Voltage-gated sodium channels regulating action potential generation in itch-, nociceptive-, and low-threshold mechanosensitive cutaneous C-fibers

Abstract

We evaluated the effect of voltage-gated sodium channel 1 (Na V 1) blockers in three nonoverlapping C-fiber subtypes in the mouse skin: chloroquine (CQ)-sensitive C-fibers with high mechanical thresholds—itch C-fibers; second, CQ-insensitive, capsaicin-sensitive C-fibers with high mechanical thresholds—nociceptors; and CQ and capsaicin-insensitive C-fibers with a very low mechanical threshold—C-LTMs. Na V 1-blocking drugs were applied to the nerve terminal receptive fields using an innervated isolated dorsal mouse skin-nerve preparation where the drugs are delivered into the skin intra-arterially. We combined these studies with an analysis of the mRNA expression of the a-subunits of Na V 1 in individual dorsal root ganglia neurons labeled from the same region of the skin. Our results show that virtually all nociceptors and itch C-fibers expressed the tetrodotoxin (TTX)-resistant channels Na V 1.8 and Na V 1.9. However, TTX applied selectively into the skin abolished the action potential firing in response to mechanical stimulation in 75% of the itch C-fibers, 100% of the nociceptors, and 100% of C-LTMs. Na V 1.7 was the most commonly expressed TTX-sensitive Na V 1 in all three C-fiber subtypes innervating the dorsal skin. Selectively blocking Na V 1.7 abolished responses in about 40% of itch C-fibers, 65% of nociceptors, but only 20% of C-LTMs. Blocking Na V 1.8 alone had no affect on the firing sensitivity of the C-fibers. However, in itch and nociceptive C-fibers where the activation was not inhibited with a Na V 1.7 blocker, adding the Na V 1.8 blocker silenced action potential discharge.