Central Nervous System Targets: Glial Cell Mechanisms in Chronic Pain

Research progress in neuropathic pain after spinal cord injury: a bibliometric study from 2013 to 2024

Background

The incidence of neuropathic pain (NP) after spinal cord injury (SCI) is quite high. This pain is clinically challenging to treat and has an debilitating effect on patients. In recent years, NP is a popular topic of research and a number of relevant articles have been published in academic journals. The purpose of this article is to analyze the global research trend of NP after SCI using bibliometric methods.

Methods

The literature was screened from 2013 to 2024 based on the Web of Science core collection (WOSCC). These publications, including annual publications, journals, authors, references, and keywords via CiteSpace, were analyzed in order to help understand the current research direction and hotspots in this field.

Results

A total of 2022 publications were included in the analysis. The results showed that an overall upward trend in the number of publications in the study period. The top five productive journals are Spinal Cord, Journal of Neurotrauma, Pain, Experimental Neurology, and Journal of Spinal Cord Medicine, the journals related to spinal cord or pain. The top five most productive scholars are Armin Curt, Michael G. Fehlings, Wu Junfang, John L. K. Kramer, and Farinaz Nasirinezhad. Keyword bursts showed that signaling pathway, neuroinflammation, neuralgia, spinal cord stimulation, inhibition, and depression have become new research hotspots in the field of NP after SCI.

Conclusion

This study provides a basis for the study of pain after SCI. It summarizes past research on NP following SCI and offers valuable reference data for further exploration of research trends and issues of focus in this field.

The Role of Mechanosensitive Piezo Channels in Chronic Pain

Purpose of Review

Mechanosensitive Piezo channels are ion channels activated by mechanical stimuli, playing a crucial role in mechanotransduction processes and mechanical hypersensitivity. When these channels are subjected to mechanical loading, membrane currents rise instantaneously, depolarizing and activating voltage-gated calcium channels. This results in an increase in intracellular Ca2+, which contributes to heightened sensitivity to mechanical stimuli. This review delves into the characteristics and mechanisms of Piezo channels in chronic pain.

Recent Findings

The findings suggest that Piezo channels are integral to the occurrence and development of chronic pain, including neuropathic pain, visceral pain, musculoskeletal pain, headache or orofacial pain, and inflammatory pain. Piezo channels significantly impact pain perception and transmission. These channels' critical involvement in various pain types highlights their potential as promising targets for chronic pain therapy.

Summary

This review discusses the role of Piezo channels in chronic pain. By understanding these pain mechanisms, new therapeutic strategies can be developed to alleviate chronic pain, offering hope for patients suffering from these debilitating conditions.

Long-term optical imaging of the spinal cord in awake behaving mice

Advances in optical imaging and fluorescent biosensors enable study of the spatiotemporal and long-term neural dynamics in the brain of awake animals. However, methodological difficulties and fibrosis limit similar advances in the spinal cord. Here, to overcome these obstacles, we combined in vivo application of fluoropolymer membranes that inhibit fibrosis, a redesigned implantable spinal imaging chamber and improved motion correction methods that together permit imaging of the spinal cord in awake behaving mice, for months to over a year. We demonstrated a robust ability to monitor axons, identified a spinal cord somatotopic map, performed months-long imaging in freely moving mice, conducted Ca2+ imaging of neural dynamics in behaving mice responding to pain-provoking stimuli and observed persistent microglial changes after nerve injury. The ability to couple in vivo imaging and behavior at the spinal cord level will drive insights not previously possible at a key location for somatosensory transmission to the brain.

Is chronic pain caused by central sensitization? A review and critical point of view

Chronic pain causes disability and loss of health worldwide. Yet, a mechanistic explanation for it is still missing. Frequently, neural phenomena, and among them, Central Sensitization (CS), is presented as causing chronic pain. This narrative review explores the evidence substantiating the relationship between CS and chronic pain: four expert researchers were divided in two independent teams that reviewed the available evidence. Three criteria were established for a study to demonstrate a causal relationship: (1) confirm presence of CS, (2) study chronic pain, and (3) test sufficiency or necessity of CS over chronic pain symptoms. No study met those criteria, failing to demonstrate that CS can cause chronic pain. Also, no evidence reporting the occurrence of CS in humans was found. Worryingly, pain assessments are often confounded with CS measures in the literature, omitting that the latter is a neurophysiological and not a perceptual phenomenon. Future research should avoid this misconception to directly interrogate what is the causal contribution of CS to chronic pain to better comprehend this problematic condition.

Deciphering pain: molecular mechanisms and neurochemical pathways-challenges and future opportunities

This review delves into the intricate biological underpinnings of pain perception. It encompasses nociceptive signaling pathways, the molecular mechanisms involved, and the subjective experience of discomfort in humans. The initial focus is on nociceptor transduction, where specialized neurons transform noxious stimuli into electrical impulses. Subsequently, the review explores the central nervous system, elucidating how these signals are processed and modulated by critical elements such as ion channels, receptors, and neurotransmitters (e.g., substance P, glutamate, GABA). Shifting gears toward chronic pain, the review examines the concept of neuroplasticity, highlighting its potential to induce maladaptive responses through alterations in neural networks. The burgeoning field of pain genomics, alongside established genetic research, offers valuable insights that could pave the way for a framework of personalized pain management strategies. Finally, the review emphasizes the significance of these molecular insights in facilitating accurate therapeutic interventions. The overarching objective is to establish an integrative framework for precision medicine in pain management by incorporating this information alongside biopsychosocial models. This framework serves to translate the heterogeneous landscape of pain mechanisms into a coherent roadmap for the development of effective therapies.

Navigating the nano-bio immune interface: advancements and challenges in CNS nanotherapeutics

In recent years, significant advancements have been made in utilizing nanoparticles (NPs) to modulate immune responses within the central nervous system (CNS), offering new opportunities for nanotherapeutic interventions in neurological disorders. NPs can serve as carriers for immunomodulatory agents or platforms for delivering nucleic acid-based therapeutics to regulate gene expression and modulate immune responses. Several studies have demonstrated the efficacy of NP-mediated immune modulation in preclinical models of neurological diseases, including multiple sclerosis, stroke, Alzheimer's disease, and Parkinson's disease. While challenges remain, advancements in NPs engineering and design have led to the development of NPs using diverse strategies to overcome these challenges. The nano-bio interface with the immune system is key in the conceptualization of NPs to efficiently act as nanotherapeutics in the CNS. The biomolecular corona plays a pivotal role in dictating NPs behavior and immune recognition within the CNS, giving researchers the opportunity to optimize NPs design and surface modifications to minimize immunogenicity and enhance biocompatibility. Here, we review how NPs interact with the CNS immune system, focusing on immunosurveillance of NPs, NP-induced immune reprogramming and the impact of the biomolecular corona on NPs behavior in CNS immune responses. The integration of NPs into CNS nanotherapeutics offers promising opportunities for addressing the complex challenges of acute and chronic neurological conditions and pathologies, also in the context of preventive and rehabilitative medicine. By harnessing the nano-bio immune interface and understanding the significance of the biomolecular corona, researchers can develop targeted, safe, and effective nanotherapeutic interventions for a wide range of CNS disorders to improve treatment and rehabilitation. These advancements have the potential to revolutionize the treatment landscape of neurological diseases, offering promising solutions for improved patient care and quality of life in the future.

Progress in treatment of pathological neuropathic pain after spinal cord injury

Pathological neuropathic pain is a common complication following spinal cord injury. Due to its high incidence, prolonged duration, tenacity, and limited therapeutic efficacy, it has garnered increasing attention from both basic researchers and clinicians. The pathogenesis of neuropathic pain after spinal cord injury is multifaceted, involving factors such as structural and functional alterations of the central nervous system, pain signal transduction, and inflammatory effects, posing significant challenges to clinical management. Currently, drugs commonly employed in treating spinal cord injury induced neuropathic pain include analgesics, anticonvulsants, antidepressants, and antiepileptics. However, a subset of patients often experiences suboptimal therapeutic responses or severe adverse reactions. Therefore, emerging treatments are emphasizing a combination of pharmacological and non-pharmacological approaches to enhance neuropathic pain management. We provide a comprehensive review of past literature, which aims to aim both the mechanisms and clinical interventions for pathological neuropathic pain following spinal cord injury, offering novel insights for basic science research and clinical practice in spinal cord injury treatment.

