Neuromodulation, Specialized Proresolving Mediators, and Resolution of Pain

Periodontitis is associated with decreased experimental pressure pain tolerance: The Tromsø Study 2015-2016

AIM

To assess the relationship between periodontitis and experimental pain tolerance.

MATERIALS AND METHODS

Participants from the population-based seventh survey of the Tromsø Study with data on periodontitis were included (n = 3666, 40-84 years old, 51.6% women). Pain tolerance was assessed through (i) pressure pain tolerance (PPT) test with a computerized cuff pressure algometry on the leg, and (ii) cold-pressor tolerance (CPT) test where one hand was placed in circulating 3°C water. Cox proportional hazard regression was used to assess the association between periodontitis and pain tolerance adjusted for age, sex, education, smoking and obesity.

RESULTS

In the fully adjusted model using the 2012 Centers for Disease Control/American Academy of Periodntology case definitions for surveillance of periodontitis, moderate (hazard ratio [HR] = 1.09; 95% confidence interval [CI]: 1.01, 1.18) and severe (HR = 1.25, 95% CI: 1.11, 1.42) periodontitis were associated with decreased PPT. Using the 2018 classification of periodontitis, having Stage II/III/IV periodontitis was significantly associated with decreased PPT (HR = 1.09; 95% CI: 1.01, 1.18) compared with having no or stage I periodontitis. There were no significant associations between periodontitis and CPT in fully adjusted models.

CONCLUSIONS

Moderate and severe periodontitis was associated with experimental PPT.

Anatomo-physiological basis and applied techniques of electrical neuromodulation in chronic pain

Chronic pain, a complex and debilitating condition, poses a significant challenge to both patients and healthcare providers worldwide. Conventional pharmacological interventions often prove inadequate in delivering satisfactory relief while carrying the risks of addiction and adverse reactions. In recent years, electric neuromodulation emerged as a promising alternative in chronic pain management. This method entails the precise administration of electrical stimulation to specific nerves or regions within the central nervous system to regulate pain signals. Through mechanisms that include the alteration of neural activity and the release of endogenous pain-relieving substances, electric neuromodulation can effectively alleviate pain and improve patients' quality of life. Several modalities of electric neuromodulation, with a different grade of invasiveness, provide tailored strategies to tackle various forms and origins of chronic pain. Through an exploration of the anatomical and physiological pathways of chronic pain, encompassing neurotransmitter involvement, this narrative review offers insights into electrical therapies' mechanisms of action, clinical utility, and future perspectives in chronic pain management.

Exploring Localized Provoked Vulvodynia: Insights from Animal Model Research

Provoked vulvodynia represents a challenging chronic pain condition, characterized by its multifactorial origins. The inherent complexities of human-based studies have necessitated the use of animal models to enrich our understanding of vulvodynia's pathophysiology. This review aims to provide an exhaustive examination of the various animal models employed in this research domain. A comprehensive search was conducted on PubMed, utilizing keywords such as "vulvodynia", "chronic vulvar pain", "vulvodynia induction", and "animal models of vulvodynia" to identify pertinent studies. The search yielded three primary animal models for vulvodynia: inflammation-induced, allergy-induced, and hormone-induced. Additionally, six agents capable of triggering the condition through diverse pathways were identified, including factors contributing to hyperinnervation, mast cell proliferation, involvement of other immune cells, inflammatory cytokines, and neurotransmitters. This review systematically outlines the various animal models developed to study the pathogenesis of provoked vulvodynia. Understanding these models is crucial for the exploration of preventative measures, the development of novel treatments, and the overall advancement of research within the field.

High-intensity swimming alleviates nociception and neuroinflammation in a mouse model of chronic post-ischemia pain by activating the resolvin E1-chemerin receptor 23 axis in the spinal cord

Physical exercise effectively alleviates chronic pain associated with complex regional pain syndrome type-I. However, the mechanism of exercise-induced analgesia has not been clarified. Recent studies have shown that the specialized pro-resolving lipid mediator resolvin E1 promotes relief of pathologic pain by binding to chemerin receptor 23 in the nervous system. However, whether the resolvin E1-chemerin receptor 23 axis is involved in exercise-induced analgesia in complex regional pain syndrome type-I has not been demonstrated. In the present study, a mouse model of chronic post-ischemia pain was established to mimic complex regional pain syndrome type-I and subjected to an intervention involving swimming at different intensities. Chronic pain was reduced only in mice that engaged in high-intensity swimming. The resolvin E1-chemerin receptor 23 axis was clearly downregulated in the spinal cord of mice with chronic pain, while high-intensity swimming restored expression of resolvin E1 and chemerin receptor 23. Finally, shRNA-mediated silencing of chemerin receptor 23 in the spinal cord reversed the analgesic effect of high-intensity swimming exercise on chronic post-ischemic pain and the anti-inflammatory polarization of microglia in the dorsal horn of the spinal cord. These findings suggest that high-intensity swimming can decrease chronic pain via the endogenous resolvin E1-chemerin receptor 23 axis in the spinal cord.

