Crosstalk between P2Y receptors and cyclooxygenase activity in inflammation and tissue repair

The role of extracellular nucleotides as modulators of inflammation and cell stress is well established. One of the main actions of these molecules is mediated by the activation of purinergic receptors (P2) of the plasma membrane. P2 receptors can be classified according to two different structural families: P2X ionotropic ion channel receptors, and P2Y metabotropic G protein-coupled receptors. During inflammation, damaged cells release nucleotides and purinergic signaling occurs along the temporal pattern of the synthesis of pro-inflammatory and pro-resolving mediators by myeloid and lymphoid cells. In macrophages under pro-inflammatory conditions, the expression and activity of cyclooxygenase 2 significantly increases and enhances the circulating levels of prostaglandin E2 (PGE2), which exerts its effects both through specific plasma membrane receptors (EP1-EP4) and by activation of intracellular targets. Here we review the mechanisms involved in the crosstalk between PGE2 and P2Y receptors on macrophages, which is dependent on several isoforms of protein kinase C and protein kinase D1. Due to this crosstalk, a P2Y-dependent increase in calcium is blunted by PGE2 whereas, under these conditions, macrophages exhibit reduced migratory capacity along with enhanced phagocytosis, which contributes to the modulation of the inflammatory response and tissue repair.

Amyotrophic Lateral Sclerosis and Pain: A Narrative Review from Pain Assessment to Therapy

Amyotrophic lateral sclerosis (ALS) is the most frequent neurodegenerative disease of the motor system that affects upper and lower motor neurons, leading to progressive muscle weakness, spasticity, atrophy, and respiratory failure, with a life expectancy of 2-5 years after symptom onset. In addition to motor symptoms, patients with ALS have a multitude of nonmotor symptoms; in fact, it is currently considered a multisystem disease. The purpose of our narrative review is to evaluate the different types of pain, the correlation between pain and the disease's stages, the pain assessment tools in ALS patients, and the available therapies focusing above all on the benefits of cannabis use. Pain is an underestimated and undertreated symptom that, in the last few years, has received more attention from research because it has a strong impact on the quality of life of these patients. The prevalence of pain is between 15% and 85% of ALS patients, and the studies on the type and intensity of pain are controversial. The absence of pain assessment tools validated in the ALS population and the dissimilar study designs influence the knowledge of ALS pain and consequently the pharmacological therapy. Several studies suggest that ALS is associated with changes in the endocannabinoid system, and the use of cannabis could slow the disease progression due to its neuroprotective action and act on pain, spasticity, cramps, sialorrhea, and depression. Our research has shown high patients' satisfaction with the use of cannabis for the treatment of spasticity and related pain. However, especially due to the ethical problems and the lack of interest of pharmaceutical companies, further studies are needed to ensure the most appropriate care for ALS patients.

ATP-mediated signalling in the central synapses

ATP released from the synaptic terminals and astrocytes can activate neuronal P2 receptors at a variety of locations across the CNS. Although the postsynaptic ATP-mediated signalling does not bring a major contribution into the excitatory transmission, it is instrumental for slow and diffuse modulation of synaptic dynamics and neuronal firing in many CNS areas. Neuronal P2X and P2Y receptors can be activated by ATP released from the synaptic terminals, astrocytes and microglia and thereby can participate in the regulation of synaptic homeostasis and plasticity. There is growing evidence of importance of purinergic regulation of synaptic transmission in different physiological and pathological contexts. Here, we review the main mechanisms underlying the complexity and diversity of purinergic signalling and purinergic modulation in central neurons.

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.

The Roles of Imaging Biomarkers in the Management of Chronic Neuropathic Pain

Chronic neuropathic pain (CNP) affects around 10% of the general population and has a significant social, emotional, and economic impact. Current diagnosis techniques rely mainly on patient-reported outcomes and symptoms, which leads to significant diagnostic heterogeneity and subsequent challenges in management and assessment of outcomes. As such, it is necessary to review the approach to a pathology that occurs so frequently, with such burdensome and complex implications. Recent research has shown that imaging methods can detect subtle neuroplastic changes in the central and peripheral nervous system, which can be correlated with neuropathic symptoms and may serve as potential markers. The aim of this paper is to review available imaging methods used for diagnosing and assessing therapeutic efficacy in CNP for both the preclinical and clinical setting. Of course, further research is required to standardize and improve detection accuracy, but available data indicate that imaging is a valuable tool that can impact the management of CNP.

Potential Role of Yoga Intervention in the Management of Chronic Non-malignant Pain

Pain is an unpleasant and upsetting experience. Persistent pain has an impact on an individual's quality of life which causes stress and mood disorders. There are currently no pain-relieving techniques available that can eliminate pain and offer relief without causing any adverse effects. These factors draw attention to traditional treatments like yoga and meditation, which can reduce biological stress and hence increase immunity, as well as alleviate the psychological and emotional suffering produced by pain. Yoga reduces the stress response and the pain cascade via the downregulation of the hypothalamus-pituitary-adrenal (HPA) axis and vagal stimulation. Yoga is a cost-effective growing health practice that, unlike pharmaceuticals, has no side effects and can help patients stay in remission for longer periods of time with fewer relapses. Yoga not only reduces stress and depression severity but also improves functional status and reduces pain perception. This article highlights the impact of yoga on pain management and on a malfunctioning immune system, which leads to improved health, pain reduction, disease management, and improvement in overall quality of life.

Hedgehog signaling plays a crucial role in hyperalgesia associated with neuropathic pain in mice

Neuropathic pain is a debilitating chronic syndrome of the nervous system caused by nerve injury. In Drosophila, the Hedgehog (Hh) signaling pathway is related to increased pain sensitivity (hyperalgesia) but does not affect the baseline nociceptive threshold. In general, the contribution of the Hh signaling pathway to neuropathic pain in vertebrates is a highly debated issue. Alternatively, we investigated the potential role of Hh signaling in mechanical allodynia using a mouse model of neuropathic pain. Seven days after spinal nerve-transection (SNT) surgery, microglial activation increased in the ipsilateral spinal dorsal horn compared with that in the sham group; however, 21 days after surgery, microglial activation decreased. Contrastingly, astrocyte activation in the spinal cord did not differ between the groups. On day 21 of postsurgery, the SNT group showed marked upregulation of sonic hedgehog expression in peripheral glial cells but not in dorsal root ganglion (DRG) neurons. Intrathecal administration of the Hh signaling inhibitor vismodegib attenuated the mechanical allodynia observed on day 21 postsurgery. Conversely, intrathecal treatment with the Hh signaling activator smoothened agonist in naive mice induced mechanical allodynia, which was abolished by the ATP transporter inhibitor clodronate. Moreover, inhibition of Hh signaling by pretreatment with vismodegib significantly reduced ATP secretion and the frequency/number of spontaneous elevations of intracellular calcium ion levels in cultured DRG cells. Thus, the Hh signaling pathway appears to modulate the neural activity of DRG neurons via ATP release, and it plays an important role in sustaining mechanical allodynia and hypersensitivity in a mouse model of neuropathic pain.

