Changes in sensitivity to the inhibitory effects of adrenergic agonists on intestinal motor activity after chronic sympathetic denervation

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.

Species dependent differences in the actions of sympathetic nerves and noradrenaline in the internal anal sphincter

Excitatory motor innervation to the internal anal sphincter (IAS) of the monkey, the rabbit and mouse were compared. Contractile responses to electrical field stimulation of nerves (EFS, atropine 1 micromol L(-1) and N(omega)-nitro-L-arginine 100 micromol L(-1) present throughout) were examined in isolated strips of IAS. In the monkey IAS, EFS caused frequency dependent (1-30 Hz) contractions which were abolished by guanethidine (10 micromol L(-1)) or phentolamine (3 micromol L(-1)). The sympathetic neurotransmitter noradrenaline (NA) also caused concentration-dependent (10 nmol L(-1)-100 micromol L(-1)) contractions which were abolished by phentolamine revealing a small relaxation that was abolished by propranolol (3 micromol L(-1)). In contrast, EFS caused only relaxation of the mouse and rabbit IAS which was not affected by guanethidine. Furthermore, NA relaxed these muscles and relaxation was nearly abolished by combined addition of phentolamine and propranolol. In conclusion, the monkey IAS is functionally innervated by sympathetic nerves that contract the muscle via excitatory alpha-adrenergic receptors. In contrast, no significant motor function could be identified for sympathetic nerves in the rabbit or mouse IAS although adrenergic receptors linked to muscle inhibition are present. These data reveal species dependent differences in sympathetic motor innervation and suggest that some species are more appropriate than others as models for motor innervation to the human IAS.

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.

Differential reversal of drug-induced small bowel paralysis by cerulein and neostigmine

OBJECTIVE

Cerulein and neostigmine are prokinetic drugs whose potency and effective dose range are barely known. The aim of this study was to assess their benefit for normal and compromised peristalsis.

DESIGN

In vitro, isolated segments of guinea pig small intestine. Setting : University laboratory.

INTERVENTIONS

Small bowel segments were mounted in tissue baths and luminally perfused with Tyrode solution. Test drugs (prokinetic: cerulein, neostigmine; inhibitory: atropine, hexamethonium, epinephrine, sufentanil) were added to the tissue bath.

MEASUREMENTS AND RESULTS

Peristalsis was quantified via changes in the peristaltic pressure threshold. One-way and two-way analysis of variance (ANOVA) were used for statistical analysis. Cerulein (0.03-100 nM) stimulated normal peristalsis in a concentration-dependent manner and reversed paralysis of peristalsis induced by all inhibitory test drugs to a similar extent. The properistaltic effect of neostigmine was limited to a narrow concentration range (0.03-0.1 micro M), whereas concentrations >0.3 micro M inhibited peristalsis. Neostigmine more effectively counteracted blockage of peristalsis caused by atropine than that caused by hexamethonium. The inhibitory effects of epinephrine and sufentanil on peristalsis were reversed only at the concentration range of 0.1-0.3 micro M neostigmine.

CONCLUSIONS

Cerulein stimulates normal peristalsis in vitro at a wide concentration range and reverses blockage of peristalsis caused by drugs with a site of action either on the enteric nervous system or intestinal smooth muscle. Neostigmine's prokinetic effect, to the contrary, is limited to a small concentration range and best seen when peristalsis is depressed by blockage of cholinergic muscle activation.

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.

Neuronal adrenergic and muscular cholinergic contractile hypersensitivity in canine jejunum after extrinsic denervation

Extrinsic denervation may be responsible for motor dysfunction after small bowel transplantation. The aim of this study was to examine the role of extrinsic innervation of canine jejunum on contractile activity. An in vitro dose response of cholinergic and adrenergic agonists was evaluated in canine jejunal strips of circular muscle at 0, 2, and 8 weeks in a control group and after jejunoileal extrinsic denervation (EX DEN). Neurons in circular muscle were quantitated by means of immunohistochemical techniques. Adrenergic and cholinergic responses did not differ at any time in the control group. However, at 2 and 8 weeks, extrinsic denervation caused an increased sensitivity to the procontractile effects of the cholinergic agonist bethanechol at the level of the smooth muscle cells, and increased sensitivity to the inhibitory effects of the adrenergic agent norepinephrine mediated at the level of the enteric nervous system. Immunohistochemical analysis showed a reduction in all neurons and a complete lack of adrenergic fibers in the EX DEN group after 2 and 8 weeks. Extrinsic denervation induces enteric neuronal cholinergic and adrenergic smooth muscle hypersensitivity in canine jejunal circular muscle.