M2 macrophage polarization: a potential target in pain relief

Pain imposes a significant urden on patients, affecting them physically, psychologically, and economically. Despite numerous studies on the pathogenesis of pain, its clinical management remains suboptimal, leading to the under-treatment of many pain patients. Recently, research on the role of macrophages in pain processes has been increasing, offering potential for novel therapeutic approaches. Macrophages, being indispensable immune cells in the innate immune system, exhibit remarkable diversity and plasticity. However, the majority of research has primarily focused on the contributions of M1 macrophages in promoting pain. During the late stage of tissue damage or inflammatory invasion, M1 macrophages typically transition into M2 macrophages. In recent years, growing evidence has highlighted the role of M2 macrophages in pain relief. In this review, we summarize the mechanisms involved in M2 macrophage polarization and discuss their emerging roles in pain relief. Notably, M2 macrophages appear to be key players in multiple endogenous pathways that promote pain relief. We further analyze potential pathways through which M2 macrophages may alleviate pain.

Inflammation, lipids, and pain in vulvar disease

Localized provoked vulvodynia (LPV) affects ∼14 million people in the US (9% of women), destroying lives and relationships. LPV is characterized by chronic pain (>3 months) upon touch to the vulvar vestibule, which surrounds the vaginal opening. Many patients go months or years without a diagnosis. Once diagnosed, the treatments available only manage the symptoms of disease and do not correct the underlying problem. We have focused on elucidating the underlying mechanisms of chronic vulvar pain to speed diagnosis and improve intervention and management. We determined the inflammatory response to microorganisms, even members of the resident microflora, sets off a chain of events that culminates in chronic pain. This agrees with findings from several other groups, which show inflammation is altered in the painful vestibule. The vestibule of patients is acutely sensitive to inflammatory stimuli to the point of being deleterious. Rather than protect against vaginal infection, it causes heightened inflammation that does not resolve, which coincides with alterations in lipid metabolism that favor production of proinflammatory lipids and not pro-resolving lipids. Lipid dysbiosis in turn triggers pain signaling through the transient receptor potential vanilloid subtype 4 receptor (TRPV4). Treatment with specialized pro-resolving mediators (SPMs) that foster resolution reduces inflammation in fibroblasts and mice and vulvar sensitivity in mice. SPMs, specifically maresin 1, act on more than one part of the vulvodynia mechanism by limiting inflammation and acutely inhibiting TRPV4 signaling. Therefore, SPMs or other agents that target inflammation and/or TRPV4 signaling could prove effective as new vulvodynia therapies.

Research Status and Prospects of Acupuncture in Perioperative Medicine Over the Past Decade: A Bibliometric Analysis

Background

Over the past decade, acupuncture in the perioperative period has attracted great interest, and a growing number of related literature has been published.

Purpose

To analyze the general information and identify the research hotspots and trends of acupuncture in perioperative medicine in the last 10 years by bibliometric analysis.

Methods

We searched the Web of Science Core Collection for publications on acupuncture in perioperative medicine from 2013 to 2023. The articles and reviews were collected with no language restriction. CiteSpace and VOSviewer software were used for bibliometric and visual analysis of relevant literature.

Results

A total of 814 bibliographic records were retrieved. Overall, the annual number of publications showed an increasing trend. China and its institutions were in a leading position regarding the publication number. With comparatively more scientific collaboration with China, the USA ranked second. Shanghai University of Traditional Chinese Medicine was the most prolific institution. Ha, In-Hyuk had the most publications, and Han JS and Lee A were the most cited authors. Medicine was the most popular journal and Journal of Clinical Oncology had the highest impact factor. "Acupuncture", "electroacupuncture" and "postoperative pain" were the top three keywords. The most popular topics were postoperative pain, postoperative ileus, and postoperative nausea and vomiting according to the keywords and references. And the clusters of postoperative cognitive dysfunction, anxiety, and breast cancer attracted relatively more attention recently.

Conclusion

This study summarized the research status, hotspots, and trends of acupuncture in perioperative medicine in the past decade, which may aid researchers in better understanding this field. The research hotspots primarily focused on postoperative pain management and postoperative gastrointestinal function. The research of acupuncture for postoperative cognitive dysfunction, cancer-related surgery, and psychological states were the main frontiers topics and may be the focus in the future.

