Characterization of muscarinic receptors in guinea-pig uterus

Elucidating the role of muscarinic acetylcholine receptor (mAChR) signaling in efferent mediated responses of vestibular afferents in mammals

The peripheral vestibular system detects head position and movement through activation of vestibular hair cells (HCs) in vestibular end organs. HCs transmit this information to the CNS by way of primary vestibular afferent neurons. The CNS, in turn, modulates HCs and afferents via the efferent vestibular system (EVS) through activation of cholinergic signaling mechanisms. In mice, we previously demonstrated that activation of muscarinic acetylcholine receptors (mAChRs), during EVS stimulation, gives rise to a slow excitation that takes seconds to peak and tens of seconds to decay back to baseline. This slow excitation is mimicked by muscarine and ablated by the non-selective mAChR blockers scopolamine, atropine, and glycopyrrolate. While five distinct mAChRs (M1-M5) exist, the subtype(s) driving EVS-mediated slow excitation remain unidentified and details on how these mAChRs alter vestibular function is not well understood. The objective of this study is to characterize which mAChR subtypes drive the EVS-mediated slow excitation, and how their activation impacts vestibular physiology and behavior. In C57Bl/6J mice, M3mAChR antagonists were more potent at blocking slow excitation than M1mAChR antagonists, while M2/M4 blockers were ineffective. While unchanged in M2/M4mAChR double KO mice, EVS-mediated slow excitation in M3 mAChR-KO animals were reduced or absent in irregular afferents but appeared unchanged in regular afferents. In agreement, vestibular sensory-evoked potentials (VsEP), known to be predominantly generated from irregular afferents, were significantly less enhanced by mAChR activation in M3mAChR-KO mice compared to controls. Finally, M3mAChR-KO mice display distinct behavioral phenotypes in open field activity, and thermal profiles, and balance beam and forced swim test. M3mAChRs mediate efferent-mediated slow excitation in irregular afferents, while M1mAChRs may drive the same process in regular afferents.

Multitargeting nature of muscarinic orthosteric agonists and antagonists

Muscarinic receptors (mAChRs) are typical members of the G protein-coupled receptor (GPCR) family and exist in five subtypes from M₁ to M₅. Muscarinic receptor subtypes do not sufficiently differ in affinity to orthosteric antagonists or agonists; therefore, the analysis of receptor subtypes is complicated, and misinterpretations can occur. Usually, when researchers mainly specialized in CNS and peripheral functions aim to study mAChR involvement in behavior, learning, spinal locomotor networks, biological rhythms, cardiovascular physiology, bronchoconstriction, gastrointestinal tract functions, schizophrenia, and Parkinson’s disease, they use orthosteric ligands and they do not use allosteric ligands. Moreover, they usually rely on manufacturers’ claims that could be misleading. This review aimed to call the attention of researchers not deeply focused on mAChR pharmacology to this fact. Importantly, limited selective binding is not only a property of mAChRs but is a general attribute of most neurotransmitter receptors. In this review, we want to give an overview of the most common off-targets for established mAChR ligands. In this context, an important point is a mention the tremendous knowledge gap on off-targets for novel compounds compared to very well-established ligands. Therefore, we will summarize reported affinities and give an outline of strategies to investigate the subtype’s function, thereby avoiding ambiguous results. Despite that, the multitargeting nature of drugs acting also on mAChR could be an advantage when treating such diseases as schizophrenia. Antipsychotics are a perfect example of a multitargeting advantage in treatment. A promising strategy is the use of allosteric ligands, although some of these ligands have also been shown to exhibit limited selectivity. Another new direction in the development of muscarinic selective ligands is functionally selective and biased agonists. The possible selective ligands, usually allosteric, will also be listed. To overcome the limited selectivity of orthosteric ligands, the recommended process is to carefully examine the presence of respective subtypes in specific tissues via knockout studies, carefully apply “specific” agonists/antagonists at appropriate concentrations and then calculate the probability of a specific subtype involvement in specific functions. This could help interested researchers aiming to study the central nervous system functions mediated by the muscarinic receptor.

M1 and M3 muscarinic receptors may play a role in the neurotoxicity of anhydroecgonine methyl ester, a cocaine pyrolysis product

The smoke of crack cocaine contains cocaine and its pyrolysis product, anhydroecgonine methyl ester (AEME). AEME possesses greater neurotoxic potential than cocaine and an additive effect when they are combined. Since atropine prevented AEME-induced neurotoxicity, it has been suggested that its toxic effects may involve the muscarinic cholinergic receptors (mAChRs). Our aim is to understand the interaction between AEME and mAChRs and how it can lead to neuronal death. Using a rat primary hippocampal cell culture, AEME was shown to cause a concentration-dependent increase on both total [³H]inositol phosphate and intracellular calcium, and to induce DNA fragmentation after 24 hours of exposure, in line with the activation of caspase-3 previously shown. Additionally, we assessed AEME activity at rat mAChR subtypes 1–5 heterologously expressed in Chinese Hamster Ovary cells. l-[N-methyl-³H]scopolamine competition binding showed a preference of AEME for the M₂ subtype; calcium mobilization tests revealed partial agonist effects at M₁ and M₃ and antagonist activity at the remaining subtypes. The selective M₁ and M₃ antagonists and the phospholipase C inhibitor, were able to prevent AEME-induced neurotoxicity, suggesting that the toxicity is due to the partial agonist effect at M₁ and M₃ mAChRs, leading to DNA fragmentation and neuronal death by apoptosis.

Functional characterization of acetylcholine receptors and calcium signaling in rat testicular capsule contraction

The motor activity of mammalian testicular capsule (TC) contributes to male fertility and infertility, but the acetylcholine receptors related to the contractions induced by cholinergic drugs are poorly known. Indeed to characterize the acetylcholine receptors and cellular signaling by Ca(2+) involved in TC motor activity of rats, the potency of agonists (pD₂) and antagonists (pA₂) of acetylcholine receptors, and effects of Ca(2+) cellular transport blockers on the cholinergic contractions were evaluated. pD₂ values of acetylcholine (5.98) were ten-fold higher than that of carbachol (4.99). Efficacy (Emax) of acetylcholine and carbachol to induce contractions corresponded to 95% and 97% of Emax for KCl, but Emax for nicotine was very low (8% of Emax for KCl). Further, physostigmine did not affect the acetylcholine potency. Contractions induced by acetylcholine or carbachol were antagonized by muscarinic but not nicotinic antagonist. The order of pA₂ values obtained for muscarinic antagonists, namely atropine>4-DAMP>AF-DX116>pirenzepine, corresponded to a typical profile of M3 receptors. Contractions induced by acetylcholine or carbachol were inhibited by blockers of Ca(2+) influx through voltage-dependent calcium channels (nifedipine and Ni(2+)), Ca(2+) reuptake by sarco-endoplasmic reticulum (cyclopiazonic acid) and mitochondria (FCCP). The protein kinase C (PKC) inhibitor chelerythrine only affected the acetylcholine-induced contraction. These results suggest that TC motor activity of rats are mediated mainly by M₃ receptors followed by the increase of cytosolic Ca(2+) concentration regulated by voltage-dependent calcium channels, sarco-endoplasmic reticulum and mitochondria. Furthermore, the differential effects of chelerythrine in the acetylcholine or carbachol-induced contractions are discussed.

Evaluation of In Vitro Uterotonic Activities of Fruit Extracts of Ficus asperifolia in Rats

The aim of the present study was to determine the uterotonic activities of Ficus asperifolia and investigate its mechanism. The effects of aqueous and methanol extracts of the dried fruits of F. asperifolia (0.05–1.60 mg mL⁻¹) were evaluated on estrogenized isolated rat uterus in the presence and absence of atropine (1.73–55.27 nM), pyrilamine maleate (1.25 × 10⁻³ to 40 × 10⁻³ M), indomethacin (0.06 × 10⁻⁵ to 2.00 × 10⁻⁵ M) or hexamethonium (0.66 × 10⁻⁴ to 21.43 × 10⁻⁴ M). Aqueous (EC₅₀, 0.36 mg mL⁻¹) and methanol (EC₅₀, 0.22 mg mL⁻¹) extracts as well as oxytocin (EC₅₀, 0.02 nM), acetylcholine (EC₅₀, 7.87 nM) and histamine (EC₅₀, 0.76 nM) evoked concentration-dependent contractions of the uterus. Atropine, pyrilamine maleate and indomethacin concentration dependently blocked the response of the uterus to acetylcholine (IC₅₀, 4.82 nM), histamine (IC₅₀, 2.49 nM) and oxytocin (IC₅₀, 0.07 nM), respectively, and to aqueous extract. Hexamethonium produced graded decreases in oxytocin-induced uterine contractions (IC₅₀, 0.37 μM), but did not prevent the contractile effects of the aqueous extract (IC₅₀, 9.88 μM). These results suggest that F. asperifolia-induced uterotonic effect is related to the release of prostaglandins and contraction of the myometrial cells through muscarinic, oxytocic and H₁ histamine receptors. These data further give added value to the ethnic use of F. asperifolia for its abortificient and contraceptive properties.

