Age differences in muscarinic cholinergic receptors assayed with (+)N-[(11)C]methyl-3-piperidyl benzilate in the brains of conscious monkeys

Imaging Cholinergic Receptors in the Brain by Positron Emission Tomography

Cholinergic receptors represent a promising class of diagnostic and therapeutic targets due to their significant involvement in cognitive decline associated with neurological disorders and neurodegenerative diseases as well as cardiovascular impairment. Positron emission tomography (PET) is a noninvasive molecular imaging tool that has helped to shed light on the roles these receptors play in disease development and their diverse functions throughout the central nervous system (CNS). In recent years, there has been a notable advancement in the development of PET probes targeting cholinergic receptors. The purpose of this review is to provide a comprehensive overview of the recent progress in the development of these PET probes for cholinergic receptors with a specific focus on ligand structure, radiochemistry, and pharmacology as well as in vivo performance and applications in neuroimaging. The review covers the structural design, pharmacological properties, radiosynthesis approaches, and preclinical and clinical evaluations of current state-of-the-art PET probes for cholinergic receptors.

Imaging mitochondrial complex I activation during a vibrotactile stimulation: A PET study using [18F]BCPP-EF in the conscious monkey brain

In order to evaluate the capability of 2-tert-butyl-4-chloro-5-{6-[2-(2-[¹⁸F]fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ([¹⁸F]BCPP-EF), a novel positron emission tomography (PET) probe for mitochondrial complex I (MC-I) activity, to assess neuronal activation, an activation PET study was conducted in the conscious monkey brain with a continuous unilateral vibrotactile stimulation. PET scans with [¹⁵O]H₂O, [¹⁸F]BCPP-EF, or 2-deoxy-2-[¹⁸F]fluoroglucose ([¹⁸F]FDG) were conducted under: (1) resting conditions; (2) a continuous vibration stimulation; (3) a continuous vibration stimulation after 15-min pre-vibration; and (4) a continuous vibration stimulation after 30-min pre-vibration. The contralateral/ipsilateral ratio (CIR) in the somatosensory cortex showed significant increases in the uptake of [¹⁵O]H₂O, [¹⁸F]BCPP-EF, and [¹⁸F]FDG with the vibration stimulation. The longer pre-vibration duration induced significantly lower CIR in regional cerebral blood flow (rCBF) measured using [¹⁵O]H₂O, whereas it did not affect the CIR in [¹⁸F]BCPP-EF or the regional cerebral metabolic rate of glucose (rCMRglc) measured using [¹⁸F]FDG 30-60 min after the injection. These results suggest that the [¹⁸F]BCPP-EF response in the later phase of scans was not influenced by the increase in rCBF, indicating the capability of [¹⁸F]BCPP-EF to detect acute changes in MC-I activity induced by neuronal activation. However, the metabolic shift from glycolysis to oxidation was not observed under the stimulation used here.

Comparing α7 nicotinic acetylcholine receptor binding, amyloid-β deposition, and mitochondria complex-I function in living brain: A PET study in aged monkeys

This study was aimed to assess the correlations among α7 nicotinic acetylcholine receptor (α7-nAChR) binding, amyloid-β (Aβ) deposition, and mitochondrial complex I (MC-I) activity in the brain of aged monkeys (Macaca mulatta). Positron emission tomography (PET) measurements with [(11) C](R)-MeQAA, [(11) C]PIB, and [(18) F]BCPP-EF were conducted in monkeys in a conscious condition. [(11) C](R)-MeQAA binding was analyzed by a simplified reference tissue model to calculate nondisplaceable binding potential (BPND), [(11) C]PIB uptake was calculated by standard uptake value ratio (SUVR), and [(18) F]BCPP-EF binding was determined by Logan graphical analysis to calculate total distribution volume (VT) with arterial blood sampling. Higher brain uptake was determined in the thalamus, hippocampus, striatum, and cortical regions for [(11) C](R)-MeQAA, while being lower in the cerebellum. Significant age-related reduction of [(11) C](R)-MeQAA binding to α7-nAChR was determined only in the occipital cortex. The plot of Vt of [(18) F]BCPP-EF against BPND of [(11) C](R)-MeQAA indicated a significant negative correlation in the hippocampus and cortical regions in aged animals. Plotting of SUVR of [(11) C]PIB against BPND of [(11) C](R)-MeQAA showed a positive correlation. The in vivo binding of [(11) C](R)-MeQAA could reflect the upregulation of α7-nAChR induced by neurodegenerative damage determined by Aβ deposition as well as impaired MC-I activity in living brain.

