Targeting ASIC3 for Relieving Mice Fibromyalgia Pain: Roles of Electroacupuncture, Opioid, and Adenosine

Pathology and physiology of acid-sensitive ion channels in the bladder

Acid-sensitive ion channels (ASICs) are sodium-permeable channels activated by extracellular acidification. They can be activated and trigger the inward flow of Na+ when the extracellular environment is acidic, leading to membrane depolarization and thus inducing action potentials in neurons. There are four ASIC genes in mammals (ASIC1-4). ASIC is widely expressed in humans. It is closely associated with pain, neurological disorders, multiple sclerosis, epilepsy, migraines, and many other disorders. Bladder pain syndrome/interstitial cystitis (BPS/IC) is a specific syndrome characterized by bladder pain. Recent studies have shown that ASICs are closely associated with the development of BPS/IC. A study revealed that ASIC levels are significantly elevated in a BPS/IC model. Additionally, researchers have reported differential changes in ASICs in the bladders of patients with neurogenic lower urinary tract dysfunction (NLUTD) caused by spinal cord injury (SCI). In this review, we summarize the structure and physiological functions of ASICs and focus on the mechanisms by which ASICs mediate bladder disease.

The role of purinergic signaling in acupuncture-mediated relief of neuropathic and inflammatory pain

Acupuncture is a traditional medicinal practice in China that has been increasingly recognized in other countries in recent decades. Notably, several reports have demonstrated that acupuncture can effectively aid in pain management. However, the analgesic mechanisms through which acupuncture provides such benefits remain poorly understood. Purinergic signaling, which is mediated by purine nucleotides and purinergic receptors, has been proposed to play a central role in acupuncture analgesia. On the one hand, acupuncture affects the transmission of nociception by increasing adenosine triphosphate dephosphorylation and thereby decreasing downstream P2X3, P2X4, and P2X7 receptors signaling activity, regulating the levels of inflammatory factors, neurotrophic factors, and synapsin I. On the other hand, acupuncture exerts analgesic effects by promoting the production of adenosine, enhancing the expression of downstream adenosine A1 and A2A receptors, and regulating downstream inflammatory factors or synaptic plasticity. Together, this systematic overview of the field provides a sound, evidence-based foundation for future research focused on the application of acupuncture as a means of relieving pain.

Knowledge Mapping of Acupuncture for Fibromyalgia from 1990 to 2022: A Bibliometric Analysis

Background

Fibromyalgia is a rheumatic disease with no specific laboratory markers and is insensitive to hormonal drugs and nonsteroidal anti-inflammatory drugs commonly used to treat rheumatism. Guidelines recommend that non-pharmacological therapy should be the first-line treatment for fibromyalgia. Since the publication of the first diagnostic criteria for fibromyalgia in 1990, studies on acupuncture for fibromyalgia have been reported periodically. This study aims to explore the intellectual landscape of acupuncture for fibromyalgia since 1990, and to identify research trends and fronts in this field.

Methods

The Web of Science Core Collection Database was searched for publications on acupuncture for fibromyalgia from 1990 to 2022. VOSviewer and CiteSpace were used to analyze the annual publication, countries, institutions, authors and cited authors, journals and cited journals, references and keywords.

Results

A total of 280 publications were retrieved, and the number of publications showed an overall upward trend. The United States was the most productive country. China Medical University was the institution with the most publications. Lin Yi-wen was the most prolific author, while Wolfe was the most cited author. Evidence-Based Complementary and Alternative Medicine was the journal in which most of the research was published, while Pain was the most cited journal. An article by Wolfe (1990) had the most citations, but an article by Crofford (2001) had the highest centrality. The four most frequently used keywords in the included articles were mechanism, spinal cord, activation and sensitivity.

Conclusion

Acupuncture can effectively relieve pain in patients with fibromyalgia and improve accompanying symptoms such as anxiety and depression. However, the design of clinical trials still needs to be optimized to better verify the efficacy of acupuncture on various clinical symptoms of fibromyalgia. Exploring the central analgesic mechanism of acupuncture on fibromyalgia is also the focus research direction now and future.

Long-lasting reflexive and nonreflexive pain responses in two mouse models of fibromyalgia-like condition

Nociplastic pain arises from altered nociception despite no clear evidence of tissue or somatosensory system damage, and fibromyalgia syndrome can be highlighted as a prototype of this chronic pain subtype. Currently, there is a lack of effective treatments to alleviate both reflexive and nonreflexive pain responses associated with fibromyalgia condition, and suitable preclinical models are needed to assess new pharmacological strategies. In this context, although in recent years some remarkable animal models have been developed to mimic the main characteristics of human fibromyalgia, most of them show pain responses in the short term. Considering the chronicity of this condition, the present work aimed to develop two mouse models showing long-lasting reflexive and nonreflexive pain responses after several reserpine (RIM) or intramuscular acid saline solution (ASI) injections. To our knowledge, this is the first study showing that RIM6 and ASI mouse models show reflexive and nonreflexive responses up to 5-6 weeks, accompanied by either astro- or microgliosis in the spinal cord as pivotal physiopathology processes related to such condition development. In addition, acute treatment with pregabalin resulted in reflexive pain response alleviation in both the RIM6 and ASI models. Consequently, both may be considered suitable experimental models of fibromyalgia-like condition, especially RIM6.

