The effects of MOR KO on drug reward were not limited to opiates. The conditioned place preference induced by Δ9-tetrahydrocannabinol (THC) (Ghozland, et al., 2002), ethanol (Hall, Sora, & Uhl, 2001), nicotine (Berrendero, Kieffer, & Maldonado, 2002) and cocaine (Becker, et al., 2002; Hall, Goeb, Li, Sora, & Uhl, 2004) are reduced or eliminated in MOR KO mice. Some studies have failed to observe effects of MOR deletion on conditioned place preference for MDMA (Robledo, et al., 2004), cocaine (Contarino, et al., 2002) or ethanol (Becker, et al., 2002), although limited dose ranges, the sex of the subjects and differences in breeding strategies may have affected the results of those studies (see discussion in (Hall & Uhl, 2006)). Not all studies have examined heterozygous MOR KO mice, but the conditioned place preferences induced by ethanol and cocaine were as affected in heterozygous MOR KO mice as in homozygous MOR KO mice (Hall, et al., 2004; Hall, et al., 2001). Ethanol consumption has also been shown to be reduced in MOR KO mice (Becker, et al., 2002), although this reduction was
