Signaling through GPCRs can lead to a variety of cellular responses via changing intracellular levels of second messengers and gene transcription (reviewed by (29)). Activation of MOR that is coupled to Gi inhibits adenylate cyclase (30), decreasing the production of cAMP (31). Due to variable levels of cAMP in cultured cells, the activity of GPCRs coupled to Gi is assayed in cells with artificially elevated cAMP levels. The levels of cAMP are usually stimulated by either PGE2 of FSK. A mu-opioid receptor agonist is then administered at varying concentrations. The extent of decrease in cAMP levels indicates the efficacy of receptor–ligand interaction.
