GDNF and Parkinson disease: GDNF has been shown to produce significant improvements in motor activity in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated rhesus monkeys, a model of PD [89,90], and improvements in motor impairment and reductions in l-dopa-induced dyskinesia in marmosets [91].