Muscle fatigue increases the probability of developing hyperalgesia in mice

Psychological treatments for temporomandibular disorder pain-A systematic review

OBJECTIVE

Temporomandibular disorders (TMD) are common. They affect abilities for carrying out daily tasks and influence different psychological aspects. In addition to standard treatment, psychological treatments have been suggested. The aim was to investigate the effects of psychological treatments on patients with painful TMD in a short- and long-term perspective.

MATERIALS AND METHODS

An electronic search was conducted in the databases MEDLINE, CINAHL, EMBASE, the Cochrane Central Registry of Controlled Trials (CENTRAL), and Web of Science for randomized clinical trials (RCTs) reporting psychological interventions for TMD. Registered beforehand in PROSPERO (CRD42022320106). In total, 18 RCTs were included; six RCTs that could be used in the meta-analysis, and all 18 RCTs were used in the narrative synthesis. Risk of bias was assessed by the Cochrane's tool for assessing risk of bias and certainty of evidence by GRADE.

RESULTS

The narrative synthesis indicates that psychological treatment options seem equivalent to standard treatment for painful TMD. The meta-analysis showed that a combination of psychological treatment and standard treatment and manual treatment (very low-quality evidence) are significantly better in pain reduction than just counselling and standard treatments of TMD.

CONCLUSION

This study indicates that psychological treatments seem to reduce pain intensity in individuals with painful TMD, and that the effect seems to be equally good as standard treatment. However, a combination of psychological treatments and standard treatments seems to have an even better effect. This indicates that psychological treatments are promising as an additional treatment approach for painful TMDs.

The Fibromyalgia Pain Experience: A Scoping Review of the Preclinical Evidence for Replication and Treatment of the Affective and Cognitive Pain Dimensions

Fibromyalgia is a chronic, widespread pain disorder that is strongly represented across the affective and cognitive dimensions of pain, given that the underlying pathophysiology of the disorder is yet to be identified. These affective and cognitive deficits are crucial to understanding and treating the fibromyalgia pain experience as a whole but replicating this multidimensionality on a preclinical level is challenging. To understand the underlying mechanisms, animal models are used. In this scoping review, we evaluate the current primary animal models of fibromyalgia regarding their translational relevance within the affective and cognitive pain realms, as well as summarize treatments that have been identified preclinically for attenuating these deficits.

Chocolate intake and muscle pain sensation: A randomized experimental study

BACKGROUND

Chocolate, as a cocoa-derived product rich in flavanols, has been used for medical and anti-inflammatory purposes. Therefore, the aim of this study was to investigate if the ingestion of different percentages of cocoa products affects the experimentally induced pain caused by intramuscular hypertonic saline injections in the masseter muscle of healthy men and women.

METHODS

This experimental randomized, double-blind, and controlled study included 15 young, healthy, and pain-free men and 15 age-matched women and involved three visits with at least a 1-week washout. Pain was induced twice at each visit with intramuscular injections of 0.2 mL hypertonic saline (5%), before and after intake of one of the different chocolate types: white (30% cocoa content), milk (34% cocoa content), and dark (70% cocoa content). Pain duration, pain area, peak pain, and pressure pain threshold (PPT) were assessed every fifth minute after each injection, up until 30 min after the initial injection. Descriptive and inferential statistics were performed using IBM® SPSS (Version 27); significance level was set to p<0.05.

RESULTS

This study showed that intake of chocolate, no matter the type, reduced the induced pain intensity significantly more than no intake of chocolate (p<0.05, Tukey test). There were no differences between the chocolate types. Further, men showed a significantly greater pain reduction than women after intake of white chocolate (p<0.05, Tukey test). No other differences between pain characteristics or sexes were revealed.

CONCLUSION

Intake of chocolate before a painful stimulus had a pain-reducing effect no matter the cocoa concentration. The results indicate that perhaps it is not the cocoa concentration (e.g., flavanols) alone that explains the positive effect on pain, but likely a combination of preference and taste-experience. Another possible explanation could be the composition of the chocolate, i.e. the concentration of the other ingredients such as sugar, soy, and vanilla. ClinicalTrials.gov Identifier: NCT05378984.

Fibromyalgia: Pathogenesis, Mechanisms, Diagnosis and Treatment Options Update

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

pH Mapping of Skeletal Muscle by Chemical Exchange Saturation Transfer (CEST) Imaging

Magnetic resonance imaging (MRI) is extensively used in clinical and basic biomedical research. However, MRI detection of pH changes still poses a technical challenge. Chemical exchange saturation transfer (CEST) imaging is a possible solution to this problem. Using saturation transfer, alterations in the exchange rates between the solute and water protons because of small pH changes can be detected with greater sensitivity. In this study, we examined a fatigued skeletal muscle model in electrically stimulated mice. The measured CEST signal ratio was between 1.96 ppm and 2.6 ppm in the z-spectrum, and this was associated with pH values based on the ratio between the creatine (Cr) and the phosphocreatine (PCr). The CEST results demonstrated a significant contrast change at the electrical stimulation site. Moreover, the pH value was observed to decrease from 7.23 to 7.15 within 20 h after electrical stimulation. This pH decrease was verified by 31P magnetic resonance spectroscopy and behavioral tests, which showed a consistent variation over time.

