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Keywords:

  • chemosensitivity;
  • functional dyspepsia;
  • sensitization;
  • TRPV1

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Disclosures
  9. Author Contribution
  10. Competing interests
  11. References

Background  Sensory sensitization is one of the main pathophysiological hypotheses in functional gastrointestinal disorders (FGIDs). As sensitization may affect various sensory modalities, we aimed to develop a reproducible gastric pain model utilizing polymodal pathways for use in functional and other pain disorders.

Methods  In this double-blind, cross-over study 42 healthy subjects swallowed one capsule containing either capsaicin 0.5 mg or nocebo every 15 min until moderate pain (intensity >30 on 100 mm visual analogue scale) was attained for at least 5 min. Pain was rated every minute. Capsaicin titration was repeated thrice for reliability calculation.

Key Results  Moderate pain in the upper abdomen was successfully achieved in 38 of 42 subjects (90%) with capsaicin titration and in one of 42 (2%) with nocebo. The median dosage required to induce moderate pain for at least 5 min was two capsules (interquartile range 1–3) and the median gastric pain intensity was 47 (41–53). The median duration of moderate pain was 8 min (5–12). Moderate pain was successfully reproduced with capsaicin in all subjects on study days 2 and 3, with an excellent Cronbach reliability coefficient of >0.8.

Conclusions & Inferences  Standardized gastric pain can be conveniently achieved in a majority of healthy subjects using a simple oral capsaicin titration, with minimal adverse events. The between-test reproducibility is high and nocebo responses are negligible. This technique stimulating a multimodal physiological pathway will be useful in the investigation of sensory changes in FGIDs, including functional dyspepsia.


Abbreviations:
CGRP

calcitonin-gene-related peptide

cGMP

cyclic guanosine monophosphate

FD

functional dyspepsia

GI

gastrointestinal

H. pylori

Helicobacter pylori

IBD

inflammatory bowel disease

IBS

irritable bowel syndrome

NKA

neurokinin A

SP

substance P

TRPV1

transient receptor potential ion channel of the vanilloid type 1

VAS

visual analogue scale

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Disclosures
  9. Author Contribution
  10. Competing interests
  11. References

Upper abdominal pain or discomfort without any discernable cause is the hallmark of functional dyspepsia (FD) (1). Patients with FD have a poor quality of life, which is even more impaired than patients with organic upper gastrointestinal (GI) disease (2). Its treatment is largely unsatisfactory, mainly due to a poor understanding of the underlying pathophysiology. A standardized and near-physiological experimental human gastric pain model would be helpful in achieving a better understanding of the sensory mechanisms involved in the pathogenesis of FD.

Barostat distension using an intragastric balloon has been used to study mechanical hypersensitivity, abnormal meal-related accommodation, and tone in FD (3). It currently remains unclear if sensitization of other sensory modalities also exists in FD. Chemosensitivity is a physiological mechanism central to every ingestion and symptoms in FD (4,5). The transient receptor potential vanilloid type 1 (TRPV1) receptor, an important chemosensor, is expressed widely on primary afferent neurons innervating the GI tract and many other organs (6). It is an established polymodal nociceptor not only involved in chemosensing, but is also activated by varied stimuli, including acid, intracellular lipid mediators, noxious heat, and other painful stimuli (6). Its potential role in mechanosensory transduction in the GI tract has been explored recently (7). Notably, TRPV1 is significantly involved in the link between immune activation, neuroinflammation, and nociceptive sensitization, one of the main etiological hypotheses in functional GI disorders (6,8). Capsaicin, the pungent ingredient in hot pepper, is a ligand of the TRPV1 receptor. Therefore, in contrast to barostat distension, TRPV1-stimulation by capsaicin is likely to represent a polymodal stimulation involving several physiological pathways central to sensitization.

