Oral tryptophan supplementation to suckling piglets affecting hypothalamic serotonin metabolism, behavior, and growth in the nursery period under social mixing stress

Tryptophan (Trp) is the rate-limiting substrate for the synthesis of serotonin (5-hydroxytryptamine; 5-HT) in the brain, which plays a central role in the regulation of stress sensitivity and feed intake. The objective of this study was to evaluate the effect of oral Trp supplementation to suckling piglets on 5-HT metabolism, growth performance, and behavior during the suckling (14 to 21 d of age) and nursery (21 to 56 d of age) periods, under social mixing stress at weaning. Eighty 14-day-old suckling piglets from 10 sows (8 piglets per litter) were allotted to 2 treatments. The 2 treatments (0.389 g alanine + 0.430 g glucose/kg BW/d or 0.467 g Trp/kg BW/d) were given orally to piglets 7 times daily, beginning at 0700 h in 2 h intervals, from d 14 to d 21 of age (suckling period). Alanine (Ala) was used to balance nitrogen contents when Trp was added. Growth performance was measured from d 14 to d 21 of age (suckling period) and from d 21 to d 56 of age (nursery period) in 2 phases. On d 21, 12 piglets from 12 litters (6 pigs/treatment) were selected and euthanized to collect hypothalamus to measure 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) concentrations. The remaining piglets were weaned and moved to nursery pens. Pigs within the same treatment and sex, but from different litters, were paired and housed together (2 pigs/pen). All pigs were fed the same diet during the nursery period. Normal and aggressive behaviors were recorded for 24 hr immediately after weaning. Oral Trp supplementation during the suckling period increased (P < 0.05) hypothalamic concentration of 5-HT and 5-HIAA at weaning, whereas growth performance was not affected. However, oral Trp supplementation tended to increase (P = 0.057) the frequency of visits to the feeders during social mixing stress. In conclusion, oral Trp supplementation during the suckling period increased hypothalamic serotonin and could influence eating behavior post-weaning.