Development of novel PET probe [¹¹C](R,R)HAPT and its stereoisomer [¹¹C](S,S)HAPT for vesicular acetylcholine transporter imaging: a PET study in conscious monkey

Carbon-11-labeled (R,R)trans-8-methyl-2-hydroxy-3-[4-[2-aminophenyl]piperizinyl]-tetralin ([(11)C](R,R)HAPT) and its stereoisomer [(11)C](S,S)HAPT were developed for imaging vesicular acetylcholine transporters (VAChTs), exclusively located in presynaptic cholinergic neurons. Both positron emission tomography (PET) probes were evaluated in the brain of conscious monkey (Macaca mulatta) using high-resolution PET. Time-activity curves (TACs) of [(11)C](R,R)HAPT peaked within 5 min after the injection in all regions except the caudate and putamen, both of which showed peaks around 20 min postinjection. The regional distribution patterns of [(11)C](R,R)HAPT determined as total distribution volume (V(t)) were highest in the putamen, high in the caudate, intermediate in the amygdala, hippocampus, and thalamus, lower in the cingulate gyrus and frontal, temporal, and occipital cortices, and lowest in the cerebellum. In contrast, the distribution and TACs of [(11)C](S,S)HAPT were homogeneous in all regions. The uptake of [(11)C](R,R)HAPT was reduced by 1 mg/kg (-)-vesamicol, a specific VAChT antagonist, in all regions except the cerebellum, but not by 0.1 mg/kg SA4503, a specific sigma-1 receptor agonist. These results well reflect the in vitro affinity assessments using rat cerebral membranes. They also demonstrate that [(11)C](R,R)HAPT is a potential PET probe for noninvasive and quantitative imaging of VAChT in the living brain.

Muscarinic receptor occupancy and cognitive impairment: a PET study with [11C](+)3-MPB and scopolamine in conscious monkeys

The muscarinic cholinergic receptor (mAChR) antagonist scopolamine was used to induce transient cognitive impairment in monkeys trained in a delayed matching to sample task. The temporal relationship between the occupancy level of central mAChRs and cognitive impairment was determined. Three conscious monkeys (Macaca mulatta) were subjected to positron emission tomography (PET) scans with the mAChR radioligand N-[(11)C]methyl-3-piperidyl benzilate ([(11)C](+)3-MPB). The scan sequence was pre-, 2, 6, 24, and 48 h post-intramuscular administration of scopolamine in doses of 0.01 and 0.03 mg/kg. Occupancy levels of mAChR were maximal 2 h post-scopolamine in cortical regions innervated primarily by the basal forebrain, thalamus, and brainstem, showing that mAChR occupancy levels were 43-59 and 65-89% in doses of 0.01 and 0.03 mg/kg, respectively. In addition, dose-dependent impairment of working memory performance was measured 2 h after scopolamine. A positive correlation between the mAChR occupancy and cognitive impairment 2 and 6 h post-scopolamine was the greatest in the brainstem (P<0.00001). Although cognitive impairment was not observed 24 h post-scopolamine, sustained mAChR occupancy (11-24%) was found with both doses in the basal forebrain and thalamus, but not in the brainstem. These results indicate that a significant degree of mAChRs occupancy is needed to produce cognitive impairment by scopolamine. Furthermore, the importance of the brainstem cholinergic system in working memory in monkey is described.

