Cerebral lactate turnover after electroshock: in vivo measurements by 1H/13C magnetic resonance spectroscopy

Detection of 13C labeling of glutamate and glutamine in human brain by proton magnetic resonance spectroscopy

A proton magnetic resonance spectroscopy (MRS) technique was used to measure ¹³C enrichments of glutamate and glutamine in a 3.5 × 1.8 × 2 cm³ voxel placed in the dorsal anterior cingulate cortex of five healthy participants after oral administration of [U-¹³C]glucose. Strong pseudo singlets of glutamate and glutamine were induced to enhance the signal strength of glutamate and glutamine. This study demonstrated that ¹³C labeling of glutamate and glutamine can be measured with the high sensitivity and spatial resolution of ¹H MRS using a proton-only MRS technique with standard commercial hardware. Furthermore, it is feasible to measure ¹³C labeling of glutamate and glutamine in limbic structures, which play major roles in behavioral and emotional responses and whose abnormalities are involved in many neuropsychiatric disorders.

Magnetic resonance spectroscopy in animal models of epilepsy

The noninvasive localization of the epileptogenic zone continues to be a challenge in many patients that present as candidates for possible epilepsy surgery. Magnetic resonance imaging (MRI) techniques provide accurate anatomical definition, but despite their high resolution, these techniques fail to visualize the pathological neocortical and hippocampal changes in a sizable number of patients with focal pathologies. Further, visualized lesions on MRI may not all produce seizures. One of the keys to the understanding of the epileptogenic zone lies in the recognition of the metabolic alterations that occur in the setting of epileptic seizures. Magnetic resonance spectroscopy (MRS) is a valuable tool that can be used to study the metabolic changes seen in both acute and chronic animal models of epilepsy. Such study allows for the identification of epileptic tissue with high sensitivity and specificity. We present here a review of the use of MRS in animal models of epilepsy.

Clinical applications of MR spectroscopy in epilepsy

1H and 31P spectroscopy detects relevant metabolite changes in patients with TLE. Numerous studies confirm reduction in NAA and in the ratio of PCr/Pi. In his 1999 review, Kuzniecky concluded that proton MRS, using single-voxel or chemical shift imaging, lateralizes temporal lobe epilepsy in 65% to 96% of cases, with bilateral changes seen in 35% to 45% of cases, whereas phosphorus MRS shows a lateralizing PCr/Pi ratio in 65% to 75% of the TLE patients. There are indications that these changes are reversible with seizure treatment. Improvements in MRS technology, such as the ability to calculate absolute concentrations, to account for differences be-tween gray and white matter and to achieve better spectral resolution by use of a higher magnetic field strength, will now allow more extensive use of this technique for patients with epilepsy.

Estimation of metabolic flux from dominant rate constants in vivo: application to brain and heart

In an earlier paper (Cohen and Bergman, Am. J. Physiol. 268 (1995) E397), we explored the relationship between the exponents in the exponential curve fit to isotopic enrichment versus time and the fractional turnover rate of the largest metabolic pool in the pathway. Here we present the analysis on a more rigorous footing and apply it to questions of cerebral and cardiac metabolism. Our emphasis in this paper is to describe and justify mathematically an approach for analysis of metabolic dynamics, not with the intention of replacing the use of numerical software for estimation of flux rates but for giving the scientist the opportunity to examine the system in an approximate manner, and thereby to check not only that the results of the numerical solution are the correct solutions to the equations but also that the equations portray the correct simplification of the metabolic pathway. We introduce the "dominant rate constant" as a tool for deriving algebraic formulas relating rates of metabolic flux, sizes of metabolic pools, and the dynamics of isotopic enrichment. Illustrations of such algebraic formulas are provided for the rates of the citric acid cycle (CAC), glycolysis and glutamine synthesis in brain, as well as the rate of the CAC in heart. In addition, we prove that formulas for estimation of rates of glycolysis and of the CAC depend critically on the fractional turnover rates of lactate and glutamate, respectively. The justification for analysis of simulated data is that we are studying the effects of simplifications of metabolic models on the accuracy of estimation of metabolic pathways. Our use of the dominant rate constant is an analytical convenience that allows us to assess proposed simplifications of metabolic pathways.

