Drug effects on discrimination performance at two levels of stimulus control

The effects of several doses of d-amphetamine, chlordiazepoxide (CDP), chlorpromazine (CPZ), LSD, pentobarbital, and scopolamine were examined in rats trained to respond to the brighter of two keys. On each of the 100 trials during a daily session, the rat pressed the key that was brighter (correct key) and received a food pellet, or pressed the incorrect key and terminated the trial without food, or pressed neither key for 10s, allowing the trial to terminate. Within a session, trials were mixed randomly such that on 50 trials the incorrect key was not lit (easy trials) and on 50 trials the incorrect key was dimly lit (difficult trials). Amphetamine (0.5-2.0 mg/kg) reduced percent correct responses, with a greater effect on difficult than on easy trials. CDP (4.0-16.0 mg/kg) and pentobarbital (2.0-16.0 mg/kg) reduced percent correct responses on the difficult trials at the highest doses tested. Scopolamine (0.12-1.0 mg/kg) reduced both percent correct (more so on the difficult trials) and percent of trials on which a response was made, in a dose-related fashion. CPZ (1.0-4.0 mg/kg) reduced trial responding at 2.0 and 4.0 mg/kg and reduced percent correct on the difficult trials at 4.0 mg/kg. LSD (0.08-0.32 mg/kg) did not significantly alter behavior in this study.