Retinal light damage vs. normal aging of rats: altered morphology, intermediate filament expression, and nuclear organization of Müller (glial) cells.

Abstract

In retinal light damage, degeneration of photoreceptors results in alterations of glial (Müller) cells. In particular, Müller cells show signs of gliosis such as thickening of their stem processes, and expression of glial fibrillary acidic protein (GFAP) which is normally not detectable by immunocytochemistry. We were interested in a quantification of these morphological alterations, and in possible effects of an application of free radical scavengers (Ginkgobiloba extract EGb 761). For this purpose, we studied Müller cells in retinae of albino rats exposed to enhanced illumination for 24 months, a procedure which causes a complete loss of photoreceptor cells. The cells were labeled by (i) bulk filling with the fluorescent dye, Procion yellow, and by (ii) immunocytochemical demonstration of vimentin and GFAP. One group of rats was fed daily with EGb 761 during the last 8 months of life when the remaining photoreceptors (about 50%) died. The retinae were compared with retinae from 3 months-old albino rats, serving as normal young controls, and with retinae from 24 month-old pigment rats, representing normal aging processes. As age-related changes of the ultrastructure of glial cell (astrocytic) nuclei have been described in the literature, the organization of Müller cell nuclei was also studied by an argyrophilic stain, and by electron microscopy. We found that in the thin light-damaged retinae, Müller cells were shorter but thicker than in age-matched control retinae. The volumes of their vitread stem processes were almost unchanged. Müller cells were GFAP-immunoreactive in the light-damaged retinae but not in the controls. The application of EGb 761 prevented the expression by Müller cells of (detectable levels of) GFAP. By contrast, in retinae from EGb 761-treated animals the volumes of the vitread stem processes were significantly increased in comparison to untreated animals. The number of nuclear organization regions was significantly enhanced in Müller cell nuclei from light-damaged untreated albino rats, as compared with the young controls. Application of EGb 761 prevented much of this increase. Thus, exogeneous free radical scavengers do not prevent the occurrence of an reactive hypertrophy but inhibit the expression of "pathological marker molecules", and the (accompanying) signs of enhanced nuclear activity.