Suppression of ASIC activity by the activation of A1 adenosine receptors in rat primary sensory neurons

Peripheral A1 adenosine receptor signaling has been shown to have analgesic effects in a variety of pain conditions. However, it is not yet fully elucidated for the precise molecular mechanisms. Acid sensing ion channels (ASICs) are expressed predominantly in nociceptive sensory neurons responding to protons. Given that both A1 adenosine receptors and ASICs are present in dorsal root ganglia (DRG) neurons, we therefore investigated whether there was a cross-talk between the two types of receptors. Herein, electrophysiological recordings showed that the A1 adenosine receptor agonist N⁶-cyclopentyladenosine (CPA) suppressed acid-induced currents and action potentials, which were mediated by ASICs, in rat DRG neurons. CPA inhibited the maximum response to protons, as shown a downward shift of concentration-response curve for protons. The CPA-induced suppression of ASIC currents was blocked by the A1 adenosine receptor antagonist KW-3902 and also prevented by intracellular application of the G_i/o-protein inhibitor pertussis toxin, the adenylate cyclase activator forskolin, and the cAMP analog 8-Br-cAMP. Finally, intraplantar pretreatment of CPA dose-dependently relieved acid-induced nociceptive responses in rats through peripheral A1 adenosine receptors. These results suggested that CPA suppressed ASICs via A1 adenosine receptors and intracellular G_i/o-proteins and cAMP signaling cascades in rat DRG neurons, which was a novel potential mechanism underlying analgesia of peripheral A1 adenosine receptors.

Molecular mechanism in trigeminal nerve and treatment methods related to orthodontic pain

BACKGROUND

Orthodontic treatment is the main treatment approach for malocclusion. Orthodontic pain is an inevitable undesirable adverse reaction during orthodontic treatment. It is reported orthodontic pain has become one of the most common reason that patients withdraw from orthodontic treatment. Therefore, understanding the underlying mechanism and finding treatment of orthodontic pain are in urgent need.

AIMS

This article aims to sort out the mechanisms and treatments of orthodontic pain, hoping to provide some ideas for future orthodontic pain relief.

MATERIALS

Tooth movement will cause local inflammation. Certain inflammatory factors and cytokines stimulating the trigeminal nerve and further generating pain perception, as well as drugs and molecular targeted therapy blocking nerve conduction pathways, will be reviewed in this article.

METHOD

We review and summaries current studies related to molecular mechanisms and treatment approaches in orthodontic pain control.

RESULTS

Orthodontics pain related influencing factors and molecular mechanisms has been introduced. Commonly used clinical methods in orthodontic pain control has been evaluated.

DISCUSSION

With the clarification of more molecular mechanisms, the direction of orthodontic pain treatment will shift to targeted drugs.

Electroacupuncture Attenuates Chronic Inflammatory Pain and Depression Comorbidity through Transient Receptor Potential V1 in the Brain

Chronic pain is one of the highest costs in clinical therapy, often appearing comorbid with depression. They present with overlapping clinical conditions and common pathological pathways especially in neuroinflammation, both of which can be reversed by electroacupuncture (EA). Transient receptor potential V1 receptor (TRPV1) is a Ca[Formula: see text] permeable ion channel that responds to brain inflammation and has a known role in the development of chronic pain and depression. Here, we investigate the role of TRPV1 and its related molecules in a mouse model of inflammation-induced chronic pain and depression using Complete Freund's adjuvant (CFA). We measured inflammatory mediators in plasma and evaluated the TRPV1 signaling pathway in the medial prefrontal cortex (mPFC), hypothalamus, and periaqueductal gray (PAG) of the mouse brain. Mechanical and thermal hyperalgesia as well as depressive-like behaviors were induced using the open field test and forced swimming test. Therapeutic effects were observed in EA and Trpv1[Formula: see text] mice in measures of chronic pain and depression. Inflammatory mediators induced by CFA injection were attenuated by EA and Trpv1 deletion. TRPV1 and downstream molecules were significantly decreased in the mPFC, hypothalamus, and PAG of mice, effects which were reversed by EA and Trpv1 knockout. We provide novel evidence that these inflammatory mediators modulate the TRPV1 signaling pathway and suggest new potential therapeutic targets for chronic pain and depression.

Passive transfer of fibromyalgia symptoms from patients to mice

Fibromyalgia syndrome (FMS) is characterized by widespread pain and tenderness, and patients typically experience fatigue and emotional distress. The etiology and pathophysiology of fibromyalgia are not fully explained and there are no effective drug treatments. Here we show that IgG from FMS patients produced sensory hypersensitivity by sensitizing nociceptive neurons. Mice treated with IgG from FMS patients displayed increased sensitivity to noxious mechanical and cold stimulation, and nociceptive fibers in skin-nerve preparations from mice treated with FMS IgG displayed an increased responsiveness to cold and mechanical stimulation. These mice also displayed reduced locomotor activity, reduced paw grip strength, and a loss of intraepidermal innervation. In contrast, transfer of IgG-depleted serum from FMS patients or IgG from healthy control subjects had no effect. Patient IgG did not activate naive sensory neurons directly. IgG from FMS patients labeled satellite glial cells and neurons in vivo and in vitro, as well as myelinated fiber tracts and a small number of macrophages and endothelial cells in mouse dorsal root ganglia (DRG), but no cells in the spinal cord. Furthermore, FMS IgG bound to human DRG. Our results demonstrate that IgG from FMS patients produces painful sensory hypersensitivities by sensitizing peripheral nociceptive afferents and suggest that therapies reducing patient IgG titers may be effective for fibromyalgia.

