Studies on mechanism of retching and vomiting in dogs. Effect of peripheral dopamine blocker on myoelectric changes in antrum and upper small intestine

Physiological changes associated with copper sulfate-induced nausea and retching in felines

Nausea is a common disease symptom, yet there is no consensus regarding its physiological markers. In contrast, the process of vomiting is well documented as sequential muscular contractions of the diaphragm and abdominal muscles and esophageal shortening. Nausea, like other self-reported perceptions, is difficult to distinguish in preclinical models, but based on human experience emesis is usually preceded by nausea. Here we focused on measuring gastrointestinal and cardiorespiratory changes prior to emesis to provide additional insights into markers for nausea. Felines were instrumented to chronically record heart rate, respiration, and electromyographic (EMG) activity from the stomach and duodenum before and after intragastric delivery of saline or copper sulfate (CuSO4, from 83 to 322 mg). CuSO4 is a prototypical emetic test agent that triggers vomiting primarily by action on GI vagal afferent fibers when administered intragastrically. CuSO4 infusion elicited a significant increase in heart rate, decrease in respiratory rate, and a disruption of gastric and intestinal EMG activity several minutes prior to emesis. The change in EMG activity was most consistent in the duodenum. Administration of the same volume of saline did not induce these effects. Increasing the dose of CuSO4 did not alter the physiologic changes induced by the treatment. It is postulated that the intestinal EMG activity was related to the retrograde movement of chyme from the intestine to the stomach demonstrated to occur prior to emesis by other investigators. These findings suggest that monitoring of intestinal EMG activity, perhaps in combination with heart rate, may provide the best indicator of the onset of nausea following treatments and in disease conditions, including GI disease, associated with emesis.

Abnormalities on Electrogastrography in Nausea and Vomiting Syndromes: A Systematic Review, Meta-Analysis, and Comparison to Other Gastric Disorders

BACKGROUND

Functional nausea and vomiting syndromes and gastroparesis, collectively grouped as nausea and vomiting syndromes (NVS), are overlapping conditions with incompletely understood pathophysiology. Gastric slow wave abnormalities are thought to contribute.

AIMS

This study aimed to systematically review and meta-analyze the prevalence of slow wave abnormalities measured by electrogastrography (EGG) in patients with NVS.

METHODS

MEDLINE, EMBASE, EMBASE classic, and CENTRAL databases were systematically searched for articles using EGG in adults (≥ 18 years) with NVS. EGG metrics of interest were percentage time in bradygastria, normogastria, and tachygastria as well as dominant frequency and dominant power. Outcomes were also compared with functional dyspepsia (FD), gastroesophageal reflux disease (GORD), and control cohorts.

RESULTS

Seven hundred and sixty NVS patients and 308 controls were included from 24 studies. Overall, 64% of patients had EGG abnormalities. Average percent time in normogastria was low during fasting (50%; 95% CI 40-63%) and fed (53%; 95% CI 41-68%) states in patients, with substantial periods in fasting bradygastria (34.1%; 95% CI 25-47%) and postprandial tachygastria (21%; 95% CI 17-26%). Across gastric disorders, pooling of 84 studies showed a comparably high prevalence of EGG abnormalities in NVS (24 studies; n = 760) and GORD (13 studies; n = 427), compared to FD (47 studies; n = 1751) and controls (45 studies; n = 1027).

CONCLUSIONS

Frequency-based gastric slow wave abnormalities are prominent in NVS. The strength and consistency of these associations across many studies suggests that gastric dysrhythmia may be an important factor in NVS, motivating the development of more reliable methods for their clinical assessment.

Pathophysiology of Vomiting and Esophageal Perforation in Boerhaave's Syndrome

BACKGROUND AND AIMS

Boerhaave's syndrome, involving esophagus rupture, is considered a pathological response to vomiting that may occur just before perforation. However, the mechanism of vomiting and occurrence of this disease have not been clearly demonstrated.

METHODS

We identified patients with esophageal perforation between 1995 and 2017 and reviewed endoscopic findings at retching during upper gastrointestinal endoscopy. Finally, we proposed a theory for the underlying pathological mechanism.

RESULTS

We retrospectively investigated 10 patients with esophageal perforation between 1995 and 2017. All patients presented after vomiting associated with large volumes of food and alcohol intake. Nine were treated by primary closure of the perforation and drainage of the thoracic cavity, and one was conservatively treated. In all cases, the perforations were longitudinal tears (1-4 cm) and located in the left of the esophagus, just above the gastric cardia.

