Assessment of GABA concentration in human brain using two-dimensional proton magnetic resonance spectroscopy

Genetically encoded fluorescent sensors for imaging neuronal dynamics in vivo

The brain relies on many forms of dynamic activities in individual neurons, from synaptic transmission to electrical activity and intracellular signaling events. Monitoring these neuronal activities with high spatiotemporal resolution in the context of animal behavior is a necessary step to achieve a mechanistic understanding of brain function. With the rapid development and dissemination of highly optimized genetically encoded fluorescent sensors, a growing number of brain activities can now be visualized in vivo. To date, cellular calcium imaging, which has been largely used as a proxy for electrical activity, has become a mainstay in systems neuroscience. While challenges remain, voltage imaging of neural populations is now possible. In addition, it is becoming increasingly practical to image over half a dozen neurotransmitters, as well as certain intracellular signaling and metabolic activities. These new capabilities enable neuroscientists to test previously unattainable hypotheses and questions. This review summarizes recent progress in the development and delivery of genetically encoded fluorescent sensors, and highlights example applications in the context of in vivo imaging.

Adaptable Singlet-Filtered Nuclear Magnetic Resonance Spectroscopy for Chemical and Biological Applications

Proton nuclear magnetic resonance (¹H NMR) spectroscopy presents a powerful detection tool for studying chemical compositions and molecular structures. In practical chemical and biological applications, ¹H NMR experiments are generally confronted with the challenge of spectral congestions caused by abundant observable components and intrinsic limitations of a narrow frequency distribution range and extensive J coupling splitting. Herein, a one-dimensional (1D) general NMR method is proposed to individually extract the signals of targeted proton groups based on their endogenous spin singlet states excited from J coupling interactions, and it is suitable for high-resolution detections on complex chemical and biological samples. The applicability of the proposed method is demonstrated by experimental observations on chemical solutions containing different coupled components, intact grape tissues subjected to crowded resonances, and in vitro pig brain with various metabolites. Moreover, the proposed method is further exploited for magnetic resonance spectroscopy applications by directly combining the spatial localization module, showing promise in in vivo biological metabolite studies.

Proton nuclear magnetic resonance J-spectroscopy of phantoms containing brain metabolites on a portable 0.05 T MRI scanner

We examined approaches for obtaining ¹H NMR spectra of brain metabolites on a low-field (B₀ = 0.05 T) portable MRI scanner, which was developed in our laboratory with the aim of bringing cost-effective radiological services to populations in underserved, remote regions. The low static magnetic field B₀ dictates low signal to noise ratio for metabolites in the mM concentration range, and results in an overall spectral region for the ¹H resonances of these metabolites narrower than the linewidth obtainable in our scanner. The narrow spectral range also precludes the possibility of suppressing the large contribution of the water resonance at the acquisition stage. We used a spectroscopic Carr-Purcell-Meiboom-Gill (CPMG) sequence to acquire multiecho data from solutions of J-coupled brain metabolites, focusing on lactic acid, a metabolite whose concentration is negligible in the healthy brain and increases significantly in several disease conditions. The J spectra we obtained for lactate from the Fourier transformation of the multiecho data are spectrally well-resolved for a range of echo spacing values. We show that the J spectra at different echo spacings fit well with simulations of the evolution of echo train signal of the lactate under the same conditions. Applying a J-refocused variant of the CPMG sequence, the J modulation of the echo decay is removed, providing a way for subtracting the large contribution of the non-modulated component in the J spectrum in conditions where notching it using post-processing methods is impossible. We also demonstrate by means of experimental data and simulations that in our experimental conditions, J-spectra of other prominent brain metabolites, such as the neurotransmitter glutamate, do not yield discernible peaks and only contribute to a broad peak at zero frequency.

Transcranial magnetic stimulation and magnetic resonance spectroscopy: Opportunities for a bimodal approach in human neuroscience

Over the last decade, there has been an increasing number of studies combining transcranial magnetic stimulation (TMS) and magnetic resonance spectroscopy (MRS). MRS provides a manner to non-invasively investigate molecular concentrations in the living brain and thus identify metabolites involved in physiological and pathological processes. Particularly the MRS-detectable metabolites glutamate, the major excitatory neurotransmitter, and gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter, are of interest when combining TMS and MRS. TMS is a non-invasive brain stimulation technique that can be applied either as a neuromodulation or neurostimulation tool, specifically targeting glutamatergic and GABAergic mechanisms. The combination of TMS and MRS can be used to evaluate alterations in brain metabolite levels following an interventional TMS protocol such as repetitive TMS (rTMS) or paired associative stimulation (PAS). MRS can also be combined with a variety of non-interventional TMS protocols to identify the interplay between brain metabolite levels and measures of excitability or receptor-mediated inhibition and facilitation. In this review, we provide an overview of studies performed in healthy and patient populations combining MRS and TMS, both as a measurement tool and as an intervention. TMS and MRS may reveal complementary and comprehensive information on glutamatergic and GABAergic neurotransmission. Potentially, connectivity changes and dedicated network interactions can be probed using the combined TMS-MRS approach. Considering the ongoing technical developments in both fields, combined studies hold future promise for investigations of brain network interactions and neurotransmission.

Analysis of chemical exchange saturation transfer contributions from brain metabolites to the Z-spectra at various field strengths and pH

Chemical exchange saturation transfer (CEST) exploits the chemical exchange of labile protons of an endogenous or exogenous compound with water to image the former indirectly through the water signal. Z-spectra of the brain have traditionally been analyzed for two most common saturation phenomena: downfield amide proton transfer (APT) and upfield nuclear Overhauser enhancement (NOE). However, a great body of brain metabolites, many of interest in neurology and oncology, contributes to the downfield saturation in Z-spectra. The extraction of these “hidden” metabolites from Z-spectra requires careful design of CEST sequences and data processing models, which is only possible by first obtaining CEST signatures of the brain metabolites possessing labile protons. In this work, we measured exchange rates of all major-for-CEST brain metabolites in the physiological pH range at 37 °C. Analysis of their contributions to Z-spectra revealed that regardless of the main magnetic field strength and pH, five main contributors, i.e. myo-inositol, creatine, phosphocreatine, glutamate, and mobile (poly)peptides, account for ca. 90% of downfield CEST effect. The fundamental CEST parameters presented in this study can be exploited in the design of novel CEST sequences and Z-spectra processing models, which will enable simultaneous and quantitative CEST imaging of multiple metabolites: multicolor CEST.

