Nonneuronal central mechanisms of pain: glia and immune response

Pramipexole decreases allodynia and hyperalgesia via NF-κB in astrocytes in rats with Parkinson's disease

Pain is one of the principal non-motor symptoms of Parkinson's disease (PD), negatively impacting the patient's quality of life. This study aimed to demonstrate whether an effective dose of pramipexole (PPX) can modulate the NF-κB/p-p65 activation in glial cells (astrocytes and microglia) and diminish the hypersensitivity (allodynia and hyperalgesia) in male Wistar rats with PD. For this, 2 μl of 6-hydroxydopamine (6-OHDA, 8 μg/μL/0.2 μl/min) was administered unilaterally in the Substantia Nigra of the Pars Compacta (SNpc) to establish a PD model rat. Motor behavioral tests were used to validate the PD model, and von Frey filaments were used to evaluate allodynia and hyperalgesia. Immunohistochemical and immunofluorescence were used to analyze the level of tyrosine hydroxylase in SNpc and striatum as well as the expression of GFAP, Iba-1, NF-κB/p-65 in the L4-L6 spinal cord dorsal horn. Unilateral 6-OHDA-lesion reduces motor capacity and produces long-term allodynia and hyperalgesia in both hind paws. L4-L6 spinal cord dorsal horn astrocytes and microglia were active in these 6-OHDA-lesioned rats. Moreover, PPX (1 and 3 mg/Kg, i.p./10 days, n = 10 per group) inhibited the bilateral mechanical hypersensitivity, and PPX (3 mg/Kg/i.p./10 days) reduced 6-OHDA-induced astrocyte and microglia activation, as well as reduced NF-κB/p-p65 expression only in astrocytes of dorsal horn spinal cord in the L5-L6. These findings suggest that PPX could alleviate pain by decreasing the activation of microglia and astrocytes through the NF-κB/p-p65 pathway in the dorsal horn spinal cord. Therefore, PPX could be considered an optional tool for improving pain hypersensitivity in PD patients.

Low-dose naltrexone, an opioid-receptor antagonist, is a broad-spectrum analgesic: a retrospective cohort study

Aim: To evaluate the use of low-dose naltrexone (LDN) as a broad-spectrum analgesic. Methods: Retrospective cohort study from a single pain management practice using data from 2014 to 2020. Thirty-six patients using LDN for ≥2 months were matched to 42 controls. Pain scores were assessed at initial visit and at most recent/final documented visit using a 10-point scale. Results: Cases reported significantly greater pain reduction (-37.8%) than controls (-4.3%; p < 0.001). Whole sample multivariate modeling predicts 33% pain reduction with LDN, with number needed to treat (for 50% pain reduction) of 3.2. Patients with neuropathic pain appeared to benefit even more than those with ‘nociceptive’/inflammatory pain. Conclusion: LDN is effective in a variety of chronic pain states, likely mediated by TLR-4 antagonism.

Pharmacologic agents directed at the treatment of pain associated with maladaptive neuronal plasticity

INTRODUCTION

The definition of nociplastic pain in 2016 has changed the way maladaptive chronic pain is viewed in that it may emerge without neural lesions or neural disease. Many endogenous and pharmacologic substances are being investigated for their role in treating the pain associated with neuronal plasticity.

AREAS COVERED

The authors review promising pharmacologic agents for the treatment of pain associated with maladaptive neuronal plasticity. The authors then provide the reader with their expert opinion and provide their perspectives for the future.

EXPERT OPINION

An imbalance between the amplification of ascending pain signals and the poor activation of descending inhibitory signals may be at the root of many chronic pain syndromes. The inhibitory activity of noradrenaline reuptake may play a role in neuropathic and nociplastic analgesia. A better understanding of the brain's pain matrix, its signaling cascades, and the complex bidirectional communication between the immune system and the nervous system may help meet the urgent and unmet medical need for safe, effective chronic pain treatment, particularly for pain with a neuropathic and/or nociplastic component.