CONCLUSIONS

We hypothesize that vomiting occurred by retrograde propagation of gastrointestinal motor contraction from the jejunum to the gastric antrum, followed by prolapse of the gastric fornix mucosal into the esophagus. Subsequent esophageal perforation probably resulted from excessive prolapse due to strong contraction and destruction of the muscularis mucosa of the left side of abdominal esophagus, with longitudinal stretching of the whole left esophageal wall due to traction. We also propose that Boerhaave's syndrome is defined as "post-emetic esophageal perforation" to ensure broader recognition and more expedient diagnosis and treatment. Remaining conditions without any definite causes may be labeled "idiopathic" or "spontaneous" rupture of the esophagus.

Recent progress in gastric arrhythmia: pathophysiology, clinical significance and future horizons

Gastric arrhythmia continues to be of uncertain diagnostic and therapeutic significance. However, recent progress has been substantial, with technical advances, theoretical insights and experimental discoveries offering new translational opportunities. The discoveries that interstitial cells of Cajal (ICC) generate slow waves and that ICC defects are associated with dysmotility have reinvigorated gastric arrhythmia research. Increasing evidence now suggests that ICC depletion and damage, network disruption and channelopathies may lead to aberrant slow wave initiation and conduction. Histological and high-resolution (HR) electrical mapping studies have now redefined the human 'gastric conduction system', providing an improved baseline for arrhythmia research. The application of HR mapping to arrhythmia has also generated important new insights into the spatiotemporal dynamics of arrhythmia onset and maintenance, resulting in the emergence of new provisional classification schemes. Meanwhile, the strong associations between gastric functional disorders and electrogastrography (EGG) abnormalities (e.g. in gastroparesis, unexplained nausea and vomiting and functional dyspepsia) continue to motivate deeper inquiries into the nature and causes of gastrointestinal arrhythmias. In future, technical progress in EGG methods, new HR mapping devices and software, wireless slow wave acquisition systems and improved gastric pacing devices may achieve validated applications in clinical practice. Neurohormonal factors in arrhythmogenesis also continue to be elucidated and a deepening understanding of these mechanisms may open opportunities for drug design for treating arrhythmias. However, for all translational goals, it remains to be seen whether arrhythmia can be corrected in a way that meaningfully improves organ function and symptoms in patients.

Mechanism of gastrointestinal abnormal motor activity induced by cisplatin in conscious dogs

AIM: To investigate whether 5-hydroxytryptamine (serotonin; 5-HT) is involved in mediating abnormal motor activity in dogs after cisplatin administration.

METHODS: After the dogs had been given a 2-wk recovery period, all of them were administered cisplatin, and the motor activity was recorded using strain gauge force transducers. Blood and intestinal fluid samples were collected to measure 5-HT for 24 h. To determine whether 5-HT in plasma or that in intestinal fluids is more closely related to abnormal motor activity we injected 5-HT into the bloodstream and the intestinal tract of the dogs.

RESULTS: Cisplatin given intravenously produced abnormal motor activity that lasted up to 5 h. From 3 to 4 h after cisplatin administration, normal intact dogs exhibited retropropagation of motor activity accompanied by emesis. The concentration of 5-HT in plasma reached the peak at 4 h, and that in intestinal fluids reached the peak at 3 h. In normal intact dogs with resection of the vagus nerve that were administered kytril, cisplatin given intravenously did not produce abnormal motor activity. Intestinal serotonin administration did not produce abnormal motor activity, but intravenous serotonin administration did.

CONCLUSION: After the intravenous administration of cisplatin, abnormal motor activity was produced in the involved vagus nerve and in the involved serotonergic neurons via another pathway. This study was the first to determine the relationship between 5-HT and emesis-induced motor activity.

Pathology of emesis: its autonomic basis

Vagal and non-vagal pathways as well as several brainstem nuclei participate in vomiting in response to different emetic stimuli. Autonomic pathways involved in nausea are less well understood. Numerous gastrointestinal disorders with prominent nausea and vomiting including gastroparesis, cyclic vomiting syndrome, and motion sickness have associated autonomic nervous system dysfunction. Autonomic disturbances are also seen with non-gastrointestinal diseases with gut manifestations such as migraine headaches, orthostatic intolerance, and familial dysautonomia. Stimulation of emetic pathways involves activation of a range of receptor subtypes. Agents acting on these receptors form the basis for antiemetic therapies. Chemotherapy-induced nausea and vomiting, a prevalent and severe consequence of anticancer treatment, is preventable in many instances by agents acting on the autonomic nervous system. Likewise, non-medication therapies may act in part via modulation of some of these same autonomic pathways.

