The tetrodotoxin-resistant Na+ channel Nav1.8 reduces the potency of local anesthetics in blocking C-fiber nociceptors
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The generation of action potentials in nociceptive neurons is accomplished by the tetrodotoxin-resistant (TTXr) Na+ channel Nav1.8. Following nerve injury, a redistribution of Nav1.8 from dorsal root ganglion (DRG) neurons into peripheral axons contributes to hyperexcitability and possibly to neuropathic pain. Nav1.8 has been reported to display a lower sensitivity to block by Na+ channel blockers as compared to TTX-sensitive (TTXs) Nav subunits. Furthermore, the antinociceptive efficacy of lidocaine is increased in Nav1.8-knockout mice. Here, we asked if Nav1.8 expression can reduce the susceptibility of sensory neurons to block by lidocaine. Employing wild-type and Nav1.8-knockout mice, we examined C-fibers in the skin-nerve preparation and Na+ currents in DRG neurons by patch-clamp recordings. Deletion of Nav1.8 resulted in an enhanced tonic block of Na+ currents in DRG neurons held at −80 mV but not at −140 mV. Accordingly, lower concentrations of lidocaine were required for a conduction block of C-fibers from Nav1.8-knockout as compared to wild-type mice. The efficacy of lidocaine on neurons lacking Nav1.8 was further increased by cold temperatures, due to a synergistic hyperpolarizing shift of the slow inactivation of TTXs Na+ channels by lidocaine and cooling. Finally, the ∼90% reduction of TTXr Na+ currents in injured neurons from mice with a peripheral nerve injury was accompanied with an enhanced tonic block by lidocaine. In conclusion, our data demonstrate that the expression of Nav1.8 in sensory neurons can confine the antinociceptive efficacy of lidocaine and other Na+ channel blockers employed for pain treatment.
KeywordsSodium channel Tetrodotoxin Sensory neuron Pain Lidocaine Neuropathic pain
NaN or SNS2
Slowly inactivating TTX-resistant current
Persistent TTX-resistant current
Total current in intact neurons
Total current in injured neurons
Spared nerve injury
Dorsal root ganglion
We thank John Wood (University College London) for providing the Nav.1.8-knockout mice and Iwona Izydorczyk for excellent technical assistance. We also thank Gerd Geisslinger (pharmazentrum frankfurt/ZAFES) for support and helpful discussions. This work was supported by a local grant of the Medical Faculty of the University of Erlangen-Nuremberg (ELAN NO. 56810013) and by BMBF 01EM0511 (Deutscher Forschungsverbund Neuropathischer Schmerz).
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