Timing constraints of action potential evoked Ca2+ current and transmitter release at a central nerve terminal

The waveform of presynaptic action potentials (APs) regulates the magnitude of Ca2+ currents (I-Ca) and neurotransmitter release. However, how APs control the timing of synaptic transmission remains unclear. Using the calyx of Held synapse, we find that Na+ and K+ channels affect the timing by changing the AP waveform. Specifically, the onset of I-Ca depends on the repolarization but not depolarization rate of APs, being near the end of repolarization phase for narrow APs and advancing to the early repolarization phase for wide APs. Increasing AP amplitude has little effect on the activation but delays the peak time of I-Ca. Raising extracellular Ca2+ concentration increases the amplitude of I-Ca yet does not alter their onset timing. Developmental shortening of APs ensures I-Ca as a tail current and faithful synaptic delay, which is particularly important at the physiological temperature (35 degrees C) as I-Ca evoked by broad pseudo-APs can occur in the depolarization phase. The early onset of I-Ca is more prominent at 35 degrees C than at 22 degrees C, likely resulting from a temperature-dependent shift in the activation threshold and accelerated gating kinetics of Ca2+ channels. These results suggest that the timing of Ca2+ influx depends on the AP waveform dictated by voltage-gated channels and temperature.