Molecular Imaging of mGluR5 Availability with [11C]ABP68 in Glutaminase Heterozygous Mice.

Many PET tracers enable determination of fluctuations in neurotransmitter release, yet glutamate specifically can not be visualized in a noninvasive manner. Several studies point to the possibility of visualizing fluctuations in glutamate release by changes in affinity of the mGluR5 radioligand [11C]ABP688. These studies use pharmacological challenges to alter glutamate levels, and so probe release, but have not measured chronic alterations in receptor occupancy due to altered neurotransmission relevant to chronic neuropsychiatric disorders or their treatment. In this regard, the GLS1 heterozygous mouse has known reductions in activity of the glutamate-synthetic enzyme glutaminase, brain glutamate levels and release. We imaged this model to elucidate glutamatergic systems. Dynamic [11C]ABP688 microPET scans were performed for mGluR5. Western blot was used as an ex vivo validation. No significant differences were found in BPND between WT and GLS1 Hets. SPM showed voxel-wise increased in BPND in GLS1 Hets compared to WT consistent with lower synaptic glutamate. This was not due to alterations in mGluR5 levels, as western blot results showed lower mGluR5 levels in GLS1 Hets. We conclude that because of the chronic glutaminase deficiency and subsequent decrease in glutamate, the mGluR5 protein levels are lowered. Due to these decreased endogenous glutamate levels, however, there is increased [11C]ABP688 binding to the allosteric site in selected regions. We speculate that lower endogenous glutamate leads to less conformational change to the receptors, and thus higher availability of the binding site. The lower mGluR5 levels, however, lessen [11C]ABP688 binding in GLS1 Hets, in part masking the increase in binding due to diminished endogenous glutamate levels as confirmed with voxel-wise analysis.

State-associated changes in longitudinal [18F]-PBR111 TSPO PET imaging of psychosis patients: Evidence for the accelerated ageing hypothesis?

OBJECTIVE: To determine whether state-associated changes in microglial activity, measured with translocator-protein positron emission tomography (TSPO PET), can be identified in psychosis patients through longitudinal evaluation of their regional tracer uptake over the clinical course from acute psychosis to post-treatment follow-up, and comparison to healthy controls. We also evaluated the relation between tracer uptake, clinical symptoms and peripheral immunological markers. METHOD: Second-generation radioligand [18F]-PBR111 TSPO PET-CT was used for longitudinal dynamic imaging in 14 male psychosis patients and 17 male age-matched healthy control subjects. Patients were first scanned during an acute psychotic episode followed by a second scan after treatment. Prior genotyping of subjects for the rs6917 polymorphism distinguished high- and mixed-affinity binders. The main outcome was regional volume of distribution (VT), representing TSPO binding. Plasma concentrations of CRP, cytokines and kynurenines were measured at each timepoint. RESULTS: We found a significant three-way interaction between time of scan, age and cohort (cortical grey matter F6.50, p.020). Age-dependent differences in VT existed between cohorts during the psychotic state, but not at follow-up. Patients' relative change in VT over time correlated with age (cortical grey matter Pearson's r.574). PANSS positive subscale scores correlated with regional VT during psychosis (cortical grey matter r.767). Plasma CRP and quinolinic acid were independently associated with lower VT. CONCLUSIONS: We identified a differential age-dependent pattern of TSPO binding from psychosis to follow-up in our cohort of male psychosis patients. We recommend future TSPO PET studies in psychosis patients to differentiate between clinical states and consider potential age-related effects.

Acute Ketamine Infusion in Rat Does Not Affect In Vivo [11C]ABP688 Binding to Metabotropic Glutamate Receptor Subtype 5.

Detecting changes in metabotropic glutamate receptor 5 (mGluR5) availability through molecular imaging with the positron emission tomography (PET) tracer [11C]ABP688 is valuable for studying dysfunctional glutamate transmission associated with neuropsychiatric disorders. Using an infusion protocol in rats, we visualized the acute effect of subanesthetic doses of ketamine on mGluR5 in rat brain. Ketamine is an N-methyl-D-aspartate (NMDA) receptor antagonist known to increase glutamate release. Imaging was performed with a high-affinity PET ligand [11C]ABP688, a negative allosteric modulator of mGluR5. Binding did not change significantly from baseline to ketamine in any region, thereby confirming previous literature with other NMDA receptor antagonists in rodents. Hence, in rats, we could not reproduce the findings in a human setup showing significant decreases in the [11C]ABP688 binding after a ketamine bolus followed by ketamine infusion. Species differences may have contributed to the different findings in the present study of rats. In conclusion, we could not confirm in rats that endogenous glutamate increases by ketamine infusion are reflected in [11C]ABP688 binding decreases as was previously shown for humans.

