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

The influence of sex on activity in voluntary wheel running, forced treadmill running, and open field testing in mice

Physical activity is commonly used for both measuring and treating dysfunction. While preclinical work has been historically biased towards males, the inclusion of both males and females is gaining popularity. With the increasing inclusion of both sexes, it is imperative to determine sex differences in common behavioral assays. This was a secondary analysis of healthy naïve mice to determine baseline sex differences in three activity assays: voluntary wheel running (32 mice), forced treadmill running (178 mice), and open field (88 mice). In voluntary wheel running, females showed greater distance run, running time, bout duration, and speed, but no difference in total bouts. In forced treadmill running, females showed greater time to exhaustion, but no difference in maximum speed attained. In open field, males showed greater active time but no difference in distance and speed over 30 min; however, male mice showed a downward trajectory in distance and speed over the final 20 min of testing, whereas females did not. These data suggest that male mice demonstrate comparable activity intensity to female mice but do not match females' duration of activity, especially for volitional tasks. Researchers utilizing these assays should account for sex differences as they could mask true findings in an experiment.

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

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

Exercise as a Therapeutic Strategy for Obesity: Central and Peripheral Mechanisms

Obesity is a complex, multifactorial condition involving excessive fat accumulation due to an imbalance between energy intake and expenditure, with its global prevalence steadily rising. This condition significantly increases the risk of chronic diseases, including sarcopenia, type 2 diabetes, and cardiovascular diseases, highlighting the need for effective interventions. Exercise has emerged as a potent non-pharmacological approach to combat obesity, targeting both central and peripheral mechanisms that regulate metabolism, energy expenditure, and neurological functions. In the central nervous system, exercise influences appetite, mood, and cognitive functions by modulating the reward system and regulating appetite-controlling hormones to manage energy intake. Concurrently, exercise promotes thermogenesis in adipose tissue and regulates endocrine path-ways and key metabolic organs, such as skeletal muscle and the liver, to enhance fat oxidation and support energy balance. Despite advances in understanding exercise's role in obesity, the precise interaction between the neurobiological and peripheral metabolic pathways remains underexplored, particularly in public health strategies. A better understanding of these interactions could inform more comprehensive obesity management approaches by addressing both central nervous system influences on behavior and peripheral metabolic regulation. This review synthesizes recent insights into these roles, highlighting potential therapeutic strategies targeting both systems for more effective obesity interventions.

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.

Brain-derived neurotrophic factor contributes to activity-induced muscle pain in male but not female mice

Activity-induced muscle pain increases interleukin-1β (IL-1β) release from muscle macrophages and the development of hyperalgesia is prevented by blockade of IL-1β in muscle. Brain derived neurotrophic factor (BDNF) is released from sensory neurons in response to IL-1β and mediates both inflammatory and neuropathic pain. Thus, we hypothesize that in activity-induced pain, fatigue metabolites combined with IL-1β activate sensory neurons to increase BDNF release, peripherally in muscle and centrally in the spinal dorsal horn, to produce hyperalgesia. We tested the effect of intrathecal or intramuscular injection of BDNF-Tropomyosin receptor kinase B (TrkB) inhibitors, ANA-12 or TrkB-Fc, on development of activity-induced pain. Both inhibitors prevented the hyperalgesia when given before or 24hr after induction of the model in male but not female mice. BDNF messenger ribonucleic acid (mRNA) and protein were significantly increased in dorsal root ganglion (DRG) 24hr after induction of the model in both male and female mice. Blockade of IL-1β in muscle had no effect on the increased BNDF mRNA observed in the activity-induced pain model, while IL-1β applied to cultured DRG significantly induced BDNF expression, suggesting IL-1β is sufficient but not necessary to induce BNDF. Thus, fatigue metabolites, combined with IL-1β, upregulate BDNF in primary DRG neurons in both male and female mice, but contribute to activity-induced pain only in males.

