Is there an optimal age for recovery from motor cortex lesions? II. behavioural and anatomical consequences of unilateral motor cortex lesions in perinatal, infant, and adult rats

Purpose. The purpose of this study was to compare the behavioural and anato mical effects of unilateral motor cortex ablation in neonatal, infant, and adult rats. Methods: Rats were given unilateral lesions of the motor cortex on the day of birth (P1), at ten days of age (P10), or in adulthood. They were trained on several motor tasks (skilled forelimb reaching, beam traversing, tongue extension), general motor activity, and a test of spatial learning (Morris water task). Results: Although all lesion groups were equally impaired at skilled reachi ng with the forelimb contralateral to the lesion, rats with P1 lesions also were impaired at traversing a narrow beam and at learning the Morris task. Gross anatomical analyses revealed that the P1 rats had smaller brains tha n the other groups, a result that may account for the larger behavioural de ficits in the P1 group. Analysis of Golgi-Cox stained neurons showed that r elative to control groups, all lesion groups showed an increase in dendriti c length in the basilar dendrites of layer III pyramidal cells and, paradox ically a decrease in length of the apical dendrites of the same cells. Conclusions: The bilateral alterations in dendritic organization following the motor cortex lesions suggest that there has been a bilateral reorganiza tion of intrinsic cortical connectivity following motor cortex lesions at a ny age. These alterations in connectivity are likely not identical in the y oung and adult animals, however, because relative to controls, both the you ng operated groups, but not the adult group, showed a bilateral drop in spi ne density in the basilar dendrites of layer V pyramidal cells. These findi ngs are discussed with respect to the idea that there may be critical ages in development in which animals can use anatomical modifications to compens ate for deficits produced by cortical injury.