[H-3] 5,7-DICHLOROKYNURENIC ACID, A NOVEL RADIOLIGAND LABELS NMDA RECEPTOR-ASSOCIATED GLYCINE BINDING-SITES

A strychnine-insensitive glycine binding site is located on the N-methyl-D-aspartate (NMDA)-preferring glutamate receptor complex. Kynurenic acid analogs are antagonists at this binding site. A derivative of kynurenic acid, 5,7-dichlorokynurenic acid (5,7-DCKA) was radiolabeled with H-3 and used to study antagonist binding to the glycine recognition site. This ligand ([H-3]5,7-DCKA) showed high affinity (K(d) = 69 nM), saturable (B(max) = 14.5 pmol/mg protein) binding to rat brain membranes. A variety of agonists and antagonists inhibited the binding of [H-3]5,7-DCKA and [H-3]glycine in a similar fashion (r = 0.93). In addition, glutamate site agonists and antagonists exerted opposite allosteric effects on [H-3]5,7-DCKA binding suggesting that [H-3]5,7-DCKA preferentially binds to the agonist-activated conformation of the receptor.