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QX-314 (N-ethyl-lidocaine) is a cationic lidocaine derivative that blocks voltage-dependent sodium channels when applied internally to axons or neuronal cell bodies. Coapplication of external QX-314 with the transient receptor potential vanilloid 1 protein (TRPV1) agonist capsaicin produces long-lasting sodium channel inhibition in TRPV1-expressing neurons, suggestive of QX-314 entry into the neurons. We asked whether QX-314 entry occurs directly through TRPV1 channels or through a different pathway (e.g., pannexin channels) activated downstream of TRPV1 and whether QX-314 entry requires the phenomenon of "pore dilation" previously reported for TRPV1. With external solutions containing 10 or 20 mM QX-314 as the only cation, inward currents were activated by stimulation of both heterologously expressed and native TRPV1 channels in rat dorsal root ganglion neurons. QX-314-mediated inward current did not require pore dilation, as it activated within several seconds and in parallel with Cs-mediated outward current, with a reversal potential consistent with PQX-314/PCs = 0.12. QX-314-mediated current was no different when TRPV1 channels were expressed in C6 glioma cells, which lack expression of pannexin channels. Rapid addition of QX-314 to physiological external solutions produced instant partial inhibition of inward currents carried by sodium ions, suggesting that QX-314 is a permeant blocker. Maintained coapplication of QX-314 with capsaicin produced slowly developing reduction of outward currents carried by internal Cs, consistent with intracellular accumulation of QX-314 to concentrations of 50-100 μM. We conclude that QX-314 is directly permeant in the "standard" pore formed by TRPV1 channels and does not require either pore dilation or activation of additional downstream channels for entry.

Permeation and block of TRPV1 channels by the cationic lidocaine derivative QX-314