1. Departments of Radiology and Pharmacology, University of Pennsylvania, 3700 Market Street, Room 305, Philadelphia, Pennsylvania 19104, Department of Psychiatry, University of Toronto, PET Centre, Centre for Addiction and Mental Health, 250 College Street, Toronto, Ontario, Canada M5T 1R8, and Departments of Neurology, Molecular Biology and Pharmacology, Washington University, St. Louis, Missouri 63110