Early exercise in spinal cord injured rats induces allodynia through TrkB signaling

Rehabilitation is important for the functional recovery of patients with spinal cord injury. However, neurological events associated with rehabilitation remain unclear. Herein, we investigated neuronal regeneration and exercise following spinal cord injury, and found that assisted stepping exercise of spinal cord injured rats in the inflammatory phase causes allodynia. Sprague–Dawley rats with thoracic spinal cord contusion injury were subjected to assisted stepping exercise 7 days following injury. Exercise promoted microscopic recovery of corticospinal tract neurons, but the paw withdrawal threshold decreased and C-fibers had aberrantly sprouted, suggesting a potential cause of the allodynia. Tropomyosin-related kinase B (TrkB) receptor for brain-derived neurotrophic factor (BDNF) was expressed on aberrantly sprouted C-fibers. Blocking of BDNF–TrkB signaling markedly suppressed aberrant sprouting and decreased the paw withdrawal threshold. Thus, early rehabilitation for spinal cord injury may cause allodynia with aberrant sprouting of C-fibers through BDNF–TrkB signaling.