ETAR and protein kinase A pathway mediate ET-1 sensitization of TRPA1 channel: A molecular mechanism of ET-1-induced mechanical hyperalgesia

Involvement of interaction of Cav3.2 and nociceptive TRPA1 in pathological pain transmission

T-type Ca2+ channels and TRPA1 expressed in sensory neurons are involved in pain. We previously demonstrated a functional interaction of these channels under physiological conditions. Here we investigated the possible involvement of these channels in inflammatory pain condition. We also evaluated the relationship of these channels endogenously expressed in RIN-14B, a rat pancreatic islet tumor cell line. In dorsal root ganglion (DRG) neurons innervated inflammatory side, [Ca2+]i increases induced by 15 mM KCl (15K) were enhanced in neurons responded to AITC. This enhancement was not observed in genetically TRPA1-deficient neurons. The T-type and AITC-induced currents were larger in neurons of the inflammatory side than in those of the control one. In DRGs of the inflammatory side, the protein expression of Cav3.2, but not TRPA1, was increased. In RIN-14B, 15K-induced [Ca2+]i increases were decreased by blockers of T-type Ca2+ channel and TRPA1, and by TRPA1-silencing. Immunoprecipitation suggested the coexistent of these channels in sensory neurons and RIN-14B. In mice with inflammation, mechanical hypersensitivity was suppressed by blockers of both channels. These data suggest that the interaction of Cav3.2 with TRPA1 in sensory neurons is enhanced via the augmentation of the activities of both channels under inflammatory conditions, indicating that both channels are therapeutic targets for inflammatory pain.

Paclitaxel Regulates TRPA1 Function and Expression Through PKA and PKC

Paclitaxel (PTX) is a frequently used anticancer drug that causes peripheral neuropathy. Transient receptor potential ankyrin 1 (TRPA1), a plasma membrane calcium channel, has been associated with PTX toxicity and with other chemotherapy agents such as oxaliplatin and vincristine. However, the effect of PTX on the functional expression and calcium currents of TRPA1 has not been determined. The present study shows the effect of PTX on TRPA1 activity in a neuronal cell line (SH-SY5Y). The effect of PTX on the expression of TRPA1 was assessed through quantitative PCR and Western blot analyses to determine the relative mRNA and protein expression levels. To assess the effect on calcium flux and currents, cells were exposed to PTX; simultaneously, a specific agonist and antagonist of TRPA1 were added to evaluate the differential response in exposed versus control cells. To assess the effect of PKA, PKC and PI3K on PTX-induced TRPA1 increased activity, selective inhibitors were added to these previous experiments. PTX increased the mRNA and protein expression of TRPA1 as well as the TRPA1-mediated Ca2+ currents and intracellular Ca2+ concentrations. This effect was dependent on AITC (a selective specific agonist) and was abolished with HC-030031 (a selective specific antagonist). The inhibition of PKA and PKC reduced the effect of PTX on the functional expression of TRPA1, whereas the inhibition of PI3K had no effects. PTX-induced neuropathy involves TRPA1 activity through an increase in functional expression and is regulated by PKA and PKC signaling. These findings support the role of the TRPA1 channel in the mechanisms altered by PTX, which can be involved in the process that lead to chemotherapy-induced neuropathy.

Estrogen metabolites increase nociceptor hyperactivity in a mouse model of uterine pain

Pain emanating from the female reproductive tract is notoriously difficult to be treated and the prevalence of transient pelvic pain has been placed as high as 70-80% in women surveyed. Although sex hormones, especially estrogen, are thought to underlie enhanced pain perception in females, the underlying molecular and cellular mechanisms are not completely understood. Here we show that the pain-initiating TRPA1 channel is required for pain-related behaviors in a mouse model of estrogen-induced uterine pain in ovariectomized female mice. Surprisingly, 2- and 4-hydroxylated estrogen metabolites (HEMs) in the estrogen hydroxylation pathway, but not estrone, estradiol and 16-HEMs, directly increase nociceptor hyperactivity through TRPA1 and TRPV1 channels, and picomolar concentrations of 2- and 4-hydroxylation estrone (OHE1) can sensitize TRPA1 channel function. Moreover, both TRPA1 and TRPV1 are expressed in uterine-innervating primary nociceptors and their expressions are increased in the estrogen-induced uterine pain model. Importantly, pretreatment of 2- or 4-OHE1 recapitulates estrogen-induced uterine pain-like behaviors and intraplantar injections of 2- and 4-OHE1 directly produce a TRPA1-dependent mechanical hypersensitivity. Our findings demonstrate that TRPA1 is critically involved in estrogen-induced uterine pain-like behaviors, which may provide a potential drug target for treating female reproductive tract pain.

