Morphology of identified corticospinal cells in the rat following motor cortex injury: absence of use-dependent change

Abstract

After unilateral injury to the forelimb area of the motor cortex, rats are reported to show relatively increased use of the ipsilateral forelimb during exploratory behavior and a concomitant transient increase in dendritic growth in neurons in the undamaged motor cortex. To identify the specific population of cells in motor cortex that undergo such use-dependent change, we examined the morphology of corticospinal pyramidal cells in the intact hemisphere 18 days following unilateral motor cortex damage. Corticospinal neurons in the motor cortex were retrogradely labeled with injections of the fluorescent tracer, DiO, into the cervical enlargement of the spinal cord. Seven days later, the rats received a lesion in the forelimb area of the contralateral motor cortex and 18 days following the lesion, limb use was assessed in two behavioral tests after which the rats were sacrificed. Under fluorescent light, corticospinal cells were visualized and injected with a horseradish peroxidase-fluorescenn conjugate that was then reacted with diaminobenzidine. The labeled cells were reconstructed and the number and centrifugal order of the branches were analyzed. The increased use of the paw contralateral to the intact motor cortex was not associated with an increase in dendritic arborization in corticospinal motor neurons in the intact motor cortex. The results are discussed in light of methodological and theoretical considerations relevant to the study of neural plasticity in the motor system.