Optogenetic Inhibition of CGRPα Sensory Neurons Reveals Their Distinct Roles in Neuropathic and Incisional Pain

Benzofuran Iboga-Analogs Modulate Nociception and Inflammation in an Acute Mouse Pain Model

Pain management following acute injury or post-operative procedures is highly necessary for proper recovery and quality of life. Opioids and non-steroidal anti-inflammatory drugs (NSAIDS) have been used for this purpose, but opioids cause addiction and withdrawal symptoms whereas NSAIDS have several systemic toxicities. Derivatives of the naturally occurring iboga alkaloids have previously shown promising behavior in anti-addiction of morphine by virtue of their interaction with opioid receptors. On this frontier, four benzofuran analogs of the iboga family have been synthesized and their analgesic effects have been studied in formalin induced acute pain model in male Swiss albino mice at 30 mg/kg of body weight dose administered intraperitoneally. The antioxidant, anti-inflammatory and neuro-modulatory effects of the analogs were analyzed. Reversal of tail flick latency, restricted locomotion and anxiogenic behavior were observed in iboga alcohol, primary amide and secondary amide. Local neuroinflammatory mediators' substance P, calcitonin gene related peptide, cyclooxygenase-2 and p65 were significantly decreased whereas the depletion of brain derived neurotrophic factor and glia derived neurotrophic factor was overturned on iboga analog treatment. Behavioral patterns after oral administration of the best analog were also analyzed. Taken together, these results show that the iboga family of alkaloid has huge potential in pain management.

An improved conflict avoidance assay reveals modality-specific differences in pain hypersensitivity across sexes

ABSTRACT

Abnormal encoding of somatosensory modalities (ie, mechanical, cold, and heat) are a critical part of pathological pain states. Detailed phenotyping of patients' responses to these modalities have raised hopes that analgesic treatments could one day be tailored to a patient's phenotype. Such precise treatment would require a profound understanding of the underlying mechanisms of specific pain phenotypes at molecular, cellular, and circuitry levels. Although preclinical pain models have helped in that regard, the lack of a unified assay quantifying detailed mechanical, cold, and heat pain responses on the same scale precludes comparing how analgesic compounds act on different sensory phenotypes. The conflict avoidance assay is promising in that regard, but testing conditions require validation for its use with multiple modalities. In this study, we improve upon the conflict avoidance assay to provide a validated and detailed assessment of all 3 modalities within the same animal, in mice. We first optimized testing conditions to minimize the necessary amount of training and to reduce sex differences in performances. We then tested what range of stimuli produce dynamic stimulus-response relationships for different outcome measures in naive mice. We finally used this assay to show that nerve injury produces modality-specific sex differences in pain behavior. Our improved assay opens new avenues to study the basis of modality-specific abnormalities in pain behavior.

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.

Updating perspectives on spinal cord function: motor coordination, timing, relational processing, and memory below the brain

Those studying neural systems within the brain have historically assumed that lower-level processes in the spinal cord act in a mechanical manner, to relay afferent signals and execute motor commands. From this view, abstracting temporal and environmental relations is the province of the brain. Here we review work conducted over the last 50 years that challenges this perspective, demonstrating that mechanisms within the spinal cord can organize coordinated behavior (stepping), induce a lasting change in how pain (nociceptive) signals are processed, abstract stimulus-stimulus (Pavlovian) and response-outcome (instrumental) relations, and infer whether stimuli occur in a random or regular manner. The mechanisms that underlie these processes depend upon signal pathways (e.g., NMDA receptor mediated plasticity) analogous to those implicated in brain-dependent learning and memory. New data show that spinal cord injury (SCI) can enable plasticity within the spinal cord by reducing the inhibitory effect of GABA. It is suggested that the signals relayed to the brain may contain information about environmental relations and that spinal cord systems can coordinate action in response to descending signals from the brain. We further suggest that the study of stimulus processing, learning, memory, and cognitive-like processing in the spinal cord can inform our views of brain function, providing an attractive model system. Most importantly, the work has revealed new avenues of treatment for those that have suffered a SCI.

Neuronal-immune axis alters pain and sensory afferent damage during dental pulp injury

ABSTRACT

Dental pulp tissue is densely innervated by afferent fibers of the trigeminal ganglion. When bacteria cause dental decay near the pulpal tissue, a strong neuronal and immune response occurs, creating pulpitis, which is associated with severe pain and pulp tissue damage. Neuroimmune interactions have the potential to modulate both the pain and pathological outcome of pulpitis. We first investigated the role of the neuropeptide calcitonin gene-related peptide (CGRP), released from peptidergic sensory afferents, in dental pain and immune responses by using Calca knockout (Calca -/- ) and wild-type (Calca +/+ ) mice, in a model of pulpitis by creating a mechanical exposure of the dental pulp horn. We found that the neuropeptide CGRP, facilitated the recruitment of myeloid cells into the pulp while also increasing spontaneous pain-like behavior 20% to 25% at an early time point. Moreover, when we depleted neutrophils and monocytes, we found that there was 20% to 30% more sensory afferent loss and increased presence of bacteria in deeper parts of the tissue, whereas there was a significant reduction in mechanical pain response scores compared with the control group at a later time point. Overall, we showed that there is a crosstalk between peptidergic neurons and neutrophils in the pulp, modulating the pain and inflammatory outcomes of the disease.

Optogenetic Neuromodulation in Inflammatory Pain

Inflammatory pain is one of the most prevalent forms of pain and negatively influences the quality of life. Neuromodulation has been an expanding field of pain medicine and is accepted by patients who have failed to respond to several conservative treatments. Despite its effectiveness, neuromodulation still lacks clinically robust evidence on inflammatory pain management. Optogenetics, which controls particular neurons or brain circuits with high spatiotemporal accuracy, has recently been an emerging area for inflammatory pain management and studying its mechanism. This review considers the fundamentals of optogenetics, including using opsins, targeting gene expression, and wavelength-specific light delivery techniques. The recent evidence on application and development of optogenetic neuromodulation in inflammatory pain is also summarised. The current limitations and challenges restricting the progression and clinical transformation of optogenetics in pain are addressed. Optogenetic neuromodulation in inflammatory pain has many potential targets, and developing strategies enabling clinical application is a desirable therapeutic approach and outcome.

The ratio of alpha-calcitonin gene-related peptide to substance P is associated with the transition of bone metabolic states during aging and healing

Alpha-calcitonin gene-related peptide (αCGRP) and substance P (SP) are functionally correlated sensory neuropeptides deeply involved in bone homeostasis. However, they are usually studied individually rather than as an organic whole. To figure out whether they are interdependent, we firstly recorded the real-time αCGRP and SP levels in aging bone and healing fracture, which revealed a moderate to high level of αCGRP coupled with a low αCGRP/SP ratio in an anabolic state, and a high level of αCGRP coupled with a high αCGRP/SP ratio in a catabolic state, suggesting the importance of αCGRP/SP ratio in driving aging and healing scenarios. During facture healing, increase in αCGRP/SP ratio by adding αCGRP led to better callus formation and faster callus remodeling, while simultaneous addition of αCGRP and SP resulted in hypertrophic callus and delayed remodeling. The characteristics in inflammation and osteoclast activation further confirmed the importance of high αCGRP/SP ratio during catabolic bone remodeling. In vitro assays using different mixtures of αCGRP-SP proved that the osteogenic potential of the mixtures depended mostly on αCGRP, while their effects on osteoclasts and neutrophils relied on both peptides. These results demonstrated that αCGRP and SP were spatiotemporally interdependent. The αCGRP/SP ratio may be more important than the dose of a single neuropeptide in managing age-related and trauma-related bone diseases.

