Dynamic monitoring of cerebral metabolites during and after transient global ischemia in rats by quantitative proton NMR spectroscopy in vivo

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

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

Sign-trackers have elevated myo-inositol in the nucleus accumbens and ventral hippocampus following Pavlovian conditioned approach

Pavlovian conditioned approach (PCA) is a behavioral procedure that can be used to assess individual differences in the addiction vulnerability of drug-naïve rats and identify addiction vulnerability factors. Using proton magnetic resonance spectroscopy (¹ H-MRS) ex vivo, we simultaneously analyzed concentrations of multiple neurochemicals throughout the mesocorticolimbic system 2 weeks after PCA training in order to identify potential vulnerability factors to addiction in drug-naïve rats for future investigations. Levels of myo-inositol (Ins), a ¹ H-MRS-detectable marker of glial activity/proliferation, were increased in the nucleus accumbens (NAc) and ventral hippocampus, but not dorsal hippocampus or medial prefrontal cortex, of sign-trackers compared to goal-trackers or intermediate responders. In addition, Ins levels positively correlated with PCA behavior in the NAc and ventral hippocampus. Because the sign-tracker phenotype is associated with increased drug-seeking behavior, these results observed in drug-naïve rats suggest that alterations in glial activity/proliferation within these regions may represent an addiction vulnerability factor. Sign-tracking rats preferentially approach reward cues during Pavlovian conditioning, while goal-trackers instead approach the location of impending reward. Sign-trackers are also more prone to cue-induced drug-seeking behavior. We used magnetic resonance spectroscopy to show that myo-inositol levels are higher in the ventral hippocampus and nucleus accumbens of sign-trackers relative to goal-trackers. Thus, elevated myo-inositol may be a vulnerability factor for addiction.

Dynamic correlations between hemodynamic, metabolic, and neuronal responses to acute whole-brain ischemia

Cerebral ischemia sets off a cascade of neuronal and metabolic responses to preserve brain viability. An understanding of the temporal evolution of these changes during and after ischemia, and their correlation with hemodynamic changes, is essential. In this study, a 12-min whole-brain ischemia based on the four-blood-vessel occlusion model was employed in rats. Using a high-temporal-resolution simultaneous (1)H-(31)P MRS acquisition sequence at 9.4 T, we investigated dynamic occlusion and reperfusion responses in cerebral lactate (Lac), phosphocreatine (PCr), adenosine triphosphate (ATP), pH, and blood oxygenation level dependence (BOLD), together with changes in neuronal field potential activity. We reveal tightly coupled dynamics between hemodynamic, metabolic, and neuronal responses to ischemia. Neuronal activity, BOLD, PCr, Lac, and pH changed immediately following occlusion, indicating reduced energy substrates and consumption, and increased glycolysis to maintain cellular ATP levels, which started to decrease 2.2 min after the onset of occlusion. ATP stores were then gradually consumed to maintain a minimum housekeeping neuronal activity level. By correlating dynamic changes of brain activity, BOLD, and energy metabolism, new insights into the brain's survival ability and mechanisms during an acute ischemic attack from the perspectives of cerebral metabolism, neuroenergetics, and neuronal activity were gained.

Therapeutic benefits of human mesenchymal stem cells derived from bone marrow after global cerebral ischemia

Although intravenous delivery of mesenchymal stem cells (MSCs) prepared from adult bone marrow reduces infarction size and ameliorates functional deficits in rat middle cerebral artery occlusion models, there are few reports of MSC treatment in global cerebral ischemia. We utilized a global cerebral ischemia model induced by arresting the heart with a combination of hypovolemia and intracardiac injections of a cold potassium chloride solution in order to study the potential therapeutic benefits of human mesenchymal stem cells (hMSCs) on global cerebral ischemia. hMSCs were intravenously injected into the rats 3 h after resuscitation from cardiac arrest. The effects on structural and functional outcome of hMSC were assessed at 5 h and 1, 3, and 7 days using magnetic resonance spectroscopy (MRS), histology, and cognitive functional analysis. Intravenous delivery of hMSCs reduced the Lac/Cr ratios, nuclear DNA fragmentation, neuronal loss, and elicited functional improvement compared with the control sham group. Enzyme-linked immunosorbent assay (ELISA) of the hippocampus revealed an increase in BDNF in hMSC-treated group. These data suggest that intravenous delivery of hMSC may have a therapeutic effect in global cerebral ischemia.