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.

Low potential of dobutamine and dopexamine to block intestinal peristalsis as compared with other catecholamines

OBJECTIVE

Catecholamines are frequently used in critically ill patients to restore stable hemodynamics and to improve organ perfusion. One effect of short-term or long-term administration of catecholamines may be inhibition of propulsive motility in the intestine. We therefore analyzed the effect of dopexamine, dobutamine, and dopamine on ileal peristalsis and compared their action with that of epinephrine and norepinephrine, which have long been known to suppress intestinal peristalsis.

DESIGN

In vitro study on excised guinea pig ileum segments.

SETTING

Laboratory for experimental studies at the University.

SUBJECTS

Isolated guinea pig ileum.

INTERVENTIONS

Segments of ileum excised from guinea pigs were mounted in a tissue bath in Krebs-Henseleit solution and bubbled with 95% oxygen/5% CO2. Luminal perfusion with the same solution was performed at a rate of 0.35 mL/min. The bath temperature was kept at 36.5 degrees C. Peristalsis was recorded via changes in the intraluminal pressure. The drugs under investigation (dopamine, epinephrine, norepinephrine, dobutamine, and dopexamine) were added to the tissue bath.

MEASUREMENTS AND MAIN RESULTS

Low concentrations of each catecholamine, except epinephrine, caused a decrease in the pressure threshold, which reflects a stimulatory effect on peristalsis. Higher catecholamine concentrations caused a concentration-related increase in the threshold, cumulating in a complete block of peristalsis. The rank order of inhibitory potency was epinephrine > norepinephrine > dopamine > dobutamine approximately dopexamine. Dobutamine and dopexamine were about 500-fold less active than epinephrine in suppressing peristalsis.

CONCLUSIONS

This study shows that dobutamine and dopexamine have the least potential to block propulsive motility in the intestine, whereas epinephrine demonstrates the most adverse inhibitory effect. Because at low concentrations dobutamine and dopexamine even stimulate peristalsis, these drugs appear to be superior compared with other catecholamines with regard to their direct effects on intestinal motility.

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.

Adrenergic mechanisms in the control of gastrointestinal motility: from basic science to clinical applications

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.

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

We have studied the effects of mu- and kappa-opioid receptor blockade on endogenous acetylcholine and noradrenaline overflow from the myenteric plexus of the guinea-pig isolated colon. Cyprodime (putative mu-selective antagonist) and nor-binaltorphimine (kappa-selective antagonist) had a concentration-dependent facilitatory effect on both acetylcholine and noradrenaline overflow. Moreover, in colonic specimens obtained from sympathetically denervated animals, the effect of opioid antagonists on acetylcholine overflow was significantly higher with respect to normal preparations. Evidence is thus given in favour of an involvement of mu- and kappa-opioid receptor pathways in the tonic modulation of neurotransmitter release at the colonic level. Enhanced sensitivity to the effect of mu and kappa antagonists after chronic sympathetic denervation is strongly suggestive for the existence of a functional link between opioid and adrenergic pathways in this model.

N-methyl-D-aspartate receptors modulate neurotransmitter release and peristalsis in the guinea pig isolated colon

To assess the role of NMDA receptors in modulating neurotransmitter release in the myenteric plexus, we studied the effects of L-glutamic acid and NMDA on endogenous acetylcholine and noradrenaline overflow (assayed by HPLC) from the guinea pig isolated distal colon. L-Glutamic acid and NMDA enhanced electrically evoked acetylcholine and noradrenaline overflow and these effects were reversed by selective NMDA receptor antagonists. The possible functional significance of these findings was studied by measuring the efficiency of the colonic peristaltic reflex in the presence of NMDA receptor agonists. NMDA inhibited propulsion velocity at all concentrations tested, this effect being antagonized by (+/-)-2-amino-5-phosphonopentanoic acid and virtually abolished in sympathetically denervated animals. In conclusion, the inhibitory effect of NMDA on peristalsis, being almost entirely dependent on the integrity of sympathetic pathways, could be, at least in part, due to NMDA-induced noradrenaline release.

