ERK is sequentially activated in neurons, microglia, and astrocytes by spinal nerve ligation and contributes to mechanical allodynia in this neuropathic pain model

Resident immune responses to spinal cord injury: role of astrocytes and microglia

Spinal cord injury can be traumatic or non-traumatic in origin, with the latter rising in incidence and prevalence with the aging demographics of our society. Moreover, as the global population ages, individuals with co-existent degenerative spinal pathology comprise a growing number of traumatic spinal cord injury cases, especially involving the cervical spinal cord. This makes recovery and treatment approaches particularly challenging as age and comorbidities may limit regenerative capacity. For these reasons, it is critical to better understand the complex milieu of spinal cord injury lesion pathobiology and the ensuing inflammatory response. This review discusses microglia-specific purinergic and cytokine signaling pathways, as well as microglial modulation of synaptic stability and plasticity after injury. Further, we evaluate the role of astrocytes in neurotransmission and calcium signaling, as well as their border-forming response to neural lesions. Both the inflammatory and reparative roles of these cells have eluded our complete understanding and remain key therapeutic targets due to their extensive structural and functional roles in the nervous system. Recent advances have shed light on the roles of glia in neurotransmission and reparative injury responses that will change how interventions are directed. Understanding key processes and existing knowledge gaps will allow future research to effectively target these cells and harness their regenerative potential.

Light-Inducible Activation of TrkA for Probing Chronic Pain in Mice

Chronic pain is a prevalent problem that plagues modern society, and better understanding its mechanisms is critical for developing effective therapeutics. Nerve growth factor (NGF) and its primary receptor, Tropomyosin receptor kinase A (TrkA), are known to be potent mediators of chronic pain, but there is a lack of established methods for precisely perturbing the NGF/TrkA signaling pathway in the study of pain and nociception. Optobiological tools that leverage light-induced protein-protein interactions allow for precise spatial and temporal control of receptor signaling. Previously, our lab reported a blue light-activated version of TrkA generated using light-induced dimerization of the intracellular TrkA domain, opto-iTrkA. In this work, we show that opto-iTrkA activation is able to activate endogenous ERK and Akt signaling pathways and causes the retrograde transduction of phospho-ERK signals in dorsal root ganglion (DRG) neurons. Opto-iTrkA activation also sensitizes the transient receptor potential vanilloid 1 (TRPV1) channel in cellular models, further corroborating the physiological relevance of the optobiological stimulus. Finally, we show that opto-iTrkA enables light-inducible potentiation of mechanical sensitization in mice. Light illumination enables nontraumatic and reversible (<2 days) sensitization of mechanical pain in mice transduced with opto-iTrkA, which provides a platform for dissecting TrkA pathways for nociception in vitro and in vivo.

Cannabidiol and pain

Chronic pain presents significant personal, psychological, and socioeconomic hurdles, impacting over 30% of adults worldwide and substantially contributing to disability. Unfortunately, current pharmacotherapy often proves inadequate, leaving fewer than 70% of patients with relief. This shortfall has sparked a drive to seek alternative treatments offering superior safety and efficacy profiles. Cannabinoid-based pharmaceuticals, notably cannabidiol (CBD), hold promise in pain management, driven by their natural origins, versatility, and reduced risk of addiction. As we navigate the opioid crisis, ongoing research plunges into CBD's therapeutic potential, buoyed by animal studies revealing its pain-relieving prowess through various system tweaks. However, the efficacy of cannabis in chronic pain management remains a contentious and stigmatized issue. The International Association for the Study of Pain (IASP) presently refrains from endorsing cannabinoid use for pain relief. Nevertheless, evidence indicates their potential in alleviating cancer-related, neuropathic, arthritis, and musculoskeletal pain, necessitating further investigation. Crucially, our comprehension of CBD's role in pain management is a journey still unfolding, with animal studies illustrating its analgesic effects through interactions with the endocannabinoid, inflammatory, and nociceptive systems. As the plot thickens, it's clear: the saga of chronic pain and CBD's potential offers a compelling narrative ripe for further exploration and understanding.

RTA-408 Regulates p-NF-κB/TSLP/STAT5 Signaling to Ameliorate Nociceptive Hypersensitivity in Chronic Constriction Injury Rats

Neuropathic pain following nerve injury is a complex condition, which often puts a negative impact on life and remains a sustained problem. To make pain management better is of great significance and unmet need. RTA 408 (Omaveloxone) is a traditional Asian medicine with a valid anti-inflammatory property. Thus, we aim to investigate the therapeutic effect of RTA-408 on mechanical allodynia in chronic constriction injury (CCI) rats as well as the underlying mechanisms. Neuropathic pain was induced by using CCI of the rats' sciatic nerve (SN) and the behavior testing was measured by calibrated forceps testing. Activation of Nrf-2, the phosphorylation of nuclear factor-κB (NF-κB), and the inflammatory response were assessed by western blots. The number of apoptotic neurons and degree of glial cell reaction were examined by immunofluorescence assay. RTA-408 exerts an analgesic effect on CCI rats. RTA-408 reduces neuronal apoptosis and glial cell activation by increasing Nrf-2 expression and decreasing the inflammatory response (TNF-α/ p-NF-κB/ TSLP/ STAT5). These data suggest that RTA-408 is a candidate with potential to reduce nociceptive hypersensitivity after CCI by targeting TSLP/STAT5 signaling.

Spinal microglial M1 polarization contributes paclitaxel-induced neuropathic pain by triggering cells necroptosis

Paclitaxel (PTX) is a chemotherapeutic agent that is widely used for the treatment of several types of tumors. However, PTX-induced peripheral neuropathy (PIPN) is an adverse effect generally induced by long-term PTX use that significantly impairs the quality of life. Necroptosis has been implicated in various neurodegenerative disorders. Necroptosis of dorsal root ganglion neurons triggers the pathogenesis of PIPN. Therefore, the present study aims to investigate the role of spinal neuronal necroptosis in PIPN. It also explores the potential role of microglial polarization in necroptosis. We established rat models of PIPN via quartic PTX administration on alternate days (accumulated dose: 8 mg/kg). PTX induced obvious neuronal necroptosis and upregulated the expression of receptor-interacting protein kinase (RIP3) and mixed lineage kinase domain-like protein (MLKL) in the spinal dorsal horn. These effects were inhibited with a necroptosis pathway inhibitor, necrostatin-1 (Nec-1). The effect of microglial polarization on the regulation of spinal necroptosis was elucidated by administering minocycline to inhibit PTX-induced M1 polarization of spinal microglia caused by PTX. We observed a significant inhibitory effect of minocycline on PTX-induced necroptosis in spinal cord cells, based on the downregulation of RIP3 and MLKL expression, and suppression of tumor necrosis factor-α and IL-β synthesis. Additionally, minocycline improved hyperalgesia symptoms in PIPN rats. Overall, this study suggests that PTX-induced polarization of spinal microglia leads to RIP3/MLKL-regulated necroptosis, resulting in PIPN. These findings suggest a potential target for the prevention and treatment of neuropathic pain.

Cytochrome P450 26A1 Contributes to the Maintenance of Neuropathic Pain

The cytochrome P450 proteins (CYP450s) have been implicated in catalyzing numerous important biological reactions and contribute to a variety of diseases. CYP26A1, a member of the CYP450 family, carries out the oxidative metabolism of retinoic acid (RA), the active metabolite of vitamin A. Here we report that CYP26A1 was dramatically upregulated in the spinal cord after spinal nerve ligation (SNL). CYP26A1 was mainly expressed in spinal neurons and astrocytes. HPLC analysis displayed that the content of all-trans-RA (at-RA), the substrate of CYP26A1, was reduced in the spinal cord on day 7 after SNL. Inhibition of CYP26A1 by siRNA or inhibition of CYP26A1-mediated at-RA catabolism by talarozole relieved the SNL-induced mechanical allodynia during the maintenance phase of neuropathic pain. Talarozole also reduced SNL-induced glial activation and proinflammatory cytokine production but increased anti-inflammatory cytokine (IL-10) production. The RA receptors RARα, RXRβ, and RXRγ were expressed in spinal neurons and glial cells. The promoter of Il-10 has several binding sites for RA receptors, and at-RA directly increased Il-10 mRNA expression in vitro. Finally, intrathecal IL-10 attenuated SNL-induced neuropathic pain and reduced the activation of astrocytes and microglia. Collectively, the inhibition of CYP26A1-mediated at-RA catabolism alleviates SNL-induced neuropathic pain by promoting the expression of IL-10 and suppressing glial activation. CYP26A1 may be a potential therapeutic target for the treatment of neuropathic pain.

SPOCK2 modulates neuropathic pain by interacting with MT1-MMP to regulate astrocytic MMP-2 activation in rats with chronic constriction injury

BACKGROUND

Neuropathic pain (NP) is a kind of intractable pain. The pathogenesis of NP remains a complicated issue for pain management practitioners. SPARC/osteonectin, CWCV, and Kazal-like domains proteoglycan 2 (SPOCK2) are members of the SPOCK family that play a significant role in the development of the central nervous system. In this study, we investigated the role of SPOCK2 in the development of NP in a rat model of chronic constriction injury (CCI).

METHODS

Sprague-Dawley rats were randomly grouped to establish CCI models. We examined the effects of SPOCK2 on pain hpersensitivity and spinal astrocyte activation after CCI-induced NP. Paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) were used to reflects the pain behavioral degree. Molecular mechanisms involved in SPOCK2-mediated NP in vivo were examined by western blot analysis, immunofluorescence, immunohistochemistry, and co-immunoprecipitation. In addition, we examined the SPOCK2-mediated potential protein-protein interaction (PPI) in vitro coimmunoprecipitation (Co-IP) experiments.

