Effects of d-amphetamine, morphine, naloxone, and drug combinations on visual discrimination in rats.

The effects of d-amphetamine, morphine, and naloxone on visual discrimination were investigated using a two-choice discrete-trial procedure in which rats were trained to discriminate the position of a lightflash. Morphine (0.3-5.6 mg/kg) but not amphetamine (0.1-1.0 mg/kg) caused a significant dose-dependent disruption in discriminative performance. Both amphetamine and morphine increased response latencies. Naloxone (1.0 mg/kg) prevented the disruption of any aspect of performance by up to 100 mg/kg morphine. Performance after naloxone/amphetamine co-administration was not significantly different from that observed after amphetamine alone. Naloxone alone (0.3-10 mg/kg) had no effect on discrimination, spatial bias or response latencies. These results suggest that morphine and amphetamine affect different components of discrimination performance.