Quantitation of ischemic damage in the rat retina

In order to determine thresholds for irreversible cellular injury in the rat retina, timed acute no-flow ischemic episodes of 30-180 min duration were produced by elevation of intraocular pressure (IOP) above systolic pressure. Quantitation of irreversible degeneration and cell loss following a 2-week post-ischemic interval was performed by computer-assisted measurements from histologic sections. Alterations of thickness of retinal layers and linear cell density were determined for ischemia of selected durations (30, 60, 80, 90, 120 and 180 min). Different thresholds were evident for inner and outer retinal damage. Neurons of the inner nuclear layers showed extensive loss with episodes at 60 min. Decrease in the thickness of the inner plexiform layer provided the best index of this inner nuclear damage. The outer retina was more resistant, with photoreceptors showing extensive damage only after 90 min in conjunction with pigment epithelial metaplasia and degeneration. Two-hour episodes produced full-thickness degeneration with loss of pigment epithelium and sparing of the peripheral retina. Greater sensitivity of the inner retina suggested problems with restoration of the retinal circulation. Horseradish peroxidase infusions did reveal central microcirculatory defects in retinal wholemounts of some specimens with episodes longer than 60 min. Refinements of the methods resulted in outcomes sufficiently reproducible for quantitative assessment of acute ischemic injury. The rat retina provides an economical basic tissue model of acute ischemic injury affecting neurons, glia, and microvasculature. Quantitation of this injury promises great utility in testing agents with potentially protective effects on acute ischemic injury.