Action of agonists and antagonists at muscarinic receptors present on ileum and atria in vitro

Identification and pharmacological profile of SPP1, a potent, functionally selective and brain penetrant agonist at muscarinic M1 receptors

BACKGROUND AND PURPOSE

We aimed to identify and develop novel, selective muscarinic M1 receptor agonists as potential therapeutic agents for the symptomatic treatment of Alzheimer's disease.

EXPERIMENTAL APPROACH

We developed and utilized a novel M1 receptor occupancy assay to drive a structure activity relationship in a relevant brain region while simultaneously tracking drug levels in plasma and brain to optimize for central penetration. Functional activity was tracked in relevant native in vitro assays allowing translational (rat-human) benchmarking of structure-activity relationship molecules to clinical comparators.

KEY RESULTS

Using this paradigm, we identified a series of M1 receptor selective molecules displaying desirable in vitro and in vivo properties and optimized key features, such as central penetration while maintaining selectivity and a partial agonist profile. From these compounds, we selected spiropiperidine 1 (SPP1). In vitro, SPP1 is a potent, partial agonist of cortical and hippocampal M1 receptors with activity conserved across species. SPP1 displays high functional selectivity for M1 receptors over native M2 and M3 receptor anti-targets and over a panel of other targets. Assessment of central target engagement by receptor occupancy reveals SPP1 significantly and dose-dependently occupies rodent cortical M1 receptors.

CONCLUSIONS AND IMPLICATIONS

We report the discovery of SPP1, a novel, functionally selective, brain penetrant partial orthosteric agonist at M1 receptors, identified by a novel receptor occupancy assay. SPP1 is amenable to in vitro and in vivo study and provides a valuable research tool to further probe the role of M1 receptors in physiology and disease.

Activation of muscarinic acetylcholine receptors elevates intracellular Ca(2+) concentrations in accessory lobe neurons of the chick

Accessory lobes are protrusions located at the lateral sides of the spinal cord of chicks and it has been proposed that they play a role as a sensory organ for equilibrium during walking. We have reported that functional neurons exist in the accessory lobe. As there is histological evidence that synaptic terminals of cholinergic nerves exist near the somata of accessory lobe neurons, we examined the effects of acetylcholine on changes in intracellular Ca2+ concentrations ([Ca2+]i), as an index of cellular activities. Acetylcholine (0.1-100 µM) caused a transient rise in the [Ca2+]i. Acetylcholine-evoked [Ca2+]i rises were observed in the absence of extracellular Ca2+, and they were abolished in the presence of cyclopiazonic acid, an inhibitor of Ca2+-ATPase of intracellular Ca2+ stores or atropine, a muscarinic receptor antagonist. mRNAs coding M3 and M5 isoforms of the muscarinic receptors were detected in accessory lobes by the RT-PCR. These results indicate that chick accessory lobe neurons express functional muscarinic acetylcholine receptors, and that acetylcholine stimulates Ca2+ mobilization from intracellular Ca2+ stores, which elevates the [Ca2+]i in the somata of accessory lobe neurons, through activation of these receptors. Cholinergic synaptic transmission to the accessory lobe neurons may regulate some cellular functions through muscarinic receptors.

In vitro isolated tissue functional muscarinic receptor assays

Muscarinic receptor (mAChRs) subtypes are viable targets for the design of novel agents for use in a number of central and peripheral disorders. In vitro isolated tissue functional assays for muscarinic receptor subtypes have played an invaluable role in basic research and drug discovery. The availability of biological assays for generation of quantitative estimates of affinity and potency of ligands allows evaluation of the contribution of a given mAChR to the functional end organ response and also enables drug discovery by facilitating the iterative process of screening and optimization of chemical leads. This unit describes isolated tissue functional assays for the quantification of ligand affinity and efficacy at the M(1), M(2), M(3), M(4), and M(5) muscarinic receptor subtypes in tissues expressing the native receptor using organ bath techniques.

