| The impact of climate change on broiler industry has been witnessed all over the world,especially in developing countries,because the population in these areas largely depend on animal husbandry for their livelihoods,and the facilities and equipment of animal husbandry is relatively backward,and it is hard to handle with climate change there.Environmental factors include environmental temperature,relative humidity,solar radiation,wind speed and so on,these factors affect animal production in many ways.Among them,high environmental temperature and high solar radiation often caused adverse effects on the production,reproduction,metabolism,and immune response of broilers due to the poor heat dissipation under stress.Especially in tropical and subtropical areas,heat stress has become the main limiting factor of broiler production,which has caused huge economic losses to local animal husbandry.Taurine is a multifunctional feed additive,which often supplemented in the aquatic feed as a phagostimulant.It has been reported that taurine can be used to alleviate the effects of heat stress,but how does it work on heat-stressed broiler is not clear.Therefore,it is important to explore the effects of continuous heat stress on feeding regulation and the alleviation effects of taurine supplementation in broilers.1.Effects of chronic heat stress on the growth performance and the development of digestive organs in the broilersExperiment 1:A total of 144 chicks on 28 d with an average initial body weight of 1.28±0.05 kg were picked out and then were divided into three groups,each group includes 6 cages,consisting of 8 chickens per cage.The three groups containing normal control(NC,22?),heat stress(HS,32?),and pair-fed(PF,22?)group.Broilers in NC and HS groups were supplied ad libitum access to diet.In pair-fed(PF)group,birds were maintained at 22?,acquired the same amount of diet as the diet consumed in the previous day of the HS group each day.The results showed that compared with NC group,the cloacal temperature of broilers in HS group increased at 28,30,35 and 42 days(P<0.05),and the respiratory frequency of broilers increased at 28,30 and 35 days(P<0.05).Meanwhile,the average feed intake(ADFI)and average daily gain(ADG)of HS group decreased(P<0.05)at 28-35 d,35-42 d and 28-42 d,and the ratios of feed to gain(F/G)increased at 28-35 d and 28-42 d(P<0.05).In addition,compared with HS group,the ADG of 28-42 d increased(P<0.05)and the F/G of 35-42 d showed a decline tendency in the PF group(P=0.062).Compared with NC group,HS decreased the relative weight of duodenum and ileum after 7 days of heat stress treatment,but had no significant effect on the relative length of intestine;HS increased the relative weight of proventriculus and duodenum significantly after 14 days of heat stress treatment(P<0.05),meanwhile,the relative length of duodenum,jejunum and ileum increased significantly(P<0.05).Compared with HS group,the relative weight of ileum in PF group increased significantly after 7 days of heat stress treatment(P<0.05);the relative weight of duodenum in PF group decreased significantly after 14 days of heat stress treatment(P<0.05),while the relative length of duodenum,jejunum and ileum decreased significantly(P<0.05).Compared with NC group,HS significantly decreased VH/CD in duodenum,jejunum and ileum after 7 days of heat stress,increased CD in jejunum and ileum,and decreased VH/CD in jejunum after 14 days of heat stress.Compared with HS group,the VH/CD in duodenum,jejunum and ileum increased significantly in PF group(P<0.05),and the CD in jejunum decreased significantly in PF group after 7 days of heat stress(P<0.05);the VH and CD in jejunum increased significantly in PF group after 14 days of heat stress.In general,chronic heat stress damaged intestinal morphology,reduced intestinal villus height and increased crypt depth,thus affected the function of intestinal tract and impaired broilers'growth performance.2.Effects of chronic heat stress on gut hormones,appetite-related genes,taste receptors expressions and AMPK pathway in the intestineThe experiment design was as same as experiment 1.The results showed that the concentration of CCK and ghrelin in serum of broilers were increased after 7 days of heat stress(P<0.05).Compared with the NC group,HS increased the concentration of SS,ghrelin,CCK and PYY in duodenal mucosa,SS,ghrelin and PYY in jejunal mucosa,ghrelin and PYY in ileal mucosa(P<0.05)after 7 days;the concentrations of SS,CCK and PYY on duodenal mucosa,SS and PYY on jejunal mucosa,as well as SS and ghrelin on jejunal mucosa were increased(P<0.05)after 14 days.The activity of trypsin in pancreas was decreased(P<0.05),the activity of trypsin in jejunal digesta was increased(P<0.05),and the activity of lipase in jejunal digesta was lower(P<0.05)after 7 days of heat stress;after 14 days of heat stress,the activity of amylase and lipase in pancreas decreased(P<0.05),and the activity of trypsin in duodenal and