TRPV1 Channels and Gastric Vagal Afferent Signalling in Lean and High Fat Diet Induced Obese Mice

CCK-expressing neurons in the NTS are directly activated by CCK-sensitive C-type vagal afferents

Vagal sensory afferents carrying information from the gastrointestinal tract (GI) terminate in the nucleus of the solitary tract (NTS). Different subpopulations of NTS neurons then relay this information throughout the brain. Cholecystokinin (CCK) is a satiety peptide that activates vagal afferents in the GI. However, CCK is also expressed by neurons in the NTS, and activation of these neurons decreases food intake. What is less clear is how these NTS CCK neurons are activated by vagal afferents and what type of information they integrate about meal size and content. To address this, we identified NTS-CCK neurons by crossing CCK-IRES-Cre mice with floxed-Rosa-tdtomato mice and made a horizontal brain slice containing vagal afferents in the solitary tract (ST). Voltage clamp recordings of NTS-CCK neurons show that activation of the ST evokes excitatory postsynaptic currents (EPSCs) mediated by both α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartate (NMDA) receptors. Analysis of these EPSCs revealed that 80% of NTS-CCK neurons receive direct, monosynaptic inputs, with many also receiving indirect, or polysynaptic, inputs. NTS-CCK neurons are sensitive to the transient receptor potential vanilloid type 1 agonist capsaicin, suggesting that they are downstream of C-fibers. In addition, both CCK and a 5 hydroxytryptamine 3 receptor (5-HT3R) agonist increased spontaneous EPSC (sEPSC) frequency in NTS-CCK neurons, with 69% of NTS-CCK neurons sensitive to CCK and 42% to the 5-HT3 receptor agonist, as well as 45% sensitive to both and 10% to neither. Taken together with previous studies, this suggests that NTS-CCK neurons are driven primarily by vagal afferents that are sensitive to CCK and are only weakly driven by those sensitive to serotonin.NEW & NOTEWORTHY Nucleus of the solitary tract (NTS) cholecystokinin (CCK) expressing neurons are directly activated by glutamate released from vagal afferents. They are downstream of primarily C-type CCK-sensitive afferents, with a small proportion also downstream of serotonin-sensitive afferents. These findings suggest that NTS-CCK neurons integrate signals from the gut about ingestion of fats and proteins as well as stretch of the stomach, which they then relay to other brain regions important for the control of food intake.

Plasticity of gastrointestinal vagal afferents in terms of feeding-related physiology and pathophysiology

Gastrointestinal vagal afferents play an important role in communicating food related information from the gut to the brain. This information initiates vago-vagal reflexes essential for gut functions, including gut motility and secretions. These afferents also play a role in energy homeostasis, signalling the arrival, amount and nutrient composition of a meal to the central nervous system where it is processed ultimately leading to termination of a meal. Vagal afferent responses to food related stimuli demonstrate a high degree of plasticity, responding to short term changes in nutritional demand, such as the fluctuations that occur across a 24-hr or in response to a fast, as well as long term changes in energy demand, such as occurs during pregnancy. This plasticity is disrupted in disease states, such as obesity or chronic stress where there is hypo- and hypersensitivity of these afferents, respectively. Improved understanding of the plasticity of these afferents will enable identification of new treatment options for diseases associated with vagal afferent function.

Gaps in our understanding of how vagal afferents to the small intestinal mucosa detect luminal stimuli

Vagal afferents to the gastrointestinal tract are crucial for the regulation of food intake, signaling negative feedback that contributes to satiation and positive feedback that produces appetition and reward. Vagal afferents to the small intestinal mucosa contribute to this regulation by sensing luminal stimuli and reporting this information to the brain. These afferents respond to mechanical, chemical, thermal, pH, and osmolar stimuli, as well as to bacterial products and immunogens. Surprisingly, little is known about how these stimuli are transduced by vagal mucosal afferents or how their transduction is organized among these afferents' terminals. Furthermore, the effects of stimulus concentration ranges or physiological stimuli on vagal activity have not been examined for some of these stimuli. Also, detection of luminal stimuli has rarely been examined in rodents, which are most frequently used for studying small intestinal innervation. Here we review what is known about stimulus detection by vagal mucosal afferents and illustrate the complexity of this detection using nutrients as an exemplar. The accepted model proposes that nutrients bind to taste receptors on enteroendocrine cells (EECs), which excite them, causing the release of hormones that stimulate vagal mucosal afferents. However, evidence reviewed here suggests that although this model accounts for many aspects of vagal signaling about nutrients, it cannot account for all aspects. A major goal of this review is therefore to evaluate what is known about nutrient absorption and detection and, based on this evaluation, identify candidate mucosal cells and structures that could cooperate with EECs and vagal mucosal afferents in stimulus detection.

Time-dependent changes in feeding behavior and energy balance associated with weight gain in mice fed obesogenic diets

OBJECTIVE

Obesity is characterized by dysregulated homeostatic mechanisms resulting in positive energy balance; however, when this dysregulation occurs is unknown. We assessed the time course of alterations to behaviors promoting weight gain in male and female mice switched to an obesogenic high-fat diet (HFD).

METHODS

Male and female C57BL/6J mice were housed in metabolic chambers and were switched from chow to a 60% or 45% HFD for 4 and 3 weeks, respectively. Food intake, meal patterns, energy expenditure (EE), and body weight were continuously measured. A separate cohort of male mice was switched from chow to a 60% HFD and was given access to locked or unlocked running wheels.

RESULTS

Switching mice to obesogenic diets promotes transient bouts of hyperphagia during the first 2 weeks followed by persistent caloric hyperphagia. EE increases but not sufficiently enough to offset increased caloric intake, resulting in a sustained net positive energy balance. Hyperphagia is associated with consumption of calorically larger meals (impaired satiation) more frequently (impaired satiety), particularly during the light cycle. Running wheel exercise delays weight gain in male mice fed a 60% HFD by enhancing satiation and increasing EE. However, exercise effects on satiation are no longer apparent after 2 weeks, coinciding with weight gain.

CONCLUSIONS

Exposure to obesogenic diets engages homeostatic regulatory mechanisms for ~2 weeks that ultimately fail, and consequent weight gain is characterized by impaired satiation and satiety. Insights into the etiology of obesity can be obtained by investigating changes to satiation and satiety mechanisms during the initial ~2 weeks of HFD exposure.

Orally Ingested Self-Powered Stimulators for Targeted Gut-Brain Axis Electrostimulation to Treat Obesity and Metabolic Disorders

Obesity is a significant health concern that often leads to metabolic dysfunction and chronic diseases. This study introduces a novel approach to combat obesity using orally ingested self-powered electrostimulators. These electrostimulators consist of piezoelectric BaTiO3 (BTO) particles conjugated with capsaicin (Cap) and aim to activate the vagus nerve. Upon ingestion by diet-induced obese (DIO) mice, the BTO@Cap particles specifically target and bind to Cap-sensitive sensory nerve endings in the gastric mucosa. In response to stomach peristalsis, these particles generate electrical signals. The signals travel via the gut-brain axis, ultimately influencing the hypothalamus. By enhancing satiety signals in the brain, this neuromodulatory intervention reduces food intake, promotes energy metabolism, and demonstrates minimal toxicity. Over a 3-week period of daily treatments, DIO mice treated with BTO@Cap particles show a significant reduction in body weight compared to control mice, while maintaining their general locomotor activity. Furthermore, this BTO@Cap particle-based treatment mitigates various metabolic alterations associated with obesity. Importantly, this noninvasive and easy-to-administer intervention holds potential for addressing other intracerebral neurological diseases.

