Axonal sprouting and changes in fibre types after running-induced muscle damage

The Role of Physical Exercise and Rehabilitative Implications in the Process of Nerve Repair in Peripheral Neuropathies: A Systematic Review

BACKGROUND

The various mechanisms involved in peripheral nerve regeneration, induced by exercise and electrical nerve stimulation, are still unclear.

OBJECTIVE

The aim of this review was to summarize the influence of physical exercise and/or electrical stimulation on peripheral nerve repair and regeneration and the variation of impact of intervention depending on timing, as well as kind and dosage of the intervention. A literature survey was conducted on PubMed, Scopus, and Web of Science, between February 2021 to July 2021, with an update in September 2022.

METHODOLOGY

The literature search identified 101,386 articles with the keywords: "peripheral nerve" OR "neuropathy" AND "sprouting" OR "neuroapraxia" OR "axonotmesis" OR "neurotmesis" OR "muscle denervation" OR "denervated muscle" AND "rehabilitation" OR "physical activity" OR "physical exercise" OR "activity" OR "electrical stimulation". A total of 60 publications were included. Eligible studies were focused on evaluating the process of nerve repair (biopsy, electromyographic parameters or biomarker outcomes) after electrical stimulation or physical exercise interventions on humans or animals with peripheral sensory or motor nerve injury.

SYNTHESIS

This study shows that the literature, especially regarding preclinical research, is mainly in agreement that an early physical program with active exercise and/or electrical stimulation promotes axonal regenerative responses and prevents maladaptive response. This was evaluated by means of changes in electrophysiological recordings of CMAPs for latency amplitude, and the sciatic functional index (SFI). Furthermore, this type of activity can cause an increase in weight and in muscle fiber diameter. Nevertheless, some detrimental effects of exercising and electrical stimulation too early after nerve repair were recorded.

CONCLUSION

In most preclinical studies, peripheral neuropathy function was associated with improvements after physical exercise and electrical stimulation. For humans, too little research has been conducted on this topic to reach a complete conclusion. This research supports the need for future studies to test the validity of a possible rehabilitation treatment in humans in cases of peripheral neuropathy to help nerve sprouting.

Systematic Review and Meta-Analysis of Endurance Exercise Training Protocols for Mice

Inbred and genetically modified mice are frequently used to investigate the molecular mechanisms responsible for the beneficial adaptations to exercise training. However, published paradigms for exercise training in mice are variable, making comparisons across studies for training efficacy difficult. The purpose of this systematic review and meta-analysis was to characterize the diversity across published treadmill-based endurance exercise training protocols for mice and to identify training protocol parameters that moderate the adaptations to endurance exercise training in mice. Published studies were retrieved from PubMed and EMBASE and reviewed for the following inclusion criteria: inbred mice; inclusion of a sedentary group; and exercise training using a motorized treadmill. Fifty-eight articles met those inclusion criteria and also included a "classical" marker of training efficacy. Outcome measures included changes in exercise performance, V ˙ O_2max, skeletal muscle oxidative enzyme activity, blood lactate levels, or exercise-induced cardiac hypertrophy. The majority of studies were conducted using male mice. Approximately 48% of studies included all information regarding exercise training protocol parameters. Meta-analysis was performed using 105 distinct training groups (i.e., EX-SED pairs). Exercise training had a significant effect on training outcomes, but with high heterogeneity (Hedges' g=1.70, 95% CI=1.47-1.94, Tau²=1.14, I² =80.4%, prediction interval=-0.43-3.84). Heterogeneity was partially explained by subgroup differences in treadmill incline, training duration, exercise performance test type, and outcome variable. Subsequent analyses were performed on subsets of studies based on training outcome, exercise performance, or biochemical markers. Exercise training significantly improved performance outcomes (Hedges' g=1.85, 95% CI=1.55-2.15). Subgroup differences were observed for treadmill incline, training duration, and exercise performance test protocol on improvements in performance. Biochemical markers also changed significantly with training (Hedges' g=1.62, 95% CI=1.14-2.11). Subgroup differences were observed for strain, sex, exercise session time, and training duration. These results demonstrate there is a high degree of heterogeneity across exercise training studies in mice. Training duration had the most significant impact on training outcome. However, the magnitude of the effect of exercise training varies based on the marker used to assess training efficacy.

