Treadmill training promotes axon regeneration in injured peripheral nerves
Introduction
Exercise has been shown to improve motor function following spinal cord injury both in animal models and in clinical studies (Edgerton et al., 1997, Hutchinson et al., 2004, Skinner et al., 1996). The effect of exercise on recovery from peripheral nerve injury has received less attention. Damaged axons in peripheral nerves are capable of significant regeneration but functional recovery in human patients with peripheral nerve injuries is poor (Brushart, 1998). The reason most often cited for poor recovery is that axon regeneration is slow (Fawcett and Keynes, 1990). Seven days of running-wheel training prior to peripheral nerve injury in rats has been found to increase afferent axonal outgrowth as measured from cultured DRG neurons (Molteni et al., 2004). The extent of enhancement was related to the extent of running-wheel usage. Pre-injury exercise may “prime” adult dorsal root ganglion neurons for increased axon regeneration. In another recent study, treadmill training was found to enhance sensory functional recovery, as measured from nerve recordings (Marqueste et al., 2004). However, there is little direct evidence of an effect of post-injury exercise on axon regeneration in injured peripheral nerves.
The data from studies of electrical stimulation, neurotrophins, and peripheral nerve injury are encouraging. If the proximal stump of a transected nerve is stimulated for as little as 1 h at the time of surgical repair, axon regeneration is enhanced (Al-Majed et al., 2000a, Al-Majed et al., 2000b, English et al., 2006, Al-Majed et al., 2000b). This enhancement is associated with an increase in BDNF and trkB in the regenerating neurons (Al-Majed et al., 2000a, Al-Majed et al., 2000b) and is dependent on neuronal neurotrophins (English et al., 2006, Al-Majed et al., 2000b). Recent experiments have also shown that exercise affects an increase in neurotrophins (e.g., BDNF) (Berchtold et al., 2005, Gomez-Pinilla et al., 2002) and that neurotrophins are required for even control levels of axon regeneration after peripheral nerve injury (English et al., 2005). Therefore, there is sufficient background to hypothesize that exercise should also enhance axon regeneration after peripheral nerve injury.
Treadmill training is readily applied and can be used by both laboratory animals and human subjects. In clinical studies of exercise with human subjects, continuous treadmill locomotion at a moderate to brisk pace is commonly used. When allowed to exercise voluntarily, laboratory mice utilize a markedly different pattern. This pattern is characterized by repeated short-duration runs (2 min) at speeds that are close to maximum, with rest periods interspersed between runs (De Bono et al., 2006), a form of interval training. In this study we provide direct evidence that treadmill training enhances the growth of regenerating axons in injured peripheral nerves. We also show that there are differences in the effects of these two forms of treadmill training on axon regeneration in the first two weeks following injury. A preliminary report of some of these findings has been made (English et al., 2006, Al-Majed et al., 2000b).
Section snippets
Animals and surgical procedures
All experimental procedures conformed to the Guidelines for the Use of Animals in Research of the Society for Neuroscience and were approved by the Institutional Animal Care and Use Committee of Emory University. All experiments were conducted using thy-1-YFP-H mice (Feng et al., 2000) on a C57BL/6J background. The thy-1-YFP-H mice are maintained as heterozygotes (YFP+), so that half of all litters born to YFP+ mice will not contain the transgene. In these transgenic mice, yellow fluorescent
Results
Typical results from a treadmill trained and an unexercised mouse are shown in Fig. 1. The two panels in this figure are each a montage of images from several microscope fields taken at the same optical plane through a surgically repaired CF nerve, two weeks after nerve repair. The proximal stump of the nerve is toward the top in each panel, and the proximal surgical repair site is indicated by the upper arrows. The distal extent of the nerve allograft used to repair the cut nerve is indicated
Discussion
The main finding of this study is that treadmill exercise, even in very small doses, enhances axon regeneration in the peripheral nervous system. As little as 1 h daily of continuous treadmill locomotion at a slow speed, or two two-min bouts of more intense treadmill locomotion at near maximal treadmill running speed (Lerman et al., 2002) performed during the first two weeks after peripheral nerve transection and repair enhances growth of regenerating axons. Exercise, and in particular
Acknowledgments
Thanks are due to W. L. Gore & Associates, Inc., for the gift of the Gore-Tex tubing; and to Dr. Robert McKeon and Dr. Robert Gregor, both of whom read and commented on earlier versions of the manuscript. This work was supported with funding from grant HD43596. Manning J Sabatier was supported by the USPHS NIH Institutional Research and Academic Career Development grant, #K12 GM00680-05.
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2021, Experimental GerontologyCitation Excerpt :This finding is partly in accordance with some previous studies that reported enhanced NCV of sensory and motor nerves following aerobic exercise training (Dixit et al., 2014; Gholami et al., 2018). The evidence showed that the neural system responds positively to physical activity through morphological and functional adaptations to the training stimulus (Shokouhi et al., 2008; Bobinski et al., 2011; Gardiner et al., 2006; Sabatier et al., 2008). Exercise training has been shown to decrease apoptosis of Schwann cells and increase myelin thickening (Shokouhi et al., 2008), increase Schwann cell proliferation (Bobinski et al., 2011), enhance axonal regeneration of the peroneal nerve (Sabatier et al., 2008) and improve axon transport and electrophysiological properties (Gardiner et al., 2006).
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