Gastric relaxation and vomiting by apomorphine, morphine and fentanyl in the conscious dog

Comparison of the effects of morphine-lidocaine-ketamine and fentanyl-lidocaine-ketamine combinations administered as constant rate infusions on postprocedure rectal temperature in dogs

OBJECTIVE

To compare the effects of morphine-lidocaine-ketamine (MLK) and fentanyl-lidocaine-ketamine (FLK) combinations administered as constant rate infusions (CRIs) during and after veterinary procedures on postprocedure rectal temperature in dogs.

ANIMALS

32 clinically normal client-owned dogs undergoing nonemergent procedures.

PROCEDURES

Dogs were randomly assigned to receive an MLK or FLK combination (16 dogs/group). During the procedure, each dog received 2% lidocaine hydrochloride (1 mg/kg/h; both groups), ketamine hydrochloride (0.6 mg/kg/h; both groups), and morphine (0.36 mg/kg/h; MLK group) or fentanyl (4 μg/kg/h; FLK group) via CRI for analgesia; esophageal temperature was maintained at 37° to 39°C. At extubation, each drug dose in each assigned combination was halved and administered (via CRI) for 12 additional hours for postprocedure analgesia. Rectal temperature and other data were recorded at baseline (prior to administration of premedicants), extubation (0 hours), and 0.5, 1.5, 3, 6, and 12 hours thereafter.

RESULTS

Mean postprocedure rectal temperature was significantly lower at each postextubation time point for the MLK group, compared with corresponding values for the FLK group. Compared with the baseline value, mean postprocedure rectal temperature was significantly lower at 0, 0.5, 1.5, and 3 hours for the FLK group and at all postprocedure time points for the MLK group. Hypothermia (rectal temperature < 37°C) was detected at ≥ 1 postprocedure time point more often in dogs in the MLK group (9/16) than in the FLK group (1/16).

CONCLUSIONS AND CLINICAL RELEVANCE

Dogs that received an MLK combination for analgesia during and after a veterinary procedure developed hypothermia more commonly than did dogs that received an FLK combination under similar conditions.

Anaesthesia of brachycephalic dogs

Brachycephalic breeds of dog have grown in popularity in the UK and so form an increasing proportion of cases requiring anaesthesia. These breeds are predisposed to several conditions, notably brachycephalic obstructive airway syndrome and gastro-oesophageal reflux, that have important implications for anaesthetic management and carry high risk for complications. This review incorporates peer-reviewed veterinary literature with clinical experience in a discussion on perioperative management of brachycephalic dogs. We focus on preoperative identification of common concurrent conditions, practical strategies for reducing anaesthetic risk and improving postoperative management. Comparisons of brachycephalic obstructive airway syndrome with the human condition of obstructive sleep apnoea are included where appropriate.

Description of a regional anaesthesia technique for the dorsal cranium in the dog: a cadaveric study

OBJECTIVE

To identify landmarks and to describe a technique for nerve blockade of the dorsal cranium in dogs.

STUDY DESIGN

Anatomic cadaveric study.

ANIMALS

A total of 39 dog cadavers, weighing 18.0 ± 9.7 kg (mean ± standard deviation).

METHODS

The study was performed in three parts. In the initial part, cadavers were dissected to determine the location of the frontal, zygomaticotemporal, and major occipital nerves, and to identify prominent landmarks for their blockade. In the second part, one technique was developed to block each of the frontal and zygomaticotemporal nerves, and two techniques, rostral and caudal, were developed to block the major occipital nerve. Injection solution was 0.05% methylene blue in 0.5% bupivacaine. In the third part, cadavers were used to test the techniques developed in the second part with 0.04 mL kg^-1 of the same injectate administered at each site (maximal volume 0.5 mL per site). The length of nerve stained was measured, with a length ≥6 mm considered successful. Confidence intervals were calculated using Fisher's exact test.

RESULTS

Success rates (95% confidence interval) for the frontal, zygomaticotemporal, and rostral and caudal locations for the major occipital nerve were 94% (80-99%), 91% (76-98%), 74% (58-86%) and 77% (59-89%), respectively. With a combination of both locations, the success rate for the major occipital nerve was 100% (90-100%).

CONCLUSION AND CLINICAL RELEVANCE

This study describes a simple regional anaesthesia technique using palpable anatomical landmarks that may provide analgesia for dogs undergoing craniotomy.

