Vagus nerve stimulation increases energy expenditure: relation to brown adipose tissue activity

Vagus Nerve and Gut-Brain Communication

The vagus nerve, as an important component of the gut-brain axis, plays a crucial role in the communication between the gut and brain. It influences food intake, fat metabolism, and emotion by regulating the gut-brain axis, which is closely associated with the development of gastrointestinal, psychiatric, and metabolism-related disorders. In recent years, significant progress has been made in understanding the vagus-mediated regulatory pathway, highlighting its profound implications in the development of many diseases. Here, we summarize the latest advancements in vagus-mediated gut-brain pathways and the novel interventions targeting the vagus nerve. This will provide valuable insights for future research on treatment of obesity and gastrointestinal and depressive disorders based on vagus nerve stimulation.

Identification of metabolic biomarkers of chronic vagus nerve stimulation (VNS) in subjects with drug-resistant epilepsy (DRE)

Neuromodulation by means of vagus nerve stimulation (VNS) therapy, reduces seizure frequency and improves quality of life in subjects with drug-resistant epilepsy (DRE), yet its molecular mechanism remains unclear. This study investigates the impact of chronic VNS on lipid bioactive metabolites and fatty acids (FA) in the plasma and red blood cells of seven subjects with DRE. By measuring expression levels of peroxisome proliferator-activated receptor α (PPARα) and sirtuin1 (SIRT1) genes-key regulators in energy and lipid metabolism-and lipid profiles before and after various stages of VNS, this study identifies potential mechanisms by which VNS may reduce seizure frequency. Blood samples collected before VNS device implantation, after acute VNS stimulus, and following gradual intensity increments up to therapeutic levels revealed that VNS increases SIRT1 and PPARα expression and erythrocyte concentrations of PPARα ligands. Additionally, we observe reduced de novo lipogenesis biomarkers in erythrocytes, indicating that VNS may influence systemic lipid and energy metabolism. Our findings suggest that VNS could enhance neuronal function by modulating energy metabolism, thus potentially reducing seizure frequency in subjects with DRE. Future research targeting SIRT1 and PPARα may provide innovative therapeutic strategies for managing DRE. Plain Language Summary: The exact mechanism of VNS is still unknown. This study investigated the effects of VNS Therapy on energetic metabolism, suggesting possible novel biomarkers for DRE subjects and neuromodulation therapies.

Vagal Stimulation Ameliorates Non-Alcoholic Fatty Liver Disease in Rats

BACKGROUND

The harmful consequences of non-alcoholic fatty liver disease (NAFLD) are posing an increasing threat to public health as the incidence of diabetes and obesity increases globally. A non-invasive treatment with a range of autonomic and metabolic benefits is transcutaneous vagus nerve stimulation (tVNS).

AIM OF THE STUDY

To investigate the possible preventive impacts of VNS against adult rats' NAFLD caused by a high-fat diet (HFD) and to clarify the underlying mechanisms.

METHODS

A total of thirty-two adult male rats were split into two groups: the HFD-induced NAFLD group (n = 24) and the control normal group (n = 8). The obesogenic diet was maintained for 12 weeks to induce hepatic steatosis. The HFD-induced NAFLD group (n = 24) was separated into three groups: the group without treatment (n = 8), the group with sham stimulation (n = 8), and the group with VNS treatment (n = 8). VNS was delivered for 30 min per day for 6 weeks after the establishment of NAFLD using a digital TENS device. The subsequent assessments included hepatic triglyceride, cholesterol content, serum lipid profile, and liver function testing. In this context, inflammatory biomarkers (TNF-α, IL-6) and hepatic oxidative stress (MDA, SOD, and GPx) were also assessed. To clarify the possible mechanisms behind the protective benefits of VNS, additional histological inspection and immunohistochemistry analysis of TNF-α and Caspase-3 were performed.

RESULTS

In the NAFLD-affected obese rats, VNS markedly decreased the rats' body mass index (BMI) and abdominal circumference (AC). Liver function markers (albumin, ALT, and AST) and the serum lipid profile-which included a notable decrease in the amounts of hepatic triglycerides and cholesterol-were both markedly improved. Additionally, oxidative stress and inflammatory indicators showed a considerable decline with VNS. Notably, the liver tissues examined by histopathologists revealed that there is evidence of the protective impact of VNS on the oxidative and inflammatory states linked to HFD-induced NAFLD while maintaining the architectural and functional condition of the liver.

CONCLUSIONS

Our findings suggest that VNS may represent a promising therapeutic candidate for managing NAFLD induced by obesity. It can be considered to be an effective adjuvant physiological intervention for the obese population with NAFLD to spare the liver against obesity-induced deleterious injury.

Vagal Nerve Therapy in the Management of Obesity: A Systematic Review and Meta-Analysis

INTRODUCTION

The vagus nerve has an important role in satiety, metabolism, and autonomic control in upper gastrointestinal function. However, the role and effects of vagal nerve therapy on weight loss remain controversial. This systematic review and meta-analysis assessed the effects of vagal nerve therapy on weight loss, body mass index (BMI), and obesity-related conditions.

METHODS

MEDLINE, EMBASE, and CINAHL databases were searched for studies up to April 2022 that reported on percentage excess weight loss (%EWL) or BMI at 12 months or remission of obesity-related conditions following vagal nerve therapy from January 2000 to April 2022. Weighted mean difference (WMD) was calculated, meta-analysis was performed using random-effects models, and between-study heterogeneity was assessed.

RESULTS

Fifteen studies, of which nine were randomised controlled trials, of 1,447 patients were included. Vagal nerve therapy led to some improvement in %EWL (WMD 17.19%; 95% confidence interval [CI]: 10.94-23.44; p < 0.001) and BMI (WMD -2.24 kg/m2; 95% CI: -4.07 to -0.42; p = 0.016). There was a general improvement found in HbA1c following vagal nerve therapy when compared to no treatment given. No major complications were reported.

CONCLUSIONS

Vagal nerve therapy can safely result in a mild-to-moderate improvement in weight loss. However, further clinical trials are required to confirm these results and investigate the possibility of the long-term benefit of vagal nerve therapy as a dual therapy combined with standard surgical bariatric interventions.

Application of Vagus Nerve Stimulation in Spinal Cord Injury Rehabilitation

Spinal cord injury (SCI) is a prevalent devastating condition causing significant morbidity and mortality, especially in developing countries. The pathophysiology of SCI involves ischemia, neuroinflammation, cell death, and scar formation. Due to the lack of definitive therapy for SCI, interventions mainly focus on rehabilitation to reduce deterioration and improve the patient's quality of life. Currently, rehabilitative exercises and neuromodulation methods such as functional electrical stimulation, epidural electrical stimulation, and transcutaneous electrical nerve stimulation are being tested in patients with SCI. Other spinal stimulation techniques are being developed and tested in animal models. However, often these methods require complex surgical procedures and solely focus on motor function. Vagus nerve stimulation (VNS) is currently used in patients with epilepsy, depression, and migraine and is being investigated for its application in other disorders. In animal models of SCI, VNS significantly improved locomotor function by ameliorating inflammation and improving plasticity, suggesting its use in human subjects. SCI patients also suffer from nonmotor complications, including pain, gastrointestinal dysfunction, cardiovascular disorders, and chronic conditions such as obesity and diabetes. VNS has shown promising results in alleviating these conditions in non-SCI patients, which makes it a possible therapeutic option in SCI patients.

