The evaluation of insulin as a metabolic signal influencing behavior via the brain

Regulation of Memory Function by Feeding-Relevant Biological Systems: Following the Breadcrumbs to the Hippocampus

The hippocampus (HPC) controls fundamental learning and memory processes, including memory for visuospatial navigation (spatial memory) and flexible memory for facts and autobiographical events (declarative memory). Emerging evidence reveals that hippocampal-dependent memory function is regulated by various peripheral biological systems that are traditionally known for their roles in appetite and body weight regulation. Here, we argue that these effects are consistent with a framework that it is evolutionarily advantageous to encode and recall critical features surrounding feeding behavior, including the spatial location of a food source, social factors, post-absorptive processing, and other episodic elements of a meal. We review evidence that gut-to-brain communication from the vagus nerve and from feeding-relevant endocrine systems, including ghrelin, insulin, leptin, and glucagon-like peptide-1 (GLP-1), promote hippocampal-dependent spatial and declarative memory via neurotrophic and neurogenic mechanisms. The collective literature reviewed herein supports a model in which various stages of feeding behavior and hippocampal-dependent memory function are closely linked.

Vagal Interoceptive Modulation of Motivated Behavior

In addition to regulating the ingestion and digestion of food, sensory feedback from gut to brain modifies emotional state and motivated behavior by subconsciously shaping cognitive and affective responses to events that bias behavioral choice. This focused review highlights evidence that gut-derived signals impact motivated behavior by engaging vagal afferents and central neural circuits that generally serve to limit or terminate goal-directed approach behaviors, and to initiate or maintain behavioral avoidance.

Effects of zinc amino acid complex on biomarkers of gut integrity and metabolism during and following heat stress or feed restriction in pigs

Study objectives were to determine the effects of zinc (Zn) amino acid complex (Availa Zn, Zinpro Corporation, Eden Prairie, MN) on metabolism, biomarkers of leaky gut, and inflammation during and following heat stress (HS) and nutrient restriction. Crossbred gilts (n = 50; 50 ± 2 kg BW) were blocked by initial BW and randomly assigned to one of five treatments: 1) thermoneutral (TN) and ad libitum fed a control diet (TNCtl), 2) TN and pair-fed a control diet (PFCtl), 3) TN and pair-fed a Zn-supplemented diet (PFZn), 4) HS and ad libitum fed a control diet (HSCtl), and 5) HS and ad libitum fed a Zn-supplemented diet (HSZn). The study consisted of 3 experimental periods (P): during P1 (7 d), all pigs were fed their respective diets ad libitum and housed in TN conditions (20.84 ± 0.03 °C, 47.11 ± 0.42% relative humidity). During P2 (7 d), HSCtl and HSZn pigs were exposed to progressive cyclical HS conditions (27 to 30 °C, 41.9 ± 0.5% relative humidity), while TNCtl, PFCtl, and PFZn pigs remained in TN conditions and were fed ad libitum or pair-fed to their respective HSCtl and HSZn counterparts. During P3 (5 d; "recovery phase"), all pigs were housed in TN conditions and fed ad libitum. Pigs exposed to HS had overall increased rectal temperature, skin temperature, and respiration rate (0.33 °C, 3.76 °C, and 27 bpm, respectively; P < 0.01). Relative to TN controls, HS decreased ADFI and ADG (28 and 35%, respectively; P < 0.05), but these variables were unaffected by dietary treatment. Additionally, circulating insulin did not differ between HS and TN pigs (P = 0.41), but was decreased in PF relative to TN pigs (P < 0.01). During recovery, no differences were observed in rectal temperature or respiration rate across treatments, but HSZn pigs had decreased skin temperature relative to TN, PF, and HSCtl pigs (P < 0.01). During P3, no Zn effects were observed in production parameters; however, PF pigs had increased ADFI and ADG relative to TN and HS treatments (P < 0.01). During P3, circulating insulin was increased in pigs that were HS relative to TN and PF pigs (75%, P < 0.05). Interestingly, tumor necrosis factor alpha (TNFα) levels were decreased during P3 (P = 0.04) in Zn relative to Ctl-fed pigs. Circulating lipopolysaccharide-binding protein was not different among periods (P > 0.10). In summary, Zn reduced TNFα (regardless of HS), and the stimulatory effect of HS on insulin secretion is amplified during HS recovery.

Neural, Hormonal, and Cognitive Correlates of Metabolic Dysfunction and Emotional Reactivity

OBJECTIVE

Prediabetes and type 2 diabetes (i.e., hyperglycemia) are characterized by insulin resistance. These problems with energy metabolism may exacerbate emotional reactivity to negatively valenced stimuli and related phenomena such as predisposition toward negative affect, as well as cognitive deficits. Higher emotional reactivity is seen with hyperglycemia and insulin resistance. However, it is largely unknown how metabolic dysfunction correlates with related neural, hormonal, and cognitive outcomes.

METHODS

Among 331 adults from the Midlife in the United States study, eye-blink response (EBR) we cross sectionally examined to gauge reactivity to negative, positive, or neutrally valenced pictures from international affect picture system stimuli proximal to an acoustic startle probe. Increased EBR to negative stimuli was considered an index of stress reactivity. Frontal alpha asymmetry, a biomarker of negative affect predisposition, was determined using resting electroencephalography. Baseline urinary cortisol output was collected. Cognitive performance was gauged using the Brief Test of Adult Cognition by telephone. Fasting glucose and insulin characterized hyperglycemia or the homeostatic model assessment of insulin resistance.

RESULTS

Higher homeostatic model assessment of insulin resistance corresponded to an increased startle response, measured by EBR magnitude, for negative versus positive stimuli (R = 0.218, F(1,457) = 5.48, p = .020, euglycemia: M(SD) = .092(.776), hyperglycemia: M(SD) = .120(.881)). Participants with hyperglycemia versus euglycemia showed greater right frontal alpha asymmetry (F(1,307) = 6.62, p = .011, euglycemia: M(SD) = .018(.167), hyperglycemia: M(SD) = -.029(.160)), and worse Brief Test of Adult Cognition by telephone arithmetic performance (F(1,284) = 4.25, p = .040, euglycemia: M(SD) = 2.390(1.526), hyperglycemia: M(SD) = 1.920(1.462)). Baseline urinary cortisol (log10 μg/12 hours) was also dysregulated in individuals with hyperglycemia (F(1,324) = 5.09, p = .025, euglycemia: M(SD) = 1.052 ± .332, hyperglycemia: M(SD) = .961 (.362)).

CONCLUSIONS

These results suggest that dysmetabolism is associated with increased emotional reactivity, predisposition toward negative affect, and specific cognitive deficits.

