1. Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX 75390–9111, USA.
2. Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390–9111, USA.
3. Intramural Research Program, National Institute on Drug Abuse (NIDA), National Institutes of Health (NIH), Baltimore, MD 21224, USA.
4. Department of Neurobiology, Northwestern University, Evanston, IL 60208, USA.
5. Department of Genetics, University of North Carolina–Chapel Hill, Chapel Hill, NC 27599, USA.
6. Jackson Laboratory, Bar Harbor, ME 04609, USA.
7. Solomon H. Snyder Department of Neuroscience, Johns Hopkins School of Medicine, Baltimore, MD 21205, USA.