494 Table 1 Effects of morphine (MOR), naloxone (NAL) and their combination on measures of attentional function and mo- tivation in the 5-CSRTT In total, n=16 animals were included in the analyses and data depict mean±SEM *p < 0.05, **p < 0.005 (versus vehicle); ***p < 0.05, ****p < 0.005 (compared to vehicle/ve- hicle and NAL/MOR drug com- binations) Psychopharmacology (2009) 205:489–502 Accuracy (%) Response latency (ms) Omissions (%) Feeder latency (ms) MOR Vehicle 0.3 mg/kg 1.0 mg/kg 3.0 mg/kg 6.0 mg/kg NAL Vehicle 0.3 mg/kg 1.0 mg/kg 3.0 mg/kg NAL+3.0 mg/kg MOR Vehicle–vehicle Vehicle–MOR 0.3 mg/kg–MOR 1.0 mg/kg–MOR 87.1± 1.3 87.0± 1.3 85.7± 1.6 85.8± 1.8 88.7± 1.5 87.7± 1.5 87.4± 1.2 89.1± 1.3 88.8± 1.3 86.8± 1.3 85.2± 1.2 87.6± 1.1 88.8± 1.4 289±7 307±9 303±15 320±13 358±24* 293±7 290±8 300±8 316±8* 294±7 292±5 300±8 290±9 7.9±1.8 8.1±1.6 11.3±3.5 17.6±5.0 35.8±5.9** 12.5±4.1 11.8±3.4 9.5±1.4 11.1±1.5 5.3±1.1 10.4±1.4*** 6.0±1.3 7.1±1.0 769 ±27 853±93 732±27 696±29 695±36 823±53 760±29 795±28 792±27 727±20 637±20**** 725±23 732±22 choice was not significantly changed by morphine [F(4,60)= 2.67, ε = 0.40, p = 0.10]. The effects of naloxone on performance in the 5-CSRTT are summarized in Fig. 1b, d and Table 1. Naloxone increased correct response latencies [F(3,45) = 4.59, p = 0.007], and further comparisons revealed that only the highest dose significantly increased latencies by approxi- mately 20 ms. In addition, the lowest dose of 0.3 mg/kg naloxone significantly reduced the number of perseverative responses after correct choice [F(3,45) = 2.96, p = 0.042]. Other performance measures in the 5-CSRTT reflecting impulsive action, attentional performance, and motivation were not affected by naloxone [accurate choice: F(3,45) = 0.60, p = 0.62; premature responses: F(3,45) = 2.46, p = 0.075; omissions: F(3,45) = 0.26, ε = 0.81, p = 0.81; feeder latency: F(3,45) = 1.28, ε = 0.43, p = 0.29]. In the morphine/naloxone combination study (morphine 3.0 mg/kg and naloxone 0.3 and 1.0 mg/kg), one animal was excluded from the analyses due to high number of omissions displayed by this animal on two different test days (68 and 72 omissions, respectively). The effects of 3.0 mg/kg morphine on impulsive action in the 5-CSRTT were fully antagonized by both doses of naloxone (Fig. 1e) [F(3,42) = 6.98, ε = 0.66, p = 0.004]. Likewise, 0.3 and 1.0 mg/kg naloxone blocked the small but significant increase in the number of omissions by 3.0 mg/kg morphine [F(3,42) = 3.44, ε = 0.77, p = 0.038]. In contrast to the morphine alone study, in the combination study, 3.0 mg/kg morphine slightly but significantly decreased the number of perseverative responses after correct choice from approxi- mately seven to five responses, which was antagonized by both doses naloxone (Fig. 1f) [F(3,42)=5.78, p = 0.002]. In addition, in the combined morphine/ naloxone experiments, morphine significantly speeded the latency to collect food reward after correct choice, and these effects of morphine were antagonized by both doses of naloxone [F(3,42)= 18.34, p < 0.001]. Attentional parameters in the 5-CSRTT were not altered by the combination of morphine and naloxone [accurate choice: F(3,42)= 2.26, p =0.095; correct response latency: F(3,42)= 0.49, p = 0.69]. Effects of morphine on response inhibition in the stop-signal task In the stop-signal task experiments, one animal was excluded from all analyses because it did not acquire stable performance. Moreover, two additional animals were excluded from all analyses because of their low go trial accuracy and high omission rate of around 50% of all go trials during training and test sessions. Morphine did not alter the estimated SSRT, the primary measure of response inhibition, and, in addition, mGoRTs were also not changed by morphine (Fig. 2 a, c) [F(3,36) = 0.76, p = 0.52 and F(3,36) = 2.54, ε = 0.63, p = 0.10, respec- tively]. Only the high dose of 3.0 mg/kg morphine did, however, significantly affect behavior during the go trials and decreased go trial accuracy from 77% under vehicle conditions to 57% [F(3,36) = 6.17, ε = 0.55, p = 0.012]. Further inspection of the data revealed that this decrease was primarily caused by two rats displaying a go trial accuracy of 17% and 21%, respectively, only at the high (3.0 mg/kg) dose morphine. In order to explore whether morphine had differentially affected response inhibition depending on baseline SSRT under vehicle condition, a correlation analysis was performed. This analysis revealed that the effect size of morphine on SSRT (averaged over the