Gut vagal afferent lesions increase meal size but do not block gastric preload-induced feeding suppression

Acts of appetite: neural circuits governing the appetitive, consummatory, and terminating phases of feeding

The overconsumption of highly caloric and palatable foods has caused a surge in obesity rates in the past half century, thereby posing a healthcare challenge due to the array of comorbidities linked to heightened body fat accrual. Developing treatments to manage body weight requires a grasp of the neurobiological basis of appetite. In this Review, we discuss advances in neuroscience that have identified brain regions and neural circuits that coordinate distinct phases of eating: food procurement, food consumption, and meal termination. While pioneering work identified several hypothalamic nuclei to be involved in feeding, more recent studies have explored how neuronal populations beyond the hypothalamus, such as the mesolimbic pathway and nodes in the hindbrain, interconnect to modulate appetite. We also examine how long-term exposure to a calorically dense diet rewires feeding circuits and alters the response of motivational systems to food. Understanding how the nervous system regulates eating behaviour will bolster the development of medical strategies that will help individuals to maintain a healthy body weight.

Glial Modulation of Energy Balance: The Dorsal Vagal Complex Is No Exception

The avoidance of being overweight or obese is a daily challenge for a growing number of people. The growing proportion of people suffering from a nutritional imbalance in many parts of the world exemplifies this challenge and emphasizes the need for a better understanding of the mechanisms that regulate nutritional balance. Until recently, research on the central regulation of food intake primarily focused on neuronal signaling, with little attention paid to the role of glial cells. Over the last few decades, our understanding of glial cells has changed dramatically. These cells are increasingly regarded as important neuronal partners, contributing not just to cerebral homeostasis, but also to cerebral signaling. Our understanding of the central regulation of energy balance is part of this (r)evolution. Evidence is accumulating that glial cells play a dynamic role in the modulation of energy balance. In the present review, we summarize recent data indicating that the multifaceted glial compartment of the brainstem dorsal vagal complex (DVC) should be considered in research aimed at identifying feeding-related processes operating at this level.

Peter JV. Sugars and Sweet Taste: Addictive or Rewarding?

The notion of food "addiction" often focuses on the overconsumption of sweet tasting foods or so-called sugar "addiction". In the extreme, some have suggested that sugar and sweet tastes elicit neural and behavioral responses analogous to those observed with drugs of abuse. These concepts are complicated by the decades long uncertainty surrounding the validity and reproducibility of functional magnetic resonance imaging (fMRI) methodologies used to characterize neurobiological pathways related to sugar and sweet taste stimuli. There are also questions of whether sweet taste or post-ingestion metabolic consequences of sugar intake would lead to addiction or excessive caloric intake. Here, we present a focused narrative review of literature related to the reward value of sweet taste which suggests that reward value can be confounded with the construct of "addictive potential". Our review seeks to clarify some key distinctions between these constructs and questions the applicability of the addiction construct to human over-eating behaviors. To adequately frame this broad discussion requires the flexibility offered by the narrative review paradigm. We present selected literature on: techniques used to link sugar and sweet tastes to addiction neurobiology and behaviors; sugar and sweet taste "addiction"; the relationship of low calorie sweetener (LCS) intake to addictive behaviors and total calorie intake. Finally, we examined the reward value of sweet tastes and contrasted that with the literature describing addiction. The lack of reproducibility of fMRI data remains problematic for attributing a common neurobiological pathway activation of drugs and foods as conclusive evidence for sugar or sweet taste "addiction". Moreover, the complicated hedonics of sweet taste and reward value are suggested by validated population-level data which demonstrate that the consumption of sweet taste in the absence of calories does not increase total caloric intake. We believe the neurobiologies of reward value and addiction to be distinct and disagree with application of the addiction model to sweet food overconsumption. Most hypotheses of sugar "addiction" attribute the hedonics of sweet foods as the equivalent of "addiction". Further, when addictive behaviors and biology are critically examined in totality, they contrast dramatically from those associated with the desire for sweet taste. Finally, the evidence is strong that responses to the palatability of sweets rather than their metabolic consequences are the salient features for reward value. Thus, given the complexity of the controls of food intake in humans, we question the usefulness of the "addiction" model in dissecting the causes and effects of sweet food over-consumption.

Intact vagal gut-brain signalling prevents hyperphagia and excessive weight gain in response to high-fat high-sugar diet

Aim

The tools that have been used to assess the function of the vagus nerve lack specificity. This could explain discrepancies about the role of vagal gut‐brain signalling in long‐term control of energy balance. Here we use a validated approach to selectively ablate sensory vagal neurones that innervate the gut to determine the role of vagal gut‐brain signalling in the control of food intake, energy expenditure and glucose homoeostasis in response to different diets.

Methods

Rat nodose ganglia were injected bilaterally with either the neurotoxin saporin conjugated to the gastrointestinal hormone cholecystokinin (CCK), or unconjugated saporin as a control. Food intake, body weight, glucose tolerance and energy expenditure were measured in both groups in response to chow or high‐fat high‐sugar (HFHS) diet. Willingness to work for fat or sugar was assessed by progressive ratio for orally administered solutions, while post‐ingestive feedback was tested by measuring food intake after an isocaloric lipid or sucrose pre‐load.

Results

Vagal deafferentation of the gut increases meal number in lean chow‐fed rats. Switching to a HFHS diet exacerbates overeating and body weight gain. The breakpoint for sugar or fat solution did not differ between groups, suggesting that increased palatability may not drive HFHS‐induced hyperphagia. Instead, decreased satiation in response to intra‐gastric infusion of fat, but not sugar, promotes hyperphagia in CCK‐Saporin‐treated rats fed with HFHS diet.

Conclusions

We conclude that intact sensory vagal neurones prevent hyperphagia and exacerbation of weight gain in response to a HFHS diet by promoting lipid‐mediated satiation.

The Phantom Satiation Hypothesis of Bariatric Surgery

The excitation of vagal mechanoreceptors located in the stomach wall directly contributes to satiation. Thus, a loss of gastric innervation would normally be expected to result in abrogated satiation, hyperphagia, and unwanted weight gain. While Roux-en-Y-gastric bypass (RYGB) inevitably results in gastric denervation, paradoxically, bypassed subjects continue to experience satiation. Inspired by the literature in neurology on phantom limbs, I propose a new hypothesis in which damage to the stomach innervation during RYGB, including its vagal supply, leads to large-scale maladaptive changes in viscerosensory nerves and connected brain circuits. As a result, satiation may continue to arise, sometimes at exaggerated levels, even in subjects with a denervated or truncated stomach. The same maladaptive changes may also contribute to dysautonomia, unexplained pain, and new emotional responses to eating. I further revisit the metabolic benefits of bariatric surgery, with an emphasis on RYGB, in the light of this phantom satiation hypothesis.

Loss of CTRP4 alters adiposity and food intake behaviors in obese mice

Central and peripheral mechanisms are both required for proper control of energy homeostasis. Among circulating plasma proteins, C1q/TNF-related proteins (CTRPs) have recently emerged as important regulators of sugar and fat metabolism. CTRP4, expressed in brain and adipose tissue, is unique among the family members in having two tandem globular C1q domains. We previously showed that central administration of recombinant CTRP4 suppresses food intake, suggesting a central nervous system role in regulating ingestive physiology. Whether this effect is pharmacological or physiological remains unclear. We used a loss-of-function knockout (KO) mouse model to clarify the physiological role of CTRP4. Under basal conditions, CTRP4 deficiency increased serum cholesterol levels and impaired glucose tolerance in male but not female mice fed a control low-fat diet. When challenged with a high-fat diet, male and female KO mice responded differently to weight gain and had different food intake patterns. On an obesogenic diet, male KO mice had similar weight gain as wild-type littermates. When fed ad libitum, KO male mice had greater meal number, shorter intermeal interval, and reduced satiety ratio. Female KO mice, in contrast, had lower body weight and adiposity. In the refeeding period following food deprivation, female KO mice had significantly higher food intake due to longer meal duration and reduced satiety ratio. Collectively, our data provide genetic evidence for a sex-dependent physiological role of CTRP4 in modulating food intake patterns and systemic energy metabolism.

