Correlation between N-acetylaspartate levels and histopathologic changes in cortical infarcts of mice after middle cerebral artery occlusion

N-acetyl aspartate levels early after ischemic stroke accurately reflect long-term brain damage

Background

Estimation of brain damage following an ischemic stroke is most often performed within the first few days after the insult, where large amounts of oedematous fluid have accumulated. This can potentially hamper correct measurement of infarcted area, since oedema formation poorly reflects infarct size. This study presents a non-invasive, easily applicable and reliable method to accurately predict long-term evolution and late-stage infarction.

Objective

We performed a longitudinal analysis of brain infarct evolution after MCAO in mice, in order to determine whether water-compensated N-Acetylaspartate (NAA) levels in the infarct area, measured 24 h after the insult, is a suitable marker for late-stage infarction and thereby prognosis.

Methods

Twenty mice were divided into 4 groups and scanned longitudinally at different time-points after MCAO, followed by euthanisation for histology: Group 1) MRI/MRS at day 1 after MCAO (n = 4), Group 2) MRI/MRS at days 1 and 7 after MCAO (n = 5), Group 3) MRI/MRS at days 1, 7, and 14 after MCAO (n = 3), and Group 4) MRI/MRS at days 1, 7, 14, and 28 after MCAO (n = 4). At days 1, 7, 14, and 28, NAA levels were correlated with histological determination of neuronal death based on Nissl and H&E stainings.

Results

Twenty-four hours after the insult, NAA levels in the infarcted area decreased by 35 %, but steadily returned to normal after 28 days. In the acute phases, NAA levels strongly correlated with loss of Nissl substance (r2 = -0.874, p = 0.002), whereas NAA levels in later stages reflect glial metabolism and tissue reorganisation. Most importantly, NAA levels 24 h after MCAO was highly correlated with late stage infarction at days 14 and 28 (r2 = 0.73, p = 0.01), in contrast to T2 (r2 = 0.06, p = 0.59).

Conclusions

By using a fixed voxel, which is easily positioned in the affected area, it is possible to obtain reliable measures of the extent of neuronal loss at early time points independent of oedema and brain deformation. Importantly, NAA levels 24 h after MCAO accurately reflects late-stage infarction, suggesting that NAA is a useful prognostic biomarker early after an ischemic stroke.

Predicting the Onset of Ischemic Stroke With Fast High-Resolution 3D MR Spectroscopic Imaging

BACKGROUND

Neurometabolite concentrations provide a direct index of infarction progression in stroke. However, their relationship with stroke onset time remains unclear.

PURPOSE

To assess the temporal dynamics of N-acetylaspartate (NAA), creatine, choline, and lactate and estimate their value in predicting early (<6 hours) vs. late (6-24 hours) hyperacute stroke groups.

STUDY TYPE

Cross-sectional cohort.

POPULATION

A total of 73 ischemic stroke patients scanned at 1.8-302.5 hours after symptom onset, including 25 patients with follow-up scans.

FIELD STRENGTH/SEQUENCE

A 3 T/magnetization-prepared rapid acquisition gradient echo sequence for anatomical imaging, diffusion-weighted imaging and fluid-attenuated inversion recovery imaging for lesion delineation, and 3D MR spectroscopic imaging (MRSI) for neurometabolic mapping.

ASSESSMENT

Patients were divided into hyperacute (0-24 hours), acute (24 hours to 1 week), and subacute (1-2 weeks) groups, and into early (<6 hours) and late (6-24 hours) hyperacute groups. Bayesian logistic regression was used to compare classification performance between early and late hyperacute groups by using different combinations of neurometabolites as inputs.

STATISTICAL TESTS

Linear mixed effects modeling was applied for group-wise comparisons between NAA, creatine, choline, and lactate. Pearson's correlation analysis was used for neurometabolites vs. time. P < 0.05 was considered statistically significant.

RESULTS

Lesional NAA and creatine were significantly lower in subacute than in acute stroke. The main effects of time were shown on NAA (F = 14.321) and creatine (F = 12.261). NAA was significantly lower in late than early hyperacute patients, and was inversely related to time from symptom onset across both groups (r = -0.440). The decrease of NAA and increase of lactate were correlated with lesion volume (NAA: r = -0.472; lactate: r = 0.366) in hyperacute stroke. Discrimination was improved by combining NAA, creatine, and choline signals (area under the curve [AUC] = 0.90).

DATA CONCLUSION

High-resolution 3D MRSI effectively assessed the neurometabolite changes and discriminated early and late hyperacute stroke lesions.

EVIDENCE LEVEL

1.

TECHNICAL EFFICACY

Stage 2.

Multinuclear MRI Reveals Early Efficacy of Stem Cell Therapy in Stroke

Compromised adult human mesenchymal stem cells (hMSC) can impair cell therapy efficacy and further reverse ischemic recovery. However, in vitro assays require extended passage to characterize cells, limiting rapid assessment for therapeutic potency. Multinuclear magnetic resonance imaging and spectroscopy (MRI/S) provides near real-time feedback on disease progression and tissue recovery. Applied to ischemic stroke, 23Na MRI evaluates treatment efficacy within 24 h after middle cerebral artery occlusion, showing recovery of sodium homeostasis and lesion reduction in specimens treated with hMSC while 1H MRS identifies reduction in lactate levels. This combined metric was confirmed by evaluating treatment groups receiving healthy or compromised hMSC versus vehicle (sham saline injection) over 21 days. Behavioral tests to assess functional recovery and cell analysis for immunomodulatory and macrophage activity to detect hMSC potency confirm MR findings. Clinically, these MR metrics may prove critical to early evaluations of therapeutic efficacy and overall stroke recovery.

