Optic nerve degeneration in experimental glaucoma is associated with microglial activation.

Abstract

Abstract: : Purpose:While neuroglial factors are thought to be important in retinal ganglion cell axon damage during glaucoma, relatively little is know about the microglial response in glaucomatous optic neuropathy. We have investigated the role that microglia might play in axon loss during experimental glaucoma. Methods:Unilateral experimental glaucoma was induced in Norwegian Brown rats (n=6) by injection of 1.75M hypertonic saline into the episcleral venous system. IOP was elevated for 7 to 14 days (untreated contralateral eyes were used as controls). Optic nerve head and retrobulbar optic nerve samples were snap frozen for light microscopy, with retrobulbar optic nerves 1mm distal to the globe embedded in resin for electron microscopy. Antibodies directed against CD200, CD200 ligand, CD45, MHCII, OX33, OX41, OX42, (microglial and immune cell markers) and GFAP (astrocyte marker) were used to probe optic nerve head and retrobulbar regions. Labelling was visualised using appropriate fluorescence conjugated secondary antibodies. Immunolabelling in the optic nerve head and retrobulbar optic nerve was quantified using Leica QWin. Results:Significant alterations in optic nerve head and retrobulbar optic nerve immunofluorescence were consistently observed with all markers. Ocular hypertension induced an increase in astrocytic GFAP levels as well as both amoeboid and dendritic microglia (p<0.01). Immunofluorescent labelling for macrophages was elevated, indicating a greater number of active microglial and/or infiltration of peripheral phagocytic cells. General immune cell complement and B lymphocyte infiltration were also increased during experimental glaucoma, as indicated by increased immunolabelling. Decreased CD200 ligand (p<0.05) and increased CD200 receptor (p<0.05) immunoreactivity suggested compromised down–regulation of microglial activation as a consequence of ocular hypertension. Optic nerve degeneration and myelin damage was observed following experimental glaucoma. Conclusions:Disrupted neuro–glial interactions are likely to influence microglial activation in this glaucoma model. Microglial activation may play a role in initiating RGC death in experimental glaucoma.

Keywords: microglia • intraocular pressure • pathology: experimental