Role of Vagus Nerve Stimulation in the Treatment of Chronic Pain

Vagus nerve stimulation (VNS) can modulate vagal activity and neuro-immune communication. Human and animal studies have provided growing evidence that VNS can produce analgesic effects in addition to alleviating refractory epilepsy and depression. The vagus nerve (VN) projects to many brain regions related to pain processing, which can be affected by VNS. In addition to neural regulation, the anti-inflammatory property of VNS may also contribute to its pain-inhibitory effects. To date, both invasive and noninvasive VNS devices have been developed, with noninvasive devices including transcutaneous stimulation of auricular VN or carotid VN that are undergoing many clinical trials for chronic pain treatment. This review aimed to provide an update on both preclinical and clinical studies of VNS in the management for chronic pain, including fibromyalgia, abdominal pain, and headaches. We further discuss potential underlying mechanisms for VNS to inhibit chronic pain.

Pain-resolving immune mechanisms in neuropathic pain

Interactions between the immune and nervous systems are of central importance in neuropathic pain, a common and debilitating form of chronic pain caused by a lesion or disease affecting the somatosensory system. Our understanding of neuroimmune interactions in pain research has advanced considerably. Initially considered as passive bystanders, then as culprits in the pathogenesis of neuropathic pain, immune responses in the nervous system are now established to underpin not only the initiation and progression of pain but also its resolution. Indeed, immune cells and their mediators are well-established promoters of neuroinflammation at each level of the neural pain pathway that contributes to pain hypersensitivity. However, emerging evidence indicates that specific subtypes of immune cells (including antinociceptive macrophages, pain-resolving microglia and T regulatory cells) as well as immunoresolvent molecules and modulators of the gut microbiota-immune system axis can reduce the pain experience and contribute to the resolution of neuropathic pain. This Review provides an overview of the immune mechanisms responsible for the resolution of neuropathic pain, including those involved in innate, adaptive and meningeal immunity as well as interactions with the gut microbiome. Specialized pro-resolving mediators and therapeutic approaches that target these neuroimmune mechanisms are also discussed.

Resolution Pharmacology: Focus on Pro-Resolving Annexin A1 and Lipid Mediators for Therapeutic Innovation in Inflammation

Chronic diseases that affect our society are made more complex by comorbidities and are poorly managed by the current pharmacology. While all present inflammatory etiopathogeneses, there is an unmet need for better clinical management of these diseases and their multiple symptoms. We discuss here an innovative approach based on the biology of the resolution of inflammation. Studying endogenous pro-resolving peptide and lipid mediators, how they are formed, and which target they interact with, can offer innovative options through augmenting the expression or function of pro-resolving pathways or mimicking their actions with novel targeted molecules. In all cases, resolution offers innovation for the treatment of the primary cause of a given disease and/or for the management of its comorbidities, ultimately improving patient quality of life. By implementing resolution pharmacology, we harness the whole physiology of inflammation, with the potential to bring a marked change in the management of inflammatory conditions.

Specialized Pro-Resolving Mediators as Resolution Pharmacology for the Control of Pain and Itch

Specialized pro-resolving mediators (SPMs), including resolvins, protectins, and maresins, are endogenous lipid mediators that are synthesized from omega-3 polyunsaturated fatty acids during the acute phase or resolution phase of inflammation. Synthetic SPMs possess broad safety profiles and exhibit potent actions in resolving inflammation in preclinical models. Accumulating evidence in the past decade has demonstrated powerful analgesia of exogenous SPMs in rodent models of inflammatory, neuropathic, and cancer pain. Furthermore, endogenous SPMs are produced by sham surgery and neuromodulation (e.g., vagus nerve stimulation). SPMs produce their beneficial actions through multiple G protein-coupled receptors, expressed by immune cells, glial cells, and neurons. Notably, loss of SPM receptors impairs the resolution of pain. I also highlight the emerging role of SPMs in the control of itch. Pharmacological targeting of SPMs or SPM receptors has the potential to lead to novel therapeutics for pain and itch as emerging approaches in resolution pharmacology.

Inflammation in pathogenesis of chronic pain: Foe and friend

Chronic pain is a refractory health disease worldwide causing an enormous economic burden on individuals and society. Accumulating evidence suggests that inflammation in the peripheral nervous system (PNS) and central nervous system (CNS) is the major factor in the pathogenesis of chronic pain. The inflammation in the early- and late phase may have distinctive effects on the initiation and resolution of pain, which can be viewed as friend or foe. On the one hand, painful injuries lead to the activation of glial cells and immune cells in the PNS, releasing pro-inflammatory mediators, which contribute to the sensitization of nociceptors, leading to chronic pain; neuroinflammation in the CNS drives central sensitization and promotes the development of chronic pain. On the other hand, macrophages and glial cells of PNS and CNS promote pain resolution via anti-inflammatory mediators and specialized pro-resolving mediators (SPMs). In this review, we provide an overview of the current understanding of inflammation in the deterioration and resolution of pain. Further, we summarize a number of novel strategies that can be used to prevent and treat chronic pain by controlling inflammation. This comprehensive view of the relationship between inflammation and chronic pain and its specific mechanism will provide novel targets for the treatment of chronic pain.

