GDNF and Parkinson disease: Since dopaminergic neurons express both GFRα and Ret in the rodent (Jing et al., 1997; Walker et al., 1998) and human (Walker et al., 1998; Quartu et al., 2007) brain, it was soon hoped that GDNF could have a therapeutic potential for Parkinson's disease (PD) (Tomac et al., 1995).