The contribution of TRPM8 and TRPA1 channels to cold allodynia and neuropathic pain

Sida cordifolia L. attenuates behavioral hypersensitivity by interfering with KIF17-NR2B signaling in rat model of neuropathic pain

ETHNOPHARMACOLOGICAL RELEVANCE

Sida cordifolia L., a perennial subshrub belonging to the Malvaceae family, holds noteworthy significance in the Indian Ayurvedic System and global texts. Roots of this plant are reported to be useful in neurodegenerative disorders, facial paralysis, and treating several neuropathic pain conditions such as neuralgia, and sciatica. However, despite these claims, there remains a dearth of experimental evidence showcasing the effectiveness of Sida cordifolia L. roots in mitigating neuropathic pain.

AIM OF THE STUDY

The primary objective of this study was to assess the analgesic properties of the whole extract (SCE) obtained from the roots of Sida cordifolia L., as well as its aqueous fraction (SAF) in rat model of chronic constriction injury (CCI)-induced neuropathic pain. Furthermore, in-depth phytochemical and molecular biology studies were conducted to identify the potential phytoconstituents and unveil the underlying mechanisms of action.

MATERIAL AND METHODS

DCM: Methanol (1:1) was used to extract the roots of Sida cordifolia L. to get whole extract (SCE) and was subjected to phytochemical investigations including LC-MS analysis. Analgesic potential of SCE was evaluated in chronic constriction injury (CCI) model of neuropathic pain in rats followed by its bioactivity guided fractionation using in-vitro anti-inflammatory assay and assessment of most potent fraction (SAF) in in-vivo pain model. We have also performed the detailed phytochemical and molecular biology investigations to delineate the mechanism of action of Sida cordifolia root extract.

RESULTS

Chronic constriction injury leads to significant decrease in paw withdrawal threshold and paw withdrawal latency indicating development of hypersensitivity in rodents. Treatment with SCE and its most potent aqueous fraction (SAF) leads to significant and dose-dependent reduction in pain-like behavior of nerve injured rats. Pro-inflammatory cytokines (TNF-α, IL-1β), glia cell markers (Iba1, ICAM1), neuropeptides (CGRP and Substance P), KIF-17 and NR2B expressions were found to be significantly upregulated in DRG and spinal cord of nerve injured rats. Treatment with SCE and SAF suppressed oxido-inflammatory cascade along with attenuation of KIF-17 mediated NR2B trafficking and neuroinflammation in DRG and spinal tissues of neuropathic rats. HPTLC and HR-MS analysis suggest betaine as major constituent in SAF which along with other phytoconstituents.

CONCLUSIONS

Both the whole extract (SCE) and the aqueous fraction (SAF) demonstrate a significant reduction in mechanical and thermal hypersensitivity by inhibiting KIF-17 mediated NR2B signaling in nerve injured rats and may be used as a potential alternative for the treatment of chronic pain. Our findings support the use of roots of Sida cordifolia L. in neuropathic pain conditions as acclaimed by its traditional use.

Targeting TRP channels: recent advances in structure, ligand binding, and molecular mechanisms

Transient receptor potential (TRP) channels are a large and diverse family of transmembrane ion channels that are widely expressed, have important physiological roles, and are associated with many human diseases. These proteins are actively pursued as promising drug targets, benefitting greatly from advances in structural and mechanistic studies of TRP channels. At the same time, the complex, polymodal activation and regulation of TRP channels have presented formidable challenges. In this short review, we summarize recent progresses toward understanding the structural basis of TRP channel function, as well as potential ligand binding sites that could be targeted for therapeutics. A particular focus is on the current understanding of the molecular mechanisms of TRP channel activation and regulation, where many fundamental questions remain unanswered. We believe that a deeper understanding of the functional mechanisms of TRP channels will be critical and likely transformative toward developing successful therapeutic strategies targeting these exciting proteins. This endeavor will require concerted efforts from computation, structural biology, medicinal chemistry, electrophysiology, pharmacology, drug safety and clinical studies.

Current Update on Categorization of Migraine Subtypes on the Basis of Genetic Variation: a Systematic Review

Migraine is a complex neurovascular disorder that is characterized by severe behavioral, sensory, visual, and/or auditory symptoms. It has been labeled as one of the ten most disabling medical illnesses in the world by the World Health Organization (Aagaard et al Sci Transl Med 6(237):237ra65, 2014). According to a recent report by the American Migraine Foundation (Shoulson et al Ann Neurol 25(3):252-9, 1989), around 148 million people in the world currently suffer from migraine. On the basis of presence of aura, migraine is classified into two major subtypes: migraine with aura (Aagaard et al Sci Transl Med 6(237):237ra65, 2014) and migraine without aura. (Aagaard K et al Sci Transl Med 6(237):237ra65, 2014) Many complex genetic mechanisms have been proposed in the pathophysiology of migraine but specific pathways associated with the different subtypes of migraine have not yet been explored. Various approaches including candidate gene association studies (CGAS) and genome-wide association studies (Fan et al Headache: J Head Face Pain 54(4):709-715, 2014). have identified the genetic markers associated with migraine and its subtypes. Several single nucleotide polymorphisms (Kaur et al Egyp J Neurol, Psychiatry Neurosurg 55(1):1-7, 2019) within genes involved in ion homeostasis, solute transport, synaptic transmission, cortical excitability, and vascular function have been associated with the disorder. Currently, the diagnosis of migraine is majorly behavioral with no focus on the genetic markers and thereby the therapeutic intervention specific to subtypes. Therefore, there is a need to explore genetic variants significantly associated with MA and MO as susceptibility markers in the diagnosis and targets for therapeutic interventions in the specific subtypes of migraine. Although the proper characterization of pathways based on different subtypes is yet to be studied, this review aims to make a first attempt to compile the information available on various genetic variants and the molecular mechanisms involved with the development of MA and MO. An attempt has also been made to suggest novel candidate genes based on their function to be explored by future research.

A novel animal model of neuropathic corneal pain-the ciliary nerve constriction model

Introduction

Neuropathic pain arises as a result of peripheral nerve injury or altered pain processing within the central nervous system. When this phenomenon affects the cornea, it is referred to as neuropathic corneal pain (NCP), resulting in pain, hyperalgesia, burning, and photoallodynia, severely affecting patients' quality of life. To date there is no suitable animal model for the study of NCP. Herein, we developed an NCP model by constriction of the long ciliary nerves innervating the eye.

Methods

Mice underwent ciliary nerve constriction (CNC) or sham procedures. Safety was determined by corneal fluorescein staining to assess ocular surface damage, whereas Cochet-Bonnet esthesiometry and confocal microscopy assessed the function and structure of corneal nerves, respectively. Efficacy was assessed by paw wipe responses within 30 seconds of applying hyperosmolar (5M) saline at Days 3, 7, 10, and 14 post-constriction. Additionally, behavior was assessed in an open field test (OFT) at Days 7, 14, and 21.

Results

CNC resulted in significantly increased response to hyperosmolar saline between groups (p < 0.0001), demonstrating hyperalgesia and induction of neuropathic pain. Further, animals that underwent CNC had increased anxiety-like behavior in an open field test compared to controls at the 14- and 21-Day time-points (p < 0.05). In contrast, CNC did not result in increased corneal fluorescein staining or decreased sensation as compared to sham controls (p > 0.05). Additionally, confocal microscopy of corneal whole-mounts revealed that constriction resulted in only a slight reduction in corneal nerve density (p < 0.05), compared to naïve and sham groups.

Discussion

The CNC model induces a pure NCP phenotype and may be a useful model for the study of NCP, recapitulating features of NCP, including hyperalgesia in the absence of ocular surface damage, and anxiety-like behavior.

Endogenous Inflammatory Mediators Produced by Injury Activate TRPV1 and TRPA1 Nociceptors to Induce Sexually Dimorphic Cold Pain That Is Dependent on TRPM8 and GFRα3

The detection of environmental temperatures is critical for survival, yet inappropriate responses to thermal stimuli can have a negative impact on overall health. The physiological effect of cold is distinct among somatosensory modalities in that it is soothing and analgesic, but also agonizing in the context of tissue damage. Inflammatory mediators produced during injury activate nociceptors to release neuropeptides, such as calcitonin gene-related peptide (CGRP) and substance P, inducing neurogenic inflammation, which further exasperates pain. Many inflammatory mediators induce sensitization to heat and mechanical stimuli but, conversely, inhibit cold responsiveness, and the identity of molecules inducing cold pain peripherally is enigmatic, as are the cellular and molecular mechanisms altering cold sensitivity. Here, we asked whether inflammatory mediators that induce neurogenic inflammation via the nociceptive ion channels TRPV1 (vanilloid subfamily of transient receptor potential channel) and TRPA1 (transient receptor potential ankyrin 1) lead to cold pain in mice. Specifically, we tested cold sensitivity in mice after intraplantar injection of lysophosphatidic acid or 4-hydroxy-2-nonenal, finding that each induces cold pain that is dependent on the cold-gated channel transient receptor potential melastatin 8 (TRPM8). Inhibition of CGRP, substance P, or toll-like receptor 4 (TLR4) signaling attenuates this phenotype, and each neuropeptide produces TRPM8-dependent cold pain directly. Further, the inhibition of CGRP or TLR4 signaling alleviates cold allodynia differentially by sex. Last, cold pain induced by both inflammatory mediators and neuropeptides requires TRPM8, as well as the neurotrophin artemin and its receptor GDNF receptor α3 (GFRα3). These results are consistent with artemin-induced cold allodynia requiring TRPM8, demonstrating that neurogenic inflammation alters cold sensitivity via localized artemin release that induces cold pain via GFRα3 and TRPM8.SIGNIFICANCE STATEMENT The cellular and molecular mechanisms that generate pain are complex with a diverse array of pain-producing molecules generated during injury that act to sensitize peripheral sensory neurons, thereby inducing pain. Here we identify a specific neuroinflammatory pathway involving the ion channel TRPM8 (transient receptor potential cation channel subfamily M member 8) and the neurotrophin receptor GFRα3 (GDNF receptor α3) that leads to cold pain, providing select targets for potential therapies for this pain modality.