Leptin suppresses development of GLP-1 inputs to the paraventricular nucleus of the hypothalamus

The nucleus of the solitary tract (NTS) is critical for the central integration of signals from visceral organs and contains preproglucagon (PPG) neurons, which express leptin receptors in the mouse and send direct projections to the paraventricular nucleus of the hypothalamus (PVH). Here, we visualized projections of PPG neurons in leptin-deficient Lep^ob/ob mice and found that projections from PPG neurons are elevated compared with controls, and PPG projections were normalized by targeted rescue of leptin receptors in LepRb^TB/TB mice, which lack functional neuronal leptin receptors. Moreover, Lep^ob/ob and LepRb^TB/TB mice displayed increased levels of neuronal activation in the PVH following vagal stimulation, and whole-cell patch recordings of GLP-1 receptor-expressing PVH neurons revealed enhanced excitatory neurotransmission, suggesting that leptin acts cell autonomously to suppress representation of excitatory afferents from PPG neurons, thereby diminishing the impact of visceral sensory information on GLP-1 receptor-expressing neurons in the PVH.

Fat, carbohydrate and protein by oral gavage in the rat can be equally effective for satiation

This study aimed to determine the relative efficacy of the macronutrients, protein, fat and carbohydrate to induce satiation and satiety in rats in relation to macronutrient activation of neurons in the nucleus of the solitary tract (NTS). Male Sprague Dawley rats were schedule-fed twice a day for 2 h, receiving 100% of daily ad-libitum energy intake. On test day 1, 30 min before the first scheduled meal of the day, rats were gavaged with an 8 kcal isocaloric, isovolumetric solution of a glucose, lipid or peptone macronutrient solution or a non-caloric saline solution. To assess satiation, thirty minutes later rats were given access to food for 2 h and food intake determined. A second 2 h food access period 3 h later was used for assessment of satiety. On the second test day, rats were gavaged as before and killed 90 min after food presentation. Blood was collected for measurement of circulating metabolic markers. Brains were removed for analysis of c-Fos expression by in situ hybridization in the NTS. Rats which received saline consumed a similar amount of food compared to pre-gavage intakes. However, rats gavaged with a caloric macronutrient solution all reduced food intake by 18-20 kcal. Interestingly, the reduction in caloric intake was greater than the caloric value of the macronutrient solution gavaged and was sustained following the second scheduled meal. Quantification by in situ hybridization of c-Fos mRNA expression in the NTS 90 min post-gavage, showed a significant increase with each macronutrient, but was 24-29% higher with a lipid or peptone gavage compared to a glucose gavage. In conclusion, when delivered directly to the stomach, all macronutrients can be equally effective in inducing satiation with significant neuronal activation in the NTS of the hindbrain.

Roles for gut vagal sensory signals in determining energy availability and energy expenditure

The gut sensory vagus transmits a wide range of meal-related mechanical, chemical and gut peptide signals from gastrointestinal and hepatic tissues to the central nervous system at the level of the caudal brainstem. Results from studies using neurophysiological, behavioral physiological and metabolic approaches that challenge the integrity of this gut-brain axis support an important role for these gut signals in the negative feedback control of energy availability by limiting food intake during a meal. These experimental approaches have now been applied to identify important and unanticipated contributions of the vagal sensory gut-brain axis to the control of two additional effectors of overall energy balance: the feedback control of endogenous energy availability through hepatic glucose production and metabolism, and the control of energy expenditure through brown adipose tissue thermogenesis. Taken together, these studies reveal the pleiotropic influences of gut vagal meal-related signals on energy balance, and encourage experimental efforts aimed at understanding how the brainstem represents, organizes and coordinates gut vagal sensory signals with these three determinants of energy homeostasis.

Endocannabinoids in Body Weight Control

Maintenance of body weight is fundamental to maintain one's health and to promote longevity. Nevertheless, it appears that the global obesity epidemic is still constantly increasing. Endocannabinoids (eCBs) are lipid messengers that are involved in overall body weight control by interfering with manifold central and peripheral regulatory circuits that orchestrate energy homeostasis. Initially, blocking of eCB signaling by first generation cannabinoid type 1 receptor (CB1) inverse agonists such as rimonabant revealed body weight-reducing effects in laboratory animals and men. Unfortunately, rimonabant also induced severe psychiatric side effects. At this point, it became clear that future cannabinoid research has to decipher more precisely the underlying central and peripheral mechanisms behind eCB-driven control of feeding behavior and whole body energy metabolism. Here, we will summarize the most recent advances in understanding how central eCBs interfere with circuits in the brain that control food intake and energy expenditure. Next, we will focus on how peripheral eCBs affect food digestion, nutrient transformation and energy expenditure by interfering with signaling cascades in the gastrointestinal tract, liver, pancreas, fat depots and endocrine glands. To finally outline the safe future potential of cannabinoids as medicines, our overall goal is to address the molecular, cellular and pharmacological logic behind central and peripheral eCB-mediated body weight control, and to figure out how these precise mechanistic insights are currently transferred into the development of next generation cannabinoid medicines displaying clearly improved safety profiles, such as significantly reduced side effects.

Harnessing Gut Microbes for Mental Health: Getting From Here to There

There has been an explosion of interest in the study of microorganisms inhabiting the gastrointestinal tract (gut microbiota) and their impact on host health and physiology. Accumulating data suggest that altered communication between gut microbiota and host systems could participate in disorders such as obesity, diabetes mellitus, and autoimmune disorders as well as neuropsychiatric disorders, including autism, anxiety, and major depressive disorders. The conceptual development of the microbiome-gut-brain axis has facilitated understanding of the complex and bidirectional networks between gastrointestinal microbiota and their host, highlighting potential mechanisms through which this environment influences central nervous system physiology. Communication pathways between gut microbiota and the central nervous system could include autonomic, neuroendocrine, enteric, and immune systems, with pathology resulting in disruption to neurotransmitter balance, increases in chronic inflammation, or exacerbated hypothalamic-pituitary-adrenal axis activity. However, uncertainty remains regarding the generalizability of controlled animal studies to the more multifaceted pattern of human pathophysiology, especially with regard to the therapeutic potential for neuropsychiatric health. This narrative review summarizes current understanding of gut microbial influence over physiological function, with an emphasis on neurobehavioral and neurological impairment based on growing understanding of the gut-brain axis. Experimental and clinical data regarding means of therapeutic manipulation of gut microbiota as a novel treatment option for mental health are described, and important knowledge gaps are identified and discussed.

Nav1.8 neurons are involved in limiting acute phase responses to dietary fat

OBJECTIVE AND METHODS

Metabolic viscera and their vasculature are richly innervated by peripheral sensory neurons. Here, we examined the metabolic and inflammatory profiles of mice with selective ablation of all Nav1.8-expressing primary afferent neurons.

RESULTS

While mice lacking sensory neurons displayed no differences in body weight, food intake, energy expenditure, or body composition compared to controls on chow diet, ablated mice developed an exaggerated inflammatory response to high-fat feeding characterized by bouts of weight loss, splenomegaly, elevated circulating interleukin-6 and hepatic serum amyloid A expression. This phenotype appeared to be directly mediated by the ingestion of saturated lipids.

