Voltage-gated sodium channel expression in rat and human epidermal keratinocytes: Evidence for a role in pain

Activation of G protein gated inwardly rectifying potassium (GIRK) channels in keratinocytes mediates peripheral kappa opioid receptor-mediated antinociception

Kappa opioid receptors (KOR) expressed by peripheral pain-sensing neurons (nociceptors) are a promising target for development of effective and safer analgesics for inflammatory pain that are devoid of central nervous system adverse effects. Here we sought to delineate the signaling pathways that underlie peripheral KOR-mediated antinociception in adult male and female Sprague-Dawley rats. In an inflammatory model of pain, local intraplantar (i.pl.) injection of pertussis toxin prevented antinociception induced by the KOR agonist, U50488, indicating that members of the Gi/o family mediate the antinociceptive response. Furthermore, i.pl. injection of the G protein-coupled inward-rectifying potassium (GIRK) channel blocker, TPNQ, as well as GIRK2 subunit-targeted siRNA abolished U50488-mediated antinociceptive behavioral responses in both male and female rats. Consistent with these data, i.pl. injection of ML297, a direct activator of GIRK1 subunit-containing channels, elicited peripheral antinociceptive behavior. It is well known that intraepidermal nerve fibers (IENF) that innervate the hindpaw propagate nociceptive signals to the spinal cord. However, recent studies suggest that keratinocytes, the major cell type in the epidermis, also play an active role in pain and sensory processing. Results from RT-qPCR, RNAscope and immunohistochemistry experiments confirmed that both KOR and GIRK are expressed in keratinocytes in the epidermal layer of the rat hindpaw. Knockdown of either KOR or GIRK2 subunits selectively in keratinocytes by i.pl. injection of shRNA plasmids, prevented the antinociceptive response to U50488. Taken together, these data suggest that KOR-mediated activation of GIRK channels in keratinocytes is required for peripherally-mediated antinociception.

Fast Analgesic Effect in Response Test with Topical Phenytoin Cream Correlates with Prolonged Pain Relief After Extended Use in Painful Diabetic Neuropathy

Background: Treatment of painful diabetic neuropathy (PDN) poses several challenges due to the limited effectiveness, high incidence of side effects, and potential drug interactions of oral neuropathic pain medication. Lacking systemic side effects, topical phenytoin cream offers a promising innovative approach to addressing unmet needs in neuropathic pain treatment. In this retrospective study in patients with PDN, we evaluated the analgesic effect of topical phenytoin cream in response tests and after extended use. Methods: We collected data from PDN patients who, prior to prolonged use of phenytoin 10% or 20% cream, either had an open response test (ORET), a single-blind (SIBRET), or a double-blind (DOBRET) placebo-controlled response test with phenytoin cream between November 2016 and February 2023. A positive ORET was defined as pain reduction of at least two points on the 11-point numerical scale (NRS) within 30 min after phenytoin cream application. A positive SIBRET or DOBRET required an additional pain reduction of 1 NRS point in the phenytoin treated area compared to the placebo. In patients with a positive response test, we evaluated the sustained pain reduction and the proportion of patients experiencing minimum pain relief of at least 30% (MPR30: moderate pain relief) and 50% (MPR50: considerable pain relief) after the extended use of phenytoin cream. We also assessed the correlation between the response test analgesic effect and the sustained pain relief. Results: We identified 65 patients with PDN of whom 31 (47.7%) had a positive response test. The median pain reduction in response tests was 3.0 NRS points (IQR 2.0-4.0). Extended use (median 3.3 months, IQR 1.5-12.1]) resulted in a median pain reduction of 4.0 NRS points (IQR 3.0-5.0); 26/31 (83.9%) of patients achieved MPR30, and 21/31 (67.7%) MPR50 achieved pain relief. The response test analgesic effect correlated significantly with sustained pain relief after extended use (τ = 0.72, p < 0.0001). Conclusions: In PDN patients who had a positive phenytoin cream response test, extended use of phenytoin cream provided a significant sustained pain relief.

The Role of TRPV1/CGRP Pathway Activated by Prevotella melaninogenica in Pathogenesis of Oral Lichen Planus

The distinctive clinicopathologic characteristics of OLP indicated that both microbial dysbiosis and neurogenic inflammation may be jointly involved in its progression, and transient receptor potential vanilloid receptor-1 (TRPV1) may be a crucial element. The purpose of this study was to explore how TRPV1 mediated P. melaninogenica-induced inflammation. Meanwhile, we aimed to unravel how IL-36γ dysregulated the barrier function in oral keratinocytes. Here, the expression of TRPV1, calcitonin gene-related peptide (CGRP), and its receptor receptor activity-modifying protein 1 (RAMP1) in OLP patients were detected. Prevotella melaninogenica (P. melaninogenica) was used to build a mouse model of oral chronic inflammation. Normal human oral keratinocytes (NHOKs) stimulated by P. melaninogenica were used to examine TRPV1 activation and CGRP release. To investigate the effect of exogenous CGRP on Interleukin-36 gamma (IL-36γ) expression in NHOKs and bacterial viability, P. melaninogenica and NHOKs were treated with it, respectively. Recombinant IL-36γ protein was used to probe its regulation of oral epithelial barrier function. TRPV1, CGRP, and RAMP1 were substantially expressed in OLP. P. melaninogenica increased TRPV1 expression in mice and caused the release of CGRP and an increase in pro-inflammatory cytokines via activating TRPV1 in NHOKs. Blockade of TRPV1 suppressed P. melaninogenica-induced inflammation. CGRP boosted the production of IL-36γ released by NHOKs, resulting in lower expression of zonula occludens-1 (ZO-1). Also, CGRP can decrease the viability of P. melaninogenica. Together, these findings provide fresh insight into the vital role performed by P. melaninogenica-induced functional changes in oral epithelial cells and neurons in an intricate OLP inflammatory process.

Sodium channels in non-excitable cells: powerful actions and therapeutic targets beyond Hodgkin and Huxley

Voltage-gated sodium channels (VGSCs) are best known for their role in the generation and propagation of action potentials in neurons, muscle cells, and cardiac myocytes, which have traditionally been labeled as 'excitable'. However, emerging evidence challenges this traditional perspective. It is now clear that VGSCs are also expressed in a broad spectrum of cells outside the neuromuscular realm, where they regulate diverse cellular functions. In this review, we summarize current knowledge on the expression, regulation, and function of VGSCs in non-neuromuscular cells, highlighting their contributions to physiological processes and pathological conditions. Dynamic expression patterns of VGSCs in different cell types, involvement of VGSCs in cellular functions, such as phagocytosis, motility, and cytokine release, and their potential as therapeutic targets for diseases that include inflammatory disorders, osteoarthritis (OA), and cancer, are discussed. This new understanding of VGSCs and their effects on cells outside the neuromuscular realm opens new avenues for research and therapeutic interventions.

Increased keratinocyte activity and PIEZO1 signaling contribute to paclitaxel-induced mechanical hypersensitivity

Recent work demonstrates that epidermal keratinocytes are critical for normal touch sensation. However, it is unknown whether keratinocytes contribute to touch-evoked pain and hypersensitivity after tissue injury. Here, we used a mouse model of paclitaxel treatment to determine the extent to which keratinocyte activity contributes to the severe neuropathic pain that accompanies chemotherapy. We found that keratinocyte inhibition by either optogenetic or chemogenetic methods largely alleviated paclitaxel-induced mechanical hypersensitivity across acute and persistent time points from 2 days through 3 weeks. Furthermore, we found that paclitaxel exposure sensitized mouse and human keratinocytes to mechanical stimulation and enhanced currents of PIEZO1, a mechanosensitive channel highly expressed in keratinocytes. Deletion of PIEZO1 from keratinocytes alleviated paclitaxel-induced mechanical hypersensitivity in mice. These findings suggest that nonneuronal cutaneous cells contribute substantially to neuropathic pain and pave the way for the development of new pain relief strategies that target epidermal keratinocytes and PIEZO1.

Exploring neuroinflammation: A key driver in neuropathic pain disorders

Inflammation is a fundamental part of the body's natural defense mechanism, involving immune cells and inflammatory mediators to promote healing and protect against harm. In the event of a lesion or disease of the somatosensory nervous system, inflammation, however, triggers a cascade of changes in both the peripheral and central nervous systems, ultimately contributing to chronic neuropathic pain. Substantial evidence links neuroinflammation to various conditions associated with neuropathic pain. This chapter will explore the role of neuroinflammation in the initiation, maintenance, and resolution of peripheral and central neuropathic pain. Additionally, biomarkers of neuroinflammation in humans will be examined, emphasizing their relevance in different neuropathic pain disorders.

