DRD1 and major depressive disorder: Recently, an increasing number of articles have highlighted the ability of both DRD1 and DRD2 to form heterodimers, and a growing body of evidence has linked D1-D2 heterodimers to drug addiction, Parkinson’s disease, schizophrenia, depression, and anhedonia (Shen et al., 2015; Hasbi et al., 2020b; Hasbi et al., 2020c; Noori et al., 2020).