Messenger RNA Levels and Binding Sites of Muscarinic Acetylcholine Receptors in Gastrointestinal Muscle Layers from Healthy Dairy Cows

Acetylcholine interacts with muscarinic receptors (M) to mediate gastrointestinal (GI) smooth muscle contractions. We have compared mRNA levels and binding sites of M(1)to M(5) in muscle tissues from fundus abomasi, pylorus, ileum, cecum, proximal loop of the ascending colon (PLAC), and external loop of the spiral colon (ELSC) of healthy dairy cows. The mRNA levels were measured by quantitative RT-PCR. The inhibition of [(3)H]-QNB (1-quinuclidinyl-[phenyl-4-(3)H]-benzilate) binding by M antagonists [atropine (M(1 - 5)), pirenzepine (M(1)), methoctramine (M(2)), 4-DAMP (M(3)), and tropicamide (M(4))] was used to identify receptors at the functional level. Maximal binding (B(max)) was determined through saturation binding with atropine as a competitor. The mRNA levels of M(1), M(2), M(3), and M(5) represented 0.2, 48, 50, and 1.8%, respectively, of the total M population, whereas mRNA of M(4) was undetectable. The mRNA levels of M(2) and of M(3) in the ileum were lower (P < 0.05) than in other GI locations, which were similar among each other. Atropine, pirenzepine, methoctramine, and 4-DAMP inhibited [(3)H]-QNB binding according to an either low- or high-affinity receptor pattern, whereas tropicamide had no effect on [(3)H]-QNB binding. The [(3)H]-QNB binding was dose-dependent and saturable. B(max) in fundus, pylorus, and PLAC was lower (P < 0.05) than in the ELSC, and in the pylorus lower (P < 0.05) than in the ileum. B(max) and mRNA levels were negatively correlated (r = -0.3; P < 0.05). In conclusion, densities of M are different among GI locations, suggesting variable importance of M for digestive functions along the GI tract.

The detection of the non-M2 muscarinic receptor subtype in the rat heart atria and ventricles

Mammal heart tissue has long been assumed to be the exclusive domain of the M(2) subtype of muscarinic receptor, but data supporting the presence of other subtypes also exist. We have tested the hypothesis that muscarinic receptors other than the M(2) subtype are present in the heart as minor populations. We used several approaches: a set of competition binding experiments with pirenzepine, AFDX-116, 4-DAMP, PD 102807, p-F-HHSiD, AQ-RA 741, DAU 5884, methoctramine and tripinamide, blockage of M(1) muscarinic receptors using MT7 toxin, subtype-specific immunoprecipitation experiments and determination of phospholipase C activity. We also attempted to block M(1)-M(4) receptors using co-treatment with MT7 and AQ-RA 741. Our results show that only the M(2) subtype is present in the atria. In the ventricles, however, we were able to determine that 20% (on average) of the muscarinic receptors were subtypes other than M(2), with the majority of these belonging to the M(1) subtype. We were also able to detect a marginal fraction (6 +/- 2%) of receptors that, based on other findings, belong mainly to the M(5) muscarinic receptors. Co-treatment with MT7 and AQ-RA 741 was not a suitable tool for blocking of M(1)-M(4) receptors and can not therefore be used as a method for M(5) muscarinic receptor detection in substitution to crude venom. These results provide further evidence of the expression of the M(1) muscarinic receptor subtype in the rat heart and also show that the heart contains at least one other, albeit minor, muscarinic receptor population, which most likely belongs to the M(5) muscarinic receptors but not to that of the M(3) receptors.

Insect muscarinic acetylcholine receptor: pharmacological and toxicological profiles of antagonists and agonists

The insect muscarinic acetylcholine receptor (mAChR) is evaluated as a potential target for insecticide action. The mammalian M2/M4-selective antagonist radioligand [3H]AF-DX 384 (a pirenzepine analogue) binds to Drosophila mAChR at a single high-affinity site identical to that for the nonselective antagonist [3H]quinuclidinyl benzilate (QNB) and with a pharmacological profile distinct from that of all mammalian mAChR subtypes. Three nonselective antagonists (QNB, scopolamine, and atropine) show the highest affinity (Ki=0.5-2.4 nM) at the Drosophila target, and AF-DX 384 and M3-selective 4-DAMP (dimethyl-4-(diphenylacetoxy)piperidinium iodide) rank next in potency (Ki=5-18 nM). Eleven muscarinic antagonists generally exhibit higher affinity than eight agonists. On injection into houseflies, the antagonists 4-DAMP and (S)-(+)-dimethindene produce suppressed movement, the agonist (methyloxadiazolyl)quinuclidine causes knockdown and tremors, and all of them inhibit [3H]QNB binding ex vivo, indicating possible mAChR-mediated intoxication. The insect mAChR warrants continuing study in lead generation to discover novel insecticides.

Methoctramine and gallamine inhibit PI hydrolysis in guinea-pig gallbladder

The present study aimed to determine the effect of two M2/M4-selective muscarinic receptor antagonists on blocking the hydrolysis of carbachol (CCh) stimulated phospho-inositide (PI) breakdown in order to address the possibility that a muscarinic receptor other than the M(3) receptor is involved in PI hydrolysis in this tissue. Gallbladder tissue slices labeled with myo-[2-3H] inositol were incubated with increasing concentrations of antagonists and agonist. After the reactions were terminated by the addition of chloroform/methanol, labeled inositol phosphates were separated using anion exchange chromatography. Muscarinic M2 antagonists methoctramine and gallamine both inhibited carbachol-induced PI breakdown at high concentrations, with log IC50 values of -5.145 and -6.049, respectively. Gallamine at 10(-5)M concentration failed to displace the dose-response curve for carbachol-induced accumulation of inositol triphosphate (IP3). Our data suggest that M(3) receptors play a major role in stimulation of PI hydrolysis in the guinea-pig gallbladder.

Experimental analysis of antimicrobial action of dicyclomine hydrochloride

Dicyclomine hydrochloride is an antispasmodic agent. The MIC of dicyclomine against standard strains of Gram positive and Gram negative bacteria were performed by NCCLS broth dilution technique. These drugs showed a rapid killing action on Gram positive bacteria, Staphylococcus aureus NCTC 6571, 8530 and several other reference strains. The killing effect against Gram negative bacteria, Shigella boydii 8 NCTC 254/66 and Salmonella typhimurium NCTC 74 showed that the drug was bacteriostatic with respect to these strains. High rate of killing was achieved for most strains of Gram positive bacteria within 2 h. When administered to Swiss strain of white mice at doses of 30 and 60 microg/g of mouse, the drug could significantly protect the animals challenged with 50 MLD of Salmonella typhimurium NCTC 74. According to chi2 test, the in vivo data were highly significant (p<0.001). Since dicyclomine showed a remarkable inhibitory action against several pathogenic bacteria, in the course of time, it may be developed as a potent antimicrobial agent for many bacterial infections.

Muscarinic M2 and M4 receptors in anterior cingulate cortex: relation to neuropsychiatric symptoms in dementia with Lewy bodies

Alterations in cholinergic functions have been reported to be associated with neuropsychiatric symptoms in dementia. Increased M1 muscarinic receptor binding in temporal cortex is associated with delusions in dementia with Lewy bodies (DLB) patients and increased M2/M4 receptor binding with psychosis in Alzheimer's disease. However, the relation between M2 and M4 muscarinic receptor and psychotic symptoms in DLB is unknown. The aim of this study was to measure M2 and M4 receptors in the anterior cingulate cortex in DLB and to correlate the neurochemical findings with neuropsychiatric symptoms. Muscarinic M2 and M4 receptor levels in the anterior cingulate cortex and adjacent cortex (Brodmann's area [BA] 32) were measured separately by using a radioligand binding protocol based on binding of [(3)H]AF-DX 384 in the presence and absence of dicyclomine, a potent M4 receptor antagonist. M2 receptor binding was significantly increased, while M4 receptor binding was unchanged in the cingulate cortex and BA32 of DLB patients compared with age-matched controls. Impaired consciousness was significantly associated with increased M4 binding and delusions were significantly associated with increased M2 binding. Increased M2 and M4 receptor binding in DLB was also associated with visual hallucinations. Upregulation of M2 and M4 muscarinic receptors in cingulate and adjacent cortex may thus contribute to the development of psychosis in DLB, with potential implications for treatments with drugs acting on these receptors.

