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Previously, we showed that dopamine protects cultured retinal neurons from N-methyl-D-aspartate (NMDA) receptor mediated-glutamate excitotoxicity via dopamine D1 receptor. This study has demonstrated for the first time that nitric oxide synthase (NOS) plays a crucial role in dopamine-induced neuroprotection. Our patch clamp study has shown that dopamine does not affect the NMDA-induced whole cell current. Dopamine or SKF38393 (D1 receptor agonist) inhibited ionomycin (calcium ionophore)-induced toxicity, while dopamine did not affect S-nitrosocysteine (NO donor)-induced toxicity. Biochemical analysis on enzymatic activities has shown that dopamine or cAMP (which is generated through D1 receptor stimulation) inhibits glutamate induced-NOS activation. These results suggest that dopamine inhibits glutamate induced-NOS activation via D1 receptor, resulting in the protection of retinal neurons.

Purpose: To examine the effects of ifenprodil on glutamate-induced neurotoxicity in cultured retinal neurons., Methods: Primary cultures obtained from the fetal rat retina (gestation day 17-19) were used for the experiment. Neurotoxicity effects on retinal cultures were quantitatively assessed by the trypan blue exclusion method. The cells were exposed briefly (10 min) to excitatory amino acids (EAA, 1 mM) and then were incubated for 1 h in an EAA-free medium. Ifenprodil (10 mM) was added for the 10-min exposure to EAA and the subsequent 60-min incubation in an EAA-free medium., Results: Ifenprodil dose-dependently prevented cell death induced by glutamate or NMDA, but did not affect that induced by kainate. The protective effects of ifenprodil against glutamate neurotoxicity were significantly reduced by spermidine, a polyamine modulatory site agonist, but not by glycine, a strychnine-insensitive glycine site agonist., Conclusion: The findings suggest that ifenprodil protected the cultured retinal cells we used in this study against glutamate neurotoxicity by its inhibitory action on the polyamine modulatory site of the NMDA receptor.

Background: Both in vivo and in vitro studies suggest that excess stimulation of non-NMDA receptors can result in massive neuronal death in the retina. In particular, murine amacrine neurons have been known to show marked susceptibility to the toxic effects of kainate., Purpose: This study was designed to examine and characterize the role of N-methyl-D-aspartate (NMDA) receptor vs non-NMDA receptor in glutamate-induced neurotoxicity in the retina., Methods: Primary cultures obtained from fetal rat retina (gestation day 16-19) were used for the experiment. The neurotoxicity was assessed quantitatively using the trypan blue exclusion method. Electrophysiological studies using patch-clamp techniques were performed to record whole-cell currents evoked by these excitatory amino acids., Results: Removal of extracellular Ca2+ from the medium or application of MK-801 reduced the extent of cell death induced by the brief exposure to glutamate, NMDA, and kainate. By contrast, cell death induced by a 60-min exposure to kainate was not affected by MK-801. The electrophysiological study demonstrated that MK-801 abolished the whole-cell currents evoked by NMDA but had no effect on those induced by kainate or AMPA., Conclusion: These findings demonstrate that brief exposure to kainate induces cell death by way of activating NMDA receptors in cultured fetal retinal neurons and that NMDA receptors are the predominant route of fetal retinal neurotoxicity induced by brief glutamate exposure.