1. Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K., Dipartimento di Chimica, Universita di Modena, Modena, Italy, and Receptor Systems Unit, Glaxo-Wellcome Research and Development, Stevenage SG1 2NY, England, U.K.