Heat stress is a critical challenge in animal health, and adversely affects animal growth performance, productivity, and meat quality (Lara and Rostagno 2013; Nawab et al. 2018; Gonzalez-Rivas et al. 2020). Traditionally, blood pH, rectal temperature, and respiratory rate were considered as the indicators of response to heat stress in broilers (Chen et al. 2013; Mutibvu et al. 2017; Shakeri et al. 2019) In the present study, the rectal temperature of broilers in the HS group was higher than that in the TN group, in agreement with previous studies (Zhang et al. 2018; Vesco et al. 2020). It suggested that the heat stress model was established successfully. Notably, it is urgent to develop nutritional strategies to prevent the detrimental effects of heat stress. Of note, the current study observed that tryptophan supplementation decreased the rectal temperature in broilers exposed to heat stress. Similarly, previous work indicated that intravenous administration of tryptophan reduced rectal temperature in steers reared under heat stress (Madoka et al. 2018), and dietary supplementation of Tryptophan improved the intestinal barrier and immune functions in other animals subjected to darkness, drive, and other stress (Yue et al. 2017). Acute heat stress activated the autonomic nervous system (ANS) and hypothalamic-pituitary-adrenal (HPA) axis and increased the levels of cortisol, catecholamine, and glucocorticoids (Kadim et al. 2006; Gonzalez-Rivas et al. 2020) Meanwhile, an increase of cortisol affected the pituitary and hypothalamus and stimulated the levels of adrenocorticotrophic hormone and corticotropin-releasing hormone. In addition, higher cortisol level affected the HPA axis, and induced dysfunctions to increase the risk of diseases (Luo et al. 2019). In the present work, heat stress increased the levels of cortisol, dopamine, adrenaline, adrenocorticotrophic hormone, and corticotropin-releasing hormone in the serum of broilers. However, dietary supplementation of Tryptophan decreased the concentrations of dopamine, adrenaline, noradrenaline, and corticotropin-releasing hormone in the serum of broilers exposed to heat stress. Consistently, the effect of heat stress on these hormones was abolished by tryptophan treatment (Yue et al. 2017; Alhassan et al. 2018). Moreover, dietary supplementation of tryptophan may be a beneficial strategy against weaning stress by decreasing adrenaline, noradrenaline, and 5-HT (Liu et al. 2013). These indicated that tryptophan supplementation may promote the healthy function in broilers exposed to heat stress.
The TDO expression is under the control of glucocorticoids and glucagon (Castro-Portuguez and Sytphin 2020). Our results found that heat stress enhanced the level of glucocorticoid and was associated with an increase of TDO and IDO concentrations in serum. During inflammation, the expression of IDO increased and stimulated the kynurenine metabolic pathway (Breda et al. 2016; Sun et al. 2020). Indeed, the concentration of kynurenine in the HS group was higher than that in the TN group. An increase of kynurenine activated the aryl hydrocarbon receptor, then suppressed the T cell proliferation and induced the apoptosis of T/B cells (Fuertig et al. 2016; Platten et al. 2019). Importantly, the current study found that tryptophan supplementation reduced the level of IDO, TDO, and kynurenine in the serum of broilers subjected to heat stress. 5-HT is a type of neurotransmitter involved in the HPA axis with hypothalamic corticotropin-releasing hormone (Dinan 1996; Winberg et al. 1997) and participated in the body behaviors and neuroendocrine system in response to stress. 5-HIAA is an indicator reflecting 5-HT conversion serotonin activity in the nervous system (Hierden et al. 2004). In this study, heat stress elevated serum 5-HIAA/5-HT and decreased hypothalamic 5-HT level significantly, which was consistent with the previous work (Calefi et al.2019).
Emerging evidence reported that increasing the 5-HT level was a benefit for alleviating heat stress in animals (Koopmans et al. 2006; Shen et al. 2012a; Shen et al. 2012b). Noteworthy, dietary supplementation of tryptophan increased the concentration of 5-HT in serum and hypothalamus, which alleviated the high-density feeding-induced stress (Liu et al. 2015). In the present study, tryptophan supplementation, especially at the 0.18% level significantly enhanced the concentrations of tryptophan and 5-HT in serum and hypothalamus and decreased the 5-HIAA/5-HT in the hypothalamus. In addition, a previous study reported that heat stress damaged the intestinal barrier function by increasing the levels of TNF-α and IL-1β (Siddiqui et al. 2020). Similarly, the current work found that heat stress reduced the serum IL-22 level in broilers. IL-22 is engaged in homeostasis balance between the immunity and microorganism, with involvement of antimicrobial peptide release based on microbial abundance and diversity (Zelante et al. 2013; Behnsen et al. 2014). Notably, tryptophan plays a critical role in the production of IL-22 in the intestine (Lee et al. 2011; Gao et al. 2018). Consistently, in this study, dietary supplementation of 0.18% tryptophan decreased the level of TNF-α in the serum of broilers. These findings suggested that tryptophan supplementation may alleviate the heat stress in broilers, and the optimal level was 0.18%.
In summary, dietary tryptophan supplementation could alleviate the heat stress in broilers, which is associated with decreasing rectal temperature, suppressing the kynurenine metabolic pathway, increasing the 5-HT synthesis, and enhancing the anti-inflammatory ability. Moreover, dietary supplementation of 0.18% tryptophan was the optimal level for broilers subjected to acute heat stress. These results demonstrate the beneficial effect of tryptophan on the healthy function in broilers reared under acute heat stress.