3-methylglutamic-acid and 2-3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline

The glutamate analog (+/-) threo-3-methylglutamate (T3MG) has recently been reported to inhibit the EAAT2 but not EAAT1 subtype of high-affinity, Na(+)-dependent excitatory amino acid transporter (EAAT). We have examined the effects of T3MG on glutamate-elicited currents mediated by EAATs 1-4 expressed in Xenopus oocytes and on the transport of radiolabeled substrate in mammalian cell lines expressing EAATs 1-3. T3MG was found to be an inhibitor of EAAT2 and EAAT4 but a weak inhibitor of EAAT1 and EAAT3. T3MG competitively inhibited uptake of D-[(3)H]-aspartate into both cortical and cerebellar synaptosomes with a similar potency, consistent with its inhibitory activity on the cloned EAAT2 and EAAT4 subtypes. In addition, T3MG produced substrate-like currents in oocytes expressing EAAT4 but not EAAT2. However, T3MG was unable to elicit heteroexchange of preloaded D-[(3)H]-aspartate in cerebellar synaptosomes, inconsistent with the behavior of a substrate inhibitor. Finally, T3MG acts as a poor ionotropic glutamate receptor agonist in cultured hippocampal neurons: concentrations greater than 100 microM T3MG were required to elicit significant NMDA receptor-mediated currents. Thus, T3MG represents a pharmacological tool for the study of not only the predominant EAAT2 subtype but also the EAAT4 subtype highly expressed in cerebellum.

Topics: 2-Amino-5-phosphonovalerate; Action Potentials; Amino Acid Transport System X-AG; Animals; Aspartic Acid; ATP-Binding Cassette Transporters; Carrier Proteins; Cell Line; Cerebellum; Cerebral Cortex; Dogs; Excitatory Amino Acid Transporter 1; Excitatory Amino Acid Transporter 2; Excitatory Amino Acid Transporter 3; Excitatory Amino Acid Transporter 4; Glutamate Plasma Membrane Transport Proteins; Glutamic Acid; Hippocampus; Kidney; Kinetics; Neurotransmitter Uptake Inhibitors; Oocytes; Quinoxalines; Rats; Rats, Sprague-Dawley; Receptors, Glutamate; Receptors, Neurotransmitter; Recombinant Fusion Proteins; Symporters; Synaptosomes; Xenopus laevis