The role of the neuronal microenvironment in sensory function and pain pathophysiology

The high prevalence of pain and the at times low efficacy of current treatments represent a significant challenge to healthcare systems worldwide. Effective treatment strategies require consideration of the diverse pathophysiologies that underlie various pain conditions. Indeed, our understanding of the mechanisms contributing to aberrant sensory neuron function has advanced considerably. However, sensory neurons operate in a complex dynamic microenvironment that is controlled by multidirectional interactions of neurons with non-neuronal cells, including immune cells, neuronal accessory cells, fibroblasts, adipocytes, and keratinocytes. Each of these cells constitute and control the microenvironment in which neurons operate, inevitably influencing sensory function and the pathology of pain. This review highlights the importance of the neuronal microenvironment for sensory function and pain, focusing on cellular interactions in the skin, nerves, dorsal root ganglia, and spinal cord. We discuss the current understanding of the mechanisms by which neurons and non-neuronal cells communicate to promote or resolve pain, and how this knowledge could be used for the development of mechanism-based treatments.

Modeling neuropathic pain in a dish

The study of pain mechanisms has advanced significantly with the development of innovative in vitro models. This chapter explores those already used in or potentially useful for neuropathic pain research, emphasizing the complementary roles of animal and human cellular models to enhance translational success. Traditional animal models have provided foundational insights into the neurobiology of pain and remain invaluable for understanding complex pain pathways. However, integrating human cellular models addresses the need for better replication of human nociceptors. The chapter details methodologies for culturing rodent and human primary sensory neurons, including isolation and culture techniques, advantages, and limitations. It highlights the application of these models in neuropathic pain research, such as identifying pain-associated receptors and ion channels. Recent advancements in using induced pluripotent stem cell (iPSC)-derived sensory neurons are also discussed. Finally, the chapter explores advanced in vitro models, including 2D co-cultures and 3D organoids, and their implications for studying neuropathic pain. These models offer significant advantages for drug screening and ethical research practices, providing a more accurate representation of human pain pathways and paving the way for innovative therapeutic strategies. Despite challenges such as limited access to viable human tissue and variability between samples, these in vitro models, alongside traditional animal models, are indispensable for advancing our understanding of neuropathic pain and developing effective treatments.

Clodronate Reduces ATP-Containing Microvesicle Releasing Induced by Nociceptive Stimuli in Human Keratinocytes

Clodronate (Clod), a first-generation bisphosphonate, acts as a natural analgesic inhibiting vesicular storage of the nociception mediator ATP by vesicular nucleotide transporter (VNUT). Epidermal keratinocytes participate in cutaneous nociception, accumulating ATP within vesicles, which are released following different stimulations. Under stress conditions, keratinocytes produce microvesicles (MVs) by shedding from plasma membrane evagination. MV secretion has been identified as a novel and universal mode of intercellular communication between cells. The aim of this project was to evaluate if two nociceptive stimuli, Capsaicin and Potassium Hydroxide (KOH), could stimulate MV shedding from human keratinocytes, if these MVs could contain ATP, and if Clod could inhibit this phenomenon. In our cellular model, the HaCaT keratinocyte monolayer, both Capsaicin and KOH stimulated MV release after 3 h incubation, and the released MVs contained ATP. Moreover, Clod (5 µM) was able to reduce Caps-induced MV release and abolish the one KOH induced, while the Dansylcadaverine, an endocytosis inhibitor of Clod uptake, partially failed to block the bisphosphonate activity. Based on these new data and given the role of the activation of ATP release by keratinocytes as a vehicle for nociception and pain, the "old" bisphosphonate Clodronate could provide the pharmacological basis to develop new local analgesic drugs.

Neuroanatomy of the Cutaneous Nervous System Regarding Wound Healing

Wound healing is an important topic in modern medicine across many disciplines. Healing of all cutaneous wounds, whether accidentally sustained or intentionally created, requires the common yet complex set of interactions between the immune, circulatory, nervous, endocrine, and integumentary systems. Deficits in any of these systems or the molecular factors that mediate their communications can contribute to impaired healing of cutaneous wounds. While the stages of wound repair, angiogenesis, growth factors, and cytokines involved have been extensively studied, the role of the cutaneous nervous system in wound healing has not been well outlined. We have provided a basic overview of cutaneous innervation and wound repair for the dermatologic surgeon by outlining the normal cutaneous nervous anatomy and function and discussing the most important neuropeptides that mediate the wound healing process.

Dendrite intercalation between epidermal cells tunes nociceptor sensitivity to mechanical stimuli in Drosophila larvae

An animal's skin provides a first point of contact with the sensory environment, including noxious cues that elicit protective behavioral responses. Nociceptive somatosensory neurons densely innervate and intimately interact with epidermal cells to receive these cues, however the mechanisms by which epidermal interactions shape processing of noxious inputs is still poorly understood. Here, we identify a role for dendrite intercalation between epidermal cells in tuning sensitivity of Drosophila larvae to noxious mechanical stimuli. In wild-type larvae, dendrites of nociceptive class IV da neurons intercalate between epidermal cells at apodemes, which function as body wall muscle attachment sites, but not at other sites in the epidermis. From a genetic screen we identified miR-14 as a regulator of dendrite positioning in the epidermis: miR-14 is expressed broadly in the epidermis but not in apodemes, and miR-14 inactivation leads to excessive apical dendrite intercalation between epidermal cells. We found that miR-14 regulates expression and distribution of the epidermal Innexins ogre and Inx2 and that these epidermal gap junction proteins restrict epidermal dendrite intercalation. Finally, we found that altering the extent of epidermal dendrite intercalation had corresponding effects on nociception: increasing epidermal intercalation sensitized larvae to noxious mechanical inputs and increased mechanically evoked calcium responses in nociceptive neurons, whereas reducing epidermal dendrite intercalation had the opposite effects. Altogether, these studies identify epidermal dendrite intercalation as a mechanism for mechanical coupling of nociceptive neurons to the epidermis, with nociceptive sensitivity tuned by the extent of intercalation.

Cutaneous targets for topical pain medications in patients with neuropathic pain: individual differential expression of biomarkers supports the need for personalized medicine

Introduction

Numerous potential cutaneous targets exist for treating chronic pain with topically applied active pharmaceutical ingredients. This preliminary human skin tissue investigation was undertaken to characterize several key biomarkers in keratinocytes and provide proof-of-principle data to support clinical development of topical compounded formulations for peripheral neuropathic pain syndromes, such as postherpetic neuralgia (PHN).

Objectives

The study intended to identify objective biomarkers in PHN skin on a patient-by-patient personalized medicine platform. The totality of biopsy biomarker data can provide a tissue basis for directing individualized compounded topical preparations to optimize treatment efficacy.

Methods

Referencing 5 of the most common actives used in topical pain relief formulations (ketamine, gabapentin, clonidine, baclofen, and lidocaine), and 3 well-established cutaneous mediators (ie, neuropeptides, cannabinoids, and vanilloids), comprehensive immunolabeling was used to quantify receptor biomarkers in skin biopsy samples taken from ipsilateral (pain) and contralateral (nonpain) dermatomes of patients with PHN.

Results

Epidermal keratinocyte labeling patterns were significantly different among the cohort for each biomarker, consistent with potential mechanisms of action among keratinocytes. Importantly, the total biomarker panel indicates that the enriched PHN cohort contains distinct subgroups.

Conclusion

The heterogeneity of the cohort differences may explain studies that have not shown statistical group benefit from topically administered compounded therapies. Rather, the essential need for individual tissue biomarker evaluations is evident, particularly as a means to direct a more accurately targeted topical personalized medicine approach and generate positive clinical results.

Pathophysiological Roles of Ion Channels in Epidermal Cells, Immune Cells, and Sensory Neurons in Psoriasis

Psoriasis is a chronic inflammatory skin disease characterized by the rapid abnormal growth of skin cells in the epidermis, driven by an overactive immune system. Consequently, a complex interplay among epidermal cells, immune cells, and sensory neurons contributes to the development and progression of psoriasis. In these cellular contexts, various ion channels, such as acetylcholine receptors, TRP channels, Ca2+ release-activated channels, chloride channels, and potassium channels, each serve specific functions to maintain the homeostasis of the skin. The dysregulation of ion channels plays a major role in the pathophysiology of psoriasis, affecting various aspects of epidermal cells, immune responses, and sensory neuron signaling. Impaired function of ion channels can lead to altered calcium signaling, inflammation, proliferation, and sensory signaling, all of which are central features of psoriasis. This overview summarizes the pathophysiological roles of ion channels in epidermal cells, immune cells, and sensory neurons during early and late psoriatic processes, thereby contributing to a deeper understanding of ion channel involvement in the interplay of psoriasis and making a crucial advance toward more precise and personalized approaches for psoriasis treatment.

