Inactivation of presynaptic calcium current contributes to synaptic depression at a fast central synapse

Voltage-gated calcium channels are well characterized at neuronal somata but less thoroughly understood at the presynaptic terminal where they trigger transmitter release. In order to elucidate how the intrinsic properties of presynaptic calcium channels influence synaptic function, we have made direct recordings of the presynaptic calcium current (I-pCa) in a brainstem giant synapse called the calyx of Held. The current was pharmacologically classified as P-type and exhibited marked inactivation. The inactivation was largely dependent upon the inward calcium current magnitude rather than the membrane potential, displayed little selectivity between divalent charge carriers (Ca2+, Ba2+ and Sr2+), and exhibited slow recovery. Simultaneous pre-and postsynaptic whole-cell recording revealed that I-pCa inactivation predominantly contributes to posttetanic depression of EPSCs. Thus, because of its slow recovery, I-pCa inactivation underlies this short-term synaptic plasticity.