GDNF and Parkinson disease: In contrast, in PD and other dementias with parkinsonian symptoms, a number of dopaminergic disorders are observed, such as increased susceptibility of DA neurons to toxic insults, substantial loss of DA neurons in SN and VTA, significant reduction of dopaminergic projection in the striatum, a severe decline in striatal dopamine transporter level, polymorphism of dopamine β-hydroxylase, accumulation of neuromelanin and β-synuclein in DA neurons of SN, and loss of trophic support specific for dopaminergic neurons by GDNF, BDNF, and CDNF.