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

A search for presynaptic inhibitory histamine receptors in guinea-pig tissues: Further H3 receptors but no evidence for H4 receptors

The histamine H4 receptor is coupled to Gi/o proteins and expressed on inflammatory cells and lymphoid tissues; it was suggested that this receptor also occurs in the brain or on peripheral neurones. Since many Gi/o protein-coupled receptors, including the H3 receptor, serve as presynaptic inhibitory receptors, we studied whether the sympathetic neurones supplying four peripheral tissues and the cholinergic neurones in the hippocampus from the guinea-pig are equipped with release-modulating H4 and H3 receptors. For this purpose, we preincubated tissue pieces from the aorta, atrium, renal cortex and vas deferens with (3)H-noradrenaline and hippocampal slices with (3)H-choline and determined the electrically evoked tritium overflow. The stimulation-evoked overflow in the five superfused tissues was inhibited by the muscarinic receptor agonist oxotremorine, which served as a positive control, but not affected by the H4 receptor agonist 4-methylhistamine. The H3 receptor agonist R-α-methylhistamine inhibited noradrenaline release in the peripheral tissues without affecting acetylcholine release in the hippocampal slices. Thioperamide shifted the concentration-response curve of histamine in the aorta and the renal cortex to the right, yielding apparent pA2 values of 8.0 and 8.1, respectively, which are close to its affinity at other H3 receptors but higher by one log unit than its pKi at the H4 receptor of the guinea-pig. In conclusion, histamine H4 receptors could not be identified in five experimental models of the guinea-pig that are suited for the detection of presynaptic inhibitory receptors whereas H3 receptors could be shown in the peripheral tissues but not in the hippocampus. This article is part of the Special Issue entitled 'Histamine Receptors'.

Cannabinoid CB1 receptor activation, pharmacological blockade, or genetic ablation affects the function of the muscarinic auto- and heteroreceptor

Different types of presynaptic inhibitory Gα(i/o) protein-coupled receptors usually do not act independently of each other but rather pre-activation of receptor X impairs the effect mediated via receptor Y. It is, however, unknown whether this interaction extends to the cannabinoid CB(1) receptor on cholinergic neurones and hence we studied whether its activation, pharmacological blockade, or genetic inactivation affects the function of other presynaptic inhibitory receptors. The electrically evoked acetylcholine or noradrenaline release was determined in superfused rodent tissues preincubated with (3)H-choline or (3)H-noradrenaline. The muscarinic M(2) receptor, Gα(i), and Gα(o) proteins were determined in hippocampal synaptosomes by Western blotting. Hippocampal anandamide and 2-arachidonoyl glycerol levels were determined by LC-MS/MS. The inhibitory effect of the muscarinic receptor agonist oxotremorine on acetylcholine release in hippocampal slices was increased by genetic CB(1) receptor ablation (mouse) and the CB(1) antagonist rimonabant (rat but not mouse) and decreased by a cannabinoid receptor agonist (mouse). In mouse tissues, CB(1) receptor ablation also increased the effect of a δ opioid receptor agonist on acetylcholine release in the hippocampus and the effect of oxotremorine on noradrenaline release in the vas deferens. CB(1) receptor ablation, to a very slight extent, increased Gα(o) protein levels without affecting either Gα(i) and M(2) receptor protein or the levels of anandamide and 2-arachidonoyl glycerol in the hippocampus. In conclusion, the CB(1) receptor shows an inhibitory interaction with the muscarinic and δ opioid receptor on cholinergic neurones in the rodent hippocampus and with the muscarinic receptor on noradrenergic neurones in the mouse vas deferens.

Non-neuronal release of ACh plays a key role in secretory response to luminal propionate in rat colon

