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 axo tomized immature neurons. Transplants of fetal spinal cord tissue resc ue axotomized rubrospinal neurons and promote their axonal regeneratio n. Rubrospinal neurons develop many of their axon collaterals postnata lly. The present study tests the hypothesis that the axotomized rubros pinal neurons that are rescued by transplants and regenerate their axo ns are those neurons that have established axon collaterals to targets rostral to the lesion. Neonatal rats received a transplant of fetal s pinal cord tissue placed into a midthoracic spinal cord hemisection. O ne month after transplantation, the retrogradely transported fluoresce nt tracers fast blue (FB) and diamidino yellow (DY) were used to ident ify rubrospinal neurons with collaterals to particular targets. FB was injected either into the interpositus nucleus of the cerebellum or in to the gray matter of the cervical enlargement to identify collaterals to these targets, and DY was injected into the spinal cord approximat ely 5 mm caudal to the transplant and lesion site to label retrogradel y the neurons that regenerated their axons. Double labeling was observ ed in the axotomized neurons of the red nucleus after tracer injection s into the cervical spinal cord but not after injections into the cere bellum. This labeling pattern indicates that axotomized rubrospinal ne urons that are rescued and regenerate axons caudal to the transplant m aintain axon collaterals at cervical spinal cord levels. Cerebellar co llaterals do not appear to play a role in the survival and regrowth of axotomized rubro-spinal neurons. (C) 1997 Academic Press.