DRD1 and Parkinson disease: This notion corroborates the evidence that there is a marked increase in the responsiveness of the striatonigral MSNs to D1R activation in PD, as determined by the fos induction experiments (Engber et al., 1989; Asin et al., 1995; Kashihara et al., 2000; Xu et al., 2003; Morigaki et al., 2017).