CCL2 is a key mediator of microglia activation in neuropathic pain states

Key features of the innate immune response is mediated by the immunoproteasome in microglia

Microglia are the resident immune cells of the central nervous system (CNS). We and others have shown that the inflammatory response of microglia is partially regulated by the immunoproteasome, an inducible form of the proteasome responsible for the generation of major histocompatibility complex (MHC) class I epitopes. While the role of the proteasome in the adaptive immune system is well established, emerging evidence suggests the immunoproteasome may have discrete functions in the innate immune response. Here, we show that inhibiting the immunoproteasome reduces the IFNγ-dependent induction of complement activator C1q, suppresses phagocytosis, and alters the cytokine expression profile in a microglial cell line and microglia derived from human inducible pluripotent stem cells. Moreover, we show that the immunoproteasome regulates the degradation of IκBα, a modulator of NF-κB signaling. Finally, we demonstrate that NADH prevents induction of the immunoproteasome, representing a potential pathway to suppress immunoproteasome-dependent immune responses.

Sodium tanshinone IIA sulfonate suppresses microglia polarization and neuroinflammation possibly via regulating miR-125b-5p/STAT3 axis to ameliorate neuropathic pain

The spinal cord microglia play a pivotal role in neuroinflammation and neuropathic pain (NP). Sodium tanshinone IIA sulfonate (STS), a derivative of tanshinone IIA, has anti-inflammatory and anti-hyperalgesic effects. However, its underlying mechanism in NP remains unclear. This study aimed to investigate the effect of STS and elucidate possible mechanisms in a rat model of spared nerve injury. In vivo experiments, STS and AG490 were administered intraperitoneally once daily for 14 consecutive days after surgery. The results showed that the expression of miR-125b-5p in the spinal dorsal horn was substantially reduced, whereas signal transducer and activator of transcription 3 (STAT3) signaling was increased. After treatment with STS, the mechanical thresholds, expression of miR-125b-5p, and microglial M2 marker such as Arg-1 in the spinal cord horn increased significantly, whereas multiple pro-inflammatory cytokines and apoptosis were significantly reduced. Moreover, STAT3 pathway-related proteins and expression of the microglial M1 marker, CD68, were appreciably inhibited. In vitro, lipopolysaccharide (LPS) was used to induce an inflammatory response in BV-2 microglial cells. STS pretreatment inhibited LPS-stimulated pro-inflammatory cytokine secretion, reduced STAT3 pathway related-proteins and apoptosis, increased miR-125b-5p and proopiomelanocortin expression, and enhanced microglia transformation from M1 to M2 phenotype in BV-2 cells. These effects were reversed after the inhibition of miR-125b-5p expression in BV-2 cells. A dual-luciferase reporter assay confirmed that STAT3 binds to miR-125b-5p. In summary, these results suggest that STS exerts anti-hyperalgesic and anti-neuroinflammatory effects in rats with NP possibly via the miR-125b-5p/STAT3 axis.

CC Chemokine Family Members' Modulation as a Novel Approach for Treating Central Nervous System and Peripheral Nervous System Injury-A Review of Clinical and Experimental Findings

Despite significant progress in modern medicine and pharmacology, damage to the nervous system with various etiologies still poses a challenge to doctors and scientists. Injuries lead to neuroimmunological changes in the central nervous system (CNS), which may result in both secondary damage and the development of tactile and thermal hypersensitivity. In our review, based on the analysis of many experimental and clinical studies, we indicate that the mechanisms occurring both at the level of the brain after direct damage and at the level of the spinal cord after peripheral nerve damage have a common immunological basis. This suggests that there are opportunities for similar pharmacological therapeutic interventions in the damage of various etiologies. Experimental data indicate that after CNS/PNS damage, the levels of 16 among the 28 CC-family chemokines, i.e., CCL1, CCL2, CCL3, CCL4, CCL5, CCL6, CCL7, CCL8, CCL9, CCL11, CCL12, CCL17, CCL19, CCL20, CCL21, and CCL22, increase in the brain and/or spinal cord and have strong proinflammatory and/or pronociceptive effects. According to the available literature data, further investigation is still needed for understanding the role of the remaining chemokines, especially six of them which were found in humans but not in mice/rats, i.e., CCL13, CCL14, CCL15, CCL16, CCL18, and CCL23. Over the past several years, the results of studies in which available pharmacological tools were used indicated that blocking individual receptors, e.g., CCR1 (J113863 and BX513), CCR2 (RS504393, CCX872, INCB3344, and AZ889), CCR3 (SB328437), CCR4 (C021 and AZD-2098), and CCR5 (maraviroc, AZD-5672, and TAK-220), has beneficial effects after damage to both the CNS and PNS. Recently, experimental data have proved that blockades exerted by double antagonists CCR1/3 (UCB 35625) and CCR2/5 (cenicriviroc) have very good anti-inflammatory and antinociceptive effects. In addition, both single (J113863, RS504393, SB328437, C021, and maraviroc) and dual (cenicriviroc) chemokine receptor antagonists enhanced the analgesic effect of opioid drugs. This review will display the evidence that a multidirectional strategy based on the modulation of neuronal-glial-immune interactions can significantly improve the health of patients after CNS and PNS damage by changing the activity of chemokines belonging to the CC family. Moreover, in the case of pain, the combined administration of such antagonists with opioid drugs could reduce therapeutic doses and minimize the risk of complications.

Vagus nerve stimulation rescues persistent pain following orthopedic surgery in adult mice

ABSTRACT

Postoperative pain is a major clinical problem imposing a significant burden on patients and society. In a survey 2 years after orthopedic surgery, 57% of patients reported persisting postoperative pain. However, only limited progress has been made in the development of safe and effective therapies to prevent the onset and chronification of pain after orthopedic surgery. We established a tibial fracture mouse model that recapitulates clinically relevant orthopedic trauma surgery, which causes changes in neuropeptide levels in dorsal root ganglia and sustained neuroinflammation in the spinal cord. Here, we monitored extended pain behavior in this model, observing chronic bilateral hindpaw mechanical allodynia in both male and female C57BL/6J mice that persisted for >3 months after surgery. We also tested the analgesic effects of a novel, minimally invasive, bioelectronic approach to percutaneously stimulate the vagus nerve (termed percutaneous vagus nerve stimulation [pVNS]). Weekly pVNS treatment for 30 minutes at 10 Hz for 3 weeks after the surgery strongly reduced pain behaviors compared with untreated controls. Percutaneous vagus nerve stimulation also improved locomotor coordination and accelerated bone healing. In the dorsal root ganglia, vagal stimulation inhibited the activation of glial fibrillary acidic protein-positive satellite cells but without affecting microglial activation. Overall, these data provide novel evidence supportive of the use of pVNS to prevent postoperative pain and inform translational studies to test antinociceptive effects of bioelectronic medicine in the clinic.

Neural Circuitry Polarization in the Spinal Dorsal Horn (SDH): A Novel Form of Dysregulated Circuitry Plasticity during Pain Pathogenesis

Pathological pain emerges from nociceptive system dysfunction, resulting in heightened pain circuit activity. Various forms of circuitry plasticity, such as central sensitization, synaptic plasticity, homeostatic plasticity, and excitation/inhibition balance, contribute to the malfunction of neural circuits during pain pathogenesis. Recently, a new form of plasticity in the spinal dorsal horn (SDH), named neural circuit polarization (NCP), was discovered in pain models induced by HIV-1 gp120 and chronic morphine administration. NCP manifests as an increase in excitatory postsynaptic currents (EPSCs) in excitatory neurons and a decrease in EPSCs in inhibitory neurons, presumably facilitating hyperactivation of pain circuits. The expression of NCP is associated with astrogliosis. Ablation of reactive astrocytes or suppression of astrogliosis blocks NCP and, concomitantly, the development of gp120- or morphine-induced pain. In this review, we aim to compare and integrate NCP with other forms of plasticity in pain circuits to improve the understanding of the pathogenic contribution of NCP and its cooperation with other forms of circuitry plasticity during the development of pathological pain.

CCL2 Potentiates Inflammation Pain and Related Anxiety-Like Behavior Through NMDA Signaling in Anterior Cingulate Cortex

Previous studies have shown that the C-C motif chemokine ligand 2 (CCL2) is widely expressed in the nervous system and involved in regulating the development of chronic pain and related anxiety-like behaviors, but its precise mechanism is still unclear. This paper provides an in-depth examination of the involvement of CCL2-CCR2 signaling in the anterior cingulate cortex (ACC) in intraplantar injection of complete Freund's adjuvant (CFA) leading to inflammatory pain and its concomitant anxiety-like behaviors by modulation of glutamatergic N-methyl-D-aspartate receptor (NMDAR). Our findings suggest that local bilateral injection of CCR2 antagonist in the ACC inhibits CFA-induced inflammatory pain and anxiety-like behavior. Meanwhile, the expression of CCR2 and CCL2 was significantly increased in ACC after 14 days of intraplantar injection of CFA, and CCR2 was mainly expressed in excitatory neurons. Whole-cell patch-clamp recordings showed that the CCR2 inhibitor RS504393 reduced the frequency of miniature excitatory postsynaptic currents (mEPSC) in ACC, and CCL2 was involved in the regulation of NMDAR-induced current in ACC neurons in the pathological state. In addition, local injection of the NR2B inhibitor of NMDAR subunits, Ro 25-6981, attenuated the effects of CCL2-induced hyperalgesia and anxiety-like behavior in the ACC. In summary, CCL2 acts on CCR2 in ACC excitatory neurons and participates in the regulation of CFA-induced pain and related anxiety-like behaviors through upregulation of NR2B. CCR2 in the ACC neuron may be a potential target for the treatment of chronic inflammatory pain and pain-related anxiety.

