SLC6A3 and Parkinson disease: These studies also can rule out the possibilities that these effects occur only (a) secondary to dopamine receptor sensitization, through chronic exposure to dopaminergic therapy, or (b) due to the fact that PD-associated as well as aging-related reductions in DAT levels, which clears and regulates dopamine at the synapse, predisposing to dopamine overdose (Harrington et al., 1996; Voon et al., 2009; Kordower et al., 2013; Kalia and Lang, 2015).