The diversity of neuronal glutamate receptors continues to increase with the discovery of multiple subunits and subunit families. The significance of this potential receptor heterogeneity is unknown because pharmacological tools that could clearly distinguish between different structural isoforms have not yet been identified. A novel glutamate receptor antagonist, 5-nitro-6,7,8,9-tetrahydrobenzo[g]indole-2,3-dione-3-oxime (NS-102), has been shown previously to selectively block the low affinity [3H]kainate binding site in rat brain. We have examined the effect of NS-102 on receptors expressed in fibroblasts from either glur6 subunits or a combination of glurB and glurD (glurB/D receptors). NS-102 (3 microM) reduced currents mediated by glur6 receptors and had very little effect on currents mediated by glurB/D receptors. The binding of [3H]kainate to glur6 receptors showed properties similar to those of the brain low affinity [3H]kainate binding site, and NS-102 inhibited specific binding to glur6 receptors with a potency nearly identical to those sites in brain membranes. Our findings suggest that NS-102 will be useful in identifying the functional role of native receptors containing a glur6 subunit.