Delayed treatment with Rho-kinase inhibitor does not enhance axonal regeneration or functional recovery after spinal cord injury in rats

Axonal regeneration in the central nervous system is blocked by many different growth inhibitory factors. Some of these inhibitors act on neurons by activating RhoA and Rho-kinase, an effector of RhoA. Several studies have shown that Rho-kinase inhibition immediately after spinal cord injury enhances axonal sprouting and functional recovery. In this study, we ask whether delayed treatment with Rho-kinase inhibitor is effective in promoting regeneration and functional recovery. We administered Fasudil, a Rho-kinase inhibitor, locally to the injury site 4 weeks or immediately after contusion of the thoracic spinal cord in rats. Although the immediate treatment significantly stimulated axonal sprouting and recovery of hindlimb function, treatment started 4 weeks after surgery had no effect on fiber sprouting or locomotor recovery. Our findings suggest that RhoA/Rho-kinase alone may not account for the irreversible arrest of axon outgrowth in the chronic stage of injury in the central nervous system.