Ognen A. C. Petroff

High‐field proton magnetic resonance spectroscopy of human cerebrum obtained during surgery for epilepsy

We analyzed specimens of histologically normal human cerebrum obtained at surgery for medically refractory epilepsy using proton magnetic resonance spectroscopy. Perchloric acid extracts of anterolateral temporal lobe cortex contained greater concentrations of creatine, N-acetylaspartate, γ-aminobutyric acid, alanine, and glutamate than the underlying white matter, which contained more acetate. Frontal and temporal lobe specimens composed of both gray and white matter failed to show statistically significant differences in the concentrations of creatine, N-acetylaspartate, alanine, aspartate, γ-aminobutyric acid, glutamate, glycine, taurine, threonine, valine, acetate, choline, β-hydroxybutyric acid, inositols, lactate, pyruvate, or succinate.

Proton magnetic resonance spectroscopy of cerebral lactate and other metabolites in stroke patients.

Background and Purpose Proton magnetic resonance spectroscopy can measure in vivo brain lactate and other metabolites noninvasively. We measured the biochemical changes accompanying stroke in 16 human subjects with cortical or deep cerebral infarcts within the first 3 weeks after symptom onset, and performed follow-up studies on six. Methods One-dimensional proton spectroscopic imaging encompassing the infarct region was performed with a 2.1-T whole-body magnet using the stimulated echo pulse sequence and an echo time of 270 msec. Results All but one of the cortical stroke patients had increased lactate within or near the infarct. Persistently elevated cerebral lactate was documented in five of six cases studied serially as long as 251 days after infarction. iV-acetylaspartate levels were decreased in most cortical strokes. Elevated lactate, accompanied by minimal reduction in iV-acetylaspartate, was recorded in two of four patients in the first week following a small subcortical infarct. Conclusions Long-term elevation of lactate commonly occurs after stroke. This lactate may arise from ongoing ischemia or infiltrating leukocytes, or it may be a residual of the lactate formed during the initial insult. The ability to observe stroke-elevated lactate pools at any time after lesion onset provides an approach to distinguishing among these possibilities in the future.

Spectroscopic imaging of stroke in humans: Histopathology correlates of spectral changes

Previous studies of human stroke by 1H nuclear magnetic resonance spectroscopy have shown elevation of lactate lasting 3 to 6 months. Complete metabolic turnover of the elevated lactate pool has been demonstrated 5 weeks after a stroke. Its cellular localization is among the first questions requiring clarification. Information pertinent to this question came to us from a patient with a 2-week-old stroke by 1H nuclear magnetic resonance spectroscopic imaging 1 week before his death led to neuropathologic examination of the brain. 1H spectra from voxels including the infarcts showed increased lactate and decreased N-acetylaspartate. Histopathology showed sheets of foamy macrophages in the infarct, but few neurons. Macrophage density ranged from 196 cells/mm2 near the surface of the infarct to 788 near its medial margin. Glial density was 500 to 800 cells/mm2. Lactate concentration in voxels including portions of the infarct was estimated at 7 to 14 mM. Voxels showing low N-acetylaspartate and high lactate on spectroscopic imaging were associated with histopathologic sections containing foamy macrophages. Brain macrophages—which begin to appear 3 days after infarction and gradually disappear over several months—could be a major source of elevated lactate signals that persist for months after stroke.

High‐Resolution Proton Magnetic Resonance Spectroscopy of Rabbit Brain: Regional Metabolite Levels and Postmortem Changes

The changes in 16 cerebral metabolites produced by cardiac arrest and subsequent room temperature auto‐lysis were studied using high‐resolution proton nuclear magnetic resonance spectroscopy. Biopsies of rabbit cerebral cortex, cerebral white matter, and cerebellum were quantitatively analyzed for acetate, alanine, γ‐aminobu‐tyric acid, creatine, glutamate, glycine, inositol, lactate, N‐acetylaspartate, phosphocreatine, succinate, taurine, and threonine. Of these, N‐acetylaspartate and the total creatine pool are the best candidates for use as concentration reference standards linking in vitro to in vivo 1H nuclear magnetic resonance measurements. Both changed little immediately after death, and they varied in a distinctive way among cortex, white matter, and cerebellum.

