Enhancement of colonic motor response to feeding by central endogenous opiates in the dog

Trimebutine: mechanism of action, effects on gastrointestinal function and clinical results

The actions of trimebutine [3,4,5-trimethoxybenzoic acid 2-(dimethylamino)-2-phenylbutylester] on the gastrointestinal tract are mediated via (i) an agonist effect on peripheral mu, kappa and delta opiate receptors and (ii) release of gastrointestinal peptides such as motilin and modulation of the release of other peptides, including vasoactive intestinal peptide, gastrin and glucagon. Trimebutine accelerates gastric emptying, induces premature phase III of the migrating motor complex in the intestine and modulates the contractile activity of the colon. Recently, trimebutine has also been shown to decrease reflexes induced by distension of the gut lumen in animals and it may therefore modulate visceral sensitivity. Clinically, trimebutine has proved to be effective in the treatment of both acute and chronic abdominal pain in patients with functional bowel disorders, especially irritable bowel syndrome, at doses ranging from 300 to 600 mg/day. It is also effective in children presenting with abdominal pain.

Central effects of neuropeptide FF on intestinal motility in naive and morphine-dependent rats

The effects on intestinal myoelectric activity of (1DME)Y8Fa (D-Tyr-D-Leu[N-Me]-Phe-Gln-Pro-Gln-Arg-Phe-NH2), a synthetic analog of the neuropeptide FF (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2) were examined in rats after central (i.c.v.) administration performed before acute morphine and after chronic morphine treatment. The acute administration of morphine sulphate (5 mg/kg s.c.) inhibited the occurrence of intestinal migrating myoelectric complexes (MMC) for 46.0 +/- 15.2 min and increased the number of contractions on the proximal colon (13.5 +/- 2.9 vs 8.1 +/- 0.6/10 min). The duration of the MMC disruption was significantly (P < 0.05) reduced (19.0 +/- 5.6 min) by central administration of naloxone (100 micrograms/kg) but not of (1DME)Y8Fa. In rats rendered tolerant to morphine by injections of a slow-release emulsion containing morphine (75 mg/rat over 48 h), intestinal MMC were disrupted and replaced by a continuous irregular activity. Central administration of naloxone (100 micrograms/kg) restored MMC after having induced for 38.3 +/- 7.3 min a motor pattern typical of the diarrhoeal state, termed minute rhythm. No change in colonic motility was observed despite the occurrence of dirrahoea. The effects of naloxone were reproduced by (1DME)Y8Fa (100 micrograms/kg) that induced a pattern of minute rhythm for 48.9 +/- 15.7 min. These features indicate that neuropeptide FF has no action on the acute effects of morphine on intestinal motility but exerts anti-opioid activities in digestive motor alterations associated to morphine withdrawal.

Action of enkephalinase (EC 3.4.24.11) inhibition on the pre- and post-prandial electromyographic patterns of colon in rats

Endogenous opioids are an important regulatory factor for the digestive tract and specially for its motility pattern. Enkephalin degradation includes an enkephalinase (EC 3.4.24. 11) and the effects on the colic electromyographic profile of its inhibition by acetorphan has been investigated in the unrestrained rat. Electromyogram consisted of Long Spike Bursts (LSB). In fasted state, they propagated indifferently in both aboral or oral directions from any point of the colon. Feeding privileges LSB which start near the cecal junction and propagated aborally to the distal colon. The acetorphan treatment (A) increases the percentage of LSB propagating aborally on the entire colon in fasted state and (B) reinforces the increased percentage of LSB which propagated down on the entire colon induced by feeding. All the actions of acetorphan on colic motility pattern disappear after inhibition of opioid receptors by naloxone. That may account for involvement of enkephalins in acetorphan properties on the pattern of the colic electrical activity.

Immunoautoradiographic localisation of enkephalinase (EC 3.4.24.11) in rat gastrointestinal tract

Enkephalinase (EC 3.4.24.11, membrane metalloendopeptidase) is a zinc peptidase expressed by neurons and a variety of epithelial cells, and responsible for the inactivation of enkephalins in brain. Its functions in the gastrointestinal (GI) tract are less well understood although enkephalinase inhibitors were reported to induce a constellation of antisecretory and motor responses. Its localisation in various segments of the rat GI tract was established autoradiographically using a 125I-labelled monoclonal antibody. All along the GI tract, the highest immunoreactivity was found in mucosal layers e.g., in intestinal villi, basal epithelial layers of the oesophagus or gastric cardia, muscularis mucosae of the stomach and large intestine. The immunoreactivity was also high in the stomach submucosae and moderate in the muscularis propria of the caecum. A faint patchy immunoreactivity was also observed in several other layers. This distribution suggests that the membrane peptidase is expressed by enterocytes and a variety of other cells. Its high expression in mucosal layers is consistent with its participation in protein digestion and also in the inactivation of endogenous peptides, particularly the enkephalins, acting at this level to control secretory mechanisms and hydroelectrolytic fluxes. Its presence in submucosal layers may account for some naloxone-reversible motor responses elicited by enkephalinase inhibitors.