CCK-sensitive C fibers activate NTS leptin receptor-expressing neurons via NMDA receptors

The hormone leptin reduces food intake through actions in the peripheral and central nervous systems, including in the hindbrain nucleus of the solitary tract (NTS). The NTS receives viscerosensory information via vagal afferents, including information from the gastrointestinal tract, which is then relayed to other central nervous system (CNS) sites critical for control of food intake. Leptin receptors (lepRs) are expressed by a subpopulation of NTS neurons, and knockdown of these receptors increases both food intake and body weight. Recently, we demonstrated that leptin increases vagal activation of lepR-expressing neurons via increased NMDA receptor (NMDAR) currents, thereby potentiating vagally evoked firing. Furthermore, chemogenetic activation of these neurons was recently shown to inhibit food intake. However, the vagal inputs these neurons receive had not been characterized. Here we performed whole cell recordings in brain slices taken from lepRCre × floxedTdTomato mice and found that lepR neurons of the NTS are directly activated by monosynaptic inputs from C-type afferents sensitive to the transient receptor potential vanilloid type 1 (TRPV1) agonist capsaicin. CCK administered onto NTS slices stimulated spontaneous glutamate release onto lepR neurons and induced action potential firing, an effect mediated by CCKR1. Interestingly, NMDAR activation contributed to the current carried by spontaneous excitatory postsynaptic currents (EPSCs) and enhanced CCK-induced firing. Peripheral CCK also increased c-fos expression in these neurons, suggesting they are activated by CCK-sensitive vagal afferents in vivo. Our results indicate that the majority of NTS lepR neurons receive direct inputs from CCK-sensitive C vagal-type afferents, with both peripheral and central CCK capable of activating these neurons and NMDARs able to potentiate these effects.

TRPV1: Receptor structure, activation, modulation and role in neuro-immune interactions and pain

In the 1990s, the identification of a non-selective ion channel, especially responsive to capsaicin, revolutionized the studies of somatosensation and pain that were to follow. The TRPV1 channel is expressed mainly in neuronal cells, more specifically, in sensory neurons responsible for the perception of noxious stimuli. However, its presence has also been detected in other non-neuronal cells, such as immune cells, β- pancreatic cells, muscle cells and adipocytes. Activation of the channel occurs in response to a wide range of stimuli, such as noxious heat, low pH, gasses, toxins, endocannabinoids, lipid-derived endovanilloid, and chemical agents, such as capsaicin and resiniferatoxin. This activation results in an influx of cations through the channel pore, especially calcium. Intracellular calcium triggers different responses in sensory neurons. Dephosphorylation of the TRPV1 channel leads to its desensitization, which disrupts its function, while its phosphorylation increases the channel's sensitization and contributes to the channel's rehabilitation after desensitization. Kinases, phosphoinositides, and calmodulin are the main signaling pathways responsible for the channel's regulation. Thus, in this review we provide an overview of TRPV1 discovery, its tissue expression as well as on the mechanisms by which TRPV1 activation (directly or indirectly) induces pain in different disease models.

Time dependent changes in feeding behavior and energy balance associated with weight gain in mice fed obesogenic diets

Obesity is characterized by dysregulated homeostatic mechanisms resulting in positive energy balance, yet when this dysregulation occurs is unknown. We assessed the time course of alterations to behaviors promoting weight gain in male and female mice switched to obesogenic 60% or 45% high fat diet (HFD). Switching mice to obesogenic diets promotes transient bouts of hyperphagia during the first 2 weeks followed by persistent caloric hyperphagia. Energy expenditure increases but not sufficiently to offset increased caloric intake, resulting in a sustained net positive energy balance. Hyperphagia is associated with consumption of calorically larger meals (impaired satiation) more frequently (impaired satiety) particularly during the light-cycle. Running wheel exercise delays weight gain in 60% HFD-fed male mice by enhancing satiation and increasing energy expenditure. However, exercise effects on satiation are no longer apparent after 2 weeks, coinciding with weight gain. Thus, exposure to obesogenic diets engages homeostatic regulatory mechanisms for ∼2 weeks that ultimately fail, and consequent weight gain is characterized by impaired satiation and satiety. Insights into the etiology of obesity can be obtained by investigating changes to satiation and satiety mechanisms during the initial ∼2 weeks of HFD exposure.

What is already known about this subject?

Obesity is associated with dysregulated homeostatic mechanisms.Increased caloric consumption contributes to obesity.Obese rodents tend to eat larger, more frequent meals.

What are the new findings in your manuscript?

Exposure to obesogenic diets promotes transient attempts to maintain weight homeostasis.After ∼2 weeks, caloric hyperphagia exceeds increased energy expenditure, promoting weight gain.This is associated with consumption of larger, more frequent meals.

How might your results change the direction of research or the focus of clinical practice?

Our findings suggest that molecular studies focusing on mechanisms that regulate meal size and frequency, particularly those engaged during the first ∼2 weeks of obesogenic diet feeding that eventually fail, can provide unique insight into the etiology of obesity.

Bilateral Subdiaphragmatic Vagal Nerve Stimulation Using a Novel Waveform Decreases Body Weight, Food Consumption, Adiposity, and Activity in Obesity-Prone Rats

INTRODUCTION

Obesity affects millions of Americans. The vagal nerves convey the degree of stomach fullness to the brain via afferent visceral fibers. Studies have found that vagal nerve stimulation (VNS) promotes reduced food intake, causes weight loss, and reduces cravings and appetite.

METHODS

Here, we evaluate the efficacy of a novel stimulus waveform applied bilaterally to the subdiaphragmatic vagal nerve stimulation (sVNS) for almost 13 weeks. A stimulating cuff electrode was implanted in obesity-prone Sprague Dawley rats maintained on a high-fat diet. Body weight, food consumption, and daily movement were tracked over time and compared against three control groups: sham rats on a high-fat diet that were implanted with non-operational cuffs, rats on a high-fat diet that were not implanted, and rats on a standard diet that were not implanted.

RESULTS

Results showed that rats on a high-fat diet that received sVNS attained a similar weight to rats on a standard diet due primarily to a reduction in daily caloric intake. Rats on a high-fat diet that received sVNS had significantly less body fat than other high-fat controls. Rats receiving sVNS also began moving a similar amount to rats on the standard diet.

CONCLUSION

Results from this study suggest that bilateral subdiaphragmatic vagal nerve stimulation can alter the rate of growth of rats maintained on a high-fat diet through a reduction in daily caloric intake, returning their body weight to that which is similar to rats on a standard diet over approximately 13 weeks.

Mechanosensitive enteric neurons in the guinea pig gastric fundus and antrum

BACKGROUND

Coping with the ingested food, the gastric regions of fundus, corpus, and antrum display different motility patterns. Intrinsic components of such patterns involving mechanosensitive enteric neurons (MEN) have been described in the guinea pig gastric corpus but are poorly understood in the fundus and antrum.

METHODS

To elucidate mechanosensitive properties of myenteric neurons in the gastric fundus and antrum, membrane potential imaging using Di-8-ANEPPS was applied. A small-volume injection led to neuronal compression. We analyzed the number of MEN and their firing frequency in addition to the involvement of selected mechanoreceptors. To characterize the neurochemical phenotype of MEN, we performed immunohistochemistry.

KEY RESULTS

In the gastric fundus, 16% of the neurons reproducibly responded to mechanical stimulation and thus were MEN. Of those, 83% were cholinergic and 19% nitrergic. In the antrum, 6% of the neurons responded to the compression stimulus, equally distributed among cholinergic and nitrergic MEN. Defunctionalizing the sensory extrinsic afferents led to a significant drop in the number of MEN in both regions.

