BDNF and depressive symptom measurement: The sex differences revealed in the present study, emanating from the higher levels of D1-D2 heteromer, coupled to lower density of D1 receptor in NAc of female rat, and the resulting sex differences in basal and D1-D2 heteromer-induced signaling through BDNF/TrkB and GSK3/β-catenin pathways may be at the origin, or at least represent an essential role, in the usually reported sex difference in female susceptibility to stress, anxiety, and depression.