Extracellular N-acetylaspartate in the rat brain: in vivo determination of basal levels and changes evoked by high K+

Cellular and molecular mechanisms of aspartoacylase and its role in Canavan disease

Canavan disease is an autosomal recessive and lethal neurological disorder, characterized by the spongy degeneration of the white matter in the brain. The disease is caused by a deficiency of the cytosolic aspartoacylase (ASPA) enzyme, which catalyzes the hydrolysis of N-acetyl-aspartate (NAA), an abundant brain metabolite, into aspartate and acetate. On the physiological level, the mechanism of pathogenicity remains somewhat obscure, with multiple, not mutually exclusive, suggested hypotheses. At the molecular level, recent studies have shown that most disease linked ASPA gene variants lead to a structural destabilization and subsequent proteasomal degradation of the ASPA protein variants, and accordingly Canavan disease should in general be considered a protein misfolding disorder. Here, we comprehensively summarize the molecular and cell biology of ASPA, with a particular focus on disease-linked gene variants and the pathophysiology of Canavan disease. We highlight the importance of high-throughput technologies and computational prediction tools for making genotype-phenotype predictions as we await the results of ongoing trials with gene therapy for Canavan disease.

Rat Brain Global Ischemia-Induced Diffusion Changes Revisited: Biophysical Modeling of the Water and NAA MR "Diffusion Signal"

PURPOSE

To assess changes in intracellular diffusion as a mechanism for the reduction in water ADC that accompanies brain injury. Using NAA as a marker of neuronal cytoplasmic diffusion, NAA diffusion was measured before and after global ischemia (immediately postmortem) in the female Sprague-Dawley rat.

METHODS

Diffusion-weighted PRESS spectra, with diffusion encoding in a single direction, were acquired from large voxels of rat brain gray matter in vivo and postischemia employing either pairs of pulsed half-sine-shaped gradients (in vivo and postischemia, b_max  = 19 ms/μm² ) or sinusoidal oscillating gradients (in vivo only) with frequencies of 99.2-250 Hz. A 2D randomly oriented cylinder (neurite) model gave estimates of longitudinal and transverse diffusivities (D_L and D_T , respectively). In this model, D_L represents the "free" diffusivity of NAA, whereas D_T reflects highly restricted diffusion. Using oscillating gradients, the frequency dependence of D_T [D_T (ω)] gave estimates of the cylinder (axon/dendrite) radius.

RESULTS

A 10% decrease in D_L,NAA followed global ischemia, dropping from 0.391 ± 0.012 μm² /ms to 0.350 ± 0.009 μm² /ms. Modeling D_T,NAA (ω) provided an estimate of the neurite radius of 1.0 ± 0.6 μm.

CONCLUSION

Whereas the increase in apparent intraneuronal viscosity suggested by changes in D_L,NAA may contribute to the overall reduction in water ADC associated with brain injury, it is not sufficient to be the sole explanation. Estimates of neurite radius based on D_T (ω) were consistent with literature values.

Brain Metabolite, N-Acetylaspartate Is a Potent Protein Aggregation Inhibitor

Deposition of toxic protein inclusions is a common hallmark of many neurodegenerative disorders including Alzheimer's disease, Parkinson disease etc. N-acetylaspartate (NAA) is an important brain metabolite whose levels got altered under various neurodegenerative conditions. Indeed, NAA has been a widely accepted biological marker for various neurological disorders. We have also reported that NAA is a protein stabilizer. In the present communication, we investigated the role of NAA in modulating the aggregation propensity on two model proteins (carbonic anhydrase and catalase). We discovered that NAA suppresses protein aggregation and could solubilize preformed aggregates.

Neurochemical Modifications in the Hippocampus, Cortex and Hypothalamus of Mice Exposed to Long-Term High-Fat Diet

Metabolic syndrome and diabetes impact brain function and metabolism. While it is well established that rodents exposed to diets rich in saturated fat develop brain dysfunction, contrasting results abound in the literature, likely as result of exposure to different high-fat diet (HFD) compositions and for varied periods of time. In the present study, we investigated alterations of hippocampal-dependent spatial memory by measuring Y-maze spontaneous alternation, metabolic profiles of the hippocampus, cortex and hypothalamus by ¹H magnetic resonance spectroscopy (MRS), and levels of proteins specific to synaptic and glial compartments in mice exposed for 6 months to different amounts of fat (10, 45, or 60% of total energy intake). Increasing the dietary amount of fat from 10 to 45% or 60% resulted in obesity accompanied by increased leptin, fasting blood glucose and insulin, and reduced glucose tolerance. In comparison to controls (10%-fat), only mice fed the 60%-fat diet showed increased fed glycemia, as well as plasma corticosterone that has a major impact on brain function. HFD-induced metabolic profile modifications measured by ¹H MRS were observed across the three brain areas in mice exposed to 60%- but not 45%-fat diet, while both HFD groups displayed impaired hippocampal-dependent memory. HFD also affected systems involved in neuro- or gliotransmission in the hippocampus. Namely, relative to controls, 60%-fat-fed mice showed reduced SNAP-25, PSD-95 and syntaxin-4 immunoreactivity, while 45%-fat-fed mice showed reduced gephyrin and syntaxin-4 immunoreactivity. For both HFD levels, reductions of the vesicular glutamate transporter vGlut1 and levels of the vesicular GABA transporter were observed in the hippocampus and hypothalamus, relative to controls. Immunoreactivity against GFAP and/or Iba-1 in the hypothalamus was higher in mice exposed to HFD than controls, suggesting occurrence of gliosis. We conclude that different levels of dietary fat result in distinct neurochemical alterations in the brain.

Newfound effect of N-acetylaspartate in preventing and reversing aggregation of amyloid-beta in vitro

Although N-acetylaspartate (NAA) has long been recognized as the most abundant amino acid in neurons by far, its primary role has remained a mystery. Based on its unique tertiary structure, we explored the potential of NAA to modulate aggregation of amyloid-beta (Aβ) peptide 1-42 via multiple corroborating aggregation assays along with electron microscopy. Thioflavin-T fluorescence assay demonstrated that at physiological concentrations, NAA substantially inhibited the initiation of Aβ fibril formation. In addition, NAA added after 25 min of Aβ aggregation was shown to break up preformed fibrils. Electron microscopy analysis confirmed the absence of mature fibrils following NAA treatment. Furthermore, fluorescence correlation spectroscopy and dynamic light scattering measurements confirmed significant reductions in Aβ fibril hydrodynamic radius following treatment with NAA. These results suggest that physiological levels of NAA could play an important role in controlling Aβ aggregation in vivo where they are both found in the same neuronal compartments.