Apolipoprotein A4 Elevates Sympathetic Activity and Thermogenesis in Male Mice

Long-chain fatty acids induce apolipoprotein A4 (APOA4) production in the small intestine and activate brown adipose tissue (BAT) thermogenesis. The increase in BAT thermogenesis enhances triglyceride clearance and insulin sensitivity. Acute administration of recombinant APOA4 protein elevates BAT thermogenesis in chow-fed mice. However, the physiological role of continuous infusion of recombinant APOA4 protein in regulating sympathetic activity, thermogenesis, and lipid and glucose metabolism in low-fat-diet (LFD)-fed mice remained elusive. The hypothesis of this study was that continuous infusion of mouse APOA4 protein would increase sympathetic activity and thermogenesis in BAT and subcutaneous inguinal white adipose tissue (IWAT), attenuate plasma lipid levels, and improve glucose tolerance. To test this hypothesis, sympathetic activity, BAT temperature, energy expenditure, body weight, fat mass, caloric intake, glucose tolerance, and levels of BAT and IWAT thermogenic and lipolytic proteins, plasma lipids, and markers of fatty acid oxidation in the liver in mice with APOA4 or saline treatment were measured. Plasma APOA4 levels were elevated, BAT temperature and thermogenesis were upregulated, and plasma triglyceride (TG) levels were reduced, while body weight, fat mass, caloric intake, energy expenditure, and plasma cholesterol and leptin levels were comparable between APOA4- and saline-treated mice. Additionally, APOA4 infusion stimulated sympathetic activity in BAT and liver but not in IWAT. APOA4-treated mice had greater fatty acid oxidation but less TG content in the liver than saline-treated mice had. Plasma insulin in APOA4-treated mice was lower than that in saline-treated mice after a glucose challenge. In conclusion, continuous infusion of mouse APOA4 protein stimulated sympathetic activity in BAT and the liver, elevated BAT thermogenesis and hepatic fatty acid oxidation, and consequently attenuated levels of plasma and hepatic TG and plasma insulin without altering caloric intake, body weight gain and fat mass.

GLP-1 Increases Circulating Leptin Levels in Truncal Vagotomized Rats

GLP-1 is a gastro-intestinal hormone acting within the gut/brain axis for energy balance regulation. We aimed to evaluate the role of the vagus nerve in whole-body energy homeostasis and in mediating GLP-1 effects. For this, rats submitted to truncal vagotomy and sham-operated controls underwent a comprehensive evaluation, including eating behavior, body weight, percentage of white (WAT) and brown adipose tissue (BAT), resting energy expenditure (REE) and acute response to GLP-1. Truncal vagotomized rats had significantly lower food intake, body weight, body weight gain, WAT and BAT, with a higher BAT/WAT ratio, but no significant difference in REE when compared to controls. Vagotomized rats also had significantly higher fasting ghrelin and lower glucose and insulin levels. After GLP-1 administration, vagotomized rats depicted a blunted anorexigenic response and higher plasma leptin levels, as compared to controls. However, in vitro stimulation of VAT explants with GLP-1 resulted in no significant changes in leptin secretion. In conclusion, the vagus nerve influences whole-body energy homeostasis by modifying food intake, body weight and body composition and by mediating the GLP-1 anorectic response. The higher leptin levels in response to acute GLP-1 administration observed after truncal vagotomy suggest the existence of a putative GLP-1-leptin axis that relies on the integrity of gut-brain vagal pathway.

Increased BAT Thermogenesis in Male Mouse Apolipoprotein A4 Transgenic Mice

Dietary lipids induce apolipoprotein A4 (APOA4) production and brown adipose tissue (BAT) thermogenesis. Administration of exogenous APOA4 elevates BAT thermogenesis in chow-fed mice, but not high-fat diet (HFD)-fed mice. Chronic feeding of HFD attenuates plasma APOA4 production and BAT thermogenesis in wildtype (WT) mice. In light of these observations, we sought to determine whether steady production of APOA4 could keep BAT thermogenesis elevated, even in the presence of HFD consumption, with an aim toward eventual reduction of body weight, fat mass and plasma lipid levels. Transgenic mice with overexpression of mouse APOA4 in the small intestine (APOA4-Tg mice) produce greater plasma APOA4 than their WT controls, even when fed an atherogenic diet. Thus, we used these mice to investigate the correlation of levels of APOA4 and BAT thermogenesis during HFD consumption. The hypothesis of this study was that overexpression of mouse APOA4 in the small intestine and increased plasma APOA4 production would increase BAT thermogenesis and consequently reduce fat mass and plasma lipids of HFD-fed obese mice. To test this hypothesis, BAT thermogenic proteins, body weight, fat mass, caloric intake, and plasma lipids in male APOA4-Tg mice and WT mice fed either a chow diet or a HFD were measured. When fed a chow diet, APOA4 levels were elevated, plasma triglyceride (TG) levels were reduced, and BAT levels of UCP1 trended upward, while body weight, fat mass, caloric intake, and plasma lipids were comparable between APOA4-Tg and WT mice. After a four-week feeding of HFD, APOA4-Tg mice maintained elevated plasma APOA4 and reduced plasma TG, but UCP1 levels in BAT were significantly elevated in comparison to WT controls; body weight, fat mass and caloric intake were still comparable. After 10-week consumption of HFD, however, while APOA4-Tg mice still exhibited increased plasma APOA4, UCP1 levels and reduced TG levels, a reduction in body weight, fat mass and levels of plasma lipids and leptin were finally observed in comparison to their WT controls and independent of caloric intake. Additionally, APOA4-Tg mice exhibited increased energy expenditure at several time points when measured during the 10-week HFD feeding. Thus, overexpression of APOA4 in the small intestine and maintenance of elevated levels of plasma APOA4 appear to correlate with elevation of UCP1-dependent BAT thermogenesis and subsequent protection against HFD-induced obesity in mice.

Thermogenic Brown Fat in Humans: Implications in Energy Homeostasis, Obesity and Metabolic Disorders

In mammals including humans, there are two types of adipose tissue, white and brown adipose tissues (BATs). White adipose tissue is the primary site of energy storage, while BAT is a specialized tissue for non-shivering thermogenesis to dissipate energy as heat. Although BAT research has long been limited mostly in small rodents, the rediscovery of metabolically active BAT in adult humans has dramatically promoted the translational studies on BAT in health and diseases. It is now established that BAT, through its thermogenic and energy dissipating activities, plays a role in the regulation of body temperature, whole-body energy expenditure, and body fatness. Moreover, increasing evidence has demonstrated that BAT secretes various paracrine and endocrine factors, which influence other peripheral tissues and control systemic metabolic homeostasis, suggesting BAT as a metabolic regulator, other than for thermogenesis. In fact, clinical studies have revealed an association of BAT not only with metabolic disorders such as insulin resistance, diabetes, dyslipidemia, and fatty liver, but also with cardiovascular diseases including hypertension and atherosclerosis. Thus, BAT is an intriguing tissue combating obesity and related metabolic diseases. In this review, we summarize current knowledge on human BAT, focusing its patho-physiological roles in energy homeostasis, obesity and related metabolic disorders. The effects of aging and sex on BAT are also discussed.

Sexual Dimorphism in Brown Adipose Tissue Activation and White Adipose Tissue Browning

The present narrative review gathers the studies reported so far, addressing sex differences in the effects of cold exposure, feeding pattern and age on brown adipose tissue (BAT) thermogenesis and white adipose tissue (WAT) browning. In rodents, when exposed to decreasing temperatures, females activate thermogenesis earlier. Results obtained in humans go in the same line, although they do not provide results as solid as those obtained in rodents. Regarding the effects of overfeeding, interesting sex differences on BAT thermogenic capacity have been reported, and the greater or lower sensitivity of each sex to this dietary situation seems to be dependent on the type of feeding. In the case of energy restriction, females are more sensitive than males. In addition, sex differences have also been observed in thermogenesis changes induced by phenolic compound administration. During sexual development, an increase in BAT mass and BAT activity takes place. This phenomenon is greater in boys than in girls, probably due to its relation to muscle-mass growth. The opposite situation takes place during ageing, a lifespan period where thermogenic capacity declines, this being more acute in men than in women. Finally, the vast majority of the studies have reported a higher susceptibility to developing WAT browning amongst females. The scarcity of results highlights the need for further studies devoted to analysing this issue, in order to provide valuable information for a more personalised approach.

