Limbic Structures Show Altered Glial–Neuronal Metabolism in the Chronic Phase of Kainate Induced Epilepsy

Abstract

A better understanding is needed of how glutamate metabolism is affected in mesial temporal lobe epilepsy (MTLE). Here we investigated glial-neuronal metabolism in the chronic phase of the kainate (KA) model of MTLE. Thirteen weeks following systemic KA, rats were injected i.p. with [1-(13)C]glucose. Brain extracts from hippocampal formation, entorhinal cortex, and neocortex, were analyzed by (13)C and (1)H magnetic resonance spectroscopy to quantify (13)C labeling and concentrations of metabolites, respectively. The amount and (13)C labeling of glutamate were reduced in the hippocampal formation and entorhinal cortex of epileptic rats. Together with the decreased concentration of NAA, these results indicate neuronal loss. Additionally, mitochondrial dysfunction was detected in surviving glutamatergic neurons in the hippocampal formation. In entorhinal cortex glutamine labeling and concentration were unchanged despite the reduced glutamate content and label, possibly due to decreased oxidative metabolism and conserved flux of glutamate through glutamine synthetase in astrocytes. This mechanism was not operative in the hippocampal formation, where glutamine labeling was decreased. In neocortex labeling and concentration of GABA were increased in epileptic rats, possibly representing a compensatory mechanism. The changes in the hippocampus might be of pathophysiological importance and merit further studies aiming at resolving metabolic causes and consequences of MTLE.

A Subconvulsive Dose of Kainate Selectively Compromises Astrocytic Metabolism in the Mouse Brain In Vivo

“…Seizure severity was graded according to a modified Racine scale. 4,14 Fifteen minutes after kainate injection, the mice were injected intraperitoneally with a combination of [1-13 C]glucose (543 mg/kg) and [1,[2][3][4][5][6][7][8][9][10][11][12][13] C]acetate (504 mg/kg) and after another 15 minutes, the brains of the mice were subjected to microwave fixation at 4 kW for 1.7 seconds (model GA5013, Gerling Applied Engineering, Modesto, CA, USA). This is the most humane euthanization method instantly inactivating proteins in the brain.…”

“…Singlets obtained in the 13 C spectrum were corrected for naturally abundant 13 C and amounts calculated from 1 H spectra were corrected for 13 C containing metabolites and number of protons. 13 [4,[5][6][7][8][9][10][11][12][13] C]a-ketoglutarate is in rapid equilibrium with [4,[5][6][7][8][9][10][11][12][13] C]glutamate, which is a precursor for [4,[5][6][7][8][9][10][11][12][13] C]glutamine. If [4,[5][6][7][8][9][10][11][12][13] C]a-ketoglutarate remains in the astrocytic TCA cycle for further metabolism via this pathway, it will, after condensation with unlabeled acetyl CoA, give rise to [3-13 C]-and [1,2-13 C]a-ketoglutarate in the subsequent turn, which may give rise to glutamate and glutamine displaying the same labeling patterns.…”

“…If [4,[5][6][7][8][9][10][11][12][13] C]a-ketoglutarate remains in the astrocytic TCA cycle for further metabolism via this pathway, it will, after condensation with unlabeled acetyl CoA, give rise to [3-13 C]-and [1,2-13 C]a-ketoglutarate in the subsequent turn, which may give rise to glutamate and glutamine displaying the same labeling patterns. Based on the distinct 13 C labeling in glutamine after the first and second turn of TCA cycle metabolism, a number indicative of astrocytic TCA cycle activity can be calculated by relating the amount of isotopomers generated in the second turn of TCA cycle metabolism to the amount of that generated in the first turn, i.e., ( [1,[2][3][4][5][6][7][8][9][10][11][12][13] …”

“…7 Later, alterations within both astrocytes and neurons were observed in the latent phase while the chronic phase was characterized predominantly by neuronal changes. 8,9 However, little is known about the changes in cerebral metabolism immediately after kainate treatment, although this might provide valuable information with regard to determining the initial cause of seizure activity underlying subsequent propagation of epileptic disorders and may provide a unique diagnostic feature. An early diagnosis could potentially provide a therapeutic possibility during the latent phase, thereby preventing epileptogenesis from progressing into the chronic convulsive phase.…”

“…A combination of [1,2-13 C]acetate and [1-13 C]glucose was injected and subsequent nuclear magnetic resonance spectroscopy of cortical extracts was employed to distinctively map astrocytic and neuronal metabolism. The subconvulsive dose of kainate led to an instantaneous increase in the cortical lactate content, a subsequent reduction in the amount of [4,[5][6][7][8][9][10][11][12][13] C]glutamine and an increase in the calculated astrocytic TCA cycle activity. In contrast, the convulsive dose led to decrements in the cortical content and 13 C labeling of glutamate, glutamine, GABA, and aspartate.…”