Resolvin D1 attenuates mechanical allodynia after burn injury: Involvement of spinal glia, p38 mitogen-activated protein kinase, and brain-derived neurotrophic factor/tropomyosin-related kinase B signaling

Resolvin D1 (RvD1) suppresses inflammatory, postoperative, and neuropathic pain. The present study assessed the roles and mechanisms of RvD1 in mechanical allodynia after burn injury. A rat model of burn injury was established for analyses, and RvD1 was injected intraperitoneally. Pain behavior and the expression levels of spinal dorsal horn Iba-1 (microglia marker), GFAP (astrocyte marker), p-p38 mitogen-activated protein kinase (MAPK), brain-derived neurotrophic factor (BDNF), and tropomyosin-related kinase B (TrkB) were detected by behavioral and immunocytochemical assays. The results showed that RvD1 attenuated mechanical allodynia after burn injury, prevented microglial and astroglial activation, and downregulated p-p38 MAPK in microglia and BDNF/TrkB following burn injury. Similarly, inhibition of p38 MAPK and BDNF/TrkB signaling attenuated mechanical allodynia after burn injury. In addition, inhibition of p38 MAPK prevented spinal microglial activation and downregulated BDNF/TrkB following burn injury. Furthermore, inhibition of BDNF/TrkB signaling prevented spinal microglial activation and downregulated p-p38 MAPK within spinal microglia. Taken together, this study demonstrated that RvD1 might attenuate mechanical allodynia after burn injury by inhibiting spinal cord glial activation, microglial p38 MAPK, and BDNF/TrkB signaling in the spinal dorsal horn.

Lidocaine alleviates inflammation and pruritus in atopic dermatitis by blocking different population of sensory neurons

BACKGROUND AND PURPOSE

Atopic dermatitis is a common chronic pruritic inflammatory disease of the skin involving neuro-immune communication. Neuronal mechanism-based therapeutic treatments remain lacking. We investigated the efficacy of intravenous lidocaine therapy on atopic dermatitis and the underlying neuro-immune mechanism.

EXPERIMENTAL APPROACH

Pharmacological intervention, immunofluorescence, RNA-sequencing, genetic modification and immunoassay were performed to dissect the neuro-immune basis of itch and inflammation in atopic dermatitis-like mouse model and in patients.

KEY RESULTS

Lidocaine alleviated skin lesions and itch in both atopic dermatitis patients and calcipotriol (MC903)-induced atopic dermatitis model by blocking subpopulation of sensory neurons. QX-314, a charged Na_V blocker that enters through pathologically activated large-pore ion channels and selectivity inhibits a subpopulation of sensory neurons, has the same effects as lidocaine in atopic dermatitis model. Genetic silencing Na_V 1.8-expressing sensory neurons was sufficient to restrict cutaneous inflammation and itch in the atopic dermatitis model. However, pharmacological blockade of TRPV1-positive nociceptors only abolished persistent itch but did not affect skin inflammation in the atopic dermatitis model, indicating a difference between sensory neuronal modulation of skin inflammation and itch. Inhibition of activity-dependent release of calcitonin gene-related peptide (CGRP) from sensory neurons by lidocaine largely accounts for the therapeutic effect of lidocaine in the atopic dermatitis model.

CONCLUSION AND IMPLICATIONS

Na_V 1.8⁺ sensory neurons play a critical role in pathogenesis of atopic dermatitis and lidocaine is a potential anti-inflammatory and anti-pruritic agent for atopic dermatitis. A dissociable difference for sensory neuronal modulation of skin inflammation and itch contributes to further understanding of pathogenesis in atopic dermatitis.

Is depression the missing link between inflammatory mediators and cancer?

Patients with cancer are at greater risk of developing depression in comparison to the general population and this is associated with serious adverse effects, such as poorer quality of life, worse prognosis and higher mortality. Although the relationship between depression and cancer is now well established, a common underlying pathophysiological mechanism between the two conditions is yet to be elucidated. Existing theories of depression, based on monoamine neurotransmitter system dysfunction, are insufficient as explanations of the disorder. Recent advances have implicated neuroinflammatory mechanisms in the etiology of depression and it has been demonstrated that inflammation at a peripheral level may be mirrored centrally in astrocytes and microglia serving to promote chronic levels of inflammation in the brain. Three major routes to depression in cancer in which proinflammatory mediators are implicated, seem likely. Activation of the kynurenine pathway involving cytokines, increases tryptophan catabolism, resulting in diminished levels of serotonin which is widely acknowledged as being the hallmark of depression. It also results in neurotoxic effects on brain regions thought to be involved in the evolution of major depression. Proinflammatory mediators also play a crucial role in impairing regulatory glucocorticoid mediated feedback of the hypothalamic-pituitary-adrenal axis, which is activated by stress and considered to be involved in both depression and cancer. The third route is via the glutamatergic pathway, whereby glutamate excitotoxicity may lead to depression associated with cancer. A better understanding of the mechanisms underlying these dysregulated and other newly emerging pathways may provide a rationale for therapeutic targeting, serving to improve the care of cancer patients.

