An improved cobalt labeling technique with complex compounds.

Results of axonal labelings with CoCl2, cobaltous lysine and cobaltic lysine complexes are compared on dorsal roots and nerves of the spinal cord and brain stem in the living frog. The most satisfactory staining of fibres and terminals is given by CoCl2; its application however, is limited by its rather short (6--10 mm) axonal transport. Cobaltous lysine is transported somewhat better, but it gives a poor fibre staining in the spinal cord. The axonal transport of cobaltic lysine is the best, covering a distance of 40--50 mm. Combination of cobaltic lysine with 2--5% dimethyl sulphoxide greatly enhances the axonal uptake of cobalt and extends the distance of transport to 70--80 mm. It is assumed that better transport of cobalt complexes is achieved by their less toxic effect on the nerve cell.