Therefore, it appears that on one side, there are antioxidant systems and growth factors promoting plasticity, such as Nrf2–HO-1, BDNF, Akt, TrkB, and CREB, which are protective from depression and, on the other side, GSK-3β and NF-κB, which promote depressive behaviour (Figure 2). Here, NTRK2 is linked to major depressive disorder.