Quantification of Metabotropic Glutamate Receptor 5 Availability With Both [11C]ABP688 and [18F]FPEB Positron Emission Tomography in the Sapap3 Knockout Mouse Model for Obsessive-Compulsive-like Behavior

BACKGROUND

This study provides a first direct comparison between positron emission tomography radioligands targeting the allosteric site of the metabotropic glutamate receptor 5 (mGluR5): [¹¹C]ABP688 and [¹⁸F]FPEB. A blocking paradigm was set up to substantiate the common binding site of both radioligands. Second, both radioligands were applied in Sapap3 knockout (KO) mice showing compulsive-like behavior characterized by a lower in vivo mGluR5 availability.

METHODS

First, wild-type mice (n = 7) received four position emission tomography/computed tomography scans: a [¹¹C]ABP688 scan, a [¹⁸F]FPEB scan, and two blocking scans using cold FPEB and cold ABP688, respectively. A second experiment compared both radioligands in wild-type (n = 7) and KO (n = 10) mice. The simplified reference tissue model was used to calculate the nondisplaceable binding potential representing the in vivo availability of mGluR5 in the brain.

RESULTS

Using cold FPEB as a blocking compound for [¹¹C]ABP688 micro-positron emission tomography and vice versa, we observed averaged global reductions in mGluR5 availability of circa 98% for [¹¹C]ABP688 and 82%-96% for [¹⁸F]FPEB. For KOs, the [¹¹C]ABP688 nondisplaceable binding potential was on average 25% lower compared with wild-type control mice (p < .0001-.001), while this was about 17% for [¹⁸F]FPEB (p < .05).

CONCLUSIONS

The current findings substantiate a common binding site and suggest a strong relationship between mGluR5 availability levels measured with both radioligands. In Sapap3 KO mice, a reduced mGluR5 availability could therefore be demonstrated with both radioligands. With [¹¹C]ABP688, higher significance levels were achieved in more brain regions. These findings suggest [¹¹C]ABP688 as a preferable radiotracer to quantify mGluR5 availability, as exemplified here in a model for compulsive-like behavior.

Kinetics and 28-day test-retest repeatability and reproducibility of [11C]UCB-J PET brain imaging

[11C]UCB-J is a novel radioligand that binds to synaptic vesicle glycoprotein 2A (SV2A). The main objective of this study was to determine the 28-day test-retest repeatability (TRT) of quantitative [11C]UCB-J brain positron emission tomography (PET) imaging in Alzheimer's disease (AD) patients and healthy controls (HCs). Nine HCs and eight AD patients underwent two 60 min dynamic [11C]UCB-J PET scans with arterial sampling with an interval of 28 days. The optimal tracer kinetic model was assessed using the Akaike criteria (AIC). Micro-/macro-parameters such as tracer delivery (K1) and volume of distribution (VT) were estimated using the optimal model. Data were also analysed for simplified reference tissue model (SRTM) with centrum semi-ovale (white matter) as reference region. Based on AIC, both 1T2k_VB and 2T4k_VB described the [11C]UCB-J kinetics equally well. Analysis showed that whole-brain grey matter TRT for VT, DVR and SRTM BPND were -2.2% ± 8.5, 0.4% ± 12.0 and -8.0% ± 10.2, averaged over all subjects. [11C]UCB-J kinetics can be well described by a 1T2k_VB model, and a 60 min scan duration was sufficient to obtain reliable estimates for both plasma input and reference tissue models. TRT for VT, DVR and BPND was <15% (1SD) averaged over all subjects and indicates adequate quantitative repeatability of [11C]UCB-J PET.

