Central insulin modulates food valuation via mesolimbic pathways

DAF-16/FOXO promotes taste avoidance learning independently of axonal insulin-like signaling

The avoidance of starvation is critical for the survival of most organisms, thus animals change behavior based on past nutritional conditions. Insulin signaling is important for nutritional state-dependent behavioral plasticity, yet the underlying regulatory mechanism at the cellular level remains unclear. Previous studies showed that insulin-like signaling is required for taste avoidance learning, in which the nematode Caenorhabditis elegans avoids salt concentrations encountered under starvation conditions. DAF-2c, a splice isoform of the DAF-2 insulin receptor, functions in the axon of the ASER sensory neuron, which senses changes in salt concentrations. In addition, mutants of a major downstream factor of DAF-2, the forkhead transcription factor O (FOXO) homolog DAF-16, show defects in taste avoidance learning. Interestingly, the defect of the daf-2 mutant is not suppressed by daf-16 mutations in the learning, unlike those in other phenomena, such as longevity and development. Here we show that multiple DAF-16 isoforms function in ASER. By epistasis analysis using a DAF-2c isoform-specific mutant and an activated form of DAF-16, we found that DAF-16 acts in the nucleus in parallel with the DAF-2c-dependent pathway in the axon, indicating that insulin-like signaling acts both in the cell body and axon of a single neuron, ASER. Starvation conditioning induces nuclear translocation of DAF-16 in ASER and degradation of DAF-16 before starvation conditioning causes defects in taste avoidance learning. Forced nuclear localization of DAF-16 in ASER biased chemotaxis towards lower salt concentrtions and this effect required the Gq/PKC pathway and neuropeptide processing enzymes. These data imply that DAF-16/FOXO transmits starvation signals and modulates neuropeptide transmission in the learning.

Animals change behavior based on remembered experiences of hunger and appetite. Signaling by insulin and insulin-like peptides in the nervous system plays key roles in behavioral responses to hunger and satiety. In C. elegans, insulin-like signaling in the gustatory sensory neuron ASER regulates learned avoidance of salt concentrations experienced during fasting, which we call taste avoidance learning. DAF-2c, an isoform of the insulin receptor homolog, is localized to the axon of ASER and regulates taste avoidance learning. Here, we show that DAF-16, the forkhead transcription factor O (FOXO) homolog, translocates into the nucleus of ASER during fasting and promotes taste avoidance learning. DAF-16 is negatively regulated by insulin-like signaling independently of axonal DAF-2c signaling. This dual function of insulin-like signaling in the cell body and the axon ensures dynamic changes in behavioral responses after experience of hunger. By genetic analyses using constitutively nuclear-translocated DAF-16, we show that DAF-16 in ASER regulates taste avoidance learning via modulating neuropeptide signaling in the nervous system, which is reminiscent of the function of FOXO in the hypothalamus in the regulation of food-seeking behavior in mammals.

Reduced Hedonic Valuation of Rewards and Unaffected Cognitive Regulation in Chronic Stress

Cognition can influence choices by modulation of decision-making processes. This cognitive regulation is defined as processing information, applying knowledge, and changing preferences to consciously modulate decisions. While cognitive regulation of emotions has been extensively studied in psychiatry, few works have detailed cognitive regulation of decision-making. Stress may influence emotional behavior, cognition, and decision-making. In addition, the brain regions responsible for decision-making are sensitive to stress-induced changes. Thus, we hypothesize that chronic stress may disrupt the ability to regulate choices. Herein, we used a functional magnetic resonance imaging task where fourteen control and fifteen chronically stressed students had to cognitively upregulate or downregulate their craving before placing a bid to obtain food. We found that stressed participants placed lower bids to get the reward and chose less frequently higher bid values for food. Nevertheless, we did not find neural and behavioral differences during cognitive regulation of craving. Our outcomes revealed that chronic stress impacts decision-making after cognitive regulation of craving by reducing the valuation of food rewards but not cognitive modulation itself. Importantly, our results need further validation with larger sample sizes.