CONCLUSION

We provided evidence for MEN in the gastric fundus and antrum and further investigated mechanoreceptors. However, the proportions of the chemical phenotypes of the MEN differed significantly between both regions. Further investigations of synaptic connections of MEN are crucial to understand the hardwired neuronal circuits in the stomach.

TRPV1 Dysfunction Impairs Gastric Nitrergic Neuromuscular Relaxation in High-Fat Diet-Induced Diabetic Gastroparesis Mice

Diabetic gastroparesis (DGP) is characterized by delayed gastric emptying of solid food. Nitrergic neuron-mediated fundus relaxation and intragastric peristalsis are pivotal for gastric emptying and are impaired in DGP. Transient receptor potential vanilloid 1 (TRPV1) ion channels are expressed in gastrointestinal vagal afferent nerves and have a potential role in relevant gastrointestinal disorders. In this study, mice with high-fat diet (HFD)-induced type 2 diabetes mellitus (T2DM), associated with gastroparesis, were used to determine the role of TRPV1 in DGP. After feeding with HFD, mice exhibited obesity, hyperglycemia, insulin resistance, and delayed gastric emptying. Cholinergic- and nitrergic neuron-mediated neuromuscular contractions and relaxation were impaired. The antral tone of the DGP mice was attenuated. Interestingly, activating or suppressing TRPV1 facilitated or inhibited gastric fundus relaxation in normal mice. These effects were neutralized by using a nitric oxide synthase (NOS) inhibitor. Activation or suppression of TRPV1 also increased or reduced NO release. TRPV1 was specifically localized with neuronal NOS in the gastric fundus. These data suggest that TRPV1 activation facilitates gastric fundus relaxation by regulating neuronal NOS and promoting NO release. However, these effects and mechanisms disappeared in mice with DGP induced by HFD diet. TRPV1 expression was only marginally decreased in the fundus of DGP mice. TRPV1 dysfunction may be a potential mechanism underlying the dysfunction of DGP gastric nitrergic neuromuscular relaxation.

Role of Ion Channels in the Chemotransduction and Mechanotransduction in Digestive Function and Feeding Behavior

The gastrointestinal tract constantly communicates with the environment, receiving and processing a wide range of information. The contents of the gastrointestinal tract and the gastrointestinal tract generate mechanical and chemical signals, which are essential for regulating digestive function and feeding behavior. There are many receptors here that sense intestinal contents, including nutrients, microbes, hormones, and small molecule compounds. In signal transduction, ion channels are indispensable as an essential component that can generate intracellular ionic changes or electrical signals. Ion channels generate electrical activity in numerous neurons and, more importantly, alter the action of non-neurons simply and effectively, and also affect satiety, molecular secretion, intestinal secretion, and motility through mechanisms of peripheral sensation, signaling, and altered cellular function. In this review, we focus on the identity of ion channels in chemosensing and mechanosensing in the gastrointestinal tract.

Neural signalling of gut mechanosensation in ingestive and digestive processes

Eating and drinking generate sequential mechanosensory signals along the digestive tract. These signals are communicated to the brain for the timely initiation and regulation of diverse ingestive and digestive processes - ranging from appetite control and tactile perception to gut motility, digestive fluid secretion and defecation - that are vital for the proper intake, breakdown and absorption of nutrients and water. Gut mechanosensation has been investigated for over a century as a common pillar of energy, fluid and gastrointestinal homeostasis, and recent discoveries of specific mechanoreceptors, contributing ion channels and the well-defined circuits underlying gut mechanosensation signalling and function have further expanded our understanding of ingestive and digestive processes at the molecular and cellular levels. In this Review, we discuss our current understanding of the generation of mechanosensory signals from the digestive periphery, the neural afferent pathways that relay these signals to the brain and the neural circuit mechanisms that control ingestive and digestive processes, focusing on the four major digestive tract parts: the oral and pharyngeal cavities, oesophagus, stomach and intestines. We also discuss the clinical implications of gut mechanosensation in ingestive and digestive disorders.

Modulatory role of the endocannabinoidome in the pathophysiology of the gastrointestinal tract

Originating from Eastern Asia, the plant Cannabis sativa has been used for centuries as a medicinal treatment. The unwanted psychotropic effects of one of its major components, Δ⁹-tetrahydrocannabinol, discouraged its therapeutic employment until, recently, the discovery of cannabinoids receptors and their endogenous ligands endocannabinoids reignited the interest. The endocannabinoid system has lately been found to play an important role in the maintenance of human health, both centrally and peripherally. However, the initial idea of the endocannabinoid system structure has been quickly understood to be too simplistic and, as new receptors, mediators, and enzymes have been discovered to participate in a complex relationship, the new, more comprehensive term "expanded endocannabinoid system" or "endocannabinoidome", has taken over. The discovery of other endocannabinoid-like receptors, such as the G protein-coupled receptor 119 and G protein-coupled receptor 55, has opened the way to the development of potential therapeutic targets for the treatment of various metabolic disorders. In addition, recent findings have also provided evidence suggesting the potential therapeutic link between the endocannabinoidome and various inflammatory-based gut diseases, such as inflammatory bowel disease and cancer. This review will provide an introduction to the endocannabinoidome, focusing on its modulatory role in the gastrointestinal tract and on the interest generated by the link between gut microbiota, the endocannabinoid system and metabolic diseases such as inflammatory bowel disease, type-2 diabetes and obesity. In addition, we will look at the potential novel aspects and benefits of drugs targeting the endocannabinoid system.

Signaling Targets Related to Antiobesity Effects of Capsaicin: A Scoping Review

The search for new antiobesogenic agents is increasing because of the current obesity pandemic. Capsaicin (Caps), an exogenous agonist of the vanilloid receptor of transient potential type 1 (TRPV1), has shown promising results in the treatment of obesity. This scoping review aims to verify the pathways mediating the effects of Caps in obesity and the different methods adopted to identify these pathways. The search was carried out using data from the EMBASE, MEDLINE (PubMed), Web of Science, and SCOPUS databases. Studies considered eligible evaluated the mechanisms of action of Caps in obesity models or cell types involved in obesity. Nine studies were included and 100% (n = 6) of the in vivo studies showed a high risk of bias. Of the 9 studies, 66.6% (n = 6) administered Caps orally in the diet and 55.5% (n = 5) used a concentration of Caps of 0.01% in the diet. In vitro, the most tested concentration was 1 μM (88.9%; n = 8). Capsazepine was the antagonist chosen by 66.6% (n = 6) of the studies. Seven studies (77.8%) linked the antiobesogenic effects of Caps to TRPV1 activation and 3 (33.3%) indicated peroxisome proliferator-activated receptor (PPAR) involvement as an upstream connection to TRPV1, rather than a direct metabolic target of Caps. The main secondary effects of Caps were lower weight gain (33.3%; n = 3) or loss (22.2%; n = 2), greater improvement in lipid profile (33.3%; n = 3), lower white adipocyte adipogenesis (33.3%; n = 3), browning process activation (44.4%; n = 4), and higher brown adipocyte activity (33.3%; n = 3) compared with those of the control treatment. Some studies have shown that PPAR agonists modulate TRPV1 activity, and no study has evaluated the simultaneous antagonism of these 2 receptors. Consequently, further studies are necessary to elucidate the role of each of these signaling molecules in the antiobesogenic effects of Caps.

