A gut-brain neural circuit for nutrient sensory transduction

Gut-brain axis and environmental factors in Parkinson's disease: bidirectional link between disease onset and progression

Parkinson's disease has long been considered a disorder that primarily affects the brain, as it is defined by the dopaminergic neurodegeneration in the substantia nigra and the brain accumulation of Lewy bodies containing α-synuclein protein. In recent decades, however, accumulating research has revealed that Parkinson's disease also involves the gut and uncovered an intimate and important bidirectional link between the brain and the gut, called the "gut-brain axis." Numerous clinical studies demonstrate that gut dysfunction frequently precedes motor symptoms in Parkinson's disease patients, with findings including impaired intestinal permeability, heightened inflammation, and distinct gut microbiome profiles and metabolites. Furthermore, α-synuclein deposition has been consistently observed in the gut of Parkinson's disease patients, suggesting a potential role in disease initiation. Importantly, individuals with vagotomy have a reduced Parkinson's disease risk. From these observations, researchers have hypothesized that α-synuclein accumulation may initiate in the gut and subsequently propagate to the central dopaminergic neurons through the gut-brain axis, leading to Parkinson's disease. This review comprehensively examines the gut's involvement in Parkinson's disease, focusing on the concept of a gut-origin for the disease. We also examine the interplay between altered gut-related factors and the accumulation of pathological α-synuclein in the gut of Parkinson's disease patients. Given the accessibility of the gut to both dietary and pharmacological interventions, targeting gut-localized α-synuclein represents a promising avenue for developing effective Parkinson's disease therapies.

Bidirectional regulation of the brain-gut-microbiota axis following traumatic brain injury

JOURNAL/nrgr/04.03/01300535-202508000-00002/figure1/v/2024-09-30T120553Z/r/image-tiff Traumatic brain injury is a prevalent disorder of the central nervous system. In addition to primary brain parenchymal damage, the enduring biological consequences of traumatic brain injury pose long-term risks for patients with traumatic brain injury; however, the underlying pathogenesis remains unclear, and effective intervention methods are lacking. Intestinal dysfunction is a significant consequence of traumatic brain injury. Being the most densely innervated peripheral tissue in the body, the gut possesses multiple pathways for the establishment of a bidirectional "brain-gut axis" with the central nervous system. The gut harbors a vast microbial community, and alterations of the gut niche contribute to the progression of traumatic brain injury and its unfavorable prognosis through neuronal, hormonal, and immune pathways. A comprehensive understanding of microbiota-mediated peripheral neuroimmunomodulation mechanisms is needed to enhance treatment strategies for traumatic brain injury and its associated complications. We comprehensively reviewed alterations in the gut microecological environment following traumatic brain injury, with a specific focus on the complex biological processes of peripheral nerves, immunity, and microbes triggered by traumatic brain injury, encompassing autonomic dysfunction, neuroendocrine disturbances, peripheral immunosuppression, increased intestinal barrier permeability, compromised responses of sensory nerves to microorganisms, and potential effector nuclei in the central nervous system influenced by gut microbiota. Additionally, we reviewed the mechanisms underlying secondary biological injury and the dynamic pathological responses that occur following injury to enhance our current understanding of how peripheral pathways impact the outcome of patients with traumatic brain injury. This review aimed to propose a conceptual model for future risk assessment of central nervous system-related diseases while elucidating novel insights into the bidirectional effects of the "brain-gut-microbiota axis."

A 40-year analysis of central neuroanatomical and neurochemical circuits mediating homeostatic intake and hedonic intake and preferences in rodents

This perspective review was written in response to the celebration of the 60th anniversary of the journal, Brain Research, and covers the evolving focus of my laboratory's work over 40 years in the neurobiological substrates of ingestive behavior in rodents. Following our initial work examining the effects of systemic and ventricular administration of general and selective opioid receptor agonists and antagonists on food intake under spontaneous, deprivation, glucoprivic and hedonic conditions, my laboratory in close collaboration with Drs. Gavril Pasternak and Ying-Xian Pan utilized an antisense oligodoxynucleotide knock-down technique affecting MOR-1, DOR-1, KOR-1 and ORL-1 genes as well as against G-protein subunits to study receptor mediation of opioid receptor agonist-induced feeding as well as feeding following regulatory challenges. Our laboratory employed intracerebral microinjection techniques to map limbic nucleus accumbens and ventral tegmental area central brain circuits mediating homeostatic and hedonic feeding responses through the use of selective mu, delta1, delta2 and kappa opioid receptor subtype agonists in combination with general and selective opioid, dopamineric, glutamatergic and GABAergic antagonists administered into the same site or the reciprocal site, allowing for the identification of a distributed brain network mediating these ingestive effects. Our laboratory in close collaboration with Dr. Anthony Sclafani then focused on the pharmacological, neuroanatomical and learning mechanisms related to the development of sugar- (sucrose, glucose and fructose) and fat- (corn oil) conditioned flavor preferences (CFP) in rats, and on murine genetic variance in food intake, preferences and the process of appetition.

GLP-1 and ghrelin inversely regulate insulin secretion and action in pancreatic islets, vagal afferents, and hypothalamus for controlling glycemia and feeding

Glucagon-like peptide-1 (GLP-1) was discovered as an incretin hormone, which is released from the intestine upon nutrient intake and stimulates insulin secretion from the pancreatic islet β-cells. Subsequently, its ability to suppress appetite was recognized. Ghrelin, discovered as the ligand for growth hormone secretagogue-receptor (GHS-R), is released from the stomach and produces appetite. Later, its ability to inhibit insulin secretion and elevate blood glucose was found. Thus, GLP-1 and ghrelin regulate insulin secretion and appetite toward opposite directions. The receptor agonists for GLP-1 and ghrelin have been developed and are now used to treat metabolic diseases, in which insulin plays a key role. However, underlying action mechanism and possible interplay of these hormones have remained elusive. Here, we describe that GLP-1 and ghrelin reciprocally regulate the insulin system. GLP-1 enhances and ghrelin suppresses insulin secretion in pancreatic β-cells. Moreover, GLP-1 cooperates with and ghrelin counteracts insulin action in the vagal afferent and hypothalamic arcuate nucleus (ARC) neurons, the interfaces between the peripheral metabolism and brain. Notably, ghrelin rises and works preprandially and GLP-1 rises and works postprandially. The interplay of ghrelin, GLP-1, and insulin leads to optimal circadian control of feeding, glycemia, and metabolism.

Interaction between nutritional factors and the enteric nervous system in inflammatory bowel diseases

The enteric nervous system (ENS) is a highly conserved, yet complicated network of neurons and glial cells located throughout the gut wall that controls digestive processes and gastrointestinal (GI) homeostasis. The intestinal epithelium, the immune system, and the gut microbiota are just a few examples of the cellular networks that the ENS interacts with on a variety of levels to maintain GI function. The presence or absence of nutrients in the intestinal lumen may cause short- and/or long-term changes in neurotransmitter expression, excitability, and neuronal survival, which ultimately affect gut motility, secretion, and permeability. Hence, the ENS should be identified as a key factor in initiating coordinated responses to nutrients. In this review we summarize current knowledge on nutrient-dependent ENS activity and how ENS secondary to nutrition may affect likelihood of developing inflammatory bowel disease. Our findings highlight that nutrients interact with enteroendocrine cells in the gut, triggering hormone secretion that plays a crucial role in signaling food-related information to the brain and regulating metabolic processes such as feeding behavior, insulin secretion, and energy balance; however, the complex interactions between nutrients, the ENS, and the immune system require further research to understand their contributions to GI disorders and potential therapeutic applications in treating obesity and metabolic diseases. Lay Summary: The enteric nervous system (ENS) controls digestion and interacts with nutrients in the gut to regulate processes like gut movement and hormone release, affecting metabolism and overall gut health. This review highlights the need for further research on how nutrient-ENS interactions contribute to conditions like inflammatory bowel disease, obesity, and metabolic disorders.

