Photoperiod-Dependent Changes in Melatonin Synthesis in the Turkey Pineal Gland and Retina

Abstract

The effect of photoperiod on melatonin content and the activity of the melatonin-synthesizing enzymes, namely, serotonin N-acetyltransferase (AANAT) and hydroxyindole-O-methyltransferase, were investigated in the pineal gland and retina of turkeys. The birds were adapted to 3 different lighting conditions: 16L:8D (long photoperiod), 12L:12D (regular photoperiod), and 8L:16D (short photoperiod). Pineal, retinal, and plasma melatonin concentrations oscillated with a robust diurnal rhythm, with high values during darkness. The duration of elevated nocturnal melatonin levels in the turkey pineal gland, retina, and plasma changed markedly in response to the length of the dark phase, being longest during the short photoperiod with 16 h of darkness. These photoperiodic variations in melatonin synthesis appear to be driven by AANAT, because changes in the activity of this enzyme were closely correlated with changes in melatonin. By contrast, pineal and retinal hydroxyindole-O-methyltransferase activities failed to exhibit any significant 24-h variation in the different photoperiods. A marked effect of photoperiod on the level of melatonin production was also observed. Peak values of melatonin and AANAT activity in the pineal gland (but not in the retina) were highest during the long photoperiod. During the light phase, mean melatonin concentrations in the pineal gland and retina of turkeys kept under the long photoperiod were significantly higher compared with those from birds maintained under the regular and short photoperiods. In addition, mean circulating melatonin levels were lowest in the short photoperiod. Finally, the magnitude of the light-evoked suppression of nighttime pineal AANAT activity was also influenced by photoperiod, with suppression being smallest under the long photoperiod. These findings show that in the turkey, photoperiod plays an important role in regulating the melatonin signal.

Dopamine in the Turkey Retina—An Impact of Environmental Light, Circadian Clock, and Melatonin

“…1). Under the three photoperiodic regimes, diurnal dopamine and DOPAC rhythms were out of phase with the demonstrated by us earlier (Zawilska et al 2007) oscillations in retinal melatonin content (Fig. 1).…”

“…In retinas of turkeys held under different light-dark illumination cycles (16L:8D, 12L:12D, and 8L:16D), levels of dopamine and its metabolite DOPAC (present results), and melatonin content (Zawilska et al 2007) showed wellexpressed daily rhythms with opposite phases. High levels of dopamine and DOPAC were observed during the light hours and low levels during the dark hours.…”

“…We have previously demonstrated that melatonin production in the turkey retina is governed by the circadian pacemaker (Zawilska et al 2006) and potently influenced by changes in day length (Zawilska et al 2007). In the present work, we asked a question what is an impact of environmental lighting conditions and biological clock on dopamine level and metabolism in the turkey retina.…”

Emerging Targets for the Pharmacological Treatment of Depression: Focus on Melatonergic System

“…However, the key regulatory enzyme in the melatonin biosynthetic pathway is the enzyme AANAT. In fact, biochemical studies revealed the presence of high AANAT mRNA levels in the pineal glands and retinas of vertebrates [47], and changes in melatonin levels reflect oscillations in its activity [43,48]. The enzyme belongs to a superfamily of GCN5-related N-acetyltransferases (GNAT) and need acetyl coenzyme A (AcCoA) as an acetyl group donor [43].…”

Melatonin treatment during early life interacts with restraint to alter neuronal morphology and provoke depressive-like responses

“…Successful treatment of SAD with bright light therapy, suggests that prolonged melatonin signals after waking contribute to depressive symptoms [18,19]. Individuals with SAD display prolonged periods of elevated melatonin during the winter relative to the summer, similar to changes observed in animals that use photoperiod to time reproduction [18,22,23]. Nondepressed individuals did not display seasonal differences in circulating melatonin concentrations, suggesting a role for melatonin in SAD etiology [18].…”