Circadian rhythms are driven by oscillating expression of a family of transcription factors called clock genes. In rodents, clock genes drive circadian rhythms in white blood cell function, and glucocorticoids are believed to regulate these rhythms. Little is known about circadian rhythms of cattle white blood cells. The objectives of this study were: (1) to quantify mRNA levels of clock genes in neutrophils and lymphocytes over 24h in healthy steers; and (2) to quantify effects of dexamethasone on clock gene mRNA levels in bovine neutrophils and lymphocytes. We hypothesized that bovine neutrophils and lymphocytes would display 24h variations in clock gene mRNA levels and that those patterns would be disrupted by glucocorticoid treatment. Six Holstein steers were injected with 0 or 0.10mg/kg body weight dexamethasone according to a crossover design. Neutrophils and lymphocytes were collected from jugular blood at 0, 4, 8, 12, 16, 20, and 24h relative to treatment administration. Neutrophil and lymphocyte mRNA levels of the clock genes Clock, Bmal1, Per1, Per2, Cry1, Cry2, Rev-erbα, and CK1ɛ were quantified. For neutrophils, an interaction between treatment and time was found for Clock, Cry1, and CK1ɛ. Time affected Clock, Per1, Cry1, Rev-erbα, and CK1ɛ. For all of those genes except Per1, neutrophils from control steers displayed 24h changes of mRNA levels characteristic of circadian regulated cells. The dexamethasone treatment increased neutrophil mRNA levels of Per1, decreased Clock, Cry1, Cry2, and Rev-erbα, and tended to decrease Bmal1. These results suggest that circadian rhythms have the potential to impact bovine neutrophil function, and that glucocorticoid-induced disruption of neutrophil circadian rhythms may contribute to periparturient immunosuppression. For lymphocytes, an interaction between treatment and time was observed for Per1 and tended to occur for Per2 and Cry2. Although time affected Per1 and Rev-erbα, distinct 24h patterns of lymphocyte clock gene mRNA levels were not evident as they were in neutrophils. Treatment increased Per1 and decreased Cry2, but the magnitude of the treatment effect was small. In summary, 24h patterns in clock gene mRNA levels were observed in bovine neutrophils and to some degree in lymphocytes, and these patterns were disrupted by dexamethasone administration. Although further research is needed, individual variation in white blood cell circadian rhythms and glucocorticoid responsiveness may help to explain individual differences in periparturient disease susceptibility.