Mechanisms and Therapeutic Prospects of Microglia-Astrocyte Interactions in Neuropathic Pain Following Spinal Cord Injury

Neuropathic pain is a prevalent and debilitating condition experienced by the majority of individuals with spinal cord injury (SCI). The complex pathophysiology of neuropathic pain, involving continuous activation of microglia and astrocytes, reactive gliosis, and altered neuronal plasticity, poses significant challenges for effective treatment. This review focuses on the pivotal roles of microglia and astrocytes, the two major glial cell types in the central nervous system, in the development and maintenance of neuropathic pain after SCI. We highlight the extensive bidirectional interactions between these cells, mediated by the release of inflammatory mediators, neurotransmitters, and neurotrophic factors, which contribute to the amplification of pain signaling. Understanding the microglia-astrocyte crosstalk and its impact on neuronal function is crucial for developing novel therapeutic strategies targeting neuropathic pain. In addition, this review discusses the fundamental biology, post-injury pain roles, and therapeutic prospects of microglia and astrocytes in neuropathic pain after SCI and elucidates the specific signaling pathways involved. We also speculated that the extracellular matrix (ECM) can affect the glial cells as well. Furthermore, we also mentioned potential targeted therapies, challenges, and progress in clinical trials, as well as new biomarkers and therapeutic targets. Finally, other relevant cell interactions in neuropathic pain and the role of glial cells in other neuropathic pain conditions have been discussed. This review serves as a comprehensive resource for further investigations into the microglia-astrocyte interaction and the detailed mechanisms of neuropathic pain after SCI, with the aim of improving therapeutic efficacy.

Impact of Chondrocyte Inflammation on Glial Cell Activation: The Mediating Role of Nitric Oxide

OBJECTIVE

This study investigates how the inflammatory response of ATDC5 murine chondrogenic cells influences the activity of C6 (rat) and GL261 (mouse) glial cell lines. Prior research suggested nitric oxide (NO) involvement in cartilage-immune crosstalk. The current study explores whether NO, produced by inflamed chondrocytes, mediates signaling between chondrocytes and glial cells.

DESIGN

Pre-challenged ATDC5 cells with 250 ng/ml of lipopolysaccharide (LPS) were cocultured with GL261 or C6 glioma cells for 24 h with a transwell culture system. Cell viability was assessed using MTT assay. Gene and protein expression were evaluated by qRT-PCR and WB, respectively.

RESULTS

Real-time reverse transcription-polymerase chain reaction (RT-qPCR) indicated statistically significant upregulation of LCN2, IL-6, TNF-α, IL-1β, and GFAP in glial cells following 24-h coculture with challenged ATDC5 cells. Suppression of LPS-induced NO production by aminoguanidine decreased LPS-mediated LCN2 and IL-6 expression in glioma cells. We identified also the involvement of the ERK1/2 and AKT signaling pathways in the glial neuroinflammatory response.

CONCLUSIONS

This study demonstrates, for the first time, that NO produced by inflamed murine chondrocytes mediated pro-inflammatory responses in glial cells via ERK1/2 and AKT signaling, highlighting a potential mechanism linking cartilage NO to neuroinflammation and chronic pain in osteoarthritis.

Discordance between preclinical and clinical testing of NaV1.7-selective inhibitors for pain

ABSTRACT

The voltage-gated sodium channel NaV1.7 plays an important role in pain processing according to genetic data. Those data made NaV1.7 a popular drug target, especially since its relatively selective expression in nociceptors promised pain relief without the adverse effects associated with broader sodium channel blockade. Despite encouraging preclinical data in rodents, NaV1.7-selective inhibitors have not yet proven effective in clinical trials. Discrepancies between preclinical and clinical results should raise alarms. We reviewed preclinical and clinical reports on the analgesic efficacy of NaV1.7-selective inhibitors and found critical differences in several factors. Putting aside species differences, most preclinical studies tested young male rodents with limited genetic variability, inconsistent with the clinical population. Inflammatory pain was the most common preclinical chronic pain model whereas nearly all clinical trials focused on neuropathic pain despite some evidence suggesting NaV1.7 channels are not essential for neuropathic pain. Preclinical studies almost exclusively measured evoked pain whereas most clinical trials assessed average pain intensity without distinguishing between evoked and spontaneous pain. Nearly all preclinical studies gave a single dose of drug unlike the repeat dosing used clinically, thus precluding preclinical data from demonstrating whether tolerance or other slow processes occur. In summary, preclinical testing of NaV1.7-selective inhibitors aligned poorly with clinical testing. Beyond issues that have already garnered widespread attention in the pain literature, our results highlight the treatment regimen and choice of pain model as areas for improvement.

Interplay between cannabinoids and the neuroimmune system in migraine

Migraine is a common and complex neurological disorder that has a high impact on quality of life. Recent advances with drugs that target the neuropeptide calcitonin gene-related peptide (CGRP) have helped, but treatment options remain insufficient. CGRP is released from trigeminal sensory fibers and contributes to peripheral sensitization, perhaps in part due to actions on immune cells in the trigeminovascular system. In this review, we will discuss the potential of cannabinoid targeting of immune cells as an innovative therapeutic target for migraine treatment. We will cover endogenous endocannabinoids, plant-derived phytocannabinoids and synthetically derived cannabinoids. The focus will be on six types of immune cells known to express multiple cannabinoid receptors: macrophages, monocytes, mast cells, dendritic cells, B cells, and T cells. These cells also contain receptors for CGRP and as such, cannabinoids might potentially modulate the efficacy of current CGRP-targeting drugs. Unfortunately, to date most studies on cannabinoids and immune cells have relied on cell cultures and only a single preclinical study has tested cannabinoid actions on immune cells in a migraine model. Encouragingly, in that study a synthetically created stable chiral analog of an endocannabinoid reduced meningeal mast cell degranulation. Likewise, clinical trials evaluating the safety and efficacy of cannabinoid-based therapies for migraine patients have been limited but are encouraging. Thus, the field is at its infancy and there are significant gaps in our understanding of the impact of cannabinoids on immune cells in migraine. Future research exploring the interactions between cannabinoids and immune cells could lead to more targeted and effective migraine treatments.

Glial-modulating agents for the treatment of pain: a systematic review

ABSTRACT

Preclinical research supports a critical role for nervous system glia in pain pathophysiology. This systematic review of human trials of potential glia-modulating drugs for the prevention or treatment of pain followed a predefined search strategy and protocol registration. We searched for English language, randomized, double-blind trials comparing putative glia-modulating drugs to placebo or other comparators. The primary outcomes included validated participant-reported measures of pain intensity or relief and, in studies of opioid administration, measures of opioid consumption and/or opioid-related adverse effects. Twenty-six trials (2132 participants) of glial modulators (12 minocycline, 11 pentoxifylline, and 3 ibudilast) were included. Because of clinical heterogeneity related to study drug, participant population, outcome measures, and trial design, no meta-analysis was possible. Only 6 trials reported a positive effect of the treatment (pentoxifylline-4 trials; minocycline-2 trials), whereas 11 trials reported mixed results and 9 trials reported no effect. This review does not provide convincing evidence of efficacy of current pharmacological targets of nervous system glial function for pain treatment or prevention. However, in light of ample preclinical evidence of the importance of neuroimmune signalling and glial functions in pain pathophysiology, continued strategic human research is anticipated to identify (1) drugs with maximal activity as selectively targeted glial modulators, (2) the necessary timing and duration of pharmacological glial modulation needed for pain prevention or treatment for specific injuries or pain conditions, and (3) the best design of future clinical trials of glial-targeted drugs for pain treatment and/or prevention.

Changes in Cx43 and AQP4 Proteins, and the Capture of 3 kDa Dextran in Subpial Astrocytes of the Rat Medial Prefrontal Cortex after Both Sham Surgery and Sciatic Nerve Injury

A subpopulation of astrocytes on the brain's surface, known as subpial astrocytes, constitutes the "glia limitans superficialis" (GLS), which is an interface between the brain parenchyma and the cerebrospinal fluid (CSF) in the subpial space. Changes in connexin-43 (Cx43) and aquaporin-4 (AQP4) proteins in subpial astrocytes were examined in the medial prefrontal cortex at postoperative day 1, 3, 7, 14, and 21 after sham operation and sciatic nerve compression (SNC). In addition, we tested the altered uptake of TRITC-conjugated 3 kDa dextran by reactive subpial astrocytes. Cellular immunofluorescence (IF) detection and image analysis were used to examine changes in Cx43 and AQP4 protein levels, as well as TRITC-conjugated 3 kDa dextran, in subpial astrocytes. The intensity of Cx43-IF was significantly increased, but AQP4-IF decreased in subpial astrocytes of sham- and SNC-operated rats during all survival periods compared to naïve controls. Similarly, the uptake of 3 kDa dextran in the GLS was reduced following both sham and SNC operations. The results suggest that both sciatic nerve injury and peripheral tissue injury alone can induce changes in subpial astrocytes related to the spread of their reactivity across the cortical surface mediated by increased amounts of gap junctions. At the same time, water transport and solute uptake were impaired in subpial astrocytes.

