1. Howard Hughes Medical Institute and Department of Physiology, University of Wisconsin, Madison, WI 53706, USA.
2. Molecular and Cellular Pharmacology Program, University of Wisconsin, Madison, WI 53706, USA.
3. Department of Food Microbiology and Toxicology, University of Wisconsin, Madison, WI 53706, USA.
4. W. M. Keck Center for Learning and Memory and Department of Neurobiology and Anatomy, University of Texas–Houston Medical School, Houston, TX 77030, USA.