Neuroimaging as a basis for rational stem cell therapy

Neonatal global or focal hypoxic-ischemic brain injury remains a frequent and devastating condition, with serious long-term sequelae. An important issue in any neonatal clinical trial of neuroprotective agents relates to developing accurate measures of injury severity and also suitable measures of the response to treatment. Advanced magnetic resonance imaging techniques can acquire serial and noninvasive data about brain structure, metabolic activity, and the response to injury or treatment. These imaging methods need validation in appropriate animal models for translational research studies in human newborns. This review describes several approaches that use imaging as well as proton magnetic resonance spectroscopy to assess the severity of ischemic injury (e.g., for possible candidate selection) and for monitoring the progression and evolution of injury over time and as an indicator of recovery or response to treatment. Preliminary data are presented on how imaging can be used after neural stem cell implantation to characterize the migration rate, the magnitude of stem cell proliferation, and their final location. Imaging has the potential to allow monitoring of many dimensions of neuroprotective treatments and can be expected to contribute to efficacy and safety when clinical trials using neural stem cells or other neuroprotective agents become available.

The effects of repeated endotoxin exposure on rat brain metabolites as measured by ex vivo 1HMRS

Abnormalities in brain chemistry induced by acute or chronic treatment with LPS were studied in the rat model. Ex vivo brain metabolites were measured using proton magnetic resonance spectroscopy, whereas serum corticosterone levels were determined using radioimmunoassay. We observed increased lactate levels in all measured brain regions and decreased choline in the hypothalamus after chronic LPS treatment. Acute LPS treatment led to an elevation of corticosterone, whereas chronic LPS treatment led to attenuation of the HPA response. These findings suggest that chronic inflammation induced by LPS could lead to cell loss/dysfunction, and hence, desensitization of the HPA axis, particularly in the hypothalamus.

Localized proton MRS of cerebral metabolite profiles in different mouse strains

Localized proton MR spectroscopy (MRS) was used to quantify cerebral metabolite concentrations in NMRI (n = 8), BALB/c (n = 7), and C57BL/6 (n = 8) mice in vivo and 1 hr after global irreversible ischemia (2.35 T, STEAM, TR/TE/TM = 6000/20/10 ms, 4 x 3 x 4 mm(3) volume, corrections for cerebrospinal fluid). Anatomical MRI and proton MRS revealed significant differences of the C57BL/6 strain in comparison with both BALB/c and NMRI mice. While MRI volumetry yielded larger ventricular spaces of the C57BL/6 strain, proton MRS resulted in elevated concentrations of N-acetylaspartate (tNAA), creatine and phosphocreatine (tCr), choline-containing compounds (Cho), glucose (Glc), and lactate (Lac) relative to BALB/c mice and elevated Glc relative to NMRI mice. Apart from the expected decrease of Glc and increase of Lac 1 hr post mortem, C57BL/6 mice presented with significant reductions of tNAA, tCr, and Cho, whereas these metabolites remained unchanged in BALB/c and NMRI mice. The results support the hypothesis that the more pronounced vulnerability of C57BL/6 mice to brain ischemia is linked to strain-dependent differences of the cerebral energy metabolism.

Effect of melatonin and nifedipine on some antioxidant enzymes and different energy fuels in the blood and brain of global ischemic rats

