5-Hydroxytryptamine: new receptors and novel drugs for gastrointestinal motor disorders

Effects of ramosetron on gastrointestinal transit of Guinea pig

BACKGROUND/AIMS

A selective 5-hydroxytryptamine (5-HT) type 3 receptor antagonist, ramosetron, inhibits stress-induced abnormal defecation in animals and is currently used as a therapeutic drug for irritable bowel syndrome with diarrhea. The aim of this study is to investigate the effect of ramosetron on altered gastrointestinal (GI) transit.

METHODS

Male guinea pigs weighing approximately 300 g were used. The effect of ramosetron was investigated on altered GI transit induced by thyrotropin-releasing hormone (TRH), 5-HT, or mustard oil (MO). GI transit was evaluated by the migration of charcoal mixture from the pylorus to the most distal point, and expressed as a percentage (%) of charcoal migration (cm) of the total length of total small intestine (cm).

RESULTS

The average charcoal transit was 51.3 ± 20.1% in the control (vehicle) group, whereas in the ramosetron group charcoal moved 56.6 ± 21.9%, 46.9 ± 9.14% and 8.4 ± 5.6% of the total small intestine at the concentrations of 10, 30 and 100 µg/kg, respectively. GI transit after administration of TRH (100 µg/kg), 5-HT (10 mg/kg) or MO (10 mg/kg) was accelerated compared to vehicle (5-HT, 94.9 ± 9.22%; TRH, 73.4 ± 14.7%; MO, 81.0 ± 13.7%). Ramosetron inhibited GI transit altered by 5-HT, TRH or MO.

CONCLUSIONS

Ramosetron modulated GI transit. We suggest that ramosetron may be therapeutically useful for those with accelerated upper GI transit.

Postoperative colonic motility after tropisetron and a standardized meal in patients undergoing conventional colorectal surgery

BACKGROUND

Early postoperative enteral nutrition is advantageous for the recovery of colonic motility but may be limited by abdominal distension, nausea, and vomiting. We aimed to investigate the tolerance of a standardized meal after pretreatment with the 5-hydroxytryptamine-3-receptor antagonist tropisetron and to study the concomitant colonic motility.

METHODS

Colonic motility and tone were recorded on postoperative day 1 to 3 with a combined manometry/barostat recording catheter in 12 patients who underwent open colorectal surgery with an anastomosis in the distal colon or rectum. The study protocol consisted of 30 min of baseline recordings followed by 5 mg of tropisetron intravenously. Then, motility was recorded for another 30 min before patients ingested a standardized meal to trigger the gastrocolonic response. Postprandial motility was recorded for the subsequent 60 min.

RESULTS

The colonic motility index increased after administration of tropisetron on all three postoperative days (day 1: 34+/-11 vs 122+/-48, day 2: 55+/-19 vs 101+/-25, and day 3: 42+/-16 vs 93+/-33 mmHg/min; p<0.05). No further increase of the motility index was observed postprandially. Frequency and amplitude of contractions were virtually unaffected by tropisetron and the meal. Barostat bag volume decreased postprandially in the proximal bag on the third, and in the distal bag on the first and second postoperative day (p<0.05). Patients' condition was unaffected by the standardized meal after tropisetron administration.

CONCLUSIONS

Tropisetron may enhance colonic motility in the early postoperative period; however, the gastrocolonic response was impaired thereafter. High caloric food intake is well tolerated early after surgery after tropisetron pretreatment.

Serotonergic modulation and irritable bowel syndrome

Irritable bowel syndrome (IBS) is a debilitating disease, which is characterised by recurrent abdominal cramping and pain, and is associated with either constipation and/or diarrhoea. It is approximately twice as prevalent in women as it is in men and is among the most common gastrointestinal (GI) disorders encountered in primary care. The aetiology of the disease is poorly understood but may include motility dysregulation, visceral sensitivity, inflammation, bacterial infection, dietary antigens, psychological stress, GI surgery or a gut-brain phenomenon. At present, there is no acceptable treatment for IBS, although recent advances indicate that some relief may be achieved by the administration of compounds that act on 5-HT (serotonin) receptors. This suggestion is the result of numerous studies which have shown that 5-HT may exert a number of diverse effects on human GI tissues. In addition, it has emerged that the levels of the 5-HT metabolite (5-HIAA) are raised in the plasma of IBS patients and that administration of 5-HT-like compounds may mimic the symptoms of IBS. It has therefore been proposed that therapy with compounds that act at 5-HT receptors will return the intestine to normal activity and alleviate the pain experienced by these patients. One compound (alosetron, a 5-HT3 receptor antagonist) has already been released onto the market but showed benefit in female patients only and only in those whose primary symptom was diarrhoea. In addition, the compound was recently withdrawn following concerns over its safety. The reasons why alosetron only appears to show efficacy in females, why these treatments are only effective in a subset of the population of IBS patients and why alosetron elicits its particular side effect profile have not been elucidated. One further serotonergic compound, tegaserod (Zelmac, a 5-HT4 receptor agonist), has shown promise for the treatment of patients with constipation-predominant IBS and is currently in pre-registration for this indication. It is clear, however, that further research will have to take place before the utility of serotonergic modulation in the treatment of IBS can be fully validated.

Regional gastric contractility alterations in a diabetic gastroparesis mouse model: effects of cholinergic and serotoninergic stimulation

The C57BLKS/J db/db mouse develops hyperglycemia and has delayed gastric emptying that is improved with tegaserod, a partial 5-HT4 agonist. Our aims here were to determine regional gastric contractility alterations in C57BLKS/J db/db mice and to determine the effects of serotonin and tegaserod. The contractile effects of bethanechol, serotonin, and tegaserod in fundic, antral, and pyloric circular muscle were compared in C57BLKS/J db/db mice and normal littermates. The effects of tetrodotoxin, atropine, and 5-HT receptor antagonists were studied. Contractions in response to bethanechol were decreased in the fundus, similar in the antrum, but increased in the pylorus in diabetic mice compared with controls. Serotonin and, to a lesser extent, tegaserod caused contractions that were more pronounced in the fundus than in the antrum and pylorus in both diabetic and normal mice. Serotonin-induced contractions were partially inhibited by atropine, the 5-HT4 antagonist GR113808, and the 5-HT2 antagonist cinanseron but not tetrodotoxin. Regional gastric contractility alterations are present in this diabetic gastroparesis mouse model. Fundic contractility was decreased, but pyloric contractility was increased in the pylorus to cholinergic stimulation in diabetic mice. Serotonin's contractile effect is mediated, in part, through muscarinic, 5-HT2, and 5-HT4 receptors. This study suggests that fundic hypomotility and pyloric hypercontractility, rather than antral hypomotility, play important roles for the gastric dysmotility that occurs in diabetes.