RESULTS

We founded the expression level of SPOCK2 in rat spinal cord was markedly increased after CCI-induced NP, while SPOCK2 downregulation could partially relieve pain caused by CCI. Our research showed that SPOCK2 expressed significantly increase in spinal astrocytes when CCI-induced NP. In addition, SPOCK2 could act as an upstream signaling molecule to regulate the activation of matrix metalloproteinase-2 (MMP-2), thus affecting astrocytic ERK1/2 activation and interleukin (IL)-1β production in the development of NP. Moreover, in vitro coimmunoprecipitation (Co-IP) experiments showed that SPOCK2 could interact with membrane-type 1 matrix metalloproteinase (MT1-MMP/MMP14) to regulate MMP-2 activation by the SPARC extracellular (SPARC_EC) domain.

CONCLUSIONS

Research shows that SPOCK2 can interact with MT1-MMP to regulate MMP-2 activation, thus affecting astrocytic ERK1/2 activation and IL-1β production to achieve positive promotion of NP.

Annulus Fibrosus Injury Induces Acute Neuroinflammation and Chronic Glial Response in Dorsal Root Ganglion and Spinal Cord-An In Vivo Rat Discogenic Pain Model

Chronic painful intervertebral disc (IVD) degeneration (i.e., discogenic pain) is a major source of global disability needing improved knowledge on multiple-tissue interactions and how they progress in order improve treatment strategies. This study used an in vivo rat annulus fibrosus (AF) injury-driven discogenic pain model to investigate the acute and chronic changes in IVD degeneration and spinal inflammation, as well as sensitization, inflammation, and remodeling in dorsal root ganglion (DRG) and spinal cord (SC) dorsal horn. AF injury induced moderate IVD degeneration with acute and broad spinal inflammation that progressed to DRG to SC changes within days and weeks, respectively. Specifically, AF injury elevated macrophages in the spine (CD68) and DRGs (Iba1) that peaked at 3 days post-injury, and increased microglia (Iba1) in SC that peaked at 2 weeks post-injury. AF injury also triggered glial responses with elevated GFAP in DRGs and SC at least 8 weeks post-injury. Spinal CD68 and SC neuropeptide Substance P both remained elevated at 8 weeks, suggesting that slow and incomplete IVD healing provides a chronic source of inflammation with continued SC sensitization. We conclude that AF injury-driven IVD degeneration induces acute spinal, DRG, and SC inflammatory crosstalk with sustained glial responses in both DRGs and SC, leading to chronic SC sensitization and neural plasticity. The known association of these markers with neuropathic pain suggests that therapeutic strategies for discogenic pain need to target both spinal and nervous systems, with early strategies managing acute inflammatory processes, and late strategies targeting chronic IVD inflammation, SC sensitization, and remodeling.

MiR-31-5p regulates the neuroinflammatory response via TRAF6 in neuropathic pain

BACKGROUND

Neuropathic pain is chronic pain and has few effective control strategies. Studies have demonstrated that microRNAs have functions in neuropathic pain. However, no study has been conducted to demonstrate the role and mechanism of microRNA (miR)-31-5p in neuropathic pain. Accordingly, this study sought to determine the pathological role of miR-31-5p in chronic constriction injury (CCI) -induced neuropathic pain mouse models.

METHODS

We used CCI surgery to establish mouse neuropathic pain model. Behavioral tests were performed to evaluate pain sensitivity of mice. Expressions of miR-31-5p and inflammatory cytokines in dorsal root ganglion (DRG) were examined by polymerase chain reaction. Animals or cells were received with/without miR-31-5p mimic or inhibitor to investigate its role in neuropathic pain. The mechanism of miR-31-5p was assayed using western blotting, immunofluorescence staining and dual-luciferase reporter assay.

RESULTS

We found that CCI led to a significant decrease in miR-31-5p levels. Knockout of miR-31-5p and administration of miPEP31 exacerbated pain in C57BL/6 mice. Meanwhile, miR-31-5p overexpression increased the paw withdrawal threshold and latency. TRAF6 is one of the target gene of miR-31-5p, which can trigger a complex inflammatory response. TRAF6 was associated with pain and that reducing the DRG expression of TRAF6 could alleviate pain. In addition, miR-31-5p overexpression inhibited the TRAF6 expression and reduced the neuroinflammatory response.

CONCLUSIONS

All the results reveal that miR-31-5p could potentially alleviate pain in CCI mouse models by inhibiting the TRAF6 mediated neuroinflammatory response. MiR-31-5p upregulation is highlighted here as new target for CCI treatment.

Regulation of Pain Perception by Microbiota in Parkinson Disease

Pain perception involves current stimulation in peripheral nociceptive nerves and the subsequent stimulation of postsynaptic excitatory neurons in the spinal cord. Importantly, in chronic pain, the neural activity of both peripheral nociceptors and postsynaptic neurons in the central nervous system is influenced by several inflammatory mediators produced by the immune system. Growing evidence has indicated that the commensal microbiota plays an active role in regulating pain perception by either acting directly on nociceptors or indirectly through the modulation of the inflammatory activity on immune cells. This symbiotic relationship is mediated by soluble bacterial mediators or intrinsic structural components of bacteria that act on eukaryotic cells, including neurons, microglia, astrocytes, macrophages, T cells, enterochromaffin cells, and enteric glial cells. The molecular mechanisms involve bacterial molecules that act directly on neurons, affecting their excitability, or indirectly on non-neuronal cells, inducing changes in the production of proinflammatory or anti-inflammatory mediators. Importantly, Parkinson disease, a neurodegenerative and inflammatory disorder that affects mainly the dopaminergic neurons implicated in the control of voluntary movements, involves not only a motor decline but also nonmotor symptomatology, including chronic pain. Of note, several recent studies have shown that Parkinson disease involves a dysbiosis in the composition of the gut microbiota. In this review, we first summarize, integrate, and classify the molecular mechanisms implicated in the microbiota-mediated regulation of chronic pain. Second, we analyze the changes on the commensal microbiota associated to Parkinson disease and propose how these changes affect the development of chronic pain in this pathology. SIGNIFICANCE STATEMENT: The microbiota regulates chronic pain through the action of bacterial signals into two main locations: the peripheral nociceptors and the postsynaptic excitatory neurons in the spinal cord. The dysbiosis associated to Parkinson disease reveals increased representation of commensals that potentially exacerbate chronic pain and reduced levels of bacteria with beneficial effects on pain. This review encourages further research to better understand the signals involved in bacteria-bacteria and bacteria-host communication to get the clues for the development of probiotics with therapeutic potential.

Ropivacaine-loaded hydrogels for prolonged relief of chemotherapy-induced peripheral neuropathic pain and potentiated chemotherapy

Chemotherapy can cause severe pain for patients, but there are currently no satisfactory methods of pain relief. Enhancing the efficacy of chemotherapy to reduce the side effects of high-dose chemotherapeutic drugs remains a major challenge. Moreover, the treatment of chemotherapy-induced peripheral neuropathic pain (CIPNP) is separate from chemotherapy in the clinical setting, causing inconvenience to cancer patients. In view of the many obstacles mentioned above, we developed a strategy to incorporate local anesthetic (LA) into a cisplatin-loaded PF127 hydrogel for painless potentiated chemotherapy. We found that multiple administrations of cisplatin-loaded PF127 hydrogels (PFC) evoked severe CIPNP, which correlated with increased pERK-positive neurons in the dorsal root ganglion (DRG). However, incorporating ropivacaine into the PFC relieved PFC-induced CIPNP for more than ten hours and decreased the number of pERK-positive neurons in the DRG. Moreover, incorporating ropivacaine into the PFC for chemotherapy is found to upregulate major histocompatibility complex class I (MHC-I) expression in tumor cells and promote the infiltration of cytotoxic T lymphocytes (CD8+ T cells) in tumors, thereby potentiating chemotherapy efficacy. This study proposes that LA can be used as an immunemodulator to enhance the effectiveness of chemotherapy, providing new ideas for painless cancer treatment.

Evaluation of pain related behaviors and disease related outcomes in an immunocompetent mouse model of prostate cancer induced bone pain

Cancer-induced bone pain (CIBP) is the most common and devastating symptom of bone metastatic cancer that substantially disrupts patients' quality of life. Currently, there are few effective analgesic treatments for CIBP other than opioids which come with severe side effects. In order to better understand the factors and mechanisms responsible for CIBP it is essential to have clinically relevant animal models that mirror pain-related symptoms and disease progression observed in patients with bone metastatic cancer. In the current study, we characterize a syngeneic mouse model of prostate cancer induced bone pain. We transfected a prostate cancer cell line (RM1) with green fluorescent protein (GFP) and luciferase reporters in order to visualize tumor growth longitudinally in vivo and to assess the relationship between sensory neurons and tumor cells within the bone microenvironment. Following intra-femoral injection of the RM1 prostate cancer cell line into male C57BL/6 mice, we observed a progressive increase in spontaneous guarding of the inoculated limb between 12 and 21 days post inoculation in tumor bearing compared to sham operated mice. Daily running wheel performance was evaluated as a measure of functional impairment and potentially movement evoked pain. We observed a progressive reduction in the distance traveled and percentage of time at optimal velocity between 12 and 21 days post inoculation in tumor bearing compared to sham operated mice. We utilized histological, radiographic and μCT analysis to examine tumor induced bone remodeling and observed osteolytic lesions as well as extra-periosteal aberrant bone formation in the tumor bearing femur, similar to clinical findings in patients with bone metastatic prostate cancer. Within the tumor bearing femur, we observed reorganization of blood vessels, macrophage and nerve fibers within the intramedullary space and periosteum adjacent to tumor cells. Tumor bearing mice displayed significant increases in the injury marker ATF3 and upregulation of the neuropeptides SP and CGRP in the ipsilateral DRG as well as increased measures of central sensitization and glial activation in the ipsilateral spinal cord. This immunocompetent mouse model will be useful when combined with cell type selective transgenic mice to examine tumor, immune cell and sensory neuron interactions in the bone microenvironment and their role in pain and disease progression associated with bone metastatic prostate cancer.

