ASICs are required for immediate exercise-induced muscle pain and are downregulated in sensory neurons by exercise training

Mechanosensitive channels in the mechanical component of the exercise pressor reflex

The cardiovascular response is appropriately regulated during exercise to meet the metabolic demands of the active muscles. The exercise pressor reflex is a neural feedback mechanism through thin-fiber muscle afferents activated by mechanical and metabolic stimuli in the active skeletal muscles. The mechanical component of this reflex is referred to as skeletal muscle mechanoreflex. Its initial step requires mechanotransduction mediated by mechanosensors, which convert mechanical stimuli into biological signals. Recently, various mechanosensors have been identified, and their contributions to muscle mechanoreflex have been actively investigated. Nevertheless, the mechanosensitive channels responsible for this muscular reflex remain largely unknown. This review discusses progress in our understanding of muscle mechanoreflex under healthy conditions, focusing on mechanosensitive channels.

ASIC3 plays a protective role in delayed-onset muscle soreness (DOMS) through muscle acid sensation during exercise

Immediate exercise-induced pain (IEIP) and DOMS are two types of exercise-induced muscle pain and can act as barriers to exercise. The burning sensation of IEIP occurs during and immediately after intensive exercise, whereas the soreness of DOMS occurs later. Acid-sensing ion channels (ASICs) within muscle afferents are activated by H+ and other chemicals and have been shown to play a role in various chronic muscle pain conditions. Here, we further defined the role of ASICs in IEIP, and also tested if ASIC3 is required for DOMS. After undergoing exhaustive treadmill exercise, exercise-induced muscle pain was assessed in wild-type (WT) and ASIC3-/- mice at baseline via muscle withdrawal threshold (MWT), immediately, and 24 h after exercise. Locomotor movement, grip strength, and repeat exercise performance were tested at baseline and 24 h after exercise to evaluate DOMS. We found that ASIC3-/- had similar baseline muscle pain, locomotor activity, grip strength, and exercise performance as WT mice. WT showed diminished MWT immediately after exercise indicating they developed IEIP, but ASIC3-/- mice did not. At 24 h after baseline exercise, both ASIC3-/- and WT had similarly lower MWT and grip strength, however, ASIC3-/- displayed significantly lower locomotor activity and repeat exercise performance at 24 h time points compared to WT. In addition, ASIC3-/- mice had higher muscle injury as measured by serum lactate dehydrogenase and creatine kinase levels at 24 h after exercise. These results show that ASIC3 is required for IEIP, but not DOMS, and in fact might play a protective role to prevent muscle injury associated with strenuous exercise.

Regulation of pain neurotransmitters and chondrocytes metabolism mediated by voltage-gated ion channels: A narrative review

Osteoarthritis (OA) is one of the leading causes of chronic pain and dysfunction. It is essential to comprehend the nature of pain and cartilage degeneration and its influencing factors on OA treatment. Voltage-gated ion channels (VGICs) are essential in chondrocytes and extracellular matrix (ECM) metabolism and regulate the pain neurotransmitters between the cartilage and the central nervous system. This narrative review focused primarily on the effects of VGICs regulating pain neurotransmitters and chondrocytes metabolism, and most studies have focused on voltage-sensitive calcium channels (VSCCs), voltage-gated sodium channels (VGSCs), acid-sensing ion channels (ASICs), voltage-gated potassium channels (VGKCs), voltage-gated chloride channels (VGCCs). Various ion channels coordinate to maintain the intracellular environment's homeostasis and jointly regulate metabolic and pain under normal circumstances. In the OA model, the ion channel transport of chondrocytes is abnormal, and calcium influx is increased, which leads to increased neuronal excitability. The changes in ion channels are strongly associated with the OA disease process and individual OA risk factors. Future studies should explore how VGICs affect the metabolism of chondrocytes and their surrounding tissues, which will help clinicians and pharmacists to develop more effective targeted drugs to alleviate the progression of OA disease.

The Effectiveness of Rehabilitation Interventions on Pain and Disability for Complex Regional Pain Syndrome: A Systematic Review and Meta-analysis

OBJECTIVES

To summarize and critically appraise the body of evidence on conservative management of complex regional pain syndrome (CRPS), we conducted a systematic review and meta-analysis of randomized controlled trials (RCTs).

