Brain glucose sensing and body energy homeostasis: role in obesity and diabetes

Exploring metabolomic dynamics in acute stress disorder: amino acids, lipids, and carbohydrates

Acute Stress Disorder (ASD) is a psychiatric condition that can develop shortly after trauma exposure. Although molecular studies of ASD are only beginning, groups of metabolites have been found to be significantly altered with acute stress phenotypes in various pre-clinical and clinical studies. ASD implicated metabolites include amino acids (β-hydroxybutyrate, glutamate, 5-aminovalerate, kynurenine and aspartate), ketone bodies (β-hydroxybutyrate), lipids (cortisol, palmitoylethanomide, and N-palmitoyl taurine) and carbohydrates (glucose and mannose). Network and pathway analysis with the most prominent metabolites shows that Extracellular signal-regulated kinases and c-AMP response element binding (CREB) protein can be crucial players. After highlighting main recent findings on the role of metabolites in ASD, we will discuss potential future directions and challenges that need to be tackled. Overall, we aim to showcase that metabolomics present a promising opportunity to advance our understanding of ASD pathophysiology as well as the development of novel biomarkers and therapeutic targets.

Investigating the role of an immediate early gene FOS as a potential regulator of autophagic response to hypoglycemia in embryonic hypothalamic neurons

Hypoglycemia-associated autonomic failure (HAAF) is a well-established complication of diabetes. Although HAAF has serious outcomes such as recurrent morbidity, coma, and death, the mechanisms of HAAF and its pathological components are largely unknown. Our previous studies have revealed that hypoglycemia is associated with the upregulation of an immediate early gene - FOS. In addition, it is documented that glucose deprivation activates neuronal autophagic activities. Therefore, the present study aimed to identify the role of FOS and one of the core components of the autophagy pathway, Beclin-1 (encoded by the BECN1 gene), in the regulation of autophagic mechanisms in embryonic hypothalamic neurons in response to hypoglycemic conditions. Embryonic Mouse Hypothalamic Cell Line N39 (mHypoE-N39 or N39) was cultured in reduced concentrations of glucose (2000, 900, 500, and 200 mg/L). Gene and protein expression, as well as immunofluorescence studies on autophagy were conducted under different reduced glucose concentrations in N39 hypothalamic neurons with and without FOS and BECN1 gene knockdowns (KD). The outcomes of the present study have demonstrated a significant increase in autophagosome formation and subsequent lysosomal degradation in the hypothalamic neurons in response to reduced glucose concentrations. This hypoglycemic response appears to be lowered to a similar extent in the FOS KD and BECN1 KD cells, albeit insignificantly from the negative control, is indicative of the involvement of FOS in the autophagic response of hypothalamic neurons to hypoglycemia. Moreover, the KD cells exhibited a change in morphology and reduced cell viability compared with the control cells. Our findings suggest that reduced FOS expression could potentially be associated with impaired autophagic activities that are dependent on BECN1, which could lead to decreased or blunted hypothalamic activation in response to hypoglycemia, and this, in turn, may contribute to the development of HAAF.

Hypothalamic steroid receptor coactivator-2 regulates adaptations to fasting and overnutrition

The neuroendocrine system coordinates metabolic and behavioral adaptations to fasting, including reducing energy expenditure, promoting counterregulation, and suppressing satiation and anxiety to engage refeeding. Here, we show that steroid receptor coactivator-2 (SRC-2) in pro-opiomelanocortin (POMC) neurons is a key regulator of all these responses to fasting. POMC-specific deletion of SRC-2 enhances the basal excitability of POMC neurons; mutant mice fail to efficiently suppress energy expenditure during food deprivation. SRC-2 deficiency blunts electric responses of POMC neurons to glucose fluctuations, causing impaired counterregulation. When food becomes available, these mutant mice show insufficient refeeding associated with enhanced satiation and discoordination of anxiety and food-seeking behavior. SRC-2 coactivates Forkhead box protein O1 (FoxO1) to suppress POMC gene expression. POMC-specific deletion of SRC-2 protects mice from weight gain induced by an obesogenic diet feeding and/or FoxO1 overexpression. Collectively, we identify SRC-2 as a key molecule that coordinates multifaceted adaptive responses to food shortage.

E. coli Nissle 1917 modulates host glucose metabolism without directly acting on glucose

Managing postprandial glycemic response, or the increase in blood sugar following a meal, is a crucial component to maintaining healthy blood sugar in patients with diabetes. To test whether oral probiotics can impact postprandial glycemic response, E. coli Nissle 1917 (EcN) was evaluated in an oral glucose tolerance test. Oral gavage of EcN concurrent with a glucose bolus reduced the post-gavage glycemic response in mice. However, there was no difference in glycemic response when comparing EcN to a mutant deficient in glucose metabolism. This suggests that while EcN can alter glycemic response to a glucose bolus, this effect is not mediated by direct uptake of glucose. Of the possible indirect effects EcN could have, gastric emptying rate was highlighted as a likely cause, but EcN had no effect on gastric emptying rate in mice. This leaves many more possible indirect explanations for the interaction between EcN and host glucose metabolism to be explored in future work.

Hypothalamic detection of macronutrients via multiple gut-brain pathways

Food intake is tightly regulated by complex and coordinated gut-brain interactions. Nutrients rapidly modulate activity in key populations of hypothalamic neurons that regulate food intake, including hunger-sensitive agouti-related protein (AgRP)-expressing neurons. Because individual macronutrients engage specific receptors in the gut to communicate with the brain, we reasoned that macronutrients may utilize different pathways to reduce activity in AgRP neurons. Here, we revealed that AgRP neuron activity in hungry mice is inhibited by site-specific intestinal detection of different macronutrients. We showed that vagal gut-brain signaling is required for AgRP neuron inhibition by fat. In contrast, spinal gut-brain signaling relays the presence of intestinal glucose. Further, we identified glucose sensors in the intestine and hepatic portal vein that mediate glucose-dependent AgRP neuron inhibition. Therefore, distinct pathways are activated by individual macronutrients to inhibit AgRP neuron activity.

17β-estradiol promotes acute refeeding in hungry mice via membrane-initiated ERα signaling

Objective

Estrogen protects animals from obesity through estrogen receptor α (ERα), partially by inhibiting overeating in animals fed ad libitum. However, the effects of estrogen on feeding behavior in hungry animals remain unclear. In this study, we examined the roles of 17β-estradiol (E2) and ERα in the regulation of feeding in hungry female animals and explored the underlying mechanisms.

Methods

Wild-type female mice with surgical depletion of endogenous estrogens were used to examine the effects of E2 supplementation on acute refeeding behavior after starvation. ERα-C451A mutant mice deficient in membrane-bound ERα activity and ERα-AF2⁰ mutant mice lacking ERα transcriptional activity were used to further examine mechanisms underlying acute feeding triggered by either fasting or central glucopenia (induced by intracerebroventricular injections of 2-deoxy-D-glucose). We also used electrophysiology to explore the impact of these ERα mutations on the neural activities of ERα neurons in the hypothalamus.

Results

In the wild-type female mice, ovariectomy reduced fasting-induced refeeding, which was restored by E2 supplementation. The ERα-C451A mutation, but not the ERα-AF2⁰ mutation, attenuated acute feeding induced by either fasting or central glucopenia. The ERα-C451A mutation consistently impaired the neural responses of hypothalamic ERα neurons to hypoglycemia.

Conclusion

In addition to previous evidence that estrogen reduces deviations in energy balance by inhibiting eating at a satiated state, our findings demonstrate the unexpected role of E2 that promotes eating in hungry mice, also contributing to the stability of energy homeostasis. This latter effect specifically requires membrane-bound ERα activity.

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Endogenous E2 is required to maintain acute refeeding in hungry female mice after starvation.

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Membrane-bound ERα activity in female mice is required for efficient refeeding after starvation.

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Membrane-bound ERα activity is required for hypothalamic ERα neurons to respond to hypoglycemia.

Dishonesty is more affected by BMI status than by short-term changes in glucose

There is evidence that human decision-making is affected by current body energy levels and physiological states. There is less clear evidence linking decision-making to long-term changes in energy, as those associated with obesity. We explore the link between energy, obesity and dishonesty by comparing the behaviour of obese and lean subjects when hungry or sated while playing an anonymous die-under-cup task. Participants performed the task either before or after breakfast. We find that short-term switches in energy have only a mild effect on dishonesty, as only lean females lie less when sated. By contrast, obese subjects lie more than lean subjects in both conditions, and they lie more to avoid the lowest payoff than to get the highest payoff. Our findings suggest that the observed patterns are more likely mediated by factors associated with obesity than by short term energy dynamics, and call for a better integration of the psychological, economic and biological drivers of moral behaviour.

