Effect of chronic oxytocin-treatment on the bovine mammary gland immune system

The aim of this study was to evaluate the effect of chronic oxytocin (OT) treatment on the mammary gland immune system. In Experiment I fourteen healthy cows were used to study the effect of chronic intramuscular (im) OT administration on concentration of milk somatic cells and white blood cells (WBC). Cows in the OT-group (6) were im injected with 50 IU OT (5 ml) whereas animals of the C-group (6) were im injected with 5 ml of saline (9 g/l) for eight days (Day 1-8) before each milking. Milk samples were taken during normal milking time on Day 0-3, 5, 7, 9-11 and 18. Blood samples were taken immediately after each milking and analysed for WBC count, polymorphonuclear neutrophils, potassium (K), sodium (Na) and chloride (Cl) ions, and blood lactose. All milk samples were analysed for somatic cell counts (SCC), lactose, Na, Cl and electrical conductivity (EC). Furthermore mRNA expression of tumor necrosis factor-α (TNFα), interleukin (IL)-1β, IL-6, IL-8 and cyclooxygenase-2 (COX2) in milk cells were measured via real-time RT-PCR. None of the investigated milk and blood parameters changed significantly in response to the OT treatment. The mRNA-expression of TNFα decreased (P < 0.05) to a minimum on Day 3 in response to OT administration. IL-1β and IL-6-mRNA expression decreased (P < 0.05) to a minimum within three day. IL-8 and COX2 expression did not change in response to OT treatment. In Experiment II twelve cows, randomly divided into two groups of six, were used to investigate the effect of chronic im OT administration on mammary tissue. Cows were im administered 50 IU OT (OT-group) or 5 ml saline (9 g/l; C-group) before each milking during eight days. Biopsy samples were taken after every morning milking. The mRNA expression of various inflammatory factors and the tight junction (TJ) proteins occludin (OCLN) and zonula occludens (ZO)-1, ZO-2 and ZO-3 were measured via real-time RT-PCR. TNFα-mRNA expression decreased (Day 2 with P < 0.05) within the first four days of OT administration and increased (P < 0.05) in the C-group on Day 2. IL-1β expression levels of the OT-group increased transiently and decreased on Day 3 and in the C-group values increased significantly on Day 3 as compared to Day 0. IL-6 expression in the OT-group decreased (P < 0.05) to a minimum on Day 1 and increased (P < 0.05) as compared to Day 0 on Day 7 and increased significantly on Day 1 and Day 5 compared to Day 0 in group C. IL-8 and COX2 expression did not change in response to OT administration. The mRNA-expression of OCLN and ZO-3 decreased (P < 0.05) as compared to Day 0 with a minimum on Day 7. ZO-1 and ZO-2 expression did not change due to OT administration. ZO-2-mRNA expression in C-group decreased significantly on Day 2 compared to Day 0. In conclusion, chronic OT administration induced increasing SCC and EC levels in milk as well as K and lactose in blood while nearly all investigated cytokines in milk cells and mammary tissue were down regulated. The mRNA expressions of the TJ proteins OCLN and ZO-3 were down-regulated in response to the OT treatment what indicates an increasing TJ permeability. Besides the effect on TJ proteins there was no obvious change of the immunological competence of the mammary gland in response to OT. However, a more complete milk ejection should help to remove pathogens during milking.