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

Beneficial Effects of Exercise in Neuropathic Pain: An Overview of the Mechanisms Involved

Neuropathic pain is a prevalent issue that often arises following injuries to the peripheral or central nervous system. Unfortunately, there is currently no definitive and flawless treatment available to alleviate this type of pain. However, exercise has emerged as a promising nonpharmacological and adjunctive approach, demonstrating a significant impact in reducing pain intensity. This is why physical therapy is considered a beneficial approach for diminishing pain and promoting functional recovery following nerve injuries. Regular physical activity exerts its hypoalgesic effects through a diverse array of mechanisms. These include inhibiting oxidative stress, suppressing inflammation, and modulating neurotransmitter levels, among others. It is possible that multiple activated mechanisms may coexist within an individual. However, the priming mechanism does not need to be the same across all subjects. Each person's response to physical activity and pain modulation may vary depending on their unique physiological and genetic factors. In this review, we aimed to provide a concise overview of the mechanisms underlying the beneficial effects of regular exercise on neuropathic pain. We have discussed several key mechanisms that contribute to the improvement of neuropathic pain through exercise. However, it is important to note that this is not an exhaustive analysis, and there may be other mechanisms at play. Our goal was to provide a brief yet informative exploration of the topic.

Female specific interactions of serotonin and testosterone in the rostral ventromedial medulla after activity-induced muscle pain

Classical preclinical studies show that serotonin (5-HT) injected into the rostral ventromedial medulla (RVM) produces analgesia that is blocked by 5-HT2 receptor antagonists. One key modulator of 5-HT activity is the serotonin transporter (SERT) which reduces serotonergic signaling through reuptake into the presynaptic terminal. In the activity-induced muscle pain model, females show widespread pain and increased SERT expression in the RVM whereas males show localized pain and no changes in SERT expression. Since prior studies show testosterone protects from the development of widespread pain, and females have widespread pain in the activity-induced pain model, we hypothesized that testosterone modulates serotonin signaling to enhance analgesia in female mice with widespread pain. We showed that testosterone reduced the enhanced SERT protein expression and increased 5-HT2A receptor mRNA expression in the RVM normally observed in the activity-induced pain model in females, but not males. Inhibition of SERT in the RVM was analgesic in both female and male mice; this analgesia was blocked by co-administration of 5-HT2A antagonist. Next, using in situ hybridization, we demonstrated co-expression of SERT, 5-HT2A receptor, and androgen receptor mRNA in cells within the RVM in female mice. Lastly, activation of androgen receptors using dihydrotestosterone reduced hyperalgesia in female mice. These data therefore show for the first time expression of androgen receptors in the RVM in female mice, that activation of androgen receptors reduces nociceptive behaviors, and endogenous testosterone modulates SERT and 5-HT2 receptor expression. Thus, we show a sex-specific role for how testosterone modulates analgesia in mice. PERSPECTIVE: This article presents novel mechanisms testosterone's protection against muscle pain in female mice showing modulation of the serotonin system in the rostral ventromedial medulla. Understanding the relationship between testosterone and serotonin could lead to better treatment of individuals with muscle pain.

Effects of resistance training on pain, functionality and quality of life in women with fibromyalgia: A systematic review

INTRODUCTION

Fibromyalgia is a chronic syndrome characterized by constant and generalized pain associated with sleep disturbance, depression, muscle stiffness, fatigue and cognitive disorders. Among non-pharmacological treatments, physical exercise stands out as a low-cost approach.

AIM

To summarize and analyze evidence on the effects of resistance training on pain, functionality and quality of life in women with fibromyalgia.

METHODS

Following the PRISMA method, this systematic review included clinical trials assessing the effects of resistance training on pain, quality of life and functionality in female patients with fibromyalgia, regardless age. The researches were conducted in April 2021 in PubMed, Cochrane, Web of Science and Scopus databases, using the search strategy: ("fibromyalgia") AND ("strength training" OR "resistance training") AND ("quality of life" OR "pain" OR "functionality"). This study was registered in PROSPERO (CRD number: 42,021,246,245), and the risk of bias was assessed using the Version 2 of the Cochrane Risk-of-Bias tool (RoB 2).

RESULTS

The search resulted in 125 studies (760 women), of which 16 were eligible for this review. Risk of bias assessment resulted in high (n = 5), moderate (n = 6) and low (n = 5) risks. Resistance training has proven to be an important non-pharmacological treatment tool for fibromyalgia, reducing pain, and improving patients' functionality and quality of life.

CONCLUSION

The available evidence suggests that resistance training performed twice weekly, with progressive loads ranging from 40 to 80% of one-repetition maximum and a total duration of 4-24 weeks, appears to be an effective and safe therapeutic approach.

Physical Therapist Interventions Versus or Combined With Surgical Treatment in Nontraumatic Degenerative Meniscal Pathology: A Systematic Review and Network Meta-Analysis

OBJECTIVE

This study aimed to synthesize the evidence from randomized clinical trials in people with nontraumatic degenerative meniscal pathology by comparing physical therapist interventions versus or combined with arthroscopic partial meniscectomy (APM).

