1. Drug Discovery & Medicinal Chemistry, Department of Pharmacology, University of Oxford, Mansfield Road, Oxford OX1 3QT, U. K.
2. Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge CB2 1PD, U. K.
3. School of Biological Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, U. K.