The primary goal is to determine if the antidepressant effects of Ketamine are mediated by an opiate mechanism.~Primary Objective:~To determine if the opioid properties of Ketamine are responsible for it's antidepressant effects by potentially blocking the antidepressant effects with a opioid antagonist naltrexone.~We will measure this objective by looking at the response on a scale called the 6- item Hamilton Rating Scale for Depression (HAM-D-6). Response is defined as a statistically significant greater decrease on the overall score on this scale, post infusion.~Secondary Objective:~This includes comparing a scale called Clinician Administered Dissociative States Scale (CADSS) on both of our patient groups, one group receiving Ketamine plus Naltrexone compared to the other group receiving Ketamine plus placebeo, to determine if naltrexone has any effect on CADSS as well as to determine if CADSS is associated with antidepressant response~Another secondary objective is to assess ketamine craving using the Visual Analog Craving Scale for Ketamine (VASK), after infusion and determine if there is a change in level of craving for the group that receives naltrexone.~Number of Subjects:~i) 30~ii) The subjects will be drawn from an outpatient sample of patients with MDD, diagnosed with the use of the Structured Clinical Interview for DSM-IV Axis I Disorders(SCID-I/P), currently on a stable, adequate dose of antidepressant therapy, as defined by the MGH ATRQ, for at least 4 weeks or a history of intolerance to at least 2 antidepressant treatments.
The primary objective is to determine if the opioid properties of ketamine are responsible for its antidepressant effects. Since naltrexone can block opiate actions, the investigators will determine if naltrexone can effectively block ketamine's effects.