Modest Amounts of Voluntary Exercise Reduce Pain- and Stress-Related Outcomes in a Rat Model of Persistent Hind Limb Inflammation

Physical activity and pain in people with and without cancer

BACKGROUND

Performing physical activity may provide analgesic benefit, although this effect is more established for noncancer pain rather than cancer pain. The relationship between physical activity and pain outcomes in adults with and without a history of cancer was examined.

METHODS

Totals of 51,439 adults without a cancer history and 10,651 adults with a cancer history from the Cancer Prevention Study II Nutrition Cohort were included. Exposures included self-reported moderate to vigorous physical activity (MVPA) as well as 2-year change in MVPA. Pain outcomes included pain intensity (primary outcome) and analgesic use (secondary outcome).

RESULTS

MVPA was inversely associated with pain intensity for adults with (odds ratio [OR], 0.84 [≥15 metabolic equivalent of task (MET) h/week vs. <7.5 MET h/week]; 95% confidence interval [CI], 0.76-0.93) and without (OR, 0.79; 95% CI, 0.75-0.82) a history of cancer. Compared to remaining inactive, participants who became sufficiently active (cancer: OR, 0.76; 95% CI, 0.68-0.86; no cancer: OR, 0.73; 95% CI, 0.69-0.77), became inactive (cancer: OR, 0.79; 95% CI, 0.71-0.88; no cancer: OR, 0.84; 95% CI, 0.80-0.89), or remained sufficiently active (cancer: OR, 0.66; 95% CI, 0.60-0.72; no cancer: OR, 0.62; 95% CI, 0.60-0.65) also reported less pain. Physical activity was not related to analgesic use.

CONCLUSIONS

The relationship between physical activity and pain intensity was not substantially different between people with and without a history of cancer. Cancer survivors who perform more activity, or who increase their activity, may experience less pain than cancer survivors who consistently perform less.

PLAIN LANGUAGE SUMMARY

People who have had cancer often experience ongoing pain. Being physically active may help reduce the intensity of the pain they experience.

Potential mechanisms of exercise for relieving inflammatory pain: a literature review of animal studies

Inflammatory pain (IP) is one of the most prevalent and intractable human conditions, and it leads to progressive dysfunction and reduced quality of life. Additionally, IP is incredibly challenging to treat successfully with drugs or surgery. The development of IP is complex and multifactorial, and peripheral and central sensitization may influence chronicity and treatment resistance in IP. Understanding the mechanisms underlying IP is vital for developing novel therapies. Strong evidence suggests that exercise can be a first-line relief for patients with IP during rehabilitation. However, the mechanisms through which exercise improves IP remain unclear. Here, we reviewed the current animal experimental evidence for an exercise intervention in IP and proposed biological mechanisms for the effects of synaptic plasticity in the anterior cingulate cortex, endocannabinoids, spinal dorsal horn excitability balance, immune cell polarization balance, cytokines, and glial cells. This information will contribute to basic science and strengthen the scientific basis for exercise therapy prescriptions for IP in clinical practice.

Physical activity and pain in people with cancer: a systematic review and meta-analysis

PURPOSE

Physical activity can provide analgesic benefit but its effect on cancer-related pain is unclear. This review synthesised and appraised the evidence for the effect of physical activity on pain in people living with or beyond cancer.

METHODS

A systematic search of Ovid Medline and Embase was performed to identify randomised controlled trials (RCTs), randomised cross-over studies (RXTs), and prospective observational studies that examined physical activity and pain outcomes in adults living with or beyond cancer. Meta-analyses were performed to generate effect estimates. Risk of bias was assessed, and the GRADE system was used to assess evidence quality.

RESULTS

One hundred twenty-one studies (n = 13,806), including 102 RCTs, 6 RXTs, and 13 observational studies, met the criteria for inclusion. Meta-analyses of RCTs identified a decrease in pain intensity (n = 3734; standardised mean difference (SMD) - 0.30; 95% confidence interval (CI) - 0.45, - 0.15) and bodily pain (n = 1170; SMD 0.28; 95% CI 0.01, 0.56) but not pain interference (n = 207; SMD - 0.13, 95% CI - 0.42, 0.15) following physical activity interventions. Individual studies also identified a reduction in pain sensitivity but not analgesic use, although meta-analysis was not possible for these outcomes. High heterogeneity between studies, low certainty in some effect estimates, and possible publication bias meant that evidence quality was graded as very low to low.