jejunal digesta increased(P<0.05).HS decreased maltase activity in duodenal mucosa and amylase activity in jejunal mucosa on after 7 days(P<0.05),and increased Na+/K+-ATPase activity in jejunal mucosa after 14 days of heat stress(P<0.05)in comparision with the NC group.The concentrations of Val and Ile in plasma were higher after 14 days of heat stress(P<0.05).The number of PCNA in jejunum was decreased after 14 days of heat stress(P<0.05).HS increased the mRNA expression of ghrelin in duodenum,jejunum and ileum(P<0.05),and upreguated the mRNA expression of CCK in jejunum after 7 days(P<0.05);HS upregulated the mRNA expression of CCK,ghrelin,T1R1 and T1R3 in duodenum(P<0.05),as well as the mRNA expression of T1R1 and T1R3 in jejunum and CCK in ileum(P<0.05)after 14 days compared with the NC group.The expression of HIF-1? in duodenum and ileum mucosa and the expression of AMPKal,LKB1 and HIF-1? in jejunum mucosa were significantly increased after 14 days of heat stress(P<0.05).Compared with HS group,the expression of LKB1 in duodenal mucosa of PF group increased significantly after 7 days(P<0.05),and the expression of HIF-1? in duodenal and ileal mucosa,and LKB1 and HIF-1? in jejunal mucosa of PF group decreased significantly after 14 days.HS significantly increased p-AMPKa and p-LKB1 protein expression in jejunal mucosa after 7 days compared with NC group,and p-LKB1 protein expression in jejunal mucosa after 14 days heat stress(P<0.05).Compared with HS group,the expression level of p-AMPKa in jejunal mucosa of PF group increased after 7 days of heat stress(P<0.05),meanwhile the p-LKB1 in jejunal mucosa of PF group increased after 14 days of heat stress(P<0.05).Overall,chronic heat stress leaded to excessive secretion of satiety hormones in intestinal mucosa,overexpression of satiety hormone genes and taste receptor genes,decreased the number of PCNA,increased the apoptosis in jejunum,disturbed amino acid homeostasis,upregulated of hypoxia-inducible factor in intestine,induced energy imbalance and activated the pathway of AMPK in intestine.3.Effects of chronic heat stress on the appetite-related genes and the level of differentially expressed genes based on transcriptome in the hypothalamusThe design of the experiment was as same as experiment 1.The results showed that the concentration of CORT in serum was higher after 7 days of heat stress(P<0.05);after 14 days of heat stress,the heat surface temperature of broiler was increased(P<0.05),the levels of FT4 and FT3 in serum increased(P<0.05).The mRNA expression of orexigenic gene NPY was downregulated after 14 days of heat stress(P<0.05).In addition,compared with the NC group,the mRNA expression of AgRP and POMC in the PF group were higher(P<0.05),there was no significant difference on the reactive oxygen species(ROS),as well as the mRNA expression of AMPKal and LKB1 in the hypothalamus.Heat stress decreased the protein expression of NPY compared to the NC group after 14 days.Compared to the NC group,the protein expression of POMC in the PF group was increased after 14 days of heat stress.Based on transcriptome technology,heat stress downregulated the genes related in sodium ion transporting,neuron cell differentiation and central nervous system development,while upregulated the expression of anorexia genes,combined with these factors that resulting appetite diminished.4.Effects of dietary taurine supplementation on the growth performance,instestinal hormones and appetite-related genes expressions in heat-stressed broilersExperiment 2:A total of 144 broilers with an average initial body weight of 1.10±0.02 kg on 28 d were pick out and then separated into three groups with 6 cages each group,8 broilers per cage.The experiment period lasting for 14 days.Broilers in normal control(NC)group,were kept at 22? ambient temperature(thermoneutral)and fed a basal diet;chickens in the heat stress(HS)group were raised to chronic heat stress at 32? and received a basal diet;chickens in the heat stress+taurine(HT)group,were kept in chronic heat stress at 32?,fed a basal diet with 0.5%taurine supplementation,broilers in all three groups were supplied ad libitum access to diet and water.The result showed that HS decreased the ADFI during 28-35 d,35-42 d and 28-42 d,and the ADG during 28-35 d and 28-42 d,as well as increased the F/G during 28-35 and 35-42 d compared with the NC group(P<0.05),whereas taurine supplementation did not ameliorate the lowered growth performance.Cloacal temperatures and respiration rates in the HS and HT group were higher(P<0.05)than those in the NC group.The relative weights of gizzard and ileum in broilers were decreased(P<0.05)after 7 days of heat stress.The relative lengths of jejunum and ileum were increased(P<0.05).After 14 days of heat stress,the relative weights of proventriculus and duodenum in broiler chickens were increased(P<0.05),and the relative lengths of duodenum,jejunum and ileum were increased(P<0.05).Compared with the NC group,the