P2X4 receptors induced in spinal microglia gate tactile allodynia after nerve injury

Goshajinkigan attenuates paclitaxel-induced neuropathic pain via cortical astrocytes

The anticancer agents platinum derivatives and taxanes such as paclitaxel (PCX) often cause neuropathy known as chemotherapy‐induced peripheral neuropathy with high frequency. However, the cellular and molecular mechanisms underlying such neuropathy largely remain unknown. Here, we show new findings that the effect of Goshajinkigan (GJG), a Japanese KAMPO medicine, inhibits PCX‐induced neuropathy by acting on astrocytes. The administration of PCX in mice caused the sustained neuropathy lasting at least 4 weeks, which included mechanical allodynia and thermal hyperalgesia but not cold allodynia. PCX‐evoked pain behaviors were associated with the sensitization of all primary afferent fibers. PCX did not activate microglia or astrocytes in the spinal cord. However, it significantly activated astrocytes in the primary sensory (S1) cortex without affecting S1 microglial activation there. GJG significantly inhibited the PCX‐induced mechanical allodynia by 50% and thermal hyperalgesia by 90%, which was in accordance with the abolishment of astrocytic activation in the S1 cortex. Finally, the inhibition of S1 astrocytes by an astrocyte‐toxin L‐alpha‐aminoadipic acid abolished the PCX‐induced neuropathy. Our findings suggest that astrocytes in the S1 cortex would play an important role in the pathogenesis of PCX‐induced neuropathy and are a potential target for its treatment.

Role of neuroglia in neuropathic pain and depression

Patients with neuropathic pain induced by nerve injury usually present with co-morbid affective changes, such as depression. Neuroglia was reported to play an important role in the development and maintenance of neuropathic pain both centrally and peripherally. Meanwhile, there have been studies showing that neuroglia participated in the development of depression. However, the specific role of neuroglia in neuropathic pain and depression has not been reviewed comprehensively. Therefore, we summarized the recent findings on the role of neuroglia in neuropathic pain and depression. Based on this review, we found a bridge-like role of neuroglia in neuropathic pain co-morbid with depression. This review may provide therapeutic implications in the treatment of neuropathic pain and offer potential help in the studies of mechanisms in the future.

MicroRNA-133b-3p targets purinergic P2X4 receptor to regulate central poststroke pain in rats

Central poststroke pain (CPSP) is a neuropathic pain syndrome that usually occurs after cerebrovascular accidents. Currently, the pathogenesis of CPSP is not fully understood. Purinergic P2X4 receptor (P2X4R) is implicated in neuropathic pain including CPSP. Herein, we demonstrated that the levels of microRNA-133b-3p (miR-133b-3p), which targets P2X4R transcripts, were significantly downregulated in the ventral posterolateral nucleus of the thalamus (VPL), cerebrospinal fluid (CSF), and plasma of CPSP rats. The expression levels of miR-133b-3p negatively correlated with the severity of allodynia. Genetic knockdown of P2X4R in the VPL protected CPSP rats against allodynia. Similarly, genetic overexpression of miR-133b-3p in the VPL reversed the allodynia established in CPSP rats via downregulation of P2X4R expression. Treatment using gabapentin in CPSP rats significantly restored the decreased miR-133b-3p expression in the VPL, CSF, and plasma and blocked allodynia in CPSP rats. The administration of an miR-133b-3p inhibitor into the VPL abolished the antiallodynic activity of gabapentin. This mechanism was associated with P2X4R expression and involved the endogenous opioid system. Human patients with CPSP showed decreased plasma levels of miR-133b-3p compared with those of control participants. Logistic regression analysis of our patient cohort showed that determining plasma levels of miR-133b-3p may be useful for CPSP diagnosis and treatment.

β2-and β3-Adrenergic Receptors Contribute to Cancer-Evoked Pain in a Mouse Model of Osteosarcoma via Modulation of Neural Macrophages

The mechanisms involved in the development and maintenance of cancer pain remain largely unidentified. Recently, it has been reported that β-adrenergic receptors (β-ARs), mainly β2-and β3-ARs, contribute to tumor proliferation and progression and may favor cancer-associated pain and neuroinflammation. However, the mechanism underlying β-ARs in cancer pain is still unknown. Here, we investigated the role of β1-, β2-and β3-ARs in a mouse model of cancer pain generated by the para-tibial injection of K7M2 osteosarcoma cells. Results showed a rapid tumor growth in the soft tissue associated with the development of mechanical allodynia in the hind paw ipsilateral to the injected site. In addition to reduce tumor growth, both propranolol and SR59230A, β1-/β2-and β3-AR antagonists, respectively, attenuated mechanical allodynia, the number of macrophages and an oxidative stress by-product accumulated in the ipsilateral tibial nerve. The selective β1-AR antagonist atenolol was able to slightly reduce the tumor growth but showed no effect in reducing the development of mechanical allodynia. Results suggest that the development of the mechanical allodynia in K7M2 osteosarcoma-bearing mice is mediated by oxidative stress associated with the recruitment of neural macrophages, and that antagonism of β2-and β3-ARs contribute not solely to the reduction of tumor growth, but also in cancer pain. Thus, the targeting of the β2-and β3-ARs signaling may be a promising therapeutic strategy against both tumor progression and the development of cancer-evoke pain in osteosarcoma.

P2X4 Receptors Mediate Ca2+ Release from Lysosomes in Response to Stimulation of P2X7 and H1 Histamine Receptors

The P2X4 purinergic receptor is targeted to endolysosomes, where it mediates an inward current dependent on luminal ATP and pH. Activation of P2X4 receptors was previously shown to trigger lysosome fusion, but the regulation of P2X4 receptors and their role in lysosomal Ca²⁺ signaling are poorly understood. We show that lysosomal P2X4 receptors are activated downstream of plasma membrane P2X7 and H₁ histamine receptor stimulation. When P2X4 receptors are expressed, the increase in near-lysosome cytosolic [Ca²⁺] is exaggerated, as detected with a low-affinity targeted Ca²⁺ sensor. P2X4-dependent changes in lysosome properties were triggered downstream of P2X7 receptor activation, including an enlargement of lysosomes indicative of homotypic fusion and a redistribution of lysosomes towards the periphery of the cell. Lysosomal P2X4 receptors, therefore, have a role in regulating lysosomal Ca²⁺ release and the regulation of lysosomal membrane trafficking.

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

BACKGROUND

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSION

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

Microglial P2Y14 receptor contributes to central sensitization following repeated inflammatory dural stimulation

BACKGROUND

Recent studies have indicated that P2Y receptors in spinal microglia play a role in the development of neuropathic and inflammatory pain. However, it remains unclear whether P2Y receptors in microglia are involved in the pathogenesis of migraine. Therefore, the aim of this study was to investigate the role of microglial P2Y14 receptor in trigeminal cervical complex (TCC) in migraine.

METHODS

We used a rat model of migraine induced by repeated inflammatory stimulation of the dura and examined the expression of P2Y14 receptor in the TCC in migraine rats by Western Blotting and immunofluorescence staining. Then, we determined the effect of P2Y14 antagonist PPTN on inflammatory soup (IS)-induced mechanical allodynia, microglial activation and ERK expression in TCC.

RESULTS

The expression level of P2Y14 receptor increased significantly in microglia in TCC after 4 or 7 days of repeated IS stimulation of the dura. Application of PPTN significantly attenuated the decrease of periorbital pain threshold in migraine model rats. In addition, repeated IS stimulation of the dura induced the activation of microglia and the phosphorylation of the ERK1/2 in microglia in TCC, which were abolished by the application of PPTN.

CONCLUSION

Our findings suggest that the increased P2Y14 receptor in microglia in TCC play a crucial role in the generation of mechanical allodynia in migraine rat model. Furthermore, the activation of the P2Y14 receptor is involved in microglial activation and ERK phosphorylation as well. The P2Y14 receptor in microglia might be used as a potential target for migraine treatment.

