Dissociation between sensing and metabolism of glucose in sugar sensing neurones

ADGRL1 is a glucose receptor involved in mediating energy and glucose homeostasis

AIMS/HYPOTHESIS

The brain is a major consumer of glucose as an energy source and regulates systemic glucose as well as energy balance. Although glucose transporters such as GLUT2 and sodium-glucose cotransporter 2 (SGLT2) are known to regulate glucose homeostasis and metabolism, the identity of a receptor that binds glucose to activate glucose signalling pathways in the brain is unknown. In this study, we aimed to discover a glucose receptor in the mouse hypothalamus.

METHODS

Here we used a high molecular mass glucose-biotin polymer to enrich glucose-bound mouse hypothalamic neurons through cell-based affinity chromatography. We then subjected the enriched neurons to proteomic analyses and identified adhesion G-protein coupled receptor 1 (ADGRL1) as a top candidate for a glucose receptor. We validated glucose-ADGRL1 interactions using CHO cells stably expressing human ADGRL1 and ligand-receptor binding assays. We generated and determined the phenotype of global Adgrl1-knockout mice and hypothalamus-specific Adgrl1-deficient mice. We measured the variables related to glucose and energy homeostasis in these mice. We also generated an Adgrl1Cre mouse model to investigate the role of ADGRL1 in sensing glucose using electrophysiology.

RESULTS

Adgrl1 is highly expressed in the ventromedial nucleus of the hypothalamus (VMH) in mice. Lack of Adgrl1 in the VMH in mice caused fasting hyperinsulinaemia, enhanced glucose-stimulated insulin secretion and insulin resistance. In addition, the Adgrl1-deficient mice had impaired feeding responses to glucose and fasting coupled with abnormal glucose sensing and decreased physical activity before development of obesity and hyperglycaemia. In female mice, ovariectomy was necessary to reveal the contribution of ADGRL1 to energy and glucose homeostasis.

CONCLUSIONS/INTERPRETATION

Altogether, our findings demonstrate that ADGRL1 binds glucose and is involved in energy as well as glucose homeostasis in a sex-dependent manner. Targeting ADGRL1 may introduce a new class of drugs for the treatment of type 2 diabetes and obesity.

Orexin and MCH neurons: regulators of sleep and metabolism

Sleep-wake and fasting-feeding are tightly coupled behavioral states that require coordination between several brain regions. The mammalian lateral hypothalamus (LH) is a functionally and anatomically complex brain region harboring heterogeneous cell populations that regulate sleep, feeding, and energy metabolism. Significant attempts were made to understand the cellular and circuit bases of LH actions. Rapid advancements in genetic and electrophysiological manipulation help to understand the role of discrete LH cell populations. The opposing action of LH orexin/hypocretin and melanin-concentrating hormone (MCH) neurons on metabolic sensing and sleep-wake regulation make them the candidate to explore in detail. This review surveys the molecular, genetic, and neuronal components of orexin and MCH signaling in the regulation of sleep and metabolism.

Effects of Short-Term Food Deprivation on Catecholamine and Metabolic-Sensory Biomarker Gene Expression in Hindbrain A2 Noradrenergic Neurons Projecting to the Forebrain Rostral Preoptic Area: Impact of Negative versus Positive Estradiol Feedback

Hindbrain A2 noradrenergic neurons assimilate estrogenic and metabolic cues. In female mammals, negative- versus positive-feedback patterns of estradiol (E) secretion impose divergent regulation of the gonadotropin-releasing hormone (GnRH)-pituitary-gonadal (HPG) neuroendocrine axis. Current research used retrograde tracing, dual-label immunocytochemistry, single-cell laser-microdissection, and multiplex qPCR methods to address the premise that E feedback modes uniquely affect metabolic regulation of A2 neurons involved in HPG control. Ovariectomized female rats were given E replacement to replicate plasma hormone levels characteristic of positive (high-E dose) or negative (low-E dose) feedback. Animals were either full-fed (FF) or subjected to short-term, e.g., 18-h food deprivation (FD). After FF or FD, rostral preoptic area (rPO)-projecting A2 neurons were characterized by the presence or absence of nuclear glucokinase regulatory protein (nGKRP) immunostaining. FD augmented or suppressed mRNAs encoding the catecholamine enzyme dopamine-beta-hydroxylase (DβH) and the metabolic-sensory biomarker glucokinase (GCK), relative to FF controls, in nGKRP-immunoreactive (ir)-positive A2 neurons from low-E or high-E animals, respectively. Yet, these transcript profiles were unaffected by FD in nGKRP-ir-negative A2 neurons at either E dosage level. FD altered estrogen receptor (ER)-alpha and ATP-sensitive potassium channel subunit sulfonylurea receptor-1 gene expression in nGKRP-ir-positive neurons from low-E, but not high-E animals. Results provide novel evidence that distinct hindbrain A2 neuron populations exhibit altered versus unaffected transmission to the rPO during FD-associated metabolic imbalance, and that the direction of change in this noradrenergic input is controlled by E feedback mode. These A2 cell types are correspondingly distinguished by FD-sensitive or -insensitive GCK, which correlates with the presence versus absence of nGKRP-ir. Further studies are needed to determine how E signal volume regulates neurotransmitter and metabolic sensor responses to FD in GKRP-expressing A2 neurons.

Hindbrain lactate regulation of hypoglycemia-associated patterns of catecholamine and metabolic-sensory biomarker gene expression in A2 noradrenergic neurons innervating the male versus female ventromedial hypothalamic nucleus

Caudal hindbrain A2 noradrenergic neurons provide critical metabolic-sensory input to the brain glucostatic circuitry. In males, insulin-induced hypoglycemia (IIH)-associated patterns of A2 cell dopamine-beta-hydroxylase (DβH) protein expression reflect diminution of the oxidizable fuel L-lactate, yet DβH exhibits sex-dimorphic responses to IIH. Here, retrograde tracing and combinatory single-cell laser-microdissection/multiplex qPCR techniques were used to examine whether lactate imposes sex-specific control of hypoglycemia-associated metabolic-sensory function and noradrenergic neurotransmission in A2 neurons that innervate the ventromedial hypothalamic nucleus (VMN), a key glucose-regulatory structure. VMN-projecting A2 neurons from each sex were characterized by presence or absence of nuclear glucokinase regulatory protein (nGKRP) immunoreactivity (-ir). IIH caused lactate-reversible up- or down-regulation of DβH mRNA in male and female nGKRP-ir-positive A2 neurons, respectively, and stimulated glucokinase (GCK) and sulfonylurea receptor-1 (SUR-1) gene expression in these cells in each sex. Hypoglycemia did not alter DβH, GCK, and SUR-1 transcript profiles in nGKRP-ir-negative male or female A2 neurons innervating the VMN. Estrogen receptor (ER) gene profiles in nGKRP-ir-positive neurons showed sex-specific [ER-alpha; G-protein-coupled estrogen-receptor-1 (GPER)] or sex-monomorphic (ER-beta) transcriptional responses to IIH. Fewer ER gene profiles were affected by IIH in nGKRP-ir-negative A2 neurons from male or female rats. Results show that during IIH, VMN-projecting A2 neurons may deliver altered, sex-dependent (nGKRP-positive) or unaffected (nGKRP-negative) noradrenergic input to the VMN. In each sex, metabolic-sensory gene profiles were reactive to hypoglycemia in nGKRP-ir-positive, not -negative A2 cells. Further studies are needed to elucidate the role of GKRP in transduction of metabolic imbalance into noradrenergic signaling, and to determine if input by one or more ER variants establishes sex differences in DβH transcriptional sensitivity to IIH.

Do orexin/hypocretin neurons signal stress or reward?

