Psychopharmacology (2012) 219:313–326 of impulsive action in the 5-CSRTT (Dalley et al. 2002; Navarra et al. 2008; Paterson et al. 2011). Not only does a long ITI evoke higher levels of premature responding, it may also be that performance under baseline conditions relies on different neural and cognitive processes than responding under novel, challenging circumstances. For example, neurotransmission appears to be especially engaged when task contingencies unexpectedly change (Cole and Robbins 1992; Dalley et al. 2001; Sirviö et al. 1993). Thus, drug effects under baseline and long ITI conditions in the 5-CSRTT may be qualitatively or quantitatively different. Materials and methods Subjects Male Lister Hooded rats (Harlan CPB, Horst, The Nether- lands), weighing 200–250 g at the beginning of the experiment, were housed two per cage under reversed lighting conditions (lights on from 19.00 to 07.00 h). After 2 weeks of habituation, rats were placed on a restricted diet of 14 g of standard rat chow per day and body weights were monitored on a weekly basis. Feeding occurred in the rats' home cages at the end of the experimental day. All experiments were approved by the Animal Ethics Committee of Utrecht University and were conducted in agreement with Dutch laws (Wet op de Dierproeven, 1996) and European regulations (Guideline 86/609/EEC). Behavioral apparatus Behavioral testing for all experiments was conducted in 16 identical operant conditioning chambers (30.5×24×21 cm; Med Associates, St. Albans, VT, USA) enclosed in sound attenuating boxes. The boxes were equipped with a fan to provide ventilation and to mask extraneous noise. Set in the curved wall of each box was an array of five holes. Each nose poke unit was equipped with an infrared detector and a yellow light-emitting diode stimulus light. Food pellets (45 mg, Formula P; Bio-Serv) could be delivered at the opposite wall via a dispenser. The chamber could be illuminated by a white house light mounted in the center of the roof. Online control of the apparatus and data collection were performed using MED- PC version 1.17 (Med Associates). Behavioral procedures A detailed description of the 5-CSRTT and DRT procedure has been provided previously (Van Gaalen et al. 2006a; Van Gaalen et al. 2006b, respectively). Separate groups of animals were trained for each experiment. For both 315 behavioral paradigms, similar habituation and magazine training protocols were followed. This protocol consisted of a habituation exposure to the operant chambers for two daily 30-min sessions, during which sucrose pellets were placed in the response holes and food magazine. Subse- quently, in the next two sessions, a total of 75 pellets were delivered with a random interval to allow the animals to associate the sound of pellet delivery with reward. This procedure was followed by magazine shaping, in which animals were trained to make a nose poke into an illuminated response hole to earn reward. The spatial location of the stimulus lights was adjusted according to the experimental paradigm. Each session of magazine training consisted of 100 trials and lasted approximately 30 min. Five sessions were scheduled per week (one session per day, Monday–Friday) during the dark phase of the light/dark cycle. Five-choice serial reaction time task Rats were trained to detect and respond to a brief visual stimulus presented randomly in one of the five nose poke units to obtain a food reward. A trial started with an ITI of 5 s, followed by 1-s illumination of one of the five apertures and 2-s limited hold. Following a nose poke in the illuminated aperture, i.e., a correct response, animals were rewarded with the delivery of one food pellet (45 mg, Formula P, Research Diets) in the food magazine. During the training session, stimulus duration was set at 32 s and was gradually decreased over sessions to 1 s until animals reached stable baseline performance (accuracy, >80% correct choice and <20% errors of omission). Each daily session consisted of 100 discrete trials or 30 min, which- ever occurred first. A nose-poke response into a non- illuminated aperture, i.e., an incorrect response as well as failure to respond within 5 s after the onset of the stimulus, i.e., an error of omission, resulted in no food delivery and a time-out period with the house light extinguished for 5 s. Nose pokes made during the ITI, i.e., before the onset of the stimulus (premature responses) were recorded as a measure of impulsivity and resulted in a 5 s time-out and no food reward. Perseverative responses, i.e., repeated responding during the presentation of the stimulus, were measured but did not have any programmed consequences. The following behavioral measures were recorded: (1) premature responses, i.e., number of responses into one of the holes during the ITI preceding stimulus presentation; (2) accuracy, i.e., percentage of correct responses [(number correct responses) / (correct+incorrect responses)×100]; (3) latency of correct responses, i.e., the mean time between stimulus onset and nose poke in the illuminated unit; (4) omission errors, i.e., the total number of omitted trials during a session; and (5) perseverative responses after correct choice.