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Abstract: Repeated-acquisition procedures that include performance controls for effects not specific to acquisition permit the assessment of drug effects on learning on a within-subject, within-session basis. Despite the advantages of this methodology, few studies have examined effects of psychomotor stimulants on repeated acquisition in rodents. The purpose of the present study was to investigate the effects of methylenedioxymethamphetamine (MDMA, 0.3–10mg/kg), methamphetamine (MA, 0.1–3mg/kg) and methylphenidate (MPD,1–17mg/kg) using repeated-acquisition procedures with performance controls in rats using a touch-screen apparatus. Rats were presented a 2×3 array of stimuli using a computer touch-screen and nose-pokes to target locations within the array were reinforced. In the acquisition component, the correct location changed across sessions, whereas during the performance component, the correct location was constant across sessions. All three drugs reduced accuracy of responding to target locations in a dose-dependent fashion. None of the compounds enhanced learning at any dose. MPD and MA produced significant disruptions of acquisition accuracy only at doses that also disrupted performance, but the 3mg/kg dose of MDMA impaired acquisition of target responding without affecting performance. The selective impairment of acquisition found in the present study adds to the evidence of learning and memory disruption produced by acute MDMA administration and raise questions about the mechanisms for these actions. [Copyright &y& Elsevier]

Some classes of drugs can selectively affect learning (i.e., acquisition of behavior) at doses that do not affect performance (i.e., previously learned behavior). Some drugs (e.g., benzodiazepines) show selective effects on acquisition across a wide variety of tasks. Other drugs [e.g., N-methyl- d-aspartate (NMDA) antagonists and opiate agonists], however, show selective effects under some tasks, but not others. The purpose of this study was to examine the effects of the NMDA-antagonist dizocilpine (0.01–0.3 mg/kg), the opiate-agonist morphine (1.0–17.0 mg/kg), and the benzodiazepine chlordiazepoxide (3.0–30.0 mg/kg) in rats under a novel repeated-acquisition and performance task. Nose pokes to a correct location within a 2×3 stimulus array on a computer touch screen were reinforced with food. In the acquisition component, the correct location changed across sessions but remained constant within sessions; in the performance component, the correct location was constant both across and within sessions. Both chlordiazepoxide and dizocilpine selectively impaired accuracy in the acquisition component at doses that did not affect accuracy in the performance component or overall response speed. Morphine, however, did not affect acquisition without affecting performance or response speed. These results with rats resembled those previously obtained for response-sequence learning in primates, rather than those previously reported for spatial learning in rats. Therefore, previous discrepancies in results for NMDA antagonists and opiate agonists across tasks probably were not a function of the species studied, but, rather, they more likely were a function of unique variables controlling acquisition within each task. [ABSTRACT FROM AUTHOR]