Effects of dietary tryptophan supplementation on body temperature, hormone, and cytokine levels in broilers exposed to acute heat stress

The present study was conducted to investigate the effects of tryptophan (Trp) supplementation on rectal temperature, hormone, and cytokine levels in broilers subjected to acute heat stress. A total of 300 18-day-old female Arbor Acres broilers were randomly allocated to five dietary treatment groups with six replicates per treatment group and ten birds per replicate. Broilers were fed a basal diet and in the thermoneutral conditions (TN, 23 ± 1 °C) was considered as the TN group. Broilers were fed a basal diet and exposed to acute heat stress (HS, 34 ± 1 °C) was regarded as the HS group, and other broilers exposed to acute heat stress (34 ± 1°C) were fed a basal diet supplemented with 0.09%, 0.18%, and 0.27% Trp. Results indicated that acute heat stress increased the rectal temperature ( P < 0.05), enhanced the concentrations of corticosterone (CORT), dopamine (DA), adrenaline (Adr), adrenocorticotropic hormone (ACTH), and corticotropin-releasing hormone (CRH) in serum ( P < 0.05), and elevated the levels of serum tryptophan hydroxylase (TPH)1, tryptophan 2, 3-dioxygenase (TDO), indoleamine 2, 3-dioxygenase (IDO), and kynurenic acid ( P < 0.05), compared with the TN group. Meanwhile, acute heat stress increased the levels of serum Trp, hypothalamic Trp, 5-hydroxytryptophan (5-HT), and interleukin-22 ( P < 0.05) relative to the TN group. However, compared with the heat stress group, Trp supplementation decreased the rectal temperature of heat-stressed broilers and dietary 0.09% Trp supplementation decreased the levels of serum CRH and TDO ( P < 0.05), increased the levels of serum Trp and IL-22 ( P < 0.05) in heat-stressed broilers. In addition, dietary supplemented with 0.18% Trp reduced the levels of serum DA, Adr, noradrenaline (NA), CRH, TDO, IDO, kynurenic acid, IL-1β, and hypothalamic 5-HIAA/5-HT ( P < 0.05), increased the levels of serum Trp, 5-HT, and IL-22, and upregulated the concentrations of hypothalamic Trp and 5-HT in heat-stressed broilers ( P < 0.05). Moreover, dietary 0.27% Trp supplementation decreased the levels of serum DA, CRH, TDO, and hypothalamic 5-HIAA/5-HT ( P < 0.05), and upregulated the levels of serum Trp, 5-HT, IL-22, hypothalamic Trp and 5-HT in heat-stressed broilers ( P < 0.05). Taken together, dietary 0.18% Trp supplementation may be the optimal level for broilers reared under acute heat stress.