Inhibition of glutamate-induced nitric oxide synthase activation by dopamine in cultured rat retinal neurons.

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.

Mitochondrial ATP-sensitive potassium channel: a novel site for neuroprotection.

PURPOSE: It has been shown that bradykinin (BK) protects retinal neurons against glutamate excitotoxicity, but it was not clear how BK inhibits glutamate excitotoxicity. The purpose of this study was to investigate the effect of opening the mitochondrial adenosine triphosphate (ATP)-sensitive potassium (Mit K (ATP)) channel on glutamate excitotoxicity and the protective effect of BK using cultured retinal neurons. METHODS: Primary cultures were obtained from the retina of fetal rats (gestation days 17-19). Glutamate neurotoxicity was assessed by 10-minute exposure to 1 mM glutamate followed by 1-hour incubation in glutamate-free medium, using the trypan blue exclusion method. BK, diazoxide (the opener of the Mit K (ATP) channel), 5HD, and glibenclamide (blockers of the Mit K (ATP) channel) were applied simultaneously with glutamate. Mitochondrial membrane potential was measured as the ratio of 590:527 nm fluorescence of JC-1. RESULTS: Cell viability was markedly reduced by 10-minute exposure to 1 mM glutamate followed by 1-hour incubation in glutamate-free medium, and glutamate induced mitochondrial depolarization of retinal neurons. BK and diazoxide protected retinal neurons against glutamate excitotoxicity and inhibited glutamate-induced mitochondrial depolarization. These actions of BK and diazoxide were inhibited by the coapplication of 5HD and glibenclamide. Furthermore, diazoxide inhibited the sodium nitroprusside (SNP, NO donor) toxicity, but did not inhibit the 3-morpholinosydnonimine (SIN-1, NO, and superoxide donor) toxicity. CONCLUSIONS: These results suggest that BK and diazoxide protect retinal neurons against glutamate excitotoxicity by opening the Mit K (ATP) channel. It is suggested that opening of the Mit K (ATP) channel inhibited glutamate-induced generation of superoxide.

New insight into the functional role of acetylcholine in developing embryonic rat retinal neurons.

PURPOSE: To examine the effects of acetylcholine (ACh) on glutamate-induced neurotoxicity in embryonic rat retinal neurons. METHODS: Primary cultures were obtained from rat retinas at embryonic days 17 to 19. Cultured cells were exposed to glutamate for 10 minutes, followed by incubation in glutamate-free medium for 1 hour. Drugs were added to the incubation medium for 1 to 24 hours until immediately before glutamate exposure and were removed from culture medium during glutamate exposure and the postincubation period. The neurotoxic effects on retinal cultures were quantitatively assessed by the trypan blue exclusion method. RESULTS: Cell viability was markedly reduced by 10-minute exposure to 500 microM glutamate followed by a 1-hour incubation in glutamate-free medium. Incubating the cultures with 1 microM ACh for 12 hours before glutamate exposure reduced glutamate neurotoxicity. A similar effect was induced by application of carbachol (1 microM). The protective effect of ACh against glutamate neurotoxicity was inhibited by a nicotinic acetylcholine receptor (nAChR) antagonist, mecamylamine (0.5 microM), whereas a muscarinic acetylcholine receptor (mAChR) antagonist, atropine (0.5 microM) did not affect ACh-induced protection. In addition, a similar protection was induced by application of nicotine (1 microM), but not by muscarine (1 microM). Pretreatment with nicotine induced a protective effect in a time-dependent manner, ranging from 1 to 12 hours. Pretreatment with nicotine at concentrations ranging from 0.001 to 1 microM induced dose-dependent protection against glutamate neurotoxicity. Furthermore, the protective action of nicotine was inhibited by simultaneous application of dopamine D1 receptor antagonist, SCH23390 (1 microM), with nicotine, whereas a dopamine D2 receptor antagonist, domperidone (1 microM), did not affect nicotine-induced protection. CONCLUSIONS: These results suggest that pretreatment of cultured rat retinal neurons with ACh or the nAChR agonists, nicotine and carbachol, has a protective action against glutamate neurotoxicity through nAChRs and that the dopamine release induced by nicotinic stimulation subsequently protects the retinal neurons by way of dopamine D1 receptors.