Mapping and Modulating the Stomach-Brain Neuroaxis

The stomach and the brain interact closely with each other. Their interactions are central to digestive functions and the “gut feeling”. The neural pathways that mediate the stomach-brain interactions include the vagus nerve and the thoracic nerve. Through these nerves, the stomach can relay neural signals to a number of brain regions that span a central gastric network. This gastric network allows the brain to monitor and regulate gastric physiology and allows the stomach to influence emotion and cognition. Impairment of this gastric network may lead to both gastric and neurological disorders, e.g., anxiety, gastroparesis, functional dyspepsia, and obesity. However, the structural constituents and functional roles of the central gastric network remain unclear. In my dissertation research, I leveraged complementary techniques to characterize the central gastric network in rats across a wide range of scales and different gastric states. I used functional magnetic resonance imaging (fMRI) to map blood-oxygen-level-dependent (BOLD) activity synchronized with gastric electrical activity and to map brain activations induced by electrical stimulation applied to the vagus nerve or its afferent terminals on the stomach. I also used neurophysiology to characterize gastric neurons in the brainstem in response to gastric electrical stimulation. My results suggest that gastric neurons in the brainstem are selective to the orientation of gastric electrical stimulation. This electrical stimulation can also evoke neural activity beyond the brainstem and drive fast blood oxygenation level dependent (BOLD) activity in the central gastric network, primarily covering the cingulate cortex, somatosensory cortex, motor cortex, and insular cortex. Stimulating the vagus nerve – the primary neural pathway between the stomach and the brain, can evoke BOLD responses across widespread brain regions partially overlapped with the brain network evoked by gastric electrical stimulation. BOLD activity within the gastric network is also coupled to intrinsic gastric activity. Specifically, gastric slow waves are synchronized with the BOLD activity in the central gastric network. The synchronization manifests itself as the phase-coupling between BOLD activity and gastric slow waves as well as the correlation between BOLD activity and power fluctuations of gastric slow waves. This synchronization is primarily supported by the vagus nerve and varies across the postprandial and fasting states. My dissertation research contributes to the foundation of mapping and characterizing the central and peripheral mechanisms of gastric interoception and sheds new light on where and how to stimulate the peripheral nerves to modulate stomach-brain interactions.PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/170007/1/jccao_1.pd

Interaction of psychological, physiological and neuronal processes in functional dyspepsia

Functional dyspepsia is characterized by postprandial fullness, early satiation, epigastric pain, bloating, and nausea symptoms in the absence of structural changes in the gastrointestinal tract. Numerous works have been performed to identify the peripheral characteristics of functional dyspepsia and its association with dyspeptic symptoms, including changes of gastric motility, visceral sensitivity, secretion of hormones, functions of immune system. However, the pathophysiological mechanisms involved and standard treatment strategies are still lacking. The role of the dysfunction of the brain-gut axis and the effect of the food ingestion in the gastrointestinal symptoms of functional dyspepsia patients have therefore been attracting more interest in recent years. How the food is processed differently in the peripheral and in the central nervous system in functional dyspepsia has, however, received little attention in comparison to other functional gastrointestinal disorders. In this thesis, we used various approaches to examine the physiological and neuronal mechanisms in functional dyspepsia patients. We commenced by summarizing previous functional neuroimaging studies to establish their limitations. To bridge the resulting research gap, we investigated physiological and attentional responses to visual food cues, and measured the altered brain activity before and after the food ingestion in functional dyspepsia patients. In the paper I, we reviewed the current status of brain research related to functional dyspepsia and were able to clearly show a knowledge gap regarding neural mechanisms of food-related factors in functional dyspepsia patients. In paper II, we introduced how to design the neuroimaging study and interpret the results of it to clinicians. In paper III, we report findings of an eyetracking and behavioral study on functional dyspepsia patients. The patients showed 1) greater dyspeptic symptoms even after ingestion of a lower calorie and food intake from standard breakfast; 2) decreased pleasantness ratings to food images; and 3) reduced visual attention to food images in comparison to healthy controls. In paper IV, we report findings of a functional magnetic resonance imaging study during meal ingestion (yoghurt with different fat content and label info) in functional dyspepsia patients. The patients showed 1) greater abdominal pain, burning, and discomfort after high fat labeled yogurt ingestion than after low fat labeled yogurt ingestion irrespective of fat content, 2) increased activity in occipital areas before and after ingestion irrespective of fat content and label and increased activity in the middle frontal gyrus before ingestion, 3) increased functional connectivity between the insula and the precuneus after ingestion of yogurt with low fat label, and 4) greater nausea-related increased functional connectivity between the insula and the occipital gyrus after ingestion of high fat yogurt than of low fat yogurt. Furthermore, bidirectional influences between quality of life and depression, as mediated by dyspeptic symptoms and the impact of food craving on the amplitude of brain activity in the middle frontal gyrus, as mediated by depression in functional dyspepsia patients were recorded. In conclusion, the abnormal dietary behavior, reduced positive emotional response and visual attention to food images, and the role of cognitive perception of fat on the aggravation of dyspeptic symptoms should be considered in clinics and in research for functional dyspepsia

