Spinal injection of TNF-α-activated astrocytes produces persistent pain symptom mechanical allodynia by releasing monocyte chemoattractant protein-1

Neuroinflammatory Pathways Associated with Chronic Post-Thoracotomy Pain: A Review of Current Literature

Chronic post-thoracotomy pain (CPTP) is a major clinical problem that affects up to 35-55% of patients undergoing thoracic incisions. Evidence suggests that multiple cellular signaling pathways and neuro-inflammatory mediators may play an essential role in the pathogenesis of CPTP. In this comprehensive review, we present the current evidence on the cellular signaling pathways and inflammatory changes associated with the initiation and maintenance of CPTP, focusing on the potential application of these findings in the clinical setting. An electronic search of Medline, EMBASE, Cochrane, Google Scholar, and ClinicalTrials.gov was performed, and 3652 abstracts were identified. After an initial abstract screening, 131 studies underwent a full-text review, and nine papers were eventually included in this review. Studies were included if they assessed the cellular signaling pathways or inflammatory processes associated with the induction and/or maintenance of CPTP. All the identified studies were pre-clinical studies conducted on animal models. Our search identified seven cellular pathways (NK-1 receptor (NK-1), Glutaminase 1, Toll-like receptor 4 (TLR4), Resolvins, Ror-2, Sonic hedgehog signaling (Shh), and Wnt5a/Wnts) and six cytokines (IL-1β, IL-6, IL-8, IL-10, IFN-γ, and TNF-α) that were investigated in the context of CPTP. Multiple cellular signaling pathways and inflammatory cytokines may play an important role in the neuroinflammatory changes associated with the induction and maintenance of chronic post-thoracotomy pain in animal models. However, the clinical impact and therapeutic utility of these neuroinflammatory changes in routine clinical practice have yet to be demonstrated.

Synergistic action of vitamin D3 and A on motor activity regulation in mice model of extrapyramidal syndrome: Correlational insights into astrocyte regulation, cytokine modulation, and dopaminergic activity

BACKGROUND

Extrapyramidal syndromes (EPS) represent neurological side effects of antipsychotic medications, characterized by motor disturbances. While previous studies have indicated the neuroprotective effects of vitamin D and A against EPS, the underlying mechanisms of this protection remain unclear.

METHODS

Twenty-four adult mice were categorized into four groups: positive and negative control groups, one receiving a dopamine antagonist, and the other receiving both a dopamine antagonist and vitamins D and A. Sections of the corticobasal loop, specifically the motor cortex (M1) and basal nuclei (CPu), were prepared for Immunohistochemistry (IHC) and stained with Glial Fibrillary Acidic Protein (GFAP) to visualize reactive astrocytes. ELISA assays for TNF-α, IL-6, IL-4, IL-13, and dopamine levels were performed on homogenized brain sections.

RESULTS

The EPS group exhibited a significant increase in TNF-α and IL-6 levels in M1 and CPu. Treatment with dopamine agonists and vitamin D&A resulted in significant reductions in IL-6 levels. Only the Vitamin D&A group showed a significant decline in TNF-α. The EPS group recorded significant decreases in IL-4 and IL-13, with IL-13 significantly elevated in the dopamine agonist and Vitamin D&A groups. IL-4 was notably increased in the Vitamin D&A groups. Dopamine concentration significantly declined in the EPS group, with improvements observed in the groups treated with dopamine agonists, and vitamin D&A. Reactive astrocytes were significantly expressed in the M1 and CPu of the EPS group but poorly expressed in other groups.

CONCLUSIONS

EPS is linked to astrocyte activation, an upsurge in pro-inflammatory cytokines, a decline in anti-inflammatory cytokines, and dopamine in the corticobasal loop. Administration of vitamin D3 and A was found to suppres pro-inflammatory cytokines and repress anti-inflammatory cytokines associated with astrocyte activation.

Tonic excitation by astrocytic GABA causes neuropathic pain by augmenting neuronal activity and glucose metabolism

Neuropathic pain is a debilitating condition caused by the hyperexcitability of spinal dorsal horn neurons and is often characterized by allodynia. Although neuron-independent mechanisms of hyperexcitability have been investigated, the contribution of astrocyte-neuron interactions remains unclear. Here, we show evidence of reactive astrocytes and their excessive GABA release in the spinal dorsal horn, which paradoxically leads to the tonic excitation of neighboring neurons in a neuropathic pain model. Using multiple electrophysiological methods, we demonstrated that neuronal hyperexcitability is attributed to both increased astrocytic GABA synthesis via monoamine oxidase B (MAOB) and the depolarized reversal potential of GABA-mediated currents (EGABA) via the downregulation of the neuronal K+/Cl- cotransporter KCC2. Furthermore, longitudinal 2-deoxy-2-[18F]-fluoro-D-glucose microPET imaging demonstrated increased regional glucose metabolism in the ipsilateral dorsal horn, reflecting neuronal hyperexcitability. Importantly, inhibiting MAOB restored the entire astrocytic GABA-mediated cascade and abrogated the increased glucose metabolism and mechanical allodynia. Overall, astrocytic GABA-mediated tonic excitation is critical for neuronal hyperexcitability, leading to mechanical allodynia and neuropathic pain.

Transcrocetin Meglumine Salt Inhibits Spinal Glial Cell-Mediated Proinflammatory Cytokines and Attenuates Complete Freund's Adjuvant-Induced Inflammatory Pain

INTRODUCTION

Inflammatory pain is a significant global clinical challenge that involves both unpleasant sensory and emotional experiences. The treatment of pain is imminent, and we are committed to seeking new analgesics for pain relief. Transcrocetin meglumine salt (TCMS), a saffron metabolite derived from the crocin apocarotenoids, has exhibited the ability to cross the blood-brain barrier and exert neuroprotective effects. In this study, we aimed to investigate whether TCMS could ameliorate complete Freund's adjuvant (CFA)-induced inflammatory pain in mice and elucidate its underlying mechanisms.

METHODS

Here, we established an inflammatory pain model in mice by injecting CFA into the left hind paw. Three days later, we administered intraperitoneal injections of TCMS (10 mg/kg) or saline to the animals. We examined mechanical allodynia, thermal hypersensitivity, and anxiety behavior. Furthermore, the activation of glial cells and proinflammatory cytokines in the spinal cord were detected.

RESULTS

Our results showed that TCMS significantly reversed the mechanical allodynia and thermal hypersensitivity in the CFA-injected mice. Furthermore, TCMS administration effectively inhibited the activation of microglia and astrocytes in the spinal cord induced by CFA. Additionally, TCMS suppressed the production and release of spinal proinflammatory cytokines, including TNF-α, IL-1β, and IL-6, in CFA-injected mice.

CONCLUSION

Taken together, our findings demonstrate that TCMS holds promise as an innovative analgesic due to its ability to ameliorate inflammatory reactions.

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

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

Targeting the chemokine ligand 2-chemokine receptor 2 axis provides the possibility of immunotherapy in chronic pain

Chronic pain affects patients' physical and psychological health and quality of life, entailing a tremendous public health challenge. Currently, drugs for chronic pain are usually associated with a large number of side effects and poor efficacy. Chemokines in the neuroimmune interface combine with their receptors to regulate inflammation or mediate neuroinflammation in the peripheral and central nervous system. Targeting chemokines and their receptor-mediated neuroinflammation is an effective means to treat chronic pain. In recent years, growing evidence has shown that the expression of chemokine ligand 2 (CCL2) and its main chemokine receptor 2 (CCR2) is involved in its occurrence, development and maintenance of chronic pain. This paper summarises the relationship between the chemokine system, CCL2/CCR2 axis, and chronic pain, and the CCL2/CCR2 axis changes under different chronic pain conditions. Targeting chemokine CCL2 and its chemokine receptor CCR2 through siRNA, blocking antibodies, or small molecule antagonists may provide new therapeutic possibilities for managing chronic pain.