Water extract of Notopterygium incisum alleviates cold allodynia in neuropathic pain by regulation of TRPA1

ETHNOPHARMACOLOGICAL RELEVANCE

Neuropathic pain can be debilitating and drastically affects the quality of life of those patients suffering from this condition. The Chinese herb Notopterygium incisum Ting ex H.T. Chang has long been used to disperse "cold". One under examined clinical feature of neuropathic pain is sensitivity to cold. Patients with neuropathic pain or arthritis usually describe a worsening of symptoms during the winter.

AIMS OF THIS STUDY

We proposed to test the hypothesis that Notopterygium incisum has a positive effect on the cold sensitivity found in neuropathic pain.

MATERIALS AND METHODS

In this study, we established chronic constriction injury (CCI) and cisplatin induced neuropathic pain mice models. Behavioral experiments and physiological examination methods were employed to investigate the effect of water extract of Notopterygium incisum (WN) on cold pain.

RESULTS

We found WN reduced cold pain and allyl isothiocyanate (AITC, Transient Receptor Potential A1 (TRPA1 agonist)) induced pain. WN inhibited AITC induced calcium response in HEK 293 cells transfected with TRPA1 and dorsal root ganglion (DRG) neurons. Moreover, we found that oral administration of WN reduced cold allodynia and mechanical allodynia caused by (CCI) and cisplatin induced neuropathic pain. We also observed that oral administration of WN decreased responses to AITC in DRG neurons as well as expression of TRPA1 in the WN treated neuropathic pain model.

CONCLUSIONS

The present study provide evidence that Notopterygium incisum alleviates cold allodynia in CCI and cisplatin induced neuropathic pain mouse models. WN alleviated neuropathic pain induced cold allodynia via directly modulating TRPA1. Our findings identify WN as a promising candidate for treating neuropathic pain that highlights a new mechanism of Notopterygium incisum on 'disperse cold'.

Reversal of cold intolerance by testosterone in orchiectomized mice after tibial nerve transection

Cold intolerance is a debilitating effect of nerve injury, has a strong impact on the life of patients and no advisable treatment exists against it. Testosterone influences pain pathways and has analgesic effects. A recent study showed testosterone as being an agonist of TRPM8, the predominant ion channel that contributes to cold hypersensitivity after injury. We investigated the effect of testosterone on cold sensitivity after nerve injury. Specifically, using the double plate test (DPT) (thermo-neutral-plate: 31 ºC and cold-plate: 18 ºC) we determined the thermal preference of mice at different points during the study design consisting of: orchiectomy, tibial nerve transection (TNT) (30 days after orchiectomy), 15-days-repeated subcutaneous injections of testosterone enanthate (250 or 500 µg/kg/day) or vehicle (started 12 h after TNT surgery). Different parameters such as time spent on cold plates, distance traveled, animal speed on the cold- and thermo-neutral-plates were determined in naïve, sham and neuropathic animals. Neither orchiectomy nor sham TNT surgery generate effects on cold intolerance and animal activity while TNT surgery decreased the time spent on the cold-plate and the distance traveled during DPT. Testosterone administration reversed the effect of nerve injury, decreasing the cold hypersensitivity and increasing activity of TNT mice. However, the effect of testosterone on cold avoidance reduced with time and at 14 days after TNT surgery, a higher dose was needed to reverse the effect generated by nerve injury. This indicates that although testosterone administration has a positive effect on cold intolerance, it might not be suitable for prolongated treatment.

Endogenous inflammatory mediators produced by injury activate TRPV1 and TRPA1 nociceptors to induce sexually dimorphic cold pain that is dependent on TRPM8 and GFRα3

The detection of environmental temperatures is critical for survival, yet inappropriate responses to thermal stimuli can have a negative impact on overall health. The physiological effect of cold is distinct among somatosensory modalities in that it is soothing and analgesic, but also agonizing in the context of tissue damage. Inflammatory mediators produced during injury activate nociceptors to release neuropeptides, such as CGRP and substance P, inducing neurogenic inflammation which further exasperates pain. Many inflammatory mediators induce sensitization to heat and mechanical stimuli but, conversely, inhibit cold responsiveness, and the identity of molecules inducing cold pain peripherally is enigmatic, as are the cellular and molecular mechanisms altering cold sensitivity. Here, we asked if inflammatory mediators that induce neurogenic inflammation via the nociceptive ion channels TRPV1 and TRPA1 lead to cold pain in mice. Specifically, we tested cold sensitivity in mice after intraplantar injection of lysophosphatidic acid (LPA) or 4-hydroxy-2-nonenal (4HNE), finding each induces cold pain that is dependent on the cold-gated channel TRPM8. Inhibition of either CGRP, substance P, or toll-like receptor 4 (TLR4) signaling attenuates this phenotype, and each neuropeptide produces TRPM8-dependent cold pain directly. Further, the inhibition of CGRP or TLR4 signaling alleviates cold allodynia differentially by sex. Lastly, we find that cold pain induced by inflammatory mediators and neuropeptides requires the neurotrophin artemin and its receptor GFRα3. These results demonstrate that tissue damage alters cold sensitivity via neurogenic inflammation, likely leading to localized artemin release that induces cold pain via GFRα3 and TRPM8.

Significance Statement

The cellular and molecular mechanisms that generate pain are complex with a diverse array of pain-producing molecules generated during injury that act to sensitize peripheral sensory neurons, thereby inducing pain. Here we identify a specific neuroinflammatory pathway involving the ion channel TRPM8 and the neurotrophin receptor GFRα3 that leads to cold pain, providing select targets for potential therapies for this pain modality.

IQGAP1 promotes chronic pain by regulating the trafficking and sensitization of TRPA1 channels

TRPA1 channels have been implicated in mechanical and cold hypersensitivity in chronic pain. But how TRPA1 mediates this process is unclear. Here we show that IQ-motif containing GTPase activating protein 1 (IQGAP1) is responsible using a combination of biochemical, molecular, Ca2+ imaging and behavioural approaches. TRPA1 and IQGAP1 bind to each other and are highly colocalised in sensory DRG neurons in mice. The expression of IQGAP1 but not TRPA1 is increased in chronic inflammatory and neuropathic pain. However, TRPA1 undergoes increased trafficking to the membrane of DRG neurons catalysed by the small GTPase Cdc42 associated with IQGAP1, leading to functional sensitization of the channel. Activation of PKA is also sufficient to evoke TRPA1 trafficking and sensitization. All these responses are, however, completely prevented in the absence of IQGAP1. Concordantly, deletion of IQGAP1 markedly reduces mechanical and cold hypersensitivity in chronic inflammatory and neuropathic pain in mice. IQGAP1 thus promotes chronic pain by coupling the trafficking and signalling machineries to TRPA1 channels.

Angiotensin type 2 receptor antagonism as a new target to manage gout

BACKGROUND

There is a growing search for therapeutic targets in the treatment of gout. The present study aimed to evaluate the analgesic and anti-inflammatory potential of angiotensin type 2 receptor (AT₂R) antagonism in an acute gout attack mouse model.

METHODS

Male wild-type (WT) C57BL/6 mice either with the AT₂R antagonist, PD123319 (10 pmol/joint), or with vehicle injections, or AT₂R KO mice, received intra-articular (IA) injection of monosodium urate (MSU) crystals (100 µg/joint), that induce the acute gout attack, and were tested for mechanical allodynia, thermal hyperalgesia, spontaneous nociception and ankle edema development at several times after the injections. To test an involvement of AT₂R in joint pain, mice received an IA administration of angiotensin II (0.05-5 nmol/joint) with or without PD123319, and were also evaluated for pain and edema development. Ankle joint tissue samples from mice undergoing the above treatments were assessed for myeloperoxidase activity, IL-1β release, mRNA expression analyses and nitrite/nitrate levels, 4 h after injections.

RESULTS

AT₂R antagonism has robust antinociceptive effects on mechanical allodynia (44% reduction) and spontaneous nociception (56%), as well as anti-inflammatory effects preventing edema formation (45%), reducing myeloperoxidase activity (54%) and IL-1β levels (32%). Additionally, Agtr2^tm1a mutant mice have largely reduced painful signs of gout. Angiotensin II administration causes pain and inflammation, which was prevented by AT₂R antagonism, as observed in mechanical allodynia 4 h (100%), spontaneous nociception (46%), cold nociceptive response (54%), edema formation (83%), myeloperoxidase activity (48%), and IL-1β levels (89%). PD123319 treatment also reduces NO concentrations (74%) and AT₂R mRNA levels in comparison with MSU untreated mice.

CONCLUSION

Our findings show that AT₂R activation contributes to acute pain in experimental mouse models of gout. Therefore, the antagonism of AT₂R may be a potential therapeutic option to manage gout arthritis.

