Two types of neuronal muscarine receptors modulating acetylcholine release from guinea-pig myenteric plexus

Muscarinic M(3) facilitation of acetylcholine release from rat myenteric neurons depends on adenosine outflow leading to activation of excitatory A(2A) receptors

Acetylcholine (ACh) is a major excitatory neurotransmitter in the myenteric plexus, and it regulates its own release acting via muscarinic autoreceptors. Adenosine released from stimulated myenteric neurons modulates ACh release preferentially via facilitatory A(2A) receptors. In this study, we investigated how muscarinic and adenosine receptors interplay to regulate ACh from the longitudinal muscle-myenteric plexus of the rat ileum. Blockade of the muscarinic M(2) receptor with 11-[[2-1[(diethylamino) methyl-1-piperidinyl]- acetyl]]-5,11-dihydro-6H-pyrido [2,3-b][1,4] benzodiazepine-6-one (AF-DX 116), 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and atropine facilitated [3H]ACh release evoked by short stimulation trains (5 Hz, 200 pulses). Prolonging stimulus train length (>750 pulses) shifted muscarinic autoinhibition towards facilitatory M(3) receptors activation, as predicted by blockade with J104129 (a selective M(3) antagonist), 4-DAMP and atropine, whereas the selective M(2) antagonist, AF-DX 116, was without of effect. Blockade of A(2A) receptors with ZM 241385, inhibition of adenosine transport with dipyridamole, and inhibition of ecto-5'-nucleotidase with concanavalin A, all attenuated release inhibition caused by 4-DAMP. J104129 and 4-DAMP, but not AF-DX 116, decreased ( approximately 60%) evoked adenosine outflow (5 Hz, 3000 pulses). Oxotremorine (300 micromol L(-1)) facilitated the release of [3H]ACh (34 +/- 4%, n = 5) and adenosine (57 +/- 3%, n = 6) from stimulated myenteric neurons. 4-DAMP, dipyridamole and concanavalin A prevented oxotremorine-induced facilitation. ZM 241385 blocked oxotremorine facilitation of [3H]ACh release, but kept adenosine outflow unchanged. Thus, ACh modulates its own release from myenteric neurons by activating inhibitory M(2) and facilitatory M(3) autoreceptors. While the M(2) inhibition is prevalent during brief stimulation periods, muscarinic M(3) facilitation is highlighted during sustained nerve activity as it depends on extracellular adenosine accumulation leading to activation of facilitatory A(2A) receptors.

Relative contribution of ecto-ATPase and ecto-ATPDase pathways to the biphasic effect of ATP on acetylcholine release from myenteric motoneurons

BACKGROUND AND PURPOSE

The relative contribution of distinct ecto-nucleotidases to the modulation of purinergic signalling may depend on differential tissue distribution and substrate preference.

EXPERIMENTAL APPROACH

Extracellular ATP catabolism (assessed by high-performance liquid chromatography) and its influence on [(3)H]acetylcholine ([(3)H]ACh) release were investigated in the myenteric plexus of rat ileum in vitro.

KEY RESULTS

ATP was primarily metabolized via ecto-ATPDase (adenosine 5'-triphosphate diphosphohydrolase) into AMP, which was then dephosphorylated into adenosine by ecto-5'-nucleotidase. Alternative conversion of ATP into ADP by ecto-ATPase (adenosine 5'-triphosphatase) was more relevant at high ATP concentrations. ATP transiently increased basal [(3)H]ACh outflow in a 2',3'-O-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate (TNP-ATP)-dependent, tetrodotoxin-independent manner. ATP and ATPgammaS (adenosine 5'-[gamma-thio]triphosphate), but not alpha,beta-methyleneATP, decreased [(3)H]ACh release induced by electrical stimulation. ADP and ADPbetaS (adenosine 5'[beta-thio]diphosphate) only decreased evoked [(3)H]ACh release. Inhibition by ADPbetaS was prevented by MRS 2179 (2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate diammonium salt, a selective P2Y(1) antagonist); blockade of ADP inhibition required co-application of MRS 2179 plus adenosine deaminase (which inactivates endogenous adenosine). Blockade of adenosine A(1) receptors with 1,3-dipropyl-8-cyclopentyl xanthine enhanced ADPbetaS inhibition, indicating that P2Y(1) stimulation is cut short by tonic adenosine A(1) receptor activation. MRS 2179 facilitated evoked [(3)H]ACh release, an effect reversed by the ecto-ATPase inhibitor, ARL67156, which delayed ATP conversion into ADP without affecting adenosine levels.

CONCLUSIONS AND IMPLICATIONS

ATP transiently facilitated [(3)H]ACh release from non-stimulated nerve terminals via prejunctional P2X (probably P2X(2)) receptors. Hydrolysis of ATP directly into AMP by ecto-ATPDase and subsequent formation of adenosine by ecto-5'-nucleotidase reduced [(3)H]ACh release via inhibitory adenosine A(1) receptors. Stimulation of inhibitory P2Y(1) receptors by ADP generated alternatively via ecto-ATPase might be relevant in restraining ACh exocytosis when ATP saturates ecto-ATPDase activity.

Immunohistochemical localisation of cholinergic muscarinic receptor subtype 1 (M1r) in the guinea pig and human enteric nervous system

Little is known regarding the location of cholinergic muscarinic receptor 1 (M1r) in the ENS, even though physiological data suggest that M1rs are central to cholinergic neurotransmission. This study localised M1rs in the ENS of the guinea pig ileum and human colon using fluorescence immunohistochemistry and RT-PCR in human colon. Double labelling using antibodies against neurochemical markers was used to identify neuron subytpes bearing M1r. M1r immunoreactivity (IR) was present on neurons in the myenteric and submucosal ganglia. The two antibodies gave similar M1r-IR patterns and M1r-IR was abolished upon antibody preabsorption. M1r-IR was present on cholinergic and nNOS-IR nerve cell bodies in both guinea pig and human myenteric neurons. Presynaptic M1r-IR was present on NOS-IR and VAChT-IR nerve fibres in the circular muscle in the human colon. In the submucosal ganglia, M1r-IR was present on a population of neurons that contained cChAT-IR, but did not contain NPY-IR or calretinin-IR. M1r-IR was present on endothelial cells of blood vessels in the submucosal plexus. The localisation of M1r-IR in the guinea pig and human ENS shown in this study agrees with physiological studies. M1r-IR in cholinergic and nitrergic neurons and nerve fibres indicate that M1rs have a role in both cholinergic and nitrergic transmission. M1r-IR present in submucosal neurons suggests a role in mediating acetylcholine's effect on submucosal sensory and secretomotor/vasodilator neurons. M1r-IR present on blood vessel endothelial cells suggests that M1rs may also mediate acetylcholine's direct effect on vasoactivation.

Increased cholinergic contractions of jejunal smooth muscle caused by a high cholesterol diet are prevented by the 5-HT4 agonist--tegaserod

Background

Excess cholesterol in bile and in blood is a major risk factor for the respective development of gallbladder disease and atherosclerosis. This lipid in excess negatively impacts the functioning of other smooth muscles, including the intestine. Serotonin is an important mediator of the contractile responses of the small intestine. Drugs targeting the serotonin receptor are used as prokinetic agents to manage intestinal motor disorders, in particular irritable bowel syndrome. Thus, tegaserod, acting on 5-HT₄ receptor, ideally should obviate detrimental effects of excessive cholesterol on gastrointestinal smooth muscle. In this study we examined the effect of tegaserod on cholesterol-induced changes in the contractile responses of intestinal smooth muscle.

Methods

The effects of a high cholesterol (1%) diet on the in vitro contractile responses of jejunal longitudinal smooth muscle from Richardson ground squirrels to the cholinergic agonist carbachol were examined in the presence or absence of tetrodrodotoxin (TTX). Two groups of animals, fed either low (0.03%) or high cholesterol rat chow diet, were further divided into two subgroups and treated for 28 days with either vehicle or tegaserod.

