The Effect of Acute Tryptophan Depletion (ATD) on Esophageal Sensitivity in Healthy Volunteers
Condition: Esophageal Sensitivity
MeSH terms: Hypersensitivity
Study type: Interventional

Summary: 
      Esophageal hypersensitivity is considered an important pathophysiological mechanism in
      patients suffering form non-erosive gastro-esophageal reflux disease. Serotonin (5-HT) is
      predominantly found in the central nervous system and in the gastro-intestinal (GI) tract.
      5-HT plays a major role in the regulation of GI secretion, motility and sensitivity, and has
      been associated with emotion regulation. Acute tryptophan depletion (ATD) temporarily reduces
      the availability of tryptophan (TRP), thereby decreasing central and peripheral 5-HT
      synthesis. From previous studies, ATD is known to affect GI physiology and enhance visceral
      pain perception in the colon. The aim of the study was to investigate the effect of ATD on
      esophageal sensitivity in healthy volunteers (HV).

      Esophageal multimodal sensitivity was assessed after intragastric infusion of an amino-acid
      mixture (AA-mix) containing 15 AAs with TRP (control condition) or without TRP (ATD
      condition). After an incubation period of 5 hours, a probe with a polyurethane bag was
      positioned in the distal esophagus. Thermal (recirculating a heated saline solution through
      the bag), mechanical (increasing bag volume), electrical (2 stimulation electrodes) and
      chemical sensitivity (modified Bernstein) were tested and at 3 time points blood samples were
      collected for biochemical analysis. General mood was assessed by the Positive and Negative
      Affect Schedule (PANAS) and the State-Trait Anxiety Inventory (STAI) questionnaires.
    

Detailed description: 
      1. INTRODUCTION Gastro-esophageal reflux disease (GERD), defined as the presence of
           symptoms or lesions that can be attributed to reflux of gastric contents into the
           esophagus, is an increasingly prevalent condition in Western societies. Typical reflux
           symptoms are heartburn and regurgitation, but GERD can manifest itself through a variety
           of esophageal and extra-esophageal (atypical) symptoms (e.g. chronic cough).

           In humans, pain is a multimodal experience composed of sensory, physiological and
           psychological aspects. In order to mimic the clinical situation, experimental models
           should be based on multimodal testing regimens in which different receptors and central
           nervous system mechanisms are activated.

           Advances in esophageal sensory stimulation have established that both typical and
           atypical symptoms may not only arise from acid reflux, but also from reflux events with
           less acidic pH (pH 4-7). In GERD patients with persisting symptoms in spite of proton
           pump inhibitor (PPI) treatment (refractory GERD), ongoing weakly acidic reflux is well
           established as the main underlying factor.

           The basis for symptom generation during weakly-acidic reflux events remains to be
           elucidated, but acid sensitivity in the pH range 4-7, mechanical distention (enhanced by
           air in the refluxate), sensitivity to other chemical factors (e.g. bile) and esophageal
           hypersensitivity to physiological levels of reflux have all been proposed.

           The investigators speculate that visceral hypersensitivity plays an important role in
           esophageal symptom perception. This is suggested by the reflux parameters that are usual
           within the physiological number during PPI therapy. Also, previous studies demonstrated
           that refractory GERD patients on PPI therapy have increased visceral hypersensitivity
           for thermal, chemical and mechanical esophageal stimulation compared to healthy
           subjects.

           Serotonin or 5-hydroxytryptamine (5-HT) is a major neurotransmitter predominantly found
           in the central nervous system and in the gastro-intestinal (GI) tract (mucosal
           enterochromaffin cells). 5-HT is derived from tryptophan and plays a pivotal role in the
           regulation of GI secretion, motility and has long been associated with emotion
           regulation and psychological problems such as depression, anxiety and phobia.

           Acute tryptophan depletion (ATD), which temporarily reduces the availability of the
           essential amino acid tryptophan (TRP), decreases 5-HT synthesis and is a validated
           technique to acutely lower central and peripheral 5-HT concentrations. This is
           accomplished by administration of an amino acid mixture lacking TRP.

