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.

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

https://www.termis.org/docs/eu10_abstracts/TERMIS2010_0064.pdf

Controlling dispersion of axonal regeneration using a multichannel collagen nerve conduit

The present study aims to investigate the potential of brief electrical stimulation (ES; 3 V, 20 Hz, 20 min) in improving functional recovery in delayed nerve injury repair (DNIR). The sciatic nerve of Sprague Dawley rats was transected, and the repair of nerve injury was delayed for different time durations (2, 4, 12 and 24 weeks). Brief depolarizing ES was applied to the proximal nerve stump when the transected nerve stumps were bridged with a hollow nerve conduit (5 mm in length) after delayed periods. We found that the diameter and number of regenerated axons, the thickness of myelin sheath, as well as the number of Fluoro-Gold retrograde-labeled motoneurons and sensory neurons were significantly increased by ES, suggesting that brief ES to proximal nerve stumps is capable of promoting nerve regeneration in DNIR with different delayed durations, with the longest duration of 24 weeks. In addition, the amplitude of compound muscle action potential (gastrocnemius muscle) and nerve conduction velocity were also enhanced, and gastrocnemius muscle atrophy was partially reversed by brief ES, indicating that brief ES to proximal nerve stump was able to improve functional recovery in DNIR. Furthermore, brief ES was capable of increasing brain-derived neurotrophic factor (BDNF) expression in the spinal cord in DNIR, suggesting that BDNF-mediated neurotrophin signaling might be one of the contributing factors to the beneficial effect of brief ES on DNIR. In conclusion, the present findings indicate the potential of using brief ES as a useful method to improve functional recovery for delayed repair of peripheral nerve lesions.

Electrical stimulation accelerates nerve regeneration and functional recovery in delayed peripheral nerve injury in rats.

Netrin-1 was the first axon guidance molecule to be discovered in vertebrates and has a strong chemotropic function for axonal guidance, cell migration, morphogenesis and angiogenesis. It is a secreted axon guidance cue that can trigger attraction by binding to its canonical receptors Deleted in Colorectal Cancer (DCC) and Neogenin or repulsion through binding the DCC/Uncoordinated (Unc5) A–D receptor complex. The crystal structures of Netrin-1/receptor complexes have recently been revealed. These studies have provided a structure based explanation of Netrin-1 bi-functionality. Netrin-1 and its receptor are continuously expressed in the adult nervous system and are differentially regulated after nerve injury. In the adult spinal cord and optic nerve, Netrin-1 has been considered as an inhibitor that contributes to axon regeneration failure after injury. In the peripheral nervous system, Netrin-1 receptors are expressed in Schwann cells, the cell bodies of sensory neurons and the axons of both motor and sensory neurons. Netrin-1 is expressed in Schwann cells and its expression is up-regulated after peripheral nerve transection injury. Recent studies indicated that Netrin-1 plays a positive role in promoting peripheral nerve regeneration, Schwann cell proliferation and migration. Targeting of the Netrin-1 signaling pathway could develop novel therapeutic strategies to promote peripheral nerve regeneration and functional recovery.

https://www.mdpi.com/1422-0067/18/3/491/pdf

Role of Netrin-1 Signaling in Nerve Regeneration.

Sciatic nerve injury: a simple and subtle model for investigating many aspects of nervous system damage and recovery.

Misdirection of regenerating motor axons after nerve injury and repair in the rat sciatic nerve model

Ankle motion analysis may provide a better method to assess function in the rat sciatic nerve model than the standard method, the sciatic functional index (SFI), but it is not widely used in experiments on nerve regeneration possibly because of complicated analysis. In this study, we investigated the practical use of a two-dimensional (2D) digital video motion analysis system. Reproducibility was investigated in normal rats. Recovery of ankle motion was analyzed after sciatic, tibial, and peroneal nerve crush injury. Results were compared with scores for the SFI. Results were not significantly different from animal-to-animal and day-to-day. Interobserver variability also was small. In the analysis of recovery after separate nerve crush injuries, subtle differences in ankle plantar flexion and dorsiflexion could be detected. The method was also more sensitive than the SFI: whereas scores for the SFI had returned to normal 4 weeks after sciatic nerve crush injury, the ankle angle at mid-stance was still significantly different from that in sham-operated animals 6 weeks after the injury. 2D digital video ankle motion analysis is a practical and sensitive method to assess function in the rat sciatic nerve model.

Two-dimensional digital video ankle motion analysis for assessment of function in the rat sciatic nerve model.

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Misdirection of regenerating motor axons after nerve injury and repair in the rat sciatic nerve model

Two-dimensional digital video ankle motion analysis for assessment of function in the rat sciatic nerve model.