Lung-innervating nociceptor sensory neurons promote pneumonic sepsis during carbapenem-resistant Klebsiella pneumoniae lung infection.

Carbapenem-resistant Klebsiella pneumoniae (CRKP) causes Gram-negative lung infections and fatal pneumonic sepsis for which limited therapeutic options are available. The lungs are densely innervated by nociceptor sensory neurons that mediate breathing, cough, and bronchoconstriction. The role of nociceptors in defense against Gram-negative lung pathogens is unknown. Here, we found that lung-innervating nociceptors promote CRKP pneumonia and pneumonic sepsis. Ablation of nociceptors in mice increased lung CRKP clearance, suppressed trans-alveolar dissemination of CRKP, and protected mice from hypothermia and death. Furthermore, ablation of nociceptors enhanced the recruitment of neutrophils and Ly6Chi monocytes and cytokine induction. Depletion of Ly6Chi monocytes, but not of neutrophils, abrogated lung and extrapulmonary CRKP clearance in ablated mice, suggesting that Ly6Chi monocytes are a critical cellular population to regulate pneumonic sepsis. Further, neuropeptide calcitonin gene-related peptide suppressed the induction of reactive oxygen species in Ly6Chi monocytes and their CRKP-killing abilities. Targeting nociceptor signaling could be a therapeutic approach for treating multidrug-resistant Gram-negative infection and pneumonic sepsis.

Presynaptic Enhancement of Transmission from Nociceptors Expressing Nav1.8 onto Lamina-I Spinothalamic Tract Neurons by Spared Nerve Injury in Mice.

Alteration of synaptic function in the dorsal horn (DH) has been implicated as a cellular substrate for the development of neuropathic pain, but certain details remain unclear. In particular, the lack of information on the types of synapses that undergo functional changes hinders the understanding of disease pathogenesis from a synaptic plasticity perspective. Here, we addressed this issue by using optogenetic and retrograde tracing ex vivo to selectively stimulate first-order nociceptors expressing Nav1.8 (NRsNav1.8) and record the responses of spinothalamic tract neurons in spinal lamina I (L1-STTNs). We found that spared nerve injury (SNI) increased excitatory postsynaptic currents (EPSCs) in L1-STTNs evoked by photostimulation of NRsNav1.8 (referred to as Nav1.8-STTN EPSCs). This effect was accompanied by a significant change in the failure rate and paired-pulse ratio of synaptic transmission from NRsNav1.8 to L1-STTN and in the frequency (not amplitude) of spontaneous EPSCs recorded in L1-STTNs. However, no change was observed in the ratio of AMPA to NMDA receptor-mediated components of Nav1.8-STTN EPSCs or in the amplitude of unitary EPSCs constituting Nav1.8-STTN EPSCs recorded with extracellular Ca2+ replaced by Sr2+ In addition, there was a small increase (approximately 10%) in the number of L1-STTNs showing immunoreactivity for phosphorylated extracellular signal-regulated kinases in mice after SNI compared with sham. Similarly, only a small percentage of L1-STTNs showed a lower action potential threshold after SNI. In conclusion, our results show that SNI induces presynaptic modulation at NRNav1.8 (consisting of both peptidergic and nonpeptidergic nociceptors) synapses on L1-STTNs forming the lateral spinothalamic tract.

Experimental nerve block study on painful withdrawal reflex responses in humans.

The nociceptive withdrawal reflex (NWR) is a protective limb withdrawal response triggered by painful stimuli, used to assess spinal nociceptive excitability. Conventionally, the NWR is understood as having two reflex responses: a short-latency Aß-mediated response, considered tactile, and a longer-latency Aδ-mediated response, considered nociceptive. However, nociceptors with conduction velocities similar to Aß tactile afferents have been identified in human skin. In this study, we investigated the effect of a preferential conduction block of Aß fibers on pain perception and NWR signaling evoked by intradermal electrical stimulation in healthy participants. We recorded a total of 198 NWR responses in the intact condition, and no dual reflex responses occurred within our latency bandwidth (50-150 ms). The current required to elicit the NWR was higher than the perceptual pain threshold, indicating that NWR did not occur before pain was felt. In the block condition, when the Aß-mediated tuning fork sensation was lost while Aδ-mediated nonpainful cooling was still detectable (albeit reduced), we observed that the reflex was abolished. Further, short-latency electrical pain intensity at pre-block thresholds was greatly reduced, with any residual pain sensation having a longer latency. Although electrical pain was unaffected at suprathreshold current, the reflex could not be evoked despite a two-fold increase in the pre-block current and a five-fold increase in the pre-block pulse duration. These observations lend support to the possible involvement of Aß-fiber inputs in pain and reflex signaling.

