Slow depolarizing afterpotentials in neocortical neurons are sodium and calcium dependent

Depolarizing afterpotentials (DAPs) were studied in intracellular recordings from neocortical slices bathed in tetrodotoxin (TTX) (1 microM) and tetraethylammonium chloride (TEA) (24 mM), to block voltage-dependent Na+ currents and most K+ currents. The DAP was Ca(2+)-dependent, in that its magnitude varied as a function of the duration of the preceding Ca2+ plateau. It had an apparent reversal potential of between -40 and -5 mV. The DAP was blocked when choline replaced all extracellular Na+; there was a hyperpolarizing shift in apparent reversal potential when extracellular Na+ was lowered. The DAP was blocked by amiloride (1 mM), which also decreased the preceding Ca2+ plateau. The data are consistent with the hypothesis that the DAP is due to electrogenic Na+/Ca2+ exchange.