Nutrient detection pathways for food reinforcement and satiation

Ingested food is broken down into macronutrient components that are detected by gut-brain signaling pathways that play a vital role in feeding behavior. These specialized mechanisms both promote food intake (via appetition and food reinforcement pathways) and terminate food intake (via satiation pathways). Despite recent significant advances in our understanding of gut-brain signaling and behavior, questions remain about the distinct mechanisms mediating food reinforcement and satiation. Here, we review the receptors/transporters and gut-brain pathways that contribute to nutrient sensing and feeding behavior, and highlight key knowledge gaps that will guide future research on the complex gut-brain systems that influence food intake.

Pro-inflammatory macrophages contribute to developing comorbid anxiety-like behaviors through gastrointestinal vagal afferent signaling in experimental colitis mice

Anxiety symptoms are commonly observed in individuals with inflammatory bowel disease (IBD), but the mechanistic link between IBD and comorbid anxiety remains incompletely understood. Our previous study revealed that vagal gut-brain signaling contributes to driving comorbid anxiety-like behaviors in dextran sulfate sodium (DSS)-induced colitis mice, but how vagus nerve senses and transmits information to the brain in response to changes in the colonic microenvironment following DSS treatment remain elusive. Here, we identify a critical contribution of pro-inflammatory CD86+ macrophages to activate gut-innervating vagal afferents and ultimately drive anxiety-like behaviors in DSS-treated mice. An increased number of F4/80+ macrophages accumulated closely with gut-innervating vagal afferent fibers following DSS treatment. Depletion of macrophages alleviated DSS-induced anxiety-like behaviors, whereas peripheral delivery of lipopolysaccharide-activated M1 macrophages promoted anxiety-like behaviors, which were prevented by bilateral vagal afferent ablation. Moreover, differential expression levels of anxiety-like behaviors were positively correlated with neuronal activity changes in the nucleus tractus solitarius, locus coeruleus, and basolateral amygdala. Finally, treatment with either anti-α4β7 integrin antagonist vedolizumab or neutralizing anti-interleukin-1β monoclonal antibody effectively alleviated DSS-induced anxiety-like behaviors. Collectively, these findings unravel a mechanism of macrophage-to-vagus nerve communication via cytokine signaling responsible for comorbid anxiety associated with experimental colitis and suggest that pro-inflammatory CD86+ macrophages may represent a potential therapeutic target for psychological comorbidities in patients with IBD.

Galactooligosaccharides Attenuate Behavioural, Haematological and Immunological Abnormalities and Influence Gut Microbiota in Rats with Amygdala Hyperactivation Induced by Electrical Stimulation

The amygdala, especially its central nucleus (CeA), is one of the key brain structures regulating fear, anxiety and stress responses and is also involved in gut microbiota signal processing. Amygdala hyperactivity, as well as microbiota alterations, plays an important role in the pathophysiology of anxiety disorders, depression or post-traumatic stress disorder (PTSD). The present study determines whether 3 weeks of galactooligosaccharide (GOS) supplementation alleviates behavioural, haematological, immunological and gut microbiota disturbances induced by long-term electrical stimulation of the CeA in rats (Stim). The unsupplemented Stim group showed locomotor hyperactivity and higher anxiety (measured with an actometer and the elevated plus maze, respectively), as well as a decrease in white blood cells (WBCs), lymphocytes (LYMs), red blood cells (RBCs) and platelets (PLTs); an elevation of TNFα; a reduction in IL-10 concentration in plasma; and microbiota alterations as compared to the control (Sham) group. GOS supplementation alleviated all these Stim-induced adverse effects or even normalised them to the sham group level. The effect of GOS was comparable to citalopram and even more effective in WBC and PLT normalisation and IL-10 induction. The obtained results indicate the high therapeutic potential of GOS in anxiety and stress-related disorders. GOS supplementation may support conventional therapy or the prevention of PTSD, depression and anxiety disorders.

The gut-brain axis mechanism of normal appetite induced by kynurenic acid

Feeding is essential for both host-organism survival and gut-microbiota maintenance. Our research focuses on how kynurenic acid (KYNA), a gut-microbiota metabolite, regulates appetite during fasting. We find that fasting significantly raises KYNA levels in the intestine, which increases short-term food intake by inhibiting vagal afferent nerve in the nodose ganglion (NG) and activating AgRP neurons in arcuate nucleus (ARCAgRP). The orexigenic effects of KYNA are abolished by subdiaphragmatic vagotomy (sdVx), chemogenetic activation/inhibition of glutamatergic NG/ARCAgRP neurons, inhibiting the nucleus of the solitary tract (NTS) to ARCAgRP inputs, or knockdown of GPR35 (a KYNA receptor) in the intestinal vagal afferent nerve. Our data support a model in which KYNA acts through the GPR35 receptor to inhibit vagal afferent signaling and subsequently activate ARCAgRP neurons, which leads to increased food intake. These findings reveal a mechanism by which gut microbiota controls appetite during fasting, highlighting the complex relationship between microbial and host feeding behavior.

Smart Bioelectronics for Real-Time Diagnosis and Therapy of Body Organ Functions

Noncommunicable diseases (NCDs) associated with cardiovascular, neurological, and gastrointestinal disorders remain a leading cause of global mortality, sounding the alarm for the urgent need for better diagnostic and therapeutic solutions. Wearable and implantable biointegrated electronics offer a groundbreaking solution, combining real-time, high-resolution monitoring with innovative treatment capabilities tailored to specific organ functions. In this comprehensive review, we focus on the diseases affecting the brain, heart, gastrointestinal organs, bladder, and adrenal gland, along with their associated physiological parameters. Additionally, we provide an overview of the characteristics of these parameters and explore the potential of bioelectronic devices for in situ sensing and therapeutic applications and highlight the recent advancements in their deployment across specific organs. Finally, we analyze the current challenges and prospects of implementing closed-loop feedback control systems in integrated sensor-therapy applications. By emphasizing organ-specific applications and advocating for closed-loop systems, this review highlights the potential of future bioelectronics to address physiological needs and serves as a guide for researchers navigating the interdisciplinary fields of diagnostics, therapeutics, and personalized medicine.

GLP-1 receptor agonist exendin-4 suppresses food intake by inhibiting hindbrain orexigenic NPY neurons

Peripherally delivered glucagon-like peptide-1 (GLP-1)-based drugs suppress eating through their action in the brain. However, the specific neuronal mechanisms, especially their impacts on the orexigenic circuit, remain largely elusive. Neuropeptide Y (NPY) neurons in the nucleus tractus solitarius (NTS) are newly identified as orexigenic neurons with a potent eating-stimulating effect, but their responses to GLP-1 drugs are unknown. Through ex vivo electrophysiological recordings, we study the impacts of GLP-1 receptor (GLP-1R) agonist exendin-4 on NTSNPY neurons. We discovered that the GLP-1R agonist exendin-4 inhibits NTSNPY neuronal activity via GABAb receptors by augmenting presynaptic GABA release. We also explored the contribution of NTSNPY neurons to exendin-4-mediated eating suppression. Interestingly, chemogenetic activation of NTSNPY neurons effectively counteracted exendin-4-induced anorexigenic effect. Moreover, chemogenetic inhibition of NTSNPY neurons mimicked the eating-suppressing effect of exendin-4. Collectively, our findings highlight a population of orexigenic NTSNPY neurons that may be targeted by a GLP-1R agonist to suppress food intake, suggesting that this neuronal population has translational importance as a potential therapeutic target for obesity treatment.NEW & NOTEWORTHY This study discovers that the glucagon-like peptide-1 (GLP-1) receptor agonist exendin-4 indirectly inhibits the majority of orexigenic hindbrain NPY neurons via GABAb receptors by augmenting presynaptic GABA release. Chemogenetic activation of these NPY neurons effectively counteracts exendin-4 (Exn-4)-induced anorexigenic effect, whereas chemogenetic inhibition of them mimics the eating-suppressing effect of exendin-4. This study uncovers a mechanism by which Exn-4 inhibits orexigenic hindbrain NPY neurons, thereby providing new insights into how GLP-1 drugs suppress food intake.

