Augmented mechanical response of muscle thin-fiber sensory receptors recorded from rat muscle-nerve preparations in vitro after eccentric contraction

Evaluation of curcumin intake in reducing exercise-induced muscle damage in athletes: a systematic review

BACKGROUND

Sports practice, particularly eccentric exercises, induces significant muscular changes, including muscle fiber injuries, strength loss, pain, and increased permeability of the muscle membrane. The duration of muscle recovery depends on factors such as exercise intensity and the specific muscle groups engaged. The inflammatory response plays a crucial role in muscle regeneration, involving various cell types. Curcumin, especially when its stability is enhanced through encapsulation, exhibits potent antioxidant and anti-inflammatory properties. Supplementing with curcumin can reduce muscle damage and inflammation caused by eccentric exercise, making it a potential remedy for athletes.

OBJECTIVE

The objective of this systematic review is to assess the scientific evidence supporting the efficacy of curcumin in reducing muscle damage caused by sports.

METHODS

A structured search in SCOPUS, Medline, and Web of Science databases was conducted in March 2023, including all available articles. The strategy involved selecting English articles without time constraints, using the search terms "curcumin" AND "Exercise-Induced Muscle Damage" (ALL(curcumin AND "Exercise-Induced Muscle Damage")). Titles and abstracts were screened to assess eligibility. Studies were chosen based on PICOS criteria, and quality was evaluated using the reliable PEDro scale. The eligibility criteria included adults without any diagnosed diseases who regularly exercise (at least three times per week) and follow a consistent pattern of curcumin intake before, during, or after exercise.

RESULTS

The comprehensive search identified 11 relevant studies investigating the effects of curcumin supplementation in sport-simulated interventions. These studies suggest that curcumin intake may help reduce muscle symptoms associated with eccentric exercises, thereby improving pain perception. Effective use of curcumin depends on factors such as dosage, bioavailability, and timing, with post-exercise ingestion appearing to be more beneficial.

CONCLUSIONS

Curcumin demonstrates a significant potential to relieve muscle-related symptoms, especially delayed-onset muscle soreness (DOMS) that arises from eccentric exercises, thus potentially improving the well-being of those who are trained. It also appears to have the capability to lower biomarkers associated with inflammation and boost antioxidant levels. Nevertheless, for future studies, the bioavailability of curcumin must be considered, as it is a key factor in its efficacy.

Mechanical and thermal stimulation for studying the somatosensory system: a review on devices and methods

The somatosensory system is widely studied to understand its functioning mechanisms. Multiple tests, based on different devices and methods, have been performed not only on humans but also on animals andex-vivomodels. Depending on the nature of the sample under analysis and on the scientific aims of interest, several solutions for experimental stimulation and for investigations on sensation or pain have been adopted. In this review paper, an overview of the available devices and methods has been reported, also analyzing the representative values adopted during literature experiments. Among the various physical stimulations used to study the somatosensory system, we focused only on mechanical and thermal ones. Based on the analysis of their main features and on literature studies, we pointed out the most suitable solution for humans, rodents, andex-vivomodels and investigation aims (sensation and pain).

Eccentric Exercise-Induced Muscle Damage Reduces Gross Efficiency

INTRODUCTION

The effect of eccentric exercise-induced muscle damage (EIMD) on cycling efficiency is unknown. The aim of the present study was to assess the effect of EIMD on gross and delta efficiency and the cardiopulmonary responses to cycle ergometry.

METHODS

Twenty-one recreational athletes performed cycling at 70%, 90%, and 110% of the gas exchange threshold (GET) under control conditions (Control) and 24 h following an eccentric damaging protocol (Damage). Knee extensor isometric maximal voluntary contraction, potentiated twitch ( Qtw,pot ), and voluntary activation were assessed before Control and Damage. Gross and delta efficiency were assessed using indirect calorimetry, and cardiopulmonary responses were measured at each power output. Electromyography root-mean-square (EMG RMS ) during cycling was also determined.

RESULTS

Maximal voluntary contraction was 25% ± 18% lower for Damage than Control ( P < 0.001). Gross efficiency was lower for Damage than Control ( P < 0.001) by 0.55% ± 0.79%, 0.59% ± 0.73%, and 0.60% ± 0.87% for 70%, 90%, and 110% GET, respectively. Delta efficiency was unchanged between conditions ( P = 0.513). Concurrently, cycling EMG RMS was higher for Damage than Control ( P = 0.004). An intensity-dependent increase in breath frequency and V̇ E /V̇CO 2 was found, which were higher for Damage only at 110% GET ( P ≤ 0.019).

CONCLUSIONS

Thus, gross efficiency is reduced following EIMD. The concurrently higher EMG RMS suggests that increases in muscle activation in the presence of EIMD might have contributed to reduced gross efficiency. The lack of change in delta efficiency might relate to its poor reliability hindering the ability to detect change. The findings also show that EIMD-associated hyperventilation is dependent on exercise intensity, which might relate to increases in central command with EIMD.

Delayed-Onset Muscle Soreness Alters Mechanical Sensitivity, but Not Thermal Sensitivity or Pain Modulatory Function

Introduction

Many clinical musculoskeletal pain conditions are characterized by chronic inflammation that sensitizes nociceptors. An unresolved issue is whether inflammation affects all nociceptors in a similar manner. Exercise-induced muscle damage (EIMD) has been proposed as a model for simulating clinical inflammatory pain in healthy samples. We sought to test the effect of EIMD on various painful stimuli (pressure and thermal), central pain processing (via the nociceptive flexion reflex) and endogenous pain modulation via conditioned pain modulation and exercise-induced hypoalgesia.

Methods

Eighteen participants (9F, age: 24.6 ± 3.3) were recruited for repeated measures testing and each completed pain sensitivity testing prior to and 48 hours after an eccentric exercise protocol. The participants performed a minimum of 6 rounds of 10 eccentric knee extension exercises to induce muscle damage and localized inflammation in the right quadriceps. Force decrements, knee range-of-motion, and delayed onset muscle soreness (DOMS) were used to quantify EIMD.

Results

There was a significant main effect of time for pressure pain (%diff; -58.9 ± 23.1; p = 0.02, ηp2 = 0.28) but no significant main effect was observed for limb (%diff; -15.5 ± 23.9; p = 0.53, ηp2 = 0.02). In contrast, there was a significant interaction between time and limb (p < 0.001, ηp2 = 0.47) whereby participants had lower pressure pain sensitivity in the right leg only after the damage protocol (%diff; -105.9 ± 29.2; p = 0.002).

Discussion

Individuals with chronic inflammatory pain usually have an increased sensitivity to pressure, thermal, and electrical stimuli, however, our sample, following muscle damage to induce acute inflammation only had sensitivity to mechanical pain. Exercise induced inflammation may reflect a peripheral sensitivity localized to the damaged muscle rather than a global sensitivity like those with chronic pain display.

Neurochemical mechanism of muscular pain: Insight from the study on delayed onset muscle soreness

We reviewed fundamental studies on muscular pain, encompassing the characteristics of primary afferent fibers and neurons, spinal and thalamic projections, several muscular pain models, and possible neurochemical mechanisms of muscle pain. Most parts of this review were based on data obtained from animal experiments, and some researches on humans were also introduced. We focused on delayed-onset muscle soreness (DOMS) induced by lengthening contractions (LC), suitable for studying myofascial pain syndromes. The muscular mechanical withdrawal threshold (MMWT) decreased 1-3 days after LC in rats. Changing the speed and range of stretching showed that muscle injury seldom occurred, except in extreme conditions, and that DOMS occurred in parameters without muscle damage. The B2 bradykinin receptor-nerve growth factor (NGF) route and COX-2-glial cell line-derived neurotrophic factor (GDNF) route were involved in the development of DOMS. The interactions between these routes occurred at two levels. A repeated-bout effect was observed in MMWT and NGF upregulation, and this study showed that adaptation possibly occurred before B2 bradykinin receptor activation. We have also briefly discussed the prevention and treatment of DOMS.

