Quantitative behavioral study of the acute and long-term effects of two-stage bilateral 6-OHDA-induced lesions of the nigrostriatal dopaminergic system in monkeys.

Bilateral lesions of the nigrostriatal dopaminergic system (2-stage lesions separated by 5-6 months) were induced in 3 monkeys trained to initiate forelimb-reaching movements toward a visual target. After each lesion, analysis of the task performance over several months of regular testing showed that the latency to initiate the movement was permanently prolonged in monkeys showing 90% or more striatal dopamine depletion, whereas animals with less severe depletion completely recovered the task performance. Several months after a unilateral nigrostriatal damage, a lesion on the other side produced impairments only on the side of the body contralateral to that second lesion and did not reinstate the deficits on the side previously affected by the first lesion. This suggests that the remaining intact nigrostriatal dopaminergic system may not be involved in the long-term behavioral recovery observed in monkeys with a unilateral lesion.