The insulin effect on cerebrocortical theta activity is associated with serum concentrations of saturated nonesterified Fatty acids

The insulin resistant brain: impact on whole-body metabolism and body fat distribution

Insulin exerts its actions not only on peripheral organs but is also transported into the brain where it performs distinct functions in various brain regions. This review highlights recent advancements in our understanding of insulin's actions within the brain, with a specific emphasis on investigations in humans. It summarises current knowledge on the transport of insulin into the brain. Subsequently, it showcases robust evidence demonstrating the existence and physiological consequences of brain insulin action, while also introducing the presence of brain insulin resistance in humans. This pathophysiological condition goes along with an impaired acute modulation of peripheral metabolism in response to brain insulin action, particularly in the postprandial state. Furthermore, brain insulin resistance has been associated with long-term adiposity and an unfavourable adipose tissue distribution, thus implicating it in the pathogenesis of subgroups of obesity and (pre)diabetes that are characterised by distinct patterns of body fat distribution. Encouragingly, emerging evidence suggests that brain insulin resistance could represent a treatable entity, thereby opening up novel therapeutic avenues to improve systemic metabolism and enhance brain functions, including cognition. The review closes with an outlook towards prospective research directions aimed at further elucidating the clinical implications of brain insulin resistance. It emphasises the critical need to establish feasible diagnostic measures and effective therapeutic interventions.

Brain insulin action on peripheral insulin sensitivity in women depends on menstrual cycle phase

Insulin action in the human brain modulates eating behaviour, whole-body metabolism and body fat distribution1,2. In particular, brain insulin action increases whole-body insulin sensitivity, but these studies were mainly performed in lean men3,4. Here we investigate metabolic and hypothalamic effects of brain insulin action in women with a focus on the impact of menstrual cycle ( ClinicalTrials.gov registration: NCT03929419 ).Eleven women underwent four hyperinsulinemic-euglycemic clamps, two in the follicular phase and two in the luteal phase. Brain insulin action was introduced using nasal insulin spray5-7 and compared to placebo spray in a fourfold crossover design with change in glucose infusion rate as the primary endpoint. Here we show that during the follicular phase, more glucose has to be infused after administration of nasal insulin than after administration of placebo. This remains significant after adjustment for blood glucose and insulin. During the luteal phase, no significant influence of brain insulin action on glucose infusion rate is detected after adjustment for blood glucose and insulin (secondary endpoint). In 15 other women, hypothalamic insulin sensitivity was assessed in a within-subject design by functional magnetic resonance imaging with intranasal insulin administration8. Hypothalamus responsivity is influenced by insulin in the follicular phase but not the luteal phase.Our study therefore highlights that brain insulin action improves peripheral insulin sensitivity also in women but only during the follicular phase. Thus, brain insulin resistance could contribute to whole-body insulin resistance in the luteal phase of the menstrual cycle.

Exercise restores brain insulin sensitivity in sedentary adults who are overweight and obese

BACKGROUNDInsulin resistance of the brain can unfavorably affect long-term weight maintenance and body fat distribution. Little is known if and how brain insulin sensitivity can be restored in humans. We aimed to evaluate the effects of an exercise intervention on insulin sensitivity of the brain and how this relates to exercise-induced changes in whole-body metabolism and behavior.METHODSIn this clinical trial, sedentary participants who were overweight and obese underwent an 8-week supervised aerobic training intervention. Brain insulin sensitivity was assessed in 21 participants (14 women, 7 men; age range 21-59 years; BMI range 27.5-45.5 kg/m2) using functional MRI, combined with intranasal administration of insulin, before and after the intervention.RESULTSThe exercise program resulted in enhanced brain insulin action to the level of a person of healthy weight, demonstrated by increased insulin-induced striatal activity and strengthened hippocampal functional connectivity. Improved brain insulin action correlated with increased mitochondrial respiration in skeletal muscle, reductions in visceral fat and hunger, as well as improved cognition. Mediation analyses suggest that improved brain insulin responsiveness helps mediate the peripheral exercise effects leading to healthier body fat distribution and reduced perception of hunger.CONCLUSIONOur study demonstrates that an 8-week exercise intervention in sedentary individuals can restore insulin action in the brain. Hence, the ameliorating benefits of exercise toward brain insulin resistance may provide an objective therapeutic target in humans in the challenge to reduce diabetes risk factors.TRIAL REGISTRATIONClinicalTrials.gov (NCT03151590).FUNDINGBMBF/DZD 01GI0925.