Role of Primary Afferents in Arthritis Induced Spinal Microglial Reactivity

Increased afferent input resulting from painful injury augments the activity of central nociceptive circuits via both neuron-neuron and neuron-glia interactions. Microglia, resident immune cells of the central nervous system (CNS), play a crucial role in the pathogenesis of chronic pain. This study provides a framework for understanding how peripheral joint injury signals the CNS to engage spinal microglial responses. During the first week of monosodium iodoacetate (MIA)-induced knee joint injury in male rats, inflammatory and neuropathic pain were characterized by increased firing of peripheral joint afferents. This increased peripheral afferent activity was accompanied by increased Iba1 immunoreactivity within the spinal dorsal horn indicating microglial activation. Pharmacological silencing of C and A afferents with co-injections of QX-314 and bupivacaine, capsaicin, or flagellin prevented the development of mechanical allodynia and spinal microglial activity after MIA injection. Elevated levels of ATP in the cerebrospinal fluid (CSF) and increased expression of the ATP transporter vesicular nucleotide transporter (VNUT) in the ipsilateral spinal dorsal horn were also observed after MIA injections. Selective silencing of primary joint afferents subsequently inhibited ATP release into the CSF. Furthermore, increased spinal microglial reactivity, and alleviation of MIA-induced arthralgia with co-administration of QX-314 with bupivacaine were recapitulated in female rats. Our results demonstrate that early peripheral joint injury activates joint nociceptors, which triggers a central spinal microglial response. Elevation of ATP in the CSF, and spinal expression of VNUT suggest ATP signaling may modulate communication between sensory neurons and spinal microglia at 2 weeks of joint degeneration.

The role of microglia in chronic pain and depression: innocent bystander or culprit?

Clinical evidence shows that chronic pain and depression often accompany each other, but the underlying pathogenesis of comorbid chronic pain and depression remains mostly undetermined. Biotechnology is gradually revealing the phenotype and function of microglia, with great progress regarding microglia's role in neurodegeneration, depression, chronic pain, and other conditions. This article summarizes the role of microglia in chronic pain, depression, and comorbidities, which is conducive to finding new targets to treat chronic pain and depression.

Single-cell transcriptomic analysis of somatosensory neurons uncovers temporal development of neuropathic pain

Peripheral nerve injury could lead to chronic neuropathic pain. Understanding transcriptional changes induced by nerve injury could provide fundamental insights into the complex pathogenesis of neuropathic pain. Gene expression profiles of dorsal root ganglia (DRG) in neuropathic pain condition have been studied. However, little is known about transcriptomic changes in individual DRG neurons after peripheral nerve injury. Here we performed single-cell RNA sequencing on dissociated mouse DRG cells after spared nerve injury (SNI). In addition to DRG neuron types that are found under physiological conditions, we identified three SNI-induced neuronal clusters (SNIICs) characterized by the expression of Atf3/Gfra3/Gal (SNIIC1), Atf3/Mrgprd (SNIIC2) and Atf3/S100b/Gal (SNIIC3). These SNIICs originated from Cldn9⁺/Gal⁺, Mrgprd⁺ and Trappc3l⁺ DRG neurons, respectively. Interestingly, SNIIC2 switched to SNIIC1 by increasing Gal and reducing Mrgprd expression 2 days after nerve injury. Inferring the gene regulatory networks after nerve injury, we revealed that activated transcription factors Atf3 and Egr1 in SNIICs could enhance Gal expression while activated Cpeb1 in SNIIC2 might suppress Mrgprd expression within 2 days after SNI. Furthermore, we mined the transcriptomic changes in the development of neuropathic pain to identify potential analgesic targets. We revealed that cardiotrophin-like cytokine factor 1, which activates astrocytes in the dorsal horn of spinal cord, was upregulated in SNIIC1 neurons and contributed to SNI-induced mechanical allodynia. Therefore, our results provide a new landscape to understand the dynamic course of neuron type changes and their underlying molecular mechanisms during the development of neuropathic pain.

Sex-dependent role of microglia in disulfide high mobility group box 1 protein-mediated mechanical hypersensitivity

ABSTRACT

High mobility group box 1 protein (HMGB1) is increasingly regarded as an important player in the spinal regulation of chronic pain. Although it has been reported that HMGB1 induces spinal glial activation in a Toll-like receptor (TLR)4-dependent fashion, the aspect of sexual dimorphisms has not been thoroughly addressed. Here, we examined whether the action of TLR4-activating, partially reduced disulfide HMGB1 on microglia induces nociceptive behaviors in a sex-dependent manner. We found disulfide HMGB1 to equally increase microglial Iba1 immunoreactivity in lumbar spinal dorsal horn in male and female mice, but evoke higher cytokine and chemokine expression in primary microglial culture derived from males compared to females. Interestingly, TLR4 ablation in myeloid-derived cells, which include microglia, only protected male mice from developing HMGB1-induced mechanical hypersensitivity. Spinal administration of the glial inhibitor, minocycline, with disulfide HMGB1 also prevented pain-like behavior in male mice. To further explore sex difference, we examined the global spinal protein expression using liquid chromatography-mass spectrometry and found several antinociceptive and anti-inflammatory proteins to be upregulated in only male mice subjected to minocycline. One of the proteins elevated, alpha-1-antitrypsin, partially protected males but not females from developing HMGB1-induced pain. Targeting downstream proteins of alpha-1-antitrypsin failed to produce robust sex differences in pain-like behavior, suggesting that several proteins identified by liquid chromatography-mass spectrometry are required to modulate the effects. Taken together, the current study highlights the importance of mapping sex dimorphisms in pain mechanisms and point to processes potentially involved in the spinal antinociceptive effect of microglial inhibition in male mice.

Teriparatide relieves ovariectomy-induced hyperalgesia in rats, suggesting the involvement of functional regulation in primary sensory neurons by PTH-mediated signaling

Clinical studies have reported that teriparatide (TPTD), a human parathyroid hormone analog, reduces back pain in osteoporotic patients. However, the mechanistic insights of this pharmacological action remain elusive. This study investigated the antinociceptive effect of TPTD mainly on primary sensory neurons in ovariectomized (OVX) rats. The plantar test showed thermal hyperalgesia in the OVX rats, which was significantly, but not fully, recovered immediately after the initial TPTD administration. The von Frey test also demonstrated reduced withdrawal threshold in the OVX rats. This was partially recovered by TPTD. Consistently, the number and size of spinal microglial cells were significantly increased in the OVX rats, while TPTD treatment significantly reduced the number but not size of these cells. RNA sequencing-based bioinformatics of the dorsal root ganglia (DRG) demonstrated that changes in neuro-protective and inflammatory genes were involved in the pharmacological effect of TPTD. Most neurons in the DRG expressed substantial levels of parathyroid hormone 1 receptor. TPTD treatment of the cultured DRG-derived neuronal cells reduced the cAMP level and augmented the intracellular calcium level as the concentration increased. These findings suggest that TPTD targets neuronal cells as well as bone cells to exert its pharmacological action.

Nerve Injury-Induced Neuronal PAP-I Maintains Neuropathic Pain by Activating Spinal Microglia

Pancreatitis-associated proteins (PAPs) display multiple functions in visceral diseases. Previous studies showed that the expression level of PAP-I was low in the DRG of naive rats but was de novo expressed after peripheral nerve injury. However, its role in neuropathic pain remains unknown. We found that PAP-I expression was continuously upregulated in the DRG neurons from rat spared nerve injury models, and transported toward the spinal dorsal horn to act as a proinflammatory factor. Intrathecal delivery of PAP-I enhanced sensory hyperalgesia, whereas PAP-I deficiency by either gene knockout or antibody application alleviated tactile allodynia at the maintenance phase after spared nerve injury. Furthermore, PAP-I functioned by activating the spinal microglia via C-C chemokine receptor Type 2 that participated in neuropathic pain. Inhibition of either microglial activation or C-C chemokine receptor Type 2 abolished the PAP-I-induced hyperalgesia. Thus, PAP-I mediates the neuron-microglial crosstalk after peripheral nerve injury and contributes to the maintenance of neuropathic pain.SIGNIFICANCE STATEMENT Neuropathic pain is maladaptive pain condition, and the maintaining mechanism is largely unclear. Here we reveal that, after peripheral nerve injury, PAP-I can be transported to the spinal dorsal horn and is crucial in the progression of neuropathic pain. Importantly, we prove that PAP-I mainly functions through activating the spinal microglia via the CCR2-p38 MAPK pathway. Furthermore, we confirm that the proinflammatory effect of PAP-I is more prominent after the establishment of neuropathic pain, thus indicating that microglia also participate in the maintenance phase of neuropathic pain.

