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M. Manookin, M. Weick, J. B. Demb; AMPA- and NMDA-receptor-mediated Responses in Off Alpha Retinal Ganglion Cells Encode Different Ranges of Visual Contrast. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1633.
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© ARVO (1962-2015); The Authors (2016-present)
Bipolar cells excite OFF Alpha (Y-type) retinal ganglion cells by releasing glutamate onto both AMPA- and NMDA-type glutamate receptors. Here, we tested how each receptor type is used to process visual contrast.
We made whole-cell recordings of OFF ganglion cells while presenting 9-100% contrast decrements from the mean luminance (~5 x 10^3 R*/M-cone/sec). The stimulus was a spot (diameter, 0.2 mm) presented for 83 msec. Contrast responses were recorded at 10 holding potentials between -80 and +40 mV. For each contrast level, the current-voltage (I-V) relationship for the response was modeled as the sum of three underlying ligand-gated conductances: AMPA, NMDA and GABA/glycine. These underlying conductances were measured in separate experiments where receptor agonists were applied via a puffer pipette while blocking synaptic transmission with Cd2+.
AMPA-mediated responses increased approximately linearly with increasing negative contrast. However, NMDA-mediated responses became asymptotic at ~40% contrast. A half-maximal response was reached at a lower contrast level for NMDA responses (26 ± 6%) relative to AMPA responses (54 ± 1%). At 100% contrast, the AMPA conductance (10.9 ± 2.3 nS) was larger than the NMDA conductance (2.1 ± 0.4 nS; n = 7 cells).
We conclude that the different sensitivities to glutamate of AMPA and NMDA receptors confer different sensitivities to visual contrast. NMDA receptors, with their high glutamate sensitivity, primarily encode low contrasts, whereas AMPA receptors, with their lower glutamate sensitivity, encode a wide range of contrasts.
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