Two-Dimensional Proton Magnetic Resonance Spectroscopy versus J-Editing for GABA Quantification in Human Brain: Insights from a GABA-Aminotransferase Inhibitor Study

Metabolite-specific, scalar spin-spin coupling constant (J)-editing ¹H MRS methods have become gold-standard for measuring brain γ-amino butyric acid (GABA) levels in human brain. Localized, two-dimensional (2D) ¹H MRS technology offers an attractive alternative as it significantly alleviates the problem of severe metabolite signal overlap associated with standard 1D MRS and retains spectroscopic information for all MRS-detectable species. However, for metabolites found at low concentration, a direct, in vivo, comprehensive methods comparison is challenging and has not been reported to date. Here, we document an assessment of comparability between 2D ¹H MRS and J-editing methods for measuring GABA in human brain. This clinical study is unique in that it involved chronic administration a GABA-amino transferase (AT) inhibitor (CPP-115), which induces substantial increases in brain GABA concentration, with normalization after washout. We report a qualitative and quantitative comparison between these two measurement techniques. In general, GABA concentration changes detected using J-editing were closely mirrored by the 2D ¹H MRS time courses. The data presented are particularly encouraging considering recent 2D ¹H MRS methodological advances are continuing to improve temporal resolution and spatial coverage for achieving whole-brain, multi-metabolite mapping.

A Novel Microbiosensor Microarray for Continuous ex Vivo Monitoring of Gamma-Aminobutyric Acid in Real-Time

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter that is essential for normal brain function. It is involved in multiple neuronal activities, including plasticity, information processing, and network synchronization. Abnormal GABA levels result in severe brain disorders and therefore GABA has been the target of a wide range of drug therapeutics. GABA being non-electroactive is challenging to detect in real-time. To date, GABA is detected mainly via microdialysis with a high-performance liquid chromatography (HPLC) system that employs electrochemical (EC) and spectroscopic methodology. However, these systems are bulky and unsuitable for real-time continuous monitoring. As opposed to microdialysis, biosensors are easy to miniaturize and are highly suitable for in vivo studies; they selectively oxidize GABA into a secondary electroactive product (usually hydrogen peroxide, H₂O₂) in the presence of enzymes, which is then detected by amperometry. Unfortunately, this method requires a rather cumbersome process with prereactors and relies on externally applied reagents. Here, we report the design and implementation of a GABA microarray probe that operates on a newly conceived principle. It consists of two microbiosensors, one for glutamate (Glu) and one for GABA detection, modified with glutamate oxidase and GABASE enzymes, respectively. By simultaneously measuring and subtracting the H₂O₂ oxidation currents generated from these microbiosensors, GABA and Glu can be detected continuously in real-time in vitro and ex vivo and without the addition of any externally applied reagents. The detection of GABA by this probe is based upon the in-situ generation of α-ketoglutarate from the Glu oxidation that takes place at the Glu microbiosensor. A GABA sensitivity of 36 ± 2.5 pA μM^-1cm^-2, which is 26-fold higher than reported in the literature, and a limit of detection of 2 ± 0.12 μM were achieved in an in vitro setting. The GABA probe was successfully tested in an adult rat brain slice preparation. These results demonstrate that the developed GABA probe constitutes a novel and powerful neuroscientific tool that could be employed in the future for in vivo longitudinal studies of the combined role of GABA and Glu (a major excitatory neurotransmitter) signaling in brain disorders, such as epilepsy and traumatic brain injury, as well as in preclinical trials of potential therapeutic agents for the treatment of these disorders.

GABA type a receptor trafficking and the architecture of synaptic inhibition

Ubiquitous expression of GABA type A receptors (GABAA R) in the central nervous system establishes their central role in coordinating most aspects of neural function and development. Dysregulation of GABAergic neurotransmission manifests in a number of human health disorders and conditions that in certain cases can be alleviated by drugs targeting these receptors. Precise changes in the quantity or activity of GABAA Rs localized at the cell surface and at GABAergic postsynaptic sites directly impact the strength of inhibition. The molecular mechanisms constituting receptor trafficking to and from these compartments therefore dictate the efficacy of GABAA R function. Here we review the current understanding of how GABAA Rs traffic through biogenesis, plasma membrane transport, and degradation. Emphasis is placed on discussing novel GABAergic synaptic proteins, receptor and scaffolding post-translational modifications, activity-dependent changes in GABAA R confinement, and neuropeptide and neurosteroid mediated changes. We further highlight modern techniques currently advancing the knowledge of GABAA R trafficking and clinically relevant neurodevelopmental diseases connected to GABAergic dysfunction. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 238-270, 2018.

Cognitive effects of creatine monohydrate adjunctive therapy in patients with bipolar depression: Results from a randomized, double-blind, placebo-controlled trial

BACKGROUND

Depressive episodes and cognitive impairment are major causes of morbidity and dysfunction in individuals suffering from bipolar disorder (BD). Novel treatment approaches that target clinical and cognitive aspects of bipolar depression are needed, and research on pathophysiology suggests that mitochondrial modulators such as the nutraceutical creatine monohydrate might have a therapeutic role for this condition.

METHODS

Eighteen (N=18) patients with bipolar depression according to DSM-IV criteria who were enrollled in a 6-week, randomized, double-blind, placebo-controlled trial of creatine monohydrate 6g daily as adjunctive therapy were submitted to neuropsychological assessments (Wisconsin Card Sorting Test, Digit Span subtest of the Wechsler Adult Intelligence Scale-Third Edition, Stroop Color-Word Test, Rey-Osterrieth complex figure test, FAS Verbal Fluency Test) at baseline and week 6.

RESULTS

There was a statistically significant difference between the treatment groups of the change on the total scores after 6 weeks in the verbal fluency test, with improvement in the group receiving adjunctive treatment with creatine. We did not find significant differences between the groups of the changes on other neuropsychological tests.

LIMITATIONS

Small sample and lack of a control group of healthy subjects.

CONCLUSIONS

Our trial, which was the first to investigate the cognitive effects of creatine monohydrate on bipolar depression, indicates that supplementation with this nutraceutical for 6 weeks is associated with improvement in verbal fluency tests in patients with this condition.

Localized MRS reliability of in vivo glutamate at 3 T in shortened scan times: a feasibility study

Glutamate is the prime excitatory neurotransmitter in the mammalian brain and has been implicated in a wide range of psychiatric conditions. To improve the applicability and clinical reach of magnetic resonance spectroscopy (MRS), research is needed to develop shortened, yet reliable, MRS scanning procedures for standard 1.5-3-T clinical magnetic resonance imaging (MRI) systems, particularly with young or vulnerable populations unable to tolerate longer protocols. To this end, we evaluated the test-retest reliability of a shortened J-resolved MRS sequence in healthy adolescents (n = 22) aged 12-14 years. Participants underwent a series of sequential 6-min MRS scans, with the participants remaining in situ between successive scans. Glutamate and other metabolites were acquired from the rostral anterior cingulate cortex, as glutamatergic function in this region has been implicated in a number of psychiatric illnesses. Thirteen neurochemicals were quantified as ratios to total creatine, and reliability scores were expressed as the percentage difference between the two scans for each metabolite. Test-retest assessment of glutamate was reliable, as scores were less than 10% different (7.1 ± 4.2%), and glutamate values across scans were significantly correlated (Pearson r = 0.680, p < 10^-4 ). Several other neurochemicals demonstrated satisfactory reliability, including choline (Cho) (7.4 ± 5.6%), glutathione (GSH) (8.6 ± 4.1%), myo-inositol (mI) (6.5 ± 7.1%) and N-acetylaspartate (NAA) (3.5 ± 3.6%), with test-retest correlations ranging from 0.747 to 0.953. A number of metabolites, however, did not demonstrate acceptable test-retest reliability using the current J-resolved MRS sequence, ranging from 13.8 ± 13.7% (aspartate, Asp) to 45.9 ± 38.3% (glycine, Gly). Collectively, test-retest analyses suggest that clinically viable quantitative data can be obtained on standard MRI systems for glutamate, as well as the other metabolites, during short scan times in a traditionally challenging brain region.