Ectopic fat accumulation in human astrocytes impairs insulin action

Astrocytes provide neurons with structural support and energy in form of lactate, modulate synaptic transmission, are insulin sensitive and act as gatekeeper for water, ions, glutamate and second messengers. Furthermore, astrocytes are important for glucose sensing, possess neuroendocrine functions and also play an important role in cerebral lipid metabolism. To answer the question, if there is a connection between lipid metabolism and insulin action in human astrocytes, we investigated if storage of ectopic lipids in human astrocytes has an impact on insulin signalling in those cells. Human astrocytes were cultured in the presence of a lipid emulsion, consisting of fatty acids and triglycerides, to induce ectopic lipid storage. After several days, cells were stimulated with insulin and gene expression profiling was performed. In addition, phosphorylation of Akt as well as glycogen synthesis and cell proliferation was assessed. Ectopic lipid storage was detected in human astrocytes after lipid exposure and lipid storage was persistent even when the fat emulsion was removed from the cell culture medium. Chronic exposure to lipids induced profound changes in the gene expression profile, whereby some genes showed a reversible gene expression profile upon removal of fat, and some did not. This included FOXO-dependent expression patterns. Furthermore, insulin-induced phosphorylation of Akt was diminished and also insulin-induced glycogen synthesis and proliferation was impaired in lipid-laden astrocytes. Chronic lipid exposure induces lipid storage in human astrocytes accompanied by insulin resistance. Analyses of the gene expression pattern indicated the potential of a partially reversible gene expression profile. Targeting astrocytic insulin resistance by reducing ectopic lipid load might represent a promising treatment target for insulin resistance of the brain in obesity, diabetes and neurodegeneration.

BMI-Independent Effects of Gestational Diabetes on Human Placenta

PURPOSE

Recently, alterations in maternal lipid metabolism were associated with gestational diabetes mellitus (GDM). However, detailed plasma lipid profiles and their relevance for placental and fetal metabolism are currently not understood.

METHODS

Maternal and placental lipid profiles were characterized in women with GDM and women with normal glucose tolerance (NGT). Inflammatory gene expression was compared in placentas and primary term trophoblasts between the groups. In addition, trophoblasts were stimulated with nonesterified fatty acids (NEFAs), and effects on gene expression were quantified. Finally, placental macrophage content and cord blood concentrations of inflammatory parameters and NEFAs were compared between women with GDM and women with NGT with similar body mass index (BMI).

RESULTS

Palmitate and stearate levels were elevated in both maternal plasma and placental tissue of women with GDM. Placental GDM-associated elevations of IL6, IL8, and TLR2 expression were reflected in trophoblasts derived from women with GDM. Stimulation of primary trophoblasts with palmitate led to increased mRNA expression and protein release of the cytokine IL6 and the chemokine IL8. In line with this, elevated amounts of CD68-positive cells were quantified in the placental tissue of women with GDM. No GDM-associated elevations in a range of inflammatory parameters and NEFAs in cord blood of NGT vs GDM neonates was found.

CONCLUSIONS

GDM, independently of BMI, altered maternal plasma NEFAs and the placental lipid profile. GDM was associated with trophoblast and whole-placenta lipoinflammation; however, this was not accompanied by elevated concentrations of inflammatory cytokines or NEFAs in neonatal cord blood.

Leptin Replacement Reestablishes Brain Insulin Action in the Hypothalamus in Congenital Leptin Deficiency

OBJECTIVE

Human obesity is associated with impaired central insulin signaling, and in very rare cases, severe obesity can be caused by congenital leptin deficiency. In such patients, leptin replacement results in substantial weight loss and improvement in peripheral metabolism.

RESEARCH DESIGN AND METHODS

In a leptin-deficient patient, we investigated the impact of leptin substitution on central insulin action, as quantified by changes in neuronal activity after intranasal insulin application. This was assessed before and during the first year of metreleptin substitution.

RESULTS

After only 1 year, treatment with metreleptin reestablishes brain insulin sensitivity, particularly in the hypothalamus and, to a lesser degree, in the prefrontal cortex. Results are depicted in comparison with a control group. In our patient, brain activation changes were accompanied by substantial weight loss, reduced visceral adipose tissue, reduced intrahepatic lipid content, and improved whole-body insulin sensitivity.

CONCLUSIONS

Leptin replacement and weight loss improved homeostatic insulin action in the patient in question.

Neuroendocrine Regulation of Food Intake in Polycystic Ovary Syndrome

Several peripheral and central signals are involved in the sophisticated regulation of food intake. Women with polycystic ovary syndrome (PCOS) are prone to consume a diet higher in saturated fat and foods with high glycemic index and show impaired appetite regulation and measures of satiety. As a consequence, obesity, mostly of the central type, is prevalent in the syndrome and worsens the endocrine and metabolic profile of the affected patients. This review article briefly analyzes the current knowledge about the neuroendocrine mechanisms underlying the interplay between feeding behavior, obesity, and reproductive abnormalities in PCOS.