Variability in serotonin and enterochromaffin cells in patients with colonic inertia and idiopathic diarrhoea as compared to normal controls

AIM: To evaluate differences in distribution, density and staining intensity of enterochromaffin cells (EC) and serotonin cells (SC) in the colonic mucosa of patients with colonic inertia (CI), idiopathic diarrhoea (ID) and a control group. METHODS: Three groups were studied: 19 patients' colons after subtotal colectomy for CI, and 17 patients' biopsies for diarrhoea (>3 bowel movements/day) with histological findings of normal mucosa (excluding microscopic, eosinophillic and collagenous colitis). The third group included 15 patients who underwent colonoscopy and biopsy for indications other than constipation, inflammatory bowel disease, diarrhoea or neoplasm (control group). Specimen blocks were obtained in each case from the right and left colon. Immunohistochemical staining for EC and SC were done on 4 micro m sections from Hollandes fixed, paraffin embedded tissues with primary rabbit antibody against chromagranin A or serotonin, and biotynylated secondary antibody and enzyme labelled streptavidin. RESULTS: The number of EC in the mucosa of the left colon in patients with CI (16.8 +/- 10.2) and ID (19.9 +/- 9.7) were significantly higher than they were on the right side (CI: 9.4 +/- 6.0, ID: 12.1 +/- 5.3). However, there were no significant differences between the left and right sides in the control group (L: 10.3 +/- 5.3; R: 13.4 +/- 7.6). Although the quantity of EC in the left colon in both patients with CI (P < 0.05) and ID (P < 0.01) were significantly higher than in the controls, there was no significant difference between CI and ID. In both the right and left colon, the percentage of EC with low positive density was significantly higher (P < 0.01) while those cells with moderate or low staining intensity were significantly lower in patients with CI than in either patients with ID or control group. In patients with CI, the quantity of SC in the mucosa of the left colon (12.1 +/- 6.4) was higher than in the right (CI: 7.9 +/- 3.6; control 4.6 +/- 3.3; ID 4.6 +/- 2.9) (P = 0.0057). In contrast there was no significant difference in SC in either the ID or control groups. The quantity of SC in both sides of the colon was significantly higher both in patients with CI as compared to the control group (P < 0.01) and patients with CI vs. patients with ID (L = P < 0.01; R = P < 0.05). There was a significantly positive correlation between the numbers of EC and SC in patients with CI (L: r = 0.5425, P < 0.05; R: r = 0.745, P < 0.01). CONCLUSION: In patients with CI, EC increases possibly due to an increase in SC. Conversely, in patients with ID, the EC increase results from peptides other than SC. Our results suggest that different aetiological factors contribute to ID and CI.

Effects of motilin on human interdigestive gastrointestinal and gallbladder motility, and involvement of 5HT3 receptors

A plasma motilin peak and a partial gallbladder emptying precede the antral phase III of the migrating motor complex (MMC). To clarify the causal relationship between these factors, we aimed to study the role of motilin in interdigestive gastrointestinal and gallbladder motility simultaneously. In addition, involvement of 5HT3 receptors in the action of motilin was studied. Eight fasting, healthy male volunteers received 13Leu-motilin or 0.9% NaCl i.v. for 30 min, in randomized order on two separate occasions, from 30 min after phase III. Seven of the eight subjects also received the 5HT3 receptor antagonist ondansetron in addition to motilin, on a third occasion. Antroduodenal motility, gallbladder volumes and plasma motilin were measured. The interval between the start of infusion and phase III was 95.0 (57.6-155.7) min for saline, 28.7 (21.0-33.2) min for motilin, and 39.3 (30.7-100.5) min for motilin + ondansetron (P < 0.05). Gallbladder volume decreased by one-third from 10 min after both motilin and motilin + ondansetron infusion (P < 0.05), and returned to baseline with duodenal passage of phase III. In two of the seven subjects phase III was absent after motilin + ondansetron, although gallbladder volume decreased and only refilled during a later spontaneous phase III. We conclude that motilin induces both partial gallbladder emptying and antral phase III. Indeed, although gallbladder emptying clearly precedes antral phase III, ondansetron only prevented phase III in some cases and had no effect on gallbladder emptying. Passage of phase III in the duodenum makes an important contribution to gallbladder refilling.

Long-term fluoxetine produces behavioral anxiolytic effects without inhibiting neuroendocrine responses to conditioned stress in rats

The aim of the present study was to investigate the anxiolytic effects of long-term treatment with fluoxetine in rats. Selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine, are used to treat anxiety and panic disorders, in addition to treating depression. A major concern with SSRIs is a 2-3-week delay in their therapeutic effects. SSRIs share with anxiolytic 5-HT(1A) agonists the ability to produce desensitization of post-synaptic 5-HT(1A) receptors. To investigate the anxiolytic effects of fluoxetine, rats were treated for 14 days with fluoxetine (10 mg kg(-1) day(-1), i.p. ). The rats were stressed using a conditioned stress procedure and tested one day after the last fluoxetine injection. Fluoxetine decreased stress-induced defecation (by 60%), reversed the stress-induced suppression of exploring behavior (by 59%) and shortened the duration of stress-induced freezing behavior (by 11. 5%). However, the stress-induced increase in plasma levels of ACTH, corticosterone, oxytocin, prolactin and renin were not inhibited by fluoxetine treatment. These findings suggest that neuroadaptive changes induced by sustained inhibition of serotonin (5-HT) reuptake, contribute to the mechanism of the anxiolytic effects of fluoxetine. In contrast, the neuroendocrine responses to conditioned stress are not affected by these neuroadaptive changes.

Characterization of the 5-hydroxytryptamine receptors mediating contraction in the intestine of Suncus murinus

1. The effects of 5-HT and 5-HT agonists to induce contraction and the 5-HT receptors mediating these effects were investigated in the proximal, central and terminal intestinal segments of Suncus murinus. 2. The contraction curves to 5-HT (3 nM - 30 microM) were shifted to the right by methysergide (1 microM) and ritanserin (0.1 microM), without affecting the maximum response. 3. In the central and terminal segments (but not the proximal segments) ondansetron (1 microM) and atropine (1 microM) significantly attenuated the contractions to higher concentrations of 5-HT. The selective 5-HT4 receptor antagonist SB204070 (1 nM), failed to modify 5-HT induced contractions in any segment examined. 4. 5-carboxamidotryptamine, alpha-methyl-5-HT and 5-methoxytryptamine (0.003 - 3.0 microM) induced contractions but unlike 5-HT, higher concentrations of these three agents failed to increase the response or were associated with a decrease in response. 2-methyl-5-HT (0.03 - 1.0 microM) was ten times less potent than 5-HT to induce contraction but achieved the same maximum response. 5. The contractions induced by the lower concentrations of 2-methyl-5-HT (0.03 - 1.0 microM) in all segments were markedly reduced or abolished by methysergide (1.0 microM); the response to the higher concentrations of 2-methyl-5-HT (3 - 30.0 microM) were markedly reduced by atropine (1.0 microM) and ondansetron (1.0 microM). 6. In all segments examined, tetrodotoxin (1 microM) significantly reduced the 5-HT-induced contraction. 7. It is concluded that the 5-HT-induced contraction was mediated via 5-HT2 (ritanserin sensitive) receptors in all regions of the intestine, with 5-HT3 (ondansetron sensitive) receptors mediating an additional major component in the central and terminal regions.

Role of nitric oxide in fasting gastric fundus tone and in 5-HT1 receptor-mediated relaxation of gastric fundus

Fasting gastric fundus tone is maintained by continuous cholinergic input. 5-Hydroxytryptamine-1 (5-HT1) receptor activation decreases gastric fundus tone in humans. Whether this fundus-relaxing effect is mediated via inhibition of cholinergic input or via activation of a nitrergic pathway is unknown. The aim of the present study was to determine the effect of nitrergic inhibition on feline gastric fundus tone and on 5-HT1 receptor-mediated relaxation of the fundus. Administration of Nomega-nitro-L-arginine methyl ester (L-NAME) alone caused a significant decrease of the mean baseline volume (P < 0.005), which was restored completely by addition of L-arginine. Sumatriptan caused a dose-dependent relaxation of the gastric fundus (P < 0.0005). This relaxation was inhibited by L-NAME (P < 0.02) and was restored by prior administration of L-arginine. Buspirone did not cause any change in mean baseline volume, whereas the sumatriptan-induced relaxation was not affected by prior administration of NAN-190. Our data indicate that fasting fundus tone relies not only on continuous cholinergic input but also on continuous nitrergic input, implying that fasting fundus tone is maintained by the balance of a cholinergic and nitrergic drive. Furthermore, fundus relaxation via 5-HT1 receptor activation is mediated through activation of a nitrergic pathway.

