Abstract
Pre-clinical studies of individual differences in addiction vulnerability have been increasing over recent years, but the amphetamine derivative 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) has received relatively little attention in this regard. Previously, we reported large individual differences both in rats' initial behavioral response to experimenter-administered MDMA and their degree of behavioral sensitization to repeated administration. To determine whether these differences could predict subsequent patterns of MDMA-taking or -seeking behaviors we used the self-administration-extinction-reinstatement model to examine addiction-like behavior (i.e., escalation of MDMA self-administration and cue-induced reinstatement of MDMA seeking) in rats a priori characterized for either locomotor sensitization or tolerance to MDMA. Rats that developed tolerance to the locomotor-activating effects of MDMA had a significantly larger locomotor response to the first MDMA injection relative to rats that developed sensitization. Importantly, rats that developed tolerance subsequently displayed an escalation of MDMA self-administration over days, as well as clear cue-induced reinstatement of MDMA seeking following extinction. Conversely, rats that developed locomotor sensitization to MDMA subsequently maintained relatively stable levels of MDMA self-administration over days and showed no cue-induced reinstatement of MDMA seeking. These results show that differences in the level of psychomotor activation following acute and repeated MDMA administration can reliably predict two important addiction-like behaviors in rats, which may have implications in the prediction of compulsive MDMA use in humans.