1. Department of Pharmaceutical Biosciences, Division of Pharmacology, Uppsala University, Box 591, Biomedicum, SE-751 24 Uppsala, Sweden, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, S-171 77 Stockholm, Sweden, Institute of Organic and Bioorganic Chemistry, Tartu University, Jakobi 2, 51014 Tartu, Estonia, Department of Cell and Molecular Biology, Uppsala University, Box 596, SE-751 24 Uppsala, Sweden, and Latvian Institute of Organic Synthesis, LV-1006, Riga, Latvia