The c-Jun N-terminal kinase 1 (JNK1) in spinal astrocytes is required for the maintenance of bilateral mechanical allodynia under a persistent inflammatory pain condition

Mirror-Image Pain Update: Complex Interactions Between Central and Peripheral Mechanisms

The appearance of contralateral effects after unilateral injury has been shown in various experimental pain models, as well as in clinics. They consist of a diversity of phenomena in contralateral peripheral nerves, sensory ganglia, or spinal cord: from structural changes and altered gene or protein expression to functional consequences such as the development of mirror-image pain (MP). Although MP is a well-documented phenomenon, the exact molecular mechanism underlying the induction and maintenance of mirror-like spread of pain is still an unresolved challenge. MP has generally been explained by central sensitization mechanisms leading to facilitation of pain impulse transfer through neural connections between the two sides of the central nervous system. On the contrary, the peripheral nervous system (PNS) was usually regarded unlikely to evoke such a symmetrical phenomenon. However, recent findings provided evidence that events in the PNS could play a significant role in MP induction. This manuscript provides an updated and comprehensive synthesis of the MP phenomenon and summarizes the available data on the mechanisms. A more detailed focus is placed on reported evidence for peripheral mechanisms behind the MP phenomenon, which were not reviewed up to now.

Genetic insights into associations of multisite chronic pain with common diseases and biomarkers using data from the UK Biobank

BACKGROUND

Chronic pain is a common, complex, and challenging condition, for which specialised healthcare is required. We investigated the relationship between multisite chronic pain (MCP) and different disease traits identify safe biomarker interventions that can prevent MCP.

METHODS

Univariable and multivariable Mendelian randomisation (MR) analysis were conducted to investigate associations between MCP and 36 common diseases in the UK Biobank. Subsequently, we estimated the potential effect of expression of 4774 proteins on MCP utilising existing plasma protein quantitative trait locus data. For the significant biomarkers, we performed phenome-wide MR (Phe-MR) with 1658 outcomes to predict potential safety profiles linked to biomarker intervention.

RESULTS

Multisite chronic pain had a substantial impact on psychiatric and neurodevelopmental traits (major depression and attention deficit hyperactivity disorder), cardiovascular diseases (myocardial infarction, coronary artery disease, and heart failure), respiratory outcomes (asthma, chronic obstructive pulmonary disease, and sleep apnoea), arthropathies, type 2 diabetes mellitus, and cholelithiasis. Higher genetically predicted levels of S100A6, DOCK9, ferritin, and ferritin light chain were associated with a risk of MCP, whereas PTN9 and NEUG were linked to decreased MCP risk. Phe-MR results suggested that genetic inhibition of DOCK9 increased the risk of 21 types of disease, whereas the other biomarker interventions were relatively safe.

CONCLUSIONS

We established that MCP has an effect on health conditions covering various physiological systems and identified six novel biomarkers for intervention. In particular, S100A6, PTN9, NEUG, and ferritin light chain represent promising targets for MCP prevention, as no significant side-effects were predicted in our study.

Epitranscriptomic Reader YTHDF2 Regulates SEK1( MAP2K4 )-JNK-cJUN Inflammatory Signaling in Astrocytes during Neurotoxic Stress

As the most abundant glial cells in the CNS, astrocytes dynamically respond to neurotoxic stress, however, the key molecular regulators controlling the inflammatory status of these sentinels during neurotoxic stress have remained elusive. Herein, we demonstrate that the m6A epitranscriptomic mRNA modification tightly regulates the pro-inflammatory functions of astrocytes. Specifically, the astrocytic neurotoxic stresser, manganese (Mn), downregulated the m6A reader YTHDF2 in human and mouse astrocyte cultures and in the mouse brain. Functionally, YTHDF2 knockdown augmented, while its overexpression dampened, neurotoxic stress induced proinflammatory response, suggesting YTHDF2 serves as a key upstream regulator of inflammatory responses in astrocytes. Mechnistically, YTHDF2 RIP-sequencing identified MAP2K4 ( MKK4; SEK1) mRNA as a YTHDF2 target influencing inflammatory signaling. Our target validation revealed Mn-exposed astrocytes mediates proinflammatory response by activating the phosphorylation of SEK1, JNK, and cJUN signaling. Collectively, YTHDF2 serves a key upstream 'molecular switch' controlling SEK1( MAP2K4 )-JNK-cJUN proinflammatory signaling in astrocytes.

Spinal apolipoprotein E is involved in inflammatory pain via regulating lipid metabolism and glial activation in the spinal dorsal horn

INTRODUCTION

Inflammation and nerve injury promote astrocyte activation, which regulates the development and resolution of pain, in the spinal dorsal horn. APOE regulates lipid metabolism and is predominantly expressed in the astrocytes. However, the effect of astrocytic APOE and lipid metabolism on spinal cellular function is unclear. This study aimed to investigate the effect of spinal Apoe on spinal cellular functions using the complete Freund's adjuvant (CFA)-induced inflammatory pain mouse model.

METHODS

After intraplantar injection of CFA, we assessed pain behaviors in C57BL6 and Apoe knockout (Apoe-/-) mice using von Frey and Hargreaves' tests and analyzed dorsal horn samples (L4-5) using western blotting, immunofluorescence, quantitative real-time polymerase chain reaction, and RNA sequencing.

RESULTS

The Apoe levels were markedly upregulated at 2 h and on days 1 and 3 post-CFA treatment. Apoe was exclusively expressed in the astrocytes. Apoe-/- mice exhibited decreased pain on day 1, but not at 2 h, post-CFA treatment. Apoe-/- mice also showed decreased spinal neuron excitability and paw edema on day 1 post-CFA treatment. Global transcriptomic analysis of the dorsal horn on day 1 post-CFA treatment revealed that the differentially expressed mRNAs in Apoe-/- mice were associated with lipid metabolism and the immune system. Astrocyte activation was impaired in Apoe-/- mice on day 1 post-CFA treatment. The intrathecal injection of Apoe antisense oligonucleotide mitigated CFA-induced pain hypersensitivity.

CONCLUSIONS

Apoe deficiency altered lipid metabolism in astrocytes, exerting regulatory effects on immune response, astrocyte activation, and neuronal activity and consequently disrupting the maintenance of inflammatory pain after peripheral inflammation. Targeting APOE is a potential anti-nociception and anti-inflammatory strategy.

Upregulation of Calhm2 in the anterior cingulate cortex contributes to the maintenance of bilateral mechanical allodynia and comorbid anxiety symptoms in inflammatory pain conditions

Peripheral inflammation-induced chronic pain tends to evoke concomitant anxiety disorders. It's common knowledge that the anterior cingulate cortex (ACC) plays a vital role in maintaining pain modulation and negative emotions. However, the potential mechanisms of chronic inflammation pain and pain-related anxiety remain elusive. Here, it was reported that injecting complete Freund's adjuvant (CFA) unilaterally resulted in bilateral mechanical allodynia and anxiety-like symptoms in mice via behavioral tests. In addition, CFA induced the bilateral upregulation and activation of calcium homeostasis modulator 2 (Calhm2) in ACC pyramidal neurons by quantitative analysis and double immunofluorescence staining. The knockdown of Calhm2 in the bilateral ACC by a lentiviral vector harboring ribonucleic acid (RNA) interference sequence reversed CFA-induced pain behaviors and neuronal sensitization. Furthermore, the modulating of ACC pyramidal neuronal activities via a designer receptor exclusively activated by designer drugs (DREADD)-hM4D(Gi) greatly changed Calhm2 expression, mechanical paw withdrawal thresholds (PWTs) and comorbid anxiety symptoms. Moreover, it was found that Calhm2 regulates inflammation pain promoting the upregulation of N-methyl-D-aspartic acid (NMDA) receptor 2B (NR2B) subunits. Calhm2 knockdown in ACC exhibited a significant decrease in NR2B expression. These results demonstrated that Calhm2 in ACC pyramidal neurons modulates chronic inflammation pain and pain-related anxiety symptoms, which provides a novel underlying mechanism for the development of inflammation pain.

Microglial Depletion does not Affect the Laterality of Mechanical Allodynia in Mice

Mechanical allodynia (MA), including punctate and dynamic forms, is a common and debilitating symptom suffered by millions of chronic pain patients. Some peripheral injuries result in the development of bilateral MA, while most injuries usually led to unilateral MA. To date, the control of such laterality remains poorly understood. Here, to study the role of microglia in the control of MA laterality, we used genetic strategies to deplete microglia and tested both dynamic and punctate forms of MA in mice. Surprisingly, the depletion of central microglia did not prevent the induction of bilateral dynamic and punctate MA. Moreover, in dorsal root ganglion-dorsal root-sagittal spinal cord slice preparations we recorded the low-threshold Aβ-fiber stimulation-evoked inputs and outputs of superficial dorsal horn neurons. Consistent with behavioral results, microglial depletion did not prevent the opening of bilateral gates for Aβ pathways in the superficial dorsal horn. This study challenges the role of microglia in the control of MA laterality in mice. Future studies are needed to further understand whether the role of microglia in the control of MA laterality is etiology-or species-specific.