CONCLUSION

Physical activity may decrease pain in people living with and beyond cancer; however, high heterogeneity limits the ability to generalise this finding to all people with cancer or to specific types of cancer-related pain.

Impact of continuous testosterone exposure on reproductive physiology, activity, and pain-related behavior in young adult female rats

Testosterone may reduce pain in cisgender women and transgender men. Rodents can provide a useful model for investigating physiological effects of hormone therapy. To this end, continuous-release testosterone or blank (placebo) capsules were implanted s.c. into young adult female rats, and three weeks later rats were either ovariectomized or sham-ovariectomized. Testosterone treatment that mimicked previously reported endogenous levels in males eliminated estrous cycling and decreased uterine weight. Testosterone also significantly increased body weight and suppressed the increases in daily wheel running observed in placebo controls over time. Subsequent ovariectomy or sham-ovariectomy decreased wheel running in all groups, but testosterone-treated rats recovered significantly more quickly than did placebo-treated rats. Neither testosterone nor ovariectomy significantly altered hindpaw mechanical threshold. Two weeks after sham/ovariectomy surgery, injection of Complete Freund Adjuvant (CFA) into one hindpaw reduced wheel running and mechanical threshold in all groups; running significantly decreased from the first to second day after CFA in testosterone- but not in placebo-treated rats. Morphine 1.0 but not 3.2 mg/kg increased CFA-suppressed wheel running similarly in all groups, whereas both doses of morphine increased CFA-suppressed mechanical threshold. These data suggest that weeks-long testosterone treatment with or without ovariectomy may provide a useful physiological model of testosterone therapy as used in human gender transition. Although testosterone administered at levels similar to those in gonadally intact males tended to hasten female rats' recovery from surgery, it did not decrease maximal pain-related behaviors after surgery or hindpaw inflammatory insult, nor did it alter opioid antinociception.

Analgesic Effect of Exercise on Neuropathic Pain via Regulating the Complement Component 3 of Reactive Astrocytes

BACKGROUND

Exercise has been proven to be an efficient intervention in attenuating neuropathic pain. However, the underlying mechanisms that drive exercise analgesia remain unknown. In this study, we aimed to examine the role of complement component 3 (C3) in neuropathic pain and whether antinociceptive effects are produced by exercise via regulating C3 in mice.

METHODS

In this study, using a spared nerve injury (SNI)-induced neuropathic pain mice model, C57BL/6J mice were divided into 3 groups: Sham mice, SNI mice, and SNI + Exercise (Ex) mice with 30-minute low-intensity aerobic treadmill running (10 m/min, no inclination). Paw withdrawal threshold; thermal withdrawal latency; and glial fibrillary acidic protein, C3, tumor necrosis factor-α, and interlukin-1β expression in the spinal cord were monitored. C3 knockout (KO) mice were further used to verify the role of C3 in neuropathic pain.

RESULTS

von Frey test, acetone test, and CatWalk gait analysis revealed that treadmill exercise for 4 weeks reversed pain behaviors. In addition, exercise reduced astrocyte reactivity (SNI mean = 14.5, 95% confidence interval [CI], 12.7-16.3; SNI + Ex mean = 10.3, 95% CI, 8.77-11.9, P = .0003 SNI + Ex versus SNI) and inflammatory responses in the spinal cord after SNI. Moreover, it suppressed the SNI-induced upregulation of C3 expression in the spinal cord (SNI mean = 5.46, 95% CI, 3.39-7.53; SNI + Ex mean = 2.41, 95% CI, 1.42-3.41, P = .0054 SNI + Ex versus SNI in Western blot). C3 deficiency reduced SNI-induced pain and spinal astrocyte reactivity (wild type mean = 7.96, 95% CI, 6.80-9.13; C3 KO mean = 5.98, 95% CI, 5.14-6.82, P = .0052 C3 KO versus wild type). Intrathecal injection of recombinant C3 (rC3) was sufficient to produce mechanical (rC3-Ex mean = 0.77, 95% CI, 0.15-1.39; rC3 mean = 0.18, 95% CI, -0.04 to 0.41, P = .0168 rC3-Ex versus rC3) and cold (rC3-Ex mean = 1.08, 95% CI, 0.40-1.77; rC3 mean = 3.46, 95% CI, 1.45-5.47, P = .0025 rC3-Ex versus rC3) allodynia in mice. Importantly, exercise training relieved C3-induced mechanical and cold allodynia, and the analgesic effect of exercise was attenuated by a subeffective dose of intrathecal injection of C3.

