Perhaps one of the most unsurprising series of studies using transgenic mice was the demonstration that the effects of most opiates are eliminated in MOR KO mice. It must be noted that these mice were exceedingly useful in examining the specificity of opiate actions at MOR and in identifying more complex sorts of interactions between opioid receptor subtypes. As for DAT and VMAT2, several groups constructed MOR KO mice at about the same time (Loh, et al., 1998; Matthes, et al., 1996; Sora, et al., 1997; Tian, et al., 1997). The first of these strains demonstrated that MOR KO mice did not exhibit morphine induced conditioned place preference, analgesia or tolerance (Matthes, et al., 1996). However, two important differences from studies of psychostimulants were observed in these strains of MOR KO mice: heterozygous KO mice showed far more effects than many other KO strains, and deletion of MOR affected responses to a much wider range of addictive drugs than most other KO strains that have been studied. This last idea was summarized in a review previously (Hall & Uhl, 2006) so those findings will be addressed only briefly here, concentrating on studies most relevant to addiction.