Altered Dopamine Synaptic Markers in Postmortem Brain of Obese Subjects

Dopaminergic signaling in the reward pathway in the brain has been shown to play an important role in food intake and the development of obesity. Obese rats release less dopamine (DA) in the nucleus accumbens (NAc) after food intake, and amphetamine stimulated striatal DA release is reduced in vivo in obese subjects. These studies suggest that DA hypofunction associated with hedonic dysregulation is involved in the pathophysiology of obesity. To identify brain changes in obesity, quantitative measures of DA synaptic markers were compared in postmortem brain tissues of normal weight and obese subjects over a range of increasing body mass indices (BMI). DA transporter (DAT) numbers in the striatum were compared to the relative expression of DAT, tyrosine hydroxylase (TH) and D2 dopamine receptors (DRD2) in midbrain DA neurons. Radioligand binding assays of [3H]WIN35,428 demonstrated that the number of striatal DAT binding sites was inversely correlated with increasing BMI (r = -0.47; p < 0.01). DAT and TH gene expression were significantly decreased in the somatodendritic compartment of obese subjects (p < 0.001), with no significant change in DRD2 compared to normal weight subjects. The reduced density of striatal DAT with corresponding reductions in DAT and TH gene expression in substantia nigra (SN) suggests, that obesity is associated with hypodopaminergic function. A DA reward deficiency syndrome has been suggested to underlie abnormal eating behavior that leads to obesity. Neurobiological changes in presynaptic DA markers demonstrated postmortem in human brain support a link between hedonic DA dysregulation and obesity.

Intranasal insulin enhances brain functional connectivity mediating the relationship between adiposity and subjective feeling of hunger

Brain insulin sensitivity is an important link between metabolism and cognitive dysfunction. Intranasal insulin is a promising tool to investigate central insulin action in humans. We evaluated the acute effects of 160 U intranasal insulin on resting-state brain functional connectivity in healthy young adults. Twenty-five lean and twenty-two overweight and obese participants underwent functional magnetic resonance imaging, on two separate days, before and after intranasal insulin or placebo application. Insulin compared to placebo administration resulted in increased functional connectivity between the prefrontal regions of the default-mode network and the hippocampus as well as the hypothalamus. The change in hippocampal functional connectivity significantly correlated with visceral adipose tissue and the change in subjective feeling of hunger after intranasal insulin. Mediation analysis revealed that the intranasal insulin induced hippocampal functional connectivity increase served as a mediator, suppressing the relationship between visceral adipose tissue and hunger. The insulin-induced hypothalamic functional connectivity change showed a significant interaction with peripheral insulin sensitivity. Only participants with high peripheral insulin sensitivity showed a boost in hypothalamic functional connectivity. Hence, brain insulin action may regulate eating behavior and facilitate weight loss by modifying brain functional connectivity within and between cognitive and homeostatic brain regions.

Sustained Treatment with Insulin Detemir in Mice Alters Brain Activity and Locomotion

Aims

Recent studies have identified unique brain effects of insulin detemir (Levemir^®). Due to its pharmacologic properties, insulin detemir may reach higher concentrations in the brain than regular insulin. This might explain the observed increased brain stimulation after acute insulin detemir application but it remained unclear whether chronic insulin detemir treatment causes alterations in brain activity as a consequence of overstimulation.

Methods

In mice, we examined insulin detemir’s prolonged brain exposure by continuous subcutaneous (s.c.) application using either micro-osmotic pumps or daily s.c. injections and performed continuous radiotelemetric electrocorticography and locomotion recordings.

Results

Acute intracerebroventricular injection of insulin detemir activated cortical and locomotor activity significantly more than regular insulin in equimolar doses (0.94 and 5.63 mU in total), suggesting an enhanced acute impact on brain networks. However, given continuously s.c., insulin detemir significantly reduced cortical activity (theta: 21.3±6.1% vs. 73.0±8.1%, P<0.001) and failed to maintain locomotion, while regular insulin resulted in an increase of both parameters.

Conclusions

The data suggest that permanently-increased insulin detemir levels in the brain convert its hyperstimulatory effects and finally mediate impairments in brain activity and locomotion. This observation might be considered when human studies with insulin detemir are designed to target the brain in order to optimize treatment regimens.

Brain Insulin Resistance at the Crossroads of Metabolic and Cognitive Disorders in Humans

Ever since the brain was identified as an insulin-sensitive organ, evidence has rapidly accumulated that insulin action in the brain produces multiple behavioral and metabolic effects, influencing eating behavior, peripheral metabolism, and cognition. Disturbances in brain insulin action can be observed in obesity and type 2 diabetes (T2D), as well as in aging and dementia. Decreases in insulin sensitivity of central nervous pathways, i.e., brain insulin resistance, may therefore constitute a joint pathological feature of metabolic and cognitive dysfunctions. Modern neuroimaging methods have provided new means of probing brain insulin action, revealing the influence of insulin on both global and regional brain function. In this review, we highlight recent findings on brain insulin action in humans and its impact on metabolism and cognition. Furthermore, we elaborate on the most prominent factors associated with brain insulin resistance, i.e., obesity, T2D, genes, maternal metabolism, normal aging, inflammation, and dementia, and on their roles regarding causes and consequences of brain insulin resistance. We also describe the beneficial effects of enhanced brain insulin signaling on human eating behavior and cognition and discuss potential applications in the treatment of metabolic and cognitive disorders.

