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Alloknesis, itch due to light mechanical stimulation, is frequently associated with dry skin and inflammatory skin disorders (e.g., atopic dermatitis, psoriasis). Recent studies have shown that mechanical itch is regulated by neuropeptide Y-positive spinal inhibitory interneurons that are innervated by low-threshold mechanoreceptors (LTMRs) (
). In hairy skin, Aβ-slowly adapting type Ι LTMRs and Merkel cells compose touch domes to detect skin identification. In dry skin and aged skin, a reduction of the sustained firing of Aβ-slowly adapting type Ι LTMRs due to a loss of Merkel cells relates to alloknesis (
). We investigated whether the number of Merkel cells or innervation of Merkel cells by Aβ-fibers is decreased in the imiquimod (IMQ)-induced psoriatic mouse model. In addition, we tested whether the targeted silencing of Aβ-fibers is sufficient to produce alloknesis.
To develop the mouse model for psoriasis, IMQ cream was applied on the rostral back skin for 7 consecutive days. For further information on materials and methods, see the Supplementary Materials online. All experimental procedures were approved by the University of Miami Institutional Animal Care and Use Committee. In naïve mice, myelinated A-fibers labeled with neurofilament H (NFH) contacted keratin 8-positive Merkel cells in the epidermis (Figure 1a). In IMQ-treated mice, the alloknesis score was markedly increased compared with vehicle (VH)-treated mice (control) (Figure 1b). Although the loss of Merkel cells in aged skin and dry skin was reported (
), the number of keratin 8-positive Merkel cells was comparable between the IMQ and VH groups (Figure 1c and d). NFH-positive epidermal nerve fiber density was decreased in the IMQ group compared with the VH group (VH, 19.6 ± 0.9 nerve fiber counts/mm; IMQ, 9.7 ± 2.1 counts/mm) (Figure 1e and f).
Toll-like receptor 5 (TLR5) was recently proposed as a marker for Aβ-fibers (
). We observed TLR5 expression in the majority of NFH-positive fibers in the rostral back skin (Figure 2a). Moreover, TLR5 is expressed by NFH-positive fibers innervating the touch dome (see Supplementary Figure S1 online). TLR5-positive epidermal nerve fiber density was decreased in the IMQ group compared with the VH group (VH, 16.8 ± 0.8 nerve fiber counts/mm; IMQ, 8.4 ± 1.2 counts/mm) (Figure 2b). To directly test the contribution of myelinated Aβ-fibers in regulating alloknesis, we used a strategy of the activity-dependent silencing of Aβ-fibers by targeted delivery of a membrane-impermeable lidocaine derivative, N-ethyl-lidocaine (QX-314) (
). In line with this finding, co-injection of QX-314 and flagellin abolished responses of low-threshold mechanosensitive fibers to brush stimuli without affecting responses of high-threshold mechanosensitive fibers to pinch stimuli (see Supplementary Figure S2 online). Given the decrease in TLR5-positive epidermal nerve fiber density in the mouse model of IMQ-induced psoriasis, we hypothesized that inhibition of TLR5-positive nerves induces a mechanical itch phenotype resembling the alloknesis seen in the psoriasis mice. Intradermal co-application of QX-314 and flagellin produced touch-evoked scratching (Figure 2c). The alloknesis score became increased at 15 minutes, peaked at 0.5–1 hours, was maintained at 3 hours, and recovered at 24 hours. Intradermal injection of either flagellin or QX-314 alone did not induce mechanical itch phenotype in naïve mice (Figure 2c). Touch-evoked scratching was observed only when the mechanical stimulus was delivered to skin adjacent to a site of injection but not skin in the center of the injection site. Consistently, it was previously reported that touch-evoked scratching was elicited only by stimulation at the skin adjacent to a site of acute itch or the border of the dry skin-treated area on the rostral back (
). Therefore, it appears that, although Aβ-fibers innervating the chemical-injected site or the skin within the treatment area drive a circuit for antimechanical itch, a subset of LTMRs innervating the adjacent skin mediates touch-evoked itch.
In summary, we show that alloknesis in an IMQ-induced psoriatic mouse model is associated with a loss of the epidermal nerve endings of Aβ-fibers but not Merkel cells. By contrast, alloknesis in dry skin and aged skin is associated with a loss of Merkel cells (
). Considering that the Merkel cells are innervated by Aβ-fibers, the loss of sustained discharge of Aβ-fibers from Merkel cells is essential for alloknesis. Aβ-fibers in hairy skin likely innervate neuropeptide Y-positive inhibitory interneurons in the spinal cord to gate mechanical itch in normal conditions (