Cocaine enhances the conditioned rewarding effects of MDMA in adolescent mice

• Cocaine enhances the conditioned rewarding effects of MDMA in adolescent mice.
• Co-administration of sub-threshold doses of cocaine and MDMA induces CPP.
• Pre-treatment with cocaine induces a sensitisation of the rewarding effects of MDMA.
• Cocaine induces few changes in the levels of biogenic amines of MDMA-treated mice.

Although the consumption of cocaine is frequent in young users of MDMA (3,4-methylenedioxymethamphetamine), the influence of exposure to cocaine on the rewarding effects of MDMA in adolescents has not been studied. The purpose of the present work was to evaluate the effect of co-administration of cocaine (1 and 10 mg/kg) and a sub-threshold dose of MDMA (1.25 mg/kg) on the acquisition of conditioned place preference (CPP) (experiment 1). In addition, the effect of pre-treatment with cocaine on MDMA-induced CPP was evaluated (experiment 2). Levels of monoamines in striatum, hippocampus and cortex were measured in both experiments. Our hypotheses were that cocaine co-administration or pre-treatment would increase the rewarding effects of MDMA, and that these effects would be related with changes in brain monoamine levels. Our results showed that cocaine potentiated the rewarding effects of MDMA, since a sub-threshold dose of MDMA, which did not induce CPP by itself, induced a significant CPP in adolescent mice when administered along with cocaine during conditioning (experiment 1). Moreover, pre-treatment with cocaine several days before conditioning also increased the rewarding effects of MDMA (experiment 2). No significant changes in the levels of biogenic amines, which correlated with these behavioural effects, were observed. Our results confirm the involvement of the dopaminergic system in MDMA-induced CPP in adolescent mice and suggest that combined consumption with or pre-exposure to cocaine increases the conditioned rewarding effects of MDMA, which may enhance the capacity of MDMA to induce dependence.