The effect of isoflurane on 18F-FDG uptake in the rat brain: a fully conscious dynamic PET study using motion compensation

Effect of reducing isoflurane level on glucosamine uptake in the mouse brain during magnetic resonance imaging studies

Anesthesia is often required during magnetic resonance imaging (MRI) examinations in animal studies. Anesthetic drugs differ in their capacity to interfere with homeostatic mechanisms responsible for glucose metabolism in the brain, which may create a constraint in the study design. Recent studies suggest that the chemical exchange saturation transfer (CEST) MRI scanning technique can detect localized metabolic changes in rodent brains induced by the uptake of glucose or its analogs; however, most of these studies do not account for the impact of anesthesia type on the brain metabolism. Herein, we aimed to evaluate the effect of reduced isoflurane levels on the preclinical imaging of glucosamine (GlcN) uptake in healthy mouse brains to establish optimal conditions for future brain imaging studies using the CEST MRI technique. The commonly used anesthesia protocol for longitudinal MRI examinations using 1.5% isoflurane level was compared to that using a mixture of low isoflurane (0.8%) level combined with midazolam (2 mg/kg, SC). Magnetization transfer ratio asymmetry (MTRasym) and area under the curve (AUC) analyses were used to characterize GlcN signals in the brain. The results indicated that mice injected with GlcN and anesthetized with 1.5% isoflurane exhibited low and insignificant changes in the MTRasym and AUC signals in the frontal cortex, whereas mice administered with 0.8% isoflurane combined with midazolam demonstrated a significant increase in these signals in the frontal cortex. This study highlights the diverse GlcN metabolic changes observed in mouse brains under variable levels of isoflurane anesthesia using the CEST MRI method. The results suggest that it is feasible to maintain anesthesia with low-dose isoflurane by integrating midazolam, which may enable the investigation of GlcN uptake in the brain. Thus, reducing isoflurane levels may support studies into mouse brain metabolism using the CEST MRI method and should be considered in future studies.

Dynamic neuroreceptor positron emission tomography in non-anesthetized rats using point source based motion correction: A feasibility study with [11C]ABP688

To prevent motion artifacts in small animal positron emission tomography (PET), animals are routinely scanned under anesthesia or physical restraint. Both may potentially alter metabolism and neurochemistry. This study investigates the feasibility of fully awake acquisition and subsequent absolute quantification of dynamic brain PET data via pharmacokinetic modelling in moving rats using the glutamate 5 receptor radioligand [11C]ABP688 and point source based motion correction. Five male rats underwent three dynamic [11C]ABP688 PET scans: two test-retest awake PET scans and one scan under anesthesia for comparison. Specific radioligand binding was determined via the simplified reference tissue model (reference: cerebellum) and outcome parameters BPND and R1 were evaluated in terms of stability and reproducibility. Test-retest measurements in awake animals gave reliable results with high correlations of BPND (y = 1.08 × -0.2, r = 0.99, p < 0.01) and an acceptable variability (mean over all investigated regions 15.7 ± 2.4%). Regional [11C]ABP688 BPNDs under awake and anesthetized conditions were comparable although in awake scans, absolute radioactive peak uptakes were lower and relative blood flow in terms of R1 was higher. Awake small animal PET with absolute quantification of neuroreceptor availability is technically feasible and reproducible thereby providing a suitable alternative whenever effects of anesthesia are undesirable, e.g. in sleep research.

In Vivo Evaluation of Brain [18F]F-FDG Uptake Pattern Under Different Anaesthesia Protocols

BACKGROUND/AIM

Since the use of anaesthetics has the drawback of altering radiotracer distribution, preclinical positron emission tomography (PET) imaging findings of anaesthetised animals must be carefully handled. This study aimed at assessing the cerebral [18F]F-FDG uptake pattern in healthy Wistar rats under four different anaesthesia protocols using microPET/magnetic resonance imaging (MRI) examinations.

MATERIALS AND METHODS

Post-injection of 15±1.2 MBq of [18F]F-FDG, either while awake or during the isoflurane-induced incubation phase was applied. Prior to microPET/MRI imaging, one group of the rats was subjected to forane-only anaesthesia while the other group was anaesthetised with the co-administration of forane and dexmedetomidine/Dexdor® Results: While as for the whole brain it was the addition of dexmedetomidine/Dexdor® to the anaesthesia protocol that generated the differences between the radiotracer concentrations of the investigated groups, regarding the cortex, the [18F]F-FDG accumulation was rather affected by the way of incubation. To ensure the most consistent and highest uptake, forane-induced anaesthesia coupled with an awake uptake condition seemed to be most suitable method of anaesthetisation for cerebral metabolic assessment. Diminished whole brain and cortical tracer accumulation detected upon dexmedetomidine/Dexdor® administration highlights the significance of the mechanism of action of different anaesthetics on radiotracer pharmacokinetics.

CONCLUSION

Overall, the standardization of PET protocols is of utmost importance to avoid the confounding factors derived from anaesthesia.