C3aR in astrocytes mediates post-thoracotomy pain by inducing A1 astrocytes in male rats

BACKGROUND

Astrocyte activation, which is polarized into classical neurotoxic A1, neuroprotective A2, A-pan, etc., is thought to be involved in the transition from acute to chronic post-thoracotomy pain. The C3aR receptor associated with astrocyte-neuron and -microglia interactions is necessary for A1 astrocytes polarization. This study aimed to determine whether C3aR in astrocytes mediates post-thoracotomy pain by inducing A1 expression in a rat thoracotomy pain model.

METHODS

A rat thoracotomy pain model was employed. The mechanical withdraw threshold was measured to evaluate pain behavior. Lipopolysaccharide (LPS) was injected intraperitoneally to induce A1. Intrathecal injection of AAV2/9-rC3ar1 shRNA-GFAP was used to knock down in vivo C3aR expression in astrocytes. The expression of associated phenotypic markers before and after intervention was assessed by RT-PCR, western blot, co-immunofluorescence, and single-cell RNA sequencing.

RESULTS

C3aR downregulation was found to inhibit LPS-induced A1 astrocytes activation, decrease the expression of C3aR, C3, and GFAP, which were activated from acute to chronic pain, and alleviate the mechanical withdrawal threshold and chronic pain incidence. In addition, more A2 astrocytes were activated in the model group that did not develop chronic pain. C3aR downregulation increased the number of A2 astrocytes upon LPS exposure. Knockdown of C3aR also decreased the activation of M1 microglia induced by LPS or thoracotomy.

CONCLUSIONS

Our study confirmed that C3aR-induced A1 polarization contributes to chronic post-thoracotomy pain. Inhibition of A1 activation via C3aR downregulation increases anti-inflammatory A2 and decreases pro-inflammatory M1 activation, which may also be involved in the mechanism of chronic post-thoracotomy pain.

The core of maintaining neuropathic pain: Crosstalk between glial cells and neurons (neural cell crosstalk at spinal cord)

BACKGROUND

Neuropathic pain (NP) caused by the injury or dysfunction of the nervous system is a chronic pain state accompanied by hyperalgesia, and the available clinical treatment is relatively scarce. Hyperalgesia mediated by pro-inflammatory factors and chemokines plays an important role in the occurrence and maintenance of NP.

DATA TREATMENT

Therefore, we conducted a systematic literature review of experimental NP (PubMed Medline), in order to find the mechanism of inducing central sensitization and explore the intervention methods of hyperalgesia caused by real or simulated injury.

RESULT

In this review, we sorted out the activation pathways of microglia, astrocytes and neurons, and the process of crosstalk among them. It was found that in NP, the microglia P2X4 receptor is the key target, which can activate the mitogen-activated protein kinase pathway inward and then activate astrocytes and outwardly activate neuronal tropomyosin receptor kinase B receptor to activate neurons. At the same time, activated neurons continue to maintain the activation of astrocytes and microglia through chemokines on CXCL13/CXCR5 and CX3CL1/CX3CR1. This crosstalk process is the key to maintaining NP.

CONCLUSION

We summarize the further research on crosstalk among neurons, microglia, and astrocytes in the central nervous system, elaborate the ways and connections of relevant crosstalk, and find potential crosstalk targets, which provides a reference for drug development and preclinical research.

Astrocytes in Chronic Pain: Cellular and Molecular Mechanisms

Chronic pain is challenging to treat due to the limited therapeutic options and adverse side-effects of therapies. Astrocytes are the most abundant glial cells in the central nervous system and play important roles in different pathological conditions, including chronic pain. Astrocytes regulate nociceptive synaptic transmission and network function via neuron–glia and glia–glia interactions to exaggerate pain signals under chronic pain conditions. It is also becoming clear that astrocytes play active roles in brain regions important for the emotional and memory-related aspects of chronic pain. Therefore, this review presents our current understanding of the roles of astrocytes in chronic pain, how they regulate nociceptive responses, and their cellular and molecular mechanisms of action.

The Role of Neuro-Immune Interactions in Chronic Pain: Implications for Clinical Practice

Chronic pain remains a public health problem and contributes to the ongoing opioid epidemic. Current pain management therapies still leave many patients with poorly controlled pain, thus new or improved treatments are desperately needed. One major challenge in pain research is the translation of preclinical findings into effective clinical practice. The local neuroimmune interface plays an important role in the initiation and maintenance of chronic pain and is therefore a promising target for novel therapeutic development. Neurons interface with immune and immunocompetent cells in many distinct microenvironments along the nociceptive circuitry. The local neuroimmune interface can modulate the activity and property of the neurons to affect peripheral and central sensitization. In this review, we highlight a specific subset of many neuroimmune interfaces. In the central nervous system, we examine the interface between neurons and microglia, astrocytes, and T lymphocytes. In the periphery, we profile the interface between neurons in the dorsal root ganglion with T lymphocytes, satellite glial cells, and macrophages. To bridge the gap between preclinical research and clinical practice, we review the preclinical studies of each neuroimmune interface, discuss current clinical treatments in pain medicine that may exert its action at the neuroimmune interface, and highlight opportunities for future clinical research efforts.

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.

Neuropathogenesis of HIV and emerging therapeutic targets

INTRODUCTION

HIV infection causes a wide range of neurological complications, many of which are among the most common complications of chronic HIV infection in the era of combined antiretroviral therapy. These neurological conditions arise due to complex interactions between HIV viral proteins and neuronal and glial cells that lead to the activation of various inflammatory and neurotoxic pathways across the nervous system.

AREAS COVERED

This review summarizes the current literature on the pathogenesis and clinical manifestations of neurological injuries associated with HIV in the brain, spinal cord, and peripheral nervous system. Molecular pathways relevant for possible therapeutic targets or advancements are emphasized. Gaps in knowledge and current challenges in therapeutic design are also discussed.

EXPERT OPINION

Several challenges exist in the development of therapeutic targets for HIV-associated cognitive impairments. However, recent developments in drug delivery systems and treatment strategies are encouraging. Treatments for HIV-associated pain and peripheral sensory neuropathies currently consist of symptomatic management, but a greater understanding of their pathogenesis can lead to the development of targeted molecular therapies and disease-modifying therapies. HIV-associated autonomic dysfunction may affect the course of systemic disease via disrupted neuro-immune interactions; however, more research is needed to facilitate our understanding of how these processes present clinically.

Molecular and cellular mechanisms of itch and pain in atopic dermatitis and implications for novel therapeutics

Atopic dermatitis is a chronic inflammatory skin disease. Patients with atopic dermatitis experience inflammatory lesions associated with intense itch and pain, which lead to sleep disturbance and poor mental health and quality of life. We review the molecular mechanisms underlying itch and pain symptoms in atopic dermatitis and discuss the current clinical development of treatments for moderate-to-severe atopic dermatitis. The molecular pathology of atopic dermatitis includes aberrant immune activation involving significant cross-talk among the skin and immune and neuronal cells. Exogenous and endogenous triggers modulate stimulation of mediators including cytokine/chemokine expression/release by the skin and immune cells, which causes inflammation, skin barrier disruption, activation and growth of sensory neurons, itch and pain. These complex interactions among cell types are mediated primarily by cytokines, but also involve chemokines, neurotransmitters, lipids, proteases, antimicrobial peptides, agonists of ion channels or various G protein-coupled receptors. Patients with atopic dermatitis have a cytokine profile characterised by abnormal levels of interleukins 4, 12, 13, 18, 22, 31 and 33; thymic stromal lymphopoietin; and interferon gamma. Cytokine receptors mainly signal through the Janus kinase/signal transducer and activator of transcription pathway. Among emerging novel therapeutics, several Janus kinase inhibitors are being developed for topical or systemic treatment of moderate-to-severe atopic dermatitis because of their potential to modulate cytokine expression and release. Janus kinase inhibitors lead to changes in gene expression that have favourable effects on local and systemic cytokine release, and probably other mediators, thus successfully modulating molecular mechanisms responsible for itch and pain in atopic dermatitis.