P2X4 Receptors on Muscle Macrophages Are Required for Development of Hyperalgesia in an Animal Model of Activity-Induced Muscle Pain

Activity-induced pain is common in those with chronic musculoskeletal pain and limits participation in daily activities and exercise. Our laboratory developed a model of activity-induced pain and shows that depletion of muscle macrophages prevents development of hyperalgesia. Adenosine triphosphate (ATP) is released from fatiguing muscle and activates purinergic receptors (P2X), and P2X4 receptors are expressed on macrophages. We hypothesized that exercise releases ATP to activate P2X4 receptors on muscle macrophages, which subsequently release interleukin-1β (IL-1β) to produce hyperalgesia. In an animal model of activity-induced pain, using male and female C57BL6/J mice, we show increased expression of P2X4 on muscle macrophages, and blockade of P2X4 receptors in muscle prevented development of hyperalgesia. Using a lentivirus expressing an artificial micro-RNA to P2X4 under the control of a CD68 promoter, we decreased expression of P2X4 mRNA in cultured macrophages, decreased expression of P2X4 protein in muscle macrophages in vivo, and prevented development of activity-induced hyperalgesia. We further show that macrophages primed with LPS differentially released IL-1β when treated with ATP in neutral or acidic pH. Lastly, blockade of IL-1β in muscle prevented development of hyperalgesia in this model. Thus, our data suggest that P2X4 receptors could be a valid pharmacological target to control activity-induced muscle pain experienced by patients with chronic musculoskeletal 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.

Acid Sensing Ion Channel 1a (ASIC1a) Mediates Activity-induced Pain by Modulation of Heteromeric ASIC Channel Kinetics

Chronic muscle pain is acutely worsened by exercise. Acid sensing ion channels (ASIC) are heteromeric channels expressed in muscle sensory neurons that detect decreases in pH. We have previously shown ASIC3 is important in activity-induced hyperalgesia. However, ASICs form heteromers with ASIC1a being a key component in sensory neurons. Therefore, we studied the role of ASIC1a in mice using behavioral pharmacology and genetic deletion in a model of activity-induced hyperalgesia. We found ASIC1a-/- mice developed mechanical hyperalgesia similar to wild-type mice, but antagonism of ASIC1a, with psalmotoxin, prevented development of mechanical hyperalgesia in wild-type mice, but not in ASIC1a-/- mice. To explain this discrepancy, we then performed electrophysiology studies of ASICs and examined the effects of psalmotoxin on ASIC heteromers. We expressed ASIC1a, 2 and 3 heteromers or ASIC1 and 3 heteromers in CHO cells, and examined the effects of psalmotoxin on pH sensitivity. Psalmotoxin significantly altered the properties of ASIC hetomeric channels. Specifically, in ASIC1a/2/3 heteromers, psalmotoxin slowed the kinetics of desensitization, slowed the recovery from desensitization, and inhibited pH-dependent steady-state desensitization, but had no effect on pH-evoked current amplitudes. We found a different pattern in ASIC1a/3 heteromers. There was a significant leftward shift in the pH dose response of steady-state desensitization and decrease in pH-evoked current amplitudes. These results suggest that blockade of ASIC1a modulates the kinetics of heteromeric ASICs to prevent development of activity-induced hyperalgesia. These data suggest ASIC1a is a key subunit in heteromeric ASICs and may be a pharmacological target for treatment of musculoskeletal pain.

Macrophage polarization contributes to local inflammation and structural change in the multifidus muscle after intervertebral disc injury

PURPOSE

Intervertebral disk (IVD) lesion and its subsequent degeneration have a profound effect on the multifidus muscle. The subacute/early chronic phase of multifidus remodeling after IVD lesion has been proposed to be regulated by inflammatory processes. The balance between pro-inflammatory (M1) and anti-inflammatory (M2) macrophages plays an important role in maintaining tissue integrity after injury. The localization, polarization of macrophage subtypes and their mediation of the pro-inflammatory cytokine tumor necrosis factor (TNF) are unknown in paraspinal muscles during IVD degeneration. A sheep model of IVD degeneration was used to investigate the role of macrophages and TNF in the structural alterations that occur within the multifidus muscle.

METHODS

Anterolateral lesions were induced at L3-4 IVD in sheep. Multifidus muscle tissue at L4 was harvested 3 and 6 months after lesion and used for immunofluorescence assays to examine total macrophage number, macrophage polarization between M1 and M2, and to assess the localization of TNF expression in muscle, adipose and connective tissues from injured and naïve control animals.

RESULTS

A greater proportion of M1 macrophages is present in muscle at both 3 and 6 months after IVD lesion, and adipose tissue at 6 months. Total number of macrophages is unchanged. At 6 months, expression of TNF is increased in adipose and connective tissue and the proportion of TNF expressed by M1 macrophages is increased.

CONCLUSIONS

These data support the proposal that macrophages and TNF (pro-inflammatory cytokine) play an active role in the subacute/early chronic phase of remodeling in muscle, adipose and connective tissues of the multifidus during IVD degeneration. This presents a novel target for treatment. These slides can be retrieved under Electronic Supplementary Material.

The Genetics of Pain: Implications for Therapeutics

Pain is an increasing clinical challenge affecting about half the population, with a substantial number of people suffering daily intense pain. Such suffering can be linked to the dramatic rise in opioid use and associated deaths in the United States. There is a pressing need for new analgesics with limited side effects. Here, we summarize what we know about the genetics of pain and implications for drug development. We make the case that chronic pain is not one but a set of disease states, with peripheral drive a key element in most. We argue that understanding redundancy and plasticity, hallmarks of the nervous system, is critical in developing analgesic drug strategies. We describe the exploitation of monogenic pain syndromes and genetic association studies to define analgesic targets, as well as issues associated with animal models of pain. We appraise present-day screening technologies and describe recent approaches to pain treatment that hold promise.