Reprimo (RPRM) as a Potential Preventive and Therapeutic Target for Radiation-Induced Brain Injury via Multiple Mechanisms

Patients receiving cranial radiotherapy for primary and metastatic brain tumors may experience radiation-induced brain injury (RIBI). Thus far, there has been a lack of effective preventive and therapeutic strategies for RIBI. Due to its complicated underlying pathogenic mechanisms, it is rather difficult to develop a single approach to target them simultaneously. We have recently reported that Reprimo (RPRM), a tumor suppressor gene, is a critical player in DNA damage repair, and RPRM deletion significantly confers radioresistance to mice. Herein, by using an RPRM knockout (KO) mouse model established in our laboratory, we found that RPRM deletion alleviated RIBI in mice via targeting its multiple underlying mechanisms. Specifically, RPRM knockout significantly reduced hippocampal DNA damage and apoptosis shortly after mice were exposed to whole-brain irradiation (WBI). For the late-delayed effect of WBI, RPRM knockout obviously ameliorated a radiation-induced decline in neurocognitive function and dramatically diminished WBI-induced neurogenesis inhibition. Moreover, RPRM KO mice exhibited a significantly lower level of acute and chronic inflammation response and microglial activation than wild-type (WT) mice post-WBI. Finally, we uncovered that RPRM knockout not only protected microglia against radiation-induced damage, thus preventing microglial activation, but also protected neurons and decreased the induction of CCL2 in neurons after irradiation, in turn attenuating the activation of microglial cells nearby through paracrine CCL2. Taken together, our results indicate that RPRM plays a crucial role in the occurrence of RIBI, suggesting that RPRM may serve as a novel potential target for the prevention and treatment of RIBI.

Peripheral CCL2-CCR2 signalling contributes to chronic headache-related sensitization

Migraine, especially chronic migraine, is highly debilitating and still lacks effective treatment. The persistent headache arises from activation and sensitization of primary afferent neurons in the trigeminovascular pathway, but the underlying mechanisms remain incompletely understood. Animal studies indicate that signalling through chemokine C-C motif ligand 2 (CCL2) and C-C motif chemokine receptor 2 (CCR2) mediates the development of chronic pain after tissue or nerve injury. Some migraine patients had elevated CCL2 levels in CSF or cranial periosteum. However, whether the CCL2-CCR2 signalling pathway contributes to chronic migraine is not clear. Here, we modelled chronic headache with repeated administration of nitroglycerin (NTG, a reliable migraine trigger in migraineurs) and found that both Ccl2 and Ccr2 mRNA were upregulated in dura and trigeminal ganglion (TG) tissues that are implicated in migraine pathophysiology. In Ccl2 and Ccr2 global knockout mice, repeated NTG administration did not evoke acute or persistent facial skin hypersensitivity as in wild-type mice. Intraperitoneal injection of CCL2 neutralizing antibodies inhibited chronic headache-related behaviours induced by repeated NTG administration and repetitive restraint stress, suggesting that the peripheral CCL2-CCR2 signalling mediates headache chronification. We found that CCL2 was mainly expressed in TG neurons and cells associated with dura blood vessels, whereas CCR2 was expressed in subsets of macrophages and T cells in TG and dura but not in TG neurons under both control and disease states. Deletion of Ccr2 gene in primary afferent neurons did not alter NTG-induced sensitization, but eliminating CCR2 expression in either T cells or myeloid cells abolished NTG-induced behaviours, indicating that both CCL2-CCR2 signalling in T cells and macrophages are required to establish chronic headache-related sensitization. At cellular level, repeated NTG administration increased the number of TG neurons that responded to calcitonin-gene-related peptide (CGRP) and pituitary adenylate cyclase activating polypeptide (PACAP) as well as the production of CGRP in wild-type but not Ccr2 global knockout mice. Lastly, co-administration of CCL2 and CGRP neutralizing antibodies was more effective in reversing NTG-induced behaviours than individual antibodies. Taken together, these results suggest that migraine triggers activate CCL2-CCR2 signalling in macrophages and T cells. This consequently enhances both CGRP and PACAP signalling in TG neurons, ultimately leading to persistent neuronal sensitization underlying chronic headache. Our work not only identifies the peripheral CCL2 and CCR2 as potential targets for chronic migraine therapy, but also provides proof-of-concept that inhibition of both peripheral CGRP and CCL2-CCR2 signalling is more effective than targeting either pathway alone.

Transforming Growth Factor-β-Activated Kinase 1 (TAK1) Mediates Chronic Pain and Cytokine Production in Mouse Models of Inflammatory, Neuropathic, and Primary Pain

The origin of chronic pain is linked to inflammation, characterized by increased levels of proinflammatory cytokines in local tissues and systemic circulation. Transforming growth factor beta-activated kinase 1 (TAK1) is a key regulator of proinflammatory cytokine signaling that has been well characterized in the context of cancer and autoimmune disorders, yet its role in chronic pain is less clear. Here, we evaluated the ability of our TAK1 small-molecule inhibitor, takinib, to attenuate pain and inflammation in preclinical models of inflammatory, neuropathic, and primary pain. Inflammatory, neuropathic, and primary pain was modeled using intraplantar complete Freund's adjuvant (CFA), chronic constriction injury (CCI), and systemic delivery of the catechol-O-methyltransferase (COMT) inhibitor OR486, respectively. Behavioral responses evoked by mechanical and thermal stimuli were evaluated in separate groups of mice receiving takinib or vehicle prior to pain induction (baseline) and over 12 days following CFA injection, 4 weeks following CCI surgery, and 6 hours following OR486 delivery. Hindpaw edema was also measured prior to and 3 days following CFA injection. Upon termination of behavioral experiments, dorsal root ganglia (DRG) were collected to measure cytokines. We also evaluated the ability of takinib to modulate nociceptor activity via in vitro calcium imaging of neurons isolated from the DRG of Gcamp3 mice. In all 3 models, TAK1 inhibition significantly reduced hypersensitivity to mechanical and thermal stimuli and expression of proinflammatory cytokines in DRG. Furthermore, TAK1 inhibition significantly reduced the activity of tumor necrosis factor (TNF)-primed/capsaicin-evoked DRG nociceptive neurons. Overall, our results support the therapeutic potential of TAK1 as a novel drug target for the treatment of chronic pain syndromes with different etiologies. PERSPECTIVE: This article reports the therapeutic potential of TAK1 inhibitors for the treatment of chronic pain. This new treatment has the potential to provide a greater therapeutic offering to physicians and patients suffering from chronic pain as well as reduce the dependency on opioid-based pain treatments.

Neat1 lncRNA organizes the inflammatory gene expressions in the dorsal root ganglion in neuropathic pain caused by nerve injury

Primary sensory neurons regulate inflammatory processes in innervated regions through neuro-immune communication. However, how their immune-modulating functions are regulated in concert remains largely unknown. Here, we show that Neat1 long non-coding RNA (lncRNA) organizes the proinflammatory gene expressions in the dorsal root ganglion (DRG) in chronic intractable neuropathic pain in rats. Neat1 was abundantly expressed in the DRG and was upregulated after peripheral nerve injury. Neat1 overexpression in primary sensory neurons caused mechanical and thermal hypersensitivity, whereas its knockdown alleviated neuropathic pain. Bioinformatics analysis of comprehensive transcriptome changes indicated the inflammatory response was the most relevant function of genes upregulated through Neat1. Consistent with this, upregulation of proinflammatory genes in the DRG following nerve injury was suppressed by Neat1 knockdown. Expression changes of these proinflammatory genes were regulated through Neat1-mRNA interaction-dependent and -independent mechanisms. Notably, Neat1 increased proinflammatory genes by stabilizing its interacting mRNAs in neuropathic pain. Finally, Neat1 in primary sensory neurons contributed to spinal inflammatory processes that mediated peripheral neuropathic pain. These findings demonstrate that Neat1 lncRNA is a key regulator of neuro-immune communication in neuropathic pain.

Inhibition of Tumor-Derived C-C Motif Chemokine Ligand 2 Expression Attenuates Tactile Allodynia in NCTC 2472 Fibrosarcoma-Inoculated Mice

Neuropathic pain associated with cancers is caused by tumor growth compressing and damaging nerves, which would also be enhanced by inflammatory factors through sensitizing nociceptor neurons. A troublesome hallmark symptom of neuropathic pain is hypersensitivity to innocuous stimuli, a condition known as "tactile allodynia", which is often refractory to NSAIDs and opioids. The involvement of chemokine CCL2 (monocyte chemoattractant protein-1) in cancer-evoked neuropathic pain is well established, but opinions remain divided as to whether CCL2 is involved in the production of tactile allodynia with tumor growth. In this study, we constructed Ccl2 knockout NCTC 2472 (Ccl2-KO NCTC) fibrosarcoma cells and conducted pain behavioral test using Ccl2-KO NCTC-implanted mice. Implantation of naïve NCTC cells around the sciatic nerves of mice produced tactile allodynia in the inoculated paw. Although the growth of Ccl2 KO NCTC-formed tumors was comparable to that of naïve NCTC-formed tumors, Ccl2-KO NCTC-bearing mice failed to show tactile pain hypersensitivity, suggesting the involvement of CCL2 in cancer-induced allodynia. Subcutaneous administration of controlled-release nanoparticles containing the CCL2 expression inhibitor NS-3-008 (1-benzyl-3-hexylguanidine) significantly attenuated tactile allodynia in naïve NCTC-bearing mice accompanied by a reduction of CCL2 content in tumor masses. Our present findings suggest that inhibition of CCL2 expression in cancer cells is a useful strategy to attenuate tactile allodynia induced by tumor growth. Development of a controlled-release system of CCL2 expression inhibitor may be a preventative option for the treatment of cancer-evoked neuropathic pain. SIGNIFICANCE STATEMENT: The blockade of chemokine/receptor signaling, particularly for C-C motif chemokine ligand 2 (CCL2) and its high-affinity receptor C-C chemokine receptor type 2 (CCR2), has been implicated to attenuate cancer-induced inflammatory and nociceptive pain. This study demonstrated that continuous inhibition of CCL2 production from cancer cells also prevents the development of tactile allodynia associated with tumor growth. Development of a controlled-release system of CCL2 expression inhibitor may be a preventative option for management of cancer-evoked tactile allodynia.

