Sparse genetic tracing reveals regionally specific functional organization of mammalian nociceptors

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.

The Mas-related G protein-coupled receptor d (Mrgprd) mediates pain hypersensitivity in painful diabetic neuropathy

ABSTRACT

Painful diabetic neuropathy (PDN) is one of the most common and intractable complications of diabetes. Painful diabetic neuropathy is characterized by neuropathic pain accompanied by dorsal root ganglion (DRG) nociceptor hyperexcitability, axonal degeneration, and changes in cutaneous innervation. However, the complete molecular profile underlying the hyperexcitable cellular phenotype of DRG nociceptors in PDN has not been elucidated. This gap in our knowledge is a critical barrier to developing effective, mechanism-based, and disease-modifying therapeutic approaches that are urgently needed to relieve the symptoms of PDN. Using single-cell RNA sequencing of DRGs, we demonstrated an increased expression of the Mas-related G protein-coupled receptor d (Mrgprd) in a subpopulation of DRG neurons in the well-established high-fat diet (HFD) mouse model of PDN. Importantly, limiting Mrgprd signaling reversed mechanical allodynia in the HFD mouse model of PDN. Furthermore, in vivo calcium imaging allowed us to demonstrate that activation of Mrgprd-positive cutaneous afferents that persist in diabetic mice skin resulted in an increased intracellular calcium influx into DRG nociceptors that we assess in vivo as a readout of nociceptors hyperexcitability. Taken together, our data highlight a key role of Mrgprd-mediated DRG neuron excitability in the generation and maintenance of neuropathic pain in a mouse model of PDN. Hence, we propose Mrgprd as a promising and accessible target for developing effective therapeutics currently unavailable for treating neuropathic pain in PDN.

The Dorsal Column Nuclei Scale Mechanical Sensitivity in Naive and Neuropathic Pain States

Tactile perception relies on reliable transmission and modulation of low-threshold information as it travels from the periphery to the brain. During pathological conditions, tactile stimuli can aberrantly engage nociceptive pathways leading to the perception of touch as pain, known as mechanical allodynia. Two main drivers of peripheral tactile information, low-threshold mechanoreceptors (LTMRs) and postsynaptic dorsal column neurons (PSDCs), terminate in the brainstem dorsal column nuclei (DCN). Activity within the DRG, spinal cord, and DCN have all been implicated in mediating allodynia, yet the DCN remains understudied at the cellular, circuit, and functional levels compared to the other two. Here, we show that the gracile nucleus (Gr) of the DCN mediates tactile sensitivity for low-threshold stimuli and contributes to mechanical allodynia during neuropathic pain in mice. We found that the Gr contains local inhibitory interneurons in addition to thalamus-projecting neurons, which are differentially innervated by primary afferents and spinal inputs. Functional manipulations of these distinct Gr neuronal populations resulted in bidirectional changes to tactile sensitivity, but did not affect noxious mechanical or thermal sensitivity. During neuropathic pain, silencing Gr projection neurons or activating Gr inhibitory neurons was able to reduce tactile hypersensitivity, and enhancing inhibition was able to ameliorate paw withdrawal signatures of neuropathic pain, like shaking. Collectively, these results suggest that the Gr plays a specific role in mediating hypersensitivity to low-threshold, innocuous mechanical stimuli during neuropathic pain, and that Gr activity contributes to affective, pain-associated phenotypes of mechanical allodynia. Therefore, these brainstem circuits work in tandem with traditional spinal circuits underlying allodynia, resulting in enhanced signaling of tactile stimuli in the brain during neuropathic pain.

A mouse DRG genetic toolkit reveals morphological and physiological diversity of somatosensory neuron subtypes

Dorsal root ganglia (DRG) somatosensory neurons detect mechanical, thermal, and chemical stimuli acting on the body. Achieving a holistic view of how different DRG neuron subtypes relay neural signals from the periphery to the CNS has been challenging with existing tools. Here, we develop and curate a mouse genetic toolkit that allows for interrogating the properties and functions of distinct cutaneous targeting DRG neuron subtypes. These tools have enabled a broad morphological analysis, which revealed distinct cutaneous axon arborization areas and branching patterns of the transcriptionally distinct DRG neuron subtypes. Moreover, in vivo physiological analysis revealed that each subtype has a distinct threshold and range of responses to mechanical and/or thermal stimuli. These findings support a model in which morphologically and physiologically distinct cutaneous DRG sensory neuron subtypes tile mechanical and thermal stimulus space to collectively encode a wide range of natural stimuli.

Pain and itch coding mechanisms of polymodal sensory neurons

Pain and itch coding mechanisms in polymodal sensory neurons remain elusive. MrgprD+ neurons represent a major polymodal population and mediate both mechanical pain and nonhistaminergic itch. Here, we show that chemogenetic activation of MrgprD+ neurons elicited both pain- and itch-related behavior in a dose-dependent manner, revealing an unanticipated compatibility between pain and itch in polymodal neurons. While VGlut2-dependent glutamate release is required for both pain and itch transmission from MrgprD+ neurons, the neuropeptide neuromedin B (NMB) is selectively required for itch signaling. Electrophysiological recordings further demonstrated that glutamate synergizes with NMB to excite NMB-sensitive postsynaptic neurons. Ablation of these spinal neurons selectively abolished itch signals from MrgprD+ neurons, without affecting pain signals, suggesting a dedicated itch-processing central circuit. These findings reveal distinct neurotransmitters and neural circuit requirements for pain and itch signaling from MrgprD+ polymodal sensory neurons, providing new insights on coding and processing of pain and itch.