Role of innate immunity in chemotherapy-induced peripheral neuropathy

It has become increasingly clear that the innate immune system plays an essential role in the generation of many types of neuropathic pain including that which accompanies cancer treatment. In this article we review current findings of the role of the innate immune system in contributing to cancer treatment pain at the distal endings of peripheral nerve, in the nerve trunk, in the dorsal root ganglion and in the spinal dorsal horn.

SUR1, newly expressed in astrocytes, mediates neuropathic pain in a mouse model of peripheral nerve injury

BACKGROUND

Neuropathic pain following peripheral nerve injury (PNI) is linked to neuroinflammation in the spinal cord marked by astrocyte activation and upregulation of interleukin 6 (IL-6), chemokine (C-C motif) ligand 2 (CCL2) and chemokine (C-X-C motif) ligand 1 (CXCL1), with inhibition of each individually being beneficial in pain models.

METHODS

Wild type (WT) mice and mice with global or pGfap-cre- or pGFAP-cre/ERT2-driven Abcc8/SUR1 deletion or global Trpm4 deletion underwent unilateral sciatic nerve cuffing. WT mice received prophylactic (starting on post-operative day [pod]-0) or therapeutic (starting on pod-21) administration of the SUR1 antagonist, glibenclamide (10 µg IP) daily. We measured mechanical and thermal sensitivity using von Frey filaments and an automated Hargreaves method. Spinal cord tissues were evaluated for SUR1-TRPM4, IL-6, CCL2 and CXCL1.

RESULTS

Sciatic nerve cuffing in WT mice resulted in pain behaviors (mechanical allodynia, thermal hyperalgesia) and newly upregulated SUR1-TRPM4 in dorsal horn astrocytes. Global and pGfap-cre-driven Abcc8 deletion and global Trpm4 deletion prevented development of pain behaviors. In mice with Abcc8 deletion regulated by pGFAP-cre/ERT2, after pain behaviors were established, delayed silencing of Abcc8 by tamoxifen resulted in gradual improvement over the next 14 days. After PNI, leakage of the blood-spinal barrier allowed entry of glibenclamide into the affected dorsal horn. Daily repeated administration of glibenclamide, both prophylactically and after allodynia was established, prevented or reduced allodynia. The salutary effects of glibenclamide on pain behaviors correlated with reduced expression of IL-6, CCL2 and CXCL1 by dorsal horn astrocytes.

CONCLUSION

SUR1-TRPM4 may represent a novel non-addicting target for neuropathic pain.

Annexin-1 Mimetic Peptide Ac2-26 Suppresses Inflammatory Mediators in LPS-Induced Astrocytes and Ameliorates Pain Hypersensitivity in a Rat Model of Inflammatory Pain

Ac2-26, a mimetic peptide of Annexin-A1, plays a vital role in the anti-inflammatory response mediated by astrocytes. In this study, we aimed to explore the underlying mechanisms of Ac2-26-mediated anti-inflammatory effect. Specifically, we investigated the inhibitory effects of Ac2-26 on lipopolysaccharide (LPS)-induced astrocyte migration and on pro-inflammatory cytokines and chemokines expressions, as well as one glutathione (GSH) reductase mRNA and total intracellular GSH levels in LPS-induced astrocytes. Additionally, we investigated whether mitogen-activated protein kinases (MAPK) and nuclear factor kappa-B (NF-κB) signaling pathway were involved in this process. Finally, we evaluated the analgesic effect of Ac2-26 in complete Freund's adjuvant (CFA)-induced inflammatory pain model. Our results demonstrated that Ac2-26 inhibited LPS-induced astrocytes migration, reduced the production of pro-inflammatory mediators [tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-1 (MIP-1α)] and upregulated GSH reductase mRNA and GSH levels in LPS-induced astrocytes in vitro. This process was mediated through the p38, JNK-MAPK signaling pathway, but not dependent on the NF-κB pathway. Furthermore, the p38 and JNK inhibitors mimicked the effects of Ac2-26, whereas a p38 and JNK activator anisomycin partially reversed its function. Finally, Ac2-26 treatment reduced CFA-induced activation of astrocytes and production of inflammatory mediators in the spinal cord. These results suggest that Ac2-26 attenuates pain by inhibiting astrocyte activation and the production of inflammatory mediators; thus, this work presents Ac2-26 as a potential drug to treat neuropathic pain.

