Müller cell GFAP expression exhibits gradient from focus of photoreceptor light damage

High intensity (ca. 150 foot-candles), cumulative fluorescent light exposure regimes of 40 or 60 minutes to pigmented Long Evans rats were sufficient to elicit glial fibrillary acidic protein immunoreactivity (GFAP-IR) in Müller cells, when the animals are sacrificed 7 days post-exposure. Exposure to only 20 minutes of cumulative light or sacrifice immediately after exposure was not sufficient to initiate GFAP-IR in Müller cells. A gradient of GFAP-IR was observed extending from an approximately circular focus superior to the optic disc to the peripheral retina, whether or not there was morphological damage to the photoreceptors observable at the light microscopic level. Photoreceptor lesions produced by laser photocoagulation elicited the same gradient of GFAP-IR, and showed that GFAP-IR was not a reflection of a central to peripheral gradient of light received by the retina. Excessive light exposure initiated a signal which induced GFAP expression in Müller cells. This signal appeared to require a dark period and may be a diffusible factor that moves through extracellular pathways.