The main facts supporting the neurotrophic hypothesis of depression are as follows: a decrease in the brain-derived neurotrophic factor (BDNF) level in the brain and blood of depressed patients, an increase in the BDNF level after effective antidepressant therapy, and a decrease in the BDNF level in the brains of experimental animals exhibiting depression-like behavior [212,213,214]. Here, BDNF is linked to depressive symptom measurement.