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To understand in detail the functional morphology of neuronal circuits it is important to identify at the ultrastructural level the incoming axon, its target neuron, and members of the signaling cascades involved. This, however, represents a formidable task, requiring highly sophisticated electron microscopic multiplelabeling techniques. To extend available double-labeling procedures such as combinations of immunogold and peroxidase methods, an additional, gold-and peroxidaseindependent procedure would represent a considerable advantage. The present investigation therefore aimed to use alkaline phosphatase as the immunoenzymatic label at the electron microscopic level via cerium phosphate precipitates. To our surprise we found that available techniques, which are well established for the visualization of endogenous enzymes in sections from various tissues, are not suitable for application to immunocytochemistry. Careful characterization of the individual reaction conditions, however, resulted in an optimized procedure with largely increased sensitivity. The novel technique yields cerium-containing precipitates which are massive enough to allow the detection of the immunoenzymatic reaction product in the electron microscope. Using the rat olfactory bulb as the model system we showed further that our technique allows the combination with the peroxidase/diaminobenzidine system for ultrastructural double labeling. For this purpose, the alkaline phosphatase product is identified by its cerium content via energy-filtered transmission electron microscopy and thereby differentiated from cerium-free peroxidase-derived precipitates. Doing so, we found that ascending serotoninergic fibers do not establish synapses with dopaminergic periglomerular cells in the rat olfactory bulb.

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