Roles of Na+-Ca2+ exchange and of mitochondria in the regulation of presynaptic Ca2+ and spontaneous glutamate release

The release of neurotransmitter from presynaptic terminals depends on an increase in the intracellular Ca2+ concentration ([Ca2+](i)). In addition to the opening of presynaptic Ca2+ channels during excitation, other Ca2+ transport systems may be involved in changes in [Ca2+]i. We have studied the regulation of [Ca2+](i) in nerve terminals of hippocampal cells in culture by the Na+-Ca2+ exchanger and by mitochondria. In addition, we have measured changes in the frequency of spontaneous excitatory postsynaptic currents (sEPSC) before and after the inhibition of the exchanger and of mitochondrial metabolism. We found rather heterogeneous [Ca2+](i) responses of individual presynaptic terminals after inhibition of Na+-Ca2+ exchange. The increase in [Ca2+](i) became more uniform and much larger after additional treatment of the cells with mitochondrial inhibitors. Correspondingly, sEPSC frequencies changed very little when only Na+-Ca2+ exchange was inhibited, hut increased dramatically after additional inhibition of mitochondria. Our results provide evidence for prominent roles of Na+-Ca2+ exchange and mitochondria in presynaptic Ca2+ regulation and spontaneous glutamate release.