Characterization of 5-HT receptors in rat proximal colon

1. In view of its multiple sites of action, we investigated the activity of 5-hydroxytryptamine (5-HT, serotonin) on various potential receptors in the isolated proximal colon of rats. 2. 5-HT induced concentration-dependent contractions of colonic strips (pEC50=7.54+/-0.12). 3. The 5-HT1 receptor agonist, 5-carboxamidotryptamine, induced concentration-dependent contractions (pEC50=5.93+/-0.27); however, neither the 5-HT3 receptor-agonist, phenylbiguanide, nor the 5-HT4 receptor-agonist, renzapride, caused contractions at concentrations as high as 10(-4) M. 4. The 5-HT 1/2 receptor antagonist, methiothepin, caused concentration-dependent nonsurmountable antagonism. The 5-HT3 receptor antagonist, tropisetron, inhibited the contractions to a concentration of 5-HT> or =10(-6) M. Ketanserin had no effect on responses to 5-HT. 5. Tetrodotoxin and atropine had no effect on responses to 5-HT. 6. We conclude that contractions to 5-HT are mediated by 5-HT1-like and probably 5-HT3 receptors that activate tetrodotoxin insensitive mechanisms.

Effects of ondansetron on electrically evoked contraction in rat stomach fundus: possible involvement of 5-HT2B receptors

This study examined the effects of ondansetron, an antagonist of the 5-hydroxytryptamine (5-HT3) receptor without 5-HT4 receptor agonistic activity. on electrically evoked contractions and acetylcholine release in rat stomach fundus strips. Ondansetron (10(-8)-10(-4) M) produced a concentration-dependent increase in the magnitude of the electrically evoked contraction, while it inhibited the release of acetylcholine induced by electrical field stimulation. The stimulatory effect of ondansetron (10(-6) M) on electrically evoked contractions was antagonized by yohimbine, a 5-HT2B receptor antagonist, but not by SDZ 205-557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-[diethylamino] ethyl ester), a 5-HT4 receptor antagonist. In the presence of tetrodotoxin, ondansetron (10(-7)-10(-5) M) enhanced the contractions induced by acetylcholine. This stimulatory effect of ondansetron on acetylcholine-induced contractions was antagonized by yohimbine. These data suggest that ondansetron potentiates the contractile response to acetylcholine in the rat stomach fundus through the activation of 5-HT2B receptors.

Zamifenacin (UK-76, 654) a potent gut M3 selective muscarinic antagonist, reduces colonic motor activity in patients with irritable bowel syndrome

BACKGROUND

Zamifenacin is a new potent gut M3 selective muscarinic antagonist developed for possible use in irritable bowel syndrome.

METHODS

In this multicentre, double-blind, parallel group, placebo-controlled study, the effect of a single dose of zamifenacin 10 mg or 40 mg on both fasting (30 min) and fed (60 min) colonic motor activity was assessed in 36 patients with irritable bowel syndrome (aged 25-68 years; 19 male). Colonic motility was recorded using a five-channel solid-state catheter introduced by colonoscopy to a depth of 35 cm in an unprepared colon.

RESULTS

Zamifenacin 40 mg profoundly reduced colonic motility, particularly after the meal (P < 0.05). This was reflected by a significant reduction in the mean amplitude of contractions, number of contractions, percentage duration of contractions, activity index and the motility index (P < 0.05). A smaller reduction in all the motility parameters was obtained with 10 mg zamifenacin, but these changes were not statistically significant. Three patients each on placebo and zamifenacin reported side-effects, but these were mild and transient.

CONCLUSION

A single 40 mg dose of zamifenacin significantly reduces colonic motility in irritable bowel syndrome patients without significant antimuscarinic effects. The results of this study confirm that the concept of developing selective antimuscarinic agents may be a promising approach to the treatment of irritable bowel syndrome. Not only would such compounds benefit from not having the usual side-effects of anticholinergics but they might also offer much more in the way of dose flexibility.

Amino Acid Effects on Post-Stress Ulcers: Relationship to Brain Serotonin, 5-HIAA and Norepinephrine

To replicate and extend results of earlier studies on amino acid effects on post-stress ulcers, rats were subjected to i.p. injections of (a) saline, (b) tryptophan, (c) tyrosine + valine or(d) tryptophan + tyrosine + valine, either 30 minutes before or immediately after one hour of water restraint stress. Gastric lesions, brain norepinephrine, serotonin and 5-hydroxyindoleacetic acid (5-HIAA) were examined after one hour of poststress rest. We hypothesised that post-stress lesions could be aggravated by central noradrenergic hypoactivity and serotonergic hyperactivity during the post-stress period. Other studies have indicated that tyrosine + valine reduces central serotonergic activity, while additional tryptophan blocks this effect. We therefore expected post-stress lesions to be reduced in tyrosine + valine but not in tryptophan + tyrosine + valine treated animals. Although these expectations were met tentatively in animals injected prior to stress, thus replicating tyrosine + valine effects we had observed earlier, opposite results were found in animals treated post-stress. The brain analyses indicate that the data cannot be explained by a norepinephrine/serotonin imbalance hypothesis. The time dependency of the effects underlines the need for caution in clinical applications of these amino acid treatments.

Antiemetic effects of N-3389, a newly synthesized 5-HT3 and 5-HT4 receptor antagonist, in ferrets

The antiemetic activity of N-3389 (endo-3,9-dimethyl-3,9-diazabicyclo[3,3,1]non-7-yl-1 H-indazole-3-carboxamide dihydrochloride), a new 5-HT3 and 5-HT4 receptor antagonist, against cisplatin-, cyclophosphamide- and copper sulfate-induced emesis was investigated using ferrets. We also examined the effects of these agents on abdominal afferent vagus nerve activity in anesthetized ferrets. Both intraperitoneal (0.1-1.0 mg/kg) and oral (0.1-1.0 mg/kg) administration of N-3389 produced dose-dependent antiemetic effects. The time-course of cisplatin (10 mg/kg, i.p.)-induced emesis in another group of ferrets paralleled the increase in abdominal afferent vagus nerve activity induced by cisplatin (10 mg/kg, i.p.) and was inhibited by pretreatment with N-3389 (1.0 mg/kg, i.v.). Furthermore, the cisplatin (10 mg/kg, i.p.)-induced increase in abdominal afferent vagus nerve activity was markedly reduced by an additional injection of N-3389 (0.1-1.0 mg/kg, i.v.) in a dose-dependent manner. The antiemetic effects exhibited by N-3389 are probably due to the inhibition of 5-HT3 and 5-HT4 receptors on the abdominal afferent vagus nerves.

VB20B7, a novel 5-HT-ergic agent with gastrokinetic activity. I. Interaction with 5-HT3 and 5-HT4 receptors