Increasing N-acetylaspartate in the Brain during Postnatal Myelination Does Not Cause the CNS Pathologies of Canavan Disease

Canavan disease is caused by mutations in the gene encoding aspartoacylase (ASPA), a deacetylase that catabolizes N-acetylaspartate (NAA). The precise involvement of elevated NAA in the pathogenesis of Canavan disease is an ongoing debate. In the present study, we tested the effects of elevated NAA in the brain during postnatal development. Mice were administered high doses of the hydrophobic methyl ester of NAA (M-NAA) twice daily starting on day 7 after birth. This treatment increased NAA levels in the brain to those observed in the brains of Nur7 mice, an established model of Canavan disease. We evaluated various serological parameters, oxidative stress, inflammatory and neurodegeneration markers and the results showed that there were no pathological alterations in any measure with increased brain NAA levels. We examined oxidative stress markers, malondialdehyde content (indicator of lipid peroxidation), expression of NADPH oxidase and nuclear translocation of the stress-responsive transcription factor nuclear factor (erythroid-derived 2)-like 2 (NRF-2) in brain. We also examined additional pathological markers by immunohistochemistry and the expression of activated caspase-3 and interleukin-6 by Western blot. None of the markers were increased in the brains of M-NAA treated mice, and no vacuoles were observed in any brain region. These results show that ASPA expression prevents the pathologies associated with excessive NAA concentrations in the brain during postnatal myelination. We hypothesize that the pathogenesis of Canavan disease involves not only disrupted NAA metabolism, but also excessive NAA related signaling processes in oligodendrocytes that have not been fully determined and we discuss some of the potential mechanisms.

Stimulation-induced transient changes in neuronal activity, blood flow and N-acetylaspartate content in rat prefrontal cortex: a chemogenetic fMRS-BOLD study

Brain activation studies in humans have shown the dynamic nature of neuronal N-acetylaspartate (NAA) and N-acetylaspartylglutamate (NAAG) based on changes in their MRS signals in response to stimulation. These studies demonstrated that upon visual stimulation there was a focal increase in cerebral blood flow (CBF) and a decrease in NAA or in the total of NAA and NAAG signals in the visual cortex, and that these changes were reversed upon cessation of stimulation. In the present study we have developed an animal model in order to explore the relationships between brain stimulation, neuronal activity, CBF and NAA. We use "designer receptor exclusively activated by designer drugs" (DREADDs) technology for site-specific neural activation, a local field potential electrophysiological method for measurement of changes in the rate of neuronal activity, functional MRS for measurement of changes in NAA and a blood oxygenation level-dependent (BOLD) MR technique for evaluating changes in CBF. We show that stimulation of the rat prefrontal cortex using DREADDs results in the following: (i) an increase in level of neuronal activity; (ii) an increase in BOLD and (iii) a decrease in the NAA signal. These findings show for the first time the tightly coupled relationships between stimulation, neuron activity, CBF and NAA dynamics in brain, and also provide the first demonstration of the novel inverse stimulation-NAA phenomenon in an animal model.

Whole-Brain N-Acetylaspartate Concentration Is Preserved during Mild Hypercapnia Challenge

BACKGROUND AND PURPOSE

Although NAA is often used as a marker of neuronal health and integrity in neurologic disorders, its normal response to physiologic challenge is not well-established and its changes are almost always attributed exclusively to brain pathology. The purpose of this study was to test the hypothesis that the neuronal cell marker NAA, often used to assess neuronal health and integrity in neurologic disorders, is not confounded by (possibly transient) physiologic changes. Therefore, its decline, when observed by using (1)H-MR spectroscopy, can almost always be attributed exclusively to brain pathology.

MATERIALS AND METHODS

Twelve healthy young male adults underwent a transient hypercapnia challenge (breathing 5% CO2 air mixture), a potent vasodilator known to cause a substantial increase in CBF and venous oxygenation. We evaluated their whole-brain NAA by using nonlocalizing proton MR spectroscopy, venous oxygenation with T2-relaxation under spin-tagging MR imaging, CBF with pseudocontinuous arterial spin-labeling, and the cerebral metabolic rate of oxygen, during normocapnia (breathing room air) and hypercapnia.

RESULTS

There was insignificant whole-brain NAA change (P = .88) from normocapnia to hypercapnia and back to normocapnia in this cohort, as opposed to highly significant increases: 28.0 ± 10.3% in venous oxygenation and 49.7 ± 16.6% in global CBF (P < 10(-4)); and a 6.4 ± 10.9% decrease in the global cerebral metabolic rate of oxygen (P = .04).

CONCLUSIONS

Stable whole-brain NAA during normocapnia and hypercapnia, despite significant global CBF and cerebral metabolic rate of oxygen changes, supports the hypothesis that global NAA changes are insensitive to transient physiology. Therefore, when observed, they most likely reflect underlying pathology resulting from neuronal cell integrity/viability changes, instead of a response to physiologic changes.

Extracellular N-Acetylaspartate in Human Traumatic Brain Injury

N-acetylaspartate (NAA) is an amino acid derivative primarily located in the neurons of the adult brain. The function of NAA is incompletely understood. Decrease in brain tissue NAA is presently considered symptomatic and a potential biomarker of acute and chronic neuropathological conditions. The aim of this study was to use microdialysis to investigate the behavior of extracellular NAA (eNAA) levels after traumatic brain injury (TBI). Sampling for this study was performed using cerebral microdialysis catheters (M Dialysis 71) perfused at 0.3 μL/min. Extracellular NAA was measured in microdialysates by high-performance liquid chromatography in 30 patients with severe TBI and for comparison, in radiographically “normal” areas of brain in six non-TBI neurosurgical patients. We established a detailed temporal eNAA profile in eight of the severe TBI patients. Microdialysate concentrations of glucose, lactate, pyruvate, glutamate, and glycerol were measured on an ISCUS clinical microdialysis analyzer. Here, we show that the temporal profile of microdialysate eNAA was characterized by highest levels in the earliest time-points post-injury, followed by a steady decline; beyond 70 h post-injury, average levels were 40% lower than those measured in non-TBI patients. There was a significant inverse correlation between concentrations of eNAA and pyruvate; eNAA showed significant positive correlations with glycerol and the lactate/pyruvate (L/P) ratio measured in microdialysates. The results of this on-going study suggest that changes in eNAA after TBI relate to the release of intracellular components, possibly due to neuronal death or injury, as well as to adverse brain energy metabolism.

