Oxidative stress in the spinal cord is an important contributor in capsaicin-induced mechanical secondary hyperalgesia in mice

Nitroxidative Stress, Cell-Signaling Pathways, and Manganese Porphyrins: Therapeutic Potential in Neuropathic Pain

Neuropathic pain, a debilitating condition arising from somatosensory system damage, significantly impacts quality of life, leading to anxiety, self-mutilation, and depression. Oxidative and nitrosative stress, an imbalance between reactive oxygen and nitrogen species (ROS/RNS) and antioxidant defenses, plays a crucial role in its pathophysiology. While reactive species are essential for physiological functions, excessive levels can cause cellular component damage, leading to neuronal dysfunction and pain. This review highlights the complex interactions between reactive species, antioxidant systems, cell signaling, and neuropathic pain. We discuss the physiological roles of ROS/RNS and the detrimental effects of oxidative and nitrosative stress. Furthermore, we explore the potential of manganese porphyrins, compounds with antioxidant properties, as promising therapeutic agents to mitigate oxidative stress and alleviate neuropathic pain by targeting key cellular pathways involved in pain. Further research is needed to fully understand their therapeutic potential in managing neuropathic pain in human and non-human animals.

Formoterol alters chemokine expression and ameliorates pain behaviors after moderate spinal cord injury in female mice

Secondary spinal cord injury (SCI) is characterized by increased cytokines and chemokines at the site of injury that have been associated with the development of neuropathic pain. Nearly 80% of SCI patients report suffering from chronic pain, which is poorly managed with available analgesics. While treatment with the US Food and Drug Administration-approved β2-adrenergic receptor agonist formoterol improves various aspects of recovery post-SCI in vivo, its effects on cytokines, chemokines, and neuropathic pain remain unknown. Female mice were subjected to moderate (60 kilodynes [kdyn]) or severe (80 kdyn) SCI followed by daily treatment with vehicle or formoterol (0.3 mg/kg, i.p.) beginning 8 hours after injury. The expression of proinflammatory cytokines/chemokines, such as interferon gamma-induced protein 10, macrophage inflammatory protein 1a, monocyte chemoattractant protein 1, B-cell attracting chemokine 1, and nuclear factor kappa-light-chain-enhancer of activated B-cells, was increased in the injury site of vehicle-treated mice 24 hours post-SCI, which was ameliorated with formoterol treatment, regardless of injury severity. Thermal hyperalgesia and mechanical allodynia, as measured by Hargreaves infrared apparatus and von Frey filaments, respectively, were assessed prior to SCI and then weekly beginning 21 days post-injury (DPI). While all injured mice exhibited decreased withdrawal latency following thermal stimulation compared with baseline, formoterol treatment reduced this response ∼15% by 35 DPI. Vehicle-treated mice displayed significant mechanical allodynia, as evidenced by a 55% decrease in withdrawal threshold from baseline. In contrast, mice treated with formoterol maintained a consistent withdrawal time at all times tested. These data indicate that formoterol reduces inflammation post-SCI, likely contributing to mitigation of neuropathic pain and further supporting the therapeutic potential of this treatment strategy. SIGNIFICANCE STATEMENT: Chronic pain is a detrimental consequence of spinal cord injury (SCI). We show that treatment with the US Food and Drug Administration-approved drug formoterol after SCI decreases injury site proinflammatory chemo-/cytokines and alters markers of glial cell activation and infiltration. Additionally, formoterol treatment improves locomotor function and body composition, and decreases lesion volume. Finally, formoterol treatment decreased mechanical allodynia and thermal hyperalgesia post-SCI. These data are suggestive of the mechanism of formoterol-induced recovery, and further indicate its potential as a therapeutic strategy for SCI.

Proprotein convertase subtilisin/kexin type 9 and apelin in fibromyalgia syndrome

OBJECTIVES

This study aimed to investigate the potential roles of proprotein convertase subtilisin/ kexin type 9 (PCSK9) and apelin in the etiology of fibromyalgia syndrome (FS).

PATIENTS AND METHODS

The retrospective study was conducted between May 2022 and February 2023. Fifty-eight female FS patients (mean age: 45.2±9.9 years; range, 25 to 66 years) and 30 age- and body mass index-matched control subjects (mean age: 43.1±9.9 years; range, 26 to 67 years) were included in the study. Apelin and PCSK9 levels of all individuals were measured using appropriate methods.

RESULTS

The levels of PCSK9 (173.2±62.2 vs. 75.1±44.1, p<0.001) and apelin (354.6±195.5 vs. 229.0±83.2, p<0.001) were significantly higher in patients with FS compared to the control group. A positive correlation was found between PCSK9 and apelin levels and various measures, including the Fibromyalgia Impact Questionnaire (FIQ), Symptom Severity Scale (SSS), Pittsburgh Sleep Quality Index (PSQI), and Beck Depression Inventory (BDI). Additionally, there was a positive correlation between apelin levels and FIQ, SSS, PSQI, Beck Anxiety Inventory, and BDI scores. The optimal cutoff value for PCSK9 in predicting FS was 110.0 ng/mL, with a sensitivity of 84.5% and specificity of 83.9% (area under the curve [AUC]=0.920, 95% confidence interval [CI]: 0.852-0.987, p<0.001). For apelin, the optimal cutoff value for predicting FS was 258.8 ng/L, with a sensitivity of 63.8% and specificity of 64.5% (AUC=0.732, 95% CI: 0.623-0.840, p<0.001).

CONCLUSION

Our findings suggest that PCSK9 may play a role in FS etiology and potentially contribute to oxidative stress. Increased apelin levels may be a compensatory response to high oxidative stress, possibly leading to hyperalgesia. Both PCSK9 and apelin can be predictive markers for FS.

SIRT1-regulated ROS generation activates NMDAR2B phosphorylation to promote central sensitization and allodynia in a male chronic migraine rat model

Background

Central sensitization is one of the pivotal pathological mechanisms in chronic migraine (CM). Silent information regulator 1 (SIRT1) was shown to be involved in CM, but its specific mechanism is unclear. Reactive oxygen species (ROS) are increasingly regarded as important signaling molecules in several models of pain. However, studies about the role of ROS in the central sensitization of CM model are rare. We thus explored the specific process of SIRT1 involvement in the central sensitization of CM, focusing on the ROS pathway.

Methods

Inflammatory soup was repeatedly administered to male Sprague-Dawley rats to establish a CM model. The SIRT1 expression level in trigeminal nucleus caudalis (TNC) tissues was assessed by qRT-PCR and Western blotting analysis. The levels of ROS were detected by a Tissue Reactive Oxygen Detection Kit, DHE staining, and the fluorescence signal intensity of 8-OHdG. A ROS scavenger (tempol), a SIRT1 activator (SRT1720), a SIRT1 inhibitor (EX527), and a mitochondrial fission inhibitor (Mdivi-1) were used to investigate the specific molecular mechanisms involved. NMDAR2B, CGRP, ERK, and mitochondrial fission-related protein were evaluated by Western blotting, and the CGRP level in frozen sections of the TNC was detected via immunofluorescence staining.

Results

After repeated inflammatory soup infusion and successful establishment of the CM rat model, SIRT1 expression was found to be significantly reduced, accompanied by elevated ROS levels. Treatment with Tempol, SRT1720, or Mdivi-1 alleviated allodynia and reduced the increase in NMDAR2B phosphorylation and CGRP and ERK phosphorylation in the CM rat. In contrast, EX527 had the opposite effect in CM rat. SRT1720 and EX527 decreased and increased ROS levels, respectively, in CM rats, and tempol reversed the aggravating effect of EX527 in CM rats. Furthermore, the regulatory effect of SIRT1 on ROS may include the involvement of the mitochondrial fission protein DRP1.

Conclusion

The results indicate the importance of SIRT1 in CM may be due to its role in regulating the production of ROS, which are involved in modulating central sensitization in CM. These findings could lead to new ideas for CM treatment with the use of SIRT1 agonists and antioxidants.

Histone deacetylase as emerging pharmacological therapeutic target for neuropathic pain: From epigenetic to selective drugs

BACKGROUND

Neuropathic pain remains a formidable challenge for modern medicine. The first-line pharmacological therapies exhibit limited efficacy and unfavorable side effect profiles, highlighting an unmet need for effective therapeutic medications. The past decades have witnessed an explosion in efforts to translate epigenetic concepts into pain therapy and shed light on epigenetics as a promising avenue for pain research. Recently, the aberrant activity of histone deacetylase (HDAC) has emerged as a key mechanism contributing to the development and maintenance of neuropathic pain.

AIMS

In this review, we highlight the distinctive role of specific HDAC subtypes in a cell-specific manner in pain nociception, and outline the recent experimental evidence supporting the therapeutic potential of HDACi in neuropathic pain.

METHODS

We have summarized studies of HDAC in neuropathic pain in Pubmed.

RESULTS

HDACs, widely distributed in the neuronal and non-neuronal cells of the dorsal root ganglion and spinal cord, regulate gene expression by deacetylation of histone or non-histone proteins and involving in increased neuronal excitability and neuroinflammation, thus promoting peripheral and central sensitization. Importantly, pharmacological manipulation of aberrant acetylation using HDAC-targeted inhibitors (HDACi) has shown promising pain-relieving properties in various preclinical models of neuropathic pain. Yet, many of which exhibit low-specificity that may induce off-target toxicities, underscoring the necessity for the development of isoform-selective HDACi in pain management.

CONCLUSIONS

Abnormally elevated HDACs promote neuronal excitability and neuroinflammation by epigenetically modulating pivotal gene expression in neuronal and immune cells, contributing to peripheral and central sensitization in the progression of neuropathic pain, and HDACi showed significant efficacy and great potential for alleviating neuropathic pain.

