ABSTRACT
Episcleral vein cauterization (EVC) is used in rats to generate a glaucoma model with high intraocular pressure (IOP). The long-term retinal damage in this glaucoma model, however, has not been accurately quantified. We report the location and amount of retinal ganglion cell (RGC) damage caused by (EVC) induced IOP elevation in two rat strains. IOP was raised in one eye of Wistar (N = 5) and Brown-Norway(B-N)(N = 7) rats by EVC and monitored monthly until IOP in contralateral eyes equalized at 5 months post-surgery. Animals were maintained for 3.5-4.5 additional months. B-N rats (N = 7) that had no EVC served as controls for this strain. Scotopic flash ERGs were recorded at baseline and just prior to euthanasia. Automated counts of all retrogradely labeled RGCs in retinal flat-mounts were determined and compared between contralateral eyes. RGC density maps were constructed and RGC size distribution was determined. Oscillatory potentials in the group of eyes which had elevated IOP were decreased at the time of euthanasia, when IOP had returned to normal. The group of normal B-N rats had similar RGC counts between contralateral eyes. In the experimental group the mean number of RGCs was not significantly different between control and experimental eyes, but 1 of 5 Wistar and 2 of 7 B-N experimental eyes had at least 30% fewer RGCs than contralateral control eyes. Total retinal area in B-N experimental eyes was higher compared to contralateral eyes. Cumulative IOP exposure of the experimental eyes was modestly correlated with RGC loss while oscillatory potentials appeared to be inversely related to RGC loss. In retinas with extensive (> 30% RGC loss) but not complete damage, smaller cells were preserved better than larger ones. The above results indicate that RGC loss in both Wistar and B-N strains is variable after a prolonged elevation of IOP via EVC. Such variability despite equivalent IOP levels and ERG abnormalities, suggests unknown factors that can protect IOP-stressed RGCs. Identification and enhancement of such factors could prove useful for glaucoma therapy.
Subject(s)
Glaucoma/pathology , Retinal Ganglion Cells/pathology , Animals , Cautery , Cell Count , Electroretinography , Female , Image Interpretation, Computer-Assisted , Intraocular Pressure , Microscopy, Fluorescence , Models, Animal , Rats , Rats, Inbred BN , Rats, Wistar , Sclera/blood supply , VeinsABSTRACT
PURPOSE: To quantify the extent and pattern of retinal ganglion cell (RGC) loss in the DBA2/NNia glaucomatous mouse strain as a function of age and compare it with ganglion cell loss in a nonglaucomatous strain. METHODS: All the ganglion cells in retinas of DBA/2NNia and C57/BL6 mice of various ages (five eyes per age group in 3-month intervals from 3 to 18 months of age) were counted. A novel counting method that does not rely on sampling and that uses retrograde labeling of RGCs with Fluorogold (Fluorochrome; Englewood, CO) was used. RGC loss in the glaucomatous DBA/2NNia mouse strain was contrasted to RGC loss in C57 mice at the same ages. The total number of Fluorogold-labeled cells per retina was compared within and among the two strains as a function of age. In addition, RGC density maps were constructed for each retina, and the range of densities for each age group was compared within and among the two strains. IOP in awake, nonsedated DBA/2NNia mice was measured with a rebound tonometer. RESULTS: RGC loss started between 12 and 15 months of age in C57 mice and led to an approximate 46% reduction by 18 months of age. The rate of loss was best approximated by a second-order polynomial curve. In comparison, DBA/2NNia mice also began showing RGC loss at approximately 12 months of age, but it proceeded at a much faster rate, with approximately 64% of their RGCs dying by the 15th month of age but little additional loss thereafter. RGC loss in the DBA animals had a focal pattern that appeared more patchy and showed greater variability than the age-related loss in C57 mice, which was more diffuse. IOP and total retinal area in DBA/2NNia mice began to increase at approximately 6 months of age. IOP normalized after the 12th month of age. CONCLUSIONS: Age-related RGC loss of up to 50% can occur in the C57 mouse by 18 months of age. The loss does not proceed linearly with age, as is often assumed, and differs both in extent and locational pattern from pathologic RGC loss secondary to glaucoma in DBA/2NNia mouse retinas.
