Morphine reward and withdrawal data are shown for the six KO lines in Table 1. Locomotor effects of morphine are presented in Table 6 together with stimulant effects of other drugs of abuse. Genetic studies have definitely established that the mu opioid receptor is required for therapeutic effects as well as unwanted effects of morphine (see (Contet et al., 2004). Hence, morphine (Matthes et al., 1996; Nguyen et al., 2012a; Nguyen et al., 2012b; Sora et al., 2001) and heroin (Contarino 2002) CPP were abolished in mu KO mice at all the tested doses. Intravenous as well as intra-VTA infusions of the drug observed in wild type animals were also abolished in mutants (Sora 2001); David 2008). In another study, mu KO mice self-administered morphine at levels lower than control mice self-administering saline, perhaps unmasking a kappa/dynorphin-mediated aversive state in these mutants (Becker et al., 2000). Locomotor responses to morphine (Tian 1997; Sora 2001; Chefer 2003; Yoo 2003 and 2006, Becker 2000) and heroin administration (Contarino et al., 2002) were eliminated in mu KO animals (see Table 6). Together all