A helping hand: roles for accessory cells in the sense of touch across species

During touch, mechanical forces are converted into electrochemical signals by tactile organs made of neurons, accessory cells, and their shared extracellular spaces. Accessory cells, including Merkel cells, keratinocytes, lamellar cells, and glia, play an important role in the sensation of touch. In some cases, these cells are intrinsically mechanosensitive; however, other roles include the release of chemical messengers, the chemical modification of spaces that are shared with neurons, and the tuning of neural sensitivity by direct physical contact. Despite great progress in the last decade, the precise roles of these cells in the sense of touch remains unclear. Here we review the known and hypothesized contributions of several accessory cells to touch by incorporating research from multiple organisms including C. elegans, D. melanogaster, mammals, avian models, and plants. Several broad parallels are identified including the regulation of extracellular ions and the release of neuromodulators by accessory cells, as well as the emerging potential physical contact between accessory cells and sensory neurons via tethers. Our broader perspective incorporates the importance of accessory cells to the understanding of human touch and pain, as well as to animal touch and its molecular underpinnings, which are underrepresented among the animal welfare literature. A greater understanding of touch, which must include a role for accessory cells, is also relevant to emergent technical applications including prosthetics, virtual reality, and robotics.

Hyperexcitability of muscle spindle afferents in jaw-closing muscles in experimental myalgia: Evidence for large primary afferents involvement in chronic pain

The goals of this review are to improve understanding of the aetiology of chronic muscle pain and identify new targets for treatments. Muscle pain is usually associated with trigger points in syndromes such as fibromyalgia and myofascial syndrome, and with small spots associated with spontaneous electrical activity that seems to emanate from fibers inside muscle spindles in EMG studies. These observations, added to the reports that large-diameter primary afferents, such as those innervating muscle spindles, become hyperexcitable and develop spontaneous ectopic firing in conditions leading to neuropathic pain, suggest that changes in excitability of these afferents might make an important contribution to the development of pathological pain. Here, we review evidence that the muscle spindle afferents (MSAs) of the jaw-closing muscles become hyperexcitable in a model of chronic orofacial myalgia. In these afferents, as in other large-diameter primary afferents in dorsal root ganglia, firing emerges from fast membrane potential oscillations that are supported by a persistent sodium current (INaP ) mediated by Na+ channels containing the α-subunit NaV 1.6. The current flowing through NaV 1.6 channels increases when the extracellular Ca2+ concentration decreases, and studies have shown that INaP -driven firing is increased by S100β, an astrocytic protein that chelates Ca2+ when released in the extracellular space. We review evidence of how astrocytes, which are known to be activated in pain conditions, might, through their regulation of extracellular Ca2+ , contribute to the generation of ectopic firing in MSAs. To explain how ectopic firing in MSAs might cause pain, we review evidence supporting the hypothesis that cross-talk between proprioceptive and nociceptive pathways might occur in the periphery, within the spindle capsule.

Keratinocyte Piezo1 drives paclitaxel-induced mechanical hypersensitivity

Recent work demonstrates that epidermal keratinocytes are critical for normal touch sensation. However, it is unknown if keratinocytes contribute to touch evoked pain and hypersensitivity following tissue injury. Here, we used inhibitory optogenetic and chemogenetic techniques to determine the extent to which keratinocyte activity contributes to the severe neuropathic pain that accompanies chemotherapeutic treatment. We found that keratinocyte inhibition largely alleviates paclitaxel-induced mechanical hypersensitivity. Furthermore, we found that paclitaxel exposure sensitizes mouse and human keratinocytes to mechanical stimulation through the keratinocyte mechanotransducer Piezo1. These findings demonstrate the contribution of non-neuronal cutaneous cells to neuropathic pain and pave the way for the development of new pain-relief strategies that target epidermal keratinocytes and Piezo1.

Maintaining and Restoring Gradients of Ions in the Epidermis: The Role of Ion and Water Channels in Acute Cutaneous Wound Healing

Significance: Aquaporins and ion channels establish and regulate gradients of calcium, sodium, potassium, chloride, water, and protons in the epidermis. These elements have been found to play significant roles in skin biology and wound healing. In this study, we review our understanding of these channels and ion gradients, with a special emphasis on their role in acute wound healing. Recent Advances: Specifically, we assess the temporal and spatial arrangements of ions and their respective channels in the intact skin and during wound and healing to provide a novel perspective of the role of ionic gradients through the various stages of wound healing. Critical Issues: The roles of gradients of ions and channels in wound healing are currently not well understood. A collective analysis of their traits and arrangements in the skin during wound healing may provide a new perspective and understanding of the functionality of gradients of ions and channels in skin biology and wound healing. Future Directions: It is important to elucidate how the gradients of ions and ion channels regulate and facilitate wound healing. A better understanding of the ionic environments may identify novel therapeutic targets and improved strategies to promote wound healing and possibly treat other cutaneous diseases.

Increased expression of Nav1.6 of reactive astrocytes in the globus pallidus is closely associated with motor deficits in a model of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disease in elderly people, which is characterized by motor disabilities in PD patients. Nav1.6 is the most abundant subtype of voltage-gated sodium channels (VGSCs) in the brain of adult mammals and rodents. Here we investigated the role of Nav1.6 in the external globus pallidus (GP) involved in the pathogenesis of motor deficits in unilateral 6-OHDA(6-hydroxydopamine)lesioned rats. The results show that Nav1.6 is dramatically increased in reactive astrocytes of the ipsilateral GP in the middle stage, but not different from the control rats in the later stage of the pathological process in 6-OHDA lesioned rats. Furthermore, the down-regulation of Nav1.6 expression in the ipsilateral GP can significantly improve motor deficits in 6-OHDA lesioned rats in the middle stage of the pathological process. The electrophysiological experiments show that the down-regulation of Nav1.6 expression in the ipsilateral GP significantly decreases the abnormal high synchronization between the ipsilateral M1 (the primary motor cortex) and GP in 6-OHDA lesioned rats. Ca2+ imaging reveals that the down-regulation of Nav1.6 expression reduces the intracellular concentration of Ca2+ ([Ca2+ ]i) in primary cultured astrocytes. These findings suggest that the increased Nav1.6 expression of reactive astrocytes in the GP play an important role in the pathogenesis of motor dysfunction in the middle stage in 6-OHDA lesioned rats, which may participate in astrocyte-neuron communication by regulating [Ca2+ ]i of astrocytes, thereby contributing to the formation of abnormal electrical signals of the basal ganglia (BG) in 6-OHDA lesioned rats.

Cutaneous nerve fiber and peripheral Nav1.7 assessment in a large cohort of patients with postherpetic neuralgia

ABSTRACT

The mechanisms of pain in postherpetic neuralgia (PHN) are still unclear, with some studies showing loss of cutaneous sensory nerve fibers that seemed to correlate with pain level. We report results of skin biopsies and correlations with baseline pain scores, mechanical hyperalgesia, and the Neuropathic Pain Symptom Inventory (NPSI) in 294 patients who participated in a clinical trial of TV-45070, a topical semiselective sodium 1.7 channel (Nav1.7) blocker. Intraepidermal nerve fibers and subepidermal Nav1.7 immunolabeled fibers were quantified in skin punch biopsies from the area of maximal PHN pain, as well as from the contralateral, homologous (mirror image) region. Across the entire study population, a 20% reduction in nerve fibers on the PHN-affected side compared with that in the contralateral side was noted; however, the reduction was much higher in older individuals, approaching 40% in those aged 70 years or older. There was a decrease in contralateral fiber counts as well, also noted in prior biopsy studies, the mechanism of which is not fully clear. Nav1.7-positive immunolabeling was present in approximately one-third of subepidermal nerve fibers and did not differ on the PHN-affected vs contralateral sides. Using cluster analysis, 2 groups could be identified, with the first cluster showing higher baseline pain, higher NPSI scores for squeezing and cold-induced pain, higher nerve fiber density, and higher Nav1.7 expression. While Nav1.7 varies from patient to patient, it does not seem to be a key pathophysiological driver of PHN pain. Individual differences in Nav1.7 expression, however, may determine the intensity and sensory aspects of pain.