CONCLUSIONS

Overall, these results suggest that exercise suppresses neuropathic pain by regulating astroglial C3 expression and function, thereby providing a rationale for the analgesic effect of exercise as an acceptable alternative approach for treating neuropathic pain.

Effect of Moderate Exercise on the Superficial Zone of Articular Cartilage in Age-Related Osteoarthritis

This study aimed to evaluate the effect of exercise on the superficial zone of the osteoarticular cartilage during osteoarthritis progression. Three-month-old, nine-month-old, and eighteen-month-old Sprague Dawley rats were randomly divided into two groups, moderate exercise and no exercise, for 10 weeks. Histological staining, immunostaining, and nanoindentation measurements were conducted to detect changes in the superficial zone. X-ray and micro-CT were quantitated to detect alterations in the microarchitecture of the tibial subchondral bone. Cells were extracted from the superficial zone of the cartilage under fluid-flow shear stress conditions to further verify changes in vitro. The number of cells and proteoglycan content in the superficial zone increased more in the exercise group than in the control group. Exercise can change the content and distribution of collagen types I and III in the superficial layer. In addition, TGFβ/pSmad2/3 and Prg4 expression levels increased under the intervention of exercise on the superficial zone. Exercise can improve the Young's modulus of the cartilage and reduce the abnormal subchondral bone remodeling which occurs after superficial zone changes. Moderate exercise delays the degeneration of the articular cartilage by its effect on the superficial zone, and the TGFβ/pSmad2/3 signaling pathways and Prg4 play an important role.

The Role of Exercise in the Alleviation of Neuropathic Pain Following Traumatic Spinal Cord Injuries: A Systematic Review and Meta-analysis

OBJECTIVE

The objective of this systematic review and meta-analysis was to assess the efficacy of exercise in neuropathic pain following traumatic spinal cord injuries.

METHODS

The search was conducted in MEDLINE, Embase, Scopus, and Web of Science by the end of 2022. Two independent researchers included the articles based on the inclusion and exclusion criteria. A standardized mean difference was calculated for each data and they were pooled to calculate an overall effect size. To assess the heterogeneity between studies, I2 and chi-square tests were utilized. In the case of heterogeneity, meta-regression was performed to identify the potential source.

RESULTS

Fifteen preclinical studies were included. Meta-analysis demonstrated that exercise significantly improves mechanical allodynia (standardized mean difference [SMD], -1.59; 95% confidence interval [CI], -2.16 to -1.02; p < 0.001; I2 = 90.37%), thermal hyperalgesia (SMD, 1.95; 95% CI, 0.96-2.94; p < 0.001), and cold allodynia (SMD, -2.92; 95% CI, -4.4 to -1.43; p < 0.001). The improvement in mechanical allodynia is significantly more in animals with a compression model of SCI (meta-regression coefficient, -1.33; 95% CI, -1.84 to -0.57; p < 0.001) and in mild SCI (p < 0.001). Additionally, the improvement was more prominent if the training was started 7 to 8 days postinjury (coefficient, -2.54; 95% CI, -3.85 to -1.23; p < 0.001) and was continued every day (coefficient, -1.99; 95% CI, -3.07 to -0.9; p < 0.001). Likewise, voluntary exercise demonstrated a significantly more effect size (coefficient, -1.45; 95% CI, -2.67 to -0.23; p = 0.02).

CONCLUSION

Exercise is effective in the amelioration of neuropathic pain. This effect in mechanical allodynia is more prominent if voluntary, continuous training is initiated in the subacute phase of mild SCI.

Voluntary running-induced activation of ventral hippocampal GABAergic interneurons contributes to exercise-induced hypoalgesia in neuropathic pain model mice

The exact mechanism of exercise-induced hypoalgesia (EIH) in exercise therapy to improve chronic pain has not been fully clarified. Recent studies have suggested the importance of the ventral hippocampus (vHPC) in inducing chronic pain. We investigated the effects of voluntary running (VR) on FosB⁺ cells and GABAergic interneurons (parvalbumin-positive [PV⁺] and somatostatin-positive [SOM⁺]) in the vHPC-CA1 in neuropathic pain (NPP) model mice. VR significantly improved thermal hyperalgesia in the NPP model. The number of the FosB⁺ cells was significantly higher in partial sciatic nerve ligation-sedentary mice than in Sham and Naive mice, whereas VR significantly suppressed the FosB⁺ cells in the vHPC-CA1. Furthermore, VR significantly increased the proportion of activated PV⁺ and SOM⁺ interneurons in the vHPC-CA1, and tracer experiments indicated that approximately 24% of neurons projecting from the vHPC-CA1 to the basolateral nucleus of amygdala were activated in NPP mice. These results indicate that feedforward suppression of the activated neurons via VR-induced activation of GABAergic interneurons in the vHPC-CA1 may be a mechanism to produce EIH effects, and suggested that disappearance of negative emotions such as fear and anxiety by VR may play a critical role in improving chronic pain.

