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E.B. Trexler, W. Li, S.C. Massey; Expression of NMDA-Sensitive Glutamate Receptors on AII Amacrine Cells . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2070.
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Purpose: AII amacrine cells relay glutamatergic signals from rod bipolars to ON and OFF cone bipolar cells. Synaptic transmission from rod bipolar ribbons to AII dendrites in sublamina 5 of the inner plexiform layer (IPL) is probably mediated by AMPA receptors, given the current immunohistochemical (Ghosh et. al. 2001. J Neurosci. 21:8636-47, Li et. al. 2002. J Comp Neurol. 448:230-48) and pharmacological evidence (Cohen and Miller. 1999. Brain Res. 831:206-28, Morkve et. al. J Physiol. 2002. 542:147-65). However, NMDA receptor subunit mRNAs have been detected by in situ hybridization in amacrine cells (Brandstatter et. al. 1994. Eur J Neurosci. 6:1100-12), but the precise cellular location of protein expression has not been determined. There are conflicting electrophysiological data regarding the expression of NMDA receptors on AIIs (Hartveit and Veruki. 1997. Neuroreport. 8:1219-23, Boos et. al. 1993. J Neurosi. 13:2874-88). Using electrophysiological measurements and confocal microscopy, we undertook studies to determine the pharmacological activation and expression pattern of NMDAR1 subunit proteins on AII amacrine cells. Methods: All NDMA receptors contain NMDAR1 subunits in addition to one or more NMDAR2 subunits. We immunocytochemically determined the expression pattern of NMDAR1 subunits on calretinin labeled AII amacrines in the rabbit retina using confocal microscopy. We also measured the expression of NMDA sensitive receptors from whole cell recordings of AIIs in rabbit retina slices by perfusing exogenous NMDA together with glycine in the presence of Co2+ or a cocktail of antagonists of other receptors. Results: AII amacrine cells were well labeled by calretinin and cell processes in both sublaminas of the IPL were clearly visible. NMDAR1 immunoreactivity was abundant in the IPL, but clear punctate staining was localized to the dendrites of AII amacrine cells in sublamina b of the IPL. In whole cell patch recordings, application of NMDA together with glycine elicits a conductance increase primarily at positive potentials, which can be blocked by the specific NMDA antagonist, CPP. The current-voltage relationship of the NMDA current was outwardly rectifying, as expected, due to voltage dependent block of NMDA receptors by Mg2+ and Co2+ ions. Conclusions: These experiments demonstrate that NMDAR1 subunit proteins are expressed in AII amacrine membranes and that functional receptors do exist on AII amacrine cells. Thus, NMDA receptors may contribute to the light response of AIIs if activated when the cells are significantly depolarized.
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