Time-dependent differences in the rat's motor response to amphetamine.

The dose-related motor effects of d-amphetamine given at the beginning of the light and dark cycle of rats were investigated using a computerized activity-monitoring system that recorded five different motor behavior indices. After 7 days of acclimatization and 2 days of baseline monitoring, rats were randomized into either a no-treatment time control group (n = 12), or to receive 0 (vehicle), 0.6, 1.25, 2.5, or 10 mg/kg d-amphetamine (n = 8 each) either 1 h into the light phase (0800) or another five groups at 1 h into the dark phase (2000) of day 3. The time control group exhibited a stable baseline level of activity for the length of the experiment. All doses (0.6, 1.25, 2.5, and 10 mg/kg) significantly elevated (p < 0.01) locomotor activity compared to baseline at both times of administration, but not all motor indices followed the same pattern of response. At both injection times, the maximum increase over baseline generally occurred following the 1.25 mg/kg dose of amphetamine (p < 0.001). The duration of the drug effect also increased with each dose. The stereotypic effects produced by high doses of AMP (10 mg/kg) was different when applied at the light phase compared to the dark phase, but the amphetamine effect on locomotor behavior remained the same regardless of the difference in motor activity baseline between the activity phases.