Validation of reference tissue model of PET ligand [¹¹C]+3-MPB for the muscarinic cholinergic receptor in the living brain of conscious monkey

N-[¹¹C]methyl-3-piperidyl benzilate ([¹¹C]+3-MPB) was developed as a positron emission tomography (PET) ligand for muscarinic cholinergic receptor (mAChR). The aim of the present study was to validate a Logan reference tissue method as an analytical method for in vivo binding of [¹¹C]+3-MPB to mAChR. Seven monkeys (Macaca mulatta) underwent [¹¹C]+3-MPB PET scans with an arterial blood sampling. Logan plot with arterial input function (Logan arterial input method) was performed to determine the binding potential (BP(ND)). The BP(ND) was also determined by Logan plot with the cerebellum as the reference region (Logan reference tissue method). BP(ND) values determined by Logan arterial input method and Logan reference tissue method showed a significant linear relationship. The present study suggests that the cerebellum is a suitable reference region for quantification of mAChR in the living brain with [¹¹C]+3-MPB and PET.

Differential effects of stress on [¹¹C]raclopride and [¹¹C]MNPA binding to striatal D₂/D₃ dopamine receptors: a PET study in conscious monkeys

It has been reported that stress and facilitation of dopamine neuronal system are closely related. In the present study, the effects of stress on the binding of antagonist-based [¹¹C]raclopride and agonist-based (R)-2-CH3O-N-n- propylnorapomorphine ([¹¹C]MNPA) to D₂/D₃ receptors were evaluated in the striatum of conscious monkey brain. The stress state assessed from plasma cortisol level was negatively correlated with [¹¹C]raclopride binding as expected. It was noteworthy that [¹¹C]MNPA binding exhibited a positive correlation with stress state; thus, the animals with higher cortisol levels showed higher binding to D₂/D₃ receptors.

Binding kinetics of11c-n-methyl piperidyl benzilate (11c-nmpb) in a rhesus monkey brain using the cerebellum as a reference region

The binding kinetics of 11C-N-methyl piperidyl benzilate (11C-NMPB) in rhesus monkey brain were studied using animal positron emission tomography (PET) (SHR2000). This study is intended to assess the validity of the method using the cerebellum as a reference region, and to evaluate the effects of anesthesia on 11C-NMPB binding. Two monkeys, anesthetized with ketamine, received intravenous 11C-NMPB alone (370-760 MBq, < 1 microg/kg) or mixed with varying doses of nonradioactive NMPB (3 microg/kg, 10 microg/kg, 30 microg/kg) and were subjected to PET scans for 60 minutes. Regions of interest (ROI) were drawn on reconstructed PET images and a time-activity curve was obtained for each region. 11C-NMPB accumulated densely in the striatum and cerebral cortex with time. In contrast, the tracer accumulation significantly decreased with increased doses of nonradioactive NMPB. In the cerebellum, on the other hand, the accumulation of 11C-NMPB remained low and the tracer was slowly eliminated from the brain following the injection. 11C-NMPB binding in the cerebellum was barely affected by the increased dose of nonradioactive NMPB. We thus concluded that the specific 11C-NMPB binding was negligible in the cerebellum, and performed simplified evaluation of 11C-NMPB binding in each brain region by a graphical method using the cerebellum as a reference region. PET was conducted 26 times, in total both in ketamine-anesthetized and awake monkeys (n = 3 each). Measurements of 11C-NMPB binding showed good run-to-run reproducibility within individual animals. When 11C-NMPB binding was compared between ketamine-treated and awake animals, a significant increase in 11C-NMPB binding was observed in the striatum but not in other brain regions of ketamine-treated animals.

