Voluntary exercise increases axonal regeneration from sensory neurons

Does Endurance Training Compensate for Neurotrophin Deficiency Following Diabetic Neuropathy?

BACKGROUND

A lack of neurotrophic support is believed to contribute to the development of diabetic neuropathy. On the other hand, neurotrophins have consistently been shown to increase in the central and peripheral nervous system following exercise, but the effects of exercise intervention on brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in diabetic neuropathy are not understood.

OBJECTIVES

This experimental study was designed and carried out at the Tarbiat Modares university (TMU) in Tehran, Iran, to investigate the hypothesis that increased activity as endurance training can help to increase the endogenous expression of neurotrophins in diabetic rats.

METHODS

This was an experimental study with 2 × 2 factorial plans performed at TMU in Iran. Sampling was accidental and 28 adult male Wistar rats in the body mass range of 326.3 ± 8.4 g comprised the sample, with each rat randomly assigned to four groups: diabetic control (DC), diabetic training (DT), healthy control (HC), and healthy training (HT). To induce diabetic neuropathy, after 12 hours of food deprivation, an intraperitoneal injection of streptozotocin (STZ) solution (45 mg/Kg) method was used. Two weeks after STZ injection, the endurance training protocol was performed for 6 weeks; 24 hours after the last training session, the rats were sacrificed. Real-time PCR was used for BDNF and NGF expression.

RESULTS

The data indicate that diabetes decreases BDNF and NGF expression in sensory (92%, P = 0.01; 90%, P = 0.038, respectively) and motor (93%, P = 0.05; 60%, P = 0.029, respectively) roots. However, NGF mRNA levels in the DT group were significantly higher than in the HC group ((7.1-fold), P = 0.01; (2.2-fold), P = 0.001, respectively, for sensory and motor roots), but this was not shown for BDNF. In addition, endurance training can increase NGF expression in healthy rats ((7.4-fold), P = 0.01; (3.8-fold), P = 0.001, respectively, for sensory and motor roots).

CONCLUSIONS

This study shows that BDNF and NGF expression decreases in diabetic neuropathy. However, this decrease can be reversed through endurance training. These results also indicate that endurance training may have a potential role in compensating for neurotrophin deficiency following diabetic neuropathy.

Exercise dependent increase in axon regeneration into peripheral nerve grafts by propriospinal but not sensory neurons after spinal cord injury is associated with modulation of regeneration-associated genes

Insufficient regeneration of central nervous system (CNS) axons contributes to persisting neurological dysfunction after spinal cord injury (SCI). Peripheral nerve grafts (PNGs) support regeneration by thousands of injured intraspinal axons and help them bypass some of the extracellular barriers that form after SCI. However this number represents but a small portion of the total number of axons that are injured. Here we tested if rhythmic sensory stimulation during cycling exercise would boost the intrinsic regenerative state of neurons to enhance axon regeneration into PNGs after a lower thoracic (T12) spinal transection of adult rats. Using True Blue retrograde tracing, we show that 4 weeks of cycling improves regeneration into a PNG from lumbar interneurons but not by primary sensory neurons. The majority of neurons that regenerate their axon are within 5 mm of the lesion and their number increased 70% with exercise. Importantly propriospinal neurons in more distant regions (5-20 mm from the lesion) that routinely exhibit very limited regeneration responded to exercise by increasing the number of regenerating neurons by 900%. There was no exercise-associated increase in regeneration from sensory neurons. Analyses using fluorescent in situ hybridization showed that this increase in regenerative response is associated with changes in levels of mRNAs encoding the regeneration associated genes (RAGs) GAP43, β-actin and Neuritin. While propriospinal neurons showed increased mRNA levels in response to SCI alone and then to grafting and exercise, sensory neurons did not respond to SCI, but there was a response to the presence of a PNG. Thus, exercise is a non-invasive approach to modulate gene expression in injured neurons leading to an increase in regeneration. This sets the stage for future studies to test whether exercise will promote axon outgrowth beyond the PNG and reconnection with spinal cord neurons, thereby demonstrating a potential clinical application of this combined therapeutic intervention.

Exercise, neurotrophins, and axon regeneration in the PNS

Electrical stimulation and exercise are treatments to enhance recovery from peripheral nerve injuries. Brain-derived neurotrophic factor and androgen receptor signaling are requirements for the effectiveness of these treatments. Increased neuronal activity is adequate to promote regeneration in injured nerves, but the dosing of activity and its relationship to neurotrophins and sex steroid hormones is less clear. Translation of these therapies will require principles associated with their cellular mechanisms.

Strategies to promote peripheral nerve regeneration: electrical stimulation and/or exercise

Enhancing the regeneration of axons is often considered to be a therapeutic target for improving functional recovery after peripheral nerve injury. In this review, the evidence for the efficacy of electrical stimulation (ES), daily exercise and their combination in promoting nerve regeneration after peripheral nerve injuries in both animal models and in human patients is explored. The rationale, effectiveness and molecular basis of ES and exercise in accelerating axon outgrowth are reviewed. In comparing the effects of ES and exercise in enhancing axon regeneration, increased neural activity, neurotrophins and androgens are considered to be common requirements. Similarly, there are sex-specific requirements for exercise to enhance axon regeneration in the periphery and for sustaining synaptic inputs onto injured motoneurons. ES promotes nerve regeneration after delayed nerve repair in humans and rats. The effectiveness of exercise is less clear. Although ES, but not exercise, results in a significant misdirection of regenerating motor axons to reinnervate different muscle targets, the loss of neuromuscular specificity encountered has only a very small impact on resulting functional recovery. Both ES and exercise are promising experimental treatments for peripheral nerve injury that seem to be ready to be translated to clinical use.

