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