Initial experience with seven tesla magnetic resonance spectroscopy of hypothalamic GABA during hyperinsulinemic euglycemia and hypoglycemia in healthy humans

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.

Monitoring the Neurotransmitter Response to Glycemic Changes Using an Advanced Magnetic Resonance Spectroscopy Protocol at 7T

The primary excitatory and inhibitory neurotransmitters glutamate (Glu) and gamma-aminobutyric acid (GABA) are thought to be involved in the response of the brain to changes in glycemia. Therefore, their reliable measurement is critical for understanding the dynamics of these responses. The concentrations of Glu and GABA, as well as glucose (Glc) in brain tissue, can be measured in vivo using proton (¹H) magnetic resonance spectroscopy (MRS). Advanced MRS methodology at ultrahigh field allows reliable monitoring of these metabolites under changing metabolic states. However, the long acquisition times needed for these experiments while maintaining blood Glc levels at predetermined targets present many challenges. We present an advanced MRS acquisition protocol that combines commercial 7T hardware (Siemens Scanner and Nova Medical head coil), BaTiO₃ dielectric padding, optical motion tracking, and dynamic frequency and B₀ shim updates to ensure the acquisition of reproducibly high-quality data. Data were acquired with a semi-LASER sequence [repetition time/echo time (TR/TE) = 5,000/26 ms] from volumes of interest (VOIs) in the prefrontal cortex (PFC) and hypothalamus (HTL). Five healthy volunteers were scanned to evaluate the effect of the BaTiO₃ pads on B 1 + distribution. Use of BaTiO₃ padding resulted in a 60% gain in signal-to-noise ratio in the PFC VOI over the acquisition without the pad. The protocol was tested in six patients with type 1 diabetes during a clamp study where euglycemic (~100 mg/dL) and hypoglycemic (~50 mg/dL) blood Glc levels were maintained in the scanner. The new protocol allowed retention of all HTL data compared with our prior experience of having to exclude approximately half of the HTL data in similar clamp experiments in the 7T scanner due to subject motion. The advanced MRS protocol showed excellent data quality (reliable quantification of 11-12 metabolites) and stability (p > 0.05 for both signal-to-noise ratio and water linewidths) between euglycemia and hypoglycemia. Decreased brain Glc levels under hypoglycemia were reliably detected in both VOIs. In addition, mean Glu level trended lower at hypoglycemia than euglycemia for both VOIs, consistent with prior observations in the occipital cortex. This protocol will allow robust mechanistic investigations of the primary neurotransmitters, Glu and GABA, under changing glycemic conditions.

Current Methods of Magnetic Resonance for Noninvasive Assessment of Molecular Aspects of Pathoetiology in Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune disease with expanding axonal and neuronal degeneration in the central nervous system leading to motoric dysfunctions, psychical disability, and cognitive impairment during MS progression. The exact cascade of pathological processes (inflammation, demyelination, excitotoxicity, diffuse neuro-axonal degeneration, oxidative and metabolic stress, etc.) causing MS onset is still not fully understood, although several accompanying biomarkers are particularly suitable for the detection of early subclinical changes. Magnetic resonance (MR) methods are generally considered to be the most sensitive diagnostic tools. Their advantages include their noninvasive nature and their ability to image tissue in vivo. In particular, MR spectroscopy (proton 1H and phosphorus 31P MRS) is a powerful analytical tool for the detection and analysis of biomedically relevant metabolites, amino acids, and bioelements, and thus for providing information about neuro-axonal degradation, demyelination, reactive gliosis, mitochondrial and neurotransmitter failure, cellular energetic and membrane alternation, and the imbalance of magnesium homeostasis in specific tissues. Furthermore, the MR relaxometry-based detection of accumulated biogenic iron in the brain tissue is useful in disease evaluation. The early description and understanding of the developing pathological process might be critical for establishing clinically effective MS-modifying therapies.

Metabolic and perfusion responses to acute hypoglycemia in the rat cortex: A non-invasive magnetic resonance approach

Hypoglycemia is critical condition during diabetic treatment that involves intensive insulin therapy, and it may impair brain function. We aimed to compare cortical responses of three hypoglycemic phases and the restoration of glycemia to control levels after a severe episode in rats using non-invasive perfusion magnetic resonance (MR) imaging and localized ¹ H MR spectroscopy. Under light α-chloralose anesthesia, cortical blood flow (cCBF) was 42 ± 3 ml/100 g/min at euglycemia (~ 5 mM plasma glucose), was not altered at mild hypoglycemia I (42 ± 4 ml/100 g/min, 2-3.5 mM), increased to 60 ± 8 ml/100 g/min under moderate hypoglycemia II (1-2 mM) and amplified to 190 ± 35 ml/100 g/min at severe hypoglycemia III (< 1 mM). ¹ H MRS revealed metabolic changes at hypoglycemia I without any perfusion alteration. At hypoglycemia III, glutamine and glutamate decreased, whereas aspartate increased. When animals subsequently regained glycemic control, not all metabolites returned to their control levels, for example, glutamine. Meanwhile, ascorbate was increased with amplified hypoglycemic severity, whereas glutathione was reduced; these compounds did not return to normal levels upon the restoration of glycemia. Our study is the first to report cCBF and neurochemical changes in cortex upon five glycemic stages. The cortical responses of different hypoglycemic phases would explain variable neuronal damages after hypoglycemia and might help identify the degrees of hypoglycemic insults and further improve alternative therapies.

