490 To date, the effects of opioid drugs on impulsivity have been less well documented, although recent evidence points towards a role for the opioid system in impulsivity. For instance, abstinent alcoholics have been shown to make impulsive decisions in a delay discounting task (Mitchell et al. 2005), operationalized as a preference for smaller and sooner over larger and later hypothetical monetary reward. Interest- ingly, in these subjects, the opioid receptor antagonist naltrexone reduced impulsive decisions in this task depend- ing on the subjects' locus of control personality trait (Mitchell et al. 2007). Also, in control subjects, naltrexone improved performance on a measure of inhibitory motor control in this task. Collectively, these findings suggest that opioid activity might modulate aspects of impulsive behav- ior. In support of this, in rats, acute challenges with the μ- opioid receptor agonist morphine induce impulsive decisions in an adjusting amount procedure (Kieres et al. 2004) and seem to induce delay aversion in a delay discounting task (Pitts and McKinney 2005). Thus, these observations confirm a role for µ-opioid receptors in impulsive decision making. In addition to the aforementioned acute opioid drug challenges on measures of impulsivity, several studies have reported increased risky decision making (Odum et al. 2000), as well as impulsive decision making in opiate users (Clark et al. 2006; Madden et al. 1997; Petry et al. 1998; Kirby and Petry 2004). To date, there is little evidence of chronic opiate use on perturbations in measures of inhibitory response control (Verdejo-Garcia and Perez- Garcia 2007; Verdejo-Garcia et al. 2007; Passetti et al. 2008). In contrast to acute opioid effects on impulsivity, elevated impulsivity in opiate addicts might also have resulted from neuroadaptations following prolonged opiate use. In this regard, alterations in brain opioid and α2 adrenergic receptor densities have been reported in opiate addicts (Gabilondo et al. 1994; Kling et al. 2000). Studying the acute effects of opioids on impulsivity might help to interpret the aforementioned observations in opiate addicts. There is broad consensus that impulsivity is multifacto- rial with distinct behavioral expressions that might be under control of partly separate underlying neuroanatomical circuits and neurotransmitter systems (Evenden 1999; Pattij and Vanderschuren 2008; Winstanley et al. 2006). Poor inhibitory response control (impulsive action) and impul- sive decision making (impulsive choice) are prominent behavioral phenomena of impulsivity, the latter often operationalized as delay aversion in delay discounting tasks. The aforementioned observations primarily suggest a role for the opioid system in impulsive decision making. Therefore, the purpose of the present experiments was to further examine the role of μ-opioid receptors in distinct forms of impulsive behavior. To this extent, we investigated the acute effects of morphine and naloxone in three Psychopharmacology (2009) 205:489–502 translational rodent paradigms: (1) the five-choice serial reaction time task (5-CSRTT; Bari et al. 2008) to measure aspects of inhibitory response control, reflected by the ability to withhold inappropriate responses; (2) the stop- signal task (Eagle et al. 2008) to measure response inhibition, i.e., the ability to inhibit a response that already has been initiated; and (3) the delayed reward paradigm (Cardinal 2006) to measure delay aversion. Materials and methods Subjects Male Wistar rats were obtained from Harlan CPB (Horst, The Netherlands). At the start of the experiments, animals were 12 weeks old, weighed approximately 250 g, and were housed in pairs in macrolon cages (42.5 ×26.6× 18.5 cm; l ×w ×h) under a reversed 12-h light/dark cycle (lights on at 7:00P.M.) at controlled room temperature (21 ± 2°C) and relative humidity of 60±15%. Animals were maintained at approximately 90% of their free-feeding weight, starting 1 week prior to the beginning of the experiments by restricting the amount of standard rodent food pellets (Harlan Teklad Global Diet, Blackthorn, UK). Water was available ad libitum throughout the entire experiment. All experiments were conducted with the approval of the animal ethical committee of the Vrije Universiteit, Amsterdam, The Netherlands. Apparatus Experiments were conducted in identical rat five-hole nose poke operant chambers with stainless steel grid floors (MED-NPW-5L, Med Associates, St. Albans, VT, USA) housed in sound-insulating and ventilated cubicles. 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. Rodent food pellets (45 mg, Formula P, Bioserv, Frenchtown, USA) could be delivered at the opposite wall via a dispenser. In addition, a white house light could illuminate the chamber and a 4,500-Hz tone could be delivered through a speaker. A computer equipped with MED-PC version 1.17 (Med Associates) controlled experimental sessions and recorded data. Animals were tested once daily from Monday until Friday, during the dark phase of the light/dark cycle. Behavioral procedures Separate groups of n = 16 animals were trained for all paradigms for which similar habituation and magazine