Highly resolved in vivo1H NMR spectroscopy of the mouse brain at 9.4 T

High-resolution 1H-MRSI at 9.4 T by integrating relaxation enhancement and subspace imaging

Achieving high-resolution and high signal-to-noise ratio (SNR) in vivo metabolic imaging via fast magnetic resonance spectroscopic imaging (MRSI) has been a longstanding challenge. This study combines the methods of relaxation enhancement (RE) and subspace imaging for the first time, enabling high-resolution and high-SNR in vivo MRSI of rodent brains at 9.4 T. Specifically, an RE-based chemical shift imaging sequence, which combines a frequency-selective pulse to excite only the metabolite frequencies with minimum perturbation of the water spins and a pair of adiabatic pulses to spatially localize the slice of interest, is designed and evaluated in vivo. This strategy effectively shortens the apparent T1 of metabolites, thereby increasing the SNR during relatively short repetition time ((TR) compared with acquisitions with only spatially selective wideband excitations, and does not require water suppression. The SNR was further enhanced via a state-of-the-art subspace reconstruction method. A novel subspace learning strategy tailored for 9.4 T and RE acquisitions is developed. In vivo, high-resolution (e.g., voxel size of 0.6 × 0.6 × 1.5 mm3) MRSI of both healthy mouse brains and a glioma-bearing mouse brain in 12.5 min has been demonstrated.

Observation of magnetic amplification using dark spins

Quantum amplification enables the enhancement of weak signals and is of great importance for precision measurements, such as biomedical science and tests of fundamental symmetries. Here, we observe a previously unexplored magnetic amplification using dark noble-gas nuclear spins in the absence of pump light. Such dark spins exhibit remarkable coherence lasting up to 6 min and the resilience against the perturbations caused by overlapping alkali-metal gas. We demonstrate that the observed phenomenon, referred to as "dark spin amplification," significantly magnifies magnetic field signals by at least three orders of magnitude. As an immediate application, we showcase an ultrasensitive magnetometer capable of measuring subfemtotesla fields in a single 500-s measurement. Our approach is generic and can be applied to a wide range of noble-gas isotopes, and we discuss promising optimizations that could further improve the current signal amplification up to [Formula: see text] with [Formula: see text]Ne, [Formula: see text] with [Formula: see text]Xe, and [Formula: see text] with [Formula: see text]He. This work unlocks opportunities in precision measurements, including searches for ultralight dark matter with sensitivity well beyond the supernova-observation constraints.

Maternal exposure to polystyrene nanoplastics alters fetal brain metabolism in mice

INTRODUCTION

Plastics used in everyday materials accumulate as waste in the environment and degrade over time. The impacts of the resulting particulate micro- and nanoplastics on human health remain largely unknown. In pregnant mice, we recently demonstrated that exposure to nanoplastics throughout gestation and during lactation resulted in changes in brain structure detected on MRI. One possible explanation for this abnormal postnatal brain development is altered fetal brain metabolism.

OBJECTIVES

To determine the effect of maternal exposure to nanoplastics on fetal brain metabolism.

METHODS

Healthy pregnant CD-1 mice were exposed to 50 nm polystyrene nanoplastics at a concentration of 106 ng/L through drinking water during gestation. Fetal brain samples were collected at embryonic day 17.5 (n = 18-21 per group per sex) and snap-frozen in liquid nitrogen. Magic angle spinning nuclear magnetic resonance was used to determine metabolite profiles and their relative concentrations in the fetal brain.

RESULTS

The relative concentrations of gamma-aminobutyric acid (GABA), creatine and glucose were found to decrease by 40%, 21% and 30% respectively following maternal nanoplastic exposure when compared to the controls (p < 0.05). The change in relative concentration of asparagine with nanoplastic exposure was dependent on fetal sex (p < 0.005).

CONCLUSION

Maternal exposure to polystyrene nanoplastics caused abnormal fetal brain metabolism in mice. The present study demonstrates the potential impacts of nanoplastic exposure during fetal development and motivates further studies to evaluate the risk to human pregnancies.

Event-related functional magnetic resonance spectroscopy

Proton-Magnetic Resonance Spectroscopy (MRS) is a non-invasive brain imaging technique used to measure the concentration of different neurochemicals. "Single-voxel" MRS data is typically acquired across several minutes, before individual transients are averaged through time to give a measurement of neurochemical concentrations. However, this approach is not sensitive to more rapid temporal dynamics of neurochemicals, including those that reflect functional changes in neural computation relevant to perception, cognition, motor control and ultimately behaviour. In this review we discuss recent advances in functional MRS (fMRS) that now allow us to obtain event-related measures of neurochemicals. Event-related fMRS involves presenting different experimental conditions as a series of trials that are intermixed. Critically, this approach allows spectra to be acquired at a time resolution in the order of seconds. Here we provide a comprehensive user guide for event-related task designs, choice of MRS sequence, analysis pipelines, and appropriate interpretation of event-related fMRS data. We raise various technical considerations by examining protocols used to quantify dynamic changes in GABA, the primary inhibitory neurotransmitter in the brain. Overall, we propose that although more data is needed, event-related fMRS can be used to measure dynamic changes in neurochemicals at a temporal resolution relevant to computations that support human cognition and behaviour.

Neonatal hyperbilirubinemia differentially alters the neurochemical profiles of the developing cerebellum and hippocampus in a preterm Gunn rat model

Neonatal hyperbilirubinemia (NHB) can lead to brain injury in newborn infants by affecting specific regions including the cerebellum and hippocampus. Extremely preterm infants are more vulnerable to bilirubin neurotoxicity, but the mechanism and extent of injury is not well understood. A preterm version of the Gunn rat model was utilized to investigate severe preterm NHB. Homozygous/jaundiced Gunn rat pups were injected (i.p.) on postnatal day (P) 5 with sulfadimethoxine, which increases serum free bilirubin capable of crossing the blood-brain barrier and causing brain injury. The neurochemical profiles of the cerebellum and hippocampus were determined using in vivo ¹ H MRS at 9.4 T on P30 and compared with those of heterozygous/non-jaundiced control rats. Transcript expression of related genes was determined by real-time quantitative PCR. MRI revealed significant morphological changes in the cerebellum of jaundiced rats. The concentrations of myo-inositol (+54%), glucose (+51%), N-acetylaspartylglutamate (+21%), and the sum of glycerophosphocholine and phosphocholine (+17%) were significantly higher in the cerebellum of the jaundiced group compared with the control group. Despite the lack of morphologic changes in the hippocampus, the concentration of myo-inositol (+9%) was higher and the concentrations of creatine (-8%) and of total creatine (-3%) were lower in the jaundiced group. In the hippocampus, expression of calcium/calmodulin dependent protein kinase II alpha (Camk2a), glucose transporter 1 (Glut1), and Glut3 transcripts were downregulated in the jaundiced group. In the cerebellum, glial fibrillary acidic protein (Gfap), myelin basic protein (Mbp), and Glut1 transcript expression was upregulated in the jaundiced group. These results indicate osmotic imbalance, gliosis, and changes in energy utilization and myelination, and demonstrate that preterm NHB critically affects brain development in a region-specific manner, with the cerebellum more severely impacted than the hippocampus.

Myeloid cell subpopulations compensate each other for Ccr2-deficiency in glioblastoma

AIMS

Glioblastomas are high-grade brain tumours that are characterised by the accumulation of brain-resident microglia and peripheral macrophages. Recruitment of these myeloid cells can be facilitated by CCR2/CCL2 signalling. Besides the well-known CCR2⁺ macrophages, we have identified microglia expressing CCR2 in glioma tissues. Thus, we investigated how Ccr2-deficiency of one of the myeloid cell populations affects the other population and tumour biology.

METHODS

We generated four chimeric groups to analyse single and combined Ccr2-deficiency of microglia and macrophages. On day 21 after tumour cell implantation (GL261), we conducted flow cytometry, immunofluorescence and real-time polymerase chain reaction analyses. Tumour volume and metabolism were determined by magnetic resonance imaging and magnetic resonance spectroscopy. Moreover, in vitro studies were performed with primary microglia and bone marrow-derived macrophages.

