Chronically injured supraspinal neurons exhibit only modest axonal dieback in response to a cervical hemisection lesion.

This study examined the extent of axon retraction (dieback) exhibited by injured brain stem neurons in a chronic spinal cord injury (SCI) condition. Adult female rats subjected to a cervical (C3) hemisection lesion were sacrificed 1, 4, 8, or 14 weeks after injury and the spinal cord from C1 to the lesion cavity was removed. One week prior to sacrifice, a microinjection of biotinylated dextran amine (BDA, 0.5 microliter) was made into the red nucleus, lateral vestibular nucleus, or medullary reticular formation of each animal. Horizontal cryostat sections were processed with avidin-HRP to detect supraspinal axons anterogradely labeled with BDA. Terminal end bulbs of axons were identified and their distance from the lesion site was measured by a computerized image analysis program. At all postinjury intervals, numerous rubrospinal, vestibulospinal, and reticulospinal tract axons were found immediately adjacent to the lesion site and over 60% of all terminals were within 500 micrometer at 1 and 4 weeks. The mean axonal distance of 450-500 micrometer from the lesion indicated that many injured axons had retracted farther than 500 micrometer from the lesion site; however, long-term maintenance of the mean axonal distance from the lesion at less than 500 micrometer indicated the absence of progressive dieback after SCI. While some modest changes occur in specific supraspinal pathways following SCI, axonal retraction does not appear to be a contributing factor to the diminished regenerative effort by certain brain stem neurons that has been observed at long postinjury intervals. Copyright 2001 Academic Press.