Metabolic sensors: viewing glucosensing neurons from a broader perspective

Hindbrain glucoregulatory mechanisms: Critical role of catecholamine neurons in the ventrolateral medulla

Glucose is the required metabolic substrate for the brain. Yet the brain stores very little glucose. Therefore, the brain continuously monitors glucose availability to detect hypoglycemia and to mobilize system-wide responses to protect and restore euglycemia. Catecholamine (CA) neurons in the hindbrain are critical elements of the brain's glucoregulatory mechanisms. They project widely throughout the brain and spinal cord, innervating sites controlling behavioral, endocrine and visceral responses. Hence, CA neurons are capable of triggering a rapid, coordinated and multifaceted response to glucose challenge. This article reviews experimental data that has begun to elucidate the importance of CA neurons for glucoregulation, the functions of specific CA subpopulations in the ventrolateral medulla, and the extended circuitry through which they engage other levels of the nervous system to accomplish their essential glucoregulatory task. Hopefully, this review also suggests the vast amount of work yet to be done in this area and the justification for engaging in that effort.

Circulating glucose levels inversely correlate with Drosophila larval feeding through insulin signaling and SLC5A11

In mammals, blood glucose levels likely play a role in appetite regulation yet the mechanisms underlying this phenomenon remain opaque. Mechanisms can often be explored from Drosophila genetic approaches. To determine if circulating sugars might be involved in Drosophila feeding behaviors, we scored hemolymph glucose and trehalose, and food ingestion in larvae subjected to various diets, genetic mutations, or RNAi. We found that larvae with glucose elevations, hyperglycemia, have an aversion to feeding; however, trehalose levels do not track with feeding behavior. We further discovered that insulins and SLC5A11 may participate in glucose-regulated feeding. To see if food aversion might be an appropriate screening method for hyperglycemia candidates, we developed a food aversion screen to score larvae with abnormal feeding for glucose. We found that many feeding defective larvae have glucose elevations. These findings highlight intriguing roles for glucose in fly biology as a potential cue and regulator of appetite.

Rupali Ugrankar et al. show that Drosophila larvae with high levels of circulating glucose, but not trehalose, don’t eat much. This study suggests that circulating glucose communicates with insulin signaling and the sodium/solute co-transporter SLC5A11 in the brain to suppress larval appetite.

BAD and KATP channels regulate neuron excitability and epileptiform activity

Brain metabolism can profoundly influence neuronal excitability. Mice with genetic deletion or alteration of Bad (BCL-2 agonist of cell death) exhibit altered brain-cell fuel metabolism, accompanied by resistance to acutely induced epileptic seizures; this seizure protection is mediated by ATP-sensitive potassium (K_ATP) channels. Here we investigated the effect of BAD manipulation on K_ATP channel activity and excitability in acute brain slices. We found that BAD’s influence on neuronal K_ATP channels was cell-autonomous and directly affected dentate granule neuron (DGN) excitability. To investigate the role of neuronal K_ATP channels in the anticonvulsant effects of BAD, we imaged calcium during picrotoxin-induced epileptiform activity in entorhinal-hippocampal slices. BAD knockout reduced epileptiform activity, and this effect was lost upon knockout or pharmacological inhibition of K_ATP channels. Targeted BAD knockout in DGNs alone was sufficient for the antiseizure effect in slices, consistent with a ‘dentate gate’ function that is reinforced by increased K_ATP channel activity.

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.

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.

Pro-Opiomelanocortin (POMC) Neurones, POMC-Derived Peptides, Melanocortin Receptors and Obesity: How Understanding of this System has Changed Over the Last Decade

Following the cloning of the melanocortin receptor and agouti protein genes, a model was developed for the central melanocortin system with respect to the regulation of energy and glucose homeostasis. This model comprised leptin regulation of melanocortin peptides and agouti-related peptide (AgRP) produced from central pro-opiomelanocortin (POMC) and AgRP neurones, respectively, as well as AgRP competitive antagonism of melanocortin peptides activating melanocortin 4 receptor (MC4R) to Gαs and the cAMP signalling pathway. In the last decade, there have been paradigm shifts in our understanding of the central melanocortin system as a result of the application of advanced new technologies, including Cre-LoxP transgenic mouse technology, pharmacogenetics and optogenetics. During this period, our understanding of G protein coupled receptor signal transduction has also dramatically changed, such that these receptors are now known to exist in the plasma membrane oscillating between various inactive and active conformational states, and the active states signal through G protein-dependent and G protein-independent pathways. The present review focuses on evidence obtained over the past decade that has changed our understanding of POMC gene expression and regulation in the central nervous system, POMC and AgRP neuronal circuitry, neuroanatomical functions of melanocortin receptors, melanocortin 3 receptor (MC3R) and MC4R, and signal transduction through MC3R and MC4R.

Peripheral and central glucose sensing in hypoglycemic detection

Hypoglycemia poses a serious threat to the integrity of the brain, owing to its reliance on blood glucose as a fuel. Protecting against hypoglycemia is an extended network of glucose sensors located within the brain and in the periphery that serve to mediate responses restoring euglycemia, i.e., counterregulatory responses. This review examines the various glucose sensory loci involved in hypoglycemic detection, with a particular emphasis on peripheral glucose sensory loci and their contribution to hypoglycemic counterregulation.

Sweet taste receptor signaling network: possible implication for cognitive functioning

Sweet taste receptors are transmembrane protein network specialized in the transmission of information from special "sweet" molecules into the intracellular domain. These receptors can sense the taste of a range of molecules and transmit the information downstream to several acceptors, modulate cell specific functions and metabolism, and mediate cell-to-cell coupling through paracrine mechanism. Recent reports indicate that sweet taste receptors are widely distributed in the body and serves specific function relative to their localization. Due to their pleiotropic signaling properties and multisubstrate ligand affinity, sweet taste receptors are able to cooperatively bind multiple substances and mediate signaling by other receptors. Based on increasing evidence about the role of these receptors in the initiation and control of absorption and metabolism, and the pivotal role of metabolic (glucose) regulation in the central nervous system functioning, we propose a possible implication of sweet taste receptor signaling in modulating cognitive functioning.

Hypothalamic glucose sensing: making ends meet

The neuroendocrine system governs essential survival and homeostatic functions. For example, growth is needed for development, thermoregulation maintains optimal core temperature in a changing environment, and reproduction ensures species survival. Stress and immune responses enable an organism to overcome external and internal threats while the circadian system regulates arousal and sleep such that vegetative and active functions do not overlap. All of these functions require a significant portion of the body's energy. As the integrator of the neuroendocrine system, the hypothalamus carefully assesses the energy status of the body in order to appropriately partition resources to provide for each system without compromising the others. While doing so the hypothalamus must ensure that adequate glucose levels are preserved for brain function since glucose is the primary fuel of the brain. To this end, the hypothalamus contains specialized glucose sensing neurons which are scattered throughout the nuclei controlling distinct neuroendocrine functions. We hypothesize that these neurons play a key role in enabling the hypothalamus to partition energy to meet these peripheral survival needs without endangering the brain's glucose supply. This review will first describe the varied mechanisms underlying glucose sensing in neurons within discrete hypothalamic nuclei. We will then evaluate the way in which peripheral energy status regulates glucose sensitivity. For example, during energy deficit such as fasting specific hypothalamic glucose sensing neurons become sensitized to decreased glucose. This increases the gain of the information relay when glucose availability is a greater concern for the brain. Finally, changes in glucose sensitivity under pathological conditions (e.g., recurrent insulin-hypoglycemia, diabetes) will be addressed. The overall goal of this review is to place glucose sensing neurons within the context of hypothalamic control of neuroendocrine function.