CONCLUSIONS

These data demonstrate that the Nav1.8-expressing afferent neurons are not essential for energy balance but are required for limiting the acute phase response caused by an obesogenic diet.

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.

Gut-brain nutrient sensing in food reward

For the past several decades, vagal and hormonal gut-brain negative feedback signaling mechanisms that promote satiety and subsequent suppression of food intake have been explored. In addition, a separate positive feedback process termed "appetition," involving postoral signaling from the gut to the brain, has been shown to promote food intake and produce flavor-nutrient preference conditioning. Afferent fibers emerging from the vagus nerve form the main pathway by which information is relayed from the abdominal viscera to the hindbrain and eventually other higher brain regions involved in food intake. Using a specialized subdiaphragmatic vagal deafferentation technique, it was observed that gut vagal and splanchnic afferents play a role in the negative feedback control of satiety after nutrient intake; however, these afferents are not required for nutrient reinforcement or flavor-nutrient preference conditioning, thereby highlighting the distinction between the processes of satiation and appetition. By linking these physiological and behavioral processes to a neurochemical mechanism, it was found that striatal dopamine release induced by intragastric glucose infusion is involved in sweet appetite conditioning. The mechanisms underlying appetition are still being investigated but may involve other nondopaminergic neurochemical systems and/or presently undiscovered hormonal mediators. Future work to delineate the biological mechanisms whereby appetition drives increased intake and conditioned food preference in response to ingestion should take a multifaceted approach by integrating hormonal, neurophysiological, and behavioral techniques.

Parabrachial CGRP Neurons Control Meal Termination

The lateral parabrachial nucleus is a conduit for visceral signals that cause anorexia. We previously identified a subset of neurons located in the external lateral parabrachial nucleus (PBel) that express calcitonin gene-related peptide (CGRP) and inhibit feeding when activated by illness mimetics. We report here that in otherwise normal mice, functional inactivation of CGRP neurons markedly increases meal size, with meal frequency being reduced in a compensatory manner, and renders mice insensitive to the anorexic effects of meal-related satiety peptides. Furthermore, CGRP neurons are directly innervated by orexigenic hypothalamic AgRP neurons, and photostimulation of AgRP fibers supplying the PBel delays satiation by inhibiting CGRP neurons, thereby contributing to AgRP-driven hyperphagia. By establishing a role for CGRP neurons in the control of meal termination and as a downstream mediator of feeding elicited by AgRP neurons, these findings identify a node in which hunger and satiety circuits interact to control feeding behavior.

Vagal innervation of intestine contributes to weight loss After Roux-en-Y gastric bypass surgery in rats

BACKGROUND

It is conceivable that overstimulation of chemo- and mechano-sensors in the Roux and common limbs by uncontrolled influx of undigested nutrients after Roux-en-Y gastric bypass surgery (RYGB) could lead to exaggerated satiety signaling via vagal afferents and contribute to body weight loss. Because previous clinical and preclinical studies using vagotomy came to different conclusions, the aim was to examine the effects of selective and histologically verified celiac branch vagotomy on reduced food intake and body weight loss induced by RYGB.

METHODS

Male Sprague-Dawley rats underwent either RYGB + celiac branch vagotomy (RYGB/VgX, n=15), RYGB + sham celiac branch vagotomy (RYGB/Sham VgX; n=6), Sham RYGB + celiac branch vagotomy (Sham/VgX; n=6), or sham RYGB + sham celiac branch vagotomy (Sham/Sham; n=6), and body weight, body composition, and food choice were monitored for 3 months after intervention.

RESULTS

In rats with RYGB, histologically confirmed celiac branch vagotomy significantly moderated weight loss during the first 40 days after surgery, compared to either sham or failed vagotomy (P<0.05). In contrast, celiac branch vagotomy slightly, but non-significantly, reduced body weight gain in sham RYGB rats compared to sham/sham rats. Furthermore, the significant food intake suppression during the first 32 days after RYGB (P<0.05) was also moderated in rats with verified celiac branch vagotomy.

CONCLUSIONS

The results suggest that signals carried by vagal afferents from the mid and lower intestines contribute to the early RYGB-induced body weight loss and reduction of food intake.

Parabrachial Nucleus Contributions to Glucagon-Like Peptide-1 Receptor Agonist-Induced Hypophagia

Exendin-4 (Ex4), a glucagon-like peptide-1 receptor (GLP-1R) agonist approved to treat type 2 diabetes mellitus, is well known to induce hypophagia in human and animal models. We evaluated the contributions of the hindbrain parabrachial nucleus (PBN) to systemic Ex4-induced hypophagia, as the PBN receives gustatory and visceral afferent relays and descending input from several brain nuclei associated with feeding. Rats with ibotenic-acid lesions targeted to the lateral PBN (PBNx) and sham controls received Ex4 (1 μg/kg) before 24 h home cage chow or 90 min 0.3 M sucrose access tests, and licking microstructure was analyzed to identify components of feeding behavior affected by Ex4. PBN lesion efficacy was confirmed using conditioned taste aversion (CTA) tests. As expected, sham control but not PBNx rats developed a CTA. In sham-lesioned rats, Ex4 reduced chow intake within 4 h of injection and sucrose intake within 90 min. PBNx rats did not show reduced chow or sucrose intake after Ex4 treatment, indicating that the PBN is necessary for Ex4 effects under the conditions tested. In sham-treated rats, Ex4 affected licking microstructure measures associated with hedonic taste evaluation, appetitive behavior, oromotor coordination, and inhibitory postingestive feedback. Licking microstructure responses in PBNx rats after Ex4 treatment were similar to sham-treated rats with the exception of inhibitory postingestive feedback measures. Together, the results suggest that the PBN critically contributes to the hypophagic effects of systemically delivered GLP-1R agonists by enhancing visceral feedback.

Reduced intestinal brain-derived neurotrophic factor increases vagal sensory innervation of the intestine and enhances satiation

Brain-derived neurotrophic factor (BDNF) is produced by developing and mature gastrointestinal (GI) tissues that are heavily innervated by autonomic neurons and may therefore control their development or function. To begin investigating this hypothesis, we compared the morphology, distribution, and density of intraganglionic laminar endings (IGLEs), the predominant vagal GI afferent, in mice with reduced intestinal BDNF (INT-BDNF(-/-)) and controls. Contrary to expectations of reduced development, IGLE density and longitudinal axon bundle number in the intestine of INT-BDNF(-/-) mice were increased, but stomach IGLEs were normal. INT-BDNF(-/-) mice also exhibited increased vagal sensory neuron numbers, suggesting that their survival was enhanced. To determine whether increased intestinal IGLE density or other changes to gut innervation in INT-BDNF(-/-) mice altered feeding behavior, meal pattern and microstructural analyses were performed. INT-BDNF(-/-) mice ate meals of much shorter duration than controls, resulting in reduced meal size. Increased suppression of feeding in INT-BDNF(-/-) mice during the late phase of a scheduled meal suggested that increased satiation signaling contributed to reduced meal duration and size. Furthermore, INT-BDNF(-/-) mice demonstrated increases in total daily intermeal interval and satiety ratio, suggesting that satiety signaling was augmented. Compensatory responses maintained normal daily food intake and body weight in INT-BDNF(-/-) mice. These findings suggest a target organ-derived neurotrophin suppresses development of that organ's sensory innervation and sensory neuron survival and demonstrate a role for BDNF produced by peripheral tissues in short-term controls of feeding, likely through its regulation of development or function of gut innervation, possibly including augmented intestinal IGLE innervation.

