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

Homocysteine and A2A-D2 Receptor-Receptor Interaction at Striatal Astrocyte Processes

The interaction between adenosine A2A and dopamine D2 receptors in striatal neurons is a well-established phenomenon and has opened up new perspectives on the molecular mechanisms involved in Parkinson's disease. However, it has barely been investigated in astrocytes. Here, we show by immunofluorescence that both A2A and D2 receptors are expressed in adult rat striatal astrocytes in situ, and investigate on presence, function, and interactions of the receptors in the astrocyte processes-acutely prepared from the adult rat striatum-and on the effects of homocysteine on the A2A-D2 receptor-receptor interaction. We found that A2A and D2 receptors were co-expressed on vesicular glutamate transporter-1-positive astrocyte processes, and confirmed that A2A-D2 receptor-receptor interaction controlled glutamate release-assessed by measuring the [³H]D-aspartate release-from the processes. The complexity of A2A-D2 receptor-receptor interaction is suggested by the effect of intracellular homocysteine, which reduced D2-mediated inhibition of glutamate release (homocysteine allosteric action on D2), without interfering with the A2A-mediated antagonism of the D2 effect (maintained A2A-D2 interaction). Our findings indicate the crucial integrative role of A2A-D2 molecular circuits at the plasma membrane of striatal astrocyte processes. The fact that homocysteine reduced D2-mediated inhibition of glutamate release could provide new insights into striatal astrocyte-neuron intercellular communications. As striatal astrocytes are recognized to be involved in Parkinson's pathophysiology, these findings may shed light on the pathogenic mechanisms of the disease and contribute to the development of new drugs for its treatment.

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.

Differential peristaltic motor effects of prostanoid (DP, EP, IP, TP) and leukotriene receptor agonists in the guinea-pig isolated small intestine

1. Since the role of prostanoid receptors in intestinal peristalsis is largely unknown, the peristaltic motor effects of some prostaglandin (DP, EP, IP), thromboxane (TP) and leukotriene (LT) receptor agonists and antagonists were investigated. 2. Propulsive peristalsis in fluid-perfused segments from 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. Alterations of distension sensitivity were deduced from alterations of the peristaltic pressure threshold and modifications of peristaltic performance were reflected by modifications of the amplitude, maximal acceleration and residual baseline pressure of the peristaltic waves. 3. Four categories of peristaltic motor effects became apparent: a decrease in distension sensitivity and peristaltic performance as induced by the EP1/EP3 receptor agonist sulprostone and the TP receptor agonist U-46619 (1-1000 nM); a decrease in distension sensitivity without a major change in peristaltic performance as induced by PGD(2) (3-300 nM) and LTD(4) (10-100 nM); a decrease in peristaltic performance without a major change in distension sensitivity as induced by PGE(1), PGE(2) (1-1000 nM) and the EP1/IP receptor agonist iloprost (1-100 nM); and a decrease in peristaltic performance associated with an increase in distension sensitivity as induced by the EP2 receptor agonist butaprost (1-1000 nM). The DP receptor agonist BW-245 C (1-1000 nM) was without effect. 4. The peristaltic motor action of sulprostone remained unchanged by the EP1 receptor antagonist SC-51089 (1 micro M) and the DP/EP1/EP2 receptor antagonist AH-6809 (30 micro M), whereas that of U-46619 and LTD(4) was prevented by the TP receptor antagonist SQ-29548 (10 micro M) and the cysteinyl-leukotriene(1) (cysLT(1)) receptor antagonist tomelukast (10 micro M), respectively. 5. These observations and their pharmacological analysis indicate that activation of EP2, EP3, IP, TP and cysLT(1) receptors, but not DP receptors, modulate intestinal peristalsis in a receptor-selective manner, whereas activation of EP1 seems to be without influence on propulsive peristalsis. In a wider perspective it appears as if the effect of prostanoid receptor agonists to induce diarrhoea is due to their prosecretory but not peristaltic motor action.

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.

Altered neuropeptide content and cholinergic enzymatic activity in the inflamed guinea pig jejunum during parasitism

We investigated the effects of an enteric infection with the parasitic nematode, Trichinella spiralis, on peptidergic and cholinergic neural pathways of the guinea pig jejunum. The content of the enteric neuropeptides, substance P (SP) and vasoactive intestinal peptide (VIP), and the activities of the key cholinergic enzymes, acetylcholinesterase (AChE) and choline acetyltransferase (ChAT), were measured and compared in extracts of jejunal muscularis externa (ME) obtained from uninfected jejunum and T. spiralis-inflamed jejunum. Significant decreases were detected in both SP immunoreactivity and AChE activity on days 6 and 10 postinfection (PI) in nematode-infected guinea pig jejunum compared to uninfected controls. The maximum changes observed for SP and AChE both occurred on day 10 PI and were evident as decreases of 37% and 48%, respectively, from the mean uninfected control values for SP and AChE. In contrast, VIP immunoreactivity and ChAT activity showed no significant changes during the enteric phase of T. spiralis infection. Nematode-evoked histopathological changes in jejunal tissues from infected animals were associated with significant increases in myeloperoxidase (MPO) activity, an index of inflammation intensity, which occurred on day 6 PI (885% of mean control) and day 10 PI (469% of mean control) coinciding temporally with the significant decrease in SP content and AChE activity during infection. Thus, intestinal motor disturbances observed in mammalian hosts during enteric nematode infections involve inflammation-generated changes in the neurohumoral control of smooth muscle function.

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.

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.

[3H]acetylcholine release from the guinea-pig distal colon: comparison with ileal [3H]acetylcholine release and effect of adrenoceptor stimulation

To study cholinergic function in the guinea-pig colon, resting and electrically evoked 3H release after preincubation with [3H] choline has been compared in colonic and ileal myenteric plexus preparations. Fractional spontaneous colonic 3H release was significantly higher than ileal 3H release, while the reverse was true for electrically evoked 3H outflow. Electrically evoked 3H outflow in the colon was linearly related to stimulation frequency (0.2-3 Hz range) and current intensity (300-600 mA range), while 3H outflow per pulse was inversely related to stimulation frequency. Electrically evoked 3H outflow was prevented in Ca2(+)-free solution, indicating that it probably mirrored neuronal exocytotic [3H]acetylcholine release. Both noradrenaline and clonidine concentration-dependently inhibited electrically evoked 3H outflow, clonidine being more potent but less efficacious than noradrenaline. For both noradrenaline and clonidine, the potency and efficacy for inhibition of 3H outflow were close to the values previously reported for the inhibition of electrically evoked endogenous acetylcholine output from colonic preparations. In conclusion, these data indicate that 3H release after incubation with [3H]choline is a valid alternative to measurement of endogenous acetylcholine output to study colonic cholinergic neuronal function in the guinea-pig.

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.