Deep RNA-seq of male and female murine sensory neuron subtypes after nerve injury

ABSTRACT

Dorsal root ganglia (DRG) neurons have been well described for their role in driving both acute and chronic pain. Although nerve injury is known to cause transcriptional dysregulation, how this differs across neuronal subtypes and the impact of sex is unclear. Here, we study the deep transcriptional profiles of multiple murine DRG populations in early and late pain states while considering sex. We have exploited currently available transgenics to label numerous subpopulations for fluorescent-activated cell sorting and subsequent transcriptomic analysis. Using bulk tissue samples, we are able to circumvent the issues of low transcript coverage and drop-outs seen with single-cell data sets. This increases our power to detect novel and even subtle changes in gene expression within neuronal subtypes and discuss sexual dimorphism at the neuronal subtype level. We have curated this resource into an accessible database for other researchers ( https://livedataoxford.shinyapps.io/drg-directory/ ). We see both stereotyped and unique subtype signatures in injured states after nerve injury at both an early and late timepoint. Although all populations contribute to a general injury signature, subtype enrichment changes can also be seen. Within populations, there is not a strong intersection of sex and injury, but previously unknown sex differences in naïve states-particularly in Aβ-RA + Aδ-low threshold mechanoreceptors-still contribute to differences in injured neurons.

Harmonized cross-species cell atlases of trigeminal and dorsal root ganglia

Peripheral sensory neurons in the dorsal root ganglion (DRG) and trigeminal ganglion (TG) are specialized to detect and transduce diverse environmental stimuli including touch, temperature, and pain to the central nervous system. Recent advances in single-cell RNA-sequencing (scRNA-seq) have provided new insights into the diversity of sensory ganglia cell types in rodents, non-human primates, and humans, but it remains difficult to compare transcriptomically defined cell types across studies and species. Here, we built cross-species harmonized atlases of DRG and TG cell types that describe 18 neuronal and 11 non-neuronal cell types across 6 species and 19 studies. We then demonstrate the utility of this harmonized reference atlas by using it to annotate newly profiled DRG nuclei/cells from both human and the highly regenerative axolotl. We observe that the transcriptomic profiles of sensory neuron subtypes are broadly similar across vertebrates, but the expression of functionally important neuropeptides and channels can vary notably. The new resources and data presented here can guide future studies in comparative transcriptomics, simplify cell type nomenclature differences across studies, and help prioritize targets for future pain therapy development.

Involvement of Mrgprd-expressing nociceptors-recruited spinal mechanisms in nerve injury-induced mechanical allodynia

Mechanical allodynia and hyperalgesia are intractable symptoms lacking effective clinical treatments in patients with neuropathic pain. However, whether and how mechanically responsive non-peptidergic nociceptors are involved remains elusive. Here, we showed that von Frey-evoked static allodynia and aversion, along with mechanical hyperalgesia after spared nerve injury (SNI) were reduced by ablation of Mrgprd^CreERT2-marked neurons. Electrophysiological recordings revealed that SNI-opened Aβ-fiber inputs to laminae I-II_o and _vII_i, as well as C-fiber inputs to _vII_i, were all attenuated in Mrgprd-ablated mice. In addition, priming chemogenetic or optogenetic activation of Mrgprd⁺ neurons drove mechanical allodynia and aversion to low-threshold mechanical stimuli, along with mechanical hyperalgesia. Mechanistically, gated Aβ and C inputs to _vII_i were opened, potentially via central sensitization by dampening potassium currents. Altogether, we uncovered the involvement of Mrgprd⁺ nociceptors in nerve injury-induced mechanical pain and dissected the underlying spinal mechanisms, thus providing insights into potential therapeutic targets for pain management.

Distinct Local and Global Functions of Aβ Low-Threshold Mechanoreceptors in Mechanical Pain Transmission

The roles of Aβ low-threshold mechanoreceptors (LTMRs) in transmitting mechanical hyperalgesia and in alleviating chronic pain have been of great interest but remain contentious. Here we utilized intersectional genetic tools, optogenetics, and high-speed imaging to specifically examine functions of Split Cre labeled Aβ-LTMRs in this regard. Genetic ablation of Split Cre -Aβ-LTMRs increased mechanical pain but not thermosensation in both acute and chronic inflammatory pain conditions, indicating their modality-specific role in gating mechanical pain transmission. Local optogenetic activation of Split Cre -Aβ-LTMRs triggered nociception after tissue inflammation, whereas their broad activation at the dorsal column still alleviated mechanical hypersensitivity of chronic inflammation. Taking all data into consideration, we propose a new model, in which Aβ-LTMRs play distinctive local and global roles in transmitting and alleviating mechanical hyperalgesia of chronic pain, respectively. Our model suggests a new strategy of global activation plus local inhibition of Aβ-LTMRs for treating mechanical hyperalgesia.