Role of FAT/CD36 in fatty acid sensing, energy, and glucose homeostasis regulation in DIO and DR rats

Hypothalamic fatty acid (FA) sensing neurons alter their activity utilizing the FA translocator/receptor, FAT/CD36. Depletion of ventromedial hypothalamus (VMH) CD36 with adeno-associated viral vector expressing CD36 shRNA (AAV CD36 shRNA) leads to redistribution of adipose stores and insulin resistance in outbred rats. This study assessed the requirement of VMH CD36-mediated FA sensing for the regulation of energy and glucose homeostasis in postnatal day 5 (P5) and P21 selectively bred diet-induced obese (DIO) and diet-resistant (DR) rats using VMH AAV CD36 shRNA injections. P5 CD36 depletion altered VMH neuronal FA sensing predominantly in DIO rats. After 10 wk on a 45% fat diet, DIO rats injected with VMH AAV CD36 shRNA at P21 ate more and gained more weight than DIO AAV controls, while DR AAV CD36 shRNA-injected rats gained less weight than DR AAV controls. VMH CD36 depletion increased inguinal fat pad weights and leptin levels in DIO and DR rats. Although DR AAV CD36 shRNA-injected rats became as obese as DIO AAV controls, only DIO control and CD36 depleted rats became insulin-resistant on a 45% fat diet. VMH CD36 depletion stunted linear growth in DIO and DR rats. DIO rats injected with AAV CD36 shRNA at P5 had increased fat mass, mostly due to a 45% increase in subcutaneous fat. They were also insulin-resistant with an associated 71% increase of liver triglycerides. These results demonstrate that VMH CD36-mediated FA sensing is a critical factor in the regulation of energy and glucose homeostasis and fat deposition in DIO and DR rats.

1H-Nuclear magnetic resonance-based metabolomic analysis of brain in mice with nicotine treatment

Background

Nicotine is rapidly absorbed from cigarette smoke and therefore induces a number of chronic illnesses with the widespread use of tobacco products. Studies have shown a few cerebral metabolites modified by nicotine; however, endogenous metabolic profiling in brain has not been well explored.

Results

H NMR-based on metabonomics was applied to investigate the endogenous metabolic profiling of brain hippocampus, nucleus acumens (NAc), prefrontal cortex (PFC) and striatum. We found that nicotine significantly increased CPP in mice, and some specific cerebral metabolites differentially changed in nicotine-treated mice. These modified metabolites included glutamate, acetylcholine, tryptamine, glucose, lactate, creatine, 3-hydroxybutyrate and nicotinamide-adenine dinucleotide (NAD), which was closely associated with neurotransmitter and energy source. Additionally, glutathione and taurine in hippocampus and striatum, phosphocholine in PFC and glycerol in NAc were significantly modified by nicotine, implying the dysregulation of anti-oxidative stress response and membrane metabolism.

Conclusions

Nicotine induces significant metabonomic alterations in brain, which are involved in neurotransmitter disturbance, energy metabolism dysregulation, anti-oxidation and membrane function disruptions, as well as amino acid metabolism imbalance. These findings provide a new insight into rewarding effects of nicotine and the underlying mechanism.

FAT/CD36: a major regulator of neuronal fatty acid sensing and energy homeostasis in rats and mice

Hypothalamic "metabolic-sensing" neurons sense glucose and fatty acids (FAs) and play an integral role in the regulation of glucose, energy homeostasis, and the development of obesity and diabetes. Using pharmacologic agents, we previously found that ~50% of these neurons responded to oleic acid (OA) by using the FA translocator/receptor FAT/CD36 (CD36). For further elucidation of the role of CD36 in neuronal FA sensing, ventromedial hypothalamus (VMH) CD36 was depleted using adeno-associated viral (AAV) vector expressing CD36 short hairpin RNA (shRNA) in rats. Whereas their neuronal glucosensing was unaffected by CD36 depletion, the percent of neurons that responded to OA was decreased specifically in glucosensing neurons. A similar effect was seen in total-body CD36-knockout mice. Next, weanling rats were injected in the VMH with CD36 AAV shRNA. Despite significant VMH CD36 depletion, there was no effect on food intake, body weight gain, or total carcass adiposity on chow or 45% fat diets. However, VMH CD36-depleted rats did have increased plasma leptin and subcutaneous fat deposition and markedly abnormal glucose tolerance. These results demonstrate that CD36 is a critical factor in both VMH neuronal FA sensing and the regulation of energy and glucose homeostasis.

Mitochondrial fatty acid oxidation in obesity

SIGNIFICANCE

Current lifestyles with high-energy diets and little exercise are triggering an alarming growth in obesity. Excess of adiposity is leading to severe increases in associated pathologies, such as insulin resistance, type 2 diabetes, atherosclerosis, cancer, arthritis, asthma, and hypertension. This, together with the lack of efficient obesity drugs, is the driving force behind much research.

RECENT ADVANCES

Traditional anti-obesity strategies focused on reducing food intake and increasing physical activity. However, recent results suggest that enhancing cellular energy expenditure may be an attractive alternative therapy.

CRITICAL ISSUES

This review evaluates recent discoveries regarding mitochondrial fatty acid oxidation (FAO) and its potential as a therapy for obesity. We focus on the still controversial beneficial effects of increased FAO in liver and muscle, recent studies on how to potentiate adipose tissue energy expenditure, and the different hypotheses involving FAO and the reactive oxygen species production in the hypothalamic control of food intake.

FUTURE DIRECTIONS

The present review aims to provide an overview of novel anti-obesity strategies that target mitochondrial FAO and that will definitively be of high interest in the future research to fight against obesity-related disorders.

Insulin attenuates the acquisition and expression of ethanol-induced locomotor sensitization in DBA/2J mice

AIM

Ethanol-induced locomotor sensitization is a behavioral manifestation of physiological responses to repeated ethanol exposures. While ethanol exerts direct effects on multiple neurotransmitter systems in the brain, ethanol-induced changes in metabolic state, including acute hyperglycemia and inhibition of insulin signaling, also have plausible roles in the expression of ethanol-related behaviors through direct and indirect effects on brain function. The current experiments examined whether insulin administration or the resultant hypoglycemia might attenuate the development of sensitization to the locomotor stimulant effect of ethanol.

MAIN METHODS

Male and female DBA/2J mice received daily injections of 5 or 10 IU/kg insulin before or after a stimulating dose of ethanol and subsequent testing in an automated activity monitor. Blood glucose levels were determined upon the completion of the experiments.

KEY FINDINGS

Insulin injected prior to ethanol blunted the acute stimulant response as well as the acquisition and expression of locomotor sensitization, while insulin given after ethanol did not affect the development of the sensitized response. In a separate experiment, mice given glucose concurrently with insulin developed ethanol-induced locomotor sensitization normally.

SIGNIFICANCE

These experiments suggest that insulin attenuates the development of ethanol-induced locomotor sensitization, and that blood glucose levels can largely account for this effect. Further studies of the role of ethanol-induced metabolic states should provide novel information on the expression of ethanol-related behaviors.

