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PubMed1992

Influence of temperature on the effects of mu-, delta- and kappa-opioid receptor agonists in the guinea-pig ileum myenteric plexus.

Electrically stimulated guinea-pig ileum myenteric plexus-longitudinal muscle was used to determine if changes in temperature alter the inhibitory effects of DAGO ([D-Ala2,N-MePhe4,Gly5-ol]enkephalin, mu-agonist), DPDPE ([D-Pen2,-Pen5] enkephalin, delta-agonist) and U-50,488H (trans-3,4-dichloro)-N-methyl-N-[2-(1- pyrrolidynyl)cyclohexyl]benzeneacetamide methane sulfonate, kappa-agonist). The potency (expressed as the concentration which produces 50% inhibition, IC50) of DAGO and DPDPE was significantly (P < 0.05) decreased at 30 degrees C (8.8 +/- 2.7 x 10(-9) and 8325.2 +/- 1070 x 10(-9) M), when compared to the potency at 37 degrees C (3.8 +/- 0.3 x 10(-9) and 6298.6 +/- 320 x 10(-9) M). Higher temperature (40 degrees C) did not modify the potency of DAGO or DPDPE compared to that at 37 degrees C. However, the potency of U-50,488H was significantly (P < 0.01) increased at 40 degrees C (0.7 +/- 0.0 x 10(-9) M) versus 37 degrees C (2.4 +/- 0.9 x 10(-9) M) or 30 degrees C (2.5 +/- 0.3 x 10(-9) M). The kappa-agonist was more potent than DAGO or DPDPE at 30 or 40 degrees C. These data demonstrate that changes in temperature can alter the potency of opioid agonists.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗

PubMed1996

Regional differences in the number of neurons in the myenteric plexus of the guinea pig small intestine and colon: an evaluation of markers used to count neurons.

BACKGROUND: Subsets of myenteric neurons have been identified. To determine the proportional representation of neurons in each, it is necessary to relate the number of neurons in the subset to that of the complete set. Prior estimates of total numbers of neurons, obtained with many different markers, have varied widely. METHODS: Markers were compared for counting myenteric neurons in dissected laminar preparations of guinea pig duodenum, jejunum-ileum, and colon; the effect of stretching preparations on these counts was also determined. Markers included the visualization of single-stranded nucleic acid with cuprolinic blue and the immunocytochemical demonstration of neuron specific enolase (NSE), PGP9.5, S-100, and the constitutive expression of a Fos related antigen (FRA). RESULTS: Neurons could not be counted accurately by demonstrating NSE, PGP9.5, or S-100. The number of neurons detected by demonstrating FRA was consistently less than that determined with cuprolinic blue (approximately 65%). Cuprolinic blue-derived estimates of neuron numbers were higher than most reported in the literature, but comparable to those recently obtained with "a nerve cell body" antiserum. Ganglionic area was found to be stretch independent. The rank order of neurons/cm2 and ganglionic area/ unit resting length was colon > duodenum >> jejunum-ileum; more neurons were found in the myenteric plexus of the colon (7.3 x 10(6)) than in that of the entire small intestine (6.5 x 10(6)). CONCLUSIONS: Prior studies that have obtained denominators for estimating the proportions of myenteric neuronal subsets with markers that do not reveal the entire population should be re-evaluated. The guinea pig colon contains a surprisingly large number of neurons, the physiological significance of which must be determined.

Animals↗

PubMed1991

Effects of calcitonin gene-related peptide on longitudinal muscle and myenteric plexus of guinea pig ileum.

Effects of rat calcitonin gene-related peptide (rCGRP) on the longitudinal muscle-myenteric plexus (lm-mp) have been investigated in the present study. rCGRP was shown to have a biphasic effect on the nonstimulated lm-mp, i.e., a transient contraction followed by a longer lasting relaxation. This biphasic action was dose-dependent over the concentration range of 10(-10) to 10(-5) M. Tetrodotoxin (1 microM), atropine (1 microM), 2-chloroadenosine (1 microM) and clonidine (1 microM) inhibited the stimulatory effect. Tetrodotoxin and clonidine also partially inhibited the relaxing effect of rCGRP. This biphasic action of rCGRP was also seen on the contraction of the lm-mp elicited by electrical field stimulation. rCGRP showed only a relaxing effect on the nonstimulated plexus-free longitudinal muscle. rCGRP (2.6 x 10(-10) to 2.6 x 10(-7) M) inhibited both the phasic and tonic contractions induced by histamine in a competitive way and showed a more potent effect on the tonic contraction than that on the phasic contraction. rCGRP at the same dose range also inhibited the tonic but not phasic contraction evoked by membrane depolarization with the maximal stimulation of KCl (30 mM). rCGRP inhibited KCl-, but not histamine-induced, extracellular calcium-dependent contraction. The peptide did not affect the dose-response curve of oxotremorine. These results suggest that rCGRP may exert its stimulatory actions on the lm-mp via actions on the plexus nerves, whereas the depressant actions may be mediated via the nerves, as well as directly on the smooth muscle.

