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Oxytocin (OXT), a nine-amino-acid peptide produced in the hypothalamus and released by the posterior pituitary, has well-known actions in parturition, lactation and social behaviour, and has become an intriguing therapeutic target for conditions such as autism and schizophrenia. Exogenous OXT has also been shown to have effects on body weight, lipid levels and glucose homeostasis, suggesting that it may also have therapeutic potential for metabolic disease. It is unclear, however, whether endogenous OXT participates in metabolic homeostasis. Here we show that OXT is a critical regulator of adipose tissue lipolysis in both mice and humans. In addition, OXT serves to facilitate the ability of β-adrenergic agonists to fully promote lipolysis. Most surprisingly, the relevant source of OXT in these metabolic actions is a previously unidentified subpopulation of tyrosine hydroxylase-positive sympathetic neurons. Our data reveal that OXT from the peripheral nervous system is an endogenous regulator of adipose and systemic metabolism.