Hypothalamic neurons that produce the peptide transmitters orexins/hypocretins (HONs) broadcast their predominantly neuroexcitatory outputs to the entire brain via their extremely wide axonal projections. HONs were originally reported to be activated by food deprivation, and to stimulate arousal, energy expenditure, and eating. This led to extensive studies of HONs in the context of nutrient-sensing and energy balance control. While activation of HONs by body energy depletion continues to be supported by experimental evidence, it has also become clear that HONs are robustly activated not only by nutrient depletion, but also by diverse sensory stimuli (both neutral and those associated with rewarding or aversive events), seemingly unrelated to each other or to energy balance. One theory that could unify these findings is that all these stimuli signal "stress" - defined either as a potentially harmful state, or an awareness of reward deficiency. If HON activity is conceptualized as a cumulative representation of stress, then many of the reported HONs outputs - including EEG arousal, sympathetic activation, place avoidance, and exploratory behaviours - could be viewed as logical stress-counteracting responses. We discuss evidence for and against this unifying theory of HON function, including the alterations in HON activity observed in anxiety and depression disorders. We propose that, in order to orchestrate stress-countering responses, HONs need to coactivate motivation and aversion brain systems, and the impact of HON stimulation on affective states may be perceived as rewarding or aversive depending on the baseline HON activity.

Mizagliflozin, a selective SGLT1 inhibitor, improves vascular cognitive impairment in a mouse model of small vessel disease

Previously, we showed that sodium/glucose cotransporter 1 (SGLT1) participates in vascular cognitive impairment in small vessel disease. We hypothesized that SGLT1 inhibitors can improve the small vessel disease induced-vascular cognitive impairment. We examined the effects of mizagliflozin, a selective SGLT1 inhibitor, and phlorizin, a non-selective SGLT inhibitor, on vascular cognitive impairment in a mouse model of small vessel disease. Small vessel disease was created using a mouse model of asymmetric common carotid artery surgery (ACAS). Two and/or 4 weeks after ACAS, all experiments were performed. Cerebral blood flow (CBF) was decreased in ACAS compared with sham-operated mice. Phlorizin but not mizagliflozin reversed the decreased CBF of ACAS mice. Both mizagliflozin and phlorizin reversed the ACAS-induced decrease in the latency to fall in a wire hang test of ACAS mice. Moreover, they reversed the ACAS-induced longer escape latencies in the Morris water maze test of ACAS mice. ACAS increased SGLT1 and proinflammatory cytokine gene expressions in mouse brains and phlorizin but not mizagliflozin normalized all gene expressions in ACAS mice. Hematoxylin/eosin staining demonstrated that they inhibited pyknotic cell death in the ACAS mouse hippocampus. In PC12HS cells, IL-1β increased SGLT1 expression and decreased survival rates of cells. Both mizagliflozin and phlorizin increased the survival rates of IL-1β-treated PC12HS cells. These results suggest that mizagliflozin and phlorizin can improve vascular cognitive impairment through the inhibition of neural SGLT1 and phlorizin also does so through the improvement of CBF in a mouse model of small vessel disease.

Nutrient infusion in the dorsal vagal complex controls hepatic lipid and glucose metabolism in rats

Hypothalamic regulation of lipid and glucose homeostasis is emerging, but whether the dorsal vagal complex (DVC) senses nutrients and regulates hepatic nutrient metabolism remains unclear. Here, we found in rats DVC oleic acid infusion suppressed hepatic secretion of triglyceride-rich very-low-density lipoprotein (VLDL-TG), which was disrupted by inhibiting DVC long-chain fatty acyl-CoA synthetase that in parallel disturbed lipid homeostasis during intravenous lipid infusion. DVC glucose infusion elevated local glucose levels similarly as intravenous glucose infusion and suppressed hepatic glucose production. This was independent of lactate metabolism as inhibiting lactate dehydrogenase failed to disrupt glucose sensing and neither could DVC lactate infusion recapitulate glucose effect. DVC oleic acid and glucose infusion failed to lower VLDL-TG secretion and glucose production in high-fat fed rats, while inhibiting DVC farnesoid X receptor enhanced oleic acid but not glucose sensing. Thus, an impairment of DVC nutrient sensing may lead to the disruption of lipid and glucose homeostasis in metabolic syndrome.

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DVC oleic acid infusion lowers hepatic secretion of VLDL-TG in chow but not HF rats

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Inhibition of ACSL in the DVC negates lipid sensing

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DVC glucose infusion lowers hepatic glucose production in chow but not HF rats

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Inhibition of FXR in the DVC enhances oleic acid but not glucose sensing in HF rats

Central administration of sodium-glucose cotransporter-2 inhibitors increases food intake involving adenosine monophosphate-activated protein kinase phosphorylation in the lateral hypothalamus in healthy rats

INTRODUCTION

Sodium glucose cotransporter-2 (SGLT2) inhibitors are widely used for diabetes treatment. Although SGLT2 inhibitors have been clinically observed to increase food intake, roles or even the presence of SGLT2 in the central nervous system (CNS) has not been established. We aimed to elucidate potential functions of SGLT2 in the CNS, and the effects of CNS-targeted SGLT2 inhibitors on food intake.

RESEARCH DESIGN AND METHODS

We administered three kinds of SGLT2 inhibitors, tofogliflozin, dapagliflozin, and empagliflozin, into the lateral ventricle (LV) in rats and evaluated their effects on food intake. We also evaluated the effects of tofogliflozin administration in the third (3V) and fourth ventricle (4V). Intraperitoneal administration of liraglutide, a glucagon-like peptide-1 (GLP-1) receptor agonist known to suppress food intake, was combined with central tofogliflozin to elucidate whether GLP-1 signaling antagonizes the effect of central SGLT2 inhibitors on food intake. To elucidate potential molecular mechanisms mediating changes in feeding, hypothalamic areas associated with food intake regulation were harvested and analyzed after intracerebroventricular administration (ICV) of tofogliflozin.

RESULTS

Bolus ICV injection of tofogliflozin induced a robust increase in food intake starting at 1.5 hours postinjection, and lasting for 5 days. No effect was observed when the same dose of tofogliflozin was administered intraperitoneally. ICV dapagliflozin and empagliflozin significantly enhanced food intake, although the strength of these effects varied among drugs. Food intake was most markedly enhanced when tofogliflozin was infused into the LV. Fewer or no effects were observed with infusion into the 3V or 4V, respectively. Systemic administration of liraglutide suppressed the effect of ICV tofogliflozin on food intake. ICV tofogliflozin increased phosphorylation of AMPK and c-fos expression in the lateral hypothalamus.

CONCLUSIONS

SGLT2 inhibitors in the CNS increase food intake. SGLT2 activity in the CNS may regulate food intake through AMPK phosphorylation in the lateral hypothalamic area.

Orexin/Hypocretin and MCH Neurons: Cognitive and Motor Roles Beyond Arousal

The lateral hypothalamus (LH) is classically implicated in sleep-wake control. It is the main source of orexin/hypocretin and melanin-concentrating hormone (MCH) neuropeptides in the brain, which have been both implicated in arousal state switching. These neuropeptides are produced by non-overlapping LH neurons, which both project widely throughout the brain, where release of orexin and MCH activates specific postsynaptic G-protein-coupled receptors. Optogenetic manipulations of orexin and MCH neurons during sleep indicate that they promote awakening and REM sleep, respectively. However, recordings from orexin and MCH neurons in awake, moving animals suggest that they also act outside sleep/wake switching. Here, we review recent studies showing that both orexin and MCH neurons can rapidly (sub-second-timescale) change their firing when awake animals experience external stimuli, or during self-paced exploration of objects and places. However, the sensory-behavioral correlates of orexin and MCH neural activation can be quite different. Orexin neurons are generally more dynamic, with about 2/3rds of them activated before and during self-initiated running, and most activated by sensory stimulation across sensory modalities. MCH neurons are activated in a more select manner, for example upon self-paced investigation of novel objects and by certain other novel stimuli. We discuss optogenetic and chemogenetic manipulations of orexin and MCH neurons, which combined with pharmacological blockade of orexin and MCH receptors, imply that these rapid LH dynamics shape fundamental cognitive and motor processes due to orexin and MCH neuropeptide actions in the awake brain. Finally, we contemplate whether the awake control of psychomotor brain functions by orexin and MCH are distinct from their “arousal” effects.