Running and Swimming Differently Adapt the BDNF/TrkB Pathway to a Slow Molecular Pattern at the NMJ

Physical exercise improves motor control and related cognitive abilities and reinforces neuroprotective mechanisms in the nervous system. As peripheral nerves interact with skeletal muscles at the neuromuscular junction, modifications of this bidirectional communication by physical activity are positive to preserve this synapse as it increases quantal content and resistance to fatigue, acetylcholine receptors expansion, and myocytes' fast-to-slow functional transition. Here, we provide the intermediate step between physical activity and functional and morphological changes by analyzing the molecular adaptations in the skeletal muscle of the full BDNF/TrkB downstream signaling pathway, directly involved in acetylcholine release and synapse maintenance. After 45 days of training at different intensities, the BDNF/TrkB molecular phenotype of trained muscles from male B6SJLF1/J mice undergo a fast-to-slow transition without affecting motor neuron size. We provide further knowledge to understand how exercise induces muscle molecular adaptations towards a slower phenotype, resistant to prolonged trains of stimulation or activity that can be useful as therapeutic tools.

Lateral epicondylalgia: A primary nervous system disorder

Lateral epicondylalgia (LE) is the most common chronic painful condition affecting the elbow in the general population. Although major advances have been accomplished in recent years in the understanding of LE, the underlying physiopathology is still a reason for debate. Differences in clinical presentation and evolution of the symptoms among patients, suggest the need for revisiting the current knowledge about subjacent mechanisms that attempt to explain pain and functional loss. Previous models have suggested that the condition is mainly a degenerative tendinopathy, associated with changes in pain pathways and the motor system. The hypothesis of this work is that LE is the clinical manifestation of a primary nervous system disorder, characterized by an abnormal increase in neuronal activity and a subsequent loss of homeostasis, which secondarily affects the musculoskeletal tissues of the elbow-forearm-hand complex. A new model for LE is presented, supported by an in-deep analysis of basic sciences, epidemiological and clinical studies.

Passive and active exercise improve regeneration and muscle reinnervation after peripheral nerve injury in the rat

INTRODUCTION

Lesions of peripheral nerves cause loss of motor and sensory function and also lead to hyperreflexia and hyperalgesia. Activity-dependent therapies promote axonal regeneration and functional recovery and may improve sensory-motor coordination and restoration of adequate circuitry at the spinal level.

METHODS

We compared the effects of passive (bicycle) and active (treadmill) exercise on nerve regeneration and modulation of the spinal H reflex after transection and repair of the rat sciatic nerve. Animals were evaluated during 2 months using electrophysiological, functional, and histological methods.

RESULTS

Moderate exercise for 1 hour/day, either active treadmill walking or passive cycling, improved muscle reinnervation, increased the number of regenerated axons in the distal nerve, and reduced the increased excitability of spinal reflexes after nerve lesion.

DISCUSSION

Maintenance of denervated muscle activity and afferent input, by active or passive exercise, may increase trophic factor release to act on regenerating axons and to modulate central neuronal plasticity.

Effect of one stretch a week applied to the immobilized soleus muscle on rat muscle fiber morphology

We determined the effect of stretching applied once a week to the soleus muscle immobilized in the shortened position on muscle fiber morphology. Twenty-six male Wistar rats weighing 269 +/- 26 g were divided into three groups. Group I, the left soleus was immobilized in the shortened position for 3 weeks; group II, the soleus was immobilized in the shortened position and stretched once a week for 3 weeks; group III, the soleus was submitted only to stretching once a week for 3 weeks. The medial part of the soleus muscle was frozen for histology and muscle fiber area evaluation and the lateral part was used for the determination of number and length of serial sarcomeres. Soleus muscle submitted only to immobilization showed a reduction in weight (44 +/- 6%, P = 0.002), in serial sarcomere number (23 +/- 15%) and in cross-sectional area of the fibers (37 +/- 31%, P < 0.001) compared to the contralateral muscles. The muscle that was immobilized and stretched showed less muscle fiber atrophy than the muscles only immobilized (P < 0.05). Surprisingly, in the muscles submitted only to stretching, fiber area was decreased compared to the contralateral muscle (2548 +/- 659 vs 2961 +/- 806 microm(2), respectively, P < 0.05). In conclusion, stretching applied once a week for 40 min to the soleus muscle immobilized in the shortened position was not sufficient to prevent the reduction of muscle weight and of serial sarcomere number, but provided significant protection against muscle fiber atrophy. In contrast, stretching normal muscles once a week caused a reduction in muscle fiber area.

