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Matthew J. Rowan, Wen Shen; Light Suppresses Synaptic Glutamate Transporter (EAAT2) Activation in Cones via a Proton-sensitive Mechanism. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2569.
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Purposes: Fluctuations in Excitatory Amino Acid Transporter (EAAT) activity in dark- and light-adapted conditions may contribute to cone synaptic plasticity. We focused upon a neuronal modulation of cone EAATs by transient light adaptation, through observing synaptic transporter currents.
Methods: Whole cell patch clamp was performed on cones in dark- and transiently light-adapted tiger salamander retinal slices. Cells were held at -70mV. Endogenous and EAAT currents were recorded in cones with a short depolarization to -10mV/2ms. DHKA, a specific transporter inhibitor, was used to identify EAAT2 currents and to evaluate overall EAAT currents. GABA and glycine network inputs were always blocked with picrotoxin and strychnine. For the light adaptation, slices were exposed to a red light stimulus of varying intensities for 2-4 seconds. Control HEPES was set to 3mM.
Results: Synaptic EAAT currents were observed from cones following the short depolarizing step, which were significantly blocked (80%) with 300uM DHKA. These currents did not change with different holding voltages of -50 or -70mV prior to the depolarizing step. Interestingly, synaptic EAAT currents could be inhibited with a preceding light step or 2-4 seconds; the inhibition became more pronounced with higher light intensities. These light steps did not inhibit glutamate release trigged by the depolarizing step, measured with cone capacitance. The light inhibition was mimicked by a high HEPES buffer (12mM) in the dark. As network proton release likely halts with background light, and because EAAT2s likely utilize synaptic protons to bolster transport, we hypothesized that these currents were inhibited with light due to a lack of available protons.
Conclusions: We determined that cone-specific EAAT2 activity is inhibited via transient light adaptation. Previously, we demonstrated that presynaptic EAAT2 activity in cones is necessary to limit glutamate signals in 2nd order neurons in dark conditions. Our data suggests that transient light adaptation suppresses EAAT2 activation by altering extracellular proton levels, resulting in largely enhanced cone-specific signals in the 2nd order neurons.
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