Resident glial cell activation in response to perispinal inflammation leads to acute changes in nociceptive sensitivity: implications for the generation of neuropathic pain

Conditional Astrocyte Rac1KO Attenuates Hyperreflexia after Spinal Cord Injury

Spasticity is a hyperexcitability disorder that adversely impacts functional recovery and rehabilitative efforts after spinal cord injury (SCI). The loss of evoked rate-dependent depression (RDD) of the monosynaptic H-reflex is indicative of hyperreflexia, a physiological sign of spasticity. Given the intimate relationship between astrocytes and neurons, that is, the tripartite synapse, we hypothesized that astrocytes might have a significant role in post-injury hyperreflexia and plasticity of neighboring neuronal synaptic dendritic spines. Here, we investigated the effect of selective Rac1KO in astrocytes (i.e., adult male and female mice, transgenic cre-flox system) on SCI-induced spasticity. Three weeks after a mild contusion SCI, control Rac1wt animals displayed a loss of H-reflex RDD, that is, hyperreflexia. In contrast, transgenic animals with astrocytic Rac1KO demonstrated near-normal H-reflex RDD similar to pre-injury levels. Reduced hyperreflexia in astrocytic Rac1KO animals was accompanied by a loss of thin-shaped dendritic spine density on α-motor neurons in the ventral horn. In SCI-Rac1wt animals, as expected, we observed the development of dendritic spine dysgenesis on α-motor neurons associated with spasticity. As compared with WT animals, SCI animals with astrocytic Rac1KO expressed increased levels of the glial-specific glutamate transporter, glutamate transporter-1 in the ventral spinal cord, potentially enhancing glutamate clearance from the synaptic cleft and reducing hyperreflexia in astrocytic Rac1KO animals. Taken together, our findings show for the first time that Rac1 activity in astrocytes can contribute to hyperreflexia underlying spasticity following SCI. These results reveal an opportunity to target cell-specific molecular regulators of H-reflex excitability to manage spasticity after SCI.Significance Statement Spinal cord injury leads to stretch reflex hyperexcitability, which underlies the clinical symptom of spasticity. This study shows for the first time that astrocytic Rac1 contributes to the development of hyperreflexia after SCI. Specifically, astrocytic Rac1KO reduced SCI-related H-reflex hyperexcitability, decreased dendritic spine dysgenesis on α-motor neurons, and elevated the expression of the astrocytic glutamate transporter-1 (GLT-1). Overall, this study supports a distinct role for astrocytic Rac1 signaling within the spinal reflex circuit and the development of SCI-related spasticity.

Involvement of microglia in chronic neuropathic pain associated with spinal cord injury - a systematic review

In recent decade microglia have been found to have a central role in the development of chronic neuropathic pain after injury to the peripheral nervous system. It is widely accepted that peripheral nerve injury triggers microglial activation in the spinal cord, which contributes to heightened pain sensation and eventually chronic pain states. The contribution of microglia to chronic pain arising after injury to the central nervous system, such as spinal cord injury (SCI), has been less studied, but there is evidence supporting microglial contribution to central neuropathic pain. In this systematic review, we focused on post-SCI microglial activation and how it is linked to emergence and maintenance of chronic neuropathic pain arising after SCI. We found that the number of studies using animal SCI models addressing microglial activity is still small, compared with the ones using peripheral nerve injury models. We have collected 20 studies for full inclusion in this review. Many mechanisms and cellular interactions are yet to be fully understood, although several studies report an increase of density and activity of microglia in the spinal cord, both in the vicinity of the injury and in the spared spinal tissue, as well as in the brain. Changes in microglial activity come with several molecular changes, including expression of receptors and activation of signalling pathways. As with peripheral neuropathic pain, microglia seem to be important players and might become a therapeutic target in the future.

Role of the blood-spinal cord barrier: An adheren junction regulation mechanism that promotes chronic postsurgical pain

Chronic postsurgical pain (CPSP) is a serious postoperative complication with high incidence, and its pathogenesis involves neuroimmune interactions and the breakdown of the blood-spinal cord barrier (BSCB), the decreased level of adheren junction (AJ)-related proteins is an important cause of BSCB injury. Vascular endothelial-cadherin (VE-cadherin) and p120 catenin (p120) constitute the endothelial barrier adheren junction. The Src/p120/VE-cadherin pathway is involved in the regulation of the endothelial barrier function. However, the role of the BSCB-AJ regulatory mechanism in CPSP has not been reported. In this study, we established a skin/muscle incision and retraction (SMIR) model and evaluated the paw withdrawal threshold (PWT), the effects of an Src inhibitor and p120 knockdown on p-Src, p120 and VE-cadherin expression, as well as BSCB-AJ function in rat spinal cord were observed to explore the regulation of BSCB-AJ function by the p-Src/p120/VE-cadherin pathway in promoting SMIR-induced CPSP. The levels of p-Src, p120 and VE-cadherin in the spinal cord were detected by Western blot. Meanwhile, BSCB permeability test was used to detect the changes in BCSB function. Finally, the spatial and temporal localization of p120 in spinal cord was detected by immunofluorescence. Our findings indicated that p-Src/p120/VE-cadherin could induce BSCB-AJ dysfunction and promote the development of CPSP.