Nrf2 Activation Mediates Antiallodynic Effect of Electroacupuncture on a Rat Model of Complex Regional Pain Syndrome Type-I through Reducing Local Oxidative Stress and Inflammation

Complex regional pain syndrome type-I (CRPS-I) represents a type of neurovascular condition featured by severe pain in affected extremities. Few treatments have proven effective for CRPS-I. Electroacupuncture (EA) is an effective therapy for pain relief. We explored the mechanism through which EA ameliorates pain in a rat CRPS-I model. The chronic postischemic pain (CPIP) model was established using Sprague-Dawley rats to mimic CRPS-I. We found that oxidative stress-related biological process was among the predominant biological processes in affected hindpaw of CPIP rats. Oxidative stress occurred primarily in local hindpaw but not in the spinal cord or serum of model rats. Antioxidant N-acetyl cysteine (NAC) attenuated mechanical allodynia and spinal glia overactivation in CPIP model rats, whereas locally increasing oxidative stress is sufficient to induce chronic pain and spinal glia overactivation in naive rats. EA exerted remarkable antiallodynia on CPIP rats by reducing local oxidative stress via enhancing nuclear factor erythroid 2-related factor 2 (Nrf2) expression. Pharmacological blocking Nrf2 abolished antioxidative and antiallodynic effects of EA. EA reduced spinal glia overactivation, attenuated the upregulation of inflammatory cytokines, reduced the enhanced TRPA1 channel activity in dorsal root ganglion neurons innervating the hindpaws, and improved blood flow dysfunction in hindpaws of CPIP rats, all of which were mimicked by NAC treatment. Thus, we identified local oxidative injury as an important contributor to pathogenesis of animal CRPS-I model. EA targets local oxidative injury by enhancing endogenous Nrf2-mediated antioxidative mechanism to relieve pain and inflammation. Our study indicates EA can be an alternative option for CRPS-I management.

Presence of TRPA1 Modifies CD4+/CD8+ T Lymphocyte Ratio and Activation

Transient Receptor Potential Ankyrin 1 (TRPA1) has been reported to influence neuroinflammation and lymphocyte function. We analysed the immune phenotype and activation characteristics of TRPA1-deficient mice (knockout—KO) generated by targeted deletion of the pore-loop domain of the ion channel. We compared TRPA1 mRNA and protein expression in monocyte and lymphocyte subpopulations isolated from primary and secondary lymphatic organs of wild type (WT) and KO mice. qRT-PCR and flow cytometric studies indicated a higher level of TRPA1 in monocytes than in lymphocytes, but both were orders of magnitude lower than in sensory neurons. We found lower CD4+/CD8+ thymocyte ratios, diminished CD4/CD8 rates, and B cell numbers in the KO mice. Early activation marker CD69 was lower in CD4+ T cells of KO, while the level of CD8+/CD25+ cells was higher. In vitro TcR-mediated activation did not result in significant differences in CD69 level between WT and KO splenocytes, but lower cytokine (IL-1β, IL-6, TNF-α, IL-17A, IL-22, and RANTES) secretion was observed in KO splenocytes. Basal intracellular Ca²⁺ level and TcR-induced Ca²⁺ signal in T lymphocytes did not differ significantly, but interestingly, imiquimod-induced Ca²⁺ level in KO thymocytes was higher. Our results support the role of TRPA1 in the regulation of activation, cytokine production, and T and B lymphocytes composition in mice.

Endothelin receptor type A is involved in the development of oxaliplatin-induced mechanical allodynia and cold allodynia acting through spinal and peripheral mechanisms in rats

Oxaliplatin, a platinum-based chemotherapeutic agent, frequently causes severe neuropathic pain typically encompassing cold allodynia and long-lasting mechanical allodynia. Endothelin has been shown to modulate nociceptive transmission in a variety of pain disorders. However, the action of endothelin varies greatly depending on many variables, including pain causes, receptor types (endothelin type A (ET_A) and B (ET_B) receptors) and organs (periphery and spinal cord). Therefore, in this study, we investigated the role of endothelin in a Sprague–Dawley rat model of oxaliplatin-induced neuropathic pain. Intraperitoneal administration of bosentan, a dual ET_A/ET_B receptor antagonist, effectively blocked the development or prevented the onset of both cold allodynia and mechanical allodynia. The preventive effects were exclusively mediated by ET_A receptor antagonism. Intrathecal administration of an ET_A receptor antagonist prevented development of long-lasting mechanical allodynia but not cold allodynia. In marked contrast, an intraplantar ET_A receptor antagonist had a suppressive effect on cold allodynia but only had a partial and transient effect on mechanical allodynia. In conclusion, ET_A receptor antagonism effectively prevented long-lasting mechanical allodynia through spinal and peripheral actions, while cold allodynia was prevented through peripheral actions.

