QUALITATIVE-ANALYSIS OF MEMBRANE CURRENTS IN GLIAL-CELLS FROM NORMAL AND GLIOTIC TISSUE IN-SITU - DOWN-REGULATION OF NA-2 PURINERGIC RESPONSES( CURRENT AND LACK OF P)

To date, the electrophysiological properties of glial cells located in reactive scar tissue are unknown. To address this issue two subtypes of hippocampal glial cells, located in thin vital slices of normal or gliotic brain tissue, were analysed for their voltage controlled ion c hannels using the patch-clamp technique. Reactive gliosis was induced in adult rats by a single peritoneal injection of kainic acid. The int ensity of the following seizures was rated ascending from 1 to 6. Rats which exhibited seizures of level 3 or higher showed, within three da ys, a marked loss of pyramidal cells (60% in CA1 and CA3) and an incre ase in the density of glial fibrillary acidic protein immunostaining, representing an apparent increase in the number and size of astrocytes in all layers of the hippocampal CAI subfield. Reactive and normal as trocytes of one subtype, electrophysiologically characterized by time- independent potassium currents, did not significantly differ in membra ne potential and potassium conductivity. Glutamine synthetase-positive , but mostly glial fibrillary acidic protein-negative, glial cells (pr esumably representing immature astrocytes) were also included in this study. This subtype of glial cells showed several voltage- and time-de pendent potassium currents and, under control conditions, tetrodotoxin -sensitive voltage-gated Na+ channels, which were almost completely lo st after reactive gliosis. Another part of this study focuses on the s ensitivity of reactive and control glial cells for extracellular ATP. Several in vitro studies suggest that P-2 purinergic receptors on glia l cells could trigger the induction of reactive gliosis. In contrast t o results described on cultured astrocytes, we found in situ that hipp ocampal glial cells were not sensitive to ATP or stable P-2 receptor a gonists in control or in gliotic brain slices. In summary, the presenc e of at least two different subtypes of hippocampal astrocytes was dem onstrated for control as well as for gliotic brain tissue. A dramatic down-regulation of tetrodotoxin-sensitive sodium channels in one subpo pulation of reactive astrocytes was shown. This result supports the hy pothesis that the presence of active neurons could be required to main tain glial voltage-gated sodium channels. Furthermore, we conclude tha t there is no longtime expression of P2 purinoceptors on hippocampal a strocytes in situ, and therefore the involvement of astrocytic ATP rec eptors in the genesis of reactive gliosis is unlikely. (C) 1997 IBRO. Published by Elsevier Science Ltd.