Loss of self-sustained plateau depolarization, burst firing, and conditional bistable behaviors in Nav1.9^−/− DRG neurons. (A) Family of superimposed membrane potential changes evoked by a series of 150-ms current pulse injection in a Nav1.9^−/− DRG neuron (22 pF). Low-intensity pulses evoked passive responses whereas large stimuli evoked action potentials and slow after-depolarization potentials that outlasted the stimulus (arrow). Injected current was varied in amplitude with 5-pA increments. (Inset) The same cell was subjected to slow voltage ramp from −100 to 0 mV. Note the “N-shaped” I-V relationship (compare with Fig. 1 C). (B) Lack of spontaneous activity in a Nav1.9^−/− DRG neuron (31 pF) held at ∼−85 mV (bias current, −35 pA). The cell was challenged by 100-ms depolarizing step currents and responded by short spike volleys. (C) Families of superimposed membrane potential changes evoked by a series of current pulse injection in a Nav1.9^−/− DRG neuron (31 pF). Membrane potential was adjusted manually to between −35 and −40 mV by DC bias, immediately after conditioning V_m at −90 mV. Note that current pulses evoked responses with exponential on and off trajectories indicating a passive RC membrane behavior (compare with Fig. 1 E). Recordings made using the intracellular solution 2 (Table I; 0 mM CsCl, 30 mM CsF, 100 mM KCl).