Distribution of glutamate receptor subunits in the primate temporal cortex and hippocampus

Abstract

The distribution of subunits for the N-methyl- d-aspartate (NR1, NR2A/B), α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (GluR1, GluR2/3, GluR4) and low affinity kainate (GluR5/6/7) ionotropic glutamate receptors was examined by immunocytochemistry in the temporal cortex and hippocampus of the rhesus macaque ( Macaca mulatta). Neurons expressing NR1, NR2A/B, GluR2/3, and GluR4 subunits were widely distributed in all of the cortical layers but the overall density of the GluR4-immunopositive neurons was very low. Neurons expressing the GluR1 subunit were found predominantly in cortical layers V and VI while those expressing the GluR5/6/7 subunits were concentrated in layer V and were readily distinguishable by the thick elongate shape of their primary apical dendrites. Subcellular differences in the immunostaining pattern were also noted between the different glutamate receptor subunits. NR1 and NR2A/B immunoreactivity was most pronounced in somatic and primary dendritic compartments and to a lesser extent in cortical and hippocampal molecular layers. GluR1 immunoreactivity was more intense than GluR2/3 in the hippocampal molecular layers whereas GluR4 was undetectable. GluR5/6/7 immunoreactivity was very intense in the dentate molecular layer, and the CA1 pyramidal cells had a subcellular distribution of GluR5/6/7 that was similar to the cortical neurons. Overall, the distribution patterns of the different glutamate receptor subunits was identical in animals that had been ovariectomized and in ovariectomized animals that had subsequently undergone estradiol or estradiol/progesterone hormone replacement. Taken together, these findings demonstrate a differential spatial arrangement of glutamate receptor subunits in the primate temporal cortex and hippocampus, which may have functional significance for the integration of excitatory inputs to these areas. Furthermore, they show that in adult macaques, sex steroids do not play a major role in determining the distribution patterns of these receptor subunits.