Results

The high cholesterol diet increased, by nearly 2-fold, contractions of the jejunal longitudinal smooth muscle elicited by carbachol. These cholinergic contractions were mediated by muscarinic receptors since they were blocked by scopolamine, a muscarinic receptor antagonist, but not by the nicotinic receptor antagonist, hexamethonium. Tegaserod treatment, which did not affect cholinergic contractions of tissues from low cholesterol fed animals, abrogated the increase caused by the high cholesterol diet. With low cholesterol diet TTX enhanced carbachol-evoked contractions, whereas this action potential blocker did not affect the augmented cholinergic contractions seen with tissues from animals on the high cholesterol diet. Tegaserod-treatment removed the effects of a high cholesterol diet on neuronal muscarinic receptors, as the potentiating effect of TTX on carbachol-elicited contractions was maintained in these animals.

Conclusion

A high cholesterol diet causes significant changes to cholinergic neurotransmission in the enteric nerves of the jejunum. The mechanisms by which these effects of cholesterol are reversed by tegaserod are unknown, but relate to removal of an inhibitory effect of cholesterol on enteric nerves.

Muscarinic modulation of nitrergic neurotransmission in guinea-pig gastric fundus

Muscarinic receptor activation by (4-Hydroxy-2-butynyl)-1-trimethylammonium-m-chlorocarbanilate chloride (McN-A-343) was investigated both on NADPH-d staining and on electrically induced responses in guinea-pig gastric fundus. McN-A-343 (10 micromol L(-1)) significantly increased the optical density of NADPH-d positive neurones, while blockade of nitric oxide synthase with N(omega)-nitro-L-arginine (L-NA) decreased it, suggesting facilitation of nitric oxide (NO) production. Electrical field stimulation (EFS; 2 Hz, 0.2 ms, supramaximal current intensity, 10 s train duration) elicited on-contraction followed by off-relaxation in the circular muscle strips. McN-A-343 (10 micromol L(-1)) transformed the EFS-evoked response from on-contraction into on-relaxation, which was neurogenic, tetrodotoxin-sensitive and hexamethonium-resistant. L-NA partly reduced the EFS-evoked relaxation, revealing two components: a nitrergic and a non-nitrergic one. The effect of McN-A-343 on the amplitude of the EFS-evoked relaxation was not changed by the M(3) receptor antagonist para-fluoro-hexahydro-sila-difenidol hydrochloride, but was significantly enhanced by M(1) receptor blockade with telenzepine. In the presence of telenzepine, the L-NA-dependent nitrergic component of the EFS-induced relaxation predominates. We suggest that cholinergic receptor activation has a dual effect on nitrergic neurotransmission: (i) stimulation of NOS by muscarinic receptor(s) different from M(1) and M(3) subtype, (ii) prejunctional inhibition of NO-mediated relaxation via M(1) receptors. In addition, M(1) receptors may facilitate the non-nitrergic relaxation.

Modulation by adenosine of both muscarinic M1-facilitation and M2-inhibition of [3H]-acetylcholine release from the rat motor nerve terminals

The crosstalk between adenosine and muscarinic autoreceptors regulating evoked [3H]-acetylcholine ([3H]-ACh) release was investigated on rat phrenic nerve-hemidiaphragm preparations. Motor nerve terminals possess facilitatory M1 and inhibitory M2 autoreceptors that can be activated by McN-A-343 (1-30 microm) and oxotremorine (0.3-100 microm), respectively. The muscarinic receptor antagonist, dicyclomine (3 nm-10 microm), caused a biphasic (inhibitory/facilitatory) effect, indicating that M1-facilitation prevails during 5 Hz stimulation trains. Concomitant activation of AF-DX 116-sensitive M2 receptors was partially attenuated, as pretreatment with M1 antagonists, muscarinic toxin 7 (MT-7, 0.1 nm) and pirenzepine (1 nm), significantly enhanced inhibition by oxotremorine. Activation of A2A-adenosine receptors with CGS 21680C (2 nm) (i) potentiated oxotremorine inhibition, and (ii) shifted McN-A-343-induced facilitation into a small inhibitory effect. Conversely, the A1-receptor agonist, R-N6-phenylisopropyl adenosine (R-PIA, 100 nm), attenuated the inhibitory effect of oxotremorine, without changing facilitation by McN-A-343. Synergism between A2A and M2 receptors is regulated by a reciprocal interaction with facilitatory M1 receptors, which may be prevented by pirenzepine (1 nm). During 50 Hz-bursts, facilitation (M1) of [3H]-ACh release by McN-A-343 disappeared, while the inhibitory (M2) effect of oxotremorine became predominant. This muscarinic shift results from the interplay with A2A receptors, as it was precluded by the selective A2A receptor antagonist, ZM 241385 (10 nm). In conclusion, when the muscarinic M1 positive feedback loop is fully operative, negative regulation of ACh release is mediated by adenosine A1 receptors. During high frequency bursts, tonic activation of A2A receptors promotes M2 autoinhibition by braking the M1 receptor operated counteraction.

Muramyl dipeptide (MDP) and 5-HT receptors. Neuroimmunomodulatory effects of MDP are probably not mediated through 5-HT4 or 5-HT1A receptors

A possible interaction of immunomodulator muramyl dipeptide (MDP) with 5-HT4 and 5-HT1A receptors was investigated. The activation of 5-HT4 receptors releases acetylcholine from nerve terminals, thereby contracting the guinea-pig distal ileum. The whole ileum segments were therefore cut and placed into the bath. The preparations were contracted by 5-HT (10 nM-3.2 microM); these contractions were totally abolished in the presence of atropine (1 microM) and significantly attenuated in the presence of SDZ-205,557 (320 nM). The 5-HT evoked contractions remained unchanged in the presence of MDP (5, 50 or 500 nM). MDP (l0 nM-3.2 microM) could not directly contract the preparations. In further experiments, the possible interaction of MDP with 5-HT1A receptors was investigated. The activation of 5-HT1A receptors inhibits the release of acetylcholine from nerve terminals, thereby decreasing the height of electrically evoked neurogenic twitches of guinea-pig ileum. The whole ileum segments were cut, placed into the bath and stimulated electrically. Selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) decreased the height of twitches and this effect was significantly attenuated in the presence of 5-HT antagonist metergoline (1 microM). The effect of 8-OH-DPAT remained unchanged in the presence of MDP (5, 50 or 500 nM). MDP (10 nM-3.2 microM) did not exert any direct effect on the preparations. These results suggest that MDP interacts with neither 5-HT4 nor 5-HT1A receptors.

Ascending neural pathways in the isolated guinea-pig ileum: effect of muscarinic M1, M2 and M3 cholinergic antagonists

The effect of muscarinic cholinoceptor antagonists was investigated on the ascending neural pathways activated by electrical stimulation in the guinea-pig ileum. For comparison, prejunctional and postjunctional effects of muscarinic cholinoceptor antagonists were also studied on circular smooth muscle. A two-compartment (oral and anal compartments) bath was used to study the ascending neural pathways. These were activated by electrical field stimulation in the anal compartment and the resulting contraction of the intestinal circular muscle in the oral compartment was recorded isotonically. Pirenzepine (10-300 nM), a muscarinic M1 cholinoceptor antagonist, reduced the ascending neural contractions in a concentration-dependent fashion when applied either to the oral or anal compartments (11-52% and 13-55% inhibition, respectively, P < 0.05). Pirenzepine inhibited (31+/-7%, P < 0.05) the acetylcholine (100 nM)-induced contractions at a higher non-selective concentration (300 nM), while its effect on the electrically-induced contractions was biphasic (10 and 30nM: 8-15% increase, P<0.05; 100 and 300 nM: 16-28% inhibition, P<0.05). The muscarinic M2 cholinoceptor antagonist methoctramine (3-100 nM) did not modify the contractions produced by 100 nM acetylcholine, electrically-induced contractions and the ascending neural contractions (when applied to either compartment). Parafluorohexahydrosiladifenidol (3-100 nM), a muscarinic M3 cholinoceptor antagonist, inhibited the contractions produced by 100 nM acetylcholine (19-81% and 15-69%), electrically-induced contractions (11-71% and 12-72%) and the ascending neural contractions (13-76% and 866%) when applied to the oral compartment, but it was without effect when applied to the anal compartment. These studies suggest that in the enteric ascending neural pathway, muscarinic M1 receptors are involved in neuroneuronal transmission, muscle contraction is mediated by muscarinic M3 cholinergic receptors, whereas muscarinic M2 receptors do not seem to participate.