Effects of intranasal insulin application on the hypothalamic BOLD response to glucose ingestion

The hypothalamus is a crucial structure in the brain that responds to metabolic cues and regulates energy homeostasis. Patients with type 2 diabetes demonstrate a lack of hypothalamic neuronal response after glucose ingestion, which is suggested to be an underlying cause of the disease. In this study, we assessed whether intranasal insulin can be used to enhance neuronal hypothalamic responses to glucose ingestion. In a randomized, double-blinded, placebo-controlled 4-double cross-over experiment, hypothalamic activation was measured in young non- diabetic subjects by determining blood-oxygen-level dependent MRI signals over 30 minutes before and after ingestion of 75 g glucose dissolved in 300 ml water, under intranasal insulin or placebo condition. Glucose ingestion under placebo condition lead to an average 1.4% hypothalamic BOLD decrease, under insulin condition the average response to glucose was a 2.2% decrease. Administration of water did not affect the hypothalamic BOLD responses. Intranasal insulin did not change circulating glucose and insulin levels. Still, circulating glucose levels showed a significant dampening effect on the BOLD response and insulin levels a significant strengthening effect. Our data provide proof of concept for future experiments testing the potential of intranasal application of insulin to ameliorate defective homeostatic control in patients with type 2 diabetes.

Effects of an oat-based high-fibre diet on insulin, glucose, cortisol and free fatty acid concentrations in gilts

To understand the mechanism underlying the effect of high-fibre diets on feeding motivation and stereotypic behaviour in pigs, non-pregnant gilts were given either concentrate or a high-fibre diet based on oat hulls and blood samples were assayed for cortisol, glucose, insulin and free fatty acids. The duration of eating was much longer for gilts on the high-fibre diet than gilts given concentrates. Concentrations of cortisol increased with food delivery and this was most pronounced with the concentrate diet. Concentrations of free fatty acids decreased before feeding and those of glucose and insulin decreased temporarily immediately after feeding began. The diet had no marked effect on these changes, although the decrease for insulin was less evident for the concentrate diet. As feeding continued, free fatty acid concentrations decreased, while concentrations of insulin and glucose increased. The increase in glucose and insulin was fastest with the concentrate diet, although post-feeding peak values did not differ. Postprandial cortisol and free fatty acid concentrations did not differ between diets. There was no evidence of a pre-prandial cephalic phase insulin or glucose response to feeding in the gilts although concentrations of both hormones decreased immediately after the gilts began to eat. The high-fibre diet tended to delay the peak glucose and insulin response to meals, suggesting that the reduced feeding motivation following consumption of high-fibre diets does not involve the same mechanism as the increased satiety following increased energy intake. As well as reducing the occurrence of ster eoty pies, high-fibre diets decrease cortisol concentrations at feeding.

Position on zinc delivery to olfactory nerves in intranasal insulin phase I-III clinical trials

Zinc in pancreatic insulin is essential for processing and action of the peptide, while in commercial preparations zinc promotes hexameric structure and prevents aggregate formation. In 2002, for the first time, insulin was delivered to humans intranasally with resulting cerebrospinal fluid insulin increases, but steady peripheral insulin levels. The novel method of increasing brain insulin levels without changes in the periphery resulted in an expansion of brain insulin research in clinical trials. As pre-clinical research has shown that brain insulin modulates a number functions, including food cravings and eating behavior, learning and memory functions, stress and mood regulation; realization of beneficial effects of insulin in modulating these functions in clinical populations became a possibility with the new direct-to-brain insulin delivery methodology. However, zinc, being integral to insulin structure and function, is neurotoxic, and has resulted in adverse effects to human health. In the last century, intranasal zinc was given preventively during the time of polio outbreak, and in the 21st century intranasal zinc was widely used over the counter to prevent common cold. In both cases, patients experienced partial or complete loss of smell. This paper is the first one to analyze zinc salts and concentrations of those two epidemiological adversities and directly compare formulations distributed to the public with animal toxicity data. The information gained from animal and epidemiological data provides a foundation for the formation of opinion given in this paper regarding safety of intranasal zinc in emerging clinical trials with intranasal insulin.

Differential effects of fructose versus glucose on brain and appetitive responses to food cues and decisions for food rewards

Prior studies suggest that fructose compared with glucose may be a weaker suppressor of appetite, and neuroimaging research shows that food cues trigger greater brain reward responses in a fasted relative to a fed state. We sought to determine the effects of ingesting fructose versus glucose on brain, hormone, and appetitive responses to food cues and food-approach behavior. Twenty-four healthy volunteers underwent two functional magnetic resonance imaging (fMRI) sessions with ingestion of either fructose or glucose in a double-blinded, random-order cross-over design. fMRI was performed while participants viewed images of high-calorie foods and nonfood items using a block design. After each block, participants rated hunger and desire for food. Participants also performed a decision task in which they chose between immediate food rewards and delayed monetary bonuses. Hormones were measured at baseline and 30 and 60 min after drink ingestion. Ingestion of fructose relative to glucose resulted in smaller increases in plasma insulin levels and greater brain reactivity to food cues in the visual cortex (in whole-brain analysis) and left orbital frontal cortex (in region-of-interest analysis). Parallel to the neuroimaging findings, fructose versus glucose led to greater hunger and desire for food and a greater willingness to give up long-term monetary rewards to obtain immediate high-calorie foods. These findings suggest that ingestion of fructose relative to glucose results in greater activation of brain regions involved in attention and reward processing and may promote feeding behavior.

Mechanism of programmed obesity: altered central insulin sensitivity in growth-restricted juvenile female rats

Intrauterine growth-restricted (IUGR) offspring are at increased risk of adult obesity, as a result of changes in energy balance mechanisms. We hypothesized that impairment of hypothalamic insulin signaling contributes to hyperphagia in IUGR offspring. Study pregnant dams were 50% food restricted from days 10 to 21 to create IUGR newborns. At 5 weeks of age, food intake was measured following intracerebroventricular (icv) injection of vehicle or insulin (10 mU) in control and IUGR pups. At 6 weeks of age, with pups in fed or fasted (48 h) states, pups received icv vehicle or insulin after which they were decapitated, and hypothalamic arcuate (ARC) nucleus dissected for RNA and protein expression. IUGR rats consumed more food than controls under basal conditions, consistent with upregulated ARC phospho AMP-activated protein kinase (pAMPK) and neuropeptide Y (NPY). Insulin acutely reduced food intake in both control and IUGR rats. Consistent with anorexigenic stimulation, central insulin decreased AMP-activated protein kinase and NPY mRNA expression and increased proopiomelanocortin mRNA expression and pAkt, with significantly reduced responses in IUGR as compared with controls. Despite feeding, IUGR offspring exhibit a persistent state of orexigenic stimulation in the ARC nucleus and relative resistance to the anorexigenic effects of icv insulin. These results suggest that impaired insulin signaling contributes to hyperphagia and obesity in IUGR offspring.

Effect of streptozotocin-induced diabetes on performance on a progressive ratio schedule

RATIONALE

It has been suggested that streptozotocin (STZ)-induced diabetes causes a motivational deficit in rodents. However, some of the evidence adduced in support of this suggestion may be interpreted in terms of a motor impairment rather than a motivational deficit.

OBJECTIVE

This experiment examined the effect of STZ-induced diabetes on performance on a progressive ratio schedule. The data were analysed using a new model derived from Killeen's (Behav Brain Sci 17:105-172, 1994) Mathematical Principles of Reinforcement model which enables the effects of interventions on motivation or incentive value to be separated from effects on motor function.

METHOD

Animals were trained under a progressive ratio schedule using food-pellet reinforcement. Then they received a single intraperitoneal injection of 50 mg/kg of STZ or the vehicle. Training continued for 30 sessions after treatment. Running and overall response rates in successive ratios were analysed using the new model, and estimates of the model's parameters were compared between groups.