Glutamate-glutamine cycling in the epileptic human hippocampus.

Summary:  Purpose: Several findings suggest that energy metabolism and the glutamate–glutamine cycle may be impaired in epilepsy. Positron emission tomography often shows interictal hypometabolism of the epileptogenic hippocampus. In vivo microdialysis studies show that seizure‐associated glutamate release is doubled, and clearance is slowed. We hypothesized that the glutamate–glutamine cycle between neurons and glia may be decreased in the epileptic human hippocampus.

Symbiosis between in vivo and in vitro NMR spectroscopy: The creatine, N-acetylaspartate, glutamate, and GABA content of the epileptic human brain

High resolution 1H NMR spectroscopy was used to analyze temporal lobe biopsies obtained from patients with epilepsy. Heat-stabilized cerebrum, dialyzed cytosolic macromolecules, and perchloric acid extracts were studied using one- and two-dimensional spectroscopy. Anterior temporal lobe neocortex was enriched in GABA, glutamate, alanine, N-acetylaspartate, and creatine. Subjacent white matter was enriched in aspartate, glutamine, and inositol. The N-acetylaspartate/creatine mole ratio was lower in anterior temporal neocortex with mesial (0.66) than neocortical (0.80) temporal lobe epilepsy. Human brain biopsy samples were separated into crude and refined synaptosomes, neuronal cell bodies, and glia using density gradient centrifugation. Neuronal fractions were enriched in glutamate and N-acetylaspartate. Glial cell fractions were enriched in lactate, glutamine, and inositol. The creatine content was the same in biopsied epileptic cortex (8.8-8.9 mmol/kg) and normal in vivo occipital lobe (8.9 mmol/kg). Glutamate content was higher in epileptic cortex at biopsy (10.1-10.5 mmol/kg) than normal in vivo occipital lobe (8.8 mmol/kg). GABA content was higher in biopsies of epileptic cortex (2.3-2.2 mmol/kg) than in normal in vivo occipital lobe (1.2 mmol/kg). N-acetylaspartate content was lower in biopsied epileptic temporal cortex (5.8-6.8 mmol/kg) than normal in vivo occipital lobe (8.9 mmol/kg). Paired in vivo and ex vivo measurements are critical for a firm understanding of the changes seen in the 1H-spectra from patients with epilepsy.

Effect of hypoglycemic encephalopathy upon amino acids, high-energy phosphates, and pHi in the rat brain in vivo: detection by sequential 1H and 31P NMR spectroscopy.

Abstract: Metabolic alterations in amino acids, high‐energy phosphates, and intracellular pH during and after insulin hypoglycemia in the rat brain was studied in vivo by 1H and 31P nuclear magnetic resonance (NMR) spectroscopy. Sequential accumulations of 1H and 31P spectra were obtained from a double‐tuned surface coil positioned over the exposed skull of a rat while the electroencephalogram was recorded continuously. The transition to EEG silence was accompanied by rapid declines in phosphocreatine, nucleoside triphosphate, and an increase in inorganic orthophosphate in 31P spectra. In 1H spectra acquired during the same time interval, the resonances of glutamate and glutamine decreased in intensity while a progressive increase in aspartate was observed. Following glucose administration, glutamate and aspartate returned to control levels (recovery half‐time, 8 min); recovery of glutamine was incomplete. An increase in lactate was detected in the 1H spectrum during recovery but it was not associated with any change in the intracellular pH as assessed in the corresponding 31P spectrum. Phosphocreatine returned to control levels following glucose administration, in contrast to nucleoside triphosphate and inorganic orthophosphate which recovered to only 80% and 200% of their control levels, respectively. These results show that the changes in cerebral amino acids and high‐energy phosphates detected by alternating the collection of 1H and 31P spectra allow for a detailed assessment of the metabolic response of the hypoglycemic brain in vivo.

Glutamate and astrocytes—Key players in human mesial temporal lobe epilepsy?