γ1 GABAA Receptors in Spinal Nociceptive Circuits

GABAergic neurons and GABAA receptors (GABAARs) are critical elements of almost all neuronal circuits. Most GABAARs of the CNS are heteropentameric ion channels composed of two α, two β, and one γ subunits. These receptors serve as important drug targets for benzodiazepine (BDZ) site agonists, which potentiate the action of GABA at GABAARs. Most GABAAR classifications rely on the heterogeneity of the α subunit (α1-α6) included in the receptor complex. Heterogeneity of the γ subunits (γ1-γ3), which mediate synaptic clustering of GABAARs and contribute, together with α subunits, to the benzodiazepine (BDZ) binding site, has gained less attention, mainly because γ2 subunits greatly outnumber the other γ subunits in most brain regions. Here, we have investigated a potential role of non-γ2 GABAARs in neural circuits of the spinal dorsal horn, a key site of nociceptive processing. Female and male mice were studied. We demonstrate that besides γ2 subunits, γ1 subunits are significantly expressed in the spinal dorsal horn, especially in its superficial layers. Unlike global γ2 subunit deletion, which is lethal, spinal cord-specific loss of γ2 subunits was well tolerated. GABAAR clustering in the superficial dorsal horn remained largely unaffected and antihyperalgesic actions of HZ-166, a nonsedative BDZ site agonist, were partially retained. Our results thus suggest that the superficial dorsal horn harbors functionally relevant amounts of γ1 subunits that support the synaptic clustering of GABAARs in this site. They further suggest that γ1 containing GABAARs contribute to the spinal control of nociceptive information flow.

The role and treatment potential of the complement pathway in chronic pain

The role of the complement system in pain syndromes has garnered attention on the back of preclinical and clinical evidence supporting its potential as a target for new analgesic pharmacotherapies. Of the components that make up the complement system, component 5a (C5a) and component 3a (C3a) are most strongly and consistently associated with pain. Receptors for C5a are widely found in immune resident cells (microglia, astrocytes, sensory neuron-associated macrophages (sNAMs)) in the central nervous system (CNS) as well as hematogenous immune cells (mast cells, macrophages, T-lymphocytes, etc.). When active, as is often observed in chronic pain conditions, these cells produce various inflammatory mediators including pro-inflammatory cytokines. These events can trigger nervous tissue inflammation (neuroinflammation) which coexists with and potentially maintains peripheral and central sensitization. C5a has a likely critical role in initiating this process highlighting its potential as a promising non-opioid target for treating pain. This review summarizes the most up-to-date research on the role of the complement system in pain with emphasis on the C5 pathway in peripheral tissue, dorsal root ganglia (DRG) and the CNS, and explores advances in complement-targeted drug development and sex differences. A perspective on the optimal application of different C5a inhibitors for different types (e.g., neuropathic, post-surgical and chemotherapy-induced pain, osteoarthritis pain) and stages (e.g., acute, subacute, chronic) of pain is also provided to help guide future clinical trials. PERSPECTIVE: This review highlights the role and mechanisms of complement components and their receptors in physiological and pathological pain. The potential of complement-targeted therapeutics for the treatment of chronic pain is also explored with a focus on C5a inhibitors to help guide future clinical trials.

Regulatory role of electroacupuncture on satellite glial cell activity in the colon and dorsal root ganglion of rats with irritable bowel syndrome

OBJECTIVE

To investigate the role of satellite glial cells in irritable bowel syndrome (IBS) and the effect of electroacupuncture (EA) at the Tianshu (ST25) and Shangjuxu (ST37) combination.

METHODS

A model for visceral hypersensitivity in IBS was induced through colorectal distension (CRD) stimulation. Clean-grade male Sprague-Dawley (SD) rats were randomly divided into four groups: a normal group (NG), a model group (MG), an electroacupuncture group (EA), and a glial cell inhibitor group (FCA). Bilateral EA (2/100 Hz, 1 mA, 30 min) was administered at the Tianshu (ST25) and Shangjuxu (ST37) in week 6. Abdominal withdrawal reflex (AWR) scores were used to assess the behavioral response associated with visceral hyperalgesia, while hematoxylin-eosin staining was employed to evaluate pathological changes in the colon. The protein and mRNA levels of glial fibrillary acidic protein (GFAP) in the colon and colon-related dorsal root ganglion (DRG) were analyzed using immun-ofluorescence, immun-ohistochemistry, Western blotting, real-time polymerase chain reaction. The impact of EA on electrophysiological properties of colon-related DRG neurons was observed through whole-cell patch clamp analysis.

RESULTS

EA significantly reduced the visceral pain behavior scores in rats with IBS in response to graded (20, 40, 60, 80 mm Hg) CRD stimulation. Additionally, EA downregulated the protein and mRNA expression levels of GFAP in the colon and colon-related DRG of rats with IBS. EA also regulated the resting membrane potential, rheobase and action potential of colon-related DRG neurons in rats with IBS.

CONCLUSIONS

EA can regulate the excitatory properties of colon-related DRG neurons by downregulating the protein and mRNA expression of GFAP in the colon and colon-related DRG, indicating a potential neurobiological mechanism by which EA relieves visceral hypersensitivity in rats with IBS.

Recent developments and challenges in positron emission tomography imaging of gliosis in chronic neuropathic pain

ABSTRACT

Understanding the mechanisms that underpin the transition from acute to chronic pain is critical for the development of more effective and targeted treatments. There is growing interest in the contribution of glial cells to this process, with cross-sectional preclinical studies demonstrating specific changes in these cell types capturing targeted timepoints from the acute phase and the chronic phase. In vivo longitudinal assessment of the development and evolution of these changes in experimental animals and humans has presented a significant challenge. Recent technological advances in preclinical and clinical positron emission tomography, including the development of specific radiotracers for gliosis, offer great promise for the field. These advances now permit tracking of glial changes over time and provide the ability to relate these changes to pain-relevant symptomology, comorbid psychiatric conditions, and treatment outcomes at both a group and an individual level. In this article, we summarize evidence for gliosis in the transition from acute to chronic pain and provide an overview of the specific radiotracers available to measure this process, highlighting their potential, particularly when combined with ex vivo / in vitro techniques, to understand the pathophysiology of chronic neuropathic pain. These complementary investigations can be used to bridge the existing gap in the field concerning the contribution of gliosis to neuropathic pain and identify potential targets for interventions.

Causal links between blood inflammation markers and postherpetic neuralgia risk: insights from a two-sample Mendelian randomization study

Introduction

Previous studies have suggested an association between blood inflammation-related factors and postherpetic neuralgia. However, the causal relationship between blood inflammation-related factors and postherpetic neuralgia remains unclear.

Methods

We employed a bidirectional Two-sample Mendelian randomization (MR) analysis to explore the causal relationship between blood inflammation-related factors and postherpetic neuralgia. The instrumental variables were obtained from a large Genome-wide association study (GWAS) meta-analysis dataset of European descent. The instrumental variables of the blood inflammation-related factors come from the database numbers GCST004420 to GCST004460 and GCST90029070. Postherpetic neuralgia has 195,191 samples with a total of 16,380,406 single nucleotide polymorphisms (SNPs). MR analyses were performed using inverse-variance weighted, MR-Egger, and weighted median methods.

Results

The MR results revealed a significant causal effect of Macrophage Inflammatory Protein 1 Beta (MIP1β) on reducing the risk of postherpetic neuralgia (95%CI = 0.492-0.991, p = 0.044). Additionally, higher levels of interleukin (IL)-10 (95%CI = 0.973-0.998, p = 0.019) and IL-12p70 (95%CI = 0.973-0.997, p = 0.013) were associated with a lower risk of postherpetic neuralgia. Other inflammatory markers showed no significant causal relationship with this condition.

Conclusion

This study identifies MIP1β, IL-10, and IL-12p70 as potential therapeutic targets for preventing or treating postherpetic neuralgia, underscoring the need for further research in this area.

Cells and circuits for amygdala neuroplasticity in the transition to chronic pain

Maladaptive plasticity is linked to the chronification of diseases such as pain, but the transition from acute to chronic pain is not well understood mechanistically. Neuroplasticity in the central nucleus of the amygdala (CeA) has emerged as a mechanism for sensory and emotional-affective aspects of injury-induced pain, although evidence comes from studies conducted almost exclusively in acute pain conditions and agnostic to cell type specificity. Here, we report time-dependent changes in genetically distinct and projection-specific CeA neurons in neuropathic pain. Hyperexcitability of CRF projection neurons and synaptic plasticity of parabrachial (PB) input at the acute stage shifted to hyperexcitability without synaptic plasticity in non-CRF neurons at the chronic phase. Accordingly, chemogenetic inhibition of the PB→CeA pathway mitigated pain-related behaviors in acute, but not chronic, neuropathic pain. Cell-type-specific temporal changes in neuroplasticity provide neurobiological evidence for the clinical observation that chronic pain is not simply the prolonged persistence of acute pain.

Role of Exercise on Neuropathic Pain in Preclinical Models: Perspectives for Neuroglia

The benefits of exercise on neuropathic pain (NP) have been demonstrated in numerous studies. In recent studies, inflammation, neurotrophins, neurotransmitters, and endogenous opioids are considered as the main mechanisms. However, the role of exercise in alleviating NP remains unclear. Neuroglia, widely distributed in both the central and peripheral nervous systems, perform functions such as support, repair, immune response, and maintenance of normal neuronal activity. A large number of studies have shown that neuroglia play an important role in the occurrence and development of NP, and exercise can alleviate NP by regulating neuroglia. This article reviewed the involvement of neuroglia in the development of NP and their role in the exercise treatment of NP, intending to provide a theoretical basis for the exercise treatment strategy of NP.