Advances in Neuropathic Pain Research: Selected Intracellular Factors as Potential Targets for Multidirectional Analgesics

Neuropathic pain is a complex and debilitating condition that affects millions of people worldwide. Unlike acute pain, which is short-term and starts suddenly in response to an injury, neuropathic pain arises from somatosensory nervous system damage or disease, is usually chronic, and makes every day functioning difficult, substantially reducing quality of life. The main reason for the lack of effective pharmacotherapies for neuropathic pain is its diverse etiology and the complex, still poorly understood, pathophysiological mechanism of its progression. Numerous experimental studies, including ours, conducted over the last several decades have shown that the development of neuropathic pain is based on disturbances in cell activity, imbalances in the production of pronociceptive factors, and changes in signaling pathways such as p38MAPK, ERK, JNK, NF-κB, PI3K, and NRF2, which could become important targets for pharmacotherapy in the future. Despite the availability of many different analgesics, relieving neuropathic pain is still extremely difficult and requires a multidirectional, individual approach. We would like to point out that an increasing amount of data indicates that nonselective compounds directed at more than one molecular target exert promising analgesic effects. In our review, we characterize four substances (minocycline, astaxanthin, fisetin, and peimine) with analgesic properties that result from a wide spectrum of actions, including the modulation of MAPKs and other factors. We would like to draw attention to these selected substances since, in preclinical studies, they show suitable analgesic properties in models of neuropathy of various etiologies, and, importantly, some are already used as dietary supplements; for example, astaxanthin and fisetin protect against oxidative stress and have anti-inflammatory properties. It is worth emphasizing that the results of behavioral tests also indicate their usefulness when combined with opioids, the effectiveness of which decreases when neuropathy develops. Moreover, these substances appear to have additional, beneficial properties for the treatment of diseases that frequently co-occur with neuropathic pain. Therefore, these substances provide hope for the development of modern pharmacological tools to not only treat symptoms but also restore the proper functioning of the human body.

Macrophages and microglia in inflammation and neuroinflammation underlying different pain states

Pain is a main symptom in inflammation, and inflammation induces pain via inflammatory mediators acting on nociceptive neurons. Macrophages and microglia are distinct cell types, representing immune cells and glial cells, respectively, but they share similar roles in pain regulation. Macrophages are key regulators of inflammation and pain. Macrophage polarization plays different roles in inducing and resolving pain. Notably, macrophage polarization and phagocytosis can be induced by specialized pro-resolution mediators (SPMs). SPMs also potently inhibit inflammatory and neuropathic pain via immunomodulation and neuromodulation. In this review, we discuss macrophage signaling involved in pain induction and resolution, as well as in maintaining physiological pain. Microglia are macrophage-like cells in the central nervous system (CNS) and drive neuroinflammation and pathological pain in various inflammatory and neurological disorders. Microglia-produced inflammatory cytokines can potently regulate excitatory and inhibitory synaptic transmission as neuromodulators. We also highlight sex differences in macrophage and microglial signaling in inflammatory and neuropathic pain. Thus, targeting macrophage and microglial signaling in distinct locations via pharmacological approaches, including immunotherapies, and non-pharmacological approaches will help to control chronic inflammation and chronic pain.

Acamprosate effect on neuropathic pain in rats: With emphasis on the role of ERK/MAPK pathway and SCN9A sodium channel

BACKGROUND

Neuropathic pain is a chronic pain owing to nerve damage or diseases of the central nervous system (CNS). The expression of SCN9A, which encodes the Nav1.7 voltage-gated sodium channel and ERK have been found to change significantly in many cases of neuropathic pain. Here, we investigated effects of acamprosate on neuropathic pain, taking into account the crucial roles of SCN9A, the ERK signaling pathway, and inflammatory markers in a rat model of chronic constriction injury (CCI).

METHODS

Acamprosate (300 mg/kg) was injected intraperitoneally (i.p.) for 14 days. The tail-immersion, acetone, and formalin tests were used to determine behavioral tests such as heat allodynia, cold allodynia, and chemical hyperalgesia, respectively. Lumbar spinal cord was extracted and processed for Nissl staining. The amount of spinal SCN9A expression and ERK phosphorylation were examined using ELISA assay.

RESULTS

The expression of SCN9A, ERK, inflammatory cytokines (IL-6 and TNF-α), allodynia and hyperalgesia significantly increased on days 7 and 14 following CCI. The treatment not only reduced neuropathic pain but also blocked CCI's effects on SCN9A upregulation and ERK phosphorylation.

CONCLUSION

This research demonstrated that acamprosate reduces the neuropathic pain induced by CCI of the sciatic nerve in rats by preventing cell loss, inhibiting spinal SCN9A expression, ERK phosphorylation, and inflammatory cytokines, suggesting potential therapeutic implications of acamprosate administration for the treatment of neuropathic pain.

An update to pain management after spinal cord injury: from pharmacology to circRNAs

Neuropathic pain (NP) following a spinal cord injury (SCI) is often hard to control and therapies should be focused on the physical, psychological, behavioral, social, and environmental factors that may contribute to chronic sensory symptoms. Novel therapeutic treatments for NP management should be based on the combination of pharmacological and nonpharmacological options. Some of them are addressed in this review with a focus on mechanisms and novel treatments. Several reports demonstrated an aberrant expression of non-coding RNAs (ncRNAs) that may represent key regulatory factors with a crucial role in the pathophysiology of NP and as potential diagnostic biomarkers. This review analyses the latest evidence for cellular and molecular mechanisms associated with the role of circular RNAs (circRNAs) in the management of pain after SCI. Advantages in the use of circRNA are their stability (up to 48 h), and specificity as sponges of different miRNAs related to SCI and nerve injury. The present review discusses novel data about deregulated circRNAs (up or downregulated) that sponge miRNAs, and promote cellular and molecular interactions with mRNAs and proteins. This data support the concept that circRNAs could be considered as novel potential therapeutic targets for NP management especially after spinal cord injuries.

Spinal CBX2 contributes to neuropathic pain by activating ERK signaling pathway in male mice

The deregulated spinal cord proteins induced by nerve injury are the key to neuropathic pain. Integrated transcriptome and translatome analyses can screen out deregulated proteins controlled by only post-transcriptional regulation. By comparing RNA sequencing (RNA-seq) and ribosome profiling sequencing (Ribo-seq) data, we identified an upregulated protein, chromobox 2 (CBX2), with its mRNA level unchanged in the spinal cord after peripheral nerve injury. CBX2 was mainly distributed in the spinal cord neurons. Blocking the SNL-induced increase of spinal CBX2 attenuated the neuronal and astrocytes hyperactivities and pain hypersensitivities in both the development and maintenance phases. Conversely, mimicking the upregulation of CBX2 in the spinal cord facilitated the activities of neurons and astrocytes and produced evoked nociceptive hypersensitivity and spontaneous pain. Our results also revealed that activating the ERK pathway, upregulating CXCL13 in neurons, and CXCL13 further inducing astrocyte activation were possible downstream signaling mechanisms of CBX2 in pain processing. In conclusion, upregulation of CBX2 after nerve injury leads to nociceptive hyperalgesia by promoting neuronal and astrocyte hyperactivities through the ERK pathway. Inhibiting CBX2 upregulation may be therapeutically beneficial.

Global Trends and Hotspots on Microglia Associated with Pain from 2002 to 2022: A Bibliometric Analysis

Background

Researchers have made significant progress in microglia associated with pain in recent years. However, more relevant bibliometric analyses are still needed on trends and directions in this field. The aim of this study is to provide a comprehensive perspective and to predict future directions of pain-related microglia research via bibliometric tools.

Methods

English articles and reviews related with pain and microglia were extracted from the Web of Science core collection (WosCC) database between 2002 to 2022. Bibliometric tools such as VOSviewer, CiteSpace, and Bibliometrix R package were used to analyze publication characteristics, countries, authors, institutions, journals, research hotspots, and trend topics.

Results

A total of 2761 articles were included in this analysis. Research on microglia associated with pain has increased significantly over the last two decades. China (n = 1020, 36.94%) and the United States (n = 751, 27.20%) contributed the most in terms of publications and citations, respectively. Kyushu University published the most articles in this field compared to other institutions, and Professor Inoue Kazuhide (n = 54) at this university made outstanding contributions in this field. Molecular Pain (n = 113) was the journal with the most publication, while Journal of Neuroscience had the highest number of citations. According to the authors keywords analysis, the research in this area can be summarized into 7 clusters such as "microglia activation pathways", "pain treatment research", "mental symptoms of chronic pain", and so on.

Conclusion

This study provides a comprehensive analysis of pain-related microglia research in the past two decades. We identified the countries, institutions, scholars, and journals with the highest number of publications and the most influence in the field, and the research trends identified in this paper may provide new insights for future research.

Effects of Natural Product-Derived Compounds on Inflammatory Pain via Regulation of Microglial Activation

Inflammatory pain is a type of pain caused by tissue damage associated with inflammation and is characterized by hypersensitivity to pain and neuroinflammation in the spinal cord. Neuroinflammation is significantly increased by various neurotransmitters and cytokines that are expressed in activated primary afferent neurons, and it plays a pivotal role in the development of inflammatory pain. The activation of microglia and elevated levels of pro-inflammatory cytokines are the hallmark features of neuroinflammation. During the development of neuroinflammation, various intracellular signaling pathways are activated or inhibited in microglia, leading to the regulation of inflammatory proteins and cytokines. Numerous attempts have been conducted to alleviate inflammatory pain by inhibiting microglial activation. Natural products and their compounds have gained attention as potential candidates for suppressing inflammatory pain due to verified safety through centuries of use. Many studies have also shown that natural product-derived compounds have the potential to suppress microglial activation and alleviate inflammatory pain. Herein, we review the literature on inflammatory mediators and intracellular signaling involved in microglial activation in inflammatory pain, as well as natural product-derived compounds that have been found to suppress microglial activation. This review suggests that natural product-derived compounds have the potential to alleviate inflammatory pain through the suppression of microglial activation.