Chemosensory dysfunction as a marker of global disease: Investigating the role of taste and smell signalling in obesity and COVID-19

The work in this thesis examines chemosensory dysfunction related to obesity and COVID-19, two global pandemics markedly impacting on health. Following the COVID-19 outbreak, reports emerged that loss of smell and/or taste may be caused by SARS-CoV-2. The work in Chapter 2 was undertaken before loss of smell and/or taste were recognised COVID-19 symptoms and aimed to determine the seroprevalence of SARS-CoV-2 antibodies in people with acute taste and/or smell loss, characterise the loss of chemosensory function and identify factors affecting their recovery. Overall, 78% of people with taste and/or smell loss had positive SARS-CoV-2 IgG/IgM antibodies. Female sex and altered smell/taste perceptions were identified as predictors for persistent loss of sense of taste and/or smell and long COVID. Furthermore, objective smell testing and quantitative MRI brain imaging were undertaken to investigate the underlying pathophysiology. Early results suggest ongoing neuroinflammation in people with persistent smell loss. Obesity, a chronic disease with multiple associated co-morbidities, is associated with chemosensory dysfunction, particularly toward dietary fat. The obesity section of this thesis used different modalities, including functional taste assessment, salivary and circulating biomarkers and functional brain imaging to characterise chemosensory dysfunction in obesity. Findings from these studies demonstrated a reduced ability to taste fat in the fed state, as well as increased taste-stimulated activity in reward-related brain regions in people with obesity. Altered adipocytokines and inflammation are postulated to underlie the increased risk of critical illness from COVID-19 in people living with obesity. In Chapter 6 inflammatory adipocytokines and metabolomic markers were measured in people with obesity before and after bariatric surgery. A substantial inter-individual variability was identified in circulating levels of these markers, which may underlie some of the susceptibility to infection-induced critical illness. Importantly, the results indicated improvement in inflammation markers following bariatric surgery, suggesting the potential for reducing obesity-associated risks

Investigating Serotonin Receptor Expression in Single Homologous Neurons Underlying Independently Evolved and Species-Specific Behaviors

Serotonin (5-HT) receptors modulate neuronal and synaptic properties, altering the functional output of neural circuits. Changing the functions of a neural circuit can alter behavior. Over evolutionary time, species differences in neuromodulation could allow for species-specific behaviors to evolve. To investigate this idea, this dissertation compared neuromodulatory receptor gene expression underlying species-specific swimming behaviors in sea slugs. The sea slug Tritonia diomedea (Mollusca, Gastropoda, Nudipleura, Nudibranchia), performs a rhythmic dorsal-ventral (DV) escape swim behavior. The behavior is controlled by a central pattern generator (CPG), composed of a small number of large, identifiable neurons. During swimming, 5-HT enhances the synaptic strength of a neuron in the swim CPG, called C2. In contrast, the nudibranch Hermissenda crassicornis does not swim in this manner. It has C2 homologues, and 5-HT is present, however, 5-HT does not modulate C2 synaptic strength. Pleurobranchaea californica, a Nudipleura species belonging to a sister clade of Nudibranchia, swims with DV flexions, although in this species swimming varies within individuals. 5-HT enhances Pleurobranchaea C2 homologue synaptic strength in swimming animals, only. Phylogenetic analysis showed that Tritonia and Pleurobranchaea independently evolved DV-swimming. Thus, there is a correlation between independently evolved swimming and serotonergic modulation of C2 homologues. It was hypothesized that 5-HT receptor differences in C2 neurons underlie species-specific swimming and modulation. To test this hypothesis, 5-HT receptor genes were identified in each species. A total of seven receptor subtypes, from five gene families, were found to be expressed in the brains of each species. Using single-cell quantitative PCR (qPCR), 5-HT receptor expression profiles were determined in C2 homologues. Genes known as 5-HT2a and 5-HT7 were expressed in C2 homologues from Tritonia and swimming Pleurobranchaea, only. Single-neuron transcriptome sequencing verified these results. The expression profiles of neuromodulatory receptor genes in single, homologous neurons correlated with species-specific swimming and modulation. The results illustrate how differences in neuromodulatory gene expression may alter the functional output of homologous neural structures, shedding light on a means by which neuromodulation can alter the brain to facilitate the evolution of species-specific behaviors. Evolution, Mollusc, Neuromodulation, Serotonin, Receptor, Behavior, Next-Generation Sequencing, Transcriptomic