Antiallodynic effects of KDS2010, a novel MAO-B inhibitor, via ROS-GABA inhibitory transmission in a paclitaxel-induced tactile hypersensitivity model

Monoamine oxidase (MAO) inhibitors have been investigated for the treatment of neuropathic pain. Here, we assessed the antiallodynic effects of a novel MAO-B inhibitor, KDS2010, on paclitaxel (PTX)-induced mechanical hypersensitivity. Oral administration of KDS2010 effectively relieved PTX-induced mechanical hypersensitivity in a dose-dependent manner. KDS2010 (25 mg/Kg) significantly prevented and suppressed PTX-induced pain responses with minimal effects on the body weight, motor activity, and working memory. KDS2010 significantly reduced reactive astrocytosis and reactive oxygen species (ROS) level in the L4–L6 spinal cord of PTX-treated mice. Furthermore, KDS2010 reversed the attenuation of GABAergic spontaneous inhibitory postsynaptic current (sIPSC) frequency in spinal dorsal horn neurons, although it failed to restore the reduced tonic GABA_A inhibition nor the increased GABA transporter 1 (GAT1) expression in PTX-treated mice. In addition, bath application of a reactive oxygen species (ROS) scavenger (PBN) restored the sIPSC frequency in PTX-treated mice but not in control and PTX + KDS2010-treated mice. These results indicated that the antiallodynic effect of KDS2010 is not due to a MAO-B-dependent GABA production. Finally, PBN alone also exerted a similar analgesic effect as KDS2010, but a co-treatment of PBN with KDS2010 showed no additive effect, suggesting that inhibition of MAO-B-dependent ROS production is responsible for the analgesic effect by KDS2010 on PTX-induced allodynia. Overall, KDS2010 attenuated PTX-induced pain behaviors by restoring the altered ROS level and GABAergic inhibitory signaling in the spinal cord, suggesting that KDS2010 is a promising therapeutic strategy for chemotherapy-induced peripheral neuropathy.

The consequence of endothelial remodelling on the blood spinal cord barrier and nociception

Nociception is a fundamental acute protective mechanism that prevents harm to an organism. Understanding the integral processes that control nociceptive processing are fundamental to our appreciation of which cellular and molecular features underlie this process. There is an extensive understanding of how sensory neurons interpret differing sensory modalities and intensities. However, it is widely appreciated that the sensory neurons do not act alone. These work in harmony with inflammatory and vascular systems to modulate pain perception. The spinal cord has an extensive interaction with the capillary network in the form of a blood spinal cord barrier to ensure homeostatic control of the spinal cord neuron milieu. However, there is an extensive appreciation that disturbances in the blood spinal cord barrier contribute to the onset of chronic pain. Enhanced vascular permeability and impaired blood perfusion have both been highlighted as contributors to chronic pain manifestation. Here, we discuss the evidence that demonstrates alterations in the blood spinal cord barrier influences nociceptive processing and perception of pain.

P2X7 receptor induces microglia polarization to the M1 phenotype in cancer-induced bone pain rat models

Background

The transition from pro-inflammatory M1 phenotype to anti-inflammatory M2 phenotype presents a novel therapeutic strategy for chronic pain.

Objective

We investigated the role of microglia polarization in cancer-induced bone pain (CIBP), as well as the role of the P2X7 receptor in modulating M1 to M2 polarization.

Methods

Walker-256 breast cancer cells were administered into tibias of female rats to induce bone cancer–associated cancer.

Results

During bone cancer development, the P2X7 receptor and M1 microglia markers were upregulated. In contrast, inhibition of the P2X7 receptor by BBG, a blood-brain barrier-permeable P2X7R-specific antagonist, alleviated the pain and promoted microglia polarization toward the M2 phenotype, while suppressing the M1 phenotype in vivo and in vitro.

Conclusion

P2X7 receptor-mediated spinal microglia polarization is involved in alleviation of CIBP. Therefore, P2X7R is a potential option for CIBP treatment.

Ror2 mediates chronic post-thoracotomy pain by inducing the transformation of A1/A2 reactive astrocytes in rats

Ror2 plays an important role in neuronal development, neuronal plasticity, and neuropathic pain. In our previous pilot study, we found that Ror2 and GFAP (a marker of astrocytes) protein levels increased in thoracic dorsal root ganglia from postoperative day (POD) 7 to POD 21 in rats with chronic post-thoracotomy pain (CPTP). In the present study, we aimed to further explore the roles of Ror2 and activated astrocytes during CPTP development. Ror2, c-JUN, and C3aR levels increased and the activated astrocytes were mainly expressed as the A1 phenotype in the spinal cord dorsal horn of the rats with CPTP. The knockdown of Ror2 in the spinal cord astrocytes alleviated thoracotomy-induced mechanical hyperalgesia and cold allodynia as well as reverted the A1/A2 ratio of the reactive astrocytes, downregulating the expression of c-JUN and C3aR in rats with CPTP. These results suggest that Ror2 in the spinal cord astrocytes mediates the transformation of A1/A2 reactive astrocytes via regulating the expressions of the c-JUN and C3aR in CPTP. Furthermore, the suppression of Ror2 could be utilized as a new strategy to help prevent CPTP.

Role of Matrix Metalloproteinases in Myelin Abnormalities and Mechanical Allodynia in Rodents with Diabetic Neuropathy

The treatment of diabetic neuropathic pain (DNP) is a major clinical challenge. The underlying mechanisms of diabetic neuropathy remain unclear, and treatment approaches are limited. Here, we report that the gelatinases MMP-9 and MMP-2 play a critical role in axonal demyelination and DNP in rodents. MMP-9 may contribute to streptozotocin (STZ)-induced DNP via inducing axonal demyelination and spinal central sensitization, while MMP-2 may serve as a negative regulator. In STZ-induced DNP rats, the activity of MMP-9 was increased, while MMP-2 was decreased in the dorsal root ganglion and spinal cord. Spinal inhibition of MMP-9, but not MMP-2, greatly suppressed the behavioral and neurochemical signs of DNP, while administration of MMP-2 alleviated mechanical allodynia. In mice, STZ treatment resulted in axonal demyelination in the peripheral sciatic nerves and spinal dorsal horn, in addition to mechanical allodynia. These neuropathic alterations were significantly reduced in MMP-9^-/- mice. Finally, systematic administration of α-lipoic acid significantly suppressed STZ-induced mechanical allodynia by inhibiting MMP-9 and rescuing MMP-2 activity. These findings support a new mechanism underlying the pathogenesis of diabetic neuropathy and suggest a potential target for DNP treatment. Gelatinases MMP-9 and MMP-2 play a critical role in the pathogenesis of diabetic neuropathy and may serve as a potential treatment target. MMP-9/2 underlies the mechanism of α-lipoic acid in diabetic neuropathy, providing a potential target for the development of novel analgesic and anti-inflammatory drugs.

