Retinal Glutamate Upregulation in Chronic Experimental Glaucoma (EG) with or without Prior Hemiretinal Endodiathermy Axotomy (HEA)

Abstract

Purpose : Controversy regarding the role of glutamate excitotoxicity in glaucoma has resulted from contradictory reports of vitreal glutamate measurements in humans and macaques with chronic EG. In contrast, we have reported that measurement of retinal glutamate by immunohistochemistry (IHC) in perfusion-fixed eyes may be a better biomarker for studying the possible role glutamate plays in glaucoma (Nork TM, et al, IOVS 2016;57:ARVO E-Abstract 2573). Here, we hypothesize that glutamate in the retina would be upregulated in a model of EG with or without prior HEA.

Methods : Three rhesus macaques underwent HEA to selectively remove the retinal ganglion cell (RGC) layer of the inferior retina of the right eye. Those 3 animals underwent laser trabecular meshwork destruction (LTD) in the same eye 8 weeks later to produce intraocular pressure (IOP) elevation. Four additional macaques (1 rhesus, 3 cynomolgus) underwent LTD without prior HEA. Intraocular pressures were elevated and remained above 25 mmHg for 6 months to 8 years. All 7 animals were perfused with glutaraldehyde at the time of sacrifice. IHC staining for glutamate was performed on epon embedded segments of retina from both eyes, including superior and inferior regions of the animals with HEA.

Results : IHC glutamate staining showed upregulation in all 7 of the EG eyes in the region of the rod spherules and cone pedicles. Increased glutamate staining was observed in the inner plexiform layer (IPL) as well. In the 3 HEA plus EG animals, IHC did not reveal significant staining variability between the non-axotomized superior and axotomized inferior region. IHC did not reveal significant upregulation in the fellow control eye. While present in the fellow control eye, the nerve fiber (NF) and RGC layers of the EG eyes were, in most of the animals, nearly absent.

Conclusions : The IHC staining results for glutamate on perfusion-fixed retinas have consistently demonstrated upregulation of glutamate in rod spherules and cone pedicles as well as the IPL of the retina. This upregulation of glutamate is present whether or not the RGCs are removed by HEA prior to IOP elevation. Based on this upregulation of glutamate, especially in the region of the photoreceptor synapses, excitotoxicity could be involved in RGC injury in glaucoma.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.