Roles of Proton-Sensing Receptors in the Transition from Acute to Chronic Pain

The protective effects of orexin B in neuropathic pain by suppressing inflammatory response

Chronic pain induced by pathological insults to the sensorimotor system is a typical form of neuropathic pain (NP), and the underlying mechanism is complex. Currently, there are no successful therapeutic interventions for NP. Orexin B is a neuropeptide with a wide range of biological functions. However, the pharmacological function of orexin B in chronic neuropathic pain has been less studied. Here, we aim to examine the neuroprotective effects of orexin B in chronic constriction injury (CCI)- induced NP. Firstly, we found that orexin type 2 receptor (OX2R) but not orexin type 1 receptor (OX1R) was reduced in the spinal cord (SC) of CCI-treated rats. Mechanical withdrawal threshold and thermal withdrawal latency assays display that administration of orexin B clearly ameliorated CCI-evoked neuropathic pain dose-dependently. Notably, orexin B treatment also effectively prevented microglia activation by reducing the levels of IBA1. Additionally, orexin B was also found to suppress the inflammatory response in the SC tissue by reducing the levels of IL-6, TNF-α, iNOS, and COX-2 as well as the production of NO and PGE2 in CCI-treated rats. Furthermore, orexin B administration attenuated oxidative stress (OS) by increasing the activity of SOD and the levels of GSH. Mechanically, orexin B prevented activation of JNK/NF-κB signaling in the SC of CCI-treated rats. Based on these findings, we conclude that orexin B might have a promising role in ameliorating CCI-evoked neuropathic pain through the inhibition of microglial activation and inflammatory response.

Bidirectional sensitivity of CALHM1 channel to protons from both sides of plasma membrane

Calcium homeostasis modulator 1 (CALHM1), a newly discovered voltage-dependent nonselective ion channel, has drawn attention for its role in neuronal activity and taste sensation. Its sluggish voltage-dependent activation is facilitated by lowering extracellular Ca²⁺ concentration ([Ca²⁺]_e). Here, we investigated the effects of extracellular and intracellular pH (pH_e and pH_i) on human CALHM1. When normalized to the amplitude of the CALHM1 current (I_CALHM1) under whole cell patch clamp at symmetrical pH 7.4, I_CALHM1 decreased at acidic pH_e or pH_i, whereas it sharply increased at alkaline pH_e or pH_i. The effects of pH were preserved in the inside-out configuration. The voltage dependence of I_CALHM1 showed leftward and rightward shifts at alkaline and acidic pH_e and pH_i, respectively. Site-directed mutagenesis of the water-accessible charged residues of the pore and nearby domains revealed that E17, K229, E233, D257, and E259 are nonadditively responsible for facilitation at alkaline pH_i. Identification of the pH_e-sensing residue was not possible because mutation of putative residues impaired membrane expression, resulting in undetectable I_CALHM1. Alkaline pH_e-dependent facilitation appeared gradually with depolarization, suggesting that the sensitivity to pH_e might be due to H⁺ diffusion through the open-state CALHM1. At pH_e 6.2, decreased [Ca²⁺]_e could not recover the inhibited I_CALHM1 but further augmented the increased I_CALHM1 at pH_e 8.6, suggesting that unidentified common residues might contribute to the [Ca²⁺]_e and acidic pH_e. This study is the first, to our knowledge, to demonstrate the remarkable pH sensitivity of CALHM1, which might contribute to the pH-dependent modulation of neuronal excitability or taste sensation.

Lactic Acidosis in Patients with Solid Cancer

Significance: Cancer-associated tissue-specific lactic acidosis stimulates and mediates tumor invasion and metastasis and is druggable. Rarely, malignancy causes systemic lactic acidosis, the role of which is poorly understood. Recent Advances: The understanding of the role of lactate has shifted dramatically since its discovery. Long recognized as only a waste product, lactate has become known as an alternative metabolism substrate and a secreted nutrient that is exchanged between the tumor and the microenvironment. Tissue-specific lactic acidosis is targeted to improve the host body's anticancer defense and serves as a tool that allows the targeting of anticancer compounds. Systemic lactic acidosis is associated with poor survival. In patients with solid cancer, systemic lactic acidosis is associated with an extremely poor prognosis, as revealed by the analysis of 57 published cases in this study. Although it is considered a pathology worth treating, targeting systemic lactic acidosis in patients with solid cancer is usually inefficient. Critical Issues: Research gaps include simple questions, such as the unknown nuclear pH of the cancer cells and its effects on chemotherapy outcomes, pH sensitivity of glycosylation in cancer cells, in vivo mechanisms of response to acidosis in the absence of lactate, and overinterpretation of in vitro results that were obtained by using cells that were not preadapted to acidic environments. Future Directions: Numerous metabolism-targeting anticancer compounds induce lactatemia, lactic acidosis, or other types of acidosis. Their potential to induce acidic environments is largely overlooked, although the acidosis might contribute to a substantial portion of the observed clinical effects. Antioxid. Redox Signal. 37, 1130-1152.

Advances in the Understanding of Two-Pore Domain TASK Potassium Channels and Their Potential as Therapeutic Targets

TWIK-related acid-sensitive K⁺ (TASK) channels, including TASK-1, TASK-3, and TASK-5, are important members of the two-pore domain potassium (K_2P) channel family. TASK-5 is not functionally expressed in the recombinant system. TASK channels are very sensitive to changes in extracellular pH and are active during all membrane potential periods. They are similar to other K_2P channels in that they can create and use background-leaked potassium currents to stabilize resting membrane conductance and repolarize the action potential of excitable cells. TASK channels are expressed in both the nervous system and peripheral tissues, including excitable and non-excitable cells, and are widely engaged in pathophysiological phenomena, such as respiratory stimulation, pulmonary hypertension, arrhythmia, aldosterone secretion, cancers, anesthesia, neurological disorders, glucose homeostasis, and visual sensitivity. Therefore, they are important targets for innovative drug development. In this review, we emphasized the recent advances in our understanding of the biophysical properties, gating profiles, and biological roles of TASK channels. Given the different localization ranges and biologically relevant functions of TASK-1 and TASK-3 channels, the development of compounds that selectively target TASK-1 and TASK-3 channels is also summarized based on data reported in the literature.