Functional neuroimaging in obesity and the potential for development of novel treatments

Recently, exciting progress has been made in understanding the role of the CNS in controlling eating behaviour and in the development of overeating. Regions and networks of the human brain involved in eating behaviour and appetite control have been identified with neuroimaging techniques such as functional MRI, PET, electroencephalography, and magnetoencephalography. Hormones that regulate our drive to eat (eg, leptin, insulin, and glucagon-like peptide-1) can affect brain function. Defects in central hunger signalling are present in many pathologies. On the basis of an understanding of brain mechanisms that lead to overeating, powerful neuroimaging protocols could be a future clinical approach to allow individually tailored treatment options for patients with obesity. The aim of our Review is to provide an overview of neuroimaging approaches for obesity (ie, neuroimaging study design, questions which can be answered by neuroimaging, and limitations of neuroimaging techniques), examine current models of central nervous processes regulating eating behaviour, summarise and review important neuroimaging studies investigating therapeutic approaches to treat obesity or to control eating behaviour, and to provide a perspective on how neuroimaging might lead to new therapeutic approaches to obesity.

Impaired insulin action in the human brain: causes and metabolic consequences

Over the past few years, evidence has accumulated that the human brain is an insulin-sensitive organ. Insulin regulates activity in a limited number of specific brain areas that are important for memory, reward, eating behaviour and the regulation of whole-body metabolism. Accordingly, insulin in the brain modulates cognition, food intake and body weight as well as whole-body glucose, energy and lipid metabolism. However, brain imaging studies have revealed that not everybody responds equally to insulin and that a substantial number of people are brain insulin resistant. In this Review, we provide an overview of the effects of insulin in the brain in humans and the relevance of the effects for physiology. We present emerging evidence for insulin resistance of the human brain. Factors associated with brain insulin resistance such as obesity and increasing age, as well as possible pathogenic factors such as visceral fat, saturated fatty acids, alterations at the blood-brain barrier and certain genetic polymorphisms, are reviewed. In particular, the metabolic consequences of brain insulin resistance are discussed and possible future approaches to overcome brain insulin resistance and thereby prevent or treat obesity and type 2 diabetes mellitus are outlined.

Weight-sparing effect of insulin detemir: a consequence of central nervous system-mediated reduced energy intake?

Insulin therapy is often associated with adverse weight gain. This is attributable, at least in part, to changes in energy balance and insulin's anabolic effects. Adverse weight gain increases the risk of poor macrovascular outcomes in people with diabetes and should therefore be mitigated if possible. Clinical studies have shown that insulin detemir, a basal insulin analogue, exerts a unique weight-sparing effect compared with other basal insulins. To understand this property, several hypotheses have been proposed. These explore the interplay of efferent and afferent signals between the muscles, brain, liver, renal and adipose tissues in response to insulin detemir and comparator basal insulins. The following models have been proposed: insulin detemir may reduce food intake through direct or indirect effects on the central nervous system (CNS); it may have favourable actions on hepatic glucose metabolism through a selective effect on the liver, or it may influence fluid homeostasis through renal effects. Studies have consistently shown that insulin detemir reduces energy intake, and moreover, it is clear that this shift in energy balance is not a consequence of reduced hypoglycaemia. CNS effects may be mediated by direct action, by indirect stimulation by peripheral mediators and/or via a more physiological counter-regulatory response to insulin through restoration of the hepatic-peripheral insulin gradient. Although the precise mechanism remains unclear, it is likely that the weight-sparing effect of insulin detemir can be explained by a combination of mechanisms. The evidence for each hypothesis is considered in this review.

Insulin action in the human brain: evidence from neuroimaging studies

Thus far, little is known about the action of insulin in the human brain. Nonetheless, recent advances in modern neuroimaging techniques, such as functional magnetic resonance imaging (fMRI) or magnetoencephalography (MEG), have made it possible to investigate the action of insulin in the brain in humans, providing new insights into the pathogenesis of brain insulin resistance and obesity. Using MEG, the clinical relevance of the action of insulin in the brain was first identified, linking cerebral insulin resistance with peripheral insulin resistance, genetic predisposition and weight loss success in obese adults. Although MEG is a suitable tool for measuring brain activity mainly in cortical areas, fMRI provides high spatial resolution for cortical as well as subcortical regions. Thus, the action of insulin can be detected within all eating behaviour relevant regions, which include regions deeply located within the brain, such as the hypothalamus, midbrain and brainstem, as well as regions within the striatum. In this review, we outline recent advances in the field of neuroimaging aiming to investigate the action of insulin in the human brain using different routes of insulin administration. fMRI studies have shown a significant insulin-induced attenuation predominantly in the occipital and prefrontal cortical regions and the hypothalamus, successfully localising insulin-sensitive brain regions in healthy, mostly normal-weight individuals. However, further studies are needed to localise brain areas affected by insulin resistance in obese individuals, which is an important prerequisite for selectively targeting brain insulin resistance in obesity.

