Vagal Nerve Stimulation for Intestinal Barrier Dysfunction in Healthy Volunteers
Condition: Intestinal Barrier Dysfunction
Study type: Interventional
Phase: Phase 1

Summary: 
      This is a proof of concept randomised placebo controlled crossover trial to evaluate the
      effect of transcutaneous vagal nerve stimulation on a stress model of increased intestinal
      permeability in healthy human subjects.

      Intestinal permeability perturbation is a phenomenon which is being increasingly recognised
      as a contributing factor to a multitude of diseases - including inflammatory bowel disease
      (IBD) and irritable bowel syndrome (IBS). There are currently limited effective treatment
      methods known to improve this intestinal permeability perturbation and the use of vagal nerve
      stimulation would present itself as an inexpensive, non-invasive and non-pharmacological
      method of reversing this dysfunction. Vagal nerve stimulation efficacy in reversing stress
      related intestinal barrier dysfunction is available from proof of concept animal models.

      This mechanistic project is an important first step in this field of research and will serve
      as a basis for further research into the role of vagal nerve stimulation in intestinal
      barrier dysfunction.
    

Detailed description: 
      Stress - a state which evokes a 'fight or flight' or sympathetic response from the human
      organism - has been implicated in the propagation of inflammation in patients with
      inflammatory bowel disease (IBD). Indeed, the risk of relapse in both Crohns disease (CD) and
      Ulcerative colitis (UC) increases in the presence of both acute and chronic stress. The
      mechanism proposed for this phenomenon includes the ability of stress to induce a breakdown
      in intestinal barrier function via mast cell- and corticotropin releasing hormone
      (CRH)-dependent pathways. This intestinal barrier perturbation has been demonstrated
      experimentally in humans using stress paradigms which could be equated to the relatively mild
      stressful events experienced in every-day life.

      The autonomic nervous system (ANS) appears to be playing a crucial role in stress-induced
      changes in intestinal permeability. Multiple in vitro and in vivo murine model studies have
      confirmed that both acute and chronic stress (induced in a mouse by various stress paradigms
      such as restraint and water emersion) can promote increased intestinal permeability. The
      underlying mechanism seems to be via the disruption of tight junction proteins along the
      paracellular space of the intestinal epithelium. The disruption of tight junctions in these
      stress paradigm models are dependent on acetylcholine and CRH. This effect on intestinal
      permeability appears to recover within 4 days in murine models.

      Similar changes have been described in human models of acute stress. Barclay and Turnberg
      first showed that intestinal absorption of salt and water was reduced in healthy participants
      in response to acute stress induced by a short-term stressor paradigm. In this study, they
      used dichotomous listening as the physiological stressor, whereby the participant is asked to
      listen to two types of different genres of music in each ear whilst they completed a mentally
      challenging task. These physiological absorption changes to the intestine's barrier function
      were found to be mediated by a cholinergic-dependent mechanism. The same authors later showed
      similar effects with the physical stress paradigm of cold-induced pain in healthy volunteers.
      Although actual intestinal permeability changes were not measured at the time, one likely
      mechanism of permeability changes related to these alterations in electrolyte flux is via the
      claudin-2 paracellular channel, which has since been described. The configuration of this
      channel is related to ionic flux across the intestinal epithelium and, therefore, would
      render the epithelium more permeable in times of electrolyte flux. In more recent studies
      utilising the same cold-pain paradigm, changes in intestinal permeability were paralleled by
      the release of products of mast cell degranulation, tryptase and histamine. Such changes in
      intestinal permeability induced by acute stress were mimicked by peripheral administration of
      CRH and could be reversed by the administration of mast cell stabilisers.

