Ketamine Biomarker Validation

Inclusion Criteria: * Physically healthy patients who meet DMS-5 criteria for a major depressive episode (MDE) in the context of MDD and who score at least 22 on the Montgomery-Åsberg Depression Rating Scale (MADRS) * If taking antidepressants, dose is stable for at least 6 weeks. Exclusion Criteria: * Lifetime history of bipolar disorder, schizoaffective disorder, schizophrenia, or any other psychotic disorder, including MDD with psychotic features * A first-degree relative with bipolar disorder, schizoaffective disorder, or schizophrenia, with the potential participant younger than 33 years (i.e., still at age of risk for a psychotic disorder) * Receipt of electroconvulsive therapy within 3 months of enrolling in the study * History of IV drug use * Nonresponse or intolerance to ketamine administered either clinically or as part of a prior research study * Pregnancy, planning to conceive, or sexually active but not using adequate birth control. * Actively suicidal (CSSRS≥3) * No antipsychotics or medications (e.g. benzodiazepines, anti-epileptics) that are likely to interact with GABA or glutamate,