NTRK2 and major depressive disorder: In depressed patients or in the depression model induced by CUMS, the expression of brain-derived neurotrophic factor (BDNF) and its high affinity receptor TrkB (related to neuronal survival) are decreased, while its low affinity receptor P75 (related to neuronal apoptosis) is increased in the limbic system of the brain, which could contribute to neuronal apoptosis [41].