In this paper evidence that supports a new role for melatonin as a negative endogenous regulator of cytosolic phospholipase A2 (cPLA2) is presented. When rat pineal glands were incubated in culture, time-dependent release of arachidonic acid (AA) was observed, which was significantly inhibited by a known 85-kDa cPLA2 inhibitor, methyl arachidonyl fluorophosphonate. Co-incubation with melatonin inhibited the AA release in a concentration-dependent manner, and this decrease was accompanied by a reduction of cPLA2 protein and mRNA expression. Melatonin-receptor agonists, 2-iodo-N-butanoyl-5-methoxytryptamine and 5-methoxycarbonylamino-N-acetyltryptamine, also decreased AA release and cPLA2 protein and mRNA levels, while pre-incubation with the melatonin receptor antagonists luzindole and 2-phenylmelatonin abolished the melatonin effect. In vivo, as melatonin production reflected a typical diurnal variation, endogenous non-esterified AA and cPLA2 mRNA levels in the rat pineal gland showed an off-phase diurnal pattern in relation to melatonin levels. Intravenous administration of isoproterenol, which has been shown to elevate melatonin production, also decreased the levels of non-esterified AA and cPLA2 mRNA significantly. Direct administration of melatonin to rats by intravenous injection decreased the levels of non-esterified AA, cPLA2 protein and mRNA in rat pineal glands. In conclusion, melatonin endogenously down-regulates cPLA2 expression, presumably through melatonin-receptor-mediated processes.

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