Oxidized Lipids in Persistent Pain States

Mechanisms for Orofacial Pain: Roles of Immunomodulation, Metabolic Reprogramming, Oxidative Stress and Epigenetic Regulation

Background and Objectives: Orofacial pain corresponds to pain sensitization originating from the facial and oral regions, often accompanied by diagnostic complexity due to a multitude of contributory factors, leading to significant patient distress and impairment. Here, we have reviewed current mechanistic pathways and biochemical aspects of complex orofacial pain pathology, highlighting recent advancements in understanding its multifactorial regulation and signaling and thus providing a holistic approach to challenging it. Materials and Methods: Studies were identified from an online search of the PubMed database without any search time range. Results: We have discussed neuron-glia interactions and glial cell activation in terms of immunomodulatory effects, metabolism reprogramming effects and epigenetic modulatory effects, in response to orofacial pain sensitization comprising different originating factors. We have highlighted the fundamental role of oxidative stress affecting significant cellular pathways as well as cellular machinery, which renders pain pathology intricate and multidimensional. Emerging research on the epigenetic modulation of pain regulatory genes in response to molecular and cellular environmental factors is also discussed, alongside updates on novel diagnostic and treatment approaches. Conclusions: This review deliberates the integrative perspectives and implications of modulation in the immune system, glucose metabolism, lipid metabolism and redox homeostasis accompanied by mitochondrial dysfunction as well as epigenetic regulation accommodating the effect of dysregulated non-coding RNAs for an interdisciplinary understanding of pain pathology at the molecular level, aiming to improve patient outcomes with precise diagnosis offering improved pain management and treatment.

The Octadecanoids: Synthesis and Bioactivity of 18-Carbon Oxygenated Fatty Acids in Mammals, Bacteria, and Fungi

The octadecanoids are a broad class of lipids consisting of the oxygenated products of 18-carbon fatty acids. Originally referring to production of the phytohormone jasmonic acid, the octadecanoid pathway has been expanded to include products of all 18-carbon fatty acids. Octadecanoids are formed biosynthetically in mammals via cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP) activity, as well as nonenzymatically by photo- and autoxidation mechanisms. While octadecanoids are well-known mediators in plants, their role in the regulation of mammalian biological processes has been generally neglected. However, there have been significant advancements in recognizing the importance of these compounds in mammals and their involvement in the mediation of inflammation, nociception, and cell proliferation, as well as in immuno- and tissue modulation, coagulation processes, hormone regulation, and skin barrier formation. More recently, the gut microbiome has been shown to be a significant source of octadecanoid biosynthesis, providing additional biosynthetic routes including hydratase activity (e.g., CLA-HY, FA-HY1, FA-HY2). In this review, we summarize the current field of octadecanoids, propose standardized nomenclature, provide details of octadecanoid preparation and measurement, summarize the phase-I metabolic pathway of octadecanoid formation in mammals, bacteria, and fungi, and describe their biological activity in relation to mammalian pathophysiology as well as their potential use as biomarkers of health and disease.

Oxylipin Receptors and Their Role in Inter-Partner Signalling in a Model Cnidarian-Dinoflagellate Symbiosis

Oxylipin signalling is central in biology, mediating processes such as cellular homeostasis, inflammation and molecular signalling. It may also facilitate inter-partner communication in the cnidarian-dinoflagellate symbiosis, though this aspect remains understudied. In this study, four oxylipin receptors were characterised using immunohistochemistry and immunoblotting in the sea anemone Exaiptasia diaphana ('Aiptasia'): Prostaglandin E2 receptor 2 (EP2) and 4 (EP4), Transient Receptor Potential cation channel A1 (TRPA1) and Glutamate Receptor Ionotropic, Kainate 2 (GRIK2). Receptor abundance and localisation were compared between aposymbiotic anemones and symbiotic anemones hosting either native Breviolum minutum or non-native Durusdinium trenchii. All receptors were localised to the putative symbiosome of freshly isolated symbionts, suggesting a role in host-symbiont crosstalk. EP2, EP4 and TRPA1 abundance decreased in the gastrodermis of anemones hosting B. minutum, indicating potential downregulation of pathways mediated by these receptors. In contrast, GRIK2 abundance increased in anemones hosting D. trenchii in both the epidermis and gastrodermis; GRIK2 acts as a chemosensor of potential pathogens in other systems and could play a similar role here given D. trenchii's reputation as a sub-optimal partner for Aiptasia. This study contributes to the understanding of oxylipin signalling in the cnidarian-dinoflagellate symbiosis and supports further exploration of host-symbiont molecular signalling.

Machine learning and biological validation identify sphingolipids as potential mediators of paclitaxel-induced neuropathy in cancer patients

Background

Chemotherapy-induced peripheral neuropathy (CIPN) is a serious therapy-limiting side effect of commonly used anticancer drugs. Previous studies suggest that lipids may play a role in CIPN. Therefore, the present study aimed to identify the particular types of lipids that are regulated as a consequence of paclitaxel administration and may be associated with the occurrence of post-therapeutic neuropathy.

Methods

High-resolution mass spectrometry lipidomics was applied to quantify d=255 different lipid mediators in the blood of n=31 patients drawn before and after paclitaxel therapy for breast cancer treatment. A variety of supervised statistical and machine-learning methods was applied to identify lipids that were regulated during paclitaxel therapy or differed among patients with and without post-therapeutic neuropathy.

Results

Twenty-seven lipids were identified that carried relevant information to train machine learning algorithms to identify, in new cases, whether a blood sample was drawn before or after paclitaxel therapy with a median balanced accuracy of up to 90%. One of the top hits, sphinganine-1-phosphate (SA1P), was found to induce calcium transients in sensory neurons via the transient receptor potential vanilloid 1 (TRPV1) channel and sphingosine-1-phosphate receptors.SA1P also showed different blood concentrations between patients with and without neuropathy.

Conclusions

Present findings suggest a role for sphinganine-1-phosphate in paclitaxel-induced biological changes associated with neuropathic side effects. The identified SA1P, through its receptors, may provide a potential drug target for co-therapy with paclitaxel to reduce one of its major and therapy-limiting side effects.

Funding

This work was supported by the Deutsche Forschungsgemeinschaft (German Research Foundation, DFG, Grants SFB1039 A09 and Z01) and by the Fraunhofer Foundation Project: Neuropathic Pain as well as the Fraunhofer Cluster of Excellence for Immune-Mediated Diseases (CIMD). This work was also supported by the Leistungszentrum Innovative Therapeutics (TheraNova) funded by the Fraunhofer Society and the Hessian Ministry of Science and Arts. Jörn Lötsch was supported by the Deutsche Forschungsgemeinschaft (DFG LO 612/16-1).

Oleic acid released by sensory neurons inhibits TRPV1-mediated thermal hypersensitivity via GPR40

Noxious stimuli activate nociceptive sensory neurons, causing action potential firing and the release of diverse signaling molecules. Several peptides have already been identified to be released by sensory neurons and shown to modulate inflammatory responses and inflammatory pain. However, it is still unclear whether lipid mediators can be released upon sensory neuron activation to modulate intercellular communication. Here, we analyzed the lipid secretome of capsaicin-stimulated nociceptive neurons with LC-HRMS, revealing that oleic acid is strongly released from sensory neurons by capsaicin. We further demonstrated that oleic acid inhibits capsaicin-induced calcium transients in sensory neurons and reverses bradykinin-induced TRPV1 sensitization by a calcineurin (CaN) and GPR40 (FFAR1) dependent pathway. Additionally, oleic acid alleviated zymosan-mediated thermal hypersensitivity via the GPR40, suggesting that the capsaicin-mediated oleic acid release from sensory neurons acts as a protective and feedback mechanism, preventing sensory neurons from nociceptive overstimulation via the GPR40/CaN/TRPV1-axis.

The analgesic effects of Yu-Xue-Bi tablet (YXB) on mice with inflammatory pain by regulating LXA4-FPR2-TRPA1 pathway

BACKGROUND

Oxylipins including lipoxin A4 (LXA4) facilitate the resolution of inflammation and possess analgesic properties by inhibiting macrophage infiltration and transient receptor potential (TRP) protein expression. Yu-Xue-Bi Tablet (YXB) is a traditional Chinese patent medicine used to relieve inflammatory pain. Our previous research has shown that the analgesic effect of YXB is related to inhibiting peripheral inflammation and regulating macrophage infiltration, but the mechanism is not yet clear. The purpose of this study is to explore the mechanisms of YXB on mice models with Complete Freund's Adjuvant (CFA)-induced inflammatory pain from the perspective at the resolution of inflammation.