Transcutaneous auricular vagus nerve stimulation augments postprandial inhibition of ghrelin

Vagus nerve stimulation (VNS) facilitates weight loss in animals and patients treated with VNS for depression or epilepsy. Likewise, chronic transcutaneous auricular VNS (taVNS) reduces weight gain and improves glucose tolerance in Zucker diabetic fatty rats. If these metabolic effects of taVNS observed in rats translate to humans is unknown. Therefore, the hypothesis of this study was that acute application of taVNS affects glucotropic and orexigenic hormones which could potentially facilitate weight loss and improve glucose tolerance if taVNS were applied chronically. In two single-blinded randomized cross-over protocols, blood glucose levels, plasma concentrations of insulin, C-peptide, glucagon, leptin, and ghrelin, together with heart rate variability and baroreceptor-heart rate reflex sensitivity were determined before and after taVNS (left ear, 10 Hz, 300 µs, 2.0-2.5 mA, 30 min) or sham-taVNS (electrode attached to ear with the stimulator turned off). In a first protocol, subjects (n = 16) were fasted throughout the protocol and in a second protocol, subjects (n = 10) received a high-calorie beverage (220 kCal) after the first blood sample, just before initiation of taVNS or sham-taVNS. No significant effects of taVNS on heart rate variability and baroreceptor-heart rate reflex sensitivity and only minor effects on glucotropic hormones were observed. However, in the second protocol taVNS significantly lowered postprandial plasma ghrelin levels (taVNS: -115.5 ± 28.3 pg/ml vs. sham-taVNS: -51.2 ± 30.6 pg/ml, p < 0.05). This finding provides a rationale for follow-up studies testing the hypothesis that chronic application of taVNS may reduce food intake through inhibition of ghrelin and, therefore, may indirectly improve glucose tolerance through weight loss.

Five weeks of intermittent transcutaneous vagus nerve stimulation shape neural networks: a machine learning approach

Invasive and transcutaneous vagus nerve stimulation [(t)-VNS] have been used to treat epilepsy, depression and migraine and has also shown effects on metabolism and body weight. To what extent this treatment shapes neural networks and how such network changes might be related to treatment effects is currently unclear. Using a pre-post mixed study design, we applied either a tVNS or sham stimulation (5 h/week) in 34 overweight male participants in the context of a study designed to assess effects of tVNS on body weight and metabolic and cognitive parameters resting state (rs) fMRI was measured about 12 h after the last stimulation period. Support vector machine (SVM) classification was applied to fractional amplitude low-frequency fluctuations (fALFF) on established rs-networks. All classification results were controlled for random effects and overfitting. Finally, we calculated multiple regressions between the classification results and reported food craving. We found a classification accuracy (CA) of 79 % in a subset of four brainstem regions suggesting that tVNS leads to lasting changes in brain networks. Five of eight salience network regions yielded 76,5 % CA. Our study shows tVNS’ post-stimulation effects on fALFF in the salience rs-network. More detailed investigations of this effect and their relationship with food intake seem reasonable for future studies.

Acute vagus nerve stimulation does not affect liking or wanting ratings of food in healthy participants

The vagus nerve plays a vital role in the regulation of food intake and vagal afferent signals may help regulate food cue reactivity by providing negative homeostatic feedback. Despite strong evidence from preclinical studies on vagal afferent "satiety" signals in guiding food intake, evidence from human studies is largely inconclusive to date. Here, we investigated the acute effects of left or right transcutaneous auricular vagus nerve stimulation (taVNS) on subjective ratings of wanting and liking of various food and non-food items in 82 healthy participants (46 women, M_BMI = 23.1 kg/m²). In contrast to previous reports in patients with depression, we found moderate to anecdotal evidence supporting the absence of taVNS-induced changes in food ratings. To test whether the absence of taVNS effects on food ratings is due to heterogeneity in the sample, we conducted post hoc subgroup analyses by splitting the data according to stimulation side and sex (between-subject factors) as well as caloric density, perceived healthiness, and flavor (sweet vs. savory) of the food (within-subject factors). This multiverse analysis largely supported the absence of taVNS-induced changes since the strongest subgroup effects provided only anecdotal evidence in favor of taVNS-induced changes. We conclude that acute taVNS only has a marginal effect on subjective ratings of food, suggesting that it is an unlikely mechanism for the reported long-term effects of VNS on body weight. In light of an absence of acute taVNS effects on conscious food liking and wanting, our results call for future research on the correspondence between acute and chronic effects of vagal afferent stimulation.

Acute vagus nerve stimulation enhances reversal learning in rats

Cognitive flexibility is a prefrontal cortex-dependent neurocognitive process that enables behavioral adaptation in response to changes in environmental contingencies. Electrical vagus nerve stimulation (VNS) enhances several forms of learning and neuroplasticity, but its effects on cognitive flexibility have not been evaluated. In the current study, a within-subjects design was used to assess the effects of VNS on performance in a novel visual discrimination reversal learning task conducted in touchscreen operant chambers. The task design enabled simultaneous assessment of acute VNS both on reversal learning and on recall of a well-learned discrimination problem. Acute VNS delivered in conjunction with stimuli presentation during reversal learning reliably enhanced learning of new reward contingencies. Enhancement was not observed, however, if VNS was delivered during the session but was not coincident with presentation of to-be-learned stimuli. In addition, whereas VNS delivered at 30 HZ enhanced performance, the same enhancement was not observed using 10 or 50 Hz. Together, these data show that acute VNS facilitates reversal learning and indicate that the timing and frequency of the VNS are critical for these enhancing effects. In separate rats, administration of the norepinephrine reuptake inhibitor atomoxetine also enhanced reversal learning in the same task, consistent with a noradrenergic mechanism through which VNS enhances cognitive flexibility.

Mitochondrial dynamics keep balance of nutrient combustion in thermogenic adipocytes

Non-shivering thermogenesis takes place in brown and beige adipocytes and facilitates cold tolerance and acclimation. However, thermogenesis in adipose tissue also was found to be activated in metabolic overload states for fast utilization of nutrients excess. This observation spurred research interest in mechanisms of thermogenesis regulation for metabolic overload and obesity prevention. One of proposed regulators of thermogenic efficiency in adipocytes is the dynamics of mitochondria, where thermogenesis takes place. Indeed, brown and beige adipocytes exhibit fragmented round-shaped mitochondria, while white adipocytes have elongated organelles with high ATP synthesis. Mitochondrial morphology can determine uncoupling protein 1 (UCP1) content, efficiency of catabolic pathways and electron transport chain, supplying thermogenesis. This review will highlight the co-regulation of mitochondrial dynamics and thermogenesis and formulate hypothetical ways for excessive nutrients burning in response to mitochondrial morphology manipulation.

Central Apolipoprotein A-IV Stimulates Thermogenesis in Brown Adipose Tissue

Stimulation of thermogenesis in brown adipose tissue (BAT) could have far-reaching health benefits in combatting obesity and obesity-related complications. Apolipoprotein A-IV (ApoA-IV), produced by the gut and the brain in the presence of dietary lipids, is a well-known short-term satiating protein. While our previous studies have demonstrated reduced diet-induced thermogenesis in ApoA-IV-deficient mice, it is unclear whether this reduction is due to a loss of peripheral or central effects of ApoA-IV. We hypothesized that central administration of ApoA-IV stimulates BAT thermogenesis and that sympathetic and sensory innervation is necessary for this action. To test this hypothesis, mice with unilateral denervation of interscapular BAT received central injections of recombinant ApoA-IV protein or artificial cerebrospinal fluid (CSF). The effects of central ApoA-IV on BAT temperature and thermogenesis in mice with unilateral denervation of the intrascapular BAT were monitored using transponder probe implantation, qPCR, and immunoblots. Relative to CSF, central administration of ApoA-IV significantly increased temperature and UCP expression in BAT. However, all of these effects were significantly attenuated or prevented in mice with unilateral denervation. Together, these results clearly demonstrate that ApoA-IV regulates BAT thermogenesis centrally, and this effect is mediated through sympathetic and sensory nerves.

Vagus Nerve Stimulation Attenuates Early Traumatic Brain Injury by Regulating the NF-κB/NLRP3 Signaling Pathway

BACKGROUND

Traumatic brain injury (TBI) is a major cause of death and disability worldwide. Oxidative stress, inflammation, and apoptosis are vital pathophysiological features post-TBI.

OBJECTIVES

Research has shown that vagus nerve stimulation (VNS) can attenuate oxidative stress in various diseases. However, the critical role of VNS in TBI is still not completely understood. This study investigated the protective effects and potential mechanism of VNS on TBI.