Dendrite intercalation between epidermal cells tunes nociceptor sensitivity to mechanical stimuli in Drosophila larvae

An animal's skin provides a first point of contact with the sensory environment, including noxious cues that elicit protective behavioral responses. Nociceptive somatosensory neurons densely innervate and intimately interact with epidermal cells to receive these cues, however the mechanisms by which epidermal interactions shape processing of noxious inputs is still poorly understood. Here, we identify a role for dendrite intercalation between epidermal cells in tuning sensitivity of Drosophila larvae to noxious mechanical stimuli. In wild-type larvae, dendrites of nociceptive class IV da neurons intercalate between epidermal cells at apodemes, which function as body wall muscle attachment sites, but not at other sites in the epidermis. From a genetic screen we identified miR-14 as a regulator of dendrite positioning in the epidermis: miR-14 is expressed broadly in the epidermis but not in apodemes, and miR-14 inactivation leads to excessive apical dendrite intercalation between epidermal cells. We found that miR-14 regulates expression and distribution of the epidermal Innexins ogre and Inx2 and that these epidermal gap junction proteins restrict epidermal dendrite intercalation. Finally, we found that altering the extent of epidermal dendrite intercalation had corresponding effects on nociception: increasing epidermal intercalation sensitized larvae to noxious mechanical inputs and increased mechanically evoked calcium responses in nociceptive neurons, whereas reducing epidermal dendrite intercalation had the opposite effects. Altogether, these studies identify epidermal dendrite intercalation as a mechanism for mechanical coupling of nociceptive neurons to the epidermis, with nociceptive sensitivity tuned by the extent of intercalation.

Integrative miRNA-mRNA profiling of human epidermis: unique signature of SCN9A painful neuropathy

Personalized management of neuropathic pain is an unmet clinical need due to heterogeneity of the underlying aetiologies, incompletely understood pathophysiological mechanisms and limited efficacy of existing treatments. Recent studies on microRNA in pain preclinical models have begun to yield insights into pain-related mechanisms, identifying nociception-related species differences and pinpointing potential drug candidates. With the aim of bridging the translational gap towards the clinic, we generated a human pain-related integrative miRNA and mRNA molecular profile of the epidermis, the tissue hosting small nerve fibres, in a deeply phenotyped cohort of patients with sodium channel-related painful neuropathy not responding to currently available therapies. We identified four miRNAs strongly discriminating patients from healthy individuals, confirming their effect on differentially expressed gene targets driving peripheral sensory transduction, transmission, modulation and post-transcriptional modifications, with strong effects on gene targets including NEDD4. We identified a complex epidermal miRNA-mRNA network based on tissue-specific experimental data suggesting a cross-talk between epidermal cells and axons in neuropathy pain. Using immunofluorescence assay and confocal microscopy, we observed that Nav1.7 signal intensity in keratinocytes strongly inversely correlated with NEDD4 expression that was downregulated by miR-30 family, suggesting post-transcriptional fine tuning of pain-related protein expression. Our targeted molecular profiling advances the understanding of specific neuropathic pain fine signatures and may accelerate process towards personalized medicine in patients with neuropathic pain.

Phenytoin Decreases Pain-like Behaviors and Improves Opioid Analgesia in a Rat Model of Neuropathic Pain

Neuropathic pain remains a clinical challenge due to its complex and not yet fully understood pathomechanism, which result in limited analgesic effectiveness of the management offered, particularly for patients with acute, refractory neuropathic pain states. In addition to the introduction of several modern therapeutic approaches, such as neuromodulation or novel anti-neuropathic drugs, significant efforts have been made in the repurposing of well-known substances such as phenytoin. Although its main mechanism of action occurs at sodium channels in excitable and non-excitable cells and is well documented, how the drug affects the disturbed neuropathic interactions at the spinal cord level and how it influences morphine-induced analgesia have not been clarified, both being crucial from a clinical perspective. We demonstrated that single and repeated systemic administrations of phenytoin decreased tactile and thermal hypersensitivity in an animal model of neuropathic pain. Importantly, we observed an increase in the antinociceptive effect on thermal stimuli with repeated administrations of phenytoin. This is the first study to report that phenytoin improves morphine-induced antinociceptive effects and influences microglia/macrophage activity at the spinal cord and dorsal root ganglion levels in a neuropathic pain model. Our findings support the hypothesis that phenytoin may represent an effective strategy for neuropathic pain management in clinical practice, particularly when combination with opioids is needed.

N-type calcium channel blockers: a new approach towards the treatment of chronic neuropathic pain

Neuropathic pain (NP) remains maltreated for a wide number of patients by the currently available treatments and little research has been done in finding new drugs for treating NP. Ziconotide (PrialtTM) had been developed as the new drug, which belongs to the class of ω-conotoxin MVIIA. It inhibits N-type calcium channels. Ziconotide is under the last phase of the clinical trial, a new non-narcotic drug for the management of NP. Synthetically it has shown the similarities with ω-conotoxin MVIIA, a constituent of poison found in fish hunting snails (Conus magus). Ziconotide acts by selectively blocking neural N-type voltage-sensitized Ca2+ channels (NVSCCs). Certain herbal drugs also have been studied but no clinical result is there and the study is only limited to preclinical data. This review emphasizes the N-type calcium channel inhibitors, and their mechanisms for blocking calcium channels with their remedial prospects for treating chronic NP.

A mechanistic understanding of the relationship between skin innervation and chemotherapy-induced neuropathic pain

Neuropathic pain is a frequent complication of chemotherapy-induced peripheral neurotoxicity (CIPN). Chemotherapy-induced peripheral neuropathies may serve as a model to study mechanisms of neuropathic pain, since several other common causes of peripheral neuropathy like painful diabetic neuropathy may be due to both neuropathic and non-neuropathic pain mechanisms like ischemia and inflammation. Experimental studies are ideally suited to study changes in morphology, phenotype and electrophysiologic characteristics of primary afferent neurons that are affected by chemotherapy and to correlate these changes to behaviors reflective of evoked pain, mainly hyperalgesia and allodynia. However, hyperalgesia and allodynia may only represent one aspect of human pain, i.e., the sensory-discriminative component, while patients with CIPN often describe their pain using words like annoying, tiring and dreadful, which are affective-emotional descriptors that cannot be tested in experimental animals. To understand why some patients with CIPN develop neuropathic pain and others not, and which are the components of neuropathic pain that they are experiencing, experimental and clinical pain research should be combined. Emerging evidence suggests that changes in subsets of primary afferent nerve fibers may contribute to specific aspects of neuropathic pain in both preclinical models and in patients with CIPN. In addition, the role of cutaneous neuroimmune interactions is considered. Since obtaining dorsal root ganglia and peripheral nerves in patients is problematic, analyses performed on skin biopsies from preclinical models as well as patients provide an opportunity to study changes in primary afferent nerve fibers and to associate these changes to human pain. In addition, other biomarkers of small fiber damage in CIPN, like corneal confocal microscope and quantitative sensory testing, may be considered.

Enriched enrollment randomized double-blind placebo-controlled cross-over trial with phenytoin cream in painful chronic idiopathic axonal polyneuropathy (EPHENE): a study protocol

Background

Patients with chronic idiopathic axonal polyneuropathy (CIAP) can have neuropathic pain that significantly impacts quality of life. Oral neuropathic pain medication often has insufficient pain relief and side effects. Topical phenytoin cream could circumvent these limitations.

The primary objectives of this trial are to evaluate (1) efficacy in pain reduction and (2) safety of phenytoin cream in patients with painful CIAP. The main secondary objective is to explore the usefulness of a double-blind placebo-controlled response test (DOBRET) to identify responders to sustained pain relief with phenytoin cream.

Methods

This 6-week, enriched enrollment randomized double-blind, placebo-controlled triple cross-over trial compares phenytoin 20%, 10% and placebo cream in 48 participants with painful CIAP. Enriched enrollment is based on a positive DOBRET in 48 participants who experience within 30 minutes ≥2 points pain reduction on the 11-point numerical rating scale (NRS) in the phenytoin 10% cream applied area and ≥1 point difference in pain reduction on the NRS between phenytoin 10% and placebo cream applied area, in favour of the former. To explore whether DOBRET has predictive value for sustained pain relief, 24 DOBRET-negative participants will be included.

An open-label extension phase is offered with phenytoin 20% cream for up to one year, to study long-term safety.

The main inclusion criteria are a diagnosis of CIAP and symmetrical neuropathic pain with a mean weekly pain score of ≥4 and <10 on the NRS.

The primary outcome is the mean difference between phenytoin 20% versus placebo cream in 7-day average pain intensity, as measured by the NRS, over week 2 in DOBRET positive participants.

Key secondary outcomes include the mean difference in pain intensity between phenytoin 10% and phenytoin 20% cream, and between phenytoin 10% and placebo cream. Furthermore, differences between the 3 interventions will be evaluated on the Neuropathic Pain Symptom Inventory, EuroQol EQ5-5D-5L, and evaluation of adverse events.

