GDNF and Parkinson disease: Recent work has shown that the lack of efficacy of GDNF and NTN may result from alpha-synuclein-induced down-regulation of their common co-receptor, Ret, which is crucial for GDNF and NTN signalling [22] as GDNF did not confer neuroprotection in the α-synuclein rat model of PD [23].