Early classification of Alzheimer's disease phenotype based on hippocampal electrophysiology in the TgF344-AD rat model.

The hippocampus plays a vital role in navigation, learning, and memory, and is affected in Alzheimer's disease (AD). This study investigated the classification of AD-transgenic rats versus wild-type littermates using electrophysiological activity recorded from the hippocampus at an early, presymptomatic stage of the disease (6 months old) in the TgF344-AD rat model. The recorded signals were filtered into low frequency (LFP) and high frequency (spiking activity) signals, and machine learning classifiers were employed to identify the rat genotype (TG vs. WT). By analyzing specific frequency bands in the low frequency signals and calculating distance metrics between spike trains in the high frequency signals, accurate classification was achieved. Gamma band power emerged as a valuable signal for classification, and combining information from both low and high frequency signals improved the accuracy further. These findings provide valuable insights into the early stage effects of AD on different regions of the hippocampus.

Combining magnetic resonance imaging with readout and/or perturbation of neural activity in animal models: Advantages and pitfalls.

One of the main challenges in brain research is to link all aspects of brain function: on a cellular, systemic, and functional level. Multimodal neuroimaging methodology provides a continuously evolving platform. Being able to combine calcium imaging, optogenetics, electrophysiology, chemogenetics, and functional magnetic resonance imaging (fMRI) as part of the numerous efforts on brain functional mapping, we have a unique opportunity to better understand brain function. This review will focus on the developments in application of these tools within fMRI studies and highlight the challenges and choices neurosciences face when designing multimodal experiments.

Glutaminase activity in GLS1 Het mouse brain compared to putative pharmacological inhibition by ebselen using ex vivo MRS.

Glutaminase mediates the recycling of neurotransmitter glutamate, supporting most excitatory neurotransmission in the mammalian central nervous system. A constitutive heterozygous reduction in GLS1 engenders in mice a model of schizophrenia resilience and associated increases in Gln, reductions in Glu and activity-dependent attenuation of excitatory synaptic transmission. Hippocampal brain slices from GLS1 heterozygous mice metabolize less Gln to Glu. Whether glutaminase activity is diminished in the intact brain in GLS1 heterozygous mice has not been assessed, nor the regional impact. Moreover, it is not known whether pharmacological inhibition would mimic the genetic reduction. We addressed this using magnetic resonance spectroscopy to assess amino acid content and 13C-acetate loading to assess glutaminase activity, in multiple brain regions. Glutaminase activity was reduced significantly in the hippocampus of GLS1 heterozygous mice, while acute treatment with the putative glutaminase inhibitor ebselen did not impact glutaminase activity, but did significantly increase GABA. This approach identifies a molecular imaging strategy for testing target engagement by comparing genetic and pharmacological inhibition, across brain regions.

Noninvasive Relative Quantification of [11C]ABP688 PET Imaging in Mice Versus an Input Function Measured Over an Arteriovenous Shunt.

