AMPA GluR2 subunit is differentially distributed on GABAergic neurons and pyramidal cells in the macaque monkey visual cortex

Abstract

The cellular and synaptic distribution of the AMPA receptor subunit GluR2 was analyzed in the monkey primary visual cortex (area V1), by immunocytochemistry and postembedding immunogold methods. GluR2 immunoreactivity was widely distributed in all of the layers of area V1. A quantitative double labeling analysis in layers II and III revealed that the vast majority of GABAergic interneurons in this area also contained GluR2. Postembedding immunogold analysis revealed that GluR2 immunoreactivity was present at asymmetric synapses on both GABAergic interneurons and pyramidal cells. A quantitative study indicated that the number of GluR2 immunogold particles at asymmetric synapses on pyramidal cells was significantly higher than that on GABAergic interneurons. These results from the primate neocortex are in agreement with and extend our previous studies on the rat hippocampus and amygdala. In view of the dominant role of the GluR2 subunit in regulating calcium flux through AMPA receptors, the differential synaptic distribution of GluR2 on different neuronal types might provide a mechanism for cell-specific response properties to glutamate as well as clues to selective neuronal vulnerability and cell death mediated by calcium-dependent excitotoxic mechanisms.

Introduction

The excitatory neurotransmitter glutamate can act through a variety of postsynaptic receptors, including cation-specific ion channels, the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), N-methyl-d-aspartate (NMDA), and kainate ionotropic receptors, and G protein-coupled metabotropic receptors [28], [29], [32]. GluR2 is the most important regulatory subunit in the AMPA family of glutamate receptors (GluRs), in that its presence inhibits calcium flux and dominates the current/voltage characteristics of AMPA receptors [20]. Previous studies from our laboratory have shown that GluR2 immunoreactivity in the rat hippocampus and amygdala is frequently located at asymmetric synapses on both pyramidal cell spines and dendritic processes as well as on somata of GABAergic interneurons, but that the labeled synapses on GABAergic neurons have a considerably lower number of immunogold particles than those on pyramidal cells [14], [15].

In primary visual cortical circuits, inputs from the periphery to cortical cells are primarily excitatory and arise from a patterned thalamic activity, as well as from massive convergent networks from other excitatory cortical cells [1], [2], [30], [40], [42]. Excitatory activity from the thalamocortical projections to area V1 is integrated with the activity of a series of layer-specific cortical connections that progressively transform and relay visual information [18], [41]. Recently, immunocytochemical studies revealed that single AMPA subunits are differentially distributed in the rat and monkey cerebral cortex [2], [3], [26], [27], [34]. At the light microscopic level, Gutierrez-Ibarluzea et al. [10] using a polyclonal antibody to GluR2/3, found that approximately 10% of GABAergic neurons contained GluR2/3, whereas most studies showed that GluR2/3 is preferentially distributed in pyramidal cells [2], [3], [16], [26], [27], [34]. In contrast, using a subunit specific monoclonal antibody to GluR2, Vissavajjhala et al. [44] reported that calcium-binding protein-containing neurons in the rat cerebral cortex are widely colocalized with GluR2, and a preliminary study indicated that most calcium-binding protein-containing neurons in the monkey visual cortex also contain GluR2 [31]. However, the detailed distribution of GluR2 at the level of single asymmetric synapses on GABAergic neurons or pyramidal cells has not yet been determined. The present study attempts to elucidate GluR2 immunoreactivity in a population of asymmetric synapses on dendrites of GABAergic neurons and on dendritic spines of pyramidal neurons in the macaque monkey primary visual cortex by using postembedding electron microscopy. The resultant data are consistent with our previous data on rat hippocampus [14] and amygdala [15], and suggest that differential synaptic representation of GluR2 in interneurons and pyramidal cells may be a general principle that transcends region and species.