Targeting Members of the Chemokine Family as a Novel Approach to Treating Neuropathic Pain

Neuropathic pain is a debilitating condition that affects millions of people worldwide. Numerous studies indicate that this type of pain is a chronic condition with a complex mechanism that tends to worsen over time, leading to a significant deterioration in patients' quality of life and issues like depression, disability, and disturbed sleep. Presently used analgesics are not effective enough in neuropathy treatment and may cause many side effects due to the high doses needed. In recent years, many researchers have pointed to the important role of chemokines not only in the development and maintenance of neuropathy but also in the effectiveness of analgesic drugs. Currently, approximately 50 chemokines are known to act through 20 different seven-transmembrane G-protein-coupled receptors located on the surface of neuronal, glial, and immune cells. Data from recent years clearly indicate that more chemokines than initially thought (CCL1/2/3/5/7/8/9/11, CXCL3/9/10/12/13/14/17; XCL1, CX3CL1) have pronociceptive properties; therefore, blocking their action by using neutralizing antibodies, inhibiting their synthesis, or blocking their receptors brings neuropathic pain relief. Several of them (CCL1/2/3/7/9/XCL1) have been shown to be able to reduce opioid drug effectiveness in neuropathy, and neutralizing antibodies against them can restore morphine and/or buprenorphine analgesia. The latest research provides irrefutable evidence that chemokine receptors are promising targets for pharmacotherapy; chemokine receptor antagonists can relieve pain of different etiologies, and most of them are able to enhance opioid analgesia, for example, the blockade of CCR1 (J113863), CCR2 (RS504393), CCR3 (SB328437), CCR4 (C021), CCR5 (maraviroc/AZD5672/TAK-220), CXCR2 (NVPCXCR220/SB225002), CXCR3 (NBI-74330/AMG487), CXCR4 (AMD3100/AMD3465), and XCR1 (vMIP-II). Recent research has shown that multitarget antagonists of chemokine receptors, such as CCR2/5 (cenicriviroc), CXCR1/2 (reparixin), and CCR2/CCR5/CCR8 (RAP-103), are also very effective painkillers. A multidirectional strategy based on the modulation of neuronal-glial-immune interactions by changing the activity of the chemokine family can significantly improve the quality of life of patients suffering from neuropathic pain. However, members of the chemokine family are still underestimated pharmacological targets for pain treatment. In this article, we review the literature and provide new insights into the role of chemokines and their receptors in neuropathic pain.

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.

SAFit2 ameliorates paclitaxel-induced neuropathic pain by reducing spinal gliosis and elevating pro-resolving lipid mediators

BACKGROUND

Chemotherapy-induced neuropathic pain (CIPN) describes a pathological pain state that occurs dose-dependently as a side effect and can limit or even impede an effective cancer therapy. Unfortunately, current treatment possibilities for CIPN are remarkably confined and mostly inadequate as CIPN therapeutics themselves consist of low effectiveness and may induce severe side effects, pointing out CIPN as pathological entity with an emerging need for novel treatment targets. Here, we investigated whether the novel and highly specific FKBP51 inhibitor SAFit2 reduces paclitaxel-induced neuropathic pain.

METHODS

In this study, we used a well-established multiple low-dose paclitaxel model to investigate analgesic and anti-inflammatory properties of SAFit2. For this purpose, the behavior of the mice was recorded over 14 days and the mouse tissue was then analyzed using biochemical methods.

RESULTS

Here, we show that SAFit2 is capable to reduce paclitaxel-induced mechanical hypersensitivity in mice. In addition, we detected that SAFit2 shifts lipid levels in nervous tissue toward an anti-inflammatory and pro-resolving lipid profile that counteracts peripheral sensitization after paclitaxel treatment. Furthermore, SAFit2 reduced the activation of astrocytes and microglia in the spinal cord as well as the levels of pain-mediating chemokines. Its treatment also increased anti-inflammatory cytokines levels in neuronal tissues, ultimately leading to a resolution of neuroinflammation.

CONCLUSIONS

In summary, SAFit2 shows antihyperalgesic properties as it ameliorates paclitaxel-induced neuropathic pain by reducing peripheral sensitization and resolving neuroinflammation. Therefore, we consider SAFit2 as a potential novel drug candidate for the treatment of paclitaxel-induced neuropathic pain.

Vagus nerve stimulation rescues persistent pain following orthopedic surgery in adult mice

Postoperative pain is a major clinical problem imposing a significant burden on our patients and society. Up to 57% of patients experience persistent postoperative pain 2 years after orthopedic surgery [49]. Although many studies have contributed to the neurobiological foundation of surgery-induced pain sensitization, we still lack safe and effective therapies to prevent the onset of persistent postoperative pain. We have established a clinically relevant orthopedic trauma model in mice that recapitulates common insults associated with surgery and ensuing complications. Using this model, we have started to characterize how induction of pain signaling contributes to neuropeptides changes in dorsal root ganglia (DRG) and sustained neuroinflammation in the spinal cord [62]. Here we have extended the characterization of pain behaviors for >3 months after surgery, describing a persistent deficit in mechanical allodynia in both male and female C57BL/6J mice after surgery. Notably, we have applied a novel minimally invasive bioelectronic approach to percutaneously stimulate the vagus nerve (termed pVNS) [24] and tested its anti-nociceptive effects in this model. Our results show that surgery induced a strong bilateral hind-paw allodynia with a slight decrease in motor coordination. However, treatment with pVNS for 30-minutes at10 Hz weekly for 3 weeks prevented pain behavior compared to naïve controls. pVNS also improved locomotor coordination and bone healing compared to surgery without treatment. In the DRGs, we observed that vagal stimulation fully rescued activation of GFAP positive satellite cells but did not affect microglial activation. Overall, these data provide novel evidence for the use of pVNS to prevent postoperative pain and may inform translational studies to test anti-nociceptive effects in the clinic.

Analysis of the Influence of IL-6 and the Activation of the Jak/Stat3 Pathway in Fibromyalgia

Background: Fibromyalgia is a medical condition that affects a small percentage of the population, with no known effective treatment. There is evidence to suggest that inflammation is a key factor in the nerve sensitization that characterizes the disorder. Therefore, this paper concentrates on the role of IL-6 in fibromyalgia and the related pain-like symptoms. Methods: This work aimed to evaluate Sprague–Dawley rats, which were injected for three consecutive days with 1 mg/kg of reserpine; IL-6-R Ab was intraperitoneally injected at 1.5 mg/kg seven days after the first reserpine injection. Behavioral analyses were conducted at the beginning of the experiment and at seven and twenty-one days from the first reserpine injection. At this timepoint, the animals were sacrificed, and tissues were collected for molecular and histological analysis. Results: Our data showed the analgesic effect of IL-6-R-Ab administration on mechanical allodynia and thermal hyperalgesia. Additionally, the reserpine + IL-6-R-Ab group showed a reduced expression of the pain-related mediators cFOS and NFG and reduced levels of pro-inflammatory cytokines (TNF-α, IL-1β and IL-6) and chemokines (Cxcl5, Cxcl10 and Cx3cl1). From the molecular point of view, the IL-6-R-Ab administration reduced the gp130 phosphorylation and the activation of the Jak/STAT3 pathway. Additionally, the IL-6-R Ab reduced the activation of neuroinflammatory cells. Conclusions: Our study showed that IL-6 plays a crucial role in fibromyalgia by triggering the Jak/STAT3 pathway, leading to an increase in chemokine levels and activating glial cells.

Pharmacological Evidence of the Important Roles of CCR1 and CCR3 and Their Endogenous Ligands CCL2/7/8 in Hypersensitivity Based on a Murine Model of Neuropathic Pain

Neuropathic pain treatment remains a challenging issue because the therapies currently used in the clinic are not sufficiently effective. Moreover, the mechanism of neuropathy is still not entirely understood; however, much evidence indicates that chemokines are important factors in the initial and late phases of neuropathic pain. To date, the roles of CCR1, CCR3 and their endogenous ligands have not been extensively studied; therefore, they have become the subject of our research. In the present comprehensive behavioral and biochemical study, we detected significant time-dependent and long-lasting increases in the mRNA levels of CCR1 and/or CCR3 ligands, such as CCL2/3/4/5/6/7/8/9, in the murine spinal cord after chronic constriction injury of the sciatic nerve, and these increases were accompanied by changes in the levels of microglial/macrophage, astrocyte and neutrophil cell markers. ELISA results suggested that endogenous ligands of CCR1 and CCR3 are involved in the development (CCL2/3/5/7/8/9) and persistence (CCL2/7/8) of neuropathic pain. Moreover, intrathecal injection of CCL2/3/5/7/8/9 confirmed their possible strong influence on mechanical and thermal hypersensitivity development. Importantly, inhibition of CCL2/7/8 production and CCR1 and CCR3 blockade by selective/dual antagonists effectively reduced neuropathic pain-like behavior. The obtained data suggest that CCL2/7/8/CCR1 and CCL7/8/CCR3 signaling are important in the modulation of neuropathic pain in mice and that these chemokines and their receptors may be interesting targets for future investigations.

