Early on, researchers did not relate any damage of the dopamine system to AD: Dopamine transporter sites, tyrosine hydroxylase (TH), and D2 autoreceptors were not changed in the substantia nigra, and ventral tegmental area of postmortem AD subjects, as well as loss of striatal dopamine transporter sites, occurred only in relation to clinical parkinsonian symptoms [131]. Here, TH is linked to Alzheimer disease.