[Pre-clinical evaluation of effects of acetylcholinesterase inhibition on the cerebral cholinergic neuronal system and cognitive function: PET study in conscious monkeys]

The present review described the effects of acetylcholinesterase (AChE) inhibition on the cerebral cholinergic neuronal system in the conscious monkey brains with PET. Somatosensory stimulation induced a regional cerebral blood flow (rCBF) response, revealed with [(15)O]H(2)O, in the contralateral somatosensory cortex. Scopolamine resulted in an abolished rCBF response to stimulation, and this abolished rCBF response was recovered by physostigmine, donepezil, and tacrine. Donepezil suppressed AChE activity, analyzed by [(11)C]MP4A, in all cortical regions in a dose-dependent manner. AChE inhibition by donepezil resulted in a dose-dependent increase in acetylcholine levels in the prefrontal cortex as measured by microdialysis. Binding of [(11)C](+)3-PPB to cortical muscarinic receptors was reduced by donepezil, probably in a competitive inhibition manner. Aged monkeys showed less reduction of [(11)C](+)3-PPB binding than young animals. As evaluated by an oculomotor delayed response task, aged monkeys showed impaired working memory performance compared to young monkeys, and the impaired performance was partly improved by the administration of donepezil, due to the facilitation of the cholinergic neuronal system by AChE inhibition by donepezil. These results demonstrated that PET imaging with specifically labeled compounds in combination with microdialysis and a behavioral cognition task could be a useful tool for pre-clinical evaluation of novel drugs.

Normal genetic variation, cognition, and aging

This article reviews the modulation of cognitive function by normal genetic variation. Although the heritability of "g" is well established, the genes that modulate specific cognitive functions are largely unidentified. Application of the allelic association approach to individual differences in cognition has begun to reveal the effects of single nucleotide polymorphisms on specific and general cognitive functions. This article proposes a framework for relating genotype to cognitive phenotype by considering the effect of genetic variation on the protein product of specific genes within the context of the neural basis of particular cognitive domains. Specificity of effects is considered, from genes controlling part of one receptor type to genes controlling agents of neuronal repair, and evidence is reviewed of cognitive modulation by polymorphisms in dopaminergic and cholinergic receptor genes, dopaminergic enzyme genes, and neurotrophic genes. Although allelic variation in certain genes can be reliably linked to cognition--specifically to components of attention, working memory, and executive function in healthy adults--the specificity, generality, and replicability of the effects are not fully known.

Effects of acute acetylcholinesterase inhibition on the cerebral cholinergic neuronal system and cognitive function: Functional imaging of the conscious monkey brain using animal PET in combination with microdialysis

This study demonstrated the effects of acute acetylcholinesterase (AChE) inhibition by donepezil (Aricept) on the cerebral cholinergic neuronal system in the brains of young (5.2 +/- 1.1 years old) and aged (20.3 +/- 2.6 years old) monkeys (Macaca mulatta) in the conscious state. Donepezil at doses of 50 and 250 microg/kg suppressed AChE activity, analyzed by metabolic rate (k(3)) of N-[(11)C]methyl-4-piperidyl acetate ([(11)C]MP4A), in all cortical regions in a dose-dependent manner in both age groups. However, the suppression degree was more marked in young than in aged monkeys. AChE inhibition by donepezil resulted in a dose-dependent increase in acetylcholine levels in the prefrontal cortex of young animals as measured by microdialysis. Binding of (+)N-[(11)C]propyl-3-piperidyl benzilate ([(11)C](+)3-PPB) to cortical muscarinic receptors was reduced by donepezil, probably in a competitive inhibition manner. Aged monkeys showed less reduction of [(11)C](+)3-PPB binding than young animals. As evaluated by an oculomotor delayed response task, aged monkeys showed impaired working memory performance compared to young monkeys, and the impaired performance was partly improved by the administration of donepezil, due to the facilitation of the cholinergic neuronal system by AChE inhibition. These results demonstrate that the PET imaging technique with specific labeled compounds in combination with microdialysis and a behavioral cognition task could be a useful method to clarify the mechanism of drugs in the living brains of experimental animals.