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.

Inhibition of the RNA Regulator HuR by SRI-42127 Attenuates Neuropathic Pain After Nerve Injury Through Suppression of Neuroinflammatory Responses

Microglial activation with the production of pro-inflammatory mediators such as IL-6, TNF-α, and IL-1β, is a major driver of neuropathic pain (NP) following peripheral nerve injury. We have previously shown that the RNA binding protein, HuR, is a positive node of regulation for many of these inflammatory mediators in glia and that its chemical inhibition or genetic deletion attenuates their production. In this report, we show that systemic administration of SRI-42127, a novel small molecule HuR inhibitor, attenuates mechanical allodynia, a hallmark of NP, in the early and chronic phases after spared nerve injury in male and female mice. Flow cytometry of lumbar spinal cords in SRI-42127-treated mice shows a reduction in infiltrating macrophages and a concomitant decrease in microglial populations expressing IL-6, TNF-α, IL-1β, and CCL2. Immunohistochemistry, ELISA, and qPCR of lumbar spinal cord tissue indicate suppression of these cytokines and other inflammatory mediators. ELISA of plasma samples in the acute phase also shows attenuation of inflammatory responses. In summary, inhibition of HuR by SRI-42127 leads to the suppression of neuroinflammatory responses and allodynia after nerve injury and represents a promising new direction in the treatment of NP.

Role of Etanercept and Infliximab on Nociceptive Changes Induced by the Experimental Model of Fibromyalgia

Background: Fibromyalgia is a clinical condition that affects 1% to 5% of the population. No proper therapy has been currently found. It has been described that inflammation plays a central role in the nerve sensitizations that characterize the pathology. Methods: This paper aimed to evaluate the efficacy of etanercept and infliximab in the management of pain sensitization. Fibromyalgia was induced by three injections once a day of reserpine at the dose of 1 mg/kg. Etanercept (3 mg/kg) and infliximab (10 mg/kg) were administered the day after the last reserpine injection and then 5 days after that. Behavioral analyses were conducted once a week, and molecular investigations were performed at the end of the experiment. Results: Our data confirmed the major effect of infliximab administration as compared to etanercept: infliximab administration strongly reduced pain sensitization in thermal hyperalgesia and mechanical allodynia. From the molecular point of view, infliximab reduced the activation of microglia and astrocytes and the expression of the purinergic P2X7 receptor ubiquitously expressed on glia and neurons. Downstream of the P2X7 receptor, infliximab also reduced p38-MAPK overexpression induced by the reserpine administration. Conclusion: Etanercept and infliximab treatment caused a significant reduction in pain. In particular, rats that received infliximab showed less pain sensitization. Moreover, infliximab reduced the activation of microglia and astrocytes, reducing the expression of the purinergic receptor P2X7 and p38-MAPK pathway.

Advances in Visualizing Microglial Cells in Human Central Nervous System Tissue

Neuroinflammation has recently been identified as a fundamentally important pathological process in most, if not all, CNS diseases. The main contributor to neuroinflammation is the microglia, which constitute the innate immune response system. Accurate identification of microglia and their reactivity state is therefore essential to further our understanding of CNS pathophysiology. Many staining techniques have been used to visualise microglia in rodent and human tissue, and immunostaining is currently the most frequently used. Historically, identification of microglia was predominantly based on morphological structure, however, recently there has been a reliance on selective antigen expression, and microglia-specific markers have been identified providing increased certainty that the cells observed are in fact microglia, rather than the similar yet distinct macrophages. To date, the most microglia-specific markers are P2Y12 and TMEM119. However, other microglia-related markers can also be useful for demonstrating activation state, phagocytic state, and for neuroimaging purposes in longitudinal studies. Overall, it is important to be aware of the microglia-selectivity issues of the various stains and immunomarkers used by researchers to distinguish microglia in CNS tissue to avoid misinterpretation.

Glycine Receptor Subtypes and Their Roles in Nociception and Chronic Pain

Disruption of the inhibitory control provided by the glycinergic system is one of the major mechanisms underlying chronic pain. In line with this concept, recent studies have provided robust proof that pharmacological intervention of glycine receptors (GlyRs) restores the inhibitory function and exerts anti-nociceptive effects on preclinical models of chronic pain. A targeted regulation of the glycinergic system requires the identification of the GlyR subtypes involved in chronic pain states. Nevertheless, the roles of individual GlyR subunits in nociception and in chronic pain are yet not well defined. This review aims to provide a systematic outline on the contribution of GlyR subtypes in chronic pain mechanisms, with a particular focus on molecular pathways of spinal glycinergic dis-inhibition mediated by post-translational modifications at the receptor level. The current experimental evidence has shown that phosphorylation of synaptic α1β and α3β GlyRs are involved in processes of spinal glycinergic dis-inhibition triggered by chronic inflammatory pain. On the other hand, the participation of α2-containing GlyRs and of β subunits in pain signaling have been less studied and remain undefined. Although many questions in the field are still unresolved, future progress in GlyR research may soon open new exciting avenues into understanding and controlling chronic pain.

Peripherally administered cannabinoid receptor 2 (CB2R) agonists lose anti-allodynic effects in TRPV1 knockout mice, while intrathecal administration leads to anti-allodynia and reduced GFAP, CCL2 and TRPV1 expression in the dorsal spinal cord and DRG

The transient receptor potential (TRP) superfamily of cation channels, of which the TRP vanilloid type 1 (TRPV1) receptor plays a critical role in inflammatory and neuropathic pain, is expressed on nociceptors and spinal cord dorsal horn neurons. TRPV1 is also expressed on spinal astrocytes and dorsal root ganglia (DRG) satellite cells. Agonists of the cannabinoid type 2 receptor (CB2R) suppress allodynia, with some that can bind TRPV1. The neuroimmune C-C class chemokine-2 (CCL2) expressed on injured DRG nociceptor cell bodies, Schwann cells and spinal astrocytes, stimulates immune cell accumulation in DRG and spinal cord, a known critical element in chronic allodynia. The current report examined whether two CB2R agonists, AM1710 and AM1241, previously shown to reverse light touch mechanical allodynia in rodent models of sciatic neuropathy, require TRPV1 activation that leads to receptor insensitivity resulting in reversal of allodynia. Global TRPV1 knockout (KO) mice with sciatic neuropathy given intrathecal or intraperitoneal AM1710 were examined for anti-allodynia followed by immunofluorescent microscopy analysis of lumbar spinal cord and DRG of astrocyte and CCL2 markers. Additionally, immunofluorescent analysis following intrathecal AM1710 and AM1241 in rat was performed. Data reveal that intrathecal AM1710 resulted in mouse anti-allodynia, reduced spinal astrocyte activation and CCL2 expression independent of TRPV1 gene deletion. Conversely, peripheral AM1710 in TRPV1-KO mice failed to reverse allodynia. In rat, intrathecal AM1710 and AM1241 reduced spinal and DRG TRPV1 expression, with CCL2-astrocyte and -microglial co-expression. These data support that CB2R agonists can impact spinal and DRG TRPV1 expression critical for anti-allodynia.

Sympathectomy decreases pain behaviors and nerve regeneration by downregulating monocyte chemokine CCL2 in dorsal root ganglia in the rat tibial nerve crush model

ABSTRACT

Peripheral nerve regeneration is associated with pain in several preclinical models of neuropathic pain. Some neuropathic pain conditions and preclinical neuropathic pain behaviors are improved by sympathetic blockade. In this study, we examined the effect of a localized "microsympathectomy," ie, cutting the gray rami containing sympathetic postganglionic axons where they enter the L4 and L5 spinal nerves, which is more analogous to clinically used sympathetic blockade compared with chemical or surgical sympathectomy. We also examined manipulations of CCL2 (monocyte chemoattractant protein 1), a key player in both regeneration and pain. We used rat tibial nerve crush as a neuropathic pain model in which peripheral nerve regeneration can occur successfully. CCL2 in the sensory ganglia was increased by tibial nerve crush and reduced by microsympathectomy. Microsympathectomy and localized siRNA-mediated knockdown of CCL2 in the lumbar dorsal root ganglion had very similar effects: partial improvement of mechanical hypersensitivity and guarding behavior, reduction of regeneration markers growth-associated protein 43 and activating transcription factor 3, and reduction of macrophage density in the sensory ganglia and regenerating nerve. Microsympathectomy reduced functional regeneration as measured by myelinated action potential propagation through the injury site and denervation-induced atrophy of the tibial-innervated gastrocnemius muscle at day 10. Microsympathectomy plus CCL2 knockdown had behavioral effects similar to microsympathectomy alone. The results show that local sympathetic effects on neuropathic pain may be mediated in a large part by the effects on expression of CCL2, which in turn regulates the regeneration process.

Chemokine CCL2 prevents opioid-induced inhibition of nociceptive synaptic transmission in spinal cord dorsal horn

Background

Opioid analgesics remain widely used for pain treatment despite the related serious side effects. Some of those, such as opioid tolerance and opioid-induced hyperalgesia may be at least partially due to modulation of opioid receptors (OR) function at nociceptive synapses in the spinal cord dorsal horn. It was suggested that increased release of different chemokines under pathological conditions may play a role in this process. The goal of this study was to investigate the crosstalk between the µOR, transient receptor potential vanilloid 1 (TRPV1) receptor and C–C motif ligand 2 (CCL2) chemokine and the involvement of spinal microglia in the modulation of opioid analgesia.

Methods

Patch-clamp recordings of miniature excitatory postsynaptic currents (mEPSCs) and dorsal root evoked currents (eEPSC) in spinal cord slices superficial dorsal horn neurons were used to evaluate the effect of µOR agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO), CCL2, TRPV1 antagonist SB366791 and minocycline. Paw withdrawal test to thermal stimuli was combined with intrathecal (i.t.) delivery of CCL2 and DAMGO to investigate the modulation in vivo.