Gastric and lower esophageal sphincter pressures during nausea: a study using visual motion-induced nausea and high-resolution manometry

Nausea is the subjective unpleasant sensation that immediately precedes vomiting. Studies using barostats suggest that gastric fundus and lower esophageal sphincter (LES) relaxation precede vomiting. Unlike barostat, high-resolution manometry allows less invasive, detailed measurements of fundus pressure (FP) and axial movement of the gastroesophageal junction (GEJ). Nausea was induced in 12 healthy volunteers by a motion video and rated on a visual analog scale. FP was measured as the mean value of the five pressure channels that were clearly positioned below the LES. After intubation, a baseline (BL) recording of 15 min was obtained. This was followed by presentation of the motion video (at least 10 min, maximum 20 min) followed by 30 min recovery recording. Throughout the experiment we recorded autonomic nervous system (ANS) parameters [blood pressure, heart rate (HR), and cardiac vagal tone (CVT), which reflects efferent vagal activity]. Ten out of 12 subjects showed a drop in FP during peak nausea compared with BL (-4.0 ± 0.8 mmHg; P = 0.005), and 8/10 subjects showed a drop in LES pressure (-8.8 ± 2.5 mmHg; P = 0.04). Peak nausea preceded peak fundus and LES pressure drop. Nausea was associated with configuration changes at the GEJ such as LES shortening and esophageal lengthening. During nausea we observed a significantly increased HR and decreased CVT. In conclusion, nausea is associated with a drop in fundus and LES pressure, configuration changes at the GEJ as well as changes in the ANS activity such as an increased sympathetic tone (increased HR) and decreased parasympathetic tone (decreased CVT).

Why can't rodents vomit? A comparative behavioral, anatomical, and physiological study

The vomiting (emetic) reflex is documented in numerous mammalian species, including primates and carnivores, yet laboratory rats and mice appear to lack this response. It is unclear whether these rodents do not vomit because of anatomical constraints (e.g., a relatively long abdominal esophagus) or lack of key neural circuits. Moreover, it is unknown whether laboratory rodents are representative of Rodentia with regards to this reflex. Here we conducted behavioral testing of members of all three major groups of Rodentia; mouse-related (rat, mouse, vole, beaver), Ctenohystrica (guinea pig, nutria), and squirrel-related (mountain beaver) species. Prototypical emetic agents, apomorphine (sc), veratrine (sc), and copper sulfate (ig), failed to produce either retching or vomiting in these species (although other behavioral effects, e.g., locomotion, were noted). These rodents also had anatomical constraints, which could limit the efficiency of vomiting should it be attempted, including reduced muscularity of the diaphragm and stomach geometry that is not well structured for moving contents towards the esophagus compared to species that can vomit (cat, ferret, and musk shrew). Lastly, an in situ brainstem preparation was used to make sensitive measures of mouth, esophagus, and shoulder muscular movements, and phrenic nerve activity–key features of emetic episodes. Laboratory mice and rats failed to display any of the common coordinated actions of these indices after typical emetic stimulation (resiniferatoxin and vagal afferent stimulation) compared to musk shrews. Overall the results suggest that the inability to vomit is a general property of Rodentia and that an absent brainstem neurological component is the most likely cause. The implications of these findings for the utility of rodents as models in the area of emesis research are discussed.

Apomorphine is a bimodal modulator of TRPA1 channels

Apomorphine is a non-narcotic derivative of morphine, which acts as a dopamine agonist and is clinically used to treat "off-states" in patients suffering from Parkinson's disease. Adverse effects of apomorphine treatment include severe emesis and nausea, and ulceration and pain at the injection site. We wanted to test whether sensory transient receptor potential (TRP) channels are a molecular target for apomorphine. Here, we show that rTRPV1, rTRPV2, rTRPV3, and mTRPV4, as well as hTRPM8, and rTRPM3, which are expressed in dorsal root ganglion neurons, are insensitive toward apomorphine treatment. This also applied to the cellular redox sensor hTRPM2. On the contrary, human TRPA1 could be concentration-dependently modulated by apomorphine. Whereas the addition of apomorphine in the low micromolar range produced an irreversible activation of the channel, application of higher concentrations caused a reversible voltage-dependent inhibition of heterologously expressed TRPA1 channels, resulting from a reduction of single-channel open times. In addition, we provide evidence that apomorphine also acts on endogenous TRPA1 in cultured dorsal root ganglion neurons from rats and in the enterochromaffin model cell line QGP-1, from which serotonin is released upon activation of TRPA1. Our study shows that human TRPA1 is a target for apomorphine, suggesting that an activation of TRPA1 might contribute to adverse side effects such as nausea and painful injections, which can occur during treatment with apomorphine.

Neuroactive substances in the dorsal vagal complex of the medulla oblongata: nucleus of the tractus solitarius, area postrema, and dorsal motor nucleus of the vagus

The distributions of classical and putative neurotransmitters within somata and fibres of the dorsal vagal complex are reviewed. The occurrence within the dorsal medulla oblongata of receptors specific for some of these substances is examined, and possible functional correlations of the specific neurochemicals with respect to their distribution within the dorsal vagal complex are discussed. Many of the known transmitters and putative transmitters are represented in the dorsal vagal complex, particularly within various subnuclei of the nucleus of the solitary tract, the main vagal afferent nucleus. In a few cases, some of these have been examined in detail, particularly with respect to their possible mediation of cardiovascular or gastrointestinal functions. For example, the catecholamines, substance P and angiotensin II in the nucleus of the solitary tract have all been strongly implicated as playing a role in the central control of cardiovascular function. Other neurotransmitters or putative transmitters may be involved as well, but probably to a lesser extent. Similarly, the roles in the dorsal vagal complex of dopamine, the endorphins and cholecystokinin in control of the gut have been studied in some detail. Future investigations of the distributions of and electrophysiological parameters of neurotransmitters at the cellular level should provide much needed clues to advance our knowledge of the correlations between anatomical distributions of specific neurochemicals and physiological functions mediated by them.

