BDNF and major depressive disorder: 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).