Dopaminergic systems, and in particular the mesolimbic dopamine system, have long been considered to have a central role in drug reward, and by implication, addiction. Numerous pieces of evidence lead to this central tenant of addiction theory, including the recognition that all drugs of abuse, either directly or indirectly, stimulate brain dopaminergic pathways (Wise & Bozarth, 1987). Indeed, dopaminergic systems, and the neural circuitry with which they integrate, have been shown to be involved in a wide variety of reward related behavior that might be expected to be involved in addiction, including reward prediction error (Schultz, 2006), cue learning (Ito, Dalley, Howes, Robbins, & Everitt, 2000), incentive motivation (Berridge & Robinson, 1998), the induction and expression of behavioural sensitization (Cador, Bjijou, & Stinus, 1995), habit formation (Yin & Knowlton, 2006) and reinstatement of drug seeking behavior (Shaham, Shalev, Lu, De Wit, & Stewart, 2003).
