1. Graduate School Neurosciences Amsterdam, Research Institute Neurosciences Vrije Universiteit, Department of Molecular and Cellular Neurobiology, Faculty of Biology, De Boelelaan 1087, 1081 HV Amsterdam, The Netherlands, Micromass, Floats Road, Wytheshawe, Manchester M23 9LZ, England, and Department of Pharmaceutical Chemistry, Mass Spectrometric Facility, University of California, San Francisco, California