Differential effects of 3,4-methylenedioxymethamphetamine, methamphetamine, meta-Chlorophenylpiperazine, and scopolamine on behavioral repetition versus variation in rats

Acute administration of drugs of abuse, such as MDMA and methamphetamine, disrupts performance on many operant tasks, for example, those used to study memory. This might occur in part because drugs make behavior, in general, more repetitive or more variable, or because they produce a more global disruption to performance. The current study explored this across two experiments by employing Neuringer's 'reinforced variability' procedure. Varied behavior was reinforced at some times during the session and repetitive behavior at other times; lights signalled the behavior required. This procedure allowed an investigation of whether a particular drug made behavior more variable (affected behavior when repetition was required), more repetitive (affected behavior when variability was required), or produced a global disruption (affected both components). In Experiment 1, MDMA increased variability while methamphetamine affected both components. In Experiment 2, m-CPP affected both components while scopolamine affected both components at lower doses and increased variability at higher doses. These results indicate both that the reinforced variability procedure can be used to isolate the specific effects of drugs of abuse on the variability of behavior, and that the specific impact of a given drug needs to be considered when interpreting pharmacological disruptions to operant task performance.