METHODS

Seven electronic databases were searched. Methodological quality was evaluated using the Physiotherapy Evidence Database scale. Data synthesis was performed with random-effects network meta-analysis, and results were summarized using the standardized mean differences.

RESULTS

From 2103 studies, 10 randomized clinical trials comprising 1411 individuals were included. Ninety percent of the selected randomized clinical trials were classified as good quality according to the Physiotherapy Evidence Database scale. All interventions (physical therapist interventions, APM, and APM plus physical therapist interventions) showed reduced pain and physical impairments at 3-month follow-up. However, when a physical therapist intervention was included, greater reductions in pain at rest (APM vs physical therapist interventions: 0.73 [95% CI = 0.20 to 1.26]; APM vs APM plus physical therapist interventions: 0.59 [95% CI = 0.15 to 1.03]) and greater increases in the strength of knee extensor muscles (APM vs physical therapist interventions: 0.44 [95% CI = 0.07 to 0.80]; APM vs APM plus physical therapist interventions: 0.73 [95% CI = 0.29 to 1.16]) were observed at 3 months. By contrast, no differences were found between treatments beyond 3 months.

CONCLUSION

Physical therapist interventions based on exercise programs demonstrate superior short-term outcomes in pain reduction and knee extensor strength compared to surgical treatment.

IMPACT

For nontraumatic degenerative meniscal pathology, conservative treatment utilizing a physical therapist intervention approach should be prioritized as the first choice over surgical treatment. It offers comparable or superior short-term pain reduction and strength improvements, with a lower risk of side effects. In cases where surgery is deemed necessary, including postsurgical, physical therapist interventions are highly recommended to enhance muscle strength and alleviate pain.

Daoyin therapy in chronic neck pain: study protocol for a randomized controlled trial

BACKGROUND

Daoyin therapy (DT), an ancient therapeutic approach with a history spanning thousands of years, has traditionally been employed to address musculoskeletal pain and psychosomatic disorders. However, the application of DT for chronic neck pain (CNP) has received limited attention in the existing literature, and systematic randomized clinical trials (RCTs) in this context remain scarce. This manuscript outlines an RCT protocol designed to investigate whether DT is more effective at alleviating CNP in adult individuals compared to other interventions.

METHODS

A 12-week RCT was conducted, with participants undergoing randomization into one of three groups: DT, Meditation + Fitness Exercise (M+FE), or a control group. Participants in the DT and M + FE groups attended their respective training classes three times per week for 12 weeks. Participants in the control group were required to attend health education workshops every 2 weeks. Following the 12-week intervention period, all participants underwent follow-up assessments at the 16th week. Outcome measures encompassed the Simplified Chinese Neck Pain and Disability Scale (SC-NPAD) and Visual Analog Scale (VAS) for pain assessment, Static Neck Posture Assessment (SNPA) to evaluate neck and shoulder posture and function, Short Form-36 (SF-36) to assess quality of life, and blood tests measuring 5-Hydroxytryptamine (5-HT), Norepinephrine/Noradrenaline (NE/NA), γ-aminobutyric acid (GABA), Adreno-Cortico-Tropic-Hormone (ACTH), β-Endorphin (β-EP), and Calcitonin-Gene-Related Peptide (CGRP) levels via high-performance liquid chromatography (HPLC), chemiluminescence immunoassay (CLIA), enzyme-linked immunosorbent assay (ELISA), and radioimmunoassay (RIA). Brain activity changes were monitored through MRI scans. Repeated measures analyses of variance (ANOVAs) will be used to evaluate the outcomes at baseline, at the 12th week, and at the 16th week. Generalized Estimating Equation (GEE) models will be applied to analyze changes in outcomes over time and differences between groups.

DISCUSSION

This trial aims to evaluate the efficacy of DT in comparison to other interventions and explore the neuroendocrine mechanisms underlying its effects in adults with CNP. If the intervention and procedures demonstrate feasibility and acceptability, there are plans to conduct a more extensive controlled trial. This could potentially pave the way for the broader application of DT, not only in the context of CNP but also for other chronic diseases.

TRIAL REGISTRATION

This trial has been registered with the Chinese Clinical Trial Registry (Registration ID: [ChiCTR2400079571]).

The impact of sex and physical activity on the local immune response to muscle pain

Induction of muscle pain triggers a local immune response to produce pain and this mechanism may be sex and activity level dependent. The purpose of this study was to measure the immune system response in the muscle following induction of pain in sedentary and physically active mice. Muscle pain was produced via an activity-induced pain model using acidic saline combined with fatiguing muscle contractions. Prior to induction of muscle pain, mice (C57/BL6) were sedentary or physically active (24hr access to running wheel) for 8 weeks. The ipsilateral gastrocnemius was harvested 24hr after induction of muscle pain for RNA sequencing or flow cytometry. RNA sequencing revealed activation of several immune pathways in both sexes after induction of muscle pain, and these pathways were attenuated in physically active females. Uniquely in females, the antigen processing and presentation pathway with MHC II signaling was activated after induction of muscle pain; activation of this pathway was blocked by physical activity. Blockade of MHC II attenuated development of muscle hyperalgesia exclusively in females. Induction of muscle pain increased the number of macrophages and T-cells in the muscle in both sexes, measured by flow cytometry. In both sexes, the phenotype of macrophages shifted toward a pro-inflammatory state after induction of muscle pain in sedentary mice (M1 + M1/2) but toward an anti-inflammatory state in physically active mice (M2 + M0). Thus, induction of muscle pain activates the immune system with sex-specific differences in the transcriptome while physical activity attenuates immune response in females and alters macrophage phenotype in both sexes.

