for 4 s (HFS). CaTs recorded at 6 synapses (green lines) and an averaged Δ[Ca²⁺]_i trace (open circles with error bars) are superimposed. Presynaptic [Ca²⁺]_i during and after HFS were measured by fluorescence imaging the presynaptic terminal loaded with 100 µM Fura-4F via a patch pipette. (B) The mean peak value of Δ[Ca²⁺]_i evoked by HFS measured using dual WCR (3.04 ± 0.27 µM, n = 6) is compared with that evoked by AFS at 100 Hz for 4 s (2.77 ± 0.63 µM, n = 5). (C) CaTs are the same as in A but on expanded scales of time and Δ[Ca²⁺]_i. (D) Mean values for resting [Ca²⁺]_i before and 40 s after HFS (39.5 ± 16.4 nM and 47.8 ± 15.3 nM, respectively, n = 6, P = 0.25, paired t test). The black arrow in C indicates the time point of 40 s after HFS. Mean ± SEM; n.s., not significant (P > 0.05, paired t test).