Reduced Expression of Cerebral Metabotropic Glutamate Receptor Subtype 5 in Men with Fragile X Syndrome

Glutamatergic receptor expression is mostly unknown in adults with fragile X syndrome (FXS). Favorable behavioral effects of negative allosteric modulators (NAMs) of the metabotropic glutamate receptor subtype 5 (mGluR5) in fmr1 knockout (KO) mouse models have not been confirmed in humans with FXS. Measurement of cerebral mGluR5 expression in humans with FXS exposed to NAMs might help in that effort. We used positron emission tomography (PET) to measure the mGluR5 density as a proxy of mGluR5 expression in cortical and subcortical brain regions to confirm target engagement of NAMs for mGluR5s. The density and the distribution of mGluR5 were measured in two independent samples of men with FXS (N = 9) and typical development (TD) (N = 8). We showed the feasibility of this complex study including MRI and PET, meaning that this challenging protocol can be accomplished in men with FXS with an adequate preparation. Analysis of variance of estimated mGluR5 expression showed that mGluR5 expression was significantly reduced in cortical and subcortical regions of men with FXS in contrast to age-matched men with TD.

Recovery of Decreased Metabotropic Glutamate Receptor 5 Availability in Abstinent Alcohol-Dependent Patients

Animal models of alcohol dependence and relapse demonstrate an important role of the glutamatergic system, in particular, cerebral metabotropic glutamate receptor 5 (mGluR5). ¹⁸F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile (¹⁸F-FPEB) PET has revealed that chronic alcohol use leads to decreased limbic mGluR5 availability, which was associated with less craving. Here, we tested whether the state of decreased mGluR5 availability in alcohol-dependent patients normalizes during abstinence (at 2 and 6 mo of detoxification) and whether initial mGluR5 imaging parameters can predict individual relapse. Methods: ¹⁸F-FPEB scans were performed for 16 recently detoxified alcohol-dependent patients (baseline condition), 2 mo after detoxification (n = 10), and 6 mo after detoxification (n = 8); 32 age- and sex-matched controls were included for comparison. mGluR5 availability was quantified by the ¹⁸F-FPEB total distribution volume using both voxel-by-voxel and volume-of-interest analyses. During follow-up, craving was assessed using the Desire for Alcohol Questionnaire, and alcohol consumption was assessed using the timeline follow-back method and monitored by hair ethyl glucuronide analysis. Results: During abstinence, alcohol-dependent patients showed sustained recovered mGluR5 availability in cortical and subcortical regions compared with the baseline, up to the levels observed in controls, after 6 mo in most areas except for the hippocampus, nucleus accumbens, and thalamus. Higher striatopallidal mGluR5 availability was observed at the baseline in patients who had a relapse during the 6-mo follow-up period (+25.1%). Also, normalization of striatal mGluR5 to control levels was associated with reduced craving ("desire and intention to drink" and "negative reinforcement"; r = 0.72-0.94). Conclusion: Reduced cerebral mGluR5 availability in alcohol-dependent patients recovers during abstinence and is associated with reduced craving. Higher striatal mGluR5 availability in alcohol-dependent users may be associated with long-term relapse.

Metabotropic Glutamate Receptor Subtype 5 in Alcohol-Induced Negative Affect

Allosteric modulators of metabotropic glutamate 5 receptors (mGlu5 receptors) have been identified as a promising treatment to independently alleviate both negative affective states and ethanol-seeking and intake. However, these conditions are often comorbid and might precipitate one another. Acute and protracted ethanol withdrawal can lead to negative affective states. In turn, these states are primary drivers of alcohol relapse, particularly among women. The current review synthesizes preclinical studies that have observed the role of mGlu5 receptor modulation in negative affective states following ethanol exposure. The primary behavioral assays discussed are ethanol-seeking and intake, development and extinction of ethanol-associated cues and contexts, behavioral despair, and anxiety-like activity. The work done to-date supports mGlu5 receptor modulation as a promising target for mediating negative affective states to reduce ethanol intake or prevent relapse. Limitations in interpreting these data include the lack of models that use alcohol-dependent animals, limited use of adolescent and female subjects, and a lack of comprehensive evaluations of negative affective-like behavior.