The functional microscopic neuroanatomy of the human subthalamic nucleus

The subthalamic nucleus (STN) is successfully used as a surgical target for deep brain stimulation in the treatment of movement disorders. Interestingly, the internal structure of the STN is still incompletely understood. The objective of the present study was to investigate three-dimensional (3D) immunoreactivity patterns for 12 individual protein markers for GABA-ergic, serotonergic, dopaminergic as well as glutamatergic signaling. We analyzed the immunoreactivity using optical densities and created a 3D reconstruction of seven postmortem human STNs. Quantitative modeling of the reconstructed 3D immunoreactivity patterns revealed that the applied protein markers show a gradient distribution in the STN. These gradients were predominantly organized along the ventromedial to dorsolateral axis of the STN. The results are of particular interest in view of the theoretical underpinning for surgical targeting, which is based on a tripartite distribution of cognitive, limbic and motor function in the STN.

Metabolomic Profiling Reveals That Reprogramming of Cerebral Glucose Metabolism Is Involved in Ischemic Preconditioning-Induced Neuroprotection in a Rodent Model of Ischemic Stroke

Ischemic tolerance renders the brain resistant to ischemia-reperfusion (I/R) injury as a result of the activation of endogenous adaptive responses triggered by various types of preconditioning. The complex underlying metabolic mechanisms responsible for the neuroprotection of cerebral ischemic preconditioning (IPC) remain elusive. Herein, gas chromatography-mass spectrometry (GC-MS) technique was applied to delineate the dynamic changes of brain metabolome in a rodent model of ischemic stroke (transient occlusion of the middle cerebral artery, tMCAO), alone or after pretreatment with nonlethal ischemic tolerance induction (transient occlusion of the bilateral common carotid arteries, tBCCAO). Metabolomic analysis showed that accumulation of glucose (concentration increased more than 4 fold) and glycolytic intermediates is the prominent feature of brain I/R-induced metabolic disturbance. IPC attenuated brain I/R damage by subduing postischemic hyperglycolysis, increasing the pentose phosphate pathway (PPP) flux and promoting the utilization of β-hydroxybutyrate. The expression analysis of pivotal genes and proteins involved in relevant metabolic pathways revealed that the downregulation of AMP-activated protein kinase (AMPK)-mediated glucose transporter-1 (GLUT-1) and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3 (PFKFB3) and reduced mRNA levels of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) subunits were associated with IPC-induced metabolic flexibility, which allows the brain to be more capable of withstanding severe I/R insults. The present study provided mechanistic insights into the metabolic signature of IPC and indicated that adaptively modulating brain glucose metabolism could be an effective approach for the therapeutic intervention of ischemic stroke.

Histology of Brain Trauma and Hypoxia-Ischemia

Forensic pathologists encounter hypoxic-ischemic (HI) brain damage or traumatic brain injuries (TBI) on an almost daily basis. Evaluation of the findings guides decisions regarding cause and manner of death. When there are gross findings of brain trauma, the cause of death is often obvious. However, microscopic evaluation should be used to augment the macroscopic diagnoses. Histology can be used to seek evidence for TBI in the absence of gross findings, e.g., in the context of reported or suspected TBI. Estimating the survival interval after an insult is often of medicolegal interest; this requires targeted tissue sampling and careful histologic evaluation. Retained tissue blocks serve as forensic evidence and also provide invaluable teaching and research material. In certain contexts, histology can be used to demonstrate nontraumatic causes of seemingly traumatic lesions. Macroscopic and histologic findings of brain trauma can be confounded by concomitant HI brain injury when an individual survives temporarily after TBI. Here we review the histologic approaches for evaluating TBI, hemorrhage, and HI brain injury. Amyloid precursor protein (APP) immunohistochemistry is helpful for identifying damaged axons, but patterns of damage cannot unambiguously distinguish TBI from HI. The evolution of hemorrhagic lesions will be discussed in detail; however, timing of any lesion is at best approximate. It is important to recognize artifactual changes (e.g., dark neurons) that can resemble HI damage. Despite the shortcomings, histology is a critical adjunct to the gross examination of brains.

Electrophysiology of cerebral ischemia and reperfusion: First evidence for the role of synapse in ischemic tolerance

OBJECTIVE

The subthreshold brain-damaging stimulus may protect the brain from subsequent ischemia; this phenomenon has been named "ischemic tolerance" (IT). We focused on the synaptic properties of the neurons after mild and severe ischemia to determine the association between IT and synaptic efficacy.

EXPERIMENTAL DESIGN

Adult male rats were randomly divided into four experimental groups including control, sham, permanent ischemia (pI/R), and mild ischemia (mI/R). Middle cerebral artery occlusion (MCAO) method was applied to induce brain ischemia. Seven days after the insult, long-term potentiation (LTP) induced by high-frequency stimulation (HFS) and paired-pulse ratio (PPR) were monitored before and after the HFS delivery.

RESULTS

The field potential recording demonstrated that mild ischemia significantly increased the basal synaptic transmission. Additionally, the HFS produced a significant potentiation compared to its baseline level in the mI/R group. Moreover, mild ischemia prevented depression of PPR by HFS. This effect was accompanied by a significant increase in the normalized PPR (PPR after HFS/PPR before HFS) in this group.

CONCLUSIONS

Our data indicated that a mild reduction in brain perfusion without permanent lesion can dramatically increase the basal synaptic transmission. This effect may be associated with an increase in the neurotransmitter content of the pre-synaptic neurons. This hypothesis could provide a new insight into the relationship between IT and synaptic efficacy. Synapse 70:351-360, 2016. © 2016 Wiley Periodicals, Inc.

