Anatomical and behavioral recovery from the effects of spinal cord transection: dependence on metamorphosis in anuran larvae

Abstract

This study of spinal cord injury in bullfrog (Rana catesbeiana) tadpoles using the neuroanatomical tracer horseradish peroxidase (HRP) was undertaken to determine (1) whether the same anatomical regions that normally give rise to ascending or descending spinal tracts do so following complete spinal cord transection and (2) whether the course of behavioral recovery could be related to the anatomical results. The results of this study show that (1) spinal cord continuity is readily restored in tadpoles subjected to spinal cord transection, but nerve fibers crossing the site of injury end within 1 to 2 mm of the lesion site; (2) tadpoles with spinal cord transections held through metamorphosis show, as juvenile frogs, restoration of lumbar projections from all brainstem regions that normally project to the lumbar spinal cord; (3) neither long ascending projections from dorsal root ganglion cells nor those from spinal neurons caudal to the transection traverse the transection site, even after metamorphosis; and (4) consistent with the anatomical results, tadpoles show only minimal behavioral recovery, but these same animals as juvenile frogs show recovery of behaviors that are dependent upon connections to supraspinal regions. In other experiments, [3H]thymidine or [3H]apo-HRP was combined with HRP histochemistry to determine if new brainstem neurons projecting to the spinal cord are born in the metamorphic period and if, in normal animals, brainstem projections to the lumbar spinal cord persist through metamorphosis. We found no evidence that neurons with lumbar spinal cord projections are born during metamorphosis; however, evidence was found that most brainstem neurons that project to the lumbar spinal cord before metamorphosis retain this projection in the juvenile frog.