MrgprdCre lineage neurons mediate optogenetic allodynia through an emergent polysynaptic circuit

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.

Nerve injury disrupts temporal processing in the spinal cord dorsal horn through alterations in PV+ interneurons

How mechanical allodynia following nerve injury is encoded in patterns of neural activity in the spinal cord dorsal horn (DH) remains incompletely understood. We address this in mice using the spared nerve injury model of neuropathic pain and in vivo electrophysiological recordings. Surprisingly, despite dramatic behavioral over-reactivity to mechanical stimuli following nerve injury, an overall increase in sensitivity or reactivity of DH neurons is not observed. We do, however, observe a marked decrease in correlated neural firing patterns, including the synchrony of mechanical stimulus-evoked firing, across the DH. Alterations in DH temporal firing patterns are recapitulated by silencing DH parvalbumin+ (PV+) interneurons, previously implicated in mechanical allodynia, as are allodynic pain-like behaviors. These findings reveal decorrelated DH network activity, driven by alterations in PV+ interneurons, as a prominent feature of neuropathic pain and suggest restoration of proper temporal activity as a potential therapeutic strategy to treat chronic neuropathic pain.

Reproducible and fully automated testing of nocifensive behavior in mice

Pain in rodents is often inferred from their withdrawal from noxious stimulation. Threshold stimulus intensity or response latency is used to quantify pain sensitivity. This usually involves applying stimuli by hand and measuring responses by eye, which limits reproducibility and throughput. We describe a device that standardizes and automates pain testing by providing computer-controlled aiming, stimulation, and response measurement. Optogenetic and thermal stimuli are applied using blue and infrared light, respectively. Precise mechanical stimulation is also demonstrated. Reflectance of red light is used to measure paw withdrawal with millisecond precision. We show that consistent stimulus delivery is crucial for resolving stimulus-dependent variations in withdrawal and for testing with sustained stimuli. Moreover, substage video reveals "spontaneous" behaviors for consideration alongside withdrawal metrics to better assess the pain experience. The entire process was automated using machine learning. RAMalgo (reproducible automated multimodal algometry) improves the standardization, comprehensiveness, and throughput of preclinical pain testing.

GluCl.Cre ON enables selective inhibition of molecularly defined pain circuits

ABSTRACT

Insight into nociceptive circuits will ultimately build our understanding of pain processing and aid the development of analgesic strategies. Neural circuit analysis has been advanced greatly by the development of optogenetic and chemogenetic tools, which have allowed function to be ascribed to discrete neuronal populations. Neurons of the dorsal root ganglion, which include nociceptors, have proved challenging targets for chemogenetic manipulation given specific confounds with commonly used DREADD technology. We have developed a cre/lox dependant version of the engineered glutamate-gated chloride channel (GluCl) to restrict and direct its expression to molecularly defined neuronal populations. We have generated GluCl.Cre ON that selectively renders neurons expressing cre-recombinase susceptible to agonist-induced silencing. We have functionally validated our tool in multiple systems in vitro, and subsequently generated viral vectors and tested its applicability in vivo. Using Nav1.8 Cre mice to restrict AAV-GluCl.Cre ON to nociceptors, we demonstrate effective silencing of electrical activity in vivo and concomitant hyposensitivity to noxious thermal and noxious mechanical pain, whereas light touch and motor function remained intact. We also demonstrated that our strategy can effectively silence inflammatory-like pain in a chemical pain model. Collectively, we have generated a novel tool that can be used to selectively silence defined neuronal circuits in vitro and in vivo. We believe that this addition to the chemogenetic tool box will facilitate further understanding of pain circuits and guide future therapeutic development.

S. aureus drives itch and scratch-induced skin damage through a V8 protease-PAR1 axis

Itch is an unpleasant sensation that evokes a desire to scratch. The skin barrier is constantly exposed to microbes and their products. However, the role of microbes in itch generation is unknown. Here, we show that Staphylococcus aureus, a bacterial pathogen associated with itchy skin diseases, directly activates pruriceptor sensory neurons to drive itch. Epicutaneous S. aureus exposure causes robust itch and scratch-induced damage. By testing multiple isogenic bacterial mutants for virulence factors, we identify the S. aureus serine protease V8 as a critical mediator in evoking spontaneous itch and alloknesis. V8 cleaves proteinase-activated receptor 1 (PAR1) on mouse and human sensory neurons. Targeting PAR1 through genetic deficiency, small interfering RNA (siRNA) knockdown, or pharmacological blockade decreases itch and skin damage caused by V8 and S. aureus exposure. Thus, we identify a mechanism of action for a pruritogenic bacterial factor and demonstrate the potential of inhibiting V8-PAR1 signaling to treat itch.