Key clinical benefits of neuroimaging at 7T

The growing interest in ultra-high field MRI, with more than 35.000 MR examinations already performed at 7T, is related to improved clinical results with regard to morphological as well as functional and metabolic capabilities. Since the signal-to-noise ratio increases with the field strength of the MR scanner, the most evident application at 7T is to gain higher spatial resolution in the brain compared to 3T. Of specific clinical interest for neuro applications is the cerebral cortex at 7T, for the detection of changes in cortical structure, like the visualization of cortical microinfarcts and cortical plaques in Multiple Sclerosis. In imaging of the hippocampus, even subfields of the internal hippocampal anatomy and pathology may be visualized with excellent spatial resolution. Using Susceptibility Weighted Imaging, the plaque-vessel relationship and iron accumulations in Multiple Sclerosis can be visualized, which may provide a prognostic factor of disease. Vascular imaging is a highly promising field for 7T which is dealt with in a separate dedicated article in this special issue. The static and dynamic blood oxygenation level-dependent contrast also increases with the field strength, which significantly improves the accuracy of pre-surgical evaluation of vital brain areas before tumor removal. Improvement in acquisition and hardware technology have also resulted in an increasing number of MR spectroscopic imaging studies in patients at 7T. More recent parallel imaging and short-TR acquisition approaches have overcome the limitations of scan time and spatial resolution, thereby allowing imaging matrix sizes of up to 128×128. The benefits of these acquisition approaches for investigation of brain tumors and Multiple Sclerosis have been shown recently. Together, these possibilities demonstrate the feasibility and advantages of conducting routine diagnostic imaging and clinical research at 7T.

Measurement of Hypothalamic Glucose Under Euglycemia and Hyperglycemia by MRI at 3T

PURPOSE

To evaluate the feasibility of using a clinical magnetic resonance (MR) system and MR spectroscopy (MRS) to measure glucose concentration changes in the human hypothalamus, a structure central to whole-body glucose regulation.

SUBJECTS AND METHODS

A time series of MR spectra (semi-LASER, TE = 28 msec), localized to the bilateral hypothalamus (∼1.6 ml) were obtained at 3T in six healthy subjects at baseline (euglycemia) and during a ∼65-70-minute-long hyperglycemic clamp in 11-minute blocks with interleaved T1 FLASH images to retrospectively assess head motion, and track changes in cerebrospinal fluid (CSF) partial volume. The LCModel was used to quantify the sum of glucose and taurine concentrations, [Glc+Tau], along with their associated Cramér-Rao lower bounds (CRLB).

RESULTS

Spectral quality allowed quantification of [Glc+Tau] (sum reported due to high negative correlation between these metabolites) with CRLB <25% in 35/36 timepoints during hyperglycemia. Increased [Glc+Tau] was observed with hyperglycemia in all subjects, but most reliably in those with plasma glucose targets ≥300 mg/dl. For these subjects, [Glc+Tau]baseline (n = 4) was 1.5 (±0.3, SD) mM, and increased to 4.5 (±1.1) mM (n = 16) for timepoints acquired ≥25 minutes after onset of the clamp, with 15/16 timepoints having no overlap of 95% confidence intervals (CIs) between baseline and hyperglycemia. Preliminary analysis revealed a linear (1:5) relationship between hypothalamus-blood glucose concentrations.

CONCLUSION

It is feasible to measure glucose concentration changes in the human hypothalamus using a standard 3T scanner and a short-echo semi-LASER sequence by utilizing retrospective motion tracking, CSF correction, predetermined quality acceptance criteria, and hyperglycemic blood glucose levels ≥300 mg/dl.

LEVEL OF EVIDENCE

2 J. Magn. Reson. Imaging 2017;45:681-691.

Brain Insulin Resistance at the Crossroads of Metabolic and Cognitive Disorders in Humans

Ever since the brain was identified as an insulin-sensitive organ, evidence has rapidly accumulated that insulin action in the brain produces multiple behavioral and metabolic effects, influencing eating behavior, peripheral metabolism, and cognition. Disturbances in brain insulin action can be observed in obesity and type 2 diabetes (T2D), as well as in aging and dementia. Decreases in insulin sensitivity of central nervous pathways, i.e., brain insulin resistance, may therefore constitute a joint pathological feature of metabolic and cognitive dysfunctions. Modern neuroimaging methods have provided new means of probing brain insulin action, revealing the influence of insulin on both global and regional brain function. In this review, we highlight recent findings on brain insulin action in humans and its impact on metabolism and cognition. Furthermore, we elaborate on the most prominent factors associated with brain insulin resistance, i.e., obesity, T2D, genes, maternal metabolism, normal aging, inflammation, and dementia, and on their roles regarding causes and consequences of brain insulin resistance. We also describe the beneficial effects of enhanced brain insulin signaling on human eating behavior and cognition and discuss potential applications in the treatment of metabolic and cognitive disorders.

