Effector Pathway-Dependent Relative Efficacy at Serotonin Type 2A and 2C Receptors: Evidence for Agonist-Directed Trafficking of Receptor Stimulus

  1. Kelly A. Berg1,
  2. Saul Maayani2,3,
  3. Joseph Goldfarb3,
  4. Clare Scaramellini4,
  5. Paul Leff4 and
  6. William P. Clarke1
  1. 1Department of Pharmacology, University of Texas Health Science Center at San Antonio, San Antonio, Texas 78284-7764 (K.A.B., W.P.C.), Departments of 2Anesthesiology (S.M.) and 3Pharmacology (S.M., J.G.), Mount Sinai School of Medicine, City University of New York, New York, New York 10029-6574, and 4Department of Pharmacology, Astra Charnwood, Loughborough, Leics LE11 5RH, England (C.S., P.L.)

    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 Sci16:232–238 (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

    • 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

    • ↵1 The treatment of CHO-1C19 cells with staurosporine did not enhance 5-HT2C-mediated IP accumulation (percent above basal, 314 ± 115 versus 361 ± 110, DOI (1 μm) in the absence and presence of staurosporine (1 μm), respectively, mean ± standard deviation, two experiments). Together with the lack of effect of PLC inhibitors on 5-HT2A-mediated IP accumulation in CHO-FA4 cells, these data suggest that 5-HT2Areceptors in CHO-FA4 cells couple to a PLC with different characteristics (sensitivity to PKC-mediated negative feedback and lack of sensitivity to PLC inhibitors) than the PLC to which 5-HT2C receptors couple in CHO-1C19 cells.

    • This work was supported by United States Public Health Service Grants DA09094 (K.A.B., S.M.) and HD26437 (W.P.C.).

    • 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).

    • Abbreviations:
      5-HT
      5-hydroxytryptamine (serotonin)
      AA
      arachidonic acid
      BSA
      bovine serum albumin
      BAPTA
      1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid
      [Ca2+]i
      intracellular calcium concentration
      CHO
      Chinese hamster ovary
      DOI
      (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane
      AM
      acetoxymethyl ester
      HBSS
      Hanks’ balanced salt solution
      HEPES
      4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
      IP
      inositol phosphates
      d-LSD
      lysergic acid diethylamide
      PACAP
      pituitary adenylate cyclase activating polypeptide
      PBZ
      phenoxybenzamine
      PLA2
      phospholipase A2
      PLC
      phospholipase C
      PTX
      pertussis toxin
      TFMPP
      3-trifluoromethylphenyl-piperazine
      EGTA
      ethylene glycol bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid
      • Received October 2, 1997.
      • Accepted March 18, 1998.
    « Previous | Next Article »Table of Contents

    This Article

    1. Molecular Pharmacology July 1, 1998 vol. 54 no. 1 94-104
    1. » Abstract
    2. Full Text
    3. Full Text (PDF)

    Classifications

    Responses

    1. Submit a response
    2. No responses published

    Current Issue

    1. November 2009, 76 (5)
    1. Current Issue
    1. Alert me to new issues of Molecular Pharmacology