The Underlying Mechanisms Of Differential Body Temperature Changes Induced By Yeast And Lipopolysaccharide In Mice

Background:Obesity is one of the major risk factors for non-communicable diseases and has become a global public health problem.The underlying mechanisms of obesity and the prevention approach for obesity have become a research priority topic in this field.Energy imbalance is the cause of the development of obesity,including eating too much or spending too little.The increase in exercise and reduction in food intake have been used to prevent obesity in previous clinical practice,however,poor compliance and high risk of recurrence make the actual outcome unsatisfactory.Currently,the research on the prevention of obesity has shifted to pharmacological interventions to inhibit fat absorption or increase energy expenditure.Therefore,it is very crucial to enhance energy expenditure by increasing basic metabolism and adaptive thermogenesis.In the process,enhancement of body temperature is particularly important to increase energy expenditure.Studies have shown that the metabolic rate will be increased by 10%-13%when the core temperature increases by 1℃,and 46.3 kcal/m2/d is consumed when the ambient temperature decreases by 1℃.Therefore,exploring the underlying mechanism of body temperature regulation is helpful to find out new ideas for the treatment of obesity.The febrile reaction is the result of increased body temperature,which is regulated by the brain when the body responds to various stimuli from the external environment.The preoptic area of the hypothalamus is the thermoregulatory center,which plays a major role in the regulation of body temperature caused by ambient temperature change and infection,including the medial preoptic area(MPA),lateral preoptic area(LPO),ventromedial preoptic area(VMPO),median preoptic nucleus(MnPO),medial preoptic area(MPO)and ventral preoptic area(VLPO).Currently,the mechanism of fever caused by different pathogens is considered similar in the peripheral and central areas.However,it has been found that bacterial infection can lead to transient high fever or low body temperature,while fungal infection leads to prolonged-low level fever without hypothermia phenomenon.Both clinical and laboratory studies have shown that the bacteria-induced transition from fever to hypothermia is associated with low ambient temperature,and a sign of poor prognosis.However,it is unclear why fungal fever is prone to persistent recurrence,whether fungal infections can also cause hypothermia in dependent on ambient temperature manner,and what is the mechanism of the difference in body temperature changes between bacteria and fungi infections.Methods:1.Experimental animals:C57BL/6N male mice(8 weeks),12 h:12 h dark-light environment,free diet,22℃,and 50%humidity for a week,all injections were uniformly performed at 9:00 AM.2.Animal model:50 μg/kg of Gram-negative bacterial lipopolysaccharide(LPS)solution was injected through the tail vein to induce bacterial infectious body temperature change;20%Angel yeast suspension was administered by subcutaneous injection into the neck and back at a dosage of 10 mL/kg to induce change in the body temperature of fungal infection,which was divided into two groups:fasting for 12 h before the experiment and free diet throughout the experiment.In the control group,the same amount of saline was given.The experimental animals were divided into the LPS group,yeast group,and saline group.3.Temperature measurement methods:Rectal thermometer,abdominal microchip telemetry,and infrared imager were used to monitor rectal temperature,abdominal core temperature,back skin and tail temperature,and scapular region heat production of mice.4.Immunoblotting technique:The expression levels of uncoupled protein(UCP-1)and tyrosine hydroxylase(TH)in the brown adipose tissue of the scapular region were detected to evaluate the thermogenesis of brown adipose tissue.5.Calculation of fever rate:The results of all LPS and yeast-induced body temperature changes were summarized,and the effects of three different temperature measurement methods on LPS and yeast thermogenesis were compared.6.Body temperature changes induced by LPS and yeast were compared at 30℃,26℃,and 22℃.7.Immunohistochemical staining:When the body temperature rose to the highest or lowest point,the brain tissues of mice were completely removed,and four 30 μm-thick coronary brain sections(Bregma 0.38 mm-0.02 mm)were collected to monitor the expression of c-Fos in the POA region of the hypothalamus.The brain map was combined into the whole brain stain map with software such as Adobe Photoshop and Image Fiji,to divide the brain regions and count the number of c-Fos+ cells in the POA region of the hypothalamus and its nucleus.8.Statistical processing:IBM SPSS 22.0 software was used to make statistics and analysis of experimental data.The two-tailed unpaired student t-test was used to compare different groups.P*<0.05,P**<0.01 indicated statistical difference,and all results were expressed as Mean ± SEM.Results:1.LPS-induced body temperature in mice shows that lower concentration of LPS and higher ambient temperature are more likely to induce fever.The rectal temperature,abdominal core temperature,and back infrared temperature of mice are increased by LPS of 50 μg/kg,and the highest temperature values were 1℃,30C,and 2℃,respectively.The peak temperature occurred 4 h after injection.2.The yeast-induced mice body temperature changes show that fungal fever is not affected by feeding status.The rectal temperature and back infrared temperature of mice in the yeast-induced fasting group and free diet group increased by 2℃,and the abdominal core temperature increased by 3℃.In the fasting group,no peak value is detected in the rectal temperature and back infrared temperature,while the peak value of core temperature appeared 5 h after injection;In the free diet group,the peak values of rectal temperature,core temperature,and back infrared temperature are observed at 7 h,5 h,and 6 h after injection,respectively.3.Results of Western Blotting and scapular region temperature show that LPS and yeastinduced body temperature are both associated with increased brown fat thermogenesis in the scapular region.LPS causes increased expressions of UCP-1 and TH,but yeast does not.4.The fever rate of all experimental results demonstrates that yeast-induced body temperature rise is more stable and more tolerant to experimental manipulation stimuli.5.The results showed that LPS causes fever at 30℃,and hypothermia at 26℃ or 22℃.Yeast is not subject to changes in ambient temperature and always causes persistent fever.6.Immunohistochemical staining shows that LPS and yeast activate more POA neurons at 30℃ compared with the saline group;At 22℃,yeast alone activated more POA neurons.VMPO is obviously activated in three febrile conditions.MPO is significantly activated by LPS-induced hypothermia.Conclusion:1.Yeast has a stronger and more stable pyrogenic effect than LPS,which is not affected by experimental manipulation stimulation,dietary status,and environmental temperature changes,which is consistent with the prolonged fever often presented by clinical fungal infections.2.The thermogenic effect of yeast and LPS is due to the increasing heat production of brown fat in the scapular region,and reducing heat dissipation at the tail.3.LPS results in hypothermia rather than hyperthermia at low ambient temperature,which is likely due to the loss of activation of POA neurons in the hypothalamus,mainly the failure of activation of VMPO neurons and the supernormal activation of MPO neurons.4.Infrared thermometry is more objective than rectal thermometry and more convenient than abdominal telemetry.