Spinal sirtuin 2 attenuates bone cancer pain by deacetylating FoxO3a

Bone cancer pain (BCP) is refractory to currently used analgesics. Recently, sirtuin 2 (SIRT2) was reported to play a vital role in neuropathic pain but its role in BCP remains unknown. It was hypothesized that spinal SIRT2 attenuates BCP by deacetylating FoxO3a and suppressing oxidative stress. The mouse model of BCP established by injecting tumor cells into the intramedullary space of the femur demonstrated that spinal SIRT2 and FoxO3a were downregulated in BCP development. Intrathecal administration of LV-SIRT2 reduced pain hypersensitivity (mechanical and thermal nociception) in BCP mice. Spinal SIRT2 overexpression upregulated FoxO3a and antioxidant genes (SOD2 and catalase) and inhibited FoxO3a acetylation, phosphorylation, and ubiquitination. Moreover, intrathecal administration of SIRT2 shRNA induced pain hypersensitivity in normal mice. Spinal SIRT2 knockdown downregulated FoxO3a and antioxidant genes and increased FoxO3a acetylation, phosphorylation, and ubiquitination. In summary, spinal SIRT2 increases FoxO3a expression in BCP mice and inhibits oxidative stress by deacetylating FoxO3a and further reducing FoxO3a phosphorylation, ubiquitination, and degradation, leading to BCP relief.

Amelioration of pain and anxiety in sleep-deprived rats by intra-amygdala injection of cinnamaldehyde

Background

Sleep disorders are accompanied by increased anxiety and somatic pain. In addition, it has been observed that anxiety and pain have a boosting effect on each other, resulting in continued sleep disturbances. Amygdala's (CeA) central nucleus plays a crucial role in these processes. Cinnamaldehyde (Cinn) is an aromatic compound with anti-anxiety, antioxidant, and sleep-promoting properties. The present study uses sleep-deprived rats to examine the effects of an intra-CeA injection of Cinn on pain and anxiety.

Methods

Sleep deprivation (SD) was induced using the platform technique. 35 male Wistar rats were divided into five groups. Anxiety state and nociception were evaluated among groups using formalin test (F.T.), open field test (OFT), and elevated plus maze (EPM). Anxiety tests (OFT and EPM) were conducted in all groups. The first group was undergone FT without induction of SD (SD-FT+). The second group received SD without FT(SD+FT-). The third group received both SD and FT(SD+FT+). The treatment and vehicle groups have undergone both SD and FT in addition to the respectively intra-CeA injection of Cinn (SD+FT+ Cinn) and Cinn vehicle (SD+FT+ VC). The recorded behaviors were analyzed between groups using IBM SPSS 24th version.

Results

SD did not lead to any significant difference in nociceptive behaviors in FT between groups SD-FT+ and SD+FT+ (P ≥ 0.05). At the same time, there was a considerable discrepancy in rearing behaviors (P < 0.006) and the number of fecal boli (P < 0.004) recorded in OFM between these groups. Treatment with Cinn led to decreased nociception (P < 0.038), decreased rearing behaviors (P < 0.01), and reduced defecation (P < 0.004) in group SD + FT+ Cinn in comparison to the group SD+FT+. There were no differences in anxiety test results between the first and second groups (P ≥ 0.05).

Conclusion

SD can lead to elevated anxiety, while intra-CeA injection of Cinn ameliorated both perceptions of acute pain and anxiety. Besides, the conduction of FT before the anxiety test led to no disturbance in the results of anxiety tests.

Role of DNA methylation during recovery from spinal cord injury with and without β2-adrenergic receptor agonism

Daily treatment with the FDA-approved β2-adrenergic receptor agonist formoterol beginning 8 h after severe spinal cord injury (SCI) induces mitochondrial biogenesis and improves recovery in mice. We observed decreased DNA methyltransferase (DNMT) expression, global DNA methylation and methylation of the mitochondrial genes PGC-1α and NDUFS1 in the injury site of formoterol-treated mice 1 DPI, but this effect was lost by 7 DPI. To investigate the role of DNA methylation on recovery post-SCI, injured mice were treated daily with formoterol or vehicle, plus the DNMT inhibitor decitabine (DAC) on days 7-9. While DAC had no apparent effect on formoterol-induced recovery, mice treated with vehicle plus DAC exhibited increased BMS scores compared to vehicle alone beginning 15 DPI, reaching a degree of functional recovery similar to that of formoterol-treated mice by 21 DPI. Furthermore, DAC treatment increased injury site mitochondrial protein expression in vehicle-treated mice to levels comparable to that of formoterol-treated mice. The effect of DNMT inhibition on pain response with and without formoterol was assessed following moderate SCI. While all injured mice not treated with DAC displayed thermal hyperalgesia by 21 DPI, mice treated with formoterol exhibited decreased thermal hyperalgesia compared to vehicle-treated mice by 35 DPI. Injured mice treated with DAC, regardless of formoterol treatment, did not demonstrate thermal hyperalgesia at any time point assessed. Although these data do not suggest enhanced formoterol-induced recovery with DNMT inhibition, our findings indicate the importance of DNA methylation post-SCI and support both DNMT inhibition and formoterol as potential therapeutic avenues.

Mapping the neuroethological signatures of pain, analgesia, and recovery in mice

Ongoing pain is driven by the activation and modulation of pain-sensing neurons, affecting physiology, motor function, and motivation to engage in certain behaviors. The complexity of the pain state has evaded a comprehensive definition, especially in non-verbal animals. Here, in mice, we used site-specific electrophysiology to define key time points corresponding to peripheral sensitivity in acute paw inflammation and chronic knee pain models. Using supervised and unsupervised machine learning tools, we uncovered sensory-evoked coping postures unique to each model. Through 3D pose analytics, we identified movement sequences that robustly represent different pain states and found that commonly used analgesics do not return an animal's behavior to a pre-injury state. Instead, these analgesics induce a novel set of spontaneous behaviors that are maintained even after resolution of evoked pain behaviors. Together, these findings reveal previously unidentified neuroethological signatures of pain and analgesia at heightened pain states and during recovery.

Crosstalk of Mast Cells and Natural Killer Cells with Neurons in Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a major comorbidity of cancer. Multiple clinical interventions have been studied to effectively treat CIPN, but the results have been disappointing, with no or little efficacy. Hence, understanding the pathophysiology of CIPN is critical to improving the quality of life and clinical outcomes of cancer patients. Although various mechanisms of CIPN have been described in neuropathic anti-cancer agents, the neuroinflammatory process involving cytotoxic/proinflammatory immune cells remains underexamined. While mast cells (MCs) and natural killer (NK) cells are the key innate immune compartments implicated in the pathogenesis of peripheral neuropathy, their role in CIPN has remained under-appreciated. Moreover, the biology of proinflammatory cytokines associated with MCs and NK cells in CIPN is particularly under-evaluated. In this review, we will focus on the interactions between MCs, NK cells, and neuronal structure and their communications via proinflammatory cytokines, including TNFα, IL-1β, and IL-6, in peripheral neuropathy in association with tumor immunology. This review will help lay the foundation to investigate MCs, NK cells, and cytokines to advance future therapeutic strategies for CIPN.

Role of Tumor Necrosis Factor Receptor 1-Reactive Oxygen Species-Caspase 11 Pathway in Neuropathic Pain Mediated by HIV gp120 With Morphine in Rats

BACKGROUND

Recent clinical research suggests that repeated use of opioid pain medications can increase neuropathic pain in people living with human immunodeficiency virus (HIV; PLWH). Therefore, it is significant to elucidate the exact mechanisms of HIV-related chronic pain. HIV infection and chronic morphine induce proinflammatory factors, such as tumor necrosis factor (TNF)α acting through tumor necrosis factor receptor I (TNFRI). HIV coat proteins and/or chronic morphine increase mitochondrial superoxide in the spinal cord dorsal horn (SCDH). Recently, emerging cytoplasmic caspase-11 is defined as a noncanonical inflammasome and can be activated by reactive oxygen species (ROS). Here, we tested our hypothesis that HIV coat glycoprotein gp120 with chronic morphine activates a TNFRI-mtROS-caspase-11 pathway in rats, which increases neuroinflammation and neuropathic pain.

METHODS

Neuropathic pain was induced by repeated administration of recombinant gp120 with morphine (gp120/M) in rats. Mechanical allodynia was assessed using von Frey filaments, and thermal latency using hotplate test. Protein expression of spinal TNFRI and cleaved caspase-11 was examined using western blots. The image of spinal mitochondrial superoxide was examined using MitoSox Red (mitochondrial superoxide indicator) image assay. Immunohistochemistry was used to examine the location of TNFRI and caspase-11 in the SCDH. Intrathecal administration of antisense oligodeoxynucleotide (AS-ODN) against TNFRI, caspase-11 siRNA, or a scavenger of mitochondrial superoxide was given for antinociceptive effects. Statistical tests were done using analysis of variance (1- or 2-way), or 2-tailed t test.

RESULTS

Intrathecal gp120/M induced mechanical allodynia and thermal hyperalgesia lasting for 3 weeks ( P < .001). Gp120/M increased the expression of spinal TNFRI, mitochondrial superoxide, and cleaved caspase-11. Immunohistochemistry showed that TNFRI and caspase-11 were mainly expressed in the neurons of the SCDH. Intrathecal administration of antisense oligonucleotides against TNFRI, Mito-Tempol (a scavenger of mitochondrial superoxide), or caspase-11 siRNA reduced mechanical allodynia and thermal hyperalgesia in the gp120/M neuropathic pain model. Spinal knockdown of TNFRI reduced MitoSox profile cell number in the SCDH; intrathecal Mito-T decreased spinal caspase-11 expression in gp120/M rats. In the cultured B35 neurons treated with TNFα, pretreatment with Mito-Tempol reduced active caspase-11 in the neurons.

