Evaluation of Two Radioactive Chemicals to Image mGluR5 Receptors in Brain

Background: \- A small brain protein called the metabotropic glutamate receptor subtype 5 (mGluR5) may affect several brain diseases such as autism and depression. Researchers will use 2 radioactive chemicals (\[11C\]SP203 and \[11C\]FPEB) and a research drug (STX107) that can attach to the receptor, to figure out the best way to use positron emission tomography (PET) to see the mGluR5 receptor. They will use scans to monitor where the radioactivity goes. Objectives: \- To find the best way to image the mGluR5 receptor in the brain. Eligibility: \- Healthy volunteers ages 18 to 55. Design: * All participants will be screened with a medical exam at visit 1. In later visits, they may have a PET scan, when two small tubes are placed under the skin and they lie down in a scanner. They may have an MRI scan, when they lie down in a scanner. * Part 1 participants will have 2 more visits. They will have a PET brain scan using \[11C\]FPEB and will have blood drawn. Then they will have an MRI brain scan. * Part 2 participants will have 1 more visit, with a whole body PET scan using \[11C\]FPEB and blood drawn. * Part 3 participants will have 4 more visits, including 1 overnight stay at a hospital. Over all the visits, they will have 4 PET scans and 1 MRI brain scan. They will receive the research drug and injections of both chemicals. Blood will be drawn during the scans.