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Santhosh Sethuramanujam, Malcolm M. Slaughter; Supralinear Summation Of Currents From NMDA And AMPA/kainate Receptors During Light Evoked EPSCs In The Vertebrate Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4576.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the role of NMDA receptors in light response of the vertebrate retina.
Whole cell voltage and current clamp recordings were done from the ganglion cell layer (GCL) in larval tiger salamander retina slices. Light evoked EPSCs (L-EPSC) were evoked by a red LED (660 nm, 200µm diameter, 2 s). The total charge transfer across the cell membrane during the light stimulus was used as a measure of the light response. Drugs were bath applied.
D-AP5 (50µM), a NMDA receptor (NMDAR) antagonist, blocked 79% of the ON L-EPSC (mean ± SEM: 78.9±3.0%; n = 30) and 60% of the OFF L-EPSC (59.8±5.7%; n = 29). Block of the ON L-EPSC is significantly higher than the OFF L-EPSC (p <0.005). The effect of D-AP5 was replicated with MK-801 (100µM), a non-competitive NMDAR antagonist. D-AP5 eliminated ON and OFF light response action potentials in current clamp mode (n = 6). NBQX (25µM), an AMPA/kainate receptor (AMPA/K-R) antagonist, in a magnesium-free external solution blocked 72% of the ON L-EPSC (72.2±10.6%; n = 10) and 80% of the OFF L-EPSC (79.8±7.9%; n = 10). However, the block was not significantly different between the ON and OFF L-EPSCs (p = 0.66). In order to compare the effects of the NMDAR and AMPAR/K-R antagonists in the inner retina, only the ON L-EPSC was considered because AMPAR/K-Rs also mediate the light response of OFF bipolar cells. The partial block by the two antagonists was not significantly different (p = 0.87). This indicates that currents through NMDARs and AMPAR/K-Rs are supralinear. NBQX (25µM) in the presence of magnesium in the external solution blocked 69% of the ON L-EPSC (68.5±15.3%; n = 5) and 89% of the OFF L-EPSC (88.8±10.1%; n = 4). DNQX (50µM), another AMPAR/K-R antagonist blocked 95% of ON L-EPSC (n = 3) and 100% of OFF L-EPSC (n = 2).
These results reveal that NMDARs and AMPA/K-Rs act synergistically to generate the light-evoked synaptic input to cells in the GCL. This synergy is not due to the magnesium block of the NMDARs as commonly perceived. The amount of current through NMDARs during ON L-EPSC is significantly higher than during OFF L-EPSC, and this also likely to be true of the AMPA/K-R currents.
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