METHODS

Mechanical allodynia thresholds and heat hypersensitivity were measured using the Von Frey test and the hot plate test respectively. The open field test and the tail suspension test were employed to measure anxiety and depressive behaviors respectively. The expression of CD68+ and the proportion of F4/80+CD11b+ cells were measured by immunofluorescence staining and flow cytometry. The expression of transient receptor potential ankyrin 1(TRPA1) was measured by immunofluorescence staining and western blotting. Oxylipins omics analysis provided quantitative data on oxylipins in the paws, and enzyme linked immunosorbent assay (ELISA) was used to measure the levels of LXA4 there. Immunofluorescence staining was used to perform the expression of Leukotriene A4 hydroxylase (LTA4H) in the paws of mice. The impact of injecting the formyl peptide receptor 2(FPR2) antagonist WRW4 and the TRPA1 agonist AITC into the left paws was observed, focusing on the expression of mechanical allodynia thresholds, the expression of CD68+, TRPA1 in the paws, and Calcitonin gene-related peptide (CGRP) in the L5 spinal dorsal horn.

RESULTS

YXB elevated mechanical allodynia thresholds, alleviated heat hypersensitivity and anxiety and depressive behaviors in CFA mice. It significantly reduced the number of CD68+ and proportion of F4/80+CD11b+ within the paws, thereby decreasing macrophage infiltration. Additionally, it diminished the expression of TRPA1 in the paws and TRPV1 in the DRG, leading to an inhibition of peripheral sensitization. Through quantitative analysis, it was found that YXB could modulate DHA-derived oxylipins and LXA4. ELISA results indicated that YXB elevated the levels of LXA4 and inhibited the expression of LAT4H in the paws. Furthermore, the pro-resolution and analgesic effects of YXB were hindered after administration of the FPR2 antagonist. Compared with the AITC group, YXB showed no significant improvement in anti-inflammatory and analgesic effects.

CONCLUSIONS

YXB can regulate the oxylipins of paws in CFA mice to promote the resolution of inflammation. The LXA4-FPR2-TRPA1 pathway is a key mechanism for the resolution of inflammation and analgesic effects.

Synovial 5-Lipoxygenase-Derived Oxylipins Define a Lympho-Myeloid-Enriched Synovium

OBJECTIVE

Oxylipins are bioactive lipids derived from polyunsaturated fatty acids (PUFAs) that modulate inflammation and may remain overexpressed in refractory synovitis. In plasma, they could also be biomarkers of synovial pathology. The aim of this study is to determine if synovial oxylipins in inflamed joints correlate with plasma oxylipins and with synovial histologic patterns.

METHODS

Patients with established rheumatoid or psoriatic arthritis with active disease despite treatment were recruited, and paired synovial tissue (ST) and plasma were collected. Oxylipins were determined by liquid chromatography with tandem mass spectrometry and were classified into groups according to their PUFA precursor and enzyme. The expression of CD20, CD68, CD3, and CD138 was obtained to describe synovial histology. Cell-specific expression of oxylipin-related genes was identified by examining available synovial single-cell RNA sequencing data.

RESULTS

We included a total of 32 ST and 26 paired-plasma samples. A total of 71 oxylipins were identified in ST, but only 24 were identified in plasma. Only levels of 9,10-dihydroxyoctadecenoic acid and tetranor-Prostaglandin FM had a significant positive correlation between plasma and ST. Several oxylipins and oxylipin-related genes were differentially expressed among synovial phenotypes. Specifically, several 5-lipoxygenase (LOX)-derived oxylipins were statistically elevated in the lympho-myeloid phenotype and associated with B cell expression in rheumatoid arthritis samples.

CONCLUSION

The lack of correlation between ST and plasma oxylipins suggests that ST lipid profiling better characterizes active pathways in treated joints. Synovial 5-LOX-derived oxylipins were highly expressed in lympho-myeloid-enriched synovium. Combination therapy with 5-LOX inhibitors to improve refractory inflammation may be needed in patients with this histologic group.

The role of orphan G protein-coupled receptors in pain

G protein-coupled receptors (GPCRs), which form the largest family of membrane protein receptors in humans, are highly complex signaling systems with intricate structures and dynamic conformations and locations. Among these receptors, a specific subset is referred to as orphan GPCRs (oGPCRs) and has garnered significant interest in pain research due to their role in both central and peripheral nervous system function. The diversity of GPCR functions is attributed to multiple factors, including allosteric modulators, signaling bias, oligomerization, constitutive signaling, and compartmentalized signaling. This review primarily focuses on the recent advances in oGPCR research on pain mechanisms, discussing the role of specific oGPCRs including GPR34, GPR37, GPR65, GPR83, GPR84, GPR85, GPR132, GPR151, GPR160, GPR171, GPR177, and GPR183. The orphan receptors among these receptors associated with central nervous system diseases are also briefly described. Understanding the functions of these oGPCRs can contribute not only to a deeper understanding of pain mechanisms but also offer a reference for discovering new targets for pain treatment.

Neural circuits of long-term thermoregulatory adaptations to cold temperatures and metabolic demands

The mammalian brain controls heat generation and heat loss mechanisms that regulate body temperature and energy metabolism. Thermoeffectors include brown adipose tissue, cutaneous blood flow and skeletal muscle, and metabolic energy sources include white adipose tissue. Neural and metabolic pathways modulating the activity and functional plasticity of these mechanisms contribute not only to the optimization of function during acute challenges, such as ambient temperature changes, infection and stress, but also to longitudinal adaptations to environmental and internal changes. Exposure of humans to repeated and seasonal cold ambient conditions leads to adaptations in thermoeffectors such as habituation of cutaneous vasoconstriction and shivering. In animals that undergo hibernation and torpor, neurally regulated metabolic and thermoregulatory adaptations enable survival during periods of significant reduction in metabolic rate. In addition, changes in diet can activate accessory neural pathways that alter thermoeffector activity. This knowledge may be harnessed for therapeutic purposes, including treatments for obesity and improved means of therapeutic hypothermia.

Eicosanoid profiles in an arthritis model: Effects of a soluble epoxide hydrolase inhibitor

Rheumatoid arthritis is a common systemic inflammatory autoimmune disease characterized by damage to joints, inflammation and pain. It is driven by an increase of inflammatory cytokines and lipids mediators such as prostaglandins. Epoxides of polyunsaturated fatty acids (PUFAs) are lipid chemical mediators in a group of regulatory compounds termed eicosanoids. These epoxy fatty acids (EpFA) have resolutive functions but are rapidly metabolized by the soluble epoxide hydrolase enzyme (sEH) into the corresponding diols. The pharmacological inhibition of sEH stabilizes EpFA from hydrolysis, improving their half-lives and biological effects. These anti-inflammatory EpFA, are analgesic in neuropathic and inflammatory pain conditions. Nonetheless, inhibition of sEH on arthritis and the resulting effects on eicosanoids profiles are little explored despite the physiological importance. In this study, we investigated the effect of sEH inhibition on collagen-induced arthritis (CIA) and its impact on the plasma eicosanoid profile. We measured the eicosanoid metabolites by LC-MS/MS-based lipidomic analysis. The treatment with a sEH inhibitor significantly modulated 11 out of 69 eicosanoids, including increased epoxides 12(13)-EpODE, 12(13)-EpOME, 13-oxo-ODE, 15-HEPE, 20-COOH-LTB4 and decreases several diols 15,6-DiHODE, 12,13-DiHOME, 14,15-DiHETrE, 5,6-DiHETrE and 16,17-DiHDPE. Overall the inhibition of sEH in the rheumatoid arthritis model enhanced epoxides generally considered anti-inflammatory or resolutive mediators and decreased several diols with inflammatory features. These findings support the hypothesis that inhibiting the sEH increases systemic EpFA levels, advancing the understanding of the impact of these lipid mediators as therapeutical targets.

9-HODE and 9-HOTrE alter mitochondrial metabolism, increase triglycerides, and perturb fatty acid uptake and synthesis associated gene expression in HepG2 cells

Non-Alcoholic Fatty Liver Disease (NAFLD) prevalence is rising and can lead to detrimental health outcomes such as Non-Alcoholic Steatohepatitis (NASH), cirrhosis, and cancer. Recent studies have indicated that Cytochrome P450 2B6 (CYP2B6) is an anti-obesity CYP in humans and mice. Cyp2b-null mice are diet-induced obese, and human CYP2B6-transgenic (hCYP2B6-Tg) mice reverse the obesity or diabetes progression, but with increased liver triglyceride accumulation in association with an increase of several oxylipins. Notably, 9-hydroxyoctadecadienoic acid (9-HODE) produced from linoleic acid (LA, 18:2, ω-6) is the most prominent of these and 9-hydroxyoctadecatrienoic acid (9-HOTrE) from alpha-linolenic acid (ALA, 18:3, ω-3) is the most preferentially produced when controlling for substrate concentrations in vitro. Transactivation assays indicate that 9-HODE and 9-HOTrE activate PPARα and PPARγ. In Seahorse assays performed in HepG2 cells, 9-HOTrE increased spare respiratory capacity, slightly decreased palmitate metabolism, and increased non-glycolytic acidification in a manner consistent with slightly increased glutamine utilization; however, 9-HODE exhibited no effect on metabolism. Both compounds increased triglyceride and pyruvate concentrations, most strongly by 9-HOTrE, consistent with increased spare respiratory capacity. qPCR analysis revealed several perturbations in fatty acid uptake and metabolism gene expression. 9-HODE increased expression of CD36, FASN, PPARγ, and FoxA2 that are involved in lipid uptake and production. 9-HOTrE decreased ANGPTL4 expression and increased FASN expression consistent with increased fatty acid uptake, fatty acid production, and AMPK activation. Our findings support the hypothesis that 9-HODE and 9-HOTrE promote steatosis, but through different mechanisms as 9-HODE is directly involved in fatty acid uptake and synthesis; 9-HOTrE weakly inhibits mitochondrial fatty acid metabolism while increasing glutamine use.