Analgesic Effect of Human Placenta Hydrolysate on CFA-Induced Inflammatory Pain in Mice

To evaluate the efficacy of human placenta hydrolysate (HPH) in a mice model of CFA-induced inflammatory pain. TNF-α, IL-1β, and IL-6 are key pro-inflammatory cytokine factors for relieving inflammatory pain. Therefore, this study investigates whether HPH suppresses CFA-induced pain and attenuates the inflammatory process by regulating cytokines. In addition, the relationship between neuropathic pain and HPH was established by staining GFAP and Iba-1 in mice spinal cord tissues. This study was conducted for a total of day 28, and inflammatory pain was induced in mice by injecting CFA into the right paw at day 0 and day 14, respectively. 100 μL of 20% glucose and polydeoxyribonucleotide (PDRN) and 100, 200, and 300 μL of HPH were administered intraperitoneally twice a week. In the CFA-induced group, cold and mechanical allodynia and pro-inflammatory cytokine factors in the spinal cord and plantar tissue were significantly increased. The five groups of drugs evenly reduced pain and gene expression of inflammatory factors, and particularly excellent effects were confirmed in the HPH 200 and 300 groups. Meanwhile, the expression of GFAP and Iba-1 in the spinal cord was increased by CFA administration but decreased by HPH administration, which was confirmed to suppress damage to peripheral ganglia. The present study suggests that HPH attenuates CFA-induced inflammatory pain through inhibition of pro-inflammatory cytokine factors and protection of peripheral nerves.

Structural reorganization of medullary dorsal horn astrocytes in a rat model of neuropathic pain

Multiple studies have shown that astrocytes in the medullary dorsal horn (MDH) play an important role in the development of pathologic pain. However, little is known about the structural reorganization of the peripheral astrocytic processes (PAP), the main functional part of the astrocyte, in MDH in neuropathic state. For this, we investigated the structural relationship between PAP and their adjacent presynaptic axon terminals and postsynaptic dendrites in the superficial laminae of the MDH using electron microscopical immunohistochemistry for ezrin, a marker for PAP, and quantitative analysis in a rat model of neuropathic pain following chronic constriction injury of the infraorbital nerve (CCI-ION). We found that, compared to controls, in rats with CCI-ION, (1) the number, % area, surface density, and volume fraction of ezrin-positive (+) PAP, as well as the fraction of synaptic edge apposed by ezrin + PAP and the degree of its coverage of presynaptic axon terminals and postsynaptic dendrites increased significantly, (2) these effects were abolished by administration of the mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine (MPEP). These findings indicate that PAP undergoes structural reorganization around the central synapses of sensory afferents following nerve injury, suggest that it may be mediated by mGluR5, and may represent the structural basis for enhancing astrocyte-neuron interaction in neuropathic pain.

Glial activation in pain and emotional processing regions in the nitroglycerin mouse model of chronic migraine

OBJECTIVE

Our aim was to survey astrocyte and microglial activation across four brain regions in a mouse model of chronic migraine.

BACKGROUND

Chronic migraine is a leading cause of disability, with higher rates in females. The role of central nervous system neurons and glia in migraine pathophysiology is not fully elucidated. Preclinical studies have shown abnormal glial activation in the trigeminal nucleus caudalis of male rodents. No current reports have investigated glial activation in both sexes in other important brain regions involved with the nociceptive and emotional processing of pain.

METHODS

The mouse nitroglycerin model of migraine was used, and nitroglycerin (10 mg/kg) or vehicle was administered every other day for 9 days. Prior to injections on days 1, 5, and 9, cephalic allodynia was determined by periorbital von Frey hair testing. Immunofluorescent staining of astrocyte marker, glial fibrillary protein (GFAP), and microglial marker, ionized calcium binding adaptor molecule 1 (Iba1), in male and female trigeminal nucleus caudalis, periaqueductal gray, somatosensory cortex, and nucleus accumbens was completed.

RESULTS

Behavioral testing demonstrated increased cephalic allodynia in nitroglycerin- versus vehicle-treated mice. An increase in the percent area covered by GFAP+ cells in the trigeminal nucleus caudalis and nucleus accumbens, but not the periaqueductal gray or somatosensory cortex, was observed in response to nitroglycerin. No significant differences were observed for Iba1 staining across brain regions. We did not detect significant sex differences in GFAP or Iba1 quantification.

CONCLUSIONS

Immunohistochemical analysis suggests that, at the time point tested, immunoreactivity of GFAP+ astrocytes, but not Iba1+ microglia, changes in response to chronic migraine-associated pain. Additionally, there do not appear to be significant differences between males and females in GFAP+ or Iba1+ cells across the four brain regions analyzed.

Molecular targets in bone cancer pain: a systematic review of inflammatory cytokines

Bone cancer pain (BCP) profoundly impacts patient's quality of life, demanding more effective pain management strategies. The aim of this systematic review was to investigate the role of inflammatory cytokines as potential molecular targets in BCP. A systematic search for animal rodent models of bone cancer pain studies was conducted in PubMed, Scopus, and Web of Science. Methodological quality and risk of bias were assessed using the SYRCLE RoB tool. Twenty-five articles met the inclusion criteria, comprising animal studies investigating molecular targets related to inflammatory cytokines in BCP. A low to moderate risk of bias was reported. Key findings in 23 manuscripts revealed upregulated classic pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, IL-17, IL-18, IL-33) and chemokines in the spinal cord, periaqueductal gray, and dorsal root ganglia. Interventions targeting these cytokines consistently mitigated pain behaviors. Additionally, it was demonstrated that glial cells, due to their involvement in the release of inflammatory cytokines, emerged as significant contributors to BCP. This systematic review underscores the significance of inflammatory cytokines as potential molecular targets for alleviating BCP. It emphasizes the promise of targeted interventions and advocates for further research to translate these findings into effective therapeutic strategies. Ultimately, this approach holds the potential to enhance the patient's quality of life.

rTMS applied to the PFC relieves neuropathic pain and modulates neuroinflammation in CCI rats

The study aimed to assess the analgesic effect of 10 Hz repetitive transcranial magnetic stimulation (rTMS) targeted to the prefrontal cortex (PFC) region on neuropathic pain (NPP) in rats with chronic constriction injury (CCI) of the sciatic nerve, and to investigate the possible underlying mechanism. Rats were randomly divided into three groups: sham operation, CCI, and rTMS. In the latter group, rTMS was applied to the left PFC. Von Frey fibres were used to measure the paw withdrawal mechanical threshold (PWMT). At the end of the treatment, immunofluorescence and western blotting were applied to detect the expression of M1 and M2 polarisation markers in microglia in the left PFC and sciatic nerve. ELISA was further used to detect the concentrations of inflammation-related cytokines. The results showed that CCI caused NPP in rats, reduced the pain threshold, promoted microglial polarisation to the M1 phenotype, and increased the secretion of pro-inflammatory and anti-inflammatory factors. Moreover, 10 Hz rTMS to the PFC was shown to improve NPP induced by CCI, induce microglial polarisation to M2, reduce the secretion of pro-inflammatory factors, and further increase the secretion of anti-inflammatory factors. Our data suggest that 10 Hz rTMS can alleviate CCI-induced neuropathic pain, while the underlying mechanism may potentially be related to the regulation of microglial M1-to-M2-type polarisation to regulate neuroinflammation.

Vagus nerve stimulation rescues persistent pain following orthopedic surgery in adult mice

ABSTRACT

Postoperative pain is a major clinical problem imposing a significant burden on patients and society. In a survey 2 years after orthopedic surgery, 57% of patients reported persisting postoperative pain. However, only limited progress has been made in the development of safe and effective therapies to prevent the onset and chronification of pain after orthopedic surgery. We established a tibial fracture mouse model that recapitulates clinically relevant orthopedic trauma surgery, which causes changes in neuropeptide levels in dorsal root ganglia and sustained neuroinflammation in the spinal cord. Here, we monitored extended pain behavior in this model, observing chronic bilateral hindpaw mechanical allodynia in both male and female C57BL/6J mice that persisted for >3 months after surgery. We also tested the analgesic effects of a novel, minimally invasive, bioelectronic approach to percutaneously stimulate the vagus nerve (termed percutaneous vagus nerve stimulation [pVNS]). Weekly pVNS treatment for 30 minutes at 10 Hz for 3 weeks after the surgery strongly reduced pain behaviors compared with untreated controls. Percutaneous vagus nerve stimulation also improved locomotor coordination and accelerated bone healing. In the dorsal root ganglia, vagal stimulation inhibited the activation of glial fibrillary acidic protein-positive satellite cells but without affecting microglial activation. Overall, these data provide novel evidence supportive of the use of pVNS to prevent postoperative pain and inform translational studies to test antinociceptive effects of bioelectronic medicine in the clinic.