The brain response to peripheral insulin declines with age: a contribution of the blood-brain barrier?

Objectives

It is a matter of debate whether impaired insulin action originates from a defect at the neural level or impaired transport of the hormone into the brain. In this study, we aimed to investigate the effect of aging on insulin concentrations in the periphery and the central nervous system as well as its impact on insulin-dependent brain activity.

Methods

Insulin, glucose and albumin concentrations were determined in 160 paired human serum and cerebrospinal fluid (CSF) samples. Additionally, insulin was applied in young and aged mice by subcutaneous injection or intracerebroventricularly to circumvent the blood-brain barrier. Insulin action and cortical activity were assessed by Western blotting and electrocorticography radiotelemetric measurements.

Results

In humans, CSF glucose and insulin concentrations were tightly correlated with the respective serum/plasma concentrations. The CSF/serum ratio for insulin was reduced in older subjects while the CSF/serum ratio for albumin increased with age like for most other proteins. Western blot analysis in murine whole brain lysates revealed impaired phosphorylation of AKT (P-AKT) in aged mice following peripheral insulin stimulation whereas P-AKT was comparable to levels in young mice after intracerebroventricular insulin application. As readout for insulin action in the brain, insulin-mediated cortical brain activity instantly increased in young mice subcutaneously injected with insulin but was significantly reduced and delayed in aged mice during the treatment period. When insulin was applied intracerebroventricularly into aged animals, brain activity was readily improved.

Conclusions

This study discloses age-dependent changes in insulin CSF/serum ratios in humans. In the elderly, cerebral insulin resistance might be partially attributed to an impaired transport of insulin into the central nervous system.

Maternal insulin sensitivity is associated with oral glucose-induced changes in fetal brain activity

AIMS/HYPOTHESIS

Fetal programming plays an important role in the pathogenesis of type 2 diabetes. The aim of the present study was to investigate whether maternal metabolic changes during OGTT influence fetal brain activity.

METHODS

Thirteen healthy pregnant women underwent an OGTT (75 g). Insulin sensitivity was determined by glucose and insulin measurements at 0, 60 and 120 min. At each time point, fetal auditory evoked fields were recorded with a fetal magnetoencephalographic device and response latencies were determined.

RESULTS

Maternal insulin increased from a fasting level of 67 ± 25 pmol/l (mean ± SD) to 918 ± 492 pmol/l 60 min after glucose ingestion and glucose levels increased from 4.4 ± 0.3 to 7.4 ± 1.1 mmol/l. Over the same time period, fetal response latencies decreased from 297 ± 99 to 235 ± 84 ms (p = 0.01) and then remained stable until 120 min (235 ± 84 vs 251 ± 91 ms, p = 0.39). There was a negative correlation between maternal insulin sensitivity and fetal response latencies 60 min after glucose ingestion (r = 0.68, p = 0.02). After a median split of the group based on maternal insulin sensitivity, fetuses of insulin-resistant mothers showed a slower response to auditory stimuli (283 ± 79 ms) than those of insulin-sensitive mothers (178 ± 46 ms, p = 0.03).

CONCLUSIONS/INTERPRETATION

Lower maternal insulin sensitivity is associated with slower fetal brain responses. These findings provide the first evidence of a direct effect of maternal metabolism on fetal brain activity and suggest that central insulin resistance may be programmed during fetal development.

Cinnamon extract improves insulin sensitivity in the brain and lowers liver fat in mouse models of obesity

Objectives

Treatment of diabetic subjects with cinnamon demonstrated an improvement in blood glucose concentrations and insulin sensitivity but the underlying mechanisms remained unclear. This work intends to elucidate the impact of cinnamon effects on the brain by using isolated astrocytes, and an obese and diabetic mouse model.

Methods

Cinnamon components (eugenol, cinnamaldehyde) were added to astrocytes and liver cells to measure insulin signaling and glycogen synthesis. Ob/ob mice were supplemented with extract from cinnamomum zeylanicum for 6 weeks and cortical brain activity, locomotion and energy expenditure were evaluated. Insulin action was determined in brain and liver tissues.

Results

Treatment of primary astrocytes with eugenol promoted glycogen synthesis, whereas the effect of cinnamaldehyde was attenuated. In terms of brain function in vivo, cinnamon extract improved insulin sensitivity and brain activity in ob/ob mice, and the insulin-stimulated locomotor activity was improved. In addition, fasting blood glucose levels and glucose tolerance were greatly improved in ob/ob mice due to cinnamon extracts, while insulin secretion was unaltered. This corresponded with lower triglyceride and increased liver glycogen content and improved insulin action in liver tissues. In vitro, Fao cells exposed to cinnamon exhibited no change in insulin action.