T Cell Subpopulations in the Physiopathology of Fibromyalgia: Evidence and Perspectives

Fibromyalgia is one of the most important “rheumatic” disorders, after osteoarthritis. The etiology of the disease is still not clear. At the moment, the most defined pathological mechanism is the alteration of central pain pathways, and emotional conditions can trigger or worsen symptoms. Increasing evidence supports the role of mast cells in maintaining pain conditions such as musculoskeletal pain and central sensitization. Importantly, mast cells can mediate microglia activation through the production of proinflammatory cytokines such as IL-1β, IL-6, and TNFα. In addition, levels of chemokines and proinflammatory cytokines are enhanced in serum and could contribute to inflammation at systemic level. Despite the well-characterized relationship between the nervous system and inflammation, the mechanism that links the different pathological features of fibromyalgia, including stress-related manifestations, central sensitization, and dysregulation of the innate and adaptive immune responses is largely unknown. This review aims to provide an overview of the current understanding of the role of adaptive immune cells, in particular T cells, in the physiopathology of fibromyalgia. It also aims at linking the latest advances emerging from basic science to envisage new perspectives to explain the role of T cells in interconnecting the psychological, neurological, and inflammatory symptoms of fibromyalgia.

Intrathecal injection of bone marrow stromal cells attenuates neuropathic pain via inhibition of P2X4R in spinal cord microglia

Background

Neuropathic pain is one of the most debilitating of all chronic pain syndromes. Intrathecal (i.t.) bone marrow stromal cell (BMSC) injections have a favorable safety profile; however, results have been inconsistent, and complete understanding of how BMSCs affect neuropathic pain remains elusive.

Methods

We evaluated the analgesic effect of BMSCs on neuropathic pain in a chronic compression of the dorsal root ganglion (CCD) model. We analyzed the effect of BMSCs on microglia reactivity and expression of purinergic receptor P2X₄ (P2X₄R). Furthermore, we assessed the effect of BMSCs on the expression of transient receptor potential vanilloid 4 (TRPV4), a key molecule in the pathogenesis of neuropathic pain, in dorsal root ganglion (DRG) neurons.

Results

I.t. BMSC transiently but significantly ameliorated neuropathic pain behavior (37.6% reduction for 2 days). We found no evidence of BMSC infiltration into the spinal cord parenchyma or DRGs, and we also demonstrated that intrathecal injection of BMSC-lysates provides similar relief. These findings suggest that the analgesic effects of i.t. BMSC were largely due to the release of BMSC-derived factors into the intrathecal space. Mechanistically, we found that while i.t. BMSCs did not change TRPV4 expression in DRG neurons, there was a significant reduction of P2X₄R expression in the spinal cord microglia. BMSC-lysate also reduced P2X₄R expression in activated microglia in vitro. Coadministration of additional pharmacological interventions targeting P2X₄R confirmed that modulation of P2X₄R might be a key mechanism for the analgesic effects of i.t. BMSC.

Conclusion

Altogether, our results suggest that i.t. BMSC is an effective and safe treatment of neuropathic pain and provides novel evidence that BMSC’s analgesic effects are largely mediated by the release of BMSC-derived factors resulting in microglial P2X₄R downregulation.

Systemic Injection of Thalidomide Prevent and Attenuate Neuropathic Pain and Alleviate Neuroinflammatory Response in the Spinal Dorsal Horn

BACKGROUND AND OBJECTIVE

Thalidomide (Tha) has been shown to exert immunomodulatory and anti-inflammatory properties. Whether Tha can alleviate spinal nerve ligation (SNL)-induced neuropathic pain (NP) is still unclear. This study aimed to investigate the therapeutic effect of Tha on the SNL-induced NP and further explore the potential analgesic mechanisms of Tha.

METHODS

The effects of Tha on SNL-induced mechanical allodynia were assessed by pain behavioral testing. The expressions of the astrocyte marker glial fibrillary acidic protein (GFAP) and the microglia marker Iba1 in the spinal dorsal horn were evaluated by immunofluorescence histochemistry. Protein expressions of the tumor necrosis factor alpha (TNF-α) in the spinal dorsal horn were tested by Western blot assay. Data were analyzed using one-way ANOVA or two-way ANOVA.

