Quantitative physiological characterization of a quinoxalinedione non-NMDA receptor antagonist

The effects of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, or FG 9065) on excitatory amino acid responses in cultured neurons from rat hippocampus were studied using tight-seal whole-cell recording techniques. CNQX reduced the magnitude of peak inward currents produced by exogenously applied kainate, quisqualate, and N-methyl-D-aspartate (NMDA) with Ki's of 2.5, 3.5, and 96 microM, respectively. The antagonism was competitive against kainate and quisqualate, but noncompetitive against NMDA. Glycine markedly reduced CNQX antagonism of NMDA responses. The same recording technique using pairs of monosynaptically connected neurons demonstrated reversible diminution of excitatory postsynaptic potentials in 7 of 7 pairs, using CNQX at concentrations as low as 10 microM. CNQX applied alone did not evoke inward or outward currents at membrane potentials near the resting membrane potential and did not affect the current-voltage relationship at membrane potentials between -90 and -30 mV. These observations represent the first quantitative characterization of glutamate receptor antagonism by CNQX with respect to physiological rather than biochemical parameters and demonstrate that CNQX is far more potent and more selective than currently available non-NMDA antagonists. The results suggest that CNQX will be a useful pharmacologic tool for the study of synaptic transmission in a variety of systems in which glutamate or related excitatory amino acids are involved.