CONCLUSIONS

These results suggest that spinal TNFRI-mtROS-caspase 11 signal pathway plays a critical role in the HIV-associated neuropathic pain state, providing a novel approach to treating chronic pain in PLWH with opioids.

P66shc in the spinal cord is an important contributor in complete Freund's adjuvant induced inflammatory pain in mice

PURPOSE

The aim of this study is to investigate whether p66shc is involved in inflammatory pain and the potential molecular mechanisms of p66shc in inflammatory pain.

METHODS

Inflammatory pain model was established by complete Freund's adjuvant (CFA) injection. Paw withdrawal latency (PWL) and paw withdrawal frequency (PWF) was recorded. The expression of spinal p66shc were determined by immunohistochemical staining, immunofluorescence staining. P66shc knockdown was performed by an adeno-associated virus (AAV) vector infusion. NLRP3 inflammasome complexes were determined by Western blot. DHE staining was used to evaluate reactive oxygen species (ROS) generation.

RESULTS

P66Shc expression was progressively elevated in spinal cord of inflammatory pain mice, and p66Shc knockdown in vivo significantly attenuated CFA injection triggers hyperalgesia. Furthermore, knockdown of p66Shc significantly inhibited ROS production and NOD-like receptor protein 3 (NLRP3) inflammasome activation, which were reversed by a ROS donor (t-BOOH). However, post-treatment with nigericin, a agonist of NLRP3, reversed AAV-shP66shc analgesic effect.

CONCLUSION

Spinal p66shc may facilitate the development of inflammatory pain by promoting the activation of NLRP3 inflammasome through ROS.

BDNF as a biomarker for neuropathic pain: Consideration of mechanisms of action and associated measurement challenges

INTRODUCTION

The primary objective of this paper is to (1) provide a summary of human studies that have used brain derived neurotrophic factor (BDNF) as a biomarker, (2) review animal studies that help to elucidate the mechanistic involvement of BDNF in the development and maintenance of neuropathic pain (NP), and (3) provide a critique of the existing measurement techniques to highlight the limitations of the methods utilized to quantify BDNF in different biofluids in the blood (i.e., serum and plasma) with the intention of presenting a case for the most reliable and valid technique. Lastly, this review also explores potential moderators that can influence the measurement of BDNF and provides recommendations to standardize its quantification to reduce the inconsistencies across studies.

METHODS

In this manuscript we examined the literature on BDNF, focusing on its role as a biomarker, its mechanism of action in NP, and critically analyzed its measurement in serum and plasma to identify factors that contribute to the discrepancy in results between plasma and serum BDNF values.

RESULTS

A large heterogenous literature was reviewed that detailed BDNF's utility as a potential biomarker in healthy volunteers, patients with chronic pain, and patients with neuropsychiatric disorders but demonstrated inconsistent findings. The literature provides insight into the mechanism of action of BDNF at different levels of the central nervous system using animal studies. We identified multiple factors that influence the measurement of BDNF in serum and plasma and based on current evidence, we recommend assessing serum BDNF levels to quantify peripheral BDNF as they are more stable and sensitive to changes than plasma BDNF.

CONCLUSION

Although mechanistic studies clearly explain the role of BDNF, results from human studies are inconsistent. More studies are needed to evaluate the methodological challenges in using serum BDNF as a biomarker in NP.

Association between urinary phthalate levels and chronic pain in US adults, 1999-2004: A nationally representative survey

Introduction

Chronic pain is a public health concern throughout the world. Ascertaining and managing its risk factors helps develop well-directed treatment plans and prevention strategies. Phthalates (PAEs) exposure leads to various health problems. The present study aims to explore the potential correlation between urinary PAEs metabolites and chronic pain in adults.

Methods

The study population data were extracted from the National Health and Nutrition Examination Survey (NHANES) conducted from 1999 to 2004 in the United States. Seven urinary PAEs metabolites were used to assess long-term PAEs exposure. The assessment of chronic pain was determined by a self-report questionnaire. Weighted analyses were conducted to consider the complex sampling design. Models were adjusted by demographic data and lifestyle factors. Urinary PAEs metabolites were assessed as both continuous and categorical variables. Tertile 1 was considered as the reference. Stratified analyses were performed by gender and pain site. All data analyses were conducted with STATA, version 15.1. P < 0.05 was considered with statistical significance.

Results

A total of 4,196 participants were considered in our final analysis. Chronic pain prevalence reached 52.19% (n = 2,138) among the participants, with women accounting for a large proportion (57.75% vs. 42.25%). After multivariable logistic regression analysis, a higher prevalence of chronic pain was observed among participants in the third tertile of mono-(2-ethyl)-hexyl phthalate (MEHP) (OR = 1.23, 95% CI = 1.02-1.48, P = 0.034) and mono-benzyl phthalate (MBzP) (OR = 1.28, 95% CI = 1.04-1.58, P = 0.022) in our adjusted model. The logtransformed concentration of MBzP also showed a significant association with chronic pain prevalence (OR = 1.09, 95% CI = 1.01-1.18, P = 0.036) in the adjusted model. In further analysis, the positive correlations of urinary phthalate metabolites with chronic pain remained robust when stratified by gender and chronic pain site.

Conclusions

Our findings presented a positive correlation between urinary PAEs metabolites and chronic pain among adult participants, and more causal research should be conducted to ascertain the interactions between the two and to expound their underlying mechanisms.

Oxidative Stress Biomarkers and Quality of Life Are Contributing Factors of Muscle Pain and Lean Body Mass in Patients with Fibromyalgia

Simple Summary

Fibromyalgia (FM) is a disease that primarily affects women and causes pain all over the body, as well as anxiety, depression, fatigue, weight gain, a decreased quality of life, and difficulties doing daily duties. Although the cause of this disease has yet to be identified, research have been completed or are under underway with the goal of uncovering clues that can explain the disease’s symptoms and proper treatment. Our research looked into whether factors that increase inflammation in the body cause disease symptoms to worsen. Pain, lean mass, quality of life, sleep quality, muscle strength, depression, and probable factors that cause these symptoms to aggravate were assessed in the blood. Women with FM with more pain had a lower quality of life, and women with FM with lower lean mass had muscle weakness in addition to a lower quality of life. Our results recommend that initiatives be implemented to reduce inflammation, improve muscle mass and strength gain and increase the quality of life of these women.

Abstract

(1) The evidence points to an increase in oxygen reactive species as one of the possible causes of fibromyalgia (FM). In addition, it is plausible that an imbalance in redox markers can be associated with pain amplification and dynapenia in FM patients. The aim of our study was to investigate possible factors associated with muscle pain and lean body mass in FM patients. (2) Methods: This was a quantitative, exploratory and cross-sectional study of 47 patients with FM (53.45 + 7.32 years). We evaluated self-perceptions of muscle pain, lean body mass, body composition, quality of life, sleep quality, depression index, muscle performance and oxidative stress biomarkers. (3) Results: We observed that lower blood levels of antioxidants and poor quality of life explained 21% of the greater muscle pain. In addition, high blood levels of oxidative stress, worse muscle performance and poor quality of life explained 27% of the lower lean mass in patients with FM. (4) Conclusions: Larger amounts of lipid peroxidation and reductions in antioxidant levels, in addition to lower muscle performance and poor life quality, are possible independent contributors to greater muscle pain and lower lean body mass in FM patients.

NADPH Oxidases in Pain Processing

Inflammation or injury to the somatosensory nervous system may result in chronic pain conditions, which affect millions of people and often cause major health problems. Emerging lines of evidence indicate that reactive oxygen species (ROS), such as superoxide anion or hydrogen peroxide, are produced in the nociceptive system during chronic inflammatory and neuropathic pain and act as specific signaling molecules in pain processing. Among potential ROS sources in the somatosensory system are NADPH oxidases, a group of electron-transporting transmembrane enzymes whose sole function seems to be the generation of ROS. Interestingly, the expression and relevant function of the Nox family members Nox1, Nox2, and Nox4 in various cells of the nociceptive system have been demonstrated. Studies using knockout mice or specific knockdown of these isoforms indicate that Nox1, Nox2, and Nox4 specifically contribute to distinct signaling pathways in chronic inflammatory and/or neuropathic pain states. As selective Nox inhibitors are currently being developed and investigated in various physiological and pathophysiological settings, targeting Nox1, Nox2, and/or Nox4 could be a novel strategy for the treatment of chronic pain. Here, we summarize the distinct roles of Nox1, Nox2, and Nox4 in inflammatory and neuropathic processing and discuss the effectiveness of currently available Nox inhibitors in the treatment of chronic pain conditions.

Effects of administration of α2 adrenergic receptor agonist into psoas major muscle on inflammatory pain induced by injection of complete Freund's adjuvant in rats

α₂ adrenergic agonists are widely used in clinical anesthesia and ICU sedation owing to their effective sedative and analgesic effects. Lumbago and leg pain is the most common clinical pain disease. Studies have reported that lumbago and leg pain is associated with dysregulation of paravertebral muscles, especially psoas major muscles. In the present study, a unilateral lower extremity chronic inflammation and pain model was established by subcutaneous administration of low-dose complete Fredrin’s adjuvant (CFA) into the posterior paw of rats. α₂ adrenalin receptor agonist was then injected into the psoas major muscle. Behavioral tests were conducted for 21 days. Psoas major muscle tissue was harvested for evaluation of biochemical indexes related to pain. The effect of α₂ adrenergic receptor agonist injected into psoas major muscle on chronic inflammatory pain of lower extremities in rats was explored. The results showed that injection of α₂ adrenergic receptor agonist into the psoas major muscle relieved CFA-induced mechanical hyperalgesia. Administration of α₂ adrenergic receptor antagonist yohimbine reversed the analgesic effect of α₂ adrenergic receptor agonists. Administration of dexmedetomidine into psoas major muscle downregulated the levels of norepinephrine, interleukin-6 and tumor necrosis factor-α in tissues. The findings of the present study show that administration of α₂ adrenoceptor agonists into the psoas major muscle relieves chronic inflammatory pain induced by CFA. Local injection of dexmedetomidine also exerted anti-inflammatory and anti-sympathetic effect by activating α₂-adrenoceptor in the psoas major muscle.

