الفهرس 
Structure of the retinaOnly 14 pages are availabe for public view
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Abstract
Retinal ganglion cells (RGCs) degeneration occurs in numerous retinal diseases leading to blindness. The protection of RGCs has been an important strategy for combating retinopathies, but little clinical success has been reported.
Neurotoxicity in the central nervous system including the retina has been linked to glutamate. It is the principle excitatory neurotransmitter in the retina. However, excessive accumulation of glutamate leads to excitotoxic neuronal cell death.
Taurine has been recently studied and used for the treatment of various neurodegenerative diseases. It is the most abundant free amino acid in the retina and the second most abundant free amino acid in the brain after glutamate. It has been reported to have antiapoptotic and antioxidative actions.
The aim of the current work was to investigate the possible neuroprotective role of taurine on the rat’s retinal ganglion cells against glutamate-induced toxicity.
Fourty-eight Spargue Dawley adult male albino rats were used in this experiment and divided into four groups (12 rats each). Group 1: Control group; was subdivided into two subgroups (six rats each); in subgroup 1a, the rats had no intervention, while in subgroup 1b, the rats were injected intravitreally with 0.1 ml. of saline in only one eye (right eye). Group 2: Monosodium glutamate (MSG) treated group; rats were injected intravitreally with MSG at a single dose of 40 nmol dissolved in saline in only one eye (right eye). Group 2 was further subdivided into two subgroups (six rats per subgroup); in subgroup 2a, the rats were sacrificed after one day from the injection, while in subgroup 2b, the rats were sacrificed three days after the injection. Group 3: Taurine treated group; rats were injected intraperitoneally with taurine at a single dose of 25 mg/kg body weight dissolved in saline. Group 3 was further subdivided into two subgroups (six rats per subgroup); in subgroup 3a, the rats were sacrificed after one day from the injection, while in subgroup 3b, the rats were sacrificed three days after the injection. Group 4: Combined MSG and taurine treated group; rats were injected with MSG by the same route, and at the same dose as in group 2 in only one eye (right eye), with co-administration of taurine by the same route, and at the same dose as in group 3. Group 4 was further subdivided into two subgroups (six rats per subgroup); in subgroup 4a, the rats were sacrificed after one day from injection, while in subgroup 4b, the rats were sacrificed three days after the injection.
At the end of each experiment, the animals were sacrificed and the eyeballs were enucleated and the retina was dissected and processed for both paraffin and epon blocks. Paraffin sections were stained with haematoxylin and eosin, while semithin sections were stained with 1% toluidine blue. Every fifth paraffin section was immunohistochemically stained to show either glial fibrillary acidic protein (GFAP), synaptophysin (SYN), vascular endothelial growth factor (VEGF), or caspase-3.
Sections were examined, and photographed by the light microscope.
Quantitative studies using the image analyzer computer system was performed for measuring the total retinal thickness, the thickness of the outer nuclear layer (ONL), and the thickness of the inner nuclear layer (INL). Also, the retinal ganglion cells (RGCs) were counted. Statistical analysis was done using ANOVA test.
Examination of group 1 (control group) revealed that both subgroups; 1a and 1b, showed similar results. The retina in these groups appeared normal with its ten constituent layers.
Measurement of the total retinal thickness in the different groups showed a high statistically significant decrease in the total thickness of the retina in group 2 when compared to the control, while the total retinal thickness was preserved in group 4 and showed a high statistically significant increase when compared to group 2.
Several degenerative changes were noted in the retina of group 2. In subgroup 2a, retinal affection was manifested in histological sections, but these changes increased in severity and become more aggrevated in subgroup 2b.
Examination of retinal sections of group 2 rats showed changes in its different layers. The retinal pigment epithelium (RPE) cells showed degenerative changes with cytoplasmic vacuolations, and accumulation of darkly stained pigment granules throughout the cytoplasm. The photoreceptor layer showed an irregular appearance, with vacuole formation and focal loss. The outer limiting membrane (OLM) showed irregularity and undulations. The outer nuclear layer (ONL), showed a high statistically significant decrease in thickness when compared to the control group, an apparent decrease in the cell density with spacing between the cells, and loss of cell discrimination. The outer plexiform layer (OPL) appeared distorted and markedly thinned out. The inner nuclear layer (INL), showed an apparent decrease in cell density resulting in a high statistically significant decrease in thickness when compared to the control group. It also showed degenerative neuronal cell loss with loss of discrimination between its four cell types (horizontal, bipolar, Müller, and amacrine). Many cells exhibited cytoplasmic vacuolations. The inner plexiform layer (IPL) showed apparent decrease in thickness, with widening of the spaces between the fibers giving a spongiform appearance. The ganglion cell layer (GCL) showed a high statistically significant decrease in the number of ganglion cells when compared to the control group, to the extent of complete absence of the ganglion cells in some areas. This was manifested statistically by a 53.6% reduction in the RGCs count (subgroup 2a), and 88.6% reduction in the RGCs count (subgroup 2b). Degenerative neuronal cell loss was dominant. The nerve fiber layer (NFL) showed thinning out, and in some areas, it showed either partial or total loss. The inner limiting membrane (ILM) also showed partial loss. Neovascularization of the retina was shown in group 2 in the OPL, the INL, and the GCL.
Examination of retinal sections of group 3 showed almost a similar histological picture as the control group, except that in most sections there was marked appearance of blood vessels in the OPL, the INL, the IPL, and the GCL. Statistically, the total retinal thickness, the ONL thickness, and the INL thickness in this group (both subgroup 3a, and subgroup 3b) showed no significant difference when compared to the control group. Also, the RGCs count showed no significant difference when compared to the control group.
Examination of retinal sections of group 4 showed a histological picture of the retina similar to that of the control group. Most of the retinal changes induced by MSG were not observed in this group. However, few blood vessels appeared in the OPL and some of these extended into the INL. These blood vessels were most probably formed under the effect of taurine. Statistically, the total retinal thickness, the ONL thickness, and the INL thickness in this group (both subgroup 4a, and subgroup 4b) showed high significant increase in thickness when compared to group 2 (both subgroup 2a, and subgroup 2b, respectively). Also, the RGCs count showed no significant difference when compared to the control group.
Regarding the immunohistochemical staining, collectively, the retinae of group 1 (control group), and group 3 (taurine treated group), showed similar results. The immunohistochemical staining for GFAP, SYN, VEGF and caspase-3 were markedly increased in group 2 (MSG treated group) when compared to the control group. However, in group 4 the immunohistochemical staining either looked similar to the staining pattern of the control group or showed decrease in the staining intensity.
The results were discussed. The present study clarified that damage in the retina was observed in all its layers especially in the ganglion cell layer after treatment with MSG, whereas co-administration of taurine prevented most of the damage caused by MSG.
Therefore, it can be suggested that taurine may initiate a new neuroprotective approach for the retinal ganglion cells against glutamate-induced toxicity.