Effect of estrogen on muscarinic acetylcholine receptor expression in rat myometrium

We report the effect of acute estrogen treatment in the expression of muscarinic acetylcholine receptors (mAChRs) in myometrium. Strips were obtained from rats in estrus (control) and treated with estrogen, 24h before the experiments. Reverse transcriptase-polymerase chain reaction (RT-PCR) was performed and m2, m3 and m5 mAChR mRNA subtypes were detected in myometrium from both groups. [(3)H]Quinuclidinyl benzilate [(3)HQNB] binding studies indicated that estrogen treatment did not change the affinity and density of mAChRs in myometrial membranes. Displacement curves of [(3)HQNB] with different mAChRs antagonists indicated a one-site fit for all antagonists tested. Comparison of pK(i) values indicated a significant correlation to M(2)-mAChR subtype. Functional studies, however, showed that estrogen treatment increased myometrium sensitivity to carbachol and the calculated apparent affinity values were significantly correlated to M(3)-mAChR. Furthermore, the pharmacological profile of the two populations of mAChR was not affected by estrogen. In conclusion, these results provide evidence for the presence of M(2)- and M(3)-mAChR, at the mRNA and protein level, in the rat myometrium and indicate that estrogen induces an increase in myometrial responsiveness to mAChR agonists.

Characterization of pre- and postsynaptic muscarinic receptors in circular muscle of pig gastric fundus

1. This study investigated the subtype of muscarinic receptors on the cholinergic neurones and smooth muscle in the circular muscle of the pig gastric fundus. 2. Muscarinic antagonists, except MT-3, concentration-dependently inhibited the contractions induced by a given concentration of acetylcholine. Concentration-response curves by acetylcholine were shifted rightwards in a parallel manner without depression of the maximum by the muscarinic antagonists, except by MT-3 that induced a leftward shift. Correlation of the pIC(50) and pA(2) values with published pK(i) values for the five muscarinic receptor subtypes suggests that the muscarinic receptors on pig gastric fundus circular muscle belong to the M(3) subtype. 3. Electrically-evoked contractions (40 V, 4 Hz, 0.25 ms, 2 min) were concentration-dependently inhibited by the muscarinic antagonists except for methoctramine and AF-DX 116, that increased the amplitude of the electrically-induced contractions in lower concentrations. MT-3 tended to increase the electrically-induced contractions. 4. The antagonists, except MT-3, concentration-dependently increased the electrically-induced tritium outflow (40 V, 4 Hz, 0.25 ms, 2 min) after incubation of the tissues with [(3)H]-choline. MT-3 (3 x 10(-8) and 10(-7) M) decreased the electrically-induced tritium release. Correlation of the pIC(50) values with published pK(i) values for the different muscarinic receptor subtypes yielded a significant and comparable correlation for M(1), M(3), M(4) and M(5) receptors. 5. These results suggest that the postsynaptic receptors in circular muscle of the pig gastric fundus belong to the M(3) subtype. However, the presynaptic receptor could not be clearly defined, although it does certainly not belong to the M(2) subtype.

Characterization of muscarinic acetylcholine receptors in the rat epididymis

The aim of the present study was to characterize the muscarinic acetylcholine receptor subtypes present in the caput and cauda of rat epididymis. The specific binding of [3H]quinuclidinyl benzilate ([3H]QNB) to epididymal membranes was time dependent, temperature dependent, and saturable. The cauda epididymis showed higher affinity to [3H]QNB and higher muscarinic receptor density when compared to the caput region. The [3H]QNB binding was tested in competition studies with different muscarinic receptor antagonists. Each antagonist tested displaced [3H]QNB bound to caput and cauda epididymal membrane with similar affinity. Correlation among the negative logarithm of inhibition constant values (pK(i)) for these antagonists obtained in the epididymis with their correspondent published pK(i) values obtained in tissues that expressed each receptor subtype (M1, M2, M3, and M4) indicated that the muscarinic receptors present in caput and cauda epididymis belong to the muscarinic M2 receptor subtype. When reverse transcription-polymerase chain reaction was used to identify muscarinic receptor mRNA subtypes in the epididymis, only m2 transcripts were detected in the caput region, while both m2 and m3 mRNA subtypes were observed in the cauda region. In conclusion, these results demonstrate that muscarinic receptors are present in the rat epididymis, with expression levels dependent on the region of the epididymis analyzed. Thus, the cholinergic neurotransmitter in the epididymis may be a factor controlling contractility and/or the luminal fluid microenvironment.

Characterisation of the prejunctional inhibitory muscarinic receptor on cholinergic nerves in the rat urinary bladder

The nature of the prejunctional inhibitory muscarinic receptor on cholinergic nerve endings in the rat urinary bladder was investigated by measuring stimulated endogenous acetylcholine release via high pressure liquid chromatography (HPLC), in the presence of various selective muscarinic antagonists. The rank order of potencies for the antagonists used was: atropine (-log concentration = 7.8) > 4-DAMP (4-diphenylacetoxy-N-methylpiperidine) (7.6) > tripitramine (7.3) = HHD (hexahydrodifenidol) (7.3) > pFHHSiD (p-fluoro-hexahydrosiladifenidol hydrochloride) (7.0) > himbacine (6.5) > methoctramine (5.9) > or = pirenzepine (5.8) > gallamine (4.3). A comparison of the antagonist potencies obtained, with affinity constants at muscarinic M(1) to M(5) receptors, suggests that the prejunctional inhibitory muscarinic receptor is of the M(4) receptor subtype.

Cholinergic facilitation of neurotransmission to the smooth muscle of the guinea-pig prostate gland

1. Functional experiments have been conducted to assess the effects of acetylcholine and carbachol, and the receptors on which they act to facilitate neurotransmission to the stromal smooth muscle of the prostate gland of the guinea-pig. 2. Acetylcholine and carbachol (0.1 microM - 0.1 mM) enhanced contractions evoked by trains of electrical field stimulation (20 pulses of 0.5 ms at 10 Hz every 50 s with a dial setting of 60 V) of nerve terminals within the guinea-pig isolated prostate. In these concentrations they had negligible effects on prostatic smooth muscle tone. 3. The facilitatory effects of acetylcholine, but not those of carbachol, were further enhanced in the presence of physostigmine (10 microM). 3. The facilitatory effects of carbachol were unaffected by the neuropeptide Y Y(1) receptor antagonist BIBP 3226 ((R)-N(2)-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]-arginina mide) (0.3 microM, n=3) or suramin (100 microM, n=5). Prazosin (0.1 microM, n=5) and guanethidine (10 microM, n=5) alone and in combination (n=4), reduced responses to field stimulation and produced rightward shifts of the log concentration-response curves to carbachol. 4. The rank orders of potency of subtype-preferring muscarinic receptor antagonists in inhibiting the facilitatory actions of acetylcholine and carbachol were: pirenzepine > HHSiD (hexahydrosiladifenidol) > pF-HHSiD (para-fluoro-hexahydrosiladifenidol)>/= 5 himbacine, and pirenzepine > HHSiD > himbacine>/= 5 pF-HHSiD, respectively. These profiles suggest that muscarinic receptors of the M(1)-subtype mediate the facilitatory effects of acetylcholine and carbachol on neurotransmission to the smooth muscle of the guinea-pig prostate.