Animals↗

PubMed1996

Effect of chronic administration of dihydropyridine Ca2+ channel ligands on sufentanil-induced tolerance to mu- and kappa-opioid agonists in the guinea pig ileum myenteric plexus.

The present investigation was aimed at elucidating if the entry of Ca2+ plays a role in the development of tolerance to mu- and kappa-opioid agonists in the guinea pig ileum myenteric plexus. For this purpose, the influence of the L-type Ca2+ channel modulators nimodipine (Ca2+ blocker) and Bay K 8644 (Ca2+ activator) on the expression of tolerance to the inhibitory effects of mu- and kappa-opioid agonists in the ileum of guinea pigs rendered tolerant to sufentanil was investigated. Chronic perfusion of guinea pigs with nimodipine (2 micrograms/microliter/h for 7 days) or Bay K 8644 (0.5 microgram/microliter/h for 7 days) did not cause any modification of the height of contractions induced by electrical stimulation of the myenteric plexus-longitudinal muscle (MPLM) strip from naive animals. Tolerance to sufentanil (a selective mu-agonist) was induced by s.c. implantation of osmotic minipumps for 7 days, which deliver at 2 micrograms/microliter/h. Control groups received saline. Tolerance to sufentanil as well as to U-50,488H (selective kappa-agonist) was observed following chronic treatment with sufentanil and was revealed as a rightward shift of the concentration-response curves. Chronic perfusion of guinea pigs with the Ca2+ antagonist nimodipine concurrently with chronic sufentanil, markedly blocked the expression of tolerance to sufentanil, as well as the cross-tolerance between sufentanil and U-50,488H. On the contrary, when guinea pigs were perfused with the Ca2+ agonist Bay K 8644 concurrently with sufentanil, it enhanced the magnitude of tolerance to both sufentanil and U-50,488H. These results suggest that, in guinea pig ileum, chronic exposure to opioids may involve the activation of L-type Ca2+ channel, which would indicate that intracellular Ca2+ may be one of the final pathways through which myenteric neurons adapt to the chronic opioid exposure.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗

PubMed1988

Evidence for a non-opioid sigma binding site in the guinea-pig myenteric plexus.

The presence of a binding site to (+)-(3H)SKF 10,047 was demonstrated in a guinea-pig myenteric plexus (MYP) membrane preparation. Specific binding to this receptor was saturable, reversible, linear with protein concentration and consisted of two components, a high affinity site (KD = 46 +/- 5 nM; Bmax = 3.4 +/- 0.5 pmole/g wet weight) and a low affinity site (KD= = 342 +/- 72 nM; Bmax = 22 +/- 3 pmole/g wet weight). Morphine and naloxone 10(-4) M were unable to displace (+)-(3H)SKF 10,047 binding. Haloperidol, imipramine, ethylketocyclazocine and propranolol were among the most potent compounds to inhibit this specific binding. These results suggest the presence of a non-opioid haloperidol sensitive sigma receptor in the MYP of the guinea-pig.

Animals↗

PubMed1985

Effects of adenylate cyclase inhibitors, phosphodiesterase inhibitors, and dibutyryl cyclic AMP on spontaneous and various stimuli-induced acetylcholine release from guinea pig ileum myenteric plexus.

