[3H] 5,7-Dichlorokynurenic Acid, a High Affinity Ligand for the NMDA Receptor Glycine Regulatory Site

The NMDA subtype of glutamate receptors is allosterically linked to a strychnine-insensitive glycine regulatory site. Kynurenic acid and its halogenated derivatives are non-competitive NMDA antagonists acting at the glycine site. We have prepared [3H] 5,7-dichlorokyrurenic acid (DCKA) as an antagonist radioligand and have characterized its binding. 3-Bromo-5,7-DCKA was catalytically dehalogenated in the presence of tritium gas and HPLC purified to yield [3H] 5,7-DCKA with a specific activity of 17.6 Ci/mmol. [3H] 5,7-DCKA bound to rat brain synaptosomes with a Kd of 69 +/- 23 nM and Bmax = 14.5 +/- 3.2 pmoles/mg protein. Binding was 65-70% specific at 10 nM [3H] 5,7-DCKA. This ligand is thus more selective and has higher affinity than [3H] glycine, in addition to being an antagonist.