Background Although the cytokine, interleukin-31 (IL-31), has been implicated in inflammatory and lymphoma-associated itch, the cellular basis for its pruritic action is yet unclear. Objective To determine whether immune cell-derived IL-31 directly stimulates sensory neurons, and to identify the molecular basis of IL-31-induced itch. Methods We used immunohistochemistry and qRTPCR to determine IL-31 expression levels in mice and humans. Immunohistochemistry, immunofluorescence, qRTPCR, in vivo pharmacology, western blotting, single cell calcium and electrophysiology were used to examine the distribution, functionality and cellular basis of the neuronal IL-31 receptor (IL-31RA) in mice and humans. Results Among all immune and resident skin cells examined, IL-31 was predominantly produced by TH2 and to a significantly lesser extend by mature dendritic cells. Cutaneous and intrathecal injections of IL-31 evoked intense itch, and its concentration increased significantly in murine atopic-like dermatitis skin. Both human and mouse DRG neurons express IL-31RA, largely in neurons that co-express TRPV1. IL-31-induced itch was significantly reduced in TRPV1- and TRPA1-deficient mice, not c-kit or PAR-2 mice. In cultured primary sensory neurons, IL-31 triggered Ca2+-release and ERK1/2 phosphorylation, Inhibition of which blocked IL-31 signaling in vitro and reduced IL-31-induced scratching in vivo. Conclusion IL-31RA is a functional receptor expressed by a small subpopulation of IL-31RA+/TRPV1+/TRPA1+ neurons, and is a critical neuro-immune link between TH2 cells and sensory nerves for the generation of T cell-mediated itch. Thus, targeting neuronal IL-31RA may be effective in the management of TH2-mediated itch, including atopic dermatitis and cutaneous T cell lymphoma.

SUMMARYAlthough microglia are implicated in nerve injury-induced neuropathic pain, how injured sensory neurons engage microglia is unclear. Here we demonstrate that peripheral nerve injury induces de novo expression of colony-stimulating factor 1 (CSF1) in injured sensory neurons. The CSF1 is transported to the spinal cord where it targets the microglial CSF1 receptor (CSF1R). Cre-mediated sensory neuron deletion of Csf1 completely prevented nerve injury-induced mechanical hypersensitivity and reduced microglia activation and proliferation. In contrast, intrathecal injection of CSF1 induces mechanical hypersensitivity and microglial proliferation. Nerve injury also upregulated CSF1 in motoneurons, where it is required for ventral horn microglial activation and proliferation. Downstream of CSF1R, we found that the microglial membrane adapter protein DAP12 is required for both nerve injury- and intrathecal CSF1-induced upregulation of pain-related microglial genes and the ensuing pain, but not for microglia proliferation. Thus, both CSF1 and DAP12 are potential targets for the pharmacotherapy of neuropathic pain.

Voltage-gated sodium (Nav) channels initiate action potentials in most neurons, including primary afferent nerve fibers of the pain pathway. Local anesthetics block pain through non-specific actions at all Nav channels, but the discovery of selective modulators would facilitate the analysis of individual subtypes and their contributions to chemical, mechanical, or thermal pain. Here, we identify and characterize spider toxins that selectively activate the Nav1.1 subtype, whose role in nociception and pain has not been explored. We exploit these probes to demonstrate that Nav1.1-expressing fibers are modality-specific nociceptors: their activation elicits robust pain behaviors without neurogenic inflammation and produces profound hypersensitivity to mechanical, but not thermal, stimuli. In the gut, high-threshold mechanosensitive fibers also express Nav1.1 and show enhanced toxin sensitivity in a model of irritable bowel syndrome. Altogether, these findings establish an unexpected role for Nav1.1 in regulating the excitability of sensory nerve fibers that underlie mechanical pain.

The original formulation of Gate Control Theory (GCT) proposed that the perception of pain produced by spinal cord signaling to the brain depends on a balance of activity generated in large (non-nociceptive) and small (nociceptive) diameter primary afferent fibers. GCT proposed that activation of the large diameter afferent “closes” the gate by engaging a superficial dorsal horn interneuron that inhibits projection neuron firing. Activation of the nociceptors “opens” the gate through concomitant excitation of projection neurons and inhibition of inhibitory interneurons. Sixty years after publication of the GCT, we are faced with an ever-growing list of morphologically and neurochemically distinct spinal cord interneurons. This review highlights the complexity of superficial dorsal horn circuitry and addresses whether the premises outlined in GCT still have relevance today. By examining the dorsal horn circuits underlying the transmission of “pain” and “itch” messages, we also address the extent to which labeled lines can be incorporated into a contemporary view of GCT.