Identification of brain-to-spinal circuits controlling the laterality and duration of mechanical allodynia in mice

Mechanical allodynia (MA) represents one prevalent symptom of chronic pain. Previously we and others have identified spinal and brain circuits that transmit or modulate the initial establishment of MA. However, brain-derived descending pathways that control the laterality and duration of MA are still poorly understood. Here we report that the contralateral brain-to-spinal circuits, from Oprm1 neurons in the lateral parabrachial nucleus (lPBNOprm1), via Pdyn neurons in the dorsal medial regions of hypothalamus (dmHPdyn), to the spinal dorsal horn (SDH), act to prevent nerve injury from inducing contralateral MA and reduce the duration of bilateral MA induced by capsaicin. Ablating/silencing dmH-projecting lPBNOprm1 neurons or SDH-projecting dmHPdyn neurons, deleting Dyn peptide from dmH, or blocking spinal κ-opioid receptors all led to long-lasting bilateral MA. Conversely, activation of dmHPdyn neurons or their axonal terminals in SDH can suppress sustained bilateral MA induced by lPBN lesion.

Astrocytic purinergic signalling contributes to the development and maintenance of neuropathic pain via modulation of glutamate release

Although activation of astrocytes is critical in developing neuropathic pain (NP) following nerve injury, the underlying mechanisms of NP and therapeutic management for NP are still vague. Importantly, the decreases in the levels of astrocytic glutamate transporter-1 (GLT-1) in the spinal dorsal horn result in enhanced excitatory transmission and cause persistent pain. P2Y1 purinergic receptor (P2Y1R) has been shown to enhance many inflammatory processes. The up-regulated expression of astrocytic P2Y1R is crucial to participate in pain transduction under conditions of nerve injury and peripheral inflammation considering that P2Y1R is potentially involved in glutamate release and synaptic transmission. This study indicates that the expression of P2Y1R in the spinal cord was increased accompanied by the activation of A1 phenotype astrocytes in the rat model of spinal nerve ligation (SNL). Astrocyte-specific knockdown of P2Y1R alleviated SNL-induced nociceptive responses and mitigated A1 reactive astrocytes, which subsequently increased GLT-1 expression. Conversely, in naïve rats, P2Y1R over-expression induced a canonical NP-like phenotype and spontaneous hypernociceptive responses and increased the concentration of glutamate in the spinal dorsal horn. Besides, our in vitro data showed that the proinflammatory cytokine tumour necrosis factor-alpha contributes to A1/A2 astrocyte reactivity and Ca²⁺ -dependent release of glutamate. Conclusively, our results provide novel insights that as a significant regulator of astrocytic A1/A2 polarization and neuroinflammation, P2Y1R may represent a potential target for the treatment of SNL-induced NP.

Ventrolateral Periaqueductal Gray Astrocytes Regulate Nociceptive Sensation and Emotional Motivation in Diabetic Neuropathic Pain

Diabetic neuropathic pain (DNP) is a diabetes complication experienced by many patients. Ventrolateral periaqueductal gray (vlPAG) neurons are essential mediators of the descending pain modulation system, yet the role of vlPAG astrocytes in DNP remains unclear. The present study applied a multidimensional approach to elucidate the role of these astrocytes in DNP. We verified the activation of astrocytes in different regions of the PAG in male DNP-model rats. We found that only astrocytes in the vlPAG exhibited increased growth. Furthermore, we described differences in vlPAG astrocyte activity at different time points during DNP progression. After the 14th day of modeling, vlPAG astrocytes exhibited obvious activation and morphologic changes. Furthermore, activation of Gq-designer receptors exclusively activated by a designer drug (Gq-DREADDs) in vlPAG astrocytes in naive male rats induced neuropathic pain-like symptoms and pain-related aversion, whereas activation of Gi-DREADDs in vlPAG astrocytes in male DNP-model rats alleviated sensations of pain and promoted pain-related preference behavior. Thus, bidirectional manipulation of vlPAG astrocytes revealed their potential to regulate pain. Surprisingly, activation of Gi-DREADDs in vlPAG astrocytes also mitigated anxiety-like behavior induced by DNP. Thus, our results provide direct support for the hypothesis that vlPAG astrocytes regulate diabetes-associated neuropathic pain and concomitant anxiety-like behavior.SIGNIFICANCE STATEMENT Many studies examined the association between the ventrolateral periaqueductal gray (vlPAG) and neuropathic pain. However, few studies have focused on the role of vlPAG astrocytes in diabetic neuropathic pain (DNP) and DNP-related emotional changes. This work confirmed the role of vlPAG astrocytes in DNP by applying a more direct and robust approach. We used chemogenetics to bidirectionally manipulate the activity of vlPAG astrocytes and revealed that vlPAG astrocytes regulate DNP and pain-related behavior. In addition, we discovered that activation of Gi-designer receptors exclusively activated by a designer drug in vlPAG astrocytes alleviated anxiety-like behavior induced by DNP. Together, these findings provide new insights into DNP and concomitant anxiety-like behavior and supply new therapeutic targets for treating DNP.

Feasibility of quantitative sensory testing in juvenile idiopathic arthritis

OBJECTIVE

Juvenile Idiopathic Arthritis (JIA) is a childhood-rheumatic disease with pain as a major early complaint, and in 10-17% pain remains a major symptom. Very few data exist on sensory threshold changes at the knee in JIA, a location in which inflammation often manifests. We determined whether JIA is associated with sensory threshold changes at the knee by using Quantitative Sensory Testing (QST) and established reference values at the knee of children.

METHODS

Sixteen patients with JIA aged 9-18 years with one affected knee and a patient-reported pain by Visual Analog Scale (VAS) > 10 on a 0-100 scale, and 16 healthy controls completed the study and were included for the analysis. QST was assessed in compliance with the German Research Network on Neuropathic Pain (DFNS) standard. Disease severity was determined using Juvenile Disease Activity Score (JADAS. Perceived pain was assessed with a visual analogue scale(0-100). Feasibility of QST was tested in patients aged 6-9.

RESULTS

Under the age of 9, QST testing showed not to be feasible in 3 out of 5 JIA patients. Patients with JIA aged 9 and older reported an average VAS pain score of 54.3. QST identified a significant reduction in pressure pain threshold (PPT) and increase in cold detection threshold (CDT) compared to healthy controls. PPT is reduced in both the affected and the unaffected knee, CDT is reduced in the unaffected knee, not the affected knee.

CONCLUSION

In a Dutch cohort of Patients with JIA, QST is only feasible from 9 years and up. Also, sensory threshold changes at the knee are restricted to pressure pain and cold detection thresholds in Patients with JIA.

PERSPECTIVE

This article shows that in a Dutch population, the extensive QST protocol is only feasible in the age group from 9 years and older, and a reduced set of QST tests containing at least pressure pain thresholds and cold detection thresholds could prove to be better suited to the pediatric setting with arthritis.

Toll-like receptor 4 activation enhances Orai1-mediated calcium signal promoting cytokine production in spinal astrocytes

Toll-like receptor 4 (TLR4) has been implicated in pathological conditions including chronic pain. Activation of astrocytic TLRs leads to the synthesis of pro-inflammatory cytokines like interleukin 6 (IL-6) and tumor necrosis factor-ɑ (TNF-α), which can cause pathological inflammation and tissue damage in the central nervous system. However, the mechanisms of TLR4-mediated cytokine releases from astrocytes are incomplete understood. Our previous study has shown that Orai1, a key component of calcium release activated calcium channels (CRACs), mediates Ca²⁺ entry in astrocytes. How Orai1 contributes to TLR4 signaling remains unclear. Here we show that Orai1 deficiency drastically attenuated lipopolysaccharides (LPS)-induced TNF-α and IL-6 production in astrocytes. Acute LPS treatment did not induce Ca²⁺ response and had no effect on thapsigargin (Ca²⁺-ATPase inhibitor)-induced store-dependent Ca²⁺ entry. Inhibition or knockdown of Orai1 showed no reduction in LPS-induced p-ERK1/2, p-c-Jun N-terminal kinase, or p-p38 MAPK activation. Interestingly, Orai1 protein level was significantly increased after LPS exposure, which was blocked by inhibition of NF-κB activity. LPS significantly increased basal Ca²⁺ level and SOCE after exposure to astrocytes. Moreover, elevating extracellular Ca²⁺ concentration increased cytosolic Ca²⁺ level, which was almost eliminated in Orai1 KO astrocytes. Our study reports novel findings that Orai1 acts as a Ca²⁺ leak channel regulating the basal Ca²⁺ level and enhancing cytokine production in astrocytes under the inflammatory condition. These findings highlight an important role of Orai1 in astrocytic TRL4 function and may suggest that Orai1 could be a potential therapeutic target for neuroinflammatory disorders including chronic pain.

Neuroinflammation in the anterior cingulate cortex: the potential supraspinal mechanism underlying the mirror-image pain following motor fiber injury

Background

Peripheral nerve inflammation or lesion can affect contralateral healthy structures, and thus result in mirror-image pain. Supraspinal structures play important roles in the occurrence of mirror pain. The anterior cingulate cortex (ACC) is a first-order cortical region that responds to painful stimuli. In the present study, we systematically investigate and compare the neuroimmune changes in the bilateral ACC region using unilateral- (spared nerve injury, SNI) and mirror-(L5 ventral root transection, L5-VRT) pain models, aiming to explore the potential supraspinal neuroimmune mechanism underlying the mirror-image pain.

Methods

The up-and-down method with von Frey hairs was used to measure the mechanical allodynia. Viral injections for the designer receptors exclusively activated by designer drugs (DREADD) were used to modulate ACC glutamatergic neurons. Immunohistochemistry, immunofluorescence, western blotting, protein microarray were used to detect the regulation of inflammatory signaling.

