Agonist signalling properties of radiotracers used for imaging of dopamine D2/3 receptors
Jan-Peter van Wieringen
0
Martin C Michel
2
Henk M Janssen
1
Anton G Janssen
4
Philip H Elsinga
3
Jan Booij
0
0
Department of Nuclear Medicine, Academic Medical Center, University of Amsterdam
, Meibergdreef 9, 1105 AZ Amsterdam,
The Netherlands
1
SyMO-Chem BV, Eindhoven,
The Netherlands
2
Department of Pharmacology, Johannes Gutenberg University
, Mainz,
Germany
3
Department of Nuclear Medicine and Molecular Imaging, University Medical Center Groningen, University of Groningen
, Groningen,
The Netherlands
4
GE Healthcare
, Eindhoven,
The Netherlands
Background: Dopamine D2/3 receptor (D2/3R) agonist radiopharmaceuticals are considered superior to antagonists to detect dopamine release, e.g. induced by amphetamines. Agonists bind preferentially to the high-affinity state of the dopamine D2R, which has been proposed as the reason why agonists are more sensitive to detect dopamine release than antagonist radiopharmaceuticals, but this theory has been challenged. Interestingly, not all agonists similarly activate the classic cyclic adenosine mono phosphate (cAMP) and the -arrestin-2 pathway, some stimulate preferentially one of these pathways; a phenomenon called biased agonism. Because these pathways can be affected separately by pathologies or drugs (including dopamine releasers), it is important to know how agonist radiotracers act on these pathways. Therefore, we characterized the intracellular signalling of the well-known D2/3R agonist radiopharmaceuticals NPA and PHNO and of several novel D2/3R agonists. Methods: cAMP accumulation and -arrestin-2 recruitment were measured on cells expressing human D2R. Results: All tested agonists showed (almost) full agonism in both pathways. Conclusions: The tested D2/3R agonist radiopharmaceuticals did not exhibit biased agonism in vitro. Consequently, it is likely that drugs (including psychostimulants like amphetamines) and/or pathologies that influence the cAMP and/or the -arrestin-2 pathway may influence the binding of these radiopharmaceuticals.
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Background
The dopamine (DA) system plays a central role in several
neuropsychiatric disorders including Parkinson's disease
and schizophrenia. DA D2 receptor (D2R) antagonists are
used to reduce psychotic symptoms, whereas D2R agonists
are commonly used in the treatment of Parkinson's
disease. Consequently, radiopharmaceuticals targeting D2Rs
are of value to obtain insight in the pathophysiology of
these brain disorders.
The D2R is a subfamily within the superfamily of
Gprotein-coupled receptors (GPCRs) and contains several
subtypes including some in the D2-like subfamily, the D2
(splice variants D2short (D2S) and D2long (D2L) [1]), D3
and D4 receptors [2]. They primarily couple to the Gi/o
type of G-proteins to inhibit the enzyme adenylyl cyclase
in producing cyclic adenosine mono phosphate (cAMP)
[2]. Like other GPCRs, they exhibit interconvertible
high- and low-affinity states for agonists in vitro [3]. In
the high-affinity state, the receptor is coupled to the
Gprotein and this is considered to be the active state of
the receptor. In the low-affinity state, the receptor is
uncoupled and consequently inactive.
Dopamine D2/3 receptor (D2/3R) agonist
radiopharmaceuticals for positron emission tomography (PET) have
been developed successfully (e.g. 11C-NPA
(N-propylnorapomorphine) and 11C-PHNO
((+)-4-propyl-9-hydroxynaphthoxazine)), and such agonists are more sensitive in
detecting DA release in humans compared to
antagonists like 11C-raclopride [4-7]. It has been proposed that
the reason for this increased sensitivity may be that D2/3R
agonist radiopharmaceuticals bind preferentially to the
high-affinity state of the dopamine D2R, whereas
antagonists do not differentiate between the high- and
lowaffinity states [8-10]. However, this proposal depends upon
the existence of two affinity states in vivo and the results
of recent studies challenged this theory [11-13].
Originally, DA receptors (and other GPCRs) were
thought to signal intracellularly only through their
Gproteins. However, it was shown recently that besides
this (canonical) pathway, DA receptors can also exert
effects through proteins of which it was initially thought
that they regulate receptor desensitization (non-canonical
pathway, Figure 1). This cAMP-independent mechanism
involves the adaptor protein -arrestin-2. Compounds can
have distinct patterns of responses on these pathways; this
phenomenon is called biased agonism or ligand-directed
signalling [14,15].
Activation of the -arrestin-2 pathway may play a role
in the increased sensitivity to detect dopamine release
in vivo. Activation of this pathway eventually leads to
the regulation of glycogen synthase kinase 3 (GSK3), a
protein that is involved in many DA-dependent
behaviours [16]. Many drugs (antipsychotics, antidepressants,
lithium) affect this cascade, and recently compounds,
based on the aripiprazole scaffold, were discovered that
are functionally selective for the -arrestin-2 pathway
[ (...truncated)