Discussion

This study will provide evidence on the efficacy and safety of phenytoin cream in patients with painful CIAP and will give insight into the usefulness of DOBRET as a way of personalized medicine to identify responders to sustained pain relief with phenytoin cream.

Trial registration

ClinicalTrials.gov NCT04647877. Registered on 1 December 2020.

Emerging roles of keratinocytes in nociceptive transduction and regulation

Keratinocytes are the predominant block-building cells in the epidermis. Emerging evidence has elucidated the roles of keratinocytes in a wide range of pathophysiological processes including cutaneous nociception, pruritus, and inflammation. Intraepidermal free nerve endings are entirely enwrapped within the gutters of keratinocyte cytoplasm and form en passant synaptic-like contacts with keratinocytes. Keratinocytes can detect thermal, mechanical, and chemical stimuli through transient receptor potential ion channels and other sensory receptors. The activated keratinocytes elicit calcium influx and release ATP, which binds to P2 receptors on free nerve endings and excites sensory neurons. This process is modulated by the endogenous opioid system and endothelin. Keratinocytes also express neurotransmitter receptors of adrenaline, acetylcholine, glutamate, and γ-aminobutyric acid, which are involved in regulating the activation and migration, of keratinocytes. Furthermore, keratinocytes serve as both sources and targets of neurotrophic factors, pro-inflammatory cytokines, and neuropeptides. The autocrine and/or paracrine mechanisms of these mediators create a bidirectional feedback loop that amplifies neuroinflammation and contributes to peripheral sensitization.

Keratinocyte PIEZO1 modulates cutaneous mechanosensation

Epidermal keratinocytes mediate touch sensation by detecting and encoding tactile information to sensory neurons. However, the specific mechanotransducers that enable keratinocytes to respond to mechanical stimulation are unknown. Here, we found that the mechanically-gated ion channel PIEZO1 is a key keratinocyte mechanotransducer. Keratinocyte expression of PIEZO1 is critical for normal sensory afferent firing and behavioral responses to mechanical stimuli in mice.

Nav1.8 in keratinocytes contributes to ROS-mediated inflammation in inflammatory skin diseases

Reactive oxygen species (ROS)-activated proinflammatory signals in keratinocytes play a crucial role in the immunoregulation of inflammatory skin diseases, including rosacea and psoriasis. Nav1.8 is a voltage-gated sodium ion channel, and its abnormal expression in the epidermal layer contributes to pain hypersensitivity in the skin. However, whether and how epidermal Nav1.8 is involved in skin immunoregulation remains unclear. This study was performed to identify the therapeutic role of Nav1.8 in inflammatory skin disorders. We found that Nav1.8 expression was significantly upregulated in the epidermis of rosacea and psoriasis skin lesions. Nav1.8 knockdown ameliorated skin inflammation in LL37-and imiquimod-induced inflammation mouse models. Transcriptome sequencing results indicated that Nav1.8 regulated the expression of pro-inflammatory mediators (IL1β and IL6) in keratinocytes, thereby contributing to immune infiltration in inflammatory skin disorders. In vitro, tumor necrosis factor alpha (TNFα), a cytokine that drives the development of various inflammatory skin disorders, increased Nav1.8 expression in keratinocytes. Knockdown of Nav1.8 eliminated excess ROS production, thereby attenuating the TNFα-induced production of inflammatory mediators; however, a Nav1.8 blocker did not have the same effect. Mechanistically, Nav1.8 reduced superoxide dismutase 2 (SOD2) activity by directly binding to SOD2 to prevent its deacetylation and mitochondrial localization, subsequently inducing ROS accumulation. Collectively, our study describes a central role for Nav1.8 in regulating pro-inflammatory responses in the skin and indicates a novel therapeutic strategy for rosacea and psoriasis.

Peripheral mechanisms of chronic pain

Acutely, pain serves to protect us from potentially harmful stimuli, however damage to the somatosensory system can cause maladaptive changes in neurons leading to chronic pain. Although acute pain is fairly well controlled, chronic pain remains difficult to treat. Chronic pain is primarily a neuropathic condition, but studies examining the mechanisms underlying chronic pain are now looking beyond afferent nerve lesions and exploring new receptor targets, immune cells, and the role of the autonomic nervous system in contributing chronic pain conditions. The studies outlined in this review reveal how chronic pain is not only confined to alterations in the nervous system and presents findings on new treatment targets and for this debilitating disease.

A peripheral CB2 cannabinoid receptor mechanism suppresses chemotherapy-induced peripheral neuropathy: evidence from a CB2 reporter mouse

ABSTRACT

CB2 cannabinoid receptors (CB2) are a promising therapeutic target that lacks unwanted side effects of CB1 activation. However, the cell types expressing CB2 that mediate these effects remain poorly understood. We used transgenic mice with CB2 promoter-driven expression of enhanced green fluorescent protein (EGFP) to study cell types that express CB2 and suppress neuropathic nociception in a mouse model of chemotherapy-induced peripheral neuropathy. Structurally distinct CB2 agonists (AM1710 and LY2828360) suppressed paclitaxel-induced mechanical and cold allodynia in CB2EGFP reporter mice with established neuropathy. Antiallodynic effects of AM1710 were blocked by SR144528, a CB2 antagonist with limited CNS penetration. Intraplantar AM1710 administration suppressed paclitaxel-induced neuropathic nociception in CB2EGFP but not CB2 knockout mice, consistent with a local site of antiallodynic action. mRNA expression levels of the anti-inflammatory cytokine interleukin-10 were elevated in the lumbar spinal cord after intraplantar AM1710 injection along with the proinflammatory cytokine tumor necrosis factor alpha and chemokine monocyte chemoattractant protein-1. CB2EGFP, but not wildtype mice, exhibited anti-GFP immunoreactivity in the spleen. However, the anti-GFP signal was below the threshold for detection in the spinal cord and brain of either vehicle-treated or paclitaxel-treated CB2EGFP mice. EGFP fluorescence was coexpressed with CB2 immunolabeling in stratified patterns among epidermal keratinocytes. EGFP fluorescence was also expressed in dendritic cells in the dermis, Langerhans cells in the epidermis, and Merkel cells. Quantification of the EGFP signal revealed that Langerhans cells were dynamically increased in the epidermis after paclitaxel treatment. Our studies implicate CB2 expressed in previously unrecognized populations of skin cells as a potential target for suppressing chemotherapy-induced neuropathic nociception.

Functional Voltage-Gated Sodium Channels Are Present in the Human B Cell Membrane

B cells express various ion channels, but the presence of voltage-gated sodium (NaV) channels has not been confirmed in the plasma membrane yet. In this study, we have identified several NaV channels, which are expressed in the human B cell membrane, by electrophysiological and molecular biology methods. The sensitivity of the detected sodium current to tetrodotoxin was between the values published for TTX-sensitive and TTX-insensitive channels, which suggests the co-existence of multiple NaV1 subtypes in the B cell membrane. This was confirmed by RT-qPCR results, which showed high expression of TTX-sensitive channels along with the lower expression of TTX-insensitive NaV1 channels. The biophysical characteristics of the currents also supported the expression of multiple NaV channels. In addition, we investigated the potential functional role of NaV channels by membrane potential measurements. Removal of Na+ from the extracellular solution caused a reversible hyperpolarization, supporting the role of NaV channels in shaping and maintaining the resting membrane potential. As this study was mainly limited to electrophysiological properties, we cannot exclude the possible non-canonical functions of these channels. This work concludes that the presence of voltage-gated sodium channels in the plasma membrane of human B cells should be recognized and accounted for in the future.

No Detectable Phenytoin Plasma Levels After Topical Phenytoin Cream Application in Chronic Pain: Inferences for Mechanisms of Action

Purpose

Topical phenytoin can act as an analgesic in chronic pain, but it is unclear if topical phenytoin gives rise to systemic side effects. Therefore, the aim of this study is: 1) to evaluate safety in chronic pain patients who used topical phenytoin up to 30% applied daily on intact skin and mucous membrane, through determining phenytoin plasma levels; and 2) to elaborate on the analgesic mechanism of action.

Patients and Methods

In this retrospective study, we collected demographic and clinical data from 33 chronic pain patients who used 10% to 30% phenytoin cream, and in whom blood samples were drawn for phenytoin concentration measurement between January 2017 until September 2020. The instruction was to withdraw blood 1 to 4 hours after the last topical phenytoin application. The primary outcome was the detectability of plasma phenytoin after daily use of topical phenytoin.

Results

Blood withdrawal was carried out after on average 14 treatment days with topical phenytoin and on average 2.5 hours after topical phenytoin application. The median daily applied amount of phenytoin cream was 1.2 grams, resulting in a median daily amount of 120 mg phenytoin on the skin. Phenytoin levels were below the limit of detection in all patients and no side effects were reported.

