1. University of Pittsburgh 1 Department of Pharmacology and Chemical Biology , , Pittsburgh, PA 15261 , USA
2. Carnegie Mellon University 2 Department of Chemistry , , Pittsburgh, PA 15213 , USA
3. Carnegie Mellon University 3 Department of Chemistry , , Pittsburgh, PA 15213 , USA
4. Carnegie Mellon University 4 Department of Biology , , Pittsburgh, PA 15213 , USA
5. Carnegie Mellon University 5 Molecular Biosensor and Imaging Center , , Pittsburgh, PA 15213 , USA
6. Cornell University 6 Department of Neurobiology and Behavior , , Ithaca, NY 14853 , USA