BDNF and Parkinson disease: It has been well-established that decreased levels of BDNF in the brain are involved in the pathogenesis of neurodegenerative diseases including AD, PD, and HD because BDNF and its related molecules are essential for various neural events including cell proliferation, survival, synaptic function, and neurogenesis by activating the intracellular signaling such as MAPK signaling pathway [1].