Local Synthesis of Estradiol in the Rostral Ventromedial Medulla Protects against Widespread Muscle Pain in Male Mice

Animal studies consistently demonstrate that testosterone is protective against pain in multiple models, including an animal model of activity-induced muscle pain. In this model, females develop widespread muscle hyperalgesia, and reducing testosterone levels in males results in widespread muscle hyperalgesia. Widespread pain is believed to be mediated by changes in the central nervous system, including the rostral ventromedial medulla (RVM). The enzyme that converts testosterone to estradiol, aromatase, is highly expressed in the RVM. Therefore, we hypothesized that testosterone is converted by aromatase to estradiol locally in the RVM to prevent development of widespread muscle hyperalgesia in male mice. This was tested through pharmacological inhibition of estrogen receptors (ERs), aromatase, or ER-α in the RVM which resulted in contralateral hyperalgesia in male mice (C57BL/6J). ER inhibition in the RVM had no effect on hyperalgesia in female mice. As prior studies show modulation of estradiol signaling alters GABA receptor and transporter expression, we examined if removal of testosterone in males would decrease mRNA expression of GABA receptor subunits and vesicular GABA transporter (VGAT). However, there were no differences in mRNA expression of GABA receptor subunits of VGAT between gonadectomized and sham control males. Lastly, we used RNAscope to determine expression of ER-α in the RVM and show expression in inhibitory (VGAT+), serotonergic (tryptophan hydroxylase 2+), and μ-opioid receptor expressing (MOR+) cells. In conclusion, testosterone protects males from development of widespread hyperalgesia through aromatization to estradiol and activation of ER-α which is widely expressed in multiple cell types in the RVM.

The role of androgens in migraine pathophysiology

Migraine affects ∼12 % of the worldwide population and is more prevalent in females, which suggests a role of sex hormones in migraine pathophysiology. Most studies have focused on estrogen and progesterone, and the involvement of androgens has been less studied. However, due to the recent advances in androgen interventions, which could advance new androgen-based migraine treatments, it is critical to better understand the role of androgens in migraine. Testosterone, the most studied androgen, was found to have an antinociceptive effect in various animal and human pain studies. Thus, it could also have a protective effect related to lower migraine severity and prevalence. In this review, we discuss studies examining the role of androgens on migraine-related symptoms in migraine animal models. Additionally, we summarize the results of human studies comparing androgen levels between patients with migraine and healthy controls, studies assessing the relationships between androgen levels and migraine severity, and intervention studies examining the impact of testosterone treatment on migraine severity. Many of the studies have limitations, however, the results suggest that androgens may have a minor effect on migraine. Still, it is possible that androgens are involved in migraine pathophysiology in a sub-group of patients such as in adolescents or postmenopausal women. We discuss potential mechanisms in which testosterone, as the main androgen tested, can impact migraine. These mechanisms range from the cellular level to systems and behavior and include the effect of testosterone on sensory neurons, the immune and vascular systems, the stress response, brain function, and mood. Lastly, we suggest future directions to advance this line of research.

Reduced sympathetic activity is associated with the development of pain and muscle atrophy in a female rat model of fibromyalgia

Fibromyalgia (FM) is characterized by chronic widespread musculoskeletal pain accompanied by fatigue and muscle atrophy. Although its etiology is not known, studies have shown that FM patients exhibit altered function of the sympathetic nervous system (SNS), which regulates nociception and muscle plasticity. Nevertheless, the precise SNS-mediated mechanisms governing hyperalgesia and skeletal muscle atrophy in FM remain unclear. Thus, we employed two distinct FM-like pain models, involving intramuscular injections of acidic saline (pH 4.0) or carrageenan in prepubertal female rats, and evaluated the catecholamine content, adrenergic signaling and overall muscle proteolysis. Subsequently, we assessed the contribution of the SNS to the development of hyperalgesia and muscle atrophy in acidic saline-injected rats treated with clenbuterol (a selective β2-adrenergic receptor agonist) and in animals maintained under baseline conditions and subjected to epinephrine depletion through adrenodemedullation (ADM). Seven days after inducing an FM-like model with acidic saline or carrageenan, we observed widespread mechanical hyperalgesia along with loss of strength and/or muscle mass. These changes were associated with reduced catecholamine content, suggesting a common underlying mechanism. Notably, treatment with a β2-agonist alleviated hyperalgesia and prevented muscle atrophy in acidic saline-induced FM-like pain, while epinephrine depletion induced mechanical hyperalgesia and increased muscle proteolysis in animals under baseline conditions. Together, the results suggest that reduced sympathetic activity is involved in the development of pain and muscle atrophy in the murine model of FM analyzed.

