PARAVENTRICULOSPINAL TRACT AS A MODEL FOR AXON INJURY - SPINAL-CORD

The response of immunocytochemically identified hypothalamic axons inn ervating the rat spinal cord was examined at varying times after cord hemisection in a model of axonal injury using the paraventriculospinal projection. The purpose was to determine whether these long descendin g peptidergic axons would show signs of regrowth after injury. From I to 180 days after hemisection, horizontal sections of the spinal cord were stained with peroxidase immunocytochemistry. Antiserum against ne urophysin was used to identify axons projecting from the hypothalamic paraventricular nucleus to the spinal cord. The paraventricular nucleu s innervates all rostrocaudal segments of the cord, yet the projection is not massive, allowing the trauma response of individual axons to b e studied.Immediately caudal to a T4 hemisection, axons began decreasi ng in number by 2 days after surgery. Ten days postoperatively, only a few axons could be found caudal to the cut; these remaining axons aro se from the contralateral cord. A substantial increase in the number o f stained axons was found rostral to the hemisection 3-12 weeks after surgery. In that an increase in axon number could be due to both incre asing staining efficacy and sprouting, the orientation of axons in con trol and hemisected rats was studied. Three millimeters rostral to the hemisection, axons had a greater variance in orientation and were mor e likely to project medially out of the dorsolateral white matter comp ared with the contralateral control side. Rostral to the hemisection, a statistically significant two- to fourfold increase in the number of branches per axon was found in comparison to the contralateral contro l side. Axons were found in the dorsal white matter 4 months after sur gery; in controls, immunostained axons were not found here. At all int ervals after surgery, structures suggestive of growth were found, incl uding terminal growth cones and lateral filopodia and lamellipodia ext ending from axons whose distal ends had been severed by hemisection. S imilar structures were not found in control spinal cord. Together, the se data suggest that after cord hemisection, axons from the paraventri cular nucleus sprout rostral to the injury. (C) 1994 Wiley-Liss, Inc.