Circadian Rhythms of the Hypothalamus: From Function to Physiology

The nearly ubiquitous expression of endogenous 24 h oscillations known as circadian rhythms regulate the timing of physiological functions in the body. These intrinsic rhythms are sensitive to external cues, known as zeitgebers, which entrain the internal biological processes to the daily environmental changes in light, temperature, and food availability. Light directly entrains the master clock, the suprachiasmatic nucleus (SCN) which lies in the hypothalamus of the brain and is responsible for synchronizing internal rhythms. However, recent evidence underscores the importance of other hypothalamic nuclei in regulating several essential rhythmic biological functions. These extra-SCN hypothalamic nuclei also express circadian rhythms, suggesting distinct regions that oscillate either semi-autonomously or independent of SCN innervation. Concurrently, the extra-SCN hypothalamic nuclei are also sensitized to fluctuations in nutrient and hormonal signals. Thus, food intake acts as another powerful entrainer for the hypothalamic oscillators' mediation of energy homeostasis. Ablation studies and genetic mouse models with perturbed extra-SCN hypothalamic nuclei function reveal their critical downstream involvement in an array of functions including metabolism, thermogenesis, food consumption, thirst, mood and sleep. Large epidemiological studies of individuals whose internal circadian cycle is chronically disrupted reveal that disruption of our internal clock is associated with an increased risk of obesity and several neurological diseases and disorders. In this review, we discuss the profound role of the extra-SCN hypothalamic nuclei in rhythmically regulating and coordinating body wide functions.

An Overview of Similarities and Differences in Metabolic Actions and Effects of Central Nervous System Between Glucagon-Like Peptide-1 Receptor Agonists (GLP-1RAs) and Sodium Glucose Co-Transporter-2 Inhibitors (SGLT-2is)

GLP-1 receptor agonists (GLP-1RAs) and SGLT-2 inhibitors (SGLT-2is) are novel antidiabetic medications associated with considerable cardiovascular benefits therapying treatment of diabetic patients. GLP-1 exhibits atherosclerosis resistance, whereas SGLT-2i acts to ameliorate the neuroendocrine state in the patients with chronic heart failure. Despite their distinct modes of action, both factors share pathways by regulating the central nervous system (CNS). While numerous preclinical and clinical studies have demonstrated that GLP-1 can access various nuclei associated with energy homeostasis and hedonic eating in the CNS via blood-brain barrier (BBB), research on the activity of SGLT-2is remains limited. In our previous studies, we demonstrated that both GLP-1 receptor agonists (GLP-1RAs) liraglutide and exenatide, as well as an SGLT-2i, dapagliflozin, could activate various nuclei and pathways in the CNS of Sprague Dawley (SD) rats and C57BL/6 mice, respectively. Moreover, our results revealed similarities and differences in neural pathways, which possibly regulated different metabolic effects of GLP-1RA and SGLT-2i via sympathetic and parasympathetic systems in the CNS, such as feeding, blood glucose regulation and cardiovascular activities (arterial blood pressure and heart rate control). In the present article, we extensively discuss recent preclinical studies on the effects of GLP-1RAs and SGLT-2is on the CNS actions, with the aim of providing a theoretical explanation on their mechanism of action in improvement of the macro-cardiovascular risk and reducing incidence of diabetic complications. Overall, these findings are expected to guide future drug design approaches.

Multiple organs involved in the pathogenesis of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) represents the leading cause of chronic liver disease worldwide and the anticipated health burden is huge. There are limited therapeutic approaches for NAFLD now. It’s imperative to get a better understanding of the disease pathogenesis if new treatments are to be discovered. As the hepatic manifestation of metabolic syndrome, this disease involves complex interactions between different organs and regulatory pathways. It’s increasingly clear that brain, gut and adipose tissue all contribute to NAFLD pathogenesis and development, in view of their roles in energy homeostasis. In the present review, we try to summarize currently available data regarding NAFLD pathogenesis and to lay a particular emphasis on the inter-organ crosstalk evidence.

Cellular and systemic mechanisms for glucose sensing and homeostasis

Glucose is a major source of energy in animals. Maintaining blood glucose levels within a physiological range is important for facilitating glucose uptake by cells, as required for optimal functioning. Glucose homeostasis relies on multiple glucose-sensing cells in the body that constantly monitor blood glucose levels and respond accordingly to adjust its glycemia. These include not only pancreatic β-cells and α-cells that secrete insulin and glucagon, but also central and peripheral neurons regulating pancreatic endocrine function. Different types of cells respond distinctively to changes in blood glucose levels, and the mechanisms involved in glucose sensing are diverse. Notably, recent studies have challenged the currently held views regarding glucose-sensing mechanisms. Furthermore, peripheral and central glucose-sensing cells appear to work in concert to control blood glucose level and maintain glucose and energy homeostasis in organisms. In this review, we summarize the established concepts and recent advances in the understanding of cellular and systemic mechanisms that regulate glucose sensing and its homeostasis.

Central KATP Channels Modulate Glucose Effectiveness in Humans and Rodents

Hyperglycemia is a potent regulator of endogenous glucose production (EGP). Loss of this "glucose effectiveness" is a major contributor to elevated plasma glucose concentrations in type 2 diabetes (T2D). K_ATP channels in the central nervous system have been shown to regulate EGP in humans and rodents. We examined the contribution of central K_ATP channels to glucose effectiveness. Under fixed hormonal conditions (studies using a pancreatic clamp), hyperglycemia suppressed EGP by ∼50% in both humans without diabetes and normal Sprague-Dawley rats. By contrast, antagonism of K_ATP channels with glyburide significantly reduced the EGP-lowering effect of hyperglycemia in both humans and rats. Furthermore, the effects of glyburide on EGP and gluconeogenic enzymes were abolished in rats by intracerebroventricular administration of the K_ATP channel agonist diazoxide. These findings indicate that about half of the suppression of EGP by hyperglycemia is mediated by central K_ATP channels. These central mechanisms may offer a novel therapeutic target for improving glycemic control in subjects with T2D.

In vitro enzyme inhibitory and cytotoxic studies with Evolvulus alsinoides (Linn.) Linn. Leaf extract: a plant from Ayurveda recognized as Dasapushpam for the management of Alzheimer's disease and diabetes mellitus

BACKGROUND

Evolvulus alsinoides (Linn.) Linn. (Convolvulaceae) is a therapeutic herb alleviating brain patterns associated with three categories of regulatory principles of the body, mind, and behaviour. In the current research, enzyme inhibition and cytotoxic potentials of E. alsinoides (L.) L. leaf extract has been studied validating its potential application.

METHODS

The plant phenolics in the leaf extracts obtained via cold-maceration with solvents viz.: n-hexane, chloroform, ethyl acetate, methanol, and water were quantitatively analyzed. The antioxidant potency was evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and Ferric Reducing Ability of Plasma (FRAP) assays at five concentrations (100-500 μg). The enzyme inhibition potential was performed with α-amylase, α-glucosidase, and acetylcholinesterase at seven concentrations (25-500 μg). The experiments were done in triplicates and statistically validated using Minitab-17 and SPSS 22.

RESULTS

Water extract contain 45.08 ± 0.02 mg GAE/g, 49.30 ± 0.07 mg GAE/g, 211.21 ± 0.02 mg QE/g tannins, phenolics, flavonoids respectively. Its antioxidant activity was supported by IC₅₀ 52.43 ± 0.2 μg/mL (DPPH assay) and 41.58 ± 0.03 (FRAP assay). Methanolic extract inhibits α-amylase with IC₅₀ 1.33 ± 0.05 μg/mL. Water extract inhibits α-glucosidase and acetylcholinesterase with IC₅₀ 3.58 ± 0.02 μg/mL and 4.46 ± 0.03 μg/mL. Cytotoxicity studies with SH-SY5Y cell-line substantiate the inhibition potential of water extract with IC₅₀ 103.0035 μg/mL.