Sestrin2 overexpression attenuates osteoarthritis pain via induction of AMPK/PGC-1α-mediated mitochondrial biogenesis and suppression of neuroinflammation

BACKGROUND

Our previous study indicated that reactive oxygen species (ROS) are critically involved in chronic pain. Sestrin2 (Sesn2), a novel stress-inducible protein, is evidenced to reduce the generation of ROS. The study examined the role of Sesn2 in osteoarthritis (OA) pain and delineated the underlying molecular mechanisms.

METHODS

In the present study, we investigated the impact of Sesn2 on mitochondrial biogenesis in a rat model of OA pain. After adeno-associated viral (AAV)-Sesn2EGFP was injected for 14 days, OA was induced by intra-articular injection of monosodium iodoacetate (MIA). We assessed pain behaviors (weight-bearing asymmetry and paw withdrawal threshold) and explored possible mechanisms in the L4-6 spinal cord.

RESULTS

Our results showed that overexpression of Sesn2 in the spinal cord alleviated pain behaviors in OA rats. Moreover, overexpression of Sesn2 increased the activity of AMP-activated protein kinase (AMPK) signaling and significantly restored mitochondrial biogenesis. Besides, Sesn2 overexpression inhibited the activation of astrocytes and microglia, and decreased the production of interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) in the spinal cord of the OA pain rats. These effects were significantly reversed by an AMPK inhibitor.

CONCLUSIONS

Collectively, these results suggest that Sesn2 overexpression ameliorates mechanical allodynia and weight-bearing asymmetry in OA rats via activation of AMPK/PGC-1α-mediated mitochondrial biogenesis in the spinal cord. Moreover, Sesn2 overexpression attenuates OA-induced neuroinflammation at least partly by activating AMPK signaling. Sesn2 may become an encouraging therapeutic strategy for OA pain relief and other disorders.

Inhibiting Hv1 channel in peripheral sensory neurons attenuates chronic inflammatory pain and opioid side effects

Both opioids and nonsteroidal anti-inflammatory drugs (NSAIDS) produce deleterious side effects and fail to provide sustained relief in patients with chronic inflammatory pain. Peripheral neuroinflammation (PN) is critical for initiation and development of inflammatory pain. A better understanding of molecular mechanisms underlying PN would facilitate the discovery of new analgesic targets and the development of new therapeutics. Emerging evidence suggests that peripheral sensory neurons are not only responders to painful stimuli, but are also actively engaged in inflammation and immunity, whereas the intrinsic regulatory mechanism is poorly understood. Here we report the expression of proton-selective ion channel Hv1 in peripheral sensory neurons in rodents and humans, which was previously shown as selectively expressed in microglia in mammalian central nervous system. Neuronal Hv1 was up-regulated by PN or depolarizing stimulation, which in turn aggravates inflammation and nociception. Inhibiting neuronal Hv1 genetically or by a newly discovered selective inhibitor YHV98-4 reduced intracellular alkalization and ROS production in inflammatory pain, mitigated the imbalance in downstream SHP-1-pAKT signaling, and also diminished pro-inflammatory chemokine release to alleviate nociception and morphine-induced hyperalgesia and tolerance. Thus, our data reveal neuronal Hv1 as a novel target in analgesia strategy and managing opioids-related side effects.

The impact of SBF2 on taxane-induced peripheral neuropathy

Taxane-induced peripheral neuropathy (TIPN) is a devastating survivorship issue for many cancer patients. In addition to its impact on quality of life, this toxicity may lead to dose reductions or treatment discontinuation, adversely impacting survival outcomes and leading to health disparities in African Americans (AA). Our lab has previously identified deleterious mutations in SET-Binding Factor 2 (SBF2) that significantly associated with severe TIPN in AA patients. Here, we demonstrate the impact of SBF2 on taxane-induced neuronal damage using an ex vivo model of SBF2 knockdown of induced pluripotent stem cell-derived sensory neurons. Knockdown of SBF2 exacerbated paclitaxel changes to cell viability and neurite outgrowth while attenuating paclitaxel-induced sodium current inhibition. Our studies identified paclitaxel-induced expression changes specific to mature sensory neurons and revealed candidate genes involved in the exacerbation of paclitaxel-induced phenotypes accompanying SBF2 knockdown. Overall, these findings provide ex vivo support for the impact of SBF2 on the development of TIPN and shed light on the potential pathways involved.

Targeted up-regulation of Drp1 in dorsal horn attenuates neuropathic pain hypersensitivity by increasing mitochondrial fission

Mitochondria play an essential role in pathophysiology of both inflammatory and neuropathic pain (NP), but the mechanisms are not yet clear. Dynamin-related protein 1 (Drp1) is broadly expressed in the central nervous system and plays a role in the induction of mitochondrial fission process. Spared nerve injury (SNI), due to the dysfunction of the neurons within the spinal dorsal horn (SDH), is the most common NP model. We explored the neuroprotective role of Drp1 within SDH in SNI. SNI mice showed pain behavior and anxiety-like behavior, which was associated with elevation of Drp1, as well as increased density of mitochondria in SDH. Ultrastructural analysis showed SNI induced damaged mitochondria into smaller perimeter and area, tending to be circular. Characteristics of vacuole in the mitochondria further showed SNI induced the increased number of vacuole, widened vac-perimeter and vac-area. Stable overexpression of Drp1 via AAV under the control of the Drp1 promoter by intraspinal injection (Drp1 OE) attenuated abnormal gait and alleviated pain hypersensitivity of SNI mice. Mitochondrial ultrastructure analysis showed that the increased density of mitochondria induced by SNI was recovered by Drp1 OE which, however, did not change mitochondrial morphology and vacuole parameters within SDH. Contrary to Drp1 OE, down-regulation of Drp1 in the SDH by AAV-Drp1 shRNA (Drp1 RNAi) did not alter painful behavior induced by SNI. Ultrastructural analysis showed the treatment by combination of SNI and Drp1 RNAi (SNI + Drp1 RNAi) amplified the damages of mitochondria with the decreased distribution density, increased perimeter and area, as well as larger circularity tending to be more circular. Vacuole data showed SNI + Drp1 RNAi increased vacuole density, perimeter and area within the SDH mitochondria. Our results illustrate that mitochondria within the SDH are sensitive to NP, and targeted mitochondrial Drp1 overexpression attenuates pain hypersensitivity. Drp1 offers a novel therapeutic target for pain treatment.

In Vivo Investigation of the Ameliorating Effect of Tempol against MIA-Induced Knee Osteoarthritis in Rats: Involvement of TGF-β1/SMAD3/NOX4 Cue

Osteoarthritis (OA) is a complex disease characterized by structural, functional, and metabolic deteriorations of the whole joint and periarticular tissues. In the current study, we aimed to investigate the possible effects of tempol on knee OA induced by the chemical chondrotoxic monosodium iodoacetate (MIA) which closely mimics both the pain and structural changes associated with human OA. Rats were administrated oral tempol (100 mg/kg) one week post-MIA injection (3 mg/50 μL saline) at the right knee joints for 21 consecutive days. Tempol improved motor performance and debilitated the MIA-related radiological and histological alterations. Moreover, it subsided the knee joint swelling. Tempol decreased the cartilage degradation-related biomarkers as matrix metalloproteinase-13, bone alkaline phosphatase (bone ALP), and fibulin-3. The superoxide dismutase mimetic effect of tempol was accompanied by decreased NADPH oxidase 4 (NOX4), inflammatory mediators, nuclear factor-kappa B (NF-κB), over-released transforming growth factor-β1 (TGF-β1). Tempol decreased the expression of chemokine (C-C motif) ligand 2 (CCL2). On the molecular level, tempol reduced the phosphorylated protein levels of p38 mitogen-activated protein kinase (MAPK), and small mother against decapentaplegic 3 homologs (SMAD3). These findings suggest the promising role of tempol in ameliorating MIA-induced knee OA in rats via collateral suppression of the catabolic signaling cascades including TGF-β1/SMAD3/NOX4, and NOX4/p38MAPK/NF-κB and therefore modulation of oxidative stress, catabolic inflammatory cascades, chondrocyte metabolic homeostasis.

The 5-HT1A receptor agonist, 8-OH-DPAT, attenuates long-lasting pain in imiquimod-induced psoriasis in mice

Psoriasis pain is a common symptom underestimated and rarely evaluated in psoriasis clinical trials. This work aimed to investigate whether the development of secondary chronic allodynia and hyperalgesia in the imiquimod (IMQ)-induced psoriasis mice model could be modulated by anti-inflammatory agents and compound 48/80 (C48/80) and to determine whether the activation of 5-HT_1A receptor modulates these nociceptive behaviours. C57BL/6 male mice were treated with 5% IMQ for 7 days. The paw withdrawal responses to von Frey filaments (10 and 250 mN) were used to assess the allodynia and hyperalgesia. Nociceptive behaviours were also evaluated using ketorolac 15 mg/kg s.c., adalimumab 10 mg/kg s.c. and C48/80 10 mg/kg i.p. Then, the serum serotonin and the impact of 8-OH-DPAT (1 mg/kg s.c), a 5-HT_1A receptor agonist, on long-lasting pain were examined. Mice receiving IMQ showed enhanced nociception, which decreased with all tested compounds. The serum serotonin in the IMQ group showed a significant decrease (947.042 ng/ml) regarding the control group (1143.68 ng/ml). The pretreatment with 8-OH-DPAT alleviated pain-related behaviours. These results suggest that the long-lasting pain resulting from psoriasis inflammation is also associated with the serotonergic system. The 5-HT1A receptor should be further explored as a potential therapeutic target for psoriasis pain modulation.

