High fat diet attenuates glucose-dependent facilitation of 5-HT3 -mediated responses in rat gastric vagal afferents

Vagus Nerve and Gut-Brain Communication

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

Microfluidic compartmentalization of rat vagal afferent neurons to model gut-brain axis

BACKGROUND

Vagal afferent neurons represent the key neurosensory branch of the gut-brain axis, which describes the bidirectional communication between the gastrointestinal system and the brain. These neurons are important for detecting and relaying sensory information from the periphery to the central nervous system to modulate feeding behavior, metabolism, and inflammation. Confounding variables complicate the process of isolating the role of the vagal afferents in mediating these physiological processes. Therefore, we developed a microfluidic model of the sensory branch of the gut-brain axis. We show that this microfluidic model successfully compartmentalizes the cell body and neurite terminals of the neurons, thereby simulates the anatomical layout of these neurons to more accurately study physiologically-relevant processes.

METHODS

We implemented a primary rat vagal afferent neuron culture into a microfluidic platform consisting of two concentric chambers interconnected with radial microchannels. The microfluidic platform separated cell bodies from neurite terminals of vagal afferent neurons. We then introduced physiologically-relevant gastrointestinal effector molecules at the nerve terminals and assessed their retrograde transport along the neurite or capacity to elicit an electrophysiological response using live cell calcium imaging.

RESULTS

The angle of microchannel outlets dictated the probability of neurites growing into a chamber versus tracking along chamber walls. When the neurite terminals were exposed to fluorescently-labeled cholera toxin subunit B, the proteins were taken up and retrogradely transported along the neurites over the course of 24 h. Additionally, mechanical perturbation (e.g., rinsing) of the neurite terminals significantly increased intracellular calcium concentration in the distal soma. Finally, membrane-displayed receptor for capsaicin was expressed and trafficked along newly projected neurites, as revealed by confocal microscopy.

CONCLUSIONS

In this work, we developed a microfluidic device that can recapitulate the anatomical layout of vagal afferent neurons in vitro. We demonstrated two physiologically-relevant applications of the platforms: retrograde transport and electrophysiological response. We expect this tool to enable controlled studies on the role of vagal afferent neurons in the gut-brain axis.

High-calorie diets uncouple hypothalamic oxytocin neurons from a gut-to-brain satiation pathway via κ-opioid signaling

Oxytocin-expressing paraventricular hypothalamic neurons (PVNOT neurons) integrate afferent signals from the gut, including cholecystokinin (CCK), to adjust whole-body energy homeostasis. However, the molecular underpinnings by which PVNOT neurons orchestrate gut-to-brain feeding control remain unclear. Here, we show that mice undergoing selective ablation of PVNOT neurons fail to reduce food intake in response to CCK and develop hyperphagic obesity on a chow diet. Notably, exposing wild-type mice to a high-fat/high-sugar (HFHS) diet recapitulates this insensitivity toward CCK, which is linked to diet-induced transcriptional and electrophysiological aberrations specifically in PVNOT neurons. Restoring OT pathways in diet-induced obese (DIO) mice via chemogenetics or polypharmacology sufficiently re-establishes CCK's anorexigenic effects. Last, by single-cell profiling, we identify a specialized PVNOT neuronal subpopulation with increased κ-opioid signaling under an HFHS diet, which restrains their CCK-evoked activation. In sum, we document a (patho)mechanism by which PVNOT signaling uncouples a gut-brain satiation pathway under obesogenic conditions.

Cultured Vagal Afferent Neurons as Sensors for Intestinal Effector Molecules

The gut-brain axis embodies the bi-directional communication between the gastrointestinal tract and the central nervous system (CNS), where vagal afferent neurons (VANs) serve as sensors for a variety of gut-derived signals. The gut is colonized by a large and diverse population of microorganisms that communicate via small (effector) molecules, which also act on the VAN terminals situated in the gut viscera and consequently influence many CNS processes. However, the convoluted in vivo environment makes it difficult to study the causative impact of the effector molecules on VAN activation or desensitization. Here, we report on a VAN culture and its proof-of-principle demonstration as a cell-based sensor to monitor the influence of gastrointestinal effector molecules on neuronal behavior. We initially compared the effect of surface coatings (poly-L-lysine vs. Matrigel) and culture media composition (serum vs. growth factor supplement) on neurite growth as a surrogate of VAN regeneration following tissue harvesting, where the Matrigel coating, but not the media composition, played a significant role in the increased neurite growth. We then used both live-cell calcium imaging and extracellular electrophysiological recordings to show that the VANs responded to classical effector molecules of endogenous and exogenous origin (cholecystokinin serotonin and capsaicin) in a complex fashion. We expect this study to enable platforms for screening various effector molecules and their influence on VAN activity, assessed by their information-rich electrophysiological fingerprints.