METHODS

Male Sprague-Dawley rats were randomized into 3 groups: sham, TBI, and TBI + VNS. The TBI model was induced in rats by the free-fall drop method. The vagal nerve trunk was separated, and VNS was performed after establishing the TBI model.

RESULTS

The results showed that VNS significantly ameliorated tissue damage, neurological deficits, and cerebral edema, compared with the sham VNS group. Additionally, VNS alleviated oxidative stress, inflammation, and apoptosis in the pericontusive cortex of rats after TBI. VNS also significantly suppressed expression of the nuclear factor-κB (NF-κB) protein in the nucleus and activation of the nucleotide-binding domain-like receptor protein 3 (NLRP3) inflammasome.

CONCLUSIONS

Taken together, the present study indicates that VNS may attenuate brain damage after TBI by inhibiting oxidative stress, inflammation, and apoptosis, possibly through the NF-κB/NLRP3 signaling pathway.

A Brain-Melanocortin-Vagus Axis Mediates Adipose Tissue Expansion Independently of Energy Intake

The melanocortin system is a brain circuit that influences energy balance by regulating energy intake and expenditure. In addition, the brain-melanocortin system controls adipose tissue metabolism to optimize fuel mobilization and storage. Specifically, increased brain-melanocortin signaling or negative energy balance promotes lipid mobilization by increasing sympathetic nervous system input to adipose tissue. In contrast, calorie-independent mechanisms favoring energy storage are less understood. Here, we demonstrate that reduction of brain-melanocortin signaling actively promotes fat mass gain by activating the lipogenic program and adipocyte and endothelial cell proliferation in white fat depots independently of caloric intake via efferent nerve fibers conveyed by the common hepatic branch of the vagus nerve. Those vagally regulated obesogenic signals also contribute to the fat mass gain following chronic high-fat diet feeding. These data reveal a physiological mechanism whereby the brain controls energy stores that may contribute to increased susceptibility to obesity.

Brain-melanocortin signaling controls fat mass indirectly by regulating energy balance and by direct control of lipid mobilization from adipose tissue via sympathetic nervous system activity. Holland et al. show that reduced brain-melanocortin signaling promotes white adipose tissue expansion via signals conveyed by efferent innervation of the vagus nerve.

Effect of Short-Term Transcutaneous Vagus Nerve Stimulation (tVNS) on Brain Processing of Food Cues: An Electrophysiological Study

Background: The vagus nerve plays an important role in the regulation of food intake. Modulating vagal activity via electrical stimulation (VNS) in patients and animal studies caused changes in food intake, energy metabolism, and body weight. However, the moderating impact of cognitive processes on VNS effects on eating behavior has not been investigated so far.

Hypothesis: We hypothesized that transcutaneous VNS (tVNS) affects food intake by altering cognitive functions relevant to the processing of food-related information.

Methods: Using a repeated-measurement design, we applied tVNS and a sham stimulation for 2 h on two different days in normal-weight subjects. We recorded standard scalp EEG while subjects watched food and object pictures presented in an oddball task. We analyzed the event-related potentials (ERPs) P1, P2, N2, and LPP and also examined the amount of consumed food and eating duration in a free-choice test meal.

Results: Significant differences between stimulations were observed for the P1, P2, and N2 amplitudes. However, we found no tVNS-dependent modulation of food intake nor a specific food-related stimulation effect on the ERPs. Further analyses revealed a negative relationship between P2 amplitude and food intake for the sham stimulation. Significant effects are additionally confirmed by Bayesian statistics.

Conclusion: Our study demonstrates tVNS’ impact on visual processing. Since the effects were similar between food and object stimuli, a general effect on visual perceptual processing can be assumed. More detailed investigations of these effects and their relationship with food intake and metabolism seem reasonable for future studies.

Incendiary Leptin

Leptin is a hormone released by adipose tissue that plays a key role in the control of energy homeostasis through its binding to leptin receptors (LepR), mainly expressed in the hypothalamus. Most scientific evidence points to leptin’s satiating effect being due to its dual capacity to promote the expression of anorexigenic neuropeptides and to reduce orexigenic expression in the hypothalamus. However, it has also been demonstrated that leptin can stimulate (i) thermogenesis in brown adipose tissue (BAT) and (ii) the browning of white adipose tissue (WAT). Since the demonstration of the importance of BAT in humans 10 years ago, its study has aroused great interest, mainly in the improvement of obesity-associated metabolic disorders through the induction of thermogenesis. Consequently, several strategies targeting BAT activation (mainly in rodent models) have demonstrated great potential to improve hyperlipidemias, hepatic steatosis, insulin resistance and weight gain, leading to an overall healthier metabolic profile. Here, we review the potential therapeutic ability of leptin to correct obesity and other metabolic disorders, not only through its satiating effect, but by also utilizing its thermogenic properties.

Brown Adipose Tissue, Diet-Induced Thermogenesis, and Thermogenic Food Ingredients: From Mice to Men

Since the recent rediscovery of brown adipose tissue (BAT) in adult humans, this thermogenic tissue has been attracting increasing interest. The inverse relationship between BAT activity and body fatness suggests that BAT, because of its energy dissipating activity, is protective against body fat accumulation. Cold exposure activates and recruits BAT, resulting in increased energy expenditure and decreased body fatness. The stimulatory effects of cold exposure are mediated through transient receptor potential (TRP) channels and the sympathetic nervous system (SNS). Most TRP members also function as chemesthetic receptors for various food ingredients, and indeed, agonists of TRP vanilloid 1 such as capsaicin and its analog capsinoids mimic the effects of cold exposure to decrease body fatness through the activation and recruitment of BAT. The antiobesity effect of other food ingredients including tea catechins may be attributable, at least in part, to the activation of the TRP-SNS-BAT axis. BAT is also involved in the facultative thermogenesis induced by meal intake, referred to as diet-induced thermogenesis (DIT), which is a significant component of the total energy expenditure in our daily lives. Emerging evidence suggests a crucial role for the SNS in BAT-associated DIT, particularly during the early phase, but several gut-derived humoral factors may also participate in meal-induced BAT activation. One intriguing factor is bile acids, which activate BAT directly through Takeda G-protein receptor 5 (TGR5) in brown adipocytes. Given the apparent beneficial effects of some TRP agonists and bile acids on whole-body substrate and energy metabolism, the TRP/TGR5-BAT axis represents a promising target for combating obesity and related metabolic disorders in humans.

Electrical Neurostimulation Promotes Brown Adipose Tissue Thermogenesis

BACKGROUND

Brown adipose tissue (BAT) is present in humans and rodents, and contributes to energy expenditure by converting energy stored in lipids and glucose into heat. Beta adrenergic receptor (β-AR) agonists have been proposed as pharmacological tools to activate BAT, but they lack selectivity for this tissue. This study aimed to investigate the possibility to apply electrical neurostimulation as a novel approach to activate BAT by promoting the sympathetic outflow towards BAT.

METHODS

Male C57BL/6J mice were treated with either unilateral electrical neurostimulation of interscapular BAT or with the β3-AR agonist CL316,243. Thermogenesis, nutrient uptake by BAT and downstream signaling of adrenergic receptors in BAT were examined.

RESULTS

Electrical neurostimulation and β3-AR agonism acutely increased heat production by BAT, as evidenced by an increase in local temperature in BAT, without influencing the core body temperature. Both treatments acutely increased tyrosine hydroxylase content in the nerve terminals thereby confirming enhanced sympathetic activity. In addition, we identified increased phosphorylation of hormone-sensitive lipase coinciding with reduced intracellular lipids in BAT, without affecting acute nutrient uptake from plasma. The increased BAT temperature as induced by electrical neurostimulation was reversed by β3-AR antagonism.

CONCLUSION

Electrical neurostimulation acutely promotes thermogenesis in BAT as dependent on β3-AR signaling. We anticipate that electrical neurostimulation may be further developed as a novel strategy to activate BAT and thereby combat (cardio)metabolic diseases.

