Role of brainstem serotonin in analgesia produced by low-intensity exercise on neuropathic pain after sciatic nerve injury in mice

IL-5 mediates monocyte phenotype and pain outcomes in fibromyalgia

ABSTRACT

Fibromyalgia (FM) is characterized by widespread chronic pain, fatigue, and somatic symptoms. The influence of phenotypic changes in monocytes on symptoms associated with FM is not fully understood. The primary aim of this study was to take a comprehensive whole-body to molecular approach in characterizing relationships between monocyte phenotype and FM symptoms in relevant clinical populations. Lipopolysaccharide-evoked and spontaneous secretion of IL-5 and other select cytokines from circulating monocytes was higher in women with FM compared to women without pain. In addition, greater secretion of IL-5 was significantly associated with pain and other clinically relevant psychological and somatic symptoms of FM. Furthermore, higher levels of pain and pain-related symptoms were associated with a lower percentage of intermediate monocytes (CD14++/CD16+) and a greater percentage of nonclassical monocytes (CD14+/CD16++) in women with FM. Based on findings from individuals with FM, we examined the role of IL-5, an atypical cytokine secreted from monocytes, in an animal model of widespread muscle pain. Results from the animal model show that IL-5 produces analgesia and polarizes monocytes toward an anti-inflammatory phenotype (CD206+). Taken together, our data suggest that monocyte phenotype and their cytokine profiles are associated with pain-related symptoms in individuals with FM. Furthermore, our data show that IL-5 has a potential role in analgesia in an animal model of FM. Thus, targeting anti-inflammatory cytokines such as IL-5 secreted by circulating leukocytes could serve as a promising intervention to control pain and other somatic symptoms associated with FM.

Testosterone protects against the development of widespread muscle pain in mice

Chronic widespread pain conditions are more prevalent in women than men, suggesting a role for gonadal hormones in the observed differences. Previously, we showed that female mice, compared to male, develop widespread, more severe, and longer-duration hyperalgesia in a model of activity-induced muscle pain. We hypothesized testosterone protects males from developing the female pain phenotype. We tested whether orchiectomy of males before induction of an activity-induced pain model produced a female phenotype and whether testosterone administration produced a male phenotype in females. Orchiectomy produced longer-lasting, more widespread hyperalgesia, similar to females. Administration of testosterone to females or orchiectomized males produced unilateral, shorter-lasting hyperalgesia. Prior studies show that the serotonin transporter (SERT) is increased in the nucleus raphe magnus (NRM) in models of chronic pain, and that blockade of SERT in the NRM reduces hyperalgesia. We examined potential sex differences in the distribution of SERT across brain sites involved in nociceptive processing using immunohistochemistry. A sex difference in SERT was found in the NRM in the activity-induced pain model; females had greater SERT immunoreactivity than males. This suggests that testosterone protects against development of widespread, long-lasting muscle pain and that alterations in SERT may underlie the sex differences.

Fibromyalgia: Pathogenesis, Mechanisms, Diagnosis and Treatment Options Update

Fibromyalgia is a syndrome characterized by chronic and widespread musculoskeletal pain, often accompanied by other symptoms, such as fatigue, intestinal disorders and alterations in sleep and mood. It is estimated that two to eight percent of the world population is affected by fibromyalgia. From a medical point of view, this pathology still presents inexplicable aspects. It is known that fibromyalgia is caused by a central sensitization phenomenon characterized by the dysfunction of neuro-circuits, which involves the perception, transmission and processing of afferent nociceptive stimuli, with the prevalent manifestation of pain at the level of the locomotor system. In recent years, the pathogenesis of fibromyalgia has also been linked to other factors, such as inflammatory, immune, endocrine, genetic and psychosocial factors. A rheumatologist typically makes a diagnosis of fibromyalgia when the patient describes a history of pain spreading in all quadrants of the body for at least three months and when pain is caused by digital pressure in at least 11 out of 18 allogenic points, called tender points. Fibromyalgia does not involve organic damage, and several diagnostic approaches have been developed in recent years, including the analysis of genetic, epigenetic and serological biomarkers. Symptoms often begin after physical or emotional trauma, but in many cases, there appears to be no obvious trigger. Women are more prone to developing the disease than men. Unfortunately, the conventional medical therapies that target this pathology produce limited benefits. They remain largely pharmacological in nature and tend to treat the symptomatic aspects of various disorders reported by the patient. The statistics, however, highlight the fact that 90% of people with fibromyalgia also turn to complementary medicine to manage their symptoms.

Bilirubin Induces Pain Desensitization in Cholestasis by Activating 5-Hydroxytryptamine 3A Receptor in Spinal Cord

Background

Cholestasis patients often suffer from pain desensitization, resulting in serious complications in perioperative period. This study was aim to investigate the mechanism of bilirubin in cholestasis mediating pain desensitization through 5-hydroxytryptamine 3A (5-HT_3A ) receptor activation in spinal dorsal horn (SDH).

Methods

A cholestasis model was established by bile duct ligation (BDL) in rats. Pain thresholds of rats were measured after BDL or intrathecally injecting bilirubin in the presence or absence of agonist (mCPBG) and antagonists (ondansetron, bicuculline, or CGP55845). Expression of 5-HT₃ receptors, and the affinity and binding mode of bilirubin to 5-HT_3A receptor were determined. Effects of bilirubin on γ-aminobutyric acid (GABA) pathway and the interactions with 5-HT_3A receptor were tested.

Results

Bilirubin was elevated significantly in both serum and CSF in BDL rats, accompanied with the up-regulation of pain thresholds. Both of 5-HT_3A receptor and GABA _A receptor antagonists could reverse the increased pain threshold in BDL rats. Further, 5-HT_3A and GABA _A receptor expressions were increased in BDL rats or intervention with bilirubin. Molecular docking suggested that bilirubin entered the hydrophobic pocket pre-formed in 5-HT_3A receptor with potential hydrogen bonding. Bilirubin also increased GABA concentrations in CSF and GABAergic spontaneous inhibitory postsynaptic current in spinal cord, and directly induced inward currents in HEK293 cells which were overexpressed 5-HT_3A receptor by lentivirus.

Conclusion

In conclusion, bilirubin induced pain desensitization in cholestasis by activating 5-HT_3A receptor in spinal cord. The activation of 5-HT_3A receptor might regulate pain threshold by acting on the GABA pathway.

Exercise and Neuropathic Pain: A General Overview of Preclinical and Clinical Research

Neuropathic pain is a disease of the somatosensory system that is characterized by tingling, burning, and/or shooting pain. Medication is often the primary treatment, but it can be costly, thus there is an interest in understanding alternative low-cost treatments such as exercise. The following review includes an overview of the preclinical and clinical literature examining the influence of exercise on neuropathic pain. Preclinical studies support the hypothesis that exercise reduces hyperalgesia and allodynia in animal models of neuropathic pain. In human research, observational studies suggest that those who are more physically active have lower risk of developing neuropathic pain compared to those who are less active. Exercise studies suggest aerobic exercise training (e.g., 16 weeks); a combination of aerobic and resistance exercise training (e.g., 10–12 weeks); or high-intensity interval training (e.g., 15 weeks) reduces aspects of neuropathic pain such as worst pain over the past month, pain over the past 24 h, pain scores, or pain interference. However, not all measures of pain improve following exercise training (e.g., current pain, heat pain threshold). Potential mechanisms and future directions are also discussed to aid in the goal of understanding the role of exercise in the management of neuropathic pain. Future research using standardized methods to further understanding of the dose of exercise needed to manage neuropathic pain is warranted.

