Comparison of Methamphetamine and MDMA Extended Access Self-administration: Acquisition, Maintenance, and Response Patterns

Rationale. 3,4-methylenedioxymethamphetamine (MDMA) and methamphetamine are two amphetamine derivatives with contrasting pharmacological profiles. Therefore, self- administration profiles might be expected to reflect these differences. Objectives. This study compared the latency and proportion to acquire self-administration, maintenance of self-administration, and within-session response patterns. Methods. Rats were given extended access (8-hour daily sessions) to either methamphetamine, MDMA or vehicle self-administration over a period of 10 consecutive days. A criterion based on the performance of the vehicle control group was used to determine acquisition of reliable MDMA and methamphetamine self-administration. In conjunction, for MDMA self-administration the infusion dose was halved for each rat that achieved a total of 85mg/kg for the remaining sessions. Temporal patterns of responding were assessed using hourly data of the first day of self-administration, the day following acquisition, and the final day of self-administration. Results. A greater proportion of rats in the methamphetamine group acquired self- administration and self-administration was acquired with a shorter latency compared to the MDMA group. Responding maintained by methamphetamine on day one was high. By the third day a pattern developed that was maintained throughout testing. The greatest proportion of responding occurring within the first hour of each daily test session. A progressive escalation of intake was also observed within the methamphetamine group. Responding maintained by MDMA was low on the first day, but by day 5 responding had increased with most of the responding within the session occurring during the first three hours. On day 10 the greatest amount of responding occurred during the first hour. No escalation of intake as a function of test day was observed for MDMA self-administration.