Conclusions

Together, cinnamon extract improved insulin action in the brain as well as brain activity and locomotion. This specific effect may represent an important central feature of cinnamon in improving insulin action in the brain, and mediates metabolic alterations in the periphery to decrease liver fat and improve glucose homeostasis.

Olive oil aroma extract modulates cerebral blood flow in gustatory brain areas in humans

BACKGROUND

Low- and high-fat meals affect homeostatic and gustatory brain areas differentially. In a previous study, we showed that a high-fat meal decreased cerebral blood flow (CBF) in homeostatic brain areas (hypothalamus), whereas a low-fat meal increased CBF in gustatory regions (anterior insula).

OBJECTIVE

The aim of this study was to investigate the long-lasting effect of fat-free flavor-active compounds of olive oil on the brain and whether those aroma components can trigger fat-associated brain responses in homeostatic and gustatory regions.

DESIGN

Eleven healthy male subjects participated in a functional magnetic resonance imaging study. On 2 measurement days, subjects consumed single-blinded a plain low-fat yogurt or low-fat yogurt mixed with a fat-free aroma extract of olive oil. Resting CBF was measured before and 30 and 120 min after yogurt intake. Hunger was rated before each measurement. Blood samples were collected at 6 time points.

RESULTS

The extract-containing yogurt elicited higher CBF in the frontal operculum 30 and 120 min after a meal. Furthermore, the activity change in the anterior insula after 30 min correlated positively with the glucose change in the extract condition only. No effects were observed in the hypothalamus.

CONCLUSIONS

The anterior insula and the frontal operculum are regarded as the primary taste cortex. Modulation of the frontal operculum by the yogurt containing the olive oil extract suggests that it might be possible to simulate fat-triggered sensations in the brain on the gustatory level, possibly by ingredients the body implicitly associates with fat. This trial was registered at clinicaltrials.gov as NCT01716286.

Is obesity a brain disease?

That the brain is involved in the pathogenesis and perpetuation of obesity is broadly self-intuitive, but traditional evaluation of this relationship has focused on psychological and environment-dependent issues, often referred to as the "it's all in the head" axiom. Here we review evidence that excessive nutrition or caloric flux, regardless of its primary trigger, elicits a biological trap which imprints aberrant energy control circuits that tend to worsen with the accumulation of body fat. Structural and functional changes in the brain can be recognized, such as hypothalamic inflammation and gliosis, reduction in brain volume, reduced regional blood flow or diminished hippocampal size. Such induced changes collectively translate into a vicious cycle of deranged metabolic control and cognitive deficits, some of which can be traced back even to childhood or adolescence. Much like other components of the obese state, brain disease is inseparable from obesity itself and requires better recognition to allow future therapeutic targeting.

Monounsaturated fatty acids prevent the aversive effects of obesity on locomotion, brain activity, and sleep behavior

Fat and physical inactivity are the most evident factors in the pathogenesis of obesity, and fat quality seems to play a crucial role for measures of glucose homeostasis. However, the impact of dietary fat quality on brain function, behavior, and sleep is basically unknown. In this study, mice were fed a diet supplemented with either monounsaturated fatty acids (MUFAs) or saturated fatty acids (SFAs) and their impact on glucose homeostasis, locomotion, brain activity, and sleep behavior was evaluated. MUFAs and SFAs led to a significant increase in fat mass but only feeding of SFAs was accompanied by glucose intolerance in mice. Radiotelemetry revealed a significant decrease in cortical activity in SFA-mice whereas MUFAs even improved activity. SFAs decreased wakefulness and increased non-rapid eye movement sleep. An intracerebroventricular application of insulin promoted locomotor activity in MUFA-fed mice, whereas SFA-mice were resistant. In humans, SFA-enriched diet led to a decrease in hippocampal and cortical activity determined by functional magnetic resonance imaging techniques. Together, dietary intake of MUFAs promoted insulin action in the brain with its beneficial effects for cortical activity, locomotion, and sleep, whereas a comparable intake of SFAs acted as a negative modulator of brain activity in mice and humans.

Fat intake modulates cerebral blood flow in homeostatic and gustatory brain areas in humans

BACKGROUND

The hypothalamus is the central homeostatic control region of the brain and, therefore, highly influenced by nutrients such as glucose and fat. Immediate and prolonged homeostatic effects of glucose ingestion have been well characterized. However, studies that used stimulation with fat have mainly investigated immediate perceptional processes. Besides homeostatic processes, the gustatory cortex, including parts of the insular cortex, is crucial for the processing of food items.

OBJECTIVE

The aim of this study was to investigate the effect of high- compared with low-fat meals on the hypothalamus and the insular cortex.