Estrogen receptor-α expressing neurons in the ventrolateral VMH regulate glucose balance

Brain glucose-sensing neurons detect glucose fluctuations and prevent severe hypoglycemia, but mechanisms mediating functions of these glucose-sensing neurons are unclear. Here we report that estrogen receptor-α (ERα)-expressing neurons in the ventrolateral subdivision of the ventromedial hypothalamic nucleus (vlVMH) can sense glucose fluctuations, being glucose-inhibited neurons (GI-ERαvlVMH) or glucose-excited neurons (GE-ERαvlVMH). Hypoglycemia activates GI-ERαvlVMH neurons via the anoctamin 4 channel, and inhibits GE-ERαvlVMH neurons through opening the ATP-sensitive potassium channel. Further, we show that GI-ERαvlVMH neurons preferentially project to the medioposterior arcuate nucleus of the hypothalamus (mpARH) and GE-ERαvlVMH neurons preferentially project to the dorsal Raphe nuclei (DRN). Activation of ERαvlVMH to mpARH circuit and inhibition of ERαvlVMH to DRN circuit both increase blood glucose. Thus, our results indicate that ERαvlVMH neurons detect glucose fluctuations and prevent severe hypoglycemia in mice.

The effect of adolescent inhalant abuse on energy balance and growth

The abuse of volatile solvents such as toluene is a significant public health concern, predominantly affecting adolescents. To date, inhalant abuse research has primarily focused on the central nervous system; however, inhalants also exert effects on other organ systems and processes, including metabolic function and energy balance. Adolescent inhalant abuse is characterized by a negative energy balance phenotype, with the peak period of abuse overlapping with the adolescent growth spurt. There are multiple components within the central and peripheral regulation of energy balance that may be affected by adolescent inhalant abuse, such as impaired metabolic signaling, decreased food intake, altered dietary preferences, disrupted glucose tolerance and insulin release, reduced adiposity and skeletal density, and adrenal hypertrophy. These effects may persist into abstinence and adulthood, and the long-term consequences of inhalant-induced metabolic dysfunction are currently unknown. The signs and symptoms resulting from chronic adolescent inhalant abuse may result in a propensity for the development of adult-onset metabolic disorders such as type 2 diabetes, however, further research investigating the long-term effects of inhalant abuse upon energy balance and metabolism are needed. This review addresses several aspects of the short- and long-term effects of inhalant abuse relating to energy and metabolic processes, including energy balance, intake and expenditure; dietary preferences and glycemic control; and the dysfunction of metabolic homeostasis through altered adipose tissue, bone, and hypothalamic-pituitary-adrenal axis function.

Characteristic Metabolic Alterations Identified in Primary Neurons Under High Glucose Exposure

Cognitive dysfunction is a central nervous system (CNS) complication of diabetes mellitus (DM) that is characterized by impaired memory and cognitive ability. An in-depth understanding of metabolic alterations in the brain associated with DM will facilitate our understanding of the pathogenesis of cognitive dysfunction. The present study used an in vitro culture of primary neurons in a high-glucose (HG) environment to investigate characteristic alterations in neuron metabolism using nuclear magnetic resonance (NMR)-based metabonomics. High performance liquid chromatography (HPLC) was also used to measure changes in the adenosine phosphate levels in the hippocampal regions of streptozotocin (STZ)-induced diabetic rats. Our results revealed significant elevations in phosphocholine and ATP production in neurons and decreased formate, nicotinamide adenine dinucleotide (NAD⁺), tyrosine, methionine, acetate and phenylalanine levels after HG treatment. However, the significant changes in lactate, glutamate, taurine and myo-inositol levels in astrocytes we defined previously in astrocytes, were not found in neurons, suggested cell-specific metabolic alterations. We also confirmed an astrocyte-neuron lactate shuttle between different compartments in the brain under HG conditions, which was accompanied by abnormal acetate transport. These alterations reveal specific information on the metabolite levels and transport processes related to neurons under diabetic conditions. Our findings contribute to the understanding of the metabolic alterations and underlying pathogenesis of cognitive decline in diabetic patients.

A protonic biotransducer controlling mitochondrial ATP synthesis

In nature, protons (H⁺) play an important role in biological activities such as in mitochondrial ATP synthesis, which is driven by a H⁺ gradient across the inner membrane, or in the activation of acid sensing ion channels in neuron cells. Bioprotonic devices directly interface with the H⁺ concentration (pH) to facilitate engineered interactions with these biochemical processes. Here we develop a H⁺ biotransducer that changes the pH in a mitochondrial matrix by controlling the flow of H⁺ between a conductive polymer of sulfonated polyaniline and solution. We have successfully modulated the rate of ATP synthesis in mitochondria by altering the solution pH. Our H⁺ biotransducer provides a new way to monitor and modulate pH dependent biological functions at the interface between the electronic devices and biological materials.

Review of the role of the nervous system in glucose homoeostasis and future perspectives towards the management of diabetes

Diabetes is a disease caused by a breakdown in the glucose metabolic process resulting in abnormal blood glucose fluctuations. Traditionally, control has involved external insulin injection in response to elevated blood glucose to substitute the role of the beta cells in the pancreas which would otherwise perform this function in a healthy individual. The central nervous system (CNS), however, also plays a vital role in glucose homoeostasis through the control of pancreatic secretion and insulin sensitivity which could potentially be used as a pathway for enhancing glucose control. In this review, we present an overview of the brain regions, peripheral nerves and molecular mechanisms by which the CNS regulates glucose metabolism and the potential benefits of modulating them for diabetes management. Development of technologies to interface to the nervous system will soon become a reality through bioelectronic medicine and we present the emerging opportunities for the treatment of type 1 and type 2 diabetes.

Potential approaches to prevent hypoglycemia-associated autonomic failure

Clear health benefits are associated with intensive glucose control in type 1 diabetes mellitus (T1DM). However, maintaining near-normal glycemia remains an elusive goal for many patients, in large part owing to the risk of severe hypoglycemia. In fact, recurrent episodes of hypoglycemia lead to 'hypoglycemia-associated autonomic failure' (HAAF), characterized by defective counter-regulatory responses to hypoglycemia. Extensive studies to understand the mechanisms underlying HAAF have revealed multiple potential etiologies, suggesting various approaches to prevent the development of HAAF. In this review, we present an overview of the literature focused on pharmacological approaches that may prevent the development of HAAF. The purported underlying mechanisms of HAAF include: 1) central mechanisms (opioid receptors, ATP-sensitive K+(KATP) channels, adrenergic receptors, serotonin selective receptor inhibitors, γ-aminobuyric acid receptors, N-methyl D-aspartate receptors); 2) hormones (cortisol, estrogen, dehydroepiandrosterone (DHEA) or DHEA sulfate, glucagon-like peptide-1) and 3) nutrients (fructose, free fatty acids, ketones), all of which have been studied vis-à-vis their ability to impact the development of HAAF. A careful review of the current literature reveals many promising therapeutic approaches to treat or reduce this important limitation to optimal glycemic control.

Self-regulation resources and physical activity participation among adults with type 2 diabetes

Physical activity plays a crucial role in the prevention and treatment of type 2 diabetes. Therefore, it is important to understand why so few adults with type 2 diabetes regularly engage in physical activity. The role of self-regulation in the context of health-related behavior adherence, especially in terms of physical activity engagement and adherence, has largely been reviewed based on the strength energy model. Building on this line of research, the aim of this theoretical work was to highlight how self-regulation and ego depletion can influence the lower rate of physical activity participation among adults with type 2 diabetes, compared to adults from the general population.