5-HT3 Receptors on Mitochondria Influence Mitochondrial Function

The 5-hydroxytryptamine 3 (5-HT₃) receptor belongs to the pentameric ligand-gated cation channel superfamily. Humans have five different 5-HT₃ receptor subunits: A to E. The 5-HT₃ receptors are located on the cell membrane, but a previous study suggested that mitochondria could also contain A subunits. In this article, we explored the distribution of 5-HT₃ receptor subunits in intracellular and cell-free mitochondria. Organelle prediction software supported the localization of the A and E subunits on the inner membrane of the mitochondria. We transiently transfected HEK293T cells that do not natively express the 5-HT₃ receptor with an epitope and fluorescent protein-tagged 5HT3A and 5HT3E subunits. Fluorescence microscopy and cell fractionation indicated that both subunits, A and E, localized to the mitochondria, while transmission electron microscopy revealed the location of the subunits on the mitochondrial inner membrane, where they could form heteromeric complexes. Cell-free mitochondria isolated from cell culture media colocalized with the fluorescent signal for A subunits. The presence of A and E subunits influenced changes in the membrane potential and mitochondrial oxygen consumption rates upon exposure to serotonin; this was inhibited by pre-treatment with ondansetron. Therefore, it is likely that the 5-HT₃ receptors present on mitochondria directly impact mitochondrial function and that this may have therapeutic implications.

Brainstem astrocytes control homeostatic regulation of caloric intake

Prolonged high-fat diet (HFD) exposure is associated with hyperphagia, excess caloric intake and weight gain. After initial exposure to a HFD, a brief (24-48 h) period of hyperphagia is followed by the regulation of caloric intake and restoration of energy balance within an acute (3-5 day) period. Previous studies have demonstrated this occurs via a vagally mediated signalling cascade that increases glutamatergic transmission via activation of NMDA receptors located on gastric-projecting neurons of the dorsal motor nucleus of the vagus (DMV). The present study used electrophysiological recordings from thin brainstem slice preparations, in vivo recordings of gastric motility and tone, measurement of gastric emptying rates, and food intake studies to investigate the hypothesis that activation of brainstem astrocytes in response to acute HFD exposure is responsible for the increased glutamatergic drive to DMV neurons and the restoration of caloric balance. Pharmacological and chemogenetic inhibition of brainstem astrocytes reduced glutamatergic signalling and DMV excitability, dysregulated gastric tone and motility, attenuated the homeostatic delay in gastric emptying, and prevented the decrease in food intake that is observed during the period of energy regulation following initial exposure to HFD. Understanding the mechanisms involved in caloric regulation may provide critical insights into energy balance as well as into the hyperphagia that develops as these mechanisms are overcome. KEY POINTS: Initial exposure to a high fat diet is associated with a brief period of hyperphagia before caloric intake and energy balance is restored. This period of homeostatic regulation is associated with a vagally mediated signalling cascade that increases glutamatergic transmission to dorsal motor nucleus of the vagus (DMV) neurons via activation of synaptic NMDA receptors. The present study demonstrates that pharmacological and chemogenetic inhibition of brainstem astrocytes reduced glutamatergic signalling and DMV neuronal excitability, dysregulated gastric motility and tone and emptying, and prevented the regulation of food intake following high-fat diet exposure. Astrocyte regulation of glutamatergic transmission to DMV neurons appears to involve release of the gliotransmitters glutamate and ATP. Understanding the mechanisms involved in caloric regulation may provide critical insights into energy balance as well as into the hyperphagia that develops as these mechanisms are overcome.

NPFF signalling is critical for thermosensory and dietary regulation of thermogenesis

Thermogenesis is a centrally regulated physiological process integral for thermoregulation and energy homeostasis. However, the mechanisms and pathways involved remain poorly understood. Importantly, in this study we uncovered that in an environment of 28 °C that is within the mouse thermoneutral zone, lack of NPFF signalling leads to significant increases in energy expenditure, resting metabolic rate and brown adipose tissue (BAT) thermogenesis, which is associated with decreased body weight gain and lean tissue mass. Interestingly, when exposed to a high-fat diet (HFD) at 28 °C, Npff^-/- mice lost the high energy expenditure phenotype observed under chow condition and exhibited an impaired diet-induced thermogenesis. On the other hand, under conditions of increasing levels of thermal demands, Npff^-/- mice exhibited an elevated BAT thermogenesis at mild cold condition (22 °C), but initiated comparable BAT thermogenic responses as WT mice when thermal demand increased, such as an exposure to 4 °C. Together, these results reveal NPFF signalling as a novel and critical player in the control of thermogenesis, where it regulates thermosensory thermogenesis at warm condition and adjusts thermoregulation under positive energy balance to regulate diet-induced thermogenesis.

Oxytocin and cardiometabolic interoception: Knowing oneself affects ingestive and social behaviors

Maintaining homeostasis while navigating one's environment involves accurately assessing and interacting with external stimuli while remaining consciously in tune with internal signals such as hunger and thirst. Both atypical social interactions and unhealthy eating patterns emerge as a result of dysregulation in factors that mediate the prioritization and attention to salient stimuli. Oxytocin is an evolutionarily conserved peptide that regulates attention to exteroceptive and interoceptive stimuli in a social environment by functioning in the brain as a modulatory neuropeptide to control social behavior, but also in the periphery as a hormone acting at oxytocin receptors (Oxtr) expressed in the heart, gut, and peripheral ganglia. Specialized sensory afferent nerve endings of Oxtr-expressing nodose ganglia cells transmit cardiometabolic signals via the Vagus nerve to integrative regions in the brain that also express Oxtr(s). These brain regions are influenced by vagal sensory pathways and coordinate with external events such as those demanding attention to social stimuli, thus the sensations related to cardiometabolic function and social interactions are influenced by oxytocin signaling. This review investigates the literature supporting the idea that oxytocin mediates the interoception of cardiovascular and gastrointestinal systems, and that the modulation of this awareness likewise influences social cognition. These concepts are then considered in relation to Autism Spectrum Disorder, exploring how atypical social behavior is comorbid with cardiometabolic dysfunction.