A DRG genetic toolkit reveals molecular, morphological, and functional diversity of somatosensory neuron subtypes

Mechanical and thermal stimuli acting on the skin are detected by morphologically and physiologically distinct sensory neurons of the dorsal root ganglia (DRG). Achieving a holistic view of how this diverse neuronal population relays sensory information from the skin to the central nervous system (CNS) has been challenging with existing tools. Here, we used transcriptomic datasets of the mouse DRG to guide development and curation of a genetic toolkit to interrogate transcriptionally defined DRG neuron subtypes. Morphological analysis revealed unique cutaneous axon arborization areas and branching patterns of each subtype. Physiological analysis showed that subtypes exhibit distinct thresholds and ranges of responses to mechanical and/or thermal stimuli. The somatosensory neuron toolbox thus enables comprehensive phenotyping of most principal sensory neuron subtypes. Moreover, our findings support a population coding scheme in which the activation thresholds of morphologically and physiologically distinct cutaneous DRG neuron subtypes tile multiple dimensions of stimulus space.

Mechanoreceptor signal convergence and transformation in the dorsal horn flexibly shape a diversity of outputs to the brain

The encoding of touch in the spinal cord dorsal horn (DH) and its influence on tactile representations in the brain are poorly understood. Using a range of mechanical stimuli applied to the skin, large-scale in vivo electrophysiological recordings, and genetic manipulations, here we show that neurons in the mouse spinal cord DH receive convergent inputs from both low- and high-threshold mechanoreceptor subtypes and exhibit one of six functionally distinct mechanical response profiles. Genetic disruption of DH feedforward or feedback inhibitory motifs, comprised of interneurons with distinct mechanical response profiles, revealed an extensively interconnected DH network that enables dynamic, flexible tuning of postsynaptic dorsal column (PSDC) output neurons and dictates how neurons in the primary somatosensory cortex respond to touch. Thus, mechanoreceptor subtype convergence and non-linear transformations at the earliest stage of the somatosensory hierarchy shape how touch of the skin is represented in the brain.

Measuring Mouse Somatosensory Reflexive Behaviors with High-speed Videography, Statistical Modeling, and Machine Learning

Objectively measuring and interpreting an animal's sensory experience remains a challenging task. This is particularly true when using preclinical rodent models to study pain mechanisms and screen for potential new pain treatment reagents. How to determine their pain states in a precise and unbiased manner is a hurdle that the field will need to overcome. Here, we describe our efforts to measure mouse somatosensory reflexive behaviors with greatly improved precision by high-speed video imaging. We describe how coupling sub-second ethograms of reflexive behaviors with a statistical reduction method and supervised machine learning can be used to create a more objective quantitative mouse "pain scale." Our goal is to provide the readers with a protocol of how to integrate some of the new tools described here with currently used mechanical somatosensory assays, while discussing the advantages and limitations of this new approach.

Parallel Spinal Pathways for Transmitting Reflexive and Affective Dimensions of Nocifensive Behaviors Evoked by Selective Activation of the Mas-Related G Protein-Coupled Receptor D-Positive and Transient Receptor Potential Vanilloid 1-Positive Subsets of Nociceptors

The high incidence of treatment-resistant pain calls for the urgent preclinical translation of new analgesics. Understanding the behavioral readout of pain in animals is crucial for efficacy evaluation when developing novel analgesics. Mas-related G protein-coupled receptor D-positive (Mrgprd⁺) and transient receptor potential vanilloid 1-positive (TRPV1⁺) sensory neurons are two major non-overlapping subpopulations of C-fiber nociceptors. Their activation has been reported to provoke diverse nocifensive behaviors. However, what kind of behavior reliably represents subjectively conscious pain perception needs to be revisited. Here, we generated transgenic mice in which Mrgprd⁺ or TRPV1⁺ sensory neurons specifically express channelrhodopsin-2 (ChR2). Under physiological conditions, optogenetic activation of hindpaw Mrgprd⁺ afferents evoked reflexive behaviors (lifting, etc.), but failed to produce aversion. In contrast, TRPV1⁺ afferents activation evoked marked reflexive behaviors and affective responses (licking, etc.), as well as robust aversion. Under neuropathic pain conditions induced by spared nerve injury (SNI), affective behaviors and avoidance can be elicited by Mrgprd⁺ afferents excitation. Mechanistically, spinal cord-lateral parabrachial nucleus (lPBN) projecting neurons in superficial layers (lamina I–II_o) were activated by TRPV1⁺ nociceptors in naïve conditions or by Mrgprd⁺ nociceptors after SNI, whereas only deep spinal cord neurons were activated by Mrgprd⁺ nociceptors in naïve conditions. Moreover, the excitatory inputs from Mrgprd⁺ afferents to neurons within inner lamina II (II_i) are partially gated under normal conditions. Altogether, we conclude that optogenetic activation of the adult Mrgprd⁺ nociceptors drives non-pain-like reflexive behaviors via the deep spinal cord pathway under physiological conditions and drives pain-like affective behaviors via superficial spinal cord pathway under pathological conditions. The distinct spinal pathway transmitting different forms of nocifensive behaviors provides different therapeutic targets. Moreover, this study appeals to the rational evaluation of preclinical analgesic efficacy by using comprehensive and suitable behavioral assays, as well as by assessing neural activity in the two distinct pathways.