Non-drug pain relievers active on non-opioid pain mechanisms

This review is aimed to summarize the pain-relieving effect of non-drug substances, mostly prescribed as integrators in treatment of pain, including especially in chronic postoperative pain (CPSP) and in chronic back pain after acute episodes. Their use reflects the fact that the current treatments for these syndromes continue to pose problems of unsatisfactory responses in a significant portion of patients and/or of an excess of side effects like those noted in the present opioid crisis. As integrators are frequently introduced into the market without adequate clinical testing, this review is aimed to collect the present scientific evidence either preclinical or clinical for their effectiveness. In particular, we reviewed the data on the use of: B vitamins; vitamin C; vitamin D; alpha lipoic acid (ALA); N-acetylcysteine; acetyl L-carnitine; curcumin; boswellia serrata; magnesium; coenzyme Q10, and palmitoylethanolamide. The combination of preclinical findings and clinical observations strongly indicate that these compounds deserve more careful attention, some of them having interesting clinical potentials also in preventing chronic pain after an acute episode. In particular, examining their putative mechanisms of action it emerges that combinations of few of them may exert an extraordinary spectrum of activities on a large variety of pain-associated pathways and may be eventually used in combination with more traditional pain killers in order to extend the duration of the effect and to lower the doses. Convincing examples of effective combinations against pain are vitamin B complex plus gabapentin for CPSP, including neuropathic pain; vitamin B complex plus diclofenac against low back pain and also in association with gabapentin, and ALA for burning mouth syndrome. These as well as other examples need, however, careful controlled independent clinical studies confirming their role in therapy.

Antihypersensitivity effect of betanin (red beetroot extract) via modulation of microglial activation in a mouse model of neuropathic pain

BACKGROUND

Neuropathic pain (NeP) medications have several side effects that affect NeP patients' quality of life. Betanin, the most common betacyanin pigment, has been shown to have potent antioxidant and anti-inflammatory properties in vivo; thus, it has potential as a healthcare treatment. In this study, we focused on betanin (red beetroot extract) as a potential therapy for NeP.

METHODS

Mice model of NeP were made by chronic constriction injury (CCI), and the development of mechanical hypersensitivity was confirmed using the von Frey test. Motor coordination and locomotor activity were assessed using open field tests and rotarod tests, respectively. The expression level of glial markers in the spinal cords was analyzed by immunostaining. The direct effects of betanin on microglial cells were investigated using primary cultured microglial cells.

RESULTS

In CCI model mice, repeated betanin treatment, both intraperitoneally and orally, attenuated developing mechanical hypersensitivity in a dose-dependent manner without impairing motor coordination. Betanin treatment also attenuated mechanical hypersensitivity that had developed and prevented the onset of mechanical hypersensitivity in CCI mice. Microglial activation in the spinal cord is known to play a key role in the development of NeP; betanin treatment reduced CCI-induced microglial activation in the spinal cord of model mice. Moreover, in primary microglia cultured cells, the activation of microglia by lipopolysaccharide application was suppressed by betanin treatment.

CONCLUSION

Betanin treatment appears to ameliorate mechanical hypersensitivity related to CCI-induced NeP in mice by inhibiting microglial activation.

SIGNIFICANCE

This article supports findings of the effect of betanin on NeP and provides a potential therapeutic candidate for NeP. Furthermore, elucidating the underlying mechanism of the effect of betanin on microglial activation could assist the development of new treatments for chronic pain.

Mitochondrial biogenesis factor PGC-1α suppresses spinal morphine tolerance by reducing mitochondrial superoxide

Opioid use disorders (OUDs) have reached an epidemic level in the United States. The opioid epidemic involves illicit opioid use, prescription opioids for analgesia, counterfeit opioids, new psychoactive substances, and diverted opioids. Opioids remain the last option for the treatment of intractable clinical pain, but chronic use of opioids are limited in part due to antinociceptive/analgesic tolerance. Peroxisome proliferator-activated receptor (PPAR)-gamma coactivator-1alpha (PGC-1α), a mitochondrial biogenesis factor can reduce toxic reactive oxygen species (ROS) that play a role in morphine tolerance (MT). Decreased PGC-1α expression has been shown to contribute to various metabolic disorders or neurodegeneration diseases through increasing ROS. We examined the relationship of PGC-1α and ROS in MT. To induce MT, adult Sprague-Dawley rats received intrathecal morphine for 7 days. Mechanical threshold was measured using the von Frey test and thermal latency was examined using the heat plate test. Expression of PGC-1α in the spinal cord dorsal horn (SCDH) was examined using RT-PCR and western blots. Mitochondrial superoxide was detected using MitoSox Red, a mitochondrial superoxide indicator. The antinociceptive effect of recombinant PGC-1α (rPGC-1α) or Mito-Tempol (a mitochondria-targeted superoxide scavenger) was determined using the von Frey test and hot plate test. Furthermore, we examined the effect of rPGC-1α on mitochondrial superoxide using cultured neurons. Our findings include that: (i) spinal MT decreased the expression of spinal PGC-1α in the SCDH neurons; (ii) rPGC-1α increased mechanical threshold and thermal latency in MT animals; (iii) Mito-Tempol reduced MT behavioral response; (iv) rPGC-1α reduced MT-induced mitochondria-targeted superoxide; and (v) cultured neuronal cells treated with TNFα increased mitochondria-targeted superoxide that can be inhibited by rPGC-1α. The present findings suggest that spinal PGC-1α reduce MT through decreasing mitochondria-targeted superoxide in the SCDH.

Nicotinamide adenine dinucleotide phosphate oxidase 2-derived reactive oxygen species contribute to long-term potentiation of C-fiber-evoked field potentials in spinal dorsal horn and persistent mirror-image pain following high-frequency stimulus of the sciatic nerve

High-frequency stimulation (HFS) of the sciatic nerve has been reported to produce long-term potentiation (LTP) and long-lasting pain hypersensitivity in rats. However, the central underlying mechanism remains unclear. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) belongs to a group of electron-transporting transmembrane enzymes that produce reactive oxygen species (ROS). Here, we found that NOX2 was upregulated in the lumbar spinal dorsal horn after HFS of the left sciatic nerve, which induced bilateral pain and spinal LTP in both male and female rats. Blocking NOX2 with blocking peptide or shRNA prevented the development of bilateral mechanical allodynia, the induction of spinal LTP, and the phosphorylation of N-methyl-d-aspartate (NMDA) receptor 2B (GluN2B) and nuclear factor kappa-B (NF-κB) p65 after HFS. Moreover, NOX2 shRNA reduced the frequency and amplitude of both spontaneous excitatory postsynaptic currents and miniature excitatory postsynaptic currents in laminar II neurons. Furthermore, 8-hydroxyguanine (8-OHG), an oxidative stress marker, was increased in the spinal dorsal horn. Spinal application of ROS scavenger, Phenyl-N-tert-butylnitrone (PBN), depressed the already established spinal LTP. Spinal application of H2O2, one ROS, induced LTP and bilateral mechanical allodynia, increased the frequency and amplitude of spontaneous excitatory postsynaptic currents in laminar II neurons, and phosphorylated GluN2B and p65 in the dorsal horn. This study provided electrophysiological and behavioral evidence that NOX2-derived ROS in the spinal cord contributed to persistent mirror-image pain by enhancing the synaptic transmission, which was mediated by increasing presynaptic glutamate release and activation of NMDA receptor and NF-κB in the spinal dorsal horn.

Neurophysiological mechanisms of chiropractic spinal manipulation for spine pain

Together, neck pain and back pain are the first cause of disability worldwide, accounting for more than 10% of the total years lived with disability. In this context, chiropractic care provides a safe and effective option for the management of a large proportion of these patients. Chiropractic is a healthcare profession mainly focused on the spine and the treatment of spinal disorders, including spine pain. Basic studies have examined the influence of chiropractic spinal manipulation (SM) on a variety of peripheral, spinal and supraspinal mechanisms involved in spine pain. While spinal cord mechanisms of pain inhibition contribute at least partly to the pain-relieving effects of chiropractic treatments, the evidence is weaker regarding peripheral and supraspinal mechanisms, which are important components of acute and chronic pain. This narrative review highlights the most relevant mechanisms of pain relief by SM and provides a perspective for future research on SM and spine pain, including the validation of placebo interventions that control for placebo effects and other non-specific effects that may be induced by SM. SIGNIFICANCE: Spinal manipulation inhibits back and neck pain partly through spinal segmental mechanisms and potentially through peripheral mechanisms regulating inflammatory responses. Other mechanisms remain to be clarified. Controls and placebo interventions need to be improved in order to clarify the contribution of specific and non-specific effects to pain relief by spinal manipulative therapy.

The Interplay between Oxidative Stress, Exercise, and Pain in Health and Disease: Potential Role of Autonomic Regulation and Epigenetic Mechanisms

Oxidative stress can be induced by various stimuli and altered in certain conditions, including exercise and pain. Although many studies have investigated oxidative stress in relation to either exercise or pain, the literature presents conflicting results. Therefore, this review critically discusses existing literature about this topic, aiming to provide a clear overview of known interactions between oxidative stress, exercise, and pain in healthy people as well as in people with chronic pain, and to highlight possible confounding factors to keep in mind when reflecting on these interactions. In addition, autonomic regulation and epigenetic mechanisms are proposed as potential mechanisms of action underlying the interplay between oxidative stress, exercise, and pain. This review highlights that the relation between oxidative stress, exercise, and pain is poorly understood and not straightforward, as it is dependent on the characteristics of exercise, but also on which population is investigated. To be able to compare studies on this topic, strict guidelines should be developed to limit the effect of several confounding factors. This way, the true interplay between oxidative stress, exercise, and pain, and the underlying mechanisms of action can be revealed and validated via independent studies.

