A New Approach to Visualize Endogenously Expressed G Protein-Coupled Receptors in Tissues and Living Cells

G protein-coupled receptors (GPCRs) represent the largest family of membrane receptors. These proteins respond to a broad diversity of environmental stimuli and ligands, modulate most physiological processes, and represent prime therapeutic targets. Detecting GPCRs in vivo , however, remains a challenge and this limitation hampers our knowledge of receptor physiology. Autoradiographic ligand binding procedures provide low-resolution information, and the development of specific antibodies for immunohistochemistry has proven difficult. Tagged GPCRs have mainly been used in heterologous overexpression systems and cellular models. Here we describe an innovative approach where a fluorescent protein is fused to a GPCR in vivo . Using a knockin methodology, one can produce mutant mice that express a functional fluorescent receptor in place of the native receptor, and at physiological levels. We have pioneered this approach with the delta opioid receptor, implicated in both pain and emotional disorders. Here we describe these unique knockin reporter mice, and address potential pitfalls of the strategy. We report our first observations using this tool, and exemplify its usefulness at the level of receptor anatomy, function, and adaptations to drugs, with a particular focus on pain processes. This approach is potentially applicable to any GPCR, using an increasing choice among fluorescent reporter proteins, and offers unprecedented perspectives toward understanding GPCR biology and developing novel drugs of therapeutic interest.