Future aspects of epilepsy research

This contribution in honour of Prof. Gerhard Pendl first reviews some recent studies on resected tissue, migrational disorders, and Rasmussen's Syndrome. These areas of basic research profit from recent advances of molecular biology and genetics. On the clinical side, some studies dealing with proton magnetic resonance spectroscopy are reviewed. In order to highlight the progress in clinical epilepsy research using modern methods of structural and functional imaging, functional outcome prediction is also reviewed. This kind of advanced clinical research is dealt with by discussing risk factor assessment associated with postsurgical decrements in memory. With regard to motor functions, we compare the yield of functional MR and intraoperative cortical stimulation in patients with lesions in or close to the Rolandic cortex. Progress in the field of advanced EEG analysis is reviewed in the context of "seizure prediction" and cognitive event-related potentials. Finally some of the new epilepsy treatment options, such as Gamma Knife treatment, where Prof. Pendl's group made pioneering contributions, are dealt with.

Proton magnetic resonance spectroscopy characteristics of a focal cortical dysgenesis during status epilepticus and in the interictal state

We report the magnetic resonance imaging and proton magnetic resonance spectroscopic findings ((1)HMRS) in a patient with a focal cortical dysgenesis in the right superior frontal gyrus during intermittent frontal status epilepticus (IFSE) with simple partial seizures, and after she had become seizure free. During the status epilepticus, demonstrated by simultaneous behavioural and electroencephalographic telemetric long-term monitoring with scalp electrodes and ictal SPECT, we performed a single voxel spectroscopy of the dysgenic cortex. The(1)HMRS was repeated after 20 days when the patient's seizures were controlled. The N-acetyl-aspartate concentration in the focal dysgenic cortex was decreased in the interictal state but more during IFSE. The creatine/phosphocreatine concentration was normal in both instances. There was a clear lactate signal during IFSE, which was no longer visible in the interictal state. To our knowledge this is the first report of a(1)HMRS study of a focal cortical dysgenesis during an intermittent status epilepticus. We interpret the observed changes as signs of histopathological changes inherent to a cortical malformation and of an impaired energy metabolism due to the partial status epilepticus.

Similar increases in extracellular lactic acid in the limbic system during epileptic and/or olfactory stimulation

Previous studies have shown that physiological stimulation of brain activity increases anaerobic glucose consumption, both in humans and in experimental animals. To investigate this phenomenon further, we measured extracellular lactate levels within different rat brain regions, using microdialysis. Experiments were performed comparing the effects of natural, physiological olfactory stimulation of the limbic system with experimental limbic seizures. Olfactory stimulation was carried out by using different odors (i.e. both conventional odors: 2-isobutyl-3-methoxypyrazine, green pepper essence; thymol; and 2-sec-butylthiazoline, a sexual pheromone). Limbic seizures were either induced by systemic injection of pilocarpine (200-400 mg/kg) or focally elicited by microinfusions of chemoconvulsants (bicuculline 118 pmol and cychlothiazide 1.2 nmol) within the anterior piriform cortex. Seizures induced by systemic pilocarpine tripled lactic acid within the hippocampus, whereas limbic seizures elicited by focal microinfusion of chemoconvulsants within the piriform cortex produced a less pronounced increase in extracellular lactic acid. Increases in extracellular lactate occurring during olfactory stimulation with the sexual pheromone (three times the baseline levels) were non-significantly different from those occurring after systemic pilocarpine. Increases in lactic acid following natural olfactory stimulation were abolished both by olfactory bulbectomy and by the focal microinfusion of tetrodotoxin, while they were significantly attenuated by the local application of the N-methyl-D-aspartate antagonist AP-5. Increases in hippocampal lactate induced by short-lasting stimuli (olfactory stimulation or microinfusion of subthreshold doses of chemoconvulsants, bicuculline 30 pmol) were reproducible after a short delay (1 h) and cumulated when applied sequentially. In contrast, limbic status epilepticus led to a long-lasting refractoriness to additional lactate-raising stimuli and there was no further increase in lactate levels when the olfactory stimulation was produced during status epilepticus. Increases in lactic acid following olfactory stimulation occurred with site specificity in the rhinencephalon (hippocampus, piriform and entorhinal cortex) but not in the dorsal striatum. Site specificity crucially relied on the quality of the stimulus. For instance, other natural stimuli (i.e. tail pinch) produced a similar increase in extracellular lactate in all brain areas under investigation. The major conclusion of this work is that the presentation of an odor known to be a rat pheromone results in lactate production as great as that induced by the systemic convulsant pylocarpine (maximum: 2.286+/-0.195 mM and 1.803+/-0.108 mM, respectively). This supports the notion that the great magnitude of lactate production known to accompany seizures can result from the intensified neural activity per se ("aerobic gycolysis"), not merely from local anoxia or other pathological changes.