Facilitation by endogenous acetylcholine and nitric oxide of luminal serotonin release from the guinea-pig colon

The present study was designed to determine the influence of endogenous acetylcholine and nitric oxide (NO) on spontaneous luminal serotonin (5-hydroxytryptamine, 5-HT) release in the luminally perfused isolated guinea-pig proximal colon in vitro. 5-HT was determined by high-performance liquid chromatography with electro-chemical detection. The luminal outflow of 5-HT was significantly reduced by atropine (0.2 microM), hexamethonium (100 microM), the NO synthase inhibitor NG-nitro-L-arginine (L-NNA, 10 microM) and the NO-trapping agent 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (carboxy-PTIO, 30 microM). Addition of excess L-arginine (300 microM) reversed the inhibitory effect of L-NNA on the 5-HT outflow. Physostigmine (1 microM) caused a great increase (atropine-sensitive) in 5-HT outflow. The enhancing action of physostigmine on 5-HT outflow was partially inhibited by L-NNA (100 microM) or carboxy-PTIO (30 microM), but was unaffected by the muscarinic M1 receptor antagonist pirenzepine (0.2 microM) or a muscarinic M3 receptor antagonist 4-diphenyl-acetoxy-N-methyl-piperidine methiodide (0.2 microM). These results suggest that 5-HT release from luminally perfused proximal colon of the guinea pig is stimulated via a NO pathway and cholinergic pathways which utilize muscarinic synapses and nicotinic synapses. Further, an intrinsic cholinergic-NO link appears to play a role in the stimulation of luminal 5-HT release, which may reflect the release of 5-HT from entero-chromaffin cells.

Effect of butanedione monoxime on the contractility of guinea pig ileum and on the electrophysiological activity of myenteric S-type neurones

2,3-Butanedione monoxime (BDM) has demonstrated protective effects on isolated cardiac tissues, and on smooth muscle but its mechanism of action is not fully understood. To simultaneously study the effect of BDM on muscle contractility and on neuronal activity, the effect of BDM was tested in the contractile force of myenteric plexus-longitudinal muscle strips and in electrophysiological activity of myenteric S-type neurones of guinea pig ileum. BDM reduces, in a dose-dependent manner, the force of the spontaneous motility and the contractions induced by acetylcholine, bethanechol and electrical stimulation. The same BDM concentrations depolarize the neuronal membrane and reduce the rate of evoked firing. The effect of BDM can be attributed to a direct effect on the smooth muscle and to modifications of the neuronal activity.

Dependence of gamma-aminobutyric acid modulation of cholinergic transmission on nitric oxide and purines in cat terminal ileum

The possible involvement of purines and/or nitric oxide (NO) in the gamma-aminobutyric acid (GABA)A receptor-mediated effects on the spontaneous activity of isolated preparations from longitudinal and circular muscles of cat terminal ileum was investigated. GABA had biphasic effects, which were neurogenic and muscarinic. ATP and adenosine dose dependently inhibited the activity of the muscles. A contractile response evoked by the nucleotide only was also observed. The effects of the purines were equipotent and resistant to Nomega-nitro-L-arginine (L-NNA), tetrodotoxin and to desensitization by alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP), except for the contractile effect of ATP, which was abolished by alpha,beta-meATP. Pretreatment of the preparations with ATP or adenosine produced: (i) desensitization to the effects of the respective purinoceptor agonist only; and (ii) suppression of the GABA-induced responses of longitudinal and circular muscles. Hemoglobin and L-NNA greatly reduced or completely blocked the GABA(A)-induced relaxation and decreased the GABA(A)-induced contraction. Our results indicate that purines and NO, to a different extent, mediate the relaxant phase of the GABA effects in both layers. Interactions between muscarinic cholinoceptors and GABA-nitrergic pathway and a concomitant activation of postjunctional P1 and P2y purinoceptors are suggested to explain the prejunctional biphasic effects of GABA.

Differential effects of nitric oxide donors on basal and electrically evoked release of acetylcholine from guinea-pig myenteric neurones

1. The effects of the nitric oxide (NO) donors, 3-morpholino-sydnonimine (SIN-1), S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside on basal and electrically evoked release of [3H]-acetylcholine were studied in myenteric plexus longitudinal muscle preparations of the guinea-pig small intestine preincubated with [3H]-choline. 2. The NO donors concentration-dependently increased basal release of [3H]-acetylcholine. The increase in release was calcium-dependent and was prevented in the presence of tetrodotoxin. Superoxide dismutase (150 u ml-1) potentiated the effect of SIN-1. The selective inhibitor of soluble guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ, 0.01-1 microM), antagonized the facilitatory effect of SNAP. 8-Bromo cyclic GMP and the cyclic GMP-specific phosphodiesterase inhibitor, zaprinast (both 0.1-1 mM), also enhanced basal [3H]-acetylcholine release. The effect of 10 microM SNAP was significantly enhanced in the presence of zaprinast. 3. The NO donors concentration-dependently inhibited the electrically evoked release of [3H]-acetylcholine, whereas 8-bromo cyclic GMP and zaprinast enhanced the evoked release. The inhibition of acetylcholine release by SNAP was not affected by ODQ (0.01-1 microM). 4. It is concluded that NO stimulates basal acetylcholine release from myenteric neurones through activation of guanylyl cyclase. In addition, NO inhibits the depolarization evoked release of acetylcholine by a presynaptic mechanism unrelated to cyclic GMP. The data imply that NO is not only an inhibitory transmitter to intestinal smooth muscles but also a modulator of cholinergic neurotransmission in the myenteric plexus.

Release of somatostatin-like immunoreactivity from enriched enteric nerve varicosities of rat ileum

Synaptosomes were isolated from rat ileum by various steps of differential centrifugation. The peptide content for somatostatin-like immunoreactivity was used as marker for neuronal membranes. The enriched synaptosomal fraction (P2) showed a good enrichment of somatostatin content (4-fold) in comparison to the post-nuclear supernatant. The basal release of somatostatin-like immunoreactivity was 26 +/- 3 pg/mg tissue protein. KCl-evoked depolarization (65 mM) caused a significant increase of somatostatin-like immunoreactivity release (72 +/- 11 pg/mg, n = 12, P < 0.001) compared to basal release. In Ca(2+)-free medium the evoked release of somatostatin-like immunoreactivity was abolished. A substantial increase of somatostatin-like immunoreactivity release (52 +/- 7 pg/mg, n = 12, P < 0.05) was also observed in the presence of the Ca2+ ionophore A-23187. The cholinergic agonist carbachol elicited a dose-dependent release of somatostatin-like immunoreactivity (10(-7) M: 54 +/- 8 pg/mg, 10(-6) M: 63 +/- 6 pg/mg, 10(-5) M: 53 +/- 5 pg/mg, n = 12, P < 0.001), which was blocked by atropine (10(-6) M: 35 +/- 6 pg/mg, n = 12, P < 0.001), but not by hexamethonium. Other presynaptic modulating substances such as serotonin, the selective neurokinin-B agonist [beta Asp4,MePhe7]neurokinin B-(4-10), neurotensin, cholecystokinin-8, caerulein and pentagastrin had no stimulatory effect on release of somatostatin-like immunoreactivity. In summary, somatostatin-like immunoreactivity can be released from enteric synaptosomes by both depolarization with KCl and cholinergic stimulation via a muscarinic mechanism. The synaptosomes of intrinsic nerves offer an approach to study release of neuronal somatostatin on the subcellular level.

