Previous studies have shown that when cortisol levels become extremely high and the brain is unable to control the excessive production of cortisol, a decrease in the concentration of brain-derived neurotrophic factor (BDNF) is detected, which affects the development of depression through tyrosine kinase receptor (TrkB) and p75 neurotrophic factor receptor (p75NTR) (30, 31). Here, BDNF is linked to depressive disorder.