Axotomized rubrospinal neurons rescued by fetal spinal cord transplants maintain axon collaterals to rostral CNS targets.

Neurons that maintain extensive axon collaterals proximal to the site of axotomy may be better able to survive injury. Early lesions of the rubrospinal tract lead to retrograde cell death of the majority of axotomized immature neurons. Transplants of fetal spinal cord tissue rescue axotomized rubrospinal neurons and promote their axonal regeneration. Rubrospinal neurons develop many of their axon collaterals postnatally. The present study tests the hypothesis that the axotomized rubrospinal neurons that are rescued by transplants and regenerate their axons are those neurons that have established axon collaterals to targets rostral to the lesion. Neonatal rats received a transplant of fetal spinal cord tissue placed into a midthoracic spinal cord hemisection. One month after transplantation, the retrogradely transported fluorescent tracers fast blue (FB) and diamidino yellow (DY) were used to identify rubrospinal neurons with collaterals to particular targets. FB was injected either into the interpositus nucleus of the cerebellum or into the gray matter of the cervical enlargement to identify collaterals to these targets, and DY was injected into the spinal cord approximately 5 mm caudal to the transplant and lesion site to label retrogradely the neurons that regenerated their axons. Double labeling was observed in the axotomized neurons of the red nucleus after tracer injections into the cervical spinal cord but not after injections into the cerebellum. This labeling pattern indicates that axotomized rubrospinal neurons that are rescued and regenerate axons caudal to the transplant maintain axon collaterals at cervical spinal cord levels. Cerebellar collaterals do not appear to play a role in the survival and regrowth of axotomized rubrospinal neurons.