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...

Serotonin (5-hydroxytryptamine, 5-HT) is a monoaminergic neurotransmitter that is believed to modulate numerous sensory, motor and behavioural processes in the mammalian nervous system. These diverse responses are elicited through the activation of a large family of receptor subtypes. The complexity of this signalling system and the paucity of selective drugs have made it difficult to define specific roles for 5-HT receptor subtypes, or to determine how serotonergic drugs modulate mood and behaviour. To address these issues, we have generated mutant mice lacking functional 5-HT2C receptors (previously termed 5-HT1C), prominent G-protein-coupled receptors that are widely expressed throughout the brain and spinal cord and which have been proposed to mediate numerous central nervous system (CNS) actions of serotonin. Here we show that 5-HT2C receptor-deficient mice are overweight as a result of abnormal control of feeding behaviour, establishing a role for this receptor in the serotonergic control of appetite. Mutant animals are also prone to spontaneous death from seizures, suggesting that 5-HT2C receptors mediate tonic inhibition of neuronal network excitability.

During pregnancy, the energy requirements of the fetus impose changes in maternal metabolism. Increasing insulin resistance in the mother maintains nutrient flow to the growing fetus, while prolactin and placental lactogen counterbalance this resistance and prevent maternal hyperglycemia by driving expansion of the maternal population of insulin-producing β-cells1–3. However, the exact mechanisms by which the lactogenic hormones drive β-cell expansion remain uncertain. Here we show that serotonin acts downstream of lactogen signaling to drive β-cell proliferation. Serotonin synthetic enzyme Tph1 and serotonin production increased sharply in β-cells during pregnancy or after treatment with lactogens in vitro. Inhibition of serotonin synthesis by dietary tryptophan restriction or Tph inhibition blocked β-cell expansion and induced glucose intolerance in pregnant mice without affecting insulin sensitivity. Expression of the Gαq-linked serotonin receptor Htr2b in maternal islets increased during pregnancy and normalized just prior to parturition, while expression of the Gαi-linked receptor Htr1d increased at the end of pregnancy and postpartum. Blocking Htr2b signaling in pregnant mice also blocked β-cell expansion and caused glucose intolerance. These studies reveal an integrated signaling pathway linking β-cell mass to anticipated insulin need during pregnancy. Modulators of this pathway, including medications and diet, may affect the risk of gestational diabetes4.

Leptin inhibition of bone mass accrual requires the integrity of specific hypothalamic neurons but not expression of its receptor on these neurons. The same is true for its regulation of appetite and energy expenditure. This suggests that leptin acts elsewhere in the brain to achieve these three functions. We show here that brainstem-derived serotonin (BDS) favors bone mass accrual following its binding to Htr2c receptors on ventromedial hypothalamic neurons and appetite via Htr1a and 2b receptors on arcuate neurons. Leptin inhibits these functions and increases energy expenditure because it reduces serotonin synthesis and firing of serotonergic neurons. Accordingly, while abrogating BDS synthesis corrects the bone, appetite and energy expenditure phenotypes caused by leptin deficiency, inactivation of the leptin receptor in serotonergic neurons recapitulates them fully. This study modifies the map of leptin signaling in the brain and identifies a molecular basis for the common regulation of bone and energy metabolisms.

The brain serotonin (5-hydroxytryptamine; 5-HT) system is a powerful modulator of emotional processes and a target of medications used in the treatment of psychiatric disorders. To evaluate the contribution of serotonin 5-HT 1A receptors to the regulation of these processes, we have used gene-targeting technology to generate 5-HT 1A receptormutant mice. These animals lack functional 5-HT 1A receptors as indicated by receptor autoradiography and by resistance to the hypothermic effects of the 5-HT 1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). Homozygous mutants display a consistent pattern of responses indicative of elevated anxiety levels in open-field, elevated-zero maze, and novel-object assays. Moreover, they exhibit antidepressant-like responses in a tail-suspension assay. These results indicate that the targeted disruption of the 5-HT 1A receptor gene leads to heritable perturbations in the serotonergic regulation of emotional state. 5-HT 1A receptor-null mutant mice have potential as a model for investigating mechanisms through which serotonergic systems modulate affective state and mediate the actions of psychiatric drugs. The brain serotonin (5-hydroxytryptamine; 5-HT) system has been strongly implicated in the neural regulation of mood and anxiety state. Accordingly, many commonly used antidepressant and antianxiety medications target this system (1). The complex physiological actions of serotonin are mediated by a heterogeneous family of at least 14 distinct receptor subtypes (2). Although the relative contributions of individual receptor subtypes to the serotonergic regulation of mood are incompletely understood, particular attention has focused on the 5-HT 1A receptor subtype. Partial agonists at this receptor, such as buspirone, are in clinical use as anxiolytics (3), and 5-HT 1A receptor antagonists are reported to accelerate the therapeutic effects of antidepressant medications (4).These compounds produce complex effects on brain function through interactions with functionally distinct populations of 5-HT 1A receptors. 5-HT 1A receptors located on serotonergic neuronal cell bodies and dendrites are the predominant somatodendritic autoreceptors of these neurons; their activation suppresses serotonergic neuronal activity (5, 6). In addition, postsynaptic 5-HT 1A receptors are expressed in numerous serotonergic projection sites such as the cerebral cortex, septal nuclei, hippocampus, and amygdala (7). The relatively selective 5-HT 1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and antagonist WAY 100635 (8) have been used as pharmacological probes of 5-HT 1A receptor function. Systemic administration of 8-OH-DPAT produces hyperphagia, hypothermia, and an anxiolytic-like effect in rodents (9-12). The behavioral and physiological effects of 8-OH-DPAT are blocked by pretreatment with WAY 100635 (12-14).To complement pharmacological approaches to the study of 5-HT 1A receptor function, we have used a gene-targeting strategy to generate a line of m...