DRD2 and Parkinson disease: In neurodegenerative Parkinson's Disease (PD), these patients classically display upregulated expression of D1R and D2R with hypersensitive response to dopamine (Seeman and Niznik, 1990; Kaasinen et al., 2000; Hurley et al., 2001; Gerfen et al., 2002; Zhu et al., 2002; Chu et al., 2004; Hisahara and Shimohama, 2011).