Antibodies against the extracellular domains of the chicken leptin receptor were used to study the biological function of leptin in growing chickens.Both polyclonal and monoclonal antibodies were administered intramuscularly to 30-d-old Chinese indigenous Gushi pullets.Both antibody preparations increased feed intake for 6 h after injection and reduced plasma concentrations of glucose,triglycerides,and both high-and low-density lipoproteins.The antibody treatments also upregulated agouti-related peptide and neuropeptide Y in the hypothalamus and downregulated proopiomelanocortin,melanocortin 4 receptor,and leptin receptor.The treatments also upregulated leptin receptor,acetyl CoA carboxylase beta,and acyl-CoA oxidase in the liver,abdominal fat,and breast muscle and downregulated sterol regulatory element-binding protein-1 and fatty acid synthase.Furthermore,even though the anti-leptin receptor antibodies failed to affect leptin receptor signaling transduction when administered alone,they did augment the induction of leptin receptor signaling transduction by leptin.These results demonstrate that antibodies against the extracellular domains of leptin-specific receptor enhance,but do not mimic,the ability of leptin to activate receptors.Furthermore,the enhanced leptin bioactivity observed after the intramuscular injection of anti-LEPR antibodies confirmed the occurrence of de novo leptin in the peripheral tissues and blood of treated chickens.