1. Istituto Di Ricerche Di Biologia Molecolare, P. Angeletti S.p.A. (Merck Research Laboratories, Rome), Via Pontina Km 30,600, 00040 Pomezia, Italy, and Merck Sharp & Dohme Research Laboratories, The Neuroscience Research Centre, Terling's Park, Harlow, Eastwick Road, Essex, CM20 2QR, United Kingdom