Ionotropic glutamate receptor GluR1 in the visual cortex of hamster: distribution and co-localization with calcium-binding proteins and GABA

Abstract

The subunit composition of the AMPA receptor is critical to its function. AMPA receptors that display very low calcium permeability include the GluR2 subunit, while AMPA receptors that contain other subunits, such as GluR1, display high calcium permeability. We have studied the distribution and morphology of neurons containing GluR1 in the hamster visual cortex with antibody immunocytochemistry. We compared this labeling to that for calbindin D28K, parvalbumin, and GABA. Anti-GluR1-immunoreactive (IR) neurons were located in all layers. The highest density of GluR1-IR neurons was found in layers II/III. The labeled neurons were non-pyramidal neurons, but were varied in morphology. The majority of the labeled neurons were round or oval cells. However, stellate, vertical fusiform, pyriform, and horizontal neurons were also labeled with the anti-GluR1 antibody. Two-color immunofluorescence revealed that many of the GluR1-IR neurons in the hamster visual cortex were double-labeled with either calbindin D28K (31.50%), or parvalbumin (22.91%), or GABA (63.89%). These results indicate that neurons in the hamster visual cortex express GluR1 differently according to different layers and selective cell types, and that many of the GluR1-IR neurons are limited to neurons that express calbindin D28K, parvalbumin, or GABA. The present study elucidates the neurochemical structure of GluR1, a useful clue in understanding the differential vulnerability of GluR1-containing neurons with regard to calcium-dependent excitotoxic mechanisms.

Fig. 1

Low power photomicrographs of the distribution of GluR1-IR neurons in the hamster visual cortex. (A) Thionin-stained section shows the cortical lamination. (B) Anti-GluR1-IR neurons. (C, D) Control sections used to show the specificity of GluR1 antibody. In C, section was incubated without the addition of the primary antibody. In D, the GluR1 antibody was preabsorbed with antigen prior to tissue incubation. I–VI: cortical layers. Bar=100 µm.

Fig. 2

Histogram of the distribution of neurons labeled by anti-GluR1 antibody in the hamster visual cortex. Anti-GluR1-IR neurons were predominantly located in layers II/III, V and VI. Error bars: SD.

Fig. 3

High power differential interference contrast (DIC) photomicrographs of some GluR1-IR neurons in the hamster visual cortex. (A, B arrow) Multipolar round or oval neurons in cortical layers II/III. The large majority of anti-GluR1-IR neurons were round or oval cells with many dendrites coursing in all directions. (B arrowhead, D) Multipolar stellate neurons. Stellate neurons had polygonally-shaped cell bodies with numerous dendrites coursing in all directions. (B asterisk, C) Vertical fusiform neurons with its longitudinal axis perpendicular to the pial surface. (E) Pyriform neuron with a thick primary dendrite oriented toward the pial surface. This ascending process had many small branches, forming a dendritic bouquet. (F) A horizontal neuron with a horizontal fusiform cell body with horizontally oriented processes. Bar=20 µm.

Fig. 4

Fluorescence confocal photomicrographs of hamster visual cortex immunostained for (A, D, G) GluR1 (B) calbindin D28K, (E) parvalbumin, and (H) GABA. (C) Superimposition of images in GluR1 and calbindin D28K. (F) Superimposition of images in GluR1 and parvalbumin. (I) Superimposition of images in GluR1 and GABA. Arrowheads indicate some co-localized neurons. The double-labeled neurons were found in all layers except in layer I. Bar=50 µm.