Oxytocin's neurochemical effects in the medial prefrontal cortex underlie recovery of task-specific brain activity in autism: a randomized controlled trial

The neuropeptide oxytocin may be an effective therapeutic strategy for the currently untreatable social and communication deficits associated with autism. Our recent paper reported that oxytocin mitigated autistic behavioral deficits through the restoration of activity in the ventromedial prefrontal cortex (vmPFC), as demonstrated with functional magnetic resonance imaging (fMRI) during a socio-communication task. However, it is unknown whether oxytocin exhibited effects at the neuronal level, which was outside of the specific task examined. In the same randomized, double-blind, placebo-controlled, within-subject cross-over clinical trial in which a single dose of intranasal oxytocin (24 IU) was administered to 40 men with high-functioning autism spectrum disorder (UMIN000002241/000004393), we measured N-acetylaspartate (NAA) levels, a marker for neuronal energy demand, in the vmPFC using (1)H-magnetic resonance spectroscopy ((1)H-MRS). The differences in the NAA levels between the oxytocin and placebo sessions were associated with oxytocin-induced fMRI signal changes in the vmPFC. The oxytocin-induced increases in the fMRI signal could be predicted by the NAA differences between the oxytocin and placebo sessions (P=0.002), an effect that remained after controlling for variability in the time between the fMRI and (1)H-MRS scans (P=0.006) and the order of administration of oxytocin and placebo (P=0.001). Furthermore, path analysis showed that the NAA differences in the vmPFC triggered increases in the task-dependent fMRI signals in the vmPFC, which consequently led to improvements in the socio-communication difficulties associated with autism. The present study suggests that the beneficial effects of oxytocin are not limited to the autistic behavior elicited by our psychological task, but may generalize to other autistic behavioral problems associated with the vmPFC.

Mitochondrial Aspartate/Glutamate Carrier SLC25A12 and Autism Spectrum Disorder: a Meta-Analysis

Mitochondrial dysfunction has been reported to be involved in the pathophysiology of autism spectrum disorder (ASD). Studies investigating the possible association between ASD and polymorphism in SLC25A12, which encodes the mitochondrial aspartate/glutamate carrier, have yielded inconsistent results. We conducted a systematic review and meta-analysis of such studies to elucidate if and which SLC25A12 single nucleotide polymorphisms (SNPs) are associated with ASD. We searched PubMed, Ovid, Web of Science, and ERIC databases through September 20th, 2014. Odds ratios (ORs) were aggregated using random effect models. Sensitivity analyses were conducted based on study design (family-based or case-control). Fifteen out of 79 non-duplicate records were retained for qualitative synthesis. We pooled 10 datasets from 9 studies with 2001 families, 735 individuals with ASD and 632 typically developing (TD) individuals for the meta-analysis of rs2292813, as well as 11 datasets from 10 studies with 2016 families, 852 individuals with ASD and 1058 TD individuals for the meta-analysis of rs2056202. We found a statistically significant association between ASD and variant in rs2292813 (OR = 1.190, 95% CI 1.052-1.346, P = 0.006) as well as in rs2056202 (OR = 1.206, 95% CI 1.035-1.405, P = 0.016). Sensitivity analyses including only studies with family-based design demonstrated significant association between ASD and polymorphism in rs2292813 (OR = 1.216, 95% CI 1.075-1.376, P = 0.002) and rs2056202 (OR = 1.267, 95% CI 1.041-1.542, P = 0.018). In contrast, sensitivity analyses including case-control design studies only failed to find a significant association. Further research on the role of SLC25A12 and ASD may pave the way for potential innovative therapeutic interventions.

Non-invasive diagnostic biomarkers for estimating the onset time of permanent cerebral ischemia

The treatments for ischemic stroke can only be administered in a narrow time-window. However, the ischemia onset time is unknown in ~30% of stroke patients (wake-up strokes). The objective of this study was to determine whether MR spectra of ischemic brains might allow the precise estimation of cerebral ischemia onset time. We modeled ischemic stroke in male ICR-CD1 mice using a permanent middle cerebral artery filament occlusion model with laser Doppler control of the regional cerebral blood flow. Mice were then subjected to repeated MRS measurements of ipsilateral striatum at 14.1 T. A striking initial increase in γ-aminobutyric acid (GABA) and no increase in glutamine were observed. A steady decline was observed for taurine (Tau), N-acetyl-aspartate (NAA) and similarly for the sum of NAA+Tau+glutamate that mimicked an exponential function. The estimation of the time of onset of permanent ischemia within 6 hours in a blinded experiment with mice showed an accuracy of 33±10 minutes. A plot of GABA, Tau, and neuronal marker concentrations against the ratio of acetate/NAA allowed precise separation of mice whose ischemia onset lay within arbitrarily chosen time-windows. We conclude that (1)H-MRS has the potential to detect the clinically relevant time of onset of ischemic stroke.

Reversible loss of N-acetylaspartate after 15-min transient middle cerebral artery occlusion in rat: a longitudinal study with in vivo proton magnetic resonance spectroscopy

It is generally accepted that N-acetylaspartate (NAA) can be used a biochemical marker for assessing neuronal viability/integrity after cerebral ischemia. However, this view has recently been questioned based on observations showing that after a photothrombotic permanent ischemia the acute decline of NAA in the infracted regions, where massive neuronal loss persists, is reversible over time. In this study, we measured the longitudinal changes of NAA and total creatine (Cr) in ischemic rat brain after a 15-min transient middle cerebral artery occlusion (MCAO) by in vivo (1)H magnetic resonance spectroscopy. The results showed that the levels of NAA and total Cr in the ischemic lesion decrease significantly at 1 day post-ichemia, followed by spontaneous recovery to the control levels by 2 weeks and remained stable thereafter up to 16 weeks. The normalization of NAA and total Cr levels was associated histologically with persisted neuronal loss up to 90 % in the ischemic core, and accompanied by marked reactive astrocytic responses occurring with a similar time course. The absolute T(2) relaxation time in the ischemic lesion increased during acute phase, and declined afterwards during subacute and chronic phases of 15-min MCAO. The delayed decreases of T(2) in the ischemic lesion might be associated with deposition of paramagnetic species, such as manganese and iron originated from chronic inflammation, vascular degradation and/or hemorrhagic transformation. The results of this study give further support to the hypothesis that the recovery of NAA after cerebral ischemia might have contributions from reactive glia cells, and caution the use of NAA as a specific neuronal marker during the chronic stage of cerebral ischemia.

