SEARCH

SEARCH BY CITATION

Keywords:

  • Heartburn;
  • capsaicin;
  • TRPV1;
  • acid perfusion;
  • oesophagus

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. References

Abstract  Heartburn is the most typical gastro-oesophageal reflux disease (GERD) symptom. The transient receptor potential vanilloid receptor-1 (TRPV1) is a candidate mediator of heartburn. Exposure of TRPV1 to capsaicin is characterized by activation, followed by desensitization. Our aim was to investigate the effect of intra-oesophageal capsaicin instillation on oesophageal symptom perception (activation) and on sensitivity to oesophageal acid perfusion and oesophageal balloon distention (desensitization). In a first protocol (= 10), saline or capsaicin solution were instilled in the mid-oesophagus and symptoms were rated at 5-min intervals for 60 min. In a second study (n = 10), oesophageal 0.1 N hydrochloric acid perfusion was performed 60 min after pretreatment with saline, low or high dose capsaicin. In a third study (= 10), sensitivity to oesophageal balloon distention was determined before and at 30-min intervals up to 90 min after pretreatment with saline, low or high dose capsaicin. Areas under the curve (AUC) for symptom intensities under different conditions were calculated and compared with Kruskal–Wallis test. Oesophageal capsaicin instillation induced transient symptoms of retrosternal and epigastric burning in a dose-dependent fashion. After oesophageal capsaicin or saline instillation, there was no difference in symptom pattern and intensities induced by oesophageal acid perfusion. After oesophageal capsaicin or saline instillation, sensitivity to oesophageal balloon distention and oesophageal compliance were not significantly altered. Oesophageal instillation of the TRPV1 receptor agonist capsaicin induces symptoms of retrosternal and epigastric burning in a dose-dependent fashion. Pretreatment with capsaicin does not desensitize the oesophagus to acid perfusion or to balloon distention.


Abbreviations:
ASIC

acid sensing ion channel

AUC

area under the curve

GERD

gastro-oesophageal reflux disease

NS

not significant

PPI

proton pump inhibitor

TRPV1

transient receptor potential vanilloid receptor 1

VAS

visual analogue scale score

VR-1

vanilloid receptor 1

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. References

Gastro-oesophageal reflux disease (GERD) is a condition that develops when the reflux of gastric content causes troublesome symptoms or complications.1 The typical reflux syndrome, characterized by heartburn and regurgitation, and the reflux chest pain syndrome are the symptomatic oesophageal syndromes associated with GERD.1 Reflux of gastric acid is widely accepted as the main mechanism underlying heartburn, as it can be mimicked by oesophageal acid perfusion, correlates closely with reflux events detected on pH monitoring and responds to acid-suppressive therapy.2–5

The receptor that mediates the sensation of heartburn in response to acid exposure has not been identified with certainty. Several types of acid-sensitive ion channel are expressed by sensory afferent neurons in the gut, including transient receptor potential cation channels (TRP channels), acid-sensing ion channels, ionotropic purinoreceptor channels and acid-sensitive tandem-pore channels.6 The vanilloid receptor 1 (TRPV1) seems a particularly attractive candidate as the receptor that mediates the sensation of heartburn. The TRPV1 receptor is a cation channel, is expressed by sensory neurons, which triggers a burning and painful sensation when activated by acid pH, heat, lipoxygenase products, anandamide or ethanol.6–11 The TRPV1 receptor has been implicated in gastrointestinal inflammatory processes, and TRPV1 deficient mice are more resistant to acid-induced oesophagitis.12,13

In the human oesophagus, the TRPV1 receptor is expressed on oesophageal nerve fibres, and increased expression on nerve fibres was found in oesophageal mucosal biopsies from patients with reflux oesophagitis compared with healthy controls.14 It is well established that exposure of TRPV1 to capsaicin, the active ingredient of chilli peppers, is characterized by receptor activation, followed by receptor desensitization.15 Previous studies have reported changes in oesophageal motility after the administration of capsaicin solutions,16–18 but the effect on acid sensitivity has not been addressed. The aim of the present study was to investigate (i) whether oesophageal capsaicin instillation induce symptoms reminiscent of heartburn, (ii) whether oesophageal sensitivity to acid perfusion is altered after capsaicin instillation, and (iii) whether oesophageal sensitivity to balloon distention is altered after capsaicin instillation.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. References