Astrocytes and Microglia-Mediated Immune Response in Maladaptive Plasticity is Differently Modulated by NGF in the Ventral Horn of the Spinal Cord Following Peripheral Nerve Injury

Reactive astrocytes and activated microglia are the key players in several pathophysiologic modifications of the central nervous system. We used the spared nerve injury (SNI) of the sciatic nerve to induce glial maladaptive response in the ventral horn of lumbar spinal cord and examine its role in the remodeling of the tripartite synapse plasticity. Imaging the ventral horn revealed that SNI was associated with both an early microglial and astrocytic activation, assessed, respectively, by analysis of Iba1 and GFAP expression. Microglia, in particular, localized peculiarly surrounding the motor neurons somata. Perineuronal astrocytes, which play a key role in maintaining the homeostasis of neuronal circuitry, underwent a substantial phenotypic change following peripheral axotomy, producing reactive gliosis. The gliosis was associated with the reduction of glial aminoacid transporters (GLT1 and GlyT1) and increase of neuronal glutamate transporter EAAC1. Although the expression of GABAergic neuronal marker GAD65/67 showed no change, glutamate increase, as demonstrated by HPLC analysis, shifted the excitatory/inhibitory balance as showed by the net increase of the glutamate/GABA ratio. Moreover, endogenous NGF levels were altered in SNI animals and not restored by the intrathecal NGF administration. This treatment reverted phenotypic changes associated with reactive astrocytosis, but failed to modify microglia activation. These findings on one hand confirm the correlation between gliopathy and maladaptive plasticity of the spinal synaptic circuitry, on the other hand add new data concerning the complex peculiar behavior of different glial cells in neuronal degenerative processes, defining a special role of microglia in sustaining the inflammatory response.

The effect of weight-bearing exercise and non-weight-bearing exercise on gait in rats with sciatic nerve crush injury

[Purpose] The purpose of this study was to access the effect of weight bearing exercise (treadmill exercise) and non-weight-bearing exercise (swimming exercise) on gait in the recovery process after a sciatic nerve crush injury. [Subjects and Methods] Rats were randomly divided into a swimming group (n=3) with non-weight-bearing exercise after a sciatic nerve crush and a treadmill group (n=3) with weight bearing exercise after a sciatic nerve crush. Dartfish is a program that can analyze and interpret motion through video images. The knee lateral epicondyle, lateral malleolus, and metatarsophalangeal joint of the fifth toe were marked by black dots before recording. [Results] There were significant differences in TOK (knee angle toe off) and ICK (knee angle at initial contact) in the swimming group and in TOK, ICA (ankle angle at initial contact), and ICK in the treadmill group. In comparison between groups, there were significant differences in TOA (ankle angle in toe off) and ICA at the 7th day. [Conclusion] There was no difference between weight bearing and non-weight-bearing exercise in sciatic nerve damage, and both exercises accelerated the recovery process in this study.

Cardiorespiratory fitness is associated with white matter integrity in aging

Objective

Aging is associated with reduced neural integrity, yet there are remarkable individual differences in brain health among older adults (OA). One factor that may attenuate age-related neural decline is cardiorespiratory fitness (CRF). The primary aim of this study was to link CRF to neural white matter microstructure using diffusion tensor imaging in OA.

Methods

Young adults (YA; n = 32) and OA (n = 27) completed a graded maximal exercise test to evaluate CRF and diffusion tensor magnetic resonance imaging to examine neural white matter integrity.

Results

As expected, pervasive age-related declines in white matter integrity were observed when OA were compared to YA. Further, peak VO₂ was positively associated with fractional anisotropy (FA), an indicator of white matter integrity, in multiple brain regions in OA, but not YA. In multiple posterior regions such as the splenium, sagittal stratum, posterior corona radiata, and superior parietal white matter, FA values were similar in YA and OA classified as higher fit, with both groups having greater FA than lower fit OA. However, age-related differences in FA values remained in other regions, including the body and genu of the corpus callosum, precuneus, and superior frontal gyrus.

Interpretation

CRF is positively associated with neural white matter microstructure in aging. The relationship between peak VO₂ and FA appears to be tract-specific, as equivalent FA values were observed in higher fit OA and YA in some white matter tracts, but not others. Further, the association between peak VO₂ and FA appears to be age-dependent.

Activity dependent therapies modulate the spinal changes that motoneurons suffer after a peripheral nerve injury

Injury of a peripheral nerve not only leads to target denervation, but also induces massive stripping of spinal synapses on axotomized motoneurons, with disruption of spinal circuits. Even when regeneration is successful, unspecific reinnervation and the limited reconnection of the spinal circuits impair functional recovery. The aim of this study was to describe the changes that axotomized motoneurons suffer after peripheral nerve injury and how activity-dependent therapies and neurotrophic factors can modulate these events. We observed a marked decrease in glutamatergic synapses, with a maximum peak at two weeks post-axotomy, which was only partially reversed with time. This decrease was accompanied by an increase in gephyrin immunoreactivity and a disintegration of perineuronal nets (PNNs) surrounding the motoneurons. Direct application of neurotrophins at the proximal stump was not able to reverse these effects. In contrast, activity-dependent treatment, in the form of treadmill running, reduced the observed destructuring of perineuronal nets and the loss of glutamatergic synapses two weeks after injury. These changes were proportional to the intensity of the exercise protocol. Blockade of sensory inputs from the homolateral hindlimb also reduced PNN immunoreactivity around intact motoneurons, and in that case treadmill running did not reverse that loss, suggesting that the effects of exercise on motoneuron PNN depend on increased sensory activity. Preservation of motoneuron PNN and reduction of synaptic stripping by exercise could facilitate the maintenance of the spinal circuitry and benefit functional recovery after peripheral nerve injury.