Tapping into 5-HT3 Receptors to Modify Metabolic and Immune Responses

5-hydroxytryptamine type 3 (5-HT₃) receptors are ligand gated ion channels, which clearly distinguish their mode of action from the other G-protein coupled 5-HT or serotonin receptors. 5-HT₃ receptors are well established targets for emesis and gastrointestinal mobility and are used as adjunct targets in treating schizophrenia. However, the distribution of these receptors is wider than the nervous system and there is potential that these additional sites can be targeted to modulate inflammatory and/or metabolic conditions. Recent progress in structural biology and pharmacology of 5-HT₃ receptors have provided profound insights into mechanisms of their action. These advances, combined with insights into clinical relevance of mutations in genes encoding 5-HT₃ subunits and increasing understanding of their implications in patient's predisposition to diseases and response to the treatment, open new avenues for personalized precision medicine. In this review, we recap on the current status of 5-HT₃ receptor-based therapies using a biochemical and physiological perspective. We assess the potential for targeting 5-HT₃ receptors in conditions involving metabolic or inflammatory disorders based on recent findings, underscoring the challenges and limitations of this approach.

Brain-gut communication: vagovagal reflexes interconnect the two "brains"

The gastrointestinal tract has its own "brain," the enteric nervous system or ENS, that executes routine housekeeping functions of digestion. The dorsal vagal complex in the central nervous system (CNS) brainstem, however, organizes vagovagal reflexes and establishes interconnections between the entire neuroaxis of the CNS and the gut. Thus, the dorsal vagal complex links the "CNS brain" to the "ENS brain." This brain-gut connectome provides reflex adjustments that optimize digestion and assimilation of nutrients and fluid. Vagovagal circuitry also generates the plasticity and adaptability needed to maintain homeostasis to coordinate among organs and to react to environmental situations. Arguably, this dynamic flexibility provided by the vagal circuitry may, in some circumstances, lead to or complicate maladaptive disorders.

Gut-brain communication by distinct sensory neurons differently controls feeding and glucose metabolism

Sensory neurons relay gut-derived signals to the brain, yet the molecular and functional organization of distinct populations remains unclear. Here, we employed intersectional genetic manipulations to probe the feeding and glucoregulatory function of distinct sensory neurons. We reconstruct the gut innervation patterns of numerous molecularly defined vagal and spinal afferents and identify their downstream brain targets. Bidirectional chemogenetic manipulations, coupled with behavioral and circuit mapping analysis, demonstrated that gut-innervating, glucagon-like peptide 1 receptor (GLP1R)-expressing vagal afferents relay anorexigenic signals to parabrachial nucleus neurons that control meal termination. Moreover, GLP1R vagal afferent activation improves glucose tolerance, and their inhibition elevates blood glucose levels independent of food intake. In contrast, gut-innervating, GPR65-expressing vagal afferent stimulation increases hepatic glucose production and activates parabrachial neurons that control normoglycemia, but they are dispensable for feeding regulation. Thus, distinct gut-innervating sensory neurons differentially control feeding and glucoregulatory neurocircuits and may provide specific targets for metabolic control.

Reciprocal control of obesity and anxiety-depressive disorder via a GABA and serotonin neural circuit

The high comorbidity between obesity and mental disorders, such as depression and anxiety, often exacerbates metabolic and neurological symptoms significantly. However, neural mechanisms that underlie reciprocal control of feeding and mental states are largely elusive. Here we report that melanocortin 4 receptor (MC4R) neurons located in the dorsal bed nucleus of the stria terminus (dBNST) engage in the regulation of mentally associated weight gain by receiving GABAergic projections from hypothalamic AgRP neurons onto α5-containing GABAA receptors and serotonergic afferents onto 5-HT3 receptors. Chronic treatment with a high-fat diet (HFD) significantly blunts the hyperexcitability of AgRP neurons in response to not only hunger but also anxiety and depression-like stimuli. Such HFD-mediated desensitization reduces GABAergic outputs from AgRP neurons to downstream MC4RdBNST neurons, resulting in severe mental dysregulation. Genetic enhancement of the GABAAR-α5 or suppression of the 5-HT3R within the MC4RdBNST neurons not only abolishes HFD-induced anxiety and depression but also robustly reduces body weight by suppression of food intake. To gain further translational insights, we revealed that combined treatment of zonisamide (enhancing the GABAAR-α5 signaling) and granisetron (a selective 5-HT3R antagonist) alleviates mental dysfunction and yields a robust reversal of diet-induced obesity by reducing total calorie intake and altering food preference towards a healthy low-fat diet. Our results unveil a neural mechanism for reciprocal control of appetite and mental states, which culminates in a novel zonisamide-granisetron cocktail therapy for potential tackling the psychosis-obesity comorbidity.

