Opioid mu and kappa receptors on axons of cholinergic excitatory motor neurons supplying the circular muscle of guinea-pig ileum

The effects of loperamide on excitatory and inhibitory neuromuscular function in the human colon

BACKGROUND

In most animal species, opioids alter colonic motility via the inhibition of excitatory enteric motor neurons. The mechanisms by which opioids alter human colonic motility are unclear. The aim of this study was to describe the effects of loperamide on neuromuscular function in the human colon.

METHODS

Tissue specimens of human colon from 10 patients undergoing an anterior resection were divided into three inter-taenial circular muscle strips. Separate organ baths were used to assess: (1) excitatory transmission (selective blockade of inhibitory transmission: L-NOARG/MRS2179); (2) inhibitory transmission (selective blockade of excitatory transmission: hyoscine hydrobromide); and (3) a control bath (no drug additions). Neuromuscular function was assessed using force transducer recordings and electrical field stimulation (EFS; 20 V, 10 Hz, 0.5 ms, 10 s) prior to and following loperamide and naloxone.

KEY RESULTS

In human preparations with L-NOARG/MRS2179, loperamide had no significant effects on isometric contractions. In preparations with hyoscine hydrobromide, loperamide reduced isometric relaxation during EFS (median difference + 0.60 g post-loperamide, Z = -2.35, p = 0.019).

CONCLUSIONS AND INFERENCES

Loperamide had no effect on excitatory neuromuscular function in human colonic circular muscle. These findings suggest that loperamide alters colonic function by acting primarily on inhibitory motor neurons, premotor enteric neurons, or via alternative non-opioid receptor pathways.

The NOP receptor involvement in both withdrawal- and CCk-8-induced contracture responses of guinea pig isolated ileum after acute activation of κ-opioid receptor

In isolated guinea-pig ileum (GPI), the κ-opioid acute withdrawal response is under the control of several neuronal signaling systems, including the μ-opioid, the A(1)-adenosine and the CB(1) receptors, which are involved in the inhibitory control of the κ-withdrawal response. After κ-opioid system stimulation, indirect activation of μ-opioid, A(1)-adenosine and CB(1) systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated whether the NOP system is also involved in the regulation of the acute κ-withdrawal response. Interestingly, we found that in GPI preparation, the NOP system is not indirectly activated by the κ-opioid receptor stimulation, but instead this system is able by itself to directly regulate the acute κ-withdrawal response. Specifically, our results clearly highlight first the existence of an endogenous tone of the NOP system in GPI, and second that it behaves as a functional anti-opioid system. We also found that, the NOP receptor system is involved in the regulation of the CCk-8-induced contracture intensity, only when in the presence of the κ-opioid receptor stimulation. This effect seems to be regulated by an activation threshold mechanism. In conclusion, the NOP system could act as neuromodulatory system, whose action is strictly related to the modulation of both excitatory and inhibitory neurotransmitters released in GPI enteric nervous system.

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.

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.

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.

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.

Prejunctional M1 and postjunctional M3 muscarinic receptors in the circular muscle of the guinea-pig ileum

The effects of subtype-selective muscarinic receptor antagonists on electrically evoked release of acetylcholine and muscle contraction were compared in circular muscle preparations of the guinea-pig ileum. Incubation of the preparation with [3H]choline resulted in the formation of [3H]acetylcholine. Electrical stimulation caused the release of [3H]acetylcholine which was abolished by tetrodotoxin and omission of calcium from the medium. 5-Hydroxytryptamine (10 microM) and the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (300 microM) did not change acetylcholine release. The muscarinic antagonists pirenzepine (M1 selective), AF-DX 116 (M2 selective) and hexahydrosiladifenidol (M3 selective) caused concentration-dependent increases in the evoked release of acetylcholine, and inhibitions of the circular muscle contraction. The postjunctional affinity constants (pA2 values) obtained for hexahydrosiladifenidol (8.06), pirenzepine (6.95) and AF-DX 116 (6.60) identified the muscular receptor as an M3 subtype. Pirenzepine was more potent in facilitating the evoked release than hexahydrosiladifenidol and AF-DX 116. These findings suggest that the release of acetylcholine in the circular muscle is inhibited by M1 muscarinic autoreceptors whereas muscle contraction is mediated by M3 receptors.

The kappa-opioid receptor agonist U50,488H induces acute physical dependence in guinea-pigs

The present study was undertaken to determine whether acute physical dependence occurred in guinea-pigs in vivo and guinea-pig isolated ileum following a single dose of the kappa-opioid receptor agonist U50,488H. Administration of naloxone hydrochloride, 15 and 30 mg/kg s.c., to guinea-pigs treated 1 h before with U50,488H, 10 mg/kg s.c., induced increased locomotor activity accompanied by behavioural responses which differed from those previously found in this species with morphine withdrawal. Nor-binaltorphimine, 10 mg/kg s.c., given 1 h after administration of U50,488H, 10 mg/kg s.c., produced a small but significant increase in locomotor activity but no other withdrawal behaviours. The morphine withdrawal response was not significantly affected by U50,488H, 1 or 10 mg/kg s.c. On the guinea-pig isolated ileum, nor-binaltorphimine, 1 microM, produced a withdrawal contracture following 2 min contact of the ileum with U50,488H 1 microM. U50,488H, 1 microM, abolished the [Met5]enkephalin withdrawal response of the ileum. It is concluded that dependence occurs following activation of kappa-opioid receptors, which is largely non-morphine-like in the central nervous system, but which is morphine-like in the enteric nervous system.