A combination of Schwann-cell grafts and aerobic exercise enhances sciatic nerve regeneration

Background

Despite the regenerative potential of the peripheral nervous system, severe nerve lesions lead to loss of target-organ innervation, making complete functional recovery a challenge. Few studies have given attention to combining different approaches in order to accelerate the regenerative process.

Objective

Test the effectiveness of combining Schwann-cells transplantation into a biodegradable conduit, with treadmill training as a therapeutic strategy to improve the outcome of repair after mouse nerve injury.

Methods

Sciatic nerve transection was performed in adult C57BL/6 mice; the proximal and distal stumps of the nerve were sutured into the conduit. Four groups were analyzed: acellular grafts (DMEM group), Schwann cell grafts (3×10⁵/2 µL; SC group), treadmill training (TMT group), and treadmill training and Schwann cell grafts (TMT + SC group). Locomotor function was assessed weekly by Sciatic Function Index and Global Mobility Test. Animals were anesthetized after eight weeks and dissected for morphological analysis.

Results

Combined therapies improved nerve regeneration, and increased the number of myelinated fibers and myelin area compared to the DMEM group. Motor recovery was accelerated in the TMT + SC group, which showed significantly better values in sciatic function index and in global mobility test than in the other groups. The TMT + SC group showed increased levels of trophic-factor expression compared to DMEM, contributing to the better functional outcome observed in the former group. The number of neurons in L4 segments was significantly higher in the SC and TMT + SC groups when compared to DMEM group. Counts of dorsal root ganglion sensory neurons revealed that TMT group had a significant increased number of neurons compared to DMEM group, while the SC and TMT + SC groups had a slight but not significant increase in the total number of motor neurons.

Conclusion

These data provide evidence that this combination of therapeutic strategies can significantly improve functional and morphological recovery after sciatic injury.

Treadmill exercise induced functional recovery after peripheral nerve repair is associated with increased levels of neurotrophic factors

Benefits of exercise on nerve regeneration and functional recovery have been reported in both central and peripheral nervous system disease models. However, underlying molecular mechanisms of enhanced regeneration and improved functional outcomes are less understood. We used a peripheral nerve regeneration model that has a good correlation between functional outcomes and number of motor axons that regenerate to evaluate the impact of treadmill exercise. In this model, the median nerve was transected and repaired while the ulnar nerve was transected and prevented from regeneration. Daily treadmill exercise resulted in faster recovery of the forelimb grip function as evaluated by grip power and inverted holding test. Daily exercise also resulted in better regeneration as evaluated by recovery of compound motor action potentials, higher number of axons in the median nerve and larger myofiber size in target muscles. Furthermore, these observations correlated with higher levels of neurotrophic factors, glial derived neurotrophic factor (GDNF), brain derived neurotrophic factor (BDNF) and insulin-like growth factor-1 (IGF-1), in serum, nerve and muscle suggesting that increase in muscle derived neurotrophic factors may be responsible for improved regeneration.

White matter connectivity and aerobic fitness in male adolescents

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DTI was collected for 34 male adolescents, ages 15–17.

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Aerobic fitness related to white matter connectivity in frontal and motor tracts.

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HF had higher tractography streamline counts in CST and Fminor compared to LF.

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A negative relationship was seen between VO₂ peak and FA in the L CST.

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Exercise is an important environmental factor to consider during neurodevelopment.

Exercise has been shown to have positive effects on the brain and behavior throughout various stages of the lifespan. However, little is known about the impact of exercise on neurodevelopment during the adolescent years, particularly with regard to white matter microstructure, as assessed by diffusion tensor imaging (DTI). Both tract-based spatial statistics (TBSS) and tractography-based along-tract statistics were utilized to examine the relationship between white matter microstructure and aerobic exercise in adolescent males, ages 15–18. Furthermore, we examined the data by both (1) grouping individuals based on aerobic fitness self-reports (high fit (HF) vs. low fit (LF)), and (2) using VO₂ peak as a continuous variable across the entire sample. Results showed that HF youth had an overall higher number of streamline counts compared to LF peers, which was driven by group differences in corticospinal tract (CST) and anterior corpus callosum (Fminor). In addition, VO₂ peak was negatively related to FA in the left CST. Together, these results suggest that aerobic fitness relates to white matter connectivity and microstructure in tracts carrying frontal and motor fibers during adolescence. Furthermore, the current study highlights the importance of considering the environmental factor of aerobic exercise when examining adolescent brain development.

The redox-associated adaptive response of brain to physical exercise

Reactive oxygen species (ROS) are continuously generated during metabolism. ROS are involved in redox signaling, but in significant concentrations they can greatly elevate oxidative damage leading to neurodegeneration. Because of the enhanced sensitivity of brain to ROS, it is especially important to maintain a normal redox state in brain and spinal cord cell types. The complex effects of exercise benefit brain function, including functional enhancement as well as its preventive and therapeutic roles. Exercise can induce neurogenesis via neurotrophic factors, increase capillarization, decrease oxidative damage, and enhance repair of oxidative damage. Exercise is also effective in attenuating age-associated loss in brain function, which suggests that physical activity-related complex metabolic and redox changes are important for a healthy neural system.