α-MSH Influences the Excitability of Feeding-Related Neurons in the Hypothalamus and Dorsal Vagal Complex of Rats

Alpha-melanocyte-stimulating hormone (α-MSH) is processed from proopiomelanocortin (POMC) and acts on the melanocortin receptors, MC3 and MC4. α-MSH plays a key role in energy homeostasis. In the present study, to shed light on the mechanisms by which α-MSH exerts its anorectic effects, extracellular neuronal activity was recorded in the hypothalamus and the dorsal vagal complex (DVC) of anesthetized rats. We examined the impact of α-MSH on glucose-sensing neurons and gastric distension (GD) sensitive neurons. In the lateral hypothalamus (LHA), α-MSH inhibited 75.0% of the glucose-inhibited (GI) neurons. In the ventromedial nucleus (VMN), most glucose-sensitive neurons were glucose-excited (GE) neurons, which were mainly activated by α-MSH. In the paraventricular nucleus (PVN), α-MSH suppressed the majority of GI neurons and excited most GE neurons. In the DVC, among the 20 GI neurons examined for a response to α-MSH, 1 was activated, 16 were depressed, and 3 failed to respond. Nineteen of 24 GE neurons were activated by α-MSH administration. Additionally, among the 42 DVC neurons examined for responses to GD, 23 were excited (GD-EXC) and 19 were inhibited (GD-INH). Fifteen of 20 GD-EXC neurons were excited, whereas 11 out of 14 GD-INH neurons were suppressed by α-MSH. All these responses were abolished by pretreatment with the MC3/4R antagonist, SHU9119. In conclusion, the activity of glucose-sensitive neurons and GD-sensitive neurons in the hypothalamus and DVC can be modulated by α-MSH.

The effects of graded levels of calorie restriction: IX. Global metabolomic screen reveals modulation of carnitines, sphingolipids and bile acids in the liver of C57BL/6 mice

Calorie restriction (CR) remains the most robust intervention to extend lifespan and improve health span. Using a global mass spectrometry-based metabolomic approach, we identified 193 metabolites that were significantly differentially expressed (SDE) in the livers of C57BL/6 mice, fed graded levels of CR (10, 20, 30 and 40% CR) compared to mice fed ad libitum for 12 h a day. The differential expression of metabolites also varied with the different feeding groups. Pathway analysis revealed that graded CR had an impact on carnitine synthesis and the carnitine shuttle pathway, sphingosine-1-phosphate (S1P) signalling and methionine metabolism. S1P, sphingomyelin and L-carnitine were negatively correlated with body mass, leptin, insulin-like growth factor- 1 (IGF-1) and major urinary proteins (MUPs). In addition, metabolites which showed a graded effect, such as ceramide, S1P, taurocholic acid and L-carnitine, responded in the opposite direction to previously observed age-related changes. We suggest that the modulation of this set of metabolites may improve liver processes involved in energy release from fatty acids. S1P also negatively correlated with catalase activity and body temperature, and positively correlated with food anticipatory activity. Injecting mice with S1P or an S1P receptor 1 agonist did not precipitate changes in body temperature, physical activity or food intake suggesting that these correlations were not causal relationships.

A High-fat Diet Induces a Loss of Midbrain Dopaminergic Neuronal Function That Underlies Motor Abnormalities

Movement defects in obesity are associated with peripheral muscle defects, arthritis, and dysfunction of motor control by the brain. Although movement functionality is negatively correlated with obesity, the brain regions and downstream signaling pathways associated with movement defects in obesity are unclear. A dopaminergic neuronal pathway from the substantia nigra (SN) to the striatum is responsible for regulating grip strength and motor initiation through tyrosine hydroxylase (TH) activity-dependent dopamine release. We found that mice fed a high-fat diet exhibited decreased movement in open-field tests and an increase in missteps in a vertical grid test compared with normally fed mice. This motor abnormality was associated with a significant reduction of TH in the SN and striatum. We further found that phosphorylation of c-Jun N-terminal kinase (JNK), which modulates TH expression in the SN and striatum, was decreased under excess-energy conditions. Our findings suggest that high calorie intake impairs motor function through JNK-dependent dysregulation of TH in the SN and striatum.

Neuronal inhibition and seizure suppression by acetoacetate and its analog, 2-phenylbutyrate

OBJECTIVE

The ketogenic diet is clinically used to treat drug-resistant epilepsy. The diet treatment markedly increases ketone bodies (acetoacetate and β-hydroxybutyrate), which work as energy metabolites in the brain. Here, we investigated effects of acetoacetate on voltage-dependent Ca²⁺ channels (VDCCs) in pyramidal cells of the hippocampus. We further explored an acetoacetate analog that inhibited VDCCs in pyramidal cells, reduced excitatory postsynaptic currents (EPSCs), and suppressed seizures in vivo.

METHODS

The effects of acetoacetate and its analogs on VDCCs and EPSCs were evaluated using patch-clamp recordings from CA1 pyramidal cells of mouse hippocampal slices. The in vivo effects of these reagents were also evaluated using a chronic seizure model induced by intrahippocampal injection of kainate.

RESULTS

Acetoacetate inhibited VDCCs in pyramidal cells of hippocampal slices, and reduced EPSCs in slices exhibiting epileptiform activity. More potent EPSC inhibitors were then explored by modifying the chemical structure of acetoacetate, and 2-phenylbutyrate was identified as an acetoacetate analog that inhibited VDCCs and EPSCs more potently. Although acetoacetate is known to inhibit vesicular glutamate transporters (VGLUTs), 2-phenylbutyrate did not inhibit VGLUTs, showing that 2-phenylbutyrate is an acetoacetate analog that preferably inhibits VDCCs. In addition, 2-phenylbutyrate markedly reduced EPSCs in slices exhibiting epileptiform activity, and suppressed hippocampal seizures in vivo in a mouse model of epilepsy. The in vivo antiseizure effects of 2-phenylbutyrate were more potent than those of acetoacetate. Finally, intraperitoneal 2-phenylbutyrate was delivered to the brain, and its brain concentration reached the level enough to reduce EPSCs.

SIGNIFICANCE

These results demonstrate that 2-phenylbutyrate is an acetoacetate analog that inhibits VDCCs and EPSCs in pyramidal cells, suppresses hippocampal seizures in vivo, and has brain penetration ability. Thus 2-phenylbutyrate provides a useful chemical structure as a lead compound to develop new antiseizure drugs originating from ketone bodies.

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.

Fatty acid-induced astrocyte ketone production and the control of food intake

Obesity and Type 2 diabetes are major worldwide public health issues today. A relationship between total fat intake and obesity has been found. In addition, the mechanisms of long-term and excessive high-fat diet (HFD) intake in the development of obesity still need to be elucidated. The ventromedial hypothalamus (VMH) is a major site involved in the regulation of glucose and energy homeostasis where "metabolic sensing neurons" integrate metabolic signals from the periphery. Among these signals, fatty acids (FA) modulate the activity of VMH neurons using the FA translocator/CD36, which plays a critical role in the regulation of energy and glucose homeostasis. During low-fat diet (LFD) intake, FA are oxidized by VMH astrocytes to fuel their ongoing metabolic needs. However, HFD intake causes VMH astrocytes to use FA to generate ketone bodies. We postulate that these astrocyte-derived ketone bodies are exported to neurons where they produce excess ATP and reactive oxygen species, which override CD36-mediated FA sensing and act as a signal to decrease short-term food intake. On a HFD, VMH astrocyte-produced ketones reduce elevated caloric intake to LFD levels after 3 days in rats genetically predisposed to resist (DR) diet-induced obesity (DIO), but not leptin-resistant DIO rats. This suggests that, while VMH ketone production on a HFD can contribute to protection from obesity, the inherent leptin resistance overrides this inhibitory action of ketone bodies on food intake. Thus, astrocytes and neurons form a tight metabolic unit that is able to monitor circulating nutrients to alter food intake and energy homeostasis.

Rapid-onset hypoglycemia suppresses Fos expression in discrete parts of the ventromedial nucleus of the hypothalamus

The consensus view of the ventromedial nucleus of the hypothalamus (VMH) is that it is a key node in the rodent brain network controlling sympathoadrenal counterregulatory responses to hypoglycemia. To identify the location of hypoglycemia-responsive neurons in the VMH, we performed a high spatial resolution Fos analysis in the VMH of rats made hypoglycemic with intraperitoneal injections of insulin. We examined Fos expression in the four constituent parts of VMH throughout its rostrocaudal extent and determined their relationship to blood glucose concentrations. Hypoglycemia significantly decreased Fos expression only in the dorsomedial and central parts of the VMH, but not its anterior or ventrolateral parts. Moreover, the number of Fos-expressing neurons was significantly and positively correlated in the two responsive regions with terminal blood glucose concentrations. We also measured Fos responses in the paraventricular nucleus of the hypothalamus (PVH) and in several levels of the periaqueductal gray (PAG), which receives strong projections from the VMH. We found the expected and highly significant increase in Fos in the neuroendocrine PVH, which was negatively correlated to terminal blood glucose concentrations, but no significant differences were seen in any part of the PAG. Our results show that there are distinct populations of VMH neurons whose Fos expression is suppressed by hypoglycemia, and their numbers correlate with blood glucose. These findings support a clear division of glycemic control functions within the different parts of the VMH.