In a Free-Living Setting, Obesity Is Associated with Greater Food Intake in Response to a Similar Pre-Meal Glucose Nadir

PURPOSE

Changes in blood glucose levels have been shown to influence eating in healthy individuals; however, less is known about glucose's effects on food intake in obese (OB) individuals. The goal of this study was to determine the predictive effect of circulating glucose levels on eating in free-living OB and normal weight (NW) individuals.

METHODS

Interstitial glucose levels, measured with a continuous glucose monitor (CGM) system, were obtained from 15 OB and 16 NW volunteers (age: 40±14, 37±12 yr; weight: 91±13, 68±12 kg; HbA1c: 5.1±0.7, 5.2±0.4%, respectively). While wearing the CGM participants filled up a food log (meal time, hunger rating and amount of food). Glucose profiles were measured in relation to their meals (macro program (CGM Peak and Nadir Analysis (CPNA)) in Microsoft® Excel).

RESULTS

OB and NW individuals showed comparable CGM glucose levels: mean (OB=99±13, NW=100±8 mg/dL, p=NS) and standard deviation (OB=18±4, NW=18±5 mg/dL, p=NS). Obesity was associated with slower post-prandial rate of changing glucose levels (p=0.04). Pre-prandial nadir glucose levels predicted hunger and food intake in both groups (p<0.0001), although hunger was associated with greater food intake in OB, in comparison to NW individuals (p=0.008 for group interaction).

CONCLUSIONS

Pre-meal glucose nadir predicted hunger and food intake in a group of free-living healthy non-diabetic NW and OB individuals, however for a similar low glucose level stimulus, greater hunger-induced food intake was recorded by OB in comparison to NW individuals.

Modulation of midbrain neurocircuitry by intranasal insulin

Insulin modulates dopamine neuron activity in midbrain and affects processes underlying food intake behaviour, including impulsivity and reward processing. Here, we used intranasal administration and task-free functional MRI in humans to assess time- and dose-dependent effects of insulin on functional connectivity of the dopaminergic midbrain - and how these effects varied depending on systemic insulin sensitivity as measured by HOMA-IR. Specifically, we used a repeated-measures design with factors dose (placebo, 40 IU, 100 IU, 160 IU), time (7 time points during a 90 min post-intervention interval), and group (low vs. high HOMA-IR). A factorial analysis identified a three-way interaction (with whole-brain significance) with regard to functional connectivity between midbrain and the ventromedial prefrontal cortex. This interaction demonstrates that systemic insulin sensitivity modulates the temporal course and dose-dependent effects of intranasal insulin on midbrain functional connectivity. It suggests that altered insulin sensitivity may impact on dopaminergic projections of the midbrain and might underlie the dysregulation of reward-related and motivational behaviour in obesity and diabetes. Perhaps most importantly, the time courses of midbrain functional connectivity we present may provide useful guidance for the design of future human studies that utilize intranasal insulin administration.

Outcomes and clinical implications of intranasal insulin administration to the central nervous system

Insulin signaling in the brain plays a critical role in metabolic control and cognitive function. Targeting insulinergic pathways in the central nervous system via peripheral insulin administration is feasible, but associated with systemic effects that necessitate tight supervision or countermeasures. The intranasal route of insulin administration, which largely bypasses the circulation and thereby greatly reduces these obstacles, has now been repeatedly tested in proof-of-concept studies in humans as well as animals. It is routinely used in experimental settings to investigate the impact on eating behavior, peripheral metabolism, memory function and brain activation of acute or long-term enhancements in central nervous system insulin signaling. Epidemiological and experimental evidence linking deteriorations in metabolic control such as diabetes with neurodegenerative diseases imply pathophysiological relevance of dysfunctional brain insulin signaling or brain insulin resistance, and suggest that targeting insulin in the brain holds some promise as a therapy or adjunct therapy. This short narrative review gives an overview over recent findings on brain insulin signaling as derived from human studies deploying intranasal insulin, and evaluates the potential of therapeutic interventions that target brain insulin resistance.

