GDNF and Parkinson disease: Furthermore, intracranial infusions of GDNF [20], [21] and BDNF [22] have been shown to provide protection and restoration of DA neurons in rodent models of PD, and in 1-methyl-4-phenyl-1,2,3,6-tetrahydro-pyridine (MPTP)- or 6-hydroxydopamine (6-OHDA)-lesioned and intact aged primates [15], [17], [23]–[25].