Post-anesthesia vomiting: impact of isoflurane and morphine on ferrets and musk shrews

Although partially controlled with antiemetic drugs, postoperative nausea and vomiting (PONV) continues to be a problem for many patients. Clinical research suggests that opioid analgesics and volatile anesthetics are the main triggers of PONV. The aim of this study was to develop an animal model for post-anesthesia vomiting for future studies to further determine mechanisms and preclinical drug efficacy. Ferrets (N=34) were initially used because they have served as a gold standard for emesis research. Ferrets were tested with several doses of morphine, inhaled isoflurane, and a positive control injection of cisplatin (a chemotherapy agent) to induce emesis. Musk shrews (a small animal model; N=36) were also tested for emesis with isoflurane exposure. A control injection of cisplatin produced emesis in ferrets (ip, 129.8±22.0 retches; 13.7±2.3 vomits; mean±SEM). Morphine also produced a dose-response on emesis in ferrets, with maximal responses at 0.9 mg/kg (sc, 29.6±12.6 retches; 1.8±0.9, vomits). Isoflurane exposure (2-4% for 10 min to 6h exposure) failed to induce vomiting, was not associated with an increased frequency in emesis when combined with a low dose of morphine (0.1 mg/kg, sc), and failed to produce consistent effects on food and water intake. In contrast to ferrets, musk shrews were very sensitive to isoflurane-induced emesis (0.5 to 3%, 10 min exposure; up to 11.8±2.4 emetic episodes). Overall, these results indicate that ferrets will not be useful for delineating mechanisms responsible for isoflurane-induced emesis; however, musk shrews may prove to be a model for vomiting after inhalation of volatile agents.

Emesis in dogs: a review

Emesis is a common presenting sign in small animal practice. It requires a rational approach to management that is based upon a sound understanding of pathophysiology combined with logical decision making. This review, which assesses the weight of available evidence, outlines the physiology of the vomiting reflex, causes of emesis, the consequences of emesis and the approach to clinical management of the vomiting dog. The applicability of diagnostic testing modalities and the merit of traditional approaches to management, such as dietary changes, are discussed. The role and usefulness of both traditional and novel anti-emetic drugs is examined, including in specific circumstances such as following cytotoxic drug treatment. The review also examines areas in which common clinical practice is not necessarily supported by objective evidence and, as such, highlights questions worthy of further clinical research.

Diminished functional association between proximal and distal gastric motility in critically ill patients

OBJECTIVE

This study examined the effects of critical illness on the relationship between proximal and distal gastric motor activity during fasting and duodenal nutrient stimulation.

DESIGN

Prospective, case-controlled study.

PATIENTS AND PARTICIPANTS

Ten critically ill patients and ten healthy volunteers.

INTERVENTIONS

Concurrent proximal gastric (barostat) and antro-pyloro-duodenal (manometry) motility were recorded during fasting and during two 60-min duodenal nutrient infusions (Ensure at 1 kcal/min and 2 kcal/min) in random order, separated by a 2-h wash-out period.

RESULTS

Baseline proximal gastric volumes were similar between the two groups. At 10 min nutrient-induced fundic relaxation was lower in patients than healthy subjects (45 +/- 26 vs. 196 +/- 29 ml). In patients the frequency and volume amplitude of fundic waves were also lower. There were fewer propagated antral waves in patients than in healthy subjects during both fasting and nutrient infusion. These were more retrograde, shorter in length and associated with a pyloric contraction. The proportion of fundic waves followed by a distally propagated antral wave was significantly less in patients (0%, 0-8%) than controls 36% (11-44%).

CONCLUSIONS

In critical illness, in addition to impairment of proximal and distal gastric motor activity, the association between the two gastric regions is abnormal. This disturbance may interfere with meal distribution and further contribute to slow gastric emptying in these patients.

