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Keywords:

  • 5-HT3;
  • food intake;
  • functional dyspepsia;
  • gastric barostat;
  • Ondansetron;
  • satiation;
  • visceral sensitivity

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Author contribution
  9. References

Background  Serotonin is believed to be involved in the regulation of the gastric accommodation reflex in man however which receptor subtype(s) are involved remains to be elucidated.

Methods  Eleven healthy subjects (nine men, age 19–30) underwent a gastric barostat and a drinking test after treatment with either placebo or ondansetron (8 mg intravenously). During the barostat protocol an intragastric flaccid bag was stepwise distended (2 mmHg increments 2 min) to determine gastric compliance and sensitivity to distention. Subsequently, the pressure level was set at intra-abdominal pressure +2 mmHg while volume was followed before and after administration of a liquid meal (200 mL; 300 kcal). During the drink test volunteers drank at a rate of 15 mL min−1 until maximal satiation. Results (mean ± SEM) were compared using t-tests and mixed model analysis.

Key Results  Gastric compliance was not significantly altered by ondansetron (51.5 ± 5.6 vs 49.2 ± 5.2 mL mmHg−1), neither were the pressure thresholds for first perception or discomfort. Ondansetron treatment did not affect basal gastric tone (173 ± 14 vs 156 ± 12 mL), neither did it affect the amplitude of the meal-induced relaxation (160 ± 52 vs 131 ± 43 mL) or the maximum volume increase after the meal (264 ± 54 mL vs 234 ± 51 mL). During the drinking test the amount of liquid meal ingested at maximum satiation was significantly increased by ondansetron (784 ± 74 vs 907 ± 64 mL, P < 0.05).

Conclusions & Inferences  These data suggest that 5-HT acting at 5-HT3 receptors is not involved in the control of gastric sensorimotor function, but contributes to the regulation of hunger and satiation in man.


Abbreviations:
5-HT

serotonin

HV

healthy volunteer

MDP

minimal distending pressure

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Author contribution
  9. References

During fasting, the proximal stomach maintains a high resting tone, whereas after the ingestion of a meal, a relaxation occurs, which provides the meal with a reservoir and enables a gastric volume increase without a rise in intragastric pressure.1 Abnormalities of this meal-induced relaxation, also referred to as gastric accommodation, have been implicated in the pathogenesis of symptoms in conditions such as functional dyspepsia, diabetic gastropathy and postfundoplication symptoms, and restoration of accommodation is considered a valid therapeutic target.2,3

Studies in animals and in man have shown that this accommodation reflex is mediated via a vago-vagal reflex pathway, which activates nitrergic neurones in the gastric wall.1,4,5 Moreover, studies in animals suggest that the gastric accommodation reflex involves release of serotonin (5-HT) which activates nitrergic nerves in the proximal stomach.6,7 Using the selective serotonin reuptake inhibitors paroxetine and citalopram, we demonstrated that 5-HT is involved in the gastric accommodation in man.8,9 The type of 5-HT receptor involved and its localization are unknown. Ondansetron is a selective 5-HT3 receptor antagonist, clinically used in the treatment of chemotherapy-induced nausea and vomiting.10,11 Previously, we already demonstrated the potential for acute administration ondansetron to alter interdigestive gastric motility in man.12

In the present study, we want to use the selectivity of ondansetron to test the hypothesis that a 5-HT3 receptor is involved in the control of gastric fundus tone in man. We used an electronic barostat to register variations in gastric fundus tone in healthy volunteers before and after the administration of ondansetron. We also assessed the effect of ondansetron on gastric compliance and on the perception of gastric distension, and on meal-induced gastric accommodation. Finally, we studied the influence of ondansetron on meal-induced satiation evoked by a slow nutrient drink test.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Author contribution
  9. References

Study subjects

Eleven healthy volunteers (HVs, nine men and two women, age 19–30 years) participated in this study. None of the subjects had a history of gastrointestinal or endocrinological diseases (e.g. diabetes) or surgery. Further, exclusion criteria were any other significant disease, psychological disorders or pregnancy. None of the volunteers had drug allergies or were taking any medication. Subjects were asked to refrain from alcohol, thee and coffee and intense physical activity at least 12 h before participation, moreover were they asked to refrain from smoking cigarettes at least 1 h before the start of the experiment. Informed consent was obtained from each participant. The protocol had been previously approved by the Ethics Committee of the University Hospital.

