Preconditioning selective ventral root injury promotes plasticity of ascending sensory neurons in the injured spinal cord of adult rats – possible roles of brain‐derived neurotrophic factor, TrkB and p75 neurotrophin receptor

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Abstract
Preconditioning sciatic nerve injury enhances axonal regeneration of ascending sensory neurons after spinal cord injury. A key question is whether direct injury of sensory nerves is necessary for the enhanced regeneration. The lumbar 5 ventral root transection (L5 VRT) model, a model of selective motor nerve injury, provides a useful tool to address this question. Here we examined the effects of a preconditioning L5 VRT on the regeneration after a subsequent dorsal column transection (DCT) in adult Sprague–Dawley rats. We found that L5 VRT 1 week before DCT increased the number of Fast Blue (FB)‐labeled neurons in the L5 dorsal root ganglia (DRG) and promoted sprouting/regenerating axons to grow into the glial scar. L5 VRT also induced a dramatic upregulation of expression of brain‐derived neurotrophic factor (BDNF) in the preconditioned DRG and in the injured spinal cord. Moreover, almost all of the FB‐labeled sprouting/regenerating neurons expressed BDNF, and approximately 55% of these neurons were surrounded by p75 neurotrophin receptor‐positive glial cells. This combined injury led to an increase in the number of BDNF‐ and TrkB‐immunoreactive nerve fibers in the dorsal column caudal to the lesion site. Taken together, these findings demonstrate that L5 VRT promotes sprouting/regeneration of ascending sensory neurons, indicating that sensory axotomy may not be essential for the plasticity of injured dorsal column axons. Thus, the sensory neurons could be preprimed in the regenerative milieu of Wallerian degeneration and neuroinflammation, which might alter the expression of neurotrophic factors and their receptors, facilitating sprouting/regeneration of ascending sensory neurons.

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