Influence of routine exercise on the peripheral immune system to prevent and alleviate pain

Routine physical activity reduces the onset of pain and exercise is a first line treatment for individuals who develop chronic pain. In both preclinical and clinical research regular exercise (routine exercise sessions) produces pain relief through multiple mechanisms such as alterations in the central and peripheral nervous system. More recently, it has been appreciated that exercise can also alter the peripheral immune system to prevent or reduce pain. In animal models, exercise can alter the immune system at the site of injury or pain model induction, in the dorsal root ganglia, and systemically throughout the body to produce analgesia. Most notably exercise shows the ability to dampen the presence of pro-inflammatory immune cells and cytokines at these locations. Exercise decreases M1 macrophages and the cytokines IL-6, IL-1β, and TFNα, while increasing M2 macrophages and the cytokines IL-10, IL-4, and IL-1ra. In clinical research, a single bout of exercise produces an acute inflammatory response, however repeated training can lead to an anti-inflammatory immune profile leading to symptom relief. Despite the clinical and immune benefits of routine exercise, the direct effect of exercise on immune function in clinical pain populations remains unexplored. This review will discuss in more detail the preclinical and clinical research which demonstrates the numerous ways through which multiple types of exercise alter the peripheral immune system. This review closes with the clinical implications of these findings along with suggestions for future research directions.

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.

Biochemical and behavioral effects of decreasing dietary linoleic acid and increasing eicosapentaenoic acid and docosahexaenoic acid in a rat chronic monoarthrits model

Clinical studies have demonstrated that decreasing linoleic acid (LA) while increasing eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in diets evokes an analgesic effect in headache sufferers. We utilized a rat chronic monoarthritis model to determine if these analgesic effects can be reproduced in rats and to and further probe potential analgesic mechanisms. We fed 8 rats a control diet (with fatty acid levels similar to standard US diets) and 8 rats a low LA diet with added EPA and DHA (H3L6 diet) and after 10 weeks, performed a unilateral intraarticular injection of Complete's Freund Adjuvant (CFA). We evaluated thermal and mechanical sensitivity as well as hind paw weight bearing prior to and at 4 and 20 days post CFA injection. At 28 days post CFA injection rats were euthanized and tissue collected. H3L6 diet fed rats had higher concentrations of EPA and DHA, as well as higher concentrations of oxidized lipids derived from these fatty acids, in hind paw and plasma, compared to control fed rats. LA and oxidized LA metabolites were lower in the plasma and hind paw of H3L6 compared to control fed rats. Diet did not affect thermal or mechanical sensitivity, nor did it affect hind paw weight bearing. In conclusion, the H3L6 diet evoked biochemical changes in rats but did not impact pain related behavioral measures in this chronic monoarthritis model.

Brain Mechanisms of Exercise-Induced Hypoalgesia: To Find a Way Out from "Fear-Avoidance Belief"

It is well known that exercise produces analgesic effects (exercise-induced hypoalgesia (EIH)) in animal models and chronic pain patients, but the brain mechanisms underlying these EIH effects, especially concerning the emotional aspects of pain, are not yet fully understood. In this review, we describe drastic changes in the mesocorticolimbic system of the brain which permit the induction of EIH effects. The amygdala (Amyg) is a critical node for the regulation of emotions, such as fear and anxiety, which are closely associated with chronic pain. In our recent studies using neuropathic pain (NPP) model mice, we extensively examined the association between the Amyg and EIH effects. We found that voluntary exercise (VE) activated glutamate (Glu) neurons in the medial basal Amyg projecting to the nucleus accumbens (NAc) lateral shell, while it almost completely suppressed NPP-induced activation of GABA neurons in the central nucleus of the Amyg (CeA). Furthermore, VE significantly inhibited activation of pyramidal neurons in the ventral hippocampus-CA1 region, which play important roles in contextual fear conditioning and the retrieval of fear memory. This review describes novel information concerning the brain mechanisms underlying EIH effects as a result of overcoming the fear-avoidance belief of chronic pain.

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.