Study design

The study used a randomized, placebo-controlled, double-blind, cross-over design in which subjects underwent a gastric barostat study and a slow nutrient drinking test on four separate occasions each at least 1 week apart. For the barostat experiments, the order of treatment was randomized by drawing cards from a box of cards determining the sequence. Six subjects received saline first and five received ondansetron first. The dose of ondansetron was based on a previous study where an 8 mg intravenous dose was able to alter interdigestive gastroduodenal motility.12 The gastric barostat protocol is summarized in Fig. 1. It consisted of four parts: the first part studied acute effects on gastric resting tone, the second consisted of a distension protocol evaluating visceral sensitivity to distension and gastric compliance, the third part evaluated meal-induced gastric relaxation, and in the fourth part the distension protocol was repeated to evaluate visceral sensitivity to distension and gastric compliance postprandially.

image

Figure 1.  Outline of the barostat study.

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To evaluate the effects of ondansetron on nutrient meal tolerance, a drink test was performed in the same HVs on two separate occasions, at least 1 week apart, after ondansetron and saline. The treatment order was randomized in a similar fashion to the barostat protocol.

Barostat procedure

Following an overnight fast of at least 12 h, a double lumen polyvinyl tube (Salem sump tube 14 Ch; Sherwood Medical, Petit Rechain, Belgium) with an adherent plastic bag (1200 mL capacity; 17 cm maximal diameter) finely folded, was introduced through the mouth and secured to the subject’s chin with adhesive tape. The position of the bag in the gastric fundus was checked fluoroscopically.

The polyvinyl tube was then connected to a computer-driven programmable volume-displacement barostat device (Synectics Visceral Stimulator, Stockholm, Sweden). The barostat device can deliver volume ramps or pressure steps at different rates, while simultaneously monitoring pressure and volume at a sampling rate of 8 s. Pressure is monitored within the inflation device. To unfold the intragastric bag, it was inflated with a fixed volume of 500 mL of air for 2 min with the study subject in a recumbent position, and again deflated completely. After a 10 min equilibration period, the subjects were positioned in a comfortable sitting position with the knees slightly bent (80°) in a bed, specifically designed for that purpose.

After a 30 min equilibration period, minimal intragastric distending pressure (MDP) was determined as the lowest pressure level that provided an intrabag volume of 30 mL or more. This pressure level equilibrates the intra-abdominal pressure. To evaluate the effect of placebo or ondansetron treatment on basal gastric tone the barostat was set at a fixed intrabag pressure of 2 mmHg above MDP for 45 min, and after 15 min ondansetron (Zofran®, Glaxo Wellcome, Belgium) 8 mg or saline placebo was administered intravenously.

Subsequently, we performed a sequence of stepwise isobaric distentions. Sequential distentions were performed in stepwise increments of 2 mmHg starting from MDP, each lasting for 2 min, while the corresponding intragastric volume was recorded. Subjects were instructed to score their perception of upper abdominal sensations induced by each distending stimulus at the end of every distending step, using a graphic rating scale that combined verbal descriptors on a scale graded from 0 to 6.13 The end point of each sequence of distentions was established at an intrabag volume of 1 L, or when the subjects reported discomfort or pain (score 5 or 6). When this endpoint was reached, the balloon was deflated completely.

After another 30 min equilibration period, the intra-balloon pressure level was set at MDP + 2 mmHg for 90 min. After 30 min, a mixed liquid meal (200 mL; 300 kcal; 13% proteins, 48% carbohydrates, 39% lipids; Nutridrink®, Nutricia, Bornem, Belgium) was administered. Measurement continued for 60 min after the administration of the meal.

Hereafter a second isobaric distension protocol was performed to determine the postprandial compliance and sensitivity to distension analog to preprandial distension protocol. Subsequently, the barostat balloon was deflated and removed.

