P2X7-dependent release of interleukin-1beta and nociception in the spinal cord following lipopolysaccharide

Peripheral Inflammatory Hyperalgesia Depends on P2X7 Receptors in Satellite Glial Cells

Peripheral inflammatory hyperalgesia depends on the sensitization of primary nociceptive neurons. Inflammation drives molecular alterations not only locally but also in the dorsal root ganglion (DRG) where interleukin-1 beta (IL-1β) and purinoceptors are upregulated. Activation of the P2X7 purinoceptors by ATP is essential for IL-1β maturation and release. At the DRG, P2X7R are expressed by satellite glial cells (SGCs) surrounding sensory neurons soma. Although SGCs have no projections outside the sensory ganglia these cells affect pain signaling through intercellular communication. Therefore, here we investigated whether activation of P2X7R by ATP and the subsequent release of IL-1β in DRG participate in peripheral inflammatory hyperalgesia. Immunofluorescent images confirmed the expression of P2X7R and IL-1β in SGCs of the DRG. The function of P2X7R was then verified using a selective antagonist, A-740003, or antisense for P2X7R administered in the L5-DRG. Inflammation was induced by CFA, carrageenan, IL-1β, or PGE₂ administered in rat's hind paw. Blockage of P2X7R at the DRG reduced the mechanical hyperalgesia induced by CFA, and prevented the mechanical hyperalgesia induced by carrageenan or IL-1β, but not PGE₂. It was also found an increase in P2X7 mRNA expression at the DRG after peripheral inflammation. IL-1β production was also increased by inflammatory stimuli in vivo and in vitro, using SGC-enriched cultures stimulated with LPS. In LPS-stimulated cultures, activation of P2X7R by BzATP induced the release of IL-1β, which was blocked by A-740003. In summary, our data suggest that peripheral inflammation leads to the activation of P2X7R expressed by SGCs at the DRG. Then, ATP-induced activation of P2X7R mediates the release of IL-1β from SGC. This evidence places the SGC as an active player in the establishment of peripheral inflammatory hyperalgesia and highlights the importance of the events in DRG for the treatment of inflammatory diseases.

Microglia-Mediated Regulation of Neuropathic Pain: Molecular and Cellular Mechanisms

Pain is a defense system that responds rapidly to harmful internal and external stimuli through the somatosensory neuronal pathway. However, damage to the nervous system through cancer, diabetes, infection, autoimmune disease, chemotherapy or trauma often leads to neuropathic pain, a debilitating chronic pain condition. Neuropathic pain is not simply a temporal continuum of acute nociceptive signals from the periphery, but rather due to pathologically altered functions in the nervous system, which shift the net neuronal excitatory balance toward excitation. Although alterations were long thought to be a result of changes in neurons, but an increasing body of evidence over the past decades indicates the necessity and sufficiency of microglia, the tissue-resident macrophages of the spinal cord and brain, for nerve injury-induced malfunction of the nervous system. In this review article, I describe our current understanding of the molecular and cellular mechanisms underlying the role of microglia in the pathogenesis of neuropathic pain and discuss the therapeutic potential of microglia from recent advances in the development of new drugs targeting microglia.

NeuroHeal Treatment Alleviates Neuropathic Pain and Enhances Sensory Axon Regeneration

Peripheral nerve injury (PNI) leads to the loss of motor, sensory, and autonomic functions, and often triggers neuropathic pain. During the last years, many efforts have focused on finding new therapies to increase axonal regeneration or to alleviate painful conditions. Still only a few of them have targeted both phenomena. Incipient or aberrant sensory axon regeneration is related to abnormal unpleasant sensations, such as hyperalgesia or allodynia. We recently have discovered NeuroHeal, a combination of two repurposed drugs; Acamprosate and Ribavirin. NeuroHeal is a neuroprotective agent that also enhances motor axon regeneration after PNI. In this work, we investigated its effect on sensory fiber regeneration and PNI-induced painful sensations in a rat model of spare nerve injury and nerve crush. The follow up of the animals showed that NeuroHeal treatment reduced the signs of neuropathic pain in both models. Besides, the treatment favored sensory axon regeneration, as observed in dorsal root ganglion explants. Mechanistically, the effects observed in vivo may improve the resolution of cell-protective autophagy. Additionally, NeuroHeal treatment modulated the P2X4-BDNF-KCC2 axis, which is an essential driver of neuropathic pain. These data open a new therapeutic avenue based on autophagic modulation to foster endogenous regenerative mechanisms and reduce the appearance of neuropathic pain in PNI.

Functional and Metabolic Characterization of Microglia Culture in a Defined Medium

Microglia are the endogenous immune cells of the brain and act as sensor of infection and pathologic injury to the brain, leading to a rapid plastic process of activation that culminates in the endocytosis and phagocytosis of damaged tissue. Microglia cells are the most plastic cells in the brain. Microglia isolation from their environment as well as culturing them in the presence of serum alter their function and lead to a rapid loss of their signature gene expression. Previous studies have identified pivotal factors allowing microglia culture in the absence of serum. Here, we have further characterized the function, expression of markers, metabolic status and response to pro and anti-inflammatory stimulus of microglia isolated by magnetic-activated cell sorting and cultured in a chemically defined medium. We have compared this new method with previous traditional protocols of culturing microglia that use high concentrations of serum.

Microbes, microglia, and pain

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Explore the connection between the gut microbiome and microglia in chronic pain.

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Discuss mechanisms by which gut bacteria might influence microglia to contribute to chronic pain.

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Highlight gaps in knowledge and discuss future directions for the field.

Globally, it is estimated that one in five people suffer from chronic pain, with prevalence increasing with age. The pathophysiology of chronic pain encompasses complex sensory, immune, and inflammatory interactions within both the central and peripheral nervous systems. Microglia, the resident macrophages of the central nervous system (CNS), are critically involved in the initiation and persistence of chronic pain. Microglia respond to local signals from the CNS but are also modulated by signals from the gastrointestinal tract. Emerging data from preclinical and clinical studies suggest that communication between the gut microbiome, the community of bacteria residing within the gut, and microglia is involved in producing chronic pain. Targeted strategies that manipulate or restore the gut microbiome have been shown to reduce microglial activation and alleviate symptoms associated with inflammation. These data indicate that manipulations of the gut microbiome in chronic pain patients might be a viable strategy in improving pain outcomes. Herein, we discuss the evidence for a connection between microglia and the gut microbiome and explore the mechanisms by which commensal bacteria might influence microglial reactivity to drive chronic pain.

Interleukin-1 receptor antagonist ameliorates the pain hypersensitivity, spinal inflammation and oxidative stress induced by systemic lipopolysaccharide in neonatal rats

Perinatal inflammation-induced reduction in pain threshold may alter pain sensitivity to hyperalgesia or allodynia which may persist into adulthood. In this study, we investigated the anti-inflammatory protective effect of interleukin-1 receptor antagonist (IL-1ra), an anti-inflammatory cytokine, on systemic lipopolysaccharide (LPS)-induced spinal cord inflammation and oxidative stress, thermal hyperalgesia, and mechanical allodynia in neonatal rats. Intraperitoneal (i.p.) injection of LPS (2 mg/kg) or sterile saline was performed in postnatal day 5 (P5) rat pups, and IL-1ra (100 mg/kg) or saline was administered (i.p.) 5 min after LPS injection. Pain reflex behavior, spinal cord inflammation and oxidative stress were examined 24 h after LPS administration. Systemic LPS exposure led to a reduction of tactile threshold in the von Frey filament tests (mechanical allodynia) and pain response latency in the tail-flick test (thermal hyperalgesia) of P6 neonatal rats. Spinal cord inflammation was indicated by the increased numbers of activated glial cells including microglia (Iba1+) and astrocytes (GFAP+), and elevated levels of pro-inflammatory cytokine interleukin-1β (IL-1β), cyclooxygenase-2 (COX-2), and prostaglandin E2 (PGE2) 24 h after LPS treatment. LPS treatment induced spinal oxidative stress as evidenced by the increase in thiobarbituric acid reactive substances (TBARS) content in the spinal cord. LPS exposure also led to a significant increase in oligodendrocyte lineage population (Olig2+) and mature oligodendrocyte cells (APC+) in the neonatal rat spinal cord. IL-1ra treatment significantly reduced LPS-induced effects including hyperalgesia, allodynia, the increased number of activated microglia, astrocytes and oligodendrocytes, and elevated levels of IL-1β, COX-2, PGE2, and lipid peroxidation (TBARS) in the neonatal rat spinal cord. These data suggest that IL-1ra provides a protective effect against the development of pain hypersensitivity, spinal cord inflammation and oxidative stress in the neonatal rats following LPS exposure, which may be associated with the blockade of LPS-induced pro-inflammatory cytokine IL-1β.

