Elsevier

Biochemical Pharmacology

Volume 83, Issue 2, 15 January 2012, Pages 193-198
Biochemical Pharmacology

Commentary
Imaging the high-affinity state of the dopamine D2 receptor in vivo: Fact or fiction?

https://doi.org/10.1016/j.bcp.2011.09.008Get rights and content

Abstract

Positron Emission Tomography (PET) has been used for more than three decades to image and quantify dopamine D2 receptors (D2R) in vivo with antagonist radioligands but in the recent years agonist radioligands have also been employed. In vitro competition studies have demonstrated that agonists bind to both a high and a low-affinity state of the D2Rs, of which the high affinity state reflects receptors that are coupled to G-proteins and the low-affinity state reflects receptors uncoupled from G-proteins. In contrast, antagonists bind with uniform affinity to the total pool of receptors. Results of these studies led to the proposal that D2Rs exist in high and low-affinity states for agonists in vivo and sparked the development and use of agonist radioligands for PET imaging with the primary purpose of measuring the proportion of receptors in the high-affinity (activating) state. Although several lines of research support the presence of high and low-affinity states of D2Rs and their detection by in vivo imaging paradigms, a growing body of controversial data has now called this into question. These include both in vivo and ex vivo studies of anesthesia effects, rodent models with increased proportions of high-affinity state D2Rs as well as the molecular evidence for stable receptor–G-protein complexes. In this commentary we review these data and discuss the evidence for the in vivo existence of D2Rs configured in high and low-affinity states and whether or not the high-affinity state of the D2R can, in fact, be imaged in vivo with agonist radioligands.

Introduction

Neuroreceptor imaging techniques such as Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) have been used for more than three decades to image and quantify dopamine D2 receptors (D2R) in the primate brain. Neuroreceptor imaging has also been used to assess endogenous dopamine release indirectly by measuring the dopamine displacement of D2R radioligand binding after a pharmacological challenge with psychostimulants (e.g., amphetamines or methylphenidate). The theoretical justification for this approach to measuring dopamine release in vivo was provided by the classical occupancy model. Briefly, amphetamine-induced release of endogenous dopamine will increase occupancy of the D2Rs by dopamine, thereby decreasing the binding potential (BPND) of the radiotracer, a parameter that is measured in PET imaging and is proportional to the product of receptor density (Bmax) and the affinity (1/Kd) of the radiotracer [1]. Several imaging studies have provided support for the occupancy model using benzamide antagonist radioligands and amphetamine challenge. However, even at high doses of amphetamine, D2R radioligand binding is not reduced beyond ∼50%, a phenomenon referred to as the ceiling effect [1], [2], [3], [4]. Several explanations have been proposed for this ceiling effect: (1) receptors located extrasynaptically are less accessible to competition from synaptically released dopamine, or maybe there is not enough dopamine to fully displace the radioligand, (2) internalized receptors are inaccessible to dopamine competition but still accessible to the relatively lipophilic radioligand, and/or (3) since D2Rs are configured in high and low-affinity state for agonist binding, dopamine competes primarily at the high-affinity sites of D2R but spares the low-affinity sites [1]. Although several lines of research support the presence of high and low-affinity state D2Rs and their detection by in vivo imaging paradigms, a growing body of evidence has now called this into question. The purpose of this commentary is to review these data and promote discussion about the existence in vivo of two populations of the D2R configured in high and low-affinity states for agonist binding, and to address whether a high-affinity state of the D2R can in fact be imaged with agonist radioligands.

Section snippets

The two-state occupancy model

The experimental basis for imaging high-affinity state D2Rs with agonist radioligands was provided by competition binding assays in washed brain membrane homogenates. These assays demonstrated that agonists bind with both high and low-affinity to the D2R in the absence of guanine nucleotide triphosphate (GTP), but with low-affinity in the presence of GTP [5], [6], [7]. GTP binding to Gα subunit promotes G-protein activation and dissociation from the receptor, resulting in a loss of

PET studies of D2Rs in the high-affinity state

Since the initial proposal of the two-state occupancy model, multiple studies have aimed at imaging the D2R in the high-affinity state with agonist radioligands [9], [10], [11], [12]. A review of 13 PET studies reveals inconclusive evidence for the two-state occupancy model (Table 1). Seven studies (all used anesthesia) supported the ability to image high-affinity state D2Rs with agonist radioligands while six studies (four used anesthesia) failed to support this hypothesis. Consistent with the

Ex vivo studies of D2Rs in the high-affinity state

The possibility of measuring high-affinity state D2Rs with agonist radioligands has also been investigated in a number of ex vivo studies. Briefly, these studies were performed by intravenous administration of radioligands in either conscious or anesthetized rodents. The rodents were euthanized by decapitation at various time points and the radioactivity was measured in the brain tissue. As was the case for the PET investigations, a review of 12 ex vivo studies revealed inconclusive evidence

Molecular mechanisms of agonist binding at the dopamine D2 receptor

The high and low-affinity states of the D2R are often referred to as G-protein coupled and uncoupled, respectively. While this makes sense in terms of the ternary complex model and under GTP depleted and stabilized conditions in membrane binding assays, evidence for the existence of pre-coupled complexes in living cells is conflicting. Two different theories have been proposed to explain receptor–G-protein interaction in the absence and presence of agonist (Fig. 2):

  • (1)

    The pre-coupled theory

Acknowledgements

The authors thank Mark Slifstein for fruitful discussions and feedback.

