Long-term effects of a lumbosacral ventral root avulsion injury on axotomized motor neurons and avulsed ventral roots in a non-human primate model of cauda equina injury.

Here, we have translated from the rat to the non-human primate a unilateral lumbosacral injury as a model for cauda equina injury. In this morphological study, we have investigated retrograde effects of a unilateral L6-S2 ventral root avulsion (VRA) injury as well as the long-term effects of Wallerian degeneration on avulsed ventral roots at 6-10 months post-operatively in four adult male rhesus monkeys. Immunohistochemistry for choline acetyl transferase and glial fibrillary acidic protein demonstrated a significant loss of the majority of the axotomized motoneurons in the affected L6-S2 segments and signs of an associated astrocytic glial response within the ventral horn of the L6 and S1 spinal cord segments. Quantitative analysis of the avulsed ventral roots showed that they exhibited normal size and were populated by a normal number of myelinated axons. However, the myelinated axons in the avulsed ventral roots were markedly smaller in caliber compared to the fibers of the intact contralateral ventral roots, which served as controls. Ultrastructural studies confirmed the presence of small myelinated axons and a population of unmyelinated axons within the avulsed roots. In addition, collagen fibers were readily identified within the endoneurium of the avulsed roots. In summary, a lumbosacral VRA injury resulted in retrograde motoneuron loss and astrocytic glial activation in the ventral horn. Surprisingly, the Wallerian degeneration of motor axons in the avulsed ventral roots was followed by a repopulation of the avulsed roots by small myelinated and unmyelinated fibers. We speculate that the small axons may represent sprouting or axonal regeneration by primary afferents or autonomic fibers.

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.

Regenerating supernumerary axons are cholinergic and emerge from both autonomic and motor neurons in the rat spinal cord.

Multipolar neurons in the mammalian nervous system normally exhibit one axon and several dendrites. However, in response to an axonal injury, adult motoneurons may regenerate supernumerary axons. Supernumerary axons emerge from the cell body or dendritic trees in addition to the stem motor axon. It is not known whether these regenerating axons contain neurotransmitters for synaptic transmission at their terminals. Here, using immunohistochemistry for choline acetyltransferase, an enzyme that synthesizes acetylcholine, we demonstrate the emergence of cholinergic supernumerary axons at 6 weeks after a unilateral L5-S2 ventral root avulsion and acute implantation of the avulsed L6 ventral root into the adult rat spinal cord. Light microscopic serial reconstruction of choline acetyltransferase immunoreactive arbors shows that these supernumerary axons originate from both autonomic and motor neurons. The supernumerary axons emerge from the cell body or dendrites, exhibit an abnormal projection pattern within the intramedullary gray and white matters, make frequent abrupt turns in direction, and form bouton-like swellings as well as growth cone-like terminals. Double labeling immunohistochemistry studies show that the choline acetyltransferase immunoreactive supernumerary axons co-localized with two proteins associated with axonal growth and elongation, growth-associated protein 43 and p75, the low affinity neurotrophic factor receptor. Our findings suggest that regenerating supernumerary axons selectively transport and store choline acetyltransferase, supporting the notion that supernumerary axons may develop functional and active synaptic transmission. Therefore, regenerating supernumerary axons may contribute to the plasticity in neural circuits following injury in the adult nervous system.