BDNF and depressive disorder: Postmortem studies have demonstrated that patients with depression and related mental illnesses have lower BDNF and/or TRKB mRNA levels (75–77), reduced cortical neuron size (75, 78, 79), lower synaptic protein levels (80), decreased mTOR signaling (81), and fewer dendritic spines/synapses (82, 83) in the PFC.