Study population

A total of three different study protocols were used. In each of the protocols, ten healthy volunteers participated. None of them had symptoms or a history of gastrointestinal disease or drug allergy, nor were any of them taking any medication. Written informed consent was obtained from each participant and the study protocol was approved by the Ethics Committee of the University Hospital Gasthuisberg, Leuven, Belgium.

Study 1: symptoms induced by capsaicin perfusion

Ten healthy subjects (four men and six women; mean age, 27 ± 1 year) participated in this protocol. Each participant was studied on two occasions with at least a 1-week interval. After an overnight fast, a manometry catheter with an attached 1.5 mm diameter perfusion catheter was inserted trough the nasal orifice. Correct position of the manometry catheter was confirmed by fluoroscopy and manometrical recordings. The perfusion catheter was attached to the manometry catheter enabling it to be positioned in the middle third of the oesophagus. The entire protocol was performed in a semi-recumbent position. A test solution containing 0 or 3 mL of a 0.17 mg mL−1 capsaicin solution, diluted with saline to a total volume of 10 mL, were randomly tested in each participant. The study protocol is represented schematically in Fig. 1A. The capsaicin dose was based on previous studies in the stomach and oesophagus.16–19 After an adaptation period, the 10 mL test solution was infused into the mid-oesophagus. At 5-min intervals, starting 15 min prior to perfusion until 60 min after perfusion, 100-mm visual analogue scale (VAS) scores were used to record the severity of eight oesophageal and epigastric symptoms (retrosternal burning, retrosternal pain, epigastric burning, nausea, fullness, bloating, belching and satiety).

image

Figure 1.  Schematic representation of the three study protocols (A, B and C).

Download figure to PowerPoint

Study 2: influence of capsaicin perfusion on sensitivity to acid perfusion

Ten healthy subjects (four men and six women; mean age, 24 ± 1 year) participated in this protocol. Each participant was studied on three occasions with at least a 1-week interval. After an overnight fast, a manometry catheter with an attached 1.5 mm diameter perfusion catheter was inserted trough the nasal orifice. Correct position of the manometry catheter was confirmed by fluoroscopy and manometrical recordings. The perfusion catheter was attached to the manometry catheter enabling it to be positioned in the middle third of the oesophagus. The entire protocol was performed in a semi-recumbent position. A test solution containing different doses of a 0.17 mg mL−1 capsaicin [0 (placebo), 0.5 or 3.0 mL] diluted with saline to a total volume of 10 mL were randomly tested in each participant. The study protocol is represented schematically in Fig. 1B. After an adaptation period, the 10 mL test solution was infused into the mid-oesophagus. At 5-min intervals, starting 15 min prior to perfusion until 60 min after perfusion, 100-mm visual analogue scale (VAS) scores were used to record the severity of eight oesophageal and epigastric symptoms (retrosternal burning, retrosternal pain, epigastric burning, nausea, fullness, bloating, belching and satiety).

Sixty minutes later, 0.1 N HCl perfusion was started at a rate of 6 mL min−1 for 30 min. Immediately prior to and every 5 min after injection of capsaicin and until the end of acid perfusion visual analogue scale (VAS) scores of ten different symptoms (retrosternal burning, retrosternal pain, retrosternal cramps, epigastric discomfort, epigastric burning, nausea, fullness, bloating, belching and satiety) were recorded.

