We report that astrocytic insulin signaling co-regulates hypothalamic glucose sensing and systemic glucose metabolism. Postnatal ablation of insulin receptors (IRs) in glial fibrillary acidic protein (GFAP)-expressing cells affects hypothalamic astrocyte morphology, mitochondrial function, and circuit connectivity. Accordingly, astrocytic IR ablation reduces glucose-induced activation of hypothalamic pro-opio-melanocortin (POMC) neurons and impairs physiological responses to changes in glucose availability. Hypothalamus-specific knockout of astrocytic IRs, as well as postnatal ablation by targeting glutamate aspartate transporter (GLAST)-expressing cells, replicates such alterations. A normal response to altering directly CNS glucose levels in mice lacking astrocytic IRs indicates a role in glucose transport across the blood-brain barrier (BBB). This was confirmed in vivo in GFAP-IR KO mice by using positron emission tomography and glucose monitoring in cerebral spinal fluid. We conclude that insulin signaling in hypothalamic astrocytes co-controls CNS glucose sensing and systemic glucose metabolism via regulation of glucose uptake across the BBB.

Comment in Sensory systems: the hungry sense. [Nat Rev Neurosci. 2014] Inhaling: endocannabinoids and food intake. [Nat Neurosci. 2014]International audienceHunger arouses sensory perception, eventually leading to an increase in food intake, but the underlying mechanisms remain poorly understood. We found that cannabinoid type-1 (CB1) receptors promote food intake in fasted mice by increasing odor detection. CB1 receptors were abundantly expressed on axon terminals of centrifugal cortical glutamatergic neurons that project to inhibitory granule cells of the main olfactory bulb (MOB). Local pharmacological and genetic manipulations revealed that endocannabinoids and exogenous cannabinoids increased odor detection and food intake in fasted mice by decreasing excitatory drive from olfactory cortex areas to the MOB. Consistently, cannabinoid agonists dampened in vivo optogenetically stimulated excitatory transmission in the same circuit. Our data indicate that cortical feedback projections to the MOB crucially regulate food intake via CB1 receptor signaling, linking the feeling of hunger to stronger odor processing. Thus, CB1 receptor-dependent control of cortical feedback projections in olfactory circuits couples internal states to perception and behavior

The endocannabinoid system (ECS) plays a critical role in obesity development. The pharmacological blockade of cannabinoid receptor type 1 (CB(1)) has been shown to reduce body weight and to alleviate obesity-related metabolic disorders. An unsolved question is at which anatomical level CB(1) modulates energy balance and the mechanisms involved in its action. Here, we demonstrate that CB(1) receptors expressed in forebrain and sympathetic neurons play a key role in the pathophysiological development of diet-induced obesity. Conditional mutant mice lacking CB(1) expression in neurons known to control energy balance, but not in nonneuronal peripheral organs, displayed a lean phenotype and resistance to diet-induced obesity. This phenotype results from an increase in lipid oxidation and thermogenesis as a consequence of an enhanced sympathetic tone and a decrease in energy absorption. In conclusion, CB(1) signaling in the forebrain and sympathetic neurons is a key determinant of the ECS control of energy balance.

The mineralocorticoid receptor (MR) controls adipocyte function, but its role in the conversion of white adipose tissue (WAT) into thermogenic fat has not been elucidated. We investigated responses to the MR antagonists spironolactone (spiro; 20 mg/kg/d) and drospirenone (DRSP; 6 mg/kg/d) in C57BL/6 mice fed a high-fat (HF) diet for 90 d. DRSP and spiro curbed HF diet-induced impairment in glucose tolerance, and prevented body weight gain and white fat expansion. Notably, either MR antagonist induced up-regulation of brown adipocyte-specific transcripts and markedly increased protein levels of uncoupling protein 1 (UCP1) in visceral and inguinal fat depots when compared with the HF diet group. Positron emission tomography and magnetic resonance spectroscopy confirmed acquisition of brown fat features in WAT. Interestingly, MR antagonists markedly reduced the autophagic rate both in murine preadipocytes in vitro (10(-5) M) and in WAT depots in vivo, with a concomitant increase in UCP1 protein expression. Moreover, the autophagy repressor bafilomycin A1 (10(-8) M) mimicked the effect of MR antagonists, increasing UCP1 protein expression in primary preadipocytes. Hence, we showed that adipocyte MR regulates brown remodeling of WAT through a modulation of autophagy. These results provide a rationale for the use of MR antagonists to prevent the adverse metabolic consequences of adipocyte dysfunction.

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Complex interactions between periphery and the brain regulate food intake in mammals. Cannabinoid type-1 (CB 1 ) receptor antagonists are potent hypophagic agents, but the sites where this acute action is exerted and the underlying mechanisms are not fully elucidated. To dissect the mechanisms underlying the hypophagic effect of CB 1 receptor blockade, we combined the acute injection of the CB 1 receptor antagonist rimonabant with the use of conditional CB 1 -knockout mice, as well as with pharmacological modulation of different central and peripheral circuits. Fasting/refeeding experiments revealed that CB 1 receptor signaling in many specific brain neurons is dispensable for the acute hypophagic effects of rimonabant. CB 1 receptor antagonist-induced hypophagia was fully abolished by peripheral blockade of β-adrenergic transmission, suggesting that this effect is mediated by increased activity of the sympathetic nervous system. Consistently, we found that rimonabant increases gastrointestinal metabolism via increased peripheral β-adrenergic receptor signaling in peripheral organs, including the gastrointestinal tract. Blockade of both visceral afferents and glutamatergic transmission in the nucleus tractus solitarii abolished rimonabant-induced hypophagia. Importantly, these mechanisms were specifically triggered by lipid-deprivation, revealing a nutrientspecific component acutely regulated by CB 1 receptor blockade. Finally, peripheral blockade of sympathetic neurotransmission also blunted central effects of CB 1 receptor blockade, such as fear responses and anxiety-like behaviors. These data demonstrate that, independently of their site of origin, important effects of CB 1 receptor blockade are expressed via activation of peripheral sympathetic activity. Thus, CB 1 receptors modulate bidirectional circuits between the periphery and the brain to regulate feeding and other behaviors.fear and anxiety | sympathetic system

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