TH and Parkinson disease: Presently there is a body of evidence revealing that BDNF and GDNF has strong neuroprotective effects on nigrostriatal dopaminergic neurons of animal PD models with 6-OHDA or MPTP, led to a substantial increase in the number of TH-positive neurons in SNpc as well as ameliorated behavioral disorders, stimulated TH positive fiber sprouting and elevated DA content (Biju et al., 2010; Ren et al., 2013).