Chemotherapy-Induced Peripheral Neuropathy: Nursing Implications

Chemotherapy-induced peripheral neuropathy (CIPN) is an unsolved and potentially life-compromising problem for most patients receiving neurotoxic chemotherapy. It manifests with numbness, tingling, and possibly neuropathic pain and motor and autonomic symptoms. This review aims to provide an evidence synthesis that prepares nurses to comprehensively assess, provide supportive care for, and critically evaluate the literature on CIPN. The prevalence, significance, characteristics, mechanisms, and risk factors of CIPN will be discussed, as well as nursing-relevant evidence on the assessment, prevention, and management of CIPN. The importance of critical literature evaluation before clinical implementation to reduce physical and financial harms to patients will also be highlighted.

The influence of physical activity on the nociceptive flexion reflex in healthy people

BACKGROUND

The nociceptive flexion reflex (NFR) is a spinal reflex induced by painful stimuli resulting in an appropriate withdrawal response. The NFR is considered to be an objective physiological correlate of spinal nociception. Previous research has already demonstrated that physical activity (PA) can influence pain assessments. To date, no studies have directly examined the relationship between PA and spinal nociception. Hence, this study aimed to investigate whether the NFR threshold can be predicted by report-based and monitor-based measures of PA in healthy adults.

METHODS

PA and the NFR threshold of 58 healthy adults were assessed. PA was evaluated by self-report using the International Physical Activity Questionnaire and by monitor-based accelerometry data. The NFR threshold was elicited through transcutaneous electrical stimulation of the sural nerve and quantified by the biceps femoris muscle electromyogram. Hierarchical linear regression analyses were performed to determine the relationship between PA and the NFR, while controlling for confounders.

RESULTS

Monitor-based measured step count and activities of moderate- to vigorous-intensity predicted the NFR threshold accounting for 23.0% (p = .047) to 37.1% (p = .002) of the variance. Larger amounts of step counts and higher participation in moderate- to vigorous-intensity activities predicted higher NFR thresholds. Monitor-based activities of sedentary intensity predicted the NFR threshold accounting for 35.8% (p = .014) to 35.9% (p = .014) of the variance. Spending more time per day on activities of sedentary intensity predicted lower NFR thresholds.

CONCLUSIONS

The study provides preliminary evidence indicating that a physically active lifestyle may reduce spinal nociception in healthy adults, while a sedentary lifestyle enhances spinal nociception.

SIGNIFICANCE

The present study provides preliminary evidence that the influencing effects of physical activity on pain are the result of a strong descending control and do not purely rely on supraspinal mechanisms. These study results highlight the importance of considering physical activity levels when evaluating nociceptive processing, given the prognostic value of physical activity in spinal nociception. Furthermore, this study encourages future research to examine the effects of moderate- to vigorous-intensity exercise programmes on spinal nociception in chronic pain populations.

Vortioxetine reduces pain hypersensitivity and associated depression-like behavior in mice with oxaliplatin-induced neuropathy

Chronic pain and depression commonly occur together so dual-acting agents might be particularly useful. The population of patients with chemotherapy-induced neuropathy is increasing in parallel with the increase of population of cancer survivors and there is a compelling need for satisfactory treatment of symptoms of neuropathy and concomitant depression. We examined the effects of vortioxetine, a novel antidepressant with unique mechanism of action, on pain hypersensitivity and depression-like behavior in oxaliplatin-induced neuropathy model in mice (OIPN). Vortioxetine (1-10 mg/kg, p.o.) significantly and dose-dependently reduced mechanical allodynia in von Frey test and cold allodynia in acetone test in OIPN mice, in both repeated prophylactic and acute therapeutic treatment regimens. It also reduced depression-like behavior in the forced swimming test in OIPN mice, in both treatment paradigms. Its antiallodynic and antidepressive-like effects were comparable to those exerted by duloxetine (1-15 mg/kg, p.o.). The antiallodynic and antidepressive-like effects of repeatedly administered vortioxetine might be related to the increased content of 5-hydroxytryptamine (5-HT) and noradrenaline (NA), detected in the brainstem of treated OIPN mice. These results indicate that vortioxetine could be potentially useful in prevention and treatment of chemotherapy-induced neuropathy, for the relief of pain and concomitant depressive symptoms. It should be further tested to this regard in clinical settings.

Regular physical activity reduces the percentage of spinally projecting neurons that express mu-opioid receptors from the rostral ventromedial medulla in mice

Introduction

Regular physical activity/exercise is an effective nonpharmacological treatment for individuals with chronic pain. Central inhibitory mechanisms, involving serotonin and opioids, are critical to analgesia produced by regular physical activity. The rostral ventromedial medulla (RVM) sends projections to the spinal cord to inhibit or facilitate nociceptive neurons and plays a key role in exercise-induced analgesia.

Objective

The goal of these studies was to examine if regular physical activity modifies RVM-spinal cord circuitry.

Methods

Male and female mice received Fluoro-Gold placed on the spinal cord to identify spinally projecting neurons from the RVM and the nucleus raphe obscurus/nucleus raphe pallidus, dermorphin-488 into caudal medulla to identify mu-opioid receptors, and were immunohistochemically stained for either phosphorylated-N-methyl-d-aspartate subunit NR1 (p-NR1) to identify excitatory neurons or tryptophan hydroxylase (TPH) to identify serotonin neurons. The percentage of dermorphin-488-positive cells that stained for p-NR1 (or TPH), and the percentage of dermorphin-488-positive cells that stained for p-NR1 (or TPH) and Fluoro-Gold was calculated. Physically active animals were provided running wheels in their cages for 8 weeks and compared to sedentary animals without running wheels. Animals with chronic muscle pain, induced by 2 intramuscular injections of pH 4.0, were compared to sham controls (pH 7.2).

Results

Physically active animals had less mu-opioid-expressing neurons projecting to the spinal cord when compared to sedentary animals in the RVM, but not the nucleus raphe obscurus/nucleus raphe pallidus. No changes were observed for TPH.

Conclusions

These data suggest that regular exercise alters central facilitation so that there is less descending facilitation to result in a net increase in inhibition.

Bioinformatics analysis of long non-coding RNAs involved in nerve regeneration following sciatic nerve injury

Little is known about the role of epigenetic modification in axon regeneration following peripheral nerve injury. The purpose of the present study was to investigate the role of long non-coding RNAs (lncRNAs) in the regulation of axon regeneration. We used bioinformatics to perform microarray analysis and screened total 476 lncRNAs and 129 microRNAs (miRNAs) of differentially expressed genes after sciatic nerve injury in mice. lncRNA-GM4208 and lncRNA-GM30085 were examined, and the changes in lncRNA expression in the L4–L6 dorsal root ganglia (DRG) following sciatic nerve crush injury were analyzed by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The expression of lncRNAs in the DRG changed, indicating that they might be related to nerve regeneration in the DRG following peripheral nerve injury.

Animal models of pain: Diversity and benefits

Chronic pain is a maladaptive neurological disease that remains a major health problem. A deepening of our knowledge on mechanisms that cause pain is a prerequisite to developing novel treatments. A large variety of animal models of pain has been developed that recapitulate the diverse symptoms of different pain pathologies. These models reproduce different pain phenotypes and remain necessary to examine the multidimensional aspects of pain and understand the cellular and molecular basis underlying pain conditions. In this review, we propose an overview of animal models, from simple organisms to rodents and non-human primates and the specific traits of pain pathologies they model. We present the main behavioral tests for assessing pain and investing the underpinning mechanisms of chronic pathological pain. The validity of animal models is analysed based on their ability to mimic human clinical diseases and to predict treatment outcomes. Refine characterization of pathological phenotypes also requires to consider pain globally using specific procedures dedicated to study emotional comorbidities of pain. We discuss the limitations of pain models when research findings fail to be translated from animal models to human clinics. But we also point to some recent successes in analgesic drug development that highlight strategies for improving the predictive validity of animal models of pain. Finally, we emphasize the importance of using assortments of preclinical pain models to identify pain subtype mechanisms, and to foster the development of better analgesics.