DISCUSSION AND CONCLUSIONS

The extracts with potent antioxidant and enzyme-inhibiting activity were determined. The findings of the research are the first report about the inhibition effects of Evolvulus alsinoides (Linn.) Linn extracts against α-amylase, α-glucosidase and acetylcholinesterase. The extracts shall be examined in future studies to evaluate its pharmaceutical potential.

Dissecting the Brain/Islet Axis in Metabesity

The high prevalence of type 2 diabetes mellitus (T2DM), together with the fact that current treatments are only palliative and do not avoid major secondary complications, reveals the need for novel approaches to treat the cause of this disease. Efforts are currently underway to identify therapeutic targets implicated in either the regeneration or re-differentiation of a functional pancreatic islet β-cell mass to restore insulin levels and normoglycemia. However, T2DM is not only caused by failures in β-cells but also by dysfunctions in the central nervous system (CNS), especially in the hypothalamus and brainstem. Herein, we review the physiological contribution of hypothalamic neuronal and glial populations, particularly astrocytes, in the control of the systemic response that regulates blood glucose levels. The glucosensing capacity of hypothalamic astrocytes, together with their regulation by metabolic hormones, highlights the relevance of these cells in the control of glucose homeostasis. Moreover, the critical role of astrocytes in the response to inflammation, a process associated with obesity and T2DM, further emphasizes the importance of these cells as novel targets to stimulate the CNS in response to metabesity (over-nutrition-derived metabolic dysfunctions). We suggest that novel T2DM therapies should aim at stimulating the CNS astrocytic response, as well as recovering the functional pancreatic β-cell mass. Whether or not a common factor expressed in both cell types can be feasibly targeted is also discussed.

To eat or to sleep: That is a lateral hypothalamic question

The lateral hypothalamus (LH) is a functionally and anatomically complex brain region that is involved in the regulation of many behavioral and physiological processes including feeding, arousal, energy balance, stress, reward and motivated behaviors, pain perception, body temperature regulation, digestive functions and blood pressure. Despite noteworthy experimental efforts over the past decades, the circuit, cellular and synaptic bases by which these different processes are regulated by the LH remains incompletely understood. This knowledge gap links in large part to the high cellular heterogeneity of the LH. Fortunately, the rapid evolution of newer genetic and electrophysiological tools is now permitting the selective manipulation, typically genetically-driven, of discrete LH cell populations. This, in turn, permits not only assignment of function to discrete cell groups, but also reveals that considerable synergistic and antagonistic interactions exist between key LH cell populations that regulate feeding and arousal. For example, we now know that while LH melanin-concentrating hormone (MCH) and orexin/hypocretin neurons both function as sensors of the internal metabolic environment, their roles regulating sleep and arousal are actually opposing. Additional studies have uncovered similarly important roles for subpopulations of LH GABAergic cells in the regulation of both feeding and arousal. Herein we review the role of LH MCH, orexin/hypocretin and GABAergic cell populations in the regulation of energy homeostasis (including feeding) and sleep-wake and discuss how these three cell populations, and their subpopulations, may interact to optimize and coordinate metabolism, sleep and arousal. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Activation of c-Jun N-terminal kinase and p38 after cerebral ischemia upregulates cerebral sodium-glucose transporter type 1

Cerebral ischemic stress increases cerebral sodium-glucose transporter type 1 (SGLT-1). However, the mechanism by which cerebral ischemia leads to the up-regulation of SGLT-1 remains unclear. In peripheral tissue, the activation of mitogen-activated protein kinases (MAPKs) increases SGLT-1. MAPK pathways [c-Jun N-terminal kinase (JNK), p38 MAPK, and extracellular signal-regulated protein kinase (ERK)] are activated by cerebral ischemic stress. Therefore, we confirmed the involvement of MAPKs in the up-regulation of cerebral SGLT-1 after cerebral ischemia. Male ddY mice were subjected to middle cerebral artery occlusion (MCAO). Protein expression was assessed by western blotting. Mice received an intracerebroventricular (i.c.v.) injection of SP600125 (JNK inhibitor), SB203580 (p38 inhibitor), and PD98059 (MEK inhibitor) immediately after reperfusion. The infarction and behavioral abnormalities were assessed on days 1 and 3 after MCAO. The MAPK inhibitors suppressed the activation of JNK, p38, and ERK 3 h after MCAO. SP600125 and SB203580 administration ameliorated cerebral ischemic neuronal damage, whereas PD98059 administration exacerbated cerebral ischemic neuronal damage. SP600125 and SB203580 significantly suppressed the increase in SGLT-1 12 h after MCAO. PD98059 had no effect on SGLT-1 expression after MCAO. Our results indicate that the activation of JNK and p38 participate in the up-regulation of cerebral SGLT-1 after MCAO.

Brain Glucose-Sensing Mechanism and Energy Homeostasis

The metabolic and energy state of the organism depends largely on the availability of substrates, such as glucose for ATP production, necessary for maintaining physiological functions. Deregulation in glucose levels leads to the appearance of pathological signs that result in failures in the cardiovascular system and various diseases, such as diabetes, obesity, nephropathy, and neuropathy. Particularly, the brain relies on glucose as fuel for the normal development of neuronal activity. Regions adjacent to the cerebral ventricles, such as the hypothalamus and brainstem, exercise central control in energy homeostasis. These centers house nuclei of neurons whose excitatory activity is sensitive to changes in glucose levels. Determining the different detection mechanisms, the phenotype of neurosecretion, and neural connections involving glucose-sensitive neurons is essential to understanding the response to hypoglycemia through modulation of food intake, thermogenesis, and activation of sympathetic and parasympathetic branches, inducing glucagon and epinephrine secretion and other hypothalamic-pituitary axis-dependent counterregulatory hormones, such as glucocorticoids and growth hormone. The aim of this review focuses on integrating the current understanding of various glucose-sensing mechanisms described in the brain, thereby establishing a relationship between neuroanatomy and control of physiological processes involved in both metabolic and energy balance. This will advance the understanding of increasingly prevalent diseases in the modern world, especially diabetes, and emphasize patterns that regulate and stimulate intake, thermogenesis, and the overall synergistic effect of the neuroendocrine system.

Central regulation of food intake in fish: an evolutionary perspective

Evidence indicates that central regulation of food intake is well conserved along the vertebrate lineage, at least between teleost fish and mammals. However, several differences arise in the comparison between both groups. In this review, we describe similarities and differences between teleost fish and mammals on an evolutionary perspective. We focussed on the existing knowledge of specific fish features conditioning food intake, anatomical homologies and analogies between both groups as well as the main signalling pathways of neuroendocrine and metabolic nature involved in the homeostatic and hedonic central regulation of food intake.

Glucosensing capacity of rainbow trout telencephalon

To assess the hypothesis of glucosensing systems present in fish telencephalon, we first demonstrated in rainbow trout, by in situ hybridisation, the presence of glucokinase (GK). Then, we assessed the response of glucosensing markers in rainbow trout telencephalon 6 hours after i.c.v. treatment with glucose or 2-deoxyglucose (inducing glucoprivation). We evaluated the response of parameters related to the mechanisms dependent on GK, liver X receptor (LXR), mitochondrial activity, sweet taste receptor and sodium-glucose linked transporter 1 (SGLT-1). We also assessed mRNA abundance of neuropeptides involved in the metabolic control of food intake (agouti-related protein, neuropeptide Y, pro-opiomelanocortin, and cocaine- and amphetamine-related transcript), as well as the abundance and phosphorylation status of proteins possibly involved in linking glucosensing with neuropeptide expression, such as protein kinase B (AkT), AMP-activated protein kinase (AMPK), mechanistic target of rapamycin and cAMP response element-binding protein (CREB). The responses obtained support the presence in the telencephalon of a glucosensing mechanism based on GK and maybe one based on LXR, although they do not support the presence of mechanisms dependent on mitochondrial activity and SGLT-1. The mechanism based on sweet taste receptor responded to glucose but in a converse way to that characterised previously in the hypothalamus. In general, systems responded only to glucose but not to glucoprivation. Neuropeptides did not respond to glucose or glucoprivation. By contrast, the presence of glucose activates Akt and inhibits AMPK, CREB and forkhead box01. This is the first study in any vertebrate species in which the response to glucose of putative glucosensing mechanisms is demonstrated in the telencephalon. Their role might relate to processes other than homeostatic control of food intake, such as the hedonic and reward system.

