strains, Wilhelm and Mitchell (2009) found significantly greater discounting in Fischer rats compared to Copenhagen and Noble rats, but not Lewis rats, although a recent study suggests that this failure to replicate may be a function of procedure rather than strain (Stein et al., 2012). Studies using mice have revealed similar results. In an initial investigation of four strains, significant between-strain differences were present as was covariation between discounting behavior and locomotor activity (Isles, Humby, Walters, & Wilkinson, 2004). Most recently, no differences in discounting were found between dopamine D4 receptor knock-out mice and wild-type controls (Helms, Gubner, Wilhelm, Mitchell, & Grandy, 2008), suggesting that the dopamine D4 receptor is not playing a primary role.