Mini-review: The nociceptive sensory functions of the polymodal receptor Transient Receptor Potential Ankyrin Type 1 (TRPA1)

Over the last 17 years since its cloning in 2003, the receptor-channel TRPA1 has received increasing attention due to its polymodal features and prominent role in pain signaling in a variety of human disease states. While evidence has been accumulating for non-neuronal TRPA1 expression, it is the presence of this channel in nociceptive nerve endings which has taken centre stage, due to its potential clinical ramifications. As a consequence, we shall focus in this review on the sensory functions of TRPA1 related to its expression in the peripheral nervous system. While substantial research has been focused on the putative role of TRPA1 in detecting irritant compounds, noxious cold and mechanical stimuli, the current overall picture is, to some extent, still cloudy. The chemosensory function of the channel is well demonstrated, as well as its involvement in the detection of oxidative and nitrosative stress; however, the other sensory features of TRPA1 have not been fully elucidated yet. The current state of the experimental evidence for these physiological roles of TRPA1 in mammals, and particularly in humans, will be discussed in this review.

Oxaliplatin Depolarizes the IB4- Dorsal Root Ganglion Neurons to Drive the Development of Neuropathic Pain Through TRPM8 in Mice

Use of chemotherapy drug oxaliplatin is associated with painful peripheral neuropathy that is exacerbated by cold. Remodeling of ion channels including TRP channels in dorsal root ganglion (DRG) neurons contribute to the sensory hypersensitivity following oxaliplatin treatment in animal models. However, it has not been studied if TRP channels and membrane depolarization of DRG neurons serve as the initial ionic/membrane drives (such as within an hour) that contribute to the development of oxaliplatin-induced neuropathic pain. In the current study, we studied in mice (1) in vitro acute effects of oxaliplatin on the membrane excitability of IB4⁺ and IB4^- subpopulations of DRG neurons using a perforated patch clamping, (2) the preventative effects of a membrane-hyperpolarizing drug retigabine on oxaliplatin-induced sensory hypersensitivity, and (3) the preventative effects of TRP channel antagonists on the oxaliplatin-induced membrane hyperexcitability and sensory hypersensitivity. We found (1) IB4⁺ and IB4^- subpopulations of small DRG neurons displayed previously undiscovered, substantially different membrane excitability, (2) oxaliplatin selectively depolarized IB4^- DRG neurons, (3) pretreatment of retigabine largely prevented oxaliplatin-induced sensory hypersensitivity, (4) antagonists of TRPA1 and TRPM8 channels prevented oxaliplatin-induced membrane depolarization, and (5) the antagonist of TRPM8 largely prevented oxaliplatin-induced sensory hypersensitivity. These results suggest that oxaliplatin depolarizes IB4^- neurons through TRPM8 channels to drive the development of neuropathic pain and targeting the initial drives of TRPM8 and/or membrane depolarization may prevent oxaliplatin-induce neuropathic pain.

The NLRP3 inflammasome: an emerging therapeutic target for chronic pain

Chronic pain affects the life quality of the suffering patients and posts heavy problems to the health care system. Conventional medications are usually insufficient for chronic pain management and oftentimes results in many adverse effects. The NLRP3 inflammasome controls the processing of proinflammatory cytokine interleukin 1β (IL-1β) and is implicated in a variety of disease conditions. Recently, growing number of evidence suggests that NLRP3 inflammasome is dysregulated under chronic pain condition and contributes to pathogenesis of chronic pain. This review provides an up-to-date summary of the recent findings of the involvement of NLRP3 inflammasome in chronic pain and discussed the expression and regulation of NLRP3 inflammasome-related signaling components in chronic pain conditions. This review also summarized the successful therapeutic approaches that target against NLRP3 inflammasome for chronic pain treatment.