Muscarinic M(2) receptors are not primarily involved in the contraction of guinea-pig gallbladder smooth muscle

The presence of M(1)-M(4) receptors in guinea-pig gallbladder smooth muscle cells has been reported recently. The majority of these receptors are said to be of M(2) subtype. However, there are controversial reports about the functional muscarinic receptors that mediate contraction in this tissue. Similar to gallbladder, it was claimed that M(4) receptors mediate guinea-pig uterine contractions, but these receptors have appeared to be of M(2) subtypes later. Therefore, the antagonistic affinities of three M(2)-selective muscarinic antagonists were determined in contraction and radioligand binding experiments in guinea-pig gallbladder in the present study. The antagonistic affinity values (p K(i)) of gallamine, tripitramine and imperialine were as follows, respectively: 6.28+/-0.15, 8.65+/-0.10 and 6.55+/-0.07 against 0.250 n m [(3)H]QNB binding. All three antagonists displaced the concentration- response curves to carbachol to the right in parallel without affecting the maximum responses. The p A(2) values obtained from constrained Schild plots (-log K(B)) were 4.14+/-0.18 for gallamine, 6.79+/-0.09 for tripitramine, and 7.02+/-0.09 for imperialine. The antagonistic affinity values of gallamine, tripitramine and imperialine for M(2) receptors are reported to be 6. 3, 9.6, 7.7, respectively. The p A(2) values obtained in this study clearly indicate that the primary muscarinic receptors involved in carbachol-induced guinea-pig gallbladder contraction are not of M(2) subtype. The poor correlation between the antagonistic affinity values of these antagonists obtained at radioligand binding (p K(i)) and contraction (p A(2)) experiments also support the conclusion that the majority of muscarinic receptors which have been reported to be of M(2) do not mediate the contractile responses.

Characterization of the muscarinic receptor in isolated uterus of sham operated and ovariectomized rats

1. The pharmacological characteristics of muscarinic receptors in rat isolated uterus were studied in ovariectomized (ov.) and sham operated (sh.) animals. 2. Competition radioligand binding studies, using uterine membranes and [3H]-NMS, were undertaken with several muscarinic receptor antagonists. Most of the antagonists indicated a one-site fit with apparent affinity estimates (pKi) unchanged by ovariectomy. The selective M2 antagonist, tripitramine revealed high (representing 33+/-8 and 38+/-2%) and low (67+/-8 and 62+/-2%) affinity binding sites in both sh. and ov. rat uterus, respectively. These sites likely represented muscarinic M2 and M3 receptors and the proportions were not significantly different in the two conditions. 3. Carbachol induced concentration-dependent contractions which were surmountably antagonized by several muscarinic receptor antagonists (pKB, sh.; ov.): zamifenacin (9.19; 9.18), p-F-HHSiD (8. 50; 9.06), tripitramine (7.23; 7.54), himbacine (7.21; 7.41), methoctramine (6.79; 7.49), pirenzepine (6.48; 7.21), AF DX 116 (6. 26; 6.61), MTx 3 (<7.00; <7.00) and PD 102807 (<7.00; <7.00). 4. The apparent affinity values obtained in functional studies using the uteri from both sh. and ov. animals correlated most closely with values reported at human recombinant muscarinic M3 receptors. This suggests that the muscarinic M3 receptor mediates contraction under both conditions. 5. Radioligand binding experiments indicate the presence of M2 receptors, in addition to M3 receptors, which probably explains the discrepancies between functional and binding affinities. These data further suggest that the pharmacological profile and proportions of the two populations of muscarinic receptors are unaffected by ovariectomy.

Subtypes of muscarinic receptors in rat duodenum: a comparison with rabbit vas deferens, rat atria, guinea-pig ileum and gallbladder by using imperialine

The specific binding of [3H]QNB to rat duodenum smooth muscle membranes was a saturable process and Scatchard transformation of the saturation curves indicated a linear plot (nH = 1.017+/-0.071). The K(D) and Bmax values were 0.168+/-0.025 nM and 46.7+/-8.6 fmol/mg protein, respectively. Analyses of competition curves using pirenzepine and guanylpirenzepine indicated more than one class of binding site. A minor population of muscarinic binding sites showed high affinity (M1) for both pirenzepine (19.3+/-1.2%; pKi = 8.29+/-0.36) and guanylpirenzepine (29.4+/-2.0%; pKi = 7.28+/-0.11). The antagonistic affinity values of pirenzepine and guanylpirenzepine for the remaining low affinity binding sites, and that of methoctramine indicated the presence of both M2 and M3 subtypes. McN-A-343 produced relaxations in rat duodenum and inhibited twitch contractions of rabbit vas deferens induced by electrical stimulation in a concentration dependent manner. Carbachol (Cch) exerted concentration-dependent negative inotropic effect in rat atria and contractile effects in guinea-pig gallbladder and ileum longitudinal muscle-myenteric plexus preparation. Imperaline displaced the concentration-response curves to McN-A-343 and Cch to the right in parallel, without affecting the maximum responses in all tissues studied. The rank order of the pA2 values was rabbit vas deferens > rat atria > guinea-pig gallbladder = guinea-pig ileum > rat duodenum. The presynaptic muscarinic receptors at the rat duodenum and rabbit vas deferens were concluded to be of M1 and M4 subtypes, respectively.

Characterization of the muscarinic receptor subtype that mediates the contractile response of acetylcholine in the swine myometrium

The aim of the present study was to characterize the subtype of muscarinic receptor that mediates acetylcholine-induced contractions in the nonpregnant proestrus swine myometrium by means of mechanical, radioligand ([3H]quinuclidinyl benzilate) binding and biochemical (measurement of cyclic AMP) approaches. Acetylcholine (-logEC50, 6.12), oxotremorine-methiodide (6.47), methacholine (6.35), carbachol (6.18) and muscarine (6.33) caused contractile responses of the uterine circular muscle, with a similar maximum amplitude, but pilocarpine and McN-A-343 (4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium) were ineffective in causing contraction. The contractile response to acetylcholine was antagonized by the following muscarinic receptor antagonists in a competitive manner (with pA2 values in parentheses): atropine (8.95), 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, 8.83), tropicamide (7.07), himbacine (7.01), pirenzepine (6.42) and 11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyri do[2,3 b][1,4]benzodiazepin-6-one (AF-DX116, 5.96). Electrical field stimulation (10 Hz) caused tetrodotoxin- and atropine-sensitive contractions in the circular muscle. All muscarinic receptor antagonists decreased the electrical field stimulation-induced contraction in a concentration-dependent manner. The order of inhibition (-logIC50) was 4-DAMP (8.35) > tropicamide (6.72) > himbacine (6.54) > pirenzepine (6.31)> AF-DX116 (6.13). Acetylcholine did not affect the cytoplasmic cyclic AMP level, regardless of the presence or absence of forskolin, suggesting the absence of functional muscarinic M2 and/or M4 receptors in the swine myometrium. The receptor binding study indicated that circular muscle layers of the swine myometrium contained a single class of [3H]quinuclidinyl benzilate binding site (Kd = 0.92 nM; Bmax = 126.6 fmol/mg protein). Specific binding was displaced by muscarinic receptor antagonists in the following order (with pKi value and Hill coefficient in parentheses): atropine (8.22 and 0.93) > 4-DAMP (8.18 and 0.94) > tropicamide (6.78 and 0.93) > pirenzepine (5.46 and 0.92) > AF-DX116 (5.12 and 0.94). The present results suggest that in circular muscle layers of the swine myometrium, exogenous and endogenous acetylcholine cause contraction through activation of muscarinic M3 receptors present on smooth muscle cells.

Characterization of an atypical muscarinic cholinoceptor mediating contraction of the guinea-pig isolated uterus

In many smooth muscle tissues a minor M3-muscarinic acetylcholine (mACh) receptor population mediates contraction, despite the presence of a larger M2-mACh receptor population. However, this is not the case for guinea-pig uterus where radioligand binding and functional studies exclude a dominant role for M3-mACh receptors. Using tissue from animals pre-treated with diethylstilboestrol, estimates of antagonist affinity were made before and after selective alkylation procedures, together with estimates of agonist affinity to characterise the mACh receptor population mediating carbachol-induced contraction of guinea-pig isolated uterus. Antagonist affinity estimates made at 'protected' receptors were not significantly different from those made in untreated tissues. However all estimations were significantly different from those reported in guinea-pig ileum and atria. The rank order of affinities were atropine>zamifenacin=tripitramine> methoctramine. Carbachol-induced contractions were insensitive to the M4-selective muscarinic toxin MTx-3, or PD102807 (0.1 microM) ruling out a role for M4-mACh receptors. The agonist affinity value for L-660,863, a putative 'M2-selective' agonist of 5.44+/-0.30 (n=6) was significantly different from that reported in guinea-pig atria. In contrast, the pKA value for carbachol (4.22+/-0.17 n = 8) agrees with that reported for guinea-pig ileum. Carbachol-induced contractions were insensitive to pertussis toxin although carbachol-induced inhibition of forskolin-stimulated cyclic AMP production was attenuated, ruling out the involvement of Gi-proteins in contraction. Radioligand binding studies revealed a KD for N-[3H]-methylscopolamine of 0.12+/-0.05 nM and a Bmax of 147+/-18 fmol mg protein(-1). Antagonist affinity estimates made using competition binding studies supported previous data suggesting the presence of a homogenous population of M2-mACh receptors. These data suggest a small population of mACh receptors with an atypical operational profile which can not be distinguished using radioligand binding studies may mediate carbachol-induced contraction of guinea-pig isolated uterus.