Ecophysiology of cognition: How do environmentally induced changes in physiology affect cognitive performance?

Cognitive performance is based on brain functions, which have energetic demands and are modulated by physiological parameters such as metabolic hormones. As both environmental demands and environmental energy availability change seasonally, we propose that cognitive performance in free-living animals might also change seasonally due to phenotypic plasticity. This is part of an emerging research field, the 'ecophysiology of cognition': environmentally induced changes in physiological traits, such as blood glucose and hormone levels, are predicted to influence cognitive performance in free-living animals. Energy availability for the brain might change, and as such cognition, with changing energetic demands (e.g. reproduction) and changes of energy availability in the environment (e.g. winter, drought). Individuals spending more energy than they can currently obtain from their environment (allostatic overload type I) are expected to trade off energy investment between cognition and other life-sustaining processes or even reproduction. Environmental changes reducing energy availability might thus impair cognition. However, selection pressures such as predation risk, mate choice or social demands may act on the trade-off between energy saving and cognition. We assume that different environmental conditions can lead to three different trade-off outcomes: cognitive impairment, resilience or enhancement. Currently we cannot understand these trade-offs, because we lack information about changes in cognitive performance due to seasonal changes in energy availability and both the resulting changes in homeostasis (for example, blood glucose levels) and the associated changes in the mechanisms of allostasis (for example, hormone levels). Additionally, so far we know little about the fitness consequences of individual variation in cognitive performance. General cognitive abilities, such as attention and associative learning, might be more important in determining fitness than complex and specialized cognitive abilities, and easier to use for comparative study in a large number of species. We propose to study seasonal changes in cognitive performance depending on energy availability in populations facing different predation risks, and the resulting fitness consequences.

Leptin acts in the brain to influence hypoglycemic counterregulation: disparate effects of acute and recurrent hypoglycemia on glucagon release

Leptin has been shown to diminish hyperglycemia via reduced glucagon secretion, although it can also enhance sympathoadrenal responses. However, whether leptin can also inhibit glucagon secretion during insulin-induced hypoglycemia or increase epinephrine during acute or recurrent hypoglycemia has not been examined. To test whether leptin acts in the brain to influence counterregulation, hyperinsulinemic hypoglycemic (∼45 mg/dl) clamps were performed on rats exposed to or not exposed to recurrent hypoglycemia (3 days, ∼40 mg/dl). Intracerebroventricular artificial cerebral spinal fluid or leptin was infused during the clamp. During acute hypoglycemia, leptin decreased glucagon responses by 51% but increased epinephrine and norepinephrine by 24 and 48%, respectively. After recurrent hypoglycemia, basal plasma leptin levels were undetectable. Subsequent brain leptin infusion during hypoglycemia paradoxically increased glucagon by 45% as well as epinephrine by 19%. In conclusion, leptin acts within the brain to diminish glucagon secretion during acute hypoglycemia but increases epinephrine, potentially limiting its detrimental effects during hypoglycemia. Exposure to recurrent hypoglycemia markedly suppresses plasma leptin, whereas exogenous brain leptin delivery enhances both glucagon and epinephrine release to subsequent hypoglycemia. These data suggest that recurrent hypoglycemia may diminish counterregulatory responses in part by reducing brain leptin action.

High fat diet attenuates glucose-dependent facilitation of 5-HT3 -mediated responses in rat gastric vagal afferents

KEY POINTS

Glucose regulates the density and function of 5-HT3 receptors on gastric vagal afferent neurones. Diet-induced obesity compromises the excitability and responsiveness of vagal afferents. In this study, we assessed whether exposure to a high fat diet (HFD) compromises the glucose-dependent modulation of 5-HT responses in gastric vagal afferents prior to the development of obesity. We show that HFD does not alter the response of gastric vagal afferent nerves and neurones to 5-HT but attenuates the ability of glucose to amplify 5-HT3-induced responses. These results suggest that glucose-dependent vagal afferent signalling is compromised by relatively short periods of exposure to HFD well in advance of the development of obesity or glycaemic dysregulation. Glucose regulates the density and function of 5-HT3 receptors on gastric vagal afferent neurones. Since diet-induced obesity attenuates the responsiveness of gastric vagal afferents to several neurohormones, the aim of the present study was to determine whether high fat diet (HFD) compromises the glucose-dependent modulation of 5-HT responses in gastric vagal afferents prior to the development of obesity. Rats were fed control or HFD (14% or 60% kilocalories from fat, respectively) for up to 8 weeks. Neurophysiological recordings assessed the ability of 5-HT to increase anterior gastric vagal afferent nerve (VAN) activity in vivo before and after acute hyperglycaemia, while electrophysiological recordings from gastric-projecting nodose neurones assessed the ability of glucose to modulate the 5-HT response in vitro. Immunocytochemical studies determined alterations in the neuronal distribution of 5-HT3 receptors. 5-HT and cholecystokinin (CCK) induced dose-dependent increases in VAN activity in all rats; HFD attenuated the response to CCK, but not 5-HT. The 5-HT-induced response was amplified by acute hyperglycaemia in control, but not HFD, rats. Similarly, although 5-HT induced an inward current in both control and HFD gastric nodose neurones in vitro, the 5-HT response and receptor distribution was amplified by acute hyperglycaemia only in control rats. These data suggest that, while HFD does not affect the response of gastric-projecting vagal afferents to 5-HT, it attenuates the ability of glucose to amplify 5-HT effects. This suggests that glucose-dependent vagal afferent signalling is compromised by short periods of exposure to HFD well in advance of obesity or glycaemic dysregulation.

Pharmacological and Morphological Evidence of AMPK-Mediated Energy Sensing in the Lower Brain Stem Ependymocytes to Control Reproduction in Female Rodents

Ependymocytes are one of the energy-sensing cells that regulate animal reproduction through their responsiveness to changes in extracellular glucose levels and the expression of pancreatic-type glucokinase and glucose transporter 2, which play a critical role in sensing blood glucose levels in pancreatic β-cells. Molecular mechanisms underlying glucose sensing in the ependymocytes remain poorly understood. The AMP-activated protein kinase (AMPK), a serine/threonine kinase highly conserved in all eukaryotic cells, has been suggested to be an intracellular fuel gauge that detects cellular energy status. The present study aims to clarify the role AMPK of the lower brainstem ependymocytes has in sensing glucose levels to regulate reproductive functions. First, we will show that administration of 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside, an AMPK activator, into the 4th ventricle suppressed pulsatile LH release in female rats. Second, we will demonstrate the presence of AMPK catalytic subunit immunoreactivities in the rat lower brainstem ependymocytes. Third, transgenic mice were generated to visualize the ependymocytes with Venus, a green fluorescent protein, expressed under the control of the mouse vimentin promoter for further in vitro study. The Venus-labeled ependymocytes taken from the lower brainstem of transgenic mice revealed that AMPK activation by 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside, an AMPK activator, increased in vitro intracellular calcium concentrations. Taken together, malnutrition-induced AMPK activation of ependymocytes of the lower brainstem might be involved in suppression of GnRH/LH release and then gonadal activities.

Fluctuations in nucleus accumbens extracellular glutamate and glucose during motivated glucose-drinking behavior: dissecting the neurochemistry of reward

While motivated behavior involves multiple neurochemical systems, few studies have focused on the role of glutamate, the brain's excitatory neurotransmitter, and glucose, the energetic substrate of neural activity in reward-related neural processes. Here, we used high-speed amperometry with enzyme-based substrate-sensitive and control, enzyme-free biosensors to examine second-scale fluctuations in the extracellular levels of these substances in the nucleus accumbens shell during glucose-drinking behavior in trained rats. Glutamate rose rapidly after the presentation of a glucose-containing cup and before the initiation of drinking (reward seeking), decreased more slowly to levels below baseline during consumption (sensory reward), and returned to baseline when the ingested glucose reached the brain (metabolic reward). When water was substituted for glucose, glutamate rapidly increased with cup presentation and in contrast to glucose drinking, increased above baseline after rats tasted the water and refused to drink further. Therefore, extracellular glutamate show distinct changes associated with key events of motivated drinking behavior and opposite dynamics during sensory and metabolic components of reward. In contrast to glutamate, glucose increased at each stimulus and behavioral event, showing a sustained elevation during the entire behavior and a robust post-ingestion rise that correlated with the gradual return of glutamate levels to their baseline. By comparing active drinking with passive intra-gastric glucose delivery, we revealed that fluctuations in extracellular glucose are highly dynamic, reflecting a balance between rapid delivery because of neural activity, intense metabolism, and the influence of ingested glucose reaching the brain.

