Glutamate uptake in retinal glial cells during diabetes.

Abstract

Abstract: : Purpose:Neuronal dysfunction is known to occur in the early stages of diabetes. Anomalies in glutamate turnover could play a significant role in the aetiology of neuronal dysfunction. The aim of this study was to evaluate the expression and function of glutamate transporters during diabetes. Methods:Sprague–Dawley rats were injected with either a single dose of STZ (50mg/kg), or citrate buffer. Following 12 weeks of diabetes, the change in mRNA expression of the two glial cell transporters, GLAST and EAAT4 was evaluated using real–time PCR. Localization of the two transporters was evaluated using the indirect immunofluorescence method in lightly fixed (4% paraformaldehyde) retinae using antisera directed against GLAST (Chemicon Int, CA, USA) and EAAT4 (Alpha Diagnostic Int. San Antonia, Texas, USA). The function of glutamate transport was investigated at 1, 4 and 12 weeks following diabetes by monitoring the level of D–aspartate accumulation within Müller cells. Briefly, this involved injecting 2mL of 2.5mM D–aspartate intravitreally, followed by fixation (1% paraformaldehyde, 2.5% glutaraldehyde), embedment in Epon and processing for serial section immunocytochemistry. The level of D–aspartate within Müller cell somata was quantified using NIH Image. Results:Immunofluorescence confirmed that GLAST was localized to Müller cells and astrocytes, whilst EAAT4 was localized exclusively within astrocytes. There was no apparent difference in the labelling of GLAST or EAAT4 during diabetes. RT–PCR revealed that the level of mRNA expression of GLAST was 30% increased following 12 weeks of diabetes, whereas EAAT4 was unchanged. At 1, 4 and 12 weeks following diabetes, D–aspartate immunoreactivity was significantly increased in Müller cells of diabetic rats compared to controls (12 weeks p<0.001). Moreover, D–aspartate transport by GLAST was found to be concentration dependent and saturable, with an EC₅₀ value of 49mM. Conclusions:This data suggests that there are alterations in glutamate transport during diabetes. However, these changes are unlikely to play a significant role in glutamate induced neuronal excitoxicity during diabetes. These results suggest that although Müller cells undergo gliosis at an early stage of diabetes, one of the most important functions for maintaining normal retinal function is preserved within the retina.

Keywords: Muller cells • diabetic retinopathy • neurotransmitters/neurotransmitter systems