Results

Increased expressions of tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6) and chemokine CX3CL1 in ACC induced by unilateral nerve injury were observed on the contralateral side in the SNI group but on the bilateral side in the L5-VRT group, representing a stronger immune response to L5-VRT surgery. In remote ACC, both SNI and L5-VRT induced robust bilateral increase in the protein level of Nav1.6 (SCN8A), a major voltage-gated sodium channel (VGSC) that regulates neuronal activity in the mammalian nervous system. However, the L5-VRT-induced Nav1.6 response occurred at PO 3d, earlier than the SNI-induced one, 7 days after surgery. Modulating ACC glutamatergic neurons via DREADD-Gq or DREADD-Gi greatly changed the ACC CX3CL1 levels and the mechanical paw withdrawal threshold. Neutralization of endogenous ACC CX3CL1 by contralateral anti-CX3CL1 antibody attenuated the induction and the maintenance of mechanical allodynia and eliminated the upregulation of CX3CL1, TNF-α and Nav1.6 protein levels in ACC induced by SNI. Furthermore, contralateral ACC anti-CX3CL1 also inhibited the expression of ipsilateral spinal c-Fos, Iba1, CD11b, TNF-α and IL-6.

Conclusions

The descending facilitation function mediated by CX3CL1 and its downstream cascade may play a pivotal role, leading to enhanced pain sensitization and even mirror-image pain. Strategies that target chemokine-mediated ACC hyperexcitability may lead to novel therapies for the treatment of neuropathic pain.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02525-8.

Spinal microglia-derived TNF promotes the astrocytic JNK/CXCL1 pathway activation in a mouse model of burn pain

Burn injury-induced pain (BIP) is an extremely complicated condition usually resistant to analgesic drugs, while its pathogenesis remains unknown. Considerable attention has been attracted to elucidate the glial mechanisms in chronic pain. In this study, we initiatively used a mouse model of second-degree BIP to investigate the underlying non-neuronal mechanisms at the spinal cord level. Our behavioral results showed that hind-paw burn injury caused persistent allodynia and hyperalgesia for 2 weeks in mice. Further studies revealed that both microglia and astrocytes activated in a spatially- and temporally-dependent manner in spinal cord after burn injury. In addition, the phosphorylated p38 mitogen-activated protein kinase (MAPK)-mediated tumor necrosis factor (TNF) release in spinal microglia is essentially attributed to the early stage of BIP, while the c-Jun N-terminal kinase (JNK) MAPK-dependent chemokine CXCL1 expression is mainly involved in the maintenance of pain hypersensitivity. Most strikingly, burn injury-induced pain symptoms and the activation of astrocytes were significantly suppressed by TNF inhibitor Thalidomide. On the contrary, intrathecal injection of TNF caused apparent pain hypersensitivity, accompanied by the activation of astrocytes and the upregulation of CXCL1 via the JNK MAPK signaling pathway, indicating that TNF is the key cytokine in the interaction between microglia and astrocytes at the spinal level. Moreover, treatment with the CXCR2 receptor antagonist SB225002 to block the biological activities of CXCL1 significantly attenuated the mechanical allodynia and thermal hyperalgesia in this BIP model. Taken together, this study indicates that intervention of glial pathways provides a new perspective in the management of BIP.

Activation of translocator protein alleviates mechanical allodynia and bladder dysfunction in cyclophosphamide-induced cystitis through repression of BDNF-mediated neuroinflammation

BACKGROUND

Bladder pain syndrome/interstitial cystitis (BPS/IC) is a refractory disease accompanied by bladder-related pain and hyperactivity. Studies have shown that the translocator protein (TSPO) modulates neuroinflammation and central sensitisation associated with pain. Moreover, we previously demonstrated that brain-derived neurotrophic factor (BDNF) regulates neuroinflammation and mechanical allodynia in cyclophosphamide (CYP)-induced cystitis through activation of glial cells. Here, we aimed to explore whether activation of TSPO attenuates mechanical allodynia and bladder dysfunction by regulating BDNF induced neuroinflammation in a CYP-induced cystitis model.

METHODS

Injection of CYP was performed to form a rat model of BPS/IC. The expression of TSPO was regulated by intrathecal injection of the TSPO agonist Ro5-4864. The von Frey filament test was applied to evaluate suprapubic allodynia. Bladder function was assessed using filling cystometry. Western blotting was used to detect the expression of TSPO, BDNF, GFAP, Iba-1, p-p38, p-JNK, TNF-α, and IL-1β, and double immunofluorescence was performed to localise TSPO in the L6-S1 spinal dorsal horn (SDH).

RESULTS

TSPO was activated in the SDH after CYP injection and was primarily colocalised with astrocytes. Ro5-4864 reversed mechanical allodynia and bladder dysfunction induced by CYP. Moreover, the upregulation of BDNF and activation of astrocytes and microglia was suppressed by Ro5-4864, resulting in downregulation of p-p38, p-JNK, TNF-α, and IL-1β.

CONCLUSIONS

Ro5-4864 alleviated mechanical allodynia and bladder dysfunction in the CYP model, possibly by inhibiting the elevation of BDNF and consequent activation of astrocytes and microglia induced neuroinflammation. TSPO may be a potential target for the treatment of BPS/IC.

A compendium of validated pain genes

The development of novel pain therapeutics hinges on the identification and rigorous validation of potential targets. Model organisms provide a means to test the involvement of specific genes and regulatory elements in pain. Here we provide a list of genes linked to pain-associated behaviors. We capitalize on results spanning over three decades to identify a set of 242 genes. They support a remarkable diversity of functions spanning action potential propagation, immune response, GPCR signaling, enzymatic catalysis, nucleic acid regulation, and intercellular signaling. Making use of existing tissue and single-cell high-throughput RNA sequencing datasets, we examine their patterns of expression. For each gene class, we discuss archetypal members, with an emphasis on opportunities for additional experimentation. Finally, we discuss how powerful and increasingly ubiquitous forward genetic screening approaches could be used to improve our ability to identify pain genes. This article is categorized under: Neurological Diseases > Genetics/Genomics/Epigenetics Neurological Diseases > Molecular and Cellular Physiology.

Glial IL-33 signaling through an ST2-to-CXCL12 pathway in the spinal cord contributes to morphine-induced hyperalgesia and tolerance

[Figure: see text].

Common and discrete mechanisms underlying chronic pain and itch: peripheral and central sensitization

Normally, an obvious antagonism exists between pain and itch. In normal conditions, painful stimuli suppress itch sensation, whereas pain killers often generate itch. Although pain and itch are mediated by separate pathways under normal conditions, most chemicals are not highly specific to one sensation in chronic pathologic conditions. Notably, in patients with neuropathic pain, histamine primarily induces pain rather than itch, while in patients with atopic dermatitis, bradykinin triggers itch rather than pain. Accordingly, repetitive scratching even enhances itch sensation in chronic itch conditions. Physicians often prescribe pain relievers to patients with chronic itch, suggesting common mechanisms underlying chronic pain and itch, especially peripheral and central sensitization. Rather than separating itch and pain, studies should investigate chronic itch and pain including neuropathic and inflammatory conditions. Here, we reviewed chronic sensitization leading to chronic pain and itch at both peripheral and central levels. Studies investigating the connection between pain and itch facilitate the development of new therapeutics against both chronic dysesthesias based on the underlying pathophysiology.

Systems and Circuits Linking Chronic Pain and Circadian Rhythms

Research over the last 20 years regarding the link between circadian rhythms and chronic pain pathology has suggested interconnected mechanisms that are not fully understood. Strong evidence for a bidirectional relationship between circadian function and pain has been revealed through inflammatory and immune studies as well as neuropathic ones. However, one limitation of many of these studies is a focus on only a few molecules or cell types, often within only one region of the brain or spinal cord, rather than systems-level interactions. To address this, our review will examine the circadian system as a whole, from the intracellular genetic machinery that controls its timing mechanism to its input and output circuits, and how chronic pain, whether inflammatory or neuropathic, may mediate or be driven by changes in these processes. We will investigate how rhythms of circadian clock gene expression and behavior, immune cells, cytokines, chemokines, intracellular signaling, and glial cells affect and are affected by chronic pain in animal models and human pathologies. We will also discuss key areas in both circadian rhythms and chronic pain that are sexually dimorphic. Understanding the overlapping mechanisms and complex interplay between pain and circadian mediators, the various nuclei they affect, and how they differ between sexes, will be crucial to move forward in developing treatments for chronic pain and for determining how and when they will achieve their maximum efficacy.

Reactive Astrocytes: Critical Players in the Development of Chronic Pain

Chronic pain is associated with long term plasticity of nociceptive pathways in the central nervous system. Astrocytes can profoundly affect synaptic function and increasing evidence has highlighted how altered astrocyte activity may contribute to the pathogenesis of chronic pain. In response to injury, astrocytes undergo a shift in form and function known as reactive astrogliosis, which affects their release of cytokines and gliotransmitters. These neuromodulatory substances have been implicated in driving the persistent changes in central nociceptive activity. Astrocytes also release lactate which neurons can use to produce energy during synaptic plasticity. Furthermore, recent research has provided insight into lactate's emerging role as a signaling molecule in the central nervous system, which may be involved in directly modulating neuronal and astrocytic activity. In this review, we present evidence for the involvement of astrocyte-derived tumor necrosis factor alpha in pain-associated plasticity, in addition to research suggesting the potential involvement of gliotransmitters D-serine and adenosine-5'-triphosphate. We also discuss work implicating astrocyte-neuron metabolic coupling, and the possible role of lactate, which has been sparsely studied in the context of chronic pain, in supporting pathological changes in central nociceptive activity.