Activation of corticotropin-releasing factor receptors in the rostral ventrolateral medulla is required for glucose-induced sympathoexcitation

Energy expenditure is determined by metabolic rate and diet-induced thermogenesis. Normally, energy expenditure increases due to neural mechanisms that sense plasma levels of ingested nutrients/hormones and reflexively increase sympathetic nerve activity (SNA). Here, we investigated neural mechanisms of glucose-driven sympathetic activation by determining contributions of neuronal activity in the hypothalamic paraventricular nucleus (PVN) and activation of corticotropin-releasing factor (CRF) receptors in the rostral ventrolateral medulla (RVLM). Glucose was infused intravenously (150 mg/kg, 10 min) in male rats to raise plasma glucose concentration to a physiological postprandial level. In conscious rats, glucose infusion activated CRF-containing PVN neurons and TH-containing RVLM neurons, as indexed by c-Fos immunofluorescence. In α-chloralose/urethane-anesthetized rats, glucose infusion increased lumbar and splanchnic SNA, which was nearly prevented by prior RVLM injection of the CRF receptor antagonist astressin (10 pmol/50 nl). This cannot be attributed to a nonspecific effect, as sciatic afferent stimulation increased SNA and ABP equivalently in astressin- and aCSF-injected rats. Glucose-stimulated sympathoexcitation was largely reversed during inhibition of PVN neuronal activity with the GABA-A receptor agonist muscimol (100 pmol/50 nl). The effects of astressin to prevent glucose-stimulated sympathetic activation appear to be specific to interruption of PVN drive to RVLM because RVLM injection of astressin prior to glucose infusion effectively prevented SNA from rising and prevented any fall of SNA in response to acute PVN inhibition with muscimol. These findings suggest that activation of SNA, and thus energy expenditure, by glucose is initiated by activation of CRF receptors in RVLM by descending inputs from PVN.

A mutation causing increased KATP channel activity leads to reduced anxiety in mice

Activating mutations in the Kir6.2 (KCNJ11) subunit of the ATP-sensitive potassium channel cause neonatal diabetes. Many patients also suffer from neurological complications. By using mice carrying a human Kir6.2 mutation (Val(59) to Met(59); nV59M mice) targeted to neurones, we show that these mutations also result in altered anxiety behaviour. The light/dark box, successive alleys and elevated plus maze tasks revealed that nV59M mice have reduced anxiety related responses. Additionally, nV59M mice displayed enhanced basal locomotor activity and exploratory behaviour, as assessed by the low anxiety open-field test. These findings, in combination with previously reported hyperactivity of nV59M mice, appear to correlate with the increased impulsivity and inattentiveness reported in iDEND/DEND patients.

Ovarian steroids influence cerebral glucose transporter expression in a region- and isoform-specific pattern

Cerebral glucose uptake is mediated by several members of the family of facilitated glucose transporters (protein nomenclature GLUT; gene nomenclature solute carrier family 2 Slc2a). Glucose uptake differs between the sexes and also varies with menstrual status in women and across the rodent oestrous cycle. The present study demonstrates the extent to which hormonal variation across the four stages of the rat oestrous cycle affects the mRNA abundance of four members of the GLUT family, including the most well characterised cerebral transporters Slc2a1 and Slc2a3, as well as the insulin-sensitive transporters Slc2a4 and Slc2a8 in the hypothalamus, hippocampus and prefrontal cortex. Slc2a1 varied significantly across the cycle in the hippocampus and prefrontal cortex, and Slc2a3 and Slc2a4 also showed significant fluctuation in the hippocampus. Transporter expression significantly increased during pro-oestrus in both the hippocampus and prefrontal cortex. Furthermore, ovarian hormones are critical for normal expression of GLUT mRNA, as demonstrated by reduced expression of Slc2a1, Slc2a3 and Sl2a8 in the hippocampus after ovariectomy. Collectively, the data reported in the present study demonstrate that glucose transporters are highly sensitive to hormonal variation and that this sensitivity is regionally distinct; thereby fluctuations likely have specific phenotypic implications.

Chronic stress modulates regional cerebral glucose transporter expression in an age-specific and sexually-dimorphic manner

Facilitative glucose transporters (GLUT) mediate glucose uptake across the blood-brain-barrier into neurons and glia. Deficits in specific cerebral GLUT isoforms are linked to developmental and neurological dysfunction, but less is known about the range of variation in cerebral GLUT expression in normal conditions and the effects of environmental influences on cerebral GLUT expression. Knowing that puberty is a time of increased cerebral plasticity, metabolic demand, and shifts in hormonal balance for males and females, we first assessed gene expression of five GLUT subtypes in four brain regions in male and female adolescent and adult Wistar rats. The data indicated that sex differences in GLUT expression were most profound in the hypothalamus, and the transition from adolescence to adulthood had the most profound effect on GLUT expression in the hippocampus. Next, given the substantial energetic demands during adolescence and prior demonstrations of the adverse effects of adolescent stress, we determined the extent to which chronic stress altered GLUT expression in males and females in both adolescence and adulthood. Chronic stress significantly altered cerebral GLUT expression in males and females throughout both developmental stages but in a sexually dimorphic and brain region-specific manner. Collectively, our data demonstrate that cerebral GLUTs are expressed differentially based on brain region, sex, age, and stress exposure. These results suggest that developmental and environmental factors influence GLUT expression in multiple brain regions. Given the importance of appropriate metabolic balance within the brain, further assessment of the functional implications of life stage and environmentally-induced changes in GLUTs are warranted.

Oral leucine supplementation is sensed by the brain but neither reduces food intake nor induces an anorectic pattern of gene expression in the hypothalamus

Leucine activates the intracellular mammalian target of the rapamycin (mTOR) pathway, and hypothalamic mTOR signaling regulates food intake. Although central infusion of leucine reduces food intake, it is still uncertain whether oral leucine supplementation is able to affect the hypothalamic circuits that control energy balance. We observed increased phosphorylation of p70s6k in the mouse hypothalamus after an acute oral gavage of leucine. We then assessed whether acute oral gavage of leucine induces the activation of neurons in several hypothalamic nuclei and in the brainstem. Leucine did not induce the expression of Fos in hypothalamic nuclei, but it increased the number of Fos-immunoreactive neurons in the area postrema. In addition, oral gavage of leucine acutely increased the 24 h food intake of mice. Nonetheless, chronic leucine supplementation in the drinking water did not change the food intake and the weight gain of ob/ob mice and of wild-type mice consuming a low- or a high-fat diet. We assessed the hypothalamic gene expression and observed that leucine supplementation increased the expression of enzymes (BCAT1, BCAT2 and BCKDK) that metabolize branched-chain amino acids. Despite these effects, leucine supplementation did not induce an anorectic pattern of gene expression in the hypothalamus. In conclusion, our data show that the brain is able to sense oral leucine intake. However, the food intake is not modified by chronic oral leucine supplementation. These results question the possible efficacy of leucine supplementation as an appetite suppressant to treat obesity.

c-Jun amino-terminal kinase-1 mediates glucose-responsive upregulation of the RNA editing enzyme ADAR2 in pancreatic beta-cells

A-to-I RNA editing catalyzed by the two main members of the adenosine deaminase acting on RNA (ADAR) family, ADAR1 and ADAR2, represents a RNA-based recoding mechanism implicated in a variety of cellular processes. Previously we have demonstrated that the expression of ADAR2 in pancreatic islet β-cells is responsive to the metabolic cues and ADAR2 deficiency affects regulated cellular exocytosis. To investigate the molecular mechanism by which ADAR2 is metabolically regulated, we found that in cultured β-cells and primary islets, the stress-activated protein kinase JNK1 mediates the upregulation of ADAR2 in response to changes of the nutritional state. In parallel with glucose induction of ADAR2 expression, JNK phosphorylation was concurrently increased in insulin-secreting INS-1 β-cells. Pharmacological inhibition of JNKs or siRNA knockdown of the expression of JNK1 prominently suppressed glucose-augmented ADAR2 expression, resulting in decreased efficiency of ADAR2 auto-editing. Consistently, the mRNA expression of Adar2 was selectively reduced in the islets from JNK1 null mice in comparison with that of wild-type littermates or JNK2 null mice, and ablation of JNK1 diminished high-fat diet-induced Adar2 expression in the islets from JNK1 null mice. Furthermore, promoter analysis of the mouse Adar2 gene identified a glucose-responsive region and revealed the transcription factor c-Jun as a driver of Adar2 transcription. Taken together, these results demonstrate that JNK1 serves as a crucial component in mediating glucose-responsive upregulation of ADAR2 expression in pancreatic β-cells. Thus, the JNK1 pathway may be functionally linked to the nutrient-sensing actions of ADAR2-mediated RNA editing in professional secretory cells.

