Daily Oscillation in Melatonin Synthesis in The Turkey Pineal Gland and Retina: Diurnal and Circadian Rhythms

May 12, 2006Chronobiology international

Daily rhythms of melatonin production in the Turkey's pineal gland and retina

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Abstract

Melatonin levels in the turkey pineal gland and retina were high during the dark phase and low during the light phase.

The pineal gland and retina of turkeys rhythmically produce melatonin, reflecting changes in light exposure.
Melatonin concentrations in plasma also exhibited rhythmic oscillations, with elevated levels at night.
AANAT activity, which is crucial for melatonin production, showed circadian rhythms that persisted even in constant darkness.
Transitioning from a light-dark cycle to constant darkness resulted in a significant decline in the amplitude of AANAT rhythm within the first day.
By the sixth day in constant darkness, AANAT activity in the pineal gland remained higher during the subjective dark phase compared to the subjective light phase, while retinal AANAT activity did not show significant oscillations.

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The aim of the present study was to examine arylalkylamine N-acetyltransferase (AANAT) activity and melatonin content in the pineal gland and retina as well as the melatonin concentration in plasma of the turkey (Meleagris gallopavo), an avian species in which several physiological processes, including reproduction, are controlled by day length. In order to investigate whether the analyzed parameters display diurnal or circadian rhythmicity, we measured these variables in tissues isolated at regular time intervals from birds kept either under a regular light-dark (LD) cycle or under constant darkness (DD). The pineal gland and retina of the turkey rhythmically produced melatonin. In birds kept under a daily LD cycle, melatonin levels in the pineal gland and retina were high during the dark phase and low during the light phase. Rhythmic oscillations in melatonin, with high night-time concentrations, were also found in the plasma. The pineal and retinal melatonin rhythms mirrored oscillations in the activity of AANAT, the penultimate enzyme in the melatonin biosynthetic pathway. Rhythmic oscillations in AANAT activity in the turkey pineal gland and retina were circadian in nature, as they persisted under conditions of constant darkness (DD). Transferring birds from LD into DD, however, resulted in a potent decline in the amplitude of the AANAT rhythm from the first day of DD. On the sixth day of DD, pineal AANAT activity was still markedly higher during the subjective dark than during the subjective light phase; whereas, AANAT activity in the retina did not exhibit significant oscillations. The results indicate that melatonin rhythmicity in the turkey pineal gland and retina is regulated both by light and the endogenous circadian clock. The findings suggest that environmental light may be of primary importance in the maintenance of the high-amplitude melatonin rhythms in the turkey.

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Daily Oscillation in Melatonin Synthesis in The Turkey Pineal Gland and Retina: Diurnal and Circadian Rhythms

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