Longitudinal relationships between habitual physical activity and pain tolerance in the general population

Physical activity (PA) might influence the risk or progression of chronic pain through pain tolerance. Hence, we aimed to assess whether habitual leisure-time PA level and PA change affects pain tolerance longitudinally in the population. Our sample (n = 10,732; 51% women) was gathered from the sixth (Tromsø6, 2007-08) and seventh (Tromsø7, 2015-16) waves of the prospective population-based Tromsø Study, Norway. Level of leisure-time PA (sedentary, light, moderate, or vigorous) was derived from questionnaires; experimental pain tolerance was measured by the cold-pressor test (CPT). We used ordinary, and multiple-adjusted mixed, Tobit regression to assess 1) the effect of longitudinal PA change on CPT tolerance at follow-up, and 2) whether a change in pain tolerance over time varied with level of LTPA. We found that participants with high consistent PA levels over the two surveys (Tromsø6 and Tromsø7) had significantly higher tolerance than those staying sedentary (20.4 s. (95% CI: 13.7, 27.1)). Repeated measurements show that light (6.7 s. (CI 3.4, 10.0)), moderate (CI 14.1 s. (9.9, 18.3)), and vigorous (16.3 s. (CI 6.0, 26.5)) PA groups had higher pain tolerance than sedentary, with non-significant interaction showed slightly falling effects of PA over time. In conclusion, being physically active at either of two time points measured 7-8 years apart was associated with higher pain tolerance compared to being sedentary at both time-points. Pain tolerance increased with higher total activity levels, and more for those who increased their activity level during follow-up. This indicates that not only total PA amount matters but also the direction of change. PA did not significantly moderate pain tolerance change over time, though estimates suggested a slightly falling effect possibly due to ageing. These results support increased PA levels as a possible non-pharmacological pathway towards reducing or preventing chronic pain.

Bulbospinal nociceptive ON and OFF cells related neural circuits and transmitters

The rostral ventromedial medulla (RVM) is a bulbospinal nuclei in the descending pain modulation system, and directly affects spinal nociceptive transmission through pronociceptive ON cells and antinociceptive OFF cells in this area. The functional status of ON and OFF neurons play a pivotal role in pain chronification. As distinct pain modulative information converges in the RVM and affects ON and OFF cell excitability, neural circuits and transmitters correlated to RVM need to be defined for an in-depth understanding of central-mediated pain sensitivity. In this review, neural circuits including the role of the periaqueductal gray, locus coeruleus, parabrachial complex, hypothalamus, amygdala input to the RVM, and RVM output to the spinal dorsal horn are discussed. Meanwhile, the role of neurotransmitters is concluded, including serotonin, opioids, amino acids, cannabinoids, TRPV1, substance P and cholecystokinin, and their dynamic impact on both ON and OFF cell activities in modulating pain transmission. Via clarifying potential specific receptors of ON and OFF cells, more targeted therapies can be raised to generate pain relief for patients who suffer from chronic pain.

Therapeutic exercise interventions in rat models of arthritis

Arthritis is the leading cause of musculoskeletal pain and disability worldwide. Nearly 50% of individuals over the age of 65 have arthritis, which contributes to limited function, articular pain, physical inactivity, and diminished quality of life. Therapeutic exercise is often recommended in clinical settings for patients experiencing arthritic pain, however, there is little practical guidance regarding the use of therapeutic exercise to alleviate arthritic musculoskeletal pain. Rodent models of arthritis allow researchers to control experimental variables, which cannot be done with human participants, providing an opportunity to test therapeutic approaches in preclinical models. This literature review provides a summary of published findings in therapeutic exercise interventions in rat models of arthritis as well as gaps in the existing literature. We reveal that preclinical research in this field has yet to adequately investigate the impact of experimental variables in therapeutic exercise including their modality, intensity, duration, and frequency on joint pathophysiology and pain outcomes.