Egg-derived tri-peptide IRW exerts antihypertensive effects in spontaneously hypertensive rats

Background

There is a growing interest in using functional food components as therapy for cardiovascular diseases such as hypertension. We have previously characterized a tri-peptide IRW (Ile-Arg-Trp) from egg white protein ovotransferrin; this peptide showed anti-inflammatory, anti-oxidant and angiotensin converting enzyme (ACE) inhibitor properties invitro. Given the pathogenic roles played by angiotensin, oxidative stress and inflammation in the spontaneously hypertensive rat (SHR), we tested the therapeutic potential of IRW in this well-established model of hypertension.

Methods and Results

16–17 week old male SHRs were orally administered IRW at either a low dose (3 mg/Kg BW) or a high dose (15 mg/Kg BW) daily for 18 days. Blood pressure (BP) and heart rate were measured by telemetry. Animals were sacrificed at the end of the treatment for vascular function studies and measuring markers of inflammation. IRW treatment attenuated mean BP by ~10 mmHg and ~40 mmHg at the low- and high-dose groups respectively compared to untreated SHRs. Heart rate was not affected. Reduction in BP was accompanied by the restoration of diurnal variations in BP, preservation of nitric oxide dependent vasorelaxation, as well as reduction of plasma angiotensin II, other inflammatory markers and tissue fibrosis.

Conclusion

Our results demonstrate anti-hypertensive effects of IRW invivo likely mediated through ACE inhibition, endothelial nitric oxide synthase and anti-inflammatory properties.

Loss of neurotrophin-3 from smooth muscle disrupts vagal gastrointestinal afferent signaling and satiation

A large proportion of vagal afferents are dependent on neurotrophin-3 (NT-3) for survival. NT-3 is expressed in developing gastrointestinal (GI) smooth muscle, a tissue densely innervated by vagal mechanoreceptors, and thus could regulate their survival. We genetically ablated NT-3 from developing GI smooth muscle and examined the pattern of loss of NT-3 expression in the GI tract and whether this loss altered vagal afferent signaling or feeding behavior. Meal-induced c-Fos activation was reduced in the solitary tract nucleus and area postrema in mice with a smooth muscle-specific NT-3 knockout (SM-NT-3(KO)) compared with controls, suggesting a decrease in vagal afferent signaling. Daily food intake and body weight of SM-NT-3(KO) mice and controls were similar. Meal pattern analysis revealed that mutants, however, had increases in average and total daily meal duration compared with controls. Mutants maintained normal meal size by decreasing eating rate compared with controls. Although microstructural analysis did not reveal a decrease in the rate of decay of eating in SM-NT-3(KO) mice, they ate continuously during the 30-min meal, whereas controls terminated feeding after 22 min. This led to a 74% increase in first daily meal size of SM-NT-3(KO) mice compared with controls. The increases in meal duration and first meal size of SM-NT-3(KO) mice are consistent with reduced satiation signaling by vagal afferents. This is the first demonstration of a role for GI NT-3 in short-term controls of feeding, most likely involving effects on development of vagal GI afferents that regulate satiation.

Vagal afferent NMDA receptors modulate CCK-induced reduction of food intake through synapsin I phosphorylation in adult male rats

Vagal afferent nerve fibers transmit gastrointestinal satiation signals to the brain via synapses in the nucleus of the solitary tract (NTS). Despite their pivotal role in energy homeostasis, little is known about the cellular mechanisms enabling fleeting synaptic events at vagal sensory endings to sustain behavioral changes lasting minutes to hours. Previous reports suggest that the reduction of food intake by the satiation peptide, cholecystokinin (CCK), requires activation of N-methyl-D-aspartate-type glutamate receptors (NMDAR) in the NTS, with subsequent phosphorylation of ERK1/2 (pERK1/2) in NTS vagal afferent terminals. The synaptic vesicle protein synapsin I is phosphorylated by pERK1/2 at serines 62 and 67. This pERK1/2-catalyzed phosphorylation increases synaptic strength by increasing the readily releasable pool of the neurotransmitter. Conversely, dephosphorylation of serines 62 and 67 by calcineurin reduces the size of the readily releasable transmitter pool. Hence, the balance of synapsin I phosphorylation and dephosphorylation can modulate synaptic strength. We postulated that CCK-evoked activation of vagal afferent NMDARs results in pERK1/2-catalyzed phosphorylation of synapsin I in vagal afferent terminals, leading to the suppression of food intake. We found that CCK injection increased the phosphorylation of synapsin I in the NTS and that this increase is abolished after surgical or chemical ablation of vagal afferent fibers. Furthermore, fourth ventricle injection of an NMDAR antagonist or the mitogen-activated ERK kinase inhibitor blocked CCK-induced synapsin I phosphorylation, indicating that synapsin phosphorylation in vagal afferent terminals depends on NMDAR activation and ERK1/2 phosphorylation. Finally, hindbrain inhibition of calcineurin enhanced and prolonged synapsin I phosphorylation and potentiated reduction of food intake by CCK. Our findings are consistent with a mechanism in which NMDAR-dependent phosphorylation of ERK1/2 modulates satiation signals via synapsin I phosphorylation in vagal afferent endings.

Molecular anatomy of the gut-brain axis revealed with transgenic technologies: implications in metabolic research

Neurons residing in the gut-brain axis remain understudied despite their important role in coordinating metabolic functions. This lack of knowledge is observed, in part, because labeling gut-brain axis neurons and their connections using conventional neuroanatomical methods is inherently challenging. This article summarizes genetic approaches that enable the labeling of distinct populations of gut-brain axis neurons in living laboratory rodents. In particular, we review the respective strengths and limitations of currently available genetic and viral approaches that permit the marking of gut-brain axis neurons without the need for antibodies or conventional neurotropic tracers. Finally, we discuss how these methodological advances are progressively transforming the study of the healthy and diseased gut-brain axis in the context of its role in chronic metabolic diseases, including diabetes and obesity.

Obesity and appetite control

Obesity is one of the major challenges to human health worldwide; however, there are currently no effective pharmacological interventions for obesity. Recent studies have improved our understanding of energy homeostasis by identifying sophisticated neurohumoral networks which convey signals between the brain and gut in order to control food intake. The hypothalamus is a key region which possesses reciprocal connections between the higher cortical centres such as reward-related limbic pathways, and the brainstem. Furthermore, the hypothalamus integrates a number of peripheral signals which modulate food intake and energy expenditure. Gut hormones, such as peptide YY, pancreatic polypeptide, glucagon-like peptide-1, oxyntomodulin, and ghrelin, are modulated by acute food ingestion. In contrast, adiposity signals such as leptin and insulin are implicated in both short- and long-term energy homeostasis. In this paper, we focus on the role of gut hormones and their related neuronal networks (the gut-brain axis) in appetite control, and their potentials as novel therapies for obesity.

Obesity and appetite control

Obesity is one of the major challenges to human health worldwide; however, there are currently no effective pharmacological interventions for obesity. Recent studies have improved our understanding of energy homeostasis by identifying sophisticated neurohumoral networks which convey signals between the brain and gut in order to control food intake. The hypothalamus is a key region which possesses reciprocal connections between the higher cortical centres such as reward-related limbic pathways, and the brainstem. Furthermore, the hypothalamus integrates a number of peripheral signals which modulate food intake and energy expenditure. Gut hormones, such as peptide YY, pancreatic polypeptide, glucagon-like peptide-1, oxyntomodulin, and ghrelin, are modulated by acute food ingestion. In contrast, adiposity signals such as leptin and insulin are implicated in both short- and long-term energy homeostasis. In this paper, we focus on the role of gut hormones and their related neuronal networks (the gut-brain axis) in appetite control, and their potentials as novel therapies for obesity.

Vagal afferent controls of feeding: a possible role for gastrointestinal BDNF

INTRODUCTION

Vagal gastrointestinal (GI) afferents do not appear to contribute to long-term controls of feeding, despite downstream connections that could support such a role. This view is largely attributable to a lack of evidence for long-term effects, especially the failure of vagal afferent lesions to produce hyperphagia or obesity.