Central effects of morphine on GI motility in conscious dogs

It has been suggested that morphine has dual effects; emetic effects and anti-emetic effects. The chemoreceptor trigger zone, which is outside the BBB, mediates the emetic effect. In contrast, the vomiting center mediates the anti-emetic effect of opioids. Thus, naloxone methiodide, which does not cross the BBB, antagonizes emetic effects of opioids. We studied whether naloxone methiodide alters abnormal motility pattern induced by morphine in gastrointestinal (GI) tract. Strain gauge force transducers were sutured on the serosal surface of upper GI tract to record the circular muscle contractions in eight dogs. The ventricular access system was implanted to inject morphine intracerebroventricularly (icv). Effects of icv-injection of morphine (0.3-3.0 mug/kg, bolus) on GI motility were studied during intravenous infusion of naloxone hydrochloride or naloxone methiodide. Icv-injection of morphine (3.0 mug/kg) induced retching and vomiting in all dogs tested. Phasic contractions of the jejunum were observed after icv-injection of morphine. These contractions in the jejunum migrated orally to the antrum (retrograde peristaltic contractions; RPCs). Both naloxone hydrochloride and naloxone methiodide treatment virtually abolished the emetic effects of morphine. Naloxone hydrochloride completely abolished morphine-induced RPCs in all dogs, whereas naloxone methiodide converted morphine-induced RPCs to anterograde peristaltic contractions (APCs) in 6 of 8 dogs. Our current study suggests that central opioids may induce APCs and prevent emesis in conscious dogs. Naloxone methiodide may be useful to prevent the undesired side effects of morphine.

Proximal gastric motility in critically ill patients with type 2 diabetes mellitus

AIM: To investigate the proximal gastric motor response to duodenal nutrients in critically ill patients with long-standing type 2 diabetes mellitus.

METHODS: Proximal gastric motility was assessed (using a barostat) in 10 critically ill patients with type 2 diabetes mellitus (59 ± 3 years) during two 60-min duodenal infusions of Ensure^® (1 and 2 kcal/min), in random order, separated by 2 h fasting. Data were compared with 15 non-diabetic critically ill patients (48 ± 5 years) and 10 healthy volunteers (28 ± 3 years).

RESULTS: Baseline proximal gastric volumes were similar between the three groups. In diabetic patients, proximal gastric relaxation during 1 kcal/min nutrient infusion was similar to non-diabetic patients and healthy controls. In contrast, relaxation during 2 kcal/min infusion was initially reduced in diabetic patients (p < 0.05) but increased to a level similar to healthy humans, unlike non-diabetic patients where relaxation was impaired throughout the infusion. Duodenal nutrient stimulation reduced the fundic wave frequency in a dose-dependent fashion in both the critically ill diabetic patients and healthy subjects, but not in critically ill patients without diabetes. Fundic wave frequency in diabetic patients and healthy subjects was greater than in non-diabetic patients.

CONCLUSION: In patients with diabetes mellitus, proximal gastric motility is less disturbed than non-diabetic patients during critical illness, suggesting that these patients may not be at greater risk of delayed gastric emptying.

Proximal gastric response to small intestinal nutrients is abnormal in mechanically ventilated critically ill patients

AIM: To determine the response of the proximal stomach to small intestinal nutrients in critically ill patients.

METHODS: Proximal gastric motility was measured in 13 critically ill patients (49.3 ± 4.7 years) and 12 healthy volunteers (27.7 ± 2.9 years) using a barostat technique. Recordings were performed at baseline, during a 60-min intra-duodenal infusion of Ensure^® (2 kcal/min), and for 2 h following the infusion. Minimum distending pressure (MDP), intra-bag volume and fundic wave activity were determined.

RESULTS: The MDP was higher in patients (11.7 ± 1.1 vs 7.8 ± 0.7 mmHg; P < 0.01). Baseline intra-bag volumes were similar in the 2 groups. In healthy subjects, a ‘bimodal’ proximal gastric volume response was observed. In patients, the initial increase in proximal gastric volume was small and delayed, but eventually reached a maximal volume similar to that of healthy subjects. In healthy subjects, the proximal gastric volume rapidly returned to baseline level after nutrient infusion (median 18 min). In contrast, the recovery of volume to baseline was delayed in critically ill patients (median 106 min). In 6 patients, the volume had not returned to baseline level 2 hours after nutrient infusion. In patients, fundic volume waves were less frequent (P < 0.05) and had lower amplitude (P < 0.001), compared to healthy subjects.

CONCLUSION: In critical illness, proximal gastric motor responses to small intestinal nutrient stimulation are abnormal.

