Supplemental dietary tryptophan modifies behavior, concentrations of salivary cortisol, plasma epinephrine, norepinephrine and hypothalamic 5-hydroxytryptamine in weaning piglets

Stress behaviors often result when piglets are abruptly weaned and mixed. Surplus dietary tryptophan (Trp) may reduce stress. In this study, three levels of dietary Trp were used, representing the standard requirement for growth (control), supplemental dietary Trp (1 g/kg and 2 g/kg of feed). Piglets were fed the diets for 28 d, during which observations were divided into three phases: prophase (4-6 d), metaphase (15-17 d), anaphase (26-28 d). Daily behavioral observations were made 14:00-16:00 during each phase afternoon. Saliva was collected from multiple pigs in each pen from 16:00 to 17:00, and body weight of each piglet was individually recorded at d 1 and 28. Aggressive behaviors of piglets were recorded for 8 h immediately after remixing, and saliva per pen was collected from 16:00 to 17:00. Subsequently, 2 pigs per pen were sampled for blood via the jugular vein and subsequently killed to obtain hypothalamus. Salivary cortisol, plasma norepinephrine and epinephrine, hypothalamic 5-hydroxytryptamine concentrations were measured using enzyme linked immunosorbent assay (ELISA) kit. High-Trp diet significantly raised hypothalamic 5-hydroxytryptamine concentration (P<0.05), and diminished salivary cortisol, plasma norepinephrine and epinephrine concentrations (P<0.05). High-Trp dietary treatment induced more lying and less standing compared with control (P<0.05) during daily behavioral observations of metaphase and anaphase. After remixing, relative to pigs fed the basal diet, pigs fed the Trp supplemented diets engaged in fewer fights and spent time fighting (P<0.05). There were no differences between dietary treatments for growth performance (ADG, ADFI, F/C) of piglets (P>0.05). Consequently, excessively supplemental Trp may modify the behavior reactivity of piglets during weaning and mixing, increase hypothalamic 5-hydroxytryptamine concentration and diminish salivary cortisol, plasma norepinephrine and epinephrine concentrations.