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Lauren Purpura, Harris Ripps, Wen Shen; Fast Glutamate Uptake by EAAT2 Prevent Glutamate Depletion in Rod Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2490.
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© ARVO (1962-2015); The Authors (2016-present)
Previously, we demonstrated that EAAT2A (excitatory amino acid transporter 2A) is expressed predominantly in the photoreceptor terminals, and showed that its function is important in shaping the cone-mediated light-offset response in bipolar cells. In this study we examined how EAAT2A prevents glutamate depletion in rods, and the contribution of EAAT2A to synaptic plasticity in the distal retina.
Whole cell patch-clamp recordings were obtained from rod photoreceptors in tiger salamander retinal slices. Single and multiple depolarizing pulses were applied to trigger glutamate release from the photoreceptors. After release, EEAT2A serves to restore glutamate in the receptor terminals, and the synaptic EAAT currents were recorded in rods with electrodes containing a high intracellular concentration of CsNO3. DHKA, a specific inhibitor of the EAAT2 subtype, was used to block the synaptic EAAT2A currents.
We find that DHKA blocks 80% of the EAAT2A-mediated current in the rod terminals, an indication that the transporter effectively takes up glutamate in dark. However, DHKA only blocked 35-40% of the transporter current in cones, and the DHKA- insensitive currents were blocked by TBOA, a broad-spectrum EAAT inhibitor,. EAAT2A activation in rods was tightly controlled by glutamate levels in the synaptic cleft, whereas the kinetics of the transporter was not affected by glutamate levels. After applying a 2ms depolarizing pre-pulse to trigger a rapid release of glutamate from rod terminals, we found that EAAT2A activates in less that a millisecond. Our results also show that internal Ca2+ levels, protons, and the cell’s ECl can affect the action of EAAT2A in rod terminals.
Our study reveals a role for EAAT2A in preventing glutamate depletion at the rod photoreceptor terminals. The fast reuptake of glutamate by EAAT2A may help to sustain glutamate release from rods in darkness.
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