Multi-omics analyses reveal fecal microbial community and metabolic alterations in finishing cattle fed probiotics-fermented distiller's grains diets

Distiller's grains (DG) are a potential source of animal feeds, and many studies have indicated positive regulatory roles of feeding DG diets in animal breeding. However, there is currently a dearth of research on the actions and underlying mechanisms of probiotics-fermented distiller's grains (FDG)-based diets in cattle breeding. This study aimed to assess the impact of integrating FDG into the diet of finishing cattle on their fecal microbial community and metabolites. Thirty Simmental crossbred cattle (local yellow cattle × Simmental cattle, 8.5 months old, 420.38 ± 68.11 kg) were selected and randomly divided into three dietary treatments, including the basal diet group (CON group), the FDG replacing 10% concentrate (FDG-10%) group, and the FDG replacing 20% concentrate (FDG-20%) group. 16S and ITS sequencing of fecal samples collected from each group on the 30th day of the formal feeding suggested that feeding FDG diets had little effect on the composition and diversity of fecal bacterial and fungal communities in finishing cattle. However, the relative abundance of cellulose-degrading bacteria, including the Christensenellaceae R-7 group and Ruminococcaceae family was significantly higher in both the FDG-20% vs CON comparison and the FDG-20% vs FDG-10% comparison. Besides, the FDG-10% group had a significant drop in the relative abundance of Aspergillus and a noteworthy increase in the relative abundance of Candida when compared to the CON group. Non-targeted metabolomics analysis showed that the addition of FDG modified the levels of organoheterocyclic compounds, lipids and lipid-like molecules, and benzenoids in the feces of finishing cattle and significantly enhanced the metabolic pathway of bile secretion. Further correlation analyses suggested a close association between the significantly differential fecal microbiota and metabolites. In conclusion, these results suggest that FDG supplementation has little effect on the structure and diversity of the fecal microbiota in finishing cattle, but alters intestinal metabolite profiles and influences bile secretion pathways by modulating the relative abundance of genera of fecal bacteria and fungi Christensenellaceae R-7 group, Lachnospiraceae_NK3A20_group, Mucor, and Candida. These findings provide a scientific theoretical basis for the use of FDG in animal feeds.

IMPORTANCE

Probiotics-fermented distiller's grains (FDG) are potential feed sources for livestock. Here, we investigated the effects of partially replacing concentrates with FDG on fecal bacterial and fungal community structure and metabolic profiles in finishing cattle. The results reveal that feeding FDG-based diets alters intestinal metabolite profiles and up-regulates bile secretion pathways through the regulation of relative abundance of certain fecal genera. These findings provide some new insights into clarifying the role and potential mechanisms of FDG diets and also offer a scientific basis for the development of FDG into functional feed resources.

Great minds think alike: Glutamatergic signaling in the second brain

The enteric nervous system is comprised of interconnected diverse cell types that control sensory and motor functions of the gut. In this issue of Neuron, Hamnett et al. demonstrate a novel role for distinct glutamatergic putative interneuron subtypes in colonic motility.

Nociceptive neurons promote gastric tumour progression via a CGRP-RAMP1 axis

Cancer cells have been shown to exploit neurons to modulate their survival and growth, including through the establishment of neural circuits within the central nervous system1-3. Here we report a distinct pattern of cancer-nerve interactions between the peripheral nervous system and gastric cancer. In multiple mouse models of gastric cancer, nociceptive nerves demonstrated the greatest degree of nerve expansion in an NGF-dependent manner. Neural tracing identified CGRP+ peptidergic neurons as the primary gastric sensory neurons. Three-dimensional co-culture models showed that sensory neurons directly connect with gastric cancer spheroids. Chemogenetic activation of sensory neurons induced the release of calcium into the cytoplasm of cancer cells, promoting tumour growth and metastasis. Pharmacological ablation of sensory neurons or treatment with CGRP inhibitors suppressed tumour growth and extended survival. Depolarization of gastric tumour membranes through in vivo optogenetic activation led to enhanced calcium flux in jugular nucleus complex and CGRP release, defining a cancer cell-peptidergic neuronal circuit. Together, these findings establish the functional connectivity between cancer and sensory neurons, identifying this pathway as a potential therapeutic target.

The neurobiology of overeating

Food intake serves to maintain energy homeostasis; however, overeating can result in obesity, which is associated with serious health complications. In this review, we explore the intricate relationship between overeating, obesity, and the underlying neurobiological mechanisms. We review the homeostatic and hedonic feeding systems, highlighting the role of the hypothalamus and reward systems in controlling food intake and energy balance. Dysregulation in both these systems leads to overeating, as seen in genetic syndromes and environmental models affecting appetite regulation when consuming highly palatable food. The concept of "food addiction" is examined, drawing parallels to drug addiction. We discuss the cellular substrate for addiction-related behavior and current pharmacological obesity treatments-in particular, GLP-1 receptor agonists-showcasing synaptic plasticity in the context of overeating and palatable food exposure. A comprehensive model integrating insights from addiction research is proposed to guide effective interventions for maladaptive feeding behaviors. Ultimately, unraveling the neurobiological basis of overeating holds promise for addressing the pressing public health issue of obesity.

Topological segregation of stress sensors along the gut crypt-villus axis

The crypt-villus structure of the small intestine serves as an essential protective barrier. The integrity of this barrier is monitored by the complex sensory system of the gut, in which serotonergic enterochromaffin (EC) cells play an important part1,2. These rare sensory epithelial cells surveil the mucosal environment for luminal stimuli and transmit signals both within and outside the gut3-6. However, whether EC cells in crypts and villi detect different stimuli or produce distinct physiological responses is unknown. Here we address these questions by developing a reporter mouse model to quantitatively measure the release and propagation of serotonin from EC cells in live intestines. Crypt EC cells exhibit a tonic low-level mode that activates epithelial serotonin 5-HT4 receptors to modulate basal ion secretion and a stimulus-induced high-level mode that activates 5-HT3 receptors on sensory nerve fibres. Both these modes can be initiated by the irritant receptor TRPA1, which is confined to crypt EC cells. The activation of TRPA1 by luminal irritants is enhanced when the protective mucus layer is compromised. Villus EC cells also signal damage through a distinct mechanism, whereby oxidative stress activates TRPM2 channels, which leads to the release of both serotonin and ATP and consequent excitation of sensory nerve fibres. This topological segregation of EC cell functionality along the mucosal architecture constitutes a mechanism for the surveillance, maintenance and protection of gut integrity under diverse physiological conditions.

Vagal Sensory Gut-Brain Pathways That Control Eating-Satiety and Beyond

The vagus nerve is the body's primary sensory conduit from gut to brain, traditionally viewed as a passive relay for satiety signals. However, emerging evidence reveals a far more complex system-one that actively encodes diverse aspects of meal-related information, from mechanical stretch to nutrient content, metabolic state, and even microbial metabolites. This review challenges the view of vagal afferent neurons (VANs) as simple meal-termination sensors and highlights their specialized subpopulations, diverse sensory modalities, and downstream brain circuits, which shape feeding behavior, metabolism, and cognition. We integrate recent advances from single-cell transcriptomics, neural circuit mapping, and functional imaging to examine how VANs contribute to gut-brain communication beyond satiety, including their roles in food reward and memory formation. By synthesizing the latest research and highlighting emerging directions for the field, this review provides a comprehensive update on vagal sensory pathways and their role as integrators of meal information.

Stress, microbiota, and the gut-brain axis in mental and digestive health

The prevailing mind-body dualism in contemporary medicine, rooted in reductionism and the fragmentation of knowledge, has impeded the development of a conceptual model that can adequately address the complexity of illnesses. Integrating biomedical data into a cohesive model that considers the mind-body-context interconnections is essential. This integration is not merely theoretical; rather, it has significant clinical implications. This is exemplified by chronic stress-related mental and digestive disorders. The onset and development of these disorders are intimately linked to chronic psychological stress via the brain-gut-microbiota axis. The present article examines the evidence and mechanisms indicating that stress is a primary factor and a potentiator of symptom severity in common mental health and digestive diseases, with a particular focus on human studies. However, due to space limitations, only a very general overview of preventive and therapeutic clinical strategies is provided. It is hoped that the recurring phrase, "Everything that happens to you is due to stress," will become more comprehensible to the physician after reading this manuscript.

