Bulbospinal and intraspinal connections in normal and regenerated salamander spinal cord

The salamander is the only limbed adult vertebrate which can regenerate portions of cervical, thoracic, or lumbar spinal cord. While the salamander has been a popular model for regeneration of the spinal cord, it is still not known what portions of the nervous system participate in the regeneration process. In the experiments reported here we examine the bulbospinal and intraspinal projections to the lumbar spinal cord in normal and regenerated salamanders (Notophthalmus viridescens). HRP application to the lumbar enlargement of normal salamanders labeled cells in the ventral thalamus, the rostral tegmentum in the proposed homolog of the red nucleus, the reticular neurons of the rhombencephalon, and the midline regions of the rhombencephalon which are possibly equivalent to raphe nuclei of other vertebrates. In the brachial spinal cord HRP-labeled cells were located in dorsal, intermediate, and ventral regions of the spinal gray matter and tended to be located at the periphery of the gray matter. To examine the spinal circuitry of regenerated salamanders, animals received complete spinal transections at the junction of the thoracic and lumbar spinal cord, abolishing all spontaneous coordinated hindlimb and tail movements. Animals exhibited walking and swimming within 60 days at which time a pledget of HRP was inserted into a gap in the spinal cord made by a transection 10.0 mm (six animals) or 5.0 mm (one animal) caudal to the first lesion. On average, the number of HRP labeled brain stem neurons in regenerated animals was 40% of that found in normal animals. The number of labeled cells in the brachial spinal cord was within the range of normal animals.(ABSTRACT TRUNCATED AT 250 WORDS)