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Michael James Neal, Joanna Ruth Cunningham, Kim Lisa Matthews; Release of Endogenous Ascorbic Acid Preserves Extracellular Dopamine in the Mammalian Retina. Invest. Ophthalmol. Vis. Sci. 1999;40(12):2983-2987.
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purpose. To investigate whether the inhibitory effect of nitric oxide (NO) on
dopamine release from the retina is due to chemical oxidation of
dopamine in the extracellular medium rather than to an inhibitory
effect on dopamine release from retinal neurons.
methods. Dopamine was incubated in Krebs bicarbonate medium and its rate of
chemical degradation measured by high-performance liquid chromatography
(HPLC). The effects of NO donors and antioxidants on dopamine were
assessed by comparing dopamine degradation in the presence and absence
of drug. The effects of NO donors on the K-evoked release of[ 3H]dopamine were measured from isolated superfused
rabbit retinas. The release of ascorbic acid from the isolated rat
retina and from an eyecup preparation in anesthetized rabbits was
measured by HPLC.
results. After 10 minutes’ incubation in Krebs bicarbonate medium, the dopamine
concentration decreased by 20%. This decline increased to 80% in the
or sodium nitroprusside (SNP). The increased rate of dopamine
degradation was abolished if retina was incubated in the medium and
then removed before the incubation of dopamine. The protective effect
of preincubation with tissue was lost in the presence of ascorbate
oxidase suggesting the release of ascorbic acid. HPLC analysis
confirmed a substantial release of ascorbic acid from both rabbit and
rat retinas. The K-evoked release of [3H]dopamine from
the rabbit retina was inhibited by SNP.
conclusions. NO can rapidly oxidize dopamine in physiological medium, but in the
presence of retina, sufficient endogenous antioxidants (mainly
ascorbate) are released to prevent this chemical reaction. Thus, the
inhibitory action of NO on dopamine release results from an action on
retinal neurons. Ascorbate release in the retina may have an important
physiological role in prolonging the life of dopamine, which often has
to diffuse long distances from axons in the inner plexiform layer to
receptors in other retinal layers.
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