1. Psychopharmacology Unit, University of Bristol, Bristol BS8 1TD, U.K., Department of Pharmacy and Pharmacology, University of Bath, Bath BA2 7AY, U.K., Neurobiology Unit, Roche Bioscience, Palo Alto, California 94304-1397, School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and School of Biological Sciences, University of Southampton, Bassett Crescent East SO16 7PX, U.K.