Effects of isoflurane anaesthesia depth and duration on renal function measured with [99mTc]Tc-mercaptoacetyltriglycine SPECT in mice

BACKGROUND

The influence of anaesthetic depth and the potential influence of different anaesthetic beds and thus different handling procedures were investigated in 86 severe combined immunodeficient (SCID) mice using semi-stationary dynamic single photon emission computed tomography (SPECT) for kidney scintigraphy. Therefore, isoflurane concentrations were adjusted using respiratory rate for low (80-90 breath/min) and deep anaesthesia (40-45 breath/min). At low anaesthesia, we additionally tested the influence of single bed versus 3-mouse bed hotel; the hotel mice were anaesthetized consecutively at ~ 30, 20, and 10 min before tracer injections for positions 1, 2, and 3, respectively. Intravenous [99mTc]Tc-MAG3 injection of ~ 28 MBq was performed after SPECT start. Time-activity curves were used to calculate time-to-peak (Tmax), T50 (50% clearance) and T25 (75% clearance).

RESULTS

Low and deep anaesthesia corresponded to median isoflurane concentrations of 1.3% and 1.5%, respectively, with no significant differences in heart rate (p = 0.74). Low anaesthesia resulted in shorter aortic blood clearance half-life (p = 0.091) and increased relative renal tracer influx rate (p = 0.018). A tendency toward earlier Tmax occurred under low anaesthesia (p = 0.063) with no differences in T50 (p = 0.40) and T25 (p = 0.24). Variance increased with deep anaesthesia. Compared to single mouse scans, hotel mice in position 1 showed a delayed Tmax, T50, and T25 (p < 0.05 each). Furthermore, hotel mice in position 1 showed delayed Tmax versus position 3, and delayed T50 and T25 versus position 2 and 3 (p < 0.05 each). No difference occurred between single bed and positions 2 (p = 1.0) and 3 (p = 1.0).

CONCLUSIONS

Deep anaesthesia and prolonged low anaesthesia should be avoided during renal scintigraphy because they result in prolonged blood clearance half-life, delayed renal influx and/or later Tmax. Vice versa, low anaesthesia with high respiratory rates of 80-90 rpm and short duration (≤ 20 min) should be preferred to obtain representative data with low variance.

Metabolic brain imaging with glucosamine CEST MRI: in vivo characterization and first insights

The utility of chemical exchange saturation transfer (CEST) MRI for monitoring the uptake of glucosamine (GlcN), a safe dietary supplement, has been previously demonstrated in detecting breast cancer in both murine and human subjects. Here, we studied and characterized the detectability of GlcN uptake and metabolism in the brain. Following intravenous GlcN administration in mice, CEST brain signals calculated by magnetization transfer ratio asymmetry (MTRasym) analysis, were significantly elevated, mainly in the cortex, hippocampus, and thalamus. The in vivo contrast remained stable during 40 min of examination, which can be attributed to GlcN uptake and its metabolic products accumulation as confirmed using 13C NMR spectroscopic studies of brain extracts. A Lorentzian multi-pool fitting analysis revealed an increase in the hydroxyl, amide, and relayed nuclear Overhauser effect (rNOE) signal components after GlcN treatment. With its ability to cross the blood-brain barrier (BBB), the GlcN CEST technique has the potential to serve as a metabolic biomarker for the diagnosis and monitoring various brain disorders.

Validation of a pharmacological imaging challenge using 11C-buprenorphine and 18F-2-fluoro-2-deoxy-D-glucose positron emission tomography to study the effects of buprenorphine to the rat brain

Aim

Buprenorphine mainly acts as an agonist of mu-opioid receptors (mu-OR). High dose buprenorphine does not cause respiratory depression and can be safely administered to elicit typical opioid effects and explore pharmacodynamics. Acute buprenorphine, associated with functional and quantitative neuroimaging, may therefore provide a fully translational pharmacological challenge to explore the variability of response to opioids in vivo. We hypothesized that the CNS effects of acute buprenorphine could be monitored through changes in regional brain glucose metabolism, assessed using ¹⁸F-FDG microPET in rats.

Materials and methods

First, level of receptor occupancy associated with a single dose of buprenorphine (0.1 mg/kg, s.c) was investigated through blocking experiments using ¹¹C-buprenorphine PET imaging. Behavioral study using the elevated plus-maze test (EPM) was performed to assess the impact of the selected dose on anxiety and also locomotor activity. Then, brain PET imaging using ¹⁸F-FDG was performed 30 min after injection of unlabeled buprenorphine (0.1 mg/kg, s.c) vs. saline. Two different ¹⁸F-FDG PET acquisition paradigms were compared: (i) ¹⁸F-FDG injected i.v. under anesthesia and (ii) ¹⁸F-FDG injected i.p. in awake animals to limit the impact of anesthesia.

