Allergen-induced histaminergic and non-histaminergic activation of itch C-fiber nerve terminals in mouse skin.

Acute cutaneous exposure to allergen often leads to itch, but seldom pain. The effect of mast cell activation on cutaneous C-fibers was studied using innervated isolated mouse skin preparation that allows for intra-arterial delivery of chemicals to the nerve terminals in the skin. Allergen (ovalbumin) injection into the isolated skin of actively sensitized mice strongly stimulated chloroquine (CQ)-sensitive C-fibers (also referred to as "itch" nerves); on the other hand, CQ-insensitive C-fibers were activated only modestly, if at all. The histamine H1 receptor antagonist pyrilamine abolished itch C-fibers response to histamine, but failed to significantly reduce the response to ovalbumin. Ovalbumin also strongly activated itch C-fibers in skin isolated from Mrgpr-cluster Δ-/- mice. When pyrilamine was studied in the Mrgpr-cluster Δ-/- mice thereby eliminating the influence of both histamine H1 and Mrgpr receptors (MrgprA3 and C11 are selectively expressed by itch nerves), the ovalbumin response was very nearly eliminated. The data indicate that the acute activation of itch C-fibers in mouse skin is largely secondary to the combined effect of activation of histamine H1 and Mrpgr receptors.

Voltage-Gated Sodium Channels Regulating Action Potential Generation in Itch-, Nociceptive-, and Low-Threshold Mechanosensitive Cutaneous C-Fibers.

We evaluated the effect of voltage-gated sodium channel 1 (NaV1) 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. NaV1-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 α-subunits of NaV1 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 NaV1.8 and NaV1.9. 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. NaV1.7 was the most commonly expressed TTX-sensitive NaV1 in all three C-fiber subtypes innervating the dorsal skin. Selectively blocking NaV1.7 abolished responses in about 40% of itch C-fibers, 65% of nociceptors, but only 20% of C-LTMs. Blocking NaV1.8 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 NaV1.7 blocker, adding the NaV1.8 blocker silenced action potential discharge.