Metabolic effects of the gastrointestinal hormone secretin with focus on brown adipose tissue

In recent years, brown adipose tissue (BAT) has been of interest in metabolic research, because its activity is associated with reduced cardiovascular risk, insulin resistance and obesity. Since cardiovascular disease accounts for the greatest cause of death globally, new treatments and methods of prevention are needed. BAT can burn fat instead of storing it, but it is likely that energy dissipation alone does not explain its health benefits. Meal induced BAT thermogenesis has recently been shown in humans, but its regulators and significance have never been pursued in detail. Since some gastric hormones have already shown both cardioprotective and appetite suppressing effects, they provide an interesting target for further studies. Secretin is the oldest known hormone and belongs to a family of gastrointestinal peptides, secreted during feeding. The aim of this thesis was to study the metabolic effects of secretin beyond its well-known pancreatic exocrine stimulation. Mouse models were first implemented by collaborators at the Technical University of Munich to investigate the potential of using secretin to activate BAT thermogenesis and induce satiation. A placebo-controlled crossover study was then conducted at the Turku PET Centre with healthy male volunteers to study BAT activation, but also the broader pleiotropic effects of secretin. Tissue metabolic activity was quantified with positron emission tomography, using a fluoride labelled glucose tracer. Appetite was studied with functional magnetic resonance imaging. Mouse models showed that secretin directly activates BAT thermogenesis through binding to the secretin receptor and that BAT thermogenesis in turn suppresses appetite. This novel satiation stimulating gut – BAT – brain axis was then shown in this thesis work to also translate to humans. Furthermore, myocardial glucose uptake was increased by secretin in humans. Previous studies have shown increased cardiac output by secretin and this thesis supports an inotropic effect. All in all, these results indicate that further clinical trials on secretin are warranted, as it could have potential applications in weight control and both preventing and treating heart disease.Ruoansulatushormoni sekretiinin aineenvaihdunnalliset vaikutukset. Painopiste ruskeassa rasvakudoksessa. Ruskea rasva on herättänyt kiinnostusta viime vuosina, koska sen aktiivisuuden on osoitettu olevan yhteydessä insuliiniherkkyyteen, normaaliin painoon ja pienentyneeseen sydän- ja verisuonitauti riskiin. Näistä viimeinen on suurin kuolinsyy maailmanlaajuisesti. Ruskea rasva polttaa rasvaa sen varastoimisen sijaan, mutta todennäköisesti sen terveysedut eivät johdu ainoastaan lisääntyneestä energiankulutuksesta. Ruokailu aktivoi ruskeaa rasvaa, mutta ilmiön säätelyä ja merkitystä ei ole aiemmin selvitetty tarkasti. Ruoansulatushormonit muodostavat mielenkiintoisen tutkimuskohteen ruskean rasvan aktivoimisen suhteen, koska osalla niistä tiedetään olevan sydäntä suojaavia ja ruokahalua vähentäviä vaikutuksia. Sekretiini on vanhin tunnettu hormoni. Sitä erittyy verenkiertoon ruokaillessa ja se aktivoi haiman eksokriinistä eritystä. Tässä väitöskirjassa tutkittiin sekretiinin muita mahdollisia aineenvaihdunnallisia vaikutuksia. Sen vaikutus ruskean rasvan aktivaatioon ja ruokahaluun selvitettiin ensin hiirimalleilla Münchenin teknillisessä yliopistossa, jonka jälkeen Turun PET-keskuksessa toteutettiin lumekontrolloitu, tapaus-ristikkäistutkimus terveillä miehillä. Kudosten aineenvaihduntaa tutkittiin fluorileimatulla glukoosimerkkiaineella ja positroniemissiotomografialla. Ruokahalua tutkittiin funktionaalisella magneettikuvantamisella. Hiirimallilla osoitettiin, että sekretiini aktivoi suoraan ruskean rasvan lämmöntuotantoa sitoutumalla sekretiinireseptoriin, mikä puolestaan johti ruokahalun vähenemiseen. Tämä uusi ruokahalua säätelevä mekanismi osoitettiin myös ihmisillä. Sen lisäksi sekretiini lisäsi ihmisillä sydämen glukoosin soluunottoa. Aikaisemmissa tutkimuksissa on osoitettu, että sekretiini lisää sydämen minuuttitilavuutta, joten väitöstyö vahvistaa, että sekretiinillä on inotrooppinen vaikutus. Tulokset osoittavat, että sekretiiniä tulisi tutkia kliinisesti laajemmin, sillä se voisi auttaa painonhallinnassa, sekä estää ja hoitaa sydänsairauksia

