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Two different types of action potentials were observed among the pyramidal cells and interneurons in cat motor cortex: the narrow action potentials and the wide action potentials. These two types of action potentials had similar rising phases (528.8 +/- 77.0 vs 553.1 +/- 71.8 mV/ms for the maximal rising rate), but differed in spike duration (0.44 +/- 0.09 vs 1.40 +/- 0.39 ms) and amplitude (57.31 +/- 8.22 vs 72.52 +/- 8.31 mV), implying that the ionic currents contributing to repolarization of these action potentials are different. Here we address this issue by pharmacological manipulation and using voltage-clamp technique in slices of cat motor cortex. Raising extracellular K+ concentration (from 3 mM to 10 mM), applying a low dose of 4-aminopyridine (2-200 microM) or administering a low concentration of tetraethylammonium (0.2-1.0 mM) each not only broadened the narrow action potentials, but also increased their amplitudes. In contrast, high K+ medium or low dose of tetraethylammonium only broadened the wide action potentials, leaving their amplitudes unaffected, and 4-aminopyridine had only a slight broadening effect on the wide spikes. These results implied that K+ currents were involved in the repolarization of both types of action potentials, and that the K+ currents in the narrow action potentials seemed to activate much earlier than those in the wide spikes. This early activated K+ current may counteract the rapid sodium current, yielding the extremely brief duration and small amplitude of the narrow spikes. The sensitivity of the narrow spikes to 4-aminopyridine may not be mainly attributed to blockade of the classical A current (IA), because depolarizing the membrane potential to inactivate IA did not reproduce the effects of 4-aminopyridine. Blockade of Ca2+ influx slowed the last two-thirds repolarization of the wide action potentials. On the contrary, the narrow action potentials were not affected by Ca(2+)-current blockers, but if they were first broadened by 4-aminopyridine or tetraethylammonium, subsequent application of Ca(2+)-free medium caused further broadening, suggesting that the narrow action potentials were too brief to activate the Ca(2+)-activated potassium currents for their repolarization. Therefore, the effects of low concentrations of tetraethylammonium on the narrow spikes appeared to be mainly due to blockade of an outward current that was different from the tetraethylammonium-sensitive Ca(2+)-activated potassium current (IC). In the neurons with the narrow spikes, voltage-clamp experiments revealed two voltage-gated outward currents that were sensitive to tetraethylammonium and 4-aminopyridine, respectively. Both currents were activated rapidly following the onset of depolarizing steps. Interestingly, the tetraethylammonium-sensitive current was a transient outward current that inactivated rapidly (tau < or = 5 ms), while the 4-aminopyridine-sensitive current was relatively persistent during maintained depolarization. The 4-aminopyridine-sensitive current did not show obvious inactivation even at membrane potential of -40 mV, which completely inactivated the transient tetraethylammonium-sensitive, current. The results indicate that different potassium currents are involved in the repolarization of the narrow and wide action potentials in cat motor cortex. A novel tetraethylammonium-sensitive transient outward current and a 4-aminopyridine-sensitive outward current are responsible for the short duration and small amplitude of the narrow action potentials in the interneurons and some of the layer V pyramidal cells. These two currents are voltage-gated and Ca(2+)-independent. For the wide action potentials that characterize most pyramidal neurons, a Ca(2+)-independent tetraethylammonium-sensitive outward current and a Ca(2+)-activated potassium current are the main contributors to their repolarization.