SLC6A3 and attention deficit-hyperactivity disorder: People with ADHD have lower [18F]DOPA intake, decreased DAT binding and decreased D2/3 receptor binding, implying increased extracellular DA tone (153, 158, 162–165).Several studies have found no differences in endogenous DA between ADHD and healthy controls within striatal and midbrain regions, failing to replicate early findings (158–161).