The distribution of several glutamate receptor subunits was investigated in cultured rat hippocampal neurons by in situ hybridization and immunocytochemistry. The AMPA/kainate-selective receptors GIuR1-6 exhibited two patterns of mRNA expression: most neurons expressed GluR1, R2, and R6, whereas only about 20% expressed significant levels of GIuR3, R4, and R5. By immunocytochemistry, the metabotropic glutamate receptor mGluR1α was detectable only in a subpopulation of GABAergic interneurons. GluR1 and GIuR2/3 segregated to the somatodendritic domain within the first week in culture, even in the absence of synaptogenesis. Glutamate receptor-enriched spines developed later and were present only on presumptive pyramidal cells, not on GABAergic interneurons. Clusters of GIuR1 and GluR2/3 completely colocalized and were restricted to a subset of postsynaptic sites. Thus, glutamate receptor subunits exhibit both a cell type-specific expression and a selective subcellular localization.
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