Glucose-responsive neurons in the subfornical organ of the rat--a novel site for direct CNS monitoring of circulating glucose

Glucose-sensitive neurons have been identified in a number of CNS regions including metabolic control centers of the hypothalamus. The location of these regions behind the blood-brain barrier restricts them to sensing central, but not circulating glucose concentrations. In this study, we have used patch-clamp electrophysiology to examine whether neurons in a specialized region lacking the blood-brain barrier, the subfornical organ (SFO), are also glucose sensitive. In dissociated SFO neurons, altering the bath concentration of glucose (1 mM, 5 mM, 10 mM) influenced the excitability of 49% of neurons tested (n=67). Glucose-inhibited (GI) neurons depolarized in response to decreased glucose (n=10; mean, 4.6±1.0 mV) or hyperpolarized in response to increased glucose (n=8; mean,-4.4±0.8 mV). In contrast, glucose-excited (GE) neurons depolarized in response to increased glucose (n=9; mean, 6.4±0.4 mV) or hyperpolarized in response to decreased glucose (n=6; mean,-4.8±0.6 mV). Using voltage-clamp recordings, we also identified GI (outward current to increased glucose) and GE (inward current to increased glucose) SFO neurons. The mean glucose-induced inward current had a reversal potential of -24±12 mV (n=5), while GE responses were maintained during sodium-dependent glucose transporter inhibition, supporting the conclusion that GE properties result from the activation of a nonselective cation conductance (NSCC). The glucose-induced outward current had a mean reversal potential of -78±1.2 mV (n=5), while GI responses were not observed in the presence of glibenclamide, suggesting that these properties result from the modulation of K(ATP) channels. These data demonstrate that SFO neurons are glucose responsive, further emphasizing the potential roles of this circumventricular organ as an important sensor and integrator of circulating signals of energy status.

Glucosensing and glucose homeostasis: from fish to mammals

This review is focused on two topics related to glucose in vertebrates. In a first section devoted to glucose homeostasis we describe how glucose levels fluctuate and are regulated in different classes of vertebrates. The detection of these fluctuations is essential for homeostasis and for other physiological processes such as regulation of food intake. The capacity of that detection is known as glucosensing, and the different mechanisms through which it occurs are known as glucosensors. Different glucosensor mechanisms have been demonstrated in different tissues and organs of rodents and humans whereas the information obtained for other vertebrates is scarce. In the second section of the review we describe the present knowledge regarding glucosensor mechanisms in different groups of vertebrates, with special emphasis in fish.

Hyperphagia induced by sucrose: relation to circulating and CSF glucose and corticosterone and orexigenic peptides in the arcuate nucleus

Sucrose-rich diets compared to starch-rich diets are known to stimulate overeating under chronic conditions. The present study in normal-weight rats established an acute "preload-to-test meal" paradigm for demonstrating sucrose-induced hyperphagia and investigating possible mechanisms that mediate this behavioral phenomenon. In this acute paradigm, the rats were first given a small (15 kcal) sucrose preload (30% sucrose) for 30 min compared to an equicaloric, starch preload (25% starch with 5% sucrose) and then allowed to freely consume a subsequent test meal of lab chow. The sucrose preload, when compared to a starch preload equal in energy density and palatability, consistently increased food intake in the subsequent test meal occurring between 60 and 120 min after the end of the preload. Measurements of hormones, metabolites and hypothalamic peptides immediately preceding this hyperphagia revealed marked differences between the sucrose vs starch groups that could contribute to the increase in food intake. Whereas the sucrose group compared to the starch group immediately after the preload (at 10 min) had elevated levels of glucose in serum and cerebrospinal fluid (CSF) along with reduced expressions of neuropeptide Y (NPY) and agouti-related protein (AgRP) in the arcuate nucleus (ARC), the subsequent effects (at 30-60 min) just preceding the test meal hyperphagia were the reverse. Along with lower levels of glucose, they included markedly elevated serum and CSF levels of corticosterone and mRNA levels of NPY and AgRP in the ARC. In addition to establishing an animal model for sucrose-induced hyperphagia, these results demonstrate peripheral and central mechanisms that may mediate this behavioral phenomenon.

Characteristics and mechanisms of hypothalamic neuronal fatty acid sensing

We assessed the mechanisms by which specialized hypothalamic ventromedial nucleus (VMN) neurons utilize both glucose and long-chain fatty acids as signaling molecules to alter their activity as a potential means of regulating energy homeostasis. Fura-2 calcium (Ca(2+)) and membrane potential dye imaging, together with pharmacological agents, were used to assess the mechanisms by which oleic acid (OA) alters the activity of dissociated VMN neurons from 3- to 4-wk-old rats. OA excited up to 43% and inhibited up to 29% of all VMN neurons independently of glucose concentrations. In those neurons excited by both 2.5 mM glucose and OA, OA had a concentration-dependent effective excitatory concentration (EC(50)) of 13.1 nM. Neurons inhibited by both 2.5 mM glucose and OA had an effective inhibitory concentration (IC(50)) of 93 nM. At 0.5 mM glucose, OA had markedly different effects on these same neurons. Inhibition of carnitine palmitoyltransferase, reactive oxygen species formation, long-chain acetyl-CoA synthetase and ATP-sensitive K(+) channel activity or activation of uncoupling protein 2 (UCP2) accounted for only approximately 20% of OA's excitatory effects and approximately 40% of its inhibitory effects. Inhibition of CD36, a fatty acid transporter that can alter cell function independently of intracellular fatty acid metabolism, reduced the effects of OA by up to 45%. Thus OA affects VMN neuronal activity through multiple pathways. In glucosensing neurons, its effects are glucose dependent. This glucose-OA interaction provides a potential mechanism whereby such "metabolic sensing" neurons can respond to differences in the metabolic states associated with fasting and feeding.

Neurocircuits integrating hormone and nutrient signaling in control of glucose metabolism

As obesity, diabetes, and associated comorbidities are on a constant rise, large efforts have been put into better understanding the cellular and molecular mechanisms by which nutrients and metabolic signals influence central and peripheral energy regulation. For decades, peripheral organs as a source and a target of such cues have been the focus of study. Their ability to integrate metabolic signals is essential for balanced energy and glucose metabolism. Only recently has the pivotal role of the central nervous system in the control of fuel partitioning been recognized. The rapidly expanding knowledge on the elucidation of molecular mechanisms and neuronal circuits involved is the focus of this review.

The brain, appetite, and obesity

Food intake and energy expenditure are controlled by complex, redundant, and distributed neural systems that reflect the fundamental biological importance of adequate nutrient supply and energy balance. Much progress has been made in identifying the various hormonal and neural mechanisms by which the brain informs itself about availability of ingested and stored nutrients and, in turn, generates behavioral, autonomic, and endocrine output. While hypothalamus and caudal brainstem play crucial roles in this homeostatic function, areas in the cortex and limbic system are important for processing information regarding prior experience with food, reward, and emotion, as well as social and environmental context. Most vertebrates can store a considerable amount of energy as fat for later use, and this ability has now become one of the major health risks for many human populations. The predisposition to develop obesity can theoretically result from any pathological malfunction or lack of adaptation to changing environments of this highly complex system.

The ketogenic diet and epilepsy

PURPOSE OF REVIEW

The ketogenic diet has long been used to treat medically refractory epilepsy. The mechanisms underlying its clinical effects, however, have remained a mystery. The evidence to date suggests that a fundamental shift from glycolysis to intermediary metabolism induced by the ketogenic diet is necessary and sufficient for clinical efficacy. This notion is supported by a growing number of studies indicating that glucose restriction, ketone bodies and polyunsaturated fatty acids may all play mechanistic roles, possibly by enhancing mitochondrial respiration and ATP production, and decreasing reactive oxygen species production.

