Diurnal variation in clock gene expression and progesterone secretion is inhibited under constant light conditions in Japanese Black cows

The circadian clock is constructed by a transcription-translation feedback loop system of clock genes and regulates various reproductive physiological functions. Understanding the rhythms of clock gene expression is important for understanding biological rhythms. However, the in vivo clock gene expression rhythm in Japanese Black breeding cows has not yet been clarified. Herein, we investigated the circadian expression rhythm of clock genes in hair follicle cells in the hair roots of Japanese Black breeding cows. We found that the clock genes of Japanese Black breeding cows exhibit an expression rhythm with a period of approximately 24 h under natural light, similar to the pattern observed in mice and humans, and that this expression rhythm disappears under constant light conditions. Furthermore, since the expression levels of clock genes are significantly reduced in pregnant cows exposed to constant light conditions, we analysed the diurnal variation in the plasma concentration of progesterone, which plays an important role in maintaining pregnancy. We found that the progesterone secretion rhythm observed under natural light in pregnant cows also disappeared under constant light conditions. These results indicate that in Japanese Black breeding cows, clock genes exhibit a diurnal expression rhythm in response to light; this rhythm disappears under constant light conditions. These clock genes play important roles in the diurnal variation in hormone secretion by interacting with progesterone in the blood.