Transsynaptic BMP Signaling Regulates Fine-Scale Topography between Adjacent Sensory Neurons.

Sensory axons projecting to the central nervous system are organized into topographic maps that represent the locations of sensory stimuli. In some sensory systems, even adjacent sensory axons are arranged topographically, forming "fine-scale" topographic maps. Although several broad molecular gradients are known to instruct coarse topography, we know little about the molecular signaling that regulates fine-scale topography at the level of two adjacent axons. Here, we provide evidence that transsynaptic bone morphogenetic protein (BMP) signaling mediates local interneuronal communication to regulate fine-scale topography in the nociceptive system of Drosophila larvae. We first show that the topographic separation of the axon terminals of adjacent nociceptors requires their common postsynaptic target, the A08n neurons. This phenotype is recapitulated by knockdown of the BMP ligand, Decapentaplegic (Dpp), in these neurons. In addition, removing the Type 2 BMP receptors or their effector (Mad transcription factor) in single nociceptors impairs the fine-scale topography, suggesting the contribution of BMP signaling originated from A08n. This signaling is likely mediated by phospho-Mad in the presynaptic terminals of nociceptors to ensure local interneuronal communication. Finally, reducing Dpp levels in A08n reduces the nociceptor-A08n synaptic contacts. Our data support that transsynaptic BMP signaling establishes the fine-scale topography by facilitating the formation of topographically correct synapses. Local BMP signaling for synapse formation may be a developmental strategy that independently regulates neighboring axon terminals for fine-scale topography.

HfAlOx-based ferroelectric memristor for nociceptor and synapse functions.

Efficient data processing is heavily reliant on prioritizing specific stimuli and categorizing incoming information. Within human biological systems, dorsal root ganglions (particularly nociceptors situated in the skin) perform a pivotal role in detecting external stimuli. These neurons send warnings to our brain, priming it to anticipate potential harm and prevent injury. In this study, we explore the potential of using a ferroelectric memristor device structured as a metal-ferroelectric-insulator-semiconductor as an artificial nociceptor. The aim of this device is to electrically receive external damage and interpret signals of danger. The TiN/HfAlOx (HAO)/HfSiOx (HSO)/n+ Si configuration of this device replicates the key functions of a biological nociceptor. The emulation includes crucial aspects, such as threshold reactivity, relaxation, no adaptation, and sensitization phenomena known as "allodynia" and "hyperalgesia." Moreover, we propose establishing a connection between nociceptors and synapses by training the Hebbian learning rule. This involves exposing the device to injurious stimuli and using this experience to enhance its responsiveness, replicating synaptic plasticity.

High-sensitive sensory neurons exacerbate rosacea-like dermatitis in mice by activating γδ T cells directly.

Rosacea patients show facial hypersensitivity to stimulus factors (such as heat and capsaicin); however, the underlying mechanism of this hyperresponsiveness remains poorly defined. Here, we show capsaicin stimulation in mice induces exacerbated rosacea-like dermatitis but has no apparent effect on normal skin. Nociceptor ablation substantially reduces the hyperresponsiveness of rosacea-like dermatitis. Subsequently, we find that γδ T cells express Ramp1, the receptor of the neuropeptide CGRP, and are in close contact with these nociceptors in the skin. γδ T cells are significantly increased in rosacea skin lesions and can be further recruited and activated by neuron-secreted CGRP. Rosacea-like dermatitis is reduced in T cell receptor δ-deficient (Tcrd-/-) mice, and the nociceptor-mediated aggravation of rosacea-like dermatitis is also reduced in these mice. In vitro experiments show that CGRP induces IL17A secretion from γδ T cells by regulating inflammation-related and metabolism-related pathways. Finally, rimegepant, a CGRP receptor antagonist, shows efficacy in the treatment of rosacea-like dermatitis. In conclusion, our findings demonstrate a neuron-CGRP-γδT cell axis that contributes to the hyperresponsiveness of rosacea, thereby showing that targeting CGRP is a potentially effective therapeutic strategy for rosacea.