Selective stimulation of colonic motor response to a meal by sigma ligands in dogs

The influence of central vs. peripheral administration of sigma ligands (dl- and l-N-allylnormetazocine, 1-3-di-o-tolylguanidine, (+) cinnamyl-1-phenyl-1-N-methyl-N-cyclopropylene and phencyclidine on colonic motility was investigated in fasted and fed dogs equipped with strain-guage transducers implanted on proximal and transverse colon. When injected intravenously at a dose of 0.25 mg/kg just before feeding, dl- or d-N-allylnormetazocine, 1-3-di-o-tolylguanidine, and (+) cinnamyl-1-phenyl-1-N-methyl-N-cyclopropylene (but not phencyclidine) enhanced the colonic motor response to a meal by increasing the 0-4-hour motility indexes from 64.1%-159.3% in both the proximal and transverse colon but had no effect on colonic motility in fasted animals or animals injected intracerebroventricularly. The motor-stimulatory effects of d-N-allylnormetazocine (1 mg/kg), 1-3-di-o-tolylguanidine (0.25 mg/kg), and (+) cinnamyl-1-phenyl-1-N-methyl-N-cyclopropylene (1 mg/kg) were abolished after previous treatment with haloperidol (0.5 mg/kg, intravenous) but not after sulpiride (0.1 mg/kg) or (+) R-(+)-8-chloro-2,3,4,5-tetrahydro-3- methyl-5-phenyl-1-H-3-benzozepine-OH. Prazosin (0.1 mg/kg, intravenous) and 1-methyl-3-(2-indolyl)amino-5-phenyl-3H-1,4-benzodiazepin-2-one (0.01 mg/kg) also suppressed the enhancement of the colonic motor response to eating induced by d-N-allylnormetazocine, 1-3-di-o-tolylguanidine, and (+)cinnamyl-1-phenyl-1-N-methyl-N-cyclopropylene whereas naltrexone did not affect their effects. It is concluded that d-N-allylnormetazocine, 1-3-di-o-tolylguanidine, and (+)cinnamyl-1-phenyl-1-N-methyl-N-cyclopropylene stimulate the postprandial colonic motility in dogs by acting selectively on sigma receptors located peripherally and probably by affecting the release of cholecystokinin octapeptide through a central adrenergic mechanism.

Central nervous system action of peptides to influence gastrointestinal motor function

The central action of peptides to influence GI motility in experimental animals is summarized in Table 1. TRH stimulates gastric, intestinal, and colonic contractility in rats and in several experimental species. A number of peptides including calcitonin, CGRP, neurotensin, NPY, and mu opioid peptides act centrally to induce a fasted MMC pattern of intestinal motility in fed animals while GRF and substance P shorten its duration. The dorsal vagal complex is site of action for TRH-, bombesin-, and somatostatin-induced stimulation of gastric contractility, and for CCK-, oxytocin- and substance P-induced decrease in gastric contractions or intraluminal pressure. The mechanisms through which TRH, bombesin, calcitonin, neurotensin, CCK, and oxytocin alter GI motility are vagally mediated. An involvement of central peptidergic neurons in the regulation of gut motility has recently been demonstrated in Aplysia, indicating that such regulatory mechanisms are important in the phylogenesis. Alterations of the pattern of GI motor activity are associated with functional changes in transit. TRH is so far the only centrally acting peptide stimulating simultaneously gastric, intestinal, and colonic transit in various animals species. Opioid peptides acting on mu receptor subtypes in the brain exert the opposite effect and inhibit concomitantly gastric, intestinal, and colonic transit. Bombesin and CRF were found to act centrally to inhibit gastric and intestinal transit and to stimulate colonic transit in the rat. The antitransit effect of calcitonin and CGRP is limited to the stomach and small intestine. The delay in GI transit is associated with reduced GI contractility for most of the peptides except central bombesin that increases GI motility. Nothing is known about brain sites through which these peptides act to alter gastric emptying and colonic transit. Regarding brain sites influencing intestinal transit, TRH-induced stimulation of intestinal transit in the rat is localized in the lateral and medial hypothalamus and medial septum. The periaqueductal gray matter is a responsive site for mu receptor agonist- and neurotensin-induced inhibition of intestinal transit. The neural pathways from the brain to the gut whereby these peptides express their stimulatory or inhibitory effects on GI transit is vagal dependent with the exception of calcitonin. It is not known whether the vagally mediated inhibition of GI transit by these peptides results from a decrease activity of vagal preganglionic fibers synapsing with excitatory myenteric neurons or an activation of vagal preganglionic neurons synapsing with inhibitory myenteric neurons. The lack of specific antagonists for these peptides has hampered the assessment of their physiological role.(ABSTRACT TRUNCATED AT 400 WORDS)

