Psychopharmacology (2009) 205:489–502 training protocols were followed. This protocol consisted of a habituation exposure to the operant chambers for 20 min with the houselight on and the food cup containing three food pellets for two consecutive sessions. Subsequently, in the next two sessions, pellets (100 per session) were delivered with an average delay of 15 s to allow the animals to associate the sound of pellet delivery with reward. Five-choice serial reaction time task experiments A detailed description of the 5-CSRTT behavioral procedure in our laboratory has been provided previ- ously (Van Gaalen et al. 2006a). In short, rats were trained to detect and respond to a brief visual stimulus in one of five nose poke units in order to obtain a food reward. Each session terminated after 100 trials or 30 min, whichever occurred first. Initially, the duration of this stimulus was 32 s and was gradually decreased to 1 s over sessions until animals reached stable baseline performance (accuracy >80% correct choice and <20% errors of omission). Responding during stimulus presentation or within the limited hold (LH) period of 2 s was counted as a correct response. Incorrect, premature responses during the fixed 5-s intertrial interval and errors of omission (no responses or a response after the LH) did not lead to the delivery of a food reward and resulted in a 5-s timeout period during which the houselight was extinguished, whereas perseverative responses, i.e., repeated responding during the presentation of the stimulus, were measured but did not have any programmed consequences. Two differ- ent measures of inhibitory control were measured, namely, (1) the number of premature responses before the onset of the visual stimulus, reflecting aspects of loss of inhibitory control and (2) the number of perseverative responses into the stimulus unit after correct choice, a measure of compulsive behavior. In addition, the following other behavioral parameters were measured that reflect task performance, namely, (3) accurate choice, i.e., percentage correct responses calculated as [number correct trials/ (correct + incorrect trials)] × 100; (4) latency to make a correct choice, i.e., the mean time between stimulus onset and nose poke in the illuminated unit; (5) omission errors, i.e., the number of omitted trials during a session; and (6) feeder latency, i.e., the latency to collect a pellet following correct choice. Stop-signal task experiments Training A detailed description of training in the stop- signal task in our laboratory has been provided previously with some minor modifications in the current behavioral procedure (Pattij et al. 2007a). In the current 491 study, the outer nose poke units were used instead of those immediately adjacent to the middle nose poke unit as described previously. In terms of behavioral performance, this modification primarily resulted in higher mean reaction times, and therefore, stop-signal delays (SSD) were also adjusted accordingly as described below. In brief, rats were trained in the same boxes used in the 5- CSRTT experiments and a typical stop-signal task session consisted of 200 trials; 25% of these trials were stop trials and the remainder go trials. To start a go trial, rats were trained to respond into the middle illuminated nose poke unit stimulus light (start stimulus) under a variable ratio 2 schedule which subsequently switched off this stimulus light and resulted into illumination of the outer left (or right, counterbalanced across rats) nose poke unit stimulus light (go stimulus). Rats were then required to respond as quickly as possible during a LH period to this go stimulus in order to obtain a food reward. Initially, the LH period was set at 5 s and in subsequent sessions was individually titrated to meet go-trial accuracy criterion of approximately 80% successful hits. A stop trial did not differ from a go trial; however, contingent with the illumination of the go stimulus or following a SSD, a stop signal (duration 50 ms; frequency 4500 Hz; intensity 80 dB) was presented and successfully refrain- ing from responding during the LH period of a stop trial resulted in delivery of a food reward. In the final stages of training, the LH periods during both go and stop trials were equal for each individual rat. The SSDs including zero delays were presented in a pseudorandom order within a session. To compensate for differences between rats, SSDs were based on each individual rat's mean reaction time on go trials (mGoRT) in the preceding drug-free training session. Thus, SSDs were calculated as follows: mGoRT minus either 25, 75, 150, 300, or 600 ms. During all stop trials, responding during the onset of the stop signal or during the LH immediately extinguished the go stimulus, houselight, and, if applicable, turned off the stop signal and was followed by a 5-s timeout period. Estimation stop-signal reaction time Calculations to esti- mate the stop-signal reaction times (SSRT) and a correction for omission errors were adapted from Logan (1994) and Solanto et al. (2001). For estimating the SSRT, data of three SSDs (mRT minus 300, 150, and 75 ms) were used, as the probability of correct inhibition on these SSDs was within the range of 0.2 < p < 0.8 and thus most informative for estimation of SSRT (Band et al. 2003). For each of the three SSDs, the probability of responding was calculated including a correction for non-responses based on the number of omissions during the go trials, the latter since omissions cannot be distinguished from successful inhibitions during stop trials. The following