Non-invasive stimulation of vagal afferents reduces gastric frequency

Metabolic feedback between the gut and the brain relayed via the vagus nerve contributes to energy homeostasis. We investigated in healthy adults whether non-invasive stimulation of vagal afferents impacts energy homeostasis via efferent effects on metabolism or digestion. In a randomized crossover design, we applied transcutaneous auricular vagus nerve stimulation (taVNS) while recording efferent metabolic effects using simultaneous electrogastrography (EGG) and indirect calorimetry. We found that taVNS reduced gastric myoelectric frequency (p = .008), but did not alter resting energy expenditure. We conclude that stimulating vagal afferents induces gastric slowing via vagal efferents without acutely affecting net energy expenditure at rest. Collectively, this highlights the potential of taVNS to modulate digestion by activating the dorsal vagal complex. Thus, taVNS-induced changes in gastric frequency are an important peripheral marker of brain stimulation effects.

Short-term discontinuation of vagal nerve stimulation alters 18F-FDG blood pool activity: an exploratory interventional study in epilepsy patients

Background

Vagus nerve activation impacts inflammation. Therefore, we hypothesized that vagal nerve stimulation (VNS) influenced arterial wall inflammation as measured by ¹⁸F-FDG uptake.

Results

Ten patients with left-sided VNS for refractory epilepsy were studied during stimulation (VNS-on) and in the hours after stimulation was switched off (VNS-off). In nine patients, ¹⁸F-FDG uptake was measured in the right carotid artery, aorta, bone marrow, spleen, and adipose tissue. Target-to-background ratios (TBRs) were calculated to normalize the respective standardized uptake values (SUVs) for venous blood pool activity. Median values are shown with interquartile range and compared using the Wilcoxon signed-rank test. Arterial SUVs tended to be higher during VNS-off than VNS-on [SUV_max all vessels 1.8 (1.5–2.2) vs. 1.7 (1.2–2.0), p = 0.051]. However, a larger difference was found for the venous blood pool at this time point, reaching statistical significance in the vena cava superior [_meanSUV_mean 1.3 (1.1–1.4) vs. 1.0 (0.8–1.1); p = 0.011], resulting in non-significant lower arterial TBRs during VNS-off than VNS-on. Differences in the remaining tissues were not significant. Insulin levels increased after VNS was switched off [55.0 pmol/L (45.9–96.8) vs. 48.1 pmol/L (36.9–61.8); p = 0.047]. The concurrent increase in glucose levels was not statistically significant [4.8 mmol/L (4.7–5.3) vs. 4.6 mmol/L (4.5–5.2); p = 0.075].

Conclusions

Short-term discontinuation of VNS did not show a consistent change in arterial wall ¹⁸F-FDG-uptake. However, VNS did alter insulin and ¹⁸F-FDG blood levels, possibly as a result of sympathetic activation.

Vagal mechanisms as neuromodulatory targets for the treatment of metabolic disease

With few effective treatments available, the global rise of metabolic diseases, including obesity, type 2 diabetes mellitus, and cardiovascular disease, seems unstoppable. Likely caused by an obesogenic environment interacting with genetic susceptibility, the pathophysiology of obesity and metabolic diseases is highly complex and involves crosstalk between many organs and systems, including the brain. The vagus nerve is in a key position to bidirectionally link several peripheral metabolic organs with the brain and is increasingly targeted for neuromodulation therapy to treat metabolic disease. Here, we review the basics of vagal functional anatomy and its implications for vagal neuromodulation therapies. We find that most existing vagal neuromodulation techniques either ignore or misinterpret the rich functional specificity of both vagal efferents and afferents as demonstrated by a large body of literature. This lack of specificity of manipulating vagal fibers is likely the reason for the relatively poor beneficial long‐term effects of such therapies. For these therapies to become more effective, rigorous validation of all physiological endpoints and optimization of stimulation parameters as well as electrode placements will be necessary. However, given the large number of function‐specific fibers in any vagal branch, genetically guided neuromodulation techniques are more likely to succeed.

Reduced Diet-induced Thermogenesis in Apolipoprotein A-IV Deficient Mice

In the presence of dietary lipids, both apolipoprotein A-IV (ApoA-IV) production and brown adipose tissue (BAT) thermogenesis are increased. The effect of dietary lipid-induced AproA-IV on BAT thermogenesis and energy expenditure remains unknown. In the present study, we hypothesized that ApoA-IV knockout (ApoA-IV-KO) mice exhibited decreased BAT thermogenesis to affect energy homeostasis. To test this hypothesis, BAT thermogenesis in wildtype (WT) and ApoA-IV-KO mice fed either a standard low-fat chow diet or a high-fat diet (HFD) was investigated. When fed a chow diet, energy expenditure and food intake were comparable between WT and ApoA-IV-KO mice. After 1 week of HFD consumption, ApoA-IV-KO mice had comparable energy intake but produced lower energy expenditure relative to their WT controls in the dark phase. After an acute feeding of dietary lipids or 1-week HFD feeding, ApoA-IV-KO mice produced lower levels of uncoupling protein 1 (UCP1) and exhibited reduced expression of thermogenic genes in the BAT compared with WT controls. In response to cold exposure, however, ApoA-IV-KO mice had comparable energy expenditure and BAT temperature relative to WT mice. Thus, ApoA-IV-KO mice exhibited reduced diet-induced BAT thermogenesis and energy expenditure.

Hypothermia Masks Most of the Effects of Rapid Cycling VNS on Rat Hippocampal Electrophysiology

AIM. Vagus nerve stimulation (VNS) modulates hippocampal dentate gyrus (DG) electrophysiology and induces hypothermia in freely moving rats. This study evaluated whether hippocampal (CA1) electrophysiology is similarly modulated and to what extent this is associated with VNS-induced hypothermia. METHODS. Six freely moving rats received a first 4h session of rapid cycling VNS (7s on/18s off), while CA1 evoked potentials, EEG and core temperature were recorded. In a second 4h session, external heating was applied during the 3rd and 4thh of VNS counteracting VNS-induced hypothermia. RESULTS. VNS decreased the slope of the field excitatory postsynaptic potential (fEPSP), increased the population spike (PS) amplitude and latency, decreased theta (4-12Hz) and gamma (30-100Hz) band power and theta peak frequency. Normalizing body temperature during VNS through external heating abolished the effects completely for fEPSP slope, PS latency and gamma band power, partially for theta band power and theta peak frequency and inverted the effect on PS amplitude. CONCLUSIONS. Rapid cycle VNS modulates CA1 electrophysiology similarly to DG, suggesting a wide-spread VNS-induced effect on hippocampal electrophysiology. Normalizing core temperature elucidated that VNS-induced hypothermia directly influences several electrophysiological parameters but also masks a VNS-induced reduction in neuronal excitability.

Chronic abdominal vagus stimulation increased brain metabolic connectivity, reduced striatal dopamine transporter and increased mid-brain serotonin transporter in obese miniature pigs

Background/objective

Changes in brain metabolism has been investigated thoroughly during unilateral cervical chronic vagal stimulation in epileptic or depressive patients. Bilateral stimulation of the abdominal vagus (_aVNS) has received less attention despite the reduction in body weight and an altered feeding behavior in obese animals that could be clinically relevant in obese individuals. Our study aims to examine the changes in brain glucose metabolism (CMR_glu) induced by _aVNS in obese adult miniature pigs. Dopamine (DAT) and serotonin transporters (SERT) were also quantified to further understand the molecular origins of the alterations in brain metabolism.

Subjects/methods

Pairs of stimulating electrodes were implanted during laparoscopy on both abdominal vagal trunks in 20 obese adult’s miniature pigs. Half of the animals were permanently stimulated while the remaining were sham stimulated. Two months after the onset of stimulation, dynamic ₁₈FDG PET and ¹²³I-ioflupane SPECT were performed. Food intake, resting energy expenditure and fat deposition were also assessed longitudinally.

Results

Food intake was halved and resting energy expenditure was increased by 60% in _aVNS group compared to sham. The gain in body weight was also 38% less in _aVNS group compared to sham. Brain metabolic connectivity increased between numerous structures including striatum, mid-brain, amygdala and hippocampus. On the contrary, increased CMR_glu were restricted to the thalamus, the periaqueducal grey and the amygdala. DAT binding potential was decreased by about one third in the striatum while SERT was about doubled in the midbrain.

