1. The Picower Institute for Learning and Memory, Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2. Department of Pharmacology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
3. Vanderbilt Center for Addiction Research, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
4. The Salk Institute for Biological Studies, La Jolla, CA 92037, USA.