References (71)

  • N. Audet et al.

    Bioluminescence resonance energy transfer assays reveal ligand-specific conformational changes within preformed signaling complexes containing delta-opioid receptors and heterotrimeric G proteins

    J Biol Chem

    (2008)
  • M.L. Toews et al.

    Agonist-induced changes in beta-adrenergic receptors on intact cells

    J Biol Chem

    (1984)
  • N. Vasdev et al.

    Syntheses and in vitro evaluation of fluorinated naphthoxazines as dopamine D2/D3 receptor agonists: radiosynthesis, ex vivo biodistribution and autoradiography of [18F]F-PHNO

    Nucl Med Biol

    (2007)
  • M. Laruelle et al.

    In vivo quantification of dopamine D2 receptor parameters in nonhuman primates with [123I]iodobenzofuran and single photon emission computerized tomography

    Eur J Pharmacol

    (1994)
  • J.M. Weiss et al.

    The cubic ternary complex receptor-occupancy model. III. Resurrecting efficacy

    J Theor Biol

    (1996)
  • U. Gether et al.

    G protein-coupled receptors. II. Mechanism of agonist activation

    J Biol Chem

    (1998)
  • R.J. Lefkowitz et al.

    Constitutive activity of receptors coupled to guanine nucleotide regulatory proteins

    Trends Pharmacol Sci

    (1993)
  • M. Laruelle

    Imaging synaptic neurotransmission with in vivo binding competition techniques: a critical review

    J Cereb Blood Flow Metab

    (2000)
  • A. Breier et al.

    Schizophrenia is associated with elevated amphetamine-induced synaptic dopamine concentrations: evidence from a novel positron emission tomography method

    Proc Natl Acad Sci USA

    (1997)
  • M. Laruelle et al.

    Microdialysis and SPECT measurements of amphetamine-induced dopamine release in nonhuman primates

    Synapse

    (1997)
  • A. De Lean et al.

    Dopamine receptor of the porcine anterior pituitary gland. Evidence for two affinity states discriminated by both agonists and antagonists

    Mol Pharmacol

    (1982)
  • Y.G. Gao et al.

    Synthesis and dopamine receptor affinities of enantiomers of 2-substituted apomorphines and their N-n-propyl analogues

    J Med Chem

    (1990)
  • A.A. Wilson et al.

    Radiosynthesis and evaluation of [11C]-(+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol as a potential radiotracer for in vivo imaging of the dopamine D2 high-affinity state with positron emission tomography

    J Med Chem

    (2005)
  • R. Narendran et al.

    In vivo vulnerability to competition by endogenous dopamine: comparison of the D2 receptor agonist radiotracer (−)-N-[11C]propyl-norapomorphine ([11C]NPA) with the D2 receptor antagonist radiotracer [11C]-raclopride

    Synapse

    (2004)
  • R. Narendran et al.

    Measurement of the proportion of D2 receptors configured in state of high affinity for agonists in vivo: a positron emission tomography study using [11C]N-propyl-norapomorphine and [11C]raclopride in baboons

    J Pharmacol Exp Ther

    (2005)
  • N. Ginovart et al.

    Binding characteristics and sensitivity to endogenous dopamine of [11C]-(+)-PHNO, a new agonist radiotracer for imaging the high-affinity state of D2 receptors in vivo using positron emission tomography

    J Neurochem

    (2006)
  • N. Seneca et al.

    Effect of amphetamine on dopamine D2 receptor binding in nonhuman primate brain: a comparison of the agonist radioligand [11C]MNPA and antagonist [11C]raclopride

    Synapse

    (2006)
  • H. Ohba et al.

    Ketamine/xylazine anesthesia alters [11C]MNPA binding to dopamine D2 receptors and response to methamphetamine challenge in monkey brain

    Synapse

    (2009)
  • D.R. Hwang et al.

    Positron-labeled dopamine agonists for probing the high affinity states of dopamine subtype 2 receptors

    Bioconjug Chem

    (2005)
  • R. Narendran et al.

    A comparative evaluation of the dopamine D(2/3) agonist radiotracer [11C](−)-N-propyl-norapomorphine and antagonist [11C]raclopride to measure amphetamine-induced dopamine release in the human striatum

    J Pharmacol Exp Ther

    (2010)
  • H. Tsukada et al.

    Ketamine decreased striatal [(11)C]raclopride binding with no alterations in static dopamine concentrations in the striatal extracellular fluid in the monkey brain: multiparametric PET studies combined with microdialysis analysis

    Synapse

    (2000)
  • G.J. Kilpatrick et al.

    The thermodynamics of agonist and antagonist binding to dopamine D-2 receptors

    Mol Pharmacol

    (1986)
  • T. Agui et al.

    Binding of [125I]-N-(p-aminophenethyl)spiroperidol to the D-2 dopamine receptor in the neurointermediate lobe of the rat pituitary gland: a thermodynamic study

    Mol Pharmacol

    (1988)
  • W. Hassoun et al.

    PET study of the [11C]raclopride binding in the striatum of the awake cat: effects of anaesthetics and role of cerebral blood flow

    Eur J Nucl Med Mol Imaging

    (2003)
  • S.J. Finnema et al.

    Dopamine D(2/3) receptor occupancy of apomorphine in the nonhuman primate brain-A comparative PET study with [11C]raclopride and [11C]MNPA

    Synapse

    (2009)
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