Results

The selected dose of buprenorphine fully blocked the binding of ¹¹C-buprenorphine in brain regions, suggesting complete receptor occupancy. This dose had no significant impact on behavioral tests used, regardless of the anesthetized/awake handling paradigm. In anesthetized rats, injection of unlabeled buprenorphine decreased the brain uptake of ¹⁸F-FDG in most brain regions except in the cerebellum which could be used as a normalization region. Buprenorphine treatment significantly decreased the normalized brain uptake of ¹⁸F-FDG in the thalamus, striatum and midbrain (p < 0.05), where binding of ¹¹C-buprenorphine was the highest. The awake paradigm did not improve sensitivity and impact of buprenorphine on brain glucose metabolism could not be reliably estimated.

Conclusion

Buprenorphine (0.1 mg/kg, s.c) combined with ¹⁸F-FDG brain PET in isoflurane anesthetized rats provides a simple pharmacological imaging challenge to investigate the CNS effects of full receptor occupancy by this partial mu-OR agonist. Sensitivity of the method was not improved in awake animals. This strategy may be useful to investigate de desensitization of mu-OR associated with opioid tolerance in vivo.

Analysis of cerebral glucose metabolism following experimental subarachnoid hemorrhage over 7 days

Little is known about changes in brain metabolism following SAH, possibly leading towards secondary brain damage. Despite sustained progress in the last decade, analysis of in vivo acquired data still remains challenging. The present interdisciplinary study uses a semi-automated data analysis tool analyzing imaging data independently from the administrated radiotracer. The uptake of 2-[¹⁸F]Fluoro-2-deoxy-glucose ([¹⁸F]FDG) was evaluated in different brain regions in 14 male Sprague-Dawley rats, randomized into two groups: (1) SAH induced by the endovascular filament model and (2) sham operated controls. Serial [¹⁸F]FDG-PET measurements were carried out. Quantitative image analysis was performed by uptake ratio using a self-developed MRI-template based data analysis tool. SAH animals showed significantly higher [¹⁸F]FDG accumulation in gray matter, neocortex and olfactory system as compared to animals of the sham group, while white matter and basal forebrain region showed significant reduced tracer accumulation in SAH animals. All significant metabolic changes were visualized from 3 h, over 24 h (day 1), day 4 and day 7 following SAH/sham operation. This [¹⁸F]FDG-PET study provides important insights into glucose metabolism alterations following SAH-for the first time in different brain regions and up to day 7 during course of disease.

Imaging Synaptic Density: The Next Holy Grail of Neuroscience?

The brain is the central and most complex organ in the nervous system, comprising billions of neurons that constantly communicate through trillions of connections called synapses. Despite being formed mainly during prenatal and early postnatal development, synapses are continually refined and eliminated throughout life via complicated and hitherto incompletely understood mechanisms. Failure to correctly regulate the numbers and distribution of synapses has been associated with many neurological and psychiatric disorders, including autism, epilepsy, Alzheimer’s disease, and schizophrenia. Therefore, measurements of brain synaptic density, as well as early detection of synaptic dysfunction, are essential for understanding normal and abnormal brain development. To date, multiple synaptic density markers have been proposed and investigated in experimental models of brain disorders. The majority of the gold standard methodologies (e.g., electron microscopy or immunohistochemistry) visualize synapses or measure changes in pre- and postsynaptic proteins ex vivo. However, the invasive nature of these classic methodologies precludes their use in living organisms. The recent development of positron emission tomography (PET) tracers [such as (¹⁸F)UCB-H or (¹¹C)UCB-J] that bind to a putative synaptic density marker, the synaptic vesicle 2A (SV2A) protein, is heralding a likely paradigm shift in detecting synaptic alterations in patients. Despite their limited specificity, novel, non-invasive magnetic resonance (MR)-based methods also show promise in inferring synaptic information by linking to glutamate neurotransmission. Although promising, all these methods entail various advantages and limitations that must be addressed before becoming part of routine clinical practice. In this review, we summarize and discuss current ex vivo and in vivo methods of quantifying synaptic density, including an evaluation of their reliability and experimental utility. We conclude with a critical assessment of challenges that need to be overcome before successfully employing synaptic density biomarkers as diagnostic and/or prognostic tools in the study of neurological and neuropsychiatric disorders.

Choice of anesthesia and data analysis method strongly increases sensitivity of 18F-FDG PET imaging during experimental epileptogenesis

Purpose

Alterations in brain glucose metabolism detected by 2-deoxy-2-[¹⁸F]-fluoro-D-glucose (¹⁸F-FDG) positron emission tomography (PET) may serve as an early predictive biomarker and treatment target for epileptogenesis. Here, we aimed to investigate changes in cerebral glucose metabolism before induction of epileptogenesis, during epileptogenesis as well as during chronic epilepsy. As anesthesia is usually unavoidable for preclinical PET imaging and influences the distribution of the radiotracer, four different protocols were compared.

