GDNF and Parkinson disease: A directdemonstration that these steady-state levels of GDNF are neuroprotective requiresaccurate titration experiments with viral vectors, and is technically difficult.However, it is remarkable that neuroprotective doses of compound PYM50028, which isin clinical development for the treatment of Parkinson's disease, induceincreases in striatal GDNF levels in MPTP-treated mice similar to those induced byLY379268 in our study [47].