A novel spinal neuron connection for heat sensation

Heat perception enables acute avoidance responses to prevent tissue damage and maintain body thermal homeostasis. Unlike other modalities, how heat signals are processed in the spinal cord remains unclear. By single-cell gene profiling, we identified ErbB4, a transmembrane tyrosine kinase, as a novel marker of heat-sensitive spinal neurons in mice. Ablating spinal ErbB4+ neurons attenuates heat sensation. These neurons receive monosynaptic inputs from TRPV1+ nociceptors and form excitatory synapses onto target neurons. Activation of ErbB4+ neurons enhances the heat response, while inhibition reduces the heat response. We showed that heat sensation is regulated by NRG1, an activator of ErbB4, and it involves dynamic activity of the tyrosine kinase that promotes glutamatergic transmission. Evidence indicates that the NRG1-ErbB4 signaling is also engaged in hypersensitivity of pathological pain. Together, these results identify a spinal neuron connection consisting of ErbB4+ neurons for heat sensation and reveal a regulatory mechanism by the NRG1-ErbB4 signaling.

Identification of MrgprD expression in mouse enteric neurons

Mas-related G protein-coupled receptor D (MrgprD) was first identified in small-diameter sensory neurons of mouse dorsal root ganglion (DRG). The role of MrgprD has been studied in somatosensation, especially in pain and itch response. We recently showed that MrgprD also participated in the modulation of murine intestinal motility. The treatment of MrgprD receptor agonist suppressed the spontaneous contractions in the isolated intestinal rings of mice, indicating the intrinsic expression of MrgprD in the murine gastrointestinal (GI) tract. Although the expression of Mrgprd in GI tract has been previously detected by the way of quantitative real-time PCR, the cell-type-specific expression of MrgprD in GI tract is no yet determined. Herein, we employed Mrgprd-tdTomato reporter mouse line and the whole-mount immunohistochemistry to observe the localization of MrgprD in the smooth muscle layers of ileum and colon. We show that tdTomato-positive cells colocalized with NeuN-immunostaining in the myenteric plexus in the whole-mount preparations of the ileum and the colon. Further immunohistochemistry using the commercially available MrgprD antibody revealed the expression of MrgprD in NeuN-labeled enteric neurons in the myenteric plexus. Our results demonstrate the expression of MrgprD in the enteric neurons in the murine GI tract, highlighting the implications of MrgprD in the physiology and pathophysiology of the GI tract.

Glutamate in primary afferents is required for itch transmission

Whether glutamate or itch-selective neurotransmitters are used to confer itch specificity is still under debate. We focused on an itch-selective population of primary afferents expressing MRGPRA3, which highly expresses Vglut2 and the neuropeptide neuromedin B (Nmb), to investigate this question. Optogenetic stimulation of MRGPRA3⁺ afferents triggers scratching and other itch-related avoidance behaviors. Using a combination of optogenetics, spinal cord slice recordings, Vglut2 conditional knockout mice, and behavior assays, we showed that glutamate is essential for MRGPRA3⁺ afferents to transmit itch. We further demonstrated that MRGPRA3⁺ afferents form monosynaptic connections with both NMBR⁺ and NMBR^- neurons and that NMB and glutamate together can enhance the activity of NMBR⁺ spinal DH neurons. Moreover, Nmb in MRGPRA3⁺ afferents and NMBR⁺ DH neurons are required for chloroquine-induced scratching. Together, our results establish a new model in which glutamate is an essential neurotransmitter in primary afferents for itch transmission, whereas NMB signaling enhances its activities.

A functional subdivision within the somatosensory system and its implications for pain research

Somatosensory afferents are traditionally classified by soma size, myelination, and their response specificity to external and internal stimuli. Here, we propose the functional subdivision of the nociceptive somatosensory system into two branches. The exteroceptive branch detects external threats and drives reflexive-defensive reactions to prevent or limit injury. The interoceptive branch senses the disruption of body integrity, produces tonic pain with strong aversive emotional components, and drives self-caring responses toward to the injured region to reduce suffering. The central thesis behind this functional subdivision comes from a reflection on the dilemma faced by the pain research field, namely, the use of reflexive-defensive behaviors as surrogate assays for interoceptive tonic pain. The interpretation of these assays is now being challenged by the discovery of distinct but interwoven circuits that drive exteroceptive versus interoceptive types of behaviors, with the conflation of these two components contributing partially to the poor translation of therapies from preclinical studies.

Influencers in the Somatosensory System: Extrinsic Control of Sensory Neuron Phenotypes

Somatosensory neurons in dorsal root ganglia (DRG) comprise several main subclasses: high threshold nociceptors/thermoceptors, high- and low-threshold mechanoreceptors, and proprioceptors. Recent years have seen an explosion in the identification of molecules that underlie the functional diversity of these sensory modalities. They also have begun to reveal the developmental mechanisms that channel the emergence of this subtype diversity, solidifying the importance of peripheral instructive signals. Somatic sensory neurons collectively serve numerous essential physiological and protective roles, and as such, an increased understanding of the processes that underlie the specialization of these sensory subtypes is not only biologically interesting but also clinically relevant.