METHODS

We conducted a literature search from inception to November 2021 in the following databases: Embase, Medline, CINAHL, Google Scholar, PEDRO, and Psychinfo. Two independent reviewers conducted risk of bias and quality assessment. Qualitative synthesis and meta-analysis were the methods for summarizing the findings of the RCTs. The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach was used to rate the overall quality and certainty of the evidence on each treatment outcome.

RESULT

Through a database search, 751 records were found, and 33 RCTs were eligible for inclusion. Studies were published between 1995 and 2021. The overall risk of bias for 2 studies was low, 8 studies were unclear, and 23 studies were high.Low-quality evidence suggests that mirror therapy (as an addition to conventional stroke rehabilitation interventions) and graded motor imagery program (compared with routine rehabilitation interventions) may result in a large improvement in pain and disability up to 6-month follow-up in poststroke CRPS-1 patients. Low-quality evidence suggests that pain exposure therapy and aerobic exercises as an additive treatment to physical therapy interventions may result in a large improvement in pain up to a 6-month follow-up. The evidence is very uncertain about the effect of all other targeted interventions over conventional physical therapy or sham treatments on pain and disability.

DISCUSSION

There is an ongoing need for high-quality studies to inform conservative management choices in CRPS.

A 10-mg dose of amiloride increases time to failure during blood-flow-restricted plantar flexion in healthy adults without influencing blood pressure

Amiloride has been shown to inhibit acid-sensing ion channels (ASICs), which contribute to ischemia-related muscle pain during exercise. The purpose of this study was to determine if a single oral dose of amiloride would improve exercise tolerance and attenuate blood pressure during blood-flow-restricted (BFR) exercise in healthy adults. Ten subjects (4 females) performed isometric plantar flexion exercise with BFR (30% maximal voluntary contraction) after ingesting either a 10-mg dose of amiloride or a volume-matched placebo (random order). Time to failure, time-tension index (TTI), and perceived pain (visual analog scale) were compared between the amiloride and placebo trials. Mean blood pressure, heart rate, blood pressure index (BPI), and BPI normalized to TTI (BPInorm) were also compared between trials using both time-matched (TM50 and TM100) and effort-matched (T50 and T100) comparisons. Time to failure (+69.4 ± 63.2 s, P < 0.01) and TTI (+1,441 ± 633 kg·s, P = 0.02) were both significantly increased in the amiloride trial compared with placebo, despite no increase in pain (+0.4 ± 1.7 cm, P = 0.46). In contrast, amiloride had no significant influence on the mean blood pressure or heart rate responses, nor were there any significant differences in BPI or BPInorm between trials when matched for time (all P ≥ 0.13). When matched for effort, BPI was significantly greater in the amiloride trial (+5,300 ± 1,798 mmHg·s, P = 0.01), likely owing to an increase in total exercise duration. In conclusion, a 10-mg oral dose of amiloride appears to significantly improve the tolerance to BFR exercise in healthy adults without influencing blood pressure responses.

Translating Outcomes from the Clinical Setting to Preclinical Models: Chronic Pain and Functionality in Chronic Musculoskeletal Pain

Fibromyalgia (FM) is a chronic pain disorder characterized by chronic widespread musculoskeletal pain (CWP), resting pain, movement-evoked pain (MEP), and other somatic symptoms that interfere with daily functioning and quality of life. In clinical studies, this symptomology is assessed, while preclinical models of CWP are limited to nociceptive assays. The aim of the study was to investigate the human-to-model translatability of clinical behavioral assessments for spontaneous (or resting) pain and MEP in a preclinical model of CWP. For preclinical measures, the acidic saline model of FM was used to induce widespread muscle pain in adult female mice. Two intramuscular injections of acidic or neutral pH saline were administered following baseline measures, 5 days apart. An array of adapted evoked and spontaneous pain measures and functional assays were assessed for 3 weeks. A novel paradigm for MEP assessment showed increased spontaneous pain following activity. For clinical measures, resting and movement-evoked pain and function were assessed in adult women with FM. Moreover, we assessed correlations between the preclinical model of CWP and in women with fibromyalgia to examine whether similar relationships between pain assays that comprise resting and MEP existed in both settings. For both preclinical and clinical outcomes, MEP was significantly associated with mechanical pain sensitivity. Preclinically, it is imperative to expand how the field assesses spontaneous pain and MEP when studying multi-symptom disorders like FM. Targeted pain assessments to match those performed clinically is an important aspect of improving preclinical to clinical translatability of animal models.