Neuropathic Pain and Spinal Cord Injury: Management, Phenotypes, and Biomarkers

Chronic neuropathic pain after a spinal cord injury (SCI) continues to be a complex condition that is difficult to manage due to multiple underlying pathophysiological mechanisms and the association with psychosocial factors. Determining the individual contribution of each of these factors is currently not a realistic goal; however, focusing on the primary mechanisms may be more feasible. One approach used to uncover underlying mechanisms includes phenotyping using pain symptoms and somatosensory function. However, this approach does not consider cognitive and psychosocial mechanisms that may also significantly contribute to the pain experience and impact treatment outcomes. Indeed, clinical experience supports that a combination of self-management, non-pharmacological, and pharmacological approaches is needed to optimally manage pain in this population. This article will provide a broad updated summary integrating the clinical aspects of SCI-related neuropathic pain, potential pain mechanisms, evidence-based treatment recommendations, neuropathic pain phenotypes and brain biomarkers, psychosocial factors, and progress regarding how defining neuropathic pain phenotypes and other surrogate measures in the neuropathic pain field may lead to targeted treatments for neuropathic pain after SCI.

Magnolin Inhibits Paclitaxel-Induced Cold Allodynia and ERK1/2 Activation in Mice

Chemotherapy-induced peripheral neuropathy (CIPN) is a common side effect of anti-cancer drugs. The main symptoms often include sensory disturbances and neuropathic pain, and currently there is no effective treatment for this condition. This study aimed to investigate the suppressive effects of magnolin, an extracellular signal-regulated kinase (ERK) inhibitor substance derived from a 95% EtOH extract of the seeds of Magnolia denudata, on the symptoms of CIPN. A taxol-based anti-cancer drug paclitaxel (PTX) was repeatedly injected (2 mg/kg/day, total 8 mg/kg) into mice to induce CIPN. A neuropathic pain symptom was assessed using a cold allodynia test that scores behaviors of licking and shaking paw after plantar administration of acetone drop. Magnolin was administered intraperitoneally (0.1, 1, or 10 mg/kg) and behavioral changes to acetone drop were measured. The effect of magnolin administration on ERK expression in the dorsal root ganglion (DRG) was investigated using western blot analysis. The results showed that the repeated injections of PTX induced cold allodynia in mice. Magnolin administration exerted an analgesic effect on the PTX-induced cold allodynia and inhibited the ERK phosphorylation in the DRG. These results suggest that magnolin could be developed as an alternative treatment to suppress paclitaxel-induced neuropathic pain symptoms.

Analgesic Effects of Fisetin, Peimine, Astaxanthin, Artemisinin, Bardoxolone Methyl and 740 Y-P and Their Influence on Opioid Analgesia in a Mouse Model of Neuropathic Pain

Treatment of neuropathic pain remains a challenge for modern medicine due to the insufficiently understood molecular mechanisms of its development and maintenance. One of the most important cascades that modulate the nociceptive response is the family of mitogen-activated protein (MAP) kinases and phosphatidylinositol-3-kinase (PI3K), as well as nuclear factor erythroid 2-related factor 2 (Nrf2). The aim of this study was to determine the effect of nonselective modulators of MAP kinases-fisetin (ERK1/2 and NFκB inhibitor, PI3K activator), peimine (MAPK inhibitor), astaxanthin (MAPK inhibitor, Nrf2 activator) and artemisinin (MAPK inhibitor, NFκB activator), as well as bardoxolone methyl (selective activator of Nrf2) and 740 Y-P (selective activator of PI3K)-in mice with peripheral neuropathy and to compare their antinociceptive potency and examine their effect on analgesia induced by opioids. The study was performed using albino Swiss male mice that were exposed to chronic constriction injury of the sciatic nerve (CCI model). Tactile and thermal hypersensitivity was measured using von Frey and cold plate tests, respectively. Single doses of substances were administered intrathecally on day 7 after CCI. Among the tested substances, fisetin, peimine, and astaxanthin effectively diminished tactile and thermal hypersensitivity in mice after CCI, while artemisinin did not exhibit analgesic potency in this model of neuropathic pain. Additionally, both of the activators tested, bardoxolone methyl and 740 Y-P, also showed analgesic effects after intrathecal administration in mice exposed to CCI. In the case of astaxanthin and bardoxolone methyl, an increase in analgesia after combined administration with morphine, buprenorphine, and/or oxycodone was observed. Fisetin and peimine induced a similar effect on tactile hypersensitivity, where analgesia was enhanced after administration of morphine or oxycodone. In the case of 740 Y-P, the effects of combined administration with each opioid were observed only in the case of thermal hypersensitivity. The results of our research clearly indicate that substances that inhibit all three MAPKs provide pain relief and improve opioid effectiveness, especially if they additionally block NF-κB, such as peimine, inhibit NF-κB and activate PI3K, such as fisetin, or activate Nrf2, such as astaxanthin. In light of our research, Nrf2 activation appears to be particularly beneficial. The abovementioned substances bring promising results, and further research on them will broaden our knowledge regarding the mechanisms of neuropathy and perhaps contribute to the development of more effective therapy in the future.

Astrocytes in the spinal cord contributed to acute stress-induced gastric damage via the gap junction protein CX43

Rat restraint water-immersion stress (RWIS) is a compound stress of high intensity and is widely used to study the pathological mechanisms of stress gastric ulcers. The spinal cord, as a part of the central nervous system, plays a dominant role in the gastrointestinal tract, but whether the spinal cord is involved in rat restraint water-immersion stress (RWIS)-induced gastric mucosal damage has not been reported. In this study, we examined the expression of spinal astrocytic glial fibrillary acidic protein (GFAP), neuronal c-Fos, connexin 43 (Cx43), and p-ERK1/2 during RWIS by immunohistochemistry and Western blotting. In addition, we intrathecally injected the astrocytic toxin L-a-aminoadipate (L-AA), gap junction blocker carbenoxolone (CBX), and ERK1/2 signaling pathway inhibitor PD98059 to explore the role of astrocytes in the spinal cord in RWIS-induced gastric mucosal damage and its possible mechanism in rats. The results showed that the expression of GFAP, c-Fos, Cx43, and p-ERK1/2 was significantly elevated in the spinal cord after RWIS. Intrathecal injection of both the astrocyte toxin L-AA and the gap junction blocker CBX significantly attenuated RWIS-induced gastric mucosal damage and decreased the activation of astrocytes and neurons induced in the spinal cord. Meanwhile, the ERK1/2 signaling pathway inhibitor PD98059 significantly inhibited gastric mucosal damage, gastric motility and RWIS-induced activation of spinal cord neurons and astrocytes. These results suggest that spinal astrocytes may regulate the RWIS-induced activation of neurons via CX43 gap junctions and play a critical role in RWIS-induced gastric mucosa damage through the ERK1/2 signaling pathway.

GABAB receptor activation alters astrocyte phenotype changes induced by trimethyltin via ERK signaling in the dentate gyrus of mice

AIMS

We examined the effect of γ-aminobutyric acid (GABA)_B receptor activation on astrocyte phenotype changes induced by trimethyltin (TMT) in the dentate gyrus of mice.

MAIN METHODS

Male C57BL/6N mice received TMT (2.6 mg/kg, i.p.), and the expression of GABA_B receptors was evaluated in the hippocampus. The GABA_B receptor agonist baclofen (2.5, 5, or 10 mg/kg, i.p. × 5 at 12-h intervals) was administered 3-5 days after TMT treatment, and the expression of Iba-1, GFAP, and astrocyte phenotype markers was evaluated 6 days after TMT. SL327 (30 mg/kg, i.p.), an extracellular signal-related kinase (ERK) inhibitor, was administered 1 h after each baclofen treatment.

KEY FINDINGS

TMT insult significantly induced the astroglial expression of GABA_B receptors in the dentate molecular layer. Baclofen significantly promoted the expression of S100A10, EMP1, and CD109, but not that of C3, GGTA1, and MX1 induced by TMT. In addition, baclofen significantly increased the TMT-induced expression of p-ERK in the dentate molecular layer. Interestingly, p-ERK was more colocalized with S100A10 than with C3 after TMT insult, and a significant positive correlation was found between the expression of p-ERK and S100A10. Consistently, SL327 reversed the effect of baclofen on astrocyte phenotype changes. Baclofen also enhanced the TMT-induced astroglial expression of glial cell-derived neurotrophic factor (GDNF), an anti-inflammatory astrocytes-to-microglia mediator, and consequently attenuated Iba-1 expression and delayed apoptotic neuronal death.

SIGNIFICANCE

Our results suggest that GABA_B receptor activation increases S100A10-positive anti-inflammatory astrocytes and astroglial GDNF expression via ERK signaling after TMT excitotoxicity in the dentate molecular layer of mice.

The Similar and Distinct Roles of Satellite Glial Cells and Spinal Astrocytes in Neuropathic Pain

Preclinical studies have identified glial cells as pivotal players in the genesis and maintenance of neuropathic pain after nerve injury associated with diabetes, chemotherapy, major surgeries, and virus infections. Satellite glial cells (SGCs) in the dorsal root and trigeminal ganglia of the peripheral nervous system (PNS) and astrocytes in the central nervous system (CNS) express similar molecular markers and are protective under physiological conditions. They also serve similar functions in the genesis and maintenance of neuropathic pain, downregulating some of their homeostatic functions and driving pro-inflammatory neuro-glial interactions in the PNS and CNS, i.e., "gliopathy". However, the role of SGCs in neuropathic pain is not simply as "peripheral astrocytes". We delineate how these peripheral and central glia participate in neuropathic pain by producing different mediators, engaging different parts of neurons, and becoming active at different stages following nerve injury. Finally, we highlight the recent findings that SGCs are enriched with proteins related to fatty acid metabolism and signaling such as Apo-E, FABP7, and LPAR1. Targeting SGCs and astrocytes may lead to novel therapeutics for the treatment of neuropathic pain.

Insular cortex stimulation alleviates neuropathic pain via ERK phosphorylation in neurons

AIMS

The clinical use of brain stimulation is attractive for patients who have side effects or tolerance. However, studies on insular cortex (IC) stimulation are lacking in neuropathic pain. The present study aimed to investigate the effects of IC stimulation (ICS) on neuropathic pain and to determine how ICS modulates pain.

