Inhibition of PGE1 induced intestinal secretion by the synthetic enkephalin analogue FK 33-824

Interaction between antisecretory opioid and sympathetic mechanisms in the rat small intestine

The putative existence of an endogenous opioid antisecretory mechanism in the small intestine was tested in anaesthetized Sprague-Dawley rats. Cholera secretion was elicited with cholera toxin and net fluid secretion was measured in vivo using a gravimetric technique allowing on line registrations. Opioid blockade with naloxone (10 mg kg-1 i.v.) increased the cholera secretion significantly but had no effect on control absorption. Pretreatment with phentolamine (2 mg kg-1 i.v.) eliminated the effect of opioid blockade indicating an interaction between the opioid and the adrenergic mechanisms. The effect of naloxone on cholera secretion was unchanged after acute division of the sympathetic nerves to the intestine and removal of the adrenals. Chronic sympathetic denervation of the small intestine, on the other hand, abolished the effect of opiate blockade on the secretion. The antisecretory effect of sympathetic nerve stimulation was unchanged by opiate blockade.

CONCLUSION

An intrinsic antisecretory opioid mechanism has been demonstrated in the small intestine of the rat. This endogenous opioid mechanism seems to decrease secretion indirectly via peripheral activation of the sympathetic antisecretory system.

Enkephalin affects ion transport via the enteric nervous system in guinea-pig ileum

The endogenous opioid enkephalin drives ion transport towards absorption. To determine the site and mechanism of this effect, fractionated stripping of guinea-pig ileum was carried out. The muscularis propria, including myenteric plexus, was removed by partial stripping. The submucosa, including the submucosal plexus, plus the muscularis mucosae were removed by total stripping. For binding studies, epithelial cells were removed by the method of Weiser leaving the lamina propria mucosae with the mucosal plexus. Radio-receptor-assay with (3H)2-D-ala-5-D-leu-enkephalin revealed enkephalin binding sites in the submucosa plus muscularis mucosae (KD = 3.6 nmol l-1; Vmax = 7.3 fmol mg-1) and in the lamina propria mucosae (KD = 4.2 nmol l-1; Vmax = 5.1 fmol mg-1. The binding was stereospecific in both layers. No binding was detected on epithelial cells. In the Ussing chamber, partially stripped ileum exhibited spontaneous ISC which was abolished by addition of tetrodotoxin (TTX) or by total stripping indicating that this ISC was neuronally stimulated by the submucosal plexus. Electrogenic chloride secretion was identified as contributing to this ISC, since the TTX-sensitive part of ISC in the partially stripped ileum was lacking in Cl- and HCO3-free medium, reappeared after addition of Cl consistent with Michaelis-Menten kinetics (Km = 19 nmol l-1) and was reversed by serosal addition of bumetanide. In addition, enkephalin increased electroneutral NaCl-absorption as obtained by Na- and Cl-flux measurements. Enkephalin decreased this spontaneous neuronally stimulated electrogenic Cl-secretion in the partially stripped ileum, but had no effect in totally stripped ileum if ISC was stimulated at the cellular level by theophylline or PGE1. We conclude that ganglia located in the submucosal plexus regulate intestinal ion transport. Enkephalin acts by presynaptic inhibition via receptors on these neurons in the submucosa and/or via receptors on their neurites in the lamina propria mucosae.

Opioid action of the intestine: the importance of the intestinal mucosa

Drug effects on the intestine are traditionally explained in terms of action on the muscle layers and the nerves that control them. This is particularly true in the case of the opioids but research starting two decades ago has identified the intestinal mucosa as the site of action of the antidiarrhoeal opioids. Continued research using the intestinal mucosa offers a fresh approach to solving some old problems. For example it could lead to more confident predictions to be made about the wanted and unwanted effects of opioid drugs on the intestine and may help to find better drug treatments for alleviating withdrawal diarrhoea in addicts. Eventually it may help to explain how the general process of opioid dependence occurs at a cellular level.

Evidence for tryptaminergic and noradrenergic involvement in the antisecretory action of morphine in the rat jejunum