Eccentric muscle contractions: their contribution to injury, prevention, rehabilitation, and sport

Muscles operate eccentrically to either dissipate energy for decelerating the body or to store elastic recoil energy in preparation for a shortening (concentric) contraction. The muscle forces produced during this lengthening behavior can be extremely high, despite the requisite low energetic cost. Traditionally, these high-force eccentric contractions have been associated with a muscle damage response. This clinical commentary explores the ability of the muscle-tendon system to adapt to progressively increasing eccentric muscle forces and the resultant structural and functional outcomes. Damage to the muscle-tendon is not an obligatory response. Rather, the muscle can hypertrophy and a change in the spring characteristics of muscle can enhance power; the tendon also adapts so as to tolerate higher tensions. Both basic and clinical findings are discussed. Specifically, we explore the nature of the structural changes and how these adaptations may help prevent musculoskeletal injury, improve sport performance, and overcome musculoskeletal impairments.

Voluntary exercise induces a BDNF-mediated mechanism that promotes neuroplasticity

We have investigated potential mechanisms by which exercise can promote changes in neuronal plasticity via modulation of neurotrophins. Rodents were exposed to voluntary wheel running for 3 or 7 days, and their lumbar spinal cord and soleus muscle were assessed for changes in brain-derived neurotrophic factor (BDNF), its signal transduction receptor (trkB), and downstream effectors for the action of BDNF on synaptic plasticity. Exercise increased the expression of BDNF and its receptor, synapsin I (mRNA and phosphorylated protein), growth-associated protein (GAP-43) mRNA, and cyclic AMP response element-binding (CREB) mRNA in the lumbar spinal cord. Synapsin I, a synaptic mediator for the action of BDNF on neurotransmitter release, increased in proportion to GAP-43 and trkB mRNA levels. CREB mRNA levels increased in proportion to BDNF mRNA levels. In separate experiments, the soleus muscle was paralyzed unilaterally via intramuscular botulinum toxin type A (BTX-A) injection to determine the effects of reducing the neuromechanical output of a single muscle on the neurotrophin response to motor activity. In sedentary BTX-A-treated rats, BDNF and synapsin I mRNAs were reduced below control levels in the spinal cord and soleus muscle. Exercise did not change the BDNF mRNA levels in the spinal cord of BTX-A-treated rats but further reduced the BDNF mRNA levels in the paralyzed soleus relative to the levels in sedentary BTX-A-treated rats. Exercise also restored synapsin I to near control levels in the spinal cord. These results indicate that basal levels of neuromuscular activity are required to maintain normal levels of BDNF in the neuromuscular system and the potential for neuroplasticity.

Histochemical differences in the responses of predominantly fast-twitch glycolytic muscle and slow-twitch oxidative muscle to veratrine

The aim of this study was to investigate if the Na(+)-channel activating alkaloid veratrine is able to change the oxidative and m-ATPase activities of a fast-twitch glycolytic muscle (EDL, extensor digitorum longus) and slow-twitch oxidative muscle (SOL, soleus) in mice. Oxidative fibers and glycolytic fibers were more sensitive to veratrine than oxidative-glycolytic fibers 15, 30 and 60 min after the i.m. injection of veratrine (10 ng/kg) with both showing an increase in their metabolic activity in both muscles. In EDL, the m-ATPase reaction revealed a significant (p < 0.001) decrease (50%) in the number of type IIB fibers after 30 min while the number of type I fibers increased by 550%. Type I fibers decreased from 34% in control SOL to 17% (50% decrease) in veratrinized muscles, with a 10% decrease in type IIA fibers within 15 min. A third type of fiber appeared in SOL veratrinized muscle, which accounted for 28% of the fibers. Our work gives evidence that the changes in the percentage of the fiber types induced by veratrine may be the result, at least partially, from a direct effect of veratrine on muscle fibers and else from an interaction with the muscle type influencing distinctively the response of a same fiber type. Based on the results obtained in the present study the alterations in EDL may be related to the higher number of Na(+) channels present in this muscle whereas those in SOL may involve an action of veratrine on mitochondria. Although it is unlikely that the shift of enzymes activities induced by veratrine involves genotypic expression changes an alternative explanation for the findings cannot be substantiated by the present experimental approach.

