Effects of a novel glutamate transporter blocker, (2S, 3S)-3-{3-[4-(trifluoromethyl)benzoylamino]benzyloxy}aspartate (TFB-TBOA), on activities of hippocampal neurons

Glutamate transporters rapidly take up synaptically released glutamate and maintain the glutamate concentration in the synaptic cleft at a low level. (2S, 3S)-3-{3-[4-(trifluoromethyl)benzoylamino]benzyloxy}aspartate (TFB-TBOA) is a novel glutamate transporter blocker that potently suppresses the activity of glial transporters. TFB-TBOA inhibited synaptically activated transporter currents (STCs) in astrocytes in the stratum radiatum in rat hippocampal slices in a dose-dependent manner with an IC50 of 13 nM, and reduced them to approximately 10% of the control at 100 nM. We investigated the effects of TFB-TBOA on glutamatergic synaptic transmission and cell excitability in CA1 pyramidal cells. TFB-TBOA (100 nM) prolonged the decay of N-methyl-d-aspartic acid receptor (NMDAR)-mediated excitatory postsynaptic currents (EPSCs), whereas it prolonged that of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated EPSCs only when the desensitization of AMPARs was reduced by cyclothiazide (CTZ). Furthermore, long-term application of TFB-TBOA induced spontaneous epileptiform discharges with a continuous depolarization shift of membrane potential. These epileptiform activities were mainly attributed to NMDAR activation. Even after pharmacological block of NMDARs, however, TFB-TBOA induced similar changes by activating AMPARs in the presence of CTZ. Thus, the continuous uptake of synaptically released glutamate by glial transporters is indispensable for protecting hippocampal neurons from glutamate receptor-mediated hyperexcitabilities.