Application of PLGA nanoparticles to enhance the action of duloxetine on microglia in neuropathic pain

Duloxetine (DLX) is a selective serotonin and noradrenaline reuptake inhibitor (SNRI) used for the treatment of pain, but it has been reported to show side effects in 10-20% of patients. Its analgesic efficacy in central pain is putatively related to its influence on descending inhibitory neuronal pathways. However, DLX can also affect the activation of microglia. This study was performed to investigate whether PLGA nanoparticles (NPs), which are expected to enhance targeting to microglia, can improve the analgesic efficacy and limit the side effects of DLX. PLGA NPs encapsulating a low dose of DLX (DLX NPs) were synthesized and characterized and their localization was determined. The analgesic and anti-inflammatory effects of DLX NPs were evaluated in a spinal nerve ligation (SNL)-induced neuropathic pain model. The analgesic effect of DLX lasted for only a few hours and disappeared within 1 day. However, DLX NPs alleviated mechanical allodynia, and the effect was maintained for 1 week. DLX NPs were localized to the spinal microglia and suppressed microglial activation, phosphorylation of p38/NF-κB-mediated pathways and the production of inflammatory cytokines in the spinal dorsal horn of SNL rats. We demonstrated that DLX NPs can provide a prolonged analgesic effect by enhanced targeting of microglia. Our observations imply that DLX delivery through nanoparticle encapsulation allows drug repositioning with a prolonged analgesic effect, and reduces the potential side effects of abuse and overdose.

Fragile X mental retardation protein-regulated proinflammatory cytokine expression in the spinal cord contributes to the pathogenesis of inflammatory pain induced by complete Freund's adjuvant

Studies have verified that Fragile X mental retardation protein (FMRP), an RNA-binding protein, plays a potential role in the pathogenesis of formalin- and (RS)-3,5-dihydroxyphenylglycine (DHPG)-induced abnormal pain sensations. However, the role of FMRP in inflammatory pain has not been reported. Here, we showed an increase in FMRP expression in the spinal dorsal horn (SDH) in a rat model of inflammatory pain induced by complete Freund's adjuvant (CFA). Double immunofluorescence staining revealed that FMRP was mainly expressed in spinal neurons and colocalized with proinflammatory cytokines [tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6)]. After consecutive intrathecal injection of fragile X mental retardation 1 (Fmr1) small interfering RNA (siRNA) for 3 days post-CFA injection, FMRP expression in the SDH was reduced, and CFA-induced hyperalgesia was decreased. In addition, the CFA-induced increase in spinal TNF-α and IL-6 production was significantly suppressed by intrathecal administration of Fmr1 siRNA. Together, these results suggest that FMRP regulates TNF-α and IL-6 levels in the SDH and plays an important role in inflammatory pain.

Astrocytic c-Jun N-terminal kinase-histone deacetylase-2 cascade contributes to glutamate transporter-1 decrease and mechanical allodynia following peripheral nerve injury in rats

Decrease of glutamate transporter-1 (GLT-1) in the spinal dorsal horn after nerve injury induces enhanced excitatory transmission and causes persistent pain. Histone deacetylases (HDACs)-catalyzed deacetylation might contribute to the decrease of GLT-1, while the detailed mechanisms have yet to be fully elaborated. Spinal nerve ligation (SNL) induced significant increases of HDAC2 and decreases of GLT-1 in spinal astrocytes. Intrathecal infusion of the HDAC2 inhibitors attenuated the decrease of GLT-1 and enhanced phosphorylation of glutamate receptors. GLT-1 and phosphorylated c-Jun N-terminal kinase (JNK) were highly colocalized in the spinal cord, and a large number of pJNK positive cells were HDAC2 positive. Intrathecally infusion of the JNK inhibitor SP600125 significantly inhibited SNL-induced upregulation of HDAC2. SNL-induced HDAC2 up-regulation could be inhibited by the neutralizing anti-tumor necrosis factor-α (TNF-α) binding protein etanercept or the microglial inhibitor minocycline. In cultured astrocytes, TNF-α induced enhanced phosphorylation of JNK and a significant increase of HDAC2, as well as a remarkable decrease of GLT-1, which could be prevented by SP600125 or the HDAC2 specific inhibitor CAY10683. Our data suggest that astrocytic JNK-HDAC2 cascade contributes to GLT-1 decrease and mechanical allodynia following peripheral nerve injury. Neuroimmune activation after peripheral nerve injury could induce epigenetic modification changes in astrocytes and contribute to chronic pain maintenance.

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.

Neuropathic pain: Spotlighting anatomy, experimental models, mechanisms, and therapeutic aspects

The International Association for the Study of Pain defines neuropathic pain as "pain arising as a direct consequence of a lesion or disease affecting the somatosensory system". The associated changes can be observed in the peripheral as well as the central nervous system. The available literature discusses a wide variety of causes as predisposing for the development and amplification of neuropathic pain. Further, key interactions within sensory pathways have been discovered, but no common molecular mechanism leading to neuropathic pain has been identified until now. In the first part of this review, the pain mediating lateral spinothalamic tract is described. Different in vivo models are presented that allow studying trauma-, chemotherapy-, virus-, and diabetes-induced neuropathic pain in rodents. We furthermore discuss approaches to assess neuropathic pain in these models. Second, the current knowledge about cellular and molecular mechanisms suggested to underlie the development of neuropathic pain is presented and discussed. A summary of established therapies that are already applied in the clinic and novel, promising approaches closes the paper. In conclusion, the established animal models are able to emulate the diversity of neuropathic pain observed in the clinics. However, the assessment of neuropathic pain in the presented in vivo models should be improved. The determination of common molecular markers with suitable in vitro models would simplify the assessment of neuropathic pain in vivo. This would furthermore provide insights into common molecular mechanisms of the disease and establish a basis to search for satisfying therapeutic approaches.

Neuroinflammation in HIV-Related Neuropathic Pain

In the management of human immunodeficiency virus (HIV) infection around the world, chronic complications are becoming a new problem along with the prolonged life expectancy. Chronic pain is widespread in HIV infected patients and even affects those with a low viral load undergoing long-term treatment with antiviral drugs, negatively influencing the adherence to disease management and quality of life. A large proportion of chronic pain is neuropathic pain, which defined as chronic pain caused by nervous system lesions or diseases, presenting a series of nervous system symptoms including both positive and negative signs. Injury caused by HIV protein, central and peripheral sensitization, and side effects of antiretroviral therapy lead to neuroinflammation, which is regarded as a maladaptive mechanism originally serving to promote regeneration and healing, constituting the main mechanism of HIV-related neuropathic pain. Gp120, as HIV envelope protein, has been found to be the major toxin that induces neuropathic pain. Particularly, the microglia, releasing numerous pro-inflammatory substances (such as TNFα, IL-1β, and IL-6), not only sensitize the neurons but also are the center part of the crosstalk bridging the astrocytes and oligodendrocytes together forming the central sensitization during HIV infection, which is not discussed detailly in recent reviews. In the meantime, some NRTIs and PIs exacerbate the neuroinflammation response. In this review, we highlight the importance of clarifying the mechanism of HIV-related neuropathic pain, and discuss about the limitation of the related studies as future research directions.

