BDNF and major depressive disorder: However, meta-analysis of numerous studies shows that BDNF is generally decreased in the blood of patients with MDD,61, 62, 63 in both manic and depressive episodes of BPD64 and in schizophrenia.65 Thus, with the general decrease in peripheral levels across diagnostic groups in contrast to the specificity of brain deficits to particular areas, lamina and diagnosis, it is unclear as to how informative peripheral levels of BDNF will be.