1. Department of Physiology, Faculty of Medicine, University of Toronto Centre for the Study of Pain, University of Toronto, 1 King's College Circle, Toronto, Ontario M5S 1A8, Canada
2. Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA
3. National Creative Research Initiative Center for Memory, Department of Biological Sciences, Seoul National University, San 56–1 Silim-dong Gwanak-gu, Seoul 151–747, Korea