           ATD is widely used in psychiatric research to investigate the role of central 5-HT in
           affective disorders but further research demonstrated that ATD also affects GI
           physiology by delaying gastric emptying and enhancing visceral pain perception during
           rectal balloon distention. Furthermore, ATD has been shown to alter gastric postprandial
           motor function and distension-induced nausea. These findings establish involvement of
           5-HT in the control of gastric accommodation and sensitivity.

        2. RATIONALE AND OBJECTIVES Multiple studies show that 5-HT is an important player in the
           brain-gut axis, but its exact role is currently not clear. It was previously shown that
           acute administration of citalopram, a selective serotonin reuptake inhibitor (SSRI),
           significantly lowers chemical and mechanical sensitivity in hypersensitive healthy
           volunteers. Buspirone, a partial 5-HT1A-receptor agonist, is able to modify esophageal
           motility. The aim of the current study is to investigate the effect of acute tryptophan
           depletion (ATD) on esophageal sensitivity in a group of healthy volunteers and to
           evaluate its role in symptom perception.

        3. GENERAL DESCRIPTION OF THE STUDY All participants will receive and sign the informed
           consent before initiation of the study. Esophageal sensitivity will be tested by
           multimodal stimulation on two sessions (after placebo amino acid solution and after
           ATD), with an interval of at least one week.

        4. MATERIALS AND METHODS Studies will be performed using a multimodal esophageal
           stimulation probe which allows thermal, mechanical, electrical and chemical stimulations
           of the esophagus.

      During each stimulation, subjects will be instructed to record perception of symptoms using
      an electronic Visual Analogue Scale (VAS) system. This device allows the subject to scale
      perception and pain on a scale from 0 to 10.

      First perception (VAS=1), pain perception threshold (VAS=5) and pain tolerance threshold
      (VAS=7) will be recorded. All types of esophageal stimulations will be immediately terminated
      when the pain tolerance threshold is reached. At the time when the pain tolerance threshold
      is reached (VAS=7), the subjects will be asked to draw the referred pain area, to identify
      location and referral area of the pain.

      Thermal stimulation will be performed by re-circulating a saline solution (NaCl 0.09%),
      heated by a water bath, through the balloon mounted on the probe. Stimulation temperature
      will be steadily increased by increasing the flow rate from the water bath to the balloon.
      Flow rate will be controlled by a computer operated pump. The volume in the balloon will be
      kept constant at 5ml to avoid mechanical stimulation of the esophagus. A temperature sensor
      present in the balloon will continuously monitor the stimulation temperature, which will be
      displayed online on a computer display throughout the study.

      Mechanical stimulation will be performed by distention of the balloon mounted on the probe.
      The flow of saline (NaCl 0.09%) into the balloon, inducing the distention, is regulated by a
      computer controlled pump. The volume in the balloon is displayed online on the computer
      screen throughout the stimulation. Mechanical stimulations will be performed with water of
      37°C, to avoid thermal stimulation of the esophagus.

      Mechanical stimulation will be preceded by a preconditioning period during which the balloon
      will be distended until the pain perception threshold (VAS=5) is reached. This
      preconditioning period is used to precondition the esophageal tissue and to allow the subject
      to get used to the feeling of mechanical distention.

      Electrical stimulation will be performed by 2 stimulation electrodes mounted on proximal to
      the balloon. Electrical block pulses will be given using a standard electrical stimulator.
      Single burst pulses will be given with duration of 1ms at 200Hz.The amplitude of the pulses
      will steadily increase, with steps of 0.5mA and an interval of 15 seconds. For safety, the
      maximum intensity is limited to 50 mA. ECG monitoring will be performed as a safety measure
      during the electrical stimulations of the esophagus.

      Chemical stimulation will be performed in the distal esophagus by infusing an acidic solution
      (HCl 0.1N) in the esophagus. Infusion rate is controlled by a peristaltic infusion pump with
      a flow rate of 2ml/min. The stimulation will last for a maximum of 30 minutes.