Two types of functionally different GABAA receptors mediate GABA modulation of cholinergic transmission in cat terminal ileum

1. The effects of GABA (1 microM-2 mM) on longitudinally or circularly oriented organ bath preparations of cat terminal ileum consisted of a relaxation phase with an inhibition of the rhythmic spontaneous phasic contractions, followed by a phase of contractions characterized by an elevation in basal tone and an increase in amplitude of the spontaneous phasic contractions. 2. Muscimol (100 microM), but not baclofen (100 microM), mimicked the relaxation phase of the response to applied GABA (100 microM) in all tissue preparations. In addition, muscimol induced a phase of contractile activity in the circular muscle layer whilst baclofen exerted a 'GABA-like' contractile effect on the longitudinal muscle layer. Bicuculline (30 microM) or picrotoxinin (30 microM) antagonized the GABA- or muscimol-induced relaxations in all preparations and decreased the GABA- but not the baclofen-induced contractions of the longitudinal muscle layer. 3. Tetrodotoxin (0.5 microM) or atropine (0.1 microM) prevented the bicuculline-sensitive phases of the GABA or muscimol effects on both muscle layers but not the contractile effect of baclofen on the longitudinal muscle layer. 4. The bicuculline-sensitive phases of the GABA effect on both muscle layers were almost completely eliminated by 1 nM pirenzepine. At this concentration pirenzepine did not affect the electrically-evoked cholinergic twitch contractions or contractile responses to applied acetylcholine of both muscle layers. 5. During electrically-evoked cholinergic twitch contractions of both muscle layers, GABA (100 microM) had an inhibitory effect. The inhibition occurred in the presence of pirenzepine (1 nM) but not of bicuculline (30 microM). 6. It is suggested that two types of functionally different bicuculline-sensitive GABAA receptors mediate an exitatory presynaptic and an inhibitory prejunctional action of GABA on the cholinergic transmission in cat terminal ileum.

Prejunctional M1 and postjunctional M3 muscarinic receptors in the circular muscle of the guinea-pig ileum

The effects of subtype-selective muscarinic receptor antagonists on electrically evoked release of acetylcholine and muscle contraction were compared in circular muscle preparations of the guinea-pig ileum. Incubation of the preparation with [3H]choline resulted in the formation of [3H]acetylcholine. Electrical stimulation caused the release of [3H]acetylcholine which was abolished by tetrodotoxin and omission of calcium from the medium. 5-Hydroxytryptamine (10 microM) and the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (300 microM) did not change acetylcholine release. The muscarinic antagonists pirenzepine (M1 selective), AF-DX 116 (M2 selective) and hexahydrosiladifenidol (M3 selective) caused concentration-dependent increases in the evoked release of acetylcholine, and inhibitions of the circular muscle contraction. The postjunctional affinity constants (pA2 values) obtained for hexahydrosiladifenidol (8.06), pirenzepine (6.95) and AF-DX 116 (6.60) identified the muscular receptor as an M3 subtype. Pirenzepine was more potent in facilitating the evoked release than hexahydrosiladifenidol and AF-DX 116. These findings suggest that the release of acetylcholine in the circular muscle is inhibited by M1 muscarinic autoreceptors whereas muscle contraction is mediated by M3 receptors.

Cholinergic activation of Cl- secretion in rat colonic epithelia

Acetylcholine receptor agonists and antagonists were used in a pharmacological analysis to identify which muscarinic receptor(s) may be involved in cholinergic regulation of Cl- secretion across rat colonic mucosa in vitro. A comparative ligand binding analysis for each of the antagonists was carried out in parallel. Both studies elicited identical rank order potencies (atropine > or = 4-diphenyl-acetoxy-N-piperidine methiodide (4-DAMP) > pirenzepine > 11-[[2[(diethylamino)methyl]-1-pipiridinyl]acetyl[5,11- dihydro-6H-pyrido[2,3-b]]1,4]benzodiazepine-6-one (AF-DX 116). Cholinomimetic-induced Cl- secretion was predominantly mediated by activation of muscarinic receptors in rat isolated colonic mucosa, with only a modest contribution from nicotinic receptors. Short circuit current responses evoked by the selective muscarinic M1 receptor agonist 4-[[(3-chlorophenyl)amino]carbonyl]-N,N,N-trimethyl-2-butyn-1-a minium chloride (McN-A-343) suggest that this receptor subtype, which is thought to be neuronally sited, also plays a minor role in regulation of intestinal ion transport. The principal epithelial cell receptors responsible for acetylcholine receptor-mediated Cl- secretion appear to belong to the M3 class.

Vagal influence on the motility of the feline jejunum

1. The effects of electrical stimulation of the peripheral end of the cervical vagal nerve on jejunal motility were investigated in anaesthetized cats, pretreated with guanethidine, with sectioned splanchnic nerves and ligated adrenal vessels. Motility was monitored as volume changes of an intraluminal balloon. 2. Vagal stimulation elicited frequency-dependent hypermotility with a short latency. Relaxatory events were also observed, which could indicate the presence of a non-adrenergic inhibitory pathway. 3. After atropine treatment, contractions and relaxations could still be elicited. The former were compared to cholinergic contractions and showed a lower maximal amplitude and a longer latency to onset. Moreover, they were antagonized by 80-100% by the opioid receptor antagonist, naloxone. 4. Vagal stimulation after hemicholinium, given in order to deplete the preganglionic acetylcholine content, elicited naloxone-sensitive contractions. This suggests that a subpopulation of the vagal preganglionic fibres is non-cholinergic. 5. Isolation of the balloon-containing segment did not qualitatively alter the responses, indicating that the vagal fibres reach the small intestine via the paravascular mesenteric nerves. 6. It is concluded that cholinergic and non-adrenergic, non-cholinergic (NANC) contractions, as well as relaxations, could be elicited by efferent vagal stimulation. The NANC contractions seem to result from the activation of opioid receptors causing disinhibition of a tonic neurogenic restraint on the gut muscle.

Chronic alcohol intake and carbachol-induced acute pancreatitis in the rat

Alcohol-induced changes in cholinergic and pancreozymin pathways regulating exocrine pancreatic secretion have been proposed to play a crucial role in the pathogenesis of acute alcoholic pancreatitis. In the present study we investigated the role of chronic alcohol intake in an experimental acute pancreatitis induced in rats by cholinergic hyperstimulation. Chronic alcohol intake interfered with the function of rat pancreatic muscarinic receptors in carbachol-induced acute pancreatitis. However, chronic alcohol intake did not sensitize the experimental animals to cholinergic hyperstimulation. Whether this increased resistance at the level of pancreatic muscarinic receptors contributes to acute alcoholic pancreatitis is discussed in the present article.

Dopamine-induced inhibition of endogenous acetylcholine release from the isolated ileal synaptosomal preparations of guinea-pig mediated via alpha-adrenoceptors

1. The effect of exogenous dopamine on the release of endogenous acetylcholine (ACh) from isolated ileal synaptosomal guinea-pig preparations was examined by means of high pressure liquid chromatography with electrochemical detection. 2. Release of ACh was induced by substance P or by depolarization with high potassium (50 mM) in a medium containing atropine propranolol and naloxone. 3. Dopamine produced a concentration-dependent inhibition of the evoked ACh release induced by substance P or in samples depolarized by high potassium. This action of dopamine was not reversed by the dopamine receptor antagonists either for the DA2 subtype domperidone, or for the DA1 subtype, SCH23390. Fenoldopam, the agonist of dopamine DA1 receptors, or quinpirole, the agonist of dopamine DA2 receptors, reduced the evoked ACh release, although only in high, non-dopamine-specific concentrations. 4. Failure of guanethidine or desipramine to inhibit this effect of dopamine ruled out mediation by endogenous noradrenaline. 5. Idazoxan and yohimbine reversed this dopamine-induced inhibition at concentration sufficient to abolish the action of clonidine. Influx of (45)Ca stimulated by substance P or high potassium into synaptosomal preparations was attenuated in the presence of dopamine. This inhibition by dopamine was also reversed by idazoxan or yohimbine but not by dopamine receptor antagonists. Moreover, the dopamine-induced inhibitions of both the ACh release and the influx of (45)Ca disappeared in the samples treated with pertussis toxin at a dose sufficient to abolish the action of clonidine. 6. It is concluded that dopamine suppresses the influx of calcium ions into cholinergic nerve terminals via an activation of alpha2-adrenoceptors coupled with a pertussis toxin-sensitive GTP-binding protein, resulting in the decrease of ACh release from ileal synaptosomes of guinea-pigs.