RESULTS

By the pretreatment with a single intraperitoneal injection, the PWMT in SNL+Tha group was significantly increased from day 1 to day 2 after SNL (P < 0.05 compared with SNL+Veh group). By the posttreatment with a single intraperitoneal injection, the PWMT in SNL+Tha group was also significantly increased from day 3 to day 4 after SNL (P < 0.05 compared with SNL+Veh group). By the posttreatment with multiple intraperitoneal injection, both the PWMT and the PWTL in SNL+Tha group were similarly significantly increased from day 3 to day 14 after SNL (P < 0.05 compared with SNL+Veh group). Furthermore, the GFAP and Iba1 expressions and TNF-α levels of the ipsilateral spinal dorsal horn in SNL+Tha group were significantly weaker from day 3 to day 14 after SNL than those in SNL+Veh group (P < 0.05).

CONCLUSION

Tha can significantly alleviate NP induced by SNL. The analgesic mechanism may be related to inhibition of astrocyte and microglia activation as well as down-regulation of TNF-α levels in the spinal dorsal horn.

Human dorsal root ganglion pulsed radiofrequency treatment modulates cerebrospinal fluid lymphocytes and neuroinflammatory markers in chronic radicular pain

Radicular pain is a common cause of disability. Traditionally treatment has been either epidural steroid injection providing short-term relief or surgery with associated complications. Pulsed radiofrequency (PRF) applied to the dorsal root ganglion (DRG) is a minimally invasive day-care treatment, which is gaining significant clinical acceptance in a selective group of patients with pure radicular pain. Greater insights into the immunomodulatory effects of this procedure may help to further optimise its application and find alternative treatment options. We have examined it's effect on lymphocyte frequencies and secreted inflammatory markers in the cerebrospinal fluid (CSF) and correlated this with clinical outcome to identify clinical markers of chronic radicular pain. Ten patients were recruited for the study. CSF lymphocyte frequencies and levels of cytokines, chemokines and growth factors were quantified using flow cytometry and enzyme-linked immunosorbent assay (ELISA), respectively. Clinical assessment utilised Brief Pain Inventory scores. Nine out of ten patients (90%) demonstrated significant reduction in pain severity (p = 0.0007) and pain interference scores (p = 0.0015) three months post-treatment. Our data revealed significant reductions in CD56⁺, CD3^-, NK cell frequencies (p = 0.03) and IFN-γ levels (p = 0.03) in treatment responders, while CD8⁺ T cell frequencies (p = 0.02) and IL-6 levels were increased (p = 0.05). IL-17 inversely correlated with post-treatment pain severity score (p = 0.01) and pre and post-treatment pain interference scores (p = 0.03, p = 0.01). These results support the concept that chronic radicular pain is a centrally mediated neuroimmune phenomenon and the mechanism of action of DRG PRF treatment is immunomodulatory.

Increased Central Nervous System Interleukin-8 in a Majority Postlaminectomy Syndrome Chronic Pain Population

Background and Objectives

Multiple processes have been identified as potential contributors to chronic pain, with increasing evidence illustrating an association with aberrant levels of neuroimmune mediators. The primary objectives of the present study were to examine central nervous system cytokines, chemokines, and growth factors present in a chronic pain population and to explore patterns of the same mediator molecules over time. Secondary objectives explored the relationship of central and peripheral neuroimmune mediators while examining the levels of anxiety, depression, sleep quality, and perception of pain associated with the chronic pain patient experience.

Methods

Cerebrospinal fluid (CSF) from a population of majority postlaminectomy syndrome patients (N = 8) was compared with control CSF samples (N = 30) to assess for significant differences in 10 cytokines, chemokines, and growth factors. The patient population was then followed over time, analyzing CSF, plasma, and psychobehavioral measures.