NFAT5 Deficiency Alleviates Formalin-Induced Inflammatory Pain Through mTOR

Nuclear factor of activated T cells (NFAT5) is a well-known transcription factor that regulates the expression of genes involved in osmotic stress. However, the role of NFAT5 in inflammatory pain remains unknown. Here, we studied the function of NFAT5 in inflammatory pain using NFAT5-heterozygous (Het) mice. To study inflammatory pain, we injected 10 µL of 2% formalin into the right hind paws of mice and monitored pain behaviors, such as licking, lifting, and flinching, for 60 min. After the first 15 min (phase I), there were no significant differences in pain behaviors between wild-type (WT) and NFAT5-Het mice. However, from 15–60 min (phase II), NFAT5-Het mice displayed significantly fewer pain behaviors compared to WT mice. Further, the expression levels of inflammatory-pain-related factors, including c-Fos, phosphorylated extracellular signal-regulated kinase (p-ERK), and phosphorylated n-methyl-D-aspartate receptor subunit 2B (p-NR2B), were significantly elevated in the spinal dorsal neurons of formalin-treated WT mice but was not elevated in NFAT5-Het mice. Similarly, c-Fos, p-ERK, and p-NR2B levels were significantly higher in glutamate-treated PC12 neuronal cells but were not affected by Nfat5 silencing in glutamate-treated PC12 cells. Altogether, our findings suggest that NFAT5 deficiency may mitigate formalin-induced inflammatory pain by upregulating mammalian target of rapamycin (mTOR) expression and downregulating its downstream factors in spinal dorsal neurons. Therefore, NFAT5 is a potential therapeutic target for the treatment of inflammatory pain.

TRPM3 Channels Play Roles in Heat Hypersensitivity and Spontaneous Pain after Nerve Injury

Transient receptor potential melastatin 3 (TRPM3) is a heat-activated ion channel in primary sensory neurons of the dorsal root ganglia (DRGs). Pharmacological and genetic studies implicated TRPM3 in various pain modalities, but TRPM3 inhibitors were not validated in TRPM3^-/- mice. Here we tested two inhibitors of TRPM3 in male and female wild-type and TRPM3^-/- mice in nerve injury-induced neuropathic pain. We found that intraperitoneal injection of either isosakuranetin or primidone reduced heat hypersensitivity induced by chronic constriction injury (CCI) of the sciatic nerve in wild-type, but not in TRPM3^-/- mice. Primidone was also effective when injected locally in the hindpaw or intrathecally. Consistently, intrathecal injection of the TRPM3 agonist CIM0216 reduced paw withdrawal latency to radiant heat in wild-type, but not in TRPM3^-/- mice. Intraperitoneal injection of 2 mg/kg, but not 0.5 mg/kg isosakuranetin, inhibited cold and mechanical hypersensitivity in CCI, both in wild-type and TRPM3^-/- mice, indicating a dose-dependent off-target effect. Primidone had no effect on cold sensitivity, and only a marginal effect on mechanical hypersensitivity. Genetic deletion or inhibitors of TRPM3 reduced the increase in the levels of the early genes c-Fos and pERK in the spinal cord and DRGs in CCI mice, suggesting spontaneous activity of the channel. Intraperitoneal isosakuranetin also inhibited spontaneous pain related behavior in CCI in the conditioned place preference assay, and this effect was eliminated in TRPM3^-/- mice. Overall, our data indicate a role of TRPM3 in heat hypersensitivity and in spontaneous pain after nerve injury.SIGNIFICANCE STATEMENT Neuropathic pain is a major unsolved medical problem. The heat-activated TRPM3 ion channel is a potential target for novel pain medications, but the pain modalities in which it plays a role are not clear. Here we used a combination of genetic and pharmacological tools to assess the role of this channel in spontaneous pain, heat, cold, and mechanical hypersensitivity in a nerve injury model of neuropathic pain in mice. Our findings indicate a role for TRPM3 in heat hyperalgesia, and spontaneous pain, but not in cold and mechanical hypersensitivity. We also find that not only TRPM3 located in the peripheral nerve termini, but also TRPM3 in the spinal cord or proximal segments of DRG neurons are important for heat hypersensitivity.

Bromelain reduced pro-inflammatory mediators as a common pathway that mediate antinociceptive and anti-anxiety effects in sciatic nerve ligated Wistar rats

The involvement of pro-inflammatory mediators complicates the complex mechanism in neuropathic pain (NP). This study investigated the roles of bromelain against pro-inflammatory mediators as a mechanism that underpins its antinociceptive and anti-anxiety effects in the peripheral model of NP. Sixty-four male Wistar rats randomly divided into eight groups, were used for the study. A chronic constriction injury model of peripheral neuropathy was used to induce NP. Tail-immersion and von Frey filaments tests were used to assess hyperalgesia while open field and elevated plus mazes were used to assess anxiety-like behaviour. NF-кB, iNOS, nitrate, and pro-inflammatory cytokines were investigated in the plasma, sciatic nerve, and brain tissues using ELISA, spectrophotometer, and immunohistochemistry techniques after twenty-one days of treatment. Bromelain significantly (p < 0.05) improved the cardinal signs of NP and inhibited anxiety-like behaviours in ligated Wistar rats. It mitigated the increases in cerebral cortex interleukin (IL) -1β, IL-6, and PGE₂ levels. Bromelain reduced NF-кB, IL-1β, IL-6, TNF-α, PGE_2, and nitrate concentrations as well as the expression of iNOS in the sciatic nerve. Hence, the antinociceptive and anxiolytic effects of bromelain in the sciatic nerve ligation model of NP is in part due to its ability to reduce nitrosative and inflammatory activities.

Astrocytic Calcium Dynamics Along the Pain Pathway

Astrocytes, once thought to be passive cells merely filling the space between neurons in the nervous system, are receiving attention as active modulators of the brain and spinal cord physiology by providing nutrients, maintaining homeostasis, and modulating synaptic transmission. Accumulating evidence indicates that astrocytes are critically involved in chronic pain regulation. Injury induces astrocytes to become reactive, and recent studies suggest that reactive astrocytes can have either neuroprotective or neurodegenerative effects. While the exact mechanisms underlying the transition from resting astrocytes to reactive astrocytes remain unknown, astrocytic calcium increase, coordinated by inflammatory molecules, has been suggested to trigger this transition. In this mini review article, we will discuss the roles of astrocytic calcium, channels contributing to calcium dynamics in astrocytes, astrocyte activations along the pain pathway, and possible relationships between astrocytic calcium dynamics and chronic pain.

TAP2, a peptide antagonist of Toll-like receptor 4, attenuates pain and cartilage degradation in a monoiodoacetate-induced arthritis rat model

Because inflammation in osteoarthritis (OA) is related to the Toll-like receptor 4 (TLR4) signaling cascades, TLR4 is a reasonable target for developing therapeutics for OA. Thus, we investigated whether TAP2, a peptide antagonist of TLR4, reduces the monoiodoacetate (MIA)-induced arthritic pain and cartilage degradation in rats. TLR4 expression of human OA chondrocytes and synoviocytes and the knee joint tissue of MIA-induced arthritis were evaluated. MIA-induced arthritic model using Sprague–Dawley rats (6 week-old-male) were treated with TAP2, a TLR4 antagonist, and evaluated with behavioral test, immunohistochemistry, and quantitative PCR. TLR4 was highly expressed in the knee joints of patients with OA and the MIA-induced rat model. Further, a single intraarticular injection of TAP2 (25 nmol/rat) molecules targeting TLR4 on day 7 after MIA injection dramatically attenuated pain behavior for about 3 weeks and reduced cartilage loss in the knee joints and microglial activation in the spinal dorsal horns. Likewise, the mRNA levels of TNFα and IL-1β, reactive oxygen species, and the expression of MMP13 in the knee joints of TAP2-treated rats was significantly decreased by TAP2 treatment compared with the control. Moreover, interestingly, the duration of OA pain relief by TAP2 was much longer than that of chemical TLR4 antagonists, such as C34 and M62812. In conclusion, TAP2 could effectively attenuate MIA-induced arthritis in rats by blocking TLR4 and its successive inflammatory cytokines and MMP13. Therefore, TAP2 could be a prospective therapeutic to treat patients with OA.