RESULTS

We demonstrated reduced infiltration of macrophages and microglia depending on the lack of Ccr2. However, the total number of myeloid cells remained constant except for the animals with dual Ccr2-knockout. Both microglia and macrophages with Ccr2-deficiency showed impaired expression of proinflammatory molecules and altered phagocytic activity. Despite the altered immunologic phenotype caused by Ccr2-deficiency, glioma progression and metabolism were hardly affected. Alterations were detected solely in apoptosis and proliferation of tumours from animals with specific Ccr2-deficient microglia, whereas vessel stability was increased in mice with Ccr2-knockout in both cell populations.

CONCLUSION

These results indicate that microglia and macrophages provide a homoeostatic balance within glioma tissue and compensate for the lack of the corresponding counterpart. Moreover, we identified that the CCR2/CCL2 axis is involved in the immunologic function of microglia and macrophages beyond its relevance for migration.

Fiber-based lactate recordings with fluorescence resonance energy transfer sensors by applying an magnetic resonance-informed correction of hemodynamic artifacts

Significance: Fluorescence resonance energy transfer (FRET) sensors offer enormous benefits when studying neurophysiology through confocal microscopy. Yet, their use for fiber-based in vivo recordings is hampered by massive confounding effects and has therefore been scarcely reported. Aim: We aim to investigate whether in vivo fiber-based lactate recordings in the rodent brain are feasible with FRET sensors and implement a correction algorithm for the predominant hemodynamic artifact. Approach: We performed fiber-based FRET recordings of lactate (Laconic) and calcium (Twitch-2B) simultaneously with functional MRI and pharmacological MRI. MR-derived parameters were applied to correct hemodynamic artifacts. Results of FRET measurements were validated by local field potential, magnetic resonance spectroscopy, and blood analysis. Results: Hemodynamic artifacts dominated fiber-based in vivo FRET measurements with both Laconic and Twitch-2B. Our MR-based correction algorithm enabled to remove the artifacts and detect lactate and calcium changes during sensory stimulation or intravenous lactate injections. Conclusions: In vivo fiber-based lactate recordings are feasible using FRET-based sensors. However, signal corrections are required. MR-derived hemodynamic parameters can successfully be applied for artifact correction.

CryptoCEST: A promising tool for spatially resolved identification of fungal brain lesions and their differentiation from brain tumors with MRI

•

The fungal disaccharide trehalose generates a concentration-dependent CEST MRI contrast.

•

CEST MRI can detect endogenous trehalose in Cryptococcus neoformans and C. gattii cells.

•

This enables spatially resolved identification of fungal lesions in the mouse brain.

•

The CryptoCEST contrast can differentiate cryptococcal brain lesions from gliomas.

•

CryptoCEST holds potential for non-invasive differential diagnosis of cryptococcomas.

Infectious brain lesions caused by the pathogenic fungi Cryptococcus neoformans and C. gattii, also referred to as cryptococcomas, could be diagnosed incorrectly as cystic brain tumors if only based on conventional magnetic resonance (MR) images. Previous MR spectroscopy (MRS) studies showed high local concentrations of the fungal disaccharide trehalose in cryptococcomas. The aim of this study was to detect and localize fungal brain lesions caused by Cryptococcus species based on Chemical Exchange Saturation Transfer (CEST) MR imaging of endogenous trehalose, and hereby to distinguish cryptococcomas from gliomas. In phantoms, trehalose and cryptococcal cells generated a concentration-dependent CEST contrast in the 0.2 – 2 ppm chemical shift range, similar to glucose, but approximately twice as strong. In vivo single voxel MRS of a murine cryptococcoma model confirmed the presence of trehalose in cryptococcomas, but mainly for lesions that were large enough compared to the size of the MRS voxel. With CEST MRI, combining the more specific CEST signal at 0.7 ppm with the higher signal-to-noise ratio signal at 4 ppm in the CryptoCEST contrast enabled localization and distinction of cryptococcomas from the normal brain and from gliomas, even for lesions smaller than 1 mm³. Thanks to the high endogenous concentration of the fungal biomarker trehalose in cryptococcal cells, the CryptoCEST contrast allowed identification of cryptococcomas with high spatial resolution and differentiation from gliomas in mice. Furthermore, the CryptoCEST contrast was tested to follow up antifungal treatment of cryptococcomas. Translation of this non-invasive method to the clinic holds potential for improving the differential diagnosis and follow-up of cryptococcal infections in the brain.

Long-term ovarian hormone deprivation alters functional connectivity, brain neurochemical profile and white matter integrity in the Tg2576 amyloid mouse model of Alzheimer's disease

Premenopausal bilateral ovariectomy is considered to be one of the risk factors of Alzheimer's disease (AD). However, the underlying mechanisms remain unclear. Here, we aimed to investigate long-term neurological consequences of ovariectomy in a rodent AD model, TG2576 (TG), and wild-type mice (WT) that underwent an ovariectomy or sham-operation, using in vivo MRI biomarkers. An increase in osmoregulation and energy metabolism biomarkers in the hypothalamus, a decrease in white matter integrity, and a decrease in the resting-state functional connectivity was observed in ovariectomized TG mice compared to sham-operated TG mice. In addition, we observed an increase in functional connectivity in ovariectomized WT mice compared to sham-operated WT mice. Furthermore, genotype (TG vs. WT) effects on imaging markers and GFAP immunoreactivity levels were observed, but there was no effect of interaction (Genotype × Surgery) on amyloid-beta-and GFAP immunoreactivity levels. Taken together, our results indicated that both genotype and ovariectomy alters imaging biomarkers associated with AD.

B0 shimming for in vivo magnetic resonance spectroscopy: Experts' consensus recommendations

Magnetic resonance spectroscopy (MRS) and spectroscopic imaging (MRSI) allow the chemical analysis of physiological processes in vivo and provide powerful tools in the life sciences and for clinical diagnostics. Excellent homogeneity of the static B₀ magnetic field over the object of interest is essential for achieving high-quality spectral results and quantitative metabolic measurements. The experimental minimization of B₀ variation is performed in a process called B₀ shimming. In this article, we summarize the concepts of B₀ field shimming using spherical harmonic shimming techniques, specific strategies for B₀ homogenization and crucial factors to consider for implementation and use in both brain and body. In addition, experts' recommendations are provided for minimum requirements for B₀ shim hardware and evaluation criteria for the primary outcome of adequate B₀ shimming for MRS and MRSI, such as the water spectroscopic linewidth.

High temporal resolution functional magnetic resonance spectroscopy in the mouse upon visual stimulation

Functional magnetic resonance spectroscopy (fMRS) quantifies metabolic variations upon presentation of a stimulus and can therefore provide complementary information compared to activity inferred from functional magnetic resonance imaging (fMRI). Improving the temporal resolution of fMRS can be beneficial to clinical applications where detailed information on metabolism can assist the characterization of brain function in healthy and sick populations as well as for neuroscience applications where information on the nature of the underlying activity could be potentially gained. Furthermore, fMRS with higher temporal resolution could benefit basic studies on animal models of disease and for investigating brain function in general. However, to date, fMRS has been limited to sustained periods of activation which risk adaptation and other undesirable effects. Here, we performed fMRS experiments in the mouse with high temporal resolution (12 s), and show the feasibility of such an approach for reliably quantifying metabolic variations upon activation. We detected metabolic variations in the superior colliculus of mice subjected to visual stimulation delivered in a block paradigm at 9.4 T. A robust modulation of glutamate is observed on the average time course, on the difference spectra and on the concentration distributions during active and recovery periods. A general linear model is used for the statistical analysis, and for exploring the nature of the modulation. Changes in NAAG, PCr and Cr levels were also detected. A control experiment with no stimulation reveals potential metabolic signal "drifts" that are not correlated with the functional activity, which should be taken into account when analyzing fMRS data in general. Our findings are promising for future applications of fMRS.

