Neurovascular coupling is optimized to compensate for the increase in proton production from nonoxidative glycolysis and glycogenolysis during brain activation and maintain homeostasis of pH, pCO2 , and pO2.

During transient brain activation cerebral blood flow (CBF) increases substantially more than cerebral metabolic rate of oxygen consumption (CMRO2 ) resulting in blood hyperoxygenation, the basis of BOLD fMRI contrast. Explanations for the high CBF vs. CMRO2 slope, termed neurovascular coupling (NVC) constant, focused on maintainenance of tissue oxygenation to support mitochondrial ATP production. However, paradoxically the brain has a 3-fold lower oxygen extraction fraction (OEF) than other organs with high energy requirements, like heart and muscle during exercise. Here, we hypothesize that the NVC constant and the capillary oxygen mass transfer coefficient (which in combination determine OEF) are co-regulated during activation to maintain simultaneous homeostasis of pH and partial pressure of CO2 and O2 (pCO2 and pO2 ). To test our hypothesis, we developed an arteriovenous flux balance model for calculating blood and brain pH, pCO2 , and pO2 as a function of baseline OEF (OEF0 ), CBF, CMRO2 , and proton production by nonoxidative metabolism coupled to ATP hydrolysis. Our model was validated against published brain arteriovenous difference studies and then used to calculate pH, pCO2, and pO2 in activated human cortex from published calibrated fMRI and PET measurements. In agreement with our hypothesis, calculated pH, pCO2, and pO2 remained close to constant independently of CMRO2 in correspondence to experimental measurements of NVC and OEF0 . We also found that the optimum values of the NVC constant and OEF0 that ensure simultaneous homeostasis of pH, pCO2, and pO2 were remarkably similar to their experimental values. Thus, the high NVC constant is overall determined by proton removal by CBF due to increases in nonoxidative glycolysis and glycogenolysis. These findings resolve the paradox of the brain's high CBF yet low OEF during activation, and may contribute to explaining the vulnerability of brain function to reductions in blood flow and capillary density with aging and neurovascular disease.

The early days of ex vivo 1 H, 13 C, and 31 P nuclear magnetic resonance in the laboratory of Dr. Robert G. Shulman from 1975 to 1995.

This paper provides a brief description of the early use of ex vivo nuclear magnetic resonance (NMR) studies of tissue and tissue extracts performed in the laboratory of Dr. Robert G. Shulman from 1975 through 1995 at Bell Laboratories, then later at Yale University. During that period, ex vivo NMR provided critical information in support of resonance assignments and the quantitation of concentrations for magnetic resonance spectroscopy studies. The period covered saw rapid advances in magnet technology, starting with studies of microorganisms in vertical bore high-resolution NMR studies, then by 1981 studies of small mammals in a horizontal bore magnet, and then studies of humans in 1984. Ex vivo NMR played a critical role in all these studies. A general strategy developed in the lab for using ex vivo NMR to support in vivo studies is presented, as well as illustrative examples.

Measurement of neuro-energetics and neurotransmission in the rat olfactory bulb using 1 H and 1 H-[13 C] NMR spectroscopy.

The olfactory bulb (OB) plays a fundamental role in the sense of smell and has been implicated in several pathologies, including Alzheimer's disease. Despite its importance, high metabolic activity and unique laminar architecture, the OB is not frequently studied using MRS methods, likely due to the small size and challenging location. Here we present a detailed metabolic characterization of OB metabolism, in terms of both static metabolite concentrations using 1 H MRS and metabolic fluxes associated with neuro-energetics and neurotransmission by tracing the dynamic 13 C flow from intravenously administered [1,6-13 C2 ]-glucose, [2-13 C]-glucose and [2-13 C]-acetate to downstream metabolites, including [4-13 C]-glutamate, [4-13 C]-glutamine and [2-13 C]-GABA. The unique laminar architecture and associated metabolism of the OB, distinctly different from that of the cerebral cortex, is characterized by elevated GABA and glutamine levels, as well as increased GABAergic and astroglial energy metabolism and neurotransmission. The results show that, despite the technical challenges, high-quality 1 H and 1 H-[13 C] MR spectra can be obtained from the rat OB in vivo. The derived metabolite concentrations and metabolic rates demonstrate a unique metabolic profile for the OB. The metabolic model provides a solid basis for future OB studies on functional activation or pathological conditions.

