GDNF and Parkinson disease: Intracerebroventricular GDNF delivered monthly in MPTP-treated monkeys resulted in decreased parkinsonism in a dose responsive manner, attenuated LIDs (Miyoshi et al., 1997; Zhang et al., 1997), and is correlated with increased levels of the DA metabolites 3,4-Dihyroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) (Gerhardt et al., 1999).