Anti-inflammatory and analgesic properties of Polyphyllin VI revealed by network pharmacology and RNA sequencing

Inflammatory pain, sustained by a complex network of inflammatory mediators, is a severe and persistent illness affecting many of the general population. We explore possible anti-inflammatory pathways of Polyphyllin VI (PPVI) based on our prior study, which showed that PPVI reduces inflammation in mice to reduce pain. Network pharmacology and RNA-Seq identified the contribution of the MAPK signaling pathway to inflammatory pain. In the LPS/ATP-induced RAW264.7 cell model, pretreatment with PPVI for 1 h inhibited the release of IL-6 and IL-8, down-regulated expression of the P2X7 receptor(P2X7R), and decreased phosphorylation of p38 and ERK1/2 components of the MAPK pathway. Moreover, PPVI decreased expression of IL-6 and IL-8 was observed in the serum of the inflammatory pain mice model and reduced phosphorylation of p38 and ERK1/2 in the dorsal root ganglia while the reductions of expression of IL-6 and phosphorylation of ERK1/2 were not observed after the pre-treatment with A740003 (an antagonist of the P2X7R). These results suggest that PPVI may inhibit the release of IL-8 by regulating P2X7R to reduce the phosphorylation of p38. However, the modulation of PPVI on the release of IL-6 and phosphorylation of ERK1/2 may mediated by other P2X7R-independent signals.

Peripheral and central neurobiological effects of botulinum toxin A (BoNT/A) in neuropathic pain: a systematic review

ABSTRACT

Botulinum toxin (BoNT), a presynaptic inhibitor of acetylcholine (Ach) release at the neuromuscular junction (NMJ), is a successful and safe drug for the treatment of several neurological disorders. However, a wide and recent literature review has demonstrated that BoNT exerts its effects not only at the "periphery" but also within the central nervous system (CNS). Studies from animal models, in fact, have shown a retrograde transport to the CNS, thus modulating synaptic function. The increasing number of articles reporting efficacy of BoNT on chronic neuropathic pain (CNP), a complex disease of the CNS, demonstrates that the central mechanisms of BoNT are far from being completely elucidated. In this new light, BoNT might interfere with the activity of spinal, brain stem, and cortical circuitry, modulating excitability and the functional organization of CNS in healthy conditions. Botulinum toxins efficacy on CNP is the result of a wide and complex action on many and diverse mechanisms at the basis of the maladaptive plasticity, the core of the pathogenesis of CNP. This systematic review aims to discuss in detail the BoNT's mechanisms and effects on peripheral and central neuroplasticity, at the basis for the clinical efficacy in CNP syndromes.

Flavonoids as Potential Therapeutics Against Neurodegenerative Disorders: Unlocking the Prospects

Neurodegeneration, the decline of nerve cells in the brain, is a common feature of neurodegenerative disorders (NDDs). Oxidative stress, a key factor in NDDs such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease can lead to neuronal cell death, mitochondria impairment, excitotoxicity, and Ca2+ stress. Environmental factors compromising stress response lead to cell damage, necessitating novel therapeutics for preventing or treating brain disorders in older individuals and an aging population. Synthetic medications offer symptomatic benefits but can have adverse effects. This research explores the potential of flavonoids derived from plants in treating NDDs. Flavonoids compounds, have been studied for their potential to enter the brain and treat NDDs. These compounds have diverse biological effects and are currently being explored for their potential in the treatment of central nervous system disorders. Flavonoids have various beneficial effects, including antiviral, anti-allergic, antiplatelet, anti-inflammatory, anti-tumor, anti-apoptotic, and antioxidant properties. Their potential to alleviate symptoms of NDDs is significant.

Analgesic Effect of SKI306X on Chronic Postischemic Pain and Spinal Nerve Ligation-Induced Neuropathic Pain in Mice

Neuropathic pain (NP) results from lesions or diseases affecting the peripheral or central somatosensory system. However, there are currently no drugs that are particularly effective in treating this condition. SKI306X is a blend of purified extracts of three oriental herbs (Clematis mandshurica, Trichosanthes kirilowii, and Prunella vulgaris) commonly used to treat osteoarthritis for their chondroprotective effects. Chronic postischemic pain (CPIP) and spinal nerve ligation (SNL) models were created by binding the upper left ankle of mice with an O-ring for 3 h and ligating the L5 spinal nerve, respectively. Mice with allodynia were injected intraperitoneally with 0.9% normal saline (NS group) or different doses (25, 50, or 100 mg/kg) of SKI306X (SKI groups). We assessed allodynia using von Frey filaments before injection and 30, 60, 90, 120, 180, and 240 min and 24 h after injection to confirm the antiallodynic effect of SKI306X. We also measured glial fibrillary acidic protein (GFAP) levels in the spinal cord and dorsal root ganglia to confirm the change of SKI306X administration. Both models exhibited significant mechanical allodynia. The intraperitoneal injection of SKI306X significantly increased the paw withdrawal threshold in a dose-dependent manner, as the paw withdrawal threshold was significantly increased after SKI306X administration compared with at baseline or after NS administration. GFAP levels in the SKI group decreased significantly (p < 0.05). Intraperitoneal administration of SKI306X dose-dependently attenuated mechanical allodynia and decreased GFAP levels, suggesting that GFAP is involved in the antiallodynic effect of SKI306X in mice with CPIP and SNL-induced NP.

New Insight into Neuropathic Pain: The Relationship between α7nAChR, Ferroptosis, and Neuroinflammation

Neuropathic pain, which refers to pain caused by a lesion or disease of the somatosensory system, represents a wide variety of peripheral or central disorders. Treating neuropathic pain is quite demanding, primarily because of its intricate underlying etiological mechanisms. The central nervous system relies on microglia to maintain balance, as they are associated with serving primary immune responses in the brain next to cell communication. Ferroptosis, driven by phospholipid peroxidation and regulated by iron, is a vital mechanism of cell death regulation. Neuroinflammation can be triggered by ferroptosis in microglia, which contributes to the release of inflammatory cytokines. Conversely, neuroinflammation can induce iron accumulation in microglia, resulting in microglial ferroptosis. Accumulating evidence suggests that neuroinflammation, characterized by glial cell activation and the release of inflammatory substances, significantly exacerbates the development of neuropathic pain. By inhibiting microglial ferroptosis, it may be possible to prevent neuroinflammation and subsequently alleviate neuropathic pain. The activation of the homopentameric α7 subtype of the neuronal nicotinic acetylcholine receptor (α7nAChR) has the potential to suppress microglial activation, transitioning M1 microglia to an M2 phenotype, facilitating the release of anti-inflammatory factors, and ultimately reducing neuropathic pain. Recent years have witnessed a growing recognition of the regulatory role of α7nAChR in ferroptosis, which could be a potential target for treating neuropathic pain. This review summarizes the mechanisms related to α7nAChR and the progress of ferroptosis in neuropathic pain according to recent research. Such an exploration will help to elucidate the relationship between α7nAChR, ferroptosis, and neuroinflammation and provide new insights into neuropathic pain management.

Baricitinib ameliorates inflammatory and neuropathic pain in collagen antibody-induced arthritis mice by modulating the IL-6/JAK/STAT3 pathway and CSF-1 expression in dorsal root ganglion neurons

BACKGROUND

Janus kinase (JAK) inhibitors, such as baricitinib, are widely used to treat rheumatoid arthritis (RA). Clinical studies show that baricitinib is more effective at reducing pain than other similar drugs. Here, we aimed to elucidate the molecular mechanisms underlying the pain relief conferred by baricitinib, using a mouse model of arthritis.

METHODS

We treated collagen antibody-induced arthritis (CAIA) model mice with baricitinib, celecoxib, or vehicle, and evaluated the severity of arthritis, histological findings of the spinal cord, and pain-related behaviours. We also conducted RNA sequencing (RNA-seq) to identify alterations in gene expression in the dorsal root ganglion (DRG) following baricitinib treatment. Finally, we conducted in vitro experiments to investigate the direct effects of baricitinib on neuronal cells.

RESULTS

Both baricitinib and celecoxib significantly decreased CAIA and improved arthritis-dependent grip-strength deficit, while only baricitinib notably suppressed residual tactile allodynia as determined by the von Frey test. CAIA induction of inflammatory cytokines in ankle synovium, including interleukin (IL)-1β and IL-6, was suppressed by treatment with either baricitinib or celecoxib. In contrast, RNA-seq analysis of the DRG revealed that baricitinib, but not celecoxib, restored gene expression alterations induced by CAIA to the control condition. Among many pathways changed by CAIA and baricitinib treatment, the interferon-alpha/gamma, JAK-signal transducer and activator of transcription 3 (STAT3), and nuclear factor kappa B (NF-κB) pathways were considerably decreased in the baricitinib group compared with the celecoxib group. Notably, only baricitinib decreased the expression of colony-stimulating factor 1 (CSF-1), a potent cytokine that causes neuropathic pain through activation of the microglia-astrocyte axis in the spinal cord. Accordingly, baricitinib prevented increases in microglia and astrocytes caused by CAIA. Baricitinib also suppressed JAK/STAT3 pathway activity and Csf1 expression in cultured neuronal cells.