Microglia-Astrocyte Communication via C1q Contributes to Orofacial Neuropathic Pain Associated with Infraorbital Nerve Injury

Trigeminal nerve injury causes a distinct time window of glial activation in the trigeminal spinal subnucleus caudalis (Vc), which are involved in the initiation and maintenance phases of orofacial neuropathic pain. Microglia-derived factors enable the activation of astrocytes. The complement component C1q, which promotes the activation of astrocytes, is known to be synthesized in microglia. However, it is unclear whether microglia–astrocyte communication via C1q is involved in orofacial neuropathic pain. Here, we analyzed microglia-astrocyte communication in a rat model with infraorbital nerve injury (IONI). The orofacial mechanical hypersensitivity induced by IONI was significantly attenuated by preemptive treatment with minocycline. Immunohistochemical analyses revealed that minocycline inhibited the increase in c-Fos immune-reactive (IR) cells and the fluorescence intensity of both Iba1 and glial fibrillary acidic protein (GFAP) in the Vc following IONI. Intracisternal administration of C1q caused orofacial mechanical hypersensitivity and an increase in the number of c-Fos-IR cells and fluorescence intensity of GFAP. C1q-induced orofacial mechanical hypersensitivity was completely abrogated by intracisternal administration of fluorocitrate. The present findings suggest that the enhancement in the excitability of Vc nociceptive neurons is produced by astrocytic activation via the signaling of C1q released from activated microglia in the Vc following IONI, resulting in persistent orofacial neuropathic pain.

Conditional expression of HIV-1 tat in the mouse alters the onset and progression of tonic, inflammatory and neuropathic hypersensitivity in a sex-dependent manner

BACKGROUND

At least one-third of HIV-1-afflicted individuals experience peripheral neuropathy. Although the underlying mechanisms are not known, they may involve neurotoxic HIV-1 proteins.

METHODS

We assessed the influence of the neurotoxic HIV-1 regulatory protein, Tat, on inflammatory and neuropathic nociceptive behaviours using transgenic male and female mice that conditionally expressed (or did not express) HIV-1 Tat1-86 in fibrillary acidic protein-expressing glia in the central and peripheral nervous systems.

RESULTS

Tat induction significantly attenuated the time spent paw-licking following formalin injection (2.5%, i.pl.) in both male and female mice. However, significant sex differences were observed in the onset and magnitude of inflammation and sensory sensitivity following complete Freund's adjuvant (CFA) injection (10%, i.pl.) after Tat activation. Unlike female mice, male mice showed a significant attenuation of paw swelling and an absence of mechanical/thermal hypersensitivity in response to CFA after Tat induction. Male Tat(+) mice also showed accelerated recovery from chronic constrictive nerve injury (CCI)-induced neuropathic mechanical and thermal hypersensitivity compared to female Tat(+) mice. Morphine (3.2 mg/kg) fully reversed CCI-induced mechanical hypersensitivity in female Tat(-) mice, but not in Tat(+) females.

CONCLUSIONS

The ability of Tat to decrease oedema, paw swelling, and limit allodynia suggests a sequel of events in which Tat-induced functional deficits precede the onset of mechanical hypersensitivity. Moreover, HIV-1 Tat attenuated responses to inflammatory and neuropathic insults in a sex-dependent manner. HIV-1 Tat appears to directly contribute to HIV sensory neuropathy and reveals sex differences in HIV responsiveness and/or the underlying peripheral neuroinflammatory and nociceptive mechanisms.

Physiology and Pathophysiology of Itch

Itch is a topic to which everyone can relate. The physiological roles of itch are increasingly understood and appreciated. The pathophysiological consequences of itch impact quality of life as much as pain. These dynamics have led to increasingly deep dives into the mechanisms that underlie and contribute to the sensation of itch. When the prior review on the physiology of itching was published in this journal in 1941, itch was a black box of interest to a small number of neuroscientists and dermatologists. Itch is now appreciated as a complex and colorful Rubik's cube. Acute and chronic itch are being carefully scratched apart and reassembled by puzzle solvers across the biomedical spectrum. New mediators are being identified. Mechanisms blur boundaries of the circuitry that blend neuroscience and immunology. Measures involve psychophysics and behavioral psychology. The efforts associated with these approaches are positively impacting the care of itchy patients. There is now the potential to markedly alleviate chronic itch, a condition that does not end life, but often ruins it. We review the itch field and provide a current understanding of the pathophysiology of itch. Itch is a disease, not only a symptom of disease.

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.

Paclitaxel-activated astrocytes produce mechanical allodynia in mice by releasing tumor necrosis factor-α and stromal-derived cell factor 1

BACKGROUND

Paclitaxel is a widely used and potent chemotherapeutic agent for the treatment of cancer. However, patients receiving paclitaxel often develop an acute pain syndrome for which there are few treatment options. Astrocytes play an important role in the pathogenesis of pain in multiple preclinical models, as well as in paclitaxel-treated rodents. However, it is still unclear what the exact contribution of astrocytes may be in paclitaxel-associated acute pain syndrome (P-APS).

METHODS

P-APS was modeled by a single systemic or intrathecal injection of paclitaxel and astrocyte contribution tested by immunohistochemical, pharmacological, and behavioral approaches. Cell cultures were also prepared to assess whether paclitaxel treatment directly activates astrocytes and whether intrathecal injection of paclitaxel-treated astrocytes produces pain that is reminiscent of P-APS.

RESULTS

Systemic injection of paclitaxel resulted in increased expression of glial fibrillary acidic protein (a common marker of astrocytic activation), as well as both systemic or intrathecal injection of paclitaxel induced pain hypersensitivity indicated by the development of mechanical allodynia, which was significantly reversed by the astrocytic inhibitor L-α-AA. Cultured astrocytes were activated by paclitaxel with significant increases in protein levels for tumor necrosis factor-α (TNF-α) and stromal-derived cell factor 1 (SDF-1). Importantly, intrathecal injection of paclitaxel-activated astrocytes produced mechanical allodynia that was reversed by TNF-α and SDF-1 neutralizing antibodies.

CONCLUSION

Our results suggest for the first time that paclitaxel can directly activate astrocytes, which are sufficient to produce acute pain by releasing TNF-α and SDF-1. Targeting astrocytes and these cytokines may offer new treatments for P-APS.

Pathological pain processing in mouse models of multiple sclerosis and spinal cord injury: contribution of plasma membrane calcium ATPase 2 (PMCA2)

Background

Neuropathic pain is often observed in individuals with multiple sclerosis (MS) and spinal cord injury (SCI) and is not adequately alleviated by current pharmacotherapies. A better understanding of underlying mechanisms could facilitate the discovery of novel targets for therapeutic interventions. We previously reported that decreased plasma membrane calcium ATPase 2 (PMCA2) expression in the dorsal horn (DH) of healthy PMCA2^+/− mice is paralleled by increased sensitivity to evoked nociceptive pain. These studies suggested that PMCA2, a calcium extrusion pump expressed in spinal cord neurons, plays a role in pain mechanisms. However, the contribution of PMCA2 to neuropathic pain processing remains undefined. The present studies investigated the role of PMCA2 in neuropathic pain processing in the DH of wild-type mice affected by experimental autoimmune encephalomyelitis (EAE), an animal model of MS, and following SCI.