Mechanically evoked defensive attack is controlled by GABAergic neurons in the anterior hypothalamic nucleus

Innate defensive behaviors triggered by environmental threats are important for animal survival. Among these behaviors, defensive attack toward threatening stimuli (for example, predators) is often the last line of defense. How the brain regulates defensive attack remains poorly understood. Here we show that noxious mechanical force in an inescapable context is a key stimulus for triggering defensive attack in laboratory mice. Mechanically evoked defensive attacks were abrogated by photoinhibition of vGAT⁺ neurons in the anterior hypothalamic nucleus (AHN). The vGAT⁺ AHN neurons encoded the intensity of mechanical force and were innervated by brain areas relevant to pain and attack. Activation of these neurons triggered biting attacks toward a predator while suppressing ongoing behaviors. The projection from vGAT⁺ AHN neurons to the periaqueductal gray might be one AHN pathway participating in mechanically evoked defensive attack. Together, these data reveal that vGAT⁺ AHN neurons encode noxious mechanical stimuli and regulate defensive attack in mice.

Mechanoreceptor synapses in the brainstem shape the central representation of touch

Mammals use glabrous (hairless) skin of their hands and feet to navigate and manipulate their environment. Cortical maps of the body surface across species contain disproportionately large numbers of neurons dedicated to glabrous skin sensation, in part reflecting a higher density of mechanoreceptors that innervate these skin regions. Here, we find that disproportionate representation of glabrous skin emerges over postnatal development at the first synapse between peripheral mechanoreceptors and their central targets in the brainstem. Mechanoreceptor synapses undergo developmental refinement that depends on proximity of their terminals to glabrous skin, such that those innervating glabrous skin make synaptic connections that expand their central representation. In mice incapable of sensing gentle touch, mechanoreceptors innervating glabrous skin still make more powerful synapses in the brainstem. We propose that the skin region a mechanoreceptor innervates controls the developmental refinement of its central synapses to shape the representation of touch in the brain.

TRPC3 Antagonizes Pruritus in a Mouse Contact Dermatitis Model

Contact dermatitis (CD), including allergic and irritant CD, are common dermatological diseases and are characterized by an erythematous rash and severe itch. In this study, we investigated the function of TRPC3, a canonical transient receptor potential channel highly expressed in type 1 nonpeptidergic (NP1) nociceptive primary afferents and other cell types, in a mouse CD model. Although TrpC3 null mice had little deficits in acute somatosensation, they showed significantly increased scratching with CD. In addition, TrpC3 null mice displayed no differences in mechanical and thermal hypersensitivity in an inflammatory pain model, suggesting that this channel preferentially functions to antagonize CD-induced itch. Using dorsal root ganglia and panimmune-specific TrpC3 conditional knockout mice, we determined that TrpC3 in dorsal root ganglia neurons but not in immune cells is required for this phenotype. Furthermore, the number of MRGPRD⁺ NP1 afferents in CD-affected dorsal root ganglia is significantly reduced in TrpC3-mutant mice. Taken together, our results suggest that TrpC3 plays a critical role in NP1 afferents to cope with CD-induced excitotoxicity and that the degeneration of NP1 fibers may lead to an increased itch of CD. Our study identified a role of TrpC3 and NP1 afferents in CD pathology.

The structure of sensory afferent compartments in health and disease

Primary sensory neurons are a heterogeneous population of cells able to respond to both innocuous and noxious stimuli. Like most neurons they are highly compartmentalised, allowing them to detect, convey and transfer sensory information. These compartments include specialised sensory endings in the skin, the nodes of Ranvier in myelinated axons, the cell soma and their central terminals in the spinal cord. In this review, we will highlight the importance of these compartments to primary afferent function, describe how these structures are compromised following nerve damage and how this relates to neuropathic pain.

The mechanosensory neurons of touch and their mechanisms of activation

Our sense of touch emerges from an array of mechanosensory structures residing within the fabric of our skin. These tactile end organ structures convert innocuous forces acting on the skin into electrical signals that propagate to the CNS via the axons of low-threshold mechanoreceptors (LTMRs). Our rich capacity for tactile discrimination arises from the dissimilar intrinsic properties of the LTMR subtypes that innervate different regions of the skin and the structurally distinct end organ complexes with which they associate. These end organ structures comprise a range of non-neuronal cell types, which may themselves actively contribute to the transformation of tactile forces into neural impulses within the LTMR afferents. Although the mechanism and the site of transduction across end organs remain unclear, PIEZO2 has emerged as the principal mechanosensitive channel involved in light touch of the skin. Here we review the physiological properties of LTMR subtypes and discuss how features of their cutaneous end organ complexes shape subtype-specific tuning.

Transparent Touch: Insights From Model Systems on Epidermal Control of Somatosensory Innervation

Somatosensory neurons (SSNs) densely innervate our largest organ, the skin, and shape our experience of the world, mediating responses to sensory stimuli including touch, pressure, and temperature. Historically, epidermal contributions to somatosensation, including roles in shaping innervation patterns and responses to sensory stimuli, have been understudied. However, recent work demonstrates that epidermal signals dictate patterns of SSN skin innervation through a variety of mechanisms including targeting afferents to the epidermis, providing instructive cues for branching morphogenesis, growth control and structural stability of neurites, and facilitating neurite-neurite interactions. Here, we focus onstudies conducted in worms (Caenorhabditis elegans), fruit flies (Drosophila melanogaster), and zebrafish (Danio rerio): prominent model systems in which anatomical and genetic analyses have defined fundamental principles by which epidermal cells govern SSN development.

