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Obesity,diabetes and fatty liver disease are extremely common in leptin-resistant patients.Dysfunction of leptin or its receptor is associated with obesity.The present study aimed to assess the effects of intramuscular injection of exogenous leptin or its receptor on fat deposition and leptin-insulin feedback regulation.Forty-five 40-day old female Sprague Dawley(SD)rats were injected thrice with leptin or its receptor intramuscularly.Adiposity and fat deposition were assessed by assessing the Lees index,body weight,food intake,and total cholesterol,high density lipoprotein,low density lipoprotein,and triglyceride levels,as well as histological properties(liver and adipose tissue).Serum glucose,leptin,and insulin amounts were evaluated,and glucose tolerance assessed to monitor glucose metabolism in SD rats; pancreas specimens were analyzed immunohistochemically.Hypothalamic phosphorylated Janus kinase 2(p-JAK2),phosphorylated signal transducer and activator of transcription 3(p-STAT3),and phosphatidylinositol-3-kinase(PI3K)signaling,and hepatic sterol regulatory element binding protein-1(SREBP-1)were qualified by Western blotting.Leptin receptor immunogen reduced fat deposition,increased appetite,and lowered serum leptin levels,enhancing STAT3 signaling in hypothalamus and down-regulating hepatic SREBP-1.In contrast,SD rats administered leptin immunogen displayed significantly increased body weight and fat deposition,with up-regulated SREBP-1,indicating adiposity occurrence.SD rats administered leptin immunogen also showed glucose intolerance,~cell reduction in the pancreas,and deregulation of JAK2-STAT3/PI3K signaling,indicating that Lep rats were at risk of diabetes.In conclusion,intramuscular injection of exogenous leptin or its receptor,a novel rat model approach,can be used in obesity pathogenesis and therapeutic studies.