Conclusions

Our findings demonstrated that _aVNS reduced weight gain as a consequence of diminished daily food intake and increased resting energy expenditure. These changes were associated with enhanced connectivity between several brain areas. A lower striatal DAT together with a doubled mid-brain SERT were likely causative for these changes.

Electronic supplementary material

The online version of this article (10.1186/s12967-019-1831-5) contains supplementary material, which is available to authorized users.

Body thermal responses and the vagus nerve

While thermosensation from external environment has been extensively studied, physiological responses to temperature changes inside the body and the underlying regulatory mechanisms are less understood. As a critical link between body and brain that relays visceral organ information and regulates numerous physiological functions, the vagus nerve has been proposed to mediate diverse visceral thermal reflexes and indirectly regulate body temperature. However, the precise role of the vagus nerve in body thermal responses or visceral organ-related thermoregulation is still under debate due to extensive contradictory results. This data discrepancy is likely due to the high cell heterogeneity in the vagus nerve, as diverse vagal neuron types mediate numerous and sometimes opposite physiological functions. Here, we will review evidences that support and against the role of the vagus nerve in body thermosensation and thermoregulation and discuss potential future approaches for better understanding of this critical issue.

Electric stimulation of ears accelerates body weight loss mediated by high-fat to low-fat diet switch accompanied by increased white adipose tissue browning in C57BL/6 J mice

Background

Weight reduction frequently occurs in patients receiving vagus nerve stimulation (VNS) therapy. Therefore, we hypothesized that during dietary intervention for weight loss, auricular electric stimulation (AES), an alternative of VNS, accelerates weight loss by increasing white adipose tissue (WAT) browning and increases energy expenditure.

Methods

C57BL/6J male mice were fed a high-fat diet for 5 wk. to induce obesity, then switched to a low-fat diet for 5 wk. and allocated into 3 groups to receive 2 Hz electric stimulation on ears, electrode clamps only, or nothing (AES, Sham and Ctrl, respectively).

Results

Switching to a low-fat diet reduced body weight progressively in all 3 groups, with the greatest reduction in the AES group. In accordance with a mild decrease in feed intake, hypothalamus mRNA levels of Npy, AgRP tended to be reduced, while Pomc tended to be increased by AES. Mice in the AES group had the highest concentrations of norepinephrine in serum and inguinal WAT, and expression levels of uncoupling protein-1 (UCP-1) and tyrosine hydroxylase in inguinal WAT. Furthermore, their subcutaneous adipocytes had multilocular and UCP-1⁺ characteristics, along with a smaller cell size.

Conclusion

AES, by increasing WAT browning, could be used in conjunction with a low-fat diet to augment weight loss in addition to suppressing appetite.

Effects of vagal neuromodulation on feeding behavior

Implanted vagus nerve stimulation (VNS) for obesity was recently approved by the FDA. However, its efficacy and mechanisms of action remain unclear. Herein, we synthesize clinical and preclinical effects of VNS on feeding behavior and energy balance and discuss engineering considerations for understanding and improving the therapy. Clinical cervical VNS (≤30 Hz) to treat epilepsy or depression has produced mixed effects on weight loss as a side effect, albeit in uncontrolled, retrospective studies. Conversely, preclinical studies (cervical and subdiaphragmatic VNS) mostly report decreased food intake and either decreased weight gain or weight loss. More recent clinical studies report weight loss in response to kilohertz frequency VNS applied to the subdiaphragmatic vagi, albeit with a large placebo effect. Rather than eliciting neural activity, this therapy putatively blocks conduction in the vagus nerves. Overall, stimulation parameters lack systematic exploration, optimization, and justification based on target nerve fibers and therapeutic outcomes. The vagus nerve transduces, transmits, and integrates important neural (efferent and afferent), humoral, energetic, and inflammatory information between the gut and brain. Thus, improved understanding of the biophysics, electrophysiology, and (patho)physiology has the potential to advance VNS as an effective therapy for a wide range of diseases.

Brown Adipose Tissue Energy Metabolism in Humans

The demonstration of metabolically active brown adipose tissue (BAT) in humans primarily using positron emission tomography coupled to computed tomography (PET/CT) with the glucose tracer 18-fluorodeoxyglucose (18FDG) has renewed the interest of the scientific and medical community in the possible role of BAT as a target for the prevention and treatment of obesity and type 2 diabetes (T2D). Here, we offer a comprehensive review of BAT energy metabolism in humans. Considerable advances in methods to measure BAT energy metabolism, including nonesterified fatty acids (NEFA), chylomicron-triglycerides (TG), oxygen, Krebs cycle rate, and intracellular TG have led to very good quantification of energy substrate metabolism per volume of active BAT in vivo. These studies have also shown that intracellular TG are likely the primary energy source of BAT upon activation by cold. Current estimates of BAT's contribution to energy expenditure range at the lower end of what would be potentially clinically relevant if chronically sustained. Yet, 18FDG PET/CT remains the gold-standard defining method to quantify total BAT volume of activity, used to calculate BAT's total energy expenditure. Unfortunately, BAT glucose metabolism better reflects BAT's insulin sensitivity and blood flow. It is now clear that most glucose taken up by BAT does not fuel mitochondrial oxidative metabolism and that BAT glucose uptake can therefore be disconnected from thermogenesis. Furthermore, BAT thermogenesis is efficiently recruited upon repeated cold exposure, doubling to tripling its total oxidative capacity, with reciprocal reduction of muscle thermogenesis. Recent data suggest that total BAT volume may be much larger than the typically observed 50-150 ml with 18FDG PET/CT. Therefore, the current estimates of total BAT thermogenesis, largely relying on total BAT volume using 18FDG PET/CT, may underestimate the true contribution of BAT to total energy expenditure. Quantification of the contribution of BAT to energy expenditure begs for the development of more integrated whole body in vivo methods.

Recent advances in the detection of brown adipose tissue in adult humans: a review

The activation of brown adipose tissue (BAT) is associated with reductions in circulating lipids and glucose in rodents and contributes to energy expenditure in humans indicating the potential therapeutic importance of targetting this tissue for the treatment of a variety of metabolic disorders. In order to evaluate the therapeutic potential of human BAT, a variety of methodologies for assessing the volume and metabolic activity of BAT are utilized. Cold exposure is often utilized to increase BAT activity but inconsistencies in the characteristics of the exposure protocols make it challenging to compare findings. The metabolic activity of BAT in response to cold exposure has most commonly been measured by static positron emission tomography of ¹⁸F-fluorodeoxyglucose in combination with computed tomography (¹⁸F-FDG PET-CT) imaging, but recent studies suggest that under some conditions this may not always reflect BAT thermogenic activity. Therefore, recent studies have used alternative positron emission tomography and computed tomography (PET-CT) imaging strategies and radiotracers that may offer important insights. In addition to PET-CT, there are numerous emerging techniques that may have utility for assessing BAT metabolic activity including magnetic resonance imaging (MRI), skin temperature measurements, near-infrared spectroscopy (NIRS) and contrast ultrasound (CU). In this review, we discuss and critically evaluate the various methodologies used to measure BAT metabolic activity in humans and provide a contemporary assessment of protocols which may be useful in interpreting research findings and guiding the development of future studies.

Brown adipose tissue plays a central role in systemic inflammation-induced sleep responses

We previously identified brown adipose tissue (BAT) as a source of sleep-inducing signals. Pharmacological activation of BAT enhances sleep while sleep loss leads to increased BAT thermogenesis. Recovery sleep after sleep loss is diminished in mice that lack uncoupling protein 1 (UCP-1), and also in wild-type (WT) mice after sensory denervation of the BAT. Systemic inflammation greatly affects metabolism and the function of adipose tissue, and also induces characteristic sleep responses. We hypothesized that sleep responses to acute inflammation are mediated by BAT-derived signals. To test this, we determined the effects of systemic inflammation on sleep and body temperature in UCP-1 knockout (KO) and WT mice. Intraperitoneal injections of lipopolysaccharide, tumor necrosis factor-α, interleukin-1 beta and clodronate containing liposomes were used to induce systemic inflammation. In WT animals, non-rapid-eye movement sleep (NREMS) was elevated in all four inflammatory models. All NREMS responses were completely abolished in UCP-1 KO animals. Systemic inflammation elicited an initial hypothermia followed by fever in WT mice. The hypothermic phase, but not the fever, was abolished in UCP-1 KO mice. The only recognized function of UCP-1 is to promote thermogenesis in brown adipocytes. Present results indicate that the presence of UCP-1 is necessary for increased NREMS but does not contribute to the development of fever in systemic inflammation.