Mechanistic stoichiometric relationship between the rates of neurotransmission and neuronal glucose oxidation: Reevaluation of and alternatives to the pseudo-malate-aspartate shuttle model.

The ~1:1 stoichiometry between the rates of neuronal glucose oxidation (CMRglc-ox-N ) and glutamate (Glu)/γ-aminobutyric acid (GABA)-glutamine (Gln) neurotransmitter (NT) cycling between neurons and astrocytes (VNTcycle ) has been firmly established. However, the mechanistic basis for this relationship is not fully understood, and this knowledge is critical for the interpretation of metabolic and brain imaging studies in normal and diseased brain. The pseudo-malate-aspartate shuttle (pseudo-MAS) model established the requirement for glycolytic metabolism in cultured glutamatergic neurons to produce NADH that is shuttled into mitochondria to support conversion of extracellular Gln (i.e., astrocyte-derived Gln in vivo) into vesicular neurotransmitter Glu. The evaluation of this model revealed that it could explain half of the 1:1 stoichiometry and it has limitations. Modifications of the pseudo-MAS model were, therefore, devised to address major knowledge gaps, that is, submitochondrial glutaminase location, identities of mitochondrial carriers for Gln and other model components, alternative mechanisms to transaminate α-ketoglutarate to form Glu and shuttle glutamine-derived ammonia while maintaining mass balance. All modified models had a similar 0.5 to 1.0 predicted mechanistic stoichiometry between VNTcycle and the rate of glucose oxidation. Based on studies of brain ß-hydroxybutyrate oxidation, about half of CMRglc-ox-N may be linked to glutamatergic neurotransmission and localized in pre-synaptic structures that use pseudo-MAS type mechanisms for Glu-Gln cycling. In contrast, neuronal compartments that do not participate in transmitter cycling may use the MAS to sustain glucose oxidation. The evaluation of subcellular compartmentation of neuronal glucose metabolism in vivo is a critically important topic for future studies to understand glutamatergic and GABAergic neurotransmission.

Cell-type specific modulation of NMDA receptors triggers antidepressant actions.

Both the NMDA receptor (NMDAR) positive allosteric modulator (PAM), and antagonist, can exert rapid antidepressant effects as shown in several animal and human studies. However, how this bidirectional modulation of NMDARs causes similar antidepressant effects remains unknown. Notably, the initial cellular trigger, specific cell-type(s), and subunit(s) of NMDARs mediating the antidepressant-like effects of a PAM or an antagonist have not been identified. Here, we used electrophysiology, microdialysis, and NMR spectroscopy to evaluate the effect of a NMDAR PAM (rapastinel) or NMDAR antagonist, ketamine on NMDAR function and disinhibition-mediated glutamate release. Further, we used cell-type specific knockdown (KD), pharmacological, and behavioral approaches to dissect the cell-type specific role of GluN2B, GluN2A, and dopamine receptor subunits in the actions of NMDAR PAM vs. antagonists. We demonstrate that rapastinel directly enhances NMDAR activity on principal glutamatergic neurons in medial prefrontal cortex (mPFC) without any effect on glutamate efflux, while ketamine blocks NMDAR on GABA interneurons to cause glutamate efflux and indirect activation of excitatory synapses. Behavioral studies using cell-type-specific KD in mPFC demonstrate that NMDAR-GluN2B KD on Camk2a- but not Gad1-expressing neurons blocks the antidepressant effects of rapastinel. In contrast, GluN2B KD on Gad1- but not Camk2a-expressing neurons blocks the actions of ketamine. The results also demonstrate that Drd1-expressing pyramidal neurons in mPFC mediate the rapid antidepressant actions of ketamine and rapastinel. Together, these results demonstrate unique initial cellular triggers as well as converging effects on Drd1-pyramidal cell signaling that underlie the antidepressant actions of NMDAR-positive modulation vs. NMDAR blockade.

NMR visibility of deuterium-labeled liver glycogen in vivo.

