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Melatonin Increases Serotonin N ‐Acetyltransferase Activity and Decreases Dopamine Synthesis in Light‐Exposed Chick Retina: In Vivo Evidence Supporting Melatonin‐Dopamine Interaction in Retina
Author(s) -
Nowak Jerzy Z.,
Kazula Arkadiusz,
Gotembiowska Krystyna
Publication year - 1992
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1992.tb08466.x
Subject(s) - melatonin , dopamine , endocrinology , medicine , dopaminergic , pineal gland , serotonin , biology , dopamine receptor , retina , retinal , chemistry , receptor , biochemistry , neuroscience
The administration of melatonin, either peripherally (0.01–10 mg/kg) or intraocularly (0.001–10 μmol/eye), to light‐exposed chicks dose‐dependently increased serotonin N ‐acetyltransferase (NAT) activity in retina but not in pineal gland. The effect of melatonin was slightly but significantly reduced by luzindole (2‐benzyl‐ N ‐acetyltryptamine), and not affected by two other purported melatonin antagonists, N ‐acetyltryptamine and N ‐(2,4‐dinitrophenyl)‐5‐methoxytryptamine (ML‐23). The elevation of the enzyme activity induced by melatonin was substantially stronger than that evoked by 5‐hydroxytryptamine, N ‐acetyl‐5‐hydroxytryptamine, or 5‐methoxytryptamine. The melatonin‐evoked rise in the retinal NAT activity was counteracted by two dopamine D 2 receptor agonists, quinpirole and apomorphine, and prevented by the dopamine D 2 receptor blocker spiroperidol, and by an inhibitor of dopamine synthesis, α‐methyl‐ p ‐tyrosine. Melatonin (0.1–10 mgJ.kg i.p.) dose‐dependently decreased the levels of dopamine and 3,4‐dihydroxyphenylacetic acid (DOPAC), as well as the DOPACJ.dopamine ratio, in chick retina but not in forebrain. The results obtained (1) indicate that melatonin in vivo potently inhibits dopamine synthesis selectively in retina, and (2) suggest that the increase in retinal NAT activity evoked by melatonin in light‐exposed chicks is an indirect action of the compound, and results from the disinhibition of the NAT induction process from the dopaminergic (inhibitory) signal. The results provide in vivo evidence supporting the idea (derived on the basis of in vitro findings) that a mutually antagonistic interaction between melatonin and dopamine operates in retinas of living animals.