The brain normally derives most of its energy from the aerobic oxidation of glucose and therefore it must be nourished with a rich supply of both glucose and oxygen. Interference with the blood supply, such as in ischemia, could shift the brain to search for another source of energy and to spare its own glucose. Ischemia results not only in energy fuel disturbance, but also in free radical formation and Ca(2+) homeostasis disruption. Therefore, our investigations studied the influence of ischemia on energy fuels, on some natural free radical scavengers, and the relationship between the changes of these parameters in brain and blood. Each of these was also studied under the influence of melatonin, a well-known free radical scavenger, and nifedipine, a Ca(2+)-channel blocker and antioxidant, during ischemia followed by reperfusion (I/R). Adult male Wistar rats were subjected to global ischemia by occlusion of the two carotid arteries for 1 hr (group I), followed by reperfusion for another hour in group II. Drugs were injected after ischemia (group I), and before or after reperfusion onset in the second group. Two series of animals were used. In the first series the effect of the two drugs on the activity of superoxide dismutase (SOD), glutathione reductase (GR), and lactate dehydrogenase (LDH) was investigated in the cytosolic fraction of four brain areas, viz., cortex (CC), thalamus/hypothalamus (T/TH), midbrain (MB) and medulla, pons and cerebellum (MPC). Moreover, the level of both SOD and GR in the erythocytes of these rats was also estimated. In the second series, we studied the effect of each drug on the content of glucose and beta-hydroxybutyrate (beta-HB) in whole brain, in addition to the plasma levels of glucose, beta-HB and lactate. The results showed that (i), ischemia elevated the brain levels of LDH and beta-HB, as well as the plasma level of glucose, beta-HB, lactate and erythocytic GR. Conversely, it lowered glucose, SOD and GR levels in the brain; (ii), reperfusion reversed the ischemic effect on all the previously altered parameters except for plasma levels of lactate and glucose; (iii), melatonin (10 mg/kg, i.p) and nifedipine (1.5 mg/kg, i.p), restored the energy fuel levels in the brain of ischemic and I/R rats, as well as the ischemic effect on the erythocyte activities of SOD and GR. Furthermore, both drugs reversed I/R effect on the cytosolic activities of the antioxidant enzymes. We conclude that melatonin and nifedipine are both neuroprotective with improvement in the antioxidant system and energy fuels.

Automatic quantitation of localized in vivo 1H spectra with LCModel

The LCModel method analyzes an in vivo spectrum as a Linear Combination of Model in vitro spectra from individual metabolite solutions. Complete model spectra, rather than individual resonances, are used in order to incorporate maximum prior information into the analysis. A nearly model-free constrained regularization method automatically accounts for the baseline and lineshape in vivo without imposing a restrictive parameterized form on them. LCModel is automatic (non-interactive) with no subjective input. Approximately maximum-likelihood estimates of the metabolite concentrations and their uncertainties (Cramér-Rao lower bounds) are obtained. LCModel analyses of spectra from users with fields from 1.5 to 9.4 T and a wide range of sequences, particularly with short TE, are used here to illustrate the capabilities and limitations of LCModel and proton MRS.

Evaluation of CA1 damage using single-voxel 1H-MRS and un-biased stereology: Can non-invasive measures of N-acetyl-asparate following global ischemia be used as a reliable measure of neuronal damage?

Global brain ischemia provoked by transient occlusion of the carotid arteries (2VO) in gerbils results in a severe loss of neurons in the hippocampal CA1 region. We measured the concentration of the neuron specific N-acetyl-aspartate, [NAA], in the gerbil dorsal hippocampus by proton MR spectroscopy (1H-MRS) in situ, and HPLC, 4 days after global ischemia. The [NAA] was correlated with graded hippocampus damage scoring and stereologically determined neuronal density. A basal hippocampal [NAA] of 8.37+/-0.10 and 9.81+/-0.44 mmol/l were found from HPLC and 1H-MRS, respectively. HPLC measurements of [NAA] obtained from hippocampus 4 days after 2VO showed a 20% reduction in the [NAA] following 4 min of ischemia (P<0.001). 1H-MRS measurements on gerbils subjected to 4 or 8 min of ischemia showed a similar 24% decline in the [NAA] (P<0.05). Thus, there was correlation between the HPLC and 1H-MRS determined NAA decline. There was also a significant correlation between 1H-MRS [NAA] and the corresponding reduction in CA1 neuronal density (P<0.004). In summary our findings show that single voxel 1H-MRS can be used as a supplement to histological evaluation of neuronal injury in studies after global brain ischemia. Accordingly, volume selective spectroscopy has a potential for assessment of neuroprotective therapeutic compounds/strategies with respect to neuronal rescue for delayed ischemic brain damage.

