Activation of Nociceptive C‐fiber Subtypes in Mouse Skin by Bee Venoms

Abstract

Bee stings are extremely painful and in allergic subjects a cause of life‐threatening anaphylaxis. We have previously developed a dorsal thoracic skin‐nerve preparation to define sub‐groups of afferent C‐fibers innervating mouse skin. These comprise low mechanical threshold C‐fibers (C‐LTMs) which failed to directly respond to inflammatory mediators, and high mechanical threshold C‐fibers (C‐HTMs). The C‐HTMs could be further subdivided into two subgroups, one responding strongly to pruritogens such as histamine, chloroquine (CQ), BAM8‐22, and mast cells activation; the other failing to respond to these pruritogens, but respond strongly to 5‐HT and ATP (Mol Pharmacol. 2018, 94:1047–1056). Both types of C‐HTMs are activated by capsaicin. Here we evaluated the response of the two subgroups of C‐HTMs to venoms applied directly to the receptive fields in isolate skin. We evaluated the venom of honey bees, wasps and yellow jackets. Venom from each activated both types of C‐HTMs. Based on peak Hz of C‐fiber discharge, the relative strength of activation was: yellow jacket > honey bee ≈ wasp. CQ‐sensitive C‐fibers were activated by venoms to a greater extent than CQ‐insensitive C‐fibers. For example, bee venom evoked 6 ± 2 Hz (n=3) and 2 ± 0.4 Hz (n=7) in CQ‐sensitive vs CQ‐insensitive C‐fibers, respectively. Melittin, an active ingredient in bee venom mimicked the venoms in strongly activating C‐HTMs, with CQ‐sensitive being more strongly stimulated than CQ‐insensitive (9 ± 4 Hz (n=3) vs 1 ± 0.2 Hz (n=3), respectively). We hypothesize that CQsensitive C‐fibers are more strongly activated because unlike the CQ‐insensitive C‐HTMs, they can be stimulated as a consequence of mast cell activation (Neuroscience. 2019; 410:55–58). Indeed, melittin (10 μg/ml) caused mast cell activation (histamine release) in isolated skin fragments (76 ± 19 ng/g, above baseline, n=4); for comparison, compound 48/80 (30 μg/ml) evoked a net 203 ± 46 ng/g of histamine. The degree of melittin‐induced histamine release was not different between wildtype and MrgprB2 knockout mice, whereas compound 48/80 evoked histamine release only from the skin of wildtype mice. The data show that bee venom directly and strongly activates the terminals of virtually all high threshold nociceptive C‐fibers in mouse skin. The prurigentic subtype of C‐fibers are the most strongly activated, and this may be due to the additional indirect effect of mast cell activation, that occurs independently of MrgprB2 receptors.Support or Funding InformationThis work was supported by Department of Education, Slovakia (VEGA 1/0306/18).