Voltage-gated
sodium (Na(V)1) channels play a critical role in modulating the excitability of sensory neurons, and human genetic evidence points to Na(V)1.7 as an essential contributor to
pain signaling. Human loss-of-function mutations in SCN9A, the gene encoding Na(V)1.7, cause
channelopathy-associated indifference to
pain (CIP), whereas gain-of-function mutations are associated with two inherited painful neuropathies. Although the human genetic data make Na(V)1.7 an attractive target for the development of
analgesics, pharmacological proof-of-concept in experimental
pain models requires Na(V)1.7-selective channel blockers. Here, we show that the
tarantula venom peptide ProTx-II selectively interacts with Na(V)1.7 channels, inhibiting Na(V)1.7 with an IC(50) value of 0.3 nM, compared with IC(50) values of 30 to 150 nM for other heterologously expressed Na(V)1 subtypes. This subtype selectivity was abolished by a point mutation in DIIS3. It is interesting that application of ProTx-II to desheathed cutaneous nerves completely blocked the C-fiber compound action potential at concentrations that had little effect on Abeta-fiber conduction. ProTx-II application had little effect on action potential propagation of the intact nerve, which may explain why ProTx-II was not efficacious in rodent models of acute and inflammatory
pain. Mono-iodo-ProTx-II ((125)I-ProTx-II) binds with high affinity (K(d) = 0.3 nM) to recombinant hNa(V)1.7 channels. Binding of (125)I-ProTx-II is insensitive to the presence of other well characterized Na(V)1 channel modulators, suggesting that ProTx-II binds to a novel site, which may be more conducive to conferring subtype selectivity than the site occupied by traditional
local anesthetics and
anticonvulsants. Thus, the (125)I-ProTx-II binding assay, described here, offers a new tool in the search for novel Na(V)1.7-selective blockers.