RESULTS

The parameter expressing incentive value was reduced in the group treated with STZ, whereas the parameters expressing motor capacity and post-reinforcement pausing were not affected by the treatment. Blood glucose concentration was significantly elevated in the STZ-treated group compared to the vehicle-treated group.

CONCLUSIONS

The results are consistent with the suggestion that STZ-induced diabetes is associated with a reduction of the incentive value of food.

Divergent and convergent roles for insulin-like peptides in the worm, fly and mammalian nervous systems

Insulin signaling plays a critical role in coupling external changes to animal physiology and behavior. Despite remarkable conservation in the insulin signaling pathway components across species, divergence in the mechanism and function of the signal is evident. Focusing on recent findings from C. elegans, D. melanogaster and mammals, we discuss the role of insulin signaling in regulating adult neuronal function and behavior. In particular, we describe the transcription-dependent and transcription-independent aspects of insulin signaling across these three species. Interestingly, we find evidence of diverse mechanisms underlying complex networks of peptide action in modulating nervous system function.

From conditioned hypoglycemia to obesity: following the data

While a graduate student in the late 1960s I trained rats to lower their blood glucose in response to an arbitrary cue, a phenomenon called conditioned hypoglycemia. Over many years as my colleagues and I attempted to understand the underlying physiology of conditioned insulin secretion and conditioned hypoglycemia, it became clear that there were many implications that were highly important, including the entry of insulin into the brain, the existence of insulin receptors in certain brain areas, neural reflexes that project to insulin-secreting B-cells in the pancreas, the entrainment of those reflexes to improve the efficiency of meal-taking, and the possibility of adiposity signals from the body to the brain that influence behavior and metabolism. This article summarizes how we tackled each of these areas of research.

Effects of fructose vs glucose on regional cerebral blood flow in brain regions involved with appetite and reward pathways

IMPORTANCE

Increases in fructose consumption have paralleled the increasing prevalence of obesity, and high-fructose diets are thought to promote weight gain and insulin resistance. Fructose ingestion produces smaller increases in circulating satiety hormones compared with glucose ingestion, and central administration of fructose provokes feeding in rodents, whereas centrally administered glucose promotes satiety.

OBJECTIVE

To study neurophysiological factors that might underlie associations between fructose consumption and weight gain.

DESIGN, SETTING, AND PARTICIPANTS

Twenty healthy adult volunteers underwent 2 magnetic resonance imaging sessions at Yale University in conjunction with fructose or glucose drink ingestion in a blinded, random-order, crossover design.

MAIN OUTCOME MEASURES

Relative changes in hypothalamic regional cerebral blood flow (CBF) after glucose or fructose ingestion. Secondary outcomes included whole-brain analyses to explore regional CBF changes, functional connectivity analysis to investigate correlations between the hypothalamus and other brain region responses, and hormone responses to fructose and glucose ingestion.

RESULTS

There was a significantly greater reduction in hypothalamic CBF after glucose vs fructose ingestion (-5.45 vs 2.84 mL/g per minute, respectively; mean difference, 8.3 mL/g per minute [95% CI of mean difference, 1.87-14.70]; P = .01). Glucose ingestion (compared with baseline) increased functional connectivity between the hypothalamus and the thalamus and striatum. Fructose increased connectivity between the hypothalamus and thalamus but not the striatum. Regional CBF within the hypothalamus, thalamus, insula, anterior cingulate, and striatum (appetite and reward regions) was reduced after glucose ingestion compared with baseline (P < .05 significance threshold, family-wise error [FWE] whole-brain corrected). In contrast, fructose reduced regional CBF in the thalamus, hippocampus, posterior cingulate cortex, fusiform, and visual cortex (P < .05 significance threshold, FWE whole-brain corrected). In whole-brain voxel-level analyses, there were no significant differences between direct comparisons of fructose vs glucose sessions following correction for multiple comparisons. Fructose vs glucose ingestion resulted in lower peak levels of serum glucose (mean difference, 41.0 mg/dL [95% CI, 27.7-54.5]; P < .001), insulin (mean difference, 49.6 μU/mL [95% CI, 38.2-61.1]; P < .001), and glucagon-like polypeptide 1 (mean difference, 2.1 pmol/L [95% CI, 0.9-3.2]; P = .01).

CONCLUSION AND RELEVANCE

In a series of exploratory analyses, consumption of fructose compared with glucose resulted in a distinct pattern of regional CBF and a smaller increase in systemic glucose, insulin, and glucagon-like polypeptide 1 levels.

Insulin suppresses ghrelin-induced calcium signaling in neuropeptide Y neurons of the hypothalamic arcuate nucleus

Neuropeptide Y (NPY) neurons in the hypothalamic arcuate nucleus (ARC) play an important role in feeding regulation. Plasma levels of ghrelin and insulin show reciprocal dynamics before and after meals. We hypothesized that ghrelin and insulin also exert reciprocal effects on ARC NPY neurons. Cytosolic Ca²⁺ concentration ([Ca²⁺]_i) was measured by fura-2 microfluorometry in single neurons isolated from ARC of adult rats, followed by immunocytochemical identification of NPY neurons. Ghrelin at 10⁻¹⁰ M increased [Ca²⁺]_i in isolated ARC neurons, and co-administration of insulin concentration-dependently suppressed the ghrelin-induced [Ca²⁺]_i increases. Insulin at 10⁻¹⁶ M, 10⁻¹⁴ M, 10⁻¹² M and 10⁻¹⁰ M counteracted ghrelin action in 26%, 41%, 61% and 53% of ghrelin-responsive neurons, respectively, showing a maximal effect at 10⁻¹² M, the estimated postprandial concentration of insulin in the brain. The majority (>70%) of the ghrelin-activated insulin-inhibited neurons were shown to contain NPY. Double-immunohistochemistry revealed that 85% of NPY neurons in ARC express insulin receptors. These data demonstrate that insulin directly interacts with ARC NPY neurons and counteracts ghrelin action. Our results suggest that postprandial increase in plasma insulin/ghrelin ratio and insulin inhibition of ghrelin action on ARC NPY neurons cooperate to effectively inhibit the neuron activity and terminate feeding.

Insulin signaling and addiction

Across species, the brain evolved to respond to natural rewards such as food and sex. These physiological responses are important for survival, reproduction and evolutionary processes. It is no surprise, therefore, that many of the neural circuits and signaling pathways supporting reward processes are conserved from Caenorhabditis elegans to Drosophilae, to rats, monkeys and humans. The central role of dopamine (DA) in encoding reward and in attaching salience to external environmental cues is well recognized. Less widely recognized is the role of reporters of the "internal environment", particularly insulin, in the modulation of reward. Insulin has traditionally been considered an important signaling molecule in regulating energy homeostasis and feeding behavior rather than a major component of neural reward circuits. However, research over recent decades has revealed that DA and insulin systems do not operate in isolation from each other, but instead, work together to orchestrate both the motivation to engage in consummatory behavior and to calibrate the associated level of reward. Insulin signaling has been found to regulate DA neurotransmission and to affect the ability of drugs that target the DA system to exert their neurochemical and behavioral effects. Given that many abused drugs target the DA system, the elucidation of how dopaminergic, as well as other brain reward systems, are regulated by insulin will create opportunities to develop therapies for drug and potentially food addiction. Moreover, a more complete understanding of the relationship between DA neurotransmission and insulin may help to uncover etiological bases for "food addiction" and the growing epidemic of obesity. This review focuses on the role of insulin signaling in regulating DA homeostasis and DA signaling, and the potential impact of impaired insulin signaling in obesity and psychostimulant abuse.