A New Personalized Cooling Protocol to Activate Brown Adipose Tissue in Young Adults

Brown adipose tissue (BAT) activity is induced when humans are exposed to cold. Therefore, cold exposure prior to the ¹⁸F-FDG-PET/CT scan is used as a tool to quantify BAT. Several cooling protocols, including fixed and personalized ones are currently in use. The aim of the present study was to determine the effect of a new personalized cooling protocol where the shivering threshold was measured on a separate day, on BAT volume and activity in young adults. A total of 47 adults (n = 28 women) aged 22 ± 2 years participated in the study. We determined participants' shivering threshold (visually and self-reported) using a water perfused cooling vest in an air-conditioned cold room. 48–72 h later, participants wore the cooling vest set at ~4°C above the shivering threshold for 60 min prior to injection of ¹⁸F-FDG and ~5°C above the shivering threshold for ~60 min after injection, until PET/CT scan. We quantified BAT following BARCIST 1.0 recommendations. We identified 40 participants (85%, n = 25 women) as PET+ and 7 (n = 3 women) as PET–. The PET+ group presented significantly higher BAT volume and activity than PET– group (all P < 0.05). PET+ women had higher BAT mean activity than PET+ men (SUV_mean: 5.0 ± 1.6 vs. 3.6 ± 0.9 g/ml respectively, P = 0.003), and there were no significant sex differences in BAT volume (P = 0.161). A total of 9 out of 47 participants did not shiver during the shivering threshold test. Our findings are similar to previous cold-stimulated human BAT studies; therefore, we conclude that our personalized cooling protocol is able to activate BAT in young adults.

Constant hepatic ATP concentrations during prolonged fasting and absence of effects of Cerbomed Nemos® on parasympathetic tone and hepatic energy metabolism

Objective

Brain insulin-induced improvement in glucose homeostasis has been proposed to be mediated by the parasympathetic nervous system. Non-invasive transcutaneous auricular vagus nerve stimulation (taVNS) activating afferent branches of the vagus nerve may prevent hyperglycemia in diabetes models. We examined the effects of 14-min taVNS vs sham stimulation by Cerbomed Nemos^® on glucose metabolism, lipids, and hepatic energy homeostasis in fasted healthy humans (n = 10, age 51 ± 6 yrs, BMI 25.5 ± 2.7 kg/m²).

Methods

Heart rate variability (HRV), reflecting sympathetic and parasympathetic nerve activity, was measured before, during and after taVNS or sham stimulation. Endogenous glucose production was determined using [6,6-²H₂]glucose, and hepatic concentrations of triglycerides (HCL), adenosine triphosphate (ATP), and inorganic phosphate (Pi) were quantified from ¹H/³¹P magnetic resonance spectroscopy at baseline and for 180 min following stimulation.

Results

taVNS did not affect circulating glucose, free fatty acids, insulin, glucagon, or pancreatic polypeptide. Rates of endogenous glucose production (P = 0.79), hepatic HCL, ATP, and Pi were also not different (P = 0.91, P = 0.48 and P = 0.24) between taVNS or sham stimulation. Hepatic HCL, ATP, and Pi remained constant during prolonged fasting for 3 h. No changes in heart rate or shift in cardiac autonomic function from HRV towards sympathetic or parasympathetic predominance were detected.

Conclusion

Non-invasive vagus stimulation by Cerbomed Nemos^® does not acutely modulate the autonomic tone to the visceral organs and thereby does not affect hepatic glucose and energy metabolism. This technique is therefore unable to mimic brain insulin-mediated effects on peripheral homeostasis in humans.

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Constant hepatic energy metabolism during prolonged fasting.

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Vagus stimulation with Cerbomed Nemos^® does not alter parasympathetic tone.

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Cerbomed Nemos^® does not modulate hepatic glucose and energy metabolism in humans.

The Role of the Autonomic Nervous System in the Pathophysiology of Obesity

Obesity is reaching epidemic proportions globally and represents a major cause of comorbidities, mostly related to cardiovascular disease. The autonomic nervous system (ANS) dysfunction has a two-way relationship with obesity. Indeed, alterations of the ANS might be involved in the pathogenesis of obesity, acting on different pathways. On the other hand, the excess weight induces ANS dysfunction, which may be involved in the haemodynamic and metabolic alterations that increase the cardiovascular risk of obese individuals, i.e., hypertension, insulin resistance and dyslipidemia. This article will review current evidence about the role of the ANS in short-term and long-term regulation of energy homeostasis. Furthermore, an increased sympathetic activity has been demonstrated in obese patients, particularly in the muscle vasculature and in the kidneys, possibily contributing to increased cardiovascular risk. Selective leptin resistance, obstructive sleep apnea syndrome, hyperinsulinemia and low ghrelin levels are possible mechanisms underlying sympathetic activation in obesity. Weight loss is able to reverse metabolic and autonomic alterations associated with obesity. Given the crucial role of autonomic dysfunction in the pathophysiology of obesity and its cardiovascular complications, vagal nerve modulation and sympathetic inhibition may serve as therapeutic targets in this condition.

Single vagus nerve stimulation reduces early postprandial C-peptide levels but not other hormones or postprandial metabolism

A recent study in rheumatoid arthritis (RA) patients using electrical vagus nerve stimulation (VNS) to activate the inflammatory reflex has shown promising effects on disease activity. Innervation by the autonomic nerve system might be involved in the regulation of many endocrine and metabolic processes and could therefore theoretically lead to unwanted side effects. Possible effects of VNS on secretion of hormones are currently unknown. Therefore, we evaluated the effects of a single VNS on plasma levels of pituitary hormones and parameters of postprandial metabolism. Six female patients with RA were studied twice in balanced assignment (crossover design) to either VNS or no stimulation. The patients selected for this substudy had been on VNS therapy daily for at least 3 months and at maximum of 24 months. We compared 10-, 20-, and 30-min poststimulus levels to baseline levels, and a 4-h mixed meal test was performed 30 min after VNS. We also determined energy expenditure (EE) by indirect calorimetry before and after VNS. VNS did not affect pituitary hormones (growth hormone, thyroid stimulating hormone, adrenocorticotropic hormone, prolactin, follicle-stimulating hormone, and luteinizing hormone), postprandial metabolism, or EE. Of note, VNS reduced early postprandial insulin secretion, but not AUC of postprandial plasma insulin levels. Cortisol and catecholamine levels in serum did not change significantly. Short stimulation of vagal activity by VNS reduces early postprandial insulin secretion, but not other hormone levels and postprandial response. This suggests VNS as a safe treatment for RA patients.

Obesity-Associated Alterations in Glucose Metabolism Are Reversed by Chronic Bilateral Stimulation of the Abdominal Vagus Nerve

Acute vagal stimulation modifies glucose and insulin metabolism, but the effect of chronic bilateral vagal stimulation is not known. Our aim was to quantify the changes in whole-body and organ-specific insulin sensitivities 12 weeks after permanent, bilateral, vagal stimulation performed at the abdominal level in adult mini-pigs. In 15 adult mini-pigs, stimulating electrodes were placed around the dorsal and ventral vagi using laparoscopy and connected to a dual-channel stimulator placed subcutaneously. Animals were divided into three groups based on stimulation and body weight (i.e., lean nonstimulated, obese nonstimulated, and obese stimulated). Twelve weeks after surgery, glucose uptake and insulin sensitivity were measured using positron emission tomography during an isoglycemic clamp. Mean whole-body insulin sensitivity was lower by 34% (P < 0.01) and the hepatic glucose uptake rate was lower by 33% (P < 0.01) in obese-nonstimulated mini-pigs but was no different in obese-stimulated compared with lean mini-pigs. An improvement in skeletal glucose uptake rate was also observed in obese-stimulated compared with obese-nonstimulated groups (P < 0.01). Vagal stimulation was associated with increased glucose metabolism in the cingulate and prefrontal brain areas. We conclude that chronic vagal stimulation improves insulin sensitivity substantially in diet-induced obesity by both peripheral and central mechanisms.