The influence of sex on activity in voluntary wheel running, forced treadmill running, and open field testing

Introduction

Physical activity is commonly used for both measuring and treating dysfunction. While preclinical work has been historically biased towards males, the use of both male and female animals is gaining popularity after multiple NIH initiatives. With increasing inclusion of both sexes, it has become imperative to determine sex differences in common behavioral assays. The purpose of this study was to determine baseline sex differences in 3 activity assays: voluntary wheel running, forced treadmill running, and open field testing.

Methods

This was a secondary analysis of sex differences in healthy mice in 3 different assays: Separate mice were used for each assay. Specifically, 16 mice underwent 28 days of voluntary wheel running, 178 mice underwent forced treadmill running, and 88 mice underwent open field testing. Differences between sex across several activity parameters were examined for each assay.

Results

In voluntary wheel running, sex differences with larger effect sizes were observed in distance run, running time, and bout duration, with smaller effect size differences in speed, and no difference in total bouts. In forced treadmill running, differences were shown in time to exhaustion, but no difference in max speed attained. In open field, there were sex differences in active time but not in distance and speed in data aggregated over 30 minutes; however, distance and speed in male mice showed a downward trajectory over the final 20 minutes of testing, whereas females maintained the same trajectory.

Conclusion

These data suggest that male mice demonstrate comparable activity intensity as female mice but do not match female's duration of activity, especially for volitional tasks. Researchers utilizing these assays should account for sex differences as they could potentially mask true findings in an experiment.

Plain English Summary

Physical activity is a common measure to examine function in human subjects with and without disease. Animal models often use measures of physical activity to assess function, yet most of these measures have been done in males only, making interpretation and translation to females and humans difficult. Several measures have been used to measure activity in animals, including those examining voluntary running behavior, maximum capacity, and general activity levels; sex differences between these measures are unclear. We discovered sex differences throughout each of three activity tests. In voluntary running behavior there were large differences between sexes with females running a greater distance and spending more time running. There were small differences in the maximum capacity with females running for a longer period at high intensity. General activity levels showed small differences with females being less active than males. Thus, the greatest differences were found for voluntary running and small differences were found for maximum capacity and general activity levels; differences observed were dependent on the task. Researchers utilizing these assays should account for sex differences as they could potentially mask true findings in an experiment.

Brain-derived neurotrophic factor contributes to activity-induced muscle pain in male but not female mice

Activity-induced muscle pain increases release of interleukin-1β (IL-1β) in muscle macrophages and the development of pain is prevented by blockade of IL-1β. Brain derived neurotrophic factor (BDNF) is released from sensory neurons in response to IL-1β and mediates both inflammatory and neuropathic pain. Thus, we hypothesized that metabolites released during fatiguing muscle contractions activate macrophages to release IL-1β, which subsequently activate sensory neurons to secrete BDNF. To test this hypothesis, we used an animal model of activity-induced pain induced by repeated intramuscular acidic saline injections combined with fatiguing muscle contractions. Intrathecal or intramuscular injection of inhibitors of BDNF-Tropomyosin receptor kinase B (TrkB) signaling, ANA-12 or TrkB-Fc, reduced the decrease in muscle withdrawal thresholds in male, but not in female, mice when given before or 24hr after, but not 1 week after induction of the model. BDNF messenger ribonucleic acid (mRNA) was significantly increased in L4-L6 dorsal root ganglion (DRG), but not the spinal dorsal horn or gastrocnemius muscle, 24hr after induction of the model in either male or female mice. No changes in TrkB mRNA or p75 neurotrophin receptor mRNA were observed. BDNF protein expression via immunohistochemistry was significantly increased in L4-L6 spinal dorsal horn and retrogradely labelled muscle afferent DRG neurons, at 24hr after induction of the model in both sexes. In cultured DRG, fatigue metabolites combined with IL-1β significantly increased BDNF expression in both sexes. In summary, fatigue metabolites release, combined with IL-1β, BDNF from primary DRG neurons and contribute to activity-induced muscle pain only in males, while there were no sex differences in the changes in expression observed in BDNF.

