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 receptor reserve in different MOR circuits (Sora, Elmer, et al., 2001). In any case, an important point is that heterozygous MOR KO mice, which are in the range of normal levels of variation in the expression of MOR in humans (G. R. Uhl, Sora, & Wang, 1999), exhibit differences in addiction related phenotypes.
