New insights into the mechanisms of itch: Are pain and itch controlled by distinct mechanisms?
Pain Signaling and Plasticity Laboratory, Department of Anesthesiology and Neurobiology, Duke University Medical Center, 595 LaSalle Street, GSRB-I, Room 1027A, DUMC 3094, Durham, NC, 27710, USA, .Pflügers Archiv - European Journal of Physiology (Impact Factor: 4.1). 05/2013; 465(12). DOI: 10.1007/s00424-013-1284-2
Itch and pain are closely related but distinct sensations. They share largely overlapping mediators and receptors, and itch-responding neurons are also sensitive to pain stimuli. Itch-mediating primary sensory neurons are equipped with distinct receptors and ion channels for itch transduction, including Mas-related G protein-coupled receptors (Mrgprs), protease-activated receptors, histamine receptors, bile acid receptor, toll-like receptors, and transient receptor potential subfamily V1/A1 (TRPV1/A1). Recent progress has indicated the existence of an itch-specific neuronal circuitry. The MrgprA3-expressing primary sensory neurons exclusively innervate the epidermis of skin, and their central axons connect with gastrin-releasing peptide receptor (GRPR)-expressing neurons in the superficial spinal cord. Notably, ablation of MrgprA3-expressing primary sensory neurons or GRPR-expressing spinal cord neurons results in selective reduction in itch but not pain. Chronic itch results from dysfunction of the immune and nervous system and can manifest as neural plasticity despite the fact that chronic itch is often treated by dermatologists. While differences between acute pain and acute itch are striking, chronic itch and chronic pain share many similar mechanisms, including peripheral sensitization (increased responses of primary sensory neurons to itch and pain mediators), central sensitization (hyperactivity of spinal projection neurons and excitatory interneurons), loss of inhibitory control in the spinal cord, and neuro-immune and neuro-glial interactions. Notably, painful stimuli can elicit itch in some chronic conditions (e.g., atopic dermatitis), and some drugs for treating chronic pain are also effective in chronic itch. Thus, itch and pain have more similarities in pathological and chronic conditions.
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- "Increasing evidence suggests that pain and itch are mediated by similar but distinct pathways in the spinal cord, and pain is known to suppress itch via inhibitory neurons in physiological conditions (Akiyama and Carstens, 2013; Liu and Ji, 2013; Ma, 2010; Ross et al., 2010). Importantly, excitatory interneurons in the superficial dorsal horn are required for the full behavioral expression of both pain and itch (Wang et al., 2013). "
ABSTRACT: Voltage-gated sodium (NaV) channels control the upstroke of the action potentials in excitable cells. Multiple studies have shown distinct roles of NaV channel subtypes in human physiology and diseases, but subtype-specific therapeutics are lacking and the current efforts have been limited to small molecules. Here, we present a monoclonal antibody that targets the voltage-sensor paddle of NaV1.7, the subtype critical for pain sensation. This antibody not only inhibits NaV1.7 with high selectivity, but also effectively suppresses inflammatory and neuropathic pain in mice. Interestingly, the antibody inhibits acute and chronic itch despite well-documented differences in pain and itch modulation. Using this antibody, we discovered that NaV1.7 plays a key role in spinal cord nociceptive and pruriceptive synaptic transmission. Our studies reveal that NaV1.7 is a target for itch management, and the antibody has therapeutic potential for suppressing pain and itch. Our antibody strategy may have broad applications for voltage-gated cation channels.Cell 05/2014; 157(6). DOI:10.1016/j.cell.2014.03.064 · 32.24 Impact Factor
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- "Recently, we have shown that TLR7 is expressed by smalldiameter DRG neurons (Liu et al., 2010) that are responsible for the sensations of pain and itch (Basbaum et al., 2009; Liu and Ji, 2013; Woolf and Ma, 2007). Furthermore, the synthetic TLR7 ligands imiquimod and loxoribine induced rapid inward currents and action potentials in DRG nociceptor neurons (Liu et al., 2010). "
ABSTRACT: Intracellular microRNAs (miRNAs) are key regulators of gene expression. The role of extracellular miRNAs in neuronal activation and sensory behaviors are unknown. Here we report an unconventional role of extracellular miRNAs for rapid excitation of nociceptor neurons via toll-like receptor-7 (TLR7) and its coupling to TRPA1 ion channel. miRNA-let-7b induces rapid inward currents and action potentials in dorsal root ganglion (DRG) neurons. These responses require the GUUGUGU motif, only occur in neurons coexpressing TLR7 and TRPA1, and are abolished in mice lacking Tlr7 or Trpa1. Furthermore, let-7b induces TLR7/TRPA1-dependent single-channel activities in DRG neurons and HEK293 cells overexpressing TLR7/TRPA1. Intraplantar injection of let-7b elicits rapid spontaneous pain via TLR7 and TRPA1. Finally, let-7b can be released from DRG neurons by neuronal activation, and let-7b inhibitor reduces formalin-induced TRPA1 currents and spontaneous pain. Thus, secreted extracellular miRNAs may serve as novel pain mediators via activating TLR7/TRPA1 in nociceptor neurons.Neuron 04/2014; 82(1):47-54. DOI:10.1016/j.neuron.2014.02.011 · 15.05 Impact Factor
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ABSTRACT: Although the cytokine IL-31 has been implicated in inflammatory and lymphoma-associated itch, the cellular basis for its pruritic action is yet unclear. We sought to determine whether immune cell-derived IL-31 directly stimulates sensory neurons and to identify the molecular basis of IL-31-induced itch. We used immunohistochemistry and quantitative real-time PCR to determine IL-31 expression levels in mice and human subjects. Immunohistochemistry, immunofluorescence, quantitative real-time PCR, in vivo pharmacology, Western blotting, single-cell calcium imaging, and electrophysiology were used to examine the distribution, functionality, and cellular basis of the neuronal IL-31 receptor α in mice and human subjects. Among all immune and resident skin cells examined, IL-31 was predominantly produced by TH2 and, to a significantly lesser extent, mature dendritic cells. Cutaneous and intrathecal injections of IL-31 evoked intense itch, and its concentrations increased significantly in murine atopy-like dermatitis skin. Both human and mouse dorsal root ganglia neurons express IL-31RA, largely in neurons that coexpress transient receptor potential cation channel vanilloid subtype 1 (TRPV1). IL-31-induced itch was significantly reduced in TRPV1-deficient and transient receptor channel potential cation channel ankyrin subtype 1 (TRPA1)-deficient mice but not in c-kit or proteinase-activated receptor 2 mice. In cultured primary sensory neurons IL-31 triggered Ca(2+) release and extracellular signal-regulated kinase 1/2 phosphorylation, inhibition of which blocked IL-31 signaling in vitro and reduced IL-31-induced scratching in vivo. IL-31RA is a functional receptor expressed by a small subpopulation of IL-31RA(+)/TRPV1(+)/TRPA1(+) neurons and is a critical neuroimmune link between TH2 cells and sensory nerves for the generation of T cell-mediated itch. Thus targeting neuronal IL-31RA might be effective in the management of TH2-mediated itch, including atopic dermatitis and cutaneous T-cell lymphoma.The Journal of allergy and clinical immunology 12/2013; 133(2). DOI:10.1016/j.jaci.2013.10.048 · 11.48 Impact Factor
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