PURPOSE: Deuterium metabolic imaging (DMI) combined with [6,6'-2 H2 ]-glucose has the potential to detect glycogen synthesis in the liver. However, the similar chemical shifts of [6,6'-2 H2 ]-glucose and [6,6'-2 H2 ]-glycogen in the 2 H NMR spectrum make unambiguous detection and separation difficult in vivo, in contrast to comparable approaches using 13 C MRS. Here the NMR visibility of 2 H-labeled glycogen is investigated to better understand its potential contribution to the observed signal in liver following administration of [6,6'-2 H2 ]-glucose. METHODS: Mice were provided drinking water containing 2 H-labeled glucose. High-resolution NMR analyses was performed of isolated liver glycogen in solution, before and after the addition of the glucose-releasing enzyme amyloglucosidase. RESULTS: 2 H-labeled glycogen was barely detectable in solution using 2 H NMR because of the very short T2 (<2 ms) of 2 H-labeled glycogen, giving a spectral line width that is more than five times as broad as that of 13 C-labeled glycogen (T2 = ~10 ms). CONCLUSION: 2 H-labeled glycogen is not detectable with 2 H MRS(I) under in vivo conditions, leaving 13 C MRS as the preferred technique for in vivo detection of glycogen.

Contribution of macromolecules to brain 1 H MR spectra: Experts' consensus recommendations.

Proton MR spectra of the brain, especially those measured at short and intermediate echo times, contain signals from mobile macromolecules (MM). A description of the main MM is provided in this consensus paper. These broad peaks of MM underlie the narrower peaks of metabolites and often complicate their quantification but they also may have potential importance as biomarkers in specific diseases. Thus, separation of broad MM signals from low molecular weight metabolites enables accurate determination of metabolite concentrations and is of primary interest in many studies. Other studies attempt to understand the origin of the MM spectrum, to decompose it into individual spectral regions or peaks and to use the components of the MM spectrum as markers of various physiological or pathological conditions in biomedical research or clinical practice. The aim of this consensus paper is to provide an overview and some recommendations on how to handle the MM signals in different types of studies together with a list of open issues in the field, which are all summarized at the end of the paper.

In vivo 13 C and 1 H-[13 C] MRS studies of neuroenergetics and neurotransmitter cycling, applications to neurological and psychiatric disease and brain cancer.

In the last 25 years 13 C MRS has been established as the only noninvasive method for measuring glutamate neurotransmission and cell specific neuroenergetics. Although technically and experimentally challenging 13 C MRS has already provided important new information on the relationship between neuroenergetics and neuronal function, the high energy cost of brain function in the resting state and the role of altered neuroenergetics and neurotransmitter cycling in disease. In this paper we review the metabolic and neurotransmitter pathways that can be measured by 13 C MRS and key findings on the linkage between neuroenergetics, neurotransmitter cycling, and brain function. Applications of 13 C MRS to neurological and psychiatric disease as well as brain cancer are reviewed. Recent technological developments that may help to overcome spatial resolution and brain coverage limitations of 13 C MRS are discussed.

Subanesthetic ketamine reverses neuronal and astroglial metabolic activity deficits in a social defeat model of depression.

Depression is one of the most debilitating neuropsychiatric disorders. Most of the current antidepressants have long remission time and low recovery rate. This study explores the impact of ketamine on neuronal and astroglial metabolic activity in prefrontal cortex in a social defeat (SD) model of depression. C57BL/6 mice were subjected to a social defeat paradigm for 5 min a day for 10 consecutive days. Ketamine (10 mg/kg, intraperitoneal) was administered to mice for two consecutive days following the last defeat stress. Mice were infused with [1,6-13 C2 ]glucose or [2-13 C]acetate to assess neuronal and astroglial metabolic activity, respectively, together with proton-observed carbon-edited nuclear magnetic resonance spectroscopy in prefrontal cortex tissue extract. The 13 C labeling of amino acids from glucose and acetate was decreased in SD mice. Ketamine treatment in SD mice restored sucrose preference, social interaction and immobility time to control values. Acute subanesthetic ketamine restored the 13 C labeling of brain amino acids from glucose as well as acetate in SD mice to the respective control values, suggesting that rates of neuronal and astroglial tricarboxylic acid (TCA) cycle and neurotransmitter cycling were re-established to normal levels. The finding of improved energy metabolism in SD mice suggests that fast anti-depressant action of ketamine is linked with improved neurotransmitter cycling.

Detection of cerebral NAD+ in humans at 7T.

