demonstrated that DAT KO mice could develop both a conditioned place preference for cocaine and self-administer cocaine, at least under some conditions, using strains developed at the the Molecular Neurobiology Branch (MNB) at the National Institute on Drug Abuse, (USA) and at Duke University, respectively (Rocha, et al., 1998; Sora, et al., 1998). It must be noted however, that although DAT KO mice did develop a significant conditioned place preference for cocaine at the high dose, there was a slight diminution in sensitivity to cocaine in that there was not a significant place preference at the low dose as there was in wildtype (WT) mice. In any case, the result was so surprising that the evidence that dopamine might partially mediate cocaine reward was largely ignored because it had been demonstrated that it was not necessary for drug reward as had been presumed. This issue was revisited recently and it was shown that although DAT KO mice do self-administer cocaine, when subsequently challenged under circumstances that demand more effort to receive the cocaine, such as a progressive ratio, they respond far less (Thomsen, Hall, Uhl, & Caine, 2009).
