SLC6A4 and Parkinson disease: Serotoninergic neurons can uptake dopamine through SERT, a phenomenon that is often linked to motor complications associated with PD treatment.28 While more work is needed to explore the serotoninergic and noradrenergic systems, retromer recycling of DAT may be particularly important in actively releasing dopaminergic terminals29 and in VPS35 parkinsonism.