INDIVIDUAL CORTICORUBRAL NEURONS PROJECT BILATERALLY DURING POSTNATAL-DEVELOPMENT AND FOLLOWING EARLY CONTRALATERAL CORTICAL-LESIONS

The corticorubral projections in adult cats are primarily uncrossed. H owever, early in development and after early unilateral lesions of the sensorimotor cortex, crossed corticorubral projections are also obser ved. The present study was performed to disclose (1) whether the cross ed projections originate from neuronal subpopulations different from t hose producing uncrossed ones and (2) how the neurons that give rise t o the crossed projections in the lesioned animals are related to those occurring in normal development. We injected fluorescent latex micros pheres into the red nucleus of two groups of animals: (1) intact kitte ns at postnatal week 3 and, (2) kittens that had received unilateral a blation of the cerebral cortex at this stage and were then allowed to survive for at least 4 weeks. Red fluorescing microspheres were inject ed on one side and green ones on the other. In both normal and lesione d kittens, a number of cells in the cortex were labeled as a result of the contralateral as well as the ipsilateral injections, and no diffe rence in size or distribution was found between the cells labeled from contralateral and ipsilateral injections. More than half of the cells labeled from contralateral injections were double-labeled in both gro ups of animals. These results indicate that individual corticorubral c ells project bilaterally in normal development as well as following un ilateral lesions of the cortex. With respect to the cells producing cr ossed projections, they were similar in both laminar and regional dist ributions between the intact and lesioned animal, suggesting that the crossed projections arise from the same neuronal subpopulation before and after cortical lesions. This view was supported by sequential inje ctions of the tracers, which indicated that cells normally projecting contralaterally maintained the crossed projection after the lesions. T aking into account our previous observations that growth and prolifera tion of crossed corticorubral axons took place in the red nucleus (Mur akami et al. 1991a), it is likely that growth and proliferation of the axons in denervated targets play a major role in lesion-induced estab lishment of aberrant projections.