Elucidation of Glutamate Transporter Functions Using Selective Inhibitors

L-Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system (CNS). Termination of glutamate receptor activation and maintenance of low extracellular glutamate concentrations are mainly achieved by glutamate transporters (excitatory amino acid transporters 1-5: EAATs1-5) located in nerve endings and surrounding glia cells. Selective and potent inhibitors are needed to identify the physiological roles of transporters in the regulation of synaptic transmission or in the pathogenesis of neurological diseases. Glutamate or aspartate analogs such as threo-β-hydroxyaspartate (THA) and pyrrolidine dicarboxylate (PDC) derivatives have served as important experimental tools. Pharmacologically useful probes have emerged from modification of known inhibitors, such as threo-β- benzyloxyaspartate (DL-TBOA) which functions as a non-transportable blocker for all subtypes of EAATs. Nontransportable blockers are indispensable because, unlike substrates, they do not cause heteroexchange. By comparing the effects of substrates and non-transportable blockers, physiological roles of EAATs have been revealed. In this review, we will describe the functions of EAATs elucidated using these inhibitors. EAATs not only remove transmitter from synaptic clefts but also actively modulate neurotransmission. Moreover, high affinity ligands have been developed as novel pharmacological tools. TBOA analogs possessing a bulky substituent on their benzene ring significantly inhibited labeled glutamate uptake, the most potent of compound being (2S, 3S)-3-{3-[4-(trifluoromethyl)benzoyl-amino]benzyloxy}aspartate (TFB-TBOA). The pharmacological characterization of TFB-TBOA is also presented in this review.