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.

Pain persists in mice lacking both Substance P and CGRPα signaling

The neuropeptides Substance P and CGRPα have long been thought important for pain sensation. Both peptides and their receptors are expressed at high levels in pain-responsive neurons from the periphery to the brain making them attractive therapeutic targets. However, drugs targeting these pathways individually did not relieve pain in clinical trials. Since Substance P and CGRPα are extensively co-expressed we hypothesized that their simultaneous inhibition would be required for effective analgesia. We therefore generated Tac1 and Calca double knockout (DKO) mice and assessed their behavior using a wide range of pain-relevant assays. As expected, Substance P and CGRPα peptides were undetectable throughout the nervous system of DKO mice. To our surprise, these animals displayed largely intact responses to mechanical, thermal, chemical, and visceral pain stimuli, as well as itch. Moreover, chronic inflammatory pain and neurogenic inflammation were unaffected by loss of the two peptides. Finally, neuropathic pain evoked by nerve injury or chemotherapy treatment was also preserved in peptide-deficient mice. Thus, our results demonstrate that even in combination, Substance P and CGRPα are not required for the transmission of acute and chronic pain.

Peripheral mechanisms of peripheral neuropathic pain

Peripheral neuropathic pain (PNP), neuropathic pain that arises from a damage or disease affecting the peripheral nervous system, is associated with an extremely large disease burden, and there is an increasing and urgent need for new therapies for treating this disorder. In this review we have highlighted therapeutic targets that may be translated into disease modifying therapies for PNP associated with peripheral neuropathy. We have also discussed how genetic studies and novel technologies, such as optogenetics, chemogenetics and single-cell RNA-sequencing, have been increasingly successful in revealing novel mechanisms underlying PNP. Additionally, consideration of the role of non-neuronal cells and communication between the skin and sensory afferents is presented to highlight the potential use of drug treatment that could be applied topically, bypassing drug side effects. We conclude by discussing the current difficulties to the development of effective new therapies and, most importantly, how we might improve the translation of targets for peripheral neuropathic pain identified from studies in animal models to the clinic.

Repetitive nociceptive stimulation elicits complex behavioral changes in Hirudo: evidence of arousal and motivational adaptations

Appropriate responses to real or potential damaging stimuli to the body (nociception) are critical to an animal's short- and long-term survival. The initial goal of this study was to examine habituation of withdrawal reflexes (whole-body and local shortening) to repeated mechanical nociceptive stimuli (needle pokes) in the medicinal leech, Hirudo verbana, and assess whether injury altered habituation to these nociceptive stimuli. While repeated needle pokes did reduce shortening in H. verbana, a second set of behavior changes was observed. Specifically, animals began to evade subsequent stimuli by either hiding their posterior sucker underneath adjacent body segments or engaging in locomotion (crawling). Animals differed in terms of how quickly they adopted evasion behaviors during repeated stimulation, exhibiting a multi-modal distribution for early, intermediate and late evaders. Prior injury had a profound effect on this transition, decreasing the time frame in which animals began to carry out evasion and increasing the magnitude of these evasion behaviors (more locomotory evasion). The data indicate the presence in Hirudo of a complex and adaptive defensive arousal process to avoid noxious stimuli that is influenced by differences in internal states, prior experience with injury of the stimulated areas, and possibly learning-based processes.

Peripheral transient receptor potential vanilloid type 4 hypersensitivity contributes to chronic sickle cell disease pain

ABSTRACT

Debilitating pain affects the lives of patients with sickle cell disease (SCD). Current pain treatment for patients with SCD fail to completely resolve acute or chronic SCD pain. Previous research indicates that the cation channel transient receptor potential vanilloid type 4 (TRPV4) mediates peripheral hypersensitivity in various inflammatory and neuropathic pain conditions that may share similar pathophysiology with SCD, but this channel's role in chronic SCD pain remains unknown. Thus, the current experiments examined whether TRPV4 regulates hyperalgesia in transgenic mouse models of SCD. Acute blockade of TRPV4 alleviated evoked behavioral hypersensitivity to punctate, but not dynamic, mechanical stimuli in mice with SCD. TRPV4 blockade also reduced the mechanical sensitivity of small, but not large, dorsal root ganglia neurons from mice with SCD. Furthermore, keratinocytes from mice with SCD showed sensitized TRPV4-dependent calcium responses. These results shed new light on the role of TRPV4 in SCD chronic pain and are the first to suggest a role for epidermal keratinocytes in the heightened sensitivity observed in SCD.

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.

Synergistic role of resveratrol and exercise training in management of diabetic neuropathy and myopathy via SIRT1/NGF/GAP43 linkage

AIMS

Oxidative stress also plays an important role in the pathogenesis of diabetic neuropathy (DN). Both resveratrol (RES) and exercise (EX) have potent anti-oxidative benefits. Low levels of nerve growth factor (NGF) and SIRT1 (a member of sirtuin family) have been reported in patients with DN. The current study has been designed to investigate the role of serum NGF and SIRT1 on DN-induced hyperalgesia and motor incoordination and to evaluate the possible protective role of RES and/or EX.

MAIN METHODS

A total of 40 male adult albino rats divided into five groups; control, DN, DN + RES, DN + EX, and DN + RES and EX. DN was confirmed by sensorimotor disturbance and diminished nerve conduction velocity (NCV). NGF and SIRT1 levels were measured by western blot. Calcitonin gene-related peptide (CGRP) was measured by PCR. Myofibrillar degeneration and inflammation scores were revealed via H&E microscopic analysis of the gastrocnemius muscle. Immunohistochemical evaluation of caspase3 and TNF-α was performed in the lumber segment of spinal cord and gastrocnemius muscle sections. Ultrastructural evaluation of sciatic nerve axonal degeneration has also been assessed.

KEY FINDINGS

DN group showed decreased SIRT1 level, decreased NGF level and correlated with CGRP level and Na+/K+ ATPase. Treatment with RES and/or EX resulted in improvement of sensorimotor disturbance. DN characterized by reduced SOD level, whereas RES and/or EX could limit oxidative damage by up-regulation Bcl2, Akt and GAP-43 and down-regulation of caspase3 and TNF-α. In conclusion, increased level of SIRT1and NGF by incorporation of RES (natural supplementation) and EX (life style modification) could improve the neuroinflammatory state in DN.

