The localization and density of G-protein-coupled receptors (GPCRs) on the cell surface is a critical factor for specifying signaling within and between neurons. GPCRs and their associated signaling components are localized to specific neuronal compartments that give rise to a variety of functional implications. Therefore, information regarding the precise localization of GPCRs is a prerequisite for studies designed to elucidate their contribution to neuronal function, and can be achieved only by immunoelectron microscopy. Three main techniques for immunoelectron microscopy exist: the preembedding immunoperoxidase, the preembedding immunogold, and the postembedding immunogold techniques. The preembedding immunogold method is reliable for the localization of receptors at extrasynaptic and perisynaptic sites. The preembedding immunoperoxidase method provides valuable information on regional distribution of receptors. Finally, to localize any receptor at synaptic sites the only reliable method is the postembedding immunogold technique. Therefore, the three immunocytochemical methods provide complementary information about the cellular, subcellular, and subsynaptic location of any receptor.