In order to examine a possible contribution of cyclic AMP to acetylcholine (ACh) release from guinea pig ileum myenteric plexus, effects of adenylate cyclase inhibitors, phosphodiesterase (PDE) inhibitors and dibutyryl cyclic AMP on the spontaneous and the various stimuli-induced ACh release were investigated. A PDE inhibitor, theophylline (1 mM) increased the ACh release induced by nicotine (6.16 microM) significantly. Another PDE inhibitor, 3-isobutyl-1-methylxanthine (IBMX, 1 mM) and dibutyryl cyclic AMP (4 mM) had no effect. The adenylate cyclase inhibitors dithiobisnitrobenzoic acid (DTNB, 1 mM) and alloxan (4 mM) both decreased the nicotine-induced ACh release remarkably. PDE inhibitors increased and adenylate cyclase inhibitors decreased the high-K+-induced ACh release. Dibutyryl cyclic AMP brought about a slight but significant increase of the high-K+-induced ACh release. All the drugs failed to alter the ACh release induced by electrical field stimulation (EFS) at 10 Hz. Effects of all drugs except dibutyryl cyclic AMP on the spontaneous ACh release were the same as those on the nicotine-induced one. Dibutyryl cyclic AMP decreased it significantly. These results suggest that the cyclic AMP system is involved in the spontaneous, the nicotine-induced and the high-K+-induced ACh release and that the EFS-induced ACh release is independent of cyclic AMP.

1-Methyl-3-isobutylxanthine↗

PubMed1987

Inhibitory effects of opioids in a circular muscle-myenteric plexus preparation of guinea-pig ileum.

The actions of opioids were examined in a strip preparation of the external muscle and myenteric plexus of the guinea-pig ileum cut parallel to the circular muscle. Contractions of the circular muscle induced by electrical stimulation of myenteric neurons were depressed in a concentration-dependent manner by the mu agonists, morphine and DAGO, and by the kappa agonist, U-50,488H. The concentrations of morphine, DAGO and U-50,488H which depressed nerve-mediated contractions by 50% (IC50) were 86 nM, 11 nM and 5.0 nM, respectively. The equilibrium dissociation constants (KD) for naloxone as an antagonist of the inhibitory effects of DAGO and of U-50,488H were 5.6 nM and 29.4 nM, respectively. In contrast to the potent inhibitory effects of mu and kappa agonists, the delta-selective agonist, D-Pen-L-Pen, produced only weak inhibition of nerve-mediated contractions. Even at a concentration of 3 microM, there was less than 50% inhibition, which was not antagonised by the delta receptor antagonist, ICI 174864. The experiments indicate that both mu and kappa opioid receptors are present on the myenteric neurons supplying the circular muscle and that delta receptors are either absent or ineffectively activated.

Animals↗

PubMed1987

Modulation of acetylcholine release from guinea-pig Ileum myenteric plexus by arachidonic acid cascade inhibitors.

Effects of arachidonic acid and mepacrine on ACh release from guinea-pig ileum myenteric plexus were investigated. Mepacrine (1-8 microM) inhibited the ACh release in a concentration-dependent manner. Arachidonic acid counteracted the inhibitory effect of mepacrine, but PGE2 did not. The inhibition induced by a combination of mepacrine and indomethacin on nicotine-induced ACh release was prevented by arachidonic acid, while that on spontaneous ACh release was prevented by arachidonic acid and PGE2 added simultaneously. The roles of arachidonic acid and PGs in the ACh release will be discussed.

Acetylcholine↗

PubMed1984

Development of 5-hydroxytryptamine-like immunoreactive neurones in cultures of the myenteric plexus from the guinea-pig caecum.

5-Hydroxytryptamine (5-HT)-like immunoreactive neurones were studied during the development of myenteric plexus explant cultures from the guinea-pig taenia coli over a period of 3 weeks in vitro, using immunofluorescence histochemistry. Brightly fluorescent 5-HT-like immunoreactive neuronal cell bodies and fibres were found in all ages of cultures examined. In mature cultures, where the histotypic organization resembles that of the plexus in vivo, the pattern of immunoreactivity was strikingly similar to that previously described for in situ preparations. These culture preparations may therefore be useful models for the study of the development of putative serotonergic neurones. High-performance liquid chromatography was used to measure 5-HT levels in fetal calf serum, a supplement of the culture medium used in this study. 5-HT levels of 0.48 X 10(-5) M to 1.74 X 10(-4) M were found in 4 batches of this serum, indicating that some of the immunoreactive neurones observed in the cultures may have selectively taken up 5-HT during development in vitro.

Animals↗

PubMed1984

Comparative anatomy of the myenteric plexus of the distal colon in eight mammals.

The behavior of the most distal part of the colon in a variety of species suggests that the innervation of this part may differ from that of more proximal parts. Silver impregnation was used to demonstrate the arrangement of the myenteric plexus of the distal colon in eight species (rat, guinea pig, rabbit, Australian possum, American opossum, cat, dog, and monkey). A distal zone, approximately 5%-20% of the total length of the colon above the anal verge in the nonrodents, was characterized by a plexus of very irregularly disposed intersecting nerve bundles of highly variable size with few and small ganglia; this zone was absent in the three rodent species. A next most distal zone, approximately 10%-65% of the total colonic length, contained a stellate plexus of large, regularly disposed ganglia interconnected by small nerve fiber bundles upon which were superimposed large dark-staining nerve bundles; these bundles began to be seen at the level of the irregular rectal plexus and ran cephalad, bypassing some ganglia but giving off branches to others. These, called shunt fascicles, contained many myelinated nerve fibers. Above this zone, the plexus was a stellate plexus throughout the remainder of the colon.

