TH and Parkinson disease: Since neuronal loss occurs concomitantly with the loss of components of the dopaminergic pathway in PD, we examined levels of 4 proteins involved in this pathway, tyrosine hydroxylase (TH), dopa decarboxylase (DDC), solute carrier family 18 member A2 (SLC18A2) and solute carrier family 6 member 3 (SLC6A3) (Fig. 3a).