Effect of a neuronal sodium channel blocker on magnetic resonance derived indices of brain water content during global cerebral ischemia

Diffusion-weighted magnetic resonance imaging (DWI) with calculation of the apparent diffusion coefficient (ADC) of water is a widely used noninvasive method to measure movement of water from the extracellular to the intracellular compartment during cerebral ischemia. Lamotrigine, a neuronal Na(+) channel blocker, has been shown to attenuate the increase in extracellular concentrations of excitatory amino acids (EAA) during ischemia and to improve neurological and histological outcome. Because of its proven ability to reduce EAA levels during ischemia, lamotrigine should also minimize excitotoxic-induced increases in intracellular water content and therefore attenuate changes in the ADC. In this study, we sought to determine the effect of lamotrigine on intra- and extracellular water shifts during transient global cerebral ischemia. Fifteen New Zealand white rabbits were anesthetized and randomized to one of three groups: a control group, a lamotrigine-treated group, or a sham group. After being positioned in the bore of the magnet, a 12-min 50-s period of global cerebral ischemia was induced by inflating a neck tourniquet. During ischemia and early reperfusion there was a similar and significant decrease of the ADC in both the lamotrigine and control group. The ADC in the sham ischemia group remained at baseline throughout the experiment. Lamotrigine-mediated blockade of voltage-gated sodium channels did not prevent the intracellular movement of water during 12 min 50 s of global ischemia, as measured by the ADC, suggesting that the ADC decline may not be mediated by voltage-gated sodium influx and glutamate release.

Probability of metabolic tissue recovery after thrombolytic treatment of experimental stroke: a magnetic resonance spectroscopic imaging study in rat brain

The effect of thrombolytic therapy on metabolic changes was studied in rats submitted to thromboembolic stroke. Reperfusion was initiated at three different time points, 1.5, 3, and 4.5 hours after embolism (n = 3 each), by injection of recombinant tissue-type plasminogen activator (rt-PA). Recovery was observed during 5 hours of reperfusion using perfusion-weighted images and a two-dimensional 1H magnetic resonance spectroscopic imaging (MRSI) technique. Temporal evolution of the cerebral metabolites lactate and N-acetyl-aspartate (NAA) was determined. To analyze the chances of metabolic tissue recovery, the outcome of treatment, defined by a reversal of lactate concentration, was compared with the lactate intensity before treatment. In untreated animals (n = 4), clot embolism resulted in a drop of perfusion signal intensity in the occluded hemisphere followed by an increase of lactate concentration and a decrease of NAA that persisted throughout the observation period. Thrombolysis partially restored blood flow, but the mean lactate concentration decreased only slightly after successful lysis in animals treated 1.5 hours after embolism. If treatment was initiated later, no decline of lactate level was observed. Five hours after initiation of thrombolysis, the average tissue lactate amounted to 95 +/- 6, 111 +/- 17, and 139 +/- 60% of the early ischemic value (40 minutes after embolization) if treatment began 1.5, 3, and 4.5 hours after embolism, respectively. The NAA level declined slightly but never showed a recovery after rt-PA treatment. In individual pixels, the probability of metabolic tissue recovery clearly declined with increasing lactate concentration before thrombolysis. Interestingly, this probability was independent of treatment delay, but the number of pixels with low lactate declined with increasing ischemia time. Potential clinical applications of MRSI include monitoring of therapeutic intervention as well as support for prognosis of outcome after rt-PA treatment.

Toward an in vivo neurochemical profile: quantification of 18 metabolites in short-echo-time (1)H NMR spectra of the rat brain

Localized in vivo (1)H NMR spectroscopy was performed with 2-ms echo time in the rat brain at 9.4 T. Frequency domain analysis with LCModel showed that the in vivo spectra can be explained by 18 metabolite model solution spectra and a highly structured background, which was attributed to resonances with fivefold shorter in vivo T(1) than metabolites. The high spectral resolution (full width at half maximum approximately 0.025 ppm) and sensitivity (signal-to-noise ratio approximately 45 from a 63-microL volume, 512 scans) was used for the simultaneous measurement of the concentrations of metabolites previously difficult to quantify in (1)H spectra. The strongly represented signals of N-acetylaspartate, glutamate, taurine, myo-inositol, creatine, phosphocreatine, glutamine, and lactate were quantified with Cramér-Rao lower bounds below 4%. Choline groups, phosphorylethanolamine, glucose, glutathione, gamma-aminobutyric acid, N-acetylaspartylglutamate, and alanine were below 13%, whereas aspartate and scyllo-inositol were below 22%. Intra-assay variation was assessed from a time series of 3-min spectra, and the coefficient of variation was similar to the calculated Cramér-Rao lower bounds. Interassay variation was determined from 31 pooled spectra, and the coefficient of variation for total creatine was 7%. Tissue concentrations were found to be in very good agreement with neurochemical data from the literature.

