Figure 4.

Facilitation in the predepressed state depends on presynaptic residual Ca²⁺. A, Presynaptic spatially averaged [Ca²⁺]_i imaged by the low-affinity Ca²⁺ indicator fura-6F (top; average of n = 4 traces), EPSCs (middle), and average peak EPSC amplitudes (bottom) in response to presynaptic voltage-clamp stimulation (10 stimuli at 20 Hz, followed by 30 stimuli at 200 Hz). The presynaptic solution contained 100 μm fura-6F and 75 μm EGTA. B, C, Presynaptic spatially averaged [Ca²⁺]_I (top) and average EPSC amplitudes (bottom) in response to the same stimulation protocol as in A. The presynaptic solution now contained 100 μm fura-6F and 1 mm EGTA (B) or 1 mm fura-2 (C). D, Normalized (norm.) average peak EPSC amplitudes during the conditioned 200 Hz train for the recordings shown in A–C. Note the nearly complete absence of facilitation in the presence of the BAPTA-like Ca²⁺ buffer fura-2. E1, E2, Average maximal (max.) facilitation (E1) and average [Ca²⁺]_i increase measured at the point of maximal facilitation during the 200 Hz train (Δ[Ca²⁺]_i) (E2). Black bars: 75 μm EGTA and fura-6F (n = 6 paired recordings); blue bars: 1 mm EGTA and fura-6F (n = 3 pairs); red bars: 1 mm fura-2 (n = 5 pairs). Note that both the maximal facilitation and the increase in spatially averaged [Ca²⁺]_i were significantly smaller with 1 mm fura-2 as compared with the two other conditions.