1. Department of Organic Chemistry, Vrije Universiteit Brussel, B-1050 Brussels, Belgium
2. Department of Chemical Biology and Peptide Research, Clinical Research Institute, Montreal, Québec H2W 1R7, Canada
3. Department of Pharmacology, University of Arizona, Tucson, Arizona 85724, United States
4. Animal Physiology and Neurobiology, Zoological Institute, Katholieke Universiteit Leuven, B-3000 Leuven, Belgium