The effect of TNFα on food intake and central insulin sensitivity in rats

Circulating and tissue levels of the proinflammatory cytokine tumor necrosis factor α (TNFα) are elevated in obesity. TNFα interferes with insulin signaling in many tissues and also plays a causal role in the anorexia that accompanies severe challenges to the immune system. The interactions between TNFα and insulin in the control of eating are less well known. The present study evaluated the role of TNFα in the central nervous system control of food intake by insulin in adult male Long Evans rats. We first determined the ability of several doses of TNFα injected into the 3rd cerebral ventricle (i3vt) to reduce food intake in male rats. Subsequently, we assessed the ability of a subthreshold dose of TNFα to modulate the effect of i3vt insulin on food intake in male rats fed a low-fat chow or a high-fat (HF) diet. TNFα administered i3vt dose-dependently reduced food intake in rats fed a standard low-fat chow diet. Moreover, a low, sub-threshold dose of TNFα diminished the reduction in food intake by insulin in rats maintained on a chow diet, but enhanced insulin action in rats maintained on a HF diet. These data suggest that the interaction of TNFα with central insulin varies with nutritional and/or dietary conditions.

Weight gain, schizophrenia and antipsychotics: new findings from animal model and pharmacogenomic studies

Excess body weight is one of the most common physical health problems among patients with schizophrenia that increases the risk for many medical problems, including type 2 diabetes mellitus, coronary heart disease, osteoarthritis, and hypertension, and accounts in part for 20% shorter life expectancy than in general population. Among patients with severe mental illness, obesity can be attributed to an unhealthy lifestyle, personal genetic profile, as well as the effects of psychotropic medications, above all antipsychotic drugs. Novel "atypical" antipsychotic drugs represent a substantial improvement on older "typical" drugs. However, clinical experience has shown that some, but not all, of these drugs can induce substantial weight gain. Animal models of antipsychotic-related weight gain and animal transgenic models of knockout or overexpressed genes of antipsychotic receptors have been largely evaluated by scientific community for changes in obesity-related gene expression or phenotypes. Moreover, pharmacogenomic approaches have allowed to detect more than 300 possible candidate genes for antipsychotics-induced body weight gain. In this paper, we summarize current thinking on: (1) the role of polymorphisms in several candidate genes, (2) the possible roles of various neurotransmitters and neuropeptides in this adverse drug reaction, and (3) the state of development of animal models in this matter. We also outline major areas for future research.

Insulin and hippocampus activation in response to images of high-calorie food in normal weight and obese adolescents

Responsiveness to food cues, especially those associated with high-calorie nutrients may be a factor underlying obesity. An increased motivational potency of foods appears to be mediated in part by the hippocampus. To clarify this, we investigated by means of 3-T magnetic resonance imaging (MRI) the activation of the hippocampus and associated brain structures in response to pictures of high-calorie and low-calorie foods in 12 obese and 12 normal-weight adolescents. To investigate the relationship between neuronal activation patterns (e.g., hippocampus) to the caloric content of food images and plasma insulin levels, we performed a multiple regression analysis. Interestingly, a significant positive correlation between fasting plasma levels of insulin, waist circumference, and right hippocampal activation was seen after stimulation with high-caloric food images. BMI values did not correlate significantly with the hippocampal activation. On the other hand, we found a significant negative correlation in response to high-caloric food images and the plasma levels of insulin in the medial right gyrus frontalis superior and in the left thalamus. In summary, our data show that insulin is importantly involved in the central regulation of food intake. The significant positive relationship between hippocampal activation after stimulation with high-caloric food images, plasma insulin levels, and waist circumference suggests a permissive role of insulin signaling pathways in the hippocampal control of eating behavior. Interestingly, only the waist circumference, as a main indicator of abdominal obesity, correlated significantly with the hippocampal activation patterns, and not the BMI.

Segregation of acute leptin and insulin effects in distinct populations of arcuate proopiomelanocortin neurons

Acute leptin administration results in a depolarization and concomitant increase in the firing rate of a subpopulation of arcuate proopiomelanocortin (POMC) cells. This rapid activation of POMC cells has been implicated as a cellular correlate of leptin effects on energy balance. In contrast to leptin, insulin inhibits the activity of some POMC neurons. Several studies have described a "cross talk" between leptin and insulin within the mediobasal hypothalamus via the intracellular enzyme, phosphoinositol-3-kinase (PI3K). Interestingly, both insulin and leptin regulate POMC cellular activity by activation of PI3K; however, it is unclear whether leptin and insulin effects are observed in similar or distinct populations of POMC cells. We therefore used dual label immunohistochemistry/in situ hybridization and whole-cell patch-clamp electrophysiology to map insulin and leptin responsive arcuate POMC neurons. Leptin-induced Fos activity within arcuate POMC neurons was localized separate from POMC neurons that express insulin receptor. Moreover, acute responses to leptin and insulin were largely segregated in distinct subpopulations of POMC cells. Collectively, these data suggest that cross talk between leptin and insulin occurs within a network of cells rather than within individual POMC neurons.

Central insulin sensitivity in male and female juvenile rats

The incidence of juvenile obesity is increasing at an alarming rate. In adults, central insulin administration decreases hypothalamic orexigenic neuropeptides, food intake and body weight more effectively in males than females. Mechanisms regulating energy balance in juvenile animals are inherently different from those in adults due to differences in growth rates and hormonal milieu. Therefore, we sought to determine if central insulin treatment in juvenile rats (4 wk) would have similar sex-dependent effects on food intake as those reported in adult rats. Twenty-four hour food intake was measured following icv saline or insulin (0.01 or 0.1 U) prior to the onset of dark phase of the light cycle. An additional set of animals was used to assess the effects of central insulin on hypothalamic orexigenic (NPY, AgRP) and anorexigenic (POMC) neuropeptide mRNA expression. In both males and females, insulin reduced meal size initially (first 4 h) and later decreased meal frequency (4-24 h) to reduce cumulative food intake. Consistent with this, central insulin decreased hypothalamic NPY and AgRP and increased POMC mRNA expression. In contrast to adult studies, there were no demonstrated sex differences. These studies indicate that juvenile females and males are equally sensitive to central insulin anorexigenic effects, perhaps due to a lack of circulating gonadal hormones. The anorexigenic responsiveness of both genders suggests a potential pharmacologic approach to childhood obesity.

Hypothalamic reactive oxygen species are required for insulin-induced food intake inhibition: an NADPH oxidase-dependent mechanism

OBJECTIVE

Insulin plays an important role in the hypothalamic control of energy balance, especially by reducing food intake. Emerging data point to a pivotal role of reactive oxygen species (ROS) in energy homeostasis regulation, but their involvement in the anorexigenic effect of insulin is unknown. Furthermore, ROS signal derived from NADPH oxidase activation is required for physiological insulin effects in peripheral cells. In this study, we investigated the involvement of hypothalamic ROS and NADPH oxidase in the feeding behavior regulation by insulin.

