Increase in endothelin B receptor expression in optic nerve astrocytes in endothelin-1 induced chronic experimental optic neuropathy

The purpose of this study was to determine whether endothelin B (ETB) receptor levels in the optic nerve are related to retinal ganglion cell (RGC) loss in a model of chronic endothelin-1 (ET-1) induced optic neuropathy. RGCs of adult Brown Norway rats were first retrogradely labeled with fluorochrome from the superior colliculi. An osmotic minipump was surgically implanted 7 days later to deliver 10−11 M (n = 9), 10−9 M (n = 12) or 10−7 M (n = 9) ET-1 to the retrobulbar optic nerve for 28 days. RGC survival was expressed as the ratio of RGC counts in experimental versus control eyes in wholemounted retinas. Optic nerves were used for either ETB western blot analysis (n = 24) or immunohistochemistry (n = 6) for ETB and glial fibrillary acidic protein (GFAP) to localize astrocytes. ETB expression was higher in the experimental nerve compared to the fellow untreated control nerve in 19 (79%) of the 24 animals with a mean increase of 16.7 ± 4.5% in densitometric analyses of the immunoblots. Experimental nerves showed stronger labeling for both ETB and GFAP compared to control nerves. ETB-positive cells almost completely co-localized with GFAP-positive cells in both experimental and untreated control nerves, however, ETB expression was stronger in the astrocyte soma and proximal processes, while GFAP was expressed more strongly in the distal processes. There was a weak relationship between RGC loss and increase in ETB expression (r = −0.417, p = 0.076). There is an upregulation of ETB expression in optic nerve astrocytes in ET-1 induced chronic optic neuropathy causing RGC loss.