Cell-signaling assays

A method of testing a compound for modulation of a receptor or enzyme involved in generation of a cyclic nucleotide, the method comprising:

• contacting a sample with a tracer comprising a cyclic nucleotide coupled to a luminophore, and with an opposite member of a specific binding pair to the cyclic nucleotide;
• illuminating the sample with polarized light, wherein the light is capable of inducing emission of polarized light from the luminophore;
• measuring the extent of polarization of light emitted from the luminophore in the presence and in the absence of a compound;
• and
• identifying the compound as a modulator of the receptor or enzyme if the extent of polarization changes in the presence relative to the absence of the compound.