Magnetic resonance characterization of ischemic tissue metabolism

Magnetic resonance imaging (MRI) and spectroscopy (MRS) are versatile diagnostic techniques capable of characterizing the complex stroke pathophysiology, and hold great promise for guiding stroke treatment. Particularly, tissue viability and salvageability are closely associated with its metabolic status. Upon ischemia, ischemic tissue metabolism is disrupted including altered metabolism of glucose and oxygen, elevated lactate production/accumulation, tissue acidification and eventually, adenosine triphosphate (ATP) depletion and energy failure. Whereas metabolism impairment during ischemic stroke is complex, it may be monitored non-invasively with magnetic resonance (MR)-based techniques. Our current article provides a concise overview of stroke pathology, conventional and emerging imaging and spectroscopy techniques, and data analysis tools for characterizing ischemic tissue damage.

Early Predictive Biomarkers for Lesion After Transient Cerebral Ischemia

Background and Purpose—

Despite the improving imaging techniques, it remains challenging to predict the outcome early after transient cerebral ischemia. The aim of this study was thus to identify early metabolic biomarkers for outcome prediction.

Methods—

We modeled transient ischemic attacks and strokes in mice. Using high-field MR spectroscopy, we correlated early changes in the neurochemical profile of the ischemic striatum with histopathologic alterations at a later time point.

Results—

A significant increase in glutamine was measured between 3 hours and 8 hours after all ischemic events followed by reperfusion independently of the outcome and can thus be considered as an indicator of recent transient ischemia. On the other hand, a reduction of the score obtained by summing the concentrations of N-acetyl aspartate, glutamate, and taurine was a good predictor of an irreversible lesion as early as 3 hours after ischemia.

Conclusions—

We identified biomarkers of reversible and irreversible ischemic damage, which can be used in an early predictive evaluation of stroke outcome.

Brain injury and recovery following binge ethanol: evidence from in vivo magnetic resonance spectroscopy

BACKGROUND

The binge-drinking model in rodents using intragastric injections of ethanol (EtOH) for 4 days results in argyrophilic corticolimbic tissue classically interpreted as indicating irreversible neuronal degeneration. However, recent findings suggest that acquired argyrophilia can also identify injured neurons that have the potential to recover. The current in vivo magnetic resonance (MR) imaging and spectroscopy study was conducted to test the hypothesis that binge EtOH exposure would injure but not cause the death of neurons as previously ascertained postmortem.

METHODS

After baseline MR scanning, 11 of 19 rats received a loading dose of 5 g/kg EtOH via oral gavage, then a maximum of 3 g/kg every 8 hours for 4 days, for a total average cumulative EtOH dose of 43 +/- 1.2 g/kg and average blood alcohol levels of 258 +/- 12 mg/dL. All animals were scanned after 4 days of gavage (post-gavage scan) with EtOH (EtOH group) or dextrose (control [Con] group) and again after 7 days of abstinence from EtOH (recovery scan).

RESULTS

Tissue shrinkage at the post-gavage scan was reflected by significantly increased lateral ventricular volume in the EtOH group compared with the Con group. At the post-gavage scan, the EtOH group had lower dorsal hippocampal N-acetylaspartate and total creatine and higher choline-containing compounds than the Con group. At the recovery scan, neither ventricular volume nor metabolite levels differentiated the groups.

CONCLUSIONS

Rapid recovery of ventricular volume and metabolite levels with removal of the causative agent argues for transient rather than permanent effects of a single EtOH binge episode in rats.

Neuronal and axonal degeneration in experimental spinal cord injury: in vivo proton magnetic resonance spectroscopy and histology

Longitudinal in vivo proton magnetic resonance spectroscopy (1H-MRS) and immunohistochemistry were performed to investigate the tissue degeneration in traumatically injured rat spinal cord rostral and caudal to the lesion epicenter. On 1H-MRS significant decreases in N-acetyl aspartate (NAA) and total creatine (Cr) levels in the rostral, epicenter, and caudal segments were observed by 14 days, and levels remained depressed up to 56 days post-injury (PI). In contrast, the total choline (Cho) levels increased significantly in all three segments by 14 days PI, but recovered in the epicenter and caudal, but not the rostral region, at 56 days PI. Immunohistochemistry demonstrated neuronal cell death in the gray matter, and reactive astrocytes and axonal degeneration in the dorsal, lateral, and ventral white-matter columns. These results suggest delayed tissue degeneration in regions both rostrally and caudally from the epicenter in the injured spinal cord tissue. A rostral-caudal asymmetry in tissue recovery was seen both on MRI-observed hyperintense lesion volume and the Cho, but not NAA and Cr, levels at 56 days PI. These studies suggest that dynamic metabolic changes take place in regions away from the epicenter in injured spinal cord.

