We investigated the contribution of corticospinal tract (CST) plasticity to training-induced recovery and side effects following spinal cord injury (SCI). Rats were divided into three lesion groups: a unilateral lesion of the dorsal funiculus, the lateral funiculus or a lesion of the entire dorsolateral quadrant (DLQ). Following surgery, rats were distributed into a training group and an untrained group. Trained rats received rehabilitative training in skilled reaching 6 days a week, starting 4 days post-lesion. Following 6 weeks, all rats were tested in reaching (trained task) and crossing a horizontal ladder (untrained task). We found that trained rats with a lesion involving the dorsal column were significantly better in reaching compared to untrained animals. However, when crossing the horizontal ladder, trained rats made significantly more mistakes than untrained animals. Interestingly, rats with a lateral funiculus lesion did not show either effect. A subsequent ablation of the pyramidal tract (pyramidotomy) in rats with a DLQ-lesion significantly reduced but did not eliminate the reaching success. This spared function suggests that other descending systems contributed to the training-induced recovery. In addition, motor-evoked potentials (MEP) from cortical stimulation could still be evoked after pyramidotomy. Further, blocking synaptic transmission passing through the red nucleus using muscimol did not influence the occurrence of MEP's, suggesting that other descending pathways, like the reticulospinal tract, were involved in functional recovery. In summary, this study demonstrates that training-induced CST plasticity may contribute to recovery of motor function, but may also negatively affect untrained tasks as previously reported.
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