Recent Advances in the Cellular and Molecular Mechanisms of Hypothalamic Neuronal Glucose Detection

The hypothalamus have been recognized for decades as one of the major brain centers for the control of energy homeostasis. This area contains specialized neurons able to detect changes in nutrients level. Among them, glucose-sensing neurons use glucose as a signaling molecule in addition to its fueling role. In this review we will describe the different sub-populations of glucose-sensing neurons present in the hypothalamus and highlight their nature in terms of neurotransmitter/neuropeptide expression. This review will particularly discuss whether pro-opiomelanocortin (POMC) neurons from the arcuate nucleus are directly glucose-sensing. In addition, recent observations in glucose-sensing suggest a subtle system with different mechanisms involved in the detection of changes in glucose level and their involvement in specific physiological functions. Several data point out the critical role of reactive oxygen species (ROS) and mitochondria dynamics in the detection of increased glucose. This review will also highlight that ATP-dependent potassium (K_ATP) channels are not the only channels mediating glucose-sensing and discuss the new role of transient receptor potential canonical channels (TRPC). We will discuss the recent advances in the determination of glucose-sensing machinery and propose potential line of research needed to further understand the regulation of brain glucose detection.

Nutrient Sensing: Another Chemosensitivity of the Olfactory System

Olfaction is a major sensory modality involved in real time perception of the chemical composition of the external environment. Olfaction favors anticipation and rapid adaptation of behavioral responses necessary for animal survival. Furthermore, recent studies have demonstrated that there is a direct action of metabolic peptides on the olfactory network. Orexigenic peptides such as ghrelin and orexin increase olfactory sensitivity, which in turn, is decreased by anorexigenic hormones such as insulin and leptin. In addition to peptides, nutrients can play a key role on neuronal activity. Very little is known about nutrient sensing in olfactory areas. Nutrients, such as carbohydrates, amino acids, and lipids, could play a key role in modulating olfactory sensitivity to adjust feeding behavior according to metabolic need. Here we summarize recent findings on nutrient-sensing neurons in olfactory areas and delineate the limits of our knowledge on this topic. The present review opens new lines of investigations on the relationship between olfaction and food intake, which could contribute to determining the etiology of metabolic disorders.

Hypothalamic Integration of Metabolic, Endocrine, and Circadian Signals in Fish: Involvement in the Control of Food Intake

The regulation of food intake in fish is a complex process carried out through several different mechanisms in the central nervous system (CNS) with hypothalamus being the main regulatory center. As in mammals, a complex hypothalamic circuit including two populations of neurons: one co-expressing neuropeptide Y (NPY) and Agouti-related peptide (AgRP) and the second one population co-expressing pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) is involved in the integration of information relating to food intake control. The production and release of these peptides control food intake, and the production results from the integration of information of different nature such as levels of nutrients and hormones as well as circadian signals. The present review summarizes the knowledge and recent findings about the presence and functioning of these mechanisms in fish and their differences vs. the known mammalian model.

A sweet taste receptor-dependent mechanism of glucosensing in hypothalamic tanycytes

Hypothalamic tanycytes are glial‐like glucosensitive cells that contact the cerebrospinal fluid of the third ventricle, and send processes into the hypothalamic nuclei that control food intake and body weight. The mechanism of tanycyte glucosensing remains undetermined. While tanycytes express the components associated with the glucosensing of the pancreatic β cell, they respond to nonmetabolisable glucose analogues via an ATP receptor‐dependent mechanism. Here, we show that tanycytes in rodents respond to non‐nutritive sweeteners known to be ligands of the sweet taste (Tas1r2/Tas1r3) receptor. The initial sweet tastant‐evoked response, which requires the presence of extracellular Ca²⁺, leads to release of ATP and a larger propagating Ca²⁺ response mediated by P2Y1 receptors. In Tas1r2 null mice the proportion of glucose nonresponsive tanycytes was greatly increased in these mice, but a subset of tanycytes retained an undiminished sensitivity to glucose. Our data demonstrate that the sweet taste receptor mediates glucosensing in about 60% of glucosensitive tanycytes while the remaining 40% of glucosensitive tanycytes use some other, as yet unknown mechanism.

Oleate induces KATP channel-dependent hyperpolarization in mouse hypothalamic glucose-excited neurons without altering cellular energy charge

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Oleate and low glucose hyperpolarize and inhibit GT1-7 and mouse GE neurons by activation of K_ATP.

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Oleate inhibition of GT1-7 neuron activity is not mediated by AMPK or fatty acid oxidation.

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Activation of K_ATP by oleate requires ATP hydrolysis but does not reduce the levels ATP or the ATP:ADP ratio.

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GT1-7 hyperpolarization by oleate is not dependent on UCP2.

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Oleate and low glucose depolarize a subpopulation of hypothalamic GI neurons.

The unsaturated fatty acid, oleate exhibits anorexigenic properties reducing food intake and hepatic glucose output. However, its mechanism of action in the hypothalamus has not been fully determined. This study investigated the effects of oleate and glucose on GT1-7 mouse hypothalamic cells (a model of glucose-excited (GE) neurons) and mouse arcuate nucleus (ARC) neurons. Whole-cell and perforated patch-clamp recordings, immunoblotting and cell energy status measures were used to investigate oleate- and glucose-sensing properties of mouse hypothalamic neurons. Oleate or lowered glucose concentration caused hyperpolarization and inhibition of firing of GT1-7 cells by the activation of ATP-sensitive K⁺ channels (K_ATP). This effect of oleate was not dependent on fatty acid oxidation or raised AMP-activated protein kinase activity or prevented by the presence of the UCP2 inhibitor genipin. Oleate did not alter intracellular calcium, indicating that CD36/fatty acid translocase may not play a role. However, oleate activation of K_ATP may require ATP metabolism. The short-chain fatty acid octanoate was unable to replicate the actions of oleate on GT1-7 cells. Although oleate decreased GT1-7 cell mitochondrial membrane potential there was no change in total cellular ATP or ATP/ADP ratios. Perforated patch and whole-cell recordings from mouse hypothalamic slices demonstrated that oleate hyperpolarized a subpopulation of ARC GE neurons by K_ATP activation. Additionally, in a separate small population of ARC neurons, oleate application or lowered glucose concentration caused membrane depolarization. In conclusion, oleate induces K_ATP-dependent hyperpolarization and inhibition of firing of a subgroup of GE hypothalamic neurons without altering cellular energy charge.

Nutrient Sensing Systems in Fish: Impact on Food Intake Regulation and Energy Homeostasis

Evidence obtained in recent years in a few species, especially rainbow trout, supports the presence in fish of nutrient sensing mechanisms. Glucosensing capacity is present in central (hypothalamus and hindbrain) and peripheral [liver, Brockmann bodies (BB, main accumulation of pancreatic endocrine cells in several fish species), and intestine] locations whereas fatty acid sensors seem to be present in hypothalamus, liver and BB. Glucose and fatty acid sensing capacities relate to food intake regulation and metabolism in fish. Hypothalamus is as a signaling integratory center in a way that detection of increased levels of nutrients result in food intake inhibition through changes in the expression of anorexigenic and orexigenic neuropeptides. Moreover, central nutrient sensing modulates functions in the periphery since they elicit changes in hepatic metabolism as well as in hormone secretion to counter-regulate changes in nutrient levels detected in the CNS. At peripheral level, the direct nutrient detection in liver has a crucial role in homeostatic control of glucose and fatty acid whereas in BB and intestine nutrient sensing is probably involved in regulation of hormone secretion from endocrine cells.