Automated whole-brain N-acetylaspartate proton MRS quantification

Concentration of the neuronal marker, N-acetylaspartate (NAA), a quantitative metric for the health and density of neurons, is currently obtained by integration of the manually defined peak in whole-head proton ((1) H)-MRS. Our goal was to develop a full spectral modeling approach for the automatic estimation of the whole-brain NAA concentration (WBNAA) and to compare the performance of this approach with a manual frequency-range peak integration approach previously employed. MRI and whole-head (1) H-MRS from 18 healthy young adults were examined. Non-localized, whole-head (1) H-MRS obtained at 3 T yielded the NAA peak area through both manually defined frequency-range integration and the new, full spectral simulation. The NAA peak area was converted into an absolute amount with phantom replacement and normalized for brain volume (segmented from T1 -weighted MRI) to yield WBNAA. A paired-sample t test was used to compare the means of the WBNAA paradigms and a likelihood ratio test used to compare their coefficients of variation. While the between-subject WBNAA means were nearly identical (12.8 ± 2.5 mm for integration, 12.8 ± 1.4 mm for spectral modeling), the latter's standard deviation was significantly smaller (by ~50%, p = 0.026). The within-subject variability was 11.7% (±1.3 mm) for integration versus 7.0% (±0.8 mm) for spectral modeling, i.e., a 40% improvement. The (quantifiable) quality of the modeling approach was high, as reflected by Cramer-Rao lower bounds below 0.1% and vanishingly small (experimental - fitted) residuals. Modeling of the whole-head (1) H-MRS increases WBNAA quantification reliability by reducing its variability, its susceptibility to operator bias and baseline roll, and by providing quality-control feedback. Together, these enhance the usefulness of the technique for monitoring the diffuse progression and treatment response of neurological disorders.

Long-term follow-up after gene therapy for canavan disease

Canavan disease is a hereditary leukodystrophy caused by mutations in the aspartoacylase gene (ASPA), leading to loss of enzyme activity and increased concentrations of the substrate N-acetyl-aspartate (NAA) in the brain. Accumulation of NAA results in spongiform degeneration of white matter and severe impairment of psychomotor development. The goal of this prospective cohort study was to assess long-term safety and preliminary efficacy measures after gene therapy with an adeno-associated viral vector carrying the ASPA gene (AAV2-ASPA). Using noninvasive magnetic resonance imaging and standardized clinical rating scales, we observed Canavan disease in 28 patients, with a subset of 13 patients being treated with AAV2-ASPA. Each patient received 9 × 10(11) vector genomes via intraparenchymal delivery at six brain infusion sites. Safety data collected over a minimum 5-year follow-up period showed a lack of long-term adverse events related to the AAV2 vector. Posttreatment effects were analyzed using a generalized linear mixed model, which showed changes in predefined surrogate markers of disease progression and clinical assessment subscores. AAV2-ASPA gene therapy resulted in a decrease in elevated NAA in the brain and slowed progression of brain atrophy, with some improvement in seizure frequency and with stabilization of overall clinical status.

Metabolic changes in concussed American football players during the acute and chronic post-injury phases

Background

Despite negative neuroimaging findings many athletes display neurophysiological alterations and post-concussion symptoms that may be attributable to neurometabolic alterations.

Methods

The present study investigated the effects of sports concussion on brain metabolism using ¹H-MR Spectroscopy by comparing a group of 10 non-concussed athletes with a group of 10 concussed athletes of the same age (mean: 22.5 years) and education (mean: 16 years) within both the acute and chronic post-injury phases. All athletes were scanned 1-6 days post-concussion and again 6-months later in a 3T Siemens MRI.

Results

Concussed athletes demonstrated neurometabolic impairment in prefrontal and motor (M1) cortices in the acute phase where NAA:Cr levels remained depressed relative to controls. There was some recovery observed in the chronic phase where Glu:Cr levels returned to those of control athletes; however, there was a pathological increase of m-I:Cr levels in M1 that was only present in the chronic phase.

Conclusions

These results confirm cortical neurometabolic changes in the acute post-concussion phase as well as recovery and continued metabolic abnormalities in the chronic phase. The results indicate that complex pathophysiological processes differ depending on the post-injury phase and the neurometabolite in question.

Upregulation of N-acetylaspartic acid induces oxidative stress to contribute in disease pathophysiology

N-acetylaspartic acid (NAA) is predominantly present in brain and also present in lower amount in peripheral organs. The role of NAA in pathophysiology is poorly understood. Therefore the review was aimed to understand contribution of NAA in disease process. Amniotic fluid of mothers with Canavan disease (CD) fetus and patients with the disease show increased levels of NAA. Increase of this pathway is also reported in Parkinson's disease and type 2 diabetes. In HIV-related dementia, NAA is affected. Recent studies suggest that upregulation of NAA leads to oxidative stress including upregulation of nitric oxide and reducing potential antioxidants. NAA also leads to physiological abnormalities including walking disorder. These changes suggest that NAA contributes in disease pathophysiology.

Overview of brain microdialysis

The technique of microdialysis enables sampling and collecting of small-molecular-weight substances from the interstitial space. It is a widely used method in neuroscience and is one of the few techniques available that permits quantification of neurotransmitters, peptides, and hormones in the behaving animal. More recently, it has been used in tissue preparations for quantification of neurotransmitter release. This unit provides a brief review of the history of microdialysis and its general application in the neurosciences. The authors review the theoretical principles underlying the microdialysis process, methods available for estimating extracellular concentration from dialysis samples (i.e., relative recovery), the various factors that affect the estimate of in vivo relative recovery, and the importance of determining in vivo relative recovery to data interpretation. Several areas of special note, including impact of tissue trauma on the interpretation of microdialysis results, are discussed. Step-by-step instructions for the planning and execution of conventional and quantitative microdialysis experiments are provided.

