The neurochemical profile quantified by in vivo 1H NMR spectroscopy

Sub-Microliter 1H Magnetic Resonance Spectroscopy for In Vivo High-Spatial Resolution Metabolite Quantification in the Mouse Brain

Proton magnetic resonance spectroscopy (MRS) offers a non-invasive, repeatable, and reproducible method for in vivo metabolite profiling of the brain and other tissues. However, metabolite fingerprinting by MRS requires high signal-to-noise ratios for accurate metabolite quantification, which has traditionally been limited to large volumes of interest, compromising spatial fidelity. In this study, we introduce a new optimized pipeline that combines LASER MRS acquisition at 11.7 T with a cryogenic coil and advanced offline pre- and post-processing. This approach achieves a signal-to-noise ratio sufficient to reliably quantify 19 distinct metabolites in a volume as small as 0.7 μL within the mouse brain. The resulting high spatial resolution and spectral quality enable the identification of distinct metabolite fingerprints in small, specific regions, as demonstrated by characteristic differences in N-acetylaspartate, glutamate, taurine, and myo-inositol between the motor and somatosensory cortices. We demonstrated a decline in taurine and glutamate in the primary motor cortex between 5 and 11 months of age, against the stability of other metabolites. Further exploitation to cortical layer-specific metabolite fingerprinting of layer I-III to layer VI-V in the primary motor cortex, with the latter showing reduced taurine and phosphoethanolamine levels, demonstrates the potential of this pipeline for detailed in vivo metabolite fingerprinting of cortical areas and subareas.

Simultaneous noninvasive quantification of redox and downstream glycolytic fluxes reveals compartmentalized brain metabolism

Brain metabolism across anatomic regions and cellular compartments plays an integral role in many aspects of neuronal function. Changes in key metabolic pathway fluxes, including oxidative and reductive energy metabolism, have been implicated in a wide range of brain diseases. Given the complex nature of the brain and the need for understanding compartmentalized metabolism noninvasively in vivo, new tools are required. Herein, using hyperpolarized (HP) magnetic resonance imaging coupled with in vivo isotope tracing, we develop a platform to simultaneously probe redox and energy metabolism in the murine brain. By combining HP dehydroascorbate and pyruvate, we are able to visualize increased lactate production in the white matter and increased redox capacity in the deep gray matter. Leveraging positional labeling, we show differences in compartmentalized tricarboxylic acid cycle entry versus downstream flux to glutamate. These findings lay the foundation for clinical translation of the proposed approach to probe brain metabolism.

Neurochemical changes in the progression of Huntington's disease: A meta-analysis of in vivo1H-MRS studies

Proton magnetic resonance spectroscopy (1H-MRS) allows measuring specific brain metabolic alterations in Huntington's disease (HD), and these metabolite profiles may serve as non-invasive biomarkers associated with disease progression. Despite this potential, previous findings are inconsistent. Accordingly, we performed a meta-analysis on available in vivo1H-MRS studies in premanifest (Pre-HD) and symptomatic HD stages (Symp-HD), and quantified neurometabolic changes relative to controls in 9 Pre-HD studies (227 controls and 188 mutation carriers) and 14 Symp-HD studies (326 controls and 306 patients). Our results indicated decreased N-acetylaspartate and creatine in the basal ganglia in both Pre-HD and Symp-HD. The overall level of myo-inositol was decreased in Pre-HD while increased in Symp-HD. Besides, Symp-HD patients showed more severe metabolism disruption than Pre-HD patients. Taken together, 1H-MRS is important for elucidating progressive metabolite changes from Pre-HD to clinical conversion; N-acetylaspartate and creatine in the basal ganglia are already sensitive at the preclinical stage and are promising biomarkers for tracking disease progression; overall myo-inositol is a possible characteristic metabolite for distinguishing HD stages.

Associations Between Brain Metabolites Measured With MR Spectroscopy and Head Impacts in High School American Football Athletes

BACKGROUND

While changes in brain metabolites after injury have been reported, relationships between metabolite changes and head impacts are less characterized.

PURPOSE

To investigate alterations in neurochemistry in high school athletes as a function of head impacts, concussion, and the use of a jugular vein compression (JVC) collar.

STUDY TYPE

Prospective controlled trial.

SUBJECTS

A total of 284 male American football players, divided into JVC collar and noncollar groups; 215 included in final analysis (age = 15.9 ± 1.0 years; 114 in collar group).

FIELD STRENGTH/SEQUENCE

3 Tesla/T1-weighted gradient echo, 1H point resolved spectroscopy, acquired between August and November 2018.

ASSESSMENT

Head impacts were quantified using accelerometers. Concussion was diagnosed by medical professionals for each team. Pre- to postseason differences in total N-acetylaspartate (tNAA), total choline (tCho), myo-inositol (myoI), and glutamate + glutamine (Glx), in primary motor cortex (M1) and anterior cingulate cortex (ACC), relative to total creatine (tCr), were determined.

STATISTICAL TESTS

Group-wise comparisons were performed using Wilcoxon signed-rank, Friedman's, and Mann-Whitney U tests. Relationships between ∆metabolite/tCr and mean g-force were analyzed using linear regressions accounting for concussion and JVC collar. Significance was set at P ≤ 0.05.

RESULTS

In participants without concussion, a significant decrease in tCho/tCr (0.233 ± 1.40 × 10-3 to 0.227 ± 1.47 × 10-7) and increase in Glx/tCr (1.60 ± 8.75 × 10-3 to 1.63 ± 1.08 × 10-2) in ACC were observed pre- to postseason. The relationship between ∆tCho/tCr in M1 and ACC and mean g-force from >80 g to >140 g differed significantly between participants with and without concussion (M1 β ranged from 3.9 × 10-3 to 2.1 × 10-3; ACC β ranged from 2.7 × 10-3 to 2.1 × 10-3). Posthoc analyses revealed increased tCho/tCr in M1 was positively associated with mean g-force >100 g (β = 3.6 × 10-3) and >110 g (β = 2.9 × 10-3) in participants with concussion. Significant associations between∆ myoI / tCr $$ \Delta \mathrm{myoI}/\mathrm{tCr} $$ in ACC and mean g-force >110 g (β = -1.1 × 10-3) and >120 g (β = -1.1 × 10-3) were observed in the collar group only.

DATA CONCLUSION

Diagnosed concussion and the use of a JVC collar result in distinct neurochemical trends after repeated head impacts.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY: Stage 3.

Concentrations of glutamate and N-acetylaspartate detected by magnetic resonance spectroscopy in the rat hippocampus correlate with hippocampal-dependent spatial memory performance

Magnetic resonance spectroscopy (MRS) has been employed to investigate brain metabolite concentrations in vivo, and they vary during neuronal activation, across brain activity states, or upon disease with neurological impact. Whether resting brain metabolites correlate with functioning in behavioral tasks remains to be demonstrated in any of the widely used rodent models. This study tested the hypothesis that, in the absence of neurological disease or injury, the performance in a hippocampal-dependent memory task is correlated with the hippocampal levels of metabolites that are mainly synthesized in neurons, namely N-acetylaspartate (NAA), glutamate and GABA. Experimentally naïve rats were tested for hippocampal-dependent spatial memory performance by measuring spontaneous alternation in the Y-maze, followed by anatomical magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) in the hippocampus and cortex. Memory performance correlated with hippocampal concentrations of NAA (p = 0.024) and glutamate (p = 0.014) but not GABA. Concentrations of glutamate in the cortex also correlated with spatial memory (p = 0.035). In addition, memory performance was also correlated with the relative volume of the hippocampus (p = 0.041). Altogether, this exploratory study suggests that levels of the neuronal maker NAA and the main excitatory neurotransmitter glutamate are associated with physiological functional capacity.

Investigation of metabolite correlates of CEST in the human brain at 7 T

Metabolite-weighted chemical exchange saturation transfer MRI can be used to indirectly image metabolites such as creatine and glutamate. This study aims to further explore the contrast of CEST at 2 ppm in the human brain at 7T and investigate the metabolite correlates of CEST at 2 ppm via correlations with magnetic resonance spectroscopy (MRS). Simulations were performed to establish the optimal acquisition parameters, such as total saturation time (tsat) and B1 root mean squared (B1rms) for CEST at 2 ppm in the human brain. Parameters were validated via in vitro phantom studies at 7T using concentrations, pH and temperature comparable to what is found in the human brain. Finally, 10 healthy volunteers were scanned at 7T for comparison with MRS. Our results show that the optimal parameters to acquire CEST at 2 ppm images are: B1rms = 2.14 μT & tsat = 1500 ms, respectively. Comparison with MRS showed no significant correlation between CEST at 2 ppm and total Creatine measured by MRS (R = 0.19; p-value = 0.273). However, a significant correlation was found between CEST at 2 ppm and Glu (R = 0.39; p-value = 0.033), indicating the broad Glutamate-weighted CEST as the main measurable contributor to CEST at 2 ppm. We identified and confirmed optimal CEST at 2 ppm sequence parameters and validated CEST at 2 ppm measurements in a controlled in vitro environment. Our findings suggest that glutamate is a substantial contributor to the CEST at 2 ppm contrast observed in the human brain, whereas the creatine contribution to CEST at 2 ppm in the brain did not show a measurable contribution.