Molecular biomarkers in stroke diagnosis and prognosis

Serum biomarkers related to the cascade of inflammatory, hemostatic, glial and neuronal perturbations have been identifed to diagnose and characterize intracerebral hemorrhage and cerebral ischemia. Interpretation of most markers is confounded by their latent rise, blood-brain barrier effects, the heterogeneity of etiologies and the wide range of normal values, limiting their application for early diagnosis, lesion size estimation and long-term outcome prediction. Certain hemostatic and inflammatory constituents have been found to predict response to thrombolysis and worsening due to infarct progression and secondary hemorrhage, offering a potential role for improved treatment selection and individualization of therapy. Biomarkers will become increasingly relevant for developing targets for neuroprotective therapies, monitoring response to treatment and as surrogate end points for treatment trials.

NEOSTRIATAL CYTOSKELETON CHANGES FOLLOWING PERINATAL ASPHYXIA: EFFECT OF HYPOTHERMIA TREATMENT

Long-term changes of different types of neurofilaments (NF) and glial fibrillar acid protein (GFAP) were studied in neostriatal rat subjected to perinatal asphyxia (PA) under normothermic and hypothermic (15 degrees C) conditions, using immunohistochemistry for light and electron microscopy. Neostriatal neurons of 6-month-old rats that were subjected to 19 and 20 min of PA, showed an increase of NF 200 kDa immunostaining mainly in the axon fascicles in comparison with the control and hypothermia groups. In contrast, no alterations were seen with NF68 and NF160 neurofilament antibodies. Furthermore, the same PA groups showed astroglial cells with enhanced GFAP immunoreactivity, evidencing a typical astroglial reaction with a clear hypertrophy of these cells. A quantitative image analysis confirmed these observations. Hypothermic treated animals did show neither astroglial nor neuronal cytoskeletal changes in comparison to the control group. These findings showed that PA produces chronic cytoskeletal alterations in the neostriatum cells that can be prevented by hypothermia.

Assessment of mitochondrial impairment and cerebral blood flow in severe brain injured patients

BACKGROUND

We believe that in traumatic brain injury (TBI), the reduction of N-acetyl aspartate (NAA) occurs in the presence of adequate cerebral blood flow (CBF) which would lend support to the concept of mitochondrial impairment. The objective of this study was to test this hypothesis in severely injured patients (GCS 8 or less) by obtaining simultaneous measures of CBF and NAA.

METHODS

Fourteen patients were studied of which six patients presented as diffuse injury at admission CT, while focal lesions were present in eight patients. CBF using stable xenon method was measured at the same time that NAA was measured by magnetic resonance proton spectroscopy (1HMRS) in the MR suite. Additionally, diffusion weighted imaging (DWI) and maps of the apparent diffusion coefficient (ADC) were assessed.

FINDINGS

In diffuse injury, NAA/Cr reduction occurred uniformly throughout the brain where the values of CBF in all patients were well above ischemic threshold. In focal injury, we observed ischemic CBF values in the core of the lesions. However, in areas other than the core, CBF was above ischemic levels and NAA/Cr levels were decreased.

CONCLUSIONS

Considering the direct link between energy metabolism and NAA synthesis in the mitochondria, this study showed that in the absence of an ischemic insult, reductions in NAA concentration reflects mitochondrial dysfunction.

Associations between diffusion and perfusion parameters, N-acetyl aspartate, and lactate in acute ischemic stroke

BACKGROUND AND PURPOSE

In acute ischemic stroke, the amount of neuronal damage in hyperintense areas on MR diffusion imaging (DWI) is unclear. We used spectroscopic imaging to measure N-acetyl aspartate (NAA, a marker of normal neurons) and lactate (a marker of ischemia) to compare with diffusion and perfusion values in the diffusion lesion in acute ischemic stroke.

METHODS

We recruited patients with acute ischemic stroke prospectively and performed MR diffusion weighted (DWI), perfusion, and spectroscopic imaging. We coregistered the images, outlined the visible diffusion lesion, and extracted metabolite, perfusion, and apparent diffusion coefficient (ADC) values from the diffusion lesion.

RESULTS

42 patients were imaged, from 1.5 to 24 hours after stroke. In the DWI lesion, although NAA was reduced, there was no correlation between NAA and ADC or perfusion values. However, raised lactate correlated with reduced ADC (Spearman rho=0.32, P=0.04) and prolonged mean transit time (MTT, rho=0.31, P=0.04). Increasing DWI lesion size was associated with lower NAA and higher lactate (rho=-0.44, P=0.003; rho=0.49, P=0.001 respectively); NAA fell with increasing times to imaging (rho=-0.3, P=0.03), but lactate did not change.

CONCLUSIONS

Although larger confirmatory studies are needed, the correlation of ADC and MTT with lactate but not NAA suggests that ADC and MTT are better markers of the presence of ischemia than of cumulative neuronal loss. Further studies should define more precisely the rate of neuronal loss and relationship to diffusion and perfusion parameters with respect to the depth and duration of ischemia.

