Two experiments examined the effects of various doses of l-amphetamine on locomotor activity, two-way avoidance, and intertrial responses in rats that were 15, 17, 21, 36, and 90 days of age. In the first experiment, testing began 20 min after drug administration and 100 avoidance trials were given. In the second experiment, testing began immediately after drug treatment and 200 trials were given. In the two youngest groups, shuttle crossings during adaptation were increased by the highest doses. The typical inverted U-shaped function between amphetamine dose and locomotor activity found in adults began to emerge at 36 days of age. Generally, at the two youngest ages, there was a dose-related increase in avoidance and intertrial responses on the first few blocks of trials and then a decrease across trials. In 21-day rats, the drug increased avoidance on the first few trial blocks but acquisition was altered only slightly. Acquisition of 36-and 90-day rats was increased, but this increase was greater in adults. It appears likely that development of CNS catecholaminergic neurons is responsible for the age-dependent behavioral effects of l-amphetamine.
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