Abstract
The microbiome has emerged as a new player on behavior, physiology and stress because of its significant effects on the brain-gut axis. The aim of this study was to increase our understanding of brain-gut function in dairy cows. We investigated the effects of a heat-stress (HS) environment and individual differences of heat sensitivity (IH) on bovine physiological characteristics and microbial composition. Results indicate that both HS and IH increased rectal temperature (RT) (P < 0.05). An HS environment increased plasma, as well as milk cortisol and cytokines in plasma; however, it decreased plasma, and milk oxytocin, triiodothyronine, and thyroxine (P < 0.05) levels. Exposure to an HS environment reduced the diversity of the fecal microbial population, and resulted in a higher expression of diseases, the environmental adaptation pathway, and the immune related pathway, whereas it lowered the expression of metabolic pathways (P < 0.05). High heat sensitive cows have upregulated metabolisms, environmental adaptation and cellular process pathways, and a downregulated neurodegenerative disease pathway (P < 0.05). Thus, we conclude that exposure to an HS environment modulates physiological characteristics, which may interplay with microbial activity, and in turn, alter the circulation levels of cytokines, implicating the role of the brain-gut axis in dairy cows. The HS environment affected physiological characteristics, cytokine levels, and microbial composition, but IH influenced RT and fecal microbial functions.
Highlights
The microbiome and its metabolites have a crucial role in the maintenance of host homeostasis[1], and have become a booming area in both human[2] and animal studies[3]
High-heat-sensitivity cows under HS environment (H-HS) had higher rectal temperature (RT) than low-heat-sensitivity cows under HS environment (L-HS) (P < 0.01), and high-heat-sensitivity cows under none/slight-HS environment (H-nHS) and low-heat-sensitivity cows under non/slight-HS environment (L-nHS) did not show a difference
The microbiome is a new indicator of the various effects on the brain-gut axis. The mechanism of this axis has not yet been completely understood, it is hypothesized that stressor-induced alterations of the gut microbiome result in the translocation of bacterial products to act as stimuli for the innate immune system[5]
Summary
The microbiome and its metabolites have a crucial role in the maintenance of host homeostasis[1], and have become a booming area in both human[2] and animal studies[3]. The gut microbiome is associated with the circulation levels of cytokines like interleukin-65, and the modulation of the brain–gut axis function is subjected to stress and behavioral responses[8]. Exposure to an HS environment has effects on physiological characteristics and circulation of the cytokines, as well as gut microbial composition, but there is still a lack of understanding of these interactions in dairy cows. By taking the characteristics of heat sensitivity into consideration, we investigated the effect of HS on the physiological characteristics and circulation levels of cytokines and microbial composition at a flock level (HS or none/slight-HS environments) and the individual level (high and low heat sensitivity). The aim was to better understand the effects of exposure to an HS environment on bovine physiology and immune activities, and how these responses interplay with the gut microbiome in dairy cows
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