Clinical Research Directory

Inclusion Criteria: 1. Patients aged 18-65 years old; 2. current diagnosis of unipolar, major depressive disorder (MDD) without psychosis as defined by the Mini-International Neuropsychiatric Interview (MINI) and The Fourth Edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) DSM-IV determined by clinical interview and by a baseline score of ≥18 on the HDRS-21 (Hamilton Depression Rating Scale 21-item; 3. moderate to severe SI defined as a score ≥3 on the HDRS-21 suicide item (item #3); 4. will have good command of the English language; 5. will have been voluntarily admitted to the inpatient Psychiatric Units at Barnes-Jewish Hospital/Washington University in St. Louis, MO. Exclusion Criteria: 1. Lifetime DSM-IV (obtained via MINI) diagnoses of schizophrenia, schizoaffective disorders, bipolar disorder, obsessive-compulsive disorder, evidence of severe personality disorder (e.g., history of recurrent self-mutilation/cutting, significant and recurrent tumultuous relationships), and panic disorders; 2. past 12-month substance abuse/dependence other than nicotine; 3. active psychotic symptoms; 4. significant pulmonary disease and/or requiring supplemental oxygen; 5. administration of other N-Methyl-D-aspartate (NMDA)-receptor antagonist treatment (e.g., ketamine) within two weeks of entry into study; 6. currently receiving electroconvulsive therapy; 7. contraindications for nitrous oxide (pneumothorax, bowel obstruction, middle ear occlusion, elevated intracranial pressure, chronic cobalamin and/or folate deficiency treated with folic acid or vitamin B12); 8. past 12-month history of head trauma; 9. women who are pregnant or breastfeeding; 10. any other factor that in the investigators' judgment may affect patient safety or compliance.