Kinesin family member 2A gates nociception

Nociceptive axons undergo remodeling as they innervate their targets during development and in response to environmental insults and pathological conditions. How is nociceptive morphogenesis regulated? Here, we show that the microtubule destabilizer kinesin family member 2A (Kif2a) is a key regulator of nociceptive terminal structures and pain sensitivity. Ablation of Kif2a in sensory neurons causes hyperinnervation and hypersensitivity to noxious stimuli in young adult mice, whereas touch sensitivity and proprioception remain unaffected. Computational modeling predicts that structural remodeling is sufficient to explain the phenotypes. Furthermore, Kif2a deficiency triggers a transcriptional response comprising sustained upregulation of injury-related genes and homeostatic downregulation of highly specific channels and receptors at the late stage. The latter effect can be predicted to relieve the hyperexcitability of nociceptive neurons, despite persisting morphological aberrations, and indeed correlates with the resolution of pain hypersensitivity. Overall, we reveal a critical control node defining nociceptive terminal structure, which is regulating nociception.

Members of the CUGBP Elav-like family of RNA-binding proteins are expressed in distinct populations of primary sensory neurons

Primary sensory dorsal root ganglia (DRG) neurons are diverse, with distinct populations that respond to specific stimuli. Previously, we observed that functionally distinct populations of DRG neurons express mRNA transcript variants with different 3' untranslated regions (3'UTRs). 3'UTRs harbor binding sites for interaction with RNA-binding proteins (RBPs) for transporting mRNAs to subcellular domains, modulating transcript stability, and regulating the rate of translation. In the current study, analysis of publicly available single-cell RNA-sequencing data generated from adult mice revealed that 17 3'UTR-binding RBPs were enriched in specific populations of DRG neurons. This included four members of the CUG triplet repeat (CUGBP) Elav-like family (CELF): CELF2 and CELF4 were enriched in peptidergic, CELF6 in both peptidergic and nonpeptidergic, and CELF3 in tyrosine hydroxylase-expressing neurons. Immunofluorescence studies confirmed that 60% of CELF4+ neurons are small-diameter C fibers and 33% medium-diameter myelinated (likely Aδ) fibers and showed that CELF4 is distributed to peripheral termini. Coexpression analyses using transcriptomic data and immunofluorescence revealed that CELF4 is enriched in nociceptive neurons that express GFRA3, CGRP, and the capsaicin receptor TRPV1. Reanalysis of published transcriptomic data from macaque DRG revealed a highly similar distribution of CELF members, and reanalysis of single-nucleus RNA-sequencing data derived from mouse and rat DRG after sciatic injury revealed differential expression of CELFs in specific populations of sensory neurons. We propose that CELF RBPs may regulate the fate of mRNAs in populations of nociceptors, and may play a role in pain and/or neuronal regeneration following nerve injury.

Long-term tactile hypersensitivity after nerve crush injury in mice is characterized by the persistence of intact sensory axons

ABSTRACT

Traumatic peripheral nerve injuries are at high risk of neuropathic pain for which novel effective therapies are urgently needed. Preclinical models of neuropathic pain typically involve irreversible ligation and/or nerve transection (neurotmesis). However, translation of findings to the clinic has so far been unsuccessful, raising questions on injury model validity and clinically relevance. Traumatic nerve injuries seen in the clinic commonly result in axonotmesis (ie, crush), yet the neuropathic phenotype of "painful" nerve crush injuries remains poorly understood. We report the neuropathology and sensory symptoms of a focal nerve crush injury using custom-modified hemostats resulting in either complete ("full") or incomplete ("partial") axonotmesis in adult mice. Assays of thermal and mechanically evoked pain-like behavior were paralleled by transmission electron microscopy, immunohistochemistry, and anatomical tracing of the peripheral nerve. In both crush models, motor function was equally affected early after injury; by contrast, partial crush of the nerve resulted in the early return of pinprick sensitivity, followed by a transient thermal and chronic tactile hypersensitivity of the affected hind paw, which was not observed after a full crush injury. The partially crushed nerve was characterized by the sparing of small-diameter myelinated axons and intraepidermal nerve fibers, fewer dorsal root ganglia expressing the injury marker activating transcription factor 3, and lower serum levels of neurofilament light chain. By day 30, axons showed signs of reduced myelin thickness. In summary, the escape of small-diameter axons from Wallerian degeneration is likely a determinant of chronic pain pathophysiology distinct from the general response to complete nerve injury.

