Inclusion Criteria:

- Meet DSM-IV TR criteria for MDD, Recurrent

- Montgomery-Asberg Depression Rating Scale (MADRS) score > 18

Exclusion Criteria:

- Any coexisting psychiatric illness other than generalized anxiety disorder, panic
disorder, or social/specific phobias

- Any history of substance dependence

- Substance abuse within the past 6 months

- Significant risk of suicide, as defined by score >4 on item 10 of the MADRS or in the
clinical judgment of the study physician

- Any significant medical or neurological condition which is likely to impact the
central nervous system and/or affect the results of MRS or PET imaging

- For the subset of unmedicated MDD patients, any psychotropic medications within 4
weeks prior to scanning. For the subgroup of medicated patients, they may be taking
a stable dose (i.e., same dose for at least 4 weeks at the time of scanning) of
standard antidepressant medications, but may not be taking any other psychotropic
medication.

- Inability to give informed consent

- Contraindication to MRI (e.g., pacemaker, ferromagnetic implants in the body)