Blood Pressure Responses to Postexercise Circulatory Occlusion Are Attenuated After Exercise-Induced Muscle Weakness

PURPOSE

Exercise blood pressure (BP) responses are thought to be determined by relative exercise intensity (percent maximal voluntary contraction (MVC) strength). However, cross-sectional studies report that during a static contraction, higher absolute force is associated with greater BP responses to relative intensity exercise and subsequent muscle metaboreflex activation with postexercise circulatory occlusion (PECO). We hypothesized that a bout of unaccustomed eccentric exercise would reduce knee extensor MVC and subsequently attenuate BP responses to PECO.

METHODS

Continuous BP, heart rate, muscle oxygenation, and knee extensor electromyography were recorded in 21 young healthy individuals (female, n = 10) during 2 min of 20% MVC static knee extension exercise and 2 min of PECO, performed before and 24 h after 300 maximal knee extensor eccentric contractions to cause exercise-induced muscle weakness. As a control, 14 participants repeated the eccentric exercise 4 wks later to test whether BP responses were altered when exercise-induced muscle weakness was attenuated via the protective effects of the repeated bout effect.

RESULTS

Eccentric exercise reduced MVC in all participants (144 ± 43 vs 110 ± 34 N·m, P < 0.0001). BP responses to matched relative intensity static exercise (lower absolute force) were unchanged after eccentric exercise ( P > 0.99) but were attenuated during PECO (systolic BP: 18 ± 10 vs 12 ± 9 mm Hg, P = 0.02). Exercise-induced muscle weakness modulated the deoxygenated hemoglobin response to static exercise (64% ± 22% vs 46% ± 22%, P = 0.04). When repeated after 4 wks, exercise-induced weakness after eccentric exercise was attenuated (-21.6% ± 14.3% vs -9.3 ± 9.7, P = 0.0002) and BP responses to PECO were not different from control values (all, P > 0.96).

CONCLUSIONS

BP responses to muscle metaboreflex activation, but not exercise, are attenuated by exercise-induced muscle weakness, indicating a contribution of absolute exercise intensity on muscle metaboreflex activation.

Vacuolar-ATPase-mediated muscle acidification caused muscular mechanical nociceptive hypersensitivity after chronic stress in rats, which involved extracellular matrix proteoglycan and ASIC3

Although widespread pain, such as fibromyalgia, is considered to have a central cause, peripheral input is important. We used a rat repeated cold stress (RCS) model with many characteristics common to fibromyalgia and studied the possible involvement of decreased muscle pH in muscle mechanical hyperalgesia. After a 5-day RCS, the muscle pH and the muscular mechanical withdrawal threshold (MMWT) decreased significantly. Subcutaneously injected specific inhibitor of vacuolar ATPase (V-ATPase), bafilomycin A1, reversed both changes almost completely. It also reversed the increased mechanical response of muscle thin-fibre afferents after RCS. These results show that V-ATPase activation caused muscle pH drop, which led to mechanical hypersensitivity after RCS. Since extracellular matrix proteoglycan and acid sensitive ion channels (TRPV1 and ASIC3) have been considered as possible mechanisms for sensitizing/activating nociceptors by protons, we investigated their involvement. Manipulating the extracellular matrix proteoglycan with chondroitin sulfate and chondroitinase ABC reversed the MMWT decrease after RCS, supporting the involvement of the extracellular mechanism. Inhibiting ASIC3, but not TRPV1, reversed the decreased MMWT after RCS, and ASIC3 mRNA and protein in the dorsal root ganglia were upregulated, indicating ASIC3 involvement. These findings suggest that extracellular mechanism and ASIC3 play essential roles in proton-induced mechanical hyperalgesia after RCS.

Neural discharge of muscle afferents and pressor response to mechanical stimulation are associated with muscle deformation velocity in rats

Skeletal muscle reflexes play a crucial role in determining the magnitude of the cardiovascular response to exercise. However, evidence supporting an association between the magnitude of the pressor response and the velocity of muscle deformation has remained to be elucidated. Thus, we investigated the impact of different muscle deformation rates on the neural discharge of muscle afferents and pressor and sympathetic responses in Sprague-Dawley rats. In an ex vivo muscle-nerve preparation, action potentials elicited by sinusoidal mechanical stimuli (137 mN) at different frequencies (0.01, 0.05, 0.1, 0.2, and 0.25 Hz) were recorded in mechanosensitive group III and IV fibers. The afferent response magnitude to sine-wave stimulation significantly varied at different frequencies (ANOVA, P = 0.01). Specifically, as compared with 0.01 Hz (0.83 ± 0.96 spikes/s), the response magnitudes were significantly greater at 0.20 Hz (4.07 ± 5.04 spikes/s, P = 0.031) and 0.25 Hz (4.91 ± 5.30 spikes/s, P = 0.014). In an in vivo decerebrated rat preparation, renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) responses to passive stretch (1 kg) of hindlimb skeletal muscle at different velocities of loading (slow, medium, and fast) were measured. Pressor responses to passive stretch were significantly associated with the velocity of muscle deformation (ANOVA, P < 0.001). The MAP response to fast stretch (Δ 56 ± 12 mmHg) was greater than slow (Δ 33 ± 11 mmHg, P = 0.006) or medium (Δ 30 ± 11 mmHg, P < 0.001) stretch. Likewise, the RSNA response was related to deformation velocity (ANOVA, P = 0.024). These findings suggest that the muscle neural afferent discharge and the cardiovascular response to mechanical stimulation are associated with muscle deformation velocity.

Antagonism of TRPV4 channels partially reduces mechanotransduction in rat skeletal muscle afferents

Mechanical distortion of working skeletal muscle induces sympathoexcitation via thin fibre afferents, a reflex response known as the skeletal muscle mechanoreflex. However, to date, the receptor ion channels responsible for mechanotransduction in skeletal muscle remain largely undetermined. Transient receptor potential vanilloid 4 (TRPV4) is known to sense mechanical stimuli such as shear stress or osmotic pressure in various organs. It is hypothesized that TRPV4 in thin-fibre primary afferents innervating skeletal muscle is involved in mechanotransduction. Fluorescence immunostaining revealed that 20.1 ± 10.1% of TRPV4 positive neurons were small dorsal root ganglion (DRG) neurons that were DiI-labelled, and among them 9.5 ± 6.1% of TRPV4 co-localized with the C-fibre marker peripherin. In vitro whole-cell patch clamp recordings from cultured rat DRG neurons demonstrated that mechanically activated current amplitude was significantly attenuated after the application of the TRPV4 antagonist HC067047 compared to control (P = 0.004). Such reductions were also observed in single-fibre recordings from a muscle-nerve ex vivo preparation where HC067047 significantly decreased afferent discharge to mechanical stimulation (P = 0.007). Likewise, in an in vivo decerebrate rat preparation, the renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) responses to passive stretch of hindlimb muscle were significantly reduced by intra-arterial injection of HC067047 (ΔRSNA: P = 0.019, ΔMAP: P = 0.002). The findings suggest that TRPV4 plays an important role in mechanotransduction contributing to the cardiovascular responses evoked by the skeletal muscle mechanoreflex during exercise. KEY POINTS: Although a mechanical stimulus to skeletal muscle reflexively activates the sympathetic nervous system, the receptors responsible for mechanotransduction in skeletal muscle thin fibre afferents have not been fully identified. Evidence suggests that TRPV4 is a mechanosensitive channel that plays an important role in mechanotransduction within various organs. Immunocytochemical staining demonstrates that TRPV4 is expressed in group IV skeletal muscle afferents. In addition, we show that the TRPV4 antagonist HC067047 decreases the responsiveness of thin fibre afferents to mechanical stimulation at the muscle tissue level as well as at the level of dorsal root ganglion neurons. Moreover, we demonstrate that intra-arterial HC067047 injection attenuates the sympathetic and pressor responses to passive muscle stretch in decerebrate rats. These data suggest that antagonism of TRPV4 attenuates mechanotransduction in skeletal muscle afferents. The present study demonstrates a probable physiological role for TRPV4 in the regulation of mechanical sensation in somatosensory thin fibre muscle afferents.