Lipid mediators of inhalation exposure-induced pulmonary toxicity and inflammation

Many inhalation exposures induce pulmonary inflammation contributing to disease progression. Inflammatory processes are actively regulated via mediators including bioactive lipids. Bioactive lipids are potent signaling molecules involved in both pro-inflammatory and resolution processes through receptor interactions. The formation and clearance of lipid signaling mediators are controlled by multiple metabolic enzymes. An imbalance of these lipids can result in exacerbated and sustained inflammatory processes which may result in pulmonary damage and disease. Dysregulation of pulmonary bioactive lipids contribute to inflammation and pulmonary toxicity following exposures. For example, inhalation of cigarette smoke induces activation of pro-inflammatory bioactive lipids such as sphingolipids, and ceramides contributing to chronic obstructive pulmonary disease. Additionally, exposure to silver nanoparticles causes dysregulation of inflammatory resolution lipids. As inflammation is a common consequence resulting from inhaled exposures and a component of numerous diseases it represents a broadly applicable target for therapeutic intervention. With new appreciation for bioactive lipids, technological advances to reliably identify and quantify lipids have occurred. In this review, we will summarize, integrate, and discuss findings from recent studies investigating the impact of inhaled exposures on pro-inflammatory and resolution lipids within the lung and their contribution to disease. Throughout the review current knowledge gaps in our understanding of bioactive lipids and their contribution to pulmonary effects of inhaled exposures will be presented. New methods being employed to detect and quantify disruption of pulmonary lipid levels following inhalation exposures will be highlighted. Lastly, we will describe how lipid dysregulation could potentially be addressed by therapeutic strategies to address inflammation.

Metabolite Biomarkers of Prolonged and Intensified Pain and Distress in Head and Neck Cancer Patients Undergoing Radio- or Chemoradiotherapy by Means of NMR-Based Metabolomics-A Preliminary Study

Treatment of head and neck squamous cell carcinoma (HNSCC) has a detrimental impact on patient quality of life. The rate of recognized distress/depression among HNSCC patients ranges from 9.8% to 83.8%, and the estimated prevalence of depression among patients receiving radiotherapy is 63%. Shorter overall survival also occurs in preexisting depression or depressive conditions. The present study analyzes the nuclear magnetic resonance (NMR) blood serum metabolic profiles during radio-/chemoradiotherapy and correlates the detected alterations with pain and/or distress accumulated with the disease and its treatment. NMR spectra were acquired on a Bruker 400 MHz spectrometer and analyzed using multivariate methods. The results indicate that distress and/or pain primarily affect the serum lipids and metabolites of energy (glutamine, glucose, lactate, acetate) and one-carbon (glycine, choline, betaine, methanol, threonine, serine, histidine, formate) metabolism. Sparse disturbances in the branched-chain amino acids (BCAA) and in the metabolites involved in protein metabolism (lysine, tyrosine, phenylalanine) are also observed. Depending on the treatment modality-radiotherapy or concurrent chemoradiotherapy-there are some differences in the altered metabolites.

Biofilm-derived oxylipin 10-HOME-mediated immune response in women with breast implants

This study investigates a mechanistic link of bacterial biofilm-mediated host-pathogen interaction leading to immunological complications associated with breast implant illness (BII). Over 10 million women worldwide have breast implants. In recent years, women have described a constellation of immunological symptoms believed to be related to their breast implants. We report that periprosthetic breast tissue of participants with symptoms associated with BII had increased abundance of biofilm and biofilm-derived oxylipin 10-HOME compared with participants with implants who are without symptoms (non-BII) and participants without implants. S. epidermidis biofilm was observed to be higher in the BII group compared with the non-BII group and the normal tissue group. Oxylipin 10-HOME was found to be immunogenically capable of polarizing naive CD4+ T cells with a resulting Th1 subtype in vitro and in vivo. Consistently, an abundance of CD4+Th1 subtype was observed in the periprosthetic breast tissue and blood of people in the BII group. Mice injected with 10-HOME also had increased Th1 subtype in their blood, akin to patients with BII, and demonstrated fatigue-like symptoms. The identification of an oxylipin-mediated mechanism of immune activation induced by local bacterial biofilm provides insight into the possible pathogenesis of the implant-associated immune symptoms of BII.

SAFit2 ameliorates paclitaxel-induced neuropathic pain by reducing spinal gliosis and elevating pro-resolving lipid mediators

BACKGROUND

Chemotherapy-induced neuropathic pain (CIPN) describes a pathological pain state that occurs dose-dependently as a side effect and can limit or even impede an effective cancer therapy. Unfortunately, current treatment possibilities for CIPN are remarkably confined and mostly inadequate as CIPN therapeutics themselves consist of low effectiveness and may induce severe side effects, pointing out CIPN as pathological entity with an emerging need for novel treatment targets. Here, we investigated whether the novel and highly specific FKBP51 inhibitor SAFit2 reduces paclitaxel-induced neuropathic pain.

METHODS

In this study, we used a well-established multiple low-dose paclitaxel model to investigate analgesic and anti-inflammatory properties of SAFit2. For this purpose, the behavior of the mice was recorded over 14 days and the mouse tissue was then analyzed using biochemical methods.

RESULTS

Here, we show that SAFit2 is capable to reduce paclitaxel-induced mechanical hypersensitivity in mice. In addition, we detected that SAFit2 shifts lipid levels in nervous tissue toward an anti-inflammatory and pro-resolving lipid profile that counteracts peripheral sensitization after paclitaxel treatment. Furthermore, SAFit2 reduced the activation of astrocytes and microglia in the spinal cord as well as the levels of pain-mediating chemokines. Its treatment also increased anti-inflammatory cytokines levels in neuronal tissues, ultimately leading to a resolution of neuroinflammation.

CONCLUSIONS

In summary, SAFit2 shows antihyperalgesic properties as it ameliorates paclitaxel-induced neuropathic pain by reducing peripheral sensitization and resolving neuroinflammation. Therefore, we consider SAFit2 as a potential novel drug candidate for the treatment of paclitaxel-induced neuropathic pain.

Endogenous Derivatives of Linoleic Acid and their Stable Analogs Are Potential Pain Mediators

Psoriasis is characterized by intense pruritus, with a subset of individuals with psoriasis experiencing thermal hypersensitivity. However, the pathophysiology of thermal hypersensitivity in psoriasis and other skin conditions remains enigmatic. Linoleic acid is an omega-6 fatty acid that is concentrated in the skin, and oxidation of linoleic acid into metabolites with multiple hydroxyl and epoxide functional groups has been shown to play a role in skin barrier function. Previously, we identified several linoleic acid‒derived mediators that were more concentrated in psoriatic lesions, but the role of these lipids in psoriasis remains unknown. In this study, we report that two such compounds-9,10-epoxy-13-hydroxy-octadecenoate and 9,10,13-trihydroxy-octadecenoate-are present as free fatty acids and induce nociceptive behavior in mice but not in rats. By chemically stabilizing 9,10-epoxy-13-hydroxy-octadecenoate and 9,10,13-trihydroxy-octadecenoate through the addition of methyl groups, we observed pain and hypersensitization in mice. The nociceptive responses suggest an involvement of the TRPA1 channel, whereas hypersensitive responses induced by these mediators may require both TRPA1 and TRPV1 channels. Furthermore, we showed that 9,10,13-trihydroxy-octadecenoate‒induced calcium transients in sensory neurons are mediated through the Gβγ subunit of an unidentified G-protein coupled receptor (GPCR). Overall, mechanistic insights from this study will guide the development of potential therapeutic targets for the treatment of pain and hypersensitivity.

Iron Metabolism and Ferroptosis in Peripheral Nerve Injury

Peripheral nerve injury (PNI) is a major clinical problem that may lead to different levels of sensory and motor dysfunction including paralysis. Due to the high disability rate and unsatisfactory prognosis, the exploration and revealment of the mechanisms involved in the PNI are urgently required. Ferroptosis, a recently identified novel form of cell death, is an iron-dependent process. It is a unique modality of cell death, closely associated with iron concentrations, generation of reactive oxygen species, and accumulation of the lipid reactive oxygen species. These processes are regulated by multiple cellular metabolic pathways, including iron overloading, lipid peroxidation, and the glutathione/glutathione peroxidase 4 pathway. Furthermore, ferroptosis is accompanied by morphological changes in the mitochondria, such as increased membrane density and shrunken mitochondria; this association between ferroptosis and mitochondrial damage has been detected in various diseases, including spinal cord injury and PNI. The inhibition of ferroptosis can promote the repair of damaged peripheral nerves, reduce mitochondrial damage, and promote the recovery of neurological function. In this review, we intend to discuss the detailed mechanisms of ferroptosis and summarize the current researches on ferroptosis with respect to nerve injury. This review also aims at providing new insights on targeting ferroptosis for PNI treatment.

