Distinct NMDA receptor subpopulations contribute to long-term potentiation and long-term depression induction.

Long-term potentiation (LTP) and long-term depression (LTD) are persistent modifications of synaptic strength that have been implicated in learning, memory, and neuronal development. Despite their opposing effects, both forms of plasticity can be triggered by the activation of NMDA receptors. One mechanism proposed for this bidirectional response is that the specific patterns of afferent stimulation producing LTP and LTD activate to different degrees a uniform receptor population. A second possibility is that these patterns activate separate receptor subpopulations composed of different NMDA receptor (NR) subunits. To test this hypothesis we examined the inhibition of LTP and LTD by a series of competitive NMDA receptor antagonists that varied in their affinities for NR2A/B and NR2C/D subunits. The potency for the inhibition of LTP compared with inhibition of LTD varied widely among the agents. Antagonists with higher affinity for NR2A/B subunits relative to NRC/D subunits showed more potent inhibition of LTP than of LTD. D-3-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid, which binds to NR2A/B with very high affinity relative to NR2C/D, showed an approximately 1000-fold higher potency for LTP than for LTD. These results show that distinct subpopulations of NMDA receptors characterized by different NR2 subunits contribute to the induction mechanisms of potentiation and depression.

[3H]homoquinolinate binds to a subpopulation of NMDA receptors and to a novel binding site.

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.