Insulin potentiates the response to capsaicin in dorsal root ganglion neurons in vitro and muscle afferents ex vivo in normal healthy rodents

Systemic insulin administration evokes sympathoexcitatory actions, but the mechanisms underlying these observations are unknown. We reported that insulin sensitizes the response of thin-fibre primary afferents, as well as the dorsal root ganglion (DRG) that subserves them, to mechanical stimuli. However, little is known about the effects of insulin on primary neuronal responses to chemical stimuli. TRPV1, whose agonist is capsaicin (CAP), is widely expressed on chemically sensitive metaboreceptors and/or nociceptors. The aim of this investigation was to determine the effects of insulin on CAP-activated currents in small DRG neurons and CAP-induced action potentials in thin-fibre muscle afferents of normal healthy rodents. Additionally, we investigated whether insulin potentiates sympathetic nerve activity (SNA) responses to CAP. In whole-cell patch-clamp recordings from cultured mice DRG neurons in vitro, the fold change in CAP-activated current from pre- to post-application of insulin (n = 13) was significantly (P < 0.05) higher than with a vehicle control (n = 14). Similar results were observed in single-fibre recording experiments ex vivo as insulin potentiated CAP-induced action potentials compared to vehicle controls (n = 9 per group, P < 0.05). Furthermore, insulin receptor blockade with GSK1838705 significantly suppressed the insulin-induced augmentation in CAP-activated currents (n = 13) as well as the response magnitude of CAP-induced action potentials (n = 9). Likewise, the renal SNA response to CAP after intramuscular injection of insulin (n = 8) was significantly (P < 0.05) greater compared to vehicle (n = 9). The findings suggest that insulin potentiates TRPV1 responsiveness to CAP at the DRG and muscle tissue levels, possibly contributing to the augmentation in sympathoexcitation during activities such as physical exercise. KEY POINTS: Evidence suggests insulin centrally activates the sympathetic nervous system, and a chemical stimulus to tissues activates the sympathetic nervous system via thin fibre muscle afferents. Insulin is reported to modulate putative chemical-sensitive channels in the dorsal root ganglion neurons of these afferents. In the present study, it is demonstrated that insulin potentiates the responsiveness of thin fibre afferents to capsaicin at muscle tissue levels as well as at the level of dorsal root ganglion neurons. In addition, it is demonstrated that insulin augments the sympathetic nerve activity response to capsaicin in vivo. These data suggest that sympathoexcitation is peripherally mediated via insulin-induced chemical sensitization. The present study proposes a possible physiological role of insulin in the regulation of chemical sensitivity in somatosensory thin fibre muscle afferents.

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

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

TRPV1 (Transient Receptor Potential Vanilloid 1) Sensitization of Skeletal Muscle Afferents in Type 2 Diabetic Rats With Hyperglycemia

[Figure: see text].

Mechanism of exercise-induced analgesia: what we can learn from physically active animals

Physical activity has become a first-line treatment in rehabilitation settings for individuals with chronic pain. However, research has only recently begun to elucidate the mechanisms of exercise-induced analgesia. Through the study of animal models, exercise has been shown to induce changes in the brain, spinal cord, immune system, and at the site of injury to prevent and reduce pain. Animal models have also explored beneficial effects of exercise through different modes of exercise including running, swimming, and resistance training. This review will discuss the central and peripheral mechanisms of exercise-induced analgesia through different modes, intensity, and duration of exercise as well as clinical applications of exercise with suggestions for future research directions.

Skeletal Muscle Reflex-Induced Sympathetic Dysregulation and Sensitization of Muscle Afferents in Type 1 Diabetic Rats

The blood pressure response to exercise is exaggerated in the type 1 diabetes mellitus (T1DM). An overactive exercise pressor reflex (EPR) contributes to the potentiated pressor response. However, the mechanism(s) underlying this abnormal EPR activity remains unclear. This study tested the hypothesis that the heightened blood pressure response to exercise in T1DM is mediated by EPR-induced sympathetic overactivity. Additionally, the study examined whether the single muscle afferents are sensitized by PKC (protein kinase C) activation in this disease. Sprague-Dawley rats were intraperitoneally administered either 50 mg/kg streptozotocin (T1DM) or saline (control). At 1 to 3 weeks after administration, renal sympathetic nerve activity and mean arterial pressure responses to activation of the EPR, mechanoreflex, and metaboreflex were measured in decerebrate animals. Action potential responses to mechanical and chemical stimulation were determined in group IV afferents with pPKCα (phosphorylated-PKCα) levels assessed in dorsal root ganglia. Compared with control, EPR (58±18 versus 96±33%; P<0.05), mechanoreflex (21±13 versus 51±20%; P<0.05), and metaboreflex (40±20 versus 88±39%; P<0.01) activation in T1DM rats evoked significant increases in renal sympathetic nerve activity as well as mean arterial pressure. The response of group IV afferents to mechanical (18±24 versus 61±45 spikes; P<0.01) and chemical (0.3±0.4 versus 1.6±0.8 Hz; P<0.01) stimuli were significantly greater in T1DM than control. T1DM rats showed markedly increased pPKCα levels in dorsal root ganglia compared with control. These data suggest that in T1DM, abnormally muscle reflex-evoked increases in sympathetic activity mediate exaggerations in blood pressure. Further, sensitization of muscle afferents, potentially via PKC activation, may contribute to this abnormal circulatory responsiveness.

Peripheral nociceptive mechanisms in an experimental rat model of fibromyalgia induced by repeated cold stress

Fibromyalgia (FM) is a debilitating disease characterized by generalized and persistent musculoskeletal pain. Although central mechanisms are strongly implicated in the pathogenesis of FM, the involvement of peripheral mechanisms is poorly understood. To understand the peripheral nociceptive mechanisms, we examined muscular nociceptors in an FM model, which was made by exposing rats to repeated cold stress (RCS). A single muscle C-fiber nociceptors were identified through the teased fiber technique using ex vivo muscle-nerve preparations. Response properties of C-fibers to noxious stimuli were systematically analyzed. Messenger RNA expression of neurotrophic factors and inflammatory mediators were also studied in the muscle. In the RCS group, the mechanical response threshold of C-fibers, measured using a ramp mechanical stimulus, was significantly decreased, and the response magnitude was significantly increased in the RCS group when compared with the SHAM group, where the environmental temperature was not altered. The general characteristics of C-fibers and the responsiveness to noxious cold and heat stimuli were similar between the two groups. Messenger RNAs of neurotrophic factors and inflammatory mediators were not changed in the muscle during and after RCS. These results suggest that augmentation of the mechanical response of muscle C-fiber nociceptors contributes to hyperalgesia in the RCS model.