The FKBP51 Inhibitor SAFit2 Restores the Pain-Relieving C16 Dihydroceramide after Nerve Injury

Neuropathic pain is a pathological pain state with a broad symptom scope that affects patients after nerve injuries, but it can also arise after infections or exposure to toxic substances. Current treatment possibilities are still limited because of the low efficacy and severe adverse effects of available therapeutics, highlighting an emerging need for novel analgesics and for a detailed understanding of the pathophysiological alterations in the onset and maintenance of neuropathic pain. Here, we show that the novel and highly specific FKBP51 inhibitor SAFit2 restores lipid signaling and metabolism in nervous tissue after nerve injury. More specifically, we identify that SAFit2 restores the levels of the C16 dihydroceramide, which significantly reduces the sensitization of the pain-mediating TRPV1 channel and subsequently the secretion of the pro-inflammatory neuropeptide CGRP in primary sensory neurons. Furthermore, we show that the C16 dihydroceramide is capable of reducing acute thermal hypersensitivity in a capsaicin mouse model. In conclusion, we report for the first time the C16 dihydroceramide as a novel and crucial lipid mediator in the context of neuropathic pain as it has analgesic properties, contributing to the pain-relieving properties of SAFit2.

Long COVID and the Neuroendocrinology of Microbial Translocation Outside the GI Tract: Some Treatment Strategies

Similar to previous pandemics, COVID-19 has been succeeded by well-documented post-infectious sequelae, including chronic fatigue, cough, shortness of breath, myalgia, and concentration difficulties, which may last 5 to 12 weeks or longer after the acute phase of illness. Both the psychological stress of SARS-CoV-2 infection and being diagnosed with COVID-19 can upregulate cortisol, a stress hormone that disrupts the efferocytosis effectors, macrophages, and natural killer cells, leading to the excessive accumulation of senescent cells and disruption of biological barriers. This has been well-established in cancer patients who often experience unrelenting fatigue as well as gut and blood–brain barrier dysfunction upon treatment with senescence-inducing radiation or chemotherapy. In our previous research from 2020 and 2021, we linked COVID-19 to myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) via angiotensin II upregulation, premature endothelial senescence, intestinal barrier dysfunction, and microbial translocation from the gastrointestinal tract into the systemic circulation. In 2021 and 2022, these hypotheses were validated and SARS-CoV-2-induced cellular senescence as well as microbial translocation were documented in both acute SARS-CoV-2 infection, long COVID, and ME/CFS, connecting intestinal barrier dysfunction to disabling fatigue and specific infectious events. The purpose of this narrative review is to summarize what is currently known about host immune responses to translocated gut microbes and how these responses relate to fatiguing illnesses, including long COVID. To accomplish this goal, we examine the role of intestinal and blood–brain barriers in long COVID and other illnesses typified by chronic fatigue, with a special emphasis on commensal microbes functioning as viral reservoirs. Furthermore, we discuss the role of SARS-CoV-2/Mycoplasma coinfection in dysfunctional efferocytosis, emphasizing some potential novel treatment strategies, including the use of senotherapeutic drugs, HMGB1 inhibitors, Toll-like receptor 4 (TLR4) blockers, and membrane lipid replacement.

Excessive dietary linoleic acid promotes plasma accumulation of pronociceptive fatty acyl lipid mediators

Various fatty acyl lipid mediators are derived from dietary polyunsaturated fatty acids (PUFAs) and modulate nociception. The modern diet is rich in linoleic acid, which is associated with nociceptive hypersensitivities and may present a risk factor for developing pain conditions. Although recommendations about fatty acid intake exist for some diseases (e.g. cardiovascular disease), the role of dietary fatty acids in promoting pain disorders is not completely understood. To determine how dietary linoleic acid content influences the accumulation of pro- and anti-nociceptive fatty acyl lipid mediators, we created novel rodent diets using custom triglyceride blends rich in either linoleic acid or oleic acid. We quantified the fatty acyl lipidome in plasma of male and female rats fed these custom diets from the time of weaning through nine weeks of age. Dietary fatty acid composition determined circulating plasma fatty acyl lipidome content. Exposure to a diet rich in linoleic acid was associated with accumulation of linoleic and arachidonic acid-derived pro-nociceptive lipid mediators and reduction of anti-nociceptive lipid mediators derived from the omega-3 PUFAs. Our findings provide mechanistic insights into exaggerated nociceptive hypersensitivity associated with excessive dietary linoleic acid intake and highlight potential biomarkers for pain risk stratification.

Characterization of plasma metabolites and proteins in patients with herpetic neuralgia and development of machine learning predictive models based on metabolomic profiling

Herpes zoster (HZ) is a localized, painful cutaneous eruption that occurs upon reactivation of the herpes virus. Postherpetic neuralgia (PHN) is the most common chronic complication of HZ. In this study, we examined the metabolomic and proteomic signatures of disease progression in patients with HZ and PHN. We identified differentially expressed metabolites (DEMs), differentially expressed proteins (DEPs), and key signaling pathways that transition from healthy volunteers to the acute or/and chronic phases of herpetic neuralgia. Moreover, some specific metabolites correlated with pain scores, disease duration, age, and pain in sex dimorphism. In addition, we developed and validated three optimal predictive models (AUC > 0.9) for classifying HZ and PHN from healthy individuals based on metabolic patterns and machine learning. These findings may reveal the overall metabolomics and proteomics landscapes and proposed the optimal machine learning predictive models, which provide insights into the mechanisms of HZ and PHN.

A Metabolomic Study of the Analgesic Effect of Lappaconitine Hydrobromide (LAH) on Inflammatory Pain

Lappaconitine (LA) is a C-18 diterpene alkaloid isolated from Aconitum sinomontanum Nakai that has been shown to relieve mild to moderate discomfort. Various researchers have tried to explain the underlying mechanism of LA's effects on chronic pain. This article uses metabolomics technology to investigate the metabolite alterations in the dorsal root ganglion (DRG) when lappaconitine hydrobromide (LAH) was injected in an inflammatory pain model, to explain the molecular mechanism of its analgesia from a metabolomics perspective. The pain model used in this study was a complete Freund's adjuvant (CFA)-induced inflammatory pain model in rats. There were two treatment groups receiving different dosages of LAH (4 mg/kg LAH and 8 mg/kg LAH). The analgesic mechanism of LAH was investigated with an analgesic behavioral test, tissue sections, and metabolomics. The results of the analgesic behavioral experiment showed that both 4 mg/kg LAH and 8 mg/kg LAH could significantly improve the paw withdrawal latency (PWL) of rats. The tissue section results showed that LAH could reduce the inflammatory response and enlargement of the paw and ankle of rats and that there was no significant difference in the tissue sections of the DRG. The metabolomics results showed that retinol metabolism and glycerophospholipid metabolism in the CFA-induced inflammatory pain model were significantly affected and may exacerbate the inflammatory reactions and initiate persistent pain; in addition, the linoleic acid metabolism, arachidonic acid metabolism, and alanine, aspartate, and glutamate metabolism were also slightly affected. Among them, the alpha-linolenic acid metabolism was up-regulated after LAH treatment, while the retinol metabolism was down-regulated. These results suggest that LAH could effectively reduce inflammatory pain and might achieve this by regulating the lipid metabolism in the rat DRG.

Circulating polyunsaturated fatty acids, pressure pain thresholds, and nociplastic pain conditions

OBJECTIVE

Polyunsaturated fatty acids (PUFAs) play a role in pain regulation. This study sought to determine whether free PUFAs found in red blood cells also play a role in nociceptive processing. We examined associations between circulating PUFAs and nociceptive thresholds to noxious mechanical stimuli. We also determined whether nociceptive thresholds were associated with nociplastic pain conditions.

METHODS

This cross-sectional study used stored red bloods cells and data from 605 adult participants in the OPPERA-2 study of chronic overlapping pain conditions. In OPPERA-2 adults completed quantitative sensory testing in which pressure algometry measured deep muscular tissue sensitivity at six anatomical sites. Standardized protocols classified adults for presence or absence of five nociplastic pain conditions: temporomandibular disorder, headache, low back pain, irritable bowel syndrome and fibromyalgia. Liquid chromatography tandem mass spectroscopy quantified erythrocyte PUFAs. We conducted three sets of analyses. First, a multivariable linear regression model assessed the association between n-6/n-3 PUFA ratio and the number of overlapping nociplastic pain conditions. Second, a series of 36 multivariable linear regression models assessed covariate-adjusted associations between PUFAs and nociceptive thresholds at each of six anatomical sites. Third, a series of 30 multivariable linear regression models assessed covariate-adjusted associations between nociceptive thresholds at six anatomical sites and each of five pain conditions.