RESEARCH DESIGN AND METHODS

We first measured hypothalamic ROS levels and food intake after acute intracerebroventricular injection of insulin. Second, effect of pretreatment with a ROS scavenger or an NADPH oxidase inhibitor was evaluated. Third, we examined the consequences of two nutritional conditions of central insulin unresponsiveness (fasting or short-term high-fat diet) on the ability of insulin to modify ROS level and food intake.

RESULTS

In normal chow-fed mice, insulin inhibited food intake. At the same dose, insulin rapidly and transiently increased hypothalamic ROS levels by 36%. The pharmacological suppression of this insulin-stimulated ROS elevation, either by antioxidant or by an NADPH oxidase inhibitor, abolished the anorexigenic effect of insulin. Finally, in fasted and short-term high-fat diet-fed mice, insulin did not promote elevation of ROS level and food intake inhibition, likely because of an increase in hypothalamic diet-induced antioxidant defense systems.

CONCLUSIONS

A hypothalamic ROS increase through NADPH oxidase is required for the anorexigenic effect of insulin.

Central and peripheral effects of chronic food restriction and weight restoration in the rat

Previous studies have demonstrated that some endocrine consequences of long-term caloric restriction persist after weight restoration in human subjects. Here we evaluate effects of chronic food restriction in rats that were restricted to 70% of control kcal for 4 wk and subsequently weight restored. Measures were taken from rats at 80% (chronically restricted; CR), 90% (partially weight restored; PR), 100% (fully weight restored; FR), and after 4 wk at 100% body weight of controls (extended weight restored; ER). Plasma insulin and leptin were decreased, and ghrelin was increased in CR compared with controls. Leptin and ghrelin normalized with weight restoration at PR, FR, and ER; however, baseline insulin was not normalized until the ER state. Hypothalamic mRNA expression levels for proopiomelanocortin (POMC), agouti-related protein (AgRP), and neuropeptide Y (NPY) revealed significantly less POMC mRNA expression in CR and PR rats, and significantly less arcuate NPY mRNA in PR and FR. In the dorsomedial hypothalamus, CR, PR, and FR rats had significantly increased NPY expression that was not normalized until the ER state. In response to a test meal, insulin and ghrelin release patterns were altered through the FR stage, and ghrelin remained affected at ER. Collectively, these data demonstrate that mere weight restoration is not sufficient to normalize hypothalamic gene expression levels and endocrine responses to a meal, and that meal-related ghrelin responses persist despite weight restoration for up to 4 wk.

A breakfast with alpha-lactalbumin, gelatin, or gelatin + TRP lowers energy intake at lunch compared with a breakfast with casein, soy, whey, or whey-GMP

BACKGROUND & AIMS

Dietary protein plays a role in body weight regulation, partly due to its effects on satiety. The objective was to compare the effects of casein-, soy-, whey-, whey without glycomacropeptide (GMP)-, alpha-lactalbumin-, gelatin-, or gelatin with tryptophan (TRP)-protein breakfasts at two concentrations on subsequent satiety and energy intake (EI).

METHODS

Twenty-four healthy subjects (mean+/-SEM BMI: 24.8+/-0.5 kg/m(2); age: 25+/-2 years) received a breakfast; a custard with casein, soy, whey, whey-GMP, alpha-lactalbumin, gelatin, or gelatin+TRP as protein source with either 10/55/35 (normal) or 25/55/20 (high) En% protein/carbohydrate/fat in a randomized, single-blind design. At the precedingly determined time point for lunch, 180 min, subjects were offered an ad lib lunch. Appetite profile (Visual Analogue Scales, VAS) and EI were determined.

RESULTS

Both at the level of 10 and 25 En% from protein, EI at lunch was approximately 20% lower after an alpha-lactalbumin or gelatin (+TRP) breakfast (2.5+/-0.2 MJ) compared with after a casein, soy, or whey-GMP breakfast (3.2+/-0.3 MJ, p<0.05). Appetite ratings at 180 min differed 15-25 mm (approximately 40%, p<0.05) between types of protein. Differences in EI were a function of differences in appetite ratings (R(2)=0.4, p<0.001).

CONCLUSIONS

Different proteins (alpha-lactalbumin, gelatin, gelatin+TRP) that are approximately 40% more satiating than other proteins (casein, soy, whey, whey-GMP) induce a related approximately 20% reduction of subsequent energy intake.

Effects of high and normal soyprotein breakfasts on satiety and subsequent energy intake, including amino acid and 'satiety' hormone responses

BACKGROUND

The role of dietary protein in short term satiety is of interest with respect to body weight regulation.

AIM

To compare the effects of a high versus a normal soyprotein breakfast on satiety and subsequent energy intake (EI), including 'satiety' hormones and plasma amino acid responses.

METHODS

Twenty-five healthy subjects (mean +/- SEM, BMI: 23.9 +/- 0.3 kg/m(2); age: 22 +/- 1 years) received a subject-specific standardized breakfast: a custard with soy as single protein type with either 10/55/35 (normal-protein) or 25/55/20 (high-protein) En% protein/carbohydrate/fat in a randomized, single-blind design. Appetite profile (Visual Analogue Scale, VAS), plasma glucose, insulin, Glucagon-like Peptide 1, ghrelin, and amino acid concentrations were determined for 4 h, determining the sensitive time point to assess EI. Since at 180 min glucose and insulin concentrations still were significantly different, in a second set of experiments subjects received an ad lib lunch at 180 min after the breakfasts; EI was assessed.

RESULTS

Overall the 25 En% soy-custard was rated as being more satiating than the 10 En% soy-custard (P < 0.01) and there was a difference at 20 min after breakfast (64 +/- 5 vs. 52 +/- 5 mmVAS, P < 0.05), related to higher postprandial taurine concentrations (P < 0.05). Insulin response was increased more after the 25 En% than after the 10 En% soy-custard (AUC: 7,520 +/- 929 vs. 4,936 +/- 468 mU/l h, P < 0.001). There was no difference in EI (25 En%: 3,212 +/- 280 kJ vs. 10 En%: 3,098 +/- 286 kJ, ns).

CONCLUSION

A high soyprotein breakfast is more satiating than a normal soyprotein breakfast related to elevated taurine and insulin concentrations.

Decreased levels of ghrelin, cortisol, and fasting blood sugar, but not n-octanoylated ghrelin, in Japanese schizophrenic inpatients treated with olanzapine

The mechanism by which chronic administration of olanzapine induces a marked weight gain in patients with schizophrenia remains unknown. We examined the influence of long-term treatment with olanzapine on plasma levels of hormones regulating food intake and energy homeostasis in schizophrenia. In this study, olanzapine was administered to 28 Japanese inpatients for 16 weeks after switching from typical antipsychotic drugs or risperidone. At endpoint, no significant changes in body weight or body mass index were found. There was a significant decrease in the plasma levels of ghrelin without any accompanying change in active, n-octanoylated ghrelin. Serum levels of leptin tended to be increased and a significant reduction in plasma cortisol levels was found. In addition, the levels of fasting blood sugar as well as free fatty acid were significantly decreased. Furthermore, we did not confirm any marked weight gain induced by chronic administration of olanzapine as previously reported. The reason for this discrepancy may be due to differences in subjects and treatment settings. Based on these findings, it is unlikely that the decrease in plasma ghrelin levels by chronic administration of olanzapine affects weight gain. Further studies examining the effect of chronic olanzapine administration on weight and energy homeostasis in inpatients are required.