N-Acetylaspartate in the CNS: from neurodiagnostics to neurobiology

The brain is unique among organs in many respects, including its mechanisms of lipid synthesis and energy production. The nervous system-specific metabolite N-acetylaspartate (NAA), which is synthesized from aspartate and acetyl-coenzyme A in neurons, appears to be a key link in these distinct biochemical features of CNS metabolism. During early postnatal central nervous system (CNS) development, the expression of lipogenic enzymes in oligodendrocytes, including the NAA-degrading enzyme aspartoacylase (ASPA), is increased along with increased NAA production in neurons. NAA is transported from neurons to the cytoplasm of oligodendrocytes, where ASPA cleaves the acetate moiety for use in fatty acid and steroid synthesis. The fatty acids and steroids produced then go on to be used as building blocks for myelin lipid synthesis. Mutations in the gene for ASPA result in the fatal leukodystrophy Canavan disease, for which there is currently no effective treatment. Once postnatal myelination is completed, NAA may continue to be involved in myelin lipid turnover in adults, but it also appears to adopt other roles, including a bioenergetic role in neuronal mitochondria. NAA and ATP metabolism appear to be linked indirectly, whereby acetylation of aspartate may facilitate its removal from neuronal mitochondria, thus favoring conversion of glutamate to alpha ketoglutarate which can enter the tricarboxylic acid cycle for energy production. In its role as a mechanism for enhancing mitochondrial energy production from glutamate, NAA is in a key position to act as a magnetic resonance spectroscopy marker for neuronal health, viability and number. Evidence suggests that NAA is a direct precursor for the enzymatic synthesis of the neuron specific dipeptide N-acetylaspartylglutamate, the most concentrated neuropeptide in the human brain. Other proposed roles for NAA include neuronal osmoregulation and axon-glial signaling. We propose that NAA may also be involved in brain nitrogen balance. Further research will be required to more fully understand the biochemical functions served by NAA in CNS development and activity, and additional functions are likely to be discovered.

Magnetic resonance spectroscopy detects brain injury and predicts cognitive functioning in children with brain injuries

Proton magnetic resonance spectroscopy (1H-MRS) and neuropsychological assessment were utilized in a longitudinal investigation of traumatic brain injury (TBI) in children. A spectroscopic imaging protocol was implemented, and neurometabolite ratios of NAA/Cre and Cho/Cre were calculated for anterior and posterior halves of a supraventricular slab of brain tissue. NAA/Cre was reduced and Cho/Cre increased in TBI patients as compared to controls, for both brain regions. Each ratio predicted aspects of neuropsychological performance, though the specific relationships varied somewhat by region and function. Anterior NAA/Cre increased and anterior Cho/Cre decreased from 3 to 21 weeks post-injury, suggesting neurometabolic recovery.

A new model of thromboembolic stroke in the posterior circulation of the rat

The prognosis of vertebrobasilar occlusion is grave and therapeutic options are limited. The aim of the present study was to develop a new model of embolic hindbrain ischemia in the rat that closely resembles the clinical situation and that can be used to study pathophysiology and treatment options. After thoracotomy in 20 male Wistar rats, 15 animals received an injection of in vitro prepared autologous blood clots into the left vertebral artery. Five animals without clot injection served as controls. Neurological deficits were assessed in all animals 2 h after embolism. After 2 h, five animals were sacrificed to measure cerebral blood flow (CBF) by iodo-antipyridine autoradiography, and to calculate early cerebellar swelling by comparison of both hemispheres in brain slices. In these animals, autoradiography revealed ipsilesional brain swelling and significantly reduced blood flow values relative to the contralateral (unaffected) structures. Immunohistology showed the typical pattern of focal cerebral ischemia in the brain stem and/or cerebellum in 7 of 10 animals allowed to recover to 24 h. Hence, successful thromboembolism was achieved in 12 of 15 animals (80%). With this novel model, the pathophysiology and potential treatments of posterior circulation stroke can be investigated.

N-Acetylaspartate as a reservoir for glutamate

N-acetylaspartate (NAA) is an intermediary metabolite that is found in relatively high concentrations in the human brain. More specifically, NAA is so concentrated in the neurons that it generates one of the most visible peaks in nuclear magnetic resonance (NMR) spectra, thus allowing NAA to serve as "a neuronal marker". However, to date there is no generally accepted physiological (primary) role for NAA. Another molecule that is found at similar concentrations in the brain is glutamate. Glutamate is an amino acid and neurotransmitter with numerous functions in the brain. We propose that NAA, a six-carbon amino acid derivative, is converted to glutamate (five carbons) in an energetically favorable set of reactions. This set of reactions starts when aspartoacylase converts the six carbons of NAA to aspartate and acetate, which are subsequently converted to oxaloacetate and acetyl CoA, respectively. Aspartylacylase is found in astrocytes and oligodendrocytes. In the mitochondria, oxaloacetate and acetyl CoA are combined to form citrate. Requiring two steps, the citrate is oxidized in the Kreb's cycle to alpha-ketoglutarate, producing NADH. Finally, alpha-ketoglutarate is readily converted to glutamate by transaminating the alpha-keto to an amine. The resulting glutamate can be used by multiple cells types to provide optimal brain functional and structural needs. Thus, the abundant NAA in neuronal tissue can serve as a large reservoir for replenishing glutamate in times of rapid or dynamic signaling demands and stress. This is beneficial in that proper levels of glutamate serve critical functions for neurons, astrocytes, and oligodendrocytes including their survival. In conclusion, we hypothesize that NAA conversion to glutamate is a logical and favorable use of this highly concentrated metabolite. It is important for normal brain function because of the brain's relatively unique metabolic demands and metabolite fluxes. Knowing that NAA is converted to glutamate will be important for better understanding myriad neurodegenerative diseases such as Canavan's Disease and Multiple Sclerosis, to name a few. Future studies to demonstrate the chemical, metabolic and pathological links between NAA and glutamate will support this hypothesis.

Reduced prefrontal N-acetylaspartate in stroke patients with apathy

BACKGROUND

Although substantial numbers of stroke patients suffer from apathy, its causes are still poorly understood. Previous studies suggest that dysfunction of the frontal lobes is implicated in the pathophysiology of motivation. Our aim was to investigate the association between proton magnetic resonance spectroscopy (H1-MRS) measurements in unaffected frontal lobes and apathy in a group of first-time stroke patients.