RESULTS

In multiple linear regression, each unit increase in n-6/n-3 PUFA ratio was associated with more pain conditions (β = 0.30, 95% confidence limits: 0.07, 0.53, p = 0.012). Omega-6 linoleic acid and arachidonic acid were negatively associated with lower nociceptive thresholds at three and at five, respectively, anatomical sites. In contrast, omega-3 alpha-linolenic acid, eicosapentaenoic acid, docosahexaenoic acid and the n-6/n-3 PUFA ratio were not associated with nociceptive thresholds at any site. Pain cases had significantly lower nociceptive thresholds than non-case controls at all anatomical sites.

CONCLUSION

A higher n-6/n-3 PUFA ratio was associated with more pain conditions. Omega-6 PUFAs may promote a generalized upregulation of nociceptive processing.

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.

The Influence of Dietary Supplementations on Neuropathic Pain

Neuropathic pain is defined as pain caused by a lesion or disease of the somatosensory nervous system and affects 7-10% of the worldwide population. Neuropathic pain can be induced by the use of drugs, including taxanes, thus triggering chemotherapy-induced neuropathic pain or as consequence of metabolic disorders such as diabetes. Neuropathic pain is most often a chronic condition, and can be associated with anxiety and depression; thus, it negatively impacts quality of life. Several pharmacologic approaches exist; however, they can lead numerous adverse effects. From this perspective, the use of nutraceuticals and diet supplements can be helpful in relieve neuropathic pain and related symptoms. In this review, we discuss how diet can radically affect peripheral neuropathy, and we focus on the potential approaches to ameliorate this condition, such as the use of numerous nutritional supplements or probiotics.

Lysophosphatidylcholine: Potential Target for the Treatment of Chronic Pain

The bioactive lipid lysophosphatidylcholine (LPC), a major phospholipid component of oxidized low-density lipoprotein (Ox-LDL), originates from the cleavage of phosphatidylcholine by phospholipase A2 (PLA2) and is catabolized to other substances by different enzymatic pathways. LPC exerts pleiotropic effects mediated by its receptors, G protein-coupled signaling receptors, Toll-like receptors, and ion channels to activate several second messengers. Lysophosphatidylcholine (LPC) is increasingly considered a key marker/factor positively in pathological states, especially inflammation and atherosclerosis development. Current studies have indicated that the injury of nervous tissues promotes oxidative stress and lipid peroxidation, as well as excessive accumulation of LPC, enhancing the membrane hyperexcitability to induce chronic pain, which may be recognized as one of the hallmarks of chronic pain. However, findings from lipidomic studies of LPC have been lacking in the context of chronic pain. In this review, we focus in some detail on LPC sources, biochemical pathways, and the signal-transduction system. Moreover, we outline the detection methods of LPC for accurate analysis of each individual LPC species and reveal the pathophysiological implication of LPC in chronic pain, which makes it an interesting target for biomarkers and the development of medicine regarding chronic pain.

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.

Serum Metabolomics Reveals Dysregulation and Diagnostic Potential of Oxylipins in Tumor-induced Osteomalacia

CONTEXT

Excessive production of fibroblast growth factor 23 (FGF23) by a tumor is considered the main pathogenesis in tumor-induced osteomalacia (TIO). Despite its importance to comprehensive understanding of pathogenesis and diagnosis, the regulation of systemic metabolism in TIO remains unclear.

OBJECTIVE

We aimed to systematically characterize the metabolome alteration associated with TIO.

METHODS

By means of liquid chromatography-tandem mass spectrometry-based metabolomics, we analyzed the metabolic profile from 96 serum samples (32 from TIO patients at initial diagnosis, pairwise samples after tumor resection, and 32 matched healthy control (HC) subjects). In order to screen and evaluate potential biomarkers, statistical analyses, pathway enrichment and receiver operating characteristic (ROC) were performed.

RESULTS

Metabolomic profiling revealed distinct alterations between TIO and HC cohorts. Differential metabolites were screened and conducted to functional clustering and annotation. A significantly enriched pathway was found involving arachidonic acid metabolism. A combination of 5 oxylipins, 4-HDoHE, leukotriene B4, 5-HETE, 17-HETE, and 9,10,13-TriHOME, demonstrated a high sensitivity and specificity panel for TIO prediction screened by random forest algorithm (AUC = 0.951; 95% CI, 0.827-1). Supported vector machine modeling and partial least squares modeling were conducted to validate the predictive capabilities of the diagnostic panel.

CONCLUSION

Metabolite profiling of TIO showed significant alterations compared with HC. A high-sensitivity and high-specificity panel with 5 oxylipins was tested as diagnostic predictor. For the first time, we provide the global profile of metabolomes and identify potential diagnostic biomarkers of TIO. The present work may offer novel insights into the pathogenesis of TIO.

Clinical and Preclinical Evidence for Roles of Soluble Epoxide Hydrolase in Osteoarthritis Knee Pain

OBJECTIVE

Chronic pain due to osteoarthritis (OA) is a major clinical problem, and existing analgesics often have limited beneficial effects and/or adverse effects, necessitating the development of novel therapies. Epoxyeicosatrienoic acids (EETs) are endogenous antiinflammatory mediators, rapidly metabolized by soluble epoxide hydrolase (EH) to dihydroxyeicosatrienoic acids (DHETs). We undertook this study to assess whether soluble EH-driven metabolism of EETs to DHETs plays a critical role in chronic joint pain associated with OA and provides a new target for treatment.

METHODS

Potential associations of chronic knee pain with single-nucleotide polymorphisms (SNPs) in the gene-encoding soluble EH and with circulating levels of EETs and DHETs were investigated in human subjects. A surgically induced murine model of OA was used to determine the effects of both acute and chronic selective inhibition of soluble EH by N-[1-(1-oxopropy)-4-piperidinyl]-N'-(trifluoromethoxy)phenyl]-urea (TPPU) on weight-bearing asymmetry, hind paw withdrawal thresholds, joint histology, and circulating concentrations of EETs and DHETs.

RESULTS

In human subjects with chronic knee pain, 3 pain measures were associated with SNPs of the soluble EH gene EPHX2, and in 2 separate cohorts of subjects, circulating levels of EETs and DHETs were also associated with 3 pain measures. In the murine OA model, systemic administration of TPPU both acutely and chronically reversed established pain behaviors and decreased circulating levels of 8,9-DHET and 14,15-DHET. EET levels were unchanged by TPPU administration.

CONCLUSION

Our novel findings support a role of soluble EH in OA pain and suggest that inhibition of soluble EH and protection of endogenous EETs from catabolism represents a potential new therapeutic target for OA pain.

Drug Repurposing to Target Neuroinflammation and Sensory Neuron-Dependent Pain

Around 20% of the American population have chronic pain and estimates in other Western countries report similar numbers. This represents a major challenge for global health care systems. Additional problems for the treatment of chronic and persistent pain are the comparably low efficacy of existing therapies, the failure to translate effects observed in preclinical pain models to human patients and related setbacks in clinical trials from previous attempts to develop novel analgesics. Drug repurposing offers an alternative approach to identify novel analgesics as it can bypass various steps of classical drug development. In recent years, several approved drugs were attributed analgesic properties. Here, we review available data and discuss recent findings suggesting that the approved drugs minocycline, fingolimod, pioglitazone, nilotinib, telmisartan, and others, which were originally developed for the treatment of different pathologies, can have analgesic, antihyperalgesic, or neuroprotective effects in preclinical and clinical models of inflammatory or neuropathic pain. For our analysis, we subdivide the drugs into substances that can target neuroinflammation or substances that can act on peripheral sensory neurons, and highlight the proposed mechanisms. Finally, we discuss the merits and challenges of drug repurposing for the development of novel analgesics.

Lipidomic Profiling Identifies Serum Lipids Associated with Persistent Multisite Musculoskeletal Pain

Lipid mediators have been suggested to have a role in pain sensitivity and response; however, longitudinal data on lipid metabolites and persistent multisite musculoskeletal pain (MSMP) are lacking. This study was to identify lipid metabolic markers for persistent MSMP. Lipidomic profiling of 807 lipid species was performed on serum samples of 536 participants from a cohort study. MSMP was measured by a questionnaire and defined as painful sites ≥4. Persistent MSMP was defined as having MSMP at every visit. Logistic regression was used with adjustment for potential confounders. The Benjamini–Hochberg method was used to control for multiple testing. A total of 530 samples with 807 lipid metabolites passed quality control. Mean age at baseline was 61.54 ± 6.57 years and 50% were females. In total, 112 (21%) of the participants had persistent MSMP. Persistent MSMP was significantly associated with lower levels of monohexosylceramide (HexCer)(d18:1/22:0 and d18:1/24:0), acylcarnitine (AC)(26:0) and lysophosphatidylcholine (LPC)(18:1 [sn1], 18:2 [sn1], 18:2 [sn2], and 15-MHDA[sn1] [104_sn1]) after controlling for multiple testing. After adjustment for age, sex, body mass index, comorbidities, and physical activity, HexCer(d18:1/22:0 and d18:1/24:0) and LPC(15-MHDA [sn1] [104_sn1]) were significantly associated with persistent MSMP [Odds Ratio (OR) ranging from 0.25–0.36]. Two lipid classes—HexCer and LPC—were negatively associated with persistent MSMP after adjustment for covariates (OR = 0.22 and 0.27, respectively). This study identified three novel lipid signatures of persistent MSMP, suggesting that lipid metabolism is involved in the pathogenesis of persistent pain.

