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Abstract
This study characterizes the diurnal profiles of ten melatonin synthesis-related indoles, the quantitative relations between these compounds, and daily variations in the contents of catecholamines and their metabolites in the domestic duck pineal organ. Fourteen-week-old birds, which were reared under a 12L:12D cycle, were killed at two-hour intervals. The indole contents were measured using HPLC with fluorescence detection, whereas the levels of catecholamines and their metabolites were measured using HPLC with electrochemical detection. All indole contents, except for tryptophan, showed significant diurnal variations. The 5-hydroxytryptophan level was approximately two-fold higher during the scotophase than during the photophase. The serotonin content increased during the first half of the photophase, remained elevated for approximately 10 h and then rapidly decreased in the middle of the scotophase. N-acetylserotonin showed the most prominent changes, with a more than 15-fold increase at night. The melatonin cycle demonstrated only an approximately 5-fold difference between the peak and nadir. The 5-methoxytryptamine content was markedly elevated during the scotophase. The 5-hydroxyindole acetic acid, 5-hydroxytryptophol, 5-methoxyindole acetic acid and 5-methoxytryptophol profiles were analogous to the serotonin rhythm. The norepinephrine and dopamine contents showed no significant changes. The DOPA, DOPAC and homovanillic acid levels were higher during the scotophase than during the photophase. Vanillylmandelic acid showed the opposite rhythm, with an elevated level during the daytime.
Highlights
Indole metabolism in the pineal organ (Figure 1) commences with the active uptake of the essential amino acid tryptophan (TRP) into parenchymal cells [1,2,3]
We proposed and validated a novel HPLC method with fluorescence detection for the simultaneous determination of ten MLT synthesis-related indoles in the pineal organ
The performed experiments showed that the levels of indoles, except for 5-methoxyindole acetic acid (5-MIAA) and MLT, were similar (RSD < 5%) in the pineal homogenates deproteinized by perchloric acid precipitation and by ultrafiltration through a molecular filter with a 3-kDa molecular weight cut-off
Summary
Indole metabolism in the pineal organ (Figure 1) commences with the active uptake of the essential amino acid tryptophan (TRP) into parenchymal cells [1,2,3]. TRP is hydroxylated by tryptophan hydroxylase to 5-hydroxytryptophan (5-HTRP), which is decarboxylated by aromatic amino acid decarboxylase to serotonin (5-HT). This indoleamine is a starting point for several metabolic pathways. Among these pathways, the most investigated one is the transformation of 5-HT into melatonin (MLT). The pineal hormone is released from cells immediately after synthesis. Other indolic compounds, such as 5-hydroxyindole acetic acid (5-HIAA) and 5-hydroxytryptophol (5-HTOL), are produced after the oxidative deamination of 5-HT. The methylation of 5-HIAA and 5-HTOL leads to the formation of 5-methoxyindole acetic acid (5-MIAA) and 5-methoxytryptophol (5-MTOL), respectively
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