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Leptin is a peptide hormone produced by adipose tissue which acts centrally to decrease appetite and increase energy expenditure. Although leptin increases norepinephrine turnover in thermogenic tissues, the effects of leptin on directly measured sympathetic nerve activity to thermogenic and other tissues are not known. We examined the effects of intravenous leptin and vehicle on sympathetic nerve activity to brown adipose tissue, kidney, hindlimb, and adrenal gland in anesthetized Sprague-Dawley rats. Intravenous infusion of mouse leptin over 3 h (total dose 10-1,000 microg/kg) increased plasma concentrations of immunoreactive murine leptin up to 50-fold. Leptin slowly increased sympathetic nerve activity to brown adipose tissue (+286+/-64% at 1,000 microg/kg; P = 0.002). Surprisingly, leptin infusion also produced gradual increases in renal sympathetic nerve activity (+228+/-63% at 1,000 microg/kg; P = 0.0008).The effect of leptin on sympathetic nerve activity was dose dependent, with a threshold dose of 100 microg/kg. Leptin also increased sympathetic nerve activity to the hindlimb (+287+/-60%) and adrenal gland (388+/-171%). Despite the increase in overall sympathetic nerve activity, leptin did not increase arterial pressure or heart rate. Leptin did not change plasma glucose and insulin concentrations. Infusion of vehicle did not alter sympathetic nerve activity. Obese Zucker rats, known to possess a mutation in the gene for the leptin receptor, were resistant to the sympathoexcitatory effects of leptin, despite higher achieved plasma leptin concentrations. These data demonstrate that leptin increases thermogenic sympathetic nerve activity and reveal an unexpected stimulatory effect of leptin on overall sympathetic nerve traffic.