CommentaryImaging the high-affinity state of the dopamine D2 receptor in vivo: Fact or fiction?
Graphical abstract
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)
- et al.
Is synaptic dopamine concentration the exclusive factor which alters the in vivo binding of [11C]raclopride?: PET studies combined with microdialysis in conscious monkeys
Brain Res
(1999) - et al.
Dopaminergic inhibition of adenylate cyclase correlates with high affinity agonist binding to anterior pituitary D2 dopamine receptors
Mol Cell Endocrinol
(1984) - et al.
Anterior pituitary dopamine receptors. Demonstration of interconvertible high and low affinity states of the D2 dopamine receptor
J Biol Chem
(1982) - et al.
A ternary complex model explains the agonist-specific binding properties of the adenylate cyclase-coupled beta-adrenergic receptor
J Biol Chem
(1980) - et al.
(−)-N-[(11)C]propyl-norapomorphine: a positron-labeled dopamine agonist for PET imaging of D(2) receptors
Nucl Med Biol
(2000) - et al.
A preliminary PET evaluation of the new dopamine D2 receptor agonist [11C]MNPA in cynomolgus monkey
Nucl Med Biol
(2005) - et al.
D2 dopamine receptor internalization prolongs the decrease of radioligand binding after amphetamine: a PET study in a receptor internalization-deficient mouse model
Neuroimage
(2010) - et al.
Dopamine D2 receptor radiotracers [11C](+)-PHNO and [3H]raclopride are indistinguishably inhibited by D2 agonists and antagonists ex vivo
Nucl Med Biol
(2008) - et al.
Third-party bioluminescence resonance energy transfer indicates constitutive association of membrane proteins: application to class a g-protein-coupled receptors and g-proteins
Biophys J
(2010) - et al.
Endocytosis promotes rapid dopaminergic signaling
Neuron
(2011)