Functional relevance of presynaptic muscarinic autoreceptors

Pre- and postsynaptic muscarinic receptors have been characterized in the isolated trachea and ileal circular muscle of the guinea pig. The muscarinic autoreceptors mediating inhibition of acetylcholine release in the circular muscle belong to the M1 subtype, whereas those inhibiting acetylcholine release in the trachea are M2 or M4 receptors. In both tissues the postsynaptic muscarinic receptors are M3 receptors. Blockade of the autoreceptors by selective M1 and M2/M4 receptor antagonists leads to facilitation of cholinergic neurotransmission.

Modulation of peristalsis in the guinea-pig isolated small intestine by exogenous and endogenous opioids

1. A recording method was developed to measure physiological parameters of the preparatory and emptying phases of peristalsis in vitro. This method enabled measurement of: the compliance of the intestinal wall during the preparatory phase (a reflection of the resistance of the wall to distension); longitudinal muscle contraction during the preparatory phase; the threshold volume required to trigger the emptying phase; the maximal ejection pressure and the average power generated during the emptying phase, which reflects the rate at which the intestine performs work. Modulation of these parameters by exogenous and endogenous opioids acting at mu, kappa and delta opioid receptors was investigated. 2. The compliance of the intestinal wall during the preparatory phase was reduced by the mu opioid receptor agonist, [D-Ala2, N-methyl-Phe4, Gly5-ol] enkephalin (DAMGO) but not by the kappa agonist, dynorphin, or the delta agonist, [D-penicillamine2, D-penicillamine5] enkephalin (DPDPE). Reflex contraction of the longitudinal muscle during the preparatory phase was inhibited by DAMGO, dynorphin and DPDPE. The threshold volume required to trigger the emptying phase of peristalsis was increased by DAMGO, dynorphin and DPDPE. 3. The maximal ejection pressure generated during the emptying phase was reduced by dynorphin and DPDPE, but not by DAMGO. The average power generated by the intestine when emptying was not altered by any of the agonists. 4. Electrically stimulated contractions of longitudinal muscle in strips of longitudinal muscle-myenteric plexus were not inhibited by DPDPE. Similarly, DPDPE did not significantly inhibit electrically induced contraction of circular muscle in strips of circular muscle-myenteric plexus.5. Each of the agonist effects on peristaltic parameters was antagonized by the appropriate antagonist:D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) (mu), norbinaltorphimine (nor-BNI) (kappa), naltrindole(delta).6. It is concluded that mu and kappa agonists act primarily on excitatory circular and longitudinal muscle motor neurones. The delta agonist probably acts on enteric neurones presynaptic to excitatory circular and longitudinal muscle motor neurones.7. Antagonists for mu, delta and kappa receptors did not affect any parameters of peristalsis when the intestine emptied against a low resistance. However, when emptying against a high outflow resistance, the average power generated by the intestine was increased by the kappa antagonist, nor-BNI, but not by CTOP or naltrindole.8. It is concluded that endogenous opioids appear to have little role in peristalsis when the intestine is working against a low outflow resistance. However endogenous opioids, acting primarily at kappa receptors,provide a braking mechanism by inhibiting the emptying phase of peristalsis in conditions in which the intestine empties against a higher resistance.

Sites of action of morphine on the ascending excitatory reflex in the guinea-pig small intestine

The effect of morphine on the ascending excitatory reflex of the circular muscle elicited by radial distension of the gut wall was studied in the isolated guinea-pig small intestine. A three compartment bath, in which an intermediate compartment divided the site of intraluminal stimulation (caudal compartment) from the site of reflex contraction recording (oral compartment), was used. Morphine (0.01-10 microM) applied independently to each compartment, caused a concentration-dependent depression (up to 90%) of the amplitude of distension-evoked reflex contractions. Concentration-response curves to morphine were shifted to the right by naloxone (30 nM) with an apparent pA2 value of about 8.5, which suggests an interaction with opioid mu-receptor subtypes. Our results indicate that morphine not only depressed transmission from excitatory motor neurons to the circular muscle but also neuro-neuronal transmission along the ascending excitatory reflex pathway.

Reflex peristalsis in the guinea pig isolated ileum is endogenously controlled by kappa opioid receptors

(1) Reflex peristalsis in the circular muscle of the guinea pig ileum was elicited in vitro by sustained luminal distension of the intestinal wall according to 2 cm H2O and evaluated in terms of the number of peristaltic waves within 15 min intervals. (2) The poorly mu-selective opioid antagonist naloxone at concentrations of 10(-7) and 10(-6) mol/l increased the frequency of peristaltic contractions within the first 15 min interval, and thereafter in a declining fashion, by 68 and 88%, respectively. The highly kappa-selective opioid antagonist nor-binaltorphimine behaved similarly. It was, by one order of magnitude, more potent but a little less effective than naloxone, i.e., the maximum effect was 57% increase in peristaltic frequency at 10(-8) mol/l. Concentrations of 10(-7) and 10(-6) mol/l had the same effect as 10(-8) mol/l, and 10(-9) mol/l were ineffective. The highly mu-selective antagonist CTOP-NH2 and the highly delta-selective antagonist ICI 174,864 were ineffective up to 10(-6) mol/l. (3) It is concluded that predominantly kappa opioid receptors are used by endogenous opioids under the present conditions to inhibit reflex peristalsis.