7 T characterization of excitatory and inhibitory systems of acute pain in healthy female participants

Current understanding of the physiological underpinnings of normative pain processing is incomplete. Enhanced knowledge of these systems is necessary to advance our understanding of pain processes as well as to develop effective therapeutic interventions. Previous neuroimaging research suggests a network of interrelated brain regions that seem to be implicated in the processing and experience of pain. Among these, the dorsal anterior cingulate cortex (dACC) plays an important role in the affective aspects of pain signals. The current study leveraged functional MRS to investigate the underlying dynamic shifts in the neurometabolic signature of the human dACC at rest and during acute pain. Results provide support for increased glutamate levels following acute pain administration. Specifically, a 4.6% increase in glutamate was observed during moderate pressure pain compared with baseline. Exploratory analysis also revealed meaningful changes in dACC gamma aminobutyric acid in response to pain stimulation. These data contribute toward the characterization of neurometabolic shifts, which lend insight into the role of the dACC in the pain network. Further research in this area with larger sample sizes could contribute to the development of novel therapeutics or other advances in pain-related outcomes.

Improved reproducibility of γ-aminobutyric acid measurement from short-echo-time proton MR spectroscopy by linewidth-matched basis sets in LCModel

γ-Aminobutyric acid (GABA), as the primary inhibitory neurotransmitter, is extremely important for maintaining healthy brain function, and deviations from GABA homeostasis are related to various brain diseases. Short-echo-time (short-TE) proton MR spectroscopy (1 H-MRS) has been employed to measure GABA concentration from various human brain regions at high magnetic fields. The aim of this study was to investigate the effect of spectral linewidth on GABA quantification and explore the application of an optimized basis-set preparation approach using a spectral-linewidth-matched (LM) basis set in LCModel to improve the reproducibility of GABA quantification from short-TE 1 H-MRS. In contrast to the fixed-linewidth basis-set approach, the LM basis-set preparation approach, where all metabolite basis spectra were simulated with a linewidth 4 Hz narrower than that of water, showed a smaller standard deviation of estimated GABA concentration from synthetic spectra with varying linewidths and lineshapes. The test-retest reproducibility was assessed by the mean within-subject coefficient of variation, which improved from 19.2% to 12.0% in the thalamus, from 27.9% to 14.9% in the motor cortex, and from 9.7% to 2.8% in the medial prefrontal cortex using LM basis sets at 7 T. We conclude that spectral linewidth has a large effect on GABA quantification from short-TE 1 H-MRS data and that using LM basis sets in LCModel can improve the reproducibility of GABA quantification.

Regional GABA levels modulate abnormal resting-state network functional connectivity and cognitive impairment in multiple sclerosis

More evidence shows that changes in functional connectivity with regard to brain networks and neurometabolite levels correlated to cognitive impairment in multiple sclerosis. However, the neurological basis underlying the relationship among neurometabolite levels, functional connectivity, and cognitive impairment remains unclear. For this purpose, we used a combination of magnetic resonance spectroscopy and resting-state functional magnetic resonance imaging to study gamma-aminobutyric acid and glutamate concentrations in the posterior cingulate cortex, medial prefrontal cortex and left hippocampus, and inter-network functional connectivity in 29 relapsing-remitting multiple sclerosis patients and 34 matched healthy controls. Neuropsychological tests were used to evaluate the cognitive function. We found that relapsing-remitting multiple sclerosis patients demonstrated significantly reduced gamma-aminobutyric acid and glutamate concentrations and aberrant functional connectivity involving cognitive-related networks compared to healthy controls, and both alterations were associated with specific cognition decline. Moreover, mediation analyses indicated that decremented hippocampus gamma-aminobutyric acid levels in relapsing-remitting multiple sclerosis patients mediated the association between inter-network functional connectivity in various components of default mode network and verbal memory deficits. In summary, our findings shed new lights on the essential function of GABAergic system abnormalities in regulating network dysconnectivity and functional connectivity in relapsing-remitting multiple sclerosis patients, suggesting potential novel approach to treatment.

Brain metabolic derangements examined using 1H MRS and their (in)consistency among different rodent models of depression

Major depressive disorder (MDD) is underlined by neurochemical changes in the brain. Proton magnetic resonance spectroscopy (1H MRS) is a useful tool for their examination as it provides information about the levels of metabolites. This review summarises the current knowledge of 1H MRS findings from rodent models of MDD, assesses the results from both a biological and a technical perspective, and identifies the main sources of bias. From a technical point of view, bias-introducing factors are the diversity of the measured volumes and their positioning in the brain, the data processing, and the metabolite concentration expression. The biological variables are strain, sex, and species, as well as the model itself, and in vivo vs. ex vivo exploration. This review identified some consistency in the 1H MRS findings in the models of MDD: lower levels of glutamine, glutamate + glutamine, and higher levels of myo-inositol and taurine in most of the brain regions of MDD models. This may suggest changes in regional metabolism, neuronal dysregulation, inflammation, and a compensatory effect reaction in the MDD rodent models.

Blood and Brain Metabolites after Cerebral Ischemia

The study of an organism's response to cerebral ischemia at different levels is essential to understanding the mechanism of the injury and protection. A great interest is devoted to finding the links between quantitative metabolic changes and post-ischemic damage. This work aims to summarize the outcomes of the most studied metabolites in brain tissue-lactate, glutamine, GABA (4-aminobutyric acid), glutamate, and NAA (N-acetyl aspartate)-regarding their biological function in physiological conditions and their role after cerebral ischemia/reperfusion. We focused on ischemic damage and post-ischemic recovery in both experimental-including our results-as well as clinical studies. We discuss the role of blood glucose in view of the diverse impact of hyperglycemia, whether experimentally induced, caused by insulin resistance, or developed as a stress response to the cerebral ischemic event. Additionally, based on our and other studies, we analyze and critically discuss post-ischemic alterations in energy metabolites and the elevation of blood ketone bodies observed in the studies on rodents. To complete the schema, we discuss alterations in blood plasma circulating amino acids after cerebral ischemia. So far, no fundamental brain or blood metabolite(s) has been recognized as a relevant biological marker with the feasibility to determine the post-ischemic outcome or extent of ischemic damage. However, studies from our group on rats subjected to protective ischemic preconditioning showed that these animals did not develop post-ischemic hyperglycemia and manifested a decreased metabolic infringement and faster metabolomic recovery. The metabolomic approach is an additional tool for understanding damaging and/or restorative processes within the affected brain region reflected in the blood to uncover the response of the whole organism via interorgan metabolic communications to the stressful cerebral ischemic challenge.

Use of 31 P magnetisation transfer magnetic resonance spectroscopy to measure ATP changes after 670 nm transcranial photobiomodulation in older adults

Mitochondrial function declines with age, and many pathological processes in neurodegenerative diseases stem from this dysfunction when mitochondria fail to produce the necessary energy required. Photobiomodulation (PBM), long-wavelength light therapy, has been shown to rescue mitochondrial function in animal models and improve human health, but clinical uptake is limited due to uncertainty around efficacy and the mechanisms responsible. Using 31 P magnetisation transfer magnetic resonance spectroscopy (MT-MRS) we quantify, for the first time, the effects of 670 nm PBM treatment on healthy ageing human brains. We find a significant increase in the rate of ATP synthase flux in the brain after PBM in a cohort of older adults. Our study provides initial evidence of PBM therapeutic efficacy for improving mitochondrial function and restoring ATP flux with age, but recognises that wider studies are now required to confirm any resultant cognitive benefits.

Taurine and N-acetylcysteine treatments prevent memory impairment and metabolite profile alterations in the hippocampus of high-fat diet-fed female mice

Background: Obesity constitutes a risk factor for cognitive impairment. In rodent models, long-term exposure to obesogenic diets leads to hippocampal taurine accumulation. Since taurine has putative cyto-protective effects, hippocampal taurine accumulation in obese and diabetic models might constitute a counteracting response to metabolic stress. Objective: We tested the hypothesis that treatment with taurine or with N-acetylcysteine (NAC), which provides cysteine for the synthesis of taurine and glutathione, prevent high-fat diet (HFD)-associated hippocampal alterations and memory impairment. Methods: Female mice were fed either a regular diet or HFD. Some mice had access to 3%(w/v) taurine or 3%(w/v) NAC in the drinking water. After 2 months, magnetic resonance spectroscopy (MRS) was used to measure metabolite profiles. Memory was assessed in novel object and novel location recognition tests. Results: HFD feeding caused memory impairment in both tests, and reduced concentration of lactate, phosphocreatine-to-creatine ratio, and the neuronal marker N-acetylaspartate in the hippocampus. Taurine and NAC prevented HFD-induced memory impairment and N-acetylaspartate reduction. NAC, but not taurine, prevented the reduction of lactate and phosphocreatine-to-creatine ratio. MRS revealed NAC/taurine-induced increase of hippocampal glutamate and GABA levels. Conclusion: NAC and taurine can prevent memory impairment, while only NAC prevents alterations of metabolite concentrations in HFD-exposed female mice.

