Cerebrovascular Insult to the Rat Visual System as an Intraocular Pressure Independent Model of Injury

Abstract

Purpose: : To develop an intraocular pressure independent rat model of retinal injury by transient cerebral hypoperfusion/reperfusion.

Methods: : Transient cerebral hypoperfusion of 5 hours was induced in Wistar rats by bilateral common carotid artery occlusion (BCCAO) under deep general anesthesia. Rats survived 5 days (n=5), 14 days (n=5) and 28 days (n=5). Sham-operated rats served as controls (n=8). Retinal fundus photography, intraocular pressure (IOP) measurements with Tonolab. Visual function was assessed using optokinetic response as the frequency of extinction (FE) (cycles/sec), before BCCAO and sacrifice. Paraformaldehyde-fixed brains were cryoprotected, frozen, and serially sectioned. Superior colliculus sections were immunostained for nitrotyrosine, a footprint of peroxynitrite-mediated oxidative damage. Confocal laser images were captured in a masked fashion and % threshold area (pixels), an index of immunoreactivity intensity, was measured (Image J software, NIH). Differences in nitrotyrosine index were compared among groups using a one way ANOVA and t-test was used to compare each experimental group to controls. Immunoreactivity of the retina for a dendrite marker (MAP-2), and a retinal ganglion cell layer marker, platelet derived growth factor receptor beta (PDGFR-b) was performed.

Results: : Retinal ischemic changes by retinal fundus photos and histology were not detected. No changes in IOP and in optokinetic response were seen after BCCAO. In the superior colliculus, differences in nitrotyrosine index was significantly different among the groups (p< 0.05). Nitrotyrosine immunoreactivity was greater in BCCAO group after 14 days of survival compared to controls (12.06 ± 10.80% vs. 2.21 ± 5.15%, mean ± SD; p<0.05). Nitrotyrosine intensity index after 5 days (1.81 ± 2.42 %; p>0.05) and 28 days (3.67 ± 6.66%;p>0.05) were not significantly different than controls. The inner retina showed reduced MAP-2 and PDGFR-b immunoreactivity at 14 days following BCCAO compared to controls.

Conclusions: : A transient hypoperfusion/reperfusion model of injury to the central visual system is described. This model may help to investigate the impact of vascular instability including decreased blood pressure or stroke on the visual system, and may provide new insights into intraocular pressure independent mechanisms of injury in glaucoma.

Keywords: ischemia • oxidation/oxidative or free radical damage • superior colliculus/optic tectum