Pharmacological characterization of muscarinic receptors in the uterus of oestrogen-primed and pregnant rats

1. Radioligand binding and contractility studies were undertaken to determine the subtype/s of muscarinic receptors present in uteri of oestrogen-treated and late pregnant rats. 2. Competition binding studies with uterine membrane preparations and [3H]-QNB (quinuclidinyl benzilate) provided negative log dissociation constants (pKi) for each antagonist as follows; oestrogen-treated - atropine (7.98) > or = himbacine (7.83) > methoctramine (7.52) > or = hexahydrosiladiphenidol (HHSiD; 7.32) > or = 5,11-dihydro- 11-[[[2-[2 - [(dipropylamino)methyl] - 1 piperidinyl]ethyl]amino] - carbonyl] - 6H-pyrido-[2,3-b][1,4]- benzodiazepin-6-one (AF-DX 384; 7.10)> 11 -[[2- [(diethylamino)methyl]-1-piperidinyl]-acetyl]5,11-dihydro-6H-pyridol+ ++]2,3,-b][1,4]benzodiazepin-6-one (AF-DX 116, 6.77)>pirenzepine (6.17); late pregnant atropine (8.05)> or =methoctramine (7.95)> or =himbacine (7.71)> or =HHSiD (7.52)> or =AF-DX 384 (7.34)>AF-DX 116 (6.72)>pirenzepine (6.18). 3. The potency of carbachol in causing uterine contraction was similar in preparations from pregnant and non-pregnant animals (pD2=5.57 and 5.46, respectively). Each muscarinic antagonist caused parallel, rightward shifts of carbachol concentration-response curves. The pA2 estimates were: oestrogen-treated - atropine (9.42)> himbacine (8.73) HHSiD (8.68) methoctramine (8.49)> or =AF-DX 384 (7.91)> or =AF-DX 116 (7.36)> or =pirenzepine (7.26); late pregnant atropine (9.48)>himbacine (8.37)> or = HHSiD (8.22) > or =methoctramine (8.01) > or =AF-DX 116 (7.73)> or = AF-DX 384 (7.44)> or = pirenzepine (6.92). 4. The relative pKi estimates for antagonists obtained in membrane preparations from oestrogen-treated rats suggest the presence of muscarinic M2 subtypes. In functional studies pA2 values indicated the additional presence of muscarinic M3 receptor or, possibly an atypical receptor subtype. The similarity between pKi and pA2 estimates obtained in uteri from oestrogen-treated and pregnant animals, respectively, indicates that pregnancy does not affect myometrial muscarinic receptors in the rat.

Muscarinic cholinergic receptors and acetylcholinesterase activity in umbilical artery and vein in pregnancy-induced hypertension (pre-eclampsia)

The influence of pregnancy-induced hypertension (pre-eclampsia) on muscarinic cholinergic receptors and on acetylcholinesterase (AChE) activity was investigated using frozen sections of the umbilical artery and vein. Pre-eclamptic patients undergoing Caesarean delivery and normotensive pregnant control woman undergoing Caesarean delivery with similar parity, gestation length and age were examined. Muscarinic cholinergic receptors were assayed in frozen sections of the umbilical artery and vein by a radioligand binding assay technique, using [3H]-N-methyl scopolamine (NMS) as a ligand. AChE was demonstrated with a histochemical technique associated with microdensitometry. [3H]-NMS was specifically bound to sections of both umbilical artery and vein in a manner consistent with the labelling of muscarinic cholinergic receptors. The affinity of the radioligand was similar in the two vessels, whereas the maximum density of binding sites (Bmax) was higher in the umbilical vein than in the artery. A faint AChE reactivity was observed in the tunica media of both umbilical artery and vein. In pre-eclampsia, a loss of [3H]-NMS binding sites not accompanied by changes in the affinity of radioligand was found. The decrease of muscarinic cholinergic receptors involved to a greater extent the umbilical artery than the vein. No differences in AChE activity were found at the level of umbilical artery and vein between control and pre-eclamptic subjects. These findings suggest that pre-eclampsia is characterized by a loss of muscarinic cholinergic receptors in the umbilical circulation not accompanied by changes of the acetylcholine catabolizing enzyme AChE. It is possible that the decreased density of vascular muscarinic cholinergic receptors in pregnancy-induced hypertension contribute to the increased resistance of the umbilical circulation occurring in pre-eclampsia.

Characterization of the muscarinic receptor subtype(s) mediating contraction of the guinea-pig lung strip and inhibition of acetylcholine release in the guinea-pig trachea with the selective muscarinic receptor antagonist tripitramine

1. The muscarinic receptor subtypes mediating contraction of the guinea-pig lung strip and inhibition of the release of acetylcholine from cholinergic vagus nerve endings in the guinea-pig trachea in vitro have previously been characterized as M2-like, i.e. having antagonist affinity profiles that are qualitatively similar but quantitatively dissimilar compared to cardiac M2 receptors. The present study sought to establish definitely the identity of these receptor subtypes by using the selective muscarinic receptor antagonist, tripitramine. Guinea-pig atria and guinea-pig trachea (postjunctional contractile response) were included for reference. 2. It was found that tripitramine antagonized methacholine-induced contractions of the guinea-pig lung strip with pKB value of 8.76 +/- 0.05. Both the parallel shifts of the concentration-response curves and the slope of the Schild plot begin not significantly different from unity (when antagonist preincubation was for 2 h) indicated the involvement of a single population of receptors in the contractile response. From the pKB values obtained with tripitramine and a range of other selective muscarinic receptor antagonists (cf. Roffel et al., 1993), this single population of receptors can only be classified as M2-like. 3. Tripitramine antagonized methacholine-induced chronotropic and inotropic responses in guinea-pig right and left atria with apparent pKB values of 9.4-9.6. However, such values were only obtained when antagonist preincubation was relatively long and/or antagonist concentration relatively high (e.g with 1 h at 100 or 300 nM but 3 h at 30 nM). It thus appears that low concentrations of tripitramine do not readily equilibrate with M2 receptors in guinea-pig atria nor with M2-like receptors in the guinea-pig lung strip. 4. Tripitramine increased electrical field stimulation-induced cholinergic twitch contractions in guinea-pig trachea in concentrations of 0.3-100 nM, by blocking prejunctional muscarinic inhibitory autoreceptors; with higher concentrations, twitch contractions were progressively diminished, as a result of blocking postjunctional M3 receptors (apparent pKB value 6.07 +/- 0.15). The pEC20 value (-log concentration that increases twitch by 20% maximum) was 8.29 +/- 0.08, which would suggest that M4 receptors are involved in this response. 5. Oxotremorine-induced inhibition of the release of prelabelled [3H]-acetylcholine from guinea-pig trachea, under conditions where there is no auto-feedback, was blocked by tripitramine (2 h preincubation) with a pKB value of 8.56 +/- 0.06. The slope of the corresponding Schild plot was not significantly different from unity, which together with the parallel shifts of the concentration-response curves indicated the involvement of a single muscarinic receptor subtype. 6. Since the pKB value for tripitramine at prejunctional receptors in guinea-pig trachea is in between the affinities towards M2 and M4 receptors, correlation plots were constructed to compare the pKB values obtained with tripitramine and a range of other selective muscarinic receptor antagonists (cf. Kilbinger et al., 1995) to reported affinities at M1-M4 receptors. This showed rather similar distribution patterns of the data points around the line of equality in the case of M2 and M4 receptor subtypes. However, the correlation coefficient was markedly better for M2 (0.9667) than for M4 (0.5976). Since recent evidence suggests that M4 receptors are not expressed in cholinergic nerves from guinea-pig trachea, it is concluded that prejunctional muscarinic autoinhibitory receptors in this tissue exhibit an atypical M2 type character, with a pharmacological profile distinct from cardiac M2 receptors.