Loss of diffuse noxious inhibitory control after traumatic brain injury in rats: A chronic issue

Chronic pain is one of the most challenging and debilitating symptoms to manage after traumatic brain injury (TBI), yet the underlying mechanisms remain elusive. The disruption of normal endogenous pain control mechanisms has been linked to several forms of chronic pain and may play a role in pain after TBI. We hypothesized therefore that dysfunctional descending noradrenergic and serotonergic pain control circuits may contribute to the loss of diffuse noxious inhibitory control (DNIC), a critical endogenous pain control mechanism, weeks to months after TBI. For these studies, the rat lateral fluid percussion model of mild TBI was used along with a DNIC paradigm involving a capsaicin-conditioning stimulus. We observed sustained failure of the DNIC response up to 180-days post injury. We confirmed, that descending α2 adrenoceptor-mediated noradrenergic signaling was critical for endogenous pain inhibition in uninjured rats. However, augmenting descending noradrenergic signaling using reboxetine, a selective noradrenaline reuptake inhibitor, failed to restore DNIC after TBI. Furthermore, blocking serotonin-mediated descending signaling using selective spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine was also unsuccessful at restoring endogenous pain modulation after TBI. Unexpectedly, increasing descending serotonergic signaling using the selective serotonin reuptake inhibitor escitalopram and the serotonin-norepinephrine reuptake inhibitor duloxetine restored the DNIC response in TBI rats at both 49- and 180- days post injury. Consistent with these observations, spinal serotonergic fiber depletion with 5, 7-dihydroxytryptamine eliminated the effects of escitalopram. Intact α2 adrenoceptor signaling, however, was not required for the serotonin-mediated restoration of DNIC after TBI. These results suggest that TBI causes maladaptation of descending nociceptive signaling mechanisms and changes in the function of both adrenergic and serotonergic circuits. Such changes could predispose those with TBI to chronic pain.

Running wheel exercise induces therapeutic and preventive effects on inflammatory stimulus-induced persistent hyperalgesia in mice

Chronic pain affects significant portion of the world's population and physical exercise has been extensively indicated as non-pharmacological clinical intervention to relieve symptoms in chronic pain conditions. In general, studies on pain chronification and physical exercise intervention have focused on neuropathic pain, although chronic pain commonly results from an original inflammatory episode. Based on this, the objective of the present study was to investigate the therapeutic and preventive effect of the running wheel exercise on the persistent hyperalgesia induced by repetitive inflammatory stimulus, a rodent model that simulates clinical conditions of chronic pain that persist even with no more inflammatory stimulus present. To evaluate the therapeutic effect of physical exercise, we first induced persistent hyperalgesia through 14 days of PGE2 hind paw injections and, after that, mice have access to the regular voluntary running wheel. To evaluate the preventive effect of physical exercise, we first left the mice with access to the regular voluntary running wheel and, after that, we performed 14 days of PGE2 hind paw injection. Our results showed that voluntary running wheel exercise reduced persistent mechanical and chemical hyperalgesia intensity induced by repetitive inflammatory stimulus. In addition, we showed that this therapeutic effect is long-lasting and is observed even if started belatedly, i.e. two weeks after the development of hyperalgesia. Also, our results showed that voluntary running wheel exercise absolutely prevented persistent mechanical and chemical hyperalgesia induction. We can conclude that physical exercise has therapeutic and preventive effect on inflammatory stimulus-induced persistent hyperalgesia. Our data from animal experiments bypass placebo effects bias of the human studies and reinforce physical exercise clinical recommendations to treat and prevent chronic pain.

Exercise-induced immune system response: Anti-inflammatory status on peripheral and central organs

A wide array of molecular pathways has been investigated during the past decade in order to understand the mechanisms by which the practice of physical exercise promotes neuroprotection and reduces the risk of developing communicable and non-communicable chronic diseases. While a single session of physical exercise may represent a challenge for cell homeostasis, repeated physical exercise sessions will improve immunosurveillance and immunocompetence. Additionally, immune cells from the central nervous system will acquire an anti-inflammatory phenotype, protecting central functions from age-induced cognitive decline. This review highlights the exercise-induced anti-inflammatory effect on the prevention or treatment of common chronic clinical and experimental settings. It also suggests the use of pterins in biological fluids as sensitive biomarkers to follow the anti-inflammatory effect of physical exercise.

Mechanism of exercise-induced analgesia: what we can learn from physically active animals

Physical activity has become a first-line treatment in rehabilitation settings for individuals with chronic pain. However, research has only recently begun to elucidate the mechanisms of exercise-induced analgesia. Through the study of animal models, exercise has been shown to induce changes in the brain, spinal cord, immune system, and at the site of injury to prevent and reduce pain. Animal models have also explored beneficial effects of exercise through different modes of exercise including running, swimming, and resistance training. This review will discuss the central and peripheral mechanisms of exercise-induced analgesia through different modes, intensity, and duration of exercise as well as clinical applications of exercise with suggestions for future research directions.

Green synthesis of Zingiberis rhizoma-based carbon dots attenuates chemical and thermal stimulus pain in mice

Aim: To evaluate the analgesic activity of Zingiberis rhizoma-based carbon dots (ZR-CDs). Materials & methods: Novel ZR-CDs were prepared via a facile, green pyrolysis method. Microstructure, optical and functional group properties were characterized. Acetic acid writhing, hot-plate and tail-immersion tests were performed using mice to evaluate the analgesic activity of ZR-CDs, followed by a preliminary study on the analgesic mechanism. Results: ZR-CDs with a quantum yield of 5.2% had a diameter ranging from 2.23 to 3.77 nm. Remarkable analgesic effect of ZR-CDs was observed against both thermal and chemical stimulus tests, possibly mediated by an opioid-like mechanism and the regulation of 5-hydroxytryptamine levels. Conclusion: ZR-CDs have a promising potential for biomedical application in relieving pain-related diseases.

Endoplasmic Reticulum Stress Contributes to Nociception via Neuroinflammation in a Murine Bone Cancer Pain Model

BACKGROUND

Prolonged endoplasmic reticulum stress has been identified in various diseases. Inflammatory mediators, which have been shown to induce endoplasmic reticulum stress in several studies, have been suggested to serve as the important modulators in pain development. In this study, the authors hypothesized that the endoplasmic reticulum stress triggered by inflammatory mediators contributed to pain development.

METHODS

The authors used a male mouse model of bone cancer pain. The control mice were intrathecally injected with tumor necrosis factor-α (TNF-α) and lipopolysaccharide, the bone cancer pain mice were intrathecally injected with the endoplasmic reticulum stress inhibitors 4-PBA and GSK2606414. The nociceptive behaviors, endoplasmic reticulum stress markers, and inflammatory mediators were assessed.