ASIC2 Synergizes with TRPV1 in the Mechano-Electrical Transduction of Arterial Baroreceptors

Mechanosensitive ion channels (MSCs) are key molecules in the mechano-electrical transduction of arterial baroreceptors. Among them, acid-sensing ion channel 2 (ASIC2) and transient receptor potential vanilloid subfamily member 1 (TRPV1) have been studied extensively and documented to play important roles. In this study, experiments using aortic arch-aortic nerve preparations isolated from rats revealed that both ASIC2 and TRPV1 are functionally necessary, as blocking either abrogated nearly all pressure-dependent neural discharge. However, whether ASIC2 and TRPV1 work in coordination remained unclear. So we carried out cell-attached patch-clamp recordings in HEK293T cells co-expressing ASIC2 and TRPV1 and found that inhibition of ASIC2 completely blocked stretch-activated currents while inhibition of TRPV1 only partially blocked these currents. Immunofluorescence staining of aortic arch-aortic adventitia from rats showed that ASIC2 and TRPV1 are co-localized in the aortic nerve endings, and co-immunoprecipitation assays confirmed that the two proteins form a compact complex in HEK293T cells and in baroreceptors. Moreover, protein modeling analysis, exogenous co-immunoprecipitation assays, and biotin pull-down assays indicated that ASIC2 and TRPV1 interact directly. In summary, our research suggests that ASIC2 and TRPV1 form a compact complex and function synergistically in the mechano-electrical transduction of arterial baroreceptors. The model of synergism between MSCs may have important biological significance beyond ASIC2 and TRPV1.

Effect of high-fat diet on mechanosensitive transient receptor potential channel activation in vagal afferent neurons

Mechanical stimulation of the gastrointestinal tract is an important stimulus of satiety and can be transduced by transient receptor potential (TRP) channels. Several studies have revealed attenuated vagally-mediated satiety responses including mechanosensitivity in diet-induced obesity; however, ion channels underlying this hyposensitivity have not been fully understood. This study aimed to examine the effect of chronic high-fat diet on activation of selected mechanosensitive TRP channels in vagal afferents. C57/BL6 mice were fed on either a high-fat or low-fat diet for 6-8 weeks. An increase in the intracellular calcium to hypotonic solution and activators of TRPV1, TRPV4, and TRPA1 was measured in nodose neurons using Ca2+-imaging techniques. Jejunal afferent nerve firing induced by mechanical stimulation and TRP channel agonists was measured using in vitro extracellular multiunit afferent recording. In high-fat diet-fed mice, we observed reduced calcium influx and jejunal afferent response induced by mechanical stimuli and agonists of TRPV4 and TRPA1, but not TRPV1. Our data show diet-induced obesity disrupts the activation of TRPV4 and TRPA1, at both the cellular level and the level of nerve terminals in the small intestine, which may partly explain reduced mechanosensitivity of vagal afferents and may contribute to decreased gut-brain satiety signaling in obesity.

Both high fat and high carbohydrate diets impair vagus nerve signaling of satiety

Obesity remains prevalent in the US. One potential treatment is vagus nerve stimulation (VNS), which activates the sensory afferents innervating the stomach that convey stomach volume and establish satiety. However, current VNS approaches and stimulus optimization could benefit from additional understanding of the underlying neural response to stomach distension. In this study, obesity-prone Sprague Dawley rats consumed a standard, high-carbohydrate, or high-fat diet for several months, leading to diet-induced obesity in the latter two groups. Under anesthesia, the neural activity in the vagus nerve was recorded with a penetrating microelectrode array while the stomach was distended with an implanted balloon. Vagal tone during distension was compared to baseline tone prior to distension. Responses were strongly correlated with stomach distension, but the sensitivity to distension was significantly lower in animals that had been fed the nonstandard diets. The results indicate that both high fat and high carbohydrate diets impair vagus activity.

A High Amylose Wheat Diet Improves Gastrointestinal Health Parameters and Gut Microbiota in Male and Female Mice

High amylose wheat (HAW) contains more resistant starch than standard amylose wheat (SAW) and may have beneficial effects on gastrointestinal health. However, it is currently unclear whether these effects differ according to the level of HAW included in the diet or between males and females. Male and female C57BL/6 mice (n = 8/group/sex) were fed SAW65 (65% SAW; control), HAW35 (35% HAW), HAW50 (50% HAW) or HAW65 (65% HAW) diet for eight weeks. Female but not male, mice consuming any amount of HAW exhibited accelerated gastric emptying compared to SAW65 group. In both sexes, relative colon weights were higher in the HAW65 group compared to SAW65 group and in females, relative weights of the small intestine and cecum were also higher in the HAW65 group. In females only, colonic expression of Pyy and Ocln mRNAs were higher in the HAW65 group compared to HAW35 and HAW50 groups. In both sexes, mice consuming higher amounts of HAW (HAW50 or HAW65) had increased fecal bacterial load and relative abundance of Bacteroidetes phylum and reduced relative abundance of Firmicutes compared to SAW65 group. These data are consistent with a beneficial impact of HAW on gastrointestinal health and indicate dose-dependent and sex-specific effects of HAW consumption.

Chronic high fat diet impairs glucagon like peptide-1 sensitivity in vagal afferents

Dysfunction of the gut-brain axis is one of the potential contributors to the pathophysiology of obesity and is therefore a potential target for treatment. Vagal afferents innervating the gut play an important role in controlling energy homeostasis. There is an increasing evidence for the role of vagal afferents in mediating the anorexigenic effects of glucagon-like peptide-1 (GLP-1), an important satiety and incretin hormone. This study aimed to examine the effect of chronic high fat diet on GLP-1 sensitivity in vagal afferents. C57/BL6 mice were fed either a high-fat or low-fat diet for 6-8 weeks. To evaluate gastrointestinal afferent sensitivity and nodose neurons' response to GLP-1, extracellular afferent recordings and patch clamp were performed, respectively. Exendin-4 (Ex-4) was used as an agonist of the GLP-1 receptor. C-Fos Expression was examined as an indication of afferent input to the nucleus tractus solitarius (NTS). Food intake was monitored in real-time before and after Ex-4 treatment to monitor the consequence of the high fat diet on the satiating effect of GLP-1. In high fat fed (HFF) mice, GLP-1 caused lower activation of intestinal afferent nerves, and failed to potentiate mechanosensitive nerve responses compared to low fat fed (LFF). GLP-1 increased excitability in LFF and this effect was reduced in HFF neurons. Consistent with these findings on vagal afferent nerves, GLP-1 receptor stimulation given systemically, had a reduced satiating effect in HFF compared to LFF mice, and neuronal activation in the NTS was also reduced. The present study demonstrated chronic high fat diet impaired vagal afferent responses to GLP-1, resulting in impaired satiety signaling. GLP-1 sensitivity may account for the impairment of satiety signaling in obesity and thus a therapeutic target for obesity treatment.

High fat diet induced obesity alters endocannabinoid and ghrelin mediated regulation of components of the endocannabinoid system in nodose ganglia

BACKGROUND

Ghrelin and anandamide (AEA) can regulate the sensitivity of gastric vagal afferents to stretch, an effect mediated via the transient receptor potential vanilloid 1 (TPRV1) channel. High fat diet (HFD)-induced obesity alters the modulatory effects of ghrelin and AEA on gastric vagal afferent sensitivity. This may be a result of altered gastric levels of these hormones and subsequent changes in the expression of their receptors. Therefore, the current study aimed to determine the effects of ghrelin and AEA on vagal afferent cell body mRNA content of cannabinoid 1 receptor (CB1), ghrelin receptor (GHSR), TRPV1, and the enzyme responsible for the breakdown of AEA, fatty acid amide hydrolase (FAAH).

METHODS

Mice were fed a standard laboratory diet (SLD) or HFD for 12wks. Nodose ganglia were removed and cultured for 14 h in the absence or presence of ghrelin or methAEA (mAEA; stable analogue of AEA). Relative mRNA content of CB1, GHSR, TRPV1, and FAAH were measured.