METHODS

31 patients with a first-time ischemic stroke located outside the frontal lobes and 20 healthy subjects were included in the study. The authors performed single voxel H1-MRS in order to measure the N-acetylaspartate/creatine (NAA)/Cr, glutamate+glutamine (Glx)/Cr, choline (Cho)/Cr and myo-inositol (mI)/Cr ratios in the frontal lobes. Patients were assessed between days 7 and 12 post stroke. Diagnosis of apathy was made on the basis of clinical observation, interview and Apathy Scale.

RESULTS

13 out of 31 patients (42%) demonstrated apathy. Patients with apathy had lower NAA/Cr ratios in the right frontal lobe than non-apathetic subjects. The patient group was divided into two subgroups: Those with left hemisphere strokes, and those with right hemisphere strokes. Of these subjects, significantly lowered NAA/Cr ratios were found in the right hemispheres of apathetic patients in the subgroup with left-sided brain lesions.

CONCLUSIONS

These findings point to the association between apathy and frontal lobe integrity, suggest different reactions of the hemispheres and indicate that changes in the NAA/Cr ratio are related to the apathy.

Novel α-Amino-3-hydroxy-5-methyl-4-isoxazole Propionate (AMPA) Receptor Antagonists of 2,3-Benzodiazepine Type: Chemical Synthesis, in Vitro Characterization, and in Vivo Prevention of Acute Neurodegeneration

Under pathophysiological conditions, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor activation is considered to play a key role in several disorders of the central nervous system. In the search for AMPA receptor antagonists, the synthesis and pharmacological characterization of a series of novel compounds that are structurally related to GYKI 52466 (1), a well-known selective noncompetitive AMPA receptor antagonist, was performed. In vitro, 2,3-dimethyl-6-phenyl-12H-[1,3]dioxolo[4,5-h]imidazo[1,2-c][2,3]benzodiazepine (ZK 187638, 14a) antagonized the kainate-induced currents in cultured hippocampal neurons with an IC(50) of 3.4 microM in a noncompetitive fashion. When tested in a clinically predictive rat model of acute ischemic stroke, this noncompetitive AMPA receptor antagonist significantly reduced brain infarction, indicating that it is neuroprotective after permanent focal cerebral ischemia.

Time course of NAA T2 and ADCw in ischaemic stroke patients: 1H MRS imaging and diffusion-weighted MRI

BACKGROUND AND PURPOSE

Proton spectroscopy and quantitative diffusion-weighted imaging (DWI) were used to investigate the pertinence of N-acetyl aspartate (NAA) as a reliable marker of neuronal density in human stroke.

METHODS

The time courses of tissue water apparent diffusion coefficient (ADC(w)) and metabolite T2 were investigated on a plane corresponding to the largest area of cerebral infarction, within and outside the site of infarction in 71 patients with a large cortical middle cerebral artery territory infarction.

RESULTS

Significant reductions are seen in NAA T2 deep within the infarction during the period comprised between 5 and 20 days postinfarction; the relaxation times appearing to normalise several months after stroke. After an acute reduction in ADC(w), the pseudonormalisation of ADC(w) occurs at 8-12 days after the ischaemic insult. The minimum in N-acetyl aspartate T2 relaxation times and the pseudonormalisation of ADC(w) appear to coincide.

CONCLUSIONS

The data suggest that modifications in the behaviour of the observed proton metabolites occur during the period when the vasogenic oedema is formed and cell membrane integrity is lost. For this reason, NAA may not be a reliable marker of neuronal density during this period.

N-acetylaspartate to total creatine ratio in the hippocampal CA1 sector after transient cerebral ischemia in gerbils: influence of neuronal elements, reactive gliosis, and tissue atrophy

The authors compared temporal profiles of N-acetylaspartate (NAA) and the NAA/total creatine ratio with neuronal and astrocytic densities and with tissue atrophy in the hippocampal CA1 sector of gerbils after 5-minute bilateral forebrain ischemia and subsequent reperfusion for up to 6 months. The CA1 sector was dissected from 20- micro m lyophilized sections (n = 5) for NAA, phosphocreatine, and creatine assays using high-performance liquid chromatography. Adjacent 10- micro m sections were used for immunohistochemical analysis to follow neuronal and astrocytic responses. The NAA concentration was significantly (P<0.01) decreased after 7 days but leveled off thereafter. The NAA/total creatine (phosphocreatine + creatine) ratio was significantly decreased after 7 days and further decreased (P<0.05) after 6 months. Extensive neuronal damage developed beyond 7 days, while reactive astrogliosis progressed throughout the observation period. There was a good linear correlation (P<0.01) between astroglial density and the NAA/total creatine ratio beyond 7 days. The thickness of the CA1 sector was significantly reduced after 1 month and further reduced after 6 months. Although both NAA level and the NAA/total creatine ratio seemed to be indicators of neuronal damage, the latter could be influenced by reactive astrogliosis with progression of tissue atrophy.

Reversible loss of N-acetyl-aspartate in rats subjected to long-term focal cerebral ischemia

To evaluate the true meaning of N-acetyl-aspartate (NAA) measurements in ischemic stroke, the authors followed the temporal changes in brain NAA content in rats subjected to permanent focal ischemia. Ischemia was induced by photothrombotic cortical occlusion. At 1, 3, 8, and 30 d after onset of ischemia, NAA was measured in the infarct by high-performance liquid chromatography coupled to ultraviolet detection and histologic damage was examined. Cerebral content of NAA was markedly reduced in the lesioned tissue, reaching -90% after 3 d, a time at which viable neurons were no longer detected. N-Acetyl-aspartate concentrations after 8 and 30 d were higher than that observed after 3 d. This metabolic change coincided with an important microglial and astroglial activation. The results of this study raise questions regarding the use of NAA as a specific neuronal marker in chronic stage of stroke.

