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 °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 °C than at 22 °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.