PURPOSE: To develop 1 H-based MR detection of nicotinamide adenine dinucleotide (NAD+ ) in the human brain at 7T and validate the 1 H results with NAD+ detection based on 31 P-MRS. METHODS: 1 H-MR detection of NAD+ was achieved with a one-dimensional double-spin-echo method on a slice parallel to the surface coil transceiver. Perturbation of the water resonance was avoided through the use of frequency-selective excitation. 31 P-MR detection of NAD+ was performed with an unlocalized pulse-acquire sequence. RESULTS: Both 1 H- and 31 P-MRS allowed the detection of NAD+ signals on every subject in 16 min. Spectral fitting provided an NAD+ concentration of 107 ± 28 µM for 1 H-MRS and 367 ± 78 µM and 312 ± 65 µM for 31 P-MRS when uridine diphosphate glucose (UDPG) was excluded and included, respectively, as an overlapping signal. CONCLUSIONS: NAD+ detection by 1 H-MRS is a simple method that comes at the price of reduced NMR visibility. NAD+ detection by 31 P-MRS has near-complete NMR visibility, but it is complicated by spectral overlap with NADH and UDPG. Overall, the 1 H- and 31 P-MR methods both provide exciting opportunities to study NAD+ metabolism on human brain in vivo. © 2016 International Society for Magnetic Resonance in Medicine. Magn Reson Med 78:828-835, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Comparison of Glutamate Turnover in Nerve Terminals and Brain Tissue During [1,6-13C2]Glucose Metabolism in Anesthetized Rats.

The 13C turnover of neurotransmitter amino acids (glutamate, GABA and aspartate) were determined from extracts of forebrain nerve terminals and brain homogenate, and fronto-parietal cortex from anesthetized rats undergoing timed infusions of [1,6-13C2]glucose or [2-13C]acetate. Nerve terminal 13C fractional labeling of glutamate and aspartate was lower than those in whole cortical tissue at all times measured (up to 120 min), suggesting either the presence of a constant dilution flux from an unlabeled substrate or an unlabeled (effectively non-communicating on the measurement timescale) glutamate pool in the nerve terminals. Half times of 13C labeling from [1,6-13C2]glucose, as estimated by least squares exponential fitting to the time course data, were longer for nerve terminals (GluC4, 21.8 min; GABAC2 21.0 min) compared to cortical tissue (GluC4, 12.4 min; GABAC2, 14.5 min), except for AspC3, which was similar (26.5 vs. 27.0 min). The slower turnover of glutamate in the nerve terminals (but not GABA) compared to the cortex may reflect selective effects of anesthesia on activity-dependent glucose use, which might be more pronounced in the terminals. The 13C labeling ratio for glutamate-C4 from [2-13C]acetate over that of 13C-glucose was twice as large in nerve terminals compared to cortex, suggesting that astroglial glutamine under the 13C glucose infusion was the likely source of much of the nerve terminal dilution. The net replenishment of most of the nerve terminal amino acid pools occurs directly via trafficking of astroglial glutamine.

Direct evidence for activity-dependent glucose phosphorylation in neurons with implications for the astrocyte-to-neuron lactate shuttle.

Previous (13)C magnetic resonance spectroscopy experiments have shown that over a wide range of neuronal activity, approximately one molecule of glucose is oxidized for every molecule of glutamate released by neurons and recycled through astrocytic glutamine. The measured kinetics were shown to agree with the stoichiometry of a hypothetical astrocyte-to-neuron lactate shuttle model, which predicted negligible functional neuronal uptake of glucose. To test this model, we measured the uptake and phosphorylation of glucose in nerve terminals isolated from rats infused with the glucose analog, 2-fluoro-2-deoxy-D-glucose (FDG) in vivo. The concentrations of phosphorylated FDG (FDG6P), normalized with respect to known neuronal metabolites, were compared in nerve terminals, homogenate, and cortex of anesthetized rats with and without bicuculline-induced seizures. The increase in FDG6P in nerve terminals agreed well with the increase in cortical neuronal glucose oxidation measured previously under the same conditions in vivo, indicating that direct uptake and oxidation of glucose in nerve terminals is substantial under resting and activated conditions. These results suggest that neuronal glucose-derived pyruvate is the major oxidative fuel for activated neurons, not lactate-derived from astrocytes, contradicting predictions of the original astrocyte-to-neuron lactate shuttle model under the range of study conditions.

Oxidation of ethanol in the rat brain and effects associated with chronic ethanol exposure.