Electroacupuncture reduces cold stress-induced pain through microglial inactivation and transient receptor potential V1 in mice

Background

The treatment, and efficacy thereof, is considered to be inadequate with specificity to alleviation of Fibromyalgia and its associated pain. Fibromyalgia patients suffer from chronic and persistent widespread pain and generalized tenderness. Transient receptor potential V1 (TRPV1), which is reported as a Ca²⁺ permeable ion channel that can be activated by inflammation, is reported to be involved in the development of fibromyalgia pain.

Methods

The current study explored the TRPV1 channel functions as a noxious sensory input in mice cold stress model. It remains unknown whether electroacupuncture (EA) attenuates fibromyalgia pain or affects the TRPV1 pathway.

Results

We show that cold stress increases mechanical and thermal pain (day 7: mechanical: 1.69 ± 0.41 g; thermal: 4.68 ± 0.56 s), and that EA and Trpv1 deletion counter this increase. EA and Trpv1 deletion reduced the cold stress-induced increase in inflammatory mediators and TRPV1-related molecules in the hypothalamus, periaqueductal gray (PAG), and cerebellum of mice.

Conclusions

Our results imply that EA has an analgesic effect associated with TRPV1 downregulation. We provide novel evidence that these inflammatory mediators can modulate the TRPV1 signaling pathway and suggest new potential therapeutic targets for fibromyalgia pain.

Acid-Sensing Ion Channels and Mechanosensation

Acid-sensing ion channels (ASICs) are mainly proton-gated cation channels that are activated by pH drops and nonproton ligands. They are part of the degenerin/epithelial sodium channel superfamily due to their sodium permeability. Predominantly expressed in the central nervous system, ASICs are involved in synaptic plasticity, learning/memory, and fear conditioning. These channels have also been implicated in multiple disease conditions, including ischemic brain injury, multiple sclerosis, Alzheimer’s disease, and drug addiction. Recent research has illustrated the involvement of ASICs in mechanosensation. Mechanosensation is a form of signal transduction in which mechanical forces are converted into neuronal signals. Specific mechanosensitive functions have been elucidated in functional ASIC1a, ASIC1b, ASIC2a, and ASIC3. The implications of mechanosensation in ASICs indicate their subsequent involvement in functions such as maintaining blood pressure, modulating the gastrointestinal function, and bladder micturition, and contributing to nociception. The underlying mechanism of ASIC mechanosensation is the tether-gate model, which uses a gating-spring mechanism to activate ASIC responses. Further understanding of the mechanism of ASICs will help in treatments for ASIC-related pathologies. Along with the well-known chemosensitive functions of ASICs, emerging evidence has revealed that mechanosensitive functions of ASICs are important for maintaining homeostasis and contribute to various disease conditions.

Inhibiting BDNF/TrkB.T1 receptor improves resiniferatoxin-induced postherpetic neuralgia through decreasing ASIC3 signaling in dorsal root ganglia

BACKGROUND

Postherpetic neuralgia (PHN) is a devastating complication after varicella-zoster virus infection. Brain-derived neurotrophic factor (BDNF) has been shown to participate in the pathogenesis of PHN. A truncated isoform of the tropomyosin receptor kinase B (TrkB) receptor TrkB.T1, as a high-affinity receptor of BDNF, is upregulated in multiple nervous system injuries, and such upregulation is associated with pain. Acid-sensitive ion channel 3 (ASIC3) is involved in chronic neuropathic pain, but its relation with BDNF/TrkB.T1 in the peripheral nervous system (PNS) during PHN is unclear. This study aimed to investigate whether BDNF/TrkB.T1 contributes to PHN through regulating ASIC3 signaling in dorsal root ganglia (DRGs).

METHODS

Resiniferatoxin (RTX) was used to induce rat PHN models. Mechanical allodynia was assessed by measuring the paw withdrawal thresholds (PWTs). Thermal hyperalgesia was determined by detecting the paw withdrawal latencies (PWLs). We evaluated the effects of TrkB.T1-ASIC3 signaling inhibition on the behavior, neuronal excitability, and inflammatory response during RTX-induced PHN. ASIC3 short hairpin RNA (shRNA) transfection was used to investigate the effect of exogenous BDNF on inflammatory response in cultured PC-12 cells.

RESULTS

RTX injection induced mechanical allodynia and upregulated the protein expression of BDNF, TrkB.T1, ASIC3, TRAF6, nNOS, and c-Fos, as well as increased neuronal excitability in DRGs. Inhibition of ASIC3 reversed the abovementioned effects of RTX, except for BDNF and TrkB.T1 protein expression. In addition, inhibition of TrkB.T1 blocked RTX-induced mechanical allodynia, activation of ASIC3 signaling, and hyperexcitability of neurons. RTX-induced BDNF upregulation was found in both neurons and satellite glia cells in DRGs. Furthermore, exogenous BDNF activated ASIC3 signaling, increased NO level, and enhanced IL-6, IL-1β, and TNF-α levels in PC-12 cells, which was blocked by shRNA-ASIC3 transfection.