Highly selective microglial uptake of ceria-zirconia nanoparticles for enhanced analgesic treatment of neuropathic pain

Neuropathic pain is a chronic and pathological pain caused by injury or dysfunction in the nervous system. Pro-inflammatory microglial activation with aberrant reactive oxygen species (ROS) generation in the spinal cord plays a critical role in the development of neuropathic pain. However, the efficacy of current therapeutic methods for neuropathic pain is limited because only neurons or neural circuits involved in pain transmission are targeted. Here, an effective strategy to treat pain hypersensitivity using microglia-targeting ceria-zirconia nanoparticles (CZ NPs) is reported. The CZ NPs are coated with microglia-specific antibodies to promote their delivery to microglia, and thus to improve their therapeutic efficacy. The targeted delivery facilitates the elimination of both pro-inflammatory cytokines and ROS in microglia, enabling the rapid and effective inhibition of microglial activation. As a result, greatly ameliorated mechanical allodynia is achieved in a spinal nerve transection (SNT)-induced neuropathic pain mouse model, proving the potent analgesic effect of the microglia-targeting CZ NPs. Given the generality of the approach used in this study, the microglia-targeting CZ NPs are expected to be useful for the treatment of not only neuropathic pain but also other neurological diseases associated with the vicious activation of microglia.

Contribution of microglial reaction to increased nociceptive responses in high-fat-diet (HFD)-induced obesity in male mice

The progressive increase in the prevalence of obesity in the population can result in increased healthcare costs and demands. Recent studies have revealed a positive correlation between pain and obesity, although the underlying mechanisms still remain unknown. Here, we aimed to clarify the role of microglia in altered pain behaviors induced by high-fat diet (HFD) in male mice. We found that C57BL/6CR mice on HFD exhibited enhanced spinal microglial reaction (increased cell number and up-regulated expression of p-p38 and CD16/32), increased tumor necrosis factor-α (TNF-α) mRNA and brain-derived neurotrophic factor (BDNF) protein expression as well as a polarization of spinal microglial toward a pro-inflammatory phenotype. Moreover, we found that using PLX3397 (a selective colony-stimulating factor-1 receptor (CSF1R) kinase inhibitor) to eliminate microglia in HFD-induced obesity mice, inflammation in the spinal cord was rescued, as was abnormal pain hypersensitivity. Intrathecal injection of Mac-1-saporin (a saporin-conjugated anti-mac1 antibody) resulted in a decreased number of microglia and attenuated both mechanical allodynia and thermal hyperalgesia in HFD-fed mice. These results indicate that the pro-inflammatory functions of spinal microglia have a special relevance to abnormal pain hypersensitivity in HFD-induced obesity mice. In conclusion, our data suggest that HFD induces a classical reaction of microglia, characterized by an enhanced phosphorylation of p-38 and increased CD16/32 expression, which may in part contribute to increased nociceptive responses in HFD-induced obesity mice.

Exploring the transcriptome of resident spinal microglia after collagen antibody-induced arthritis

Supplemental Digital Content is Available in the Text.

Glial inhibitors only reverse mechanical hypersensitivity in male mice subjected to arthritis. No obvious arthritis-related transcriptional difference was identified between male and female spinal microglia.

Recent studies have suggested a sexually dimorphic role of spinal glial cells in the maintenance of mechanical hypersensitivity in rodent models of chronic pain. We have used the collagen antibody–induced arthritis (CAIA) mouse model to examine differences between males and females in the context of spinal regulation of arthritis-induced pain. We have focused on the late phase of this model when joint inflammation has resolved, but mechanical hypersensitivity persists. Although the intensity of substance P, calcitonin gene–related peptide, and galanin immunoreactivity in the spinal cord was not different from controls, the intensity of microglia (Iba-1) and astrocyte (glial fibrillary acidic protein) markers was elevated in both males and females. Intrathecal administration of the glial inhibitors minocycline and pentoxifylline reversed mechanical thresholds in male, but not in female mice. We isolated resident microglia from the lumbar dorsal horns and observed a significantly lower number of microglial cells in females by flow cytometry analysis. However, although genome-wide RNA sequencing results pointed to several transcriptional differences between male and female microglia, no convincing differences were identified between control and CAIA groups. Taken together, these findings suggest that there are subtle sex differences in microglial expression profiles independent of arthritis. Our experiments failed to identify the underlying mRNA correlates of microglial actions in the late phase of the CAIA model. It is likely that transcriptional changes are either subtle and highly localised and therefore difficult to identify with bulk isolation techniques or that other factors, such as changes in protein expression or epigenetic modifications, are at play.

The Role of CXCR3 in Neurological Diseases

BACKGROUND

Neurological diseases have become an obvious challenge due to insufficient therapeutic intervention. Therefore, novel drugs for various neurological disorders are in desperate need. Recently, compelling evidence has demonstrated that chemokine receptor CXCR3, which is a G protein-coupled receptor in the CXC chemokine receptor family, may play a pivotal role in the development of neurological diseases. The aim of this review is to provide evidence for the potential of CXCR3 as a therapeutic target for neurological diseases.

METHODS

English journal articles that focused on the invovlement of CXCR3 in neurological diseases were searched via PubMed up to May 2017. Moreover, reference lists from identified articles were included for overviews.

RESULTS

The expression level of CXCR3 in T cells was significantly elevated in several neurological diseases, including multiple sclerosis (MS), glioma, Alzheimer's disease (AD), chronic pain, human T-lymphotropic virus type 1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and bipolar disorder. CXCR3 antagonists showed therapeutic effects in these neurological diseases.

CONCLUSION

These studies provided hard evidence that CXCR3 plays a vital role in the pathogenesis of MS, glioma, AD, chronic pain, HAM/TSP and bipolar disorder. CXCR3 is a crucial molecule in neuroinflammatory and neurodegenerative diseases. It regulates the activation of infiltrating cells and resident immune cells. However, the exact functions of CXCR3 in neurological diseases are inconclusive. Thus, it is important to understand the topic of chemokines and the scope of their activity in neurological diseases.

Hyperactivation of proprioceptors induces microglia-mediated long-lasting pain in a rat model of chronic fatigue syndrome

BACKGROUND

Patients diagnosed with chronic fatigue syndrome (CFS) or fibromyalgia experience chronic pain. Concomitantly, the rat model of CFS exhibits microglial activation in the lumbar spinal cord and pain behavior without peripheral tissue damage and/or inflammation. The present study addressed the mechanism underlying the association between pain and chronic stress using this rat model.

METHODS

Chronic or continuous stress-loading (CS) model rats, housed in a cage with a thin level of water (1.5 cm in depth), were used. The von Frey test and pressure pain test were employed to measure pain behavior. The neuronal and microglial activations were immunohistochemically demonstrated with antibodies against ATF3 and Iba1. Electromyography was used to evaluate muscle activity.

RESULTS

The expression of ATF3, a marker of neuronal hyperactivity or injury, was first observed in the lumbar dorsal root ganglion (DRG) neurons 2 days after CS initiation. More than 50% of ATF3-positive neurons simultaneously expressed the proprioceptor markers TrkC or VGluT1, whereas the co-expression rates for TrkA, TrkB, IB4, and CGRP were lower than 20%. Retrograde labeling using fluorogold showed that ATF3-positive proprioceptive DRG neurons mainly projected to the soleus. Substantial microglial accumulation was observed in the medial part of the dorsal horn on the fifth CS day. Microglial accumulation was observed around a subset of motor neurons in the dorsal part of the ventral horn on the sixth CS day. The motor neurons surrounded by microglia were ATF3-positive and mainly projected to the soleus. Electromyographic activity in the soleus was two to three times higher in the CS group than in the control group. These results suggest that chronic proprioceptor activation induces the sequential activation of neurons along the spinal reflex arc, and the neuronal activation further activates microglia along the arc. Proprioceptor suppression by ankle joint immobilization significantly suppressed the accumulation of microglia in the spinal cord, as well as the pain behavior.