RECENT FINDINGS

Recent reports indicate that ketone bodies can reduce oxidative stress and that fatty acid-induced mitochondrial uncoupling may also yield similar protective effects. Ketone bodies may attenuate spontaneous firing of ATP-sensitive potassium channels in central neurons, and pharmacological inhibition of glycolysis has been shown to retard epileptogenesis in a rat kindling model.

SUMMARY

While the mechanisms underlying the broad clinical efficacy of the ketogenic diet remain unclear, there is growing evidence that the ketogenic diet alters the fundamental biochemistry of neurons in a manner that not only inhibits neuronal hyperexcitability but also induces a protective effect. Thus, the ketogenic diet may ultimately be useful in the treatment of a variety of neurological disorders.

Reduced expression of the KATP channel subunit, Kir6.2, is associated with decreased expression of neuropeptide Y and agouti-related protein in the hypothalami of Zucker diabetic fatty rats

The link between obesity and diabetes is not fully understood but there is evidence to suggest that hypothalamic signalling pathways may be involved. The hypothalamic neuropeptides, pro-opiomelanocortin (POMC), neuropeptide Y (NPY) and agouti-related protein (AGRP) are central to the regulation of food intake and have been implicated in glucose homeostasis. Therefore, the expression of these genes was quantified in hypothalami from diabetic Zucker fatty (ZDF) rats and nondiabetic Zucker fatty (ZF) rats at 6, 8, 10 and 14 weeks of age. Although both strains are obese, only ZDF rats develop pancreatic degeneration and diabetes over this time period. In both ZF and ZDF rats, POMC gene expression was decreased in obese versus lean rats at all ages. By contrast, although there was the expected increase in both NPY and AGRP expression in obese 14-week-old ZF rats, the expression of NPY and AGRP was decreased in 6-week-old obese ZDF rats with hyperinsulinaemia and in 14-week-old rats with the additional hyperglycaemia. Therefore, candidate genes involved in glucose, and insulin signalling pathways were examined in obese ZDF rats over this age range. We found that expression of the ATP-sensitive potassium (K(ATP)) channel component, Kir6.2, was decreased in obese ZDF rats and was lower compared to ZF rats in each age group tested. Furthermore, immunofluorescence analysis showed that Kir6.2 protein expression was reduced in the dorsomedial and ventromedial hypothalamic nuclei of 6-week-old prediabetic ZDF rats compared to ZF rats. The Kir6.2 immunofluorescence colocalised with NPY throughout the hypothalamus. The differences in Kir6.2 expression in ZF and ZDF rats mimic those of NPY and AGRP, which could infer that the changes occur in the same neurones. Overall, these data suggest that chronic changes in hypothalamic Kir6.2 expression may be associated with the development of hyperinsulinaemia and hyperglycaemia in ZDF rats.

Glucose-monitoring neurons in the nucleus accumbens

The nucleus accumbens, a key structure of the limbic circuitry, is involved in the regulation of motivated behaviors. The accumbens performs its roles via interconnections with brain areas where glucose-monitoring neurons have been localized. To search for such integrative chemosensory cells here, extracellular single neuron activity was recorded in the nucleus accumbens of Wistar rats by means of multibarreled glass microelectrodes during microelectrophoretic administration of D-glucose and other chemicals. Every fourth neuron tested changed in activity in response to glucose. Accumbens cells also displayed distinct gamma-amino-n-butyric acid type sensitivity. It is suggested that differential distribution of the chemosensory units, demonstrated between subdivisions of the nucleus accumbens, has particular significance with respect to functional dichotomy of the shell and core subregions.

Regulation of KATP channel subunit gene expression by hyperglycemia in the mediobasal hypothalamus of female rats

The ATP-sensitive potassium (K(ATP)) channels are gated by intracellular adenine nucleotides coupling cell metabolism to membrane potential. Channels comprised of Kir6.2 and SUR1 subunits function in subpopulations of mediobasal hypothalamic (MBH) neurons as an essential component of a glucose-sensing mechanism in these cells, wherein uptake and metabolism of glucose leads to increase in intracellular ATP/ADP, closure of the channels, and increase in neuronal excitability. However, it is unknown whether glucose and/or insulin may also regulate the gene expression of the channel subunits in the brain. The present study investigated whether regulation of K(ATP) channel subunit gene expression might be a mechanism by which neuronal populations adapt to prolonged changes in glucose and/or insulin levels in the periphery. Ovariectomized, steroid-replaced rats were fitted with indwelling jugular catheters and infused for 48 h with saline, glucose (hyperglycemia-hyperinsulinemia), insulin and glucose (hyperinsulinemia), diazoxide (control), or glucose and diazoxide (hyperglycemia). At the end of infusions, the MBH, preoptic area, and pituitary were dissected for RNA isolation and RT-PCR. Hyperglycemia decreased Kir6.2 mRNA levels in the MBH in both the presence and absence of hyperinsulinemia. These same conditions also produced a trend toward decreased SUR1 mRNA levels in the MBH; however, it did not exceed statistical significance. Hyperglycemia increased whereas hyperinsulinemia reduced neuropeptide Y mRNA levels when these groups were compared with each other. However, neither was significantly different from values observed in saline-infused controls. In conclusion, hyperglycemia per se may alter expression of K(ATP) channels and thereby induce changes in the excitability of some MBH neurons.

Glucose response to an oral glucose tolerance test predicts weight change in non-diabetic subjects

OBJECTIVE

Glucose exerts a dual action in the regulation of energy balance, consisting of inhibition of energy intake and stimulation of energy expenditure. Whether blood glucose affects long-term regulation of body weight in humans remains to be established. We sought to test the hypothesis that the post-challenge glucose response is a predictor of weight change.

RESEARCH METHODS AND PROCEDURES

We performed a prospective analysis of the impact of glucose response to an oral glucose tolerance test (OGTT) and a mixed-meal test (MT) on subsequent changes in body weight (BW) on 253 Pima Indians (166 men and 87 women) with normal glucose regulation at baseline and follow-up (follow-up: 7 +/- 4 years). Main outcome measures included BW change (total, percent, and annual), plasma glucose and insulin concentrations during OGTT and MT [total and incremental areas under the curve (AUCs)], resting metabolic rate (RMR; indirect calorimetry), and insulin action (euglycemic-hyperinsulinemic clamp).

RESULTS

Total and incremental glucose AUCs during the OGTT (but not the MT) were negatively associated with BW change (total, percent, and annual), both before and after adjusting for sex, age, initial BW, follow-up time, insulin action, RMR, fasting plasma glucose and insulin concentrations, and insulin response. Total and incremental glucose AUCs during the OGTT were independent determinants of final BW with age, initial BW, follow-up time, fasting plasma insulin concentrations, and RMR.

DISCUSSION

Higher post-challenge glucose response protects against BW gain in subjects with normal glucose regulation. We propose that this action may be because of the effect of glucose on food intake and/or thermogenesis.

