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Lesion-Induced Alterations in Astrocyte Glutamate Transporter Expression and Function in the Hippocampus

DOI: 10.1155/2013/893605

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Astrocytes express the sodium-dependent glutamate transporters GLAST and GLT-1, which are critical to maintain low extracellular glutamate concentrations. Here, we analyzed changes in their expression and function following a mechanical lesion in the CA1 area of organotypic hippocampal slices. 6-7 days after lesion, a glial scar had formed along the injury site, containing strongly activated astrocytes with increased GFAP and S100β immunoreactivity, enlarged somata, and reduced capability for uptake of SR101. Astrocytes in the scar’s periphery were swollen as well, but showed only moderate upregulation of GFAP and S100β and efficiently took up SR101. In the scar, clusters of GLT-1 and GLAST immunoreactivity colocalized with GFAP-positive fibers. Apart from these, GLT-1 immunoreactivity declined with increasing distance from the scar, whereas GLAST expression appeared largely uniform. Sodium imaging in reactive astrocytes indicated that glutamate uptake was strongly reduced in the scar but maintained in the periphery. Our results thus show that moderately reactive astrocytes in the lesion periphery maintain overall glutamate transporter expression and function. Strongly reactive astrocytes in the scar, however, display clusters of GLAST and GLT-1 immunoreactivity together with reduced glutamate transport activity. This reduction might contribute to increased extracellular glutamate concentrations and promote excitotoxic cell damage at the lesion site. 1. Introduction Glutamate reuptake represents the principal mechanism for inactivation of synaptically released glutamate [1, 2]. In the rodent hippocampus, it is mainly accomplished by astrocytic glutamate transporters (EAATs: excitatory amino acid transporters), namely, GLAST (glutamate/aspartate transporter) and GLT-1 (glutamate-transporter-1; rodent analogues of EAAT1 and EAAT2, resp.; [3–7]). Glutamate uptake is energized by the concomitant inward transport of three sodium ions and a proton, while one potassium ion is transported outward. Consequently, its activation is accompanied by an increase in the intracellular sodium concentration of astrocytes [8, 9]. Under pathological conditions, astrocytes undergo a complex reaction referred to as reactive astrogliosis, which is seen in diverse preparations and conditions ranging from primary cell culture to the intact brain [10, 11]. The hallmarks of reactive gliosis are a massive upregulation of the expression of the intermediate filament Glial Fibrillary Acidic Protein (GFAP) and a cellular hypertrophy [12, 13]. Reactive astrocytes display several

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