Astrocytes in chronic pain and itch

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

Microglia in Pain: Detrimental and Protective Roles in Pathogenesis and Resolution of Pain

The previous decade has seen a rapid increase in microglial studies on pain, with a unique focus on microgliosis in the spinal cord after nerve injury and neuropathic pain. Numerous signaling molecules are altered in microglia and contribute to the pathogenesis of pain. Here, we discuss how microglial signaling regulates spinal cord synaptic plasticity in acute and chronic pain conditions with different degrees and variations of microgliosis. We highlight that microglial mediators such as pro- and anti-inflammatory cytokines are powerful neuromodulators that regulate synaptic transmission and pain via neuron-glial interactions. We also reveal an emerging role of microglia in the resolution of pain, in part via specialized pro-resolving mediators including resolvins, protectins, and maresins. We also discuss a possible role of microglia in chronic itch.

Spinal Interleukin-1β Inhibits Astrocyte Cytochrome P450c17 Expression Which Controls the Development of Mechanical Allodynia in a Mouse Model of Neuropathic Pain

We have recently demonstrated that sciatic nerve injury increases the expression of spinal cytochrome P450c17, a key neurosteroidogenic enzyme, which plays a critical role in the development of peripheral neuropathic pain. However, the modulatory mechanisms responsible for the expression of spinal P450c17 have yet to be examined. Here we investigated the possible involvement of interleukin-1β (IL-1β) in altering P450c17 expression during the induction phase of neuropathic pain. Neuropathic pain was produced by chronic constriction injury (CCI) of the right sciatic nerve in mice and mechanical allodynia was evaluated in the hind paws using a von-Frey filament (0.16 g). Western blotting and immunohistochemistry were performed to assess the expression of spinal IL-1β, interleukin-1 receptor type 1 (IL-1R1), P450c17, and GFAP. Spinal IL-1β was significantly increased on day 1 post-surgery and its receptor, IL-1R1 was expressed in GFAP-positive astrocytes. Intrathecal administration of the recombinant interleukin-1 receptor antagonist (IL-1ra, 20 ng) on days 0 and 1 post-surgery enhanced GFAP expression on day 1 post-surgery and induced an early increase in P450c17 expression in astrocytes, but not in neurons. Administration of IL-1β (10 ng) on days 0 and 1 post-surgery blocked the enhancement of both spinal P450c17 and GFAP expression induced by IL-1ra (20 ng) administration. Intrathecal administration of IL-1ra (20 ng) on days 0 to 3 post-surgery also facilitated the CCI-induced development of mechanical allodynia, and this early developed pain was dose-dependently attenuated by the administration of the P450c17 inhibitor, ketoconazole (1, 3, or 10 nmol) or the astrocyte metabolic inhibitor, fluorocitrate (0.01, 0.03, or 0.1 nmol). These results demonstrate that early increases in spinal IL-1β temporally inhibit astrocyte P450c17 expression and astrocyte activation ultimately controlling the development of mechanical allodynia induced by peripheral nerve injury. These findings imply that spinal IL-1β plays an important role as an early, but transient, control mechanism in the development of peripheral neuropathic pain via the inhibition of astrocyte P450c17 expression and astrocyte activation.

Targeting strategies for chemotherapy-induced peripheral neuropathy: does gut microbiota play a role?

Chemotherapy-induced peripheral neuropathy (CIPN) is a progressive, often irreversible condition that produces severe neurological deficits. Emerging data suggest that chemotherapy also exerts detrimental effects on gut microbiota composition and intestinal permeability, contributing to dysbiosis and inflammation. Compared with other complications associated with chemotherapy, such as diarrhoea and mucositis, CIPN is of particular concern because it is the most common reason for terminating or suspending treatment. However, specific and effective curative treatment strategies are lacking. In this review, we provide an update on current preclinical and clinical understandings about the role of gut microbiota in CIPN. The gut microbiota serves as an intersection between the microbiome-gut-brain and the neuroimmune-endocrine axis, forming a complex network that can directly or indirectly affect key components involved in the manifestations of CIPN. Herein, we discuss several potential mechanisms within the context of the networks and summarize alterations in gut microbiome induced by chemotherapeutic drugs, providing great potential for researchers to target pathways associated with the gut microbiome and overcome CIPN.

P2Y14 receptor is functionally expressed in satellite glial cells and mediates interleukin-1β and chemokine CCL2 secretion

Satellite glial cells (SGCs) activation in the trigeminal ganglia (TG) is critical in various abnormal orofacial sensation in nerve injury and inflammatory conditions. SGCs express several subtypes of P2 purinergic receptors contributing to the initiation and maintenance of neuropathic pain. The P2Y₁₄ receptor, a G-protein-coupled receptor activated by uridine diphosphate (UDP)-glucose and other UDP sugars, mediates various physiologic events such as immune, inflammation, and pain. However, the expression, distribution, and function of P2Y₁₄ receptor in SGCs remains largely unexplored. Our study reported the expression and functional identification of P2Y₁₄ receptor in SGCs. SGCs were isolated from TG of rat, and the P2Y₁₄ receptor expression was examined using immunofluorescence technique. Cell proliferation and viability were examined via cell counting kit-8 experiment. Immunofluorescence demonstrated the presence of P2Y₁₄ receptor in SGCs. Immunofluorescence and western blot showed that UDP-glucose treatment upregulated glial fibrillary acid protein, a common marker for glial activation. Extracellular UDP-glucose enhanced the phosphorylation of extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38, which were both abolished by the P2Y₁₄ receptor inhibitor (PPTN). Furthermore, quantitative reverse transcription polymerase chain reaction and enzyme-linked immunosorbent assay demonstrated that extracellular UDP-glucose significantly enhanced interleukin-1β (IL-1β) and chemokine CCL2 (CCL2) release, which was abolished by PPTN and significantly decreased by inhibitors of MEK/ERK (U0126) and p38 (SB202190). Our findings directly proved the functional presence of P2Y₁₄ receptor in SGCs. It was also verified that P2Y₁₄ receptor activation was involved in activating SGCs, phosphorylating MAPKs, and promoting the secretion of IL-1β and CCL2 via ERK and p38 pathway.

Central inhibition of granulocyte-macrophage colony-stimulating factor is analgesic in experimental neuropathic pain

Supplemental Digital Content is Available in the Text.

GM-CSF is a proinflammatory cytokine that plays a role in central pain pathways through the modulation of spinal glial cells.

With less than 50% of patients responding to the current standard of care and poor efficacy and selectivity of current treatments, neuropathic pain continues to be an area of considerable unmet medical need. Biological therapeutics such as monoclonal antibodies (mAbs) provide better intrinsic selectivity; however, delivery to the central nervous system (CNS) remains a challenge. Granulocyte-macrophage colony-stimulating factor (GM-CSF) is well described in inflammation-induced pain, and early-phase clinical trials evaluating its antagonism have exemplified its importance as a peripheral pain target. Here, we investigate the role of this cytokine in a murine model of traumatic nerve injury and show that deletion of the GM-CSF receptor or treatment with an antagonizing mAb alleviates pain. We also demonstrate enhanced analgesic efficacy using an engineered construct that has greater capacity to penetrate the CNS. Despite observing GM-CSF receptor expression in microglia and astrocytes, the gliosis response in the dorsal horn was not altered in nerve injured knockout mice compared with wild-type littermate controls as evaluated by ionized calcium binding adapter molecule 1 (Iba1) and glial fibrillary acidic protein, respectively. Functional analysis of glial cells revealed that pretreatment with GM-CSF potentiated lipopolysaccharide-induced release of proinflammatory cytokines. In summary, our data indicate that GM-CSF is a proinflammatory cytokine that contributes to nociceptive signalling through driving spinal glial cell secretion of proinflammatory mediators. In addition, we report a successful approach to accessing CNS pain targets, providing promise for central compartment delivery of analgesics.

