GDNF and Parkinson disease: Recent developments in the treatment of PD focus on [417] immunotherapies to restrict the propagation of α-synuclein, neurotrophic factors, such as GDNF (glial cell-derived neurotrophic factor), regenerative therapies using cell-based and genetic approaches to replace the function of the lost dopaminergic neurons, re-establishing the balance between neurotransmitters by targeting non-dopaminergic neurotransmission, interfering with the neuroinflammatory response, or improving the stereotactical surgical treatments.