RESULTS

Increased expression of the p-RNA-dependent protein kinase-like endoplasmic reticulum kinase and p-eukaryotic initiation factor 2α were found in the spinal neurons during bone cancer pain, along with upregulation of inflammatory mediators (TNF-α, interleukin 1β, and interleukin 6). Intrathecal administration of TNF-α or lipopolysaccharide increased the expression of endoplasmic reticulum stress markers in control mice. Inhibition of endoplasmic reticulum stress by intrathecal administration of 4-PBA (baseline vs. 3 h: 0.34 ± 0.16 g vs. 1.65 ± 0.40 g in paw withdrawal mechanical threshold, 8.00 ± 1.20 times per 2 min vs. 0.88 ± 0.64 times per 2 min in number of spontaneous flinches, P < 0.001, n = 8) or GSK2606414 (baseline vs. 3 h: 0.37 ± 0.08 g vs. 1.38 ± 0.11 g in paw withdrawal mechanical threshold, 8.00 ± 0.93 times per 2 min vs. 3.25 ± 1.04 times per 2 min in number of spontaneous flinches, P < 0.001, n = 8) showed time- and dose-dependent antinociception. Meanwhile, decreased expression of inflammatory mediators (TNF-α, interleukin 1β, and interleukin 6), as well as decreased activation of astrocytes in the spinal cord, were found after 4-PBA or GSK2606414 treatment.

CONCLUSIONS

Inhibition of inflammatory mediator-triggered endoplasmic reticulum stress in spinal neurons attenuates bone cancer pain via modulation of neuroinflammation, which suggests new approaches to pain relief.

Exercise effects on brain and behavior in healthy mice, Alzheimer's disease and Parkinson's disease model-A systematic review and meta-analysis

This systematic review and meta-analysis examines how exercise modifies brain and behavior in healthy mice, dementia (D) and Parkinson disease (PD) models. A search was performed on the Medline and Scopus electronic databases (2008-2019). Search terms were "mice", "brain", "treadmill", "exercise", "physical exercise". In the total, 430 were found but only 103 were included. Animals n = 1,172; exercised 4-8 weeks (Range 24 h to 32 weeks), 60 min/day (Range 8-120 min per day), and 10/12 m/min (Range 0.2 m/min to 36 m/min). Hippocampus, cerebral cortex, striatum and whole brain were more frequently investigated. Exercise improved learning and memory. Meta-analysis showed that exercise increased: cerebral BDNF in health (n = 150; z = 5.8, CI 3.43-12.05; p < 0.001 I2 = 94.3 %), D (n = 124; z = 4.18, CI = 2.22-9.12; p < 0.001; I2 = 93.7 %) and PD (n = 16 z = 4.26, CI 5.03-48.73 p < 0.001 I2 = 94.8 %). TrkB improved in health (n = 84 z = 5.49, CI 3.8-17.73 p < 0.001, I2 = 0.000) and PD (n = 22; z = 3.1, CI = 2.58-67.3, p < 0.002 I2 = 93.8 %). Neurogenesis increased in health (n = 68; z = 7.08, CI 5.65-21.25 p < 0.001; I2 17.58) and D model (n = 116; z = 4.18, CI 2.22-9.12 p < 0.001 I2 93.7 %). Exercise augmented amyloid clearance (n = 166; z = 7.51 CI = 4.86-14.85, p < 0.001 I2 = 58.72) and reduced amyloid plaques in D models (n = 49; z = 4.65, CI = 3.94-15.3 p < 0.001 I2 = 0.000). In conclusion, exercise improved brain and behavior, neurogenesis in healthy and dementia models, reduced toxicity and cerebral amyloid. Evidence regarding inflammation, oxidative stress and energy metabolism were scarce. Studies examining acute vs chronic exercise, extreme training and the durability of exercise benefit were rare. Vascular or glucose metabolism changes were seldom reported.

Mild Traumatic Brain Injury Causes Nociceptive Sensitization through Spinal Chemokine Upregulation

High rates of acute and chronic pain are associated with traumatic brain injury (TBI), but mechanisms responsible for the association remain elusive. Recent data suggest dysregulated descending pain modulation circuitry could be involved. Based on these and other observations, we hypothesized that serotonin (5-HT)-dependent activation of spinal CXC Motif Chemokine Receptor 2 (CXCR2) may support TBI-related nociceptive sensitization in a mouse model of mild TBI (mTBI). We observed that systemic 5-HT depletion with p-chlorophenylalanine attenuated mechanical hypersensitivity seen after mTBI. Likewise, selective spinal 5-HT fiber depletion with 5,7-dihydroxytryptamine (5,7-DHT) reduced hypersensitivity after mTBI. Consistent with a role for spinal 5-HT₃ serotonin receptors, intrathecal ondansetron administration after TBI dose-dependently attenuated nociceptive sensitization. Also, selective CXCR2 antagonist SCH527123 treatment attenuated mechanical hypersensitivity after mTBI. Furthermore, spinal CXCL1 and CXCL2 mRNA and protein levels were increased after mTBI as were GFAP and IBA-1 markers. Spinal 5,7-DHT application reduced both chemokine expression and glial activation. Our results suggest dual pathways for nociceptive sensitization after mTBI, direct 5-HT effect through 5-HT₃ receptors and indirectly through upregulation of chemokine signaling. Designing novel clinical interventions against either the 5-HT₃ mediated component or chemokine pathway may be beneficial in treating pain frequently seen in patients after mTBI.

Sedentary behaviour facilitates conditioned pain modulation in middle-aged and older adults with persistent musculoskeletal pain: a cross-sectional investigation

INTRODUCTION

Higher physical activity (PA) and lower sedentary behaviour (SB) levels have demonstrated beneficial effects on temporal summation (TS) and conditioned pain modulation (CPM) in healthy adults. This cross-sectional study investigated the relationships between PA and SB and TS/CPM responses in individuals with chronic musculoskeletal pain.

METHODS

Sixty-seven middle-aged and older adults with chronic musculoskeletal pain were recruited from the community. Questionnaires measuring demographics, pain, and psychological measures were completed. Physical activity/SB levels were measured using the International Physical Activity Questionnaire-short form and Sedentary Behaviour Questionnaire, respectively. Semmes monofilament was used to assess mechanical TS (MTS) at the most symptomatic (MTS-S) and a reference region (MTS-R); change in the pain scores (baseline-10th application) was used for analysis. Conditioned pain modulation procedure involved suprathreshold pressure pain threshold (PPT-pain4) administered before and after (CPM30sec, CPM60sec, and CPM90sec) conditioning stimulus (2 minutes; ∼12°C cold bath immersion). For analysis, PPT-pain4 (%) change scores were used.

RESULTS

PPT-pain4 (%) change scores at CPM30sec and CPM60sec demonstrated significant weak positive correlations with SB levels and weak negative correlations with PA measures. After adjusting for confounding variables, a significant positive association was found between SB (h/d) and PPT-pain4 (%) change scores at CPM30sec and CPM60sec. No significant associations between MTS and PA/SB measures.

CONCLUSION

Sedentariness is associated with higher pain inhibitory capacity in people with chronic musculoskeletal pain. The observed relationship may be characteristic of a protective (sedentary) behaviour to enhance pain modulatory mechanism. Prospective longitudinal studies using objective PA/SB measures are required to validate the observed relationship in a larger sample size.

Voluntary exercise reduces both chemotherapy-induced neuropathic nociception and deficits in hippocampal cellular proliferation in a mouse model of paclitaxel-induced peripheral neuropathy

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Paclitaxel treatment did not alter voluntary running activity.

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Voluntary running reduced mechanical and cold allodynia induced by paclitaxel.

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Voluntary running reduced paclitaxel-induced deficits in hippocampal cellular proliferation.

