The apelin-APJ system is involved in the disorders of HPA axis caused by depression, therefore, the purpose of the this study is to characterize the expression of apelin and APJ in hypothalamus of mice with chronic unpredictable mild stress (CUMS), and observe the effect of Xiaoyaosan intervention on the apelin-APJ system to determine the anti-depressant mechanism of action of Xiaoyaosan. The gene discussed is APLN; the disease is major depressive disorder.