Profiling of fatty acid metabolism in the dorsal root ganglion after peripheral nerve injury

Peripheral nerve injury (PNI) induces neuronal hyperexcitability, which underlies neuropathic pain. The emergence of RNA sequencing technologies has enabled profiling of transcriptional changes in pathological conditions. However, these approaches do not provide information regarding metabolites such as lipids that are not directly encoded by genes. Fatty acids (FAs) are some of the essential lipids in mammalian organisms and are mainly stored as membrane phospholipids. In response to various biological stimuli, FAs are rapidly released and converted into several mediators, such as eicosanoids and docosanoids. FAs themselves or their metabolites play important roles in physiology and pathology. In this study, using a comprehensive lipidomic analysis of FA metabolites, 152 species were measured in the dorsal root ganglia of mice at multiple time points after PNI. We found that PNI increased the ω-6 FA metabolites produced by cyclooxygenases but not those produced by lipoxygenases or cytochrome P450 enzymes in the dorsal root ganglia. In contrast, ω-3 FA metabolites biosynthesized by any enzyme transiently increased after nerve injury. Overall, these findings provide a new resource and valuable insights into PNI pathologies, including pain and nerve regeneration.

Peroxisome proliferator-activated receptor-gamma (PPARγ) and its immunomodulation function: current understanding and future therapeutic implications

INTRODUCTION

: Pain is a multidimensional experience involving the biological, psychological, and social dimensions of each individual. Particularly, the biological aspects of pain conditions are a response of the neuroimmunology system and the control of painful conditions is a worldwide challenge for researchers. Although years of investigation on pain experience and treatment exist, the high prevalence of chronic pain is still a fact.

AREAS COVERED

: Peroxisome proliferator-activated receptor-gamma (PPARγ) is a ligand-activated transcription factor belonging to the nuclear hormone receptor superfamily. It regulates several metabolic pathways, including lipid biosynthesis and glucose metabolism, when activated. However, PPARγ activation also has a critical immunomodulatory and neuroprotective effect.

EXPERT OPINION

: This review summarizes the evidence of synthetic or natural PPARγ ligands such as 15d-PGJ₂, epoxyeicosatrienoic acids, thiazolidinediones, and specialized pro-resolving mediators, representing an interesting therapeutic tool for pain control.

A functional subdivision within the somatosensory system and its implications for pain research

Somatosensory afferents are traditionally classified by soma size, myelination, and their response specificity to external and internal stimuli. Here, we propose the functional subdivision of the nociceptive somatosensory system into two branches. The exteroceptive branch detects external threats and drives reflexive-defensive reactions to prevent or limit injury. The interoceptive branch senses the disruption of body integrity, produces tonic pain with strong aversive emotional components, and drives self-caring responses toward to the injured region to reduce suffering. The central thesis behind this functional subdivision comes from a reflection on the dilemma faced by the pain research field, namely, the use of reflexive-defensive behaviors as surrogate assays for interoceptive tonic pain. The interpretation of these assays is now being challenged by the discovery of distinct but interwoven circuits that drive exteroceptive versus interoceptive types of behaviors, with the conflation of these two components contributing partially to the poor translation of therapies from preclinical studies.

Human leukocytes selectively convert 4S,5S-epoxy-resolvin to resolvin D3, resolvin D4, and a cys-resolvin isomer

Human phagocytes have key functions in the resolution of inflammation. Here, we assessed the role of the proposed 4S,5S-epoxy-resolvin intermediate in the biosynthesis of both resolvin D3 and resolvin D4. We found that human neutrophils converted this synthetic intermediate to resolvin D3 and resolvin D4. M2 macrophages transformed this labile epoxide intermediate to resolvin D4 and a previously unknown cysteinyl-resolvin isomer without appreciable amounts of resolvin D3. M2 macrophages play critical roles in the resolution of inflammation and in wound healing. Human M2 macrophages also converted leukotriene A₄ to lipoxins. The cysteinyl-resolvin isomer significantly accelerated tissue regeneration of surgically injured planaria. In a model of human granuloma formation, the cysteinyl-resolvin isomer significantly inhibited granuloma development by human peripheral blood leukocytes. Together, these results provide evidence for a human cell type-specific role of 4S,5S-epoxy-resolvin in the biosynthesis of resolvin D3 by neutrophils, resolvin D4 by both M2 macrophages and neutrophils, and a unique cysteinyl-resolvin isomer produced by M2 macrophages that carries potent biological activities in granuloma formation and tissue regeneration.