Integrated 16S rRNA Gene Sequencing and Metabolomics Analysis to Investigate the Important Role of Osthole on Gut Microbiota and Serum Metabolites in Neuropathic Pain Mice

Accumulating evidence suggests that neuropathic pain (NP) is closely connected to the metabolic disorder of gut microbiota, and natural products could relieve NP by regulating gut microbiota. The purpose of this study is to investigate the important regulatory effects of osthole on gut microbiota and serum metabolites in mice with chronic constriction injury (CCI). Mice's intestinal contents and serum metabolites were collected from the sham group, CCI group, and osthole treatment CCI group. The 16S rRNA gene sequencing was analyzed, based on Illumina NovaSeq platform, and ANOVA analysis were used to analyze the composition variety and screen differential expression of intestinal bacteria in the three groups. Ultra-high-performance liquid chromatography-quadrupole time of flight-tandem mass spectrometry (UHPLC-Q-TOF-MS) was used for analyzing the data obtained from serum specimens, and KEGG enrichment analysis was used to identify pathways of differential metabolites in the treatment of neuralgia mice. Furthermore, the Pearson method and Cytoscape soft were used to analyze the correlation network of differential metabolites, gut microbiota, and disease genes. The analysis results of 16S rRNA gene sequencing displayed that Bacteroidetes, Firmicutes, and Verrucomicrobia were highly correlated with NP after osthole treatment at the phylum level. Akkermansia, Lachnospiraceae_unclassified, Lachnospiraceae_NK4A136_group, Bacteroides, Lactobacillus, and Clostridiales_unclassified exhibited higher relative abundance and were considered important microbial members at genus level in neuralgia mice. Serum metabolomics results showed that 131 metabolites were considered to be significantly different in the CCI group compared to the sham group, and 44 metabolites were significantly expressed between the osthole treatment group and the CCI group. At the same time, we found that 29 differential metabolites in the two comparison groups were overlapping. Integrated analysis results showed that many intestinal microorganisms and metabolites have a strong positive correlation. The correlation network diagram displays that 10 genes were involved in the process of osthole alleviating NP through a metabolic pathway and gut microbiota, including IGF2, GDAP1, MYLK, IL18, CD55, MIR331, FHIT, F3, ERBB4, and ITGB3. Our findings have preliminarily confirmed that NP is closely related to metabolism and intestinal microbial imbalance, and osthole can improve the metabolic disorder of NP by acting on gut microbiota.

Oxidized linoleic acid metabolites maintain mechanical and thermal hypersensitivity during sub-chronic inflammatory pain

Inflammatory pain serves as a protective defense mechanism which becomes pathological when it turns into chronic inflammatory pain. This transition is mediated by a variety of peripheral mediators that sensitize nociceptors and increase pain perception in sensory neurons. Besides cytokines, chemokines and growth factors, accumulating evidence shows that oxidized lipids, such as eicosanoids and oxidized linoleic acid metabolites, contribute to this sensitization process. Most notably, the oxidized linoleic acid metabolite and partial TRPV1 agonist 9-HODE (hydroxyoctadecadienoic acid) was shown to be involved in this sensitization process. However, it is still unknown how some of the oxidized linoleic acid metabolites are synthesized in the inflammatory environment and in which phase of inflammation they become relevant. Here we show that the concentrations of oxidized linoleic acid metabolites, especially 9-HODE and 13-HODE, are significantly increased in inflamed paw tissue and the corresponding dorsal root ganglia in the sub-chronic phase of inflammation. Surprisingly, classical inflammatory lipid markers, such as prostaglandins were at basal levels in this phase of inflammation. Moreover, we revealed the cell type specific synthesis pathways of oxidized linoleic acid metabolites in primary macrophages, primary neutrophils and dorsal root ganglia. Finally, we show that blocking the most elevated metabolites 9-HODE and 13-HODE at the site of inflammation in the sub-chronic phase of inflammation, leads to a significant relief of mechanical and thermal hypersensitivity in vivo. In summary, these data offer an approach to specifically target oxidized linoleic acid metabolites in the transition of acute inflammatory pain to chronic inflammatory pain.

Hemodialysis and Plasma Oxylipin Biotransformation in Peripheral Tissue

Factors causing the increased cardiovascular morbidity and mortality in hemodialysis (HD) patients are largely unknown. Oxylipins are a superclass of lipid mediators with potent bioactivities produced from oxygenation of polyunsaturated fatty acids. We previously assessed the impact of HD on oxylipins in arterial blood plasma and found that HD increases several oxylipins. To study the phenomenon further, we now evaluated the differences in arterial and venous blood oxylipins from patients undergoing HD. We collected arterial and venous blood samples in upper extremities from 12 end-stage renal disease (ESRD) patients before and after HD and measured oxylipins in plasma by LC-MS/MS tandem mass spectrometry. Comparison between cytochrome P450 (CYP), lipoxygenase (LOX), and LOX/CYP ω/(ω-1)-hydroxylase metabolites levels from arterial and venous blood showed no arteriovenous differences before HD but revealed arteriovenous differences in several CYP metabolites immediately after HD. These changes were explained by metabolites in the venous blood stream of the upper limb. Decreased soluble epoxide hydrolase (sEH) activity contributed to the release and accumulation of the CYP metabolites. However, HD did not affect arteriovenous differences of the majority of LOX and LOX/CYP ω/(ω-1)-hydroxylase metabolites. The HD treatment itself causes changes in CYP epoxy metabolites that could have deleterious effects in the circulation.

Unlocking the potential of TRPV1 based siRNA therapeutics for the treatment of chemotherapy-induced neuropathic pain

Chemotherapy-induced neuropathic pain (CINP) is among the most common clinical complications associated with the use of anti-cancer drugs. CINP occurs in nearly 68.1% of the cancer patients receiving chemotherapeutic drugs. Most of the clinically available analgesics are ineffective in the case of CINP patients as the pathological mechanisms involved with different chemotherapeutic drugs are distinct from each other. CINP triggers the somatosensory nervous system, increases the neuronal firing and activation of nociceptive mediators including transient receptor protein vanilloid 1 (TRPV1). TRPV1 is widely present in the peripheral nociceptive nerve cells and it has been reported that the higher expression of TRPV1 in DRGs serves a critical role in the potentiation of CINP. The therapeutic glory of TRPV1 is well recognized in clinics which gives a promising insight into the treatment of pain. But the adverse effects associated with some of the antagonists directed the scientists towards RNA interference (RNAi), a tool to silence gene expression. Thus, ongoing research is focused on developing small interfering RNA (siRNA)-based therapeutics targeting TRPV1. In this review, we have discussed the involvement of TRPV1 in the nociceptive signaling associated with CINP and targeting this nociceptor, using siRNA will potentially arm us with effective therapeutic interventions for the clinical management of CINP.

Lipid oxidation that is, and is not, inhibited by vitamin E: Consideration about physiological functions of vitamin E

Lipids are oxidized in vivo by multiple oxidizing species with different properties, some by regulated manner to produce physiological mediators, while others by random mechanisms to give detrimental products. Vitamin E plays an important role as a physiologically essential antioxidant to inhibit unregulated lipid peroxidation by scavenging lipid peroxyl radicals to break chain propagation independent of the type of free radicals which induce chain initiation. Kinetic data suggest that vitamin E does not act as an efficient scavenger of nitrogen dioxide radical, carbonate anion radical, and hypochlorite. The analysis of regio- and stereo-isomer distribution of the lipid oxidation products shows that, apart from lipid oxidation by CYP enzymes, the free radical-mediated lipid peroxidation is the major pathway of lipid oxidation taking place in humans. Compared with healthy subjects, the levels of racemic and trans,trans-hydro (pero)xyoctadecadienoates, specific biomarker of free radical lipid oxidation, are elevated in the plasma of patients including atherosclerosis and non-alcoholic fatty liver diseases. α-Tocopherol acts as a major antioxidant, while γ-tocopherol scavenges nitrogen dioxide radical, which induces lipid peroxidation, nitration of aromatic compounds and unsaturated fatty acids, and isomerization of cis-fatty acids to trans-fatty acids. It is essential to appreciate that the antioxidant effects of vitamin E depend on the nature of both oxidants and substrates being oxidized. Vitamin E, together with other antioxidants such as vitamin C, contributes to the inhibition of detrimental oxidation of biological molecules and thereby to the maintenance of human health and prevention of diseases.