Decreased cerebral blood flow during seizures with ictal SPECT injections

Increased regional cerebral blood flow (rCBF) at the epileptogenic site has been consistently reported for single photon emission computed tomography (SPECT) injections made during seizure activity, and the increased rCBF has been shown to remain elevated at the epileptogenic site in some cases, even when SPECT injections are made after seizure termination (postictal). A sustained increase in rCBF after seizure cessation was recently confirmed, but for no more than 100 s from seizure onset [Avery, R.A., Spencer, S.S., Spanaki, M.V., Corsi, M., Seibyl, J.P., Zubal, I.G., 1999. Effect of injection time on postictal SPET perfusion changes in medically refractory epilepsy. Eur. J. Nucl. Med. 26, 830-836]. In the current study, it is examined whether ictal SPECT injections demonstrate a similar change in rCBF around 100 s from seizure onset. Twenty-one patients with medically refractory epilepsy and a known area of seizure onset receiving ictal and interictal 99mTc-Hexamethyl-propyleneamineoxime (HMPAO) SPECT scans were studied. The results of SPECT subtraction analysis which visualize increased and decreased rCBF were compared to seizure duration and HMPAO injection time. Five patients received ictal SPECT injections (during ongoing seizure activity) more than 90 s after seizure onset and demonstrated decreased rCBF. Two of these patients also demonstrated areas of increased rCBF. Decreased rCBF was localized to the epileptogenic lobe in four of the five patients. By examining ictal SPECT injections made 90 s after seizure onset, evidence was found that reduced rCBF may exist during ictus. The change in rCBF around 90 s is also observed in postictal injections, suggesting a common metabolic mechanism may be responsible.

Neuroimaging in epilepsy

Neuroimaging techniques have improved the understanding, diagnosis, and management of epilepsy. By providing excellent structural information, MRI is the technique of choice in evaluating patients with epilepsy. Functional imaging techniques, including MR spectroscopy, functional MRI, positron emission tomography, and single photon emission CT, permit noninvasive assessment of the epileptic substrate, its functional status, and neuroreceptors. The MRI-based techniques will potentially assume a greater role in the cost-effective workup of the patient. Currently, newer techniques such as magnetoencephalography, magnetic source imaging, and optical imaging are research tools.

Temporal changes in proton MRS metabolites after kainic acid-induced seizures in rat brain

PURPOSE

In situ 1H-magnetic resonance spectroscopy (MRS) was used to study temporal metabolic changes in a rat model of temporal lobe epilepsy (TLE) by using kainic acid (KA).

METHODS

Rat brains were scanned at the level of the hippocampal body for MRS measurements. Relative ratios of N-acetyl groups (NA: N-acetylaspartate and N-acetylaspartyl glutamate), choline, and lactate (Lac) over creatine (Cr) were calculated.

RESULTS

NA/Cr ratios increased significantly during the ictal phase. During the postictal and interictal phases, the NA/Cr ratio decreased. There was a significant and prolonged increase of the lactate/Cr ratio in the hippocampi of rats that started 1 h after the onset of KA-induced seizure activity and persisted up to 24 h after the injection. The prolonged lactate/Cr increase in an area susceptible to neuronal damage (e.g., hippocampus) correlated with the onset of seizure activity but remained elevated thereafter.

CONCLUSIONS

The ictal and early postictal increase in lactate ratios may reflect increased cellular activity and metabolism resulting from KA excitotoxicity. Assuming that the changes in NA/Cr ratios are due to NAA increase, we speculate that an activation of the N-acetylaspartylglutamate (NAAG) dipeptidase pathway may explain the ictal increase in NA/Cr ratios. The late postictal decrease in NA/Cr ratios is a reflection of KA-induced neuronal cell loss.

