Enhancement of the conditioning lesion effect in rat sciatic motor axons after superimposition of conditioning and test lesions.

In previous studies on sensory axons we reported that the effect of a conditioning lesion on increasing regeneration rate was enhanced if the two lesions were superimposed, rather than made at separate sites on the nerve, and proposed that this was due to the growth of axons through nerve predegenerated by the conditioning lesion. We now find that the regeneration of motor axons, determined by labeling with fast axonally transported protein, is also enhanced by superimposed conditioning and test lesions, to a greater extent than by separated lesions. However, the regeneration rate of the conditioned motor axons (5.40 +/- 0.44 mm/day) was less than that of conditioned sensory axons in the same nerves (6.65 +/- 0.56 mm/day). Recovery of motor function after the test lesion was assessed by computing a "sciatic functional index" from measurements of hind footprints made by the rats while walking. Recovery began earlier in the conditioned animals, with the time to half-maximum recovery being 13 days, compared with 18 days in animals that had received a test lesion only. In both groups of animals recovery was complete. Although these results are consistent with the proposal that regenerating motor axons elongate more rapidly through nerve predegenerated following the conditioning lesion, we cannot eliminate the possibility that the enhanced regeneration rate in motoneurons was a result of a more vigorous metabolic response to the conditioning lesion when placed more proximally on their axons.