CONCLUSION

These findings demonstrate that inhibiting BDNF/TrkB.T1 reduced inflammation, decreased neuronal hyperexcitability, and improved mechanical allodynia through regulating the ASIC3 signaling pathway in DRGs, which may provide a novel therapeutic target for patients with PHN.

Satellite glia as a critical component of diabetic neuropathy: Role of lipocalin-2 and pyruvate dehydrogenase kinase-2 axis in the dorsal root ganglion

Diabetic peripheral neuropathy (DPN) is a common complication of uncontrolled diabetes. The pathogenesis of DPN is associated with chronic inflammation in dorsal root ganglion (DRG), eventually causing structural and functional changes. Studies on DPN have primarily focused on neuronal component, and there is limited knowledge about the role of satellite glial cells (SGCs), although they completely enclose neuronal soma in DRG. Lipocalin-2 (LCN2) is a pro-inflammatory acute-phase protein found in high levels in diverse neuroinflammatory and metabolic disorders. In diabetic DRG, the expression of LCN2 was increased exclusively in the SGCs. This upregulation of LCN2 in SGCs correlated with increased inflammatory responses in DRG and sciatic nerve. Furthermore, diabetes-induced inflammation and morphological changes in DRG, as well as sciatic nerve, were attenuated in Lcn2 knockout (KO) mice. Lcn2 gene ablation also ameliorated neuropathy phenotype as determined by nerve conduction velocity and intraepidermal nerve fiber density. Mechanistically, studies using specific gene KO mice, adenovirus-mediated gene overexpression strategy, and primary cultures of DRG SGCs and neurons have demonstrated that LCN2 enhances the expression of mitochondrial gate-keeping regulator pyruvate dehydrogenase kinase-2 (PDK2) through PPARβ/δ, thereby inhibiting pyruvate dehydrogenase activity and increasing production of glycolytic end product lactic acid in DRG SGCs and neurons of diabetic mice. Collectively, our findings reveal a crucial role of glial LCN2-PPARβ/δ-PDK2-lactic acid axis in progression of DPN. Our results establish a link between pro-inflammatory LCN2 and glycolytic PDK2 in DRG SGCs and neurons and propose a novel glia-based mechanism and drug target for therapy of DPN. MAIN POINTS: Diabetes upregulates LCN2 in satellite glia, which in turn increases pyruvate dehydrogenase kinase-2 (PDK2) expression and lactic acid production in dorsal root ganglia (DRG). Glial LCN2-PDK2-lactic acid axis in DRG plays a crucial role in the pathogenesis of diabetic neuropathy.

Transient receptor potential V1 (TRPV1) modulates the therapeutic effects for comorbidity of pain and depression: The common molecular implication for electroacupuncture and omega-3 polyunsaturated fatty acids

Chronic pain and depression are conditions that are highly comorbid and present with overlapping clinical presentations and common pathological biological pathways in neuroinflammation, both of which can be reversed by the use of electroacupuncture (EA) and omega-3 polyunsaturated fatty acids (PUFAs). Transient receptor potential V1 (TRPV1), a Ca²⁺ permeable ion channel that can be activated by inflammation, is reported to be involved in the development of chronic pain and depression. Here, we investigated the role of TRPV1 and its related pathways in the murine models of cold stress-induced nociception and depression. Female C57BL/6 wild type and TRPV1 knockout mice were subjected to intermittent cold-stress (ICS) to initiate depressive-like and chronic pain behaviors, respectively. The Bio-Plex ELISA technique was utilized to analyze inflammatory mediators in mice plasma. The western blot and immunostaining techniques were used to analyze the presence of TRPV1 and related molecules in the medial prefrontal cortex (mPFC), hippocampus, periaqueductal gray (PAG), and amygdala. The ICS model significantly induced chronic pain (mechanical: 2.55 ± 0.31 g; thermal: 8.12 ± 0.87 s) and depressive-like behaviors (10.95 ± 0.95% in the center zone; 53.14 ± 4.01% in immobility). The treatment efficacy of EA, docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA) were observed in both nociceptive and depression test results. Inflammatory mediators were increased after ICS induction and further reversed by the use of EA, EPA and DHA. A majority of TRPV1 proteins and related molecules were significantly decreased in the mPFC, hippocampus and PAG of mice. This decrease can be reversed by the use of EA, EPA and DHA. In contrast, these molecules were increased in the mice's amygdala, and were attenuated by the use of EA, EPA and DHA. Our findings indicate that these inflammatory mediators can regulate the TRPV1 signaling pathway and initiate new potential therapeutic targets for chronic pain and depression treatment.