Study 3: influence of capsaicin perfusion on sensitivity to oesophageal balloon distention

Ten healthy subjects (four men and six women; mean age, 28 ± 1 year) participated in this protocol. Each participant was studied on three occasions with at least a 1-week interval. After an overnight fast, a manometry catheter with an attached 1.5 mm diameter perfusion catheter was inserted trough the nasal orifice. Correct position of the manometry catheter was confirmed by fluoroscopy and manometrical recordings. The perfusion catheter was attached to the manometry catheter enabling it to be positioned in the middle third of the oesophagus. A polyvinyl balloon (length 30 mm, maximal diameter 27 mm at 10 mL distension) was positioned 3 cm distal to the perfusion port. A separate lumen and a three-way stopcock allows to inflate air into the balloon at 1 mL increments by means of a calibrated syringe. The entire protocol was performed in a semi-recumbent position.

A test solution containing different doses of a 0.17 mg mL−1 capsaicin [0 (placebo), 0.5 or 3.0 mL] diluted with saline to a total volume of 10 mL were randomly tested in each participant. The study protocol is represented schematically in Fig. 1C. After an adaptation period, the balloon attached was inflated with air by increments of 1 mL. At each distending step, the subject was asked to report perception on seven-point oesophageal discomfort/pain scale that combines verbal and numerical descriptors (0–6; 0 = no sensation; 1 = first perception; 5 = discomfort; 6 = pain), and the corresponding intra-balloon volumes were registered. The end point of the sequence of distentions was established at an intra-balloon volume of 15 mL, or when the patients reported discomfort or pain (score 5 or more).Subsequently, the 10 mL test solution was infused into the mid-oesophagus. Balloon distentions were repeated 30, 60 and 90 min later.

Data analysis

For each patient the area under the curve (AUC) during the first 15 min following the test solution and 15 min following the start of acid perfusion was calculated for each symptom. When volunteers did not complete the acid perfusion period; the last VAS rating for each symptom was carried over to the remaining data points.

During oesophageal balloon distention, oesophageal compliance was calculated as the slope of the volume–pressure relationship using a linear regression model for each subject at each distention series. The discomfort threshold was defined as the first level of intra-balloon volume that provoked a perception score of 5 or more.

Statistical analysis

Descriptive statistics were performed using Chi-square testing and paired Student’s t-Test. AUCs after perfusion of placebo, acid and capsaicin solutions were compared using the Kruskal–Wallis test, as values were not normally distributed. A P-value < 0.05 was considered significant. For balloon distentions, variables were compared before and after administration of the different test solutions. Analyses were performed with SAS 9.1 (SAS Institute, Cary, NC, USA). Results are shown as mean ± SEM.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. References

Symptoms induced by capsaicin instillation

Prior to infusion of the test solution, VAS symptom ratings did not differ on the placebo and capsaicin treatment days. Compared with baseline, instillation of capsaicin induced a significant rise in VAS scores for retrosternal burning and epigastric burning during the first 14 to 12 min (Fig. 2A), while saline had no significant effect on any of the symptom ratings. Significantly higher values for the AUCs after administration of the test solution were obtained after capsaicin for retrosternal burning, epigastric burning and nausea (Fig. 2B).

image

Figure 2.  (A) Visual analogue scale score (VAS) scores in response to oesophageal instillation of saline or capsaicin solution in the first study protocol. Only scores for retrosternal and epigastric burning are shown. *: time period with significantly higher epigastric burning scores after capsaicin. †: time period with significantly higher retrosternal burning scores after capsaicin. (B) Area under the curve (AUC) for sensations induced by saline or capsaicin solution instillation. *: P < 0.05.

Download figure to PowerPoint

Symptoms induced by acid instillation after saline or capsaicin pretreatment

Baseline infusion confirmed significantly greater AUCs for epigastric and retrosternal burning with the high dose of capsaicin compared with saline, while no significant differences with saline were observed for the lower capsaicin dose (Fig. 3A). After each of these pretreatments, oesophageal acid perfusion was associated with a significant rise in ratings for epigastric and retrosternal burning (Fig. 4). However, the type of baseline infusion had no significant influence on the AUCs during acid perfusion for any of the symptoms (Fig. 3B).

image

Figure 3.  (A) Area under the curve (AUC) for sensations induced by oesophageal instillation of saline, low or high doses capsaicin in the second study protocol. *: P < 005 compared with saline. (B) Area under the curve (AUC) for sensations induced by acid perfusion after previous instillation of test solutions with saline, low or high doses capsaicin. No significant differences were observed.