Regenerated rat skeletal muscle after periodic contusions

In the present study we evaluated the morphological aspect and changes in the area and incidence of muscle fiber types of long-term regenerated rat tibialis anterior (TA) muscle previously submitted to periodic contusions. Animals received eight consecutive traumas: one trauma per week, for eight weeks, and were evaluated one (N = 8) and four (N = 9) months after the last contusion. Serial cross-sections were evaluated by toluidine blue staining, acid phosphatase and myosin ATPase reactions. The weight of injured muscles was decreased compared to the contralateral intact one (one month: 0.77 +/- 0.15 vs 0.91 +/- 0.09 g, P = 0.03; four months: 0.79 +/- 0.14 vs 1.02 +/- 0.07 g, P = 0.0007, respectively) and showed abundant presence of split fibers and fibers with centralized nuclei, mainly in the deep portion. Damaged muscles presented a higher incidence of undifferentiated fibers when compared to the intact one (one month: 3.4 +/- 2.1 vs 0.5 +/- 0.3%, P = 0.006; four months: 2.3 +/- 1.6 vs 0.3 +/- 0.3%, P = 0.007, respectively). Injured TA evaluated one month later showed a decreased area of muscle fibers when compared to the intact one (P = 0.003). Thus, we conclude that: a) muscle fibers were damaged mainly in the deep portion, probably because they were compressed against the tibia; b) periodic contusions in the TA muscle did not change the percentage of type I and II muscle fibers; c) periodically injured TA muscles took four months to reach a muscle fiber area similar to that of the intact muscle.

Regeneration and change of muscle fiber types after injury induced by a hemorrhagic fraction isolated from Agkistrodon contortrix laticinctus venom

Tibialis anterior (TA) muscles of rats were evaluated 3h, 3 and 30days after intramuscular injection of ACL hemorrhagic toxin I (ACLHT-I, 5mg/kg), partially purified from the venom of Agkistrodon contortrix laticinctus. Contralateral muscles were injected with saline. Three hours after ACLHT-1 injection: presence of hemorrhagic areas and myonecrotic muscle fibers. Three days: injured muscles showed areas in regeneration, some regions with delay of regeneration and bundles of normal fibers. An increased TA muscle weight was found when compared with the contralateral (0.45+/-0.03g versus 0.36+/-0.04g, p=0.04). Thirty days: areas of regenerated muscle fibers presented splits and centralized nuclei. Some regions were replaced by connective tissue. All muscle fiber types were injured but only the incidence of type IIC increased (3.4+/-2.0% versus 0.2+/-0.2%, p=0.0005). Regenerated areas of muscles were exclusively composed by fiber types II and IIC. Regenerated muscles decreased the muscle weight (0.49+/-0.1g versus 0.66+/-0.05g, p=0. 03). In conclusion, ACLHT-I: (a) caused hemorrhage and muscle fiber injury; (b) injured both fiber types I and II; (c) increased the incidence of fiber type IIC and; (d) some muscle regions were replaced by connective tissue.

Downhill running preferentially increases CGRP in fast glycolytic muscle fibers

Calcitonin gene-related peptide (CGRP) is present in some spinal cord motoneurons and at neuromuscular junctions in skeletal muscle. We previously reported increased numbers of CGRP-positive (CGRP+) motoneurons supplying hindlimb extensors after downhill exercise (Homonko DA and Theriault E, Inter J Sport Med 18: 1-7, 1997). The present study identifies the responding population with respect to muscle and motoneuron pool and correlates changes in CGRP with muscle fiber type-identified end plates. Twenty seven rats were divided into the following groups: control and 72 h and 2 wk postexercise. FluoroGold was injected into the soleus, lateral gastrocnemius, and the proximal (mixed fiber type) or distal (fast-twitch glycolytic) regions of the medial gastrocnemius (MG). Untrained animals ran downhill on a treadmill for 30 min. The number of FluoroGold/CGRP+ motoneurons within proximal and distal MG increased by 72 h postexercise (P<0.05). No significant changes were observed in soleus or lateral gastrocnemius motoneurons postexercise. The number of alpha-bungarotoxin/CGRP+ motor end plates in the MG increased exclusively at fast-twitch glycolytic muscle fibers 72 h and 2 wk postexercise (P<0.05). One interpretation of these results is that unaccustomed exercise preferentially activates fast-twitch glycolytic muscle fibers in the MG.