Signals for nausea and emesis: Implications for models of upper gastrointestinal diseases

Nausea and vomiting are amongst the most common symptoms encountered in medicine as either symptoms of diseases or side effects of treatments. In a more biological setting they are also important components of an organism's defences against ingested toxins. Identification of treatments for nausea and vomiting and reduction of emetic liability of new therapies has largely relied on the use of animal models, and although such models have proven invaluable in identification of the anti-emetic effects of both 5-hydroxytryptamine(3) and neurokinin(1) receptor antagonists selection of appropriate models is still a matter of debate. The present paper focuses on a number of controversial issues and gaps in our knowledge in the study of the physiology of nausea and vomiting including: The choice of species for the study of emesis and the underlying behavioural (e.g. neophobia), anatomical (e.g. elongated, narrow abdominal oesophagus with reduced ability to shorten) and physiological (e.g. brainstem circuitry) mechanisms that explain the lack of a vomiting reflex in certain species (e.g. rats); The choice of response to measure (emesis[retching and vomiting], conditioned flavour avoidance or aversion, ingestion of clay[pica], plasma hormone levels[e.g. vasopressin], gastric dysrhythmias) and the relationship of these responses to those observed in humans and especially to the sensation of nausea; The stimulus coding of nausea and emesis by abdominal visceral afferents and especially the vagus-how do the afferents encode information for normal postprandial sensations, nausea and finally vomiting?; Understanding the central processing of signals for nausea and vomiting is particularly problematic in the light of observations that vomiting is more readily amenable to pharmacological treatment than is nausea, despite the assumption that nausea represents "low" intensity activation of pathways that can evoke vomiting when stimulated more intensely.

Area postrema mediates gastric motor response induced by apomorphine in rats

The effects of apomorphine administration on the autonomic responses were investigated in rats. Distinctive gastric motor responses were observed after the intravenous administration of apomorphine (0.1 mg/kg body weight). Gastric motor responses in the distal stomach induced by apomorphine administration were classified into two types. One type involved inhibition of phasic contractions which appeared just after the administration of apomorphine. The other involved an increase in the frequency of small phasic contractions accompanied by increased gastric tone appearing with a relatively longer delay. No relaxation was observed in either the proximal or distal stomach. These gastric motor responses showed a dose-response effect to the amount of apomorphine administered (0.002-0.1 mg/kg body weight). In addition, submandibular salivary secretion was observed in response to the intravenous administration of apomorphine at a dose of 3 or 10 mg/kg body weight. Pretreatment with domperidone (1 or 2 mg/kg body weight) or the ablation of the area postrema (AP) abolished the gastric motor response and salivary secretion induced by the administration of apomorphine. In conclusion, rats showed definitive autonomic phenomena in response to the administration of apomorphine. Dopamine 2-like receptors situated in the AP mediate apomorphine-induced autonomic phenomena in rats.

Gastrointestinal drugs

Various drugs are commonly used for the many gastrointestinal problems that are routinely seen in dogs and cats. Many of these drugs are briefly reviewed in this article. Anecdotal as well as documented findings are provided that will hopefully allow the clinician to use them wisely and effectively in the many clinical situations that arise daily. Emetics, although not used for gastrointestinal problems, are also reviewed because of their obvious connection with the gastrointestinal tract.

Absence of emetic effects of morphine and loperamide in Suncus murinus

The house musk shrew Suncus murinus recently has been introduced for the study of emesis. We investigated the emetic effects of the opioids morphine (0.1-21.5 mg/kg i.p.) and loperamide (0.01-10 mg/kg i.p.) and found a complete lack of emetogenic potential. Nicotine, however, dose dependently induced vomiting in the Suncus with an ED50 of 8.8 mg/kg s.c. and a 100% incidence at 20 mg/kg. This drug-induced vomiting was reduced by morphine or loperamide: ED50 values obtained were 1.2 mg/kg i.p. for morphine and 0.7 mg/kg i.p. for loperamide. Naloxone (2 mg/kg s.c.) antagonised the inhibitory effect of morphine (2 mg/kg i.p.) or loperamide (10 mg/kg i.p.). Serotonin (20 mg/kg s.c.) had less reliable emetogenic potency than nicotine in the Suncus with incidences between 50 and 100%. However, the serotonin-induced vomiting was abolished by morphine and loperamide and this inhibition was antagonised by naloxone. These results suggest that systemically administered opioids are pure antiemetics in Suncus murinus in contrast to other animal models and man. Naloxone antagonism indicates that this antiemetic effect is mediated by opioid receptors.

The effects of different antiemetic agents on morphine-induced emesis in ferrets

There is interest in the development of antiemetics other than dopamine receptor antagonists for the treatment of postoperative nausea and vomiting. A ferret model of morphine-induced emesis may have wider application in evaluating newer agents than the apomorphine dog model. This study describes the conditions for morphine-induced emesis in ferrets and evaluates five antiemetics that are prototypical of three different mechanisms. The average numbers of vomiting and retching episodes induced by morphine (0.1-2.5 mg/kg s.c.) were distributed as a bell-shaped curve. Maximum number of vomits occurred at 0.3 mg/kg (11.8 +/- 2.1 vomits; 45 +/- 12.5 retches). Antiemetics or vehicle were given i.v. 5 min prior to morphine while each ferret was maintained under isoflurane-O2 anesthesia. Ondansetron, a 5-HT3 receptor antagonist, reduced vomiting episodes by 47% and 70% (3 and 10 mg/kg). Granisetron, a 5-HT3 receptor antagonist was inactive at doses of 0.1, 1.0, 3.0 and 10 mg/kg. Metoclopramide reduced vomiting episodes by 48% and 82% (3 and 10 mg/kg). Droperidol reduced vomiting episodes by 84% at 3 mg/kg. Naloxone reduced vomiting episodes by 91% and 43% at doses of 0.1 and 1.0 mg/kg. In most cases, prolonged latency times to the first episodes accompanied the reduction in total numbers of episodes. The significant reduction of morphine-induced emesis in the ferret by ondansetron, metoclopramide and droperidol is consistent with the reduction of postoperative emesis in man by these compounds when morphine was a component of the anesthetic regimen. These results suggested that the morphine ferret model may be useful for evaluating compounds having the potential for preventing and treating postoperative vomiting.