Ultrasound Neuromodulation: Integrating Medicine and Engineering for Neurological Disease Treatment

Abstract Neurological diseases associated with dysfunctions of neural circuits, including Alzheimer’s disease (AD), depression and epilepsy, have been increasingly prevalent. To tackle these issues, artificial stimulation or regulation of specific neural circuits and nuclei are employed to alleviate or cure certain neurological diseases. In particular, ultrasound neuromodulation has been an emerging interdisciplinary approach, which integrates medicine and engineering methodologies in the treatment. With the development of medicine and engineering, ultrasound neuromodulation has gradually been applied in the treatment of central nervous system diseases. In this review, we aimed to summarize the mechanism of ultrasound neuromodulation and the advances of focused ultrasound (FUS) in neuromodulation in recent years, with a special emphasis on its application in central nervous system disease treatment. FUS showed great feasibility in the treatment of epilepsy, tremor, AD, depression, and brain trauma. We also suggested future directions of ultrasound neuromodulation in clinical settings, with a focus on its fusion with genetic engineering or nanotechnology.

Specialized Pro-resolving Mediators Reduce Pro-nociceptive Inflammatory Mediator Production in Models of Localized Provoked Vulvodynia

Localized provoked vulvodynia (LPV) is the most common cause of chronic dyspareunia in premenopausal women, characterized by pain with light touch to the vulvar vestibule surrounding the vaginal opening. The devastating impact of LPV includes sexual dysfunction, infertility, depression, and even suicide. Yet, its etiology is unclear. No effective medical therapy exists; surgical removal of the painful vestibule is the last resort. In LPV, the vestibule expresses a unique inflammatory profile with elevated levels of pro-nociceptive proinflammatory mediators prostaglandin E2 (PGE2) and interleukin-6 (IL-6), which are linked to lower mechanical sensitivity thresholds. Specialized pro-resolving mediators (SPMs), lipids produced endogenously within the body, hold promise as an LPV treatment by resolving inflammation without impairing host defense. Ten of 13 commercially available SPMs reduced IL-6 and PGE2 production by vulvar fibroblasts, administered either before or after inflammatory stimulation. Using a murine vulvar pain model, coupling proinflammatory mediator quantification with mechanical sensitivity threshold determination, topical treatment with the SPM, maresin 1, decreased sensitivity and suppressed PGE2 levels. Docosahexaenoic acid, a precursor of maresin 1, was also effective in reducing PGE2 in vulvar fibroblasts and rapidly restored mouse sensitivity thresholds. Overall, SPMs and their precursors may be a safe and efficacious for LPV. Perspective: Vulvodynia, like many pain conditions, is difficult to treat because disease origins are incompletely understood. Here, we applied our knowledge of more recently discovered vulvodynia disease mechanisms to screen novel therapeutics. We identified several specialized pro-resolving mediators as likely potent and safe for treating LPV with potential for broader application.

N-Acylethanolamine-Hydrolyzing Acid Amidase Inhibition, but Not Fatty Acid Amide Hydrolase Inhibition, Prevents the Development of Experimental Autoimmune Encephalomyelitis in Mice

N-acylethanolamines (NAEs) are endogenous bioactive lipids reported to exert anti-inflammatory and neuroprotective effects mediated by cannabinoid receptors and peroxisome proliferator-activated receptors (PPARs), among others. Therefore, interfering with NAE signaling could be a promising strategy to decrease inflammation in neurological disorders such as multiple sclerosis (MS). Fatty acid amide hydrolase (FAAH) and N-acylethanolamine-hydrolyzing acid amidase (NAAA) are key modulators of NAE levels. This study aims to investigate and compare the effect of NAAA inhibition, FAAH inhibition, and dual inhibition of both enzymes in a mouse model of MS, namely the experimental autoimmune encephalomyelitis (EAE). Our data show that NAAA inhibition strongly decreased the hallmarks of the pathology. Interestingly, FAAH inhibition was less efficient in decreasing inflammatory hallmarks despite the increased NAE levels. Moreover, the inhibition of both NAAA and FAAH, using a dual-inhibitor or the co-administration of NAAA and FAAH inhibitors, did not show an added value compared to NAAA inhibition. Furthermore, our data suggest an important role of decreased activation of astrocytes and microglia in the effects of NAAA inhibition on EAE, while NAAA inhibition did not affect T cell recall. This work highlights the beneficial effects of NAAA inhibition in the context of central nervous system inflammation and suggests that the simultaneous inhibition of NAAA and FAAH has no additional beneficial effect in EAE.