GaAs (904-nm) laser radiation does not affect muscle regeneration in mouse skeletal muscle

BACKGROUND AND OBJECTIVE

We evaluated the effect of GaAs (904-nm) laser, applied directly to the skin of injured muscle, in muscle regeneration.

STUDY DESIGN/MATERIAL AND METHODS

Muscle injury was induced in the Tibialis anterior (TA) muscle by ACL myotoxin (5 mg/kg). Two groups were irradiated with doses of 3 (n = 8) and 10 J/cm(2)(n = 8). GaAs laser (power 1.5 mW, intensity 7.5 mW/cm(2), spot 0.2 cm(2)) was applied daily for five days. Contralateral TA received a sham procedure.

RESULTS

Similar morphological aspects were found in both laser irradiated and sham muscles. No differences were found in the muscle weight, but animals irradiated with 10 J/cm(2) showed a significant gain of body weight (P = 0.002).

CONCLUSIONS

Doses of 3 and 10 J/cm(2) of GaAs laser were not efficient to promote significant morphological changes in the regenerated skeletal muscle, but the dose of 10 J/cm(2) promoted significant gain of body weight.

Long-term regeneration of fast and slow murine skeletal muscles after induced injury by ACL myotoxin isolated from Agkistrodon contortrix laticinctus (broad-banded copperhead) venom

The aim of the present work was to analyze the regenerated muscle types I and II fibers of the soleus and gastrocnemius muscles of mice, 8 months after damage induced by ACL myotoxin (ACLMT). Animals received 5 mg/kg of ACLMT into the subcutaneous lateral region of the right hind limb, near the Achilles tendon; contralateral muscles received saline. Longitudinal and cross sections (10 microm) of frozen muscle tissue were evaluated. Eight months after ACLMT injection, both muscle types I and II fibers of soleus and gastrocnemius muscles still showed centralized nuclei and small regenerated fibers. Compared with the left muscle, the incidence of type I fibers increased in the right muscle (21% +/- 03% versus 12% +/- 06%, P = 0.009), whereas type II fibers decreased (78% +/- 02% versus 88% +/- 06%, P = 0.01). The incidence of type IIC fibers was normal. These results confirm that ACLMT induced muscle type fiber transformation from type II to type I, through type IIC. The area analysis of types I and II fibers of the gastrocnemius revealed that injured right muscles have a higher percentage of small fibers in both types I and II fibers (0-1,500 microm2) than left muscles, which have larger normal type I and II fibers (1,500-3,500 microm2). These results indicate that ACLMT can be used as an excellent model to study the rearrangement of motor units and the transformation of muscle fiber types during regeneration.

Effect of single and periodic contusion on the rat soleus muscle at different stages of regeneration

This work analyzed the rat soleus muscle after single and recurrent contusions at different stages of regeneration. A noninvasive contusion was produced by a type of drop-mass equipment. The posterior region of the right hind limb received a trauma and both right and left soleus muscles were analyzed 1, 4, and 6 days after a single contusion (1x), and 6 and 30 days after periodic contusions (10x, one trauma per week for 10 weeks). Single contusion: there was no significant difference between right and left soleus muscle weight. All animals showed abundant signs of acute damage in the right soleus. AChE activity was identified in regeneration segments of the right soleus. Periodic contusions: there was an increase in the right soleus muscle weight (alpha = 5%) only in the animals evaluated 6 days after periodic contusions. The right soleus muscle showed a high incidence of chronic signs of damage, such as split fibers and a centralized nucleus, which predominated when compared with the acute signs. Right soleus muscles showed split fibers with AChE activity in both the proximal and middle regions. There was no difference in the incidence of muscle fiber types (I, II, and IIC) between right and left soleus muscles after periodic contusions. Skeletal muscle contusion is common in humans, especially in sport activities, where repetitive traumas are also frequent. The results of this work indicate that despite the regeneration process there is an important change in the morphological aspect of regenerated muscle fibers, which possibly affect muscle performance.