This study describes the in-vitro interaction of the gastrokinetic agent 2[1-(4-piperonyl)piperazinyl]benzothiazole (VB20B7) with the 5-hydroxytryptamine 5-HT3 and 5-HT4 receptor subtypes, using functional as well as radioligand binding studies. The benzamide derivative cisapride was used as a comparison. In radioligand binding assays VB20B7 showed, like cisapride, a weak affinity at 5-HT3 receptors from rat cerebral cortex. The new compound lacked any affinity at other 5-HT receptors or at dopaminergic D2 receptors, whereas cisapride showed high affinity for the 5-HT4 receptors from guinea-pig hippocampus and moderate affinity at dopaminergic D2 receptors. In the non-stimulated guinea-pig ileum, the concentration-response curves to the specific 5-HT3 agonist 2-Me-5-HT and to 5-HT were shifted to the right by VB20B7. In the rat oesophagus tunica muscularis mucosae preparation (TMM), VB20B7 was evaluated for its activity at 5-HT4 receptors. VB20B7 behaved as a 5-HT4 receptor agonist, inducing a concentration-dependent relaxation of the preparation precontracted with carbachol. In this preparation, VB20B7 and cisapride were able to stimulate adenylate cyclase activity, an effect probably mediated through activation of 5-HT4 receptors, as can be inferred from the blockade by the 5-HT4 antagonist, tropisetron, of the enhanced cAMP formation. However, consistent with the lack of affinity at central 5-HT4 receptors, VB20B7 did not stimulate cAMP formation in guinea-pig hippocampal slices. VB20B7 also caused an increase in the twitch response of the transmurally stimulated guinea-pig ileum, although at a concentration higher than cisapride. This effect was blocked by desensitization of the 5-HT4 receptor with 5-MeOT and also by the 5-HT4 receptor antagonist tropisetron. Both VB20B7 and cisapride increased the K(+)-evoked acetylcholine release in this preparation. The results show that VB20B7 possesses affinity for 5-HT4 receptors located in the rat TMM and guinea-pig ileum preparations, but is devoid of affinity at central 5-HT4 receptors. In addition, VB20B7 shows low to moderate affinity at both central and peripheral (enteric) 5-HT3 receptors. The interaction of VB20B7 with the peripheral 5-HT4 and 5-HT3 receptors may be relevant for the gastrokinetic effects of the new compound.

5-HT receptor subtypes involved in the serotonin-induced inhibition of L-leucine absorption in rabbit jejunum

The aim of the present study was to determine the 5-HT receptor subtypes involved in the serotonin-induced inhibition of L-leucine absorption across rabbit jejunum in vitro. A number of agonists and antagonists were used to characterize the receptors through which serotonin inhibits this absorption. The results show that 2.5x10(-6) M 5-HT inhibits the amino acid absorption by about 20%. The 5-HT receptor agonists, alpha-methyl-5-HT (5-HT2), 2-methyl-5-HT (5-HT3) and zacopride (5-HT4) at concentrations 2.5x10(-6) and 2.5x10(-5) M produced 10-30% inhibition on L-leucine intestinal absorption. 5-carboxyamidotryptamine (5-HT1) did not produce any inhibition. The 5-HT antagonists, GR 113808A (5-HT4) at 2.5x10(-6) M and ritanserin (5-HT2) and ondansetron (5-HT3) at 2.5x10(-5) M completely blocked the effect of 5-HT. However, methiothepin (5-HT1) did not produce any effect on serotonin action in the intestinal absorption of amino acid. It can be concluded that 5-HT2, 5-HT3 and 5-HT4 receptors could mediate inhibition of L-leucine absorption across rabbit jejunum.

Gastric motility patterns for digestion and vomiting evoked by sympathetic nerve stimulation and 5-hydroxytryptamine in the dogfish Scyliorhinus canicula

The sympathetic control of motor activity in the stomach of the dogfish Scyliorhinus canicula and its pharmacology was studied in freshly killed animals with the abdominal viscera superfused in situ with elasmobranch Ringer by recording gastric pressure and by quantitative analysis of video-tape recordings of the activity. Little spontaneous activity was seen in the stomach, although in two animals retrograde contractions occurred in the spiral intestine. The effects of electrical stimulation of the splanchnic (sympathetic) nerves varied markedly with frequency. At 4 Hz after a latency of about 1 min there was a slight increase in the overall level of contractile activity in both cardiac and pyloric regions of the stomach, which persisted throughout the stimulation period (5-7 min). At 16 Hz a contraction was visible in the pyloric region within 10 s of the start of stimulation. Over the next 30 s this contraction proceeded cranially becoming more powerful as it swept into the cardiac region. This contraction returned the contents of the pyloric stomach to the cardiac stomach past the valve-like junction between them. This contraction continued into the cardiac stomach and in some animals propelled the contents into the oesophagus. About 2 min after the start of stimulation there followed a series of contractions in both gastric regions, predominantly in the circular muscle. In the cardiac region these contractions occurred alternately in the proximal and distal regions. Occasionally a large contraction passed a bolus of material from the cardiac to the pyloric region. No movements or pressure changes were observed in the stomach after stimulation of the vagus, although contractions were readily induced in the oesophagus. It is proposed that the initial large retrograde contraction provides a mechanism by which the animal can vomit indigestible or accidentally ingested material. In contrast the later motor patterns suggest that this type of activity is involved in more normal digestive functions of mixing food with gastric secretions, trituration and gastric emptying. Evidence is presented which implicates 5-hydroxytryptamine as a principal neurotransmitter involved in the genesis of the retrograde contraction by the splanchnic nerve.

Response heterogeneity of 5-HT3 receptor antagonists in a rat visceral hypersensitivity model

Subcutaneous administration of granisetron (BRL 43694, endo-1-methyl-N-(9-methyl-9-azabicyclo[3.3.1.]non-3-yl-1 H-indazole-3-carboxamide) and zacopride (4-amino-N-(1-azabicyclo[2.2.2.]oct-3-yl)-5-chloro-2-methoxybenzamide), two 5-HT3 receptor antagonists, at doses ranging from 3 to 1000 micrograms/kg, inhibited abdominal contractions induced by distension (30 mmHg, 10 min) of irritated colon (0.6% acetic acid) in conscious rats with a bell-shaped dose-response curve. The ED50 of granisetron and zacopride were 17.6 and 8.2 micrograms/kg, respectively. In contrast, both tropisetron (ICS 205-930, (3-a-tropanyl)t-indole-3-carboxylic ester) and ondansetron (GR38032F, 1,2,3,9-tetrahydro-9-methyl-3-[(2-methyl-1 H-imidazol-1-yl)methyl]-4 H-carbazol-4-one hydrocloride dihydrate) were inactive in this model. These data further support the concept of a heterogeneity in the potency of 5-HT3 receptor antagonists in modulating visceral hypersensitivity in conscious rats. This finding is in agreement with a reported efficacy of granisetron but not of ondansetron in patients with irritable bowel syndrome.

5-HTP induced diarrhea as a carcinoid syndrome model in mice?

Serotonin (5-HT) is present in the gastrointestinal tract and is probably one of the compounds responsible for diarrhea in patients presenting with carcinoid syndrome. Intraperitoneal administration of L-5-hydroxytryptophan (L-5-HTP) at doses of 25 to 100 mg/kg dramatically increase defecation in mice. In this new paradigm, counting fecal boli deposited is simple and the appraised or inhibition of diarrhea induced by ip 25 mg/kg of L-5-HIP is very clear, with a good reproducibility of scores. L-5-HTP needs to be metabolized into 5-HT to be active; benserazide, an inhibitor of decarboxylase, antagonized the diarrhea induced by 5-HT. Among the 5-HT antagonists used in interaction with 5-HT, only these of the 5-HT3 type (ondansetron, granisetron, tropisetron) and, to a lesser extent 5-HT2 type (ritanserin), decreased the diarrhea induced by 5-HTP. The 5-HT4 receptor agonists from the benzamide family (metoclopramide and zacopride) increased defecation in mice but the effect failed to reach statistical significance.