Review of the neuroanatomic landscape implicated in glucose sensing and regulation of nutrient signaling: immunophenotypic localization of diabetes gene Tcf7l2 in the developing murine brain

Genetic variants in the transcription factor 7-like 2(Tcf7l2) gene have been found to confer a significant risk of type 2 diabetes and attenuated insulin secretion. Based on its genomic wide association Tcf7l2 is considered the single most important predictor of diabetes to date. Previous studies of Tcf7l2 mRNA localization in the adult brain suggest a putative role of Tcf7l2 in the CNS regulation of energy homeostasis. The present study further characterizes the immunophenotypic distribution of peptide expression in the brains of Tcf7l2 progeny during developmental time periods between E12.5 and P1. Tcf7l2(-/-) is lethal beyond P1. Results show that while negligible TCF7L2 expression is found in the developing brains of Tcf7l2(-/-)mice, TCF7L2 protein is relatively widespread and robustly expressed in the brain by E18.5 and exhibits specific expression within neuronal populations and regions of the brain in Tcf7l2(+/-) and Tcf7l2(+/+) progeny. Strong immunophenotypic labeling was found in the diencephalic structure of the thalamus that suggests a role of Tcf7l2 in the development and maintenance of thalamic activity. Strongly expressed TCF7L2 was localized in select hypothalamic and preoptic nuclei indicative of Tcf7l2 function within neurons controlling energy balance. Definitive neuronal staining for TCF7L2 within nuclei of the brain stem and circumventricular organs extends TCF7L2 localization within autonomic neurons and its potential integration with autonomic function. In addition robust TCF7L2 expression was found in the tectal and tegmental structures of the superior and inferior colliculi as well as transient expression in neuroepithelium of the cerebral and hippocampal cortices of E16 and E18.5. Patterns of TCF7L2 peptide localization when compared to the adult protein synthetic chemical/anatomical landscape of glucose sensing exhibit a good correlational fit between its expression and regions, nuclei, and pathways regulating energy homeostasis via integration and response to peripheral endocrine, metabolic and neuronal signaling. TCF was also found co-localized with peptides that regulate energy homeostasis including AgRP, POMC and NPY. TCF7l2, some variants of which have been shown to impair GLP-1-induced insulin secretion, was also found co-localize with GLP-1 in adult TCF wild type progeny. Impaired Tcf7l2-mediated neural regulation may contribute to the risk and/or underlying pathophysiology of type 2 diabetes that has found high expression in genomic studies of Tcf7l2 variants.

Roles of dorsomedial hypothalamic cholecystokinin signaling in the controls of meal patterns and glucose homeostasis

A role for dorsomedial hypothalamus (DMH) cholecystokinin (CCK) signaling in feeding control has been proposed. Administration of CCK into the DMH reduces food intake and OLETF rats lacking CCK1 receptors (CCK1R) become hyperphagic and obese. We hypothesized that site specific replenishment of CCK1R in the DMH of OLETF rats would attenuate aspects of their feeding deficits. Recombinant vectors of adeno-associated viral (AAV)-mediated expression of CCK1R (AAVCCK1R) were bilaterally delivered into the DMH of OLETF. OLETF rats with AAVCCK1R injections demonstrated a 65% replenishment of Cck1r mRNA expression in the DMH relative to lean LETO control rats. Although this level of replenishment did not significantly affect overall food intake or body weight through 14 weeks following viral injections, meal patterns were partially normalized in OLETF rats receiving AAVCCK1R with a significant decrease in dark cycle meal size and a small but significant decrease in daily food intake in the meal analysis chambers. Importantly, the elevation in blood glucose level of OLETF rats was attenuated by the AAVCCK1R injections (p=0.03), suggesting a role for DMH CCK signaling in glucose homeostasis. In support of this role, administration of CCK into the DMH of intact rats enhanced glucose tolerance, as this occurred through activation of CCK1R but not CCK2R signaling. In conclusion, partial replenishment of CCK1R in the DMH of OLETF rats, although insufficient for altering overall food intake and body weight, normalizes meal pattern changes and reduces blood glucose levels. Our study also shows a novel role of DMH CCK signaling in glucose homeostasis.

Comprehensive review on herbal medicine for energy intake suppression

The obesity drug development is present not a bright and successful story. So far, drugs reported to be effective, either from synthetic or natural sources, mostly stimulated controversy because of serious adverse effects, which ended with stopping clinical trials or even withdrawal from the market. However, obesity and its comorbidities have become rapidly a major problem in both developed and developing countries. This has encouraged pharmaceutical companies and academia to keep on struggling on developing novel effective but safe obesity drugs, and on characterizing novel obesity drug targets. From existing scientific work on obesity drug discovery and commercial slimming preparations, compounds originating from nature, especially from plants, seem to be the first choice. Traditional belief that herbal medicine is safer than synthetic ones is one of the classical arguments, although scientifically this is not always true (e.g. ban on Ephedra). But in general, it has been widely acknowledged that a plant compound, with its unique scaffolds and rich diversity is an unlimited source of novel lead compounds. This paper aims to summarize all works focused on screening plant materials by targeting important pathways related to energy intake regulation, either by in vivo or in vitro experiments.

Renal glomerular and tubular injury after gastric bypass in obese rats

OBJECTIVE

Roux-en-Y gastric bypass (RYGB) surgery is the most common surgical intervention for long-term weight loss in morbidly obese patients. By decreasing obesity-associated hyperfiltration, diabetes, and hypertension, RYGB is touted to stabilize, if not prevent, progression of chronic renal disease. To test this, the renal histology of diet-induced obese rats that underwent RYGB surgery was compared with that of pair-fed and sham obese controls.

METHODS

Sprague-Dawley rats, fed a high-fat, low-oxalate diet to induce gross obesity, were randomized to RYGB (n = 6), gastrointestinal-intact sham-operated obese controls (sham, n = 4), or gastrointestinal-intact sham-operated obese pair-fed controls (fed, n = 8). Daily body weight and food intake were recorded. On postoperative day 42, renal histology and immunohistochemistry were examined. Renal pathology was assessed by a categorical glomerular lesion score and a quantitative glomerular/tubular scoring system by experienced veterinary pathologists. Osteopontin and ED-1 (monocyte/macrophage cell) stainings were estimated by the percentage of stained area and the number of counted cells/high-power field, respectively.

RESULTS

Compared with sham and fed controls, RYGB rats had significant decreases in body weight (P < 0.001), more glomerular lesions (P = 0.02), and received higher glomerular and tubular lesion scores (P < 0.01). RYGB rodents had significantly stronger staining for osteopontin within the inner medullary region (P < 0.005) and ED-1 within the outer medullary region (P < 0.02) compared with sham and fed controls.

CONCLUSION

In this diet-induced obese rat model, RYGB is associated with chronic glomerulosclerosis and tubulointerstitial nephritis, confirmed by histology and immunohistochemistry. Prospective studies to better define the injurious mechanisms in this model are underway.

Carbohydrate reward and psychosis: an explanation for neuroleptic induced weight gain and path to improved mental health?

Evidence links dopamine release in the mid-brain to the pathophysiology of psychosis, addiction and reward. Repeated ingestion of refined carbohydrate may stimulate the same mesolimbic dopaminergic pathway, rewarding such eating behaviour and resulting in excessive food intake along with obesity. In this paper, we explore the role of dopamine in reward and psychosis, and discuss how reward pathways may contribute to the weight gain that commonly follows antipsychotic drug use, in people with psychotic illness. Our theory also explains the frequent co-occurrence of substance abuse and psychosis. From our hypothesis, we discuss the use of carbohydrate modified diets as an adjunctive treatment for people with psychosis.

