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E. Kastenhuber, C. M. Maurer, M. Gesemann, S. Frueh, S. L. Renninger, S. C. F. Neuhauss; Characterization of Glutamate Transporters (EAATs) in the Zebrafish Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4118.
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The main excitatory neurotransmitter used in the vertebrate retina is glutamate. At the first retinal synapse, between photoreceptor and bipolar cells, glutamate acts excitatory on OFF bipolar cells, while it inhibits ON bipolar cells by binding to cell-type specific glutamate receptors. Glutamate transporters, also known as excitatory amino acid transporters (EAATs), are known to be responsible for glutamate clearance at retinal synapses, thereby buffering glutamate concentrations, allowing normal neural transmission, and preventing neurotoxic effects of the neurotransmitter. Additionally, they are thought to have the potential to act as depolarizing receptors in teleost ON bipolar cells. The expression and function of glutamate transporters in the zebrafish retina are still elusive.
Phylogenetic comparison and microsynteny analysis allowed us to classify the EAAT members of the teleost lineage into subfamilies. In situ hybridization (ISH) analysis of a chosen subset of eaats revealed their expression profile in zebrafish larvae and adult retinas. Antibodies against EAATs with retinal expression are currently raised to visualize their localization. Knockdown experiments with morpholinos against the glutamate transporters will allow us to analyze their function.
In contrast to the five members of the EAAT family in mammals, we have found 11 members in the teleost Danio rerio, falling into seven subfamilies. The expression of four subfamilies (eaat2, 5, 6, and 7) comprising seven genes in total has been analyzed by ISH. Expression of the eaat7 gene is found exclusively in retinal bipolar cells, whereas eaat2b, eaat5, and eaat6 genes have shown expression in photoreceptor cells only so far.
At least five out of 11 eaat genes are expressed in a cell-type specific manner in the teleost retina, and thus are potentially involved in glutamate transmission. Further analysis is currently conducted to characterize their function in retinal signalling processes.
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