Persistent nociceptor hyperactivity as a painful evolutionary adaptation

Chronic pain caused by injury or disease of the nervous system (neuropathic pain) has been linked to persistent electrical hyperactivity of the sensory neurons (nociceptors) specialized to detect damaging stimuli and/or inflammation. This pain and hyperactivity are considered maladaptive because both can persist long after injured tissues have healed and inflammation has resolved. While the assumption of maladaptiveness is appropriate in many diseases, accumulating evidence from diverse species, including humans, challenges the assumption that neuropathic pain and persistent nociceptor hyperactivity are always maladaptive. We review studies indicating that persistent nociceptor hyperactivity has undergone evolutionary selection in widespread, albeit selected, animal groups as a physiological response that can increase survival long after bodily injury, using both highly conserved and divergent underlying mechanisms.

Nav1.7 is required for normal C-low threshold mechanoreceptor function in humans and mice

Patients with bi-allelic loss of function mutations in the voltage-gated sodium channel Nav1.7 present with congenital insensitivity to pain (CIP), whilst low threshold mechanosensation is reportedly normal. Using psychophysics (n = 6 CIP participants and n = 86 healthy controls) and facial electromyography (n = 3 CIP participants and n = 8 healthy controls), we found that these patients also have abnormalities in the encoding of affective touch, which is mediated by the specialized afferents C-low threshold mechanoreceptors (C-LTMRs). In the mouse, we found that C-LTMRs express high levels of Nav1.7. Genetic loss or selective pharmacological inhibition of Nav1.7 in C-LTMRs resulted in a significant reduction in the total sodium current density, an increased mechanical threshold and reduced sensitivity to non-noxious cooling. The behavioural consequence of loss of Nav1.7 in C-LTMRs in mice was an elevation in the von Frey mechanical threshold and less sensitivity to cooling on a thermal gradient. Nav1.7 is therefore not only essential for normal pain perception but also for normal C-LTMR function, cool sensitivity and affective touch.

Serotonin mediates stress-like effects on responses to non-nociceptive stimuli in Hirudo

Noxious stimuli can elicit stress in animals that produce a variety of adaptations including changes in responses to nociceptive and non-nociceptive sensory input. One example is stress-induced analgesia that may be mediated, in part, by the endocannabinoid system. However, endocannabinoids can also have pro-nociceptive effects. In this study, the effects of electroshock, one experimental approach for producing acute stress, were examined on responses to non-nociceptive mechanical stimuli and nociceptive thermal stimuli in the medicinal leech (Hirudo verbana). The electroshock stimuli did not alter the leeches’ responses to nociceptive stimuli, but did cause sensitization to non-nociceptive stimuli, characterized by a reduction in response threshold. These experiments were repeated with drugs that either blocked synthesis of the endocannabinoid transmitter 2-arachidonoylglycerol (2-AG) or transient receptor potential vanilloid (TRPV) channel, which is known to act as an endocannabinoid receptor. Surprisingly, neither treatment had any effect on responses following electroshock. However, the electroshock stimuli reliably increased serotonin (5-hydroxytryptamine or 5HT) levels in the H. verbana CNS. Injection of 5HT mimicked the effects of the electroshocks, sensitizing responses to non-nociceptive stimuli and having no effect on responses to nociceptive stimuli. Injections of the 5HT receptor antagonist methysergide reduced the sensitization effect to non-nociceptive stimuli after electroshock treatment. These results indicate that electroshocks enhance response to non-nociceptive stimuli but do not alter responses to nociceptive stimuli. Furthermore, while 5HT appears to play a critical role in this shock-induced sensitizing effect, the endocannabinoid system seems to have no effect.

Summary: The role of serotonin and endocannabinoids in mediating the effects of potentially stress-inducing stimuli on Hirudo verbana’s response to nociceptive and non-nociceptive input.

Innovations and advances in modelling and measuring pain in animals

Best practices in preclinical algesiometry (pain behaviour testing) have shifted over the past decade as a result of technological advancements, the continued dearth of translational progress and the emphasis that funding institutions and journals have placed on rigour and reproducibility. Here we describe the changing trends in research methods by analysing the methods reported in preclinical pain publications from the past 40 years, with a focus on the last 5 years. We also discuss how the status quo may be hampering translational success. This discussion is centred on four fundamental decisions that apply to every pain behaviour experiment: choice of subject (model organism), choice of assay (pain-inducing injury), laboratory environment and choice of outcome measures. Finally, we discuss how human tissues, which are increasingly accessible, can be used to validate the translatability of targets and mechanisms identified in animal pain models.

Painful diabetic neuropathy leads to functional CaV3.2 expression and spontaneous activity in skin nociceptors of mice

Painful diabetic neuropathy occurs in approximately 20% of diabetic patients with underlying pathomechanisms not fully understood. We evaluated the contribution of the CaV3.2 isoform of T-type calcium channel to hyperglycemia-induced changes in cutaneous sensory C-fiber functions and neuropeptide release employing the streptozotocin (STZ) diabetes model in congenic mouse strains including global knockouts (KOs). Hyperglycemia established for 3-5 weeks in male C57BL/6J mice led to major reorganizations in peripheral C-fiber functions. Unbiased electrophysiological screening of mechanosensitive single-fibers in isolated hairy hindpaw skin revealed a relative loss of (polymodal) heat sensing in favor of cold sensing. In healthy CaV3.2 KO mice both heat and cold sensitivity among the C-fibers seemed underrepresented in favor of exclusive mechanosensitivity, low-threshold in particular, which deficit became significant in the diabetic KOs. Diabetes also led to a marked increase in the incidence of spontaneous discharge activity among the C-fibers of wildtype mice, which was reduced by the specific CaV3.2 blocker TTA-P2 and largely absent in the KOs. Evaluation restricted to the peptidergic class of nerve fibers - measuring KCl-stimulated CGRP release - revealed a marked reduction in the sciatic nerve by TTA-P2 in healthy but not diabetic wildtypes, the latter showing CGRP release that was as much reduced as in healthy and, to the same extent, in diabetic CaV3.2 KOs. These data suggest that diabetes abrogates all CaV3.2 functionality in the peripheral nerve axons. In striking contrast, diabetes markedly increased the KCl-stimulated CGRP release from isolated hairy skin of wildtypes but not KO mice, and TTA-P2 reversed this increase, strongly suggesting a de novo expression of CaV3.2 in peptidergic cutaneous nerve endings which may contribute to the enhanced spontaneous activity. De-glycosylation by neuraminidase showed clear desensitizing effects, both in regard to spontaneous activity and stimulated CGRP release, but included actions independent of CaV3.2. However, as diabetes-enhanced glycosylation is decisive for intra-axonal trafficking, it may account for the substantial reorganizations of the CaV3.2 distribution. The results may strengthen the validation of CaV3.2 channel as a therapeutic target of treating painful diabetic neuropathy.

Optogenetic Inhibition of Nav1.8 Expressing Corneal Afferents Reduces Persistent Dry Eye Pain

Purpose

The aim of the present study was to investigate the contribution of Nav1.8 expressing corneal afferent neurons to the presence of ongoing pain in lacrimal gland excision (LGE)-induced dry eye.

