Seasonal variation of melatonin secretion across various segments of the gastrointestinal tract in rats

Objective: To investigate whether melatonin (MT) secretion in different parts of the gastrointestinal tract (GIT) exhibits seasonal variations and its correlation with immune regulation. Methods: Sixty Sprague-Dawley rats were divided into control and model groups, and the pineal gland was removed in the model group. Stomach, jejunum, ileum, and colon tissues were obtained during the spring equinox, summer solstice, beginning of autumn, autumn equinox, and winter solstice. The levels of MT, MT receptors (MR), arylalkylamine N-acetyltransferase (AANAT), hydroxyindole-O-methyltransferase (HIOMT), interleukin-2 (IL-2), and interleukin-10 (IL-10) in the GIT were measured using enzyme-linked immunosorbent assay. Results: Except for the stomach, the jejunum, ileum, and the colon showed seasonal tendencies in MT secretion. In the control group, MT secretion in the jejunum and ileum was the highest in the long summer, and colonic MT secretion was the highest in winter. In the model group, MT levels in the colon were highest in the summer. The seasonal rhythms of the MR, AANAT, HIOMT, IL-2, and IL-10 in the colon were roughly similar to those of MT, and changed accordingly after pinealectomy. Conclusions: Gastrointestinal MT secretion is related to seasonal changes, and MT secretion in each intestinal segment is influenced by different seasons. The biological effects of MT in the gut are inextricably linked to the mediation of MR, and a hormone-receptor linkage exists between MT and MR. The effect of seasonal changes on the gastrointestinal immune system may be mediated through the regulation of seasonal secretion of MT.