BDNF and major depressive disorder: BDNF is also likely to be involved in a number of major neurological and psychiatric conditions including Major Depressive Disorder (MDD),3 Alzheimer’s Disease,4 Parkinson’s Disease,5 and epilepsy.6 What is less clear is the degree to which measurements of BDNF levels in accessible body fluids may be used to correlate these levels with brain function and dysfunction.