Rewiring of the corticospinal tract in the adult rat after unilateral stroke and anti-Nogo-A therapy.

Adult Long Evans rats received a photothrombotic stroke that destroyed >90% of the sensorimotor cortex unilaterally; they were subsequently treated intrathecally for 2 weeks with a function blocking antibody against the neurite growth inhibitory central nervous system protein Nogo-A. Fine motor control of skilled forelimb grasping improved to 65% of intact baseline performance in the anti-Nogo-A treated rats, whereas control antibody treated animals recovered to only 20% of baseline scores. Bilateral retrograde tract tracing with two different tracers from the intact and the denervated side of the cervical spinal cord, at different time points post-lesion, indicated that the intact corticospinal tract had extensively sprouted across the midline into the denervated spinal hemicord. The original axonal arbours of corticospinal tract fibres that had recrossed the midline were subsequently withdrawn, leading to a complete side-switch in the projection of a subpopulation of contralesional corticospinal tract axons. Anterograde tracing from the contralesional cortex showed a 2-3-fold increase of midline crossing fibres and additionally a massive sprouting of the pre-existing ipsilateral ventral corticospinal tract fibres throughout the entire cervical enlargement of the anti-Nogo-A antibody-treated rats compared to the control group. The laminar distribution pattern of the ipsilaterally projecting corticospinal tract fibres was similar to that in the intact spinal cord. These plastic changes were paralleled by a somatotopic reorganization of the contralesional motor cortex where the formation of an ipsilaterally projecting forelimb area was observed. Intracortical microstimulation of the contralesional motor cortex revealed that low threshold currents evoked ipsilateral movements and electromyography responses at frequent cortical sites in the anti-Nogo-A, but not in the control antibody-treated animals. Subsequent transection of the spared corticospinal tract in chronically recovered animals, treated with anti-Nogo-A, led to a reappearance of the initial lesion deficit observed after the stroke lesion. These results demonstrate a somatotopic side switch anatomically and functionally in the projection of adult corticospinal neurons, induced by the destruction of one sensorimotor cortex and the neutralization of the CNS growth inhibitory protein Nogo-A.