The Gut-Brain Axis in Parkinson disease: Emerging Concepts and Therapeutic Implications

BACKGROUND

The gut-brain axis, i.e. the bidirectional communication system between the gut and the brain, has become of central importance in Parkinson disease (PD) research over the past 20 years.

AIMS

We aimed to describe the milestones of the gut-brain axis research in PD and the development of theories proposing the involvement of the gastrointestinal tract in PD pathogenesis.

METHODS

We searched PubMed using the terms 'gut-brain axis' AND 'Parkinson disease', and selected relevant articles to provide the foundation for reconstructing an historical overview of the gut-brain axis research in PD.

RESULTS

Mounting evidence from preclinical, clinical and post-mortem studies suggests that a subgroup of PD patients present with a range of prodromal symptoms (e.g., autonomic dysfunction, rapid eye movement sleep behaviour disorder) which reflect initial accumulation and later spread of pathological α-synuclein rostrally from the gastrointestinal tract ("body-first" PD). Through neural connections along the gut-brain axis, pathological α-synuclein may spread to the brain, producing clinically manifest disease. Recently, two mechanisms involving the gut-brain axis have attracted increasing attention for their role in PD pathogenesis and progression, namely the perturbation of the composition of the microorganisms living in the gut (the gut microbiome), and the dysfunction of enteroendocrine cells.

CONCLUSION

Treatments targeting the gut-brain axis, especially the gut microbiome and the enteroendocrine cells pathway, could potentially slow disease progression or even prevent disease onset. Among these, pre/probiotics, faecal microbiota transplantation, and glucagon-like peptide-1 receptor agonists, have entered advanced stages of clinical trials in humans and shown potential symptomatic and disease-modifying effects.

Gut Microbial Control of Neurotransmitters and Their Relation to Neurological Disorders: A Comprehensive Review

The study explores the vital role of gut microbiota in regulating neurotransmitters and its subsequent effects on brain function and mental health. It aims to unravel the mechanisms by which microbial metabolites influence neurotransmitter synthesis and signaling. The ultimate goal is to identify potential therapeutic strategies targeting gut microbiota for the management and treatment of neurological disorders, such as depression, autism spectrum disorder (ASD), anxiety, and Parkinson's disease. The review synthesizes current research on the gut-brain axis, focusing on the influence of gut microbial metabolites on key neurotransmitters, including dopamine, serotonin, and gamma-aminobutyric acid (GABA). It incorporates a multidisciplinary approach, linking microbiology, neurobiology, and clinical research. Each section presents an in-depth review of scientific studies, clinical trials, and emerging therapeutic strategies. The findings highlight the intricate interplay between gut microbiota and the central nervous system. Gut microbes significantly impact the synthesis and signaling of crucial neurotransmitters, which play a pivotal role in neurological health. Evidence supports the hypothesis that modulating gut microbiota can alter neurotransmitter output and alleviate symptoms associated with neurological disorders. Notable therapeutic potentials include microbiota-targeted interventions for managing depression, ASD, anxiety, and Parkinson's disease. This comprehensive analysis underscores the critical connection between gut microbiota and neurological health. By bridging gaps between microbiology, neurobiology, and clinical practice, the study opens avenues for innovative therapeutic approaches. It provides a valuable resource for researchers, clinicians, and students, emphasizing the need for continued investigation into gut microbiota's role in neurological disorders and its therapeutic potential.

Research progress on the kidney-gut-brain axis in brain dysfunction in maintenance hemodialysis patients

Maintenance hemodialysis (MHD) has become the primary renal replacement therapy for patients with end-stage renal disease. The kidney-gut-brain axis represents a communication network connecting the kidney, intestine and brain. In MHD patients, factors such as uremic toxins, hemodynamic changes, vascular damage, inflammation, oxidative stress, and intestinal dysbiosis in MHD patients refers to a range of clinical syndromes, including brain injury, and is manifested by conditions such as white matter disease, brain atrophy, cerebrovascular disease, cognitive impairment, depression, anxiety, and other behavioral or consciousness abnormalities. Numerous studies have demonstrated the prevalence of these brain disorders in MHD patients. Understanding the mechanisms of brain disorders in MHD patients, particularly through the lens of kidney-gut-brain axis dysfunction, offers valuable insights for future research and the development of targeted therapies. This article reviews the brain dysfunction associated with MHD, the impact of the kidney-brain axis, intestinal barrier damage, gut microbiota dysbiosis caused by MHD, and the role of the gut-brain axis in brain dysfunction.

Tryptophan and Its Metabolite Serotonin Impact Metabolic and Mental Disorders via the Brain-Gut-Microbiome Axis: A Focus on Sex Differences

The crisis of metabolic and mental disorders continues to escalate worldwide. A growing body of research highlights the influence of tryptophan and its metabolites, such as serotonin, beyond their traditional roles in neural signaling. Serotonin acts as a key neurotransmitter within the brain-gut-microbiome axis, a critical bidirectional communication network affecting both metabolism and behavior. Emerging evidence suggests that the gut microbiome regulates brain function and behavior, particularly through microbial influences on tryptophan metabolism and the serotonergic system, both of which are essential for normal functioning. Additionally, sex differences exist in multiple aspects of serotonin-mediated modulation within the brain-gut-microbiome axis, affecting feeding and affective behaviors. This review summarizes the current knowledge from human and animal studies on the influence of tryptophan and its metabolite serotonin on metabolic and behavioral regulation involving the brain and gut microbiome, with a focus on sex differences and the role of sex hormones. We speculate that gut-derived tryptophan and serotonin play essential roles in the pathophysiology that modifies neural circuits, potentially contributing to eating and affective disorders. We propose the gut microbiome as an appealing therapeutic target for metabolic and affective disorders, emphasizing the importance of understanding sex differences in metabolic and behavioral regulation influenced by the brain-gut-microbiome axis. The therapeutic targeting of the gut microbiota and its metabolites may offer a viable strategy for treating serotonin-related disorders, such as eating and affective disorders, with potential differences in treatment efficacy between men and women. This review would promote research on sex differences in metabolic and behavioral regulation impacted by the brain-gut-microbiome axis.

The Power of Exercise: Unlocking the Biological Mysteries of Peripheral-Central Crosstalk in Parkinson's Disease

BACKGROUND

Exercise is a widely recognized non-pharmacological treatment for Parkinson's Disease (PD). The bidirectional regulation between the brain and peripheral organs has emerged as a promising area of research, with the mechanisms by which exercise impacts PD closely linked to the interplay between peripheral signals and the central nervous system.

AIM OF REVIEW

This review aims to summarize the mechanisms by which exercise influences peripheral-central crosstalk to improve PD, discuss the molecular processes mediating these interactions, elucidate the pathways through which exercise may modulate PD pathophysiology, and identify directions for future research.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This review examines how exercise-induced cytokine release promotes neuroprotection in PD. It discusses how exercise can stimulate cytokine secretion through various pathways, including the gut-brain, muscle-brain, liver-brain, adipose-brain, and bone-brain axes, thereby alleviating PD symptoms. Additionally, the potential contributions of the heart-brain, lung-brain, and spleen-brain axes, as well as multi-axis crosstalk-such as the brain-gut-muscle and brain-gut-bone axes-are explored in the context of exercise therapy. The study highlights the need for further research into peripheral-central crosstalk and outlines future directions to address challenges in clinical PD therapy.

The gut-brain axis and pain signalling mechanisms in the gastrointestinal tract

Visceral pain is a major clinical problem and one of the most common reasons patients with gastrointestinal disorders seek medical help. Peripheral sensory neurons that innervate the gut can detect noxious stimuli and send signals to the central nervous system that are perceived as pain. There is a bidirectional communication network between the gastrointestinal tract and the nervous system that mediates pain through the gut-brain axis. Sensory neurons detect mechanical and chemical stimuli within the intestinal tissues, and receive signals from immune cells, epithelial cells and the gut microbiota, which results in peripheral sensitization and visceral pain. This Review focuses on molecular communication between these non-neuronal cell types and neurons in visceral pain. These bidirectional interactions can be dysregulated during gastrointestinal diseases to exacerbate visceral pain. We outline the anatomical pathways involved in pain processing in the gut and how cell-cell communication is integrated into this gut-brain axis. Understanding how bidirectional communication between the gut and nervous system is altered during disease could provide new therapeutic targets for treating visceral pain.

