Combinations of AMPA receptor subunit expression in individual cortical neurons correlate with expression of specific calcium-binding proteins.

The functional properties of AMPA-type glutamate receptors are determined by their subunit composition. We detected the expression of the AMPA receptor subunits (GluR1-GluR4) in neurons in the somatosensory cortex of adult rats by combining nonradioactive in situ hybridization using digoxigenin-labeled RNA probes of GluR1 and GluR2 with immunocytochemistry using specific antibodies against GluR1, GluR2/3, and GluR4. On the basis of differential expression of the GluR1 and GluR2 subunits, we classified the cortical neurons into four categories. To correlate the differential expression of AMPA receptor subunits in each neuron with that of two calcium-binding proteins, parvalbumin and calbindin-D28k, we used a triple-labeling method. The majority of cortical neurons ( approximately 2/3) showed expression of GluR2 and undetectable expression of GluR1. GluR1-/GluR2-expressing neurons and GluR1-expressing/GluR2-undetectable neurons comprised approximately 1/10 each. Regarding the morphology, most GluR1-undetectable/GluR2-expressing neurons were pyramidal cells in layers II/III, V, and VI, whereas most GluR1-expressing/GluR2-undetectable neurons were nonpyramidal cells in layers II-VI. The GluR1-/GluR2-expressing neurons were either pyramidal or nonpyramidal. The majority of GluR1-/GluR2-expressing nonpyramidal cells was intensely stained with monoclonal antibody against calbindin-D28k, and one-half of the GluR1-undetectable/GluR2-expressing pyramidal neurons in layer II/III were lightly stained with this antibody. Most of GluR1-expressing/GluR2-undetectable neurons possessed parvalbumin immunoreactivity. These results indicate that neurons in the rat somatosensory cortex express differential combinations of GluR subunits, which correlate with the specific expression of the calcium-binding proteins.