Restoration of neuromuscular specificity following ventral rhizotomy in the bullfrog tadpole, Rana catesbeiana.

The specificity of hindlimb reinnervation following transection of lumbar ventral roots was investigated in adult and larval bullfrogs (Rana catesbeiana). Five to 6 weeks following ventral rhizotomy, the retrogradely transported marker horseradish peroxidase (HRP) was applied to circumscribed regions of the hindlimb. The location of labeled motoneuron somata within the lumbar lateral motor column was compared with that obtained in unoperated tadpoles. Reinnervation of the hindlimb was largely specific in tadpoles operated during the first third of larval life. However, localization was largely lost in older tadpoles and adult frogs. Repeated applications of 3H-thymidine combined with retrograde labeling with HRP failed to provide evidence that newly born motoneurons contribute to reinnervation of the hindlimb. Hindlimb reinnervation thus appears to result from regeneration of transected motor axons. In contrast to the lack of neuromuscular specificity seen in older animals after transection of ventral roots, motoneuron axons disconnected from their targets by crush injury regenerate to the appropriate limb regions.