INCREASED EXPRESSION OF ENDOTHELIN B RECEPTORS PRECEDES RETINAL GANGLION CELL DEATH IN A RODENT MODEL OF GLAUCOMA

Purpose: Endothelin B (ETB) receptors have been shown to be involved in the pathogenesis of glaucoma. However, the precise sequence of molecular events involving ETB receptor expression and retinal ganglion cell death is not completely understood. This study was aimed at investigating whether the expression of ETB receptors precedes retinal ganglion cell loss in vivo in the Morrison's elevated intraocular pressure (IOP) model of glaucoma in rats. Methods: Intraocular pressure (IOP) was elevated in one eye of retired breeders Brown Norway rats using the Morrison's method (injection of hypertonic saline through episcleral veins), while the contralateral eye served as control. Retinas were collected after 1 and 2 weeks of IOP elevation, sectioned and stained for ETB receptor expression by immunohistochemistry. In a separate set of experiments, retired breeders Brown Norway rats were used for retrograde labeling of retinal ganglion cells (RGCs) with Fluoro-gold. Following retrograde labeling, IOP was elevated in one eye using the Morrison's method, while the contralateral eye served as control. After IOP was elevated, rats were maintained for 2 weeks and sacrificed. Fluoro-gold labeled retinas were isolated, flat-mounted, and images taken using a Zeiss LSM-510 confocal microscope. Fluoro-gold-labeled RGCs were counted in three eccentricities from the optic nerve head. In addition, optic nerves were isolated from Brown Norway rats following 2 weeks of IOP elevation, sectioned and stained using p-phenylenediamine. Confocal images were taken and morphology of optic nerve axons was compared. Results: Immunohistochemical analysis of retinal sections showed no appreciable change in ETB receptor immunostaining following 1 week of IOP elevation. However, IOP elevation for 2 weeks produced increased expression of ETB receptor in the RGCs, inner plexiform layer (IPL) and inner nuclear layer (INL). In addition, IOP elevation for 2 weeks resulted in 25% loss of RGCs in the first eccentricity, while no significant loss was seen in the second and third eccentricities. Moreover, there was no appreciable damage observed in the optic nerve axons after 2 weeks of IOP elevation. Conclusions: Early increase in expression of ETB receptors (at 2 weeks of IOP elevation) could contribute to RGC loss and damage to the optic nerve axons.