Voluntary Forelimbs Exercise Reduces Immobilization-Induced Mechanical Hyperalgesia in the Rat Hind Paw

Voluntary exercise is sufficient to protect against neuropathic pain. However, it is unclear whether voluntary exercise reduces immobilization-induced hyperalgesia. We examined the effect of voluntary forelimb exercise on immobilized-induced hyperalgesia in hind paws of rats. Wistar rats were randomly divided into the (1) both hind limbs immobilized group (IM group), (2) immobilization and exercise with nonimmobilized fore limbs group (EX group), and (3) control group. In the IM and EX groups, the bilateral ankle joints of each rat were immobilized in full plantar flexion with a plaster cast for eight weeks. In the EX group, voluntary exercise using nonimmobilized forelimbs in the running wheel was administered during the immobilization period, while hind limbs were kept immobilized (60 min/day, 5 days/week). Mechanical hyperalgesia in the hind paw was measured using a digital von Frey device every week. To investigate the abnormality of primary sensory neurons and central sensitization, the number of calcitonin gene-related peptide-positive cells in the dorsal root ganglion and the expression level of calcitonin gene-related peptide in the spinal dorsal horn were analyzed by immunohistochemical staining. Immobilization-induced mechanical hyperalgesia was inhibited in the EX group compared to the IM group at three weeks after immobilization. In the EX group, the number of calcitonin gene-related peptide-positive cells in the dorsal root ganglion and the expression level of calcitonin gene-related peptide were significantly decreased compared to those in the IM group. Our results therefore suggest that voluntary forelimb exercise during hind limb immobilization partially reduces immobilization-induced hyperalgesia by suppressing that the plastic changes of the primary sensory nerves that excessively transmit pain and increased responsiveness of nociceptive neurons in the spinal dorsal horn.

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.

Redox interactions of immune cells and muscle in the regulation of exercise-induced pain and analgesia: implications on the modulation of muscle nociceptor sensory neurons

The mechanistic interactions among redox status of leukocytes, muscle, and exercise in pain regulation are still poorly understood and limit targeted treatment. Exercise benefits are numerous, including the treatment of chronic pain. However, unaccustomed exercise may be reported as undesirable as it may contribute to pain. The aim of the present review is to evaluate the relationship between oxidative metabolism and acute exercise-induced pain, and as to whether improved antioxidant capacity underpins the analgesic effects of regular exercise. Preclinical and clinical studies addressing relevant topics on mechanisms by which exercise modulates the nociceptive activity and how redox status can outline pain and analgesia are discussed, in sense of translating into refined outcomes. Emerging evidence points to the role of oxidative stress-induced signaling in sensitizing nociceptor sensory neurons. In response to acute exercise, there is an increase in oxidative metabolism, and consequently, pain. Instead, regular exercise can modulate redox status in favor of antioxidant capacity and repair mechanisms, which have consequently increased resistance to oxidative stress, damage, and pain. Data indicate that acute sessions of unaccustomed prolonged and/or intense exercise increase oxidative metabolism and regulate exercise-induced pain in the post-exercise recovery period. Further, evidence demonstrates regular exercise improves antioxidant status, indicating its therapeutic utility for chronic pain disorders. An improved comprehension of the role of redox status in exercise can provide helpful insights into immune-muscle communication during pain modulatory effects of exercise and support new therapeutic efforts and rationale for the promotion of exercise.

Domain-Specific Physical Activity, Pain Interference, and Muscle Pain after Activity

PURPOSE

Using the Melbourne Collaborative Cohort Study, we examined the associations of occupation, household, transport, and leisure physical activity with pain interference with normal work and muscle pain after activity.

METHODS

This cross-sectional analysis included 7655 working and 11,766 nonworking participants. Physical activity was assessed using the long-form International Physical Activity Questionnaire. Pain interference was assessed with the Short-Form 12-Item Health Survey version 2.0, and muscle pain after activity was assessed using the 12-item Somatic and Psychological Health Report. Ordered logistic regression was used to estimate odds ratios (OR) and 95% confidence intervals (CI), and restricted cubic splines were used to graphically represent the shape of associations.

RESULTS

All physical activity domain-pain outcome associations were nonlinear. Compared with participants who reported the lowest level of activity, participants who reported the median level of transport physical activity (10 MET·h·wk) reported less pain interference (workers: OR, 0.86 [95% CI, 0.77-0.97]; nonworkers: OR, 0.88 [95% CI, 0.79-0.97]) and muscle pain after activity (workers: OR, 0.81 [95% CI, 0.70-0.95]; nonworkers: OR, 0.86 [95% CI, 0.77-0.95]). Higher levels of leisure time activity (20 MET·h·wk) were associated with less pain interference in nonworkers (OR, 0.87; 95% CI, 0.77-0.98) and muscle pain after activity in workers (OR, 0.67; 95% CI, 0.56-0.80). Workers who reported the median level of household activity (16 MET·h·wk) had increased pain interference (OR, 1.19; 95% CI, 1.07-1.32) and muscle pain after activity (OR, 1.23; 95% CI, 1.06-1.42) than did those who reported the least household activity.

