BDNF and Parkinson disease: This is evident in other neurodegenerative conditions, such as Parkinson’s disease (PD), where BDNF expression is downregulated in the pars compacta of substantia niagra (SN) in the PD patients, which resulting in the deprivation of dopaminergic neurons from trophic support and the remaining dopaminergic neurons in SN produce progressively lower BDNF levels [55, 56].