DRD2 and Parkinson disease: Training improves dopamine neurotransmission though increased dopamine release, as well as an increase in the density of postsynaptic dopamine D2 receptors in the basal ganglia (Fisher et al., 2008; Beeler et al., 2010), which may partially compensate for the loss of dopamine neurons in PD (Lees et al., 2009; Mazzoni et al., 2012; Ferrazzoli et al., 2018).