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 determi ned by their subunit composition. We detected the expression of the AM PA receptor subunits (GluR1-GluR4) in neurons in the somatosensory cor tex of adult rats by combining nonradioactive in situ hybridization us ing digoxigenin-labeled RNA probes of GluR1 and GluR2 with immunocytoc hemistry using specific antibodies against GluR1, GluR2/3, and GluR4. On the basis of differential expression of the GluR1 and GluR2 subunit s, we classified the cortical neurons into four categories. To correla te the differential expression of AMPA receptor subunits in each neuro n with that of two calcium-binding proteins, parvalbumin and calbindin -D28k, we used a triple-labeling method. The majority of cortical neur ons (similar to 2/3) showed expression of GluR2 and undetectable expre ssion of GluR1. GluR1-/GluR2-expressing neurons and GluR1-expressing/G luR2-undetectable neurons comprised similar to 1/10 each. Regarding th e morphology, most GluR1-undetectable/GluR2-expressing neurons were py ramidal cells in layers II/III, V, and VI, whereas most GluR1-expressi ng/GluR2-undetectable neurons were nonpyramidal cells in layers Il-VI. The GluR1-/GluR2-expressing neurons were either pyramidal or nonpyram idal. The majority of GluR1-/GluR2-expressing nonpyramidal cells was i ntensely 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 immunorea ctivity. These results indicate that neurons in the rat somatosensory cortex express differential combinations of GluR subunits, which corre late with the specific expression of the calcium-binding proteins.