Molecular Mechanisms of Hypothalamic Insulin Resistance

Insulin exists in the central nervous system, where it executes two important functions in the hypothalamus: the suppression of food intake and the improvement of glucose metabolism. Recent studies have shown that both are exerted robustly in rodents and humans. If intact, these functions exert beneficial effects on obesity and diabetes, respectively. Disruption of both occurs due to a condition known as hypothalamic insulin resistance, which is caused by obesity and the overconsumption of saturated fat. An enormous volume of literature addresses the molecular mechanisms of hypothalamic insulin resistance. IKKβ and JNK are major players in the inflammation pathway, which is activated by saturated fatty acids that induce hypothalamic insulin resistance. Two major tyrosine phosphatases, PTP-1B and TCPTP, are upregulated in chronic overeating. They dephosphorylate the insulin receptor and insulin receptor substrate proteins, resulting in hypothalamic insulin resistance. Prolonged hyperinsulinemia with excessive nutrition activates the mTOR/S6 kinase pathway, thereby enhancing IRS-1 serine phosphorylation to induce hypothalamic insulin resistance. Other mechanisms associated with this condition include hypothalamic gliosis and disturbed insulin transport into the central nervous system. Unveiling the precise molecular mechanisms involved in hypothalamic insulin resistance is important for developing new ways of treating obesity and type 2 diabetes.

Mesolimbic white matter connectivity mediates the preference for sweet food

Dopaminergic brain structures like the nucleus accumbens (NAc) are thought to encode the incentive salience of palatable foods motivating appetitive behaviour. Animal studies have identified neural networks mediating the regulation of hedonic feeding that comprise connections of the NAc with the ventral tegmental area (VTA) and the lateral hypothalamus (LH). Here, we investigated how structural connectivity of these pathways relates to individual variability in decisions on sweet food consumption in humans. We therefore combined probabilistic tractography on diffusion imaging data from 45 overnight fasted lean to overweight participants with real decisions about high and low sugar food consumption. Across all individuals, sugar preference and connectivity strength were not directly related, however, multiple regression analysis revealed interaction of mesolimbic structure and sugar preference to depend on individuals’ BMI score. In overweight individuals (BMI: ≥25 kg/m², N = 22) higher sugar preference was thereby specifically related to stronger connectivity within the VTA-NAc pathway while the opposite pattern emerged in participants with normal BMI (BMI: <25 kg/m², N = 23). Our structural results complement previous functional findings on the critical role of the human mesolimbic system for regulating hedonic eating in overweight individuals.

Upper airway stimulation in obstructive sleep apnea improves glucose metabolism and reduces hedonic drive for food

Upper airway stimulation is a new and effective second-line treatment for obstructive sleep apnea, but possible consequences on glucose metabolism and central regulation of food intake are unclear. Twenty patients were prospectively studied before and 12 months after obstructive sleep apnea treatment by upper airway stimulation. Respiratory parameters and daytime sleepiness were assessed to document effectiveness of treatment. Glucose metabolism was assessed by the oral glucose tolerance test, and hedonic versus homeostatic drive to eat was characterized. At 12 months, upper airway stimulation significantly improved measures of obstructive sleep apnea (all p < 0.01). Despite no change in body weight, fasting C-peptide insulin resistance index (p = 0.01) as well as insulin and C-peptide levels at 60 min during the oral glucose tolerance test (p < 0.02) were reduced. Hedonic drive to eat was strongly reduced (p < 0.05), while leptin and ghrelin remained unchanged (p > 0.15). Upper airway stimulation is effective in treatment of obstructive sleep apnea and improves glucose metabolism. Reduced hedonic drive to eat might contribute to these metabolic improvements. These promising findings are in need for long-term controlled evaluation of metabolic sequelae of upper airway stimulation and to mechanistically evaluate the metabolic benefits of upper airway stimulation in patients with obstructive sleep apnea.