CONCLUSION

Our results indicate that proprioceptor-induced microglial activation may be a key player in the initiation and maintenance of abnormal pain in patients with CFS.

Brain Microglial Activation in Chronic Pain-Associated Affective Disorder

A growing body of evidence from both clinical and animal studies indicates that chronic neuropathic pain is associated with comorbid affective disorders. Spinal cord microglial activation is involved in nerve injury-induced pain hypersensitivity characterizing neuropathic pain. However, there is a lack of thorough assessments of microglial activation in the brain after nerve injury. In the present study, we characterized microglial activation in brain sub-regions of CX3CR1^GFP/+ mice after chronic constriction injury (CCI) of the sciatic nerve, including observations at delayed time points when affective brain dysfunctions such as depressive-like behaviors typically develop. Mice manifested chronic mechanical hypersensitivity immediately after CCI and developed depressive-like behaviors 8 weeks post-injury. Concurrently, significant increases of soma size and microglial cell number were observed in the medial prefrontal cortex (mPFC), hippocampus, and amygdala 8 weeks post-injury. Transcripts of CD11b, and TNF-α, genes associated with microglial activation or depressive-like behaviors, are correspondingly upregulated in these brain areas. Our results demonstrate that microglia are activated in specific brain sub-regions after CCI at delayed time points and imply that brain microglial activation plays a role in chronic pain-associated affective disorders.

HIV-associated sensory polyneuropathy and neuronal injury are associated with miRNA-455-3p induction

Symptomatic distal sensory polyneuropathy (sDSP) is common and debilitating in people with HIV/AIDS, leading to neuropathic pain, although the condition's cause is unknown. To investigate biomarkers and associated pathogenic mechanisms for sDSP, we examined plasma miRNA profiles in HIV/AIDS patients with sDSP or without sDSP in 2 independent cohorts together with assessing related pathogenic effects. Several miRNAs were found to be increased in the Discovery Cohort (sDSP, n = 29; non-DSP, n = 40) by array analyses and were increased in patients with sDSP compared with patients without sDSP. miR-455-3p displayed a 12-fold median increase in the sDSP group, which was confirmed by machine learning analyses and verified by reverse transcription PCR. In the Validation Cohort (sDSP n = 16, non-DSP n = 20, healthy controls n = 15), significant upregulation of miR-455-3p was also observed in the sDSP group. Bioinformatics revealed that miR-455-3p targeted multiple host genes implicated in peripheral nerve maintenance, including nerve growth factor (NGF) and related genes. Transfection of cultured human dorsal root ganglia with miR-455-3p showed a concentration-dependent reduction in neuronal β-III tubulin expression. Human neurons transfected with miR-455-3p demonstrated reduced neurite outgrowth and NGF expression that was reversed by anti-miR-455-3p antagomir cotreatment. miR-455-3p represents a potential biomarker for HIV-associated sDSP and might also exert pathogenic effects leading to sDSP.

Transcription factor MafB contributes to the activation of spinal microglia underlying neuropathic pain development

Microglia, which are pathological effectors and amplifiers in the central nervous system, undergo various forms of activation. A well-studied microglial-induced pathological paradigm, spinal microglial activation following peripheral nerve injury (PNI), is a key event for the development of neuropathic pain but the transcription factors contributing to microglial activation are less understood. Herein, we demonstrate that MafB, a dominant transcriptional regulator of mature microglia, is involved in the pathology of a mouse model of neuropathic pain. PNI caused a rapid and marked increase of MafB expression selectively in spinal microglia but not in neurons. We also found that the microRNA mir-152 in the spinal cord which targets MafB expression decreased after PNI, and intrathecal administration of mir-152 mimic suppressed the development of neuropathic pain. Reduced MafB expression using heterozygous Mafb deficient mice and by intrathecal administration of siRNA alleviated the development of PNI-induced mechanical hypersensitivity. Furthermore, we found that intrathecal transfer of Mafb deficient microglia did not induce mechanical hypersensitivity and that conditional Mafb knockout mice did not develop neuropathic pain after PNI. We propose that MafB is a key mediator of the PNI-induced phenotypic alteration of spinal microglia and neuropathic pain development.

Roles of TRAF6 in Central Nervous System

Tumor necrosis factor receptor-associated factor (TRAF) is an important binding protein of tumor necrosis factor (TNF) superfamily and the toll/IL-1 receptor (TIR) superfamily, which play an important role in innate immunity and ac-quired immunity. TRAFs family have 7 members (TRAF1-7), and TRAF6 has its special facture and biological function. TRAF6 has two special domains: C-terminal domain and N-terminal domain, which could integrate with multiple kinases and regulate signaling pathway function as an E3 ubiquitin ligase. Studies have increasingly found that TRAF6 is closely re-lated to central nervous system diseases, such as stroke, Traumatic brain injury, neurodegenerative diseases and neuropathic pain. Further research on the pathophysiological mechanism may be expected to become the new targets for the treatment of central nervous system diseases

Erythropoietin reduces nerve demyelination, neuropathic pain behavior and microglial MAPKs activation through erythropoietin receptors on Schwann cells in a rat model of peripheral neuropathy

Neuroprotective effects of erythropoietin (EPO) on peripheral nerve injury remain uncertain. This study investigated the efficacy of EPO in attenuating median nerve chronic constriction injury (CCI)-induced neuropathy. Animals received an intraneural injection of EPO at doses of 1,000, 3,000, or 5,000 units/kg 15 min before median nerve CCI. Afterwards, the behavioral and electrophysiological tests were conducted. Immunohistochemistry and immunoblotting were used for qualitative and quantitative analysis of microglial and mitogen-activated protein kinases (MAPKs), including p38, JNK, and ERK, activation. Enzyme-linked immunosorbent assay and microdialysis were applied to measure pro-inflammatory cytokine and glutamate responses, respectively. EPO pre-treatment dose-dependently ameliorated neuropathic pain behavior, decreased microglial and MAPKs activation, and diminished the release of pro-inflammatory cytokines and glutamate in the ipsilateral cuneate nucleus after CCI. Moreover, EPO pre-treatment preserved myelination of the injured median nerve on morphological investigation and suppressed injury-induced discharges. We also observed that EPO receptor (EPOR) expression was up-regulated in the injured nerve after CCI. Double immunofluorescence showed that EPOR was localized to Schwann cells. Furthermore, siRNA-mediated knockdown of EPOR expression eliminated the therapeutic effects of EPO on attenuating the microglial and MAPKs activation, pro-inflammatory cytokine responses, injury discharges, and neuropathic pain behavior in CCI rats. In conclusion, binding of EPO to its receptors on Schwann cells maintains myelin integrity and blocks ectopic discharges in the injured median nerve, that in the end contribute to attenuation of neuropathic pain via reducing glutamate release from primary afferents and inhibiting activation of microglial MAPKs and production of pro-inflammatory cytokines.