New pharmacologic approaches to the prevention of space/motion sickness

Fundamental approaches in selection of new agents for evaluation in prevention of space/motion sickness (SMS) are reviewed. The discussion centers on drugs under investigation at the Johnson Space Center. Methodology that employs the rotating chair for measuring SMS symptomatology and susceptibility is described. The most obvious approach to the development of new agents relies on selection of agents from drug classes that possess pharmacologic properties of established anti-motion sickness agents. A second approach selects drugs that are used to prevent emesis caused by means other than exposure to motion. The third approach relies on basic research that characterizes individual differences in susceptibility. The hypothesis is: detection of individual differences leads to identification of specific drugs, which target physiologic systems that show individual differences. These physiologic systems are targets for therapy and may play a role in the etiology of SMS. Two drugs that reduce susceptibility to SMS include dexamethasone and d(CH2)5Tyr(Me)AVP, a vasopressin (AVP)V1 antagonist. The latter peptide has demonstrated complete blockade of emesis and other significant symptoms in squirrel monkeys. These studies were predicated on observations that subjects who were more resistant to SMS had higher plasma AVP after severe nausea than subjects with lower resistances. Investigations are underway to test a 0.5-mg intravenous dose in humans. Kappa opioid agonists inhibit AVP release and offer new therapeutic possibilities and advantages over AVP peptides. This review details the experimental data collected on AVP and adrenocorticotropin. The literature supports interrelated roles for AVP and opioid peptides in SMS. Experimental testing of kappa agonists is warranted because specific opioid agonists act at neuroanatomical sites causing nausea and vomiting. It is argued opioid receptors in the chemoreceptor trigger zone and vomiting center stimulate and inhibit the emetic response, respectively. The evidence suggests kappa and/or mu receptors at VC are involved in inhibition of emesis, whereas delta opioid receptors at CTZ are involved in stimulation of emesis.

The current status of opioid research on gastrointestinal motility

Apparently conflicting data on opioid effects on gastrointestinal motility have been reported in the literature. The current status is reviewed and an attempt is made to find a common denominator to discrepant results by suggesting functionally contrasting opioid systems modulating the same physiological functions. Upon superimposition, these contrasting systems might result in opposite opioid effects dependent on the actual functional balance between the systems at the time of drug administration. Inhibitory neuromodulation at multiple sites leading to either inhibition or disinhibition by opioids may serve as a common basis of their contrasting effects. This interpretation, though consistent with most of the currently available data, is still a working hypothesis.

Opposite effects of kappa-opioid agonists on gastric emptying of liquids and solids in dogs

The influence of oral (p.o.) administration of kappa-(U-50488, tifluadom) and mu- (morphine, DAGO) opioid substances on gastric emptying of liquids and solids in a standard canned dog food meal was evaluated using a double-radiolabeled technique in dogs fitted with gastric cannulas. One hour after feeding, 28.6% +/- 3.6% (mean +/- SD) of the solid phase and 27.1% +/- 8.6% of the liquid phase of the meal had been emptied. Both U-50488 and tifluadom given orally (0.01-0.1 mg/kg) significantly increased (p less than 0.05) the 1-h emptying of the solid phase of the meal by 23.1%-49.6%. In contrast, both drugs significantly reduced emptying of liquids. These effects were not reproduced when similar doses were given intravenously. Oral administration of morphine or DAGO (0.01-0.1 mg/kg) did not affect gastric emptying, whereas an inhibited emptying of solids was observed for morphine at a higher dose (1 mg/kg p.o.). At a dose of 100 micrograms/kg i.v. both naloxone and MR 2266 (0.1 mg/kg) abolished the effects of orally administered U-50488 on gastric emptying of solids and liquids. It is concluded that kappa- but not mu-agonists act locally to alter gastric emptying of a standard meal in dogs, having opposite effects on solid and liquid phases. A selective local stimulation of kappa mucosal or submucosal receptors of the gastroduodenal area may explain such effects.

