SUMMARY Obesity and nutrient homeostasis are linked by mechanisms that are not fully elucidated. Here we describe a secreted protein, adropin, encoded by a gene, Energy Homeostasis Associated (Enho), expressed in liver and brain. Liver Enho expression is regulated by nutrition: lean C57BL/6J mice fed high-fat diet (HFD) exhibited a rapid increase, while fasting reduced expression compared to controls. However, liver Enho expression declines with diet-induced obesity (DIO) associated with 3 months of HFD or with genetically induced obesity, suggesting an association with metabolic disorders in the obese state. In DIO mice, transgenic overexpression or systemic adropin treatment attenuated hepatosteatosis and insulin resistance independently of effects on adiposity or food intake. Adropin regulated expression of hepatic lipogenic genes and adipose tissue peroxisome proliferator-activated receptor gamma, a major regulator of lipogenesis. Adropin may therefore be a factor governing glucose and lipid homeostasis, which protects against hepatosteatosis and hyperinsulinemia associated with obesity.

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...

D-fenfluramine (d-FEN) was once widely prescribed and was among the most effective weight loss drugs, but was withdrawn from clinical use because of reports of cardiac complications in a subset of patients. Discerning the neurobiology underlying the anorexic action of d-FEN may facilitate the development of new drugs to prevent and treat obesity. Through a combination of functional neuroanatomy, feeding, and electrophysiology studies in rodents, we show that d-FEN-induced anorexia requires activation of central nervous system melanocortin pathways. These results provide a mechanistic explanation of d-FEN's anorexic actions and indicate that drugs targeting these downstream melanocortin pathways may prove to be effective and more selective anti-obesity treatments.

Summary paragraphSerotonin (5-hydroxytryptamine; 5-HT) is a neurotransmitter that has an essential role in the regulation of emotion. The precise circuits through which aversive states are orchestrated by 5-HT, however, have not yet been defined. Here we show that 5-HT from the dorsal raphe nucleus (5-HTDRN) enhances fear and anxiety and activates a subpopulation of corticotropin-releasing factor (CRF) neurons in the bed nucleus of the stria terminalis (CRFBNST). Specifically, 5-HTDRN projections to the BNST, via actions at 5-HT2C receptors (5-HT2CRs), engage a CRFBNST inhibitory microcircuit that silences anxiolytic BNST outputs to the ventral tegmental area (VTA) and lateral hypothalamus (LH). Further, we demonstrate that this CRFBNST inhibitory circuit underlies aversive behavior following acute exposure to selective serotonin reuptake inhibitors (SSRIs). This early aversive effect is mediated via the corticotrophin releasing factor type 1 receptor (CRF1R) given that CRF1R antagonism is sufficient to prevent acute SSRI-induced enhancements in aversive learning. These results reveal an essential 5-HTDRN→CRFBNST circuit governing fear and anxiety and provide a potential mechanistic explanation for the clinical observation of early adverse events to SSRI treatment in some patients with anxiety disorders1,2.

The neural pathways through which central serotonergic systems regulate food intake and body weight remain to be fully elucidated. We report that serotonin, via action at serotonin1B receptors (5-HT1BRs), modulates the endogenous release of both agonists and antagonists of the melanocortin receptors, which are a core component of the central circuitry controlling body weight homeostasis. We also show that serotonin-induced hypophagia requires downstream activation of melanocortin 4, but not melanocortin 3, receptors. These results identify a primary mechanism underlying the serotonergic regulation of energy balance and provide an example of a centrally derived signal that reciprocally regulates melanocortin receptor agonists and antagonists in a similar manner to peripheral adiposity signals.