Glutamate excitotoxicity has been implicated in epileptic seizures. Our objectives are 1) to clarify the mechanisms that control the metabolic flux and intercompartmental transport of glutamate and glutamine in the intact mammalian brain and 2) determine how glutamate excitotoxicity may cause epileptic seizures. Impaired glial uptake of GLU in the extracellular fluid (GLUECF) can lead to excessive stimulation of the GLU receptors and to neuronal degeneration (GLU excitotoxicity). The rate of uptake of neurotransmitter GLU from the extracellular fluid into astrocytes and subsequent metabolism to GLN has been measured through a) selective 13C enrichment of neurotransmitter GLU by isotope chase, b) collection of extracellular GLU by microdialysis and analysis of its 13C enrichment by gas-chromatography/mass spectrometry and c) observation of [5-13C, 5-15N] GLN formed in astrocytes using localized in vivo 13C and 15N MRS. Now, Specific Aim #1, a 'missing-link' in the GLN/GLU cycle, the kinetics and mechanism of transport of GLN from astrocytes to neurons will be determined using in vivo 15N, 1H-15N HMQC and 13C MRS. The results will clarify the roles of recently identified GLN transporters in the intact normal brain and determine whether the rate of glial GLN efflux (hypothesis #1) or of neuronal GLN uptake (hypothesis #2) limits the GLN/GLU cycle rate in vivo.
Specific Aim #2 Kinetics of GLU release and uptake in temporal lobe epilepsy will be studied using the chronic kainate-induced epileptic (KA) rats model. In vivo flux rates will be measured in the lesioned and contralateral hippocampus and correlated with electrophysiological and behavioral seizures, to determine whether the onset and propogation of seizures is the result of (hypothesis #3) abnormal GLU release, (hypothesis #4) impaired GLU clearance by glial transporters, or (hypothesis #5) a combination of these processes. Together these studies will contribute to a better understanding of the regulatory events of the GLN/GLU cycle in KA and increase potential to replace surgical management of temporal lobe epilepsy.
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