Regional sex differences in neurochemical profiles of healthy mice measured by magnetic resonance spectroscopy at 9.4 tesla

Objective

To determine sex differences in the neurochemical concentrations measured by in vivo proton magnetic resonance spectroscopy (1H MRS) of healthy mice on a genetic background commonly used for neurodegenerative disease models.

Methods

1H MRS data collected from wild type mice with C57BL/6 or related genetic backgrounds in seven prior studies were used in this retrospective analysis. To be included, data had to be collected at 9.4 tesla magnetic field using advanced 1H MRS protocols, with isoflurane anesthesia and similar animal handling protocols, and a similar number of datasets from male and female mice had to be available for the brain regions analyzed. Overall, 155 spectra from female mice and 166 spectra from male mice (321 in total), collected from six brain regions (brainstem, cerebellum, cortex, hippocampus, hypothalamus, and striatum) at various ages were included.

Results

Concentrations of taurine, total creatine (creatine + phosphocreatine), ascorbate, glucose and glutamate were consistently higher in male vs. female mice in most brain regions. Striatum was an exception with similar total creatine in male and female mice. The sex difference pattern in the hypothalamus was notably different from other regions. Interaction between sex and age was significant for total creatine and taurine in the cerebellum and hippocampus.

Conclusion

Sex differences in regional neurochemical levels are small but significant and age-dependent, with consistent male-female differences across most brain regions. The neuroendocrine region hypothalamus displays a different pattern of sex differences in neurochemical levels. Differences in energy metabolism and cellular density may underlie the differences, with higher metabolic rates in females and higher osmoregulatory and antioxidant capacity in males.

Searching for Effective Methods of Diagnosing Nervous System Lesions in Patients with Alström and Bardet-Biedl Syndromes

Bardet-Biedl syndrome (BBS) and Alström syndrome (ALMS) are rare multisystem diseases with an autosomal recessive mode of inheritance and genetic heterogeneity, characterized by visual impairment, hearing impairment, cardiomyopathy, childhood obesity, and insulin resistance. The purpose of our study was to evaluate the indicators of nervous system changes occurring in patients with ALMS and BBS using optical coherence tomography (OCT) and magnetic resonance spectroscopy (MRS) methods compared to a group of healthy subjects. The OCT results showed significantly lower macular thickness in the patient group compared to the control group (p = 0.002). The MRS study observed differences in metabolite levels between the study and control groups in brain areas such as the cerebellum, thalamus, and white matter. After summing the concentrations from all areas, statistically significant results were obtained for N-acetylaspartate, total N-acetylaspartate, and total creatine. Concentrations of these metabolites were reduced in ALMS/BBS patients by 38% (p = 0.0004), 35% (p = 0.0008), and 28% (p = 0.0005), respectively. Our results may help to understand the pathophysiology of these rare diseases and identify strategies for new therapies.

Multinuclear Magnetic Resonance Spectroscopy at Ultra-High-Field: Assessing Human Cerebral Metabolism in Healthy and Diseased States

The brain is a highly energetic organ. Although the brain can consume metabolic substrates, such as lactate, glycogen, and ketone bodies, the energy metabolism in a healthy adult brain mainly relies on glucose provided via blood. The cerebral metabolism of glucose produces energy and a wide variety of intermediate metabolites. Since cerebral metabolic alterations have been repeatedly implicated in several brain disorders, understanding changes in metabolite levels and corresponding cell-specific neurotransmitter fluxes through different substrate utilization may highlight the underlying mechanisms that can be exploited to diagnose or treat various brain disorders. Magnetic resonance spectroscopy (MRS) is a noninvasive tool to measure tissue metabolism in vivo. ¹H-MRS is widely applied in research at clinical field strengths (≤3T) to measure mostly high abundant metabolites. In addition, X-nuclei MRS including, ¹³C, ²H, ¹⁷O, and ³¹P, are also very promising. Exploiting the higher sensitivity at ultra-high-field (>4T; UHF) strengths enables obtaining unique insights into different aspects of the substrate metabolism towards measuring cell-specific metabolic fluxes in vivo. This review provides an overview about the potential role of multinuclear MRS (¹H, ¹³C, ²H, ¹⁷O, and ³¹P) at UHF to assess the cerebral metabolism and the metabolic insights obtained by applying these techniques in both healthy and diseased states.

High-field ex vivo and in vivo two-dimensional nuclear magnetic resonance spectroscopy in murine brain: Resolving and exploring the molecular environment

The structural and chemical complexities within the brain pose a challenge that few noninvasive techniques can tackle with the dexterity of nuclear magnetic resonance (NMR) spectroscopy. Still, even with the advent of ultrahigh fields and of cryogenically cooled coils for in vivo research, the superposition of metabolic resonances arising from the brain remains a challenge. The present study explores the potential to tackle this milieu using a combination of two-dimensional (2D) NMR techniques, implemented on murine brains in vivo at 15.2 T and ex vivo at 14.1 T. While both experiments were affected by substantial inhomogeneous broadenings conveying distinct elongated lineshapes to the cross-peaks, the ability of increased fields to resolve off-diagonal resonances was clear. A comparison between the corresponding conventional and double quantum-filtered correlated spectroscopy traces enabled an improved assignment of in vivo resonances on the basis of more sensitive ex vivo 2D acquisitions, foremost on the basis of homonuclear cross-relaxation-driven correlations for peaks resonating downfield from water, and of heteronuclear correlations at natural abundance for the upfield protons. With the aid of such 2D correlations approximately 29 metabolites could be resolved and identified. This enhanced resolution was used to explore features related to the metabolites' diffusivities, their exposure to water, and their facility to undergo magnetization transfers to amide/amine/hydroxyl resonances. Cross-peaks from main murine brain biomolecules, including choline, creatine, γ-aminobutyric acid, N-acetyl aspartate, glutamine, and glutamate, showed enhancements in several of these various features, opening interesting vistas about metabolite compartmentalization as viewed by these 2D NMR experiments.

Organization of neurochemical interactions in young and older brains as revealed with a network approach: Evidence from proton magnetic resonance spectroscopy (1H-MRS)

Aging is associated with alterations in the brain including structural and metabolic changes. Previous research has focused on neurometabolite level differences associated to age in a variety of brain regions, but the relationship among metabolites across the brain has been much less studied. Investigating these relationships can reveal underlying neurometabolic processes, their interdependency, and their progress throughout the lifespan. Using ¹H-MRS, we investigated the relationship among metabolite concentrations of N-acetylaspartate (NAA), creatine (Cr), choline (Cho), myo-Inositol (mIns) and glutamate-glutamine complex (Glx) in seven voxel locations, i.e., bilateral sensorimotor cortex, bilateral striatum, pre-supplementary motor area, right inferior frontal gyrus and occipital cortex. These measurements were performed on 59 human participants divided in two age groups: young adults (YA: 23.2 ± 4.3; 18-34 years) and older adults (OA: 67.5 ± 3.9; 61-74 years). Our results showed age-related differences in NAA, Cho, and mIns across brain regions, suggesting the presence of neurodegeneration and altered gliosis. Moreover, associative patterns among NAA, Cho and Cr were observed across the selected brain regions, which differed between young and older adults. Whereas most of metabolite concentrations were inhomogeneous across different brain regions, Cho levels were shown to be strongly related across brain regions in both age groups. Finally, we found metabolic associations between homologous brain regions (SM1 and striatum) in the OA group, with NAA showing a significant correlation between bilateral sensorimotor cortices (SM1) and mIns levels being correlated between the bilateral striata. We posit that a network perspective provides important insights regarding the potential interactions among neurochemicals underlying metabolic processes at a local and global level and their relationship with aging.

