Influence of 5-HT3 receptor antagonists in visceromotor and nociceptive responses to rectal distension before and during experimental colitis in rats

Lactococcus lactis NCDO2118 exerts visceral antinociceptive properties in rat via GABA production in the gastro-intestinal tract

Gut disorders associated to irritable bowel syndrome (IBS) are combined with anxiety and depression. Evidence suggests that microbially produced neuroactive molecules, like γ-aminobutyric acid (GABA), can modulate the gut-brain axis. Two natural strains of Lactococcus lactis and one mutant were characterized in vitro for their GABA production and tested in vivo in rat by oral gavage for their antinociceptive properties. L. lactis NCDO2118 significantly reduced visceral hypersensitivity induced by stress due to its glutamate decarboxylase (GAD) activity. L. lactis NCDO2727 with similar genes for GABA metabolism but no detectable GAD activity had no in vivo effect, as well as the NCDO2118 ΔgadB mutant. The antinociceptive effect observed for the NCDO2118 strain was mediated by the production of GABA in the gastro-intestinal tract and blocked by GABA_B receptor antagonist. Only minor changes in the faecal microbiota composition were observed after the L. lactis NCDO2118 treatment. These findings reveal the crucial role of the microbial GAD activity of L. lactis NCDO2118 to deliver GABA into the gastro-intestinal tract for exerting antinociceptive properties in vivo and open avenues for this GRAS (Generally Recognized As safe) bacterium in the management of visceral pain and anxious profile of IBS patients.

Visceral pain: the neurophysiological mechanism

The mechanism of visceral pain is still less understood compared with that of somatic pain. This is primarily due to the diverse nature of visceral pain compounded by multiple factors such as sexual dimorphism, psychological stress, genetic trait, and the nature of predisposed disease. Due to multiple contributing factors there is an enormous challenge to develop animal models that ideally mimic the exact disease condition. In spite of that, it is well recognized that visceral hypersensitivity can occur due to (1) sensitization of primary sensory afferents innervating the viscera, (2) hyperexcitability of spinal ascending neurons (central sensitization) receiving synaptic input from the viscera, and (3) dysregulation of descending pathways that modulate spinal nociceptive transmission. Depending on the type of stimulus condition, different neural pathways are involved in chronic pain. In early-life psychological stress such as maternal separation, chronic pain occurs later in life due to dysregulation of the hypothalamic-pituitary-adrenal axis and significant increase in corticotrophin releasing factor (CRF) secretion. In contrast, in early-life inflammatory conditions such as colitis and cystitis, there is dysregulation of the descending opioidergic system that results excessive pain perception (i.e., visceral hyperalgesia). Functional bowel disorders and chronic pelvic pain represent unexplained pain that is not associated with identifiable organic diseases. Often pain overlaps between two organs and approximately 35% of patients with chronic pelvic pain showed significant improvement when treated for functional bowel disorders. Animal studies have documented that two main components such as (1) dichotomy of primary afferent fibers innervating two pelvic organs and (2) common convergence of two afferent fibers onto a spinal dorsal horn are contributing factors for organ-to-organ pain overlap. With reports emerging about the varieties of peptide molecules involved in the pathological conditions of visceral pain, it is expected that better therapy will be achieved relatively soon to manage chronic visceral pain.

Increased 5-hydroxytryptamine mediates post-inflammatory visceral hypersensitivity via the 5-hydroxytryptamine 3 receptor in rats