Results

Application of DAMGO induced a rapid decrease of mEPSC frequency and eEPSC amplitude, followed by a delayed increase of the eESPC amplitude, which was prevented by SB366791. Chemokine CCL2 treatment significantly diminished all the DAMGO-induced changes. Minocycline treatment prevented the CCL2 effects on the DAMGO-induced eEPSC depression, while mEPSC changes were unaffected. In behavioral experiments, i.t. injection of CCL2 completely blocked DAMGO-induced thermal hypoalgesia and intraperitoneal pre-treatment with minocycline prevented the CCL2 effect.

Conclusions

Our results indicate that opioid-induced inhibition of the excitatory synaptic transmission could be severely attenuated by increased CCL2 levels most likely through a microglia activation-dependent mechanism. Delayed potentiation of neurotransmission after µOR activation is dependent on TRPV1 receptors activation. Targeting CCL2 and its receptors and TRPV1 receptors in combination with opioid therapy could significantly improve the analgesic properties of opioids, especially during pathological states.

Triptolide Attenuates Neuropathic Pain by Regulating Microglia Polarization through the CCL2/CCR2 Axis

Triptolide (T10) is a common anti-inflammatory and analgesic drug. However, the activation of microglia and elimination of the corresponding inflammatory response are new targets for the treatment of neuropathic pain. Chemokine CCL (CCL2) is a key mediator for activating microglia. In this study, the effects of triptolide on the activation and polarization of microglia cells and CCL2 and its corresponding receptor, chemokine receptor 2 (CCR2), were mainly discussed. Microglia were stimulated with 1 μg/mL lipopolysaccharide (LPS) and pretreated with 10, 20, and 40 nM T10 and CCR2 antagonist (RS102895), respectively. The quantitative polymerase chain reaction (QPCR) and western blot results showed that T10 could obviously inhibit the upregulation of CCL2 and CCR2 induced by LPS stimulation in microglia cells, inhibit the fluorescence intensity of glial fibrillary acidic protein (GFAP) and inducible nitric oxide synthase (iNOS) antibody immunostaining in cells, and upregulate the fluorescence intensity of arginase 1 antibody in cells. The expression of interleukin-6 (IL-6), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α) was inhibited in a dose-dependent manner. RS102895 can significantly reverse the activation and M2 polarization of microglia pretreated with 40 nM T10 and weaken the anti-inflammatory effect of T10. The addition of CCL2 did not extremely affect the function of RS102895. T10 may inhibit microglia activation and M1 polarization by inhibiting the expression of CCL2 and CCR2, promoting M2 polarization, reducing the level of inflammatory factors in cells, and exerting its analgesic effect, which is worthy of clinical promotion as a drug for neuropathic pain.

CXCR3 signalling partially contributes to the pathogenesis of neuropathic pain in male rodents

BACKGROUND

Currently, there is a lack of effective therapy for chronic pain. Increasing evidence has shown that chemokines and their correlative receptors involved in the neuron-glial cell cross-talk could contribute to the pathogenesis of neuropathic pain. Our previous studies suggested that CXCR3 expression was elevated in the spinal dorsal horn after nerve injury.

OBJECTIVES

In this study, we aimed to explore the role of CXCR3 signalling in chronic pain modulation.

METHODS

Reverse transcription quantitative PCR and Western blotting were used to measure the expression of CXCR3 and its ligands in the spinal cord following chronic constriction injury (CCI) of the sciatic nerve. Cxcr3 -knockout mice were used to observe the effect of the receptor on pain-related behaviour and microglial activation. Immunohistochemistry was used to investigate the expression of two activation markers for spinal microglia, Iba-1 and phosphorylated-p38 (p-p38) in these mice.

RESULTS

The expression of CXCR3 and its ligand CXCL11 was upregulated in the lumbar dorsal horn of the spinal cord in CCI models. In Cxcr3 -knockout mice, CCI-induced tactile allodynia and thermal hyperalgesia were observed to be alleviated during the early stage of pain processing. Meanwhile, the expression of the glial activation markers, namely, Iba-1 and p-p38, was decreased.

CONCLUSION

Our results demonstrate that CXCR3 could be a key modulator involved in pain modulation of the spinal cord; therefore, CXCR3-related signalling pathways could be potential targets for the treatment of intractable pathological pain.

Neuropathic Pain: Biomolecular Intervention and Imaging via Targeting Microglia Activation

Many diseases, including cancer, can lead to neuropathic pain (NP). NP is one of the accompanying symptoms of suffering in many conditions and the life quality of NP patient is seriously affected. Due to complex causes, the effects of clinical treatments have been very unsatisfactory. Many experts have found that neuron-microglia interaction plays an essential role in NP occurrence and development. Therefore, the activation of microglia, related inflammatory mediators and molecular and cellular signaling pathways have become the focus of NP research. With the help of modern functional imaging technology, advanced pre-and clinical studies have been carried out and NP interventions have been attempted by using the different pharmaceuticals and the extracted active components of various traditional herbal medicines. In this communication, we review the mechanism of microglia on NP formation and treatment and molecular imaging technology's role in the clinical diagnosis and evaluation of NP therapies.

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.

Rationally Designed, Self-Assembling, Multifunctional Hydrogel Depot Repairs Severe Spinal Cord Injury

Following severe spinal cord injury (SCI), dysregulated neuroinflammation causes neuronal and glial apoptosis, resulting in scar and cystic cavity formation during wound healing and ultimately the formation of an atrophic microenvironment that inhibits nerve regrowth. Because of this complex and dynamic pathophysiology, a systemic solution for scar- and cavity-free wound healing with microenvironment remodeling to promote nerve regrowth has rarely been explored. A one-step solution is proposed through a self-assembling, multifunctional hydrogel depot that punctually releases the anti-inflammatory drug methylprednisolone sodium succinate (MPSS) and growth factors (GFs) locally according to pathophysiology to repair severe SCI. Synergistically releasing the anti-inflammatory drug MPSS and GFs in the hydrogel depot throughout SCI pathophysiology protects spared tissues/axons from secondary injury, promotes scar boundary- and cavity-free wound healing, and results in permissive bridges for remarkable axonal regrowth. Behavioral and electrophysiological studies indicate that remnants of spared axons, not regenerating axons, mediate functional recovery, strongly suggesting that additional interventions are still required to render the rebuilt neuronal circuits functional. These findings pave the way for the development of a systemic solution to treat acute SCI.

Differential expression of cerebrospinal fluid neuroinflammatory mediators depending on osteoarthritis pain phenotype

Supplemental Digital Content is Available in the Text.

Distinct cerebrospinal fluid neuroinflammatory profiles may be associated with different objective characteristics of persistent pain in osteoarthritis patients undergoing total hip arthroplasty.

Neuroinflammation is implicated in the development and maintenance of persistent pain states, but there are limited data linking cerebrospinal fluid (CSF) inflammatory mediators with neurophysiological pain processes in humans. In a prospective observational study, CSF inflammatory mediators were compared between patients with osteoarthritis (OA) who were undergoing total hip arthroplasty due to disabling pain symptoms (n = 52) and pain-free comparison controls (n = 30). In OA patients only, detailed clinical examination and quantitative sensory testing were completed. Cerebrospinal fluid samples were analyzed for 10 proinflammatory mediators using Meso Scale Discovery platform. Compared to controls, OA patients had higher CSF levels of interleukin 8 (IL-8) (P = 0.002), intercellular adhesion molecule 1 (P = 0.007), and vascular cell adhesion molecule 1 (P = 0.006). Osteoarthritis patients with central sensitization possibly indicated by arm pressure pain detection threshold <250 kPa showed significantly higher CSF levels of Fms-related tyrosine kinase 1 (Flt-1) (P = 0.044) and interferon gamma-induced protein 10 (IP-10) (P = 0.024), as compared to subjects with PPDT above that threshold. In patients reporting pain numerical rating scale score ≥3/10 during peripheral venous cannulation, Flt-1 was elevated (P = 0.025), and in patients with punctate stimulus wind-up ratio ≥2, CSF monocyte chemoattractant protein 1 was higher (P = 0.011). Multiple logistic regression models showed that increased Flt-1 was associated with central sensitization, assessed by remote-site PPDT and peripheral venous cannulation pain, and monocyte chemoattractant protein-1 with temporal summation in the area of maximum pain. Multiple proinflammatory mediators measured in CSF are associated with persistent hip OA-related pain. Pain phenotype may be influenced by specific CSF neuroinflammatory profiles.

Role of innate immunity in chemotherapy-induced peripheral neuropathy

It has become increasingly clear that the innate immune system plays an essential role in the generation of many types of neuropathic pain including that which accompanies cancer treatment. In this article we review current findings of the role of the innate immune system in contributing to cancer treatment pain at the distal endings of peripheral nerve, in the nerve trunk, in the dorsal root ganglion and in the spinal dorsal horn.