Resistance training protects against muscle pain through activation of androgen receptors in male and female mice

ABSTRACT

Resistance training-based exercise is commonly prescribed in the clinic for the treatment of chronic pain. Mechanisms of aerobic exercise for analgesia are frequently studied, while little is known regarding resistance training mechanisms. We developed a resistance training model in mice and hypothesized resistance training would protect against development of muscle pain, mediated through the activation of androgen receptors. Activity-induced muscle hyperalgesia was produced by 2 injections of pH 5.0 stimuli with fatiguing muscle contractions. Resistance training was performed by having mice climb a ladder with attached weights, 3 times per week. Resistance training acutely increased blood lactate and prolonged training increased strength measured via forepaw grip strength and 1 repetition maximum, validating the exercise program as a resistance training model. Eight weeks of resistance training prior to induction of the pain model blocked the development of muscle hyperalgesia in both sexes. Resistance training initiated after induction of the pain model reversed muscle hyperalgesia in male mice only. A single resistance training bout acutely increased testosterone in male but not female mice. Administration of the androgen receptor antagonist flutamide (200 mg pellets) throughout the 8-week training program blocked the exercise-induced protection against muscle pain in both sexes. However, single administration of flutamide (1, 3, 10 mg/kg) in resistance-trained animals had no effect on existing exercise-induced protection against muscle pain. Therefore, resistance training acutely increases lactate and testosterone and strength overtime. Eight weeks of resistance training prevents the development of hyperalgesia through the activation of androgen receptors in an animal model of muscle pain.

Sleep Problems Mediate the Relationship Between Psychosocial Stress and Pain Facilitation in Native Americans: A Structural Equation Modeling Analysis from the Oklahoma Study of Native American Pain Risk

BACKGROUND

Native Americans (NAs) are more likely to experience chronic pain than non-Hispanic Whites (NHWs); however, the proximate causes predisposing NAs to chronic pain remain elusive. Likely due to centuries of adversity, discrimination, and marginalization, NAs report greater psychological stress than NHWs, which may place them at risk for sleep problems, a well-established risk factor for chronic pain onset.

PURPOSE

This study examined the effects of psychological stress and sleep problems on subjective and physiological measures of pain processing in NAs and NHWs.

METHODS

Structural equation modeling was used to determine whether ethnicity (NA or NHW) was associated with psychological stress or sleep problems and whether these variables were related to conditioned pain modulation of pain perception (CPM-pain) and the nociceptive flexion reflex (CPM-NFR), temporal summation of pain (TS-pain) and NFR (TS-NFR), and pain tolerance in a sample of 302 (153 NAs) pain-free participants.

RESULTS

NAs experienced more psychological stress (Estimate = 0.027, p = .009) and sleep problems (Estimate = 1.375, p = .015) than NHWs. When controlling for age, sex, physical activity, BMI, and general health, NA ethnicity was no longer related to greater sleep problems. Psychological stress was also related to sleep problems (Estimate = 30.173, p = <.001) and psychological stress promoted sleep problems in NAs (indirect effect = 0.802, p = .014). In turn, sleep problems were associated with greater TS-pain (Estimate = 0.714, p = .004), but not other pain measures.

CONCLUSIONS

Sleep problems may contribute to chronic pain risk by facilitating pain perception without affecting facilitation of spinal neurons or endogenous inhibition of nociceptive processes. Since psychological stress promoted pain facilitation via enhanced sleep problems, efforts to reduce psychological stress and sleep problems among NAs may improve health outcomes.

Regular Aerobic Exercise Attenuates Pain and Anxiety in Mice by Restoring Serotonin-modulated Synaptic Plasticity in the ACC

PURPOSE

Clinical studies found that regular aerobic exercise has analgesic and anti-anxiety effects; however, the underlying neural mechanisms remain unclear. Multiple studies have suggested that regular aerobic exercise may exert brain-protective effects by promoting the release of serotonin, which may be a pain modulator. The anterior cingulate cortex (ACC) is a key brain area for pain information processing, receiving dense serotonergic innervation. As a result, we hypothesized that exercise may increase the release of serotonin in the ACC, thus improving pain and anxiety behaviours.

METHODS

Integrative methods were used, including behavioural, electrophysiological, pharmacological, biochemical, and genetic approaches, to explore the effects of regular aerobic exercise and the underlying neural mechanisms.

RESULTS

Regular aerobic exercise in the form of voluntary wheel running for 30 minutes daily for 15 days showed significant effectiveness in relieving pain and concomitant anxiety in complete Freund's adjuvant (CFA)-induced chronic inflammation pain models. c-Fos staining and multielectrode array recordings revealed alterations in neuronal activities and synaptic plasticity in the ACC. Moreover, systemic pharmacological treatment with 4-chloro-DL-phenylalanine (PCPA) to deplete endogenous serotonin and local delivery of serotonin to the ACC revealed that exercise-related serotonin release in the ACC bidirectionally modulates pain sensitization and anxiety behaviours by modulating synaptic plasticity in the ACC. Furthermore, we found that 5-HT1A and 5-HT7 receptors mediated the serotonin modulation effects under conditions of regular aerobic exercise through local infusion of a selective antagonist and shRNA in the ACC.

CONCLUSIONS

Our results reveal that regular aerobic exercise can increase serotonin release and modulate synaptic plasticity in the ACC, ultimately improving pain and concomitant anxiety behaviours through the functions of the 5-HT1A and 5-HT7 receptors.

G protein-coupled estrogen receptor in the rostral ventromedial medulla contributes to the chronification of postoperative pain

AIMS

Chronification of postoperative pain is a common clinical phenomenon following surgical operation, and it perplexes a great number of patients. Estrogen and its membrane receptor (G protein-coupled estrogen receptor, GPER) play a crucial role in pain regulation. Here, we explored the role of GPER in the rostral ventromedial medulla (RVM) during chronic postoperative pain and search for the possible mechanism.