Creatine as a booster for human brain function. How might it work?

Creatine, a naturally occurring nitrogenous organic acid found in animal tissues, has been found to play key roles in the brain including buffering energy supply, improving mitochondrial efficiency, directly acting as an anti-oxidant and acting as a neuroprotectant. Much of the evidence for these roles has been established in vitro or in pre-clinical studies. Here, we examine the roles of creatine and explore the current status of translation of this research into use in humans and the clinic. Some further possibilities for use of creatine in humans are also discussed.

GABA concentration in posterior cingulate cortex predicts putamen response during resting state fMRI

The role of neurotransmitters in the activity of resting state networks has been gaining attention and has become a field of research with magnetic resonance spectroscopy (MRS) being one of the key techniques. MRS permits the measurement of γ-aminobutyric acid (GABA) and glutamate levels, the central biochemical constituents of the excitation-inhibition balance in vivo. The inhibitory effects of GABA in the brain have been largely investigated in relation to the activity of resting state networks in functional magnetic resonance imaging (fMRI). In this study GABA concentration in the posterior cingulate cortex (PCC) was measured using single voxel spectra acquired with standard point resolved spectroscopy (PRESS) from 20 healthy male volunteers at 3 T. Resting state fMRI was consecutively measured and the values of GABA/Creatine+Phosphocreatine ratio (GABA ratio) were included in a general linear model matrix as a step of dual regression analysis in order to identify voxels whose neuroimaging metrics during rest were related to individual levels of the GABA ratio. Our data show that the connection strength of putamen to the default-mode network during resting state has a negative linear relationship with the GABA ratio measured in the PCC. These findings highlight the role of PCC and GABA in segregation of the motor input, which is an inherent condition that characterises resting state.

The use of magnetic resonance spectroscopy in the evaluation of epilepsy

Magnetic resonance spectroscopy (MRS) is indicated in the imaging protocol of the patient with epilepsy to screen for metabolic derangements such as inborn errors of metabolism and to characterize masses that may be equivocal on conventional magnetic resonance imaging for dysplasia versus neoplasia. Single-voxel MRS with echo time of 35 milliseconds may be used for this purpose as a quick screening tool in the epilepsy imaging protocol. MRS is useful in the evaluation of both focal and generalized epilepsy.

Human breast cancer metastases to the brain display GABAergic properties in the neural niche

Dispersion of tumors throughout the body is a neoplastic process responsible for the vast majority of deaths from cancer. Despite disseminating to distant organs as malignant scouts, most tumor cells fail to remain viable after their arrival. The physiologic microenvironment of the brain must become a tumor-favorable microenvironment for successful metastatic colonization by circulating breast cancer cells. Bidirectional interplay of breast cancer cells and native brain cells in metastasis is poorly understood and rarely studied. We had the rare opportunity to investigate uncommonly available specimens of matched fresh breast-to-brain metastases tissue and derived cells from patients undergoing neurosurgical resection. We hypothesized that, to metastasize, breast cancers may escape their normative genetic constraints by accommodating and coinhabiting the neural niche. This acquisition or expression of brain-like properties by breast cancer cells could be a malignant adaptation required for brain colonization. Indeed, we found breast-to-brain metastatic tissue and cells displayed a GABAergic phenotype similar to that of neuronal cells. The GABAA receptor, GABA transporter, GABA transaminase, parvalbumin, and reelin were all highly expressed in breast cancer metastases to the brain. Proliferative advantage was conferred by the ability of breast-to-brain metastases to take up and catabolize GABA into succinate with the resultant formation of NADH as a biosynthetic source through the GABA shunt. The results suggest that breast cancers exhibit neural characteristics when occupying the brain microenvironment and co-opt GABA as an oncometabolite.

GABA-based evaluation of neurologic conditions: MR spectroscopy

SUMMARY

GABA serves as a major neurotransmitter of the brain and functions mainly to inhibit neural excitatory activity. Disruption of the GABAergic processes appears to occur in various neurologic and psychiatric conditions, including epilepsy, mood disorders, motor disorders such as focal dystonia and stiff-person syndrome, sleep disorders, neuroplasticity, and drug and alcohol dependence. These concentration differences may be ascertained by using MR spectroscopy to provide information on the concentration of different metabolites. This review briefly discusses advances in MR spectroscopy methods and explores the application of this technique to detect changes in GABA due to disease processes and medication-induced effects.

Two-dimensional J-resolved proton MR spectroscopy and prior knowledge fitting (ProFit) in the frontal and parietal lobes of healthy volunteers: assessment of metabolite discrimination and general reproducibility

PURPOSE

To investigate human brain metabolite discriminability and general measurement reproducibility of two-dimensional (2D) J-resolved (1)H MRS and Prior Knowledge Fitting (ProFit).

MATERIALS AND METHODS

2D J-resolved (1)H MRS spectra were acquired from the anterior cingulate cortex (ACC) and the parietal-occipital cortex (POC) of 10 healthy subjects at a magnetic field strength of 2.9 Tesla. Amplitude correlation matrices were constructed for each subject and brain region to assess metabolite discriminability. ProFit-estimated metabolite peak areas were normalized to a water reference signal, and intra- and inter-subject reproducibility was evaluated.

RESULTS

Favorable between-metabolite correlation coefficients (<20%) were observed for a range of metabolites. Lower correlation coefficients between a given pair of metabolite estimates were consistently observed for POC metabolites. The group mean correlation coefficient existing between glutamate and glutamine was calculated as -18% and -13% for ACC and POC, respectively. Most ACC and POC metabolites showed intra- and inter-subject CV values of <15% and <20%, respectively.

CONCLUSION

The observed Glu and Gln signal discrimination makes these techniques suitable for investigating a variety of psychiatric disorders. Intra- and inter-subject metabolite level reproducibility was comparable to the existing literature findings. These data serve as a valuable benchmark for assessing future modifications to 2D (1)H MRS data acquisition and ProFit analysis.