The mTOR inhibitor rapamycin suppresses trigeminal neuropathic pain and p-MKK4/p-p38 mitogen-activated protein kinase-mediated microglial activation in the trigeminal nucleus caudalis of mice with infraorbital nerve injury

Neuropathic pain caused by trigeminal nerve injury is a typical refractory orofacial chronic pain accompanied by the development of hyperalgesia and allodynia. We previously demonstrated that the mammalian target of rapamycin (mTOR) inhibitor rapamycin suppressed orofacial formalin injection-induced nociception; however, the underlying mechanism is unclear, and it is unknown whether it can reduce trigeminal neuropathic pain. In mice, left infraorbital nerve and partial nerve ligation (ION-pNL) was performed using a silk suture (8-0). Fourteen days after surgery, neuropathic pain behavior was examined on a whisker pad and rapamycin (0.1, 0.3, and 1.0 mg/kg) was administered intraperitoneally. Mechanical and cold sensitivities in the orofacial region were quantified using von Frey filaments and acetone solution, respectively. Changes in mTOR and related proteins, such as p-MKK3/6, p-MKK4, p-JNK, p-ERK, p-p38 MAPK, GFAP, and Iba-1, in the trigeminal nucleus caudalis (TNC) or the trigeminal ganglia (TG) tissues were examined via western blot analysis or immunohistochemistry. Mice demonstrated significant mechanical and cold allodynia 2 weeks following ION-pNL injury, both of which were significantly reduced 1 h after the administration of high-dose rapamycin (1.0 mg/kg). In the TG tissue, ION-pNL surgery or rapamycin treatment did not change p-mTOR and p-4EBP1, but rapamycin reduced the increase of p-S6 and S6 induced by ION-pNL. In the TNC tissue, neither ION-pNL surgery nor rapamycin treatment altered p-mTOR, p-S6, and p-4EBP1 expressions, whereas rapamycin significantly decreased the ION-pNL-induced increase in Iba-1 expression. In addition, rapamycin suppressed the increase in p-p38 MAPK and p-MKK4 expressions but not p-MKK3/6 expression. Moreover, p-p38 MAPK-positive cells were colocalized with increased Iba-1 in the TNC. Our findings indicate that rapamycin treatment reduces both mechanical and cold orofacial allodynia in mice with trigeminal neuropathic pain, which is closely associated with the modulation of p-MKK4/p-p38 MAPK-mediated microglial activation in the TNC.

Mangiferin ameliorates intracerebroventricular-quinolinic acid-induced cognitive deficits, oxidative stress, and neuroinflammation in Wistar rats

INTRODUCTION:

Mangiferin (MGF), a xanthonoid polyphenol, confers neuroprotection via combating oxidative stress and inflammation. The current investigation aimed to assess the neuroprotective potential of MGF on behavioral and neurochemical anomalies evoked by administration of quinolinic acid (QA) through intrastriatal injection in male Wistar rats and to reveal the associated mechanisms.

MATERIALS AND METHODS:

QA (300 nm/4 μl saline) was administered intracerebroventricular in the striatum (unilaterally) once. Thereafter, MGF 20 and 40 mg/kg (peroral) was administered to the animals for 21 days.

RESULTS:

QA administration caused marked alteration in motor activity (rotatod), footprint analysis, and cognitive function (Morris water maze test, and novel object recognition test). Furthermore, oxido-nitrosative stress (increased nitrite content, lipid peroxidation, with reduction of GSH), cholinergic dysfunction, and mitochondrial complex (I, II, and IV) dysfunction were observed in hippocampus and striatal region of QA-treated rats in comparison to normal control. Pro inflammatory mediators (tumor necrosis factor-alpha TNF-α and interleukin-1β) were noted to increase in the hippocampus and striatum of QA-treated rats. In addition, we observed BDNF depletion in both the hippocampus and striatum of QA-treated animals. MGF treatment significantly ameliorated memory and motor deficits in QA-administered rats. Moreover, MGF treatment (40 mg/kg) restored the GSH level and reduced the MDA, nitrite level, and pro-inflammatory cytokines in striatum and hippocampus. Furthermore, QA-induced cholinergic dysfunction (AChE), BDNF depletion and mitochondrial impairment were found to be ameliorated by MGF treatment.

CONCLUSION:

The results suggest that MGF offers the neuroprotective potential that may be a promising pharmacological approach to ameliorate cognitive deficits associated with neurodegeneration.