Behavioural and neuroimaging studies of food reward after bariatric surgery for obesity

BACKGROUND Roux-en-Y gastric bypass (RYGB) surgery is the most effective treatment for obesity and has greater efficacy for weight loss than gastric banding (BAND) surgery. The superior weight loss seen after RYGB may result from profoundly different effects on food hedonics and reward brought about by physiological changes secondary to the distinct manipulations of gut anatomy. AIMS To compare body mass index (BMI) matched patients after RYGB or BAND surgery and unoperated controls using comprehensive phenotyping of brain structure and function, eating behaviour and metabolism. METHODS In these cross-sectional studies, un-operated controls and patients after RYGB and BAND surgery had functional and anatomical neuroimaging of food reward systems. Reward responses to food were assessed with a functional magnetic resonance imaging (fMRI) food picture evaluation task. Anatomical differences in grey and white matter were assessed using voxel-based morphometry (VBM) and diffusion tensor imaging (DTI). Eating behaviour, food appeal and palatability, potential mediators, and post-ingestive effects were compared between groups using questionnaires, test meals, food diaries and assay of plasma hormones and metabolites. Surgical patients were compared in both the fasted and fed state, and after administration of the somatostatin analogue, Octreotide, to suppress anorexigenic gut hormone responses after RYGB. RESULTS Obese patients after RYGB had healthier gut-brain-hedonic responses to food than patients after BAND surgery. RYGB patients had lower activation than BAND patients in brain reward systems, particularly to high-calorie foods, including the orbitofrontal cortex, amygdala, caudate nucleus, nucleus accumbens and hippocampus. This was associated with lower palatability and appeal of high-calorie foods, and healthier eating behaviour, including less fat intake, in RYGB compared to BAND patients and/or BMI-matched unoperated controls. These differences were not explicable by differences in hunger or psychological traits between the surgical groups, or by differences in brain structure as measured by VBM and DTI. However anorexigenic plasma gut hormones (GLP-1 and PYY), plasma bile acids and symptoms of dumping syndrome were increased in RYGB patients. Octreotide increased nucleus accumbens activation to food pictures, increased food appeal and decreased post-meal satiety in patients after RYGB, but not BAND surgery. The preliminary nature of this small study precludes extensive interpretation especially of the difference between surgical groups. Patients in the operated groups (RYGB and BAND) had lower grey matter density in areas involved in reward processing, including the amygdala, nucleus accumbens and hippocampus compared to BMI-matched controls. There was no difference between the groups in white matter tract integrity. CONCLUSIONS Identification of these differences in the gut-brain axis and hence food hedonic responses as a result of altered gut anatomy/physiology provides a novel explanation for the more favorable long-term weight loss seen after RYGB than BAND surgery. This supports targeting of gut-brain reward systems for future treatments of obesity.Open Acces