Conclusion

Plasma phenytoin levels were below the limit of detection after topical use of phenytoin cream formulations up to 30% on intact skin and mucous membrane for the treatment of chronic pain, without side effects emerging. This finding suggests that the mechanism of analgesic action resides in the skin.

Mechanical stimulus-evoked signal transduction between keratinocytes and sensory neurons via extracellular ATP

The skin is exposed to various external stimuli. Keratinocytes, which are the main cell type in the epidermis, interact with peripheral sensory neurons and modulate neuronal activity. Recent studies have revealed that keratinocytes play crucial roles in nociception, and that ATP is one of the main mediators of signal transduction from keratinocytes to sensory neurons. However, no quantitative cellular level analyses of ATP-mediated information flow from keratinocytes to sensory dorsal root ganglion (DRG) neurons have been conducted. In this study, we performed simultaneous imaging of cell surface ATP and intracellular Ca²⁺ signals using both iATPSnFR, a genetically encoded ATP probe localized to the outside of the cell membrane, and the Ca²⁺ probe, Fura-red. Upon mechanical stimulation of the keratinocyte with a glass needle, an increase in Ca²⁺ and ATP release were observed around the stimulated area, and these phenomena were positively correlated. In cultured DRG neurons and keratinocytes neighboring the stimulated keratinocyte, increased intracellular Ca²⁺ concentration and levels of cell surface ATP on the side closer to the stimulated cell were detected. The ratio of Ca²⁺ response to input ATP signal was significantly larger in DRG neurons than in keratinocytes. We found that DRG neurons were more sensitive to ATP than keratinocytes, and therefore, only DRG neurons responded to ATP at 1 μM or lower concentrations when in co-culture with keratinocytes. Moreover, signals caused by moderate mechanical stimulation of keratinocytes were transmitted predominantly to DRG neurons. These findings would be important in the further determination of the detailed mechanism of nociception in the epidermis.

Keratinocyte Biomarkers Distinguish Painful Diabetic Peripheral Neuropathy Patients and Correlate With Topical Lidocaine Responsiveness

This study investigated quantifiable measures of cutaneous innervation and algesic keratinocyte biomarkers to determine correlations with clinical measures of patient pain perception, with the intent to better discriminate between diabetic patients with painful diabetic peripheral neuropathy (PDPN) compared to patients with low-pain diabetic peripheral neuropathy (lpDPN) or healthy control subjects. A secondary objective was to determine if topical treatment with a 5% lidocaine patch resulted in correlative changes among the quantifiable biomarkers and clinical measures of pain perception, indicative of potential PDPN pain relief. This open-label proof-of-principle clinical research study consisted of a pre-treatment skin biopsy, a 4-week topical 5% lidocaine patch treatment regimen for all patients and controls, and a post-treatment skin biopsy. Clinical measures of pain and functional interference were used to monitor patient symptoms and response for correlation with quantitative skin biopsy biomarkers of innervation (PGP9.5 and CGRP), and epidermal keratinocyte biomarkers (Nav1.6, Nav1.7, CGRP). Importantly, comparable significant losses of epidermal neural innervation (intraepidermal nerve fibers; IENF) and dermal innervation were observed among PDPN and lpDPN patients compared with control subjects, indicating that innervation loss alone may not be the driver of pain in diabetic neuropathy. In pre-treatment biopsies, keratinocyte Nav1.6, Nav1.7, and CGRP immunolabeling were all significantly increased among PDPN patients compared with control subjects. Importantly, no keratinocyte biomarkers were significantly increased among the lpDPN group compared with control. In post-treatment biopsies, the keratinocyte Nav1.6, Nav1.7, and CGRP immunolabeling intensities were no longer different between control, lpDPN, or PDPN cohorts, indicating that lidocaine treatment modified the PDPN-related keratinocyte increases. Analysis of the PDPN responder population demonstrated that increased pretreatment keratinocyte biomarker immunolabeling for Nav1.6, Nav1.7, and CGRP correlated with positive outcomes to topical lidocaine treatment. Epidermal keratinocytes modulate the signaling of IENF, and several analgesic and algesic signaling systems have been identified. These results further implicate epidermal signaling mechanisms as modulators of neuropathic pain conditions, highlight a novel potential mode of action for topical treatments, and demonstrate the utility of comprehensive skin biopsy evaluation to identify novel biomarkers in clinical pain studies.

Mechanisms of ATP release in pain: role of pannexin and connexin channels

Pain is a physiological response to bodily damage and serves as a warning of potential threat. Pain can also transform from an acute response to noxious stimuli to a chronic condition with notable emotional and psychological components that requires treatment. Indeed, the management of chronic pain is currently an important unmet societal need. Several reports have implicated the release of the neurotransmitter adenosine triphosphate (ATP) and subsequent activation of purinergic receptors in distinct pain etiologies. Purinergic receptors are broadly expressed in peripheral neurons and the spinal cord; thus, purinergic signaling in sensory neurons or in spinal circuits may be critical for pain processing. Nevertheless, an outstanding question remains: what are the mechanisms of ATP release that initiate nociceptive signaling? Connexin and pannexin channels are established conduits of ATP release and have been suggested to play important roles in a variety of pathologies, including several models of pain. As such, these large-pore channels represent a new and exciting putative pharmacological target for pain treatment. Herein, we will review the current evidence for a role of connexin and pannexin channels in ATP release during nociceptive signaling, such as neuropathic and inflammatory pain. Collectively, these studies provide compelling evidence for an important role of connexins and pannexins in pain processing.

Cutaneous pain in disorders affecting peripheral nerves

Our ability to quickly detect and respond to harmful environmental stimuli is vital for our safety and survival. This inherent acute pain detection is a "gift" because it both protects our body from harm and allows healing of damaged tissues [1]. Damage to tissues from trauma or disease can result in distorted or amplified nociceptor signaling and sensitization of the spinal cord and brain (Central Nervous System; CNS) pathways to normal input from light touch mechanoreceptors. Together, these processes can result in nagging to unbearable chronic pain and extreme sensitivity to light skin touch (allodynia). Unlike acute protective pain, chronic pain and allodynia serve no useful purpose and can severely reduce the quality of life of an affected person. Chronic pain can arise from impairment to peripheral neurons, a phenomenon called "peripheral neuropathic pain." Peripheral neuropathic pain can be caused by many insults that directly affect peripheral sensory neurons, including mechanical trauma, metabolic imbalance (e.g., diabetes), autoimmune diseases, chemotherapeutic agents, viral infections (e.g., shingles). These insults cause "acquired" neuropathies such as small-fiber neuropathies, diabetic neuropathy, chemotherapy-induced peripheral neuropathy, and post herpetic neuralgia. Peripheral neuropathic pain can also be caused by genetic factors and result in hereditary neuropathies that include Charcot-Marie-Tooth disease, rare channelopathies and Fabry disease. Many acquired and hereditary neuropathies affect the skin, our largest organ and protector of nearly our entire body. Here we review how cutaneous nociception (pain perceived from the skin) is altered following diseases that affect peripheral nerves that innervate the skin. We provide an overview of how noxious stimuli are detected and encoded by molecular transducers on subtypes of cutaneous afferent endings and conveyed to the CNS. Next, we discuss several acquired and hereditary diseases and disorders that cause painful or insensate (lack of sensation) cutaneous peripheral neuropathies, the symptoms and percepts patients experience, and how cutaneous afferents and other peripheral cell types are altered in function in these disorders. We highlight exciting new research areas that implicate non-neuronal skin cells, particularly keratinocytes, in cutaneous nociception and peripheral neuropathies. Finally, we conclude with ideas for innovative new directions, areas of unmet need, and potential opportunities for novel cutaneous therapeutics that may avoid CNS side effects, as well as ideas for improved translation of mechanisms identified in preclinical models to patients.

Fibromyalgia vs small fiber neuropathy: diverse keratinocyte transcriptome signature

ABSTRACT

Damage to thinly myelinated and unmyelinated nerve fibers causes small fiber pathology, which is increasingly found in pain syndromes such as small fiber neuropathy (SFN) and fibromyalgia syndrome (FMS). The peripheral nerve endings of the small nerve fibers terminate within the epidermis, where they are surrounded by keratinocytes that may act as primary nociceptive transducers. We performed RNA sequencing of keratinocytes obtained from patients with SFN, FMS, and healthy controls. We found 141 deregulated protein coding genes between SFN patients and healthy controls and no differentially expressed genes between patients with FMS and healthy controls. When comparing patients with SFN with patients with FMS, we detected 167 differentially expressed protein coding genes (129 upregulated and 38 downregulated). Further analysis revealed enriched inflammatory pathways. Validation of selected candidates in an independent cohort confirmed higher expression of the proinflammatory mediators interleukin-8, C-X-C motif chemokine 3, endothelin receptor type A, and the voltage-gated sodium channel 1.7 in SFN compared with patients with FMS. We provide a diverse keratinocyte transcriptome signature between patients with SFN and patients with FMS, which may hint toward distinct pathomechanisms of small fiber sensitization in both entities and lay the basis for advanced diagnostics.