Brain mGluR5 in Shank3B-/- Mice Studied With in vivo [18F]FPEB PET Imaging and ex vivo Immunoblotting

Although several studies have found that metabotropic glutamate 5 receptor (mGluR5) may play an important role in autism spectrum disorders (ASD), the mechanisms remain unclear. Here, we used a Shank3 gene complete knockout mouse model (Shank3B^-/-) to explore the change in mGluR5 in the brain. To assess whether deletion of Shank3 in mice results in ASD-like behavior, we conducted a battery of behavioral experiments to characterize Shank3B^-/- mice, including repetitive grooming behavior tests, three-chamber tests and resident-intruder tests. Wild-type C57/BL6 and Shank3B^-/- mice underwent PET scans with [¹⁸F]FPEB, which was highly specific to mGluR5. Mouse brains were extracted post-scan, and mGluR5 protein levels were assessed by immunoblotting. The binding potential (BPnd) of mGluR5 was rich in the hippocampus, thalamus, striatum, and amygdala. More importantly, Shank3B^-/- mice showed significantly increased BPnd compared to the control mice in these brain regions. Immunoblotting revealed elevated mGluR5 levels in the hippocampus, thalamus, and amygdala but not in the striatum compared with control mice. These findings indicated that [¹⁸F]FPEB could visualize mGluR5 in the mouse brain. The deficiency of Shank3 can impair mGluR5 expression in multiple brain regions. Future work is also needed to understand the reasons for different results between in vivo PET and ex vivo immunoblotting.

Cyto- and receptor architectonic mapping of the human brain

Mapping of the human brain is more than the generation of an atlas-based parcellation of brain regions using histologic or histochemical criteria. It is the attempt to provide a topographically informed model of the structural and functional organization of the brain. To achieve this goal a multimodal atlas of the detailed microscopic and neurochemical structure of the brain must be registered to a stereotaxic reference space or brain, which also serves as reference for topographic assignment of functional data, e.g., functional magnet resonance imaging, electroencephalography, or magnetoencephalography, as well as metabolic imaging, e.g., positron emission tomography. Although classic maps remain pioneering steps, they do not match recent concepts of the functional organization in many regions, and suffer from methodic drawbacks. This chapter provides a summary of the recent status of human brain mapping, which is based on multimodal approaches integrating results of quantitative cyto- and receptor architectonic studies with focus on the cerebral cortex in a widely used reference brain. Descriptions of the methods for observer-independent and statistically testable cytoarchitectonic parcellations, quantitative multireceptor mapping, and registration to the reference brain, including the concept of probability maps and a toolbox for using the maps in functional neuroimaging studies, are provided.

Test-retest variability of [11 C]ABP688 estimates of metabotropic glutamate receptor subtype 5 availability in humans

[¹¹ C]ABP688 is a positron emission tomography (PET) radioligand that binds selectively to metabotropic glutamate type 5 receptors (mGluR5). The use of this tracer has identified receptor binding changes in clinical populations, and has been informative in drug occupancy studies. However, previous studies have found significant increases in [¹¹ C]ABP688 binding in the later scan of same-day comparisons, and estimates of test-retest reliability under consistent scanning conditions are not available. The objective of this study was to assess the variability of [¹¹ C]ABP688 binding in healthy people in scans performed at the same time of day. Two [¹¹ C]ABP688 scans were acquired in eight healthy volunteers (6 women, 2 men) using a high-resolution research tomograph (HRRT). Scans were acquired 3 weeks apart with start times between 10:00am and 1:30pm. Mean mGluR5 binding potential (BP_ND ) values were calculated across cortical, striatal and limbic brain regions. Participants reported on subjective mood state after each scan and blood samples were drawn for cortisol analysis. No significant change in BP_ND between scans was observed. Variability in BP_ND values of 11-21% was observed across regions, with the greatest change in the hippocampus and amygdala. Reliability was low to moderate. BP_ND was not statistically related to scan start time, subjective anxiety, serum cortisol levels, or menstrual phase in women. Overall, [¹¹ C]ABP688 BP_ND estimates show moderate variability in healthy people. Reliability is fair in cortical and striatal regions, and lower in limbic regions. Future research using this ligand should account for this in study design and analysis.