NPY/AgRP neurons are not essential for feeding responses to glucoprivation

Animals respond to hypoglycemia by eating and by stimulating gluconeogenesis. These responses to glucose deprivation are initiated by glucose-sensing neurons in the brain, but the neural circuits that control feeding behavior are not well established. Neurons in the arcuate region of the hypothalamus that express neuropeptide Y (NPY) and agouti-related protein (AgRP) have been implicated in mediating the feeding response to glucoprivation. We devised a method to selectively ablate these neurons in neonatal mice and then tested adult mice for their feeding responses to fasting, mild hypoglycemia, 2-deoxy-d-glucose and a ghrelin receptor agonist. Whereas the feeding response to the ghrelin receptor agonist was completely abrogated, the feeding response to glucoprivation was normal. The feeding response after a fast was attenuated when standard chow was available but normal with more palatable solid or liquid diet. We conclude that NPY/AgRP neurons are not necessary for generating or mediating the orexigenic response to glucose deficiency, but they are essential for the feeding response to ghrelin and refeeding on standard chow after a fast.

Glucose sensing by hypothalamic neurones and pancreatic islet cells: AMPle evidence for common mechanisms?

A fuller understanding of the central mechanisms involved in controlling food intake and metabolism is likely to be crucial for developing treatments to combat the growing problem of obesity in Westernised societies. Within the hypothalamus, specialized neurones respond to both appetite-regulating hormones and circulating metabolites to regulate feeding behaviour accordingly. Thus, the activity of hypothalamic glucose-excited and glucose-inhibited neurones is increased or decreased, respectively, by an increase in local glucose concentration. These 'glucose-sensing' neurones may therefore play a key role in the central regulation of food intake and potentially in the regulation of blood glucose concentrations. Whilst the intracellular signalling mechanisms through which glucose-sensing neurones detect changes in the concentration of the sugar have been investigated quite extensively, many elements remain poorly understood. Furthermore, the similarities, or otherwise, with other nutrient-sensing cells, including pancreatic islet cells, are not completely resolved. In this review, we discuss recent advances in this field and explore the potential involvement of AMP-activated protein kinase and other nutrient-regulated protein kinases.

Pharmacological and molecular characterization of ATP-sensitive K(+) conductances in CART and NPY/AgRP expressing neurons of the hypothalamic arcuate nucleus

The role of hypothalamic ATP-sensitive potassium channels in the maintenance of energy homeostasis has been extensively explored. However, how these channels are incorporated into the neuronal networks of the arcuate nucleus remains unclear. Whole-cell patch-clamp recordings from rat arcuate nucleus neurons in hypothalamic slice preparations revealed widespread expression of functional ATP-sensitive potassium channels within the nucleus. ATP-sensitive potassium channels were expressed in orexigenic neuropeptide Y/agouti-related protein (NPY/AgRP) and ghrelin-sensitive neurons and in anorexigenic cocaine-and-amphetamine regulated transcript (CART) neurons. In 70% of the arcuate nucleus neurons recorded, exposure to glucose-free bathing medium induced inhibition of electrical excitability, the response being characterized by membrane hyperpolarization, a reduction in neuronal input resistance and a reversal potential consistent with opening of potassium channels. These effects were reversible upon re-introduction of glucose to the bathing medium or upon exposure to the ATP-sensitive potassium channel blockers tolbutamide or glibenclamide. The potassium channel opener diazoxide, but not pinacidil, also induced a tolbutamide and glibenclamide-sensitive inhibition of electrical excitability. Single-cell reverse transcription-polymerase chain reaction revealed expression of mRNA for sulfonylurea receptor 1 but not sulfonylurea receptor 2 subunits of ATP-sensitive potassium channels. Thus, rat arcuate nucleus neurons, including those involved in functionally antagonistic orexigenic and anorexigenic pathways express functional ATP-sensitive potassium channels which include sulfonylurea receptor 1 subunits. These data indicate a crucial role for these ion channels in central sensing of metabolic and energy status. However, further studies are needed to clarify the differential roles of these channels, the organization of signaling pathways that regulate them and how they operate in functionally opposing cell types.

Metabolic sensing neurons and the control of energy homeostasis

The brain and periphery carry on a constant conversation; the periphery informs the brain about its metabolic needs and the brain provides for these needs through its control of somatomotor, autonomic and neurohumoral pathways involved in energy intake, expenditure and storage. Metabolic sensing neurons are the integrators of a variety of metabolic, humoral and neural inputs from the periphery. Such neurons, originally called "glucosensing", also respond to fatty acids, hormones and metabolites from the periphery. They are integrated within neural pathways involved in the regulation of energy homeostasis. Unlike most neurons, they utilize glucose and other metabolites as signaling molecules to regulate their membrane potential and firing rate. For glucosensing neurons, glucokinase acts as the rate-limiting step in glucosensing while the pathways that mediate responses to metabolites like lactate, ketone bodies and fatty acids are less well characterized. Many metabolic sensing neurons also respond to insulin and leptin and other peripheral hormones and receive neural inputs from peripheral organs. Each set of afferent signals arrives with different temporal profiles and by different routes and these inputs are summated at the level of the membrane potential to produce a given neural firing pattern. In some obese individuals, the relative sensitivity of metabolic sensing neurons to various peripheral inputs is genetically reduced. This may provide one mechanism underlying their propensity to become obese when exposed to diets high in fat and caloric density. Thus, metabolic sensing neurons may provide a potential therapeutic target for the treatment of obesity.

Changes in extracellular hypothalamic glucose in relation to feeding

The aim of the present in vivo microdialysis study was to investigate the relation between feeding and changes in glucose concentrations in the rat ventromedial hypothalamus (VMH). Absolute ambient glucose concentrations in VMH were 1.43 mm in non-deprived rats as compared to 0.94 mm after 24-h food deprivation. To examine whether feeding influences hypothalamic glucose, changes of glucose concentration over time were determined relative to a baseline. Experiments were conducted in relation to both, nutritional state (food-deprived rats vs. non-deprived rats) and feeding conditions throughout the experiment (freely feeding rats vs. rats without access to food). The results of this microdialysis study show clearly that glucose concentration in the VMH of rats increases significantly in relation to food intake. The data demonstrate that a 24-h food deprivation before the experiment further augments this increase (up to 350% from baseline) as compared to non-deprived conditions (up to 60% from baseline). However, the magnitude of food related increase in VMH glucose does not correlate with the individual amount of food eaten. In conclusion, the present study shows for the first time that VMH glucose concentrations increase with food intake in the early dark phase, indicating that such changes do not only occur after pharmacological treatment, but also under physiological feeding conditions. The results further indicate that the feeding related increase in VMH glucose depends on the nutritional state of the organism.

Metabolic imprinting: critical impact of the perinatal environment on the regulation of energy homeostasis

Epidemiological studies in humans suggest that maternal undernutrition, obesity and diabetes during gestation and lactation can all produce obesity in offspring. Animal models have allowed us to investigate the independent consequences of altering the pre- versus post-natal environments on a variety of metabolic, physiological and neuroendocrine functions as they effect the development in the offspring of obesity, diabetes, hypertension and hyperlipidemia (the 'metabolic syndrome'). During gestation, maternal malnutrition, obesity, type 1 and type 2 diabetes and psychological, immunological and pharmacological stressors can all promote offspring obesity. Normal post-natal nutrition can reduce the adverse impact of some of these pre-natal factors but maternal high-fat diets, diabetes and increased neonatal access to food all enhance the development of obesity and the metabolic syndrome in offspring. The outcome of these perturbations of the perinatal environmental is also highly dependent upon the genetic background of the individual. Those with an obesity-prone genotype are more likely to be affected by factors such as maternal obesity and high-fat diets than are obesity-resistant individuals. Many perinatal manipulations appear to promote offspring obesity by permanently altering the development of central neural pathways, which regulate food intake, energy expenditure and storage. Given their strong neurotrophic properties, either excess or an absence of insulin and leptin during the perinatal period are likely to be effectors of these developmental changes. Because obesity is associated with an increased morbidity and mortality and because of its resistance to treatment, prevention is likely to be the best strategy for stemming the tide of the obesity epidemic. Such prevention should begin in the perinatal period with the identification and avoidance of factors which produce permanent, adverse alterations in neural pathways which control energy homeostasis.