Association of dyslipidemia with chronic non-malignant pain in elderly patients with femoral neck fractures treated by primary total hip arthroplasty: a retrospective study

OBJECTIVE

The association of chronic non-malignant pain (CNP) with dyslipidemia is unclear. This retrospective study was performed to evaluate the association between CNP and dyslipidemia in elderly patients with femoral neck fractures (FNFs) treated by primary unilateral total hip arthroplasty (THA).

METHODS

We retrospectively identified 521 consecutive patients with FNFs (AO/OTA type 31B) who underwent primary unilateral THA from 2009 to 2021. The study population was divided into patients with and without CNP. Serum lipids were measured for each patient. The association between CNP and dyslipidemia was assessed using a multivariate binary logistic regression model.

RESULTS

In total, 436 patients (220 with CNP, 216 without CNP) were eligible for analysis. In the quantile regression, the adverse effect of CNP was significantly attenuated by resilience in patients with a high high-density lipoprotein (HDL) concentration and low low-density lipoprotein (LDL) concentration. The multivariate binary logistic regression model showed that the HDL and LDL concentrations were the only variables significantly associated with the development of CNP.

CONCLUSION

Both a low HDL and high LDL concentration may result in the occurrence of CNP in elderly patients with FNFs treated by primary unilateral THA.

Pain in the Cancer Survivor

Recent decades have demonstrated significant strides in cancer screening, diagnostics and therapeutics. As such there have been dramatic changes in survival following a diagnosis of cancer.

Novel treatments for chronic pain: moving beyond opioids

It is essential that safe and effective treatment options be available to patients suffering from chronic pain. The emergence of an opioid epidemic has shaped public opinions and created stigmas surrounding the use of opioids for the management of pain. This reality, coupled with high risk of adverse effects from chronic opioid use, has led chronic pain patients and their healthcare providers to utilize nonopioid treatment approaches. In this review, we will explore a number of cellular reorganizations that are associated with the development and progression of chronic pain. We will also discuss the safety and efficacy of opioid and nonopioid treatment options for chronic pain. Finally, we will review the evidence for adenylyl cyclase type 1 (AC1) as a novel target for the treatment of chronic pain.

Dietary alteration of n-3 and n-6 fatty acids for headache reduction in adults with migraine: randomized controlled trial

OBJECTIVE

To determine whether dietary interventions that increase n-3 fatty acids with and without reduction in n-6 linoleic acid can alter circulating lipid mediators implicated in headache pathogenesis, and decrease headache in adults with migraine.

DESIGN

Three arm, parallel group, randomized, modified double blind, controlled trial.

SETTING

Ambulatory, academic medical center in the United States over 16 weeks.

PARTICIPANTS

182 participants (88% women, mean age 38 years) with migraines on 5-20 days per month (67% met criteria for chronic migraine).

INTERVENTIONS

Three diets designed with eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and linoleic acid altered as controlled variables: H3 diet (n=61)-increase EPA+DHA to 1.5 g/day and maintain linoleic acid at around 7% of energy; H3-L6 diet (n=61)-increase n-3 EPA+DHA to 1.5 g/day and decrease linoleic acid to ≤1.8% of energy; control diet (n=60)-maintain EPA+DHA at <150 mg/day and linoleic acid at around 7% of energy. All participants received foods accounting for two thirds of daily food energy and continued usual care.

MAIN OUTCOME MEASURES

The primary endpoints (week 16) were the antinociceptive mediator 17-hydroxydocosahexaenoic acid (17-HDHA) in blood and the headache impact test (HIT-6), a six item questionnaire assessing headache impact on quality of life. Headache frequency was assessed daily with an electronic diary.

RESULTS

In intention-to-treat analyses (n=182), the H3-L6 and H3 diets increased circulating 17-HDHA (log ng/mL) compared with the control diet (baseline-adjusted mean difference 0.6, 95% confidence interval 0.2 to 0.9; 0.7, 0.4 to 1.1, respectively). The observed improvement in HIT-6 scores in the H3-L6 and H3 groups was not statistically significant (-1.6, -4.2 to 1.0, and -1.5, -4.2 to 1.2, respectively). Compared with the control diet, the H3-L6 and H3 diets decreased total headache hours per day (-1.7, -2.5 to -0.9, and -1.3, -2.1 to -0.5, respectively), moderate to severe headache hours per day (-0.8, -1.2 to -0.4, and -0.7, -1.1 to -0.3, respectively), and headache days per month (-4.0, -5.2 to -2.7, and -2.0, -3.3 to -0.7, respectively). The H3-L6 diet decreased headache days per month more than the H3 diet (-2.0, -3.2 to -0.8), suggesting additional benefit from lowering dietary linoleic acid. The H3-L6 and H3 diets altered n-3 and n-6 fatty acids and several of their nociceptive oxylipin derivatives in plasma, serum, erythrocytes or immune cells, but did not alter classic headache mediators calcitonin gene related peptide and prostaglandin E2.

CONCLUSIONS

The H3-L6 and H3 interventions altered bioactive mediators implicated in headache pathogenesis and decreased frequency and severity of headaches, but did not significantly improve quality of life.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02012790.

Activation of Transient Receptor Potential Vanilloid 1 Channels in the Nucleus of the Solitary Tract and Activation of Dynorphin Input to the Median Preoptic Nucleus Contribute to Impaired BAT Thermogenesis in Diet-Induced Obesity

The impairment of cold-evoked activation of brown adipose tissue (BAT) in rats fed a high-fat diet (HFD) requires the activity of a vagal afferent to the medial nucleus of the solitary tract (mNTS). We determined the role of transient receptor potential vanilloid 1 (TRPV1) activation in the mNTS, and of a dynorphin input to the median preoptic nucleus (MnPO) in the impaired BAT thermogenic response to cold in HFD-fed rats. The levels of some linoleic acid (LA) metabolites, which can act as endogenous TRPV1 agonists, were elevated in the NTS of HFD rats compared with chow-fed rats. In HFD rats, nanoinjections of the TRPV1 antagonist, capsazepine (CPZ) in the NTS rescued the impaired BAT sympathetic nerve activity (BAT SNA) and thermogenic responses to cold. In contrast, in chow-fed rats, cold-evoked BAT SNA and BAT thermogenesis were not changed by nanoinjections of CPZ into the NTS. Axon terminals of NTS neurons that project to the dorsal lateral parabrachial nucleus (LPBd) were closely apposed to LPBd neurons that project to the MnPO. Many of the neurons in the LPBd that expressed c-fos during cold challenge were dynorphinergic. In HFD rats, nanoinjections of the κ opioid receptor (KOR) antagonist, nor-binaltorphimine (nor-BNI), in the MnPO rescued the impaired BAT SNA and thermogenic responses to cold. These data suggest that HFD increases the content of endogenous ligands of TRPV1 in the NTS, which increases the drive to LPBd neurons that in turn release dynorphin in the MnPO to impair activation of BAT.

Dietary Interventions Are Beneficial for Patients with Chronic Pain: A Systematic Review with Meta-Analysis

Background

The standard Western diet is high in processed hyperpalatable foods that displace nutrient-dense whole foods, leading to inflammation and oxidative stress. There is limited research on how these adverse metabolic drivers may be associated with maladaptive neuroplasticity seen in chronic pain and whether this could be attenuated by a targeted nutritional approach. The aim of this study was to review the evidence for whole-food dietary interventions in chronic pain management.

Method

A structured search of eight databases was performed up to December 2019. Two independent reviewers screened studies and evaluated risk of bias by using the National Institutes of Health assessment tool for controlled or pre–post studies and the Joanna Briggs checklist for case reports. A meta-analysis was performed in Review Manager.

Results

Forty-three studies reporting on 48 chronic pain groups receiving a whole-food dietary intervention were identified. These included elimination protocols (n = 11), vegetarian/vegan diets (n = 11), single-food changes (n = 11), calorie/macronutrient restriction (n = 8), an omega-3 focus (n = 5), and Mediterranean diets (n = 2). A visual analog scale was the most commonly reported pain outcome measure, with 17 groups reporting a clinically objective improvement (a two-point or 33% reduction on the visual analog scale). Twenty-seven studies reported significant improvement on secondary metabolic measures. Twenty-five groups were included in a meta-analysis that showed a significant finding for the effect of diet on pain reduction when grouped by diet type or chronic pain type.

Conclusion

There is an overall positive effect of whole-food diets on pain, with no single diet standing out in effectiveness. This suggests that commonalities among approaches (e.g., diet quality, nutrient density, weight loss) may all be involved in modulating pain physiology. Further research linking how diet can modulate physiology related to pain (such as inflammation, oxidative stress, and nervous system excitability) is required.

Early evidence of efficacy for orally administered SPM-enriched marine lipid fraction on quality of life and pain in a sample of adults with chronic pain

BACKGROUND

Marine lipids contain omega-3 fatty acids that can be metabolized into anti-inflammatory and pro-resolving mediators-namely 17-HDHA and 18-HEPE-which can serve as modulators of the pain experience. The purpose of this study was to determine the impact of 4 weeks of oral supplementation with a fractionated marine lipid concentration, standardized to 17-HDHA and 18-HEPE, on health-related quality of life and inflammation in adults with chronic pain.

