As an example, the motivation to use drugs has been linked to the mesolimbic and mesocortical networks in the brain (the neural substrates that putatively underlie the attribution of incentive motivation to drugs of abuse) and is thought to be an important factor in the etiology of addiction (Berridge & Robinson, 1998; Kalivas & Volkow, 2005; Robinson & Berridge, 1993; Wise, 1988). The most widely used animal model for investigating the neurobiology of drug-seeking behavior is the drug reinstatement model (Epstein, Preston, Stewart, & Shaham, 2006). Here, the animal is exposed chronically to a drug and a drug-related cue, withdrawn from the drug, and then subsequently re-exposed to the cue. Although some concerns regarding the validity of the model have been raised (see Epstein et al., 2006), this approach has been extremely useful for identifying the neurobiological mechanisms that underlie drug-seeking behavior. To examine a gene by environment interaction, animals that have been genetically modified by “knocking in” human genes with different versions of a specific polymorphism or more generally “knocking out” the gene can be exposed to specific environmental