Cerebral dopaminergic and glutamatergic transmission relate to different subjective responses of acute alcohol intake: an in vivo multimodal imaging study

Converging preclinical evidence links extrastriatal dopamine release and glutamatergic transmission via the metabotropic glutamate receptor 5 (mGluR5) to the rewarding properties of alcohol. To date, human evidence is lacking on how and where in the brain these processes occur. Mesocorticolimbic dopamine release upon intravenous alcohol administration and mGluR5 availability were measured in 11 moderate social drinkers by single-session [¹⁸ F]fallypride and [¹⁸ F]FPEB positron emission tomography, respectively. Additionally, baseline and postalcohol glutamate and glutamine levels in the anterior cingulate cortex (ACC) were measured by using proton-magnetic resonance spectroscopy. To investigate differences in reward domains linked to both neurotransmitters, regional imaging data were related to subjective alcohol responses. Alcohol induced significant [¹⁸ F]fallypride displacement in the prefrontal cortex (PFC), temporal and parietal cortices and thalamus (P < 0.05, corrected for multiple comparisons). Dopamine release in the ACC and orbitofrontal and ventromedial PFCs were correlated with subjective 'liking' and 'wanting' effects (P < 0.05). In contrast, baseline mGluR5 availability was positively correlated with the 'high' effect of alcohol in dorsolateral, ventrolateral and ventromedial PFCs and in the medial temporal lobe, thalamus and caudate nucleus (P < 0.05). Although neither proton-magnetic resonance spectroscopy glutamate nor glutamine levels were affected by alcohol, baseline ACC glutamate levels were negatively associated with the alcohol 'liking' effect (P < 0.003). These data reveal new mechanistic understanding and differential neurobiological underpinnings of the effects of acute alcohol consumption on human behavior. Specifically, prefrontal dopamine release may encode alcohol 'liking' and 'wanting' effects in specific areas underlying value processing and motivation, whereas mGluR5 availability in distinct prefrontal-temporal-subcortical regions is more related to the alcohol 'high' effect.

Lower Limbic Metabotropic Glutamate Receptor 5 Availability in Alcohol Dependence

Animal studies suggest an important role for the metabotropic glutamate receptor subtype 5 (mGlu5) in the pathophysiology of alcohol dependence, but direct human evidence is lacking. The goal of this study was to investigate cerebral mGlu5 availability in alcohol-dependent subjects versus controls using ¹⁸F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile (¹⁸F-FPEB) PET. Methods: Dynamic 90-min ¹⁸F-FPEB scans combined with arterial blood sampling were acquired for 16 recently abstinent alcohol-dependent subjects and 32 age-matched controls. Regional mGlu5 availability was quantified by the ¹⁸F-FPEB total distribution volume using both a voxel-by-voxel and a volume-of-interest analysis with partial-volume effect correction. Alcohol consumption within the last 3 mo was assessed by questionnaires and by hair ethyl glucuronide analysis. Craving was assessed using the Desire for Alcohol Questionnaire. Results: mGlu5 availability was lower in mainly limbic regions of alcohol-dependent subjects than in controls (P < 0.05, familywise error-corrected), ranging from 14% in the posterior cingulate cortex to 36% in the caudate nucleus. Lower mGlu5 availability was associated with higher hair ethyl glucuronide levels for most regions and was related to a lower level of craving specifically in the middle frontal gyrus, cingulate cortex, and inferolateral temporal lobe. Conclusion: These findings provide human in vivo evidence that limbic mGlu5 has a role in the pathophysiology of alcohol dependence, possibly involved in a compensatory mechanism helping to reduce craving during abstinence.