Upregulation of N-acetylaspartic acid alters inflammation, transcription and contractile associated protein levels in the stomach and smooth muscle contractility

N-acetylaspartic acid (NAA) is converted into aspartate and acetate by aspartoacylase. Abnormal levels of the enzyme leads to accumulation of NAA and these changes have been observed in Canavan disease and type 2 diabetes. How upregulation of NAA affect the gastrointestine protein levels and the function is not known. Incubation of rat stomach tissue with NAA 1.5 mM, 1.5 microM and 1.5 nM induced inflammatory agents TNFalpha, p38MAPK, iNOS, PKC, COX2 and ICAM3; transcription factors phospho-NF-kBp65, cjun and cfos; contractile proteins MLCK and phospho MLC; and calcium channel alpha1C and calcium channel, voltage-dependent, beta 3 subunit compared to their respective control. Incubation of circular smooth muscle cells with the above doses of NAA induced contractility compared to the control. These studies suggest that NAA alters proteins levels and smooth muscle contractility and these changes likely to contribute to gastrointestinal disorder seen in these diseases.

In Vitro Studies on the Putative Function of N-acetylaspartate as an Osmoregulator

Efflux and tissue content of N-acetylaspartate (NAA) and amino acids were evaluated from cultured and acutely prepared hippocampal slices in response to changes in osmolarity. The osmoregulator taurine, but not NAA, was lost from both types of slices after moderate reductions in extracellular osmolarity (-60 mOsm) for 10-48 h. Hypoosmotic shock (-166 mOsm) for 5 min resulted in unselective efflux of several amino acids from acutely prepared slices. Notably, the efflux of taurine, but not NAA, was prominent also after the shock. Efflux of NAA was markedly enhanced by NMDA and high K(+), in particular after the stimulation period. The high K(+)-mediated efflux was decreased by high extracellular osmolarity and a NMDA-receptor antagonist. The results indicate that NAA efflux can be induced by a sudden non-physiological decrease in extracellular osmolarity but not by prolonged more moderate changes in osmolarity. The mechanisms behind the efflux of NAA by high K(+) are complex and may involve both swelling and activation of NMDA-receptors.

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.

MR spectroscopy: its potential role for drug development for the treatment of psychiatric diseases

Magnetic resonance spectroscopy (MRS) is likely in the near future to play a key role in the process of drug discovery and evaluation. As the pharmaceutical industry seeks biochemical markers of drug delivery, efficacy and toxicity, this non-invasive technique offers numerous ways to study adults and children repeatedly and without ionizing radiation. In this article, we survey an array of the information that MRS offers about neurochemistry in general and psychiatric disorders and their treatment in particular. We also present growing evidence of glial abnormalities in neuropsychiatric disorders and discuss what MRS is contributing to that line of investigation. The third major direction of this article is the discussion of where MRS techniques are headed and how those new techniques can contribute to studies of mechanisms of psychiatric disease and drug discovery.

Novel role for aspartoacylase in regulation of BDNF and timing of postnatal oligodendrogenesis

Neuronal growth factors are thought to exert a significant degree of control over postnatal oligodendrogenesis, but mechanisms by which these factors coordinateoligodendrocyte development with the maturation of neural networks are poorly characterized. We present here a developmental analysis of aspartoacylase (Aspa)-null tremor rats and show a potential role for this hydrolytic enzyme in the regulation of a postnatal neurotrophic stimulus that impacts on early stages of oligodendrocyte differentiation. Abnormally high levels of brain-derived neurotrophic factor (BDNF) expression in the Aspa-null Tremor brain are associated with dysregulated oligodendrogenesis at a stage in development normally characterized by high levels of Aspa expression. BDNF promotes the survival of proliferating cells during the early stages of oligodendrocyte maturation in vitro, but seems to compromise the ability of these cells to populate the cortex in vivo. Aspartoacylase activity in oligodendrocytes is shown to provide for the negative regulation of BDNF in neurons, thereby determining the availability of a developmental stimulus via a mechanism that links oligodendroglial differentiation with neuronal maturation.

Assessment of glutamate and glutamine contribution to in vivo N-acetylaspartate quantification in human brain by (1)H-magnetic resonance spectroscopy

N-Acetylaspartate (NAA) is one of the most important metabolites detectable by brain (1)H-MRS being considered an index of neuronal integrity. At the low magnetic field used in most clinical settings beta,gamma-glutamate/glutamine (Glx) resonances are very close and partially overlap the methyl-NAA resonance interfering with NAA quantification especially at low TE and in the presence of increased Glx signals. NAA overestimation due to Glx on a set of model solutions containing NAA, glutamate, and glutamine in variable amounts was evaluated and the result tested in vivo in six healthy controls and five age- and sex-matched patients with hepatic encephalopathy (HE), the latter having an increased Glx content. A method to assess in vivo the NAA overestimation caused by Glx is proposed. A perfect match was obtained between the assessment of Glx contamination on the NAA of healthy controls and that obtained on the model solutions. However, a substantial difference in NAA overestimation was found between controls and HE patients that cannot be explained by our model. An interpretative hypothesis is provided.

Extracellular N-acetylaspartate depletion in traumatic brain injury

N-Acetylaspartate (NAA) is almost exclusively localized in neurons in the adult brain and is present in high concentration in the CNS. It can be measured by proton magnetic resonance spectroscopy and is seen as a marker of neuronal damage and death. NMR spectroscopy and animal models have shown NAA depletion to occur in various types of chronic and acute brain injury. We investigated 19 patients with traumatic brain injury (TBI). Microdialysis was utilized to recover NAA, lactate, pyruvate, glycerol and glutamate, at 12-h intervals. These markers were correlated with survival and a 6-month Glasgow Outcome Score. Eleven patients died and eight survived. A linear mixed model analysis showed a significant effect of outcome and of the interaction between time of injury and outcome on NAA levels (p = 0.009 and p = 0.004, respectively). Overall, extracellular NAA was 34% lower in non-survivors. A significant non-recoverable fall was observed in this group from day 4 onwards, with a concomitant rise in lactate-pyruvate ratio and glycerol. These results suggest that mitochondrial dysfunction is a significant contributor to poor outcome following TBI and propose extracellular NAA as a potential marker for monitoring interventions aimed at preserving mitochondrial function.

