BDNF and Parkinson disease: The importance of the TrkB receptor is also proven by the fact that its signaling pathways (triggered by brain-derived neurotrophic factor (BDNF) or neurotrophin 4/5 (NT-4/5)~13 kDa large proteins) are impaired in many neurodegenerative diseases (like Alzheimer’s disease [6,7,8,9], Parkinson’s disease [10,11], and Huntington’s disease [12]).