Mechanisms of hypoglycemia unawareness and implications in diabetic patients

Hypoglycemia unawareness (HU) is defined at the onset of neuroglycopenia before the appearance of autonomic warning symptoms. It is a major limitation to achieving tight diabetes and reduced quality of life. HU occurs in approximately 40% of people with type 1 diabetes mellitus (T1DM) and with less frequency in T2DM. Though the aetiology of HU is multifactorial, possible mechanisms include chronic exposure to low blood glucose, antecedent hypoglycaemia, recurrent severe hypoglycaemia and the failure of counter-regulatory hormones. Clinically it manifests as the inability to recognise impeding hypoglycaemia by symptoms, but the mechanisms and mediators remain largely unknown. Prevention and management of HU is complex, and can only be achieved by a multifactorial intervention of clinical care and structured patient education by the diabetes team. Less know regarding the impact of medications on the development or recognition of this condition in patients with diabetes. Several medications are thought to worsen or promote HU, whereas others may have an attenuating effect on the problem. This article reviews recent advances in how the brain senses and responds to hypoglycaemia, novel mechanisms by which people with insulin-treated diabetes develop HU and impaired counter-regulatory responses. The consequences that HU has on the person with diabetes and their family are also described. Finally, it examines the evidence for prevention and treatment of HU, and summarizes the effects of medications that may influence it.

Role of γ-aminobutyric acid signalling in the attenuation of counter-regulatory hormonal responses after antecedent hypoglycaemia in healthy men

The attenuated counter-regulatory response to hypoglycaemia after antecedent hypoglycaemic episodes has been observed in animals to be associated with an increase in γ-aminobutyric acid (GABA) signalling. We therefore tested the hypothesis that the pharmacological suppression of GABAergic activity during a repeated hypoglycaemic episode enhances counter-regulatory responses. Fourteen healthy men participated in two experimental sessions each comprising three insulin-induced hypoglycaemic episodes. Before the third hypoglycaemic episode, participants received the GABA-antagonistic drug modafinil (200 mg orally) and placebo, respectively. In the placebo condition, the secretion of norepinephrine, adrenocorticotropic hormone, cortisol and growth hormone, and the perception of neuroglycopenic symptoms were attenuated during the third as compared with the first hypoglycaemic episode (each p < 0.05). Modafinil reversed this effect for the noradrenergic response (p < 0.05), while not significantly altering the attenuation of other hormonal responses and symptom perception (p > 0.3). Our findings indicate that increased GABAergic transmission could contribute to aspects of the attenuated counter-regulatory response after recurrent hypoglycaemia in humans.

Changes in human brain glutamate concentration during hypoglycemia: insights into cerebral adaptations in hypoglycemia-associated autonomic failure in type 1 diabetes

Hypoglycemia-associated autonomic failure (HAAF) is a condition in which patients with type 1 diabetes (T1D) who experience frequent hypoglycemia develop defective glucose counter-regulation and become unable to sense hypoglycemia. Brain glutamate may be involved in the mechanism of HAAF. The goal of this study was to follow the human brain glutamate concentration during experimentally induced hypoglycemia in subjects with and without HAAF. (1)H magnetic resonance spectroscopy was used to track the occipital cortex glutamate concentration throughout a euglycemic clamp followed immediately by a hypoglycemic clamp. T1D patients with HAAF were studied in comparison to two control groups, i.e., T1D patients without HAAF and healthy controls (n=5 per group). Human brain glutamate concentration decreased (P ≤ 0.01) after the initiation of hypoglycemia in the two control groups, but a smaller trend toward a decrease in patients with HAAF did not reach significance (P>0.05). These findings are consistent with a metabolic adaptation in HAAF to provide higher glucose and/or alternative fuel to the brain, eliminating the need to oxidize glutamate. In an exploratory analysis, we detected additional metabolite changes in response to hypoglycemia in the T1D patient without HAAF control group, namely, increased aspartate and decreased lactate.

Influence of VMH fuel sensing on hypoglycemic responses

Hypoglycemia produces complex neural and hormonal responses that restore glucose levels to normal. Glucose, metabolic substrates and their transporters, neuropeptides and neurotransmitters alter the firing rate of glucose-sensing neurons in the ventromedial hypothalamus (VMH); these monitor energy status and regulate the release of neurotransmitters that instigate a suitable counter-regulatory response. Under normal physiological conditions, these mechanisms maintain blood glucose concentrations within narrow margins. However, antecedent hypoglycemia and diabetes can lead to adaptations within the brain that impair counter-regulatory responses. Clearly, the mechanisms employed to detect and regulate the response to hypoglycemia, and the pathophysiology of defective counter-regulation in diabetes, are complex and need to be elucidated to permit the development of therapies that prevent or reduce the risk of hypoglycemia.