Cellular mechanisms underlying central sensitization in a mouse model of chronic muscle pain

Chronic pain disorders are often associated with negative emotions, including anxiety and depression. The central nucleus of the amygdala (CeA) has emerged as an integrative hub for nociceptive and affective components during central pain development. Prior adverse injuries are precipitating factors thought to transform nociceptors into a primed state for chronic pain. However, the cellular basis underlying the primed state and the subsequent development of chronic pain remains unknown. Here, we investigated the cellular and synaptic alterations of the CeA in a mouse model of chronic muscle pain. In these mice, local infusion of pregabalin, a clinically approved drug for fibromyalgia and other chronic pain disorders, into the CeA or chemogenetic inactivation of the somatostatin-expressing CeA (CeA-SST) neurons during the priming phase prevented the chronification of pain. Further, electrophysiological recording revealed that the CeA-SST neurons had increased excitatory synaptic drive and enhanced neuronal excitability in the chronic pain states. Finally, either chemogenetic inactivation of the CeA-SST neurons or pharmacological suppression of the nociceptive afferents from the brainstem to the CeA-SST neurons alleviated chronic pain and anxio-depressive symptoms. These data raise the possibility of targeting treatments to CeA-SST neurons to prevent central pain sensitization.

Transcriptional Alterations of Mouse Trigeminal Ganglion Neurons Following Orofacial Inflammation Revealed by Single-Cell Analysis

Orofacial inflammation leads to transcriptional alterations in trigeminal ganglion (TG) neurons. However, diverse alterations and regulatory mechanisms following orofacial inflammatory pain in different types of TG neurons remain unclear. Here, orofacial inflammation was induced by injection of complete Freund’s adjuvant (CFA) in mice. After 7 days, we performed single-cell RNA-sequencing on TG cells of mice from control and treatment groups. We identified primary sensory neurons, Schwann cells, satellite glial cells, oligodendrocyte-like cells, immune cells, fibroblasts, and endothelial cells in TG tissue. After principal component analysis and hierarchical clustering, we identified six TG neuronal subpopulations: peptidergic nociceptors (PEP1 and PEP2), non-peptidergic nociceptors (NP1 and NP2), C-fiber low-threshold mechanoreceptors (cLTMR) and myelinated neurons (Nefh-positive neurons, NF) based on annotated marker gene expression. We also performed differential gene expression analysis among TG neuronal subtypes, identifying several differential genes involved in the inflammatory response, neuronal excitability, neuroprotection, and metabolic processes. Notably, we identified several potential novel targets associated with pain modulation, including Arl6ip1, Gsk3b, Scn7a, and Zbtb20 in PEP1, Rgs7bp in PEP2, and Bhlha9 in cLTMR. The established protein–protein interaction network identified some hub genes, implying their critical involvement in regulating orofacial inflammatory pain. Our study revealed the heterogeneity of TG neurons and their diverse neuronal transcriptomic responses to orofacial inflammation, providing a basis for the development of therapeutic strategies for orofacial inflammatory pain.

Phosphorylation at Ser 727 Increases STAT3 Interaction with PKCε Regulating Neuron–Glia Crosstalk via IL-6-Mediated Hyperalgesia In Vivo and In Vitro

Methods

A rat hyperalgesia model was induced using an intraplantar injection of Freund's complete adjuvant (FCA) or an intrathecal injection of IL-6. Mechanical allodynia was evaluated using von Frey filament tests after intrathecal injections of T-5224 (c-Fos/AP-1 inhibitor), minocycline (Mino, a specific microglia inhibitor), L-2-aminoadipic acid (LAA, an astroglial toxin), PKCε inhibitor peptide, APTSTAT3-9R (STAT3 inhibitor), or anti-IL-6 antibody. The c-Fos, GFAP, Iba-1, PKCε, STAT3, pSTAT3^Tyr705 and pSTAT3^Ser727, and IL-6 expression at the spinal cord level was assessed by Western blot analysis. The interactive effects of PKCε and STAT3 were determined using immunofluorescence staining and immunoprecipitation in vivo and in vitro. Interleukin-6 promoter activity was examined using luciferase assays.

Results

T-5224, Mino, and LAA attenuated FCA- or IL-6-mediated inflammatory pain, with a decrease in c-Fos, GFAP, Iba-1, PKCε, and IL-6 expression. PKCε inhibitor peptide and APTSTAT3-9R reversed FCA-induced nociceptive behavior, while decreasing pSTAT3^Ser727, IL-6, c-Fos, GFAP, and Iba-1 expression and PKCε and STAT3 coexpression. Interleukin-6 promoter activity increased in the presence of PKCε and STAT3. The interaction with PKCε increased on phosphorylating STAT3 at Ser727 but not at Tyr705.

Conclusion

STAT3 phosphorylation at Ser 727 and the interaction with PKCε contribute to hyperalgesia via the IL-6-mediated signaling pathway, thus regulating neuron–glia crosstalk during inflammatory pain.

Periodontal acidification contributes to tooth pain hypersensitivity during orthodontic tooth movement

Tooth movements associated with orthodontic treatment often cause tooth pain. However, the detailed mechanism remains unclear. Here, we examined the involvement of periodontal acidification caused by tooth movement in mechanical tooth pain hypersensitivity. Elastics were inserted between the first and second molars to move the teeth in Sprague-Dawley rats. Mechanical head-withdrawal reflex threshold to first molar stimulation and the pH of the gingival sulcus around the tooth were measured. The expression of acid-sensing ion channel 3 (ASIC3) in trigeminal ganglion neurons and phosphorylation of ASIC3 in the periodontal tissue were analyzed. The mechanical head-withdrawal reflex threshold to first molar stimulation and pH in the gingival sulcus decreased on day 1 after the elastic insertion. These decreases recovered to the sham level by buffering periodontal acidification. Periodontal inhibition of ASIC3 channel activity reversed the decreased mechanical head-withdrawal reflex threshold to first molar stimulation. On day 1 after elastic insertion, the tooth movement did not change the number of ASIC3 immunoreactive trigeminal ganglion neurons innervating the periodontal tissue but increased phosphorylated-ASIC3 levels in the periodontal tissue. Periodontal acidification induced by tooth movement causes phosphorylation of ASIC3, resulting in mechanical pain hypersensitivity in mechanically forced tooth.