Quantitative analysis in magnetic resonance spectroscopy: from metabolic profiling to in vivo biomarkers

Nuclear magnetic resonance spectroscopy (called NMR for ex vivo techniques and MRS for in vivo techniques) has become a useful analytical and diagnostic tool in biomedicine. In the past two decades, an MR-based spectroscopic approach for translational and clinical research has emerged that allows for biochemical characterization of the tissue of interest either ex vivo (NMR-based metabolomics) or in vivo (localized MRS-single voxel or multivoxel-spectroscopic imaging). The greatest advantages of MRS techniques are their ability to detect multiple tissue-specific metabolites in a single experiment, their quantitative nature and translational component (in vitro/ex vivo-discovered metabolic biomarkers can be translated into noninvasive spectroscopic imaging protocols). Disadvantages of MRS include low sensitivity and spectral resolution and, in case of NMR-metabolomics, metabolite degradation and incomplete recovery in processed samples. In vivo MRS has worse spectral resolution than ex vivo high-resolution NMR due to the inherently wider lines of metabolites in vivo and the difficulty of using traditional line-narrowing methods (e.g., sample spinning). It also suffers from poor time-resolution, therefore offering fewer metabolic biomarkers to be followed in vivo. In the present review article, we provide considerations for establishing reliable protocols (both in vivo and ex vivo) for metabolite detection, recovery and quantification from in vivo and ex vivo MR spectra.

Reproducibility of prefrontal γ-aminobutyric acid measurements with J-edited spectroscopy

γ-Aminobutyric acid (GABA) is the chief inhibitory neurotransmitter of the human brain, and GABA-ergic dysfunction has been implicated in a variety of neuropsychiatric disorders. Recent MRS techniques have allowed the quantification of GABA concentrations in vivo, and could therefore provide biologically relevant information. Few reports have formally characterized the reproducibility of these techniques, and differences in field strength, acquisition and processing parameters may result in large differences in measured GABA values. Here, we used a J-edited, single-voxel spectroscopy method of measurement of GABA + macromolecules (GABA + ) in the anterior cingulate cortex (ACC) and right frontal white matter (rFWM) at 3 T. We measured the coefficient of variation within subjects (CVw) and intra-class correlation coefficients on two repeated scans obtained from 10 healthy volunteers with processing procedures developed in-house for the quantification of GABA + and other major metabolites. In addition, by segmenting the spectroscopic voxel into cerebrospinal fluid, gray matter and white matter, and employing a linear regression technique to extrapolate metabolite values to pure gray and white matter, we determined metabolite differences between gray and white matter in ACC and rFWM. CVw values for GABA + /creatine, GABA + /H(2) O, GABA + , creatine, partially co-edited glutamate + glutamine (Glx)/creatine, partially co-edited Glx and N-acetylaspartic acid (NAA)/creatine were all below 12% in both ACC and rFWM. After extrapolation to pure gray and pure white matter, CVw values for all metabolites were below 16%. We found metabolite ratios between gray and white matter for GABA + /creatine, GABA + , creatine, partially co-edited Glx and NAA/creatine to be 0.88 ± 0.21 (standard deviation), 1.52 ± 0.32, 1.77 ± 0.4, 2.69 ± 0.74 and 0.70 ± 0.05, respectively. This study validates a reproducible method for the quantification of brain metabolites, and provides information on gray/white matter differences that may be important in the interpretation of results in clinical populations.

GABAA receptor trafficking-mediated plasticity of inhibitory synapses

Proper developmental, neural cell-type-specific, and activity-dependent regulation of GABAergic transmission is essential for virtually all aspects of CNS function. The number of GABA(A) receptors in the postsynaptic membrane directly controls the efficacy of GABAergic synaptic transmission. Thus, regulated trafficking of GABA(A) receptors is essential for understanding brain function in both health and disease. Here we summarize recent progress in the understanding of mechanisms that allow dynamic adaptation of cell surface expression and postsynaptic accumulation and function of GABA(A) receptors. This includes activity-dependent and cell-type-specific changes in subunit gene expression, assembly of subunits into receptors, as well as exocytosis, endocytic recycling, diffusion dynamics, and degradation of GABA(A) receptors. In particular, we focus on the roles of receptor-interacting proteins, scaffold proteins, synaptic adhesion proteins, and enzymes that regulate the trafficking and function of receptors and associated proteins. In addition, we review neuropeptide signaling pathways that affect neural excitability through changes in GABA(A)R trafficking.

New perspectives in glutamate and anxiety

Anxiety and stress-related disorders, namely posttraumatic stress disorder (PTSD), generalized anxiety disorder (GAD), obsessive-compulsive disorder (ODC), social and specific phobias, and panic disorder, are a major public health issue. A growing body of evidence suggests that glutamatergic neurotransmission may be involved in the biological mechanisms underlying stress response and anxiety-related disorders. The glutamatergic system mediates the acquisition and extinction of fear-conditioning. Thus, new drugs targeting glutamatergic neurotransmission may be promising candidates for new pharmacological treatments. In particular, N-methyl-d-aspartate receptors (NMDAR) antagonists (AP5, AP7, CGP37849, CGP39551, LY235959, NPC17742, and MK-801), NMDAR partial agonists (DCS, ACPC), α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPARs) antagonists (topiramate), and several allosteric modulators targeting metabotropic glutamate receptors (mGluRs) mGluR1, mGluR2/3, and mGluR5, have shown anxiolytic-like effects in several animal and human studies. Several studies have suggested that polyamines (agmatine, putrescine, spermidine, and spermine) may be involved in the neurobiological mechanisms underlying stress-response and anxiety-related disorders. This could mainly be attributed to their ability to modulate ionotropic glutamate receptors, especially NR2B subunits. The aim of this review is to establish that glutamate neurotransmission and polyaminergic system play a fundamental role in the onset of anxiety-related disorders. This may open the way for new drugs that may help to treat these conditions.

1H-MR spectroscopic detection of metabolic changes in pain processing brain regions in the presence of non-specific chronic low back pain

Reliable detection of metabolic changes in the brain in vivo induced by chronic low back pain may provide improved understanding of neurophysiological mechanisms underlying the manifestation of chronic pain. In the present study, absolute concentrations of N-acetyl-aspartate (NAA), creatine (Cr), total choline (tCho), myo-inositol (mI), glutamate (Glu) and glutamine (Gln) were measured in three different pain processing cortical regions (anterior insula, anterior cingulate cortex, and thalamus) of ten patients with non-specific chronic low back pain by means of proton MR spectroscopy ((1)H-MRS) and compared to matched healthy controls. Significant decrease of Glu was observed in the anterior cingulate cortex of patients. Patients also revealed a trend of decreasing Gln concentrations in all investigated brain areas. Reductions of NAA were observed in the patient group in anterior insula and in anterior cingulated cortex, whereas mI was reduced in anterior cingulated cortex and in thalamus of patients. Reduced concentrations of Glu and Gln may indicate disordered glutamatergic neurotransmission due to prolonged pain perception, whereas decrease of NAA and mI may be ascribed to neuron and glial cell loss. No significant changes were found for Cr. The morphological evaluation of anatomic brain data revealed a significantly decreased WM volume of 17% (p<0.05) as well as a non significant trend for GM volume increase in the anterior insula of patients.

Excitatory neurotransmitters in brain regions in interictal migraine patients

OBJECTIVE

To examine biochemical differences in the anterior cingulate cortex (ACC) and insula during the interictal phase of migraine patients. We hypothesized that there may be differences in levels of excitatory amino acid neurotransmitters and/or their derivatives in migraine group based on their increased sensitivity to pain.