A chemogenetic screen reveals that Trpv1-expressing neurons control regulatory T cells in the gut.

Neuroimmune cross-talk participates in intestinal tissue homeostasis and host defense. However, the matrix of interactions between arrays of molecularly defined neuron subsets and of immunocyte lineages remains unclear. We used a chemogenetic approach to activate eight distinct neuronal subsets, assessing effects by deep immunophenotyping, microbiome profiling, and immunocyte transcriptomics in intestinal organs. Distinct immune perturbations followed neuronal activation: Nitrergic neurons regulated T helper 17 (TH17)-like cells, and cholinergic neurons regulated neutrophils. Nociceptor neurons, expressing Trpv1, elicited the broadest immunomodulation, inducing changes in innate lymphocytes, macrophages, and RORγ+ regulatory T (Treg) cells. Neuroanatomical, genetic, and pharmacological follow-up showed that Trpv1+ neurons in dorsal root ganglia decreased Treg cell numbers via the neuropeptide calcitonin gene-related peptide (CGRP). Given the role of these neurons in nociception, these data potentially link pain signaling with gut Treg cell function.

Comparison of nociceptor properties using electrophysiology in preclinical models of osteoarthritis.

Osteoarthritis (OA) pain originates in the joint by sensitization of articular nociceptors. While behavioural assessments provide valuable information regarding pain symptoms, the techniques are subjective and open to interpretation by the experimenter. This study used in vivo electrophysiological approaches to measure objectively joint nociceptor properties in three rodent models of OA. Single unit extracellular recordings of joint mechanosensitive afferents were carried out in male and female rats following either (1) transection of the medial meniscus (MMT: post-traumatic OA), (2) intra-articular injection of sodium monoiodoacetate (MIA: chemically-induced OA), or (3) intra-articular injection of lysophosphatidic acid (LPA: neuropathic OA). In naïve male control rats, the mechanical threshold of joint mechanonociceptors (23.5 ± 1.8 mNm) was significantly reduced with MMT (9.4 ± 1.1 mNm) and MIA (15.1 ± 1.6 mNm). In females, the mechanical threshold of naïve rats (23.2 ± 3.1 mNm) was reduced following induction of MMT (8.3 ± 1.0 mNm) and LPA (10.6 ± 2.2 mNm). Afferent firing frequency increased in male MMT (∼275 %), LPA (∼175 %), MIA (225 %), and female MMT (∼146 %), LPA (∼200 %), and MIA (∼192 %). Mechanical threshold and evoked firing were negatively correlated in all models for both sexes except LPA rats (male + female) and female MMT. These data indicate that MMT, MIA, and LPA induce peripheral sensitization of joint afferents thereby validating their use in OA pain studies.

Linking severe traumatic brain injury to pulmonary Infections: Translocation of intestinal bacteria mediated by nociceptor neurons.

The prevalence of bacterial infections significantly increases among patients with severe traumatic brain injury (STBI), leading to a notable rise in mortality rates. While immune dysfunctions are linked to the incidence of pneumonia, our observations indicate that endogenous pathogens manifest in the lungs post-STBI due to the migration of gut commensal bacteria. This translocation involves gut-innervating nociceptor sensory neurons, which are crucial for host defense. Following STBI, the expression of transient receptor potential vanilloid 1 (TRPV1) in dorsal root ganglion (DRG) neurons significantly decreases, despite an initial brief increase. The timing of TRPV1 defects coincides with the occurrence of pulmonary infections post-STBI. This alteration in TRPV1+ neurons diminishes their ability to signal bacterial injuries, weakens defense mechanisms against intestinal bacteria, and increases susceptibility to pulmonary infections via bacterial translocation. Experimental evidence demonstrates that pulmonary infections can be successfully replicated through the chemical ablation and gene interference of TRPV1+ nociceptors, and that these infections can be mitigated by TRPV1 activation, thereby confirming the crucial role of nociceptor neurons in controlling intestinal bacterial migration. Furthermore, TRPV1+ nociceptors regulate the immune response of microfold cells by releasing calcitonin gene-related peptide (CGRP), thereby influencing the translocation of gut bacteria to the lungs. Our study elucidates how changes in nociceptive neurons post-STBI impact intestinal pathogen defense. This new understanding of endogenous risk factors within STBI pathology offers novel insights for preventing and treating pulmonary infections.