VH and VH/CD in duodenum and jejunum,as well as VH/CD in ileum were higher(P<0.05)in HS group,and the CD of jejunum and ileum were increased(P<0.05)after 7 days(P<0.05),taurine treatment increased the VH and VH/CD in duodenum,the VH/CD in jejunum,and decreased the CD in jejunum and ileum(P<0.05)compared with the HS group;HS decreased VH and VH/CD in jejunum(P<0.05),VHand CD in ileum(P<0.05),increased CD in duodenum(P<0.05)after 14 days,taurinesupplementation increased VH and VH/CD in jejunum(P<0.05),decreased VH/CD in duodenum(P<0.05),and VH in ileum(P<0.05)compared with the HS group.7 days of HS increase the levels of CCK,ghrelin and PYY in serum(P<0.05),and 14 days of HS increased the levels of PYY in serum(P<0.05).Heat stress significantly increased the secretion of SS,CCK and PYY in duodenal mucosa and ghrelin in ileal mucosa after 7 days(P<0.05);and HS significantly increased the secretion of ghrelin and PYY in duodenal mucosa and SS in jejunal and ileal mucosa after 14 days(P<0.05).Compared with NC group,HS increased the expression levels of CCK duodenal,TlR1 and TlR3 in jejunal mucosa,CCK,ghrelin and T1R3 in ileal mucosa(P<0.05)after 7 days;HS increased CCK and ghrelin in duodenal mucosa,CCK and T1R3 in jejunal mucosa(P<0.05)after 14 days.Compared with HS group,HT group significantly decreased the expressions of T1R3 in jejunal mucosa,as well as ghrelin and T1R1 in ileal mucosa after 7 days(P<0.05);HT group decreased the expression of CCK in duodenal and jejunal mucosa after 14 days(P<0.05),and increased the expression of ghrelin in ileal mucosa(P<0.05).The mRNA expression of LKB1 and HIF-1? in jejunal mucosa and AMPKal,LKB1 and HIF-1? in ileal mucosa were upregulated after 7 days of heat stress(P<0.05)compared with NC group.HT increased the expression of AMPK?1 and LKB1 in duodenal mucosa(P<0.05);HT upregulated the expression of AMPK?1 in duodenal mucosa(P<0.05).Overall,dietary taurine supplementation could not alleviate the negative effect of heat stress on the growth performance,but it could play a role on the impaired intestinal morphology induced by heat stress.5.Effects of dietary taurine supplementation on the endocrine hormones in serum,morphology and appetite-related genes expression in the hypothalamus of heat-stressed broilersThe design of the experiment was as same as experiment 2.The aim of the study was to investigate the effects of taurine supplementation on serum biochemical parameters,free fatty acids and expression of appetite-related genes in hypothalamus of broilers under heat stress.The results showed that the levels of CORT,NE and T4 in serum were increased after 7 days of heat stress(P<0.05),whereas taurine supplementation increased the levels of T3(P<0.05).After 14 days of heat stress,the level of NE in serum was increased(P<0.05).Heat stress for 7 or 14 days had no significant effects on ROS level in hypothalamus.The mRNA expression of oxregenic gene AgRP was downregulated after 7 days of heat stress(P<0.05),and the mRNA expression of anorexia gene POMC was upregulated(P<0.05).After 14 days of heat stress,the mRNA expression of LEPR was upregulated(P<0.05),but the expression of NPY,AgRP and POMC was not significantly affected.In conclusion,heat stress leaded to changes in serum indicators,downregulated orexigenic genes,upregulated anorexia genes,taurine supplementation had no significant improvement in serum indicators,downregulated anorexia genes,but exerted no influence on appetite stimulation.In summary,the conclusions were as follow:(1)Chronic heat stress reduced the body weight gain and feed intake of broilers,impeded the development of digestive organs,impaired the intestinal morphology via shortening the villus height and deepening the crypt depth,increased the number of apoptotic cells on the apical of villus,decreased the number of PCNA in intestinal epithelial cells,and resulted in impairment of intestinal digestive and absorption function.(2)Chronic heat stress increased gastrointestinal hormones both in serum and intestine,increased Val and Ile in plasma,disturbed the activities of digestive enzymes,upregulated the satiety hormone genes expression and taste receptors expression,induced the negative balance of intestinal energy homeostasis via activation of AMPK pathway,and upregulated hypoxia inducible factor,impaired appetite and reduced growth performance eventually.(3)Chronic heat stress increased the levels of corticosterone and thyroid hormone in serum,increased the head surface temperature of broilers,altered the hypothalamus integrity,downregulated the expression of NPY in hypothalamus,and reduced appetite of broilers.(4)The dietary supplementation of taurine did not significantly improve the growth performance of broilers under chronic heat stress,and taurine supplementation had no obvious effect on appetite stimulation except alleviated the deterioration of intestinal morphology caused by heat stress. 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