Colonic chloride secretion is induced by chemical stimuli via the enteric nervous reflex. We have previously demonstrated that propionate stimulates chloride secretion via sensory and cholinergic systems of the mucosa in rat distal colon. In this study, we demonstrate non-neuronal release of ACh in the secretory response to propionate using an Ussing chamber. Mucosa preparations from the colon, not including the myenteric and submucosal plexuses, were used. Luminal addition of propionate and serosal addition of ACh caused biphasic changes in short-circuit current (Isc). TTX (1 μm) had no effects, while atropine (10 μm) significantly inhibited the Isc response to propionate and abolished that to ACh. In response to luminal propionate stimulation, ACh was released into the serosal fluid. A linear relationship was observed between the maximal increase in Isc and the amounts of ACh released 5 min after propionate stimulation. This ACh release induced by propionate was not affected by atropine and bumetanide, although both drugs significantly reduced the Isc responses to propionate. Luminal addition of 3-chloropropionate, an inactive analogue of propionate, abolished both ACh release and Isc response produced by propionate. RT-PCR analysis indicated that isolated crypt cells from the distal colon expressed an enzyme of ACh synthesis (ChAT) and transporters of organic cation (OCTs), but not neuronal CHT1 and VAChT. The isolated crypt cells contained comparable amounts of ACh to the residual muscle tissues including nerve plexuses. In conclusion, the non-neuronal release of ACh from colonocytes coupled with propionate stimulation plays a key role in chloride secretion, via the paracrine action of ACh on muscarinic receptors of colonocytes.

Involvement of Ca2+-dependent PKCs in the adaptive changes of mu-opioid pathways to sympathetic denervation in the guinea pig colon

In the guinea pig colon, chronic sympathetic denervation entails supersensitivity to inhibitory mu-opioid agents modulating cholinergic neurons. The mechanism underlying such adaptive change has not yet been unravelled, although protein kinase C (PKC) may be involved. A previous study indirectly demonstrated that activation of mu-opioid receptors on myenteric neurons facilitates PKC activity. Such coupling may counteract the inhibitory action of mu-opioid agents on acetylcholine overflow, since PKC, per se, increases this parameter. After chronic sympathetic denervation such restraint abates, representing a possible mechanism for development of supersensitivity to mu-opioid agents. In the present study, this hypothesis was further investigated. After chronic sympathetic denervation, Ca(2+)-dependent PKC activity was reduced in colonic myenteric plexus synaptosomes. The mu-opioid agent, DAMGO, increased Ca(2+)-dependent PKC activity in synaptosomes obtained from normal, but not from denervated animals. In myenteric synaptosomes obtained from this experimental group, protein levels of Ca(2+)-dependent PKC isoforms betaI, betaII and gamma decreased, whereas alpha levels increased. In whole-mount preparations, the four Ca(2+)-dependent PKC isoforms co-localized with mu-opioid receptors on subpopulations of colonic myenteric neurons. The percentage of neurons staining for PKCbetaII, as well as the number of mu-opioid receptor-positive neurons staining for PKCbetaII, decreased in denervated preparations. The same parameters related to PKCalpha, betaI or gamma remained unchanged. Overall, the present data strengthen the concept that mu-opioid receptors located on myenteric neurons are coupled to Ca(2+)-dependent PKCs. After chronic sympathetic denervation, a reduced efficiency of this coupling may predominantly involve PKCbetaII, although also PKCbetaI and gamma, but not PKCalpha, may be implicated.

Synaptic plasticity: the new explanation of visceral hypersensitivity in rats with Trichinella spiralis infection?

BACKGROUND AND AIM

Synaptic plasticity plays an important role in affecting the intensity of visceral reflex. It may also be involved in the development of visceral hypersensitivity. The aim of this study was to investigate the role of synaptic plasticity on visceral hypersensitivity of rats infected by Trichinella spiralis.

METHODS

Thirty male Sprague-Dawley (SD) rats were randomly divided into control, acute, and chronic infection groups, and were investigated at 1 week after adaptive feeding and at 2 and 8 weeks post infection (PI) by oral administration of 1 ml phosphate-buffered saline (PBS) containing 8,000 Trichinella spiralis larvae. Visceral sensitivity was evaluated by electromyography (EMG) recording during colorectal distension. Intestinal inflammation was observed by hematoxylin-eosin (HE) staining. Synaptic ultrastructure parameters, such as postsynaptic density (PSD) length, synaptic cleft, and number of synaptic vesicles, were examined by transmission electron microscopy (TEM). The expression of protein associated with synaptic plasticity, including postsynaptic density-95 (PSD-95), synaptophysin, calbindin-28 K, N-methyl-D-aspartate receptor-1 (NMDA-R1), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPA-R), and glial cell line-derived neurotrophic factor (GDNF), were analyzed by Western blot.