News and views on in-vivo imaging of neurotransmission using PET and MRI

Molecular neuroimaging with PET is an integrated tool in psychiatry research and drug-development for as long as this modality has been available, in particular for studying neurotransmission and endogenous neurotransmitter release. Pharmacologic, behavioral and other types of challenges are currently applied to induce changes in neurochemical levels that can be inferred through their effects on changes in receptor binding and related outcome measures. Based on the availability of tracers that are sensitive for measuring neurotransmitter release these experiments have focused on the brain's dopamine system, while recent developments have extended those studies to other targets such as the serotonin or choline system. With the introduction of hybrid, truly simultaneous PET/MRI systems, in-vivo imaging of the dynamics of neuroreceptor signal transmission in the brain using PET and functional MRI (fMRI) has become possible. fMRI has the ability to provide information about the effects of receptor function that are complementary to the PET measurement. Dynamic acquisition of both PET and fMRI signals enables not only an in-vivo real-time assessment of neurotransmitter or drug binding to receptors but also dynamic receptor adaptations and receptor-specific neurotransmission. While fMRI temporal resolution is comparatively fast in relation to PET, the timescale of observable biological processes is highly dependent on the kinetics of radiotracers and study design. Overall, the combination of the specificity of PET radiotracers to neuroreceptors, fMRI signal as a functional readout and integrated study design promises to expand our understanding of the location, propagation and connections of brain activity in health and disease.

A first-in-man PET study of [18F]PSS232, a fluorinated ABP688 derivative for imaging metabotropic glutamate receptor subtype 5

PURPOSE

Non-invasive imaging of metabotropic glutamate receptor 5 (mGlu₅) in the brain using PET is of interest in e.g., anxiety, depression, and Parkinson's disease. Widespread application of the most widely used mGlu₅ tracer, [¹¹C]ABP688, is limited by the short physical half-life of carbon-11. [¹⁸F]PSS232 is a fluorinated analog with promising preclinical properties and high selectivity and specificity for mGlu₅. In this first-in-man study, we evaluated the brain uptake pattern and kinetics of [¹⁸F]PSS232 in healthy volunteers.

METHODS

[¹⁸F]PSS232 PET was performed with ten healthy male volunteers aged 20-40 years. Seven of the subjects received a bolus injection and the remainder a bolus/infusion protocol. Cerebral blood flow was determined in seven subjects using [¹⁵O]water PET. Arterial blood activity was measured using an online blood counter. Tracer kinetics were evaluated by compartment modeling and parametric maps were generated for both tracers.

RESULTS

At 90 min post-injection, 59.2 ± 11.1% of total radioactivity in plasma corresponded to intact tracer. The regional first pass extraction fraction of [¹⁸F]PSS232 ranged from 0.41 ± 0.06 to 0.55 ± 0.03 and brain distribution pattern matched that of [¹¹C]ABP688. Uptake kinetics followed a simple two-tissue compartment model. The volume of distribution of total tracer (V _T, ml/cm³) ranged from 1.18 ± 0.20 for white matter to 2.91 ± 0.51 for putamen. The respective mean distribution volume ratios (DVR) with cerebellum as the reference tissue were 0.88 ± 0.06 and 2.12 ± 0.10, respectively. The tissue/cerebellum ratios of a bolus/infusion protocol (30/70 dose ratio) were close to the DVR values.

CONCLUSIONS

Brain uptake of [¹⁸F]PSS232 matched the distribution of mGlu₅ and followed a two-tissue compartment model. The well-defined kinetics and the possibility to use reference tissue models, obviating the need for arterial blood sampling, make [¹⁸F]PSS232 a promising fluorine-18 labeled radioligand for measuring mGlu₅ density in humans.

Imaging the glutamate receptor subtypes-Much achieved, and still much to do

Functional imaging of glutamate receptors using PET imaging modality can be used to study numerous CNS disorders and also to select appropriate doses of clinically relevant glutamate-receptor-targeting candidate drugs. Great strides have been made in developing PET imaging probes for the non-invasive detection of glutamate receptors in the brain. This review highlights recent progress made towards the development of glutamatergic PET imaging agents. Focus is placed on PET imaging probes that have been labelled with either carbon-11 or fluorine-18.

