Insights into opioid action in the intestinal tract

The effects of tramadol and fentanyl on gastrointestinal motility in septic rats

In this study, we investigated the effects of tramadol and fentanyl on gastrointestinal transit (GIT) during acute systemic inflammation in an experimental model of cecal ligation and perforation (CLP). One-hundred-twenty male Swiss-Albino rats were divided randomly into 6 groups: Group I = sham-operated + saline; Group II = sham-operated + fentanyl; Group III = sham-operated + tramadol; Group IV = CLP + saline; Group V = CLP + fentanyl; Group VI = CLP + tramadol. Suspension of charcoal was administered as an intragastric meal to measure the GIT. GIT% (mean +/- sd) were 46.1% +/- 9.8%, 43.2% +/- 9.8%, 45.9% +/- 10.2%, 33.2% +/- 9.2%, 24.9% +/- 4.1%, and 31.8% +/- 8.4% in Groups I, II, III, IV, V, and VI, respectively. GIT% was significantly less in Group V than in Groups I, II, III, and IV (P < 0.05). The Group VI mean value was significantly lower than those of Groups I, II, and III (P < 0.05) but not different from those of Groups IV and V (P > 0.05). The antitransit effect of fentanyl was shown to have increased in the experimental sepsis model, but no decrease in GIT was obtained with tramadol. This was thought to be the result of an associated endogenic opioid system activation and receptor upregulation in sepsis.

Effects of mu, kappa or delta opioids administered by pellet or pump on oral Salmonella infection and gastrointestinal transit

Our laboratory has shown previously that subcutaneously implanted, slow-release morphine pellets markedly enhanced susceptibility to oral infection with Salmonella typhimurium. Further, morphine, kappa and delta opioid receptor agonists infused via osmotic minipumps were immunosuppressive. The present study compared morphine pellets to morphine pumps and also examined the differential effects of morphine versus U50,488H (kappa agonist), deltorphin II (delta2 agonist), and (D-Pen2, D-Pen5)-enkephalin (DPDPE, delta1 agonist), administered via Alzet minipumps, on oral Salmonella infection and on gastrointestinal transit. The results show that all morphine-pelleted mice (26/26) had a marked increase in Salmonella burden in the Peyer's Patches, mesenteric lymph nodes and spleen. In comparison, only 8/20 mice receiving morphine by minipump at doses ranging from 1 to 25 mg/kg/day had any culturable Salmonella in their organs and the number of bacteria was very low. The level of Salmonella colonization correlated with blood morphine levels and gut transit measured using an intragastric charcoal meal. Morphine pellets inhibited gut transit by 38%, while mice receiving morphine by minipump at doses of 1 to 25 mg/kg/day showed only a dose-dependent 7% to 17% inhibition. Mice receiving various doses of U50,488H or DPDPE had no culturable Salmonella in the three sites. Deltorphin II given by minipump resulted in a moderate level of Salmonella in the spleen. Deltorphin II and U50,488H (0.1 to 10 mg/kg/day) did not suppress gut transit. The present studies indicate that a predominantly mu opioid receptor agonist, morphine, given by slow-release pellet, potentiated Salmonella infection and inhibited gastrointestinal transit. In contrast, morphine in pumps slightly inhibited intestinal transit, but did not sensitize to Salmonella infection. A delta1 opioid receptor agonist did not sensitize to infection, and a delta2 and a kappa opioid receptor agonist had minimal effects on either parameter.

Peripheral opioids for functional GI disease: a reappraisal

Opioids have been used medicinally and recreationally for thousands of years. The clinical use of opioids for gastrointestinal conditions has been limited by central nervous system side effects. A new generation of peripheral opioid receptor ligands free of central nervous system side effects is being developed. Clinical trials with the peripherally acting mu opioid receptor antagonists' alvimopan and N-methylnaltrexone show promise for improving postoperative ileus- and opioid-induced constipation. Likewise, preliminary studies with the peripherally acting kappa opioid agonist fedotozine showed promise in the treatment of irritable bowel syndrome (IBS) and functional dyspepsia. Further studies are on hold presumably due to lack of efficacy in subsequent studies. However, clinical studies are underway for newer kappa opioid agonists such as asimadoline and ADL 10-0101.

Characterisation of opioid receptors involved in modulating circular and longitudinal muscle contraction in the rat ileum

1. The aim of the present investigation was to characterise the opioid receptor subtypes present in the rat ileum using a method that detects drug action on the enteric nerves innervating the circular and longitudinal muscles. 2. Neurogenic contractions were reversibly inhibited by morphine (circular muscle pEC50, 6.43+/-0.17, Emax 81.7+/-5.0%; longitudinal muscle pEC50, 6.65+/-0.27, Emax 59.7+/-7.8%), the mu-opioid receptor-selective agonist, DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin acetate) (circular pEC50, 7.85+/-0.04, Emax 97.8+/-3.6%; longitudinal pEC50, 7.35+/-0.09, Emax 56.0+/-6.1%), the delta-selective agonist DADLE ([D-Ala2,D-Leu5]enkephalin acetate) (circular pEC50, 7.41+/-0.17, Emax, 93.3+/-8.4%; longitudinal pEC50, 6.31+/-0.07, Emax 66.5+/-5.2%) and the kappa-selective agonist U 50488H (trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide methanesulphonate) (circular pEC50, 5.91+/-0.41, Emax, 83.5+/-26.8%; longitudinal pEC50, 5.60+/-0.08, Emax 74.3+/-7.2%). Agonist potencies were generally within expected ranges for activity at the subtype for which they are selective, except for U 50488H, which was less potent than expected. 3. The mu and delta receptor-selective antagonists, CTAP (H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2) and naltrindole, caused progressive, parallel rightward shifts in the DAMGO and DADLE curves, respectively. Analysis indicated conformity to theoretical simple competitive antagonist behaviour. U 50488H effects were insensitive to the kappa-selective antagonist, n-BNI. A high concentration (1 microM) of naltrexone caused apparent potentiation of U 50488H effects. 4. CTAP pK(B) estimates were consistent with previously reported values for mu receptor antagonism (circular 7.84+/-0.17, longitudinal 7.64+/-0.35). However, the naltrindole pK(B) estimates indicated lower antagonist potency than expected (circular 8.22+/-0.23, longitudinal 8.53+/-0.35). 5. It is concluded that mu and possibly atypical delta receptors (but not kappa receptors) mediate inhibition of contraction in this model. Nonopioid actions of U 50488H are probably responsible for the inhibitory effects seen with this compound.

Tolerability, gut effects, and pharmacokinetics of methylnaltrexone following repeated intravenous administration in humans

Previous studies have shown that a single dose of methylnaltrexone, a unique peripheral opioid antagonist, reverses opioid-induced gut hypomotility in humans. Because repeated drug doses are likely to be needed to treat patients with opioid-induced or postsurgical bowel dysfunction, the authors have now examined the safety, pharmacological activity, and pharmacokinetics of a multiple-dose regimen of methylnaltrexone, administered as 12 consecutive intravenous doses (0.3 mg/kg every 6 hours) in 12 healthy subjects. Steady state was achieved rapidly, and after repeated dosing for 3 days, methylnaltrexone decreased oral-cecal transit time from a pretreatment baseline value of 101.3 +/- 29.4 min (mean +/- SD) to 82.5 +/- 20.7 min. Maximum observed plasma concentrations, measured 5 minutes postdose, were 538 +/- 237 and 675 +/- 180 ng/mL after doses 1 and 2, respectively. Based on 6-hour sampling periods, the plasma half-life, 2.5 +/- 0.5 and 2.9 +/- 0.9 hours following the 1st and 12th doses, respectively, was unchanged at steady state. There was essentially no accumulation of methylnaltrexone, based on the ratio of AUC values after doses 12 and 1. This study showed that repeated administration of intravenous methylnaltrexone is well tolerated in humans, with no significant adverse events or changes in opioid subjective ratings and no clinically noteworthy alterations in pharmacokinetics. The observation of a significant reduction in the gut transit time after repeated administration of methylnaltrexone to these opioid-naive volunteers suggests that endogenous opioids modulate human gut motility.