METHODS

2D J-resolved proton magnetic resonance spectroscopy (1H-MRS) data were acquired at 4.0 Tesla (T) from the ACC and insula in 10 migraine patients (7 women, 3 men, age 43 +/- 11 years) and 8 age gender matched controls (7 women, 3 men, age 41 +/- 9 years).

RESULTS

Standard statistical analyses including analysis of variance (ANOVA) showed no significant metabolite differences between the two subject cohorts in the ACC nor the insula. However, linear discriminant analysis (LDA) introduced a clear separation between subject cohorts based on N-acetyl aspartylglutamate (NAAG) and glutamine (Gln) in the ACC and insula.

CONCLUSION

These results are consistent with glutamatergic abnormalities in the ACC and insula in migraine patients during their interictal period compared to healthy controls. An alteration in excitatory amino acid neurotransmitters and their derivatives may be a contributing factor for migraineurs for a decrease in sensitivity for migraine or a consequence of the chronic migraine state. Such findings, if extrapolated to other regions of the brain would offer new opportunities to modulate central system as interictal or preemptive medications in these patients.

In vivo quantification of intracerebral GABA by single-voxel (1)H-MRS-How reproducible are the results?

Gamma aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the human brain. It plays a decisive role in a variety of nervous system disorders, such as anxiety disorders, epilepsy, schizophrenia, insomnia, and many others. The reproducibility of GABA quantification results obtained with a single-voxel spectroscopy J-difference editing sequence with Point Resolved Spectroscopy localization (MEGA-PRESS) was determined on a 3.0 Tesla MR scanner in healthy adults. Eleven volunteers were measured in long- and short-term intervals. Intra- and inter-subject reproducibility were evaluated. Internal referencing of GABA+ to total creatine (tCr) and water (H(2)O), as well as two different post-processing methods for the evaluation (signal integration and time-domain fitting) were compared. In all subjects lower coefficient of variation and therefore higher reproducibility can be observed for fitting compared to integration. The GABA+/tCr ratio performs better than the GABA+/H(2)O ratio or GABA+ without internal referencing for both fitting and integration (GABA+/tCr: 13.3% and 17.0%; GABA+/H(2)O: 15.0% and 17.8%; GABA+: 19.2% and 21.7%). Four-day measurements on three subjects showed higher intra- than inter-subject reproducibility (GABA+/tCr approximately 10-12%). With a coefficient of variation of about 13% for inter-subject and 10-12% for intra-subject variability of GABA+/tCr, this technique seems to be a precise tool that can detect GABA confidently. The results of this study show the reproducibility limitations of GABA quantification in vivo, which are necessary for further clinical studies.

Reduced brain GABA in primary insomnia: preliminary data from 4T proton magnetic resonance spectroscopy (1H-MRS)

STUDY OBJECTIVES

Both basic and clinical data suggest a potential significant role for GABA in the etiology and maintenance of primary insomnia (PI). Proton magnetic resonance spectroscopy (1H-MRS) can non-invasively determine GABA levels in human brain. Our objective was to assess GABA levels in unmedicated individuals with PI, using 1H-MRS.

DESIGN AND SETTING

Matched-groups, cross-sectional study conducted at two university-based hospitals.

PARTICIPANTS

Sixteen non-medicated individuals (8 women) with PI (mean age = 37.3 +/- 8.1) and 16 (7 women) well-screened normal sleepers (mean age = 37.6 +/- 4.5).

METHODS AND MEASUREMENTS

PI was established with an unstructured clinical interview, a Structured Clinical Interview for DSM-IV (SCID), sleep diary, actigraphy and polysomnography (PSG). 1H-MRS data were collected on a Varian 4 Tesla magnetic resonance imagingl spectroscopy scanner. Global brain GABA levels were averaged from samples in the basal ganglia, thalamus, and temporal, parietal, and occipital white-matter and cortex.

RESULTS

Average brain GABA levels were nearly 30% lower in patients with PI (.18 +/- .06) compared to controls (.25 +/- .11). GABA levels were negatively correlated with wake after sleep onset (WASO) on two independent PSGs (r = -0.71, p = 0.0024 and -0.70, p = 0.0048).

CONCLUSIONS

Our preliminary finding of a global reduction in GABA in non-medicated individuals with PI is the first demonstration of a neurochemical difference in the brains of those with PI compared to normal sleeping controls. 1H-MRS is a valuable tool to assess GABA in vivo, and may provide a means to shed further light on the neurobiology of insomnia.

Abnormal glutamatergic neurotransmission and neuronal-glial interactions in acute mania

BACKGROUND

At excitatory synapses, glutamate released from neurons is taken up by glial cells and converted to glutamine, which is cycled back to neurons. Alterations in this system are believed to play a role in the pathophysiology of bipolar disorder, but they have not been characterized in vivo. We examined the glutamine/glutamate ratio and levels of other metabolites in acute mania and schizophrenia in this exploratory study.

METHODS

Data were obtained from 2 x 2 x 2 cm voxels in the anterior cingulate cortex (ACC) and parieto-occipital cortex (POC) using two-dimensional J-resolved proton magnetic resonance spectroscopy at 4 Tesla and analyzed using LCModel. Fifteen bipolar disorder patients with acute mania and 17 schizophrenia patients with acute psychosis were recruited from an inpatient unit; 21 matched healthy control subjects were also studied. Glutamine/glutamate ratio and N-acetylaspartate, creatine, choline, and myo-inositol levels were evaluated in a repeated measures design. Medication effects and relationship to demographic and clinical variables were analyzed.

RESULTS

Glutamine/glutamate ratio was significantly higher in ACC and POC in bipolar disorder, but not schizophrenia, compared with healthy control subjects. N-acetylaspartate was significantly lower in the ACC in schizophrenia. Patients on and off lithium, anticonvulsants, or benzodiazepines had similar glutamine/glutamate ratios.

CONCLUSIONS

The elevated glutamine/glutamate ratio is consistent with glutamatergic overactivity and/or defective neuronal-glial coupling in acute mania, although medication effects cannot be ruled out. Abnormalities in glutamatergic neurotransmission and glial cell function in bipolar disorder may represent targets for novel therapeutic interventions.

A detailed analysis of localized J-difference GABA editing: theoretical and experimental study at 4 T

The problem of low signal-to-noise ratio for gamma-aminobutyric acid (GABA) in vivo is exacerbated by inefficient detection schemes and non-optimal experimental parameters. To analyze the mechanisms for GABA signal loss of a MEGA-PRESS J-difference sequence at 4 T, numerical simulations were performed ranging from ideal to realistic experimental implementation, including volume selection and experimental radio frequency (RF) pulse shapes with a macromolecular minimization scheme. The simulations were found to be in good agreement with phantom and in vivo data from human brain. The overall GABA signal intensity for the simulations with realistic conditions for the MEGA-PRESS difference spectrum was calculated to be almost half of the signal simulated under ideal conditions (~43% signal loss). In contrast, creatine was reduced significantly less then GABA (~19% signal loss). The 'four-compartment' distribution due to J-coupling in the PRESS-based localization was one of the most significant sources of GABA signal loss, in addition to imperfect RF profiles for volume selection and editing. An alternative strategy that reduces signal loss due to the four-compartment distribution is suggested. In summary, a detailed analysis of J-difference editing is provided with estimates of the relative amounts of GABA signal losses due to various mechanisms. The numerical simulations presented in this study should facilitate both implementation of the more efficient acquisition and quantification process of J-coupled systems.

