Acute implantation of an avulsed lumbosacral ventral root into the rat conus medullaris promotes neuroprotection and graft reinnervation by autonomic and motor neurons

Trauma to the conus medullaris and cauda equina may result in autonomic, sensory, and motor dysfunctions. We have previously developed a rat model of cauda equina injury, where a lumbosacral ventral root avulsion resulted in a progressive and parallel death of motoneurons and preganglionic parasympathetic neurons, which are important for i.e. bladder control. Here, we report that an acute implantation of an avulsed ventral root into the rat conus medullaris protects preganglionic parasympathetic neurons and motoneurons from cell death as well as promotes axonal regeneration into the implanted root at 6 weeks post-implantation. Implantation resulted in survival of 44±4% of preganglionic parasympathetic neurons and 44±4% of motoneurons compared with 22% of preganglionic parasympathetic neurons and 16% of motoneurons after avulsion alone. Retrograde labeling from the implanted root at 6 weeks showed that 53±13% of surviving preganglionic parasympathetic neurons and 64±14% of surviving motoneurons reinnervated the graft. Implantation prevented injury-induced atrophy of preganglionic parasympathetic neurons and reduced atrophy of motoneurons. Light and electron microscopic studies of the implanted ventral roots demonstrated a large number of both myelinated axons (79±13% of the number of myelinated axons in corresponding control ventral roots) and unmyelinated axons. Although the diameter of myelinated axons in the implanted roots was significantly smaller than that of control roots, the degree of myelination was appropriate for the axonal size, suggesting normal conduction properties. Our results show that preganglionic parasympathetic neurons have the same ability as motoneurons to survive and reinnervate implanted roots, a prerequisite for successful therapeutic strategies for autonomic control in selected patients with acute conus medullaris and cauda equina injuries.