NTRK2 and major depressive disorder: In patients with schizophrenia, depression and bipolar disorder, reduced expression of BDNF and/or TrkB has been found in the hippocampus and multiple cortical areas (Autry and Monteggia, 2012; Castrén, 2014) and there is also evidence for reduced BDNF levels in the peripheral blood in these disorders (Fernandes et al., 2014).