HRP reacted with the chromogen o-tolidine produces whole-cell reaction product at light and electron microscope levels: negative effects of sucrose and Golgi staining on benzidine reactions.

Studies of pathway microcircuitry often require electron microscope analysis. To facilitate these analyses, methods for labeling cells in their entirety are extremely useful. Furthermore, such a method would be most useful if the label would be completely confined by the cell membrane so that second labels for synapse identification could be used. No existing method reliably and repeatably produces this kind of a result. In seeking to develop such a method, a seldom-used chromogen for horseradish peroxidase (HRP) was found which produced superlative results for light and electron microscope analysis. o-Tolidine (3,3'-dimethylbenzidine) reacted with HRP produces a very electron-dense reaction product distributed uniformly throughout the cytoplasm and nucleoplasm; membranes are unobscured so that mitochondria, lysosomes, Golgi apparati and endoplasmic reticula are well defined. The reaction product extends into cellular processes of all sizes, including processes with cell bodies not within the plane of section, and is easily visualized at even the lowest electron microscope magnification. The HRP reaction product is completely confined by the cell membrane, thus terminals presynaptic to labeled cells remain distinct. However, the o-tolidine/HRP reaction product is negatively affected by exposure to oxidizers. In tissue exposed to sucrose before or after being reacted with o-tolidine, the HRP reaction product is less electron dense and is found only in lysosomes outside the nucleus or occasionally in proximal cellular processes. The o-tolidine/HRP reaction product is similarly affected when exposed to potassium dichromate for Golgi staining. For some other benzidine compounds, the chromogen/HRP reaction product is also negatively affected by exposure to these chemicals. Therefore, o-tolidine is a superior chromogen for HRP, labeling cells with details similar to that found for cells intracellularly injected with HRP.