Calretinin-expressing islet cells are a source of pre- and post-synaptic inhibition of non-peptidergic nociceptor input to the mouse spinal cord

Unmyelinated non-peptidergic nociceptors (NP afferents) arborise in lamina II of the spinal cord and receive GABAergic axoaxonic synapses, which mediate presynaptic inhibition. However, until now the source of this axoaxonic synaptic input was not known. Here we provide evidence that it originates from a population of inhibitory calretinin-expressing interneurons (iCRs), which correspond to lamina II islet cells. The NP afferents can be assigned to 3 functionally distinct classes (NP1-3). NP1 afferents have been implicated in pathological pain states, while NP2 and NP3 afferents also function as pruritoceptors. Our findings suggest that all 3 of these afferent types innervate iCRs and receive axoaxonic synapses from them, providing feedback inhibition of NP input. The iCRs also form axodendritic synapses, and their targets include cells that are themselves innervated by the NP afferents, thus allowing for feedforward inhibition. The iCRs are therefore ideally placed to control the input from non-peptidergic nociceptors and pruritoceptors to other dorsal horn neurons, and thus represent a potential therapeutic target for the treatment of chronic pain and itch.

Calretinin-expressing islet cells: a source of pre- and post-synaptic inhibition of non-peptidergic nociceptor input to the mouse spinal cord

Unmyelinated non-peptidergic nociceptors (NP afferents) arborise in lamina II of the spinal cord and receive GABAergic axoaxonic synapses, which mediate presynaptic inhibition. However, until now the source of this axoaxonic synaptic input was not known. Here we provide evidence that it originates from a population of inhibitory calretinin-expressing interneurons (iCRs), which correspond to lamina II islet cells. The NP afferents can be assigned to 3 functionally distinct classes (NP1-3). NP1 afferents have been implicated in pathological pain states, while NP2 and NP3 afferents also function as pruritoceptors. Our findings suggest that all 3 of these afferent types innervate iCRs and receive axoaxonic synapses from them, providing feedback inhibition of NP input. The iCRs also form axodendritic synapses, and their targets include cells that are themselves innervated by the NP afferents, thus allowing for feedforward inhibition. The iCRs are therefore ideally placed to control the input from non-peptidergic nociceptors and pruritoceptors to other dorsal horn neurons, and thus represent a potential therapeutic target for the treatment of chronic pain and itch.

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.

Neuroimmune interactions in atopic and allergic contact dermatitis

The skin is a barrier organ populated by many types of skin-resident immune cells and sensory neurons. It has become increasingly appreciated that neuroimmune interactions are an important component of inflammatory diseases such as atopic dermatitis and allergic contact dermatitis. Neuropeptides secreted from nerve terminals play an important role in mediating cutaneous immune cell function, and soluble mediators derived from immune cells interact with neurons to induce itch. In this review article, we will explore emerging research describing neuronal effector functions on skin immune cells in mouse models of atopic and contact dermatitis. We will also discuss the contributions of both specific neuronal subsets and secreted immune factors to itch induction and the associated inflammatory processes. Finally, we will explore how treatment strategies have emerged around these findings and discuss the relationship between scratching and dermatitis.

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.

Touch neurons underlying dopaminergic pleasurable touch and sexual receptivity

Pleasurable touch is paramount during social behavior, including sexual encounters. However, the identity and precise role of sensory neurons that transduce sexual touch remain unknown. A population of sensory neurons labeled by developmental expression of the G protein-coupled receptor Mrgprb4 detects mechanical stimulation in mice. Here, we study the social relevance of Mrgprb4-lineage neurons and reveal that these neurons are required for sexual receptivity and sufficient to induce dopamine release in the brain. Even in social isolation, optogenetic stimulation of Mrgprb4-lineage neurons through the back skin is sufficient to induce a conditioned place preference and a striking dorsiflexion resembling the lordotic copulatory posture. In the absence of Mrgprb4-lineage neurons, female mice no longer find male mounts rewarding: sexual receptivity is supplanted by aggression and a coincident decline in dopamine release in the nucleus accumbens. Together, these findings establish that Mrgprb4-lineage neurons initiate a skin-to-brain circuit encoding the rewarding quality of social touch.

PIEZO1 transduces mechanical itch in mice

Itch triggers scratching, a behavioural defence mechanism that aids in the removal of harmful irritants and parasites¹. Chemical itch is triggered by many endogenous and exogenous cues, such as pro-inflammatory histamine, which is released during an allergic reaction¹. Mechanical itch can be triggered by light sensations such as wool fibres or a crawling insect². In contrast to chemical itch pathways, which have been extensively studied, the mechanisms that underlie the transduction of mechanical itch are largely unknown. Here we show that the mechanically activated ion channel PIEZO1 (ref. ³) is selectively expressed by itch-specific sensory neurons and is required for their mechanically activated currents. Loss of PIEZO1 function in peripheral neurons greatly reduces mechanically evoked scratching behaviours and both acute and chronic itch-evoked sensitization. Finally, mice expressing a gain-of-function Piezo1 allele⁴ exhibit enhanced mechanical itch behaviours. Our studies reveal the polymodal nature of itch sensory neurons and identify a role for PIEZO1 in the sensation of itch.

