Histamine activates inflammatory response and depresses casein synthesis in mammary gland of dairy cows during SARA

Background: Mounting evidences observed that subacute ruminal acidosis (SARA) induced by high concentration (HC) diet increases the translocation of histamine from digestive tract into ciculation causing a diverse of diseases in dairy cows. However, it is largely unknown how it does affect the function of mammary gland and milk quality Hence, this study aims to observe the effects of histamine derived from the digestive tract on the inflammatory response and casein synthesis in the mammary glands during SARA. Twelve cows fitted rumen fistula were randomly divided into either control group administrated low concentration (LC) diet (60% forage, n = 6) or treatment group administrated HC diet (40% forage, n = 6) for 18 weeks. Results: Our data showed that HC diet resulted in significant declines in lumen pH value, milk yield and milk quality, as well as longer duration of averaged pH value below 5.6 per day (more than 180 min) compared to LC diet, these findings confirmed SARA occurence. Our study also observed that SARA increased the content of histamine in rumen fluid, plasma, liver and mammary gland, and enhanced the mRNA expression of histamine specific receptor in the mammary gland. Additionally, we found that the mRNA expression of inflammatory response genes in mammary glands was increased, which was consistent with the protein expression results, showing that the protein kinase C(PKC) / nuclear factor kappa B (NF- kB) or protein kinase A (PKA) / NF -kB signalling pathways of the inflammatory response were activated The mRNA expression of mTOR, P70S6K and alpha S1 in mammary glands were significantly decreased with the protein expression of mTOR, P70S6K and alpha S1- casein, and the phosphorylation levels of the mTOR and P70S6K proteins were also decreased. Conclusions: Our study showed that the milk protein of lactating cows is depressed after long term feeding of HC at the individual level, which was paralleled at the gene and protein levels. The inflammatory response in mammary gland caused by histamine derived from the digestive tract is related to the decline of casein synthesis. Our findings point to a new link between the inflammatory response and casein synthesis, but the understanding of the molecular mechanisms involved in this process will require further research.