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Regional and laminar differences in synaptic localization of NMDA receptor subunit NR1 splice variants in rat visual cortex and hippocampus

Changes in N‐methyl‐D‐aspartate (NMDA) receptor expression may represent a molecular substrate for differences in synaptic plasticity between early postnatal and adult brains (Fox and Zahs [1994] Curr. Opinion Neurobiol. 4:112–119). We have, therefore, examined the regional and laminar distribution of NR1, the essential subunit of the NMDA receptor, in two regions in which synaptic plasticity has been most thoroughly studied: primary visual cortex and hippocampus. To study NR1 expression at the light and electron microscopic levels we have used a new antiserum (NR1‐C1; Sheng et al. [1994] Nature 368:144–147) directed against a differentially spliced C‐terminal exon (“C1”). The most striking result was that the pattern of NR1‐C1 labeling in the adult was more restricted than that of previously published NR1‐specific antibodies. Specifically, NR1‐C1 did not label cells in the CA3, dentate gyrus or subicular regions of the hippocampus or in layer 4 of the visual cortex. Quantitative ultrastructural analysis revealed that these differences were paralleled by differential expression of NR1‐C1 at synapses.