Results

The present observational study is the first to demonstrate increased mean CSF levels of interleukin-8 (IL-8; P < 0.001) in a small population of majority postlaminectomy syndrome patients, as compared with a control population. Over time in pain patients, CSF levels of IL-8 increased significantly (P < 0.001).

Conclusions

These data indicate that IL-8 should be further investigated and psychobehavioral components considered in the overall chronic pain paradigm. Future studies examining the interactions between these factors and IL-8 may identify novel targets for treatment of persistent pain states.

Activated Glia Increased the Level of Proinflammatory Cytokines in a Resiniferatoxin-Induced Neuropathic Pain Rat Model

BACKGROUND AND OBJECTIVES

Administration of resiniferatoxin (RTX) can mimic the clinical symptoms of postherpetic neuralgia. However, it is unclear whether activated glia contribute to the pathogenesis of RTX-induced neuropathic pain; furthermore, the relationship between p38, N-methyl-D-aspartate receptor type 2B (NR2B) as well as proinflammatory cytokines and activated glia remains unknown.

METHODS

Intraperitoneal injection of RTX was performed to induce neuropathic pain in rats. Mechanical allodynia and thermal hyperalgesia were assessed by von Frey filaments and a radiant heat stimulus, respectively. Western blot and immunofluorescence labeling examined the expression of NR2B, activated glia markers, p38, and proinflammatory cytokines in the spinal cord. We further investigated the effect of the glial inhibitors, fluorocitrate and minocycline, on nociceptive behaviors and expression of p38, NR2B, and proinflammatory cytokines.

RESULTS

Resiniferatoxin leads to an increase of paw withdrawal latency to a heat stimulus and caused a mechanical allodynia within 2 weeks. The expression of tumor necrosis factor α, IL-1β, p38, and NR2B was up-regulated in RTX-induced neuropathic pain rat model and lasted for at least 49 days. Microglia were activated at the early phase of the disease, whereas activated astrocytes were detected in the sustainment phase. Both minocycline and fluorocitrate attenuated the nociceptive behaviors and expression of related proteins.

CONCLUSIONS

Activated glia participate in the pathogenesis of RTX-induced neuropathic pain and are likely to be the source of proinflammatory cytokines. Inhibition of glia contributes to an analgesic effect. These findings provide a novel strategy for the treatment of postherpetic neuralgia.

An Inflammation-Centric View of Neurological Disease: Beyond the Neuron

Inflammation is a complex biological response fundamental to how the body deals with injury and infection to eliminate the initial cause of cell injury and effect repair. Unlike a normally beneficial acute inflammatory response, chronic inflammation can lead to tissue damage and ultimately its destruction, and often results from an inappropriate immune response. Inflammation in the nervous system (“neuroinflammation”), especially when prolonged, can be particularly injurious. While inflammation per se may not cause disease, it contributes importantly to disease pathogenesis across both the peripheral (neuropathic pain, fibromyalgia) and central [e.g., Alzheimer disease, Parkinson disease, multiple sclerosis, motor neuron disease, ischemia and traumatic brain injury, depression, and autism spectrum disorder] nervous systems. The existence of extensive lines of communication between the nervous system and immune system represents a fundamental principle underlying neuroinflammation. Immune cell-derived inflammatory molecules are critical for regulation of host responses to inflammation. Although these mediators can originate from various non-neuronal cells, important sources in the above neuropathologies appear to be microglia and mast cells, together with astrocytes and possibly also oligodendrocytes. Understanding neuroinflammation also requires an appreciation that non-neuronal cell—cell interactions, between both glia and mast cells and glia themselves, are an integral part of the inflammation process. Within this context the mast cell occupies a key niche in orchestrating the inflammatory process, from initiation to prolongation. This review will describe the current state of knowledge concerning the biology of neuroinflammation, emphasizing mast cell-glia and glia-glia interactions, then conclude with a consideration of how a cell's endogenous mechanisms might be leveraged to provide a therapeutic strategy to target neuroinflammation.