Diabetes-induced damage of gastric nitric oxide neurons mediated by P2X7R in diabetic mice

It is generally considered that enteric neuropathy is one of the causative factors in diabetic gastroparesis. Our previous study demonstrated that there is a loss of NOS neurons in diabetic mice. However, the underlying mechanism remains unclear. The present study was designed to clarify the relationship between neuronal P2X7R and NOS neuron damage. The effect of P2X7R on diabetes-induced gastric NOS neurons damage and its mechanism were investigated by using quantitative RT-PCR，immunofluorescence, western blot, isometric force recording, intracellular calcium ([Ca²⁺]i) measurement and whole-cell patch clamp techniques. The immunohistochemistry and western blot results showed that nNOS expression was significantly down-regulated in diabetic mice, meanwhile, electric field stimulation-induced NOS sensitive relaxation was significantly suppressed. Myenteric neurons expressed P2X7R and pannexin1, and the mRNA and protein level of P2X7R and pannexin1 were up-regulated in diabetic mice. BzATP, a P2X7R activator, evoked [Ca²⁺]i increase in Hek293 cells with heterologous expression of P2X7R (Hek293-P2X7R cells) and the same dose of ATP-induced [Ca²⁺]i was more obvious in Hek293-P2X7R cells than in Hek293 cells. Application of BzATP activated an inward current of Hek293-P2X7R in a dose dependent manner. Hek293-P2X7R but not untransfected Hek293 cells could take up of YO-PRO-1. In addition, the uptake of YO-PRO-1 by Hek293-P2X7R was blocked by oxATP, a P2X7 antagonist and CBX, a pannexin1 inhibitor. The results suggest that the P2X7R of enteric neurons may be involved in diabetes-induced NOS neuron damage via combining with pannexin-1 to form transmembrane pores which induce macromolecular substances and calcium into the cells.

Inhibition of spinal 15-LOX-1 attenuates TLR4-dependent, nonsteroidal anti-inflammatory drug-unresponsive hyperalgesia in male rats

Although nonsteroidal anti-inflammatory drugs are the first line of therapeutics for the treatment of mild to moderate somatic pain, they are not generally considered to be effective for neuropathic pain. In the current study, direct activation of spinal Toll-like 4 receptors (TLR4) by the intrathecal (IT) administration of KDO2 lipid A (KLA), the active component of lipopolysaccharide, elicits a robust tactile allodynia that is unresponsive to cyclooxygenase inhibition, despite elevated expression of cyclooxygenase metabolites in the spinal cord. Intrathecal KLA increases 12-lipoxygenase-mediated hepoxilin production in the lumbar spinal cord, concurrent with expression of the tactile allodynia. The TLR4-induced hepoxilin production was also observed in primary spinal microglia, but not in astrocytes, and was accompanied by increased microglial expression of the 12/15-lipoxygenase enzyme 15-LOX-1. Intrathecal KLA-induced tactile allodynia was completely prevented by spinal pretreatment with the 12/15-lipoxygenase inhibitor CDC or a selective antibody targeting rat 15-LOX-1. Similarly, pretreatment with the selective inhibitors ML127 or ML351 both reduced activity of the rat homolog of 15-LOX-1 heterologously expressed in HEK-293T cells and completely abrogated nonsteroidal anti-inflammatory drug-unresponsive allodynia in vivo after IT KLA. Finally, spinal 12/15-lipoxygenase inhibition by nordihydroguaiaretic acid (NDGA) both prevents phase II formalin flinching and reverses formalin-induced persistent tactile allodynia. Taken together, these findings suggest that spinal TLR4-mediated hyperpathic states are mediated at least in part through activation of microglial 15-LOX-1.

Targeting inflammatory components in neuropathic pain: The analgesic effect of thymulin related peptide

Neuropathic pain is considered to be pathological in nature and has been shown to involve, at least partially, dysregulated inflammatory processes. It is a severe chronic disease that can develop following lesions to the central nervous system or to peripheral nerves. The peripheral nerve damage can be caused by either diseases such as diabetes, or by trauma. A common underlying mechanism of neuropathic pain is the presence of inflammation at the site of the damaged or affected nerve(s). This inflammatory response, especially when unresolved, initiates and maintains a cascade of events resulting in the activation of innate immune cells at the site of tissue injury. The release of inflammatory mediators such as cytokines, neurotrophic factors, and chemokines initiates local actions and can result in a more generalized immune response. The resultant neuroinflammatory environment can cause activation of glial cells, which can release, in an uncontrolled manner, more of these mediators and exasperate the situation, thus having a prominent role in nociception. The neuropathic pain pathophysiology is complex and includes peripheral and central neuronal alterations as well as neuro-immune interactions, which become more prominent during inflammatory reactions. This report focuses on how targeting inflammatory mediators may result in novel therapeutic approaches to neuropathic pain management.

Enteric Glia: A New Player in Abdominal Pain

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

A P2X7 receptor antagonist reverses behavioural alterations, microglial activation and neuroendocrine dysregulation in an unpredictable chronic mild stress (UCMS) model of depression in mice

A polymorphism in the P2RX7 gene that encodes for the P2X7 ionotropic ATP-gated receptor (P2X7R) protein has been shown to be associated with an increased risk for developing depressive illnesses. However, the role of P2X7R in depression is still unclear. To better understand the role of P2X7R and its subsequent impact on microglial activation, we compared the effect of the P2X7R antagonist Brilliant Blue G (BBG) with that of fluoxetine in an unpredictable chronic mild stress (UCMS) model of depression in mice. Our results indicate that BBG (50 mg/kg body weight in 0.9% NaCl, 10 ml/kg/day) successfully reversed the degradation of coat states and nest-building scores induced by exposure to UCMS, similar to the conventional antidepressant fluoxetine (15 mg/kg body weight in 0.9% NaCl, 10 ml/kg/day). BBG also reversed the UCMS-induced microglial activation in cortical and hippocampal regions and the basal nuclei of mouse brains and corrected the UCMS-induced hypothalamo-pituitary-adrenal (HPA) axis dysregulation. In contrast to fluoxetine, however, BBG treatment did not increase the density of doublecortin-positive cells in the dentate gyrus, indicating that BBG had no impact on hippocampal neurogenesis. These results suggest that P2X7R is involved in recovery from depressive-like states caused by exposure to UCMS in a mechanism that involves restoration of the HPA axis but not hippocampal neurogenesis. These results add to the evidence that P2X7R antagonist agents may have potential value in the pharmacological management of depression.

Neuroinflammation and Central Sensitization in Chronic and Widespread Pain

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

P2X receptor channels in chronic pain pathways

Chronic pain is a highly prevalent debilitating condition for which treatment options remain limited for many patients. Ionotropic ATP signalling through excitatory and calcium-permeable P2X receptor channels is now rightfully considered as a critical player in pathological pain generation and maintenance; therefore, their selective targeting represents a therapeutic opportunity with promising yet untapped potential. Recent advances in the structural, functional and pharmacological characterization of rodent and human ATP-gated P2X receptor channels have shed brighter light on the role of specific subtypes in the pathophysiology of chronic inflammatory, neuropathic or cancer pain. Here, we will review the contribution of P2X3, P2X4 and P2X7 receptors to chronic pain and discuss the opportunities and challenges associated with the pharmacological manipulation of their function.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Intraoperative electroacupuncture relieves remifentanil-induced postoperative hyperalgesia via inhibiting spinal glial activation in rats

Background

Accumulating studies have suggested that remifentanil, the widely-used opioid analgesic in clinical anesthesia, can activate the pronociceptive systems and enhance postoperative pain. Glial cells are thought to be implicated in remifentanil-induced hyperalgesia. Electroacupuncture is a complementary therapy to relieve various pain conditions with few side effects, and glial cells may be involved in its antinociceptive effect. In this study, we investigated whether intraoperative electroacupuncture could relieve remifentanil-induced postoperative hyperalgesia by inhibiting the activation of spinal glial cells, the production of spinal proinflammatory cytokines, and the activation of spinal mitogen-activated protein kinases.