Carbachol-induced pressor responses and muscarinic M1 receptors in the central nucleus of amygdala in conscious rats

The type of muscarinic receptor in the central nucleus of the amygdala that mediates the carbachol-evoked pressor responses was investigated in conscious unrestraint Sprague-Dawley rats. Carbachol (100 ng) injected into the lateral cerebral ventricle caused a significant rise in blood pressure of 31.8+/-4.5 mmHg and a decrease in heart rate of 80.0+/-12.2 beats/min. Pirenzepine (10-75 nmol) injected into the central nucleus of the amygdala inhibited carbachol-induced pressor responses dose-dependently. The bradycardic response to carbachol was also inhibited by pirenzepine, but no dose-dependency was observed. Injection of pirenzepine into the basolateral amygdala at a dose (50 nmol) that inhibited carbachol-induced changes in mean arterial pressure and heart rate when injected into the central nucleus of the amygdala failed to exert any inhibition. Methoctramine at a dose of 50 nmol injected into both the central nucleus of the amygdala and the basolateral amygdala did not cause any significant alteration in the responses. These results indicate that muscarinic M1 receptors in the central nucleus of the amygdala are involved in cardiovascular regulation mediated by central cholinergic pathways.

Intracellular signals coupled to different rat ileal muscarinic receptor subtypes

Taking into account that the activation of different subtypes of ileal muscarinic acetylcholine receptors (mAChR) regulate gut functions such as tone, motility, and electrolyte secretion, we characterized the expression of mAChR in ileal-purified membranes. We also studied intracellular signals triggered by mAChR activation. Binding parameters obtained from saturation assays with the nonselective tritiated muscarinic antagonist, quinuclidynil benzilate ([3H]-QNB), were maximal number of binding sites (Bmax): 30 +/- 2 fmol/mg prot and dissociation constant (Kd): 0.2 +/- 0.03 nM. The competitive inhibition of [3H]-QNB specific binding by various nonlabelled muscarinic antagonists was measured and the rank order of potency was: atropine (ATROP) > 4-DAMP > AF-DX 116 > pirenzepine (PZ). The activation of mAChR by carbachol (CARB) increased ileal motility in a concentration-dependent manner (EC50 2 x 10[-7] M). The antagonists' order of potency to displace dose-response curve of CARB was: ATROP > 4-DAMP > AF-DX116 > PZ. Optimal concentration of CARB on ileal strips increased phosphoinositide turnover and cGMP levels by activating ml receptor subtype and decreased isoproterenol (ISO) stimulated levels of cAMP due to M2 receptor activation. We can conclude that the activation of different mAchR subtypes triggers different intracellular signals that could regulate intestinal tone and motility.

Contraction of guinea-pig gallbladder: muscarinic M3 or M4 receptors?

The muscarinic receptor mediating contraction of the guinea-pig isolated gallbladder, currently being disputed to belong either to the M3 or M4 subtype, was characterized by subtype-preferring agonists and discriminating antagonists. Highly significant correlations of agonist potencies to contract the gallbladder, e.g., arecaidine propargyl ester, oxotremorine, 5-methylfurtrethonium > arecoline, arecaidine 2-butyne-1,4-diyl bisester > (R)-nipecotic acid ethyl ester > 4-[[N-(4-chlorophenyl)carbamyl]oxy]-2-butynyltrimethylammonium iodide (4-Cl-McN-A-343), (S)-nipecotic acid ethyl ester > 4-[[N-(3-chlorophenyl)carbamoyl]oxy]-2-butynyltrimethylammonium chloride (McN-A-343) were found with muscarinic M3 receptors mediating contraction of the guinea-pig ileum and vasodilation in rat perfused kidney. Functional affinities at guinea-pig gallbladder muscarinic receptors of antagonists known to distinguish between native or cloned muscarinic M3/m3 and M4/m4 receptors, e.g., himbacine, methoctramine, mefurtramine, tripitramine, idaverine, zamifenacin and 11-[[4-[4-(diethylamino)butyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyr ido(2,3-b)(1,4)benzodiazepin-6-one (AQ-RA 741), were consistent with those at guinea-pig ileal muscarinic M3 receptors but not with published data at recently defined muscarinic M4 receptors in rabbit anococcygeus muscle or at muscarinic M1 and M2 receptors in rabbit vas deferens. Antagonist affinities at guinea-pig gallbladder correlated also best with published binding data on native or cloned muscarinic M3/m3 receptors but not with those for muscarinic M4/m4 receptors. The agonist potencies and antagonist affinities suggest that smooth muscle contraction elicited by muscarinic stimuli in guinea-pig gallbladder is mediated by functional muscarinic M3 receptors.

Tolterodine--a new bladder-selective antimuscarinic agent

Tolterodine is a new muscarinic receptor antagonist intended for the treatment of urinary urge incontinence and other symptoms related to an overactive bladder. The aim of the present study was to compare the antimuscarinic properties of tolterodine with those of oxybutynin, in vitro and in vivo. Tolterodine effectively inhibited carbachol-induced contractions of isolated strips of urinary bladder from guinea pigs (K(B) 3.0 nM; pA2 8.6; Schild slope 0.97) and humans (K(B) 4.0 nM; pA2 8.4; Schild slope 1.04) in a concentration-dependent, competitive manner. The affinity of tolterodine was similar to that derived for oxybutynin (K(B) 4.4 nM; pA2 8.5; Schild slope 0.89) in the guinea-pig bladder. Tolterodine (21-2103 nmol/kg (0.01-1 mg/kg); intravenous infusion) was significantly more potent in inhibiting acetylcholine-induced urinary bladder contraction than electrically-induced salivation in the anaesthetised cat. In contrast, oxybutynin displayed the opposite tissue selectivity. Radioligand binding data showed that tolterodine bound with high affinity to muscarinic receptors in urinary bladder (K(i) 2.7 nM), heart (K(i) 1.6 nM), cerebral cortex (K(i) 0.75 nM) and parotid gland (K(i) 4.8 nM) from guinea pigs and in urinary bladder from humans (K(i) 3.3 nM). Tolterodine and oxybutynin were equipotent, except in the parotid gland, where oxybutynin bound with 8-times higher affinity (K(i) 0.62 nM). Binding data on human muscarinic m1-m5 receptors expressed in Chinese hamster ovary cells showed that oxybutynin, in contrast to tolterodine, exhibits selectivity (10-fold) for muscarinic m3 over m2 receptors. The K(B) value determined for oxybutynin (4.4 nM) in functional studies on guinea-pig bladder correlated better with the binding affinity at muscarinic M2/m2 receptors (K(i) 2.8 and 6.7 nM) than at muscarinic M3/m3 receptors (K(i) 0.62 and 0.67 nM). The tissue selectivity demonstrated for tolterodine in vivo cannot be attributed to selectivity for a single muscarinic receptor subtype. However, the combined in vitro and in vivo data on tolterodine and oxybutynin may indicate either that muscarinic M3/m3 receptors in glands are more sensitive to blockade than those in bladder smooth muscle, or that muscarinic M2/m2 receptors contribute to bladder contraction.

Modulation of the pressor response elicited by carbachol and electrical stimulation of the amygdala by muscarinic antagonists in conscious rats

1. The nature of the muscarinic receptor involved in mediating cardiovascular changes caused by unilateral microinjection of carbachol (5 nmol) into, and electrical stimulation (200-300 microA) of, the amygdaloid complex was investigated in conscious, unrestrained female Sprague-Dawley rats. 2. Unilateral microinjection of carbachol (5 nmol; n = 6) and electrical stimulation (200-300 microA, 80 Hz, 30 s; n = 4) caused a significant rise in blood pressure of 21 +/- 4 mmHg and 25 +/- 5 mmHg, respectively. These changes were associated with no overall effect on heart rate. The effects of electrical stimulation were found to be repeatable. 3. Pretreatment i.c.v. with pirenzepine (5-20 mmol; n = 6-7 for each dose), dose-dependently inhibited the rise in blood pressure induced by carbachol, whereas AF-DX 116 (100 nmol; n = 6) failed to have any effect on the carbachol-induced pressure response. Neither antagonist alone had any effect on resting baseline variables. 4. Unilateral microinjections of atropine sulphate (1-100 nmol; n = 4-6 for each dose), pirenzepine (0.03-10 nmol; n = 4 for each dose) or AF-DX 116 (10-60 nmol; n = 4-5 for each dose), into the amygdala, dose-dependently inhibited the rise in blood pressure caused by electrical stimulation (200-300 microA). The ID50 values were 1.05, 0.23 and 39.5 nmol, respectively. Although pirenzepine seemed to be more potent than atropine, this difference was not significant. 5. It is concluded that the rise in blood pressure elicited by unilateral microinjection of carbachol into, or electrical stimulation of, the amygdaloid complex is mediated by M1-muscarinic receptors.