Spectroscopic data following stroke reveal tissue abnormality beyond the region of T2-weighted hyperintensity

Cerebral tissue with T2 magnetic resonance imaging (MRI) abnormalities following stroke is generally considered infarcted, while surrounding regions with normal MRI appearance are believed to be healthy. To assess whether these surrounding regions consist of normal tissue, we explored the distribution of N-acetylaspartate (NAA) and lactate within and around the hyperintense area on T2-weighted MRI using proton MR spectroscopy. The study was carried out in 25 patients with middle cerebral artery occlusion imaged between 1 and 42 days after stroke onset. NAA/choline (Cho) ratios were significantly reduced in both areas of T2 hyperintensity and in surrounding tissue. The reduction was greater in the region of T2 hyperintensity than in the surrounding region (-50% vs. -28%, respectively) and was unrelated to the delay after the ictus. Lactate/Cho ratios increased massively within the abnormal T2 area, but did not differ from control values beyond the margin of hyperintensity. Overall data indicate that T2 visible lesions on MRI do not infer the entire injured tissue.

Rapid tolerance to focal cerebral ischemia in rats is attenuated by adenosine A1 receptor antagonist

Two types of ischemic tolerance in the brain, rapid and delayed, have been reported in terms of the interval between the conditioning and test insults. Although many reports showed that delayed-phase neuroprotection evoked by preconditioning is evident after 1 week or longer, there have been a few investigations about rapidly induced tolerance, and the reported neuroprotective effects become ambiguous 7 days after the insults. The authors examined whether this rapid ischemic tolerance exists after 7 days of reperfusion in a rat focal ischemic model, and investigated modulating effects of the adenosine A 1 receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine). Preconditioning with 30 minutes of middle cerebral artery occlusion reduced infarct volume 7 days after 180 minutes of subsequent focal ischemia given after 1-hour reperfusion. The rapid preconditioning also improved neurologic outcome. These beneficial effects were attenuated by pretreatment of 0.1 mg/kg DPCPX, which did not influence the infarct volume after conditioning (30 minutes) or test (180 minutes) ischemia when given alone. The results show that preconditioning with a brief focal ischemia induces rapid tolerance to a subsequent severe ischemic insult, the effect of which is still present after 7 days of reperfusion, and that the rapid ischemic tolerance is possibly mediated through an adenosine A 1 receptor-related mechanism.

Application of magnetic resonance to animal models of cerebral ischemia

The present review has been compiled to highlight the role of magnetic resonance imaging (MRI) and MR spectroscopy (MRS) for the investigation of cerebral ischemia in the animal experimental field of basic research. We have focused on stroke investigations analyzing the pathomechanisms of the disease evolution and on new advances in both nuclear MR (NMR) methodology or genetic engineering of transgenic animals for the study of complex molecular relationships and causes of the disease. Furthermore, we have tried to include metabolic and genetic aspects, as well as the application of functional imaging, for the investigation of the disturbance or restitution of functional brain activation under pathological conditions as relates to controlled animal experiments.

1H magnetic resonance spectroscopic imaging of permanent focal cerebral ischemia in rat: longitudinal metabolic changes in ischemic core and rim

The purpose of this study was to determine whether regional differences in metabolites can be seen chronologically in permanent focal cerebral ischemia using 1H magnetic resonance spectroscopic imaging (MRSI), and whether these changes reflect pathological outcome. Regional variation in metabolites after permanent focal ischemia were investigated longitudinally in rats using 1H MRSI for a total of 7 days and then compared to histopathological findings. Four hours after the induction of ischemia, N-acetyl-L-aspartate (NAA) levels in the lateral caudo-putamen and the somatosensory cortex, core ischemic regions, decreased 22 and 40%, respectively. This reduction in NAA was coupled with a marked rise in lactate. In the medial caudo-putamen, the ischemic rim, however, NAA was preserved in spite of a marked increase in lactate. By 24 h post ischemia, the levels of NAA in medial caudo-putamen (ischemic rim in caudate) also decreased significantly. However NAA in cingulated cortex (ischemic rim in cortex) decreased more gradually between 24 and 48 h. This regional difference can reflect the severity of metabolic derangement in the acute stage. After 96 h following ischemia, the levels of all metabolites detected by 1H MRSI had decreased and the levels of NAA decline reflected the severity of histopathological damage. In conclusion, the regional metabolic differences could be assessed by 1H MRSI chronologically, and the depth of NAA decline reflected histopathological changes in the chronic stage.

N-Acetylaspartate, a marker of both cellular dysfunction and neuronal loss: its relevance to studies of acute brain injury

To evaluate the contribution of cellular dysfunction and neuronal loss to brain N-acetylaspartate (NAA) depletion, NAA was measured in brain tissue by HPLC and UV detection in rats subjected to cerebral injury, associated or not with cell death. When lesion was induced by intracarotid injection of microspheres, the fall in NAA was related to the degree of embolization and to the severity of brain oedema. When striatal lesion was induced by local injection of malonate, the larger the lesion volume, the higher the NAA depletion. However, reduction of brain oedema and striatal lesion by treatment with the lipophilic iron chelator dipyridyl (20 mg/kg, 1 h before and every 8 h after embolization) and the inducible nitric oxide synthase inhibitor aminoguanidine (100 mg/kg given 1 h before malonate and then every 9 h), respectively, failed to ameliorate the fall in NAA. Moreover, after systemic administration of 3-nitropropionic acid, a marked reversible fall in NAA striatal content was observed despite the lack of tissue necrosis. Overall results show that cellular dysfunction can cause higher reductions in NAA level than neuronal loss, thus making of NAA quantification a potential tool for visualizing the penumbra area in stroke patients.

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.