GDNF and Parkinson disease: In animals, GDNF is required for the maintenance of the nigrostriatal pathway (Pascual et al., 2008) and has been extensively used to ameliorate Parkinson’s disease models (for reviews see Pascual et al., 2011; d’Anglemont de Tassigny et al., 2015; Ibáñez and Andressoo, 2016).