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      Circadian Rhythms of Dopamine and Dihydroxyphenyl Acetic Acid in the Mouse Striatum: Effects of Pinealectomy and of Melatonin Treatment

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          The existence of dopamine (DA)-melatonin (aMT) relationships is well documented in several brain areas of the mammalian central nervous system such as the retina and hypothalamus or the nigrostriatal system. For instance, aMT tempers 1 methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced nigrostriatal damage in C57BL/6 mice. In this mouse strain however, rhythmic production of aMT and its possible interaction with striatal DA is still unclear. In the present work we investigated circadian variations in pineal production of aMT and striatal DA levels in C57BL/6 mice. Effects of pinealectomy and aMT administration were also assessed. Intact, pinealectomized and pinealectomized + aMT-treated mice and their respective control groups were sacrificed at six different times during the 24-hour period. In control animals, aMT displayed a circadian rhythm with a narrow peak at midnight. The peak of aMT coincided with the nadir of the DA rhythm present in the striatum. Shortly after the decrease of DA levels, an increase in 3,4-dihydroxyphenylacetic acid (DOPAC), the main DA metabolite, was observed. The rhythmic changes of DA and DOPAC levels in the striatum were blunted by pinealectomy, whereas administration of aMT (0.1–10 mg/kg) during 6 days to pinealectomized mice restored the rhythms in a dose-dependent manner. Striatal levels of 3-methoxytyramine and homovanillic acid did not change during the 24-hour cycle. The serotonergic system, assessed by the determination of 5-hydroxytryptamine and 5-hydroxyindole-3-acetic acid concentration in striatum, did not show significant time-dependent changes in control animals and was not affected by pinealectomy or aMT treatment. These data substantiate the existence of a link between pineal function, melatonin secretion and DA circadian rhythm in the mouse striatum.

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          Chemistry, physiology and pathology of free radicals

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            Dopamine inhibits melatonin release in the mammalian retina: in vitro evidence.

            A circadian oscillator located within the retina controls melatonin synthesis in the retina of mammals. In non-mammalian vertebrates retinal melatonin and dopamine appear to act as mutually inhibitory paracrine signals for night and day, respectively; while in mammals this mutually inhibitory capability has now been fully demonstrated. In this study using a flow-through culture apparatus we investigated melatonin release from cultured retinas of golden hamster (Mesocricetus auratus) in the presence of dopamine or dopaminergic agonists and antagonists. Neural retinas were cultured with medium 199 for 24 h in a flow-through apparatus at the temperature 33 degrees C. During the subjective night the culturing medium was supplemented with dopamine, dopamine receptor antagonists or agonists. At the concentration of 0.1 microM dopamine did not inhibit melatonin release, while at higher dopamine concentration (1 to 1000 microM) melatonin release was inhibited in a dose-dependent manner. These effects appeared to be mediated by a D2/D4 receptor, because D2 and D4 receptor agonists (100 microM), but not D1/D5 receptor agonists (100 microM), inhibited melatonin release. Furthermore, D2/D4 selective receptor antagonists (100 microM) in conjunction with 100 microM dopamine blocked melatonin suppression, whereas a D1/D5 selective receptors antagonist was completely ineffective. Taken together, these results directly demonstrate for the first time that in the retina of mammals dopamine may suppress melatonin, and that suppression is mediated by D2/D4 dopaminergic receptors.
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              Enzyme-Dependent Ascorbate Recycling in Human Erythrocytes: Role of Thioredoxin Reductase


                Author and article information

                S. Karger AG
                March 2002
                21 March 2002
                : 75
                : 3
                : 201-208
                aDepartamento de Fisiología, Instituto de Biotecnología, Universidad de Granada, y bDepartamento de Farmacología y Fisiología, Facultad de Medicina, Universidad de Zaragoza, España
                48238 Neuroendocrinology 2002;75:201–208
                © 2002 S. Karger AG, Basel

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                Page count
                Figures: 6, References: 54, Pages: 8
                Neuroendocrine Regulation of Catecholamine Neurons


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