Advances in neurobiology and genetic epidemiology highlight the role of genetic regulation of addiction etiology and provide an opportunity to develop directed behavioral and biologically based treatments. Overall, genes involved in the regulation of the dopaminergic system have been found to be important for SUD. It is known that SUD exerts broad effects that extend to several neurobiological systems including glutamate. Additionally, unique genetic influences involved in the maintenance of addiction for specific substances also appear to be important in addition to those involved in SUD overall. Moreover, examining the role of peripherial systems in SUD; e.g., brain-gut interactions (Kiraly et al., 2016), is an exciting area requiring additional study. For example, the ghrelin signaling system is expected to play a role in SUD because ghrelin activates the cholinergic-dopaminergic reward syste in rodents (Jerlhag et al., 2007). Ghrelin is a circulating orexigenic stomach-derived hormone that regulates energy homeostatsis mainly via hypothalamic growth hormone secretagogue receptor (Cheung & Mao, 2012; Horvath, Diano, Sotonyi, Heiman, & Tschöp, 2001; Kojima et al., 1999; Kojima & Kangawa, 2005; Tschöp, Smiley, & Heiman, 2000). There