Fibroblast growth factor 13 stabilizes microtubules to promote Na+ channel function in nociceptive DRG neurons and modulates inflammatory pain

Introduction

Fibroblast growth factor homologous factors (FHFs), among other fibroblast growth factors, are increasingly found to be important regulators of ion channel functions. Although FHFs have been link to several neuronal diseases and arrhythmia, its role in inflammatory pain still remains unclear.

Objectives

This study aimed to investigate the role and mechanism of FGF13 in inflammatory pain.

Methods

Fgf13 conditional knockout mice were generated and CFA-induced chronic inflammatory pain model was established to measure the pain threshold. Immunostaining, western blot and quantitative real-time reverse transcription PCR (qRT-PCR) were performed to detect the expression of FGF13 in CFA-induced inflammatory pain. Whole-cell patch clamp recording was used to record the action potential firing properties and sodium currents of DRG neurons.

Results

Conditional knockout of Fgf13 in dorsal root ganglion (DRG) neurons (Fgf13^-/Y) led to attenuated pain responses induced by complete Freund's adjuvant (CFA). FGF13 was expressed predominantly in small-diameter DRG neurons. CFA treatment resulted in an increased expression of FGF13 proteins as well as an increased excitability in nociceptive DRG neurons which was inhibited when FGF13 was absent. The role of FGF13 in neuronal excitability of DRG was linked to its modulation of voltage-gated Na⁺ channels mediated by microtubules. Overexpression of FGF13, but not FGF13 mutant which lacks the ability to bind and stabilize microtubules, rescued the decreased neuronal excitability and Na⁺ current density in DRG neurons of Fgf13^-/Y mice.

Conclusion

This study revealed that FGF13 could stabilize microtubules to modulate sodium channel function in DRG neurons and modulate inflammatory pain. This study provides a novel mechanism for FGF13 modulation of sodium channel function and suggests that FGF13 might be a novel target for inflammatory pain treatment.

IL-33/ST2 induces neutrophil-dependent reactive oxygen species production and mediates gout pain

Objective: Gout, induced by monosodium urate (MSU) crystal deposition in joint tissues, provokes severe pain and impacts life quality of patients. However, the mechanisms underlying gout pain are still incompletely understood.

Methods: We established a mouse gout model by intra-articularly injection of MSU crystals into the ankle joint of wild type and genetic knockout mice. RNA-Sequencing, in vivo molecular imaging, Ca²⁺ imaging, reactive oxygen species (ROS) generation, neutrophil influx and nocifensive behavioral assays, etc. were used.

Results: We found interleukin-33 (IL-33) was among the top up-regulated cytokines in the inflamed ankle. Neutralizing or genetic deletion of IL-33 or its receptor ST2 (suppression of tumorigenicity) significantly ameliorated pain hypersensitivities and inflammation. Mechanistically, IL-33 was largely released from infiltrated macrophages in inflamed ankle upon MSU stimulation. IL-33 promoted neutrophil influx and triggered neutrophil-dependent ROS production via ST2 during gout, which in turn, activated transient receptor potential ankyrin 1 (TRPA1) channel in dorsal root ganglion (DRG) neurons and produced nociception. Further, TRPA1 channel activity was significantly enhanced in DRG neurons that innervate the inflamed ankle via ST2 dependent mechanism, which results in exaggerated nociceptive response to endogenous ROS products during gout.

Conclusions: We demonstrated a previous unidentified role of IL-33/ST2 in mediating pain hypersensitivity and inflammation in a mouse gout model through promoting neutrophil-dependent ROS production and TRPA1 channel activation. Targeting IL-33/ST2 may represent a novel therapeutic approach to ameliorate gout pain and inflammation.

Potentiation of P2X3 receptor mediated currents by endothelin-1 in rat dorsal root ganglion neurons