Review/overview of pain in sickle cell disease

Sickle cell disease (SCD) is a highly complex inherited disorder of hemoglobin structure. Although the molecular lesion is a single-point mutation, the sickle gene is pleiotropic in nature causing multiple phenotypic expressions that constitute the various complications of the disease. Its manifestations could be acute, chronic, nociceptive, neuropathic that could occur singly or in various combinations. Pain continues to be the major factor of SCD phenotypic complications and the most common cause of admissions to the Emergency Department and/or the hospital. Although progress has been made in understanding the pathophysiology of SCD as well as in developing curative therapies such as hematopoietic stem cell transplantation and gene therapy, effective pain management continues to lag behind. Palliative therapies continue to be the major approach to the management of SCD and its complications. The advent of hydroxyurea made partial success in preventing the frequency of vaso-occlusive crises and l-glutamine awaits post-trial confirmation of benefits. The search for additional pharmacotherapeutic agents that could be used singly or in combination with hydroxyurea and/or l-glutamine awaits their dawn hopefully in the near future. The purpose of this review is to describe the various manifestations of SCD, their pathophysiology and their current management. Recent impressive advances in understanding the pathophysiology of pain promise the determination of agents that could replace or minimize the use of opioids.

P-glycoprotein Inhibitor Tariquidar Potentiates Efficacy of Astragaloside IV in Experimental Autoimmune Encephalomyelitis Mice

ATP-binding cassette (ABC) transporters, such as P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), often reduce drug efficacy and are the major cause of drug resistance. Astragaloside IV (ASIV), one of the bioactive saponins isolated from Astragalus membranaceus, has been demonstrated to alleviate the progression of experimental autoimmune encephalomyelitis (EAE) in mice, an animal model for multiple sclerosis (MS). In the present study, we found for the first time that ASIV induced the upregulation of P-gp and BCRP in the central nervous system (CNS) microvascular endothelial cells of EAE mice. Further study disclosed that tariquidar, a P-gp inhibitor, could facilitate the penetration of ASIV into CNS. On bEnd.3 cells, a mouse brain microvascular endothelial cell line, tariquidar benefited the net uptake and transport of ASIV. Additional molecular docking experiment suggested that ASIV might be a potential substrate of P-gp. In EAE mice, tariquidar was demonstrated to enhance the efficacy of ASIV, as shown by attenuated clinical symptom and reduced incidence rate as well as mitigated inflammatory infiltration and decreased demyelination in the CNS. Collectively, our findings implicate that P-gp inhibitor can promote the therapeutic efficacy of ASIV on EAE mice, which may boost its clinical usage together with ASIV in the therapy of MS.

Retracted Article: Gm5820, an antisense RNA of FGF1, suppresses FGF1 expression at the posttranscriptional level to inactivate the ERK/STAT3 pathway and alleviates neuropathic pain in mice

Emerging evidence reveals that lncRNAs play important roles in various pathological processes, but precious little indicates their regulatory role in neuropathic pain.

Reward Circuitry Plasticity in Pain Perception and Modulation

Although pain is a widely known phenomenon and an important clinical symptom that occurs in numerous diseases, its mechanisms are still barely understood. Owing to the scarce information concerning its pathophysiology, particularly what is involved in the transition from an acute state to a chronic condition, pain treatment is frequently unsatisfactory, therefore contributing to the amplification of the chronic pain burden. In fact, pain is an extremely complex experience that demands the recruitment of an intricate set of central nervous system components. This includes cortical and subcortical areas involved in interpretation of the general characteristics of noxious stimuli. It also comprises neural circuits that process the motivational-affective dimension of pain. Hence, the reward circuitry represents a vital element for pain experience and modulation. This review article focuses on the interpretation of the extensive data available connecting the major components of the reward circuitry to pain suffering, including the nucleus accumbens, ventral tegmental area, and the medial prefrontal cortex; with especial attention dedicated to the evaluation of neuroplastic changes affecting these structures found in chronic pain syndromes, such as migraine, trigeminal neuropathic pain, chronic back pain, and fibromyalgia.

Rapamycin suppresses microglial activation and reduces the development of neuropathic pain after spinal cord injury

Rapamycin is an inhibitor of the mammalian target of rapamycin (mTOR) signaling pathway, plays an important role in multiple cellular functions. Our previous study showed rapamycin treatment in acute phase reduced the neural tissue damage and locomotor impairment after spinal cord injury (SCI). However, there has been no study to investigate the therapeutic effect of rapamycin on neuropathic pain after SCI. In this study, we examined whether rapamycin reduces neuropathic pain following SCI in mice. We used a mouse model of thoracic spinal cord contusion injury, and divided the mice into the rapamycin-treated and the vehicle-treated groups. The rapamycin-treated mice were intraperitoneally injected with rapamycin (1 mg/kg) 4 h after SCI. The rapamycin treatment suppressed phosphorylated-p70S6K in the injured spinal cord that indicated inhibition of mTOR. The rapamycin treatment significantly improved not only locomotor function, but also mechanical and thermal hypersensitivity in the hindpaws after SCI. In an immunohistochemical analysis, Iba-1-stained microglia in the lumbar spinal cord was significantly decreased in the rapamycin-treated mice. In addition, the activity of p38 MAPK in the lumbar spinal cord was significantly attenuated by rapamycin treatment. Furthermore, phosphorylated-p38 MAPK-positive microglia was relatively decreased in the rapamycin-treated mice. These results indicated rapamycin administration in acute phase to reduce secondary neural tissue damage can contribute to the suppression of the microglial activation in the lumbar spinal cord and attenuate the development of neuropathic pain after SCI. The present study first demonstrated that rapamycin has significant therapeutic potential to reduce the development of neuropathic pain following SCI. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:93-103, 2017.