Experiments have been performed to determine whether the antisecretory (antidiarrhoeal) effect of morphine in the intestine is mediated by a direct action of morphine on enteric nerves. Rats were pretreated with 6-hydroxydopamine (6-OHDA) or p-chlorophenylalanine (PCPA) to deplete intestinal stores of noradrenaline and 5-hydroxytryptamine (5-HT). Intraperitoneal injection of 6-OHDA (3 doses at 50 mg kg-1) caused a selective reduction in the level of noradrenaline in the jejunum to 7.3% of control. Intraperitoneal injection of PCPA (200 mg kg-1) selectively reduced the jejunal level of 5-HT to 30.5% of control. Groups of rats that had been treated as described above were anaesthetized and then injected intravenously with saline or with blocking doses of either atropine (0.25 mg kg-1), hexamethonium (20 mg kg-1), ketanserin (30 micrograms kg-1), methysergide (30 micrograms kg-1), phentolamine (2 mg kg-1) or propranolol (1 mg kg-1). Following perfusion of the lumen of the jejunum, the rate of glucose absorption was measured to assess the integrity of the mucosa. Glucose absorption was unaltered in animals pretreated with hexamethonium and propranolol but there was a small enhancement in animals pretreated with atropine, PCPA, methysergide, 6-OHDA and phentolamine. The rate of net water absorption from the lumen of the jejunum and the rate of fluid secretion into the lumen following intra-arterial infusion of vasoactive intestinal peptide (VIP, 0.8 microgram min-1) were unaltered by any of the drug treatments. Intravenous injection of morphine (10 mg kg-1) did not alter the levels of noradrenaline or 5-HT in the whole jejunum. However, this dose of morphine did cause a 63.5% decrease in the VIP-induced change in water transport. This antisecretory effect of morphine was unaltered in animals pretreated with atropine, hexamethonium and propranolol. In contrast, methysergide, ketanserin and 6-OHDA abolished the antisecretory effect of morphine. PCPA and phentolamine produced a partial inhibition of morphine's antisecretory effect. It is concluded that morphine produces its antisecretory effect in the jejunum by activation of noradrenergic and tryptaminergic systems.

Human intestinal motor activity and transport: effects of a synthetic opiate

Effects of opiates on intestinal motor activity and transport of water and electrolytes have been studied separately in previous investigations. The aim of these experiments was to evaluate simultaneously the effects of a synthetic opiate, loperamide, on motor activity and transport in the human intestine. Jejunal, ileal, and colonic perfusions were performed in 9 healthy volunteers. After application of loperamide (12 mg), cyclically recurring migrating motor complexes in the small intestine occurred at a significantly higher frequency than after application of placebo. This was primarily due to a decrease in the duration of irregular motor activity (phase II). Loperamide increased the transit time in the jejunum but not in the ileum or in the colon. Transport rates of water and electrolytes and transmural electrical potential differences were not significantly affected by the drug. These results suggest that opiates exert their constipating effect by inhibiting phase II-related irregular motor activity.

The effects of morphine and enkephaline on gallbladder function in experimental cholecystitis. Inhibition of inflammatory gallbladder secretion

Administration of morphine or its derivatives is the traditional way to treat biliary pain. Despite the common use of morphine and its analogues in patients with cholecystitis and biliary pain, their effects on the function of the inflamed gallbladder are not known. In the present study it is demonstrated that morphine usually contracts the normal gallbladder but does not influence the fluid transport across the mucosa. In experimental cholecystitis morphine and enkephaline do not further contract the gallbladder but, by specific opioid receptors, reduce the inflammatory fluid secretion by the mucosa. In case of obstruction of the gallbladder, fluid secretion by the mucosa will distend its wall and induce biliary pain. It is suggested that the pain-relieving effect of morphine in cholecystitis and attacks of biliary pain is mediated not only by a central analgesic effect but also by an influence on the function of the inflamed gallbladder.

Nufenoxole, a new antidiarrhoeal agent, inhibits fluid secretion in the human jejunum

Nufenoxole is an orally active antidiarrhoeal agent which binds to opioid receptors in the brain and myenteric plexus of the intestine. A perfusion technique has been used to investigate the effect of nufenoxole (1 mg/kg intrajejunally) on water and solute transport stimulated by the secretagogue, dioctyl sodium sulphosuccinate, in the human jejunum in vivo. Nufenoxole reversed the direction of jejunal transport of salt and water from net secretion to net absorption. These changes in water and electrolyte transport were inhibited by intravenous naloxone, the opioid antagonist. Nufenoxole possesses potent antisecretory properties, which are mediated via opioid receptors and may contribute to its antidiarrhoeal action in man.

Studies of the mechanism of the antidiarrheal effect of codeine

To determine whether the antidiarrheal action of opiate drugs in humans is due to enhanced intestinal absorption rates, as suggested by recent experiments in animals, or is due to altered intestinal motility, as traditionally thought, we studied the effect of therapeutic doses of codeine on experimental diarrhea and on the rate of intestinal absorption of water and electrolytes in normal human subjects. Our results show that codeine (30-60 mg i.m.) markedly reduced stool volume during experimental diarrhea induced by rapid intragastric infusion of a balanced electrolyte solution. There was, however, no evidence that codeine stimulated the rate of intestinal absorption in the gut as a whole or in any segment of the gastrointestinal tract, either in the basal state or when absorption rates were reduced by intravenous infusion of vasoactive intestinal polypeptide. We also measured segmental transit times to determine whether and where codeine delayed the passage of fluid through the intestine. Codeine caused a marked slowing of fluid movement through the jejunum, but had no effect on the movement of fluid through the ileum or colon. In other studies, we found that the opiate antagonist naloxone did not significantly affect water or electrolyte absorption rates in the jejunum or ileum. We conclude (a) that therapeutic doses of codeine increase net intestinal absorption (and thereby reduce stool volume) by increasing the contact time of luminal fluid with mucosal cells, not by increasing the rate of absorption by the mucosal cells; and (b) that endogenous opiates do not regulate intestinal absorption in humans.

