Neurotrophins BDNF and NT-3 promote axonal re-entry into the distal host spinal cord through Schwann cell-seeded mini-channels

To promote axonal regeneration in the injured adult spinal cord, a two-phas e repair strategy was employed to (i) bridge a spinal cord hemilesion cavit y with a grafted Schwann cell (SC)-seeded mini-channel, and (ii) promote ax onal re-entry into the distal cord by infusing two neurotrophins, BDNF and/ or NT-3, directly into the distal cord parenchyma. Here we report that infu sion of two neurotrophins, delivered alone or in combination, effectively p romotes axonal outgrowth from SC-seeded mini-channels into the distal host spinal cord. When an anterogradely transported marker, PHA-L or BDA, was in jected into the spinal cord 3 mm rostral to the graft, a large number of ax ons was observed to regenerate from the SC graft into the distal cord in ne urotrophin-treated groups. A subpopulation of these axons was found to grow up to 6 mm within the distal spinal cord. These axons, which were confined mainly within the grey matter, arborized and formed structures which resem ble terminal boutons. In channels containing no SCs, the infusion of neurot rophins did not promote axonal ingrowth from the proximal cord stump. In ca ses which received SC grafts but no neurotrophin infusion, axonal re-entry into the distal cord was limited. Thus, the present study demonstrates that regenerating axons not only cross a lesion site when a permissive cellular bridge is provided but also penetrate into the distal host spinal cord and elongate for a distance of several cord segments after the infusion of two neurotrophins. The latter event is prerequisite for establishment of appro priate connections between regenerating axons and target neurons and thus, functional recovery.