Study Of Alleviation Effects And Mechanism Of ?-aminobutyric Acid On Heat Stress Of Huainan Partridge Chicken

In Anhui Province,high temperature in summer often lead to livestock and poultry heat stress,seriously restricting the development of livestock and poultry farming.?-aminobutyric acid(GABA)as an inhibitory neurotransmitter in the brain tissue,has the effects of anti-anxiety,anticonvulsant as well as regulating endocrine mechanism.Recent studies have found that feeding GABA can effectively relieve animal heat stress,improve animal feed intake and body weight gain,improve carcass quality and reproductive performance.In 2009,Chinese health authorities approved that GABA as an additive for food processing.Most scholars believe that excessive secretion of hypothalamic-pituitary-adrenocortical axis(HPA)can damage the animal body.The mechanism by which GABA relieves animal heat stress mainly involved in immune,anti-oxidation and intestinal digestive function,however,there is little research on the HPA axis regulated by the GABA.Only a few articles reported that the addition of GABA in the feed inhibited the secretion of HPA-axis hormones in heat-stressed pigs.Our previous study also found that the blood levels of HPA axis hormone is reduced by adding coated GABA in the cow feed,but the mechanism of GABA inhibiting HPA axis hormone secretion is not clear.In the central nervous system,the corticotropin-releasing hormone(CRH)neurons in the hypothalamic paraventricular nucleus are the highest control center of the HPA axis.Endogenous neurotransmitter GABA produced in the brain tissue can act on CRH neurons to regulate the secretion of HPA axis hormones.However,the exogenous GABA outside the brain has not been reported to regulate the secretion of HPA axis hormones.Firstly,we studied the effect of GABA on summer heat stress in partridge chicken and its outcome on HPA axis hormone secretion.One hundred and twenty 28 days old Huainan partridge chickens were randomly divided into four groups(30 chickens per group).Group I,II,III and IV were given GABA at a dose of 0 mg/kg,50 mg/kg,100 mg/kg,150 mg/kg,respectively from August 3th to September 9th in the year 2015.The pre-test continued for 7 days and the formal test lasted for 30 days.The temperature,humidity and feed consumption of each group were recorded daily.At 10 th,20th and 30 th day of the experiment,weighing and 5 chickens were randomly selected per group and the concentrations of CRH,ACTH,Cor,HSP70 and GABA were measured.Chickens in group I and III were decapitated to detect GABA levels in the brain tissue.Secondly,the effect of high temperature on the blood-brain barrier of Huainan partridge chickens and its mechanism were investigated.One hundred and twenty 100 days old Huainan partridge chickens were randomly divided into four groups(30 chickens per group).After 7 days of pre-test,chickens in group 1 and 2 were kept at the normal temperature(21~23??RH 60~70%),which in group 3 and 4 were subjected to high temperature treatment(34~37??RH 45~50%)for 3 hours.The trial lasted for 2 weeks.On the 7th and 14 th day of the experiment,5 chickens were randomly selected per group.Chickens in group 2 and 4 were injected GABA(8.87 mg/kg)intravenously.The chickens in group 1 and 3 were injected intravenously with the same dose of saline.After 0.5 h of injection,the chickens were decapitated to collect the brain.The level of GABA in the brain tissue of the four groups were detected at the 7 th and 14 th day of the experiment.The blood-brain barrier structure and the junction proteins ZO-1,Occludin and MAPK signaling pathways of group 1 and group 3 were detected on the 14 th day.During the test of GABA preventing and treating summer heat stress of Huainan partridge chicken,the temperature of the chicken coop ranges from 28°C to 39°C,and the temperature and humidity index was higher than 72,which means that the chicken had suffered from heat stress.The addition of GABA in the diet reduced the rectal temperature of the chickens,promoted chicken feeding,increased daily gain and reduced feed weight ratio.Compared with the control group,the concentration of serum CRH,ACTH,Cor,HSP70 in the test group decreased,in which,group III showed best effect.The concentration of GABA in chicken serum increased with the increasing amount of GABA in the diet and content of GABA in chicken brain in group III was higher than that in group I.The effect of high temperature on the blood-brain barrier integrity of Huainan partridge chicken showed that,observed by transmission electron microscopy(TEM),the blood-brain barrier of the chickens in the normal temperature control group was intact,but the mitochondrial ridge of the vascular endothelial cells of the brain cortex of high-temperature control group showed rupture and vacuolar degeneration,the tightly connected gap become widened,and the basement membrane was thickened with overall loose structure.The content of GABA in the brain of group 4 was higher than that in group 3,while the contents of GABA in the brain of group 1 and group 2 were similar.Compared with the normal temperature control group,the high temperature decreased the expression of close connector protein ZO-1,Occludin gene and protein.At the same time,the transcription and protein phosphorylation of p38 and JNK were elevated at high temperatures,while ERK gene transcription and protein phosphorylation were reduced.In conclusion,feeding GABA effectively relieved the summer heat stress of Huainan partridge chicken.Among them,the best dose to relieve heat stress was 100 mg/kg.The mechanism may be that high temperature activates p38 and JNK MAPK pathway in brain,restrains ERK MAPK pathway,reduces brain tight junction protein ZO-1 and Occludin expression,leading to the permeability of the blood-brain barrier increased,so that the exogenous GABA absorbed into the blood through the blood-brain barrier to play a role in the regulation of HPA axis.