Effects of metformin on the expression of AMPK and STAT3 in the spinal dorsal horn of rats with neuropathic pain

Neuropathic pain (NP) is a frustrating and burdensome problem. Current treatments for NP have unendurable side effects and/or questionable efficacy, and once these therapies are stopped, the symptoms often return. Thus, novel drugs are needed to enhance the effectiveness of treatments for NP. One novel target for pain treatments is adenosine monophosphate-activated protein kinase (AMPK), which regulates a variety of cellular processes, including protein translation, which is considered to be affected in NP. Metformin is a widely available drug that possesses the ability to activate AMPK. The signal transducer and activator of transcription 3 (STAT3) pathway plays an important role in neuroinflammation. The present study investigated the analgesic effect of metformin on NP induced by chronic constriction injury (CCI), and the influence of metformin on the expression of AMPK and STAT3 in the spinal dorsal horn (SDH). In CCI rats, paw withdrawal latencies in response to thermal hyperalgesia were significantly shorter, while phosphorylated (p)-AMPK was expressed at lower levels and p-STAT3 was expressed at higher levels in the SDH. Administering intraperitoneal injections of metformin (200 mg/kg) for 6 successive days activated AMPK and suppressed the expression of p-STAT3, in addition to reversing hyperalgesia. Finally, metformin inhibited the activation of microglia and astrocytes in the SDH, which may explain how it alleviates NP.

Neuronal-Glial Interactions Maintain Chronic Neuropathic Pain after Spinal Cord Injury

The hyperactive state of sensory neurons in the spinal cord enhances pain transmission. Spinal glial cells have also been implicated in enhanced excitability of spinal dorsal horn neurons, resulting in pain amplification and distortions. Traumatic injuries of the neural system such as spinal cord injury (SCI) induce neuronal hyperactivity and glial activation, causing maladaptive synaptic plasticity in the spinal cord. Recent studies demonstrate that SCI causes persistent glial activation with concomitant neuronal hyperactivity, thus providing the substrate for central neuropathic pain. Hyperactive sensory neurons and activated glial cells increase intracellular and extracellular glutamate, neuropeptides, adenosine triphosphates, proinflammatory cytokines, and reactive oxygen species concentrations, all of which enhance pain transmission. In addition, hyperactive sensory neurons and glial cells overexpress receptors and ion channels that maintain this enhanced pain transmission. Therefore, post-SCI neuronal-glial interactions create maladaptive synaptic circuits and activate intracellular signaling events that permanently contribute to enhanced neuropathic pain. In this review, we describe how hyperactivity of sensory neurons contributes to the maintenance of chronic neuropathic pain via neuronal-glial interactions following SCI.

Identification of the Spinal Expression Profile of Non-coding RNAs Involved in Neuropathic Pain Following Spared Nerve Injury by Sequence Analysis

Neuropathic pain (NP) is caused by damage to the nervous system, resulting in aberrant pain, which is associated with gene expression changes in the sensory pathway. However, the molecular mechanisms are not fully understood. A non-coding Ribose Nucleic Acid (ncRNA) is an RNA molecule that is not translated into a protein. NcRNAs are involved in many cellular processes, and mutations or imbalances of the repertoire within the body can cause a variety of diseases. Although ncRNAs have recently been shown to play a role in NP pathogenesis, the specific effects of ncRNAs in NP remain largely unknown. In this study, sequencing analysis was performed to investigated the expression patterns of ncRNAs in the spinal cord following spared nerve injury-induced NP. A total of 134 long non-coding RNAs (lncRNAs), 12 microRNAs (miRNAs), 188 circular RNAs (circRNAs) and 1066 mRNAs were significantly regulated at 14 days after spared nerve injury (SNI) surgery. Next, quantitative real-time polymerase chain reaction (PCR) was performed to validate the expression of selected lncRNAs, miRNAs, circRNAs, and mRNAs. Bioinformatics tools and databases were employed to explore the potential ncRNA functions and relationships. Our data showed that the most significantly involved pathways in SNI pathogenesis were ribosome, PI3K-Akt signaling pathway, focal adhesion, ECM-receptor interaction, amoebiasis and protein digestion and absorption. In addition, the lncRNA-miRNA-mRNA and circRNA-miRNA-mRNA network of NP was constructed. This is the first study to comprehensively identify regulated ncRNAs of the spinal cord and to demonstrate the involvement of different ncRNA expression patterns in the spinal cord of NP pathogenesis by sequence analysis. This information will enable further research on the pathogenesis of NP and facilitate the development of novel NP therapeutics targeting ncRNAs.

