Reduced serum BDNF levels, together with altered expression of BDNF and its high-affinity receptor tropomyosin receptor kinase B (TrkB) in the hippocampus and PFC, have been found in depression state and could be reversed by effective treatment with antidepressants (Gawali et al., 2016; Song et al., 2019; Seo et al., 2019). Here, NTRK2 is linked to major depressive disorder.