A few studies using capsaicin stimulation in the upper GI tract have been reported in healthy volunteers, and in patients with FD. Tack et al. first showed an altered sensorimotor function of the proximal stomach in healthy controls with infusion of red pepper by oral intubation (9). Subsequently, Hammer et al. demonstrated jejunal hypersensitivity to capsaicin in persons with un-investigated dyspepsia, using jejunal perfusion with capsaicin (10). Very recently, the same Austrian group presented a simple capsule-based test using a fixed dose of 0.75 mg of capsaicin to distinguish between patients with FD and healthy controls (11). A wide range of sensory responses were seen in controls and patients with FD, but most did not report pain (11). Reaching sufficient pain intensity is important in the study of sensory sensitization, as sensitization is most prominent during activation of the high-threshold C-fibers. As the capsaicin-binding receptor TRPV1 is almost exclusively seen in C-fiber neurons, adequately intense stimulation will be most likely to reveal sensitization (12). Discomfort or non-painful stimulation is less likely to reflect sensitization or hyperalgesia and may be less effective to demonstrate abnormal endogenous pain modulation (13). Therefore, in this study we aimed to develop a gastric pain model to reliably induce moderate pain intensity using individual oral titration of capsaicin. We also evaluated the reproducibility of this simple sensory test as a basis for its future use in pathophysiological and therapeutic studies.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Disclosures
  9. Author Contribution
  10. Competing interests
  11. References

This was a prospective and nocebo [induction of pain by expectation when an inert substance is given in blinded fashion (14)]-controlled methodological study performed in healthy controls without any medication in a single center at the University Hospital of Singapore, Singapore from September 2008 till November 2009.

Subjects

Forty-two healthy volunteers were recruited by word of mouth. Main exclusion criteria were past history of documented upper GI diseases, current, or past history of GI symptoms or significant abdominal pain in the last 3 months, a history of other significant systemic illness, including cardiovascular, psychiatric, neurological, and endocrine diseases, any other chronic or acute pain syndromes, bowel resections or abdominal operations except appendectomy, ongoing treatment with any drugs or complementary medication within the last 14 days before the start of the study, pregnancy or lactation, and absence of written informed consent.

The protocol of the study was approved by the Ethics Committee of the University Hospital of Singapore, Singapore (Clinical Trials.gov ID NCT00693407).

Experimental procedure

Subjects were seen on four occasions. At the first visit, written informed consent was obtained, the Leeds Dyspepsia Questionnaire (15), Hospital Anxiety and Depression (HAD) (16), Perceived Stress (17) scales, and a standard dietary questionnaire (18) were completed and subjects were familiarized with the study procedures. The actual experimental procedures were performed after an overnight fasting on the remaining three study visits, which were separated by at least 2 days (see below). Tests were performed during the morning to minimize diurnal rhythm influences and within 7–14 days after the end of the menstrual period in women to standardize the influence of the menstrual period. Identical instructions were given before each test.

Double-blind dose-titration regimen using ingested capsaicin or nocebo

On the first day, volunteers were randomized to swallow identical capsules containing either capsaicin 0.5 mg (Sigma-Aldrich™, Singapore, Singapore) or lactose monohydrate 0.5 mg as nocebo (manufacture and randomization by National University Hospital, Department of Pharmacy, Singapore) with 100 mL of water in a double-blind, cross-over fashion. The order of capsule intake was randomized using a computerized allocation method (http://www.randomization.com) without blocks. Enrollment was performed by the study investigators who had no access to the randomization code. Subjects underwent both capsaicin and nocebo stimulation on the first day, with a washout period of 2 h between tests. This interval was chosen based on the earlier results by Hammer et al (19). At the subsequent two visits, only the capsaicin capsules were administered in identical fashion, albeit openly. On each study day, the volunteers were identically seated in a comfortable, semi-recumbent position.

The dose-titration procedure was the same on every study day, irrespective of which capsule was taken and the standardized endpoint was the report of moderate pain, indicated by a pain intensity of at least 30 on a 100 mm horizontal visual analogue scale (VAS) of 0 to 100, where 0 = no pain and 100 = worst pain bearable. Throughout the study subjects rated their abdominal pain and discomfort on the VAS every minute and they mapped the area of pain on a 2D body map every 5 min until the end of testing. If pain scores did not reach a minimum level of moderate pain for at least five successive minutes within 15 min after capsule ingestion, a further capsule of the same content was swallowed. This was repeated until moderate pain of at least 5-min duration was reported or a maximum number of eight capsules were ingested. After conclusion of the titration procedure discomfort and/or pain were described by the subject as a (i) sensation of abdominal pressure or fullness, (ii) sensation of cramp or colic, (iii) stinging or sharp sensation, (iv) burning sensation, or (v) nausea. The intensity of these sensations was also reported on a Likert rating scale of 0–5 (0 = none, 5 = worst intensity bearable) (20). Any adverse events were recorded at this time. After the completion of the test subjects drank 500 mL of plain water to wash out any remaining capsaicin or nocebo. If at any time during the study the pain intensity was above 80 on the VAS, pain was immediately eliminated by the drinking of 500 mL of water. Sensory data from these patients were included for analysis up to this point.

