DRD2 and Parkinson disease: While some studies have reported that D2R agonist-mediated reinforcement is similar in both sham and PD-induced rats (Engeln et al., 2013; Zengin-Toktas et al., 2013), other studies have observed that lower doses of DAAs are sufficient to induce CPP in PD-induced rats compared with sham controls (Riddle et al., 2012; Loiodice et al., 2017).