Ascending tract neurons survive spinal cord transection in the neonatal rat

Abstract

Retrograde axonal transport was used to determine which ascending nerve tracts from the lumbosacral spinal cord are present in the cervical spinal cord of the newborn rat and if their cell bodies survive axotomy. A pledget of true blue was applied to a low cervical spinal transection in the newborn rat ( N = 4). After a 5-day survival period, neurons were labeled in the laminae of origin of all ascending nerve tracts throughout the lumbosacral spinal cord. Neurons labeled in the same way survived for at least 1 month postoperatively when the spinal cord was transected at a midthoracic level at 5 days of age ( N = 4). No neurons in the lumbosacral spinal cord were labeled if the midthoracic spinal cord was transected at the same time as application of the dye to cervical spinal cord ( N = 2). Therefore, neurons labeled with true blue from cervical spinal cord during the neonatal period are likely to have been axotomized by thoracic injury made at 5 days of age. Three months after midthoracic spinal transection of newborn rats, HRP was injected or a pledget was applied to the first spinal segment caudal to this lesion ( N = 8). The same population of neurons was labeled as in adult rats receiving application of HRP to an acute midthoracic spinal transection ( N = 4). Neurons were seldom labeled in adult rats in which HRP was injected and ascending nerve tract axons not damaged ( N = 4). These results suggest that most ascending nerve tract axons are present in cervical spinal cord during the neonatal period (by 4 to 5 days of age). A substantial number of these neurons survive neonatal midthoracic spinal transection and their axon terminals remain near this lesion site.