For patients who have two [C-11]rolipram PET scans, one before and another after SSRI treatment, previous failures of or intolerance to SSRI may not allow for treatment in the current protocol. In clinical practice, medication can be switched between sertraline and citalopram/escitalopram because sertraline and citalopram/escitalopram have somewhat different therapeutic effects and adverse reactions. Along these lines, we will consider citalopram and its enantiomer escitalopram as being equivalent to each other. Patients will therefore be excluded from the study with two [C-11]rolipram PET scans if they previously failed to respond to adequate treatment trials of all medications available for use in the study, or if they have a history of being unable to tolerate all of the study medications. Specifically, patients will be excluded from the study with two [C-11]rolipram PET scans if they: