(A) Recordings of steady-state fast inactivation of Na⁺ currents from nucleated patches of inhibitory neurons of wt/wt (top) and mut/wt (bottom) animals. Steady-state inactivation induced by 30 ms–lasting conditioning pulses to different potentials (between –120 mV and 20 mV in 10 mV increments) and 30 ms test pulse to 0 mV (holding potential –90 mV). Shown are currents elicited by test pulse to 0 mV (conditioning pulses of –100 mV, –60 mV, –50 mV, –40 mV, –30 mV, –20 mV, –10 mV shown on the left). (B) Mean voltage-dependence of steady-state Na⁺-channel activation and fast inactivation (± SEM). Lines represent Boltzmann functions fit. V_1/2 and k_V slope factor for activation and inactivation curves were not different between genotypes. Dashed box represents the part used for analyzing area under the curve (AUC) shown in C. (C) Box plot of values of AUC of steady-state fast inactivation between –50 mV and 20 mV, indicating a persistent, steady-state Na⁺ current, which was significantly increased in inhibitory neurons of mut/wt compared with wt/wt animals (P = 0.029; Mann-Whitney rank sum test; wt/wt, n = 8; mut/wt, n = 8). (D) Representative whole-cell recordings elicited by voltage ramps from –80mV to 0 mV showing ramp Na⁺ currents of fast-spiking neurons, which were blocked by TTX. Bottom traces show net ramp current after subtracting traces recorded with TTX from those without. (E) Box plots of ramp current amplitudes recorded in inhibitory neurons from wt/wt and mut/wt animals. The ramp current was significantly increased in neurons of mut/wt animals compared with wt/wt (**P < 0.01; Mann-Whitney rank sum test; wt/wt: n = 9 cells from 2 animals [9/2]; mut/wt: 7/2). (F) Box plots of integral of recorded ramp currents, which were significantly increased in neurons of mut/wt animals compared with wt/wt (***P < 0.001; Mann-Whitney rank sum test; wt/wt: 9/2; mut/wt: 7/2).