Release of bombesin-like immunoreactivity from synaptosomal membranes isolated from the rat ileum

In the enteric nervous system, direct effects on peptidergic neurotransmitter release are difficult to assess since the neuronal network predisposes to numerous interactions between the various transmitter systems. The aim of the present study was to examine the release of bombesin-like immunoreactivity from isolated nerve synapses of the enteric nervous system. Enriched synaptosomal fractions were obtained by using homogenized tissue from rat ileum, which was subjected to various steps of differential and sucrose density centrifugation. Specific binding of [3H]saxitoxin served as a marker for neuronal membranes. For comparison, the content of bombesin-like immunoreactivity was determined. Both the enriched synaptosomal fraction (mitochondrial fraction II or P2) and the purified synaptosomal fraction (F2), obtained after discontinuous sucrose density centrifugation, showed substantial enrichment of the neuronal marker [3H]saxitoxin and bombesin-like immunoreactivity. The basal release of bombesin-like immunoreactivity was 52 +/- 17 pg/mg (100%). KCl-evoked depolarization (65 mM) significantly stimulated the release of bombesin-like immunoreactivity to 142.2% (P < 0.05, n = 17). The release was abolished in Ca(2+)-free medium. Stimulation of the release of bombesin-like immunoreactivity was also observed in the presence of the Ca2+ ionophore A-23187 (10(-6) M: 129%, P < 0.05, n = 17), supporting the role of Ca2+ in the release process. Cholinergic stimulation with carbachol elicited a significant dose-dependent release of bombesin-like immunoreactivity (10(-8) M: 106%, 10(-7) M: 175%, P < 0.05, 10(-6) M: 156%, P < 0.05, 10(-5) M: 115%, n = 14), which was reduced by atropine (10(-6) M: 99%, P < 0.01, n = 14). The basal value was 67 +/- 9 pg/mg (100%). The different effects of the muscarinic M1 receptor antagonist pirenzepine, which stimulated release of bombesin-like immunoreactivity in combination with carbachol 10(-6) M (10(-6) M: 123%, n = 10), and of the muscarinic M2 receptor antagonist AFDX 116, which attenuated release of bombesin-like immunoreactivity evoked by carbachol (10(-5) M: 66%, P < 0.01, 10(-6) M: 88%, n = 10), strongly suggest modulation of the release of bombesin-like immunoreactivity at the presynaptic receptor site through an excitatory muscarinic M2 receptor. The basal value was 46 +/- 9 pg/mg (100%). In summary, bombesin-like immunoreactivity can be released from these synaptosomes by both depolarization with KCl in a Ca(2+)-dependent manner and by cholinergic stimulation. The synaptosomes of intrinsic nerves of the gut offer an approach to study the release of neuropeptides and neurotransmitters at the subcellular level independent of the ganglionic network.

Presynaptic muscarinic receptors modulate acetylcholine release from rat antral mucosal/submucosal nerves

The purpose of the present studies was to determine whether autoinhibition of acetylcholine release could be demonstrated in vitro from mucosal/submucosal neurons in rat antrum. Rat antral mucosal/submucosal tissues preloaded with [3H]choline were perifused and [3H]acetylcholine release measured under basal and stimulated conditions. Carbachol inhibited both spontaneous and evoked (electrical field stimulation, KCl) acetylcholine release from rat antral tissues: 1 x 10(-5) M carbachol inhibited basal [3H]ACh release maximally to -38.2 +/- 3.1% (P < 0.001 vs control). The nonselective muscarinic antagonist atropine enhanced both basal and stimulated acetylcholine release and abolished carbachol-induced inhibition of acetylcholine release. Pirenzepine, a muscarinic M1 receptor antagonist, inhibited acetylcholine release and did not alter carbachol-induced inhibition of acetylcholine release. In conclusion, acetylcholine release from rat antral mucosal/submucosal neurons is regulated negatively by a presynaptic feedback mechanism involving M2 and/or M3 receptors, while presynaptic M1 receptors facilitate release of neurotransmitter.

Increase by NO synthase inhibitors of acetylcholine release from guinea-pig myenteric plexus

The effects of nitric oxide (NO) synthase inhibitors on the electrically evoked release of [3H]acetylcholine were studied in guinea-pig myenteric plexus preparations preincubated with [3H]choline. NG-monomethyl-L-arginine (EC50 5.3 mumol l-1) and NG-nitro-L-arginine (EC50 1.3 mumol l-1) concentration-dependently increased the evoked release of [3H]acetylcholine without affecting the basal outflow. The facilitatory effect of NG-mono-methyl-L-arginine was prevented by L-arginine but not by D-arginine. The results suggest that endogenous NO inhibits the depolarisation-evoked release of acetylcholine.

Alcohol, pancreatic muscarinic receptors and acute pancreatitis

Recently, a new theory about the pathogenesis of acute alcoholic pancreatitis was proposed. The aim of the present work was to further study the basis of this cholinergic theory about the pathogenesis of acute alcoholic pancreatitis. The results indicated that already a short-term alcohol consumption induces in some rats a dramatic decrease in the number of pancreatic muscarinic receptors. This decrease may predispose to acute alcoholic pancreatitis by increasing the cholinergic tone, since excessive cholinergic tone invariably leads to acute pancreatitis both in experimental animals and in man. Thus, the pathogenetic mechanism triggering acute alcoholic pancreatitis might be similar to the mechanism triggering acute pancreatitis caused by the scorpion sting, intoxication with an antiacetylcholine-esterase-containing insecticides or after excessive cholinergic stimulation.

Muscarinic acetylcholine receptor subtypes in smooth muscle

Muscarinic acetylcholine M2 and M3 receptor subtypes are coexpressed in many types of smooth muscle including gastrointestinal smooth muscle, urinary bladder and vascular and airway tissue. Activation of M3 receptors, via the G protein Gq, results in increased polyphosphoinositide hydrolysis, release of Ca2+ ions from the sarcoplasmic reticulum and consequently causes contraction. Quantitation of the relative expression of M2 and M3 receptors has shown that the proportion of M2 receptors often predominates over the M3 receptor population by 4:1 or more. Although it is established that M2 receptors preferentially link, via a pertussis-toxin-sensitive G protein Gi, to inhibition of adenylate cyclase activity, relatively little is known concerning the physiological role of the M2 receptor population. In this review, Richard Eglen and colleagues discuss recent data concerning the possible role(s) of muscarinic receptor subtypes in smooth muscle and appraise the pharmacological methods for dissecting the function of muscarinic receptor subtypes in tissues co-expressing multiple receptors.

Muscarinic M1 and M3 receptor antagonist effects of a new pirenzepine analogue in isolated guinea-pig ileal longitudinal muscle-myenteric plexus

The new pirenzepine analogue DF 545 has been tested for its muscarinic M1 and M3 receptor antagonist properties in guinea-pig longitudinal muscle-myenteric plexus preparations. McN-A-343-induced inhibition of twitch contractions was taken as a parameter for muscarinic M1 receptor activation while electrical and acetylcholine-induced contractions were considered as a model for muscarinic M3 receptor stimulation. An unexpected contractile effect evoked by McN-A-343 was also investigated. In contrast to pirenzepine, DF 545 only weakly counteracted the M1-mediated McN-A-343 inhibitory effect but blocked M3-related twitch- or acetylcholine-stimulated responses with a 2-fold higher affinity than pirenzepine. Therefore, in this preparation, our findings suggest that DF 545 does not share the selectivity profile exhibited by pirenzepine at ileal muscarinic receptors. Studies on the McN-A-343 contractile effect provide evidence that this agonist may interact with ileal muscarinic effector sites in a different way from other cholinergic agents.