Recent advances in understanding neuropathic pain: glia, sex differences, and epigenetics

Neuropathic pain results from diseases or trauma affecting the nervous system. This pain can be devastating and is poorly controlled. The pathophysiology is complex, and it is essential to understand the underlying mechanisms in order to identify the relevant targets for therapeutic intervention. In this article, we focus on the recent research investigating neuro-immune communication and epigenetic processes, which gain particular attention in the context of neuropathic pain. Specifically, we analyze the role of glial cells, including microglia, astrocytes, and oligodendrocytes, in the modulation of the central nervous system inflammation triggered by neuropathy. Considering epigenetics, we address DNA methylation, histone modifications, and the non-coding RNAs in the regulation of ion channels, G-protein-coupled receptors, and transmitters following neuronal damage. The goal was not only to highlight the emerging concepts but also to discuss controversies, methodological complications, and intriguing opinions.

The analgesic effect of rolipram is associated with the inhibition of the activation of the spinal astrocytic JNK/CCL2 pathway in bone cancer pain

Bone cancer pain (BCP) is one of the most difficult and intractable tasks for pain management, which is associated with spinal 'neuron-astrocytic' activation. The activation of the c-Jun N-terminal kinase (JNK)/chemokine (C-C motif) ligand (CCL2) signaling pathway has been reported to be critical for neuropathic pain. Rolipram (ROL), a selective phosphodiesterase 4 inhibitor, possesses potent anti-inflammatory and anti-nociceptive activities. The present study aimed to investigate whether the intrathecal administration of ROL has an analgesic effect on BCP in rats, and to assess whether the inhibition of spinal JNK/CCL2 pathway and astrocytic activation are involved in the analgesic effects of ROL. The analgesic effects of ROL were evaluated using the Von Frey and Hargreaves tests. Immunofluorescence staining was used to determine the number of c-Fos immunoreactive neurons, and the expression of spinal astrocytes and microglial activation on day 14 after tumor cell inoculation. Enzyme-linked immunosorbent assay (ELISA) was used to detect the expression of pro-inflammatory cytokines [interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α] and chemokines (CCL2), and western blot analysis was then used to examine the spinal phosphodiesterase 4 (PDE4), ionized calcium binding adapter molecule-1 (IBA-1) and JNK levels on day 14 after tumor cell inoculation. The results revealed that ROL exerted a short-term analgesic effect in a dose-dependent manner, and consecutive daily injections of ROL exerted continuous analgesic effects. In addition, spinal 'neuron-astrocytic' activation was suppressed and was associated with the downregulation of spinal IL-1β, IL-6 and TNF-α expression, and the inhibition of PDE4B and JNK levels in the spine was also observed. In addition, the level of CCL2 was decreased in the rats with BCP. The JNK inhibitor, SP600125, decreased CCL2 expression and attenuated pain behavior. Following co-treatment with ROL and SP600125, no significant increases in thermal hyperalgesia and CCL2 expression were observed compared with the ROL group. Thus, our findings suggest that the analgesic effects of ROL in BCP are mainly mediated through the inhibition of 'neuron-astrocytic' activation, which occurs via the suppression of spinal JNK/CCL2 signaling.

Effect of pioglitazone on neuropathic pain and spinal expression of TLR-4 and cytokines

