Delayed and fast transient potassium currents in rat neocortical neurons in cell culture

Cultured rat neocortical neurons were subjected to the whole-cell mode of voltage clamping to study outward K⁺ currents. Tetrodotoxin and Cd²⁺ were applied extracellularly to block Na⁺ and Ca²⁺ currents. Depolarizing voltage commands from a holding potential of -90 mV evoked an outward current which peaked early and decayed over 20-50 ms to attain a steady level. At holding potentials more positive than -50 mV, the fast, transient component of the outward current was largely inactivated while the late, steady one remained present. The two components of the outward current displayed different pharmacological sensitivities: the fast, transient one was blocked by 4-aminopyridine, while the late, persistent one was reduced by tetraethylammonium. These currents were present in neocortical cells as early as the third day in culture. Our experiments indicate that as in sympathetic, hippocampal and spinal cord neurons, neocortical cells possess both a fast, transient, and a delayed K⁺ current. These currents might play an important role in controlling neocortical excitability.