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

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

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

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

Exercise prevents development of autonomic dysregulation and hyperalgesia in a mouse model of chronic muscle pain

Chronic musculoskeletal pain (CMP) conditions, like fibromyalgia, are associated with widespread pain and alterations in autonomic functions. Regular physical activity prevents the development of CMP and can reduce autonomic dysfunction. We tested if there were alterations in autonomic function of sedentary mice with CMP, and whether exercise reduced the autonomic dysfunction and pain induced by CMP. Chronic musculoskeletal pain was induced by 2 intramuscular injections of pH 5.0 in combination with a single fatiguing exercise task. A running wheel was placed into cages so that the mouse had free access to it for either 5 days or 8 weeks (exercise groups) and these animals were compared to sedentary mice without running wheels. Autonomic function and nociceptive withdrawal thresholds of the paw and muscle were assessed before and after induction of CMP in exercised and sedentary mice. In sedentary mice, we show decreased baroreflex sensitivity, increased blood pressure variability, decreased heart rate variability, and decreased withdrawal thresholds of the paw and muscle 24 hours after induction of CMP. There were no sex differences after induction of the CMP in any outcome measure. We further show that both 5 days and 8 weeks of physical activity prevent the development of autonomic dysfunction and decreases in withdrawal threshold induced by CMP. Thus, this study uniquely shows the development of autonomic dysfunction in animals with chronic muscle hyperalgesia, which can be prevented with as little as 5 days of physical activity, and suggest that physical activity may prevent the development of pain and autonomic dysfunction in people with CMP.

ASICs Mediate Pain and Inflammation in Musculoskeletal Diseases

Chronic musculoskeletal pain is debilitating and affects ∼ 20% of adults. Tissue acidosis is present in painful musculoskeletal diseases like rheumatoid arthritis. ASICs are located on skeletal muscle and joint nociceptors as well as on nonneuronal cells in the muscles and joints, where they mediate nociception. This review discusses the properties of different types of ASICs, factors affecting their pH sensitivity, and their role in musculoskeletal hyperalgesia and inflammation.

Neurobiology of fibromyalgia and chronic widespread pain

Fibromyalgia is the current term for chronic widespread musculoskeletal pain for which no alternative cause can be identified. The underlying mechanisms, in both human and animal studies, for the continued pain in individuals with fibromyalgia will be explored in this review. There is a substantial amount of support for alterations of central nervous system nociceptive processing in people with fibromyalgia, and that psychological factors such as stress can enhance the pain experience. Emerging evidence has begun exploring other potential mechanisms including a peripheral nervous system component to the generation of pain and the role of systemic inflammation. We will explore the data and neurobiology related to the role of the CNS in nociceptive processing, followed by a short review of studies examining potential peripheral nervous system changes and cytokine involvement. We will not only explore the data from human subjects with fibromyalgia but will relate this to findings from animal models of fibromyalgia. We conclude that fibromyalgia and related disorders are heterogenous conditions with a complicated pathobiology with patients falling along a continuum with one end a purely peripherally driven painful condition and the other end of the continuum is when pain is purely centrally driven.

Effect of Intramuscular Protons, Lactate, and ATP on Muscle Hyperalgesia in Rats

Chronic muscle pain is a significant health problem leading to disability[1]. Muscle fatigue can exacerbate muscle pain. Metabolites, including ATP, lactate, and protons, are released during fatiguing exercise and produce pain in humans. These substances directly activate purinergic (P2X) and acid sensing ion channels (ASICs) on muscle nociceptors, and when combined, produce a greater increase in neuron firing than when given alone. Whether the enhanced effect of combining protons, lactate, and ATP is the sum of individual effects (additive) or more than the sum of individual effects (synergistic) is unknown. Using a rat model of muscle nociceptive behavior, we tested each of these compounds individually over a range of physiologic and supra-physiologic concentrations. Further, we combined all three compounds in a series of dilutions and tested their effect on muscle nociceptive behavior. We also tested a non-hydrolyzable form of ATP (α,β-meATP) alone and in combination with lactate and acidic pH. Surprisingly, we found no dose-dependent effect on muscle nociceptive behavior for protons, lactate, or ATP when given alone. We similarly found no effect after application of each two-metabolite combination. Only pH 4 saline and α,β-meATP produced hyperalgesia when given alone. When all 3 substances were combined, however, ATP (2.4μm), lactate (10mM), and acidic pH (pH 6.0) produced an enhanced effect greater than the sum of the effects of the individual components, i.e. synergism. α,β me ATP (3nmol), on the other hand, showed no enhanced effects when combined with lactate (10mM) or acidic pH (pH 6.0), i.e. additive. These data suggest that combining fatigue metabolites in muscle produces a synergistic effect on muscle nociception.

Masticatory sensory-motor changes after an experimental chewing test influenced by pain catastrophizing and neck-pain-related disability in patients with headache attributed to temporomandibular disorders

Background

Recent research has shown a relationship of craniomandibular disability with neck-pain-related disability has been shown. However, there is still insufficient information demonstrating the influence of neck pain and disability in the sensory-motor activity in patients with headache attributed to temporomandibular disorders (TMD). The purpose of this study was to investigate the influence of neck-pain-related disability on masticatory sensory-motor variables.