Alvimopan: an oral, peripherally acting, mu-opioid receptor antagonist for the treatment of opioid-induced bowel dysfunction--a 21-day treatment-randomized clinical trial

Alvimopan has been shown to reverse the inhibitory effect of opioids on gastrointestinal transit without affecting analgesia. We evaluated oral alvimopan, 0.5 or 1 mg, versus placebo, once daily for 21 days, in 168 patients with opioid-induced bowel dysfunction (OBD) who were receiving chronic opioid therapy (minimum, 1 month) for nonmalignant pain (n = 148) or opioid dependence (n = 20). The primary outcome was the proportion of patients having at least one bowel movement (BM) within 8 hours of study drug on each day during the 21-day treatment period. Averaged over the 21-day treatment period, 54%, 43%, and 29% of patients had a BM within 8 hours after alvimopan 1 mg, 0.5 mg, or placebo, respectively (P < .001). Secondary outcomes of median times to first BM were 3, 7, and 21 hours after initial doses of 1 mg, 0.5 mg, and placebo, respectively (P < .001; 1 mg vs placebo). Weekly BMs and overall patient satisfaction were increased after the 1-mg dose (P < .001 at weeks 1 and 2 vs placebo, and P = .046, respectively). Treatment-emergent adverse events were primarily bowel-related, occurred during the first week of treatment, and were of mild to moderate severity. Alvimopan was generally well tolerated and did not antagonize opioid analgesia. Patients treated with chronic opioid therapy often experience opioid-induced bowel dysfunction as a result of undesirable effects on peripheral opioid receptors located in the gastrointestinal tract. Alvimopan, a novel peripheral opioid mu-receptor antagonist, has demonstrated significant efficacy for the management of opioid-induced bowel dysfunction without compromise of centrally mediated opioid-induced analgesia.

Effects of acupuncture at GV01 on experimentally induced colitis in rats: possible involvement of the opioid system

Oriental medicine uses acupuncture at the GV01 acupoint with great success to treat diarrhea. It significantly reduced the colonic motility and inflammation in colitic rats. Naloxone pretreatment blocked these effects. The therapeutic effects of acupuncture at GV01 in colitis may involve endogenous opioid pathways.

Alvimopan, a selective peripherally acting mu-opioid antagonist

Alvimopan is a novel, peripherally acting mu-opioid antagonist that is being developed for the management of acute postoperative ileus and for the reversal of the delayed gastrointestinal and colonic transit that result in symptoms such as constipation, nausea and motility disorders in patients treated with opiate analgesics. There is a clinical need for effective medications for the treatment of postoperative ileus and opiate-induced constipation and other motility disorders. This review addresses the basic and applied pharmacology and current evidence for the use of the medication, alvimopan, in clinical gastroenterology.

Management of opioid-induced bowel dysfunction in cancer patients

The gastrointestinal (GI) effects of morphine and other opioids may result in opioid-induced bowel dysfunction (OBD) and the need for treatment. Although OBD is very common in morphine-treated patients, it is usually under-diagnosed. Opioids deliver their GI effect through central and peripheral mechanisms. Laxatives are the pharmaceuticals prescribed most in this area. Prokinetics as well as cholinergic agonists have been used satisfactorily. One-third of patients with OBD have to be treated rectally. The use of opioid antagonists has been favored, but the bioavailability of oral forms is poor. Opioid antagonists with a quaternary structure have a high affinity for peripheral opioid receptors and therefore do not interfere with the analgesia, nor do they generate alkaloid withdrawal syndrome. Opioid rotation is another strategy for maintaining or improving analgesic quality directed toward decreasing the effects of previous opiates on the GI tract.

Peristalsis in the Guinea Pig Small Intestine In Vitro Is Impaired by Acetaminophen but Not Aspirin and Dipyrone

Inhibition of intestinal peristalsis is a major side effect of opioid analgesics. It is unknown whether non-opioid analgesics, such as acetaminophen, acetylsalicylic acid, and dipyrone, exert any effect on intestinal motility. In the current in vitro study we examined the effect of these analgesics on intestinal peristalsis and analyzed some of their mechanisms of action. In isolated segments of the guinea pig small intestine peristalsis was triggered by a perfusion-induced increase of the intraluminal pressure. The peristaltic pressure threshold (PPT) at which peristaltic waves were elicited was used to quantify drug effects on peristalsis. Vehicle (Tyrode's solution), acetaminophen (0.01-100 microM), acetylsalicylic acid (100-300 microM), and dipyrone (10-100 microM) were added extraserosally to the organ bath. Acetaminophen concentration-dependently increased PPT and abolished peristalsis in four of six segments at the concentration of 10 microM and in all segments tested at 100 microM (EC50=6.0 microM). The increase in PPT resulting from 3 microM acetaminophen was reduced by naloxone and apamin but not changed by L-nitro-arginine methylester (L-NAME), its inactive enantiomer D-NAME, acetylsalicylic acid, methysergide, or tropisetron. Acetylsalicylic acid and dipyrone did not affect peristalsis. The results reveal, for the first time, that acetaminophen concentration-dependently impairs intestinal peristalsis, whereas acetylsalicylic acid and dipyrone lacked such an effect. The inhibition caused by acetaminophen involves transmitters acting via small conductance Ca2+-activated potassium channels, endogenous opioidergic pathways, and presumably inhibition of cyclooxygenase-3.

Pharmacological characterization of a 7-benzylidenenaltrexone-preferring opioid receptor in porcine ileal submucosa

In the intestine, opioids produce antidiarrhoeal and constipating actions that are mediated by enteric neurones. Through interactions with opioid receptors (ORs) on submucosal neurones, opioids suppress active ion transport evoked by transmural electrical stimulation (TES) in mucosa-submucosa sheets from the porcine ileum. In this study, we examined the pharmacological characteristics of the previously described OR, which is sensitive to the delta1-OR antagonist 7-benzylidenenaltrexone and modulates neurogenic transepithelial ion transport in this tissue preparation. Increases in short-circuit current (Isc, a measure of active anion transport) evoked by TES in ileal mucosa-submucosa sheets were inhibited by opioid agonists possessing high selectivity for either delta- or micro-ORs including [d-Pen2,5]enkephalin (DPDPE), [d-Ala2, Glu4]deltorphin II, and [d-Ala2, N-Me-Phe4, Gly5-ol]enkephalin (DAMGO). As determined by the Schild analysis, the actions of these agonists were competitively inhibited by 7-benzylidenenaltrexone. The nonequilibrium micro-OR antagonist beta-funaltrexamine inhibited the actions of DAMGO only at a high concentration (1 microm) but did not alter DPDPE or deltorphin II action. At concentrations up to 10 microm, the nonequilibrium delta-OR antagonist naltrindole 5'-isothiocyanate did not alter the actions of delta- or micro-OR agonists. Radioligand binding analyses of neuronal homogenates from the ileal submucosa revealed that the nonselective OR ligand [3H]diprenorphine bound to two populations of specific binding sites. One of these sites possessed binding characteristics similar to the delta-OR. In summary, neurogenic ion transport in the porcine intestine is modulated by an OR which shares pharmacological characteristics of both micro- and delta-ORs and may represent a novel receptor entity.