MrgprC11+ sensory neurons mediate glabrous skin itch

Itch arising from glabrous skin (palms and soles) has attracted limited attention within the field due to the lack of methodology. This is despite glabrous itch arising from many medical conditions such as plantar and palmar psoriasis, dyshidrosis, and cholestasis. Therefore, we developed a mouse glabrous skin behavioral assay to investigate the contribution of three previously identified pruriceptive neurons in glabrous skin itch. Our results show that MrgprA3⁺ and MrgprD⁺ neurons, although key mediators for hairy skin itch, do not play important roles in glabrous skin itch, demonstrating a mechanistic difference in itch sensation between hairy and glabrous skin. We found that MrgprC11⁺ neurons are the major mediators for glabrous skin itch. Activation of MrgprC11⁺ neurons induced glabrous skin itch, while ablation of MrgprC11⁺ neurons reduced both acute and chronic glabrous skin itch. Our study provides insights into the mechanisms of itch and opens up new avenues for future glabrous skin itch research.

Pathogenic alleles in microtubule, secretory granule and extracellular matrix-related genes in familial keratoconus

Keratoconus is a common corneal defect with a complex genetic basis. By whole exome sequencing of affected members from 11 multiplex families of European ancestry, we identified 23 rare, heterozygous, potentially pathogenic variants in 8 genes. These include nonsynonymous single amino acid substitutions in HSPG2, EML6 and CENPF in two families each, and in NBEAL2, LRP1B, PIK3CG and MRGPRD in three families each; ITGAX had nonsynonymous single amino acid substitutions in two families and an indel with a base substitution producing a nonsense allele in the third family. Only HSPG2, EML6 and CENPF have been associated with ocular phenotypes previously. With the exception of MRGPRD and ITGAX, we detected the transcript and encoded protein of the remaining genes in the cornea and corneal cell cultures. Cultured stromal cells showed cytoplasmic punctate staining of NBEAL2, staining of the fibrillar cytoskeletal network by EML6, while CENPF localized to the basal body of primary cilia. We inhibited the expression of HSPG2, EML6, NBEAL2 and CENPF in stromal cell cultures and assayed for the expression of COL1A1 as a readout of corneal matrix production. An upregulation in COL1A1 after siRNA inhibition indicated their functional link to stromal cell biology. For ITGAX, encoding a leukocyte integrin, we assayed its level in the sera of 3 affected families compared with 10 unrelated controls to detect an increase in all affecteds. Our study identified genes that regulate the cytoskeleton, protein trafficking and secretion, barrier tissue function and response to injury and inflammation, as being relevant to keratoconus.

Single-cell transcriptomic analysis of somatosensory neurons uncovers temporal development of neuropathic pain

Peripheral nerve injury could lead to chronic neuropathic pain. Understanding transcriptional changes induced by nerve injury could provide fundamental insights into the complex pathogenesis of neuropathic pain. Gene expression profiles of dorsal root ganglia (DRG) in neuropathic pain condition have been studied. However, little is known about transcriptomic changes in individual DRG neurons after peripheral nerve injury. Here we performed single-cell RNA sequencing on dissociated mouse DRG cells after spared nerve injury (SNI). In addition to DRG neuron types that are found under physiological conditions, we identified three SNI-induced neuronal clusters (SNIICs) characterized by the expression of Atf3/Gfra3/Gal (SNIIC1), Atf3/Mrgprd (SNIIC2) and Atf3/S100b/Gal (SNIIC3). These SNIICs originated from Cldn9⁺/Gal⁺, Mrgprd⁺ and Trappc3l⁺ DRG neurons, respectively. Interestingly, SNIIC2 switched to SNIIC1 by increasing Gal and reducing Mrgprd expression 2 days after nerve injury. Inferring the gene regulatory networks after nerve injury, we revealed that activated transcription factors Atf3 and Egr1 in SNIICs could enhance Gal expression while activated Cpeb1 in SNIIC2 might suppress Mrgprd expression within 2 days after SNI. Furthermore, we mined the transcriptomic changes in the development of neuropathic pain to identify potential analgesic targets. We revealed that cardiotrophin-like cytokine factor 1, which activates astrocytes in the dorsal horn of spinal cord, was upregulated in SNIIC1 neurons and contributed to SNI-induced mechanical allodynia. Therefore, our results provide a new landscape to understand the dynamic course of neuron type changes and their underlying molecular mechanisms during the development of neuropathic pain.

Beyond somatosensation: Mrgprs in mucosal tissues

Mas-related G coupled receptors (Mrgprs) are a superfamily of receptors expressed in sensory neurons that are known to transmit somatic sensations from the skin to the central nervous system. Interestingly, Mrgprs have recently been implicated in sensory and motor functions of mucosal-associated neuronal circuits. The gastrointestinal and pulmonary tracts are constantly exposed to noxious stimuli. Therefore, it is likely that neuronal Mrgpr signaling pathways in mucosal tissues, akin to their family members expressed in the skin, might relay messages that alert the host when mucosal tissues are affected by damaging signals. Further, Mrgprs have been proposed to mediate the cross-talk between sensory neurons and immune cells that promotes host-protective functions at barrier sites. Although the mechanisms by which Mrgprs are activated in mucosal tissues are not completely understood, these exciting studies implicate Mrgprs as potential therapeutic targets for conditions affecting the intestinal and airway mucosa. This review will highlight the central role of Mrgpr signaling pathways in the regulation of homeostasis at mucosal tissues.