CONCLUSIONS

Our findings demonstrate the effects baricitinib has on the DRG in relation to ameliorating both inflammatory and neuropathic pain.

Sodium tanshinone IIA sulfonate suppresses microglia polarization and neuroinflammation possibly via regulating miR-125b-5p/STAT3 axis to ameliorate neuropathic pain

The spinal cord microglia play a pivotal role in neuroinflammation and neuropathic pain (NP). Sodium tanshinone IIA sulfonate (STS), a derivative of tanshinone IIA, has anti-inflammatory and anti-hyperalgesic effects. However, its underlying mechanism in NP remains unclear. This study aimed to investigate the effect of STS and elucidate possible mechanisms in a rat model of spared nerve injury. In vivo experiments, STS and AG490 were administered intraperitoneally once daily for 14 consecutive days after surgery. The results showed that the expression of miR-125b-5p in the spinal dorsal horn was substantially reduced, whereas signal transducer and activator of transcription 3 (STAT3) signaling was increased. After treatment with STS, the mechanical thresholds, expression of miR-125b-5p, and microglial M2 marker such as Arg-1 in the spinal cord horn increased significantly, whereas multiple pro-inflammatory cytokines and apoptosis were significantly reduced. Moreover, STAT3 pathway-related proteins and expression of the microglial M1 marker, CD68, were appreciably inhibited. In vitro, lipopolysaccharide (LPS) was used to induce an inflammatory response in BV-2 microglial cells. STS pretreatment inhibited LPS-stimulated pro-inflammatory cytokine secretion, reduced STAT3 pathway related-proteins and apoptosis, increased miR-125b-5p and proopiomelanocortin expression, and enhanced microglia transformation from M1 to M2 phenotype in BV-2 cells. These effects were reversed after the inhibition of miR-125b-5p expression in BV-2 cells. A dual-luciferase reporter assay confirmed that STAT3 binds to miR-125b-5p. In summary, these results suggest that STS exerts anti-hyperalgesic and anti-neuroinflammatory effects in rats with NP possibly via the miR-125b-5p/STAT3 axis.

Neuropathic pain in burn patients - A common problem with little literature: A systematic review

BACKGROUND

The prevalence of neuropathic pain (NP) in burn patients is reported in the literature to be as high as 80%1. Given the complexity of NP in burn patients and the wide range of treatments available, a systematic review of the literature is warranted to summarize our current understanding of management and treatment of NP in this population.

METHODS

This systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines. The following databases were queried to identify relevant articles: PubMed, Cochrane, Embase, Scopus, Ovid, and Web of Science. The main outcome measures were incidence and management of NP. Secondary outcomes included risk factors for NP.

RESULTS

Included articles presented findings from 11 different countries, capturing outcomes for 4366 patients. Risk factors for neuropathic pain in burn patients were identified, including older age, alcohol and substance abuse, current daily smoking, greater % total body surface area burns (TBSA), and longer hospitalizations. Pharmacologic treatments included gabapentin/pregabalin (n = 7), ascorbic acid (n = 1), and lidocaine (n = 1). Overall, the studies showed varied results regarding the efficacy of pharmacological treatments. While certain studies demonstrated gabapentanoids to be effective in reducing neuropathic symptoms, others found conflicting results. With regards to non-pharmacologic treatments, electroconvulsive therapy (n = 1), electropuncture (n = 1), nerve release/reconstruction (n = 2), and somatosensory feedback rehabilitation (n = 1) were used and demonstrated promise in reducing pain intensity and improving functionality.

CONCLUSIONS

Despite NP afflicting the majority of burn patients long after their injury, this systematic review demonstrates insufficient evidence on the pathophysiology, outcomes, and risk factors in NP, as well as the efficacy of various therapies. Future prospective and randomized studies evaluating the etiology of these factors can substantially improve our treatment strategies. This can allow for the development of well-delineated and evidence-based protocols in NP management in hopes of improving quality of life and both psychological and physical function in burn patients.

Inflammation, Autoimmunity, and Infection in Fibromyalgia: A Narrative Review

Fibromyalgia (FM) is a chronic disease characterized by widespread musculoskeletal pain of unknown etiology. The condition is commonly associated with other symptoms, including fatigue, sleep disturbances, cognitive impairment, and depression. For this reason, FM is also referred to as FM syndrome. The nature of the pain is defined as nociplastic according to the latest international classification and is characterized by altered nervous sensitization both centrally and peripherally. Psychosocial conditions have traditionally been considered critical in the genesis of FM. However, recent studies in animal models and humans have provided new evidence in favor of an inflammatory and/or autoimmune pathogenesis. In support of this hypothesis are epidemiological data of an increased female prevalence, similar to that of autoimmune diseases, and the frequent association with immune-mediated inflammatory disorders. In addition, the observation of an increased incidence of this condition during long COVID revived the hypothesis of an infectious pathogenesis. This narrative review will, therefore, discuss the evidence supporting the immune-mediated pathogenesis of FM in light of the most current data available in the literature.

Whole-Body Photobiomodulation Therapy Propels the Fibromyalgia Patient into the Recomposition Phase: A Reflexive Thematic Analysis

BACKGROUND

Recent evidence has identified great promise for the novel whole-body photobiomodulation therapy (PBMT) for individuals with fibromyalgia (FM). However, currently no evidence has documented the experiences of participants. The objective of this study was to qualitatively assess treatment experience and response in a group of participants with FM undergoing a course of whole-body PBMT.

METHODS

An interpretive hermeneutic phenomenological study situated within the worldview of pragmatism was undertaken. A convenience sample of individuals with FM were included if they had undertaken a novel 6-week trial of PBMT. Individuals undertook semi-structured interviews exploring treatment experience and multidimensional treatment responses during Week 3 and Week 6.

RESULTS

Sixteen trial participants (47.3 ± 10.9 years) took part in this study. The analysis produced three overarching themes that were previously identified from a baseline study (namely, 'Body Structure & Function', 'Activities & Participation', and 'Environment') with an additional five sub-themes that highlighted the intervention experience. Subsequently, four important processes were observed and identified: increased motivation; feeling proud; improved confidence; feeling like 'old self'. This ultimately culminated in the identification of a positive spiral, which we have termed 'recomposition'.

CONCLUSIONS

We believe our study is the first in the field of chronic pain management to utilise qualitative methodology to directly assess the acceptability and efficacy of a specific medical intervention in a clinical trial, and the first study to qualitatively assess whole-body PBMT experience. The findings are compelling and warrant further work to support the introduction of this device into the National Health Service (NHS).

Stress, pain, anxiety, and depression in endometriosis-Targeting glial activation and inflammation

Endometriosis (EM) is a gynecological inflammatory disease often accompanied by stress, chronic pelvic pain (CPP), anxiety, and depression, leading to a diminished quality of life. This review aims to discuss the relationship between systemic and local inflammatory responses in the central nervous system (CNS), focusing on glial dysfunctions (astrocytes and microglia) as in critical brain regions involved in emotion, cognition, pain processing, anxiety, and depression. The review presents that EM is connected to increased levels of pro-inflammatory cytokines in the circulation. Additionally, chronic stress and CPP as stressors may contribute to the dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, depleting the production of inflammatory mediators in the circulatory system and the brain. The systemic cytokines cause blood-brain barrier (BBB) breakdown, activate microglia in the brain, and lead to neuroinflammation. Furthermore, CPP may induce neuronal morphological alterations in critical regions through central sensitization and the activation of glial cells. The activation of glial cells, particularly the polarization of microglia, leads to the activation of the NLRP3 inflammasome and the overproduction of inflammatory cytokines. These inflammatory cytokines interact with the signaling pathways involved in neural plasticity. Additionally, persistent inflammatory conditions in the brain lead to neuronal death, which is correlated with a reduced volume of key brain regions such as the hippocampus. This review highlights the involvement of glial cells in the pathogenesis of the mental comorbidities of EM (i.e., pain, anxiety, and depression) and to discuss potential therapeutic approaches for targeting the inflammation and activation of microglia in key brain regions.

Effect of General Anesthetic Agents on Microglia

The effects of general anesthetic agents (GAAs) on microglia and their potential neurotoxicity have attracted the attention of neuroscientists. Microglia play important roles in the inflammatory process and in neuromodulation of the central nervous system. Microglia-mediated neuroinflammation is a key mechanism of neurocognitive dysfunction during the perioperative period. Microglial activation by GAAs induces anti-inflammatory and pro-inflammatory effects in microglia, suggesting that GAAs play a dual role in the mechanism of postoperative cognitive dysfunction. Understanding of the mechanisms by which GAAs regulate microglia may help to reduce the incidence of postoperative adverse effects. Here, we review the actions of GAAs on microglia and the consequent changes in microglial function. We summarize clinical and animal studies associating microglia with general anesthesia and describe how GAAs interact with neurons via microglia to further explore the mechanisms of action of GAAs in the nervous system.