Impairment of the metabotropic glutamate receptor 5 (mGluR5) has been implicated with various neurologic disorders. Although mGluR5 density can be quantified with the PET radiotracer [11C]ABP688, the methods for reproducible quantification of [11C]ABP688 PET imaging in mice have not been thoroughly investigated yet. Thus, this study aimed to assess and validate cerebellum as reference region for simplified reference tissue model (SRTM), investigate the feasibility of a noninvasive cardiac image-derived input function (IDIF) for relative quantification, to validate the use of a PET template instead of an MRI template for spatial normalization, and to determine the reproducibility and within-subject variability of [11C]ABP688 PET imaging in mice. Blocking with the mGluR5 antagonist MPEP resulted in a reduction of [11C]ABP688 binding of 41% in striatum (p < 0.0001), while no significant effect could be found in cerebellum (-4.8%, p > 0.99) indicating cerebellum as suitable reference region for mice. DVR-1 calculated using a noninvasive IDIF and an arteriovenous input function correlated significantly when considering the cerebellum as the reference region (striatum: DVR-1, r = 0.978, p < 0.0001). Additionally, strong correlations between binding potential calculated from SRTM (BPND) with DVR-1 based on IDIF (striatum: r = 0.980, p < 0.0001) and AV shunt (striatum: r = 0.987, p < 0.0001). BPND displayed higher discrimination power than VT values in determining differences between wild-types and heterozygous Q175 mice, an animal model of Huntington's disease. Furthermore, we showed high agreement between PET- and MRI-based spatial normalization approaches (striatum: r = 0.989, p < 0.0001). Finally, both spatial normalization approaches did not reveal any significant bias between test-retest scans, with a relative difference below 5%. This study indicates that noninvasive quantification of [11C]ABP688 PET imaging is reproducible and cerebellum can be used as reference region in mice.

MR-based spatial normalization improves [18F]MNI-659 PET regional quantification and detectability of disease effect in the Q175 mouse model of Huntington's disease.

The positron emission tomography (PET) tracer [18F]MNI-659, selective for phosphodiesterase 10A (PDE10A), is a promising tool to assess an early biomarker for Huntington's disease (HD). In this study we investigated [18F]MNI-659 uptake in the Q175 mouse model of HD. Given the focal striatal distribution of PDE10A as well as the striatal atrophy occurring in HD, the spatial normalization approach applied during the processing could sensibly affect the accuracy of the regional quantification. We compared the use of a magnetic resonance images (MRI) template based on individual MRI over a PET and CT templates for regional quantification and spatial normalization of [18F]MNI-659 PET images. We performed [18F]MNI-659 PET imaging in six months old heterozygous (HET) Q175 mice and wild-type (WT) littermates, followed by X-ray computed tomography (CT) scan. In the same week, individual T2-weighted MRI were acquired. Spatial normalization and regional quantification of the PET/CT images was performed on MRI, [18F]MNI-659 PET, or CT template and compared to binding potential (BPND) using volumes manually delineated on the individual MR images. Striatal volume was significantly reduced in HET mice (-7.7%, p<0.0001) compared to WT littermates. [18F]MNI-659 BPND in striatum of HET animals was significantly reduced (p<0.0001) when compared to WT littermates using all three templates. However, BPND values were significantly higher for HET mice using the PET template compared to the MRI and CT ones (p<0.0001), with an overestimation at lower activities. On the other hand, the CT template spatial normalization introduced larger variability reducing the effect size. The PET and CT template-based approaches resulted in a lower accuracy in BPND quantification with consequent decrease in the detectability of disease effect. This study demonstrates that for [18F]MNI-659 brain PET imaging in mice the use of an MRI-based spatial normalization is recommended to achieve accurate quantification and fully exploit the detectability of disease effect.

18F-PBR111 PET Imaging in Healthy Controls and Schizophrenia: Test-Retest Reproducibility and Quantification of Neuroinflammation.