HIV-1 Tat regulation of dopamine transmission and microglial reactivity is brain region specific

The transactivator of transcription protein, HIV-1 Tat, is linked to neuroAIDS, where degeneration of dopamine neurons occurs. Using a mouse model expressing GFAP-driven Tat protein under doxycycline (Dox) regulation, we investigated microglial-neuronal interactions in the rostral substantia nigra pars compacta (SNc). Immunohistochemistry for microglia and tyrosine hydroxylase (TH) showed that the ratio of microglia to dopamine neurons is smaller in the SNc than in the ventral tegmental area (VTA) and that this difference is maintained following 7-day Dox exposure in wild type animals. Administration of Dox to wild types had no effect on microglial densities. In addressing the sensitivity of neurons to potentially adverse effects of HIV-1 Tat, we found that HIV-1 Tat exposure in vivo selectively decreased TH immunoreactivity in the SNc but not in the VTA, while levels of TH mRNA in the SNc remained unchanged. HIV-1 Tat induction in vivo did not alter the total number of neurons in these brain regions. Application of Tat (5 ng) into dopamine neurons with whole-cell patch pipette decreased spontaneous firing activity. Tat induction also produced a decline in microglial cell numbers, but no microglial activation. Thus, disappearance of dopaminergic phenotype is due to a loss of TH immunoreactivity rather than to neuronal death, which would have triggered microglial activation. We conclude that adverse effects of HIV-1 Tat produce a hypodopamine state by decreasing TH immunoreactivity and firing activity of dopamine neurons. Reduced microglial numbers after Tat exposure in vivo suggest impaired microglial functions and altered bidirectional interactions between dopamine neurons and microglia.

Effect of electro-acupuncture on the BDNF-TrkB pathway in the spinal cord of CCI rats

Microglia, which comprise a sensor for pathological events in the central nervous system, may be triggered by nerve injury and transformed from a quiescent state into an activated state; ionised calcium binding adaptor molecule 1 (Iba1) is a sensitive marker associated with activated microglia. Accumulated evidence suggests that spinal activated microglia and the brain-derived neurotrophic factor (BDNF)-tyrosine kinase receptor B (TrkB) signalling pathway play major roles in the production and development of neuropathic pain. Electro-acupuncture (EA) has a positive effect on relieving chronic neuropathic pain; however, the underlying mechanisms remain unclear. To determine the significance of EA in the treatment of neuropathic pain mediated by activated microglia and the BDNF-TrkB signalling pathway in the spinal cord, the mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) values were recorded to assess hyperalgesia and allodynia. In addition, the amount of activated microglia and BDNF were assessed via immunofluorescence. Iba1, BDNF and TrkB mRNA expression levels were examined using qPCR; the protein levels of BDNF, p-TrkB and TrkB in the spinal cord were analysed via western blotting. The present study demonstrated that EA treatment increased the MWT and TWL values. EA significantly inhibited the proportion of activated microglia and BDNF expression in the spinal cord after chronic constrictive injury (CCI). Furthermore, EA decreased the expression of BDNF and TrkB at both the mRNA and protein levels in the spinal cord of CCI rats. These findings suggest that the analgesic effect of EA may be achieved by inhibiting the activation of spinal microglia and subsequently blocking the BDNF-TrkB signalling pathway.

Manual acupuncture relieves bile acid-induced itch in mice: the role of microglia and TNF-α

Pruritus, or itch, is a frequent complaint amongst patients with cholestatic hepatobiliary disease and is difficult to manage, with many patients refractory to currently available antipruritic treatments. In this study, we examined whether manual acupuncture (MA) at particular acupoints represses deoxycholic acid (DCA)-induced scratching behavior and microglial activation and compared these effects with those induced by another pruritogen, 5'-guanidinonaltrindole (GNTI, a kappa opioid receptor antagonist). MA at Hegu (LI4) and Quchi (LI11) acupoints significantly attenuated DCA- and GNTI-induced scratching, whereas no such effects were observed at the bilateral Zusanli acupoints (ST36). Interestingly, GNTI-induced scratching was reduced similarly by both MA and electroacupuncture (EA) at the LI4 and LI11 acupoints. MA at non-acupoints did not affect scratching behavior. Intraperitoneal injection of minocycline (a microglial inhibitor) reduced GNTI- and DCA-induced scratching behavior. In Western blot analysis, subcutaneous DCA injection to the back of the neck increased spinal cord expression of ionized calcium-binding adapter molecule 1 (Iba1) and tumor necrosis factor-alpha (TNF-α) as compared with saline injection, while MA at LI4 and LI11 reduced these DCA-induced changes. Immunofluorescence confocal microcopy revealed that DCA-induced Iba1-positive cells with thicker processes emanated from the enlarged cell bodies, while this effect was attenuated by pretreatment with MA. It is concluded that microglia and TNF-α play important roles in the itching sensation and MA reduces DCA-induced scratching behavior by alleviating spinal microglial activation. MA may be an effective treatment for cholestatic pruritus.

DRG Voltage-Gated Sodium Channel 1.7 Is Upregulated in Paclitaxel-Induced Neuropathy in Rats and in Humans with Neuropathic Pain

Chemotherapy-induced peripheral neuropathy (CIPN) is a common adverse effect experienced by cancer patients receiving treatment with paclitaxel. The voltage-gated sodium channel 1.7 (Na_v1.7) plays an important role in multiple preclinical models of neuropathic pain and in inherited human pain phenotypes, and its gene expression is increased in dorsal root ganglia (DRGs) of paclitaxel-treated rats. Hence, the potential of change in the expression and function of Na_v1.7 protein in DRGs from male rats with paclitaxel-related CIPN and from male and female humans with cancer-related neuropathic pain was tested here. Double immunofluorescence in CIPN rats showed that Na_v1.7 was upregulated in small DRG neuron somata, especially those also expressing calcitonin gene-related peptide (CGRP), and in central processes of these cells in the superficial spinal dorsal horn. Whole-cell patch-clamp recordings in rat DRG neurons revealed that paclitaxel induced an enhancement of ProTx II (a selective Na_v1.7 channel blocker)-sensitive sodium currents. Bath-applied ProTx II suppressed spontaneous action potentials in DRG neurons occurring in rats with CIPN, while intrathecal injection of ProTx II significantly attenuated behavioral signs of CIPN. Complementarily, DRG neurons isolated from segments where patients had a history of neuropathic pain also showed electrophysiological and immunofluorescence results indicating an increased expression of Na_v1.7 associated with spontaneous activity. Na_v1.7 was also colocalized in human cells expressing transient receptor potential vanilloid 1 and CGRP. Furthermore, ProTx II decreased firing frequency in human DRGs with spontaneous action potentials. This study suggests that Na_v1.7 may provide a potential new target for the treatment of neuropathic pain, including chemotherapy (paclitaxel)-induced neuropathic pain.SIGNIFICANCE STATEMENT This work demonstrates that the expression and function of the voltage-gated sodium channel Na_v1.7 are increased in a preclinical model of chemotherapy-induced peripheral neuropathy (CIPN), the most common treatment-limiting side effect of all the most common anticancer therapies. This is key as gain-of-function mutations in human Na_v1.7 recapitulate both the distribution and pain percept as shown by CIPN patients. This work also shows that Na_v1.7 is increased in human DRG neurons only in dermatomes where patients are experiencing acquired neuropathic pain symptoms. This work therefore has major translational impact, indicating an important novel therapeutic avenue for neuropathic pain as a class.

An autocrine purinergic signaling controls astrocyte-induced neuronal excitation

Astrocyte-derived gliotransmitters glutamate and ATP modulate neuronal activity. It remains unclear, however, how astrocytes control the release and coordinate the actions of these gliotransmitters. Using transgenic expression of the light-sensitive channelrhodopsin 2 (ChR2) in astrocytes, we observed that photostimulation reliably increases action potential firing of hippocampal pyramidal neurons. This excitation relies primarily on a calcium-dependent glutamate release by astrocytes that activates neuronal extra-synaptic NMDA receptors. Remarkably, our results show that ChR2-induced Ca²⁺ increase and subsequent glutamate release are amplified by ATP/ADP-mediated autocrine activation of P2Y1 receptors on astrocytes. Thus, neuronal excitation is promoted by a synergistic action of glutamatergic and autocrine purinergic signaling in astrocytes. This new mechanism may be particularly relevant for pathological conditions in which ATP extracellular concentration is increased and acts as a major danger signal.

