Mechanisms of H+ and Na+ changes induced by glutamate, kainate, and D- aspartate in rat hippocampal astrocytes

Abstract

The excitatory transmitter glutamate (Glu), and its analogs kainate (KA), and D-aspartate (D-Asp) produce significant pH changes in glial cells. Transmitter-induced pH changes in glial cells, generating changes in extracellular pH, may represent a special form of neuronal-glial interaction. We investigated the mechanisms underlying these changes in intracellular H+ concentration ([H+](i)) in cultured rat hippocampal astrocytes and studied their correlation with increases in intracellular Na+ concentration ([Na+](i)), using fluorescence ratio imaging with 2',7'-bis(carboxyethyl)- 5,6-carboxyfluorescein (BCECF) or sodium-binding benzofuran isophthalate (SBFI). Glu, KA, or D-Asp evoked increases in [Na+](i); Glu or D-Asp produced parallel acidifications. KA, in contrast, evoked biphasic changes in [H+](i), alkaline followed by acid shifts, which were unaltered after Ca2+ removal and persisted in 0 Cl--saline, but were greatly reduced in CO2/HCO3-free or Na+-free saline, or during 4,4'-diisothiocyanato- stilbene-2,2'-disulphonic acid (DIDS) application. The non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked KA-evoked changes in [H+](i) and [Na+](i), indicating that they were receptor- ionophore mediated. In contrast, CNQX increased the [H+](i) change and decreased the [Na+](i) change induced by Glu. D-Asp, which is transported but does not act at Glu receptors, induced [H+](i) and [Na+](i) changes that were virtually unaltered by CNQX. Our study indicates that [Na+](i) increases are not primarily responsible for Glu- or KA-induced acidifications in astrocytes. Instead, intracellular acidifications evoked by Glu or D-Asp are mainly caused by transmembrane movement of acid equivalents associated with Glu/Asp-uptake into astrocytes. KA-evoked biphasic [H+](i) changes, in contrast, are probably attributable to transmembrane ion movements mediated by inward, followed by outward, electrogenic Na+/HCO3 cotransport, reflecting KA-induced biphasic membrane potential changes.