BDNF and bipolar disorder: Additionally, other studies found lower serum BDNF protein levels in patients with attention deficit hyperactivity disorder (ADHD), schizophrenia, bipolar disorder, major depressive disorder (MDD), and autism spectrum disorder (ASD) (Akay et al., 2018; Bakirhan et al., 2017; Francis et al., 2018; Jiang et al., 2017; S. et al., 2017).