Glutamate uptake in retinal glial cells during diabetes

Aims: Glutamate recycling is a major function of retinal Muller cells. The aim of this study was to evaluate the expression and function of glutamate transporters during diabetes. Methods: Sprague - Dawley rats were rendered diabetic by a single dose of streptozotocin ( 50 mg/kg). Following 12 weeks of diabetes, immunolocalisation and mRNA expression of the two glial cell transporters, GLAST and EAAT4 were evaluated using indirect immunofluorescence and real-time PCR. The function of glutamate transport was investigated at 1, 4 and 12 weeks following induction of diabetes by measuring the level of uptake of the non-metabolisable glutamate analogue, D-aspartate, into Muller cells. Results: There was no difference in the localisation of either GLAST or EAAT4 during diabetes. Although there was a small apparent increase in expression of both GLAST and EAAT4 in diabetic retinae compared with controls this was not statistically significant. At 1, 4 and 12 weeks following diabetes, D-aspartate immunoreactivity was significantly increased in Muller cells of diabetic rats compared to controls ( p< 0.001). The EC50 was found to increase by 0.304 log units in diabetic Muller cells compared with controls, suggesting that glutamate uptake is twice as efficient. Conclusions: These data suggest 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 Muller 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.