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Allergic skin diseases, such as atopic dermatitis, are clinically characterized by severe itching and type 2 immunity-associated hypersensitivity to widely distributed allergens, including those derived from house dust mites (HDMs). Here we found that HDMs with cysteine protease activity directly activated peptidergic nociceptors, which are neuropeptide-producing nociceptive sensory neurons that express the ion channel TRPV1 and Tac1, the gene encoding the precursor for the neuropeptide substance P. Intravital imaging and genetic approaches indicated that HDM-activated nociceptors drive the development of allergic skin inflammation by inducing the degranulation of mast cells contiguous to such nociceptors, through the release of substance P and the activation of the cationic molecule receptor MRGPRB2 on mast cells. These data indicate that, after exposure to HDM allergens, activation of TRPV1+Tac1+ nociceptor–MRGPRB2+ mast cell sensory clusters represents a key early event in the development of allergic skin reactions.

Chambers et al . use a combination of small-molecule pathway inhibitors to rapidly differentiate human pluripotent stem cells into nociceptors, a type of sensory neuron. The conversion occurs about three-fold faster than during development, suggesting that pathway inhibition may offer a general approach for speeding up the generation of specific cell types in vitro . Considerable progress has been made in identifying signaling pathways that direct the differentiation of human pluripotent stem cells (hPSCs) into specialized cell types, including neurons. However, differentiation of hPSCs with extrinsic factors is a slow, step-wise process, mimicking the protracted timing of human development. Using a small-molecule screen, we identified a combination of five small-molecule pathway inhibitors that yield hPSC-derived neurons at >75% efficiency within 10 d of differentiation. The resulting neurons express canonical markers and functional properties of human nociceptors, including tetrodotoxin (TTX)-resistant, SCN10A-dependent sodium currents and response to nociceptive stimuli such as ATP and capsaicin. Neuronal fate acquisition occurs about threefold faster than during in vivo development 1 , suggesting that use of small-molecule pathway inhibitors could become a general strategy for accelerating developmental timing in vitro . The quick and high-efficiency derivation of nociceptors offers unprecedented access to this medically relevant cell type for studies of human pain.

Assuming the voltage-gated sodium channel (VGSC) NaV1.7 facilitates action potential generation upon slow electrical depolarization, we investigated protoxin II and TTX to target VGSC sub-types and to assess their role in C-fiber excitability when stimulated with sinusoidal single 1 Hz pulse (500 ms) and repetitive 4 Hz stimuli. We performed ex vivo extracellular compound potentials (CAP) recordings of pig saphenous nerve and in vivo pig single nerve fiber (SNF) recordings of heat- mechanosensitive (\"polymodal\") nociceptors (C-HT) and low- threshold mechanoresponsive C-fibers (C-LTMR) upon electrical 1 and 4 Hz sinusoidal stimulation, which evoke a discharge burst and a tonic response, respectively. Both toxins reduced C-CAP amplitudes and conduction velocity. Number of action potentials evoked by low-intensity phasic (1 Hz) or tonic (4 Hz) sinusoidal stimulation were reduced in C-HT nociceptors after protoxin. In C-LTMR fibers, protoxin reduced the number of action potentials to 4 Hz, but did not affect 1 Hz discharges. The toxins did not increase the delay of action potential initiation of C-CAPs or during SNF. Our results confirm the functional role of NaV1.7 to tonic supra-threshold electrical 4 Hz sinusoidal action potential firing in C-fibers. Protoxin reduced AP discharges to low-intensity phasic 1 Hz stimuli in C-HT nociceptors but not C-LTMR touch fibers. This finding suggests a differential functional role of NaV1.7 between C-fiber classes. Peripheral NaV1.7 blockade seems to increase the depolarization level required for C-nociceptor activation, and this might be relevant to target clinically ongoing pain.