Comparison of in vivo and in situ detection of hippocampal metabolites in mouse brain using 1 H-MRS

The study of cerebral metabolites relies heavily on detection methods and sample preparation. Animal experiments in vivo require anesthetic agents that can alter brain metabolism, whereas ex vivo experiments demand appropriate fixation methods to preserve the tissue from rapid postmortem degradation. In this study, the metabolic profiles of mouse hippocampi using proton magnetic resonance spectroscopy (¹ H-MRS) were compared in vivo and in situ with or without focused beam microwave irradiation (FBMI) fixation. Ten major brain metabolites, including lactate (Lac), N-acetylaspartate (NAA), total choline (tCho), myo-inositol (mIns), glutamine (Gln), glutamate (Glu), aminobutyric acid (GABA), glutathione (GSH), total creatine (tCr) and taurine (Tau), were analyzed using LCModel. After FBMI fixation, the concentrations of Lac, tCho and mIns were comparable with those obtained in vivo under isoflurane, whereas other metabolites were significantly lower. Except for a decrease in NAA and an increase in Tau, all the other metabolites remained stable over 41 hours in FBMI-fixed brains. Without FBMI, the concentrations of mIns (before 2 hours), tCho and GABA were close to those measured in vivo. However, higher Lac (P < .01) and lower NAA, Gln, Glu, GSH, tCr and Tau were observed (P < .01). NAA, Gln, Glu, GSH, tCr and Tau exhibited good temporal stability for at least 20 hours in the unfixed brain, whereas a linear increase of tCho, mIns and GABA was observed. Possible mechanisms of postmortem degradation are discussed. Our results indicate that a proper fixation method is required for in situ detection depending on the targeted metabolites of specific interests in the brain.

In Vivo Measurement of Neurochemical Abnormalities in the Hippocampus in a Rat Model of Cuprizone-Induced Demyelination

This study quantitatively measured the changes in metabolites in the hippocampal lesions of a rat model of cuprizone-induced demyelination as detected using in vivo 7 T proton magnetic resonance spectroscopy. Nineteen Sprague Dawley rats were randomly divided into two groups and fed a normal chow diet or cuprizone (0.2%, w/w) for 7 weeks. Demyelinated hippocampal lesions were quantitatively measured using a 7 T magnetic resonance imaging scanner. All proton spectra were quantified for metabolite concentrations and relative ratios. Compared to those in the controls, the cuprizone-induced rats had significantly higher concentrations of glutamate (p = 0.001), gamma-aminobutyric acid (p = 0.019), and glutamate + glutamine (p = 0.001); however, creatine + phosphocreatine (p = 0.006) and myo-inositol (p = 0.001) concentrations were lower. In addition, we found that the glutamine and glutamate complex/total creatine (p < 0.001), glutamate/total creatine (p < 0.001), and GABA/total creatine (p = 0.002) ratios were significantly higher in cuprizone-treated rats than in control rats. Our results showed that cuprizone-induced neuronal demyelination may influence the severe abnormal metabolism in hippocampal lesions, and these responses could be caused by microglial activation, mitochondrial dysfunction, and astrocytic necrosis.

Magnetic resonance spectroscopy in the rodent brain: Experts' consensus recommendations

In vivo MRS is a non-invasive measurement technique used not only in humans, but also in animal models using high-field magnets. MRS enables the measurement of metabolite concentrations as well as metabolic rates and their modifications in healthy animals and disease models. Such data open the way to a deeper understanding of the underlying biochemistry, related disturbances and mechanisms taking place during or prior to symptoms and tissue changes. In this work, we focus on the main preclinical 1H, 31P and 13C MRS approaches to study brain metabolism in rodent models, with the aim of providing general experts' consensus recommendations (animal models, anesthesia, data acquisition protocols). An overview of the main practical differences in preclinical compared with clinical MRS studies is presented, as well as the additional biochemical information that can be obtained in animal models in terms of metabolite concentrations and metabolic flux measurements. The properties of high-field preclinical MRS and the technical limitations are also described.

Dietary supplementation with Lactobacillus rhamnosus JB-1 restores brain neurochemical balance and mitigates the progression of mood disorder in a rat model of chronic unpredictable mild stress

Major depressive disorder is a stress-related disease associated with brain metabolic dysregulation in the glutamine-glutamate/γ-aminobutyric acid (Gln-Glu/GABA) cycle. Recent studies have demonstrated that microbiome-gut-brain interactions have the potential to influence mental health. The hypothesis of this study was that Lactobacillus rhamnosus JB-1 (LR-JB1™) dietary supplementation has a positive impact on neuro-metabolism which can be quantified in vivo using magnetic resonance spectroscopy (MRS). A rat model of depressive-like disorder, chronic unpredictable mild stress (CUMS), was used. Baseline comparisons of MRS and behavior were obtained in a control group and in a stressed group subjected to CUMS. Of the 22 metabolites measured using MRS, stressed rats had significantly lower concentrations of GABA, glutamate, glutamine + glutathione, glutamate + glutamine, total creatine, and total N-acetylaspartate (tNAA). Stressed rats were then separated into 2 groups and supplemented with either LR-JB1™ or placebo and re-evaluated after 4 weeks of continued CUMS. The LR-JB1™ microbiotic diet restored these metabolites to levels previously observed in controls, while the placebo diet resulted in further significant decrease of glutamate, total choline, and tNAA. LR-JB1™ treated animals also exhibited calmer and more relaxed behavior, as compared with placebo treated animals. In summary, significant cerebral biochemical downregulation of major brain metabolites following prolonged stress were measured in vivo using MRS, and these decreases were reversed using a microbiotic dietary supplement of LR-JB1™, even in the presence of continued stress, which also resulted in a reduction of stress-induced behavior in a rat model of depressive-like disorder.

Metabolomic study of disease progression in scrapie prion infected mice; validation of a novel method for brain metabolite extraction

INTRODUCTION

Prion disease is a form of neurodegenerative disease caused by the misfolding and aggregation of cellular prion protein (PrP^C). The neurotoxicity of the misfolded form of prion protein, PrP^Sc still remains understudied. Here we try to investigate this issue using a metabolomics approach.

OBJECTIVES

The intention was to identify and quantify the small-in-size and water-soluble metabolites extracted from mice brains infected with the Rocky Mountain Laboratory isolate of mouse-adapted scrapie prions (RML) and track changes in these metabolites during disease evolution.

METHODS

A total of 73 mice were inoculated with RML prions or normal brain homogenate control; brains were harvested at 30, 60, 90, 120 and 150 days post-inoculation (dpi). We devised a high-efficiency metabolite extraction method and used nuclear magnetic resonance spectroscopy to identify and quantify 50 metabolites in the brain extracts. Data were analyzed using multivariate approach.

RESULTS

Brain metabolome profiles of RML infected animals displayed continuous changes throughout the course of disease. Among the analyzed metabolites, the most noteworthy changes included increases in myo-inositol and glutamine as well as decreases in 4-aminobutyrate, acetate, aspartate and taurine.

CONCLUSION

We report a novel metabolite extraction method for lipid-rich tissue. As all the major metabolites are identifiable and quantifiable by magnetic resonance spectroscopy, this study suggests that tracking of neurochemical profiles could be effective in monitoring the progression of neurodegenerative diseases and useful for assessing the efficacy of candidate therapeutics.

Evaluating the potential of hyperpolarised [1-13C] L-lactate as a neuroprotectant metabolic biosensor for stroke

Cerebral metabolism, which can be monitored by magnetic resonance spectroscopy (MRS), changes rapidly after brain ischaemic injury. Hyperpolarisation techniques boost ¹³C MRS sensitivity by several orders of magnitude, thereby enabling in vivo monitoring of biochemical transformations of hyperpolarised (HP) ¹³C-labelled precursors with a time resolution of seconds. The exogenous administration of the metabolite L-lactate was shown to decrease lesion size and ameliorate neurological outcome in preclinical studies in rodent stroke models, as well as influencing brain metabolism in clinical pilot studies of acute brain injury patients. The aim of this study was to demonstrate the feasibility of measuring HP [1-¹³C] L-lactate metabolism in real-time in the mouse brain after ischaemic stroke when administered after reperfusion at a therapeutic dose. We showed a rapid, time-after-reperfusion-dependent conversion of [1-¹³C] L-lactate to [1-¹³C] pyruvate and [¹³C] bicarbonate that brings new insights into the neuroprotection mechanism of L-lactate. Moreover, this study paves the way for the use of HP [1-¹³C] L-lactate as a sensitive molecular-imaging biosensor in ischaemic stroke patients after endovascular clot removal.