METHODS AND RESULTS

Postoperative pain was induced in mice or rats via a plantar incision surgery. Behavioral tests were conducted to detect both thermal and mechanical pain, showing a small part (16.2%) of mice developed into pain persisting state with consistent low pain threshold on 14 days after incision surgery compared with the pain recovery mice. Immunofluorescent staining assay revealed that the GPER-positive neurons in the RVM were significantly activated in pain persisting rats. In addition, RT-PCR and immunoblot analyses showed that the levels of GPER and phosphorylated μ-type opioid receptor (p-MOR) in the RVM of pain persisting mice were apparently increased on 14 days after incision surgery. Furthermore, chemogenetic activation of GPER-positive neurons in the RVM of Gper-Cre mice could reverse the pain threshold of pain recovery mice. Conversely, chemogenetic inhibition of GPER-positive neurons in the RVM could prevent mice from being in the pain persistent state.

CONCLUSION

Our findings demonstrated that the GPER in the RVM was responsible for the chronification of postoperative pain and the downstream pathway might be involved in MOR phosphorylation.

Plastic changes in amygdala subregions by voluntary running contribute to exercise-induced hypoalgesia in neuropathic pain model mice

Physical exercise has been established as a low-cost, safe, and effective way to manage chronic pain, but exact mechanisms underlying such exercise-induced hypoalgesia (EIH) are not fully understood. Since a growing body of evidence implicated the amygdala (Amyg) as a critical node in emotional affective aspects of chronic pain, we hypothesized that the Amyg may play important roles to produce EIH effects. Here, using partial sciatic nerve ligation (PSL) model mice, we investigated the effects of voluntary running (VR) on the basal amygdala (BA) and the central nuclei of amygdala (CeA). The present study indicated that VR significantly improved heat hyperalgesia which was exacerbated in PSL-Sedentary mice, and that a significant positive correlation was detected between total running distances after PSL-surgery and thermal withdrawal latency. The number of activated glutamate (Glu) neurons in the medal BA (medBA) was significantly increased in PSL-Runner mice, while those were increased in the lateral BA in sedentary mice. Furthermore, in all subdivisions of the CeA, the number of activated gamma-aminobutyric acid (GABA) neurons was dramatically increased in PSL-Sedentary mice, but these numbers were significantly decreased in PSL-Runner mice. In addition, a tracer experiment demonstrated a marked increase in activated Glu neurons in the medBA projecting into the nucleus accumbens lateral shell in runner mice. Thus, our results suggest that VR may not only produce suppression of the negative emotion such as fear and anxiety closely related with pain chronification, but also promote pleasant emotion and hypoalgesia. Therefore, we conclude that EIH effects may be produced, at least in part, via such plastic changes in the Amyg.

Regular swimming exercise prevented the acute and persistent mechanical muscle hyperalgesia by modulation of macrophages phenotypes and inflammatory cytokines via PPARγ receptors

Physically active individuals are less likely to develop chronic pain, and physical exercise is an established strategy to control inflammatory diseases. Here, we hypothesized that 1) peripheral pro-inflammatory macrophages phenotype contribute to predisposition of the musculoskeletal to chronic pain, and that 2) activation of PPARγ receptors, modulation of macrophage phenotypes and cytokines through physical exercise would prevent persistent muscle pain. We tested these hypotheses using swimming exercise, pharmacological and immunochemical techniques in a rodent model of persistent muscle hyperalgesia. Swimming prevented the persistent mechanical muscle hyperalgesia most likely through activation of PPARγ receptors, as well as activation of PPARγ receptors by 15d-PGJ₂ and depletion of muscle macrophages in sedentary animals. Acute and persistent muscle hyperalgesia were characterized by an increase in pro-inflammatory macrophages phenotype, and swimming and the 15d-PGJ₂ prevented this increase and increased anti-inflammatory macrophages phenotype. Finally, IL-1β concentration in muscle increased in the acute phase, which was also prevented by PPARγ receptors activation through swimming. Besides, swimming increased muscle concentration of IL-10 in both acute and chronic phases, but only in the persistent phase through PPARγ receptors. Our findings suggest physical exercise activates PPARγ receptors and increases anti-inflammatory responses in the muscle tissue by modulating macrophages phenotypes and cytokines, thereby preventing the establishment of persistent muscle hyperalgesia. These results further highlight the potential of physical exercise to prevent chronic muscle pain.

Mechanisms, diagnosis, prevention and management of perioperative opioid-induced hyperalgesia

Opioid-induced hyperalgesia (OIH) occurs when opioids paradoxically enhance the pain they are prescribed to ameliorate. To address a lack of perioperative awareness, we present an educational review of clinically relevant aspects of the disorder. Although the mechanisms of OIH are thought to primarily involve medullary descending pathways, it is likely multifactorial with several relevant therapeutic targets. We provide a suggested clinical definition and directions for clinical differentiation of OIH from other diagnoses, as this may be confusing but is germane to appropriate management. Finally, we discuss prevention including patient education and analgesic management choices. As prevention may serve as the best treatment, patient risk factors, opioid mitigation, and both pharmacologic and non-pharmacologic strategies are discussed.

