A threshold sodium current in pyramidal cells in rat hippocampus.

Maintained, inward currents were activated by small depolarizations from the resting membrane potential (-50 to -60 mV) in voltage-clamped, pyramidal neurons in rat hippocampal slices. The currents were apparently Na currents as they were blocked by tetrodotoxin or removal of extracellular Na and were not affected by Cd. They showed little decrease in amplitude during prolonged depolarizations. The increase in Na conductance with depolarization was sigmoidal, with half-maximum conductance at about -50 mV, and saturated at -20 to -30 mV. This 'threshold' Na current may be involved in setting patterns of repetitive firing of action potentials.