Tonic modulation of neurotransmitter release in the guinea-pig myenteric plexus: effect of mu and kappa opioid receptor blockade and of chronic sympathetic denervation

Human native kappa opioid receptor functions not predicted by recombinant receptors: Implications for drug design

If activation of recombinant G protein-coupled receptors in host cells (by drugs or other ligands) has predictive value, similar data must be obtained with native receptors naturally expressed in tissues. Using mouse and human recombinant κ opioid receptors transfected into a host cell, two selectively-acting compounds (ICI204448, asimadoline) equi-effectively activated both receptors, assessed by measuring two different cell signalling pathways which were equally affected without evidence of bias. In mouse intestine, naturally expressing κ receptors within its nervous system, both compounds also equi-effectively activated the receptor, inhibiting nerve-mediated muscle contraction. However, whereas ICI204448 acted similarly in human intestine, where κ receptors are again expressed within its nervous system, asimadoline was inhibitory only at very high concentrations; instead, low concentrations of asimadoline reduced the activity of ICI204448. This demonstration of species-dependence in activation of native, not recombinant κ receptors may be explained by different mouse/human receptor structures affecting receptor expression and/or interactions with intracellular signalling pathways in native environments, to reveal differences in intrinsic efficacy between receptor agonists. These results have profound implications in drug design for κ and perhaps other receptors, in terms of recombinant-to-native receptor translation, species-dependency and possibly, a need to use human, therapeutically-relevant, not surrogate tissues.

Kynurenines and intestinal neurotransmission: the role of N-methyl-D-aspartate receptors

Gastrointestinal neuroprotection involves the net effect of many mechanisms which protect the enteral nervous system and its cells from death, dysfunction or degeneration. Neuroprotection is also a therapeutic strategy, aimed at slowing or halting the progression of primary neuronal loss following acute or chronic diseases. The neuroprotective properties of a compound clearly have implications for an understanding of the mechanism of dysfunctions and for therapeutic approaches in a number of gastrointestinal diseases.This paper focused on the roles of glutamate and N-methyl-D-aspartate (NMDA) receptors in the intrinsic neuronal control of gastrointestinal motility; the consequences of inflammation on gastrointestinal motility changes; and the involvement of tryptophan metabolites (especially kynurenic acid) in the regulatory function of the enteral nervous system and the modulation of the inflammatory response. Common features in the mechanisms of action, illustrative evidence from animal models, and experimental neuroprotective therapies making use of the currently available possibilities are also discussed.Overall, the evidence suggests that gastrointestinal neuroprotection against inflammation and glutamate-induced neurotoxicity may be mediated synergistically through the blockade of NMDA receptors and the inhibition of neuronal nitric oxide synthase activity and xanthine oxidoreductase-dependent superoxide production. These components are likewise significant factors in the pathomechanism of gastrointestinal inflammatory diseases and inflammation-linked motility alterations. Inhibition of the enteric NMDA receptors by kynurenic acid or its analogues may provide a novel option via which to influence intestinal hypermotility and inflammatory processes simultaneously.

Presynaptic neuropeptide receptors

Presynaptic receptors for four families of neuropeptides will be discussed: opioids, neuropeptide Y, adrenocorticotropic hormone (ACTH), and orexins. Presynaptic receptors for the opioids (micro, delta, kappa, and ORL(1)) and neuropeptide Y (Y(2)) inhibit transmitter release from a variety of neurones, both in the peripheral and central nervous systems. These receptors, which were also identified in human tissue, are coupled to G(i/o) proteins and block voltage-dependent Ca(2+) channels, activate voltage-dependent K(+) channels, and/or interfere with the vesicle release machinery. Presynaptic receptors for ACTH (MC(2) receptors) have so far been identified almost exclusively in cardiovascular tissues from rabbits, where they facilitate noradrenaline release; they are coupled to G(s) protein and act via stimulation of adenylyl cyclase. Presynaptic receptors for orexins (most probably OX(2) receptors) have so far almost exclusively been identified in the rat and mouse brain, where they facilitate the release of glutamate and gamma-aminobutyric acid (GABA); they are most probably linked to G(q) and directly activate the vesicle release machinery or act via a transduction mechanism upstream of the release process. Agonists and antagonists at opioid receptors owe at least part of their therapeutic effects to actions on presynaptic receptors. Therapeutic drugs targeting neuropeptide Y and orexin receptors and presynaptic ACTH receptors so far are not available.