5-HT3 and 5-HT4 receptor-mediated facilitation of the emptying phase of the peristaltic reflex in the marmoset isolated ileum

1. The patterns of intestinal motility and the effect of an increase in intraluminal pressure were studied in vitro on segments obtained from the marmoset small intestine. 2. Segments obtained from the distal half of the marmoset small intestine exhibited segmentation, consisting of narrow annular contractions (lasting for 2-3 s) interposed between the relaxed segments of varying length. The subsequent contractions occurred slightly distal to the previous contraction, with ring-like contractions appearing to move in the aboral direction. Such segmentation was infrequent or absent in the segments obtained from the proximal half of the small intestine. An increase in intraluminal pressure inhibited segmentation and finally produced peristalsis in most of the tissues. 3. The influence of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists on the threshold of the peristaltic reflex was investigated in the segments obtained from the distal half of the intestine after segmentation had subsided. The effect of drug application to the serosal surface was measured as a change in threshold pressure required to trigger the peristaltic reflex. A facilitation was defined in two ways (a) as a reduction in the threshold pressure required to trigger the reflex and (b) in those tissues that failed to respond with peristalsis on raising intraluminal pressure to the maximum attainable (1 kPa), as a reduction in threshold pressure compared to this value. 4. 5-HT (7.85 +/- 0.19), 5-methoxytryptamine (7.79 +/- 0.24), 5-carboxamidotryptamine (6.66 +/- 0.13) and 2-methyl-5-HT (6.24 +/- 0.16) caused a concentration related facilitation of the peristaltic reflex, the pD2 values (mean +/- s.e.mean) being shown in parentheses. 5. The concentration-response curves to both 5-HT and 5-methoxytryptamine were dextrally shifted in a surmountable manner in the presence of GR 113808 (30 nM). pD2 values for 5-HT and 5-methoxytryptamine were significantly decreased to 6.98 +/- 0.24 and 6.83 +/- 0.36 respectively in the presence of GR 113808 (30 nM). 6. In the presence of a high concentration of (10 microM) 5-methoxytryptamine the subsequent addition of 2-methyl-5-HT (3-10 microM) but not 5-methoxytryptamine (10 microM) facilitated peristalsis; the effect of 3 microM 2-methyl-5-HT was significantly decreased by 2 microM ondansetron. 7. It is concluded that the facilitation of the peristaltic reflex in the marmoset intestine induced by 5-HT at submicromolar concentrations involves a 5-HT4 receptor stimulation with an additional 5-HT3 receptor activation at higher concentrations.

A comparative study of functional 5-HT4 receptors in human colon, rat oesophagus and rat ileum

1. The pharmacological properties of 5-hydroxytryptamine (5-HT), the 5-HT4 receptor agonists, DAU 6236 and SC 53116 and the 5-HT4 receptor antagonist, GR 1130808, were studied in the rat oesophagus, rat ileum and human colon. 2. 5-HT relaxed the longitudinal muscle of the rat oesophagus and rat ileum and the circular muscle of the human colon. Absolute values of relaxation were measured and showed the order of the maximum responses, rat oesophagus >> human colon > rat ileum with EC50 values of 189 +/- 15 nM, 157 +/- 4 nM, 306 +/- 72 nM, respectively. 5-HT also inhibited the spontaneous contractions of the human colon with an EC50 value of 119 +/- 1 nM. The effect of 5-HT on the human colon was not affected by methysergide (10 microM) or ondansetron (1 microM). 3. The use of the uptake and metabolism inhibitors, cocaine (30 microM) and pargyline (100 microM), did not increase the potency of 5-HT in the rat oesophagus or human colon. In the rat oesophagus, cocaine (30 microM) produced a reduction in carbachol-induced tone of 22.2 +/- 0.6% and reduced the 5-HT maximum effect by 52.0 +/- 0.4%. 4. The compounds, DAU 6236 and SC 53116, showed a different pattern of potencies and efficacies in the rat oesophagus, rat ileum and human colon compared to 5-HT. DAU 6236 relaxed the human colonic circular muscle with an EC50 value of 129 +/- 16 nM but its efficacy was less than that of 5-HT. DAU 6236 (1 microM) also antagonized the 5-HT-induced relaxation of the human colon with a dose-ratio of 9.9. In the rat oesophagus and rat ileum, DAU 6236 was inactive in the majority of tissues. In the minority of oesophagus tissues that did respond the EC50 value was 1.2 +/- 0.7 microM. DAU 6236 also antagonized the effect of 5-HT in the rat oesophagus in a non-surmountable fashion. SC 53116 relaxed the rat oesophagus with an EC50 value of 91 +/- 4 nM, with an efficacy less than that observed to 5-HT; however, at 200 nM it did not antagonize the 5-HT-induced relaxation of the rat oesophagus. SC 53116 showed no agonist activity in the rat ileum and human colon, but at 1 microM it did antagonize the effect of 5-HT in the human colon with a dose-ratio of 11.3 +/- 0.3. 5. GR 113808 competitively antagonized the 5-HT4 receptor-mediated relaxation of the rat oesophagus with a pA2 value of 8.59 (8.18-9.00) against 5-HT and 9.05 (8.79-9.31) against SC 53116. GR 113808(0.01 microM) also antagonized the 5-HT-induced relaxation of human colonic circular muscle with an apparent pA2 value of 9.02 +/- 0.12. However at 1 microM the apparent pA2 value was significantly lower than that measured at 0.01 and 0.1 microM. GR 113808 (0.01 microM) antagonized the 5-HT4 receptor-mediated relaxation of the rat ileum with an apparent pA2 value of 9.30 +/- 0.21.6. In conclusion, these studies have shown that the human colon, rat oesophagus and rat ileum contain functional 5-HT4 receptors. However, the 5-HT4 receptor agonists displayed differences in these tissues making it necessary to be cautious when extrapolating from animal to human tissue. This emphasizes the importance of the use of human tissue in the development of therapeutic drugs.

Drug-induced defaecation in rats: role of central 5-HT1A receptors

1. We investigated the acute effects of 5-hydroxytryptamine (5-HT), and of the 5-HT1A receptor agonists, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), buspirone and SR 57746A, on rat faecal pellet output and water content. 2. 5-HT, 8-OH-DPAT, buspirone and SR 57746A, a new selective 5-HT1A receptor agonist, displaced [3H]-8-OH-DPAT from specific binding sites in rat hippocampus membranes (Ki, nM; 1.8, 1.2, 15, 3.1 respectively) and stimulated rat defaecation dose-dependently. SR 57746A and buspirone induced 1 g dry weight of faeces at 1.3 and 6.1 mg kg-1, p.o. (AD1) respectively. 8-OH-DPAT and 5-HT stimulated defaecation after s.c. injection (AD1, 0.07 and 7.5 mg kg-1, respectively). All these agents increased faecal water content. 3. The putative 5-HT1A receptor antagonist, pindolol, injected s.c. or i.c.v., significantly reduced the defaecation induced by systemically administered 8-OH-DPAT, buspirone or SR 57746A, but not 5-HT. 4. Pretreatment with p-chlorophenylalanine (i.p.) or 5,7-dihydroxytryptamine (i.c.v.), according to protocols designed to cause either generalized or CNS-limited 5-HT depletion respectively, also reduced the defaecation induced by buspirone or SR 57746A. 5. No specific 5-HT1A binding sites could be labelled by incubating rat colon membranes with [3H]-8-OH-DPAT, and in vitro preparations of rat colon segments showed no response to 8-OH-DPAT or SR 57746A up to 5 microM. 6. After eight days' repeated daily treatment, complete tolerance developed to the stimulant effects of SR 57746A and buspirone on faecal water content, but not on faecal pellet output. This suggests that faecal mass excretion and water exchange through the gut wall are affected by independent mechanisms.7. The present findings support the involvement of central 5-HTIA receptors in intestinal propulsion and regulation of luminal fluid content, presumably accounting for the drug-induced defaecation in rats.