An increase of sigma receptors in the aged monkey brain

We evaluated in vivo the effect of aging on the sigma(1) receptors in the monkey brain by the quantitative analysis of the binding of [11C]SA4503 to sigma(1) receptors with positron emission tomography. Based on a three-compartment model, the influx rate constant K(1) of [11C]SA4503 from plasma to brain across the blood-brain barrier in all 10 regions investigated became smaller in the aged monkeys (20-28 years old, n=5) than in the young adult monkeys (4-8 years old, n=5), but the reduction was not significant due to the individual differences. On the other hand, the binding potential, which was calculated as the ratio of the association rate constant k(3) to the dissociation rate constant k(4) for the binding of [11C]SA4503 to sigma(1) receptors in the brain, significantly increased in nine of the brain regions of the aged monkeys to the 160-210% levels of the young monkeys. We concluded that the sigma(1) receptor binding sites increased in the aging process of the monkey brain.

Age-related changes in the striatal dopaminergic system in the living brain: a multiparametric PET study in conscious monkeys

In the present study, age-related changes in the striatal dopaminergic system were examined in the living brains of conscious young (6.2 +/- 1.5 years old) and aged (20.2 +/- 2.6 years old) monkeys (Macaca mulatta) using positron emission tomography (PET). L-[beta-(11)C]DOPA and [(11)C]beta-CFT were applied to determine dopamine presynaptic functions such as synthesis rate and transporter (DAT) availability, respectively. Striatal dopamine D(1)- (D(1)R) and D(2)-like receptor (D(2)R) binding were measured with [(11)C]SCH23390 and [(11)C]raclopride, respectively. Although the markers of presynaptic terminals showed parallel age-related declines, the reduction of dopamine synthesis rate measured with L-[beta-(11)C]DOPA was slightly smaller than that of DAT determined with [(11)C]beta-CFT. The binding of [(11)C]raclopride to D(2)R in vivo was significantly reduced with aging, while that of [(11)C]SCH23390 to D(1)R showed no such marked age-related reduction. When the DAT inhibitor GBR12909 (0.5 and 5 mg/kg) was administered, DAT availability, dopamine synthesis, and D(2)R binding were significantly decreased in a dose-dependent manner in both age groups; however, the degrees of the decreases in these parameters were significantly higher in young rather than in aged animals. Dopamine concentration in the striatal extracellular fluid (ECF), as measured by microdialysis, was increased by administration of GBR12909 in a dose-dependent manner and the degree of the increase in dopamine level decreased with age. These results demonstrate that age-related changes of dopamine neuronal functions were not limited to the resting condition but were also seen in the functional responses to the neurotransmitter modulation.

Age differences in phosphodiesterase type-IV and its functional response to dopamine D1 receptor modulation in the living brain: a PET study in conscious monkeys

The present study demonstrated the age-related changes in the striatal dopamine D1 receptor binding and its related cAMP second-messenger system in the living brains of conscious young (6.4 +/- 1.8 years old) and aged (19.5 +/- 3.3 years old) monkeys (Macaca mulatta) using positron emission tomography (PET). For quantitative analysis of D1 receptors, [11C]SCH23390 was used and phosphodiesterase type-IV (PDE-IV) activity, as an index of cAMP system, was estimated by two scans with R- and S-[11C]rolipram. Significant age-related decreases in D1 receptor binding were observed in the striatum and frontal cortex. Analysis of uptake of R- and S-[11C]rolipram indicated age-related decreases in PDE-IV activity showing 22.0 and 25.2% decreases in the striatum and frontal cortex, respectively, while no significant changes were observed in the cerebellum. With systemic preadministration of the dopamine D1 receptor antagonist SCH23390 (0.2, 0.6, and 2 mg/kg), the PDE-IV activities in the striatum and frontal cortex were dose-dependently suppressed in both age groups. However, the degree of suppression by SCH23390 was more marked in young than in aged monkeys. These results demonstrate that the striatal cAMP second-messenger system activity as well as its functional response to dopamine D1 antagonist showed age-related impairment in the brain.

