Axotomy affects the retrograde labeling of cervical and lumbar-cord-projecting rubrospinal neurons differently

The effect of axotomy at cervical and lumbar spinal levels upon the ability of rubrospinal neurons to retrogradely transport tracer was compared. Unilateral rubrospinal tractotomy was performed first at C5 and, after a few days, at C2 vertebral levels. Different retrograde tracers were applied at the lesioned sites right after tractotomy. Tracer applied at C5 labeled both cervical and lumbar-cord-projecting neurons. Tracer applied at C2 also labeled both groups of neurons if performed 2 days after that at C5; however, only cervical-cord-projecting neurons were labeled when it was performed 3 or 5 days after that at C5. In another set of experiments, a T10 tractotomy without tracer application was performed 2 or 5 days prior to the C5/C2, series of tract lesions. When preceded by a T10 lesion 2 days in advance, tracer applied at C5 labeled both cervical and lumbar-cord-projecting neurons. However, a T10 lesion 5 days in advance resulted in the labeling of only cervical-cord-projecting neurons by the tracer applied at C5. In either case, tracer applied at C2 consistently labeled only cervical-cord-projecting neurons, irrespective of the intervals-2, 3, or 5 days-allowed between C5 and C2 lesions. Most neurons labeled from C2 were also double-labeled by the tracer applied at C5. Thus, unlike lumbar-cord-projecting counterparts, cervical-cord-projecting rubrospinal neurons retain the ability to uptake and/or transport retrograde tracer several days following axotomy. This implies that cervical-cord-projecting rubrospinal neurons survive in a different functional state from their lumbar-cord-projecting counterparts following axonal injury.