      Acute tryptophan depletion

      The effect of ATD on esophageal sensitivity will be studied. The amino acid mixture will be
      prepared according to a protocol previously used by the investigators. All substrates are
      commercially available with an isotopic and chemical purity of minimal 99%. The identity of
      the products will be confirmed using gas chromatography - mass spectrometry (GC-MS,
      GC-column: AT5-MS 30m x 0.25 mm iternal diameter; 0.25µm film (Grace)). The amino acid
      mixture consists of 15 amino acids, (4.1g L-alanine, 2.4g glycine, 2.4g L-histidine, 6.0g
      L-isoleucine, 10.1g L-leucine, 6.7g L-lysine, 4.3g L-phenylalanine, 9.2g L-proline, 5.2g
      L-serine, 4.3g L-threonine, 5.2g L-tyrosine, 6.7g L-valine, 3.7g L-argine, 2.0g L-cysteine,
      3.0g L-methionine and 3.0g L-trypyophan). The TRP-deficient amino acid mixture consists of
      the same 15 amino acids but lacking tryptophan.

      The amino acid mixture will be administered via a nasogastric probe to avoid nausea due to
      the unpleasant taste and smell of the mixture. During placebo sessions, a control amino acid
      mixture is used, containing the same amino acids enriched with TRP in order to prevent a
      decrease in TRP levels. Over time, each participant will receive placebo or ATD in the first
      session in a random sequence. In the second session, the subject will receive the other
      product that he/she didn't receive the first time. Because the amino acid mixture will
      influence the levels of 5-HT of the HV we will first exert the Mini International
      Neuropsychiatric Interview (dutch, version 5.0.0, DSM-IV) during the recruitment of the
      volunteers to evaluate their psychiatric condition.

      STUDY OUTLINE After an overnight fast subjects will come to the endoscopy unit of the UZ
      Gasthuisberg, where the study will be performed. Two sessions will be scheduled for every
      subject: one placebo and one ATD session, with at least one week interval between each of the
      sessions. Sessions will run in a single-blind way, the order of placebo and ATD will be
      randomized.

      Since maximal TRP depletion is obtained approximately 5 hours after intake of the amino acid
      mixture, the amino acid will be administered through nasogastric infusion 5 hours prior to
      the actual start of the esophageal stimulations.

      During the time between administration of the amino acid mixture and the actual start of
      esophageal stimulations, the subjects will be asked to watch standardized movies with a
      neutral emotional content. Five hours after the administration of the amino acid mixture, the
      multimodal stimulation probe will be positioned through the mouth. After the probe is
      positioned in the esophagus (10 cm above the lower esophageal sphincter), it will be fixed to
      the chin and the subject will remain in in semi-recumbent position for the entire study
      period. Before the start of the stimulations, there will be an adaptation period of 15
      minutes for the subjects, to get used to the feeling of the probe and to provide instructions
      for the correct use of the VAS meter.

      VAS scores will be monitored during each type of stimulation. All stimulations will be
      immediately stopped at the moment the subject reaches the pain tolerance threshold (VAS=7)
      except for the electrical stimulation, where we will stop the stimulation after reaching VAS
      5, for safety reasons.
    

Intervention name(s): Acute tryptophan depletion; Placebo; Multimodal stimulation probe
Intervention type(s): Dietary Supplement; Dietary Supplement; Device
Intervention description(s): an amino-acid mixture will be administered without tryptophan (active comparator). After an incubation period of 5 hours esophageal sensitivity will be assessed by multimodal stimulation to evaluate the effect of depleted tryptophan levels on esophageal sensitivity; an amino-acid mixture with tryptophan (placebo mixture) will be administered. After an incubation period of 5 hours esophageal sensitivity will be assessed by multimodal stimulation; The multimodal esophageal probe allows to perform 4 types of stimulations in the esophagus (thermal, mechanical, electrical and chemical stimulation). These 4 modalities will be evaluated to assess esophageal sensitivity.
Intervention MeSH terms: Tryptophan
Allocation: Randomized
Intervention model: Crossover Assignment
Masking: Single (Participant)
Primary purpose: Basic Science
Keywords: Acute tryptophan depletion, esophageal sensitivity,