Nociceptor-localized cGMP-dependent protein kinase I is a critical generator for central sensitization and neuropathic pain

Patients with neuropathic pain often experience exaggerated pain and anxiety. Central sensitization has been linked with the maintenance of neuropathic pain and may become an autonomous pain generator. Conversely, emerging evidence accumulated that central sensitization is initiated and maintained by ongoing nociceptive primary afferent inputs. However, it remains elusive what mechanisms underlie this phenomenon and which peripheral candidate contributes to central sensitization that accounts for pain hypersensitivity and pain-related anxiety. Previous studies have implicated peripherally localized cGMP-dependent protein kinase I (PKG-I) in plasticity of nociceptors and spinal synaptic transmission as well as inflammatory hyperalgesia. However, whether peripheral PKG-I contributes to cortical plasticity and hence maintains nerve injury-induced pain hypersensitivity and anxiety is unknown. Here, we demonstrated significant upregulation of PKG-I in ipsilateral L3 dorsal root ganglia (DRG), no change in L4 DRG, and downregulation in L5 DRG upon spared nerve injury. Genetic ablation of PKG-I specifically in nociceptors or post-treatment with intervertebral foramen injection of PKG-I antagonist, KT5823, attenuated the development and maintenance of spared nerve injury-induced bilateral pain hypersensitivity and anxiety. Mechanistic analysis revealed that activation of PKG-I in nociceptors is responsible for synaptic potentiation in the anterior cingulate cortex upon peripheral neuropathy through presynaptic mechanisms involving brain-derived neurotropic factor signaling. Our results revealed that PKG-I expressed in nociceptors is a key determinant for cingulate synaptic plasticity after nerve injury, which contributes to the maintenance of pain hypersensitivity and anxiety. Thereby, this study presents a strong basis for opening up a novel therapeutic target, PKG-I, in nociceptors for treatment of comorbidity of neuropathic pain and anxiety with least side effects.

TRAF6 Contributes to CFA-Induced Spinal Microglial Activation and Chronic Inflammatory Pain in Mice

Tumor necrosis factor receptor-associated factor 6 (TRAF6) has been reported to be expressed in spinal astrocytes and is involved in neuropathic pain. In this study, we investigated the role and mechanism of TRAF6 in complete Freund's adjuvant (CFA)-evoked chronic inflammatory hypersensitivity and the effect of docosahexaenoic acid (DHA) on TRAF6 expression and inflammatory pain. We found that TRAF6 was dominantly increased in microglia at the spinal level after intraplantar injection of CFA. Intrathecal TRAF6 siRNA alleviated CFA-triggered allodynia and reversed the upregulation of IBA-1 (microglia marker). In addition, intrathecal administration of DHA inhibited CFA-induced upregulation of TRAF6 and IBA-1 in the spinal cord and attenuated CFA-evoked mechanical allodynia. Furthermore, DHA prevented lipopolysaccharide (LPS)-caused increase of TRAF6 and IBA-1 in both BV2 cell line and primary cultured microglia. Finally, intrathecal DHA reduced LPS-induced upregulation of spinal TRAF6 and IBA-1, and alleviated LPS-induced mechanical allodynia. Our findings indicate that TRAF6 contributes to pain hypersensitivity via regulating microglial activation in the spinal dorsal horn. Direct inhibition of TRAF6 by siRNA or indirect inhibition by DHA may have therapeutic effects on chronic inflammatory pain.

Role of the Anterior Cingulate Cortex in Translational Pain Research

As the most common symptomatic reason to seek medical consultation, pain is a complex experience that has been classified into different categories and stages. In pain processing, noxious stimuli may activate the anterior cingulate cortex (ACC). But the function of ACC in the different pain conditions is not well discussed. In this review, we elaborate the commonalities and differences from accumulated evidence by a variety of pain assays for physiological pain and pathological pain including inflammatory pain, neuropathic pain, and cancer pain in the ACC, and discuss the cellular receptors and signaling molecules from animal studies. We further summarize the ACC as a new central neuromodulation target for invasive and non-invasive stimulation techniques in clinical pain management. The comprehensive understanding of pain processing in the ACC may lead to bridging the gap in translational research between basic and clinical studies and to develop new therapies.

Effects of Curcumin Treatment in a Diabetic Neuropathic Pain Model of Rats: Involvement of c-Jun N-Terminal Kinase Located in the Astrocytes and Neurons of the Dorsal Root Ganglion

Curcumin (diferuloylmethane) is a major component of turmeric, which is isolated from the rhizomes of Curcuma longa L. from the family Zingiberaceae. It is used as a dietary pigment for curry and in traditional Indian medicine for its anti-inflammatory and attenuating pain effects. This study aimed to evaluate the beneficial effects of curcumin in a rat model of diabetic neuropathic pain. Additionally, we investigated the involvement of the phosphorylated form of c-Jun N-terminal kinase (pJNK) located in the neurons and astrocytes of the dorsal root ganglion (DRG). To induce diabetic neuropathic pain in rats, 50 mg/kg of streptozotocin (STZ) was intraperitoneally injected. After 4 weeks, rats were administered the vehicle, 10 mg/kg/day curcumin, or 50 mg/kg/day curcumin orally for 4 consecutive weeks. One day after the final drug administration, we performed behavioral tests to measure responses of rats to mechanical, heat, cold, and acetone-induced cold stimuli. After behavioral tests, pJNK expression in the DRG was evaluated using western blot assay and immunohistochemistry. Curcumin treatment for 4 consecutive weeks in STZ-induced diabetic neuropathic pain rats improved behavioral responses to mechanical, cold, and thermal stimuli. Increased pJNK expression in the astrocytes and neurons of the DRG in STZ-induced diabetic neuropathic pain rats was reduced by curcumin treatment for 4 consecutive weeks. We suggest that curcumin can be an option for the treatment of diabetes-related neuropathic pain, and one of the mechanisms that underlie the action of curcumin may involve pJNK expression in the astrocytes and neurons of the DRG.

Intrathecal Administration of an Anti-nociceptive Non-CpG Oligodeoxynucleotide Reduces Glial Activation and Central Sensitization

Inflammatory pain associates with spinal glial activation and central sensitization. Systemic administration of IMT504, a non-CpG oligodeoxynucleotide originally designed as an immunomodulator, exerts remarkable anti-allodynic effects in rats with complete Freund´s adjuvant (CFA)-induced hindpaw inflammation. However, the anti-nociceptive mechanisms of IMT504 remain unknown. Here we evaluated whether IMT504 blocks inflammatory pain-like behavior by modulation of spinal glia and central sensitization. The study was performed in Sprague Dawley rats with intraplantar CFA, and a single lumbosacral intrathecal (i.t.) administration of IMT504 or vehicle was chosen to address if changes in glial activation and spinal sensitization relate to the pain-like behavior reducing effects of the ODN. Naïve rats were also included. Von Frey and Randall-Selitto tests, respectively, exposed significant reductions in allodynia and mechanical hypersensitivity, lasting at least 24 h after i.t. IMT504. Analysis of electromyographic responses to electrical stimulation of C fibers showed progressive reductions in wind-up responses. Accordingly, IMT504 significantly downregulated spinal glial activation, as shown by reductions in the protein expression of glial fibrillary acidic protein, CD11b/c, Toll-like receptor 4 (TLR4) and the phosphorylated p65 subunit of NFκB, evaluated by immunohistochemistry and western blot. In vitro experiments using early post-natal cortical glial cultures provided further support to in vivo data and demonstrated IMT504 internalization into microglia and astrocytes. Altogether, our study provides new evidence on the central mechanisms of anti-nociception by IMT504 upon intrathecal application, and further supports its value as a novel anti-inflammatory ODN with actions upon glial cells and the TLR4/NFκB pathway. Intrathecal administration of the non-CpG ODN IMT504 fully blocks CFA-induced mechanical allodynia and hypersensitivity, in association with reduced spinal sensitization. Administration of the ODN also results in downregulated gliosis and reduced TLR4-NF-κB pathway activation. IMT504 uptake into astrocytes and microglia support the concept of direct modulation of CFA-induced glial activation.

The Antinociceptive, Antioxidant and Anti-Inflammatory Effects of 5-Fluoro-2-Oxindole during Inflammatory Pain

Recent studies demonstrate that 5-fluoro-2-oxindole inhibits neuropathic pain but the antinociceptive actions of this drug and its effects on the plasticity, oxidative and inflammatory changes induced by peripheral inflammation as well as on the effects and expression of µ-opioid receptors (MOR) have not been evaluated. In C57BL/6 male mice with inflammatory pain provoked by the subplantar administration of complete Freund’s adjuvant (CFA), we evaluated: (1) the antinociceptive actions of 5-fluoro-2-oxindole and its reversion with the HO-1 inhibitor, tin protoporphyrin IX (SnPP); (2) the effects of 5-fluoro-2-oxindole in the protein levels of mitogen-activated protein kinase (MAPK), Nrf2, NADPH quinone oxidoreductase1 (NQO1), heme oxygenase 1 (HO-1), oxidative stress marker (4-hydroxy-2-nonenal; 4-HNE), inducible nitric oxide synthase (NOS2), microglial markers (CD11b/c and IBA-1), and MOR in the spinal cord and/or paw of animals with inflammatory pain; (3) the antinociceptive effects of morphine in 5-fluoro-2-oxindole pre-treated animals. Treatment with 5 and 10 mg/kg of 5-fluoro-2-oxindole inhibited the allodynia and hyperalgesia induced by CFA in a different, time-dependent manner. These effects were reversed by SnPP. Treatment with 5-fluoro-2-oxindole increased the expression of NQO1, HO-1 and MOR and inhibited the CFA-induced upregulation of phosphorylated MAPK, 4-HNE, NOS2, CD11b/c and IBA-1 in spinal cords and/or paws. The local effects of morphine were improved with 5-fluoro-2-oxindole. This work reveals that 5-fluoro-2-oxindole inhibits the plasticity, oxidative and inflammatory responses provoked by peripheral inflammation and potentiates the antinociceptive effects of morphine. Thus, treatment with 5-fluoro-2-oxindole alone and/or combined with morphine are two remarkable new procedures for chronic inflammatory pain management.