Effects of aging on 5-HT(1A) receptors and their functional response to 5-HT(1a) agonist in the living brain: PET study with [carbonyl-(11)C]WAY-100635 in conscious monkeys

Age-related changes in the serotonin 5-HT(1A) receptors in the living brains of conscious young (5.9 +/- 1.8 years old) and aged (19.0 +/- 3.3 years old) monkeys (Macaca mulatta) were evaluated by [carbonyl-(11)C]WAY-100635 and high-resolution positron emission tomography (PET). The regional distribution pattern of [carbonyl-(11)C]WAY-100635 at 60-91 min postinjection was the highest in the cingulate gyrus and hippocampus, high in the frontal and temporal cortices, lower in the occipital cortex, striatum, thalamus, and raphe nuclei, and lowest in the cerebellum in both young and aged monkeys. Graphical Logan plot analysis with metabolite-corrected plasma radioactivity as an input function into the brain was applied to evaluate 5-HT(1A) receptor binding in vivo. Significant age-related decreases in 5-HT(1A) receptor binding were observed only in the frontal and temporal cortices. In the hippocampus, although 5-HT(1A) receptor binding indicated no significant age-related changes, it showed an inverse correlation with individual cortisol levels in plasma. When the 5-HT(1A) receptor agonist 8-OH-DPAT was administered intravenously at a dose of 0.1, 0.3, or 1 mg/kg 30 min after tracer injection, binding of [carbonyl-(11)C]WAY-100635 was displaced in both age groups in a dose-dependent manner. However, the degree of displacement was more marked in young than in aged monkeys. These observations demonstrated the usefulness of [carbonyl-(11)C]WAY-100635 as an indicator of the age-related changes in cortical 5-HT(1A) receptors measured noninvasively by PET. In addition, these observations suggested that the age-related impairment of 5-HT(1A) receptor responses to 8-OH-DPAT might be related to the reduced efficacy of antidepressant therapy in elderly patients with depression.

Age-related changes in muscarinic cholinergic receptors in the living brain: a PET study using N-[11C]methyl-4-piperidyl benzilate combined with cerebral blood flow measurement in conscious monkeys

The effects of changes in regional cerebral blood flow (rCBF) with aging on muscarinic cholinergic receptor binding were evaluated with [15O]H(2)O and N-[11C]methyl-4-piperidyl benzilate (4-MPB) in the living brains of young (5.9+/-1.8 years old) and aged (19.0+/-3.3 years old) monkeys (Macaca mulatta) in the conscious state using high-resolution positron emission tomography (PET). For quantitative analysis of receptor binding in vivo with [11C]4-MPB, metabolite-corrected arterial plasma radioactivity curves were obtained as an input function into the brain, and graphical Patlak plot analysis was applied. In addition, two-compartment model analysis using the radioactivity curve in the cerebellum as an input function (reference analysis) was also applied to determine the distribution volume (DV=K(1)/k(2)') for [11C]4-MPB. With metabolite-corrected arterial input, Patlak plot analysis of [11C]4-MPB indicated a regionally specific decrease in muscarinic cholinergic receptor binding in vivo in the frontal and temporal cortices as well as the striatum in aged compared with young animals, showing no correlation with the degree of reduced rCBF. In contrast, on the reference analysis with cerebellar input of [11C]4-MPB, all regions assayed except the pons showed a significant age-related decrease of DV, and the degree of reduction of DV was correlated with that of rCBF. These results demonstrated the usefulness of kinetic analysis of [11C]4-MPB with metabolite-corrected arterial input, not with reference region's input, as an indicator of the aging process of cortical muscarinic cholinergic receptors in vivo measured by PET with less blood flow dependency.