1. Laboratory of Medicinal Chemistry, NIDDK, National Institutes of Health, DHHS, Bethesda, Maryland 20892, Clinical Psychopharmacology Section, NIDA, Addiction Research Center, National Institutes of Health, DHHS, Baltimore, Maryland 21224, College of Pharmacy, The University of Iowa, 115 South Grand Avenue, Iowa City, Iowa 52242-1112, and Department of Pharmaceutical Studies, Faculty of Pharmacy, Universita' degli Studi di Roma “La Sapienza”, P.le Aldo Moro 5, 00185 Rome, Italy