Increased N-acetylaspartate in rat striatum following long-term administration of haloperidol

N-acetylaspartate (NAA) is present in high concentrations in the CNS and is found primarily in neurons. NAA is considered to be a marker of neuronal viability. Numerous magnetic resonance spectroscopy (MRS) and postmortem studies have shown reductions of NAA in different brain regions in schizophrenia. Most of these studies involved patients chronically treated with antipsychotic drugs. However, the effect of chronic antipsychotic treatment on NAA remains unclear. In the present study, we measured NAA in brain tissue taken from 43 male Long-Evans rats receiving 28.5 mg/kg haloperidol decanoate i.m. every 3 weeks for 24 weeks and from 21 controls administered with vehicle. Determination of tissue concentrations of NAA was achieved by HPLC of sections of frozen tissue from several brain regions with relevance to schizophrenia. Chronic administration of haloperidol was associated with a significant increase (+23%) in NAA in the striatum (p<0.05) when compared to controls, with no significant changes in the other regions investigated (frontal and temporal cortex, thalamus, hippocampus, amygdala, and nucleus accumbens). NAA appears to be selectively increased in the striatum of rats chronically receiving haloperidol. This increase may reflect a hyperfunction of striatal neurons and relate to the reported increase in somal size of these cells and/or the increase in synaptic density seen in this region following antipsychotic administration. The lack of effect in other regions indicates that the well-documented NAA deficits seen in chronically treated schizophrenia patients is not an effect of antipsychotic medication and may in fact be related to the disease process.

Transport characteristics of N ‐acetyl‐ l ‐aspartate in rat astrocytes: involvement of sodium‐coupled high‐affinity carboxylate transporter NaC3/NaDC3‐mediated transport system

We investigated in the present study the transport characteristics of N-acetyl-L-aspartate in primary cultures of astrocytes from rat cerebral cortex and the involvement of NA+-coupled high-affinity carboxylate transporter NaC3 (formerly known as NaDC3) responsible for N-acetyl-L-aspartate transport. N-acetyl-L-aspartate transport was NA+-dependent and saturable with a Michaelis-Menten constant (Km) of approximately 110 microm. NA+-activation kinetics revealed that the NA+ to-N-acetyl-L-aspartate stoichiometry was 3 : 1 and concentration of Na+ necessary for half-maximal transport (KNA m) was 70 mm. NA+-dependent N-acetyl-L-aspartate transport was competitively inhibited by succinate with an inhibitory constant (Ki) of 14.7 microm, which was comparable to the Km value of NA+-dependent succinate transport (29.4 microm). L-aspartate also inhibited NA+-dependent [14C]N-acetyl-L-aspartate transport with relatively low affinity (Ki = 2.2 mm), whereas N-acetyl-L-aspartate was not able to inhibit NA+-dependent aspartate transport in astrocytes. In addition, Li+ was found to have a significant inhibitory effect on the NA+-dependent N-acetyl-L-aspartate transport in a concentration-dependent manner. Furthermore, RT-PCR and western blot analyses revealed that NaC3 is expressed in primary cultures of astrocytes. Taken collectively, these results indicate that NaC3 expressed in rat cerebrocortical astrocytes is responsible for NA+-dependent N-acetyl-L-aspartate transport. This transporter is likely to be an essential prerequisite for the metabolic role of N-acetyl-L-aspartate in the process of myelination.

Effects of AAV-2-mediated aspartoacylase gene transfer in the tremor rat model of Canavan disease

The tremor rat is a spontaneous epilepsy model with a seizure phenotype caused by a deletion in the aspartoacylase (ASPA) gene. The absence of ASPA expression in these animals results in undetectable levels of enzyme activity and the accumulation of the substrate N-acetyl-aspartate (NAA) in brain, leading to generalized myelin vacuolation and severe motor and cognitive impairment. In support of human gene therapy for CD, recombinant adeno-associated viral vector (AAV-2) expressing ASPA was stereotactically delivered to the tremor rat brain and effects on the mutant phenotype were measured. AAV-ASPA gene transfer resulted in elevated aspartoacylase bioactivity compared to untreated mutant animals and elicited a significant decrease in the pathologically elevated whole-brain NAA levels. Assessment of motor function via quantitative rotorod testing demonstrated that rats injected with AAV-ASPA significantly improved on tests of balance and coordinated locomotion compared to animals receiving control vectors. This study provides evidence that AAV-2-mediated aspartoacylase gene transfer to the brain improves biochemical and behavioral deficits in tremor rat mutants (tm/tm) and supports the rationale of human gene transfer for Canavan disease.

NMDA-receptor mediated efflux of N-acetylaspartate: physiological and/or pathological importance?

N-Acetylaspartate (NAA) is a largely neuron specific dianionic amino acid present in high concentration in vertebrate brain. Many fundamental questions concerning N-acetylaspartate in brain remain unanswered. One such issue is the predominantly neuronal synthesis and largely glial catabolism which implies the existence of a regulated efflux from neurons. Here we show that transient (5 min) NMDA-receptor activation (60 microM) induces a long lasting Ca2+ -dependent efflux of N-acetylaspartate from organotypic slices of rat hippocampus. The NMDA-receptor stimulated efflux was unaffected by hyper-osmotic conditions (120 mM sucrose) and no efflux of N-acetylaspartate was evoked by high K+ -depolarization (50 mM) or kainate (300 microM). These results indicate that the efflux induced by NMDA is not related directly to either cell swelling or depolarization but is coupled to Ca2+ -influx via the NMDA-receptor. The efflux of N-acetylaspartate persisted at least 20 min after the omission of NMDA, similar to the efflux of the organic anions glutathione and phosphoethanolamine. The efflux of taurine and hypotaurine was also stimulated by NMDA but returned more quickly to basal levels. The NMDA-receptor stimulated efflux of N-acetylaspartate, glutathione, phosphoethanolamine, taurine and hypotaurine correlated with delayed nerve cell death measured 24 h after the transient NMDA-receptor stimulation. However, exogenous administration of high concentrations of N-acetylaspartate to the culture medium was non-toxic. The results suggest that Ca2+ -influx via the NMDA-receptor regulates the efflux of N-acetylaspartate from neurons which may have both physiological and pathological importance.