Action of opiates on gastrointestinal function

Opioid peptides and opioid receptors are distributed along the gastrointestinal (GI) tract, indicating endogenous opiates released peripherally may modulate GI motor and secretory functions. Animal studies have revealed that the effects of opiates on gut motility depend on the nature of the subclasses of receptor involved, the species and the part of bowel. Most opiates that have a selective or predominant mu agonist activity inhibit gastric motility and delay gastric emptying by acting centrally; delta and kappa agonist are inactive when injected systemically. The effect of opiates in delaying intestinal transit observed in man, rat and other species is related to an inhibition (rat) or a stimulation (dog and man) of intestinal contractions as premature phase III-like sequences. The constipating effects of morphine probably result mainly from its action on colonic motility. Morphine stimulates colonic motility in humans by action on both central and peripheral sites. This increase in colonic motility and the delay in colonic transit is associated with a reinforcement of tonic contractions and reduced propulsive waves. Opioid peptides have been shown to participate in the colonic motor response to eating in man and animals. Both delta and mu receptors are involved in the stimulatory effects of opiates on colonic motility, while kappa receptors inhibit colonic contractions, mainly by acting centrally. The effects of opiates on gastric acid secretion are still controversial but it has been well demonstrated that opiates act centrally to reduce pancreatic secretion in rats. Opiates also inhibit intestinal secretions via an action on the enteric nervous system as well as in the CNS. All these results reinforce the hypothesis that opioid peptides have a major physiological role in the control of gut motility and secretions, and these actions explain most of the pharmacological effects of opiate substances on the digestive tract.

Naloxone-reversible antidiarrheal effects of enkephalinase inhibitors

Thiorphan and acetorphan, two potent inhibitors of enkephalinase (EC 3.4.24.11 membrane-metalloendopeptidase) significantly reduced the castor oil-induced diarrhea in rats when administered intravenously (or orally, for acetorphan) but not when administered intracerebroventricularly. These effects were more marked during the 90 min period following the castor oil challenge but were still significant up to 4-8 h after the latter. Acetorphan was about 6 times more potent than thiorphan. The antidiarrheal activity of both compounds was completely prevented in rats receiving naloxone subcutaneously but not intracerebroventricularly (in the case of thiorphan). In contrast to loperamide, a peripherally acting opiate receptor agonist, the enkephalinase inhibitors did not significantly reduce gastrointestinal transit as measured in the charcoal meal test. The antidiarrheal activity of enkephalinase inhibitors therefore seems attributable to protection of endogenous opioids, presumably outside the brain, and to reduction of intestinal secretion rather than transit.

Origin of the stimulation of food intake by oral administration of enkephalinase inhibitors in sheep

The influence of two enkephalinase inhibitors (thiorphan and acetorphan) orally, parenterally and centrally administered on food intake was tested in hay-fed ewes. When orally administered at a dose of 1 mg/kg, acetorphan, but not thiorphan, produced a biphasic increase in food intake corresponding to a 17.0% increase of daily food intake. Similarly thiorphan (0.1 mg X kg-1) IV administered increased by 19.3% the daily food intake; in contrast acetorphan IV administered produced a early (0-2 h) decrease followed by a late increase in hay consumption without significant (P greater than 0.05) change in the daily food intake. When ICV administered (10 micrograms X kg-1) thiorphan but not acetorphan at the same dose depressed the early (0-2 h) and daily food intake by 43.2% and 25.4% respectively. Pretreatment with naltrexone (0.1 mg X kg-1 IV) blocked the increased food intake induced by oral acetorphan or IV acetorphan and thiorphan but did not affect the anorectic effects of ICV thiorphan. We conclude that enkephalinase inhibitors like thiorphan and acetorphan increase daily food intake in sheep probably by increasing enkephalin levels in peripheral tissues.

Endogenous opiates: 1985

This paper is the eighth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1985. The specific topics this year include stress, tolerance and dependence, eating, drinking and alcohol consumption, gastrointestinal and renal activity, mental illness, learning and memory, cardiovascular responses, respiration and thermoregulation, seizures and neurological disorders, activity, and some other selected topics.