Dietary ʟ-Tryptophan Supplementation with Reduced Large Neutral Amino Acids Enhances Feed Efficiency and Decreases Stress Hormone Secretion in Nursery Pigs under Social-Mixing Stress

Abstract

Tryptophan (Trp), the rate-limiting substrate of serotonin [5-hydroxytryptoamine (5-HT)] synthesis in the brain, competes with large neutral amino acids (LNAA) to cross the blood-brain barrier. This study was designed to evaluate the effect of L-Trp supplementation on nursery pigs experiencing social-mixing stress and fed diets varying in LNAA concentrations. Forty-eight individually housed barrows at 6 wk of age were randomly allotted to 4 dietary treatments based on a 2 × 2 factorial arrangement, with L-Trp supplementation (0 or 0.6%) and LNAA concentrations (4.5 or 3.8%) as the 2 main factors. Pigs were fed the diets for 7 d. On d 4, pigs within a treatment were paired in a new pen to create social-mixing stress and behavior was recorded for 24 h. Body weight was measured on d 0, 4, 5, and 7. Saliva and blood were collected on d 4 and 7. On d 7, pigs were killed to obtain hypothalami. During the entire period excluding the mixing day (d 5), L-Trp supplementation improved (P < 0.01) feed efficiency of pigs and lowering the LNAA further enhanced (P < 0.05) the effects of L-Trp. Supplementation of 0.6% L-Trp increased (P < 0.001) hypothalamic 5-HT and 5-hydroxyindoleacetic acid. The salivary cortisol concentration was reduced (P < 0.05) by lowering the LNAA. Collectively, lowering the LNAA further enhanced the improvement of feed efficiency by L-Trp supplementation of nursery pigs under social-mixing stress in association with reduced stress hormones, indicating that reducing LNAA in the diet can facilitate the effect of L-Trp on the stress response of pigs.

Effects of short term supplementation of l-tryptophan and reducing large neutral amino acid along with l-tryptophan supplementation on growth and stress response in pigs

“…This result supports our previous study (Shen et al, 2012a) indicated lowering LNAA further enhanced the improvement of feed efficiency by l-Trp supplementation of nursery pigs under social-mixing stress.…”

“…Surprisingly, pigs fed a reduced LNAA diet supplemented with 0.7% l-Trp had lower PUN than pigs fed the diet supplemented with 0.8% l-Trp. This may be due to the 0.7% l-Trp supplemented reduced LNAA diet had a lower protein level compared with 0.8% l-Trp supplemented diet (Shen et al, 2012a).…”

“…In the current study, dietary LNAA was reduced from 4.5 to 3.8% to examine if lowering dietary LNAA along with 0.7% l-Trp supplementation could have similar beneficial effects on stress responses to the effects of 0.8% l-Trp with normal LNAA concentration. Previously, we have shown lowering LNAA further enhanced the improvement of feed efficiency by l-Trp supplementation of nursery pigs under social-mixing stress in association with reduced stress hormone (Shen et al, 2012a). The supplemental level of l-Trp for maximized beneficial effects of mitigating the stress might be reduced by lowering LNAA concentrations, which gives an economical advantage to this method.…”

“…In total, 80 pigs were collected on each collection day (10 replicates with 4 sub-samples for each treatment). Pigs were introduced and trained with cotton gauze at the beginning of this experiment to ensure the saliva collection procedure during the experiment period would not disturb the stress condition of the pigs (Shen et al, 2012a(Shen et al, , 2012b. Cotton gauze chewed by a pig for approximately 1 min was used to collect saliva.…”

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

“…For hypothalamus analysis, samples were analyzed in duplicate within a single assay. Hypothalamus was measured for 5-HT concentration (validated for pigs by Shen et al, 2012) using a commercially available ELISA kit (Research & Diagnostics Systems, US), conforming to the manufacturer's instructions. The intra-and inter-assay variation coefficients of the kit were 3.2% to 5.3% and 8.1% to 9.3%, respectively.…”