RESULTS

(1) Visceral hypersensitivity was noted in the chronic infection group, although the inflammation was nearly eliminated (P<0.05). Severe inflammation and downregulation of visceral sensitivity were observed in the acute infection group (P<0.05). (2) There were many more synaptic vesicles and longer PSD in the chronic infection group than in the control group (P<0.05, respectively). However, in comparison with control rats, disappearance of mitochondria cristae in the synapses, and decrease of synaptic vesicles and length of PSD were observed in the acute infection group. There was no significant difference in width of synaptic cleft among the three groups. (3) Compared with the control, the expression of proteins associated with synaptic plasticity was significantly upregulated during chronic infection phase (P<0.05), and downregulated during acute infection phase.

CONCLUSION

Synaptic plasticity was observed in SD rats infected by Trichinella spiralis and was associated with visceral sensitivity, which suggests that it may play an important role in the formation of visceral hypersensitivity.

Functional interaction between α2-adrenoceptors, μ- and κ-opioid receptors in the guinea pig myenteric plexus: Effect of chronic desipramine treatment

The existence of a functional interplay between alpha(2)-adrenoceptor and opioid receptor inhibitory pathways modulating neurotransmitter release has been demonstrated in the enteric nervous system by development of sensitivity changes to alpha(2)-adrenoceptor, mu- and kappa-opioid receptor agents on enteric cholinergic neurons after chronic sympathetic denervation. In the present study, to further examine this hypothesis we evaluated whether manipulation of alpha(2)-adrenoceptor pathways by chronic treatment with the antidepressant drug, desipramine (10 mg/kg i.p. daily, for 21 days), could entail changes in enteric mu- and kappa-opioid receptor pathways in the myenteric plexus of the guinea pig distal colon. In this region, subsensitivity to the inhibitory effect of both UK14,304 and U69,593, respectively alpha(2A)-adrenoceptor and kappa-opioid receptor agonist, on the peristaltic reflex developed after chronic desipramine treatment. On opposite, in these experimental conditions, supersensitivity developed to the inhibitory effect of [D-Ala, N-Me-Phe4-Gly-ol5]-enkephalin (DAMGO), mu-opioid receptor agonist, on propulsion velocity. Immunoreactive expression levels of alpha(2A)-adrenoceptors, mu- and kappa-opioid receptors significantly decreased in the myenteric plexus of the guinea pig colon after chronic desipramine treatment. In these experimental conditions, mRNA levels of alpha(2A)-adrenoceptors, mu- and kappa-opioid receptors significantly increased, excluding a direct involvement of transcription mechanisms in the regulation of receptor expression. Levels of G protein-coupled receptor kinase 2/3 and of inhibitory G(i/o) proteins were significantly reduced in the myenteric plexus after chronic treatment with desipramine. Such changes might represent possible molecular mechanisms involved in the development of subsensitivity to UK14,304 and U69,593 on the efficiency of peristalsis. Alternative molecular mechanisms, including a higher efficiency in the coupling between receptor activation and downstream intracellular effector systems, possibly independent from inhibitory G(i/o) proteins, may be accounted for the development of supersensitivity to DAMGO. Increased sensitivity to the mu-opioid agonist might compensate for the development of alpha(2A)-adrenoceptor and kappa-opioid receptor subsensitivity. On the whole, the present data further strengthen the concept that, manipulation of alpha(2)-adrenergic inhibitory receptor pathways in the enteric nervous system entails changes in opioid inhibitory receptor pathways, which might be involved in maintaining homeostasis as suggested for mu-opioid, but not for kappa-opioid receptors.

Vesicular glutamate transporter 1 immunoreactivity in extrinsic and intrinsic innervation of the rat esophagus

Encouraged by the recent finding of vesicular glutamate transporter 2 (VGLUT2) immunoreactivity (-ir) in intraganglionic laminar endings (IGLEs) of the rat esophagus, we investigated also the distribution and co-localization patterns of VGLUT1. Confocal imaging revealed substantial co-localization of VGLUT1-ir with selective markers of IGLEs, i.e., calretinin and VGLUT2, indicating that IGLEs contain both VGLUT1 and VGLUT2 within their synaptic vesicles. Besides IGLEs, we found VGLUT1-ir in both cholinergic and nitrergic myenteric neuronal cell bodies, in fibers of the muscularis mucosae, and in esophageal motor endplates. Skeletal neuromuscular junctions, in contrast, showed no VGLUT1-ir. We also tested for probable co-localization of VGLUT1-ir with markers of extrinsic and intrinsic esophageal innervation and glia. Within the myenteric neuropil we found, besides co-localization of VGLUT1 and substance P, no further co-localization of VGLUT1-ir with any of these markers. In the muscularis mucosae some VGLUT1-ir fibers were shown to contain neuronal nitric oxide synthase (nNOS)-ir. VGLUT1-ir in esophageal motor endplates was partly co-localized with vesicular acetylcholine transporter (VAChT)/choline acetyltransferase (ChAT)-ir, but VGLUT1-ir was also demonstrated in separately terminating fibers at motor endplates co-localized neither with ChAT/VAChT-ir nor with nNOS-ir, suggesting a hitherto unknown glutamatergic enteric co-innervation. Thus, VGLUT1-ir was found in extrinsic as well as intrinsic innervation of the rat esophagus.