Specific Gq protein involvement in muscarinic M3 receptor-induced phosphatidylinositol hydrolysis and Ca2+ release in mouse duodenal myocytes

1. Cytosolic Ca2+ concentration ([Ca2+]i) during exposure to acetylcholine or caffeine was measured in mouse duodenal myocytes loaded with fura-2. Acetylcholine evoked a transient increase in [Ca2+]i followed by a sustained rise which was rapidly terminated after drug removal. Although L-type Ca2+ currents participated in the global Ca2+ response induced by acetylcholine, the initial peak in [Ca2+]i was mainly due to release of Ca2+ from intracellular stores. 2. Atropine, 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, a muscarinic M3 antagonist), pirenzepine (a muscarinic M1 antagonist), methoctramine and gallamine (muscarinic M2 antagonists) inhibited the acetylcholine-induced Ca2+ release, with a high affinity for 4-DAMP and atropine and a low affinity for the other antagonists. Selective protection of muscarinic M2 receptors with methoctramine during 4-DAMP mustard alkylation of muscarinic M3 receptors provided no evidence for muscarinic M2 receptor-activated [Ca2+]i increase. 3. Acetylcholine-induced Ca2+ release was blocked by intracellular dialysis with a patch pipette containing either heparin or an anti-phosphatidylinositol antibody and by external application of U73122 (a phospholipase C inhibitor). 4. Acetylcholine-induced Ca2+ release was insensitive to external pretreatment with pertussis toxin, but concentration-dependently inhibited by intracellular dialysis with a patch pipette solution containing an anti-alpha q/alpha 11 antibody. An antisense oligonucleotide approach revealed that only the Gq protein was involved in acetylcholine-induced Ca2+ release. 5. Intracellular applications of either an anti-beta com antibody or a peptide corresponding to the G beta gamma binding domain of the beta-adrenoceptor kinase 1 had no effect on acetylcholine-induced Ca2+ release. 6. Our results show that, in mouse duodenal myocytes, acetylcholine-induced release of Ca2+ from intracellular stores is mediated through activation of muscarinic M3 receptors which couple with a Gq protein to activate a phosphatidylinositol-specific phospholipase C.

Transient impairment of cholinergic function in the rat insular cortex disrupts the encoding of taste in conditioned taste aversion

The muscarinic antagonist scopolamine blocks conditioned taste aversion (CTA) when microinjected bilaterally into the rat insular cortex shortly before the exposure of the rat to a novel taste (the conditioned stimulus, CS) in CTA training. Scopolamine has no effect when microinjected shortly after the exposure to the novel taste or shortly before the application of the malaise-inducing agent (unconditioned stimulus, UCS). Scopolamine does not affect sensory, motor and retrieval mechanisms required for performing the CTA task, and does not block CTA when injected into another cortical area. The effect of scopolamine is independent of the taste used as CS. Furthermore, microinjection of scopolamine into the insular cortex shortly before the pre-exposure to a new taste in a latent inhibition paradigm, impairs the attenuation of CTA by that pre-exposure. Other muscarinic antagonists, pirenzepine and AF DX-116, have an effect similar to that of scopolamine. Comparison of the dose-dependency curves of the muscarinic antagonists suggests a predominant role in CTA for M2 subtype receptors. Carbachol, a muscarinic agonist, also impairs the encoding of taste in the insular cortex, but the results are confounded by the ability of that ligand to induce seizures. Our findings suggest that cholinergic neuromodulation participates in processing the CS in the gustatory cortex in CTA, either by encoding novelty at the cellular level, or by instructing the neural circuits to store the novel taste representation.

Stereoselective interaction of uncharged esters at four muscarinic receptor subtypes

We investigated the binding and pharmacological properties of the esters of 3,3-dimethylbutan-1-ol (the carbon analogue of choline) with either diphenylglycolic acid, (R)-phenylcyclohexylglycolic acid, or (S)-phenylcyclohexylglycolic acid [BS-6181, (R)-BS-7826 and (S)-BS-7826, respectively] at muscarinic M1, M2, M3 (Hm3) and M4 receptors. The three uncharged compounds were muscarinic receptor antagonists, with pA2 or pKi values between 7.9 and 5.6. The achiral ester BS-6181 displayed highest affinity for M1, M3 (Hm3) and M4 receptors (pA2 or pKi = 7.2-7.6) and lower affinity for M2 receptors (pA2 or pKi = 6.7 and 6.8). The four muscarinic receptor subtypes were able to distinguish between the two enantiomers of the cyclohexyl derivative of BS-6181 [(R)- and (S)-BS-7826], with a preference for the (R)-isomer (up to 79-fold). Interestingly, the (S)-enantiomer of BS-7826, being the distomer, was found to be M4 selective (pKi/M4 = 6.9; pA2 or pKi/M1-M3 (Hm3) = 5.6-6.2). These results indicate that uncharged compounds may (stereo)selectively bind to muscarinic receptors via hydrophobic interactions. Thus, an ionic bond between muscarinic ligands and an anionic site of the receptor is not absolutely necessary for recognition of muscarinic receptors.

Muscarinic receptors mediating depression and long-term potentiation in rat hippocampus

1. Two concentration-dependent effects of the muscarinic agonist carbachol (CCh) were characterized in submerged slices of rat hippocampus using extracellular recordings of excitatory postsynaptic potentials (EPSPs): muscarinic long-term potentiation (LTP(m)) and depression. 2. LTP(m) of the EPSP slope was seen following long exposure (20 min) of the slice to low concentrations of CCh (0.2-0.5 microM). This LTP(m) was not accompanied by a change in the size of the afferent fibre volley or by a change in paired-pulse potentiation, consistent with a postsynaptic locus of CCh action. 3. Intracellular recordings from voltage-clamped neurons of inward current evoked by iontophoretically applied alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) revealed that, while cellular responses to NMDA rose transiently upon superfusion with 0.5 microM CCh, responses to AMPA increased gradually and remained potentiated after washout of CCh. 4. LTP(m) is mediated by an M2 muscarinic receptor. Two M2 muscarinic receptor antagonists, methoctramine and AFDX-116, blocked LTP(m). The M2 agonist oxotremorine induced LTP(m) at low agonist concentrations. None of the M1 and M3 receptor agonists and antagonists tested affected LTP(m). 5. Muscarinic fast onset depression of the EPSP was seen in response to higher concentrations of CCh (2-5 mu M). This depression was accompanied by an increase in paired-pulse potentiation, indicating a possible presynaptic locus of action. The M3 muscarinic receptor antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) blocked the muscarinic depression of the EPSP slope. M1, M2 and M4 muscarinic antagonists did not block this response. 6. Blockade of the muscarinic depression by 4-DAMP did not uncover a suppressed LTP(m). However, addition of picrotoxin facilitated the expression of LTP(m) induced by high concentrations of CCh, indicating an involvement of interneurons in regulation of LTP(m). 7. Cholinergic denervation produced by fimbria-fornix transection resulted in supersensitivity of both M2- and M3-mediated effects, indicating that the receptors mediating these effects are not located on presynaptic cholinergic fibres. In the presence of 4-DAMP and picrotoxin the dose-response curve for CCh-induced effects in slices from lesioned animals was shifted to the left relative to that of normal animals, indicating a supersensitivity of both receptor types.

