Peripheral nerve injury : primary sensory neuronal death & regeneration after chronic nerve injury
After a defined unilateral sciatic nerve transection in the rat, a novel triple staining technique was employed in order to enable the detection of neuronal death in L4 & L5 dorsal root ganglia by light microscopic morphology, and TdT Uptake Nick-End Labelling (TUNEL). Optical dissection was then used to quantify neuronal loss from statistically unbiased estimates of the number of surviving neurons. Neuronal death was demonstrated to begin within 24 hours of injury and to peak 2 weeks later, while neuronal loss plateaued 2 months after axotomy, and 39.2% of neurons died overall. Thus the most relevant experimental timepoints at which to examine the effects of putative neuroprotective strategies are 2 weeks and 2 months after axotomy, until which time a window of opportunity exists for therapeutic intervention. The principal that sensory outcome might be related to the delay between injury and nerve repair was confined by the fact that although surgical nerve repair reduced neuronal death 2 weeks after axotomy, the neuroprotective benefit depended upon how soon after injury the nerve was repaired. Even immediate repair did not entirely eliminate neuronal loss, confirming the need for an adjuvent therapy. Hence the effect of two promising agents with established clinical safety records was examined. N-acetyl-cysteine (NAC) is a clinically proven glutathione substrate antioxidant, and anti-mitotic properties. Systemic treatment caused a dose-dependent improvement in neuronal morphology, a significant reduction in the number of TUNEL positive neurons 2 weeks after axotomy (p<0.05), and 2 months after axotomy it was found to have reduced neuronal loss from 35% to only 3% (p<0.001). L-acetyl-carnitine (LAC) is a physiological peptide integral to mitochondrial aerobic glycolysis that was found to be even more neuroprotective than NAC, since after LAC treatment no neuronal loss was detected 2 months after axotomy (no treatment 35% loss; high-dose LAC -4% loss, p<0.001).