Methods

The proton pump archaerhodopsin-3/eGFP (ArchT/eGFP) was conditionally expressed in corneal afferents using Nav1.8-cre mice. Dry eye was produced by unilateral LGE. Real time place preference was assessed using a three-chamber apparatus. A neutral, unlit center chamber was flanked by one illuminated with a control light and one illuminated with an ArchT activating light. For real-time preference, animals were placed in the neutral chamber and tracked over five 10-minute sessions, with the lights turned on during the second and fourth sessions. In other studies, movement was tracked over three 10-minute sessions (the lights turned on only during the second session), with animals tested once per day over the course of 4 days. A local anesthetic was used to examine the role of ongoing corneal afferent activity in producing place preference.

Results

The corneal afferent nerves and trigeminal ganglion cell bodies showed a robust eGFP signal in Nav1.8-cre;ArchT/eGFP mice. After LGE, Nav1.8-cre;ArchT/eGFP mice demonstrated a preference for the ArchT activating light paired chamber. Preference was prevented with pre-application to the cornea of a local anesthetic. Nav1.8-cre;ArchT/eGFP mice with sham surgery and LGE wild-type control mice did not develop preference.

Conclusions

Results indicate LGE-induced persistent, ongoing pain, driven by Nav1.8 expressing corneal afferents. Inhibition of these neurons represents a potential strategy for treating ongoing dry eye-induced pain.

Inhibiting endocytosis in CGRP+ nociceptors attenuates inflammatory pain-like behavior

The advantage of locally applied anesthetics is that they are not associated with the many adverse effects, including addiction liability, of systemically administered analgesics. This therapeutic approach has two inherent pitfalls: specificity and a short duration of action. Here, we identified nociceptor endocytosis as a promising target for local, specific, and long-lasting treatment of inflammatory pain. We observed preferential expression of AP2α2, an α-subunit isoform of the AP2 complex, within CGRP⁺/IB4^- nociceptors in rodents and in CGRP⁺ dorsal root ganglion neurons from a human donor. We utilized genetic and pharmacological approaches to inhibit nociceptor endocytosis demonstrating its role in the development and maintenance of acute and chronic inflammatory pain. One-time injection of an AP2 inhibitor peptide significantly reduced acute and chronic pain-like behaviors and provided prolonged analgesia. We evidenced sexually dimorphic recovery responses to this pharmacological approach highlighting the importance of sex differences in pain development and response to analgesics.

The authors show the endocytotic adaptor subunit called AP2A2 is differentially expressed in CGRP⁺ nociceptors. Locally inhibiting nociceptor endocytosis with a lipidated AP2 inhibitor peptide reduces acute and chronic pain-like behaviour in mice and rats, indicating prolonged analgesia.

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.

Silent cold-sensing neurons contribute to cold allodynia in neuropathic pain

Patients with neuropathic pain often experience innocuous cooling as excruciating pain. The cell and molecular basis of this cold allodynia is little understood. We used in vivo calcium imaging of sensory ganglia to investigate how the activity of peripheral cold-sensing neurons was altered in three mouse models of neuropathic pain: oxaliplatin-induced neuropathy, partial sciatic nerve ligation, and ciguatera poisoning. In control mice, cold-sensing neurons were few in number and small in size. In neuropathic animals with cold allodynia, a set of normally silent large diameter neurons became sensitive to cooling. Many of these silent cold-sensing neurons responded to noxious mechanical stimuli and expressed the nociceptor markers Nav1.8 and CGRPα. Ablating neurons expressing Nav1.8 resulted in diminished cold allodynia. The silent cold-sensing neurons could also be activated by cooling in control mice through blockade of Kv1 voltage-gated potassium channels. Thus, silent cold-sensing neurons are unmasked in diverse neuropathic pain states and cold allodynia results from peripheral sensitization caused by altered nociceptor excitability.

Transient receptor potential canonical 5 mediates inflammatory mechanical and spontaneous pain in mice

Tactile and spontaneous pains are poorly managed symptoms of inflammatory and neuropathic injury. Here, we found that transient receptor potential canonical 5 (TRPC5) is a chief contributor to both of these sensations in multiple rodent pain models. Use of TRPC5 knockout mice and inhibitors revealed that TRPC5 selectively contributes to the mechanical hypersensitivity associated with CFA injection, skin incision, chemotherapy induced peripheral neuropathy, sickle cell disease, and migraine, all of which were characterized by elevated concentrations of lysophosphatidylcholine (LPC). Accordingly, exogenous application of LPC induced TRPC5-dependent behavioral mechanical allodynia, neuronal mechanical hypersensitivity, and spontaneous pain in naïve mice. Lastly, we found that 75% of human sensory neurons express TRPC5, the activity of which is directly modulated by LPC. On the basis of these results, TRPC5 inhibitors might effectively treat spontaneous and tactile pain in conditions characterized by elevated LPC.

A proteome signature for acute incisional pain in dorsal root ganglia of mice

ABSTRACT

After surgery, acute pain is still managed insufficiently and may lead to short-term and long-term complications including chronic postsurgical pain and an increased prescription of opioids. Thus, identifying new targets specifically implicated in postoperative pain is of utmost importance to develop effective and nonaddictive analgesics. Here, we used an integrated and multimethod workflow to reveal unprecedented insights into proteome dynamics in dorsal root ganglia (DRG) of mice after plantar incision (INC). Based on a detailed characterization of INC-associated pain-related behavior profiles, including a novel paradigm for nonevoked pain, we performed quantitative mass-spectrometry-based proteomics in DRG 1 day after INC. Our data revealed a hitherto unknown INC-regulated protein signature in DRG with changes in distinct proteins and cellular signaling pathways. In particular, we show the differential regulation of 44 protein candidates, many of which are annotated with pathways related to immune and inflammatory responses such as MAPK/extracellular signal-regulated kinases signaling. Subsequent orthogonal assays comprised multiplex Western blotting, bioinformatic protein network analysis, and immunolabeling in independent mouse cohorts to validate (1) the INC-induced regulation of immune/inflammatory pathways and (2) the high priority candidate Annexin A1. Taken together, our results propose novel potential targets in the context of incision and, therefore, represent a highly valuable resource for further mechanistic and translational studies of postoperative pain.

Contribution of dorsal horn CGRP-expressing interneurons to mechanical sensitivity

Primary sensory neurons are generally considered the only source of dorsal horn calcitonin gene-related peptide (CGRP), a neuropeptide critical to the transmission of pain messages. Using a tamoxifen-inducible CalcaCreER transgenic mouse, here we identified a distinct population of CGRP-expressing excitatory interneurons in lamina III of the spinal cord dorsal horn and trigeminal nucleus caudalis. These interneurons have spine-laden, dorsally directed, dendrites, and ventrally directed axons. As under resting conditions, CGRP interneurons are under tonic inhibitory control, neither innocuous nor noxious stimulation provoked significant Fos expression in these neurons. However, synchronous, electrical non-nociceptive Aβ primary afferent stimulation of dorsal roots depolarized the CGRP interneurons, consistent with their receipt of a VGLUT1 innervation. On the other hand, chemogenetic activation of the neurons produced a mechanical hypersensitivity in response to von Frey stimulation, whereas their caspase-mediated ablation led to mechanical hyposensitivity. Finally, after partial peripheral nerve injury, innocuous stimulation (brush) induced significant Fos expression in the CGRP interneurons. These findings suggest that CGRP interneurons become hyperexcitable and contribute either to ascending circuits originating in deep dorsal horn or to the reflex circuits in baseline conditions, but not in the setting of nerve injury.

