1. Departments of Medicinal Chemistry, Molecular and Cellular Neuroscience, and in Vivo Neuroscience, The Neuroscience Research Centre, Merck Sharp & Dohme Research Laboratories, Terlings Park, Eastwick Road, Harlow, Essex, CM20 2QR, U.K., and Department of Process Research, Merck Research Laboratories, 126 East Lincoln Avenue, Rahway, New Jersey 07065