The opioid system in the gastrointestinal tract

Mu-, delta- and kappa-opioid receptors (ORs) mediate the effects of endogenous opioids and opiate drugs. Here we report (1) the distribution of muOR in the guinea-pig and human gastrointestinal tract in relation to endogenous ligands, to functionally distinct structures in the gut and to deltaOR and kappaOR; and (2) the ligand-induced muOR endocytosis in enteric neurones using in vitro and in vivo models. In the guinea pig, muOR immunoreactivity is confined mainly to the myenteric plexus. MuOR myenteric neurones are most numerous in the small intestine, followed by the stomach and the proximal colon. MuOR immunoreactive fibres are dense in the muscle layer and the deep muscular plexus, where they are in close association with interstitial cells of Cajal. This distribution closely matches the pattern of enkephalin. MuOR enteric neurones comprise functionally distinct populations of neurones of the ascending and descending pathways of the peristaltic reflex. In human gut, muOR immunoreactivity is localized to myenteric and submucosal neurones and to immune cells of the lamina propria. DeltaOR immunoreactivity is located in both plexuses where it is predominantly in varicose fibres in the plexuses, muscle and mucosa, whereas kappaOR immunoreactivity appears to be confined to the myenteric plexus and to bundles of fibres in the muscle. MuOR undergoes endocytosis in a concentration-dependent manner, in vitro and in vivo. Pronounced muOR endocytosis is observed in neurones from animals that underwent abdominal surgery that has been shown to induce delay in gastrointestinal transit. We can conclude that all three ORs are localized to the enteric nervous system with differences among species, and that muOR endocytosis can be utilized as a means to visualize enteric neurones activated by opioids and sites of opioid release.

Opioids and opioid receptors in the enteric nervous system: from a problem in opioid analgesia to a possible new prokinetic therapy in humans

The gut is a neurological organ, which implies that many neuroactive drugs such as opioid analgesics can seriously disturb gastrointestinal function, because many of the transmitters and transmitter receptors present in the brain are also found in the enteric nervous system. One of the most common manifestations of opioid-induced bowel dysfunction is constipation which results from blockade of peristalsis and intestinal fluid secretion. The discovery of opioid receptor antagonists with a peripherally restricted site of action, such as N-methylnaltrexone and alvimopan, makes it possible to normalize bowel function in opiate-treated patients without compromising central opioid analgesia. There is emerging evidence that opioid receptor antagonists may also have prokinetic actions, reversing pathological states of gastrointestinal hypomotility that are due to overactivity of the enteric opioid system.

Milk bioactive peptides and beta-casomorphins induce mucus release in rat jejunum

Intestinal mucus is critically involved in the protection of the mucosa. An enzymatic casein hydrolysate and beta-casomorphin-7, a mu-opioid peptide generated in the intestine during bovine casein digestion, markedly induce mucus discharge. Because shorter mu-opioid peptides have been described, the effects of the opioid peptides in casein, beta-casomorphin-7, -6, -4, -4NH2 and -3, and of opioid neuropeptides met-enkephalin, dynorphin A and (D-Ala2,N-Me-Phe4,glycinol5)enkephalin (DAMGO) on intestinal mucus secretion were investigated. The experiments were conducted with isolated perfused rat jejunum. Mucus secretion under the influence of beta-casomorphins and opioid neuropeptides administered intraluminally or intra-arterially was evaluated using an ELISA for rat intestinal mucus. Luminal administration of beta-casomorphin-7 (1.2 x 10(-4) mol/L) provoked a mucus discharge (500% of controls) that was inhibited by naloxone, a specific opiate receptor antagonist. Luminal beta-casomorphin-6, -4 and -4NH2 did not modify basal mucus secretion, whereas intra-arterial administration of beta-casomorphin-4 (1.2 x 10(-6) mol/L) induced a mucus discharge. In contrast, intra-arterial administration of the nonopioid peptide beta-casomorphin-3 did not release mucus. Among the opioid neuropeptides, intra-arterial infusion of Met-enkephalin or dynorphin-A did not provoke mucus secretion. In contrast, beta-endorphin (1.2 x 10(-8) to 1.2 x 10(-6) mol/L) induced a dose-dependent release of mucus (maximal response at 500% of controls). DAMGO (1.2 x 10(-6) mol/L), a mu-receptor agonist, also evoked a potent mucus discharge. Our findings suggest that mu-opioid neuropeptides, as well as beta-casomorphins after absorption, modulate intestinal mucus discharge. Milk opioid-derived peptides may thus be involved in defense against noxious agents and could have dietary and health applications.

Ingested placenta blocks the effect of morphine on gut transit in Long–Evans rats

Opioids produce antinociception, and ingested placenta or amniotic fluid modifies that antinociception. More specifically, ingested placenta enhances the antinociception produced by selective activation of central kappa-opioid or delta-opioid receptors but attenuates that produced by activation of central mu-opioid receptors. Opioids also slow gut transit by acting on central or peripheral mu-opioid receptors. Therefore, we hypothesized that ingested placenta would reverse the slowing of gut transit that is produced by morphine, a preferential mu-opioid-receptor agonist. Rats were injected with morphine either centrally or systemically and fed placenta, after which gastrointestinal transit was evaluated. We report here that ingested placenta reversed the slowing of gut transit produced by centrally administered morphine but did not affect the slowing of gut transit produced by systemically administered morphine. These results suggest another likely consequence of placentophagia at parturition in mammals--reversal of opioid-mediated, pregnancy-based disruption of gastrointestinal function--as well as an important consideration in opioid-based treatments for pain in humans--enhancement of desirable effects with attenuation of adverse effects.

Opioids and the gut: pharmacology and current clinical experience

This article reviews the pharmacology and physiology of opiate receptors and the current and potential uses of opioid agonists and antagonists in clinical gastroenterology. Mu-receptors are involved in motor and sensory functions, and their modulation is established for treatment of diarrhea. Mu-antagonists have potential to reverse endogenous (e.g., postoperative ileus) or iatrogenic dysmotility (e.g., opioid bowel dysfunction). Modulation of the function of kappa-receptors may be a novel approach to control visceral pain in functional gut disorders. Results of formal testing of novel opioid modulators are keenly awaited.

Lessons from the porcine enteric nervous system

The porcine intestinal tract possesses functional and pathological similarities to the human digestive tract and the organization of the porcine enteric nervous system, like that of the human, appears to be more complex than that of commonly investigated guinea-pig intestine. Intrinsic primary afferent neurones appear to differ in the intestines of large and small animals in terms of their chemical coding, distribution over enteric neural networks, electrophysiological behaviour and synaptic properties. Opioid receptors on afferent and motor neurones in the porcine small intestine are predominately of the delta type, whereas those in guinea-pig ileum are mu. Moreover, delta-opioid receptors associated with the myenteric and submucosal plexuses of porcine ileum that, respectively, modulate neurogenic smooth muscle contractions and mucosal ion transport appear to differ in their pharmacological characteristics. These profound interspecies and interregional differences underscore the complexity of the enteric nervous system, and the development of new drugs designed to treat human neurogastrointestinal disorders should be based on the results of investigations in homologous animal models, such as the pig.

Incidence of constipation associated with long-acting opioid therapy: a comparative study

BACKGROUND

Opioid therapy plays a key role in the management of chronic pain. Constipation is one of the more frequently occurring adverse effects associated with opioid therapy.

METHODS

A retrospective cohort design study was conducted to determine the incidence of constipation in chronic pain patients who received three different long-acting opioids (transdermal fentanyl, oxycodone HCl controlled-release [CR], or morphine CR) for malignant or nonmalignant chronic pain. The data source was claims data (January 1996 through March 2001) from a 20% random sample of the California Medicaid (Medi-Cal) database. Claims data were from adult patients with chronic pain (malignant or nonmalignant) who had no prior diagnosis of constipation and no prior usage of long-acting opioids for at least 3 months before the observation period. Patients were followed for at least 3 months after the initiation of opioid therapy. ICD-9 code for diagnosis of constipation was the main outcome variable. Crude rates of constipation, annual incidence density, relative risk, and adjusted odds ratios were compared.

