The Role of HNKs in the Antidepressant Effect of Ketamine

Major depressive disorder (MDD) is a common illness, affecting over 14 million American adults each year. MDD is a leading cause of disability worldwide and is responsible for huge workplace and healthcare costs. The several week delay between onset of treatment and improvement in MDD symptoms with currently available treatments further increases the burden of the disorder. Shortening this delay is a major unmet challenge in the treatment of MDD. Studies report that a single intravenous low dose of a drug called ketamine can bring about substantial improvement in depression in hours, even in patients that have not improved with other antidepressant treatments. Certain aspects of ketamine's drug action are fairly well understood, but the question remains of how these properties relate to antidepressant effect. Preliminary data support the rapid antidepressant benefit from ketamine but do not show a relationship between clinical improvement and the amount of ketamine, norketamine or dehydronorketamine (DHNK)(two of ketamine's metabolites) in the blood. The investigators hypothesize that a different metabolite of ketamine, hydroxynorketamine (HNK), produces the antidepressant effect of ketamine. The investigators have also used a scanner to measure the effects of ketamine on two major brain chemical transmitters and found that it causes a significant increase (more than 60%) in glutamate (Glu) and gamma aminobutyric acid (GABA) levels in the front of the brain. The investigators hypothesize that this increase in Glu and GABA levels is responsible for the antidepressant action of the drug. Knowing how ketamine works could help to develop better medications that can be used orally and used for maintenance of the improvement seen with ketamine.