Metabolic fingerprints discriminating severity of acute ischemia using in vivo high-field 1 H magnetic resonance spectroscopy

Despite the improving imaging techniques, it remains challenging to produce magnetic resonance (MR) imaging fingerprints depicting severity of acute ischemia. The aim of this study was to evaluate the potential of the overall high-field ¹ H MR Spectroscopy (¹ H-MRS) neurochemical profile as a metabolic signature for acute ischemia severity in rodent brains. We modeled global ischemia with one-stage 4-vessel-occlusion (4VO) in rats. Vascular structures were assessed immediately by magnetic resonance angiography. The neurochemical responses in the bilateral cortex were measured 1 h after stroke onset by ¹ H-MRS. Then we used Partial-Least-Squares discriminant analysis on the overall neurochemical profiles to seek metabolic signatures for ischemic severity subgroups. This approach was further tested on neurochemical profiles of mouse striatum 1 h after permanent middle cerebral artery occlusion, where vascular blood flow was monitored by laser Doppler. Magnetic resonance angiography identified successful 4VO from controls and incomplete global ischemia (e.g., 3VO). ¹ H-MR spectra of rat cortex after 4VO showed a specific metabolic pattern, distinct from that of respective controls and rats with 3VO. Partial-Least-Squares discriminant analysis on the overall neurochemical profiles revealed metabolic signatures of acute ischemia that may be extended to mice after permanent middle cerebral artery occlusion. Fingerprinting severity of acute ischemia using neurochemical information may improve MR diagnosis in stroke patients.

High-resolution creatine mapping of mouse brain at 11.7 T using non-steady-state chemical exchange saturation transfer

The current study aims to optimize the acquisition scheme for the creatine chemical exchange saturation transfer weighted (CrCESTw) signal on mouse brain at 11.7 T, in which a strong magnetization transfer contrast (MTC) is present, and to further develop the polynomial and Lorentzian line-shape fitting (PLOF) method for quantifying CrCESTw signal with a non-steady-state (NSS) acquisition scheme. Studies on a Cr phantom with cross-linked bovine serum albumin (BSA) as well as on mouse brain demonstrated that the maximum CrCESTw signal was reached with a short saturation time determined by the rotating frame relaxation time of the MTC pool instead of the steady-state saturation. The saturation power for the maximal signal was around 1-1.5 μT for Cr with 20% cross-linked BSA and in vivo applications, but 2 μT was found to be most practical for signal stability. For the CrCEST acquisition with strong MTC interference, the optimal saturation power and length are completely different from those on Cr solution alone. This observation could be explained well using R_1ρ theory by incorporating the strong MTC pool. Finally, a high-resolution Cr map was obtained on mouse brain using the PLOF method with the NSS CEST acquisition and a cryogenic coil. The Cr map obtained by CEST showed homogenous intensity across the mouse brain except for regions with cerebrospinal fluid.

Multimodal Imaging of Retired Professional Contact Sport Athletes Does Not Provide Evidence of Structural and Functional Brain Damage

BACKGROUND

Long-term consequences of playing professional football and hockey on brain function and structural neuronal integrity are unknown.

OBJECTIVES

To investigate multimodal metabolic and structural brain magnetic resonance imaging (MRI) differences in retired professional contact sport athletes compared with noncontact sport athletes.

METHODS

Twenty-one male contact sport athletes and 21 age-matched noncontact sport athletes were scanned on a 3 tesla (3T) MRI using a multimodal imaging approach. The MRI outcomes included presence, number, and volume of focal white matter signal abnormalities, volumes of global and regional tissue-specific brain structures, diffusion-tensor imaging tract-based spatial statistics measures of mean diffusivity and fractional anisotropy, quantitative susceptibility mapping of deep gray matter, presence, number, and volume of cerebral microbleeds, MR spectroscopy N-acetyl-aspartate, glutamate, and glutamine concentrations relative to creatine and phosphor creatine of the corpus callosum, and perfusion-weighted imaging mean transit time, cerebral blood flow, and cerebral blood volume outcomes. Subjects were also classified as having mild cognitive impairment.

RESULTS

No significant differences were found for structural or functional MRI measures between contact sport athletes and noncontact sport athletes.

CONCLUSIONS

This multimodal imaging study did not show any microstructural, metabolic brain tissue injury differences in retired contact versus non-contact sport athletes.

Focus on the glycerophosphocholine pathway in choline phospholipid metabolism of cancer

Activated choline metabolism is a hallmark of carcinogenesis and tumor progression, which leads to elevated levels of phosphocholine and glycerophosphocholine in all types of cancer tested so far. Magnetic resonance spectroscopy applications have played a key role in detecting these elevated choline phospholipid metabolites. To date, the majority of cancer-related studies have focused on phosphocholine and the Kennedy pathway, which constitutes the biosynthesis pathway for membrane phosphatidylcholine. Fewer and more recent studies have reported on the importance of glycerophosphocholine in cancer. In this review article, we summarize the recent literature on glycerophosphocholine metabolism with respect to its cancer biology and its detection by magnetic resonance spectroscopy applications.

Evolution of the neurochemical profiles in the G93A-SOD1 mouse model of amyotrophic lateral sclerosis

In vivo ¹H magnetic resonance spectroscopy (¹H-MRS) investigations of amyotrophic lateral sclerosis (ALS) mouse brain may provide neurochemical profiles and alterations in association with ALS disease progression. We aimed to longitudinally follow neurochemical evolutions of striatum, brainstem and motor cortex of mice transgenic for G93A mutant human superoxide dismutase type-1 (G93A-SOD1), an ALS model. Region-specific neurochemical alterations were detected in asymptomatic G93A-SOD1 mice, particularly in lactate (-19%) and glutamate (+8%) of brainstem, along with γ-amino-butyric acid (-30%), N-acetyl-aspartate (-5%) and ascorbate (+51%) of motor cortex. With disease progression towards the end-stage, increased numbers of metabolic changes of G93A-SOD1 mice were observed (e.g. glutamine levels increased in the brainstem (>+66%) and motor cortex (>+54%)). Through ALS disease progression, an overall increase of glutamine/glutamate in G93A-SOD1 mice was observed in the striatum (p < 0.01) and even more so in two motor neuron enriched regions, the brainstem and motor cortex (p < 0.0001). These ¹H-MRS data underscore a pattern of neurochemical alterations that are specific to brain regions and to disease stages of the G93A-SOD1 mouse model. These neurochemical changes may contribute to early diagnosis and disease monitoring in ALS patients.

Assessing Cerebral Metabolism in the Immature Rodent: From Extracts to Real-Time Assessments

Brain development is an energy-expensive process. Although glucose is irreplaceable, the developing brain utilizes a variety of substrates such as lactate and the ketone bodies, β-hydroxybutyrate and acetoacetate, to produce energy and synthesize the structural components necessary for cerebral maturation. When oxygen and nutrient supplies to the brain are restricted, as in neonatal hypoxia-ischemia (HI), cerebral energy metabolism undergoes alterations in substrate use to preserve the production of adenosine triphosphate. These changes have been studied by in situ biochemical methods that yielded valuable quantitative information about high-energy and glycolytic metabolites and established a temporal profile of the cerebral metabolic response to hypoxia and HI. However, these analyses relied on terminal experiments and averaging values from several animals at each time point as well as challenging requirements for accurate tissue processing.More recent methodologies have focused on in vivo longitudinal analyses in individual animals. The emerging field of metabolomics provides a new investigative tool for studying cerebral metabolism. Magnetic resonance spectroscopy (MRS) has enabled the acquisition of a snapshot of the metabolic status of the brain as quantifiable spectra of various intracellular metabolites. Proton (1H) MRS has been used extensively as an experimental and diagnostic tool of HI in the pursuit of markers of long-term neurodevelopmental outcomes. Still, the interpretation of the metabolite spectra acquired with 1H MRS has proven challenging, due to discrepancies among studies, regarding calculations and timing of measurements. As a result, the predictive utility of such studies is not clear. 13C MRS is methodologically more challenging, but it provides a unique window on living tissue metabolism via measurements of the incorporation of 13C label from substrates into brain metabolites and the localized determination of various metabolic fluxes. The newly developed hyperpolarized 13C MRS is an exciting method for assessing cerebral metabolism in vivo, that bears the advantages of conventional 13C MRS but with a huge gain in signal intensity and much shorter acquisition times. The first part of this review article provides a brief description of the findings of biochemical and imaging methods over the years as well as a discussion of their associated strengths and pitfalls. The second part summarizes the current knowledge on cerebral metabolism during development and HI brain injury.

