Characterization of the opiate receptor population mediating inhibition of VIP-induced secretion from the small intestine of the rat

Comparison of opioid receptor distributions in the rat ileum

The cellular expression patterns of mu-, delta- and kappa-opioid receptors in the rat ileum were examined using fluorescence immunohistochemistry. Double-labelling was used to examine cellular receptor co-localisation as a pre-requisite for intracellular molecular interactions, such as heterodimerisation. Tissues were stained as whole-mount preparations. Strong, broadly distributed immunoreactivity (ir) was observed for each receptor in the myenteric and submucous plexuses. Although intracellular mu- and delta-ir patterns differed in ganglion neurons, mu/delta co-expression was extensive in these cells. mu/delta co-expression was also observed in interstitial cells, which were diffusely distributed in submucous plexus preparations but generally located adjacent to myenteric plexus structures. Punctate kappa-ir was seen broadly in nerve fibres in both plexuses, suggesting localisation in varicosities. Neuronal mu/kappa co-localisation was not apparent, although kappa-ir fibres were often apposed against mu-ir cells. mu/kappa co-localisation was detected in interstitial cells in submucous plexus preparations. Similarities in mu and delta expression patterns might reflect similar functional properties previously detected for these receptors. This study indicates that the rat gastrointestinal tract might provide a useful tool for the future study of molecular interactions between opioid receptor types.

Studies investigating the possible involvement of adenosine in the antisecretory action of morphine

1. Fluid secretion was induced in the jejunum of anesthetised rats using vasoactive intestinal peptide. 2. The adenosine antagonist, DPCPX (0.1 mg/kg), suppressed the antisecretory action of morphine (10 mg/kg), but naloxone (80 micrograms/kg) did not inhibit the antisecretory response of the adenosine agonist, NECA (40 micrograms/kg), at a dose previously shown to antagonize the antisecretory response of morphine. 3. NECA (40 (micrograms/kg) reversed secretion in pithed and reserpine-pretreated (5 mg/kg subcutaneously) rats. 4. It is proposed that adenosine acts as a mediator of the morphine antisecretory effect at a site distal to the noradrenergic neurons involved in the action of morphine.

Evidence for functional delta-opiate receptors in the rat intestine

The selective delta-opiate agonists D-Ser2, Leu5, Thr6-enkephalin (DSLET), D-Ala2, D-Leu5-enkephalin and D-Pen2, D-Pen5-enkephalin caused inhibition of the cholinergic contraction produced by transmural stimulation of the rat isolated jejunum. Dynorphin A, which is an agonist at both kappa- and delta-opioid receptors also inhibited the cholinergic contraction, as did leu- and met-enkephalin. The selective mu-receptor agonist D-Ala2-NMe-Phe4, Gly-ol5-enkephalin was the least potent of all peptides tested. In general, the order of potency of the peptides was similar to that reported for the delta-receptor-rich mouse vas deferens with potency values similar to those recorded previously for the hamster vas deferens. The selective delta-opioid antagonist naltrindole caused parallel shifts to the concentration-effect curve to DSLET giving a pA2 value of 10.15. The results indicate that the previously identified delta-binding sites in the rat jejunum may correspond to functional delta-opiate receptors involved in attenuating acetylcholine release.

Difenoxin and loperamide: studies on possible mechanisms of intestinal antisecretory action

Experiments have been performed to determine whether the antisecretory (antidiarrhoeal) actions of difenoxin and loperamide are mediated by enteric neurones. An iso-osmotic perfusion solution was circulated around the lumen of the jejunum of anaesthetised rats. Vasoactive intestinal peptide was infused intra-arterially to induce net fluid secretion which was inhibited by difenoxin (ED50, 0.23 mg/kg) and loperamide (ED50, 0.5 mg/kg). However, neither were able to restore the fluid transport rate to the control level of absorption. The antisecretory effects of difenoxin (0.77 mg/kg) and loperamide (0.6 mg/kg) were blocked by the opiate receptor antagonist naloxone (2 mg/kg). Their effects were also abolished by pretreatment with the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA; 200 mg/kg; with desmethylimipramine given beforehand to protect noradrenergic nerves and enhance 5-HT depletion). The effect of difenoxin was blocked with methiothepin (1 mg/kg) and methysergide (30 micrograms/kg) but not ketanserin (30 micrograms/kg), ritanserin (30 mg/kg), ondansetron (10 micrograms/kg) or ICS 205-930 (3 mg/kg). None of the above 5-HT receptor antagonists modified the antisecretory effect of loperamide. The antisecretory effect of difenoxin but not loperamide was prevented by phentolamine (2 mg/kg) and by pretreatment with 6-hydroxy-dopamine (150 mg/kg total). It is concluded that both difenoxin and loperamide inhibit net fluid secretion by indirect mechanisms. It is proposed that the initial action is on enteric mu-opiate receptors and that this results in the release of 5-HT. In the case of difenoxin, the 5-HT may act on 5-HT1-like receptors to release noradrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-clamp experiments reveal receptor type-dependent modulation of chloride secretion in the guinea pig colonic mucosa by intestinal opioids