Methods

A rat model of remifentanil-induced postoperative hyperalgesia was used in this study. Electroacupuncture during surgery was conducted at bilateral Zusanli (ST36) acupoints. Behavior tests, including mechanical allodynia and thermal hyperalgesia, were performed at different time points. Astrocytic marker glial fibrillary acidic protein, microglial marker Iba1, proinflammatory cytokines, and phosphorylated mitogen-activated protein kinases in the spinal cord were detected by Western blot and/or immunofluorescence.

Results

Mechanical allodynia and thermal hyperalgesia were induced by both surgical incision and remifentanil infusion, and remifentanil infusion significantly exaggerated and prolonged incision-induced pronociceptive effects. Glial fibrillary acidic protein, Iba1, proinflammatory cytokines (interleukin-1β and tumor necrosis factor-α), and phosphorylated mitogen-activated protein kinases (p-p38, p-JNK, and p-ERK1/2) were upregulated after surgical incision, remifentanil infusion, and especially after their combination. Intraoperative electroacupuncture significantly attenuated incision- and/or remifentanil-induced pronociceptive effects, spinal glial activation, proinflammatory cytokine upregulation, and phosphorylated mitogen-activated protein kinase upregulation.

Conclusions

Our study suggests that remifentanil-induced postoperative hyperalgesia can be relieved by intraoperative electroacupuncture via inhibiting the activation of spinal glial cells, the upregulation of spinal proinflammatory cytokines, and the activation of spinal mitogen-activated protein kinases.

The Neurotoxin DSP-4 Induces Hyperalgesia in Rats that is Accompanied by Spinal Oxidative Stress and Cytokine Production

Central neuropathic pain (CNP) a significant problem for many people, is not well-understood and difficult to manage. Dysfunction of the central noradrenergic system originating in the locus coeruleus (LC) may be a causative factor in the development of CNP. The LC is the major noradrenergic nucleus of the brain and plays a significant role in central modulation of nociceptive neurotransmission. Here, we examined CNS pathophysiological changes induced by intraperitoneal administration of the neurotoxin DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride). Administration of DSP-4 decreased levels of norepinephrine in spinal tissue and cerebrospinal fluid (CSF) and led to the development of thermal and mechanical hyperalgesia over 21 days, that was reversible with morphine. Hyperalgesia was accompanied by significant increases in noradrenochrome (oxidized norepinephrine) and expression of 4-hydroxynonenal in CSF and spinal cord tissue respectively at day 21, indicative of oxidative stress. In addition, spinal levels of pro-inflammatory cytokines (interleukins 6 and 17A, tumor necrosis factor-α), as well as the anti-inflammatory cytokine interleukin10 were also significantly elevated at day 21, indicating that an inflammatory response occurred. The inflammatory effect of DSP-4 presented in this study that includes oxidative stress may be particularly useful in elucidating mechanisms of CNP in inflammatory disease states.

Microglia in the CNS and Neuropathic Pain

Neuropathic pain occurring after peripheral nerve injury is not simply a consequence of temporal continuity of acute nociceptive signals, but rather of maladaptive nervous system function. Over the past decades, a body of literature has provided evidence for the necessity and sufficiency of microglia, the tissue-resident macrophages of the central nervous system, for nerve injury-induced alterations in synaptic function. Recent studies have also revealed active roles for microglia in brain regions important for emotion and memory. In this chapter, I highlight recent advances in our understanding of the mechanisms that underlie the role of spinal and brain microglia in neuropathic pain, with a focus on how microglia are activated and alter synaptic function. I also discuss the therapeutic potential of microglia from recent advances in the development of new drugs targeting microglia, which may facilitate translation from the bench to bedside.

Purinergic receptors P2Y12R and P2X7R: potential targets for PET imaging of microglia phenotypes in multiple sclerosis

Background

Microglia are major players in the pathogenesis of multiple sclerosis (MS) and may play a dual role in disease progression. The activation status of microglia in vivo is highly dynamic and occurs as a continuum, with the pro-inflammatory and anti-inflammatory phenotypes on either end of this spectrum. Little is known about in vivo dynamics of microglia phenotypes in MS due to the lack of diagnostic tools. Positron emission tomography (PET) imaging is a powerful non-invasive technique that allows real-time imaging of microglia activation phenotypes in the central nervous system, depending on the availability of selective PET tracers. Our objective is to investigate and characterize the expression of the purinergic receptors P2Y12R and P2X7R as potential targets for PET tracer development and subsequent PET imaging in order to evaluate the dynamics of microglia status in vivo.

Methods

We used immunohistochemical analysis to explore the expression of P2Y12R and P2X7R in experimental autoimmune encephalomyelitis (EAE) post-mortem tissues and different stages of well-characterized MS lesions. We evaluated by quantitative real-time polymerase chain reaction the expression of P2Y12R and P2X7R in human polarized microglia, and we performed autoradiography binding assay with radiolabeled P2Y12R and P2X7R antagonists using MS and rat EAE tissues.

Results

Here, we demonstrate that P2X7R is associated with a pro-inflammatory phenotype of human microglia in vitro, and is highly expressed in microglia in MS lesions as well as during the peak of EAE. In contrast, P2Y12R was associated with an anti-inflammatory phenotype in human microglia in vitro and was expressed at lower levels in active inflammatory MS lesions compared to normal-appearing white matter (NAWM) and similarly in EAE, while its expression increased in the remission phase of EAE. Binding of radiolabeled tracers specific for P2Y12R and P2X7R on ex vivo tissues validated the value of these receptors as PET imaging targets for microglia phenotypes in vivo.

Conclusion

Our results suggest that P2Y12R and P2X7R are excellent targets for PET imaging to discriminate distinct microglia phenotypes in MS. Ultimately, this may provide insight into the role of microglia in disease progression and monitor novel treatment strategies to alter microglia phenotype.

Electronic supplementary material

The online version of this article (10.1186/s12974-017-1034-z) contains supplementary material, which is available to authorized users.

A cyclic pathway of P2 × 7, bradykinin, and dopamine receptor activation induces a sustained articular hyperalgesia in the knee joint of rats

OBJECTIVE

We investigated whether: (1) P2 × 7 receptor activation by its agonist (BzATP) induces articular hyperalgesia in the rat's knee joint via inflammatory mechanisms and (2) activation of P2 × 7 receptors by endogenous ATP contributes to the articular hyperalgesia induced by bradykinin, TNF-α, IL-1β, CINC-1, PGE_2, and dopamine.

METHODS

The articular hyperalgesia was quantified using the rat knee joint incapacitation test. The knee joint inflammation, characterized by the concentration of pro-inflammatory cytokines and by neutrophil migration, was quantified in the synovial lavage fluid by ELISA and myeloperoxidase enzyme activity assay, respectively.

RESULTS

BzATP induced a dose-dependent articular hyperalgesia in the rat's knee joint that was significantly reduced by the selective antagonists for P2 × 7, bradykinin B₁ or B₂ receptors, β₁ or β₂ adrenoceptors, and by pre-treatment with Indomethacin. BzATP induced a local increase of TNF-α, IL-1β, IL-6, and CINC-1 concentration and neutrophil migration into the knee joint. The co-administration of the selective P2 × 7 receptor antagonist A-740003 significantly reduced the articular hyperalgesia induced by bradykinin and dopamine, but not by TNF-α, IL-1β, CINC-1, and PGE₂.

CONCLUSIONS

P2 × 7 receptor activation induces articular hyperalgesia mediated by the previous inflammatory mediator release. P2 × 7 receptor-induced articular hyperalgesia is sustained by the involvement of this purinergic receptor in bradykinin and dopamine-induced hyperalgesia in the knee joint.

