Evaluation of regeneration of nerve and reinnervation of skeletal muscle in the hibernating ground squirrel.

The retrograde transport of HRP was used to determine the status of axonal transport in the peroneal and sciatic nerves of hibernating and nonhibernating ground squirrels following crush of the peroneal nerve at 10 to 12 mm (SNS) or sciatic nerve at 33 to 35 mm (LNS) from its entrance into the extensor muscle. The ability of the proximal segment to reestablish axonal continuity and thus neuromuscular transmission was also studied. Two weeks to 3 months after nerve crush the extensor muscles were injected with HRP. We found that during hibernation no axonal transport across the site of crush was seen even after 3 months and that regeneration of the nerve during this period was minimal. Evidence of slight regeneration seen at 90 days could be due to periods of awaking of the animals during their natural hibernation cycle. In these animals HRP deposits were seen only in the nerve distal to crush, i.e., between crush site and muscle. In the nonhibernating squirrels, axoplasmic flow was reestablished at the site of injury as early as 2 weeks after crush, and HRP could be detected in the spinal cord in motoneurons of the ipsilateral ventral horn at spinal levels L3 to L5. In one hibernating animal the peroneal nerve was crushed at the distal site (SNS) and also the spinal cord was injured by dropping a weight. After nerve crush and the spinal cord injury the hibernating state could not be maintained and the animal stayed awake 22 days. The time course of regeneration of the nerve in that animal was similar to that seen in nonhibernating squirrels. After nerve crush in nonhibernating animals, reaction product was also found in sensory cell bodies of dorsal root ganglia as well as in terminals in the substantia gelatinosa of the spinal cord at the same levels. Thus, the axonal transport occurs in hibernating and non-hibernating squirrels in both sensory and motor nerve fibers. The extensor muscle fibers of the hibernating squirrels showed substantial membrane depolarization 90 days after crush. Action potentials from these fibers could be obtained from 15 to 35 days only through stimulating the nerve segment distal to the crush. Stimulation of the proximal nerve segment did not evoke muscle activity. These results demonstrate that nerve regeneration was nearly abolished during hibernation and that blockade of axonal transport continued across a region of nerve crush for the duration of the hibernating period.