In vivo study of the role of muscarinic receptors in the parasympathetic control of rabbit colonic motility

The aim of the present study was to elucidate the role of the non-M1 muscarinic receptors, in the extrinsic and intrinsic nerve control of in vivo colonic motility. Experiments were performed on the proximal colon of anaesthetized rabbits. In this species, the parasympathetic innervation of the proximal colon originates from the vagus nerves. The action of methoctramine and 4-diphenyl-acetoxy-N-methylpiperidine methobromide (4-DAMP) was studied on excitatory junction potentials (EJPs), and on inhibitory junction potentials (IJPs) elicited in smooth muscle cells by stimulating parasympathetic efferents. The effects of the same drugs on spontaneous spiking activity were also investigated. The EJPs either decreased or disappeared after intra-arterial (i.a.) administration of 4-DAMP (45 pg to 450 ng). In the presence of 4-DAMP, further intravenous (i.v.) administration of pirenzepine (0.1 mg.kg-1) had facilitatory effects on the inhibitory pathway, i.e., after abolition of the EJPs, vagal stimulation elicited IJPs. With the highest dose of 4-DAMP, vagal stimulation immediately elicited IJPs the amplitude of which still increased after pirenzepine. In the presence of 4-DAMP, the spontaneous spike discharge was not noticeably altered. Methoctramine (0.37 to 75 micrograms, i.a. or 50 micrograms to 0.2 mg.kg-1, i.v.) increased the amplitude of the EJPs, whereas it decreased that of the IJPs. In addition, at the same doses, it either initiated or increased spike discharges that were not altered by pirenzepine up to 0.2 mg.kg-1, i.v. The so-called rebound excitation occurring after IJPs was not affected by methoctramine. No change in the EJP or IJP amplitude was observed with gallamine at sufficiently high doses to paralyse striated muscles (up to 3 mg.kg-1.h-1). It is concluded that the parasympathetic excitatory pathway to smooth muscle is blocked by 4-DAMP, whereas it is facilitated by methoctramine. 4-DAMP has no effect on the inhibitory pathway which is strongly depressed by methoctramine; however, the fact that these two drugs have opposite effects indicates that 4-DAMP and methoctramine may act on different muscarinic receptor subtypes. In addition, the facilitatory effects of pirenzepine on IJPs observed in animals pre-treated with 4-DAMP, indicates that the latter drug may act on non-M1 and non-M2 (presumably M3) muscarinic receptors. Methoctramine acts on non-M1 and non-M3 (presumably M2) receptors. The spike discharge induced by methoctramine is presumably due to an increased release of acetylcholine, and possibly also of a non-cholinergic transmitter which has excitatory effects on smooth muscle, the identification of which requires further investigations.(ABSTRACT TRUNCATED AT 400 WORDS)

Possible mechanisms of the effect of physostigmine on the facilitation of acetylcholine release in the guinea pig myenteric plexus

The automodulation of acetylcholine (ACh) release in the guinea pig myenteric plexus-longitudinal muscle preparation was investigated by studying the electric stimulation-evoked release of radiolabeled ACh. When the release associated with neuronal activity was challenged by the muscarinic antagonist atropine, the release was not significantly enhanced. When the acetylcholinesterase (AChE) blocker physostigmine was present, the well-established muscarinic receptor-mediated autoinhibition was operative, i.e., the release was significantly reduced. However, when both drugs were added together, the release was much higher than under control conditions. Therefore, it seems likely that there is also a facilitatory system. We made an effort to clear up the mechanism of this facilitation by blocking possible nicotinic presynaptic receptors, by excluding the alpha 2-adrenoceptor-mediated masking effect of noradrenergic heteromodulation, by preventing a possible ATP-mediated mechanism, and by attempting to prevent the direct effect of physostigmine. None of these manipulations resulted in a decrease of the surplus release. It is concluded, that when the negative feedback modulation of ACh is inhibited and AChE activity is reduced, high levels of ACh facilitates additional release of ACh from the nerve terminals, possibly through a not yet verified class of receptors.

Characterization of the prejunctional muscarinic receptors mediating inhibition of evoked release of endogenous noradrenaline in rabbit isolated vas deferens

The aim of the present study was to characterize the prejunctional modulation of evoked release of endogenous noradrenaline in rabbit vas deferens by the use of muscarinic receptor agonists and subtype-preferring antagonists. Vasa deferentia of the rabbit were stimulated electrically by trains of 120 pulses delivered at 4 Hz or trains of 30 pulses at 1 Hz. The inhibition by muscarinic agonists of the stimulation-evoked overflow of endogenous noradrenaline in the absence and presence of antagonists was used to determine affinity constants for antagonists. These values were compared with those observed at putative M1 receptors inhibiting neurogenic twitch contractions in the rabbit vas deferens and with affinity data obtained at M1(m1)-M4(m5) receptors in functional studies and binding experiments. The evoked overflow of noradrenaline from sympathetic nerves was enhanced by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the P2 purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) and indomethacin, indicating a tonic inhibition by endogenous A1 and P2 purinoceptor agonists and prostanoids, respectively. The stimulation-evoked overflow at 4 Hz was not sensitive to inhibition by the muscarinic agonists methacholine or 4-(4-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium iodide (4-Cl-McN-A-343). In contrast, at a stimulation frequency of 1 Hz the evoked noradrenaline release was decreased by muscarinic agonists (EC50): arecaidine propargyl ester (0.062 microM), 4-Cl-McN-A-343 (0.32 microM), 4-(4-fluorophenylcarbamoyloxy)-2-butynyl-N-methyl-pyrrolidinium tosylate (4-F-PyMcN+; 0.48 microM) and methacholine (0.86 microM). The affinity constants of most of the muscarinic antagonists [atropine: pKB = 9.47; (R)-trihexyphenidyl: pKB = 9.18; pirenzepine: pA2 = 7.68; methoctramine: pKB = 6.90] are consistent with estimates of these antagonists at M1(m1) receptors determined in various functional and binding studies. The high antagonistic potency of pirenzepine and (R)-trihexyphenidyl and the agonistic activity of 4-F-PyMcN+ argue for the involvement of M1, and against that of M2 and M3 receptors in the inhibition of evoked noradrenaline overflow. However, the high apparent pKB of 8.30 for himbacine is not in accordance with an M1 receptor; by contrast, it would be compatible with the presence of M2 or M4 receptors. The potencies of the tested muscarinic agonists and antagonists largely agree with those obtained for the inhibition of neurogenic twitch responses (0.05 Hz) in the rabbit vas deferens.(ABSTRACT TRUNCATED AT 400 WORDS)

Opposite modulation by muscarinic M1 and M3 receptors of acetylcholine release from guinea pig ileum as measured directly

Muscarinic receptor agonist and antagonist effects on acetylcholine release evoked by electrical or dimethylphenylpiperazinium stimulation from guinea pig ileum were evaluated by measuring acetylcholine with a high performance liquid chromatography-electrochemical detector system. AF102B (cis-2-methylspiro-(1,3-oxathiolane-5,3')-quinuclidine), a muscarinic M1 receptor agonist, increased markedly the evoked release of acetylcholine. In contrast, pirenzepine decreased the evoked acetylcholine release. 4-DAMP (4-diphenyl-acetoxy-N-methylpiperidine methiodide) and p-F-HHSiD (p-fluoro-hexahydrosiladifenidol), muscarinic M3 antagonists, increased the release of acetylcholine. Atropine enhanced acetylcholine release to a similar extent while bethanechol reduced the electrically evoked acetylcholine release. This reduction was virtually unaffected by methoctramine, but was antagonized by 4-DAMP or p-F-HHSiD. The results from direct determination of acetylcholine suggest that, in contrast to autoinhibition by stimulation of muscarinic M3 receptors, stimulation of presynaptic muscarinic M1 receptors is predominantly involved in enhancement of the acetylcholine release from guinea pig ileal nerves, and that AF102B functions as a muscarinic M1 agonist in this peripheral neuron.