AIMS

Here, the possibility is evaluated that "side effects" of vagal lesion methods resulting largely from complexities of vagal organization would probably suppress long-term effects. Criteria based on knowledge of vagal organization were utilized to critique and compare vagal lesion methods and to interpret their effects on GI function, feeding and body weight.

RESULTS AND CONCLUSIONS

This analysis suggested that it was premature to eliminate a long-term vagal GI afferent role based on the effects of these lesions and highlighted aspects of vagal organization that must be addressed to reduce the problematic side effects of vagal lesions. The potential of "genetic" lesions that alter vagal sensory development to address these aspects, examination of the feasibility of this approach, and the properties of brain-derived neurotrophic factor (BDNF) that made it an attractive candidate for application of this approach are described. BDNF knockout from GI smooth muscle unexpectedly demonstrated substantial overeating and weight gain associated with increased meal size and frequency. The decay of eating rate during a scheduled meal was also reduced. However, meal-induced c-Fos activation was increased in the dorsal motor nucleus of the vagus, suggesting that the effect on eating rate was due to augmentation of GI reflexes by vagal afferents or other neural systems.

Early postnatal overnutrition: potential roles of gastrointestinal vagal afferents and brain-derived neurotrophic factor

Abnormal perinatal nutrition (APN) results in a predisposition to develop obesity and the metabolic syndrome and thus may contribute to the prevalence of these disorders. Obesity, including that which develops in organisms exposed to APN, has been associated with increased meal size. Vagal afferents of the gastrointestinal (GI) tract contribute to regulation of meal size by transmitting satiation signals from gut-to-brain. Consequently, APN could increase meal size by altering this signaling, possibly through changes in expression of factors that control vagal afferent development or function. Here two studies that addressed these possibilities are reviewed. First, meal patterns, meal microstructure, and the structure and density of vagal afferents that innervate the intestine were examined in mice that experienced early postnatal overnutrition (EPO). These studies provided little evidence for EPO effects on vagal afferents as it did not alter meal size or vagal afferent density or structure. However, these mice exhibited modest hyperphagia due to a satiety deficit. In parallel, the possibility that brain-derived neurotrophic factor (BDNF) could mediate APN effects on vagal afferent development was investigated. Brain-derived neurotrophic factor was a strong candidate because APN alters BDNF levels in some tissues and BDNF knockout disrupts development of vagal sensory innervation of the GI tract. Surprisingly, smooth muscle-specific BDNF knockout resulted in early-onset obesity and hyperphagia due to increases in meal size and frequency. Microstructure analysis revealed decreased decay of intake rate during a meal in knockouts, suggesting that the loss of vagal negative feedback contributed to their increase in meal size. However, meal-induced c-Fos activation within the dorsal vagal complex suggested this effect could be due to augmentation of vago-vagal reflexes. A model is proposed to explain how high-fat diet consumption produces increased obesity in organisms exposed to APN, and may be required to reveal effects of EPO on vagal function.

Obesity surgery and gut-brain communication

The prevalence of obesity, and the cluster of serious metabolic diseases it is associated with, continues to rise globally, and hopes for effective treatment with drugs have been considerably set back. Thus, success with bariatric surgeries to induce sustained body weight loss and effectively cure most of the associated co-morbidities appears almost "miraculous" and systematic investigation of the mechanisms at work has gained momentum. Here, we will discuss the basic organization of gut-brain communication and review clinical and pre-clinical investigations on the potential mechanisms by which gastric bypass surgery leads to its beneficial effects on energy balance and glucose homeostasis. Although a lot has been learned regarding changes in energy intake and expenditure, secretion of gut hormones, and improvement in glucose homeostasis, there has not yet been the "breakthrough observation" of identifying a key signaling component common to the beneficial effects of the surgery. However, given the complexity and redundancy of gut-brain signaling and gut signaling to other relevant organs, it is perhaps more realistic to expect a number of key signaling changes that act in concert to bring about the "miracle".

The gut hormones in appetite regulation

Obesity has received much attention worldwide in association with an increased risk of cardiovascular diseases, diabetes, and cancer. At present, bariatric surgery is the only effective treatment for obesity in which long-term weight loss is achieved in patients. By contrast, pharmacological interventions for obesity are usually followed by weight regain. Although the exact mechanisms of long-term weight loss following bariatric surgery are yet to be fully elucidated, several gut hormones have been implicated. Gut hormones play a critical role in relaying signals of nutritional and energy status from the gut to the central nervous system, in order to regulate food intake. Cholecystokinin, peptide YY, pancreatic polypeptide, glucagon-like peptide-1, and oxyntomodulin act through distinct yet synergistic mechanisms to suppress appetite, whereas ghrelin stimulates food intake. Here, we discuss the role of gut hormones in the regulation of food intake and body weight.

Sensing the fuels: glucose and lipid signaling in the CNS controlling energy homeostasis

The central nervous system (CNS) is capable of gathering information on the body's nutritional state and it implements appropriate behavioral and metabolic responses to changes in fuel availability. This feedback signaling of peripheral tissues ensures the maintenance of energy homeostasis. The hypothalamus is a primary site of convergence and integration for these nutrient-related feedback signals, which include central and peripheral neuronal inputs as well as hormonal signals. Increasing evidence indicates that glucose and lipids are detected by specialized fuel-sensing neurons that are integrated in these hypothalamic neuronal circuits. The purpose of this review is to outline the current understanding of fuel-sensing mechanisms in the hypothalamus, to integrate the recent findings in this field, and to address the potential role of dysregulation in these pathways in the development of obesity and type 2 diabetes mellitus.

The role of gut hormones and the hypothalamus in appetite regulation

The World Health Organisation has estimated that by 2015 approximately 2.3 billion adults will be overweight and more than 700 million obese. Obesity is associated with an increased risk of diabetes, cardiovascular events, stroke and cancer. The hypothalamus is a crucial region for integrating signals from central and peripheral pathways and plays a major role in appetite regulation. In addition, there are reciprocal connections with the brainstem and higher cortical centres. In the arcuate nucleus of the hypothalamus, there are two major neuronal populations which stimulate or inhibit food intake and influence energy homeostasis. Within the brainstem, the dorsal vagal complex plays a role in the interpretation and relaying of peripheral signals. Gut hormones act peripherally to modulate digestion and absorption of nutrients. However, they also act as neurotransmitters within the central nervous system to control food intake. Peptide YY, pancreatic polypeptide, glucagon-like peptide-1 and oxyntomodulin suppress appetite, whilst ghrelin increases appetite through afferent vagal fibres to the caudal brainstem or directly to the hypothalamus. A better understanding of the role of these gut hormones may offer the opportunity to develop successful treatments for obesity. Here we review the current understanding of the role of gut hormones and the hypothalamus on food intake and body weight control.

A high-fat diet attenuates the central response to within-meal satiation signals and modifies the receptor expression of vagal afferents in mice

During digestion, macronutrients are sensed within the small intestine. This sensory process is dependent upon the action of gut mediators, such as cholecystokinin (CCK) or serotonin (5-HT), on vagal afferents that, in turn, convey peripheral information to the brain to influence the control of food intake. Recent studies have suggested that dietary conditions alter vagal sensitivity to CCK and 5-HT. This phenomenon may be of importance to the onset of eating disorders. The aim of the present study was thus to investigate the effects of subjecting mice to 15 days of either an HF diet (30% fat, 54% carbohydrate) or an NF diet (10% fat, 74% carbohydrate) on 1) daily and short-term food intake, 2) vagal sensitivity to peripheral anorectic factors and macronutrient loads, and 3) vagal afferent neuron receptor expression. The results indicated that compared with an NF diet, and while increasing food intake and body weight gain, an HF diet altered the short-term response to CCK-8 and intragastric macronutrient loads, while decreasing vagal activation by CCK-8 and modifying the receptor expression of vagal neurons. These findings, therefore, suggest that dietary intervention effect on food intake could be linked to changes in vagal afferent receptor profiles.