Investigating the effect of emetic compounds on chemotaxis in Dictyostelium identifies a non-sentient model for bitter and hot tastant research

Novel chemical entities (NCEs) may be investigated for emetic liability in a range of unpleasant experiments involving retching, vomiting or conditioned taste aversion/food avoidance in sentient animals. We have used a range of compounds with known emetic /aversive properties to examine the possibility of using the social amoeba, Dictyostelium discoideum, for research into identifying and understanding emetic liability, and hence reduce adverse animal experimentation in this area. Twenty eight emetic or taste aversive compounds were employed to investigate the acute (10 min) effect of compounds on Dictyostelium cell behaviour (shape, speed and direction of movement) in a shallow chemotaxic gradient (Dunn chamber). Compound concentrations were chosen based on those previously reported to be emetic or aversive in in vivo studies and results were recorded and quantified by automated image analysis. Dictyostelium cell motility was rapidly and strongly inhibited by four structurally distinct tastants (three bitter tasting compounds--denatonium benzoate, quinine hydrochloride, phenylthiourea, and the pungent constituent of chilli peppers--capsaicin). In addition, stomach irritants (copper chloride and copper sulphate), and a phosphodiesterase IV inhibitor also rapidly blocked movement. A concentration-dependant relationship was established for five of these compounds, showing potency of inhibition as capsaicin (IC(50) = 11.9 ± 4.0 µM) > quinine hydrochloride (IC(50) = 44.3 ± 6.8 µM) > denatonium benzoate (IC(50) = 129 ± 4 µM) > phenylthiourea (IC(50) = 366 ± 5 µM) > copper sulphate (IC(50) = 1433 ± 3 µM). In contrast, 21 compounds within the cytotoxic and receptor agonist/antagonist classes did not affect cell behaviour. Further analysis of bitter and pungent compounds showed that the effect on cell behaviour was reversible and not cytotoxic, suggesting an uncharacterised molecular mechanism of action for these compounds. These results therefore demonstrate that Dictyostelium has potential as a non-sentient model in the analysis of the molecular effects of tastants, although it has limited utility in identification of emetic agents in general.

[Management of chemotherapy-induced emesis: what is the standard after 20 years of clinical research]

BACKGROUND

The knowledge of the importance, the physiopathological mechanisms, and the management of the chemotherapy-induced emesis has increased exponentially during the last 20 years. High-dosage metoclopramide (MCP) therapy has been introduced in the eighties and serotonine type-3 receptor antagonists (5-HT(3) antagonists) have been used since the late eighties and early nineties. Due to both classes of substances the results of the antiemetic therapies have improved drastically. After 20 years of intensive clinical research it seems to be appropriate to come to an intermediate conclusion.

METHOD

With the aid of an overview and a new analysis of the literature published on this topic so far, the current state of research is shown (including the fields in which further improvement will be necessary), and suggestions are made, wherever it seemed possible, to attain the "gold standard" in antiemetic therapy.

RESULTS AND CONCLUSIONS

In connection with all highly or very highly emetogenic chemotherapies, an antiemetic prophylaxis should be initiated on the day of therapy, especially when using platinum or most of the cyclophosphamide-based regimes for cancer treatment. The recommended prophylaxis consists of a combination of 5-HT(3) antagonists with a corticosteroid. To combat the so-called delayed emesis on the days following therapy, all patients should undergo an oral corticoid therapy, possibly in combination with MCP (especially platinum-therapy patients), less frequently with 5-HT(3) antagonists. With these means of prophylaxis emesis can be prevented/avoided completely in most patients, and nausea can at least be reduced. It is sufficient to administer a single dose of 5-HT(3) antagonists prior to chemotherapy. For ondansetron and granisetron, the best documented substances within this class of drugs, 8 mg (ondansteron) and 3 mg (granisetron) are considered standard dosages. Among the corticoids, most data have been accumulated for dexamethasone. A standard dose of 10 to 20 mg can be administered prior to chemotherapy. Right after and especially on the days following chemotherapy higher dosages seem to be indicated.