Prospects for management of gastrointestinal injury associated with the acute radiation syndrome

The effect of total-body ionizing radiation on the digestive tract is dose-dependent and time-dependent. At low doses (1.5 Gy), one observes only a short prodromal syndrome consisting of nausea, vomiting, and gastric suppression. At doses greater than 6 Gy, the prodromal syndrome is more marked, and it is followed after a 2-5-day remission period by a subacute syndrome, characterized by diarrhea and hematochezia. This gastrointestinal syndrome is superimposed onto a radiation-induced bone marrow suppression. The combination of intestinal and hemopoietic syndromes results in dehydration, anemia, and infection, leading eventually to irreversible shock and death. The treatment of prodromal symptoms is based on the administration of antiemetics and gastrokinetics, although an effective treatment devoid of side effects is not yet available for human therapy. The treatment of the gastrointestinal subacute syndrome remains difficult and unsuccessful after exposure to total body doses greater than 8-10 Gy. Supportive therapy to prevent infection and dehydration may be effective if restoration or repopulation of the intestinal and bone marrow stem cells does occur. In addition, bone marrow transplantation may improve the prospect of treating the hemopoietic syndrome, although the experience gained in Chernobyl suggests that this treatment is difficult to apply in the case of nuclear accidents. Administration of radioprotectants before irradiation decreases damage to healthy cells, while not protecting cancerous tissues. In the future, stimulation of gastrointestinal and hemopoietic progenitor cells may be possible using cell growth regulators, but much remains to be done to improve the treatment of radiation damage to the gastrointestinal tract.

Initiation of motility in canine ileum by short chain fatty acids and inhibition by pharmacological agents

We have previously shown that short chain fatty acids (SCFA) stimulate motility in the canine ileum. Concentrations of SCFA in the ileum are normally low but would be expected to increase after coloileal reflux; thus, this phenomenon could have pathophysiological relevance. The present studies were designed to seek pharmacological means by which this response could be blocked. Four dogs were prepared with isolated, ileocolonic fistulae into which physiological concentrations of SCFA could be instilled so as to stimulate ileal motility. Pretreatment of the ileum with topical lidocaine abolished the response to luminal SCFA but general anaesthesia did not. Indomethacin stimulated ileal motility and prostacyclin abolished the ileal response to SCFA. Naloxone and a calcium channel blocker also negated the response to SCFA; blockage of muscarinic, adrenergic and 5H-T receptors did not. We conclude that the motor response to SCFA is probably a local neural reflex which is sensitive to local anaesthetics, opiates and the prostanoids.

Gastric emptying and small bowel transit times in volunteers after intravenous morphine and nalbuphine

Gastric emptying half-times and small intestinal transit times were measured in a double-blind crossover study of 17 volunteers who received an intravenous injection of nalbuphine (5 or 10 mg), morphine (5 mg) or placebo. Both times were monitored using a gamma camera after a radioactive test meal and gastric emptying half-time was calculated. Small intestinal transit time was measured by the appearance of radioactivity in the caecum and also of hydrogen in end tidal air. Gastric emptying was prolonged over placebo by nalbuphine 10 mg, which had more effect than nalbuphine 5 mg or morphine 5 mg; morphine 5 mg had less effect than nalbuphine 5 mg. Small intestinal transit time was prolonged over placebo by nalbuphine 10 mg more than by nalbuphine 5 mg or morphine 5 mg, which had approximately equal effects. In these respects, the potency ratio of nalbuphine appears roughly equivalent to morphine. Small intestinal transit times measured by end tidal hydrogen concentration and gamma camera showed close agreement.

Gastric tone modifies the responses to extrinsic neural stimuli in the anaesthetized ferret

To determine the relation between resting intragastric pressure and neuronally evoked motility responses, the stomach of the anaesthetized ferret was inflated with two volumes within the physiological range. Vagal evoked contractions decreased as the resting pressure was increased. The decrease affected responses to high, but not low, intensity stimulation of the vagus. Vagal evoked relaxation of the stomach increased as a linear function of resting pressure. Atropine lowered the resting pressure in vagotomized ferrets but did not alter the relation between vagal evoked relaxation and resting pressure. Intra-arterial acetylcholine evoked a contraction followed by a relaxation. The contraction was reduced at higher resting pressures but the relaxation increased. Hexamethonium had no effect on the contraction but substantially reduced the relaxation. Stimulation of the greater splanchnic nerve evoked two responses. The first, direct relaxation of the stomach (independent of cholinergic activity), bore the same relation to resting pressure as vagal evoked relaxation. The second, inhibition of vagal evoked contraction, had no relation to resting pressure. The degree of reduction in the amplitude of vagal evoked contraction by an immediately preceding vagal stimulation decreased at higher resting pressures. This was the opposite of vagal evoked relaxation. The capacity of stimulation of one cervical vagus to evoke a response as large as that from stimulation of two cervical vagi was shown to depend on the use of near-maximal levels of stimulation. At low levels of stimulation the responses to two vagi were additive, and at the lowest levels there was facilitation. It was concluded that the effect of resting pressure on the amplitude of evoked contractions was mediated mainly by smooth muscle, although modulation of the quantities of transmitter released from nerves could play a minor part. The practical implications of manipulating resting pressure by changes in volume were discussed. In particular, the manipulation of resting pressure as a method of differentiating the mechanisms of central nervous influence on gastric motility was compared to surgical and pharmacological nerve block.