Inhibition of lncRNA DILC attenuates neuropathic pain via the SOCS3/JAK2/STAT3 pathway

Long noncoding RNAs (lncRNAs) have been involved in the development of multiple pathological processes including neuropathic pain. The aim of the present study is to investigate the role of lncRNA down-regulated in liver cancer stem cells (DILC) in the progression of neuropathic pain and its underlying mechanism. Neuropathic pain rat model was established with the bilateral chronic constriction injury (bCCI) method. The results from quantitative PCR analysis in the spinal cord showed that DILC was significantly up-regulated in rats with bCCI compared with the sham group. DILC down-regulation mediated by intrathecal administration of DILC siRNA significantly increased the mechanical shrinkage threshold (MWT) and paw withdrawal threshold latency (PWTL), decreased the positive frequency for nerve sensitivity to cold and suppressed the expression of inflammatory genes in bCCI rats. Down-regulation of DILC induced suppressor of cytokine signaling (SOCS3) expression and inhibited the phosphorylation of signal transducer and activator of transcription 3 (p-STAT3) in spinal cord tissues. Western blotting showed that down-regulation of DILC by DILC siRNA transfection induced SOCS3 expression and inhibited the expression of p-Janus kinase 2 (p-JAK2) and p-STAT3 and their downstream genes in primary microglia. Furthermore, down-regulation of DILC increased the viability of primary microglia, suppressed apoptosis, and inhibited the production of interleukin (IL)-6 and IL-1β in microglia. In contrast, overexpression of DILC showed the opposite functions to those of DILC knockdown. In conclusion, silence of lncRNA DILC attenuates neuropathic pain via SOCS3-induced suppression of the JAK2/STAT3 pathway.

Mechanistic insights into the role of the chemokine CCL2/CCR2 axis in dorsal root ganglia to peripheral inflammation and pain hypersensitivity

BACKGROUND

Pain is reported as the leading cause of disability in the common forms of inflammatory arthritis conditions. Acting as a key player in nociceptive processing, neuroinflammation, and neuron-glia communication, the chemokine CCL2/CCR2 axis holds great promise for controlling chronic painful arthritis. Here, we investigated how the CCL2/CCR2 system in the dorsal root ganglion (DRG) contributes to the peripheral inflammatory pain sensitization.

METHODS

Repeated intrathecal (i.t.) administration of the CCR2 antagonist, INCB3344 was tested for its ability to reverse the nociceptive-related behaviors in the tonic formalin and complete Freund's adjuvant (CFA) inflammatory models. We further determined by qPCR the expression of CCL2/CCR2, SP and CGRP in DRG neurons from CFA-treated rats. Using DRG explants, acutely dissociated primary sensory neurons and calcium mobilization assay, we also assessed the release of CCL2 and sensitization of nociceptors. Finally, we examined by immunohistochemistry following nerve ligation the axonal transport of CCL2, SP, and CGRP from the sciatic nerve of CFA-treated rats.

RESULTS

We first found that CFA-induced paw edema provoked an increase in CCL2/CCR2 and SP expression in ipsilateral DRGs, which was decreased after INCB3344 treatment. This upregulation in pronociceptive neuromodulators was accompanied by an enhanced nociceptive neuron excitability on days 3 and 10 post-CFA, as revealed by the CCR2-dependent increase in intracellular calcium mobilization following CCL2 stimulation. In DRG explants, we further demonstrated that the release of CCL2 was increased following peripheral inflammation. Finally, the excitation of nociceptors following peripheral inflammation stimulated the anterograde transport of SP at their peripheral nerve terminals. Importantly, blockade of CCR2 reduced sensory neuron excitability by limiting the calcium mobilization and subsequently decreased peripheral transport of SP towards the periphery. Finally, pharmacological inhibition of CCR2 reversed the pronociceptive action of CCL2 in rats receiving formalin injection and significantly reduced the neurogenic inflammation as well as the stimuli-evoked and movement-evoked nociceptive behaviors in CFA-treated rats.

CONCLUSIONS

Our results provide significant mechanistic insights into the role of CCL2/CCR2 within the DRG in the development of peripheral inflammation, nociceptor sensitization, and pain hypersensitivity. We further unveil the therapeutic potential of targeting CCR2 for the treatment of painful inflammatory disorders.

Sensory neuron-associated macrophages as novel modulators of neuropathic pain

The peripheral nervous system comprises an infinity of neural networks that act in the communication between the central nervous system and the most diverse tissues of the body. Along with the extension of the primary sensory neurons (axons and cell bodies), a population of resident macrophages has been described. These newly called sensory neuron-associated macrophages (sNAMs) seem to play an essential role in physiological and pathophysiological processes, including infection, autoimmunity, nerve degeneration/regeneration, and chronic neuropathic pain. After different types of peripheral nerve injury, there is an increase in the number and activation of sNAMs in the sciatic nerve and sensory ganglia. The activation of sNAMs and their participation in neuropathic pain development depends on the stimulation of pattern recognition receptors such as Toll-like receptors and Nod-like receptors, chemokines/cytokines, and microRNAs. On activation, sNAMs trigger the production of critical inflammatory mediators such as proinflammatory cytokines (eg, TNF and IL-1β) and reactive oxygen species that can act in the amplification of primary sensory neurons sensitization. On the other hand, there is evidence that sNAMs can produce antinociceptive mediators (eg, IL-10) that counteract neuropathic pain development. This review will present the cellular and molecular mechanisms behind the participation of sNAMs in peripheral nerve injury-induced neuropathic pain development. Understanding how sNAMs are activated and responding to nerve injury can help set novel targets for the control of neuropathic pain.

Prior perineural or neonatal treatment with capsaicin does not alter the development of spinal microgliosis induced by peripheral nerve injury

Peripheral nerve injury is associated with spinal microgliosis which plays a pivotal role in the development of neuropathic pain behavior. Several agents of primary afferent origin causing the microglial reaction have been identified, but the type(s) of primary afferents that release these mediators are still unclear. In this study, specific labeling of C-fiber spinal afferents by lectin histochemistry and selective chemodenervation by capsaicin were applied to identify the type(s) of primary afferents involved in the microglial response. Comparative quantitative morphometric evaluation of the microglial reaction in central projection territories of intact and injured peripheral nerves in the superficial (laminae I and II) and deep (laminae III and IV) spinal dorsal horn revealed a significant, about three-fold increase in microglial density after transection of the sciatic or the saphenous nerve. Prior perineural treatment of these nerves with capsaicin, resulting in a selective defunctionalization of C-fiber afferent fibers failed to affect spinal microgliosis. Similarly, peripheral nerve injury-induced increase in microglial density was unaffected in rats treated neonatally with capsaicin known to result in a near-total loss of C-fiber dorsal root fibers. Perineural treatment with capsaicin per se did not evoke a significant increase in microglial density. These observations indicate that injury-induced spinal microgliosis may be attributed to phenotypic changes in injured myelinated primary afferent neurons, whereas the contribution of C-fiber primary sensory neurons to this neuroimmune response is negligible. Spinal myelinated primary afferents may play a hitherto unrecognized role in regulation of neuroimmune and perisynaptic microenvironments of the spinal dorsal horn.

SUR1, newly expressed in astrocytes, mediates neuropathic pain in a mouse model of peripheral nerve injury

BACKGROUND

Neuropathic pain following peripheral nerve injury (PNI) is linked to neuroinflammation in the spinal cord marked by astrocyte activation and upregulation of interleukin 6 (IL-6), chemokine (C-C motif) ligand 2 (CCL2) and chemokine (C-X-C motif) ligand 1 (CXCL1), with inhibition of each individually being beneficial in pain models.

METHODS

Wild type (WT) mice and mice with global or pGfap-cre- or pGFAP-cre/ERT2-driven Abcc8/SUR1 deletion or global Trpm4 deletion underwent unilateral sciatic nerve cuffing. WT mice received prophylactic (starting on post-operative day [pod]-0) or therapeutic (starting on pod-21) administration of the SUR1 antagonist, glibenclamide (10 µg IP) daily. We measured mechanical and thermal sensitivity using von Frey filaments and an automated Hargreaves method. Spinal cord tissues were evaluated for SUR1-TRPM4, IL-6, CCL2 and CXCL1.

RESULTS

Sciatic nerve cuffing in WT mice resulted in pain behaviors (mechanical allodynia, thermal hyperalgesia) and newly upregulated SUR1-TRPM4 in dorsal horn astrocytes. Global and pGfap-cre-driven Abcc8 deletion and global Trpm4 deletion prevented development of pain behaviors. In mice with Abcc8 deletion regulated by pGFAP-cre/ERT2, after pain behaviors were established, delayed silencing of Abcc8 by tamoxifen resulted in gradual improvement over the next 14 days. After PNI, leakage of the blood-spinal barrier allowed entry of glibenclamide into the affected dorsal horn. Daily repeated administration of glibenclamide, both prophylactically and after allodynia was established, prevented or reduced allodynia. The salutary effects of glibenclamide on pain behaviors correlated with reduced expression of IL-6, CCL2 and CXCL1 by dorsal horn astrocytes.

CONCLUSION

SUR1-TRPM4 may represent a novel non-addicting target for neuropathic pain.

CB2 Receptor in Microglia: The Guardian of Self-Control

Microglia are key to maintaining the homeostasis of the brain. These immune cells of the brain can be our biggest ally in fighting infections, but can worsen pathology or hinder recovery when uncontrolled. Thus, understanding how microglia contribute to neuroinflammatory processes and how their activity can be controlled is of great importance. It is known that activation of endocannabinoid system, and especially the cannabinoid type 2 receptor (CB2R), decreases inflammation. Alongside its non-psychoactive effect, it makes the CB2R receptor a perfect target for treating diseases accompanied by neuroinflammation including neurodegenerative diseases. However, the exact mechanisms by which CB2R regulates microglial activity are not yet understood. Here, we review the current knowledge on the roles of microglial CB2R from in vitro and in vivo studies. We look into CB2R function under physiological and pathological conditions and focus on four different disease models representing chronic and acute inflammation. We highlight open questions and controversies and provide an update on the latest discoveries that were enabled by the development of novel technologies. Also, we discuss the recent findings on the role of microglia CB2R in cognition and its role in neuron–microglia communication.