Ethanol differentially modulates P2X4 and P2X7 receptor activity and function in BV2 microglial cells

Neuroinflammation is one of the mechanisms leading to neurodegenerative brain damage induced by chronic alcohol (ethanol) exposure. Microglia play a major role in the development of innate immune responses to environmental injuries including ethanol. Adenosine 5″-triphosphate (ATP)-activated purinergic P2X receptor (P2XR) subtypes, P2X4Rs and P2X7Rs, are endogenously expressed in microglia and can modulate their activity. These 2 P2XR subtypes differ pharmacologically and functionally: 1) P2X4Rs are activated at lower (≤0.1 mM) whereas P2X7Rs - at higher (≥1.0 mM) ATP concentrations; 2) P2X4R activation contributes to the release of brain derived neurotrophic factor and its role in tactile allodynia and neuropathic pain is demonstrated; 3) Due to its role in the secretion of pro-inflammatory IL-1β, P2X7Rs have been implicated in the development of neurodegenerative pathologies, pain and morphine tolerance. To date, the roles of individual P2XR subtypes in ethanol effects on microglia and the functional consequences are not completely understood. Based on the existing knowledge on the pharmacological and functional differences between P2X4Rs and P2X7Rs, the present work tested the hypothesis that P2X4Rs and P2X7Rs play differential roles in ethanol action in microglia. Effects of ethanol on P2X4R and P2X7R activity, expression and functional consequences were determined using murine BV2 microglial cells. Ethanol (≥100 mM) inhibited P2X4Rs but was inactive on P2X7 channel activity. Ethanol (25, 100 mM) inhibited P2X4R-mediated microglia migration whereas it potentiated pore formation in P2X7Rs. Furthermore, ethanol (25, 100 mM) potentiated P2X7R-mediated IL-1β secretion from BV2 microglia. Ethanol also induced protein expression for both P2XR subtypes. Overall, the findings identify differential roles for P2X4Rs and P2X7Rs in regards to ethanol effects on microglia which may be linked to different stages of ethanol exposure.

Site-Specific Regulation of P2X7 Receptor Function in Microglia Gates Morphine Analgesic Tolerance

Tolerance to the analgesic effects of opioids is a major problem in chronic pain management. Microglia are implicated in opioid tolerance, but the core mechanisms regulating their response to opioids remain obscure. By selectively ablating microglia in the spinal cord using a saporin-conjugated antibody to Mac1, we demonstrate a causal role for microglia in the development, but not maintenance, of morphine tolerance in male rats. Increased P2X7 receptor (P2X7R) activity is a cardinal feature of microglial activation, and in this study we found that morphine potentiates P2X7R-mediated Ca²⁺ responses in resident spinal microglia acutely isolated from morphine tolerant rats. The increased P2X7R function was blocked in cultured microglia by PP2, a Src family protein tyrosine kinase inhibitor. We identified Src family kinase activation mediated by μ-receptors as a key mechanistic step required for morphine potentiation of P2X7R function. Furthermore, we show by site-directed mutagenesis that tyrosine (Y_382-384) within the P2X7R C-terminus is differentially modulated by repeated morphine treatment and has no bearing on normal P2X7R function. Intrathecal administration of a palmitoylated peptide corresponding to the Y_382-384 site suppressed morphine-induced microglial reactivity and preserved the antinociceptive effects of morphine in male rats. Thus, site-specific regulation of P2X7R function mediated by Y_382-384 is a novel cellular determinant of the microglial response to morphine that critically underlies the development of morphine analgesic tolerance.SIGNIFICANCE STATEMENT Controlling pain is one of the most difficult challenges in medicine and its management is a requirement of a large diversity of illnesses. Although morphine and other opioids offer dramatic and impressive relief of pain, their impact is truncated by loss of efficacy (analgesic tolerance). Understanding why this occurs and how to prevent it are of critical importance in improving pain therapies. We uncovered a novel site (Y_382-384) within the P2X7 receptor that can be targeted to blunt the development of morphine analgesic tolerance, without affecting normal P2X7 receptor function. Our findings provide a critical missing mechanistic piece, site-specific modulation by Y_382-384, that unifies P2X7R function to the activation of spinal microglia and the development of morphine tolerance.

Neuronal P2X7 Receptors Revisited: Do They Really Exist?

P2X7 receptors (Rs) constitute a subclass of ATP-sensitive ionotropic receptors (P2X1-P2X7). P2X7Rs have many distinguishing features, mostly based on their long intracellular C terminus regulating trafficking to the cell membrane, protein-protein interactions, and post-translational modification. Their C-terminal tail is especially important in enabling the transition from the nonselective ion channel mode to a membrane pore allowing the passage of large molecules. There is an ongoing dispute on the existence of neuronal P2X7Rs with consequences for our knowledge on their involvement in neuroinflammation, aggravating stroke, temporal lobe epilepsy, neuropathic pain, and various neurodegenerative diseases. Whereas early results appeared to support the operation of P2X7Rs at neurons, more recently glial P2X7Rs are increasingly considered as indirect causes of neuronal effects. Specific tools for P2X7Rs are of limited value because of the poor selectivity of agonists, and the inherent failure of antibodies to differentiate between the large number of active and inactive splice variants, or gain-of-function and loss-of-function small nucleotide polymorphisms of the receptor. Unfortunately, the available P2RX7 knock-out mice generated by pharmaceutical companies possess certain splice variants, which evade inactivation. In view of the recently discovered bidirectional dialogue between astrocytes and neurons (and even microglia and neurons), we offer an alternative explanation for previous data, which assumedly support the existence of P2X7Rs at neurons. We think that the unbiased reader will follow our argumentation on astrocytic or microglial P2X7Rs being the primary targets of pathologically high extracellular ATP concentrations, although a neuronal localization of these receptors cannot be fully excluded either.

P2Y12 and P2Y13 receptors involved in ADPβs induced the release of IL-1β, IL-6 and TNF-α from cultured dorsal horn microglia

Objective

P2 receptors have been implicated in the release of neurotransmitter and pro-inflammatory cytokines due to their response to neuroexcitatory substances in the microglia. Dorsal horn P2Y₁₂ and P2Y₁₃ receptors are involved in the development of pain behavior induced by peripheral nerve injury. However, it is not known whether P2Y₁₂ and P2Y₁₃ receptors activation is associated with the expression and the release of interleukin-1B (IL-1β), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) in cultured dorsal spinal cord microglia. For this reason, we examined the effects of ADPβs (ADP analog) on the expression and the release of IL-1β, IL-6, and TNF-α.

Methods and results

In this study, we observed the effect of P2Y receptor agonist ADPβs on the expression and release of IL-1β, IL-6 and TNF-α by using real-time fluorescence quantitative polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA). ADPβs induced the increased expression of Iba-1, IL-1β, IL-6 and TNF-α at the level of messenger RNA (mRNA). ADPβs-evoked increase in Iba-1, IL-1β, IL-6 and TNF-α mRNA expression was inhibited only partially by P2Y₁₂ receptor antagonist MRS2395 or P2Y₁₃ receptor antagonist MRS2211, respectively. Similarly, ADPβs-evoked release of IL-1β, IL-6 and TNF-α was inhibited only partially by MRS2395 or MRS2211. Furthermore, ADPβs-evoked increased expression of Iba-1, IL-1β, IL-6 and TNF-α mRNA, and release of IL-1β, IL-6 and TNF-α were nearly all blocked after co-administration of MRS2395 plus MRS2179. Further evidence indicated that P2Y₁₂ and P2Y₁₃ receptor-evoked increased gene expression of IL-1β, IL-6 and TNF-α were inhibited by Y-27632 (ROCK inhibitor), SB203580 (P38MAPK inhibitor) and PDTC (NF-κb inhibitor), respectively. Subsequently, P2Y₁₂ and P2Y₁₃ receptor-evoked release of IL-1β, IL-6 and TNF-α, were also inhibited by Y-27632, SB203580 and PDTC, respectively.

