G protein-coupled receptors (GPCRs) represent one of the most important classes of drug targets in the pharmaceutical industry. Due to a limited number of well-characterized downstream signaling pathways, GPCR signaling can be monitored with a relatively small standard set of assay formats, most of which have been adapted for high-throughput screening. These formats include, among others, real-time Ca2+ release assays for Gaq coupling, cAMP assays for Gas and Gai coupling, and GTPyS assays, mainly for Gai coupling. New assay technologies are continuously developed, thereby creating a variety of possibilities to choose the appropriate assay platform for every specific need or budget. Nevertheless, most of them are variations of well-known signaling pathways, and the majority of these assays are based on the use of dyes, antibodies, or reporter gene constructs. These conventional assay technologies often rely on the use of highly expressed recombinant receptors or signal transducers in a cellular background that is compatible with the assay but, in many instances, nonphysiological. In addition, the majority of assays are end point assays, thereby preventing a continued monitoring of cell signaling. It is therefore a challenge to develop label-free, real-time detection assays to characterize GPCR signaling and to circumvent some of the drawbacks of traditional assay technologies. Recent developments, such as the introduction of automated microscopy, impedance, or resonant waveguide grating tech-

GPCR Molecular Pharmacology and Drug Targeting: Shifting Paradigms and New Directions,

Edited by Annette Gilchrist

Copyright © 2010 John Wiley & Sons, Inc.

nologies, allow high-throughput-compatible, label-free, real-time GPCR assays with the required accuracy and robustness to make them amenable to drug discovery. In this review, these novel assay principles are discussed in the context of GPCR receptor pharmacology, especially information content and comparability with conventional assay methods.

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