Therapeutic potential of drugs with mixed 5-HT4 agonist/5-HT3 antagonist action in the control of emesis

Drugs interacting with serotonin (5-hydroxytryptamine, 5-HT) receptors are of value in the treatment of several gastrointestinal disturbances. Selective 5-HT3 receptor antagonists (ondansetron, granisetron, tropisetron) are widely utilized to control emesis induced by chemotherapy and radiation, while agonists at 5-HT4 receptors (cisapride, renzapride, BIMU compounds) are endowed with gastrointestinal prokinetic action. Here we overview the therapeutic potential of drugs with potent mixed 5-HT4 agonist/5-HT3 antagonist properties (i.e. BIMU 1) in the management of anticancer therapy-induced emesis and of intestinal adynamic post-operative conditions associated with vomiting. In the former situation, the agonism at 5-HT4 receptors is expected to be of benefit via two possible mechanism: (i) inhibition of 5-HT release from enterochromaffin cells; (ii) restoration of anally driven peristaltic waves in the upper gastrointestinal tract. Moreover, 5-HT4 receptor-induced prokinetic activity may counteract colonic constipation, an unwanted effect which occurs in a number of patients treated with pure 5-HT3 receptor antagonists. Additionally, the above mentioned drugs might be of value in post-operative conditions associated with intestinal adynamia and emesis sensitive to 5-HT3 receptor blockade.

Visceral perception in irritable bowel syndrome. Rectal and gastric responses to distension and serotonin type 3 antagonism

We wished to determine if visceral perception in the rectum and stomach is altered in patients with irritable bowel syndrome and to evaluate the effects on visceral sensation of 5-HT3 receptor blockade. Twelve community patients with diarrhea-predominant irritable bowel syndrome and 10 healthy controls were studied in a double-blind, randomized, placebo-controlled study. Using two barostats, the stomach and rectum were distended, with pressure increments of 4 mm Hg, from 10 to 26 mm Hg; visceral perception was measured on an ordinal scale of 0-10. Personality traits were measured using standard psychological methods, and somatic pain was evaluated by immersion of the nondominant hand in cold water. The effect of 5-HT3 antagonism was tested with a single intravenous dose of ondansetron at 0.15 mg/kg. Gastric perception was higher in irritable bowel syndrome, but rectal distension was perceived similarly in irritable bowel syndrome and controls. Pain tolerance to cold water was also similar in irritable bowel syndrome and controls. Ondansetron induced rectal relaxation and increased rectal compliance but did not significantly alter gastric compliance or visceral perception. Psychological test scores were similar in patients and controls. We conclude that in this group of psychologically normal patients with irritable bowel syndrome, who were not chronic health-care seekers, visceral perception was normal. Ondansetron did not alter gut perception in health or in irritable bowel syndrome.

Projections of 5-hydroxytryptamine-immunoreactive neurons in guinea-pig distal colon

The presence of 5-hydroxytryptamine in enteric neurons of the guinea-pig distal colon was demonstrated by immunohistochemistry and the projections of the neurons were determined. 5-Hydroxytryptamine-containing nerve cells were observed in the myenteric plexus but no reactive nerve cells were found in submucous ganglia. Varicose reactive nerve fibres were numerous in the ganglia of both the myenteric and submucous plexuses, but were infrequent in the longitudinal muscle, circular muscle, muscularis mucosae and mucosa. Reactivity also occurred in enterochromaffin cells. Lesion studies showed that the axons of myenteric neurons projected anally to provide innervation to the circular muscle and submucosa and to other more anally located myenteric ganglia. The results suggest that a major population of 5-hydroxytryptamine neurons in the colon is descending interneurons, most of which extend for 10 to 15 mm in the myenteric plexus and innervate both 5-hydroxytryptamine and non-5-hydroxytryptamine neurons.

Characterization of the 5-hydroxytryptamine receptor type involved in inhibition of spontaneous activity of human isolated colonic circular muscle

1. Experiments were carried out to characterize pharmacologically the 5-hydroxytryptamine (5-HT) receptor types which mediate inhibition of spontaneous contractions of the intertaenial circular muscle in human isolated colon. 2. 5-HT caused a reproducible concentration-dependent inhibition of spontaneous contractions of the circular muscle of human colon in vitro with a mean EC50 value of 0.2 microM and 95% confidence limits of 0.1-0.5 microM. No evidence for a contractile action of 5-HT was found. Tetrodotoxin (TTX, 1.5 microM) caused a rightward shift of the concentration-response curve of 5-HT with a concentration-ratio of 2.9. 3. The inhibitory response to 5-HT was mimicked by several indoles with the rank order of potency 5-HT > 5-methoxytryptamine = alpha-methyl-5-HT > 5-carboxamidotryptamine >> 2-methyl-5-HT. 5-Hydroxyindalpine was inactive. 4. The substituted benzamides were agonists with the following rank order of potency, 5-HT > renzapride > zacopride > metoclopramide > cisapride. 5. The inhibitory responses to 5-HT were not inhibited by methysergide (10 microM) or methiothepin (1 microM), which are antagonists selective for 5-HT1-like and 5-HT2 receptors, nor by ondansetron (10 microM) which is an antagonist at 5-HT3 receptors. 6. The inhibitory responses induced by 5-HT and 5-methoxytryptamine were competitively antagonized by a weak 5-HT4 receptor antagonist, tropisetron, with pKB values of approximately 6. Tropisetron had no significant effect on the inhibitory response curve produced by isoprenaline (0.01-100 microM). 7. The pharmacological profile of the 5-HT-evoked relaxations of human colon circular muscle are consistent with activation of a 5-HT4-like receptor.

Investigation of the role of 5-HT3 and 5-HT4 receptors in ascending and descending reflexes to the circular muscle of guinea-pig small intestine

1. The present study was undertaken to ascertain whether 5-hydroxytryptamine (5-HT) acting at either 5-HT3 or 5-HT4 receptors plays a significant role in motility reflexes in the guinea-pig small intestine. 2. An isolated segment of small intestine was opened along its mesenteric border and pinned, mucosa uppermost, in a three chambered organ bath so that the oral, middle and anal regions of a single preparation could be separately superfused. 3. Conventional intracellular recording methods were used to monitor the responses of the circular muscle in the oral or the anal end chambers when distension was applied in either of the other two chambers or the mucosal villi were compressed in the middle chamber. Drugs were added to the middle chamber. 4. 5-HT3 receptor antagonists (tropisetron, 0.1-10 microM; granisetron, 1 microM and BRL 46470, 1 microM) depressed the ascending excitatory reflex evoked by these stimuli but had no effect on the descending inhibitory reflex. The depression of the excitatory reflex was observed whether the reflex was evoked from the chamber containing the drug or was simply conducted, via interneurones, through this chamber. 5. The 5-HT4 receptor antagonist, SDZ 205-557 (1 microM), had no significant effect on either the ascending or descending reflex pathways. However, 5-HT4 receptors were present as cisapride (0.1 microM) significantly enhanced the ascending excitation without affecting the descending inhibition. This effect of cisapride was converted to a significant depression of the ascending reflex by SDZ 205-557. 6. The results suggest that 5-HT3, but not 5-HT4, receptors play an important role in the ascending excitatory reflex and that these receptors may be on interneurones in the reflex pathway.

Drugs affecting serotonin receptors

A greater understanding of the various serotonin receptor subtypes has led to a clearer appreciation of the role of serotonin in gastrointestinal motility, sensation and secretion. Serotonin is definitely involved in the aetiopathogenesis of cisplatin-induced emesis and carcinoid diarrhoea. The application of serotonergic drugs in clinical therapeutics for gut disturbances is presently dominated by the use of 5-HT3 antagonists for acute chemotherapy-induced nausea and vomiting, and the use of substituted benzamides which are 5-HT4 agonists stimulating gut motor function through 5-HT4 neuronal receptors. The best-studied 5-HT4 agonist is cisapride, which has been shown to stimulate motility at several levels of the gut. Cisapride is approved for healing and maintenance treatment of reflux oesophagitis and is used in several countries for the alleviation of symptoms consistent with regional stasis, from dyspepsia to constipation. Carcinoid diarrhoea is a prototypic disease associated with deranged serotonin metabolism, and a rationale for using 5-HT3 or 5-HT4 antagonists is based on the recent appreciation of the important role of impaired gut motor function in carcinoid diarrhoea. In the future, greater understanding of the serotonin receptor subtypes and their role in gut disorders may lead to novel approaches to alleviate increased visceral perception of functional gastrointestinal disorders, to correct changes in colonic capacitance, or to alter gastrointestinal motility that contributes to diarrhoea or constipation. However, at the present time, it must be stressed that these uses are still at an experimental stage and that careful validation and proper controlled studies are still required.