Procedures

We investigated ¹⁸F-FDG uptake phase in conscious rats followed by a static scan as well as dynamic scans under continuous isoflurane, medetomidine-midazolam-fentanyl (MMF), or propofol anesthesia. Furthermore, we applied different analysis approaches: atlas-based regional analysis, statistical parametric mapping, and kinetic analysis.

Results

At baseline and compared to uptake in conscious rats, isoflurane and propofol anesthesia resulted in decreased cortical ¹⁸F-FDG uptake while MMF anesthesia led to a globally decreased tracer uptake. During epileptogenesis, MMF anesthesia was clearly best distinctive for visualization of prominently increased glucometabolism in epilepsy-related brain areas. Kinetic modeling further increased sensitivity, particularly for continuous isoflurane anesthesia. During chronic epilepsy, hypometabolism affecting more or less the whole brain was detectable with all protocols.

Conclusion

This study reveals evaluation of anesthesia protocols for preclinical ¹⁸F-FDG PET imaging as a critical step in the study design. Together with an appropriate data analysis workflow, the chosen anesthesia protocol may uncover otherwise concealed disease-associated regional glucometabolic changes.

Translation of Preclinical PET Imaging Findings: Challenges and Motion Correction to Overcome the Confounding Effect of Anesthetics

Preclinical brain positron emission tomography (PET) in animals is performed using anesthesia to avoid movement during the PET scan. In contrast, brain PET scans in humans are typically performed in the awake subject. Anesthesia is therefore one of the principal limitations in the translation of preclinical brain PET to the clinic. This review summarizes the available literature supporting the confounding effect of anesthesia on several PET tracers for neuroscience in preclinical small animal scans. In a second part, we present the state-of-the-art methodologies to circumvent this limitation to increase the translational significance of preclinical research, with an emphasis on motion correction methods. Several motion tracking systems compatible with preclinical scanners have been developed, each one with its advantages and limitations. These systems and the novel experimental setups they can bring to preclinical brain PET research are reviewed here. While technical advances have been made in this field, and practical implementations have been demonstrated, the technique should become more readily available to research centers to allow for a wider adoption of the motion correction technique for brain research.

Conscious rat PET imaging with soft immobilization for quantitation of brain functions: comprehensive assessment of anesthesia effects on cerebral blood flow and metabolism

BACKGROUND

Animal brain functions evaluated by in vivo imaging under anesthesia can be affected by anesthetic agents, resulting in incorrect assessment of physiological brain function. We therefore performed dynamic positron emission tomography (PET) imaging of conscious rats using recently reported soft immobilization to validate the efficacy of the immobilization for brain function assessments. We also determined the effects of six anesthetic agents-a mixed anesthetic agent (MMB), ketamine + xylazine (KX), chloral hydrate (Chloral), pentobarbital (PTB), propofol (PF), and isoflurane (IFL)-on brain function by comparison with conscious rats.

RESULTS

The immobilization enabled 45-min dynamic [¹⁸F]FDG-PET acquisition with arterial blood sampling using conscious rats without the use of special techniques or invasive surgery. The spatial resolution and quantitativity of [¹⁸F]FDG-PET were not significantly lower for conscious rats than for anesthetized rats. While MMB, Chloral, PTB, and PF showed ubiquitous reduction in the cerebral metabolic rates of glucose (CMR_glu) in brain regions, KX and IFL showed higher reductions in cerebellum and interbrain, and cerebellum, respectively. Cerebral blood flow (CBF) was reduced by MMB, KX, PTB, and PF; increased by IFL; and unaltered by Chloral. The magnitude of decrease in CMR_glu and CBF for MMB were not larger than for other five anesthetic agents, although blood glucose levels and body temperature can be easily affected by MMB.

CONCLUSION

The six anesthetic agents induced various effects on CMR_glu and CBF. The immobilization technique presented here is a promising tool for noninvasive brain functional imaging using conscious rats to avoid the effects of anesthetic agents.

Elucidating the complementarity of resting-state networks derived from dynamic [18F]FDG and hemodynamic fluctuations using simultaneous small-animal PET/MRI

Functional connectivity (FC) and resting-state network (RSN) analyses using functional magnetic resonance imaging (fMRI) have evolved into a growing field of research and have provided useful biomarkers for the assessment of brain function in neurological disorders. However, the underlying mechanisms of the blood oxygen level-dependant (BOLD) signal are not fully resolved due to its inherent complexity. In contrast, [¹⁸F]fluorodeoxyglucose positron emission tomography ([¹⁸F]FDG-PET) has been shown to provide a more direct measure of local synaptic activity and may have additional value for the readout and interpretation of brain connectivity. We performed an RSN analysis from simultaneously acquired PET/fMRI data on a single-subject level to directly compare fMRI and [¹⁸F]FDG-PET-derived networks during the resting state. Simultaneous [¹⁸F]FDG-PET/fMRI scans were performed in 30 rats. Pairwise correlation analysis, as well as independent component analysis (ICA), were used to compare the readouts of both methods. We identified three RSNs with a high degree of similarity between PET and fMRI-derived readouts: the default-mode-like network (DMN), the basal ganglia network and the cerebellar-midbrain network. Overall, [¹⁸F]FDG connectivity indicated increased integration between different, often distant, brain areas compared to the results indicated by the more segregated fMRI-derived FC. Additionally, several networks exclusive to either modality were observed using ICA. These networks included mainly bilateral cortical networks of a limited spatial extent for fMRI and more spatially widespread networks for [¹⁸F]FDG-PET, often involving several subcortical areas.

