1. Drug Discovery Program, Department of Neurology, Georgetown University Medical Center, 3900 Reservoir Road, N.W., Washington, D.C. 20007, Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, Texas 77555-1031, and Department of Pharmacology, Georgetown University Medical Center, 3900 Reservoir Road, N.W., Washington, D.C. 20007