Randomized Cross Over Study Assessing the Efficacy of Non-invasive Stimulation of the Vagus Nerve in Patients With Axial Spondyloarthritis Resistant to Biotherapies: The ESNV-SPA Study Protocol

Axial spondyloarthritis (SpA), is a major cause of chronic pain and disability that profoundly alters the quality of life of patients. Nearly half of patients with SpA usually develop drug resistance. Non-pharmacological treatments targeting inflammation are an attractive alternative to drug administration. Vagus nerve stimulation (VNS), by promoting a cholinergic anti-inflammatory reflex holds promise for treating inflammatory disease. Inflammatory reflex signaling, which is enhanced by electrically stimulating the vagus nerve, significantly reduces cytokine production and attenuates disease severity in animal models of endotoxemia, sepsis, colitis, and other preclinical models of inflammatory diseases. It has been proposed that vagal efferent fibers release acetylcholine (Ach), which can interact with α7-subunit-containing nicotinic receptors expressed by tissue macrophages and other immune cells to rapidly inhibit the synthesis/release of pro-inflammatory cytokines such as TNFα, IL-1β, IL-6, and IL-18. External vagal nerve stimulation devices are now available that do not require surgery nor implantation to non-invasively stimulate the vagal nerve. This double-blind randomized cross-over clinical trial aims to study the change in SpA disease activity, according to Assessment in Ankylosing Spondylitis 20 (ASAS20) definition, after 12 weeks of non-invasive VNS treatment vs. non-specific dummy stimulation (control group). One hundred and twenty adult patients with drug resistant SpA, meeting the ASAS classification criteria, will be included in the study. Patients will be randomized into two parallel groups according to a cross over design: either active VNS for 12 weeks, then dummy stimulation for 12 weeks, or dummy stimulation for 12 weeks, then active VNS for 12 weeks. The two stimulation periods will be separated by a 4 weeks wash-out period. A transcutaneous auricular vagus nerve stimulator Tens Eco Plus SCHWA MEDICO^TM France will be used in this study. The active VNS stimulation will be applied in the cymba conchae of the left ear upon the auricular branch of the vagus nerve, using low intensity (2–5 mA), once à week, during 1 h. Dummy stimulation will be performed under the same conditions and parameters as active VNS stimulation, but at an irrelevant anatomical site: the left ear lobule. This multicenter study was registered on ClinicalTrials.gov: NCT04286373.

Spinal Cord Stimulation Attenuates Mechanical Allodynia and Increases Central Resolvin D1 Levels in Rats With Spared Nerve Injury

Mounting evidence from animal models of inflammatory and neuropathic pain suggests that inflammation regulates the resolution of pain by producing specialized pro-resolving mediators (SPMs), such as resolvin D1 (RvD1). However, it remains unclear how SPMs are induced in the central nervous system and whether these mechanisms can be reconciled with outcomes of neuromodulation therapies for pain, such as spinal cord stimulation. Here, we show that in a male rat model of neuropathic pain produced by spared nerve injury (SNI), 1 kHz spinal cord stimulation (1 kHz SCS) alone was sufficient to reduce mechanical allodynia and increase RvD1 in the cerebrospinal fluid (CSF). SNI resulted in robust and persistent mechanical allodynia and cold allodynia. Spinal cord electrode implantation was conducted at the T11-T13 vertebral level 1 week after SNI. The spinal locations of the implanted electrodes were validated by X-Ray radiography. 1 kHz SCS was applied for 6 h at 0.1 ms pulse-width, and this stimulation alone was sufficient to effectively reduce nerve injury-induced mechanical allodynia during stimulation without affecting SNI-induced cold allodynia. SCS alone significantly reduced interleukin-1β levels in both serum and CSF samples. Strikingly, SCS significantly increased RvD1 levels in the CSF but not serum. Finally, intrathecal injection of RvD1 (100 and 500 ng, i.t.) 4 weeks after nerve injury reduced SNI-induced mechanical allodynia in a dose-dependent manner. Our findings suggest that 1 kHz SCS may alleviate neuropathic pain via reduction of IL-1β and via production and/or release of RvD1 to control SNI-induced neuroinflammation.

Specialized pro-resolving lipid mediators in endodontics: a narrative review

Endodontics is the branch of dentistry concerned with the morphology, physiology, and pathology of the human dental pulp and periradicular tissues. Human dental pulp is a highly dynamic tissue equipped with a network of resident immunocompetent cells that play major roles in the defense against pathogens and during tissue injury. However, the efficiency of these mechanisms during dental pulp inflammation (pulpitis) varies due to anatomical and physiological restrictions. Uncontrolled, excessive, or unresolved inflammation can lead to pulp tissue necrosis and subsequent bone infections called apical periodontitis. In most cases, pulpitis treatment consists of total pulp removal. Although this strategy has a good success rate, this treatment has some drawbacks (lack of defense mechanisms, loss of healing capacities, incomplete formation of the root in young patients). In a sizeable number of clinical situations, the decision to perform pulp extirpation and endodontic treatment is justifiable by the lack of therapeutic tools that could otherwise limit the immune/inflammatory process. In the past few decades, many studies have demonstrated that the resolution of acute inflammation is necessary to avoid the development of chronic inflammation and to promote repair or regeneration. This active process is orchestrated by Specialized Pro-resolving lipid Mediators (SPMs), including lipoxins, resolvins, protectins and maresins. Interestingly, SPMs do not have direct anti-inflammatory effects by inhibiting or directly blocking this process but can actively reduce neutrophil infiltration into inflamed tissues, enhance efferocytosis and bacterial phagocytosis by monocytes and macrophages and simultaneously inhibit inflammatory cytokine production. Experimental clinical application of SPMs has shown promising result in a wide range of inflammatory diseases, such as renal fibrosis, cerebral ischemia, marginal periodontitis, and cancer; the potential of SPMs in endodontic therapy has recently been explored. In this review, our objective was to analyze the involvement and potential use of SPMs in endodontic therapies with an emphasis on SPM delivery systems to effectively administer SPMs into the dental pulp space.

