A time-dependent loss of retrograde transport ability in distally axotomized rubrospinal neurons

Studies on the effect of axotomy on adult intrinsic central projection neurons have generally assumed that the severed proximal axonal stumps were still capable of retrogradely transporting tracer at varying times after injury. Failure of transport was interpreted as neuronal death, which is at odds with current understanding that central projection neurons survive distal axotomy. We used lumbar spinal cord-projecting rubrospinal neurons of the rat as a model to evaluate the ability of injured neurons to transport tracer retrogradely at different times after distal axotomy. We examined only the caudal part of the red nucleus, since rubrospinal neurons are concentrated here. In control animals, tracer applied to the rubrospinal tract at the T10 vertebral level labeled ventrolateral rubral neurons, while C3 application marked all rubral neurons. From 3 days after a T10 axotomy and tracer application, most ventrolateral neurons were no longer labeled by another tracer application at the C3 vertebral level via an axonal cut. The phenomenon was not caused by tracer toxicity, since a T10 tractotomy without tracer application also prevented these axotomized neurons from being labeled when treated similarly. Thus, neuronal retrograde transport capability was seriously retarded 3 days after a distal axotomy. Loss of retrograde transport may merely suggest that a mechanism no longer in service has been switched off, or perhaps it may insulate injured neurons from the effect of lesion site-derived factors. Using this property, we were able to localize cervical spinal cord-projecting rubrospinal neurons in the caudal red nucleus. Results show that although they concentrate in the dorsomedial region, some neurons were found to extend into the ventrolateral part of the nucleus.