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 and University College Medical School, Rowland Hill Street, London NW3 2PF, U.K., Department of Pharmacology, University of North Carolina School of Medicine, Chapel Hill, North Carolina 27599-7365, and Department of Physiology and Biophysics, Case Western Reserve University School of Medicine,...