Daily rhythms of two melatonin-making enzymes in the pineal gland of pike and zebrafish, but not 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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