Treatment of opioid-induced gut dysfunction

Opioid analgesics are the mainstay in the treatment of moderate-to-severe pain, yet their use is frequently associated with adverse effects, the most common and debilitating being constipation. Opioid-induced motor stasis results from blockade of gastrointestinal peristalsis and fluid secretion, and reflects the action of the endogenous opioid system in the gut. Methylnaltrexone and alvimopan are new investigational drugs that selectively target peripheral mu-opioid receptors because they are poorly absorbed in the intestine and do not enter the brain. Clinical studies have proved the concept that these drugs prevent opioid-induced bowel dysfunction without interfering with analgesia. As reviewed in this article, opioid receptor antagonists with a peripherally restricted site of action also hold therapeutic promise in postoperative ileus and chronic constipation due to the fact that they have been found to stimulate intestinal transit.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

N-methyl-D-aspartate receptors mediate endogenous opioid release in enteric neurons after abdominal surgery

BACKGROUND & AIMS

We tested the hypothesis that N-methyl-D-aspartate (NMDA) receptors mediate surgery-induced opioid release in enteric neurons.

METHODS

We used mu opioid receptor (muOR) internalization as a measure of opioid release with immunohistochemistry and confocal microscopy. MuOR internalization was quantified in enteric neurons from nondenervated and denervated ileal segments of guinea pig after abdominal laparotomy with and without pretreatment with NMDA-receptor antagonists acting at different recognition sites (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,b] cyclohepten-5,10-imine (MK-801) or (D) 2-amino-5-phosphopenoic acid (AP-5) at .5, 1 mg/kg; 8-chloro-4-hydroxy-1-oxo-1,2-dihydropyridazinol [4,5-]quinoline-5-oxide choline (MRZ 2/576) or 8-chloro-1,4-dioxo-1,2,3,4-tetrahydropyridazinol [4,5-]quinoline choline salt (MRZ 2/596) at .3, 1 mg/kg, or with an antagonist for the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (1, 3 mg/kg). To determine whether NMDA stimulation induces opioid release, (1) ilea were exposed to NMDA (100 micromol/L) and D-serine (10 micromol/L) with or without the antagonist MK-801 or AP-5 (50 micromol/L); and (2) neuromuscular preparations of the ileum were stimulated electrically (20 Hz, 20 min) with or without MK-801 or AP-5 (50 micromol/L).

RESULTS

MuOR endocytosis induced by abdominal laparotomy was inhibited significantly by NMDA-receptor antagonists in nondenervated and denervated ileal segments, but not by the AMPA-receptor antagonist. MuOR endocytosis in neurons exposed to NMDA or electrical stimulation was prevented by NMDA-R antagonists.

CONCLUSIONS

Abdominal laparotomy evokes local release of glutamate that results in endogenous opioid release through the activation of peripheral NMDA receptors. This suggests an interaction between the glutamatergic and opioid systems in response to the noxious and perhaps mechanosensory stimulation of surgery.

The role of endogenous opioids in the control of gastrointestinal motility: predictions from in vitro modelling

Gastrointestinal motility can be assessed in vitro by investigating the effects of drugs or gene knockouts on intestinal propulsion, and on neurone-mediated responses evoked by electrical field stimulation (EFS). The latter predominantly measure enteric motor activity and can detect prokinetic activity of exogenous agents. Some evidence suggests that naloxone has prokinetic activity when evaluated for an ability to modulate responses to EFS, but the effects are inconsistent across different species or intestinal regions. Models of intestinal peristalsis measure an integrated sensory-motor nerve function and possess more intact neuro-neuronal connections. In such preparations, the effects of naloxone also suggest a prokinetic property but again, this is inconsistent. By contrast, consistent prokinetic activity of naloxone is apparent in models where peristalsis is compromised by drug-induced suppression of motor nerve activity or by modulation of endogenous processes using receptor antagonists or inappropriate intraluminal distension. These data suggest that endogenous opioids play little or no role in normal intestinal physiology, but suppress intestinal motility when motor function is compromised. Consequently, drugs that antagonize opioid receptors may exert prokinetic activity in conditions where intestinal motility is reduced, such as constipation. Further work is required to elucidate the opiate receptor(s) involved.

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.

