In the self-control baseline condition (≥ 80% choice of the LL reinforcer), an average of 79.2 (SE = 16.5) sessions were required to reach an average stable adjusted-delay of 6.4 s (SE = 1.2 see ). In the impulsive baseline condition, an average of 89.3 (SE = 13.4) sessions were required to achieve a stable adjusted-delay of 24.0 s (SE = 1.9). In one condition, two rats’ (B1P1 & G1B2) stable baseline choices could not be recovered during no-injection days between the second and third series of doses. These rats did not complete the third dose series and missing data from this series were interpolated by SPSS using the between-subjects average. These interpolations did not affect the outcome of any analysis.
The top panel of shows the average percent of trials on which the rat failed to respond within 30 s (an omission) in the vehicle (V) and pramipexole sessions. When compared with vehicle, pramipexole dose-dependently increased the number of trials scored as omissions in both the impulsive (F(3, 48) = 30.8, p < .001, ηp2= 0.79; linear contrast: p < .001) and self-control (F(1.5, 17.4) = 15.3, p = .001, ηp2= 0.66; linear contrast: p = .001) baselines. The difference in omission frequencies across the two baseline conditions approached, but did not achieve conventional levels of significance (main effect of baseline, p = .06). No interactions were significant. A separate two-way ANOVA revealed no significant main effect of series (i.e., the first, second, or third pass through the sequence of saline and active injections, p = .17) and the dose × series interaction was not significant (p = .45).
Average (SEM) percent of trials omitted per session (top panel) and mean center lever response latency (bottom panel) as a function of saline and pramipexole doses in Experiment 1.
As shown in the bottom panel of , pramipexole similarly affected center-lever response latencies on trials in which a response was made. The main effect of dose was significant in both the impulsive (F(3, 48) = 17.3, p < .001, ηp2= 0.68; linear contrast: p = .001) and self-control baselines (F(3, 48) = 16.9, p < .001, ηp2= 0.68; linear contrast: p < .001); however, no significant difference in latencies was observed across baselines (p = .17). A separate two-way ANOVA revealed no significant main effect of series (p = .93) and the dose × series interaction was not significant (p = .71).
In five sessions in which injections were administered (2.2% of all injection sessions), four rats failed to complete any free- or forced-choice trials (one rat did this twice at the highest dose). This occurred twice in the self-control baseline and three times in the impulsive baseline and was restricted to the two highest doses (0.18 and 0.3 mg/kg). In these cases, the missing data were interpolated from between-subject averages; these interpolations did not affect the outcome of any analysis.
The top panel of shows the average percentage of impulsive choices made in the self-control baseline condition. Separate data paths illustrate the effects of pramipexole in the three dosing series. In this condition, impulsive choices were infrequent on vehicle (V) days. By contrast, pramipexole increased impulsive choices above saline levels (F(3,48) = 21.3, p < .001; ηp2= 0.73; linear contrast: p < .001). The main effect of dosing series was not significant (p < .48). However the dose × series interaction approached significance (p = .06), reflecting the higher prevalence of impulsive choices on the final dosing series at the highest dose.
Average (SEM) percent choice of the impulsive (SS) alternative in impulsive and self-control baseline conditions of Experiment 1.
In the impulsive baseline condition (bottom panel of ), impulsive choices predominated in vehicle sessions. No significant main effect of pramipexole was detected (p = .34); however, there was a significant effect of dosing series (F(2,48) = 9.5, p < .01; ηp2= 0.54) reflecting a reduction in impulsive choice at the 0.3 mg/kg dose in the first, relative to the second and third, series of pramipexole doses. Where this reduction was observed (three sessions), it occurred in sessions in which more than 60% of the trials were omitted (i.e., the trial was terminated because the rat failed to respond within 30 s).
To reduce the probability that conclusions about the effects of pramipexole on impulsive choice would be influenced by sessions in which few choice responses occurred, a second series of analyses were conducted in which data were excluded if the rat did not complete at least 50% of the free-choice trials. This exclusion criterion resulted in the exclusion of 7.4% of the data in the self-control baseline condition and 10.8% in the impulsive baseline condition. To allow statistical analyses with these missing data, a single average was calculated at each dose for each rat across dosing series. These data are shown in .
Average (SEM) percent choice of the impulsive (SS) alternative in impulsive and self-control baseline conditions of Experiment 1. Data were excluded from analysis if the rats completed fewer than 50% of free-choice trials in a session.
As before, a significant effect of pramipexole was observed in the self-control baseline condition (F(3,24) = 16.1, p < .001; ηp2= 0.67), but not in the impulsive baseline (p = .12). Post-hoc comparisons of self-control baseline choices revealed significant differences between saline and all pramipexole doses (p < .03 in all cases); however, no significant differences were detected between doses. In the impulsive baseline, post-hoc comparisons revealed no differences between doses or between any single dose and saline.