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 the data demonstrate that mu receptors indeed represent the primary in vivo molecular target for both most clinically useful (morphine) and most largely abused (heroin) opiates.
