<qa>  What is the main reason for the decrease of M2 receptor V?This application proposes a pilot PET study of M2 receptor distribution volume in currently depressed subjects with major depressive disorder n30, currently depressed subjects with bipolar disorder n30, and psychiatrically healthy controls n30. The proposed pilot study will test the central hypothesis that M2 receptor V is decreased in regions where they are primarily located presynaptically in depressed subjects relative to healthy controls. The proposed study will advance knowledge regarding the pathophysiology of depression. 