Exercise changes the intrahepatic immune cell profile and inhibits the progression of nonalcoholic steatohepatitis in a mouse model

BACKGROUND

NASH is an increasingly common cause of chronic liver disease and can progress to cirrhosis and HCC. Although exercise suppresses inflammation during acute hepatitis, its impact on the progression of chronic liver disease remains unclear. Here, we investigated the effects of exercise on disease progression and intrahepatic immune cell composition in a mouse model of NASH.

METHOD

Mice were assigned to 4 groups: 2 control groups (normal diet) and 2 NASH groups (western diet and low-dose carbon tetrachloride injection). One of each group remained sedentary and one was exercised on a treadmill for 12 weeks (60 min/d, 5 times/wk). All mice were then analyzed for liver histomorphology, steatosis, inflammation, and fibrosis; liver, adipose tissue, and skeletal muscle expression of genes related to metabolism and inflammation; and intrahepatic immune cell composition.

RESULT

Compared with the normal diet mice, NASH mice exhibited enhanced liver steatosis, inflammation, and fibrosis; upregulated expression of liver lipogenesis-related and inflammation-related genes; and increased frequencies of intrahepatic F4/80 int CD11b hi bone marrow-derived macrophages and programmed death receptor-1 (PD-1) + CD8 + T cells. Expression of inflammatory cytokines and the frequencies of bone marrow-derived macrophages and PD-1 + CD8 + T cells correlated positively with liver steatosis, inflammation, and fibrosis. Exercise was shown to reduce NASH-induced hepatic steatosis, liver inflammation, and fibrosis; induce alterations in metabolism-related genes and inflammatory cytokines in the liver; and suppress accumulation of liver bone marrow-derived macrophages and PD-1 + CD8 + T cells. In addition, we showed that exercise induced increased expression of IL-15 in muscle and its deficiency exacerbated the pathology of NASH.

CONCLUSIONS

Exercise alters the intrahepatic immune cell profile and protects against disease progression in a mouse model of NASH.

The potential protective effects of pre-injury exercise on neuroimmune responses following experimentally-induced traumatic neuropathy: a systematic review with meta-analysis

Pre-clinical evidence shows that neuropathy is associated with complex neuroimmune responses, which in turn are associated with increased intensity and persistence of neuropathic pain. Routine exercise has the potential to mitigate complications of future nerve damage and persistence of pain through neuroimmune regulation. This systematic review aimed to explore the effect of pre-injury exercise on neuroimmune responses, and other physiological and behavioural reactions following peripheral neuropathy in animals. Three electronic databases were searched from inception to July 2022. All controlled animal studies assessing the influence of an active exercise program prior to experimentally-induced traumatic peripheral neuropathy compared to a non-exercise control group on neuroimmune, physiological and behavioural outcomes were selected. The search identified 17,431 records. After screening, 11 articles were included. Meta-analyses showed that pre-injury exercise significantly reduced levels of IL-1β (SMD: -1.06, 95% CI: -1.99 to -0.13, n=40), but not iNOS (SMD: -0.71 95% CI: -1.66 to 0.25, n=82). From 72 comparisons of different neuroimmune outcomes at different anatomical locations, vote counting revealed reductions in 23 pro-inflammatory and increases in 6 anti-inflammatory neuroimmune outcomes. For physiological outcomes, meta-analyses revealed that pre-injury exercise improved one out of six nerve morphometric related outcomes (G-ratio; SMD: 1.95, 95%CI: 0.77 to 3.12, n=20) and one out of two muscle morphometric outcomes (muscle fibre cross-sectional area; SMD: 0.91, 95%CI: 0.27 to 1.54, n=48). For behavioural outcomes, mechanical allodynia was significantly less in the pre-injury exercise group (SMD -1.24, 95%CI: -1.87 to -0.61) whereas no overall effect was seen for sciatic function index. Post hoc subgroup analysis suggests that timing of outcome measurement may influence the effect of pre-injury exercise on mechanical allodynia. Risk of bias was unclear in most studies, as the design and conduct of the included experiments were poorly reported. Preventative exercise may have potential neuroprotective and immunoregulatory effects limiting the sequalae of nerve injury, but more research in this field is urgently needed.

