BDNF and depressive disorder: Even if it is still entirely unclear why BDNF levels in platelets or serum should reflect brain levels (see Discussion), these levels have been associated with a number of conditions including depression (Karege et al., 2002; Sen et al., 2008), Huntington’s disease (Ciammola et al., 2007; Zuccato et al., 2011), and Alzheimer’s disease (Laske et al., 2006).