Experiments in mice show that the mechanically activated ion channel PIEZO1 is expressed in itch-specific sensory neurons and has a role in transducing mechanical itch.

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.

Evidence for a phenotypic switch in corneal afferents after lacrimal gland excision

Dry eye is a common cause of ocular pain. The aim of this study was to investigate corneal innervation, ongoing pain, and alterations in corneal afferent phenotypes in a mouse model of severe aqueous tear deficiency. Chronic dry eye was produced by ipsilateral excision of the extra- and intraorbital lacrimal glands in male and female mice. Tearing was measured using a phenol thread and corneal epithelial damage assessed using fluorescein. Changes in corneal ongoing ocular pain was evaluated by measuring palpebral opening ratio. Corneal axons were visualized using Nav1.8-Cre;tdTomato reporter mice. Immunohistochemistry was performed to characterize somal expression of calcitonin gene-related peptide (CGRP), the capsaicin sensitive transient receptor potential vanilloid 1 (TRPV1), and activating transcription factor-3 (ATF-3) in tracer labeled corneal neurons following lacrimal gland excision (LGE). LGE decreased tearing, created severe epithelial damage, and decreased palpebral opening, indicative of chronic ocular irritation, over the 28-day observation period. Corneal axon terminals exhibited an acute decrease in density after LGE, followed by a regenerative process over the course of 28 days that was greater in male animals. Corneal neurons expressing CGRP, TRPV1, and ATF3 increased following injury, corresponding to axonal injury and regeneration processes observed during the same period. CGRP and TRPV1 expression was notably increased in IB4-positive cells following LGE. These results indicate that dry eye-induced damage to corneal afferents can result in alterations in IB4-positive neurons that may enhance neuroprotective mechanisms to create resiliency after chronic injury.

Dissecting the Neural Circuitry for Pain Modulation and Chronic Pain: Insights from Optogenetics

Pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage. The processing of pain involves complicated modulation at the levels of the periphery, spinal cord, and brain. The pathogenesis of chronic pain is still not fully understood, which makes the clinical treatment challenging. Optogenetics, which combines optical and genetic technologies, can precisely intervene in the activity of specific groups of neurons and elements of the related circuits. Taking advantage of optogenetics, researchers have achieved a body of new findings that shed light on the cellular and circuit mechanisms of pain transmission, pain modulation, and chronic pain both in the periphery and the central nervous system. In this review, we summarize recent findings in pain research using optogenetic approaches and discuss their significance in understanding the pathogenesis of chronic pain.

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.

Pain modulation in the spinal cord

The sensory inflow from the periphery that triggers innocuous and painful sensations is highly complex, capturing key elements of the nature of any stimulus, its location, intensity, and duration, and converting this to dynamic action potential firing across a wide population of afferents. While sensory afferents are highly specialized to detect these features, their input to the spinal cord also triggers active processing and modulation there which determines its output, to drive the sensory percept experienced and behavioral responses. Focus on such active spinal modulation was arguably first introduced by Melzack and Wall in their Spinal Cord Gate Control theory. This theory has had a profound influence on our understanding of pain, and especially its processing, as well as leading directly to the development of clinical interventions, and its historical importance certainly needs to be fully recognized. However, the enormous progress we are making in the understanding of the function of the somatosensory system, means that it is time to incorporate these newly discovered features into a more complex and accurate model of spinal sensory modulation.

The cellular and molecular basis of somatosensory neuron development

Primary somatosensory neurons convey salient information about our external environment and internal state to the CNS, allowing us to detect, perceive, and react to a wide range of innocuous and noxious stimuli. Pseudo-unipolar in shape, and among the largest (longest) cells of most mammals, dorsal root ganglia (DRG) somatosensory neurons have peripheral axons that extend into skin, muscle, viscera, or bone and central axons that innervate the spinal cord and brainstem, where they synaptically engage the central somatosensory circuitry. Here, we review the diversity of mammalian DRG neuron subtypes and the intrinsic and extrinsic mechanisms that control their development. We describe classical and contemporary advances that frame our understanding of DRG neurogenesis, transcriptional specification of DRG neurons, and the establishment of morphological, physiological, and synaptic diversification across somatosensory neuron subtypes.