Mechanisms of pruritogen‐induced activation of itch nerves in isolated mouse skin

Abstract

Chloroquine (CQ) and histamine are pruritogens commonly used to study itch in the mouse. A novel skin-nerve preparation was used to evaluate chloroquine (CQ)- and histamine-induced activation of afferent nerves in the dorsal thoracic skin of the mouse. All CQ sensitive nerves were C-fibres, and were also sensitive to histamine. The response to CQ, but not histamine, was largely absent in mrgpr-cluster Δ mice, supporting the hypothesis that CQ evokes itch largely via stimulation of MrgprA3 receptors. The CQ-induced action potential discharge was largely absent in phospholipase Cβ3 knockout animals. The CQ and histamine responses were not influenced by removal of TRPA1, TRPV1, TRPC3 or TRPC6, nor by the TRP channel blocker Ruthenium Red. The bouts of scratching in response to CQ were not different between wild-type and TRPA1-deficient mice. A selective inhibitor of the calcium-activated chloride channel TMEM16A, N-((4-methoxy)-2-naphthyl)-5-nitroanthranilic acid (MONNA), inhibited CQ-induced action potential discharge at itch nerve terminals and bouts of scratching by about 50%. Although TRPA1 and TRPV1 channels may be involved in the scratching responses to intradermal pruritogens, this is unlikely to be due to an effect at the nerve terminals, where chloride channels may play a more important role.

Itchy channels and where to find them

“…Interestingly, M channel inhibition with the selective blocker XE991 did not induce itch fibre activity in vitro; however, the in vivo effect of XE991 has not been tested. The excitatory action of the Cl − channel TMEM16A in sensory afferents, suggested in this (Ru et al 2017) and previous (Liu et al 2010;Cho et al 2012;Jin et al 2013;Takayama et al 2015) studies is at odds with known inhibitory action of another Cl − channel abundantly expressed in sensory afferents, the GABA A channel (see e.g. Takkala et al 2016;Du et al 2017).…”

“…The excitatory action of the Cl − channel TMEM16A in sensory afferents, suggested in this (Ru et al . ) and previous (Liu et al . ; Cho et al .…”

“…In the new study published in the current issue of The Journal of Physiology, Ru and colleagues developed an improved skin-nerve preparation in which electrophysiological recordings from intact itch afferents are combined with fast, reliable and repeatable subcutaneous application of drugs (Ru et al 2017). In this preparation the dorsal side of the mouse skin is dissected out along with the blood vessels (subscapular artery and its branches) and the nerves (with their associated T7-T10 dorsal root ganglia).…”

Itching, chloroquine, and malaria: a review of recent molecular and neuroscience advances and their contribution to mechanistic understanding and therapeutics of chronic non‐histaminergic pruritus

“…They were named mas‐related because they were responsive to chemical mediators released after mast cell degranulation . The CQ itch sensitive pruriceptors are of the subfamily mrgprA3 in DRG of mice and mrgprX1+ in DRG of humans …”

“…Very recent evidence suggests that the mrgprA3/X1 coupling to TRPA1 may be restricted to noncutaneous, nonprimary afferent sites in CQ pruritus . Experiments in mice on pruritogen activation of itch dorsal‐skin nerves (C fibers) show that phospholipase C β 3 knockout abolished the CQ activation of action potential generation, whilst double knockout of TRPA1, TRPV1 did not affect action potential discharge after CQ in the skin, even though mrgpr cluster knockout mice do not itch to CQ.…”