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Retinal neurochemical changes following application of glutamate as a metablolic substrate
Michael Kalloniatis1, Genevieve A Napper
1Department of Optometry and Vision Sciences, The University of Melbourne, Parkville, VIC, 3010, Australia.
Background:
Retinal neural and glial cells share an intricate relationship that includes uptake and recycling of the amino acid neurotransmitters, glutamate and gamma-amino butyric acid (GABA), as well as metabolic links. The aim of this work was to determine the neurochemical and morphological changes induced by the removal of glucose but with the provision of exogenous glutamate in the isolated retinal preparation incubated under aerobic conditions. The carbon skeleton of glutamate can enter the tricarboxylic acid cycle as alpha-ketogluterate, providing an alternative metabolic substrate in cases of glucose deprivation.
Methods:
Isolated rat retinas were incubated in physiological media with and without glucose, using a range of glutamate concentrations to provide an alternative source of metabolic substrate. We conducted post-embedding immunocytochemistry and quantified the change in glutamate and GABA immunoreactivity within Müller cells under these different incubation conditions.
Results:
The provision of glutamate with normal (6 mM) glucose levels resulted in a gradual accumulation of glutamate and GABA in Müller cells, with Müller loading when exogenous glutamate concentrations were above 0.1 mM. However, when these varying levels of glutamate were applied in the absence of glucose, glutamate accumulation in Müller cells was decreased compared to the 6 mM glucose condition and GABA accumulation in Müller cells was at a minimum at moderate (0.5 and 1 mM) glutamate levels. Under hypoglycaemic conditions, exogenous glutamate (0.5 to 1 mM) is rapidly metabolised by Müller cells to the extent that no glial loading is evident, despite the high concentrations.
Conclusions:
Normal neurochemical function appears to be maintained secondary to exogenous glutamate provision of 0.5 to 1 mM when glucose is not in the incubation medium, implying that glutamate can be used as an alternative metabolic substrate. We also show that Müller cells possess more rapid glutamate metabolic capabilities compared to the metabolism of GABA.