Strength gains after 12 weeks of resistance training correlate with neurochemical markers of brain health in older adults: a randomized control 1H-MRS study

Physical exercise is considered a potent countermeasure against various age-associated physiological deterioration processes. We therefore assessed the effect of 12 weeks of resistance training on brain metabolism in older adults (age range: 60-80 years). Participants either underwent two times weekly resistance training program which consisted of four lower body exercises performed for 3 sets of 6-10 repetitions at 70-85% of 1 repetition maximum (n = 20) or served as the passive control group (n = 21). The study used proton magnetic resonance spectroscopy to quantify the ratio of total N-acetyl aspartate, total choline, glutamate-glutamine complex, and myo-inositol relative to total creatine (tNAA/tCr, tCho/tCr, Glx/tCr, and mIns/tCr respectively) in the hippocampus (HPC), sensorimotor (SM1), and prefrontal (dlPFC) cortices. The peak torque (PT at 60°/s) of knee extension and flexion was assessed using an isokinetic dynamometer. We used repeated measures time × group ANOVA to assess time and group differences and correlation coefficient analyses to examine the pre-to-post change (∆) associations between PT and neurometabolite variables. The control group showed significant declines in tNAA/tCr and Glx/tCr of SM1, and tNAA/tCr of dlPFC after 12 weeks, which were not seen in the experimental group. A significant positive correlation was found between ∆PT knee extension and ∆SM1 Glx/tCr, ∆dlPFC Glx/tCr and between ∆PT knee flexion and ∆dlPFC mIns/tCr in the experimental group. Overall, findings suggest that resistance training seems to elicit alterations in various neurometabolites that correspond to exercise-induced "preservation" of brain health, while simultaneously having its beneficial effect on augmenting muscle functional characteristics in older adults.

Functional MRS studies of GABA and glutamate/Glx - A systematic review and meta-analysis

Functional magnetic resonance spectroscopy (fMRS) can be used to investigate neurometabolic responses to external stimuli in-vivo, but findings are inconsistent. We performed a systematic review and meta-analysis on fMRS studies of the primary neurotransmitters Glutamate (Glu), Glx (Glutamate + Glutamine), and GABA. Data were extracted, grouped by metabolite, stimulus domain, and brain region, and analysed by determining standardized effect sizes. The quality of individual studies was rated. When results were analysed by metabolite type small to moderate effect sizes of 0.29-0.47 (p < 0.05) were observed for changes in Glu and Glx regardless of stimulus domain and brain region, but no significant effects were observed for GABA. Further analysis suggests that Glu, Glx and GABA responses differ by stimulus domain or task and vary depending on the time course of stimulation and data acquisition. Here, we establish effect sizes and directionality of GABA, Glu and Glx response in fMRS. This work highlights the importance of standardised reporting and minimal best practice for fMRS research.

Alterations to Cerebral Perfusion, Metabolite Profiles, and Neuronal Morphology in the Hippocampus and Cortex of Male and Female Mice during Chronic Exposure to a High-Salt Diet

Excess dietary salt reduces resting cerebral blood flow (CBF) and vascular reactivity, which can limit the fueling of neuronal metabolism. It is hitherto unknown whether metabolic derangements induced by high-salt-diet (HSD) exposure during adulthood are reversed by reducing salt intake. In this study, male and female mice were fed an HSD from 9 to 16 months of age, followed by a normal-salt diet (ND) thereafter until 23 months of age. Controls were continuously fed either ND or HSD. CBF and metabolite profiles were determined longitudinally by arterial spin labeling magnetic resonance imaging and magnetic resonance spectroscopy, respectively. HSD reduced cortical and hippocampal CBF, which recovered after dietary salt normalization, and affected hippocampal but not cortical metabolite profiles. Compared to ND, HSD increased hippocampal glutamine and phosphocreatine levels and decreased creatine and choline levels. Dietary reversal only allowed recovery of glutamine levels. Histology analyses revealed that HSD reduced the dendritic arborization and spine density of cortical and hippocampal neurons, which were not recovered after dietary salt normalization. We conclude that sustained HSD exposure throughout adulthood causes permanent structural and metabolic alterations to the mouse brain that are not fully normalized by lowering dietary salt during aging.

Short-term high-fat diet alters the mouse brain magnetic resonance imaging parameters consistently with neuroinflammation on males and metabolic rearrangements on females. A pre-clinical study with an optimized selection of linear mixed-effects models

Introduction

High-fat diet (HFD) consumption is known to trigger an inflammatory response in the brain that prompts the dysregulation of energy balance, leads to insulin and leptin resistance, and ultimately obesity. Obesity, at the same, has been related to cerebral magnetic resonance imaging (MRI) alterations, but the onset of HFD-induced neuroinflammation, however, has been principally reported on male rodents and by ex vivo methods, with the effects on females and the origin of MRI changes remaining unassessed.

Methods

We characterized the onset and evolution of obesity on male and female mice during standard or HFD administration by physiological markers and multiparametric MRI on four cerebral regions involved in appetite regulation and energy homeostasis. We investigated the effects of diet, time under diet, brain region and sex by identifying their significant contributions to sequential linear mixed-effects models, and obtained their regional neurochemical profiles by high-resolution magic angle spinning spectroscopy.

Results

Male mice developed an obese phenotype paralleled by fast increases in magnetization transfer ratio values, while females delayed the obesity progress and showed no MRI-signs of cerebral inflammation, but larger metabolic rearrangements on the neurochemical profile.

Discussion

Our study reveals early MRI-detectable changes compatible with the development of HFD-induced cerebral cytotoxic inflammation on males but suggest the existence of compensatory metabolic adaptations on females that preclude the corresponding detection of MRI alterations.

Altered prefrontal neurochemistry in the DJ-1 knockout mouse model of Parkinson's disease: complementary semi-quantitative analyses with in vivo magnetic resonance spectroscopy and MALDI-MSI

In vivo proton magnetic resonance spectroscopy (¹H-MRS) and matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) are two semi-quantitative analytical methods commonly used in neurochemical research. In this study, the two methods were used complementarily, in parallel, to investigate neurochemical perturbations in the medial prefrontal cortex (mPFC) of 9-month-old DJ-1 knockout mice, a well-established transgenic model for Parkinson's diseases. Convergingly, the results obtained with the two methods demonstrated that, compared with the wild-type (WT) mice, the DJ-1 knockout mice had significantly increased glutathione (GSH) level and GSH/glutamate (Glu) ratio in the mPFC, which likely presented an astrocytic compensatory mechanism in response to elevated regional oxidative stress induced by the loss of DJ-1 function. The results from this study also highlighted (1) the need to be cautious when interpreting the in vivo ¹H-MRS results obtained from aged transgenic animals, in which the concentration of internal reference, being whether water or total creatine, could no longer be assumed to be the same as that in the age-matched WT animals, and (2) the necessity and importance of complementary analyses with more than one method under such circumstances.

Hippocampal metabolic recovery as a manifestation of the protective effect of ischemic preconditioning in rats

The ever-present risk of brain ischemic events in humans and its full prevention make the detailed studies of an organism's response to ischemia at different levels essential to understanding the mechanism of the injury as well as protection. We used the four-vessel occlusion as an animal model of forebrain ischemia to investigate its impact on the metabolic alterations in both the hippocampus and the blood plasma to see changes on the systemic level. By inducing sublethal ischemic stimuli, we focused on the endogenous phenomena known as ischemic tolerance. NMR spectroscopy was used to analyze relative metabolite levels in tissue extracts from rats' hippocampus and blood plasma in three various ischemic/reperfusion times: 3 h, 24 h, and 72 h. Hippocampal tissues were characterized by postischemically decreased glutamate and GABA (4-aminobutyrate) tissue content balanced with increased glutamine level, with most pronounced changes at 3 h reperfusion time. Glutamate (as well as glutamine) levels recovered towards the control levels on the third day, as if the glutamate re-synthesis would be firstly preferred before GABA. These results are indicating the higher feasibility of re-establishing of glutamatergic transmission three days after an ischemic event, in contrast to GABA-ergic. Tissue levels of N-acetylaspartate (NAA), as well as choline, were decreased without the tendency to recover three days after the ischemic event. Metabolomic analysis of blood plasma revealed that ischemically preconditioned rats, contrary to the non-preconditioned animals, did not show hyperglycemic conditions. Ischemically induced semi-ketotic state, manifested in increased plasma ketone bodies 3-hydroxybutyrate and acetoacetate, seems to be programmed to support the brain tissue revitalization after the ischemic event. These and other metabolites changes found in blood plasma as well as in the hippocampus were observed to a lower extent or recovered faster in preconditioned animals. Some metabolomic changes in hippocampal tissue extract were so strong that even single metabolites were able to differentiate between ischemic, ischemically preconditioned, and control brain tissues.

Swimming Training Mitigates Neurological Impairment of Intracerebral Haemorrhage in Mice via the Serine-Threonine Kinase/Glycogen Synthase Kinase 3β Signalling Pathway

Swimming training (ST) can mitigate functional disorders in neurological diseases, but the effect and mechanism of ST in improving the neurological function of intracerebral haemorrhage (ICH) have not been reported. Our study aimed to explore the protective effect of early ST on ICH mice and its relationship with the serine-threonine kinase (Akt)/glycogen synthase kinase 3β (GSK3β) pathway. Our findings showed that the ICH model mice had poor behavioural manifestations in the Y maze test and open field test compared to the ST group and sham group. The modified neurological severity score was increased in the ICH mice, and 7 days of ST intervention significantly attenuated the neurological deficits. The ratios of myo-inositol/creatine, lactate/creatine and glutamate/creatine were decreased, and the ratios of N-acetylaspartate/creatine and choline/creatine were increased in the ICH mice with ST intervention. ST intervention decreased the expression of Iba1 and GFAP. Seven days of ST significantly increased the expression of p-Akt/Akt compared to that in the ICH mice. Furthermore, the Akt kinase inhibitor GSK690693 exacerbated neurological impairment, increased the expression of Iba1, GFAP and Bax/Bcl-2, and reversed the anti-apoptotic effects and anti-glia activation of ST, which was associated with the inhibition of p-Akt/Akt and p-GSK3β/GSK3β expression. These results indicated that the protective role of ST in ICH was mediated via the Akt/GSK3β pathway. In conclusion, ST displayed neuroprotection by inhibiting apoptosis and glial activation in ICH mice by activating the Akt/GSK3β signalling pathway.