Recent advances in understanding the neurobiology of pediatric functional neurological disorder

INTRODUCTION

Functional neurological disorder (FND) is a neuropsychiatric disorder that manifests in a broad array of functional motor, sensory, or cognitive symptoms, which arise from complex interactions between brain, mind, body, and context. Children with FND make up 10%-20% of presentations to neurology services in children's hospitals and up to 20% of adolescents admitted to hospital for the management of intractable seizures.

AREAS COVERED

The current review focuses on the neurobiology of pediatric FND. The authors present an overview of the small but growing body of research pertaining to the biological, emotion-processing, cognitive, mental health, physical health, and social system levels.

EXPERT OPINION

Emerging research suggests that pediatric FND is underpinned by aberrant changes within and between neuron-glial (brain) networks, with a variety of factors - on multiple system levels - contributing to brain network changes. In pediatric practice, adverse childhood experiences (ACEs) are commonly reported, and activation or dysregulation of stress-system components is a frequent finding. Our growing understanding of the neurobiology of pediatric FND has yielded important flow-on effects for assessing and diagnosing FND, for developing targeted treatment interventions, and for improving the treatment outcomes of children and adolescents with FND.

Neuropathic Pain After Burn Injury: A Severe and Common Problem in Recovery

OBJECTIVE

The aim of this study was to address the limited understanding of neuropathic pain (NP) among burn survivors by comprehensively examining its prevalence and related factors on a national scale using the Burn Model System (BMS) National Database.

BACKGROUND

NP is a common but underexplored complaint among burn survivors, greatly affecting their quality of life and functionality well beyond the initial injury. Existing data on NP and its consequences in burn survivors are limited to select single-institution studies, lacking a comprehensive national perspective.

METHODS

The BMS National Database was queried to identify burn patients responding to NP-related questions at enrollment, 6 months, 12 months, 2 years, and 5 years postinjury. Descriptive statistics and regression analyses were used to explore associations between demographic/clinical characteristics and self-reported NP at different time points.

RESULTS

There were 915 patients included for analysis. At discharge, 66.5% of patients experienced NP in their burn scars. Those with NP had significantly higher Patient-Reported Outcomes Measurement Information System 29 (PROMIS-29) pain inference, itch, anxiety, depression, and sleep disturbance scores and were less able to partake in social roles. Multiple logistic regression revealed male sex, % total body surface area, and moderate-to-severe pain as predictors of NP at 6 months. At 12 months, % total body surface area and moderate-to-severe pain remained significant predictors, while ethnicity and employment status emerged as significant predictors at 24 months.

CONCLUSIONS

This study highlights the significant prevalence of NP in burn patients and its adverse impacts on their physical, psychological, and social well-being. The findings underscore the necessity of a comprehensive approach to NP treatment, addressing both physical symptoms and psychosocial factors.

PD-L1/PD-1 pathway: a potential neuroimmune target for pain relief

Pain is a common symptom of many diseases with a high incidence rate. Clinically, drug treatment, as the main method to relieve pain at present, is often accompanied by different degrees of adverse reactions. Therefore, it is urgent to gain a profound understanding of the pain mechanisms in order to develop advantageous analgesic targets. The PD-L1/PD-1 pathway, an important inhibitory molecule in the immune system, has taken part in regulating neuroinflammation and immune response. Accumulating evidence indicates that the PD-L1/PD-1 pathway is aberrantly activated in various pain models. And blocking PD-L1/PD-1 pathway will aggravate pain behaviors. This review aims to summarize the emerging evidence on the role of the PD-L1/PD-1 pathway in alleviating pain and provide an overview of the mechanisms involved in pain resolution, including the regulation of macrophages, microglia, T cells, as well as nociceptor neurons. However, its underlying mechanism still needs to be further elucidated in the future. In conclusion, despite more deep researches are needed, these pioneering studies indicate that PD-L1/PD-1 may be a potential neuroimmune target for pain relief.

A mouse model of chronic primary pain that integrates clinically relevant genetic vulnerability, stress, and minor injury

Chronic primary pain conditions (CPPCs) affect over 100 million Americans, predominantly women. They remain ineffectively treated, in large part because of a lack of valid animal models with translational relevance. Here, we characterized a CPPC mouse model that integrated clinically relevant genetic (catechol-O-methyltransferase; COMT knockdown) and environmental (stress and injury) factors. Compared with wild-type mice, Comt+/- mice undergoing repeated swim stress and molar extraction surgery intervention exhibited pronounced multisite body pain and depressive-like behavior lasting >3 months. Comt+/- mice undergoing the intervention also exhibited enhanced activity of primary afferent nociceptors innervating hindpaw and low back sites and increased plasma concentrations of norepinephrine and pro-inflammatory cytokines interleukin-6 (IL-6) and IL-17A. The pain and depressive-like behavior were of greater magnitude and longer duration (≥12 months) in females versus males. Furthermore, increases in anxiety-like behavior and IL-6 were female-specific. The effect of COMT genotype × stress interactions on pain, IL-6, and IL-17A was validated in a cohort of 549 patients with CPPCs, demonstrating clinical relevance. Last, we assessed the predictive validity of the model for analgesic screening and found that it successfully predicted the lack of efficacy of minocycline and the CB2 agonist GW842166X, which were effective in spared nerve injury and complete Freund's adjuvant models, respectively, but failed in clinical trials. Yet, pain in the CPPC model was alleviated by the beta-3 adrenergic antagonist SR59230A. Thus, the CPPC mouse model reliably recapitulates clinically and biologically relevant features of CPPCs and may be implemented to test underlying mechanisms and find new therapeutics.

Olfactory ensheathing cells as candidate cells for chronic pain treatment

Chronic pain is often accompanied by tissue damage and pain hypersensitivity. It easily relapses and is challenging to cure, which seriously affects the patients' quality of life and is an urgent problem to be solved. Current treatment methods primarily rely on morphine drugs, which do not address the underlying nerve injury and may cause adverse reactions. Therefore, in recent years, scientists have shifted their focus from chronic pain treatment to cell transplantation. This review describes the classification and mechanism of chronic pain through the introduction of the characteristics of olfactory ensheathing cells (OECs), an in-depth discussion of special glial cells through the phagocytosis of nerve debris, receptor-ligand interactions, providing nutrition, and other inhibition of neuroinflammation, and ultimately supporting axon regeneration and mitigation of chronic pain. This review summarizes the potential and limitations of OECs for treating chronic pain by objectively analyzing relevant clinical trials and methods to enhance efficacy and future development prospects.

Cellular and molecular mechanisms involved in the analgesic effects of botulinum neurotoxin: A literature review

Botulinum neurotoxins (BoNTs), derived from Clostridium botulinum, have been employed to treat a range of central and peripheral neurological disease. Some studies indicate that BoNT may be beneficial for pain conditions as well. It has been hypothesized that BoNTs may exert their analgesic effects by preventing the release of pain-related neurotransmitters and neuroinflammatory agents from sensory nerve endings, suppressing glial activation, and inhibiting the transmission of pain-related receptors to the neuronal cell membrane. In addition, there is evidence to suggest that the central analgesic effects of BoNTs are mediated through their retrograde axonal transport. The purpose of this review is to summarize the experimental evidence of the analgesic functions of BoNTs and discuss the cellular and molecular mechanisms by which they can act on pain conditions. Most of the studies reviewed in this article were conducted using BoNT/A. The PubMed database was searched from 1995 to December 2022 to identify relevant literature.

Satellite Glial Cells in Human Disease

Satellite glial cells (SGCs) are the main type of glial cells in sensory ganglia. Animal studies have shown that these cells play essential roles in both normal and disease states. In a large number of pain models, SGCs were activated and contributed to the pain behavior. Much less is known about SGCs in humans, but there is emerging recognition that SGCs in humans are altered in a variety of clinical states. The available data show that human SGCs share some essential features with SGCs in rodents, but many differences do exist. SGCs in DRG from patients suffering from common painful diseases, such as rheumatoid arthritis and fibromyalgia, may contribute to the pain phenotype. It was found that immunoglobulins G (IgG) from fibromyalgia patients can induce pain-like behavior in mice. Moreover, these IgGs bind preferentially to SGCs and activate them, which can sensitize the sensory neurons, causing nociception. In other human diseases, the evidence is not as direct as in fibromyalgia, but it has been found that an antibody from a patient with rheumatoid arthritis binds to mouse SGCs, which leads to the release of pronociceptive factors from them. Herpes zoster is another painful disease, and it appears that the zoster virus resides in SGCs, which acquire an abnormal morphology and may participate in the infection and pain generation. More work needs to be undertaken on SGCs in humans, and this review points to several promising avenues for better understanding disease mechanisms and developing effective pain therapies.