RESULTS

In nodose cells from SLD-mice, mAEA increased TRPV1 and FAAH mRNA content, and decreased CB1 and GHSR mRNA content. Ghrelin decreased TRPV1, CB1, and GHSR mRNA content. In nodose cells from HFD-mice, mAEA had no effect on TRPV1 mRNA content, and increased CB1, GHSR, and FAAH mRNA content. Ghrelin decreased TRPV1 mRNA content and increased CB1 and GHSR mRNA content.

CONCLUSIONS

AEA and ghrelin modulate receptors and breakdown enzymes involved in the mAEA-vagal afferent satiety signalling pathways. This was disrupted in HFD-mice, which may contribute to the altered vagal afferent signalling in obesity.

Dissecting the Role of Subtypes of Gastrointestinal Vagal Afferents

Gastrointestinal (GI) vagal afferents convey sensory signals from the GI tract to the brain. Numerous subtypes of GI vagal afferent have been identified but their individual roles in gut function and feeding regulation are unclear. In the past decade, technical approaches to selectively target vagal afferent subtypes and to assess their function has significantly progressed. This review examines the classification of GI vagal afferent subtypes and discusses the current available techniques to study vagal afferents. Investigating the distribution of GI vagal afferent subtypes and understanding how to access and modulate individual populations are essential to dissect their fundamental roles in the gut-brain axis.

Modulatory effect of methanandamide on gastric vagal afferent satiety signals depends on nutritional status

SIGNIFICANCE STATEMENT

Gastric vagal afferent responses to tension are dampened in high fat diet-induced obesity. Endocannabinoids are known to dose-dependently inhibit and excite gastric vagal afferents but their effect on gastric vagal afferents in diet-induced obesity are unknown. In individual gastric vagal afferent neurons of diet-induced obese mice the co-expression of components of the endocannabinoid system, including CB1, GHSR, TRPV1 and FAAH, was increased compared with lean mice. In high fat diet-induced obese mice, methanandamide only inhibited gastric vagal afferent responses to tension, possibly due to the observed change in the balance of receptors, hormones and breakdown enzymes in this system. Collectively, these data suggest that endocannabinoid signalling, by gastric vagal afferents, is altered in diet-induced obesity which may impact satiety and gastrointestinal function.

ABSTRACT

Gastric vagal afferents (GVAs) play a role in appetite regulation. The endocannabinoid anandamide (AEA) dose-dependently inhibits and excites tension-sensitive GVAs. However, it is also known that high fat diet (HFD) feeding alters GVA responses to stretch. The aim of this study was to determine the role of AEA in GVA signalling in lean and HFD-induced obese mice. Male C57BL/6 mice were fed (12 weeks) a standard laboratory diet (SLD) or HFD. Protein and mRNA expression of components of the cannabinoid system was determined in individual GVA cell bodies and the gastric mucosa. An in vitro GVA preparation was used to assess the effect of methanandamide (mAEA) on tension-sensitive GVAs and the second messenger pathways involved. In individual GVA cell bodies, cannabinoid 1 (CB1) and ghrelin (GHSR) receptor mRNA was higher in HFD mice than SLD mice. Conversely, gastric mucosal AEA and ghrelin protein levels were lower in HFD mice than SLD mice. In SLD mice, mAEA exerted dose-dependent inhibitory and excitatory effects on tension-sensitive GVAs. Only an inhibitory effect of mAEA was observed in HFD mice. The excitatory effect of mAEA was dependent on CB1, transient receptor potential vanilloid 1 (TRPV1) and the protein kinase C. Conversely, the inhibitory effect was dependent on CB1, growth hormone secretagogue receptor, TRPV1 and the protein kinase A. Endocannabinoids, acting through CB1 and TRPV1, have a pivotal role in modulating GVA satiety signals depending on the second messenger pathway utilised. In HFD mice only an inhibitory effect was observed. These changes may contribute to the development and/or maintenance of obesity.

Evidence for acupoint catgut embedding treatment and TRPV1 gene deletion increasing weight control in murine model

Obesity is a global health problem affecting the general population. Acupoint catgut embedding (ACE) is an alternative treatment that involves the implantation of absorbable catgut suture at acupoints. The transient receptor vanilloid member 1 (TRPV1) is a calcium ion channel that responds to several chemical ligands and is identified in numerous locations throughout the body. The aim of the present study was to examine the effect of ACE treatment on obesity and its associated complications through various neural mechanisms in a murine model. A C57/BL6 wild type (WT) and TRPV1‑/‑ (KO) mouse model was utilized to exclude any psychological factors associated with obesity. The WT‑HFD‑ACE and WT‑HFD‑SHAM groups received weekly ACE or placebo treatments at the bilateral ST36 acupoint. The mice were fed with a normal mice chow diet (ND) or a high‑fat food diet (HFD; 45 kcal%), and their body weights were recorded once a week. After 8 weeks, the subjects were sacrificed and changes in the levels of a number of biomarkers were investigated using ELISA, immunoblotting and immunofluorescence. The results indicated a significant decrease in body weight variation for the WT‑HFD‑ACE group compared with the WT‑HFD and WT‑HFD‑SHAM groups, using the WT‑ND group as the body weight baseline. By contrast, KO mice fed with ND or HFD demonstrated notable body weight maintenance throughout the experimental period. Similar patterns were observed in adipose tissue mass, glucose, leptin and insulin plasma levels, and protein molecule density of TRPV1 and its associated molecules in the hypothalamus and nucleus tractus solitarii. In contrast, in the prefrontal cortex, significant decreases in the concentrations of MAPK pathway proteins in the WT‑HFD and WT‑HFD‑SHAM groups were observed. The levels of these proteins were significantly increased in the WT‑HFD‑ACE and KO‑HFD groups. These results suggested that TRPV1 and its associated pathways may be involved in body weight maintenance, and may be controlled through ACE treatment or genetic manipulation.

Obesity Affects the Microbiota-Gut-Brain Axis and the Regulation Thereof by Endocannabinoids and Related Mediators

The hypothalamus regulates energy homeostasis by integrating environmental and internal signals to produce behavioral responses to start or stop eating. Many satiation signals are mediated by microbiota-derived metabolites coming from the gastrointestinal tract and acting also in the brain through a complex bidirectional communication system, the microbiota–gut–brain axis. In recent years, the intestinal microbiota has emerged as a critical regulator of hypothalamic appetite-related neuronal networks. Obesogenic high-fat diets (HFDs) enhance endocannabinoid levels, both in the brain and peripheral tissues. HFDs change the gut microbiota composition by altering the Firmicutes:Bacteroidetes ratio and causing endotoxemia mainly by rising the levels of lipopolysaccharide (LPS), the most potent immunogenic component of Gram-negative bacteria. Endotoxemia induces the collapse of the gut and brain barriers, interleukin 1β (IL1β)- and tumor necrosis factor α (TNFα)-mediated neuroinflammatory responses and gliosis, which alter the appetite-regulatory circuits of the brain mediobasal hypothalamic area delimited by the median eminence. This review summarizes the emerging state-of-the-art evidence on the function of the “expanded endocannabinoid (eCB) system” or endocannabinoidome at the crossroads between intestinal microbiota, gut-brain communication and host metabolism; and highlights the critical role of this intersection in the onset of obesity.

Chronic stress induces hypersensitivity of murine gastric vagal afferents

BACKGROUND

Stress exposure is known to trigger and exacerbate functional dyspepsia (FD) symptoms. Increased gastric sensitivity to food-related stimuli is widely observed in FD patients and is associated with stress and psychological disorders. The mechanisms underlying the hypersensitivity are not clear. Gastric vagal afferents (GVAs) play an important role in sensing meal-related mechanical stimulation to modulate gastrointestinal function and food intake. This study aimed to determine whether GVAs display hypersensitivity after chronic stress, and whether its interaction with leptin was altered by stress.