Proton magnetic resonance spectroscopic imaging for discrimination of absence and complex partial seizures

We performed proton magnetic resonance spectroscopic imaging of the temporal lobes between, during, and soon after nonconvulsive seizures in 20 patients with documented temporal lobe epilepsy, 5 patients with primary generalized epilepsy, and 2 patients with secondary generalized epilepsy. Our objective was to determine whether there were metabolic changes observable by magnetic resonance spectroscopic imaging during seizures and whether these changes were specific for focal or generalized nonconvulsive seizures. We found a significant increase in lactate to creatine plus phosphocreatine (lactate/creatine) values, reflecting an imbalance in energy supply and demand or an adaptation in response to ictal neuronal discharges, during and soon after complex partial seizures, but not during or soon after absence seizures associated with generalized epilepsy. In patients with temporal lobe epilepsy, the N-acetylaspartate resonance relative to creatine plus phosphocreatine was low in one or both temporal lobes, indicating neuronal loss or damage. This was not observed in patients with primary generalized epilepsy. The regions with abnormal lactate/creatine and N-acetylaspartate/creatine values corresponded to the epileptogenic focus as defined by clinical-electroencephalographic investigation. There was no change in the N-acetylaspartate/creatine values in the temporal lobes between the interictal, ictal, or postictal states. We conclude that (1) partial seizures are associated with abnormally high lactate levels, but absence seizures are not, and (2) no short-term changes of N-acetylaspartate occur during or soon after complex partial seizures or absence seizures. These findings may be related to the lack of postictal confusion in patients with absence seizures, as well as with the more benign course of primary generalized epilepsy with nonconvulsive attacks.

Reversible, reproducible reduction of brain water apparent diffusion coefficient by cortical electroshocks

Rat brains were imaged after cortical electroshock pulse trains (1 ms pulses at 100 Hz) of varying durations (0.1-10 s), with diffusion-weighted echo planar imaging sequences at 2.0 T. The apparent water diffusion coefficient (ADC) decreased after either single or repeat electroshock trains. ADC reductions were observed within 6 s after the first shock. The size of the affected area of the brain increased in subsequent images during the 1st min after a 10-pulse (0.1 s) train, and also increased with the duration of electroshock trains. ADC reduction was reproducible in extent and time course after single 10-shock trains and was reversible. In the affected pixels the mean ADC reduction was 4% for a single shock train (0.1 s), and 7-8% for trains repeated once a minute, independent of electroshock train duration. The results indicate that neuronal activity associated with electrostimulation may be monitored with water diffusion measurements, and they may be useful for measuring the severity of seizure activity in patients with medically intractable epilepsy.

The mechanisms controlling physiologically stimulated changes in rat brain glucose and lactate: a microdialysis study

1. This study is concerned with the supply of metabolic substrates for neuronal metabolism. Experiments were carried out to investigate whether mechanisms demonstrated in cultured astrocytes also occurred in vivo; these were cAMP-mediated breakdown of glycogen and glutamate uptake-stimulated release of lactate. 2. In vivo microdialysis was used in freely moving rats. Lactate and glucose in the dialysate were assayed using enzyme-based on-line assays. Drugs were given locally through the dialysis probe. Regional cerebral blood flow was measured using the hydrogen clearance method. 3. There was an increase in dialysate glucose in response to the beta-adrenoceptor agonist isoprenaline and to 8-bromo-cAMP, an analogue of cAMP, the second messenger of beta-adrenoceptor stimulation. The effect of isoprenaline was blocked by the antagonist propranolol. Isoprenaline had no effect on dialysate lactate, which was increased by the glutamate uptake blocker beta-D,L-threohydroxyaspartate (THA). 4. Physiological stimulation of neuronal activity produced an increase in both lactate and glucose. The increase in lactate was depressed in the presence of THA but was unaffected by propranolol. The increase in glucose was blocked by propranolol. Regional cerebral blood flow was increased by physiological stimulation but was unaffected by propranolol. 5. These results demonstrate that physiologically stimulated increases in glucose and lactate in the brain are mediated by different mechanisms.

Magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS) is noninvasive and may be readily combined with magnetic resonance imaging (MRI). Attention has focussed on proton (1H) and phosphorus (31P) MRS, and studies have been undertaken by using single voxels or many voxels simultaneously (chemical-shift imaging, magnetic resonance spectroscopic imaging). The latter is more difficult and prone to artefact but potentially yields significantly more information. 1H MRS has principally yielded data on concentrations of N-acetyl aspartate (NAA), choline, creatine, and phosphocreatine. NAA is located primarily within neurons, and reduction of the ratio of NAA to choline, creatine, and phosphocreatine is a marker of neuronal loss and dysfunction. This technique may be useful as a noninvasive tool for localizing epileptogenic foci, but its role requires further evaluation. As with all functional imaging methods, coregistration with high-quality MRI is essential for interpreting data. 1H MRS can be used also to estimate cerebral concentrations of several neurotransmitters: glutamate, glutamine, and gamma-aminobutyric acid (GABA). This may prove useful for characterizing the neurometabolic profiles of patients with different epilepsy syndromes and for evaluating the effects of medical and surgical treatments. 31P MRS can detect adenosine triphosphate, phosphodiesters, phosphomonoesters, phosphocreatine, and inorganic phosphate, and estimate intracerebral pH. Abnormalities that have been associated with epileptogenic brain areas include increased inorganic phosphate, reduced phosphomonoesters, and increased pH. Only small numbers of patients have been studied, however, so that conclusions are not definitive, and the clinical role of this technique is not yet established.

Development and applications of in vivo clinical magnetic resonance spectroscopy

4.1 CURRENT STATUS. While an extensive clinical literature of MRS of muscle, brain, heart and liver has been achieved, the MRS technique is not considered essential for routine diagnosis because it is inherently insensitive and metabolic changes tend to be small. However, MRS techniques have proven to be of considerable value for prognosis in some circumstances, notably for predicting outcome following hypoxic-ischaemic injury in the newborn and also in predicting graft viability following organ transplantation. The chemical specificity of MRS has been illustrated, and exploiting the non-invasive nature of the technique, metabolic fingerprinting of pathophysiological processes throughout the natural history of a wide variety of diseases is now being accomplished. Particularly exciting are the applications of 13C MRS for measuring hepatic and muscle glycogen levels, for example in diabetics, and the use of hepatic 31P MRS for assessing liver function in cirrhosis. Other areas of excitement are the applications of 1H MRS in assessing neuronal function in epilepsy and stroke, and for measuring the evolution of lactate in stroke and hypoxic-ischaemic encephalopathy. Emphasis on technique development continues, and applications still tend to be technology-led. The availability of routine clinical MRI systems with spectroscopy capabilities has given MRS studies wider applicability. The recent improvements in spatial resolution have been impressive and the technique is slowly becoming more quantitative. 4.2. FUTURE PERSPECTIVES. Given the flexibility of clinical magnetic resonance techniques, particularly magnetic resonance imaging, it is likely that MRI will be the diagnostic tool of choice in a wider range of diseases, such as multiple sclerosis, stroke, neurodegenerative conditions, sports injuries and in staging malignancies. Since proton magnetic resonance spectroscopy packages have become a routine addition to many MRI systems, it is feasible to select the MRI sequences of most value in highlighting anatomical and pathological abnormalities and to incorporate specifically selected MRS sequences to emphasize biochemical differences. Improvements in technical methodologies are central to further developments. For example, use of internal coils, such as implantable or endoscopic coils, will enable small regions of tissue to be studied in considerable detail, which may otherwise be inaccessible to measurement. Chemical MRS studies have benefited from the use of higher magnetic fields, and the same may be expected for clinical MRS studies. Whole-body magnets up to 4 T have been used in a few centres, and certainly 3 T systems are becoming more widely available with the recent tremendous interest in functional imaging. Certainly, better control of artefacts can be expected; for example, improved definition of spectral changes due to voluntary or involuntary movements. Wider use of proton decoupling methods will improve the specificity of the spectra, by allowing definitive assignments of overlapping resonances, as well as the sensitivity. Comparing PET and MRS studies, it is becoming increasingly obvious that both will be required in parallel to explore parameters of brain metabolism and function. The ability to measure 13C MR signals in the brain has been demonstrated, which allows measurements of glutamate and glucose turnover. MRS measurements have the advantage of not requiring a radioactive isotope, as well as being insensitive to activity-related changes in regional cerebral blood flow. Also the study of cerebral glucose metabolism by MRS is very promising, allowing a resolution and sensitivity comparable to PET. A combination of MRS and PET studies will allow the pathogenesis of neuropsychiatric disorders to be better understood. (ABSTRACT TRUNCATED)

Brain proton magnetic resonance spectroscopy studies in recently abstinent alcoholics

