Low potency inhibition of Na_V1.7 by externally applied QX-314 via a depolarizing shift in the voltage-dependence of activation

QX-314 is a quaternary permanently charged lidocaine derivative that inhibits voltage-gated sodium channels (Na_V). As it is membrane impermeable, it is generally considered that QX-314 applied externally is inactive, unless it can gain access to the internal local anesthetic binding site via another entry pathway. Here, we characterized the electrophysiological effects of QX-314 on Na_V1.7 heterologously expressed in HEK293 cells, and found that at high concentrations, external QX-314 inhibited Na_V1.7 current (IC₅₀ 2.0 ± 0.3 mM) and shifted the voltage-dependence to more depolarized potentials (ΔV₅₀ +10.6 mV). Unlike lidocaine, the activity of external QX-314 was not state- or use-dependent. The effect of externally applied QX-314 on Na_V1.7 channel biophysics differed to that of internally applied QX-314, suggesting QX-314 has an additional externally accessible site of action. In line with this hypothesis, disruption of the local anesthetic binding site in a [F1748A]Na_V1.7 mutant reduced the potency of lidocaine by 40-fold, but had no effect on the potency or activity of externally applied QX-314. Therefore, we conclude, using an expression system where QX-314 was unable to cross the membrane, that externally applied QX-314 is able to inhibit Na_V1.7 peak current at low millimolar concentrations.