Haematopoietic stem cells (HSCs) reside in specialized microenvironments in the bone marrow—often referred to as ‘niches’—that represent complex regulatory milieux influenced by multiple cellular constituents, including nerves 1 , 2 . Although sympathetic nerves are known to regulate the HSC niche 3 – 6 , the contribution of nociceptive neurons in the bone marrow remains unclear. Here we show that nociceptive nerves are required for enforced HSC mobilization and that they collaborate with sympathetic nerves to maintain HSCs in the bone marrow. Nociceptor neurons drive granulocyte colony-stimulating factor (G-CSF)-induced HSC mobilization via the secretion of calcitonin gene-related peptide (CGRP). Unlike sympathetic nerves, which regulate HSCs indirectly via the niche 3 , 4 , 6 , CGRP acts directly on HSCs via receptor activity modifying protein 1 (RAMP1) and the calcitonin receptor-like receptor (CALCRL) to promote egress by activating the Gα s /adenylyl cyclase/cAMP pathway. The ingestion of food containing capsaicin—a natural component of chili peppers that can trigger the activation of nociceptive neurons—significantly enhanced HSC mobilization in mice. Targeting the nociceptive nervous system could therefore represent a strategy to improve the yield of HSCs for stem cell-based therapeutic agents. Stimulation of pain-sensing neurons, which can be achieved in mice by the ingestion of capsaicin, promotes the migration of haematopoietic stem cells from the bone marrow into the blood.

Voltage-gated sodium (Na v ) channels initiate action potentials in most neurons, including primary afferent nerve fibres of the pain pathway. Local anaesthetics block pain through non-specific actions at all Na v channels, but the discovery of selective modulators would facilitate the analysis of individual subtypes of these channels and their contributions to chemical, mechanical, or thermal pain. Here we identify and characterize spider ( Heteroscodra maculata ) toxins that selectively activate the Na v 1.1 subtype, the role of which in nociception and pain has not been elucidated. We use these probes to show that Na v 1.1-expressing fibres are modality-specific nociceptors: their activation elicits robust pain behaviours without neurogenic inflammation and produces profound hypersensitivity to mechanical, but not thermal, stimuli. In the gut, high-threshold mechanosensitive fibres also express Na v 1.1 and show enhanced toxin sensitivity in a mouse model of irritable bowel syndrome. Together, these findings establish an unexpected role for Na v 1.1 channels in regulating the excitability of sensory nerve fibres that mediate mechanical pain. Two spider toxins are shown to target the Na v 1.1 subtype of sodium channel specifically, shedding light on the role of these channels in mechanical pain signalling. Na v 1.1 channels mediate mechanical pain Mutations affecting several Na v 1 subtype voltage-gated sodium channels have been shown to be associated with insensitivity to pain or persistent pain syndromes. Na v 1.1 is expressed by somatosensory neurons, but no direct link has been established between this subtype and nociception. Further studies have been hampered by a paucity of pharmacological agents that discriminate between the closely related members of the Na v 1 family. Now David Julius and colleagues have identified two spider toxins specifically targeting Na v 1.1, and use them to show that this channel is key to the specific transduction of mechanical but not thermal pain by myelinated Aδ sensory fibres. Previous genetic studies of Na v 1.1 indicate that such selective agents may open therapeutic avenues in disorders associated with the central nervous system, such as epilepsy, autism and Alzheimer disease. The involvement of Na v 1.1 channels in mediating mechanical pain reported here was unexpected.

Specialized peripheral sensory neurons known as nociceptors alert us to potentially damaging stimuli at the skin by detecting extremes in temperature and pressure and injury-related chemicals, and transducing these stimuli into long-ranging electrical signals that are relayed to higher brain centers. The activation of functionally distinct cutaneous nociceptor populations and the processing of information they convey provide a rich diversity of pain qualities. Current work in this field is providing researchers with a more thorough understanding of nociceptor cell biology at molecular and systems levels and insight that will allow the targeted design of novel pain therapeutics.