Neuro-immune signatures in chronic low back pain subtypes

We recently showed that patients with different chronic pain conditions (such as chronic low back pain, fibromyalgia, migraine, and Gulf War Illness) demonstrated elevated brain and/or spinal cord levels of the glial marker 18 kDa translocator protein, which suggests that neuroinflammation might be a pervasive phenomenon observable across multiple etiologically heterogeneous pain disorders. Interestingly, the spatial distribution of this neuroinflammatory signal appears to exhibit a degree of disease specificity (e.g. with respect to the involvement of the primary somatosensory cortex), suggesting that different pain conditions may exhibit distinct "neuroinflammatory signatures". To further explore this hypothesis, we tested whether neuroinflammatory signal can characterize putative etiological subtypes of chronic low back pain patients based on clinical presentation. Specifically, we explored neuroinflammation in patients whose chronic low back pain either did or did not radiate to the leg (i.e. "radicular" vs. "axial" back pain). Fifty-four chronic low back pain patients, twenty-six with axial back pain (43.7 ± 16.6 y.o. [mean±SD]) and twenty-eight with radicular back pain (48.3 ± 13.2 y.o.), underwent PET/MRI with [11C]PBR28, a second-generation radioligand for the 18 kDa translocator protein. [11C]PBR28 signal was quantified using standardized uptake values ratio (validated against volume of distribution ratio; n = 23). Functional MRI data were collected simultaneously to the [11C]PBR28 data 1) to functionally localize the primary somatosensory cortex back and leg subregions and 2) to perform functional connectivity analyses (in order to investigate possible neurophysiological correlations of the neuroinflammatory signal). PET and functional MRI measures were compared across groups, cross-correlated with one another and with the severity of "fibromyalgianess" (i.e. the degree of pain centralization, or "nociplastic pain"). Furthermore, statistical mediation models were employed to explore possible causal relationships between these three variables. For the primary somatosensory cortex representation of back/leg, [11C]PBR28 PET signal and functional connectivity to the thalamus were: 1) higher in radicular compared to axial back pain patients, 2) positively correlated with each other and 3) positively correlated with fibromyalgianess scores, across groups. Finally, 4) fibromyalgianess mediated the association between [11C]PBR28 PET signal and primary somatosensory cortex-thalamus connectivity across groups. Our findings support the existence of "neuroinflammatory signatures" that are accompanied by neurophysiological changes, and correlate with clinical presentation (in particular, with the degree of nociplastic pain) in chronic pain patients. These signatures may contribute to the subtyping of distinct pain syndromes and also provide information about inter-individual variability in neuro-immune brain signals, within diagnostic groups, that could eventually serve as targets for mechanism-based precision medicine approaches.

Brain Immune Interactions-Novel Emerging Options to Treat Acute Ischemic Brain Injury

Ischemic stroke is still among the leading causes of mortality and morbidity worldwide. Despite intensive advancements in medical sciences, the clinical options to treat ischemic stroke are limited to thrombectomy and thrombolysis using tissue plasminogen activator within a narrow time window after stroke. Current state of the art knowledge reveals the critical role of local and systemic inflammation after stroke that can be triggered by interactions taking place at the brain and immune system interface. Here, we discuss different cellular and molecular mechanisms through which brain–immune interactions can take place. Moreover, we discuss the evidence how the brain influence immune system through the release of brain derived antigens, damage-associated molecular patterns (DAMPs), cytokines, chemokines, upregulated adhesion molecules, through infiltration, activation and polarization of immune cells in the CNS. Furthermore, the emerging concept of stemness-induced cellular immunity in the context of neurodevelopment and brain disease, focusing on ischemic implications, is discussed. Finally, we discuss current evidence on brain–immune system interaction through the autonomic nervous system after ischemic stroke. All of these mechanisms represent potential pharmacological targets and promising future research directions for clinically relevant discoveries.

Anti-AQP4 autoantibodies promote ATP release from astrocytes and induce mechanical pain in rats

Background

Intractable neuropathic pain is a common symptom of neuromyelitis optica spectrum disorder (NMOSD). However, the underlying mechanism of NMOSD pain remains to be elucidated. In this study, we focused on ATP, which is one of the damage-associated molecular patterns, and also a well-recognized molecule involved in peripheral neuropathic pain.

Methods

We assessed the development of pain symptoms by injecting anti-AQP4 recombinant autoantibodies (rAQP4 IgG) into rat spinal cords. We incubated HEK293 cells expressing AQP4 (HEK-AQP4) and rat astrocytes with rAQP4 IgG and assessed the level of ATP in the supernatant. We performed transcriptome analysis of the spinal cords injected with rAQP4 IgG. Pharmacological inhibition was also applied to investigate the involvement of ATP in the development of neuropathic pain in our rat model. The ATP concentration within the cerebrospinal fluid was examined in patients with NMOSD and other neurological diseases.

Results

Development of mechanical allodynia was confirmed in rAQP4 IgG–treated rats. AQP4-Ab–mediated extracellular ATP release from astrocytes was observed in vitro, and pharmacological inhibition of ATP receptor reversed mechanical allodynia in the rAQP4 IgG–treated rats. Furthermore, transcriptome analysis revealed elevation of gene expressions related to several ATP receptors including P2rx4 and IL1B in the spinal cord of rAQP4 IgG–treated rats. In patients, CSF ATP concentration was significantly higher in the acute and remission phase of NMOSD than in multiple sclerosis or other neurological disorders.

Conclusion

Anti-AQP4 antibody was shown to induce the release of extracellular ATP from astrocytes. The ATP-mediated development of mechanical allodynia was also suggested in rats treated with anti-AQP4 antibody. Our study indicates the pivotal role of ATP in the pain mechanism of NMOSD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02232-w.

The anatomy of pain and suffering in the brain and its clinical implications

Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage. Chronic pain, with a prevalence of 20-30 % is the major cause of human suffering worldwide, because effective, specific and safe therapies have yet to be developed. It is unevenly distributed among sexes, with women experiencing more pain and suffering. Chronic pain can be anatomically and phenomenologically dissected into three separable but interacting pathways, a lateral 'painfulness' pathway, a medial 'suffering' pathway and a descending pain inhibitory pathway. One may have pain(fullness) without suffering and suffering without pain(fullness). Pain sensation leads to suffering via a cognitive, emotional and autonomic processing, and is expressed as anger, fear, frustration, anxiety and depression. The medial pathway overlaps with the salience and stress networks, explaining that behavioural relevance or meaning determines the suffering associated with painfulness. Genetic and epigenetic influences trigger chronic neuroinflammatory changes which are involved in transitioning from acute to chronic pain. Based on the concept of the Bayesian brain, pain (and suffering) can be regarded as the consequence of an imbalance between the two ascending and the descending pain inhibitory pathways under control of the reward system. The therapeutic clinical implications of this simple pain model are obvious. After categorizing the working mechanisms of each of the available treatments (pain killers, psychopharmacology, psychotherapy, neuromodulation, psychosurgery, spinal cord stimulation) to 1 or more of the 3 pathways, a rational combination can be proposed of activating the descending pain inhibitory pathway in combination with inhibition of the medial and lateral pathway, so as to rebalance the pain (and suffering) pathways.

HMGB1 released from nociceptors mediates inflammation

Inflammation, the body's primary defensive response system to injury and infection, is triggered by molecular signatures of microbes and tissue injury. These molecules also stimulate specialized sensory neurons, termed nociceptors. Activation of nociceptors mediates inflammation through antidromic release of neuropeptides into infected or injured tissue, producing neurogenic inflammation. Because HMGB1 is an important inflammatory mediator that is synthesized by neurons, we reasoned nociceptor release of HMGB1 might be a component of the neuroinflammatory response. In support of this possibility, we show here that transgenic nociceptors expressing channelrhodopsin-2 (ChR2) directly release HMGB1 in response to light stimulation. Additionally, HMGB1 expression in neurons was silenced by crossing synapsin-Cre (Syn-Cre) mice with floxed HMGB1 mice (HMGB1f/f). When these mice undergo sciatic nerve injury to activate neurogenic inflammation, they are protected from the development of cutaneous inflammation and allodynia as compared to wild-type controls. Syn-Cre/HMGB1fl/fl mice subjected to experimental collagen antibody-induced arthritis, a disease model in which nociceptor-dependent inflammation plays a significant pathological role, are protected from the development of allodynia and joint inflammation. Thus, nociceptor HMGB1 is required to mediate pain and inflammation during sciatic nerve injury and collagen antibody-induced arthritis.