Conclusion

These observations suggest that P2Y₁₂ and P2Y₁₃ receptor-evoked gene expression and release of IL-1β, IL-6 and TNF-α are associated with ROCK/P38MAPK/NF-κb signaling pathway.

Inhibition of the P2X7-PANX1 complex suppresses spreading depolarization and neuroinflammation

Spreading depolarization is a wave of neuronal and glial depolarization. Within minutes after spreading depolarization, the neuronal hemichannel pannexin 1 (PANX1) opens and forms a pore complex with the ligand-gated cation channel P2X7, allowing the release of excitatory neurotransmitters to sustain spreading depolarization and activate neuroinflammation. Here, we explore the hypothesis that the P2X7-PANX1 pore complex is a critical determinant of spreading depolarization susceptibility with important consequences for neuroinflammation and trigeminovascular activation. We found that genetic loss of function or ablation of the P2x7 gene inhibits spreading depolarization. Moreover, pharmacological suppression of the P2X7-PANX1 pore complex inhibits spreading depolarization in mice carrying the human familial hemiplegic migraine type 1 R192Q missense mutation as well as in wild-type mice and rats. Pore inhibitors elevate the electrical threshold for spreading depolarization, and reduce spreading depolarization frequency and amplitude. Pore inhibitors also suppress downstream consequences of spreading depolarization such as upregulation of interleukin-1 beta, inducible nitric oxide synthase and cyclooxygenase-2 in the cortex after spreading depolarization. In addition, they inhibit surrogates for trigeminovascular activation, including expression of calcitonin gene-related peptide in the trigeminal ganglion and c-Fos in the trigeminal nucleus caudalis. Our results are consistent with the hypothesis that the P2X7-PANX1 pore complex is a critical determinant of spreading depolarization susceptibility and its downstream consequences, of potential relevance to its signature disorders such as migraine.

Neuronal P2X7 receptor-induced reactive oxygen species production contributes to nociceptive behavior in mice

ATP can activate a variety of pathways through P2 purinoreceptors, leading to neuroprotection and pathology in the CNS. Among all P2X receptors, the P2X7 receptor (P2X7R) is a well-defined therapeutic target for inflammatory and neuropathic pain. Activation of P2X7R can generate reactive oxygen species (ROS) in macrophages and microglia. However, the role of ROS in P2X7R–induced pain remains unexplored. Here, we investigated the downstream effects of neuronal P2X7R activation in the spinal cord. We found that ATP induces ROS production in spinal cord dorsal horn neurons, an effect eliminated by ROS scavenger N-tert-butyl-α-phenylnitrone (PBN) and P2X7R antagonist A438079. A similar effect was observed with a P2X7R agonist, BzATP, and was attenuated by a NADPH oxidase inhibitor apocynin. Intrathecal administration of BzATP resulted in ROS production in the spinal cord and oxidative DNA damage in dorsal horn neurons. BzATP also induced robust biphasic spontaneous nociceptive behavior. Pre-treatment with A438079 abolished all BzATP-induced nociceptive behaviors, while ROS scavengers dose-dependently attenuated the secondary response. Here, we provide evidence that neuronal P2X7R activation leads to ROS production and subsequent nociceptive pain in mice. Together, the data indicate that P2X7R-induced ROS play a critical role in the P2X7R signaling pathway of the CNS.

Activation of P2X7 receptor and NLRP3 inflammasome assembly in hippocampal glial cells mediates chronic stress-induced depressive-like behaviors

Background

In recent years, proinflammatory cytokine interleukin-1β (IL-1β) was considered to play a critical role in the pathogenesis of depression. In addition, P2X7 receptor (P2X7R), a member of the purinergic receptor family, which is predominantly present on microglia, as well as on astrocytes and neurons in lesser amounts in the central nervous system, was suggested to be involved in the processing and releasing of IL-1β. Here, we investigated the role of P2X7R in the pathogenesis of depression.

Methods

Male Sprague-Dawley rats were subjected to chronic unpredictable stressors (CUS) for 3 weeks. At the end of week 1, 2, and 3, extracellular ATP, caspase 1, IL-1β, and components and activation of NLRP3 inflammasome (nucleotide-binding, leucine-rich repeat, pyrin domain containing 3) were evaluated as biomarker of neuroinflammation. In separate experiments, the rats were microinjected with P2X7R agonists ATP, BzATP, and saline into the hippocampus, respectively, or exposed to CUS combined with hippocampal microinjection with P2X7R antagonist, BBG and A438079, and saline, respectively, for 3 weeks, followed by exposed to forced swimming test and open-field test. Moreover, we also evaluated the depressive and anxiety-like behavior of P2X7-null mice in forced swimming test, open-field test, and elevated plus maze.

Results

Along with stress accumulation, extracellular ATP, cleaved-caspase 1, IL-1β, and ASC were significantly enhanced in the hippocampus, but P2X7R and NLRP3 were not. Immunoprecipitation assay indicated that along with the accumulation of stress, assembly of NLRP3 inflammasome and cleaved caspase 1 in NLRP3 inflammasome were significantly increased. Moreover, antagonists of P2X7R, either BBG or A438079, prevented the development of depressive-like behaviors induced by chronic unpredictable stress in rats. Meanwhile, we could not observe any depressive-like or anxiety-like behaviors of P2X7-null mice after they had been exposed to CUS. The results implied that P2X7 knockout could impede the development of depressive-like and anxiety-like behaviors induced by CUS. In contrast, chronic administration of agonists of P2X7R, either ATP or BzATP, could induce depressive-like behaviors.

Conclusions

The activation of P2X7R and subsequent NLRP3 inflammasome in hippocampal microglial cells could mediate depressive-like behaviors, which suggests a new therapeutic target for the prevention and treatment of depression.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0865-y) contains supplementary material, which is available to authorized users.

Bovine lactoferrin reduces extra-territorial facial allodynia/hyperalgesia following a trigeminal nerve injury in the rat

There is an urgent clinical need for an effective therapeutic agent to treat neuropathic pain. This study explored whether intrathecal administration of bovine lactoferrin (bLF), in combination with signal transduction pathway inhibition or an inflammatory cytokine production, results in reduced allodynia/hyperalgesia in the whisker pad area following mental nerve transection (MNT) in rats. Rats were intrathecally infused with bLF, lipopolysaccharide from Rhodobacter sphaeroides (LPS-RS), an antagonist of Toll-like receptor 4 (TLR4), or interleukin (IL)-18 binding protein (BP). bLF attenuated allodynia/hyperalgesia and blocked upregulation of phosphorylated (p)-p38 mitogen-activated protein kinase (MAPK), p-nuclear factor (NF)-κB p65, p-IκB kinase, and IL-18 in the trigeminal subnucleus caudalis (Vc). Microglia expressed p-p38 and astrocytes expressed p-NF-κB p65 in the Vc following MNT. LPS-RS had the same effects as bLF, except for attenuation of p-NF-κB p65. IL-18BP attenuated allodynia/hyperalgesia and IL-18 upregulation in the Vc. These results suggest that bLF suppresses IL-18 production, which is involved in allodynia/hyperalgesia following MNT, by inhibiting TLR4-derived p38 MAPK activation in microglia. Additionally, binding of bLF to tumor necrosis factor receptor-associated factor 6 might result in inhibition of p38 MAPK and NF-κB activation. The findings suggest that bLF could serve as a potent therapeutic agent for neuropathic pain.