This is the first study using simultaneous PET/fMRI to report RSNs subject-wise from dynamic [¹⁸F]FDG tracer delivery and BOLD fluctuations with both independent component analysis (ICA) and pairwise correlation analysis in small animals. Our findings support previous studies, which show a close link between local synaptic glucose consumption and BOLD-fMRI-derived FC. However, several brain regions were exclusively attributed to either [¹⁸F]FDG or BOLD-derived networks underlining the complementarity of this hybrid imaging approach, which may contribute to the understanding of brain functional organization and could be of interest for future clinical applications.

Estimation of the net influx rate Ki and the cerebral metabolic rate of glucose MRglc using a single static [18F]FDG PET scan in rats

Since accurate quantification of 2-deoxy-2-¹⁸F-fluoro-D-glucose ([¹⁸F]FDG) positron emission tomography (PET) requires dynamic acquisition with arterial input function, more practical semi-quantitative (static) approaches are often preferred. However, static standardized uptake values (SUV) are typically biased due to large variations in body weight (BW) occurring over time in animal studies. This study aims to improve static [¹⁸F]FDG PET SUV quantification by better accounting for BW variations in rats. We performed dynamic [¹⁸F]FDG PET imaging with arterial blood sampling in rats (n = 27) with different BW (range 0.230-0.487 kg). By regressing the area under the curve of the input function divided by injected activity against BW (r²=0.697), we determined a conversion factor f(BW) to be multiplied with SUV and SUV_glc to obtain ratSUV and ratSUV_glc, providing an improved estimate of the net influx rate K_i (r = 0.758, p<0.0001) and cerebral metabolic rate of glucose MR_glc (r = 0.906, p<0.0001), respectively. In conclusion, the proposed ratSUV and ratSUV_glc provide a proxy for the K_i and MR_glc based on a single static [¹⁸F]FDG PET SUV measurement improving clinical significance and translation of rodent studies. Given a defined strain, sex, age, diet, and weight range, this method is applicable for future experiments by converting SUV with the derived f(BW).

Effects of animal handling on striatal DAT availability in rats

OBJECTIVE

Positron emission tomography (PET) is a non-invasive technique measuring quantification of physiological and biochemical processes in the living organism. However, there are many considerations including anesthesia and fasting to acquire small animal imaging. We aimed to evaluate the effects of anesthesia and fasting of rats in dopamine transporter (DAT) imaging acquisition.

METHODS

Male Sprague Dawley (SD) rats aged 7 weeks and weighing 180-260 g were used in this study. Rats were randomly divided by 4 groups. Group A was kept under anesthesia for 40 min and fasted over 12 h. Group B was only fasted over 12 h. Group C was only kept under anesthesia for 40 min. Group D was neither kept under anesthesia nor fasted over 12 h. PET scans were started at 40 min after ¹⁸F-FP-CIT injection and obtained for 20 min. Volumes-of-interest for striatum and extrastriatal area were used for ¹⁸F-FP-CIT PET analysis. Cerebellum was considered as a reference region. Specific binding ratio (SBR) was calculated as follows: [(uptake of target-uptake of cerebellum)]/(uptake of cerebellum).

RESULTS

SBR without fasting and anesthesia (group D) was significantly lower than those of other groups (vs group A, p = 0.0004; vs group B, p = 0.0377; vs group C, p = 0.0134). However, SBRs of extrastriatal area (p = 0.5120) were not affected by fasting and anesthesia.

CONCLUSIONS

In conclusion, the SBR of striatum was increased after anesthesia by isoflurane and fasting. When designing an experiment using DAT imaging, the effects of isoflurane and fasting should be considered.

Digital autoradiography for efficient functional imaging without anesthesia in experimental animals: Reversing phencyclidine-induced functional alterations using clozapine

Autoradiography (ARG) is a high-resolution imaging method for localization of radiolabeled biomarkers in ex vivo specimen. ARG using 2-deoxy-d-glucose (2-DG) method is used in to study drug actions on brain functional activity, as it provides results comparable to clinically used functional positron-emission tomography (PET). The requirement of slow analog detection methods and emerging advances in small animal PET imaging have, however, reduced the interest in ARG. In contrast to ARG, experimental animals need to be restrained or sedated/anesthetized for PET imaging, which strongly influence functional activity and thus complicate the interpretation of the results. Digital direct particle-counting ARG systems have gained attraction during the last decade to overcome the caveats of conventional ARG methods. Here we demonstrate that the well-established 2-DG imaging method can be adapted into use with contemporary digital detectors. This method readily and rapidly captures the characteristic effects of phencyclidine (5 mg/kg, i.p.), a dissociative agent targeting the NMDAR (N-methyl-d-aspartate receptor), on regional glucose utilization in the adult mouse brain. Pretreatment with antipsychotic drug clozapine (6 mg/kg, i.p.) essentially abolishes these effects of phencyclidine on brain functional activity. Digital ARG produces viable data for the regional analysis of functional activity in a fraction of time required for film development. These results support the use of digital ARG in preclinical drug research, where high throughput and response linearity are preferred and use of sedation/anesthesia has to be avoided.