The distinctive role of menthol in pain and analgesia: Mechanisms, practices, and advances

Menthol is an important flavoring additive that triggers a cooling sensation. Under physiological condition, low to moderate concentrations of menthol activate transient receptor potential cation channel subfamily M member 8 (TRPM8) in the primary nociceptors, such as dorsal root ganglion (DRG) and trigeminal ganglion, generating a cooling sensation, whereas menthol at higher concentration could induce cold allodynia, and cold hyperalgesia mediated by TRPM8 sensitization. In addition, the paradoxical irritating properties of high concentrations of menthol is associated with its activation of transient receptor potential cation channel subfamily A member 1 (TRPA1). Under pathological situation, menthol activates TRPM8 to attenuate mechanical allodynia and thermal hyperalgesia following nerve injury or chemical stimuli. Recent reports have recapitulated the requirement of central group II/III metabotropic glutamate receptors (mGluR) with endogenous κ-opioid signaling pathways for menthol analgesia. Additionally, blockage of sodium channels and calcium influx is a determinant step after menthol exposure, suggesting the possibility of menthol for pain management. In this review, we will also discuss and summarize the advances in menthol-related drugs for pathological pain treatment in clinical trials, especially in neuropathic pain, musculoskeletal pain, cancer pain and postoperative pain, with the aim to find the promising therapeutic candidates for the resolution of pain to better manage patients with pain in clinics.

Systemic DNA/RNA heteroduplex oligonucleotide administration for regulating the gene expression of dorsal root ganglion and sciatic nerve

Neuropathic pain, a heterogeneous condition, affects 7%–10% of the general population. To date, efficacious and safe therapeutic approaches remain limited. Antisense oligonucleotide (ASO) therapy has opened the door to treat spinal muscular atrophy, with many ongoing clinical studies determining its therapeutic utility. ASO therapy for neuropathic pain and peripheral nerve disease requires efficient gene delivery and knockdown in both the dorsal root ganglion (DRG) and sciatic nerve, key tissues for pain signaling. We previously developed a new DNA/RNA heteroduplex oligonucleotide (HDO) technology that achieves highly efficient gene knockdown in the liver. Here, we demonstrated that intravenous injection of HDO, comprising an ASO and its complementary RNA conjugated to α-tocopherol, silences endogenous gene expression more than 2-fold in the DRG, and sciatic nerve with higher potency, efficacy, and broader distribution than ASO alone. Of note, we observed drastic target suppression in all sizes of neuronal DRG populations by in situ hybridization. Our findings establish HDO delivery as an investigative and potentially therapeutic platform for neuropathic pain and peripheral nerve disease.

Slack Potassium Channels Modulate TRPA1-Mediated Nociception in Sensory Neurons

The transient receptor potential (TRP) ankyrin type 1 (TRPA1) channel is highly expressed in a subset of sensory neurons where it acts as an essential detector of painful stimuli. However, the mechanisms that control the activity of sensory neurons upon TRPA1 activation remain poorly understood. Here, using in situ hybridization and immunostaining, we found TRPA1 to be extensively co-localized with the potassium channel Slack (K_Na1.1, Slo2.2, or Kcnt1) in sensory neurons. Mice lacking Slack globally (Slack^−/−) or conditionally in sensory neurons (SNS-Slack^−/−) demonstrated increased pain behavior after intraplantar injection of the TRPA1 activator allyl isothiocyanate. By contrast, pain behavior induced by the TRP vanilloid 1 (TRPV1) activator capsaicin was normal in Slack-deficient mice. Patch-clamp recordings in sensory neurons and in a HEK cell line transfected with TRPA1 and Slack revealed that Slack-dependent potassium currents (I_KS) are modulated in a TRPA1-dependent manner. Taken together, our findings highlight Slack as a modulator of TRPA1-mediated, but not TRPV1-mediated, activation of sensory neurons.

Genetic and functional evidence for gp130/IL6ST-induced transient receptor potential ankyrin 1 upregulation in uninjured but not injured neurons in a mouse model of neuropathic pain

ABSTRACT

Peripheral nerve injuries result in pronounced alterations in dorsal root ganglia, which can lead to the development of neuropathic pain. Although the polymodal mechanosensitive transient receptor potential ankyrin 1 (TRPA1) ion channel is emerging as a relevant target for potential analgesic therapies, preclinical studies do not provide unequivocal mechanistic insight into its relevance for neuropathic pain pathogenesis. By using a transgenic mouse model with a conditional depletion of the interleukin-6 (IL-6) signal transducer gp130 in Nav1.8 expressing neurons (SNS-gp130-/-), we provide a mechanistic regulatory link between IL-6/gp130 and TRPA1 in the spared nerve injury (SNI) model. Spared nerve injury mice developed profound mechanical hypersensitivity as indicated by decreased withdrawal thresholds in the von Frey behavioral test in vivo, as well as a significant increase in mechanosensitivity of unmyelinated nociceptive primary afferents in ex vivo skin-nerve recordings. In contrast to wild type and control gp130fl/fl animals, SNS-gp130-/- mice did not develop mechanical hypersensitivity after SNI and exhibited low levels of Trpa1 mRNA in sensory neurons, which were partially restored by adenoviral gp130 re-expression in vitro. Importantly, uninjured but not injured neurons developed increased responsiveness to the TRPA1 agonist cinnamaldehyde, and neurons derived from SNS-gp130-/- mice after SNI were significantly less responsive to cinnamaldehyde. Our study shows for the first time that TRPA1 upregulation is attributed specifically to uninjured neurons in the SNI model, and this depended on the IL-6 signal transducer gp130. We provide a solution to the enigma of TRPA1 regulation after nerve injury and stress its significance as an important target for neuropathic pain disorders.

β-Lactam TRPM8 Antagonist RGM8-51 Displays Antinociceptive Activity in Different Animal Models

Transient receptor potential melastatin subtype 8 (TRPM8) is a cation channel extensively expressed in sensory neurons and implicated in different painful states. However, the effectiveness of TRPM8 modulators for pain relief is still a matter of discussion, since structurally diverse modulators lead to different results, depending on the animal pain model. In this work, we described the antinociceptive activity of a β–lactam derivative, RGM8-51, showing good TRPM8 antagonist activity, and selectivity against related thermoTRP channels and other pain-mediating receptors. In primary cultures of rat dorsal root ganglion (DRG) neurons, RGM8-51 potently reduced menthol-evoked neuronal firing without affecting the major ion conductances responsible for action potential generation. This compound has in vivo antinociceptive activity in response to cold, in a mouse model of oxaliplatin-induced peripheral neuropathy. In addition, it reduces cold, mechanical and heat hypersensitivity in a rat model of neuropathic pain arising after chronic constriction of the sciatic nerve. Furthermore, RGM8-51 exhibits mechanical hypersensitivity-relieving activity, in a mouse model of NTG-induced hyperesthesia. Taken together, these preclinical results substantiate that this TRPM8 antagonist is a promising pharmacological tool to study TRPM8-related diseases.

The Role of TRPA1 in Skin Physiology and Pathology

The transient receptor potential ankyrin 1 (TRPA1), a member of the TRP superfamily of channels, acts as 'polymodal cellular sensor' on primary sensory neurons where it mediates the peripheral and central processing of pain, itch, and thermal sensation. However, the TRPA1 expression extends far beyond the sensory nerves. In recent years, much attention has been paid to its expression and function in non-neuronal cell types including skin cells, such as keratinocytes, melanocytes, mast cells, dendritic cells, and endothelial cells. TRPA1 seems critically involved in a series of physiological skin functions, including formation and maintenance of physico-chemical skin barriers, skin cells, and tissue growth and differentiation. TRPA1 appears to be implicated in mechanistic processes in various immunological inflammatory diseases and cancers of the skin, such as atopic and allergic contact dermatitis, psoriasis, bullous pemphigoid, cutaneous T-cell lymphoma, and melanoma. Here, we report recent findings on the implication of TRPA1 in skin physiology and pathophysiology. The potential use of TRPA1 antagonists in the treatment of inflammatory and immunological skin disorders will be also addressed.

Sensory neuron-associated macrophages as novel modulators of neuropathic pain

The peripheral nervous system comprises an infinity of neural networks that act in the communication between the central nervous system and the most diverse tissues of the body. Along with the extension of the primary sensory neurons (axons and cell bodies), a population of resident macrophages has been described. These newly called sensory neuron-associated macrophages (sNAMs) seem to play an essential role in physiological and pathophysiological processes, including infection, autoimmunity, nerve degeneration/regeneration, and chronic neuropathic pain. After different types of peripheral nerve injury, there is an increase in the number and activation of sNAMs in the sciatic nerve and sensory ganglia. The activation of sNAMs and their participation in neuropathic pain development depends on the stimulation of pattern recognition receptors such as Toll-like receptors and Nod-like receptors, chemokines/cytokines, and microRNAs. On activation, sNAMs trigger the production of critical inflammatory mediators such as proinflammatory cytokines (eg, TNF and IL-1β) and reactive oxygen species that can act in the amplification of primary sensory neurons sensitization. On the other hand, there is evidence that sNAMs can produce antinociceptive mediators (eg, IL-10) that counteract neuropathic pain development. This review will present the cellular and molecular mechanisms behind the participation of sNAMs in peripheral nerve injury-induced neuropathic pain development. Understanding how sNAMs are activated and responding to nerve injury can help set novel targets for the control of neuropathic pain.

