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 stump s were still capable of retrogradely transporting tracer at varying ti mes after injury. Failure of transport was interpreted as neuronal dea th, which is at odds with current understanding that central projectio n neurons survive distal axotomy. We used lumbar spinal cord-projectin g rubrospinal neurons of the rat as a model to evaluate the ability of injured neurons to transport tracer retrogradely at different times a fter distal axotomy. We examined only the caudal part of the red nucle us, since rubrospinal neurons are concentrated here. In control animal s, tracer applied to the rubrospinal tract at the T10 vertebral level labeled ventrolateral rubral neurons, while C3 application marked ail 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 phenomeno n was not caused by tracer toxicity, since a T10 tractotomy without tr acer application also prevented these axotomized neurons from being la beled when treated similarly. Thus, neuronal retrograde transport capa bility was seriously retarded 3 days after a distal axotomy. Loss of r etrograde transport may merely suggest that a mechanism no longer in s ervice has been switched off, or perhaps it may insulate injured neuro ns 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 co ncentrate in the dorsomedial region, some neurons were found to extend into the ventrolateral part of the nucleus.