1. Contribution from the Department Chemie, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany, Department of Molecular Pharmacology, Physiology and Biotechnology, Box B-G4, 171 Meeting Street, Brown University, Providence, Rhode Island 02912, and Forschungsinstitut für Molekulare Pharmakologie, Robert-Rössle-Strasse 10, 13125 Berlin-Buch, Germany