Characterization of the functional muscarinic receptors in the rat urinary bladder

1. Muscarinic receptors mediating contraction of the rat urinary bladder were characterized functionally in vitro by use of atropine, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP methiodide), 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride (4-DAMP mustard), hexahydro-sila-diphenidol hydrochloride (HHSiD), the p-fluoro analogue of hexahydro-sila-diphenidol hydrochloride (p-F-HHSiD), methoctramine, and pirenzepine. 2. (+)-cis-Dioxolane contracted bladder strips in a concentration-dependent manner with an EC50 of 0.169 +/- 0.018 microM and an Emax of 7.84 +/- 0.67 g. 3. Concentration-effect curves to (+)-cis-dioxolane were shifted to the right in the presence of the antagonists in a concentration-dependent manner. The rank order of antagonist affinities against the (+)-cis-dioxolane response was (pA2 values in the parentheses) atropine (9.28) > or = 4-DAMP methiodide (9.04) > HHSiD (8.01) > p-F-HHSiD (7.28) = pirenzepine (7.12) > or = methoctramine (6.77, 7.25). The profile resembles that associated with the M3 receptor subtype. 4. Atropine, 4-DAMP methiodide, pirenzepine, and methoctramine had no effects on the contractile response to 120 mM KCl. However, HHSiD and p-F-HHSiD decreased the response to KCl, and 4-DAMP mustard increased it. 5. Contractile responses to electrical field stimulation (1-32 Hz, 0.05 ms pulse duration) were biphasic in nature. The tonic response was suppressed more than the phasic response by all antagonists except methoctramine. The suppression was not always concentration-dependent, and did not seem to be related to antagonism of any one receptor subtype. 6. Our findings are consistent with the minority M3 receptors mediating the contractile response to muscarinic stimulation by (+)-cis-dioxolane in the rat bladder.

Muscarinic receptors and drugs in cardiovascular medicine

The parasympathetic system and its associated muscarinic receptors have been the subject of a renaissance of interest for the following two main reasons: (1) the association of endothelial muscarinic receptors and the nitric oxide (NO) pathway; (2) the discovery of several muscarinic receptor subtypes and drugs interacting with them. In the present survey modern insights into the subdivision of muscarinic receptors have been dealt with as the basis for a description of the muscarinic receptor agonists and antagonists thus far known. There are at least four pharmacologically defined M receptors (M1, M2, M3, M4) in primary tissues, and five muscarinic receptors have been cloned (m1, m2, m3, m4, m5). Selective agonists for M-receptor subtypes hardly exist, and all classical agonists (acetylcholine, carbachol, etc.) are clearly nonselective. A few selective antagonists for M1 (pirenzepine) and M2 receptors (AF-DX 116) have been introduced, although selective M3 receptors are hardly available. Finally, the potential therapeutic use of M-receptor agonists (myocardial ischemia, hypertension) and muscarinic antagonists (certain forms of bradycardia, coronary spasm) has been critically discussed. Although only in a preliminary stage, this development appears to be promising and at least of great fundamental interest.

Characterization of the interaction between muscarinic M2 receptors and beta-adrenoceptor subtypes in guinea-pig isolated ileum

1. Contraction of guinea-pig ileum to muscarinic agonists is mediated by M3 receptors, even though they account for only 30% of the total muscarinic receptor population. The aim of this study was to characterize the biochemical and functional effects of stimulation of the predominant M2 muscarinic receptor (70%) and to investigate the hypothesis that M2 receptors specifically oppose beta-adrenoceptor-mediated effects in the ileum. 2. In guinea-pig ileal longitudinal smooth muscle slices, isoprenaline, a non-selective beta-adrenoceptor agonist, and BRL 37344 (sodium-4-[2-[2-hydroxy-2-(3- chlorophenyl)ethylamino]propyl]-phenoxyacetate sesquihydrate), a beta 3-adrenoceptor selective agonist, increased cyclic AMP accumulation with -log EC50 values of 6.6 +/- 0.1 and 5.8 +/- 0.1 respectively. Maximal stimulation by BRL 37344 (10 microM) was 26.4 +/- 5.2% of that observed with isoprenaline (10 microM). Isoprenaline (10 microM)-stimulated cyclic AMP accumulation was significantly, but not completely, inhibited by propranolol (5 microM), with a propranolol-resistant component of 28.2 +/- 6.8% of the maximal stimulation to isoprenaline. In contrast, basal and BRL 37344 responses were resistant to this antagonist. These data provide evidence that both beta 1- and beta 3-adrenoceptors activate adenylyl cyclase in guinea-pig ileum. 3. Isoprenaline (10 microM)-stimulated cyclic AMP accumulation was inhibited (67.4 +/- 0.9%) by the muscarinic agonist (+)-cis-dioxolane (-log EC50 = 7.3 +/- 0.1). The rank order of antagonist affinities against the (+)-cis-dioxolane response was (-log KB values in parentheses): atropine (9.0 +/- 0.2)>methoctramine (7.1 +/- 0.1) >p-fluoro-hexa-hydrosilaphenidol (p-F-HHSiD; 6.5 +/- 0.2) ) pirenzepine(6.3 +/- 0.2). (+)-cis-dioxolane also significantly inhibited BRL 37344 (10 IM; 56.5 +/-2.4%) stimulated cyclic AMP accumulation. These data suggest that M2 receptors mediate inhibition of cyclic AMP accumulation in response to both beta l- and beta 3-adrenoceptor stimulation in guinea-pig ileum.4. 5-Hydroxytryptamine (5-HT), vasoactive intestinal peptide, prostaglandins E2 and E1, all at 10 micro M,significantly increased cyclic AMP accumulation. (+)-cis-Dioxolane (10 micro M) inhibited both basal and agonist-induced cyclic AMP accumulation. Thus the inhibitory effect of M2 receptor agonism does not appear to be restricted to beta-adrenoceptor-stimulated cyclic AMP accumulation.5. The potential for involvement of activation of M2 receptors on responses to beta-adrenoceptor agonists was also studied functionally. Selective M3 receptor alkylation was achieved by pretreatment of tissues with 4-DAMP mustard (40 nM), in the presence of methoctramine (1 micro M; to protect M2 receptors). After washing, tissues were pre-contracted with histamine (0.3 micro M) and relaxed with isoprenaline (0.6 micro M).Under these conditions, oxotremorine M caused concentration-dependent contractions (-log EC50 of 7.8 +/- 0.1), that were surmountably antagonized by methoctramine (1 microM) with a - log KB estimate of 7.4 +/- 0.1. Similar observations were seen versus relaxation produced by BRL 37344 (1 micro M), where the-log KB value for methoctramine was 7.8 +/- 0.2. These data suggest that M2 receptors mediate a functional inhibition of relaxant responses to isoprenaline and BRL 37344.6. These findings are consistent with beta l- and beta 3-adrenoceptors coupling to stimulation of a denylylcyclase in guinea-pig ileum; a response that is inhibited by M2 receptor stimulation. Concordantly, M2 receptor stimulation also inhibits relaxation to both beta l- and beta 3-adrenoceptor stimulation. These results implicate M2 receptors in the modulation of sympathetic control of ileal motility.

Signal transduction pathways of muscarinic receptors in circular smooth muscle from the rabbit caecum

The effects of muscarinic acetylcholine receptor stimulation on phosphoinositides breakdown and adenylate cyclase activity were examined in the circular smooth muscle of the rabbit caecum. In Myo-[3H]inositol-labeled circular smooth muscle cells, carbachol caused a concentration-dependent increase in [3H]inositol phosphates ([3H]IPs) accumulation (EC50 of 3 +/- 1 microM). The M1-selective antagonist pirenzepine (PRZ), the M2-selective AF-DX 116 (11-2[[2-[(diethyl-amino)methyl]-1-piperidinyl]acetyl]-5, 11-dihydro-6Hypyrido[2,3-b][1,4]benzodiazepin-6-one) and the M3-selective para-fluoro-hexahydrosiladifenidol (p-F-HHSiD) inhibited the carbachol-induced [3H]inositol phosphates accumulation with the following order of potency; p-F-HHSiD > PRZ > AF-DX 116. In saponin-permeabilized circular smooth muscle cells, carbachol and GTP gamma [S] elicited a concentration-dependent increase in [3H]inositol phosphates accumulation. The concentration-response curve for GTP gamma [S] was shifted to the left when cells were incubated with 1 microM carbachol. The [3H]inositol phosphates accumulation elicited by simultaneous addition of 0.1 microM GTP gamma [S] and 1 microM carbachol to permeabilized cells was significantly decreased (78.28 +/- 18.23% inhibition) when cells were preincubated for 5 min with 0.1 mM GDP beta [S]. In nonpermeabilized cells, pertussis toxin did not alter the carbachol-induced increase in [3H]inositol phosphates accumulation. On the other hand, the 0.1 mM carbachol-induced inhibition of forskolin-stimulated adenylate cyclase activity in circular smooth muscle homogenates was significantly reversed by atropine and AF-DX 116, whereas PRZ and p-F-HHSiD were ineffective (muscarinic antagonists were used at 1 microM final concentration).(ABSTRACT TRUNCATED AT 250 WORDS)