1. Station Biologique de Roscoff, C.N.R.S., BP 74, 29682 Roscoff Cedex, Bretagne, France, Laboratory of Molecular & Cellular Neuroscience, The Rockefeller University, 1230 York Avenue, New York, New York 10021-6399, and Department of Medicine, Institut Gustave Roussy, 39, rue Camille Desmoulins, 94815 Villejuif Cedex, France