Visceral hypersensitivity often develops after intestinal inflammation, but the pathogenic mechanism has not been clearly elucidated. We investigated whether this post-inflammatory visceral hypersensitivity is mediated by 5-hydroxytryptamine through activation of the 5-hydroxytryptamine 3 receptor. In male Sprague-Dawley rats recovered from acetic acid-induced colitis, we monitored visceral nociceptive response by scoring the abdominal withdrawal reflex and simultaneously measuring the changes in arterial pulse rate. Seven days after induction of colitis, 52% of the rats showed an increased abdominal withdrawal reflex score and arterial pulse rate changes to colorectal distension, indicating that they had post-inflammatory visceral hypersensitivity. The 5-hydroxytryptamine 3 receptor antagonists, alosetron (20 mg/kg, p.o.) and granisetron (10 microg/kg, s.c.), inhibited post-inflammatory visceral hypersensitivity. Administration of a 5-hydroxytryptamine precursor, 5-hydroxytryptophan; 10 mg/kg, s.c.), induced visceral hypersensitivity in naïve rats, which was antagonized by granisetron. Increase in 5-hydroxytryptamine immunoreactive cells in colonic mucosal layer was found both in the rats with post-inflammatory visceral hypersensitivity and in the 5-hydroxytryptophan-treated rats. These results suggest that increased 5-hydroxytryptamine in colonic mucosa mediates post-inflammatory visceral hypersensitivity through activation of the 5-hydroxytryptamine 3 receptor.

Serotonin pharmacology in the gastrointestinal tract: a review

Serotonin (5-hydroxytryptamine or 5-HT) plays a critical physiological role in the regulation of gastrointestinal (GI) function. 5-HT dysfunction may also be involved in the pathophysiology of a number of functional GI disorders, such as chronic constipation, irritable bowel syndrome and functional dyspepsia. This article describes the role of 5-HT in the enteric nervous system (ENS) of the mammalian GI tract and the receptors with which it interacts. Existing serotonergic therapies that have proven effective in the treatment of GI functional disorders and the potential of drugs currently in development are also highlighted. Advances in our understanding of the physiological and pathophysiological roles of 5-HT in the ENS and the identification of selective receptor ligands bodes well for the future development of more efficacious therapies for patients with functional GI disorders.

Cilansetron in the treatment of diarrhea-predominant irritable bowel syndrome?

Information on the 5-hydroxytryptamine type 3 receptor antagonist cilansetron is scarce and most studies have only been published in abstract form. Results from preclinical and two dose-finding studies have suggested that cilansetron could be effective in the treatment of patients with diarrhea-predominant irritable bowel syndrome. Two large efficacy and safety trials extending over 3 and 6 months revealed a superiority of cilansetron 2 mg orally three-times daily over placebo reflected by numbers needed to treat of 4.8 and 5.6, respectively, for the parameter proportion of patients reporting adequate symptom relief. Dose-ranging studies showed no dose-response relationship. Cilansetron tended to induce constipation but, apart from transient ischemic colitis in four out of 1484 cases, no serious adverse effects were observed. Further trials are underway to fully determine the role of cilansetron in the treatment of diarrhea-predominant irritable bowel syndrome.

Importance of 5-hydroxytryptamine receptors on intestinal afferents in the regulation of visceral sensitivity

Serotonin (5-HT) plays an important role as a signalling molecule in the gastrointestinal (GI) tract. The regulation of GI sensitivity via 5-HT is mediated by specific 5-HT receptor subytypes on intrinsic and extrinsic afferents. This review discusses visceral afferent hypersensitivity in irritable bowel syndrome (IBS) and the importance of 5HT(3), 5HT(4), and 5HT(2B) receptor-mediated mechanisms in the regulation of visceral sensitivity.

Serotonergic and non-serotonergic targets in the pharmacotherapy of visceral hypersensitivity

Visceral hypersensitivity is considered a key mechanism in the pathogenesis of functional gastrointestinal (GI) disorders. Targeting visceral hypersensitivity seems an attractive approach to the development of drugs for functional GI disorders. This review summarizes current knowledge on targets for the treatment of visceral hypersensitivity, and the status of current and future drug and probiotic treatment development, and the role of pharmacogenomic factors.