Anesthesia and Preconditioning Induced Changes in Mouse Brain [18F] FDG Uptake and Kinetics

PURPOSE

2-Deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) has been widely used for imaging brain metabolism. Tracer injection in anesthetized animals is a prerequisite for performing dynamic positron emission tomography (PET) scanning. Since preconditioning, as well as anesthesia, has been described to potentially influence brain [¹⁸F] FDG levels, this study evaluated how these variables globally and regionally affect both [¹⁸F] FDG uptake and kinetics in murine brain.

PROCEDURES

Sixty-minute dynamic [¹⁸F] FDG PET scans were performed in adult male C57BL/6 mice anesthetized with isoflurane [control (in 100 % O₂), in medical air, in 100 % O₂ + insulin pre-treatment, and in 100 % O₂ after 18 h fasting], ketamine/xylazine, sevoflurane, and chloral hydrate. An additional group was scanned after awake uptake. Blood glucose levels were determined, and data was analyzed by comparing percent injected dose per cc tissue (%ID/cc) and glucose influx rate and metabolic rate (MRGlu) calculated by Patlak plot.

RESULTS

Ketamine/xylazine and chloral hydrate anesthesia induced a lower whole-brain uptake of [¹⁸F] FDG (2.86 ± 0.67 %ID/cc, p < 0.001; 4.25 ± 0.28 %ID/cc, p = 0.0179, respectively) compared to isoflurane anesthesia (5.04 ± 0.19 %ID/cc). In addition, protocols affected differently distribution of [¹⁸F] FDG uptake in brain regions. Ketamine/xylazine reduced [¹⁸F] FDG influx rate in murine brain (0.0135 ± 0.0009 vs 0.0247 ± 0.0014 ml/g/min; p < 0.005) and chloral hydrate increased MRGlu (66.72 ± 3.75 vs 41.55 ± 3.06 μmol/min/100 ml; p < 0.01) compared to isoflurane. Insulin-pretreated animals showed a higher influx rate (0.0477 ± 0.0101 ml/min/g; p < 0.05) but a reduced MRGlu (21.92 ± 3.12 μmol/min/100 ml; p < 0.01). Blood glucose levels were negatively correlated to [¹⁸F] FDG uptake and influx rate, but positively correlated to MRGlu.

CONCLUSIONS

Choice of anesthesia and pre-conditioning affect not only [¹⁸F] FDG uptake but also kinetics and regional distribution in the mouse brain. Both anesthesia and pre-conditioning should be carefully considered in the interpretation of [¹⁸F] FDG studies due to its great influence on the uptake and distribution of the tracer along the brain regions.

Optimizing clozapine for chemogenetic neuromodulation of somatosensory cortex

Clozapine (CLZ) has been proposed as an agonist for Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), to replace Clozapine-N-oxide (CNO); however, there are no reliable guidelines for the use of CLZ for chemogenetic neuromodulation. We titrated the optimal dose of CLZ required to evoke changes in neural activity whilst avoiding off-target effects. We also performed [¹⁸F]Fluoro-deoxy-glucose micro positron emission tomography (FDG-microPET) scans to determine the global effect of CLZ-induced hM3D(Gq) DREADD activation in the rat brain. Our results show that low doses of CLZ (0.1 and 0.01 mg/kg) successfully induced neural responses without off-target effects. CLZ at 1 mg/kg evoked a stronger and longer-lasting neural response but produced off-target effects, observed as changes in locomotor behavior and FDG-microPET imaging. Unexpectedly, FDG-microPET imaging failed to demonstrate an increase in regional glucose metabolism in the stimulated cortex during CLZ chemogenetic neuromodulation. Therefore, caution should be used when interpreting FDG-PET images in the context of cortical chemogenetic activation.

