[3H]Homoquinolinate Binds to a Subpopulation of NMDA Receptors and to a Novel Binding Site

Abstract: NMDA receptors mediate several important functions in the CNS; however, little is known about the pharmacology, biochemistry, and function of distinct NMDA receptor subtypes in brain tissue. To facilitate the study of native NMDA receptor subpopulations, we have determined the radioligand binding properties of ³H]homoquinolinate, a potential subtype-selective ligand. Using quantitative receptor autoradiography, NMDA-specific ³H]homoquinolinate binding selectively labeled brain regions expressing NR2B mRNA (layers I–III of cerebral cortex, striatum, hippocampus, and septum). NMDA-specific ³H]homoquinolinate binding was low in brain regions that express NR2C and NR2D mRNA (cerebellar granular cell layer, NR2C; glomerular layer of olfactory bulb, NR2C/NR2D; and midline thalamic nuclei, NR2D). In forebrain, the pattern of NMDA-specific ³H]homoquinolinate binding paralleled NR2B and not NR2A distribution. In addition to NMDA-displaceable binding, there was a subpopulation of ³H]homoquinolinate binding sites in the forebrain, cerebellum, and choroid plexus that was not displaced by NMDA or l -glutamate. In contrast, we found that the derivative of homoquinolinate, 2-carboxy-3-carboxymethylquinoline, markedly inhibited the NMDA-insensitive binding of ³H]homoquinolinate without inhibiting the NMDA-sensitive population. ³H]Homoquinolinate may be useful for selectively characterizing NR2B-containing NMDA receptors in a preparation containing multiple receptor subtypes and for characterizing a novel binding site of unknown function.