Implications of the magnetic susceptibility difference between grey and white matter for single-voxel proton spectroscopy at 7 T

Magnetic susceptibility differences between grey matter (GM) and white matter (WM) can potentially affect lineshapes and chemical shifts in single-voxel spectroscopy. This study aimed to investigate the consequences and potential utility of these effects. Spectroscopy voxels were segmented into GM, WM, and cerebrospinal fluid based on T₁-weighted images. GM and WM lineshapes were computed using multi-echo gradient-echo images to measure the frequency distribution. Twenty 7 Tesla single voxel spectra with corresponding T₁-weighted images were acquired from the frontal and parietal lobes from five healthy human volunteers. Consistent frequency shifts (mean [±SD] 4.9 ± 2.0 Hz) and linewidth differences (2.4 ± 1.5 Hz) between the two tissue types were observed. Directly visible metabolites (creatine, choline, and myo-inositol) exhibited frequency shifts and linewidth differences that were consistent with a linear-weighted summation of their expected GM and WM distribution ratios. The magnetic susceptibility difference between GM and WM had a detectable effect on single-voxel proton spectra, which results in both frequency shifts and lineshape broadening. This effect can be used to estimate the relative metabolic distribution in the GM and WM for directly observable metabolites. Fractional distributions estimated with this method demonstrated good agreement with literature values for the selected metabolites.

High-fat diet consumption alters energy metabolism in the mouse hypothalamus

BACKGROUND/OBJECTIVES

High-fat diet consumption is known to trigger an inflammatory response in the hypothalamus, which has been characterized by an initial expression of pro-inflammatory genes followed by hypothalamic astrocytosis, microgliosis, and the appearance of neuronal injury markers. The specific effects of high-fat diet on hypothalamic energy metabolism and neurotransmission are however not yet known and have not been investigated before.

SUBJECTS/METHODS

We used ¹H and ¹³C magnetic resonance spectroscopy (MRS) and immunofluorescence techniques to evaluate in vivo the consequences of high-saturated fat diet administration to mice, and explored the effects on hypothalamic metabolism in three mouse cohorts at different time points for up to 4 months.

RESULTS

We found that high-fat diet increases significantly the hypothalamic levels of glucose (P < 0.001), osmolytes (P < 0.001), and neurotransmitters (P < 0.05) from 2 months of diet, and alters the rates of metabolic (P < 0.05) and neurotransmission fluxes (P < 0.001), and the contribution of non-glycolytic substrates to hypothalamic metabolism (P < 0.05) after 10 weeks of high-fat feeding.

CONCLUSIONS/INTERPRETATION

We report changes that reveal a high-fat diet-induced alteration of hypothalamic metabolism and neurotransmission that is quantifiable by ¹H and ¹³C MRS in vivo, and present the first evidence of the extension of the inflammation pathology to a localized metabolic imbalance.

In vivo 13C MRS in the mouse brain at 14.1 Tesla and metabolic flux quantification under infusion of [1,6-13C2]glucose

In vivo ¹³C magnetic resonance spectroscopy (MRS) enables the investigation of cerebral metabolic compartmentation while, e.g. infusing ¹³C-labeled glucose. Metabolic flux analysis of ¹³C turnover previously yielded quantitative information of glutamate and glutamine metabolism in humans and rats, while the application to in vivo mouse brain remains exceedingly challenging. In the present study, ¹³C direct detection at 14.1 T provided highly resolved in vivo spectra of the mouse brain while infusing [1,6-¹³C₂]glucose for up to 5 h. ¹³C incorporation to glutamate and glutamine C4, C3, and C2 and aspartate C3 were detected dynamically and fitted to a two-compartment model: flux estimation of neuron-glial metabolism included tricarboxylic acid cycle (TCA) flux in astrocytes (V_g = 0.16 ± 0.03 µmol/g/min) and neurons (V_TCAⁿ= 0.56 ± 0.03 µmol/g/min), pyruvate carboxylase activity (V_PC = 0.041 ± 0.003 µmol/g/min) and neurotransmission rate (V_NT = 0.084 ± 0.008 µmol/g/min), resulting in a cerebral metabolic rate of glucose (CMR_glc) of 0.38 ± 0.02 µmol/g/min, in excellent agreement with that determined with concomitant ¹⁸F-fluorodeoxyglucose positron emission tomography (¹⁸FDG PET).We conclude that modeling of neuron-glial metabolism in vivo is accessible in the mouse brain from ¹³C direct detection with an unprecedented spatial resolution under [1,6-¹³C₂]glucose infusion.

Teriflunomide's Effect on Glia in Experimental Demyelinating Disease: A Neuroimaging and Histologic Study

BACKGROUND AND PURPOSE

Teriflunomide reduces disability progression and brain atrophy in multiple sclerosis patients. The exact mechanism of action by which teriflunomide exerts these effects is currently unknown. We assessed the effect of teriflunomide on brain glial cells in the Theiler's murine encephalomyelitis virus (TMEV) by using a histological approach in combination with neuroimaging.

METHODS

Forty-eight SJL female mice received an intracerebral injection of TMEV at 6-8 weeks of age and were then treated with teriflunomide (n = 24) or placebo (n = 24) for 9 months. They were examined with MRI and behavioral testing at 2, 6, and 9 months postinduction (mPI). Of those, 18 teriflunomide-treated and 17 controls mice were analyzed histologically at 9 mPI to sample from different brain regions for myelination status, microglial density, and oligodendroglial lineage. The histological and MRI outcomes were correlated.

RESULTS

Corpus callosum microglial density was numerically lower in the teriflunomide-treated mice compared to the control group (141.1 ± 21.7 SEM vs. 214.74 ± 34.79 SEM, Iba1⁺ cells/mm² , P = .087). Basal ganglia (BG) microglial density in the teriflunomide group exhibited a negative correlation with fractional anisotropy (P = .021) and a positive correlation with mean diffusivity (P = .034), indicating less inflammation and axonal damage. Oligodendroglial lineage cell and myelin density were not significantly different between treatment groups. However, a significant positive correlation between BG oligodendrocytes and BG volume (P = .027), and with N-acetyl aspartate concentration (P = .008), was found in the teriflunomide group, indicating less axonal loss.

CONCLUSION

Teriflunomide altered microglia density and oligodendrocytes differentiation, which was associated with less evident microstructural damage on MRI.

Improved localization, spectral quality, and repeatability with advanced MRS methodology in the clinical setting

PURPOSE

To investigate the utility of an advanced magnetic resonance spectroscopy (MRS) protocol in the clinical setting, and to compare the localization accuracy, spectral quality, and quantification repeatability between this advanced and the conventional vendor-provided MRS protocol on a clinical 3T platform.

METHODS

Proton spectra were measured from the posterior cingulate cortices in 30 healthy elderly subjects by clinical MR technologists using a vendor-provided (point resolved spectroscopy with advanced 3D gradient-echo B0 shimming) and an advanced (semi-LASER with FAST(EST)MAP shimming) protocol, in random order. Spectra were quantified with LCModel using standard pipelines for the clinical and research settings, respectively.

RESULTS

The advanced protocol outperformed the vendor-provided protocol in localization accuracy (chemical-shift-displacement error: 2.0%/ppm, semi-LASER versus 11.6%/ppm, point resolved spectroscopy), spectral quality (water linewidth: 6.1 ± 1.8 Hz, FAST(EST)MAP versus 10.5 ± 3.7 Hz, 3D gradient echo; P < 7e-6; residual water: 0.08 ± 0.12%, VAPOR versus 0.45 ± 0.50%, WET; P < 2e-5) and within-session repeatability of metabolite concentrations, particularly of low signal-to-noise ratio data with two to eight averages (test-retest coefficients of variance of metabolite concentrations, P < 0.01). Concentrations of J-coupled metabolites such as γ-aminobutyric acid and glutamate were biased when using the default pipeline with simulated macromolecules.