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.

DMV extrasynaptic NMDA receptors regulate caloric intake in rats

Acute high-fat diet (aHFD) exposure induces a brief period of hyperphagia before caloric balance is restored. Previous studies have demonstrated that this period of regulation is associated with activation of synaptic N-methyl-D-aspartate (NMDA) receptors on dorsal motor nucleus of the vagus (DMV) neurons, which increases vagal control of gastric functions. Our aim was to test the hypothesis that activation of DMV synaptic NMDA receptors occurs subsequent to activation of extrasynaptic NMDA receptors. Sprague-Dawley rats were fed a control or high-fat diet for 3-5 days prior to experimentation. Whole-cell patch-clamp recordings from gastric-projecting DMV neurons; in vivo recordings of gastric motility, tone, compliance, and emptying; and food intake studies were used to assess the effects of NMDA receptor antagonism on caloric regulation. After aHFD exposure, inhibition of extrasynaptic NMDA receptors prevented the synaptic NMDA receptor-mediated increase in glutamatergic transmission to DMV neurons, as well as the increase in gastric tone and motility, while chronic extrasynaptic NMDA receptor inhibition attenuated the regulation of caloric intake. After aHFD exposure, the regulation of food intake involved synaptic NMDA receptor-mediated currents, which occurred in response to extrasynaptic NMDA receptor activation. Understanding these events may provide a mechanistic basis for hyperphagia and may identify novel therapeutic targets for the treatment of obesity.

Glutamatergic plasticity within neurocircuits of the dorsal vagal complex and the regulation of gastric functions

The meticulous regulation of the gastrointestinal (GI) tract is required for the coordination of gastric motility and emptying, intestinal secretion, absorption, and transit as well as for the overarching management of food intake and energy homeostasis. Disruption of GI functions is associated with the development of severe GI disorders and the alteration of food intake and caloric balance. Functional GI disorders as well as the dysregulation of energy balance and food intake are frequently associated with, or result from, alterations in the central regulation of GI control. The faithful and rapid transmission of information from the stomach and upper GI tract to second-order neurons of the nucleus of the tractus solitarius (NTS) relies on the delicate modulation of excitatory glutamatergic transmission, as does the relay of integrated signals from the NTS to parasympathetic efferent neurons of the dorsal motor nucleus of the vagus (DMV). Many studies have focused on understanding the physiological and pathophysiological modulation of these glutamatergic synapses, although their role in the control and regulation of GI functions has lagged behind that of cardiovascular and respiratory functions. The purpose of this review is to examine the current literature exploring the role of glutamatergic transmission in the DVC in the regulation of GI functions.

Central Neurocircuits Regulating Food Intake in Response to Gut Inputs-Preclinical Evidence

The regulation of energy balance requires the complex integration of homeostatic and hedonic pathways, but sensory inputs from the gastrointestinal (GI) tract are increasingly recognized as playing critical roles. The stomach and small intestine relay sensory information to the central nervous system (CNS) via the sensory afferent vagus nerve. This vast volume of complex sensory information is received by neurons of the nucleus of the tractus solitarius (NTS) and is integrated with responses to circulating factors as well as descending inputs from the brainstem, midbrain, and forebrain nuclei involved in autonomic regulation. The integrated signal is relayed to the adjacent dorsal motor nucleus of the vagus (DMV), which supplies the motor output response via the efferent vagus nerve to regulate and modulate gastric motility, tone, secretion, and emptying, as well as intestinal motility and transit; the precise coordination of these responses is essential for the control of meal size, meal termination, and nutrient absorption. The interconnectivity of the NTS implies that many other CNS areas are capable of modulating vagal efferent output, emphasized by the many CNS disorders associated with dysregulated GI functions including feeding. This review will summarize the role of major CNS centers to gut-related inputs in the regulation of gastric function with specific reference to the regulation of food intake.

Systemic serotonin inhibits brown adipose tissue sympathetic nerve activity via a GABA input to the dorsomedial hypothalamus, not via 5HT1A receptor activation in raphe pallidus

AIM

Serotonin (5-hydroxytryptamine, 5-HT), an important neurotransmitter and hormone, modulates many physiological functions including body temperature. We investigated neural mechanisms involved in the inhibition of brown adipose tissue (BAT) sympathetic nerve activity (SNA) and BAT thermogenesis evoked by 5-HT.

METHODS

Electrophysiological recordings, intravenous (iv) injections and nanoinjections in the brains of anaesthetized rats.

RESULTS

Cooling-evoked increases in BAT SNA were inhibited by the intra-rostral raphé pallidus (rRPa) and the iv administration of the 5-HT_1A receptor agonist, 8-OH-DPAT or 5-HT. The intra-rRPa 5-HT, the intra-rRPa and the iv 8-OH-DPAT, but not the iv 5-HT-induced inhibition of BAT SNA were prevented by nanoinjection of a 5-HT_1A receptor antagonist in the rRPa. The increase in BAT SNA evoked by nanoinjection of NMDA in the rRPa was not inhibited by iv 5-HT, indicating that iv 5-HT does not inhibit BAT SNA by acting in the rRPa or in the sympathetic pathway distal to the rRPa. In contrast, under a warm condition, blockade of 5HT_1A receptors in the rRPa increased BAT SNA and BAT thermogenesis, suggesting that endogenous 5-HT in the rRPa contributes to the suppression of BAT SNA and BAT thermogenesis. The increases in BAT SNA and BAT thermogenesis evoked by nanoinjection of NMDA in the dorsomedial hypothalamus (DMH) were inhibited by iv 5-HT, but those following bicuculline nanoinjection in the DMH were not inhibited.

