Differential distribution of ionotropic glutamate receptor subunits in the rat olfactory bulb

The subcellular localization of ionotropic glutamate receptor (GluR) subunits was examined with light and electron microscopy in the rat olfactory bulb by using antibodies to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunits: GluR1, GluR2/3, and GluR4; and kainate (KA) receptor subunits: GluR5/6/7. Immunoreactivity to GluR1 was heavy in the glomerular layer, moderate in the external plexiform layer, and localized to periglomerular somata and dendrites, short axon somata and dendrites, mitral cell somata, and mitral/tufted dendrites. GluR2/3 immunoreactivity was heavy in the external plexiform and glomerular layers and localized to periglomerular somata and dendrites, mitral cell somata, mitral/tufted dendrites, granule cell somata, and olfactory nerve-associated glia. GluR4 immunoreactivity showed heavy staining in the external plexiform and olfactory nerve layers with localization to mitral cells, mitral/tufted dendritic processes, and olfactory nerve glial processes. GluR5/6/7 immunoreactivity was heavy in the external plexiform layer, moderate in the olfactory nerve and glomerular layers, and localized to granule cells, mitral cells, and mitral/tufted dendritic processes. Ultrastructural immunolabeling for all antibodies examined showed immunoreactivity in the postsynaptic membrane and densities, adjacent dendritic cytoplasm, and somatic cytoplasm. These data demonstrate a highly specific laminar, cellular, and subcellular distribution of ionotropic GluR subunits within the primary afferent and local synaptic circuits of the olfactory bulb. The results are consistent with the notion that the different roles subserved by glutamate in the olfactory bulb are actuated, in part, by a differential distribution of GluR subunits.