Intrathecal Injection of Human Umbilical Cord-Derived Mesenchymal Stem Cells Ameliorates Neuropathic Pain in Rats

Neuropathic pain (NP) is a clinically incurable disease with miscellaneous causes, complicated mechanisms and available therapies show poor curative effect. Some recent studies have indicated that neuroinflammation plays a vital role in the occurrence and promotion of NP and anti-inflammatory therapy has the potential to relieve the pain. During the past decades, mesenchymal stem cells (MSCs) with properties of multipotentiality, low immunogenicity and anti-inflammatory activity have showed excellent therapeutic effects in cell therapy from animal models to clinical application, thus aroused great attention. However there are no reports about the effect of intrathecal human umbilical cord-derived mesenchymal stem cells (HUC-MSCs) on NP which is induced by peripheral nerve injury. Therefore, in this study, intrathecally transplanted HUC-MSCs were utilized to examine the effect on neuropathic pain induced by a rat model with spinal nerve ligation (SNL), so as to explore the possible mechanism of those effects. As shown in the results, the HUC-MSCs transplantation obviously ameliorated SNL-induced mechanical allodynia and thermal hyperalgesia, which was related to the inhibiting process of neuroinflammation, including the suppression of activated astrocytes and microglia, as well as the significant reduction of pro-inflammatory cytokines Interleukin-1β (IL-1β) and Interleukin -17A (IL-17A) and the up-regulation of anti-inflammatory cytokine Interleukin -10 (IL-10). Therefore, through the effect on glial cells, pro-inflammatory and anti-inflammatory cytokine, the targeting intrathecal HUC-MSCs may offer a novel treatment strategy for NP.

Activated brain mast cells contribute to postoperative cognitive dysfunction by evoking microglia activation and neuronal apoptosis

Background

Neuroinflammation plays a key role in the occurrence and development of postoperative cognitive dysfunction (POCD). Microglia, the resident immune cells in the brain, has been increasingly recognized to contribute to neuroinflammation. Although brain mast cells (MCs) are the “first responder” in the brain injury rather than microglia, little is known about the functional aspects of MCs-microglia interactions.

Methods

Male Sprague-Dawley (SD) rats were injected intracerebroventricular with MC stabilizer Cromolyn (100 μg/μl), MC stimulator C48/80 (1 μg/μl), or sterile saline 30 min before open tibial fracture surgery, and the levels of neuroinflammation and memory dysfunction were tested 1 and 3 days after surgery. In addition, the effect of activated MCs on microglia and neurons was determined in vitro.

Results

Tibial fracture surgery induced MCs degranulation, microglia activation, and inflammatory factors production, which initiated the acute brain inflammatory response and neuronal death and exhibited cognitive deficit. Site-directed preinjection of the “MCs stabilizer” disodium cromoglycate (Cromolyn) inhibited this effect, including decrease of inflammatory cytokines, reduced MCs degranulation, microglia activation, neuronal death, and improved cognitive function 24 h after the surgery. In vitro study, we found that the conditioned medium from lipopolysaccharide (LPS)-stimulated mast cells line (P815) could induce primary microglia activation through mitogen-activated protein kinase (MAPK) pathway signaling and subsequent production of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6). In addition, the activated P815 could directly induce neuronal apoptosis and synapse injury with microglia independently. Cromolyn could inhibit P815 activation following improved microglia activation and neuronal loss.

Conclusions

These results implicate that activated MCs could trigger microglia activation and neuronal damage, resulting in central nervous system (CNS) inflammation, and communications of MCs with microglia and neuron could constitute a new and unique therapeutic target for CNS immune inflammation-related diseases.

Glial Modulation by N-acylethanolamides in Brain Injury and Neurodegeneration

Neuroinflammation involves the activation of glial cells and represents a key element in normal aging and pathophysiology of brain damage. N-acylethanolamides (NAEs), naturally occurring amides, are known for their pro-homeostatic effects. An increase in NAEs has been reported in vivo and in vitro in the aging brain and in brain injury. Treatment with NAEs may promote neuroprotection and exert anti-inflammatory actions via PPARα activation and/or by counteracting gliosis. This review aims to provide an overview of endogenous and exogenous properties of NAEs in neuroinflammation and to discuss their interaction with glial cells.

