Aldosterone: a mediator of retinal ganglion cell death and the potential role in the pathogenesis in normal-tension glaucoma

Abstract

Glaucoma is conventionally defined as a chronic optic neuropathy characterized by progressive loss of retinal ganglion cells (RGCs) and optic nerve fibers. Although glaucoma is often associated with elevated intraocular pressure (IOP), significant IOP reduction does not prevent progression of the disease in some glaucoma patients. Thus, exploring IOP-independent mechanisms of RGC loss is important. We describe chronic systemic administration of aldosterone and evaluate its effect on RGCs in rat. Aldosterone was administered via an osmotic minipump that was implanted subcutaneously into the mid-scapular region. Although systemic administration of aldosterone caused RGC loss associated with thinning of the retinal nerve fiber layer without elevated IOP, the other cell layers appeared to be unaffected. After chronic administration of aldosterone, RGC loss was observed at 2 weeks in the peripheral retina and at 4 weeks in the central retina. However, administration of mineralocorticoid receptor blocker prevented RGC loss. These results demonstrate aldosterone is a critical mediator of RGC loss that is independent of IOP. We believe this rat normal-tension glaucoma (NTG) animal model not only offers a powerful system for investigating the mechanism of neurodegeneration in NTG, but can also be used to develop therapies directed at IOP-independent mechanisms of RGC loss.

Figure 1

Effect of aldosterone on retinal ganglion cell (RGC) death. (a) Retrograde labeling of RGCs treated with 8 μg/kg/day or 80 μg/kg/day aldosterone for 6 weeks, and a normal control eye. Micrographs of the central and peripheral areas were taken ∼1 and 4 mm away from the optic nerve head. Scale bar, 100 μm. RGCs were counted in the central (b) and peripheral (c) areas. Results are expressed as the mean±SEM (n=4 in each group). *P<0.001 versus control (Dunnett's multiple comparison test)

Figure 2

The number of RGCs in the eyes treated weekly with 80 μg/kg/day aldosterone. (a) In the central retina, RGC was decreased 4 weeks after the administration of aldosterone. (b) In the peripheral retina, however, reduction in the number of RGCs was already detected at 2 weeks after the administration of aldosterone. Results are expressed as the mean±S.E.M. (n=4 in each week). ^#P<0.05, *P<0.001 versus control (Dunnett's multiple comparison test)

Figure 3

Retinal thickness layer analysis. (a) Light micrographs of the retina of an eye treated with 8 μg/kg/day or 80 μg/kg/day aldosterone for 6 weeks, and a normal control eye. Change in mean thickness of the (b) inner plexiform layer (IPL), (c) inner nuclear layer (INL), (d) outer plexiform layer (OPL), and (e) outer nuclear layer (ONL). Results are expressed as the mean±S.E.M. (n=4 in each group). Scale bar, 20 μm

Figure 4

Optic nerve degeneration in aldosterone-treated rats. Representative photos of HE-stained sections in normal (a) and aldosterone-treated (b) rats. Sections illustrate the thinning of the retinal nerve fiber layer (arrowheads) in the aldosterone-treated rats as compared with normal rats. Cupping extends to the posterior aspect of the inner retinal layer (allows). Scale bar, 20 μm. (c) Quantitative analysis was performed in order to show the significant difference in the thickness of the nerve fiber layer in the aldosterone-treated rats. Results are expressed as the mean±S.E.M. (n=4 in each group). *P<0.01 versus control (independent Student's t-test)

Figure 5

Normal intraocular pressure (IOP) in aldosterone-treated rats. There were no significant differences in the IOP between the vehicle- and 80 μg/kg/day aldosterone- treated rats. ●: vehicle-treated rats, ▪: aldosterone-treated rats. Results are expressed as the mean±S.E.M. (n=4 in each week)

Figure 6

Effect of spironolactone on aldosterone-induced retinal ganglion cell death. (a) Retrograde labeling of RGCs in a normal control eye and aldosterone-treated rats with or without spironolactone for 6 weeks. Scale bar, 100 μm. RGCs were counted in the central (b) and peripheral (c) areas. Results are expressed as the mean±S.E.M. (n=4 in each group). *P<0.05 versus aldosterone-treated rats (Tukey's honestly significant difference test)