Inhibitory effects of (2S, 3S)-3-[3-[4-(trifluoromethyl) tun benzoylamino]benzyloxy]aspartate (TFB-TBOA) on the astrocytic sodium responses to glutamate

Astrocytes are responsible for the majority of the clearance of extracellular glutamate released during neuronal activity. DL-threo-beta-benzyloxyaspartate (TBOA) is extensively used as inhibitor of glutamate transport activity, but suffers from relatively low affinity for the transporter. Here, we characterized the effects of (2S, 3S)-3-[3-[4-(trifluoromethyl) benzoylamino]benzyloxy]aspartate (TFB-TBOA), a recently developed inhibitor of the glutamate transporter on mouse cortical astrocytes in primary culture. The glial Na+-glutamate transport system is very efficient and its activation by glutamate causes rapid intracellular Na+ concentration (Na-i(+)) changes that enable real time monitoring of transporter activity. Na-i(+) was monitored by fluorescence microscopy in single astrocytes using the fluorescent Na+-sensitive probe sodium-binding benzofuran isophtalate. when applied alone, TFB-TBOA, at a concentration of 1 mu M, caused small alterations of Na-i(+). TFB-TBOA inhibited the Na-i(+) response evoked by 200 mu M glutamate in a concentration-dependent manner with IC50 value of 43 +/- 9 nM, as measured on the amplitude of the Na-i(+) response. The maximum inhibition of glutamate-evoked Na-i(+) increase by TFB-TBOA was >80%, but was only partly reversible. The residual response persisted in the presence of the AMPA/kainate receptor antagonist CNQX. TFB-TBOA also efficiently inhibited Na-i(+) elevations caused by the application of D-aspartate, a transporter substrate that does not activate non-NMDA ionotropic receptors. TFB-TBOA was found not to influence the membrane properties of cultured cortical neurons recorded in whole-cell patch clamp. Thus, TFB-TBOA, with its high potency and its apparent lack of neuronal effects, appears to be one of the most useful pharmacological tools available so far for studying glial glutamate transporters. (C) 2009 Elsevier B.V. All rights reserved.