1. Departments of Biochemistry & Molecular Biology and Medicinal Chemistry, Merck Sharp & Dohme Research Laboratories, The Neuroscience Research Centre, Terlings Park, Harlow, Essex CM20 2QR, United Kingdom, Department of Biological Chemistry, Merck Research Laboratories, West Point, Pennsylvania, and Department of Pathology, University of Melbourne, and Neuropathology Laboratory, Mental Health Institute of Victoria, Parkville, Victoria 3010, Australia