Methods

An experimental case–control study investigated 83 patients with headache attributed to TMD and 39 healthy controls. Patients were grouped according to their scores on the neck disability index (NDI) (mild and moderate neck disability). Initial assessment included the pain catastrophizing scale and the Headache Impact Test-6. The protocol consisted of baseline measurements of pressure pain thresholds (PPT) and pain-free maximum mouth opening (MMO). Individuals were asked to perform the provocation chewing test, and measurements were taken immediately after and 24 hours later. During the test, patients were assessed for subjective feelings of fatigue (VAFS) and pain intensity.

Results

VAFS was higher at 6 minutes (mean 51.7; 95% CI: 50.15-53.26) and 24 hours after (21.08; 95% CI: 18.6-23.5) for the group showing moderate neck disability compared with the mild neck disability group (6 minutes, 44.16; 95% CI 42.65-45.67/ 24 hours after, 14.3; 95% CI: 11.9-16.7) and the control group. The analysis shows a decrease in the pain-free MMO only in the group of moderate disability 24 hours after the test. PPTs of the trigeminal region decreased immediately in all groups, whereas at 24 hours, a decrease was observed in only the groups of patients. PPTs of the cervical region decreased in only the group with moderate neck disability 24 hours after the test. The strongest negative correlation was found between pain-free MMO immediately after the test and NDI in both the mild (r = −0.49) and moderate (r = −0.54) neck disability groups. VAFS was predicted by catastrophizing, explaining 17% of the variance in the moderate neck disability group and 12% in the mild neck disability group.

Conclusion

Neck-pain-related disability and pain catastrophizing have an influence on the sensory-motor variables evaluated in patients with headache attributed to TMD.

ASIC3 Is Required for Development of Fatigue-Induced Hyperalgesia

An acute bout of exercise can exacerbate pain, hindering participation in regular exercise and daily activities. The mechanisms underlying pain in response to acute exercise are poorly understood. We hypothesized that proton accumulation during muscle fatigue activates acid-sensing ion channel 3 (ASIC3) on muscle nociceptors to produce hyperalgesia. We investigated the role of ASIC3 using genetic and pharmacological approaches in a model of fatigue-enhanced hyperalgesia. This model uses two injections of pH 5.0 saline into muscle in combination with an electrically induced fatigue of the same muscle just prior to the second injection of acid to induce mechanical hyperalgesia. We show a significant decrease in muscle force and decrease in muscle pH after 6 min of electrical stimulation. Genetic deletion of ASIC3 using knockout mice and pharmacological blockade of ASIC3 with APETx2 in muscle prevents the fatigue-enhanced hyperalgesia. However, ASIC3(-/-) mice and APETx2 have no effect on the fatigue response. Genetic deletion of ASIC3 in primary afferents innervating muscle using an HSV-1 expressing microRNA (miRNA) to ASIC3 surprisingly had no effect on the development of the hyperalgesia. Muscle fatigue increased the number of macrophages in muscle, and removal of macrophages from muscle with clodronate liposomes prevented the development of fatigue-enhanced hyperalgesia. Thus, these data suggest that fatigue reduces pH in muscle that subsequently activates ASIC3 on macrophages to enhance hyperalgesia to muscle insult.

Animal models of fibromyalgia

Animal models of disease states are valuable tools for developing new treatments and investigating underlying mechanisms. They should mimic the symptoms and pathology of the disease and importantly be predictive of effective treatments. Fibromyalgia is characterized by chronic widespread pain with associated co-morbid symptoms that include fatigue, depression, anxiety and sleep dysfunction. In this review, we present different animal models that mimic the signs and symptoms of fibromyalgia. These models are induced by a wide variety of methods that include repeated muscle insults, depletion of biogenic amines, and stress. All potential models produce widespread and long-lasting hyperalgesia without overt peripheral tissue damage and thus mimic the clinical presentation of fibromyalgia. We describe the methods for induction of the model, pathophysiological mechanisms for each model, and treatment profiles.

Anatomical and physiological factors contributing to chronic muscle pain

Chronic muscle pain remains a significant source of suffering and disability despite the adoption of pharmacologic and physical therapies. Muscle pain is mediated by free nerve endings distributed through the muscle along arteries. These nerves project to the superficial dorsal horn and are transmitted primarily through the spinothalamic tract to several cortical and subcortical structures, some of which are more active during the processing of muscle pain than other painful conditions. Mechanical forces, ischemia, and inflammation are the primary stimuli for muscle pain, which is reflected in the array of peripheral receptors contributing to muscle pain-ASIC, P2X, and TRP channels. Sensitization of peripheral receptors and of central pain processing structures are both critical for the development and maintenance of chronic muscle pain. Further, variations in peripheral receptors and central structures contribute to the significantly greater prevalence of chronic muscle pain in females.

An overview of animal models of pain: disease models and outcome measures

Pain is ultimately a perceptual phenomenon. It is built from information gathered by specialized pain receptors in tissue, modified by spinal and supraspinal mechanisms, and integrated into a discrete sensory experience with an emotional valence in the brain. Because of this, studying intact animals allows the multidimensional nature of pain to be examined. A number of animal models have been developed, reflecting observations that pain phenotypes are mediated by distinct mechanisms. Animal models of pain are designed to mimic distinct clinical diseases to better evaluate underlying mechanisms and potential treatments. Outcome measures are designed to measure multiple parts of the pain experience, including reflexive hyperalgesia measures, sensory and affective dimensions of pain, and impact of pain on function and quality of life. In this review, we discuss the common methods used for inducing each of the pain phenotypes related to clinical pain syndromes as well as the main behavioral tests for assessing pain in each model.

