Neuropharmacologic Imaging and Biomarker Assessments of Response to Acute and Repeated-Dosed Ketamine Infusions in Major Depressive Disorder

Background: Most medications that treat depression take weeks or months to work. Researchers want to develop fast-acting treatments. One dose of ketamine has a rapid antidepressant effect. For most people, this lasts a week or less. Repeated doses of ketamine may help maintain this effect. Objective: Main Study: To study the effects of ketamine in treating depression. Ketamine Metabolites Substudy: To study how ketamine effects brain chemistry. To study how ketamine effects the brain. This is done by looking at metabolites, which are created when a drug is broken down. Eligibility: Main Study: People ages 18-65 with major depressive disorder and healthy volunteers Ketamine Metabolites Substudy: Healthy volunteers ages 18-65 Design: Main Study: Participants will be screened in another study, with: * Medical and psychiatric history * Psychiatric and physical exam * Blood, urine, and heart tests Participants will be inpatients at NIH for 4 phases totaling 14-20 weeks. Phase I (2-7 weeks): * Gradually stop current medications * MRI: Participants lie and perform tasks in a machine that takes pictures of the body. * Mood and thinking tests * Blood and urine tests * Sleep test: Monitors on the skin record brain waves, breathing, heart rate, and movement during sleep. * Transcranial magnetic stimulation: A coil on the scalp gives an electrical current that affects brain activity. * Stress tests: Electrodes on the skin measure reactions to loud noises or electric shocks. Phase I tests are repeated in Phases II and III and in the final visit. Phase II (4-5 weeks): * 4 weekly IV infusions of ketamine or a placebo during an MRI or MEG. For the MEG, a cone over the head records brain activity. Phase III (optional): * 8 infusions of ketamine over 4 weeks Phase IV (optional): * Symptoms monitoring for 4 weeks * Participants will have a final visit. They will be offered standard treatment at NIH for up to 2 months. Ketamine Metabolites Substudy: Participants