Data and statistical analysis

All statistical analyses were performed using the standard spss package (version 17.0 for Windows, Chicago, IL, USA). The inclusion of 42 healthy subjects provided 95% confidence that the current capsaicin titration pain model was able to induce moderate gastric pain in 90 ± 9.1% of the entire Singapore population of around 4.4 million in 2010. The primary study variable was the number of capsules required to induce moderate pain lasting at least 5 min. Further outcome variables were the time taken to reach the first mention of pain and the pain intensities at these two points in titration. Moderate pain referred to pain VAS score was between 30 and 54 (21). The amount of chili used per month was quantified using the published questionnaire by Kang et al (18). Continuous variables are expressed as arithmetic means with 95% confidence interval if they are normally distributed; otherwise, median values with interquartile ranges are given for skewed data. Categorical variables are expressed as frequencies and percentages. For statistical comparisons between two groups, Student’s t-test or Mann–Whitney U-test were used as appropriate. For the statistical comparison of more than two groups, anova and post hoc analysis using Tukey’s honest significance difference test were used. A two-tailed P-value <0.05 was considered statistically significant. Linear regression between demographic, psychological variables and gastric pain induction outcomes were performed. A P-value <0.05 suggested that, the slope of the regression line differed significantly from zero. The reliability was tested by calculating the value of Cronbach’s alpha, the coefficient of reliability, and measuring the internal consistency. A ‘high’ value of alpha is consistent with a high internal consistency. A reliability coefficient of ≥0.7 is considered as evidence of good reproducibility (22).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Disclosures
  9. Author Contribution
  10. Competing interests
  11. References

Characterization of healthy subjects

The demographic data and further characteristics of the 42 healthy participants are listed in Table 1. The majority of studied subjects were middle-aged Chinese with levels of anxiety and depression within the normal range. The mean stress scores of 18.3 (Table 1) were higher than the established community norm of 12 in Caucasians (17). However, the present stress score values are comparable to those from a healthy Asian cohort, where mean stress levels of 18 were reported (23). Most of the healthy volunteers led a healthy lifestyle with regular exercise and with no smoking or alcoholic drinking habit. Around half of the participants regularly consumed caffeine. In the chili dietary examination, only a small minority avoided chili meals, due to a dislike of chili.

Table 1.   Characteristics of participants
 Healthy subjects
  1. HAD, hospital anxiety and depression.

  2. *Scores of stress levels range from 0 to 40.

  3. Scores of anxiety and depression range from 0 to 21, with <7 defined as the normal range.

Number of subjects, n42
Age (95% CI), years39.1 (35.8–42.3)
Female subjects, n (%)25/42 (59.5)
Chinese subjects, n (%)31/42 (73.8)
Stress scores*, mean (95% CI)18.3 (16.6–20.0)
Anxiety HAD scores, mean (95% CI)3.7 (2.9–4.4)
Depression HAD scores, mean (95% CI)2.1 (1.5–2.7)
Regular Chili ingestors, n (%)39/42 (92.9)
 Chili units eaten per month, mean (95% CI)29.8 (20.7–39.0)
Smokers, n (%)1/42 (2.3)
Regular alcohol consumers, n (%)3/42 (7.0)
Regular caffeine consumers, n (%)24/42 (57.1)
Regular exercisers, n (%)31/42 (73.8)

Characterization of gastric pain induced by capsaicin titration

Results of capsule titration  Moderate pain was successfully achieved with the standardized capsaicin titration in 38 of the 42 (90.5%) healthy participants. Of the four subjects (9.5%) who failed to reach moderate pain intensity of 5 min duration, two were men, reaching only mild pain with VAS scores of 4 and 23, respectively, despite eight capsaicin capsules, and two were women reporting strong nausea before pain was attained, who consequently dropped out of the study. During nocebo titration, only one male participant (1/42, 2.4%) attained moderate pain with a VAS score of 33 after ingesting two capsules.

