(A) Top: Voltage-clamp (VC) recordings showing representative traces of ICa++pre upon depolarization to 0 mV for various durations (1, 2, 5, 10, 20, and 50 ms). Bottom: Average ICa++pre amplitude as a function of stimulus duration (mean ± SEM). Red line is the fit to the data with an exponential function. Number of presynaptic boutons analyzed: 1 ms = 4; 2 ms = 8; 5 ms = 12; 10 ms = 16; 20 ms = 16; 50 ms = 16. (B) Top: The black circles show the current to voltage (I/V) relationship of the ICa++pre (mean ± standard deviation [SD]; n = 16) and the continuous line the simulated I/V relationship, which yielded the following parameters (VCav50 = −17 mV [the half-maximal Cav voltage] and α = 0.2 [steepness of the voltage-dependent activation]). Middle: The probabilities of the different states of the Cav channels are shown as a function of voltage. Bottom: Different states of the Cav channels based on activation of independent four voltage sensors. (C) Top: Representative ICa++pre (black trace) upon a depolarization to 0 mV for 20 ms and the simulated data (red trace). Middle: Subtracted data, which indicates that the difference between the experimental and simulated data probably corresponding to a GABAA autoreceptor current. Bottom: The probabilities of the different states of the Cav channels upon the depolarizing pulse shown on the top, as a function of time. For the color code please see (B), middle and bottom schemes. (D) Left: Simulated data showing the presynaptic ICa++ induced by a single presynaptic action potential (AP) and the effect of EPSPs right before the AP. Right: Effect of the interval between the prior EPSPs and the presynaptic AP on the ICa++ pre augmentation. Please look at Figure 5A–C for a comparison with experimental data. (E) Simulated data showing the effect of a suprathreshold square depolarization on the presynaptic ICa++ pre and the effect of pre-pulse, subthreshold depolarizations to various potentials. Please look at Figure 5D, E for a comparison with experimental data. (F) Effect of the duration and interval of pre-pulse depolarizations on the presynaptic ICa++ pre augmentation. An interval between the subthreshold and suprathreshold pulses dramatically weakens the effect. (G) Effect of a change in the kon and koff rate constants on the presynaptic ICa++ enhancement induced by a 20-mV subthreshold depolarization.