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Rotational behavior was examined prior to (Exp. 1) and following unilateral lesions (Exp. 2) in the substantia nigra of young and old rats. Before lesioning no preferences for turning to the left or right were seen in either age group following graded doses of amphetamine. However, after radiofrequency lesions of the left substantia nigra both young and old rats turned predominantly to the left, and the ratio of left to right turns increased as a function of amphetamine dose. However, there were significant differences between young and old rats in the effect of amphetamine on rotational behavior, with old rats showing decided decrements in response strength. Direct dopaminergic stimulation under apomorphine or dopaminergic receptor blockade with haloperidol produced no such age related deficits. The results are discussed in terms of possible presynaptic alterations in dopaminergic functioning with aging.

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