Enhanced excitability of MRGPRA3- and MRGPRD-positive nociceptors in a model of inflammatory itch and pain

Abstract

Itch is a common symptom of diseases of the skin but can also accompany diseases of other tissues including the nervous system. Acute itch from chemicals experimentally applied to the skin is initiated and maintained by action potential activity in a subset of nociceptive neurons. But whether these pruriceptive neurons are active or might become intrinsically more excitable under the pathological conditions that produce persistent itch and nociceptive sensations in humans is largely unexplored. Recently, two distinct types of cutaneous nociceptive dorsal root ganglion neurons were identified as responding to pruritic chemicals and playing a role in itch sensation. One expressed the mas-related G-coupled protein receptor MRGPRA3 and the other MRGPRD (MRGPRA3+ and MRGPRD+ neurons, respectively). Here we tested whether these two distinct pruriceptive nociceptors exhibited an enhanced excitability after the development of contact hypersensitivity, an animal model of allergic contact dermatitis, a common pruritic disorder in humans. The characteristics of increased excitability of pruriceptive neurons during this disorder may also pertain to the same types of neurons active in other pruritic diseases or pathologies that affect the nervous system and other tissues or organs. We found that challenging the skin of the calf of the hind paw or the cheek of previously sensitized mice with the hapten, squaric acid dibutyl ester, produced symptoms of contact hypersensitivity including an increase in skin thickness and site-directed spontaneous pain-like (licking or wiping) and itch-like (biting or scratching) behaviours. Ablation of MRGPRA3+ neurons led to a significant reduction in spontaneous scratching of the hapten-challenged nape of the neck of previously sensitized mice. In vivo, electrophysiological recordings revealed that MRGPRA3+ and MRGPRD+ neurons innervating the hapten-challenged skin exhibited a greater incidence of spontaneous activity and/or abnormal after-discharges in response to mechanical and heat stimuli applied to their receptive fields compared with neurons from the vehicle-treated control animals. Whole-cell recordings in vitro showed that both MRGPRA3+ and MRGPRD+ neurons from hapten-challenged mice displayed a significantly more depolarized resting membrane potential, decreased rheobase, and greater number of action potentials at twice rheobase compared with neurons from vehicle controls. These signs of neuronal hyperexcitability were associated with a significant increase in the peak amplitude of tetrodotoxin-sensitive and resistant sodium currents. Thus, the hyperexcitability of MRGPRA3+ and MRGPRD+ neurons, brought about in part by enhanced sodium currents, may contribute to the spontaneous itch- and pain-related behaviours accompanying contact hypersensitivity and/or other inflammatory diseases in humans.

Psychophysical Measurements of Itch and Nociceptive Sensations in an Experimental Model of Allergic Contact Dermatitis

“…For example, the onset of spontaneous sensory behaviors appears similar for humans and mice during the development of ACD. Both the spontaneous itch- and pain-like behaviors of mice 25 and the ratings of spontaneous itch and nociceptive sensations of our human subjects were directed toward the site of SADBE challenge. Electrophysiologically, enhanced spontaneous activity was observed in a subset of pruriceptive MrgprA3- and MrgprD-expressing neurons innervating the SADBE-challenged mouse skin 25 .…”

“…Both the spontaneous itch- and pain-like behaviors of mice 25 and the ratings of spontaneous itch and nociceptive sensations of our human subjects were directed toward the site of SADBE challenge. Electrophysiologically, enhanced spontaneous activity was observed in a subset of pruriceptive MrgprA3- and MrgprD-expressing neurons innervating the SADBE-challenged mouse skin 25 . Increased sodium currents in these subsets of pruriceptive neurons likely contributed to spontaneous itch and pain responses following ACD 25 .…”

Mast cells: Versatile gatekeepers of pain

“…In our studies, we defined pain (nociceptive) responses as licking or jumping following mechanical stimulation [54]. Licking of the affected region or wiping it with forepaws has been shown to correlate with pain sensation, while biting correlated more with itch [58, 59]. Our unpublished data suggest that Mas-related G-protein coupled receptor A3 (MrgprA3) + expression is not significantly enhanced in painful sites multiply challenged with oxazolone in mice exhibiting vulvar tactile sensitivity.…”

Clonal differences in IgE antibodies affect cutaneous anaphylaxis-associated thermal sensitivity in mice

“…We also demonstrate a novel functional heterogeneity of IgE/Ag-induced pain outcomes in PCA reactions mediated by different clones against the same antigen in vivo . The intersection of allergies and pain is an area of growing interest with emerging epidemiological [16], clinical [17] and experimental [15,18] evidence that atopic signaling can drive changes in pain sensitivity through a variety of mechanisms. Our data indicate that the nature of the participating IgE molecules can also be a differentiating factor in anaphylaxis-associated pain in mice.…”