Sympathetic denervation-induced changes in G protein expression in enteric neurons of the guinea pig colon

Chronic sympathetic denervation entails subsensitivity to alpha(2)-adrenoceptor agonists and supersensitivity to kappa- and mu-opioid receptor agonists modulating cholinergic neurons in the guinea pig colon. A possible role for signal transduction G proteins in contributing to development of these sensitivity changes was investigated. Pertussis toxin (PTX), a blocker of the G(i/o)-type family of G proteins significantly reduced the inhibitory effects of UK14,304 (alpha(2)-adrenoceptor agonist), U69593 (kappa-opioid receptor agonist) and DAMGO (mu-opioid receptor agonist) on acetylcholine (ACh) overflow in preparations obtained from normal animals, but not in those obtained from sympathetically denervated animals. In this experimental condition, immunoblot analysis revealed reduced levels of G(alphao), G(alphai2), G(alphai3) and G(beta) in myenteric plexus synaptosomes. On reverse, synaptosomal levels of G(alphai1) and G(alphaz), a PTX-insensitive G-protein, increased after chronic ablation of the sympathetic pathways. These data suggest that changes in the function and expression of inhibitory G proteins coupled to alpha(2)-adrenoceptors, kappa- and mu-opioid receptors occur in the myenteric plexus of the guinea pig colon after chronic sympathetic denervation. The possibility that regulation of G proteins represents one of the biochemical mechanisms at the basis of the changes in sensitivity of enteric cholinergic neurons to alpha(2)-adrenoceptor, kappa- and mu-opioid receptor agonists is discussed.

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.

Plasticity in the enteric nervous system

Enteric ganglia can maintain integrated functions, such as the peristaltic reflex, in the absence of input from the central nervous system, which has a modulatory role. Several clinical and experimental observations suggest that homeostatic control of gut function in a changing environment may be achieved through adaptive changes occurring in the enteric ganglia. A distinctive feature of enteric ganglia, which may be crucial during the development of adaptive responses, is the vicinity of the final effector cells, which are an important source of mediators regulating cell growth. The aim of this review is to focus on the possible mechanisms underlying neuronal plasticity in the enteric nervous system and to consider approaches to the study of plasticity in this model. These include investigations of neuronal connectivity during development, adaptive mechanisms that maintain function after suppression of a specific neural input, and the possible occurrence of activity-dependent modifications of synaptic efficacy, which are thought to be important in storage of information in the brain. One of the applied aspects of the study of plasticity in the enteric nervous system is that knowledge of the underlying mechanisms may eventually enable us to develop strategies to correct neuronal alterations described in several diseases.

Modulation of enteric cholinergic neurons by hetero- and autoreceptors: cooperation among inhibitory inputs

In the guinea-pig colon, acetylcholine (ACh) release from intrinsic cholinergic motor neurons is inhibited by adrenoceptors, opioid and muscarinic receptors. Chronic sympathetic denervation resulted in supersensitivity to the inhibitory effect of DAMGO (mu-opioid agonist) on ACh release and on the peristaltic reflex. After chronic treatment with naltrexone (NTX) supersensitivity to DAMGO and subsensitivity to UK14,304 (alpha2-adrenoceptor agonist) developed for both functional parameters. The facilitatory effect of scopolamine on ACh release remained unchanged after chronic NTX treatment, whereas it was potentiated after chronic sympathetic denervation. These data suggest the existence of a functional interaction between different inhibitory pathways modulating cholinergic motor neurons in the guinea-pig colon. Namely, chronic manipulation of an inhibitory pathway may entail adaptive sensitivity changes in another inhibitory pathway so that homeostasis can be maintained.

Modulation of neurotransmitter release by opioid mu- and kappa-receptors from adrenergic terminals in the myenteric plexus of the guinea-pig colon: effect of alpha 2-autoreceptor blockade

We have studied the effect of [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO, opioid mu-receptor agonist) and ICI-204,448 (kappa-receptor agonist) on endogenous noradrenaline release in the guinea-pig isolated distal colon. DAMGO enhances noradrenaline over-flow and this effect is antagonized by naloxone (pIC50 = 10.27) and nor-binaltorphimine (pIC50 = 7.97), and concentration-dependently turned into inhibition by yohimbine. ICI-204,448 inhibits noradrenaline overflow and is antagonized by naloxone (pIC50 = 9.38) and nor-binaltorphimine (pIC50 = 10.48), but is not affected by yohimbine. Evidence is thus given that mu- and kappa-opioid receptors modulate noradrenaline release in the guinea-pig colon. Modifications by yohimbine of the effect of DAMGO indicate the existence of a functional relationship between mu-receptors and alpha(2)-autoreceptors in this model.