In clinics, BDNF depletion in brain (up to 70% in SN and 80% in DOPAn) has been correlated with DOPAn loss [56], and BDNF decrease in the serum of PD patients has been reported correlating with the timing from diagnosis, pathology severity [58], and cognitive deficit [86,87]. Here, BDNF is linked to Cognitive impairment.