Chemotherapy-induced peripheral neuropathy (CIPN) is a common dose-limiting side-effect of all major chemotherapeutic agents. Here, we explored efficacy of voluntary exercise as a nonpharmacological strategy for suppressing two distinct adverse side effects of chemotherapy treatment. We evaluated whether voluntary running would suppress both neuropathic pain and deficits in hippocampal cell proliferation in a mouse model of CIPN induced by the taxane chemotherapeutic agent paclitaxel. Mice were given free access to running wheels or were housed without running wheels during one of three different intervention phases: 1) during the onset (i.e. development phase) of paclitaxel-induced neuropathy, 2) prior to dosing with paclitaxel or its vehicle, or 3) following the establishment (i.e. maintenance phase) of paclitaxel-induced neuropathy. Paclitaxel treatment did not alter running wheel behavior relative to vehicle-treated animals in any study. Animals that engaged in voluntary running during the development phase of paclitaxel-induced neuropathy failed to display mechanical or cold hypersensitivities relative to sedentary control animals that did not have access to running wheels. A prior history of voluntary running delayed the onset of, but did not fully prevent, development of paclitaxel-induced neuropathic pain behavior. Voluntary running reduced already established mechanical and cold allodynia induced by paclitaxel. Importantly, voluntary running did not alter mechanical or cold responsivity in vehicle-treated animals, suggesting that the observed antinociceptive effect of exercise was dependent upon the presence of the pathological pain state. In the same animals evaluated for nociceptive responding, paclitaxel also reduced cellular proliferation but not cellular survival in the dentate gyrus of the hippocampus, as measured by immunohistochemistry for Ki67 and BrdU expression, respectively. Voluntary running abrogated paclitaxel-induced reductions in cellular proliferation to levels observed in vehicle-treated mice and also increased BrdU expression levels irrespective of chemotherapy treatment. Our studies support the hypothesis that voluntary exercise may be beneficial in suppressing both neuropathic pain and markers of hippocampal cellular function that are impacted by toxic challenge with chemotherapeutic agents.

Environmental enrichment improves pain sensitivity, depression-like phenotype, and memory deficit in mice with neuropathic pain: role of NPAS4

Patients suffering from neuropathic pain have a higher incidence of depression and cognitive decline. Although environment enrichment (EE) may be effective in the treatment of neuropathic pain, the precise mechanisms underlying its actions remain determined. The aim of the study was to examine the molecular mechanisms underlying the EE's beneficial effects in mice with neuropathic pain. EE attenuated the pain threshold reduction, depression-like phenotype, and memory deficit in mice after chronic constriction injury (CCI). Furthermore, EE attenuated decreased neurogenesis and increased inflammation in the hippocampus of mice with neuropathic pain after CCI. Moreover, the suppression of adult hippocampal neurogenesis by temozolomide antagonized the beneficial effects of EE on depression-like phenotype and cognitive deficit in the mice with neuropathic pain. In addition, lipopolysaccharide-induced increase in tumor necrosis factor-α (TNF-α) in the hippocampus antagonized the beneficial effects of EE for these behavioral abnormalities in mice with neuropathic pain. Knock-down of NPAS4 (neuronal PAS domain protein 4) in the hippocampus by lentivirus targeting NPAS4 blocked these beneficial effects of EE in the mice with neuropathic pain. These all findings suggest that hippocampal NPAS4 plays a key role in the beneficial effects of EE on the pain sensitivity, depression-like phenotype, and memory deficit in mice with neuropathic pain. Therefore, it is likely that NPAS4 would be a new therapeutic target for perceptional, affective, and cognitive dimensions in patients with chronic pain.

Effects of Salmon Calcitonin on the Concentrations of Monoamines in Periaqueductal Gray in Formalin Test

Background:

The receptors of salmon calcitonin, located on certain areas of the brain such as the periaqueductal gray matter, are responsible for pain modulation.

Aims:

The effects of intracerebroventricular injection of salmon calcitonin on the behavioral response to pain and on the levels of monoamines in the periaqueductal gray were explored using a biphasic animal model of pain.

Study Design:

Animal experiment.

Methods:

A total of 45 male rats were divided into four groups (n=6). Salmon calcitonin was injected into the lateral ventricle of the brain (1.5 nmol, with a volume of 5 μL). After 20 min, 2.5% formalin was subcutaneously injected into the right leg claw, and pain behavior was recorded on a numerical basis. At the time of the formalin test, the periaqueductal gray area was microdialized. High-performance liquid chromatography method was used to gauge the levels of monoamines and their metabolites.

Results:

Intracerebroventricular injections of salmon calcitonin resulted in pain reduction in the formalin test (p<0.05). The dialysate concentrations of serotonin, dopamine, norepinephrine, 5-hydroxyindoleacetic acid, 3,4-dihydroxyphenylacetic, and 4-hydroxy-3-methoxyphenylglycol increased in the periaqueductal gray area in different phases of the formalin pain test (p<0.05).

Conclusion:

Salmon calcitonin reduced pain by increasing the concentrations of monoamines and the metabolites derived from them in the periaqueductal gray area.

Interleukin-4 mediates the analgesia produced by low-intensity exercise in mice with neuropathic pain

Peripheral nerve injury (PNI) activates the immune system, resulting in increased proinflammatory cytokines at the site of injury and in the spinal cord dorsal horn. Exercise modulates the immune system promoting an anti-inflammatory phenotype of macrophages in uninjured muscle, and increases in anti-inflammatory cytokines can promote healing and analgesia. We proposed that PNI will decrease, and treadmill exercise will increase, release of anti-inflammatory cytokines at the site of injury and in the spinal cord. We show that 2 weeks of treadmill exercise improves neuropathic pain behaviors in mice: mechanical hyperalgesia, escape and avoidance behavior, and spontaneous locomotor activity. Peripheral nerve injury reduced anti-inflammatory cytokines (interleukin-4 [IL-4], IL-1ra, and IL-5) at the site of nerve injury and in the spinal dorsal horn, whereas exercise restored IL-4, IL-1ra, and IL-5 concentrations to preinjury levels. IL4 mice and mice treated with IL-4 antibody did not develop analgesia to treadmill exercise. Using immunohistochemical staining of the sciatic nerve, treadmill exercise increased the percentage of M2 macrophages (secretes anti-inflammatory cytokines) and decreased M1 macrophages (secretes proinflammatory cytokines) when compared with sedentary mice. The increased M2 and decreased M1 macrophages in exercised mice did not occur in IL-4 mice. In the spinal cord, PNI increased glial cell activation, brain-derived neurotrophic factor and β-nerve growth factor levels, and decreased IL-4 and IL-1ra levels, whereas treadmill exercise suppressed glial cells activation (Glial Fibrillary Acidic Protein and Iba1 immunoreactivity), reduced brain-derived neurotrophic factor and β-nerve growth factor, and increased IL-4, IL-1ra, and IL-5 concentrations. Our results suggest that IL-4 mediates the analgesia produced by low-intensity exercise by modulating peripheral and central neuroimmune responses in mice with neuropathic pain.

[Effect of long-term resistance exercise on masseter muscle mechanical hyperalgesia in rats]

OBJECTIVE

To investigate the effect of long-term resistance exercise of hindlimb on mechanical hyperalgesia of bilateral masseter muscle in rats with or without occlusal interference.

