Eligibility Criteria:
* INCLUSION CRITERIA:
For healthy volunteers
* Age 18 or older
* No serious current medical condition
* Able to give written informed consent.
* No prior diagnosis of drug or alcohol dependence.
For patients
* Age 18 or older
* Able to give written informed consent.
Subjects will have met DSM-IV criteria for recurrent MDD in a current major depressive episode.
-No prior diagnosis of drug or alcohol dependence.
EXCLUSION CRITERIA:
For healthy volunteers
* Any current Axis I diagnosis
* Clinically significant laboratory abnormalities other than CRP.
* Subjects with autoimmune disorders
* HIV positive
* Subjects with current infections
* Recent peripheral injury
* Smoking in the last 6 months, because smoking may cause a inflammatory responses
* Risk for MRI scan, such as a pacemaker or other implanted electrical devices, brain stimulators, some types of dental implants, aneurysm clips (metal clips on the wall of a large artery), metallic prostheses (including metal pins and rods, heart valves, and cochlear implants), permanent eyeliner, implanted delivery pump, or shrapnel fragments. Welders and metal workers are also at risk for injury because of possible small metal fragments in the eye of which they may be unaware.
* History of neurologic illness or injury with the potential to affect study data interpretation.
* Recent exposure to radiation (i.e., PET from other research) which when combined with this study would be above the allowable limits.
* Inability to lie flat on camera bed for at least two hours.
* Pregnancy or breast feeding.
* Able to get pregnant but does not use birth control.
* Non binder to PBR28 determined with a blood test or genotyping
* Positive urine drug screen on the day of the scan
* Subjects receiving treatment likely to impact inflammation (e.g., chronic antinflammatory drug treatment), statins or corticosteroids
* Significant MRI abnormalities of the brain.
* History of seizures, other than in childhood and related to fever.
* A history of drug or alcohol abuse within 6 months or a history of alcohol or drug dependence (excepting nicotine) within 3 years (DSM-IV criteria)
For patients
* Previous radiation exposure (X-rays, PET scans etc.) that, together with study procedures, would exceed NIH RSC research limits.
* Claustrophobia to a degree that the subject would feel uncomfortable in the MRI machine.
* Cannot lie on their back for at least two hours.
* Smoking in the last 6 months, because smoking may cause a inflammatory responses
* Brain abnormality such as a brain tumor, stroke, brain damage from head trauma or blood vessel abnormalities, on an MRI scan.
* Risk for MRI scan, such as a pacemaker or other implanted electrical devices, brain stimulators, some types of dental implants, aneurysm clips (metal clips on the wall of a large artery), metallic prostheses (including metal pins and rods, heart valves, and cochlear implants), permanent eyeliner, implanted delivery pump, or shrapnel fragments. Welders and metal workers are also at risk for injury because of possible small metal fragments in the eye of which they may be unaware.
* Subjects meet criteria for bipolar disorder or schizophrenia
* Current serious suicidal ideation (e.g., thoughts of attempting suicide along with the intent or plan to attempt suicide) or recent suicidal behavior (i.e., a suicide attempt within the past six months)
* Current psychosis
* Medical conditions or current medications that are likely to influence cerebral function or radiotracer delivery including cardiovascular, cerebrovascular, endocrine or neurological diseases
* A history of drug or alcohol abuse within 6 months or a history of alcohol or drug dependence (excepting nicotine) within 3 years (DSM-IV criteria)
* Current pregnancy or breastfeeding
* Able to get pregnant but does not use birth control
* Subjects with autoimmune disorders
* Subjects with current infections
* HIV positive
* Recent peripheral injury
* Subjects receiving treatment likely to impact inflammation (e.g., chronic antinflammatory drug treatment), statins or corticosteroids
* Subjects unable to travel independently to participate in the research study because of the severity of their depression.
* Non binder to PBR28 determined with a blood test or genotyping
* Positive urine drug screen on the day of the scan.