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.

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.

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.

Chronic Pain and Premature Aging - The Moderating Role of Physical Exercise

Chronic pain induces a multitude of harmful effects; recently it has been suggested that chronic pain is also associated with premature aging, manifested in shortened telomere length (TL). However, evidence for this hypothesis is scarce and inconsistent. The aim was twofold: 1) Investigate whether chronic pain is associated with premature aging, and 2) Determine whether physical exercise (PE) moderates this association if it exists. Participants were 116 male subjects, with (n = 67) and without chronic pain (n = 49). Blood samples for TL analysis were collected and participants were interviewed and completed questionnaires. As a part of the cohort, we included people with physical disability; this variable was controlled in the analysis. The TL of individuals with chronic pain was significantly shorter than that of pain-free individuals. Regression analysis revealed a significant moderating effect of PE on chronic pain and TL, above and beyond the effects of disability, age, and weight. Whereas chronic pain was associated with shorter telomeres in participants who did not exercise, this association was nonsignificant among participants who did exercise. The results suggest that chronic pain is associated with premature ageing; however, PE may mitigate this association and may protect individuals against the harmful effects of chronic pain. PERSPECTIVE: The study suggests that it is important to monitor signs of premature ageing among chronic pain patients as they are at risk. However, chronic pain patients may benefit from regular PE in this respect as it may moderate premature ageing.

Longitudinal associations of changes in physical activity and TV viewing with chronic musculoskeletal pain in Brazilian schoolteachers

This study analyzed the longitudinal association of changes in leisure-time physical activity (LTPA) practice and television viewing (TV viewing) with chronic musculoskeletal pain (CMP). The data about LTPA, TV viewing, and CMP were obtained in 2012 and after 24 months through individual interviews with schoolteachers from elementary and secondary education public schools in a large city in the southern region of Brazil. The statistical analysis was performed using generalized estimating equation regression models adjusted for sex, age, body mass index and depression. A total of 527 schoolteachers were studied, among which 66.6% were women, and the median age was 42 years (interquartile range: 34 to 49). A total of 170 (32.3%) participants reported CMP at baseline and 130 (24.7%) at follow-up. Both LTPA and TV viewing were independently and significantly associated with CMP regardless of all adjustment variables. Concretely, increasing LTPA by 60 minutes/week was associated with a 6.2% lower likelihood of CMP, and increasing TV viewing by 30 minutes/day was associated with a 5.1% higher likelihood of having CMP among the participants. In summary, this study showed that LTPA and TV viewing have independent and opposite relationships with the longitudinal risk of CMP, which suggests that the potential benefits obtained from practicing more LTPA are insufficient to compensate for the potential detrimental effect of viewing TV for longer with respect to the CMP.

Longitudinal study of the relationship between physical activity and knee pain and functional limitation in community-dwelling older adults

AIMS

To describe the associations between interindividual (between-person) and intraindividual (within-person) variability in physical activity (PA) and knee pain and functional limitation among older adults. We also investigated the potential bidirectional association of between-person and within-person variability in knee pain and functional limitation with PA.

METHOD

Participants (N = 1064; 51% women; mean age 63 ± 7.4 years) were measured at baseline, 2.5, and 5 years. PA was measured using pedometers. Knee pain and functional limitation were assessed using the WOMAC scale. A two-part hurdle model, with adjustment for confounders, estimated the association of between-person and within-person variability in PA with knee pain/functional limitation (as the outcome). Linear mixed effect regression models described the association of between-person and within-person variability in knee pain and functional limitation with PA (as the outcome).

RESULTS

Between-person effects showed that participants with a higher 5-year average PA had lower average WOMAC scores (β= -1.17, 95% CI: -1.82, -0.51). Within-person effects showed that at time-points when participants had a higher PA level than average, they also had lower WOMAC scores (β= -0.85, 95% CI: -1.36, -0.35). Conversely, both between-person (β= -15.6, 95% CI: -22.5, -8.8) and within-person increase (β= -7.4, 95% CI: -13.5, -1.4) in WOMAC scores were associated with lower PA.

CONCLUSION

These findings suggest that PA and knee pain/dysfunctional contribute to the development of one another. Pain can lead to changes in inter- and intraindividual PA levels, but the reverse is also possible - changes in PA results in changes in inter- and intraindividual pain/dysfunctional levels.

The search for translational pain outcomes to refine analgesic development: Where did we come from and where are we going?

Pain measures traditionally used in rodents record mere reflexes evoked by sensory stimuli; the results thus may not fully reflect the human pain phenotype. Alterations in physical and emotional functioning, pain-depressed behaviors and facial pain expressions were recently proposed as additional pain outcomes to provide a more accurate measure of clinical pain in rodents, and hence to potentially enhance analgesic drug development. We aimed to review how preclinical pain assessment has evolved since the development of the tail flick test in 1941, with a particular focus on a critical analysis of some nonstandard pain outcomes, and a consideration of how sex differences may affect the performance of these pain surrogates. We tracked original research articles in Medline for the following periods: 1973-1977, 1983-1987, 1993-1997, 2003-2007, and 2014-2018. We identified 606 research articles about alternative surrogate pain measures, 473 of which were published between 2014 and 2018. This indicates that preclinical pain assessment is moving toward the use of these measures, which may soon become standard procedures in preclinical pain laboratories.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Physical activity level and temporomandibular disorders in South Koreans

OBJECTIVE

This study aimed to investigate the association between general physical activity level and TMD pain in Koreans in a large-scale national database established through a nationwide survey.