Drinking test procedure

After an overnight fast, volunteers underwent a drinking test to quantitate nutrient tolerance and the occurrence of meal-induced satiation as previously described.14 Twenty minute before the start of the nutrient drink ingestion either saline or ondansetron (8 mg) was administered intravenously. A peristaltic pump (MINIpuls2; Gilson, Villiers-Le-Bel, France) filled one of two beakers at a rate of 15 mL min−1 with a liquid meal (Nutridrink®). Subjects were requested to maintain intake at the filling rate, thereby alternating the beakers as they were filled and emptied. At 5-min intervals, they were asked to score their satiation on a scale graded 0–5 (1 = threshold, 5 = maximum satiation). Meal intake was stopped when a score of 5 was reached.

Analysis

All analyses were performed using sas statistical software version 9.1.3. (SAS Institute, Cary, NC, USA). Data are presented as the mean ± SEM. Differences were considered to be significant at the 5% level.

Barostat experiments  To study gastric tone balloon volumes were calculated as the mean volume over consecutive 5-min intervals. To evaluate the effect of ondansetron on basal gastric tone the average volumes before and after administration of ondansetron and placebo were compared using the paired Student’s t-test.

During the isobaric distension sequences to determine compliance and sensitivity to distension, before and after the meal, the dependent variable was measured by averaging the recordings during every 2-min distending step. Gastric compliance was calculated as the slope of the pressure–volume curve obtained by stepwise distentions using linear regression analysis and compared after ondansetron or placebo administration using the Student’s t-test. Perception threshold was defined as the first level of pressure that evoked a perception score of 1 or more. Discomfort threshold was defined as the first level of pressure that provoked a perception score of 5 or more. Perception and discomfort thresholds were compared using the paired Student’s t-test.

Meal-induced gastric relaxation at each time interval of the postprandial period was quantified by subtracting the average preprandial volume (in the 30 min before meal intake) from the postprandial volume. To compare postprandial relaxation over time between treatments, delta values (relaxationplacebo– relaxationondansetron) were calculated at each time point; these delta values were used as the dependent variable in all analyses described below. Mixed model analysis (using ‘proc mixed’ in sas) with time as a categorical fixed effect was used to test the hypothesis that this delta-relaxation curve was significantly different from zero over time, this corresponds to a treatment effect over time and will be referred to accordingly. Random and/or repeated effects were added to the model to account for the between- and within-subject variability structure of the data in the best possible way. The model that fitted the data best (based on Akaike’s Information Criterion) was reported.

The amplitude of the meal-induced gastric relaxation was quantified by calculating the difference between the preprandial volume and the average volume in the 60 min after the administration of the meal.15 The maximum postprandial volume increase and the time needed to reach the maximum postprandial volume were also calculated. Comparisons were made using the paired Student’s t-test.

Drink test  Satiation scores were represented as the average satiation score per 5 min. The difference in satiation score over time between both treatments was analyzed using mixed model analysis as described above. Furthermore, the volume at which HVs stopped drinking was compared using Student’s t-tests.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Author contribution
  9. References

Influence on basal gastric fundus tone

The MDP did not significantly differ between both study conditions (6.8 ± 0.7 vs 6.6 ± 0.7 mmHg). Basal intragastric balloon volumes were not significantly affected by saline (184 ± 20 vs 170 ± 22 mL before and after saline administration respectively) or by ondansetron (173 ± 14 vs 156 ± 12 mL before and after ondansetron respectively).

Influence on fasting gastric distentions

Gastric compliance was not altered by ondansetron (51.5 ± 5.6 vs 49.2 ± 5.2 mL mmHg−1, NS), and this is reflected in the pressure–volume curves after saline and ondansetron (Fig. 2A). The perception scores during gastric distention were not affected by ondansetron (Fig. 2B). The pressure thresholds for first perception (3.6 ± 0.4 vs 4.0 ± 0.6 mmHg above MPD) or discomfort (respectively 12.5 ± 1.4 vs 11.2 ± 1.4 mmHg above MPD) during gastric distentions did not differ significantly after saline and ondansetron treatment respectively. The corresponding intra-balloon volumes at first perception (269 ± 35 vs 242 ± 56 mL, NS) or discomfort thresholds (648 ± 63 vs 591 ± 53 mL, NS) were also similar after saline and ondansetron treatment respectively.

image

Figure 2.  (A) Pressure–volume relationship during fasting isobaric gastric distensions after treatment with placebo or ondansetron (n = 11). No significant differences were observed. (B) Corresponding mean perception scores for fasting isobaric gastric distensions after treatment with placebo or ondansetron (n = 11). No significant differences were observed.