RESULTS

A total of 1,836 patients (601 receiving transdermal fentanyl, 721 receiving oxycodone CR, and 514 receiving morphine CR) were included in the analysis. Crude (unadjusted) rates of constipation were 3.7% for transdermal fentanyl, 6.1% for oxycodone CR, and 5.1% for morphine CR (P > 0.05). Transdermal fentanyl had a lower annual incidence density and risk of constipation than oxycodone CR and morphine CR (P > 0.05). After adjusting for confounding variables, including race and supplemental opioid use, the adjusted risk of constipation was 78% greater in the oxycodone CR group (P = 0.0337) and 44% greater in the morphine CR group (P = 0.2242) than in the transdermal fentanyl group.

CONCLUSION

In this population, patients receiving transdermal fentanyl had a lower risk of developing constipation compared with those receiving oxycodone CR or morphine CR.

Reduction of Saquinavir Exposure by Coadministration of Loperamide

OBJECTIVE

To assess any pharmacokinetic interactions between loperamide and saquinavir.

DESIGN

Double-blind, double-dummy, randomised, placebo-controlled, three-way crossover trial.

PARTICIPANTS

Twelve healthy male and female volunteers, aged 24-46 years.

METHODS

Saquinavir and loperamide pharmacokinetics were determined over a 72-hour period after single dose administration of saquinavir 600mg and/or loperamide 16mg. Plasma and urine concentrations of loperamide, its metabolites, and saquinavir were analysed using a single liquid chromatography/tandem mass spectrometry method for all compounds.

RESULTS

Saquinavir exposure was reduced by 54% when given with loperamide (median area under the concentration-time curve from zero to infinity [range], 1189 [243-2113] vs 550 [234-1468] pmol . h/mL; p = 0.016) with unchanged renal clearance. In contrast, loperamide concentrations increased and desmethylloperamide concentrations decreased during saquinavir coadministration, resulting in a reduced metabolic clearance of loperamide (median [range], 544 [224-1393] vs 443 [238-692] mL/min; p = 0.016).

CONCLUSIONS

Whereas the effect of saquinavir on loperamide disposition is unlikely to be of clinical relevance, the reduced drug exposure of saquinavir when loperamide is coadministered is worrisome because a relationship between protease inhibitor drug exposure and antiviral response has been reported. Patients receiving saquinavir monotherapy should be advised not to combine these drugs, especially for prolonged periods of time because a reduction in therapeutic efficacy may result.

Abdominal surgery induces μ opioid receptor endocytosis in enteric neurons of the guinea-pig ileum

Immunohistochemistry and confocal microscopy were used to investigate mu opioid receptor (muOR) internalization in enteric neurons of the guinea-pig ileum following abdominal surgery. The following surgical procedures were performed under halothane or isofluorane anesthesia: a) midline abdominal skin incision, b) laparotomy or c) laparotomy with intestinal manipulation. Gastrointestinal transit was evaluated by using a non-absorbable marker and measuring fecal pellet output. In neurons from normal and control (anesthesia alone) animals, muOR was predominantly at the cell surface. muOR endocytosis following skin incision was not significantly different from controls (21.2+/-3.5% vs. 13.7+/-2.1%, mean+/-S.E.M.), whereas it was significantly increased by laparotomy (46.5+/-6.1%; P<0.01 vs. controls) or laparotomy plus intestinal manipulation (40.5+/-6.1%; P<0.01 vs. controls) 30 min following surgery compared with controls. muOR endocytosis remained elevated at 4 h (38.6+/-1.2%; P<0.01 vs. controls), whereas it was similar to controls at 6 and 12 h (17.5+/-5.8% and 11.2+/-3.0%). muOR endocytosis occurred in cholinergic and nitrergic neurons. Gastrointestinal transit was significantly delayed by laparotomy or laparotomy plus intestinal manipulation (12.8+/-1.2 and 13.8+/-0.6 h vs. 7.0+/-0.5 in controls; P<0.01), but was not significantly changed by skin incision (8.2+/-0.6 h). The findings of the present study support the concept that the noxious stimulation caused by abdominal surgery induces release of endogenous opioids thus resulting in muOR endocytosis in neurochemically distinct enteric neurons. muOR internalization can serve as indirect evidence of opioid release and as a means to visualize neuronal pathways activated by opioids.

Are Peripheral Opioid Antagonists the Solution to Opioid Side Effects?

Opioid medication is the mainstay of therapy for severe acute and chronic pain. Unfortunately, the side effects of these medications can affect patient comfort and safety, thus limiting their proven therapeutic potential. Whereas the main analgesic effects of opioids are centrally mediated, many of the common side effects are mediated via peripheral receptors. Novel peripheral opioid antagonists have been recently introduced that can block the peripheral actions of opioids without affecting centrally mediated analgesia. We review the clinical and experimental evidence of their efficacy in ameliorating opioid side effects and consider what further information might be useful in defining their role.

IMPLICATIONS

The major analgesic effects of opioid medication are mediated within the brain and spinal cord. Many of the side effects of opioids are caused by activation of receptors outside these areas. Recently developed peripherally restricted opioid antagonists have the ability to block many opioid side effects without affecting analgesia.

Characterization of specific delta-opioid binding sites in the distal small intestine of swine

Delta-opioid receptors modulate neurogenic smooth muscle contractions and mucosal ion transport in the porcine distal small intestine. We compared specific delta-opioid binding sites in neuronal membranes isolated from the inner submucosal plexus and myenteric plexus of this gut region. In both preparations, the delta-opioid receptor antagonist naltrindole displaced [3H]diprenorphine from two binding sites. [3H]Naltrindole bound specifically to a single high-affinity site that was displaced by delta-opioid receptor ligands with the rank order of affinity: naltriben >7-benzylidenenaltrexone>deltorphin II>[D-Pen(2), D-Pen(5)] enkephalin. Relative decreases in delta-opioid receptor agonist affinity in the presence of Na(+) and other ions differed in submucosal and myenteric membranes. The kappa-opioid receptor antagonist norbinaltorphimine displaced [3H]naltrindole binding in myenteric membranes (K(i)=7.2 nM). Delta-opioid receptors in submucosal and myenteric plexuses of porcine ileum appear to differ in some respects.

Evidence for a glutamatergic modulation of the cholinergic function in the human enteric nervous system via NMDA receptors

Several reports suggest that enteric cholinergic neurons are subject to a tonic inhibitory modulation, whereas few studies are available concerning the role of facilitatory pathways. Glutamate, the main excitatory neurotransmitter in the central nervous system (CNS), has recently been described as an excitatory neurotransmitter also in the guinea-pig enteric nervous system (ENS). The present study aimed at investigating the presence of glutamatergic neurons in the ENS of the human colon. At this level, the presence of ionotropic glutamate receptors of the NMDA type, and their possible interaction with the enteric cholinergic function was also studied. In the human colon, L-glutamate and NMDA concentration dependently enhance spontaneous endogenous acetylcholine overflow in Mg2+-free buffer, both effects being significantly reduced by the antagonists, (+/-)-2-amino-5-phosphonopentanoic acid (+/- AP5) and 5,7-diCl-kynurenic acid. In the presence of Mg2+, the facilitatory effect of L-glutamate changes to inhibition, while the effect of NMDA is significantly reduced. In addition, morphological investigations reveal that glutamate- and NR1-immunoreactivities are present in enteric cholinergic neurons and glial cells in both myenteric and submucosal plexus. These findings suggest that, as described for the guinea-pig ileum, glutamatergic neurons are present in enteric plexuses of the human colon. Modulation of the cholinergic function can be accomplished through NMDA receptors.