Inhibitory effect of octopamine on the release of endogenous acetylcholine from isolated myenteric synaptosomes of guinea-pig

1. The effect of octopamine on the release of endogenous acetylcholine (ACh) from isolated ileal synaptosomal preparations of guinea-pigs was examined using high pressure liquid chromatography with electrochemical detection. Release of ACh was induced by substance P or by depolarization with high potassium (50 mmol/L) in medium containing atropine, propranolol and naloxone. 2. Octopamine produced a dose-dependent inhibition of substance P-induced ACh release. A similar inhibitory action of octopamine was found in the samples depolarized by high potassium as a reference. 3. The action of octopamine was not reversed by the dopamine receptor antagonists either for the DA-2 subtype, domperidone, or for the DA-1 subtype, SCH23390, or by haloperidol. However, idazoxan and yohimbine antagonized this octopamine-induced inhibition at concentrations sufficient to abolish the action of clonidine. 4. Failure of guanethidine or nomifensine to inhibit octopamine ruled out mediation by noradrenergic neurotransmitters. 5. Octopamine decreased the influx of [45Ca] stimulated by substance P into synaptosomal preparations and this was reversed by idazoxan or yohimbine at concentrations sufficient to block the action of clonidine. 6. Pertussis toxin abolished the inhibitory action of octopamine at a dose high enough to block the action of clonidine. 7. These results indicate that octopamine suppresses the influx of calcium ions into cholinergic nerve terminals of ileal synaptosomes of guinea-pigs via an activation of alpha 2-adrenoceptors coupled with a pertussis toxin-sensitive GTP-binding protein which results in a decrease of ACh release.

SDZ ENS 163 a novel pilocarpine like drug: pharmacological in vitro and in vivo profile

The thiolactone analogue of pilocarpine, SDZ ENS 163, acts in vitro and in vivo as a partial agonist at M1/M3 and as an antagonist at M2 muscarinic receptors. In vitro, the properties of SDZ ENS 163 have been investigated in several functional models for muscarinic receptors: it is a full agonist at M1 (rat superior cervical ganglion, carbachol = 100%) and a partial agonist at M3 receptors (guinea pig ileum). However, the drug shows antagonistic properties at M2 receptors (rat atria). Radioligand binding studies with 3H-N-methylscopolamine (3H-NMS) using CHO cells expressing m1 or m3 receptors indicate that SDZ ENS 163 does not discriminate between m1 and m3 receptors (Ki 1.5 and 2.4 microM respectively). Regarding phosphoinositide (PI) turnover in A9L cells, SDZ ENS 163 is a partial agonist at m1 receptors. In ex vivo neurochemical studies in rats SDZ ENS 163 displays effects characteristic of muscarinic antagonists regarding the turnover of ACh which is increased in the brain. At a similar dose-range SDZ ENS 163 accelerates PI metabolism in the rat brain in vivo and increases the energy of the low frequency band (2-5 Hz) in the rat hippocampal EEG. These effects observed in vivo are consistent with postsynaptic M1 agonistic and presynaptic M2 antagonistic activities. Since SDZ ENS 163 at centrally active doses exerts no peripheral cholinergic effects, it may be useful for the symptomatic treatment of Alzheimer's disease.

Role of M1 muscarinic receptors in the parasympathetic control of colonic motility in cats and rabbits

1. The effects of pirenzepine (a selective blocking agent of M1 muscarinic receptors) were studied on excitatory junction potentials (EJPs) and inhibitory junction potentials (IJPs) elicited on colonic smooth muscle by stimulation of efferent parasympathetic nerve fibres in anaesthetized cats and rabbits. 2. Pirenzepine (25 micrograms kg-1 to 0.2 mg kg-1, i.v.) decreased the amplitude of EJPs or abolished them. In pirenzepine, parasympathetic stimulation elicited IJPs in most cases. 3. In both species, pirenzepine initiated spontaneous IJPs, indicating an increase in the activity of the non-adrenergic, non-cholinergic (NANC) inhibitory neurones. 4. The results suggest that M1 muscarinic receptors are involved in synaptic transmission within intramural plexuses, at interneuronal synapses in the parasympathetic excitatory pathway to colonic smooth muscle, but are not involved in the pathway to the NANC inhibitory neurones. The facilitation of IJPs by pirenzepine suggests that, under normal physiological conditions, NANC neurones are tonically inhibited by an intramural nervous circuit involving M1 muscarinic receptors.

Motility of rabbit proximal colon. Relevance of cholinergic pathways and role of different muscarinic receptor subtypes

To better define the physiologic relevance of the cholinergic muscarinic input to the rabbit colon and the role of different muscarinic receptor subtypes, we studied the effects of atropine, telenzepine (MI antagonist) and DF594 (M3 antagonist) on colonic motility in eight conscious rabbits fitted with bipolar electrodes and strain gauges along the proximal colon. In some experiments, the chronotropic and mydriatic effect of the pharmacological agents were also assessed. Two main patterns of spike activity were identified: short spike bursts (SSBs), which were usually stationary, and long spike bursts (LSBs), which were usually propagated. Both myoelectrical patterns were dose-dependently inhibited by atropine (0.06-4 mumol/kg). Atropine, at the doses of 2-4 mumol/kg, abolished both myoelectrical and mechanical activity. Telenzepine (0.008-0.125 mumol/kg) dose-dependently inhibited migrating LSBs without significant effect on SSBs. Higher doses (0.25-0.5 mumol/kg) inhibited both LSBs and SSBs. DF594 (0.06-2 mumol/kg) dose-dependently inhibited both migrating LSBs and SSBs. The three antimuscarinic agents, at doses that inhibited colonic spike activity by approximately 80% (equiactive doses), behaved as follows on heart rate and pupil diameter: atropine induced tachycardia and mydriasis, telenzepine had no effect, and DF594 induced slight mydriasis with no effect on heart rate. We conclude that spontaneous motility in the rabbit proximal colon depends on a muscarinic excitatory input. M3 receptors are involved in the control of both LSBs and SSBs, while M1 receptors play an important role in the regulation of LSBs. The development of selective antimuscarinic drugs, acting on a given motility pattern and with minimal side effects, may offer new perspectives in the treatment of functional bowel motor disorders.

Muscarinic autoreceptors of Torpedo electric organ are of the M1 subtype: evidence by radioligand binding using selective antagonists

The presynaptic muscarinic autoreceptor of Torpedo marmorata electric organ has been characterised by radioligand binding studies using the subtype-selective antagonists pirenzepine, (+)-telenzepine, methoctramine, and AF-DX 116. The presynaptic receptor had relatively high affinity for the M1 antagonists pirenzepine and (+)-telenzepine (Ki = 35 and 7 nM, respectively) and lower affinities for the M2 antagonists AF-DX 116 and methoctramine (Ki = 311 and 277 nM, respectively). Comparison of these binding data with those from an M2 receptor (rat heart membranes) assayed under identical conditions and with data in the recent literature suggests that the Torpedo muscarinic autoreceptor has a pharmacology most similar to the M1 pharmacological subtype of muscarinic acetylcholine receptor.

Evidence for inhibitory nicotinic and facilitatory muscarinic receptors in cholinergic nerve terminals of the rat urinary bladder

Cholinergic prejunctional modulatory receptors on parasympathetic nerves in the rat urinary bladder were studied by measuring 3H-acetylcholine (ACh) release in muscle strips from the bladder body. Electrical field stimulation markedly increased 3H-ACh overflow in strips preloaded with 3H-choline. Oxotremorine (1 microM), an M2 receptor agonist and DMPP (10 microM) a nicotinic (N) receptor agonist decreased the release of ACh (50% and 55% respectively); whereas McN-A 343 (50 microM) an M1 receptor agonist increased the release (33%), indicating the presence of three types of modulatory receptors. The anticholinesterase agent, physostigmine in concentrations of 1, 5 and 25 microM and neostigmine (5 microM) increased ACh release (44-710%). However a low concentration of physostigmine (0.05 microM) decreased release. Pirenzepine, an M1 muscarinic antagonist or atropine blocked the increased ACh release in physostigmine-treated strips, but in normal strips pirenzepine did not change release and atropine increased release. McN-A 343 or prolonged application (15 min) of DMPP increased ACh release (376% and 391% respectively) in physostigmine-treated strips. The response to McN-A 343 was blocked by pirenzepine. d-Tubocurarine (DTC), a nicotinic receptor blocker, enhanced ACh release in the presence of physostigmine but proved to be ineffective in normal preparations. These findings suggest that all three cholinergic receptors (M1 facilitatory, N inhibitory and M2 inhibitory) are activated by endogenous ACh in physostigmine treated preparations whereas only M2-inhibitory receptors are activated in normal preparations. It will be important in future studies to determine whether M1 and M2 mechanisms can also be activated under more physiological conditions in the bladder and whether they are present at other cholinergic synapses.

