1. Departments of Medicinal Chemistry and CNS Pharmacology, Solvay Pharmaceuticals Research Laboratories, P.O. Box 900, 1380 DA Weesp, The Netherlands, Bijvoet Center for Biomolecular Research, Laboratory of Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands, and Division of Medicinal Chemistry, Leiden/Amsterdam Center for Drug Research, P.O. Box 9502, 2300 RA Leiden, The Netherlands