Magnetic Resonance Imaging and Its Clinical Correlation in Spinocerebellar Ataxia Type 3: A Systematic Review

Background

Spinocerebellar ataxia type 3 (SCA3) is a complex cerebrocerebellar disease primarily characterized by ataxia symptoms alongside motor and cognitive impairments. The heterogeneous clinical presentation of SCA3 necessitates correlations between magnetic resonance imaging (MRI) and clinical findings in reflecting progressive disease changes. At present, an attempt to systematically examine the brain-behavior relationship in SCA3, specifically, the correlation between MRI and clinical findings, is lacking.

Objective

We investigated the association strength between MRI abnormality and each clinical symptom to understand the brain-behavior relationship in SCA3.

Methods

We conducted a systematic review on Medline and Scopus to review studies evaluating the brain MRI profile of SCA3 using structural MRI (volumetric, voxel-based morphometry, surface analysis), magnetic resonance spectroscopy, and diffusion tensor imaging, including their correlations with clinical outcomes.

Results

Of 1,767 articles identified, 29 articles met the eligibility criteria. According to the National Institutes of Health quality assessment tool for case-control studies, all articles were of excellent quality. This systematic review found that SCA3 neuropathology contributes to widespread brain degeneration, affecting the cerebellum and brainstem. The disease gradually impedes the cerebral cortex and basal ganglia in the late stages of SCA3. Most findings reported moderate correlations (r = 0.30–0.49) between MRI features in several regions and clinical findings. Regardless of the MRI techniques, most studies focused on the brainstem and cerebellum.

Conclusions

Clinical findings suggest that rather than individual brain regions, the connectivity between different brain regions in distributed networks (i.e., cerebellar-cerebral network) may be responsible for motor and neurocognitive function in SCA3. This review highlights the importance of evaluating the progressive changes of the cerebellar-cerebral networks in SCA3 patients, specifically the functional connectivity. Given the relative lack of knowledge about functional connectivity on SCA3, future studies should investigate possible functional connectivity abnormalities in SCA3 using fMRI.

Current Status of Our Understanding for Brain Integrated Functions and its Energetics

Human/animal brain is a unique organ with substantially high metabolism but it contains no energy reserve that is the reason it requires continuous supply of O₂ and energy fluxes through CBF. The main source of energy remains glucose as the other biomolecules do not able to cross the blood-brain barrier. The speed of glucose metabolism is heterogeneous throughout the brain. One of the major flux consumption is Neuron-astrocyte cycling of glutamate and glutamine in glutamatergic neurons (approximately 80% of glucose metabolism in brain). The quantification of cellular glucose and other related substrate in resting, activated state can be analyzed through [¹⁸ F]FDG -positron-emission tomography (studying CMRglc) and [¹³ C/³¹P -MRS: for neuroenergetics & neurotransmitter cycling &³¹P-MRS: for energy induction & redox state). Merging basic in vitro studies with these techniques will help to develop new treatment paradigms for human brain diseased conditions.

Increased prefrontal GABA concentrations in adults with autism spectrum disorders

The excitatory-inhibitory imbalance hypothesis postulates dysregulation of the gamma-aminobutyric acid (GABA) and glutamate (Glu) neurotransmitter systems as a common underlying deficit in individuals with autism spectrum disorders (ASD). Previous studies suggest an important role of these systems in the pathophysiology of ASD, including a study of our group reporting decreased glutamate concentrations in the pregenual anterior cingulate cortex (ACC) of adults with ASD. The aim of this study was to replicate our previous findings of impaired glutamate metabolism in ASD in a new sample and to additionally quantify GABA in the ACC and dorsolateral prefrontal cortex (dlPFC). Concentrations of GABA and glutamate-glutamine (Glx; combined glutamate and glutamine signal) were quantified in the ACC and dlPFC of 43 adults with ASD and 43 neurotypical controls (NTC) by magnetic resonance spectroscopy (MRS). The ASD group showed increased absolute GABA concentrations and elevated GABA/creatine ratios in the left dlPFC compared to NTC, while no group differences were detected in the pregenual and dorsal ACC. Previous findings of altered Glx concentration in the pregenual ACC of the ASD group could not be replicated. Regarding Glx concentrations and Glx/creatine ratios, there were no significant differences in the dlPFC and ACC either. The study supports the hypothesis of an altered GABA and glutamate equilibrium, indicating an imbalance between excitatory and inhibitory metabolism in ASD patients. However, inconsistent results across studies and brain regions suggest a complex underlying phenomenon. LAY SUMMARY: Adults of the autism spectrum exhibit elevated levels of the inhibitory neurotransmitter GABA in the left dorsolateral prefrontal cortex. This finding supports the hypothesis of an imbalance between excitatory and inhibitory equilibrium in patients with autism spectrum disorders.

Taurine Supplementation as a Neuroprotective Strategy upon Brain Dysfunction in Metabolic Syndrome and Diabetes

Obesity, type 2 diabetes, and their associated comorbidities impact brain metabolism and function and constitute risk factors for cognitive impairment. Alterations to taurine homeostasis can impact a number of biological processes, such as osmolarity control, calcium homeostasis, and inhibitory neurotransmission, and have been reported in both metabolic and neurodegenerative disorders. Models of neurodegenerative disorders show reduced brain taurine concentrations. On the other hand, models of insulin-dependent diabetes, insulin resistance, and diet-induced obesity display taurine accumulation in the hippocampus. Given the possible cytoprotective actions of taurine, such cerebral accumulation of taurine might constitute a compensatory mechanism that attempts to prevent neurodegeneration. The present article provides an overview of brain taurine homeostasis and reviews the mechanisms by which taurine can afford neuroprotection in individuals with obesity and diabetes. We conclude that further research is needed for understanding taurine homeostasis in metabolic disorders with an impact on brain function.

Proton magnetic resonance spectroscopy detects cerebral metabolic derangement in a mouse model of brain coenzyme a deficiency

Background

Pantothenate kinase (PANK) is the first and rate-controlling enzymatic step in the only pathway for cellular coenzyme A (CoA) biosynthesis. PANK-associated neurodegeneration (PKAN), formerly known as Hallervorden–Spatz disease, is a rare, life-threatening neurologic disorder that affects the CNS and arises from mutations in the human PANK2 gene. Pantazines, a class of small molecules containing the pantazine moiety, yield promising therapeutic effects in an animal model of brain CoA deficiency. A reliable technique to identify the neurometabolic effects of PANK dysfunction and to monitor therapeutic responses is needed.

Methods

We applied ¹H magnetic resonance spectroscopy as a noninvasive technique to evaluate the therapeutic effects of the newly developed Pantazine BBP-671.

Results

¹H MRS reliably quantified changes in cerebral metabolites, including glutamate/glutamine, lactate, and N-acetyl aspartate in a neuronal Pank1 and Pank2 double-knockout (SynCre⁺Pank1,2 dKO) mouse model of brain CoA deficiency. The neuronal SynCre⁺Pank1,2 dKO mice had distinct decreases in Glx/tCr, NAA/tCr, and lactate/tCr ratios compared to the wildtype matched control mice that increased in response to BBP-671 treatment.

Conclusions

BBP-671 treatment completely restored glutamate/glutamine levels in the brains of the mouse model, suggesting that these metabolites are promising clinically translatable biomarkers for future therapeutic trials.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-022-03304-y.