Challenges in translational drug research in neuropathic and inflammatory pain: the prerequisites for a new paradigm

AIM

Despite an improved understanding of the molecular mechanisms of nociception, existing analgesic drugs remain limited in terms of efficacy in chronic conditions, such as neuropathic pain. Here, we explore the underlying pathophysiological mechanisms of neuropathic and inflammatory pain and discuss the prerequisites and opportunities to reduce attrition and high-failure rate in the development of analgesic drugs.

METHODS

A literature search was performed on preclinical and clinical publications aimed at the evaluation of analgesic compounds using MESH terms in PubMed. Publications were selected, which focused on (1) disease mechanisms leading to chronic/neuropathic pain and (2) druggable targets which are currently under evaluation in drug development. Attention was also given to the role of biomarkers and pharmacokinetic-pharmacodynamic modelling.

RESULTS

Multiple mechanisms act concurrently to produce pain, which is a non-specific manifestation of underlying nociceptive pathways. Whereas these manifestations can be divided into neuropathic and inflammatory pain, it is now clear that inflammatory mechanisms are a common trigger for both types of pain. This has implications for drug development, as the assessment of drug effects in experimental models of neuropathic and chronic pain is driven by overt behavioural measures. By contrast, the use of mechanistic biomarkers in inflammatory pain has provided the pharmacological basis for dose selection and evaluation of non-steroidal anti-inflammatory drugs (NSAIDs).

CONCLUSION

A different paradigm is required for the identification of relevant targets and candidate molecules whereby pain is coupled to the cause of sensorial signal processing dysfunction rather than clinical symptoms. Biomarkers which enable the characterisation of drug binding and target activity are needed for a more robust dose rationale in early clinical development. Such an approach may be facilitated by quantitative clinical pharmacology and evolving technologies in brain imaging, allowing accurate assessment of target engagement, and prediction of treatment effects before embarking on large clinical trials.

The Dorsolateral Prefrontal Cortex in Acute and Chronic Pain

The dorsolateral prefrontal cortex (DLPFC) is a functionally and structurally heterogeneous region and a key node of several brain networks, implicated in cognitive, affective, and sensory processing. As such, the DLPFC is commonly activated in experimental pain studies, and shows abnormally increased function in chronic pain populations. Furthermore, several studies have shown that some chronic pains are associated with decreased left DLPFC gray matter and that successful interventions can reverse this structural abnormality. In addition, studies have indicated that noninvasive stimulation of the left DLPFC effectively treats some chronic pains. In this article, we review the neuroimaging literature regarding the role of the DLPFC and its potential as a therapeutic target for chronic pain conditions, including studies showing the involvement of the DLPFC in encoding and modulating acute pain and studies demonstrating the reversal of DLPFC functional and structural abnormalities after successful interventions for chronic pain. We also review studies of noninvasive brain stimulation of the DLPFC showing acute pain modulation and some effectiveness as a treatment for certain chronic pain conditions. We further discuss the network architecture of the DLPFC, and postulate mechanisms by which DLPFC stimulation alleviates chronic pain. Future work testing these mechanisms will allow for more effective therapies.

PERSPECTIVE

The structure and function of the DLPFC is abnormal in some chronic pain conditions. Upon successful resolution of pain, these abnormalities are reversed. Understanding the underlying mechanisms and the role of this region can lead to the development of an effective therapeutic target for some chronic pain conditions.

Vimentin Promotes Astrocyte Activation After Chronic Constriction Injury

Vimentin, among the family of the intermediate filament, plays as the organizer of some critical proteins involved in migration, attachment, and cell signaling. In this study, the role of vimentin in chronic constriction injury (CCI) was investigated. Western blot revealed increased protein level of vimentin following CCI, peaking at 7 days. Double immunofluorescent staining showed that vimentin was mostly co-localized with astrocytes, not with neurons or microglia. In vitro, sensory neuronal injury stimulated astrocytes to produce more pro-inflammation cytokines, p-ERK (phosphorylated extracellular signal-regulated protein kinase), and vimentin. However, vimentin knockdown by siRNA (small interfering RNA) reversed the upregulation of p-ERK and vimentin expression and reduced the release of pro-inflammatory cytokines. Overall, stimulated astrocytes might release pro-inflammatory cytokines to promote the development of neuropathic pain via vimentin/ERK signaling.

Effects of Treadmill Exercise on Advanced Osteoarthritis Pain in Rats

OBJECTIVE

Exercise is commonly recommended for patients with osteoarthritis (OA) pain. However, whether exercise is beneficial in ameliorating ongoing pain that is persistent, resistant to nonsteroidal antiinflammatory drugs (NSAIDs), and associated with advanced OA is unknown.

METHODS

Rats treated with intraarticular (IA) monosodium iodoacetate (MIA) or saline underwent treadmill exercise or remained sedentary starting 10 days postinjection. Tactile sensory thresholds and weight bearing were assessed, followed by radiography at weekly intervals. After 4 weeks of exercise, ongoing pain was assessed using conditioned place preference (CPP) to IA or rostral ventromedial medulla (RVM)-administered lidocaine. The possible role of endogenous opioids in exercise-induced pain relief was examined by systemic administration of naloxone. Knee joints were collected for micro-computed tomography (micro-CT) analysis to examine pathologic changes to subchondral bone and metaphysis of the tibia.

RESULTS

Treadmill exercise for 4 weeks reversed MIA-induced tactile hypersensitivity and weight asymmetry. Both IA and RVM lidocaine D35, administered post-MIA, induced CPP in sedentary but not exercised MIA-treated rats, indicating that exercise blocks MIA-induced ongoing pain. Naloxone reestablished weight asymmetry in MIA-treated rats undergoing exercise and induced conditioned place aversion, indicating that exercise-induced pain relief is dependent on endogenous opioids. Exercise did not alter radiographic evidence of OA. However, micro-CT analysis indicated that exercise did not block lateral subchondral bone loss or trabecular bone loss in the metaphysis, but did block MIA-induced medial bone loss.

CONCLUSION

These findings support the conclusion that exercise induces pain relief in advanced, NSAID-resistant OA, likely through increased endogenous opioid signaling. In addition, treadmill exercise blocked MIA-induced bone loss in this model, indicating a potential bone-stabilizing effect of exercise on the OA joint.

Levo-Corydalmine Alleviates Neuropathic Cancer Pain Induced by Tumor Compression via the CCL2/CCR2 Pathway

Background: Tumor compression-induced pain (TCIP) is a complex pathological cancer pain. Spinal glial cells play a critical role in maintenance of cancer pain by releasing proinflammatory cytokines and chemokines. In this study, we verified the role of levo-corydalmine (l-CDL) on TCIP. Methods: Spontaneous pain, paw withdrawal threshold and latency were assessed using TCIP mouse model. Immunofluorescence was used to identify the reactions of glia. RT-PCR and western blot or ELISA were used to determine mRNA or protein expression of tumor necrosis factor-α (TNF-α), interlukin-1β (IL-1β), CC chemokine ligand 2 (CCL2) and chemotactic cytokine receptor 2 (CCR2) in vivo and in vitro. Results: l-CDL significantly attenuated TCIP hypersensitivity, accompanying with downregulation of TNF-α and IL-1β expression levels and declined astrocytes and microglial activation. It also significantly decreased the expression of the mRNA and protein level for CCL2 and CCR2. Further, l-CDL could suppress TNF-α-induced astrocytes activation and IL-1β expression through downregulating the CCL2/CCR2. Besides, CCL2-induced BV-microglia activation and inflammatory factors secretion were suppressed by l-CDL via CCR2. Conclusions: Suppression of CCL2/CCR2 by l-CDL may contribute to alleviate TCIP, offering an alternative medication for TCIP.