METHODS

Changes in pain behaviors were observed following ICS with various parameters in neuropathic rats. Western blotting was performed to assess molecular changes in the expression levels of phosphorylated extracellular signal-regulated kinase (pERK), neurons, astrocytes, and microglia between experimental groups. Immunohistochemistry was performed to investigate the colocalization of pERK with different cell types.

RESULTS

The most effective pain-relieving effect was induced at 50 Hz-120 μA in single trial of ICS and it maintained 4 days longer after the termination of repetitive ICS. The expression levels of pERK, astrocytes, and microglia were increased in neuropathic rats. However, after ICS, the expression levels of pERK were decreased, and colocalization of pERK and neurons was reduced in layers 2-3 of the IC.

CONCLUSION

These results indicated that ICS attenuated neuropathic pain by the regulation of pERK in neurons located in layers 2-3 of the IC. This preclinical study may enhance the potential use of ICS and identify the therapeutic mechanisms of ICS in neuropathic pain.

Bone marrow mesenchymal stem cells alleviate stress-induced hyperalgesia via restoring gut microbiota and inhibiting neuroinflammation in the spinal cord by targeting the AMPK/NF-κB signaling pathway

Aim Spinal neuroinflammation contributes to the mechanism of stress-induced hyperalgesia (SIH). Recent research has demonstrated that bone marrow mesenchymal stem cells (BMSCs) alleviate chronic pain. However, what remains unidentified is whether BMSCs could improve hyperalgesia induced by chronic restraint stress (CRS). In another dimension, our previous study proved that gut microbiota played an important role in CRS-induced hyperalgesia in mice. Yet, whether BMSCs treatments change gut microbiota composition in CRS mice remains unexplored.

MAIN METHODS

Mechanical allodynia and thermal hyperalgesia were used to assess pain behavior. Composition of fecal samples were verified by 16S rRNA analysis. Western blot was used to investigate the expression of adenosine monophosphate-activated protein kinase (AMPK)/ nuclear factor kappa B (NF-κB) signaling pathway, pro-inflammatory cytokines [interleukin-1 beta (IL-1β), tumor necrosis factor alpha (TNF-α), IL-6], and the markers of microglia and astrocytes. The morphology of glia cells was evaluated by immunofluorescence staining.

KEY FINDINGS

CRS down-regulated phosphorylated AMPK (p-AMPK), up-regulated phosphorylated NF-κB p65 (p-NF-κB p65), activated microglia and astrocytes and promoted the secretion of IL-1β, TNF-α and IL-6 in the spinal cord. BMSCs alleviated CRS-induced hyperalgesia by inhibiting the activation of microglia and astrocytes and by reducing neuroinflammation via improving the disrupted AMPK/NF-κB pathway. Furthermore, BMSCs also raised the relative abundance of Muribaculaceae and Lachnospiraceae in CRS mice feces, which was significantly related to its effect of relieving hyperalgesia.

SIGNIFICANCE

Our results support that BMSCs could alleviate CRS-induced hyperalgesia by reducing AMPK/NF-κB-dependent neuroinflammation in the spinal cord and restoring the homeostasis of gut microbiota.

Microglial Cannabinoid CB2 Receptors in Pain Modulation

Pain, especially chronic pain, can strongly affect patients' quality of life. Cannabinoids ponhave been reported to produce potent analgesic effects in different preclinical pain models, where they primarily function as agonists of G_i/o protein-coupled cannabinoid CB₁ and CB₂ receptors. The CB₁ receptors are abundantly expressed in both the peripheral and central nervous systems. The central activation of CB₁ receptors is strongly associated with psychotropic adverse effects, thus largely limiting its therapeutic potential. However, the CB₂ receptors are promising targets for pain treatment without psychotropic adverse effects, as they are primarily expressed in immune cells. Additionally, as the resident immune cells in the central nervous system, microglia are increasingly recognized as critical players in chronic pain. Accumulating evidence has demonstrated that the expression of CB₂ receptors is significantly increased in activated microglia in the spinal cord, which exerts protective consequences within the surrounding neural circuitry by regulating the activity and function of microglia. In this review, we focused on recent advances in understanding the role of microglial CB₂ receptors in spinal nociceptive circuitry, highlighting the mechanism of CB₂ receptors in modulating microglia function and its implications for CB₂ receptor- selective agonist-mediated analgesia.

Neuropathic Pain: Mechanisms, Sex Differences, and Potential Therapies for a Global Problem

The study of chronic pain continues to generate ever-increasing numbers of publications, but safe and efficacious treatments for chronic pain remain elusive. Recognition of sex-specific mechanisms underlying chronic pain has resulted in a surge of studies that include both sexes. A predominant focus has been on identifying sex differences, yet many newly identified cellular mechanisms and alterations in gene expression are conserved between the sexes. Here we review sex differences and similarities in cellular and molecular signals that drive the generation and resolution of neuropathic pain. The mix of differences and similarities reflects degeneracy in peripheral and central signaling processes by which neurons, immune cells, and glia codependently drive pain hypersensitivity. Recent findings identifying critical signaling nodes foreshadow the development of rationally designed, broadly applicable analgesic strategies. However, the paucity of effective, safe pain treatments compels targeted therapies as well to increase therapeutic options that help reduce the global burden of suffering.

Resolvin D1 attenuates mechanical allodynia after burn injury: Involvement of spinal glia, p38 mitogen-activated protein kinase, and brain-derived neurotrophic factor/tropomyosin-related kinase B signaling

Resolvin D1 (RvD1) suppresses inflammatory, postoperative, and neuropathic pain. The present study assessed the roles and mechanisms of RvD1 in mechanical allodynia after burn injury. A rat model of burn injury was established for analyses, and RvD1 was injected intraperitoneally. Pain behavior and the expression levels of spinal dorsal horn Iba-1 (microglia marker), GFAP (astrocyte marker), p-p38 mitogen-activated protein kinase (MAPK), brain-derived neurotrophic factor (BDNF), and tropomyosin-related kinase B (TrkB) were detected by behavioral and immunocytochemical assays. The results showed that RvD1 attenuated mechanical allodynia after burn injury, prevented microglial and astroglial activation, and downregulated p-p38 MAPK in microglia and BDNF/TrkB following burn injury. Similarly, inhibition of p38 MAPK and BDNF/TrkB signaling attenuated mechanical allodynia after burn injury. In addition, inhibition of p38 MAPK prevented spinal microglial activation and downregulated BDNF/TrkB following burn injury. Furthermore, inhibition of BDNF/TrkB signaling prevented spinal microglial activation and downregulated p-p38 MAPK within spinal microglia. Taken together, this study demonstrated that RvD1 might attenuate mechanical allodynia after burn injury by inhibiting spinal cord glial activation, microglial p38 MAPK, and BDNF/TrkB signaling in the spinal dorsal horn.

Sciatic nerve block downregulates the BDNF pathway to alleviate the neonatal incision-induced exaggeration of incisional pain via decreasing microglial activation

Sciatic nerve block is under investigation as a possible therapeutic strategy for neonatal injury-induced exaggeration of pain responses to reinjury. Spinal microglial priming, brain-derived neurotrophic factor (BDNF) and Src homology-2 domain-containing protein tyrosine phosphatase-2 (SHP2) participate in exaggerated incisional pain induced by neonatal incision. However, effects of sciatic nerve block on exacerbated incisional pain and underlying mechanisms remain unclear. Here, we demonstrated that sciatic nerve block alleviates pain hypersensitivity and microglial activation in rats subjected to neonatal incision and adult incision (nIN-IN). Chemogenetic activation or inhibition of spinal microglia attenuates or mimics effects of sciatic nerve block on pain hypersensitivity, respectively. Moreover, α-amino-3-hydroxy- 5-methy- 4-isoxazole propionate (AMPA) receptor subunit GluA1 contributes to the exaggeration of incisional pain. The inhibition of BDNF or SHP2 blocks upregulations of downstream molecules in nIN-IN rats. Knockdown of SHP2 attenuates the increase of GluA1 induced by injection of BDNF in adult rats with only neonatal incision. The inhibition of microglia or ablation of microglial BDNF attenuates upregulations of SHP2 and GluA1. Additionally, sciatic nerve block downregulates the expression of these three molecules. Upregulation of BDNF, SHP2 or AMPA receptor attenuates sciatic nerve block-induced reductions of downstream molecules and pain hypersensitivity. Microglial activation abrogates reductions of these three molecules induced by sciatic nerve block. These results suggest that decreased activation of spinal microglia contributes to beneficial effects of sciatic nerve block on the neonatal incision-induced exaggeration of incisional pain via downregulating BDNF/SHP2/GluA1-containing AMPA receptor signaling. Thus, sciatic nerve block may be a promising therapy.

NS5806 inhibits ERK activation to attenuate pain induced by peripheral nerve injury

Neuropathic pain is a serious health problem, but optimal drug treatments remain lacking. It has been known that the compound NS5806 is a Kv4.3 activator, which increases Kv4.3-mediated K⁺ current to reduce neuronal excitability. In this study, we investigated the molecular and cellular mechanisms underlying the analgesic effect of NS5806 in neuropathic pain induced by peripheral nerve injury. Using lumbar (L)5/L6 spinal nerve ligation (SNL) in rats, we found that, without changing the basal nociception, the analgesic effect of NS5806 (220 μg/kg) peaked at 4 h and lasted for 8 h after intraperitoneal injection. Multiple doses of NS5806 reduced not only SNL-upregulated proinflammatory mediators in the DRG and spinal cord on day 1 and day 4 after L5/L6 SNL, but also SNL-evoked expansion of DRG macrophages and spinal microglia on day 4. Furthermore, at 10 min after L5 SNL, NS5806 pretreatment for 4 h suppressed SNL-induced phosphorylated extracellular signal-regulated kinase (pERK) in both Kv4.3⁺ and Kv4.3^- neurons in the dorsal root ganglion (DRG) and superficial spinal dorsal horn, indicating that the action of NS5806 is not restricted to Kv4.3⁺ neurons. In vitro kinase activity assays revealed that NS5806 weakly inhibited ERK2, MEK1, MEK2, and c-Raf in the ERK pathway. Since NS5806 and the ERK pathway inhibitors have similar antinociceptive characteristics, this study suggests that NS5806 also acts as an ERK pathway inhibitor to attenuate neuropathic pain.

