Mechanism of Action Underlying Ketamine's Antidepressant Effects: The AMPA Throughput Theory in Patients With Treatment-Resistant Major Depression

Background: Most drugs that treat mood disorders take a long time to work. Ketamine works within hours. A dose can last for a week or more. Certain receptors in the brain might help ketamine work. A drug that blocks these receptors might affect how it works. Objective: To see if the antidepressant response of ketamine is linked to AMPA receptors. Eligibility: Adults ages 18-70 with major depression disorder without psychotic features Design: Participants will be screened under protocol 01-M-0254. They will have blood tests and a physical exam. Participants will stay at the NIH Clinical Center for 5 weeks. Phase 1 lasts 4 weeks. For 2 weeks, participants will taper off their psychiatric medicine. Then they will have the following tests: * Blood draws * Psychological tests * MRI: Participants will lie in a machine that takes pictures of their brain. * MEG: Participants will lie down and do tasks. A cone lowered on their head will record brain activity. * Optional sleep tests: Electrodes on the scalp and body and belts around the body will monitor participants while they sleep. * Optional TMS: Participants will do tasks while a wire coil is held on their scalp. An electrical current will pass through the coil that affects brain activity. For phase 2, on day 0 participants will take the study drug or a placebo orally. While having a MEG, they will get ketamine infused into a vein in one arm while blood is drawn from a vein in the other arm. On day 1, participants will again take the study drug or a placebo orally. On days 3-7, they will repeat many of the phase 1 tests. Days 8 and 9 are optional and include an open label ketamine treatment and many of the phase 1 tests.