Modulation of Pain and Itch by Spinal Glia

Chronic pain and itch are a pathological operation of the somatosensory system at the levels of primary sensory neurons, spinal cord and brain. Pain and itch are clearly distinct sensations, and recent studies have revealed the separate neuronal pathways that are involved in each sensation. However, the mechanisms by which these sensations turn into a pathological chronic state are poorly understood. A proposed mechanism underlying chronic pain and itch involves abnormal excitability in dorsal horn neurons in the spinal cord. Furthermore, an increasing body of evidence from models of chronic pain and itch has indicated that synaptic hyperexcitability in the spinal dorsal horn might not be a consequence simply of changes in neurons, but rather of multiple alterations in glial cells. Thus, understanding the key roles of glial cells may provide us with exciting insights into the mechanisms of chronicity of pain and itch, and lead to new targets for treating chronic pain and itch.

Neuropathic pain

Neuropathic pain is caused by a lesion or disease of the somatosensory system, including peripheral fibres (Aβ, Aδ and C fibres) and central neurons, and affects 7-10% of the general population. Multiple causes of neuropathic pain have been described and its incidence is likely to increase owing to the ageing global population, increased incidence of diabetes mellitus and improved survival from cancer after chemotherapy. Indeed, imbalances between excitatory and inhibitory somatosensory signalling, alterations in ion channels and variability in the way that pain messages are modulated in the central nervous system all have been implicated in neuropathic pain. The burden of chronic neuropathic pain seems to be related to the complexity of neuropathic symptoms, poor outcomes and difficult treatment decisions. Importantly, quality of life is impaired in patients with neuropathic pain owing to increased drug prescriptions and visits to health care providers, as well as the morbidity from the pain itself and the inciting disease. Despite challenges, progress in the understanding of the pathophysiology of neuropathic pain is spurring the development of new diagnostic procedures and personalized interventions, which emphasize the need for a multidisciplinary approach to the management of neuropathic pain.

Unconventional Role of Caspase-6 in Spinal Microglia Activation and Chronic Pain

Chronic pain affects ~20% of the worldwide population. The clinical management of chronic pain is mostly palliative and results in limited success. Current treatments mostly target the symptoms or neuronal signaling of chronic pain. It has been increasingly recognized that glial cells, such as microglia, and inflammatory signaling play a major role in the pathogenesis of chronic pain. Caspases (CASPs) are a family of protease enzymes involved in apoptosis and inflammation. They are pivotal components in a variety of neurological diseases. However, little is known about the role of CASPs in microglial modulation as to chronic pain. In particular, our recent studies have shown that CASP6 regulates chronic pain via microglial inflammatory signaling. Inhibition of microglia and CASP signaling might provide a new strategy for the prevention and treatment of chronic pain.

Src-family kinases activation in spinal microglia contributes to central sensitization and chronic pain after lumbar disc herniation

Background Lumbar disc herniation is a major cause of radicular pain, but the underlying mechanisms remain largely unknown. Spinal activation of src-family kinases are involved in the development of chronic pain from nerve injury, inflammation, and cancer. In the present study, the role of src-family kinases activation in lumbar disc herniation-induced radicular pain was investigated. Results Lumbar disc herniation was induced by implantation of autologous nucleus pulposus, harvest from tail, in lumbar 4/5 spinal nerve roots of rat. Behavior test and electrophysiologic data showed that nucleus pulposus implantation induced persistent mechanical allodynia and thermal hyperalgesia and increased efficiency of synaptic transmission in spinal dorsal horn which underlies central sensitization of pain sensation. Western blotting and immunohistochemistry staining revealed that the expression of phosphorylated src-family kinases was upregulated mainly in spinal microglia of rats with nucleus pulposus. Intrathecal delivery of src-family kinases inhibitor PP2 alleviated pain behaviors, decreased efficiency of spinal synaptic transmission, and reduced phosphorylated src-family kinases expression. Furthermore, we found that the expression of ionized calcium-binding adapter molecule 1 (marker of microglia), tumor necrosis factor-α, interleukin 1 -β in spinal dorsal horn was increased in rats with nucleus pulposus. Therapeutic effect of PP2 may be related to its capacity in reducing the expression of these factors. Conclusions These findings suggested that central sensitization was involved in radicular pain from lumbar disc herniation; src-family kinases-mediated inflammatory response may be responsible for central sensitization and chronic pain after lumbar disc herniation.

Peripheral and Central Neuroinflammatory Changes and Pain Behaviors in an Animal Model of Multiple Sclerosis

Pain is a widespread and debilitating symptom of multiple sclerosis (MS), a chronic inflammatory demyelinating disease of the central nervous system. Although central neuroinflammation and demyelination have been implicated in MS-related pain, the contribution of peripheral and central mechanisms during different phases of the disease remains unclear. In this study, we used the animal model experimental autoimmune encephalomyelitis (EAE) to examine both stimulus-evoked and spontaneous pain behaviors, and neuroinflammatory changes, over the course of chronic disease. We found that mechanical allodynia of the hind paw preceded the onset of clinical EAE but was unmeasurable at clinical peak. This mechanical hypersensitivity coincided with increased microglial activation confined to the dorsal horn of the spinal cord. The development of facial mechanical allodynia also emerged in preclinical EAE, persisted at the clinical peak, and corresponded with pathology of the peripheral trigeminal afferent pathway. This included T cell infiltration, which arose prior to overt central lesion formation and specific damage to myelinated neurons during the clinical peak. Measurement of spontaneous pain using the mouse grimace scale, a facial expression-based coding system, showed increased facial grimacing in mice with EAE during clinical disease. This was associated with multiple peripheral and central neuroinflammatory changes including a decrease in myelinating oligodendrocytes, increased T cell infiltration, and macrophage/microglia and astrocyte activation. Overall, these findings suggest that different pathological mechanisms may underlie stimulus-evoked and spontaneous pain in EAE, and that these behaviors predominate in unique stages of the disease.

Neuropathic pain following traumatic spinal cord injury: Models, measurement, and mechanisms

Neuropathic pain following spinal cord injury (SCI) is notoriously difficult to treat and is a high priority for many in the SCI population. Resolving this issue requires animal models fidelic to the clinical situation in terms of injury mechanism and pain phenotype. This Review discusses the means by which neuropathic pain has been induced and measured in experimental SCI and compares these with human outcomes, showing that there is a substantial disconnection between experimental investigations and clinical findings in a number of features. Clinical injury level is predominantly cervical, whereas injury in the laboratory is modeled mainly at the thoracic cord. Neuropathic pain is primarily spontaneous or tonic in people with SCI (with a relatively smaller incidence of allodynia), but measures of evoked responses (to thermal and mechanical stimuli) are almost exclusively used in animals. There is even the question of whether pain per se has been under investigation in most experimental SCI studies rather than simply enhanced reflex activity with no affective component. This Review also summarizes some of the problems related to clinical assessment of neuropathic pain and how advanced imaging techniques may circumvent a lack of patient/clinician objectivity and discusses possible etiologies of neuropathic pain following SCI based on evidence from both clinical studies and animal models, with examples of cellular and molecular changes drawn from the entire neuraxis from primary afferent terminals to cortical sensory and affective centers. © 2016 Wiley Periodicals, Inc.