Sex differences in the effectiveness of treadmill training in enhancing axon regeneration in injured peripheral nerves

Exercise in the form of daily treadmill training results in significant enhancement of axon regeneration following peripheral nerve injury. Because androgens are also linked to enhanced axon regeneration, we wanted to investigate whether sex differences in the effect of treadmill training might exist. The common fibular nerves of thy-1-YFP-H mice were cut and repaired with a graft of the same nerve from a strain-matched wild-type donor mouse. Animals were treated with one of two daily treadmill training paradigms: slow continuous walking for 1 h or four higher intensity intervals of 2 min duration separated by 5-min rest periods. Training was begun on the third day following nerve injury and continued 5 days per week for 2 weeks. Effects on regeneration were evaluated by measuring regenerating axon profile lengths in optical sections through the repair sites and grafts at the end of the training period. No sex differences were found in untrained control mice. Continuous training resulted in significant enhancement of axon regeneration only in males. No effect was found in females or in castrated males. Interval training was effective in enhancing axon regeneration only in females and not in intact males or castrated males. Untrained females treated with the aromatase inhibitor, anastrozole, had significant enhancement of axon regeneration without increasing serum testosterone levels. Two different mechanisms exist to promote axon regeneration in a sex-dependent manner. In males treadmill training uses testicular androgens. In females, a different cellular mechanism for the effect of treadmill training must exist.

The influence of exercise on cognitive abilities

Scientific evidence based on neuroimaging approaches over the last decade has demonstrated the efficacy of physical activity improving cognitive health across the human lifespan. Aerobic fitness spares age-related loss of brain tissue during aging, and enhances functional aspects of higher order regions involved in the control of cognition. More active or higher fit individuals are capable of allocating greater attentional resources toward the environment and are able to process information more quickly. These data are suggestive that aerobic fitness enhances cognitive strategies enabling to respond effectively to an imposed challenge with a better yield in task performance. In turn, animal studies have shown that exercise has a benevolent action on health and plasticity of the nervous system. New evidence indicates that exercise exerts its effects on cognition by affecting molecular events related to the management of energy metabolism and synaptic plasticity. An important instigator in the molecular machinery stimulated by exercise is brain-derived neurotrophic factor, which acts at the interface of metabolism and plasticity. Recent studies show that exercise collaborates with other aspects of lifestyle to influence the molecular substrates of cognition. In particular, select dietary factors share similar mechanisms with exercise, and in some cases they can complement the action of exercise. Therefore, exercise and dietary management appear as a noninvasive and effective strategy to counteract neurological and cognitive disorders.

Differential effects of activity dependent treatments on axonal regeneration and neuropathic pain after peripheral nerve injury

Activity treatments are useful strategies to increase axonal regeneration and functional recovery after nerve lesions. They are thought to benefit neuropathy by enhancing neurotrophic factor expression. Nevertheless the effects on sensory function are still unclear. Since neurotrophic factors also play a fundamental role in peripheral and central sensitization, we studied the effects of acute electrical stimulation and early treadmill exercise on nerve regeneration and on neuropathic pain, and the relation with the expression of neurotrophins. After sciatic nerve section and suture repair, rats were subjected to electrical stimulation (ES) for 4h after injury, forced treadmill running (TR) for 5 days, or both treatments combined. Sciatic nerve section induced hyperalgesia in the medial area of the plantar skin in the injured paw. TR and ES differently but positively reduced adjacent neuropathic pain before and after sciatic reinnervation. ES enhanced motor and sensory reinnervation, and combination with TR induced strong agonistic effects in relieving pain. The differential effects of these activity treatments were related to changes in neurotrophic factor mRNA levels in sensory and motor neurons. ES speeded up expression of BDNF and GDNF in DRG, and of BDNF and NT3 in the ventral horn. TR reduced the levels of pro-nociceptive factors such as BDNF, NGF and GDNF in DRG. Combination of ES and TR induced intermediate levels suggesting an optimal balancing of treatment effects.

The effect of exercise on mobilization of hematopoietic progenitor cells involved in the repair of sciatic nerve crush injury

Object Mobilization of hematopoietic progenitor cells (HPCs) from bone marrow involved in the process of peripheral nerve regeneration occurs mostly through deposits of CD34(+) cells. Treadmill exercise, with either differing intensity or duration, has been shown to increase axon regeneration and sprouting, but the effect of mobilization of HPCs on peripheral nerve regeneration due to treadmill exercise has not yet been elucidated. Methods Peripheral nerve injury was induced in Sprague-Dawley rats by crushing the left sciatic nerve using a vessel clamp. The animals were categorized into 2 groups: those with and without treadmill exercise (20 m/min for 60 minutes per day for 7 days). Cytospin and flow cytometry were used to determine bone marrow progenitor cell density and distribution. Neurobehavioral analysis, electrophysiological study, and regeneration marker expression were investigated at 1 and 3 weeks after exercise. The accumulation of HPCs, immune cells, and angiogenesis factors in injured nerves was determined. A separate chimeric mice study was conducted to assess CD34(+) cell distribution according to treadmill exercise group. Results Treadmill exercise significantly promoted nerve regeneration. Increased Schwann cell proliferation, increased neurofilament expression, and decreased Schwann cell apoptosis were observed 7 days after treadmill exercise. Elevated expression of S100 and Luxol fast blue, as well as decreased numbers of vacuoles, were identified in the crushed nerve 3 weeks after treadmill exercise. Significantly increased numbers of mononuclear cells, particularly CD34(+) cells, were induced in bone marrow after treadmill exercise. The deposition of CD34(+) cells was abolished by bone marrow irradiation. In addition, deposits of CD34(+) cells in crushed nerves paralleled the elevated expressions of von Willebrand factor, isolectin B4, and vascular endothelial growth factor. Conclusions Bone marrow HPCs, especially CD34(+) cells, were able to be mobilized by low-intensity treadmill exercise, and this effect paralleled the significant expression of angiogenesis factors. Treadmill exercise stimulation of HPC mobilization during peripheral nerve regeneration could be used as a therapy in human beings.

