Neuro-inflammatory Process And Depression-like Behavior In Hypothyroid Rats:A [¹⁸F]DPA-714 Micro Positron Emission Tomography Study

Objective:To explore the mechanism of depression-like behavior in hypothyroid rats.Positron emission tomography（PET）is used to observe the activation of microglia in the hippocampus of hypothyroid rats.By analyzing the correlation between the behavioral changes of hypothyroid rats and the activation of microglia,we can verify the role of neuro-inflammatory in the depressive behavior of hypothyroid rats.So as to provide a new target for the diagnosis and treatment of hypothyroidism patients with depression-like behavior.Materials and Methods:In this study,20 healthy adult male Wistar rats were used,and 0.05%（w/v）propylthiouracil（PTU）was used to establish hypothyroidism model.The whole modeling process lasted for 5 weeks.After model establishment,all rats were subjected to behavioral tests,including open field test,forced swimming test and tail suspension test to evaluate depressive behavior.After the behavioral experiment,[¹⁸F]DPA-714 Micro PET/CT imaging was performed to evaluate the activation of microglia in hippocampus region of rats.At the end of the study,all rats were euthanized,blood samples were collected through cardiac puncture.The levels of total thyroxine（TT₄）and total 3,5,3’-triiodothyronine（TT₃）were analyzed by radioimmunoassay.Finally,the brain tissues of hippocampus were taken for immunohistochemistry to detect the expression of TSPO,Iba-1 and NLRP3.Descriptive data were expressed as mean±mean standard error（S.E.M.）.Body weight and DPA-714 intake were compared using repeated measures ANOVA,and t-test was used to compare the mean between the two groups.Pearson correlation analysis was used to determine the relationship between behavioral tests and brain DPA-714 uptake.The significant level was p<0.05.Results:Open field test results:at baseline,no significant difference was found between the two groups in any inspection index.After 5 weeks of administration of PTU,there were no differences in total activity and defecation between the two groups.In addition,there was no significant difference between the control group and hypothyroid group in the number of trans central cells and outer cells.Compared with the control group,the first minute activity（t=3.511,p<0.01）and vertical standing of hind legs（t=2.632,p<0.05）of hypothyroid rats in the fifth week were significantly reduced.Tail suspension test results:At baseline,there was no significant difference in immobility time between the two groups.Five weeks after PTU administration,the resting time of hypothyroid rats（140.0±5.3 s）was significantly longer than that of control rats（102.1±4.7 s）（t=5.353,p<0.001）.In the hypothyroidism group,there was a significant difference in immobility time between baseline and week 5.Results of forced swimming test:At baseline,there was no significant difference in floating time and climbing time between the two groups.Five weeks after PTU administration,the immobility time（163.4±3.9s）of hypothyroid rats was significantly longer than that of control rats（116.8±4.8s）（t=7.524,p<0.001）,and the struggling time（78.9±5.3s）of hypothyroid rats was significantly shorter than that of control rats（99.7±5.3s）（t=3.329,p<0.01）.In the hypothyroidism group,there were significant differences in immobility time and struggle time between baseline and week5.Micro PET/CT results:Hypothyroidism induced increased uptake of radioactivity in hippocampus（p<0.05）.According to Pearson correlation test,the radioactivity uptake in hippocampus was negatively correlated with the first minute activity（p<0.05）in open field test,while the radioactivity uptake in hippocampus was positively correlated with immobility time（p<0.05）in forced swimming test and tail suspension test.Analysis of variance of repeated measurements in hippocampus showed significant time effect,inter group effect and time inter group interaction effect（all p<0.001）.Serum results:There was no significant difference in TT₃ and TT₄ levels between the control group and hypothyroid group at baseline.Five weeks after PTU administration,the TT₃ level of hypothyroid rats was 0.24±0.02 nmol/l,and the TT₄ level was not detected,while the TT₃ level of CON rats was 1.86±0.14 nmol/l,and the TT₄ level was 99.68±7.41 nmol/l.Compared with the control group,after 5 weeks of PTU administration,TT₃and TT₄ in hypothyroid rats were significantly reduced（t=11.45,p<0.001 and t=13.45,p<0.001,respectively）.Immunohistochemical results:The levels of TSPO,Iba-1 and NLRP3 in hippocampus of hypothyroidism group were significantly higher than those of control group（t=3.085,p<0.05;t=3.300,p<0.05 and t=2.812,p<0.05,respectively）.Conclusion:1.Through the modeling of 0.05%（w/v）PTU for 5 weeks,adult male Wistar rats had hypothyroidism,accompanied by depressive behavior,which was manifested by a significant reduction in the first minute activity and rearing in the open field test,and a significant increase in the immobility time in the forced swimming test and tail suspension test.2.The uptake of TSPO radioligand in the hippocampus of hypothyroid rats increased significantly,and the more the uptake of radioligand,the more obvious the depressive behavior.At the same time,the immunohistochemical results confirmed that the high expression of TSPO was mainly due to the activation of microglia in the hippocampus.3.Neuro-inflammatory process is the basis of depression-like behavior in hypothyroidism rats,depression-like behavior associated with hypothyroidism and depression may have a common mechanism.This provides a new target for the diagnosis and treatment of depression-like in patients with hypothyroidism.