Recent advances in magnetic resonance neurospectroscopy

Over the past two decades, proton magnetic resonance spectroscopy (proton MRS) of the brain has made the transition from research tool to a clinically useful modality. In this review, we first describe the localization methods currently used in MRS studies of the brain and discuss the technical and practical factors that determine the applicability of the methods to particular clinical studies. We also describe each of the resonances detected by localized solvent-suppressed proton MRS of the brain and discuss the metabolic and biochemical information that can be derived from an analysis of their concentrations. We discuss spectral quantitation and summarize the reproducibility of both single-voxel and multivoxel methods at 1.5 and 3-4 T. We have selected three clinical neurologic applications in which there has been a consensus as to the diagnostic value of MRS and summarize the information relevant to clinical applications. Finally, we speculate about some of the potential technical developments, either in progress or in the future, that may lead to improvements in the performance of proton MRS.

Reproducibility of GABA measurements using 2D J-resolved magnetic resonance spectroscopy

We determined the reproducibility of GABA (gamma-aminobutyric acid) measurements using 2D J-resolved magnetic resonance spectroscopy (MRS) on a clinical 1.5-T MR imaging scanner. Two-dimensional J-resolved spectra were acquired in vitro across five GABA concentrations using a volume head coil and a 5-in. surface coil. Additional spectra using a sixth GABA phantom with a very low concentration and from a healthy volunteer were recorded in the 5-in. surface coil only. In each case, the 3.01-ppm GABA resonance was quantified; for comparison, the peak integrals of choline (3.2 ppm) and creatine (3.03 ppm) were recorded. At a physiological concentration (1.2 mM), in vitro GABA measurement was significantly more reproducible in the surface coil than in the volume coil (P=.005), with coefficients of variation (CVs) being less than 16% with the surface coil and up to 68% with the volume head coil. At the smallest concentration of in vivo GABA reported using other spectroscopy techniques (0.8 mM) and detected only using the surface coil, the CV for GABA was 23% and was less than 10% for choline and creatine, which compare favorably with results from published studies. In vivo, the CV for GABA measurement was 26%, suggesting that 2D J-resolved MRS would be suitable for detecting physiological changes in GABA, similar to those reported using other methods.

Elimination of spatial interference in PRESS-localized editing spectroscopy

Unambiguous detection of gamma-amino butyric acid (GABA) in the human brain is hindered by low concentration and spectral overlap with other metabolites. The popular MEGA-PRESS (PRESS: point-resolved spectroscopic sequence) method allows spectral separation of GABA from other metabolites, but suffers from a significant signal-to-noise ratio (SNR) reduction due to the 4-compartment artifact. An alternative PRESS localization technique (PRESS+4) was investigated and compared to MEGA-PRESS using numerical simulations, phantom, and in vivo experiments. It was shown that while the MEGA-PRESS method suffers significant signal loss ( approximately equal 20% for the difference spectrum), GABA signal intensity in PRESS+4 is reduced by only 2% compared to the nonlocalized condition at 4T. The improved method retains important features of the popular MEGA-PRESS such as additional water suppression and macromolecular elimination as demonstrated in human brain experiments. This method is not limited to GABA J-difference editing, but can be applied in any PRESS-based experiments. It should prove particularly useful at higher field, where the 4-compartment artifact is especially detrimental.

Topiramate raises anterior cingulate cortex glutamine levels in healthy men; a 4.0 T magnetic resonance spectroscopy study

RATIONALE

Potential mechanisms of action of topiramate include alterations of glutamatergic and GABAergic systems. In particular, topiramate has been shown to increase occipital cortex GABA levels, as measured using proton magnetic resonance spectroscopy (MRS).

OBJECTIVES

The purpose of this study was to measure the effect of acute oral topiramate on the GABA precursors glutamate and glutamine in the anterior cingulate cortex (ACC) and occipital lobe (OL) using high-field (4.0 T) proton MRS (1H MRS).

METHODS

Proton MR spectra were acquired from healthy men at three times: at baseline and 2 and 6 h after ingesting 50 (N=5) or 100 mg (N=5) of topiramate. Blood samples were acquired prior to each scan for the purpose of obtaining serum topiramate levels.

RESULTS

A 100-mg dose of topiramate significantly increased ACC glutamine levels within 2 h of ingestion and OL glutamine levels within 6 h of ingestion. There were no measured significant effects of topiramate on ACC or OL glutamate levels.

CONCLUSIONS

A 100-mg dose of oral topiramate increased serum topiramate and ACC glutamine levels within 2 h. OL glutamine levels increased within 6 h. Increased brain glutamine levels may be a consequence of topiramate positively modulating GABAA receptors. This result is of interest given the possible role for topiramate in the treatment of epilepsy, migraine headache, bipolar disorder, eating disorders, and alcohol dependence.

In vivo detection of gray and white matter differences in GABA concentration in the human brain

A novel selective multiple quantum filtering-based chemical shift imaging method was developed for acquiring GABA images in the human brain at 3 T. This method allows a concomitant acquisition of an interleaved total creatine image with the same spatial resolution. Using T(1)-based image segmentation and a nonlinear least square regression analysis of GABA-to-total creatine concentration ratios in frontal and parietal lobes of healthy adult volunteers as a function of the tissue gray matter fraction, the mean GABA concentration in gray and white matter was determined to be 1.30+/-0.36 micromol/g and 0.16+/-0.16 micromol/g (mean+/-SD, n=13), respectively. It is expected that this method will become a useful tool for studying GABAergic function in the human brain in vivo.

Improved two-dimensional J-resolved spectroscopy

Localised two-dimensional J-resolved spectroscopy (JPRESS) is optimised for the in vivo detection of J-coupled metabolites using magnetic resonance spectroscopy at 3 T. The acquisition of echo signals starts as early as possible (i.e. maximum-echo sampling). This sampling scheme increases sensitivity and decreases overlap of peak tails, hence alleviating baseline problems. Reconstruction issues are discussed and the sensitivity is compared analytically with that of 1D PRESS. The qualitative behaviour of eddy currents in JPRESS is outlined and a 2D eddy current correction procedure based on the 1D phase deconvolution method is proposed.

Normal prefrontal gamma-aminobutyric acid levels in remitted depressed subjects determined by proton magnetic resonance spectroscopy

BACKGROUND

There is growing evidence that the brain gamma-aminobutyric acid (GABA) system is involved in depression. Lowered plasma GABA levels were identified as a traitlike abnormality found in patients with remitted unipolar depression and in healthy first-degree relatives of patients with unipolar depression. Major depressive disorder has been associated with neuroimaging and neuropathological abnormalities in the prefrontal cortex by various types of evidence. As a result, the current study investigates whether GABA levels in the prefrontal cortex differ between unmedicated subjects with remitted major depressive disorder (rMDD) and healthy control subjects.

