MORPHOLOGY OF IDENTIFIED CORTICOSPINAL CELLS IN THE RAT FOLLOWING MOTOR CORTEX INJURY - ABSENCE OF USE-DEPENDENT CHANGE

After unilateral injury to the forelimb area of the motor cortex, rats are reported to show relatively increased use of the ipsilateral fore limb during exploratory behavior and a concomitant transient increase in dendritic growth in neurons in the undamaged motor cortex. To ident ify the specific population of cells in motor cortex that undergo such use-dependent change, we examined the morphology of corticospinal pyr amidal cells in the intact hemisphere 18 days following unilateral mot or cortex damage. Corticospinal neurons in the motor cortex were retro gradely labeled with injections of the fluorescent tracer, DiO, into t he cervical enlargement of the spinal cord. Seven days later, the rats received a lesion in the forelimb area of the contralateral motor cor tex and 18 days following the lesion, limb use was assessed in two beh avioral tests after which the rats were sacrificed. Under fluorescent light, corticospinal cells were visualized and injected with a horsera dish peroxidase-fluorescein conjugate that was then reacted with diami nobenzidine. The labeled cells were reconstructed and the number and c entrifugal order of the branches were analyzed. The increased use of t he paw contralateral to the intact motor cortex was not associated wit h an increase in dendritic arborization in corticospinal motor neurons in the intact motor cortex. The results are discussed in light of met hodological and theoretical considerations relevant to the study of ne ural plasticity in the motor system.