“…Thus, Lewis rats acquire cocaine, morphine, methamphetamine, nicotine and heroin self-administration or conditioned place preference more rapidly and/or at higher rates/doses compared with Fischer rats (Ambrosio et al, 1995; Cadoni et al, 2015; Kosten et al, 1994; Kosten et al, 1997; Kruzich and Xi, 2006a, b; Nylander et al, 1995; Picetti et al, 2012; Picetti et al, 2010). Fischer and Lewis rats differ also in their stress response (Dhabhar et al, 1993; Ergang et al, 2015) and impulsivity score (Hamilton et al, 2014; Madden et al, 2008), two features related to vulnerability to addiction. Overall, comparisons between Fischer and Lewis rats in cocaine self-administration have been carried out primarily in relatively short access self-administration paradigm (≤ 6h), rather than extended access (Freeman et al, 2009; Miguens et al, 2015; Rivera et al, 2013).…”