Approximately one‐third of all patients with epilepsy continue to suffer from seizures even after appropriate treatment with antiepileptic drugs. Medically refractory epilepsies are associated with considerable morbidity and mortality, and more efficacious therapies against these disorders are clearly needed. However, the discovery of better therapies has been lagging due to an incomplete understanding of the mechanisms underlying the development of epilepsy (epileptogensis) and seizures (ictogenesis) in humans. An increasing number of studies have suggested that an abnormal amplification of glutamatergic activity—often referred to as the “glutamate hypothesis”—is involved in the pathophysiology of seizures and certain types of medically refractory epilepsies. For example, elevated levels of extracellular glutamate in hyperexcitable areas of the brain, up‐regulation of glutamate receptors, and loss of the glutamate‐metabolizing enzyme, glutamine synthetase (GS), have all been reported in patients with mesial temporal lobe epilepsy (MTLE). Moreover, it appears that glial cells, particularly the astrocyte, may play a key role in the glutamate overflow in MTLE. Proliferation of astrocytes is a hallmark of the epileptogenic focus in MTLE, and the proliferated cells are characterized by several unique features that are permissive for the excessive accumulation and release of astrocytic glutamate. Here, we assess recent data regarding the glutamate excess in epilepsy, review the role of glutamine synthetase, and discuss the implications of astrocytes in the pathophysiology of MTLE.

Topiramate increases brain GABA, homocarnosine, and pyrrolidinone in patients with epilepsy

Objective: To measure the effects of topiramate on brain gamma-aminobutyric acid (GABA) in patients with epilepsy. Background: Topiramate is a new antiepileptic medication with multiple putative mechanisms of action. In a recent meta-analysis of the newer antiepileptic drugs, topiramate was the most potent. Homocarnosine and pyrrolidinone are important metabolites of GABA with antiepileptic actions. Methods: In vivo measurements of GABA, homocarnosine, and pyrrolidinone were made of a 14-cm3 volume in the occipital cortex using 1H spectroscopy with a 2.1-Tesla magnetic resonance spectrometer and an 8-cm surface coil. Twelve patients (eight women) with refractory complex partial seizures were studied while using topiramate. Nine epilepsy-free, drug-free volunteers served as control subjects. Results: Topiramate increased mean brain GABA, homocarnosine, and pyrrolidinone concentrations in all patients. In paired measurements, brain GABA increased by 0.7 μmol/g (SD 0.3, n 7, 95% CI 0.4 to 1.0, p < 0.01). Homocarnosine increased by 0.5 μmol/g (SD 0.2, n 7, 95% CI 0.3 to 0.7, p < 0.001). Pyrrolidinone increased by 0.21 μmol/g (SD 0.06, n 7, 95% CI 0.16 to 0.27, p < 0.01). In two additional patients, GABA, homocarnosine, and pyrrolidinone increased after they were switched from vigabatrin to topiramate. Conclusions: Topiramate increased brain GABA, homocarnosine, and pyrrolidinone to levels that could contribute to its potent antiepileptic action in patients with complex partial seizures.

Initial Observations on Effect of Vigabatrin on In Vivo 1H Spectroscopic Measurements of γ‐Aminobutyric Acid, Glutamate, and Glutamine in Human Brain

Summary: Recent developments involving 1H nuclear magnetic resonance (NMR) spectroscopic editing techniques have allowed noninvasive measurements of γ‐aminobutyric acid (GABA) in human cerebrum. The additional information gained from GABA and macromolecule measurements permitted more precise glutamate (Glu) and glutamine (Gln) measurements. Occipital lobe GABA in 10 nonepileptic, healthy subjects was 1.0 μmol/g brain [95% confidence interval (CI) 0.9–1.1]. Vigabatrin (VGB) is a safe and effective antiepileptic drug (AED)that irreversibly inhibits neuronal and glial GABA transaminase. GABA levels were increased in all patients treated with VGB. With a standard dose of 3–6 g/day, GABA levels were 2.6 μmol/g (95% CI 2.3–2.8).Mean occipital GABA level measured in epileptic patients not receiving VGB was 0.9 μmol/g (95% CI 0.7–1.1). Gln was increased by 1.9 μmol/g and Glu was decreased by 0.8 μmol/g in patients receiving VGB as compared with patients receiving standard medications alone.