Unlocking the Energy Dynamics of Executive Functioning: Linking Executive Functioning to Brain Glycogen

Past work suggests that executive functioning relies on glucose as a depletable energy, such that executive functioning uses a relatively large amount of glucose and is impaired when glucose is low. Glucose from the bloodstream is one energy source for the brain, and glucose stored in the brain as glycogen is another. A review of the literature on glycogen suggests that executive functioning uses it in much the same way as glucose, such that executive functioning uses glycogen and is impaired when glycogen is low. Findings on stress, physical persistence, glucose tolerance, diabetes, sleep, heat, and other topics provide general support for this view.

Effects of glucose-to-fructose ratios in solutions on subjective satiety, food intake, and satiety hormones in young men

BACKGROUND

The greater prevalence of obesity and the metabolic syndrome in the past 35 y has been attributed to the replacement of sucrose in the food supply with high-fructose corn syrup (HFCS).

OBJECTIVE

Two experiments were conducted to determine the effect of solutions containing sucrose, HFCS, or various ratios of glucose to fructose (G:F) on food intake (FI), average appetite (AA), blood glucose (BG), plasma insulin, ghrelin, and uric acid (UA) in men.

DESIGN

Sugar solutions (300 kcal/300 mL) were (in %) G20:F80, HFCS 55 (G45:F55), sucrose, and G80:F20 (experiment 1, n = 12) and G20:F80, G35:F65, G50:F50, sucrose, and G80:F20 (experiment 2, n = 19). The controls were a sweet energy-free control (experiment 1) and water (both experiments). Solutions were provided in a repeated-measures design. AA, BG, and FI were measured in all subjects. Hormonal responses and UA were measured in 7 subjects in experiment 2. Measurements were taken from baseline to 75 min. FI was measured at 80 min.

RESULTS

Sucrose and HFCS (experiment 1) and sucrose and G50:F50 (experiment 2) had similar effects on all dependent measures. All sugar solutions similarly reduced the AA area under the curve (AUC). FI and plasma UA concentrations were significantly (P < 0.05) lower after high-glucose solutions than after low-glucose solutions. The lower FI was associated with a greater BG AUC (P < 0.05) and smaller AA and ghrelin AUCs (P < 0.01). Insulin and BG AUCs were positively associated (P < 0.001).

CONCLUSION

Sucrose, HFCS, and G50:F50 solutions do not differ significantly in their short-term effects on subjective and physiologic measures of satiety, UA, and FI at a subsequent meal.

Effects of retrograde gastric electrical stimulation with pulse trains on gastric emptying of solids and plasma hormones in dogs

BACKGROUND

Retrograde gastric electrical stimulation (RGES) is proposed as a novel therapy for obesity. However, mechanisms of RGES are not fully investigated. The aim of this study was to investigate the effects of RGES with trains of pulses on gastric slow waves, gastric emptying of solids, and plasma concentrations of satiety-related peptides and glucose.

METHODS

Seven female beagle dogs implanted with 4 pairs of gastric electrodes on the gastric serosa were studied (control and RGES). Gastric emptying, gastric slow waves, and signs were recorded in each session. Plasma leptin, insulin, glucagons, and glucose were also measured.

RESULTS

RGES with pulse trains (a pulse width of 2 milliseconds) significantly decreased gastric emptying of solids and plasma insulin but has no effect on plasma leptin, glucagons, and glucose.

CONCLUSION

Acute gastric electrical stimulation with pulse trains is able to decrease gastric emptying of solids and plasma insulin but has no effects on plasma leptin, glucagons, and glucose.

Effects of high-fructose corn syrup and sucrose consumption on circulating glucose, insulin, leptin, and ghrelin and on appetite in normal-weight women

OBJECTIVE

Fructose has been implicated in obesity, partly due to lack of insulin-mediated leptin stimulation and ghrelin suppression. Most work has examined effects of pure fructose, rather than high-fructose corn syrup (HFCS), the most commonly consumed form of fructose. This study examined effects of beverages sweetened with HFCS or sucrose (Suc), when consumed with mixed meals, on blood glucose, insulin, leptin, ghrelin, and appetite.

METHODS

Thirty lean women were studied on two randomized 2-d visits during which HFCS- and Suc-sweetened beverages were consumed as 30% of energy on isocaloric diets during day 1 while blood was sampled. On day 2, food was eaten ad libitum. Subjects rated appetite at designated times throughout visits.

RESULTS

No significant differences between the two sweeteners were seen in fasting plasma glucose, insulin, leptin, and ghrelin (P > 0.05). The within-day variation in all four items was not different between the two visits (P > 0.05). Net areas under the curve were similar for glucose, insulin, and leptin (P > 0.05). There were no differences in energy or macronutrient intake on day 2. The only appetite variable that differed between sweeteners was desire to eat, which had a higher area under the curve the day after Suc compared with HFCS.

CONCLUSION

These short-term results suggest that, when fructose is consumed in the form of HFCS, the measured metabolic responses do not differ from Suc in lean women. Further research is required to examine appetite responses and to determine if these findings hold true for obese individuals, males, or longer periods.

Role of the autonomic nervous system in the development of hyperinsulinemia by high-carbohydrate formula feeding to neonatal rats

An early dietary intervention in the form of a high-carbohydrate (HC) milk formula in neonatal rat pups results in immediate onset of hyperinsulinemia. While increased insulin secretion in HC rats has been shown to be related to hypersensitivity to glucose, the immediate onset of hyperinsulinemia and its persistence throughout the suckling period suggest involvement of multiple systems that enhance insulin secretion in response to increased demand. Evidence presented here in 12-day-old HC rats indicates that altered activity of the autonomic nervous system contributes to enhanced insulin secretory responses to glucose stimulation through increased parasympathetic and decreased sympathetic signaling. Both in vivo and in vitro studies have shown that HC rats secrete significantly higher levels of insulin in response to glucose in the presence of acetylcholine, a cholinergic agonist, while sensitivity to inhibition of insulin secretion by oxymetazoline, an alpha(2a)-adrenergic receptor (alpha(2a)AR) agonist, was reduced. In addition, HC rats showed increased sensitivity to blockade of cholinergic-induced insulin secretion by the muscarinic type 3 receptor (M3R) antagonist 4-diphenylacetoxy-N-methylpiperidine methobromide, as well as increased potentiation of glucose-stimulated insulin secretion by treatment with yohimbine. Increases in islets levels of M3R, phospholipase C-beta1, and protein kinase Calpha mRNAs, as well as decreased alpha(2a)AR mRNA, in 12-day-old HC rats provide a mechanistic connection to the changes in insulin secretion seen in HC rats. In conclusion, altered autonomic regulation of insulin secretion, due to the HC nutritional intervention, contributes to the development of hyperinsulinemia in 12-day-old HC rats.