Endothelin-1 (ET-1), an endogenous vasoconstrictor, has been known as a pro-nociceptive agent involved in multitude of pain. ET-1 acts on endothelin receptors on vascular endothelial cells, sensitizes release of ATP, which then acts on P2X3 receptors on nociceptors and results in mechanical hyperalgesia. Both endothelin receptors and P2X3 receptors are present in primary sensory neuron, where it remains unclear whether there is an interaction between them. Herein, we reported that ET-1 potentiated the electrophysiological activity of P2X3 receptors in rat dorsal root ganglia (DRG) neurons. ET-1 concentration-dependently increased α,β-methylene-ATP (α,β-meATP)-evoked inward currents, which were mediated by P2X3 receptors. ET-1 shifted the α,β-meATP concentration-response curve upwards, with an increase of 34.38 ± 4.72% in the maximal current response to α,β-meATP in the presence of ET-1. ET-1 potentiation of α,β-meATP-evoked currents was voltage-independent. ET-1 potentiated P2X3 receptor-mediated currents through endothelin-A receptors (ET_AR), but not endothelin-B receptors (ET_BR). ET-1 potentiation was supressed by blockade of intracellular G-protein or protein kinase C (PKC) signaling. Moreover, there is a synergistic effect on mechanical allodynia induced by intraplantar injection of ET-1 and α,β-meATP in rats. Pharmacological blockade of P2X3 receptors also alleviated ET-1-induced mechanical allodynia. These results suggested that ET-1 sensitized P2X3 receptors in primary sensory neurons via an ET_AR and PKC signaling pathway. Our data provide evidence that cutaneous ET-1 induced mechanical allodynia not only by increasing the release of ATP from vascular endothelial cells, but also by sensitizing P2X3 receptors on nociceptive DRG neurons.

Endothelin-1 enhances acid-sensing ion channel currents in rat primary sensory neurons

Endothelin-1 (ET-1), an endogenous vasoactive peptide, has been found to play an important role in peripheral pain signaling. Acid-sensing ion channels (ASICs) are key sensors for extracellular protons and contribute to pain caused by tissue acidosis. It remains unclear whether an interaction exists between ET-1 and ASICs in primary sensory neurons. In this study, we reported that ET-1 enhanced the activity of ASICs in rat dorsal root ganglia (DRG) neurons. In whole-cell voltage-clamp recording, ASIC currents were evoked by brief local application of pH 6.0 external solution in the presence of TRPV1 channel blocker AMG9810. Pre-application with ET-1 (1−100 nM) dose-dependently increased the proton-evoked ASIC currents with an EC₅₀ value of 7.42 ± 0.21 nM. Pre-application with ET-1 (30 nM) shifted the concentration–response curve of proton upwards with a maximal current response increase of 61.11% ± 4.33%. We showed that ET-1 enhanced ASIC currents through endothelin-A receptor (ET_AR), but not endothelin-B receptor (ET_BR) in both DRG neurons and CHO cells co-expressing ASIC3 and ET_AR. ET-1 enhancement was inhibited by blockade of G-protein or protein kinase C signaling. In current-clamp recording, pre-application with ET-1 (30 nM) significantly increased acid-evoked firing in rat DRG neurons. Finally, we showed that pharmacological blockade of ASICs by amiloride or APETx2 significantly alleviated ET-1-induced flinching and mechanical hyperalgesia in rats. These results suggest that ET-1 sensitizes ASICs in primary sensory neurons via ET_AR and PKC signaling pathway, which may contribute to peripheral ET-1-induced nociceptive behavior in rats.

Electroacupuncture Alleviates Paclitaxel-Induced Peripheral Neuropathic Pain in Rats via Suppressing TLR4 Signaling and TRPV1 Upregulation in Sensory Neurons

Paclitaxel-induced peripheral neuropathy is a common adverse effect during paclitaxel treatment resulting in sensory abnormalities and neuropathic pain during chemotherapy and in cancer survivors. Conventional therapies are usually ineffective and possess adverse effects. Here, we examined the effects of electroacupuncture (EA) on a rat model of paclitaxel-induced neuropathic pain and related mechanisms. EA robustly and persistently alleviated paclitaxel-induced pain hypersensitivities. Mechanistically, TLR4 (Toll-Like Receptor 4) and downstream signaling MyD88 (Myeloid Differentiation Primary Response 88) and TRPV1 (Transient Receptor Potential Vallinoid 1) were upregulated in dorsal root ganglion (DRGs) of paclitaxel-treated rats, whereas EA reduced their overexpression. Ca²⁺ imaging further indicated that TRPV1 channel activity was enhanced in DRG neurons of paclitaxel-treated rats whereas EA suppressed the enhanced TRPV1 channel activity. Pharmacological blocking of TRPV1 mimics the analgesic effects of EA on the pain hypersensitivities, whereas capsaicin reversed EA’s effect. Spinal astrocytes and microglia were activated in paclitaxel-treated rats, whereas EA reduced the activation. These results demonstrated that EA alleviates paclitaxel-induced peripheral neuropathic pain via mechanisms possibly involving suppressing TLR4 signaling and TRPV1 upregulation in DRG neurons, which further result in reduced spinal glia activation. Our work supports EA as a potential alternative therapy for paclitaxel-induced neuropathic pain.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.