Age related sprouting of dorsal root axons after sensory denervation

To test for sprouting of dorsal root axons, 3 consecutive dorsal roots were sectioned above and 3 were sectioned below a spared root in rats 1 month and 24 months old. At various survival times, myelinated and unmyelinated axons were counted in the spared root, the first cranial root and the first caudal root, and these counts were compared to similar counts from the same roots on the unoperated side. The counts of myelinated axons in all roots and of unmyelinated axons in the caudal roots were not different from the counts in the same roots on the unoperated side. Thus there is no evidence for sprouting of these axons in our system. By contrast, the unmyelinated axons in the cranial and spared roots were significantly increased as compared to the unmyelinated axons in the same roots on the unoperated side. We interpret this increase to be sprouting of dorsal root axons. To determine the effects of age, animals operated at 1 month and sacrificed 1 month, 3 months or 18 months later were compared with animals operated at 2 years and sacrificed 1 month later. The unmyelinated axons were greater by approximately 20% on the operated side in the cranial and spared roots in the young rats and by approximately 8% in old rats. Thus the sprouting of unmyelinated axons is more vigorous when the surgery is performed in young animals, but there still seems to be a response in older animals.