Gut Microbiota-Derived Extracellular Vesicles Influence Alcohol Intake Preferences in Rats

Growing preclinical and clinical evidence suggests a link between gut microbiota dysbiosis and problematic alcohol consumption. Extracellular vesicles (EVs) are key mediators involved in bacteria-to-host communication. However, their potential role in mediating addictive behaviour remains unexplored. This study investigates the role of gut microbiota-derived bacterial extracellular vesicles (bEVs) in driving high alcohol consumption. bEVs were isolated from the gut microbiota of a high alcohol-drinking rat strain (UChB rats), either ethanol-naïve or following chronic alcohol consumption and administered intraperitoneally or orally to alcohol-rejecting male and female Wistar rats. Both types of UChB-derived bEVs increased Wistar's voluntary alcohol consumption (three bottle choice test) up to 10-fold (p < 0.0001), indicating that bEVs are able and sufficient to transmit drinking behaviour across different rat strains. Molecular analysis revealed that bEVs administration did not induce systemic or brain inflammation in the recipient animals, suggesting that the increased alcohol intake triggered by UChB-derived bEVs operates through an inflammation-independent mechanism. Furthermore, we demonstrate that the vagus nerve mediates the bEV-induced increase in alcohol consumption, as bilateral vagotomy completely abolished the high drinking behaviour induced by both intraperitoneally injected and orally administered bEVs. Thus, this study identifies bEVs as a novel mechanism underlying gut microbiota-induced high alcohol intake in a vagus nerve-dependent manner.

Role of glutamatergic signaling in the acquisition and expression of learned sugar preferences in C57BL/6 mice

C57BL/6 (B6) mice learn to prefer glucose or sucrose to initially isopreferred or even more preferred nonnutritive sweeteners due to the postoral appetite stimulating (appetition) actions of glucose. Recent evidence indicates that specific duodenal neuropod cells transmit the glucose appetition signal to the brain via glutamatergic synaptic connections with vagal afferents. The present study found that intraperitoneal pretreatment with a glutamatergic receptor antagonist cocktail (kynurenic acid (KA)/D-2-amino-3-phosphonopentanoic acid (AP3)) in B6 mice did not block the expression of their learned preference for 8% glucose solution over an initially-preferred 0.1% sucralose + 0.1% saccharin solution. However, acquisition of the glucose preference was blocked by drug treatment during 1-h training sessions with the two sweeteners. Systemic KA/AP3 injections also did not block the expression of the learned preference for a 10.6% sucrose solution over a 0.6% sucralose solution. Drug effects on the acquisition of the sucrose preference were not determined because sucrose, unlike glucose conditioning, required 24-h training trials. The findings that the 1-h training regimen conditioned 8% glucose, but not 10.6% sucrose, preferences suggest that glucose has more potent appetition actions. This was confirmed by the finding that B6 mice learned to prefer 10.6% glucose to 10.6% sucrose after 1-h or 24-h training despite an initial strong sucrose preference. This action can be explained by 10.6% sucrose's digestion in the gut to glucose and fructose with only glucose activating the gut-brain appetition pathway.

I "Gut" Rhythm: the microbiota as a modulator of the stress response and circadian rhythms

Modern habits are becoming more and more disruptive to health. As our days are often filled with circadian disruption and stress exposures, we need to understand how our responses to these external stimuli are shaped and how their mediators can be targeted to promote health. A growing body of research demonstrates the role of the gut microbiota in influencing brain function and behavior. The stress response and circadian rhythms, which are essential to maintaining appropriate responses to the environment, are known to be impacted by the gut microbiota. Gut microbes have been shown to alter the host's response to stress and modulate circadian rhythmicity. Although studies demonstrated strong links between the gut microbiota, circadian rhythms and the stress response, such studies were conducted in an independent manner not conducive to understanding the interface between these factors. Due to the interconnected nature of the stress response and circadian rhythms, in this review we explore how the gut microbiota may play a role in regulating the integration of stress and circadian signals in mammals and the consequences for brain health and disease.

Transcutaneous Vagus Nerve Stimulation Effects on Flavor-Evoked Electroencephalogram and Eye-Blink Rate

INTRODUCTION

Chemosensory food signals are carried by the vagus nerve (VN) from the gut to the brain and these signals contribute to communicating fullness and caloric value of the consumed food in regulatory and reward-related contexts. Here, we aimed to explore whether neural responses to flavor can be modulated through noninvasive VN stimulation, which can be done transcutaneously (transcutaneous vagus nerve stimulation [tVNS]) on the outer ear via the auricular branch of VN. The ideal stimulation location on the outer ear for tVNS is not agreed on but two candidate locations are cymba conchae and tragus.

METHODS

In this study, we explore the optimal stimulation location for tVNS (cymba conchae, tragus, and cymba conchae and tragus) and timing of tVNS relative to chocolate milk presentation (during, after) in a within-participants design (15 participants). We examined various measures of efficacy; event-related potential from electroencephalogram, eye-blink rate, perceptual and hedonic aspects of flavor, swallowing behavior, and consumption behavior.

RESULTS

We observed no effect of stimulation location on any of the dependent variables. Unexpectedly, we observed a large effect of food consumption on spontaneous eye-blink rate.

CONCLUSION

In conclusion, overall we did not observe a clear optimal ear location for tVNS-induced modulation of neurophysiological, perceptual, and behavioral variables. Future studies may confirm whether spontaneous eye-blink rate can be a sensitive proxy for food reward-related phasic dopamine shifts.

Vagus Nerve Mediated Liver-Brain-Axis Is a Major Regulator of the Metabolic Landscape in the Liver

The liver serves as a major energetic reservoir for other tissues and its metabolic function is controlled by humoral and neural factors. The vagus nerve innervating the gastrointestinal tract plays an important role in regulating peripheral metabolism and energy expenditure. Although the liver receives vagus nerve fibers, the impact of this circuitry in the regulation of hepatic metabolism is still poorly understood. Herein, we used a combination of quantitative proteomics and in vivo imaging techniques to investigate the impact of the vagus nerve on liver metabolism in male mice. Liver-brain axis was impaired by vagotomy (VNX) or knocking down of the vesicular acetylcholine transporter (VAChT-KD). Mice were challenged with high carbohydrate or high-fat feeding. The vagus nerve shapes the metabolic framework of the liver, as vagotomy led to a significant alteration of the hepatic proteome landscape. Differential protein expression and pathway enrichment analyses showed that glycolytic and fatty acid biosynthesis were increased following VNX, whereas β-oxidation was decreased. These results were corroborated in VAChT-KD mice. This metabolic shift facilitated lipid accumulation in hepatocytes in mice fed with a standard commercial diet. Furthermore, VNX worsened liver steatosis following high-carbohydrate or high-fat dietary challenges. This study describes the liver-brain axis mediated by the vagus nerve as an important regulator of the hepatic metabolic landscape.

Heterocyclic Antidepressants with Antimicrobial and Fungicide Activity

In this review, the presence of antimicrobial and fungicidal activity in heterocyclic antidepressants was investigated. The already proven connection between the intestinal microbiome and mental health prompted the idea of whether these drugs disrupt the normal intestinal microflora. In addition, there is a serious problem of increasing resistance of microorganisms to antibiotics. In this article, we found that almost all of the antidepressants considered (except moclobemide, haloperidol, and doxepin) have antimicrobial activity and can suppress the growth of not only pathogenic microorganisms but also the growth of bacteria that directly affect mental health (such as Lactobacillus, Lactococcus, Streptococcus, Enterococcus, and Bifidobacterium).