In vivo variation in same-day estimates of metabotropic glutamate receptor subtype 5 binding using [11C]ABP688 and [18F]FPEB

Positron emission tomography tracers [11C]ABP688 and [18F]FPEB target the metabotropic glutamate receptor subtype 5 providing quantification of the brain glutamatergic system in vivo. Previous [11C]ABP688 positron emission tomography human test-retest studies indicate that, when performed on the same day, significant binding increases are observed; however, little deviation is reported when scans are >7 days apart. Due to the small cohorts examined previously (eight and five males, respectively), we aimed to replicate the same-day test-retest studies in a larger cohort including both males and females. Results confirmed large within-subject binding differences (ranging from -23% to 108%), suggesting that measurements are greatly affected by study design. We further investigated whether this phenomenon was specific to [11C]ABP688. Using [18F]FPEB and methodology that accounts for residual radioactivity from the test scan, four subjects were scanned twice on the same day. In these subjects, binding estimates increased between 5% and 39% between scans. Consistent with [11C]ABP688, mean absolute test-retest variability was previously reported as <12% when scans were >21 days apart. This replication study and pilot extension to [18F]FPEB suggest that observed within-day binding variation may be due to characteristics of mGluR5; for example, diurnal variation in mGluR5 may affect measurement of this receptor.

Small Molecule Receptor Ligands for PET Studies of the Central Nervous System-Focus on G Protein Coupled Receptors

G protein-coupled receptors (GPRCs) are a class of proteins that are expressed in high abundance and are responsible for numerous signal transduction pathways in the central nervous system. Consequently, alterations in GPRC function have been associated with a wide variety of neurologic and neuropsychiatric disorders. The development of PET probes for imaging GPRCs has served as a major emphasis of PET radiotracer development and PET imaging studies over the past 30 years. In this review, a basic description of the biology of G proteins and GPRCs is provided. This includes recent evidence of the existence of dimeric and multimeric species of GPRCs that have been termed "receptor mosaics," with an emphasis on the different GPRCs that form complexes with the dopamine D2 receptor. An overview of the different PET radiotracers for imaging the component GPRC within these different multimeric complexes of the D2 receptor is also provided.

Positive Association Between Limbic Metabotropic Glutamate Receptor 5 Availability and Novelty-Seeking Temperament in Humans: An 18F-FPEB PET Study

Heritable temperament traits have been linked to several neuropsychiatric illnesses, including disorders associated with metabotropic glutamate receptor 5 (mGluR5) and dopaminergic dysfunctions. Considering its modulating effect on neurotransmission, we hypothesized that cerebral mGluR5 availability is associated with temperament traits in healthy humans.

METHODS

Forty-four nonsmoking healthy volunteers (mean age ± SD, 40 ± 14 y; age range, 22-66 y; 22 women) were included in this cross-sectional investigation. Brain mGluR5 availability was quantified on both a voxel-by-voxel and a volume-of-interest basis using the total distribution volume of the radioligand ¹⁸F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile (¹⁸F-FPEB) with 90-min dynamic PET and arterial input function. Moreover, glutamate-glutamine concentrations in the anterior cingulate cortex were measured using MR spectroscopy. These measures were related to the temperament traits of the 240-item Cloninger temperament and character inventory using a regression analysis with age and sex as nuisance variables.

RESULTS

High novelty-seeking temperament was robustly associated with increased mGluR5 availability in various regions including the thalamus (r = 0.71; the strongest association), amygdala, parahippocampus, insula, anterior and posterior cingulate cortex, and several primary sensory areas (all r > 0.58; P < 0.05, corrected for familywise error). These associations were specific because no correlations were found with other temperament scales or with spectroscopic measures of glutamatergic transmission.

CONCLUSION

Overall, these data posit mGluR5 in key paralimbic areas as a strong determinant of the temperament trait novelty seeking. These data add to our understanding of how brain neurochemistry accounts for the variation in human behavior and strongly support further research on mGluR5 as a potential therapeutic target in neuropsychiatric disorders associated with abnormal novelty-seeking behaviors.