Evaluation of Peristalsis in Multiple Segments of the Guinea-pig Isolated Small Intestine: Optimisation of Tissue Use by Refined In Vitro Methodology

Peristalsis is the aboral movement by which the intestine propels its contents. Since pharmacological research requires an experimental model with which drug-induced modifications of peristalsis can be reliably quantified, we set out to develop and validate an in vitro method for studying peristalsis in multiple gut segments. In our arrangement, up to four 10cm segments isolated from the guinea-pig jejunum and ileum can be set up in parallel and their lumens perfused. Peristalsis was elicited by pressure-evoked wall distension, and the peristalsis-induced changes in the intraluminal pressure were evaluated with software that determined the peristaltic pressure threshold, the frequency, maximal acceleration and amplitude of the peristaltic waves, and the residual baseline pressure. Validation experiments showed that the peristalsis parameters at baseline and after modification by morphine (0.01–10μM) did not differ between segments from the jejunum and ileum, or between segments examined in a consecutive manner. In conclusion, our work succeeded in optimising the use of the guinea-pig jejunum and ileum for multiple recordings of peristalsis in vitro, and in refining the recording and evaluation of peristaltic motility. This system promises to be particularly useful in the pharmacological screening and testing of drugs which modify peristalsis.

Opioid-induced bowel dysfunction: pathophysiology and potential new therapies

Opioid treatment for postoperative or chronic pain is frequently associated with adverse effects, the most common being dose-limiting and debilitating bowel dysfunction. Postoperative ileus, although attributable to surgical procedures, is often exacerbated by opioid use during and following surgery. Postoperative ileus is marked by increased inhibitory neural input, heightened inflammatory responses, decreased propulsive movements and increased fluid absorption in the gastrointestinal tract. The use of opioids for chronic pain is characterised by a constellation of symptoms including hard dry stools, straining, incomplete evacuation, bloating, abdominal distension and increased gastroesophageal reflux. The current management of opioid-induced bowel dysfunction among patients receiving opioid analgesics consists primarily of nonspecific ameliorative measures. Intensive investigations into the mode of action of opioids have characterised three opioid receptor classes -mu, delta and kappa- that mediate the myriad of peripheral and central actions of opioids. Activation of mu-opioid receptors in the gastrointestinal tract is responsible for inhibition of gut motility, whereas receptors in the central nervous system mediate the analgesic actions of opioids. Blocking peripheral opioid receptors in the gut is therefore a logical therapeutic target for managing opioid-induced bowel dysfunction. Available opioid antagonists such as naloxone are of limited use because they are readily absorbed, cross the blood-brain barrier, and act at central opioid receptors to reverse analgesia and elicit opioid withdrawal. Methylnaltrexone and alvimopan are recently developed opioid antagonists with activity that is restricted to peripheral receptors. Both have recently shown the ability to reverse opioid-induced bowel dysfunction without reversing analgesia or precipitating central nervous system withdrawal signs in non-surgical patients receiving opioids for chronic pain. In addition, recent clinical studies with alvimopan suggest that it may normalise bowel function without blocking opioid analgesia in abdominal laparotomy patients with opioid-related postoperative ileus.

Kinin-induced anion-dependent secretion in porcine ileum: characterization and involvement of opioid- and cannabinoid-sensitive enteric neural circuits

The intestinal secretory actions of the proinflammatory peptide kallidin (lysyl-bradykinin) are mediated partially by enteric neurons. We hypothesized that kallidin produces neurogenic anion secretion through opioid- and cannabinoid-sensitive enteric neural pathways. Changes in short-circuit current (I(sc)) across sheets of porcine ileal mucosa-submucosa mounted in Ussing chambers were measured in response to kallidin (1 microM) or drugs added to the contraluminal bathing medium. Kallidin transiently increased I(sc), an effect reduced after inhibition of neuronal conduction by 0.1 microM saxitoxin, cyclooxygenase inhibition by 10 microM indomethacin, or kinin B(2) receptor blockade by 1 microM d-arginyl-l-arginyl-l-prolyl-trans-4-hydroxy-l-prolylglycyl-3-(2-thienyl)-l-alanyl-l-seryl-d-1,2,3,4-tetrahydro-3-isoquinolinecarbonyl-l-(2alpha,3beta,7alphabeta)-octahydro-1H-indole-2-carbonyl-l-arginine (HOE-140). Its action was dependent upon extracellular Cl(-) or HCO(3)(-) ions, but was resistant to 10 microM bumetanide or 0.3 mM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, and seemed to involve luminal alkalinization as measured by pH-stat titration. Kallidin-induced I(sc) elevations were sensitive to saxitoxin in tissues bathed in Cl(-)-, but not HCO(3)(-)-deficient media. Tissues pretreated with 0.1 microM [d-Pen(2,5)]-enkephalin, a selective delta-opioid agonist, displayed reduced I(sc) responses to kallidin; this effect was prevented by the delta-opioid antagonist naltrindole. At a contraluminal concentration of 1 microM, the cannabinoid receptor agonist (6aR)-trans-3-(1,1-dimethylheptyl)-6a,7,10,10a-tetrahydro-1-hydroxy-6,6-dimethyl-6H-dibenzo[b,d]pyran-9-methanol (HU-210) also attenuated responses to kallidin. Proinflammatory kinins seem to stimulate neurogenic anion secretion in porcine ileum by activating enteric neural circuits expressing inhibitory opioid and possibly cannabinoid receptors.

Anticipating and treating opioid-associated adverse effects

Opioids are frequently avoided as viable tools in the management of pain due to perceived dangerous or untoward adverse drug events. Whilst they are relatively safe options for the treatment of pain, side effects and toxicities do exist and should be anticipated by the provider. The central nervous, gastrointestinal, genito-urinary, integumentary, metabolic/endocrine, cardiovascular, pulmonary, hepatic/renal, ocular and immune systems all manifest changes associated with opioid therapy. These adverse events, ranging from nuisance to therapy-limiting, are manageable when addressed quickly and appropriately. Opioids are safe and efficacious analgesics when these effects are considered.

Relative efficacy of some prokinetic drugs in morphine-induced gastrointestinal transit delay in mice

AIM: To study the relative efficacy of cisapride, metoclopramide, domperidone, erythromycin and mosapride on gastric emptying (GE) and small intestinal transit (SIT) in morphine treated mice.

METHODS: Phenol red marker meal was employed to estimate GE and SIT in Swiss albino mice of either sex. The groups included were control, morphine 1 mg/kg (s.c. 15 min before test meal) alone or with (45 min before test meal p.o.) cisapride 10 mg/kg, metoclopramide 20 mg/kg, domperidone 20 mg/kg, erythromycin 6 mg/kg and mosapride 20 mg/kg.

RESULTS: Cisapride, metoclopramide and mosapride were effective in enhancing gastric emptying significantly (P < 0.001) whereas other prokinetic agents failed to do so in normal mice. Metoclopramide completely reversed morphine induced delay in gastric emptying followed by mosapride. Metoclopramide alone was effective when given to normal mice in increasing the SIT. Cisapride, though it did not show any significant effect on SIT in normal mice, was able to reverse morphine induced delay in SIT significantly (P < 0.001) followed by metoclopramide and mosapride.

CONCLUSION: Metoclopramide and cisapride are most effective in reversing morphine-induced delay in gastric emptying and small intestinal transit in mice respectively.

