The avian pineal gland, apart from the hypothalamic master clock (suprachiasmatic nuclei, SCN) and retina, functions as an independent circadian oscillator, receiving external photic cues that it translates into the rhythmical synthesis of melatonin, a biochemical signal of darkness. Functional similarity to the mammalian SCN makes the avian pineal gland a convenient model for studies on biological clock mechanisms in general. Pineal melatonin is produced not only in a light-dependent manner but also remains under the control of the endogenous oscillator, while the possible involvement of melatonin in maintaining cyclic expression of the avian clock genes remains to be elucidated. The aim of the present study was to characterize the diurnal profiles of main clock genes transcription in the pineal glands of chickens exposed to continuous light (LL) and supplemented with exogenous melatonin. We hypothesized that rearing chickens from the day of hatch under LL conditions would evoke a functional pinealectomy, influencing, in turn, pineal clock function. To verify this hypothesis, we examined the diurnal transcriptional profiles of selected clock genes as well as the essential parameters of pineal gland function: transcription of the genes encoding arylalkylamine N-acetyltransferase (Aanat), a key enzyme in melatonin biosynthesis, and the melatonin receptor (Mel1c), along with the blood melatonin level. Chickens hatched in summer or winter were maintained under LD 16:8 and 8:16, corresponding to the respective photoperiods, as the seasonal control groups. Another set of chickens was kept in parallel under LL conditions and some were supplemented with melatonin to check the ability of exogenous hormone to antagonize the effects evoked by continuous light. Twelve-day-old chickens were sacrificed every 3 h over a 24-h period and the mRNAs of selected clock genes, Bmal1, Cry1, Per3, E4bp4, together with those of Aanat and Mel1c, were quantified in the isolated pineal glands. Our results indicate that the profiles of clock gene transcription are not dependent on the duration of the light phase, while LL conditions decrease the amplitude of diurnal changes, but do not abolish them entirely. Melatonin supplied in drinking water to the birds kept in LL seems to desynchronize transcription of the majority of clock genes in the summer, while in the winter, it restores the pattern, but not the diurnal rhythmicity. Rhythmic expression of Bmal1 appears to provide a direct link between the circadian clock and the melatonin output pathway, while the availability of cyclic melatonin is clearly involved in the canonical transcription pattern of Per3 in the chicken pineal gland. Regardless of the experimental conditions, a negative correlation was identified between the transcription of genes involved in melatonin biosynthesis (Aanat) and melatonin signal perception (Mel1c receptor).

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Previously, we have demonstrated the postembryonic development of chicken (Gallus gallus domesticus L.) pineal gland functions expressed as changes in melatonin (MEL) biosynthesis. Pineal concentrations of MEL and its precursor serotonin (5-HT) were shown to increase between the 2nd and 16th day of life. We also found that levels of the mRNAs encoding the enzymes participating in the final two steps of MEL biosynthesis from 5-HT: arylalkylamine-N-acetyltransferase (AANAT) and hydroxyindole-O-methyltransferase (HIOMT), as well as their enzymatic activities, were raised during postembryonic development. Moreover, the manner of these changes was season-of-hatch dependent, even in animals kept under constant laboratory conditions (L:D 12:12). The most pronounced changes were seen in the concentrations of 5-HT and MEL, as well as in Aanat mRNA level and its enzymatic activity. The high daily variability in 5-HT content suggested that season- and age-dependent changes in the activity of the chicken pineal gland might rely on the availability of 5-HT, i.e. it may be limited by changes in pineal tryptophan (TRP) and/or 5-hydroxytryptophan (5-HTP) levels as well as by the activity of tryptophan hydroxylase (TPH) and aromatic l-amino acid decarboxylase (AADC): two enzymes participating in the conversion of TRP to 5-HT. The present study was undertaken with the following objectives: (1) to examine whether the pineal concentration of the 5-HT precursors TRP and 5-HTP exhibit age- and season-related changes; (2) to look for season-related differences in the transcription of the Tph1 and Ddc genes encoding enzymes TPH and AADC; (3) to identify the step(s) in postembryonic development in which these season-related variations in pineal gland function are most pronounced. Male Hy-line chickens hatched in the summer or winter, from eggs laid by hens held in L:D 16:8 conditions were kept from the day of hatch in L:D 12:12 conditions. At the age of 2 or 9 days, animals were sacrificed every 2 or 4 h over a 24-h period and their pineal glands were isolated under dim red light and processed for the measurement of (i) the pineal content of TRP, 5-HTP and 5-HT, and (ii) the level of Tph1 and Ddc mRNAs. Circadian rhythmicity of all the measured parameters was evaluated by the cosinor method. The pineal levels of TRP and 5-HT as well as the Tph1 and Ddc transcripts changed during postembryonic development in a season-related way. Whereas, the 5-HTP concentration did not vary between animals from both age groups, regardless of the season. Circadian rhythmicity of all the measured parameters was dependent on both the age and the season of hatch, and was greatest in older animals in the summer. These findings indicated that the efficiency of season-related MEL biosynthesis, reported previously, is limited by 5-HT availability and this limitation depends on the transcription of both the Tph1 and Ddc genes. Moreover, Ddc mRNA level in 9-d-old birds changed rhythmically, even though this gene is generally considered to be arrhythmic.

Immune system activity in higher vertebrates (mammals and birds) varies over the year, mainly in correlation with the external light phase duration and intensity, as well as accompanying environmental cues. Light influences immunity mainly via retina-SCN-pineal gland-melatonin pathway. Diurnal rhythm of melatonin biosynthesis, adjusted to the changing external day length, acts as a message of darkness for the target cells/organs in the body, including the immune system. Some of melatonin effects are receptor mediated, while others are receptor independent. According to the trade-off hypothesis, seasonal changes in immunity are inversely related to reproduction, as both physiological functions are energetically costly while the resources are limited, especially in adverse environmental conditions. In the present review, the examples of the seasonal changes of immunity in mammalian and avian species are described and discussed in the context of the pineal gland biosynthetic activity. Current knowledge on the communication between the pineal gland/melatonin and immune system indicates that these relationships are complex and the precise mechanism(s) involved are still under investigation.

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