Impaired amygdala astrocytic signaling worsens neuropathic pain-associated neuronal functions and behaviors

Introduction: Pain is a clinically relevant health care issue with limited therapeutic options, creating the need for new and improved analgesic strategies. The amygdala is a limbic brain region critically involved in the regulation of emotional-affective components of pain and in pain modulation. The central nucleus of amygdala (CeA) serves major output functions and receives nociceptive information via the external lateral parabrachial nucleus (PB). While amygdala neuroplasticity has been linked causally to pain behaviors, non-neuronal pain mechanisms in this region remain to be explored. As an essential part of the neuroimmune system, astrocytes that represent about 40-50% of glia cells within the central nervous system, are required for physiological neuronal functions, but their role in the amygdala remains to be determined for pain conditions. In this study, we measured time-specific astrocyte activation in the CeA in a neuropathic pain model (spinal nerve ligation, SNL) and assessed the effects of astrocyte inhibition on amygdala neuroplasticity and pain-like behaviors in the pain condition. Methods and Results: Glial fibrillary acidic protein (GFAP, astrocytic marker) immunoreactivity and mRNA expression were increased at the chronic (4 weeks post-SNL), but not acute (1 week post-SNL), stage of neuropathic pain. In order to determine the contribution of astrocytes to amygdala pain-mechanisms, we used fluorocitric acid (FCA), a selective inhibitor of astrocyte metabolism. Whole-cell patch-clamp recordings were performed from neurons in the laterocapsular division of the CeA (CeLC) obtained from chronic neuropathic rats. Pre-incubation of brain slices with FCA (100 µM, 1 h), increased excitability through altered hyperpolarization-activated current (Ih) functions, without significantly affecting synaptic responses at the PB-CeLC synapse. Intra-CeA injection of FCA (100 µM) had facilitatory effects on mechanical withdrawal thresholds (von Frey and paw pressure tests) and emotional-affective behaviors (evoked vocalizations), but not on facial grimace score and anxiety-like behaviors (open field test), in chronic neuropathic rats. Selective inhibition of astrocytes by FCA was confirmed with immunohistochemical analyses showing decreased astrocytic GFAP, but not NeuN, signal in the CeA. Discussion: Overall, these results suggest a complex modulation of amygdala pain functions by astrocytes and provide evidence for beneficial functions of astrocytes in CeA in chronic neuropathic pain.

Inhibition of NFAT5-Dependent Astrocyte Swelling Alleviates Neuropathic Pain

Astrocyte swelling is implicated in various neurological disorders. However, whether astrocyte swelling contributes to neuropathic pain remains elusive. This study elucidates the pivotal role of the nuclear factor of activated T-cells 5 (NFAT5) emerges as a master regulator of astrocyte swelling in the spinal dorsal horn (SDH) during neuropathic pain. Despite the ubiquitous expression of NFAT5 protein in SDH cell types, it selectively induces swelling specifically in astrocytes, not in microglia. Mechanistically, NFAT5 directly controls the expression of the water channel aquaporin-4 (AQP4), a key regulator exclusive to astrocytes. Additionally, aurora kinase B (AURKB) orchestrates NFAT5 phosphorylation, enhancing its protein stability and nuclear translocation, thereby regulating AQP4 expression. The findings establish NFAT5 as a crucial regulator for neuropathic pain through the modulation of astrocyte swelling. The AURKB-NFAT5-AQP4 pathway in astrocytes emerges as a potential therapeutic target to combat neuropathic pain.

Cytochrome P450 26A1 Contributes to the Maintenance of Neuropathic Pain

The cytochrome P450 proteins (CYP450s) have been implicated in catalyzing numerous important biological reactions and contribute to a variety of diseases. CYP26A1, a member of the CYP450 family, carries out the oxidative metabolism of retinoic acid (RA), the active metabolite of vitamin A. Here we report that CYP26A1 was dramatically upregulated in the spinal cord after spinal nerve ligation (SNL). CYP26A1 was mainly expressed in spinal neurons and astrocytes. HPLC analysis displayed that the content of all-trans-RA (at-RA), the substrate of CYP26A1, was reduced in the spinal cord on day 7 after SNL. Inhibition of CYP26A1 by siRNA or inhibition of CYP26A1-mediated at-RA catabolism by talarozole relieved the SNL-induced mechanical allodynia during the maintenance phase of neuropathic pain. Talarozole also reduced SNL-induced glial activation and proinflammatory cytokine production but increased anti-inflammatory cytokine (IL-10) production. The RA receptors RARα, RXRβ, and RXRγ were expressed in spinal neurons and glial cells. The promoter of Il-10 has several binding sites for RA receptors, and at-RA directly increased Il-10 mRNA expression in vitro. Finally, intrathecal IL-10 attenuated SNL-induced neuropathic pain and reduced the activation of astrocytes and microglia. Collectively, the inhibition of CYP26A1-mediated at-RA catabolism alleviates SNL-induced neuropathic pain by promoting the expression of IL-10 and suppressing glial activation. CYP26A1 may be a potential therapeutic target for the treatment of neuropathic pain.

Annulus Fibrosus Injury Induces Acute Neuroinflammation and Chronic Glial Response in Dorsal Root Ganglion and Spinal Cord-An In Vivo Rat Discogenic Pain Model

Chronic painful intervertebral disc (IVD) degeneration (i.e., discogenic pain) is a major source of global disability needing improved knowledge on multiple-tissue interactions and how they progress in order improve treatment strategies. This study used an in vivo rat annulus fibrosus (AF) injury-driven discogenic pain model to investigate the acute and chronic changes in IVD degeneration and spinal inflammation, as well as sensitization, inflammation, and remodeling in dorsal root ganglion (DRG) and spinal cord (SC) dorsal horn. AF injury induced moderate IVD degeneration with acute and broad spinal inflammation that progressed to DRG to SC changes within days and weeks, respectively. Specifically, AF injury elevated macrophages in the spine (CD68) and DRGs (Iba1) that peaked at 3 days post-injury, and increased microglia (Iba1) in SC that peaked at 2 weeks post-injury. AF injury also triggered glial responses with elevated GFAP in DRGs and SC at least 8 weeks post-injury. Spinal CD68 and SC neuropeptide Substance P both remained elevated at 8 weeks, suggesting that slow and incomplete IVD healing provides a chronic source of inflammation with continued SC sensitization. We conclude that AF injury-driven IVD degeneration induces acute spinal, DRG, and SC inflammatory crosstalk with sustained glial responses in both DRGs and SC, leading to chronic SC sensitization and neural plasticity. The known association of these markers with neuropathic pain suggests that therapeutic strategies for discogenic pain need to target both spinal and nervous systems, with early strategies managing acute inflammatory processes, and late strategies targeting chronic IVD inflammation, SC sensitization, and remodeling.

Spinal interleukin-24 contributes to neuropathic pain after peripheral nerve injury through interleukin-20 receptor2 in mice

Neuroinflammation is critically involved in nerve injury-induced neuropathic pain, characterized by local and systemic increased levels of proinflammatory cytokines. Interleukin-24 (IL-24), a key member of the IL-10 family, has been extensively studied for its therapeutic potential in various diseases, including cancer, autoimmune disorders, and bacterial infections, but whether it is involved in the regulation of neuropathic pain caused by peripheral nerve injury (PNI) has not been well established. In this study, we reported that spared nerve injury (SNI) induced a significant upregulation of IL-24 in fibroblasts, neurons, and oligodendrocyte precursor cells (OPCs, also called NG2-glia) in the affected spinal dorsal horns (SDHs), as well as dorsal root ganglions (DRGs). We also found that tumor necrosis factor α (TNF-α) induced the transcriptional expression of IL-24 in cultured fibroblasts, neurons, and NG2-glia; in addition, astrocytes, microglia, and NG2-glia treated with TNF-α exhibited a prominent increase in interleukin-20 receptor 2 (IL-20R2) expression. Furthermore, we evaluated the ability of IL-24 and IL-20R2 to attenuate pain in preclinical models of neuropathic pain. Intrathecal (i.t.) injection of IL-24 neutralizing antibody or IL-20R2 neutralizing antibody could effectively alleviate mechanical allodynia and thermal hyperalgesia after PNI. Similarly, intrathecal injection of IL-24 siRNA or IL-20R2 siRNA also alleviated mechanical allodynia after SNI. The inhibition of IL-24 reduced SNI-induced proinflammatory cytokine (IL-1β and TNF-α) production and increased anti-inflammatory cytokine (IL-10) production. Meanwhile, the inhibition of IL-20R2 also decreased IL-1β mRNA expression after SNI. Collectively, our findings revealed that IL-24/IL-20R might contribute to neuropathic pain through inflammatory response. Therefore, targeting IL-24 could be a promising strategy for treating neuropathic pain induced by PNI.