Meta-analysis: The treatment of irritable bowel syndrome

To evaluate therapies available for the treatment of irritable bowel syndrome, and provide consensus recommendations for their use, a total of 51 double-blind clinical trials using bulking agents, prokinetics, antispasmodics, alosetron, tegaserod and antidepressants were selected. The quality of studies was assessed using 5-point scale. Meta-analyses were performed on all studies, and on 'high-quality studies'. The efficacy of fibre in the global irritable bowel syndrome symptoms relief (OR: 1.9; 95% CI:1.5-2.4) was lost after exclusion of low-quality trials (OR: 1.4; 95% CI: 1.0-2.0, P = 0.06). When excluding the low-quality trials, an improvement of global irritable bowel syndrome symptoms with all antispasmodics (OR: 2.1; 95% CI:1.8-2.9) was maintained only for octylonium bromide, but on the basis of only two studies. Antidepressants were effective (OR: 2.6, 95% CI: 1.9-3.5), even after exclusion of low-quality studies (OR: 1.9, 95% CI: 1.3-2.7). Alosetron (OR: 2.2; 95% CI: 1.9-2.6) and tegaserod (OR: 1.4; 95% CI: 1.2-1.5) showed a significant effect in women. We recommend the use of tegaserod for women with irritable bowel syndrome with constipation and alosetron for women with severe irritable bowel syndrome with diarrhoea. Antidepressants can be beneficial for irritable bowel syndrome with diarrhoea patients with severe symptoms. Loperamide can be recommended in painless diarrhoea. Evidence is weak to recommend the use of bulking agents in the treatment of irritable bowel syndrome with constipation.

5-HT3-Receptor Antagonist Inhibits Visceral Pain Differently in Chemical and Mechanical Stimuli in Rats

The present study was designed to compare the effects of a selective 5-HT(3)-receptor antagonist, alosetron, on the glycerol-and colorectal distention (CRD)-induced visceral nociception as measured by changes in EMG of the external oblique muscle in conscious rats. Both glycerol and CRD evoked the EMG response, and these amplified EMG were attenuated by morphine, indicating that these responses might reflect visceral nociceptive responses. In the present study, we showed that alosetron significantly attenuated the glycerol-induced visceral pain, but not that of CRD. These results suggest that the mechanism of glycerol-induced visceral nociception are apparently different from that of CRD.

A model for chronic quantitative studies of colorectal sensitivity using balloon distension in conscious mice -- effects of opioid receptor agonists

In the current study, colorectal distension (CRD) was performed in conscious mice, in order to study visceral (colon) sensitivity. Electrodes were chronically implanted into the external oblique muscle to obtain the electromyographic (EMG) response to CRD. CRD was performed using a computerized system, which inflated the balloon with air to the desired pressures. An increasing (10-80 mmHg) and a repeated (12 x 55 mmHg) phasic paradigm with distensions lasting 10 s and with 5-min intervals were used. The EMG recordings were linearly correlated to intracolonic pressures between 10 and 80 mmHg, which are characteristic of the visceromotor response (VMR). Repeated phasic distensions at 55 mmHg resulted in a stable VMR in female mice, but an increasing VMR in male mice. Interestingly, the duration of the VMR was about 5 s, which is shorter than the actual duration of the distension. U-69593 and fentanyl (selective kappa and mu opioid receptor agonists) significantly reduced the VMR at subcutaneous doses of 0.5 and 0.05 mg x kg-1, respectively. In conclusion, a CRD model for repetitive quantitative studies of colorectal sensitivity and evaluation of pharmacological modulation of visceral sensitivity in conscious mice is presented.