Activated microglia express the translocator protein (TSPO) on the outer mitochondrial membrane. 18F-PBR111 is a second-generation PET ligand that specifically binds the TSPO, allowing in vivo visualization and quantification of neuroinflammation. The aim of this study was to evaluate whether the test-retest variability of 18F-PBR111 in healthy controls is acceptable to detect a psychosis-associated neuroinflammatory signal in schizophrenia. Methods: Dynamic 90-min 18F-PBR111 scans were obtained in 17 healthy male controls (HCs) and 11 male schizophrenia patients (SPs) during a psychotic episode. Prior genotyping for the rs6917 polymorphism distinguished high-affinity binders (HABs) and mixed-affinity binders (MABs). Total volume of distribution (VT) was determined from 2-tissue-compartment modeling with vascular trapping and a metabolite-corrected plasma input function. A subgroup of HCs (n = 12; 4 HABs and 8 MABs) was scanned twice to assess absolute test-retest variability and intraclass correlation coefficients of the regional VT values. Differences in TSPO binding between HC and SP were assessed using mixed model analysis adjusting for age, genotype, and age*cohort. The effect of using different scan durations (VT-60 min versus VT-90 min) was determined based on Pearson r. Data were mean ± SD. Results: Mean absolute variability in VT ranged from 16% ± 14% (19% ± 20% HAB; 15% ± 11% MAB) in the cortical gray matter to 22% ± 15% (23% ± 15% HAB; 22% ± 16% MAB) in the hippocampus. Intraclass correlation coefficients were consistently between 0.64 and 0.82 for all tested regions. TSPO binding in SP compared with HC depended on age (cohort*age: P < 0.05) and was increased by +14% ± 4% over the regions. There was a significant effect of genotype on TSPO binding, and VT of HABs was 31% ± 8% (HC: 17% ± 5%, SP: 61% ± 14%) higher than MABs. Across all clinical groups, VT-60 min and VT-90 min were strongly correlated (r > 0.7, P < 0.0001). Conclusion:18F-PBR111 can be used for monitoring of TSPO binding, as shown by medium test-retest variability and reliability of VT in HCs. Microglial activation is present in SPs depending on age and needs to be adjusted for genotype.

Longitudinal Characterization of mGluR5 Using 11C-ABP688 PET Imaging in the Q175 Mouse Model of Huntington Disease.

Metabotropic glutamate receptor 5 (mGluR5) represents a potential therapeutic target for Huntington disease. Using 11C-ABP688 (3-(6-methyl-pyridin-2-ylethynyl)-cyclohex-2-enone-O-11C-methyl-oxime), a noncompetitive and highly selective antagonist for mGluR5, we aimed to longitudinally characterize in vivo changes in mGluR5 by means of PET imaging in the Q175 mouse model of Huntington disease. Methods:11C-ABP688 PET imaging, followed by a CT scan, was performed on 18 heterozygous mice and 18 wild-type (WT) littermates at 3 different time points (6, 9, and 13 mo old). 11C-ABP688 nondisplaceable binding potential (BPND) was calculated for each time point in striatum and cortex using the cerebellum as the reference region. In addition, voxel-based statistical parametric mapping (SPM) analysis was performed on BPND images. Postmortem validation of mGluR5 level and neuronal density was performed on the mice at 6 mo old. Results: The 11C-ABP688 BPND of heterozygous animals was significantly reduced at all time points in the striatum (-13.1%, -13.5%, and -14.2% at 6, 9, and 13 mo, respectively; P < 0.001 for all) and in the cortex (-9.8%, -10.2%, and -10.6%, respectively; P < 0.01 for all), when compared with WT animals. Longitudinal changes in 11C-ABP688 BPND were also found in heterozygous mice, showing a reduction at 13 mo compared with 6 mo (-10.4%, P < 0.05). SPM analysis confirmed reduced BPND in heterozygous compared with WT mice, as well as a time-related decline in 11C-ABP688 binding in the striatum of heterozygous mice. Postmortem analysis confirmed a mGluR5 decrease in both striatum (-36.6%; P < 0.01) and cortex (-16.6%; P < 0.05) in heterozygous mice, whereas no difference in neuronal density was found. Conclusion: In vivo imaging of mGluR5 using 11C-ABP688 PET/CT revealed a marked reduction in ligand binding in the striatum and cortex of heterozygous mice, compared with WT mice, as well as a temporal decline. This study suggests that 11C-ABP688 PET imaging is a potential biomarker to monitor the progression of, and therapeutic strategies for, Huntington disease.

Multiprobe molecular imaging of an NMDA receptor hypofunction rat model for glutamatergic dysfunction.

