The gastrointestinal tract is unique among the organs of the body for its ability to exhibit a vast repertoire of functions which occur independently from the central nervous system (CNS). These include transport of luminal content, secretion and absorption of ions, water, and nutrients, blood flow, defensive mechanisms against pathogens and elimination of waste and/or noxious substances are under refined regulation of a dense intrinsic innervation referred to as enteric nervous system (ENS). Notably, the ENS consisting of many different specialized subtypes of neurons and glial cells (reminiscent of CNS astrocytes), shares many common features with the CNS leading enteric neuroscientists to define it as “the brain of the gut” or the mammalian “second brain”. In the ENS, neural activity is initiated by the luminal contents via local sensory neurons (IPANS) detecting mechanical changes from the smooth muscle and chemical luminal changes from the mucosa generating adaptive responses. In addition, ongoing spontaneous activity of enteric circuits generate cyclic motor and secretory behaviour. Several features of enteric neurons contribute to their ability to control gastrointestinal functions. These include: their specific neurochemistry, their polarized axonal projections in the oral or anal direction, their synaptic interactions and their pronounced sensitivity to modulation by non-neuronal mediators. More than 50 bioactive substances have been identified as putative enteric neurotransmitters, neuromodulators and mediators from immune competent cells and the list is still growing.
In addition to maintaining and regulating gut physiology and luminal homeostasis, a number of functional and structural gut disorders are associated with malfunctions of the ENS. Thus, inflammatory conditions are associated with defined alterations of the ENS. Functional imaging and other sophisticated methodological approaches have allowed to characterize neurophysiological and neuropharmacological features of the human ENS in health and disease. Alterations in ENS circuits are linked to gut dysfunctions observed in patients with various disorders spanning from common conditions such as irritable bowel syndrome or functional dyspepsia to the most severe forms of gut dysmotility, i.e. chronic intestinal pseudo-obstruction. This symposium is aimed at enhancing interest of the neuroscientist community on the multifunctional role of the ENS / second brain playing in concert with a wide array of specialized cells integrating signals that are involved in the spatio-temporal control of digestive functions. Also, the putative mechanisms leading to ENS dysfunction and disease will be highlighted.
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2:30pm - 2:50pmHow the Enteric Nervous System is Organized and How it Works
Marcello Costa
Flinders University, Adelaide, South Australia, Australia
The Enteric Nervous System consists of neural circuits which shares many common features with the CNS but that can operate independently from it. Enteric circuits include several classes of local sensory neurons (IPANS), inter and motor neurons. They contain (neurochemical coding) and utilize multiple bioactive transmitter substances (plurichemical transmission). Intestinal motor functions involve polarized sensory-motor circuits which generate propulsive movements in an aboral direction, in response to mechanical and chemical stimuli of its luminal contents by self-sustained neuromechanical loops. In addition ongoing spontaneous activity of enteric circuits generates slowly migrating cyclic motor patterns which extend along the entire digestive tract from the stomach to the colon.
These two fundamental neural mechanisms, interacting with the underlying rhythmic activity generated by the net of pacemaker cells (ICCs), generate motor patterns of the digestive tract. These have been studied by novel combinations of recording methods linking kinematics with kinetics. Spatio-temporal maps of changes in diameter (kinematics) combined with spatiotemporal maps of intraluminal pressure (kinetics) and with electrophysiological recording of the intestinal smooth muscle and of enteric neurons, enabled to identify the role of enteric neural circuits and of the pacemaker cells.
These novel methods have been applied to reveal in detail neurogenic and myogenic mechanisms in determining very specific motor patterns along the intestine. In most species the proximal and the distal colon show distinct motor patterns suitable for the slow advancing of faeces, shaping them leading to the formation of more solid faeces to be excreted.
The proposal that intestinal propulsion is a form of locomotion rather than a simple reflex and opens a new conceptual frame with neuro-mechanical activity represented as four dimensional structures, applicable to the functions of entire nervous system from the spinal cord neuromechanical loop to the highest mental functions, in search for a neural correlate of consciousness.