Intraganglionic laminar endings and their relationships with neuronal and glial structures of myenteric ganglia in the esophagus of rat and mouse

Intraganglionic laminar endings (IGLEs) represent the only vagal mechanosensory terminals in the tunica muscularis of the esophagus and may be involved in local reflex control. We recently detected extensive though not complete colocalization of the vesicular glutamate transporter 2 (VGLUT2) with markers for IGLEs. To elucidate this colocalization mismatch, this study aimed at identifying markers for nitrergic, cholinergic, peptidergic, and adrenergic neurons and glial cells, which may colocalize with VGLUT2 outside of IGLEs. Confocal imaging revealed, besides substantial colocalization of VGLUT2 and substance P (SP), no other significant colocalizations of VGLUT2 and immunoreactivity for any of these markers within the same varicosities. However, we found close contacts of VGLUT2-positive structures to vesicular acetylcholine transporter, choline acetyltransferase, neuronal nitric oxide synthase, galanin, neuropeptide Y, and vasoactive intestinal peptide immunoreactive cell bodies and varicosities, as well as to glial cells. Neuronal perikarya were never positive for VGLUT2. Thus, VGLUT2 was almost exclusively found in IGLEs and may serve as a specific marker for them. In addition, many IGLEs also contained SP. The close contacts established by IGLEs to myenteric cell bodies, dendrites, and varicose fibers suggest that IGLEs modulate various types of enteric neurons and vice versa.

Involvement of protein kinase C in the adaptive changes of cholinergic neurons to sympathetic denervation in the guinea pig myenteric plexus

Supersensitivity to muscarinic, kappa- and mu-opioid agents modulating cholinergic neurons in the guinea pig colon develops after chronic sympathetic denervation. A possible role for protein kinase C (PKC) in contributing to development of these sensitivity changes was investigated. The PKC activator, phorbol-12-myristate-13-acetate (PMA), enhanced acetylcholine (ACh) overflow in preparations obtained from normal animals. The facilitatory effect of PMA was significantly reduced after prolonged exposure to the phorbol ester and by the PKC inhibitors, chelerythrine and calphostin C. Subsensitivity to the facilitatory effect of PMA developed after chronic sympathetic denervation. In this experimental condition, immunoblot analysis revealed reduced levels of PKC in myenteric plexus synaptosomes. The facilitatory effect of the muscarininc antagonist, scopolamine, on ACh overflow was significantly reduced by the phospolipase C (PLC) inhibitor, U73122, chelerythrine and calphostin C, both in normal and denervated animals. However, in both experimental groups, PLC antagonists and PKC antagonists did not affect the inhibitory effect of the muscarinic agonist, oxotremorine-M on ACh overflow. The inhibitory effects of U69593 (kappa-opioid receptor agonist) and DAMGO (mu-opioid receptor agonist) on ACh overflow significantly increased in the presence of U73122, chelerythrine and calphostin C in preparations obtained from normal animals, but not in those obtained from sympathetically denervated animals. These results indicate that activation of PKC enhances ACh release in the myenteric plexus of the guinea pig colon. At this level, chronic sympathetic denervation entails a reduced efficiency of the enzyme. In addition, PKC is involved in the inhibitory modulation of ACh release mediated by muscarinic-, kappa- and mu-opioid receptors, although with different modalities. Muscarinic receptors inhibit PKC activity, whereas kappa- and mu-opioid receptors increase PKC activity. Both the inhibitory and the facilitatory effect on PKC involve modulation of PLC activity. The possibility that the change in PKC activity represents one of the biochemical mechanisms at the basis of development of sensitivity changes to opioid and muscarinic agents after chronic sympathetic denervation is discussed.

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.

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.