PERSPECTIVE

Understanding animal models and outcome measures in animals will assist in translating data from basic science to the clinic.

Fatigue-enhanced hyperalgesia in response to muscle insult: induction and development occur in a sex-dependent manner

Chronic muscle pain affects 20-50% of the population, is more common in women than men, and is associated with increased pain during physical activity and exercise. Muscle fatigue is common in people with chronic muscle pain, occurs in response to exercise, and is associated with release of fatigue metabolites. Fatigue metabolites can sensitize muscle nociceptors, which could enhance pain with exercise. Using a mouse model we tested whether fatigue of a single muscle, induced by electrical stimulation, resulted in enhanced muscle hyperalgesia and if the enhanced hyperalgesia was more pronounced in female mice. Muscle fatigue was induced in combination with a sub-threshold muscle insult (2 injections of pH 5.0 saline) in male and female mice. We show that male and female mice, fatigued immediately prior to muscle insult in the same muscle, develop similar muscle hyperalgesia 24 hours later. However, female mice also develop hyperalgesia when muscle fatigue and muscle insult occur in different muscles, and when muscle insult is administered 24 hours after fatigue in the same muscle. Further, hyperalgesia lasts significantly longer in females. Finally, muscle insult with or without muscle fatigue results in minimal inflammatory changes in the muscle itself, and sex differences are not related to estradiol (ovariectomy) or changes in brainstem activity (pNR1). Thus, the current model mimics muscle fatigue-induced enhancement of pain observed in chronic muscle pain conditions in the human population. Interactions between fatigue and muscle insult may underlie the development of chronic widespread pain with an associated female predominance observed in human subjects.

Assessment of avoidance behaviors in mouse models of muscle pain

Pain encompasses both a sensory as well as an affective dimension and these are differentially processed in the cortex. Animal models typically use reflexive behaviors to test nociceptive responses; these are thought to reflect the sensory dimension of pain. While several behavioral tests are available for examining the affective dimension of pain it is unclear if these are appropriate in animal models of muscle pain. We therefore tested the utility of existing paradigms as well as new avoidance paradigms in animal models of muscle pain in mice. Specifically we used an escape-avoidance test to noxious mechanical stimuli, a learned avoidance test to noxious mechanical stimuli, and avoidance of physical activity. We used three animal models of muscle pain: carrageenan-induced inflammation, non-inflammatory muscle pain, and exercise-enhanced pain. In the carrageenan model of inflammation mice developed escape-avoidance behaviors to mechanical stimuli, learned avoidance to mechanical stimulation and avoidance of physical activity - these models are associated with unilateral hyperalgesia. When both muscles were inflamed, escape-avoidance behaviors did not develop suggesting that equivalent bilateral pain behaviors cannot be tested with an escape-avoidance test. In the non-inflammatory muscle pain model mice did not show significant changes in escape-avoidance behaviors or learned avoidance, but did avoid physical activity. In the exercise-enhanced pain model, there were no changes in escape-avoidance, learned avoidance of noxious or physical activity In conclusion, we developed several testing protocols that assess supraspinal processing of pain behaviors in models of muscle pain and that are most sensitive in animals with unilateral hyperalgesia.

Prefrontal lactate predicts exercise-induced cognitive dysfunction in Gulf War Illness

BACKGROUND

25% to 30% of Veterans deployed to the 1990 to 1991 Persian Gulf War exhibit an idiopathic syndrome of chronic fatigue, exertional exhaustion, pain, hyperalgesia, cognitive and affective dysfunction known as Gulf War Illness (GWI).

METHODS

Gulf War veterans (n=15) and sedentary veteran and civilian controls (n=11) completed a 2-back working memory test in an fMRI before and after two bicycle exercise stress test. We performed single voxel (1)H MRS to evaluate brain metabolic differences in the left anterior cingulate cortex and the changes associated with exercise.

RESULTS

Eight GWI subjects increased their 2-back scores after exercise (labelled increasers) and seven GWI subjects decreased their 2-back scores after exercise (labelled decreasers). These phenotypic responses were absent for controls. Decreasers had significantly elevated prefrontal lactate levels compared to Increasers prior to completion of the exercise stress tests. Evaluation of prefrontal lactate levels prior to exercise demonstrated predictability (ROC analysis) of the two diametrically opposed subgroups.

CONCLUSION

Prefrontal lactate levels may be a potential biomarker for exercise-induced subgroups in GWI. The alterations in brain energetics may be in part responsible for a subgroup of GWI and underlie some of the symptoms present in the patient population.

Regular physical activity prevents development of chronic pain and activation of central neurons

Chronic musculoskeletal pain is a significant health problem and is associated with increases in pain during acute physical activity. Regular physical activity is protective against many chronic diseases; however, it is unknown if it plays a role in development of chronic pain. The current study induced physical activity by placing running wheels in home cages of mice for 5 days or 8 wk and compared these to sedentary mice without running wheels in their home cages. Chronic muscle pain was induced by repeated intramuscular injection of pH 4.0 saline, exercise-enhanced pain was induced by combining a 2-h fatiguing exercise task with a low-dose muscle inflammation (0.03% carrageenan), and acute muscle inflammation was induced by 3% carrageenan. We tested the responses of the paw (response frequency) and muscle (withdrawal threshold) to nociceptive stimuli. Because the rostral ventromedial medulla (RVM) is involved in exercise-induced analgesia and chronic muscle pain, we tested for changes in phosphorylation of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptor in the RVM. We demonstrate that regular physical activity prevents the development of chronic muscle pain and exercise-induced muscle pain by reducing phosphorylation of the NR1 subunit of the NMDA receptor in the central nervous system. However, regular physical activity has no effect on development of acute pain. Thus physical inactivity is a risk factor for development of chronic pain and may set the nervous system to respond in an exaggerated way to low-intensity muscle insults.