CONCLUSIONS

The quality of MRS data can be improved using advanced acquisition and analysis protocols on standard 3T hardware in the clinical setting, which can facilitate robust applications in central nervous system diseases. Magn Reson Med 79:1241-1250, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Diffusion MRI and MR spectroscopy reveal microstructural and metabolic brain alterations in chronic mild stress exposed rats: A CMS recovery study

Chronic mild stress (CMS) induced depression elicits several debilitating symptoms and causes a significant economic burden on society. High variability in the symptomatology of depression poses substantial impediment to accurate diagnosis and therapy outcome. CMS exposure induces significant metabolic and microstructural alterations in the hippocampus (HP), prefrontal cortex (PFC), caudate-putamen (CP) and amygdala (AM), however, recovery from these maladaptive changes are limited and this may provide negative effects on the therapeutic treatment and management of depression. The present study utilized anhedonic rats from the unpredictable CMS model of depression to study metabolic recovery in the ventral hippocampus (vHP) and microstructural recovery in the HP, AM, CP, and PFC. The study employed 1H MR spectroscopy (1H MRS) and in-vivo diffusion MRI (d-MRI) at the age of week 18 (week 1 post CMS exposure) week 20 (week 3 post CMS) and week 25 (week 8 post CMS exposure) in the anhedonic group, and at the age of week 18 and week 22 in the control group. The d-MRI data have provided an array of diffusion tensor metrics (FA, MD, AD, and RD), and fast kurtosis metrics (MKT, WL and WT). CMS exposure induced a significant metabolic alteration in vHP, and significant microstructural alterations were observed in the HP, AM, and PFC in comparison to the age match control and within the anhedonic group. A significantly high level of N-acetylaspartate (NAA) was observed in vHP at the age of week 18 in comparison to age match control and week 20 and week 25 of the anhedonic group. HP and AM showed significant microstructural alterations up to the age of week 22 in the anhedonic group. PFC showed significant microstructural alterations only at the age of week 18, however, most of the metrics showed significantly higher value at the age of week 20 in the anhedonic group. The significantly increased NAA concentration may indicate impaired catabolism due to astrogliosis or oxidative stress. The significantly increased WL in the AM and HP may indicate hypertrophy of AM and reduced volume of HP. Such metabolic and microstructural alterations could be useful in disease diagnosis and follow-up treatment intervention in depression and similar disorders.

In Vivo Heteronuclear Magnetic Resonance Spectroscopy

Magnetic Resonance Spectroscopy is a technique that has the capability of measuring metabolites in vivo and, in appropriate conditions, to infer its metabolic rates. The success of MRS depends a lot on its sensitivity, which limits the usage of X-nuclei MRS. However, technological developments and refinements in methods have made in vivo heteronuclear MRS possible in humans and in small animals. This chapter provides detailed descriptions of the main procedures needed to perform successful in vivo heteronuclear MRS experiments, with a particular focus on experimental setup in ¹³C MRS experiments in rodents.

Improved resolution of glutamate, glutamine and γ-aminobutyric acid with optimized point-resolved spectroscopy sequence timings for their simultaneous quantification at 9.4 T

Glutamine (Gln), glutamate (Glu) and γ-aminobutyric acid (GABA) are relevant brain metabolites that can be measured with magnetic resonance spectroscopy (MRS). This work optimizes the point-resolved spectroscopy (PRESS) sequence echo times, TE₁ and TE₂ , for improved simultaneous quantification of the three metabolites at 9.4 T. Quantification was based on the proton resonances of Gln, Glu and GABA at ≈2.45, ≈2.35 and ≈2.28 ppm, respectively. Glu exhibits overlap with both Gln and GABA; in addition, the Gln peak is contaminated by signal from the strongly coupled protons of N-acetylaspartate (NAA), which resonate at about 2.49 ppm. J-coupling evolution of the protons was characterized numerically and verified experimentally. A {TE₁ , TE₂ } combination of {106 ms, 16 ms} minimized the NAA signal in the Gln spectral region, whilst retaining Gln, Glu and GABA peaks. The efficacy of the technique was verified on phantom solutions and on rat brain in vivo. LCModel was employed to analyze the in vivo spectra. The average T₂ -corrected Gln, Glu and GABA concentrations were found to be 3.39, 11.43 and 2.20 mM, respectively, assuming a total creatine concentration of 8.5 mM. LCModel Cramér-Rao lower bounds (CRLBs) for Gln, Glu and GABA were in the ranges 14-17%, 4-6% and 16-19%, respectively. The optimal TE resulted in concentrations for Gln and GABA that agreed more closely with literature concentrations compared with concentrations obtained from short-TE spectra acquired with a {TE₁ , TE₂ } combination of {12 ms, 9 ms}. LCModel estimations were also evaluated with short-TE PRESS and with the optimized long TE of {106 ms, 16 ms}, using phantom solutions of known metabolite concentrations. It was shown that concentrations estimated with LCModel can be inaccurate when combined with short-TE PRESS, where there is peak overlap, even when low (<20%) CRLBs are reported.

Investigation of the contribution of total creatine to the CEST Z-spectrum of brain using a knockout mouse model

The current study aims to assign and estimate the total creatine (tCr) signal contribution to the Z-spectrum in mouse brain at 11.7 T. Creatine (Cr), phosphocreatine (PCr) and protein phantoms were used to confirm the presence of a guanidinium resonance at this field strength. Wild-type (WT) and knockout mice with guanidinoacetate N-methyltransferase deficiency (GAMT-/-), which have low Cr and PCr concentrations in the brain, were used to assign the tCr contribution to the Z-spectrum. To estimate the total guanidinium concentrations, two pools for the Z-spectrum around 2 ppm were assumed: (i) a Lorentzian function representing the guanidinium chemical exchange saturation transfer (CEST) at 1.95 ppm in the 11.7-T Z-spectrum; and (ii) a background signal that can be fitted by a polynomial function. Comparison between the WT and GAMT-/- mice provided strong evidence for three types of contribution to the peak in the Z-spectrum at 1.95 ppm, namely proteins, Cr and PCr, the latter fitted as tCr. A ratio of 20 ± 7% (protein) and 80 ± 7% tCr was found in brain at 2 μT and 2 s saturation. Based on phantom experiments, the tCr peak was estimated to consist of about 83 ± 5% Cr and 17 ± 5% PCr. Maps for tCr of mouse brain were generated based on the peak at 1.95 ppm after concentration calibration with in vivo magnetic resonance spectroscopy.

Effect of teriflunomide on cortex-basal ganglia-thalamus (CxBGTh) circuit glutamatergic dysregulation in the Theiler's Murine Encephalomyelitis Virus mouse model of multiple sclerosis

BACKGROUND

Pathology of gray matter is associated with development of physical and cognitive disability in patients with multiple sclerosis. In particular, glutamatergic dysregulation in the cortex-basal ganglia-thalamus (CxBGTh) circuit could be associated with decline in these behaviors.

OBJECTIVES

To investigate the effect of an immunomodulatory therapy (teriflunomide, Aubagio®) on changes of the CxBGTh loop in the Theiler's Murine Encephalomyelitis Virus, (TMEV) mouse model of MS.

METHODS

Forty-eight (48) mice were infected with TMEV, treated with teriflunomide (24) or control vehicle (24) and followed for 39 weeks. Mice were examined with MRS and volumetric MRI scans (0, 8, 26, and 39 weeks) in the cortex, basal ganglia and thalamus, using a 9.4T scanner, and with behavioral tests (0, 4, 8, 12, 17, 26, and 39 weeks). Within conditions, MRI measures were compared between two time points by paired samples t-test and across multiple time points by repeated measures ANOVA (rmANOVA), and between conditions by independent samples t-test and rmANOVA, respectively. Data were considered as significant at the p<0.01 level and as a trend at p<0.05 level.

RESULTS

In the thalamus, the teriflunomide arm exhibited trends toward decreased glutamate levels at 8 and 26 weeks compared to the control arm (p = 0.039 and p = 0.026), while the control arm exhibited a trend toward increased glutamate between 0 to 8 weeks (p = 0.045). In the basal ganglia, the teriflunomide arm exhibited a trend toward decreased glutamate earlier than the control arm, from 0 to 8 weeks (p = 0.011), resulting in decreased glutamate compared to the control arm at 8 weeks (p = 0.016).

CONCLUSIONS

Teriflunomide may reduce possible excitotoxicity in the thalamus and basal ganglia by lowering glutamate levels.