Download figure to PowerPoint

image

Figure 4.  Symptom intensities for retrosternal burning (A) and epigastric burning (B) during initial instillation of test solutions with saline, low or high doses capsaicin and subsequent acid perfusion.

Download figure to PowerPoint

Comparison of symptoms induced by saline, capsaicin and acid instillation

Acid perfusion induced more retrosternal burning compared with saline (< 0.01) or compared with the low dose of capsaicin (P < 0.005), while the difference with high dose capsaicin was not significant (P = 0.07). Symptom intensities between saline, capsaicin or acid perfusion did not differ significantly for any of the other symptoms. No inverse correlations for symptom intensities after pretreatment or acid were seen, for any of the symptoms. Correlations between intensities of retrosternal and epigastric burning after acid or after high dose capsaicin perfusion tended to be significant (r = 0.58 and 0.59 respectively, P = 0.08 for both).

Influence of capsaicin on sensitivity to oesophageal balloon distention

The threshold to induce discomfort at baseline on the placebo-perfusion day was a balloon volume of 12.2 ± 1.2 mL, at a corresponding intra-balloon pressure of 69.9 ± 8.2 mm Hg, and a balloon compliance of 5.5 ± 0.7 mm Hg mL−1. These parameters were not changed 30, 60 and 90 min after saline instillation (Fig. 5A,B; pressures respectively 71.6 ± 7.6, 71.4 ± 7.0 and 70.9 ± 8.7 mmHg mL−1, NS). Similarly, instillation of low or high doses of capsaicin failed to induce changes in compliance or thresholds to oesophageal balloon distention (Fig. 5A,B).

image

Figure 5.  (A) Discomfort threshold for oesophageal balloon distention before and after oesophageal instillation of test solutions with saline, low or high doses capsaicin. (B) Oesophageal compliance during oesophageal balloon distention before and after instillation of test solutions with saline, low or high doses capsaicin.

Download figure to PowerPoint

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. References

In this study, we demonstrated that capsaicin induced the sensations of retrosternal and epigastric burning in healthy volunteers. These findings are in agreement with the demonstrated expression of TRPV1 receptors in the human oesophagus, and perhaps the gastric mucosa.13,20 However, we found no alteration of oesophageal sensitivity to acid perfusion or to oesophageal balloon distention after capsaicin pretreatment, although the TRPV1 receptor desensitizes rapidly to administration of capsaicin.15

While the first observation is supportive of a role for TRPV1 receptors in the induction of heartburn, the lack of desensitization of acid sensitivity after capsaicin might seem to argue against their involvement. In vitro, the desensitizing properties of capsaicin occur rapidly and may result from dephosphorylation of TRPV1 by calcium-induced calcineurin activation.15,20,21 Desensitization to chemical pain stimuli after brief administration of capsaicin has been reported in animal models and in man, lasting from several hours to days.22–26 The desensitizing potential of capsaicin has been demonstrated for the skin, the nasal mucosa and the tongue in man.24–26 Evidence of capsaicin-induced desensitization in the gastrointestinal tract in man is less clear, as a number of studies demonstrated that administration of capsaicin induces acute pain and discomfort, followed by hypersensitivity in some studies. In the oesophagus, thresholds to oesophageal balloon distention were lowered during acute instillation of a capsaicin-containing red pepper sauce suspension, but the effect on subsequent distentions was not assessed.16 Ingestion of a capsaicin-containing capsule with a meal enhanced initial postprandial sensations of heartburn and dyspepsia in GERD patients, but tended to decrease heartburn sensations in the later postprandial phase although acid exposure was not different.27 In the stomach and in the ileum, administration of capsaicin increased sensitivity to balloon distention.19,28 Acute jejunal or duodenal instillation of capsacin induces burning sensations and pain, but does not alter sensitivity to jejunal balloon distention.29,30 Repeated administration of capsaicin-containing capsules is associated with decreased sensitivity to intestinal balloon distention.31,32 These studies show that acute administration of capsaicin induces chemical nociception, while the short- and longer-term effect on mechanosensitivity seems to be variable. None of these studies assessed the effect of capsaicin administration on sensitivity to acid, but the stomach and small bowel lack the prominent sensitivity to acid that is found in the oesophagus.

