Functional studies reveal pronounced gain-of-function changes for the V423L and F1597L mutations and loss-of-function changes for the P1622S and G899S mutations. (A) Schematic of the human Na_v1.2 α-subunit together with β₁ and β₂ subunits showing the locations of the four functionally studied mutations (V423L green square; F1597L blue diamond; G899S orange inverted triangle; P1622S red triangle). (B) Representative current traces of whole-cell Na⁺ currents recorded from tsA201 cells transfected with either Na_v1.2 wild-type or mutant channels. (C) Voltage dependence of steady state Na⁺ channel activation and inactivation revealing a significant depolarizing shift of the activation curve for G899S (loss-of-function) as well as a significant hyperpolarizing shift of the inactivation curve for P1622S (loss-of-function) in comparison with the wild-type. Lines represent fits of Boltzmann functions. (D) Voltage dependence of the fast inactivation time constant for wild-type and mutant channels revealing a slowing of fast inactivation for F1597L and an acceleration for P1622S. (E) The time course of recovery from fast inactivation determined at −100 mV showed significant changes between wild-type and mutant channels. F1597L mutant channels showed a significantly faster recovery (gain-of-function), whereas P1622S mutant channels showed a significant slowing of the recovery from fast inactivation compared to wild-type channels (loss-of-function). Lines represent fits of exponential functions yielding the time constant τ_rec. (F) Voltage dependence of the persistent sodium current showing a large increase for V423L compared to the wild-type. Current amplitudes recorded at the end of a 70-ms depolarization were normalized to the peak current amplitude (steady state current/initial peak current). Shown are means ± SEM.