A novel 5-HT3 receptor agonist, YM-31636, increases gastrointestinal motility without increasing abdominal pain

We examined the effects of YM-31636 (2-(1H-imidazol-4-ylmethyl)-8H-indeno[1,2-d]thiazole monofumarate), a novel 5-HT3 receptor agonist, on gastrointestinal functions including visceral pain reflex in rats. Injection of YM-31636 increased the number of fecal pellets. This effect was completely inhibited by ramosetron, a 5-HT3 receptor antagonist. YM-31636 also increased the intracolonic pressure measured in both conscious and anesthetized rats. In isolated distal colon, YM-31636 increased the short-circuit current response. This effect was abolished by ramosetron. Both the maximal response and the potency of YM-31636 were weaker than those of other 5-HT3 receptor agonists. In two visceral pain reflex models, YM-31636 neither changed the magnitude of pressor response to colonic distension in anesthetized rats nor affected the visceromotor threshold to colorectal distension in conscious rats. In conclusion, YM-31636 facilitated defecation without increasing visceral pain. Consequently, 5-HT3 receptor agonists like YM-31636 would be promising in the treatment of chronic constipation.

Role of Barrington's nucleus in the activation of rat locus coeruleus neurons by colonic distension

The locus coeruleus (LC)-noradrenergic system, which has been implicated in arousal and attention, is activated by visceral stimuli such as colon and bladder distension. Neurons of Barrington's nucleus (the pontine micturition center) have been identified which project to both the LC and preganglionic column of the lumbosacral spinal cord. Thus, Barrington's nucleus is positioned to coordinate brain noradrenergic activity with pelvic visceral functions. The aim of this study was to determine whether LC activation by colonic distension was mediated by projections from Barrington's nucleus to the LC in the rat. Lesions of Barrington's nucleus were performed unilaterally by local injection of ibotenic acid (microg/microl, 90 nl) 10 days prior to recording: (i) ipsilateral spontaneous LC discharge rate; (ii) LC responses to colonic distension; and (iii) LC responses to sciatic nerve stimulation. In some rats LC activation by hypotensive challenge was also examined. Lesions of Barrington's nucleus significantly reduced LC activation by colon distension from a magnitude of 26.6+/-6% increase in discharge rate (n=8) to 6.9+/-3% (n=6), while having no effect on basal LC discharge rate. In contrast, LC responses to sciatic nerve stimulation were not altered in rats with lesions of Barrington's nucleus and LC neurons were still activated by hypotensive challenge. These results support the hypothesis that Barrington's nucleus selectively relays input from pelvic visceral afferents to the LC. This may serve as a limb in a circuit designed to coordinate central and peripheral responses to pelvic visceral stimuli.

Rectal antinociceptive properties of alverine citrate are linked to antagonism at the 5-HT1A receptor subtype

Serotonin (5-HT) is considered as a major mediator causing hyperalgesia and is involved in inflammatory reactions and irritable bowel syndrome. Alverine citrate may possess visceral antinociceptive properties in a rat model of rectal distension-induced abdominal contractions. This study was designed to evaluate the pharmacological properties of alverine citrate in a rat model of rectal hyperalgesia induced by 5-HTP (5-HT precursor) and by a selective 5-HT1A agonist (8-OH-DPAT) and to compare this activity with a reference 5-HT1A antagonist (WAY 100635). At 4 h after their administration, 5-HTP and 8-OH-DPAT increased the number of abdominal contractions in response to rectal distension at the lowest volume of distension (0.4 mL). When injected intraperitoneally before 8-OH-DPAT and 5-HTP, WAY 100635 (1 mg kg(-1)) blocked their nociceptive effect, but also reduced the response to the highest volume of distension (1.6 mL). Similarly, when injected intraperitoneally, alverine citrate (20 mg kg(-1)) suppressed the effect of 5-HTP, but not that of 8-OH-DPAT. However, when injected intracerebroventricularly (75 microg/rat) alverine citrate reduced 8-OH-DPAT-induced enhancement of rectal distension-induced abdominal contractions. In-vitro binding studies revealed that alverine citrate had a high affinity for 5-HT1A receptors and a weak affinity for 5-HT3 and 5-HT4 subtypes. These results suggest that 5-HTP-induced rectal hypersensitivity involves 5-TH1A receptors and that alverine citrate acts as a selective antagonist at the 5-HT1A receptor subtype to block both 5-HTP and 8-OH-DPAT-induced rectal hypersensitivity.

