EFFECTS OF AMPHETAMINE AT THE BEGINNING OF THE LIGHT CYCLE ON MULTIPLE INDEXES OF MOTOR-ACTIVITY IN THE RAT

The motor effects of a single dose of d-amphetamine on internally sync hronized male Sprague Dawley rats and its dose response relationship a t the beginning of the light cycle was investigated using a computeriz ed monitoring system. After 7 days of acclimatization to light/dark cy cle and 2 days of baseline monitoring, rats were randomized to a no-tr eatment 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 cyc le 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/k g) significantly elevated (P < 0.01) locomotor activity compared to ba seline, but not all activity parameters (horizontal activity, total di stance, vertical activity, stereotypic activity, and number of stereot ypic movements) followed the same pattern of response. The maximum inc rease 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 param eters returning to baseline levels on the day after treatment. The ran ge of variables and the establishment of baseline values at the time o f injection for each rat provides the potential to characterize circad ian patterns of locomotor activity and chronopharmacologic effects of drugs on motor activity, including sensitization and tolerance.