METHODS

Six-teen male Sprague-Dawley rats (220-250 g) were randomly divided into four groups: the naive control group, naive exercise group, occlusal interference control group, and occlusal interference exercise group. The rats in occlusal interference groups (occlusal interference control group and occlusal interference exercise group) obtained occlusal interference with 0.4 mm-thick crowns bonded to the right maxillary first molars. The rats in exercise groups (naive exercise group and occlusal interference exercise group) performed squat-type resistance exercises for 30 minutes, once a day, 5 days/week, lasting for 14 weeks. Resistance exercise was recorded every day. Mechanical withdrawal thresholds of bilateral masseter muscle were tested per week by use of modified electronic von-frey anesthesiometer. The rats were weighed per week. After the 14-week exercise, the muscle strength of the hindlimb was tested with a grip strength meter. Muscle (gastrocnemius and soleus) weight of bilateral hindlimb and length of bilateral fibula of the rats were obtained. The muscle-mass/body-mass ratios and muscle-mass/fibula-length ratios were calculated.

RESULTS

Between the naive control group and naive exercise group, there was no significant difference in the mechanical withdrawal thresholds of bilateral masseter muscle for the 0-4 weeks (P>0.05). During the 5-14 weeks, the mechanical withdrawal thresholds of the rats in the naive exercise group were higher than those in the naive control group (P<0.05). Between the occlusal interference control group and occlusal interference exercise group, there was no significant difference in the mechanical withdrawal thresholds of bilateral masseter muscle for the 0-6 weeks (P>0.05). During the 7-14 weeks, the mechanical withdrawal thresholds of rats in the naive exercise group were higher than those in the occlusal interference control group (P<0.05). After the 14week exercise, the body mass of the rats in nonexercise group (the naive control group and occlusal interference control group) were larger than those in exercise group [(462±6) g vs. (418±14) g, P<0.05]. And the muscle strength of hindlimb of the rats in exercise group were bigger than those in non-exercise group [(6.75±0.13) N vs. (5.41±0.15) N, P<0.01].

CONCLUSION

long-term resistance exercise can increase mechanical withdrawal thresholds of the bilateral masseter muscle in rats with or without masseter muscle mechanical hyperalgesia.

Physiological Response to Opioids

The clinical setting in which women's health physicians practice, whether as generalist, obstetricians and gynecologists, or subspecialists, dictates our frequent clinical interaction with "pain." Opioid-containing medications are frequently prescribed within our specialty as a means of immediate pain relief. Opioid-containing medication causes a deep physiological alteration of several systems resulting in potential harm to acute and chronic opioid users. This article includes a thorough system-based review of opioid-containing medications on physiological systems. Women's health providers should have an in-depth understanding of such reverberations on patients' wellbeing to maintain the safest level of care. A solid grasp of physiological repercussions of opioid use would encourage physicians to seek alternative treatment options. Such practice is essential in curbing the opioid epidemic our patients are facing.

Interventional and preventive effects of aripiprazole and ceftriaxone used alone or in combination on oxaliplatin-induced tactile and cold allodynia in mice

BACKGROUND AND PURPOSE

Chemotherapy-induced peripheral neuropathy (CIPN) is a pharmacoresistant neurological complication induced by some antitumor drugs. This study aimed to assess antiallodynic properties of aripiprazole and ceftriaxone used alone or in combination to attenuate neuropathic pain related to CIPN caused by oxaliplatin.

METHODS

Neuropathic pain was induced in mice by a single intraperitoneal dose of oxaliplatin (10 mg/kg). Aripiprazole and ceftriaxone were used in a single- or repeated dosing protocol. Their antiallodynic activity was assessed using von Frey and cold plate tests on the day of oxaliplatin injection and after 7 days. The influence of aripiprazole and ceftriaxone on animals' locomotor activity and motor coordination was also assessed.

RESULTS

Single-dose and repeated-dose aripiprazole 10 mg/kg and ceftriaxone 200 mg/kg used alone and in combination attenuated early-phase and late-phase tactile allodynia in oxaliplatin-treated mice. Repeated administrations of ceftriaxone 200 mg/kg prevented the development of late-phase tactile allodynia. Both drugs showed no antiallodynic properties in the cold plate test. Single-dose aripiprazole 1 and 10 mg/kg but not its repeated administration significantly decreased locomotor activity of oxaliplatin-treated mice. Single-dose aripiprazole 1 and 10 mg/kg, aripiprazole 1 mg/kg + ceftriaxone 50 mg/kg and aripiprazole 1 mg/kg + ceftriaxone 200 mg/kg impaired motor coordination in the rotarod test.

CONCLUSIONS

In mice, neither ceftriaxone nor aripiprazole attenuated cold allodynia. Ceftriaxone alone could attenuate tactile allodynia caused by oxaliplatin without inducing motor adverse effects. Although the administration of aripiprazole reduced tactile allodynia, this effect seems to be limited considering severe motor deficits induced by this drug.

Association of Serum Serotonin and Pain in Patients with Chronic Low Back Pain before and after Spinal Surgery

Introduction

In this study we are aiming to evaluate the changes of serum serotonin and its association with pain in patients suffering from chronic low back pain before and after lumbar discectomy surgery.

Patients and Methods

A prospective study was performed on the patients referring to the outpatient clinic in Besat hospital, Hamadan University of Medical Sciences, Hamadan, Iran, during 2016. A 2 mL fasting blood sample was collected from each patient at preoperative day 1 and postoperative day 14 and they were measured for level of serum serotonin. Besides, all patients were asked for severity of their low back pain in preoperative day 1 and postoperative day 14 and scored their pain from zero to ten using a Numerical Rating Scale.

Results

Forty patients with the mean age of 47 ± 13 yrs/old (range 25–77) including 15 (37.5%) males were enrolled into the study. The overall mean score of preoperative pain was significantly decreased from 7.4 ± 2.18 (range 4–10) to the postoperative pain score 3.87 ± 2.92 (range 0–10) (P < .001). The overall levels of pre- and postoperative serum serotonin were 3.37 ± 1.27 (range 1.1–6.4) and 3.58 ± 1.32 (range .94–7.1) ng/mL, respectively, with no significant difference (P = .09). The levels of pre- and postoperative serum serotonin were significantly higher in males and patients older than 50 yrs/old compared to the females and patients younger than 50 yrs/old, respectively (P = .03 and .005, respectively). A significant inverse correlation between the postoperative levels of pain and serum serotonin was observed (r = -.36 and P = .02).

Conclusion

A negative medium strength linear relationship may exist between the postoperative serum serotonin and low back pain.

The Impact of Exercise in Rodent Models of Chronic Pain

PURPOSE OF REVIEW

Physical activity is increasingly recommended for chronic pain. In this review, we briefly survey recent, high-quality meta-analyses on the effects of exercise in human chronic pain populations, followed by a critical discussion of the rodent literature.

RECENT FINDINGS

Most meta-analytical studies on the effects of exercise in human chronic pain populations describe moderate improvements in various types of chronic pain, despite substantial variability in the outcomes reported in the primary literature. The most consistent findings suggest that while greater adherence to exercise programs produces better outcomes, there is minimal support for the superiority of one type of exercise over another. The rodent literature similarly suggests that while regular exercise reduces hypersensitivity in rodent models of chronic pain, exercise benefits do not appear to relate to either the type of injury or any particular facet of the exercise paradigm. Potential factors underlying these results are discussed, including the putative involvement of stress-induced analgesic effects associated with certain types of exercise paradigms. Exercise research using rodent models of chronic pain would benefit from increased attention to the role of stress in exercise-induced analgesia, as well as the incorporation of more clinically relevant exercise paradigms.