METHODS

Data from the Fourth Korean National Health and Nutrition Examination Survey (KNHANES IV), which was conducted from 2007 to 2009, were analysed. In total, 16 941 participants were included in this cross-sectional study. Data on sociodemographic characteristics, TMD-related variables, and general physical activity level were collected. Participants were divided into moderate- and low-intensity exercise groups according to their physical activity levels. Multivariate logistic regression analyses were performed, adjusting for sociodemographic and other covariates.

RESULTS

The adjusted odds ratio (OR) (95% confidence interval; CI) for TMD pain was 1.373 (1.017-1.854) for people doing moderate-intensity exercises and 0.797 (0.629-1.008) among people doing low-intensity exercises. Those who did moderate-intensity exercises had significantly more TMD pain. This was higher in the 30- to 39-year age group (OR: 1.991, 95% CI: 1.137-3.488), with significantly higher risk for TMD pain in those who did moderate-intensity exercise, whereas low-intensity exercise significantly decreased the risk for TMD pain in the same age group (OR: 0.625, 95% CI: 0.409-0.958).

CONCLUSIONS

Moderate-intensity physical activity is associated with more TMD pain. Patients with TMD should avoid high-intensity level exercises and continue low-intensity exercises to prevent pain aggravation.

Physical Activity Induces Nucleus Accumbens Genes Expression Changes Preventing Chronic Pain Susceptibility Promoted by High-Fat Diet and Sedentary Behavior in Mice

Recent findings from rodent studies suggest that high-fat diet (HFD) increases hyperalgesia independent of obesity status. Furthermore, weight loss interventions such as voluntary physical activity (PA) for adults with obesity or overweight was reported to promote pain reduction in humans with chronic pain. However, regardless of obesity status, it is not known whether HFD intake and sedentary (SED) behavior is underlies chronic pain susceptibility. Moreover, differential gene expression in the nucleus accumbens (NAc) plays a crucial role in chronic pain susceptibility. Thus, the present study used an adapted model of the inflammatory prostaglandin E2 (PGE2)-induced persistent hyperalgesia short-term (PH-ST) protocol for mice, an HFD, and a voluntary PA paradigm to test these hypotheses. Therefore, we performed an analysis of differential gene expression using a transcriptome approach of the NAc. We also applied a gene ontology enrichment tools to identify biological processes associated with chronic pain susceptibility and to investigate the interaction between the factors studied: diet (standard diet vs. HFD), physical activity behavior (SED vs. PA) and PH-ST (PGE vs. saline). Our results demonstrated that HFD intake and sedentary behavior promoted chronic pain susceptibility, which in turn was prevented by voluntary physical activity, even when the animals were fed an HFD. The transcriptome of the NAc found 2,204 differential expression genes and gene ontology enrichment analysis revealed 41 biologic processes implicated in chronic pain susceptibility. Taking these biological processes together, our results suggest that genes related to metabolic and mitochondria stress were up-regulated in the chronic pain susceptibility group (SED-HFD-PGE), whereas genes related to neuroplasticity were up-regulated in the non-chronic pain susceptibility group (PA-HFD-PGE). These findings provide pieces of evidence that HFD intake and sedentary behavior provoked gene expression changes in the NAc related to promotion of chronic pain susceptibility, whereas voluntary physical activity provoked gene expression changes in the NAc related to prevention of chronic pain susceptibility. Finally, our findings confirmed previous literature supporting the crucial role of voluntary physical activity to prevent chronic pain and suggest that low levels of voluntary physical activity would be helpful and highly recommended as a complementary treatment for those with chronic pain.

BMP-7 protects male and female rodents against neuropathic pain induced by nerve injury through a mechanism mediated by endogenous opioids

Neuropathic pain is highly prevalent in pathological conditions such as diabetes, herpes zoster, trauma, etc. The severity and refractoriness to treatments make neuropathic pain a significant health concern. The transforming growth factor (TGF-β) family of cytokines is involved in pain modulation. Bone morphogenetic proteins (BMPs) constitute the largest subgroup within the TGF-β family. BMP-7 induces the transcription of genes coding endogenous opioid precursors in vitro. However, a nociception modulatory function for this cytokine remains unexplored in vivo. Herein, we show that BMP-7 and its type I receptors were detected in regions of the nervous system involved in pain transmission, processing, and modulation. BMP-7 haploinsufficiency confers to male and female mice a tactile hyperalgesia phenotype to mechanical stimuli, both at baseline and after sciatic nerve injury (SNI). The administration of recombinant BMP-7 (rBMP-7) reduced the severity of the allodynia after SNI in rodents without sexual dimorphism. Central administration of rBMP-7 delayed allodynia development after SNI and reduced the severity of allodynia. The opioid antagonist naloxone antagonized the antinociceptive effect of rBMP-7 in rats. The analgesic effect of morphine was significantly attenuated in BMP-7^+/- mice. The antiallodynic effect of voluntary exercise after SNI, whose mechanism involves the endogenous opioid system, was hampered by BMP-7 deficiency while potentiated by rBMP-7. Our results suggest that BMP-7 may constitute a novel therapeutic target for the treatment of neuropathic pain, which improves the function of the endogenous pain-resolution mechanisms to alleviate chronic pain.

