Differential Activation of Itch vs Pain C‐fibers by Mast Cell Stimulation in Mouse Skin

Abstract

Allergic skin reactions are often itchy, but seldom painful. We addressed the hypothesis that mast cell activation preferentially stimulates itch nerves in the mouse skin. Nerves that evoke itch in mouse skin can be identified by their expression of MrgprA3 and selective activation by chloroquine (CQ) (Cell. 2009,139:1353–65). We have developed a unique innervated isolated dorsal skin preparation in which action potential discharge in single afferent nerves terminating in the skin can be evaluated with an extracellular electrode positioned in the dorsal root ganglia. The dissected section skin includes the main arterial supply through which chemical stimuli are delivered directly to the receptive field. CQ strongly stimulated (12 ± 1Hz) only C‐fibers (0.8 ± 0.01 m/s). Among 228 C‐fibers from 138 mice, 26% were CQ and histamine sensitive; so‐called “itch C‐fibers”. Capsaicin stimulated 73% of the CQ sensitive C‐fibers. The response to CQ, but not histamine was absent in Mrgpr cluster knock out animals. The majority of capsaicin‐sensitive C‐fibers are not stimulated by CQ, but are activated by ATP or 5‐HT; a population likely comprising “pain C‐fibers”. Mice were actively sensitized to ovalbumin (OVA). OVA added to the sensitized skin activates mast cells leading to mast cell mediator release strong activation of 12 of 12 CQ‐sensitive C‐fibers (6 ± 2 Hz). Conversely OVA‐activation of mast cells did not or only very modestly activated the non‐itch, pain population of C‐fibers (1.6 ± 2 Hz, n=30). The itch fibers do not respond to 5‐HT (0 of 24 responded), so it is unlikely that 5‐HT contributes to the activation of itch nerves. CQ‐sensitive C‐fibers respond to histamine 100 μM with only 4 ± 1 Hz discharge, so it is likely that mediators in addition to histamine are involved. We have previously noted that mast cells can release MrgprC11 agonists (J Immunol. 2008, 180:2251–5). BAM8‐22, a MrgprC11 agonist, stimulated 13 of 13 CQ‐sensitive C‐fibers (12 ± 2 Hz), but not CQ‐insensitive fibers (only 3 of 29 stimulated) thereby mimicking the effect of mast cells activation. We hypothesize that mast cell mediators that stimulate Mrgpr C11 may contribute to the response.Support or Funding InformationSupprted by NIH