Taspine is a natural product that suppresses P2X4 receptor activity via phosphoinositide 3-kinase inhibition

BACKGROUND & PURPOSE

P2X4 is a ligand-gated cation channel activated by extracellular ATP, involved in neuropathic pain, inflammation and arterial tone.

EXPERIMENTAL APPROACH

Natural products were screened against human or mouse P2X4 activity using fura-2 loaded 1321N1 cells for measurement of intracellular Ca²⁺ responses; whole-cell currents were measured by patch clamp electrophysiological. Human primary macrophage chemokine release was used to assess effect of taspine on inflammatory cell function. An enzymatic assay was performed to assess the effect of taspine on recombinant PI3-kinase.

KEY RESULTS

A natural product screen identified taspine as an inhibitor of human P2X4 activity. Taspine inhibits human and mouse P2X4-mediated Ca²⁺ influx in 1321N1 cells expressing receptors (IC₅₀ 1.6±0.4 μM and 1.6±0.3 μM, respectively), but lacked activity at human P2X2, P2X3, P2X2/3 and P2X7 receptors. Taspine inhibited the maximal response at human and mouse P2X4 but had no effect on ATP potency. Taspine has a slow onset rate (~15 mins for half-maximal inhibition), irreversible over 30 minutes of washout. Taspine inhibits P2X4-mediated Ca²⁺ signalling in mouse BV-2 microglia cells and human primary macrophage. Taspine inhibited P2X4-mediated CXCL5 secretion in human primary macrophage. Taspine reversed ivermectin-induced potentiation of P2X4 currents in 1321N1 stably expressing cells. The known PI3-kinase inhibitor LY294002 mimicked the properties of taspine on P2X4-mediated Ca²⁺ influx and whole-cell currents. Taspine directly inhibited the enzymatic activity of recombinant PI3-kinase in a competitive manner.

CONCLUSIONS AND IMPLICATIONS

Taspine is a novel natural product P2X4 inhibitor, mediating its effect through PI3-kinase inhibitor rather than receptor antagonism. Taspine can inhibit the pro-inflammatory signalling by P2X4 in human primary macrophage.

Age-related gene expression changes in lumbar spinal cord: Implications for neuropathic pain

Clinically, pain has an uneven incidence throughout lifespan and impacts more on the elderly. In contrast, preclinical models of pathological pain have typically used juvenile or young adult animals to highlight the involvement of glial populations, proinflammatory cytokines, and chemokines in the onset and maintenance of pathological signalling in the spinal dorsal horn. The potential impact of this mismatch is also complicated by the growing appreciation that the aged central nervous system exists in a state of chronic inflammation because of enhanced proinflammatory cytokine/chemokine signalling and glial activation. To address this issue, we investigated the impact of aging on the expression of genes that have been associated with neuropathic pain, glial signalling, neurotransmission and neuroinflammation. We used qRT-PCR to quantify gene expression and focussed on the dorsal horn of the spinal cord as this is an important perturbation site in neuropathic pain. To control for global vs region-specific age-related changes in gene expression, the ventral half of the spinal cord was examined. Our results show that expression of proinflammatory chemokines, pattern recognition receptors, and neurotransmitter system components was significantly altered in aged (24-32 months) versus young mice (2-4 months). Notably, the magnitude and direction of these changes were spinal-cord region dependent. For example, expression of the chemokine, Cxcl13, increased 119-fold in dorsal spinal cord, but only 2-fold in the ventral spinal cord of old versus young mice. Therefore, we propose the dorsal spinal cord of old animals is subject to region-specific alterations that prime circuits for the development of pathological pain, potentially in the absence of the peripheral triggers normally associated with these conditions.

Microglial heterogeneity in chronic pain

In this review, we report existing preclinical evidence on how the CNS compartment as well as sex affect microglia functions in health. We highlight that recent advances in transcriptomics analyses have led to thorough characterization of disease-associated microglial states in mice and humans. We then consider the specific scenario of peripheral nerve or tissue injury which induce expression of a specific subset of genes in microglia in the dorsal horn of the spinal cord. We suggest the intriguing possibility that future studies may disclose the existence of a unique microglia transcriptional profile that is associated with chronic pain conditions. We also collect evidence that microglial activation in pain-related areas of the brain can be observed in models of neuropathic pain in agreement with recent neuroimaging studies in chronic pain patients. Based on the evidence discussed here, we predict that future studies on the neuroimmune interactions in chronic pain should complement our current understanding of microglia functions, but also adventure in using novel approaches such as scRNA-seq, spatial transcriptomics, CYTOF and transmission electron microscopy to provide a more complete characterization of the function, transcriptome and structure of microglia in chronic pain.

Sex differences in neuroimmune and glial mechanisms of pain

ABSTRACT

Pain is the primary motivation for seeking medical care. Although pain may subside as inflammation resolves or an injury heals, it is increasingly evident that persistency of the pain state can occur with significant regularity. Chronic pain requires aggressive management to minimize its physiological consequences and diminish its impact on quality of life. Although opioids commonly are prescribed for intractable pain, concerns regarding reduced efficacy, as well as risks of tolerance and dependence, misuse, diversion, and overdose mortality rates limit their utility. Advances in development of nonopioid interventions hinge on our appreciation of underlying mechanisms of pain hypersensitivity. For instance, the contributory role of immunity and the associated presence of autoimmune syndromes has become of particular interest. Males and females exhibit fundamental differences in innate and adaptive immune responses, some of which are present throughout life, whereas others manifest with reproductive maturation. In general, the incidence of chronic pain conditions, particularly those with likely autoimmune covariates, is significantly higher in women. Accordingly, evidence is now accruing in support of neuroimmune interactions driving sex differences in the development and maintenance of pain hypersensitivity and chronicity. This review highlights known sexual dimorphisms of neuroimmune signaling in pain states modeled in rodents, which may yield potential high-value sex-specific targets to inform future analgesic drug discovery efforts.

Spared Nerve Injury Causes Sexually Dimorphic Mechanical Allodynia and Differential Gene Expression in Spinal Cords and Dorsal Root Ganglia in Rats

Neuropathic pain is more prevalent in women. However, females are under-represented in animal experiments, and the mechanisms of sex differences remain inadequately understood. We used the spared nerve injury (SNI) model in rats to characterize sex differences in pain behaviour, unbiased RNA-Seq and proteomics to study the mechanisms. Male and female rats were subjected to SNI- and sham-surgery. Mechanical and cold allodynia were assessed. Ipsilateral lumbar dorsal root ganglia (DRG) and spinal cord (SC) segments were collected for RNA-seq analysis with DESeq2 on Day 7. Cerebrospinal fluid (CSF) samples for proteomic analysis and DRGs and SCs for analysis of IB-4 and CGRP, and IBA1 and GFAP, respectively, were collected on Day 21. Females developed stronger mechanical allodynia. There were no differences between the sexes in CGRP and IB-4 in the DRG or glial cell markers in the SC. No CSF protein showed change following SNI. DRG and SC showed abundant changes in gene expression. Sexually dimorphic responses were found in genes related to T-cells (cd28, ctla4, cd274, cd4, prf1), other immunological responses (dpp4, c5a, cxcr2 and il1b), neuronal transmission (hrh3, thbs4, chrna4 and pdyn), plasticity (atf3, c1qc and reg3b), and others (bhlhe22, mcpt1l, trpv6). We observed significantly stronger mechanical allodynia in females and numerous sexually dimorphic changes in gene expression following SNI in rats. Several genes have previously been linked to NP, while some are novel. Our results suggest gene targets for further studies in the development of new, possibly sex-specific, therapies for NP.

Effects of Dorsal Column Spinal Cord Stimulation on Neuroinflammation: Revisiting Molecular Mechanisms and Clinical Outcomes on Chronic Lumbar/Leg Pain and Failed Back Surgery Syndrome

OBJECTIVE

In this narrative review, we reviewed and discussed current literature describing the molecular mechanisms leading to neuroinflammation and its role in the onset and progression of chronic neuropathic lumbar and leg pain in patients with persistent spinal pain syndrome. In addition, we reviewed the proposed mechanisms and impact of spinal cord stimulation (SCS) on neuroinflammation.