[Acute pain measurement in animals. Part 1]

OBJECTIVE

To describe tests of nociception which appear in the "pre-clinical" literature.

DATA SOURCES

References obtained by computerized bibliographic research (Medline) and the authors' personal data.

DATA SYNTHESIS

Ethical problems arising from the study of the pain in awake animals, problems arising from the choice of stimulus and stimulus parameters and the quantification of responses are presented. Pain in animals can be estimated only by examining their reactions, but at the same time, the existence of a reaction does not necessarily mean that there is a concomitant sensation. A description of the signs of pain in mammals is proposed. A noxious stimulus can be defined by its physical nature, its site of application and what has previously happened to the tissues at this site. Electrical stimulation short-circuits the process of transduction at free nerve endings and is not specific; however it has the advantage that it can be applied suddenly and briefly and thus results in synchronised signals in the relevant primary afferent fibres which can be differentiated into A delta and C fibres. Heat selectively stimulates thermoreceptors and nociceptors, but the low calorific power of conventional stimulators restricts their usefulness. Radiant sources have the disadvantage of emitting waves in the visible and the adjacent infrared spectra, for which the skin is a poor absorber and good reflector. Thermodes have the disadvantage of activating mechanoreceptors and thermoreceptors simultaneously; furthermore, their capacity for transferring heat depends on the quality of contact with skin and thus on the pressure with which they are applied. These problems can be overcome by using CO2 lasers but even today, the cost of these is a major disadvantage. Chemical stimuli differ from those mentioned above by the progressive onset of their effectiveness, their duration of action and the fact that they are of an inescapable nature. Experimental models employing chemical stimuli are undoubtedly the most similar to acute clinical pain. A wide spectrum of reactions are observed in nociceptive tests, but in almost every case they involve motor responses. After defining the ideal characteristics of a nociceptive test, tests based on the use of short duration and longer duration stimuli are presented. In tests of phasic pain, reactions are evoked by thermal (tail-flick test, hot-plate test), mechanical or electrical (flinch-jump test, vocalisation test) stimuli. Tests of tonic pain employ injections of algogenic agents intradermally (formalin test) or intraperitoneally (writhing test) or even the dilation of hollow organs. All these tests will be critically appraised in a subsequent paper [1].

CONCLUSION

The tail-flick and hot-plate tests are the most used, but there is an increasing recourse to the formalin test and tests involving foot withdrawal after mechanical stimulation.

Effects of the 5-HT3 antagonist cilansetron vs placebo on phasic sigmoid colonic motility in healthy man: a double-blind crossover trial

AIMS

5-hydroxytryptamine3 receptor antagonists act antiemetically and slow colonic transit. This study evaluated effects of the high-affinity 5-HT3 antagonist, cilansetron, on fasting, meal-and anticholinesterase-stimulated phasic contractile activity of the human sigmoid colon as well as on bowel habits and stool consistency.

METHODS

Five female and seven male healthy volunteers received, during three 7 day periods separated by 7 day wash-out periods, 4 mg cilansetron, 8 mg cilansetron or placebo three times daily orally under random, double-blind, crossover conditions. On day 8 of each treatment period, motility 20-40 cm from the anal verge was recorded using five pressure sensors spaced at 5 cm intervals. After a basal 30 min, subjects swallowed a further dose of the scheduled treatment; 60 min later, blood was taken for the determination of plasma cilansetron levels. Thereafter, subjects ingested a 4200 kJ meal and 250 ml sweetened mallow tea (166 kJ); 90 min after meal onset, 1 mg neostigmine was administered intramuscularly and motility recording was continued for 60 min

RESULTS

Phasic contractile activity and intraluminal base-line pressure increased postprandially and more so after neostigmine. With cilansetron, the area under the pressure curve as the primary outcome variable and the number of contractions were significantly greater than with placebo (P = 0.005), amplitude and duration of contractions and base-line pressure were not affected. The effects of the two cilansetron dosages did not differ. With cilansetron, stool tended to become firmer. No adverse effects were observed. Plasma levels were highest with 8 mg cilansetron.

