The melanocortin-4 receptor (MC4R) is critically involved in regulating energy balance, and obesity has been observed in mice with mutations in the gene for brain-derived neurotrophic factor (BDNF). Here we report that BDNF is expressed at high levels in the ventromedial hypothalamus (VMH) where its expression is regulated by nutritional state and by MC4R signaling. In addition, similar to MC4R mutants, mouse mutants that expresses the BDNF receptor TrkB at a quarter of the normal amount showed hyperphagia and excessive weight gain on higher-fat diets. Furthermore, BDNF infusion into the brain suppressed the hyperphagia and excessive weight gain observed on higherfat diets in mice with deficient MC4R signaling. These results show that MC4R signaling controls BDNF expression in the VMH and support the hypothesis that BDNF is an important effector through which MC4R signaling controls energy balance.Neurotrophins are a family of structurally related growth factors, including brain-derived neurotrophic factor (BDNF), that exert many of their effects on neurons through Trk receptor tyrosine kinases. Among these, BDNF and its receptor TrkB are the most widely and abundantly expressed in the brain. BDNF has been shown to regulate neuronal development and to modulate synaptic plasticity 1 . Obesity phenotypes have been observed in BDNF heterozygous mice and in mice in which the BDNF gene has been deleted in excitatory neurons in the brain 2-4 . These mutants also show hyperactivity, hyperleptinaemia, hyperinsulinaemia, hyperglycemia and increased linear growth 3,4 . Moreover, both central and peripheral administration of BDNF decrease food intake, increase energy expenditure and ameliorate hyperinsulinaemia and hyperglycemia in diabetic db/db mice 5-8 . However, the means by which BDNF alters energy balance and the relationship of TrkB signaling to the major pathways previously shown to be involved in the regulation of energy balance remain unknown. COMPETING INTERESTS STATEMENTThe authors declare that they have no competing financial interests. The status of energy balance is communicated to the hypothalamus through neuronal and hormonal signals. Several hypothalamic nuclei are involved in the regulation of energy balance, including the ventromedial (VMH), paraventricular (PVN) and arcuate (ARC) hypothalamic nuclei and the lateral hypothalamic area (LH). Although the observation that bilateral VMH lesions produce hyperphagia and obesity has implicated this region in the regulation of energy balance 9-11 , the neural mechanisms through which the VMH functions to influence energy balance are not clear. NIH Public AccessOne of the major signals that serves as a monitor of energy balance is leptin, a polypeptide generated in adipocytes 12 . One of the main targets of leptin in the hypothalamus is the ARC, a region containing at least two distinct populations of neurons 13-15 . One population of neurons expresses two orexigenic polypeptides, neuropeptide Y (NPY) and agouti-related protein (AgRP), whereas the other...

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