Cognitive Impairment and Metabolite Profile Alterations in the Hippocampus and Cortex of Male and Female Mice Exposed to a Fat and Sugar-Rich Diet are Normalized by Diet Reversal

Diabetes impacts on brain metabolism, structure, and function. Alterations in brain metabolism have been observed in obesity and diabetes models induced by exposure to diets rich in saturated fat and/or sugar and have been linked to memory impairment. However, it remains to be determined whether brain dysfunction induced by obesogenic diets results from permanent brain alterations. We tested the hypothesis that an obesogenic diet (high-fat and high-sucrose diet; HFHSD) causes reversible changes in hippocampus and cortex metabolism and alterations in behavior. Mice were exposed to HFHSD for 24 weeks or for 16 weeks followed by 8 weeks of diet normalization. Development of the metabolic syndrome, changes in behavior, and brain metabolite profiles by magnetic resonance spectroscopy (MRS) were assessed longitudinally. Control mice were fed an ingredient-matched low-fat and low-sugar diet. Mice fed the HFHSD developed obesity, glucose intolerance and insulin resistance, with a more severe phenotype in male than female mice. Relative to controls, both male and female HFHSD-fed mice showed increased anxiety-like behavior, impaired memory in object recognition tasks, but preserved working spatial memory as evaluated by spontaneous alternation in a Y-maze. Alterations in the metabolite profiles were observed both in the hippocampus and cortex but were more distinct in the hippocampus. HFHSD-induced metabolic changes included altered levels of lactate, glutamate, GABA, glutathione, taurine, N-acetylaspartate, total creatine and total choline. Notably, HFHSD-induced metabolic syndrome, anxiety, memory impairment, and brain metabolic alterations recovered upon diet normalization for 8 weeks. In conclusion, cortical and hippocampal derangements induced by long-term HFHSD consumption are reversible rather than being the result of permanent tissue damage.

Emerging methods and applications of ultra-high field MR spectroscopic imaging in the human brain

Magnetic Resonance Spectroscopic Imaging (MRSI) of the brain enables insights into the metabolic changes and fluxes in diseases such as tumors, multiple sclerosis, epilepsy, or hepatic encephalopathy, as well as insights into general brain functionality. However, the routine application of MRSI is mostly hampered by very low signal-to-noise ratios (SNR) due to the low concentrations of metabolites, about 10000 times lower than water. Furthermore, MRSI spectra have a dense information content with many overlapping metabolite resonances, especially for proton MRSI. MRI scanners at ultra-high field strengths, like 7 T or above, offer the opportunity to increase SNR, as well as the separation between resonances, thus promising to solve both challenges. Yet, MRSI at ultra-high field strengths is challenged by decreased B0- and B1-homogeneity, shorter T2 relaxation times, stronger chemical shift displacement errors, and aggravated lipid contamination. Therefore, to capitalize on the advantages of ultra-high field strengths, these challenges must be overcome. This review focuses on the challenges MRSI of the human brain faces at ultra-high field strength, as well as the possible applications to this date.

Mitochondrial dysfunction, oxidative stress, neuroinflammation, and metabolic alterations in the progression of Alzheimer's disease: A meta-analysis of in vivo magnetic resonance spectroscopy studies

Accumulating evidence demonstrates that metabolic changes in the brain associated with neuroinflammation, oxidative stress, and mitochondrial dysfunction play an important role in the pathophysiology of mild cognitive impairment (MCI) and Alzheimer's disease (AD). However, the neural signatures associated with these metabolic alterations and underlying molecular mechanisms are still elusive. Accordingly, we reviewed the literature on in vivo human brain 1H and 31P-MRS studies and use meta-analyses to identify patterns of brain metabolic alterations in MCI and AD. 40 and 39 studies on MCI and AD, respectively, were classified according to brain regions. Our results indicate decreased N-acetyl aspartate and creatine but increased myo-inositol levels in both MCI and AD, decreased glutathione level in MCI as well as disrupted energy metabolism in AD. In addition, the hippocampus shows the strongest alterations in most of these metabolites. This meta-analysis also illustrates progressive metabolite alterations from MCI to AD. Taken together, it suggests that 1) neuroinflammation and oxidative stress may occur in the early stages of AD, and likely precede neuron loss in its progression; 2) the hippocampus is a sensitive region of interest for early diagnosis and monitoring the response of interventions; 3) targeting bioenergetics associated with neuroinflammation/oxidative stress is a promising approach for treating AD.

Dynamic alterations in the central glutamatergic status following food and glucose intake: in vivo multimodal assessments in humans and animal models

Fluctuations of neuronal activities in the brain may underlie relatively slow components of neurofunctional alterations, which can be modulated by food intake and related systemic metabolic statuses. Glutamatergic neurotransmission plays a major role in the regulation of excitatory tones in the central nervous system, although just how dietary elements contribute to the tuning of this system remains elusive. Here, we provide the first demonstration by bimodal positron emission tomography (PET) and magnetic resonance spectroscopy (MRS) that metabotropic glutamate receptor subtype 5 (mGluR5) ligand binding and glutamate levels in human brains are dynamically altered in a manner dependent on food intake and consequent changes in plasma glucose levels. The brain-wide modulations of central mGluR5 ligand binding and glutamate levels and profound neuronal activations following systemic glucose administration were further proven by PET, MRS, and intravital two-photon microscopy, respectively, in living rodents. The present findings consistently support the notion that food-associated glucose intake is mechanistically linked to glutamatergic tones in the brain, which are translationally accessible in vivo by bimodal PET and MRS measurements in both clinical and non-clinical settings.

Ultrastructural Evidence for a Role of Astrocytes and Glycogen-Derived Lactate in Learning-Dependent Synaptic Stabilization

Long-term memory formation (LTM) is a process accompanied by energy-demanding structural changes at synapses and increased spine density. Concomitant increases in both spine volume and postsynaptic density (PSD) surface area have been suggested but never quantified in vivo by clear-cut experimental evidence. Using novel object recognition in mice as a learning task followed by 3D electron microscopy analysis, we demonstrate that LTM induced all aforementioned synaptic changes, together with an increase in the size of astrocytic glycogen granules, which are a source of lactate for neurons. The selective inhibition of glycogen metabolism in astrocytes impaired learning, affecting all the related synaptic changes. Intrahippocampal administration of l-lactate rescued the behavioral phenotype, along with spine density within 24 hours. Spine dynamics in hippocampal organotypic slices undergoing theta burst-induced long-term potentiation was similarly affected by inhibition of glycogen metabolism and rescued by l-lactate. These results suggest that learning primes astrocytic energy stores and signaling to sustain synaptic plasticity via l-lactate.

Metabolomic Recovery as a Result of Ischemic Preconditioning Was More Pronounced in Hippocampus than in Cortex That Appeared More Sensitive to Metabolomic Blood Components

The study of an organism's response to ischemia at different levels is essential to understand the mechanism of the injury as well as protection. We used the occlusion of four vessels as an animal model of global cerebral ischemia to investigate metabolic alterations in cerebral cortex, hippocampus, blood plasma, as well as in a remote organ, the heart, in rats undergoing 24 h postischemic reperfusion. By inducing sublethal ischemic stimuli, we focused on endogenous phenomena known as ischemic tolerance that is currently the best known and most effective way of protecting against ischemic injury. NMR spectroscopy was used to analyze relative metabolite levels in homogenates from rats' cerebral cortex, hippocampus, and heart together with deproteinized blood plasma. In individual animals subjected to global cerebral ischemia, relative concentrations of the essential amino acids isoleucine, valine, phenylalanine, and tyrosine in cerebral cortex correlated with those in blood plasma (p < 0.05, or boundary significant p < 0.09). This did not apply for the hippocampus, suggesting a closer relation between ischemic cortex and metabolomic blood components. Hippocampal non-participation on correlation with blood components may emphasize the observed partial or full normalization the post-ischemically altered levels of a number of metabolites in the preconditioned animals. Remarkably, that was observed for cortex to a lesser extent. As a response to the global cerebral ischemia in heart tissue, we observed decreased glutamate and increased 3-hydroxybutyrate. Ischemically induced semi-ketotic state and other changes found in blood plasma partially normalized when ischemic preconditioning was introduced. Some metabolomic changes were so strong that even individual metabolites were able to differentiate between ischemic, ischemically preconditioned, and control brain tissues.

Asymmetric left-right hippocampal glutamatergic modulation of cognitive control in ApoE-isoform subjects is unrelated to neuroinflammation

The glutamatergic cycle is essential in modulating memory processing by the hippocampal circuitry. Our combined proton magnetic resonance spectroscopy (¹H‐MRS) and task‐based functional magnetic resonance imaging (fMRI) study (using face‐name paired‐associates encoding and retrieval task) of a cognitively normal cohort of 67 healthy adults (18 ApoE4 carriers and 49 non‐ApoE4 carriers) found altered patterns of relationships between glutamatergic‐modulated synaptic signalling and neuronal activity or functional hyperaemia in the ApoE4 isoforms. Our study highlighted the asymmetric left–right hippocampal glutamatergic system in modulating neuronal activities in ApoE4 carriers versus non‐carriers. Such brain differentiation might be developmental cognitive advantages or compensatory due to impaired synaptic integrity and plasticity in ApoE4 carriers. As there was no difference in myoinositol levels measured by MRS between the ApoE4 and non‐ApoE4 subgroups, the mechanism is unlikely to be a response to neuroinflammation.

