State-dependent enhancement of subthreshold A-type potassium current by 4-aminopyridine in tuberomammillary nucleus neurons.

A-type potassium current (I(A)) both activates and inactivates at subthreshold voltages. We asked whether there is steady-state I(A) at subthreshold voltages, using dissociated mouse tuberomammillary nucleus neurons, pacemaking neurons with large I(A) currents in which subthreshold I(A) might regulate firing frequency. With slow depolarizing voltage ramps (20 mV/s), there was no discernible component of steady-state outward current in the range of -70 to -40 mV. However, faster ramps of 50-100 mV/s, similar to the rate of spontaneous depolarization during pacemaking, did evoke subthreshold outward currents. Ramp-evoked current at subthreshold voltages was unaffected by 10 mM tetraethylammonium and likely represents I(A), because its voltage dependence overlaps that of I(A) activation (midpoint near -44 mV) and inactivation (midpoint near -85 mV). However, although 4-aminopyridine (4-AP) inhibited peak I(A) activated by step depolarizations as expected (IC50, approximately 1 mM), ramp-evoked current was instead dramatically enhanced (current at -40 mV evoked by 50 mV/s ramp enhanced >15-fold by 10 mM 4-AP). In cell-attached recordings of spontaneous pacemaking, 10 mM 4-AP slowed rather than speeded firing, consistent with enhancement of subthreshold I(A). Also consistent with such enhancement, 4-AP also greatly increased the latency to first spike after long hyperpolarizations. The striking enhancement of I(A) during depolarizing ramps can be explained by a model in which 4-AP binds tightly to closed channels but must unbind before channels can inactivate. Thus, the state dependence of 4-AP binding to the channels underlying I(A) can result in effects on firing patterns opposite to those expected from simple block of I(A).