Peripheral versus central mechanisms of the cannabinoid type 2 receptor agonist AM1710 in a mouse model of neuropathic pain

The CB₂ R agonist AM1710, examined in animal models of peripheral neuropathy, is effective in controlling aberrant light touch sensitivity, referred to as mechanical allodynia. However, nonspecific binding of AM1710 to CB₁ R, either peripherally or centrally, could be partially responsible for the analgesic effects of AM1710. Thus, we sought to determine in mice whether spinal (intrathecal; i.t.) or peripheral AM1710 administration could lead to anti-allodynia by reducing the protein expression of spinal and dorsal root ganglia (DRG) proinflammatory cytokines and elevating the anti-inflammatory cytokine interleukin-10 (IL-10) in the absence of CB₁ R. Macrophage cell cultures were examined to characterize AM1710-mediated suppression of the proinflammatory cytokine tumor necrosis factor-alpha (TNF-α). Either i.p. or i.t. AM1710 reversed CCI-induced mechanical allodynia to sham levels in CB₁ R (-/-), (+/-), (+/+) mice. CCI-induced neuropathy decreased IL-10 immunoreactivity (IR) in the dorsal root ganglia (DRG) and the dorsal horn of the spinal cord, with i.t. AM1710 restoring basal IL-10 IR. CCI-induced elevations in proinflammatory cytokine IR were decreased within the spinal cord only after i.t. AM1710 in all mouse genotypes. Meanwhile, within DRG tissue from neuropathic mice, proinflammatory cytokines were decreased following either i.p. or i.t. AM1710. Analysis of cultured supernatants revealed AM1710 decreased TNF-alpha protein. We conclude that CB₁ R is dispensable for either peripheral or central anti-allodynic actions of AM1710 in neuropathic mice. Cannabinoid CB₂ R agonists produce heightened spinal IL-10 which may be clinically relevant to successfully treat neuropathic pain.

CCL2 facilitates spinal synaptic transmission and pain via interaction with presynaptic CCR2 in spinal nociceptor terminals

Previous studies have shown that CCL2 may cause chronic pain, but the exact mechanism of central sensitization is unclear. In this article, we further explore the presynaptic role of CCL2. Behavioral experiments show that intervertebral foramen injection CCR2 antagonists into dorsal root ganglion (DRG) can inhibit the inflammatory pain caused by CCL2 in spinal cord. We raised the question of the role of presynaptic CCR2 in the spinal dorsal horn. Subsequent electron microscopy experiments showed that CCR2 was expressed in the presynaptic CGRP terminal in the spinal dorsal horn. CCL2 can enhance presynaptic calcium signal. Whole-cell patch-clamp recordings showed that CCL2 can enhance NMDAR-eEPSCs through presynaptic effects, and further application of glutamate sensor method proved that CCL2 can act on presynaptic CCR2 to increase the release of presynaptic glutamate. In conclusion, we suggest that CCL2 can directly act on the CCR2 on presynaptic terminals of sensory neurons in the spinal dorsal horn, leading to an increase in the release of presynaptic glutamate and participate in the formation of central sensitization.

The CCL2/CCL7/CCL12/CCR2 pathway is substantially and persistently upregulated in mice after traumatic brain injury, and CCL2 modulates the complement system in microglia

Traumatic brain injury (TBI) is the leading cause of death in the global population. Disturbed inflammatory processes after TBI exacerbate secondary brain injury and contribute to unfavorable outcomes. Multiple inflammatory events that accompany brain trauma, such as glial activation, chemokine release, or the initiation of the complement system cascade, have been identified as potential targets for TBI treatment. However, the participation of chemokines in the complement activation remains unknown. Our studies sought to determine the changes in the expression of the molecules involved in the CCL2/CCL7/CCL12/CCR2 pathway in the injured brain and the effect of CCL2, CCL7, and CCL12 (10, 100, and 500 ng/mL) on the classic and lectin complement pathways and inflammatory factors in microglial cell cultures. Brain injury in mice was modeled by controlled cortical impact (CCI). Our findings indicate a time-dependent upregulation of CCL2, CCL7, and CCL12 at the mRNA and protein levels within the cortex, striatum, and/or thalamus beginning 24 h after the trauma. The analysis of the expression of the receptor of the tested chemokines, CCR2, revealed its substantial upregulation within the injured brain areas mainly on the mRNA level. Using primary cortical microglial cell cultures, we observed a substantial increase in the expression of CCL2, CCL7, and CCL12 after 24 h of LPS (100 ng/mL) treatment. CCL2 stimulation of microglia increased the level of IL-1β mRNA but did not influence the expression of IL-18, IL-6, and IL-10. Moreover, CCL2 significantly increased the expression of Iba1, a marker of microglia activation. CCL2 and CCL12 upregulated the expression of C1qa but did not influence the expression of C1ra and C1s1 (classical pathway); moreover, CCL2 increased ficolin A expression and reduced collectin 11 expression (lectin pathway). Additionally, we observed the downregulation of pentraxin 3, a modulator of the complement cascade, after CCL2 and CCL12 treatment. We did not detect the expression of ficolin B, Mbl1, and Mbl2 in microglial cells. Our data identify CCL2 as a modulator of the classical and lectin complement pathways suggesting that CCL2 may be a promising target for pharmacological intervention after brain injury. Moreover, our study provides evidence that CCL2 and two other CCR2 ligands may play a role in the development of changes in TBI.

STAU1 selectively regulates the expression of inflammatory and immune response genes and alternative splicing of the nerve growth factor receptor signaling pathway

Double‑stranded RNA‑binding protein Staufen homolog 1 (STAU1) is a highly conserved multifunctional double‑stranded RNA‑binding protein, and is a key factor in neuronal differentiation. RNA sequencing was used to analyze the overall transcriptional levels of the upregulated cells by STAU1 and control cells, and select alternative splicing (AS). It was determined that the high expression of STAU1 led to changes in the expression levels of a variety of inflammatory and immune response genes, including IFIT2, IFIT3, OASL, and CCL2. Furthermore, STAU1 was revealed to exert a significant regulatory effect on the AS of genes related to the 'nerve growth factor receptor signaling pathway'. This is of significant importance for neuronal survival, differentiation, growth, post‑damage repair, and regeneration. In conclusion, overexpression of STAU1 was associated with immune response and regulated AS of pathways related to neuronal growth and repair. In the present study, the whole transcriptome of STAU1 expression was first analyzed, which laid a foundation for further understanding the key functions of STAU1.

Local Sympathectomy Promotes Anti-inflammatory Responses and Relief of Paclitaxel-induced Mechanical and Cold Allodynia in Mice

BACKGROUND

Patients undergoing cancer treatment often experience chemotherapy-induced neuropathic pain at their extremities, for which there is no U.S. Food and Drug Administration-approved drug. The authors hypothesized that local sympathetic blockade, which is used in the clinic to treat various pain conditions, can also be effective to treat chemotherapy-induced neuropathic pain.

METHODS

A local sympathectomy (i.e., cutting the ipsilateral gray rami entering the spinal nerves near the L3 and L4 dorsal root ganglia) was performed in mice receiving intraperitoneal injections every other day of the chemotherapeutic drug paclitaxel. Sympathectomy effects were then assessed in chemotherapy-induced pain-like behaviors (i.e., mechanical and cold allodynia) and neuroimmune and electrophysiologic responses.

RESULTS

Local microsympathectomy produced a fast recovery from mechanical allodynia (mean ± SD: sympathectomy vs. sham at day 5, 1.07 ± 0.34 g vs. 0.51 ± 0.17g, n = 5, P = 0.030 in male mice, and 1.08 ± 0.28 g vs. 0.62 ± 0.16 g, n = 5, P = 0.036 in female mice) and prevented the development of cold allodynia in both male and female mice after paclitaxel. Mechanistically, microsympathectomy induced transcriptional increases in dorsal root ganglia of macrophage markers and anti-inflammatory cytokines, such as the transforming growth factor-β. Accordingly, depletion of monocytes/macrophages and blockade of transforming growth factor-β signaling reversed the relief of mechanical allodynia by microsympathectomy. In particular, exogenous transforming growth factor-β was sufficient to relieve mechanical allodynia after paclitaxel (transforming growth factor-β 100 ng/site vs. vehicle at 3 h, 1.21 ± 0.34g vs. 0.53 ± 0.14 g, n = 5, P = 0.001 in male mice), and transforming growth factor-β signaling regulated neuronal activity in dorsal root ganglia.

CONCLUSIONS

Local sympathetic nerves control the progression of immune responses in dorsal root ganglia and pain-like behaviors in mice after paclitaxel, raising the possibility that clinical strategies already in use for local sympathetic blockade may also offer an effective treatment for patients experiencing chemotherapy-induced neuropathic pain.

Spinal CCL2 Promotes Pain Sensitization by Rapid Enhancement of NMDA-Induced Currents Through the ERK-GluN2B Pathway in Mouse Lamina II Neurons

Previous studies have shown that CCL2 (C–C motif chemokine ligand 2) induces chronic pain, but the exact mechanisms are still unknown. Here, we established models to explore the potential mechanisms. Behavioral experiments revealed that an antagonist of extracellular signal-regulated kinase (ERK) inhibited not only CCL2-induced inflammatory pain, but also pain responses induced by complete Freund’s adjuvant. We posed the question of the intracellular signaling cascade involved. Subsequent experiments showed that CCL2 up-regulated the expression of phosphorylated ERK (pERK) and N-methyl D-aspartate receptor [NMDAR] subtype 2B (GluN2B); meanwhile, antagonists of CCR2 and ERK effectively reversed these phenomena. Whole-cell patch-clamp recordings revealed that CCL2 enhanced the NMDAR-induced currents via activating the pERK pathway, which was blocked by antagonists of GluN2B and ERK. In summary, we demonstrate that CCL2 directly interacts with CCR2 to enhance NMDAR-induced currents, eventually leading to inflammatory pain mainly through the CCL2–CCR2–pERK–GluN2B pathway.