METHODS

This study was a prospective, non-randomized, open-label clinical trial. Forty-four adults with ≥ moderate pain intensity for at least 3 months were recruited. The primary outcome was change in health-related quality of life (QOL) using the Patient Reported Outcomes Measurement Information System-43 Profile (PROMIS-43) and the American Chronic Pain Association (ACPA) QOL scale. Exploratory outcomes assessed safety and tolerability, changes in anxiety and depression, levels of pain intensity and interference, patient satisfaction, and impression of change. Changes in blood biomarkers of inflammation (hs-CRP and ESR) were also explored.

RESULTS

Outcome measures were collected at Baseline, Week 2, and Week 4 (primary endpoint). At Week 4, PROMIS-43 QOL subdomains changed with significance from baseline (p < 0.05), with borderline changes in the ACPA Quality of Life scale (p < 0.052). Exploratory analyses revealed significant changes (p < 0.05) in all measures of pain intensity, pain interference, depression, and anxiety. There were no statistically significant changes in either hs-CRP or ESR, which stayed within normal limits.

CONCLUSION

We conclude that oral supplementation with a fractionated marine lipid concentration standardized to 17-HDHA and 18-HEPE may improve quality of life, reduce pain intensity and interference, and improve mood within 4 weeks in adults with chronic pain. The consistency and magnitude of these results support the need for placebo-controlled clinical trials of marine lipid concentrations standardized to 17-HDHA and 18-HEPE. Trial registration ClinicalTrials.gov: Influence of an Omega-3 SPM Supplement on Quality of Life, NCT02683850. Registered 17 February 2016-retrospectively registered, https://clinicaltrials.gov/ct2/show/NCT02683850 .

Molecular Pathways Linking Oxylipins to Nociception in Rats

Oxylipins are lipid peroxidation products that participate in nociceptive, inflammatory, and vascular responses to injury. Effects of oxylipins depend on tissue-specific differences in accumulation of precursor polyunsaturated fatty acids and the expression of specific enzymes to transform the precursors. The study of oxylipins in nociception has presented technical challenges leading to critical knowledge gaps in the way these molecules operate in nociception. We applied a systems-based approach to characterize oxylipin precursor fatty acids, and expression of genes coding for proteins involved in biosynthesis, transport, signaling and inactivation of pro- and antinociceptive oxylipins in pain circuit tissues. We further linked these pathways to nociception by demonstrating intraplantar carrageenan injection induced gene expression changes in oxylipin biosynthetic pathways. We determined functional-biochemical relevance of the proposed pathways in rat hind paw and dorsal spinal cord by measuring basal and stimulated levels of oxylipins throughout the time-course of carrageenan-induced inflammation. Finally, when oxylipins were administered by intradermal injection we observed modulation of nociceptive thermal hypersensitivity, providing a functional-behavioral link between oxylipins, their molecular biosynthetic pathways, and involvement in pain and nociception. Together, these findings advance our understanding of molecular lipidomic systems linking oxylipins and their precursors to nociceptive and inflammatory signaling pathways in rats. PERSPECTIVE: We applied a systems approach to characterize molecular pathways linking precursor lipids and oxylipins to nociceptive signaling. This systematic, quantitative evaluation of the molecular pathways linking oxylipins to nociception provides a framework for future basic and clinical research investigating the role of oxylipins in pain.

Fecal transplantation and butyrate improve neuropathic pain, modify immune cell profile, and gene expression in the PNS of obese mice

Obesity affects over 2 billion people worldwide and is accompanied by peripheral neuropathy (PN) and an associated poorer quality of life. Despite high prevalence, the molecular mechanisms underlying the painful manifestations of PN are poorly understood, and therapies are restricted to use of painkillers or other drugs that do not address the underlying disease. Studies have demonstrated that the gut microbiome is linked to metabolic health and its alteration is associated with many diseases, including obesity. Pathologic changes to the gut microbiome have recently been linked to somatosensory pain, but any relationships between gut microbiome and PN in obesity have yet to be explored. Our data show that mice fed a Western diet developed indices of PN that were attenuated by concurrent fecal microbiome transplantation (FMT). In addition, we observed changes in expression of genes involved in lipid metabolism and calcium handling in cells of the peripheral nerve system (PNS). FMT also induced changes in the immune cell populations of the PNS. There was a correlation between an increase in the circulating short-chain fatty acid butyrate and pain improvement following FMT. Additionally, butyrate modulated gene expression and immune cells in the PNS. Circulating butyrate was also negatively correlated with distal pain in 29 participants with varied body mass index. Our data suggest that the metabolite butyrate, secreted by the gut microbiome, underlies some of the effects of FMT. Targeting the gut microbiome, butyrate, and its consequences may represent novel viable approaches to prevent or relieve obesity-associated neuropathies.

The Lipid Receptor G2A (GPR132) Mediates Macrophage Migration in Nerve Injury-Induced Neuropathic Pain

Nerve injury-induced neuropathic pain is difficult to treat and mechanistically characterized by strong neuroimmune interactions, involving signaling lipids that act via specific G-protein coupled receptors. Here, we investigated the role of the signaling lipid receptor G2A (GPR132) in nerve injury-induced neuropathic pain using the robust spared nerve injury (SNI) mouse model. We found that the concentrations of the G2A agonist 9-HODE (9-Hydroxyoctadecadienoic acid) are strongly increased at the site of nerve injury during neuropathic pain. Moreover, G2A-deficient mice show a strong reduction of mechanical hypersensitivity after nerve injury. This phenotype is accompanied by a massive reduction of invading macrophages and neutrophils in G2A-deficient mice and a strongly reduced release of the proalgesic mediators TNFα, IL-6 and VEGF at the site of injury. Using a global proteome analysis to identify the underlying signaling pathways, we found that G2A activation in macrophages initiates MyD88-PI3K-AKT signaling and transient MMP9 release to trigger cytoskeleton remodeling and migration. We conclude that G2A-deficiency reduces inflammatory responses by decreasing the number of immune cells and the release of proinflammatory cytokines and growth factors at the site of nerve injury. Inhibiting the G2A receptor after nerve injury may reduce immune cell-mediated peripheral sensitization and may thus ameliorate neuropathic pain.

Exploring Associations Between Metabolites and Symptoms of Fatigue, Depression and Pain in Women With Fibromyalgia

Fibromyalgia (FM) is a chronic noncommunicable disorder characterized by a constellation of symptoms that include fatigue, depression and chronic pain. FM affects 2%-8% of the U.S. population, 2% of the global population, with 61%-90% of FM diagnoses attributed to women. Key causal factors leading to the development and severity of FM-related symptoms have not yet been identified. The purpose of this article is to report relationships among identified metabolites and levels of fatigue, depression, pain severity, and pain interference in a sample of 20 women with FM. In this secondary analysis, we conducted global metabolomic analysis and examined the data for relationships of metabolite levels with self-reported symptoms of fatigue, depression, pain severity, and pain interference. Results revealed six metabolites (6-deoxy-hexose; pantothenic acid; ergothioneine; l-carnitine; n-acetylserotonin; butyrobetaine) and their associated metabolic pathways such as carnitine synthesis, lipid oxidation, tryptophan metabolism, beta-alanine metabolism and pantothenic and Coenzyme-A biosynthesis that were either positively or inversely related to pain severity, pain interference, or both. The preliminary data presented suggest that metabolites representing energy, amino acid, or lipid classification may be associated with pain symptom severity and interference in women with FM. Future work will confirm these findings in a large, comparative cohort, targeting metabolites and metabolite pathways to better understand the relationships of metabolites and symptomology.

Soluble Epoxide Hydrolase Regulation of Lipid Mediators Limits Pain

The role of lipids in pain signaling is well established and built on decades of knowledge about the pain and inflammation produced by prostaglandin and leukotriene metabolites of cyclooxygenase and lipoxygenase metabolism, respectively. The analgesic properties of other lipid metabolites are more recently coming to light. Lipid metabolites have been observed to act directly at ion channels and G protein-coupled receptors on nociceptive neurons as well as act indirectly at cellular membranes. Cytochrome P450 metabolism of specifically long-chain fatty acids forms epoxide metabolites, the epoxy-fatty acids (EpFA). The biological role of these metabolites has been found to mediate analgesia in several types of pain pathology. EpFA act through a variety of direct and indirect mechanisms to limit pain and inflammation including nuclear receptor agonism, limiting endoplasmic reticulum stress and blocking mitochondrial dysfunction. Small molecule inhibitors of the soluble epoxide hydrolase can stabilize the EpFA in vivo, and this approach has demonstrated relief in preclinical modeled pain pathology. Moreover, the ability to block neuroinflammation extends the potential benefit of targeting soluble epoxide hydrolase to maintain EpFA for neuroprotection in neurodegenerative disease.