MCT expression and lactate influx/efflux in tanycytes involved in glia-neuron metabolic interaction

Metabolic interaction via lactate between glial cells and neurons has been proposed as one of the mechanisms involved in hypothalamic glucosensing. We have postulated that hypothalamic glial cells, also known as tanycytes, produce lactate by glycolytic metabolism of glucose. Transfer of lactate to neighboring neurons stimulates ATP synthesis and thus contributes to their activation. Because destruction of third ventricle (III-V) tanycytes is sufficient to alter blood glucose levels and food intake in rats, it is hypothesized that tanycytes are involved in the hypothalamic glucose sensing mechanism. Here, we demonstrate the presence and function of monocarboxylate transporters (MCTs) in tanycytes. Specifically, MCT1 and MCT4 expression as well as their distribution were analyzed in Sprague Dawley rat brain, and we demonstrate that both transporters are expressed in tanycytes. Using primary tanycyte cultures, kinetic analyses and sensitivity to inhibitors were undertaken to confirm that MCT1 and MCT4 were functional for lactate influx. Additionally, physiological concentrations of glucose induced lactate efflux in cultured tanycytes, which was inhibited by classical MCT inhibitors. Because the expression of both MCT1 and MCT4 has been linked to lactate efflux, we propose that tanycytes participate in glucose sensing based on a metabolic interaction with neurons of the arcuate nucleus, which are stimulated by lactate released from MCT1 and MCT4-expressing tanycytes.

ATP-sensitive potassium channel-mediated lactate effect on orexin neurons: implications for brain energetics during arousal

Active neurons have a high demand for energy substrate, which is thought to be mainly supplied as lactate by astrocytes. Heavy lactate dependence of neuronal activity suggests that there may be a mechanism that detects and controls lactate levels and/or gates brain activation accordingly. Here, we demonstrate that orexin neurons can behave as such lactate sensors. Using acute brain slice preparations and patch-clamp techniques, we show that the monocarboxylate transporter blocker alpha-cyano-4-hydroxycinnamate (4-CIN) inhibits the spontaneous activity of orexin neurons despite the presence of extracellular glucose. Furthermore, fluoroacetate, a glial toxin, inhibits orexin neurons in the presence of glucose but not lactate. Thus, orexin neurons specifically use astrocyte-derived lactate. The effect of lactate on firing activity is concentration dependent, an essential characteristic of lactate sensors. Furthermore, lactate disinhibits and sensitizes these neurons for subsequent excitation. 4-CIN has no effect on the activity of some arcuate neurons, indicating that lactate dependency is not universal. Orexin neurons show an indirect concentration-dependent sensitivity to glucose below 1 mm, responding by hyperpolarization, which is mediated by ATP-sensitive potassium channels composed of Kir6.1 and SUR1 subunits. In conclusion, our study suggests that lactate is a critical energy substrate and a regulator of the orexin system. Together with the known effects of orexins in inducing arousal, food intake, and hepatic glucose production, as well as lactate release from astrocytes in response to neuronal activity, our study suggests that orexin neurons play an integral part in balancing brain activity and energy supply.

Evidence for a relationship between body mass and energy metabolism in the human brain

Cerebral energy metabolism has been suggested to have an important function in body weight regulation. We therefore examined whether there is a relationship between body mass and adenosine triphosphate (ATP) metabolism in the human brain. On the basis of our earlier findings indicating a neuroprotective preferential energy supply of the brain, as compared with peripheral muscle on experimentally induced hypoglycemia, we examined whether this physiological response is preserved also in low-weight and obese participants. We included 45 healthy male subjects with a body mass index (BMI) ranging from 17 to 44 kg/m(2). Each participant underwent a hypoglycemic glucose-clamp intervention, and the ATP metabolism, that is, the content of high-energy phosphates phosphocreatine (PCr) and ATP, was measured repeatedly by (31)phosphor magnetic resonance spectroscopy ((31)P-MRS) in the cerebral cortex and skeletal muscle. Results show an inverse correlation between BMI and high-energy phosphate content in the brain (P<0.01), whereas there was no such relationship found between skeletal muscle and BMI. The hypoglycemic clamp intervention did not affect the ATP metabolism in both tissues. Our data show an inverse correlation between BMI and cerebral high-energy phosphate content in healthy humans, suggesting a close relationship between energetic supply of the brain and body weight regulation.

Deficiency in RNA editing enzyme ADAR2 impairs regulated exocytosis

Mammalian RNA editing catalyzed by adenosine deaminases acting on RNA (ADARs) ADAR1 and ADAR2 plays pivotal roles in the brain through functional modifications of neurotransmitter receptors and ion channels. We have demonstrated previously that RNA editing by ADAR2 is regulated metabolically in pancreatic β cells. To investigate the cellular functions of ADAR2 in professional secretory cells, we studied the effects of ADAR2 knockdown on regulated exocytosis. Selective knockdown of ADAR2 expression markedly impaired glucose-stimulated insulin secretion in the rat insulinoma INS-1 cells and primary pancreatic islets and significantly diminished KCl-stimulated secretion of exogenous human growth hormone or endogenous chromogranin B protein in the rat adrenal pheochromocytoma PC12 cells. Notably, restored overexpression of catalytically active but not editing-deficient mutant ADAR2 could rescue the impairment in stimulated secretion from ADAR2 knockdown cells. Moreover, ADAR2 suppression significantly attenuated Ca(2+)-evoked membrane capacitance increases and appreciably reduced the number of membrane-docked insulin granules in INS-1 cells. Interestingly, the secretory defects resulting from ADAR2 deficiency were coupled to decreased expression of Munc18-1 and synaptotagmin-7, two key molecules in the regulation of vesicle exocytosis. Thus, these findings reveal an important aspect of ADAR2 actions in regulated exocytosis, implicating RNA editing in the control of cellular secretory machinery.

Brain glucose overexposure and lack of acute metabolic flexibility in obesity and type 2 diabetes: a PET-[18F]FDG study in Zucker and ZDF rats

Brain glucose exposure may complicate diabetes and obesity. We used positron emission tomography with (18)F-fluorodeoxyglucose in Zucker obese, diabetic, and control rats to determine the contributions of blood glucose mass action versus local mechanisms in regulating central glucose disposal in fasted and acutely glucose-stimulated states, and their adaptations in obesity and diabetes. Our study data indicate that brain glucose uptake is dependent on both local and mass action components, and is stimulated by acute glucose intake in healthy rats. In diseased animals, the organ was chronically overexposed to glucose, due to high fasting glucose uptake, almost abolishing the physiologic response to glucose loading.

Injection of Urocortin 3 into the ventromedial hypothalamus modulates feeding, blood glucose levels, and hypothalamic POMC gene expression but not the HPA axis

Urocortin 3 (Ucn 3) is a corticotropin-releasing factor (CRF)-related peptide with high affinity for the type 2 CRF receptor (CRFR2). Central administration of Ucn 3 stimulates the hypothalamic-pituitary-adrenal axis, suppresses feeding, and elevates blood glucose levels, suggesting that activation of brain CRFR2 promotes stress-like responses. Several CRFR2-expressing brain areas, including the ventromedial hypothalamus (VMH) and the posterior amygdala (PA), may be potential sites mediating the effects of Ucn 3. In the present study, Ucn 3 or vehicle was bilaterally injected into the VMH or PA, and food intake and plasma levels of ACTH, corticosterone, glucose, and insulin were determined. Food intake was greatly reduced in rats following Ucn 3 injection into the VMH. Ucn 3 injection into the VMH rapidly elevated plasma levels of glucose and insulin but did not affect ACTH and corticosterone secretion. Injection of Ucn 3 into the PA did not alter any of the parameters measured. We determined that the majority of CRFR2-positive neurons in the VMH were excitatory glutamatergic, and a subset of these neurons project to the arcuate nucleus of the hypothalamus (ARH). Importantly, stimulation of CRFR2 in the VMH increased proopiomelanocortin mRNA expression in the ARH. In conclusion, the present study demonstrates that CRFR2 in the VMH mediates some of the central effects of Ucn 3, and the ARH melanocortin system may be a downstream target of VMH CRFR2 neurons.