The median dosage of capsaicin required to attain the moderate pain endpoint was 2 (IQR: 1–3) capsules. Among the 38 healthy controls who completed the capsaicin titration, 16 (42%) required one dose (0.5 mg) of capsaicin and the remaining needed between 2 and 6 doses to reach the endpoint (Fig. 1A).

image

Figure 1.  Characterization of gastric pain and symptoms induced by capsaicin titration. (A) Number of capsaicin capsules required to induce moderate gastric pain in 38 subjects, (B) individual and group gastric pain profiles after capsaicin titration to moderate pain threshold pain. The bold horizontal line indicates the moderate pain [visual analogue scale (VAS) 30] threshold. The bold line with dots indicates the group median pain intensity profile. Each individual thin line represents one of the subject’s pain intensity profiles, (C) Percentage of patients with capsaicin-induced pain in the shown location.

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The individual gastric pain scores after the onset of moderate pain are shown in Fig. 1B. The latency and intensity of capsaicin-induced pain are summarized in Table 2. No order effects for the sequence of capsaicin or nocebo stimulation were seen (Table 2).

Table 2.   Pain characteristics in healthy subjects during capsaicin titration and order effects when ingesting either capsaicin or nocebo first
 nCharacteristics of titration to target of moderate pain*Pain characteristics once target of moderate pain* has been reached
Time to reach first pain (min)No. of capsules to reach moderate painMedian pain intensityDuration of pain (min)
  1. Medians and interquartile ranges are shown.

  2. *Pain intensity >30 on 100 mm VAS for at least 5 min.

All subjects389.0 (7–15.5)2.0 (1.0–3.0)47.3 (40.8–53.4)8 (5–12)
Subjects ingesting capsaicin first199.0 (7.3–17.8)2.0 (1.0–3.8)48.3 (43.3–51.9)8.0 (5.3–9.0)
Subjects ingesting nocebo first198.5 (7–11.5)2.0 (1.0–3.0)43.0 (40.0–54.4)11.5 (5.8–15)
P-value for order effect capsaicin vs nocebo first0.390.350.740.16

Demographic factors and capsaicin-induced pain  The correlations between the demographic factors and the main pain variables are shown in Table 3. Caffeine consumption was associated with a 10-min longer onset time to moderate pain compared to non-consumers (< 0.05). Age was individually correlated with the intensity of first pain (r = 0.40, < 0.03). The linear regression between the amount of chili consumed monthly and gastric pain measures was weak and did not reach significance (r = 0.32, = 0.058). No other significant associations were observed between the demographic data and pain variables.

Table 3.   Associations between demographic factors and capsaicin-induced pain
 Gender (female vs male)Chinese (yes vs no)Regular caffeine consumers (yes vs no)
  1. Medians and interquartile ranges are shown.

  2. VAS, visual analogue scale.

  3. *= 0.04.

Subgroup size (n)23 vs 1526 vs 1222 vs 16
Time to reach first pain (min)9.0 (7.0–20.5) vs 8.5 (7.0–10.8)9.0 (7.0–12.0) vs 8.0 (7.0–10.5)8.5 (7.0–19.8) vs 9.0 (6.0–11.0)
Intensity of first pain (VAS)14.0 (8.0–23.0) vs 10.5 (6.3–21.5)11.0 (6.0–20.0) vs 20.0 (10–22)14.0 (7.0–25.0) vs 13.0 (8.0–20.0)
Time to reach moderate pain (min)13.0 (10.0–28.0) vs 21.5 (10.0–27.0)15.0 (10.0–36.0) vs 11.0 (8.0–27.0)23.5 (10.0–36.5) vs 11.0 (8.0–21.0)*
Intensity of moderate pain (VAS)36.0 (31.5–42.5) vs 30.5 (30.0–39.5)36.0 (30.0–42.0) vs 30.5 (30.0–37.0)35.0 (30.0–42.0) vs 36.0 (31.0–40.0)

Location of capsaicin-induced pain  The areas of pain radiation recorded during the capsaicin titration are shown in Fig. 1C. All subjects showed pain in the epigastric and umbilical areas. Twelve subjects (12/38, 32%) reported pain at other sites, including the lumbar (7/38, 18%), hypogastric (4/38, 11%), and retrosternal (2/38, 5%) regions.

