DRD2 and Parkinson disease: Given that we and others have shown that OB dopamine acts as a presynaptic inhibitor of glutamate release from the OSNs (Nickell et al., 1994; Hsia et al., 1999; Berkowicz and Trombley, 2000; Ennis et al., 2001; Liu et al., 2013; Vaaga et al., 2017), and that increasing D2R activation can lead to a decrease in odor detection (Doty and Risser, 1989; Escanilla et al., 2009), it is likely that an increase in dopamine and D2R activity can lead to increased hyposmia and anosmia during the prodromal stages of PD (Huisman et al., 2004).