Initial investigation of glucose metabolism in mouse brain using enriched 17 O-glucose and dynamic 17 O-MRS

In this initial work, the in vivo degradation of ¹⁷ O-labeled glucose was studied during cellular glycolysis. To monitor cellular glucose metabolism, direct ¹⁷ O-magnetic resonance spectroscopy (MRS) was used in the mouse brain at 9.4 T. Non-localized spectra were acquired with a custom-built transmit/receive (Tx/Rx) two-turn surface coil and a free induction decay (FID) sequence with a short TR of 5.4 ms. The dynamics of labeled oxygen in the anomeric 1-OH and 6-CH₂ OH groups was detected using a Hankel-Lanczos singular value decomposition (HLSVD) algorithm for water suppression. Time-resolved ¹⁷ O-MRS (temporal resolution, 42/10.5 s) was performed in 10 anesthetized (1.25% isoflurane) mice after injection of a 2.2 M solution containing 2.5 mg/g body weight of differently labeled ¹⁷ O-glucose dissolved in 0.9% physiological saline. From a pharmacokinetic model fit of the H₂¹⁷ O concentration-time course, a mean apparent cerebral metabolic rate of ¹⁷ O-labeled glucose in mouse brain of CMR_Glc  = 0.07 ± 0.02 μmol/g/min was extracted, which is of the same order of magnitude as a literature value of 0.26 ± 0.06 μmol/g/min reported by ¹⁸ F-fluorodeoxyglucose (¹⁸ F-FDG) positron emission tomography (PET). In addition, we studied the chemical exchange kinetics of aqueous solutions of ¹⁷ O-labeled glucose at the C1 and C6 positions with dynamic ¹⁷ O-MRS. In conclusion, the results of the exchange and in vivo experiments demonstrate that the C6-¹⁷ OH label in the 6-CH₂ OH group is transformed only glycolytically by the enzyme enolase into the metabolic end-product H₂¹⁷ O, whereas C1-¹⁷ OH ends up in water via direct hydrolysis as well as glycolysis. Therefore, dynamic ¹⁷ O-MRS of highly labeled ¹⁷ O-glucose could provide a valuable non-radioactive alternative to FDG PET in order to investigate glucose metabolism.

Brain neurochemical and hemodynamic findings in the NY1DD mouse model of mild sickle cell disease

To characterize the cerebral profile associated with sickle cell disease (SCD), we used in vivo proton MRI and MRS to quantify hemodynamics and neurochemicals in the thalamus of NY1DD mice, a mild model of SCD, and compared them with wild-type (WT) control mice. Compared with WT mice, NY1DD mice at steady state had elevated cerebral blood flow (CBF) and concentrations of N-acetylaspartate (NAA), glutamate (Glu), alanine, total creatine and N-acetylaspartylglutamate. Concentrations of glutathione (GSH) at steady state showed a negative correlation with BOLD signal change in response to 100% oxygen, a marker for oxidative stress, and mean diffusivity assessed using diffusion-tensor imaging, a marker for edematous inflammation. In NY1DD mice, elevated basal CBF was correlated negatively with [NAA], but positively with concentration of glutamine ([Gln]). Immediately after experimental hypoxia (at reoxygenation after 18 hours of 8% O₂ ), concentrations of NAA, Glu, GSH, Gln and taurine (Tau) increased only in NY1DD mice. [NAA], [Glu], [GSH] and [Tau] all returned to baseline levels two weeks after the hypoxic episode. The altered neurochemical profile in the NY1DD mouse model of SCD at steady state and following experimental hypoxia/reoxygenation suggests a state of chronic oxidative stress leading to compensatory cerebral metabolic adjustments.

Progress towards in vivo brain 13C-MRS in mice: Metabolic flux analysis in small tissue volumes

The combination of dynamic ¹³C MRS data under infusion of ¹³C-labelled substrates and compartmental models of cerebral metabolism enabled in vivo measurement of metabolic fluxes with a quantitative and distinct determination of cellular-specific activities. The non-invasive nature and the chemical specificity of the ¹³C dynamic data obtained in those tracer experiments makes it an attractive approach offering unique insights into cerebral metabolism. Genetically engineered mice present a wealth of disease models particularly interesting for the neuroscience community. Nevertheless, in vivo¹³C NMR studies of the mouse brain are only recently appearing in the field due to the numerous challenges linked to the small mouse brain volume and the difficulty to follow the mouse physiological parameters within the NMR system during the infusion experiment. This review will present the progresses in the quest for a higher in vivo¹³C signal-to-noise ratio up to the present state of the art techniques, which made it feasible to assess glucose metabolism in different regions of the mouse brain. We describe how experimental results were integrated into suitable compartmental models and how a deep understanding of cerebral metabolism depends on the reliable detection of ¹³C in the different molecules and carbon positions.

A practical guide to in vivo proton magnetic resonance spectroscopy at high magnetic fields

Localized proton magnetic resonance spectroscopy (¹H-MRS) is a noninvasive tool for measuring in vivo neurochemical information in animal and human brains. With the increase of magnetic field strength, whereas localized ¹H-MRS benefits from higher sensitivity and spectral dispersion, it is challenged by increased spatial inhomogeneity of the B₀ and B₁ fields, larger chemical shift displacement error, and shortened T₂ relaxation times of metabolites. Advanced localized ¹H-MRS methodologies developed for high magnetic fields have shown promising results and allow the measurement of neurochemical profiles with up to 19 brain metabolites, including less-abundant metabolites, such as glutathione, glycine, γ-aminobutyric acid and ascorbate. To provide a practical guide for conducting in vivo¹H-MRS studies at high magnetic field strength, we reviewed various essential technical aspects from data acquisition (hardware requirements, B₁ and B₀ inhomogeneity, water suppression, localization sequences and acquisition strategies) to data processing (frequency and phase correction, spectral quality control, spectral fitting and concentration referencing). Additionally, we proposed guidelines for choosing the most appropriate data acquisition and processing approaches to maximize the achievable neurochemical information.

Characterization of the response of taurine protons to PRESS at 9.4 T for Resolving choline and Determining taurine T2

Point-resolved spectroscopy (PRESS), characterized by two TEs (TE1 and TE2 ), can be employed to perform animal magnetic resonance spectroscopy (MRS) studies at 9.4 T. Taurine (Tau) and choline (Cho) are relevant metabolites that can be measured by MRS. In this work, the response of the J-coupled protons of Tau as a function of PRESS TE1 and TE2 was characterized at 9.4 T to achieve two objectives. The first was to determine two TE1 and TE2 combinations that could be used to obtain T2 -corrected measures of Tau (3.42 ppm) that were minimally influenced by J coupling. The second was to exploit the Tau J coupling to find a timing combination that minimized the 3.25-ppm Tau signal to enable the Cho (3.22 ppm) resonance to be resolved from the overlapping Tau signal. The response of Tau protons was investigated both numerically and experimentally. It was numerically determined that the timings {TE1 , TE2 } = {17 ms, 10 ms} and {TE1 , TE2 } = {80 ms, 70 ms} yielded similar 3.42-ppm Tau resonance areas (5% difference), rendering them suitable for Tau T2 determination. {TE1 , TE2 } = {25 ms, 50 ms} was found to yield minimal 3.25-ppm Tau signal, reducing its interference with Cho. The efficacy of the timings was demonstrated on phantom solutions and in vivo in four Sprague Dawley rats. LCModel was employed to analyse the in vivo spectra and Tau T2 values were estimated by fitting the Tau peak areas obtained with {TE1 , TE2 } = {17 ms, 10 ms} and {TE1 , TE2 } = {80 ms, 70 ms} to a monoexponentially decaying function. An average Tau T2 of 106 ms (standard deviation, 12 ms) was obtained. LCModel analysis of rat spectra obtained with {TE1 , TE2 } = {25 ms, 50 ms} demonstrated negligible levels of Tau signal, compared with that obtained with short TE.

Magnetic resonance spectroscopic analysis of neurometabolite changes in the developing rat brain at 7T

We utilized proton magnetic resonance spectroscopy to evaluate the metabolic profile of the hippocampus and anterior cingulate cortex of the developing rat brain from postnatal days 14-70. Measured metabolite concentrations were modeled using linear, exponential, or logarithmic functions and the time point at which the data reached plateau (i.e. when the portion of the data could be fit to horizontal line) was estimated and was interpreted as the time when the brain has reached maturity with respect to that metabolite. N-acetyl-aspartate and myo-inositol increased within the observed period. Gluthathione did not vary significantly, while taurine decreased initially and then stabilized. Phosphocreatine and total creatine had a tendency to increase towards the end of the experiment. Some differences between our data and the published literature were observed in the concentrations and dynamics of phosphocreatine, myo-inositol, and GABA in the hippocampus and creatine, GABA, glutamine, choline and N-acetyl-aspartate in the cortex. Such differences may be attributed to experimental conditions, analysis approaches and animal species. The latter is supported by differences between in-house rat colony and rats from Charles River Labs. Spectroscopy provides a valuable tool for non-invasive brain neurochemical profiling for use in developmental neurobiology research. Special attention needs to be paid to important sources of variation like animal strain and commercial source.