Mutual interference of myotonia and muscular dystrophy in the mouse: a study on ADR-MDX double mutants

For Duchenne muscular dystrophy (DMD, dystrophin deficiency) and Thomsen/Becker myotonia (muscular chloride channel deficiency) genetically homologous mouse models are available, the dystrophin-deficient MDX mouse and the myotonic ADR mouse. Whereas the latter shows more severe symptoms than human myotonia patients, the MDX mouse, in contrast to DMD patients, is only mildly affected. We have introduced, by appropriate breeding, the defect leading to myotonia (Clc1 null mutation, adr allele) into MDX mice, thus creating ADR-MDX double mutants. The expectation was that, due to mechanical stress during myotonic cramps, the ADR status should symptomatically aggravate the muscle fibre necrosis caused by the dystrophin deficiency. The overall symptoms of the double mutants were dominated by myotonia. Weight reduction and premature death rate were higher in ADR-MDX than in ADR mice. Sarcolemmal ruptures as indicated by influx into muscle fibres of serum globulins and injected Evans blue were found with great inter-individual variation in MDX and in ADR-MDX muscles. Affected fibres were found mainly in large groups in MDX but single or in small clusters in ADR-MDX leg muscles. The symptoms of myotonia (aftercontractions, shift towards oxidative fibres) were less pronounced in ADR-MDX than in ADR muscles. Conversely, numbers of damaged fibres as well as the percentage of central nuclei (an indicator of fibre regeneration) were significantly lower in ADR-MDX than in MDX skeletal muscles. Thus it appears that, at the level of the muscle fibre, myotonia and muscular dystrophy attenuate each other.

Injury and recovery of fast and slow skeletal muscle fibers affected by ACL myotoxin isolated from Agkistrodon contortrix laticinctus (Broad-Banded copperhead) venom

The response of different types of skeletal muscle fibers to a snake venom PLA2 myotoxin was tested in vivo by injecting ACL myotoxin (ACLMT) into mice. Both the soleus (slow-twitch) and gastrocnemius (fast-twitch) were examined at different time periods (3 h, 3 and 21 d) after the injection. All animals received 5 mg/kg myotoxin into the subcutaneous lateral region of the right hind limb, near the Achilles tendon; contralateral muscles were used as controls. Cross-sections (10 microm) of frozen muscle tissue were cut from the medial region of the muscle. Alternate serial sections were stained either with toluidine blue or for acid phosphatase, myofibrillar ATPase activity after alkali (pH 10.3) or acid preincubation (pH 4.3), succinate dehydrogenase or acetylcholinesterase. Several stages of necrosis were observed 3 h after ACLMT injection, in both superficial and deep regions of both muscles. In these same regions 3 d after injection, clusters of regenerated muscle fibers were present, and some of them presented AChE activity. Twenty-one days after ACLMT injection the muscle fibers of soleus and gastrocnemius presented only chronic signs of damage such as split fibers and centralized nuclei. Using m-ATPase reactions it was possible to determine that both muscle fiber types I and II were injured in both muscles. The number of type IIC fibers was significantly increased, and the number of type II fibers significantly decreased in the gastrocnemius 21 d after ACLMT injection, suggesting a change in muscle fiber type from type II to type I, through type IIC. The increased number of type IIC fibers and the presence of AChE activity in clusters of regenerating fibers and split fibers indicate that injury by ACLMT produces axonal remodeling and muscle fiber type change.

Effects of aging and voluntary exercise on the function of dystrophic muscle from mdx mice

To understand how exercise affects the contractile function of dystrophic muscle, we examined the effect of long-term voluntary exercise on mdx mice and related these effects to our findings in sedentary aging mice. Although the mdx mouse is the genetic homolog for Duchenne muscular dystrophy, it does not demonstrate the same progression in limb muscle dysfunction as Duchenne muscular dystrophy as it ages. We postulated that the sedentary lifestyle of this animal plays an important role in its minimal phenotypic expression. To examine the effect of exercise, eight C57BL/10 (C57) and eight mdx mice were allowed to run ad libitum for one year. Forty sedentary mdx mice and 40 sedentary C57 from one month to 18 months of age were used as controls. Contractile characteristics of the extensor digitorum longus and soleus muscles and morphometric characteristics of the mice were examined. The mdx mice ran approximately 45% fewer kilometers per day than C57 mice. Long-term voluntary running had beneficial training effects on both the old mdx mice and their C57 controls. The exercise ameliorated the age-associated loss in tension production that was observed in the soleus of sedentary mdx and sedentary C57 mice. There was a 9% reduction in the fatigability of the extensor digitorum longus muscle of the old mdx mice after the exercise. Despite these improvements, the old mdx mice exhibited significant functional deficits compared with their C57 controls. Our hypothesis, that long-term voluntary exercise would have a beneficial training effect on control mice and a deleterious effect on mdx mice as they aged, was not supported by this study. This study shows that dystrophin-less muscles from sedentary mice display significant signs of muscle damage, yet can respond beneficially to low-level voluntary running in a manner similar to that of the C57 control.