Orexin-A hyperphagia: hindbrain participation in consummatory feeding responses

Orexin-A (ORXA) is an orexigenic neuropeptide produced by the lateral hypothalamus that increases food intake when injected into the brain ventricles or forebrain nuclei. We used a licking microstructure analysis to evaluate hindbrain and forebrain ORXA effects in intact and hindbrain-lesioned rats, to identify the motivational and anatomical bases of ORXA hyperphagia. Intact rats with cannulas in the fourth brain ventricle (4V) received vehicle (artificial cerebrospinal fluid) or ORXA (0.1, 0.4, 1, or 10 nm) injections before 90 min access to 0.1 m sucrose. Meal size and frequency were increased in a double-dissociated manner by the 1 and 10 nm doses, respectively. In experiment 2, 4V 1 nm ORXA was applied to rats offered solutions varied in caloric and gustatory intensity (water and 0.1 and 1 m sucrose). ORXA increased meal frequency for all tastants. ORXA increased meal size only for 0.1 m sucrose, by prolonging the meal without affecting early ingestion rate or lick burst size, suggesting that 4V ORXA influenced inhibitory postingestive feedback rather than taste evaluation. In experiment 3, rats with cannulas in the third ventricle (3V) received dorsal medullary lesions centered on the area postrema (APX group) or sham procedures, and licking for water and 0.1 and 1 m sucrose was evaluated after 1 nm 3V ORXA/artificial cerebrospinal fluid injections. The 3V ORXA increased 0.1 m sucrose meal size and meal frequency for all tastants in the sham group, as observed after 4V ORXA in experiment 2. In the APX group, 3V ORXA injections influenced meal frequency, but they no longer increased meal size. However, the APX rats increased meal size for 0.1 m sucrose after food and water deprivation and after 3V angiotensin II injection. They also showed meal size suppression after 3V injection of the melanocortin-3/4 receptor agonist melanotan II (1 nm). These findings suggest that the area postrema and subjacent nucleus of the solitary tract are necessary for increases in consummatory (meal size) but not appetitive (meal frequency) responses to 3V ORXA. The meal size increases may be due to reduced postingestive feedback inhibition induced by ORXA delivered to either the hindbrain or forebrain ventricles.

The vagus nerve, food intake and obesity

Food interacts with sensors all along the alimentary canal to provide the brain with information regarding its composition, energy content, and beneficial effect. Vagal afferents innervating the gastrointestinal tract, pancreas, and liver provide a rapid and discrete account of digestible food in the alimentary canal, as well as circulating and stored fuels, while vagal efferents, together with the sympathetic nervous system and hormonal mechanisms, codetermine the rate of nutrient absorption, partitioning, storage, and mobilization. Although vagal sensory mechanisms play a crucial role in the neural mechanism of satiation, there is little evidence suggesting a significant role in long-term energy homeostasis. However, increasing recognition of vagal involvement in the putative mechanisms making bariatric surgeries the most effective treatment for obesity should greatly stimulate future research to uncover the many details regarding the specific transduction mechanisms in the periphery and the inter- and intra-neuronal signaling cascades disseminating vagal information across the neuraxis.

Factors regulating vagal sensory development: potential role in obesities of developmental origin

Contributors to increased obesity in children may include perinatal under- or overnutrition. Humans and rodents raised under these conditions develop obesity, which like obesities of other etiologies has been associated with increased meal size. Since vagal sensory innervation of the gastrointestinal (GI) tract transmits satiation signals that regulate meal size, one mechanism through which abnormal perinatal nutrition could increase meal size is by altering vagal development, possibly by causing changes in the expression of factors that control it. Therefore, we have begun to characterize development of vagal innervation of the GI tract and the expression patterns and functions of the genes involved in this process. Important events in development of mouse vagal GI innervation occurred between midgestation and the second postnatal week, suggesting they could be vulnerable to effects of abnormal nutrition pre- or postnatally. One gene investigated was brain- derived neurotrophic factor (BDNF), which regulates survival of a subpopulation of vagal sensory neurons. BDNF was expressed in some developing stomach wall tissues innervated by vagal afferents. At birth, mice deficient in BDNF exhibited a 50% reduction of putative intraganglionic laminar ending mechanoreceptor precursors, and a 50% increase in axons that had exited fiber bundles. Additionally, BDNF was required for patterning of individual axons and fiber bundles in the antrum and differentiation of intramuscular array mechanoreceptors in the forestomach. It will be important to determine whether abnormal perinatal environments alter development of vagal sensory innervation of the GI tract, involving effects on expression of BDNF, or other factors regulating vagal development.

Effects of hindbrain melanin-concentrating hormone and neuropeptide Y administration on licking for water, saccharin, and sucrose solutions

Melanin-concentrating hormone (MCH) and neuropeptide Y (NPY) are orexigenic peptides found in hypothalamic neurons that project throughout the forebrain and hindbrain. The effects of fourth ventricle (4V) infusions of NPY (5 microg) and MCH (5 microg) on licking for water, 4 mM saccharin, and sucrose (0.1 and 1.0 M) solutions were compared to identify the contributions of each peptide to hindbrain-stimulated feeding. NPY increased mean meal size only for the sucrose solutions, suggesting that caloric feedback or taste quality is pertinent to the orexigenic effect; MCH infusions under identical testing conditions failed to produce increases for any tastant. A second experiment also observed no intake or licking effects after MCH doses up to 15 microg, supporting the conclusion that MCH-induced orexigenic responses require forebrain stimulation. A third experiment compared the 4V NPY results with those obtained after NPY infusions (5 microg) into the third ventricle (3V). In contrast to the effects observed after the 3V NPY injections and previously reported forebrain intracerebroventricular (ICV) NPY infusion studies, 4V NPY failed to increase meal frequency for any taste solution or ingestion rate in the early phases of the sucrose meals. Overall, 4V NPY responses were limited to intrameal behavioral processes, whereas forebrain ICV NPY stimulation elicited both consummatory and appetitive responses. The dissociation between MCH and NPY effects observed for 4V injections is consistent with reports that forebrain ICV injections of MCH and NPY produced nearly dichotomous effects on the pattern of licking microstructure, and, collectively, the results indicate that the two peptides have separate sites of feeding action in the brain.

Gastrointestinal regulation of food intake

Despite substantial fluctuations in daily food intake, animals maintain a remarkably stable body weight, because overall caloric ingestion and expenditure are exquisitely matched over long periods of time, through the process of energy homeostasis. The brain receives hormonal, neural, and metabolic signals pertaining to body-energy status and, in response to these inputs, coordinates adaptive alterations of energy intake and expenditure. To regulate food consumption, the brain must modulate appetite, and the core of appetite regulation lies in the gut-brain axis. This Review summarizes current knowledge regarding the neuroendocrine regulation of food intake by the gastrointestinal system, focusing on gastric distention, intestinal and pancreatic satiation peptides, and the orexigenic gastric hormone ghrelin. We highlight mechanisms governing nutrient sensing and peptide secretion by enteroendocrine cells, including novel taste-like pathways. The increasingly nuanced understanding of the mechanisms mediating gut-peptide regulation and action provides promising targets for new strategies to combat obesity and diabetes.