P2X1, P2X4, and P2X7 Receptor Knock Out Mice Expose Differential Outcome of Sepsis Induced by α-Haemolysin Producing Escherichia coli

α-haemolysin (HlyA)-producing Escherichia coli commonly inflict severe urinary tract infections, including pyelonephritis, which comprises substantial risk for sepsis. In vitro, the cytolytic effect of HlyA is mainly mediated by ATP release through the HlyA pore and subsequent P2X₁/P2X₇ receptor activation. This amplification of the lytic process is not unique to HlyA but is observed by many other pore-forming proteins including complement-induced haemolysis. Since free hemoglobin in the blood is known to be associated with a worse outcome in sepsis one could speculate that inhibition of P2X receptors would ameliorate the course of sepsis. Surprisingly, this study demonstrates that [Formula: see text] and [Formula: see text] mice are exceedingly sensitive to sepsis with uropathogenic E. coli. These mice have markedly lower survival, higher cytokine levels and activated intravascular coagulation. Quite the reverse is seen in [Formula: see text] mice, which had markedly lower cytokine levels and less coagulation activation compared to controls after exposure to uropathogenic E. coli. The high cytokine levels in the [Formula: see text] mouse are unexpected, since P2X₇ is implicated in caspase-1-dependent IL-1β production. Here, we demonstrate that IL-1β production during sepsis with uropathogenic E. coli is mediated by caspase-8, since caspase-8 and RIPK3 double knock out mice show substantially lower cytokine during sepsis and increased survival after injection of TNFα. These data support that P2X₇ and P2X₄ receptor activation has a protective effect during severe E. coli infection.

Blockade of the kinin B1 receptor affects the cytokine/chemokine profile in rat brain subjected to autoimmune encephalomyelitis

Kinins are bioactive peptides which provide multiple functions, including critical regulation of the inflammatory response. Released during tissue injury, kinins potentiate the inflammation which represents a hallmark of numerous neurological disorders, including those of autoimmune origin such as multiple sclerosis (MS). In the present work, we assess the expression of B1 receptor (B1R) in rat brain during the course of experimental autoimmune encephalomyelitis (EAE) which is an animal model of MS. We apply pharmacological inhibition to investigate the role of this receptor in the development of neurological deficits and in shaping the cytokine/chemokine profile during the course of the disease. Overexpression of B1R is observed in brain tissue of rats subjected to EAE, beginning at the very early asymptomatic phase of the disease. This overexpression is suppressed by a specific antagonist known as DALBK. The involvement of B1R in the progression of neurological symptoms in immunized rats is confirmed. Analysis of an array of cytokines/chemokines identified a sub-group as being B1R-dependent. Increase of the protein levels for the proinflammatory cytokines (Il-6, TNF-α but not IL-1β), chemokines attracting immune cells into nervous tissue (MCP-1, MIP-3α, LIX), and protein levels of fractalkine and vascular endothelial growth factor observed in EAE rats, were significantly diminished after DALBK administration. This may indicate the protective potential of pharmacological inhibition of B1R. However, simultaneously reduced protein levels of anti-inflammatory and neuroprotective factors (IL-10, IL-4, and CNTF) was noticed. The results show that B1R-mediated signaling regulates the cellular response profile following neuroinflammation in EAE.

Spinal mechanisms of neuropathic pain: Is there a P2X4-BDNF controversy?

More than a decade ago the novel concept that glial cells are major players in the modulation of pain mechanisms in the spinal cord has started a prolific series of work addressing the modalities of neuron-glia communication. Mike Salter with Kazuhide Inoue laboratories introduced ATP as pivotal mediator for such communication via activation of P2X4 receptors expressed by microglia in the dorsal horn ipsilateral to a peripheral nerve injury. Activation of P2X4 receptors result in release of the neurotrophin BDNF, which, through the activation of neuronal TrkB receptors, alters neuronal excitability and this effect is associated with behavioural ipsilateral allodynia. This viewpoint article compares the evidence supporting a biological relevance of the P2X4 and BDNF system in neuropathic pain with recent data which question such importance. Having read this article, readers will be able to formulate their own opinion on such controversy.

Role of extracellular calcitonin gene-related peptide in spinal cord mechanisms of cancer-induced bone pain

Severe pain is a common and debilitating complication of metastatic bone cancer. Current analgesics provide insufficient pain relief and often lead to significant adverse effects. In models of cancer-induced bone pain, pathological sprouting of sensory fibers at the tumor-bone interface occurs concomitantly with reactive astrocytosis in the dorsal horn of the spinal cord. We observed that calcitonin gene-related peptide (CGRP)-fiber sprouting in the bone was associated with an increase in CGRP content in sensory neuron cell bodies in the dorsal root ganglia (DRG) and increased basal and activity-evoked release of CGRP from their central terminals in the dorsal horn. Intrathecal administration of a peptide antagonist (α-CGRP8-37) attenuated referred allodynia in the hind paw ipsilateral to bone cancer. CGRP receptor components (CLR and RAMP1) were up-regulated in dorsal horn neurons and expressed by reactive astrocytes. In primary cultures of astrocytes, CGRP incubation led to a concentration-dependent increase of forskolin-induced cAMP production, which was attenuated by pretreatment with CGRP8-37. Furthermore, CGRP induced ATP release in astrocytes, which was inhibited by CGRP8-37. We suggest that the peripheral increase in CGRP content observed in cancer-induced bone pain is mirrored by a central increase in the extracellular levels of CGRP. This increase in CGRP not only may facilitate glutamate-driven neuronal nociceptive signaling but also act on astrocytic CGRP receptors and lead to release of ATP.

Neuronal Release of Cytokine IL-3 Triggered by Mechanosensitive Autostimulation of the P2X7 Receptor Is Neuroprotective

Mechanical strain due to increased pressure or swelling activates inflammatory responses in many neural systems. As cytokines and chemokine messengers lead to both pro-inflammatory and neuroprotective actions, understanding the signaling patterns triggered by mechanical stress may help improve overall outcomes. While cytokine signaling in neural systems is often associated with glial cells like astrocytes and microglia, the contribution of neurons themselves to the cytokine response is underappreciated and has bearing on any balanced response. Mechanical stretch of isolated neurons was previously shown to trigger ATP release through pannexin hemichannels and autostimulation of P2X7 receptors (P2X7Rs) on the neural membrane. Given that P2X7Rs are linked to cytokine activation in other cells, this study investigates the link between neuronal stretch and cytokine release through a P2X7-dependent pathway. Cytokine assays showed application of a 4% strain to isolated rat retinal ganglion cells (RGCs) released multiple cytokines. The P2X7R agonist BzATP also released multiple cytokines; Interleukin 3 (IL-3), TNF-α, CXCL9, VEGF, L-selectin, IL-4, GM-CSF, IL-10, IL-1Rα, MIP and CCL20 were released by both stimuli, with the release of IL-3 greatest with either stimuli. Stretch-dependent IL-3 release was confirmed with ELISA and blocked by P2X7R antagonists A438079 and Brilliant Blue G (BBG), implicating autostimulation of the P2X7R in stretch-dependent IL-3 release. Neuronal IL-3 release triggered by BzATP required extracellular calcium. The IL-3Rα receptor was expressed on RGCs but not astrocytes, and both IL-3Rα and IL-3 itself were predominantly expressed in the retinal ganglion cell layer of adult retinal sections, implying autostimulation of receptors by released IL-3. While the number of surviving ganglion cells decreased with time in culture, the addition of IL-3 protected against this loss of neurons. Expression of mRNA for IL-3 and IL-3Rα increased in rat retinas stretched with moderate intraocular pressure (IOP) elevation; BBG blocked the rise in IL-3, implicating a role for the P2X7R in transcriptional regulation in vivo. In summary, mechanical stretch triggers release of cytokines from neurons that can convey neuroprotection. The enhancement of these signals in vivo implicates P2X7R-mediated IL-3 signaling as an endogenous pathway that could minimize damage following neuronal exposure to chronic mechanical strain.

Role of inflammatory cytokines in depression: Focus on interleukin-1β

According to the World Health Organization, major depression will become the leading cause of disability worldwide by the year 2030. Despite extensive research into the mechanisms underlying this disease, the rate, prevalence and disease burden has been on the rise, particularly in the industrialized world. Epidemiological studies have shown biological and biochemical differences in disease characteristics and treatment responses in different age groups. Notable differences have been observed in the clinical presentation, co-prevalence with other diseases, interaction with the immune system and even in the outcome. Thus, there is an increased interest in characterizing these differences, particularly in terms of contribution of different factors, including age, cytokines and immunotherapy. Research into the possible mechanisms of these interactions may reveal novel opportunities for future pharmacotherapy. The aim of the present review is to document recent literature regarding the impact of inflammatory mechanisms on the pathophysiology of the depressive disorder.