The molecular mechanisms underlying neuropathic pain have yet to be elucidated. The present study aimed to examine the modulation of neuroimmune activation in the spinal cord by the synthetic peroxisome proliferator-activated receptor gamma (PPAR-γ) agonist, pioglitazone (Pio), in a rat model of neuropathic pain induced by chronic constriction injury (CCI). Rats were randomly assigned into four groups: Sham surgery with vehicle, chronic constriction injury with vehicle or Pio (10 mg/kg), and chronic constriction injury with Pio and a PPAR-γ antagonist GW9662 (2 mg/kg). Pio or vehicle was administered 1 h prior to the surgery and continued daily until day 7 post-surgery. Paw pressure threshold was measured prior to surgery and on days 0, 1, 3 and 7 post-surgery. Microglia activation markers macrophage antigen complex-1, the mRNA expression levels of tumor necrosis factor α and interleukin-1β, and the mRNA expression levels of toll like receptor (TLR-4) in the lumbar spinal cord were determined. Administration of Pio resulted in the prominent attenuation of mechanical hyperalgesia. In addition, Pio was able to significantly inhibit neuroimmune activation characterized by glial activation, the production of cytokines and expression levels of TLR-4. Concurrent administration of a PPAR-γ antagonist, GW9662, reversed the effects of Pio. The antihyperalgesic effect of administration of Pio in rats receiving CCI may, in part, be attributed to the inhibition of neuroimmune activation associated with the sustaining of neuropathic pain.

Pain in rheumatoid arthritis: models and mechanisms

Pain is one of the most challenging symptoms for patients with rheumatoid arthritis (RA). RA-related pain is frequently considered to be solely a consequence of inflammation in the joints; however, recent studies show that multiple mechanisms are involved. Indeed, RA pain may start even before the disease manifests, and frequently does not correlate with the degree of inflammation or pharmacological management. In this aspect, animal studies have the potential to provide new insights into the pathology that initiate and maintain pain in RA. The focus of this review is to describe the most commonly used animal models for studies of RA pathology, which have also been utilized in pain research, and to summarize findings providing potential clues to the mechanisms involved in the regulation of RA-induced pain.

Chronic Pain Management: An Overview of Taxonomy, Conditions Commonly Encountered, and Assessment

Chronic pain has multiple mechanisms that result in pain amplification and maintenance, including central and peripheral sensitization and altered modulation of pain perception. Assessment of pain requires comprehensive assessment of symptoms and signs, suspected pain mechanisms, and the patient's biopsychosocial context. Multiple validated measures exist for the assessment of pain symptoms, pain-related disability, psychological impact of pain, and candidacy for opioid management.

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.

Palmitoylethanolamide in Fibromyalgia: Results from Prospective and Retrospective Observational Studies

Introduction

Fibromyalgia syndrome (FM) is characterized by persistent pain which is often refractory to common analgesic therapies and is particularly disabling. The objective of this study was to evaluate the therapeutic efficacy of duloxetine (DLX) + pregabalin (PGB) in patients suffering from FM and the possible added benefit of the lipid signaling molecule, palmitoylethanolamide (PEA). PEA is well-documented to exert anti-inflammatory, analgesic, and pain-relieving effects at both the preclinical and clinical level.

Methods

A total of 80 patients were recruited in two steps. The first was a retrospective observational study comprising 45 patients. This patient group received DLX + PGB for 6 months. The second step was a prospective observational study with 35 patients. Patients in this cohort began treatment with DLX + PGB at the same dosage as for the retrospective study plus micronized PEA (PEA-m^®; Epitech Group, Italy) and ultramicronized PEA (PEA-um^®; Epitech Group, Italy) for 3 months. Positive tender points (TPs), pain evoked, and pain intensity were evaluated at baseline and after 3 and 6 months in both studies. Statistical analyses were employed for comparison of data within the two studies and between them.

Results

The retrospective observational study (DLX + PGB), after 3 months of treatment showed a decrease of positive TPs, pain evoked, and pain intensity. After 6 months of treatment, these parameters had further improvement. In the prospective observational study (DLX + PGB + PEA), PEA introduction after 3 months of therapeutic regimen with DLX + PGB provided a significant improvement in pain symptoms, with a further reduction in the number of TPs and significant reduction in pain, compared to combined DLX + PGB only (p < 0.0001 for TPs and Visual Analog Scale comparisons). None of the patients experienced adverse side effects.

Conclusion

Our study confirms the efficacy of DLX + PGB and demonstrates as well the added benefit and safety of PEA in the treatment of pain in patients affected by FM.

Electronic supplementary material

The online version of this article (doi:10.1007/s40122-015-0038-6) contains supplementary material, which is available to authorized users.