Blockade of bradykinin receptors or angiotensin II type 2 receptor prevents paclitaxel-associated acute pain syndrome in mice

BACKGROUND

Paclitaxel (PCX) is the first-line choice for the treatment of several types of cancer, including breast, ovarian, and lung cancers. However, patients who receive even a single dose with PCX commonly develop mechanical and cold allodynia, a symptom known as PCX-associated acute pain syndrome (P-APS). Here, we assessed possible involvement of kinin-kallikrein and renin-angiotensin systems in P-APS in mice.

METHODS

Male mice C57Bl/6 wild type (WT) and knockouts for bradykinin receptors, B1 (B1^-/- ) and B2 (B2^-/- ), were used. Mechanical and cold allodynia were evaluated by using von Frey filaments and acetone test, respectively. P-APS was induced by administration of PCX 4 mg/kg, i.v.. ACE inhibitors (captopril and enalapril), antagonists for angiotensin II type 1 (losartan) and type 2 ([AT2R]; PD123319 and EMA 401) receptors were administrated prior the treatment with PCX. RT-PCR was used to analyse the expression of mRNA for B1, B2 and AT2R receptors.

RESULTS

Administration of PCX in B1^-/- and B2^-/- mice induced lower mechanical and cold allodynia compared to the WT. However, the pre-treatment with ACE inhibitors reduced the development of mechanical and cold allodynia in P-APS. Surprisingly, we found that mice pre-treatment with the PD123319 or EMA401, but not losartan, prevented the development of mechanical and cold allodynia induced by PCX.

CONCLUSION

Our results demonstrated the involvement of bradykinin receptors B1 and B2 as well as AT2R in the induction of P-APS in mice, and suggest the use of AT2R antagonists as a potential therapy for the prevention of P-APS in humans.

SIGNIFICANCE

Kinin-kallikrein and renin-angiotensin systems, through B1, B2 and AT2 receptors, potentiates paclitaxel-associated acute pain syndrome (P-APS) in mice. Antagonists for AT2R are potential alternatives to prevent P-APS.

TRPM8: A Therapeutic Target for Neuroinflammatory Symptoms Induced by Severe Dry Eye Disease

Dry eye disease (DED) is commonly associated with ocular surface inflammation and pain. In this study, we evaluated the effectiveness of repeated instillations of transient receptor potential melastatin 8 (TRPM8) ion channel antagonist M8-B on a mouse model of severe DED induced by the excision of extra-orbital lacrimal and Harderian glands. M8-B was topically administered twice a day from day 7 until day 21 after surgery. Cold and mechanical corneal sensitivities and spontaneous ocular pain were monitored at day 21. Ongoing and cold-evoked ciliary nerve activities were next evaluated by electrophysiological multi-unit extracellular recording. Corneal inflammation and expression of genes related to neuropathic pain and inflammation were assessed in the trigeminal ganglion. We found that DED mice developed a cold allodynia consistent with higher TRPM8 mRNA expression in the trigeminal ganglion (TG). Chronic M8-B instillations markedly reversed both the corneal mechanical allodynia and spontaneous ocular pain commonly associated with persistent DED. M8-B instillations also diminished the sustained spontaneous and cold-evoked ciliary nerve activities observed in DED mice as well as inflammation in the cornea and TG. Overall, our study provides new insight into the effectiveness of TRPM8 blockade for alleviating corneal pain syndrome associated with severe DED, opening a new avenue for ocular pain management.

Involvement of Neural Transient Receptor Potential Channels in Peripheral Inflammation

Transient receptor potential (TRP) channels are a superfamily of non-selective cation channels that act as polymodal sensors in many tissues throughout mammalian organisms. In the context of ion channels, they are unique for their broad diversity of activation mechanisms and their cation selectivity. TRP channels are involved in a diverse range of physiological processes including chemical sensing, nociception, and mediating cytokine release. They also play an important role in the regulation of inflammation through sensory function and the release of neuropeptides. In this review, we discuss the functional contribution of a subset of TRP channels (TRPV1, TRPV4, TRPM3, TRPM8, and TRPA1) that are involved in the body’s immune responses, particularly in relation to inflammation. We focus on these five TRP channels because, in addition to being expressed in many somatic cell types, these channels are also expressed on peripheral ganglia and nerves that innervate visceral organs and tissues throughout the body. Activation of these neural TRP channels enables crosstalk between neurons, immune cells, and epithelial cells to regulate a wide range of inflammatory actions. TRP channels act either through direct effects on cation levels or through indirect modulation of intracellular pathways to trigger pro- or anti-inflammatory mechanisms, depending on the inflammatory disease context. The expression of TRP channels on both neural and immune cells has made them an attractive drug target in diseases involving inflammation. Future work in this domain will likely yield important new pathways and therapies for the treatment of a broad range of disorders including colitis, dermatitis, sepsis, asthma, and pain.

TRPV4 and TRPM8 as putative targets for chronic low back pain alleviation

The purpose of this study is to investigate the presence of nervous fibers and expression of TRP channels in samples harvested during decompressive/fusion spine surgeries from patients affected by chronic low back pain (CLBP). The aim was to understand if members of this family of receptors played a role in detection and processing of painful stimuli, to eventually define them as potential targets for CLBP alleviation. Expression of transient receptor potential (TRP) channels (A1, V1, V2, V4, and M8) was evaluated in samples from different periarticular sites of 6 patients affected by CLBP, at both protein and transcript levels. The capsular connective pathological tissue appeared infiltrated by sensitive unmyelinated nervous fibers. An increase in TRP channel mRNAs and proteins was observed in the pathological capsule compared with tissues collected from the non-symptomatic area in five of the six analyzed patients, independently by the location and number of affected sites. In particular, TRPV4 and TRPM8 were consistently upregulated in pathological tissues. Interestingly, the only patient showing a different pattern of expression also had a different clinical history. TRPV4 and TRPM8 channels may play a role in CLBP and warrant further investigations as possible therapeutic targets.

TRPA1 as a therapeutic target for nociceptive pain

Introduction

Chronic pain affects approximatively 30–50% of the population globally. Pathologies such as migraine, diabetic neuropathy, nerve injury and treatment with chemotherapeutic agents, can induce chronic pain. Members of the transient receptor potential (TRP) channels, including the TRP ankyrin 1 (TRPA1), have a major role in pain.

Areas covered

We focus on TRPA1 as a therapeutic target for pain relief. The structure, localization, and activation of the channel and its implication in different pathways to signal pain are described. This paper underlines the role of pharmacological interventions on TRPA1 to reduce pain in numerous pain conditions. We conducted a literature search in PubMed up to and including July 2020.

Expert opinion

Our understanding of the molecular mechanisms underlying the sensitization of central and peripheral nociceptive pathways is limited. Preclinical evidence indicates that, in murine models of pain diseases, numerous mechanisms converge on the pathway that encompasses oxidative stress and Schwann cell TRPA1 to sustain chronic pain. Programs to identify and develop treatments to attenuate TRPA1-mediated chronic pain have emerged from this knowledge. Antagonists explored as a novel class of analgesics have a new and promising target in the TRPA1 expressed by peripheral glial cells.

Naked mole-rats lack cold sensitivity before and after nerve injury

Neuropathic pain is a chronic disease state resulting from injury to the nervous system. This type of pain often responds poorly to standard treatments and occasionally may get worse instead of better over time. Patients who experience neuropathic pain report sensitivity to cold and mechanical stimuli. Since the nociceptive system of African naked mole-rats contains unique adaptations that result in insensitivity to some pain types, we investigated whether naked mole-rats may be resilient to sensitivity following nerve injury. Using the spared nerve injury model of neuropathic pain, we showed that sensitivity to mechanical stimuli developed similarly in mice and naked mole-rats. However, naked mole-rats lacked sensitivity to mild cold stimulation after nerve injury, while mice developed robust cold sensitivity. We pursued this response deficit by testing behavior to activators of transient receptor potential (TRP) receptors involved in detecting cold in naïve animals. Following mustard oil, a TRPA1 activator, naked mole-rats responded similarly to mice. Conversely, icilin, a TRPM8 agonist, did not evoke pain behavior in naked mole-rats when compared with mice. Finally, we used RNAscope to probe for TRPA1 and TRPM8 messenger RNA expression in dorsal root ganglia of both species. We found increased TRPA1 messenger RNA, but decreased TRPM8 punctae in naked mole-rats when compared with mice. Our findings likely reflect species differences due to evolutionary environmental responses that are not easily explained by differences in receptor expression between the species.

TRPM8 channels: A review of distribution and clinical role

Ion channels are important therapeutic targets due to their plethoric involvement in physiological and pathological consequences. The transient receptor potential cation channel subfamily M member 8 (TRPM8) is a nonselective cation channel that controls Ca²⁺ homeostasis. It has been proposed to be the predominant thermoreceptor for cellular and behavioral responses to cold stimuli in the transient receptor potential (TRP) channel subfamilies and exploited so far to reach the clinical-stage of drug development. TRPM8 channels can be found in multiple organs and tissues, regulating several important processes such as cell proliferation, migration and apoptosis, inflammatory reactions, immunomodulatory effects, pain, and vascular muscle tension. The related disorders have been expanded to new fields ranging from cancer and migraine to dry eye disease, pruritus, irritable bowel syndrome (IBS), and chronic cough. This review is aimed to summarize the distribution of TRPM8 and disorders related to it from a clinical perspective, so as to broaden the scope of knowledge of researchers to conduct more studies on this subject.