Integrative capacity of the caudal brainstem in the control of food intake

The caudal brainstem nucleus of the solitary tract (NTS) is the initial central nervous system (CNS) terminus for a variety of gastrointestinal mechanical, nutrient chemical and gut peptide signals that limit the amount of food consumed during a meal. It receives neuroanatomical projections from gut vagal and non-vagal visceral afferents that mediate the CNS representation of these meal-stimulated gut feedback signals, and is reciprocally connected to a range of hypothalamic and limbic system sites that play significant roles in the neural processing of meal-related stimuli and in determining food consumption. Neurons in the NTS also contains elements of leptinergic and melanocortinergic signalling systems, presenting the possibility that the brainstem actions of these neuropeptides affect both the NTS processing of meal-stimulated gut afferent neural activity and its behavioural potency. Taken together, these features suggest that the NTS is ideally situated to integrate central and peripheral signals that determine meal size. This manuscript will review recent support from molecular genetic, neurophysiological and immunocytochemical studies that begin to identify and characterize the types of integrative functions performed within the NTS, and highlight the extent to which they are consistent with a causal role for NTS integration of peripheral gut and central neuropeptide signals important in the control of food intake.

Influence of nutrient preload on encapsulated macronutrient selection in European sea bass

The sea bass is a teleost that is able to regulate its energy intake by selecting from pure macronutrient sources, although the regulatory mechanisms involved in this selection are unknown. Nutrient preloads are known to reduce food intake and modify macronutrient selection patterns in mammals, but no information is available on its effects in fish. The aim of the present work was to determine the effect of orally administered macronutrient preloads of protein (P), fat (F) or carbohydrate (CH) on the subsequent macronutrient selection, using for the purpose feed consisting of CH, P or F packaged separately in gelatin capsules. The macronutrient preloads left the total food intake unaltered, but caused differential changes in the pattern of macronutrient selection. The CH preload increased the selection of CH (39%) and decreased that of P (20%), independently of the fish's previous nutritional preferences. The F preload induced an F increase (32%) and a P decrease (18%) in P-preferring fish, but not in F-preferring fish in which the macronutrient selection pattern remained unaffected. The P preload stimulated F selection by 42% in P-preferring fish, but left the macronutrient selection pattern unchanged in F-preferring fish. In conclusion, oral macronutrient preloads affected the pattern of macronutrient selection in fish, acting by post-ingestive mechanisms. The effect was influenced by the fish's previous nutritional preference and/or status, which could depend on its metabolic capacity.

Effects of melanin-concentrating hormone on licking microstructure and brief-access taste responses

The effects of intracerebroventricular application of melanin-concentrating hormone (MCH) on licking for sucrose, quinine hydrochloride (QHCl), and water solutions were evaluated in two experiments. In experiment 1, rats received 90-min access to sucrose and water solutions after MCH or vehicle microinjection to the third ventricle (3V). MCH increased intake largely through increases in the rate of licking early in the meal and in the mean duration of lick bursts, suggesting an effect on gustatory evaluation. Therefore, in experiment 2, brief access tests were used with a series of sucrose and QHCl concentrations to behaviorally isolate the effects of intracerebroventricular MCH on gustatory evaluation. MCH uniformly increased licking for all sucrose solutions, water, and weak concentrations of QHCl; however, it had no effect on licking for the strongest concentrations of QHCl, which were generally avoided under control conditions. Thus MCH did not produce nonspecific increases in oromotor activity, nor did it change the perceived intensity of the tastants. We conclude that MCH enhanced the gain of responses to normally accepted stimuli at a phase of processing after initial gustatory detection and after the decision to accept or reject the taste stimulus. A comparison of 3V NPY and MCH effects on licking microstructure indicated that these two peptides increased intake via dichotomous behavioral processes; although NPY suppressed measures associated with inhibitory feedback from the gut, MCH appeared instead to enhance measures associated with hedonic taste evaluation.

Inhibition of thirst when dehydrated rats drink water or saline

The present experiments sought to identify the physiological signals that inhibit thirst when dehydrated rats drink water or NaCl solution. Rats were deprived of drinking fluid but not food overnight. When allowed to drink again, the dehydrated animals consumed water or saline (0.05 M, 0.10 M, 0.15 M, or 0.20 M NaCl solution) almost continuously for 5–8 min before stopping. The volumes consumed were similar regardless of which fluid they ingested, but blood analyses indicated that increased plasma osmolality and decreased plasma volume, or both, still remained when drinking terminated. These results suggest that the composition of the ingested fluid is less significant than its volume in providing an early signal that inhibits thirst and fluid consumption by dehydrated rats. Analyses of the gastrointestinal tracts revealed that the cumulative volume in the stomach and small intestine correlated highly with the amount consumed regardless of which fluid was ingested. These and other results suggest that the volume of fluid ingested by dehydrated rats is sensed by stretch receptors detecting distension of the stomach and small intestine, which provide an early inhibitory stimulus of thirst.

Thoracic cross-over pathways of the rat vagal trunks

It is very difficult to study the independent contributions of the afferent and efferent pathways of the subdiaphragmatic vagus to physiology and behavior. Total subdiaphragmatic vagotomy can confound the interpretation of experimental results because it destroys both afferent and efferent vagal fibers. One approach to address this problem involves producing a total ablation of afferent (or efferent) vagal fibers while retaining half of the efferent (or afferent) vagal fibers by making a unilateral rhizotomy plus contralateral subdiaphragmatic vagotomy. However, the completeness of this afferent (or efferent) lesion is based on the assumption that there are no cross-over pathways within the thoracic cavity between the vagal trunks of the rat. To directly test for the presence of vagal cross-over pathways in the rat, we recorded the compound action potentials from the ventral and dorsal trunks of the subdiaphragmatic vagus following electrical stimulation of the left or right cervical vagi. C-fiber cross-over pathways comprised an average of 9% of the total nerve responses (range was 0 to 29%, n = 20). Direct application of the anesthetic bupivacaine to the vagus completely blocked the recorded signals. The vagal cross-over pathways were also demonstrated using capsaicin as a stimulus. These results indicate the presence of thoracic cross-over pathways between vagal trunks in the rat and demonstrate that for most animals it is not possible to produce a "complete" ablation of afferent (or efferent) components of the subdiaphragmatic vagus using unilateral rhizotomy combined with contralateral subdiaphragmatic vagotomy.

Independent ingestion and microstructure of feeding patterns in infant rats lacking CCK-1 receptors

Otsuka Long-Evans Tokushima fatty (OLETF) rats are a strain of Long-Evans Tokushima Otsuka (LETO) rats that do not express CCK-1 receptors, developing in adulthood, hyperphagia, obesity, and non-insulin-dependent diabetes mellitus (NIDDM). We examined weight gain and meal patterns during a 30-min independent ingestion test on postnatal days 2-4 and again on days 9-11 in OLETF and LETO rat pups. OLETF pups were significantly heavier compared with their LETO controls at both ages, and they consumed significantly more of the sweet milk diet. The difference in intake can be attributed to a significant increase in meal size and duration. Number of clusters and bursts of licking within a meal were greater in OLETF rat pups, with no difference between strains in burst and cluster size. Interlick interval (ILI) was not significantly different between OLETF and LETO pups. This measure decreased on days 9-11 compared with days 2-4 in both strains. Latency to start feeding was significantly shorter on days 2-4 in OLETF vs. LETO pups, but this difference disappeared at the second test at the older age. Two- to four-day-old OLETF pups consumed a larger volume of milk during the first minute of feeding, and their initial lick rate and decay of lick rate were significantly larger compared with their LETO controls. Lack of CCK-1 receptors, or other OLETF-related abnormalities, therefore, resulted in a satiation deficit, leading to increased meal size, hyperphagia, and increased weight gain as early as 2-4 postnatal days.

