Experiment 1

Self-administration training sessions were conducted daily for seven days. During these sessions, responding was reinforced on an FR1 schedule of reinforcement with 0.5 mg/kg/infusion cocaine. All sessions terminated automatically after 1 hr.

After 7 days of training, daily self-administration sessions were extended to 6 hr with no limit placed on the number of infusions that could be earned. Each session began at the start of the dark phase of the daily light/dark cycle (star time: 1700) and ended 6 hr later (end time: 2300). All other experimental events were identical to those used during training. The dose of cocaine was maintained at 0.5 mg/kg/infusion in all test sessions. Testing continued in this manner for 14 consecutive days.

Drug self-administration data were expressed as the number of infusions obtained and analyzed via two-way mixed-factor ANOVA, with group serving as a between-subjects factor and session serving as the repeated measure. Preplanned analyses comparing the first versus last 1, 3, 5 and 7 days of testing were also conducted using similar two-way, mixed-factor ANOVA. Group effects were further analyzed under conditions in which the omnibus test was significant using Fisher’s Least Significant Differences Test. Cumulative records taken from the first (Day 1) and last (Day 14) test session were also analyzed to determine relative response rates and the duration of responding over the 6-hr test session. Relative response rates were calculated for each rat by determining the slope of a regression line fitted to the data for the duration of responding. The duration of responding was operationally defined as the elapsed time between the beginning of the session and the occurrence of the final lever press emitted during the session, and thus included the time allocated to lever pressing and the inter-response intervals. These data were analyzed via two-way, mixed-factor ANOVA, with group serving as a between-subjects factor and session serving as the repeated measure. Inactive lever responding from rats without access to cocaine was examined via repeated-measures ANOVA, with session serving as the repeated measure.