Longitudinal quantification of metabolites and macromolecules reveals age- and sex-related changes in the healthy Fischer 344 rat brain

Normal aging is associated with numerous biological changes, including altered brain metabolism and tissue chemistry. In vivo characterization of the neurochemical profile during aging is possible using magnetic resonance spectroscopy, a powerful noninvasive technique capable of quantifying brain metabolites involved in physiological processes that become impaired with age. A prominent macromolecular signal underlies those of brain metabolites and is particularly visible at high fields; parameterization of this signal into components improves quantification and expands the number of biomarkers comprising the neurochemical profile. The present study reports, for the first time, the simultaneous absolute quantification of brain metabolites and individual macromolecules in aging male and female Fischer 344 rats, measured longitudinally using proton magnetic resonance spectroscopy at 7 T. We identified age- and sex-related changes in neurochemistry, with prominent differences in metabolites implicated in anaerobic energy metabolism, antioxidant defenses, and neuroprotection, as well as numerous macromolecule changes. These findings contribute to our understanding of the neurobiological processes associated with healthy aging, critical for the proper identification and management of pathologic aging trajectories. This article is part of the Virtual Special Issue titled COGNITIVE NEUROSCIENCE OF HEALTHY AND PATHOLOGICAL AGING. The full issue can be found on ScienceDirect athttps://www.sciencedirect.com/journal/neurobiology-of-aging/special-issue/105379XPWJP.

Proton functional magnetic resonance spectroscopy in rodents

Proton functional magnetic resonance spectroscopy (¹ H-fMRS) in the human brain is able to assess and quantify the metabolic response due to localized brain activity. Currently, ¹ H-fMRS of the human brain is complementary to functional magnetic resonance imaging (fMRI) and a recommended technique at high field strengths (>7 T) for the investigation of neurometabolic couplings, thereby providing insight into the mechanisms underlying brain activity and brain connectivity. Understanding typical healthy brain metabolism during a task is expected to provide a baseline from which to detect and characterize neurochemical alterations associated with various neurological or psychiatric disorders and diseases. It is of paramount importance to resolve fundamental questions related to the regulation of neurometabolic processes. New techniques such as optogenetics may be coupled to fMRI and fMRS to bring more specificity to investigations of brain cell populations during cerebral activation thus enabling a higher link to molecular changes and therapeutic advances. These rather novel techniques are mainly available for rodent applications and trigger renewed interest in animal fMRS. However, rodent fMRS remains fairly confidential due to its inherent low signal-to-noise ratio and its dependence on anesthesia. For instance, the accurate determination of metabolic concentration changes during stimulation requires robust knowledge of the physiological environment of the measured region of interest linked to anesthesia in most cases. These factors may also have a strong influence on B0 homogeneity. Therefore, a degree of calibration of the stimulus strength and duration may be needed for increased knowledge of the underpinnings of cerebral activity. Here, we propose an early review of the current status of ¹ H-fMRS in rodents and summarize current difficulties and future perspectives.

Repeated fluoxetine treatment induces transient and long-term astrocytic plasticity in the medial prefrontal cortex of normal adult rats

Fluoxetine (Flx)-induced neuronal plasticity plays an important role in the effective treatment of depression and mood disorders. It is less understood whether repeated Flx treatment induces astrocytic plasticity that outlasts the presence of the drug in the body. We showed previously that Flx-induced neuronal plasticity in the medial prefrontal cortex (mPFC) persisted up to 20 days after the treatment. In this study, adult rats were subjected to a 15-day repeated Flx treatment at a daily dose of 20 mg/kg body weight. Astrocytic metabolites and markers were assessed in the mPFC at day 1 (d1) and day 20 (d20) after the treatment. Significant transient reductions in the concentrations of astrocytic metabolites taurine and myo-inositol and the expressions of glial fibrillary acidic protein (GFAP) and aquaporin-4 (AQP4) were observed in the mPFC of Flx-treated rats at d1, which recovered to the control levels at d20. Further, Flx treatment resulted in long-lasting changes in Kir4.1 expression in the mPFC, which remained downregulated at d20. The expression of 5-HT_1A receptor in the mPFC of Flx-treated rats was downregulated at d1 but became upregulated at d20. In summary, repeated Flx treatment induces both transient and long-term astrocytic plasticity in the mPFC of adult rats. The changes observed at d1 are consistent with disturbed water homeostasis and astrocytic de-maturation in the mPFC. The persistent changes in the expressions of Kir4.1 and 5-HT_1A at d20, presumably of the astrocytic origin, might have contributed to the long-term neurotrophic effects of repeated Flx treatment in the mPFC.

Effect of Ketamine on Human Neurochemistry in Posterior Cingulate Cortex: A Pilot Magnetic Resonance Spectroscopy Study at 3 Tesla

Ketamine is a powerful glutamatergic long-lasting antidepressant, efficient in intractable major depression. Whereas ketamine's immediate psychomimetic side-effects were linked to glutamate changes, proton MRS (1H-MRS) showed an association between the ratio of glutamate and glutamine and delayed antidepressant effect emerging ∼2 h after ketamine administration. While most 1H-MRS studies focused on anterior cingulate, recent functional MRI connectivity studies revealed an association between ketamine's antidepressant effect and disturbed connectivity patterns to the posterior cingulate cortex (PCC), and related PCC dysfunction to rumination and memory impairment involved in depressive pathophysiology. The current study utilized the state-of-the-art single-voxel 3T sLASER 1H-MRS methodology optimized for reproducible measurements. Ketamine's effects on neurochemicals were assessed before and ∼3 h after intravenous ketamine challenge in PCC. Concentrations of 11 neurochemicals, including glutamate (CRLB ∼ 4%) and glutamine (CRLB ∼ 13%), were reliably quantified with the LCModel in 12 healthy young men with between-session coefficients of variation (SD/mean) <8%. Also, ratios of glutamate/glutamine and glutamate/aspartate were assessed as markers of synaptic function and activated glucose metabolism, respectively. Pairwise comparison of metabolite profiles at baseline and 193 ± 4 min after ketamine challenge yielded no differences. Minimal detectable concentration differences estimated with post hoc power analysis (power = 80%, alpha = 0.05) were below 0.5 μmol/g, namely 0.39 μmol/g (∼4%) for glutamate, 0.28 μmol/g (∼10%) for Gln, ∼14% for glutamate/glutamine and ∼8% for glutamate/aspartate. Despite the high sensitivity to detect between-session differences in glutamate and glutamine concentrations, our study did not detect delayed glutamatergic responses to subanesthetic ketamine doses in PCC.

Preclinical Assessment of a New Hybrid Compound C11 Efficacy on Neurogenesis and Cognitive Functions after Pilocarpine Induced Status Epilepticus in Mice

Status epilepticus (SE) is a frequent medical emergency that can lead to a variety of neurological disorders, including cognitive impairment and abnormal neurogenesis. The aim of the presented study was the in vitro evaluation of potential neuroprotective properties of a new pyrrolidine-2,5-dione derivatives compound C11, as well as the in vivo assessment of the impact on the neurogenesis and cognitive functions of C11 and levetiracetam (LEV) after pilocarpine (PILO)-induced SE in mice. The in vitro results indicated a protective effect of C11 (500, 1000, and 2500 ng/mL) on astrocytes under trophic stress conditions in the MTT (3-[4,5-dimethylthiazole-2-yl]-2,5-diphenyltetrazolium bromide) test. The results obtained from the in vivo studies, where mice 72 h after PILO SE were treated with C11 (20 mg/kg) and LEV (10 mg/kg), indicated markedly beneficial effects of C11 on the improvement of the neurogenesis compared to the PILO control and PILO LEV mice. Moreover, this beneficial effect was reflected in the Morris Water Maze test evaluating the cognitive functions in mice. The in vitro confirmed protective effect of C11 on astrocytes, as well as the in vivo demonstrated beneficial impact on neurogenesis and cognitive functions, strongly indicate the need for further advanced molecular research on this compound to determine the exact neuroprotective mechanism of action of C11.

Imaging Mechanisms of Disease Progression in Multiple Sclerosis: Beyond Brain Atrophy

Clinicians involved with different aspects of the care of persons with multiple sclerosis (MS) and scientists with expertise on clinical and imaging techniques convened in Dallas, TX, USA on February 27, 2019 at a North American Imaging in Multiple Sclerosis Cooperative workshop meeting. The aim of the workshop was to discuss cardinal pathobiological mechanisms implicated in the progression of MS and novel imaging techniques, beyond brain atrophy, to unravel these pathologies. Indeed, although brain volume assessment demonstrates changes linked to disease progression, identifying the biological mechanisms leading up to that volume loss are key for understanding disease mechanisms. To this end, the workshop focused on the application of advanced magnetic resonance imaging (MRI) and positron emission tomography (PET) imaging techniques to assess and measure disease progression in both the brain and the spinal cord. Clinical translation of quantitative MRI was recognized as of vital importance, although the need to maintain a relatively short acquisition time mandated by most radiology departments remains the major obstacle toward this effort. Regarding PET, the panel agreed upon its utility to identify ongoing pathological processes. However, due to costs, required expertise, and the use of ionizing radiation, PET was not considered to be a viable option for ongoing care of persons with MS. Collaborative efforts fostering robust study designs and imaging technique standardization across scanners and centers are needed to unravel disease mechanisms leading to progression and discovering medications halting neurodegeneration and/or promoting repair.

