1. Departments of Radiology, Medicinal Chemistry, and Neurosurgery, and Graduate Program in Neuroscience, University of Minnesota, Minneapolis, Minnesota 55455, Department of Chemistry, Hunter College, City University of New York, New York, New York 10021, Department of Pharmacology and Therapeutics, University of Florida College of Medicine, Gainesville, Florida 32610, and Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104