1. Department of Pharmacology, University of California, San Diego, La Jolla, CA 92093-0654
2. Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-0654
3. Molecular and Cell Biology Laboratory, Salk Institute for Biological Studies, La Jolla, CA 92037
4. Department of Neuroscience, Institut Pasteur, URA 2182, CNRS, Paris F-75015, France