Adolescent Intermittent Alcohol Exposure: Dysregulation of Thrombospondins and Synapse Formation are Associated with Decreased Neuronal Density in the Adult Hippocampus

BACKGROUND

Adolescent intermittent alcohol exposure (AIE) has profound effects on neuronal function. We have previously shown that AIE causes aberrant hippocampal structure and function that persists into adulthood. However, the possible contributions of astrocytes and their signaling factors remain largely unexplored. We investigated the acute and enduring effects of AIE on astrocytic reactivity and signaling on synaptic expression in the hippocampus, including the impact of the thrombospondin (TSP) family of astrocyte-secreted synaptogenic factors and their neuronal receptor, alpha2delta-1 (α2δ-1). Our hypothesis is that some of the influences of AIE on neuronal function may be secondary to direct effects on astrocytes.

METHODS

We conducted Western blot analysis on TSPs 1 to 4 and α2δ-1 from whole hippocampal lysates 24 hours after the 4th and 10th doses of AIE, then 24 days after the last dose (in adulthood). We used immunohistochemistry to assess astrocyte reactivity (i.e., morphology) and synaptogenesis (i.e., colocalization of pre- and postsynaptic puncta).

RESULTS

Adolescent AIE reduced α2δ-1 expression, and colocalized pre- and postsynaptic puncta after the fourth ethanol (EtOH) dose. By the 10th dose, increased TSP2 levels were accompanied by an increase in colocalized pre- and postsynaptic puncta, while α2δ-1 returned to control levels. Twenty-four days after the last EtOH dose (i.e., adulthood), TSP2, TSP4, and α2δ-1 expression were all elevated. Astrocyte reactivity, indicated by increased astrocytic volume and area, was also observed at that time.

CONCLUSIONS

Repeated EtOH exposure during adolescence results in long-term changes in specific astrocyte signaling proteins and their neuronal synaptogenic receptor. Continued signaling by these traditionally developmental factors in adulthood may represent a compensatory mechanism whereby astrocytes reopen the synaptogenic window and repair lost connectivity, and consequently contribute to the enduring maladaptive structural and functional abnormalities previously observed in the hippocampus after AIE.

Activity-triggered tetrapartite neuron-glial interactions following peripheral injury

Recent studies continue to support the proposition that non-neuronal components of the nervous system, mainly glial cells and associated chemical mediators, contribute to the development of neuronal hyperexcitability that underlies persistent pain conditions. In the event of peripheral injury, enhanced or abnormal nerve input is likely the most efficient way to activate simultaneously central neurons and glia. Injury induces phenotypic changes in glia and triggers signaling cascades that engage reciprocal interactions between presynaptic terminals, postsynaptic neurons, microglia and astrocytes. While some responses to peripheral injury may help the nervous system to adapt positively to counter the disastrous effect of injury, the net effect often leads to long-lasting sensitization of pain transmission pathways and chronic pain.

Identification of Spinal Cord MicroRNA and Gene Signatures in a Model of Chronic Stress-Induced Visceral Hyperalgesia in Rat

Introduction

Animal studies have shown that stress could induce epigenetic and transcriptomic alterations essential in determining the balance between adaptive or maladaptive responses to stress. We tested the hypothesis that chronic stress in rats deregulates coding and non-coding gene expression in the spinal cord, which may underline neuroinflammation and nociceptive changes previously observed in this model.

Methods

Male Wistar rats were exposed to daily stress or handled, for 10 days. At day 11, lumbar spinal segments were collected and processed for mRNA/miRNA isolation followed by expression profiling using Agilent SurePrint Rat Exon and Rat miRNA Microarray platforms. Differentially expressed gene lists were generated using the dChip program. Microarrays were analyzed using the Ingenuity Pathways Analysis (IPA) tool from Ingenuity Systems. Multiple methods were used for the analysis of miRNA-mRNA functional modules. Quantitative real time RT-PCR for Interleukin 6 signal transducer (gp130), the Signal Transducer And Activator Of Transcription 3 (STAT3), glial fibrillary acidic protein and mir-17-5p were performed to confirm levels of expression.

Results

Gene network analysis revealed that stress deregulated different inflammatory (IL-6, JAK/STAT, TNF) and metabolic (PI3K/AKT) signaling pathways. MicroRNA array analysis revealed a signature of 39 deregulated microRNAs in stressed rats. MicroRNA-gene network analysis showed that microRNAs are regulators of two gene networks relevant to inflammatory processes. Specifically, our analysis of miRNA-mRNA functional modules identified miR-17-5p as an important regulator in our model. We verified miR-17-5p increased expression in stress using qPCR and in situ hybridization. In addition, we observed changes in the expression of gp130 and STAT3 (involved in intracellular signaling cascades in response to gp130 activation), both predicted targets for miR-17-5p. A modulatory role of spinal mir17-5p in the modulation of visceral sensitivity was confirmed in vivo.

Conclusion

Using an integrative high throughput approach, our findings suggest a link between miR-17-5p increased expression and gp130/STAT3 activation providing new insight into the possible mechanisms mediating the effect of chronic stress on neuroinflammation in the spinal cord.

