Abstracts

Rationale: Astrocytic buffering of glutamate and potassium is critical for normal neuronal functioning. Reactive astrocytes are a prominent marker of any brain stress reaction. We hypothesize that reactive astrocytes differ from gray matter protoplasmic astrocytes in their ability to uptake extracellular glutamate and potassiumMethods: 57 samples of neocortex and hippocampi (41 with hippocampal sclerosis (HS) and 16 without sclerosis (non HS)) obtained during surgery were used for immunohistochemical investigation with confocal and electron microscopy.Results: Reactive astrocytes were defined as hypertrophied cells with high expression of GFAP, vimentin, and nestin. They differed morphologically and immunohistochemically from protoplasmic astrocytes (cells expressed variable amount of GFAP depending on a level of sclerosis), fibrous type astrocytes present in gray matter, and gliotic scar astrocytes in HS. Reactive astrocytes were observed in: (I) the molecular layer of the dentate gyrus in HS cases (19/41 cases, usually in association with granule cell layer dispersion); (II) the 'transitional' zone between sclerotic and non sclerotic areas in CA1 (5 cases); and (III) focal areas in non-sclerotic CA1 and in adjacent subiculum in HS (2 cases) and in non-HS (4 cases). Reactive astrocytes revealed downregulation (in comparison with protoplasmic astrocytes) in immunoreactivity for glutamate transporters (EAAT1 and EAAT2), glutamine synthetase (GS), and Kir 4.1 (main astrocyte isoform of inwardly rectifying potassium channel). They also showed prominent downregulation in connexin 30 and upregulation in connexin 43. No obvious neuronal abnormalities (activated caspase 3, Hsp-70, Egr-1, MAP2) or microglial activation (CD68, LN-3) were identified in areas with reactive astrocytes. Conclusions: The presence of reactive astrocytes suggests that there are ongoing pathological processes in epileptic hippocampi at the time of surgical resection. Decreased astrocytic ability to uptake glutamate and potassium may predispose to epileptiform activity.