Insulin potentiates the response to mechanical stimuli in small dorsal root ganglion neurons and thin fibre muscle afferents in vitro

KEY POINTS

Insulin is known to activate the sympathetic nervous system centrally. A mechanical stimulus to tissues activates the sympathetic nervous system via thin fibre afferents. Evidence suggests that insulin modulates putative mechanosensitive channels in the dorsal root ganglion neurons of these afferents. In the present study, we report the novel finding that insulin augments the mechanical responsiveness of thin fibre afferents not only at dorsal root ganglion, but also at muscle tissue levels. Our data suggest that sympathoexcitation is mediated via the insulin-induced mechanical sensitization peripherally. The present study proposes a novel physiological role of insulin in the regulation of mechanical sensitivity in somatosensory thin fibre afferents.

ABSTRACT

Insulin activates the sympathetic nervous system, although the mechanism underlying insulin-induced sympathoexcitation remains to be determined. A mechanical stimulus to tissues such as skin and/or skeletal muscle, no matter whether the stimulation is noxious or not, activates the sympathetic nervous system via thin fibre afferents. Evidence suggests that insulin modulates putative mechanosensitive channels in the dorsal root ganglion (DRG) neurons of these afferents. Accordingly, we investigated whether insulin augments whole-cell current responses to mechanical stimuli in small DRG neurons of normal healthy mice. We performed whole-cell patch clamp recordings using cultured DRG neurons and observed mechanically-activated (MA) currents induced by mechanical stimuli applied to the cell surface. Local application of vehicle solution did not change MA currents or mechanical threshold in cultured DRG neurons. Insulin (500 mU mL^-1 ) significantly augmented the amplitude of MA currents (P < 0.05) and decreased the mechanical threshold (P < 0.05). Importantly, pretreatment with the insulin receptor antagonist, GSK1838705, significantly suppressed the insulin-induced potentiation of the mechanical response. We further examined the impact of insulin on thin fibre muscle afferent activity in response to mechanical stimuli in normal healthy rats in vitro. Using a muscle-nerve preparation, we recorded single group IV fibre activity to a ramp-shaped mechanical stimulation. Insulin significantly decreased mechanical threshold (P < 0.05), although it did not significantly increase the response magnitude to the mechanical stimulus. In conclusion, these data suggest that insulin augments the mechanical responsiveness of small DRG neurons and potentially sensitizes group IV afferents to mechanical stimuli at the muscle tissue level, possibly contributing to insulin-induced sympathoexcitation.

Thin-fibre receptors expressing acid-sensing ion channel 3 contribute to muscular mechanical hypersensitivity after exercise

BACKGROUND

Delayed onset muscle soreness (DOMS) is characterized by mechanical hyperalgesia after lengthening contractions (LC). It is relatively common and causes disturbance for many people who require continuous exercise, yet its molecular and peripheral neural mechanisms are poorly understood.

METHODS

We examined whether muscular myelinated Aδ-fibres, in addition to unmyelinated C-fibres, are involved in LC-induced mechanical hypersensitivity, and whether acid-sensing ion channel (ASIC)-3 expressed in thin-fibre afferents contributes to this type of pain using a rat model of DOMS. The peripheral contribution of ASIC3 was investigated using single-fibre electrophysiological recordings in extensor digitorum longus muscle-peroneal nerve preparations in vitro.

RESULTS

Behavioural tests demonstrated a significant decrease of the muscular mechanical withdrawal threshold following LC to ankle extensor muscles, and it was improved by intramuscular injection of APETx2 (2.2 μM), a selective blocker of ASIC3. The lower concentration of APETx2 (0.22 µM) and its vehicle had no effect on the threshold. Intramuscular injection of APETx2 (2.2 μM) in naïve rats without LC did not affect the withdrawal threshold. In the ankle extensor muscles that underwent LC one day before the electrophysiological recordings, the mechanical response of Aδ- and C-fibres was significantly facilitated (i.e. decreased response threshold and increased magnitude of the response). The facilitated mechanical response of the Aδ- and C-fibres was significantly suppressed by selective blockade of ASIC3 with APETx2, but not by its vehicle.

CONCLUSIONS

These results clearly indicate that ASIC3 contributes to the augmented mechanical response of muscle thin-fibre receptors in delayed onset muscular mechanical hypersensitivity after LC.

SIGNIFICANCE

Here, we show that not only C- but also Aδ-fibre nociceptors in the muscle are involved in mechanical hypersensitivity after lengthening contractions, and that acid-sensing ion channel (ASIC)-3 expressed in the thin-fibre nociceptors is responsible for the mechanical hypersensitivity. ASIC3 might be a novel pharmacological target for pain after exercise.

Chondroitin sulfate attenuates acid-induced augmentation of the mechanical response in rat thin-fiber muscle afferents in vitro

Exercise-induced tissue acidosis augments the exercise pressor reflex (EPR). One reason for this may be acid-induced mechanical sensitization in thin-fiber muscle afferents, which is presumably related to EPR. Acid-induced sensitization to mechanical stimulation has been reported to be attenuated in cultured primary-sensory neurons by exogenous chondroitin sulfate (CS) and chondroitinase ABC, suggesting that the extracellular matrix CS proteoglycan is involved in this sensitization. The purpose of this study was to clarify whether acid-induced sensitization of the mechanical response in the thin-fiber muscle afferents is also suppressed by exogenous CS and chondroitinase ABC using a single-fiber recording technique. A total of 88 thin fibers (conduction velocity <15.0 m/s) dissected from 86 male Sprague-Dawley rats were identified. A buffer solution at pH 6.2 lowered their mechanical threshold and increased their response magnitude. Five minutes after CS (0.3 and 0.03%) injection near the receptive field, these acid-induced changes were significantly reduced. No significant difference in attenuation was detected between the two CS concentrations. Chondroitinase ABC also significantly attenuated this sensitization. The control solution (0% CS) did not significantly alter the mechanical sensitization. Furthermore, no significant differences were detected in this sensitization and CS-based suppression between fibers with and without acid-sensitive channels [transient receptor potential vanilloid 1 (TRPV1), acid-sensing ion channel (ASIC)]. In addition, this mechanical sensitization was not changed by TRPV1 and ASIC antagonists, suggesting that these ion channels are not involved in the acid-induced mechanical sensitization of muscle thin-fiber afferents. In conclusion, CS administration has a potential to attenuate the acidosis-induced exaggeration of muscle mechanoreflex. NEW & NOTEWORTHY We found that exogenous chondroitin sulfate attenuated acid-induced mechanical sensitization in thin-fiber muscle afferents that play a crucial role in the exercise pressor reflex. This finding suggests that extracellular matrix chondroitin sulfate proteoglycans may be involved in the mechanism of acid-induced mechanical sensitization and that daily intake of chondroitin sulfate may potentially attenuate this amplification of muscle mechanoreflex and therefore reduce muscle pain related to acidic muscle conditions.