Persistent proliferation of keratinocytes and prolonged expression of pronociceptive inflammatory mediators might be associated with the postoperative pain in KK mice

Epidermal keratinocytes play a vital role in restoration of the intact skin barrier during wound healing. The negative effect of hyperglycemia may prolong the wound healing process. Epidermal keratinocytes have been demonstrated to modulate and directly initiate nociceptive responses in rat models of fractures and chemotherapy-induced neuropathic pain. However, it is unclear whether epidermal keratinocytes are involved in the development and maintenance of incisional pain in nondiabetic or diabetic animals. In the current study, using behavioral tests and immunohistochemistry, we investigated the differential keratinocytes proliferation and expression of pronociceptive inflammatory mediators in keratinocytes in C57BL/6J mice and diabetic KK mice. Our data showed that plantar incision induced postoperative pain hypersensitivity in both C57BL/6J mice and KK mice, while the duration of postoperative pain hypersensitivity in KK mice was longer than that in C57BL/6J mice. Moreover, plantar incision induced the keratinocytes proliferation and expression of IL-1β and TNF-α in keratinocytes in both C57BL/6J mice and KK mice. Interestingly, compared to C57BL/6J mice, the slower and more persistent proliferation of keratinocytes and expression of IL-1β and TNF-α in keratinocytes were observed in KK mice. Together, our study suggested that plantar incision may induce the differential keratinocytes proliferation and expression of IL-1β and TNF-α in kertinocytes in diabetic and nondiabetic animals, which might be associated with the development and maintenance differences in diabetic and nondiabetic postoperative pain.

Topical Treatments and Their Molecular/Cellular Mechanisms in Patients with Peripheral Neuropathic Pain-Narrative Review

Neuropathic pain in humans results from an injury or disease of the somatosensory nervous system at the peripheral or central level. Despite the considerable progress in pain management methods made to date, peripheral neuropathic pain significantly impacts patients' quality of life, as pharmacological and non-pharmacological methods often fail or induce side effects. Topical treatments are gaining popularity in the management of peripheral neuropathic pain, due to excellent safety profiles and preferences. Moreover, topical treatments applied locally may target the underlying mechanisms of peripheral sensitization and pain. Recent studies showed that peripheral sensitization results from interactions between neuronal and non-neuronal cells, with numerous signaling molecules and molecular/cellular targets involved. This narrative review discusses the molecular/cellular mechanisms of drugs available in topical formulations utilized in clinical practice and their effectiveness in clinical studies in patients with peripheral neuropathic pain. We searched PubMed for papers published from 1 January 1995 to 30 November 2020. The key search phrases for identifying potentially relevant articles were "topical AND pain", "topical AND neuropathic", "topical AND treatment", "topical AND mechanism", "peripheral neuropathic", and "mechanism". The result of our search was 23 randomized controlled trials (RCT), 9 open-label studies, 16 retrospective studies, 20 case (series) reports, 8 systematic reviews, 66 narrative reviews, and 140 experimental studies. The data from preclinical studies revealed that active compounds of topical treatments exert multiple mechanisms of action, directly or indirectly modulating ion channels, receptors, proteins, and enzymes expressed by neuronal and non-neuronal cells, and thus contributing to antinociception. However, which mechanisms and the extent to which the mechanisms contribute to pain relief observed in humans remain unclear. The evidence from RCTs and reviews supports 5% lidocaine patches, 8% capsaicin patches, and botulinum toxin A injections as effective treatments in patients with peripheral neuropathic pain. In turn, single RCTs support evidence of doxepin, funapide, diclofenac, baclofen, clonidine, loperamide, and cannabidiol in neuropathic pain states. Topical administration of phenytoin, ambroxol, and prazosin is supported by observational clinical studies. For topical amitriptyline, menthol, and gabapentin, evidence comes from case reports and case series. For topical ketamine and baclofen, data supporting their effectiveness are provided by both single RCTs and case series. The discussed data from clinical studies and observations support the usefulness of topical treatments in neuropathic pain management. This review may help clinicians in making decisions regarding whether and which topical treatment may be a beneficial option, particularly in frail patients not tolerating systemic pharmacotherapy.

Peripheral Mechanisms of Neuropathic Pain-the Role of Neuronal and Non-Neuronal Interactions and Their Implications for Topical Treatment of Neuropathic Pain

Neuropathic pain in humans arises as a consequence of injury or disease of somatosensory nervous system at peripheral or central level. Peripheral neuropathic pain is more common than central neuropathic pain, and is supposed to result from peripheral mechanisms, following nerve injury. The animal models of neuropathic pain show extensive functional and structural changes occurring in neuronal and non-neuronal cells in response to peripheral nerve injury. These pathological changes following damage lead to peripheral sensitization development, and subsequently to central sensitization initiation with spinal and supraspinal mechanism involved. The aim of this narrative review paper is to discuss the mechanisms engaged in peripheral neuropathic pain generation and maintenance, with special focus on the role of glial, immune, and epithelial cells in peripheral nociception. Based on the preclinical and clinical studies, interactions between neuronal and non-neuronal cells have been described, pointing out at the molecular/cellular underlying mechanisms of neuropathic pain, which might be potentially targeted by topical treatments in clinical practice. The modulation of the complex neuro-immuno-cutaneous interactions in the periphery represents a strategy for the development of new topical analgesics and their utilization in clinical settings.

Neurological Disturbances of Ciguatera Poisoning: Clinical Features and Pathophysiological Basis

Ciguatera fish poisoning (CFP), the most prevalent seafood poisoning worldwide, is caused by the consumption of tropical and subtropical fish contaminated with potent neurotoxins called ciguatoxins (CTXs). Ciguatera is a complex clinical syndrome in which peripheral neurological signs predominate in the acute phase of the intoxication but also persist or reoccur long afterward. Their recognition is of particular importance in establishing the diagnosis, which is clinically-based and can be a challenge for physicians unfamiliar with CFP. To date, no specific treatment exists. Physiopathologically, the primary targets of CTXs are well identified, as are the secondary events that may contribute to CFP symptomatology. This review describes the clinical features, focusing on the sensory disturbances, and then reports on the neuronal targets and effects of CTXs, as well as the neurophysiological and histological studies that have contributed to existing knowledge of CFP neuropathophysiology at the molecular, neurocellular and nerve levels.

PAR2, Keratinocytes, and Cathepsin S Mediate the Sensory Effects of Ciguatoxins Responsible for Ciguatera Poisoning

Ciguatera fish poisoning is caused by the consumption of fish contaminated with ciguatoxins (CTXs). The most distressing symptoms are cutaneous sensory disturbances, including cold dysesthesia and itch. CTXs are neurotoxins known to activate voltage-gated sodium channels, but no specific treatment exists. Peptidergic neurons have been critically involved in ciguatera fish poisoning sensory disturbances. Protease-activated receptor-2 (PAR2) is an itch- and pain-related G protein‒coupled receptor whose activation leads to a calcium-dependent neuropeptide release. In this study, we studied the role of voltage-gated sodium channels, PAR2, and the PAR2 agonist cathepsin S in the cytosolic calcium increase and subsequent release of the neuropeptide substance P elicited by Pacific CTX-2 (P-CTX-2) in rat sensory neurons and human epidermal keratinocytes. In sensory neurons, the P-CTX-2‒evoked calcium response was driven by voltage-gated sodium channels and PAR2-dependent mechanisms. In keratinocytes, P-CTX-2 also induced voltage-gated sodium channels and PAR2-dependent marked calcium response. In the cocultured cells, P-CTX-2 significantly increased cathepsin S activity, and cathepsin S and PAR2 antagonists almost abolished P-CTX-2‒elicited substance P release. Keratinocytes synergistically favored the induced substance P release. Our results demonstrate that the sensory effects of CTXs involve the cathepsin S-PAR2 pathway and are potentiated by their direct action on nonexcitable keratinocytes through the same pathway.

