properties of drugs of abuse, whereas D2-receptors play a role in drug seeking mechanisms (Self et al., 1996; Self, 2010). Studies with D1 receptor and D2-receptor knockout mice provide some insight into the role of these receptors in the two MSNs. D1 knockout mice show a blunted induction of immediate early genes (IEGs) c-Fos and Zif268 in response to cocaine, a diminished response to psychostimulant-induced locomotor activity but with no alterations in cocaine-conditioned place preference (CPP) – an indirect measure of drug reward, and diminished cocaine self-administration and ethanol consumption (Miner et al., 1995; Drago et al., 1996; Crawford et al., 1997; El-Ghundi et al., 1998; Caine et al., 2007). D2 knockout mice display diminished rewarding effects to opiates and cocaine as well as decreased ethanol consumption but no reduction in cocaine taking (Maldonado et al., 1997; Cunningham et al., 2000; Risinger et al., 2000; Caine et al., 2002; Chausmer et al., 2002; Elmer et al., 2002; Welter et al., 2007). Such data support important roles for D1 and D2-receptors in the two MSNs in multiple aspects of drug abuse, however, the knockouts lack striatal specificity and occur early in development, thus one cannot rule out other brain regions and
