1. Molecular Pharmacology Laboratory and Laboratory for Molecular Graphics and Theoretical Modeling, College of Pharmacy, and Institute for Neuroscience, University of Texas, Austin, Texas 78712, Laboratory of Protein Modeling, Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167, Psychiatry Service, Department of Veterans Affairs Medical Center, Bedford, Massachusetts 01730, and Medical Research Service, Department of Veterans Affairs Medical Center, Portland, Oregon 97201