title: High-starch diets increase behavioural reactivity, alter hindgut microbiota and brain neurochemistry in horses
Creators: name: Bulmer, Louise S.
Publisher: name: University of Glasgow
Year: 2020

description: The digestive tracts of animals are home to bacterial communities comprising trillions of individual microorganisms. The community of microorganisms residing in the gut is the gut microbiota. Studies have shown that a symbiotic gut microbiota is intrinsic to the overall health and wellbeing of the individual. As well as influencing an animal’s overall health the gut microbiota affects central nervous system functioning and as such could play a role in influencing behaviour. The primary contribution of this thesis is to explain the role played by diet in altering behaviour through changes in gut microbiota and brain neurochemistry. Anecdotal evidence suggests that horses receiving starch in their diets display increased behavioural reactivity. The addition of starch to the equine diet also results in changes to hindgut microbiota. This thesis aims to: investigate the effects of a high-starch diet on the behaviour of young, unhandled ponies, describe the microbial community composition of faecal samples and different regions of the hindgut in response to diet, and explore changes in serotonin and dopamine receptor densities in brain and gut tissues in relation to diet. The use of young unhandled ponies is an important aspect of this study. Their inexperience meant that their responses to behavioural tests were natural, untrained reactions. The gut microbiota of the ponies had also not been exposed to the dietary changes experienced by mature horses. Therefore, this study gives a true reflection of the behavioural and hindgut microbial responses to a high-starch diet. This thesis first looks at the behaviour of young, unhandled ponies when receiving a high-starch (HS) or high-fibre (HF) diet. 16S rRNA gene sequencing was then used to describe changes in the faecal microbial community composition in relation to diet. The ponies were found to be behaviourally more reactive on the HS diet compared to the HF diet. There were also differences in faecal microbial community structure related to diet. A reduction in Ruminococcaceae bacteria and an increase in Streptococcus bacteria was evident in the faecal microbiota of ponies fed the HS diet. The microbial community composition of each hindgut region was then explored in relation to diet using 16S rRNA gene sequencing. Comparisons were made between faecal samples collected post-mortem and faecal samples collected during behavioural testing. For the HS and HF diets, the microbial community composition differed significantly in the ventral colon, dorsal colon and small colon regions of the hindgut. The HS diet resulted in a decrease in Ruminococcaceae-Oscillospira bacteria and an increase in Streptococcus in the dorsal and small colon regions. These changes were also observed in the faecal samples collected post-mortem and the faecal samples collected during behavioural testing. The effects of diet on the relative expression of serotonin and dopamine receptor densities in the gut and brain tissues were explored using real-time polymerase chain reaction (qPCR). There was a greater relative expression of dopamine D2-like receptor densities in the caudate region of the brain for the HS diet compared to the HF diet and for the right-hand side of the brain compared to the left. There was lower relative expression of dopamine D2-like receptor densities for the nucleus accumbens region of the brain for the HS diet and for the right-hand side of the brain. This study has shown that feeding horses starch results in increased behavioural reactivity. The addition of starch to the diet also alters the microbial community composition of more caudal regions of the hindgut and is associated with neurochemical changes in specific regions of the brain. These results lead us to question whether we should feed starch-based feeds to the horses in our care. Particularly when the energy demands of working horses could be met by other food sources.