The Functional Changes Of BAT And Its Role In WAT Lipolysis And Fat Mass Reduction In A Rat Model Of Parkinson's Disease

Objective: Loss of dopaminergic neurons in the substantia nigra and weight loss as the main pathological features of Parkinson's disease(PD),has become one of the important diseases that affect human health,which incidence and prevalence has increased year by year.Exploring factors that influence the adipose lipolysis is the key to improve the quality of life of PD patients.Brown adipose tissue as specific organ,shares a common developmental ancestry with skeletal muscle,participating in non-shivering thermogenesis and lipid metabolism by utilizing TG and FFA.In our previous study,the sympathetic nervous activity markers elevated in brown adipose tissue in 6-OHDA model compared with the sham group,suggesting that BAT has connections with 6-OHDA fat decrease and weight loss.Design and Methods: Rats were anesthetized and fixed to a stereotaxic instrument.5 ul 6-OHDA was injected into the left medial forebrain bundle(MFB)in the experimental rats,5 ul saline was injected into the sham group.We performed synaptic nerve denervation of BAT both in 6-OHDA and sham group.Nine weeks later,we housed another 6-OHDA and sham rats at 4? for 9 days and measured core body temperature every other day.10 weeks later,retroperitoneal WAT,epididymal WAT,subcutaneous WAT and Brown adipose tissue were removed and weighed.The expression of Tyrosine hydroxylase(TH),uncoupling protein-1(Ucp1),?3R were measured in BAT.TG,NEFA and lipolysis-related molecules were detected.Results: 1.Detection of sympathetic activity and thermogenesis-related molecules in BAT of6-OHDA rats In order to determine the activity of the SN that innervates the BAT,we examined the expression of Tyrosine hydroxylase(TH)in the BAT in 6-OHDA and sham groups.TH expression was significantly increased in 6-OHDA rats compared to the sham group,which suggests that degenerated dopaminergic neurons in the nigrostriatal system increased the activity of SN in BAT.Upon sympathetic activation,?3R on the BAT is activated and its downstream signaling pathway triggers the expression of UCP1.We found that UCP1 protein expression and RNA levels of Adrb3(the gene encoding ?3R)were increased in 6-OHDA rats.Immunohistochemical staining also demonstrated induced expression of UCP1 protein in 6-OHDA rats.Acute cold exposure activates BAT and promotes energy expenditure by non-shivering thermogenesis in mice.To test cold-induced BAT activation in rats,we housed rats at 4? for 9 days and measured core body temperature every other day.As expected,6-OHDA rats recovered their body temperature quicker than sham group.These results indicate that SN over activation mediates BAT thermogenesis and the adaptation to cold in 6-OHDA rats.2.BAT denervation suppressed thermogenesis and weight loss in 6-OHDA rats To test whether increased sympathetic tone was required for activated BAT thermogenesis and body weight loss in 6-OHDA rats,we performed synaptic nerve denervation of BAT in 6-OHDA rats.The expression of TH and UCP1 wassignificantly decreased in BAT-denervated rats in both sham and 6-OHDA groups.The expression of Adrb3 and Ucp1 genes in BAT-denervated group reduced to comparable levels in sham and 6-OHDA groups.As a result,body weight of BAT-denervated 6-OHDA rats was increased,compared with control 6-OHDA rats.These data demonstrate that sympathetic hyperactivation in BAT contributes to loss of body weight in 6-OHDA rats.3.Denervation suppressed lipolysis in 6-OHDA rats BAT exerts thermogenesis by utilizing NEFA generated from lipolysis in BAT and WAT.We found that,compared to the sham group,levels of TG in 6-OHDA rats were decreased while NEFA levels were increased in r WAT and epididymal WAT(e WAT),but not in subcutaneous WAT(s WAT)and BAT,suggesting increased lipolysis in selected WAT depot in PD rats.Interestingly,BAT denervation didn't change levels of TG and NEFA in sham rats,but significantly increased TG and reduced NEFA in r WAT and e WAT in PD rats.Consistently,rWAT and eWAT adipocytes in 6-OHDA rats were larger than those in sham rats,and BAT denervation reverted the size of adipocytes to the level comparable to the sham group.The expression of ?3R gene in r WAT and e WAT in 6-OHDA rats was significantly higher than sham rats,largely due to increased SN activity in WAT.Intriguingly,denervation in BAT dramatically reduced ?3R expression to similar levels as in the sham group.Signaling downstream of ?3R triggers lipolysis,thus we measured lipolysis-related signal molecules,including c AMP,P-HSL,ATGL and MAGL.We found that all these molecules were significantly increased in 6-OHDA rats compared to sham rats,and BAT denervation in 6-OHDA rats restored their levels in BAT,r WAT and e WAT,but not s WAT.Taken together,our results revealed that overactivated SN activity and thermogenesis in BAT contribute to dysregulated WAT lipolysis in the 6-OHDA rats.Conclusion: Degenerated dopaminergic neurons in the nigrostriatal system correlated with increases in sympathetic nervous system function of BAT,and BAT thermogenesis controlled by increased sympathetic activity promotes WAT lipolysis and weight loss in 6-OHDA rats.