Associations of Brain Reactivity to Food Cues with Weight Loss, Protein Intake and Dietary Restraint during the PREVIEW Intervention

The objective was to assess the effects of a weight loss and subsequent weight maintenance period comprising two diets differing in protein intake, on brain reward reactivity to visual food cues. Brain reward reactivity was assessed with functional magnetic resonance imaging in 27 overweight/obese individuals with impaired fasting glucose and/or impaired glucose tolerance (HOMA-IR: 3.7 ± 1.7; BMI: 31.8 ± 3.2 kg/m²; fasting glucose: 6.4 ± 0.6 mmol/L) before and after an 8-week low energy diet followed by a 2-year weight maintenance period, with either high protein (HP) or medium protein (MP) dietary guidelines. Brain reactivity and possible relationships with protein intake, anthropometrics, insulin resistance and eating behaviour were assessed. Brain reactivity, BMI, HOMA-IR and protein intake did not change differently between the groups during the intervention. In the whole group, protein intake during weight maintenance was negatively related to changes in high calorie images>low calorie images (H > L) brain activation in the superior/middle frontal gyrus and the inferior temporal gyrus (p < 0.005, corrected for multiple comparisons). H > L brain activation was positively associated with changes in body weight and body-fat percentage and inversely associated with changes in dietary restraint in multiple reward, gustatory and processing regions (p < 0.005, corrected for multiple comparisons). In conclusion, changes in food reward-related brain activation were inversely associated with protein intake and dietary restraint during weight maintenance after weight loss and positively associated with changes in body weight and body-fat percentage.

Current state of the use of neuroimaging techniques to understand and alter appetite control in humans

PURPOSE OF REVIEW

It is in the brain where the decision is made what and how much to eat. In the last decades neuroimaging research has contributed extensively to new knowledge about appetite control by revealing the underlying brain processes. Interestingly, there is the fast growing idea of using these methods to develop new treatments for obesity and eating disorders. In this review, we summarize the findings of the importance of the use of neuropharmacology and neuroimaging techniques in understanding and modifying appetite control.

RECENT FINDINGS

Appetite control is a complex interplay between homeostatic, hedonic, and cognitive processes. Administration of the neuropeptides insulin and oxytocin curb food intake and alter brain responses in reward and cognitive control areas. Additionally, these areas can be targeted for neuromodulation or neurofeedback to reduce food cravings and increase self-control to alter food intake.

SUMMARY

The recent findings reveal the potential of intranasal administration of hormones or modifying appetite control brain networks to reduce food consumption in volunteers with overweight and obesity or individuals with an eating disorder. Although long-term clinical studies are still needed.

Trends in Obesity and Risk of Cardiovascular Disease

Purpose of review

The obesity epidemic is a global health crisis of staggering proportion. Excess body weight is a major risk factor for the development of cardiovascular disease (CVD). We review temporal trends in obesity rates, pertinent pathophysiology to understand mechanisms of disease, and treatment strategies in the context of reducing cardiovascular risk.

Recent findings

The prevalence of obesity is increasing in recent decades and is driven by a complex interplay of economic, environmental, and biological factors. In developed countries, changes in foodintake, such as increased consumption of energy-dense and added sugar have contributed significantly to weight gain. Single nucleotide variations in genes and alterations in the gut microbiome have been associated with the obese phenotype. The description of an obesity paradox in patients with CVD may have several explanations, including limitations of body mass index (BMI) to assess adiposity, selection bias, and lead-time bias with earlier onset of disease. Evidence-based treatments for weight loss include lifestyle intervention, pharmacotherapy, and bariatric surgery. Data on the long-term effects of these therapies on cardiovascular risk are limited.

Summary

Overweight and obesity are associated with increased cardiovascular morbidity and mortality over the lifespan. Despite our increasing understanding of biological and environmental drivers of obesity, more work is needed in developing effective prevention strategies and implementation of evidence-based treatments to promote cardiovascular health and reduce cardiovascular risk. Ultimately, efforts to prevent and postpone cardiovascular morbidity should include focus on maintenance of normal BMI (primordial prevention) for a longer and healthier life, free of CVD.