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.

Proton magnetic resonance spectroscopy of the frontal lobe in schizophrenics: a critical review of the methodology

Schizophrenic patients undergoing proton magnetic resonance spectroscopy show alterations in N-acetyl aspartate levels in several brain regions, indicating neuronal dysfunction. The present review focuses on the main proton magnetic resonance spectroscopy studies in the frontal lobe of schizophrenics. A MEDLINE search, from 1991 to March 2004, was carried out using the key-words spectroscopy and schizophrenia and proton and frontal. In addition, articles cited in the reference list of the studies obtained through MEDLINE were included. As a result, 27 articles were selected. The results were inconsistent, 19 papers reporting changes in the N-acetyl aspartate levels, while 8 reported no change. Methodological analysis led to the conclusion that the discrepancy may be due the following factors: (i) number of participants; (ii) variation in the clinical and demographic characteristics of the groups; (iii) little standardization of the acquisition parameters of spectroscopy. Overall, studies that fulfill strict methodological criteria show N-acetyl aspartate decrease in the frontal lobe of male schizophrenics.

Treatment of rats with antipsychotic drugs: lack of an effect on brain N-acetyl aspartate levels

BACKGROUND

Proton magnetic resonance spectroscopy (1H-MRS) studies of schizophrenia suggest an effect of the disease or of antipsychotic medications on brain N-acetyl aspartate (NAA), a marker of neuronal viability. We studied in the rat the effect of antipsychotic drugs on NAA in several brain regions where NAA reductions have been reported in chronically medicated patients with schizophrenia.

METHODS

Three groups of nine rats each were treated with haloperidol (6 mg/kg/day), clozapine (70 mg/kg/day) and vehicle for 6 weeks and were sacrificed. Concentrations of NAA were determined by high-performance liquid chromatography (HPLC) from the following brain regions: cortex, striatum, thalamus, hippocampus and cerebellum.

RESULTS

Mixed-factorial ANOVA of NAA concentrations revealed no significant effect of drug group [F(2, 24) = 0.034; p = 0.966] or a group by brain region interaction [F(8, 44) = 0.841; p = 0.572]. There was a significant main effect of region [F(4, 21) = 6.104; p = 0.002] with higher NAA in the cortex.

CONCLUSIONS

These results are consistent with the only other study of the effect of typical and atypical antipsychotic drugs on NAA in the rat brain. The well-documented lower NAA in chronically treated schizophrenia patients is probably not a simple effect of antipsychotic medications.

Magnetic resonance spectroscopy in Alzheimer's disease: focus on N-acetylaspartate

This paper reviews published post-mortem brain and in-vivo proton magnetic resonance spectroscopy (1H-MRS) studies in Alzheimer's disease (AD) and focuses on the emerging role of N-acetylaspartate (NAA) as a prognostic marker of neuronal function. Post-mortem brain studies have reported significantly lower NAA levels in AD brains than in control brains, and some have correlated the low levels with neuropathological findings (i.e. amyloid plaques and neurofibrillary tangles). Similarly, almost all published in-vivo studies have reported lower NAA levels in AD patients compared to elderly controls. While some studies have found changes in metabolite levels that were considered useful for the diagnosis of AD, most have found that 1H-MRS provided little or no advantages over other, more common diagnostic tools. Instead, recent studies in AD and other neuropsychiatric disorders suggest that NAA may be more useful as a prognostic marker for monitoring neurodegeneration, stabilization, or improvement, and for evaluating therapeutic response to novel drugs.

Overview of microdialysis

The technique of microdialysis enables the monitoring of neurotransmitters and other molecules in the extracellular environment. This method has undergone several modifications and is now widely used for sampling and quantitating neurotransmitters, neuropeptides, and hormones in the brain and periphery. This unit describes the principles of conventional and quantitative microdialysis as well as strategies in designing a dialysis experiment. It establishes the groundwork for the basic techniques of preparation, conduct, and analysis of dialysis experiments in rodents and subhuman primates. Although the methods described are those used for monitoring CNS function, they can be easily applied with minor modification to other organ systems.

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.

The effect of N-acetyl-L-aspartic acid dilithium salt on dopamine release and synthesis in the rat striatum in vivo

The effect of the dilithium salt of N-acetyl-L-aspartic acid on release and synthesis of dopamine in the striatum was investigated using microdialysis in freely moving rats. Intrastriatal infusion of 1 mM N-methyl-D-aspartate, an NMDA receptor agonist, augmented extracellular dopamine to 215% of baseline, while 1 mM dilithium N-acetyl-L-aspartate increased dopamine release to 190% of baseline in rat striatum. Infusion of DL-2-amino-5-phosphonopentanoic acid, a competitive NMDA receptor antagonist, prior to infusion of dilithium N-acetyl-L-aspartate did not significantly alter basal levels of dopamine, but reversed the dilithium N-acetyl-L-aspartate-evoked elevation in extracellular dopamine. Intrastriatal perfusion with 6-cyano-7-nitroquinoxaline-2,3-dione, an AMPA/kainate receptors antagonist, altered neither basal levels of dopamine nor dilithium N-acetylaspartate-induced dopamine release. When the striatum was continuously perfused with the inhibitor of L-aromatic amino acid decarboxylase, 3-hydroxybenzylhydrazine dihydrochloride (100 microM), both dilithium N-acetylaspartate and NMDA added to the perfusate increased extracellular 3,4-dihydroxyphenyl-L-alanine, reflecting the effect of the compounds on the biosynthesis of dopamine. The data suggest that availability of dilithium N-acetyl-L-aspartate to activate dopamine turnover and release in the rat striatum may be mediated by presynaptic NMDA heteroreceptors located at dopaminergic neurons.

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.

Canavan's spongiform leukodystrophy: a clinical anatomy of a genetic metabolic CNS disease

Canavan disease (CD) is a globally distributed early-onset leukodystrophy. It is genetic in nature, and results from an autosomally inherited recessive trait that is characterized by loss of the axon's myelin sheath while leaving the axons intact, and spongiform degeneration especially in white matter. There is also a buildup of N-acetyl-L-aspartate (NAA) in brain, as well as NAA acidemia and NAA aciduria. The cause of the altered NAA metabolism has been traced to several mutations in the gene for the production of aspartoacylase, located on chromosome 17, which is the primary enzyme involved in the catabolic metabolism of NAA. In this review, an attempt is made to correlate the change in NAA metabolism that results from the genetic defects in CD with the processes involved in the development of the CD syndrome. In addition, present efforts to counter the results of the genetic defects in this disease are also considered.