Sodium transport through the cerebral sodium-glucose transporter exacerbates neuron damage during cerebral ischaemia

OBJECTIVES

We recently demonstrated that the cerebral sodium-glucose transporter (SGLT) is involved in postischaemic hyperglycaemia-induced exacerbation of cerebral ischaemia. However, the associated SGLT-mediated mechanisms remain unclear. Thus, we examined the involvement of cerebral SGLT-induced excessive sodium ion influx in the development of cerebral ischaemic neuronal damage.

METHODS

[Na+]i was estimated according to sodium-binding benzofuran isophthalate fluorescence. In the in vitro study, primary cortical neurons were prepared from fetuses of ddY mice. Primary cortical neurons were cultured for 5 days before each treatment with reagents, and these survival rates were assessed using biochemical assays. In in vivo study, a mouse model of focal ischaemia was generated using middle cerebral artery occlusion (MCAO).

KEY FINDINGS

In these experiments, treatment with high concentrations of glucose induced increment in [Na+]i, and this phenomenon was suppressed by the SGLT-specific inhibitor phlorizin. SGLT-specific sodium ion influx was induced using a-methyl-D-glucopyranoside (a-MG) treatments, which led to significant concentration-dependent declines in neuronal survival rates and exacerbated hydrogen peroxide-induced neuronal cell death. Moreover, phlorizin ameliorated these effects. Finally, intracerebroventricular administration of a-MG exacerbated the development of neuronal damage induced by MCAO, and these effects were ameliorated by the administration of phlorizin.

CONCLUSIONS

Hence, excessive influx of sodium ions into neuronal cells through cerebral SGLT may exacerbate the development of cerebral ischaemic neuronal damage.

The effect of diet interventions on hypothalamic nutrient sensing pathways in rodents

The hypothalamus plays a fundamental role in regulating homeostatic processes including regulation of food intake. Food intake is driven in part by energy balance, which is sensed by specific brain structures through signaling molecules such as nutrients and hormones. Both circulating glucose and fatty acids decrease food intake via a central mechanism involving the hypothalamus and brain stem. Besides playing a role in signaling energy status, glucose and fatty acids serve as fuel for neurons. This review focuses on the effects of glucose and fatty acids on hypothalamic pathways involved in regulation of energy metabolism as well as on the role of the family of peroxisome proliferator activated receptors (PPARs) which are implicated in regulation of central energy homeostasis. We further discuss the effects of different hypercaloric diets on these pathways.

In vitro evidence supports the presence of glucokinase-independent glucosensing mechanisms in hypothalamus and hindbrain of rainbow trout

We previously obtained evidence in rainbow trout for the presence and response to changes in circulating levels of glucose (induced by intraperitoneal hypoglycaemic and hyperglycaemic treatments) of glucosensing mechanisms based on liver X receptor (LXR), mitochondrial production of reactive oxygen species (ROS) leading to increased expression of uncoupling protein 2 (UCP2), and sweet taste receptor in the hypothalamus, and on sodium/glucose co-transporter 1 (SGLT-1) in hindbrain. However, these effects of glucose might be indirect. Therefore, we evaluated the response of parameters related to these glucosensing mechanisms in a first experiment using pooled sections of hypothalamus and hindbrain incubated for 6 h at 15°C in modified Hanks' medium containing 2, 4 or 8 mmol l(-1) d-glucose. The responses observed in some cases were consistent with glucosensing capacity. In a second experiment, pooled sections of hypothalamus and hindbrain were incubated for 6 h at 15°C in modified Hanks' medium with 8 mmol l(-1) d-glucose alone (control) or containing 1 mmol l(-1) phloridzin (SGLT-1 antagonist), 20 µmol l(-1) genipin (UCP2 inhibitor), 1 µmol l(-1) trolox (ROS scavenger), 100 µmol l(-1) bezafibrate (T1R3 inhibitor) and 50 µmol l(-1) geranyl-geranyl pyrophosphate (LXR inhibitor). The response observed in the presence of these specific inhibitors/antagonists further supports the proposal that critical components of the different glucosensing mechanisms are functioning in rainbow trout hypothalamus and hindbrain.

Emerging role of the brain in the homeostatic regulation of energy and glucose metabolism

Accumulated evidence from genetic animal models suggests that the brain, particularly the hypothalamus, has a key role in the homeostatic regulation of energy and glucose metabolism. The brain integrates multiple metabolic inputs from the periphery through nutrients, gut-derived satiety signals and adiposity-related hormones. The brain modulates various aspects of metabolism, such as food intake, energy expenditure, insulin secretion, hepatic glucose production and glucose/fatty acid metabolism in adipose tissue and skeletal muscle. Highly coordinated interactions between the brain and peripheral metabolic organs are critical for the maintenance of energy and glucose homeostasis. Defective crosstalk between the brain and peripheral organs contributes to the development of obesity and type 2 diabetes. Here we comprehensively review the above topics, discussing the main findings related to the role of the brain in the homeostatic regulation of energy and glucose metabolism.

Glucose Induces Slow-Wave Sleep by Exciting the Sleep-Promoting Neurons in the Ventrolateral Preoptic Nucleus: A New Link between Sleep and Metabolism

Sleep-active neurons located in the ventrolateral preoptic nucleus (VLPO) play a crucial role in the induction and maintenance of slow-wave sleep (SWS). However, the cellular and molecular mechanisms responsible for their activation at sleep onset remain poorly understood. Here, we test the hypothesis that a rise in extracellular glucose concentration in the VLPO can promote sleep by increasing the activity of sleep-promoting VLPO neurons. We find that infusion of a glucose concentration into the VLPO of mice promotes SWS and increases the density of c-Fos-labeled neurons selectively in the VLPO. Moreover, we show in patch-clamp recordings from brain slices that VLPO neurons exhibiting properties of sleep-promoting neurons are selectively excited by glucose within physiological range. This glucose-induced excitation implies the catabolism of glucose, leading to a closure of ATP-sensitive potassium (KATP) channels. The extracellular glucose concentration monitors the gating of KATP channels of sleep-promoting neurons, highlighting that these neurons can adapt their excitability according to the extracellular energy status. Together, these results provide evidence that glucose may participate in the mechanisms of SWS promotion and/or consolidation.

SIGNIFICANCE STATEMENT

Although the brain circuitry underlying vigilance states is well described, the molecular mechanisms responsible for sleep onset remain largely unknown. Combining in vitro and in vivo experiments, we demonstrate that glucose likely contributes to sleep onset facilitation by increasing the excitability of sleep-promoting neurons in the ventrolateral preoptic nucleus (VLPO). We find here that these neurons integrate energetic signals such as ambient glucose directly to regulate vigilance states accordingly. Glucose-induced excitation of sleep-promoting VLPO neurons should therefore be involved in the drowsiness that one feels after a high-sugar meal. This novel mechanism regulating the activity of VLPO neurons reinforces the fundamental and intimate link between sleep and metabolism.

Hindbrain Catecholamine Neurons Activate Orexin Neurons During Systemic Glucoprivation in Male Rats

Hindbrain catecholamine neurons are required for elicitation of feeding responses to glucose deficit, but the forebrain circuitry required for these responses is incompletely understood. Here we examined interactions of catecholamine and orexin neurons in eliciting glucoprivic feeding. Orexin neurons, located in the perifornical lateral hypothalamus (PeFLH), are heavily innervated by hindbrain catecholamine neurons, stimulate food intake, and increase arousal and behavioral activation. Orexin neurons may therefore contribute importantly to appetitive responses, such as food seeking, during glucoprivation. Retrograde tracing results showed that nearly all innervation of the PeFLH from the hindbrain originated from catecholamine neurons and some raphe nuclei. Results also suggested that many catecholamine neurons project collaterally to the PeFLH and paraventricular hypothalamic nucleus. Systemic administration of the antiglycolytic agent, 2-deoxy-D-glucose, increased food intake and c-Fos expression in orexin neurons. Both responses were eliminated by a lesion of catecholamine neurons innervating orexin neurons using the retrogradely transported immunotoxin, anti-dopamine-β-hydroxylase saporin, which is specifically internalized by dopamine-β-hydroxylase-expressing catecholamine neurons. Using designer receptors exclusively activated by designer drugs in transgenic rats expressing Cre recombinase under the control of tyrosine hydroxylase promoter, catecholamine neurons in cell groups A1 and C1 of the ventrolateral medulla were activated selectively by peripheral injection of clozapine-N-oxide. Clozapine-N-oxide injection increased food intake and c-Fos expression in PeFLH orexin neurons as well as in paraventricular hypothalamic nucleus neurons. In summary, catecholamine neurons are required for the activation of orexin neurons during glucoprivation. Activation of orexin neurons may contribute to appetitive responses required for glucoprivic feeding.