Methods

EAE was induced in female and male C57Bl/6N mice via inoculation with myelin oligodendrocyte glycoprotein fragment 35–55 (MOG_35–55) emulsified in Complete Freund’s Adjuvant (CFA). CFA-inoculated mice were used as controls. A severe SC contusion injury was induced at thoracic (T8) level in female C57Bl/6N mice. Pain was evaluated by the Hargreaves and von Frey filament tests. PMCA2 levels in the lumbar DH were analyzed by Western blotting. The effectors that decrease PMCA2 expression were identified in SC neuronal cultures.

Results

Increased pain in EAE and SCI was paralleled by a significant decrease in PMCA2 levels in the DH. In contrast, PMCA2 levels remained unaltered in the DH of mice with EAE that manifested motor deficits but not increased pain. Interleukin-1β (IL-1β), tumor necrosis factor α (TNFα), and IL-6 expression were robustly increased in the DH of mice with EAE manifesting pain, whereas these cytokines showed a modest increase or no change in mice with EAE in the absence of pain. Only IL-1β decreased PMCA2 levels in pure SC neuronal cultures through direct actions.

Conclusions

PMCA2 is a contributor to neuropathic pain mechanisms in the DH. A decrease in PMCA2 in DH neurons is paralleled by increased pain sensitivity, most likely through perturbations in calcium signaling. Interleukin-1β is one of the effectors that downregulates PMCA2 by acting directly on neurons.

Evidence for a dynorphin-mediated inner ear immune/inflammatory response and glutamate-induced neural excitotoxicity: an updated analysis

Acoustic overstimulation (AOS) is defined as the stressful overexposure to high-intensity sounds. AOS is a precipitating factor that leads to a glutamate (GLU)-induced Type I auditory neural excitotoxicity and an activation of an immune/inflammatory/oxidative stress response within the inner ear, often resulting in cochlear hearing loss. The dendrites of the Type I auditory neural neurons that innervate the inner hair cells (IHCs), and respond to the IHC release of the excitatory neurotransmitter GLU, are themselves directly innervated by the dynorphin (DYN)-bearing axon terminals of the descending brain stem lateral olivocochlear (LOC) system. DYNs are known to increase GLU availability, potentiate GLU excitotoxicity, and induce superoxide production. DYNs also increase the production of proinflammatory cytokines by modulating immune/inflammatory signal transduction pathways. Evidence is provided supporting the possibility that the GLU-mediated Type I auditory neural dendritic swelling, inflammation, excitotoxicity, and cochlear hearing loss that follow AOS may be part of a brain stem-activated, DYN-mediated cascade of inflammatory events subsequent to a LOC release of DYNs into the cochlea. In support of a DYN-mediated cascade of events are established investigations linking DYNs to the immune/inflammatory/excitotoxic response in other neural systems.

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.

Analgesic Effect of Toll-like Receptor 4 Antagonistic Peptide 2 on Mechanical Allodynia Induced with Spinal Nerve Ligation in Rats

Neuroinflammation is one of the key mechanisms of neuropathic pain, which is primarily mediated by the Toll-like receptor 4 (TLR4) signaling pathways in microglia. Therefore, TLR4 may be a reasonable target for treatment of neuropathic pain. Here, we examined the analgesic effect of TLR4 antagonistic peptide 2 (TAP2) on neuropathic pain induced by spinal nerve ligation in rats. When lipopolysaccharide (LPS)-stimulated BV2 microglia cells were treated with TAP2 (10 µM), the mRNA levels of proinflammatory mediators, such as tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, cyclooxygenase (COX)-2, and inducible nitric oxide synthase (iNOS), were markedly decreased by 54-83% as determined by quantitative PCR (qPCR) analysis. Furthermore, when TAP2 (25 nmol in 20 µL PBS) was intrathecally administered to the spinal nerve ligation-induced rats on day 3 after surgery, the mechanical allodynia was markedly decreased for approximately 2 weeks in von Frey filament tests, with a reduction in microglial activation. On immunohistochemical and qPCR analyses, both the level of reactive oxygen species and the gene expression of the proinflammatory mediators, such as TNF-α, IL-1β, IL-6, COX-2, and iNOS, were significantly decreased in the ipsilateral spinal dorsal horn. Finally, the analgesic effect of TAP2 was reproduced in rats with monoiodoacetate-induced osteoarthritic pain. The findings of the present study suggest that TAP2 efficiently mitigates neuropathic pain behavior by suppressing microglial activation, followed by downregulation of neuropathic pain-related factors, such as reactive oxygen species and proinflammatory molecules. Therefore, it may be useful as a new analgesic for treatment of neuropathic pain.

Spinal IL-33/ST2 signaling mediates chronic itch in mice through the astrocytic JAK2-STAT3 cascade

Interleukin-33 (IL-33) and its receptor ST2 contribute to spinal glial activation and chronic pain. A recent study showed that peripheral IL-33 plays a pivotal role in the pathogenesis of chronic itch induced by poison ivy. However, how IL-33/ST2 signaling in the spinal cord potentially mediates chronic itch remains elusive. Here, we determined that St2^-/- substantially reduced scratching behaviors in 2,4-dinitrofluorobenzene (DNFB)-induced allergic contact dermatitis (ACD) as well as acetone and diethylether followed by water-induced dry skin in mice. Intrathecal administration of the neutralizing anti-ST2 or anti-IL-33 antibody remarkably decreased the scratching response in DNFB-induced ACD mice. Expression of spinal IL-33 and ST2 significantly increased in ACD mice, as evidenced by increased mRNA and protein levels. Immunofluorescence and in situ hybridization demonstrated that increased expression of spinal IL-33 was predominant in oligodendrocytes and astrocytes, whereas ST2 was mainly expressed in astrocytes. Further studies showed that in ACD mice, the activation of astrocytes and increased phosphorylation of signal transducer and activator of transcription 3 (STAT3) were markedly attenuated by St2^-/- . Intrathecal injection of Janus Kinase 2 Inhibitor AG490 significantly alleviated scratching behaviors in ACD mice. rIL-33 pretreatment exacerbated gastrin-releasing peptide (GRP)-evoked scratching behaviors. This increased gastrin-releasing peptide receptor (GRPR) expression was abolished by St2^-/- . Tnf-α upregulation was suppressed by St2^-/- . Our results indicate that the spinal IL-33/ST2 signaling pathway contributes to chronic itch via astrocytic JAK2-STAT3 cascade activation, promoting TNF-α release to regulate the GRP/GRPR signaling-related itch response. Thus, these findings provide a potential therapeutic option for treating chronic pruritus.

Enteric Glia: A New Player in Abdominal Pain

Chronic abdominal pain is the most common gastrointestinal issue and contributes to the pathophysiology of functional bowel disorders and inflammatory bowel disease. Current theories suggest that neuronal plasticity and broad alterations along the brain-gut axis contribute to the development of chronic abdominal pain, but the specific mechanisms involved in chronic abdominal pain remain incompletely understood. Accumulating evidence implicates glial cells in the development and maintenance of chronic pain. Astrocytes and microglia in the central nervous system and satellite glia in dorsal root ganglia contribute to chronic pain states through reactive gliosis, the modification of glial networks, and the synthesis and release of neuromodulators. In addition, new data suggest that enteric glia, a unique type of peripheral glia found within the enteric nervous system, have the potential to modify visceral perception through interactions with neurons and immune cells. Understanding these emerging roles of enteric glia is important to fully understand the mechanisms that drive chronic pain and to identify novel therapeutic targets. In this review, we discuss enteric glial cell signaling mechanisms that have the potential to influence chronic abdominal pain.

