Modulation of transmitter release by presynaptic resting potential and background calcium levels

Activation of presynaptic ion channels alters the membrane potential of nerve terminals, leading to changes in transmitter release. To study the relationship between resting potential and exocytosis, we combined pre- and postsynaptic electrophysiological recordings with presynaptic Ca2+ measurements at the calyx of Held. Depolarization of the membrane potential to between -60 mV and -65 mV elicited P/Q-type Ca2+ currents of < 1 pA and increased intraterminal Ca2+ by < 100 nM. These small Ca2+ elevations were sufficient to enhance the probability of transmitter release up to 2-fold, with no effect on the readily releasable pool of vesicles. Moreover, the effects of mild depolarization on release had slow kinetics and were abolished by 1 mM intraterminal EGTA, suggesting that Ca2+ acted through a high-affinity binding site. Together, these studies suggest that control of resting potential is a powerful means for regulating synaptic function at mammalian synapses.