The role of tanycytes in hypothalamic glucosensing

Tanycytes are elongated hypothalamic glial cells that cover the basal walls of the third ventricle; their apical regions contact the cerebrospinal fluid (CSF), and their processes reach hypothalamic neuronal nuclei that control the energy status of an organism. These nuclei maintain the balance between energy expenditure and intake, integrating several peripheral signals and triggering cellular responses that modify the feeding behaviour and peripheral glucose homeostasis. One of the most important and well-studied signals that control this process is glucose; however, the mechanism by which this molecule is sensed remains unknown. We along with others have proposed that tanycytes play a key role in this process, transducing changes in CSF glucose concentration to the neurons that control energy status. Recent studies have demonstrated the expression and function of monocarboxylate transporters and canonical pancreatic β cell glucose sensing molecules, including glucose transporter 2 and glucokinase, in tanycytes. These and other data, which will be discussed in this review, suggest that hypothalamic glucosensing is mediated through a metabolic interaction between tanycytes and neurons through lactate. This article will summarize the recent evidence that supports the importance of tanycytes in hypothalamic glucosensing, and discuss the possible mechanisms involved in this process. Finally, it is important to highlight that a detailed analysis of this mechanism could represent an opportunity to understand the evolution of associated pathologies, including diabetes and obesity, and identify new candidates for therapeutic intervention.

Orexin-A enhances feeding in male rats by activating hindbrain catecholamine neurons

Both lateral hypothalamic orexinergic neurons and hindbrain catecholaminergic neurons contribute to control of feeding behavior. Orexin fibers and terminals are present in close proximity to hindbrain catecholaminergic neurons, and fourth ventricular (4V) orexin injections that increase food intake also increase c-Fos expression in hindbrain catecholamine neurons, suggesting that orexin neurons may stimulate feeding by activating catecholamine neurons. Here we examine that hypothesis in more detail. We found that 4V injection of orexin-A (0.5 nmol/rat) produced widespread activation of c-Fos in hindbrain catecholamine cell groups. In the A1 and C1 cell groups in the ventrolateral medulla, where most c-Fos-positive neurons were also dopamine β hydroxylase (DBH) positive, direct injections of a lower dose (67 pmol/200 nl) of orexin-A also increased food intake in intact rats. Then, with the use of the retrogradely transported immunotoxin, anti-DBH conjugated to saporin (DSAP), which targets and destroys DBH-expressing catecholamine neurons, we examined the hypothesis that catecholamine neurons are required for orexin-induced feeding. Rats given paraventricular hypothalamic injections of DSAP, or unconjugated saporin (SAP) as control, were implanted with 4V or lateral ventricular (LV) cannulas and tested for feeding in response to ventricular injection of orexin-A (0.5 nmol/rat). Both LV and 4V orexin-A stimulated feeding in SAP controls, but DSAP abolished these responses. These results reveal for the first time that catecholamine neurons are required for feeding induced by injection of orexin-A into either LV or 4V.

Evidence for the Presence of Glucosensor Mechanisms Not Dependent on Glucokinase in Hypothalamus and Hindbrain of Rainbow Trout (Oncorhynchus mykiss)

We hypothesize that glucosensor mechanisms other than that mediated by glucokinase (GK) operate in hypothalamus and hindbrain of the carnivorous fish species rainbow trout and stress affected them. Therefore, we evaluated in these areas changes in parameters which could be related to putative glucosensor mechanisms based on liver X receptor (LXR), mitochondrial activity, sweet taste receptor, and sodium/glucose co-transporter 1 (SGLT-1) 6h after intraperitoneal injection of 5 mL.Kg^-1 of saline solution alone (normoglycaemic treatment) or containing insulin (hypoglycaemic treatment, 4 mg bovine insulin.Kg^-1 body mass), or D-glucose (hyperglycaemic treatment, 500 mg.Kg^-1 body mass). Half of tanks were kept at a 10 Kg fish mass.m^-3 and denoted as fish under normal stocking density (NSD) whereas the remaining tanks were kept at a stressful high stocking density (70 kg fish mass.m^-3) denoted as HSD. The results obtained in non-stressed rainbow trout provide evidence, for the first time in fish, that manipulation of glucose levels induce changes in parameters which could be related to putative glucosensor systems based on LXR, mitochondrial activity and sweet taste receptor in hypothalamus, and a system based on SGLT-1 in hindbrain. Stress altered the response of parameters related to these systems to changes in glycaemia.

GLUT2, glucose sensing and glucose homeostasis

The glucose transporter isoform GLUT2 is expressed in liver, intestine, kidney and pancreatic islet beta cells, as well as in the central nervous system, in neurons, astrocytes and tanycytes. Physiological studies of genetically modified mice have revealed a role for GLUT2 in several regulatory mechanisms. In pancreatic beta cells, GLUT2 is required for glucose-stimulated insulin secretion. In hepatocytes, suppression of GLUT2 expression revealed the existence of an unsuspected glucose output pathway that may depend on a membrane traffic-dependent mechanism. GLUT2 expression is nevertheless required for the physiological control of glucose-sensitive genes, and its inactivation in the liver leads to impaired glucose-stimulated insulin secretion, revealing a liver-beta cell axis, which is likely to be dependent on bile acids controlling beta cell secretion capacity. In the nervous system, GLUT2-dependent glucose sensing controls feeding, thermoregulation and pancreatic islet cell mass and function, as well as sympathetic and parasympathetic activities. Electrophysiological and optogenetic techniques established that Glut2 (also known as Slc2a2)-expressing neurons of the nucleus tractus solitarius can be activated by hypoglycaemia to stimulate glucagon secretion. In humans, inactivating mutations in GLUT2 cause Fanconi-Bickel syndrome, which is characterised by hepatomegaly and kidney disease; defects in insulin secretion are rare in adult patients, but GLUT2 mutations cause transient neonatal diabetes. Genome-wide association studies have reported that GLUT2 variants increase the risks of fasting hyperglycaemia, transition to type 2 diabetes, hypercholesterolaemia and cardiovascular diseases. Individuals with a missense mutation in GLUT2 show preference for sugar-containing foods. We will discuss how studies in mice help interpret the role of GLUT2 in human physiology.

Electrical properties of kisspeptin neurons and their regulation of GnRH neurons

Kisspeptin neurons are critical components of the neuronal network controlling the activity of the gonadotropin-releasing hormone (GnRH) neurons. A variety of genetically-manipulated mouse models have recently facilitated the study of the electrical activity of the two principal kisspeptin neuron populations located in the rostral periventricular area of the third ventricle (RP3V) and arcuate nucleus (ARN) in acute brain slices. We discuss here the mechanisms and pathways through which kisspeptin neurons regulate GnRH neuron activity. We then examine the different kisspeptin-green fluorescent protein mouse models being used for kisspeptin electrophysiology and the data obtained to date for RP3V and ARN kisspeptin neurons. In light of these new observations on the spontaneous firing rates, intrinsic membrane properties, and neurotransmitter regulation of kisspeptin neurons, we speculate on the physiological roles of the different kisspeptin populations.

