Contribution of the Endothelin Receptor A to Neurodegeneration in a Rat Model of Ocular Hypertension

Purpose: The endothelin system of peptides and their receptors, primarily the ETB receptor, has been shown to have a neurodegenerative role in glaucoma. The purpose of this study was to examine alterations in ETA receptor expression within the retina following IOP elevation by the Morrison’s model of ocular hypertension in Brown Norway rats. Methods: IOP was elevated in the left eye of adult male retired breeder Brown Norway rats using the Morrison model of ocular hypertension (by injection of hypertonic saline through episcleral veins) while the contralateral eye served as the control. Rats were maintained for either two or four weeks following IOP elevation and sacrificed. Retina sections were obtained from both control and IOP-elevated eyes and analyzed for changes in ETA receptor expression by immunohistochemistry. ETA receptor immunostaining was co-localized with β-III-Tubulin, which is selectively expressed in retinal ganglion cells. In separate experiments a live/dead assay was performed using calcein AM and ethidium homodimer on stable 661W clones overexpressing the ETA receptor to determine the effect an increase in ETA receptors could have on cell viability. Results: After two weeks of IOP elevation rat eyes showed an increase in immunostaining for ETA receptors in multiple retinal layers. The most prominent increase in ETA receptor expression was observed in the outer plexiform layer and a moderate increase was seen in the ganglion cell layer and inner plexiform layer. Following four weeks of IOP elevation an increase in ETA receptor expression was observed primarily in the outer plexiform layer compared to that in the corresponding contralateral eyes. A modest increase in the ganglion cell layer was also observed. Cell culture studies showed that cells overexpressing the ETA receptor had a greater number of dead or dying cells compared to the empty vector expressing cells. Conclusion: Elevated intraocular pressure results in a appreciable change in ETA receptor expression. Overexpression of the ETA receptor results in a decrease in cell viability in cultured 661W cells. Further work is needed to understand the precise role of the ETA receptor in neurodegeneration during ocular hypertension.