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From 1 L’Institut National de la Santé et de la Recherche Médicale U 450, Laboratoire de NeuroCytologie Oculaire, Paris, France; 2 Laboratoire de Biochimie, Hôpital Lariboisière, Paris, France; 3 Laboratoire de Pharmacologie, Faculté de Pharmacie, Paris, France; and 4 Muséum National d’Histoire Naturelle, Laboratoire d’Anatomie Comparée, Paris, France.
PURPOSE. To observe the morphology and physiology of the retina in rats 11 weeks after a constant (24-hour) but moderate (500-lux) illumination for 1 week.
METHODS. Levels of aspartate, 
-aminobutyric acid (GABA), glutamate,
glutamine, and taurine were measured by high-pressure liquid
chromatography (HPLC) in the retina and vitreous humor of albino
(Wistar) and pigmented (Long-Evans) rats. Semithin sections were used
to determine retinal morphology. The TUNEL method was used to detect
cells degenerating by apoptosis. Because the GABAergic system has been
shown to be particularly sensitive to the loss of photoreceptors, an
additional immunohistochemical study using anti-GABA, anti-glutamate
decarboxylase (GAD)67 and anti-GAD65 antibodies
was performed.
RESULTS. No apparent morphologic changes were found in the retina of pigmented rats after constant illumination, whereas in albino rats disappearance of photoreceptors (except in the extreme retinal periphery) and cell bodies was observed. A significant number of TUNEL-positive nuclei also occurred in the remaining nuclear and ganglion cell layers. However, no change in the distribution of GABA, GAD67, and GAD65 immunoreactivities was found in either strain under constant illumination compared with control animals. Constant illumination affected the retinal levels of aspartate, glutamate, glutamine, glycine in both strains, whereas GABA contents did not change and taurine was decreased only in albino rats. A significant increase of vitreal glutamate levels was also found in both strains and of taurine levels only in albino rats.
CONCLUSIONS. Phototoxicity can provoke durable retinal alterations beyond the period of lighting, suggesting progressive and probably continuous modifications of retinal physiology, even in pigmented animals in which the retina seems morphologically normal.
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