Topical agents: a thoughtful choice for multimodal analgesia

For over a thousand years, various substances have been applied to the skin to treat pain. Some of these substances have active ingredients that we still use today. However, some have been discontinued due to their harmful effect, while others have been long forgotten. Recent concerns regarding the cardiovascular and renal risk from nonsteroidal anti-inflammatory drugs, and issues with opioids, have resulted in increasing demand and attention to non-systemic topical alternatives. There is increasing evidence of the efficacy and safety of topical agents in pain control. Topical analgesics are great alternatives for pain management and are an essential part of multimodal analgesia. This review aims to describe essential aspects of topical drugs that physicians should consider in their practice as part of multimodal analgesia. This review describes the mechanism of popular topical analgesics and also introduces the most recently released and experimental topical medications.

N-methyl-d-aspartate receptor subunit 2B on keratinocyte mediates peripheral and central sensitization in chronic post-ischemic pain in male rats

The spinal N-methyl-d-aspartate (NMDA) receptor, and particularly its NR2B subunit, plays a pivotal role in neuropathic pain. However, the role of peripheral NMDA receptor in neuropathic pain is less well understood. We first treated cultured human keratinocytes, HaCaT cells with NMDA or NR2B-specific antagonist, ifenprodil and evaluated the level of total and phosphorylated NR2B at 24 h using Western blot. Next, using the chronic post-ischemia pain (CPIP) model, we administered NMDA or ifenprodil subcutaneously into the hind paws of male rats. Nociceptive behaviors were assessed by measuring mechanical and thermal withdrawal thresholds. Expression and phosphorylation of NR2B on keratinocyte were analyzed at 6, 12, 18, and 24 h on day 1 (initiation of pain) as well as day 2, 6, 10 and 14 (development and maintenance of pain) after the ischemia. The level of peripheral sensitization-related proteins (nuclear factor-κB (NF-κB), extracellular regulated protein kinases (ERK), and interleukin-1β (IL-1β)) in epidermis and dorsal root ganglion (DRG) were evaluated by immunofluorescence and western blot. Central sensitization-related C-fos induction, as well as astrocytes and microglia activation in the spinal cord dorsal horn (SDH) were studied using immunofluorescence. Administration of NMDA upregulated NR2B phosphorylation on HaCaT cells. CPIP-induced mechanical allodynia and thermal hyperalgesia were intensified by NMDA and alleviated by ifenprodil. CPIP resulted in an early upregulation of NR2B (peaked at 24 h) and late phosphorylation of NR2B (peaked at 14d) in hindpaw keratinocytes. CPIP led to an upregulation and phosphorylation of NF-κB and ERK, as well as an increased IL-1β production in the ipsilateral skin and DRG. CPIP-associated c-fos induction in SDH persisted from acute to chronic stages after ischemia, while microglia and astrocyte activation were only observed in chronic phase. These CPIP-induced changes were also suppressed by ifenprodil administered subcutaneously in the hind paw. Our findings reveal a previously unrecognized role of keratinocyte NMDA receptor subunit 2B in peripheral and central nociceptive sensitization induced by CPIP.

Role of Transient Receptor Potential Vanilloid 4 Channel in Skin Physiology and Pathology

Transient receptor potential vanilloid 4 (TRPV4) channel responds to temperature, as well as various mechanical and chemical stimuli. This non-selective cation channel is expressed in several organs, including the blood vessels, kidneys, oesophagus and skin. In the skin, TRPV4 channel is present in various cell types such as keratinocytes, melanocytes and sensory neurons, as well as immune and inflammatory cells, and engages in several physiological actions, from skin homeostasis to sensation. In addition, there is substantial evidence implicating dysfunctional TRPV4 channel—in the form of either deficient or excessive channel activity—in pathological cutaneous conditions such as skin barrier compromise, pruritus, pain, skin inflammation and carcinogenesis. These varied functions, combined with the fact that TRPV4 channel owns pharmacologically-accessible sites, make this channel an attractive therapeutic target for skin disorders. In this review, we summarize the different physiological and pathophysiological effects of TRPV4 in the skin.

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.

Lifting the veil on the keratinocyte contribution to cutaneous nociception

Cutaneous nociception is essential to prevent individuals from sustaining injuries. According to the conventional point of view, the responses to noxious stimuli are thought to be exclusively initiated by sensory neurons, whose activity would be at most modulated by keratinocytes. However recent studies have demonstrated that epidermal keratinocytes can also act as primary nociceptive transducers as a supplement to sensory neurons. To enlighten our understanding of cutaneous nociception, this review highlights recent and relevant findings on the cellular and molecular elements that underlie the contribution of epidermal keratinocytes as nociceptive modulators and noxious sensors, both under healthy and pathological conditions.

Phenytoin Cream for the Treatment of Sciatic Pain: Clinical Effects and Theoretical Considerations: Case Report

Chronic sciatic pain is difficult to treat. Patients often suffer from considerable pain and are severely hampered in their everyday activities. Most pharmacologic analgesic treatments have disappointing effects, and often are limited due to adverse events. New treatments are therefore needed. Surprisingly we found fast pain reduction after applying topical phenytoin cream at the painful dermatome in a 55-year-old patient suffering from sciatic pain due to pathology of a disc. This patient was treatment resistant for 13 years. Prescribing topical analgesic cream seemed to us at first sight quite counter-intuitive. The clear response in a treatment-resistant patient however provoked us to look deeper in the pathophysiology of sciatic nerve impingement. Recently it has been documented that proximal nerve lesions are followed by small fiber pathology in the skin. This might be a responsible peripheral wind-up generator for the chronification of pain in sciatic nerve compression. Topical application of the broad-acting voltage-gated sodium channel blocker phenytoin could reduce neuropathic pain in our case completely, supporting a peripheral mechanism of action for phenytoin cream in sciatic pain.

Haplopelma hainanum venom induces inflammatory skin lesions

The Haplopelma hainanum is a species of theraphosid spider from China. Its large size and charming appearance make this species a popular pet. According to a previous study, theraphosid spider bites can induce pain, erythema, and edema in humans and can present more severely in domestic animals. The pathological consequences of envenomation by H. hainanum remain unclear. In this study, we investigated the effects and mechanisms of H. hainanum envenomation in mice. We showed that the venom induced slight swelling, intense inflammatory response, and increased the microvascular density in mice skin. Moreover, we found that 50 µg/ml of the spider’s venom induced IL-1β expression in both HaCaT cells and fibroblast cells, but repressed CXCL10 expression in fibroblasts. The venom significantly induced cell senescence and repressed cell proliferation and migration in both HaCaT cells and fibroblast cells. Finally, we examined the expression of Nav channel in HaCaT and fibroblast cells and found that H. hainanum venom effectively inhibited Na⁺ currents in HaCaT cells. Our study calls for further investigation of the pathological consequences and potential mechanisms of H. hainanum envenomation. This information might assist in the development of suitable therapy.

Diagnostic criteria for small fibre neuropathy in clinical practice and research

The diagnostic criteria for small fibre neuropathy are not established, influencing the approach to patients in clinical practice, their access to disease-modifying and symptomatic treatments, the use of healthcare resources, and the design of clinical trials. To address these issues, we performed a reappraisal study of 150 patients with sensory neuropathy and a prospective and follow-up validation study of 352 new subjects with suspected sensory neuropathy. Small fibre neuropathy diagnostic criteria were based on deep clinical phenotyping, quantitative sensory testing (QST) and intraepidermal nerve fibre density (IENFD). Small fibre neuropathy was ruled out in 5 of 150 patients (3.3%) of the reappraisal study. Small fibre neuropathy was diagnosed at baseline of the validation study in 149 of 352 patients (42.4%) based on the combination between two clinical signs and abnormal QST and IENFD (69.1%), abnormal QST alone (5.4%), or abnormal IENFD alone (20.1%). Eight patients (5.4%) had abnormal QST and IENFD but no clinical signs. Further, 38 patients complained of sensory symptoms but showed no clinical signs. Of those, 34 (89.4%) had normal QST and IENFD, 4 (10.5%) had abnormal QST and normal IENFD, and none had abnormal IENFD alone. At 18-month follow-up, 19 of them (56%) reported the complete recovery of symptoms and showed normal clinical, QST and IENFD findings. None of those with one single abnormal test (QST or IENFD) developed clinical signs or showed abnormal findings on the other test. Conversely, all eight patients with abnormal QST and IENFD at baseline developed clinical signs at follow-up. The combination of clinical signs and abnormal QST and/or IENFD findings can more reliably lead to the diagnosis of small fibre neuropathy than the combination of abnormal QST and IENFD findings in the absence of clinical signs. Sensory symptoms alone should not be considered a reliable screening feature. Our findings demonstrate that the combined clinical, functional and structural approach to the diagnosis of small fibre neuropathy is reliable and relevant both for clinical practice and clinical trial design.