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,