Psychopharmacology (2007) 193:85–96 Five-choice serial reaction time task A more detailed description of training in the 5-CSRTT in our laboratory has been reported previously (Van Gaalen et al. 2006a). In short, rats were trained 5 days per week to detect and respond to a brief visual stimulus in one of five holes 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 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 time-out period during which the houselight was extinguished, whereas persever- ative responses, i.e., repeated responding during the presentation of the stimulus, were measured but did not have any programmed consequences. Two different mea- sures 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 hole after correct choice, presumably measur- ing aspects 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 hole; (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. Delayed-reward paradigm In addition to the 5-CSRTT, the delayed reward paradigm, as employed in our laboratory, has also been described more elaborately (Van Gaalen et al. 2006b). Briefly, rats were trained 5 days per week in this paradigm. In the final stages of training and during drug testing, a session was divided into 5 blocks of 12 trials, each block starting with 2 forced trials during which, after initiating the trial through a nose poke into the central hole, either the left hole or the right hole was illuminated in a counterbalanced fashion. In the next ten trials, the animals had a free choice, and both the left and right hole were illuminated. 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 87 delayed, reinforcer (four food pellets). If an animal did not make a response during this choice phase within 10 s, an intertrial interval was initiated, and the trial was counted as an omission. The position associated with the small and large reinforcer was always the same for each individual and counterbalanced for the group. The delay for the large reinforcer progressively increased within a session per block of 12 trials as follows: 0, 10, 20, 40, and 60 s. Responding into non-illuminated holes during the test was recorded, but had no programmed consequences. 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 rein- forcers)× 100. In addition, we calculated the total number of omitted choice trials within a session. Stop-signal paradigm Shaping During initial shaping for two consecutive ses- sions, both the middle nose poke hole and the hole immediately adjacent to the right or left were illuminated (counterbalanced for all subjects). A nose poke into either one of the two active holes extinguished the visual stimuli in both holes and resulted in delivery of a pellet. After an intertrial interval of 10 s, the next trial started. Nose poking within this intertrial interval period did not have any programmed consequences. A session ended when the rat had earned 100 pellets or after 30 min, whichever occurred first. Shaping: go trials During the next phase, only the stimulus light in the middle nose poke hole was illuminated (start stimulus). A response into the active middle hole switched off the stimulus light and was followed by the illumination of the stimulus light (go stimulus) in the hole immediately adjacent to the left or right. A nose poke into this illuminated hole switched off the stimulus light and resulted in the delivery of a pellet. After an intertrial interval of 10 s, the next trial started. Responding in the start stimulus hole during presentation of the go stimulus was counted as perseverative start pokes, whereas prestimulus responses into the go stimulus hole resulted in a timeout period of 5 s. Subsequently, the response requirements into the start stimulus hole before onset of a go stimulus were varied into a variable ratio 2 schedule (VR2, i.e., either FR1, FR2, or FR3) to avoid the development of a prepotent response pattern from start stimulus to go stimulus hole and to ensure that animals waited until the appearance of a go stimulus. During this phase, rats were trained until they reliably completed 200 successful go trials. Following this phase, a LH period was introduced for the go stimulus and only during this period was the go stimulus present. Initially, the