Potentiation of histamine-induced itch and flare responses in human skin by the enkephalin analogue FK-33-824, beta-endorphin and morphine

The effect of various opioid or putative neurotransmitter peptides on histamine-induced itch and flare responses was studied in humans after intradermal injection. Significant enhancement of the histamine responses was induced by the stable methionine-enkephalin analogue FK 33-824, beta-endorphin and morphine. The putative neurotransmitters substance P and vasoactive intestinal polypeptide (VIP)--which moreover are potent histamine liberators--had no enhancing effect. The potentiation induced by FK 33-824 was induced neither by local pretreatment with Compound 48/80 to deplete the local stores of mast-cell-bound histamine, nor by oral pretreatment with indomethacin to inhibit prostaglandin formation in the skin. Thus, the enhancement did not seem to be due to histamine release or to prostaglandin formation and the mechanism of the effect remains to be shown. The specific morphine antagonist naloxone did not inhibit the potentiation by FK 33-824, which might indicate that ordinary opiate receptors were not involved. The results support the idea that pain and itch are qualitatively separate processes and suggest possible mechanisms of morphine-induced pruritus. The findings are of particular interest in view of recent reports on the presence of methionine-enkephalin in Merkel cells.

Influence of vasoactive intestinal polypeptide on net water flux and cyclic adenosine 3',5'-monophosphate formation in the rat jejunum

1. The effects were studied of vasoactive intestinal polypeptide (VIP), theophylline, and morphine on net water flux and cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels in the jejunum of anaesthetized rats in vivo and of VIP and morphine on adenylate cyclase activity in rat epithelial cell membranes in vitro. 2. Infusion of VIP (0.1-2 x 10(-9) mol/min/kg) dose dependently caused a reversal from net water absorption to net secretion; 2 x 10(-9) mol/min/kg enhanced the mucosal cyclic AMP content by 67%. 3. Theophylline (5 mg/ml, intraluminally) enhanced the effect of intra-arterial infusion of VIP (2 x 10(-9) mol/min/kg) as to net water secretion and increase in mucosal cyclic AMP content. 4. Pretreatment with morphine (5 mg/kg, s.c.) did not influence the effects of VIP on net water flux and on mucosal cyclic AMP content. 5. Atropine (2 mg/kg, s.c.) also failed to reduced the effect of VIP (0.4 x 10(-9) mol/min/kg) on net water flux. 6. Stimulation of adenylate cyclase activity was a function of VIP concentration over a range of 1 x 10(-10)-1 x 10(-7) M. Morphine (up to 1 x 10(-3) M) failed to influence stimulation of adenylate cyclase by VIP. 7. The finding that low doses of VIP, which already have an effect on net water flux, fail to increase cyclic AMP levels makes it likely that other mediators besides cyclic AMP are involved in the effect of VIP on net water flux. Some of the present results, however, support the assumption that VIP stimulates intestinal fluid secretion by increasing mucosal cyclic AMP levels.

Naloxone increases carbon dioxide stimulated respiration in the rabbit

1. The effects were studied of morphine and naloxone on the respiratory minute volume and the minute volume stimulated by carbon dioxide in the anaesthetized rabbit. 2. The respiratory minute volume is reduced by morphine (2 mg/kg) to about 60% of control. The effect of morphine is abolished by naloxone (5 mg/kg); the minute volume depressed by morphine transiently becomes larger after naloxone than prior to morphine. Also the minute volume stimulated by carbon dioxide becomes larger after naloxone than that prior to the period of depression produced by morphine. 3. Naloxone (0.5 mg and 5 mg/kg i.v.) enhances the sensitivity of respiration to carbon dioxide by about 21% and 28%, respectively. 0.5 mg/kg naloxone itself had no influence, 5 mg/kg for about 5 min increases basal respiratory minute volume. 4. It is assumed that endogenous opiates modulate respiration and, like exogenously administered opiates, reduce the sensitivity of respiration to carbon dioxide. 5. The hypothesis is discussed that endogenous opiates are involved in regulation of biological functions which are known to be inhibited by opiates and which belong to the "protective system" postulated by O. Schaumann.