NTRK2 and major depressive disorder: Social defeat stress significantly decreased levels of p-TrkB/TrkB ratio in the PFC, DG, and CA3, but not in CA1, while significantly increasing p-TrkB/TrkB ratio in the NAc (Fig. 4a), consistent with previous data from the social defeat stress model of depression (Ren et al. 2016; Yang et al. 2015b; Zhang et al. 2015b).