Hypoglycemia, an Old Tool for New Findings in the Adrenomedullary Hormonal System in Patients with Rheumatic Diseases

Over the past decades, research in patients with rheumatic disorders showed enormous progress in detecting various perturbations of the neuroendocrine system including those affecting autonomic nervous function. There is, however, a substantial lack of data on adrenomedullary hormonal system (AMHS) function in those patients. Insulin-induced hypoglycemia (IIH) represents a metabolic stressor, which elicits a counterregulatory stress response not only of the hypothalamic-pituitary axis but also of the AMHS. Therefore, in addition to traditional testing of hypothalamic-pituitary function, IIH can be used as a well-controlled functional test of the AMHS. Our recent studies showed, for the first time, attenuated epinephrine and norepinephrine responses to IIH in premenopausal females with rheumatoid arthritis (RA) and systemic sclerosis (SSc). These findings are suggestive of downregulation, or possibly defects, of the AMHS in those patients. This article reviews mechanism of the AMHS activation during IIH and demonstrates applications of the test in neuroendocrine-immune research.

Differential effects of central leptin, insulin, or glucose administration during fasting on the hypothalamic-pituitary-thyroid axis and feeding-related neurons in the arcuate nucleus

The reductions in circulating levels of leptin, insulin, and glucose with fasting serve as important homeostasis signals to neurons of the hypothalamic arcuate nucleus that synthesize neuropeptide Y (NPY)/agouti-related protein (AGRP) and alpha-MSH/cocaine and amphetamine-regulated transcript. Because the central administration of leptin is capable of preventing the inhibitory effects of fasting on TRH mRNA in hypophysiotropic neurons primarily through effects on the arcuate nucleus, we determined whether the continuous administration of 30 mU/d insulin or 648 microg/d glucose into the cerebrospinal fluid by osmotic minipump might also have similar effects on the hypothalamic-pituitary-thyroid axis. As anticipated, the intracerebroventricular infusion of leptin reduced fasting-induced elevations in NPY and AGRP mRNA and increased proopiomelanocortin and cocaine and amphetamine-regulated transcript mRNA in the arcuate nucleus. In addition, leptin prevented fasting-induced reduction in pro-TRH mRNA levels in the paraventricular nucleus and in circulating thyroid hormone levels. In contrast, whereas insulin increased proopiomelanocortin mRNA and both insulin and glucose reduced NPY mRNA in arcuate nucleus neurons, neither prevented the fasting-induced suppression in hypophysiotropic TRH mRNA or circulating thyroid hormone levels. We conclude that insulin and glucose only partially replicate the central effects of leptin and may not be essential components of the hypothalamic-pituitary-thyroid regulatory system during fasting.

Central regulation of energy balance: inputs, outputs and leptin resistance

The regulation of energy balance is complex and, in man, imprecise. Nevertheless, in many individuals intake and expenditure are balanced with <1% error with little or no conscious effect. Essential components of such a regulatory system are signals, leptin and insulin, that reflect the size of lipid stores. Leptin receptors signal via phosphatidylinositol 3-kinase (as do insulin receptors) and via the transcription factor signal transducer and activator of transcription-3 to activate various types of neurone. Obese rodents, and possibly man, are resistant to leptin; in some cases because of genetic or perinatal programming (primary resistance), but commonly in response to high leptin levels (secondary resistance). Secondary leptin resistance may be a result of reduced transport of leptin to the brain or down-regulation of leptin signalling. Signals that reflect lipid stores form the tonic homeostatic regulatory system. They interact with episodic homeostatic signals carried by neurones, hormones and metabolites to regulate meal size and frequency. They also interact with signals related to the palatability of food, biorhythms and learning. Many neurotransmitters and hormones mediate responses to more than one input (e.g. gastric and adipocyte leptin), but are nevertheless most involved with particular inputs (e.g. leptin with adipocyte fat stores). Feeding can be divided into appetitive (preparation for feeding) and consummatory phases, which can both be further subdivided. Different sets of neurotransmitters and hormones are involved at each stage. In the long term it may be possible to customise obesity therapies according to those inputs and outputs that are most disturbed and most amenable to intervention in individual subjects.

Hyperglycemia does not increase basal hypothalamo-pituitary-adrenal activity in diabetes but it does impair the HPA response to insulin-induced hypoglycemia

Recently, we established that hypothalamo-pituitary-adrenal (HPA) and counterregulatory responses to insulin-induced hypoglycemia were impaired in uncontrolled streptozotocin (STZ)-diabetic (65 mg/kg) rats and insulin treatment restored most of these responses. In the current study, we used phloridzin to determine whether the restoration of blood glucose alone was sufficient to normalize HPA function in diabetes. Normal, diabetic, insulin-treated, and phloridzin-treated diabetic rats were either killed after 8 days or subjected to a hypoglycemic (40 mg/dl) glucose clamp. Basal: Elevated basal ACTH and corticosterone in STZ rats were normalized with insulin but not phloridzin. Increases in hypothalamic corticotrophin-releasing hormone (CRH) and inhibitory hippocampal mineralocorticoid receptor (MR) mRNA with STZ diabetes were not restored with either insulin or phloridzin treatments. Hypoglycemia: In response to hypoglycemia, rises in plasma ACTH and corticosterone were significantly lower in diabetic rats compared with controls. Insulin and phloridzin restored both ACTH and corticosterone responses in diabetic animals. Hypothalamic CRH mRNA and pituitary pro-opiomelanocortin mRNA expression increased following 2 h of hypoglycemia in normal, insulin-treated, and phloridzin-treated diabetic rats but not in untreated diabetic rats. Arginine vasopressin mRNA was unaltered by hypoglycemia in all groups. Interestingly, hypoglycemia decreased hippocampal MR mRNA in control, insulin-, and phloridzin-treated diabetic rats but not uncontrolled diabetic rats, whereas glucocorticoid receptor mRNA was not altered by hypoglycemia. In conclusion, despite elevated basal HPA activity, HPA responses to hypoglycemia were markedly reduced in uncontrolled diabetes. We speculate that defects in the CRH response may be related to a defective MR response. It is intriguing that phloridzin did not restore basal HPA activity but it restored the HPA response to hypoglycemia, suggesting that defects in basal HPA function in diabetes are due to insulin deficiency, but impaired responsiveness to hypoglycemia appears to stem from chronic hyperglycemia.

A new focus on interoceptive properties of adrenal medulla

The adrenal medulla is an important part of the sympathoadrenal system. Chromaffin cells of the adrenal medulla respond to a broad spectrum of stressful situations by releasing epinephrine and norepinephrine. Originally, it was accepted that this response is controlled exclusively by central nervous system structures. However, it was also demonstrated that a surgically denervated adrenal medulla can respond directly by secreting epinephrine and norepinephrine during an imbalance of internal environment (hypoglycemia, asphyxia). Published data had documented the innervation of the adrenal medulla by sensory neurons of spinal dorsal root ganglia. In addition, recent data showed that ganglion cells of the adrenal medulla project ascending axons. These data suggested potential transmission of information from the adrenal medulla to the central nervous system regarding metabolic changes in the blood. This paper presents an overview of possible involvement of adrenal medullary chromaffin cells in the detection of changes in the internal environment and in the transmission of this information to the central nervous system.