Hedonic Eating and the "Delicious Circle": From Lipid-Derived Mediators to Brain Dopamine and Back

Palatable food can be seductive and hedonic eating can become irresistible beyond hunger and negative consequences. This is witnessed by the subtle equilibrium between eating to provide energy intake for homeostatic functions, and reward-induced overeating. In recent years, considerable efforts have been devoted to study neural circuits, and to identify potential factors responsible for the derangement of homeostatic eating toward hedonic eating and addiction-like feeding behavior. Here, we examined recent literature on “old” and “new” players accountable for reward-induced overeating and possible liability to eating addiction. Thus, the role of midbrain dopamine is positioned at the intersection between selected hormonal signals involved in food reward information processing (namely, leptin, ghrelin, and insulin), and lipid-derived neural mediators such as endocannabinoids. The impact of high fat palatable food and dietary lipids on endocannabinoid formation is reviewed in its pathogenetic potential for the derangement of feeding homeostasis. Next, endocannabinoid signaling that regulates synaptic plasticity is discussed as a key mechanism acting both at hypothalamic and mesolimbic circuits, and affecting both dopamine function and interplay between leptin and ghrelin signaling. Outside the canonical hypothalamic feeding circuits involved in energy homeostasis and the notion of “feeding center,” we focused on lateral hypothalamus as neural substrate able to confront food-associated homeostatic information with food salience, motivation to eat, reward-seeking, and development of compulsive eating. Thus, the lateral hypothalamus-ventral tegmental area-nucleus accumbens neural circuitry is reexamined in order to interrogate the functional interplay between ghrelin, dopamine, orexin, and endocannabinoid signaling. We suggested a pivotal role for endocannabinoids in food reward processing within the lateral hypothalamus, and for orexin neurons to integrate endocrine signals with food reinforcement and hedonic eating. In addition, the role played by different stressors in the reinstatement of preference for palatable food and food-seeking behavior is also considered in the light of endocannabinoid production, activation of orexin receptors and disinhibition of dopamine neurons. Finally, type-1 cannabinoid receptor-dependent inhibition of GABA-ergic release and relapse to reward-associated stimuli is linked to ghrelin and orexin signaling in the lateral hypothalamus-ventral tegmental area-nucleus accumbens network to highlight its pathological potential for food addiction-like behavior.

Insulin and Estrogen Independently and Differentially Reduce Macronutrient Intake in Healthy Men

CONTEXT

Insulin administration to the central nervous system inhibits food intake, but this effect has been found to be less pronounced in female compared with male organisms. This sex-specific pattern has been suggested to arise from a modulating influence of estrogen signaling on the insulin effect.

OBJECTIVE

We assessed in healthy young men whether pretreatment with transdermal estradiol interacts with the hypophagic effect of central nervous insulin administration via the intranasal pathway.

DESIGN, SETTING, PARTICIPANTS, AND INTERVENTION

According to a 2×2 design, two groups of men (n = 16 in each group) received a 3-day transdermal estradiol (100 µg/24 h) or placebo pretreatment and on two separate mornings were intranasally administered 160 IU regular human insulin or placebo.

MAIN OUTCOME MEASURES

We assessed free-choice ad libitum calorie intake from a rich breakfast buffet and relevant blood parameters in samples collected before and after breakfast.

RESULTS

Estrogen treatment induced a 3.5-fold increase in serum estradiol concentrations and suppressed serum testosterone concentrations by 70%. Independent of estradiol administration, intranasal insulin reduced the intake of carbohydrates during breakfast, attenuating in particular the consumption of sweet, palatable foods. Estradiol treatment per se decreased protein consumption. We did not find indicators of eating-related interactions between both hormones.

CONCLUSIONS

Results indicate that, in an acute setting, estrogen does not interact with central nervous insulin signaling in the control of eating behavior in healthy men. Insulin and estradiol rather exert independent inhibiting effects on macronutrient intake.

Metabolic and Cognitive Outcomes of Subchronic Once-Daily Intranasal Insulin Administration in Healthy Men

