Development of nutritional strategies to mitigate stress response by manipulating neurotransmitters in weaner pigs

Samantha Sterndale

Pigs grown in commercial production systems typically encounter numerous challenges that are often intrinsic to the production system(s) and, in some cases, unavoidable. Examples of these challenges include pathogenic/viral challenges and mixing/establishing a social hierarchy. These stressors have been shown to provoke various physiological and endocrinological responses, altering nutrient partitioning to address the need for various defence systems within the pig's immune system. The overall hypothesis tested in this project was that manipulating neurotransmitters via dietary additives/injectables can reduce the stress response and therefore mitigate production losses in weaner pigs. Having first developed a stress model using enterotoxigenic strains of Escherichia coli (ETEC; F4), that causes post-weaning diarrhoea (PWD) and inferior performance in weaner pigs, two experiments were undertaken with the aim to mitigate this stressor. Firstly, it was found that an increased supplementation of tryptophan (Trp) and a reduction in other large neutral amino acids (LNAA) in weaner diets increased Trp availability in plasma regardless of whether the pigs were infected with ETEC, or not. Furthermore, the increase in Trp availability improved serotonin production and performance. Due to the limited response in plasma cortisol, an indicator of stress used in the series of experiments, the second study investigated the use of an adrenocorticotrophic hormone (ACTH) injection in order to illicit a greater endocrine response. The results showed that gamma-aminobutyric acid (GABA) supplementation, a non-protein amino acid, did not improve performance. Although some changes were observed in certain stress biomarkers, both an endocrine and immunological challenge were unable to be shown consistently using the ETEC infection model. Therefore, the remaining two studies in this thesis examined the use of different treatments in newly-weaned pigs to initiate a ‘stress response’ using social and physiological stressors. GABA, Trp, glutamine (Gln) and glutamate (Glu) supplementation when mixed with unfamiliar pigs and challenged with an out-of-feed event was shown to improve feed conversion ratio following the challenges and increase plasma glucose in pigs exposed to an acute stressor, in this case feed deprivation for 12 hours. Finally, a corticosteroid (dexamethasone) injectable’s ability to attenuate physiological responses of weaner pigs subjected to different acute stressors, in this case mixing of non-littermates, was tested. The results showed that administration of dexamethasone (DEX) in weaner pigs exposed to a mixing stress increased brain-derived neurotrophic factor (BDNF), a key indicator of neural plasticity, improved nitrogen utilisation and decreased cortisol, presumably via negative feedback on the hypothalamic-pituitary- adrenal (HPA) axis. Furthermore, DEX administration minimised gut dysfunction after the mixing stress by minimising paracellular permeability. However, such alterations of endocrine and metabolic responses after DEX application were not translated to improved performance. Collectively, the hypotheses tested in these experiments partly supported the use of dietary strategies such as increasing Trp:LNAA ratio and supplementation of Trp, Gln and Glu can manipulate selective neurotransmitters and improve performance when exposed to various stressors such as ETEC infection, feed deprivation and mixing unfamiliar littermates.

Development of nutritional strategies to mitigate stress response by manipulating neurotransmitters in weaner pigs

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