1. Division of Nuclear Medicine. Johns Hopkins Medical Institutions. Baltimore. Maryland 21205
2. Institute of Chemistry, University of Uppsala, 575121 Uppsala 1, Sweden
3. Division of Neuroradiology. Johns Hopkins Medical Institutions.
4. NIDA Addiction Research Center, Baltimore City Hospitals. Baltimore. Maryland 21224.
5. Departments of Neuroscience, Pharmacology and Experimental Therapeutics, and Psychiatry and Behavioral Sciences. Johns Hopkins University School of Medicine. Baltimore. Maryland 21205.