It has been reported that chronic and acute alcohol exposure decreases cerebral glucose metabolism and increases acetate oxidation. However, it remains unknown how much ethanol the living brain can oxidize directly and whether such a process would be affected by alcohol exposure. The questions have implications for reward, oxidative damage, and long-term adaptation to drinking. One group of adult male Sprague-Dawley rats was treated with ethanol vapor and the other given room air. After 3 wk the rats received i.v. [2-(13)C]ethanol and [1, 2-(13)C2]acetate for 2 h, and then the brain was fixed, removed, and divided into neocortex and subcortical tissues for measurement of (13)C isotopic labeling of glutamate and glutamine by magnetic resonance spectroscopy. Ethanol oxidation was seen to occur both in the cortex and the subcortex. In ethanol-naïve rats, cortical oxidation of ethanol occurred at rates of 0.017 ± 0.002 µmol/min/g in astroglia and 0.014 ± 0.003 µmol/min/g in neurons, and chronic alcohol exposure increased the astroglial ethanol oxidation to 0.028 ± 0.002 µmol/min/g (P = 0.001) with an insignificant effect on neuronal ethanol oxidation. Compared with published rates of overall oxidative metabolism in astroglia and neurons, ethanol provided 12.3 ± 1.4% of cortical astroglial oxidation in ethanol-naïve rats and 20.2 ± 1.5% in ethanol-treated rats. For cortical astroglia and neurons combined, the ethanol oxidation for naïve and treated rats was 3.2 ± 0.3% and 3.8 ± 0.2% of total oxidation, respectively. (13)C labeling from subcortical oxidation of ethanol was similar to that seen in cortex but was not affected by chronic ethanol exposure.

Effects of γ-Aminobutyric acid transporter 1 inhibition by tiagabine on brain glutamate and γ-Aminobutyric acid metabolism in the anesthetized rat In vivo.

γ-Aminobutyric acid (GABA) clearance from the extracellular space after release from neurons involves reuptake into terminals and astrocytes through GABA transporters (GATs). The relative flows through these two pathways for GABA released from neurons remains unclear. This study determines the effect of tiagabine, a selective inhibitor of neuronal GAT-1, on the rates of glutamate (Glu) and GABA metabolism and GABA resynthesis via the GABA-glutamine (Gln) cycle. Halothane-anesthetized rats were administered tiagabine (30 mg/kg, i.p.) and 45 min later received an intravenous infusion of either [1,6-(13)C2]glucose (in vivo) or [2-(13)C]acetate (ex vivo). Nontreated rats served as controls. Metabolites and (13)C enrichments were measured with (1)H-[(13)C]-nuclear magnetic resonance spectroscopy and referenced to their corresponding endpoint values measured in extracts from in situ frozen brain. Metabolic flux estimates of GABAergic and glutamatergic neurons were determined by fitting a metabolic model to the (13)C turnover data measured in vivo during [1,6-(13)C2]glucose infusion. Tiagabine-treated rats were indistinguishable (P > 0.05) from controls in tissue amino acid levels and in (13)C enrichments from [2-(13)C]acetate. Tiagabine reduced average rates of glucose oxidation and neurotransmitter cycling in both glutamatergic neurons (↓18%, CMR(glc(ox)Glu): control, 0.27 ± 0.05 vs. tiagabine, 0.22 ± 0.04 µmol/g/min; ↓11%, V(cyc(Glu-Gln)): control 0.23 ± 0.05 vs. tiagabine 0.21 ± 0.04 µmol/g/min and GABAergic neurons (↓18-25%, CMR(glc(ox)GABA): control 0.09 ± 0.02 vs. tiagabine 0.07 ± 0.03 µmol/g/min; V(cyc(GABA-Gln)): control 0.08 ± 0.02 vs. tiagabine 0.07 ± 0.03 µmol/g/min), but the changes in glutamatergic and GABAergic fluxes were not significant (P > 0.10). The results suggest that any reduction in GABA metabolism by tiagabine might be an indirect response to reduced glutamatergic drive rather than direct compensatory effects.

Distribution of temperature changes and neurovascular coupling in rat brain following 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") exposure.