A Src family kinase maintains latent sensitization in rats, a model of inflammatory and neuropathic pain

Latent sensitization is a long-term model of chronic pain in which hyperalgesia is continuously suppressed by opioid receptors, as demonstrated by the induction of mechanical allodynia by opioid antagonists. Different intracellular signals may mediate the initiation, maintenance and expression of latent sensitization. Our criterion for the involvement of a signal in the maintenance of latent sensitization is that inhibitors should permanently eliminate the allodynia produced by an opioid antagonist. We hypothesized that Src family kinases (SFKs) maintain latent sensitization and tested this hypothesis by inducing latent sensitization in rats with complete Freund's adjuvant (CFA) or spared nerve injury. After measures of mechanical allodynia returned to baseline, vehicle or the SFK inhibitor PP2 were injected intrathecally. The opioid antagonist naltrexone injected intrathecally 15 min later produced allodynia in control rats but not in rats injected with PP2. Vehicle or PP2 were injected daily for two more days and naltrexone was injected five days later. Again, naltrexone induced allodynia in the control rats but not in the rats injected with PP2. Results were similar when latent sensitization was induced with CFA or spared nerve injury. We concluded that an SFK, likely Fyn, maintains latent sensitization induced by inflammation or nerve injury.

Learning to promote recovery after spinal cord injury

The present review explores the concept of learning within the context of neurorehabilitation after spinal cord injury (SCI). The aim of physical therapy and neurorehabilitation is to bring about a lasting change in function-to encourage learning. Traditionally, it was assumed that the adult spinal cord is hardwired-immutable and incapable of learning. Research has shown that neurons within the lower (lumbosacral) spinal cord can support learning after communication with the brain has been disrupted by means of a thoracic transection. Noxious stimulation can sensitize nociceptive circuits within the spinal cord, engaging signal pathways analogous to those implicated in brain-dependent learning and memory. After a spinal contusion injury, pain input can fuel hemorrhage, increase the area of tissue loss (secondary injury), and undermine long-term recovery. Neurons within the spinal cord are sensitive to environmental relations. This learning has a metaplastic effect that counters neural over-excitation and promotes adaptive learning through an up-regulation of brain-derived neurotrophic factor (BDNF). Exposure to rhythmic stimulation, treadmill training, and cycling also enhances the expression of BDNF and counters the development of nociceptive sensitization. SCI appears to enable plastic potential within the spinal cord by down-regulating the Cl- co-transporter KCC2, which reduces GABAergic inhibition. This enables learning, but also fuels over-excitation and nociceptive sensitization. Pairing epidural stimulation with activation of motor pathways also promotes recovery after SCI. Stimulating motoneurons in response to activity within the motor cortex, or a targeted muscle, has a similar effect. It is suggested that a neurofunctionalist approach can foster the discovery of processes that impact spinal function and how they may be harnessed to foster recovery after SCI.

Neuromodulation, Specialized Proresolving Mediators, and Resolution of Pain

The current crises in opioid abuse and chronic pain call for the development of nonopioid and nonpharmacological therapeutics for pain relief. Neuromodulation-based approaches, such as spinal cord stimulation, dorsal root ganglion simulation, and nerve stimulation including vagus nerve stimulation, have shown efficacy in achieving pain control in preclinical and clinical studies. However, the mechanisms by which neuromodulation alleviates pain are not fully understood. Accumulating evidence suggests that neuromodulation regulates inflammation and neuroinflammation-a localized inflammation in peripheral nerves, dorsal root ganglia/trigeminal ganglia, and spinal cord/brain-through neuro-immune interactions. Specialized proresolving mediators (SPMs) such as resolvins, protectins, maresins, and lipoxins are lipid molecules produced during the resolution phase of inflammation and exhibit multiple beneficial effects in resolving inflammation in various animal models. Recent studies suggest that SPMs inhibit inflammatory pain, postoperative pain, neuropathic pain, and cancer pain in rodent models via immune, glial, and neuronal modulations. It is noteworthy that sham surgery is sufficient to elevate resolvin levels and may serve as a model of resolution. Interestingly, it has been shown that the vagus nerve produces SPMs and vagus nerve stimulation (VNS) induces SPM production in vitro. In this review, we discuss how neuromodulation such as VNS controls pain via immunomodulation and neuro-immune interactions and highlight possible involvement of SPMs. In particular, we demonstrate that VNS via auricular electroacupuncture effectively attenuates chemotherapy-induced neuropathic pain. Furthermore, auricular stimulation is able to increase resolvin levels in mice. Thus, we propose that neuromodulation may control pain and inflammation/neuroinflammatioin via SPMs. Finally, we discuss key questions that remain unanswered in our understanding of how neuromodulation-based therapies provide short-term and long-term pain relief.

Wnt/beta-Catenin Signaling Contributes to Vincristine-Induced Neuropathic Pain

Chemotherapy-induced neuropathic pain (CNP) is the major dose-limiting factor in cancer chemotherapy. However, the mechanisms underlying CNP remain elusive. In the present study, CNP was induced by repeated intraperitoneal injection of vincristine (VCR) into male C57BL/6J mice. VCR administration caused significant activation of Wnt/beta-catenin signaling, which led to the activation of astrocytes, microglia, the release of inflammatory cytokines tumour necrosis factor (TNF)-alpha, monocyte chemoattractant protein-1 (MCP-1) and the activation of subsequent mitogen-activated protein kinase (MAPK)/extracellular signal-regulated protein kinase (ERK) signaling pathway in CNP mice. Blocking Wnt/beta-catenin signaling by intrathecal administration of the inhibitors of Wnt response (IWR) effectively attenuated VCR-induced neuropathic pain. Furthermore, IWR inhibited the activation of astrocytes, microglia, TNF-alpha, MCP-1 and MAPK/ERK signaling in the spinal cord, which was triggered by VCR-induced Wnt/beta-catenin signaling upregulation. These results suggest that Wnt/beta-catenin signaling plays a critical role in VCR-induced neuropathic pain and provides evidence for potential interfering with Wnt/beta-catenin signaling to ameliorate VCR-induced neuropathic pain.

Osthole alleviates neuropathic pain in mice by inhibiting the P2Y1-receptor-dependent JNK signaling pathway

There are many reports about natural products relieving neuralgia. Osthole is the main component of Angelica biserrata Yuan et Shan, a natural product that treats rheumatism through the elimination of inflammation and the alleviation of pain that has a long history in the clinic. The analgesic mechanism of osthole is complicated and confusing. Astrocytes have attracted increasing attention from pain researchers. Inhibitors targeting astrocytes are thought to be promising treatments for neuropathic pain. Whether osthole can alleviate neuropathic pain through astrocytes has not been elucidated in detail. In this study, CCI surgery was used to establish the neuropathic pain model in mice. The CCI mice were treated with osthole (5, 10, or 20 mg/kg/day) for 14 days in vivo. Mechanical allodynia and heat hyperalgesia were measured to evaluate the therapeutic effect of osthole. In mechanism research, the activation of astrocytes; the protein expression of P2Y₁R and p-JNK in astrocytes; the release of inflammatory factors; the variations in mEPSPs and eEPSPs; and the levels of GluA1, GluN2B, p-ERK, p-CREB and c-Fos in neurons were observed. The P2Y₁R inhibitor MRS2179 and the p-JNK inhibitor SP600125 were used to demonstrate how osthole works in neuropathic pain. In addition, astrocytes and neurons were used to estimate the direct effect of osthole on astrocyte-neuron interactions and signal transmission in vitro. Our findings suggest that osthole treatment obviously relieved mechanical allodynia and heat hyperalgesia in CCI mice. P2Y₁R is involved in CCI-induced pain hypersensitivity, and P2Y₁R is required for osthole-induced p-JNK downregulation in the spinal cord. Osthole inhibited astrocyte activation and reduced inflammatory factor expression. After osthole treatment, mEPSP frequency and eEPSP amplitude were decreased in spinal lamina I-II neurons. Downstream signaling molecules such as pGluA1, pGluN2B, p-ERK, p-CREB and c-Fos were also reduced very quickly in osthole-treated neuralgic mice. Our conclusion is that osthole alleviates neuropathic pain in mice via the P2Y₁-receptor-dependent JNK signaling pathway in spinal astrocytes, and osthole could be considered a potential pharmacotherapy to alleviate neuropathic pain.

