Characterizing Synaptic Density in Treatment Resistant Depression

This trial is studying how dopamine receptors in the brain differ between people who respond to antidepressant medications and those who do not (called treatment-resistant depression), using a specialized brain scan called PET imaging. **You may be eligible if...** - You are between 25 and 55 years old - You have been diagnosed with a major depressive episode as part of major depressive disorder (MDD) - Your depression is moderate to severe based on a standard rating scale - You have been off all psychiatric medications for a specific period before the scan - You can safely undergo an MRI scan (no metal implants, pacemakers, etc.) - Healthy volunteers without any psychiatric history are also welcome to participate as comparison participants **You may NOT be eligible if...** - You are pregnant or breastfeeding - You have an untreated or urgent medical condition - You have a recent history (within 6 months) of drug or alcohol problems - You have a history of psychosis or schizophrenia - You are currently taking any psychiatric medications Talk to your doctor to see if this trial is right for you.