Reactive Oxygen Species (ROS) are Critical for Morphine Exacerbation of HIV-1 gp120-Induced Pain

Many HIV patients develop chronic pain and use opioid-derived medicine as primary analgesics. Emerging clinical evidence suggests that chronic use of opioid analgesics paradoxically heightens pain states in patients. This side effect of opioid analgesics has a significant negative impact on clinical practice, but the underlying pathogenic mechanism remains elusive. Using a mouse model of HIV-associated pain, we simulated the development of morphine exacerbation on pain and investigated potential underlying cellular and molecular pathways. We found that repeated morphine treatment promoted astrocyte activation in the spinal dorsal horn (SDH) and up-regulation of pro-inflammatory cytokines IL-1β and TNF-α. Furthermore, we observed that morphine administration potentiated mitochondrial reactive oxygen species (ROS) in the SDH of the HIV pain model, especially on astrocytes. Systemic application of the ROS scavenger phenyl-N-t-butyl nitrone (PBN) not only blocked the enhancement of gp120-induced hyperalgesia by morphine but also astrocytic activation and cytokine up-regulation. These findings suggest a critical role of ROS in mediating the exacerbation of gp120-induced pain by morphine. Graphical abstract.

PKCγ interneurons, a gateway to pathological pain in the dorsal horn

Chronic pain is a frequent and disabling condition that is significantly maintained by central sensitization, which results in pathological amplification of responses to noxious and innocuous stimuli. As such, mechanical allodynia, or pain in response to a tactile stimulus that does not normally provoke pain, is a cardinal feature of chronic pain. Recent evidence suggests that the dorsal horn excitatory interneurons that express the γ isoform of protein kinase C (PKCγ) play a critical role in the mechanism of mechanical allodynia during chronic pain. Here, we review this evidence as well as the main aspects of the development, anatomy, electrophysiology, inputs, outputs, and pathophysiology of dorsal horn PKCγ neurons. Primary afferent high-threshold neurons transmit the nociceptive message to the dorsal horn of the spinal cord and trigeminal system where it activates second-order nociceptive neurons relaying the information to the brain. In physiological conditions, low-threshold mechanoreceptor inputs activate inhibitory interneurons in the dorsal horn, which may control activation of second-order nociceptive neurons. During chronic pain, low-threshold mechanoreceptor inputs now activate PKCγ neurons that forward the message to second-order nociceptive neurons, turning thus tactile inputs into pain. Several mechanisms may contribute to opening this gate, including disinhibition, activation of local astrocytes, release of diffusible factors such as reactive oxygen species, and alteration of the descending serotoninergic control on PKCγ neurons through 5-HT_2A serotonin receptors. Dorsal horn PKCγ neurons, therefore, appear as a relevant therapeutic target to alleviate mechanical allodynia during chronic pain.

Oxidative Stress Contributes to Hyperalgesia in Osteoporotic Mice

Purpose

Chronic pain is one of the most common complications of postmenopausal osteoporosis. Since oxidative stress is involved in the pathogenesis of postmenopausal osteoporosis, we explored whether oxidative stress contributes to postmenopausal osteoporotic pain.

Methods

Osteoporosis was induced in mice by ovariectomy (OVX). Pain-related behaviours were assessed by measuring sensitivity to mechanical, thermal and cold stimulation. The expression of pain-related transcripts, such acid-sensing ion channel 3 (ASIC3), transient receptor potential vanilloid 1 (TRPV1) and calcitonin gene-related peptide (CGRP), was evaluated. Plasma markers of oxidative stress were also measured. In addition, the effects of the reactive oxygen species scavenger phenyl N-tert-butylnitrone (PBN) on these parameters were assessed.

Results

The OVX mice presented hyperalgesia, as demonstrated by decreased paw withdrawal thresholds to mechanical stimulation and withdrawal latencies to thermal and cold stimulation, along with upregulated expression of ASIC3, TRPV1 and CGRP in the dorsal root ganglia, spinal cord and thalamus tissue. OVX elevated the plasma levels of malondialdehyde (MDA) and advanced oxidation protein products (AOPPs). However, the administration of PBN alleviated these effects.

Conclusion

Our results indicated that oxidative stress contributes to hyperalgesia in OVX mice. Enhanced oxidative stress may be associated with osteoporotic pain. Antioxidant treatment could help alleviate chronic pain in postmenopausal osteoporotic patients.

Ryanodine Receptor to Mitochondrial Reactive Oxygen Species Pathway Plays an Important Role in Chronic Human Immunodeficiency Virus gp120MN-Induced Neuropathic Pain in Rats

BACKGROUND

Chronic pain is one of the most common complaints in patients with human immunodeficiency virus (HIV)-associated sensory neuropathy. Ryanodine receptor (RyR) and mitochondrial oxidative stress are involved in neuropathic pain induced by nerve injury. Here, we investigated the role of RyR and mitochondrial superoxide in neuropathic pain induced by repeated intrathecal HIV glycoprotein 120 (gp120) injection.

METHODS

Recombinant HIV glycoprotein gp120MN was intrathecally administered to induce neuropathic pain. Mechanical threshold was tested using von Frey filaments. Peripheral nerve fiber was assessed by the quantification of the intraepidermal nerve fiber density in the skin of the hindpaw. The expression of spinal RyR was examined using Western blots. Colocalization of RyR with neuronal nuclei (NeuN; neuron marker), glial fibrillary acidic protein (GFAP; astrocyte marker), or ionizing calcium-binding adaptor molecule 1 (Iba1; microglia marker) in the spinal cord was examined using immunohistochemistry. MitoSox-positive profiles (a mitochondrial-targeted fluorescent superoxide indicator) were examined. The antiallodynic effects of intrathecal administration of RyR antagonist, dantrolene (a clinical drug for malignant hyperthermia management), or selective mitochondrial superoxide scavenger, Mito-Tempol, were evaluated in the model.

RESULTS

We found that repeated but not single intrathecal injection of recombinant protein gp120 induced persistent mechanical allodynia. Intraepidermal nerve fibers in repeated gp120 group was lower than that in sham at 2 weeks, and the difference in means (95% confidence interval) was 8.495 (4.79-12.20), P = .0014. Repeated gp120 increased expression of RyR, and the difference in means (95% confidence interval) was 1.50 (0.504-2.495), P = .007. Repeated gp120 also increased mitochondrial superoxide cell number in the spinal cord, and the difference in means (95% confidence interval) was 6.99 (5.99-8.00), P < .0001. Inhibition of spinal RyR or selective mitochondrial superoxide scavenger dose dependently reduced mechanical allodynia induced by repeated gp120 injection. RyR and mitochondrial superoxide were colocalized in the neuron, but not glia. Intrathecal injection of RyR inhibitor lowered mitochondrial superoxide in the spinal cord dorsal horn in the gp120 neuropathic pain model.

CONCLUSIONS

These data suggest that repeated intrathecal HIV gp120 injection induced an acute to chronic pain translation in rats, and that neuronal RyR and mitochondrial superoxide in the spinal cord dorsal horn played an important role in the HIV neuropathic pain model. The current results provide evidence for a novel approach to understanding the molecular mechanisms of HIV chronic pain and treating chronic pain in patients with HIV.

Manganese Oxide Nanozymes Ameliorate Mechanical Allodynia in a Rat Model of Partial Sciatic Nerve-Transection Induced Neuropathic Pain

BACKGROUND

Reactive oxygen species (ROS) induced oxidative stress is linked to numerous neurological diseases, including neuropathic pain. Natural ROS scavenging enzymes like superoxide dismutase (SOD) and catalase have been found to be efficient in alleviating neuropathic pain. However, their sensitivity towards extreme pH and a short half-life limit their efficacy in vivo. Manganese oxide nanoparticles (MONPs) are recently known to possess ROS scavenging properties. In this study, MONPs were examined for their therapeutic effect on neuropathic pain.

METHODS

The MONPs were synthesized by a hydrothermal method. The synthesized MONPs were characterized by UV/Vis, TEM, SEM, FTIR, NTA and XRD. The biocompatibility of the nanoparticles is evaluated in neural cells using LDH assay. MONPs were evaluated for their antioxidant activity by DPPH assay. In addition, in vitro ROS scavenging properties were examined in bone marrow-derived macrophage (BMDM) cells using 2',7'-dichlorofluorescin diacetate (DCFDA) assay. To evaluate the in vivo efficacy of nanoparticles, neuropathic pain was induced in Wistar rats by partial sciatic nerve transection (PSNT). On post-transection days 14 to 18, rats were intrathecally injected with MONPs and paw withdrawal threshold was measured. The spinal cords were collected and processed for Western blotting and histological analysis.

RESULTS

The synthesized MONPs were biocompatible and showed effective antioxidant activity against DPPH free radical scavenging. Further, the nanoparticles scavenged ROS efficiently in vitro in BMDM and their intrathecal administration significantly reduced mechanical allodynia as well as the expression of cyclooxygenase-2 (COX-2), an important mediator of chronic and inflammatory pain in the spinal dorsal horns of PSNT rats.

CONCLUSION

As ROS play a significant role in neuropathic pain, we expect that MONPs could be a promising tool for the treatment of various inflammatory diseases and might also serve as a potential nanocarrier for the delivery of analgesics.

Mitochondrially-targeted treatment strategies

Disruption of mitochondrial function is a common feature of inherited mitochondrial diseases (mitochondriopathies) and many other infectious and non-infectious diseases including viral, bacterial and protozoan infections, inflammatory and chronic pain, neurodegeneration, diabetes, obesity and cardiovascular diseases. Mitochondria therefore become an attractive target for developing new therapies. In this review we describe critical mechanisms involved in the maintenance of mitochondrial functionality and discuss strategies used to identify and validate mitochondrial targets in different diseases. We also highlight the most recent preclinical and clinical findings using molecules targeting mitochondrial bioenergetics, morphology, number, content and detoxification systems in common pathologies.