Changes in spatial memory and BDNF expression to simultaneous dietary restriction and forced exercise

Previous literature suggests that learning and memory formation can be influenced by diet and exercise. In the current study, we investigated the combined effects of forced swimming exercise (FSE) and every other day fasting (EODF) on spatial memory formation and on the levels of brain-derived neurotrophic factor (BDNF) in the hippocampus of Wistar male rats. The radial arm water maze (RAWM) paradigm was used to assess changes in learning and memory formation, whereas ELISA assay was used to measure BDNF protein levels. The FSE and/or EODF were simultaneously instituted for 6 weeks. Results show that FSE improved learning, short-term as well as long-term memory formation, and significantly increased BDNF protein in the hippocampus (p<0.05). However, EODF had no effect on either spatial learning and memory formation or the levels of hippocamapal BDNF protein (p>0.05). In addition, EODF did not modulate beneficial effect of swimming exercise on cognitive function (p>0.05). Thus exercise enhanced, while EODF did not affect spatial learning and memory formation.

Exercise alleviates hypoalgesia and increases the level of calcitonin gene-related peptide in the dorsal horn of the spinal cord of diabetic rats

OBJECTIVE

The aim of this study was to evaluate the effects of treadmill training on nociceptive sensitivity and immunoreactivity to calcitonin gene-related peptide in the dorsal horn of the spinal cord of diabetic rats.

METHODS

Male Wistar rats were divided into three groups: control, diabetic and trained diabetic. Treadmill training was performed for 8 weeks. The blood glucose concentrations and body weight were evaluated 48 h after diabetes induction and every 30 days thereafter. The nociceptive sensitivity was evaluated using the tail-flick apparatus. The animals were then transcardially perfused, and the spinal cords were post-fixed, cryoprotected and sectioned in a cryostat. Immunohistochemistry for calcitonin gene-related peptide analysis was performed on the dorsal horn of the spinal cord.

RESULTS

The nociceptive sensitivity analysis revealed that, compared with the control and trained diabetic animals, the latency to tail deflection on the apparatus was longer for the diabetic animals. Optical densitometry demonstrated decreased calcitonin gene-related peptide immunoreactivity in the dorsal horn of the spinal cord in diabetic animals, which was reversed by treadmill training.

CONCLUSION

We concluded that treadmill training can alleviate nociceptive hypoalgesia and reverse decreased calcitonin gene-related peptide immunoreactivity in the dorsal horn of the spinal cord of diabetic animals without pharmacological treatment.

Repercussões morfológicas e funcionais do exercício sobre a regeneração nervosa periférica

INTRODUÇÃO: A indicação de exercícios físicos como abordagem terapêutica após lesões nervosas periféricas, apesar de resultados clínicos favoráveis, ainda não é consenso na literatura pertinente. Objetivos: Este estudo buscou avaliar a interferência do exercício em roda motorizada no processo de regeneração nervosa periférica. MATERIAIS E MÉTODOS: Foram utilizados 48 ratos Wistar divididos proporcionalmente em quatro grupos de 12 ratos cada (seis controles e seis experimentais). Após treinamento, os ratos foram submetidos à axonotmese do nervo ciático direito. Todos os grupos iniciaram o regime de exercícios diários 24 horas após a lesão, porém com durações diferenciadas (G1 por 28 dias; G2 por 21 dias, G3 por 14 dias e G4 por sete dias). O exercício teve tempo e velocidade ajustados de acordo com o tempo de tratamento para cada grupo. RESULTADOS: Por meio do cálculo do Índice Funcional do Ciático, os animais submetidos ao exercício apresentaram sinais de atraso na recuperação funcional da marcha, em comparação a animais controles. Na análise histológica dos nervos lesionados, observou-se que animais exercitados apresentaram sinais de preservação dos axônios no coto proximal e de degeneração no coto distal, e os animais exercitados por somente sete dias não apresentaram a mesma característica degenerativa no coto distal. A aplicação de exercícios ativos e sem carga em animais que sofreram axonotmese interfere negativamente no processo de recuperação funcional de alguns parâmetros da marcha. CONCLUSÃO: Apesar de o exercício ativo não ter interferido na manutenção da viabilidade neuronal nos sítios proximais à lesão, sua continuidade prejudicou a viabilidade dos cotos neurais distais, provável causa do atraso na recuperação funcional da marcha.

