BDNF and major depressive disorder: The neurotrophin hypothesis, which postulates that BDNF is involved in the development of MDD, has been supported by the low BDNF levels found in postmortem brain studies and serum studies.2, 3, 4 Serum studies are further expected to determine whether low BDNF levels are a state or a trait marker.19 Our epidemiologic approaches show that the serum BDNF level is not a trait marker of MDD in old women.