Human skin is constantly exposed to solar ultraviolet radiation (UVR), the most prevalent environmental carcinogen. Humans have the unique ability among mammals to respond to UVR by increasing their skin pigmentation, a protective process driven by melanin synthesis in epidermalmelanocytes. The molecular mechanisms used by melanocytes to detect and respond to long-wavelength UVR (UVA) are not well understood. We recently identified a UVA phototransduction pathway in melanocytes that is mediated by G protein-coupled receptors and leads to rapid calcium mobilization. Here we report that in human epidermal melanocytes physiological doses of UVR activate a retinal-dependent current mediated bytransient receptor potential A1 (TRPA1) ion channels. The TRPA1 photocurrent is UVA-specific and requires G protein and phospholipase C signaling, thus contributing to UVA-induced calcium responses to mediate downstream cellular effects and providing evidence for TRPA1 function in mammalianphototransduction. Remarkably, TRPA1 activation is required for the UVR-induced and retinal-dependent early increase in cellular melanin. Our results show that TRPA1 is essential for a unique extraocular phototransduction pathway in human melanocytes that is activated by physiological doses of UVR and results in early melanin synthesis.

The sensations of pain, itch and cold often interact with each other. Pain inhibits itch, whereas cold inhibits both pain and itch. TRPV1 and TRPA1 channels transduce pain and itch, whereas TRPM8 transduces cold. The pruritogen chloroquine (CQ) was reported to excite TRPA1, leading to the sensation of itch. It is unclear how CQ excites and modulates TRPA1+, TRPV1+ and TRPM8+ neurons and thus affects the sensations of pain, itchand cold. Here, we show that only 43% of CQ-excited dorsal root ganglion (DRG) neurons express TRPA1; as expected, the responses of these neurons were completely prevented by the TRPA1 antagonist HC-030031. The remaining 57% of CQ-excited neurons did not express TRPA1, and excitation was not prevented by either a TRPA1 or a TRPV1 antagonist, but was prevented by the general TRPC channel blocker BTP2 and the selective TRPC3 inhibitor Pyr3. Furthermore, CQ caused potent sensitization of TRPV1 in 51.9% of TRPV1+ neurons, and concomitant inhibition of TRPM8 in 48.8% of TRPM8+ DRG neurons. Sensitization of TRPV1 is mainly caused by activation of the PLC-PKC pathway following activation of the CQ receptor MrgprA3. By contrast, inhibition of TRPM8 is caused by a direct action of activated Galpha q independently of the PLC pathway. Our data suggest the involvement of TRPC3 channel acting together with TRPA1 to mediate CQ-induced itch. CQ not only elicits itch by directly excitingitch-encoding neurons, but also exerts previously unappreciated widespread actions on pain-, itch- and cold-sensing neurons, leading to enhanced pain and itch.

The formalin model is widely used for evaluating the effects of analgesic compounds in laboratory animals. Injection of formalin into the hind paw induces a biphasic pain response; the first phase is thought to result from direct activation of primary afferent sensory neurons, whereas the second phase has been proposed to reflect the combined effects of afferent input and central sensitization in the dorsal horn. Here we show that formalin excites sensory neurons by directly activating TRPA1, a cation channel that plays an important role in inflammatory pain. Formalin induced robust calcium influx in cells expressing cloned or native TRPA1 channels, and these responses were attenuated by a previously undescribed TRPA1-selective antagonist. Moreover, sensory neurons from TRPA1-deficient mice lacked formalin sensitivity. At the behavioral level, pharmacologic blockade or genetic ablation of TRPA1 produced marked attenuation of the characteristic flinching, licking, and lifting responses resulting from intraplantar injection of formalin. Our results show that TRPA1 is the principal site of formalin’s pain-producing action in vivo, and that activation of this excitatory channel underlies the physiological and behavioral responses associated with this model of pain hypersensitivity.

