Propagation of action potentials in axons and dendrites increases intracellular Na+ ([Na+](i)] and Ca2+ concentrations ([Ca2+](i)]. While the importance of [Ca2+](i) in synaptic transmission is well established, a possible functional role of [Na+](i) is unclear. In cultured hippocampal cells, [Na+](i) was increased by veratridine. We have then measured spontaneous excitatory postsynaptic currents (sEPSCs) and, by means of fluorescent dyes, changes in [Na+](i), in [Ca2+](i), and in the turnover of the vesicular pool of individual boutons. An elevation of [Na+](i) and a concomitant rise in [Ca2+](i) led to a large increase in sEPSC frequency and in the turnover of the presynaptic vesicular pool. Extracellular Ca2+ was essential for these effects of elevated [Na+](i) on synaptic transmission. They probably occur via Na+/Ca2+ exchange.
Bouron, A., & Reuter, H. (1996). A role of intracellular Na+ in the regulation of synaptic transmission and turnover of the vesicular pool in cultured hippocampal cells. Neuron, 17(5), 969–978. https://doi.org/10.1016/S0896-6273(00)80227-5