(+/-)3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") is an abused psychostimulant that produces strong monoaminergic stimulation and whole-body hyperthermia. MDMA-induced thermogenesis involves activation of uncoupling proteins (UCPs), primarily a type specific to skeletal muscle (UCP-3) and absent from the brain, although other UCP types are expressed in the brain (e.g. thalamus) and might contribute to thermogenesis. Since neuroimaging of brain temperature could provide insights into MDMA action, we measured spatial distributions of systemically administered MDMA-induced temperature changes and dynamics in rat cortex and subcortex using a novel magnetic resonance method, Biosensor Imaging of Redundant Deviation in Shifts (BIRDS), with an exogenous temperature-sensitive probe (thulium ion and macrocyclic chelate 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethyl-1,4,7,10-tetraacetate (DOTMA(4-))). The MDMA-induced temperature rise was greater in the cortex than in the subcortex (1.6 ± 0.4 °C versus 1.3 ± 0.4 °C) and occurred more rapidly (2.0 ± 0.2 °C/h versus 1.5 ± 0.2 °C/h). MDMA-induced temperature changes and dynamics in the cortex and body were correlated, although the body temperature exceeded the cortex temperature before and after MDMA. Temperature, neuronal activity, and blood flow (CBF) were measured simultaneously in the cortex and subcortex (i.e. thalamus) to investigate possible differences of MDMA-induced warming across brain regions. MDMA-induced warming correlated with increases in neuronal activity and blood flow in the cortex, suggesting that the normal neurovascular response to increased neural activity was maintained. In contrast to the cortex, a biphasic relationship was seen in the subcortex (i.e. thalamus), with a decline in CBF as temperature and neural activity rose, transitioning to a rise in CBF for temperature above 37 °C, suggesting that MDMA affected CBF and neurovascular coupling differently in subcortical regions. Considering that MDMA effects on CBF and heat dissipation (as well as potential heat generation) may vary regionally, neuroprotection may require different cooling strategies.

Quantification of high-resolution ¹H-[¹³C] NMR spectra from rat brain extracts.

NMR spectroscopy in combination with (13)C-labeled substrate infusion is a unique technique to obtain information about dynamic metabolic fluxes noninvasively in vivo. In many cases, the in vivo information content obtained during dynamic (13)C studies in rodents can be enhanced by high-resolution (1)H-[(13)C] NMR spectroscopy on brain extracts. Previously, it has been shown that (1)H NMR spectra from rat brain extracts can be accurately quantified with a spectral fitting routine utilizing simulated basis sets using complete prior knowledge of chemical shifts and scalar couplings. The introduction of (13)C label into the various metabolites presents complications that demand modifications of the spectral fitting routine. As different multiplets within a given molecule accumulate various amounts of (13)C label, the fixed amplitude relationship between multiplets typical for (1)H NMR spectra must be abandoned. In addition, (13)C isotope effects lead to spectral multiplet patterns that become dependent on the amount of (13)C label accumulation, thereby preventing the use of a common basis set. Here a modified spectral fitting routine is presented that accommodates variable (13)C label accumulation and (13)C isotope effects. Spectral fitting results are quantitatively compared to manual integration on column-separated samples in which spectral overlap is minimized.

Detection of cerebral NAD(+) by in vivo (1)H NMR spectroscopy.

Nicotinamide adenine dinucleotide (NAD(+)) plays a central role in cellular metabolism both as a coenzyme for electron-transfer enzymes as well as a substrate for a wide range of metabolic pathways. In the current study NAD(+) was detected on rat brain in vivo at 11.7T by 3D localized (1)H MRS of the NAD(+) nicotinamide protons in the 8.7-9.5 ppm spectral region. Avoiding water perturbation was critical to the detection of NAD(+) as strong, possibly indirect cross-relaxation between NAD(+) and water would lead to a several-fold reduction of the NAD(+) intensity in the presence of water suppression. Water perturbation was minimized through the use of localization by adiabatic spin-echo refocusing (LASER) in combination with frequency-selective excitation. The NAD(+) concentration in the rat cerebral cortex was determined at 296 ± 28 µm, which is in good agreement with recently published (31) P NMR-based results as well as results from brain extracts in vitro (355 ± 34 µm). The T1 relaxation time constants of the NAD(+) nicotinamide protons as measured by inversion recovery were 280 ± 65 and 1136 ± 122 ms in the absence and presence of water inversion, respectively. This confirms the strong interaction between NAD(+) nicotinamide and water protons as observed during water suppression. The T2 relaxation time constants of the NAD(+) nicotinamide protons were determined at 60 ± 13 ms after confounding effects of scalar coupling evolution were taken into account. The simplicity of the MR sequence together with the robustness of NAD(+) signal detection and quantification makes the presented method a convenient choice for studies on NAD(+) metabolism and function. As the method does not critically rely on magnetic field homogeneity and spectral resolution it should find immediate applications in rodents and humans even at lower magnetic fields.

Deficits in brain glucose transport among younger adults with obesity.

