Vitreous Glutamate Concentration and Axon Loss in Monkeys With Experimental Glaucoma

Abstract

To evaluate vitreous glutamate concentration and axon loss in monkeys with experimental glaucoma. We induced unilateral chronic glaucoma by means of laser trabecular destruction in 14 rhesus and 6 cynomolgus monkeys. Intraocular pressure (IOP) was monitored weekly. We assessed optic nerve damage clinically and photographically. Vitreous, sampled immediately before enucleation, was analyzed for glutamate concentration by means of high-performance liquid chromatography. We quantified percentage of axon loss after histopathologic sectioning of the optic nerve, compared median glutamate concentration ratios, and assessed correlation of glutamate concentration, axon count, IOP, cup-disc ratio, duration of IOP elevation, and age. Median vitreous glutamate concentration in glaucomatous eyes was 7.0 micromol/L (range, 3.0-88.6 micromol/L) vs 6.7 micromol/L (range, 2.8-87.4 micromol/L) in control eyes. The ratio (glaucomatous to control eyes) was 1.08. We found no significant correlation between vitreous glutamate concentration ratio and any of the other variables. The IOP, disc cupping, and axon loss were correlated. We found no difference between vitreous glutamate concentration in glaucomatous and contralateral control monkey eyes when the entire data set was examined and no evidence of correlation between vitreous glutamate concentration and axon loss. Vitreous concentration of the excitotoxic amino acid glutamate, thought to be associated with retinal ganglion cell death in glaucoma, was not altered in this study.