Muscle-derived neurotrophin-4 as an activity-dependent trophic signal for adult motor neurons

The production of neurotrophin-4 (NT-4) in rat skeletal muscle was found to depend on muscle activity. The amounts of NT-4 messenger RNA present decreased after blockade of neuromuscular transmission with alpha-bungarotoxin and increased during postnatal development and after electrical stimulation in a dose-dependent manner. NT-4 immunoreactivity was detected in slow, type I muscle fibers. Intramuscular administration of NT-4 induced sprouting of intact adult motor nerves. Thus, muscle-derived NT-4 acted as an activity-dependent neurotrophic signal for growth and remodeling of adult motor neuron innervation. NT-4 may thus be partly responsible for the effects of exercise and electrical stimulation on neuromuscular performance.

Differential expression of tenascin after denervation, damage or paralysis of mouse soleus muscle

The expression of the extracellular matrix molecule tenascin was studied by immunocytochemistry and Western blotting in soleus muscles of adult mice after nerve damage (denervation), muscle injury (induced by enforced running or freezing) and functional block of synaptic transmission (botulinum toxin). Enhanced expression of tenascin in the extracellular spaces around focally damaged muscle fibres was found already 10 h after onset of running on a motor-driven treadmill which causes muscle injury in soleus muscle. Tenascin expression reached a peak at 2-3 days post-exercise, after which it declined gradually and became undetectable by two weeks after injury. Similarly, cryo-damage of soleus muscles in situ led to upregulation of tenascin. Chronic muscle denervation after sciatic nerve transection caused a persistent (studied up to 31 days) expression of tenascin at denervated endplates and in intramuscular nerve branches but not in other tissue compartments. Local application of botulinum toxin Type A, which results in muscle inactivity but not in tissue degeneration, however, did not induce tenascin expression 12 h to 12 days post-injection. Expression of tenascin after denervation and muscle damage, but its absence after paralysis, were verified by SDS-PAGE and Western blot analysis. Independent of the type of injury (muscle, nerve or both) the known major isoforms of mouse tenascin, as judged by M(r) comparison, were re-expressed, with no preponderance of individual M(r) forms. These results show that tenascin expression in adult muscles is induced by both axon and muscle fibre damage but not by muscle inactivity. In contrast, NCAM, in accordance with previous observations, showed enhanced expression both as a result of inactivity and in association with tissue repair.

Formation of new muscle fibres and tumours after injection of cultured myogenic cells

We examined the effects of implantation of cultured myogenic cells from a permanent cell line into soleus muscles of histocompatible adult mice. Myogenic cells (10(6) or 10(4)) were implanted into intact muscles, muscles frozen with liquid nitrogen, paralysed with botulinum toxin or reinnervated after long-term (seven months) denervation. Formation of numerous muscle fibres in myogenic cell-injected muscles raised the total number of fibres up to ten times above control by four weeks. Larger effects were found in freeze-damaged than in paralysed muscles. The new fibres had small calibers, considerable length (greater than 1.3 mm, maximum distance over which serial sections were made), were multinucleated and were oriented parallel to the large-diameter fibres of the host muscles. In some experiments beta-galactosidase, introduced into myogenic cells via retroviral transfection, was detected in small and large muscle fibres 4-20 weeks after implantation, indicating survival of the grafted cells and formation of mosaic (host-donor) and new fibres of donor origin. Muscle weight increased significantly and, rather surprisingly, a parallel increase was found in isometric tetanic tension of isolated nerve-muscle preparations; thus tension per mg muscle tissue was not different from normal. By eight weeks reduction of acetylcholine sensitivity and down-regulation of neural cell adhesion molecule to normal were observed, indicating that synaptic transmission at the new fibres was mature. After different periods of time (5-20 weeks, depending on the subclone used) tumours developed in most but not all injected limbs (37 out of 39). The tumours were destructive to the muscles and were classified as rhabdomyosarcomas. Prior to tumour formation, neural cell adhesion molecule positive cells reappeared in the muscles; since the myogenic cells initially produced differentiated muscle fibres, it appears that malignant growth is induced by factors in vivo. Thus, at present the outcome of such implantation is unpredictable.