Select microbial metabolites in the small intestinal lumen regulate vagal activity via receptor-mediated signaling

The vagus nerve is proposed to enable communication between the gut microbiome and the brain, but activity-based evidence is lacking. We find that mice reared germ-free exhibit decreased vagal tone relative to colonized controls, which is reversed via microbiota restoration. Perfusing antibiotics into the small intestines of conventional mice, but not germ-free mice, acutely decreases vagal activity which is restored upon re-perfusion with intestinal filtrates from conventional, but not germ-free, mice. Microbiome-dependent short-chain fatty acids, bile acids, and 3-indoxyl sulfate indirectly stimulate vagal activity in a receptor-dependent manner. Serial perfusion of each metabolite class activates both shared and distinct neuronal subsets with varied response kinetics. Metabolite-induced and receptor-dependent increases in vagal activity correspond with the activation of brainstem neurons. Results from this study reveal that the gut microbiome regulates select metabolites in the intestinal lumen that differentially activate vagal afferent neurons, thereby enabling the microbial modulation of chemosensory signals for gut-brain communication.

Imaging flow cytometry reveals LPS-induced changes to intracellular intensity and distribution of α-synuclein in a TLR4-dependent manner in STC-1 cells

BACKGROUND

Parkinson's disease is a chronic neurodegenerative disorder, where pathological protein aggregates largely composed of phosphorylated α-synuclein are implicated in disease pathogenesis and progression. Emerging evidence suggests that the interaction between pro-inflammatory microbial factors and the gut epithelium contributes to α-synuclein aggregation in the enteric nervous system. However, the cellular sources and mechanisms for α-synuclein pathology in the gut are still unclear.

METHODS

The STC-1 cell line, which models an enteroendocrine population capable of communicating with the gut microbiota, immune and nervous systems, was treated with a TLR4 inhibitor (TAK-242) prior to microbial lipopolysaccharide (LPS) exposure to investigate the role of TLR4 signalling in α-synuclein alterations. Antibodies targeting the full-length protein (α-synuclein) and the Serine-129 phosphorylated form (pS129) were used. Complex, multi-parametric image analysis was conducted through confocal microscopy (with Zen 3.8 analysis) and imaging flow cytometry (with IDEAS® analysis).

RESULTS

Confocal microscopy revealed heterogenous distribution of α-synuclein and pS129 in STC-1 cells, with prominent pS129 staining along cytoplasmic processes. Imaging flow cytometry further quantified the relationship between various α-synuclein morphometric features. Thereafter, imaging flow cytometry demonstrated a dose-specific effect of LPS, where the low (8 μg/mL), but not high dose (32 μg/mL), significantly altered measures related to α-synuclein intensity, distribution, and localisation. Pre-treatment with a TLR4 inhibitor TAK-242 alleviated some of these significant alterations.

CONCLUSION

This study demonstrates that LPS-TLR4 signalling alters the intracellular localisation of α-synuclein in enteroendocrine cells in vitro and showcases the utility of combining imaging flow cytometry to investigate subtle protein changes that may not be apparent through confocal microscopy alone. Further investigation is required to understand the apparent dose-dependent effects of LPS on α-synuclein in the gut epithelium in healthy states as well as conditions such as Parkinson's disease.

Gut microbiota in Alzheimer's disease: Understanding molecular pathways and potential therapeutic perspectives

Accumulating evidence suggests that gut microbiota (GM) plays a crucial role in Alzheimer's disease (AD) pathogenesis and progression. This narrative review explores the complex interplay between GM, the immune system, and the central nervous system in AD. We discuss mechanisms through which GM dysbiosis can compromise intestinal barrier integrity, enabling pro-inflammatory molecules and metabolites to enter systemic circulation and the brain, potentially contributing to AD hallmarks. Additionally, we examine other pathophysiological mechanisms by which GM may influence AD risk, including the production of short-chain fatty acids, secondary bile acids, and tryptophan metabolites. The role of the vagus nerve in gut-brain communication is also addressed. We highlight potential therapeutic implications of targeting GM in AD, focusing on antibiotics, probiotics, prebiotics, postbiotics, phytochemicals, and fecal microbiota transplantation. While preclinical studies showed promise, clinical evidence remains limited and inconsistent. We critically assess clinical trials, emphasizing challenges in translating GM-based therapies to AD patients. The reviewed evidence underscores the need for further research to elucidate precise molecular mechanisms linking GM to AD and determine whether GM dysbiosis is a contributing factor or consequence of AD pathology. Future studies should focus on large-scale clinical trials to validate GM-based interventions' efficacy and safety in AD.

Novel neural pathways targeted by GLP-1R agonists and bariatric surgery

The glucagon-like peptide 1 receptor (GLP-1R) agonist semaglutide has revolutionized the treatment of obesity, with other gut hormone-based drugs lined up that show even greater weight-lowering ability in obese patients. Nevertheless, bariatric surgery remains the mainstay treatment for severe obesity and achieves unparalleled weight loss that generally stands the test of time. While their underlying mechanisms of action remain incompletely understood, it is clear that the common denominator between GLP-1R agonists and bariatric surgery is that they suppress food intake by targeting the brain. In this Review, we highlight recent preclinical studies using contemporary neuroscientific techniques that provide novel concepts in the neural control of food intake and body weight with reference to endogenous GLP-1, GLP-1R agonists, and bariatric surgery. We start in the periphery with vagal, intestinofugal, and spinal sensory nerves and then progress through the brainstem up to the hypothalamus and finish at non-canonical brain feeding centers such as the zona incerta and lateral septum. Further defining the commonalities and differences between GLP-1R agonists and bariatric surgery in terms of how they target the brain may not only help bridge the gap between pharmacological and surgical interventions for weight loss but also provide a neural basis for their combined use when each individually fails.

Spinal sensory innervation of the intestine

Sensing our internal environment, or interoception, is essential under physiologic circumstances, such as controlling food intake, and under pathophysiologic circumstances, often triggering abdominal pain. The sensory neurons that innervate the gastrointestinal (GI) tract to mediate interoception originate in two separate parts of the peripheral nervous system: the spinal sensory neurons, whose cell bodies reside in the dorsal root ganglia (DRG), and the vagal sensory neurons, whose cell bodies reside in the nodose ganglia. While the vagal sensory neurons have been extensively studied for their roles in interoception, the roles of the DRG sensory neurons in internal gut sensing are only beginning to be uncovered. Here, we review the recent advances in understanding the diverse properties and functions of gut-innervating DRG sensory neurons and highlight the many unknowns with regards to this understudied population in regulating interoception.

The vagus nerve: An old but new player in brain-body communication

The vagus nerve is a crucial component of the parasympathetic nervous system, facilitating communication between the brain and various organs, including the ears, heart, lungs, pancreas, spleen, and gastrointestinal tract. The caudal nucleus of the solitary tract in the brainstem is the initial site regulated by the vagus nerve in brain-body communication, including the interactions with immune system. Increasing evidence suggests that the gut-brain axis, via the vagus nerve, may play a role in the development and progression of psychiatric, neurologic, and inflammation-related disorders. Population-based cohort studies indicate that truncal vagotomy may reduce the risk of neurological disorders such as Parkinson's disease and Alzheimer's disease, underscoring the vagus nerve's significance in these conditions. Given its role in the cholinergic anti-inflammatory pathway, α7 nicotinic acetylcholine receptors present a potential therapeutic target. Additionally, noninvasive transcutaneous auricular vagus nerve stimulation (taVNS) shows promise as a therapeutic tool for these disorders. This article provides a historical review of the vagus nerve and explores its role in brain-body communication. Finally, we discuss future directions, including the potential of noninvasive taVNS as a therapeutic approach.

