Expression of Fgf14^F145S results in selective changes in Nav current inactivation in cerebellar Purkinje neurons that are similar to the changes identified in cells with targeted deletion of Fgf14. A, To analyze the voltage dependences of steady-state inactivation of I_NaT in Purkinje neurons across genotypes, peak I_NaT amplitudes evoked at −20 mV from a 2.5 ms voltage step to various conditioning potentials (−120 to −10 mV) were measured and normalized to the peak I_NaT measured from the −120 mV conditioning voltage step in the same cell; the voltage-clamp paradigm is illustrated below the representative current records. B, Mean ± SEM normalized peak I_NaT amplitudes in wild-type (black; n = 11), Fgf14^F145S/+ (purple, n = 8), and Fgf14^F145S/F145S (blue, n = 10) Purkinje neurons are plotted as a function of the conditioning voltage and fitted with first-order Boltzmann functions. C, To determine the voltage dependences of I_NaT activation, the mean ± SEM normalized peak I_NaT conductances calculated (as described in Materials and Methods) in wild-type (black; n = 8), Fgf14^F145S/+ (purple, n = 7), and Fgf14^F145S/F145S (blue, n = 7) Purkinje neurons are plotted as a function of the test potential and fitted with first-order Boltzmann functions. D, The voltage dependences of steady-state inactivation of I_NaT were also measured in adult Fgf14^+/− (green, n = 10) and Fgf14^−/− (red, n = 10) Purkinje neurons, plotted and fitted (as in panel B), and compared with the steady-state inactivation data for I_NaT in wild-type (black; n = 11) Purkinje neurons. E, Mean ± SEM normalized peak Nav conductances were also determined in Fgf14^+/− (green, n = 7) and Fgf14^−/− (red, n = 7) Purkinje neurons and are plotted and fitted (similar to panel C) in comparison with data from wild-type (black, n = 8) Purkinje neurons. F, To compare the kinetics of I_NaT decay, the waveforms of the currents evoked at 0 mV in wild-type, Fgf14^−/−, and Fgf14^F145S/F145S Purkinje neurons were fitted with single exponentials. As is evident, the mean ± SEM time constants determined from these fits indicate that the kinetics of I_NaT inactivation are similar (one-way ANOVA) in wild-type, Fgf14^−/−, and Fgf14^F145S/F145S Purkinje neurons.