Common and distinct regulation of human and mouse brown and beige adipose tissues: a promising therapeutic target for obesity

Obesity, which underlies various metabolic and cardiovascular diseases, is a growing public health challenge for which established therapies are inadequate. Given the current obesity epidemic, there is a pressing need for more novel therapeutic strategies that will help adult individuals to manage their weight. One promising therapeutic intervention for reducing obesity is to enhance energy expenditure. Investigations into human brown fat and the recently discovered beige/brite fat have galvanized intense research efforts during the past decade because of their pivotal roles in energy dissipation. In this review, we summarize the evolution of human brown adipose tissue (hBAT) research and discuss new in vivo methodologies for evaluating energy expenditure in patients. We highlight the differences between human and mouse BAT by integrating and comparing their cellular morphology, function, and gene expression profiles. Although great advances in hBAT biology have been achieved in the past decade, more cellular models are needed to acquire a better understanding of adipose-specific processes and molecular mechanisms. Thus, this review also describes the development of a human brown fat cell line, which could provide promising mechanistic insights into hBAT function, signal transduction, and development. Finally, we focus on the therapeutic potential and current limitations of hBAT as an anti-glycemic, anti-lipidemic, and weight loss-inducing ‘metabolic panacea’.

Clinical Vagus Nerve Stimulation Paradigms Induce Pronounced Brain and Body Hypothermia in Rats

Vagus nerve stimulation (VNS) is a widely used neuromodulation technique that is currently used or being investigated as therapy for a wide array of human diseases such as epilepsy, depression, Alzheimer's disease, tinnitus, inflammatory diseases, pain, heart failure and many others. Here, we report a pronounced decrease in brain and core temperature during VNS in freely moving rats. Two hours of rapid cycle VNS (7s on/18s off) decreased brain temperature by around [Formula: see text]C, while standard cycle VNS (30[Formula: see text]s on/300[Formula: see text]s off) was associated with a decrease of around [Formula: see text]C. Rectal temperature similarly decreased by more than [Formula: see text]C during rapid cycle VNS. The hypothermic effect triggered by VNS was further associated with a vasodilation response in the tail, which reflects an active heat release mechanism. Despite previous evidence indicating an important role of the locus coeruleus-noradrenergic system in therapeutic effects of VNS, lesioning this system with the noradrenergic neurotoxin DSP-4 did not attenuate the hypothermic effect. Since body and brain temperature affect most physiological processes, this finding is of substantial importance for interpretation of several previously published VNS studies and for the future direction of research in the field.

Vagal afferent activation decreases brown adipose tissue (BAT) sympathetic nerve activity and BAT thermogenesis

In urethane/α-chloralose anesthetized rats, electrical stimulation of cervical vagal afferent fibers inhibited the increases in brown adipose tissue sympathetic nerve activity and brown adipose tissue thermogenesis evoked by cold exposure, by nanoinjection of the GABA_A receptor antagonist, bicuculline, in the dorsomedial hypothalamus, and by nanoinjection of N-methyl-D-aspartate in the rostral raphe pallidus. Vagus nerve stimulation-evoked inhibition of brown adipose tissue sympathetic nerve activity was prevented by blockade of ionotropic glutamate receptors in the termination site of vagal afferents in the nucleus of the solitary tract, and by nanoinjection of GABA_A receptor antagonists in the rostral raphe pallidus. In conclusion, the brown adipose tissue sympathoinhibitory effect of cervical afferent vagal nerve stimulation is mediated by glutamatergic activation of second-order sensory neurons in the nucleus of the solitary tract and by a GABAergic inhibition of brown adipose tissue sympathetic premotor neurons in the rostral raphe pallidus, but does not require GABAergic inhibition of the brown adipose tissue sympathoexcitatory neurons in the dorsomedial hypothalamus.

Comparison of Diet versus Exercise on Metabolic Function and Gut Microbiota in Obese Rats

Cardiometabolic impairments that begin early in life are particularly critical, because they often predict metabolic dysfunction in adulthood. Obesity, high-fat diet (HFD), and inactivity are all associated with adipose tissue (AT) inflammation and insulin resistance (IR), major predictors of metabolic dysfunction. Recent evidence has also associated the gut microbiome with cardiometabolic health.

PURPOSE

The objective of this study is to compare equal energy deficits induced by exercise and caloric reduction on cardiometabolic disease risk parameters including AT inflammation, IR, and gut microbiota changes during HFD consumption.

METHODS

Obesity-prone rats fed HFD were exercise trained (Ex, n = 10) or weight matched to Ex via caloric reduction although kept sedentary (WM, n = 10), and compared with ad libitum HFD-fed (Sed, n = 10) rats for IR, systemic energetics and spontaneous physical activity (SPA), adiposity, and fasting metabolic parameters. Visceral, subcutaneous, periaortic, and brown AT (BAT), liver, aorta, and cecal digesta were examined.

RESULTS

Despite identical reductions in adiposity, Ex, but not WM, improved IR, increased SPA by approximately 26% (P < 0.05 compared with WM and Sed), and reduced LDL cholesterol (P < 0.05 compared with Sed). WM and Ex both reduced inflammatory markers in all AT depots and aorta, whereas only Ex increased indicators of mitochondrial function in BAT. Ex significantly increased the relative abundance of cecal Streptococcaceae and decreased S24-7 and one undefined genus in Rikenellaceae; WM induced similar changes but did not reach statistical significance.

CONCLUSIONS

Both Ex and WM reduced AT inflammation across depots, whereas Ex caused more robust changes to gut microbial communities, improved IR, increased fat oxidation, increased SPA, and increased indices of BAT mitochondrial function. Our findings add to the growing body of literature indicating that there are weight-loss-independent metabolic benefits of exercise.

Immunity-Based Evolutionary Interpretation of Diet-Induced Thermogenesis

Diet-induced thermogenesis (DIT) has often been argued to be a physiological defense against obesity, but no empirical proof of its effectiveness in limiting human body weight gain is available. We here propose an immune explanation of DIT-i.e., that it results from the coevolution of host and gut microbiota (especially Firmicutes) that ferment ingested food and proliferate, causing periodic, vagally mediated increases in thermogenesis aimed at curtailing their expansion. Because of this evolutionary adaptive significance related to the immune system, DIT is not effective as an "adaptation" to maintain a certain body mass. Were DIT an effective adaptation to prevent obesity, the current obesity epidemic might not have occurred.

Role of the vagus nerve in the development and treatment of diet-induced obesity

This review highlights evidence for a role of the vagus nerve in the development of obesity and how targeting the vagus nerve with neuromodulation or pharmacology can be used as a therapeutic treatment of obesity. The vagus nerve innervating the gut plays an important role in controlling metabolism. It communicates peripheral information about the volume and type of nutrients between the gut and the brain. Depending on the nutritional status, vagal afferent neurons express two different neurochemical phenotypes that can inhibit or stimulate food intake. Chronic ingestion of calorie-rich diets reduces sensitivity of vagal afferent neurons to peripheral signals and their constitutive expression of orexigenic receptors and neuropeptides. This disruption of vagal afferent signalling is sufficient to drive hyperphagia and obesity. Furthermore neuromodulation of the vagus nerve can be used in the treatment of obesity. Although the mechanisms are poorly understood, vagal nerve stimulation prevents weight gain in response to a high-fat diet. In small clinical studies, in patients with depression or epilepsy, vagal nerve stimulation has been demonstrated to promote weight loss. Vagal blockade, which inhibits the vagus nerve, results in significant weight loss. Vagal blockade is proposed to inhibit aberrant orexigenic signals arising in obesity as a putative mechanism of vagal blockade-induced weight loss. Approaches and molecular targets to develop future pharmacotherapy targeted to the vagus nerve for the treatment of obesity are proposed. In conclusion there is strong evidence that the vagus nerve is involved in the development of obesity and it is proving to be an attractive target for the treatment of obesity.