1. Laboratory of Medicinal Chemistry, Building 8, Room B1-23, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, and Clinical Psychopharmacology Section, National Institute on Drug Abuse, Addiction Research Center, Baltimore, Maryland 21224, and Department of Pharmacology, College of Medicine, The University of Arizona, Tucson, Arizona 85724