Gastrointestinal tract innervation of the mouse: afferent regeneration and meal patterning after vagotomy

Mice, with the variety of genotypes they provide, should be particularly useful for studies of growth factors and gene products in regeneration of autonomic pathways such as the vagus nerve. To provide a foundation for examinations of mouse vagal reorganization, two experiments assessed the rate, extent, and accuracy of afferent reinnervation of the stomach after vagotomy and related these patterns to feeding behavior. In experiment 1, the pattern of afferent regrowth into the gut after unilateral truncal vagotomy was characterized by labeling of these afferents with wheat germ agglutinin-horseradish peroxidase and Micro-Ruby. Regenerating neurites had reached and, in some cases, already reinnervated the stomach by 4 wk after axotomy. By 8 wk, regrowth was more extensive, and many fibers had redifferentiated terminals in the smooth muscle. By 16 wk, vagal projections had reached or exceeded normal density in the corpus, density in the forestomach was still reduced, and regrowth in the antrum was minimal. At all time points, not only appropriate terminals, but also growth cones and aberrant endings, were observed. In experiment 2, meal patterns of vagotomized mice were evaluated using a solid diet over the period of regeneration; cholecystokinin suppression of a liquid meal after unilateral and bilateral truncal vagotomies was also evaluated. Unilaterally, as well as bilaterally, vagotomized animals ate smaller and more frequent meals. These disturbed patterns became more pronounced in the first 8 wk after vagotomy, during regeneration. Cholecystokinin inhibition of intake was attenuated by bilateral, but not unilateral, vagotomy. Overall, the spatial and temporal patterns of structural and functional changes observed during regeneration verify that the mouse provides a useful preparation for examining the control of vagal plasticity.

Extracellular signal-regulated kinase 1/2 signaling pathway in solitary nucleus mediates cholecystokinin-induced suppression of food intake in rats

Increased food intake is a major factor in the development of obesity, and the control of meal size is a valid approach to reduce food intake in humans. Meal termination, or satiety, is thought to be organized within the caudal brainstem where direct signals from the food handling alimentary canal and long-term signals from the forebrain converge in the solitary nucleus. Cholecystokinin (CCK) released from the gut after ingestion of food has been strongly implicated in nucleus tractus solitarius (NTS)-mediated satiation, but the exact cellular and intracellular signaling events are not understood. Using Western blotting and immunohistochemistry with phosphospecific antibodies, we demonstrate here that peripheral administration of CCK in rats leads to rapid activation of the extracellular signal-regulated kinase (ERK) signaling cascade in NTS neurons and that blockade of ERK signaling with microinfusion of a selective mitogen-activated ERK kinase inhibitor into the fourth ventricle attenuates the capacity of CCK to suppress food intake. In addition, we show that CCK-induced activation of ERK results in phosphorylation of the voltage-dependent potassium channel Kv4.2 and the nuclear transcription factor CREB (cAMP response element-binding protein). The results demonstrate that ERK signaling is necessary for exogenous CCK to suppress food intake in deprived rats and suggest that this pathway may also be involved in natural satiation and the period of satiety between meals through coupling of ERK activation to both cytosolic and nuclear effector mechanisms that have the potential to confer acute and long-term changes in neuronal functioning.

Effects of perivagal administration of capsaicin on food intake in animals after noxious gastric surgery

Previous studies have shown that the perivagal administration of capsaicin induces greater food intake vs. controls at 24 h after the surgery but a similar intake to that of controls at 48 h. The present study aimed to determine whether the nutritive effect observed after perivagal capsaicin administration is due to the interruption of noxious vagal fibers in rats. For this purpose, postsurgical food intake was analyzed in control and capsaicin-treated animals with (Experiment 2) and without (Experiment 1) noxious lesions in the gastric wall. The results of both experiments showed that the food intake of capsaicin-treated animals was greater vs. control animals at 24 h but not at 48 h after the surgery (p<0.025), as previously demonstrated. However, the food intake of the capsaicin-treated lesion animals in Experiment 2, although still greater than that of the control group, was significantly less than the intake of the capsaicin-treated animals in Experiment 1 (p<0.01). Therefore, it appears unlikely that the demonstrated effect is produced by lesion to noxious vagal fibers destroyed by the capsaicin. On the contrary, it is more likely that the vagal afferent pathways are those related to short-term nutrition.

Lipopolysaccharide fever is initiated via a capsaicin-sensitive mechanism independent of the subtype-1 vanilloid receptor

As pretreatment with intraperitoneal capsaicin (8-methyl-N-vanillyl-6-nonenamide, CAP), an agonist of the vanilloid receptor known as VR1 or transient receptor potential channel-vanilloid receptor subtype 1 (TRPV-1), has been shown to block the first phase of lipopolysaccharide (LPS) fever in rats, this phase is thought to depend on the TRPV-1-bearing sensory nerve fibers originating in the abdominal cavity. However, our recent studies suggest that CAP blocks the first phase via a non-neural mechanism. In the present work, we studied whether this mechanism involves the TRPV-1. Adult Long-Evans rats implanted with chronic jugular catheters were used. Pretreatment with CAP (5 mg kg(-1), i.p.) 10 days before administration of LPS (10 microg kg(-1), i.v.) resulted in the loss of the entire first phase and a part of the second phase of LPS fever. Pretreatment with the ultrapotent TRPV-1 agonist resiniferatoxin (RTX; 2, 20, or 200 microg kg(-1), i.p.) 10 days before administration of LPS had no effect on the first and second phases of LPS fever, but it exaggerated the third phase at the highest dose. The latter effect was presumably due to the known ability of high doses of TRPV-1 agonists to cause a loss of warm sensitivity, thus leading to uncontrolled, hyperpyretic responses. Pretreatment with the selective competitive TRPV-1 antagonist capsazepine (N-[2-(4-chlorophenyl)ethyl]-1,3,4,5-tetrahydro-7,8-dihydroxy-2H-2-benzazepine-2-carbothioamidem, CPZ; 40 mg kg(-1), i.p.) 90 min before administration of LPS (10 microg kg(-1), i.v.) or CAP (1 mg kg(-1), i.p.) did not affect LPS fever, but blocked the immediate hypothermic response to acute administration of CAP. It is concluded that LPS fever is initiated via a non-neural mechanism, which is CAP-sensitive but RTX- and CPZ-insensitive. The action of CAP on this mechanism is likely TRPV-1-independent. It is speculated that this mechanism may be the production of prostaglandin E(2) by macrophages in LPS-processing organs.

The ontogeny of the postingestive inhibitory effect of peptone in rats

The ontogeny of the postingestive inhibitory control of intake by protein digestion products was investigated by administering gastric preloads of a peptone that was a hydrolysate of meat and that decreased intake in adult rats [Am. J. Physiol., Regul. Integr. Comp. Physiol. 276 (1999) R1623; Am. J. Physiol., Regul. Integr. Comp. Physiol. 277 (1999) R1144]. Gastric preloads of saline or peptone, or sham preloads were given 5 min before a 30-min, independent ingestion test in which pups had access to a sweet, high-fat milk diet. Preloads of isotonic peptone reduced intake significantly more than preloads of isotonic saline on postnatal day (P) 18, but not on P12. Pretreatment with the CCKA receptor antagonist devazepide (600 microg/kg ip) did not change the inhibitory effect of isotonic peptone. Thus, the inhibitory effect of peptone on P18 was apparently not mediated by endogenous CCK acting at CCKA receptors. In contrast to isotonic peptone, preloads of hypertonic peptone did not decrease intake more than preloads of hypertonic saline on P12, P18, or P24. We conclude that if the isotonic peptone used in these experiments is an adequate model of the digestion products of dietary protein at these postnatal ages, then the postingestive inhibitory control of intake by digestion products of dietary protein during independent ingestion appears between P13 and P18.