Disclosed are novel compositions and methods employing cyclic nucleotide-gated channels and fluorescence dyes and other indicators in detecting changes of intracellular cAMP levels, for instance in response to the stimulation of G-protein-coupled receptors. CNG mutants comprising one or more pore mutations that enhance sensitivity to cAMP and decrease divalent cation-mediated blockage are used in a variety of assays, including detection of the G-protein-coupled receptor activity. Also disclosed are novel dye quencher formulations that are particularly suited for cell-based assays of protein interactions.