CONCLUSIONS

The systemic 5-HT-induced inhibition of BAT SNA requires a GABAergic inhibition of BAT sympathoexcitatory neurones in the DMH. In addition, during warming, 5-HT released endogenously in rRPa inhibits BAT SNA.

Stress-induced modulation of vagal afferents

Vagally dependent gastric functions, including motility, tone, compliance, and emptying rate, play an important role in the regulation of food intake and satiation. Vagal afferent fibers relay sensory information from the stomach, including meal-related information, centrally and initiate co-ordinated autonomic efferent responses that regulate upper gastrointestinal responses. The purpose of this mini-review is to highlight several recent studies which have uncovered the remarkable degree of neuroplasticity within gastric mechanosensitive vagal afferents and the recent study by Li et al, in this issue of Neurogastroenterology and Motility, who show that the mechanosensitivity of gastric vagal afferents is dysregulated in a murine model of chronic stress. The authors demonstrate that both gastric mucosal and tension afferents are hypersensitive following chronic stress, and responses to mucosal stroking and muscle stretch are enhanced significantly. As gastric distension and volumetric signaling is important in satiety signaling and meal termination, this may provide a mechanistic basis for the gastric hypersensitivity associated with stress-associated clinical problems such as functional dyspepsia.

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.

Role of astroglia in diet-induced central neuroplasticity

Obesity, characterized by increased adiposity that develops when energy intake outweighs expenditure, is rapidly becoming a serious health crisis that affects millions of people worldwide and is associated with severe comorbid disorders including hypertension, cardiovascular disease, and type II diabetes. Obesity is also associated with the dysregulation of central neurocircuits involved in the control of autonomic, metabolic, and cognitive functions. Systemic inflammation associated with diet-induced obesity (DIO) has been proposed to be responsible for the development of these comorbidities as well as the dysregulation of central neurocircuits. A growing body of evidence suggests, however, that exposure to a high-fat diet (HFD) may cause neuroinflammation and astroglial activation even before systemic inflammation develops, which may be sufficient to cause dysregulation of central neurocircuits involved in energy homeostasis before the development of obesity. The purpose of this review is to summarize the current literature exploring astroglial-dependent modulation of central circuits following exposure to HFD and DIO, including not only dysregulation of neurocircuits involved in energy homeostasis and feeding behavior, but also the dysregulation of learning, memory, mood, and reward pathways.

Sugar Responses of Human Enterochromaffin Cells Depend on Gut Region, Sex, and Body Mass

Gut-derived serotonin (5-HT) is released from enterochromaffin (EC) cells in response to nutrient cues, and acts to slow gastric emptying and modulate gastric motility. Rodent studies also evidence a role for gut-derived 5-HT in the control of hepatic glucose production, lipolysis and thermogenesis, and in mediating diet-induced obesity. EC cell number and 5-HT content is increased in the small intestine of obese rodents and human, however, it is unknown whether EC cells respond directly to glucose in humans, and whether their capacity to release 5-HT is perturbed in obesity. We therefore investigated 5-HT release from human duodenal and colonic EC cells in response to glucose, sucrose, fructose and α-glucoside (αMG) in relation to body mass index (BMI). EC cells released 5-HT only in response to 100 and 300 mM glucose (duodenum) and 300 mM glucose (colon), independently of osmolarity. Duodenal, but not colonic, EC cells also released 5-HT in response to sucrose and αMG, but did not respond to fructose. 5-HT content was similar in all EC cells in males, and colonic EC cells in females, but 3 to 4-fold higher in duodenal EC cells from overweight females (p < 0.05 compared to lean, obese). Glucose-evoked 5-HT release was 3-fold higher in the duodenum of overweight females (p < 0.05, compared to obese), but absent here in overweight males. Our data demonstrate that primary human EC cells respond directly to dietary glucose cues, with regional differences in selectivity for other sugars. Augmented glucose-evoked 5-HT release from duodenal EC is a feature of overweight females, and may be an early determinant of obesity.