CONCLUSIONS

Associations between domain-specific physical activity and pain outcomes were not uniform. Within the transport and leisure domains, physical activity was inversely associated with pain-related outcomes, whereas household physical activity was positively associated with pain scores within the working sample.

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.

Excessive walking exercise precipitates diabetic neuropathic foot pain: hind paw suspension treadmill exercise experiment in a rat model

The harmful effects of excessive mechanical loading on diabetic neuropathy and the reason diabetic neuropathic symptoms are common in feet are unclear. In this study, the hind paw suspension treadmill exercise model was used in rats to investigate whether mechanical loading applied to the front paws precipitates neuropathic pain, especially in diabetic conditions. Thirty-two rats were divided into six groups according to the presence of diabetes (DM) and the intensity of mechanical loading applied to the front paws: DM-Hi (high-intensity); DM-Lo (low-intensity); DM-No (non-mechanical loading); Sham-Hi; Sham-Lo; and Sham-No. DM was induced by streptozotocin injection. For high-intensity or low-intensity mechanical loading, treadmill walking exercise was conducted with or without hind paw suspension, respectively. The mechanical withdrawal threshold of the front paw decreased significantly after 8 weeks only in the DM mechanical loading groups (DM-Hi and DM-Lo), and high-intensity loading more significantly decreased the front-paw withdrawal threshold than low-intensity loading. In the DM-Hi group only, macrophage migration inhibitory factor (MIF) increased significantly, and intra-epidermal nerve fibers (IENF) in the front paws decreased significantly. In diabetic conditions, mechanical overloading such as excessive walking is likely to precipitate mechanical allodynia and damage IENF¸ which could explain why diabetic neuropathic symptoms are common in feet. This finding might be related to up-regulation of intracellular signaling cascades such as MIF, rather than inflammatory processes.

Voluntary exercise improves voiding function and bladder hyperalgesia in an animal model of stress-induced visceral hypersensitivity: A multidisciplinary approach to the study of urologic chronic pelvic pain syndrome research network study

OBJECTIVE

The underlying mechanism of interstitial cystitis/bladder pain syndrome (IC/BPS) is not well understood and evaluation of current therapeutic interventions has not identified any generally effective treatments. Physical activity has shown beneficial effects on individuals suffering from chronic pain. Anxiety-prone rats exposed to water avoidance stress (WAS) develop urinary frequency and lower bladder sensory thresholds with high face and construct validity for the study of IC/BPS. The aim of this study was to evaluate the role of chronic voluntary exercise on urinary frequency, voiding function, and hyperalgesia in animals exposed to WAS.

MATERIALS AND METHODS

Twenty-six female Wistar-Kyoto rats were exposed to WAS and thereafter randomized to either voluntary exercise for 3 weeks or sedentary groups. Voiding parameters were assessed at baseline, post-WAS, and weekly for 3 weeks. Before euthanasia, the animals underwent cystometrogram (CMG), external urinary sphincter electromyography, and assessment of visceromotor response (VMR) to isotonic bladder distension (IBD).

RESULTS

WAS exposure resulted in adverse changes in voiding parameters. Compared with sedentary animals, animals in the voluntary exercise group had improved voiding parameters during metabolic cage and CMG testing, as well as improved bladder sensory thresholds as determined by VMR during IBD.

CONCLUSION

Voluntary exercise in an animal model of chronic stress leads to improvement in voiding function and visceral bladder hyperalgesia.