2:50pm - 3:10pmWhen the Second Brain Gets Sick: A Journey through Enteric Neuropathies
Roberto De Giorgio
University of Ferrara, Italy
The enteric nervous system (ENS), the third division of the autonomic innervation and the largest neuronal collection outside the central nervous system (CNS), has been referred to as “the brain in the gut” or “the second brain of the mammalian body” because of its highly integrated neural circuits controlling all gut functions, including motility. The latter is the result of the ENS fine tuning over smooth musculature along with the contribution of other key cells, such as enteric glia (astrocyte-like cells supporting and contributing to neuronal activity), interstitial cells of Cajal (ICCs, the pacemaker cells of the gastrointestinal tract also involved in neuromuscular transmission), and enteroendocrine cells (releasing bioactive substances affecting gut physiology). Any noxa / insult perturbing the ENS morpho-functional integrity may result in a neuropathy with variable degree of neuro-muscular dysfunction. A classic clinical phenotype of enteric neuropathy is Hirschsprung’s disease a genetically driven condition characterized by a lack of any enteric neurons in a portion of the gastrointestinal (GI) tract (usually the descending colon). Also, genetic and non-genetic mechanisms may induce enteric neurodegeneration and neuronal loss responsible for severe GI dysmotility phenotypes, such as chronic intestinal pseudo-obstruction (CIPO). The present “journey” through enteric neuropathy will highlight research performed on RAD21 mutation (which has been demonstrated in a family whose affected members exhibited severe gut dysmotility) impairing GI motility and syndromic conditions, e.g. mitochondrial disorders including mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) and LIG3-related mitochondrial disorder, also characterized by a significant impairment of GI physiology. The balance between mechanisms involved in neurodegeneration with molecular pathways of neuroprotection will unveil strategies to better understand enteric neuropathies and pave the way to targeted therapies for such severe diseases.
3:10pm - 3:30pmThe Opioid Paradigm in the Enteric Nervous System: Insights Into New Molecular Mechanisms
Catia Sternini
University of California, Los Angeles, USA
µ opioid receptors (µOR), GPCRs which mediate opioid effects, are distributed throughout the body, including the enteric nervous system. Activation of µORs attenuates neuronal activity by inhibiting transmitter release through coupling with Gi/Go proteins. Activated µORs undergo a cascade of events controlling cell functions, including interaction with ß arrestins and dynamin-dependent endocytosis inducing desensitization, and recycling that mediates resensitization.
Using multiple immunofluorescence in the guinea pig as animal model, we showed that µORs are mostly localized at the cell surface of enteric neurons in the ileum, whereas they are largely in the cytosol in the colon. µORs are more abundant in excitatory neurons in the ileum, and in inhibitory neurons in the colon. Interestingly, µOR internalization in enteric neurons is ligand-dependent and cell-specific, which likely represents the mechanism underlying the different neurogenic response to chronic opioids with the ileum developing tolerance, whereas the colon does not. This difference likely depends on ß arrestins and downstream effectors that differ in the small and large intestine.
Our studies using mucosal biopsies of patients with irritable bowel syndrome (IBS) showed increased µOR expression in IBS mucosa vs. controls, with an increase of µORs in immune cells, suggesting an immune-related compensatory role of µORs in visceral pain perhaps through neuronal pathways. Our recent studies with surgical specimens and biopsies of human colon identified µORs in enteric neurons of the outer and inner submucosal plexuses, which are predominantly involved with motility and mucosal function, respectively. Of note, µORs is localized to early endosomes underneath the plasma membrane indicating receptor trafficking. Opioid systems have been implicated in visceral perceptions, intestinal motility and secretion, and immune response. Changes in µOR expression and µOR cellular translocation might represent novel mechanisms underlying motility impairment induced by opioids and alteration in immune response affecting visceral sensation.
3:30pm - 3:45pmThe role of the gut-brain axis in GBA1-linked Parkinson’s disease
Chiara Sinisgalli, Francesca Terrin, Davide Santinello, Luisa Dalla Valle, Stefano Campanaro, Nicoletta Plotegher
Department of Biology - University of Padova
Parkinson’s disease (PD) is a chronic neurodegenerative disease characterized by the accumulation of misfolded alpha-synuclein in the dopaminergic neurons of the substantia nigra. PD shows typical motor symptoms such as tremor, bradykinesia and muscular rigidity, but also non-motor symptoms. Among them, the gastrointestinal ones are the most frequent and may precede the onset of the motor symptoms and the diagnosis. Several studies demonstrated that gut health can affect the activity of the central nervous system through the physiological contribution of the microbiota, via the regulation of intestinal barrier function and through the activity of the peripheral neurons belonging to the enteric nervous system, supporting the idea of an active dialogue between the two compartments, the so-called gut-brain axis.
Heterozygous mutations in the GBA1 gene, encoding the lysosomal enzyme glucocerebrosidase (GCase), are the most common genetic risk factor for PD. Inoculation of preformed fibrils of alpha-synuclein in the duodenum of wild-type mice induced an inflammatory response and decreased GCase activity, but no data are available on gut function and gut microbiota when GCase is mutated.
The aim of this work is to investigate the role of gut-brain axis in the gba-/- hN370S transgenic mouse model. In this mice, GCase activity and expression change in different tissues, when collected from 8-month old mice. Hematoxilin-eosin staining showed alteration in the length of intestinal villi and alteration in submucosa and muscular layers, suggesting the alteration of intestinal barrier permeability due to activation of inflammatory response and the alteration in gut microbiota composition. Imaging experiment, western blotting, Elisa assays and 16s RNA analysis of the microbiota were performed in order to shed light on the role of the gut in our mice model. Further investigations will be needed in order to better understand if gut defects could contribute to the disease etiopathogenesis in GBA1-linked PD.
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