Pharmacogenetic inhibition of TrkB signaling in adult mice attenuates mechanical hypersensitivity and improves locomotor function after spinal cord injury

Brain-derived neurotrophic factor (BDNF) signals through tropomyosin receptor kinase B (TrkB), to exert various types of plasticity. The exact involvement of BDNF and TrkB in neuropathic pain states after spinal cord injury (SCI) remains unresolved. This study utilized transgenic TrkBF616 mice to examine the effect of pharmacogenetic inhibition of TrkB signaling, induced by treatment with 1NM-PP1 (1NMP) in drinking water for 5 days, on formalin-induced inflammatory pain, pain hypersensitivity, and locomotor dysfunction after thoracic spinal contusion. We also examined TrkB, ERK1/2, and pERK1/2 expression in the lumbar spinal cord and trunk skin. The results showed that formalin-induced pain responses were robustly attenuated in 1NMP-treated mice. Weekly assessment of tactile sensitivity with the von Frey test showed that treatment with 1NMP immediately after SCI blocked the development of mechanical hypersensitivity up to 4 weeks post-SCI. Contrastingly, when treatment started 2 weeks after SCI, 1NMP reversibly and partially attenuated hind-paw hypersensitivity. Locomotor scores were significantly improved in the early-treated 1NMP mice compared to late-treated or vehicle-treated SCI mice. 1NMP treatment attenuated SCI-induced increases in TrkB and pERK1/2 levels in the lumbar cord but failed to exert similar effects in the trunk skin. These results suggest that early onset TrkB signaling after SCI contributes to maladaptive plasticity that leads to spinal pain hypersensitivity and impaired locomotor function.

The role of astrocytes in neuropathic pain

Neuropathic pain, whose symptoms are characterized by spontaneous and irritation-induced painful sensations, is a condition that poses a global burden. Numerous neurotransmitters and other chemicals play a role in the emergence and maintenance of neuropathic pain, which is strongly correlated with common clinical challenges, such as chronic pain and depression. However, the mechanism underlying its occurrence and development has not yet been fully elucidated, thus rendering the use of traditional painkillers, such as non-steroidal anti-inflammatory medications and opioids, relatively ineffective in its treatment. Astrocytes, which are abundant and occupy the largest volume in the central nervous system, contribute to physiological and pathological situations. In recent years, an increasing number of researchers have claimed that astrocytes contribute indispensably to the occurrence and progression of neuropathic pain. The activation of reactive astrocytes involves a variety of signal transduction mechanisms and molecules. Signal molecules in cells, including intracellular kinases, channels, receptors, and transcription factors, tend to play a role in regulating post-injury pain once they exhibit pathological changes. In addition, astrocytes regulate neuropathic pain by releasing a series of mediators of different molecular weights, actively participating in the regulation of neurons and synapses, which are associated with the onset and general maintenance of neuropathic pain. This review summarizes the progress made in elucidating the mechanism underlying the involvement of astrocytes in neuropathic pain regulation.

K+ channel Kv4.1 is expressed in the nociceptors/secondary nociceptive neurons and participates in pain regulation

BACKGROUND

Kv4 channels are key components controlling neuronal excitability at membrane potentials below action potential thresholds. It remains elusive whether Kv4.1 participates in pain regulation.

METHODS

We raised a Kv4.1 antibody to map Kv4.1⁺ neurons in the superficial dorsal horn of spinal cord and dorsal root ganglion (DRG) of rats. Behavioral, biochemical, and immunohistochemical methods were used to examine whether the activity of Kv4.1⁺ neurons or Kv4.1 expression level is altered after peripheral nerve injury.

RESULTS

In lamina I of spinal cord, Kv4.1 immunoreactivity (IR) was detected in neurokinin-1 receptor positive (NK1R)⁺ projection neurons (the secondary nociceptive neurons) and NK1R⁺ excitatory interneurons. Kv4.1, KChIP2 and DPP10 were co-expressed in these neurons. Peripheral nerve injury evoked by lumbar spinal nerve ligation (SNL) immediately induced phosphorylated extracellular regulated protein kinase (pERK, an indicator of enhanced neuronal activity) in lamina I Kv4.1⁺ neurons and lamina II Kv4.2/Kv4.3⁺ neurons of the spinal cord. Furthermore, Kv4.1 appeared in 59.9% of DRG neurons with variable sizes. Kv4.1 mRNA and protein levels in DRG neurons were gradually decreased after SNL. Following intrathecal injection of Kv4.1 antisense oligodeoxynucleotide (ASO) into naive rats, Kv4.1 protein level was reduced in the DRG, and mechanical but not thermal hypersensitivity was induced.

CONCLUSIONS

Kv4.1 appears in the secondary nociceptive neurons, and peripheral nerve injury increases the activity of these neurons. Kv4.1 expression in DRG neurons (including half of the nociceptors) is gradually reduced after peripheral nerve injury, and knockdown of Kv4.1 in DRG neurons induces pain. Thus, Kv4.1 participates in pain regulation.

SIGNIFICANCE

Based on the expression of Kv4.1 and Kv4.3 in the nociceptors, Kv4.1 in the secondary nociceptive neurons, Kv4.1 in spinal lamina I excitatory interneurons that regulate the activity of the secondary nociceptive neurons, as well as Kv4.2 and Kv4.3 in spinal lamina II excitatory interneurons that also regulate the activity of the secondary nociceptive neurons, developing Kv4 activators or genetic manipulation to increase Kv4 channel activity in these pain-related Kv4⁺ neurons will be useful in future pain therapeutics.

Polyphenolic grape stalk and coffee extracts attenuate spinal cord injury-induced neuropathic pain development in ICR-CD1 female mice

More than half of spinal cord injury (SCI) patients develop central neuropathic pain (CNP), which is largely refractory to current treatments. Considering the preclinical evidence showing that polyphenolic compounds may exert antinociceptive effects, the present work aimed to study preventive effects on SCI-induced CNP development by repeated administration of two vegetal polyphenolic extracts: grape stalk extract (GSE) and coffee extract (CE). Thermal hyperalgesia and mechanical allodynia were evaluated at 7, 14 and 21 days postinjury. Then, gliosis, ERK phosphorylation and the expression of CCL2 and CX3CL1 chemokines and their receptors, CCR2 and CX3CR1, were analyzed in the spinal cord. Gliosis and CX3CL1/CX3CR1 expression were also analyzed in the anterior cingulate cortex (ACC) and periaqueductal gray matter (PAG) since they are supraspinal structures involved in pain perception and modulation. GSE and CE treatments modulated pain behaviors accompanied by reduced gliosis in the spinal cord and both treatments modulated neuron-glia crosstalk-related biomolecules expression. Moreover, both extracts attenuated astrogliosis in the ACC and PAG as well as microgliosis in the ACC with an increased M2 subpopulation of microglial cells in the PAG. Finally, GSE and CE prevented CX3CL1/CX3CR1 upregulation in the PAG, and modulated their expression in ACC. These findings suggest that repeated administrations of either GSE or CE after SCI may be suitable pharmacologic strategies to attenuate SCI-induced CNP development by means of spinal and supraspinal neuroinflammation modulation.

Astrocytic ERK/STAT1 signaling contributes to maintenance of stress-related visceral hypersensitivity in rats

The rostral anterior cingulate cortex (rACC) has been found to be an important brain region in mediating visceral hypersensitivity. However, the underlying mechanisms remain unclear. This study aimed to explore the role of astrocytes in the maintenance of visceral hypersensitivity induced by chronic water avoidance stress (WAS) as well as the potential signaling pathway that activates astrocytes in the rACC. We found that ACC-reactive astrogliosis resulted in the overexpression of c-fos, TSP-1, and BDNF in stress-related visceral hypersensitivity rats. Visceral hypersensitivity was reversed by pharmacological inhibition of astrocytic activation after WAS, as were the overexpression of c-fos, TSP-1 and BDNF. Activation of the astrocytic Gi-pathway increased the visceral sensitivity and expression of c-fos, TSP-1, and BDNF. Visceral hypersensitivity was also ameliorated by the pharmacological inhibition of ERK and STAT1 phosphorylation after WAS. Furthermore, inhibition of the ERK-STAT1 cascade reduced astrocytic activation. These findings suggest that astrocytic ERK/STAT1 signaling in the rACC contributes to the maintenance of stress-related visceral hypersensitivity. PERSPECTIVE: Visceral hypersensitivity is a key factor in the pathophysiology of irritable bowel syndrome. This study highlights the important role of astrocytic ERK/STAT1 signaling in activating astrocytes in the rostral anterior cingulate cortex, which contributes to visceral hypersensitivity.

Empagliflozin mitigates type 2 diabetes-associated peripheral neuropathy: a glucose-independent effect through AMPK signaling

Diabetic peripheral neuropathy (DPN) represents a severe microvascular condition that dramatically affects diabetic patients despite adequate glycemic control, resulting in high morbidity. Thus, recently, anti-diabetic drugs that possess glucose-independent mechanisms attracted attention. This work aims to explore the potentiality of the selective sodium-glucose cotransporter-2 inhibitor, empagliflozin (EMPA), to ameliorate streptozotocin-induced DPN in rats with insight into its precise signaling mechanism. Rats were allocated into four groups, where control animals received vehicle daily for 2 weeks. In the remaining groups, DPN was elicited by single intraperitoneal injections of freshly prepared streptozotocin and nicotinamide (52.5 and 50 mg/kg, respectively). Then EMPA (3 mg/kg/p.o.) was given to two groups either alone or accompanied with the AMPK inhibitor dorsomorphin (0.2 mg/kg/i.p.). Despite the non-significant anti-hyperglycemic effect, EMPA improved sciatic nerve histopathological alterations, scoring, myelination, nerve fibers’ count, and nerve conduction velocity. Moreover, EMPA alleviated responses to different nociceptive stimuli along with improved motor coordination. EMPA modulated ATP/AMP ratio, upregulated p-AMPK while reducing p-p38 MAPK expression, p-ERK1/2 and consequently p-NF-κB p65 as well as its downstream mediators (TNF-α and IL-1β), besides enhancing SOD activity and lowering MDA content. Moreover, EMPA downregulated mTOR and stimulated ULK1 as well as beclin-1. Likewise, EMPA reduced miR-21 that enhanced RECK, reducing MMP-2 and -9 contents. EMPA’s beneficial effects were almost abolished by dorsomorphin administration. In conclusion, EMPA displayed a protective effect against DPN independently from its anti-hyperglycemic effect, probably via modulating the AMPK pathway to modulate oxidative and inflammatory burden, extracellular matrix remodeling, and autophagy.

