GDNF Delivery Methods Investigated in Animal Models of Parkinson's Disease (PD). Several methods of GDNF delivery have proven to be effective in preventing the loss of dopaminergic neurons in neurotoxin-induced animal models of PD. Ad-GDNF: Adenoviral-expressed GDNF; Lenti-GDNF: Lentiviral-expressed GDNF; hNPCs: Human Neural Progenitor Cells; Cop-1: Copolymer-1 immune cell transfer; HACs: Human Amniotic Cells.