DESIGN

Eleven healthy men participated in a single-blinded, functional MRI study of high- and low-fat meals on 2 measurement days. Cerebral blood flow (CBF) was measured before and 30 and 120 min after intake of high- and low-fat yogurts. Hunger was rated and blood samples were taken before each CBF measurement.

RESULTS

High-fat yogurt induced a pronounced decrease in CBF in the hypothalamus, and the corresponding CBF change correlated positively with the insulin change. Furthermore, insular activity increased after 120 min in the low-fat condition only. The CBF change in both regions correlated positively in the high-fat condition.

CONCLUSIONS

The decrease in hypothalamic activity and the interaction with the insular cortex elicited by fat may contribute to an efficient energy homeostasis. Therefore, fat might be a modulator of homeostatic and gustatory brain regions and their interaction. This trial was registered at clinicaltrials.gov as NCT01516021.

Intranasal administration of insulin to the brain impacts cognitive function and peripheral metabolism

In recent years, the central nervous system (CNS) has emerged as a principal site of insulin action. This notion is supported by studies in animals relying on intracerebroventricular insulin infusion and by experiments in humans that make use of the intranasal pathway of insulin administration to the brain. Employing neurobehavioural and metabolic measurements as well as functional imaging techniques, these studies have provided insight into a broad range of central and peripheral effects of brain insulin. The present review focuses on CNS effects of insulin administered via the intranasal route on cognition, in particular memory function, and whole-body energy homeostasis including glucose metabolism. Furthermore, evidence is reviewed that suggests a pathophysiological role of impaired brain insulin signaling in obesity and type 2 diabetes, which are hallmarked by peripheral and possibly central nervous insulin resistance, as well as in conditions such as Alzheimer's disease where CNS insulin resistance might contribute to cognitive dysfunction.

Toll-like receptors 2 and 4 impair insulin-mediated brain activity by interleukin-6 and osteopontin and alter sleep architecture

Impaired insulin action in the brain represents an early step in the progression toward type 2 diabetes, and elevated levels of saturated free fatty acids are known to impair insulin action in prediabetic subjects. One potential mediator that links fatty acids to inflammation and insulin resistance is the Toll-like receptor (TLR) family. Therefore, C3H/HeJ/TLR2-KO (TLR2/4-deficient) mice were fed a high-fat diet (HFD), and insulin action in the brain as well as cortical and locomotor activity was analyzed by using telemetric implants. TLR2/4-deficient mice were protected from HFD-induced glucose intolerance and insulin resistance in the brain and displayed an improvement in cortical and locomotor activity that was not observed in C3H/HeJ mice. Sleep recordings revealed a 42% increase in rapid eye movement sleep in the deficient mice during daytime, and these mice spent 41% more time awake during the night period. Treatment of control mice with a neutralizing IL-6 antibody improved insulin action in the brain as well as cortical activity and diminished osteopontin protein to levels of the TLR2/4-deficient mice. Together, our data suggest that the lack of functional TLR2/4 protects mice from a fat-mediated impairment in insulin action, brain activity, locomotion, and sleep architecture by an IL-6/osteopontin-dependent mechanism.

High cerebral insulin sensitivity is associated with loss of body fat during lifestyle intervention

AIMS/HYPOTHESIS

Loss of weight and body fat are major targets in lifestyle interventions to prevent diabetes. In the brain, insulin modulates eating behaviour and weight control, resulting in a negative energy balance. This study aimed to test whether cerebral insulin sensitivity facilitates reduction of body weight and body fat by lifestyle intervention in humans.

METHODS

The study was performed as an additional arm of the TUebingen Lifestyle Intervention Program (TULIP). In 28 non-diabetic individuals (14 female/14 male; mean ± SE age 42 ± 2 years; mean ± SE BMI 29.9 ± 0.8 kg/m²), we measured cerebrocortical insulin sensitivity by using magnetoencephalography before lifestyle intervention. Total and visceral fat were measured by using MRI at baseline and after 9 months and 2 years of lifestyle intervention.

RESULTS

Insulin-stimulated cerebrocortical theta activity at baseline correlated with a reduction in total adipose tissue (r = -0.59, p = 0.014) and visceral adipose tissue (r = -0.76, p = 0.001) after 9 months of lifestyle intervention, accompanied by a statistical trend for reduction in body weight change (r = -0.37, p = 0.069). Similar results were obtained after 2 years.

CONCLUSIONS/INTERPRETATION

Our results suggest that high insulin sensitivity of the human brain facilitates loss of body weight and body fat during lifestyle intervention.