Chronic alcohol-dependent patients have reduced brain volumes and concomitant neurobehavioral deficits that may recover during abstinence. In 10 chronic alcoholic patients, using localized proton magnetic resonance spectroscopy, we found reliable increases during the first 3-4 weeks of abstinence in the concentrations within the superior cerebellar vermis of choline (Cho)-containing compounds relative to the neuronal marker, N-acetylaspartate (NAA). Lesser changes were observed following 1 month of abstinence, and in one of the patients studied longitudinally over 3 months, a marked reduction in the Cho/NAA ratio was associated with relapse. After detoxification, the Cho/NAA ratio correlated with a composite clinical impression of brain functions. The lowest Cho/NAA was observed in a patient with persisting alcoholic dementia, in striking contrast to reduced relative concentrations of NAA reported in dementia of the Alzheimer's type. Possible molecular explanations for these brain metabolic changes are discussed.

Nuclear magnetic resonance spectroscopy: a review of neuropsychiatric applications

1. Magnetic resonance spectroscopy (MRS) is a powerful new neuropsychiatric research tool which allows for the noninvasive investigation of in vivo biochemistry. This review focuses on the recent applications of MRS to in vivo neuropsychiatric research. 2. The history of MRS as it has progressed from an in vitro method of biochemical analysis to its current in vivo research uses is presented. 3. A brief overview of the physical principles of MRS, including methods for spectral localization, is discussed. 4. Applications of the different MRS modalities (1H, 31P, 19F, 7Li, 13C and 23Na) to various neuropsychiatric disorders such as Alzheimer's disease, schizophrenia, affective disorders, acquired immunodeficiency disease, etc. are reviewed. The study of both fluorinated neuroleptics and the antidepressant fluoxetine using 19F MRS are discussed in greater detail. 5. Finally, potential future neuropsychiatric applications of MRS and specifically 19F MRS are presented.

■REVIEW : The Nuclear Magnetic Resonance Revolution in Basic and Clinical Neuroscience

Nuclear magnetic resonance (NMR) methods for the noninvasive, chemically specific investigation of living tissue have proliferated in number and advanced greatly in power since the first NMR images of anatomy appeared in the 1970s. By 1994, about 50 different NMR measurements had become available for the study of normal and pathological brain. These include aspects of biochemistry, angiography, perfusion, activation- sensitive metabolic rates for glucose and oxygen, monitoring of function through activation-induced changes in blood flow and water diffusion, and normal and pathological anatomy with submillimeter resolution. At least as many more measurements of biomedical importance are under development. Neuroscience research and management of neurological illness will be profoundly affected by NMR methods as they mature and become routine. The Neuroscientist 1:84-94, 1995

Nuclear magnetic resonance spectroscopy of seizure states

Magnetic resonance spectroscopy (MRS) can be used for noninvasive measurement of more than two dozen small metabolites in the brains of living animals and humans. In the first decade of its use for study of seizure phenomena in animals, MRS successfully detected in vivo seizure-induced cerebral acidosis and reduction of phosphocreatine concentration, changes that had been described previously by techniques requiring destruction of tissue. Thus validated, MRS was used to reveal new aspects of epileptic pathophysiology in animals: (a) dissociation of brain lactate and pH during experimental status epilepticus of low and intermediate intensity, reflecting metabolic compartmentation; and (b) long persistence of metabolically active elevated brain lactate after brief cortical electroshock. The latter phenomenon may be an extreme form of a mechanism by which lactate production primes synaptic terminals for maximal sustained firing rates during normal brain activation. Diffusion-weighted imaging of rat brain has shown that status epilepticus apparently shortens the mean path length of water diffusion, a novel finding that provides new insight concerning the physical conditions under which the seizure-related chemical changes detected by MRS occur. MRS study of epileptic patients has been undertaken more recently as instruments large enough for observations on humans have become available. Acidosis, reduction of phosphocreatine, and elevation of lactate have all been demonstrated in the human brain during seizure discharge. Chronic reduction of N-acetylaspartate in limbic regions probably reflects neuronal loss and may correlate with mesial temporal sclerosis.

Functional study of the brain by NMR

Nuclear magnetic resonance (NMR) methods allow a wide variety of noninvasive measurements to be made on living animals and humans. The most extensively developed application of such methods is magnetic resonance imaging (MRI) of the brain and other organs, which has already come to the attention of most biomedical scholars, many physicians, and even much of the lay public because of its widespread use in neurological research and medical diagnosis.

