Conditioned place preference induced by morphine or heroin is eliminated in homozygous MOR KO mice (Contarino, et al., 2002; Hall, et al., 2003; Matthes, et al., 1996; Sora, Elmer, et al., 2001). The rewarding effects of buprenorphine, on the other hand were shown to have a partially non-MOR dependent component (Ide, Minami, et al., 2004). Heterozygous mice on a mixed genetic background have increased morphine conditioned place preference (Sora, Elmer, et al., 2001), while no difference was observed in congenic MOR KO mice on a C57Bl6/J background (Hall, et al., 2003). The rewarding effects of buprenorphine were equally affected in heterozygous MOR KO mice as they were in homozygous MOR KO mice (Ide, Minami, et al., 2004). Morphine self-administration was eliminated in homozygous MOR KO mice (Becker, et al., 2000; Sora, Elmer, et al., 2001), but also almost completely abolished in heterozygous MOR KO mice (Sora, Elmer, et al., 2001). That heterozygous MOR KO mice are unaffected under some conditions, but under other conditions are as affected as homozygous MOR KO mice, was suggested to be the result of differential