Effect of skilled and unskilled training on nerve regeneration and functional recovery

The most disabling aspect of human peripheral nerve injuries, the majority of which affect the upper limbs, is the loss of skilled hand movements. Activity-induced morphological and electrophysiological remodeling of the neuromuscular junction has been shown to influence nerve repair and functional recovery. In the current study, we determined the effects of two different treatments on the functional and morphological recovery after median and ulnar nerve injury. Adult Wistar male rats weighing 280 to 330 g at the time of surgery (N = 8-10 animals/group) were submitted to nerve crush and 1 week later began a 3-week course of motor rehabilitation involving either "skilled" (reaching for small food pellets) or "unskilled" (walking on a motorized treadmill) training. During this period, functional recovery was monitored weekly using staircase and cylinder tests. Histological and morphometric nerve analyses were used to assess nerve regeneration at the end of treatment. The functional evaluation demonstrated benefits of both tasks, but found no difference between them (P > 0.05). The unskilled training, however, induced a greater degree of nerve regeneration as evidenced by histological measurement (P < 0.05). These data provide evidence that both of the forelimb training tasks used in this study can accelerate functional recovery following brachial plexus injury.

The influence of aerobic fitness on cerebral white matter integrity and cognitive function in older adults: results of a one-year exercise intervention

Cerebral white matter (WM) degeneration occurs with increasing age and is associated with declining cognitive function. Research has shown that cardiorespiratory fitness and exercise are effective as protective, even restorative, agents against cognitive and neurobiological impairments in older adults. In this study, we investigated whether the beneficial impact of aerobic fitness would extend to WM integrity in the context of a one-year exercise intervention. Further, we examined the pattern of diffusivity changes to better understand the underlying biological mechanisms. Finally, we assessed whether training-induced changes in WM integrity would be associated with improvements in cognitive performance independent of aerobic fitness gains. Results showed that aerobic fitness training did not affect group-level change in WM integrity, executive function, or short-term memory, but that greater aerobic fitness derived from the walking program was associated with greater change in WM integrity in the frontal and temporal lobes, and greater improvement in short-term memory. Increases in WM integrity, however, were not associated with short-term memory improvement, independent of fitness improvements. Therefore, while not all findings are consistent with previous research, we provide novel evidence for correlated change in training-induced aerobic fitness, WM integrity, and cognition among healthy older adults.

Collaborative effects of diet and exercise on cognitive enhancement

Certain dietary factors, such as omega-3 fatty acids and curcumin, are reviewed in their context of stimulating molecular systems that serve synaptic function, while diets rich in saturated fats do the opposite. In turn, exercise, using similar mechanisms as healthy diets, displays healing effects on the brain such as counteracting the mental decline associated with age and facilitating functional recovery resulting from brain injury and disease. Diet and exercise are two noninvasive approaches that used together may enhance neural repair. Omega 3 fatty acids and curcumin elevate levels of molecules important for synaptic plasticity such as brain-derived neurotrophic factor (BDNF), thus benefiting normal brain function and recovery events following brain insults.

The influence of dietary factors in central nervous system plasticity and injury recovery

Although feeding is an essential component of life, it is only recently that the actions of foods on brain plasticity and function have been scrutinized. There is evidence that select dietary factors are important modifiers of brain plasticity and can have an impact on central nervous system health and disease. Results of new research indicate that dietary factors exert their effects by affecting molecular events related to the management of energy metabolism and synaptic plasticity. Recent study results show that select dietary factors have mechanisms similar to those of exercise, and that, in some cases, dietary factors can complement the action of exercise. Abundant research findings in animal models of central nervous system injury support the idea that nutrients can be taken in through whole foods and dietary supplements to reduce the consequences of neural damage. Therefore, exercise and dietary management appear as a noninvasive and effective strategy to help counteract neurologic and cognitive disorders.

Spinal cord injury and physical activity: preservation of the body

SETTING

Spinal cord injury (SCI) causes devastating loss of function and can result in serious secondary complications. Although significant advances are being made to develop cellular and molecular therapies to promote regeneration, it is important to optimize physical interventions.

OBJECTIVES

The objective of this review was to examine the evidence for the effects of physical rehabilitation strategies on health and fitness, and maintenance of target systems below the level of injury (for example, muscle, bone, circulation).

RESULTS

Exercise appears to be a potent means of achieving these goals, using a variety of strategies.

CONCLUSION

Physical rehabilitation after SCI needs to move beyond the goal of maximizing independence to focus on maintenance of optimum health and fitness as well as maintenance of target system function below the level of injury. Issues requiring further investigation include identification of the optimum dosage of interventions to achieve specific goals, for example, prevention of muscle atrophy and osteoporosis, and development and validation of simple clinical measures to monitor the changes in body composition. Adoption of a classification system for physical interventions and standardized outcome measures would facilitate large-scale observational studies to identify the critical variables contributing to better outcomes.

Activity-dependent plasticity and gene expression modifications in the adult CNS

Information processing, memory formation, or functional recovery after nervous system damage depend on the ability of neurons to modify their functional properties or their connections. At the cellular/molecular level, structural modifications of neural circuits are finely regulated by intrinsic neuronal properties and growth-regulatory cues in the extracellular milieu. Recently, it has become clear that stimuli coming from the external world, which comprise sensory inflow, motor activity, cognitive elaboration, or social interaction, not only provide the involved neurons with instructive information needed to shape connection patterns to sustain adaptive function, but also exert a powerful influence on intrinsic and extrinsic growth-related mechanisms, so to create permissive conditions for neuritic remodeling. Here, we present an overview of recent findings concerning the effects of experience on molecular mechanisms underlying CNS structural plasticity, both in physiological conditions and after damage, with particular focus on activity-dependent modulation of growth-regulatory genes and epigenetic modifications.