EXTREME CONDITIONING TRAINING: ACUTE EFFECTS ON MOOD STATE

ABSTRACT Introduction: The search for strategies aimed at reducing daily stress is increasing in the current literature. As a result, several types of fitness training are constantly being investigated for their influence on mood states. However, we know little about strategies that use higher intensities. Objective: To investigate the acute effects of an extreme conditioning training (ECT) session on the mood states of individuals with or without prior experience in this type of training. Methods: Volunteers were divided into TRAINED (n = 10) and CONDITIONED (n = 10) groups, undergoing a single 9-minute ECT session. Mood states (BRUMS scale) were analyzed at baseline, immediately afterwards, and after 30 minutes. Results: Anger, confusion and tension were significantly reduced in both groups immediately afterwards and post 30 minutes. The TRAINED group showed a significant reduction in fatigue 30 minutes after the end of the session. Effect size for all variables was considered small to moderate. The TRAINED group had significantly increased vigor immediately after the end of the session compared to baseline. However, the CONDITIONED group had significantly reduced vigor 30 minutes afterwards, in comparison to baseline. The increase in vigor in the TRAINED group was considered moderate (ES = 0.68), while the decrease in vigor in the CONDITIONED group was considered major (ES = −0.88). Conclusion: An ECT session is powerful enough to induce significant, albeit small and moderate, changes in mood states in both trained and untrained individuals in this particular type of exercise. Level of evidence II, Therapeutic studies-investigation of treatment results.

Interleukin 1β inhibition contributes to the antinociceptive effects of voluntary exercise on ischemia/reperfusion-induced hypersensitivity

Issues of peripheral circulation have been increasingly suggested as an underlying cause of musculoskeletal pain in many conditions, including sickle cell anemia and peripheral vascular disease. We have previously shown in our model of transient ischemia and reperfusion (I/R) injury of the forelimb that individual group III and IV muscle afferents display altered chemosensitivity and mechanical thresholds 1 day after injury. Functional alterations corresponded to increased evoked and spontaneous pain-related behaviors and decreased muscle strength and voluntary activity-all actions that echo clinical symptoms of ischemic myalgia. These behavioral and physiological changes appeared to originate in part from the action of increased interleukin 1β (IL1β) in the injured muscles at its upregulated IL1 receptor 1 within the dorsal root ganglion. Here, we describe that two days of voluntary wheel running prior to I/R blocks both injury-induced IL1β enhancement and the subsequent development of ischemic myalgia-like behaviors. Furthermore, the protective effects of 2 days prior exercise on the I/R-evoked increases in pain-related behaviors were also paralleled with systemic injection of the IL1 receptor antagonist during I/R. Interleukin 1 receptor antagonist treatment additionally prevented the I/R-induced changes in mechanical and chemical sensitivity of individual primary muscle afferents. Altogether, these data strengthen the evidence that transient I/R injury sensitizes group III and IV muscle afferents via increased IL1β in the muscles to stimulate ischemic myalgia development. Targeting IL1β may, therefore, be an effective treatment strategy for this insidious type of muscle pain.

Exercise Reverses Nociceptive Sensitization, Upregulated Neuropeptide Signaling, Inflammatory Changes, Anxiety, and Memory Impairment in a Mouse Tibia Fracture Model

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: This study tested the hypothesis that ad lib running wheel exercise in a tibia fracture model of complex regional pain syndrome can reverse hindlimb nociceptive sensitization and inflammation in mice.

METHODS

Three weeks after tibia fracture, the cast was removed and hindlimb von Frey thresholds and unweighting were tested; the mice were then randomized to either ad lib access to a running wheel for 4 weeks or no wheel access. After 4 weeks the behavioral testing was repeated and then skin, sciatic nerve, and spinal cord tissues collected for polymerase chain reaction and enzyme immunoassay measurements of neuropeptide and inflammatory mediator levels. A similar protocol was used in fracture mice treated with exercise for 4 weeks, and then the running wheel was removed for 2 weeks. Memory and anxiety were measured in both groups with use of open-field, zero-maze, and novel-objects recognition assays.

RESULTS

At 7 weeks postfracture the mice with no wheel access exhibited hindlimb allodynia and unweighting, anxiety, memory loss, upregulated spinal neuropeptide signaling, and increased hind paw and spinal inflammatory mediator expression, but the postfracture mice allowed to exercise for 4 weeks exhibited none of these changes (n = 12/cohort). When exercise was stopped for 2 weeks after 4 weeks of running, hindlimb allodynia and unweighting were rekindled, and this nociceptive sensitization was associated with increased sciatic nerve neuropeptide levels and hind paw skin interleukin 6 and nerve growth factor expression (n = 12/cohort).

CONCLUSIONS

Daily exercise reversed nociceptive sensitization, inflammation, anxiety, and memory loss after tibia fracture.

