SLC6A4 and Abnormal autonomic nervous system physiology: We used [123I]FP-CIT SPECT binding to presynaptic serotonin (SERT) and dopamine (DAT) transporters as a measure of the integrity of these neurotransmitter systems, and the SCOPA-AUT (Scales for Outcomes in Parkinson’s Disease—Autonomic) questionnaire to evaluate the perceived severity of autonomic dysfunction.