P2X7-NLRP3-Caspase-1 signaling mediates activity-induced muscle pain in male but not female mice

ABSTRACT

We developed an animal model of activity-induced muscle pain that is dependent on local macrophage activation and release of interleukin-1β (IL-1β). Activation of purinergic type 2X (P2X) 7 receptors recruits the NOD-like receptor protein (NLRP) 3 and activates Caspase-1 to release IL-1β. We hypothesized that pharmacological blockade of P2X7, NLRP3, and Caspase-1 would prevent development of activity-induced muscle pain in vivo and release of IL-1β from macrophages in vitro. The decrease in muscle withdrawal thresholds in male, but not female, mice was prevented by the administration of P2X7, NLRP3, and Caspase-1 inhibitors before induction of the model, whereas blockade of IL-1β before induction prevented muscle hyperalgesia in both male and female mice. Blockade of P2X7, NLRP3, Capsase-1, or IL-1β 24 hours, but not 1 week, after induction of the model alleviated muscle hyperalgesia in male, but not female, mice. mRNA expression of P2X7, NLRP3, Caspase-1, and IL-1β from muscle was increased 24 hours after induction of the model in both male and female mice. Using multiplex, increases in IL-1β induced by combining adenosine triphosphate with pH 6.5 in lipopolysaccharide-primed male and female macrophages were significantly lower with the presence of inhibitors of P2X7 (A740003), NLRP3 (MCC950), and Caspase-1 (Z-WEHD-FMK) when compared with the vehicle. The current data suggest the P2X7/NLRP3/Caspase-1 pathway contributed to activity-induced muscle pain initiation and early maintenance phases in male but not female, and not in late maintenance phases in male mice.

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

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

Selective androgen receptor modulator microparticle formulation reverses muscle hyperalgesia in a mouse model of widespread muscle pain

ABSTRACT

Chronic pain is a significant health problem associated with disability and reduced quality of life. Current management of chronic pain is inadequate with only modest effects of pharmacological interventions. Thus, there is a need for the generation of analgesics for treating chronic pain. Although preclinical and clinical studies demonstrate the analgesic effects of testosterone, clinical use of testosterone is limited by adverse androgenic effects. Selective androgen receptor modulators (SARMs) activate androgen receptors and overcome treatment limitations by minimizing androgenic side effects. Thus, we tested whether daily soluble SARMs or a SARM-loaded microparticle formulation alleviated muscle hyperalgesia in a mouse-model of widespread pain (male and female C57BL/6J mice). We tested whether the analgesic effects of the SARM-loaded microparticle formulation was mediated through androgen receptors by blocking androgen receptors with flutamide pellets. In vitro and in vivo release kinetics were determined for SARM-loaded microparticles. Safety and toxicity of SARM treatment was determined using serum cardiac and liver toxicity panels, heart histology, and conditioned place preference testing. Subcutaneous daily SARM administration, and 2 injections, 1 week apart, of SARM-loaded microparticles alleviated muscle hyperalgesia in both sexes and was prevented with flutamide treatment. Sustained release of SARM, from the microparticle formulation, was observed both in vitro and in vivo for 4 weeks. Selective androgen receptor modulator treatment produced no cardiac or liver toxicity and did not produce rewarding behaviors. These studies demonstrate that SARM-loaded microparticles, which release drug for a sustained period, alleviate muscle pain, are safe, and may serve as a potential therapeutic for chronic muscle 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.