METHODS

Eight-week-old male C57BL/6 mice were exposed to unpredictable chronic mild stress or no stress (control) for 8 weeks. The metabolic rate, gastric emptying rate, and anxiety- and depression-like behaviors were determined. GVA mechanosensitivity, and its modulation by leptin, was determined using an in vitro single fiber recording technique. QRT-PCR was used to establish the levels of leptin and leptin receptor mRNA in the stomach and nodose ganglion, respectively.

KEY RESULTS

The stressed mice had lower body weight and food intake, and increased anxiety-like behavior compared to the control mice. The mechanosensitivity of mucosal and tension-sensitive GVAs was higher in the stressed mice. Leptin potentiated mucosal GVA mechanosensitivity in control but not stressed mice. The expression of leptin mRNA in the gastric mucosa was lower in the stressed mice.

CONCLUSIONS AND INFERENCES

In conclusion, chronic stress enhances GVA mechanosensitivity, which may contribute to the gastric hypersensitivity in FD. In addition, the modulatory effect of leptin on GVA signaling is lost after chronic stress exposure.

Biphasic effects of methanandamide on murine gastric vagal afferent mechanosensitivity

KEY POINTS

The fine control of food intake is important for the maintenance of a healthy metabolic state. Gastric vagal afferents (GVAs) are involved in the peripheral regulation of food intake via signalling the degree of distension of the stomach which ultimately leads to feelings of fullness and satiety. This study provides evidence that endocannabinoids such as anandamide are capable of regulating GVA sensitivity in a concentration-dependent biphasic manner. This biphasic effect is dependent upon interactions between the CB1, TRPV1 and GHSR receptors. These data have important implications for the peripheral control of food intake.

ABSTRACT

Gastric vagal afferents (GVAs) signal to the hindbrain resulting in satiety. Endocannabinoids are endogenous ligands of cannabinoid 1 receptor (CB1) and transient receptor potential vanilloid-1 (TRPV1) channels. The endocannabinoid anandamide (AEA) is expressed in the stomach, and its receptor CB1 is expressed in ghrelin-positive gastric mucosal cells. Further, TRPV1, CB1 and growth hormone secretagogue receptor (ghrelin receptor, GHSR) are expressed in subpopulations of GVA neurons. This study aimed to determine the interaction between TRPV1, CB1, GHSR and endocannabinoids in the modulation of GVA signalling. An in vitro electrophysiology preparation was used to assess GVA mechanosensitivity in male C57BL/6 mice. Effects of methanandamide (mAEA; 1-100 nm), on GVA responses to stretch were determined in the absence and presence of antagonists of CB1, TRPV1, GHSR, protein kinase-A (PKA), protein kinase-C (PKC) and G-protein subunits Gα_i/o , or Gα_q . Low doses (1-10 nm) of mAEA reduced GVA responses to 3 g stretch, whereas high doses (30-100 nm) increased the response. The inhibitory and excitatory effects of mAEA (1-100 nm) were reduced/lost in the presence of a CB1 and TRPV1 antagonist. PKA, Gα_i/o or GHSR antagonists prevented the inhibitory effect of mAEA on GVA mechanosensitivity. Conversely, in the presence of a PKC or Gα_q antagonist the excitatory effect of mAEA was reduced or lost, respectively. Activation of CB1, by mAEA, can activate or inhibit TRPV1 to increase or decrease GVA responses to stretch, depending on the pathway activated. These interactions could play an important role in the fine control of food intake.

Gastric vagal afferent neuropathy following experimental spinal cord injury

Dramatic impairment of gastrointestinal (GI) function accompanies high-thoracic spinal cord injury (T3-SCI). The vagus nerve contains mechano- and chemosensory fibers as well as the motor fibers necessary for the central nervous system (CNS) control of GI reflexes. Cell bodies for the vagal afferent fibers are located within the nodose gangla (NG) and the majority of vagal afferent axons are unmyelinated C fibers that are sensitive to capsaicin through activation of transient receptor potential vanilloid-1 (TRPV1) channels. Vagal afferent fibers also express receptors for GI hormones, including cholecystokinin (CCK). Previously, T3-SCI provokes a transient GI inflammatory response as well as a reduction of both gastric emptying and centrally-mediated vagal responses to GI peptides, including CCK. TRPV1 channels and CCK-A receptors (CCKar) expressed in vagal afferents are upregulated in models of visceral inflammation. The present study investigated whether T3-SCI attenuates peripheral vagal afferent sensitivity through plasticity of TRPV1 and CCK receptors. Vagal afferent response to graded mechanical stimulation of the stomach was significantly attenuated by T3-SCI at 3-day and 3-week recovery. Immunocytochemical labeling for CCKar and TRPV1 demonstrated expression on dissociated gastric-projecting NG neurons. Quantitative assessment of mRNA expression by qRT-PCR revealed significant elevation of CCKar and TRPV1 in the whole NG following T3-SCI in 3-day recovery, but levels returned to normal after 3-weeks. Three days after injury, systemic administration of CCK-8 s showed a significantly diminished gastric vagal afferent response in T3-SCI rats compared to control rats while systemic capsaicin infusion revealed a significant elevation of vagal response in T3-SCI vs control rats. These findings demonstrate that T3-SCI provokes peripheral remodeling and prolonged alterations in the response of vagal afferent fibers to the physiological signals associated with digestion.

Multifunctional TRPV1 Ion Channels in Physiology and Pathology with Focus on the Brain, Vasculature, and Some Visceral Systems

TRPV1 has been originally cloned as the heat and capsaicin receptor implicated in acute pain signalling, while further research has shifted the focus to its importance in chronic pain caused by inflammation and associated with this TRPV1 sensitization. However, accumulating evidence suggests that, apart from pain signalling, TRPV1 subserves many other unrelated to nociception functions in the nervous system. In the brain, TRPV1 can modulate synaptic transmission via both pre- and postsynaptic mechanisms and there is a functional crosstalk between GABA receptors and TRPV1. Other fundamental processes include TRPV1 role in plasticity, microglia-to-neuron communication, and brain development. Moreover, TRPV1 is widely expressed in the peripheral tissues, including the vasculature, gastrointestinal tract, urinary bladder, epithelial cells, and the cells of the immune system. TRPV1 can be activated by a large array of physical (heat, mechanical stimuli) and chemical factors (e.g., protons, capsaicin, resiniferatoxin, and endogenous ligands, such as endovanilloids). This causes two general cell effects, membrane depolarization and calcium influx, thus triggering depending on the cell-type diverse functional responses ranging from neuronal excitation to secretion and smooth muscle contraction. Here, we review recent research on the diverse TRPV1 functions with focus on the brain, vasculature, and some visceral systems as the basis of our better understanding of TRPV1 role in different human disorders.

TRPV1 protects renal ischemia-reperfusion injury in diet-induced obese mice by enhancing CGRP release and increasing renal blood flow

Background

Obesity is a major risk factor for end-stage renal disease. Using transient receptor potential vanilloid 1 knockout (TRPV1^−/−) mice, we tested the hypothesis that TRPV1 protects against obesity-induced exacerbation of renal ischemia-reperfusion (I/R) injury.

Methods

TRPV1^−/− and wild-type (WT) mice were fed a chow or Western diet (WD) for 22–23 weeks. After that, mice were subjected to renal I/R injury, and renal cortical blood flow (CBF) and medullary blood flow (MBF) were measured.