Animals↗

PubMed1977

Extracellular recording from the guinea-pig myenteric plexus and the action of morphine.

Extracellular recordings were made with glass suction electrodes from neurons in isolated ganglia of the myenteric plexus of the guinea-pig ileum. The activity recorded was considered to arise in neuronal somata as a consequence of the mechanical contact with the recording electrode. The spike activity was depressed by normorphine (10 nM-1 micrometer) and this action was prevented by low concentrations of naloxone. Morphine remained effective in solutions which contained 1 or 12 mM K+. The findings support other evidence that morphine acts by hyperpolarizing the soma membrane of myenteric neurons, but do not elucidate the ionic mechanism of the effect.

Action Potentials↗

PubMed1990

Lesion patterns of vasoactive intestinal polypeptide-containing neurons in the myenteric plexus induced by clamping or transsection of rat jejunum.

The acute reactions of vasoactive intestinal polypeptide (VIP)-containing neurons in the myenteric plexus of the rat jejunum in response to circumferential clamping or transection/reanastomosis were evaluated in a detailed time course of up to ten days. Whereas in sham-operated controls VIP immunostaining was confined exclusively to varicose fibers, either clamping or transection induced the appearance of strongly VIP-immunoreactive beaded neuronal processes and neuronal perikarya in the oral part of the lesion one day postoperatively. After five days strongly immunostained varicose fibers orientated in the longitudinal axis of the gut towards the lesion reappeared suggestive of the onset of regenerative processes.

Animals↗

PubMed1992

Some morphological and histochemical features of the midgut myenteric plexus of the common European frog, Rana esculenta.

The neuron morphology and distribution of four putative transmitters were investigated in the myenteric plexus of frog (Rana esculenta) midgut. The gross morphology was revealed by NADH-diaphorase histochemistry, and the shape of the neurons by silver impregnation. Nerve cells had heterogeneous distribution: they either formed ganglia or placed as solitary neurons in the duodenum, while in the rest of the midgut only solitary neurons were observed. Three morphologically distinct cell types were revealed by silver impregnation: mainly type I and type II neurons cells were seen in the duodenum, while the rest of the intestine contained type II and III cells. Catecholamine fluorescence was revealed in nerve fibres in the duodenum, while few small nerve cells were observed in the small intestinal region. Acetylcholinesterase histochemistry showed strongly reactive nerve cells that were associated with the main fibre bundles in the duodenum. Only longitudinally oriented fibres and occasionally stained neurons were seen in the small intestine. Substance P immunocytochemistry revealed an extensive plexus, which contained a moderate number of stained perikarya in the full length of the midgut. Gamma-aminobutyric acid showed non-uniform distribution in the two parts of the midgut: a stronger and more regular fibre staining was found in the duodenum then in the rest of the intestine. Ultrastructural observations demonstrated that intrinsic neurons received synaptic inputs from the profiles contained agranular vesicles, while "P"-type profiles established close contacts with neurons. Both profile types formed close contacts with the smooth muscle cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholinesterase↗

PubMed2000

IL-1beta and IL-6 excite neurones and suppress cholinergic neurotransmission in the myenteric plexus of the guinea pig.

The effects of the inflammatory mediators interleukin-1beta (IL-1beta) and interleukin-6 (IL-6) on myenteric neurones were investigated by intracellular recordings in a conventional myenteric plexus preparation of guinea pig ileum. Micropressure ejection of IL-1beta and IL-6 (10-7 mol L-1) both caused an excitatory effect in, respectively, 19% (13/70) and 7% (5/70) of the myenteric neurones. The IL-1beta-induced depolarizations were inhibited by superfusion of the IL-1beta receptor antagonist. The responses seen were tetrodotoxin-resistant, indicating a direct neuronal effect. Responses to both cytokines were seen in nitric oxide synthase-immunoreactive as well as choline acetyltransferase-immunoreactive neurones. In addition, both IL-1beta and IL-6 reversibly caused a presynaptic inhibition of acetylcholine release from cholinergic nerve terminals. Both cytokines had no effect on the slow excitatory postsynaptic potentials. Therefore, we can conclude that the inflammatory mediators IL-1beta and IL-6 can act as excitatory neuromodulators of gastrointestinal motility through direct excitatory actions on a subset of myenteric neurones and through the presynaptic inhibition of acetylcholine release.

