Regeneration of descending projections in Xenopus laevis tadpole spinal cord demonstrated by retrograde double labeling

Xenopus laevis tadpoles functionally recover from spinal cord transection. Because this recovery requires the tadpole to metamorphose, it may result from compensatory changes initiated by de novo growth of axons involved in limb dominant locomotion rather than from regeneration of cut axons. To determine whether axonal regrowth contributes to functional recovery, sequential retrograde double labeling with two fluorescent dextran amines was used to identify neurons with regenerated axons. Rhodamine dextran amine was applied to hemisected spinal cords of prometamorphic tadpoles between the 4th and 5th vertebrae. After metamorphosis, in animals that had recovered movement, fluorescein dextran amine was applied to the lumbar spinal cord. Two weeks later, the CNS of these animals was examined for the presence of double-labeled neurons, i.e., those whose axons had regenerated. Double-labeled neurons were found in the reticular, raphe, and solitary tract nuclei, and in the interstitial nucleus of the medial longitudinal fasciculus. Because Xenopus expresses all the known mammalian molecular inhibitors of CNS axon regeneration, the determination that these phylogenetically conserved populations of neurons are indeed capable of axon regeneration should facilitate molecular studies of successful recovery from spinal cord trauma.