AXOTOMY INDUCES RETRACTION OF THE DENDRITIC ARBOR OF ADULT-RAT RUBROSPINAL NEURONS

The effect of distal axonal injury on the soma-dendritic morphology of intrinsic central neurons was examined using adult rat lumbar spinal cord-projecting rubrospinal neurons as a model, The soma-dendritic mor phology was revealed using an improved Golgi-aldehyde method. Impregna ted neurons were reconstructed in the two-dimensional plane for analys is. Four weeks after axotomy, neurons had reduced soma sizes and remai ned multipolar in shape. Some dendrites were found to end not far from their cell bodies, In addition, no long dendrite was identified follo wing axotomy, Shell's analysis [The Organization of the Cerebral Corte x. London, Methuen, 1956] revealed that axotomized neurons had fewer d endritic branches than control neurons, Total dendritic length was als o reduced, Subsequent analyses showed that the average number of dendr itic trunks was not altered however the mean number of terminal branch es per dendritic trunk was reduced. The dendritic membrane of the norm al neurons was usually smooth with occasional short protuberances on t he proximal dendrites and spines on the distal dendrites, which did no t change after axotomy. In control neurons, we identified an elaborate type of dendritic structures named dendritic appendage aggregates, Th ese aggregates were located preferentially on terminal dendrites and w ere classified into three categories according to their complexity The incidence of occurrence for these aggregates decreased following dist al axotomy. These phenomena indicate that rat lumbar spinal cord-proje cting rubrospinal neurons retract their distal dendrites in response t o distal axotomy. The observed anatomic restructuring following axonal injury is likely to be accompanied by an alteration of afferents whic h normally synapse on distal dendrites.