PROSPECT

Further therapy improvements, especially concerning emesis and nausea on the days following chemotherapy, are necessary and are currently object of clinical research.

Impact of bitter taste on gastric motility

BACKGROUND

Unexplained nausea and vomiting is often associated with delayed gastric emptying in patients with functional dyspepsia. We hypothesized that the experience of an unpleasant, nauseating taste could lead to a delay in gastric emptying.

METHODS

Sixteen healthy women consumed a bland liquid test meal on three separate study days. On two of the study days subjects sham fed either a bitter tasting, modified Slim-Fast bar or one with a pleasant strawberry flavour. The time for 50% gastric emptying (GE(50)) was non-invasively assessed by electrical impedance tomography and antral motility by electrogastrography (EGG).

RESULTS

Gastric emptying was significantly delayed by sham feeding the bitter compared with the pleasant bar, GE(50) 24.7+/-3.9 versus 17.2+/-1.8 min, P<0.05. EGG power rose significantly during both the pleasant (basal 1.46+/-0.07 to 2.33+/-0.14 log(10) microV(2)/min, P=0.000) and the bitter sham feed (basal 1.64+/-0.09 to 2.35+/-0.11 log(10) microV(2)/min, P=0.000).

CONCLUSION

An unpleasant bitter taste delays gastric emptying but does not significantly impair antral motility.

Effects of acupuncture on vasopressin-induced emesis in conscious dogs

Although acupuncture has a significant clinical benefit, the mechanism of acupuncture remains unclear. Vasopressin, a posterior pituitary hormone, is involved in nausea and vomiting in humans and dogs. To investigate the antiemetic effects of acupuncture on vasopressin-induced emesis, gastroduodenal motor activity and the frequency of retching and vomiting were simultaneously recorded in conscious dogs. In seven dogs, four force transducers were implanted on the serosal surfaces of the gastric body, antrum, pylorus, and duodenum. Gastroduodenal motility was continuously monitored throughout the experiment. Vasopressin was intravenously infused at a dose of 0.1 U x kg(-1) x min(-1) for 20 min. Electroacupuncture (EA, 1-30 Hz) at pericardium-6 (PC6), bladder-21 (BL21), or stomach-36 (ST36) was performed before, during, and after the vasopressin infusion. To investigate whether the opioid pathway is involved in EA-induced antiemetic effects, naloxone (a central and peripheral opioid receptor antagonist) or naloxone methiodide (a peripheral opioid receptor antagonist) was administered before, during, and after EA and vasopressin infusion. Intravenous infusion of vasopressin induced retching and vomiting in all dogs tested. Retrograde peristaltic contractions occurred before the onset of retching and vomiting. EA (10 Hz) at PC6 significantly reduced the number of episodes of retching and vomiting. EA at PC6 also suppressed retrograde peristaltic contractions. In contrast, EA at BL21 or ST36 had no antiemetic effects. The antiemetic effect of EA was abolished by pretreatment with naloxone but not naloxone methiodide. It is suggested that the antiemetic effect of acupuncture is mediated via the central opioid pathway.

Gastric fundus relaxation and emetic sequences induced by apomorphine and intragastric lipid infusion in healthy humans

OBJECTIVES

Experimental animal studies have suggested that gastric relaxation precedes emesis, whether induced by peripheral or central stimuli. We aimed to quantify the gastric relaxatory and symptomatic responses to a standardized emetic inductor.

METHODS

In healthy volunteers, we measured the gastric and symptomatic response to two different proemetic stimuli: a peripheral stimulus (intragastric infusion at 5 ml/min of 50% fat emulsion in water) and a central stimulus (s.c. apomorphine at 0.01 mg/kg). Proximal gastric tone was continuously recorded by an electronic barostat. Symptoms were simultaneously quantified by a graded questionnaire.