Neurosteroids and neuropathic pain management: Basic evidence and therapeutic perspectives

Complex mechanisms involved in neuropathic pain that represents a major health concern make its management complicated. Because neurosteroids are bioactive steroids endogenously synthesized in the nervous system, including in pain pathways, they appear relevant to develop effective treatments against neuropathic pain. Neurosteroids act in paracrine or autocrine manner through genomic mechanisms and/or via membrane receptors of neurotransmitters that pivotally modulate pain sensation. Basic studies which uncovered a direct link between neuropathic pain symptoms and endogenous neurosteroid production/regulation, paved the way for the investigations of neurosteroid therapeutic potential against pathological pain. Concordantly, antinociceptive properties of synthetic neurosteroids were evidenced in humans and animals. Neurosteroids promote peripheral analgesia mediated by T-type calcium and gamma-aminobutyric acid type A channels, counteract chemotherapy-induced neuropathic pain and ameliorate neuropathic symptoms of injured spinal cord animals by stimulating anti-inflammatory, remyelinating and neuroprotective processes. Together, these data open interesting perspectives for neurosteroid-based strategies to manage/alleviate efficiently neuropathic pain.

Melatonin MT2 receptor agonist IIK-7 produces antinociception by modulation of ROS and suppression of spinal microglial activation in neuropathic pain rats

Background

In recent years, several melatonin (MLT) receptor agonists have been approved by FDA for the treatment of sleep disorders and depression. Very few studies have shed light on their efficacy against neuropathic pain (NP). IIK-7 is an MT-2 agonist known to promote sleep. Whether IIK-7 suppresses NP has not been reported, and the signaling profile is unknown.

Objective

To investigate the effect of melatonin type 2 receptor agonist IIK-7 on partial sciatic nerve transection-induced NP in rats and elucidate the underlying molecular mechanisms.

Methods

NP was induced by the PSNT in the left leg of adult male Wistar rats. On post-transection day 7, rats were implanted with intrathecal (i.t) catheter connected to an infusion pump and divided in to four groups: sham-operated/vehicle, PSNT/vehicle, PSNT/0.5 μg/hr IIK-7 and PSNT/0.5 μg IIK-7/1 μg 4-p/hr. To test the MT-2 dependence on IIK-7 activity, the animals were implanted with a single i.t catheter and injected MT-2 antagonist 4-Phenyl-2-propionamidotetralin (4-p) 20 mins prior to IIK-7 injection on day 7 after PSNT. The antinociceptive response was measured using a mechanical paw withdrawal threshold. Activation of microglial cells and the expression of NP-associated proteins in the spinal cord dorsal horn was assessed by immunofluorescence assay (IFA) and Western blotting (WB). Reactive oxygen species (ROS) scavenging ability of IIK-7 was evaluated by using bone marrow-derived macrophages (BMDM).

Results

Treatment with the MT-2 agonist IIK-7 significantly alleviated PSNT-induced mechanical allodynia and glial activation along with the inhibition of P44/42 MAPK, HMGB-1, STAT3, iNOS and casp-3 proteins.

Conclusion

IIK-7 attenuates NP through the suppression of glial activation and suppression of proteins involved in inflammation and apoptosis. MT-2 receptor agonists may establish a promising and unique therapeutic approach for the treatment of NP.

PIEZO1 Is Selectively Expressed in Small Diameter Mouse DRG Neurons Distinct From Neurons Strongly Expressing TRPV1

Using a high resolution in situ hybridization technique we have measured PIEZO1, PIEZO2, and TRPV1 transcripts in mouse dorsal root ganglion (DRG) neurons. Consistent with previous studies, PIEZO2 transcripts were highly expressed in DRG neurons of all sizes, including most notably the largest diameter neurons implicated in mediating touch and proprioception. In contrast, PIEZO1 transcripts were selectively expressed in smaller DRG neurons, which are implicated in mediating nociception. Moreover, the small neurons expressing PIEZO1 were mostly distinct from those neurons that strongly expressed TRPV1, one of the channels implicated in heat-nociception. Interestingly, while PIEZO1- and TRPV1- expressing neurons form essentially non-overlapping populations, PIEZO2 showed co-expression in both populations. Using an in vivo functional test for the selective expression, we found that Yoda1, a PIEZO1-specific agonist, induced a mechanical hyperalgesia that displayed a significantly prolonged time course compared with that induced by capsaicin, a TRPV1-specific agonist. Taken together, our results indicate that PIEZO1 should be considered a potential candidate in forming the long sought channel mediating mechano-nociception.

Liposomal Curcumin Enhances the Effect of Naproxen in a Rat Model of Migraine

BACKGROUND Curcumin is an antioxidant that reduces inflammation and pain. This study aimed to assess the effect of pretreatment with naproxen and liposomal curcumin compared with naproxen and curcumin solution on oxidative stress parameters and pain in a rat model of migraine. MATERIAL AND METHODS Sixty-three male Wistar rats included a control group (n=9) and a rat model of migraine (n=54) induced by intraperitoneal injection of nitroglycerin (1 mg/0.1 kg). The rat model group was divided into an untreated control group (n=9), a group pretreated with naproxen alone (2.8 mg/kg) (n=9), a group pretreated with naproxen (2.8 mg/kg) combined with curcumin solution (1 mg/0.1 kg) (n=9), a group pretreated with naproxen (2.8 mg/kg) combined with curcumin solution (2 mg/0.1 kg) (n=9), a group pretreated with naproxen (2.8 mg/kg) combined with liposomal curcumin solution (1 mg/0.1 kg) (n=9) a group pretreated with naproxen (2.8 mg/kg) combined with liposomal curcumin solution (2 mg/0.1 kg) (n=9). Spectroscopy measured biomarkers of total oxidative status and nociception was tested using an injection of 1% of formalin into the rat paw. RESULTS Expression of biomarkers of oxidative stress and enhanced nociception were significantly increased following pretreatment with combined naproxen and liposomal curcumin compared with curcumin solution or naproxen alone (P<0.001). Combined curcumin solution and naproxen were more effective at a concentration of 2 mg/0.1kg for the first nociceptive phase (P<0.005). CONCLUSIONS In a rat model of migraine, combined therapy with liposomal curcumin and naproxen showed an improved antioxidant effect and anti-nociceptive effect.

Allopregnanolone and Progesterone in Experimental Neuropathic Pain: Former and New Insights with a Translational Perspective

In the last decades, an active and stimulating area of research has been devoted to explore the role of neuroactive steroids in pain modulation. Despite challenges, these studies have clearly contributed to unravel the multiple and complex actions and potential mechanisms underlying steroid effects in several experimental conditions that mimic human chronic pain states. Based on the available data, this review focuses mainly on progesterone and its reduced derivative allopregnanolone (also called 3α,5α-tetrahydroprogesterone) which have been shown to prevent or even reverse the complex maladaptive changes and pain behaviors that arise in the nervous system after injury or disease. Because the characterization of new related molecules with improved specificity and enhanced pharmacological profiles may represent a crucial step to develop more efficient steroid-based therapies, we have also discussed the potential of novel synthetic analogs of allopregnanolone as valuable molecules for the treatment of neuropathic pain.

Mitochondrial superoxide increases excitatory synaptic strength in spinal dorsal horn neurons of neuropathic mice

Reactive oxygen species has been suggested as a key player in neuropathic pain, causing central sensitization by changing synaptic strengths in spinal dorsal horn neurons. However, it remains unclear as to what type of reactive oxygen species changes what aspect of synaptic strengths for central sensitization in neuropathic pain conditions. In this study, we investigated whether mitochondrial superoxide affects both excitatory and inhibitory synaptic strengths in spinal dorsal horn neurons after peripheral nerve injury. Upregulation of mitochondrial superoxide level by knockout of superoxide dismutase-2 exacerbated neuropathic mechanical hypersensitivity caused by L5 spinal nerve ligation, whereas downregulation of mitochondrial superoxide level by overexpression of superoxide dismutase-2 alleviated the hypersensitivity. In spinal nerve ligation condition, the frequency of miniature excitatory postsynaptic currents increased, while that of miniature inhibitory postsynaptic currents decreased in spinal dorsal horn neurons. Superoxide dismutase-2-knockout augmented, whereas superoxide dismutase-2-overexpression prevented, the spinal nerve ligation-increased miniature excitatory postsynaptic currents frequency. However, superoxide dismutase-2-knockout had no effect on the spinal nerve ligation-decreased miniature inhibitory postsynaptic current frequency, and superoxide dismutase-2-overexpression unexpectedly decreased miniature inhibitory postsynaptic current frequency in the normal condition. When applied to the spinal cord slice during in vitro recordings, mitoTEMPO, a specific scavenger of mitochondrial superoxide, reduced the spinal nerve ligation-increased miniature excitatory postsynaptic currents frequency but failed to normalize the spinal nerve ligation-decreased miniature inhibitory postsynaptic current frequency. These results suggest that in spinal dorsal horn neurons, high levels of mitochondrial superoxide increase excitatory synaptic strength after peripheral nerve injury and contribute to neuropathic mechanical hypersensitivity. However, mitochondrial superoxide does not seem to be involved in the decreased inhibitory synaptic strength in this neuropathic pain condition.

Expression of mitochondrial dysfunction-related genes and pathways in paclitaxel-induced peripheral neuropathy in breast cancer survivors

Background

Paclitaxel is one of the most commonly used drugs to treat breast cancer. Its major dose-limiting toxicity is paclitaxel-induced peripheral neuropathy (PIPN). PIPN persists into survivorship and has a negative impact on patient’s mood, functional status, and quality of life. No interventions are available to treat PIPN. A critical barrier to the development of efficacious interventions is the lack of understanding of the mechanisms that underlie PIPN. Mitochondrial dysfunction has been evaluated in preclinical studies as a hypothesized mechanism for PIPN, but clinical data to support this hypothesis are limited. The purpose of this pilot study was to evaluate for differential gene expression and perturbed pathways between breast cancer survivors with and without PIPN.

