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C. T. Fu, D. Sretavan; Ectopic Optic Nerve Head (ONH) Accumulation of Glutamatergic Vesicles and Potential Glutamate Release in Experimental Glaucoma. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4321.
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© ARVO (1962-2015); The Authors (2016-present)
To test the hypotheses that elevated intraocular pressure (IOP) in experimental glaucoma results in ectopic accumulation of glutamatergic vesicles at the ONH, and that local glutamate release has the potential to alter axonal and glial physiology.
Laser-induced ocular hypertension (LIOH) was performed in one eye of CD-1 mice by photocoagulation of limbal and episcleral veins. The untreated eye served as control. Protein expression was examined with immunohistochemistry on cryostat sections. Glutamate release was assayed using the conversion of NAD+ to NADH by L-glutamatic dehydrogenase (GDH) in acute ONH preparations. Pharmacological experiments were performed in optic nerve (ON) explant cultures with retinas attached.
Two days after laser treatment, an accumulation of glutamatergic SV+ synaptophysin+ VGLUT2+ synaptic vesicles was detected in RGC axons at the ONH of LIOH eyes (IOP > 21 mmHg), but not in axons from control eyes. Co-localization of vesicles with SNAP-25 and Bassoon suggested their competence for exocytosis. Vesicular glutamate release in response to potassium depolarization or hypertonic sucrose was demonstrated using GDH-based assays in acute preparations of LIOH ONH. Immunolabeling identified expression of multiple AMPA and NMDA receptor subunits on ONH axons, astrocytes and microglia. ON-retina explant cultures from untreated eyes exposed to glutamate exhibited axonal degeneration in a dose-dependent manner. ON-retina explants harvested 2 days post-LIOH showed substantial degeneration of axons after 4 days in vitro compared to controls. Blockade of glutamate signaling with TTX or MK-801/NBQX attenuated axonal degeneration in LIOH ON-retina cultures.
Taken together, our data demonstrated an ONH accumulation of glutamatergic vesicles in experimental glaucoma and the capability of glutamate release in situ. Efforts are underway to examine the occurrence and effect of local ONH glutamate release in vivo, which may shed light on previously unexplored pathophysiological mechanisms in glaucoma.
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