RESULTS

Lipid-induced gastric relaxation occurred in 17/17 subjects and was followed by nausea in 12/17. The total amount of intragastric fat required to trigger gastric relaxation was 139 +/- 34 ml and to induce nausea, 258 +/- 32 ml. Gastric relaxation after subcutaneous apomorphine occurred in 13/14, and nausea/emesis, in the same 13/14. As expected, timing of events after stimulation was much shorter in response to the central than to the peripheral stimulus. Thus, gastric relaxation began 4.0 +/- 0.6 min after subcutaneous apomorphine and 28 +/- 7 min after the onset of lipid infusion (p < 0.05); likewise, the symptomatic response to central stimulation with apomorphine was 8.5 +/- 1.8 min versus 52 +/- 6 min for lipid infusion, p < 0.05. However, the magnitude of gastric relaxation was quite similar for both stimuli (apomorphine: 208 +/- 35 ml; intragastric lipid: 235 +/- 23 ml). No correlation was observed between the symptomatic response and the magnitude of the gastric relaxation, whether elicited by intragastric infusion of lipids or by subcutaneous apomorphine. Specifically, when symptomatic and gastric motor responses were compared in each of the 14 subjects who received both stimuli, no significant correlation between individual responses was detected.

CONCLUSIONS

Gastric relaxation invariably precedes nausea and emesis and is of the same magnitude whether induced by apomorphine (central stimulus) or intragastric lipid infusion (peripheral stimulus). Nevertheless, in some instances and depending on the nature of the stimulus, fundic relaxation is a physiological event unassociated with nausea.

A novel water-soluble dopamine-2 antagonist with anticholinesterase activity in gastrointestinal motor activity. Comparison with domperidone and neostigmine

A novel water-soluble dopamine-2 antagonist, N-[4-[2-(dimethylamino) ethoxy]benzyl]-3,4-dimethoxybenzamide hydrochloride (HSR-803) was synthesized and assayed for its gastrointestinal smooth muscle stimulating activity in vivo and in vitro. In the in vivo study, gastrointestinal contractile activity was measured by means of chronically implanted force transducers in conscious dogs; it was found that HSR-803 at 3.0 mg/kg IV probably stimulated gastric contractile force twice during the digestive state and significantly antagonized dopamine-(1.0 mg/kg per hour) inhibited gastric contractions in doses of 0.3, 1, and 3 mg/kg IV. With a background IV infusion of HSR-803 at 3 mg/kg per hour, the contraction-stimulating activity of acetylcholine (0.05 mg/kg per minute) was greatly enhanced while the response to bethanechol was not changed. As a result, HSR-803 was found to have a strong anticholinesterase activity besides the antidopamine-2 activity; i.e., the anticholinesterase activity of HSR-803 at 3 mg/kg per hour was equivalent to that of neostigmine at 10 micrograms/kg per hour, and dopamine-2 antagonistic activity of HSR-803 was similar to that of domperidone on a weight basis. No symptom suggesting actions on the central nervous system was noticed in HSR-803 up to 10 mg/kg IV in conscious dogs. In the in vitro study, HSR-803 inhibited cholinesterase dose-dependently, and IC50 was 2.9 x 10(-6) mol/L, while those of neostigmine and domperidone were 2.3 x 10(-8) mol/L and 1.7 x 10(-5) mol/L, respectively. In conclusion, HSR-803 stimulates endogenous acetylcholine release by antagonizing the dopamine-2 receptor on the postsynaptic cholinergic neurons, and the anticholinesterase activity of HSR-803 may cause the released acetylcholine to accumulate at cholinergic receptor sites. Thus, HSR-803 is potentially capable of enhancing cholinergic activity in the gastrointestinal region.

Relationship between small intestinal fasting motility and vomiting in dogs

Preceding vomiting, several changes in small intestinal motility have been described. They consist mainly of high-amplitude retrograde contractions and inhibition of motility before and after these contractions. The recordings of 94 episodes of emesis occurring spontaneously, during manometric studies of intestinal motility by means of infused catheters in dogs with gastric and duodenal cannulae, showed that 95.7% of all episodes developed during phase II of the migratory motor complex. In order to establish whether different phases of the fasting cyclic activity are associated with a different sensitivity to emetic stimulus, two agents, apomorphine, a centrally acting drug and copper sulfate, a peripherally acting agent, were administered at the beginning of phases I and II of the migratory motor complex. Coincident with spontaneously occurring vomiting, a statistically significative greater number of responses to both emetic agents was observed during phase II as compared to phase I. This finding suggests that cyclic changes of the small bowel motility are related to changes in the threshold of the vomiting center.