Empagliflozin mitigates type 2 diabetes-associated peripheral neuropathy: a glucose-independent effect through AMPK signaling

Diabetic peripheral neuropathy (DPN) represents a severe microvascular condition that dramatically affects diabetic patients despite adequate glycemic control, resulting in high morbidity. Thus, recently, anti-diabetic drugs that possess glucose-independent mechanisms attracted attention. This work aims to explore the potentiality of the selective sodium-glucose cotransporter-2 inhibitor, empagliflozin (EMPA), to ameliorate streptozotocin-induced DPN in rats with insight into its precise signaling mechanism. Rats were allocated into four groups, where control animals received vehicle daily for 2 weeks. In the remaining groups, DPN was elicited by single intraperitoneal injections of freshly prepared streptozotocin and nicotinamide (52.5 and 50 mg/kg, respectively). Then EMPA (3 mg/kg/p.o.) was given to two groups either alone or accompanied with the AMPK inhibitor dorsomorphin (0.2 mg/kg/i.p.). Despite the non-significant anti-hyperglycemic effect, EMPA improved sciatic nerve histopathological alterations, scoring, myelination, nerve fibers' count, and nerve conduction velocity. Moreover, EMPA alleviated responses to different nociceptive stimuli along with improved motor coordination. EMPA modulated ATP/AMP ratio, upregulated p-AMPK while reducing p-p38 MAPK expression, p-ERK1/2 and consequently p-NF-κB p65 as well as its downstream mediators (TNF-α and IL-1β), besides enhancing SOD activity and lowering MDA content. Moreover, EMPA downregulated mTOR and stimulated ULK1 as well as beclin-1. Likewise, EMPA reduced miR-21 that enhanced RECK, reducing MMP-2 and -9 contents. EMPA's beneficial effects were almost abolished by dorsomorphin administration. In conclusion, EMPA displayed a protective effect against DPN independently from its anti-hyperglycemic effect, probably via modulating the AMPK pathway to modulate oxidative and inflammatory burden, extracellular matrix remodeling, and autophagy.

CXCR4/CX43 Regulate Diabetic Neuropathic Pain via Intercellular Interactions between Activated Neurons and Dysfunctional Astrocytes during Late Phase of Diabetes in Rats and the Effects of Antioxidant N-Acetyl-L-Cysteine

Growing evidence suggests that the interactions between astrocytes and neurons exert important functions in the central sensitization of the spinal cord dorsal horn in rodents with diabetes and neuropathic pain (DNP). However, it still remains unclear how signal transmission occurs in the spinal cord dorsal horn between astrocytes and neurons, especially in subjects with DNP. Chemokine CXC receptor 4 (CXCR4) plays critical roles in DNP, and connexin 43 (CX43), which is also primarily expressed by astrocytes, contributes to the development of neuropathy. We thus postulated that astrocytic and neuronal CXCR4 induces and produces inflammatory factors under persistent peripheral noxious stimulation in DNP, while intercellular CX43 can transmit inflammatory stimulation signals. The results showed that streptozotocin-induced type 1 diabetic rats developed heat hyperalgesia and mechanical allodynia. Diabetes led to persistent neuropathic pain. Diabetic rats developed peripheral sensitization at the early phase (2 weeks) and central sensitization at the late phase (5 weeks) after diabetes induction. Both CXCR4 and CX43, which are localized and coexpressed in neurons and astrocytes, were enhanced significantly in the dorsal horn of spinal cord in rats undergoing DNP during late phase of diabetes, and the CXCR4 antagonist AMD3100 reduced the expression of CX43. The nociceptive behavior was reversed, respectively, by AMD3100 at the early phase and by the antioxidant N-acetyl-L-cysteine (NAC) at the late phase. Furthermore, rats with DNP demonstrated downregulation of glial fibrillary acidic protein (GFAP) as well as upregulation of c-fos in the spinal cord dorsal horn at the late phase compared to the controls, and upregulation of GFAP and downregulation of c-fos were observed upon treatment with NAC. Given that GFAP and c-fos are, respectively, makers of astrocyte and neuronal activation, our findings suggest that CXCR4 as an inflammatory stimulation protein and CX43 as an intercellular signal transmission protein both may induce neurons excitability and astrocytes dysfunction in developing DNP.

Sciatic nerve stimulation alleviates acute neuropathic pain via modulation of neuroinflammation and descending pain inhibition in a rodent model

Background

Neuropathic pain (NP) is characterized by abnormal activation of pain conducting pathways and manifests as mechanical allodynia and thermal hypersensitivity. Peripheral nerve stimulation is used for treatment of medically refractory chronic NP and has been shown to reduce neuroinflammation. However, whether sciatic nerve stimulation (SNS) is of therapeutic benefit to NP remains unclear. Moreover, the optimal frequency for SNS is unknown. To address this research gap, we investigated the effect of SNS in an acute NP rodent model.

Methods

Rats with right L5 nerve root ligation (NRL) or Sham surgery were used. Ipsilateral SNS was performed at 2 Hz, 20 Hz, and 60 Hz frequencies. Behavioral tests were performed to assess pain and thermal hypersensitivity before and after NRL and SNS. Expression of inflammatory proteins in the L5 spinal cord and the immunohistochemical alterations of spinal cord astrocytes and microglia were examined on post-injury day 7 (PID7) following NRL and SNS. The involvement of the descending pain modulatory pathway was also investigated.

Results

Following NRL, the rats showed a decreased pain threshold and latency on the von Frey and Hargreaves tests. The immunofluorescence results indicated hyperactivation of superficial spinal cord dorsal horn (SCDH) neurons. Both 2-Hz and 20-Hz SNS alleviated pain behavior and hyperactivation of SCDH neurons. On PID7, NRL resulted in elevated expression of spinal cord inflammatory proteins including NF-κB, TNF-α, IL-1β, and IL-6, which was mitigated by 2-Hz and 20-Hz SNS. Furthermore, 2-Hz and 20-Hz SNS suppressed the activation of spinal cord astrocytes and microglia following NRL on PID7. Activity of the descending serotoninergic pain modulation pathway showed an increase early on PID1 following 2-Hz and 20-Hz SNS.

Conclusions

Our results support that both 2-Hz and 20-Hz SNS can alleviate NP behaviors and hyperactivation of pain conducting pathways. We showed that SNS regulates neuroinflammation and reduces inflammatory protein expression, astrocytic gliosis, and microglia activation. During the early post-injury period, SNS also facilitates the descending pain modulatory pathway. Taken together, these findings support the therapeutic potential of SNS for acute NP.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02513-y.

DKK3 ameliorates neuropathic pain via inhibiting ASK-1/JNK/p-38-mediated microglia polarization and neuroinflammation

BACKGROUND

Neuropathic pain is a common and severely disabling state that affects millions of people worldwide. Microglial activation in the spinal cord plays a critical role in the pathogenesis of neuropathic pain. However, the mechanisms underlying spinal microglial activation during neuropathic pain remain incompletely understood. Here, we investigated the role of Dickkopf (DKK) 3 and its interplay with microglial activation in the spinal cord in neuropathic pain.

METHODS

In this study, we investigated the effects of intrathecal injection of recombinant DKK3 (rDKK3) on mechanical allodynia and microglial activation in the spinal cord after spared nerve injury (SNI) in rats by western blot (WB), immunofluorescence (IF), quantitative polymerase chain reaction (qPCR), and enzyme-linked immunosorbent assay (ELISA).

RESULTS

We found that SNI induced a significant decrease in the levels of DKK3, Kremen-1 and Dishevelled-1 (DVL-1) and up-regulated the expression of phosphorylated apoptosis signal-regulating kinase 1 (p-ASK1), phosphorylated c-JUN N-terminal kinase (p-JNK), phosphorylated p38 (p-p38) in the spinal cord. Moreover, our results showed that exogenous intrathecal administration of rDKK3 inhibited expression of p-ASK1, p-JNK, p-p38, promoted the transformation of microglia from M1 type to M2 type, and decreased the production of pro-inflammatory cytokines compared to the rats of SNI + Vehicle. However, these effects were reversed by intrathecal administration of Kremen-1 siRNA or Dishevelled-1 (DVL-1) siRNA.

CONCLUSIONS

These results suggest that DKK3 ameliorates neuropathic pain via inhibiting ASK-1/JNK/p-38-mediated microglia polarization and neuroinflammation, at least partly, by the Kremen-1 and DVL-1 pathways.