Revisiting PNS Plasticity: How Uninjured Sensory Afferents Promote Neuropathic Pain

Despite the widespread study of how injured nerves contribute to chronic pain, there are still major gaps in our understanding of pain mechanisms. This is particularly true of pain resulting from nerve injury, or neuropathic pain, wherein tactile or thermal stimuli cause painful responses that are particularly difficult to treat with existing therapies. Curiously, this stimulus-driven pain relies upon intact, uninjured sensory neurons that transmit the signals that are ultimately sensed as painful. Studies that interrogate uninjured neurons in search of cell-specific mechanisms have shown that nerve injury alters intact, uninjured neurons resulting in an activity that drives stimulus-evoked pain. This review of neuropathic pain mechanisms summarizes cell-type-specific pathology of uninjured sensory neurons and the sensory ganglia that house their cell bodies. Uninjured neurons have demonstrated a wide range of molecular and neurophysiologic changes, many of which are distinct from those detected in injured neurons. These intriguing findings include expression of pain-associated molecules, neurophysiological changes that underlie increased excitability, and evidence that intercellular signaling within sensory ganglia alters uninjured neurons. In addition to well-supported findings, this review also discusses potential mechanisms that remain poorly understood in the context of nerve injury. This review highlights key questions that will advance our understanding of the plasticity of sensory neuron subpopulations and clarify the role of uninjured neurons in developing anti-pain therapies.

The Input-Output Relation of Primary Nociceptive Neurons is Determined by the Morphology of the Peripheral Nociceptive Terminals

The output from the peripheral terminals of primary nociceptive neurons, which detect and encode the information regarding noxious stimuli, is crucial in determining pain sensation. The nociceptive terminal endings are morphologically complex structures assembled from multiple branches of different geometry, which converge in a variety of forms to create the terminal tree. The output of a single terminal is defined by the properties of the transducer channels producing the generation potentials and voltage-gated channels, translating the generation potentials into action potential (AP) firing. However, in the majority of cases, noxious stimuli activate multiple terminals; thus, the output of the nociceptive neuron is defined by the integration and computation of the inputs of the individual terminals. Here, we used a computational model of nociceptive terminal tree to study how the architecture of the terminal tree affects the input-output relation of the primary nociceptive neurons. We show that the input-output properties of the nociceptive neurons depend on the length, the axial resistance (Ra), and location of individual terminals. Moreover, we show that activation of multiple terminals by a capsaicin-like current allows summation of the responses from individual terminals, thus leading to increased nociceptive output. Stimulation of the terminals in simulated models of inflammatory or neuropathic hyperexcitability led to a change in the temporal pattern of AP firing, emphasizing the role of temporal code in conveying key information about changes in nociceptive output in pathologic conditions, leading to pain hypersensitivity.SIGNIFICANCE STATEMENT Noxious stimuli are detected by terminal endings of primary nociceptive neurons, which are organized into morphologically complex terminal trees. The information from multiple terminals is integrated along the terminal tree, computing the neuronal output, which propagates toward the CNS, thus shaping the pain sensation. Here, we revealed that the structure of the nociceptive terminal tree determines the output of nociceptive neurons. We show that the integration of noxious information depends on the morphology of the terminal trees and how this integration and, consequently, the neuronal output change under pathologic conditions. Our findings help to predict how nociceptive neurons encode noxious stimuli and how this encoding changes in pathologic conditions, leading to pain.

Visualizing the Itch-Sensing Skin Arbors

Diverse sensory neurons exhibit distinct neuronal morphologies with a variety of axon terminal arborizations subserving their functions. Because of its clinical significance, the molecular and cellular mechanisms of itch are being intensely studied. However, a complete analysis of itch-sensing terminal arborization is missing. Using an MrgprC11^CreERT2 transgenic mouse line, we labeled a small subset of itch-sensing neurons that express multiple itch-related molecules including MrgprA3, MrgprC11, histamine receptor H1, IL-31 receptor, 5-hydroxytryptamine receptor 1F, natriuretic precursor peptide B, and neuromedin B. By combining sparse genetic labeling and whole-mount placental alkaline phosphatase histochemistry, we found that itch-sensing skin arbors exhibit free endings with extensive axonal branching in the superficial epidermis and large receptive fields. These results revealed the unique morphological characteristics of itch-sensing neurons and provide intriguing insights into the basic mechanisms of itch transmission.

Development of a Mouse Pain Scale Using Sub-second Behavioral Mapping and Statistical Modeling

Rodents are the main model systems for pain research, but determining their pain state is challenging. To develop an objective method to assess pain sensation in mice, we adopt high-speed videography to capture sub-second behavioral features following hind paw stimulation with both noxious and innocuous stimuli and identify several differentiating parameters indicating the affective and reflexive aspects of nociception. Using statistical modeling and machine learning, we integrate these parameters into a single index and create a "mouse pain scale," which allows us to assess pain sensation in a graded manner for each withdrawal. We demonstrate the utility of this method by determining sensations triggered by three different von Frey hairs and optogenetic activation of two different nociceptor populations. Our behavior-based "pain scale" approach will help improve the rigor and reproducibility of using withdrawal reflex assays to assess pain sensation in mice.

