Diurnal and circadian rhythms in melatonin synthesis in the turkey pineal gland and retina

Melatonin levels in the turkey's pineal gland and retina were 50-80% lower under constant darkness compared to a light-dark cycle.

• Melatonin concentrations in the pineal gland and retina are low during the light phase and high during the dark phase in a light-dark cycle.
• The activity of serotonin N-acetyltransferase (AANAT) aligns with the rhythmic production of melatonin in the pineal gland and retina.
• No significant changes were observed in the enzyme hydroxyindole-O-methyltransferase (HIOMT) activity throughout the 24-hour period.
• Light exposure at night rapidly decreased melatonin levels in the pineal gland, retina, and plasma.
• AANAT activity and melatonin rhythms persist under constant darkness, indicating a circadian nature, but with reduced amplitudes.

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