Peripheral Mechanisms of Ischemic Myalgia

Musculoskeletal pain due to ischemia is present in a variety of clinical conditions including peripheral vascular disease (PVD), sickle cell disease (SCD), complex regional pain syndrome (CRPS), and even fibromyalgia (FM). The clinical features associated with deep tissue ischemia are unique because although the subjective description of pain is common to other forms of myalgia, patients with ischemic muscle pain often respond poorly to conventional analgesic therapies. Moreover, these patients also display increased cardiovascular responses to muscle contraction, which often leads to exercise intolerance or exacerbation of underlying cardiovascular conditions. This suggests that the mechanisms of myalgia development and the role of altered cardiovascular function under conditions of ischemia may be distinct compared to other injuries/diseases of the muscles. It is widely accepted that group III and IV muscle afferents play an important role in the development of pain due to ischemia. These same muscle afferents also form the sensory component of the exercise pressor reflex (EPR), which is the increase in heart rate and blood pressure (BP) experienced after muscle contraction. Studies suggest that afferent sensitization after ischemia depends on interactions between purinergic (P2X and P2Y) receptors, transient receptor potential (TRP) channels, and acid sensing ion channels (ASICs) in individual populations of peripheral sensory neurons. Specific alterations in primary afferent function through these receptor mechanisms correlate with increased pain related behaviors and altered EPRs. Recent evidence suggests that factors within the muscles during ischemic conditions including upregulation of growth factors and cytokines, and microvascular changes may be linked to the overexpression of these different receptor molecules in the dorsal root ganglia (DRG) that in turn modulate pain and sympathetic reflexes. In this review article, we will discuss the peripheral mechanisms involved in the development of ischemic myalgia and the role that primary sensory neurons play in EPR modulation.

The pain threshold of high-threshold mechanosensitive receptors subsequent to maximal eccentric exercise is a potential marker in the prediction of DOMS associated impairment

Background

Delayed-onset muscle soreness (DOMS) refers to dull pain and discomfort in people after participating in exercise, sport or recreational physical activities. The aim of this study was to detect underlying mechanical thresholds in an experimental model of DOMS.

Methods

Randomised study to detect mechanical pain thresholds in a randomised order following experimentally induced DOMS of the non-dominant arm in healthy participants. Main outcome was the detection of the pressure pain threshold (PPT), secondary thresholds included mechanical detection (MDT) and pain thresholds (MPT), pain intensity, pain perceptions and the maximum isometric voluntary force (MIVF).

Results

Twenty volunteers (9 female and 11 male, age 25.2 ± 3.2 years, weight 70.5 ± 10.8 kg, height 177.4 ± 9.4 cm) participated in the study. DOMS reduced the PPT (at baseline 5.9 ± 0.4 kg/cm²) by a maximum of 1.5 ± 1.4 kg/cm² (-24%) at 48 hours (p < 0.001). This correlated with the decrease in MIVF (r = -0.48, p = 0.033). Whereas subjective pain was an indicator of the early 48 hours, the PPT was still present after 72 hours (r = 0.48, p = 0.036). Other mechanical thresholds altered significantly due to DOMS, but did show no clinically or physiologically remarkable changes.

Conclusions

Functional impairment following DOMS seems related to the increased excitability of high-threshold mechanosensitive nociceptors. The PPT was the most valid mechanical threshold to quantify the extent of dysfunction. Thus PPT rather than pain intensity should be considered a possible marker indicating the athletes’ potential risk of injury.

Heart Rate Changes in Response to Mechanical Pressure Stimulation of Skeletal Muscles Are Mediated by Cardiac Sympathetic Nerve Activity

Stimulation of mechanoreceptors in skeletal muscles such as contraction and stretch elicits reflexive autonomic nervous system changes which impact cardiovascular control. There are pressure-sensitive mechanoreceptors in skeletal muscles. Mechanical pressure stimulation of skeletal muscles can induce reflex changes in heart rate (HR) and blood pressure, although the neural mechanisms underlying this effect are unclear. We examined the contribution of cardiac autonomic nerves to HR responses induced by mechanical pressure stimulation (30 s, ~10 N/cm²) of calf muscles in isoflurane-anesthetized rats. Animals were artificially ventilated and kept warm using a heating pad and lamp, and respiration and core body temperature were maintained within physiological ranges. Mechanical stimulation was applied using a stimulation probe 6 mm in diameter with a flat surface. Cardiac sympathetic and vagus nerves were blocked to test the contribution of the autonomic nerves. For sympathetic nerve block, bilateral stellate ganglia, and cervical sympathetic nerves were surgically sectioned, and for vagus nerve block, the nerve was bilaterally severed. In addition, mass discharges of cardiac sympathetic efferent nerve were electrophysiologically recorded. Mechanical stimulation increased or decreased HR in autonomic nerve-intact rats (range: −56 to +10 bpm), and the responses were negatively correlated with pre-stimulus HR (r = −0.65, p = 0.001). Stimulation-induced HR responses were markedly attenuated by blocking the cardiac sympathetic nerve (range: −9 to +3 bpm, p < 0.0001) but not the vagus nerve (range: −75 to +30 bpm, p = 0.17). In the experiments with cardiac sympathetic efferent nerve activity recordings, mechanical stimulation increased, or decreased the frequency of sympathetic nerve activity in parallel with HR (r = 0.77, p = 0.0004). Furthermore, the changes in sympathetic nerve activity were negatively correlated with its tonic level (r = −0.62, p = 0.0066). These results suggest that cardiac sympathetic nerve activity regulates HR responses to muscle mechanical pressure stimulation and the direction of HR responses depends on the tonic level of the nerve activity, i.e., bradycardia occurs when the tonic activity is high and tachycardia occurs when the activity is low.

No Effect of Acupuncture in the Relief of Delayed-Onset Muscle Soreness: Results of a Randomized Controlled Trial

BACKGROUND

Delayed-onset muscle soreness (DOMS) is a common symptom in people participating in exercise, sport, or recreational physical activities. Several remedies have been proposed to prevent and alleviate DOMS.

DESIGN AND METHODS

A five-arm randomized controlled study was conducted to examine the effects of acupuncture on eccentric exercise-induced DOMS of the biceps brachii muscle. Participants were recruited through convenience sampling of students and general public. Participants were randomly allocated to needle, laser, sham needle, sham laser acupuncture, and no intervention. Outcome measures included pressure pain threshold (PPT), pain intensity (visual analog scale), and maximum isometric voluntary force.

RESULTS

Delayed-onset muscle soreness was induced in 60 participants (22 females, age 23.6 ± 2.8 years, weight 66.1 ± 9.6 kg, and height 171.6 ± 7.9 cm). Neither verum nor sham interventions significantly improved outcomes within 72 hours when compared with no treatment control (P > 0.05).

CONCLUSIONS

Acupuncture was not effective in the treatment of DOMS. From a mechanistic point of view, these results have implications for further studies: (1) considering the high-threshold mechanosensitive nociceptors of the muscle, the cutoff for PPT (5 kg/cm) chosen to avoid bruising might have led to ceiling effects; (2) the traditional acupuncture regimen, targeting muscle pain, might have been inappropriate as the DOMS mechanisms seem limited to the muscular unit and its innervation. Therefore, a regionally based regimen including an intensified intramuscular needling (dry needling) should be tested in future studies, using a higher cutoff for PPT to avoid ceiling effects.