Nociceptor-localized cGMP-dependent protein kinase I is a critical generator for central sensitization and neuropathic pain

Patients with neuropathic pain often experience exaggerated pain and anxiety. Central sensitization has been linked with the maintenance of neuropathic pain and may become an autonomous pain generator. Conversely, emerging evidence accumulated that central sensitization is initiated and maintained by ongoing nociceptive primary afferent inputs. However, it remains elusive what mechanisms underlie this phenomenon and which peripheral candidate contributes to central sensitization that accounts for pain hypersensitivity and pain-related anxiety. Previous studies have implicated peripherally localized cGMP-dependent protein kinase I (PKG-I) in plasticity of nociceptors and spinal synaptic transmission as well as inflammatory hyperalgesia. However, whether peripheral PKG-I contributes to cortical plasticity and hence maintains nerve injury-induced pain hypersensitivity and anxiety is unknown. Here, we demonstrated significant upregulation of PKG-I in ipsilateral L3 dorsal root ganglia (DRG), no change in L4 DRG, and downregulation in L5 DRG upon spared nerve injury. Genetic ablation of PKG-I specifically in nociceptors or post-treatment with intervertebral foramen injection of PKG-I antagonist, KT5823, attenuated the development and maintenance of spared nerve injury-induced bilateral pain hypersensitivity and anxiety. Mechanistic analysis revealed that activation of PKG-I in nociceptors is responsible for synaptic potentiation in the anterior cingulate cortex upon peripheral neuropathy through presynaptic mechanisms involving brain-derived neurotropic factor signaling. Our results revealed that PKG-I expressed in nociceptors is a key determinant for cingulate synaptic plasticity after nerve injury, which contributes to the maintenance of pain hypersensitivity and anxiety. Thereby, this study presents a strong basis for opening up a novel therapeutic target, PKG-I, in nociceptors for treatment of comorbidity of neuropathic pain and anxiety with least side effects.

A genome-wide screen reveals microRNAs in peripheral sensory neurons driving painful diabetic neuropathy

ABSTRACT

Diabetes is a leading cause of peripheral neuropathy (diabetic peripheral neuropathy, DPN), and uncontrolled long-lasting hyperglycemia leads to severe complications. A major proportion of diabetics develop excruciating pain with a variable course. Mechanisms leading to painful DPN are not completely understood and treatment options limited. We hypothesized that epigenetic modulation at the level of microRNA (miRNA) expression triggered by metabolic imbalance and nerve damage regulates the course of pain development. We used clinically relevant preclinical models, genome-wide screening, in silico analyses, cellular assays, miRNA fluorescent in situ hybridization, in vivo molecular manipulations, and behavioral analyses in the current study. We identified miRNAs and their targets that critically impact on nociceptive hypersensitivity in painful DPN. Our analyses identify miR-33 and miR-380 expressed in nociceptive neurons as critical denominators of diabetic pain and miR-124-1 as a mediator of physiological nociception. Our comprehensive analyses on the putative mRNA targets for miR-33 or miR-124-1 identified a set of mRNAs that are regulated after miR-33 or miR-124-1 overexpression in dorsal root ganglia in vivo. Our results shed light on the regulation of DPN pathophysiology and implicate specific miRNAs as novel therapeutic targets for treating painful DPN.

Altered sensory innervation and pain hypersensitivity in a model of young painful arthritic joints: short- and long-term effects

BACKGROUND

Early life experience can cause long-term alterations in the nociceptive processes underlying chronic pain, but the consequences of early life arthritic joint inflammation upon the sensory innervation of the joint is not known. Here, we measure pain sensitivity and sensory innervation in a young, juvenile and adult rodent model of arthritic joints and test the consequences of joint inflammation in young animals upon adult arthritic pain and joint innervation.

METHODS

Unilateral ankle joint injections of complete Freund's adjuvant (CFA) (6-20 µl) were performed in young, postnatal day (P)8, adolescent (P21) and adult (P40) rats. A separate cohort of animals were injected at P8, and again at P40. Hindpaw mechanical sensitivity was assessed using von Frey monofilaments (vF) for 10 days. Nerve fibres were counted in sections through the ankle joint immunostained for calcitonin gene-related peptide (CGRP) and neurofilament 200 kDa (NF200).

RESULTS

Ankle joint CFA injection increased capsular width at all ages. Significant mechanical pain hypersensitivity and increased number of joint CGRP + ve sensory fibres occurred in adolescent and adult, but not young, rats. Despite the lack of acute reaction, joint inflammation at a young age resulted in significantly increased pain hypersensitivity and CGRP+ fibre counts when the rats were re-inflamed as adults.

CONCLUSIONS

Joint inflammation increases the sensory nociceptive innervation and induces acute pain hypersensitivity in juvenile and adult, but not in young rats. However, early life joint inflammation 'primes' the joint such that adult inflammatory pain behaviour and nociceptive nerve endings in the joint are significantly increased. Early life joint inflammation may be an important factor in the generation and maintenance of chronic arthritic pain.

Characterization of acute pain-induced behavioral passivity in mice: Insights from statistical modeling

Affective-motivational disturbances are highly inconsistent in animal pain models. The reproducibility of the open-field test in assessing anxiety, malaise or disability remains controversial despite its popularity. While traumatic, persistent or multiregional pain models are commonly considered more effective in inducing negative affect or functional impairment, the early psychobehavioral changes before pain chronification are often underexplored. Here, we aimed to clarify the fundamental relationship between hypernociception and passive distress-like behavior using a model of transient inflammatory pain. To minimize latent confounders and increase data consistency, male C57BL/6N mice were habituated to the open-field arena 6 times before receiving the unilateral intraplantar injection of prostaglandin E2 (PGE2) or vehicle. Open-field (40-min exploration) and nociceptive behavior were evaluated repeatedly along the course of hypernociception in both wild-type and transgenic mice with a known pronociceptive phenotype. To reduce subjectivity, multivariate open-field behavioral outcomes were analyzed by statistical modeling based on exploratory factor analyses, which yielded a 2-factor solution. Within 3 hr after PGE2 injection, mice developed significantly reduced center exploration (factor 1) and a marginally significant increase in their habituation tendency (factor 2), which were not apparent in vehicle-injected mice. The behavioral passivity generally improved as hypernociception subsided. Therefore, transient inflammatory irritation is sufficient to suppress mouse open-field exploratory activity. The apparent absence of late affective-motivational changes in some rodents with prolonged hypernociception may not imply a lack of preceding or underlying neuropsychological alterations. Procedural pain after invasive animal experiments, however small, should be assessed and adequately controlled as a potential research confounder.