Abnormalities of innervation of internal anal sphincter in fecal incontinence

Physiological and histological studies have shown that the internal and sphincter is abnormal in idiopathic fecal incontinence. We have recently demonstrated that the in vitro contractile response of the internal anal sphincter to the sympathetic neurotransmitter noradrenaline is decreased in incontinence. In this study we have further defined this reduced sensitivity and provided more information about the intrinsic innervation in both the normal and the incontinent sphincter muscle. Muscle strips from 12 incontinent patients undergoing post and repair and from 11 controls undergoing rectal excision for low rectal carcinoma were studied. Responses to noradrenaline were recorded initially alone and then in the presence of phentolamine, an alpha-adrenoceptor blocker. In the presence of phentolamine, noradrenaline caused relaxation: there was no significant difference in the relaxation-response curves and the EC50 was the same in the two groups. These results demonstrate that the previously documented reduced sensitivity to noradrenaline is due to an altered sensitivity of the alpha-adrenoceptors. Electrical field stimulation produced relaxations in all muscle strips, but only in the controls was the magnitude of the relaxation significantly increased in the presence of phentolamine. This indicates that there is an alpha-adrenergic excitatory component of the response to electrical field stimulation of the intramural nerves, which was present in tissues from control patients but which was absent in tissues from patients with idiopathic fecal incontinence.

Opioid pathways exert a tonic restraint in the guinea-pig isolated colon: changes after chronic sympathetic denervation

We have studied the effects of naloxone on acetylcholine and noradrenaline release in the guinea-pig isolated distal colon, and have assessed the effect of naloxone on electrically-induced contractions of the longitudinal muscle and non-adrenergic, non-cholinergic (NANC) relaxations of the circular muscle coat. Naloxone dose-dependently increased resting and electrically-evoked acetylcholine release and electrically-evoked noradrenaline release. Naloxone was more potent in increasing resting acetylcholine release in colonic specimens obtained after chronic sympathetic denervation. Naloxone (1 microM) did not affect electrically-induced contractions of the longitudinal muscle, while it enhanced NANC relaxations of the circular muscle. The effects observed with naloxone in the present experiments suggest that opioid pathways exert a tonic restraint on neurotransmission in the guinea-pig colon. After suppression of the adrenergic inhibitory tone, the functional relevance of opioid pathways seems to be increased.

Inhibition of endogenous acetylcholine release by blockade of voltage-dependent calcium channels in enteric neurons of the guinea-pig colon

The effects on acetylcholine release from the guinea-pig colon of the N-type calcium channel blocker omega-conotoxin GVIA (omega-conotoxin), the L-type calcium channel blocker nifedipine and the putative blocker of T-type channels, flunarizine, have been investigated. Endogenous basal acetylcholine release and electrically (1 Hz, 1 ms, 450 mA)-evoked overflow in the presence of cholinesterase inhibitor were studied. omega-Conotoxin (1-10 nM) and nifedipine (0.03-3 microM) dose-dependently inhibited basal and electrically-evoked acetylcholine release. Maximal inhibition of basal or electrically-evoked acetylcholine release was about 40% for nifedipine and about 75% for omega-conotoxin. The potency of nifedipine was inversely related to the external calcium concentration: its EC50 value in low-calcium medium (0.5 mM) was as low as 12 nM. Flunarizine inhibited acetylcholine release only at concentrations higher than 0.2 microM. Our results are consistent with an involvement of N- and L-type calcium channels in the control of the endogenous acetylcholine release from the guinea-pig colon.

Calcium entry blockade as a mechanism for chlordimeform-induced inhibition of motor activity in the isolated guinea-pig ileum

Central and peripheral alpha 2-adrenoceptors, including those of the gastrointestinal tract, have been indicated as a toxicity target of formamidine pesticides in mammals. In this study, the inhibitory effect of chlordimeform on twitch contractions from electrically-stimulated longitudinal muscle-myenteric plexus preparations (LMMPs) of the guinea-pig ileum was found to be resistant to the action of the alpha 2-adrenoceptor antagonist idazoxan. This drug was also ineffective on chlordimeform-induced inhibition of peristalsis recorded in whole ileal segments. As expected, idazoxan antagonized the inhibitory effect of the alpha 2-adrenoceptor agonist clonidine on twitch contractions and peristaltic activity. Chlordimeform reduced the amplitude of direct mechanical responses to a variety of spasmogens such as acetylcholine, histamine and substance P, suggesting a muscular site of action. Moreover, Ca(2+)-free, K(+)-depolarized LMMPs, chlordimeform inhibited submaximal contractions caused by addition of exogenous calcium, through an action apparently similar to that of the Ca2+ entry blocker nifedipine. Both chlordimeform- and nifedipine-induced inhibition of calcium contractions were reversed by the calcium channel activator BAY K 8644. This compound also partially prevented the inhibitory action of chlordimeform on peristaltic activity. On the whole, these results indicate that chlordimeform-induced depression of motor activity in the guinea-pig ileum is, at least in part, related to inhibition of transmembrane Ca2+ fluxes responsible for smooth muscle contraction.