Glucose injection reduces neuropeptide Y and agouti-related protein expression in the arcuate nucleus: A possible physiological role in eating behavior

Evidence suggests that neuropeptide Y (NPY) and agouti-related protein (AgRP) in the arcuate nucleus (ARC) are modulated by glucoregulatory hormones and involved in maintaining normal eating patterns and glucose homeostasis in states of energy deficiency. This study investigated whether these peptides respond to glucose itself under conditions, e.g., before the nocturnal feeding cycle, when carbohydrate stores are low. After removal of food 3 h before dark onset, Sprague-Dawley rats were given a single, intraperitoneal (i.p.) injection of saline or 10% glucose (0.13 g/kg) and were sacrificed at different intervals, from 3.5 to 90 min later, for measurements of circulating hormones and metabolites or of NPY and AgRP mRNA in the ARC. With no change in insulin, leptin, or triglycerides, glucose injection produced a 1.8-mM rise in circulating glucose during the first 15 min, followed by a 30-60% reduction in NPY and AgRP mRNA at 30 and 60 min post-injection. A similar effect was observed with intraventricular administration of 5% glucose. At 90 min, however, this suppressive effect of i.p. glucose relative to saline was lost and actually reversed into a 50% increase in NPY and AgRP, possibly attributed to a decline in circulating glucose followed by a 50% rise in corticosterone at 60 min. These biphasic shifts over a 90-min period may reflect mechanisms underlying natural eating patterns at the onset of the nocturnal cycle, when spontaneous meals are approximately 90 min apart and rich in carbohydrate, glucose levels are low, and corticosterone and ARC peptides naturally peak.

Fructose normalizes specific counterregulatory responses to hypoglycemia in patients with type 1 diabetes

We have previously reported that specific counterregulatory responses to hypoglycemia were augmented by an infusion of fructose in nondiabetic humans. We hypothesized that this effect was due to the interaction of a "catalytic" dose of fructose with the regulatory protein for glucokinase in glucose-sensing cells that drive counterregulation. To examine whether fructose could restore counterregulatory responses in type 1 diabetic patients with defective counterregulation, we performed stepped hypoglycemic clamp studies (5.0, 4.4, 3.9, and 3.3 mmol/l glucose steps, 50 min each) in eight intensively treated patients (HbA(1c) 6.4 +/- 0.7%) on two separate occasions: without (control) or with coinfusion of fructose (1.2 mg . kg(-1) . min(-1)). Fructose induced a resetting of the glycemic threshold for secretion of epinephrine to higher plasma glucose concentrations (from 3.3 +/- 0.1 to 3.9 +/- 0.1 mmol/l; P = 0.001) and markedly augmented the increment in epinephrine (by 56%; P < 0.001). The amplification of epinephrine responses was specific; plasma norepinephrine, glucagon, growth hormone, and cortisol were unaffected. Hypoglycemia-induced endogenous glucose production ([3-(3)H]-glucose) rose by 90% (P < 0.001) in the fructose studies, compared with -2.0% (NS) in control. In concert, the glucose infusion rates during the 3.9- and 3.3-mmol/l steps were significantly lower with fructose (2.3 +/- 0.6 and 0.0 +/- 0.0 vs. 5.9 +/- 1.15 and 3.9 +/- 1.0 micromol . kg(-1) . min(-1), respectively; P < 0.001 for both), indicating the more potent counterregulatory response during fructose infusion. We conclude that infusion of fructose nearly normalizes the epinephrine and endogenous glucose production responses to hypoglycemia in type 1 diabetic patients with impaired counterregulation, suggesting that defects in these responses may be dependent on glucokinase-mediated glucose sensing.

Orexin-A projections to the caudal medulla and orexin-induced c-Fos expression, food intake, and autonomic function

Orexin-expressing neurons in the hypothalamus project throughout the neuraxis and are involved in regulation of the sleep/wake cycle, food intake, and autonomic functions. Here we specifically analyze the anatomical organization of orexin projections to the dorsal vagal complex (DVC) and raphe pallidus and effects on ingestive behavior and autonomic functions of local orexin-A administration in nonanesthetized rats. Retrograde tracing experiments revealed that as many as 20% of hypothalamic orexin neurons project to the DVC, where they form straight varicose axon profiles, some of which are in close anatomical apposition with tyrosine hydroxylase (TH)-, glucagon-like peptide-1-, gamma-aminobutyric acid-, and nitric oxide synthase-immunoreactive neurons in a nonselective manner. Similar contacts were frequently observed with neurons of the nucleus of the solitary tract whose activation by gastrointestinal food stimuli was demonstrated by the expression of nuclear c-Fos immunoreactivity. Orexin-A administration to the fourth ventricle induced significant Fos-expression throughout the DVC compared with saline control injections, with about 20-25% of TH-ir neurons among the stimulated ones. Fourth ventricular orexin injections also significantly stimulated chow and water intake in nonfood-deprived rats. Direct bilateral injections of orexin into the DVC increased intake of palatable high-fat pellets. Orexin-ir fibers also innervated raphe pallidus. Fourth ventricular orexin-A (1 nmol) activated Fos expression in the raphe pallidus and C1/A1 catecholaminergic neurons in the ventral medulla and increased body temperature, heart rate, and locomotor activity. The results confirm that hypothalamomedullary orexin projections are involved in a variety of physiological functions, including ingestive behavior and sympathetic outflow.

Sensory experience of food and obesity: a positron emission tomography study of the brain regions affected by tasting a liquid meal after a prolonged fast

The sensory experience of food is a primary reinforcer of eating and overeating plays a major role in the development of human obesity. However, whether the sensory experience of a forthcoming meal and the associated physiological phenomena (cephalic phase response, expectation of reward), which prepare the organism for the ingestion of food play a role in the regulation of energy intake and contribute to the development of obesity remains largely unresolved. We used positron emission tomography (PET) and 15O-water to measure changes in regional cerebral blood flow (rCBF) and to assess the brain's response to the oral administration of 2 ml of a liquid meal (Ensure Plus, 1.5 kcal/ml) after a 36-h fast and shortly before consuming the same meal. Twenty-one obese (BMI > 35 kg/m2, 10M/11F, age 28 +/- 6 years, body fat 40 +/- 6%) and 20 lean individuals (BMI < 25 kg/m2, 10M/10F, age 33 +/- 9 years, body fat 21 +/- 7%) were studied. Compared to lean individuals, obese individuals had higher fasting plasma glucose (83.3 +/- 6.2 vs. 75.5 +/- 9.6 mg/dl; P = 0.0003) and insulin concentrations (6.1 +/- 3.5 vs. 2.5 +/- 1.7 microU/ml; P < 0.0001) and were characterized by a higher score of dietary disinhibition (i.e., the susceptibility of eating behavior to emotional factors and sensory cues, 5.7 +/- 3.6 vs. 3.5 +/- 2.7; P = 0.01) assessed by the Three Factor Eating Questionnaire. In response to the sensory experience of food, differences in rCBF were observed in several regions of the brain, including greater increases in the middle-dorsal insula and midbrain, and greater decreases in the posterior cingulate, temporal, and orbitofrontal cortices in obese compared to lean individuals (P < 0.05, after small volume correction). In a multiple regression model, percentage of body fat (P = 0.04), glycemia (P = 0.01), and disinhibition (P = 0.07) were independent correlates of the neural response to the sensory experience of the meal in the middle-dorsal insular cortex (R2 = 0.45). We conclude that obesity is associated with an abnormal brain response to the sensory aspects of a liquid meal after a prolonged fast especially in areas of the primary gustatory cortex. This is only partially explained by the elevated glycemia and high level of disinhibition which characterize individuals with increased adiposity. These results provide a new perspective on the understanding of the neuroanatomical correlates of abnormal eating behavior and their relationship with obesity in humans.

