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 sex-dependent in another study (Hall, et al., 2001). It is interesting to note that in the Becker et al., (2002) study consumption was initially similar, but MOR KO mice did not increase their consumption over the period of study as did the WT mice. Operant self-administration of ethanol is largely eliminated in homozygous MOR KO mice (Roberts, et al., 2000) and cocaine self-administration is substantially reduced (Mathon, Lesscher, et al., 2005). This latter effect was associated with reduced midbrain dopamine cell firing (Mathon, Ramakers, Pintar, & Marinelli, 2005), perhaps due to increased GABAergic inhibition (Mathon, Lesscher, et al., 2005).