METHODS

A broad search of current literature in PubMed, Embase, Scopus, Cochrane library, Medline/Ovid, and Web of Science was performed using the following terms and their combinations: "biomarkers", "chronic back and leg pain", "cytokines", "neuroinflammation", "spinal cord stimulation (scs)," and "spinal cord modulation". We selected: 1) articles published in the English language between January 2000 and July 2020 2) preclinical and clinical data 3) case reports 4) meta-analysis and systematic reviews and 5) conference abstracts. Manuscripts not disclosing methodology or without full-text availability were excluded.

DISCUSSION

SCS techniques have gradually evolved since inception to include novel methods such as burst-SCS, high frequency SCS, and differential targeted multiplexed SCS. The incidence of chronic pain after spine surgery is highly variable, with at least one third of patients developing persistent spinal pain syndrome. Novel SCS techniques have been associated with improved clinical and functional outcomes thus increasing patient quality of life.

CONCLUSION

Currently, health care providers rely on different options and methods for SCS when treating patients with refractory chronic lumbar pain and persistent spinal pain syndrome. Nevertheless, compelling clinical trials remain necessary to elucidate the long-term benefits and mechanisms of neuromodulation of all different types of SCS.

Analysis of binding residues in monoclonal antibody with high affinity for the head domain of the rat P2X4 receptor

P2X4 receptor is known to be involved in neuropathic pain. In order to detect the expression of P2X4 receptor on microglia at the time of onset of neuropathic pain, one approach consists on the preparation of the monoclonal antibodies with both selective binding and high affinity. We have recently established a monoclonal antibody (named 12-10H) which had high affinity to rat P2X4 receptor expressed in 1321N1 cells. The dissociation constants of the complex between the monoclonal antibodies obtained so far and the head domain (HD) in the rat P2X4 receptor were in the nanomolar range. To improve the affinity by rational mutations, we need to know the precious location of the binding site in these monoclonal antibodies. Here, we have analysed and identified the binding residues in the monoclonal antibody (12-10H) with high affinity for the HD of the rat P2X4 receptor by site-directed mutagenesis.

The effect of microglial inhibition on the expression of BDNF, KCC2, and GABAA receptor before and after the establishment of CCI-induced neuropathic pain model

Damage to the peripheral or central nervous system results in neuropathic pain. Based on a complicated mechanism, neuropathic pain has no efficient treatment so far. It has been well-known that the expression of some proteins (BDNF, KCC2, GABA-A) during neuropathic pain changes. Microglial cell activation is considered as a trigger to alter the expression of these proteins. In the current study, the effect of minocycline as a potent microglial activation inhibitor on the gene and protein expression of these neuropathic pain mediators was investigated. This experiment was done in two paradigms, preinjury and postinjury administration of minocycline. In each paradigm, male Wistar rats (weight 150-200 g, n = 6) were allocated to sham, control, and drug groups. Minocycline (30 mg/kg, i.p.) was injected 1 h before or at day seven after nerve injury and continued till day 14 in the preemptive or postinjury part of the study, respectively. After the last injection, the animals were decapitated and the lumbar part of the spinal cord was isolated to assess the expression of genes and proteins of interest. In the preventive study, minocycline increased the expression of KCC2 and GABA-A/γ₂ proteins and decreased BDNF expression. On the other hand, the target gene expression and protein expression were not changed when minocycline was administered after nerve injury. It seems that minocycline was able to change the expression of proteins of interest merely when used before nerve damage.

BDNF produced by cerebral microglia promotes cortical plasticity and pain hypersensitivity after peripheral nerve injury

Peripheral nerve injury–induced mechanical allodynia is often accompanied by abnormalities in the higher cortical regions, yet the mechanisms underlying such maladaptive cortical plasticity remain unclear. Here, we show that in male mice, structural and functional changes in the primary somatosensory cortex (S1) caused by peripheral nerve injury require neuron-microglial signaling within the local circuit. Following peripheral nerve injury, microglia in the S1 maintain ramified morphology and normal density but up-regulate the mRNA expression of brain-derived neurotrophic factor (BDNF). Using in vivo two-photon imaging and Cx3cr1^CreER;Bdnf^flox mice, we show that conditional knockout of BDNF from microglia prevents nerve injury–induced synaptic remodeling and pyramidal neuron hyperactivity in the S1, as well as pain hypersensitivity in mice. Importantly, S1-targeted removal of microglial BDNF largely recapitulates the beneficial effects of systemic BDNF depletion on cortical plasticity and allodynia. Together, these findings reveal a pivotal role of cerebral microglial BDNF in somatosensory cortical plasticity and pain hypersensitivity.

This study reveals that brain-derived neurotrophic factor (BDNF) from cerebral microglia contributes to nerve injury-induced synaptic remodeling and neuronal hyperactivity, and ultimately contributes to pain sensitivity in mice; removal of microglial BDNF has beneficial effects on cortical plasticity and pain.

Increased Expression of Thymic Stromal Lymphopoietin in Chronic Constriction Injury of Rat Nerve

Thymic stromal lymphopoietin (TSLP) is a well-known cytokine for T helper 2 inflammatory responses. A nerve injury activates the neuroinflammation cascade and neuron-glia interaction in dorsal root ganglions (DRG)s, leading to neuropathic pain. Therefore, this study was to investigate the role of TSLP after nerve injury. Male Sprague-Dawley rats were divided as an experimental group with chronic constriction injury (CCI) to the sciatic nerve and a control group. The mechanical pain threshold response was determined by calibration forceps. After assessment of mechanical allodynia, the ipsilateral spinal cord, DRG, sciatic nerve and skin were harvested. Immunofluorescence staining was performed to identify cell types with various markers. Western blot analyses were performed to evaluate protein expressions. Mechanical allodynia developed after CCI and persisted for the next 14 days. Astrocyte reactions occurred and continued until day 14, too. After CCI, DRG and the sciatic nerve also had significantly increased expressions of TSLP/TSLP-R/STAT5. The TSLPR was localized to sensory neuronal endings innervating the skin. This study is the first to demonstrate that the TSLP complex and the STAT5 pathway in nerve are potential therapeutic targets because of their roles in pain regulation after nerve injury.

Thalamic neurometabolite alterations in patients with knee osteoarthritis before and after total knee replacement

ABSTRACT

The weak association between disability levels and "peripheral" (ie, knee) findings suggests that central nervous system alterations may contribute to the pathophysiology of knee osteoarthritis (KOA). Here, we evaluated brain metabolite alterations in patients with KOA, before and after total knee arthroplasty (TKA), using 1H-magnetic resonance spectroscopy (MRS). Thirty-four presurgical patients with KOA and 13 healthy controls were scanned using a PRESS sequence (TE = 30 ms, TR = 1.7 seconds, voxel size = 15 × 15 × 15 mm). In addition, 13 patients were rescanned 4.1 ± 1.6 (mean ± SD) weeks post-TKA. When using creatine (Cr)-normalized levels, presurgical KOA patients demonstrated lower N-acetylaspartate (NAA) (P < 0.001), higher myoinositol (mIns) (P < 0.001), and lower Choline (Cho) (P < 0.05) than healthy controls. The mIns levels were positively correlated with pain severity scores (r = 0.37, P < 0.05). These effects reached statistical significance also using water-referenced concentrations, except for the Cho group differences (P ≥ 0.067). Post-TKA patients demonstrated an increase in NAA (P < 0.01), which returned to the levels of healthy controls (P > 0.05), irrespective of metric. In addition, patients demonstrated postsurgical increases in Cr-normalized (P < 0.001), but not water-referenced mIns, which were proportional to the NAA/Cr increases (r = 0.61, P < 0.05). Because mIns is commonly regarded as a glial marker, our results are suggestive of a possible dual role for neuroinflammation in KOA pain and post-TKA recovery. Moreover, the apparent postsurgical normalization of NAA, a putative marker of neuronal integrity, might implicate mitochondrial dysfunction, rather than neurodegenerative processes, as a plausible pathophysiological mechanism in KOA. More broadly, our results add to a growing body of literature suggesting that some pain-related brain alterations can be reversed after peripheral surgical treatment.