Protective effects of a polyphenol-enriched fraction of the fruit peel of Annona crassiflora Mart. on acute and persistent inflammatory pain

Different parts of Annona crassiflora Mart., a native species from Brazilian savanna, were traditionally used for the treatment of a wide variety of ailments including arthritis. Thus, this study aimed to investigate the possible antinociceptive and anti-inflammatory properties of a polyphenol-enriched fraction of the fruit peel of A. crassiflora, named here as EtOAc, in mice. Pro-inflammatory cytokines and nitric oxide (NO) production were evaluated in LPS-activated macrophages. Then, EtOAc fraction was administered by oral route in male C57BL/6/J mice, and the animals were submitted to glutamate-induced nociception and complete Freund's adjuvant (CFA)-induced monoarthritis tests to assess nociception (mechanical, spontaneous and cold pain) and inflammation (edema and neutrophil infiltration), and to the open-field and rotarod tests for motor performance analysis. EtOAc fraction inhibited the production of IL-6 and NO in the LPS-induced macrophages, and reduced spontaneous nociception induced by glutamate, without altering the animals' locomotor activity. In addition, the polyphenol-enriched fraction was able to revert the early and late hyperalgesia induced by CFA, as well as edema at the acute phase. Reduction of myeloperoxidase activity and inflammatory cell infiltration was observed in the paw tissue of mice injected with CFA and treated with EtOAc fraction. Together, our results support the antinociceptive and anti-inflammatory effects of the polyphenol-enriched fraction of A. crassiflora fruit peel and suggest that these effects are triggered, at least in part, by suppressing pro-inflammatory cytokines and neutrophils infiltration.

Characterization of New TRPM8 Modulators in Pain Perception

BACKGROUND

Transient Receptor Potential Melastatin-8 (TRPM8) is a non-selective cation channel activated by cold temperature and by cooling agents. Several studies have proved that this channel is involved in pain perception. Although some studies indicate that TRPM8 inhibition is necessary to reduce acute and chronic pain, it is also reported that TRPM8 activation produces analgesia. These conflicting results could be explained by extracellular Ca²⁺-dependent desensitization that is induced by an excessive activation. Likely, this effect is due to phosphatidylinositol 4,5-bisphosphate (PIP2) depletion that leads to modification of TRPM8 channel activity, shifting voltage dependence towards more positive potentials. This phenomenon needs further evaluation and confirmation that would allow us to understand better the role of this channel and to develop new therapeutic strategies for controlling pain.

EXPERIMENTAL APPROACH

To understand the role of TRPM8 in pain perception, we tested two specific TRPM8-modulating compounds, an antagonist (IGM-18) and an agonist (IGM-5), in either acute or chronic animal pain models using male Sprague-Dawley rats or CD1 mice, after systemic or topical routes of administration.

RESULTS

IGM-18 and IGM-5 were fully characterized in vivo. The wet-dog shake test and the body temperature measurements highlighted the antagonist activity of IGM-18 on TRPM8 channels. Moreover, IGM-18 exerted an analgesic effect on formalin-induced orofacial pain and chronic constriction injury-induced neuropathic pain, demonstrating the involvement of TRPM8 channels in these two pain models. Finally, the results were consistent with TRPM8 downregulation by agonist IGM-5, due to its excessive activation.

CONCLUSIONS

TRPM8 channels are strongly involved in pain modulation, and their selective antagonist is able to reduce both acute and chronic pain.

Astrocyte D-serine modulates the activation of neuronal NOS leading to the development of mechanical allodynia in peripheral neuropathy

Spinal D-serine plays an important role in nociception via an increase in phosphorylation of the N-Methyl-D-aspartate (NMDA) receptor GluN1 subunit (pGluN1). However, the cellular mechanisms underlying this process have not been elucidated. Here, we investigate the possible role of neuronal nitric oxide synthase (nNOS) in the D-serine-induced potentiation of NMDA receptor function and the induction of neuropathic pain in a chronic constriction injury (CCI) model. Intrathecal administration of the serine racemase inhibitor, L-serine O-sulfate potassium salt (LSOS) or the D-serine degrading enzyme, D-amino acid oxidase (DAAO) on post-operative days 0–3 significantly reduced the CCI-induced increase in nitric oxide (NO) levels and nicotinamide adenine dinucleotide phosphate-diaphorase staining in lumbar dorsal horn neurons, as well as the CCI-induced decrease in phosphorylation (Ser847) of nNOS (pnNOS) on day 3 post-CCI surgery. LSOS or DAAO administration suppressed the CCI-induced development of mechanical allodynia and protein kinase C (PKC)-dependent (Ser896) phosphorylation of GluN1 on day 3 post-surgery, which were reversed by the co-administration of the NO donor, 3-morpholinosydnonimine hydrochloride (SIN-1). In naïve mice, exogenous D-serine increased NO levels via decreases in pnNOS. D-serine-induced increases in mechanical hypersensitivity, NO levels, PKC-dependent pGluN1, and NMDA-induced spontaneous nociception were reduced by pretreatment with the nNOS inhibitor, 7-nitroindazole or with the NMDA receptor antagonists, 7-chlorokynurenic acid and MK-801. Collectively, we show that spinal D-serine modulates nNOS activity and concomitant NO production leading to increases in PKC-dependent pGluN1 and ultimately contributing to the induction of mechanical allodynia following peripheral nerve injury.

X-ray induces mechanical and heat allodynia in mouse via TRPA1 and TRPV1 activation

Radiotherapy-related pain is a common adverse reaction with a high incidence among cancer patients undergoing radiotherapy and remarkably reduces the quality of life. However, the mechanisms of ionizing radiation-induced pain are largely unknown. In this study, mice were treated with 20 Gy X-ray to establish ionizing radiation-induced pain model. X-ray evoked a prolonged mechanical, heat, and cold allodynia in mice. Transient receptor potential vanilloid 1 and transient receptor potential ankyrin 1 were significantly upregulated in lumbar dorsal root ganglion. The mechanical and heat allodynia could be transiently reverted by intrathecal injection of transient receptor potential vanilloid 1 antagonist capsazepine and transient receptor potential ankyrin 1 antagonist HC-030031. Additionally, the phosphorylated extracellular regulated protein kinases (ERK) and Jun NH2-terminal Kinase (JNK) in pain neural pathway were induced by X-ray treatment. Our findings indicated that activation of transient receptor potential ankyrin 1 and transient receptor potential vanilloid 1 is essential for the development of X-ray-induced allodynia. Furthermore, our findings suggest that targeting on transient receptor potential vanilloid 1 and transient receptor potential ankyrin 1 may be promising prevention strategies for X-ray-induced allodynia in clinical practice.

Potential for endocannabinoid system modulation in ocular pain and inflammation: filling the gaps in current pharmacological options

Challenges in the management of ocular pain are an underappreciated topic. Currently available therapeutics lack both efficacy and clear guidelines for their use, with many also possessing unacceptable side effects. Promising novel agents would offer analgesic, anti-inflammatory, and possibly neuroprotective actions; have favorable ocular safety profiles; and show potential in managing neuropathic pain. Growing evidence supports a link between the endocannabinoid system (ECS) and a range of physiological and disease processes, notably those involving inflammation and pain. Both preclinical and clinical data suggest analgesic and anti-inflammatory actions of cannabinoids and ECS-modifying drugs in chronic pain conditions, including those of neuropathic origin. This review will examine existing evidence for the anatomical and physiological basis of ocular pain, specifically, ocular surface disease and the development of chronic ocular pain. The mechanism of action, efficacy, and limitations of currently available treatments will be discussed, and current knowledge related to ECS-modulation of ocular pain and inflammatory disease will be summarized. A perspective will be provided on the future directions of ECS research in terms of developing cannabinoid therapeutics for ocular pain.

TRPA1: a molecular view

The transient receptor potential ankyrin 1 (TRPA1) ion channel is expressed in pain-sensing neurons and other tissues and has become a major target in the development of novel pharmaceuticals. A remarkable feature of the channel is its long list of activators, many of which we are exposed to in daily life. Many of these agonists induce pain and inflammation, making TRPA1 a major target for anti-inflammatory and analgesic therapies. Studies in human patients and in experimental animals have confirmed an important role for TRPA1 in a number of pain conditions. Over the recent years, much progress has been made in elucidating the molecular structure of TRPA1 and in discovering binding sites and modulatory sites of the channel. Because the list of published mutations and important molecular sites is steadily growing and because it has become difficult to see the forest for the trees, this review aims at summarizing the current knowledge about TRPA1, with a special focus on the molecular structure and the known binding or gating sites of the channel.

Antagonism of Transient Receptor Potential Ankyrin Type-1 Channels as a Potential Target for the Treatment of Trigeminal Neuropathic Pain: Study in an Animal Model

Transient receptor potential ankyrin type-1 (TRPA1) channels are known to actively participate in different pain conditions, including trigeminal neuropathic pain, whose clinical treatment is still unsatisfactory. The aim of this study was to evaluate the involvement of TRPA1 channels by means of the antagonist ADM_12 in trigeminal neuropathic pain, in order to identify possible therapeutic targets. A single treatment of ADM_12 in rats 4 weeks after the chronic constriction injury of the infraorbital nerve (IoN-CCI) significantly reduced the mechanical allodynia induced in the IoN-CCI rats. Additionally, ADM_12 was able to abolish the increased levels of TRPA1, calcitonin gene-related peptide (CGRP), substance P (SP), and cytokines gene expression in trigeminal ganglia, cervical spinal cord, and medulla induced in the IoN-CCI rats. By contrast, no significant differences between groups were seen as regards CGRP and SP protein expression in the pars caudalis of the spinal nucleus of the trigeminal nerve. ADM_12 also reduced TRP vanilloid type-1 (TRPV1) gene expression in the same areas after IoN-CCI. Our findings show the involvement of both TRPA1 and TRPV1 channels in trigeminal neuropathic pain, and in particular, in trigeminal mechanical allodynia. Furthermore, they provide grounds for the use of ADM_12 in the treatment of trigeminal neuropathic pain.