Characterization of gastric symptoms induced by capsaicin titration  The most prominent gastric sensations and their median intensities during capsaicin titration were burning sensation: 3 (IQR 2.3–4, on a scale of 0 = none to 5 = worst bearable intensity), abdominal pressure/fullness: 3 (0–4), abdominal cramps: 2 (0–4), abdominal stinging: 2 (0–3), and some nausea: 0 (0–2.5). During nocebo titration five participants (5/42, 12%) attained very mild abdominal fullness with median scores of 1, two subjects (2/42, 5%) reported a moderate stinging sensation with symptom scores of 2 and 3, one (1/42, 2%) had both mild burning and nausea at respective scores of 1, and no one reported cramps.

Reliability of capsaicin-induced pain  The complete analysis of reliability was evaluable in 36 subjects across three study days and in 37 across two study days. As previously mentioned, four subjects did not achieve moderate pain, the one nocebo responder was excluded, and an additional woman withdrew from the study after completion of the first two titration tests. Therefore, the number of subjects in whom moderate pain was successfully achieved were 38/42 (90.5%) on study day 1, 37/37 (100%) on study day 2, and 36/36 (100%) on study day 3. A similar proportion of subjects attained moderate pain after a single capsule on each of the three study days: 18/38 (47.4%) on study day 1, 18/37 (48.6%) on study day 2, and 17/36 (47.2%) on study day 3. All subjects reporting a burning sensation also had other symptoms, but the accompanying symptoms were variable. The median number of symptoms per subject is 3 (IQR: 3–4) on day 1, 3 (2–4) on day 2, and 3 (3–4) on day 3. The results of the reliability analysis are shown in Table 4. The time to onset of moderate pain across the 3 days was not significantly different between the repeated measurements (= 0.45). All Cronbach alpha-values were >0.7, which is considered as evidence of good reproducibility. Individual data are displayed in Figs 2 and 3.

Table 4.   Reliability of capsaicin-induced pain and gastric symptoms across three study days
 Study day 1Study day 2Study day 3Cronbach alpha-value: reliability across 3 study days
  1. Medians and interquartile ranges or number of patients (percentages) are shown.

  2. A Cronbach reliability coefficient ≥0.7 is considered as evidence of good reproducibility.

  3. *See text for exclusions.

Induction of gastric pain
 Attaining moderate pain, n (%)38/42 (90.5)*37/37 (100)*36/36 (100)*
 Capsaicin capsules to attain moderate pain2 (1–3)2 (1–2)2 (1–2)0.8
 Time to onset of first pain (min)9.0 (7.0–19.0)9.0 (7.0–12.0)9.0 (6.0–9.3)0.7
Incidence of other gastric symptoms (%)
 Pressure28/38 (73.7)22/37 (59.5)25/36 (69.4)0.8
 Cramps22/38 (57.9)20/37 (54.1)23/36 (63.9)0.8
 Stinging20/38 (52.6)22/37 (59.5)22/36 (61.1)0.8
 Burning34/38 (89.5)33/37 (89.2)32/36 (88.9)0.9
 Nausea19/38 (50)17/37 (45.9)15/36 (41.6)0.7
image

Figure 2.  Individual data: capsaicin-induced pain. (A) Capsules of capsaicin required for moderate pain, (B) the time to onset of first pain sensation.

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image

Figure 3.  Individual data: capsaicin-induced symptoms. The occurrence of (A) abdominal pressure or fullness, (B) abdominal cramps, (C) abdominal stinging, (D) abdominal burning, and (E) nausea.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Disclosures
  9. Author Contribution
  10. Competing interests
  11. References

The purpose of this study was to establish a reproducible, individualized, and simple gastric pain model. The oral capsaicin titration protocol reliably and conveniently induced moderate gastric pain with minimal adverse events in more than 90% of all healthy subjects. In addition, nocebo effects were negligible, i.e., <3%.