The complex relationship between gut microbiota and Alzheimer's disease: A systematic review

Alzheimer's disease (AD) is a progressive, degenerative disorder of the central nervous system. Despite extensive research conducted on this disorder, its precise pathogenesis remains unclear. In recent years, the microbiota-gut-brain axis has attracted considerable attention within the field of AD. The gut microbiota communicates bidirectionally with the central nervous system through the gut-brain axis, and alterations in its structure and function can influence the progression of AD. Consequently, regulating the gut microbiota to mitigate the progression of AD has emerged as a novel therapeutic approach. Currently, numerous studies concentrate on the intrinsic relationship between the microbiota-gut-brain axis and AD. In this paper, we summarize the multifaceted role of the gut microbiota in AD and present detailed therapeutic strategies targeting the gut microbiota, including the treatment of AD with Traditional Chinese Medicine (TCM), which has garnered increasing attention in recent years. Finally, we discuss potential therapeutic strategies for modulating the gut microbiota to alleviate the progression of AD, the current challenges in this area of research, and provide an outlook on future research directions in this field.

Lewy body diseases and the gut

Gastrointestinal (GI) involvement in Lewy body diseases (LBDs) has been observed since the initial descriptions of patients by James Parkinson. Recent experimental and human observational studies raise the possibility that pathogenic alpha-synuclein (⍺-syn) might develop in the GI tract and subsequently spread to susceptible brain regions. The cellular and mechanistic origins of ⍺-syn propagation in disease are under intense investigation. Experimental LBD models have implicated important contributions from the intrinsic gut microbiome, the intestinal immune system, and environmental toxicants, acting as triggers and modifiers to GI pathologies. Here, we review the primary clinical observations that link GI dysfunctions to LBDs. We first provide an overview of GI anatomy and the cellular repertoire relevant for disease, with a focus on luminal-sensing cells of the intestinal epithelium including enteroendocrine cells that express ⍺-syn and make direct contact with nerves. We describe interactions within the GI tract with resident microbes and exogenous toxicants, and how these may directly contribute to ⍺-syn pathology along with related metabolic and immunological responses. Finally, critical knowledge gaps in the field are highlighted, focusing on pivotal questions that remain some 200 years after the first descriptions of GI tract dysfunction in LBDs. We predict that a better understanding of how pathophysiologies in the gut influence disease risk and progression will accelerate discoveries that will lead to a deeper overall mechanistic understanding of disease and potential therapeutic strategies targeting the gut-brain axis to delay, arrest, or prevent disease progression.

Mechanisms by which microbiome-derived metabolites exert their impacts on neurodegeneration

Recent developments in microbiome research suggest that the gut microbiome may remotely modulate central and peripheral neuronal processes, ranging from early brain development to age-related changes. Dysbiotic microbiome configurations have been increasingly associated with neurological disorders, such as neurodegeneration, but causal understanding of these associations remains limited. Most mechanisms explaining how the microbiome may induce such remote neuronal effects involve microbially modulated metabolites that influx into the 'sterile' host. Some metabolites are able to cross the blood-brain barrier (BBB) to reach the central nervous system, where they can impact a variety of cells and processes. Alternatively, metabolites may directly signal to peripheral nerves to act as neurotransmitters or exert modulatory functions, or impact immune responses, which, in turn, modulate neuronal function and associated disease propensity. Herein, we review the current knowledge highlighting microbiome-modulated metabolite impacts on neuronal disease, while discussing unknowns, controversies and prospects impacting this rapidly evolving research field.

Perception of Enterococcus faecalis without infection induces fmo-2 in C. elegans

C. elegans pathogenic susceptibility is influenced by the worm's detection of its environment and its capacity to resist and resolve damage following infection. Here, we use a model where worms can sense, but not ingest, the pathogen Enterococcus faecalis (EF) . We identify that perception of EF without infection induces the stress-response gene fmo-2. We further identify that neural and intestinal signaling genes are necessary for fmo-2 induction without active infection. Finally, we show that fmo-2 overexpression is sufficient to extend lifespan with EF exposure, while fmo-2 KO is not detrimental, suggesting that additional fmo-2 expression benefits worms in this condition.

Microbiota-neuroepithelial signalling across the gut-brain axis

Research over the past two decades has established a remarkable ability of the gut microbiota to modulate brain activity and behaviour. Conversely, signals from the brain can influence the composition and function of the gut microbiota. This bidirectional communication across the gut microbiota-brain axis, involving multiple biochemical and cellular mediators, is recognized as a major brain-body network that integrates cues from the environment and the body's internal state. Central to this network is the gut sensory system, formed by intimate connections between chemosensory epithelial cells and sensory nerve fibres, that conveys interoceptive signals to the central nervous system. In this Review, we provide a broad overview of the pathways that connect the gut and the brain, and explore the complex dialogue between microorganisms and neurons at this emerging intestinal neuroepithelial interface. We highlight relevant microbial factors, endocrine cells and neural mechanisms that govern gut microbiota-brain interactions and their implications for gastrointestinal and neuropsychiatric health.

Neuroendocrine gut-brain signaling in obesity

The past decades have witnessed the rise and fall of several, largely unsuccessful, therapeutic attempts to bring the escalating obesity pandemic to a halt. Looking back to look ahead, the field has now put its highest hopes in translating insights from how the gastrointestinal (GI) tract communicates with the brain to calibrate behavior, physiology, and metabolism. A major focus of this review is to summarize the latest advances in comprehending the neuroendocrine aspects of this so-called 'gut-brain axis' and to explore novel concepts, cutting-edge technologies, and recent paradigm-shifting experiments. These exciting insights continue to refine our understanding of gut-brain crosstalk and are poised to promote the development of additional therapeutic avenues at the dawn of a new era of antiobesity therapeutics.

Bitter taste receptors as sensors of gut luminal contents

Taste is important in the selection of food and is orchestrated by a group of distinct receptors, the taste G protein-coupled receptors (GPCRs). Taste 1 receptors (Tas1rs in mice and TAS1Rs in humans; also known as T1Rs) detect sweet and umami tastes, and taste 2 receptors (Tas2rs in mice and TAS2Rs in humans; also known as T2Rs) detect bitterness. These receptors are also expressed in extraoral sites, including the gastrointestinal mucosa. Tas2rs/TAS2Rs have gained interest as potential targets to prevent or treat metabolic disorders. These bitter taste receptors are expressed in functionally distinct types of gastrointestinal mucosal cells, including enteroendocrine cells, which, upon stimulation, increase intracellular Ca2+ and release signalling molecules that regulate gut chemosensory processes critical for digestion and absorption of nutrients, for neutralization and expulsion of harmful substances, and for metabolic regulation. Expression of Tas2rs/TAS2Rs in gut mucosa is upregulated by high-fat diets, and intraluminal bitter 'tastants' affect gastrointestinal functions and ingestive behaviour through local and gut-brain axis signalling. Tas2rs/TAS2Rs are also found in Paneth and goblet cells, which release antimicrobial peptides and glycoproteins, and in tuft cells, which trigger type 2 immune response against parasites, thus providing a direct line of defence against pathogens. This Review will focus on gut Tas2r/TAS2R distribution, signalling and regulation in enteroendocrine cells, supporting their role as chemosensors of luminal content that serve distinct functions as regulators of body homeostasis and immune response.

Liangxue Tongyu Prescription exerts neuroprotection by regulating the microbiota-gut-brain axis of rats with acute intracerebral hemorrhage

Liangxue Tongyu Prescription (LTP) is a classic herbal formula for treating acute intracerebral hemorrhage (AICH) in China. Previous studies have shown that LTP significantly ameliorates neurological impairments and gastrointestinal dysfunction in patients with AICH. However, the underlying molecular mechanism remains unclear. The aim of this study is to investigate whether LTP exerts its neuroprotective effect on AICH rats through the microbiota-gut-brain axis and explore its potential underlying mechanism. In the current study, AICH models were established by injecting autologous whole blood into the right caudate nucleus of rats. Behavioural and pathological evaluations demonstrated that LTP ameliorated neuronal and intestinal damage in AICH rats. Analysis via western blot, quantitative real-time PCR, immunohistochemistry (IHC) and tunel staining indicated that LTP upregulated the expression of brain-derived neurotrophic factor (BDNF) and nerve growth factor(NGF) and reduced neuronal cell apoptosis. Additionally, 16S rDNA sequencing revealed that LTP mitigated dysbiosis of intestinal microbiota in AICH rats. LTP increased the levels of noradrenaline (NA), dopamine (DA), glutamate (GLU) and modulated brain-gut peptides such as gastrin (GAS), motilin (MTL), ghrelin in AICH rats. Furthermore, LTP enhanced vagus nerve discharge. In summary, this research provides evidence suggesting that LTP's influence on AICH may involve modulation of the microbiota-gut-brain axis, offering a potential scientific rationale for its therapeutic efficacy in improving outcomes of AICH.

