The Possible Neuroprotective Role of Taurine on the Rat's Retinal Ganglion Cells against Glutamate - induced Toxicity

Abstract

Purpose: Glutamate is found in a wide variety of foods, as it has been shown to have a flavor-enhancing effect. It is added to foods-either as the purified monosodium salt or as hydrolyzed protein. As Glutamate is one of contributors to retinal ganglion cells (RGCs) degeneration. Therefore the aim of the present study was to investigate the possible neuroprotective role of taurine on the rat's RGCs against glutamateinduced toxicity using histological and immunohistochemical techniques. Material and Methods: Twenty four adult Sprague Dawley male albino rats were divided into four groups. Control group: six rats were divided into two subgroups; Subgroup 1a included both eyes of three rats (6 eyes) that were left without any intervention. Subgroup 1b right eye of six rats (6 eyes) was injected intravitreally with 0.1 ml of balanced salt solution; 0.9% NaCl (BSS). Glutamate group: right eyes of 6 rats (6 eyes) were injected intravitreally with 0.1 ml of a single dose of monosodium glutamate (40 nmol/ml). Taurine group: three rats (6 eyes) received intraperitoneal injection of a single dose of taurine (25 mg / kg) B.W., which was dissolved in 1 ml of BSS and injected intraperitoneally. Combined Glutamate and taurine group: right eyes of 6 rats (6 eyes) were injected intravitreally with 0.1 ml of a single dose of monosodium glutamate (40 nmol/ml) and at the same time, each rat received a single dose of taurine intraperitoneally as in taurine group. Three days after injection, animals were sacrificed and eye balls were enucleated and processed for histological and immunohistochemical staining. Results: Extensive damage and disruption of the structure of the retina following glutamate intravitreal injection was found. The photoreceptor layer showed marked irregular appearance, vacuole formation, focal loss and wide retinal blood vessel was seen within the GCL. Complete absence of ganglion cells with the presence of small dark glial cells within the GCL were noticed. There was a statistically significant decrease in the mean total retinal thickness. Also, there was decrease in the thickness of outer nuclear layer (ONL) and inner nuclear layer (INL). In addition to decrease in ganglion cell count. A significant improvement of this picture was observed in taurine and combined groups (P-values were < 0.001). In Taurine group preservation of normal architecture of all retinal layers was observed with presence of multiple blood vessels in some retinal layers. A statistically significant increase in glial fibrillary acidic protein and synaptophysin immunostaining was seen in most retinal layers in glutamate group compared to no or weak staining in the other groups (P-values < 0.001) however, negative or faint vascular endothelial growth factor and caspase-3 immunostaining was detected in all animal groups. Conclusion: Taurine is protective to the retina against glutamate excitotoxicity and could have clinical implications in protecting the ganglion cells in several ophthalmic diseases as glaucoma and diabetic retinopathy.