Peripheral and central mechanisms of chronic musculoskeletal pain

Dr Sluka is a professor in the Department of Physical Therapy and Rehabilitation Science at the University of Iowa (IA, USA). She is also a member of the Pain Research Program and the Neuroscience Graduate Program. She received a physical therapy degree from Georgia State University (GA, USA) and practiced physical therapy pain management in Houston (TX, USA) before obtaining a PhD in Anatomy from the University of Texas Medical Branch in Galveston (TX, USA). After a postdoctoral fellowship with Dr William D Willis, she joined the faculty at the University of Iowa. Dr Sluka’s research focuses on the neurobiology of musculoskeletal pain, as well as the mechanisms and effectiveness of nonpharmacological pain treatments commonly used by physical therapists. She has published over 140 peer-reviewed manuscripts, numerous book chapters and a textbook on Pain Mechanisms and Management for the Physical Therapist. She has received numerous awards including the Marian Williams Award for Research in Physical Therapy and is a Catherine Worthingham Fellow from the American Physical Therapy Association and the Frederick WL Kerr Basic Science Research Award from the American Pain Society. She is actively involved in the International Association for the Study of Pain, the American Pain Society and the American Physical Therapy Association, serving on committees, task forces and society boards.

Exercise-induced pain requires NMDA receptor activation in the medullary raphe nuclei

PURPOSE

Pain in response to physical activity is common in people with chronic musculoskeletal pain and is likely a barrier to regular exercise, which would lead to a sedentary lifestyle. We recently developed a model of exercise-induced pain that is associated with increased activation of neurons in the medullary raphe nuclei, i.e., the nucleus raphe obscurus (NRO) and nucleus raphe pallidus (NRP). Because the NRO and NRP not only modulate motor output but also respond to noxious stimuli, we hypothesized that the NRO and NRP were key nuclei in the interaction between pain and exercise. We tested whether exercise enhances hyperalgesia through activation of N-methyl D-aspartate (NMDA) receptors in the NRO/NRP.

METHODS

Muscle insult was induced by two injections of pH 5.0 saline 5 d apart into one gastrocnemius muscle. We initially tested whether hyperalgesia developed in mice injected with acidic saline (pH 5.0) into the gastrocnemius muscle immediately after a 30-min or 2-h exercise task or 2 h after a 2-h exercise task. Next, we tested whether blockade of NMDA receptors in the NRO/NRP during the exercise task prevented the development of exercise-induced hyperalgesia. Finally, we evaluated changes in phosphorylation of the NR1 subunit of the NMDA receptor (pNR1) after the exercise task at times in which muscle insult was given in behavioral experiments, i.e., immediately after a 30-min or 2-h exercise task or 2 h after the 2-h exercise task.

RESULTS

All exercise conditions enhanced nociception (hyperalgesia) after combining with two injections of pH 5.0 saline. Microinjection of AP5 (1.0-0.1 nmol; 2-amino-5-phophonopenanoate) dose-dependently prevented the development of exercise-induced hyperalgesia. All exercise conditions increased pNR1 in the NRO and NRP.

CONCLUSIONS

Thus, exercise-induced pain in sedentary mice is associated with increased phosphorylation and activation of NMDA receptors in the NRO/NRP, suggesting that changes in central excitability mediate an interaction between unaccustomed exercise and pain.

TRPV1 is important for mechanical and heat sensitivity in uninjured animals and development of heat hypersensitivity after muscle inflammation

Inflammatory thermal hyperalgesia is principally mediated through transient receptor potential vanilloid 1 (TRPV1) channels, as demonstrated by prior studies using models of cutaneous inflammation. Muscle pain is significantly different from cutaneous pain, and the involvement of TRPV1 in hyperalgesia induced by muscle inflammation is unknown. We tested whether TRPV1 contributes to the development of mechanical and heat hypersensitivity of the paw in TRPV1(-/-) mice after muscle inflammation. Because TRPV1(-/-) mice lack TRPV1 at the site of inflammation (muscle) and at the testing site (paw), we do not know whether TRPV1 is important as a mediator of nociceptor sensitization in the muscle or as a heat sensor in the paw. Using recombinant herpesviruses, we reexpressed TRPV1 in TRPV1(-/-) mice in primary afferents innervating skin, muscle, or both to determine which sites were important for the behavioral deficits. Responses to repeated application of noxious mechanical stimuli to the hind paw were enhanced in TRPV1(-/-) mice; this was restored by reexpression of TRPV1 into skin. Withdrawal latencies to noxious heat were increased in TRPV1(-/-) mice; normal latencies were restored by reexpression of TRPV1 in both skin and muscle. Heat hypersensitivity induced by muscle inflammation did not develop in TRPV1(-/-) mice; mechanical hypersensitivity was similar between TRPV1(-/-) and TRPV1(+/+) mice. Heat hypersensitivity induced by muscle inflammation was restored by reexpression of TRPV1 into both muscle and skin of TRPV1(-/-) mice. These results suggest that TRPV1 serves as both a mediator of nociceptor sensitization at the site of inflammation and as a heat sensor at the paw.