Microglia and monocytes synergistically promote the transition from acute to chronic pain after nerve injury

Microglia and peripheral monocytes contribute to hypersensitivity in rodent models of neuropathic pain. However, the precise respective function of microglia and peripheral monocytes has not been investigated in these models. To address this question, here we combined transgenic mice and pharmacological tools to specifically and temporally control the depletion of microglia and monocytes in a mouse model of spinal nerve transection (SNT). We found that although microglia and monocytes are required during the initiation of mechanical allodynia or thermal hyperalgesia, these cells may not be as important for the maintenance of hypersensitivity. Moreover, we demonstrated that either resident microglia or peripheral monocytes are sufficient in gating neuropathic pain after SNT. We propose that resident microglia and peripheral monocytes act synergistically to initiate hypersensitivity and promote the transition from acute to chronic pain after peripheral nerve injury.

Microglia and monocytes contribute to neuropathic pain states, but the precise role of the two cell types is not clear. Here Peng et al. use temporally controlled ablation of monocytes and microglia in mice to show that these cells work together to initiate neuropathic-pain like behaviour, but are less important in the maintenance phase.

Purinergic signaling in epilepsy

Until recently, analysis of the mechanisms underlying epilepsy was centered on neuron dysfunctions. Accordingly, most of the available pharmacological treatments aim at reducing neuronal excitation or at potentiating neuronal inhibition. These therapeutic options can lead to obvious secondary effects, and, moreover, seizures cannot be controlled by any known medication in one-third of the patients. A purely neurocentric view of brain functions and dysfunctions has been seriously questioned during the past 2 decades because of the accumulation of experimental data showing the functional importance of reciprocal interactions between glial cells and neurons. In the case of epilepsy, our current knowledge of the human disease and analysis of animal models clearly favor the involvement of astrocytes and microglial cells during the progression of the disease, including at very early stages, opening the way to the identification of new therapeutic targets. Purinergic signaling is a fundamental feature of neuron-glia interactions, and increasing evidence indicates that modifications of this pathway contribute to the functional remodeling of the epileptic brain. This Review discusses the recent experimental results indicating the roles of astrocytic and microglial P2X and P2Y receptors in epilepsy. © 2016 Wiley Periodicals, Inc.

A non-aggressive, highly efficient, enzymatic method for dissociation of human brain-tumors and brain-tissues to viable single-cells

Background

Conducting research on the molecular biology, immunology, and physiology of brain tumors (BTs) and primary brain tissues requires the use of viably dissociated single cells. Inadequate methods for tissue dissociation generate considerable loss in the quantity of single cells produced and in the produced cells’ viability. Improper dissociation may also demote the quality of data attained in functional and molecular assays due to the presence of large quantities cellular debris containing immune-activatory danger associated molecular patterns, and due to the increased quantities of degraded proteins and RNA.

Results

Over 40 resected BTs and non-tumorous brain tissue samples were dissociated into single cells by mechanical dissociation or by mechanical and enzymatic dissociation. The quality of dissociation was compared for all frequently used dissociation enzymes (collagenase, DNase, hyaluronidase, papain, dispase) and for neutral protease (NP) from Clostridium histolyticum. Single-cell-dissociated cell mixtures were evaluated for cellular viability and for the cell-mixture dissociation quality. Dissociation quality was graded by the quantity of subcellular debris, non-dissociated cell clumps, and DNA released from dead cells. Of all enzymes or enzyme combinations examined, NP (an enzyme previously not evaluated on brain tissues) produced dissociated cell mixtures with the highest mean cellular viability: 93 % in gliomas, 85 % in brain metastases, and 89 % in non-tumorous brain tissue. NP also produced cell mixtures with significantly less cellular debris than other enzymes tested. Dissociation using NP was non-aggressive over time—no changes in cell viability or dissociation quality were found when comparing 2-h dissociation at 37 °C to overnight dissociation at ambient temperature.

Conclusions

The use of NP allows for the most effective dissociation of viable single cells from human BTs or brain tissue. Its non-aggressive dissociative capacity may enable ambient-temperature shipping of tumor pieces in multi-center clinical trials, meanwhile being dissociated. As clinical grade NP is commercially available it can be easily integrated into cell-therapy clinical trials in neuro-oncology. The high quality viable cells produced may enable investigators to conduct more consistent research by avoiding the experimental artifacts associated with the presence dead cells or cellular debris.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-016-0262-y) contains supplementary material, which is available to authorized users.

Medial plantar nerve ligation as a novel model of neuropathic pain in mice: pharmacological and molecular characterization

Peripheral neuropathic pain is a consequence of an injury/disease of the peripheral nerves. The mechanisms involved in its pathophysiology are not entirely understood. To better understand the mechanisms involved in the development of peripheral nerve injury-induced neuropathic pain, more experimental models are required. Here, we developed a novel peripheral neuropathic pain model in mice by using a minimally invasive surgery and medial plantar nerve ligation (MPNL). After MPNL, mechanical allodynia was established, and mice quickly recovered from the surgery without any significant motor impairment. MPNL causes an increased expression of ATF-3 in the sensory neurons. At 14 days after surgery, gabapentin was capable of reversing the mechanical allodynia, whereas anti-inflammatory drugs and opioids were ineffective. MPNL-induced neuropathic pain was mediated by glial cells activation and the production of TNF-α and IL-6 in the spinal cord. These results indicate MPNL as a reasonable animal model for the study of peripheral neuropathic pain, presenting analgesic pharmacological predictivity to clinically used drugs. The results also showed molecular phenotypic changes similar to other peripheral neuropathic pain models, with the advantage of a lack of motor impairment. These features indicate that MPNL might be more appropriate for the study of neuropathic pain than classical models.

Sodium channels in astroglia and microglia

Voltage-gated sodium channels are required for electrogenesis in excitable cells. Their activation, triggered by membrane depolarization, generates transient sodium currents that initiate action potentials in neurons, cardiac, and skeletal muscle cells. Cells that have not traditionally been considered to be excitable (nonexcitable cells), including glial cells, also express sodium channels in physiological conditions as well as in pathological conditions. These channels contribute to multiple functional roles that are seemingly unrelated to the generation of action potentials. Here, we discuss the dynamics of sodium channel expression in astrocytes and microglia, and review evidence for noncanonical roles in effector functions of these cells including phagocytosis, migration, proliferation, ionic homeostasis, and secretion of chemokines/cytokines. We also examine possible mechanisms by which sodium channels contribute to the activity of glial cells, with an eye toward therapeutic implications for central nervous system disease. GLIA 2016;64:1628-1645.

A Multiplex Protein Panel Applied to Cerebrospinal Fluid Reveals Three New Biomarker Candidates in ALS but None in Neuropathic Pain Patients

The objective of this study was to develop and apply a novel multiplex panel of solid-phase proximity ligation assays (SP-PLA) requiring only 20 μL of samples, as a tool for discovering protein biomarkers for neurological disease and treatment thereof in cerebrospinal fluid (CSF). We applied the SP-PLA to samples from two sets of patients with poorly understood nervous system pathologies amyotrophic lateral sclerosis (ALS) and neuropathic pain, where patients were treated with spinal cord stimulation (SCS). Forty-seven inflammatory and neurotrophic proteins were measured in samples from 20 ALS patients and 15 neuropathic pain patients, and compared to normal concentrations in CSF from control individuals. Nineteen of the 47 proteins were detectable in more than 95% of the 72 controls. None of the 21 proteins detectable in CSF from neuropathic pain patients were significantly altered by SCS. The levels of the three proteins, follistatin, interleukin-1 alpha, and kallikrein-5 were all significantly reduced in the ALS group compared to age-matched controls. These results demonstrate the utility of purpose designed multiplex SP-PLA panels in CSF biomarker research for understanding neuropathological and neurotherapeutic mechanisms. The protein changes found in the CSF of ALS patients may be of diagnostic interest.