Dual Modulation of Nociception and Cardiovascular Reflexes during Peripheral Ischemia through P2Y1 Receptor-Dependent Sensitization of Muscle Afferents

Numerous musculoskeletal pain disorders are based in dysfunction of peripheral perfusion and are often comorbid with altered cardiovascular responses to muscle contraction/exercise. We have recently found in mice that 24 h peripheral ischemia induced by a surgical occlusion of the brachial artery (BAO) induces increased paw-guarding behaviors, mechanical hypersensitivity, and decreased grip strength. These behavioral changes corresponded to increased heat sensitivity as well as an increase in the numbers of chemosensitive group III/IV muscle afferents as assessed by an ex vivo forepaw muscles/median and ulnar nerves/dorsal root ganglion (DRG)/spinal cord (SC) recording preparation. Behaviors also corresponded to specific upregulation of the ADP-responsive P2Y1 receptor in the DRGs. Since group III/IV muscle afferents have separately been associated with regulating muscle nociception and exercise pressor reflexes (EPRs), and P2Y1 has been linked to heat responsiveness and phenotypic switching in cutaneous afferents, we sought to determine whether upregulation of P2Y1 was responsible for the observed alterations in muscle afferent function, leading to modulation of muscle pain-related behaviors and EPRs after BAO. Using an afferent-specific siRNA knockdown strategy, we found that inhibition of P2Y1 during BAO not only prevented the increased mean blood pressure after forced exercise, but also significantly reduced alterations in pain-related behaviors. Selective P2Y1 knockdown also prevented the increased firing to heat stimuli and the BAO-induced phenotypic switch in chemosensitive muscle afferents, potentially through regulating membrane expression of acid sensing ion channel 3. These results suggest that enhanced P2Y1 in muscle afferents during ischemic-like conditions may dually regulate muscle nociception and cardiovascular reflexes.

SIGNIFICANCE STATEMENT

Our current results suggest that P2Y1 modulates heat responsiveness and chemosensation in muscle afferents to play a key role in the development of pain-related behaviors during ischemia. At the same time, under these pathological conditions, the changes in muscle sensory neurons appear to modulate an increase in mean systemic blood pressure after exercise. This is the first report of the potential peripheral mechanisms by which group III/IV muscle afferents can dually regulate muscle nociception and the exercise pressor reflex. These data provide evidence related to the potential underlying reasons for the comorbidity of muscle pain and altered sympathetic reflexes in disease states that are based in problems with peripheral perfusion and may indicate a potential target for therapeutic intervention.

Delayed onset muscle soreness: Involvement of neurotrophic factors

Delayed-onset muscle soreness (DOMS) is quite a common consequence of unaccustomed strenuous exercise, especially exercise containing eccentric contraction (lengthening contraction, LC). Its typical sign is mechanical hyperalgesia (tenderness and movement related pain). Its cause has been commonly believed to be micro-damage of the muscle and subsequent inflammation. Here we present a brief historical overview of the damage-inflammation theory followed by a discussion of our new findings. Different from previous observations, we have observed mechanical hyperalgesia in rats 1-3 days after LC without any apparent microscopic damage of the muscle or signs of inflammation. With our model we have found that two pathways are involved in inducing mechanical hyperalgesia after LC: activation of the B2 bradykinin receptor-nerve growth factor (NGF) pathway and activation of the COX-2-glial cell line-derived neurotrophic factor (GDNF) pathway. These neurotrophic factors were produced by muscle fibers and/or satellite cells. This means that muscle fiber damage is not essential, although it is sufficient, for induction of DOMS, instead, NGF and GDNF produced by muscle fibers/satellite cells play crucial roles in DOMS.

Peripheral and spinal mechanisms of nociception in a rat reserpine-induced pain model

Chronic widespread pain is a serious medical problem, yet the mechanisms of nociception and pain are poorly understood. Using a reserpine-induced pain model originally reported as a putative animal model for fibromyalgia, this study was undertaken to examine the following: (1) expression of several ion channels responsible for pain, mechanotransduction, and generation/propagation of action potentials in the dorsal root ganglion (DRG), (2) activities of peripheral nociceptive afferents, and (3) alterations in spinal microglial cells. A significant increase in mRNA expression of the acid-sensing ion channel (ASIC)-3 was detected in the DRG, and the behavioral mechanical hyperalgesia was significantly reversed by subcutaneous injection of APETx2, a selective blocker of ASIC3. Single-fiber recordings in vitro revealed facilitated mechanical responses of mechanoresponsive C-fibers both in the skin and muscle although the proportion of mechanoresponsive C-nociceptors was paradoxically decreased. In the spinal dorsal horn, microglial cells labeled with Iba1 immunoreactivity was activated, especially in laminae I-II where the nociceptive input is mainly processed compared with the other laminae. The activated microglia and behavioral hyperalgesia were significantly tranquilized by intraperitoneal injection of minocycline. These results suggest that the increase in ASIC3 in the DRG facilitated mechanical response of the remaining C-nociceptors and that activated spinal microglia may direct to intensify pain in this model. Pain may be further amplified by reserpine-induced dysfunction of the descending pain inhibitory system and by the decrease in peripheral drive to this system resulting from a reduced proportion of mechanoresponsive C-nociceptors.

ATP decreases mechanical sensitivity of muscle thin-fiber afferents in rats

ATP is an energy rich substance contained in cells in the order of mM. It is released when cells are damaged and when muscle is compressed or contracted. Subcutaneous injection of ATP induces pain-related behavior and hyperalgesia to mechanical and heat stimulation in rats. However, the effects of ATP in muscle have not been fully studied. In the present study we examined the effects of ATP on muscle C-fiber afferent activities using single fiber recordings, and on nociceptive behavior. Muscle C-fiber activities were recorded in vitro using extensor digitorum longus muscle-common peroneal nerve preparations excised from rats deeply anesthetized with pentobarbital. ATP (100 μM and 1 mM, but not 1 μM) superfused for 5 min before the mechanical stimulation suppressed the mechanical responses of muscle thin fibers irrespective of whether they excited the fiber. This suppressive effect was reversed by P2X receptor antagonists PPADS (100 μM) and suramin (300 μM). We also found that subcutaneous injection of ATP (10 mM) induced nociceptive behavior, whereas intramuscular injection had no effect. These findings showed that effects of ATP on muscle afferents differ from those on cutaneous afferents.

Interleukin-10 limits intense acute swimming-induced muscle mechanical hyperalgesia in mice

NEW FINDINGS

What is the central question of this study? This study investigated the role of the endogenous anti-inflammatory cytokine interleukin-10 in intense acute swimming-induced muscle mechanical hyperalgesia in mice. What is the main finding and its importance? Endogenous interleukin-10 has a key role in limiting exercise-induced muscle pain in a model presenting similarities to delayed-onset muscle soreness in mice. Interleukin-10 reduced muscle pain by diminishing leucocyte recruitment, hyperalgesic cytokine production, oxidative stress and myocyte damage. Interleukin-10 (IL-10) is an antihyperalgesic cytokine. In this study, IL-10-deficient (IL-10(-/-) ) mice were used to investigate the role of endogenous IL-10 in intense acute swimming-induced muscle mechanical hyperalgesia, which presents similarities with delayed-onset muscle soreness. An intense acute swimming session of 1 or 2 h induced significant muscle mechanical hyperalgesia in a time-dependent manner in wild-type mice compared with the sham group 24 h after the session, which was further increased in IL-10(-/-) mice (P ˂ 0.05). Intraperitoneal treatment of wild-type mice with IL-10 (1-10 ng) reduced muscle mechanical hyperalgesia in a dose-dependent manner and reversed the enhanced muscle hyperalgesia in IL-10(-/-) mice (P ˂ 0.05). The 2 h swimming session induced increases in tumour necrosis factor-α, interleukin-1β and IL-10 production in the soleus muscle. However, tumour necrosis factor-α and interleukin-1β production in the soleus muscle were even higher in IL-10(-/-) mice between 2 and 6 h after the stimulus (P ˂ 0.05). There was no statistical difference in the levels of the antihyperalgesic cytokines interleukin-4, interleukin-5, interleukin-13 and transforming growth factor-β between wild-type and IL-10(-/-) mice (P ˃ 0.05). Interleukin-10 deficiency also resulted in increased myeloperoxidase activity, greater depletion of reduced glutathione levels, increased superoxide anion production and the maintenance of high plasma concentrations of creatine kinase (until 24 h after the swimming session) in soleus muscle (P ˂ 0.05). These results demonstrate that endogenous IL-10 controls intense acute swimming-induced muscle mechanical hyperalgesia by limiting oxidative stress and cytokine production.

