Is there an optimal age for recovery from motor cortex lesions? I. Behavioral and anatomical sequelae of bilateral motor cortex lesions in rats on postnatal days 1, 10, and in adult hood

Rats were given bilateral lesions of the motor cortex on the day of birth ( P1), tenth day of life (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 ). Although all lesion groups were impaired at skilled reaching, the P10 gr oup was less impaired than either of the other two lesion groups. Furthermo re, on the other motor tests the P10 group did not differ from controls whe reas both P1 and adult groups were impaired. Only the P1 lesion group was i mpaired at the acquisition of the Morris water task. Anatomical analyses re vealed that the P1 and P10 rats had smaller brains than the other two group s as well as having a generalized decrease in cortical thickness. Dendritic analysis of layer III pyramidal cells in the parietal cortex revealed a de crease in apical arbor in the lesion groups and an increase in the basilar arbor of the P1 and adult lesion animals. The P1 and adult operated groups showed an increase in spine density in the basilar dendrites of layer V pyr amidal cells. Finally, analysis;of the pattern of corticospinal projections revealed that the P1 animals had a markedly wider field of corticospinal p rojection neurons than any of the other groups. The widespread anatomical c hanges in all lesion groups versus the relatively better behavioral recover y after P10 lesions suggests that day 10 represents an optimal period for a dapting to brain damage and subsequent brain reorganization. (C) 2000 Elsev ier Science B.V. All rights reserved.