Chronic non-inflammatory muscle pain: central and peripheral mediators

Conditions with chronic widespread non-inflammatory muscle pain, such as fibromyalgia, have complex etiologies with numerous proposed mechanisms for their pathophysiology of underlying chronic pain. Advancements in neuroimaging have allowed for the study of brain function and connectivity in humans with these conditions, while development of animal models have allowed for the study of both peripheral and central factors that lead to chronic pain. This article reviews the current literature surrounding the pathophysiology of chronic widespread non-inflammatory muscle pain focusing on both peripheral and central nervous system, as well as immune system, contributions to the development and maintenance of pain. A better understanding of the mechanisms underlying these conditions can allow for improvements in patient education, treatment and outcomes.

Activation of mesolimbic reward system via laterodorsal tegmental nucleus and hypothalamus in exercise-induced hypoalgesia

Ventral tegmental area (VTA) dopamine (DA) neurons are the primary source of dopamine in target structures that constitute the mesolimbic reward system. Previous studies demonstrated that voluntary wheel running (VWR) by neuropathic pain (NPP) model mice produces exercise-induced hypoalgesia (EIH), and that activation of mesolimbic reward system may lead to EIH. However, the neuronal mechanism by which the mesolimbic reward system is activated by VWR is unknown. Here, we found that VWR produces EIH effects and reverses the marked reduction in activated lateral VTA (lVTA)-DA neurons induced by NPP. The proportions of activated laterodorsal tegmental nucleus (LDT)-cholinergic and lateral hypothalamus-orexin neurons were significantly enhanced by VWR. Retrograde tracing and dual immunostaining revealed that VWR activates lVTA-projecting LDT-cholinergic/non-cholinergic and lateral hypothalamic area (LHA)-orexin/non-orexin neurons. Therefore, EIH effects may be produced, at least in part, by activation of the mesolimbic reward system via activation of LDT and LHA neurons.

Enriched Environment and Effects on Neuropathic Pain: Experimental Findings and Mechanisms

Neuropathic pain inflicts tremendous biopsychosocial suffering for patients worldwide. However, safe and effective treatment of neuropathic pain is a prominent unmet clinical need. Environmental enrichment (EE) is an emerging cost-effective nonpharmacological approach to alleviate neuropathic pain and complement rehabilitation care. We present here a review of preclinical studies in ascertaining the efficacy of EE for neuropathic pain. Their proposed mechanisms, including the suppression of ascending nociceptive signaling to the brain, enhancement of the descending inhibitory system, and neuroprotection of the peripheral and central nervous systems, may collectively reduce pain perception and improve somatic and emotional functioning in neuropathic pain. The current evidence offers critical insights for future preclinical research and the translational application of EE in clinical pain management.

Intrinsic Activity of C57BL/6 Substrains Associates with High-Fat Diet-Induced Mechanical Sensitivity in Mice

Pain is significantly impacted by the increasing epidemic of obesity and the metabolic syndrome. Our understanding of how these features impact pain is only beginning to be developed. Herein, we have investigated how small genetic differences among C57BL/6 mice from 2 different commercial vendors lead to important differences in the development of high-fat diet-induced mechanical sensitivity. Two substrains of C57BL/6 mice from Jackson Laboratories (Bar Harbor, ME; C57BL/6J and C57BL/6NIH), as well as C57BL/6 from Charles Rivers Laboratories (Wilmington, MA; C57BL/6CR) were placed on high-fat diets and analyzed for changes in metabolic features influenced by high-fat diet and obesity, as well as measures of pain-related behaviors. All 3 substrains responded to the high-fat diet; however, C57BL/6CR mice had the highest weights, fat mass, and impaired glucose tolerance of the 3 substrains. In addition, the C57BL/6CR mice were the only strain to develop significant mechanical sensitivity over the course of 8 weeks. Importantly, the C57BL/6J mice were protected from mechanical sensitivity, which may be based on increased physical activity compared with the other 2 substrains. These findings suggest that activity may play a powerful role in protecting metabolic changes associated with a high-fat diet and that these may also be protective in pain-associated changes as a result of a high-fat diet. These findings also emphasize the importance of selection and transparency in choosing C57BL/6 substrains in pain-related research. PERSPECTIVE: Obesity and the metabolic syndrome play an important role in pain. This study identifies key differences in the response to a high-fat diet among substrains of C57BL/6 mice and differences in intrinsic physical activity that may influence pain sensitivity. The results emphasize physical activity as a powerful modulator of obesity-related pain sensitivity.

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.