1. Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health, 10 Center Drive, Bethesda, Maryland 20892, Department of Radiology, University of Pennsylvania, Pennsylvania 19104, Department of Chemistry, Imperial College, London, U.K., Nuclear Medicine Department, NIH Clinical Center, National Institutes of Health, Bethesda, Maryland 20892, and Clinical Brain Disorders Branch, National Institute of Mental Health, National Institutes of Health, Bethesda, Maryland 20892