Mapping gut feelings and immune control.

A chemogenetic screen reveals links between nociceptors and gut immune function.

The role of nociceptive neurons in allergic rhinitis.

Allergic rhinitis (AR) is a chronic, non-infectious condition affecting the nasal mucosa, primarily mediated mainly by IgE. Recent studies reveal that AR is intricately associated not only with type 2 immunity but also with neuroimmunity. Nociceptive neurons, a subset of primary sensory neurons, are pivotal in detecting external nociceptive stimuli and modulating immune responses. This review examines nociceptive neuron receptors and elucidates how neuropeptides released by these neurons impact the immune system. Additionally, we summarize the role of immune cells and inflammatory mediators on nociceptive neurons. A comprehensive understanding of the dynamic interplay between nociceptive neurons and the immune system augments our understanding of the neuroimmune mechanisms underlying AR, thereby opening novel avenues for AR treatment modalities.

Activation of skeletal muscle mechanoreceptors and nociceptors reduces the exercise performance of the contralateral homologous muscles.

Increasing evidence suggests that activation of muscle nerve afferents may inhibit central motor drive, affecting contractile performance of remote exercising muscles. Although these effects are well documented for metaboreceptors, very little is known about the activation of mechano- and mechanonociceptive afferents on performance fatigability. Therefore, the purpose of the present study was to examine the influence of mechanoreceptors and nociceptors on performance fatigability. Eight healthy young males undertook four randomized experimental sessions on separate occasions in which the experimental knee extensors were the following: 1) resting (CTRL), 2) passively stretched (ST), 3) resting with delayed onset muscle soreness (DOMS), or 4) passively stretched with DOMS (DOMS+ST), whereas the contralateral leg performed an isometric time to task failure (TTF). Changes in maximal voluntary contraction (ΔMVC), potentiated twitch force (ΔQtw,pot), and voluntary muscle activation (ΔVA) were also assessed. TTF was reduced in DOMS+ST (-43%) and ST (-29%) compared with CTRL. DOMS+ST also showed a greater reduction of VA (-25% vs. -8%, respectively) and MVC compared with CTRL (-28% vs. -45%, respectively). Rate of perceived exertion (RPE) was significantly increased at the initial stages (20-40-60%) of the TTF in DOMS+ST compared with all conditions. These findings indicate that activation of mechanosensitive and mechanonociceptive afferents of a muscle with DOMS reduces TTF of the contralateral homologous exercising limb, in part, by reducing VA, thereby accelerating mechanisms of central fatigue.NEW & NOTEWORTHY We found that activation of mechanosensitive and nociceptive nerve afferents of a rested muscle group experiencing delayed onset muscle soreness was associated with reduced exercise performance of the homologous exercising muscles of the contralateral limb. This occurred with lower muscle voluntary activation of the exercising muscle at the point of task failure.

Mechanical and cold polymodality coexist in tactile peripheral afferents, and it's not mediated by TRPM8.

In the mammalian somatosensory system, polymodality is defined as the competence of some neurons to respond to multiple forms of energy (e.g., mechanical and thermal). This ability is thought to be an exclusive property of nociceptive neurons (polymodal C-fiber nociceptors) and one of the pillars of nociceptive peripheral plasticity. The current study uncovered a completely different neuronal sub-population with polymodal capabilities on the opposite mechanical modality spectrum (tactile). We have observed that several tactile afferents (1/5) can respond to cold in non-nociceptive ranges. These cells' mechanical thresholds and electrical properties are similar to any low-threshold mechano-receptors (LT), conducting in a broad range of velocities (Aδ to Aß), lacking CGRP and TRPM8 receptors. Due to its density, cold-response range, speed, and response to injury (or lack thereof), we speculate on its role in controlling reflexive behaviors (wound liking and rubbing) and modulation of nociceptive spinal cord integration. Further studies are required to understand the mechanisms behind this neuron's polymodality, central architecture, and impact on pain perception.