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Influence of ondansetron on gastric accommodation

Preprandial volumes were not different between both treatment groups (182 ± 20 vs 163 ± 12 mL for placebo and ondansetron respectively). After meal intake intragastric volumes were not significantly different between both treatment groups (Fig. 3). Ondansetron had no significant effect on the amplitude of the meal-induced fundus relaxation (160 ± 52 vs 131 ± 43 mL). The maximum volume increase after the meal (264 ± 54 mL vs 234 ± 51 mL), and the time necessary to reach the maximum postprandial relaxation (20 ± 4 and 19 ± 2 min after the meal) did not differ significantly between the placebo and ondansetron treatment group respectively.

image

Figure 3.  Influence of pretreatment with placebo or ondansetron (n = 11) on pre- and postprandial fundic tone. Before and after the meal, mean intra-balloon volumes did not differ between both treatment arms.

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Influence on postprandial gastric distentions

Postprandial gastric compliance was not altered by ondansetron (56.8 ± 6.4 vs 51.3 ± 8.2 mL mmHg−1 after saline and ondansetron respectively), and this is reflected in the postprandial pressure–volume curves after saline and ondansetron (Fig. 4A). Perception scores during postprandial gastric distention were also not affected by ondansetron (Fig. 4B). The pressure thresholds for first perception (4.6 ± 0.6 vs 4.0 ± 0.6 mmHg above MPD) or discomfort (11.1 ± 0.6 vs 10.0 ± 0.6 mmHg above MPD) during postprandial gastric distentions did not significantly differ between the saline and the ondansetron treatment group respectively. The corresponding intra-balloon volumes at first perception (352 ± 58 vs 346 ± 76 mL) or discomfort thresholds (726 ± 72 vs 676 ± 61 mL) were not significantly different after saline and ondansetron treatment respectively.

image

Figure 4.  (A) Pressure–volume relationship during postprandial isobaric gastric distensions after treatment with placebo or ondansetron (n = 11). No significant differences were observed. (B) Corresponding mean perception scores for postprandial isobaric gastric distensions after treatment with placebo or ondansetron (n = 11). No significant differences were observed.

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Effect of ondansetron on nutrient tolerance

Satiation scores after ondansetron were consistently lower compared with placebo treatment throughout the drinking test. The effect of treatment over time was significant (F18,160 = 2.22, P < 0.005). Significances of post hoc tests at individual time points are shown in Fig. 5A. The amount of liquid meal ingested at maximum satiation was significantly increased by ondansetron compared with saline (784 ± 74 vs 907 ± 64 mL, P < 0.05) (Fig. 5B).

image

Figure 5.  (A) Satiation score over time during the drink test after treatment with ondansetron was significantly lower compared with placebo (P < 0.005). Statistical differences at individual time points are indicated as follows: *P < 0.05, **P < 0.005 and ***P < 0.0005. (B) Maximum tolerated nutrient volume after treatment with ondansetron or placebo. *P < 0.05.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Author contribution
  9. References

Previous studies in animals and in man provided direct or indirect evidence for a role for 5-HT in the control of proximal stomach muscle tone.6,8,9 In the present study, the 5-HT3 receptor antagonist ondansetron did not affect fasting or postprandial tone of the proximal stomach or sensitivity to gastric distention in the fasting or postprandial state. On the other hand, we showed that ondansetron decreased feelings of satiation during food intake.

These observations argue against a role for 5-HT acting at 5-HT3 receptors in the control of proximal stomach sensorimotor function. They are in keeping with previous studies which failed to demonstrate an influence of 5-HT3 receptor antagonists on the gastric motor function in the dog as assessed with chronically implanted force transducers16 or on sensitivity to gastric distention in man as assessed with a barostat.16–18 Barostat studies assessing the effect of 5-HT3 receptor antagonists on meal-induced gastric accommodation are lacking, but ultrasonographic and SPECT studies did not observe changes in postprandial volumes after ondansetron.19,20