Active bicarbonate-dependent secretion evoked by 5-hydroxytryptamine in porcine ileal mucosa is mediated by opioid-sensitive enteric neurons

5-Hydroxytryptamine (5-HT) mediates intestinal hypersecretion associated with infection and inflammation. We tested the hypothesis that 5-HT-induced anion secretion is mediated by an opioid-sensitive enteric neural circuit. 5-HT, at a contraluminal concentration of 10 microM, increased short-circuit current by 58 +/- 7 microA/cm(2) in sheets of porcine ileal mucosa with attached inner submucosal plexus. Responses to 5-HT were inhibited by saxitoxin or indomethacin, and reduced in tissues bathed in Cl(-)- or HCO(3)(-)-deficient media. 5-HT action was attenuated by saxitoxin in tissues bathed in Cl(-)-free media, but not HCO(3)-free media. The delta-opioid receptor agonist [D-Pen(2,5)]enkephalin (0.1 microM) blunted the 5-HT change in short-circuit current by a mechanism sensitive to the delta-opioid receptor antagonist naltrindole. The inhibitory actions of [D-Pen(2,5)]enkephalin and saxitoxin were not additive. These results suggest that 5-HT stimulates HCO(3)(-)-dependent ion transport through a mechanism involving prostanoids and an enteric neural pathway modulated by opioids.

Naloxone treatment for irritable bowel syndrome--a randomized controlled trial with an oral formulation

BACKGROUND

Opioids change gut motility and secretion, causing delayed intestinal transit and constipation. Endorphins play a role in the constipation troubling some patients with irritable bowel syndrome; hence naloxone, an opioid antagonist, may have a therapeutic role.

AIM

To assess the efficacy and safety of an oral formulation of naloxone in irritable bowel syndrome patients with constipation.

METHODS

A randomized, double-blind, placebo-controlled trial was performed. Patients fulfilling the Rome II criteria for irritable bowel syndrome (constipation-predominant and alternating types) were randomized to receive 8 weeks of treatment with naloxone capsules, 10 mg twice daily, or identical placebo.

RESULTS

Twenty-eight patients entered the study, which was completed by 25. 'Adequate symptomatic relief' was recorded in six of 14 on naloxone and three of 11 on placebo. Whilst the differences were not significant, improvements in severity gradings and mean symptom scores for pain, bloating, straining and urgency to defecate were greater with naloxone than placebo for all parameters. In addition, quality of life assessments improved to a greater extent in patients taking naloxone.

CONCLUSIONS

Preliminary results suggest that naloxone is well tolerated and beneficial in patients with irritable bowel syndrome and constipation. A larger clinical trial is needed to provide sufficient statistical power to assess efficacy.

Opioid antagonists: a review of their role in palliative care, focusing on use in opioid-related constipation

Opioid antagonists have well-established indications in the reversal of life-threatening opioid toxicity, but also hold considerable promise for other applications in palliative care practice, particularly management of opioid-related constipation. We briefly review current understanding of opioid receptors, focusing on their complex role in gastrointestinal physiology. We summarize the pharmacology, conventional indications, and clinical usage of three major groups of opioid antagonists, including a promising new peripherally acting agent, methylnaltrexone, which is not commercially available. We suggest an approach to administering opioid antagonists for reduction of life-threatening opioid toxicity in patients with pain. The literature on opioid-induced constipation and its treatment with opioid-antagonists is reviewed in detail. Finally, other potential uses of opioid antagonists in palliative care are described, especially strategies for reducing such opioid side effects as nausea and pruritus and for improving analgesia or reducing tolerance by concomitantly administrating both an opioid agonist and low dosages of an antagonist.

Predominance of delta-opioid-binding sites in the porcine enteric nervous system

The antidiarrheal and constipating actions of opioids are mediated in part by enteric neurons, which lie within the wall of the small intestine and colon, but the differential expression of specific, high-affinity opioid-binding sites in ganglionated plexuses within functionally distinct intestinal segments has not been examined. We determined the saturation binding characteristics under Na+-free conditions of the nonselective opioid receptor (OPR) ligand [3H][(5alpha,7alpha)-17-(cyclopropylmethyl)-4,5-epoxy-18,19-dihydro-3-hydroxy-6-methoxy-alpha, alpha-dimethyl-6,14-ethenomorphinan-7-methanol] (diprenorphine) and the respective delta-, kappa-, and mu-OPR ligands [3H]naltrindole, D-(5alpha,7alpha,8beta)-(-)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxoaspiro-(4,5)dec-8-yl]benzeneacetamide ([3H]U-69,593), and [3H][D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) in neuronal membranes isolated from myenteric and submucosal plexuses of porcine small intestine and colon. Naloxone-displaceable [3H]diprenorphine-binding sites (KD values ranging from 0.2-0.5 nM and Bmax = 50-95 fmol/mg of protein) were found in both subregions from all gut segments examined. High-affinity [3H]naltrindole sites (KD = 60-140 pmol) were at highest densities (approximately 60 fmol/mg of protein) in submucosal plexus of the ileum and distal colon myenteric plexus and were at lowest densities (8-9 fmol/mg of protein) in the submucosal plexuses of cecum and distal colon. [3H]U-69,593 sites (KD = 0.3-4 nM) were present only in the myenteric plexuses of all segments examined, with highest densities in cecum and proximal colon (44-47 fmol/mg of protein). [3H]DAMGO-binding sites were expressed at relatively low densities in the enteric plexuses of all gut regions. These results indicate that delta-OPRs predominate in the porcine enteric nervous system with a more circumscribed expression of kappa- and mu-OPRs.

K+ channel cAMP activated in guinea pig gallbladder epithelial cells

In guinea pig gallbladder epithelial cells, an increase in intracellular cAMP levels elicits the rise of anion channel activity. We investigated by patch-clamp techniques whether K(+) channels were also activated. In a cell-attached configuration and in the presence of theophylline and forskolin or 8-Br-cAMP in the cellular incubation bath, an increase of the open probability (P(o)) values for Ca(2+)-activated K(+) channels with a single-channel conductance of about 160 pS, for inward current, was observed. The increase in P(o) of these channels was also seen in an inside-out configuration and in the presence of PKA, ATP, and cAMP, but not with cAMP alone; phosphorylation did not influence single-channel conductance. In the inside-out configuration, the opioid loperamide (10(-5) M) was able to reduce P(o) when it was present either in the microelectrode filling solution or on the cytoplasmic side. Detection in the epithelial cells by RT-PCR of the mRNA corresponding to the alpha subunit of large-conductance Ca(2+)-activated K(+) channels (BK(Ca)) indicates that this gallbladder channel could belong to the BK family. Immunohistochemistry experiments confirm that these cells express the BK alpha subunit, which is located on the apical membrane. Other K(+) channels with lower conductance (40 pS) were not activated either by 8-Br-cAMP (cell-attached) or by PKA + ATP + cAMP (inside-out). These channels were insensitive to TEA(+) and loperamide. The data demonstrate that under conditions that induce secretion, phosphorylation activates anion channels as well as Ca(2+)-dependent, loperamide-sensitive K(+) channels present on the apical membrane.

Management of opioid-induced gastrointestinal effects in patients receiving palliative care

Opioid-induced gastrointestinal side effects, namely, nausea and constipation, are bothersome yet often easy to manage. Due to their widespread frequency, it is imperative that prophylactic and treatment modalities be understood. Although many pharmacotherapeutic agents are available with which to prevent or treat these side effects, few randomized, placebo-controlled studies have been conducted in terminally ill patients, thus limiting most treatment decisions to empiric therapies based on extrapolated data. A strong understanding of the pathophysiology of the sequelae is therefore paramount. Common agents administered for nausea are butyrophenones, phenothiazines, metoclopramide, and serotonin-receptor antagonists. Those given to manage constipation are stimulant laxatives and stool softeners, individually or in combination.