Temporal changes in neuroinflammation and brain glucose metabolism in a rat model of viral vector-induced α-synucleinopathy

Rat models based on viral vector-mediated overexpression of α-synuclein are regarded as highly valuable models that closely mimic cardinal features of human Parkinson's disease (PD) such as L-DOPA-dependent motor impairment, dopaminergic neurodegeneration and α-synuclein inclusions. To date, the downstream effects of dopaminergic cell loss on brain glucose metabolism, including the neuroinflammation component, have not been phenotyped in detail for this model. Cerebral glucose metabolism was monitored throughout different stages of the disease using in vivo 2-[¹⁸F]-fluoro-2-deoxy-d-glucose ([¹⁸F]FDG) positron emission tomography (PET) and was combined with in vitro [¹⁸F]DPA-714 autoradiography to assess concomitant inflammation. Rats were unilaterally injected with recombinant adeno-associated viral vector serotype 2/7 (rAAV2/7) encoding either A53T α-synuclein or eGFP. Brain [¹⁸F]FDG microPET was performed at baseline, 1, 2, 3, 4, 6, and 9 weeks post-surgery, in combination with behavioral tests. As a second experiment, [¹⁸F]DPA-714 autoradiography was executed across the same timeline. Voxel-based analysis of relative [¹⁸F]FDG uptake showed a dynamic pattern of PD-related metabolic changes throughout the disease progression (weeks 2-9). Glucose hypermetabolism covering a large bilateral area reaching from the insular, motor- and somatosensory cortex to the striatum was observed at week 2. At week 4, hypermetabolism presented in a cluster covering the ipsilateral nigra-thalamic region, whereas hypometabolism was noted in the ipsilateral striatum at week 6. Elevated [¹⁸F]FDG uptake was seen in a cluster extending across the contralateral striatum, motor- and somatosensory cortex at week 9. Increased [¹⁸F]FDG in the region of the substantia nigra was associated with increased [¹⁸F]DPA-714 binding, and correlated significantly with motor symptoms. These findings point to disease-associated metabolic and neuroinflammatory changes taking place in the primary area of dopaminergic neurodegeneration but also closely interconnected motor and somatosensory brain regions.

Behavioral neuroimaging in birds using PET

BACKGROUND

Birds comprise the most diverse group of terrestrial vertebrates. This success likely is related to the evolution of powered flight over 75 mya. Modern approaches for studying brain function, however, have yet to be fully adapted and applied to birds, especially as they relate to specific behaviors including flight. New method: We have developed a comprehensive set of in vivo experimental methods utilizing PET imaging with F-18 labeled fluorodeoxyglucose (FDG) to study regional changes in metabolism specifically related to flight, yet applicable to other behaviors as well. It incorporates approaches for selection of species, behavioral/imaging paradigm, animal preparation, radiotracer injection route, image quantification, and image analysis via an enhanced brain atlas. We also carried out preliminary modeling studies to better understand tracer kinetics.

RESULTS

The methods were successful in identifying brain regions statistically associated with flight using only 8 animals. Peak brain uptake of FDG between birds and rodents is similar despite much higher blood glucose levels in birds. We also confirmed that brain uptake of FDG steadily decreases after the initial peak and provide evidence that it may be related to greater dephosphorylation of FDG phosphate than that observed in mammals. Comparison with existing methods: FDG PET has been used in only a few studies of the bird brain. We introduce a new species, more realistic flight behavior, paired (test/retest) design, and improved quantification and analysis approaches.

CONCLUSIONS

The proposed imaging protocol is non-invasive yet sensitive to regional metabolic changes in the bird brain related to behavior.

Alteration of FDG uptake by performing novel object recognition task in a rat model of Traumatic Brain Injury

Traumatic Brain Injury (TBI) affects approximately 2.5 million people in the United States, of which 80% are considered to be mild (mTBI). Previous studies have shown that cerebral glucose uptake and metabolism are altered after brain trauma and functional metabolic deficits observed following mTBI are associated with changes in cognitive performance. Imaging of glucose uptake using [¹⁸F] Fluorodeoxyglucose (FDG) based Positron Emission Tomography (PET) with anesthesia during the uptake period demonstrated limited variability in results, but may have depressed uptake. Anesthesia has been found to interfere with blood glucose levels, and hence, FDG uptake. Conversely, forced cognitive testing during uptake may increase glucose demand in targeted regions, such as hippocampus, allowing for better differentiation of outcomes. Therefore, the objective of this study was to investigate the influence of a directed cognitive function task during the FDG uptake period on uptake measurements both in naïve rats and at 2 days after mild lateral fluid percussion (mLFP) TBI. Adult male Sprague Dawley rats underwent FDG uptake with either cognitive testing with the Novel Object Recognition (NOR) test or No Novel Object (NNO), followed by PET scans at baseline (prior to injury) and at 2days post mLFP. At baseline, FDG uptake in the right hippocampus was elevated in rats completing the NOR in comparison to the NNO (control group). Further, the NNO group rats demonstrated a greater fold change in the FDG uptake between baseline and post injury scans than the NOR group. Overall, these data suggest that cognitive activity during FDG uptake affects the regional uptake pattern in the brain, increasing uptake at baseline and suppressing the effects of injury.

Cortico-Amygdala-Striatal Activation by Modafinil/Flecainide Combination

Background

Modafinil, a nonamphetaminic wake-promoting compound, is prescribed as first line therapy in narcolepsy, an invalidating disorder characterized by excessive daytime sleepiness and cataplexy. Although its mode of action remains incompletely known, recent studies indicated that modafinil modulates astroglial connexin-based gap junctional communication as administration of a low dose of flecainide, an astroglial connexin inhibitor, enhanced the wake-promoting and procognitive activity of modafinil in rodents and healthy volunteers. The aim of this study is to investigate changes in glucose cerebral metabolism in rodents, induced by the combination of modafinil+flecainide low dose (called THN102).