TMEM16C belongs to the TMEM16 family, which includes the Ca2+-activated Cl– channels (CaCCs) TMEM16A and TMEM16B and a small conductance Ca2+-activated, non-selective cation channel (SCAN), TMEM16F. Here we report that in rat dorsal root ganglia (DRG) TMEM16C is expressed mainly in the IB4 positive, non-peptidergic nociceptors that also express the sodium-activated potassium (KNa) channel Slack. Together these channel proteins promote KNa channel activity and dampen neuronal excitability. DRG from TMEM16C knock out rats have reduced Slack expression, broadened action potential and increased excitability. Moreover, the TMEM16C knock out rats as well as rats with Slack knockdown via intrathecal injection of siRNA exhibit increased thermal and mechanical sensitivity. Experiments involving heterologous expression in HEK293 cells further show that TMEM16C modulates the single channel activity of Slack channels and increases its sodium sensitivity. Our study thus reveals that TMEM16C enhances KNa channel activity in DRG neurons and regulate the processing of pain messages.

SUMMARY To what extent dorsal horn interneurons contribute to the modality specific processing of pain and itch messages is not known. Here we report that loxp/cre-mediated CNS deletion of TR4, a testicular orphan nuclear receptor, results in loss of many excitatory interneurons in the superficial dorsal horn, but preservation of primary afferents and spinal projection neurons. The interneuron loss is associated with a near complete absence of supraspinally-integrated pain and itch behaviors, elevated mechanical withdrawal thresholds and loss of nerve injury-induced mechanical hypersensitivity, but reflex responsiveness to noxious heat, nerve injury-induced heat hypersensitivity and tissue injury-induced heat and mechanical hypersensitivity are intact. We conclude that different subsets of dorsal horn excitatory interneurons contribute to tissue and nerve injury-induced heat and mechanical pain and that the full expression of supraspinally-mediated pain and itch behaviors cannot be generated solely by nociceptor and pruritoceptor activation of projection neurons; concurrent activation of excitatory interneurons is essential.

Dorsal root ganglia (DRG) neurons, including the nociceptors that detect painful thermal, mechanical, and chemical stimuli, transmit information to spinal cord neurons via glutamatergic and peptidergic neurotransmitters. However, the specific contribution of glutamate to pain generated by distinct sensory modalities or injuries is not known. Here we generated mice in which the vesicular glutamate transporter 2 (VGLUT2) is ablated selectively from DRG neurons. We report that conditional knockout (cKO) of the Slc17a6 gene encoding VGLUT2 from the great majority of nociceptors profoundly decreased VGLUT2 mRNA and protein in these neurons, and reduced firing of lamina I spinal cord neurons in response to noxious heat and mechanical stimulation. In behavioral assays, cKO mice showed decreased responsiveness to acute noxious heat, mechanical, and chemical (capsaicin) stimuli, but responded normally to cold stimulation and in the formalin test. Strikingly, although tissue injury-induced heat hyperalgesia was lost in the cKO mice, mechanical hypersensitivity developed normally. In a model of nerve injuryinduced neuropathic pain, the magnitude of heat hypersensitivity was diminished in cKO mice, but both the mechanical allodynia and the microgliosis generated by nerve injury were intact. These findings suggest that VGLUT2 expression in nociceptors is essential for normal perception of acute pain and heat hyperalgesia, and that heat and mechanical hypersensitivity induced by peripheral injury rely on distinct (VGLUT2 dependent and VGLUT2 independent, respectively) primary afferent mechanisms and pathways.nociceptor | inflammatory pain | electrophysiology | neuroanatomy P rimary afferent neurons of the dorsal root ganglia (DRG) detect a wide range of stimulus modalities and intensities (1). This is particularly true for nociceptors, which are the neurons specialized to detect noxious stimuli. Not only do subsets of nociceptors express different repertoires of neuropeptides, receptors, and ion channels, but they also project to different laminae in the spinal cord where they engage different CNS circuits (2-4). Importantly, studies in rodents in which different nociceptor populations have been deleted revealed remarkably selective behavioral deficits (e.g., heat, mechanical, or chemical pain), demonstrating the existence of behaviorally relevant peripheral-labeled lines for different modalities of pain (5-8).Whether the modality-specific contribution of sensory neurons to acute pain and to injury-induced hypersensitivity states is also manifest at the level of the different neurotransmitters expressed by these neurons is still not known. Interestingly, previous pharmacological and genetic studies that disrupted neuropeptide function did not find a modality-specific loss of pain processing (9). Here we turned our attention to glutamate, which is presumed to be released by all DRG neurons to activate second-order spinal cord neurons.Because pharmacological blockade of glutamate signaling would nonselectively inhibit the centr...

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