Methods

Gene expression in peripheral blood was assayed using RNA-seq. Differentially expressed genes (DEG) and pathways associated with mitochondrial dysfunction were identified between survivors who received paclitaxel and did (n = 25) and did not (n = 25) develop PIPN.

Results

Breast cancer survivors with PIPN were significantly older; more likely to be unemployed; reported lower alcohol use; had a higher body mass index and poorer functional status; and had a higher number of lower extremity sites with loss of light touch, cold, and pain sensations and higher vibration thresholds. No between-group differences were found in the cumulative dose of paclitaxel received or in the percentage of patients who had a dose reduction or delay due to PIPN. Five DEGs and nine perturbed pathways were associated with mitochondrial dysfunction related to oxidative stress, iron homeostasis, mitochondrial fission, apoptosis, and autophagy.

Conclusions

This study is the first to provide molecular evidence that a number of mitochondrial dysfunction mechanisms identified in preclinical models of various types of neuropathic pain including chemotherapy-induced peripheral neuropathy are found in breast cancer survivors with persistent PIPN and suggest genes for validation and as potential therapeutic targets.

Differential involvement of reactive oxygen species in a mouse model of capsaicin-induced secondary mechanical hyperalgesia and allodynia

Intradermally injected capsaicin induces secondary mechanical hyperalgesia and allodynia outside the primary (i.e., capsaicin-injected) site. This secondary mechanical hypersensitivity is attributed to central sensitization in which reactive oxygen species (ROS) play a key role. We examined whether ROS would be differentially involved in secondary mechanical hyperalgesia and allodynia using a mouse intraplantar capsaicin injection model. In mice, capsaicin-induced secondary mechanical hyperalgesia outlasted its allodynia counterpart. Unlike the hyperalgesia, the allodynia was temporarily abolished by an anesthetic given at the capsaicin-injected site. The ROS scavenger phenyl-N-tert-butylnitrone slowed the development of both secondary mechanical hyperalgesia and allodynia when administered before intraplantar capsaicin injection, whereas it inhibited only the allodynia when administered after capsaicin had already induced secondary mechanical hyperalgesia and allodynia. Intrathecal injection of the ROS donor KO₂ induced both mechanical hyperalgesia and allodynia with the former outlasting the latter. Metformin, an activator of redox-sensitive adenosine monophosphate-activated protein kinase, selectively inhibited capsaicin-induced secondary mechanical allodynia and intrathecal KO₂-induced mechanical allodynia. These results suggest that ROS is required for rapid activation of central sensitization mechanisms for both secondary mechanical hyperalgesia and allodynia after intraplantar capsaicin injection. Once activated, the mechanism for the hyperalgesia is long-lasting without being critically dependent on ongoing afferent activities arising from the capsaicin-injected site and the continuous presence of ROS. On the contrary, the ongoing afferent activities, ROS presence and adenosine monophosphate-activated protein kinase inhibition are indispensable for the maintenance mechanism for capsaicin-induced secondary mechanical allodynia.

A machine-learned analysis of human gene polymorphisms modulating persisting pain points to major roles of neuroimmune processes

Background

Human genetic research has implicated functional variants of more than one hundred genes in the modulation of persisting pain. Artificial intelligence and machine‐learning techniques may combine this knowledge with results of genetic research gathered in any context, which permits the identification of the key biological processes involved in chronic sensitization to pain.

Methods

Based on published evidence, a set of 110 genes carrying variants reported to be associated with modulation of the clinical phenotype of persisting pain in eight different clinical settings was submitted to unsupervised machine‐learning aimed at functional clustering. Subsequently, a mathematically supported subset of genes, comprising those most consistently involved in persisting pain, was analysed by means of computational functional genomics in the Gene Ontology knowledgebase.

Results

Clustering of genes with evidence for a modulation of persisting pain elucidated a functionally heterogeneous set. The situation cleared when the focus was narrowed to a genetic modulation consistently observed throughout several clinical settings. On this basis, two groups of biological processes, the immune system and nitric oxide signalling, emerged as major players in sensitization to persisting pain, which is biologically highly plausible and in agreement with other lines of pain research.

Conclusions

The present computational functional genomics‐based approach provided a computational systems‐biology perspective on chronic sensitization to pain. Human genetic control of persisting pain points to the immune system as a source of potential future targets for drugs directed against persisting pain. Contemporary machine‐learned methods provide innovative approaches to knowledge discovery from previous evidence.

Significance

We show that knowledge discovery in genetic databases and contemporary machine‐learned techniques can identify relevant biological processes involved in Persitent pain.

The effect of intravenous administration of liposomal curcumin in addition to sumatriptan treatment in an experimental migraine model in rats

BACKGROUND

Curcumin has antioxidative properties that could be useful in various diseases due to its ability to act on multiple targets of various cellular pathways. We aimed to assess the efficacy of liposomal curcumin compared with curcumin solution, when in addition to sumatriptan (ST) treatment, in an experimental migraine model induced with nitroglycerin (NTG) in rats.

METHODS

Seven groups of 9 rats each were investigated: control group without migraine (1 mL saline solution intraperitoneal injection [ip]), control group with induced migraine, NTG+ST group (ST), NTG+ST+curcumin1 (CC1) group - 1 mg/100 g body weight (bw), NTG+ST+CC2 - 2 mg/100 g bw, NTG+ST+liposomal curcumin1 (lCC1) group - 1 mg/100 g bw, and NTG+ST+lCC2 (lCC2) group - 2 mg/100 g bw. NTG and ST were administered as 1 mL ip NTG | 1 mg/100 g bw and 1 mL ip ST | 1 mg/100 g bw, respectively. Plasma total oxidative stress (TOS), malondialdehyde (MDA), nitric oxide (NOx), thiol levels, as well as total antioxidative capacity (TAC) were assessed. The nociception process was assessed by counting the number of flinches and shakes after the formalin test.

RESULTS

The plasma TOS, MDA, and NOx levels, as oxidative stress parameters, were significantly decreased in the curcumin-treated groups, especially where curcumin was in liposomal form. The thiol and TAC were also improved by the curcumin treatment, with the best results obtained for the liposomal curcumin. The closest number of flinches and shakes to the control group was obtained for the group treated with liposomal curcumin at a dose of 2 mg/100 g bw.

CONCLUSION

Liposomal curcumin in a dose of 2 mg/100 g bw when in addition to ST treatment could be an optimum therapeutic strategy for migraine attacks and could represent a base for future clinical research and application.

Recent development in antihyperalgesic effect of phytochemicals: anti-inflammatory and neuro-modulatory actions

INTRODUCTION

Pain is an unpleasant sensation triggered by noxious stimulation. It is one of the most prevalent conditions, limiting productivity and diminishing quality of life. Non steroidal anti inflammatory drugs (NSAIDs) are widely used as pain relievers in present day practice as pain is mostly initiated due to inflammation. However, due to potentially serious side effects, long term use of these antihyperalgesic drugs raises concern. Therefore there is a demand to search novel medicines with least side effects. Herbal products have been used for centuries to reduce pain and inflammation, and phytochemicals are known to cause fewer side effects. However, identification of active phytochemicals of herbal medicines and clear understanding of the molecular mechanism of their action is needed for clinical acceptance.

MATERIALS AND METHODS

In this review, we have briefly discussed the cellular and molecular changes during hyperalgesia via inflammatory mediators and neuro-modulatory action involved therein. The review includes 54 recently reported phytochemicals with antihyperalgesic action, as per the literature available with PubMed, Google Scholar and Scopus.

CONCLUSION

Compounds of high interest as potential antihyperalgesic agents are: curcumin, resveratrol, capsaicin, quercetin, eugenol, naringenin and epigallocatechin gallate (EGCG). Current knowledge about molecular targets of pain and their regulation by these phytochemicals is elaborated and the scope of further research is discussed.

Oxidative Stress Contributes to Fracture/Cast-Induced Inflammation and Pain in a Rat Model of Complex Regional Pain Syndrome

Clinical evidence suggests that vitamin C (Vit C) may protect against the development of complex regional pain syndrome (CRPS) after fracture or surgery. Tibia fracture followed by 4 weeks of cast immobilization (fracture/cast) in rats results in nociceptive, vascular, and bone changes resembling clinical CRPS. In this study, fracture/cast rats were treated with the oxidative stress inhibitors Vit C, N-acetyl cysteine, or 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl to examine their effects on CRPS-related nociceptive and vascular changes. Administration of these agents significantly reduced fracture/cast-induced cutaneous allodynia by 64 to 78%, muscle hyperalgesia by 34 to 40%, and hind limb unweighting by 48 to 89%. Treatments with Vit C and N-acetyl cysteine reduced the oxidative stress markers malondialdehyde in the skin, muscle, and sciatic nerve, and lactate in the gastrocnemius muscle of the fracture/cast limb. Furthermore, Vit C treatment inhibited the post-fracture upregulation of substance P and calcitonin gene-related peptide in the sciatic nerve and the increased expression of the pain-related inflammatory mediators, including interleukin (IL)-6, and nerve growth factor in the skin and IL-1β, and IL-6 in the muscle of the post-fracture/cast limb. These data suggest that oxidative stress may contribute to the nociceptive features of the rat CRPS model.

PERSPECTIVE

Vit C reduced the CRPS-like signs, oxidative stress, and the upregulation of neuropeptide production and inflammatory mediators observed after tibia fracture and casting in rats. Limiting oxidative stress by use of Vit C or alternative strategies could reduce the risk of developing CRPS after surgery or other forms of trauma.