Responses of cutaneous C-fiber afferents and spinal microglia after hindlimb cast immobilization in rats

Previous studies have shown that persistent limb immobilization using a cast increases nociceptive behavior to somatic stimuli in rats. However, the peripheral neural mechanisms of nociception remain unclear. Using single-fiber electrophysiological recordings in vitro, we examined the general characteristics of cutaneous C-fiber afferents in the saphenous nerve and their responsiveness to mechanical and heat stimuli in a rat model of immobilization-induced pain by subjecting the rats to hindlimb cast immobilization for 4 weeks. The mechanical response of C-fibers appeared to increase in the model; however, statistical analysis revealed that neither the response threshold nor the response magnitude was altered. The general characteristics and heat responses of the C-fibers were not altered. The number of microglia and cell diameters significantly increased in the superficial dorsal horn of the lumbar spinal cord. Thus, activated microglia-mediated spinal mechanisms are associated with the induction of nociceptive hypersensitivity in rats after persistent cast immobilization.

Yokukansan, a Japanese Herbal Medicine, Suppresses Substance PInduced Production of Interleukin-6 and Interleukin-8 by Human U373 MG Glioblastoma Astrocytoma Cells

BACKGROUND

Yokukansan is a traditional Japanese herbal medicine that has an antiallodynic effect in patients with chronic pain. However, the mechanisms by which yokukansan inhibits neuropathic pain are unclear.

OBJECTIVE

This study aimed to investigate the molecular effects of yokukansan on neuroinflammation in U373 MG glioblastoma astrocytoma cells, which express a functional high-affinity neurokinin 1 receptor (substance P receptor), and produce interleukin (IL)-6 and IL-8 in response to stimulation by substance P (SP).

METHODS

We assessed the effect of yokukansan on the expression of ERK1/2, P38 MAPK, nuclear factor (NF)-κB, and cyclooxygenase-2 (COX-2) in U373 cells by western blot assay. Levels of IL-6 and IL-8 in conditioned medium obtained after stimulation of cells with SP for 24 h were measured by enzyme-linked immunosorbent assay. All experiments were conducted in triplicate. Results were analyzed by one-way ANOVA, and significance was accepted at p < 0.05.

RESULTS

Yokukansan suppressed SP-induced production of IL-6 and IL-8 by U373 MG cells, and downregulated SP-induced COX-2 expression. Yokukansan also inhibited phosphorylation of ERK1/2 and p38 MAPK, as well as nuclear translocation of NF-κB, induced by SP stimulation of U373 MG cells.

CONCLUSION

Yokukansan exhibits anti-inflammatory activity by suppressing SP-induced production of IL-6 and IL-8 and downregulating COX-2 expression in U373 MG cells, possibly via inhibition of the activation of signaling molecules, such as ERK1/2, p38 MAPK, and NF-κB.

Neuropathic pain modeling: Focus on synaptic and ion channel mechanisms

Animal models of pain consist of modeling a pain-like state and measuring the consequent behavior. The first animal models of neuropathic pain (NP) were developed in rodents with a total lesion of the sciatic nerve. Later, other models targeting central or peripheral branches of nerves were developed to identify novel mechanisms that contribute to persistent pain conditions in NP. Objective assessment of pain in these different animal models represents a significant challenge for pre-clinical research. Multiple behavioral approaches are used to investigate and to validate pain phenotypes including withdrawal reflex to evoked stimuli, vocalizations, spontaneous pain, but also emotional and affective behaviors. Furthermore, animal models were very useful in investigating the mechanisms of NP. This review will focus on a detailed description of rodent models of NP and provide an overview of the assessment of the sensory and emotional components of pain. A detailed inventory will be made to examine spinal mechanisms involved in NP-induced hyperexcitability and underlying the current pharmacological approaches used in clinics with the possibility to present new avenues for future treatment. The success of pre-clinical studies in this area of research depends on the choice of the relevant model and the appropriate test based on the objectives of the study.

Contribution of P2X4 receptor in pain associated with rheumatoid arthritis: a review

Pain is the most common symptom reported by patients with rheumatoid arthritis (RA) even after the resolution of chronic joint inflammation. It is believed that RA-associated pain is not solely due to inflammation, but could also be attributed to aberrant modifications to the central nervous system. The P2X4 receptor (P2X4R) is an ATP-activated purinergic receptor that plays a significant role in the transmission of information in the nervous system and pain. The involvement of P2X4R during the pathogenesis of chronic inflammatory pain and neuropathic pain is well-established. The attenuation of this receptor alleviates disease pathogenesis and related symptoms, including hyperalgesia and allodynia. Although some studies have revealed the contribution of P2X4R in promoting joint inflammation in RA, how it implicates pain associated with RA at peripheral and central nervous systems is still lacking. In this review, the possible contributions of P2X4R in the nervous system and how it implicates pain transmission and responses were examined.

Can FDA-Approved Immunomodulatory Drugs be Repurposed/Repositioned to Alleviate Chronic Pain?

Pain is among the most widespread chronic health condition confronting society today and our inability to manage chronic pain contributes to the opioid abuse epidemic in America. The immune system is known to contribute to acute and chronic pain, but only limited therapeutic treatments such as non-steroid anti-inflammatory drugs have resulted from this knowledge. The last decade has shed light on neuro-immune interactions mediating the development, maintenance, and resolution of chronic pain. Here, we do not aim to perform a comprehensive review of all immune mechanisms involved in chronic pain, but to briefly review the contribution of the main cytokines and immune cells (macrophages, microglia, mast cells and T cells) to chronic pain. Given the urgent need to address the Pain crisis, we provocatively propose to repurpose/reposition FDA-approved immunomodulatory drugs for their potential to alleviate chronic pain. Repositioning or repurposing offers an attractive way to accelerate the arrival of new analgesics.

Alzheimer and Purinergic Signaling: Just a Matter of Inflammation?

Alzheimer's disease (AD) is a widespread neurodegenerative pathology responsible for about 70% of all cases of dementia. Adenosine is an endogenous nucleoside that affects neurodegeneration by activating four membrane G protein-coupled receptor subtypes, namely P1 receptors. One of them, the A_2A subtype, is particularly expressed in the brain at the striatal and hippocampal levels and appears as the most promising target to counteract neurological damage and adenosine-dependent neuroinflammation. Extracellular nucleotides (ATP, ADP, UTP, UDP, etc.) are also released from the cell or are synthesized extracellularly. They activate P2X and P2Y membrane receptors, eliciting a variety of physiological but also pathological responses. Among the latter, the chronic inflammation underlying AD is mainly caused by the P2X7 receptor subtype. In this review we offer an overview of the scientific evidence linking P1 and P2 mediated purinergic signaling to AD development. We will also discuss potential strategies to exploit this knowledge for drug development.

Thalamic neuroinflammation as a reproducible and discriminating signature for chronic low back pain

ABSTRACT

Using positron emission tomography, we recently demonstrated elevated brain levels of the 18 kDa translocator protein (TSPO), a glial activation marker, in chronic low back pain (cLBP) patients, compared to healthy controls (HCs). Here, we first sought to replicate the original findings in an independent cohort (15 cLBP, 37.8 ± 12.5 y/o; 18 HC, 48.2 ± 12.8 y/o). We then trained random forest machine learning algorithms based on TSPO imaging features combining discovery and replication cohorts (totaling 25 cLBP, 42.4 ± 13.2 y/o; 27 HC, 48.9 ± 12.6 y/o), to explore whether image features other than the mean contain meaningful information that might contribute to the discrimination of cLBP patients and HC. Feature importance was ranked using SHapley Additive exPlanations values, and the classification performance (in terms of area under the curve values) of classifiers containing only the mean, other features, or all features was compared using the DeLong test. Both region-of-interest and voxelwise analyses replicated the original observation of thalamic TSPO signal elevations in cLBP patients compared to HC (P < 0.05). The random forest-based analyses revealed that although the mean is a discriminating feature, other features demonstrate similar level of importance, including the maximum, kurtosis, and entropy. Our observations suggest that thalamic neuroinflammatory signal is a reproducible and discriminating feature for cLBP, further supporting a role for glial activation in human cLBP, and the exploration of neuroinflammation as a therapeutic target for chronic pain. This work further shows that TSPO signal contains a richness of information that the simple mean might fail to capture completely.

Unilateral facial injection of Botulinum neurotoxin A attenuates bilateral trigeminal neuropathic pain and anxiety-like behaviors through inhibition of TLR2-mediated neuroinflammation in mice

Objectives

In this study, we investigated the possible analgesic effects of Botulinum toxin type A (BoNT/A) on trigeminal neuralgia (TN). A modified TN mouse model was established by chronic constriction injury of the distal infraorbital nerve (dIoN-CCI) in mice, and the possible roles of microglia toll-like receptor 2 (TLR2) and neuroinflammation was investigated.