Effects of Exercise-induced Hypoalgesia and Its Neural Mechanisms

PURPOSE

Exercise-induced hypoalgesia is frequently documented in the literature. However, the underlying neural mechanism of this phenomenon remains unclear. Here, we explored the effects of different intensities of isometric exercise on pain perception with a randomized controlled design and investigated its neural mechanisms through tracing the dynamic changes of heat-evoked brain responses.

METHODS

Forty-eight participants were randomly assigned to one of the three groups with different exercise intensities (i.e., high, low, and control). Their subjective pain reports and brain responses elicited by heat stimuli before and after exercise were assessed.

RESULTS

We observed 1) the increased pressure pain thresholds and heat pain thresholds on the dorsal surface of the hand and the biceps brachii muscle of the exercised limb (closed to the contracting muscle), and the decreased pressure pain ratings at the indexed finger of the unexercised limb; 2) more reduction of pain sensitivity on both the biceps brachii muscle and the dorsal surface of the hand induced by the high-intensity isometric exercise than the low-intensity isometric exercise; and 3) both the high-intensity and the low-intensity isometric exercise induced the reduction of N2 amplitudes and N2-P2 peak-to-peak amplitudes, as well as the reduction of event-related potential magnitudes elicited by the heat stimuli on the exercised limb.

CONCLUSIONS

The hypoalgesic effects induced by the isometric exercise were not only localized to the moving part of the body but also can be extended to the distal part of the body. The exercise intensities play a vital role in modulating these effects. Exercise-induced hypoalgesia could be related to the modulation of nociceptive information transmission via a spinal gating mechanism and also rely on a top-down descending pain inhibitory mechanism.

The physiology and pathophysiology of exercise hyperpnea

In health, the near-eucapnic, highly efficient hyperpnea during mild-to-moderate intensity exercise is driven by three obligatory contributions, namely, feedforward central command from supra-medullary locomotor centers, feedback from limb muscle afferents, and respiratory CO₂ exchange (V̇CO₂). Inhibiting each of these stimuli during exercise elicits a reduction in hyperpnea even in the continuing presence of the other major stimuli. However, the relative contribution of each stimulus to the hyperpnea remains unknown as does the means by which V̇CO2 is sensed. Mediation of the hyperventilatory response to exercise in health is attributed to the multiple feedback and feedforward stimuli resulting from muscle fatigue. In patients with COPD, diaphragm EMG amplitude and its relation to ventilatory output are used to decipher mechanisms underlying the patients' abnormal ventilatory responses, dynamic lung hyperinflation and dyspnea during exercise. Key contributions to these exercise-limiting responses across the spectrum of COPD severity include high dead space ventilation, an excessive neural drive to breathe and highly fatigable limb muscles, together with mechanical constraints on ventilation. Major controversies concerning control of exercise hyperpnea are discussed along with the need for innovative research to uncover the link of metabolism to breathing in health and disease.

Paradoxical Potentiation of Acid-Sensing Ion Channel 3 (ASIC3) by Amiloride via Multiple Mechanisms and Sites Within the Channel

