IDO1 and major depressive disorder: The present study established an oestrogen withdrawal‐induced depressive animal model by conducting ovariectomy (OVX) on mice to investigate: (1) whether the occurrence of oestrogen withdrawal‐induced depression is related to the level of IDO1 in the hippocampus; (2) the effects of E2 on the OVX‐induced changes in mice and the role of ER thereof; (3) whether the possible function of E2‐ER related to the modulation of depression is difference between females and males.