Methods

Male C57BL/6 mice were divided into 3 groups, including sham group, vehicle-treated TN group and BoNT/A-treated TN group. Bilateral mechanical pain hypersensitivity, anxiety-like and depressive-like behaviors were evaluated by using von Frey test, open field, elevated plus-maze testing, and forced swimming test in mice, respectively. The mRNA or protein expression levels of toll-like receptors (TLRs), glia activation markers and proinflammatory factors in the trigeminal nucleus caudalis (TNC) were tested by RT-qPCR, immunofluorescence and Western blotting. We also tested the pain behaviors of TN in Tlr2^−/− mice.

Results

We found that unilateral subcutaneous injection of BoNT/A into the whisker pad on the ipsilateral side of dIoN-CCI mice significantly attenuated bilateral mechanical pain hypersensitivity and anxiety-like behaviors induced by dIoN-CCI surgery in mice. The dIoN-CCI surgery significantly up-regulated the expression of TLR2, MyD88, CD11b (a microglia marker), IL-1β, TNF-α and IL-6 in the ipsilateral TNC in mice, and BoNT/A injection significantly inhibited the expression of these factors. Immunostaining results confirmed that BoNT/A injection significantly inhibited the microglia activation in the ipsilateral TNC in dIoN-CCI mice. TLR2 deficiency also alleviated bilateral mechanical pain hypersensitivity and the up-regulation of MyD88 expression in the TNC of dIoN-CCI mice.

Conclusion

These results indicate that unilateral injection of BoNT/A attenuated bilateral mechanical pain hypersensitivity and anxiety-like behaviors in dIoN-CCI mice, and the analgesic effects of BoNT/A may be associated with the inhibition of TLR2-mediated neuroinflammation in the TNC.

Role of microglia and P2X4 receptors in chronic pain

This study summarizes current understanding of the role of microglia and P2X4 receptor in chronic pain including neuropathic pain and of their therapeutic potential.

Pain plays an indispensable role as an alarm system to protect us from dangers or injuries. However, neuropathic pain, a debilitating pain condition caused by damage to the nervous system, persists for a long period even in the absence of dangerous stimuli or after injuries have healed. In this condition, pain becomes a disease itself rather than the alarm system and is often resistant to currently available medications. A growing body of evidence indicates that microglia, a type of macrophages residing in the central nervous system, play a crucial role in the pathogenesis of neuropathic pain. Whenever microglia in the spinal cord detect a damaging signal within the nervous system, they become activated and cause diverse alterations that change neural excitability, leading to the development of neuropathic pain. For over a decade, several lines of molecular and cellular mechanisms that define microglial activation and subsequently altered pain transmission have been proposed. In particular, P2X4 receptors (a subtype of purinergic receptors) expressed by microglia have been investigated as an essential molecule for neuropathic pain. In this review article, we describe our understanding of the mechanisms by which activated microglia cause neuropathic pain through P2X4 receptors, their involvement in several pathological contexts, and recent efforts to develop new drugs targeting microglia and P2X4 receptors.

Lack of correlation between spinal microgliosis and long-term development of tactile hypersensitivity in two different sciatic nerve crush injury

Microglia activation following peripheral nerve injury has been shown to contribute to central sensitization of the spinal cord for the development of neuropathic pain. In a recent study, we reported that the amount of nerve damage does not necessarily correlate with chronic pain development. Here we compared the response of spinal microglia, using immunohistochemistry as a surrogate of microglial activation, in mice with two different types of crush injury of the sciatic nerve. We confirmed that incomplete crush of the sciatic nerve (partial crush injury, PCI) resulted in tactile hypersensitivity after the recovery of sensory function (15 days after surgery), whereas the hypersensitivity was not observed after the complete crush (full crush injury, FCI). We observed that immunoreactivity for Iba-1, a microglial marker, was greater in the ipsilateral dorsal horn of lumbar (L4) spinal cord of mice 2 days after FCI compared to PCI, positively correlating with the intensity of crush injury. Ipsilateral Iba-1 reactivity was comparable between injuries at 7 days with a significant increase compared to the contralateral side. By day 15 after injury, ipsilateral Iba-1 immunoreactivity was much reduced compared to day 7 and was not different between the groups. Our results suggest that the magnitude of the early microgliosis is dependent on injury severity, but does not necessarily correlate with the long-term development of chronic pain-like hypersensitivity after peripheral nerve injury.

Microglia and Inhibitory Circuitry in the Medullary Dorsal Horn: Laminar and Time-Dependent Changes in a Trigeminal Model of Neuropathic Pain

Craniofacial neuropathic pain affects millions of people worldwide and is often difficult to treat. Two key mechanisms underlying this condition are a loss of the negative control exerted by inhibitory interneurons and an early microglial reaction. Basic features of these mechanisms, however, are still poorly understood. Using the chronic constriction injury of the infraorbital nerve (CCI-IoN) model of neuropathic pain in mice, we have examined the changes in the expression of GAD, the synthetic enzyme of GABA, and GlyT2, the membrane transporter of glycine, as well as the microgliosis that occur at early (5 days) and late (21 days) stages post-CCI in the medullary and upper spinal dorsal horn. Our results show that CCI-IoN induces a down-regulation of GAD at both postinjury survival times, uniformly across the superficial laminae. The expression of GlyT2 showed a more discrete and heterogeneous reduction due to the basal presence in lamina III of 'patches' of higher expression, interspersed within a less immunoreactive 'matrix', which showed a more substantial reduction in the expression of GlyT2. These patches coincided with foci lacking any perceptible microglial reaction, which stood out against a more diffuse area of strong microgliosis. These findings may provide clues to better understand the neural mechanisms underlying allodynia in neuropathic pain syndromes.

Temporal Contribution of Myeloid-Lineage TLR4 to the Transition to Chronic Pain: A Focus on Sex Differences

Complex regional pain syndrome (CRPS) is a chronic pain disorder with a clear acute-to-chronic transition. Preclinical studies demonstrate that toll-like receptor 4 (TLR4), expressed by myeloid-lineage cells, astrocytes, and neurons, mediates a sex-dependent transition to chronic pain; however, evidence is lacking on which exact TLR4-expressing cells are responsible. We used complementary pharmacologic and transgenic approaches in mice to more specifically manipulate myeloid-lineage TLR4 and outline its contribution to the transition from acute-to-chronic CRPS based on three key variables: location (peripheral vs central), timing (prevention vs treatment), and sex (male vs female). We demonstrate that systemic TLR4 antagonism is more effective at improving chronic allodynia trajectory when administered at the time of injury (early) in the tibial fracture model of CRPS in both sexes. In order to clarify the contribution of myeloid-lineage cells peripherally (macrophages) or centrally (microglia), we rigorously characterize a novel spatiotemporal transgenic mouse line, Cx3CR1-Cre^ERT2-eYFP;TLR4^fl/fl (TLR4 cKO) to specifically knock out TLR4 only in microglia and no other myeloid-lineage cells. Using this transgenic mouse, we find that early TLR4 cKO results in profound improvement in chronic, but not acute, allodynia in males, with a significant but less robust effect in females. In contrast, late TLR4 cKO results in partial improvement in allodynia in both sexes, suggesting that downstream cellular or molecular TLR4-independent events may have already been triggered. Overall, we find that the contribution of TLR4 is time- and microglia-dependent in both sexes; however, females also rely on peripheral myeloid-lineage (or other TLR4 expressing) cells to trigger chronic pain.SIGNIFICANCE STATEMENT The contribution of myeloid cell TLR4 to sex-specific pain progression remains controversial. We used complementary pharmacologic and transgenic approaches to specifically manipulate TLR4 based on three key variables: location (peripheral vs central), timing (prevention vs treatment), and sex (male vs female). We discovered that microglial TLR4 contributes to early pain progression in males, and to a lesser extent in females. We further found that maintenance of chronic pain likely occurs through myeloid TLR4-independent mechanisms in both sexes. Together, we define a more nuanced contribution of this receptor to the acute-to-chronic pain transition in a mouse model of complex regional pain syndrome.