Glucose improvement of memory: a review

The memory-improving action of glucose has now been studied for almost 20 years and the study of this phenomenon has led to a number of important developments in the understanding of memory, brain physiology and pathological consequences of impaired glucose tolerance. Glucose improvement of memory appears to involve two optimal doses in animals (100 mg/kg and 2 g/kg) that may correspond to two physiological mechanisms underlying glucose effects on memory. In humans, there have been few dose-response studies so the existence of more than one effective dose in humans is uncertain. Many tasks are facilitated by glucose in humans but tasks that are difficult to master or involve divided attention are improved more readily that easier tasks. There are a number of hypotheses about the physiological bases of the memory-improving action of glucose. Peripheral glucose injections could alleviate localized deficits in extracellular glucose in the hippocampus. These localized deficits may be due to changes in glucose transporters in that structure. Because certain neurotransmitters such as acetylcholine are directly dependent on the glucose supply for their synthesis, glucose is thought to facilitate neurotransmitter synthesis under certain circumstances. However, these hypotheses cannot account for the specificity of the dose-response effect of glucose. A number of peripheral mechanisms have been proposed, including the possibility that glucose-sensitive neurons in the brain or in the periphery may serve as glucose sensors and eventually produce neural changes that would facilitate memory processing. These latter results could be of importance because the mechanisms they suggest appear to be dose-dependent, a crucial characteristic to explain the dose-dependent effects of glucose. There may be an advantage to develop hypotheses that include both peripheral and central actions of glucose. There is evidence that impaired glucose regulation is associated with impaired cognition, particularly episodic memory. This impairment is minimal in young people but increases in older people (65 years and over) where it may compound other aging processes leading to reduced brain function. A small number of studies showed that glucose improvement of memory is associated with poor glucose regulation although this may not be the case for diabetic patients. Results of a few studies also suggest that drug treatments that improve glucose regulation also produce cognitive improvement in diabetic patients.

Neuronal glucosensing: what do we know after 50 years?

Glucosensing neurons are specialized cells that use glucose as a signaling molecule to alter their action potential frequency in response to variations in ambient glucose levels. Glucokinase (GK) appears to be the primary regulator of most neuronal glucosensing, but other regulators almost certainly exist. Glucose-excited neurons increase their activity when glucose levels rise, and most use GK and an ATP-sensitive K(+) channel as the ultimate effector of glucose-induced signaling. Glucose-inhibited (GI) neurons increase their activity at low glucose levels. Although many use GK, it is unclear what the final pathway of GI neuronal glucosensing is. Glucosensing neurons are located in brain sites and respond to and integrate a variety of hormonal, metabolic, transmitter, and peptide signals involved in the regulation of energy homeostasis and other biological functions. Although it is still uncertain whether daily fluctuations in blood glucose play a specific regulatory role in these physiological functions, it is clear that large decreases in glucose availability stimulate food intake and counterregulatory responses that restore glucose levels to sustain cerebral function. Finally, glucosensing is altered in obesity and after recurrent bouts of hypoglycemia, and this altered sensing may contribute to the adverse outcomes of these conditions. Thus, although much is known, much remains to be learned about the physiological function of brain glucosensing neurons.

On Gerard P. Smith's scientific character and thought

A fundamental tenet of the intellectual history of science is that the examination of the intellectual qualities of individual eminent scientists is a fruitful exercise for scholars who come after. I therefore review some salient aspects of the scientific character and thought of Gerard P. Smith (GPS), as they have been expressed in his writings. The first section considers some biographical details in respect to the sort of qualities that may prepare a mind for scientific achievement. The second section considers some logical and rhetorical qualities evident in the scientific writing of GPS, especially in the period of his career before the crucial tests of the CCK satiation hypothesis. The third section then considers the character of his thought as revealed in the formal statement, the operationalization, and the critical testing of the CCK satiation hypothesis. The qualities of mind discussed certainly contributed decisively to the scientific achievements of GPS, and their emulation may well contribute to the success of others.

The role of glucosensing neurons in the detection of hypoglycemia

Hypoglycemia is a life-threatening side effect of intensive insulin therapy in Type 1 diabetic patients. The ability to detect hypoglycemia and restore blood glucose levels to normal is of critical concern to the brain since glucose is its preferred fuel. When plasma glucose levels fall, powerful hormonal and sympathoadrenal mechanisms respond to restore blood glucose levels to normal. These mechanisms are believed to be initiated by diverse populations of glucose sensors, which are located centrally as well as peripherally. The exact contribution of each of these individual glucose sensors to the regulation of glucose homeostasis is not known at this time. This review focuses on the diversity of central and peripheral glucose sensors and the mechanisms by which they sense glucose.

Clonazepam increases in vivo striatal extracellular glucose in diabetic rats after glucose overload

Hyperglycemia modulates brain function, including neuronal excitability, neurotransmitter release and behavioral changes. There may be connections between the GABAergic system, glucose sensing neurons and glucose in the neuronal environment that shed light on the mechanism by which GABA(A) agents influence depressive behavior in diabetic rats submitted to the forced swimming test. We aimed to investigate whether clonazepam (CNZ), a GABA(A) receptor positive modulator, modifies in vivo striatal extracellular glucose levels in diabetic rats under fasting condition or after oral glucose overload. Streptozotocin diabetic and nondiabetic rats were submitted to in vivo striatal microdialysis. Perfusate samples were collected at baseline, during fasting and following administration of CNZ (0.25 mg/kg) and oral glucose overload. Blood glucose and striatal extracellular glucose were measured simultaneously at several time points. Fasting striatal glucose levels were higher in diabetic than in nondiabetic rats and the differences between these animals were maintained after glucose overload. The increases in extracellular striatal glucose after glucose overload were around 40% and blood to brain transference was decreased in diabetics. CNZ treatment paradoxically increased striatal glucose after glucose overload in diabetic rats, which may mark the dysfunction in brain glucose homeostasis.

Intestinal absorption in health and disease--sugars

Carbohydrates are mostly digested to glucose, fructose and galactose before absorption by the small intestine. Absorption across the brush border and basolateral membranes of enterocytes is mediated by sodium-dependent and -independent membrane proteins. Glucose and galactose transport across the brush border occurs by a Na(+)/glucose (galactose) co-transporter (SGLT1), whereas passive fructose transport is mediated by a uniporter (GLUT5). The passive exit of all three sugars out of the cell across the basolateral membrane occurs through two uniporters (GLUT2 and GLUT5). Mutations in SGLT1 cause a major defect in glucose and galactose absorption (glucose-galactose Malabsorption), but mutations in GLUT2 do not appear to disrupt glucose and galactose absorption. Studies on GLUT1 null mice and Fanconi-Bickel patients suggest that there is another exit pathway for glucose and galactose that may involve exocytosis. There are no known defects of fructose absorption.