Role of GABAAR in the Transition From Acute to Chronic Pain and the Analgesic Effect of Electroacupuncture on Hyperalgesic Priming Model Rats

Chronic pain is a costly health problem that impairs health-related quality of life when not effectively treated. Regulating the transition from acute to chronic pain is a new therapeutic strategy for chronic pain that presents a major clinical challenge. The underlying mechanisms of pain transition are not entirely understood, and strategies for preventing this transition are lacking. Here, a hyperalgesic priming model was used to study the potential mechanism by which γ-aminobutyric acid receptor type A (GABAAR) in the dorsal root ganglion (DRG) contributes to pain transition. Furthermore, electroacupuncture (EA), a modern method of acupuncture, was administered to regulate pain transition, and the mechanism underlying EA’s regulatory effect was investigated. Hyperalgesic priming was induced by intraplanar injection of carrageenan (Car)/prostaglandin E₂ (PGE₂). The decrease in mechanical withdrawal threshold (MWT) induced by PGE₂ returned to baseline 4 h after injection in NS + PGE₂ group, and still persisted 24 h after injection in Car + PGE₂ group. Lower expression of GABAAR in the lumbar DRG was observed in the model rats. Furthermore, activating or blocking GABAAR could reversed the long-lasting hyperalgesia induced by Car/PGE₂ injection or produced a persistent hyperalgesia. In addition, GABAAR may be involved in Protein Kinase C epsilon (PKCε) activation in the DRG, a mark molecular of pain transition. EA considerably increased the mechanical pain thresholds of hyperalgesic priming model mammals in both the acute and chronic phases. Furthermore, EA upregulated the expression of GABAAR and inhibited the activation of PKCε in the DRG. In addition, peripheral administration of picrotoxin blocked the analgesic effect of EA on the model rats and abolished the regulatory effect of EA on PKCε activation. These findings suggested that GABAAR plays a key role in both the transition from acute to chronic pain and the analgesic effect of EA on hyperalgesic priming.

Electroacupuncture Regulates Pain Transition by Inhibiting the mGluR5-PKCε Signaling Pathway in the Dorsal Root Ganglia

Background

Acute pain can transition to chronic pain, presenting a major clinical challenge. Electroacupuncture (EA) can partly prevent the transition from acute to chronic pain. However, little is known about the mechanisms underlying the effect of EA. This study investigated the effect of EA on pain transition and the activation of metabotropic glutamate receptor 5 (mGluR5)–protein kinase C epsilon (PKCε) signaling pathway in the dorsal root ganglia (DRG).

Methods

The hyperalgesic priming model was established by the sequential intraplantar injection of carrageenan (1%, 100 μL) and prostaglandin E2 (PGE2) into the left hind paw of rats. EA treatment (2/100 Hz, 30 min, once/day) was applied at bilateral Zusanli (ST36) and Kunlun (BL60) acupoints in rats. Von Frey filaments were used to investigate the mechanical withdrawal threshold (MWT) at different time points. The protein expression levels of mGluR5 and PKCε in the ipsilateral L4-L6 DRGs of rats were detected by Western blot. Some pharmacological experiments were performed to evaluate the relationship between mGluR5, PKCε and the MWT. It was also used to test the effects of EA on the expression levels of mGluR5 and PKCε and changes in the MWT.

Results

Sequential injection of carrageenan and PGE2 significantly decreased the MWT of rats and up-regulated the expression level of mGluR5 and PKCε in the ipsilateral L4-L6 DRGs. EA can reverse the hyperalgesic priming induced by sequential injection of carrageenan/PGE and down-regulate the protein expression of mGluR5 and PKCε. Glutamate injection instead of PGE2 can mimic the hyperalgesic priming model. Pharmacological blocking of mGluR5 with specific antagonist MTEP can prevent the hyperalgesic priming and inhibit the activation of PKCε in DRGs. Furthermore, EA also produced analgesic effect on the hyperalgesic priming rats induced by carrageenan/mGluR5 injection and inhibited the high expression of PKCε. Sham EA produced none analgesic and regulatory effect.

Conclusion

EA can regulate pain transition and it may relate with its inhibitory effect on the activation of mGluR5-PKCε signaling pathway in the DRGs.

Neuraxial Cytokines in Pain States

A high-intensity potentially tissue-injuring stimulus generates a homotopic response to escape the stimulus and is associated with an affective phenotype considered to represent pain. In the face of tissue or nerve injury, the afferent encoding systems display robust changes in the input-output function, leading to an ongoing sensation reported as painful and sensitization of the nociceptors such that an enhanced pain state is reported for a given somatic or visceral stimulus. Our understanding of the mechanisms underlying this non-linear processing of nociceptive stimuli has led to our appreciation of the role played by the functional interactions of neural and immune signaling systems in pain phenotypes. In pathological states, neural systems interact with the immune system through the actions of a variety of soluble mediators, including cytokines. Cytokines are recognized as important mediators of inflammatory and neuropathic pain, supporting system sensitization and the development of a persistent pathologic pain. Cytokines can induce a facilitation of nociceptive processing at all levels of the neuraxis including supraspinal centers where nociceptive input evokes an affective component of the pain state. We review here several key proinflammatory and anti-inflammatory cytokines/chemokines and explore their underlying actions at four levels of neuronal organization: (1) peripheral nociceptor termini; (2) dorsal root ganglia; (3) spinal cord; and (4) supraspinal areas. Thus, current thinking suggests that cytokines by this action throughout the neuraxis play key roles in the induction of pain and the maintenance of the facilitated states of pain behavior generated by tissue injury/inflammation and nerve injury.