The observation that a short pretreatment with the selective serotonin reuptake inhibitor paroxetine and citalopram enhanced postprandial barostat balloon volumes8,9 argues for a role for 5-HT in the control of the gastric accommodation reflex in man. Based on a study with the selective 5-HT4 receptor antagonist R216073, a 5-HT4 receptor does not seem to be involved in the control of gastric tone in man.21 The 5-HT3 receptor agonist MKC-733 was shown to induce a relaxation of the proximal stomach in man,22 but the present study failed to confirm a physiological role for 5-HT acting at a 5-HT3 receptor in the control of gastric tone. The receptor through which 5-HT acts in the control of the accommodation reflex therefore remains to be identified. Experiments in dogs have characterized 5-HT1 and 5-HT7 receptors that might mediate gastric relaxation23,24 although human studies addressing this hypothesis are lacking at present.

We did observe increased tolerance of a nutrient challenge test, and lower satiation scores during meal ingestion after ondansetron pretreatment. Altered nutrient tolerance in the absence of an effect on proximal gastric tone is surprising as in previous studies we demonstrated in several studies a good concordance between the effects on gastric accommodation and on meal-induced satiation.1,15,25–27 The mechanism through which ondansetron decreases meal-induced satiation remains to be fully elucidated. Exposure of the duodenum to nutrients, especially lipids, induces sensations of fullness and satiation, and may contribute to meal-induced satiation. The mechanism involves, apart from cholecystokinin release, the release of 5-HT from enterochromaffin cells that activate vagal afferent nerves via 5-HT3 receptors.10,28 Indeed, blockage of 5-HT3 receptors with ondansetron has been shown to attenuate duodenal lipid-induced Fos-like immunoreactivity in the dorsal hindbrain of rats, a mechanism that most likely involves 5-HT3 receptors on vagal afferents.29 Given that the nucleus of the solitary tract, area postrema, and dorsal motor nucleus of the vagus nerve are known to be involved in the negative feedback control of food intake, this 5-HT3 mediated satiation mechanism can account for much of the lipid-induced satiation,28,30 and blockage of 5-HT3 receptors on vagal afferents can attenuate this important satiation mechanism. No effects on the gastric emptying rate of solids or liquids in man have been reported with 5-HT3 receptor antagonists, arguing against decreased delivery of nutrients to the duodenum as an underlying mechanism.19,20,31,32

In our study, we included both males and females. It is however known that plasma clearance is slightly slower in females as compared with males.33 Only two females were included in our study and their values all fell within the range of values obtained in males, hence we expect that the presence of the two females did not influence our results. Moreover, the plasma half-live of ondansetron ranges from 3.5 to 5.5 h,33 and all procedures in our study were performed within 2 h after ondansetron administration.

A placebo-controlled dose-finding study in functional dyspepsia suggested a potential for the 5-HT3 receptor antagonist alosetron 1 mg b.i.d. for relieving the symptoms of early satiation and postprandial fullness.34 In the absence of effects of 5-HT3 receptor antagonists on gastric sensorimotor function, the well-known effects on nausea10,35 and the effects on meal tolerance observed in the present study may have contributed to this therapeutic benefit. However, 5-HT3 receptor antagonists were not further developed for the treatment of functional dyspepsia due to an associated risk of colonic ischemia.36

In conclusion, the selective 5-HT3 receptor antagonist ondansetron does not alter fasting or postprandial gastric tone or sensitivity to gastric distention. Ondansetron decreases meal-induced satiation and increases tolerance of a nutrient meal challenge. These data suggest that 5-HT acting at 5-HT3 receptors is not involved in the control of gastric sensorimotor function, but contributes to the regulation of hunger and satiation in man. Given the fact that ondansetron is considered safe and is associated with little side effects we suggest further studies to examine whether ondansetron can be used in patients with early satiation and associated weight loss as seen in a subgroup of functional dyspeptic patients.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Author contribution
  9. References

Pieter Janssen and Lukas Van Oudenhove are postdoctoral research fellows of the FWO Flanders. This work was supported by an FWO grant and a Methusalem grant to Jan Tack, MD, PhD.

Author contribution

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Author contribution
  9. References

Pieter Janssen interpreted and analyzed the data and wrote the paper; Rita Vos performed the research; Lukas Van Oudenhove and Jan Tack analyzed the data; designed the study and revised the manuscript critically for important intellectual content.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Author contribution
  9. References