Exercise restores levels of neurotrophins and synaptic plasticity following spinal cord injury

https://doi.org/10.1016/j.expneurol.2005.01.015Get rights and content

Abstract

We have conducted studies to determine the potential of exercise to benefit the injured spinal cord using neurotrophins. Adult rats were randomly assigned to one of three groups: (1) intact control (Con); (2) sedentary, hemisected at a mid-thoracic level (Sed-Hx), or (3) exercised, hemisected (Ex-Hx). One week after surgery, the Ex-Hx rats were exposed to voluntary running wheels for 3, 7, or 28 days. BDNF mRNA levels on the lesioned side of the spinal cord lumbar region of Sed-Hx rats were ∼80% of Con values at all time points and BDNF protein levels were ∼40% of Con at 28 days. Exercise compensated for the reductions in BDNF after hemisection, such that BDNF mRNA levels in the Ex-Hx rats were similar to Con after 3 days and higher than Con after 7 (17%) and 28 (27%) days of exercise. After 28 days of exercise, BDNF protein levels were 33% higher in Ex-Hx than Con rats and were highly correlated (r = 0.86) to running distance. The levels of the downstream effectors for the action of BDNF on synaptic plasticity synapsin I and CREB were lower in Sed-Hx than Con rats at all time points. Synapsin I mRNA and protein levels were higher in Ex-Hx rats than Sed-Hx rats and similar to Con rats at 28 days. CREB mRNA values were higher in Ex-Hx than Sed-Hx rats at all time points. Hemisection had no significant effects on the levels of NT-3 mRNA or protein; however, voluntary exercise resulted in an increase in NT-3 mRNA levels after 28 days (145%). These results are consistent with the concept that synaptic pathways under the regulatory role of BDNF induced by exercise can play a role in facilitating recovery of locomotion following spinal cord injury.

Introduction

The capacity of repetitive locomotor activity to promote functional restoration after CNS injury is well recognized (Edgerton and Roy, 2002, Edgerton et al., 2004). It seems that the outcome of recovery is task specific such that rehabilitative strategies that simulate the action of walking are particularly effective in promoting the recovery of locomotion. Although neurotrophins have been identified as a molecular system with the potential to enhance spinal cord repair, most of the strategies to induce motor recovery after SCI have involved the addition of exogenous neurotrophins into the CNS. These strategies, however, ignore the intrinsic capabilities of the neural system to produce neurotrophins. Our experimental approach is to modulate the synthesis of endogenous neurotrophins in the CNS via a systemic stimulus, i.e., physical activity. This approach infers that those specific neural pathways that are activated during an exercise may have access to the highest concentration of brain-derived neurotrophic factor (BDNF) and, therefore, able to show the most synaptic plasticity.

Recent studies illustrate that physical activity increases the expression of BDNF and neurotrophin-3 (NT-3) in the intact spinal cord (Gomez-Pinilla et al., 2002a, Gomez-Pinilla et al., 2002b, Ying et al., 2003). It is our contention that a better understanding of the variables involved in the regulation of neurotrophins by exercise in the injured spinal cord will enable us to optimize the impact of this process on CNS function. BDNF delivered exogenously to the injured spinal cord can increase axonal growth (Bregman et al., 1997) and stimulate hindlimb stepping (Jakeman et al., 1998). These observations, together with evidence that BDNF is a powerful modifier of neuronal excitability and synaptic transmission (Kafitz et al., 1999, Lu and Figurov, 1997), suggest that BDNF is a crucial effector of experience-dependent plasticity. The spinal cord expresses NT-3 and its TrkC receptor (McAllister et al., 1999, Scarisbrick et al., 1999), and NT-3 plays a role in mediating synaptic transmission (Xie et al., 1997) and regeneration in the spinal cord (Ramer et al., 2000, Schecterson and Bothwell, 1992, Shibayama et al., 1998). NT-3 also is involved in the survival and function of sensory neurons, such that mice lacking the NT-3 gene show a severe loss of sensory neurons and concomitant gait abnormalities (Tessarollo et al., 1994).

BDNF affects the synthesis (Wang et al., 1995) and phosphorylation (Jovanovic et al., 1996) of synapsin I, resulting in elevated neurotransmitter release (Jovanovic et al., 2000). Therefore, synapsin I can be used as a marker to evaluate the role of BDNF on synaptic adaptation and function (Vaynman et al., 2003). Synapsin I is a member of a family of nerve terminal-specific phosphoproteins and is involved in neurotransmitter release, axonal elongation, and maintenance of synaptic contacts (Brock and O'Callaghan, 1987). The transcription factor cyclic AMP response element binding protein (CREB) is required for various forms of memory including spatial learning (Silva et al., 1998) and appears to play a role in neuronal resistance to trauma in conjunction with BDNF (Walton et al., 1999). CREB is characterized by its ability to modulate gene expression encoding BDNF and cell survival in the CNS (Tao et al., 1998, Ying et al., 2002). Growth associated protein 43 (GAP-43) is present in growing axon terminals and has important roles in axonal growth, neurotransmitter release (Oestreicher et al., 1997), and learning and memory (Routtenberg et al., 2000). In the present study, we investigate the capacity of exercise to counteract the reduced expression of BDNF and NT-3 and their effects on molecular systems related to synaptic plasticity and axonal growth in regions of the spinal cord distal to an injury. The results are consistent with the view that neurotrophin modulation induced by neuromuscular activity can play a role in facilitating functional recovery following spinal cord injury.

Section snippets

Materials and methods

Seventy-six male Sprague–Dawley rats of 2 months of age (Charles River, San Diego, CA) were housed singly in standard polyethylene cages with food and water ad libitum, and exposed to alternate light and dark periods of 12 h. After 1 week of acclimation, the animals were randomly assigned to a sedentary control (Con) or a hemisected (Hx) group.

A spinal cord hemisection at a mid-thoracic level (T7–T9) was performed under aseptic conditions. Briefly, the rats were pre-sedated with buprenorphine

BDNF levels after spinal cord injury with and without exercise (Figs. 1 and 2)

The levels of BDNF mRNA in the lumbar hemicord ipsilateral to the hemisection in Sed-Hx rats decreased to 83% (day 7, P < 0.05), 84% (day 10, P < 0.05), 84% (day 14, P < 0.05), and 85% (day 35, P < 0.05) of Con values (Fig. 1A) relative to the injury onset. The effects of injury were more dramatic at the protein level: ELISA analyses showed that BDNF levels were ∼40% of Con values 35 days after the hemisection (Fig. 1B). In the Ex-Hx rats, 3 days of exercise were sufficient to normalize the

Discussion

Abundant evidence indicates that exercise can facilitate the recovery of locomotion after spinal cord injury (Edgerton and Roy, 2002, Edgerton et al., 2004). The present studies were performed to help better understand the molecular mechanism by which exercise can modulate the plasticity of the injured spinal cord. After receiving a spinal cord hemisection, rodents were exposed to voluntary running wheel exercise for up to 28 days. Compared to controls, hemisected rats had lower levels of BDNF

Acknowledgments

This work was supported by NIH awards NS38978 and NS39522. We thank Ms. Hui Zhong for assisting in the surgeries.

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