Role of a Glial Glutamate Transporter in Metabolic Coupling in the Retina.

Abstract

Abstract: : Purpose:Metabolic coupling between neurons and glial cells has been proposed to be mediated by synaptically–released glutamate acting through a glutamate transporter. Because the glutamate/aspartate transporter (GLAST) is the predominant glutamate transporter in Müller cells, we have used GLAST knockout (GLAST^–/–) mice to examine whether GLAST is needed for metabolic coupling in the retina. Methods:Isolated retinas from wild type and GLAST^–/– mice were incubated in vitro under dark or light conditions and the lactate contents of the medium and the tissue were determined. In addition, the effects of glutamate and the glutamate transporter inhibitor, threohydroxy–aspartate (THA), on lactate release were examined under dark and light conditions. Finally, glucose uptake by isolated retinas was determined under light and dark conditions using ³H–2–deoxyglucose (³H–2–DG). Results: In both wild type and GLAST^–/– mice the levels of lactate released into the medium after 30 min, was approximately 4 to 5 fold higher than the levels in the retina, under both light and dark conditions. Also, the presence of the lactate uptake inhibitor, 4–CIN, in the medium had little affect on the amount of lactate released into the medium. Compared to wild type, there was a slight decrease in the amount of lactate released in both light and dark conditions, in GLAST^–/– mice. However, glutamate in the medium did not affect the amount of lactate release in either wild type or GLAST^–/– mice, irrespective of the light condition. Retinas from both wild type and GLAST^–/– mice showed a significant accumulation of ³H–2–DG after 30 min under our experimental conditions, indicating good viability of the retinas in vitro. Moreover, addition of 1mM glutamate did not significantly stimulate ³H–2–DG uptake. Finally lactate release was not affected by the glutamate uptake inhibitor, THA, under dark conditions in either wild type or GLAST^–/– mice. Conclusions: The results show that lactate production and release from Muller cells is not affected by the absence of the major glutamate transporter, GLAST, in Muller cells. We conclude that GLAST does not have a role in metabolic coupling between photoreceptors and Müller cells in the retina.

Keywords: Muller cells • retina