Proteomic profiling of proteins in the dorsal horn of the spinal cord in dairy cows with chronic lameness

Chronic lameness affects bovine welfare and has a negative economic impact in dairy industry. Moreover, due to the translational gap between traditional pain models and new drugs development for treating chronic pain states, naturally occurring painful diseases could be a potential translational tool for chronic pain research. We therefore employed liquid chromatography tandem mass spectrometry (LC-MS/MS) to stablish the proteomic profile of the spinal cord samples from lumbar segments (L2-L4) of chronic lame dairy cows. Data were validated and quantified through software tool (Scaffold® v 4.0) using output data from two search engines (SEQUEST® and X-Tandem®). Search Tool for the Retrieval of Interacting Genes/Proteins (STRING) analysis was performed to detect proteins interactions. LC-MS/MS identified a total amount of 177 proteins; of which 129 proteins were able to be quantified. Lame cows showed a strong upregulation of interacting proteins with chaperone and stress functions such as Hsp70 (p < 0.006), Hsc70 (p < 0.0079), Hsp90 (p < 0.015), STIP (p > 0.0018) and Grp78 (p <0.0068), and interacting proteins associated to glycolytic pathway such as; γ-enolase (p < 0.0095), α-enolase (p < 0.013) and hexokinase-1 (p < 0.028). It was not possible to establish a clear network of interaction in several upregulated proteins in lame cows. Non-interacting proteins were mainly associated to redox process and cytoskeletal organization. The most relevant down regulated protein in lame cows was myelin basic protein (MBP) (p < 0.02). Chronic inflammatory lameness in cows is associated to increased expression of stress proteins with chaperone, metabolism, redox and structural functions. A state of endoplasmic reticulum stress and unfolded protein response (UPR) might explain the changes in protein expression in lame cows; however, further studies need to be performed in order to confirm these findings.

T-cell death-associated gene 8 accelerates atherosclerosis by promoting vascular smooth muscle cell proliferation and migration

BACKGROUND AND AIMS

Atherosclerosis is a serious cardiovascular disease, featuring inflammation, abnormal proliferation and migration of vascular smooth muscle cells (VSMCs). During atherosclerosis, inflammation may cause low pH. T-cell death-associated gene 8 (Tdag8) is a proton-sensing receptor, however, the role of Tdag8 in VSMCs remains unknown. This study aimed to investigate the potential effects of Tdag8 in VSMCs during atherosclerosis.

METHODS

We examined the expression of Tdag8 in an atherosclerotic model of high-fat-diet-fed ApoE^-/- mice, while the role and mechanism of Tdag8 in phenotype transformation, proliferation and migration of VSMCs were investigated in a series of in vivo and in vitro experiments.

RESULTS

We first found that Tdag8 expression at the mRNA and protein level was significantly increased in atherosclerotic ApoE^-/- mice. Immunofluorescence staining showed that Tdag8 was primarily distributed in PCNA-positive VSMCs and the phenotype of VSMCs switching from contractile phenotype to synthetic phenotype. Additionally, the protein level of Tdag8 was upregulated in FBS-treated VSMCs. VSMCs proliferation and migration were inhibited by Tdag8 silencing and increased by Tdag8 overexpression. Further mechanistic studies showed that cAMP level was increased in Tdag8-overexpressing VSMCs and ApoE^-/- mice. However, the PKA inhibitor H-89 reversed Tdag8-induced VSMC proliferation and migration.

CONCLUSIONS

The results demonstrate that Tdag8 mediated phenotype transformation, proliferation and migration of VSMCs via the cAMP/PKA signaling pathway, thus partially contributing to atherosclerosis.

Involvement of Acid-Sensing Ion Channel 1b in the Development of Acid-Induced Chronic Muscle Pain

Acid-sensing ion channels (ASICs) are important acid sensors involved in neural modulation in the central nervous system and pain-associated tissue acidosis in the peripheral system. Among ASIC subtypes, ASIC1b is the most selectively expressed in peripheral sensory neurons. However, the role of ASIC1b is still elusive in terms of its functions and expression profile. In this study, we probed the role of ASIC1b in acid-induced muscle pain in Asic1b-knockout (Asic1b^–/–) and Asic1b-Cre transgenic (Asic1b^Cre) mice. We tested the effect of ASIC1b knockout in a mouse model of fibromyalgia induced by dual intramuscular acid injections. In this model, a unilateral acid injection to the gastrocnemius muscle induced transient bilateral hyperalgesia in wild-type (Asic1b^+/+) but not Asic1b^–/– mice; a second acid injection, spaced 1 or 5 days apart, to the same muscle induced chronic hyperalgesia lasting for 4 weeks in Asic1b^+/+ mice, but the duration of hyperalgesia was significantly shortened in Asic1b^–/– mice. Mambalgin-1, an ASIC1b-containing channel inhibitor that was mixed with acid saline at the first injection, dose-dependently blocked the acid-induced transient and chronic hyperalgesia in Asic1b^+/+ mice. In contrast, psalmotoxin 1 (PcTx1), an ASIC1a-selective antagonist, had no effect on acid-induced transient or chronic hyperalgesia. We used whole-cell patch clamp recording to study the properties of acid-induced currents in ASIC1b-expressing dorsal root ganglia (DRG) neurons from Asic1b^Cre-TdTomato reporter mice. Medium- to large-sized ASIC1b-expressing DRG neurons mainly exhibited an amiloride-sensitive ASIC-like biphasic current (I_ASIC) in response to acid stimulation, whereas small- to medium-sized ASIC1b-expressing DRG neurons predominantly exhibited an amiloride-insensitive sustained current. Specifically, mambalgin-1 selectively inhibited the I_ASIC in most ASIC1b-expressing DRG neurons. However, PcTx1 or APETx2 (an ASIC3-selective antagonist) had only a mild inhibitory effect on I_ASIC in about half of the ASIC1b-expressing DRG neurons. In situ hybridization revealed that ASIC1b-positive DRG neurons co-expressed highly with ASIC1a and ASIC2a mRNA and partially with ASIC3 and ASIC2b. Thus, ASIC1b might form a wide variety of heteromeric channels. ASIC1b-containing heteromeric channels might be promising targets for the therapeutic treatment of acid-induced chronic muscle pain.

Evolution of acid nociception: ion channels and receptors for detecting acid

Nociceptors, i.e. sensory neurons tuned to detect noxious stimuli, are found in numerous phyla of the Animalia kingdom and are often polymodal, responding to a variety of stimuli, e.g. heat, cold, pressure and chemicals, such as acid. Owing to the ability of protons to have a profound effect on ionic homeostasis and damage macromolecular structures, it is no wonder that the ability to detect acid is conserved across many species. To detect changes in pH, nociceptors are equipped with an assortment of different acid sensors, some of which can detect mild changes in pH, such as the acid-sensing ion channels, proton-sensing G protein-coupled receptors and several two-pore potassium channels, whereas others, such as the transient receptor potential vanilloid 1 ion channel, require larger shifts in pH. This review will discuss the evolution of acid sensation and the different mechanisms by which nociceptors can detect acid. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Mechanisms of Below-Level Pain Following Spinal Cord Injury (SCI)