The incidence of chronic pain is estimated to be 20–25% worldwide. Few patients with chronic pain obtain complete relief from the drugs that are currently available, and more than half report inadequate relief. Underlying the challenge of developing better drugs to manage chronic pain is incomplete understanding of the heterogeneity of mechanisms that contribute to the transition from acute tissue insult to chronic pain and to pain conditions for which the underlying pathology is not apparent. An intact central nervous system (CNS) is required for the conscious perception of pain, and changes in the CNS are clearly evident in chronic pain states. However, the blockage of nociceptive input into the CNS can effectively relieve or markedly attenuate discomfort and pain, revealing the importance of ongoing peripheral input to the maintenance of chronic pain. Accordingly, we focus here on nociceptors: their excitability, their heterogeneity and their role in initiating and maintaining pain.

Pain is a hallmark of tissue injury, inflammatory diseases, pathogen invasion and neuropathy. It is mediated by nociceptor sensory neurons that innervate the skin, joints, bones, muscles and mucosal tissues and protects organisms from noxious stimuli. Nociceptors are sensitized by inflammatory mediators produced by the immune system, including cytokines, lipid mediators and growth factors, and can also directly detect pathogens and their secreted products to produce pain during infection. Upon activation, nociceptors release neuropeptides from their terminals that potently shape the function of innate and adaptive immune cells. For some pathogens, neuron–immune interactions enhance host protection from infection, but for other pathogens, neuron–immune signalling pathways can be exploited to facilitate pathogen survival. Here, we discuss the role of nociceptor interactions with the immune system in pain and infection and how understanding these pathways could produce new approaches to treat infectious diseases and chronic pain.This Review considers the link between pain and the immune system. Nociceptors are directly activated by immune mediators and microbial products and, in turn, release neuropeptides that shape immune responses. These neuroimmune pathways can contribute to protective immunity from infections but also lead to chronic pain.

Acute pain is protective and a cardinal feature of inflammation. Chronic pain after arthritis, nerve injury, cancer, and chemotherapy is associated with chronic neuroinflammation, a local inflammation in the peripheral or central nervous system. Accumulating evidence suggests that non-neuronal cells such as immune cells, glial cells, keratinocytes, cancer cells, and stem cells play active roles in the pathogenesis and resolution of pain. We review how non-neuronal cells interact with nociceptive neurons by secreting neuroactive signaling molecules that modulate pain. Recent studies also suggest that bacterial infections regulate pain through direct actions on sensory neurons, and specific receptors are present in nociceptors to detect danger signals from infections. We also discuss new therapeutic strategies to control neuroinflammation for the prevention and treatment of chronic pain.

Fibromyalgia syndrome (FMS) is characterized by widespread pain and tenderness, and patients typically experience fatigue and emotional distress. The etiology and pathophysiology of fibromyalgia are not fully explained and there are no effective drug treatments. Here we show that IgG from FMS patients produced sensory hypersensitivity by sensitizing nociceptive neurons. Mice treated with IgG from FMS patients displayed increased sensitivity to noxious mechanical and cold stimulation, and nociceptive fibers in skin-nerve preparations from mice treated with FMS IgG displayed an increased responsiveness to cold and mechanical stimulation. These mice also displayed reduced locomotor activity, reduced paw grip strength, and a loss of intraepidermal innervation. In contrast, transfer of IgG-depleted serum from FMS patients or IgG from healthy control subjects had no effect. Patient IgG did not activate naive sensory neurons directly. IgG from FMS patients labeled satellite glial cells and neurons in vivo and in vitro, as well as myelinated fiber tracts and a small number of macrophages and endothelial cells in mouse dorsal root ganglia (DRG), but no cells in the spinal cord. Furthermore, FMS IgG bound to human DRG. Our results demonstrate that IgG from FMS patients produces painful sensory hypersensitivities by sensitizing peripheral nociceptive afferents and suggest that therapies reducing patient IgG titers may be effective for fibromyalgia.