The orexin neuropeptide system: physical activity and hypothalamic function throughout the aging process

There is a rising medical need for novel therapeutic targets of physical activity. Physical activity spans from spontaneous, low intensity movements to voluntary, high-intensity exercise. Regulation of spontaneous and voluntary movement is distributed over many brain areas and neural substrates, but the specific cellular and molecular mechanisms responsible for mediating overall activity levels are not well understood. The hypothalamus plays a central role in the control of physical activity, which is executed through coordination of multiple signaling systems, including the orexin neuropeptides. Orexin producing neurons integrate physiological and metabolic information to coordinate multiple behavioral states and modulate physical activity in response to the environment. This review is organized around three questions: (1) How do orexin peptides modulate physical activity? (2) What are the effects of aging and lifestyle choices on physical activity? (3) What are the effects of aging on hypothalamic function and the orexin peptides? Discussion of these questions will provide a summary of the current state of knowledge regarding hypothalamic orexin regulation of physical activity during aging and provide a platform on which to develop improved clinical outcomes in age-associated obesity and metabolic syndromes.

Orexin-A controls sympathetic activity and eating behavior

It is extremely important for the health to understand the regulatory mechanisms of energy expenditure. These regulatory mechanisms play a central role in the pathogenesis of body weight alteration. The hypothalamus integrates nutritional information derived from all peripheral organs. This region of the brain controls hormonal secretions and neural pathways of the brainstem. Orexin-A is a hypothalamic neuropeptide involved in the regulation of feeding behavior, sleep-wakefulness rhythm, and neuroendocrine homeostasis. This neuropeptide is involved in the control of the sympathetic activation, blood pressure, metabolic status, and blood glucose level. This minireview focuses on relationship between the sympathetic nervous system and orexin-A in the control of eating behavior and energy expenditure. The “thermoregulatory hypothesis” of food intake is analyzed, underlining the role played by orexin-A in the control of food intake related to body temperature. Furthermore, the paradoxical eating behavior induced orexin-A is illustrated in this minireview.

Contribution of glucose- and fatty acid sensing systems to the regulation of food intake in fish. A review

Food intake in fish is a complex process regulated through many different factors including abundance of energy and nutrients. In recent years, evidence have been obtained in several fishes, mainly in rainbow trout, regarding the presence and functioning in brain areas of metabolic sensors informing about changes in the levels of nutrients like glucose and fatty acids. The activity of these sensors relate to the control of food intake through changes in the expression of anorexigenic and orexigenic neuropeptides. The present review will provide a picture of the main results obtained to date in these studies, as well as perspectives for future research in the field.

CART in the brain of vertebrates: circuits, functions and evolution

Cocaine- and amphetamine-regulated transcript peptide (CART) with its wide distribution in the brain of mammals has been the focus of considerable research in recent years. Last two decades have witnessed a steady rise in the information on the genes that encode this neuropeptide and regulation of its transcription and translation. CART is highly enriched in the hypothalamic nuclei and its relevance to energy homeostasis and neuroendocrine control has been understood in great details. However, the occurrence of this peptide in a range of diverse circuitries for sensory, motor, vegetative, limbic and higher cortical areas has been confounding. Evidence that CART peptide may have role in addiction, pain, reward, learning and memory, cognition, sleep, reproduction and development, modulation of behavior and regulation of autonomic nervous system are accumulating, but an integration has been missing. A steady stream of papers has been pointing at the therapeutic potentials of CART. The current review is an attempt at piecing together the fragments of available information, and seeks meaning out of the CART elements in their anatomical niche. We try to put together the CART containing neuronal circuitries that have been conclusively demonstrated as well as those which have been proposed, but need confirmation. With a view to finding out the evolutionary antecedents, we visit the CART systems in sub-mammalian vertebrates and seek the answer why the system is shaped the way it is. We enquire into the conservation of the CART system and appreciate its functional diversity across the phyla.

Hypoglycemia-activated GLUT2 neurons of the nucleus tractus solitarius stimulate vagal activity and glucagon secretion

Glucose-sensing neurons in the brainstem participate in the regulation of energy homeostasis but have been poorly characterized because of the lack of specific markers to identify them. Here we show that GLUT2-expressing neurons of the nucleus of the tractus solitarius form a distinct population of hypoglycemia-activated neurons. Their response to low glucose is mediated by reduced intracellular glucose metabolism, increased AMP-activated protein kinase activity, and closure of leak K(+) channels. These are GABAergic neurons that send projections to the vagal motor nucleus. Light-induced stimulation of channelrhodospin-expressing GLUT2 neurons in vivo led to increased parasympathetic nerve firing and glucagon secretion. Thus GLUT2 neurons of the nucleus tractus solitarius link hypoglycemia detection to counterregulatory response. These results may help identify the cause of hypoglycemia-associated autonomic failure, a major threat in the insulin treatment of diabetes.

Stimulation of feeding by three different glucose-sensing mechanisms requires hindbrain catecholamine neurons

Previous work has shown that hindbrain catecholamine neurons are required components of the brain's glucoregulatory circuitry. However, the mechanisms and circuitry underlying their glucoregulatory functions are poorly understood. Here we examined three drugs, glucosamine (GcA), phloridzin (Phl) and 5-thio-d-glucose (5TG), that stimulate food intake but interfere in different ways with cellular glucose utilization or transport. We examined feeding and blood glucose responses to each drug in male rats previously injected into the hypothalamic paraventricular nucleus with anti-dopamine-β-hydroxylase conjugated to saporin (DSAP), a retrogradely transported immunotoxin that selectively lesions noradrenergic and adrenergic neurons, or with unconjugated saporin (SAP) control. Our major findings were 1) that GcA, Phl, and 5TG all stimulated feeding in SAP controls whether injected into the lateral or fourth ventricle (LV or 4V), 2) that each drug's potency was similar for both LV and 4V injections, 3) that neither LV or 4V injection of these drugs evoked feeding in DSAP-lesioned rats, and 4) that only 5TG, which blocks glycolysis, stimulated a blood glucose response. The antagonist of the MEK/ERK signaling cascade, U0126, attenuated GcA-induced feeding, but not Phl- or 5TG-induced feeding. Thus GcA, Phl, and 5TG, although differing in mechanism and possibly activating different neural populations, stimulate feeding in a catecholamine-dependent manner. Although results do not exclude the possibility that catecholamine neurons possess glucose-sensing mechanisms responsive to all of these agents, currently available evidence favors the possibility that the feeding effects result from convergent neural circuits in which catecholamine neurons are a required component.

Post-oral appetite stimulation by sugars and nonmetabolizable sugar analogs

Post-oral sugar actions enhance the intake of and preference for sugar-rich foods, a process referred to as appetition. Here, we investigated the role of intestinal sodium glucose cotransporters (SGLTs) in sugar appetition in C57BL/6J mice using sugars and nonmetabolizable sugar analogs that differ in their affinity for SGLT1 and SGLT3. In experiments 1 and 2, food-restricted mice were trained (1 h/day) to consume a flavored saccharin solution [conditioned stimulus (CS-)] paired with intragastric (IG) self-infusions of water and a different flavored solution (CS+) paired with infusions of 8 or 12% sugars (glucose, fructose, and galactose) or sugar analogs (α-methyl-D-glucopyranoside, MDG; 3-O-methyl-D-glucopyranoside, OMG). Subsequent two-bottle CS+ vs. CS- choice tests were conducted without coinfusions. Infusions of the SGLT1 ligands glucose, galactose, MDG, and OMG stimulated CS+ licking above CS- levels. However, only glucose, MDG, and galactose conditioned significant CS+ preferences, with the SGLT3 ligands (glucose, MDG) producing the strongest preferences. Fructose, which is not a ligand for SGLTs, failed to stimulate CS+ intake or preference. Experiment 3 revealed that IG infusion of MDG+phloridzin (an SGLT1/3 antagonist) blocked MDG appetition, whereas phloridzin had minimal effects on glucose-induced appetition. However, adding phloretin (a GLUT2 antagonist) to the glucose+phloridzin infusion blocked glucose appetition. Taken together, these findings suggest that humoral signals generated by intestinal SGLT1 and SGLT3, and to a lesser degree, GLUT2, mediate post-oral sugar appetition in mice. The MDG results indicate that sugar metabolism is not essential for the post-oral intake-stimulating and preference-conditioning actions of sugars in mice.