2009; Gelernter et al., 2006; Grucza et al., 2008; Ittiwut et al., 2011; Kalayasiri et al., 2007; Levran et al., 2015; Luo et al., 2004; Malison et al., 2006; Sherva et al., 2010; Zhang et al., 2009; Zuo et al., 2009). Genes whose products function in the monoamine serotonergic signalling pathway have also been implicated; i.e. the gene encoding the 5-HT2C receptor (5-HT2C) was associated with two subtypes of cocaine use (Bi, et al., 2014). However, a GWAS of cocaine dependence reported no genome-wide significant associations for dopamine pathway genes, and as such additional replication studies are necessary (Gelernter et al., 2014) (Table 5). Study of additional pathways may also be beneficial since associations have been detected for genes (CLOCK, PER1 and PER2) (Malison, et al., 2006; McClung, 2007) whose products function in regulating circadian rhythm which may also regulate dompaminergic activity.
