Figure 3.

Presynaptic Ca²⁺ current dynamics during facilitation in the predepressed state. A, Paired presynaptic and postsynaptic whole-cell recording (75 μm EGTA added to the presynaptic pipette solution). Presynaptic voltage-clamp protocol (top), presynaptic Ca²⁺ currents (I_Ca; middle), and EPSCs (bottom). Note the different time scales during the 20 Hz train and during the 166 Hz train. B, Average and individual peak EPSC amplitudes (ampl.) (top; dark and light symbols respectively) in response to the sequence of 20 Hz train followed by a high-frequency train (red) and to the control high-frequency trains (black). The bottom panel shows the average Ca²⁺ current integrals (Q_Ca) during the conditioned high-frequency train. The dashed red trace (top) shows the depression of EPSCs predicted by the measured Ca²⁺ currents (calculated as the 3.5th power of the relative Q_Ca; see Results). Note that Ca²⁺ current inactivation fails to predict the full magnitude of the observed EPSC depression. C, Average EPSC amplitudes (top) and Ca²⁺ current charge values (bottom) for the conditioned 200 Hz trains (red) and the control 200 Hz trains (black) normalized to the first response at 200 Hz. Note that the Ca²⁺ current facilitation time courses were very similar for control and conditioned 200 Hz trains, whereas the short-term plasticity behavior differed considerably. The red dashed line shows the EPSC facilitation predicted by Ca²⁺ current facilitation. D, Average maximal (max.) facilitation during control 200 Hz trains (open bar) and during conditioned (cond.) 200 Hz trains (red bar; n = 6 cell pairs; p < 0.001).