Josh Collins

BACKGROUND: Normal gastrointestinal function depends on an intact and coordinated enteric nervous system (ENS). While the ENS is formed during fetal life, plasticity persists in the postnatal period during which the gastrointestinal tract is colonized by bacteria. We tested the hypothesis that colonization of the bowel by intestinal microbiota influences the postnatal development of the ENS. METHODS: The development of the ENS was studied in whole mount preparations of duodenum, jejunum, and ileum of specific pathogen-free (SPF), germ-free (GF), and altered Schaedler flora (ASF) NIH Swiss mice at postnatal day 3 (P3). The frequency and amplitude of circular muscle contractions were measured in intestinal segments using spatiotemporal mapping of video recorded spontaneous contractile activity with and without exposure to lidocaine and N-nitro-L-arginine (NOLA). KEY RESULTS: Immunolabeling with antibodies to PGP9.5 revealed significant abnormalities in the myenteric plexi of GF jejunum and ileum, but not duodenum, characterized by a decrease in nerve density, a decrease in the number of neurons per ganglion, and an increase in the proportion of myenteric nitrergic neurons. Frequency of amplitude of muscle contractions were significantly decreased in the jejunum and ileum of GF mice and were unaffected by exposure to lidocaine, while NOLA enhanced contractile frequency in the GF jejunum and ileum. CONCLUSIONS & INFERENCES: These findings suggest that early exposure to intestinal bacteria is essential for the postnatal development of the ENS in the mid to distal small intestine. Future studies are needed to investigate the mechanisms by which enteric microbiota interact with the developing ENS.

BACKGROUND: Depression often coexists with the irritable bowel syndrome (IBS) which is characterized by alterations in gut function. There is emerging evidence that the microbial composition (microbiota) of the gut is altered in IBS, but the basis for this is poorly understood. The aim of this study was to determine whether the induction of chronic depression results in changes in the colonic function and in its microbial community, and to explore underlying mechanisms. METHODS: Bilateral olfactory bulbectomy (OBx) was used to induce depression-like behavior in mice. Colonic function was assessed by measuring muscle contractility, pellet excretion, c-fos activity, and serotonin levels. Microbiota profiles were obtained using denaturing gradient gel electrophoresis (DGGE). The hypothalamic-pituitary axis (HPA) was assessed by the hypothalamic expression of corticotropin-releasing hormone (CRH). In separate studies, mice without OBx received CRH via intracerebroventricular (ICV) infusion for 4 weeks prior to assessing colonic function and microbiota profiles. KEY RESULTS: Olfactory bulbectomy mice demonstrated chronic depression- and anxiety-like behaviors associated with elevated central CRH expression and increases in c-Fos activity, serotonin levels, and motility in the colon. These changes were accompanied by an altered intestinal microbial profile. Central CRH administration produced similar changes in behavior and motility and altered the microbiota profile in the colon. CONCLUSIONS & INFERENCES: The induction of chronic depression alters motor activity and the microbial profile in the colon likely via activation of the HPA. These findings provide a basis for linking the behavioral and gastrointestinal manifestations of IBS.

Emerging evidence indicates that the commensal microbiota communicates with the brain and influences behavior. In animal models, perturbation of the microbiota is accompanied by changes in brain-derived neurotrophic factor (BDNF) levels in the brain. However, underlying mechanisms are unknown. We investigated whether vagal-parasympathetic and sympathetic branches of the autonomic nervous system are involved in the microbiota-gut-brain signalling and attempt to identify specific brain regions that are responsive to alterations in the intestinal microbiota. Specific pathogen-free Balb/c mice, with or without surgical vagotomy or chemical sympathectomy, received oral non-absorbable antimicrobials (ATM) <em>ad libitum</em> for 7 days. Behavior was tested on day 7 in the light/dark preference and step-down latency tests. Specific brain regions were sectioned and stained for the neuronal activation marker, <em>c-fos</em>. Perturbation of the microbiota significantly enhanced the exploratory behavior of mice in both tests and increased the expression of <em>c-fos</em> and phosphorylated <em>c-fos</em> in the hippocampus and dentate gyrus. <em>c-fos</em> expression in the nucleus of the solitary tract was unaffected and neither vagal-parasympathetic nor sympathetic neurotransmission were required for induction of the behavioral change following perturbation of the microbiota. Instability of the commensal microbiota enhances the activation of the hippocampal formation and influences host behavior in a manner that is independent of vagal-parasympathetic and sympathetic autonomic neurotransmission.