Cerebral metabolic changes induced by MK-801: a 1D (phosphorus and proton) and 2D (proton) in vivo NMR spectroscopy study

The dynamic effects of the non-competitive NMDA receptor antagonist, MK-801 on brain metabolism were investigated over 105 minutes in unanesthetized rats by proton and phosphorus NMR spectroscopy. MK-801 (0.5 and 5 mg/kg, i.p) induced no changes in intracellular pH, and in phosphocreatine, ATP, and inorganic phosphate levels, indicating that the drug preserved energy and intracellular pH homeostasis. There were transient increases in lactate after both doses of MK-801, suggesting early activation of glycolysis, which was not immediately matched by enhanced oxidative metabolism or by enhanced blood flow. Thereafter, lactate control level was not restored after 0.5 mg/kg whereas it was restored after 5 mg/kg in spite of a sustained metabolic activation. The low dose of MK-801 also caused a continuous decrease in cerebral aspartate level (-38%) which is thought to match the enhanced energy demand, whereas the high dose caused shorter and smaller changes. The intracerebral glucose level rose after MK-801 injection, indicating that brain tissue had an adequate or even excessive supply of glucose. Glucose time course seemed to closely match the changes in blood flow elicited by MK-801. This is the first study giving the metabolic pattern of a pharmacological activation. We demonstrate an excess of glycolysis over oxidative metabolism in the early time similar to that following physiological and pathophysiological states such as photic stimulation and seizures. The difference between the effects of the two doses of MK-801 suggests that the adjustment of cerebral metabolism to MK-801 activation is faster and greater with the high dose than with the low dose.

Focal intracerebral elevation of L-lactate is anticonvulsant

Sodium lactate (pH 7.0) infused over the area tempestas, an epileptogenic site in the prepiriform cortex, protected rats from limbic motor seizures induced by infusion of a GABA receptor antagonist in area tempestas. The anticonvulsant action, which was anatomically site-specific and reversible, persisted for 90 min. Infusions of sodium acetate (pH 5.5 or 7.0) over area tempestas were not anticonvulsant. Our findings suggest that lactate can modulate neural activity and that increased cerebral lactate as occurs with epileptic seizures, may limit the duration and spread of seizure activity.

Effects of kainate-induced seizures on cerebral metabolism: a combined 1H and 31P NMR study in rat

The cerebral metabolic changes elicited by kainate-induced seizures in the rat were investigated by in vivo combined NMR spectroscopy of 31P and 1H. Systemic injection of kainate induced no significant changes in cerebral ATP or PCr levels during up to 90 min of continuous, generalised seizures, and the cerebral 31P spectra showed only a transient mild cerebral acidosis 30 min after kainate administration. In parallel with the changes in intracellular cerebral pH, the 1H spectra showed a significant increase in lactate, which remained elevated throughout the seizures. These findings indicate that oxidative metabolism does not completely match the increased glycolysis during seizures though the energy homeostasis is maintained. This suggests that oxidative metabolism has a limited capacity to satisfy the brain's energy needs during the kainate-induced seizures, but that the different pathways of energy production in the brain cells can overcome this limitation. Thus the brain damage associated with this experimental model of epilepsy is not due to extended major failure of the energy supply.

Metabolic assessment of a neuron-enriched fraction of rat cerebrum using high-resolution 1H and 13C NMR spectroscopy

This study explored the utility of 1H and 13C magnetic resonance spectroscopy to study a neuron-enriched preparation made from rat cerebrum. The preparation contained high concentrations of N-acetylaspartate and gamma-aminobutyric acid and low concentrations of glutamine, indicating that it was in fact rich in neuronal cytosol. This was confirmed by immunohistochemical studies with antibodies to neuronal and glial markers. A method of metabolite quantification based on the creatine signal yielded metabolite concentrations similar to those of rat cerebrum, whereas concentrations based on the metabolite/protein ratio were five times lower, suggesting that much protein in the preparation was not associated with functioning cytoplasm. The metabolic competence of the preparation was assessed by quantitative measurements of its ability to convert 1-13C-glucose into lactate, glutamate, aspartate, and other metabolites under well oxygenated conditions for 30 min. Calculated from the creatine standard, the mean glycolytic rate was the same as in a synaptosomal preparation studied under similar conditions and the same as rat cerebrum in vivo. Tricarboxylic acid cycle flux occurred at half the rate observed in the synaptosomal preparation and 16% of the basal cerebral metabolic rate in vivo.