Effects of Different Parameters of Continuous Training and High-Intensity Interval Training in the Chronic Phase of a Mouse Model of Complex Regional Pain Syndrome Type I

This study evaluated the effects of continuous and interval running on a treadmill on mechanical hyperalgesia in an animal model of chronic postischemia pain and analyzed the mechanism of action of this effect. Different groups of male Swiss mice with chronic postischemia pain, induced by 3 hours of paw ischemia followed by reperfusion, ran on the treadmill in different protocols-the speed (10, 13, 16, or 19 m/min), duration (15, 30, or 60 minutes), weekly frequency (3 or 5 times), weekly increase in continuous and interval running speed-were tested. Mechanical hyperalgesia was evaluated by von Frey filament 7, 14, and 21 days after paw ischemia followed by reperfusion. On day 11 after paw ischemia followed by reperfusion and after 5 days of continuous and interval running, concentrations of cytokines, oxidative stress parameters, and extracellular signal-regulated kinase 1/2 and AKT 1/2/3 expression in the spinal cord were measured. The results showed that continuous running has an antihyperalgesic effect that depends on intensity and volume. Interval running has a longer-lasting antihyperalgesic effect than continuous running. The antihyperalgesic effect depends on intensity and volume in continuous running, and increasing speed maintains the antihyperalgesic effect in both protocols. In the spinal cord, both runs decreased tumor necrosis factor-α and interleukin-6 levels and increased interleukin-10. Both running protocols reduced oxidative damage in the spinal cord. Only interval running had lower concentrations of phosphorylated extracellular signal-regulated kinase 1/2 in the spinal cord. Interval running presented a great antihyperalgesic potential with more promising results than continuous running, which may be owing to the fact that the interval running can activate different mechanisms from those activated by continuous running. PERSPECTIVE: A minimum of .5-hour sessions of moderate to high intensity ≥3 times a week are essential parameters for continuous and interval running-induced analgesia. However, interval running was shown to be more effective than continuous running and can be an important adjuvant treatment to chronic pain.

A Mechanism-Based Approach to Physical Therapist Management of Pain

Pain reduction is a primary goal of physical therapy for patients who present with acute or persistent pain conditions. The purpose of this review is to describe a mechanism-based approach to physical therapy pain management. It is increasingly clear that patients need to be evaluated for changes in peripheral tissues and nociceptors, neuropathic pain signs and symptoms, reduced central inhibition and enhanced central excitability, psychosocial factors, and alterations of the movement system. In this Perspective, 5 categories of pain mechanisms (nociceptive, central, neuropathic, psychosocial, and movement system) are defined, and principles on how to evaluate signs and symptoms for each mechanism are provided. In addition, the underlying mechanisms targeted by common physical therapist treatments and how they affect each of the 5 categories are described. Several different mechanisms can simultaneously contribute to a patient's pain; alternatively, 1 or 2 primary mechanisms may cause a patient's pain. Further, within a single pain mechanism, there are likely many possible subgroups. For example, reduced central inhibition does not necessarily correlate with enhanced central excitability. To individualize care, common physical therapist interventions, such as education, exercise, manual therapy, and transcutaneous electrical nerve stimulation, can be used to target specific pain mechanisms. Although the evidence elucidating these pain mechanisms will continue to evolve, the approach outlined here provides a conceptual framework for applying new knowledge as advances are made.

Impact of physical exercise in cancer survivors during and after antineoplastic treatments

Cancer patients experience symptoms and adverse effects of treatments that may last even after the end of treatments. Exercise is a safe, non-pharmacological and cost-effective therapy that can provide several health benefits in cancer patient and survivors, reducing cancer symptoms and cancer treatment side effects. The purpose of this review is to describe how the physical exercise is capable to reduce cancer symptoms and cancer treatment side effects. We realized a pragmatic classification of symptoms, dividing them into physical, psychological and psycho-physical aspects. For each symptom we discuss causes, therapies, we analyse the effects of physical exercise and we summarize the most effective type of exercise to reduce the symptoms. This review also points out what are the difficulties that patients and survivors face during the practice of physical activity and provides some solutions to overcome these barriers. Related to each specific cancer, it emerges that type, frequency and intensity of physical exercise could be prescribed and supervised as a therapeutic program, like it occurs for the type, dose and duration of a drug treatment.

The effect of exercise frequency on neuropathic pain and pain-related cellular reactions in the spinal cord and midbrain in a rat sciatic nerve injury model

Background

Exercise regimens are established methods that can relieve neuropathic pain. However, the relationship between frequency and intensity of exercise and multiple cellular responses of exercise-induced alleviation of neuropathic pain is still unclear. We examined the influence of exercise frequency on neuropathic pain and the intracellular responses in a sciatic nerve chronic constriction injury (CCI) model.

Materials and methods

Rats were assigned to four groups as follows: CCI and high-frequency exercise (HFE group), CCI and low-frequency exercise (LFE group), CCI and no exercise (No-Ex group), and naive animals (control group). Rats ran on a treadmill, at a speed of 20 m/min, for 30 min, for 5 (HFE) or 3 (LFE) days a week, for a total of 5 weeks. The 50% withdrawal threshold was evaluated for mechanical sensitivity. The activation of glial cells (microglia and astrocytes), expression of brain-derived neurotrophic factor (BDNF) and μ-opioid receptor in the spinal dorsal horn and endogenous opioid in the midbrain were examined using immunohistochemistry. Opioid receptor antagonists (naloxone) were administered using intraperitoneal injection.

Results

The development of neuropathic pain was related to the activation of glial cells, increased BDNF expression, and downregulation of the μ-opioid receptor in the ipsilateral spinal dorsal horn. In the No-Ex group, neuropathic pain showed the highest level of mechanical hypersensitivity at 2 weeks, which improved slightly until 5 weeks after CCI. In both exercise groups, the alleviation of neuropathic pain was accelerated through the regulation of glial activation, BDNF expression, and the endogenous opioid system. The expression of BDNF and endogenous opioid in relation to exercise-induced alleviation of neuropathic pain differed in the HFE and LFE groups. The effects of exercise-induced alleviation of mechanical hypersensitivity were reversed by the administration of naloxone.

Conclusion

The LFE and HFE program reduced neuropathic pain. Our findings indicated that aerobic exercise-induced alleviated neuropathic pain through the regulation of glial cell activation, expression of BDNF in the ipsilateral spinal dorsal horn, and the endogenous opioid system.

Monoaminergic descending pathways contribute to modulation of neuropathic pain by increasing-intensity treadmill exercise after peripheral nerve injury

This study characterizes the impact of increasing-intensity treadmill exercise (iTR) on noradrenergic (NE) and serotonergic (5HT) modulation of neuropathic pain. Following sciatic nerve transection and repair (SNTR) rats developed significant mechanical and thermal hyperalgesia that was partially prevented by iTR performed during the first 2weeks after injury. Marked decrease in the expression of 5HT_2A and α_1A and β-, but not α_2A adrenergic receptors in the spinal cord dorsal horn was associated to SNTR and recovered by iTR, particularly in lamina II. iTR significantly increased 5HT_2A in periaqueductal grey (PAG), raphe magnus (RM) and dorsal raphe nucleus (DRN), with a pattern suggesting reorganization of serotonergic excitatory interconnections between PAG and DRN. iTR also increased the expression of α_1A in locus coeruleus (LC) and DRN, and β₂ in LC, indicating that exercise enhanced activity of NE neurons, likely by activating autologous projections from DRN and PAG. iTR hypoalgesia was antagonized by blockade of β₂ and 5HT_2A receptors with administration of butoxamine and ketanserin. The neurotoxin DSP4 was injected to induce depletion of NE projections from LC before starting iTR. DSP4 treatment worsened mechanical hyperalgesia, but iTR hypoalgesia was similarly produced. Moreover, 5HT_2A expression in LC further increased after DSP4 injection, all these results suggesting an intrinsic regulation of 5HT and NE activity between PAG, DRN and LC neurons activated by iTR. Finally, iTR significantly reduced microglial reactivity in LC and increased non-microglial BDNF expression, an effect that was reverted by butoxamine, implicating BDNF regulation in central 5HT/NE actions on neuropathic pain.