Persistent inflammatory pain alters sexually-motivated behavior in male rats

Urine from pro-œstrus female rodents evokes increased levels of sexually-motivated behaviors in males, including sniffing and scent marking of the urine spot as well as activation of brain reward regions. Stressors such as social defeat can adversely impact urine scent marking behavior in male rodents, an effect that can be mitigated with anti-depressant drugs. Persistent pain is also known to be a potent stressor, producing elevated levels of plasma corticosterone as well as reduced sucrose preference and reduced social interaction. However, the effect of persistent pain on sexually-motivated behavior is unknown. Here, we compared urine scent marking behavior in male rats for up to 3 weeks following intra-articular injection of Complete Freund’s Adjuvant (CFA) or sham injection. CFA-injected rats exhibited profound and ongoing deficits in static weight bearing capacity. CFA-induced persistent inflammatory pain increased plasma corticosterone levels and reduced urine scent marking behavior in male rats. Moreover, while the vast majority of injured rats showed decreased urine scent marking preference for the pro-œstrus female urine spot, male rats with higher baseline scent marking preference also exhibited higher post-injury scent marking preference, more sniffing behavior and lower levels of plasma corticosterone, compared to those with lower baseline scent marking preference. Overall, scent marking behavior may be an ethologically relevant behavioral predictor of persistent pain-induced stress in rats, representing a novel translational approach to understanding chronic pain comorbidities.

Concentrations of oxidized linoleic acid derived lipid mediators in the amygdala and periaqueductal grey are reduced in a mouse model of chronic inflammatory pain

Chronic pain is both a global public health concern and a serious source of personal suffering for which current treatments have limited efficacy. Recently, oxylipins derived from linoleic acid (LA), the most abundantly consumed polyunsaturated fatty acid in the modern diet, have been implicated as mediators of pain in the periphery and spinal cord. However, oxidized linoleic acid derived mediators (OXLAMs) remain understudied in the brain, particularly during pain states. In this study, we employed a mouse model of chronic inflammatory pain followed by a targeted lipidomic analysis of the animals' amygdala and periaqueductal grey (PAG) using LC-MS/MS to investigate the effect of chronic inflammatory pain on oxylipin concentrations in these two brain nuclei known to participate in pain sensation and perception. From punch biopsies of these brain nuclei, we detected twelve OXLAMs in both the PAG and amygdala and one arachidonic acid derived mediator, 15-HETE, in the amygdala only. In the amygdala, we observed an overall decrease in the concentration of the majority of OXLAMs detected, while in the PAG the concentrations of only the epoxide LA derived mediators, 9,10-EpOME and 12,13-EpOME, and one trihydroxy LA derived mediator, 9,10,11-TriHOME, were reduced. This data provides the first evidence that OXLAM concentrations in the brain are affected by chronic pain, suggesting that OXLAMs may be relevant to pain signaling and adaptation to chronic pain in pain circuits in the brain and that the current view of OXLAMs in nociception derived from studies in the periphery is incomplete.

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.

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.

A dose response study of the effect of prostaglandin E2 on thermal nociceptive sensitivity

Inhibition of prostaglandin (PG) biosynthesis has been used to relieve pain for thousands of years. Today non-steroidal anti-inflammatory drugs (which largely inhibit PG synthesis) are widely used to treat pain. Four main types of PGs (PGD2, PGE2, PGF2 and PGI2) are synthesized from arachidonic acid during inflammation and have been demonstrated to impact nociception. PGE2 has been the most studied and utilized for its pain producing properties and has been demonstrated to increase hypersensitivity in rodent nociceptive behavioral models when applied centrally and/or peripherally. Surprisingly, there are no published reports that use withdrawal from radiant light beam (Hargreaves apparatus) to examine the dose response effect of peripherally applied PGE2 on thermal nociceptive hypersensitivity. To address this gap in the literature, we performed a dose response study examining the effect of PGE2 on thermal hypersensitivity (assessed using a Hargreaves apparatus) where rats were injected with 0.003-30μg of PGE2, intradermally into the hindpaw. Thermal hypersensitivity was assessed by measuring withdraw latency from a radiant light beam (Hargreaves test) and our primary objective was to determine the dose of PGE2 causing the most pronounced increase in thermal hypersensitivity (i.e. lowest withdraw latency). A secondary objective was to determine the minimum dose of PGE2 required to cause statistically significant decreases in thermal withdrawal latency as compared to rats injected with vehicle. We found that rats injected with the 30μg dose of PGE2 exhibited the most pronounced thermal nociceptive hypersensitivity though secondary analysis showed that rats injected with PGE2 doses of 0.03-30μg had lower withdrawal latencies as compared to rats injected with vehicle. This work fills an evidence gap and provides context to guide dose selection in future rodent pain behavior studies.

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.