Coupled cell networks are target cells of inflammation, which can spread between different body organs and develop into systemic chronic inflammation

Several organs in the body comprise cells coupled into networks. These cells have in common that they are excitable but do not express action potentials. Furthermore, they are equipped with Ca²⁺ signaling systems, which can be intercellular and/or extracellular. The transport of small molecules between the cells occurs through gap junctions comprising connexin 43. Examples of cells coupled into networks include astrocytes, keratinocytes, chondrocytes, synovial fibroblasts, osteoblasts, connective tissue cells, cardiac and corneal fibroblasts, myofibroblasts, hepatocytes, and different types of glandular cells. These cells are targets for inflammation, which can be initiated after injury or in disease. If the inflammation reaches the CNS, it develops into neuroinflammation and can be of importance in the development of systemic chronic inflammation, which can manifest as pain and result in changes in the expression and structure of cellular components. Biochemical parameters of importance for cellular functions are described in this review.

Toll-like receptors 2, -3 and -4 prime microglia but not astrocytes across central nervous system regions for ATP-dependent interleukin-1β release

Interleukin-1β (IL-1β) is a crucial mediator in the pathogenesis of inflammatory diseases at the periphery and in the central nervous system (CNS). Produced as an unprocessed and inactive pro-form which accumulates intracellularly, release of the processed cytokine is strongly promoted by ATP acting at the purinergic P2X₇ receptor (P2X₇R) in cells primed with lipopolysaccharide (LPS), a Toll-like receptor (TLR) 4 ligand. Microglia are central to the inflammatory process and a major source of IL-1β when activated. Here we show that purified (>99%) microglia cultured from rat cortex, spinal cord and cerebellum respond robustly to ATP-dependent IL-1β release, upon priming with a number of TLR isoform ligands (zymosan and Pam3CSK4 for TLR2, poly(I:C) for TLR3). Cytokine release was prevented by a P2X₇R antagonist and inhibitors of stress-activated protein kinases. Enriched astrocytes (≤5% microglia) from these CNS regions displayed responses qualitatively similar to microglia but became unresponsive upon eradication of residual microglia with the lysosomotropic agent Leu-Leu-OMe. Activation of multiple TLR isoforms in nervous system pathology, coupled with elevated extracellular ATP levels and subsequent P2X₇R activation may represent an important route for microglia-derived IL-1β. This phenomenon may have important consequences for neuroinflammation and its position to the common pathology of CNS diseases.

The role of the blood-brain barrier in the development and treatment of migraine and other pain disorders

The function of the blood-brain barrier (BBB) related to chronic pain has been explored for its classical role in regulating the transcellular and paracellular transport, thus controlling the flow of drugs that act at the central nervous system, such as opioid analgesics (e.g., morphine) and non-steroidal anti-inflammatory drugs. Nonetheless, recent studies have raised the possibility that changes in the BBB permeability might be associated with chronic pain. For instance, changes in the relative amounts of occludin isoforms, resulting in significant increases in the BBB permeability, have been demonstrated after inflammatory hyperalgesia. Furthermore, inflammatory pain produces structural changes in the P-glycoprotein, the major efflux transporter at the BBB. One possible explanation for these findings is the action of substances typically released at the site of peripheral injuries that could lead to changes in the brain endothelial permeability, including substance P, calcitonin gene-related peptide, and interleukin-1 beta. Interestingly, inflammatory pain also results in microglial activation, which potentiates the BBB damage. In fact, astrocytes and microglia play a critical role in maintaining the BBB integrity and the activation of those cells is considered a key mechanism underlying chronic pain. Despite the recent advances in the understanding of BBB function in pain development as well as its interference in the efficacy of analgesic drugs, there remain unknowns regarding the molecular mechanisms involved in this process. In this review, we explore the connection between the BBB as well as the blood-spinal cord barrier and blood-nerve barrier, and pain, focusing on cellular and molecular mechanisms of BBB permeabilization induced by inflammatory or neuropathic pain and migraine.

Neuroinflammatory contributions to pain after SCI: roles for central glial mechanisms and nociceptor-mediated host defense

Neuropathic pain after spinal cord injury (SCI) is common, often intractable, and can be severely debilitating. A number of mechanisms have been proposed for this pain, which are discussed briefly, along with methods for revealing SCI pain in animal models, such as the recently applied conditioned place preference test. During the last decade, studies of animal models have shown that both central neuroinflammation and behavioral hypersensitivity (indirect reflex measures of pain) persist chronically after SCI. Interventions that reduce neuroinflammation have been found to ameliorate pain-related behavior, such as treatment with agents that inhibit the activation states of microglia and/or astroglia (including IL-10, minocycline, etanercept, propentofylline, ibudilast, licofelone, SP600125, carbenoxolone). Reversal of pain-related behavior has also been shown with disruption by an inhibitor (CR8) and/or genetic deletion of cell cycle-related proteins, deletion of a truncated receptor (trkB.T1) for brain-derived neurotrophic factor (BDNF), or reduction by antisense knockdown or an inhibitor (AMG9810) of the activity of channels (TRPV1 or Nav1.8) important for electrical activity in primary nociceptors. Nociceptor activity is known to drive central neuroinflammation in peripheral injury models, and nociceptors appear to be an integral component of host defense. Thus, emerging results suggest that spinal and systemic effects of SCI can activate nociceptor-mediated host defense responses that interact via neuroinflammatory signaling with complex central consequences of SCI to drive chronic pain. This broader view of SCI-induced neuroinflammation suggests new targets, and additional complications, for efforts to develop effective treatments for neuropathic SCI pain.