Single-nucleus characterization of adult mouse spinal dynorphin-lineage cells and identification of persistent transcriptional effects of neonatal hindpaw incision

Neonatal tissue damage can have long-lasting effects on nociceptive processing in the central nervous system, which may reflect persistent injury-evoked alterations to the normal balance between synaptic inhibition and excitation in the spinal dorsal horn. Spinal dynorphin-lineage (pDyn) neurons are part of an inhibitory circuit which limits the flow of nociceptive input to the brain and is disrupted by neonatal tissue damage. To identify the potential molecular underpinnings of this disruption, an unbiased single-nucleus RNAseq analysis of adult mouse spinal pDyn cells characterized this population in depth and then identified changes in gene expression evoked by neonatal hindpaw incision. The analysis revealed 11 transcriptionally distinct subpopulations (ie, clusters) of dynorphin-lineage cells, including both inhibitory and excitatory neurons. Investigation of injury-evoked differential gene expression identified 15 genes that were significantly upregulated or downregulated in adult pDyn neurons from neonatally incised mice compared with naive littermate controls, with both cluster-specific and pan-neuronal transcriptional changes observed. Several of the identified genes, such as Oxr1 and Fth1 (encoding ferritin), were related to the cellular stress response. However, the relatively low number of injury-evoked differentially expressed genes also suggests that posttranscriptional regulation within pDyn neurons may play a key role in the priming of developing nociceptive circuits by early-life injury. Overall, the findings reveal novel insights into the molecular heterogeneity of a key population of dorsal horn interneurons that has previously been implicated in the suppression of mechanical pain and itch.

Tweety-Homolog 1 Facilitates Pain via Enhancement of Nociceptor Excitability and Spinal Synaptic Transmission

Tweety-homolog 1 (Ttyh1) is expressed in neural tissue and has been implicated in the generation of several brain diseases. However, its functional significance in pain processing is not understood. By disrupting the gene encoding Ttyh1, we found a loss of Ttyh1 in nociceptors and their central terminals in Ttyh1-deficient mice, along with a reduction in nociceptor excitability and synaptic transmission at identified synapses between nociceptors and spinal neurons projecting to the periaqueductal grey (PAG) in the basal state. More importantly, the peripheral inflammation-evoked nociceptor hyperexcitability and spinal synaptic potentiation recorded in spinal-PAG projection neurons were compromised in Ttyh1-deficient mice. Analysis of the paired-pulse ratio and miniature excitatory postsynaptic currents indicated a role of presynaptic Ttyh1 from spinal nociceptor terminals in the regulation of neurotransmitter release. Interfering with Ttyh1 specifically in nociceptors produces a comparable pain relief. Thus, in this study we demonstrated that Ttyh1 is a critical determinant of acute nociception and pain sensitization caused by peripheral inflammation.

Activation of oxytocin receptor in the trigeminal ganglion attenuates orofacial ectopic pain attributed to inferior alveolar nerve injury

This study explores the effects of oxytocin receptor (OXTR) in the trigeminal ganglion (TG) on orofacial neuropathic pain. We demonstrate that OXTR activation in the TG relieves the orofacial ectopic pain as well as inhibits the upregulated expression of calcitonin gene-related peptide (CGRP), IL-1β, and TNFα in the TG and spinal trigeminal nucleus caudalis (SpVc) of rats with inferior alveolar nerve transection. OXTR, a G protein-coupled receptor, has been demonstrated to play a significant role in analgesia after activation by its canonical agonist oxytocin (OXT) in the dorsal root ganglion. However, the role of OXTR in the trigeminal nervous system on the orofacial neuropathic pain is still little known. In the present study, we aimed to investigate the regulation effect and mechanism of OXTR in the TG) and SpVc) on orofacial ectopic pain induced by trigeminal nerve injury. The inferior alveolar nerve (IAN) was transected to establish a ectopic pain model. A behavioral test with electronic von Frey filament demonstrated IAN transection (IANX) evoked mechanical hypersensitivity in the whisker pad from day 1 to at least day 14 after surgery. In addition, administration of OXT (50 and 100 μM) into the TG attenuated the mechanical hypersensitivity induced by IANX, which was reversed by pretreatment with L-368,899 (a selective antagonist of OXTR) into the TG. In addition, immunofluorescence showed the expression of OXTR in neurons in the TG and SpVc. Furthermore, Western blot analysis indicated that the upregulated expression of OXTR, CGRP, IL-1β, and TNFα in the TG and SpVc after IANX was inhibited by the administration of OXT into the TG. And the inhibition effect of OXT on the expression of CGRP, IL-1β, and TNFα was abolished by preapplication of OXTR antagonist L-368,899 into the TG.NEW & NOTEWORTHY This study explores the effects of oxytocin receptor (OXTR) in the trigeminal ganglion (TG) on orofacial neuropathic pain. We demonstrate that OXTR activation in the TG relieves the orofacial ectopic pain as well as inhibits the upregulated expression of calcitonin gene-related peptide, IL-1β, and TNF-α in the TG and spinal trigeminal nucleus caudalis of rats with inferior alveolar nerve transection.

Mechanical Allodynia Circuitry in the Dorsal Horn Is Defined by the Nature of the Injury

The spinal dorsal horn is a major site for the induction and maintenance of mechanical allodynia, but the circuitry that underlies this clinically important form of pain remains unclear. The studies presented here provide strong evidence that the neural circuits conveying mechanical allodynia in the dorsal horn differ by the nature of the injury. Calretinin (CR) neurons in lamina II inner convey mechanical allodynia induced by inflammatory injuries, while protein kinase C gamma (PKCγ) neurons at the lamina II/III border convey mechanical allodynia induced by neuropathic injuries. Cholecystokinin (CCK) neurons located deeper within the dorsal horn (laminae III-IV) are important for both types of injuries. Interestingly, the Maf⁺ subset of CCK neurons is composed of transient vesicular glutamate transporter 3 (tVGLUT3) neurons, which convey primarily dynamic allodynia. Identification of an etiology-based circuitry for mechanical allodynia in the dorsal horn has important implications for the mechanistic and clinical understanding of this condition.

Optogenetic stimulation of the motor cortex alleviates neuropathic pain in rats of infraorbital nerve injury with/without CGRP knock-down

BACKGROUND

Previous studies have reported that electrical stimulation of the motor cortex is effective in reducing trigeminal neuropathic pain; however, the effects of optical motor cortex stimulation remain unclear.

OBJECTIVE

The present study aimed to investigate whether optical stimulation of the primary motor cortex can modulate chronic neuropathic pain in rats with infraorbital nerve constriction injury.

