In recent years, the rhythmicity of depression gradually begin to attract attention. In clinic, the diurnal rhythm (being heavy in the morning and light at night) has been reported. But in animal models, the rhythmic characteristics of the depression- like behavior and the influence of clock genes on the rhythm of depression- like behavior is not clear. The forced swimming test was used as an assay method, and ZT2、ZT6、ZT10、ZT14、ZT18 and ZT22 were chosen to be the representative of time points. To study the rhythmic characteristics of immobility behavior of forced swimming test, and to compare the differences between strains or genders, the subjects for the wild type were C57BL / 6 mice, and clock gene knockout mice (mper1-/- , mper2-/-and CLOCK△19). Through the establishment of the depression model induced by LPS in mice, we made a preliminary exploration as the differences in the depression--like behavior caused by exogenous stimulus and endogenous. The results showed that, the immobility behavior of forced swimming test of wild type mice had obvious rhythmic characteristics, which completely changed in mper1-/- and totally lost in both mper2-/- and CLOCK△19 mice. We also compared the differences between male and female mice, and the results showed that both wild type and mper1-/- mice had differences in rhythmic characteristics and the degree of immobility between male and female, while mper2-/- and CLOCK△19 mice only showed the varying levels of the immobility behavior between male and female. Under the same dose, depression behavior-like behavior induced by LPS increased significantly in wild type mice, but there was no obvious changes in mper2-/- mice. This study suggests the important role of the clock genes on the rhythm of immobility behavior of forced swimming test and the LPS-induced depression- like behavior, and providing the basic data for the future research on the regulation of clock genes on rhythmicity of depression-like behavior.
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