Effects of MPEP on expression of food-, MDMA- or amphetamine-conditioned place preference in rats

Recent studies have revealed the effectiveness of 2-methyl-6-(phenylethynyl)pyridine (MPEP), a highly selective antagonist of metabotropic glutamate receptors subtype 5 (mGluR5), in conditioned drug reward. In a previous study we showed that MPEP blocks expression of context-conditioned morphine- but not cocaine reward in the rat. The present study now examines the effectiveness of MPEP in the expression of context-conditioned food, MDMA ('ecstasy') or amphetamine reward. Therefore, three groups of rats were conditioned either to food, MDMA or amphetamine in the conditioned place preference (CPP) paradigm. After conditioning, CPP expression and locomotion were determined simultaneously in the presence and absence of the respective reward (i.e. food or drug), or after application of 50 mg/kg MPEP (the dose that was most effective in reducing morphine CPP expression in our previous study). As a result, MPEP reduced locomotion in all groups. Furthermore, only expression of amphetamine CPP was inhibited by MPEP, while expression of food and MDMA CPP was not affected, suggesting that the MPEP-induced inhibition of amphetamine CPP expression was not causally linked to the reduction of locomotion. Overall, we conclude that MPEP reduces expression of context-conditioned amphetamine but not MDMA or food reward.