Mechanisms of below-level pain are discoverable as neural adaptations rostral to spinal injury. Accordingly, the strategy of investigations summarized here has been to characterize behavioral and neural responses to below-level stimulation over time following selective lesions of spinal gray and/or white matter. Assessments of human pain and the pain sensitivity of humans and laboratory animals following spinal injury have revealed common disruptions of pain processing. Interruption of the spinothalamic pathway partially deafferents nocireceptive cerebral neurons, rendering them spontaneously active and hypersensitive to remaining inputs. The spontaneous activity among these neurons is disorganized and unlikely to generate pain. However, activation of these neurons by their remaining inputs can result in pain. Also, injury to spinal gray matter results in a cascade of secondary events, including excitotoxicity, with rostral propagation of excitatory influences that contribute to chronic pain. Establishment and maintenance of below-level pain results from combined influences of injured and spared axons in the spinal white matter and injured neurons in spinal gray matter on processing of nociception by hyperexcitable cerebral neurons that are partially deafferented. A model of spinal stenosis suggests that ischemic injury to the core spinal region can generate below-level pain. Additional questions are raised about demyelination, epileptic discharge, autonomic activation, prolonged activity of C nocireceptive neurons, and thalamocortical plasticity in the generation of below-level pain. PERSPECTIVE: An understanding of mechanisms can direct therapeutic approaches to prevent development of below-level pain or arrest it following spinal cord injury. Among the possibilities covered here are surgical and other means of attenuating gray matter excitotoxicity and ascending propagation of excitatory influences from spinal lesions to thalamocortical systems involved in pain encoding and arousal.

ASIC3-dependent metabolomics profiling of serum and urine in a mouse model of fibromyalgia

Fibromyalgia (FM) is characterized by chronic widespread pain. The pathogenesis of FM remains unclear. No specific biomarkers are available. Animal models of FM may provide an opportunity to explore potential biomarkers in a relative homogenous disease condition. Here, we probed the metabolomics profiles of serum and urine in a mouse model of FM induced by intermittent cold stress (ICS). We focused on the role of acid-sensing ion channel 3 (ASIC3) in the metabolomics profiling because ICS treatment induced chronic widespread muscle pain lasting for 1 month in wild-type (Asic3^+/+) but not Asic3-knockout (Asic3^−/−) mice. Serum and urine samples were collected from both genotypes at different ICS stages, including before ICS (basal level) and post-ICS at days 10 (middle phase, P10) and 40 (recovery phase, P40). Control naïve mice and ICS-induced FM mice differed in ¹H-NMR- and LC-MS-based metabolomics profiling. On pathway analysis, the leading regulated pathways in Asic3^+/+ mice were taurine and hypotaurine, cysteine and methionine, glycerophospholipid, and ascorbate and aldarate metabolisms, and the major pathways in Asic3^−/− mice involved amino acid-related metabolism. Finally, we developed an algorithm for the impactful metabolites in the FM model including cis-aconitate, kynurenate, taurine, pyroglutamic acid, pyrrolidonecarboxylic acid, and 4-methoxyphenylacetic acid in urine as well as carnitine, deoxycholic acid, lysoPC(16:0), lysoPC(20:3), oleoyl-L-carnitine, and trimethylamine N-oxide in serum. Asic3^−/− mice were impaired in only muscle allodynia development but not other pain symptoms in the ICS model, so the ASIC3-dependent metabolomics changes could be useful for developing diagnostic biomarkers specific to chronic widespread muscle pain, the core symptom of FM. Further pharmacological validations are needed to validate these metabolomics changes as potential biomarkers for FM diagnosis and/or treatment responses.

Roles of ASICs in Nociception and Proprioception

Acid-sensing ion channels (ASICs) are a group of proton-gated ion channels belonging to the degenerin/epithelial sodium channel (DED/ENaC) family. There are at least six ASIC subtypes - ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, and ASIC4 - all expressed in somatosensory neurons. ASIC3 is the most abundant in dorsal root ganglia (DRG) and the most sensitive to extracellular acidification. ASICs were found as the major player involved in acid-induced pain in humans. Accumulating evidence has further shown ASIC3 as the molecular determinant involved in pain-associated tissue acidosis in rodent models. Besides having a role in nociception, members of the DEG/ENaC family have been demonstrated as essential mechanotransducers in the nematode Caenorhabditis elegans and fly Drosophila melanogaster. ASICs are mammalian homologues of DEG/ENaC and therefore may play a role in mechanotransduction. However, the role of ASICs in neurosensory mechanotransduction is disputed. Here we review recent studies to probe the roles of ASICs in acid nociception and neurosensory mechanotransduction. In reviewing genetic models and delicate electrophysiology approaches, we show ASIC3 as a dual-function protein for both acid-sensing and mechano-sensing in somatosensory nerves and therefore involved in regulating both nociception and proprioception.

Targeting acidity in cancer and diabetes

While cancer is commonly described as "a disease of the genes", it is also a disease of metabolism. Indeed, carcinogenesis and malignancy are highly associated with metabolic re-programming, and there is clinical evidence that interrupting a cancer's metabolic program can improve patients' outcomes. Notably, many of the metabolic adaptations observed in cancer are similar to the same perturbations observed in diabetic patients. For example, metformin is commonly used to reduce hyperglycemia in diabetic patients, and has been demonstrated to reduce cancer incidence. Treatment with PI3K inhibitors can induce hyperinsulinemia, which can blunt therapeutic efficacy if unchecked. While commonalities between metabolism in cancer and diabetes have been extensively reviewed, here we examine a less explored and emergent convergence between diabetic and cancer metabolism: the generation of lactic acid and subsequent acidification of the surrounding microenvironment. Extracellular lactic acidosis is integral in disease manifestation and is a negative prognostic in both disease states. In tumors, this results in important sequela for cancer progression including increased invasion and metastasis, as well as inhibition of immune surveillance. In diabetes, acidosis impacts the ability of insulin to bind to its receptor, leading to peripheral resistance and an exacerbation of symptoms. Thus, acidosis may be a relevant therapeutic target, and we describe three approaches for targeting: buffers, nanomedicine, and proton pump inhibitors.

