FK506 increases the regeneration of spinal cord axons in a predegenerated peripheral nerve autograft

The authors examined the ability of FK506 to accelerate axonal regeneration of rat spinal cord axons in a peripheral nerve (PN) graft. Predegenerated autografts were produced by transecting the left tibial nerve 1 week prior to spinal cord implantation into the lumbar (L-3-L-4) spinal cord. Rats were given daily injections of either FK506 (5 mg/kg, subcutaneous) or vehicle for 21 days. The PN grafts from FK506-treated rats contained larger sized regenerating axons compared with vehicle-treated controls, and mean axonal areas increased by 25% at 7.5 mm along the PN graft. Fluoro-Gold retrograde labeling confirmed that the regenerating axons originated from the central nervous system. Unexpectedly, the majority (>50%) of neurons in the red nucleus were retrogradely labeled in the FK506-treated animals only. The results indicate that FK506 not only accelerates the elongation of spinal cord axons but also promotes regeneration of rubrospinal neurons.