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.

Role of nerve growth factor in pain

Nerve growth factor (NGF) was first identified as a substance that is essential for the development of nociceptive primary neurons and later found to have a role in inflammatory hyperalgesia in adults. Involvement of NGF in conditions with no apparent inflammatory signs has also been demonstrated. In this review we look at the hyperalgesic effects of exogenously injected NGF into different tissues, both human and animal, with special emphasis on the time course of these effects. The roles of NGF in inflammatory and neuropathic conditions as well as cancer pain are then reviewed. The role of NGF in delayed onset muscle soreness is described in more detail than its other roles based on the authors' recent observations. Acute effects are considered to be peripherally mediated, and accordingly, sensitization of nociceptors by NGF to heat and mechanical stimulation has been reported. Changes in the conductive properties of axons have also been reported. The intracellular mechanisms so far proposed for heat sensitization are direct phosphorylation and membrane trafficking of TRPV1 by TrkA. Little investigation has been done on the mechanism of mechanical sensitization, and it is still unclear whether mechanisms similar to those for heat sensitization work in mechanical sensitization. Long-lasting sensitizing effects are mediated both by changed expression of neuropeptides and ion channels (Na channels, ASIC, TRPV1) in primary afferents and by spinal NMDA receptors. Therapeutic perspectives are briefly discussed at the end of the chapter.

A review of nutritional intervention on delayed onset muscle soreness. Part I

This review is focused on the effect of nutritional intervention on delayed onset muscle soreness (DOMS) that occurs after exercise. In general, high force eccentric contractions and/or unaccustomed exercise result in DOMS attributed to reduction in performance such as muscle strength and range of motion (ROM) for both athletes and non-athletes. Nutritional intervention is one of the preventive or therapeutic ways to reduce DOMS. Previous research studies have suggested the following nutrition intervention: caffeine, omega-3 fatty acids, taurine, polyphenols, and so on. Nutritional intervention with these nutrients before and after exercise was reported to be effective in reducing DOMS. These nutritional interventions have also been reported to affect inflammatory responses and oxidative stress leading to DOMS reduction. However, other studies have reported that these nutritional interventions have no effect on DOMS. It is suggested that intake of proper nutrition intervention can effectively reduce DOMS after exercise and quickly help an athlete return to exercise or training program. In addition, nutritional intervention may help both athletes and non-athletes who engage in physical therapy or rehabilitative programs after surgery or any injurious events.

Homocysteine-induced attenuation of vascular endothelium-dependent hyperalgesia in the rat

We have recently demonstrated a role of the vascular endothelium in peripheral pain mechanism by disrupting endothelial cell function using intravascular administration of octoxynol-9, a non-selective membrane active agent. As an independent test of the role of endothelial cells in pain mechanisms, we evaluated the effect of homocysteine, an agent that damages endothelial cell function. Mechanical stimulus-induced enhancement of endothelin-1 hyperalgesia in the gastrocnemius muscle of the rat was first prevented then enhanced by intravenous administration of homocysteine, but was only inhibited by its precursor, methionine. Both homocysteine and methionine significantly attenuated mechanical hyperalgesia in two models of ergonomic muscle pain, induced by exposure to vibration, and by eccentric exercise, and cutaneous mechanical hyperalgesia in an ischemia-reperfusion injury model of Complex Regional Pain Syndrome type I, all previously shown responsive to octoxynol-9. This study provides independent support for a role of the endothelial cell in pain syndromes thought to have a vascular basis, and suggests that substances that are endothelial cell toxins can enhance vascular pain.

An in vitro adult mouse muscle-nerve preparation for studying the firing properties of muscle afferents

Muscle sensory neurons innervating muscle spindles and Golgi tendon organs encode length and force changes essential to proprioception. Additional afferent fibers monitor other characteristics of the muscle environment, including metabolite buildup, temperature, and nociceptive stimuli. Overall, abnormal activation of sensory neurons can lead to movement disorders or chronic pain syndromes. We describe the isolation of the extensor digitorum longus (EDL) muscle and nerve for in vitro study of stretch-evoked afferent responses in the adult mouse. Sensory activity is recorded from the nerve with a suction electrode and individual afferents can be analyzed using spike sorting software. In vitro preparations allow for well controlled studies on sensory afferents without the potential confounds of anesthesia or altered muscle perfusion. Here we describe a protocol to identify and test the response of muscle spindle afferents to stretch. Importantly, this preparation also supports the study of other subtypes of muscle afferents, response properties following drug application and the incorporation of powerful genetic approaches and disease models in mice.

Glial cell line-derived neurotrophic factor sensitized the mechanical response of muscular thin-fibre afferents in rats

BACKGROUND

The role of glial cell line-derived neurotrophic factor (GDNF) in pain and muscular nociceptor activities is not well understood. We examined pain-related behaviour and mechanical response of muscular thin-fibre afferents after intramuscular injection of GDNF in rats.

METHODS

GDNF and antagonist to transient receptor potential V1 or acid-sensing ion channels were injected into rat gastrocnemius muscle and muscular mechanical hyperalgesia was assessed with a Randall-Selitto analgesiometer. Activities of single C- (conduction velocity < 2.0 m/s) and Aδ-fibres (conduction velocity 2.0-12.0 m/s) were recorded from extensor digitorum longus muscle-nerve preparations in vitro. The changes in the responses to mechanical stimuli before and after GDNF injection were recorded.

RESULTS

Mechanical hyperalgesia was observed from 1 h to 1 day after GDNF (0.03 μM, 20 μL) injection. The decreased withdrawal threshold was temporarily reversed after intramuscular injection of amiloride (50 mM, 20 μL), but not capsazepine (50 μM, 20 μL). In single-fibre recordings, both phosphate buffered saline (PBS) and GDNF failed to induce any significant discharges. GDNF significantly enhanced the mechanical response when compared with the PBS group, but only in Aδ-fibre afferents. C-fibres were not affected. Significantly lowered threshold and increased response magnitude to mechanical stimuli were observed 30 or 60-120 min after injection. These times are compatible with the timing of the onset of the hyperalgesic effect of GDNF.

CONCLUSIONS

These results suggest that GDNF increased the response of muscular Aδ-fibre afferents to mechanical stimuli, resulting in muscular mechanical hyperalgesia.

Prior eccentric exercise augments muscle pain and perception of effort during cycling exercise

OBJECTIVES

This study examined the effects of exercise-induced muscle damage (EIMD) on the physiological and perceptual responses to 30 minutes of submaximal cycling at 60% of oxygen consumption (VO2 peak).

METHODS

Ten participants completed two 30-minute bouts of cycling, one before and one 48 hours after performance of strenuous (24 contractions with 120% of concentric 1-repeition maximum) eccentric exercise.

