Itch, also known as pruritus, is a common, intractable symptom of several skin diseases, such as atopic dermatitis and xerosis. TLRs mediate innate immunity and regulate neuropathic pain, but their roles in pruritus are elusive. Here, we report that scratching behaviors induced by histamine-dependent and -independent pruritogens are markedly reduced in mice lacking the Tlr3 gene. TLR3 is expressed mainly by small-sized primary sensory neurons in dorsal root ganglions (DRGs) that coexpress the itch signaling pathway components transient receptor potential subtype V1 and gastrin-releasing peptide. Notably, we found that treatment with a TLR3 agonist induces inward currents and action potentials in DRG neurons and elicited scratching in WT mice but not Tlr3–/– mice. Furthermore, excitatory synaptic transmission in spinal cord slices and long-term potentiation in the intact spinal cord were impaired in Tlr3–/– mice but not Tlr7–/– mice. Consequently, central sensitization–driven pain hypersensitivity, but not acute pain, was impaired in Tlr3–/– mice. In addition, TLR3 knockdown in DRGs also attenuated pruritus in WT mice. Finally, chronic itch in a dry skin condition was substantially reduced in Tlr3–/– mice. Our findings demonstrate a critical role of TLR3 in regulating sensory neuronal excitability, spinal cord synaptic transmission, and central sensitization. TLR3 may serve as a new target for developing anti-itch treatment.

The itch field has made great advances in recent years, building upon earlier work to form a clearer picture of the biology behind this important sensory modality. Models for how itch is encoded have emerged that fit with physiological, molecular, and behavioral data. The molecular mechanisms of itch, both peripherally and centrally, are being revealed with the aid of newer animal models. Future work must address shortcomings in our current understanding of itch including limitations of current experimental methods. Here we review what is known about the cells, molecules, and circuits involved in itch and highlight key questions that remain to be answered.

The itch field has made great advances in recent years, building upon earlier work to form a clearer picture of the biology behind this important sensory modality. Models for how itch is encoded have emerged that fit with physiological, molecular, and behavioral data. The molecular mechanisms of itch, both peripherally and centrally, are being revealed with the aid of newer animal models. Future work must address shortcomings in our current understanding of itch including limitations of current experimental methods. Here we review what is known about the cells, molecules, and circuits involved in itch and highlight key questions that remain to be answered.

Itch has been defined as an unpleasant skin sensation that triggers the urge to scratch. Primary sensory dorsal root ganglia neurons detect itch stimuli through peripheral axons in the skin, playing an important role in generating itch. Itch is broadly categorized as histaminergic (sensitive to antihistamines) or nonhistaminergic. The peptide Ser-Leu-Ile-Gly-Arg-Leu (SLIGRL) is an itch-inducing agent widely used to study histamine-independent itch. Here, we show that Mrgprs (Mas-related G protein–coupled receptors), particularly MrgprC11, rather than PAR2 (protease-activated receptor 2) as previously thought, mediate this type of itch. A shorter peptide, SLIGR, which specifically activates PAR2 but not MrgprC11, induced thermal pain hypersensitivity in mice but not a scratch response. Therefore, although both Mrgpr and PAR2 are SLIGRL-responsive G protein–coupled receptors present in dorsal root ganglia, each plays a specific role in mediating itch and pain.

Chronic itch accompanying many dermatological, neurological and systemic diseases is unresponsive to antihistamines. Our knowledge of endogenous chemicals that evoke histamine-independent itch and their molecular targets is very limited. Recently it was demonstrated in behavioral and cellular experiments that bovine adrenal medulla 8–22 peptide (BAM8–22), a proteolytically cleaved product of proenkephalin A, is a potent activator of Mas-related G protein-coupled receptors (Mrgprs), MrgprC11 and hMrgprX1, and induces scratching in mice in a Mrgpr-dependent manner. To study the sensory qualities that BAM8–22 evokes in humans we tested the volar forearm of 15 healthy volunteers with heat-inactivated cowhage spicules previously soaked in the peptide. BAM8–22 produced itch in each subject, usually accompanied by sensations of pricking/stinging and burning. The sensations were occasionally accompanied by one or more mechanically evoked dysesthesias, namely alloknesis, hyperknesis, and hyperalgesia, but no wheal or neurogenic flare in the skin surrounding the application site. The inactive truncated peptide BAM8–18 produced weak or no sensations. Pretreatment of the tested skin with an antihistamine cream (doxepin) inhibited the histamine-induced sensations, dysesthesias and skin reactions but not the sensations and dysesthesias evoked by BAM8–22. We show that BAM8–22 produces itch and nociceptive sensations in humans in a histamine-independent manner. Thus, BAM8–22 may be an endogenous itch mediator that activates, in humans, MrgprX1, a novel target for potential anti-itch treatments.