CONCLUSIONS

Cilansetron slightly augments meal-stimulated and markedly neostigmine-stimulated phasic motility of the sigmoid colon. When administered over 7 days, it tends to increase stool consistency and is well tolerated.

Pathobiology of visceral pain: molecular mechanisms and therapeutic implications. III. Visceral afferent pathways: a source of new therapeutic targets for abdominal pain

Visceral pain is the major cause of consulting in gastroenterology and the principal symptom of functional bowel disorders. This symptom is often associated with gut hypersensitivity to distension. The use of animal models has recently permitted the identification of some mediators supposed to play a pivotal role in the genesis of visceral hypersensitivity. Serotonin, through different receptor subtypes, as well as kinins and calcitonin gene-related peptide, are known to be involved, but other putative transmitters arise and are new potential targets for the development of efficacious treatments. This themes article addresses both physiological and preclinical issues of interest for the selection of active new drugs in regard to the clinical pharmacology of visceral pain.

Role of tachykinin NK2 receptors in normal and altered rectal sensitivity in rats

Irritable bowel syndrome is characterized by visceral hyperalgesia commonly associated with stress and inflammatory processes. We investigated the role of tachykinin NK2 receptors in the ability of trinitrobenzenesulphonic acid (TNBS) and stress to enhance the sensitivity of the rat rectum to distension using a selective tachykinin NK2 receptor antagonist (MEN 11420). Rats were fitted with electrodes implanted in the striated muscles of the abdomen. Rectal distension (RD) was performed with a balloon inflated by steps of 0.4 ml from 0 to 1.6 ml. Five groups were submitted to RD performed 3 days before and after intrarectal instillation of TNBS. Fifteen minutes before RD, rats were treated with saline or MEN 11420 (5 - 100 microg kg(-1) i.v.). Two other groups, submitted to 2 h restraint or sham stress sessions were randomly treated i.v. with saline or MEN 11420 (10 - 200 microg kg(-1)) prior to RD applied 20 min later. The basal response to RD was characterized by a significant increase in the number of abdominal contractions. This response occurred with a threshold volume of 0.8 ml and was dose-dependently reduced by MEN 11420 (5 - 100 microg kg(-1) i.v.). Rectal inflammation lowered the volume of distension producing abdominal contractions to 0.4 ml (allodynia). This effect was either reduced or suppressed by MEN 11420. A similar allodynia was observed after a stress session and this effect was reduced (49%) or suppressed by MEN 11420 at 200 and 100 microg kg(-1), respectively. Tachykinin NK2 receptors are involved in rectal hypersensitivity associated with inflammation and stress. British Journal of Pharmacology (2000) 129, 193 - 199

Intracolonic glycerol induces abdominal contractions in rats: role of 5-HT3 receptors

Serotonin and 5-HT3 receptors may be involved in the activation of nociceptive afferent pathways by rectal distension. In rats, intracolonic infusion of glycerol is able to trigger nociceptive inputs as evidenced by the occurrence of abdominal constrictions. This work was designed to evaluate the influence of 5-HT3 receptor antagonists on this reflex and to approach the site of action by comparing their relative efficacies according to the route of administration. Male Wistar rats (250-350 g) were surgically prepared for abdominal electromyography and a catheter was placed in the colonic lumen. Five days after surgery, electrical activity of abdominal muscles was recorded before and during (20 min) intracolonic infusion of glycerol (60% glycerol + 40% saline, rate 0.75 mL/h). Cilansetron was administered intraperitoneally, 15 min before glycerol infusion, at doses of 5 to 500 micrograms/kg. Granisetron, ondansetron and cilansetron were administered at the dose of 20 micrograms/kg by intraperitoneal (i.p.), intravenous (i.v.) or intracolonic (i.c.) routes. The number of abdominal spike bursts was used as an index of visceral nociception. Intracolonic infusion of glycerol increased significantly (P < 0.05) the number of abdominal spike bursts during the time of infusion compared with saline (30.6 +/- 6.6 vs 4.5 +/- 3.4 bursts). When administered i.p., cilansetron dose-dependently reduced the frequency of abdominal spike bursts from the dose of 20 micrograms/kg i.p. Administration i.p. of granisetron and ondansetron at this dose also significantly reduced the number of abdominal spikes (19.0 +/- 6.0 and 18.3 +/- 6.9 respectively). Cilansetron, ondansetron and granisetron were also effective by i.v. and i.c. routes, cilansetron was more active by the i.c. route. Serotonin, via 5-HT3 receptors, is involved in the mediation of abdominal contractions induced by intracolonic infusion of glycerol. 5-HT3 receptor antagonists are also active by i.c. route suggesting a local site of action.