METHODS

Sixteen rMDD subjects and 15 healthy control subjects underwent magnetic resonance spectroscopy. We used a 3 Tesla GE whole body scanner with a homogeneous resonator coil providing a homogenous radiofrequency field and capability of obtaining measurement from the prefrontal cortex. Gamma-aminobutyric acid levels were measured in the ventromedial prefrontal cortex and dorsolateral/anterior medial prefrontal cortex.

RESULTS

There was no difference in GABA concentrations between rMDD subjects and healthy control subjects in the ventromedial prefrontal cortex and dorsolateral/anterior medial prefrontal cortex. Secondary analyses provided preliminary evidence for a negative relationship between the glutamate/glutamine (Glx)/GABA ratio and age of onset of major depression in the ventromedial prefrontal cortex.

CONCLUSIONS

This result suggests that GABA levels in the prefrontal cortex, if found to be reduced in symptomatic depression, do not represent a persistent characteristic of major depression. Further research is needed to determine brain GABA levels in different brain regions, in different stages of depressive illness, and in different depressive subtypes.

Grey and white matter GABA level differences in the human brain using two-dimensional, J-resolved spectroscopic imaging

A novel, two-dimensional, J-resolved chemical-shift imaging sequence was used to collect gamma-aminobutyric acid (GABA) spectroscopic imaging data on six healthy subjects at 4 T. Using image segmentation and a linear-regression analysis relating brain GABA level to tissue-type, a consistent and significant (n = 6, p < 0.01) elevation of mean GABA levels was measured in the cortical grey matter (0.96 +/- 0.24 mm) compared with white matter (0.44 +/- 0.16 mm) across all six subjects. The results suggest an approximately two-fold elevation of GABA levels in cortical grey matter compared with white matter in vivo. Our findings are consistent with ex vivo studies in the literature of both animal and human brain and demonstrate the significant potential of this technique for detecting and quantifying tissue-specific neurochemical pathology in vivo.

Prefrontal GABA levels in cocaine-dependent subjects increase with pramipexole and venlafaxine treatment

RATIONALE

There is evidence that prefrontal lobe GABA levels are low in cocaine-dependent (CD) individuals, and treatment with GABA agonists decreases cocaine self-administration.

OBJECTIVES

The aim of the study is to measure changes in GABA levels in CD subjects at baseline and after 8 weeks of treatment with pramipexole, venlafaxine, or placebo.

METHODS

CD subjects enrolled in a treatment trial for cocaine dependence were recruited for this proton (1H) magnetic resonance spectroscopy (MRS) study. GABA levels in the prefrontal lobe were measured before and after treatment.

RESULTS

Mean percentage changes in GABA levels were as follows: pramipexole +17.0+/-28.0%, venlafaxine +13.0+/-11.0%, and placebo -2.1+/-19.5%. Pramipexole-treated subjects had significantly increased brain GABA levels compared to placebo (p=0.031). Venlafaxine treatment was nonsignificantly associated with increased GABA levels compared to placebo (p=0.16). The overall statistical model for the effect of drug treatment vs placebo on brain GABA levels, including adjustment for baseline levels, was highly significant (p=0.002). Despite significant changes in GABA levels, there were no significant differences in the number of urine samples positive for cocaine metabolites.

CONCLUSIONS

This study demonstrates that 1H MRS can measure changes in GABA levels following pharmacologic treatment. The increase in GABA levels, although significant, is modest compared to other MRS studies of depression or epilepsy associated with clinical improvements. The failure to see larger increases in GABA levels and an associated reduction in cocaine consumption may reflect the relatively low doses of medication used.

The role of glutamate in anxiety and related disorders

Anxiety, stress, and trauma-related disorders are a major public health concern in the United States. Drugs that target the gamma-aminobutyric acid or serotonergic system, such as benzodiazepines and selective serotonin reuptake inhibitors, respectively, are the most widely prescribed treatments for these disorders. However, the role of glutamate in anxiety disorders is becoming more recognized with the belief that drugs that modulate glutamatergic function through either ionotropic or metabotropic glutamate receptors have the potential to improve the current treatment of these severe and disabling illnesses. Animal models of fear and anxiety have provided a method to study the role of glutamate in anxiety. This research has demonstrated that drugs that alter glutamate transmission have potential anxiolytic action for many different paradigms including fear-potentiated startle, punished responding, and the elevated plus maze. Human clinical drug trials have demonstrated the efficacy of glutamatergic drugs for the treatment of obsessive-compulsive disorder, posttraumatic stress disorder, generalized anxiety disorder, and social phobia. Recent data from magnetic resonance imaging studies provide an additional link between the glutamate system and anxiety. Collectively, the data suggest that future studies on the mechanism of and clinical efficacy of glutamatergic agents in anxiety disorders are appropriately warranted.

Chemical-shift-selective filter for the in vivo detection of J-coupled metabolites at 3T

A chemical-shift-selective filter (CSSF) was applied to the detection of J-coupled metabolites in the human brain. This filter is an acquisition-based technique that requires the chemical shifts (CS's) of different metabolites, but not their whole multiplet structures, to be resolved. The sequence is based on the 2D constant-time spin-echo experiment, which yields pure CS spectra in the indirect dimension. Localization is achieved through point-resolved spectroscopy (PRESS). The method enables unequivocal detection of glutamate and myo-inositol, both in vitro and in vivo in the human brain, at 3T.

Ultrafast 2D NMR spectroscopy using sinusoidal gradients: principles and ex vivo brain investigations

A new methodology capable of delivering complete 2D NMR spectra within a single scan was recently introduced. The resulting potential gain in time resolution could open new opportunities for in vivo spectroscopy, provided that the technical demands of the methodology are satisfied by the corresponding hardware. Foremost among these demands are the relatively short switching times expected from the applied gradient-echo trains. These rapid transitions may be particularly difficult to accomplish on imaging systems. As a step toward solving this problem, we assessed the possibility of replacing the square-wave gradient train currently used during the course of the acquisition by a shaped sinusoidal gradient. Examples of the implementation of this protocol are given, and successful ultrafast acquisitions of 2D NMR spectra with suitable spectral widths on a microimaging probe (for both phantom solutions and ex vivo mouse brains) are demonstrated.