Medicinal Chemistry and Therapeutic Potential of Agonists, Antagonists and Allosteric Modulators of A1 Adenosine Receptor: Current Status and Perspectives

Adenosine is a purine nucleoside, responsible for the regulation of a wide range of physiological and pathophysiological conditions by binding with four G-protein-coupled receptors (GPCRs), namely A1, A2A, A2B and A3 adenosine receptors (ARs). In particular, A1 AR is ubiquitously present, mediating a variety of physiological processes throughout the body, thus represents a promising drug target for the management of various pathological conditions. Agonists of A1 AR are found to be useful for the treatment of atrial arrhythmia, angina, type-2 diabetes, glaucoma, neuropathic pain, epilepsy, depression and Huntington's disease, whereas antagonists are being investigated for the treatment of diuresis, congestive heart failure, asthma, COPD, anxiety and dementia. However, treatment with full A1 AR agonists has been associated with numerous challenges like cardiovascular side effects, off-target activation as well as desensitization of A1 AR leading to tachyphylaxis. In this regard, partial agonists of A1 AR have been found to be beneficial in enhancing insulin sensitivity and subsequently reducing blood glucose level, while avoiding severe CVS side effects and tachyphylaxis. Allosteric enhancer of A1 AR is found to be potent for the treatment of neuropathic pain, culminating the side effects related to off-target tissue activation of A1 AR. This review provides an overview of the medicinal chemistry and therapeutic potential of various agonists/partial agonists, antagonists and allosteric modulators of A1 AR, with a particular emphasis on their current status and future perspectives in clinical settings.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Preventing the induction of acid saline-induced fibromyalgia pain in mice by electroacupuncture or APETx2 injection

BACKGROUND

Fibromyalgia (FM) is a syndrome involving chronic pain, fatigue, sleep difficulties, morning stiffness and muscle cramping lasting longer than 3 months. The epidemiological prevalence is approximately 3-5% in women and increases with age. Antagonism of acid-sensing ion channel 3 (ASIC3) reportedly attenuates acid saline-induced FM pain in mice.

AIMS

Whether pre-treatment with electroacupuncture (EA) or APETx2 can attenuate mechanical hyperalgesia in this murine model remains unknown.

METHODS

Accordingly, we examined the analgesic effect of EA in a murine model of FM pain induced by dual injections of acid saline and investigated whether EA or APETx2 can attenuate FM pain via the ASIC3 channel.

RESULTS

EA significantly reduced mechanical hyperalgesia in this model. ASIC3 antagonism, induced by injecting APETx2, also significantly reduced mechanical hyperalgesia. The expression of ASIC3 in the dorsal root ganglia, spinal cord and thalamus was increased after FM model induction. Over-expression of these nociceptive channels was attenuated by pre-treatment with EA or an ASIC3 antagonist.

CONCLUSION

Our data reveal that both EA and ASIC3 blockade significantly reduce FM pain in mice via the ASIC3, Nav1.7 and Nav1.8 signalling pathways. Moreover, our findings support the potential clinical use of EA for the treatment of FM pain.

Electroacupuncture attenuates chronic fibromyalgia pain through the phosphorylated phosphoinositide 3-kinase signaling pathway in the mouse brain

Objectives

Fibromyalgia (FM) is a central nervous system disorder characterized by widespread mechanical hyperalgesia due to unknown mechanisms. Several inflammatory mediators, such as interleukin-1 (IL-1), IL-6, IL-8, and tumor necrosis factor, are increased in the serum of FM patients. Although medications including pregabalin, duloxetine, and milnacipran are used to treat FM, the results are unsatisfying. In the present study we assessed whether electroacupuncture (EA) can reduce chronic FM pain and then proposed an underlying mechanism for this effect.

Materials and Methods

Chronic FM pain was induced in mice by dual acid saline injection lasting up to 4 weeks.

Results

Chronic FM pain was treated by EA manipulation, but not in the sham operated group. Phosphorylated phosphatidylinositol 3-kinase (pPI3K), protein kinase B, mechanistic target of rapamycin, and nuclear factor kappa-light-chain-enhancer of activated B cells were unaltered in the mouse dorsal root ganglion (DRG) and spinal cord (SC) after inducing FM and administering EA treatment. The pPI3K-associated nociceptive signaling pathway was increased in the thalamus of FM mice, but reversed by EA. Similar results were observed in the mouse somatosensory cortex.

Conclusion

These data suggest that EA has a significant effect on a signaling pathway in brain areas of FM mice. These findings suggest the value of EA for clinical practice.

Acid-sensing ion channel 3 expression is increased in dorsal root ganglion, hippocampus and hypothalamus in remifentanil-induced hyperalgesia in rats

BACKGROUND

Remifentanil induces hyperalgesia, but the underlying mechanisms are not fully understood. Acid-sensing ion channel 3 (ASIC3) plays a regulatory role in the pain pathway. This study aimed to explore the effect of remifentanil administration on postoperative pain and on ASIC3 expression at the prespinal and supraspinal levels in a rat model.

METHODS

Rats were randomly allocated to the control, incision, remifentanil, and remifentanil + incision groups. Remifentanil was given by a 1-h intravenous infusion prior to plantar incision. Paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) were measured at different time points before and after incision to evaluate mechanical and thermal hyperalgesia, respectively. The dorsal root ganglion (DRG), hippocampus, and hypothalamus were obtained after sacrifice at 48 h post-incision for determination of the protein expression of ASIC3 using western blot.

RESULTS

Remifentanil administration significantly induced mechanical and thermal hyperalgesia from 2 to 48 h after incision. In addition, remifentanil exposure remarkably stimulated ASIC3 protein expression in DRG, hippocampus, and hypothalamus of rats at 48 h after incision.

CONCLUSION

Remifentanil-induced hyperalgesia is accompanied by increased ASIC3 expression at the DRG and supraspinal levels, implying a possible involvement of ASIC3 in remifentanil-induced hyperalgesia.