Involvement of mu- and kappa-, but not delta-, opioid receptors in the peristaltic motor depression caused by endogenous and exogenous opioids in the guinea-pig intestine

Opiates inhibit gastrointestinal propulsion, but it is not clear which opioid receptor types are involved in this action. For this reason, the effect of opioid receptor - selective agonists and antagonists on intestinal peristalsis was studied. Peristalsis in isolated segments of the guinea-pig small intestine was triggered by a rise of the intraluminal pressure and recorded via the intraluminal pressure changes associated with the peristaltic waves. Mu-opioid receptor agonists (DAMGO, morphine), kappa-opioid receptor agonists (ICI-204,448 and BRL-52,537) and a delta-opioid receptor agonist (SNC-80) inhibited peristalsis in a concentration-related manner as deduced from a rise of the peristaltic pressure threshold (PPT) and a diminution of peristaltic effectiveness. Experiments with the delta-opioid receptor antagonists naltrindole (30 nM) and HS-378 (1 microM), the kappa-opioid receptor antagonist nor-binaltorphimine (30 nM) and the mu-opioid receptor antagonist cyprodime (10 microM) revealed that the antiperistaltic effect of ICI-204,448 and BRL-52,537 was mediated by kappa-opioid receptors and that of morphine and DAMGO by mu-opioid receptors. In contrast, the peristaltic motor inhibition caused by SNC-80 was unrelated to delta-opioid receptor activation. Cyprodime and nor-binaltorphimine, but not naltrindole and HS-378, were per se able to stimulate intestinal peristalsis as deduced from a decrease in PPT. The results show that the neural circuits controlling peristalsis in the guinea-pig small intestine are inhibited by endogenous and exogenous opioids acting via mu- and kappa-, but not delta-, opioid receptors.

Low-dose oral naloxone reverses opioid-induced constipation and analgesia

The most common side effect of opioid therapy is constipation. It is often difficult to treat and is believed to be primarily a peripheral effect. Single large doses of oral naloxone have been shown to be efficacious in reversing opioid-induced constipation. However, they often cause the unwanted side effect of analgesia reversal. This study evaluated the effects on constipation and analgesia of low doses of oral naloxone given three times daily. Patients taking stable doses of opioids with complaints of constipation were recruited for this double-blind, randomized, placebo-controlled study. Patients were given 4 mg or 2 mg of oral naloxone, or placebo, three times daily. Stool frequency and symptoms related to constipation were recorded daily. Patients also recorded the daily amount of analgesics required to maintain pain control. Nine patients were recruited for the study. All the patients who received oral naloxone had some improvement in their bowel frequency. Three of the patients also experienced reversal of analgesia, including one who had complete reversal of analgesia. This study demonstrates that reversal of analgesia still occurred despite dividing the oral naloxone into very low doses relative to the total dose of opioid used. Patients using high doses of opioids appear to be the most vulnerable to the analgesic effect of oral naloxone.

Patient care after discharge from the ambulatory surgical center

An important and often forgotten aspect of postoperative care occurs after the patient is discharged from the ambulatory surgical center. With more than 60% of all surgeries and procedures occurring on an ambulatory basis, what happens after the patient is no longer in continuous professional care is of concern to the ambulatory nurse. Numerous physical postoperative complaints are common and expected sequelae of anesthesia and surgery in the ambulatory patient. In this article, important postdischarge complications are reviewed and contemporary management options discussed. The information contained in this review article is valuable to the provider in educating patients regarding their anticipated course of postoperative recovery.

OBJECTIVES

-Based on the content of this article, the reader should be able to (1) identify important postdischarge complications to provide patients with comprehensive discharge instructions regarding their continued recovery at home; (2) discuss contemporary management options available to treat postdischarge complications; (3) realize the incidence of specific postdischarge complications and how that relates to patient satisfaction with the surgical experience; (4) recognize signs and symptoms of postdischarge complications; and (5) identify risk factors of patients for developing specific complications in the postoperative phase.

Incidence, prevalence, and management of opioid bowel dysfunction

Opioid bowel dysfunction (OBD) is a common adverse effect associated with opioid therapy. OBD is commonly described as constipation; however, it is a constellation of adverse gastrointestinal (GI) effects, which also includes abdominal cramping, bloating, and gastroesophageal reflux. The mechanism for these effects is mediated primarily by stimulation of opioid receptors in the GI tract. In patients with pain, uncontrolled symptoms of OBD can add to their discomfort and may serve as a barrier to effective pain management, limiting therapy, or prompting discontinuation. Patients with cancer may have disease-related constipation, which is usually worsened by opioid therapy. However, OBD is not limited to cancer patients. A recent survey of patients taking opioid therapy for pain of noncancer origin found that approximately 40% of patients experienced constipation related to opioid therapy (<3 complete bowel movements per week) compared with 7.6% in a control group. Of subjects who required laxative therapy, only 46% of opioid-treated patients (control subjects, 84%) reported achieving the desired treatment results >50% of the time. Laxatives prescribed prophylactically and throughout opioid therapy may improve bowel movements in many patients. Nevertheless, a substantial number of patients will not obtain adequate relief of OBD because of its refractory nature. Naloxone and other tertiary opioid receptor antagonists effectively reduce the symptoms of constipation in opioid-treated patients. However, because they also act centrally, they may provoke opioid withdrawal symptoms or reverse analgesia in some patients. There are 2 peripherally selective opioid receptor antagonists, methylnaltrexone and ADL 8-2698 (Adolor Corporation, Exton, PA, USA), that are currently under investigation for their use in treating OBD. Early studies confirm that they are effective at normalizing bowel function in opioid-treated patients without entering the central nervous system and affecting analgesia. With a better understanding of the prevalence of OBD and its pathophysiology, a more aggressive approach to preventing and treating OBD is possible and will likely improve the quality of life of patients with pain.

Alvimopan* (ADL 8-2698) is a novel peripheral opioid antagonist

Alvimopan (ADL 8-2698; Adolor Corporation, Exton, PA, USA) is a novel, peripherally restricted opioid antagonist. After oral administration, it has activity specific to the gastrointestinal (GI) tract. ADL 8-2698 has low systemic absorption and a high affinity for mu-opioid receptors. In healthy subjects, ADL 8-2698 antagonized loperamide-induced changes in GI transit and prevented morphine-induced delays in oral-cecal transit time without antagonizing centrally mediated opioid effects, such as analgesia or pupillary constriction. In the treatment of opioid naive patients who underwent surgery and received opioids for acute pain, oral ADL 8-2698 (6.0 mg) improved the management of postoperative ileus (POI) by shortening the time to achieve normal bowel function and, ultimately, hospital stay. Postoperative nausea and vomiting and the overall incidence of all GI side effects were reduced in patients treated with ADL 8-2698 for POI. Analgesia was not compromised, because there were no changes in median opioid consumption or Visual Analog Scale (VAS) pain scores in patients treated with ADL 8-2698 versus patients treated with placebo. No drug-related side effects were observed in acute pain postsurgical patients in the initial POI study. In patients treated with opioids for chronic pain or opioid addiction, lower doses of oral ADL 8-2698 (0.5 to 3.0 mg) reversed opioid bowel dysfunction (OBD) and normalized GI activity. These effects were evident without compromising opioid analgesia or inducing central nervous system symptoms of withdrawal. Some chronic opioid patients receiving apparently supramaximal doses of ADL 8-2698 (> or = 3.0 mg) reported localized GI side effects, possibly indicative of a localized GI withdrawal response. The most common side effects of ADL 8-2698 in chronic pain patients with OBD were abdominal pain, flatulence, and diarrhea. These effects were not observed in most OBD patients receiving lower doses of ADL 8-2698. Overall, ADL 8-2698 was well tolerated in clinical trials. Further studies to evaluate the efficacy and safety of ADL 8-2698 in clinical practice are in progress.