Micronized/ultramicronized palmitoylethanolamide displays superior oral efficacy compared to nonmicronized palmitoylethanolamide in a rat model of inflammatory pain

BACKGROUND

The fatty acid amide palmitoylethanolamide (PEA) has been studied extensively for its anti-inflammatory and neuroprotective actions. The lipidic nature and large particle size of PEA in the native state may limit its solubility and bioavailability when given orally, however. Micronized formulations of a drug enhance its rate of dissolution and reduce variability of absorption when orally administered. The present study was thus designed to evaluate the oral anti-inflammatory efficacy of micronized/ultramicronized versus nonmicronized PEA formulations.

METHODS

Micronized/ultramicronized PEA was produced by the air-jet milling technique, and the various PEA preparations were subjected to physicochemical characterization to determine particle size distribution and purity. Each PEA formulation was then assessed for its anti-inflammatory effects when given orally in the carrageenan-induced rat paw model of inflammation, a well-established paradigm of edema formation and thermal hyperalgesia.

RESULTS

Intraplantar injection of carrageenan into the right hind paw led to a marked accumulation of infiltrating inflammatory cells and increased myeloperoxidase activity. Both parameters were significantly decreased by orally given micronized PEA (PEA-m; 10 mg/kg) or ultramicronized PEA (PEA-um; 10 mg/kg), but not nonmicronized PeaPure (10 mg/kg). Further, carrageenan-induced paw edema and thermal hyperalgesia were markedly and significantly reduced by oral treatment with micronized PEA-m and ultramicronized PEA-um at each time point compared to nonmicronized PeaPure. However, when given by the intraperitoneal route, all PEA formulations proved effective.

CONCLUSIONS

These findings illustrate the superior anti-inflammatory action exerted by orally administered, micronized PEA-m and ultramicronized PEA-um, versus that of nonmicronized PeaPure, in the rat paw carrageenan model of inflammatory pain.

Mast cells, glia and neuroinflammation: partners in crime?

Glia and microglia in particular elaborate pro-inflammatory molecules that play key roles in central nervous system (CNS) disorders from neuropathic pain and epilepsy to neurodegenerative diseases. Microglia respond also to pro-inflammatory signals released from other non-neuronal cells, mainly those of immune origin such as mast cells. The latter are found in most tissues, are CNS resident, and traverse the blood-spinal cord and blood-brain barriers when barrier compromise results from CNS pathology. Growing evidence of mast cell-glia communication opens new perspectives for the development of therapies targeting neuroinflammation by differentially modulating activation of non-neuronal cells that normally control neuronal sensitization - both peripherally and centrally. Mast cells and glia possess endogenous homeostatic mechanisms/molecules that can be up-regulated as a result of tissue damage or stimulation of inflammatory responses. Such molecules include the N-acylethanolamine family. One such member, N-palmitoylethanolamine is proposed to have a key role in maintenance of cellular homeostasis in the face of external stressors provoking, for example, inflammation. N-Palmitoylethanolamine has proven efficacious in mast-cell-mediated experimental models of acute and neurogenic inflammation. This review will provide an overview of recent progress relating to the pathobiology of neuroinflammation, the role of microglia, neuroimmune interactions involving mast cells and the possibility that mast cell-microglia cross-talk contributes to the exacerbation of acute symptoms of chronic neurodegenerative disease and accelerates disease progression, as well as promoting pain transmission pathways. We will conclude by considering the therapeutic potential of treating systemic inflammation or blockade of signalling pathways from the periphery to the brain in such settings.

Psychological stress as a modulator of functional recovery following spinal cord injury

There is strong evidence indicating that the social environment triggers changes to the psychological stress response and glucocorticoid receptor function. Considerable literature links the subsequent changes in stress resiliency to physical health. Here, converging evidence for the modulatory role of chronic psychological stress in the recovery process following spinal cord injury (SCI) is presented. Despite the considerable advances in SCI research, we are still unable to identify the causes of variability in patients' recovery following injury. We propose that individuals' past and present life experiences (in the form of stress exposure) may significantly modulate patients' outcome post-SCI. We propose a theoretical model to explain the negative impact of chronic psychological stress on physical and psychological recovery. The stress experienced in life prior to SCI and also as a result of the traumatic injury, could compromise glucocorticoid receptor sensitivity and function, and contribute to high levels of inflammation and apoptosis post-SCI, decreasing the tissue remaining at the injury site and undermining recovery of function. Both stress-induced glucocorticoid resistance and stress-induced epigenetic changes to the glucocorticoid receptor can modulate the nuclear factor-kappa B regulated inflammatory pathways and the Bcl-2 regulated apoptosis pathways. This model not only contributes to the theoretical understanding of the recovery process following injury, but also provides concrete testable hypotheses for future studies.