Reactive oxygen species play a role in P2X7 receptor-mediated IL-6 production in spinal astrocytes

Astrocytes mediate a remarkable variety of cellular functions, including gliotransmitter release. Under pathological conditions, high concentrations of the purinergic receptor agonist adenosine triphosphate (ATP) are released into the extracellular space leading to the activation of the purinergic P2X7 receptor, which in turn can initiate signaling cascades. It is well-established that reactive oxygen species (ROS) increase in macrophages and microglia following P2X7 receptor activation. However, direct evidence that activation of P2X7 receptor leads to ROS production in astrocytes is lacking to date. While it is known that P2X7R activation induces cytokine production, the mechanism involved in this process is unclear. In the present study, we demonstrated that P2X7 receptor activation induced ROS production in spinal astrocytes in a concentration-dependent manner. We also found that P2X7R-mediated ROS production is at least partially through NADPH oxidase. In addition, our ELISA data show that P2X7R-induced IL-6 release was dependent on NADPH oxidase-mediated production of ROS. Collectively, these results reveal that activation of the P2X7 receptor on spinal astrocytes increases ROS production through NADPH oxidase, subsequently leading to IL-6 release. Our results reveal a role of ROS in the P2X7 signaling pathway in mouse spinal cord astrocytes and may indicate a potential mechanism for the astrocytic P2X7 receptor in chronic pain.

c-Jun N terminal kinase signaling pathways mediate cannabinoid tolerance in an agonist-specific manner

Tolerance to the antinociceptive effects of cannabinoids represents a significant limitation to their clinical use in managing chronic pain. Tolerance likely results from desensitization and down-regulation of the cannabinoid type 1 receptor (CB₁R), with CB₁R desensitization occurring via phosphorylation of CB₁Rs by a G protein-coupled receptor kinase and subsequent association with an arrestin protein. Previous studies have shown that (1) desensitization-resistant S426A/S430A mice exhibit a modest delay in tolerance for Δ⁹-THC and (-)-CP55,940 but a more pronounced disruption in tolerance for WIN 55,212-2 and (2) that c-Jun N-terminal kinase (JNK) signaling may selectively mediate antinociceptive tolerance to morphine compared to other opioid analgesics. In the current study, we found that pretreatment with the JNK inhibitor SP600125 (3 mg/kg) attenuates tolerance to the antinociceptive in the formalin test and to the anti-allodynic effects of Δ⁹-THC (6 mg/kg) in cisplatin-evoked neuropathic pain using wild-type mice. We also find that SP600125 causes an especially robust reduction in tolerance to the antinociceptive effects of Δ⁹-THC (30 mg/kg), but not WIN 55,212-2 (10 mg/kg) in the tail-flick assay using S426A/S430A mice. Interestingly, SP600125 pretreatment accelerated tolerance to the antinociceptive and anti-allodynic effects of (-)-CP55,940 (0.3 mg/kg) in mice with acute and neuropathic pain. These results demonstrate that inhibition of JNK signaling pathways delay tolerance to Δ⁹-THC, but not to CP55,940 or WIN55,212-2, demonstrating that the mechanisms of cannabinoid tolerance are agonist-specific.

Moxibustion Eases Chronic Inflammatory Visceral Pain In Rats Via MAPK Signaling Pathway In The Spinal Cord

Purpose

The purpose of this study was to explore the central analgesia mechanism of moxibustion for chronic inflammatory visceral pain (CIVP).

Methods

A CIVP rat model was established by 2,4,6-trinitrobenzene sulfonic acid (TNBS) plus 50% ethanol via enema. The analgesic effect of moxibustion was evaluated using the abdominal withdrawal reflex (AWR), mechanical withdrawal threshold (MWT), and thermal withdrawal latency (TWL). The expression profile of phosphorylated proteins of the mitogen-activated protein kinase (MAPK) signaling pathway in the spinal cord was assayed by protein microarray. The differentially expressed proteins were examined by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) for functional clusters and corresponding signaling pathways.

Results

Moxibustion exerted a significant analgesic effect for CIVP rats, mainly presenting as a decrease in the AWR score (all P<0.01) under different levels of distending pressure and an increase in MWT and TWL thresholds (all P<0.05). Compared with the normal group, 76 proteins were upregulated while 15 were downregulated, and MAPK signaling pathway was activated in the model group. Compared with the model group, there were 53 downregulated and 38 upregulated proteins in the moxibustion group, and MAPK signaling pathway was inhibited. Fold change (FC)>1.3 or <0.77 was taken as the screening standard to define the differentially expressed proteins. Fifteen differentially expressed proteins upregulated in the model group were downregulated in the moxibustion group. GO analysis showed that the differentially expressed proteins mainly controlled cellular metabolism regulation, transportation, and stress reactions. KEGG analysis revealed that these differentially expressed proteins were mostly involved in the ERK, JNK, and p38 pathways, and the ERK pathway was predominant.

Conclusion

Moxibustion mitigates CIVP in rats and inhibits the phosphorylation of proteins in the spinal MAPK signaling pathway. The analgesic effect of moxibustion may be associated with the regulation of the spinal MAPK signaling pathway.

Astrocytes in chronic pain and itch

Astrocytes are critical for maintaining the homeostasis of the CNS. Increasing evidence suggests that a number of neurological and neuropsychiatric disorders, including chronic pain, may result from astrocyte 'gliopathy'. Indeed, in recent years there has been substantial progress in our understanding of how astrocytes can regulate nociceptive synaptic transmission via neuronal-glial and glial-glial cell interactions, as well as the involvement of spinal and supraspinal astrocytes in the modulation of pain signalling and the maintenance of neuropathic pain. A role of astrocytes in the pathogenesis of chronic itch is also emerging. These developments suggest that targeting the specific pathways that are responsible for astrogliopathy may represent a novel approach to develop therapies for chronic pain and chronic itch.

Antinociceptive effectiveness of the inhibition of NCX reverse-mode action in rodent neuropathic pain model

Background

Chronic neuropathic pain is a debilitating condition that remains difficult to treat. The Na⁺-Ca²⁺ exchanger (NCX) is a transporter that can exchange Ca²⁺ with Na⁺ in either direction to maintain intracellular Ca²⁺ homeostasis. However, the effect of NCX on neuropathic pain remains unclear. Therefore, in this study, we aimed to clarify whether neuropathic pain is altered by NCX.

Methods

Adult Sprague–Dawley rats and mice (NCX2 knockout and wild type) were randomized to receive spinal nerve ligation surgery or intrathecal injection. Using behavioral testing to analyze the withdrawal thresholds and thermal withdrawal latency of rats after surgery or intrathecal injection. Immunohistochemistry and Western blotting were used to analyze the changes of NCX protein and downstream signaling pathways in rats dorsal root ganglion. We isolated the dorsal root ganglion neurons of adult rats using Fluo-4AM to detect the Ca²⁺ imaging in neurons after drug treatment.

Results

NCX was expressed in the sensory neurons of rodent dorsal root ganglia. NCX expression was altered in ipsilateral L4–6 dorsal root ganglion neurons in spinal nerve ligation rats. Intrathecal injection of an inhibitor of reverse-mode NCX activity (KB-R7943 5∼20 µg) had an antinociceptive effect in spinal nerve ligation rats, and the effect lasted for 3 h. We measured the expression of signaling pathway molecules in dorsal root ganglion neurons, and only the p-extracellular signal-regulated kinase (ERK) 1/2 level was reduced after intrathecal injection in the spinal nerve ligation group compared to the control group. In cultured dorsal root ganglion neurons, inhibitors of reverse-mode NCX activity (KB-R7943 and ORM-10103) restrained Ca²⁺ overload after tumor necrosis factor alpha (TNF-α) or lipopolysaccharide (LPS) treatment. NCX2 knockout mice presented an antinociceptive effect that lasted for more than 28 days after spinal nerve ligation surgery. The p-ERK1/2 level in NCX2 knockout mice ipsilateral L4–6 dorsal root ganglion neurons was lower than that in wild-type mice.

Conclusions

NCX proteins may mediate neuropathic pain progression via the Ca²⁺ and ERK pathways. NCX represents a potential target for the treatment of neuropathic pain.

Contribution of spinal cord glial cells to L. amazonensis experimental infection-induced pain in BALB/c mice

Background

The cellular and molecular pathophysiological mecha\nisms of pain processing in neglected parasitic infections such as leishmaniasis remain unknown. The present study evaluated the participation of spinal cord glial cells in the pathophysiology of pain induced by Leishmania amazonensis infection in BALB/c mice.

Methods

Mice received intra-plantar (i.pl.) injection of L. amazonensis (1 × 10⁵) and hyperalgesia, and paw edema were evaluated bilaterally for 40 days. The levels of TNF-α and IL-1β, MPO activity, and histopathology were assessed on the 40th day. ATF3 mRNA expression was assessed in DRG cells at the 30th day post-infection. Blood TNF-α and IL-1β levels and systemic parasite burden were evaluated 5–40 days after the infection. At the 30th day post-infection L. amazonensis, the effects of intrathecal (i.t.) treatments with neutralizing antibody anti-CX₃CL1, etanercept (soluble TNFR2 receptor), and interleukin-1 receptor antagonist (IL-1ra) on infection-induced hyperalgesia and paw edema were assessed. In another set of experiments, we performed a time course analysis of spinal cord GFAP and Iba-1 (astrocytes and microglia markers, respectively) and used confocal immunofluorescence and Western blot to confirm the expression at the protein level. Selective astrocyte (α-aminoadipate) and microglia (minocycline) inhibitors were injected i.t. to determine the contribution of these cells to hyperalgesia and paw edema. The effects of i.t. treatments with glial and NFκB (PDTC) inhibitors on spinal glial activation, TNF-α, IL-1β, CX₃CR1 and CX₃CL1 mRNA expression, and NFκB activation were also evaluated. Finally, the contribution of TNF-α and IL-1β to CX₃CL1 mRNA expression was investigated.