Supersensitivity to morphine after chronic sympathetic denervation in guinea-pig colon

The possible role of opioid systems in the adaptive changes which follow chronic sympathetic denervation in the guinea-pig colon has been studied by comparing the effects of the opioid agonist morphine in control animals and after chronic sympathetic denervation. Supersensitivity to the inhibitory effects of morphine on the peristaltic reflex was observed after chronic sympathetic denervation, while the potency against acetylcholine release was unmodified. Our results suggest that a modification of the opioid system occurs after sympathetic denervation in the guinea-pig colon. Supersensitivity to endogenous opioids at a site different from that regulating acetylcholine release could account for the counter-regulation of intestinal motility after chronic sympathetic denervation.

Effects of noradrenaline and galanin on duodenal motility in the isolated perfused porcine pancreatico-duodenal block

The influence of neurotransmitters on gastrointestinal motility is different in different segments of the gastrointestinal tract. To clarify the regulation of duodenal motility, the aim of the present study was to investigate the effects of alpha-adrenoceptor agonism and blockade and of galanin on duodenal motility. The study was undertaken in the isolated perfused porcine pancreatico-duodenal block. The agents under investigation were administered arterially. Duodenal motility was measured by means of a low-compliance perfusion system using an intraluminal catheter. In addition the concentration of galanin was measured in the portal effluent. We found that spontaneous motility was abolished by noradrenaline by an effect that was counteracted by the alpha 2-adrenoceptor antagonist idazoxan. In contrast, the selective alpha 1-adrenoceptor antagonist prazosin did not influence the effect of noradrenaline. Galanin, like noradrenaline, abolished duodenal motility. Furthermore, the concentration of galanin in the portal effluent was decreased by noradrenaline by an alpha 2-adrenoceptor mediated mechanism. We conclude that alpha 2-adrenoceptor activation and galanin inhibit duodenal motility and that the release of galanin from the pancreatico-duodenal preparation is reduced by alpha 2-adrenoceptors.

Selectivity of Ca2+ channel blockers in inhibiting muscular and nerve activities in isolated colon

1. Potency and efficacy of nifedipine, verapamil and diltiazem and of Bay K 8644 in modifying propulsion and nerve or smooth muscle activities have been compared in the guinea-pig isolated distal colon. Both the neuronal and muscular effects of Ca2+ channel blockers seem to develop at concentrations that are devoid of any significant effect apart from that on Ca2+ channels. 2. Nifedipine, verapamil and diltiazem were all able to impair propulsion, resting and stimulated acetylcholine (ACh) release and smooth muscle contractility in a concentration-dependent way. However, some degree of selectivity for neuronal and muscular effects could be observed. Nifedipine was more than 500 fold more potent than verapamil in relaxing musculature but less than twice as potent in reducing ACh release. On the other hand, verapamil was the most efficacious Ca2+ channel blocker tested in inhibiting ACh release, its effects being inversely correlated to the external Ca2+ concentration, and completely abolished by Bay K 8644. 3. By comparing the potencies exhibited by each drug against peristaltic reflex, smooth muscle contractility and ACh release, verapamil proved to be almost as potent in slowing the peristaltic reflex as in reducing ACh release, while nifedipine was about 100 fold more potent against the peristaltic reflex than against ACh release, but nearly equal against the peristaltic reflex and smooth muscle tone. Therefore, interference with cholinergic neurotransmission is likely to play a major role in the antipropulsive effect of verapamil, while peristaltic reflex impairment by nifedipine is likely to be dependent on inhibition of smooth muscle. 4. A facilitatory effect of Bay K 8644 on both the efficiency of the peristaltic reflex and the nonadrenergic, non-cholinergic (NANC) nerve-mediated relaxation could be observed at concentrations at least 10 fold lower than those required to affect ACh release or smooth muscle. 5. It is concluded that the effects of Ca2+ channel blockers on neurotransmitter release may be relevant to their effects on the gastrointestinal motor function.