Safety and Efficacy of GABAergic Medications for Treating Alcoholism

This article highlights the proceedings of a symposium presented at the 27th Annual Scientific Meeting of the Research Society on Alcoholism in Vancouver, British Columbia, Canada, June 29, 2004. The organizers and co-chairs were Bankole A. Johnson, MD, PhD, and Robert M. Swift, MD, PhD. The presentations included (1) Introduction, by Bankole A. Johnson; (2) Safety, Tolerability, and Efficacy of gamma-Hydroxybutyric Acid and Baclofen in the Treatment of Alcohol Addiction, by Giovanni Addolorato; (3) Safety of Gabapentin in Treating Alcoholism, by Hugh Myrick; (4) New Data on the Safety and Effectiveness of Topiramate in the Treatment of Alcohol Dependence, by Bankole A. Johnson; (5) Evaluating the Risk of Benzodiazepine Prescription to Alcohol-Dependent Individuals, by Domenic A. Ciraulo; and (6) Safety and Efficacy of GABAergic Agents in Treating Alcoholics: Discussion, by Robert M. Swift.

Brain γ-aminobutyric acid measurement by proton double-quantum filtering with selectiveJ rewinding

An optimized single-shot proton double-quantum (DQ) filter for the quantification of gamma-aminobutyric acid (GABA) levels in human brain is reported. It is demonstrated that creation of DQ coherences following dual-resonance-selective refocusing gives a theoretical editing efficiency of 50% for the detection of the GABA resonance at 3.01 ppm. The sequence times are optimized with both numerical and experimental analyses of the editing performance, giving an experimental editing efficiency of 42%. It is acknowledged that homocarnosine is partially coedited, leading to a 20% contribution to the edited signal; however, macromolecule contamination is negligible in vivo under these experimental conditions. The GABA concentration in human prefrontal cortex is estimated to be 0.8 +/- 0.1 micromol/g (mean +/- SD, n = 6), with reference to the internal standard creatine at 9 micromol/g.

Multi-spin-echo J-resolved spectroscopic imaging without water suppression: application to a rat glioma at 7 T

Two-dimensional J-resolved spectroscopy may be used to separate resonances which overlap in 1D NMR spectra. Coupled with spectroscopic imaging (SI), it would give unequivocal information on the distribution of such resonances. Multi-echo acquisition decreases the minimum experimental time of such 4D experiments. The water peak may be used for phase and chemical-shift reference. This study aimed to demonstrate the feasibility of J-resolved SI based on a multi-echo sequence and without water suppression, and its ability to separate the peaks for lactate and mobile lipid in a rat glioma. Experiments were performed on rat brain, without water suppression, at 7 T. The water signal was used for correcting the phase of the echoes. A FOCSY-like acquisition was used to collect the first part of the echoes at short echo times. Two different data processing methods were tested to overcome the problem of contaminations of metabolite signals by the intense water signal. Maps of N-acetylaspartate, choline, creatine, lactate and mobile lipids were obtained in vivo on a rat glioma in 70 min. The in-plane resolution was 2 mm2. The 2D spatially resolved, 2D J-resolved spectra enabled the separate mapping of lactate and mobile lipids.

Advances in high-field magnetic resonance imaging

Among advances in magnetic resonance imaging (MRI), the increase of the magnetic field strength is perhaps one of the most significant. The use of high magnetic fields for in vivo magnetic resonance is motivated by a number of considerations. Advantages are increases in signal-to-noise ratio, blood-oxygenation level-dependent contrast, and spectral resolution, while disadvantages include potential reduction of contrast in anatomic imaging owing to lengthening of T1 and effects of susceptibility of high fields. To address these challenges, technical advances have been made in various aspects of MRI, allowing high-field MRI to provide exquisite morphological and functional details in clinical and research settings. This review provides an overview of technical issues and applications of high-field MRI.

A new strategy for in vivo spectral editing. Application to GABA editing using selective homonuclear polarization transfer spectroscopy

A novel single-shot in vivo spectral editing method is proposed in which the signal to be detected, is regenerated anew from the thermal equilibrium magnetization of a source to which it is J-coupled. The thermal equilibrium magnetization of the signal to be detected together with those of overlapping signals are suppressed by single-shot gradient dephasing prior to the signal regeneration process. Application of this new strategy to in vivo GABA editing using selective homonuclear polarization transfer allows complete suppression of overlapping creatine and glutathione while detecting the GABA-4 methylene resonance at 3.02 ppm with an editing yield similar to that of conventional editing methods. The NAA methyl group at 2.02 ppm was simultaneously detected and can be used as an internal navigator echo for correcting the zero order phase and frequency shifts and as an internal reference for concentration. This new method has been demonstrated for robust in vivo GABA editing in the rat brain and for study of GABA synthesis after acute vigabatrin administration.

Frontal lobe GABA levels in cocaine dependence: a two-dimensional, J-resolved magnetic resonance spectroscopy study

Non-invasive measures of brain gamma-aminobutyric acid (GABA) concentrations may be especially useful in the identification of cocaine-related changes in brain chemistry that can be used to guide the development of future treatments for cocaine-dependent persons. This study assessed whether brain GABA levels in cocaine-dependent subjects with and without an alcohol disorder differ from GABA levels in healthy comparison subjects. Two-dimensional, proton magnetic resonance spectroscopy was used to determine GABA levels in the left prefrontal lobe of cocaine-dependent subjects (N=35) recruited from a National Institute on Drug Abuse (NIDA)-sponsored treatment trial of cocaine dependence and a comparison group (N=20). At treatment baseline, mean GABA concentrations were 0.93+/-0.27 mM/kg in cocaine-dependent subjects and 1.32+/-0.44 mM/kg in the comparison sample (t [d.f.=53]=3.65, P<0.001). Cocaine-dependent subjects with a history of a co-morbid alcohol disorder (N=23) had significantly lower baseline GABA levels (0.87 mM/kg) (t [d.f.=41]=4.31, P<0.001) than the comparison group. However, cocaine-dependent subjects without an alcohol disorder (N=12) also had lower GABA levels (1.04 mM/kg) than the comparison subjects (t [d.f.=30]=2.09, P=0.045), suggesting that cocaine dependence alone can decrease GABA levels.

Single-shot two-echo technique for simultaneous measurement of GABA and creatine in the human brain in vivo

A single-shot, two-echo method for the simultaneous detection of multiple-quantum (MQ)-filtered gamma-aminobutyric acid (GABA) and creatine (Cr) was developed and demonstrated in the human brain in vivo at 3 Tesla. The simultaneously measured Cr singlet served as a navigator for the spectral phase of GABA and any frequency shift during measurements due to drift in the static magnetic field (B(0)) or subject movement, as well as an internal concentration reference. In addition, the use of a double-band frequency-selective MQ filter for C(3) and C(4) methylene protons of GABA provided a very robust measurement of GABA, with excellent suppression of overlapping metabolites such as Cr and glutathione (GSH) in each single scan. Contamination from overlapping macromolecules was also demonstrated to be negligible with this method. The GABA-to-Cr ratio was 0.09 +/- 0.03 (mean +/- SD, N = 17) and the estimated concentration of GABA in the frontoparietal region of the human brain in vivo was 0.66 +/- 0.19 micromol/g (mean +/- SD, N = 17) with the internal reference method, and 0.69 +/- 0.18 micromol/g (mean +/- SD, N = 17) with the external reference method. The observed pattern of GABA doublet was consistent among all subjects, with a frequency separation of approximately 13 Hz.