SLC6A3 and Parkinson disease: Lower synaptic density in the substantia nigra was confirmed in early PD patients [145] and may be more widespread even in drug-naive PD patients [146], while synaptic density correlated with DAT availability [145] and 11C-SA-4503 PET signal (binding to the sigma 1 receptor (σ1R), a chaperone protein present in the membrane of the mitochondrion-associated endoplasmic reticulum) in the caudate nucleus [146].