METHODS

Animals were randomly divided into a trigeminal neuralgia group, a sham group, and a control group. Trigeminal neuropathic pain was generated via constriction of the infraorbital nerve and animals were treated via selective inhibition of calcitonin gene-related peptide in the trigeminal ganglion. We assessed alterations in behavioral responses in the pre-stimulation, stimulation, and post-stimulation conditions. In vivo extracellular recordings were obtained from the ventral posteromedial nucleus of the thalamus, and viral and α-CGRP expression were investigated in the primary motor cortex and trigeminal ganglion, respectively.

RESULTS

We found that optogenetic stimulation significantly improved pain behaviors in the trigeminal neuralgia animals and it provided more significant improvement with inhibited α-CGRP state than active α-CGRP state. Electrophysiological recordings revealed decreases in abnormal thalamic firing during the stimulation-on condition.

CONCLUSION

Our findings suggest that optical motor cortex stimulation can alleviate pain behaviors in a rat model of trigeminal neuropathic pain. Transmission of trigeminal pain signals can be modulated via knock-down of α-CGRP and optical motor cortex stimulation.

Neuropathic Pain: From Mechanisms to Treatment

Neuropathic pain caused by a lesion or disease of the somatosensory nervous system is a common chronic pain condition with major impact on quality of life. Examples include trigeminal neuralgia, painful polyneuropathy, postherpetic neuralgia, and central poststroke pain. Most patients complain of an ongoing or intermittent spontaneous pain of, for example, burning, pricking, squeezing quality, which may be accompanied by evoked pain, particular to light touch and cold. Ectopic activity in, for example, nerve-end neuroma, compressed nerves or nerve roots, dorsal root ganglia, and the thalamus may in different conditions underlie the spontaneous pain. Evoked pain may spread to neighboring areas, and the underlying pathophysiology involves peripheral and central sensitization. Maladaptive structural changes and a number of cell-cell interactions and molecular signaling underlie the sensitization of nociceptive pathways. These include alteration in ion channels, activation of immune cells, glial-derived mediators, and epigenetic regulation. The major classes of therapeutics include drugs acting on α₂δ subunits of calcium channels, sodium channels, and descending modulatory inhibitory pathways.

Behavioral, Biochemical and Electrophysiological Changes in Spared Nerve Injury Model of Neuropathic Pain

Neuropathic pain is a pathological condition induced by a lesion or disease affecting the somatosensory system, with symptoms like allodynia and hyperalgesia. It has a multifaceted pathogenesis as it implicates several molecular signaling pathways involving peripheral and central nervous systems. Affective and cognitive dysfunctions have been reported as comorbidities of neuropathic pain states, supporting the notion that pain and mood disorders share some common pathogenetic mechanisms. The understanding of these pathophysiological mechanisms requires the development of animal models mimicking, as far as possible, clinical neuropathic pain symptoms. Among them, the Spared Nerve Injury (SNI) model has been largely characterized in terms of behavioral and functional alterations. This model is associated with changes in neuronal firing activity at spinal and supraspinal levels, and induces late neuropsychiatric disorders (such as anxious-like and depressive-like behaviors, and cognitive impairments) comparable to an advanced phase of neuropathy. The goal of this review is to summarize current findings in preclinical research, employing the SNI model as a tool for identifying pathophysiological mechanisms of neuropathic pain and testing pharmacological agent.

Reversal of peripheral nerve injury-induced neuropathic pain and cognitive dysfunction via genetic and tomivosertib targeting of MNK

Neuropathic pain caused by nerve injury presents with severe spontaneous pain and a variety of comorbidities, including deficits in higher executive functions. None of these clinical problems are adequately treated with current analgesics. Targeting of the mitogen-activated protein kinase-interacting kinase (MNK1/2) and its phosphorylation target, the mRNA cap binding protein eIF4E, attenuates many types of nociceptive plasticity induced by inflammatory mediators and chemotherapeutic drugs but inhibiting this pathway does not alter nerve injury-induced mechanical allodynia. We used genetic manipulations and pharmacology to inhibit MNK-eIF4E activity in animals with spared nerve injury, a model of peripheral nerve injury (PNI)-induced neuropathic pain. We assessed the presence of spontaneous pain using conditioned place preference. We also tested performance in a medial prefrontal cortex (mPFC)-dependent rule-shifting task. WT neuropathic animals showed signs of spontaneous pain and were significantly impaired in the rule-shifting task while genetic and pharmacological inhibition of the MNK-eIF4E signaling axis protected against and reversed spontaneous pain and PNI-mediated cognitive impairment. Additionally, pharmacological and genetic inhibition of MNK-eIF4E signaling completely blocked and reversed maladaptive shortening in the length of axon initial segments (AIS) in the mPFC of PNI mice. Surprisingly, these striking positive outcomes on neuropathic pain occurred in the absence of any effect on mechanical allodynia, a standard test for neuropathic pain efficacy. Our results illustrate new testing paradigms for determining preclinical neuropathic pain efficacy and point to the MNK inhibitor tomivosertib (eFT508) as an important drug candidate for neuropathic pain treatment.

Molecular mechanisms of cold pain

The sensation of cooling is essential for survival. Extreme cold is a noxious stimulus that drives protective behaviour and that we thus perceive as pain. However, chronic pain patients suffering from cold allodynia paradoxically experience innocuous cooling as excruciating pain. Peripheral sensory neurons that detect decreasing temperature express numerous cold-sensitive and voltage-gated ion channels that govern their response to cooling in health and disease. In this review, we discuss how these ion channels control the sense of cooling and cold pain under physiological conditions, before focusing on the molecular mechanisms by which ion channels can trigger pathological cold pain. With the ever-rising number of patients burdened by chronic pain, we end by highlighting the pressing need to define the cells and molecules involved in cold allodynia and so identify new, rational drug targets for the analgesic treatment of cold pain.

Gabapentin alleviates chronic spontaneous pain and acute hypoxia-related pain in a mouse model of sickle cell disease

Pain is the main complication of sickle cell disease (SCD). Individuals with SCD experience acute pain episodes and chronic daily pain, both of which are managed with opioids. Opioids have deleterious side effects and use-associated stigma that make them less than ideal for SCD pain management. After recognizing the neuropathic qualities of SCD pain, clinically-approved therapies for neuropathic pain, including gabapentin, now present unique non-opioid based therapies for SCD pain management. These experiments explored the efficacy of gabapentin in relieving evoked and spontaneous chronic pain, and hypoxia/reoxygenation (H/R)-induced acute pain in mouse models of SCD. When administered following H/R, a single dose of gabapentin alleviated mechanical hypersensitivity in SCD mice by decreasing peripheral fibre activity. Gabapentin treatment also alleviated spontaneous ongoing pain in SCD mice. Longitudinal daily administration of gabapentin failed to alleviate H/R-induced pain or chronic evoked mechanical, cold or deep tissue hypersensitivity in SCD mice. Consistent with this observation, voltage-gated calcium channel (VGCC) α2 δ1 subunit expression was similar in sciatic nerve, dorsal root ganglia and lumbar spinal cord tissue from SCD and control mice. Based on these data, gabapentin may be an effective opioid alternative for the treatment of chronic spontaneous and acute H/R pain in SCD.