The Effect of ASIC3 Knockout on Corticostriatal Circuit and Mouse Self-grooming Behavior

Stereotypic and/or repetitive behavior is one of the major symptoms of autism spectrum disorder (ASD). Increase of self-grooming behavior is a behavioral phenotype commonly observed in the mouse models for ASD. Previously, we have shown that knockout of acid-sensing ion channel 3 (ASIC3) led to the increased self-grooming behavior in resident-intruder test. Given the facts that ASIC3 is mainly expressed in the peripheral dorsal root ganglion (DRG) and conditional knockout of ASIC3 in the proprioceptors induced proprioception deficits. We speculate a hypothesis that stereotypic phenotype related to ASD, pararalled with striatal dysfunction, might be caused by proprioception defect in the peripheral sensory neuron origin. Herein, we investigate in depth whether and how ASIC3 is involved in the regulation of self-grooming behavior. First, we observed that Asic3 null mutant mice exhibited increased self-grooming in social interaction during juvenile stage. Similarly, they displayed increased self-grooming behavior in a novel cage in the absence of cagemate. To further understand the mechanism by which ASIC3 affects grooming behavior, we analyzed neurochemical, neuropathological and electrophysiological features in the dorsal striatum of Asic3 null mutant mice. Knockout of Asic3 increased dopamine (DA) activity and phospho-ERK immunoreactivities in the dorsal striatum. Furthermore, we detected a lower paired-pulse ratio (PPR) and impaired long-term potentiation (LTP) in corticostriatal circuits in Asic3 null mutant mice as compared with wild-type (WT) littermates. Moreover, knockout of Asic3 altered the medial spiny neurons in the striatum with defects in presynaptic function and decrease of dendritic spines. Lastly, genetic ablation of Asic3 specifically in parvalbumin-positive (PV+) cells resulted in the increase of self-grooming behavior in mice. These findings suggest knockout of Asic3 in the PV+ neurons alters grooming behavior by co-opting corticostriatal circuits.

Sensing acidosis: nociception or sngception?

Background

Sensing tissue acidosis is an important function of the somatosensory nervous system to response to noxious stimuli.

Main body

In the pain clinic, acid or soreness sensation is a characteristic sensory phenotype of various acute and chronic pain syndromes, such as delayed onset muscle soreness, fibromyalgia, and radicular pain. However, soreness sensation is a sign of successful analgesia for acupuncture and noxipoint therapy. Thus, the nature of acid or soreness sensation is not always nociceptive (or painful) and could be anti-nociceptive. To facilitate the investigation of the molecular and neurobiological mechanisms of soreness sensation, we propose a concept called “sngception (sng- ception)” to describe the response of the somatosensory nervous system to sense tissue acidosis and to distinguish it from nociception. “Sng” is a Taiwanese word that represents the state of soreness while at the same time imitates the natural vocalization of humans feeling sore.

Conclusion

Here we propose sngception as a specific somatosensory function that transmits the acid sensation from the peripheral to the central nervous system. Sngception could partially overlap with nociception, but it could also transmit antinociception, proprioception, and pruriception.

A new rocuronium formulation not causing vascular pain in a flexor reflex model of anesthetized rats

PURPOSE

Intravenous administration of the brand formulation of rocuronium bromide, currently used as a muscle relaxant, has been associated with vascular pain accompanied by withdrawal movements of the arm and wrist. The purpose of this study was to identify the cause of vascular pain induced by the brand formulation and to develop a new rocuronium formulation, not causing vascular pain, using a vascular pain-evoked flexor reflex response model of anesthetized rats.

METHODS

A rat flexor reflex model, monitored by electromyography, was used to evaluate a flexor reflex response as the index of vascular pain. A catheter for drug administration was inserted into the superficial caudal epigastric artery. A needle electrode was inserted into a muscle in the femoral area to obtain an electromyogram (EMG) value. The integrated EMG values obtained after the administration of each test drug were compared to the baseline value and quantified.

RESULTS

The acetate buffer contained in the solvent could cause flexor reflex response. Furthermore, the flexor reflex response increased in an acid concentration-dependent manner. Based on these results, we prepared a new rocuronium formulation using a low-acid-concentration buffer solution and found that it decreased the integrated EMG value in the rat model. The integrated EMG value acquired using the brand formulation was reduced by pretreatment with the TRPA1 channel inhibitor.

CONCLUSION

Our findings suggest that the high acid concentration in the brand formulation buffer solution is the cause of vascular pain. The rocuronium formulation developed using a low-acid-concentration buffer solution might help eliminate vascular pain in the clinic.

Acid-sensing ion channels: dual function proteins for chemo-sensing and mechano-sensing

Background

Acid-sensing ion channels (ASICs) are a group of amiloride-sensitive ligand-gated ion channels belonging to the family of degenerin/epithelial sodium channels. ASICs are predominantly expressed in both the peripheral and central nervous system and have been characterized as potent proton sensors to detect extracellular acidification in the periphery and brain.

Main body

Here we review the recent studies focusing on the physiological roles of ASICs in the nervous system. As the major acid-sensing membrane proteins in the nervous system, ASICs detect tissue acidosis occurring at tissue injury, inflammation, ischemia, stroke, and tumors as well as fatiguing muscle to activate pain-sensing nerves in the periphery and transmit pain signals to the brain. Arachidonic acid and lysophosphocholine have been identified as endogenous non-proton ligands activating ASICs in a neutral pH environment. On the other hand, ASICs are found involved in the tether model mechanotransduction, in which the extracellular matrix and cytoplasmic cytoskeletons act like a gating-spring to tether the mechanically activated ion channels and thus transmit the stimulus force to the channels. Accordingly, accumulating evidence has shown ASICs play important roles in mechanotransduction of proprioceptors, mechanoreceptors and nociceptors to monitor the homoeostatic status of muscle contraction, blood volume, and blood pressure as well as pain stimuli.

Conclusion

Together, ASICs are dual-function proteins for both chemosensation and mechanosensation involved in monitoring physiological homoeostasis and pathological signals.

Does a Better Perfusion of Deconditioned Muscle Tissue Release Chronic Low Back Pain?