Extracellular N-acetyl-aspartate as a biochemical marker of the severity of neuronal damage following experimental acute traumatic brain injury

We evaluated the acute changes in interstitial and whole brain N-acetyl-aspartate (NAA) measured by high-performance liquid chromatography in animal models of isolated traumatic brain injury (TBI) and TBI combined with secondary insult (hypotension-hypoxia [HH]). The Marmarou impact-acceleration model was used. Four groups were studied: (1) sham-operated control, (2) TBI alone (TBI 500 gm, 2 m), (3) TBI plus 30 min of hypoxia (PaO2, approximately 40 mm Hg) and hypotension (mean arterial blood pressure, approximately 40 mm Hg) (THH), and (4) HH alone. The baseline value for dialysate NAA (NAAd) in the rats was 8.17+/-1 microM. No significant difference between groups was found for this baseline value. The TBI group had a modest (100%) transient increase in NAAd after isolated TBI. The HH group had a transient (500%) increase in NAAd at 1 h, sustained for 2 h. In the THH group, there was a persistent increase in NAAd (800%) that peaked at 2.5 h. The whole brain NAA (NAAw) concentration in controls was 8.5+/-0.5 mmol/kg wet weight. There was no significant difference between TBI and controls; however, there was a significant decrease in NAAw in the THH and HH group compared to controls. Thus, in this animal model of TBI and TBI with secondary insult, we found that persistent, marked elevation in NAA is associated with TBI and secondary ischemic/hypoxic insult, but not with isolated TBI alone.

Astroglia contain a specific transport mechanism for N-acetyl-L-aspartate

N-Acetylaspartate (NAA) is the second most abundant amino acid in the adult brain. It is located and synthesized in neurons and probably degraded in the glia compartment, but the transport mechanisms are unknown. Rat primary neuron and astrocyte cell cultures were exposed to the L isomer of [3H]NAA and demonstrated concentration-dependent uptake of [3H]NAA with a Km approximately 80 microM. However, Vmax was 23+/-6.4 pmol/mg of protein/min in astrocytes but only 1.13+/-0.4 pmol/mg of protein/min in neurons. The fact that neuron cultures contain 3-5% astrocytes suggests that the uptake mechanism is expressed only in glial cells. The astrocyte uptake was temperature and sodium chloride dependent and specific for L-NAA. The affinity for structural analogues was (IC50 in mM) as follows: L-NAA (0.12) > N-acetylaspartylglutamate (0.4) > N-acetylglutamate (0.42) > L-aspartate (>1) > L-glutamate (>1) > or = DL-threo-beta-hydroxyaspartate > N-acetyl-L-histidine. The naturally occurring amino acids showed no inhibitory effect at 1 mM. The glutamate transport blocker trans-pyrrolidine-2,4-dicarboxylate exhibited an IC50 of 0.57 mM, whereas another specific glutamate transport inhibitor, DL-threo-beta-hydroxyaspartate, had an IC50 of >1 mM. The experiments suggest that NAA transport in brain parenchyma occurs by a novel type of sodium-dependent carrier that is present only in glial cells.

Biological markers in the diagnosis and treatment of ALS

The care of patients with amyotrophic lateral sclerosis (ALS), which has classically focused on treatment of symptomatology, has now entered an encouraging new era of therapy targeted at the pathophysiology of the disease. However, an objective measure of disease progression and therapeutic response is sorely needed. Quantitative neuromuscular examinations, measurement of pulmonary function, disability scales, and even survival, are limited by variability due to a number of poorly controlled factors. Quantitative electromyography, positron emission tomography scanning, and magnetic cortical stimulation, provide potential objective indicators of disease progression, but require a large number of patients and a long observation period for adequate statistical power. We have examined the role of magnetic resonance spectroscopic imaging in detecting acute changes in motor cortical metabolism in response to riluzole therapy. N-acetylaspartate (NAA), the most prominent signal in proton spectra of normal brain, is a neuron-specific molecule. ALS patients were found to experience a significant increase in the NAA/creatine ratio within 3 weeks of initiation of riluzole therapy. As glutamate can trigger the generation of reactive oxygen species in neurons, we speculate that acute changes in NAA levels may reflect oxidative injury to mitochondria where NAA is synthesised. The advent of a useful test for upper motor neuron metabolic compromise may provide an objective, non-invasive, short duration measure with which to screen the efficacy of potential therapeutic agents for ALS.

Neuroprotective electrical stimulation of cerebellar fastigial nucleus attenuates expression of periinfarction depolarizing waves (PIDs) and inhibits cortical spreading depression

In rat, electrical stimulation of the cerebellar fastigial nucleus (FN) for 1 h reduces the volume of focal ischemic infarctions produced by occluding the middle cerebral artery (MCAO), even 10 days later. The mechanism by which this 'central neurogenic neuroprotection' salvages ischemic brain is not known but does not result from changes in cerebral perfusion. MCAO also triggers periodic periinfarction depolarizing waves (PIDs) in the ischemic penumbra, the territory of salvage. These may contribute to neuronal death and promote infarct expansion. Conceivably, FN stimulation, which can otherwise modify cortical excitability, may alter the development of PIDs. We investigated in anesthetized rats whether FN stimulation modifies PIDs expression and, if so, the threshold for evoking cortical spreading depression (CSD), a process sharing characteristics with PIDs and an index of cortical excitability. Stimulation of FN immediately or 72 h before MCAO decreased infarction volumes by approximately 45% (p<0.01), increased PID latency >10-fold, and decreased the number of PIDs by >50% (p<0.001). In normal rats, stimulation of FN increased the threshold current for eliciting CSD by 175% and slowed its propagation velocity by 35% (p<0.01 for each) immediately, but not 72 h, after FN stimulation. We conclude: FN stimulation elicits long-lasting suppression of PIDs in parallel with neuroprotection. However, PIDs suppression over time is unlikely to result from a major increase in cortical tolerance to depolarization and probably is not the principal mechanism of salvage.