Influence of peripheral and central administration of apomorphine and morphine on intragastric pressure in the dog

Apomorphine was about 30 times more potent in inducing gastric relaxation when applied intracerebroventricularly (i.c.v.) than when injected intravenously (i.v.) in the conscious dog. In the anesthetized dog, the dose of apomorphine producing gastric relaxation via the vertebral artery was at least 10 times lower than that needed to produce gastric relaxation via the i.v. route. For morphine, similar doses had to be given i.c.v. and i.v. to obtain the same degree of gastric relaxation in the conscious dog; in the anesthetized dog, morphine was 3 times more potent via the vertebral artery than i.v. The results suggest that apomorphine-induced gastric relaxation in the dog is mediated via a central site located in the region supplied by the vertebral artery, but that the gastric relaxatory effect of morphine is mediated by both a peripheral and a central site of action.

Emetic effects of centrally administered angiotensin II, arginine vasopressin and neurotensin in the dog

Short-latency emetic responses were induced in dogs by injecting angiotensin II (AII), arginine vasopressin (AVP), and neurotensin (NTN) into cerebroventricular (ICV) and cisternal (ICT) sites also responsive to the emetic effects of apomorphine (APO). Angiotensin III, bradykinin, bombesin, oxytocin, adrenocorticotropic hormone, substance P, gastrin-related peptide and cholecystokinin were ineffective. The results suggest a possible dopaminergic mediation of peptide-induced emesis by receptors in the area postrema (AP).

Peptide-induced emesis in dogs

Systemic administration of apomorphine, angiotensin II, neurotensin and leucine-enkephalin induces emesis in dogs in a dose-dependent fashion. Receptors for Leu-enkephalin and angiotensin II but not apomorphine show receptor desensitization, such that a second systemic administration 5 min after the first is ineffective. Domperidone blocked the emetic response to apomorphine but not to Leu-enkephalin or angiotensin II. Naloxone selectively blocked the Leu-enkephalin response, while saralasin blocked responses to both angiotensin II and Leu-enkephalin, but not apomorphine. Chlorpromazine prevented the emetic response to all agents, suggesting a dopamine receptor in the emetic pathway on the brain side of the blood-brain barrier. In dogs with ablation of the area postrema the emetic response to apomorphine and all peptides was prevented.

Systemic naloxone increases the incidence of motion sickness in the cat

Subcutaneous injections of naloxone in a total dose of 0.4 mg or greater one hour before a swing stimulus increased the frequency of motion sickness symptoms and shortened the latency of retching and vomiting.

Effects of emetic and cathartic agents on the gastrointestinal tract and the treatment of toxic ingestion

Emetic drugs and saline cathartics produce direct or reflex changes in gastrointestinal motility. The changes in gastrointestinal smooth muscle function may be important in the rapid oral or rectal expulsion of gastrointestinal contents, effects which serve as a basis for emetic and cathartic drug use in the treatment of toxic ingestion. Because of difficulties in recording gastrointestinal smooth muscle contractile activity from the intact, unanesthetized animal or man, relatively few studies have attempted to characterize the changes in gastrointestinal motility preceding vomiting. Limited results from past studies and the results of more recent studies employing improved technology suggest that pharmacological activation of the emetic reflex is accompanied by characteristic movements of the stomach and small intestine. The gastric response consists of initial muscle relaxation and an expansion of gastric volume. The intestine responds with a contraction, which begins in the distal ileum and migrates orad over the entire small intestine immediately before active retching. The changes in gastric and intestinal motility may be initiated by structures in the central nervous system and may be an important component of the emetic reflex. This article urges more active research to characterize the gastrointestinal emetic response and to investigate more generally the therapeutic value of emesis in the treatment of toxic ingestion. Emphasis should be placed on the clinically important emetic drugs apomorphine and syrup of ipecac. Studies comparing the efficiency of removal of gastrointestinal contents, resultant blood levels of orally administered drugs with and without emesis, differences in the gastrointestinal emetic response between agents and the pharmacology of the gastrointestinal emetic response should be performed. Studies should also be conducted to determine the pharmacology of the emetic sensory receptors in the gastrointestinal tract and the intraluminal physical-chemical or gastrointestinal physiological factors influencing gastrointestinal emetic sensory receptor activation. The results would demonstrate the value of emesis in various poison cases and help establish criteria for use and selection of emetic drugs. No less experimental attention should be devoted to the cathartic drugs.(ABSTRACT TRUNCATED AT 400 WORDS)