Axon regeneration and exercise-dependent plasticity after spinal cord injury

Abstract

Current dogma states that meaningful recovery of function after spinal cord injury (SCI) will likely require a combination of therapeutic interventions comprised of regenerative/neuroprotective transplants, addition of neurotrophic factors, elimination of inhibitory molecules, functional sensorimotor training, and/or stimulation of paralyzed muscles or spinal circuits. We routinely use (1) peripheral nerve grafts to support and direct axonal regeneration across an incomplete cervical or complete thoracic transection injury, (2) matrix modulation with chondroitinase (ChABC) to facilitate axonal extension beyond the distal graft-spinal cord interface, and (3) exercise, such as forced wheel walking, bicycling, or step training on a treadmill. We and others have demonstrated an increase in spinal cord levels of endogenous neurotrophic factors with exercise, which may be useful in facilitating elongation and/or synaptic activity of regenerating axons and plasticity of spinal neurons below the level of injury.

© 2013 New York Academy of Sciences.

Figures

Figure 1

Axon regeneration into peripheral nerve grafts. (A) Diagrammatic representation of a PNG bridging a spinal cord lesion with a demonstration of axonal growth into and through the distal end of the graft where ChABC was delivered to degrade inhibitory matrix molecules. This illustration shows the flexibility of the PNG in that grafts of a specified length can be created, the distal end can be apposed to a specific region or level of the spinal cord and the graft is external to the spinal canal which facilitates isolation for electrophysiological testing and microinjection of tracers into the graft. (B) Regenerating axons immunolabeled with antibody to neurofilament are oriented in a longitudinal array within the PNG. (C) Evaluation of successful grafting includes quantitation of the number of myelinated axons observed in a transverse section through the PNG. Here myelinated axons of various diameters are seen surrounded by fibrous-like connective tissue.

Figure 2

Exercise devices used for spinal cord injured rats. (A) A motorized exercise wheel (Lafayette Instruments, Lafayette, IN) is useful for rats that have a unilateral injury so that they can maintain a slow walking speed using 3 limbs initially, until the impaired limb regains some measure of function. Changes in speed and duration of exercise are routinely implemented to encourage rats to perform at a higher level. (B) Motorized cycles (custom-made) for spinalized rats provide rhythmic sensory input to the lumbar spinal cord as they move the hind limbs through a complete range of motion. As little as 15 minutes of cycling per day is sufficient to increase intraspinal neurotrophic factor levels. (C) Step training of spinalized rats on a motorized treadmill provides loading of hind limb muscles which does not occur with cycling. Similar to treadmill training of cats (see Rossignol and Martinez, this issue), spinalized rats require assistance with balance and perineal stimulation to initiate locomotion.

Figure 3

Deafferentation abolishes exercise dependent increase in neurotrophic factor mRNA. Rats with a complete thoracic transection injury show significant increase in levels of BDNF mRNA after cycling exercise. When dorsal root axons caudal to the injury are removed bilaterally there is no benefit from exercise of these rats. This indicates the necessity of sensory information being transmitted to the spinal cord during exercise.

Figure 4

Neurotrophin expression with acute or delayed exercise after SCI. (A) Western blot data demonstrates the presence of comparable levels of BDNF in the lumbar spinal cord when cycling exercise was initiated 5 days after injury (acute) or 28 days after injury (delayed). (B) Frequency dependent depression of the H reflex was restored in spinalized rats that received acute or delayed cycling exercise. Together these observations demonstrate the potential for exercise dependent plasticity in the injured spinal cord at short and long intervals after SCI.

Figure 5

Immunolabeling for cFos after SCI and exercise. With bike or step training there is an increase in the number of neurons in the L4 dorsal horn (DH) and intermediate gray (IG) compared to neurons in untrained animals. The base level of cFos in motoneurons of the ventral horn (VH) does not appear to be changed with exercise. The expression of cFos is used as an indicator of neuron activity following hind limb exercise of spinalized rats.