1. National Center for Microscopy and Imaging Research, Center for Research in Biological Systems, Department of Neurosciences, University of California, San Diego, La Jolla, CA 92093, USA.
2. Department of Pharmacology, University of California, San Diego, La Jolla, CA 92093, USA.
3. Department of Chemistry and Biochemistry and Howard Hughes Medical Institute, University of California, San Diego, La Jolla, CA 92093, USA.