Modulation of multisensory nociceptive neurons in monkey cortical area 7b and behavioral correlates.

Wide-range thermoreceptive neurons (WRT-EN) in monkey cortical area 7b that encoded innocuous and nocuous cutaneous thermal and threatening visuosensory stimulation with high fidelity were studied to identify their multisensory integrative response properties. Emphasis was given to characterizing the spatial and temporal effects of threatening visuosensory input on the thermal stimulus-response properties of these multisensory nociceptive neurons. Threatening visuosensory stimulation was most efficacious in modulating thermal evoked responses when presented as a downward ("looming"), spatially congruent, approaching and closely proximal target in relation to the somatosensory receptive field. Both temporal alignment and misalignment of spatially aligned threatening visual and thermal stimulation significantly increased mean discharge frequencies above those evoked by thermal stimulation alone, particularly at near noxious (43°C) and mildly noxious (45°C) temperatures. The enhanced multisensory discharge frequencies were equivalent to the discharge frequency evoked by overtly noxious thermal stimulation alone at 47°C (monkey pain tolerance threshold). A significant increase in behavioral mean escape frequency with shorter escape latency was evoked by multisensory stimulation at near noxious temperature (43°C), which was equivalent to that evoked by noxious stimulation alone (47°C). The remarkable concordance of elevating both neural discharge and escape frequency from a nonnociceptive and prepain level by near noxious thermal stimulation to a nociceptive and pain level by multisensory visual and near noxious thermal stimulation and integration is an elegantly designed defensive neural mechanism that in effect lowers both nociceptive response and pain thresholds to preemptively engage nocifensive behavior and, consequently, avert impending and actual injurious noxious thermal stimulation.NEW & NOTEWORTHY Multisensory nociceptive neurons in cortical area 7b are engaged in integration of threatening visuosensory and a wide range of innocuous and nocuous somatosensory (thermoreceptive) inputs. The enhancement of neuronal activity and escape behavior in monkey by multisensory integration is consistent and supportive of human psychophysical studies. The spatial features of visuosensory stimulation in peripersonal space in relation to somatic stimulation in personal space are critical to multisensory integration, nociception, nocifensive behavior, and pain.

Non-neuronal cells act as crucial players in neuropathic orofacial pain.

BACKGROUND: Following peripheral nerve damage, various non-neuronal cells are activated, triggering accumulation in the peripheral and central nervous systems, and communicate with neurons. Evidence suggest that neuronal and non-neuronal cell communication is a critical mechanism of neuropathic pain; however, its detailed mechanisms in contributing to neuropathic orofacial pain development remain unclear. HIGHLIGHT: Neuronal and non-neuronal cell communication in the trigeminal ganglion (TG) is believed to cause neuronal hyperactivation following trigeminal nerve damage, resulting in neuropathic orofacial pain. Trigeminal nerve damage activates and accumulates non-neuronal cells, such as satellite cells and macrophages in the TG and microglia, astrocytes, and oligodendrocytes in the trigeminal spinal subnucleus caudalis (Vc) and upper cervical spinal cord (C1-C2). These non-neuronal cells release various molecules, contributing to the hyperactivation of TG, Vc, and C1-C2 nociceptive neurons. These hyperactive nociceptive neurons release molecules that enhance non-neuronal cell activation. This neuron and non-neuronal cell crosstalk causes hyperactivation of nociceptive neurons in the TG, Vc, and C1-C2. Here, we addressed previous and recent data on the contribution of neuronal and non-neuronal cell communication and its involvement in neuropathic orofacial pain development. CONCLUSION: Previous and recent data suggest that neuronal and non-neuronal cell communication in the TG, Vc, and C1-C2 is a key mechanism that causes neuropathic orofacial pain associated with trigeminal nerve damage.

STING recognition of viral dsDNA by nociceptors mediates pain in mice.