A 5HT3 antagonist corrects the postprandial colonic hypertonic response in carcinoid diarrhea

BACKGROUND/AIMS

Carcinoid patients show a hypertonic colonic motor response postprandially. Ondansetron reduces postprandial colonic tone in health. It was hypothesized that ondansetron, a selective 5HT3 antagonist, corrects the colonic motor response to eating in carcinoid diarrhea.

METHODS

The effects of ondansetron and placebo on fasting and postprandial colonic tone and motility in 10 patients with carcinoid diarrhea were compared using a manometry-barostat assembly positioned in the upper descending colon.

RESULTS

Fasting colonic tone and motility indices were similar in the placebo and ondansetron groups; ondansetron did not affect fasting motility. The placebo group showed a significant reduction in barostat balloon volume (signifying increased tone) from 207 +/- 29 mL (mean +/- SEM) during fasting to 106 +/- 14 mL postprandially (P = 0.01). With ondansetron, a tonic colonic response was induced postprandially (198 +/- 37 mL to 151 +/- 30 mL; P = 0.053). However, the increment in tone in the ondansetron group (23% +/- 7%) was significantly lower than in the placebo group (48% +/- 5%; P = 0.02) and was similar to that observed in untreated healthy subjects (24% +/- 3%). Postprandial manometric pressure activity increased significantly in the placebo group (P = 0.01); in the ondansetron group there was a trend (P = 0.09) to increased phasic activity.

CONCLUSIONS

Ondastetron reduces the postprandial colonic hypertonic response in carcinoid diarrhea to levels previously reported in health; further clinical studies of this class of antagonists in carcinoid diarrhea appear warranted.

Characterization of 5-HT receptors mediating contraction and relaxation of the longitudinal muscle of guinea-pig distal colon in vitro

A range of agonists and antagonists were used to characterize the receptors through which 5-hydroxytryptamine (5-HT) contracts and relaxes the longitudinal muscle of segments of guinea-pig distal colon, in vitro. 5-HT contracted the longitudinal muscle over the concentration range 10(-9) to 10(-4) mol/l. The 5-HT3 receptor agonist, 2-methyl-5-HT, produced concentration dependent contractions over the range 10(-6) to 10(-4) mol/l. 5-methoxytryptamine, an agonist at 5-HT4 receptors, caused contractions over a concentration range of 10(-8) to 10(-4) mol/l. The 5-HT4 antagonist, SDZ 205-557 (5 x 10(-7) mol/l) substantially suppressed the responses to low concentrations of 5-HT and to 5-methoxytryptamine, but had no effect on the responses to higher concentrations of 5-HT. In contrast, the 5-HT3 antagonist, granisetron (10(-6) mol/l), blocked the effect of 2-methyl-5-HT and substantially depressed responses to high concentrations of 5-HT, but had no effect on lower concentrations of 5-HT. Granisetron produced a small reduction in the response to 5-methoxytryptamine. Tetrodotoxin (TTX) (3 x 10(-7) mol/l) almost abolished the response to 5-methoxytryptamine and markedly suppressed the response to 2-methyl-5-HT, but the responses to 5-HT were only partially reduced. The 5-HT1 antagonist, methiothepin (10(-6) mol/l) depressed the response to 5-HT (10(-7) to 10(-4) mol/l) and blocked its TTX insensitive component. The 5-HT2 antagonist, ketanserin, in concentrations up to 10(-5) mol/l, had no effect on the contractions evoked by 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

The importance of 5-hydroxytryptamine receptors in the gut

Most of the 5-hydroxytryptamine (5-HT) present in the adult human body is located in the gastrointestinal tract. The vast majority is contained in enteroendocrine cells, the rest exists mainly in myenteric interneurons separated from the mucosa by an intraenteric barrier. Physiological studies suggest that 5-HT plays a vital role in mediating both sensory and reflex responses to gastrointestinal stimuli and, thus, this transmitter is closely implicated in gut reactions. This review outlines some of the evidence for different 5-HT receptors, summarizes the role of 5-HT in mediating gut sensitivity and motor activity, secretion and more complex activities, such as emesis and diarrhoea and identifies the clinical role of drugs acting on 5-HT receptors in the treatment of emesis, diarrhoea, the control of abdominal pain and discomfort and the rectification of gastrointestinal motility.

Motor-modifying properties of 5-HT3 and 5-HT4 receptor agonists on ovine abomasum

The effects and sites of action of 5-HT3 and 5-HT4 receptor agonists and antagonists on the abomasal myoelectric activity were examined in conscious sheep, chronically fitted with intravenous (i.v.) and intracerebroventricular (ICV) cannulas and intraparietal electrodes on the gastric body and antrum. The 5-HT3 receptor agonist 2-methylserotonin, injected either i.v. (150 micrograms/kg) or ICV (5 micrograms/kg), induced an inhibition of the spiking activity in both the gastric body and antrum. This inhibition was abolished when the 5-HT3 antagonist granisetron was preinjected either i.v. (150 micrograms/kg) or ICV (15 micrograms/kg). The i.v. injection of 5-HT4 agonist 5-methoxytryptamine (200 micrograms/kg) initially provoked stimulation and thereafter inhibition of abomasal activity, while its ICV administration (10 micrograms/kg) resulted in only inhibition of the gastric body activity. BIMU 1, another 5-HT4 agonistic substance, injected i.v. (300-1000 micrograms/kg), mimicked only the stimulatory actions of 5-methoxytryptamine, while its ICV administration (10-50 micrograms/kg) had no effect on the abomasal activity. The i.v. (2000 micrograms/kg), but not the ICV (100 micrograms/kg), pre-injection of the 5-HT4 antagonist DAU 6285 blocked the stimulation of the abomasal spiking activity resulting from the i.v. injection of either 5-methoxytryptamine or BIMU 1. These results suggest that, in sheep, inhibitory 5-HT3 and excitatory 5-HT4 receptors, located at brain and peripheral levels respectively, participate in the control of the abomasal contractions.

Gastric emptying of four liquid meals in pony foals

Water, saline and a saline/Intralipid mixture (lipid concentration 1.25 per cent) emptied very rapidly from the stomach in neonatal pony foals, and obeyed an apparently exponential pattern. Cisapride did not significantly modify the emptying of the saline/Intralipid mixture. Equine milk emptied more slowly. The data indicated that the lipid component of milk was not an important factor. Milk caused deviation from an exponential pattern in two of four foals: an initial lag phase appeared, which could represent a phase of intragastric processing. However, the rate was still rapid compared with results from human infants.

5-hydroxytryptamine 3-receptor antagonist modulates gallbladder emptying and motilin release induced by erythromycin

In the present study we evaluated the effect of ondansetron (formerly indicated as GR38032F), a potent and selective type-3 5-hydroxytryptamine receptor antagonist, on erythromycin-induced gallbladder emptying and motilin release, as well as gallbladder emptying induced by a regular meal in healthy volunteers. Gallbladder emptying was evaluated by sonography. Ondansetron, at the dose of 0.05 mg/kg, significantly reduced (P < 0.001 by ANOVA) the gallbladder emptying induced by 2 mg/kg/hr erythromycin, but did not increase basal gallbladder volume or inhibit gallbladder emptying induced by a regular meal. Ondansetron also inhibited the motilin release induced by erythromycin (P < 0.001, by ANOVA). These results suggest that serotoninergic mechanisms modulate the effects of erythromycin on the gastrointestinal tract. The exact site of action of ondansetron remains to be identified.

