1. Institute of Biochemistry, Biological Research Center, Hungarian Academy of Sciences, P.O. Box 521, H-6701 Szeged, Hungary, Special Lab Non-Invasive Brain Imaging, Leibniz-Institute for Neurobiology, Brenneckestr. 6, D-39118 Magdeburg, Germany, and Cardiovascular Research Group, Department of Biochemistry, University of Szeged, Dóm tér 9. H-6720 Szeged, Hungary