Immunocytochemical characterization of AMPA-selective glutamate receptor subunits: laminar and compartmental...

Subunit proteins that comprise functional AMPA receptors were localized by immunocytochemical methods in the adult macaque primary visual cortex (V1). GluR1, GluR2/3/4c, and GluR4 immunoreactivity consisted of rich plexuses of punctate profiles scattered throughout the neuropil, in radial arrays, and outlining the membrane of somata and proximal dendrites. Cytoplasmic immunoreactivity was limited. GluR2/3/4c immunostaining was more prominent along the somata surface and exhibited greater levels of cytoplasmic immunoreactivity than GluR1 and GluR4 immunostaining. The density of AMPA subunit immunoreactive elements also varied across layers and compartments of macaque V1. Immunoreactivity for GluR1, GluR2/3/4c, and GluR4 was densest in three bands that corresponded to layers IVA, IVC, and VI. Immunostaining for each subunit was also unevenly distributed within many of the layers. In layers II-III, patches of intense immunostaining coincided with cytochrome oxidase (CO)-rich blobs. In layer IVA, intense subunit staining formed a conspicuous honeycomb pattern. In layer IVC, subunit staining formed a radial lattice. GluR2/3/4c subunit immunostaining was also preferentially distributed within the CO-rich blobs of layers V-VI. These findings demonstrate that AMPA subunit immunoreactivity is densely concentrated in layers and compartments receiving direct geniculocortical innervation. This distribution, which differs from that of excitatory synapses, suggests that the density of AMPA receptors is unevenly distributed at synaptic and possibly extrasynaptic sites within macaque visual circuits.