Increase of preprotachykinin mRNA and substance P immunoreactivity in spared dorsal root ganglion neurons following partial sciatic nerve injury

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Abstract
Complete sciatic nerve injury reduces substance P (SP) expression in primary sensory neurons of the L4 and L5 dorsal root ganglia (DRG), due to loss of target‐derived nerve growth factor (NGF). Partial nerve injury spares a proportion of DRG neurons, whose axons lie in the partially degenerating nerve, and are exposed to elevated NGF levels from Schwann and other endoneurial cells involved in Wallerian degeneration. To test the hypothesis that SP is elevated in spared DRG neurons following partial nerve injury, we compared the effects of complete sciatic nerve transection (CSNT) with those of two types of partial injury, partial sciatic nerve transection (PSNT) and chronic constriction injury (CCI). As expected, a CSNT profoundly decreased SP expression at 4 and 14 days postinjury, but after PSNT and CCI the levels of preprotachykinin (PPT) mRNA, assessed by in situ hybridization, and the SP immunoreactivity (SP‐IR) of the L4 and L5 DRGs did not decrease, nor did dorsal horn SP‐IR decrease. Using retrograde labelling with fluorogold to identify spared DRG neurons, we found that the proportion of these neurons expressing SP‐IR 14 days after injury was much higher than in neurons of normal DRGs. Further, the highest levels of SP‐IR in individual neurons were detected in ipsilateral L4 and L5 DRG neurons after PSNT and CCI. We conclude that partial sciatic nerve injury elevates SP levels in spared DRG neurons. This phenomenon might be involved in the development of neuropathic pain, which commonly follows partial nerve injury.