Figure 6. Extracellular QX-314 specifically blocks I1706V without affecting WT channel. A, Representative inward currents recorded from a small-diameter DRG neuron expressing WT channels after 10 min wash-in of vehicle (saline), followed by another 10 min treatment of 1 mm QX-314 in the bath. Cells were held at −80 mV and stepped to membrane potentials from −60 to +50 mV for 100 ms in 5 mV increments. B, The same activation protocol was applied to a DRG neuron expressing I1706V channels in the presence of vehicle and QX-314. C, The average peak inward current of neurons expressing WT channels (n = 6) was not altered by external QX-314 (p = 0.130, paired t test). Each line connects the data points of pretreament and post-treatment QX-314 at the same neuron. D, Extracellular QX-314 significantly reduced peak current amplitude of neurons expressing I1706V channels (p = 0.001, paired t test). E, Normalized peak current–membrane voltage (I–V) relationship for activation of WT channels (n = 6) before treatment, after 10 min wash-in of vehicle, and after 10 min wash-in of 1 mm QX-314. F, Normalized I–V curved of I1706V channels (n = 9) before treatment, after 10 min wash-in of vehicle, and after 10 min wash-in of 1 mm QX-314. External QX-314 blocked I1706V currents without significant effect on WT currents. G, Normalized peak inward currents of neurons expressing WT (n = 6) or I1706V (n = 9) channels in the presence of vehicle or 1 mm QX-314. Externally applied quaternary blocker QX-314 barely altered the normalized peak current of WT (p = 0.871), but it significantly decreased the normalized current of I1706V by ∼30% (p < 0.001). There was no significant difference (p = 0.991) in the normalized peak current treated by vehicle between WT and I1706V channels. H, Action of externally applied QX-314 on WT (vehicle, n = 8; QX-314, n = 6) and I1706V (vehicle, n = 11; QX-314, n = 14) channels, respectively, for the time indicated. Currents were evoked by depolarizing pulses to 0 mV for 50 ms at 10 s intervals for 5 min. The data in each experiment were normalized with respect to the control currents. I, Representative current traces of a DRG neuron expressing WT channels treated with vehicle, followed by 1 mm QX-314 application in the bath. WT current slightly run up over time. J, Representative current traces of a DRG neuron expressing I1706V channels washed-in with vehicle for 5 min, followed by external QX-314 treatment for an additional 5 min. I1706V current (green line) was markedly reduced compared with vehicle treatment (red) or control (black).