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), andd-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, ord-Asp evoked increases in [Na+]i; Glu ord-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/HCO−3-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+]iand [Na+]i, indicating that they were receptor–ionophore mediated. In contrast, CNQX increased the [H+]ichange and decreased the [Na+]ichange induced by Glu.d-Asp, which is transported but does not act at Glu receptors, induced [H+]iand [Na+]ichanges that were virtually unaltered by CNQX. Our study indicates that [Na+]iincreases are not primarily responsible for Glu- or KA-induced acidifications in astrocytes. Instead, intracellular acidifications evoked by Glu ord-Asp are mainly caused by transmembrane movement of acid equivalents associated with Glu/Asp-uptake into astrocytes. KA-evoked biphasic [H+]ichanges, in contrast, are probably attributable to transmembrane ion movements mediated by inward, followed by outward, electrogenic Na+/HCO−3cotransport, reflecting KA-induced biphasic membrane potential changes.

A Subconvulsive Dose of Kainate Selectively Compromises Astrocytic Metabolism in the Mouse Brain In Vivo

“…10,28,29 This was supported by several studies carried out in cultured astrocytes demonstrating alterations in cellular ion homeostasis after exposure to kainate including complex changes in intracellular concentrations of Na þ and H þ (see ref. 30) and increases in the extracellular K þ concentration likely due to increased astrocytic K þ efflux. 31 As K þ is cleared from the extracellular space predominantly by ATP-dependent exchangers in the astrocytic membrane, K þ homeostasis relies on proper energy metabolism in astrocytes.…”

Heterogeneity of Astrocyte Resting Membrane Potentials and Intercellular Coupling Revealed by Whole-Cell and Gramicidin-Perforated Patch Recordings from Cultured Neocortical and Hippocampal Slice Astrocytes

“…Astrocytes are endowed with an electrogenic Na ϩ /2HCO 3 Ϫ cotransporter that has an estimated reversal potential of Ϫ95mV and helps to maintain pH homeostasis in response to neuronal activity (Rose and Ransom, 1996). Because of its electrogenic nature (1Na ϩ for 2HC O 3 Ϫ taken up by the cell), this co-transport mechanism may be involved in the regulation of glial RMP.…”

“…Cells maintained in H EPES-buffered solution have a more acidic pH i than those kept in C O 2 /HC O 3 Ϫ HC O-buffered solution (Rose and Ransom, 1996), a condition that favors uncoupling. Similarly, gap junctions are sensitive to changes in [C a 2ϩ ] i (Giaume and McCarthy, 1996).…”

“…Cell 2 is similarly sensitive to changes in E K but has a more depolarized resting membrane potential. Under these "resting" conditions, gap junctions are kept closed because of a relatively acidic pH i (Rose and Ransom, 1996). 1, Increases in [K ϩ ] out in proximity to Cell 1 cause potassium influx into Cell 1; during this accumulation process, cell depolarization will occur because of the (Figure legend continues) sudden shift in E K. 2, Escape of intracellular potassium in this cell cannot occur because of the exclusive presence of potassium currents characterized by inward-going rectification; f urthermore, electrochemical communication with Cell 2 is precluded by the low conductance of gap junctions.…”

Stimulation of steroidogenic acute regulatory protein (STAR) geneexpression by D‐aspartate in rat Leydig cells

“…In the previous report [10] it was shown that D-Asp should be taken up into Leydig cells for the stimulation of steroidogenesis. Uptake of D-Asp through the c-glutamate transporter is known to induce several changes in cells, including intracellular acidification [22] and activation of Cl conductance [23], which may play a role in the increased StAR expression. Alternatively, a specific factor(s) may interact with D-Asp and cause the observed stimulation of StAR gene expression.…”