Effect of acute gastric electrical stimulation on the systemic release of hormones and plasma glucose in dogs

This study evaluated the effect of gastric electrical stimulation (GES) with various parameters on plasma concentrations of satiety-related peptides and glucose. GES was performed in nine healthy dogs via electrodes implanted in the middle of the lesser curvature. Four sessions were performed in each animal: control, stimulation with IGS (implantable gastric stimulation for obesity, 0.3 m sec), modified IGS (2 msec), and long pulses (300 msec). Blood samples were collected at 15 and 0 min before the meal and at 15, 30, and 60 min after the meal. GES was initiated 30 min before the first blood sample and maintained throughout collection. Plasma ghrelin, leptin, insulin and glucose were measured. The total AUCs of plasma ghrelin and leptin were not significantly affected by GES. The total AUC of plasma insulin was significantly lower with IGS and long pulse parameters (P < 0.05). The total AUC for plasma glucose was significantly lower in sessions with long pulses and modified IGS parameters (P < 0.05). We conclude that acute GES is able to change the release of some satiety-related peptides. Whether this is associated with the changed eating behavior and weight loss in obese patients needs further investigation.

Mercaptoacetate fails to block the feeding-inhibitory effect of the β3-adrenergic receptor agonist CGP 12177A

Peripherally administered beta3-adrenergic receptor (beta3-AR) agonists stimulate lipolysis and inhibit food intake. To test the hypothesis that this inhibition of feeding is due to a substrate-driven increase in hepatic fatty acid oxidation (FAO), we assessed the ability of the FAO inhibitor mercaptoacetate (MA) to reverse the feeding-inhibitory effect of the beta3-AR agonist CGP 12177A (CGP). Adult male Sprague-Dawley rats received intraperitoneal injections of 1 mg/kg CGP, of 45.6 mg/kg MA, or of both drugs, and the effects on food intake, plasma free fatty acids (FFA), and plasma beta-hydroxybutyrate (BHB), an indicator for hepatic FAO, were assessed. Control rats received saline injections. CGP significantly reduced food intake after 0.5 and 6 h and increased plasma FFA and BHB at 0.5 h, suggesting increased lipolysis and hepatic FAO. MA completely reversed the increase in plasma BHB and thus appeared to effectively abolish CGP's effect on hepatic FAO, but MA failed to affect CGP's feeding-inhibitory action. These findings do not support the hypothesis that the beta3-AR agonist CGP inhibits feeding by enhancing hepatic FAO or ketogenesis. Although the beta3-AR agonist CGP reduced saccharin intake in a one-bottle condition taste aversion test, it seems unlikely that the hypophagic effect of CGP is elicited by malaise.

Pancreatic signals controlling food intake; insulin, glucagon and amylin

The control of food intake and body weight by the brain relies upon the detection and integration of signals reflecting energy stores and fluxes, and their interaction with many different inputs related to food palatability and gastrointestinal handling as well as social, emotional, circadian, habitual and other situational factors. This review focuses upon the role of hormones secreted by the endocrine pancreas: hormones, which individually and collectively influence food intake, with an emphasis upon insulin, glucagon and amylin. Insulin and amylin are co-secreted by B-cells and provide a signal that reflects both circulating energy in the form of glucose and stored energy in the form of visceral adipose tissue. Insulin acts directly at the liver to suppress the synthesis and secretion of glucose, and some plasma insulin is transported into the brain and especially the mediobasal hypothalamus where it elicits a net catabolic response, particularly reduced food intake and loss of body weight. Amylin reduces meal size by stimulating neurons in the hindbrain, and there is evidence that amylin additionally functions as an adiposity signal controlling body weight as well as meal size. Glucagon is secreted from A-cells and increases glucose secretion from the liver. Glucagon acts in the liver to reduce meal size, the signal being relayed to the brain via the vagus nerves. To summarize, hormones of the endocrine pancreas are collectively at the crossroads of many aspects of energy homeostasis. Glucagon and amylin act in the short term to reduce meal size, and insulin sensitizes the brain to short-term meal-generated satiety signals; and insulin and perhaps amylin as well act over longer intervals to modulate the amount of fat maintained and defended by the brain. Hormones of the endocrine pancreas interact with receptors at many points along the gut-brain axis, from the liver to the sensory vagus nerve to the hindbrain to the hypothalamus; and their signals are conveyed both neurally and humorally. Finally, their actions include gastrointestinal and metabolic as well as behavioural effects.

Alterations in blood glucose levels under hyperinsulinemia affect accumbens dopamine

Dopaminergic systems have been implicated in diabetes and obesity. Notwithstanding, the most basic relationship between dopamine and plasma insulin as well as glucose levels yet remains unknown. The present experiments were designed to investigate the effects of acute hyperinsulinemia on basal dopamine levels in the nucleus accumbens of the rat under chloral hydrate anesthesia using acute microdialysis in combination with the hyperinsulinemic-glycemic clamping procedure. In Experiment 1, each rat was infused with one of the three concentrations of insulin (2.4, 4.8, or 9.6 mU/kg per min) while plasma glucose levels were maintained at euglycemia (approximately 5.5 mmol/L). Dopamine, dihydroxyphenylacetic acid and homovanillic acid were not significantly different from baseline during either the clamp or post-clamp periods for all insulin concentrations. In Experiment 2, rats were infused with the highest concentration of insulin (9.6 mU/kg per min) and plasma glucose levels were maintained at either hypoglycemia (approximately 3 mmol/L) or hyperglycemia (approximately 14 mmol/L). Dopamine was elevated at 100 min (+113% above basal levels) and 120 min (+117%) in the hypoglycemic condition and at 120 min (+121%) in the hyperglycemic condition. In the hyperglycemic post-clamp period, homovanillic acid was decreased below basal levels (approximately -32%). These results together suggest that short-term blood glucose deviations coupled with acute hyperinsulinemia affect the mesoaccumbens dopamine system.

Gonadal hormones determine sensitivity to central leptin and insulin

Males have proportionally more visceral fat and are more likely to develop complications associated with obesity than females, and the male brain is relatively more sensitive to the catabolic action of insulin and less sensitive to that of leptin than the female brain. To understand the underlying mechanism, we manipulated estrogen through ovariectomy (OVX) and estradiol administration. Rats with relatively high systemic estrogen (intact females and OVX females and males administered estrogen subcutaneously) were significantly more sensitive to leptin's anorexic action in the brain (i3vt), as well as significantly less sensitive to insulin's i3vt action, than intact males. Administering estradiol directly into the brain of our females increased i3vt leptin sensitivity while decreasing i3vt insulin sensitivity and changed the body fat distribution of our females to resemble that of intact females. These data indicate that estrogen acts within the brain to increase leptin sensitivity, decrease insulin sensitivity, and favor subcutaneous over visceral fat.