Dynorphinergic system alterations in the corticostriatal circuitry of neuropathic mice support its role in the negative affective component of pain

The dynorphinergic system is involved in pain transmission at spinal level, where dynorphin exerts antinociceptive or pronociceptive effects, based on its opioid or non‐opioid actions. Surprisingly, little evidence is currently available concerning the supraspinal role of the dynorphinergic system in pain conditions. The present study aimed to investigate whether neuropathic pain is accompanied by prodynorphin (Pdyn) and κ‐opioid receptor (Oprk1) gene expression alterations in selected mouse brain areas. To this end, mice were subjected to chronic constriction injury of the right sciatic nerve and neuropathic pain behavioral signs were ascertained after 14 days. At this interval, a marked increase in Pdyn mRNA in the anterior cingulate cortex (ACC) and prefrontal cortex (PFC) was observed. Oprk1 gene expression was increased in the PFC, and decreased in the ACC and nucleus accumbens (NAc). No changes were observed in the other investigated regions. Because of the relationship between dynorphin and the brain‐derived neurotrophic factor, and the role of this neurotrophin in chronic pain‐related neuroplasticity, we investigated brain‐derived neurotrophic factor gene (Bdnf) expression in the areas showing Pdyn or Oprk1 mRNAs changes. Bdnf mRNA levels were increased in both the ACC and PFC, whereas no changes were assessed in the NAc. Present data indicate that the dynorphinergic system undergoes quite selective alterations involving the corticostriatal circuitry during neuropathic pain, suggesting a contribution to the negative affective component of pain. Moreover, parallel increases in Pdyn and Bdnf mRNA at cortical level suggest the occurrence of likely interactions between these systems in neuropathic pain maladaptive neuroplasticity.

The role and mechanism of action of menthol in topical analgesic products

WHAT IS KNOWN AND OBJECTIVE

Menthol has been used as a non-opioid pain reliever since ancient times. A modern understanding of its molecular mechanism of action could form the basis for generating targets for discovery of novel non-opioid analgesic drugs.

METHODS

The PubMed database was queried using search words related to menthol, pain and analgesia. The results were limited to relevant preclinical studies and clinical trials and reviews published in English during the past 5 years, which yielded 31 reports. The bibliographies of these articles were sources of additional supporting articles.

RESULTS

Menthol is a selective activator of transient receptor potential melastatin-8 (TRPM8) channels and is also a vasoactive compound. As a topical agent, it acts as a counter-irritant by imparting a cooling effect and by initially stimulating nociceptors and then desensitizing them. Topically applied menthol may also activate central analgesic pathways. At high concentrations, menthol may generate cold allodynia.

WHAT IS NEW AND CONCLUSIONS

Recent elucidation of TRPM8 channels has provided a molecular basis for understanding the molecular action of menthol and its ability to produce both a cooling sensation and reduction in pain associated with a wide variety of pain(ful) conditions. The more modern mechanistic understanding of menthol and its pharmacologic mechanism of action may lead to an expanded role for this substance in the search for replacements for opioid analgesics, particularly those that can be applied topically.

Optogenetic Activation of Non-Nociceptive Aβ Fibers Induces Neuropathic Pain-Like Sensory and Emotional Behaviors after Nerve Injury in Rats

Neuropathic pain is caused by peripheral nerve injury (PNI). One hallmark symptom is allodynia (pain caused by normally innocuous stimuli), but its mechanistic underpinning remains elusive. Notably, whether selective stimulation of non-nociceptive primary afferent Aβ fibers indeed evokes neuropathic pain-like sensory and emotional behaviors after PNI is unknown, because of the lack of tools to manipulate Aβ fiber function in awake, freely moving animals. In this study, we used a transgenic rat line that enables stimulation of non-nociceptive Aβ fibers by a light-activated channel (channelrhodopsin-2; ChR2). We found that illuminating light to the plantar skin of these rats with PNI elicited pain-like withdrawal behaviors that were resistant to morphine. Light illumination to the skin of PNI rats increased the number of spinal dorsal horn (SDH) Lamina I neurons positive to activity markers (c-Fos and phosphorylated extracellular signal-regulated protein kinase; pERK). Whole-cell recording revealed that optogenetic Aβ fiber stimulation after PNI caused excitation of Lamina I neurons, which were normally silent by this stimulation. Moreover, illuminating the hindpaw of PNI rats resulted in activation of central amygdaloid neurons and produced an aversion to illumination. Thus, these findings provide the first evidence that optogenetic activation of primary afferent Aβ fibers in PNI rats produces excitation of Lamina I neurons and neuropathic pain-like behaviors that were resistant to morphine treatment. This approach may provide a new path for investigating circuits and behaviors of Aβ fiber-mediated neuropathic allodynia with sensory and emotional aspects after PNI and for discovering novel drugs to treat neuropathic pain.

Distinct calcitonin gene-related peptide expression pattern in primary afferents contribute to different neuropathic symptoms following chronic constriction or crush injuries to the rat sciatic nerve

Although calcitonin gene-related peptide is a recognized pain transducer, the expression of calcitonin gene-related peptide in primary afferents may be differentially affected following different types of nerve injury. Here, we examined whether different calcitonin gene-related peptide expression patterns in primary afferents contributes to distinct sensory disturbances in three animal models of sciatic nerve injury: chronic constriction injury, mild (100g force) or strong (1000g force) transient crush in rats. Assessments of withdrawal reflexes and spontaneous behavior indicated that chronic constriction injury and mild crush resulted in positive neuropathic symptoms (static/dynamic mechanical allodynia, heat hyperalgesia, cold allodynia, spontaneous pain). However, strong crush led to both positive (dynamic mechanical allodynia, cold allodynia, spontaneous pain) and negative symptoms (static mechanical hypoesthesia, heat hypoalgesia). Calcitonin gene-related peptide immunoreactivity in dorsal root ganglia and corresponding spinal cord segments, and calcitonin gene-related peptide mRNA levels in dorsal root ganglia, indicated that the primary afferent calcitonin gene-related peptide supply was markedly reduced only after strong crush. This reduction paralleled the development of negative symptoms (static mechanical hypoesthesia and heat hypoalgesia). Administration of exogenous calcitonin gene-related peptide intrathecally after strong crush did not alter heat hypoalgesia but ameliorated static mechanical hypoesthesia, an effect blocked by a calcitonin gene-related peptide receptor antagonist. Thus, reducing the primary afferent calcitonin gene-related peptide supply contributed to subsequent negative neuropathic symptoms, especially to static mechanical stimuli. Moreover, nerve injury caused a subcellular redistribution of calcitonin gene-related peptide from small- and medium-size dorsal root ganglia neurons to large-size dorsal root ganglia neurons, which paralleled the development of positive neuropathic symptoms. Intrathecal administration of the calcitonin gene-related peptide receptor antagonist ameliorated these positive symptoms, indicating that the expression of calcitonin gene-related peptide in large-size dorsal root ganglia neurons is important for the positive neuropathic symptoms in all three models. Taken together, these results suggest that distinct calcitonin gene-related peptide expression pattern in primary afferents contribute to different neuropathic symptoms following chronic constriction or crush injuries to the rat sciatic nerve.

The Absence of Sensory Axon Bifurcation Affects Nociception and Termination Fields of Afferents in the Spinal Cord

A cGMP signaling cascade composed of C-type natriuretic peptide, the guanylyl cyclase receptor Npr2 and cGMP-dependent protein kinase I (cGKI) controls the bifurcation of sensory axons upon entering the spinal cord during embryonic development. However, the impact of axon bifurcation on sensory processing in adulthood remains poorly understood. To investigate the functional consequences of impaired axon bifurcation during adult stages we generated conditional mouse mutants of Npr2 and cGKI (Npr2^fl/fl;Wnt1^Cre and cGKI^KO/fl;Wnt1^Cre) that lack sensory axon bifurcation in the absence of additional phenotypes observed in the global knockout mice. Cholera toxin labeling in digits of the hind paw demonstrated an altered shape of sensory neuron termination fields in the spinal cord of conditional Npr2 mouse mutants. Behavioral testing of both sexes indicated that noxious heat sensation and nociception induced by chemical irritants are impaired in the mutants, whereas responses to cold sensation, mechanical stimulation, and motor coordination are not affected. Recordings from C-fiber nociceptors in the hind limb skin showed that Npr2 function was not required to maintain normal heat sensitivity of peripheral nociceptors. Thus, the altered behavioral responses to noxious heat found in Npr2^fl/fl;Wnt1^Cre mice is not due to an impaired C-fiber function. Overall, these data point to a critical role of axonal bifurcation for the processing of pain induced by heat or chemical stimuli.