Phytohormone abscisic acid ameliorates neuropathic pain via regulating LANCL2 protein abundance and glial activation at the spinal cord

Spinal neuroinflammation plays a critical role in the genesis of neuropathic pain. Accumulating data suggest that abscisic acid (ABA), a phytohormone, regulates inflammatory processes in mammals. In this study, we found that reduction of the LANCL2 receptor protein but not the agonist ABA in the spinal cord is associated with the genesis of neuropathic pain. Systemic or intrathecal administration of ABA ameliorates the development and pre-existence of mechanical allodynia and heat hyperalgesia in animals with partial sciatic nerve ligation (pSNL). LANCL2 is expressed only in microglia in the spinal dorsal horn. Pre-emptive treatment with ABA attenuates activation of microglia and astrocytes, ERK activity, and TNFα protein abundance in the dorsal horn in rats with pSNL. These are accompanied by restoration of spinal LANCL2 protein abundance. Spinal knockdown of LANCL2 gene with siRNA recapitulates the behavioral and spinal molecular changes induced by pSNL. Activation of spinal toll-like receptor 4 (TLR4) with lipopolysaccharide leads to activation of microglia, and over production of TNFα, which are concurrently accompanied by suppression of protein levels of LANCL2 and peroxisome proliferator activated-receptor γ. These changes are ameliorated when ABA is added with LPS. The anti-inflammatory effects induced by ABA do not requires G_i protein activity. Our study reveals that the ABA/LANCL2 system is a powerful endogenous system regulating spinal neuroinflammation and nociceptive processing, suggesting the potential utility of ABA as the management of neuropathic pain.

Differential Activation of pERK1/2 and c-Fos Following Injury to Different Regions of Primary Sensory Neuron

Nerve injury causes hyperexcitability of the dorsal root ganglion (DRG) and spinal dorsal horn (DH) neurons, which results in neuropathic pain. We have previously demonstrated that partial dorsal rhizotomy (PDR) produced less severe pain-like behavior than chronic constriction injury (CCI) or chronic compression of DRG (CCD) and did not enhance DRG neuronal excitability. However, the mechanisms underlying such discrepancy remain unclear. This study was designed to compare the activation of phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) in DRG and DH, and c-Fos in DH following treatments of CCI, CCD, and PDR. We confirmed that thermal hyperalgesia produced by PDR was less severe than that produced by CCI or CCD. We showed that pERK1/2 in DRG and DH was greatly activated by CCI or CCD, whereas PDR produced only transient and mild pERK1/2 activation. CCI, CCD, and PDR induced robust c-Fos expression in DH; nevertheless, c-Fos⁺ neurons following PDR were much fewer than that following CCI or CCD. Blocking retrograde axonal transport by colchicine proximal to the CCI injury site diminished thermal hyperalgesia and inhibited pERK1/2 and c-Fos activation. These findings demonstrate that less severe pain-like behavior produced by PDR than CCI or CCD attributes to less activation of pERK1/2 and c-Fos. Such neurochemical activation partially relies on retrograde axonal transport of certain “injury signals” from the peripheral injured site to DRG somata.

The analgesic effects of β-elemene in rats with neuropathic pain by inhibition of spinal astrocytic ERK activation

Neuropathic pain takes a heavy toll on individual well-being, while current therapy is far from desirable. Herein, we assessed the analgesic effect of β-elemene, a chief component in the traditional Chinese medicine Curcuma wenyujin, and explored the underlying mechanisms at the level of spinal dorsal horn (SDH) under neuropathic pain. A spared nerve injury (SNI)-induced neuropathic pain model was established in rats. Intraperitoneal injection (i.p.) of β-elemene was administered for 21 consecutive days. Mechanical allodynia was explored by von Frey filaments. The activation of the mitogen-activated protein kinase (MAPK) family (including ERK, p38, and JNK) in spinal neurons, astrocytes, and microglia was evaluated using immunostaining 29 days after SNI surgery. The expression of GFAP, Iba-1, p-ERK, p-JNK, and p-p38 within the SDH was measured using immunoblotting. The levels of proinflammatory cytokines (including TNF-α, IL-1β, and IL-6) were measured with ELISA. The levels of oxidative stress indicators (including MDA, SOD, and GSH-PX) were detected using biochemical tests. Consecutive i.p. administration of β-elemene relieved SNI-induced mechanical allodynia (with an EC50 of 16.40 mg/kg). SNI significantly increased the expression of p-ERK in spinal astrocytes but not microglia on day 29. β-elemene reversed spinal astrocytic ERK activation and subsequent upregulation of proinflammatory cytokines in SNI rats, with no effect on the expression of p38 and JNK in spinal glia. β-elemene also exerted antioxidative effects by increasing the levels of SOD and GSH-PX and decreasing the level of MDA. Our results suggest that SNI induces robust astrocytic ERK activation within the SDH in the late phase of neuropathic pain. β-elemene exerts remarkable analgesic effects on neuropathic pain, possibly by inhibiting spinal astrocytic ERK activation and subsequent neuroinflammatory processes. Our findings suggest that β-elemene might be a promising analgesic for the treatment of chronic pain.

Role of Microglia and Astrocytes in Spinal Cord Injury Induced Neuropathic Pain

Background:

Spinal cord injuries incite varying degrees of symptoms in patients, ranging from weakness and incoordination to paralysis. Common amongst spinal cord injury (SCI) patients, neuropathic pain (NP) is a debilitating medical condition. Unfortunately, there remain many clinical impediments in treating NP because there is a lack of understanding regarding the mechanisms behind SCI-induced NP (SCINP). Given that more than 450,000 people in the United States alone suffer from SCI, it is unsatisfactory that current treatments yield poor results in alleviating and treating NP.

Summary:

In this review, we briefly discussed the models of SCINP along with the mechanisms of NP progression. Further, current treatment modalities are herein explored for SCINP involving pharmacological interventions targeting glia cells and astrocytes.

Key message:

The studies presented in this review provide insight for new directions regarding SCINP alleviation. Given the severity and incapacitating effects of SCINP, it is imperative to study the pathways involved and find new therapeutic targets in coordination with stem cell research, and to develop a new gold-standard in SCINP treatment.

Inhibition of phosphodiesterase-4 in the spinal dorsal horn ameliorates neuropathic pain via cAMP-cytokine-Cx43 signaling in mice

Background

The spinal phosphodiesterase‐4 (PDE4) plays an important role in chronic pain. Inhibition of PDE4, an enzyme catalyzing the hydrolysis of cyclic adenosine monophosphate AMP (cAMP), produces potent antinociceptive activity. However, the antinociceptive mechanism remains largely unknown. Connexin43 (Cx43), a gap junction protein, has been shown to be involved in controlling pain transduction at the spinal level; restoration of Cx43 expression in spinal astrocytes to the normal levels reduces nerve injury‐induced pain. Here, we evaluate the novel mechanisms involving spinal cAMP‐Cx43 signaling by which PDE4 inhibitors produce antinociceptive activity.

Methods

First, we determined the effect of PDE4 inhibitors rolipram and roflumilast on partial sciatic nerve ligation (PSNL)‐induced mechanical hypersensitivity. Next, we observed the role of cAMP‐Cx43 signaling in the effect of PDE4 inhibitors on PSNL‐induced mechanical hypersensitivity.

Results

Single or repeated, intraperitoneal or intrathecal administration of rolipram or roflumilast significantly reduced mechanical hypersensitivity in mice following PSNL. In addition, repeated intrathecal treatment with either of PDE4 inhibitors reduced PSNL‐induced downregulation of cAMP and Cx43, and upregulation of proinflammatory cytokines tumor necrosis factor‐α (TNF‐α) and interleukin‐1β. Furthermore, the antinociceptive effects of PDE4 inhibitors were attenuated by the protein kinase A (PKA) inhibitor H89, TNF‐α, or Cx43 antagonist carbenoxolone. Finally, PSNL‐induced upregulation of PDE4B and PDE4D, especially the PDE4B subtype, was reduced by treatment with either of the PDE4 inhibitors.

Conclusions

The results suggest that the antinociceptive effect of PDE4 inhibitors is contributed by increasing Cx43 expression via cAMP‐PKA‐cytokine signaling in the spinal dorsal horn.

Qianghuo Shengshi decoction exerts anti-inflammatory and analgesic via MAPKs/CREB signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Traditional Chinese Medicine Qianghuo Shengshi decoction (QSD) is widely used in the treatment of nervous headache, rheumatoid arthritis, sciatica, allergic purpura, and other clinical diseases in China. However, the underlying mechanisms of its anti-inflammatory and analgesic effects has not been elucidated.

AIM OF THE STUDY

The aim of this study was to confirm the anti-inflammatory and analgesic effects and the underlying mechanism of QSD in vivo. In addition, this study was also to isolate and analyze the main active components of QSD by high performance liquid chromatography (HPLC).

MATERIALS AND METHODS

In this study, the acetic acid writhing test, hot plate test and ear swelling test and formalin test were carried out to explore the anti-inflammatory and analgesic effects of QSD. The doses were set to 7.8 g/kg, 15.6 g/kg and 31.2 g/kg body weight. Western blot was utilized to study further possible mechanisms of QSD. Moreover, the HPLC method was used to isolate and identify the components in the extraction of QSD.

RESULTS

Twelve characteristic peaks were recognized in the HPLC spectrum, which all were the known compounds. The QSD exhibited dose-dependent effects in anti-inflammatory and analgesic aspects. Compared with model group, the writhing times of in groups of different doses of QSD (15.6 g/kg and 31.2 g/kg (oral administration = p.o.)) were reduced by 33.0% and 45.8% and indicated the QSD showed significant (p < 0.05) peripheral analgesic effect. QSD ((31.2 g/kg), p.o.) showed significant(p < 0.05) analgesic effect in the hot plate test. Inhibition rates of QSD ((15.6 g/kg and 31.2 g/kg), p.o.) in ear swelling test induced by p-xylene were 27.5% and 54.6% and demonstrated the significant (p < 0.05) anti-inflammatory activity. QSD ((31.2 g/kg), p.o.) significantly (p < 0.05) reduced times of paw licking in formalin test, and its inhibition rates were 34.3% and 28.0% in Phase I and Phase Ⅱ response, respectively. Western blot results showed that QSD inhibited the phosphorylation of mitogen-activated protein kinase (MAPK) protein and cAMP response element-binding protein (CREB).

CONCLUSIONS

These results of this study undoubtedly confirmed that QSD expressed obvious analgesic and anti-inflammatory activities. Anti-inflammatory and analgesic effects of QSD may be achieved by regulating the MAPKs protein and further regulating the expression of CREB. In all, QSD may play an anti-inflammatory and analgesic role through a variety of active ingredients.