Effect of a 5HT3-antagonist (ondansetron) on rectal sensitivity and compliance in health and the irritable bowel syndrome

In some patients with the irritable bowel syndrome, rectal urgency and discomfort are major clinical problems and, under experimental conditions, these symptoms are perceived at lesser volumes of rectal distension than they are in asymptomatic controls. Further, a 5-hydroxytryptamine type-3 receptor antagonist increased the threshold for rectal discomfort in irritable bowel syndrome. Our aims were, (a) to measure rectal sensation during isobaric distensions of the rectum, and (b) to test the effect of another selective 5HT3-antagonist, ondansetron 0.15 mg/kg, on rectal sensitivity, colonic tone, rectal tone and manometric responses. Ten healthy volunteers and five patients with diarrhoea-predominant irritable bowel syndrome were studied. A multilumen barostat-manometric assembly was placed in the descending colon, and a second barostat balloon was positioned in the rectum. Tone in the wall of the colon and rectum was measured by the barostat balloon volume during a constant pressure clamp, while intraluminal pressures were recorded by manometry; perceived sensations were also recorded before and after the intravenous administration of ondansetron or placebo in blinded fashion. Rectal resistance to stretch was greater and rectal urgency was induced by lower distending pressures in irritable bowel syndrome, however, basal tone in the rectum was similar in health and irritable bowel syndrome. Ondansetron did not change rectal sensitivity (first sensation or urgency) or tone. Rectal distension did not alter tone in the descending colon or colonic manometry; ondansetron did not influence any index of colonic function.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of serotonin on rat ileocolonic transit and fluid transfer in vivo: possible mechanisms of action

The aim was to investigate the action of serotonin (5HT) on function of the ileocolonic junction (ICJ) in vivo. In anaesthetised rats, models were developed to study the effects of intra-aortic (ia) serotonin on ileocolonic and colonic transit, and the effects on transit of a number of 5HT receptor antagonists. In the first series of experiments, a bolus of saline labelled with 99mTc DTPA was instilled 20 cm proximal to the ICJ and transit was assessed three hours later by the geometric centre of the spread of isotope. In the second series, similar techniques were used on the postcaecal colon and transit assessed two hours later. In the third series of experiments, the effects of ia 5HT on ileal net fluid flux was evaluated by standard perfusion experiments with 14C polyethylene glycol (PEG) 4000 as a non-absorbable marker in rat plasma-like electrolyte solution. Compared with ia saline, 5HT accelerated ICJ transit significantly (p < 0.05). This acceleration was comparable with the effect of ia bethanechol. The effects of 5HT on ICJ transit were inhibited by the intraperitoneal (ip) infusion of atropine, the 5HT receptor antagonists, methysergide, ketanserin, zacopride, and the 5HT4 agonist, SC53116. Methysergide, zacopride, and SC53116 given with ia 5HT slowed ICJ transit to rates below those of ia saline alone. When these same agents were given together with ia saline, the ICJ transit was not significantly altered. Serotonin, at the dose that accelerated ICJ transit, did not significantly alter colonic transit or ileal fluid transport. In conclusion, 5HT is a potent pharmacological stimulant of transit across the rat ICJ in vivo; the action of 5HT is mediated partly through muscarinic neurones and several 5HT receptor subtypes.

Problematical binding of [3H]5-hydroxytryptamine to rat gut myenteric membranes

The binding of [3H]5-hydroxytryptamine (5-HT) to rat enteric membranes was inhibited by the inclusion of 5-HT 2-methyl-5-HT, 5-hydroxytryptophan, N,N,N-triethyltryptamine and 2-Br-N,N-diethyltryptamine in the incubation buffer. In contrast, tryptamine, 5-methoxytryptamine and 2-methyl-N,N-diethyltryptamine enhanced binding. Ascorbate and dithiothreitol facilitated and reduced binding at low and high concentrations respectively. Methysergide, tropisetron, paroxetine and pargyline failed to modify binding. However, following the establishment of [3H]5-HT binding, the subsequent addition of 5-HT and other agents failed to displace [3H]5-HT binding. Heat treatment of the membrane preparation also failed to modify [3H]5-HT binding. The latter findings of an irreversible and heat-insensitive binding indicate that the apparently high affinity 'specific' labelling of [3H]5-HT to rat enteric membranes does not represent receptor-bound [3H]5-HT. The complex interaction between [3H]5-HT and other agents may reflect oxidative events and problematic binding.

Effect of metoclopramide, ondansetron and granisetron on aldosterone secretion in man

The plasma aldosterone response following the administration of drugs with antagonist and agonist activity at Serotonin 3 and 4 (5-HT3&4) receptors has been examined in 9 healthy male volunteers receiving the following four treatments i.v. in a randomised, cross-over sequence: ondansetron 8 mg, granisetron 3 mg, metoclopramide 20 mg, and saline 20 ml. Metoclopramide significantly increased the mean plasma aldosterone level to 196% of basal level at 5 min. It rose to 234% at 15 min and remained at more than 185% of basal level for the duration of the experiment. The response to ondansetron and granisetron did not differ significantly from placebo. If dopamine antagonism is discounted, the results suggest that metoclopramide-induced aldosterone secretion results from its agonist activity at 5-HT4 receptors, although slow neuronal depolarization via an unidentified receptor remains a possibility. Antagonism at the 5-HT3 receptor plays no role, as the selective antagonist, granisetron, did not elicit a significant response. It seems unlikely that the 5-HT4 receptor is the second, low affinity binding site of ondansetron, unless it had no agonist activity at this receptor.

Labelling of 5-hydroxytryptamine3 receptors with a novel 5-HT3 receptor ligand, [3H]RS-42358-197

RS-42358-197[(S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-2,4,5,6-tetrahydro-1 H-benzo[de]isoquinolin-1-one hydrochloride] displaced the prototypic 5-hydroxytryptamine3 (5-HT3) receptor ligand [3H]quipazine in rat cerebral cortical membranes with an affinity (pKi) of 9.8 +/- 0.1, while having weak affinity (pKi < 6.0) in 23 other receptor binding assays. [3H]RS-42358-197 was then utilized to label 5-HT3 receptors in a variety of tissues. [3H]RS-42358-197 labelled high-affinity and saturable binding sites in membranes from rat cortex, NG108-15 cells, and rabbit ileal myenteric plexus with affinities (KD) of 0.12 +/- 0.01, 0.20 +/- 0.01, and 0.10 +/- 0.01 nM and densities (Bmax) of 16.0 +/- 2.0, 660 +/- 74, and 88 +/- 12 fmol/mg of protein, respectively. The density of sites labelled in each of these tissues with [3H]RS-42358-197 was similar to that labelled with [3H]GR 65630, but was significantly less than that found with [3H]-quipazine. The binding of [3H]RS-42358-197 had a pharmacological profile similar to that of [3H]quipazine, as indicated by the rank order of displacement potencies: RS-42358-197 > (S)-zacopride > tropisetron > (R)-zacopride > ondansetron > MDL72222 > 5-HT. However, differences in 5-HT3 receptors of different tissues and species were detected on the basis of statistically significant differences in the affinities of phenylbiguanide, and 1-(m-chlorophenyl)biguanide when displacing [3H]RS-42358-197 binding. [3H]RS-42358-197 also labelled a population (Bmax = 91 +/- 17 fmol/mg of protein) of binding sites in guinea pig myenteric plexus membranes, with lower affinity (KD = 1.6 +/- 0.3 nM) than those in the other preparations. Moreover, the rank order of displacement potencies of 15 5-HT3 receptor ligands in guinea pig ileum was found not to be identical to that in other tissues. Binding studies carried out with [3H]RS-42358-197 have detected differences in 5-HT3 receptor binding sites in tissues of different species and further underscore the unique nature of the guinea pig 5-HT3 receptor.