Altered Gene Expression of RNF34 and PACAP Possibly Involved in Mechanism of Exercise-Induced Analgesia for Neuropathic Pain in Rats

Despite the availability of several modalities of treatment, including surgery, pharmacological agents, and nerve blocks, neuropathic pain is often unresponsive and sometimes progresses to intractable chronic pain. Although exercise therapy is a candidate for treatment of neuropathic pain, the mechanism underlying its efficacy has not been elucidated. To clarify the molecular mechanism for pain relief induced by exercise, we measured Rnf34 and Pacap mRNA levels in the spinal cord dorsal horn of SNL rats, a model of neuropathic pain. SNL model rats exhibited stable mechanical hyperalgesia for at least 6 weeks. When the rats were forced to exercise on a treadmill, mechanical and thermal hyperalgesia were significantly ameliorated compared with the non-exercise group. Accordingly, gene expression level of Rnf34 and Pacap were also significantly altered in the time course analysis after surgery. These results suggest that exercise therapy possibly involves pain relief in SNL rats by suppressing Rnf34 and Pacap expression in the spinal cord.

Short-duration physical activity prevents the development of activity-induced hyperalgesia through opioid and serotoninergic mechanisms

Regular physical activity prevents the development of chronic muscle pain through the modulation of central mechanisms that involve rostral ventromedial medulla (RVM). We tested if pharmacological blockade or genetic deletion of mu-opioid receptors in physically active mice modulates excitatory and inhibitory systems in the RVM in an activity-induced hyperalgesia model. We examined response frequency to mechanical stimulation of the paw, muscle withdrawal thresholds, and expression of phosphorylation of the NR1 subunit of the N-methyl-D-aspartate receptor (p-NR1) and serotonin transporter (SERT) in the RVM. Mice that had performed 5 days of voluntary wheel running prior to the induction of the model were compared with sedentary mice. Sedentary mice showed significant increases in mechanical paw withdrawal frequency and a reduction in muscle withdrawal threshold; wheel running prevented the increase in paw withdrawal frequency. Naloxone-treated and MOR mice had increases in withdrawal frequency that were significantly greater than that in physically active control mice and similar to sedentary mice. Immunohistochemistry in the RVM showed increases in p-NR1 and SERT expression in sedentary mice 24 hours after the induction of the model. Wheel running prevented the increase in SERT, but not p-NR1. Physically active, naloxone-treated, and MOR mice showed significant increases in SERT immunoreactivity when compared with wild-type physically active control mice. Blockade of SERT in the RVM in sedentary mice reversed the activity-induced hyperalgesia of the paw and muscle. These results suggest that analgesia induced by 5 days of wheel running is mediated by mu-opioid receptors through the modulation of SERT, but not p-NR1, in RVM.

Regular physical activity prevents development of chronic muscle pain through modulation of supraspinal opioid and serotonergic mechanisms

The current study shows that blockade of opioid receptors systemically in the periaqueductal gray and the rostral ventromedial medulla prevents analgesia by 8 weeks of wheel running in a chronic muscle pain model. We further show increases in serotonin transporter expression and reversal of hyperalgesia with a selective reuptake inhibitor in the rostral ventromedial medulla in the chronic muscle pain model, and exercise normalizes serotonin transporter expression.

Introduction:

It is generally believed that exercise produces its effects by activating central opioid receptors; there are little data that support this claim. The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are key nuclei in opioid-induced analgesia, and opioids interact with serotonin to produce analgesia.

Objectives:

The purpose was to examine central inhibitory mechanisms involved in analgesia produced by wheel running.

Methods:

C57/Black6 mice were given access to running wheels in their home cages before induction of chronic muscle hyperalgesia and compared with those without running wheels. Systemic, intra-PAG, and intra-RVM naloxone tested the role of central opioid receptors in the antinociceptive effects of wheel running in animals with muscle insult. Immunohistochemistry for the serotonin transporter (SERT) in the spinal cord and RVM, and pharmacological blockade of SERT, tested whether the serotonin system was modulated by muscle insult and wheel running.

Results:

Wheel running prevented the development of muscle hyperalgesia. Systemic naloxone, intra-PAG naloxone, and intra-RVM naloxone reversed the antinociceptive effect of wheel running in animals that had received muscle insult. Induction of chronic muscle hyperalgesia increased SERT in the RVM, and blockade of SERT reversed the hyperalgesia in sedentary animals. Wheel running reduced SERT expression in animals with muscle insult. The serotonin transporter in the superficial dorsal horn of the spinal cord was unchanged after muscle insult, but increased after wheel running.

Conclusion:

These data support the hypothesis that wheel running produced analgesia through central inhibitory mechanisms involving opioidergic and serotonergic systems.

Exercise reverses pain-related weight asymmetry and differentially modulates trabecular bone microarchitecture in a rat model of osteoarthritis

There is great interest in developing and utilizing non-pharmacological/non-invasive forms of therapy for osteoarthritis (OA) pain including exercise and other physical fitness regimens.

AIMS

The present experiments determined the effects of prior wheel running on OA-induced weight asymmetry and trabecular bone microarchitecture.

MAIN METHODS

Wheel running included 7 or 21days of prior voluntary access to wheels followed by OA induction, followed by 21days post-OA access to wheels. OA was induced with monosodium iodoacetate (MIA), and weight asymmetry was measured using a hind limb weight bearing apparatus. Bone microarchitecture was characterized using ex vivo μCT.

KEY FINDINGS

Relative to saline controls, MIA (3.2mg/25μl) produced significant weight asymmetry measured on post-days (PDs) 3, 7, 14, 21 in sedentary rats. Seven days of prior running failed to alter MIA-induced weight asymmetry. In contrast, 21days of prior running resulted in complete reversal of MIA-induced weight asymmetry on all days tested. As a comparator, the opioid agonist morphine (3.2-10mg/kg) dose-dependently reversed weight asymmetry on PDs 3, 7, 14, but was ineffective in later-stage (PD 21) OA. In runners, Cohen's d (effect sizes) for OA vs. controls indicated large increases in bone volume fraction, trabecular number, trabecular thickness, and connective density in lateral compartment, and large decreases in the same parameters in medial compartment. In contrast, effect sizes were small to moderate for sedentary OA vs.

CONTROLS

SIGNIFICANCE

Results indicate that voluntary exercise may protect against OA pain, the effect varies as a function of prior exercise duration, and is associated with distinct trabecular bone modifications.

Does exercise increase or decrease pain? Central mechanisms underlying these two phenomena

Exercise is an integral part of the rehabilitation of patients suffering a variety of chronic musculoskeletal conditions, such as fibromyalgia, chronic low back pain and myofascial pain. Regular physical activity is recommended for treatment of chronic pain and its effectiveness has been established in clinical trials for people with a variety of pain conditions. However, exercise can also increase pain making participation in rehabilitation challenging for the person with pain. Animal models of exercise-induced pain have been developed and point to central mechanisms underlying this phenomena, such as increased activation of NMDA receptors in pain-modulating areas. Meanwhile, a variety of basic science studies testing different exercise protocols, show exercise-induced analgesia involves activation of central inhibitory pathways. Opioid, serotonin and NMDA mechanisms acting in rostral ventromedial medulla promote analgesia associated with exercise. This review explores and discusses current evidence on central mechanisms underlying exercised-induced pain and analgesia.