N-Acetylaspartate distribution in rat brain striatum during acute brain ischemia

Brain N-acetylaspartate (NAA) can be quantified by in vivo proton magnetic resonance spectroscopy (1H-MRS) and is used in clinical settings as a marker of neuronal density. It is, however, uncertain whether the change in brain NAA content in acute stroke is reliably measured by 1H-MRS and how NAA is distributed within the ischemic area. Rats were exposed to middle cerebral artery occlusion. Preischemic values of [NAA] in striatum were 11 mmol/L by 1H-MRS and 8 mmol/kg by HPLC. The methods showed a comparable reduction during the 8 hours of ischemia. The interstitial level of [NAA] ([NAA]e) was determined by microdialysis using [3H]NAA to assess in vivo recovery. After induction of ischemia, [NAA]e increased linearly from 70 micromol/L to a peak level of 2 mmol/L after 2 to 3 hours before declining to 0.7 mmol/L at 7 hours. For comparison, [NAA]e was measured in striatum during global ischemia, revealing that [NAA]e increased linearly to 4 mmol/L after 3 hours and this level was maintained for the next 4 h. From the change in in vivo recovery of the interstitial space volume marker [14C]mannitol, the relative amount of NAA distributed in the interstitial space was calculated to be 0.2% of the total brain NAA during normal conditions and only 2 to 6% during ischemia. It was concluded that the majority of brain NAA is intracellularly located during ischemia despite large increases of interstitial [NAA]. Thus, MR quantification of NAA during acute ischemia reflects primarily changes in intracellular levels of NAA.

Hypoosmolarity induces an increase of extracellular N-acetylaspartate concentration in the rat striatum

We previously showed that extracellular levels of N-acetylaspartate (NAA) increase when a medium with reduced NaCl concentration is perfused through a microdialysis probe, and proposed that NAA may be released during hypoosmotic swelling. Here, we demonstrate that this effect is due to hypoosmolarity of the perfusion medium, and not to low NaCl. NAA changes in the dialysate were compared with those of taurine as the osmoregulatory role of this amino acid is established. Reduction of the NaCl concentration in the perfusion medium increased the dialysate levels of NAA and taurine, but this effect was abolished when NaCl was replaced by sucrose to maintain isosmolarity. The NAA response to hypoosmolarity was smaller than that of taurine, but it may still be important to neurons as NAA is predominantly neuronal in the mammalian CNS.

Is high extracellular glutamate the key to excitotoxicity in traumatic brain injury?

Traumatic brain injury (TBI) increases extracellular levels of the excitatory amino acid glutamate and aspartate, and N-methyl-D aspartate (NMDA)-receptor antagonists protect against experimental TBI. These two findings have led to the prevalent hypothesis that excitatory amino acid efflux is a major contributor to the development of neuronal damage subsequent to traumatic injury. However, as with stroke, the hypothesis that high extracellular glutamate is the key to excitotoxicity in TBI conflicts with important data. For example, the initial increase in extracellular glutamate is cleared within 5 min after moderate TBI, whereas antagonists of glutamate receptors and the so- called presynaptic glutamate release inhibitors remain effective when administered 30 min after insult. In this article, we argue that the current concept of excitotoxicity in TBI, centered on high extracellular glutamate, does not withstand scientific scrutiny. As alternatives to explain the beneficial actions of glutamate antagonists in experimental TBI, we propose abnormalities of glutamatergic neurotransmission, such as deficient Mg2+ block of NMDA-receptor ionophore complexes, and phenomena such as spreading depression, which requires activation of glutamate receptors and is detrimental to neurons in damaged/vulnerable brain regions. Finally, we introduce the notion that beneficial effects of glutamate receptor antagonists in experimental models of neurological disorders do not necessarily imply the occurrence of excitotoxic processes. Indeed, glutamate-receptor blockade may be protective by reducing the energy demand required to counterbalance Na+ influx associated with glutamatergic synaptic transmission. In other words, glutamate receptor antagonists (and blockers of voltage-gated Na+-channels) may help nervous tissue to cope with increased permeability of the cellular membrane to ions and reduced efficacy of Na+ extrusion, and thus prevent the decay of transmembrane ionic concentrations gradients.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.

In vivo microdialysis and gas chromatography/mass spectrometry for studies on release of N-acetylaspartlyglutamate and N-acetylaspartate in rat brain hypothalamus

Microdialysis and gas chromatography/mass spectrometry was used for the measurement of extracellular N-acetylaspartate (NAA) and N-acetylaspartylglutamate (NAAG) in rat hypothalamus. The sensitivity of the method for each of these compounds was approximately 5 pmol/30 microliters of dialysate. Baseline NAA concentrations in dialysate were estimated to be approximately 25 pmol/36 microliters, while that for NAAG was at or below the detection limit of 5 pmol/ 36 microliters. In vivo and in vitro calibrations of microdialysis probes showed that the recovery for NAA was approximately 10 percent. For NAAG, the in vitro recovery was 6.3%, and in vivo recovery, 11%. Depolarization stimulation using 100 mM KCl in the microdialysis perfusate was employed to measure extracellular NAA and NAAG concentrations. Extracellular NAA was elevated to approximately 70 pmol/36 microliters dialysate following depolarization. No significant elevation of NAAG was observed. By infusing known amounts of stable isotopically labeled NAAG-d3 via the microdialysis probe and measuring the isotopically labeled catabolic product, NAA-d3, in collected microdialysate, we were able to confirm the existence of one or more hydrolytic enzymes active towards NAAG in the hypothalamus. This finding suggest the possible involvement of active metabolic processes in the relationship between NAAG and NAA releases.

The effect of N-acetylaspartate on the intracellular free calcium concentration in NTera2-neurons

We have studied the effect of relatively high concentrations of extracellular N-acetylaspartate (NAA) on the intracellular free calcium concentration [Ca2+]i in NTera2-neurons. While low concentrations of extracellular NAA (0.1, 1 mM) had no effect on the [Ca2+]i, high concentrations of extracellular NAA (3, 10 mM) elicited sharp and statistically significant elevations of [Ca2+]i. Different classes of antagonists of the N-methyl-D-aspartate (NMDA) receptor abolished the NAA induced elevations of the [Ca2+]i, indicating the involvement of the NMDA receptor in NAA-induced elevations of [Ca2+]i.