      A key role for the parasympathetic nervous system in the perturbation of intestinal
      permeability by severe physiological stress was derived from the effects of vagal nerve
      stimulation in animal models. Stimulation of the cervical vagal nerve for 10 minutes prior to
      giving mice a severe burn protected them from intestinal barrier perturbation and increased
      intestinal glial fibrillary acidic protein (FGAP) expression, a marker of enteric nervous
      system glial activation, in vitro and in vivo.This effect was likely, at least in part, due
      to an α7 acetylcholine receptor dependent mechanism that was demonstrated in both cultured
      intestinal epithelial cells as well as in enteric glial cells. Further studies have since
      supported this protective effect of vagal nerve stimulation, suggesting a role for the
      preservation of the number of dendritic cells in the mesenteric lymph as another potential
      mechanism.Moreover, the protective effect on the epithelial lining seemed to be via a
      locally-induced mechanism on the intestine rather than being dependent on a splenic-mediated
      process as the protective effect held true in splenectomised mice. Stimulation of the
      parasympathetic nervous system via the vagal nerve therefore seems to play a protective role
      in modulation of the intestinal barrier function with experimental evidence hinting at its
      ability to reverse stress induced changes in intestinal permeability.

      These results are of significant interest given that the last decade has seen a number of
      studies supporting the theory of intestinal barrier perturbation in the pathogenesis of
      IBD.Additionally, the detection of antibodies directed against microbial antigens (such as
      'ASCA' - epitopes of Saccharomyces cerevisiae and Flagellin CBir1) are found in the serum of
      patients prior to the development of IBD - indicating a disruption of the intestinal barrier
      function prior to the onset of disease. Furthermore, a genetic predisposition to intestinal
      barrier perturbation is considered to be a predisposing factor in the development of IBD.
      Given that it seems that the ANS has a significant role to play in the maintenance of the
      intestinal barrier function, it would be reasonable to conclude that alterations within the
      ANS in a susceptible individual may, at least in part, be implicated the development and
      flares of IBD.

      Several serological and urinary markers have been validated as useful tools in measuring
      intestinal permeability changes in vivo. These include lipopolysaccharide-binding protein
      (LBP), CD-14 and intestinal-type fatty acid-binding protein (I-FABP) in the serum, as well as
      an orally-administered dual sugar solution and its by-products in serum and urine. Soluble
      CD-14 and LBP are part of the LPS inflammatory signalling pathway, thus their levels in serum
      have been shown to be objective markers for systemic immune activation and damage to gut
      epithelium. I-FABP is an intracellular protein expressed in the epithelial cells of the
      mucosal layer of the small and large intestine tissue and is prone to leakage into the blood
      stream from the enterocytes when intestinal mucosal damage occurs. The oral dual sugar test
      uses lactulose and rhamnose that are differentially absorbed in the intestine and excreted in
      the urine. The ratio of lactulose to rhamnose in the urine and serum can be used as a measure
      of small intestinal permeability when the urine is collected for two hours post ingestion of
      the test solution.

      The therapeutic effect of vagal nerve stimulation in intestinal barrier dysfunction is an
      important area of study as it could lead to a novel and in-expensive method of treatment of
      conditions associated with intestinal barrier perturbation. In humans, the auricular branch
      of the vagus nerve is located directly under the skin, making it a suitable target for
      transcutaneous stimulation. A transcutaneous electrical nerve stimulation (TENS) device is
      thus a suitable non-invasive tool for the investigation of transcutaneous vagal nerve
      stimulation (tVNS) for reduction of intestinal permeability in humans and has been used in a
      number of studies utilizing the technique of tVNS in our laboratory.

      Corticotropin releasing hormone (CRH) is naturally secreted from the healthy human
      hypothalamus during periods of stress. Intravenously administered CRH possesses the advantage
      of having a short half-life and is routinely clinically utilised in the diagnosis of
      Cushing's syndrome. Recent evidence has shown that injecting CRH intravenously reliably and
      significantly increases intestinal permeability in healthy volunteers with no significant
      adverse effects.This model will be used to induce increased intestinal permeability in
      healthy human volunteers in order to test the hypothesis that pre-treatment with vagal nerve
      stimulation can reduce the degree of intestinal permeability induced by CRH.
    

Intervention name(s): Vagal nerve stimulation
Intervention type(s): Device
Intervention description(s): The participant will be administered electrical stimulation at a level specifically set to their comfort. This will be performed using a TENS machine device.
Allocation: Randomized
Intervention model: Crossover Assignment
Intervention model description: Proof of concept randomised, placebo controlled, single blinded, cross over study
Masking: Single (Participant)
Primary purpose: Treatment