Intra-Articular Blockade of P2X7 Receptor Reduces the Articular Hyperalgesia and Inflammation in the Knee Joint Synovitis Especially in Female Rats

Synovitis is a key factor in joint disease pathophysiology, which affects a greater proportion of women than men. P2X7 receptor activation contributes to arthritis, but whether it plays a role in articular inflammatory pain in a sex-dependent manner is unknown. We investigated whether the P2X7 receptor blockade in the knee joint of male and female rats reduces the articular hyperalgesia and inflammation induced by a carrageenan knee joint synovitis model. Articular hyperalgesia was quantified using the rat knee joint incapacitation test and the knee joint inflammation, characterized by the concentration of cytokines tumor necrosis factor-α, interleukin-1β, interleukin-6, and cytokine-induced neutrophil chemoattractant-1, and by neutrophil migration, was quantified using enzyme-linked immunosorbent assay and by myeloperoxidase enzyme activity measurement, respectively. P2X7 receptor blockade by the articular coadministration of selective P2X7 receptor antagonist A740003 with carrageenan significantly reduced articular hyperalgesia, pro-inflammatory cytokine concentrations, and myeloperoxidase activity induced by carrageenan injection into the knee joint of male and estrus female rats. However, a lower dose of P2X7 receptor antagonist was sufficient to significantly induce the antihyperalgesic and anti-inflammatory effects in estrus female but not in male rats. These results suggest that P2X7 receptor activation by endogenous adenosine 5'-triphosphate is essential to articular hyperalgesia and inflammation development in the knee joint of male and female rats. However, female rats are more responsive than male rats to the antihyperalgesic and anti-inflammatory effects induced by P2X7 receptor blockade.

PERSPECTIVE

P2X7 receptors could be promising therapeutic targets in the treatment of knee joint disease symptoms, especially in women, who are more affected than men by these conditions.

Mechanisms underlying the hyperalgesic responses triggered by joint activation of TLR4

BACKGROUND

Toll-like receptors (TLRs) including TLR4 and their signal pathways contribute to the pathogenesis of arthritis. Herein, we evaluated the mechanisms underlying the hyperalgesic response caused by TLR4 activation in the tibio-tarsal joint in mice.

METHODS

Joint inflammatory hyperalgesia was induced by intra-articular (ia) injection of LPS (lipopolysaccharide- TLR4 agonist) in C57BL/6, TLR4, TLR2, MyD88, TRIF, TNFR1/2 and IL-1R1 knockout (^-/-) mice. Joint hyperalgesia was evaluated using an electronic von Frey. Neutrophil recruitment was assessed by MPO activity. Joint levels of cytokines were measured by ELISA.

RESULTS

Firstly, it was shown that LPS injected into the joints causes a dose- and time-dependent reduction in the mechanical nociceptive threshold. The TLR4 activation in the joint triggers mechanical hyperalgesia and neutrophil migration, which was abolished in TLR4 ^-/- and MyD88^-/-, but not in TLR2^-/- and TRIF^-/- mice. Besides, joint administration of LPS increased the release of TNF-α, IL-1β, and KC/CXCL1, which were reduced in TLR4^-/- and MyD88^-/-, but not in TRIF^-/- mice. In agreement, the LPS-induced joint nociceptive effect was decreased in TNFR1/2^-/- and IL-1R1^-/- mice or in mice pre-treated with a CXCR1/2 selective antagonist (DF2156A).

CONCLUSIONS

These results suggest that TLR4 activation in the joint produces articular hyperalgesia via MyD88 signaling pathway. Moreover, this pathway is involved in the cascade of events of articular hyperalgesia through mechanisms dependent on cytokines and chemokines production. Thus, TLR4/MyD88 signaling pathway inhibitors might be useful for the treatment of inflammatory joint pain.

P2X7R modulation of visually evoked synaptic responses in the retina

P2X₇Rs are distributed throughout all layers of the retina, and thus, their localisation on various cell types puts into question their specific site(s) of action. Using a dark-adapted, ex vivo mouse retinal whole mount preparation, the present study aimed to characterise the effect of P2X₇R activation on light-evoked, excitatory RGC ON-field excitatory post-synaptic potentials (fEPSPs) and on outer retinal electroretinogram (ERG) responses under comparable conditions. The pharmacologically isolated NMDA receptor-mediated RGC ON-fEPSP was reduced in the presence of BzATP, an effect which was significantly attenuated by A438079 and other selective P2X₇R antagonists A804598 or AF27139. In physiological Krebs medium, BzATP induced a significant potentiation of the ERG a-wave, with a concomitant reduction in the b-wave and the power of the oscillatory potentials. Conversely, in the pharmacologically modified Mg²⁺-free perfusate, BzATP reduced both the a-wave and b-wave. The effects of BzATP on the ERG components were suppressed by A438079. A role for P2X₇R function in visual processing in both the inner and outer retina under physiological conditions remains controversial. The ON-fEPSP was significantly reduced in the presence of A804598 but not by A438079 or AF27139. Furthermore, A438079 did not have any effect on the ERG components in physiological Krebs but potentiated and reduced the a-wave and b-wave, respectively, when applied to the pharmacologically modified medium. Therefore, activation of P2X₇Rs affects the function in the retinal ON pathway. The presence of a high concentration of extracellular ATP would most likely contribute to the modulation of visual transmission in the retina in the pathophysiological microenvironment.

Electroacupuncture Relieves Nerve Injury-Induced Pain Hypersensitivity via the Inhibition of Spinal P2X7 Receptor-Positive Microglia

BACKGROUND

Electroacupuncture (EA) has therapeutic effects on neuropathic pain induced by nerve injury; however, the underlying mechanisms remain unclear. In this study, we examined whether EA treatment relieves pain hypersensitivity via the down-regulation of spinal P2X7 receptor-positive (P2X7R⁺) microglia-mediated overexpression of interleukin (IL)-1β and/or IL-18.

METHODS

Male Sprague-Dawley rats underwent chronic constriction injury (CCI) or 3'-O-(4-benzoylbenzoyl) adenosine 5'-triphosphate (BzATP) intrathecal injection. Von Frey and Hargreaves tests were performed to evaluate the effect of EA on pain hypersensitivity. The spinal P2X7R, IL-1β, and IL-18 expression levels were determined by real-time polymerase chain reaction, Western blot analysis, immunofluorescence staining, and enzyme-linked immunosorbent assay. The selective P2X7R antagonist A-438079 was used to examine the P2X7R⁺ microglia-dependent release of IL-1β and IL-18. Primary cultures were subsequently used to assess the P2X7R⁺ microglia-induced IL-1β and IL-18 release.

RESULTS

EA treatment significantly improved the pain thresholds and inhibited spinal P2X7R⁺ microglia activation induced by CCI or BzATP administration, which was accompanied by the suppression of spinal IL-1β and IL-18 overexpression. Moreover, A-438079 also improved pain thresholds and suppressed overexpression of IL-1β in the CCI- and BzATP-injected rats. The analysis of cultured microglia further demonstrated that A-438079 markedly decreased BzATP-induced IL-1β release.

CONCLUSIONS

EA treatment relieves nerve injury-induced tactile allodynia and thermal hyperalgesia via the inhibition of P2X7R⁺ microglia-mediated IL-1β overexpression.

Reprint of: Microglial toll-like receptors and Alzheimer's disease

Microglial activation represents an important pathological hallmark of Alzheimer's disease (AD), and emerging data highlight the involvement of microglial toll-like receptors (TLRs) in the course of AD. TLRs have been observed to exert both beneficial and detrimental effects on AD-related pathologies, and transgenic animal models have provided direct and credible evidence for an association between TLRs and AD. Moreover, analyses of genetic polymorphisms have suggested interactions between genetic polymorphisms in TLRs and AD risk, further supporting the hypothesis that TLRs are involved in AD. In this review, we summarize the key evidence in this field. Future studies should focus on exploring the mechanisms underlying the potential roles of TLRs in AD.