Minimally Elevated Intraocular Pressure Exposure Dramatically Increases Retinal Glial Fibrillary Acidic Protein mRNA and Protein Levels

Abstract

Abstract: : Purpose: The retinal glial cell responses to elevated intraocular pressure (IOP) and consequent retinal ganglion cell injury are poorly understood. Glial fibrillary acidic protein (GFAP) is a marker for retinal astrocytes and activated Muller cells. The purpose of this study was to determine the relationship between elevated IOP exposure history and the retinal glial response as determined by GFAP mRNA and protein levels. Methods: Brown Norway rats (n=70) were housed in low level constant light and IOP monitored daily by Tonopen on awake animals. Sustained elevation of IOP was produced by injection of hypertonic saline unilaterally into episcleral veins. Five weeks following injection, retinas were removed and divided into two groups with matched distributions of IOP history. RNA was extracted from one group of retinas, reverse transcribed and the cDNA amplified by semi-quantitative PCR to determine GFAP mRNA levels. The other group of retinas were homogenized for GFAP protein measurement by ELISA. All optic nerves were post-fixed, cross-sectioned and graded for nerve injury by light microscopy by 5 masked observers Results: The mean IOP for uninjected, control eyes was 28.9 mm Hg. For the experimental eyes, mean IOP was correlated to nerve injury grade (R2=0.82, third order polynomial). Compared to the control eyes, retinal GFAP mRNA and protein increased dramatically with increasing mean IOP (p<0.001 and p<0.02, respectively). In retinas exposed to only slightly elevated IOP (mean IOP between 30 and 34 mmHg) the GFAP mRNA and protein levels were 272% and 309% of control values respectively, (p<0.05 and 0.01). The optic nerves from these eyes demonstrated focal optic nerve lesions Conclusion: GFAP mRNA and protein levels are dramatically increased in retinas following an exposure to elevated IOP that results in focal optic nerve lesions. This suggests retinal glial sensitivity to either elevated IOP itself or to early retinal ganglion cell injury.

Keywords: 316 animal model • 444 intraocular pressure • 565 retinal glia