Effect of beta-casomorphins on intestinal propulsion in the guinea-pig colon

beta-Casomorphins are a family of opioid peptides originally isolated from beta-casein. In view of a possible physiological significance of these milk-derived compounds, the effects of bovine beta-casomorphin-5 (beta-CM-5), beta-casomorphin-4 (beta-CM-4) and D-Ala2-beta-casomorphin-4-NH2 (D-Ala2-beta-CM-4-NH2) have been investigated on the peristaltic reflex in the guinea-pig isolated colon and compared with morphine. beta-CM-5 and D-Ala2-beta-CM-4-NH2 each dose-dependently inhibited the velocity of propulsion of an intraluminal bolus; beta-CM-4 was ineffective. IC50 values were 0.30, 5.21 and 0.29 microM for morphine, beta-CM-5 and D-Ala2-beta-CM-4-NH2, respectively. The potency ratios vs morphine were 0.06 and 0.96 for beta-CM-5 and D-Ala2-beta-CM-4-NH2, respectively. Blockade of the peristaltic reflex by beta-CM-5 or D-Ala2-beta-CM-4-NH2 was reversed by the opioid antagonist naloxone. D-Ala2-beta-CM-4-NH2 also dose-dependently inhibited resting acetylcholine output (IC50 = 5.69 microM; potency ratio vs morphine: 0.63). In conclusion, certain beta-casomorphins inhibit intestinal propulsion and cholinergic neurotransmission in the guinea-pig colon, probably by acting at opioid receptors.

Parasympathetic denervation abolishes acetylcholine-induced relaxation in the rat iris dilator

An anomalous change in response to exogenously applied acetylcholine (ACh) was found in the rat iris dilator muscle after surgical parasympathetic denervation. The relaxation induced by nerve stimulation in the normal dilator muscles disappeared after the denervation. ACh elicited relaxation and contraction at low (less than 3 X 10(-6) M) and higher doses, respectively, in the normal muscles. The relaxing response to ACh was almost abolished after the denervation, while the maximum contractile response to ACh was not affected significantly.

A facilitatory effect of bicuculline on the enteric neurones in the guinea-pig isolated colon

Changes in the efficiency of the peristaltic reflex, acetylcholine (ACh) output and motor responses to transmural and periarterial nerve stimulation produced by bicuculline and gamma-aminobutyric acid (GABA) receptor desensitization were investigated in the guinea-pig isolated colon. Bicuculline, at concentrations unable to affect spontaneous colonic motility and lacking anticholinesterase activity, produced a dose-dependent increase of both the efficiency of the peristaltic reflex and the stimulated ACh output. Such effects could not be observed in GABA-desensitized preparations. A frequency-dependent potentiation of the cholinergic excitatory and non-adrenergic non-cholinergic (NANC) inhibitory responses to transmural stimulation was also observed in the presence of bicuculline. Conversely bicuculline exhibited an inhibitory effect on the relaxation induced by periarterial nerve stimulation. Acute GABA-desensitization was unable to affect the contractile responses to transmural stimulation, the ACh output and the efficiency of the peristaltic reflex. On the contrary, desensitization was able to mimic the effects of bicuculline on the inhibitory responses to both transmural and periarterial nerve stimulation. Our results are consistent with a significant role played by an intrinsic GABAergic pathway in the modulation of both cholinergic excitatory and NANC inhibitory neurones. The hypothesis is advanced that a feed-back modulation carried out through bicuculline-sensitive GABAergic synapses could operate during the propagation of peristaltic motor activity.

Subsensitivity of enteric cholinergic neurones to alpha 2-adrenoceptor agonists after chronic sympathetic denervation

The concentration-effect relationships of noradrenaline, dopamine and clonidine in inhibiting resting and stimulated acetylcholine output have been studied in intact and in sympathetically denervated preparations of guinea pig isolated distal colon. The order of potencies for the inhibition of resting acetylcholine release in intact preparations was clonidine greater than dopamine greater than noradrenaline while the order of intrinsic activities was noradrenaline greater than dopamine greater than clonidine. Sympathetic denervation was able to modify the potency of either clonidine, dopamine and noradrenaline. Noradrenaline was 6 times more potent in inhibiting resting acetylcholine release in denervated than in intact preparations, while clonidine and dopamine underwent a 18-fold and a 11-fold decrease in potency after denervation. The potency of clonidine relative to noradrenaline was 110 in intact preparations and only 1.2 in denervated organs. The intrinsic activities of noradrenaline, dopamine and clonidine were almost unchanged in denervated organs. A dose-dependent facilitatory effect of yohimbine on both the resting acetylcholine output and the peristaltic reflex could be observed in intact but not in sympathetically denervated preparations at concentrations ranging from 2.5 X 10(-8) M to 2.5 X 10(-7) M. Yohimbine was able to counteract the inhibitory effect of dopamine and to remove the inhibitory effect of periarterial nerve stimulation on both acetylcholine release and the peristaltic reflex. Our results are consistent with the existence of a tonic physiological modulation of enteric cholinergic neurones by postganglionic sympathetic fibres. The order of potencies of adrenoceptor agonists and the antagonism by yohimbine is consistent with such a modulation being entirely carried out through alpha 2-heteroceptors.(ABSTRACT TRUNCATED AT 250 WORDS)