Uncovering the Cells and Circuits of Touch in Normal and Pathological Settings

The sense of touch is fundamental as it provides vital, moment-to-moment information about the nature of our physical environment. Primary sensory neurons provide the basis for this sensation in the periphery; however, recent work demonstrates that touch transduction mechanisms also occur upstream of the sensory neurons via non-neuronal cells such as Merkel cells and keratinocytes. Within the spinal cord, deep dorsal horn circuits transmit innocuous touch centrally and also transform touch into pain in the setting of injury. Here non-neuronal cells play a key role in the induction and maintenance of persistent mechanical pain. This review highlights recent advances in our understanding of mechanosensation, including a growing appreciation for the role of non-neuronal cells in both touch and pain.

NOD-like receptor protein 3 inflammasome drives postoperative mechanical pain in a sex-dependent manner

Postoperative pain management continues to be suboptimal because of the lack of effective nonopioid therapies and absence of understanding of sex-driven differences. Here, we asked how the NLRP3 inflammasome contributes to postoperative pain. Inflammasomes are mediators of the innate immune system that are responsible for activation and secretion of IL-1β upon stimulation by specific molecular signals. Peripheral IL-1β is known to contribute to the mechanical sensitization induced by surgical incision. However, it is not known which inflammasome mediates the IL-1β release after surgical incision. Among the 9 known inflammasomes, the NLRP3 inflammasome is ideally positioned to drive postoperative pain through IL-1β production because NLRP3 can be activated by factors that are released by incision. Here, we show that male mice that lack NLRP3 (NLRP3) recover from surgery-induced behavioral and neuronal mechanical sensitization faster and display less surgical site inflammation than mice expressing NLRP3 (wild-type). By contrast, female NLRP3 mice exhibit minimal attenuation of the postoperative mechanical hypersensitivity and no change in postoperative inflammation compared with wild-type controls. Sensory neuron-specific deletion of NLRP3 revealed that in males, NLRP3 expressed in non-neuronal cells and potentially sensory neurons drives postoperative pain. However, in females, only the NLRP3 that may be expressed in sensory neurons contributes to postoperative pain where the non-neuronal cell contribution is NLRP3 independent. This is the first evidence of a key role for NLRP3 in postoperative pain and reveals immune-mediated sex differences in postoperative pain.

The Role of TRESK in Discrete Sensory Neuron Populations and Somatosensory Processing

Two-pore domain K⁺ (K_2P) channels generate K⁺ leak current, which serves a vital role in controlling and modulating neuronal excitability. This diverse family of K⁺ channels exhibit distinct expression and function across neuronal tissues. TWIK-related spinal cord K⁺ channel (TRESK) is a K_2P channel with a particularly enriched role in sensory neurons and in vivo pain pathways. Here, we explored the role of TRESK across molecularly distinct sensory neuron populations and assessed its contribution to different sensory modalities. We found TRESK mRNA only in select populations of C- and A-δ nociceptors, in addition to low threshold D-hair afferents. Neurons from mice in which TRESK has been ablated demonstrated marked hyperexcitability, which was amplified under inflammatory challenge. Detailed behavioral phenotyping of TRESK knockout mice revealed specific deficits in somatosensory processing of noxious and non-noxious stimuli. These results demonstrate novel roles of TRESK in somatosensory processing and offer important information to those wishing to target the channel for therapeutic means.

A Novel Sex-Dependent Target for the Treatment of Postoperative Pain: The NLRP3 Inflammasome

In recent years the innate immune system has been shown to be crucial for the pathogenesis of postoperative pain. The mediators released by innate immune cells drive the sensitization of sensory neurons following injury by directly acting on peripheral nerve terminals at the injury site. The predominate sensitization signaling pathway involves the proinflammatory cytokine interleukin-1β (IL-1β). IL-1β is known to cause pain by directly acting on sensory neurons. Evidence demonstrates that blockade of IL-1β signaling decreases postoperative pain, however complete blockade of IL-1β signaling increases the risk of infection and decreases effective wound healing. IL-1β requires activation by an inflammasome; inflammasomes are cytosolic receptors of the innate immune system. NOD-like receptor protein 3 (NLRP3) is the predominant inflammasome activated by endogenous molecules that are released by tissue injury such as that which occurs during neuropathic and inflammatory pain disorders. Given that selective inhibition of NLRP3 alleviates postoperative mechanical pain, its selective targeting may be a novel and effective strategy for the treatment of pain that would avoid complications of global IL-1β inhibition. Moreover, NLRP3 is activated in pain in a sex-dependent and cell type-dependent manner. Sex differences in the innate immune system have been shown to drive pain and sensitization through different mechanisms in inflammatory and neuropathic pain disorders, indicating that it is imperative that both sexes are studied when researchers investigate and identify new targets for pain therapeutics. This review will highlight the roles of the innate immune response, the NLRP3 inflammasome, and sex differences in neuropathic and inflammatory pain.

Mechanisms underlying mechanical sensitization induced by complement C5a: the roles of macrophages, TRPV1, and calcitonin gene-related peptide receptors

The complement system significantly contributes to the development of inflammatory and neuropathic pain, but the underlying mechanisms are poorly understood. Recently, we identified the signaling pathway responsible for thermal hypersensitivity induced by the complement system component C5a. Here, we examine the mechanisms of another important action of C5a, induction of mechanical hypersensitivity. We found that intraplantar injection of C5a produced a dose-dependent mechanical sensitization and that this effect was blocked by chemogenetic ablation of macrophages in both male and female mice. Knockout of TRPV1 or pretreatment with the TRPV1 antagonists, AMG9810 or 5'-iodoresiniferatoxin (5'-IRTX), significantly reduced C5a-induced mechanical sensitization. Notably, local administration of 5'-IRTX 90 minutes after C5a injection resulted in a slow, but complete, reversal of mechanical sensitization, indicating that TRPV1 activity was required for maintaining C5a-induced mechanical hypersensitivity. This slow reversal suggests that neurogenic inflammation and neuropeptide release may be involved. Indeed, pretreatment with a calcitonin gene-related peptide (CGRP) receptor antagonist (but not an antagonist of the neurokinin 1 receptor) prevented C5a-induced mechanical sensitization. Furthermore, intraplantar injection of CGRP produced significant mechanical sensitization in both wild-type and TRPV1 knockout mice. Taken together, these findings suggest that C5a produces mechanical sensitization by initiating macrophage-to-sensory-neuron signaling cascade that involves activation of TRPV1 and CGRP receptor as critical steps in this process.

A Mouse Model of Postoperative Pain

Postoperative pain is highly debilitating and hinders recovery. Opioids are the main pain medication used for acute postoperative pain. Given the devastating opioid addiction and overdose epidemic across the US, non-opioid pain therapeutics are desperately needed. In order to develop novel, non-opioid therapies for the treatment of postoperative pain and identify the mechanisms underlying this pain, rodent models of incisional pain have been established. The protocol herein describes in detail how to create a mouse model of postoperative pain that was adapted from established protocols. This model of postoperative pain is frequently-used, highly reproducible, and results in peripheral and central nervous system alterations.