Intestinal butyric acid-mediated disruption of gut hormone secretion and lipid metabolism in vasopressin receptor-deficient mice

OBJECTIVES

Arginine vasopressin (AVP), known as an antidiuretic hormone, is also crucial in metabolic homeostasis. Although AVP receptor-deficient mice exhibit various abnormalities in glucose and lipid metabolism, the mechanism underlying these symptoms remains unclear. This study aimed to explore the involvement of the gut hormones including glucagon-like peptide-1 (GLP-1) and microbiota as essential mediators.

METHODS

We used the mouse GLP-1-secreting cell line, GLUTag, and performed live cell imaging to examine the contribution of V1a and V1b vasopressin receptors (V1aR and V1bR, respectively) to GLP-1 secretion. We next investigated the hormone dynamics of V1aR-deficient mice (V1aR-/- mice), V1bR-deficient mice (V1bR-/- mice), and V1aR/V1bR-double deficient mice (V1aR-/-V1bR-/-mice).

RESULTS

AVP induced the increase in intracellular Ca2+ levels and GLP-1 secretion from GLUTag cells in a V1aR and V1bR-dependent manner. AVP receptor-deficient mice, particularly V1aR-/-V1bR-/- mice, demonstrated impaired secretion of GLP-1 and peptide YY secreted by enteroendocrine L cells. V1aR-/-V1bR-/-mice also exhibited abnormal lipid accumulation in the brown adipose tissue and skeletal muscle. We discovered that V1aR-/-V1bR-/- mice showed increased Paneth cell-related gene expression in the small intestine, which was attributed to increased fecal butyric acid levels. Exposure to butyric acid reduced GLP-1 secretion in L cell line. Additionally, human Paneth cell-related gene expressions negatively correlated with that of V1 receptor genes.

CONCLUSIONS

The deficiency in V1 receptor genes may increase gut butyric acid levels and impair the function of L cells, thus dysregulating lipid homeostasis in the brown adipose tissue and skeletal muscle. This study highlights the importance of appropriate control of the gut microbiota and its metabolites, including butyric acid, for the optimum functioning of enteroendocrine cells.

Microbiota-Gut-Brain Axis in Age-Related Neurodegenerative Diseases

BACKGROUND

Age-related neurodegenerative diseases (NDs) pose a formidable challenge to healthcare systems worldwide due to their complex pathogenesis, significant morbidity, and mortality. Scope and Approach: This comprehensive review aims to elucidate the central role of the microbiotagut- brain axis (MGBA) in ND pathogenesis. Specifically, it delves into the perturbations within the gut microbiota and its metabolomic landscape, as well as the structural and functional transformations of the gastrointestinal and blood-brain barrier interfaces in ND patients. Additionally, it provides a comprehensive overview of the recent advancements in medicinal and dietary interventions tailored to modulate the MGBA for ND therapy.

CONCLUSION

Accumulating evidence underscores the pivotal role of the gut microbiota in ND pathogenesis through the MGBA. Dysbiosis of the gut microbiota and associated metabolites instigate structural modifications and augmented permeability of both the gastrointestinal barrier and the blood-brain barrier (BBB). These alterations facilitate the transit of microbial molecules from the gut to the brain via neural, endocrine, and immune pathways, potentially contributing to the etiology of NDs. Numerous investigational strategies, encompassing prebiotic and probiotic interventions, pharmaceutical trials, and dietary adaptations, are actively explored to harness the microbiota for ND treatment. This work endeavors to enhance our comprehension of the intricate mechanisms underpinning ND pathogenesis, offering valuable insights for the development of innovative therapeutic modalities targeting these debilitating disorders.

A high-protein diet-responsive gut hormone regulates behavioral and metabolic optimization in Drosophila melanogaster

Protein is essential for all living organisms; however, excessive protein intake can have adverse effects, such as hyperammonemia. Although mechanisms responding to protein deficiency are well-studied, there is a significant gap in our understanding of how organisms adaptively suppress excessive protein intake. In the present study, utilizing the fruit fly, Drosophila melanogaster, we discover that the peptide hormone CCHamide1 (CCHa1), secreted by enteroendocrine cells in response to a high-protein diet (HPD), is vital for suppressing overconsumption of protein. Gut-derived CCHa1 is received by a small subset of enteric neurons that produce short neuropeptide F, thereby modulating protein-specific satiety. Importantly, impairment of the CCHa1-mediated gut-enteric neuronal axis results in ammonia accumulation and a shortened lifespan under HPD conditions. Collectively, our findings unravel the crosstalk of gut hormone and neuronal pathways that orchestrate physiological responses to prevent and adapt to dietary protein overload.

Microbial alpha diversity in the intestine negatively correlated with disease duration in patients with Meniere's disease

Ménière's disease (MD) is characterized by loss of balance and hearing disorders. Although there is known to be endolymphatic hydrops involved in the pathological process, the pathogenesis of the disease is still largely unclear. Approximately half of patients with MD suffer from depressive symptoms and high levels of several stress hormones were observed in MD and depression, simultaneously. Recently, gut microbes have been shown to influence the function of the central nervous system in humans through their metabolites. We studied the intestinal microbiota of 10 patients with MD and 11 healthy donors (HD). Significant negative correlations were found between disease duration and alpha diversity indexes of gut microbes in patients with MD. The relative abundance of the species Butyricicoccus ambiguous taxa was increased in patients with MD compared with that of HD. In contrast, Oscillospiraceae UCG-002/UCG-005 ambiguous taxa and Anaerovoracaceae (Eubacterium) brachy group uncultured bacterium were increased in the relative abundance of HD than that of patients with MD. The relative abundance of the Butyricicoccus species was positively correlated with disease duration. Thus, these compositional alterations of gut microbes in patients with MD are associated with inner ear pathologies, such as endolymphatic hydrops, by changing the metabolite profiles in the intestine.

Understanding immune system dysfunction and its context in mood disorders: psychoneuroimmunoendocrinology and clinical interventions

Mood disorders include a set of psychiatric manifestations of increasing prevalence in our society, being mainly represented by major depressive disorder (MDD) and bipolar disorder (BD). The etiopathogenesis of mood disorders is extremely complex, with a wide spectrum of biological, psychological, and sociocultural factors being responsible for their appearance and development. In this sense, immune system dysfunction represents a key mechanism in the onset and pathophysiology of mood disorders, worsening mainly the central nervous system (neuroinflammation) and the periphery of the body (systemic inflammation). However, these alterations cannot be understood separately, but as part of a complex picture in which different factors and systems interact with each other. Psychoneuroimmunoendocrinology (PNIE) is the area responsible for studying the relationship between these elements and the impact of mind-body integration, placing the immune system as part of a whole. Thus, the dysfunction of the immune system is capable of influencing and activating different mechanisms that promote disruption of the psyche, damage to the nervous system, alterations to the endocrine and metabolic systems, and disruption of the microbiota and intestinal ecosystem, as well as of other organs and, in turn, all these mechanisms are responsible for inducing and enhancing the immune dysfunction. Similarly, the clinical approach to these patients is usually multidisciplinary, and the therapeutic arsenal includes different pharmacological (for example, antidepressants, antipsychotics, and lithium) and non-pharmacological (i.e., psychotherapy, lifestyle, and electroconvulsive therapy) treatments. These interventions also modulate the immune system and other elements of the PNIE in these patients, which may be interesting to understand the therapeutic success or failure of these approaches. In this sense, this review aims to delve into the relationship between immune dysfunction and mood disorders and their integration in the complex context of PNIE. Likewise, an attempt will be made to explore the effects on the immune system of different strategies available in the clinical approach to these patients, in order to identify the mechanisms described and their possible uses as biomarkers.