Update on the relation between pain and movement: consequences for clinical practice

It is generally thought that exercise is beneficial to alleviate pain. However, prolonged movement may lead to the development of painful injuries, because of the overload of low-threshold motor units. Especially in individuals with a pain condition, exercise prescription and the impact of fatigue is less clear. This may be because of the dual effects, aggravation and relief, which fatigue has on pain. The purpose of this review is to ascertain the relation between pain and the motor system, both in the development and management of pain. Recent studies show that fatigue alters pain-induced increases in corticomotor excitability and leads to within and between-muscle adaptations. Studies of acute pain have shown complex adaptations such as increased movement variability, which may be because of a search for motor solutions to prolong overall task performance. In contrast, chronic pain seems to limit movement duration, speed, and variability which could be protective in the short term but also counterproductive over time. Owing to these adaptations in movement strategies, pain chronicity may help to dictate exercise prescription. For example, the correct dosage of multimuscle, dynamic exercises would act to promote movement variability. Thus, it seems that exercise involving the use of different movement strategies could be effective in helping people to obtain exercise-induced benefits while avoiding injury and pain reaggravation.

Changes in expression of NMDA-NR1 receptor subunits in the rostral ventromedial medulla modulate pain behaviors

NMDA receptors have an important role in pain facilitation in rostral ventromedial medulla (RVM) and the NR1 subunit is essential for its function. Studies suggest that the NMDA receptors in RVM are critical to modulate both cutaneous and muscle hypersensitivity induced by repeated intramuscular acid injections. We propose that increased expression of the NR1 subunit in the RVM is critical for the full development of hypersensitivity. To test this we used recombinant lentiviruses to over-express the NR1 subunit in the RVM and measured nociceptive sensitivity to cutaneous and muscle stimuli. We also downregulated the expression of NR1 in the RVM and measured the hyperalgesia produced by repeated-acid injections. Increasing the expression of NR1 in the RVM reduces cutaneous and muscle withdrawal threshold, and decreasing the expression of NR1 in the RVM increases the muscle withdrawal threshold and prevents the development of hyperalgesia in an animal model of muscle pain. These results suggest that the NR1 subunits in the RVM are critical for modulating NMDA receptor function, which in turn sets the 'tone' of the nervous system's response to noxious stimuli and tissue injury.

Aerobic exercise alters analgesia and neurotrophin-3 synthesis in an animal model of chronic widespread pain

BACKGROUND

Present literature and clinical practice provide strong support for the use of aerobic exercise in reducing pain and improving function for individuals with chronic musculoskeletal pain syndromes. However, the molecular basis for the positive actions of exercise remains poorly understood. Recent studies suggest that neurotrophin-3 (NT-3) may act in an analgesic fashion in various pain states.

OBJECTIVE

The purpose of the present study was to examine the effects of moderate-intensity aerobic exercise on pain-like behavior and NT-3 in an animal model of widespread pain.

DESIGN

This was a repeated-measures, observational cross-sectional study.

METHODS

Forty female mice were injected with either normal (pH 7.2; n=20) or acidic (pH 4.0; n=20) saline in the gastrocnemius muscle to induce widespread hyperalgesia and exercised for 3 weeks. Cutaneous (von Frey monofilament) and muscular (forceps compression) mechanical sensitivity were assessed. Neurotrophin-3 was quantified in 2 hind-limb skeletal muscles for both messenger RNA (mRNA) and protein levels after exercise training. Data were analyzed with 2-factor analysis of variance for repeated measures (group x time).

RESULTS

Moderate-intensity aerobic exercise reduced cutaneous and deep tissue hyperalgesia induced by acidic saline and stimulated NT-3 synthesis in skeletal muscle. The increase in NT-3 was more pronounced at the protein level compared with mRNA expression. In addition, the increase in NT-3 protein was significant in the gastrocnemius muscle but not in the soleus muscle, suggesting that exercise can preferentially target NT-3 synthesis in specific muscle types.

LIMITATIONS

Results are limited to animal models and cannot be generalized to chronic pain syndromes in humans.

CONCLUSIONS

This is the first study demonstrating the effect of exercise on deep tissue mechanical hyperalgesia in a rodent model of pain and providing a possible molecular basis for exercise training in reducing muscular pain.

Fatiguing exercise enhances hyperalgesia to muscle inflammation

Since many people with chronic fatigue present with pain and many people with chronic pain present with fatigue, we tested if fatigue would enhance the response to pain in male and female mice. We further tested for the activation of brainstem nuclei by the fatigue task using c-fos as a marker. Fatigue was induced by having mice spontaneously run in running wheel for 2h. Carrageenan (0.03%) was injected into the gastrocnemius muscle either 2h before or 2h after the fatigue task. The mechanical sensitivity of the paw (von Frey filaments), muscle (tweezers), grip force and running wheel activity was assessed before and 24h after injection of carrageenan. Both male and female mice that performed the fatigue task, either before or after intramuscular injection of carrageenan, showed an enhanced mechanical sensitivity of the paw, but not the muscle. Ovariectomized mice showed a similar response to male mice. There was a decrease in running wheel activity after carrageenan injection, but no change in grip force suggesting that mice had no deficit in motor performance induced by the carrageenan. C-fos expression was observed in the nucleus raphe pallidus, obscurus, and magnus after the fatigue task suggesting an increased activity in the raphe nuclei in response to the fatigue task. Therefore, widespread hyperalgesia is enhanced by the fatigue response but not hyperalgesia at the site of insult. We suggest that this effect is sex-dependent and involves mechanisms in the brainstem to result in an enhanced hyperalgesia.