Acute high-fat diet upregulates glutamatergic signaling in the dorsal motor nucleus of the vagus

Obesity is associated with dysregulation of vagal neurocircuits controlling gastric functions, including food intake and energy balance. In the short term, however, caloric intake is regulated homeostatically although the precise mechanisms responsible are unknown. The present study examined the effects of acute high-fat diet (HFD) on glutamatergic neurotransmission within central vagal neurocircuits and its effects on gastric motility. Sprague-Dawley rats were fed a control or HFD diet (14% or 60% kcal from fat, respectively) for 3-5 days. Whole cell patch-clamp recordings and brainstem application of antagonists were used to assess the effects of acute HFD on glutamatergic transmission to dorsal motor nucleus of the vagus (DMV) neurons and subsequent alterations in gastric tone and motility. After becoming hyperphagic initially, caloric balance was restored after 3 days following HFD exposure. In control rats, the non- N-methyl-d-aspartate (NMDA) receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), but not the NMDA receptor antagonist, amino-5-phosphonopentanoate (AP5), significantly decreased excitatory synaptic currents and action potential firing rate in gastric-projecting DMV neurons. In contrast, both AP5 and DNQX decreased excitatory synaptic transmission and action potential firing in acute HFD neurons. When microinjected into the brainstem, AP5, but not DNQX, decreased gastric motility and tone in acute HFD rats only. These results suggest that acute HFD upregulates NMDA receptor-mediated currents, increasing DMV neuronal excitability and activating the vagal efferent cholinergic pathway, thus increasing gastric tone and motility. Although such neuroplasticity may be a persistent adaptation to the initial exposure to HFD, it may also be an important mechanism in homeostatic regulation of energy balance. NEW & NOTEWORTHY Vagal neurocircuits are critical to the regulation of gastric functions, including satiation and food intake. Acute high-fat diet upregulates glutamatergic signaling within central vagal neurocircuits via activation of N-methyl-d-aspartate receptors, increasing vagal efferent drive to the stomach. Although it is possible that such neuroplasticity is a persistent adaptation to initial exposure to the high-fat diet, it may also play a role in the homeostatic control of feeding.

Perinatal high fat diet increases inhibition of dorsal motor nucleus of the vagus neurons regulating gastric functions

BACKGROUND

Previous studies suggest an increased inhibition of dorsal motor nucleus of the vagus (DMV) neurons following exposure to a perinatal high fat diet (PNHFD); the underlying neural mechanisms, however, remain unknown. This study assessed the effects of PNHFD on inhibitory synaptic inputs to DMV neurons and the vagally dependent control of gastric tone and motility.

METHODS

Whole-cell patch clamp recordings were made from DMV neurons in thin brainstem slices from Sprague-Dawley rats fed either a control diet or HFD (14 or 60% kcal from fat, respectively) from embryonic day 13 onwards; gastric tone and motility were recorded in in vivo anesthetized rats.

KEY RESULTS

The non-selective GABA_A antagonist, BIC (10 μmol L^-1 ), induced comparable inward currents in PNHFD and control DMV neurons, but a larger current in PNHFD neurons at higher concentrations (50 μmol L^-1 ). Differences were not apparent in neuronal responses to the phasic GABA_A antagonist, gabazine (GBZ), the extrasynaptic GABA_A agonist, THIP, the GABA transport blocker, nipecotic acid, or the gliotoxin, fluoroacetate, suggesting that PNHFD altered inhibitory transmission but not GABA_A receptor density or function, GABA uptake or glial modulation of synaptic strength. Similarly, the increase in gastric motility and tone following brainstem microinjection of low doses of BIC (1-10 pmoles) and GBZ (0.01-0.1 pmoles) were unchanged in PNHFD rats while higher doses of BIC (25 pmoles) induced a significantly larger increase in gastric tone compared to control.

CONCLUSIONS AND INFERENCES

These studies suggest that exposure to PNHFD increases the tonic inhibition of DMV neurons, possibly contributing to dysregulated vagal control of gastric functions.

High glucose increases action potential firing of catecholamine neurons in the nucleus of the solitary tract by increasing spontaneous glutamate inputs

Glucose is a crucial substrate essential for cell survival and function. Changes in glucose levels impact neuronal activity and glucose deprivation increases feeding. Several brain regions have been shown to respond to glucoprivation, including the nucleus of the solitary tract (NTS) in the brain stem. The NTS is the primary site in the brain that receives visceral afferent information from the gastrointestinal tract. The catecholaminergic (CA) subpopulation within the NTS modulates many homeostatic functions including cardiovascular reflexes, respiration, food intake, arousal, and stress. However, it is not known if they respond to changes in glucose. Here we determined whether NTS-CA neurons respond to changes in glucose concentration and the mechanism involved. We found that decreasing glucose concentrations from 5 mM to 2 mM to 1 mM, significantly decreased action potential firing in a cell-attached preparation, whereas increasing it back to 5 mM increased the firing rate. This effect was dependent on glutamate release from afferent terminals and required presynaptic 5-HT₃Rs. Decreasing the glucose concentration also decreased both basal and 5-HT₃R agonist-induced increase in the frequency of spontaneous glutamate inputs onto NTS-CA neurons. Low glucose also blunted 5-HT-induced inward currents in nodose ganglia neurons, which are the cell bodies of vagal afferents. The effect of low glucose in both nodose ganglia cells and in NTS slices was mimicked by the glucokinase inhibitor glucosamine. This study suggests that NTS-CA neurons are glucosensing through a presynaptic mechanism that is dependent on vagal glutamate release, 5-HT₃R activity, and glucokinase.

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.