Visceral pain models include chemical, electrical, thermal, and the most extensively validated barostat-controlled luminal distension. The ideal human visceral pain model should be sensitive, minimally invasive, physiological, reproducible, and quantifiable (24,25). The current validated pain models in the upper GI tract only partially meet these requirements. Gastric barostat studies are established methods for assessing gastric sensitivity to distension, compliance, and meal-induced accommodation. Visceral sensory abnormalities have been demonstrated in a subset of patients with FD using this technique (3). However, the technique is invasive, time-consuming, costly, relatively non-physiological, and better suited to assessing mechanical than multimodal sensory changes. Nonetheless, a recent retrospective review of the technique reported a low procedure-related dropout rate (26). In addition, a wide range of intense, non-nociceptive sensations induced by the intubation may bias measurements of tone and sensation (27,28). Other technical pitfalls of gastric barostat use include the lack of consistent reproducibility and several commonly encountered artifacts, such as low baseline volumes, air leak, and negative accommodations, which may lead to difficulties in the interpretation of results (26,27,29,30). The drink test represents a non-invasive and cheap alternative for assessing gastric accommodation with good reproducibility (25), which has also been proposed to discriminate patients with FD from normal subjects (31,32). However, these tests have limited applicability in studies of visceral nociception and sensitization, as they predominantly evoke sensations of satiety or fullness and not pain (33). Electrical stimulations are also less useful due to their very localized, non-specific and non-physiological nature, despite their ability to reliably and reproducibly evoke gut pain (24).

Oral capsaicin titration is advantageous in being a sensitive, minimally invasive, convenient, reproducible, quantifiable, and well-tolerated method to induce gastric pain in 90% of healthy subjects. The time to onset of first pain and to sustained, moderate pain was relatively short, with medians of less than 10 and 20 min, respectively. The evoked pain was primarily localized in the upper abdominal region.

The induction of a standardized level of gastric pain by capsaicin requires individual titration, as no fixed dose achieved stable moderate pain in the majority of participants. The median capsaicin dose required to establish moderate pain was 1 mg (two capsules), which is only slightly higher than the dose of 0.75 mg used in a previous study by Hammer et al (11). However in that study, the 0.75 mg dose failed to evoke pain sensations in half of the healthy participants. Moreover, in another study by Hammer et al. healthy volunteers barely developed any symptoms during ingestion of 0.50 mg capsaicin (19), whereas in our study almost half of the healthy subjects reached stable moderate pain using exactly the same dose. Possible reasons for this discrepancy may be differences in subjects’ characteristics, such as age, ethnicity, dietary habits, and the formulation of the capsaicin itself. The average age of subjects in the present study was more than 10 years older than in Hammer et al.’s fixed dosage study (11) (39.1 ± 1.7 years vs 26.5 ± 4.3 years). Moreover, increased age was shown to be significantly correlated with increased first pain intensity in our healthy participants (11). Therefore, age may be a relevant factor in the sensitivity to capsaicin-induced pain, which may be partially explained by central pain regulation, as endogenous pain modulation diminishes with increasing age (34). Hammer et al., however, found no relationship between age and overall visceral sensation (pain was not attained) (11), and levels of peripheral TRPV1 expression indicated by numbers of TRPV1-immunoreactive fibers were not significantly dependent on age (35). The absence of a significant gender difference in capsaicin response corroborates earlier data (11). Ethnic differences in both experimental and clinical pain and its management have been reported, although little data exists in Asians (36,37). Besides age, gender, and ethnicity, diet could be another factor affecting capsaicin-induced gastric pain. It is well-known that prolonged exposure to capsaicin causes reversible desensitization of sensory neurons and thereby attenuates their sensitivity to noxious stimuli (38). The traditionally high use of chili in Asian diets could be expected to influence the result of the capsaicin stimulation test, but no correlation was found between chili consumption and the pain measures. A significantly longer latency for induction of moderate, but not first pain was found in regular caffeine drinkers. Caffeine has numerous effects on physiological functions including pain regulation (39), with conflicting reports regarding its analgesic action (40–43). It is primarily an antagonist of the neurotransmitter adenosine and regular caffeine consumption leads to up-regulation of adenosine receptors (44). As the activation of TRPV1 promotes release of adenosine, which produces analgesia by activating A (1) and A (2A) adenosine receptors on central and peripheral neurons (45), it is plausible that the present observation resulted from the promotion of analgesic activity via adenosine receptors.