Proton MRS of oral creatine supplementation in rats. Cerebral metabolite concentrations and ischemic challenge

Proton magnetic resonance spectroscopy (MRS) was employed to determine the concentrations of N-acetylaspartate (NAA), total creatine (tCr), choline-containing compounds (Cho), myo-inositol (Ins), glucose (Glc), and lactate (Lac) in rat brain before and after 10 days of oral supplementation of 2.6 g Cr-monohydrate per kg body weight per day. Measurements were performed both in vitro (n = 16) and in vivo (n = 6). The neuroprotective potential of oral Cr was assessed by dynamically monitoring brain Glc and Lac in response to transient global ischemia (12 min). In comparison to controls the in vitro concentrations of Cr (13.1 +/- 9.3%) and Ins (12.7 +/- 14. 0%) were significantly increased in Cr-fed rats. Under in vivo conditions, the data revealed trends for elevated tCr (4.7%) and Ins (10.6%) which were enhanced in the concentration ratios of tCr:Cho (10.2%) and Ins:Cho (17.8%). Together with an increased Glc level (27.3%), the observation of a statistically significant decrease of brain Lac (-38.5 +/- 19.3%) in Cr-fed rats may reflect a shift of the energy metabolism from non-oxidative toward oxidative glycolysis. One hour after global ischemia most of the metabolic differences between Cr-fed rats and controls were retained. The increased Glc level (44.4 +/- 33.3%) reached statistical significance, but the accumulation of Lac and its time course during ischemia and early reperfusion showed no differences between Cr-fed rats and controls.

In vivo 1H NMR spectroscopy of rat brain at 1 ms echo time

Using optimized, asymmetric radiofrequency (RF) pulses for slice selection, the authors demonstrate that stimulated echo acquisition mode (STEAM) localization with ultra-short echo time (1 ms) is possible. Water suppression was designed to minimize sensitivity to B1 inhomogeneity using a combination of 7 variable power RF pulses with optimized relaxation delays (VAPOR). Residual water signal was well below the level of most observable metabolites. Contamination by the signals arising from outside the volume of interest was minimized by outer volume saturation using a series of hyperbolic secant RF pulses, resulting in a sharp volume definition. In conjunction with FASTMAP shimming (Gruetter Magn Reson Med 1993;29: 804-811), the short echo time of 1 msec resulted in highly resolved in vivo 1H nuclear magnetic resonance spectra. In rat brain the water linewidths of 11-13 Hz and metabolite singlet linewidths of 8-10 Hz were measured in 65 microl volumes. Very broad intense signals (delta v(1/2) > 1 kHz), as expected from membranes, for example, were not observed, suggesting that their proton T2 are well below 1 msec. The entire chemical shift range of 1H spectrum was observable, including resolved resonances from alanine, aspartate, choline group, creatine, GABA, glucose, glutamate, glutamine, myo-inositol, lactate, N-acetylaspartate, N-acetylaspartylglutamate, phosphocreatine, and taurine. At 9.4 T, peaks close to the water were observed, including the H-1 of alpha-D-glucose at 5.23 ppm and a tentative H-1 resonance of glycogen at 5.35 ppm.

Proton T2 relaxation of cerebral metabolites during transient global ischemia in rat brain

Putative changes of metabolite T2 relaxation times were investigated before and after a 20-min period of global ischemia in rat brain in vivo (n = 10) using localized proton MRS at different echo times (2.35 T). Neither absolute T2 relaxation times (TE = 20-270 ms) nor time courses of T2-weighted metabolite signals (TE = 135 ms) revealed statistically significant changes during the occlusion or early reperfusion relative to pre-ischemic baseline. These findings are in line with reports of relaxation changes at much later stages and further demonstrate that altered T2 relaxation is not a confounding factor in diffusion-weighted long-TE proton MRS during early ischemic events.