Modulation of antigen-induced chronic fatigue in mouse model of water immersion stress by naringin, a polyphenolic antioxidant

It is believed that physical stress, infection and oxidative stress are involved in the development of chronic fatigue syndrome. There is little evidence stating the beneficial role of nutritional supplements in chronic fatigue syndrome. Based on this, this study was designed to evaluate the effect of naringin, a natural polyphenol, in a mouse model of immunologically-induced fatigue, wherein purified lipopolysaccharide (LPS) as well as Brucella abortus (BA) antigen was used as immunogens. The assessment of chronic fatigue syndrome was based on chronic water-immersion stress test for 10 mins as well as measurement of hyperalgesia for 19 days. Immobility time and tail withdrawal latency as well as oxidative stress were taken as the markers of fatigue. Mice challenged with LPS or BA for 19 days showed significant increase in the immobility time, hyperalgesia and oxidative stress on 19th day. Serum tumor necrosis factor-alpha (TNF-alpha) levels markedly increased with LPS or BA challenge. Concurrent treatment with naringin resulted in the significant decrease in the immobility time as well as hyperalgesia. There was significant attenuation of oxidative stress as well as in TNF-alpha levels. Present findings strongly suggest the role of oxidative stress and immunological activation in the pathophysiology of chronic fatigue syndrome, and treatment with naringin can be a valuable option in chronic fatigue syndrome.

Sarcolemma-localized nNOS is required to maintain activity after mild exercise

Many neuromuscular conditions are characterized by an exaggerated exercise-induced fatigue response that is disproportionate to activity level. This fatigue does not necessarily correlate with elevated central or peripheral fatigue in patients1, and some patients experience severe fatigue without any demonstrable somatic disease2. Except in myopathies that are due to specific metabolic defects, the mechanism underlying this type of fatigue remains unknown2. With no treatment available, this form of inactivity is a major determinant of disability3. Here we show, using mouse models, that this exaggerated fatigue response is distinct from a loss in specific force production by muscle, and that sarcolemma-localized nNOS signaling in skeletal muscle is required to maintain activity after mild exercise. Of significance, we show that nNOS-null mice do not have muscle pathology and have no loss of muscle specific-force after exercise, but do display this exaggerated fatigue response to mild exercise. In mouse models of nNOS mislocalization from the sarcolemma, prolonged inactivity was only relieved by pharmacologically enhancing the cGMP signal that results from muscle nNOS activation during the nitric oxide signaling response to mild exercise. Our findings suggest that the mechanism underlying the exaggerated fatigue response to mild exercise is a lack of contraction-induced signaling from sarcolemma-localized nNOS, which reduces cGMP-mediated vasomodulation in the vessels that supply active muscle after mild exercise. Notably, sarcolemmal nNOS was reduced in patient biopsies from a large number of distinct myopathies, suggesting a common mechanism of fatigue. Our results suggest that patients with an exaggerated fatigue response to mild exercise would show clinical improvement in response to treatment strategies aimed at improving exercise-induced signaling.

Central mechanisms in the maintenance of chronic widespread noninflammatory muscle pain

Chronic widespread pain (CWP) conditions such as fibromyalgia and myofascial syndromes are characterized by generalized pain, tenderness, morning stiffness, disturbed sleep, and pronounced fatigue. However, CWP pathophysiology is still unclear. A number of hypotheses have been proposed as the underlying pathophysiology of CWP: muscular dysfunction/ischemia, central sensitization, and a deficit in endogenous pain-modulating systems. This article reviews the current and emerging literature about the pathophysiology and neurobiology of chronic widespread -musculoskeletal pain. Widespread musculoskeletal pain results in changes in the central nervous system in human subjects and animal models. These changes likely reflect alterations in supraspinal modulation of nociception, and include increases in excitatory and decreases in inhibitory modulation pathways. These alterations in excitation and inhibition likely drive changes observed in the spinal cord to result in central sensitization, and the consequent pain and hyperalgesia.

Enhanced muscle fatigue occurs in male but not female ASIC3-/- mice

Muscle fatigue is associated with a number of clinical diseases, including chronic pain conditions. Decreases in extracellular pH activates acid-sensing ion channel 3 (ASIC3), depolarizes muscle, protects against fatigue, and produces pain. We examined whether ASIC3-/- mice were more fatigable than ASIC3+/+ mice in a task-dependent manner. We developed two exercise protocols to measure exercise-induced muscle fatigue: (fatigue task 1, three 1-h runs; fatigue task 2, three 30-min runs). In fatigue task 1, male ASIC3+/+ mice muscle showed less fatigue than male ASIC3-/- mice and female ASIC3+/+ mice. No differences in fatigue were observed in fatigue task 2. We then tested whether the development of muscle fatigue was dependent on sex and modulated by testosterone. Female ASIC3+/+ mice that were ovariectomized and administered testosterone developed less muscle fatigue than female ASIC3+/+ mice and behaved similarly to male ASIC3+/+ mice. However, testosterone was unable to rescue the muscle fatigue responses in ovariectomized ASIC3-/- mice. Plasma levels of testosterone from male ASIC3-/- mice were significantly lower than in male ASIC3+/+ mice and were similar to female ASIC3+/+ mice. Muscle fiber types, measured by counting ATPase-stained whole muscle sections, were similar in calf muscles from male and female ASIC3+/+ mice. These data suggest that both ASIC3 and testosterone are necessary to protect against muscle fatigue in a task-dependent manner. Also, differences in expression of ASIC3 and the development of exercise-induced fatigue could explain the female predominance in clinical syndromes of pain that include muscle fatigue.