Restriction of Axonal Retraction and Promotion of Axonal Regeneration by Chronically Injured Neurons after Intraspinal Treatment with Glial Cell Line-Derived Neurotrophic Factor (GDNF)

The response of supraspinal neurons to acute or delayed treatment with GDNF following a spinal cord injury was examined. A cervical level 3 hemisection lesion cavity was created by tissue aspiration in adult, female rats. In one experiment gel foam saturated with GDNF was placed into the lesion cavity immediately after injury to determine if the extent of axonal retraction was affected by neurotrophic factor treatment. One week prior to sacrifice animals received a microinjection of biotinylated dextran amine (BDA) into the red nucleus and reticular formation to label descending spinal pathways by anterograde transport mechanisms. Animals were sacrificed 1 or 4 weeks after injury and treatment with GDNF. The terminal end of injured BDA-labeled rubrospinal and reticulospinal tract axons was identified and the distance from the lesion was measured. In comparison to PBS-treated animals, GDNF-treatment resulted in a significant decrease in the extent of axonal retraction of both rubrospinal and reticulospinal tract axons at 1 week after spinal cord injury for both tracts. At 4 weeks after injury the mean distance from the lesion was less than 240 μm following GDNF-treatment for both tracts, compared to over 480μm following PBS-treatment. In the second experiment injured supraspinal neurons were labeled by retrograde transport of True Blue that had been placed into the lesion cavity. One month later scar tissue was removed from the cavity by aspiration to enlarge the cavity by approximately 500 μm in a rostral direction. GDNF-saturated gel foam was placed into the cavity for 60 min prior to apposition of an autologous peripheral nerve (PN) graft to the rostral cavity wall. One month later Nuclear Yellow was applied to the distal end of the PN graft and animals were sacrificed after 2 days. The number of supraspinal neurons containing both True Blue and Nuclear Yellow was counted as a measure of axonal regeneration by chronically injured neurons. There was a seven-fold increase in the number of regenerating neurons after GDNF-treatment, with the majority (65%) of dual-labeled neurons located within the reticular formation. These results indicate that GDNF has neuroprotective effects when provided acutely after injury and promotes axonal regeneration when provided in a chronic injury situation.