Development of glutamate receptor subunit 2 immunoreactivity in postnatal rat retina

Previous studies have shown that the expression of glutamate receptor subunits is developmentally regulated and have been implicated in processes of cell differentiation during postnatal life. The tissue localization and developmental pattern of the glutamate receptor 2 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptor were investigated by means of immunohistochemistry and immunoblotting. Labeling of amacrine and ganglion cells and the inner plexiform layer appeared early during development, while glutamate receptor 2 subunit expression in the outer plexiform layer started after the first postnatal week. The distribution of labeling within the inner plexiform layer changed from nonorganized to laminated appearance prior to eye-opening. There was an increasing number of positive amacrine and ganglion cell somata during the first 2 weeks, but their number decreased considerably as the retina matured and were seen at least up to 35 days of postnatal development. Little labeling was found in the ganglion cell layer and in the inner plexiform layer of late postnatal and adult retina. Labeling in the outer plexiform layer and of bipolar cell somata appeared to increase in the developing retina. Glur2 labeling of these cells and the outer plexiform layer became discernible during the second postnatal week, and this labeling was present in the adult as well. Immunoblotting showed that GluR2 protein levels were similar at postnatal days 7 and 10, but slightly decreased between the second and fourth postnatal weeks. Our data imply that the immunological expression of glutamate receptor 2 subunit in the inner plexiform layer decreases as a function of age, and is correlated with developmental event(s) in the postnatal retina.