Regulation of Pain and Itch by TRP Channels

Nociception is an important physiological process that detects harmful signals and results in pain perception. In this review, we discuss important experimental evidence involving some TRP ion channels as molecular sensors of chemical, thermal, and mechanical noxious stimuli to evoke the pain and itch sensations. Among them are the TRPA1 channel, members of the vanilloid subfamily (TRPV1, TRPV3, and TRPV4), and finally members of the melastatin group (TRPM2, TRPM3, and TRPM8). Given that pain and itch are pro-survival, evolutionarily-honed protective mechanisms, care has to be exercised when developing inhibitory/modulatory compounds targeting specific pain/itch-TRPs so that physiological protective mechanisms are not disabled to a degree that stimulus-mediated injury can occur. Such events have impeded the development of safe and effective TRPV1-modulating compounds and have diverted substantial resources. A beneficial outcome can be readily accomplished via simple dosing strategies, and also by incorporating medicinal chemistry design features during compound design and synthesis. Beyond clinical use, where compounds that target more than one channel might have a place and possibly have advantageous features, highly specific and high-potency compounds will be helpful in mechanistic discovery at the structure-function level.

Peripheral and spinal TRPA1 channels contribute to formalin-induced long-lasting mechanical hypersensitivity

Background

Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel expressed by a subset of nociceptive neurons that acts as a multimodal receptor. Its activity contributes to modulate nociceptive transmission in acute inflammatory pain. However, the role of this channel in chronic pain has been less studied. The purpose of this study was to investigate the local peripheral and spinal participation of TRPA1 channels in formalin-induced long-lasting hypersensitivity.

Materials and methods

Formalin (1%)-induced chronic hypersensitivity was determined by the application of von Frey filaments to ipsilateral and contralateral paws and through pharmacological testing using a selective TRPA1 blocker (A-967079). TRPA1 expression in the dorsal root ganglion (DRG) and spinal cord was analyzed by Western blotting.

Results

Formalin (1%) injection produced acute flinching behavior (1 h) as well as secondary allodynia and hyperalgesia (12 days). Local peripheral pretreatment (10 min before) or posttreatment (6 days later) with A-967079 (1–100 µM) partially prevented and reversed, respectively, in a dose-dependent manner, long-lasting secondary mechanical allodynia and hyperalgesia evoked by 1% formalin. Likewise, intrathecal pretreatment or posttreatment with A-967079 partially prevented and reversed, respectively, formalin-induced long-lasting hypersensitivity. A-967079 (100 µM) completely abolished the pro-nociceptive effect of formalin (adjusted to pH 7.4). Finally, formalin injection increased TRPA1 protein expression in the DRG and spinal cord.

Conclusion

Results indicate that TRPA1 expressed in the DRG and spinal cord plays a relevant role in formalin-induced long-lasting secondary nociceptive hypersensitivity.

Schwann cell TRPA1 mediates neuroinflammation that sustains macrophage-dependent neuropathic pain in mice

It is known that transient receptor potential ankyrin 1 (TRPA1) channels, expressed by nociceptors, contribute to neuropathic pain. Here we show that TRPA1 is also expressed in Schwann cells. We found that in mice with partial sciatic nerve ligation, TRPA1 silencing in nociceptors attenuated mechanical allodynia, without affecting macrophage infiltration and oxidative stress, whereas TRPA1 silencing in Schwann cells reduced both allodynia and neuroinflammation. Activation of Schwann cell TRPA1 evoked NADPH oxidase 1 (NOX1)-dependent H2O2 release, and silencing or blocking Schwann cell NOX1 attenuated nerve injury-induced macrophage infiltration, oxidative stress and allodynia. Furthermore, the NOX2-dependent oxidative burst, produced by macrophages recruited to the perineural space activated the TRPA1-NOX1 pathway in Schwann cells, but not TRPA1 in nociceptors. Schwann cell TRPA1 generates a spatially constrained gradient of oxidative stress, which maintains macrophage infiltration to the injured nerve, and sends paracrine signals to activate TRPA1 of ensheathed nociceptors to sustain mechanical allodynia.

Downregulations of TRPM8 expression and membrane trafficking in dorsal root ganglion mediate the attenuation of cold hyperalgesia in CCI rats induced by GFRα3 knockdown

BACKGROUND

Cold hyperalgesia is an intractable sensory abnormality commonly seen in peripheral neuropathies. Although glial cell line-derived neurotrophic factor family receptor alpha3 (GFRα3) is required for the formation of pathological cold pain has been revealed, potential transduction mechanism is poorly elucidated. We have previously demonstrated the contribution of enhanced activity of transient receptor potential melastatin 8 (TRPM8) to cold hyperalgesia in neuropathic pain using a rat model of chronic constriction injury (CCI) to the sciatic nerve. Recently, the enhancement of TRPM8 activity is attributed to the increased TRPM8 plasma membrane trafficking. In addition, TRPM8 can be sensitized by the activation of GFRα3, leading to increased cold responses in vivo. The aim of this study was to investigate whether GFRα3 could influence cold hyperalgesia of CCI rats via modulating TRPM8 expression and plasma membrane trafficking in dorsal root ganglion (DRG).

METHODS

Mechanical allodynia, cold and heat hyperalgesia were measured on 1day before CCI and the 1st, 4th, 7th, 10th and 14th day after CCI. TRPM8 total expression and membrane trafficking as well as GFRα3 expression in DRG were detected by immunofluorescence and western blot. Furthermore, GFRα3 small interfering RNA (siRNA) was intrathecally administrated to reduce GFRα3 expression in DRG, and the effects of GFRα3 knockdown on CCI-induced behavioral sensitization as well as TRPM8 total expression and membrane trafficking in both mRNA and protein levels were investigated, and the change in coexpression of TRPM8 with GFRα3 was also evaluated. Then, the effect of GFRα3 activation with artemin on pain behavior of CCI rats pretreated with the selective TRPM8 antagonist RQ-00203078 was observed.

RESULTS

Here we found that TRPM8 total expression and plasma membrane trafficking as well as GFRα3 expression in DRG were initially increased on the 4th day after CCI, and maintained at the peak level from the 10th to the 14th day, which entirely conformed with the induction and maintenance of behavioral-reflex facilitation following CCI. The coexpression of TRPM8 with GFRα3, which was mainly located in peptidergic C-fibers DRG neurons, was also increased after CCI. Downregulation of GFRα3 protein in DRG attenuated CCI-induced cold hyperalgesia without affecting mechanical allodynia and heat hyperalgesia, and reduced the upregulations of TRPM8 total expression and plasma membrane trafficking as well as coexpression of TRPM8 with GFRα3 induced by CCI. Additionally, the inhibition of TRPM8 abolished the influence of GFRα3 activation on cold hyperalgesia after CCI.

CONCLUSION

Our results demonstrate that GFRα3 knockdown specially inhibits cold hyperalgesia following CCI via decreasing the expression level and plasma membrane trafficking of TRPM8 in DRG. GFRα3 and its downstream mediator, TRPM8, represent a new analgesia axis which can be further exploited in sensitized cold reflex under the condition of chronic pain.

Postinflammatory hyperpigmentation after human cold pain testing

After assessment of thermal sensitivity during development of a novel cold pain assay, one subject developed postinflammatory hyperpigmentation. Resultant epidermal darkening gradually faded without clinical intervention.

Changes in cold temperature sensitivity are often associated with chronic pain conditions. Progress in understanding the neurobiological mechanism underlying these changes and resulting development of effective therapies has been slowed by the accessibility and affordability of devices used to measure thermal sensitivity in humans. To address this gap, we developed an inexpensive method to measure cold pain thresholds in healthy adult volunteers using dry ice and a thermode. However, early in preliminary testing, a subject presented with epidermal postinflammatory hyperpigmentation that lasted for >200 days. Although this response was unique among the small number of subjects in development of the assay, it raised questions as to the safety of the assay design.

N/OFQ system in brain areas of nerve-injured mice: its role in different aspects of neuropathic pain

Several studies showed that chronic pain causes reorganization and functional alterations of supraspinal brain regions. The nociceptin-NOP receptor system is one of the major systems involved in pain control and much evidence also suggested its implication in stress, anxiety and depression. Therefore, we investigated the nociceptin-NOP system alterations in selected brain regions in a neuropathic pain murine model. Fourteen days after the common sciatic nerve ligature, polymerase chain reaction (PCR) analysis indicated a significant decrease of pronociceptin and NOP receptor mRNA levels in the thalamus; these alterations could contribute to the decrease of the thalamic inhibitory function reported in neuropathic pain condition. Nociceptin peptide and NOP mRNA increased in the anterior cingulate cortex (ACC) and not in the somatosensory cortex, suggesting a peculiar involvement of this system in pain regulating circuitry. Similarly to the ACC, an increase of nociceptin peptide levels was observed in the amygdala. Finally, the pronociceptin and NOP mRNAs decrease observed in the hypothalamus reflects the lack of hypothalamus-pituitary-adrenal axis activation, already reported in neuropathic pain models. Our data indicate that neuropathic pain conditions affect the supraspinal nociceptin-NOP system which is also altered in regions known to play a role in emotional aspects of pain.