Bioactive Lipid Mediators in the Initiation and Resolution of Inflammation after Spinal Cord Injury

Neuroinflammation is a prominent feature of the response to CNS trauma. It is also an important hallmark of various neurodegenerative diseases in which inflammation contributes to the progression of pathology. Inflammation in the CNS can contribute to secondary damage and is therefore an excellent therapeutic target for a range of neurological conditions. Inflammation in the nervous system is complex and varies in its fine details in different conditions. It involves a wide variety of secreted factors such as chemokines and cytokines, cell adhesion molecules, and different cell types that include resident cell of the CNS, as well as immune cells recruited from the peripheral circulation. Added to this complexity is the fact that some aspects of inflammation are beneficial, while other aspects can induce secondary damage in the acute, subacute and chronic phases. Understanding these aspects of the inflammatory profile is essential for developing effective therapies. Bioactive lipids constitute a large group of molecules that modulate the initiation and the resolution of inflammation. Dysregulation of these bioactive lipid pathways can lead to excessive acute inflammation, and failure to resolve this by specialized pro-resolution lipid mediators can lead to the development of chronic inflammation. The focus of this review is to discuss the effects of bioactive lipids in spinal cord trauma and their potential for therapies.

A new synthetic protectin D1 analog 3-oxa-PD1n-3 DPA reduces neuropathic pain and chronic itch in mice

The resolution of inflammation is a biosynthetically active process controlled by the interplay between oxygenated polyunsaturated mediators and G-protein coupled receptor-signaling pathways. These enzymatically oxygenated polyunsaturated fatty acids belong to distinct families of specialized pro-resolving autacoids. The protectin family of mediators has attracted an interest because of their potent pro-resolving and anti-inflammatory actions verified in several in vivo disease models. Herein, we present the stereoselective synthesis and biological evaluations of 3-oxa-PD1n-3 DPA, a protectin D1 analog. Results from mouse models indicate that the mediators protectin D1, PD1n-3 DPA and the new analog 3-oxa-PD1n-3 DPA all relieved streptozotocin-induced diabetic neuropathic pain at doses of 90 and 300 pmol, equivalent to 30 and 100 ng, respectively, following intrathecal (I.T.) injection. Of interest, at a low dose of only 30 pmol (10 ng; I.T.) only 3-oxa PD1n-3 DPA was able to alleviate neuropathic pain, directly compared to vehicle controls. Moreover, using a chronic itch model of cutaneous T-cell lymphoma (CTCL), all three compounds at 300 pmol (100 ng) showed a significant reduction in itching for several hours. The biomolecular information on the structure-functions of the protectins and the new synthetic analog 3-oxa-PD1n-3 DPA is of interest towards developing new immunoresolvents.

Central Nervous System Targets: Glial Cell Mechanisms in Chronic Pain

Interactions between central glial cells and neurons in the pain circuitry are critical contributors to the pathogenesis of chronic pain. In the central nervous system (CNS), two major glial cell types predominate: astrocytes and microglia. Injuries or pathological conditions which evoke pain are concurrently associated with the presence of a reactive microglia or astrocyte state, which is characterized by a variety of changes in the morphological, molecular, and functional properties of these cells. In this review, we highlight the changes that reactive microglia and astrocytes undergo following painful injuries and insults and discuss the critical and interactive role these two cell types play in the initiation and maintenance of chronic pain. Additionally, we focus on several crucial mechanisms by which microglia and astrocytes contribute to chronic pain and provide commentary on the therapeutic promise of targeting these pathways. In particular, we discuss how the inflammasome in activated microglia drives maturation and release of key pro-inflammatory cytokines, which drive pain through neuronal- and glial regulations. Moreover, we highlight several potentially-druggable hemichannels and proteases produced by reactive microglia and astrocytes in pain states and discuss how these pathways regulate distinct phases during pain pathogenesis. We also review two emerging areas in chronic pain research: 1) sexually dimorphic glial cell signaling and 2) the role of oligodendrocytes. Finally, we highlight important considerations for potential pain therapeutics targeting glial cell mediators as well as questions that remain in our conceptual understanding of glial cell activation in pain states.