Insulin acts in the brain to limit food intake and improve memory function. We have previously shown that 8 weeks of intranasal insulin delivered in four daily doses of 40 IU decrease body weight and enhance word list recall. In the present study, we investigated the effect on body composition, endocrine parameters, and memory performance of 8 weeks of once-daily administration of 160 IU in healthy men. We assumed that intranasal insulin administered before nocturnal sleep, a period of relative metabolic inactivity that moreover benefits memory formation, would be superior to insulin delivery in the morning and placebo administration. After a 2-week baseline period, healthy male normal-weight subjects (mean age, 27.1 ± 0.9 years) received either placebo, 160 IU intranasal insulin in the morning, or 160 IU in the evening (n = 12 per group) for 8 consecutive weeks. Throughout the experiment, we measured body weight and body composition as well as circulating concentrations of glucose, insulin, adrenocorticotropin, cortisol, growth hormone, insulin-like growth-factor 1, adiponectin, and leptin. Declarative and procedural memory function was repeatedly assessed by means of, respectively, word list recall and word-stem priming. We found that neither morning nor evening insulin compared to placebo administration induced discernible changes in body weight and body composition. Delayed recall of words showed slight improvements by insulin administration in the evening, and serum cortisol concentrations were reduced after 2 weeks of insulin administration in the morning compared to the other groups. Results indicate that catabolic long-term effects of central nervous insulin delivery necessitate repetitive, presumably pre-meal delivery schedules. The observed memory improvements, although generally weaker than previously found effects, suggest that sleep after intranasal insulin administration may support its beneficial cognitive impact.

Insulin-mediated synaptic plasticity in the CNS: Anatomical, functional and temporal contexts

For decades the brain was erroneously considered an insulin insensitive organ. Although gaps in our knowledge base remain, conceptual frameworks are starting to emerge to provide insight into the mechanisms through which insulin facilitates critical brain functions like metabolism, cognition, and motivated behaviors. These diverse physiological and behavioral activities highlight the region-specific activities of insulin in the CNS; that is, there is an anatomical context to the activities of insulin in the CNS. Similarly, there is also a temporal context to the activities of insulin in the CNS. Indeed, brain insulin receptor activity can be conceptualized as a continuum in which insulin promotes neuroplasticity from development into adulthood where it is an integral part of healthy brain function. Unfortunately, brain insulin resistance likely contributes to neuroplasticity deficits in obesity and type 2 diabetes mellitus (T2DM). This neuroplasticity continuum can be conceptualized by the mechanisms through which insulin promotes cognitive function through its actions in brain regions like the hippocampus, as well as the ability of insulin to modulate motivated behaviors through actions in brain regions like the nucleus accumbens and the ventral tegmental area. Thus, the goals of this review are to highlight these anatomical, temporal, and functional contexts of insulin activity in these brain regions, and to identify potentially critical time points along this continuum where the transition from enhancement of neuroplasticity to impairment may take place.

Motivated Cognition: Effects of Reward, Emotion, and Other Motivational Factors Across a Variety of Cognitive Domains

A growing body of literature has demonstrated that motivation influences cognitive processing. The breadth of these effects is extensive and span influences of reward, emotion, and other motivational processes across all cognitive domains. As examples, this scope includes studies of emotional memory, value-based attentional capture, emotion effects on semantic processing, reward-related biases in decision making, and the role of approach/avoidance motivation on cognitive scope. Additionally, other less common forms of motivation–cognition interactions, such as self-referential and motoric processing can also be considered instances of motivated cognition. Here I outline some of the evidence indicating the generality and pervasiveness of these motivation influences on cognition, and introduce the associated ‘research nexus’ at Collabra: Psychology.

Targeting Mediators of Smoking Persistence with Intranasal Insulin

Rapid-acting, non-irritating nasal treatment options for smoking cessation pharmacotherapy are lacking. The halt in development is due, in part, to difficulty in delivering compounds across the blood brain barrier. Recently, in both human and animal models, insulin was shown to be capable of being transported to the cerebrospinal fluid and various brain regions via the “nose-to-brain” pathway, which bypasses the blood brain barrier, but is not free of its own unique, though different from blood brain barrier, challenges. This review will first evaluate and critique pharmacokinetic and pharmacodynamic evidence of intranasal insulin (i.e., nose-to-brain) delivery. As intranasal insulin has been shown in clinical trials to be effective in reducing nicotine cravings, in the remainder of the review, hypothesis-generating literature for additional mediators (i.e., other than the already shown nicotine craving) of smoking persistence will be reviewed. In particular, weight gain, impulsive behavior, and anhedonia have been shown to contribute to the inability to quit smoking. For each of these, after reviewing how the mediator promotes smoking, intranasal insulin literature from animal and clinical models will be critiqued in assessing whether a hypothesis may be generated that intranasal insulin may alleviate it, thereby potentially contributing to a successful smoking cessation outcome.