In vitro studies have shown that recovery of TRPV1 from desensitization is dependent on phophatidylinositol 4,5-diphosphate synthesis, which may occur relatively rapidly.21 On the other hand, in vivo and human studies have reported durations of desensitization of several hours to a couple of weeks after acute exposure,22–26 and motor effects in the upper gastrointestinal tract, including gastric emptying delay, persisted longer after oesophageal capsaicin instillation.16 Hence, we think that the current 60-min time-frame is accurate for in vivo assessment of TRPV1 receptor desensitization. It has been argued that repeated exposure to capsaicin provides a more complete desensitization,33,34 but it is unclear whether this applies to oesophageal stimulation.

The present study therefore has a number of limitations with respect to the attempt to obtain capsaicin-induced desensitization of heartburn sensation. First of all, it is unknown which concentrations would be required to induced desensitization in vivo. In the present study, only the highest dose of capsaicin was able to induce symptom intensities that were comparable with those induced by acid instillation. The current highest dose was based on documented use in the oesophagus in man. However, it is conceivable that clinically relevant desensitization can only be obtained when capsaicin induces symptoms of a much greater magnitude, which is beyond the present experience with oesophageal applications of capsaicin in man. The time interval between capsaicin and acid instillation or subsequent capsaicin instillations is another critical factor to consider. The time frame in the present was chosen to have obtained complete disappearance of the capsaicin-induced sensations, which could last more than 30 min after the instillation. In the present study, we did not explore whether a shorter time interval before acid perfusion or balloon distention, or whether repeat administration of capsaicin would be more efficacious in inducing desensitization. While addressing these questions is relevant, they fall outside of the scope of the present, acute study.

In the absence of clear desensitization of acid sensitivity after oesophageal capsaicin instillation, the receptor that mediates acid-induced heartburn remains to be identified with certainty. Besides the TRPV1 receptor, other candidates are the acid sensing ion channels (ASICs).6 Expression of ASICs has indeed been shown on primary afferent neurons in animals, and this receptor may also be involved in transduction of acid and noxious stimuli.35

It has been reported that visceral mechanosensitivity seems to be diminished in TRPV1 receptor knockout animals.36 We therefore also studied the influence of capsaicin pretreatment on sensitivity to balloon distention, but failed to observe significant changes in compliance or sensitivity to oesophageal balloon distention. Previously, Gonzalez et al. reported lowered thresholds to oesophageal balloon distention during instillation of a capsaicin-containing solution.16 However, this likely reflect summation of different sensory modalities from the stomach, given the overlapping timeframe of capsaicin instillation and balloon distention in that study. Sensitivity to balloon distention after disappearance of capsaicin-induced sensations were not addressed in this study.16 The use of air-filled balloons to assess mechanosensitivity of hollow viscera has limitations, as this is also inducing contractions, which may alter tone of the viscous, thereby modulating sensory perception, and may also induce balloon movement. Some of these limitations may be overcome by other stimulus modalities, such as electrical or heat stimulation, which could be used in future studies.37

In conclusion, oesophageal capsaicin instillation induced symptoms of retrosternal and epigastric burning, but had no effect on sensitivity to subsequent acid perfusion or balloon distention. Our observations demonstrate that TRPV1 receptor activation has the potential to induce heartburn, but fails to confirm involvement of the same receptor to acid perfusion or oesophageal mechanosensitivity. Addressing this question will probably require the use of TRPV1 receptor antagonists.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. References