Multiple studies have pointed out a role for opioid receptors and their endogenous ligands in psychostimulant - particularly cocaine- addiction (for a recent review, see (Yoo et al., 2012), and Table 3). Cocaine self-administration was dose-dependently reduced in mu KO mice (Mathon et al., 2005), and cocaine CPP was maintained (Contarino et al., 2002; Hall et al., 2004; Nguyen et al., 2012a) or decreased (Hall et al., 2004) depending on dose and experimental conditions (number of pairings, number and duration of conditioning sessions). These data indicate that mu receptors mediate, at least in part, cocaine reward. A rightward shift of the CPP dose-response curve was observed in both mu (Becker et al., 2002) and β-endorphin (Marquez et al., 2007) KO mice, suggesting decreased cocaine sensitivity in the two lines and a possible implication of mu/βend signaling in cocaine reinforcement. Place preference studies were also conducted in mu KO for amphetamine (Marquez et al., 2007) and MDMA (Robledo et al., 2004) but no phenotype could be detected.