RESULTS

Eccentric exercise resulted in a significant delayed-onset muscle pain (1.6±1.6 mm to 44.8±20 mm on a 100-mm visual analog scale; P<0.001) and a 15% (P<0.001) reduction in maximal strength 48 hours after exercise. Ratings of quadriceps muscle pain (1.99±0.42 vs. 3.30±0.56; P=0.003) and perceived exertion (RPE; 13.0±0.30 vs. 13.8±0.61; P=0.02) were elevated during cycling after EIMD at identical work rates. No changes were observed in VO2 (29.6±4.6 vs. 30.2±4.4 mL/kg/min; P=0.41), heart rate (154±15 vs. 155±9 beats/min; P=0.58), and ventilation (57.2±12.1 vs. 59.8±12.7 L/min; P=0.13) during exercise after EIMD. The mean change in RPE was significantly correlated (r=0.56; P<0.01) with the change in muscle pain during cycling and delayed-onset pain during resistance exercise (r=0.86; P<0.01), but did not correlate with changes in VO2, heart rate, ventilation, and maximal strength.

DISCUSSION

These findings indicate the elevations in RPE after EIMD are likely a consequence of the EIMD with the most likely explanation being an increase in localized pain before and during cycling exercise.

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.

C-tactile fibers contribute to cutaneous allodynia after eccentric exercise

We recently showed that during acute muscle pain, C-tactile (CT) fibers mediate allodynia in healthy human subjects. In this study, we pursued the following questions: Do CTs contribute to allodynia observed in delayed onset muscle soreness (DOMS)? Is CT-mediated allodynia reproducible in a clinical pain state? In 30 healthy subjects, DOMS was induced in anterior compartment muscles of the leg by repeated eccentric contractions. DOMS was confirmed by mapping the emergence of tender points (decreased pressure pain thresholds). Furthermore, we measured pressure pain thresholds in a clinical subject who presented with activity-triggered heel pain but no resting pain. Cutaneous vibration (sinusoidal; 200 Hz-200 μm)--an otherwise innocuous stimulus--was applied to anterolateral leg before exercise, during DOMS, and following recovery from DOMS. The peripheral origin of allodynia was determined by employing conduction blocks of unmyelinated (intradermal anesthesia) and myelinated (nerve compression) fibers. In DOMS state, there was no resting pain, but vibration reproducibly evoked pain (allodynia). The blockade of cutaneous C fibers abolished this effect, whereas it persisted during blockade of myelinated fibers. In the clinical subject, without exposure to eccentric exercise, vibration (and brushing) produced a cognate expression of CT-mediated allodynia. These observations attest to a broader role of CTs in pain processing.

PERSPECTIVE

This is the first study to demonstrate the contribution of CT fibers to mechanical allodynia in exercise-induced as well as pathological pain states. These findings are of clinical significance, given the crippling effect of sensory impairments on the performance of competing athletes and patients with chronic pain and neurological disorders.

Decreased nerve growth factor upregulation is a mechanism for reduced mechanical hyperalgesia after the second bout of exercise in rats

Delayed onset muscle soreness (DOMS) is reduced when the same exercise is repeated after a certain interval. However, the mechanism for this adaptation, called a repeated bout effect, is still not well understood. Recently, we showed that upregulated nerve growth factor (NGF) triggered by B2 bradykinin receptor (B2R) activation in exercised muscle was responsible for DOMS. In this study, we investigated whether NGF upregulation was reduced after repeated bouts of exercise in rats, and if so, whether this change occurred upstream of B2R. A bout of 500 lengthening contractions (LC) was applied on day 0 and again 5 days later. DOMS was evaluated by the mechanical withdrawal threshold of the exercised extensor digitorum longus (EDL) muscle. Mechanical hyperalgesia and NGF mRNA upregulation in EDL were observed after the first LC, but not after the second LC. We then injected HOE140, a B2R antagonist with effects lasting only several hours, once before the first LC. This blocked the development of mechanical hyperalgesia and NGF mRNA upregulation not only after the first LC but also after the second LC. This suggests that adaptation occurred upstream of B2R, as the influence of the first LC was limited to that area by HOE140.

Central projection of pain arising from delayed onset muscle soreness (DOMS) in human subjects

Delayed onset muscle soreness (DOMS) is a subacute pain state arising 24–48 hours after a bout of unaccustomed eccentric muscle contractions. Functional magnetic resonance imaging (fMRI) was used to examine the patterns of cortical activation arising during DOMS-related pain in the quadriceps muscle of healthy volunteers evoked by either voluntary contraction or physical stimulation. The painful movement or physical stimulation of the DOMS-affected thigh disclosed widespread activation in the primary somatosensory and motor (S1, M1) cortices, stretching far beyond the corresponding areas somatotopically related to contraction or physical stimulation of the thigh; activation also included a large area within the cingulate cortex encompassing posteroanterior regions and the cingulate motor area. Pain-related activations were also found in premotor (M2) areas, bilateral in the insular cortex and the thalamic nuclei. In contrast, movement of a DOMS-affected limb led also to activation in the ipsilateral anterior cerebellum, while DOMS-related pain evoked by physical stimulation devoid of limb movement did not.

Characterization of muscle spindle afferents in the adult mouse using an in vitro muscle-nerve preparation

We utilized an in vitro adult mouse extensor digitorum longus (EDL) nerve-attached preparation to characterize the responses of muscle spindle afferents to ramp-and-hold stretch and sinusoidal vibratory stimuli. Responses were measured at both room (24°C) and muscle body temperature (34°C). Muscle spindle afferent static firing frequencies increased linearly in response to increasing stretch lengths to accurately encode the magnitude of muscle stretch (tested at 2.5%, 5% and 7.5% of resting length [Lo]). Peak firing frequency increased with ramp speeds (20% Lo/sec, 40% Lo/sec, and 60% Lo/sec). As a population, muscle spindle afferents could entrain 1:1 to sinusoidal vibrations throughout the frequency (10-100 Hz) and amplitude ranges tested (5-100 µm). Most units preferentially entrained to vibration frequencies close to their baseline steady-state firing frequencies. Cooling the muscle to 24°C decreased baseline firing frequency and units correspondingly entrained to slower frequency vibrations. The ramp component of stretch generated dynamic firing responses. These responses and related measures of dynamic sensitivity were not able to categorize units as primary (group Ia) or secondary (group II) even when tested with more extreme length changes (10% Lo). We conclude that the population of spindle afferents combines to encode stretch in a smoothly graded manner over the physiological range of lengths and speeds tested. Overall, spindle afferent response properties were comparable to those seen in other species, supporting subsequent use of the mouse genetic model system for studies on spindle function and dysfunction in an isolated muscle-nerve preparation.

Absence of mechanical hyperalgesia after exercise (delayed onset muscle soreness) in neonatally capsaicin-treated rats

Delayed onset muscle soreness (DOMS) appears with some delay after unaccustomed, strenuous exercise, especially after lengthening contraction (LC). It is characterized by tenderness and movement related pain, namely muscular mechanical hyperalgesia. To clarify the involvement of C-fibers in this mechanical hyperalgesia, we examined whether DOMS could be induced in rats treated neonatally with capsaicin. We confirmed that a large portion of unmyelinated afferent fibers were lost in capsaicin treated rats. In these animals, LC failed to induce muscular mechanical hyperalgesia. mRNA of nerve growth factor (NGF) in the muscle, which plays a pivotal role in maintaining mechanical hyperalgesia, was upregulated in the capsaicin treated animals similar to the vehicle treated animals. These results demonstrate that C-fiber afferents are essential in transmitting the nociceptive information from exercised muscle in DOMS.