316 Following acute drug administration under baseline conditions, a cohort of rats was used for pharmacological manipulation under long ITI conditions. During these sessions, the ITI was extended to 7 s to provoke impulsive behavior (Dalley et al. 2002). The long ITI session consisted of 100 trials (ITI 7 s, stimulus duration 0.5 s) per session and was repeated once a week. The long ITI days were at all times preceded and followed by two baseline days (ITI 5 s, stimulus duration 0.5 s) to avoid habituation to the long ITI. Delayed reward paradigm Rats were faced with a choice between a small, immediate food reward and a larger but delayed food reward. Ultimately, the delayed option is more beneficial, but the subjective value of the large food reward declines with increasing delay to its delivery (Logue 1988). In the final stage of the task, a session was divided into five blocks of 12 trials. Each block started with two forced trials in which, after initiating the trial by a nose poke into the center hole, either the left or the right hole was illuminated in a counterbalanced fashion. For the next 10 trials, the animals had a free choice and both the left and right units were illuminated. Nose poking into one position resulted in the immediate delivery of a small reinforcer (one food pellet), whereas a nose poke into the other position resulted in the delivery of a large, but delayed, reinforcer (four food pellets). Over sessions, the delays for the large reinforcer were progressively increased within a session from 0 to 10, 20, 40 and 60 s per block. After delivery of the reinforcer or the choice phase time elapsed, an ITI commenced until the next trial started. As the trial time was fixed, the ITI duration depended on the duration of the delay. The positions associated with the small and large reinforcers were always the same for each individual but counterbalanced for the group. The behavioral measure to assess task performance, i.e., the percentage preference for the large reinforcer as a function of delay, was calculated as the [(number of choices for the large reinforcer) / (number choices large+small reinforcers) ×100]. Furthermore, no response during the choice phase within 10 s was counted as an omission and the total number of omitted choice trials per block of 10 trials within a session was calculated. Drugs (+)-Amphetamine sulphate was purchased from O.P.G. (Utrecht, The Netherlands). GBR12909 dihydrochloride, atomoxetine hydrochloride and citalopram hydrobromide were obtained from Tocris Bioscience (UK). All drug doses were calculated as salt, dissolved in 0.9% saline (amphet- amine, atomoxetine, and citalopram) or sterile water Psychopharmacology (2012) 219:313–326 (GBR12909). Drugs were freshly prepared each day before testing and injected intraperitoneally (i.p.) in a volume of 1 ml/ kg body weight. Drug tests were conducted on Tuesdays and Fridays with baseline training sessions on the other weekdays. Before the first test day, all animals had been habituated twice to i.p. saline injections. Drugs were administered according to a Latin square design and each animal received a maximum of two different drugs. In addition, in a subgroup of rats pharmacological manipulations were tested in the 5-CSRTT under a long ITI of 7 s once a week according to a Latin square design with baseline training sessions (intertrial interval of 5 s) in between. Statistical analysis All data are presented as means and standard errors of the mean and analyzed using SPSS for Windows, version 15.0. The data of the delayed reward task were subjected to an arcsin transformation before statistical analysis (McDonald 2009). In the DRT, animals that did not show a delay- dependent curve in their choice behavior (0% choice for large reward at 0 s delay, 100% choice for large reward at 40 and 60 s delay) were excluded from the experiment. Data were analyzed by one- (5-CSRTT) or two-factor (DRT) repeated-measures ANOVAs with drug treatment (5-CSRTT, DRT) and delay to large reinforcer (DRT) as within-subjects variables. If the outcome of the repeated-measures ANOVA yielded significant effects of dose or dose×delay at p<0.05 level, further post-hoc analysis was performed using paired samples t-tests. Pharmacological effects on 5-CSRTT perfor- mance under a long ITI were analyzed using paired samples t-tests (vehicle versus drug treatment). Results Effect of amphetamine and monoamine reuptake inhibitors on 5-CSRTT performance under baseline conditions As depicted in Fig. 1a and Table 1, amphetamine significantly increased the number of premature responses and perseverative responses in the 5-CSRTT [F(3,45) = 13.12, p < 0.001 and F(3,45) = 3.64, p < 0.05, respectively]. Post-hoc analysis revealed that the increase of premature and perseverative responses was significant at all tested doses (p <0.01 and p <0.05, respectively). In addition to the effect on impulsivity, systemic administration of amphet- amine reduced accuracy, i.e., percentage of correct responses at all doses [F(3,45) = 8.82, p < 0.001], whereas the response latency was only reduced at a dose of 0.5 mg/ kg (Table 1) [F(3,45) = 3.37, p< 0.05]. Errors of omission were differentially affected by amphetamine; as 0.25 mg/kg of amphetamine reduced the amount of errors of omission,