ARA 290, a peptide derived from the tertiary structure of erythropoietin, produces long-term relief of neuropathic pain coupled with suppression of the spinal microglia response

Background

Neuropathic pain is a difficult to treat disorder arising from central or peripheral nervous system lesions. The etiology of neuropathic pain consists of several overlapping pathways converging into an exaggerated pain state with symptoms such as allodynia and hyperalgesia. One of these pathways involves activation of spinal cord microglia and astrocytes, which drive and maintain the inflammatory response following the lesion. These cells are a potential target for drugs for neuropathic pain relief. In this current study, we investigated the dose-effect relationship of the tissue protective peptide ARA 290, derived from the tertiary structure of erythropoietin, on allodynia and concurrent spinal cord microglia and astrocytes.

Results

Following a spared nerve injury in rats, vehicle or ARA290 (administered in either one of 4 doses: 3, 10, 30 and 60 μg/kg) was administered on days 1, 3, 6, 8 and 10. ARA290 exerted a dose–response effect by significantly reducing mechanical allodynia up to 20 weeks when compared to vehicle. The reduction of cold allodynia was significant up to 20 weeks for the doses 3, 10, 30 and 60 μg/kg when compared to vehicle. The effect 10 and 30 μg/kg ARA290 and vehicle on the microglia response (iba-1-immunoreactivity, iba-1-IR) and astrocyte reaction (GFAP-immunoreactivity, GFAP-IR) was investigated in animals surviving 2 (group 1) or 20 (group 2) weeks following lesion or sham surgery. In group 1, significant microglia reactivity was observed in the L5 segment of the spinal cord of animals treated with vehicle when compared to sham operated, while animals treated with 10 or 30 μg/kg did not show a increase. In group 2, a more widespread and increased microglia reactivity was observed for animals treated with 0 and 10 μg/kg when compared to sham operated animals, indicated by involvement of more spinal cord segments and higher iba-1-IR. Animals treated with 30 μg/kg did not show increased microglia reactivity. No difference in astrocyte reaction was observed.

Conclusions

The erythropoietin-analogue ARA290 dose-dependently reduced allodynia coupled to suppression of the spinal microglia response, suggestive of a mechanistic link between ARA290-induced suppression of central inflammation and relief of neuropathic pain symptoms.

Progesterone reduces the expression of spinal cyclooxygenase-2 and inducible nitric oxide synthase and prevents allodynia in a rat model of central neuropathic pain

BACKGROUND

Spinal cord injury (SCI) results in the development of chronic pain that is refractory to conventional treatment. Progesterone, a neuroprotective steroid, may offer a promising perspective in pain modulation after central injury. Here, we explore the impact of progesterone administration on the post-injury inflammatory cascade involving the enzymes cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) at the spinal cord level. We also analyse pain behaviours, the profile of glial cell activation, and IκB-α mRNA levels, as an index of NF-κB transactivation.

METHODS

We used biochemical, immunohistochemical and molecular techniques, as well as behavioural studies, to investigate the effects of progesterone in a well-characterized model of central neuropathic pain.

RESULTS

Injured animals receiving progesterone presented reduced mRNA levels of the proinflammatory enzymes, as well as decreased COX-2 activity and nitrite levels, as compared to vehicle-treated injured rats. Further, animals receiving the steroid exhibited lower levels of IκB-α mRNA, suggesting decreased NF-κB transactivation. Progesterone administration also attenuated the injury-induced increase in the number of glial fibrillary acidic protein and OX-42 positive cells both at early and late time points after injury, and prevented the development of mechanical and thermal allodynia. Further, when injured rats received early progesterone administration for a critical period of time after injury, they did not display allodynic behaviours even after the treatment had stopped.

CONCLUSIONS

Our results suggest that progesterone, by modulating early neuroinflammatory events triggered after SCI, may represent a useful strategy to prevent the development of central chronic pain.

Neurovascular unit in chronic pain

Chronic pain is a debilitating condition with major socioeconomic impact, whose neurobiological basis is still not clear. An involvement of the neurovascular unit (NVU) has been recently proposed. In particular, the blood-brain barrier (BBB) and blood-spinal cord barrier (BSCB), two NVU key players, may be affected during the development of chronic pain; in particular, transient permeabilization of the barrier is suggested by several inflammatory- and nerve-injury-based pain models, and we argue that the clarification of molecular BBB/BSCB permeabilization events will shed new light in understanding chronic pain mechanisms. Possible biases in experiments supporting this theory and its translational potentials are discussed. Moving beyond an exclusive focus on the role of the endothelium, we propose that our understanding of the mechanisms subserving chronic pain will benefit from the extension of research efforts to the NVU as a whole. In this view, the available evidence on the interaction between analgesic drugs and the NVU is here reviewed. Chronic pain comorbidities, such as neuroinflammatory and neurodegenerative diseases, are also discussed in view of NVU changes, together with innovative pharmacological solutions targeting NVU components in chronic pain treatment.