40 major depressed (MDD) and 40 healthy subjects will undergo 2 PET/MR scans on a 3T SIEMENS mMR Biograph scanner: 1. challenge with citalopram 8mg 2. placebo (saline). After structural imaging, functional MRI will be continuously acquired. [11C]DASB will be applied using a bolus + constant infusion paradigm to probe 5-HTT binding potentials and monitor 5-HTT occupancy with drug challenge, applied after 70min, in a single session. Scanning will be terminated 80min after challenge. MDD patients will receive subsequent escitalopram treatment with repeated evaluation of response for 3 months.