Results

L. amazonensis infection induced chronic mechanical and thermal hyperalgesia and paw edema in the infected paw. Mechanical hyperalgesia was also observed in the contralateral paw. TNF-α, IL-1β, MPO activity, and epidermal/dermal thickness increased in the infected paw, which confirmed the peripheral inflammation at the primary foci of this infection. ATF3 mRNA expression at the ipsilateral DRG of the infected paw was unaltered 30 days post-infection. TNF-α and IL-1β blood levels were not changed over the time course of disease, and parasitism increased in a time-dependent manner in the ipsilateral draining lymph node. Treatments targeting CX₃CL1, TNF-α, and IL-1β inhibited L. amazonensis-induced ongoing mechanical and thermal hyperalgesia, but not paw edema. A time course of GFAP, Iba-1, and CX₃CR1 mRNA expression indicated spinal activation of astrocytes and microglia, which was confirmed at the GFAP and Iba-1 protein level at the peak of mRNA expression (30th day). Selective astrocyte and microglia inhibition diminished infection-induced ipsilateral mechanical hyperalgesia and thermal hyperalgesia, and contralateral mechanical hyperalgesia, but not ipsilateral paw edema. Targeting astrocytes, microglia and NFκB diminished L. amazonensis-induced GFAP, Iba-1, TNF-α, IL-1β, CX₃CR1 and CX₃CL1 mRNA expression, and NFκB activation in the spinal cord at the peak of spinal cord glial cells activation. CX₃CL1 mRNA expression was also detected in the ipsilateral DRG of infected mice at the 30th day post-infection, and the i.t. injection of TNF-α or IL-1β in naïve animals induced CX₃CL1 mRNA expression in the spinal cord and ipsilateral DRG.

Conclusions

L. amazonensis skin infection produces chronic pain by central mechanisms involving spinal cord astrocytes and microglia-related production of cytokines and chemokines, and NFκB activation contributes to L. amazonensis infection-induced hyperalgesia and neuroinflammation.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1496-2) contains supplementary material, which is available to authorized users.

Astrocyte activation in the periaqueductal gray promotes descending facilitation to cancer-induced bone pain through the JNK MAPK signaling pathway

Descending nociceptive modulation from the supraspinal structures has an important role in cancer-induced bone pain (CIBP). Midbrain ventrolateral periaqueductal gray (vlPAG) is a critical component of descending nociceptive circuits; nevertheless, its precise cellular and molecular mechanisms involved in descending facilitation remain elusive. Our previous study has shown that the activation of p38 MAPK in vlPAG microglia is essential for the neuropathic pain sensitization. However, the existence of potential connection between astrocytes and c-Jun N-terminal kinase (JNK) pathway in CIBP has not yet been elucidated. The following study examines the involvement of astrocyte activation and upregulation of p-JNK in vlPAG, using a CIBP rat model. Briefly, CIBP was mimicked by an intramedullary injection of Walker 256 mammary gland carcinoma cells into the animal tibia. A significant increase in expression levels of astrocytes in the vlPAG of CIBP rats was observed. Furthermore, stereotaxic microinjection of the astrocytic cytotoxin L-α-aminoadipic acid decreased the mechanical allodynia as well as established and reversed the astrocyte activation in CIBP rats. A significant increase in expression levels of p-JNK in astrocytes in vlPAG of CIBP rats was also observed. Moreover, the intrathecal administration of JNK inhibitors SP600125 reduced the expression of glial fibrillary acidic protein, while microinjection of the SP600125 decreased the mechanical allodynia of CIBP rats. These results suggested that CIBP is associated with astrocyte activation in the vlPAG that probably participates in driving descending pain facilitation through the JNK MAPK signaling pathway. To sum up, these findings reveal a novel site of astrocytes modulation of CIBP.

Cortical astrocytes prime the induction of spine plasticity and mirror image pain

Peripheral nerve injury causes maladaptive plasticity in the central nervous system and induces chronic pain. In addition to the injured limb, abnormal pain sensation can appear in the limb contralateral to the injury, called mirror image pain. Because synaptic remodeling in the primary somatosensory cortex (S1) has critical roles in the induction of chronic pain, cortical reorganization in the S1 ipsilateral to the injured limb may also accompany mirror image pain. To elucidate this, we conducted in vivo 2-photon calcium imaging of neuron and astrocyte activity in the ipsilateral S1 after a peripheral nerve injury. We found that cross-callosal inputs enhanced the activity of both S1 astrocytes and inhibitory neurons, whereas activity of excitatory neurons decreased. When local inhibitory circuits were blocked, astrocyte-dependent spine plasticity and allodynia were revealed. Thus, we propose that cortical astrocytes prime the induction of spine plasticity and mirror image pain after peripheral nerve injury. Moreover, this result suggests that cortical synaptic rewiring could be sufficient to cause allodynia on the uninjured periphery.

Spinal Cord Glycine Transporter 2 Mediates Bilateral ST35 Acupoints Sensitization in Rats with Knee Osteoarthritis

The concept of “acupoint sensitization” refers to the functional status of acupoint switches from silent to active under pathological conditions. In clinic, acupoint sensitization provides important guidance for acupoints selection in different diseases. However, the mechanism behind this phenomenon remains unclear. We generated a model of knee osteoarthritis (KOA) by intra-articular injection of monosodium iodoacetate (MIA) into the left knee of rats. The paw withdrawal mechanical threshold (PWMT) and the total number of mast cells as well as mast cell degranulation rate (MCDR) of acupoint tissue were used to test whether the acupoints were sensitized. The results showed that KOA resulted in a reduced mechanical threshold and elevated total number of mast cell as well as mast cell degranulation rate at bilateral ST35 (Dubi) but not GB37 (Guangming) or nonacupoint area. The acupoint sensitization was accompanied by upregulation of glycine transporter 2 (GlyT2) and reduction of extracellular glycine levels in the bilateral dorsal horns of the spinal cord at L3-5. Selective inhibition of GlyT2 or intrathecal administration of glycine attenuated ST35 acupoint sensitization. The sensitization of bilateral ST35 was blocked after intraspinal GlyT2 short hairpin (sh) RNA (GlyT2-shRNA) microinjection to specifically downregulate GlyT2 expression in the left side (ipsilateral) L3-5 spinal cord dorsal horn before MIA injection. Moreover, electroacupuncture (EA) stimulation at ST35 ameliorated articular pathological lesions and improved KOA-related pain behaviors. GlyT2-shRNA injection reversed EA-induced pain relief but not EA-induced reduction of joint lesions. Overall, this study demonstrated that spinal GlyT2, especially elevated GlyT2 expression in the ipsilateral dorsal horn of the spinal cord, is a crucial mediator of ST35 acupoint sensitization in KOA rats.

Connexin Hemichannels in Astrocytes: Role in CNS Disorders

In the central nervous system (CNS), astrocytes form networks interconnected by gap junctions made from connexins of the subtypes Cx30 and Cx43. When unopposed by an adjoining hemichannel, astrocytic connexins can act as hemichannels to control the release of small molecules such as ATP and glutamate into the extracellular space. Accruing evidence indicates that astrocytic connexins are crucial for the coordination and maintenance of physiologic CNS activity. Here we provide an update on the role of astrocytic connexins in neurodegenerative disorders, glioma, and ischemia. In addition, we address the regulation of Cx43 in chronic pain.

Peripheral Inflammation Accelerates the Onset of Mechanical Hypersensitivity after Spinal Cord Injury and Engages Tumor Necrosis Factor α Signaling Mechanisms

Previously, we showed that noxious stimulation of the tail produces numerous detrimental effects after spinal cord injury (SCI), including an earlier onset and increased magnitude of mechanical hypersensitivity. Expanding on these observations, this study sought to determine whether localized peripheral inflammation similarly impacts the expression of mechanical hypersensitivity after SCI. Adult rats received a moderate contusion injury at the thoracic level (Tl0) or sham surgery, and were administered complete Freund's adjuvant (CFA) or vehicle in one hindpaw 24 hours later. Examination of locomotor recovery (Basso, Beattie, and Bresnahan [BBB] score) showed no adverse effect of CFA. Mechanical testing with von Frey hairs was done at time-points ranging from 1 h to 28 days after CFA or vehicle treatment, and rats were sacrificed at 1, 7, or 28 days for cellular assessment. Unlike vehicle-treated SCI rats where mechanical hypersensitivity emerged at 14 days, CFA-treated SCI rats showed mechanical hypersensitivity as early as 1 h after CFA administration, which lasted at least 28 days. CFA-treated sham subjects also showed an early onset of mechanical hypersensitivity, but this was maintained up to 7 days after treatment. Cellular assessments revealed congruent findings. Expression levels of c-fos, tumor necrosis factor α (TNFα), TNF receptors, and members of the TNFα signaling pathway such as caspase 8 and phosphorylated extracellular related kinase (pERK) were preferentially upregulated in the lumbar spinal cord of SCI-CFA rats. Meanwhile, c-jun was significantly increased in both CFA-treated groups. Overall, these results together with our previous reports, suggest that peripheral noxious input after SCI facilitates the development of pain by mechanisms that may require TNFα signaling.

Altered glial glutamate transporter expression in descending circuitry and the emergence of pain chronicity

BACKGROUND

The glutamate type 1 transporter (GLT1) plays a major role in glutamate homeostasis in the brain. Although alterations of GLT1 activity have been linked to persistent pain, the significance of these changes is poorly understood. Focusing on the rostral ventromedial medulla, a key site in pain modulation, we examined the expression and function of GLT1 and related transcription factor kappa B-motif binding phosphoprotein (KBBP) in rats after adjuvant-induced hind paw inflammation.

RESULTS

After inflammation, GLT1 and KBBP showed an early upregulation and gradual transition to downregulation that lasted throughout the eight-week observation period. Nitration of GLT1 was reduced at 30 min and increased at eight weeks after inflammation, suggesting an initial increase and later decrease in transporter activity. Mechanical hyperalgesia and paw edema exhibited an initial developing phase with peak hyperalgesia at 4 to 24 h, a subsequent attenuating phase, followed by a late persistent phase that lasted for months. The downregulation of GLT1 occurred at a time when hyperalgesia transitioned into the persistent phase. In the rostral ventromedial medulla, pharmacological block with dihydrokainic acid and RNAi of GLT1 and KBBP increased nociception and overexpression of GLT1 reversed persistent hyperalgesia. Further, the initial upregulation of GLT1 and KBBP was blocked by local anesthetic block, and pretreatment with dihydrokainic acid facilitated the development of hyperalgesia.

CONCLUSIONS

These results suggest that the initial increased GLT1 activity depends on injury input and serves to dampen the development of hyperalgesia. However, later downregulation of GLT1 fosters the net descending facilitation as injury persists, leading to the emergence of persistent pain.

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

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

Spinal IL-36γ/IL-36R participates in the maintenance of chronic inflammatory pain through astroglial JNK pathway

Emerging evidence indicates that spinal neuroinflammation contributes to the maintenance of chronic inflammatory pain. IL-36, as a novel member of the interleukin (IL)-1 super-family cytokines, plays an important role in inflammatory responses. The present study aimed to investigate the role of spinal IL-36 and IL-36 receptor (IL-36R) signaling in the pathology of chronic inflammatory pain. IL-36γ and IL-36R, but not IL-36α and IL-36β, were persistently upregulated in the spinal cord of mice with intraplantar injections of complete Freund's adjuvant (CFA). Intrathecal administration of both IL-36R antagonist (IL-36Ra) and IL-36γ siRNA significantly attenuated CFA-induced chronic inflammatory pain behaviors. Furthermore, CFA-induced IL-36γ expression was mainly observed in spinal neurons whereas IL-36R was primarily expressed in spinal astrocytes. Additionally, the intrathecal injection of IL-36γ was sufficient to induce pain hypersensitivity and astrocyte activation in naive mice, and these effects could be inhibited by blocking c-Jun N-terminal kinase (JNK) phosphorylation. In vitro experiments also demonstrated that the IL-36γ could induce astrocytic JNK activation and inflammatory cytokines release, which was mediated by IL-36R. Finally, intrathecal injection of IL-36γ-activated astrocytes in a pJNK-dependent manner induced mechanical allodynia and thermal hyperalgesia in naive mice. Collectively, these findings reveal that the neuronal/astrocytic interaction in the spinal cord by which neuronally produced IL-36γ activates astrocytes via IL-36R-mediated JNK pathway is crucial for the maintenance of chronic inflammatory pain. Thus, IL-36γ/IL-36R-mediated astrocyte signaling may be a suitable therapeutic target for chronic inflammatory pain.

Analgesic effects of the cathepsin K inhibitor L-006235 in the monosodium iodoacetate model of osteoarthritis pain

INTRODUCTION

The mounting evidence that osteoclasts play an important role in osteoarthritis (OA) pain lead us to investigate the effects of L-006235, a potent and selective inhibitor of cathepsin K, on pain behaviour and joint pathology in a model of OA pain.

METHODS

Effects of preventative (30 and 100 mg/kg) and therapeutic (100 mg/kg) oral dosing with L-006235 on weight-bearing asymmetry, hind paw withdrawal thresholds, cartilage and bone pathology, synovial inflammation, and drug exposure were studied in the monosodium iodoacetate rat model of OA pain.

RESULTS

Preventative L-006235 inhibited weight-bearing asymmetry from day 14, with this measure nearly abolished by the higher dose. In the same treatment setting, L-006235 prevented lowering of hind paw withdrawal thresholds from day 7. Exposure to L-006235 in plasma was higher for the 100 mg/kg dose, compared with 30 mg/kg. Therapeutic dosing with L-006235 from day 14 significantly inhibited weight-bearing asymmetry, compared with monosodium iodoacetate vehicle rats. Regression analysis revealed a significant interaction coefficient of the effects of L-006235 on weight-bearing asymmetry and synovitis score, but not for cartilage damage nor osteophyte scores.

CONCLUSION

Our novel finding that cathepsin K inhibition is analgesic in a clinically relevant model of OA pain provides new evidence for the therapeutic potential of this target.

Biochemical and Pharmacological Characterization of a Mice Model of Complex Regional Pain Syndrome

BACKGROUND AND OBJECTIVES

Complex regional pain syndrome is a challenging disease to treat. Recently, a mouse fracture model of complex regional pain syndrome has been developed that has many signs of the clinical syndrome. However, many aspects of the sensory neuron biochemistry and behavioral and pharmacological characterization of this model remain to be clarified.

METHODS

Mice were randomly assigned to fracture/cast or control (naive) groups. Fracture/cast mice underwent a closed distal tibia facture, with hindlimb wrapped in casting tape for 3 weeks. After cast removal, mice were tested for mechanical allodynia, burrowing behavior, and motor ability over a 12-week period. Protein immunohistochemistry was performed for substance P, calcitonin gene-related peptide, tropomyosin receptor kinase A, nerve growth factor, Nav1.7, and transient receptor potential cation-channel V1, colocalized in neurons, in the ipsilateral lumbar dorsal root ganglia (DRGs). Analgesic drugs were tested for pain-relieving efficacy.

RESULTS

Mechanical allodynia was greater in the ipsilateral hindpaw (P = 0.0002) in the fracture/cast group versus the control group, over the 3- to 12-week period. The amount of burrowing material removed was decreased (P = 0.0026), and there were deficits in spontaneous motor-rearing behavior (P = 0.018). Immunostaining of substance P, calcitonin gene-related peptide, Trk A receptor, nerve growth factor, Nav1.7, and transient receptor potential cation-channel V1 all demonstrated up-regulation in the DRGs of fracture mice versus controls (all P < 0.05). Morphine, pregabalin, ketamine, acetaminophen, and dexamethasone transiently increased force withdrawal thresholds on the ipsilateral (fracture) side and improved burrowing activity after injection (all P < 0.05). Ketorolac improved only burrowing.

CONCLUSIONS

Persistent pain-related behavior was demonstrated in this mouse fracture/cast model with wide-scale DRG up-regulation of pain mediators. Antihyperalgesic drugs reduced mechanical allodynia and improved burrowing.

Neuritis and vinblastine-induced axonal transport disruption lead to signs of altered dorsal horn excitability

BACKGROUND

Many patients with neuropathic pain present without signs of nerve injury on routine clinical examination. Some of these patients may have inflamed peripheral nerves (neuritis). In this study, we have examined whether neuritis causes changes within the dorsal horn that may contribute to a central pain mechanism. Comparisons have been made to a model of axonal transport disruption induced using vinblastine, since neuritis disrupts such processes.

RESULTS

At the peak of cutaneous hypersensitivities, recordings from wide dynamic range neurons revealed increases in wind-up following neuritis but not vinblastine treatment. Ongoing activity from these neurons was unchanged. Vinblastine treatment caused a reduction in the responses of wide dynamic range neurons to noxious mechanical stimulation of the receptive field. The response of neurons to innocuous mechanical stimulation was also reduced in wide dynamic range neurons that were at a depth ≥550 µm following vinblastine treatment. An examination of the superficial dorsal horn revealed an increase in c-Fos-positive neurons in both groups following electrical stimulation of the sciatic nerve. The area of dorsal horn expressing substance P was also decreased following vinblastine treatment.

CONCLUSION

These findings indicate that a minor nerve insult, such as neuritis, can lead to changes within the dorsal horn that are consistent with a central neuropathic pain mechanism.

Neuroinflammation and Central Sensitization in Chronic and Widespread Pain

Chronic pain is maintained in part by central sensitization, a phenomenon of synaptic plasticity, and increased neuronal responsiveness in central pain pathways after painful insults. Accumulating evidence suggests that central sensitization is also driven by neuroinflammation in the peripheral and central nervous system. A characteristic feature of neuroinflammation is the activation of glial cells, such as microglia and astrocytes, in the spinal cord and brain, leading to the release of proinflammatory cytokines and chemokines. Recent studies suggest that central cytokines and chemokines are powerful neuromodulators and play a sufficient role in inducing hyperalgesia and allodynia after central nervous system administration. Sustained increase of cytokines and chemokines in the central nervous system also promotes chronic widespread pain that affects multiple body sites. Thus, neuroinflammation drives widespread chronic pain via central sensitization. We also discuss sex-dependent glial/immune signaling in chronic pain and new therapeutic approaches that control neuroinflammation for the resolution of chronic pain.