Stochastic resonance activity influences serum tryptophan metabolism in healthy human subjects

Background

Stochastic resonance therapy (SRT) is used for rehabilitation of patients with various neuropsychiatric diseases. An alteration in tryptophan metabolism along the kynurenine pathway has been identified in the central and peripheral nervous systems in patients with neuroinflammatory and neurodegenerative diseases and during the aging process. This study investigated the effect of SRT as an exercise activity on serum tryptophan metabolites in healthy subjects.

Methods

Serum L-tryptophan, L-kynurenine, kynurenic acid, and anthranilic acid levels were measured one minute before SRT and at one, 5, 15, 30, and 60 minutes after SRT. We found that SRT affected tryptophan metabolism. Serum levels of L-tryptophan, L-kynurenine, and kynurenic acid were significantly reduced for up to 60 minutes after SRT. Anthranilic acid levels were characterized by a moderate, non significant transient decrease for up to 15 minutes, followed by normalization at 60 minutes. Tryptophan metabolite ratios were moderately altered, suggesting activation of metabolism after SRT. Lowering of tryptophan would generally involve activation of tryptophan catabolism and neurotransmitter, protein, and bone biosynthesis. Lowering of kynurenic acid by SRT might be relevant for improving symptoms in patients with neuropsychiatric disorders, such as Parkinson’s disease, Alzheimer’s disease, schizophrenia, and depression, as well as certain pain conditions.

Spinal cord injury triggers an intrinsic growth-promoting state in nociceptors

Although most investigations of the mechanisms underlying chronic pain after spinal cord injury (SCI) have examined the central nervous system (CNS), recent studies have shown that nociceptive primary afferent neurons display persistent hyperexcitability and spontaneous activity in their peripheral branches and somata in dorsal root ganglia (DRG) after SCI. This suggests that SCI-induced alterations of primary nociceptors contribute to central sensitization and chronic pain after SCI. Does SCI also promote growth of these neurons' fibers, as has been suggested in some reports? The present study tests the hypothesis that SCI induces an intrinsic growth-promoting state in DRG neurons. This was tested by dissociating DRG neurons 3 days or 1 month after spinal contusion injury at thoracic level T10 and measuring neuritic growth 1 day later. Neurons cultured 3 days after SCI exhibited longer neurites without increases in branching ("elongating growth"), compared to neurons from sham-treated or untreated (naïve) rats. Robust promotion of elongating growth was found in small and medium-sized neurons (but not large neurons) from lumbar (L3-L5) and thoracic ganglia immediately above (T9) and below (T10-T11) the contusion site, but not from cervical DRG. Elongating growth was also found in neurons immunoreactive to calcitonin gene-related peptide (CGRP), suggesting that some of the neurons exhibiting enhanced neuritic growth were nociceptors. The same measurements made on neurons dissociated 1 month after SCI revealed no evidence of elongating growth, although evidence for accelerated initiation of neurite outgrowth was found. Under certain conditions this transient growth-promoting state in nociceptors might be important for the development of chronic pain and hyperreflexia after SCI.

Enhancing recovery from peripheral nerve injury using treadmill training

Full functional recovery after traumatic peripheral nerve injury is rare. We postulate three reasons for the poor functional outcome measures observed. Axon regeneration is slow and not all axons participate. Significant misdirection of regenerating axons to reinnervate inappropriate targets occurs. Seemingly permanent changes in neural circuitry in the central nervous system are found to accompany axotomy of peripheral axons. Exercise in the form of modest daily treadmill training impacts all three of these areas. Compared to untrained controls, regenerating axons elongate considerably farther in treadmill trained animals and do so via an autocrine/paracrine neurotrophin signaling pathway. This enhancement of axon regeneration takes place without an increase in the amount of misdirection of regenerating axons found without training. The enhancement also occurs in a sex-dependent manner. Slow continuous training is effective only in males, while more intense interval training is effective only in females. In treadmill trained, but not untrained mice the extent of coverage of axotomized motoneurons is maintained, thus preserving important elements of the spinal circuitry.

Schwann-spheres derived from injured peripheral nerves in adult mice--their in vitro characterization and therapeutic potential

Multipotent somatic stem cells have been identified in various adult tissues. However, the stem/progenitor cells of the peripheral nerves have been isolated only from fetal tissues. Here, we isolated Schwann-cell precursors/immature Schwann cells from the injured peripheral nerves of adult mice using a floating culture technique that we call “Schwann-spheres." The Schwann-spheres were derived from de-differentiated mature Schwann cells harvested 24 hours to 6 weeks after peripheral nerve injury. They had extensive self-renewal and differentiation capabilities. They strongly expressed the immature-Schwann-cell marker p75, and differentiated only into the Schwann-cell lineage. The spheres showed enhanced myelin formation and neurite growth compared to mature Schwann cells in vitro. Mature Schwann cells have been considered a promising candidate for cell-transplantation therapies to repair the damaged nervous system, whereas these “Schwann-spheres" would provide a more potential autologous cell source for such transplantation.