Animals↗

PubMed1986

Assessment of acetylcholine release from myenteric plexus of guinea-pig colon.

Due to a complex mechanical tissue response to electrical stimulation, a colonic smooth muscle-myenteric plexus (SMMP) preparation, combined with radiolabelling techniques for assessing the acetylcholine release, has been developed to investigate intrinsic cholinergic nerve activity in the guinea-pig colon. Electrical field stimulation of the preparation gave reproducible release of label which was inhibited by tetrodotoxin. Release increased proportionally with the strength of stimulus (130, 150, 195 and 250 mA), but was inversely proportional to frequency of stimulation for a fixed number of pulses, 1 Hz releasing more than 10 Hz. Electrical stimulation of the tissue during incubation with [3H]choline enhanced the release by subsequent field stimulation. Release of label increased progressively towards the distal end of the colon.

Acetylcholine↗

PubMed1987

Temporal changes in mechanical properties of rat jejunal smooth muscle after myenteric plexus ablation.

We determined length-stress properties and active tension development of rat jejunal longitudinal and circular muscle at various times after myenteric neuron ablation. Myenteric neurons were destroyed by serosal application of benzalkonium chloride. Active tension generation in response to both carbachol and barium was depressed in both muscle layers up to 7 days after treatment, at which times the responses were normal. At 15 days, circular muscle responses were still comparable to control, but longitudinal muscle responses were significantly increased. Length-stress parameters of circular muscle were minimally affected, while those of longitudinal muscle were significantly altered 15 and 30 days after treatment. These alterations include changes in resting stress and increased active stress generation at both times. Our results suggest that 1) normal myenteric innervation is necessary for normal contractile activity in rat jejunal smooth muscle and 2) changes in cellular or tissue morphology or alterations in intracellular calcium homeostasis, as seen in other tissue, may occur after myenteric plexus ablation.

Animals↗

PubMed1998

Ascending choline acetyltransferase and descending nitric oxide synthase immunoreactive neurones of the myenteric plexus project to the mucosa of the guinea pig gastric corpus.

The aim of this study was to reveal mucosal projections of myenteric neurones in the stomach by using the neuronal tracer DiI (1,1'-didodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorat) in combination with immunohistochemical detection of choline acetyltransferase (ChAT) and nitric oxide synthase (NOS). The mucosal application of one DiI coated glass bead (diameter 50-100 microm) labelled on average 167 +/- 58 neurones in the myenteric plexus (n = 9 preparations). Most labelled cells were ChAT-positive (74%), the remaining cells were NOS-positive (n = 6). The vast majority of ascending DiI-labelled neurones were ChAT-positive (94%), whereas most descending neurones were NOS-positive (75%). ChAT- and NOS-positive fibers were demonstrated in the mucosa. Results suggest that ascending and descending myenteric neuronal pathways releasing acetylcholine and nitric oxide, respectively, are involved in control of mucosal functions in the stomach.

Animals↗

PubMed1998

Quantitative analysis of inputs to somatostatin-immunoreactive descending interneurons in the myenteric plexus of the guinea-pig small intestine.

Somatostatin immunoreactivity occurs in a specific subgroup of cholinergic descending interneurons in the myenteric plexus of the guinea-pig small intestine. In the present work, we made light- and electron-microscopic investigations of chemically defined inputs to these neurons, in order that the origins of the connections of other neurons with them could be deduced. Somatostatin-immunoreactive synapses and close contacts were found on the cell bodies and filamentous processes of somatostatin neurons; these were 84% of all inputs. It is thus confirmed that this class of interneuron forms chains that project anally. Descending interneurons with immunoreactivity for nitric oxide synthase provided 14% of inputs to somatostatin-immunoreactive descending interneurons. An antiserum against a calcium-binding protein, calbindin, was used as marker for the majority of intrinsic primary afferent neurons, AH/Dogiel type II neurons; this class of neurons provided only 2.5% of the inputs to somatostatin-immunoreactive descending interneurons. We conclude that somatostatin-immunoreactive descending interneurons are involved in the conduction of impulses distally along the full length of the small intestine, but receive only a minor input from calbindin-immunoreactive primary afferent neurons.

Animals↗