Stimulation of TRPV1 receptors is a physiological mechanism. The activation of this receptor leads to rapid influx of extracellular calcium, which produces nerve cell action potentials and nerve cell firing, activates synthesis of intracellular nitric oxide and cyclic guanosine monophosphate (cGMP), and also release of substance P (SP), neurokinin A (NKA), calcitonin-gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), and adenosine (46). Numerous physiological processes are affected following TRPV1 stimulation, including the regulation of GI perfusion, secretion, mucosal homeostasis, motility, neurogenic inflammation, and nociception (6). TRPV1 is activated not only by capsaicin, but also by a variety of physical stimuli, such as acid, intracellular lipid mediators, noxious heat, mechanical stimuli, and many xenobiotics. The participation of TRPV1 in GI nociception is well established in clinical visceral pain syndromes (6,8,11,47–50). There is thus ample evidence supporting an important role of TRPV1 in GI sensory function, and further physiology and pathophysiology, including the food hypersensitivity which is commonly observed in functional bowel syndromes like FD and irritable bowel syndrome (IBS). The classic postprandial symptoms in FD could be ascribed in part to elevated chemosensitivity or mechanosensitivity to ingested foods via the polymodal TRPV1 receptor. The range of pain and non-pain sensory characteristics elicited during capsaicin stimulation corresponds well with those seen clinically in FD (48) (11,19) and the non-pain sensations are in-line with previous findings (11,19,20), where oral intake of capsaicin capsules mainly induced abdominal pressure, burning sensation, and some nausea. Non-pain symptoms, such as abdominal fullness and nausea, have been consistently shown in our present and others’ oral capsaicin ingestion studies. They may be due to TPRV1’s role in the release of several important neuropeptides, e.g., SP and NKA, which mediate changes in GI physiology, including the increase of the lower esophageal sphincter pressure (51), the inhibition of gastric emptying (52,53), the contraction of smooth muscles, and nausea (54). The non-pain effects of capsaicin TRPV1 stimulation may relate to these changes. Preliminary data showing hypersensitivity in FD vs controls using this model has been published very recently (55).

Limitations of this study include the lack of direct evidence to show where the oral capsaicin capsules acted. Our body map data and the symptom and pain onset indicate the actions are likely to be evoked in the upper GI tract. However, whether the capsaicin acted primarily in the stomach or duodenum remains unsubstantiated. Evidence supporting the stomach rather than the duodenum as the main source of symptoms includes the rapid resolution of symptoms when water is ingested (11,19,20), and the endoscopic observation of pooled capsaicin in the gastric corpus but not beyond after oral ingestion (11). However, with the present understanding of the pathophysiology of FD, this may not be of prime importance, as both gastric and duodenal mechanisms are probably involved (3,56,57). Another potential confounder is the open capsaicin dose titration on study days 2 and 3, whereas the titration on day 1 was nocebo-controlled and visually blinded. However, the results of the titrations whether open or blinded, were very similar regarding time to pain onset, required dose, and pain intensity. In addition, we found no order effect for the sequence of the capsaicin or the nocebo dosing. The washout time of 2 h between capsaicin and nocebo ingestion was hereby also shown to be adequate, without any carry-over effects. Finally, Helicobacter pylori status was not controlled for and could be a confounding issue. However, H. pylori eradication does not generally reduce dyspeptic symptoms and no conclusive evidence has emerged regarding gastric hypersensitivity and H. pylori infection (58–61).

In conclusion, this study demonstrated that oral capsaicin titration can be used conveniently and reliably to induce moderate upper abdominal pain in healthy subjects. This gastric pain model will be used more widely in the study of functional GI and chronic visceral pain disorders in the near future.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Disclosures
  9. Author Contribution
  10. Competing interests
  11. References

We wish to thank Ms. Tung Siew Lai, the research assistant for this study, for her expert help in study coordinating and subject recruitment. We also wish to thank Ms. Siang Nee Teoh et al. for their expert help in manufacture and randomization of the study capsules.

Disclosures

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Disclosures
  9. Author Contribution
  10. Competing interests
  11. References

This study was funded in full by Singapore National Medical Research Council, Individual Research Grant. Grant number: NMRC/1120/2007.

Author Contribution

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Disclosures
  9. Author Contribution
  10. Competing interests
  11. References

YC acquisition of data; analysis and interpretation of data; drafting of the manuscript; statistical analysis; administrative, technical, or material support; CHW-S study concept and design; obtained funding; study supervision; critical revision of the manuscript for important intellectual content; analysis and interpretation of data; XL acquisition of data; administrative, technical, or material support; RKMW study supervision; critical revision of the manuscript for important intellectual content; JH study concept and design; critical revision of the manuscript for important intellectual content; KYH obtained funding; study supervision; critical revision of the manuscript for important intellectual content.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Disclosures
  9. Author Contribution
  10. Competing interests
  11. References