Distal Electroacupuncture at the LI4 Acupoint Reduces CFA-Induced Inflammatory Pain via the Brain TRPV1 Signaling Pathway

There is accumulating evidence supporting electroacupuncture’s (EA) therapeutic effects. In mice, local EA reliably attenuates inflammatory pain and increases the transient receptor potential cation channel, subfamily V, member 1 (TRPV1). However, the effect of distal acupoint EA on pain control has rarely been studied. We used a mouse model to investigate the analgesic effect of distal EA by measuring TRPV1 expression in the brain. Complete Freund’s adjuvant (CFA) was injected into mice’s hind paws to induce inflammatory pain. The EA-treated group received EA at the LI4 acupoint on the bilateral forefeet on the second and the third days, whereas the control group underwent sham manipulation. Mechanical and thermal pain behavior tests showed that the EA-treated group experienced inflammatory pain alleviation immediately after EA, which did not occur in the sham group. Additionally, following CFA injection, the expression of TRPV1-associated molecules such as phosphorylated protein kinase A (pPKA), extracelluar signal-regulated kinase (pERK), and cAMP-response-element-binding protein (pCREB) increased in the prefrontal cortex (PFC) and the hypothalamus but decreased in the periaqueductal gray (PAG) area. These changes were significantly attenuated by EA but not sham EA. Our results show an analgesic effect of distal EA, which is based on the traditional Chinese medicine theory. The mechanism underlying this analgesic effect involves TRPV1 in the PFC, the hypothalamus, and the PAG. These novel findings are relevant for the evaluation and the treatment of clinical inflammatory pain syndrome.

Acupuncture therapy for fibromyalgia: a systematic review and meta-analysis of randomized controlled trials

Purpose

Fibromyalgia (FM) can cause chronic widespread pain and seriously affect the quality of patient lives. Acupuncture therapy is widely used for pain management. However, the effect of acupuncture on FM is still uncertain. The aim of this review was to determine the effect and safety of acupuncture therapy on the pain intensity and quality of life in patients with FM.

Materials and methods

We searched PubMed, the Cochrane Library, Embase, the China National Knowledge Infrastructure, the Chinese Science and Technology Periodical Database, and the Chinese Biomedical Literature Database to collect randomized controlled trials (RCTs) of acupuncture for FM published before May 2018. A meta-analysis was performed according to the Cochrane systematic review method by using RevMan 5.3 software, and GRADE was used to evaluate the quality of the evidence.

Results

We identified 12 RCTs that compared acupuncture therapy to sham acupuncture or conventional medication. Meta-analysis showed that acupuncture was significantly better than sham acupuncture for relieving pain (MD =−1.04, 95% CI [−1.70, –0.38], P=0.002, I²=78%) and improving the quality of life (MD =−13.39, 95% CI [−21.69, –5.10], P=0.002, I²=82%), with low- to moderate-quality evidence in the short term. At follow-up in the long term, the effect of acupuncture was also superior to that of sham acupuncture. No serious adverse events were found during acupuncture.

Conclusion

Acupuncture therapy is an effective and safe treatment for patients with FM, and this treatment can be recommended for the management of FM.

Analgesic Activity of Acid-Sensing Ion Channel 3 (ASIС3) Inhibitors: Sea Anemones Peptides Ugr9-1 and APETx2 versus Low Molecular Weight Compounds

Acid-sensing ion channel 3 (ASIC3) makes an important contribution to the development and maintenance of inflammatory and acid-induced pain. We compared different ASIC3 inhibitors (peptides from sea anemones (APETx2 and Ugr9-1) and nonpeptide molecules (sevanol and diclofenac)) in anti-inflammatory action and analgesic effects. All tested compounds had distinct effects on pH-induced ASIC3 current. APETx2 inhibited only transient current, whereas Ugr9-1 and sevanol decreased transient and sustained components of the current. The effect on mice was evaluated after administering an intramuscular injection in the acetic acid writhing pain model and the complete Freund's adjuvant-induced thermal hyperalgesia/inflammation test. The bell-shaped dependence of the analgesic effect was observed for APETx2 in the acetic acid-induced writhing test, as well as for sevanol and peptide Ugr9-1 in the thermal hyperalgesia test. This dependence could be evidence of the nonspecific action of compounds in high doses. Compounds reducing both components of ASIC3 current produced more significant pain relief than APETx2, which is an effective inhibitor of a transient current only. Therefore, the comparison of the efficacy of ASIC3 inhibitors revealed the importance of ASIC3-sustained currents' inhibition for promotion of acidosis-related pain relief.

Involvement of adenosine A1 receptor in electroacupuncture-mediated inhibition of astrocyte activation during neuropathic pain

ABSTRACT Neuropathic pain is a chronic pain condition caused by damage or dysfunction of the central or peripheral nervous system. Electroacupuncture (EA) has an antinociceptive effect on neuropathic pain, which is partially due to inhibiting astrocyte activation in the spinal cord. We found that an intrathecal injection of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective adenosine A1 receptor antagonist, reversed the antinociceptive effects of EA in a chronic constriction injury-induced neuropathic pain model. The expression of GFAP in L4-L6 spinal cord was significantly upgraded, while DPCPX suppressed the effect of the EA-mediating inhibition of astrocyte activation, as well as wiping out the EA-induced suppression of cytokine content (TNF-α). These results indicated that the adenosine A1 receptor is involved in EA actions during neuropathic pain through suppressing astrocyte activation as well as TNF-α upregulation of EA, giving enlightenment to the mechanisms of acupuncture analgesia and development of therapeutic targets for neuropathic pain.