Integration of metabolic stimuli in the hypothalamic arcuate nucleus

Integration of peripheral and central anabolic and catabolic inputs within the hypothalamic arcuate nucleus (ARC) is believed to be central to the maintenance of energy balance. In order to perform this complex task, neurons in the ARC express receptors for all major humoral and central transmitters involved in the maintenance of energy homeostasis. The integration of these inputs occurs at the cellular and circuit level and the resulting electrical output forms the origins for the activation of feeding and energy balance-related networks. Here, we discuss the role that active intrinsic membrane conductances, K(ATP) channels and intracellular second messenger systems play in the integration of metabolic stimuli at the cellular level in the ARC. We conclude that the research into the integration of hunger and satiety signals in the ARC has made substantial progress in the last decade, but we are far from unraveling the complex neuronal networks involved in the maintenance of energy homeostasis. The diverse range of inputs, neuronal integrative properties, targets, output signals and how these signals relate to the physiological output provides us with a colossal challenge for years to come. However, to battle the current obesity epidemic, target-specific drugs need to be developed for which the knowledge of neuronal pathways involved in the maintenance of energy homeostasis will be crucial.

Dietary fructose: implications for dysregulation of energy homeostasis and lipid/carbohydrate metabolism

Fructose intake and the prevalence of obesity have both increased over the past two to three decades. Compared with glucose, the hepatic metabolism of fructose favors lipogenesis, which may contribute to hyperlipidemia and obesity. Fructose does not increase insulin and leptin or suppress ghrelin, which suggests an endocrine mechanism by which it induces a positive energy balance. This review examines the available data on the effects of dietary fructose on energy homeostasis and lipid/carbohydrate metabolism. Recent publications, studies in human subjects, and areas in which additional research is needed are emphasized.

Obesity: basic science and medical aspects relevant to anaesthetists

Obesity is becoming a major public health problem throughout the world. It is now the second leading cause of death in the United States and is associated with significant, potentially life-threatening co-morbidities. Significant advances in the understanding of the physiology of body weight regulation and the pathogenesis of obesity have been achieved. A better understanding of the physiology of appetite control has enabled advances in the medical and surgical treatment of obesity. Visceral or abdominal obesity is associated with an increased risk of cardiovascular disease and type 2 diabetes. Various drugs are used in the treatment of mild obesity but they are associated with adverse effects. Surgery has become an essential part of the treatment of morbid obesity, notwithstanding the potential adverse events that accompany it. An appreciation of these problems is essential to the anaesthetist and intensivist involved in the management of this group of patients.

The prandial insulin sensitivity-modifying effect of vagal stimulation in rats

The effect of left cervical vagal nerve stimulation was studied on insulin sensitivity to test the proposed permissive insulin-sensitizing role of hepatic vagal parasympathetic efferent pathways in fasted and fed anesthetized rats. In fed animals, electrical stimulation (square impulses: 25 V, 5 Hz, 0.5 milliseconds over 15 minutes) of the vagal nerve induced hyperglycemia and an increase in plasma insulin immunoreactivity. Atropine (1.0 mg/kg intravenously) induced insulin resistance estimated by rapid insulin sensitivity testing. This was amplified when the vagal nerve was stimulated. The insulin-resistant state developed by fasting was not modified by either treatment with atropine or electrical stimulation. We conclude that both parasympathetic cholinergic and noncholinergic vagal efferents modulate postprandial neurogenic insulin sensitivity adjustments.

The critical role of the melanocortin system in the control of energy balance

Animals have developed highly adaptive and redundant mechanisms to maintain energy balance by matching caloric intake to caloric expenditure. Recent evidence has pointed to a variety of peripheral signals that inform specific central nervous system (CNS) circuits about the status of peripheral energy stores as critical to the maintenance of energy balance. A critical component of these CNS circuits is the melanocortin system. Regulation of signaling by melanocortin 3 and melanocortin 4 receptors in the CNS is controlled via neuronal cell bodies in the arcuate nucleus of the hypothalamus that synthesize melanocortin receptor agonists such as alpha-melanocyte-stimulating hormone (alpha-MSH) or antagonists such as agouti-related protein (AgRP). The activity of these two populations of neurons is reciprocally regulated by a number of peripheral and central systems that influence energy balance. Further, increased melanocortin signaling via pharmacological or genetic means in the CNS causes potent reductions in food intake and weight loss. Decreased melanocortin signaling via pharmacological or genetic means results in increased food intake and weight gain. Reviewed here is the wide range of evidence that points to the melanocortin system as a critical node in the diverse neurocircuitry that regulates food intake and body weight.

Thermoregulation of transgenic growth hormone mice

Transgenic growth hormone (TG) mice (Mus musculus L., 1758) obtain enhanced growth via compensatory feeding at intermediate sizes and via higher growth efficiency. The latter involves diverting resources from other functions such as locomotion and wakefulness. Thermogenesis is a major expense for small mammals, so we explored whether TG mice express a trade-off between growth and thermoregulation. TG mice are hypothermic and cannot maintain their body temperature under cold stress. TG mice showed initial enlargement of brown adipose tissue and subsequent age-related decreases not seen in controls. Some TG mice became torpid after fasting durations not known to affect other mice. On a high-calorie diet, TG mice had higher body temperatures even though controls did not. Our background strain developed obesity on a high-protein and high-fat diet, and on a diet supplemented with carbohydrates, whereas TG mice never developed obesity. White adipose tissue deposits of TG females were relatively larger, but those of TG males were relatively smaller, than those of controls fed standard food. We also found significant effects of the three experimental diets, as well as gender, age, body mass, ambient temperature, and behavioural activity, on rectal temperatures of TG mice and controls in a large breeding colony. Thermogenesis of TG mice fed standard food appears energetically constrained, likely contributing to enhanced growth efficiency.

Brain glucose and insulin: effects on food intake and brain biogenic amines of rainbow trout

The effects of central (intracerebroventricular, 9 microg fish(-1)) and peripheral (intraperitoneal, 4 mg kg(-1)) administration of bovine insulin, as well as the effect of hyperglycemia (oral administration of 1 g glucose fish(-1)) and brain glucodeprivation (intracerebroventricular administration of 2-deoxy-D-glucose) on food intake and levels of brain (telencephalon, preoptic area, and hypothalamus) biogenic amines (serotonin, dopamine, noradrenaline and their metabolites 5-hydroxyindoleacetic acid, and dihydroxyphenylacetic acid) were assessed on rainbow trout ( Oncorhynchus mykiss). Treatment with insulin inhibited food intake after 26 or 52 h of administration, central or peripheral, respectively. This effect was still apparent after 74 h of central treatment. When assessing changes in the levels of biogenic amines after 26 h of central insulin administration, there was a significant increase in the levels of 5-hydroxyindoleacetic acid, and in the ratio of dihydroxyphenylacetic acid/dopamine of insulin-treated fish, in telencephalon and hypothalamus, respectively. These results suggest that peripherally administered insulin is involved in a feedback regulatory loop with food intake and body weight. Moreover, at least part of the effects of insulin could be mediated by hypothalamic dopaminergic activity. The strong hyperglycemia induced by oral administration of glucose did not induce significant changes either on food intake (control versus treated), or in brain levels of biogenic amines. The intracerebroventricular administration of 2-deoxy-D-glucose induced an increase in food intake without altering plasma glucose levels, suggesting that fish brain possesses a control system for detecting hypoglycemia in plasma and therefore keep brain glucose levels high enough for brain function.