Effects of glial glutamate transporter inhibitors on intracellular Na+ in mouse astrocytes

The effects of inhibitors of the glial Na+ /glutamate co-transporter on the intracellular Na+ concentration ([Na+](I)) were investigated in mouse cortical astrocytes. [Na+](I) was monitored by fluorescence microscopy on single astrocytes using the Na+-sensitive probe sodium-binding benzofuran isophtalate. Application of the competitive inhibitors threo-beta -hydroxyaspartate (T IA) and trans-pyrrolidine-2,4-dicarboxylic acid (t-PDC) resulted in robust and reversible increases in [Na+](I) that were comparable in shape to the response to glutamate but about twice lower in amplitude. As previously observed with glutamate, the amplitude of the [Na+](I) response to these compounds was concentration-dependent with EC50 values of 11.1 muM (THA) and 7.6 muM (t-PDC), as was the initial rate of [Na+](I) rise (EC50 values of 14.8 muM for THA and 11.5 muM for t-PDC). Both compounds diminished the response to subsequent glutamate applications, possibly because of an inhibitory effect of the intracellularly-accumulated compounds. In comparison, the newly-developed compound threo-beta -benzyloxyaspartate (TBOA) alone did not cause any significant alteration of [Na+](I) u(p) to a concentration of 500 muM. TBOA inhibited the [Na+]I response evoked by 200 muM glutamate in a concentration-dependent manner with IC50 values of 114 and 63 muM. as measured on the amplitude and the initial rate, respectively. The maximum inhibition of glutamate-evoked [Na+](I) increase by TBOA was similar to 70%. The residual response persisted in the presence of a non-NMDA receptor antagonist or the inhibitor of the GLT-1 glutamate transporters, dihydrokainate: (DHK). In view of the complete reversibility of its effects, TBOA represents a very useful pharmacological tool for studies of glutamate transporters. (C) 2001 Elsevier Science B.V. All rights reserved.