An immunoelectron microscopic method is described for sensitive high-resolution visualization of tissuebound cholera toxin. The principle is to incubate cells or tissue sections with toxin and then to localize the bound toxin with toxin-specific peroxidase (donor:hydrogen-peroxide oxidoreductase; EC 1.11.1.7)-conjugated antibody and enzyme substrate. Thin sections are examined for electron-opaque precipitates in a transmission electron microscope. Because of the specific binding of the toxin to membrane ganglioside G M1 , the method can be used for ultrastructural localization of this ganglioside. Semiquantitative data are obtained by titration of the limiting concentration of cholera toxin producing specific precipitates. The specificity of the method was controlled in various ways, including analyses of the correlation between the immunoelectron microscopy results and determinations of ganglioside G M1 in tissues with different ganglioside concentrations, tissues hydrolyzed with Vibrio cholerae sialidase, tissues in which exogenous G M1 has been incorporated, and lipid-extracted tissues. The immunoelectron microscopic method demonstrates that membrane G M1 ganglioside is positioned on the external side exclusively. Cell-bound toxin remains in its original location on the plasma membrane surface of cells below 18°, but appears to be redistributed both laterally and vertically in the membrane of cells incubated at 37° for 30 min or longer. The results of this method indicate that in the central nervous system G M1 is concentrated in the pre- and postsynaptic membranes of the synaptic terminals; a further increase in reactivity of these structures after hydrolysis of the nervous tissue with V. cholerae sialidase suggests that higher gangliosides of the same series are particularly increased in the pre- and postsynaptic junctions.

Hansson, H.-A., Holmgren, J., & Svennerholm, L. (1977). Ultrastructural localization of cell membrane G M1 ganglioside by cholera toxin. Proceedings of the National Academy of Sciences, 74(9), 3782–3786.