Abstract
There are many examples of a single receptor coupling directly to more than one cellular signal transduction pathway. Although traditional receptor theory allows for activation of multiple cellular effectors by agonists, it predicts that the relative degree of activation of each effector pathway by an agonist (relative efficacy) must be the same. In the current experiments, we demonstrate that agonists at the human serotonin2A (5-HT2A) and 5-HT2Creceptors activate differentially two signal transduction pathways independently coupled to the receptors [phospholipase C (PLC)-mediated inositol phosphate (IP) accumulation and phospholipase A2(PLA2)-mediated arachidonic acid (AA) release]. The relative efficacies of agonists differed depending on which signal transduction pathway was measured. Moreover, relative to 5-HT, some 5-HT2C agonists (e.g., 3-trifluoromethylphenyl-piperazine) preferentially activated the PLC-IP pathway, whereas others (e.g., lysergic acid diethylamide) favored the PLA2-AA pathway. In contrast, when two dependent responses were measured (IP accumulation and calcium mobilization), agonist relative efficacies were not different. These data strongly support the hypothesis termed “agonist-directed trafficking of receptor stimulus” recently proposed by Kenakin [Trends Pharmacol Sci 16:232–238 (1995)]. Concentration-response curves to 5-HT2C agonists were fit well by a three-state model of receptor activation, suggesting that two active receptor states may be sufficient to explain pathway-dependent agonist efficacy. Rational drug design that optimizes preferential effector activity within a group of receptor-selective drugs holds the promise of increased selectivity in clinically useful agents.
Footnotes
- Received October 2, 1997.
- Accepted March 18, 1998.
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Send reprint requests to: William P. Clarke, Ph.D., Department of Pharmacology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78284-7764. E-mail: clarkew{at}uthscsa.edu
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This work was supported by United States Public Health Service Grants DA09094 (K.A.B., S.M.) and HD26437 (W.P.C.).
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Portions of this work have been presented at the International Union of Pharmacology-sponsored Symposium on Serotonin Receptors (1997) and at the annual meetings of the Society for Neuroscience (1995, 1996 and 1997), the British Pharmacological Society (1996), and the American College of Neuropsychopharmacology (1997).
- The American Society for Pharmacology and Experimental Therapeutics
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