The cannabinoid system and microglia in health and disease

Recent years have yielded significant advances in our understanding of microglia, the immune cells of the central nervous system (CNS). Microglia are key players in CNS development, immune surveillance, and the maintenance of proper neuronal function throughout life. In the healthy brain, homeostatic microglia have a unique molecular signature. In neurological diseases, microglia become activated and adopt distinct transcriptomic signatures, including disease-associated microglia (DAM) implicated in neurodegenerative disorders. Homeostatic microglia synthesise the endogenous cannabinoids 2-arachidonoylglycerol and anandamide and express the cannabinoid receptors CB1 and CB2 at constitutively low levels. Upon activation, microglia significantly increase their synthesis of endocannabinoids and upregulate their expression of CB2 receptors, which promote a protective microglial phenotype by enhancing their production of neuroprotective factors and reducing their production of pro-inflammatory factors. Here, we summarise the effects of the microglial cannabinoid system in the CNS demyelinating disease multiple sclerosis, the neurodegenerative diseases Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis, chronic inflammatory and neuropathic pain, and psychiatric disorders including depression, anxiety and schizophrenia. We discuss the therapeutic potential of cannabinoids in regulating microglial activity and highlight the need to further investigate their specific microglia-dependent immunomodulatory effects.

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.

Microglia: The Missing Link to Decipher and Therapeutically Control MS Progression?

Therapeutically controlling chronic progression in multiple sclerosis (MS) remains a major challenge. MS progression is defined as a steady loss of parenchymal and functional integrity of the central nervous system (CNS), occurring independent of relapses or focal, magnetic resonance imaging (MRI)-detectable inflammatory lesions. While it clinically surfaces in primary or secondary progressive MS, it is assumed to be an integral component of MS from the very beginning. The exact mechanisms causing progression are still unknown, although evolving evidence suggests that they may substantially differ from those driving relapse biology. To date, progression is assumed to be caused by an interplay of CNS-resident cells and CNS-trapped hematopoietic cells. On the CNS-resident cell side, microglia that are phenotypically and functionally related to cells of the monocyte/macrophage lineage may play a key role. Microglia function is highly transformable. Depending on their molecular signature, microglia can trigger neurotoxic pathways leading to neurodegeneration, or alternatively exert important roles in promoting neuroprotection, downregulation of inflammation, and stimulation of repair. Accordingly, to understand and to possibly alter the role of microglial activation during MS disease progression may provide a unique opportunity for the development of suitable, more effective therapeutics. This review focuses on the current understanding of the role of microglia during disease progression of MS and discusses possible targets for therapeutic intervention.

The role of the P2X4 receptor in trigeminal neuralgia, a common neurological disorder

Neurological disorders, which include various types of diseases with complex pathological mechanisms, are more common in the elderly and have shown increased prevalence, morbidity and mortality worldwide. Unfortunately, current therapies for these diseases are usually suboptimal or have undesirable side effects. This necessitates the development of new potential targets for disease-modifying therapies. P2X4R, a type of purinergic receptor, has multiple roles in neurological disorders. In this review, we briefly introduce a neurological disorder, trigeminal neuralgia and its' symptoms, etiology and pathology. Moreover, we focused on the role of P2X4R in neurological disorders and their related pathophysiologic mechanisms. Further studies of P2X4R are required to determine potential therapeutic effects for these pathophysiologies.

Microglial Cell Morphology and Phagocytic Activity Are Critically Regulated by the Neurosteroid Allopregnanolone: A Possible Role in Neuroprotection

Microglial cells are key players in neural pathogenesis and microglial function regulation appears to be pivotal in controlling neuroinflammatory/neurological diseases. Here, we investigated the effects and mechanism of action of neurosteroid allopregnanolone (ALLO) on murine microglial BV-2 cells and primary microglia in order to determine ALLO-induced immunomodulatory potential and to provide new insights for the development of both natural and safe neuroprotective strategies targeting microglia. Indeed, ALLO-treatment is increasingly suggested as beneficial in various models of neurological disorders but the underlying mechanisms have not been elucidated. Therefore, the microglial cells were cultured with various serum concentrations to mimic the blood-brain-barrier rupture and to induce their activation. Proliferation, viability, RT-qPCR, phagocytosis, and morphology analyzes, as well as migration with time-lapse imaging and quantitative morphodynamic methods, were combined to investigate ALLO actions on microglia. BV-2 cells express subunits of GABA-A receptor that mediates ALLO activity. ALLO (10µM) induced microglial cell process extension and decreased migratory capacity. Interestingly, ALLO modulated the phagocytic activity of BV-2 cells and primary microglia. Our results, which show a direct effect of ALLO on microglial morphology and phagocytic function, suggest that the natural neurosteroid-based approach may contribute to developing effective strategies against neurological disorders that are evoked by microglia-related abnormalities.

Optogenetic activation of spinal microglia triggers chronic pain in mice

Spinal microglia are highly responsive to peripheral nerve injury and are known to be a key player in pain. However, there has not been direct evidence showing that selective microglial activation in vivo is sufficient to induce chronic pain. Here, we used optogenetic approaches in microglia to address this question employing CX3CR1creER/+: R26LSL-ReaChR/+ transgenic mice, in which red-activated channelrhodopsin (ReaChR) is inducibly and specifically expressed in microglia. We found that activation of ReaChR by red light in spinal microglia evoked reliable inward currents and membrane depolarization. In vivo optogenetic activation of microglial ReaChR in the spinal cord triggered chronic pain hypersensitivity in both male and female mice. In addition, activation of microglial ReaChR up-regulated neuronal c-Fos expression and enhanced C-fiber responses. Mechanistically, ReaChR activation led to a reactive microglial phenotype with increased interleukin (IL)-1β production, which is likely mediated by inflammasome activation and calcium elevation. IL-1 receptor antagonist (IL-1ra) was able to reverse the pain hypersensitivity and neuronal hyperactivity induced by microglial ReaChR activation. Therefore, our work demonstrates that optogenetic activation of spinal microglia is sufficient to trigger chronic pain phenotypes by increasing neuronal activity via IL-1 signaling.

Pain induces stable, active microcircuits in the somatosensory cortex that provide a therapeutic target

Sustained neuropathic pain from injury or inflammation remains a major burden for society. Rodent pain models have informed some cellular mechanisms increasing neuronal excitability within the spinal cord and primary somatosensory cortex (S1), but how activity patterns within these circuits change during pain remains unclear. We have applied multiphoton in vivo imaging and holographic stimulation to examine single S1 neuron activity patterns and connectivity during sustained pain. Following pain induction, there is an increase in synchronized neuronal activity and connectivity within S1, indicating the formation of pain circuits. Artificially increasing neuronal activity and synchrony using DREADDs reduced pain thresholds. The expression of N-type voltage-dependent Ca²⁺ channel subunits in S1 was increased after pain induction, and locally blocking these channels reduced both the synchrony and allodynia associated with inflammatory pain. Targeting these S1 pain circuits, via inhibiting N-type Ca²⁺ channels or other approaches, may provide ways to reduce inflammatory pain.

Pain in Axial Spondyloarthritis: Insights from Immunology and Brain Imaging

Inflammatory back pain is characteristic of spondyloarthritis (SpA); however, this pain may not respond to treatment with NSAIDs or biologics. Pain is multifactorial and a combination of mechanical and inflammatory factors. A growing body of literature examines the impact of emotions on pain in SpA; many patients with this condition suffer from depression and fibromyalgia. Advanced imaging techniques can investigate the interplay of various brain networks in pain perception. Animal models have helped understand the interplay between the immune and nervous systems in pain generation and have highlighted differences in pain perception between the sexes.

