1. Department of Biochemistry and Biophysics, Pharmacology Graduate Group Perelman School of Medicine at the University of Pennsylvania Philadelphia PA 19104 U.S.A.
2. Biochemistry Section, Surgical Neurology Branch National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, U.S.A.
3. Postdoctoral Research Associate Training Program National Institute of General Medical Sciences, National Institutes of Health Bethesda MD 20892 U.S.A.