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This study aimed to determine whether glucocorticoid receptors are expressed in the photosensitive trout pineal organ, and whether glucocorticoids modulate melatonin secretion. On Western blots from pineal extracts, an antibody directed against trout glucocorticoid receptor labeled a single band at the expected size (approximately 100 kDa). Dexamethasone inhibited pineal arylalkylamine N-acetyltransferase activity (AANAT2; serotonin --> N-acetylserotonin) in a dose-dependent manner after 6 h of culture in the dark (IC50 2.10(-8) M). RU486 (10(-7) M) alone had a partial agonistic activity, whereas it antagonized the effects of 10(-8) M dexamethasone. Hydroxyindole-O-methyltransferase activity (N-acetylserotonin --> melatonin) remained unaffected. This is the first demonstration that glucocorticoid receptors are present in the pineal organ and that glucocorticoids modulate melatonin production.