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.

Sampling Method Affects HR-MAS NMR Spectra of Healthy Caprine Brain Biopsies

The metabolic profiling of tissue biopsies using high-resolution–magic angle spinning (HR-MAS) ¹H nuclear magnetic resonance (NMR) spectroscopy may be influenced by experimental factors such as the sampling method. Therefore, we compared the effects of two different sampling methods on the metabolome of brain tissue obtained from the brainstem and thalamus of healthy goats by ¹H HR-MAS NMR spectroscopy—in vivo-harvested biopsy by a minimally invasive stereotactic approach compared with postmortem-harvested sample by dissection with a scalpel. Lactate and creatine were elevated, and choline-containing compounds were altered in the postmortem compared to the in vivo-harvested samples, demonstrating rapid changes most likely due to sample ischemia. In addition, in the brainstem samples acetate and inositols, and in the thalamus samples ƴ-aminobutyric acid, were relatively increased postmortem, demonstrating regional differences in tissue degradation. In conclusion, in vivo-harvested brain biopsies show different metabolic alterations compared to postmortem-harvested samples, reflecting less tissue degradation. Sampling method and brain region should be taken into account in the analysis of metabolic profiles. To be as close as possible to the actual situation in the living individual, it is desirable to use brain samples obtained by stereotactic biopsy whenever possible.

The potential of 1H-MRS in CNS drug development

RATIONALE

Proton magnetic resonance spectroscopy (¹H-MRS) is a cross-species neuroimaging technique that can measure concentrations of several brain metabolites, including glutamate and GABA. This non-invasive method has promise in developing centrally acting drugs, as it can be performed repeatedly within-subjects and be used to translate findings from the preclinical to clinical laboratory using the same imaging biomarker.

OBJECTIVES

This review focuses on the utility of single-voxel ¹H-MRS in developing novel glutamatergic or GABAergic drugs for the treatment of psychiatric disorders and includes research performed in rodent models, healthy volunteers and patient cohorts.

RESULTS

Overall, these studies indicate that ¹H-MRS is able to detect the predicted pharmacological effects of glutamatergic or GABAergic drugs on voxel glutamate or GABA concentrations, although there is a shortage of studies examining dose-related effects. Clinical studies have applied ¹H-MRS to better understand drug therapeutic mechanisms, including the glutamatergic effects of ketamine in depression and of acamprosate in alcohol dependence. There is an emerging interest in identifying patient subgroups with 'high' or 'low' brain regional ¹H-MRS glutamate levels for more targeted drug development, which may require ancillary biomarkers to improve the accuracy of subgroup discrimination.

CONCLUSIONS

Considerations for future research include the sensitivity of single-voxel ¹H-MRS in detecting drug effects, inter-site measurement reliability and the interpretation of drug-induced changes in ¹H-MRS metabolites relative to the known pharmacological molecular mechanisms. On-going technological development, in single-voxel ¹H-MRS and in related complementary techniques, will further support applications within CNS drug discovery.

Magnetic Resonance Spectroscopy in the Ventral Tegmental Area Distinguishes Responders to Suvorexant Prior to Treatment: A 4-Week Prospective Cohort Study

Background: The ventral tegmental area (VTA; a dopaminergic nucleus) plays an important role in the sleep-wake regulation system including orexin system. In addition to neuronal activity, there is increasing evidence for an important role of glial cells (i.e., astrocytes and microglia) in these systems. The present study examined the utility of magnetic resonance spectroscopy (MRS) for detecting neural and/or glial changes in the VTA to distinguish responders from non-responders before treatment with the orexin receptor antagonist suvorexant. Methods: A total of 50 patients were screened and 9 patients were excluded. The remaining 41 patients with insomnia who have or not a psychiatric disease who were expected to receive suvorexant treatment were included in this study. We compared MRS signals in the VTA between responders to suvorexant and non-responders before suvorexant use. Based on previous reports, suvorexant responders were defined as patients who improved ≥3 points on the Pittsburgh Sleep Quality Index after 4 weeks of suvorexant use. MRS data included choline (reflects non-specific cell membrane breakdown, including of glial cells) and N-acetylaspartate (a decrease reflects neuronal degeneration). Results: Among 41 examined patients, 20 patients responded to suvorexant and 21 patients did not. By MRS, the choline/creatine and phosphorylcreatine ratio in the VTA was significantly high in non-responders compared with responders (p = 0.039) before suvorexant treatment. There was no difference in the N-acetylaspartate/creatine and phosphorylcreatine ratio (p = 0.297) between the two groups. Conclusions: Changes in glial viability in the VTA might be used to distinguish responders to suvorexant from non-responders before starting treatment. These findings may help with more appropriate selection of patients for suvorexant treatment in clinical practice. Further, we provide novel possible evidence for a relationship between glial changes in the VTA and the orexin system, which may aid in the development of new hypnotics focusing on the VTA and/or glial cells.

Double diffusion encoding and applications for biomedical imaging

Diffusion Magnetic Resonance Imaging (dMRI) is one of the most important contemporary non-invasive modalities for probing tissue structure at the microscopic scale. The majority of dMRI techniques employ standard single diffusion encoding (SDE) measurements, covering different sequence parameter ranges depending on the complexity of the method. Although many signal representations and biophysical models have been proposed for SDE data, they are intrinsically limited by a lack of specificity. Advanced dMRI methods have been proposed to provide additional microstructural information beyond what can be inferred from SDE. These enhanced contrasts can play important roles in characterizing biological tissues, for instance upon diseases (e.g. neurodegenerative, cancer, stroke), aging, learning, and development. In this review we focus on double diffusion encoding (DDE), which stands out among other advanced acquisitions for its versatility, ability to probe more specific diffusion correlations, and feasibility for preclinical and clinical applications. Various DDE methodologies have been employed to probe compartment sizes (Section 3), decouple the effects of microscopic diffusion anisotropy from orientation dispersion (Section 4), probe displacement correlations, study exchange, or suppress fast diffusing compartments (Section 6). DDE measurements can also be used to improve the robustness of biophysical models (Section 5) and study intra-cellular diffusion via magnetic resonance spectroscopy of metabolites (Section 7). This review discusses all these topics as well as important practical aspects related to the implementation and contrast in preclinical and clinical settings (Section 9) and aims to provide the readers a guide for deciding on the right DDE acquisition for their specific application.

Magnetic resonance assessment of the cerebral alterations associated with obesity development

Obesity is a current threat to health care systems, affecting approximately 13% of the world's adult population, and over 18% children and adolescents. The rise of obesity is fuelled by inadequate life style habits, as consumption of diets rich in fats and sugars which promote, additionally, the development of associated comorbidities. Obesity results from a neuroendocrine imbalance in the cerebral mechanisms controlling food intake and energy expenditure, including the hypothalamus and the reward and motivational centres. Specifically, high-fat diets are known to trigger an early inflammatory response in the hypothalamus that precedes weight gain, is time-dependent, and eventually extends to the remaining appetite regulating regions in the brain. Multiple magnetic resonance imaging (MRI) and spectroscopy (MRS) methods are currently available to characterize different features of cerebral obesity, including diffusion weighted, T2 and volumetric imaging and 1H and 13C spectroscopic evaluations. In particular, consistent evidences have revealed increased water diffusivity and T2 values, decreased grey matter volumes, and altered metabolic profiles and fluxes, in the brain of animal models and in obese humans. This review provides an integrative interpretation of the physio-pathological processes associated with obesity development in the brain, and the MRI and MRS methods implemented to characterize them.

Mitochondrial gene signature in the prefrontal cortex for differential susceptibility to chronic stress

Mitochondrial dysfunction was highlighted as a crucial vulnerability factor for the development of depression. However, systemic studies assessing stress-induced changes in mitochondria-associated genes in brain regions relevant to depression symptomatology remain scarce. Here, we performed a genome-wide transcriptomic study to examine mitochondrial gene expression in the prefrontal cortex (PFC) and nucleus accumbens (NAc) of mice exposed to multimodal chronic restraint stress. We identified mitochondria-associated gene pathways as most prominently affected in the PFC and with lesser significance in the NAc. A more detailed mitochondrial gene expression analysis revealed that in particular mitochondrial DNA-encoded subunits of the oxidative phosphorylation complexes were altered in the PFC. The comparison of our data with a reanalyzed transcriptome data set of chronic variable stress mice and major depression disorder subjects showed that the changes in mitochondrial DNA-encoded genes are a feature generalizing to other chronic stress-protocols as well and might have translational relevance. Finally, we provide evidence for changes in mitochondrial outputs in the PFC following chronic stress that are indicative of mitochondrial dysfunction. Collectively, our work reinforces the idea that changes in mitochondrial gene expression are key players in the prefrontal adaptations observed in individuals with high behavioral susceptibility and resilience to chronic stress.