SUMMARY

Itch, the unpleasant sensation that evokes a desire to scratch, accompanies numerous skin and nervous system disorders. In many cases, pathological itch is insensitive to antihistamine treatment. Recent studies have identified members of the Mas-related GPCR (Mrgpr) family that are activated by mast cell mediators and promote histamine-independent itch. MrgprA3 and MrgprC11 act as receptors for the pruritogens chloroquine and BAM8–22, respectively. However, the signaling pathways and transduction channels activated downstream of these pruritogens are largely unknown. We found that TRPA1 is the downstream target of both MrgprA3 and MrgprC11, in cultured sensory neurons and heterologous cells. TRPA1 is required for Mrgpr-mediated signaling, as sensory neurons from TRPA1-deficient mice exhibited profoundly diminished responses to chloroquine and BAM8–22. Likewise, TRPA1-deficient mice displayed little to no scratching in response to these pruritogens. Our findings demonstrate that TRPA1 is an essential component of the signaling pathways that promote histamine-independent itch.

The description of itch (formally known as pruritus) as an “unpleasant sensation that elicits the desire or reflex to scratch” (Ikoma et al., 2006) is immediately familiar. Research in the field of pruritoception has added to our understanding of this area of sensory neurobiology as it pertains to both normal and pathological conditions. In particular, much progress has been made on the mechanisms and circuits of itch, which we review here.

Itch, or pruritus, is an important clinical problem whose molecular basis has yet to be understood. Recent work has begun to identify genes that contribute to detecting itch at the molecular level. Here we show that Pirt, known to play a vital part in sensing pain through modulation of the transient receptor potential vanilloid 1 (TRPV1) channel, is also necessary for proper itch sensation. Pirt−/− mice exhibit deficits in cellular and behavioral responses to various itch-inducing compounds, or pruritogens. Pirt contributes to both histaminergic and nonhistaminergic itch and, crucially, is involved in forms of itch that are both TRPV1-dependent and -independent. Our findings demonstrate that the function of Pirt extends beyond nociception via TRPV1 regulation to its role as a critical component in several itch signaling pathways.

SUMMARY

The cellular and molecular mechanisms mediating histamine-independent itch in primary sensory neurons are largely unknown. Itch induced by chloroquine (CQ) is a common side-effect of this widely used anti-malarial drug. Here we show that Mrgprs, a family of G protein-coupled receptors expressed exclusively in peripheral sensory neurons, function as itch receptors. Mice lacking a cluster of Mrgpr genes display significant deficits in itch induced by CQ but not histamine. CQ directly excites sensory neurons in an Mrgpr-dependent manner. CQ specifically activates mouse MrgprA3 and human MrgprX1. Loss- and gain-of-function studies demonstrate that MrgprA3 is required for CQ responsiveness in mice. Furthermore, MrgprA3-expressing neurons respond to histamine and co-express Gastrin-Releasing Peptide, a peptide involved in itch sensation, and MrgprC11. Activation of these neurons with MrgprC11-specific agonist BAM8-22 induces itch in wild-type but not mutant mice. Therefore, Mrgprs may provide molecular access to itch-selective neurons and constitute novel targets for itch therapeutics.

SUMMARY

Transient receptor potential vanilloid 1 (TRPV1) is a molecular sensor of noxious heat and capsaicin. Its channel activity can be modulated by several mechanisms. Here we identify a membrane protein, Pirt, as a regulator of TRPV1. Pirt is expressed in most nociceptive neurons in the dorsal root ganglia (DRG) including TRPV1-positive cells. Pirt null mice show impaired responsiveness to noxious heat and capsaicin. Noxious heat- and capsaicin-sensitive currents in Pirt-deficient DRG neurons are significantly attenuated. Heterologous expression of Pirt strongly enhances TRPV1-mediated currents. Furthermore, the C terminus of Pirt binds to TRPV1 and several phosphoinositides, including phosphatidylinositol-4,5-bisphosphate (PIP2), and can potentiate TRPV1. The PIP2 binding is dependent on the cluster of basic residues in the Pirt C terminus and is crucial for Pirt regulation of TRPV1. Importantly, the enhancement of TRPV1 by PIP2 requires Pirt. Therefore, Pirt is a key component of the TRPV1 complex and positively regulates TRPV1 activity.