Energy-dense diet triggers changes in gut microbiota, reorganization of gut‑brain vagal communication and increases body fat accumulation

Obesity is associated with consumption of energy-dense diets and development of systemic inflammation. Gut microbiota play a role in energy harvest and inflammation and can influence the change from lean to obese phenotypes. The nucleus of the solitary tract (NTS) is a brain target for gastrointestinal signals modulating satiety and alterations in gut-brain vagal pathway may promote overeating and obesity. Therefore, we tested the hypothesis that high-fat diet‑induced changes in gut microbiota alter vagal gut-brain communication associated with increased body fat accumulation. Sprague-Dawley rats consumed a low energy‑dense rodent diet (LFD; 3.1 kcal/g) or high energy‑dense diet (HFD, 5.24 kcal/g). Minocycline was used to manipulate gut microbiota composition. 16S Sequencing was used to determine microbiota composition. Immunofluorescence against IB4 and Iba1 was used to determine NTS reorganization and microglia activation. Nodose ganglia from LFD rats were isolated and co-cultured with different bacteria strains to determine neurotoxicity. HFD altered gut microbiota with increases in Firmicutes/Bacteriodetes ratio and in pro-inflammatory Proteobacteria proliferation. HFD triggered reorganization of vagal afferents and microglia activation in the NTS, associated with weight gain. Minocycline-treated HFD rats exhibited microbiota profile comparable to LFD animals. Minocycline suppressed HFD‑induced reorganization of vagal afferents and microglia activation in the NTS, and reduced body fat accumulation. Proteobacteria isolated from cecum of HFD rats were toxic to vagal afferent neurons in culture. Our findings show that diet‑induced shift in gut microbiome may disrupt vagal gut‑brain communication resulting in microglia activation and increased body fat accumulation.

The Vagus Nerve in Appetite Regulation, Mood, and Intestinal Inflammation

Although the gastrointestinal tract contains intrinsic neural plexuses that allow a significant degree of independent control over gastrointestinal functions, the central nervous system provides extrinsic neural inputs that modulate, regulate, and integrate these functions. In particular, the vagus nerve provides the parasympathetic innervation to the gastrointestinal tract, coordinating the complex interactions between central and peripheral neural control mechanisms. This review discusses the physiological roles of the afferent (sensory) and motor (efferent) vagus in regulation of appetite, mood, and the immune system, as well as the pathophysiological outcomes of vagus nerve dysfunction resulting in obesity, mood disorders, and inflammation. The therapeutic potential of vagus nerve modulation to attenuate or reverse these pathophysiological outcomes and restore autonomic homeostasis is also discussed.

Energy-dense diet triggers changes in gut microbiota, reorganization of gut‑brain vagal communication and increases body fat accumulation

Obesity is associated with consumption of energy-dense diets and development of systemic inflammation. Gut microbiota play a role in energy harvest and inflammation and can influence the change from lean to obese phenotypes. The nucleus of the solitary tract (NTS) is a brain target for gastrointestinal signals modulating satiety and alterations in gut-brain vagal pathway may promote overeating and obesity. Therefore, we tested the hypothesis that high-fat diet‑induced changes in gut microbiota alter vagal gut-brain communication associated with increased body fat accumulation. Sprague-Dawley rats consumed a low energy‑dense rodent diet (LFD; 3.1 kcal/g) or high energy‑dense diet (HFD, 5.24 kcal/g). Minocycline was used to manipulate gut microbiota composition. 16S Sequencing was used to determine microbiota composition. Immunofluorescence against IB4 and Iba1 was used to determine NTS reorganization and microglia activation. Nodose ganglia from LFD rats were isolated and co-cultured with different bacteria strains to determine neurotoxicity. HFD altered gut microbiota with increases in Firmicutes/Bacteriodetes ratio and in pro-inflammatory Proteobacteria proliferation. HFD triggered reorganization of vagal afferents and microglia activation in the NTS, associated with weight gain. Minocycline-treated HFD rats exhibited microbiota profile comparable to LFD animals. Minocycline suppressed HFD‑induced reorganization of vagal afferents and microglia activation in the NTS, and reduced body fat accumulation. Proteobacteria isolated from cecum of HFD rats were toxic to vagal afferent neurons in culture. Our findings show that diet‑induced shift in gut microbiome may disrupt vagal gut‑brain communication resulting in microglia activation and increased body fat accumulation.

Eating disinhibition and vagal tone moderate the postprandial response to glycemic load: a randomised controlled trial

Reducing the glycemic load (GL) of the diet may benefit appetite control but its utility is complicated by psychological influences on eating. Disinhibited behaviour, a risk factor for overconsumption, is characterized by reduced prefrontal cortex activity, which in turn modulates vagal tone; a phenomenon associated with glucoregulation. This double blind randomised controlled trial explored for the first time the influence of disinhibited eating and vagal tone (heart rate variability (HRV)) on hunger and the postprandial response to GL. Blood glucose (BG) and hunger were measured 30 and 150 min after consumption of water, glucose or isomaltulose (low glycemic sugar). After consuming glucose, independently of BMI or habitual diet, those with the highest levels of disinhibition had higher BG levels after thirty minutes (B = 0.192, 95% CI LL. 086, UL 0.297), and lower BG after one hundred and fifty minutes (B = −0.240, 95% CI LL −0.348, UL −0.131). BG was related to hunger but only in low disinhibited eaters. Disinhibited eaters were characterised by a reduced HRV which was related to greater BG excursions (B = 0.407, 95% CI LL 0.044, UL 1.134). These findings highlight novel mechanisms by which disinhibited eating leads to obesity and insulin resistance. This trial was registered at clinicaltrials.gov NCT02827318.

