Transmembrane receptors play a crucial role in communication between cells. By coupling to intracellular proteins and second-messenger systems, these molecular entities allow for the transduction of a biochemical message across the cell membrane. Early investigations of receptor-mediated signaling pathways in nervous system tissue were originally limited to staining techniques for neurotransmitters, retro- and orthograde transport of labeled substances or enzymes, and identification of cells by specific stains (e.g., silver impregnation, Nissl stain, and so on). The introduction of immunocytochemistry in brain research led to the discovery of numerous neurochemical pathways (1). In this regard, immunocytochemistry offers laboratories the ability to visualize receptors at a tissue, cellular, and subcellular resolution, thereby providing a powerful tool for understanding the nature and organization of receptor-based signaling mechanisms. Readily available immunological and biochemical probes for markers of cellular differentiation, cytoskeletal proteins, and signal transduction cascade components extend these techniques, allowing for an assessment of the dynamics of receptor function. Furthermore, improvements in commercial monoclonal (MAb) and polyclonal (PAb) antibodies, and the widespread use of extremely sensitive multistep developing kits have considerably simplified immunocytochemical staining procedures. Finally, advances in tissue culture technology have allowed for the isolation and manipulation of homogenous populations of nervous system cells as well as immortalized cell lines.