Simplified quantification of nicotinic receptors with 2[18F]F-A-85380 PET
Introduction
Neuronal nicotinic acetylcholine receptors (nAChRs) in mammals are found in the central and peripheral nervous system, neuromuscular junctions and adrenal glands. Cerebral nAChRs belong to the superfamily of ligand-gated cation channels and are composed of protein subunits (α and β) associated in homologous or heterologous pentameric channels, permeable to sodium (Na+), potassium (K+) and calcium (Ca2+) [1]. The majority of cerebral nAChRs are of the α4β2 subtype [1], [2], characterized by their high affinity for (−) nicotine, labeled by agonists such as 3H-acetylcholine, 3H-nicotine, 3H-cytisine and 3H-epibatidine [3], and low affinity for 125I-α-bungarotoxin [4] that binds with high affinity to α7 nAChR subtype [5].
Located on pre-, post- and extrasynaptic sites where they exert their modulatory actions, nAChRs are involved in a series of crucial physiological higher cognitive functions such as learning and memory, cognition and arousal [5], [6]. Cerebral nAChRs play multiple roles in signal transduction including fast synaptic transmission [7], auto-axonic transmission [8] and modulation of presynaptic transmitter release, including the secretion of both excitatory and inhibitory transmitters such as acetylcholine, dopamine, gamma-amino butyric acid, glutamate, norepinephrine and serotonin [9], [10].
Postmortem studies have demonstrated a decline in nAChRs with aging in the human brain [3], [5], [6], particularly frontal and temporal cortices for both [3H]-epibatidine (79% and 84%, respectively) and [3H]-nicotine binding (82% and 79% respectively) between the ages of 56 and 85 years [3], with some data suggesting a sparing of the thalamus [11], [12].
Cerebral nAChRs have been implicated in the pathophysiology and/or treatment strategies of Alzheimer's [13], [14], [15], [16] and Parkinson's diseases [17], epilepsy [18], schizophrenia [19], Tourette's syndrome [20], anxiety [21], depression [22] and nicotine dependence [23].
There is a selective loss of both high-affinity nicotinic binding sites and the α4 subunit from the cerebral cortex in AD, correlating with the severity of dementia [5], [6]. Studies have revealed deficits of nAChRs (between 25% and 75%) in temporal and frontal cortices and hippocampal regions of AD patients by in vitro binding assays [6]. In vivo assessment of nAChR in AD patients with [11C]-nicotine and PET showed a reduction in binding sites [2], [24]; however, [11C]-nicotine characteristics as a ligand for quantification of nAChRs in vivo via PET are less than ideal. This is mainly due to the fact that its uptake is mediated principally by regional cerebral blood flow [25]. The discovery of epibatidine, a potent nAChR agonist [26], stimulated development of nAChR radioligands with more favorable properties for PET studies. These ligands displayed superb binding properties toward the α4β2 subtype [27], but had a narrow safety margin that severely limited their use in humans [28].
The recently developed azetidine derivative of the 3-pyridyl ethers A85380 (3-[2(S)-2-azetidinylmethoxyl]pyridine), a weak agonist with high affinity for the α4β2 subtype nAChR [29], showed suitable properties for imaging nAChRs in vivo with both PET and single-photon emission tomography (SPECT) [30], [31]. Radiolabelling of A85380 with 18F (t1/2=109 min) was achieved without alteration of its receptor binding characteristics [31], [32]. The first human PET studies with 2[18F]F-A-85380 showed that maximal radioactivity uptake (2.5% ID) in the human brain was reached between 50 and 80 min postinjection [33], [34]. Tissue-to-plasma ratios for 2[18F]F-A-85380 stabilised very slowly for most regions except the thalamus, where no equilibrium was reached over a 4-h scanning period [35]. Positron emission tomography and SPECT studies in both primates and humans validated the use of compartmental and graphical approaches for the quantification of nAChRs with 2[18F]F-A-85380 or its analogs [36].
In this study, we aimed to develop a simplified method for quantifying nAChRs in humans using 2[18F]F-A-85380 and PET suitable for research and clinical applications, particularly in elderly or cognitively impaired subjects who may not be able to tolerate either a prolonged scan and/or arterial blood sampling.
Section snippets
Demographics
Ten normal volunteers (four females, six males) aged 63.40±9.22 years (range 49–76) were recruited from the community by advertisement. Written informed consent for participation in this study was obtained prior to the scan. Approval was obtained for the study from the Austin Health Human Research Ethics Committee, Austin Radiation Sub-committee and the Victorian Department of Human Services Radiation Safety Unit.
Subjects had no history of progressive cognitive decline, a Mini-Mental State
Regional brain kinetics
The brain uptake of 2[18F]F-A-85380 was consistent with the known distribution of nAChRs [12]. Higher uptake and slower clearance of the radioligand in the thalamus compared with the other regions were clearly evident. In most subjects, 2[18F]F-A-85380 uptake in the thalamus approached a plateau around 90 min postinjection (93±15.8 min). Cortical regions reached maximal uptake approximately at 60 min postinjection (61±9.4 min), followed by a slow washout, while the cerebellum peaked at around
Discussion and conclusion
The binding of 2[18F]F-A-85380 as visualized by PET showed the highest uptake in the thalamus, modest uptake in the cerebellum and cortex, and lowest in white matter. Though consistent with the known distribution of nAChR as determined by in vitro and in vivo studies, the uptake in the cerebellum and white matter was greater than expected from previous in vitro human and animal studies of nAChR binding with agents such as [3H]-nicotine and [3H]-epibatidine [41]. While nAChRs are not found in
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