Methods

The impact of THN102 on brain glucose metabolism was noninvasively investigated using 18F-2-fluoro-2-deoxy-D-glucose Positron Emission Tomography imaging in Sprague-Dawley male rats. Animals were injected with vehicle, flecainide, modafinil, or THN102 and further injected with 18F-2-fluoro-2-deoxy-D-glucose followed by 60-minute Positron Emission Tomography acquisition. 18F-2-fluoro-2-deoxy-D-glucose Positron Emission Tomography images were coregistered to a rat brain template and normalized from the total brain Positron Emission Tomography signal. Voxel-to-voxel analysis was performed using SPM8 software. Comparison of brain glucose metabolism between groups was then performed.

Results

THN102 significantly increased regional brain glucose metabolism as it resulted in large clusters of 18F-2-fluoro-2-deoxy-D-glucose uptake localized in the cortex, striatum, and amygdala compared with control or drugs administered alone. These regions, highly involved in the regulation of sleep-wake cycle, emotions, and cognitive functions were hence quantitatively modulated by THN102.

Conclusion

Data presented here provide the first evidence of a regional brain activation induced by THN102, currently being tested in a phase II clinical trial in narcoleptic patients.

Regional knockdown of NDUFS4 implicates a thalamocortical circuit mediating anesthetic sensitivity

Knockout of the mitochondrial complex I protein, NDUFS4, profoundly increases sensitivity of mice to volatile anesthetics. In mice carrying an Ndufs4lox/lox gene, adeno-associated virus expressing Cre recombinase was injected into regions of the brain postulated to affect sensitivity to volatile anesthetics. These injections generated otherwise phenotypically wild type mice with region-specific, postnatal inactivation of Ndufs4, minimizing developmental effects of gene loss. Sensitivities to the volatile anesthetics isoflurane and halothane were measured using loss of righting reflex (LORR) and movement in response to tail clamp (TC) as endpoints. Knockdown (KD) of Ndufs4 in the vestibular nucleus produced resistance to both anesthetics for movement in response to TC. Ndufs4 loss in the central and dorsal medial thalami and in the parietal association cortex increased anesthetic sensitivity to both TC and LORR. Knockdown of Ndufs4 only in the parietal association cortex produced striking hypersensitivity for both endpoints, and accounted for half the total change seen in the global KO (Ndufs4(KO)). Excitatory synaptic transmission in the parietal association cortex in slices from Ndufs4(KO) animals was hypersensitive to isoflurane compared to control slices. We identified a direct neural circuit between the parietal association cortex and the central thalamus, consistent with a model in which isoflurane sensitivity is mediated by a thalamic signal relayed through excitatory synapses to the parietal association cortex. We postulate that the thalamocortical circuit is crucial for maintenance of consciousness and is disrupted by the inhibitory effects of isoflurane/halothane on mitochondria.

Effects of isoflurane anesthesia and intravenous morphine self-administration on regional glucose metabolism ([18 F]FDG-PET) of male Sprague-Dawley rats

Although certain drugs of abuse are known to disrupt brain glucose metabolism (BGluM), the effects of opiates on BGluM are not well characterized. Moreover, preclinical positron emission tomography (PET) studies anesthetize animals during the scan, which limits clinical applications. We investigated the effects of (i) isoflurane anesthesia and (ii) intravenous morphine self-administration (MSA) on BGluM in rats. Jugular vein cannulated adult male Sprague-Dawley rats self-administered either saline (SSA) or morphine (0.5 mg/kg/infusion, 4 h/day for 12 days). All animals were scanned twice with [¹⁸ F]-fluoro-deoxy-glucose (FDG)-PET/CT at a baseline and at 2-day withdrawal from self-administration. After the IV injection of FDG, one batch of animals (n = 14) was anesthetized with isoflurane and the other batch (n = 16) was kept awake during the FDG uptake (45 min). After FDG uptake, all animals were anesthetized in order to perform a PET/CT scan (30 min). Isoflurane anesthesia, as compared to the awake condition, reduced BGluM in the olfactory, cortex, thalamus, and basal ganglia, while increasing BGluM in the midbrain, hypothalamus, hippocampus, and cerebellum. Morphine self-administered animals exhibited withdrawal signs (piloerection and increased defecation), drug seeking, and locomotor stimulation to morphine (0.5 mg/kg) during the 2 day withdrawal. The BGluM in the striatum was increased in the MSA group as compared to the SSA group; this effect was observed only in the isoflurane anesthesia, not the awake condition. These findings suggest that the choice of the FDG uptake condition may be important in preclinical PET studies and increased BGluM in the striatum may be associated with opiate seeking in withdrawal.