1. Laboratory of Cerebral Metabolism, National Institute of Mental Health, U.S. Department of Health and Human Services, Public Health Service, Bethesda, Maryland, U.S.A.
2. Department of Medical Chemistry and Biochemistry, University of Milan, Istituto di Neuroscienze e Bioimmagini, Consiglio Nazionale delle Ricerche, Institute H. San Raffaele, Milan, Italy