Abstract
We investigated whether there is an imbalance in the nigrostriatal pathway in unlesioned young (8 weeks) and old (20 months) female rats. Rotation behavior and stepping tests were employed to evaluate the functional laterality of this pathway. In the absence of methamphetamine, there was no preference in the rotational direction in young rats but there was temporally consistent rotational preference in old rats. Methamphetamine injection (5 mg/kg, i.p.) uncovered the imbalance of rotational direction in young rats and enhanced in old rats. Old rat showed a temporally consistent rotational direction preference, as measured weekly for three consecutive weeks. Neither the presence nor the absence of methamphetamine affected rotational direction during the three weeks. Furthermore, statistical analyses revealed lateral malfunctions, which had been predicted from the methamphetamineβinduced rotational test, to be consistent with those revealed by the stepping test in old rats. On the other hand, there was no temporal consistency in young rats. These results suggest that old female rats have a persistent functional imbalance of the nigrostriatal pathway.