Effects of amphetamine at the beginning of the light cycle on multiple indices of motor activity in the rat

The motor effects of a single dose of d-amphetamine on internally synchronized male Sprague Dawley rats and its dose response relationship at the beginning of the light cycle was investigated using a computerized monitoring system. After 7 days of acclimatization to light/dark cycle and 2 days of baseline monitoring, rats were randomized to 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) 1 h into the light cycle of day 3, and monitored for an additional post-treatment day 4. In the time control group, there was a stable baseline level of activity for both light and dark phases. All doses (0.6, 1.25, 2.5, and 10 mg/kg) significantly elevated (P < 0.01) locomotor activity compared to baseline, but not all activity parameters (horizontal activity, total distance, vertical activity, stereotypic activity, and number of stereotypic movements) followed the same pattern of response. The maximum increase in all parameters, except vertical activity, occurred at 1.25 mg/kg (P < 0.001). The duration of drug effect increased with dose, with increased activity lasting until the fifth hour after injection of 10 mg/kg. ANOVA revealed no consistent long term effects, with all parameters returning to baseline levels on the day after treatment. The range of variables and the establishment of baseline values at the time of injection for each rat provides the potential to characterize circadian patterns of locomotor activity and chronopharmacologic effects of drugs on motor activity, including sensitization and tolerance.