There are many indications of a connection between abnormal glutamate transmission through N-methyl-d-aspartate (NMDA) receptor hypofunction and the occurrence of schizophrenia. The importance of metabotropic glutamate receptor subtype 5 (mGluR5) became generally recognized due to its physical link through anchor proteins with NMDAR. Neuroinflammation as well as the kynurenine (tryptophan catabolite; TRYCAT) pathway are equally considered as major contributors to the pathology. We aimed to investigate this interplay between glutamate release, neuronal activation and inflammatory markers, by using small-animal positron emission tomography (PET) in a rat model known to induce schizophrenia-like symptoms. Daily intraperitoneal injection of MK801 or saline were administered to induce the model together with N-Acetyl-cysteine (NAc) or saline as the treatment in 24 male Sprague Dawley rats for one month. Biweekly in vivo [(11)C]-ABP688 microPET was performed together with mGluR5 immunohistochemistry. Simultaneously, weekly in vivo [(18)F]-FDG microPET imaging data for glucose metabolism was acquired and microglial activation was investigated with biweekly in vivo [(18)F]-PBR111 scans versus OX42 immunohistochemistry. Finally, plasma samples were analyzed for TRYCAT metabolites. We show that chronic MK801 administration (and thus elevated endogenous glutamate) causes significant tissue loss in rat brain, enhances neuroinflammatory pathways and may upregulate mGluR5 expression.

Impact of 24 Weeks of Resistance and Endurance Exercise on Glucose Tolerance in Persons with Multiple Sclerosis.

BACKGROUND: Recently, the authors reported an elevated prevalence of impaired glucose tolerance in individuals with multiple sclerosis (MS), compared with matched healthy controls, indicating metabolic defects that may increase comorbidity. MS also leads to a more inactive lifestyle, increasing the likelihood to develop fat accumulation, muscle wasting/weakness, and exercise intolerance. In other populations, these health complications can partly be reversed by physical exercise. OBJECTIVE: The aim of this study was to determine the impact of a mild-to-moderate-intensity exercise program on glucose tolerance, ranging between normal and impaired, in persons with MS. DESIGN: Persons with MS (mean expanded disability status scale, 3.3 ± 0.2; mean age, 48 ± 15 yrs) were randomized to an exercise group (n = 29) or a nonexercise control group (n = 15). Glucose tolerance, as well as muscle strength, exercise tolerance, and body composition to validate the applied exercise program, was determined in both groups at baseline and after 6, 12, and 24 wks of mild-to-moderate-intensity combined endurance and resistance training. RESULTS: No effects on blood glucose and serum insulin were detected. However, 6 mos of exercise improved muscle strength, exercise tolerance, and lean tissue mass within the intervention group as compared with baseline. In the control group, no changes were detected. CONCLUSION: Twenty-four weeks of mild-to-moderate-intensity combined endurance and resistance training was not able to improve glycemic control in this cohort of persons with MS. Future research is warranted to investigate the influence of higher exercise intensities on glucose tolerance, in an attempt to remediate metabolic deficits and to decrease the prevalence of comorbidities in MS.

Slowed exercise-onset Vo2 kinetics during submaximal endurance exercise in subjects with multiple sclerosis.

BACKGROUND: Low physical activity levels in persons with multiple sclerosis (MS) may reduce skeletal muscle oxidative capacity. Rehabilitation strategies might be altered by a measure of capacity that did not require invasive techniques or maximal exercise testing. For this purpose, we measured exercise onset and offset oxygen uptake (Vo(2)) kinetics during endurance exercise. OBJECTIVE: This study compared exercise-onset and -offset Vo(2) kinetics in mildly affected persons with MS with healthy matched participants. METHODS: From 38 MS patients who had a mean Expanded Disability Status Scale of 3.1 and 16 healthy participants, exercise-onset and -offset Vo(2) kinetics (mean response time [MRT]) were determined during two 6-minute submaximal bouts of exercise separated by a 6-minute recovery interval. Blood lactate, heart rate, expiratory volume, and Borg ratings of perceived exertion were assessed during exercise and compared between groups. Relationships between clinical characteristics and MRT were assessed. RESULTS: During exercise, blood lactate, heart rate, and expiratory volume did not differ between groups (P > .05), but exercise-onset MRT was significantly slower in MS versus healthy participants (P = .007). Exercise-onset MRT was independently related to having MS (P = .02). Exercise-offset MRT was not different between groups or was independently related to having MS (P > .05). No independent relationships between clinical characteristics of MS and exercise-onset or -offset MRT were found. CONCLUSIONS: Exercise-onset Vo(2) kinetics during submaximal endurance exercise are significantly slowed in mildly disabled persons with MS, suggesting low skeletal muscle oxidative capacity. Using mean response time testing, rehabilitation interventions for this reduction in exercise capacity can be assessed and targeted.