The influence of four opioid antagonists on short circuit current (Isc), transepithelial potential difference (Pdo) and tissue conductance (Gt) in the guinea pig colonic mucosa was investigated in vitro under both basal and PGE1 plus theophylline-stimulated conditions. The experiments aimed at identifying the opioid receptor type(s) endogenously activated to control chloride secretion. Under blockade of sodium-dependent Isc by amiloride (100 mumol/l), net anion secretion was regarded to equal the lumen-negative shift in Isc upon addition of 1 mumol/l PGE1 plus 100 mumol/l theophylline. It was significantly elevated by 100 nmol/l of the kappa-selective antagonist nor-binaltorphimine (nor-BNI). This augmenting effect was totally abolished in amiloride-free buffer or by omission of chloride. 1 mumol/l TTX completely prevented the effect of both PGE1 plus theophylline and nor-BNI. Both the kappa agonist U 69593 (10 nmol/l) and the calcium channel agonist Bay K 8644 (1 mumol/l) significantly depressed net anion secretion stimulated by PGE1 plus theophylline. Nor-BNI at 10 nmol/l prevented the suppressive effect of both Bay K 8644 and U 69593. This suggests release of endogenous opioids by the calcium channel agonist Bay K 8644 and competition between the kappa agonist U 69593 and the kappa antagonist nor-BNI. In contrast to the kappa antagonist nor-BNI, the mu antagonist CTOP-NH2 at 100 nmol/l significantly impaired, while the mu-selective agonist DAGO at 0.2 nmol/l augmented, net anion secretion stimulated by PGE1 plus theophylline. The effect of CTOP-NH2 was abolished in chloride-free buffer.(ABSTRACT TRUNCATED AT 250 WORDS)

[D-Ala2, Met5]-enkephalinamide: CNS-mediated inhibition of prostaglandin-stimulated intestinal fluid and ion transport in the rat

The opioid peptide [D-Ala2, Met5]-enkephalinamide (DAMA), a non-selective opioid agonist, has previously been shown to inhibit cholera toxin-induced fluid accumulation in the rat and dog small intestine after its intracerebroventricular (ICV) administration. In the present study, we examined the time course of the antisecretory/proabsorptive effects of ICV DAMA on net fluid and ion transport across the rat jejunum in situ during intravenous prostaglandin E1 (PGE) infusion. Net water and NaCl absorption were measured using a standard dilution marker technique in a 15-20 cm segment of proximal jejunum in urethane-anesthetized Sprague-Dawley rats. Infusion of PGE (5 micrograms/kg-min) over a 2 hr period produced a decrease in fluid and ion absorption that plateaued to a steady-state within 60 min. DAMA (1 and 3 micrograms/rat) administered by ICV bolus 60 min after the start of PGE infusion inhibited significantly PGE-induced decreases in water and chloride absorption relative to saline-treated controls. These dose-related peptide effects were expressed 15 min after DAMA treatment and were approximately 30 min in duration; they were antagonized by naloxone (1 mg/kg, IV) given at the time of DAMA injection. These results indicate that low concentrations of DAMA administered into the central nervous system rapidly and effectively inhibit changes in intestinal transport induced by a blood-borne secretagogue through an interaction with opiate receptors.

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.

Choice of anesthetic for intestinal absorption and secretion experiments using rats

The net rates of glucose and water absorption from, and secretion of water into, the lumen of the rat small intestine were compared using alpha-chloralose, halothane, pentobarbitone, and urethane as anesthetics. Water and glucose transport were determined by recirculation of perfusion fluid through the lumen of the jejunum. Net fluid secretion was stimulated by intraarterial (i.a.) infusion of prostaglandin E1 (PGE1) and vasoactive intestinal peptide (VIP). The results show that the anesthetics used exert only minimal influence on the physiological rates of glucose and water absorption and there is no correlation between the small differences in glucose and water absorption among the anesthetics. However, when the fluid secretory mechanism of the mucosa is stimulated with either PGE1 or VIP, pentobarbitone allows the greatest rates of secretion, but halothane prevents secretion. It is suggested that of the anesthetics used, pentobarbitone is the most appropriate for intestinal secretion experiments.