Müller and RPE cell response to photoreceptor cell degeneration in aging Fischer rats

With increasing age, retinas of male Fischer rats gradually lose photoreceptor cells beginning at the ora serrata and extending to the central retina resulting in a pronounced peripheral retinopathy. In this study, we used immunocytochemical methods for glial fibrillary acidic protein (GFAP) and carbonic anhydrase II (CA II) to study the Müller cell response to age-related photoreceptor cell degeneration in the superior retina. Retinas of Fischer rats were also examined by electron microscopy to investigate retinal pigment epithelial (RPE) cell and Bruch's membrane structural changes with advancing age. Our study showed extensive photoreceptor cell loss in the region of the ora serrata beginning by 16 months, while few photoreceptor cells were found at 23 months. Neovascularization also occurred in the area of the peripheral retinopathy at the level of the RPE cells as determined by electron microscopy, as well as a thickening of Bruch's membrane with initial signs of small breaks. Dense areas of GFAP-immunostaining of Müller cell processes were found in the superior peripheral retina of 16-30 month-old rats where photoreceptor cells were degenerating. After 21 months, Müller cell processes extended into the subretinal space. However, in the central retina, where the photoreceptor cell population was more stable, GFAP-immunolabelled Můller cells were not detected. Immunoblots of retinal homogenates confirmed elevated GFAP levels at 18-30 months when compared to homogenates from retinas of 6-month-old Fischer rats. During photoreceptor cell degeneration, Müller cell processes were also prominently immunostained for CA II, which were seen to occupy the subretinal space at 18-30 months. Our results suggest that Müller cells respond to the age-related peripheral retinopathy in Fischer rats by increasing GFAP content and growth of their processes into the subretinal space to form a glial scar, but only in the area of severe photoreceptor cell loss. In addition, RPE and Bruch's membrane of aged retinas exhibit typical early age-related changes as also reported for aged human eyes.