Effect of alosetron on responses to colonic distension in patients with irritable bowel syndrome

BACKGROUND

Visceral hypersensitivity plays a major role in the pathophysiology of irritable bowel syndrome, as shown by balloon distension studies. 5-HT3 receptors on afferent nerves may modulate visceral sensitivity and be the target of new treatments for irritable bowel syndrome.

AIM

To evaluate the effects of alosetron, a potent and selective 5-HT3 antagonist, on the perception of colonic distension by patients with irritable bowel syndrome, and on the colonic compliance to distension with a barostat.

METHODS

Twenty-five irritable bowel syndrome patients were included in a randomized double-blind parallel group trial; data were available for 22 (Rome criteria; 48 +/- 11 years: 13 men and nine women). Patients were treated for 7 days with placebo (n = 6), alosetron 0.25 mg b.d. (n = 8) or alosetron 4 mg b.d. (n = 8). On day 6, a barostat bag was placed in the left colon. On day 7, after an overnight fast, isobaric phasic distensions were performed (4 mmHg steps, 5 min) up to the step triggering a sensation of abdominal pain.

RESULTS

Groups were comparable at inclusion (age, sex, symptoms, bowel habits). There were no differences between treatment groups in pressure recorded within the bag at the time of first sensation of abdominal pain. However, bag volumes were significantly increased. At the first sensation threshold, median volume differences of 61 mL and 90 mL (P = 0.028) were recorded with alosetron 0.25 mg b.d. and 4 mg b.d., respectively. At the threshold of abdominal pain, these differences were 71 mL (P = 0.039) and 84 mL (P = 0.017). Colonic compliance increased from 5.9 mL/mmHg on placebo to 7.6 mL/mmHg on alosetron 0.25 mg b.d. and to 9.8 mL/mmHg (P = 0.034) on alosetron 4 mg b.d.

CONCLUSION

Alosetron increases the compliance of the colon to distension, and could thereby contribute to changes in perception of colonic distension and improvement in the symptoms of irritable bowel syndrome.

Influence of trimebutine on inflammation- and stress-induced hyperalgesia to rectal distension in rats

The effects of trimebutine and its major metabolite, N-desmethyltrimebutine on inflammation- and stress-induced rectal hyperalgesia have been evaluated in rats fitted with electrodes implanted in the longitudinal striated muscle of the abdomen. Intermittent rectal distension was performed before and 3 days after induction of rectal inflammation by local infusion of trinitrobenzenesulphonic acid (in ethanol). Stress consisted of 2h partial restraint and rectal distension was performed before and 30min after the end of the partial restraint session. The animals were treated intraperitoneally with trimebutine or desmethyltrimebutine (5, 10 or 20mgkg(-1)) or vehicle 15min before rectal distension. Naloxone (1mgkg(-1)) or saline was injected subcutaneously before trimebutine and desmethyltrimebutine. Before treatment trimebutine at the highest dose (20mgkg(-1)) reduced the abdominal response to rectal distension for the highest volume of distension (1.6mL) whereas desmethyltrimebutine was inactive. After rectocolitis the abdominal response to rectal distension was enhanced and trimebutine at 5mgkg(-1) reduced and at 10 mgkg(-1) suppressed inflammation-induced hyperalgesia, an effect reversed by naloxone. Desmethyltrimebutine was inactive. Stress-induced hypersensitivity was attenuated or suppressed, or both, by trimebutine and desmethyltrimebutine at doses of 5, 10 or 20mgkg(-l); greater efficacy was observed for desmethyltrimebutine and the effects were not reversed by naloxone. It was concluded that trimebutine and desmethyltrimebutine are active against inflammation- and stress-induced rectal hyperalgesia but act differently. The effect of trimebutine on inflammation-induced hyperalgesia is mediated through opioid receptors.

