Transcripts Encoding Two Melatonin Synthesis Enzymes in the Teleost Pineal Organ: Circadian Regulation in Pike and Zebrafish, But Not in Trout*

Melatonin production in trout does not exhibit daily rhythmicity in contrast to pike and zebrafish, where it is regulated by a pineal clock.

• AA-NAT mRNA, crucial for melatonin synthesis, is present in the pineal organs of pike, zebrafish, and trout.
• In pike, AA-NAT mRNA levels show a circadian rhythm under various light conditions, indicating regulation by a biological clock.
• Zebrafish also demonstrate a circadian rhythm in AA-NAT mRNA levels in both light/dark cycles and constant light.
• Trout exhibit stable levels of AA-NAT mRNA regardless of light conditions, suggesting a lack of circadian regulation.
• The abundance of mRNA for tryptophan hydroxylase, another enzyme in melatonin production, shows circadian variation in pike but not in trout.
• A hypothesis suggests that a single mutation may disrupt the circadian system regulating melatonin synthesis genes in trout.

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