Abstract
Circadian rhythmicity and melatonin secretion influence many functions in mammals, including the immune system function. The aim of our study is to investigate the effect of suppression of melatonin synthesis (caused by constant lighting) on the quantity of leucocytes in immunized BALB/c mice. The mice were kept under different lighting conditions: (1) customary environmental lighting; (2) constant exposure to light; (3) exposure to light and daily melatonin treatment. The disrupted melatonin synthesis had no effect on the number of cells in the thymus, bone marrow, spleen, lymph nodes and Peyer's patches of immunized mice. However, the mice kept under constant light had an increased number of leucocytes in the peritoneal cavity when immunizations were performed in the evening. Melatonin treatment normalized the cell number. When the immunizations were performed in the morning, the numbers of cells in peritoneum of mice kept under constant lighting conditions were lower compared to mice exposed to normal lighting conditions. The number of cells of mice kept in normal light/dark conditions was significantly higher when the immunizations were performed in the morning. The number of peritoneal cells, however, did not depend on the immunization time when mice were kept under constant lighting. In conclusion, the amount of peritoneal cells in mice immunized with T cell-dependent antigens seems to be related to circadian rhythmicity, melatonin production and immunization timing.