6‐Nitro‐7‐sulphamoylbenzo[f]quinoxaline‐2,3‐dione (NBQX) is a competitive antagonist selective for α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate (AMPA) receptors. Here we report the pharmacological characteristics and anatomical distribution of [3H]NBQX binding to rat brain. The association rate of [3H]NBQX to rat cerebrocortical membranes was rapid, with peak binding occurring within 10 min at 0°C. The off‐rate was also rapid, with near‐complete dissociation of the radioligand within 5 min of addition of 1 mM unlabelled l‐glutamate. [3H]NBQX bound to a single class of sites with KD and Bmax values of 47 nM and 2.6 pmol mg−1 of protein, respectively. The rank order of inhibition of [3H]NBQX binding by AMPA receptor ligands was NBQX ≫ 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) ≥ (S)‐5‐fluorowillardiine ≥ AMPA ≫l‐glutamate. The chaotrope KSCN had no effect on the IC50 value of unlabelled NBQX displacement of [3H]NBQX binding. The kainate receptor‐selective ligands NS102 and kainate were only very weak displacers. It is interesting that NBQX and CNQX displaced significantly more [3H]NBQX than any of the agonists tested. Autoradiographic analysis of the binding of [3H]NBQX to coronal sections showed a distribution compatible with that of [3H]AMPA binding. These data indicate that [3H]NBQX provides a useful novel tool to characterise the antagonist binding properties of AMPA receptors.

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