Identification of Orphan G Protein-Coupled Receptor Ligands Using FLIPR Assays

G protein-coupled receptors (GPCRs) make up the largest and most diverse family of transmembrane proteins and respond to a wide variety of stimuli including biogenic amines, peptides, bioactive lipids, hormones, and light (1,2). Agonist binding to these receptors activates intracellular signalling events mediated by G proteins, such as modulation of intracellular cyclic adenosine monophosphate (cAMP) levels or Ca²⁺ mobilization. To date, there are approx 250 characterized nonsensory GPCRs and a further 140 genes predicted to be GPCRs for which the endogenous or natural ligand is unknown—the “orphan” GPCRs (oGPCRs) (3–5). Historically, GPCRs, especially those in the aminergic receptor subfamily, have proved amenable to the design of synthetic agonists and antagonists of their activity. Of the top-selling prescription drugs in 2002, more than 33% act through GPCRs and provide greater than $25 billion in worldwide pharmaceutical sales. Therefore, considerable effort has been made to identify cognate ligands for oGPCRs and functionally characterize these receptors in order to elucidate their physiological and therapeutic relevance.