Obesity: Current and Potential Pharmacotherapeutics and Targets

Obesity is a global epidemic that contributes to a number of health complications including cardiovascular disease, type 2 diabetes, cancer and neuropsychiatric disorders. Pharmacotherapeutic strategies to treat obesity are urgently needed. Research over the past two decades has increased substantially our knowledge of central and peripheral mechanisms underlying homeostatic energy balance. Homeostatic mechanisms involve multiple components including neuronal circuits, some originating in hypothalamus and brain stem, as well as peripherally-derived satiety, hunger and adiposity signals that modulate neural activity and regulate eating behavior. Dysregulation of one or more of these homeostatic components results in obesity. Coincident with obesity, reward mechanisms that regulate hedonic aspects of food intake override the homeostatic regulation of eating. In addition to functional interactions between homeostatic and reward systems in the regulation of food intake, homeostatic signals have the ability to alter vulnerability to drug abuse. Regarding the treatment of obesity, pharmacological monotherapies primarily focus on a single protein target. FDA-approved monotherapy options include phentermine (Adipex-P®), orlistat (Xenical®), lorcaserin (Belviq®) and liraglutide (Saxenda®). However, monotherapies have limited efficacy, in part due to the recruitment of alternate and counter-regulatory pathways. Consequently, a multi-target approach may provide greater benefit. Recently, two combination products have been approved by the FDA to treat obesity, including phentermine/topiramate (Qsymia®) and naltrexone/bupropion (Contrave®). The current review provides an overview of homeostatic and reward mechanisms that regulate energy balance, potential therapeutic targets for obesity and current treatment options, including some candidate therapeutics in clinical development. Finally, challenges in anti-obesity drug development are discussed

Oscillatory architecture of memory circuits

The coordinated activity between remote brain regions underlies cognition and memory function. Although neuronal oscillations have been proposed as a mechanistic substrate for the coordination of information transfer and memory consolidation during sleep, little is known about the mechanisms that support the widespread synchronization of brain regions and the relationship of neuronal dynamics with other bodily rhythms, such as breathing. During exploratory behavior, the hippocampus and the prefrontal cortex are organized by theta oscillations, known to support memory encoding and retrieval, while during sleep the same structures are dominated by slow oscillations that are believed to underlie the consolidation of recent experiences. The expression of conditioned fear and extinction memories relies on the coordinated activity between the mPFC and the basolateral amygdala (BLA), a neuronal structure encoding associative fear memories. However, to date, the mechanisms allowing this long-range network synchronization of neuronal activity between the mPFC and BLA during fear behavior remain virtually unknown. Using a combination of extracellular recordings and open- and closed-loop optogenetic manipulations, we investigated the oscillatory and coding mechanisms mediating the organization and coupling of the limbic circuit in the awake and asleep brain, as well as during memory encoding and retrieval. We found that freezing, a behavioral expression of fear, is tightly associated with an internally generated brain state that manifests in sustained 4Hz oscillatory dynamics in prefrontal-amygdala circuits. 4Hz oscillations accurately predict the onset and termination of the freezing state. These oscillations synchronize prefrontal-amygdala circuits and entrain neuronal activity to dynamically regulate the development of neuronal ensembles. This enables the precise timing of information transfer between the two structures and the expression of fear responses. Optogenetic induction of prefrontal 4Hz oscillations promotes freezing behavior and the formation of long-lasting fear memory, while closed-loop phase specific manipulations bidirectionally modulate fear expression. Our results unravel a physiological signature of fear memory and identify a novel internally generated brain state, characterized by 4Hz oscillations. This oscillation enables the temporal coordination and information transfer in the prefrontal-amygdala circuit via a phase-specific coding mechanism, facilitating the encoding and expression of fear memory. In the search for the origin of this oscillation, we focused our attention on breathing, the most fundamental and ubiquitous rhythmic activity in life. Using large-scale extracellular recordings from a number of structures, including the medial prefrontal cortex, hippocampus, thalamus, amygdala and nucleus accumbens in mice we identified and characterized the entrainment by breathing of a host of network dynamics across the limbic circuit. We established that fear-related 4Hz oscillations are a state-specific manifestation of this cortical entrainment by the respiratory rhythm. We characterized the translaminar and transregional profile of this entrainment and demonstrated a causal role of breathing in synchronizing neuronal activity and network dynamics between these structures in a variety of behavioral scenarios in the awake and sleep state. We further revealed a dual mechanism of respiratory entrainment, in the form of an intracerebral corollary discharge that acts jointly with an olfactory reafference to coordinate limbic network dynamics, such as hippocampal ripples and cortical UP and DOWN states, involved in memory consolidation. Respiration provides a perennial stream of rhythmic input to the brain. In addition to its role as the condicio sine qua non for life, here we provide evidence that breathing rhythm acts as a global pacemaker for the brain, providing a reference signal that enables the integration of exteroceptive and interoceptive inputs with the internally generated dynamics of the hippocampus and the neocortex. Our results highlight breathing, a perennial rhythmic input to the brain, as an oscillatory scaffold for the functional coordination of the limbic circuit, enabling the segregation and integration of information flow across neuronal networks