Selectivity of quinoxalines and kynurenines as antagonists of the glycine site on N-methyl-D-aspartate receptors.

Xenopus oocytes injected with rat brain mRNA were used to identify and characterize the effects of compounds that are antagonists at both the glycine site on N-methyl-D-aspartate (NMDA) receptors and the quisqualate/kainate receptor. Oocytes were voltage-clamped at -60 mV and inward currents were measured at equilibrium following perfusion with agonists and antagonists. Application of 7-chlorokynurenic acid (7-Cl-Kyn) or 6,7-dichloro-3-hydroxy-2-quinoxaline carboxylic acid (6,7-diCl-HQC), each at 15 microM, produced a parallel shift to the right of the glycine concentration-response curve. Schild analysis indicated a KB of 300 nM for 6,7-diCl-HQC and 350 nM for 7-Cl-Kyn. The slopes of the Schild plots were 1.01-1.02 in each case, suggesting that both compounds are competitive glycine antagonists. Both compounds also blocked the receptor mediating kainate-induced inward current. Schild analysis of 6,7-diCl-HQC (KB = 3.0 microM, slope = 0.98) indicated competitive antagonism of kainate currents, but with a potency 10-fold lower than at the glycine site. 7-Cl-Kyn antagonized kainate-evoked currents (KB = 14.1 microM), but the slope of the Schild regression was less than 1 (0.72 +/- 0.11; p less than 0.05). Thus, 7-Cl-kyn was approximately 40-fold more potent at the glycine site than at the receptor mediating kainate currents but is probably not entirely competitive at the latter receptor. Omission of the Cl groups from these antagonists drastically reduced activity at both glycine and kainate sites. 6,7-Dinitro- and 6-cyano-7-nitro-quinoxalinedione were both more potent antagonists of kainate than glycine, but substitution of Cl at the 6-position and especially the 6- and 7-positions increased potency at the glycine site. These results suggest that the glycine coagonist site of the NMDA receptor and the agonist binding site of the quisqualate/kainate receptor have some structural similarity. Halogenated derivatives of quinoxalines and kynurenines should be useful in evaluating the function of the glycine site in synaptic transmission mediated by NMDA receptors. In this regard we found that 7-Cl-kyn (5 and 15 microM) selectively attenuated NMDA receptor-mediated epileptiform bursts in the CA1 region of hippocampal slices perfused with zero-Mg medium, without reducing the amplitude of the primary population spike. This block could be overcome by 300 microM D-serine, which alone did not influence bursting. These results together indicate that the glycine site plays a role in epileptiform bursting mediated by NMDA receptors in adult rat hippocampus.