Acid-Sensing Ion Channels (ASICs) are proton-gated sodium-selective cation channels that have emerged as metabolic and pain sensors in peripheral sensory neurons and contribute to neurotransmission in the CNS. These channels and their related degenerin/epithelial sodium channel (DEG/ENaC) family are often characterized by their sensitivity to amiloride inhibition. However, amiloride can also cause paradoxical potentiation of ASIC currents under certain conditions. Here we characterized and investigated the determinants of paradoxical potentiation by amiloride on ASIC3 channels. While inhibiting currents induced by acidic pH, amiloride potentiated sustained currents at neutral pH activation. These effects were accompanied by alterations in gating properties including (1) an alkaline shift of pH-dependent activation, (2) inhibition of pH-dependent steady-state desensitization (SSD), (3) prolongation of desensitization kinetics, and (4) speeding of recovery from desensitization. Interestingly, extracellular Ca²⁺ was required for paradoxical potentiation and it diminishes the amiloride-induced inhibition of SSD. Site-directed mutagenesis within the extracellular non-proton ligand-sensing domain (E79A, E423A) demonstrated that these residues were critical in mediating the amiloride-induced inhibition of SSD. However, disruption of the purported amiloride binding site (G445C) within the channel pore blunted both the inhibition and potentiation of amiloride. Together, our results suggest that the myriad of modulatory and blocking effects of amiloride are the result of a complex competitive interaction between amiloride, Ca²⁺, and protons at probably more than one site in the channel.

Effects of Brief Dry Cupping on Muscle Soreness in the Gastrocnemius Muscle and Flexibility of the Ankle

OBJECTIVES The purpose of this study was to analyze the effects of brief dry cupping on muscle soreness of the gastrocnemius muscle and range of motion (ROM) in the ankle.METHODS Thirty-six participants (age=29±10 yrs, ht=173.8±10.3 cm, wt=75.3±15.9 kg) were randomly assigned to three groups: A (no exercise), B (bilateral heel drops to exhaustion), and C (right unilateral heel drops to exhaustion). Dorsiflexion ROM was assessed bilaterally for all groups. All participants received the cupping protocol on the right gastrocnemius, but C also received it on the left. Dry cupping was applied using four two-inch cups in four quadrants on the calf for 90 seconds. Soreness was reported using a 10-point VAS scale at 24- and 48-hours. Repeated measures ANOVAs were used to examine the effects of cupping on soreness and range of motion, respectively. Alpha was set at 0.05 for all tests.RESULTS A significant difference in muscle soreness existed between right (3.58±1.31 VAS) and left (2.83±1.02 VAS) calves for group B at 24 hours (p = 0.029). No other significant differences existed between calves that were and were not cupped (p > 0.05). No significant differences in ROM occurred between any conditions (p > 0.05).CONCLUSION Ninety seconds of dry cupping on the calf may reduce 24-hr muscle soreness after performing heel drops to exhaustion, but has no effect on ROM.

Comparison of murine behavioural and physiological responses after forced exercise by electrical shock versus manual prodding

NEW FINDINGS

What is the central question of this study? Forced treadmill exercise using electrical shock is the most common technique in rodent exercise studies. Here, we examined how the use of electrical shock during forced treadmill exercise affects behavioural and physiological responses in comparison to a novel non-electrical shock technique. What is the main finding and its importance? In comparison to mice that underwent traditional treadmill running induced by electrical shock, mice that underwent forced running using a novel technique involving gentle prodding to induce running showed: (i) higher locomotor activity; (ii) less anxiety-like behaviour; and (iii) altered exercise-induced muscle pain immediately after exercise.

ABSTRACT

Animal models of exercise have been useful to understand underlying cellular and molecular mechanisms. Many studies have used methods of exercise that are unduly stressful (e.g., electrical shock to force running), potentially skewing results. Here, we compared physiological and behavioural responses of mice after exercise induced using a prodding technique that avoids electrical shock versus a traditional protocol using electrical shock. We found that exercise performance was similar for both techniques; however, the shock group demonstrated significantly lower locomotor activity and higher anxiety-like behaviour. We also observed divergent effects on muscle pain; the prodding group showed hyperalgesia immediately after exercise, whereas the shock group showed hypoalgesia. Corticosterone concentrations were elevated to a similar extent for both groups. In conclusion, mice that were exercised without shock generated similar maximal exercise performance, but postexercise these mice showed an increase in locomotor activity, less anxiety-like behaviour and altered muscle pain in comparison to mice that exercised with shock. Our data suggest that running of mice without the use of electrical shock is potentially less stressful and might be a better technique to study the physiological and behavioural responses to exercise.