The neuroprotective effect of trans-resveratrol against NMDA-induced retinal excitotoxicity in sprague dawley rats via NMDA-mediated calcium signalling and adenosine a1 receptor

Glaucoma is the second leading cause of irreversible blindness worldwide. Elevation of intraocular pressure (IOP) is the common risk factor in glaucoma. However, a considerable number of individuals still suffer glaucoma with normal IOP. Glutamate- mediated excitotoxicity plays a key role in development of glaucoma including in those patients with normal IOP, which is termed as normal tension glaucoma (NTG). Modulation of adenosine A1 receptor (AA1R) has been shown to counteract glutamate- mediated excitotoxicity. Therefore, compounds with AA1R agonistic property have potential to be used as antiglaucoma agents. Trans resveratrol (TR), a polyphenolic compound known for its antioxidants properties has been reported to stimulate AA1R. Meanwhile, another synthetic compound of interest named RU615, has also shown to have high affinity towards AA1R via computational analysis. Hence, the aim of this study was to investigate the effect of TR towards NMDA-induced retinal excitotoxicity rat model through AA1R modulation. In the first study, TR and RU615 were used to investigate their effect towards retinal morphology and apoptotic changes in two different doses. Whereas, for second study, in-depth mechanism on retinal oxidative stress, apoptosis, and visual-behavioural test through modulation of AA1R by TR were elucidated. In the first study, Sprague-Dawley rats were divided into six groups; group 1 received intravitreal vehicle, group 2 received intravitreal NMDA, group 3 and 4 received intravitreal NMDA and pre-treatment of TR at two different doses and group 5 and 6 received intravitreal NMDA and pre-treatment of RU615 at two different doses. Rats were euthanised seven days later and retinal morphology, TUNEL staining for retinal cell apoptosis and optic nerve degeneration were evaluated. In the second study, rats were divided into four groups; group 1 and 2 were similar as in first study, group 3 received intravitreal NMDA and pre-treatment of TR, and group 4 received intravitreal NMDA and DPCPX (an AA1R antagonist) and pre-treatment of TR. Rats were subjected to visual-behavioural test 24 hours prior to euthanisation. As in first study, rats were euthanised seven days later. Retinal tissues were collected for oxidative stress markers (catalase, superoxide dismutase, glutathione, inducible nitric oxide synthase, 3-nitrotyrosine and lipid peroxidation levels) and retinal apoptotic markers (caspase-3, BAX and BCL-2 levels) including calcium dependent markers (calcineurin, calpain-1 and cabin-1).TR and RU615 at both doses preserved retinal and optic nerve morphology against NMDA-induced retinal excitotoxicity. This is evidenced by preservation of retinal nuclei, layers and optic nerve structure. In second study, TR group preserved retinal antioxidant levels, reduced retinal lipid peroxidation, nitrosative markers and calcium mediated apoptotic markers accompanied by improved visual behavioural function. Whereas reversal of TR effects by DPCPX were observed on those parameters which suggested the role of AA1R in its protective mechanism towards NMDA-induced retinal excitotoxicity. In conclusion, intravitreal TR administration protected NMDA- induced retinal excitotoxicity changes by improving the retinal redox status, reducing retinal apoptosis, that preserved retinal morphology and leads to improvement of visual- behavioural function through AA1R modulation.