Activated microglia drive demyelination via CSF1R signaling

Microgliosis is a prominent pathological feature in many neurological diseases including multiple sclerosis (MS), a progressive auto-immune demyelinating disorder. The precise role of microglia, parenchymal central nervous system (CNS) macrophages, during demyelination, and the relative contributions of peripheral macrophages are incompletely understood. Classical markers used to identify microglia do not reliably discriminate between microglia and peripheral macrophages, confounding analyses. Here, we use a genetic fate mapping strategy to identify microglia as predominant responders and key effectors of demyelination in the cuprizone (CUP) model. Colony-stimulating factor 1 (CSF1), also known as macrophage colony-stimulating factor (M-CSF) - a secreted cytokine that regulates microglia development and survival-is upregulated in demyelinated white matter lesions. Depletion of microglia with the CSF1R inhibitor PLX3397 greatly abrogates the demyelination, loss of oligodendrocytes, and reactive astrocytosis that results from CUP treatment. Electron microscopy (EM) and serial block face imaging show myelin sheaths remain intact in CUP treated mice depleted of microglia. However, these CUP-damaged myelin sheaths are lost and robustly phagocytosed upon-repopulation of microglia. Direct injection of CSF1 into CNS white matter induces focal microgliosis and demyelination indicating active CSF1 signaling can promote demyelination. Finally, mice defective in adopting a toxic astrocyte phenotype that is driven by microglia nevertheless demyelinate normally upon CUP treatment implicating microglia rather than astrocytes as the primary drivers of CUP-mediated demyelination. Together, these studies indicate activated microglia are required for and can drive demyelination directly and implicate CSF1 signaling in these events.

New Inhibitory Effects of Cilnidipine on Microglial P2X7 Receptors and IL-1β Release: An Involvement in its Alleviating Effect on Neuropathic Pain

P2X7 receptors (P2X7Rs) belong to a family of ATP-gated non-selective cation channels. Microglia represent a major cell type expressing P2X7Rs. The activation of microglial P2X7Rs causes the release of pro-inflammatory cytokines such as interleukin-1β (IL-1β). This response has been implicated in neuroinflammatory states in the central nervous system and in various diseases, including neuropathic pain. Thus, P2X7R may represent a potential therapeutic target. In the present study, we screened a chemical library of clinically approved drugs (1979 compounds) by high-throughput screening and showed that the Ca2+ channel blocker cilnidipine has an inhibitory effect on rodent and human P2X7R. In primary cultured rat microglial cells, cilnidipine inhibited P2X7R-mediated Ca2+ responses and IL-1β release. Moreover, in a rat model of neuropathic pain, the intrathecal administration of cilnidipine produced a reversal of nerve injury-induced mechanical hypersensitivity, a cardinal symptom of neuropathic pain. These results point to a new inhibitory effect of cilnidipine on microglial P2X7R-mediated inflammatory responses and neuropathic pain, proposing its therapeutic potential.

Glial and neuroimmune cell choreography in sexually dimorphic pain signaling

Chronic pain is a major global health issue that affects all populations regardless of sex, age, ethnicity/race, or country of origin, leading to persistent physical and emotional distress and to the loss of patients' autonomy and quality of life. Despite tremendous efforts in the elucidation of the mechanisms contributing to the pathogenesis of chronic pain, the identification of new potential pain targets, and the development of novel analgesics, the pharmacological treatment options available for pain management remain limited, and most novel pain medications have failed to achieve advanced clinical development, leaving many patients with unbearable and undermanaged pain. Sex-specific susceptibility to chronic pain conditions as well as sex differences in pain sensitivity, pain tolerance and analgesic efficacy are increasingly recognized in the literature and have thus prompted scientists to seek mechanistic explanations. Hence, recent findings have highlighted that the signaling mechanisms underlying pain hypersensitivity are sexually dimorphic, which sheds light on the importance of conducting preclinical and clinical pain research on both sexes and of developing sex-specific pain medications. This review thus focuses on the clinical and preclinical evidence supporting the existence of sex differences in pain neurobiology. Attention is drawn to the sexually dimorphic role of glial and immune cells, which are both recognized as key players in neuroglial maladaptive plasticity at the origin of the transition from acute pain to chronic pathological pain. Growing evidence notably attributes to microglial cells a pivotal role in the sexually dimorphic pain phenotype and in the sexually dimorphic analgesic efficacy of opioids. This review also summarizes the recent advances in understanding the pathobiology underpinning the development of pain hypersensitivity in both males and females in different types of pain conditions, with particular emphasis on the mechanistic signaling pathways driving sexually dimorphic pain responses.

Use of low-dose naltrexone in the management of chronic pain conditions: A systematic review

BACKGROUND

The authors aimed to evaluate the efficacy of low-dose naltrexone in the management of chronic pain conditions and determine its potential use in orofacial pain management.

METHODS

A comprehensive literature review was completed in the PubMed/MEDLINE, Embase, Cumulated Index to Nursing and Allied Health Literature, Dentistry and Oral Sciences Source Library databases up through June 17, 2019, using terms such as neurogenic, inflammation, naltrexone, temporomandibular, and chronic pain. The primary outcome was reduction in pain intensity and, secondarily, improvement in quality of life.

RESULTS

A total of 793 studies were obtained with the initial search and 8 articles were selected for evaluation. Of these 8 articles, 4 were case reports, 3 were clinical studies, and 1 was a randomized controlled trial. Six studies included data on fibromyalgia, 2 studies included data on chronic regional pain syndrome, and 1 examined multiple diagnoses, including fibromyalgia, interstitial cystitis, and chronic pelvic pain. The primary outcome of all of the studies was pain intensity reduction.

CONCLUSIONS AND PRACTICAL IMPLICATIONS

Low-dose naltrexone provides an alternative in medical management of chronic pain disorders as a novel anti-inflammatory and immunomodulator. It can offer additional management options, as orofacial pain conditions share characteristics with other chronic pain disorders. Owing to the size and heterogeneity of the studies, more large-scale studies are needed, along with additional studies assessing orofacial pain response to low-dose naltrexone.

Increased surface P2X4 receptor regulates anxiety and memory in P2X4 internalization-defective knock-in mice

ATP signaling and surface P2X4 receptors are upregulated selectively in neurons and/or glia in various CNS disorders including anxiety, chronic pain, epilepsy, ischemia, and neurodegenerative diseases. However, the cell-specific functions of P2X4 in pathological contexts remain elusive. To elucidate P2X4 functions, we created a conditional transgenic knock-in P2X4 mouse line (Floxed P2X4mCherryIN) allowing the Cre activity-dependent genetic swapping of the internalization motif of P2X4 by the fluorescent mCherry protein to prevent constitutive endocytosis of P2X4. By combining molecular, cellular, electrophysiological, and behavioral approaches, we characterized two distinct knock-in mouse lines expressing noninternalized P2X4mCherryIN either exclusively in excitatory forebrain neurons or in all cells natively expressing P2X4. The genetic substitution of wild-type P2X4 by noninternalized P2X4mCherryIN in both knock-in mouse models did not alter the sparse distribution and subcellular localization of P2X4 but increased the number of P2X4 receptors at the surface of the targeted cells mimicking the pathological increased surface P2X4 state. Increased surface P2X4 density in the hippocampus of knock-in mice altered LTP and LTD plasticity phenomena at CA1 synapses without affecting basal excitatory transmission. Moreover, these cellular events translated into anxiolytic effects and deficits in spatial memory. Our results show that increased surface density of neuronal P2X4 contributes to synaptic deficits and alterations in anxiety and memory functions consistent with the implication of P2X4 in neuropsychiatric and neurodegenerative disorders. Furthermore, these conditional P2X4mCherryIN knock-in mice will allow exploring the cell-specific roles of P2X4 in various physiological and pathological contexts.

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

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

Chronic BDNF simultaneously inhibits and unmasks superficial dorsal horn neuronal activity

Brain-derived neurotrophic factor (BDNF) is critically involved in the pathophysiology of chronic pain. However, the mechanisms of BDNF action on specific neuronal populations in the spinal superficial dorsal horn (SDH) requires further study. We used chronic BDNF treatment (200 ng/ml, 5–6 days) of defined-medium, serum-free spinal organotypic cultures to study intracellular calcium ([Ca²⁺]_i) fluctuations. A detailed quantitative analysis of these fluctuations using the Frequency-independent biological signal identification (FIBSI) program revealed that BDNF simultaneously depressed activity in some SDH neurons while it unmasked a particular subpopulation of ‘silent’ neurons causing them to become spontaneously active. Blockade of gap junctions disinhibited a subpopulation of SDH neurons and reduced BDNF-induced synchrony in BDNF-treated cultures. BDNF reduced neuronal excitability assessed by measuring spontaneous excitatory postsynaptic currents. This was similar to the depressive effect of BDNF on the [Ca²⁺]_i fluctuations. This study reveals novel regulatory mechanisms of SDH neuronal excitability in response to BDNF.