The underlying mechanisms of itch are poorly understood. We have investigated a model involving the chemoattractant leukotriene B(4) (LTB(4)) that is up-regulated in common skin diseases. Intradermal injection of LTB(4) (0.1 nmol/site) into female CD1 mice induced significant scratching movements (used as an itch index) compared with vehicle-injected (0.1% bovine serum albumin-saline) mice. Intraperitoneal transient receptor potential (TRP) channel antagonist treatment significantly inhibited itch as follows: TRP vanilloid 1 (TRPV1) antagonist SB366791 (0.5 mg/kg, by 97%) and the TRP ankyrin 1 (TRPA1) antagonists TCS 5861528 (10 mg/kg; 82%) and HC-030031 (100 mg/kg; 76%). Leukotriene B(4) receptor 2 antagonism by LY255283 (5 mg/kg i.p.; 62%) reduced itch. Neither TRPV1-knockout (TRPV1-KO) nor TRPA1-knockout (TRPA1-KO mice exhibited LTB(4)-induced itch compared with their wild-type counterparts. The reactive oxygen species scavengers N-acetylcysteine (NAC; 204 mg/kg i.p.; 86%) or superoxide dismutase (SOD; 10 mg/kg i.p.; 83%) also inhibited itch. LTB(4)-induced superoxide release was attenuated by TCS 5861528 (56%) and HC-030031 (66%), NAC (58%), SOD (50%), and LY255283 (59%) but not by the leukotriene B(4) receptor 1 antagonist U-75302 (9 nmol/site) or SB366791. Itch, superoxide, and myeloperoxidase generation were inhibited by the leukocyte migration inhibitor fucoidan (10 mg/kg i.v.) by 80, 61, and 34%, respectively. Myeloperoxidase activity was also reduced by SB366791 (35%) and SOD (28%). TRPV1-KO mice showed impaired myeloperoxidase release, whereas TRPA1-KO mice exhibited diminished production of superoxide. This result provides novel evidence that TRPA1 and TRPV1 contribute to itch via distinct mechanisms.-Fernandes, E. S., Vong, C. T., Quek, S., Cheong, J., Awal, S., Gentry, C., Aubdool, A. A., Liang, L., Bodkin, J.V., Bevan, S., Heads, R., Brain, S.D. Superoxide generation and leukocyte accumulation: key elements in the mediation of leukotriene B(4)-induced itch by transient receptor potential ankyrin 1 and transient receptor potential vanilloid 1.

PMID: 23271050

Persistent pruritus is a common disabling dermatologic symptom associated with different etiologic factors. These include primary skin conditions, as well as neuropathic, psychogenic or systemic disorders like chronic liver disease. Defective clearance of potential pruritogenic substances that activate itch-specific neurons innervating the skin is thought to contribute to cholestatic pruritus. However, because the underlying disease-specific pruritogens and itch-specific neuronal pathways and mechanism(s) are unknown, symptomatic therapeutic intervention often leads to no or only limited success. In the current study, we aimed to first validate rats with bile duct ligation (BDL) as a model for hepatic pruritus, and then to evaluate the contribution of inflammation, peripheral neuronal sensitization, and specific signaling pathways and subpopulations of itch-responsive neurons to scratching behavior and thermal hypersensitivity. Chronic BDL rats displayed enhanced scratching behavior and thermal hyperalgesia indicative of peripheral neuroinflammation. BDL-induced itch and hypersensitivity involved a minor contribution of histaminergic/serotonergic receptors, but significant activation of PAR(2) receptors, prostaglandin PGE(2) formation and potentiation of TRPV1 channel activity. The sensitization of DRG nociceptors in BDL rats was associated with increased surface expression of PAR(2) and TRPV1 proteins and an increase in the number of PAR(2)- and TRPV1-expressing peptidergic neurons together with a shift of TRPV1 receptor expression to medium-sized DRG neurons. These results suggest that pruritus and hyperalgesia in chronic cholestatic BDL rats are associated with neuroinflammation and involves PAR(2)-induced TRPV1 sensitization. Thus, pharmacological modulation of PAR(2) and/or TRPV1 may be a valuable therapeutic approach for patients with chronic liver pruritus refractory to conventional treatments.