Non-specific chronic low back pain (nsCLBP) is a multifactorial condition of unknown etiology and pathogenesis. Physical and genetic factors may influence the predisposition of individuals to CLBP, which in many instances share a musculoskeletal origin. A reduced pain level in low back pain patients that participate in exercise therapy highlights that disuse-related muscle deconditioning may predispose individuals to nsCLBP. In this context, musculoskeletal pain may be the consequence of capillary rarefaction in inactive muscle as this would lower local tissue drainage and washing out of toxic waste. Muscle activity is translated into an angio-adaptative process, which implicates angiogenic-gene expression and individual response differences due to heritable modifications of such genes (gene polymorphisms). The pathophysiologic mechanism underlying nsCLBP is still largely unaddressed. We hypothesize that capillary rarefaction due to a deconditioning of dorsal muscle groups exacerbates nsCLBP by increasing noxious sensation, reducing muscle strength and fatigue resistance by initiating a downward spiral of local deconditioning of back muscles which diminishes their load-bearing capacity. We address the idea that specific factors such as angiotensin-converting enzyme and Tenascin-C might play an important role in altering susceptibility to nsCLBP via their effects on microvascular perfusion and vascular remodeling of skeletal muscle, inflammation, and pain sensation. The genetic profile may help to explain the individual predisposition to nsCLBP, thus identifying subgroups of patients, which could benefit from ad hoc treatment types. Future therapeutic approaches aimed at relieving the pain associated with nsCLBP should be based on the verification of mechanistic processes of activity-induced angio-adaptation and muscle-perfusion.

Tackling Pain Associated with Rheumatoid Arthritis: Proton-Sensing Receptors

Rheumatoid arthritis (RA), characterized by chronic inflammation of synovial joints, is often associated with ongoing pain and increased pain sensitivity. Chronic pain that comes with RA turns independent, essentially becoming its own disease. It could partly explain that a significant number (50%) of RA patients fail to respond to current RA therapies that focus mainly on suppression of joint inflammation. The acute phase of pain seems to associate with joint inflammation in early RA. In established RA, the chronic phase of pain could be linked to inflammatory components of neuron-immune interactions and noninflammatory components. Accumulating evidence suggests that the initial inflammation and autoimmunity in RA (preclinical RA) begin outside of the joint and may originate at mucosal sites and alterations in the composition of microbiota located at mucosal sites could be essential for mucosal inflammation, triggering joint inflammation. Fibroblast-like synoviocytes in the inflamed joint respond to cytokines to release acidic components, lowering pH in synovial fluid. Extracellular proton binds to proton-sensing ion channels, and G-protein-coupled receptors in joint nociceptive fibers may contribute to sensory transduction and release of neurotransmitters, leading to pain and hyperalgesia. Activation of peripheral sensory neurons or nociceptors further modulates inflammation, resulting in neuroinflammation or neurogenic inflammation. Peripheral and central nerves work with non-neuronal cells (such as immune cells, glial cells) in concert to contribute to the chronic phase of RA-associated pain. This review will discuss actions of proton-sensing receptors on neurons or non-neuronal cells that modulate RA pathology and associated chronic pain, and it will be beneficial for the development of future therapeutic treatments.

Involvement of Opioid System, TRPM8, and ASIC Receptors in Antinociceptive Effect of Arrabidaea brachypoda (DC) Bureau

Arrabidaea brachypoda (DC) Bureau is a medicinal plant found in Brazil. Known as “cipó-una”, it is popularly used as a natural therapeutic agent against pain and inflammation. This study evaluated the chemical composition and antinociceptive activity of the dichloromethane fraction from the roots of A. brachypoda (DEAB) and its mechanism of action. The chemical composition was characterized by high-performance liquid chromatography, and this fraction is composed only of dimeric flavonoids. The antinociceptive effect was evaluated in formalin and hot plate tests after oral administration (10–100 mg/kg) in male Swiss mice. We also investigated the involvement of TRPV1 (transient receptor potential vanilloid 1), TRPA1 (transient receptor potential ankyrin 1), TRPM8 (transient receptor potential melastatin 8), and ASIC (acid-sensing ion channel), as well as the opioidergic, glutamatergic, and supraspinal pathways. Moreover, the nociceptive response was reduced (30 mg/kg) in the early and late phase of the formalin test. DEAB activity appears to involve the opioid system, TRPM8, and ASIC receptors, clearly showing that the DEAB alleviates acute pain in mice and suggesting the involvement of the TRPM8 and ASIC receptors and the opioid system in acute pain relief.

A systems approach for discovering linoleic acid derivatives that potentially mediate pain and itch

Chronic pain and itch are common hypersensitivity syndromes that are affected by endogenous mediators. We applied a systems-based, translational approach to predict, discover, and characterize mediators of pain and itch that are regulated by diet and inflammation. Profiling of tissue-specific precursor abundance and biosynthetic gene expression predicted that inflamed skin would be abundant in four previously unknown 11-hydroxy-epoxy- or 11-keto-epoxy-octadecenoate linoleic acid derivatives and four previously identified 9- or 13-hydroxy-epoxy- or 9- or 13-keto-epoxy-octadecenoate linoleic acid derivatives. All of these mediators were confirmed to be abundant in rat and human skin by mass spectrometry. However, only the two 11-hydroxy-epoxy-octadecenoates sensitized rat dorsal root ganglion neurons to release more calcitonin gene-related peptide (CGRP), which is involved in pain transmission, in response to low pH (which mimics an inflammatory state) or capsaicin (which activates ion channels involved in nociception). The two 11-hydroxy-epoxy-octadecenoates share a 3-hydroxy-Z-pentenyl-E-epoxide moiety, thus suggesting that this substructure could mediate nociceptor sensitization. In rats, intradermal hind paw injection of 11-hydroxy-12,13-trans-epoxy-(9Z)-octadecenoate elicited C-fiber-mediated sensitivity to thermal pain. In a randomized trial testing adjunctive strategies to manage refractory chronic headaches, reducing the dietary intake of linoleic acid was associated with decreases in plasma 11-hydroxy-12,13-trans-epoxy-(9Z)-octadecenoate, which correlated with clinical pain reduction. Human psoriatic skin had 30-fold higher 9-keto-12,13-trans-epoxy-(10E)-octadecenoate compared to control skin, and intradermal injection of this compound induced itch-related scratching behavior in mice. Collectively, these findings define a family of endogenous mediators with potential roles in pain and itch.