Targeting TRPV1 to relieve motion sickness symptoms in mice by electroacupuncture and gene deletion

Motion sickness (MS) is an acute disorder that occurs in healthy individuals worldwide regardless of gender, age, or ethnicity. Our study used a mouse model to rule out the effects of any psychological factors related to MS and EA. Subjects were randomly separated into four groups, namely the control group (Con), motion sickness inducing group (MS), mentioning sickness inducing with electroacupuncture treatment group (EA) and motion sickness inducing only in TRPV1 knockout mice group (TRPV1-/-). The consumption of kaolin, a non-nutrient substance, was measured as a behavior observed response of an emetic reflex in a murine model. This behavior is referred to as pica behavior. Our results showed that pica behavior was observed in the MS group. Moreover, kaolin consumption in the EA group decreased to the average baseline of the control group. A similar result was observed in TRPV1 null mice. We also observed an increase of TRPV1 and related molecules in the thalamus, hypothalamic and brain stem after MS stimulation and a significant decrease in the EA and TRPV1 null groups. This is the first study to demonstrate that TRPV1 pathways are possibly associated with mechanisms of MS, and can be attended through EA or TRPV1 genetic manipulation.

Electroacupuncture at Hua Tuo Jia Ji Acupoints Reduced Neuropathic Pain and Increased GABAA Receptors in Rat Spinal Cord

Chronic constriction injury- (CCI-) induced neuropathic pain is the most similar model to hyperalgesia in clinical observation. Neuropathic pain is a neuronal dysfunction in the somatosensory system that may lead to spontaneous pain. In this study, electroacupuncture (EA) was applied at bilateral L4 and L6 of Hua Tuo Jia Ji points (EX-B2) for relieving neuropathic pain in rats. Eighteen Sprague-Dawley rats were randomly assigned to three groups: sham, 2-Hz EA, and 15-Hz EA groups. Following von Frey and cold plate tests, both the 2- and the 15-Hz EA groups had significantly lower mechanical and thermal hyperalgesia than the sham group. Western blot analysis results showed that γ-aminobutyric acid A (GABA_A), adenosine A1 receptor (A1R), transient receptor potential cation channel subfamily V member 1 (TRPV1), TRPV4, and metabotropic glutamate receptor 3 (mGluR3) were similar in the dorsal root ganglion of all three groups. Furthermore, levels of GABA_A receptors were higher in the spinal cord of rats in the 2- and 15-Hz EA groups compared with the sham control group. This was not observed for A1R, TRPV1, TRPV4, or mGluR3 receptors. In addition, all the aforementioned receptors were unchanged in the somatosensory cortex of the study rats, suggesting a central spinal effect. The study results provide evidence to support the clinical use of EA for specifically alleviating neuropathic pain.

Role of Asic3, Nav1.7 and Nav1.8 in Electroacupuncture-Induced Analgesia in a Mouse Model of Fibromyalgia Pain

Background

The mechanisms underlying fibromyalgia (FM) pain are not understood. The US Food and Drug Administration has recommended three drugs for treating FM—namely, pregabalin, duloxetine and milnacipran; however, these medications are associated with severe side effects.

Objective

To create a mouse model of FM pain using dual injections of acidic saline to cause mechanical hyperalgesia and test whether ASIC3, Nav1.7 and Nav1.8 are involved in this process and whether electroacupuncture (EA) can reverse these phenomena.

Methods

The FM model was established by injecting acidic saline twice into 40 ICR mice. The mice were assigned to subgroups (n=8 each) treated with different EA frequencies (2, 15 and 50 Hz). ASIC3, Nav1.7 and Nav1.8 expression levels were measured by Western blotting and immunohistochemistry.

Results

Significant mechanical hyperalgesia was induced on day 8 in FM mice, which was reversed by 2, 15 and 50 Hz EA. ASIC3, Nav1.7 and Nav1.8 protein levels increased significantly in both the dorsal root ganglion and in the spinal cord of FM model mice. These changes were further attenuated by 2, 15 and 50 Hz EA.

Conclusion

Reduced nociceptive ASIC3, Nav1.7 and Nav1.8 proteins are involved in the preventive effects of EA against FM, and this series of molecules may represent targets for FM treatment.

Electroacupuncture Alleviates Pain Responses and Inflammation in a Rat Model of Acute Gout Arthritis

Acute gout arthritis is one of the most painful inflammatory conditions. Treatments for gout pain are limited to colchicine, nonsteroidal anti-inflammatory drugs, and corticosteroids, which oftentimes result in severe adverse effects. Electroacupuncture (EA) has been proved to be effective in relieving many inflammatory pain conditions with few side effects. Here, we aim to investigate the therapeutic potentials of EA on pain and inflammation of a rat model of acute gout arthritis and underlying mechanisms. We found that 2/100 Hz EA produced the most robust analgesic effect on mechanical hyperalgesia of acute gout arthritis rat model compared with 2 and 100 Hz. EA produced similar analgesic effect compared with indomethacin. 2/100 Hz EA also significantly alleviates the ongoing pain behavior, thermal hyperalgesia, and ankle edema. Locally applied μ and κ-opioid receptor antagonists but not adenosine A₁ receptor antagonist significantly abolished the analgesic effect of EA. Locally applied μ and κ-opioid receptor agonists produced significant antiallodynia on acute gout arthritis rats, mimicking EA. Furthermore, 2/100 Hz EA upregulated β-endorphin expression in inflamed ankle skin tissue. Our results demonstrated, for the first time, that EA can be used for relieving acute gout arthritis with effect dependent on peripheral opioid system and comparable with the one obtained with indomethacin.