Mast cell degranulation induces delayed rectal allodynia in rats: role of histamine and 5-HT

Visceral hypersensitivity is a common feature of functional bowel disorders, where an increased number of mast cells have often been described. Thus, we investigated the effect of an experimental mast cell degranulation induced by BrX-537A on somatic (tail heating) and visceral (rectal distension) sensitivity in rats and the involvement of histamine and/or serotonin on this last response. After BrX-537A administration, the latency of tail withdrawal reflex was shortened within the 2- to 8-hr period. Moreover, BrX-537A reduced the distension volume threshold from 0.8 ml to 0.4 ml inducing allodynia, from 6 to 12 hr after its administration. This effect was suppressed by doxantrazole (mast cell stabilizing agent) and WAY 100635 (5-HT1A receptor antagonist), and reproduced by 5-HTP (5-HT precursor) and 8-OH-DPAT (5-HT1A receptor agonist). However, neither granisetron (5-HT3 receptor antagonist) nor H1, H2, or H3 histamine receptor antagonists modified the BrX-537A-induced allodynia. Consequently, mast cell degranulation initiates a delayed somatic and visceral allodynia, with the participation of serotonin, through 5-HT1A receptor activation, on the visceral response.

c-fos expression in specific rat brain nuclei after intestinal anaphylaxis: involvement of 5-HT3 receptors and vagal afferent fibers

The c-fos immediate-early gene is acutely induced in brain after various stimuli from the digestive tract. 5-HT3 receptors and vagal afferents have been found involved in intestinal motor disturbances induced by intestinal anaphylaxis. Our aim was to determine whether intestinal anaphylaxis activates brain structures, using c-fos expression, and to evaluate the modulation of c-fos induction by 5-HT3 receptors and vagal afferents. The effects of antigen challenge on intestinal motility were evaluated in ovalbumin-sensitized Hooded Lister rats chronically fitted with NiCr electrodes in the jejunal wall. Intestinal motility was assessed in conscious rats pretreated or not by perivagal capsaicin or a 5-HT3 antagonist (ondansetron). In sensitized rats, ovalbumin disrupted for 62.4 +/- 9.5 min the jejunal migrating motor complexes (MMC) and an important c-fos expression was detected in the nucleus tractus solitarius (NTS), lateral parabrachial nucleus (LPB) and paraventricular nucleus of the hypothalamus (PVN). Intraperitoneal administration of ondansetron or perivagal capsaicin treatment significantly reduced the duration of MMC disruption and attenuated markedly c-fos staining in the 3 brain sites. In contrast, intracerebroventricular administration of ondansetron significantly reduced jejunal motor alterations but did not diminish the c-fos expression, suggesting a role of central 5-HT3 receptors in the efferent control of the intestinal disturbances. Blockade of both c-fos expression and MMC disruption by systemic ondansetron and by perivagal capsaicin indicates that some brainstem nuclei are involved in digestive disturbances after intestinal anaphylaxis, and reflects an involvement of peripheral 5-HT3 receptors on vagal afferents. The reduction of c-fos staining in NTS as well as in LPB and PVN after perivagal capsaicin suggests that the NTS is the primary relay in the activation of the central nervous system during intestinal allergic challenge.