Multiple signaling states of G-protein-coupled receptors

Studies have been amassed in the past several years indicating that an agonist can conform a receptor into an activation state that is dependent upon an intrinsic property of the agonist usually based upon its chemical composition. Theoretically, each different agonist could impart its own unique activation state. Evidence for multiple signaling states for the G-protein-coupled receptors will be reviewed and is derived from many different pharmacological behaviors: efficacy, kinetics, protean agonism, differential desensitization and internalization, inverse agonism, and fusion chimeras. A recent extension of the ternary complex model is suggested by evidence that the different processes that govern deactivation, such as desensitization and internalization, is also regulated by conformers specific to the agonist. Rhodopsin may serve as a primer for the study of multiple activation states. Therapeutic implications that utilize multiple signaling states hold vast promise in the rationale design of drugs.