Inhibition of choline efflux results in enhanced acetylcholine synthesis and release in the guinea-pig corticocerebral synaptosomes

Synthesis and release of [3H]acetylcholine ([3H]ACh) were measured in synaptosomes from the guinea pig cerebral cortex after preloading with [3H]choline ([3H]Ch). We demonstrate here that inhibition of choline (Ch) efflux results in an increase in acetylcholine (ACh) synthesis and release. Our findings are as follows: (1) inhibition of [3H]Ch efflux by hemicholinium-3 (HC-3) (100 microM), increased the levels of both the released (116% of control) and the residing (115% of control) [3H]ACh. (2) The muscarinic agonist, McN-A-343 (100 microM), which was previously shown to inhibit Ch efflux, also increased the released (121% of control) and the residing (109% of control) [3H]ACh. (3) Omission of Na+ ions (which are required for Ch transport) from the incubation medium had similar effects to those observed with McN-A-343 and HC-3. These results suggest inverse relationships between Ch efflux on one hand, and ACh synthesis and release on the other hand. (4) Depolarization with 50 mM K+, or with the K+ channel blocker, 4-aminopyridine (100 microM), also increased the total level of [3H]ACh (113 and 107% of nondepolarized synaptosomes, respectively). However, whereas conditions that inhibit Ch transport such as HC-3, McN-A-343 and "no sodium" increased both the residing and the released [3H]ACh depolarization with high K+ or 4-aminopyridine reduced the residing (79 and 87% of control, respectively) and increased only the released [3H]ACh (182 and 148% of control, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of prejunctional muscarinic autoreceptors in the guinea-pig trachea

1. The effects of ten muscarinic antagonists on electrically evoked [3H]-acetylcholine release and muscle contraction were compared in an epithelium-free preparation of the guinea-pig trachea that had been preincubated with [3H]-choline. 2. The M3-selective antagonists UH-AH 37, 4-diphenyl-acetoxy-N-piperidine methobromide and para-fluorohexahydrosiladiphenidol were more potent in reducing the contractile response than in facilitating the evoked [3H]-acetylcholine release. Hexahydrosiladiphenidol did not discriminate between pre- and postjunctional effects. The rank order of the postjunctional potencies of the ten antagonists as well as the postjunctional pA2 values obtained for hexahydrosiladiphenidol (7.95) and AQ-RA (7.08) identified the muscular receptor as an M3 subtype. 3. The M2-selective antagonists methoctramine, AF-DX 116 and AQ-RA 741 were more potent in facilitating the evoked [3H]-acetylcholine release than in inhibiting the contractile response. The increase in release by low concentrations of methoctramine, AF-DX 116 and AQ-RA 741 was paralleled by an enhancement of the stimulation-evoked contractions. 4. Comparison of the pre- and postjunctional potencies of the M1-, M2- and M3-selective antagonists suggests that autoinhibition of acetylcholine release is mediated via an 'M2-like' receptor which differs from the cardiac type M2 receptor in its relatively high affinity for hexahydrosiladiphenidol.

An attempt to localize the site of action of different agents within cholinergic motor neurones of the myenteric plexus-longitudinal muscle preparation of the guinea pig ileum by the triple bath method

The site of action of cholinergic, adrenergic, peptidergic and opioid agents was studied in myenteric plexus-longitudinal muscle strips from the guinea pig ileum. A preparation in a special triple bath was drawn through two rubber membranes, dividing the strip into three segments. Neurogenic stimulation of the oral segment, set up nerve action potentials also in the neurones projecting axons up to the aboral segment. These axons, turning into varicose nerve terminals, conducted action potentials aborally across the middle segment, that was up to 10 mm wide. Finally, the nerve terminals, extending into the aboral segment, might be also invaded triggering twitches. Agents were added, either to the oral segment, to affect the genesis and spread of action potentials in the proximal parts of cholinergic neurones (cell bodies, axon hillocks, initial segments and axon preterminals) or they were added to the middle segment to affect propagation of action potentials in varicose nerve terminals. As a result, the amplitude of aboral twitches reflected their effects at each site, quantitatively. Noradrenaline and ethylketocyclazocine were more effective at the site of varicose nerve terminals, whereas substance P, acetylcholine and oxotremorine were more effective at the proximal parts; pilocarpine and nicotine were effective at both sites. Changes in membrane polarization might be the final common effect in the mechanism of action of all the stimulatory agents used.

Non-synaptic cholinergic modulation of neurogenic twitches of the guinea-pig ileum

The effect of cholinergic and anticholinergic compounds on conduction of neuronal excitation has been studied in myenteric plexus-longitudinal muscle strips from the guinea-pig ileum. A preparation in a special triple bath was drawn through two rubber membranes dividing the strip into three segments. Neurogenic stimulation of the oral segment set up nerve action potentials propagating aborally across the middle segment (10 mm) so that the aboral segment might be also invaded, eventually. Drugs were added to the middle segment to affect neuronal propagation (non-synaptic effects) which was monitored by twitch height of the aboral segment. The application of acetylcholine to the middle segment augmented aboral twitches. The effects of nicotine, pilocarpine and oxotremorine were selectively blocked by (+)-tubocurarine, pirenzepine and atropine, respectively. The effect of acetylcholine was suppressed by pirenzepine and atropine and mimicked by doubling of KCl concentration. The effect of acetylcholine may be thus explained by the facilitated propagation of nerve action potentials in partially depolarized cholinergic terminals via stimulation of muscarinic receptors. The adenylate cyclase system is not directly involved in the mechanism of muscarinic facilitation of neuronal propagation in the terminals; however, it may participate in the modulation of a final common effector mechanism.

Presynaptic effects of methoctramine on release of acetylcholine

Stores of transmitter, labelled with [3H]choline, were used to study the negative feedback modulation of the release of acetylcholine by presynaptic M2 muscarinic receptors. The release of acetylcholine was measured by radioassaying the electrical stimulation-evoked release of [3H]acetylcholine from slices of cerebral cortex of the rat and from the Auerbach plexus of the guinea pig ileum. Experimental conditions (2 Hz, 240 shocks) were chosen where the negative-feedback modulation by endogenous acetylcholine was not significant, therefore the presynaptic affinity constant for antagonists was not underestimated. The M2 agonist oxotremorine inhibited the release of acetylcholine in a concentration-dependent manner in both preparations. The IC50 values for oxotremorine were 10.8 +/- 4.89 x 10(-6) M on the cortex and 5.89 +/- 3.85 x 10(-8) M on the Auerbach plexus (n = 4). The effect of oxotremorine was blocked by atropine, similarly to methoctramine, which is a cardioselective muscarinic receptor antagonist. The dose-ratio and dissociation constant were calculated by measuring the righward shift that methoctramine and other antagonists produced on the inhibitory dose-effect curve for oxotremorine. The antagonist equilibrium dissociation constants (pKB) of methoctramine were 5.69 +/- 0.27 and 5.51 +/- 0.37 on the cortical and the myenteric plexus preparations, respectively (n = 4). Postsynaptic antimuscarinic affinity (pA2) of methoctramine on the smooth muscle of the guinea pig ileum was found to be 6.68 +/- 0.11 (n = 4). These findings indicate that, although methoctramine is a cardioselective compound, unlike pancuronium, it may not be a useful tool for discriminating between different presynaptic muscarinic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Possible existence of dopaminergic receptors on cholinergic nerve terminals in the guinea-pig Auerbach's plexus

In the Auerbach's plexus of guinea-pig ileum lower concentrations of oxotremorine (Oxo-T) produced an increase, whereas higher concentrations of Oxo-T caused inhibition of evoked acetylcholine (ACh) release, measured by bioassay using dorsal muscle of leech. Dopamine inhibited the increase in evoked release of ACh induced by Oxo-T as a function of its concentration and this inhibitory effect was nullified in presence of pimozide, the dopamine receptor antagonist. The results demonstrate existence of presynaptic dopamine receptors having inhibitory influence on excitatory presynaptic muscarinic receptors on regulation of ACh release. However, no physiological role of dopamine could be observed on ACh release in this preparation.