Sustained stimulation of β2- and β3-adrenergic receptors leads to persistent functional pain and neuroinflammation

Functional pain syndromes, such as fibromyalgia and temporomandibular disorder, are associated with enhanced catecholamine tone and decreased levels of catechol-O-methyltransferase (COMT; an enzyme that metabolizes catecholamines). Consistent with clinical syndromes, our lab has shown that sustained 14-day delivery of the COMT inhibitor OR486 in rodents results in pain at multiple body sites and pain-related volitional behaviors. The onset of COMT-dependent functional pain is mediated by peripheral β2- and β3-adrenergic receptors (β2- and β3ARs) through the release of the pro-inflammatory cytokines tumor necrosis factor α (TNFα), interleukin-1β (IL-1β), and interleukin-6 (IL-6). Here, we first sought to investigate the role of β2- and β3ARs and downstream mediators in the maintenance of persistent functional pain. We then aimed to characterize the resulting persistent inflammation in neural tissues (neuroinflammation), characterized by activated glial cells and phosphorylation of the mitogen-activated protein kinases (MAPKs) p38 and extracellular signal-regulated kinase (ERK). Separate groups of rats were implanted with subcutaneous osmotic mini-pumps to deliver OR486 (15 mg/kg/day) or vehicle for 14 days. The β2AR antagonist ICI118551 and β3AR antagonist SR59230A were co-administrated subcutaneously with OR486 or vehicle either on day 0 or day 7. The TNFα inhibitor Etanercept, the p38 inhibitor SB203580, or the ERK inhibitor U0126 were delivered intrathecally following OR486 cessation on day 14. Behavioral responses, pro-inflammatory cytokine levels, glial cell activation, and MAPK phosphorylation were measured over the course of 35 days. Our results demonstrate that systemic delivery of OR486 leads to mechanical hypersensitivity that persists for at least 3 weeks after OR486 cessation. Corresponding increases in spinal TNFα, IL-1β, and IL-6 levels, microglia and astrocyte activation, and neuronal p38 and ERK phosphorylation were observed on days 14-35. Persistent functional pain was alleviated by systemic delivery of ICI118551 and SR59230A beginning on day 0, but not day 7, and by spinal delivery of Etanercept or SB203580 beginning on day 14. These results suggest that peripheral β2- and β3ARs drive persistent COMT-dependent functional pain via increased activation of immune cells and production of pro-inflammatory cytokines, which promote neuroinflammation and nociceptor activation. Thus, therapies that resolve neuroinflammation may prove useful in the management of functional pain syndromes.

A role for pericytes in chronic pain?

PURPOSE OF REVIEW

The importance of the blood-brain barrier (BBB) and neuroinflammation in neurodegenerative conditions is becoming increasingly apparent, yet very little is known about these neurovascular functions in nonmalignant disease chronic pain. Neural tissue pericytes play critical roles in the formation and maintenance of the BBB. Herein, we review the important roles of neural pericytes and address their potential role in chronic pain.

RECENT FINDINGS

Pericytes are implicated in the function of neural microvasculature, including BBB permeability, neuroimmune factor secretion and leukocyte transmigration. In addition, the multipotent stem cell nature of pericytes affords pericytes the ability to migrate into neural parenchyma and differentiate into pain-associated cell types. These recent findings indicate that pericytes are key players in pathological BBB disruption and neuroinflammation, and as such pericytes may be key players in chronic pain states.

SUMMARY

Pericytes play key roles in pathological processes associated with chronic pain. We propose that pericytes may be a therapeutic target for painful diseases that have associated neural vascular dysfunction. Given the paucity of new pharmacotherapies for chronic pain conditions, we hope that this review inspires researchers to unearth the potential role(s) of pericytes in chronic pain sowing the seeds for future new chronic pain therapies.

Curcumin Inhibits Monocyte Chemoattractant Protein-1 Expression in TNF-α induced Astrocytes Through AMPK Pathway

Curcumin, a phenolic pigment, plays an inhibitory role in astrocytes activation, a key step for neuropathic pain (NP). The present study aimed to investigate the mechanism behind the therapeutic effect of Curcumin on NP in vitro. Specifically, we investigated the inhibitory effect of Curcumin on tumor necrosis factor-α (TNF-α)-induced astrocyte migration. We also studied the effects of Curcumin on monocyte chemoattractant protein-1(MCP-1) expression and activity, as well as super oxide dismutase-2 (SOD2) expression and activity in TNF-α-induced astrocytes. Additionally, we investigated whether the adenosine-monophosphate-activated protein kinase signaling (AMPK) pathway was involved in this process. Our data demonstrated that Curcumin inhibited TNF-α-induced astrocytes migration, decreased MCP-1 expression, and up-regulated SOD2 expression in TNF-α-induced astrocytes in vitro. Our study also indicated that this process was mediated through the AMPK signaling pathway, as addition of Curcumin significantly increased the level of phosphorylated AMPK protein. Furthermore, the specific AMPK activator AICAR (5-aminoimidazole-4-carboxamide 1-D-ribofuranoside) mimicked the effects of Curcumin, whereas a selective AMPK inhibitor Compound C (also called dorsomorphin) partially blocked its function. These results could shed light on understanding of the molecular basis for the inhibition of Curcumin on MCP-1 expression during the process of astrocyte activation, and provide a molecular mechanism for using Curcumin in neuropathic pain.

Reversible Induction of Pain Hypersensitivity following Optogenetic Stimulation of Spinal Astrocytes

While glial activation is an integral part of pain pathogenesis, the existence of a causal relationship between glia and pain processing has yet to be demonstrated in vivo. Here, we have investigated whether the activation of spinal astrocytes could directly evoke pain hypersensitivity in vivo via the use of optogenetic techniques. Optogenetic stimulation of channelrhopdopsin-2 (ChR)-expressing spinal astrocytes induced pain hypersensitivity in a reversible and time-dependent manner, which was accompanied by glial activation, NR1 phosphorylation, ATP release, and the production of proalgesic mediators. Photostimulation of ChR2-expressing astrocytes in culture and spinal slices recapitulated in vivo findings, demonstrating the release of proalgesic mediators and electrophysiological disinhibition of spinal projection neurons. These findings deepen our understanding of the role of astrocytes in pain pathogenesis and provide the scientific basis for an astrocyte-oriented pain treatment.

Laser acupuncture-induced analgesic effect and molecular alterations in an incision pain model: a comparison with electroacupuncture-induced effects

Low-level laser acupuncture (LLLA) produces photobiomodulation through acupuncture point and is an alternative to low-level laser therapy. Although the analgesic effect of LLLA on chronic pain has been proven, its effect on acute postincisional pain has yet to be investigated. A plantar incision (PI) model was used to mimic human postsurgical pain. Male adult rats received GaAlAs laser irradiation at the right ST36 acupoint immediately after operation and on the following 4 days. Three laser treatment groups (two red laser groups with a 30- or 15-min treatment duration and one 30-min near-infrared laser group) were compared with sham LLLA and naive groups and an electroacupuncture (EA) group (separate study). Behavioral withdrawal thresholds of both hind paws were measured before and after incision. Expression of mitogen-activated protein kinases (p-ERK and p-p38), inducible nitric oxide synthase (iNOS), and tumor necrosis factor (TNF) in the spinal cord was analyzed. All three LLLA treatments attenuated post-PI tactile allodynia in the ipsilateral paw, but only the 30-min red laser treatment affected the contralateral paw and had similar efficacy to that of EA. All laser treatments barely reduced heat hyperalgesia in both hind paws. At 3 days after PI, the 30-min red laser group showed reversed increases of PI-induced p-ERK, p-p38, and iNOS but not TNF expression in the spinal cord. Repetitive LLLA treatments ameliorated PI-induced mechanical pain. The inhibition of multiple sensitization signals highlights the unique clinical role of LLLA. Thus, LLLA is an alternative to EA as an adjuvant for postoperative pain control.