Unique characteristics of "the second brain" - The enteric nervous system / 生理学报

Enteric nervous system (ENS) is composed of intestinal submucosal and myenteric plexuses. ENS may independently regulate intestinal digestive and absorptive function, and it is also known as "the second brain" or gut brain. ENS has significant specificity relative to central nervous system (CNS) in properties and functional activities of neurons and neural circuits. ENS is connected with CNS through the feedback pathway (brain-gut-axis) of sympathetic and parasympathetic nerves and peripheral primary sensory afferent nerves to form the bidirectional brain-gut-axis, which may affect emotion, appetite and behavioral states of individuals. Gastrointestinal functional disorder (GIFD) induced by ENS dysfunction may not only cause abnormal gastrointestinal function but also has been implicated in cognitive and mood disorders, such as irritable bowel syndrome (IBS). GIFD would influence deeply the quality of life in patients. Nevertheless, in the worldwide, ENS has so far received much less attention as compared with CNS. The depth of research and scale of investment in ENS studies have been much lower than those in CNS studies. The situation in China is even more evident. From ENS research history, an outstanding problem is to ignore largely the unique properties of ENS and apply mechanically the hypotheses formed in CNS studies to ENS researches. In this review, the structure and function of ENS are briefly introduced, and the importance of extraordinary characteristics of ENS is illustrated by the problems encountered in our studies.

Changes in Enteric Neurons of Small Intestine in a Rat Model of Irritable Bowel Syndrome with Diarrhea

BACKGROUND/AIMS: Physical and/or emotional stresses are important factors in the exacerbation of symptoms in irritable bowel syndrome (IBS). Several lines of evidence support that a major impact of stress on the gastrointestinal tract occurs via the enteric nervous system. We aimed to evaluate histological changes in the submucosal plexus (SMP) and myenteric plexus (MP) of the distal ileum in concert with the intestinal motor function in a rat model of IBS with diarrhea. METHODS: The rat model was induced by heterotypic chronic and acute stress (CAS). The intestinal transit was measured by administering powdered carbon by gastric gavage. Double immunohistochemical fluorescence staining with whole-mount preparations of SMP and MP of enteric nervous system was used to assess changes in expression of choline acetyltransferase, vasoactive intestinal peptide, or nitric oxide synthase in relation to the pan neuronal marker, anti-Hu. RESULTS: The intestinal transit ratio increased significantly from control values of 50.8% to 60.6% in the CAS group. The numbers of enteric ganglia and neurons in the SMP were increased in the CAS group. The proportions of choline acetyltransferase- and vasoactive intestinal peptide-immunoreactive neurons in the SMP were increased (82.1 ± 4.3% vs. 76.0 ± 5.0%, P = 0.021; 40.5 ± 5.9% vs 28.9 ± 3.7%, P = 0.001), while nitric oxide synthase-immunoreactive neurons in the MP were decreased compared with controls (23.3 ± 4.5% vs 32.4 ± 4.5%, P = 0.002). CONCLUSIONS: These morphological changes in enteric neurons to CAS might contribute to the dysfunction in motility and secretion in IBS with diarrhea.

Dietary Glutamate: Interactions With the Enteric Nervous System

BACKGROUND/AIMS: Digestion of dietary protein elevates intraluminal concentrations of glutamate in the small intestine, some of which gain access to the enteric nervous system (ENS). Glutamate, in the central nervous system (CNS), is an excitatory neurotransmitter. A dogma that glutamatergic neurophysiology in the ENS recapitulates CNS glutamatergic function persists. We reassessed the premise that glutamatergic signaling in the ENS recapitulates its neurotransmitter role in the CNS. METHODS: Pharmacological analysis of actions of receptor agonists and antagonists in concert with immunohistochemical localization of glutamate transporters and receptors was used. Analysis focused on intracellularly-recorded electrical and synaptic behavior of ENS neurons, on stimulation of mucosal secretion by secretomotor neurons in the submucosal plexus and on muscle contractile behavior mediated by musculomotor neurons in the myenteric plexus. RESULTS: Immunoreactivity for glutamate was expressed in ENS neurons. ENS neurons expressed immunoreactivity for the EAAC-1 glutamate transporter. Neither L-glutamate nor glutamatergic receptor agonists had excitatory actions on ENS neurons. Metabotropic glutamatergic receptor agonists did not directly stimulate neurogenic mucosal chloride secretion. Neither L-glutamate nor the metabotropic glutamatergic receptor agonist, aminocyclopentane-1,3-dicarboxylic acid (ACPD), changed the mean amplitude of spontaneously occurring contractions in circular or longitudinal strips of intestinal wall from either guinea pig or human small intestinal preparations. CONCLUSIONS: Early discoveries, for excitatory glutamatergic neurotransmission in the CNS, inspired enthusiasm that investigation in the ENS would yield discoveries recapitulating the CNS glutamatergic story. We found this not to be the case.

Anti-Enteric Neuronal Antibodies and the Irritable Bowel Syndrome

BACKGROUND/AIMS: Functional gastrointestinal disorders are those in which no abnormal metabolic or physical processes, which can account for the symptoms, can be identified. The irritable bowel syndrome (IBS) is a significant functional disorder, which affects 10-20 percent of the population worldwide. Predominant symptoms of IBS are abnormal defecation associated with abdominal pain, both of which may be exacerbated by psychogenic stress. Our study was designed to test a hypothesis that symptoms in a subset of patients with a diagnosis of IBS are associated with an autoimmune degenerative neuropathy in the enteric nervous system. METHODS: Serum was collected from Rome II-IBS patients and controls at the University of North Carolina Functional Gastrointestinal Diseases Center. Assay procedures were immunohistochemical localization of antibody binding to enteric neurons and human protein microarray assay for antigens recognized by antibodies in the sera. RESULTS: Eighty-seven percent of IBS sera and 59% of control sera contained anti-enteric neuronal antibodies. Antibody immunostaining was seen in the nucleus and cytoplasm of neurons in the enteric nervous system. Protein microarray analysis detected antibody reactivity for autoantigens in serum with anti-enteric neuronal antibodies and no reactivity for the same autoantigens in samples not containing anti-enteric neuronal antibodies in our immunostaining assay. Antibodies in sera from IBS patients recognized only 3 antigens out of an 8,000 immunoprotein array. The 3 antigens were: (1) a nondescript ribonucleoprotein (RNP-complex); (2) small nuclear ribonuclear polypeptide A; and (3) Ro-5,200 kDa. CONCLUSIONS: Results of the present study suggest that symptoms in a subset of IBS patients might be a reflection of enteric neuronal damage or loss, caused by circulating anti-enteric autoimmune antibodies.