Background Recent reports have suggested a role for brain-derived neurotrophic factor (BDNF) in psychiatric disorders. Decreased serum BDNF levels have been reported in major depression, but the cause of this decrease has not yet been investigated. The goal of this study was to assess blood BDNF and a platelet activation index, PF4. Methods Forty-three drug-free patients (27 female, 16 male) diagnosed with major depression and 35 healthy control subjects (18 female, 17 male) were assessed for plasma, serum, and blood BDNF content. Brain-derived neurotrophic factor and PF4 were assayed with enzyme-linked immunosorbent assay methods, and severity of depression was evaluated with the Montgomery-Asberg Depression Rating Scale. Results Serum and plasma BDNF levels were decreased in depressed patients compared with control subjects. In whole blood, BDNF levels were unaltered in the depressed subjects compared with control subjects. The serum/blood BDNF ratio was lower in patients with major depression. Increased plasma but not serum PF4 levels were observed in depressed subjects compared with control subjects. Conclusions Our results suggest that an alteration of serum or plasma BDNF is not due to the change in blood BDNF but rather is probably related to mechanisms of BDNF release. Secretion of BDNF seems to be independent of platelet reactivity; other mechanisms are therefore probably involved and need to be elucidated. Recent reports have suggested a role for brain-derived neurotrophic factor (BDNF) in psychiatric disorders. Decreased serum BDNF levels have been reported in major depression, but the cause of this decrease has not yet been investigated. The goal of this study was to assess blood BDNF and a platelet activation index, PF4. Forty-three drug-free patients (27 female, 16 male) diagnosed with major depression and 35 healthy control subjects (18 female, 17 male) were assessed for plasma, serum, and blood BDNF content. Brain-derived neurotrophic factor and PF4 were assayed with enzyme-linked immunosorbent assay methods, and severity of depression was evaluated with the Montgomery-Asberg Depression Rating Scale. Serum and plasma BDNF levels were decreased in depressed patients compared with control subjects. In whole blood, BDNF levels were unaltered in the depressed subjects compared with control subjects. The serum/blood BDNF ratio was lower in patients with major depression. Increased plasma but not serum PF4 levels were observed in depressed subjects compared with control subjects. Our results suggest that an alteration of serum or plasma BDNF is not due to the change in blood BDNF but rather is probably related to mechanisms of BDNF release. Secretion of BDNF seems to be independent of platelet reactivity; other mechanisms are therefore probably involved and need to be elucidated.

Low Brain-Derived Neurotrophic Factor (BDNF) levels in serum of depressed patients probably results from lowered platelet BDNF release unrelated to platelet reactivity