High fat diet sensitizes fibromyalgia-like pain behaviors in mice via tumor necrosis factor alpha

Fibromyalgia (FM) and obesity are closely related. However, little is known about how obesity contributes to FM. Importantly, adequate evidence has shown that tumor necrosis factor alpha (TNF-α) plays a critical role in obesity. Thus, we hypothesized that obesity-induced TNF-α release may potentiate FM-associated pain. To test this hypothesis, we investigated the role of TNF-α in the development of FM-like pain in a mouse model of acid saline injection-induced FM. Consistent with previous reports, we showed that repeated acid saline injections induced bilateral mechanical hyperalgesia, and this effect lasted for at least 4 days after acid saline injections. This phenomenon was associated with increased levels of TNF-α in plasma, muscles, and spinal cord. Furthermore, we found that 24 weeks of high fat diet treatment significantly potentiated acid saline-induced bilateral mechanical hyperalgesia. High fat diet-treated mice exhibited robustly increased levels of TNF-α in plasma, muscles, and spinal cord after acid saline injections compared with low fat diet-treated mice. Additionally, using immunofluorescence staining, we found that the number of TNF-α positive cells in dorsal root ganglion (DRG) was increased after acid saline injections, and high fat diet treatment further sensitized this increase. Finally, we reported that acid saline-induced FM-like pain behaviors were abolished in TNFRp55-/- mice, confirming the critical role of TNF-α in the development of FM-like pain. Taken together, our results suggested that high fat diet treatment may sensitize acid saline-induced FM-like pain via increasing TNF-α levels in plasma, muscles, and DRG.

Role of Transient Receptor Potential Vanilloid 1 in Electroacupuncture Analgesia on Chronic Inflammatory Pain in Mice

Chronic inflammatory pain may result from peripheral tissue injury or inflammation, increasing the release of protons, histamines, adenosine triphosphate, and several proinflammatory cytokines and chemokines. Transient receptor potential vanilloid 1 (TRPV1) is known to be involved in acute to subacute neuropathic and inflammatory pain; however, its exact mechanisms in chronic inflammatory pain are not elucidated. Our results showed that EA significantly reduced chronic mechanical and thermal hyperalgesia in the chronic inflammatory pain model. Chronic mechanical and thermal hyperalgesia were also abolished in TRPV1-/- mice. TRPV1 increased in the dorsal root ganglion (DRG) and spinal cord (SC) at 3 weeks after CFA injection. The expression levels of downstream molecules such as pPKA, pPI3K, and pPKC increased, as did those of pERK, pp38, and pJNK. Transcription factors (pCREB and pNFκB) and nociceptive ion channels (Nav1.7 and Nav1.8) were involved in this process. Inflammatory mediators such as GFAP, S100B, and RAGE were also involved. The expression levels of these molecules were reduced in EA and TRPV1-/- mice but not in the sham EA group. Our data provided evidence to support the clinical use of EA for treating chronic inflammatory pain.

Nerve Stimulation: Immunomodulation and Control of Inflammation

Neuronal stimulation is an emerging field in modern medicine to control organ function and re-establish physiological homeostasis during illness. Transdermal nerve stimulation with electroacupuncture is currently endorsed by the World Health Organization (WHO) and the National Institutes of Health (NIH), and is used by millions of people to control pain and inflammation. Recent advances in electroacupuncture may permit activation of specific neuronal networks to prevent organ damage in inflammatory and infectious disorders. Experimental studies of nerve stimulation are also providing new information on the functional organization of the nervous system to control inflammation and its clinical implications in infectious and inflammatory disorders. These studies may allow the design of novel non-invasive techniques for nerve stimulation to help to control immune and organ functions.

Spotlight on acupuncture in laboratory animal medicine

Acupuncture has been practiced for thousands of years, although it is only in the past century that science has worked to unravel the mechanisms behind its use. Literature supporting the efficacious use of acupuncture to treat a variety of conditions has been and continues to be published, including the randomized controlled studies we all appreciate when practicing evidence-based medicine. The use of acupuncture in veterinary medicine has paralleled the trends observed in people, with an increasingly common use to remedy specific medical conditions. These conditions are commonly related to neurological dysfunction or orthopedic pain. Although pain relief is the most common use of acupuncture, numerous other conditions have been shown to improve with this therapy. Laboratory animals are also benefiting from acupuncture. Its use is starting to be incorporated into research settings, although there is still further progress to be made in this field. Acupuncture has been shown to improve clinical conditions and quality of life in laboratory animals, and should be considered as a tool to treat laboratory animals with conditions known to benefit from therapy. Here we review the history, mechanisms of action, and use of acupuncture to treat veterinary patients and laboratory animals.