2017; Thanos et al., 2001; Vanyukov et al., 2007). SUD is generally characterized as a signalling imbalance through striatal inhibitory D2-like dopamine receptors (DRD2) although this is not necessarily consistent across all drugs. With the execption of cannabinoids, human imaging studies consistently report that affected individuals have lower D2-like dopamine receptor occupancy by a PET imaging ligand compared to controls (Volkow, Fowler, & Wang, 2002). Receptor occupancy is a proxy for the capacity of a receptor to transduce signal that can be influenced by receptor levels as well as intracellular and extracellular factors. Individuals with low D2-like dopamine receptor occupancy are also more likely to describe psychostimulant drugs as pleasurable compared to those with high D2-like receptor occupancy (Volkow et al., 1999; Volkow et al., 2002). Moreover, decreased D2-like dopamine receptor expression is associated with increased disposition to develop alcohol use disorder (Samochowiec et al., 2000). Further, rat alcohol self-administration was decreased by overexpressing D2 dopamine receptors in the nucleus accumbens (Thanos, et al., 2001), rats that are more prone to resume heroin-seeking behavior in response to stress express less striatal D2 dopamine receptors (Zhou, Leri, Cummins, & Kreek, 2015), and D3 dopamine receptor knockout mice exhibit increased cocaine self-administration