Results

The Western diet significantly increased body weight and fasting blood glucose levels in both TRPV1^−/− and WT mice. WD-induced impairment of glucose tolerance was worsened in TRPV1^−/− mice compared with WT mice. WD intake prolonged the time required to reach peak reperfusion in the cortex and medulla (both P < 0.05), decreased the recovery rate of CBF (P < 0.05) and MBF (P < 0.05), and increased blood urea nitrogen, plasma creatinine, and urinary 8-isoprostane levels after I/R in both mouse strains, with greater effects in TRPV1^−/− mice (all P < 0.05). Renal I/R increased calcitonin gene-related peptide (CGRP) release in WT but not in TRPV1^−/− mice, and WD attenuated CGRP release in WT mice. Moreover, blockade of CGRP receptors impaired renal regional blood flow and renal function in renal I/R injured WT mice.

Conclusion

These results indicate that TRPV1 plays a protective role in WD-induced exacerbation of renal I/R injury probably through enhancing CGRP release and increasing renal blood flow.

Monoacylglycerol lipase deficiency affects diet-induced obesity, fat absorption, and feeding behavior in CB1 cannabinoid receptor-deficient mice

Excess energy intake causes obesity, which leads to insulin resistance and various other complications of metabolic syndrome, including diabetes, atherosclerosis, dyslipidemia, and nonalcoholic fatty liver disease. Although recent studies have depicted altered lipid metabolism as an underlying feature, the detailed mechanisms are still unclear. Here we describe a possible role in high-fat diet (HFD)-induced obesity for monoacylglycerol lipase (MGL), an enzyme that is also known to hydrolyze the endocannabinoid 2-arachidonoylglycerol in brain. MGL-deficient [MGL-knockout (KO)] mice fed a HFD gained less body weight than wild-type mice and were protected from insulin resistance and hepatic steatosis. Food intake and energy expenditure were not altered in MGL-KO mice, but blood triglyceride levels after oral olive oil gavage were suppressed, indicating a role for MGL in intestinal fat absorption. Experiments with cannabinoid receptor type 1 (CB₁)/MGL double-KO mice revealed that these phenotypes may include mechanisms that are independent of CB₁-receptor-mediated endocannabinoid functions. We also noted that MGL-KO mice had less preference for HFD over normal chow diet. Oral but not intraperitoneal lipid administration strongly suppressed the appetites of MGL-KO and CB₁/MGL double-KO mice, but not of wild-type and CB₁-KO mice. Appetite suppression was reversed by vagotomy, suggesting involvement of MGL in the gut-brain axis regulation of appetite. Our results provide mechanistic insights of MGL's role in diet-induced obesity, lipid metabolic disorder, and regulation of appetite.-Yoshida, K., Kita, Y., Tokuoka, S. M., Hamano, F., Yamazaki, M., Sakimura, K., Kano, M., Shimizu, T. Monoacylglycerol lipase deficiency affects diet-induced obesity, fat absorption, and feeding behavior in CB₁ cannabinoid receptor-deficient mice.

The vagus neurometabolic interface and clinical disease

The nervous system both monitors and modulates body metabolism to maintain homoeostasis. In disease states such as obesity and diabetes, the neurometabolic interface is dysfunctional and contributes to clinical illness. The vagus nerve, in particular, with both sensory and motor fibres, provides an anatomical substrate for this interface. Its sensory fibres contain receptors for important circulating metabolic mediators, including leptin and cholecystokinin, and provide real-time information about these mediators to the central nervous system. In turn, efferent fibres within the vagus nerve participate in a brain-gut axis to regulate metabolism. In this review, we describe these vagus nerve-mediated metabolic pathways and recent clinical trials of vagus nerve stimulation for the management of obesity. These early studies suggest that neuromodulation approaches that employ electricity to tune neurometabolic circuits may represent a new tool in the clinical armamentarium directed against obesity.

Meal-Sensing Signaling Pathways in Functional Dyspepsia

The upper gastrointestinal tract plays an important role in sensing the arrival, amount and chemical composition of a meal. Ingestion of a meal triggers a number of sensory signals in the gastrointestinal tract. These include the response to mechanical stimulation (e.g., gastric distension), from the presence of food in the gut, and the interaction of various dietary nutrients with specific "taste" receptors on specialized enteroendocrine cells in the small intestine culminating in the release of gut hormones. These signals are then transmitted to the brain where they contribute to food intake regulation by modulating appetite as well as feedback control of gastrointestinal functions (e.g., gut motility). There is evidence that the sensitivity to these food related stimuli is abnormally enhanced in functional dyspepsia leading to symptoms such nausea and bloating. In addition, these gut-brain signals can modulate the signaling pathways involved in visceral pain. This review will discuss the role of gut-brain signals in appetite regulation and the role dysregulation of this system play in functional dyspepsia.

Involvement of TRPV1 Channels in Energy Homeostasis

The ion channel TRPV1 is involved in a wide range of processes including nociception, thermosensation and, more recently discovered, energy homeostasis. Tightly controlling energy homeostasis is important to maintain a healthy body weight, or to aid in weight loss by expending more energy than energy intake. TRPV1 may be involved in energy homeostasis, both in the control of food intake and energy expenditure. In the periphery, it is possible that TRPV1 can impact on appetite through control of appetite hormone levels or via modulation of gastrointestinal vagal afferent signaling. Further, TRPV1 may increase energy expenditure via heat production. Dietary supplementation with TRPV1 agonists, such as capsaicin, has yielded conflicting results with some studies indicating a reduction in food intake and increase in energy expenditure, and other studies indicating the converse. Nonetheless, it is increasingly apparent that TRPV1 may be dysregulated in obesity and contributing to the development of this disease. The mechanisms behind this dysregulation are currently unknown but interactions with other systems, such as the endocannabinoid systems, could be altered and therefore play a role in this dysregulation. Further, TRPV1 channels appear to be involved in pancreatic insulin secretion. Therefore, given its plausible involvement in regulation of energy and glucose homeostasis and its dysregulation in obesity, TRPV1 may be a target for weight loss therapy and diabetes. However, further research is required too fully elucidate TRPV1s role in these processes. The review provides an overview of current knowledge in this field and potential areas for development.

The Effects of a Normal Rate versus a Slow Intervalled Rate of Oral Nutrient Intake and Intravenous Low Rate Macronutrient Application on Psychophysical Function – Two Pilot Studies

Stomach distension and energy per time are factors influencing satiety. Moreover, different rates of nutrient intake induce different stomach distension. The goal of our studies was to elucidate the influence of different oral rates of nutrient intake (normal rate versus slow intervalled rate; study I) and intravenous low rate macronutrient application (protein, carbohydrate, fat) or placebo (study II) on psychophysical function. The pilot studies investigated the effects of 1) study I: a mixed nutrient solution (1/3 protein, 1/3 fat, 1/3 carbohydrates) 2) study II: intravenous macronutrient infusions (protein, carbohydrate, fat) or placebo on psychophysical function (mood, hunger, food craving, alertness, smell intensity ratings and hedonic ratings) in human subjects. In study I 10 male subjects (age range: 21–30 years) completed the study protocol participating in both test conditions and in study II 20 male subjects (age range: 19–41 years) completed the study protocol participating in all test conditions. Additionally, metabolic function was analyzed and cognitive and olfactory tests were conducted twice starting 100 min before the beginning of the intervention and 240 min after. Psychophysical (mood, hunger, fat-, protein-, carbohydrate-, sweets- and vegetable-craving), alertness and metabolic function tests were performed seven times on each examination day. Greater effects on hunger and food cravings were observed for normal rate of intake compared to slow intervalled rate of intake and intravenous low rate macronutrient application. Our findings potentially confirm that volume of the food ingested and a higher rate of energy per time contribute to satiety during normal rate of food intake, while slow intervalled rate of food intake and intravenous low rate macronutrient application showed no effects on satiation. Our results motivate the view that a certain amount of volume of the food ingested and a certain energy per time ratio are necessary to reduce hunger and food craving.

