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ER Stevens, TA Doerr, P Kim, RF Miller, SH Snyder; D-Serine Acts as a Co-Agonist at NMDA Receptors in the Vertebrate Retina . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4764.
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Purpose: To determine if D-serine is present in the vertebrate retina, and to assess its role in shaping ganglion cell light responses. Methods: Immunohistochemical studies were performed on amphibian retinas (Necturus maculosus and Ambystoma tigrum) to establish the location of D-serine and serine racemase, the enzyme which synthesizes D-serine. In addition, the role of D-serine was evaluated using whole-cell recordings from ganglion cells in an isolated, perfused tiger salamander retina. Results: D-serine and serine racemase are present in the inner layers of the amphibian retina. Co-labeling with glutamine synthetase, a glial enzyme, shows significant overlap, localizing serine racemase in Muller cells. D-serine appears to be synthesized and stored in close proximity to NMDA receptors in the inner retina. Whole-cell recordings from ganglion cells in the tiger salmander retina demonstrate that exogenously applied D-Serine potentiates tonic inward currents and enhances the light-evoked amplitude and duration of ON and OFF responses in ganglion cells in Mg-free Ringer using holding potentials between -60 mV and -70 mV. The effects of D-serine are reversible, and blocked by the NMDA receptor antagonist, D-AP7. The potentiation by D-serine was also blocked by 7-chloro-kynurenic acid (7ClKyn), a competitive antagonist at the strychnine-insensitive, glycine binding site of the NMDA receptor. This block by 7ClKyn could be overcome by the application of additional D-serine, strongly suggesting that D-serine interacts with the glycine binding site of the NMDA receptor. Conclusion: The present studies suggest that D-serine is synthesized in Muller cell processes in the inner retinal layers adjacent to its putative site of action on the NMDA receptors of ganglion and amacrine cells. D-serine appears to act as a co-agonist at the glycine binding site of NMDA receptors and to potentiate the light-evoked activity of ganglion cells. These results also strongly suggest that the glycine binding site of the NMDA receptors is not normally saturated, indicating a role for D-serine in modulating the light-evoked activity of ganglion cells through control of NMDA receptors. This pathway demonstrates a new mechanism through which Muller cells have some control of ganglion cell excitablility.
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