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1From the Retina and Optic Nerve Research Laboratory, and the 2Departments of Ophthalmology, 3Physiology and Biophysics, and 4Pharmacology, Dalhousie University, Halifax, Nova Scotia, Canada; the 5Centre for Ophthalmology and Visual Science, Lions Eye Institute, University of Western Australia, Nedlands, Australia; and the 6Departments of Physiology and 7Clinical Pharmacology, University of Vienna Medical School, Vienna, Austria.
PURPOSE. To describe a model of chronic endothelin (ET)-1 administration to the optic nerve and evaluate its effect on retinal ganglion cell (RGC) and axon survival in rat.
METHODS. Osmotic minipumps were surgically implanted in one eye of 113 Brown Norway rats to deliver 0.05, 0.10, 0.20, or 0.40 µg ET-1 per day (3.3, 6.7, 13.4, and 26.8 µM, respectively), or balanced salt solution (BSS) to the immediate retrobulbar optic nerve; the fellow untreated eye served as the control. Before pump implantation, RGCs were retrogradely labeled with fluorochrome. Animals were killed at 21, 42, or 84 days. RGC survival was expressed as the ratio of RGC counts in experimental versus control eyes in wholemounted retinas, whereas axon survival was expressed similarly from electron micrographs of the optic nerves. Serial optic disc changes were evaluated using scanning laser tomography. The effect of ET-1 (3 µL topical application of 10-5 M) on blood flow in the surgically exposed optic nerve was measured using laser Doppler flowmetry in a separate group of five animals.
RESULTS. ET-1 led to a mean reduction in optic nerve blood flow of 68%. There were no significant differences in RGC survival among the four ET-1 doses used in this study. Pooled across all ET-1 doses, RGC survival decreased incrementally at 21, 42, and 84 days (P < 0.001; mean ± SD, 0.77 ± 0.25, 0.60 ± 0.27, and 0.50 ± 0.26, respectively) and was statistically significantly lower at each time point than in the BSS-treated animals. The axon survival data also showed a similar time-dependent loss. Only one of 21 animals showed significantly increased disc cupping, and there was no relationship between RGC survival and change in cupping.
CONCLUSIONS. Chronic administration of ET-1 to the rat optic nerve results in a time-dependent loss of RGCs and their axons without apparent change in optic disc topography.
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