Plasticity of gastrointestinal vagal afferent satiety signals

The vagal link between the gastrointestinal tract and the central nervous system (CNS) has numerous vital functions for maintaining homeostasis. The regulation of energy balance is one which is attracting more and more attention due to the potential for exploiting peripheral hormonal targets as treatments for conditions such as obesity. While physiologically, this system is well tuned and demonstrated to be effective in the regulation of both local function and promoting/terminating food intake the neural connection represents a susceptible pathway for disruption in various disease states. Numerous studies have revealed that obesity in particularly is associated with an array of modifications in vagal afferent function from changes in expression of signaling molecules to altered activation mechanics. In general, these changes in vagal afferent function in obesity further promote food intake instead of the more desirable reduction in food intake. It is essential to gain a comprehensive understanding of the mechanisms responsible for these detrimental effects before we can establish more effective pharmacotherapies or lifestyle strategies for the treatment of obesity and the maintenance of weight loss.

Novel developments in vagal afferent nutrient sensing and its role in energy homeostasis

Vagal afferent neurons (VANs) play an important role in the control of food intake by signaling nutrient type and quantity to the brain. Recent findings are broadening our view of how VANs impact not only food intake but also energy homeostasis. This review focuses exclusively on studies of the vagus nerve from the past 2 years that highlight major new advancements in the field. We firstly discuss evidence that VANs can directly sense nutrients, and we consider new insights into mechanisms affecting sensing of gastric distension and signaling by gastrointestinal hormones ghrelin and GLP1. We discuss evidence that disrupting vagal afferent signaling increases long-term control of food intake and body weight management, and the importance of this gut-brain pathway in mediating beneficial effects of bariatric surgery. We conclude by highlighting novel roles for vagal afferent neurons in circadian rhythm, thermogenesis, and reward that may provide insight into mechanisms by which VAN nutrient sensing controls long-term control of energy homeostasis.

Diet-induced obesity promotes systemic inflammation and increased susceptibility to murine visceral leishmaniasis

Obesity is the main causal factor for metabolic syndrome and chronic systemic inflammation, which impacts on immune function and increases susceptibility to pathogens. Here, we investigated the effect of obesity on the outcome of visceral leishmaniasis caused by Leishmaniasis infantum chagasi. C57BL/6 mice fed with high-sugar and butter diet (HSB) showed a significant increase in body weight, adiposity index and morphological changes in adipocyte. To investigate the consequences of obesity on the specific immunity against Leishmania, both control and HSB diet groups were infected with 107L. infantum chagasi promastigotes in the eighth-week after diet started and euthanized 4 weeks later. HSB-diet fed mice exhibited a significantly higher parasite burden in both liver and spleen compared with control- diet group. Gonadal adipocyte tissue from HSB-diet mice showed increased TNF-α, IL-6 and leptin and diminished IL-10 production compared with control. Cytokines production analysis in the spleen and liver from these animals also demonstrated higher production of IFN-γ, TNF-α, IL-6 and nitric oxide and diminished production of IL-10 and TGF-β, which correlate with inflammatory foci and the cell hyperplasia observed. Taken together, obesity can interfere with responses to pathogen-derived signals and impair the development of protective anti-Leishmania immunity.

Involvement of the TRPV1 channel in the modulation of spontaneous locomotor activity, physical performance and physical exercise-induced physiological responses

Physical exercise triggers coordinated physiological responses to meet the augmented metabolic demand of contracting muscles. To provide adequate responses, the brain must receive sensory information about the physiological status of peripheral tissues and organs, such as changes in osmolality, temperature and pH. Most of the receptors involved in these afferent pathways express ion channels, including transient receptor potential (TRP) channels, which are usually activated by more than one type of stimulus and are therefore considered polymodal receptors. Among these TRP channels, the TRPV1 channel (transient receptor potential vanilloid type 1 or capsaicin receptor) has well-documented functions in the modulation of pain sensation and thermoregulatory responses. However, the TRPV1 channel is also expressed in non-neural tissues, suggesting that this channel may perform a broad range of functions. In this review, we first present a brief overview of the available tools for studying the physiological roles of the TRPV1 channel. Then, we present the relationship between the TRPV1 channel and spontaneous locomotor activity, physical performance, and modulation of several physiological responses, including water and electrolyte balance, muscle hypertrophy, and metabolic, cardiovascular, gastrointestinal, and inflammatory responses. Altogether, the data presented herein indicate that the TPRV1 channel modulates many physiological functions other than nociception and thermoregulation. In addition, these data open new possibilities for investigating the role of this channel in the acute effects induced by a single bout of physical exercise and in the chronic effects induced by physical training.

Genetic and pharmacological evidence for low-abundance TRPV3 expression in primary vagal afferent neurons

Primary vagal afferent neurons express a multitude of thermosensitive ion channels. Within this family of ion channels, the heat-sensitive capsaicin receptor (TRPV1) greatly influences vagal afferent signaling by determining the threshold for action-potential initiation at the peripheral endings, while controlling temperature-sensitive forms of glutamate release at central vagal terminals. Genetic deletion of TRPV1 does not completely eliminate these temperature-dependent effects, suggesting involvement of additional thermosensitive ion channels. The warm-sensitive, calcium-permeable, ion channel TRPV3 is commonly expressed with TRPV1; however, the extent to which TRPV3 is found in vagal afferent neurons is unknown. Here, we begin to characterize the genetic and functional expression of TRPV3 in vagal afferent neurons using molecular biology (RT-PCR and RT-quantitative PCR) in whole nodose and isolated neurons and fluorescent calcium imaging on primary cultures of nodose ganglia neurons. We confirmed low-level TRPV3 expression in vagal afferent neurons and observed direct activation with putative TRPV3 agonists eugenol, ethyl vanillin (EVA), and farnesyl pyrophosphate (FPP). Agonist activation stimulated neurons also containing TRPV1 and was blocked by ruthenium red. FPP sensitivity overlapped with EVA and eugenol but represented the smallest percentage of vagal afferent neurons, and it was the only agonist that did not stimulate neurons from TRPV3(-/-1) mice, suggesting FPP has the highest selectivity. Further, FPP was predictive of enhanced responses to capsaicin, EVA, and eugenol in rats. From our results, we conclude TRPV3 is expressed in a discrete subpopulation of vagal afferent neurons and may contribute to vagal afferent signaling either directly or in combination with TRPV1.

TRP channel functions in the gastrointestinal tract

Transient receptor potential (TRP) channels are predominantly distributed in both somatic and visceral sensory nervous systems and play a crucial role in sensory transduction. As the largest visceral organ system, the gastrointestinal (GI) tract frequently accommodates external inputs, which stimulate sensory nerves to initiate and coordinate sensory and motor functions in order to digest and absorb nutrients. Meanwhile, the sensory nerves in the GI tract are also able to detect potential tissue damage by responding to noxious irritants. This nocifensive function is mediated through specific ion channels and receptors expressed in a subpopulation of spinal and vagal afferent nerve called nociceptor. In the last 18 years, our understanding of TRP channel expression and function in GI sensory nervous system has been continuously improved. In this review, we focus on the expressions and functions of TRPV1, TRPA1, and TRPM8 in primary extrinsic afferent nerves innervated in the esophagus, stomach, intestine, and colon and briefly discuss their potential roles in relevant GI disorders.