1. Molecular Recognition Section, Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, NIH, Bethesda, Maryland 20892-0810, Autonomic Neuroscience Institute, Royal Free Hospital School of Medicine, Rowland Hill Street, London, NW3 2PF, U.K., and Department of Pharmacology, University of North Carolina School of Medicine, Chapel Hill, North Carolina 27599-7365