1. Faculty of Mathematics and Natural Sciences, Institute of Neurobiology, Heinrich Heine University Duesseldorf, Universitätsstrasse 1; Duesseldorf D-40225 Germany
2. Institute of Anatomy II and Dr. Senckenbergisches Chronomedizinisches Institut, Goethe-University of Frankfurt, Theodor-Stern-Kai 7; Frankfurt/M D-60590 Germany
3. Medical Faculty, Institute of Cellular Neurosciences, University of Bonn; Bonn D-53105 Germany