Dopamine (DA) is an important neuromodulator of incentive salience, encoding both rewarding and aversive responses in vivo. Dysfunction in DA signaling has been implicated in a variety of psychiatric disorders including drug addiction, anxiety, and schizophrenia (Schultz, 2007). Dopaminergic neuronal cell bodies are concentrated in two main nuclei in the midbrain, the ventral tegmental area (VTA) and the substantia nigra (SN). The actions of addictive drugs converge on VTA DA neurons, where they increase DA neuron activity and the level of DA released in projection areas (Lüscher and Malenka, 2011). Exposure to addictive drugs, even acutely, produces persistent adaptations in excitatory and inhibitory pathways within the mesolimbic dopamine circuit, establishing the building blocks underlying addiction (Arora et al., 2011; Liu et al., 2005; Mameli et al., 2011; Padgett et al., 2012; Saal et al., 2003; Ungless et al., 2001).