Neurotrophin-4/5 is implicated in the enhancement of axon regeneration produced by treadmill training following peripheral nerve injury

The role of neurotrophin-4/5 (NT-4/5) in the enhancement of axon regeneration in peripheral nerves produced by treadmill training was studied in mice. Common fibular nerves of animals of the H strain of thy-1-YFP mice, in which a subset of axons in peripheral nerves is marked by the presence of yellow fluorescent protein, were cut and surgically repaired using nerve grafts from non-fluorescent mice. Lengths of profiles of fluorescent regenerating axons were measured using optical sections made through whole mounts of harvested nerves. Measurements from mice that had undergone 1 h of daily treadmill training at modest speed (10 m/min) were compared with those of untrained (control) mice. Modest treadmill training resulted in fluorescent axon profiles that were nearly twice as long as controls at 1, 2 and 4 week survival times. Similar enhanced regeneration was found when cut nerves of wild type mice were repaired with grafts from NT-4/5 knockout mice or grafts made acellular by repeated freezing/thawing. No enhancement was produced by treadmill training in NT-4/5 knockout mice, irrespective of the nature of the graft used to repair the cut nerve. Much as had been observed previously for the effects of brief electrical stimulation, the effects of treadmill training on axon regeneration in cut peripheral nerves are independent of changes produced in the distal segment of the cut nerve and depend on the promotion of axon regeneration by changes in NT-4/5 expression by cells in the proximal nerve segment.

Balance and coordination training after sciatic nerve injury

INTRODUCTION

Numerous therapeutic interventions have been tested to enhance functional recovery after peripheral nerve injuries.

METHODS

After sciatic nerve crush in rats we tested balance and coordination and motor control training in sensorimotor tests and analyzed nerve and muscle histology.

RESULTS

The balance and coordination training group and the sham group had better results than the sedentary and motor control groups in sensorimotor tests. The sham and balance and coordination groups had a significantly larger muscle area than the other groups, and the balance and coordination group showed significantly better values than the sedentary and motor control groups for average myelin sheath thickness and g-ratio of the distal portion of the nerve.

CONCLUSIONS

The findings indicate that balance and coordination training improves sciatic nerve regeneration, suggesting that it is possible to revert and/or prevent soleus muscle atrophy and improve performance on sensorimotor tests.

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.

Repairing injured peripheral nerves: Bridging the gap

Peripheral nerve injuries that induce gaps larger than 1-2 cm require bridging strategies for repair. Autologous nerve grafts are still the gold standard for such interventions, although alternative treatments, as well as treatments to improve the therapeutic efficacy of autologous nerve grafting are generating increasing interest. Investigations are still mostly experimental, although some clinical studies have been undertaken. In this review, we aim to describe the developments in bridging technology which aim to replace the autograft. A multi-disciplinary approach is of utmost importance to develop and optimise treatments of the most challenging peripheral nerve injuries.

Exercise and fitness are related to peripheral nervous system function in overweight adults

PURPOSE

This study examined the association between physical activity and fitness and peripheral nervous system (PNS) function in overweight and obese individuals.

METHODS

Forty nondiabetic overweight adults (mean +/- SD; age = 44 +/- 11 yr) were recruited for the study. Peroneal motor nerve and radial, sural, and medial plantar sensory nerve conductions were studied. Maximal oxygen uptake was measured in an incremental bicycle ergometer test. Physical activity was assessed by accelerometer and self-reporting. We analyzed the data using multiple stepwise linear regression models adjusted for age, height, and skin temperature.

RESULTS

VO2max predicted 17% of peroneal distal compound muscle action potential (CMAP) amplitude variation and 16% of peroneal proximal CMAP amplitude variation. Physical activity index at the age of 30 yr predicted 9% of peroneal motor nerve conduction velocity (NCV), 8% of peroneal F-wave maximum latency, 14% of medial plantar sensory latency, and 10% of medial plantar sensory NCV variation.

CONCLUSIONS

Physical activity and fitness are positively associated with PNS function and should be encouraged in overweight people.

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

BACKGROUND

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

OBJECTIVE

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

DESIGN

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

METHODS

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

RESULTS

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

LIMITATIONS

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

CONCLUSIONS

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

ERK1/2-mediated Schwann cell proliferation in the regenerating sciatic nerve by treadmill training

Proliferation of Schwann cells in the injured peripheral nerve supports axonal regeneration, and physical training in experimental animals has been shown to promote nerve regeneration. Extracellular signal-regulated kinase 1/2 (ERK1/2) activity can mediate neuronal responses to lesion signals, but its role in non-neuronal cells in the injured area is largely unknown. Here we report that treadmill training (TMT) facilitates axonal regeneration via the upregulation of phospho-ERK1/2 protein levels in Schwann cells in the injured sciatic nerve. Low-intensity, but not high-intensity, TMT increased neurite outgrowth of dorsal root ganglion (DRG) sensory neurons and potentiated Schwann cell proliferation. TMT elevated levels of GAP-43 mRNA and protein, and phospho-ERK1/2 protein in the injured sciatic nerves. TMT also enhanced phospho-c-Jun protein levels in the injured nerve. In-vivo administration of the ERK1/2 inhibitor PD98059 eliminated phospho-c-Jun, suggesting ERK1/2 phosphorylation of the c-Jun protein. PD98059 treatment decreased levels of BrdU-labeled proliferating Schwann cells in the distal portion of the injured nerve, and delayed the axonal regrowth that was promoted by TMT. The present data suggest that increased ERK1/2 activity in Schwann cells may play an important role in TMT-mediated enhancement of axonal regeneration in the injured peripheral nerve.