OBJECTIVE: Obesity is associated with alterations in eating behavior and neurocognitive function. In this study, we investigate the effect of obesity on brain energy utilization, including brain glucose transport and metabolism. METHODS: A total of 11 lean participants and 7 young healthy participants with obesity (mean age, 27 years) underwent magnetic resonance spectroscopy scanning coupled with a hyperglycemic clamp (target, ~180 mg/dL) using [1-13C] glucose to measure brain glucose uptake and metabolism, as well as peripheral markers of insulin resistance. RESULTS: Individuals with obesity demonstrated an ~20% lower ratio of brain glucose uptake to cerebral glucose metabolic rate (Tmax/CMRglucose) than lean participants (2.12 ± 0.51 vs. 2.67 ± 0.51; p = 0.04). The cerebral tricarboxylic acid cycle flux (VTCA) was similar between the two groups (p = 0.64). There was a negative correlation between total nonesterified fatty acids and Tmax/CMRglucose (r = -0.477; p = 0.045). CONCLUSIONS: We conclude that CMRglucose is unlikely to differ between groups due to similar VTCA, and, therefore, the glucose transport Tmax is lower in individuals with obesity. These human findings suggest that obesity is associated with reduced cerebral glucose transport capacity even at a young age and in the absence of other cardiometabolic comorbidities, which may have implications for long-term brain function and health.

Functional MRI and neural responses in a rat model of Alzheimer's disease.

Based on the hypothesis that brain plaques and tangles can affect cortical function in Alzheimer's disease (AD), we investigated functional responses in an AD rat model (called the Samaritan Alzheimer's rat achieved by ventricular infusion of amyloid peptide) and age-matched healthy control. High-field functional magnetic resonance imaging (fMRI) and extracellular neural activity measurements were applied to characterize sensory-evoked responses. Electrical stimulation of the forepaw led to BOLD and neural responses in the contralateral somatosensory cortex and thalamus. In AD brain we noted much smaller BOLD activation patterns in the somatosensory cortex (i.e., about 50% less activated voxels compared to normal brain). While magnitudes of BOLD and neural responses in the cerebral cortex were markedly attenuated in AD rats compared to normal rats (by about 50%), the dynamic coupling between the BOLD and neural responses in the cerebral cortex, as assessed by transfer function analysis, remained unaltered between the groups. However thalamic BOLD and neural responses were unaltered in AD brain compared to controls. Thus cortical responses in the AD model were indeed diminished compared to controls, but the thalamic responses in the AD and control rats were quite similar. Therefore these results suggest that Alzheimer's disease may affect cortical function more than subcortical function, which may have implications for interpreting altered human brain functional responses in fMRI studies of Alzheimer's disease.

Metabolic products of [2-(13) C]ethanol in the rat brain after chronic ethanol exposure.

Most ingested ethanol is metabolized in the liver to acetaldehyde and then to acetate, which can be oxidized by the brain. This project assessed whether chronic exposure to alcohol can increase cerebral oxidation of acetate. Through metabolism, acetate may contribute to long-term adaptation to drinking. Two groups of adult male Sprague-Dawley rats were studied, one treated with ethanol vapor and the other given room air. After 3 weeks the rats received an intravenous infusion of [2-(13) C]ethanol via a lateral tail vein for 2 h. As the liver converts ethanol to [2-(13) C]acetate, some of the acetate enters the brain. Through oxidation the (13) C is incorporated into the metabolic intermediate α-ketoglutarate, which is converted to glutamate (Glu), glutamine (Gln), and GABA. These were observed by magnetic resonance spectroscopy and found to be (13) C-labeled primarily through the consumption of ethanol-derived acetate. Brain Gln, Glu, and, GABA (13) C enrichments, normalized to (13) C-acetate enrichments in the plasma, were higher in the chronically treated rats than in the ethanol-naïve rats, suggesting increased cerebral uptake and oxidation of circulating acetate. Chronic ethanol exposure increased incorporation of systemically derived acetate into brain Gln, Glu, and GABA, key neurochemicals linked to brain energy metabolism and neurotransmission. The liver converts ethanol to acetate, which may contribute to long-term adaptation to drinking. Astroglia oxidize acetate and generate neurochemicals, while neurons and glia may also oxidize ethanol. When (13) C-ethanol is administered intravenously, (13) C-glutamine, glutamate, and GABA, normalized to (13) C-acetate, were higher in chronic ethanol-exposed rats than in control rats, suggesting that ethanol exposure increases cerebral oxidation of circulating acetate.