Expression of green fluorescent protein defines a specific population of lamina II excitatory interneurons in the GRP::eGFP mouse

Dorsal horn excitatory interneurons that express gastrin-releasing peptide (GRP) are part of the circuit for pruritogen-evoked itch. They have been extensively studied in a transgenic line in which enhanced green fluorescent protein (eGFP) is expressed under control of the Grp gene. The GRP-eGFP cells are separate from several other neurochemically-defined excitatory interneuron populations, and correspond to a class previously defined as transient central cells. However, mRNA for GRP is widely distributed among excitatory interneurons in superficial dorsal horn. Here we show that although Grp mRNA is present in several transcriptomically-defined populations, eGFP is restricted to a discrete subset of cells in the GRP::eGFP mouse, some of which express the neuromedin receptor 2 and likely belong to a cluster defined as Glut8. We show that these cells receive much of their excitatory synaptic input from MrgA3/MrgD-expressing nociceptive/pruritoceptive afferents and C-low threshold mechanoreceptors. Although the cells were not innervated by pruritoceptors expressing brain natriuretic peptide (BNP) most of them contained mRNA for NPR1, the receptor for BNP. In contrast, these cells received only ~ 10% of their excitatory input from other interneurons. These findings demonstrate that the GRP-eGFP cells constitute a discrete population of excitatory interneurons with a characteristic pattern of synaptic input.

TRPV1 activity and substance P release are required for corneal cold nociception

As a protective mechanism, the cornea is sensitive to noxious stimuli. Here, we show that in mice, a high proportion of corneal TRPM8⁺ cold-sensing fibers express the heat-sensitive TRPV1 channel. Despite its insensitivity to cold, TRPV1 enhances membrane potential changes and electrical firing of TRPM8⁺ neurons in response to cold stimulation. This elevated neuronal excitability leads to augmented ocular cold nociception in mice. In a model of dry eye disease, the expression of TRPV1 in TRPM8⁺ cold-sensing fibers is increased, and results in severe cold allodynia. Overexpression of TRPV1 in TRPM8⁺ sensory neurons leads to cold allodynia in both corneal and non-corneal tissues without affecting their thermal sensitivity. TRPV1-dependent neuronal sensitization facilitates the release of the neuropeptide substance P from TRPM8⁺ cold-sensing neurons to signal nociception in response to cold. Our study identifies a mechanism underlying corneal cold nociception and suggests a potential target for the treatment of ocular pain.

The eye shows protective responses to noxious stimuli including cold. Here, the authors show that TRPV1, found co-expressed on TRPM8 + fibres in the cornea, is necessary for cold nociception in the eye.

Mechanisms of Tactile Sensory Phenotypes in Autism: Current Understanding and Future Directions for Research

PURPOSE OF REVIEW

This review aims to summarize the current body of behavioral, physiological, and molecular knowledge concerning tactile sensitivity in autism spectrum disorder (ASD), with a focus on recent studies utilizing rodent models.

RECENT FINDINGS

Mice with mutations in the ASD-related genes, Shank3, Fmr1, UBE3A, and Mecp2, display tactile abnormalities. Some of these abnormalities appear to be caused by mutation-related changes in the PNS, as opposed to changes in the processing of touch stimuli in the CNS, as previously thought. There is also growing evidence suggesting that peripheral mechanisms may contribute to some of the core symptoms and common comorbidities of ASD. Researchers are therefore beginning to assess the therapeutic potential of targeting the PNS in treating some of the core symptoms of ASD. Sensory abnormalities are common in rodent models of ASD. There is growing evidence that sensory hypersensitivity, especially tactile sensitivity, may contribute to social deficits and other autism-related behaviors.

Somatotopic organization of central arbors from nociceptive afferents develops independently of their intact peripheral target innervation

Functionally important regions of sensory maps are overrepresented in the sensory pathways and cortex, but the underlying developmental mechanisms are not clear. In the spinal cord dorsal horn (DH), we recently showed that paw innervating Mrgprd⁺ nonpeptidergic nociceptors display distinctive central arbor morphologies that well correlate with increased synapse transmission efficiency and heightened sensitivity of distal limb skin. Given that peripheral and central arbor formation of Mrgprd⁺ neurons co-occurs around the time of birth, we tested whether peripheral cues from different skin areas and/or postnatal reorganization mechanisms could instruct this somatotopic difference among central arbors. We found that, while terminal outgrowth/refinement occurs during early postnatal development in both the skin and the DH, postnatal refinement of central terminals precedes that of peripheral terminals. Furthermore, we used single-cell ablation of Ret to genetically disrupt epidermal innervation of Mrgprd⁺ neurons and revealed that the somatotopic difference among their central arbors was unaffected by this manipulation. Finally, we saw that region-specific Mrgprd⁺ central terminal arbors are present from the earliest postnatal stages, before skin terminals are evident. In summary, we find that region-specific organization of Mrgprd⁺ neuron central arbors is present shortly after initial central terminal formation, which likely develops independently of peripheral target innervation. Our data suggest that either cell-intrinsic and/or DH prepatterning mechanisms are likely to establish this somatotopic difference.