Computer analysis of orally propagated contractions in canine small intestine after Billroth-II gastrectomy

The aims of the study were to characterize an unusual motor pattern with orally propagated intestinal contractions occurring after subtotal gastrectomy and to clarify if orally propagated contractions also occur during the normal fasted and fed motor pattern in intact dogs. Jejunal motility was recorded with multiple closely spaced extraluminal transducers. Contractile patterns were analyzed by a special computerized method. Following a Billroth-II gastrectomy without additional vagotomy, the migrating motor complex was often disrupted and replaced by a motor pattern that consisted of contractions propagating aborally and retrograde. The propagation velocity was significantly faster compared with that of phase III contraction waves. In intact dogs, both the fasted and fed motor patterns showed a negligible number of orally propagated contractions. These results show that it is possible to differentiate between aborally and orally propagating contractions by a computerized method. Orally propagated contractions occur frequently after subtotal gastrectomy with gastroenterostomy but are an uncommon feature in intact dogs.

Effect of dopamine on electrical activity of isolated stomach muscle in cats

The effect of dopamine on electrical activity (gastric slow wave) of the stomach was studied in isolated stomach muscle strips of 145 cats. The gastric slow wave was recorded by four monopolar electrodes (Ag-AgCl) in Krebs-Ringer solution (pH 7.4, temperature 36 +/- 0.5 degrees C) bubbled with 5% CO2 in O2. Dopamine induced concentration-dependent alterations in frequency and propagation of slow waves. Development of irregular propagation was significantly suppressed by domperidone pretreatment. The variation of dopamine-induced slow-wave frequency was significantly inhibited by domperidone and phentolamine but not by propranolol, hexamethonium, and tetrodotoxin. The study indicates that a dopamine receptor exists in the stomach and plays a role in the genesis of gastric electrical abnormality.

Effect of cisplatin on myoelectric activity of the stomach and small intestine in dogs

The effect of cisplatin on interdigestive myoelectric activity (IDMA) of the gastric antrum, duodenum and jejunum, plasma concentration of motilin, and animal behavior was studied in seven conscious dogs with gastric cannulas and platinum electrodes implanted on the serosal surface of antrum and upper small intestine. Cisplatin given intravenously in a dose of 2 mg/kg resulted in complete interruption of IDMA and produced predominantly phase II-like activity, lasting as long as 24 hr. Six of the seven dogs exhibited retropropagation of spike bursts from the jejunum to the antrum which was accompanied by emesis. Retropropagation of spike bursts did not occur in one dog who did not exhibit emesis. In addition, abnormal spike bursts occurred frequently, including simultaneously occurring spike bursts in both duodenum and jejunum and nonpropagating short spike bursts in isolated segments of the upper small intestine. As phase III of IDMA disappeared, plasma motilin concentrations remained comparable to those in phase II of normal IDMA but without the cyclic increases. These studies indicate that cisplatin interrupts the regular cycle of IDMA as well as the plasma motilin cycle and produces retropropagation of spike bursts and abnormal spike bursts. These changes in both motility and plasma motilin levels may play a part in the mechanism of emesis induced by cisplatin in dogs.

The neurophysiology of vomiting

Nausea and vomiting can be induced by a wide variety of stimuli such as pregnancy, space travel, raised intracranial pressure, radiation and cytotoxic drugs. The mechanisms by which all these diverse stimuli culminate in a final common act is unknown. From studies in the 1950s a model of the emetic reflex emerged consisting of a chemoreceptor trigger zone in the area postrema and a vomiting centre in the brain stem. This concept has been reviewed and revised in the light of recent studies. Many discussions of emesis involve detailed descriptions of the gastrointestinal events associated with the act of vomiting only-nausea and retching receiving little attention. Here we have tried to give a broader view by considering the neurophysiology of such events and have included nausea and retching, phenomena that are usually inseparable from vomiting. The possible biological function of these events is also discussed. The involvement of visceral systems (such as the heart, airways and gut) is included, and particular attention is paid to vagal mechanisms underlying the changes in gut motor activity. Emesis has long been thought to be organized by a 'vomiting centre'; the possibility that this vomiting centre could be the parvocellular reticular formation is reviewed, as is the concept that the 'centre' is larger than an anatomically defined single group of cells. The mechanism of action of two clinically relevant emetic stimuli--radiation and cytotoxic drugs-is considered in detail. Recent studies of the antiemetic properties of novel 5-HT-3 receptor antagonists against radiation and cytotoxic drug-induced vomiting are discussed; these studies suggest that important advances will be made in the treatment of emesis induced by these and other related agents.