TDAG8, TRPV1, and ASIC3 involved in establishing hyperalgesic priming in experimental rheumatoid arthritis

Rheumatoid arthritis (RA), characterized by chronic inflammation of synovial joints, is often associated with ongoing pain and increased pain sensitivity. High hydrogen ion concentration (acidosis) found in synovial fluid in RA patients is associated with disease severity. Acidosis signaling acting on proton-sensing receptors may contribute to inflammation and pain. Previous studies focused on the early phase of arthritis (<5 weeks) and used different arthritis models, so elucidating the roles of different proton-sensing receptors in the chronic phase of arthritis is difficult. We intra-articularly injected complete Freund’s adjuvant into mice once a week for 4 weeks to establish chronic RA pain. Mice with knockout of acid-sensing ion channel 3 (ASIC3) or transient receptor potential/vanilloid receptor subtype 1 (TRPV1) showed attenuated chronic phase (>6 weeks) of RA pain. Mice with T-cell death-associated gene 8 (TDAG8) knockout showed attenuated acute and chronic phases of RA pain. TDAG8 likely participates in the initiation of RA pain, but all three genes, TDAG8, TRPV1, and ASIC3, are essential to establish hyperalgesic priming to regulate the chronic phase of RA pain.

Peripheral sensory neuron injury contributes to neuropathic pain in experimental autoimmune encephalomyelitis

Multiple sclerosis (MS)-induced neuropathic pain deteriorates quality of life in patients but is often refractory to treatment. In experimental autoimmune encephalomyelitis (EAE), a rodent model of MS, animals develop neuropathy and inflammation-induced tissue acidosis, which suggests the involvement of acid-sensing ion channels (ASICs). Also, peripheral neuropathy is reported in MS patients. However, the involvement of the peripheral nervous system (PNS) in MS neuropathic pain remains elusive. This study investigated the contribution of ASICs and peripheral neuropathy in MS-induced neuropathic pain. Elicited pain levels were as high in Asic1a^-/-, Asic2^-/- and Asic3^-/- mice as wild-type mice even though only Asic1a^-/- mice showed reduced EAE disease severity, which indicates that pain in EAE was independent of disease severity. We thus adopted an EAE model without pertussis toxin (EAEnp) to restrain activated immunity in the periphery and evaluate the PNS contribution to pain. Both EAE and EAEnp mice showed similar pain behaviors and peripheral neuropathy in nerve fibers and DRG neurons. Moreover, pregabalin significantly reduced neuropathic pain in both EAE and EAEnp mice. Our findings highlight the essential role of the PNS in neuropathic pain in EAE and pave the way for future development of analgesics without side effects in the CNS.

TDAG8 involved in initiating inflammatory hyperalgesia and establishing hyperalgesic priming in mice

Chronic pain, resulting from injury, arthritis, and cancer, is often accompanied by inflammation. High concentrations of protons found in inflamed tissues results in tissue acidosis, a major cause of pain and hyperalgesia. Acidosis signals may mediate a transition from acute to chronic hyperalgesia (hyperalgesic priming) via proton-sensing G-protein-coupled receptors (GPCRs). The expression of T-cell death-associated gene 8 (TDAG8), a proton-sensing GPCR, is increased during inflammatory hyperalgesia. Attenuating TDAG8 expression in the spinal cord inhibits bone cancer pain, but whether TDAG8 is involved in inflammatory hyperalgesia or hyperalgesic priming remains unclear. In this study, we used TDAG8-knockout or -knockdown to explore the role of TDAG8 in pain. Suppressed TDAG8 expression delayed the onset of inflammatory hyperalgesia and shortened hyperalgesic time in mice. In a dual acid-injection model (acid [pH 5.0] injected twice, 5 days apart), shRNA inhibition of TDAG8 shortened the duration of the second hyperalgesia. Similar results were found in TDAG8-deficient mice. The dual administration of TDAG8 agonist also confirmed that TDAG8 is involved in hyperalgsic priming. Accordingly, TDAG8 may mediate acidosis signals to initiate inflammatory hyperalgesia and establish hyperalgesic priming.

TRPA1 and TRPV1 Antagonists Do Not Inhibit Human Acidosis-Induced Pain

Acidosis occurs in a variety of pathophysiological and painful conditions where it is thought to excite or contribute to excitation of nociceptive neurons. Despite potential clinical relevance the principal receptor for sensing acidosis is unclear, but several receptors have been proposed. We investigated the contribution of the acid-sensing ion channels, transient receptor potential vanilloid type 1 (TRPV1) and transient receptor potential ankyrin type 1 (TRPA1) to peripheral pain signaling. We first established a human pain model using intraepidermal injection of the TRPA1 agonist carvacrol. This resulted in concentration-dependent pain sensations, which were reduced by experimental TRPA1 antagonist A-967079. Capsaicin-induced pain was reduced by the TRPV1 inhibitor BCTC. Amiloride was used to block acid-sensing ion channels. Testing these antagonists in a double-blind and randomized experiment, we probed the contribution of the respective channels to experimental acidosis-induced pain in 15 healthy human subjects. A continuous intraepidermal injection of pH 4.3 was used to counter the buffering capacity of tissue and generate a prolonged painful stimulation. In this model, addition of A-967079, BCTC or amiloride did not reduce the reported pain. In conclusion, target-validated antagonists, applied locally in human skin, have excluded the main hypothesized targets and the mechanism of the human acidosis-induced pain remains unclear.

PERSPECTIVE

An acidic milieu is a trigger of pain in many clinical conditions. The aim of this study was to identify the contribution of the currently hypothesized sensors of acid-induced pain in humans. Surprisingly, inhibition of these receptors did not alter acidosis-induced pain.

Animal models of chronic pain: Advances and challenges for clinical translation

Chronic pain is a global problem that has reached epidemic proportions. An estimated 20% of adults suffer from pain, and another 10% are diagnosed with chronic pain each year (Goldberg and McGee, ). Despite the high prevalence of chronic pain (an estimated 1.5 billion people are afflicted worldwide), much remains to be understood about the underlying causes of this condition, and there is an urgent requirement for better pain therapies. The discovery of novel targets and the development of better analgesics rely on an assortment of preclinical animal models; however, there are major challenges to translating discoveries made in animal models to realized pain therapies in humans. This review discusses common animal models used to recapitulate clinical chronic pain conditions (such as neuropathic, inflammatory, and visceral pain) and the methods for assessing the sensory and affective components of pain in animals. We also discuss the advantages and limitations of modeling chronic pain in animals as well as highlighting strategies for improving the predictive validity of preclinical pain studies. © 2016 Wiley Periodicals, Inc.