Characterization of TRPA channels in the starfish Patiria pectinifera: involvement of thermally activated TRPA1 in thermotaxis in marine planktonic larvae

The vast majority of marine invertebrates spend their larval period as pelagic plankton and are exposed to various environmental cues. Here we investigated the thermotaxis behaviors of the bipinnaria larvae of the starfish, Patiria pectinifera, in association with TRPA ion channels that serve as thermal receptors in various animal species. Using a newly developed thermotaxis assay system, we observed that P. pectinifera larvae displayed positive thermotaxis toward high temperatures, including toward temperatures high enough to cause death. In parallel, we identified two TRPA genes, termed PpTRPA1 and PpTRPA basal, from this species. We examined the phylogenetic position, spatial expression, and channel properties of each PpTRPA. Our results revealed the following: (1) The two genes diverged early in animal evolution; (2) PpTRPA1 and PpTRPA basal are expressed in the ciliary band and posterior digestive tract of the larval body, respectively; and (3) PpTRPA1 is activated by heat stimulation as well as by known TRPA1 agonists. Moreover, knockdown and rescue experiments demonstrated that PpTRPA1 is involved in positive thermotaxis in P. pectinifera larvae. This is the first report to reveal that TRPA1 channels regulate the behavioral response of a marine invertebrate to temperature changes during its planktonic larval period.

Transplantation of Cultured Olfactory Bulb Cells Prevents Abnormal Sensory Responses During Recovery From Dorsal Root Avulsion in the Rat

The central branches of the C7 and C8 dorsal roots were avulsed close to their entry point into the spinal cord in adult rats. The forepaw responses to heat and cold stimuli were tested at 1, 2, and 3 weeks after injury. Over this period, the paws were sensitive to both stimuli at 1-2 weeks and returned toward normal at 3 weeks. Immunohistology showed no evidence of axonal regeneration into the spinal cord in a control group of rats with avulsion only, implying that adjacent dorsal roots and their corresponding dermatomes were involved in the recovery. In a further group of rats, a mixture of bulbar olfactory ensheathing cells and olfactory nerve fibroblasts were transplanted into the gap between the avulsed roots and the spinal cord at the time of avulsion. These rats showed no evidence of either loss of sensation or exaggerated responses to stimuli at any of the time points from 1 to 3 weeks. Immunohistology showed that the transplanted cells formed a complete bridge, and the central branches of the dorsal root fibers had regenerated into the dorsal horn of the spinal cord. These regenerating axons, including Tuj1 and CGRP immunoreactive fibers, were ensheathed by the olfactory ensheathing cells. This confirms our previous demonstration of central regeneration by these transplants and suggests that such transplants may provide a useful means to prevent the development of abnormal sensations such as allodynia after spinal root lesions.

Role of the Excitability Brake Potassium Current IKD in Cold Allodynia Induced by Chronic Peripheral Nerve Injury

Cold allodynia is a common symptom of neuropathic and inflammatory pain following peripheral nerve injury. The mechanisms underlying this disabling sensory alteration are not entirely understood. In primary somatosensory neurons, cold sensitivity is mainly determined by a functional counterbalance between cold-activated TRPM8 channels and Shaker-like Kv1.1-1.2 channels underlying the excitability brake current I_KD Here we studied the role of I_KD in damage-triggered painful hypersensitivity to innocuous cold. We found that cold allodynia induced by chronic constriction injury (CCI) of the sciatic nerve in mice, was related to both an increase in the proportion of cold-sensitive neurons (CSNs) in DRGs contributing to the sciatic nerve, and a decrease in their cold temperature threshold. I_KD density was reduced in high-threshold CSNs from CCI mice compared with sham animals, with no differences in cold-induced TRPM8-dependent current density. The electrophysiological properties and neurochemical profile of CSNs revealed an increase of nociceptive-like phenotype among neurons from CCI animals compared with sham mice. These results were validated using a mathematical model of CSNs, including I_KD and TRPM8, showing that a reduction in I_KD current density shifts the thermal threshold to higher temperatures and that the reduction of this current induces cold sensitivity in former cold-insensitive neurons expressing low levels of TRPM8-like current. Together, our results suggest that cold allodynia is largely due to a functional downregulation of I_KD in both high-threshold CSNs and in a subpopulation of polymodal nociceptors expressing TRPM8, providing a general molecular and neural mechanism for this sensory alteration.SIGNIFICANCE STATEMENT This paper unveils the critical role of the brake potassium current I_KD in damage-triggered cold allodynia. Using a well-known form of nerve injury and combining behavioral analysis, calcium imaging, patch clamping, and pharmacological tools, validated by mathematical modeling, we determined that the functional expression of I_KD is reduced in sensory neurons in response to peripheral nerve damage. This downregulation not only enhances cold sensitivity of high-threshold cold thermoreceptors signaling cold discomfort, but it also transforms a subpopulation of polymodal nociceptors signaling pain into neurons activated by mild temperature drops. Our results suggest that cold allodynia is linked to a reduction of I_KD in both high-threshold cold thermoreceptors and nociceptors expressing TRPM8, providing a general model for this form of cold-induced pain.

Forsythoside A exerts antipyretic effect on yeast-induced pyrexia mice via inhibiting transient receptor potential vanilloid 1 function

Transient receptor potential vanilloid 1 (TRPV1) is a non-selective cation channel gated by noxious heat, playing major roles in thermoregulation. Forsythoside A (FT-A) is the most abundant phenylethanoid glycosides in Fructus Forsythiae, which has been prescribed as a medicinal herb for treating fever in China for a long history. However, how FT-A affects pyrexia and what is the underlying molecular mechanism remain largely unknown. Here we found that FT-A exerted apparent antipyretic effect through decreasing the levels of prostaglandin E₂ (PGE₂) and interleukin 8 (IL-8) in a dose-dependent fashion on the yeast induced pyrexia mice. Interestingly, FT-A significantly downregulated TRPV1 expression in the hypothalamus and dorsal root ganglion (DRG) of the yeast induced pyrexia mice. Moreover, FT-A inhibited IL-8 and PGE₂ secretions, and calcium influx in the HEK 293T-TRPV1 cells after stimulated with capsaicin, the specific TRPV1 agonist. Further investigation of the molecular mechanisms revealed that FT-A treatment rapidly inhibited phosphorylation of extracellular signal-regulated kinase (ERK), Jun N-terminal kinase (JNK) and p38 in both yeast induced pyrexia mice and HEK 293T-TRPV1 cells. These results suggest that FT-A may serve as a potential antipyretic agent and the therapeutic action of Fructus Forsythiae on pyretic related disease is, in part, due to the FT-A activities.

Orthopedic surgery modulates neuropeptides and BDNF expression at the spinal and hippocampal levels

Pain is a critical component hindering recovery and regaining of function after surgery, particularly in the elderly. Understanding the role of pain signaling after surgery may lead to novel interventions for common complications such as delirium and postoperative cognitive dysfunction. Using a model of tibial fracture with intramedullary pinning in male mice, associated with cognitive deficits, we characterized the effects on the primary somatosensory system. Here we show that tibial fracture with pinning triggers cold allodynia and up-regulates nerve injury and inflammatory markers in dorsal root ganglia (DRGs) and spinal cord up to 2 wk after intervention. At 72 h after surgery, there is an increase in activating transcription factor 3 (ATF3), the neuropeptides galanin and neuropeptide Y (NPY), brain-derived neurotrophic factor (BDNF), as well as neuroinflammatory markers including ionized calcium-binding adaptor molecule 1 (Iba1), glial fibrillary acidic protein (GFAP), and the fractalkine receptor CX3CR1 in DRGs. Using an established model of complete transection of the sciatic nerve for comparison, we observed similar but more pronounced changes in these markers. However, protein levels of BDNF remained elevated for a longer period after fracture. In the hippocampus, BDNF protein levels were increased, yet there were no changes in Bdnf mRNA in the parent granule cell bodies. Further, c-Fos was down-regulated in the hippocampus, together with a reduction in neurogenesis in the subgranular zone. Taken together, our results suggest that attenuated BDNF release and signaling in the dentate gyrus may account for cognitive and mental deficits sometimes observed after surgery.

Challenges to develop novel anti-inflammatory and analgesic drugs

Chronic inflammatory diseases and persistent pain of different origin represent common medical, social, and economic burden, and their pharmacotherapy is still an unresolved issue. Therefore, there is a great and urgent need to develop anti-inflammatory and analgesic agents with novel mechanisms of action, but it is a very challenging task. The main problem is the relatively large translational gap between the preclinical experimental data and the clinical results due to characteristics of the models, difficulties with the investigational techniques particularly for pain, as well as species differences in the mechanisms. We summarize here the current state-of-the-art medication and related ongoing strategies, and the novel targets with lead molecules under clinical development. The first members of the gold-standard categories, such as nonsteroidal anti-inflammatory drugs, glucocorticoids, and opioids, were introduced decades ago, and since then very few drugs with novel mechanisms of action have been successfully taken to the clinics despite considerable development efforts. Several biologics targeting different key molecules have provided breakthrough in some autoimmune/inflammatory diseases, but they are expensive, only parenterally available, their long-term side effects often limit their administration, and they do not effectively reduce pain. Some kinase inhibitors and phosphodiesterase-4 blockers have recently been introduced as new directions. There are in fact some promising novel approaches at different clinical stages of drug development focusing on transient receptor potential vanilloid 1/ankyrin 1 channel antagonism, inhibition of voltage-gated sodium/calcium channels, several enzymes (kinases, semicarbazide-sensitive amine oxidases, and matrix metalloproteinases), cytokines/chemokines, transcription factors, nerve growth factor, and modulation of several G protein-coupled receptors (cannabinoids, purinoceptors, and neuropeptides). WIREs Nanomed Nanobiotechnol 2017, 9:e1427. doi: 10.1002/wnan.1427 For further resources related to this article, please visit the WIREs website.