Relationship between basal metabolic rate and cortisol secretion throughout pregnancy

The role of cortisol in mediating basal metabolic rate (BMR) changes that accompany the adjustment of maternal body weight (BW) and body composition during pregnancy is unknown. We tested whether increase in BMR during pregnancy is explained by variations in cortisol secretion. Longitudinal changes in BW, fat mass (FM), fat-free mass (FFM), BMR, hormonal, and metabolic parameters in 31 parous Caucasian women at gestational weeks 12, 26, and 36 were examined. Individual differences (Delta) between the last and the first measurement occasions for each variable were calculated. By gestational week 36, BW and BMR increased while both FFM/FM and BMR/BW ratio decreased (P < 0.001 for all) suggesting higher proportion of FM accretion. Cortisol, leptin, and insulin-like growth factor-1 (IGF-1) concentration rose, whereas non-placental growth hormone (GH) and thyroid hormones declined (P < 0.001 for all). Insulin resistance changed; basal glucose (P < 0.001) and ghrelin (P < 0.014) declined, whereas insulin (P < 0.001), homeostatic model index (HOMA-IR) (P = 0.041), and free fatty acid (FFA) concentration (P = 0.007) increased. The elevation in BMR showed inverse correlations with DeltaBW (r = 0.37, P = 0.047) and Deltacortisol (r = -0.53, P = 0.004). Significant portion (51.6%) of the variation in BMR change was explained by increases of cortisol (27.1%), FFA (13.4%), and free triiodothyronine (11.1%). In conclusion, the changes in maternal cortisol concentration are in relationship with changes in BMR and BW, further suggesting that increased cortisol secretion during pregnancy could be linked with the maintenance of maternal BW and body composition.

Silencing of ventromedial hypothalamic neurons by glucose-stimulated K(+) currents

Glucose sensing by neurons of the ventromedial hypothalamus (VMH) plays a central role in the regulation of body energy balance. Physiological rises in extracellular glucose levels hyperpolarise and inhibit a group of VMH neurons. This specialised sensing response is currently thought to involve glucose-induced activation of chloride channels, but alternative mechanisms have not been explored in detail. In this study, we converted all chloride channels from inhibitory to excitatory by filling the cytosol of VMH neurons with a high concentration of chloride. Despite this, some VMH neurons were still strongly hyperpolarised and inhibited by glucose. Voltage-clamp analysis revealed that this was due to glucose-induced activation of K(+)-selective currents of sufficient size to cause complete inhibition of whole-cell electrical activity. These K(+) currents exhibited leak-like biophysical properties and were inhibited by extracellular acidification. Our data support the idea that glucose-stimulated K(+) currents contribute to sugar-induced suppression of firing in the VMH.

Plasticity of central autonomic neural circuits in diabetes

Regulation of energy metabolism is controlled by the brain, in which key central neuronal circuits process a variety of information reflecting nutritional state. Special sensory and gastrointestinal afferent neural signals, along with blood-borne metabolic signals, impinge on parallel central autonomic circuits located in the brainstem and hypothalamus to signal changes in metabolic balance. Specifically, neural and humoral signals converge on the brainstem vagal system and similar signals concentrate in the hypothalamus, with significant overlap between both sensory and motor components of each system and extensive cross-talk between the systems. This ultimately results in production of coordinated regulatory autonomic and neuroendocrine cues to maintain energy homeostasis. Therapeutic metabolic adjustments can be accomplished by modulating viscerosensory input or autonomic motor output, including altering parasympathetic circuitry related to GI, pancreas, and liver regulation. These alterations can include pharmacological manipulation, but surgical modification of neural signaling should also be considered. In addition, central control of visceral function is often compromised by diabetes mellitus, indicating that circuit modification should be studied in the context of its effect on neurons in the diabetic state. Diabetes has traditionally been handled as a peripheral metabolic disease, but the central nervous system plays a crucial role in regulating glucose homeostasis. This review focuses on key autonomic brain areas associated with management of energy homeostasis and functional changes in these areas associated with the development of diabetes.

Neuronal functions, feeding behavior, and energy balance in Slc2a3+/- mice

Homozygous deletion of the gene of the neuronal glucose transporter GLUT3 (Slc2a3) in mice results in embryonic lethality, whereas heterozygotes (Slc2a3+/-) are viable. Here, we describe the characterization of heterozygous mice with regard to neuronal function, glucose homeostasis, and, since GLUT3 might be a component of the neuronal glucose-sensing mechanism, food intake and energy balance. Levels of GLUT3 mRNA and protein in brain were reduced by 50% in Slc2a3+/- mice. Electrographic features examined by electroencephalographic recordings give evidence for slightly but significantly enhanced cerebrocortical activity in Slc2a3+/- mice. In addition, Slc2a3+/- mice were slightly more sensitive to an acoustic startle stimulus (elevated startle amplitude and reduced prepulse inhibition). However, systemic behavioral testing revealed no other functional abnormalities, e.g., in coordination, reflexes, motor abilities, anxiety, learning, and memory. Furthermore, no differences in body weight, blood glucose, and insulin levels were detected between wild-type and Slc2a3+/- littermates. Food intake as monitored randomly or after intracerebroventricular administration of 2-deoxyglucose or d-glucose, or food choice for carbohydrates/fat was not affected in Slc2a3+/- mice. Taken together, our data indicate that, in contrast to Slc2a1, a single allele of Slc2a3 is sufficient for maintenance of neuronal energy supply, motor abilities, learning and memory, and feeding behavior.

Prior hypoglycemia enhances glucose responsiveness in some ventromedial hypothalamic glucosensing neurons

Antecedent insulin-induced hypoglycemia (IIH) reduces adrenomedullary responses (AMR) to subsequent bouts of hypoglycemia. The ventromedial hypothalamus [VMH: arcuate (ARC) + ventromedial nuclei] contains glucosensing neurons, which are thought to be mediators of these AMR. Since type 1 diabetes mellitus often begins in childhood, we used juvenile (4- to 5-wk-old) rats to demonstrate that a single bout of IIH (5 U/kg sc) reduced plasma glucose by 24% and peak epinephrine by 59% 1 day later. This dampened AMR was associated with 46% higher mRNA for VMH glucokinase, a key mediator of neuronal glucosensing. Compared with neurons from saline-injected rats, ventromedial nucleus glucose-excited neurons from insulin-injected rats demonstrated a leftward shift in their glucose responsiveness (EC50= 0.45 and 0.10 mmol/l for saline and insulin, respectively, P = 0.05) and a 31% higher maximal activation by glucose ( P = 0.05), although this maximum occurred at a higher glucose concentration (saline, 0.7 vs. insulin, 1.5 mmol/l). Although EC50values did not differ, ARC glucose-excited neurons had 19% higher maximal activation, which occurred at a lower glucose concentration in insulin- than saline-injected rats (saline, 2.5 vs. insulin, 1.5 mmol/l). In addition, ARC glucose-inhibited neurons from insulin-injected rats were maximally inhibited at a fivefold lower glucose concentration (saline, 2.5 vs. insulin, 0.5 mmol/l), although this inhibition declined at >0.5 mmol/l glucose. These data suggest that the increased VMH glucokinase after IIH may contribute to the increased responsiveness of VMH glucosensing neurons to glucose and the associated blunting of the AMR.

Aquaporins in the brain: from aqueduct to "multi-duct"

The aquaporin channel family was first considered as a family of water channels, however it is now clear that some of these channels are also permeable to small solutes such glycerol, urea and monocarboxylates. In this review, we will consider AQP4 and AQP9 expressed in the rodent brain. AQP4 is present on astrocytic end-feet in contact with brain vessels and could be involved in ionic homeostasis. However, AQP4 may also be involved in cell adhesion. AQP4 expression is highly modified in several brain disorders and it can play a key role in the cerebral edema formation. However, the exact role of AQP4 in edema formation is still debated. Recently, AQP4 has been shown to be also involved in astrocyte migration during glial scar formation. AQP9 is expressed in astrocytes and in catecholaminergic neurons. Two isoforms of AQP9 are expressed in brain cells, the shortest isoform is localized in the inner membrane of mitochondria and the longest in the cell membrane. The level of expression of AQP9 is negatively regulated by high concentrations of insulin. Taken together, these results suggest that AQP9 could be involved in brain energy metabolism. The induction of AQP9 in astrocytes is observed with time after stroke onset suggesting participation in the clearance of excess lactate in the extracellular space. These recent exciting results suggest that AQPs may not only be involved in water homeostasis in the brain but could also participate in other important physiological functions.