Functional recovery, serotonergic sprouting, and endogenous progenitor fates in response to delayed environmental enrichment after spinal cord injury.

Environmental enrichment (EE) is a way to induce voluntary locomotor training that positively affects locomotor recovery after acute spinal cord injury (SCI). The beneficial effect on SCI outcome is thought to be based on enhanced plasticity in motor pathways, triggered by locomotor-specific sensory feedback to the spinal cord circuitry for locomotion (central pattern generators [CPGs]). In view of chronic SCI, we tested the hypothesis that EE improves motor outcome after SCI in the rat when started after a clinically relevant delay of 3 weeks. At the CPG level (i.e., the spinal L1-L2 level), where EE-related sensory feedback is processed, two key mechanisms of anatomical plasticity were examined: (1) serotonergic innervation, and (2) survival and differentiation of spinal cord progenitor cells. Delayed EE improved interlimb coordination, which was associated with an increased serotonergic innervation of the ventro-lateral grey matter within the L1-L2 segments. Although spinal cord progenitor cells were found to differentiate into both neurons and glial cells, EE did not affect their survival. These results show that EE induces a substantial improvement of motor outcome after SCI when commenced after a clinically-relevant delay. Increased serotonergic innervation of the lumbar CPG area is therefore suggested to play an important role in the EE-induced recovery of interlimb coordination.