Neurochemical Changes Associated with Stress-Induced Sleep Disturbance in Rats: In Vivo and In Vitro Measurements

The goal of this study was to quantitatively assess the changes in the cerebral neurochemical profile and to identify those factors that contribute to the alteration of endogenous biomolecules when rats are subjected to stress-induced sleep disturbance. We exposed Sprague-Dawley rats (controls: n = 9; stress-induced sleep perturbation rats: n = 11) to a psychological stressor (cage exchange method) to achieve stress-induced sleep perturbation. In vivo magnetic resonance imaging assessments were carried out using a high-resolution 9.4 T system. For in vivo neurochemical analysis, a single voxel was localized in the right dorsal hippocampal region, and in vivo spectra were quantified for 17 cerebral neurochemical signals. Rats were sacrificed upon completion of the magnetic resonance spectroscopy protocol, and whole-brain tissue was harvested from twenty subjects. The dopamine and serotonin signals were obtained by performing in vitro liquid chromatography-tandem mass spectrometry on the harvested tissue. In the right dorsal hippocampal region, the gamma-aminobutyric-acid (GABA) and glutamine (Gln) concentrations were significantly higher in the sleep-perturbed rats than in the sham controls. The ratios of Gln/Glu (glutamate), Gln/tCr (total-creatine), and GABA/Glu were also significantly higher in the sleep-perturbed group, while serotonin concentrations were significantly lower in the sleep-perturbed rats. Pearson correlation results among individual rat data indicate that concentrations of dopamine (DA) and serotonin (5-HT) were significantly higher in SSP rats. A larger correlation coefficient was also observed for the SSP rats. Analysis of the correlation between the in vivo and in vitro signals indicated that the concentrations of Gln, 5-HT, and DA exhibited a significant negative correlation in the SSP rat data but not in that of control rats. The authors propose that the altered and correlated GABA, Gln, 5-HT, and DA concentrations/ratios could be considered key markers of neurological function in animal models of stress-induced sleep perturbation.

T1 mapping of the mouse brain following fractionated manganese administration using MP2RAGE

With the increasing development of transgenic mouse models of neurodegenerative diseases allowing improved understanding of the underlying mechanisms of these disorders, robust quantitative mapping techniques are also needed in rodents. MP2RAGE has shown great potential for structural imaging in humans at high fields. In the present work, MP2RAGE was successfully implemented at 9.4T and 14.1T. Following fractionated injections of MnCl₂, MP2RAGE images were acquired allowing simultaneous depiction and T₁ mapping of structures in the mouse brain at both fields. In addition, T₁ maps demonstrated significant T₁ shortenings in different structures of the mouse brain (p < 0.0008 at 9.4T, p < 0.000001 at 14.1T). T₁ values recovered to the levels of saline-injected animals 1 month after the last injection except in the pituitary gland. We believe that MP2RAGE represents an important prospective translational tool for further structural MRI.

Metabolic, synaptic and behavioral impact of 5-week chronic deep brain stimulation in hemiparkinsonian rats

The long-term effects and action mechanisms of subthalamic nucleus (STN) high-frequency stimulation (HFS) for Parkinson's disease still remain poorly characterized, mainly due to the lack of experimental models relevant to clinical application. To address this issue, we performed a multilevel study in freely moving hemiparkinsonian rats undergoing 5-week chronic STN HFS, using a portable constant-current microstimulator. In vivo metabolic neuroimaging by (1) H-magnetic resonance spectroscopy (11.7 T) showed that STN HFS normalized the tissue levels of the neurotransmission-related metabolites glutamate, glutamine and GABA in both the striatum and substantia nigra reticulata (SNr), which were significantly increased in hemiparkinsonian rats, but further decreased nigral GABA levels below control values; taurine levels, which were not affected in hemiparkinsonian rats, were significantly reduced. Slice electrophysiological recordings revealed that STN HFS was, uniquely among antiparkinsonian treatments, able to restore both forms of corticostriatal synaptic plasticity, i.e. long-term depression and potentiation, which were impaired in hemiparkinsonian rats. Behavior analysis (staircase test) showed a progressive recovery of motor skill during the stimulation period. Altogether, these data show that chronic STN HFS efficiently counteracts metabolic and synaptic defects due to dopaminergic lesion in both the striatum and SNr. Comparison of chronic STN HFS with acute and subchronic treatment further suggests that the long-term benefits of this treatment rely both on the maintenance of acute effects and on delayed actions on the basal ganglia network. We studied the effects of chronic (5 weeks) continuous subthalamic nucleus (STN) high-frequency stimulation (HFS) in hemiparkinsonian rats. The levels of glutamate and GABA in the striatum () and substantia nigra reticulata (SNr) (), measured by in vivo proton magnetic resonance spectroscopy ((1) H-MRS), were increased by 6-hydroxydopamine (6-OHDA) lesion, which also disrupted corticostriatal synaptic plasticity () and impaired forepaw skill () in the staircase test. Five-week STN HFS normalized glutamate and GABA levels and restored both synaptic plasticity and motor function. A partial behavioral recovery was observed at 2-week STN HFS.

In Vivo NMR Studies of the Brain with Hereditary or Acquired Metabolic Disorders

Metabolic disorders, whether hereditary or acquired, affect the brain, and abnormalities of the brain are related to cellular integrity; particularly in regard to neurons and astrocytes as well as interactions between them. Metabolic disturbances lead to alterations in cellular function as well as microscopic and macroscopic structural changes in the brain with diabetes, the most typical example of metabolic disorders, and a number of hereditary metabolic disorders. Alternatively, cellular dysfunction and degeneration of the brain lead to metabolic disturbances in hereditary neurological disorders with neurodegeneration. Nuclear magnetic resonance (NMR) techniques allow us to assess a range of pathophysiological changes of the brain in vivo. For example, magnetic resonance spectroscopy detects alterations in brain metabolism and energetics. Physiological magnetic resonance imaging (MRI) detects accompanying changes in cerebral blood flow related to neurovascular coupling. Diffusion and T1/T2-weighted MRI detect microscopic and macroscopic changes of the brain structure. This review summarizes current NMR findings of functional, physiological and biochemical alterations within a number of hereditary and acquired metabolic disorders in both animal models and humans. The global view of the impact of these metabolic disorders on the brain may be useful in identifying the unique and/or general patterns of abnormalities in the living brain related to the pathophysiology of the diseases, and identifying future fields of inquiry.

Magnetic resonance spectroscopy reveals oral Lactobacillus promotion of increases in brain GABA, N-acetyl aspartate and glutamate

The gut microbiome has been shown to regulate the development and functions of the enteric and central nervous systems. Its involvement in the regulation of behavior has attracted particular attention because of its potential translational importance in clinical disorders, however little is known about the pathways involved. We previously have demonstrated that administration of Lactobacillus rhamnosus (JB-1) to healthy male BALB/c mice, promotes consistent changes in GABA-A and -B receptor sub-types in specific brain regions, accompanied by reductions in anxiety and depression-related behaviors. In the present study, using magnetic resonance spectroscopy (MRS), we quantitatively assessed two clinically validated biomarkers of brain activity and function, glutamate+glutamine (Glx) and total N-acetyl aspartate+N-acetyl aspartyl glutamic acid (tNAA), as well as GABA, the chief brain inhibitory neurotransmitter. Mice received 1×10(9) cfu of JB-1 per day for 4weeks and were subjected to MRS weekly and again 4weeks after cessation of treatment to ascertain temporal changes in these neurometabolites. Baseline concentrations for Glx, tNAA and GABA were equal to 10.4±0.3mM, 8.7±0.1mM, and 1.2±0.1mM, respectively. Delayed increases were first seen for Glx (~10%) and NAA (~37%) at 2weeks which persisted only to the end of treatment. However, Glx was still elevated 4weeks after treatment had ceased. Significantly elevated GABA (~25%) was only seen at 4weeks. These results suggest specific metabolic pathways in our pursuit of mechanisms of action of psychoactive bacteria. They also offer through application of standard clinical neurodiagnostic techniques, translational opportunities to assess biomarkers accompanying behavioral changes induced by alterations in the gut microbiome.