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.

A high-fat diet impairs cooling-evoked brown adipose tissue activation via a vagal afferent mechanism

In dramatic contrast to rats on a control diet, rats maintained on a high-fat diet (HFD) failed to activate brown adipose tissue (BAT) during cooling despite robust increases in their BAT activity following direct activation of their BAT sympathetic premotor neurons in the raphe pallidus. Cervical vagotomy or blockade of glutamate receptors in the nucleus of the tractus solitarii (NTS) reversed the HFD-induced inhibition of cold-evoked BAT activity. Thus, a HFD does not prevent rats from mounting a robust, centrally driven BAT thermogenesis; however, a HFD does alter a vagal afferent input to NTS neurons, thereby preventing the normal activation of BAT thermogenesis to cooling. These results, paralleling the absence of cooling-evoked glucose uptake in the BAT of obese humans, reveal a neural mechanism through which consumption of a HFD contributes to reduced energy expenditure and thus to weight gain.

High-Fat Diet-Induced Obesity Ablates Gastric Vagal Afferent Circadian Rhythms

Rats with high-fat diet (HFD)-induced obesity increase daytime eating, suggesting an alteration in circadian food intake mechanisms. Gastric vagal afferents (GVAs) respond to mechanical stimuli to initiate satiety. These signals are dampened in HFD mice and exhibit circadian variations inversely with food intake in lean mice. Furthermore, leptin shows circadian variation in its circulating level and is able to modulate GVA mechanosensitivity. However, whether leptin's ability to modulate GVAs occurs in a circadian manner is unknown. Therefore, we investigated whether changes in the circadian intake of food in HFD-induced obesity is associated with a disruption in GVA circadian rhythms. Eight-week-old male C57BL/6 mice were fed a standard laboratory diet (SLD) or a HFD for 12 weeks. A subgroup of SLD and HFD mice were housed in metabolic cages. After 12 weeks, ex vivo GVA recordings were taken at 3 h intervals starting at zeitgeber time 0 (ZT0) and stomach content was measured. After 12 weeks, HFD mice consumed more food during the light phase through larger and more frequent meals compared with SLD mice. SLD mice exhibited circadian fluctuation in stomach content, which peaked at ZT18 and reached a nadir at ZT9. At these time points, both tension and mucosal receptor mechanosensitivity were the lowest and highest, respectively. HFD mice exhibited little circadian variation in stomach content or GVA mechanosensitivity. Leptin potentiated mucosal receptor mechanosensitivity only in SLD mice and with reduced potency during the dark phase. In conclusion, loss of circadian variation in GVA signaling may underpin changes in eating behavior in HFD-induced obesity.

SIGNIFICANCE STATEMENT

Appropriate circadian control of food intake is vital for maintaining metabolic health. Diet-induced obesity is associated with strong circadian changes in food intake, but the contributing mechanisms have yet to be determined. Vagal afferents are involved in regulation of feeding behavior, particularly meal size, and have been shown to exhibit circadian fluctuation in mechanosensitivity, potentially allowing for time of day-specific levels of satiety signaling. Our study indicates that, in diet-induced obesity, these circadian fluctuations in gastric vagal afferent mechanosensitivity are lost. This was accompanied by increased light phase eating, particularly increased meal size. This is the first evidence that diet-induced disruption to vagal afferent signaling may cause a perturbation in circadian eating patterns.

Role of central vagal 5-HT3 receptors in gastrointestinal physiology and pathophysiology

Vagal neurocircuits are vitally important in the co-ordination and modulation of GI reflexes and homeostatic functions. 5-hydroxytryptamine (5-HT; serotonin) is critically important in the regulation of several of these autonomic gastrointestinal (GI) functions including motility, secretion and visceral sensitivity. While several 5-HT receptors are involved in these physiological responses, the ligand-gated 5-HT3 receptor appears intimately involved in gut-brain signaling, particularly via the afferent (sensory) vagus nerve. 5-HT is released from enterochromaffin cells in response to mechanical or chemical stimulation of the GI tract which leads to activation of 5-HT3 receptors on the terminals of vagal afferents. 5-HT3 receptors are also present on the soma of vagal afferent neurons, including GI vagal afferent neurons, where they can be activated by circulating 5-HT. The central terminals of vagal afferents also exhibit 5-HT3 receptors that function to increase glutamatergic synaptic transmission to second order neurons of the nucleus tractus solitarius within the brainstem. While activation of central brainstem 5-HT3 receptors modulates visceral functions, it is still unclear whether central vagal neurons, i.e., nucleus of the tractus solitarius (NTS) and dorsal motor nucleus of the vagus (DMV) neurons themselves also display functional 5-HT3 receptors. Thus, activation of 5-HT3 receptors may modulate the excitability and activity of gastrointestinal vagal afferents at multiple sites and may be involved in several physiological and pathophysiological conditions, including distention- and chemical-evoked vagal reflexes, nausea, and vomiting, as well as visceral hypersensitivity.