Psychopharmacology (2009) 205:489–502 Fig. 2 Effects of morphine and naloxone on response inhibition as measured in the stop-signal paradigm. In total, n=13 ani- mals were included in the mor- phine and naloxone analyses and data are depicted as mean (±SEM) estimated stop-signal reaction times (a, b) and go reaction times (c, d). In addition, Pearson’s r indicated a negative correlation between the magni- tude morphine’s effects on SSRT averaged over all three doses and baseline SSRT under vehicle condition (e). *p <0.05 versus vehicle 495 three doses 0.3, 1.0, and 3.0 mg/kg) correlated nega- tively with baseline SSRT as shown in Fig. 2e (Pear- son’s r = −0.83, p < 0.001). The effect size of morphine on mGoRT did, however, not correlate with baseline SSRT (data not shown; Pearson’s r = −0.32, p = 0.23). Comparable to the effects of morphine, naloxone did neither affect estimated SSRTs nor mGoRTs (Fig. 2b, d) [F (3,36) = 0.69, p = 0.56 and F(3,36) = 2.73, p = 0.058, respec- tively]. Furthermore, naloxone had no effects on go trial accuracy [F(3,36) = 0.39, p = 0.76]. In contrast to the effects of morphine, there was no relationship between the effects of naloxone on response inhibition and baseline SSRT (Pearson’s r = −0.36, p = 0.089). Effects of morphine on impulsive choice in the delayed reward paradigm The percentage choice for the larger delayed reward significantly declined depending on the delay that was imposed on the animals (Fig. 3a) [delay: F(4,60) = 156.15, p < 0.001]. Morphine significantly reduced the preference for the large delayed reward [dose: F(4,60) = 3.35, ε = 0.64, p = 0.035; dose × delay: F(16,240) = 1.58, p = 0.082], and further, post hoc comparisons revealed that this effect was significant in the highest dose morphine. However, analyses of the steepness (k) of the discounting curves with morphine revealed no significant differences between