CCR4 Antagonist (C021) Administration Diminishes Hypersensitivity and Enhances the Analgesic Potency of Morphine and Buprenorphine in a Mouse Model of Neuropathic Pain

Neuropathic pain is a chronic condition that remains a major clinical problem owing to high resistance to available therapy. Recent studies have indicated that chemokine signaling pathways are crucial in the development of painful neuropathy; however, the involvement of CC chemokine receptor 4 (CCR4) has not been fully elucidated thus far. Therefore, the aim of our research was to investigate the role of CCR4 in the development of tactile and thermal hypersensitivity, the effectiveness of morphine/buprenorphine, and opioid-induced tolerance in mice exposed to chronic constriction injury (CCI) of the sciatic nerve. The results of our research demonstrated that a single intrathecal or intraperitoneal administration of C021, a CCR4 antagonist, dose dependently diminished neuropathic pain-related behaviors in CCI-exposed mice. After sciatic nerve injury, the spinal expression of CCL17 and CCL22 remained unchanged in contrast to that of CCL2, which was significantly upregulated until day 14 after CCI. Importantly, our results provide evidence that in naive mice, CCL2 may evoke pain-related behaviors through CCR4 because its pronociceptive effects are diminished by C021. In CCI-exposed mice, the pharmacological blockade of CCR4 enhanced the analgesic properties of morphine/buprenorphine and delayed the development of morphine-induced tolerance, which was associated with the silencing of IBA-1 activation in cells and decrease in CCL2 production. The obtained data suggest that the pharmacological blockade of CCR4 may be a new potential therapeutic target for neuropathic pain polytherapy.

Granulocyte Colony Stimulating Factor (GCSF) Can Attenuate Neuropathic Pain by Suppressing Monocyte Chemoattractant Protein-1 (MCP-1) Expression, through Upregulating the Early MicroRNA-122 Expression in the Dorsal Root Ganglia

Our previous animal studies and several human clinical trials have shown that granulocyte-colony stimulating factor (GCSF) can attenuate neuropathic pain through various mechanisms. GCSF itself is also a multipotent cytokine that can modulate microribonucleic acid (microRNA) expression profiles in vitro. In this study, we used the NanoString nCounter analysis system to screen the expression of different rodent microRNAs at early stage after nerve injury and studied the expression of related cytokines/chemokines in the dorsal root ganglia (DRGs) of rats that underwent chronic constriction injury (CCI) to explore the underlying mechanisms of the analgesic effects of GCSF. We found that microRNA-122 expression was downregulated by CCI; in contrast, GCSF treatment significantly upregulated microRNA-122 expression in the DRGs of CCI rats on the 1st day after nerve injury. We further studied the expression of different cytokines/chemokines (IL-1β, IL-6, and monocyte chemoattractant protein-1 (MCP-1)) that were modulated by microRNA-122. MCP-1 has been reported to participate in neuropathic pain development, and its expression on the DRGs of vehicle-treated CCI rats was significantly higher than that on the DRGs of sham-operated rats; in contrast, GCSF-treated rats exhibited significantly lower MCP-1 expression in the DRG than vehicle-treated rats on the 7th day after nerve injury. An early GCSF treatment can suppress MCP-1 expressions, through upregulating microRNA-122 expressions in the DRGs of CCI rats at an earlier stage, thus indirectly attenuating neuropathic pain development.

The high expression of the pain-related inflammatory factors in the eyes of cataract patients infected with hepatitis B virus

In cataract surgery, it is often found that patients infected hepatitis B virus (HBV) are likely to suffer from more pain than other patients. In order to assess the inflammation status of the aqueous humor in the eyes of cataract patients infected with HBV. RayBio Human Inflammation Array was used to assay aqueous humor samples collected from 14 eyes of patients infected with HBV and 14 eyes of cataract patients without HBV infection (the controls) during the cataract surgery. RayBio Human Quantibody Cutom Array was adopted for the validation of the screened cytokines, with aqueous humor samples collected from 40 eyes of patients infected with HBV and 40 eyes of the controls. A pain questionnaire survey about the surgery was conducted in all patients after operation. The results of questionnaire showed that patients infected with HBV were more likely to have pain during operation. The Human Inflammation Array revealed that the expression levels of monocyte chemoattractant protein-1 (MCP-1), tumor necrosis factor α (TNF-α) and tumor necrosis factor β (TNF-β) were very high in HBV infected patients and IL-1ra was much lower in patients infected with HBV (all, P < 0.05). In validation, the Human Quantibody Cutom Array revealed that the expression levels of MCP-1 and TNF-α were high in HBV infected patients with significant difference (all P < 0.05). These results revealed that pain-related inflammatory factors MCP-1 and TNF-α were increased in aqueous humor of cataract patients infected with HBV, which indicates that patients infected with HBV may be more prone to intraoperative pain.

Omega-3 derivatives, specialized pro-resolving mediators: Promising therapeutic tools for the treatment of pain in chronic liver disease

The main causes of liver injury are associated with inflammation and permanent damage. They can cause chronic liver disease (CLD), which is mainly related to viral hepatitis, alcohol consumption and non-alcoholic steatohepatitis, leading to fibrosis, cirrhosis and hepatocellular carcinoma. These conditions prevent the liver from working normally and make it begin to fail, which in turn may prompt a liver transplant. CLD and cirrhosis are the eleventh cause of death worldwide. At present, there are no approved pharmacological treatments to prevent, treat or resolve liver fibrosis. The prevalence of pain in the hepatic disease is elevated with ranges between 30% and 40%. Most of the pain drugs require hepatic function; therefore, the suitable control of pain is still a clinical challenge. Specialized pro-resolving mediators (SPM): lipoxins, resolvins, protectins and maresins, are potent endogenous molecules (nM concentrations) that modulate inflammatory body responses by reducing neutrophil infiltration, macrophage activity and pain sensitization. SPM have anti-inflammatory properties, stimulate tissue resolution, repair and regeneration, and exhibit anti-nociceptive actions. Furthermore, SPM were tried on different cellular, animal models and human observational data of liver injury, improving the pathogenesis of inflammation and fibrosis. In the present work, we will describe recent evidence that suggests that SPM can be used as a therapeutic option for CLD. Additionally, we will examine the role of SPM in the control of pain in pathologies associated with liver injury.

Spinal microglia-neuron interactions in chronic pain

Current deficiency in our understanding of acute-to-chronic pain transition remains a hurdle for developing effective treatments against chronic pain. Whereas neurocentric mechanisms alone are insufficient to provide satisfactory explanation for such transition, neuro-immune crosstalk has attracted attention in recent pain research. In contrast to brain microglia, spinal microglia are activated immediately in various pain states. The fast-responsive enrichment and activation of spinal microglia among different pain conditions have highlighted the crucial role of neuroinflammation caused by microglia-neuron crosstalk in pain initiation. Recent studies have revealed spinal microglia-neuron interactions are also involved in chronic pain maintenance, albeit, with different anatomic distribution, cellular and molecular mechanisms, and biologic functions. Delineating the exact temporal discrepancies of spinal microglia distribution and functions along acute-to-chronic pain transition may provide additional mechanistic insights for drug development to prevent deterioration of acute pain into the chronic state. This narrative review summerizes the longitudinal alterations of spinal microglia-neuron interactions in the initiation of pain hypersensitivity, acute-to-chronic pain progression, and chronic pain maintenance, followed by an overview of current clinical translation of preclinical studies on spinal microglia. This review highlights the crucial role of the interaction between spinal microglia and neighboring neurons in the initiation and maintenance of pain hypersensitivity, in relation to the release of cytokines, chemokines, and neuroactive substances, as well as the modulation of synaptic plasticity. Further exploration of the uncharted functions of spinal microglia-neuron crosstalk may lead to the design of novel drugs for preventing acute-to-chronic pain transition.

Neuronal/astrocytic expression of chemokine (C-C motif) ligand 2 is associated with monocyte/macrophage recruitment in male chronic pelvic pain

Chronic pelvic pain syndrome is a multisymptom syndrome with unknown etiology. The experimental autoimmune prostatitis (EAP) mouse model of chronic pelvic pain syndrome is associated with immune cell infiltration into the prostate, expression of C-C chemokine ligand 2 (CCL2), and neuroinflammation in the spinal cord. Here, we studied CCL2 expression in tissues along the nociceptive pathway and its association with neuroimmune cells during pain development. Examination of prostate tissues at days 14 and 28 after EAP induction revealed CCL2 expression was increased in epithelial cells and was associated with increased numbers of macrophages lying in close apposition to PGP9.5-positive afferent neuronal fibers. C-C Chemokine ligand 2 immunoreactivity was elevated to a similar degree in the dorsal root ganglia at day 14 and day 28. D14 of EAP was associated with elevated IBA1 cells in the dorsal root ganglia that were not evident at D28. Adoptive transfer of green fluorescent protein+ leukocytes into EAP mice demonstrated monocytes are capable of infiltrating the spinal cord from peripheral blood with what seemed to be a proinflammatory phenotype. In the lower dorsal spinal cord, CCL2 expression localized to NeuN expressing neurons and GFAP-expressing astrocytes. Myeloid derived cell infiltration into the spinal cord in EAP was observed in the L6-S2 dorsal horn. Myeloid-derived CD45 IBA1+ cells were localized with IBA1+ TMEM199+ microglia in the dorsal horn of the spinal cord in EAP, with intimate association of the 2 cell types suggesting cell-cell interactions. Finally, intrathecal administration of liposomal clodronate ameliorated pelvic pain symptoms, suggesting a mechanistic role for macrophages and microglia in chronic pelvic pain.