Pain is often one of the initial indicators of a viral infection, yet our understanding of how viruses induce pain is limited. Immune cells typically recognize viral nucleic acids, which activate viral receptors and signaling, leading to immunity. Interestingly, these viral receptors and signals are also present in nociceptors and are associated with pain. Here, we investigate the response of nociceptors to nucleic acids during viral infections, specifically focusing on the role of the viral signal, Stimulator of Interferon Genes (STING). Our research shows that cytosolic double-stranded DNA (dsDNA) from viruses, like herpes simplex virus 1 (HSV-1), triggers pain responses through STING expression in nociceptors. In addition, STING agonists alone can elicit pain responses. Notably, these responses involve the direct activation of STING in nociceptors through TRPV1. We also provided a proof-of-concept showing that STING and TRPV1 significantly contribute to the mechanical hypersensitivity induced by HSV-1 infection. These findings suggest that STING could be a potential therapeutic target for relieving pain during viral infections.

Tomivosertib reduces ectopic activity in dorsal root ganglion neurons from patients with radiculopathy.

Spontaneous activity in dorsal root ganglion (DRG) neurons is a key driver of neuropathic pain in patients suffering from this largely untreated disease. While many intracellular signalling mechanisms have been examined in preclinical models that drive spontaneous activity, none have been tested directly on spontaneously active human nociceptors. Using cultured DRG neurons recovered during thoracic vertebrectomy surgeries, we showed that inhibition of mitogen-activated protein kinase interacting kinase (MNK) with tomivosertib (eFT508, 25 nM) reversibly suppresses spontaneous activity in human sensory neurons that are likely nociceptors based on size and action potential characteristics associated with painful dermatomes within minutes of treatment. Tomivosertib treatment also decreased action potential amplitude and produced alterations in the magnitude of after hyperpolarizing currents, suggesting modification of Na+ and K+ channel activity as a consequence of drug treatment. Parallel to the effects on electrophysiology, eFT508 treatment led to a profound loss of eIF4E serine 209 phosphorylation in primary sensory neurons, a specific substrate of MNK, within 2 min of drug treatment. Our results create a compelling case for the future testing of MNK inhibitors in clinical trials for neuropathic pain.

Peripheral macrophages contribute to nociceptor priming in mice with chronic intermittent hypoxia.

Obstructive sleep apnea (OSA) is a prevalent sleep disorder that is associated with increased incidence of chronic musculoskeletal pain. We investigated the mechanism of this association in a mouse model of chronic intermittent hypoxia (CIH) that mimics the repetitive hypoxemias of OSA. After 14 days of CIH, both male and female mice exhibited behaviors indicative of persistent pain, with biochemical markers in the spinal cord dorsal horn and sensory neurons of the dorsal root ganglia consistent with hyperalgesic priming. CIH, but not sleep fragmentation alone, induced an increase in macrophage recruitment to peripheral sensory tissues (sciatic nerve and dorsal root ganglia), an increase in inflammatory cytokines in the circulation, and nociceptor sensitization. Peripheral macrophage ablation blocked CIH-induced hyperalgesic priming. The findings suggest that correcting the hypoxia or targeting macrophage signaling might suppress persistent pain in patients with OSA.

Anandamide-Mediated Modulation of Nociceptive Transmission at the Spinal Cord Level.

Three decades ago, the first endocannabinoid, anandamide (AEA), was identified, and its analgesic effect was recognized in humans and preclinical models. However, clinical trial failures pointed out the complexity of the AEA-induced analgesia. The first synapses in the superficial laminae of the spinal cord dorsal horn represent an important modulatory site in nociceptive transmission and subsequent pain perception. The glutamatergic synaptic transmission at these synapses is strongly modulated by two primary AEA-activated receptors, cannabinoid receptor 1 (CB1) and transient receptor potential vanilloid 1 (TRPV1), both highly expressed on the presynaptic side formed by the endings of primary nociceptive neurons. Activation of these receptors can have predominantly inhibitory (CB1) and excitatory (TRPV1) effects that are further modulated under pathological conditions. In addition, dual AEA-mediated signaling and action may occur in primary sensory neurons and dorsal horn synapses. AEA application causes balanced inhibition and excitation of primary afferent synaptic input on superficial dorsal horn neurons in normal conditions, whereas peripheral inflammation promotes AEA-mediated inhibition. This review focuses mainly on the modulation of synaptic transmission at the spinal cord level and signaling in primary nociceptive neurons by AEA via CB1 and TRPV1 receptors. Furthermore, the spinal analgesic effect in preclinical studies and clinical aspects of AEA-mediated analgesia are considered.