GDNF and Parkinson disease: Monteiro et al. (2015) suggested two hypotheses to explain the underlying mechanism: (a) PE reduces chronic oxidative stress along with stimulating mitochondria biogenesis and upregulation of autophagy in PD; and (b) exercise stimulates the synthesis of neuro-transmitters like dopamine and trophic factors like Glial-derived neurotrophic factor (GDNF), insulin-like growth factor-1 (IGF-1), brain derived neurotrophic factor (BDNF), and fibroblast growth factor 2 (FGF-2) [30].