Opioid, cannabinoid and vanilloid receptor localization on porcine cultured myenteric neurons

Opioids and cannabinoids have profound inhibitory actions on intestinal motility which are mediated in part by their cognate receptors in the enteric nervous system. In the present study, we examined the expression of immunoreactivity for delta- and kappa-opioid receptors, CB(1)-cannabinoid receptors and type 1 vanilloid receptors by immunocytochemistry and confocal laser scanning microscopy on ileal myenteric neurons, isolated from juvenile pigs, that were <70 microm diameter in either axis and maintained for 1-2 weeks in primary culture. Immunoreactivities for delta-opioid and cannabinoid receptors were present in neurons immunoreactive for the cholinergic marker, choline acetyltransferase. Some neurons with delta-opioid receptor-like immunoreactivity were also immunoreactive for kappa-opioid, cannabinoid or vanilloid receptors. These observations indicate that receptors for cannabinoids or vanilloids are co-localized in opioid receptor-expressing myenteric neurons which modulate intestinal sensorimotor function.

Effect of an enteric-release formulation of naloxone on intestinal transit in volunteers taking codeine

INTRODUCTION

Constipation is a common side-effect of opioid therapy; in addition to their analgesic effect, opioids reduce intestinal secretion and motility with an increase in whole-gut transit time. Naloxone, a specific opioid antagonist, reverses these effects but may also cause symptoms of opioid withdrawal in patients on long-term therapy.

AIM

To use an enteric-release formulation, designed to produce a topical effect in the gut, with minimum systemic effects.

METHODS

Naloxone 10 mg b.d. and codeine 30 mg b.d. were used with identical placebo capsules in four sets of studies; 12 male volunteers were given the drugs alone and in combination, with a control study involving double placebo, during each of four study periods. Whole-gut transit time was calculated and compared for each treatment period.

RESULTS

Naloxone, both alone and with codeine, significantly shortened the mean whole-gut transit time compared with the control period, respectively, from 53.1 to 42.1 h (P=0.005) and to 40.7 h (P=0.024). Urgency to defecate was reported by two volunteers on naloxone alone and by three on combination therapy.

CONCLUSIONS

The results show that the naloxone formulation counteracts the effect of codeine on intestinal transit, suggesting that it may have useful clinical applications.

Orphanin FQ/nociceptin: a novel neuromodulator of gastrointestinal function?

Orphanin FQ/nociceptin (OFQ/N) has been immunohistochemically localized in the rat enteric nervous system, and mRNA signals for its precursor (i.e. prepro-OFQ/-N) and cognate receptor ORL-1 are expressed in the intestinal tract. OFQ/N inhibits neurogenic contractions in a variety of stomach and small intestine preparations in vitro, but contracts rodent colon. In vivo, it acts at peripheral and central nervous system sites to stimulate or inhibit mechanical activity in the stomach and colon, respectively. Thus, OFQ/N acts as a neuromodulator of gastrointestinal motility and may have additional roles in regulation of intestinal blood flow, active ion transport, and immunity.

Opioid Type and Other Clinical Predictors of Laxative Dose in Advanced Cancer Patients: A Retrospective Study

BACKGROUND

Constipation is a frequent and underdiagnosed complication in patients with advanced cancer. Constipation in this population is multifactorial, but the use of opioids is one of the main causes. The purpose of this retrospective study was to establish the association between opioid type and laxative dose, as well as the contribution of other clinical factors in advanced cancer patients admitted to a palliative care unit.

METHODS

The records of consecutive patients admitted to the Acute Palliative Care Unit at the Grey Nuns Hospital between December 1995 and January 1997 were reviewed. Criteria of eligibility were the presence of cancer pain treated by opioids (oral and subcutaneous morphine and hydromorphone, oral methadone), oral laxative treatment capable of achieving at least one bowel movement every 3 days, and the absence of bowel obstruction or colostomy. During period(s) of stable analgesic doses, the charts were reviewed for demographic and clinical characteristics, average number of bowel movements, daily laxative doses, doses and type of opioid, laxative/opioid dose ratio (LOR) (calculated by dividing the total laxative dose by the total opioid dose), functional and cognitive status, food intake, and level of calcium, albumin, and potassium.

RESULTS

Forty-nine evaluable patients were identified. The LOR in patients receiving oral opioids was 0.15 +/- 0.19 vs. 0.18 +/- 0.17 in patients on parenteral opioids (p > 0.2). The LOR in patients receiving methadone was 0.025 +/- 0.027 as compared to 0.24 +/- 0.23 in patients receiving morphine and 0.17 +/- 0.13 in patients on hydromorphone (p < 0.0001). We found a strong association between LOR and abdominal involvement (p < 0.0006), opioid type (p < 0.0001), age (p < 0.0001), and female gender (p < 0.034). There were no significant correlation between LOR and functional status, cognitive status, food intake, and level of calcium or potassium.

CONCLUSION

We conclude that laxative dose needs to be titrated on an individualized basis. The LOR is lower in patients receiving methadone and in those of male gender, younger age, and absence of abdominal involvement.

A qualitative and quantitative study on the enkephalinergic innervation of the pig gastrointestinal tract

Enkephalins are involved in neural control of digestive functions such as motility, secretion, and absorption. To better understand their role in pigs, we analyzed the qualitative and quantitative distribution of enkephalin immunoreactivity (ENK-IR) in components of the intestinal wall from the esophagus to the anal sphincter. Immunohistochemical labelings were analyzed using conventional fluorescence and confocal microscopy. ENK-IR was compared with the synaptophysin immunoreactivity (SYN-IR). The results show that maximal ENK-IR levels in the entire digestive tract are reached in the myenteric plexuses and, to a lesser extent, in the external submucous plexus and the circular muscle layer. In the longitudinal muscle layer, ENK-IR was present in the esophagus, stomach, rectum, and anal sphincter, whereas it was absent from the duodenum to the distal colon. In the ENK-IR plexuses and muscle layers, more than 60% of the nerve fibers identified by SYN-IR expressed ENK-IR. No ENK-IR was observed in the internal submucous plexus and the mucosa; the latter was found to contain ENK-IR endocrine cells. These results strongly suggest that, in pigs, enkephalins play a major role in the regulatory mechanisms that underlie the neural control of digestive motility.

Effects of mu-opioid receptor agonists on intestinal secretion and permeability during acute intestinal inflammation in mice

We evaluated and compared the effects of mu-opioid receptor agonists on mucosal fluid transport and permeability, during acute intestinal inflammation. We hypothesized that inflammation would sensitize mu-opioid receptors in the submucosal plexus and/or enterocytes enhancing the effects of mu-opioid receptor agonists. Inflammation was induced by intragastric administration of croton oil, whereas controls received saline. Fluid transport was assessed by enteropooling, and intestinal permeability by blood-to-lumen passage of [51Cr] etylenediaminetetraacetate ([51Cr] EDTA). Intestinal inflammation induced a significant increase in enteropooling (1.9 times) and permeability (2.5 times). In saline- and croton oil-treated animals, mu-opioid receptor agonists produced dose-related inhibitions of enteropooling and intestinal permeability. During inflammation, the potency of morphine increased 4.8 and 3.7 times, inhibiting enteropooling and intestinal permeability, respectively; the potencies of fentanyl and PL017 similarly increased by approximately three (enteropooling) and two times (permeability) in croton oil animals. All effects were reversed by naloxone and naloxone methiodide. The results show that inflammation increases the inhibitory potency of mu-opioid receptor agonists on secretion and permeability, suggesting a sensitization of peripheral mu-opioid receptors.