Insulin sensitivity of the human brain

The brain is an insulin sensitive organ and insulin signaling is important to regulate feeding behavior, body weight, and cognitive processes. Insulin resistance in peripheral tissues is a hallmark in the development of type 2 diabetes mellitus (T2DM), yet the finding of insulin resistance in the brain is relatively novel. Studies in humans revealed that environmental factors like obesity, age, and the genetic background have an impact on central insulin sensitivity. According to the physiological effects of insulin in the brain, disturbances of this signaling chain lead to an impairment of cognitive functions and a deterioration of eating behavior with a potential role in the pathogenesis of obesity and T2DM. First attempts to treat insulin resistance not only in peripheral tissues but also in the CNS have therefore come on its way: Cerebral insulin resistance can at least partially be overcome by intranasal treatment with insulin or by commercial insulins that exhibit specific effects in the brain due to their pharmacokinetic properties. Despite the advances towards a better understanding of insulin function in the human brain in the last years, achieving a more profound knowledge of mechanisms behind central insulin function and identifying further strategies to overcome insulin resistance must be a main goal of future research.

The obese brain: association of body mass index and insulin sensitivity with resting state network functional connectivity

Obesity is a key risk factor for the development of insulin resistance, Type 2 diabetes and associated diseases; thus, it has become a major public health concern. In this context, a detailed understanding of brain networks regulating food intake, including hormonal modulation, is crucial. At present, little is known about potential alterations of cerebral networks regulating ingestive behavior. We used "resting state" functional magnetic resonance imaging to investigate the functional connectivity integrity of resting state networks (RSNs) related to food intake in lean and obese subjects using independent component analysis. Our results showed altered functional connectivity strength in obese compared to lean subjects in the default mode network (DMN) and temporal lobe network. In the DMN, obese subjects showed in the precuneus bilaterally increased and in the right anterior cingulate decreased functional connectivity strength. Furthermore, in the temporal lobe network, obese subjects showed decreased functional connectivity strength in the left insular cortex. The functional connectivity magnitude significantly correlated with body mass index (BMI). Two further RSNs, including brain regions associated with food and reward processing, did not show BMI, but insulin associated functional connectivity strength. Here, the left orbitofrontal cortex and right putamen functional connectivity strength was positively correlated with fasting insulin levels and negatively correlated with insulin sensitivity index. Taken together, these results complement and expand previous functional neuroimaging findings by demonstrating that obesity and insulin levels influence brain function during rest in networks supporting reward and food regulation.

Insulin modulation of magnetoencephalographic resting state dynamics in lean and obese subjects

Lean and obese subjects can exhibit differences in neuronal activity during resting state and tasks. Changes in hormonal status and their action related to increased body weight may be the determining factor for these differences. One prime candidate is insulin, which until recently was mainly related to its metabolic function for the transport and regulation of glucose in the periphery. However insulin also acts as an anorexic signal in the central nervous system contributing to the termination of food intake in the postprandial state. In our study, we examined with whole-head magnetoencephalography the effect of intranasal insulin on the dynamics of the resting state network in a placebo controlled study. Weighted clustering coefficient C, which describes local interconnectedness, and weighted path length L, a measure of global interconnectedness, were computed. These parameters showed high intraindividual reliability. However, no difference for the network dynamics was found between lean and obese subjects in the basal state. The application of insulin led to subject specific changes and we found a statistically significant positive correlation between the insulin induced change in path length in the theta band (4-8 Hz) and body mass index. The change in pathway length after insulin administration indicates a strong insulin modulation on global communication efficiency, which is probably related to the signaling between different regions involved in satiation and homeostatic control.

Insulin-mediated cortical activity in the slow frequency range is diminished in obese mice and promotes physical inactivity

AIMS/HYPOTHESIS

There is evidence from mouse models and humans that alterations in insulin action in the brain are accompanied by an obese phenotype; however, the impact of insulin with regard to behavioural aspects such as locomotion is unknown.

METHODS

To address insulin action in the brain with regard to cortical activity in distinct frequency bands and the behavioural consequences, the insulin signalling pathway was followed from the receptor to electrical activity and locomotion. Western blot analysis, electrocorticograms with intracerebroventricular (i.c.v.) application of insulin, and measurements of locomotor activity were performed in lean and obese, as well as Toll-like receptor (TLR) 2/4-deficient, mice.

RESULTS

We show that insulin application i.c.v. into lean mice was accompanied by a profound increase in cortical activity in the slow frequency range, while diet-induced obese mice displayed insulin resistance. In parallel, insulin administered i.c.v. increased locomotor activity in lean mice, whereas a phosphatidylinositol-3 (PI3) kinase inhibitor or obesity abolished insulin-mediated locomotion. A potential candidate that links insulin signalling to locomotion is the Kv1.3 channel that is activated by PI3-kinase. Pharmacological inhibition of Kv1.3 channels that bypassed insulin receptor activation promoted activity. Moreover, mice deficient in TLR2/4-dependent signalling displayed an increase in cortical activity in the slow frequency range that was correlated with improved spontaneous and insulin-mediated locomotor activity.

CONCLUSIONS/INTERPRETATION

Our data provide functional evidence for a direct effect of insulin on brain activation patterns in the slow frequency bands and locomotor activity in lean mice, while in obese mice, insulin-mediated locomotion is blunted and further aggravates physical inactivity.