FIGURE 2.

Sensitization to amphetamine-induced early-phase and late-phase locomotion occurs at different rates. (a) Measurements of distance traveled across the last saline-treatment day and following amphetamine-treatment days. Each row shows one day with averaged distance traveled measurements in 5 min bins. Injection occurred at time = 0. The average duration of confined locomotion, between the early-phase and late-phase locomotion periods, diminishes with prolonged treatments but recurs on the challenge day. (b) Average distance traveled before and after D-amphetamine injection on day 1 (magenta), day 2 (black), and day 7 (green). There was significantly less mid-phase locomotion on treatment days 2 and 7 than on day 1 (*P < 0.005 at every point marked in the comparisons between day 1 and day 7 and between day 1 and day 2). By day 7, the mice showed increased late-phase locomotion, relative to day 1 (^†P < 0.005 at every point marked in the comparison between day 1 and day 7). (c) On the challenge day (orange), mice maintained early-phase sensitization, based on their high levels of locomotion within 5 min of drug injection (*P = 8 x 10⁻⁶ by 2-tailed, paired t-test compared to day 1 data shown in panel b and File S1). During the late-phase locomotion period, however, the mice showed a diminished locomotor response on challenge day than on day 21 (blue) (^†P < 0.03). Averages and +SEM across mice are shown (n = 12 mice).