Characterization of bencycloquidium bromide, a novel muscarinic M(3) receptor antagonist in guinea pig airways

In this study we have investigated the antagonist affinity, efficacy and duration of action of bencycloquidium bromide (BCQB), a selective muscarinic M(3) receptor antagonist, as a possible clinical bronchodilator for the treatment of chronic obstructive pulmonary disease (COPD) and asthma. In competition studies, BCQB showed high affinity toward the M(3) receptor in Chinese hamster ovary (CHO) cells (M(3) pKi=8.21, M(2) pKi=7.21, and M(1) pKi=7.86); pA(2)=8.85, 8.71 and 8.57 in methacholine-induced contraction of trachea, ileum and urinary bladder, 8.19 in methacholine-induced bradycardia of right atrium in vitro, respectively. In function studies, duration of inhibition of carbachol-induced tonic contraction, BCQB and ipratropium had a very similar onset and offset of action, but onset faster and offset slower than that of tiotropium. After treatment with intratracheally instilled or the inhalation route, BCQB protects against methacholine or antigen-induced bronchoconstriction in a dose-dependent manner in the normal and sensitized guinea pigs in vivo. BCQB and ipratropium-induced inhibitory activity was short lasting, as it declined quickly when compared to tiotropium. These results suggest that BCQB bind muscarinic M(3) receptors with high affinity. On this basis we speculate that a putative BCQB-based therapy for COPD might require more than once-a-day administration to be as effective as the currently employed once-daily therapy with tiotropium. Nevertheless, Inhalable M(3)-selective compounds may spare M(2)-cardiac receptors and reduce the risks of cardiovascular events associated with the long-term treatment of these agents.

Functional M3 muscarinic acetylcholine receptors in mammalian hearts

In contrast to most peripheral tissues where multiple subtypes of muscarinic acetylcholine receptor (mAChR) coexist, with each of them playing its part in the orchestra of parasympathetic innervation, the myocardium has been traditionally considered to possess a single mAChR subtype. Although there is much evidence to support the notion that one receptor subtype (M2) orchestrates myocardial muscarinic transduction, there is emerging evidence that M1 and M3 receptors are also expressed and are of potential physiological, pathophysiological and pharmacological relevance. Clarifying this issue has a profound impact on our thinking about the cholinergic control of the heart function and disease and approaches to new drug development for the treatment of heart disease associated with parasympathetic dysfunction. This review article presents evidence for the presence of the M3 receptor subtype in the heart, and analyzes the controversial data from published pharmacological, functional and molecular studies. The potential roles of the M3 receptors, in parasympathetic control of heart function under normal physiological conditions and in heart failure, myocardial ischemia and arrhythmias, are discussed. On the basis of these considerations, we have made some proposals concerning the future of myocardial M3 receptor research.

Evaluation of an agonist index: affinity ratio for compounds active on muscarinic cholinergic M2 receptors

A protocol for predicting full agonist, partial agonist, and antagonist profiles of compounds with M2 muscarinic cholinergic receptor activity was developed using radioligand binding assay techniques with [3H]-N-methyl scopolamine (NMS) and [3H]-Oxotremorine-M (Oxo-M) as radioligands. Full muscarinic cholinergic receptor agonists such as muscarine and oxotremorine-M expressed a high agonist index (> 3000 for M1 muscarinic cholinergic receptors and > 900 for M2 muscarinic cholinergic receptor), whereas muscarinic receptor antagonists (selective or non-selective) for different receptor subtypes gave a low (0.5-10) agonist index. Functional studies performed on preparations of guinea-pig ileum and heart were consistent with radioligand binding assay experiments. The above results suggest that similarly as already established for the M1 muscarinic cholinergic receptor subtype, evaluation of the [3H]-NMS/[3H]-Oxo-M ratio may provide useful information on the profile of compounds acting at the M2 muscarinic cholinergic receptor subtype. The availability of simple and predictive techniques for the characterization of muscarinic M2 cholinergic receptor agonists, may help the identification of new compounds in therapeutic areas in which stimulation or inhibition of this receptor is desirable.

Functional effects of the muscarinic receptor agonist, xanomeline, at 5-HT1 and 5-HT2 receptors

Xanomeline [3(3-hexyloxy-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-me thylpyridine] has been reported to act as a functionally selective muscarinic partial agonist with potential use in the treatment of Alzheimer's disease. This study examined the functional activity of xanomeline at 5-HT1 and 5-HT2 receptors in native tissue and/or human cloned receptors. Xanomeline had affinity for muscarinic receptors in rat cortical membranes where the ratio of the displacement affinity of [3H]-Quinuclidinyl benzilate vs that of [3H]-Oxotremorine-M was 16, indicative of partial agonist activity. Radioligand binding studies on human cloned receptors confirmed that xanomeline had substantial affinity for M1, M2, M3, M4, M5 receptors and also for 5-HT1 and 5-HT2 receptor subtypes. Carbachol and xanomeline stimulated basal [35S]-GTPgammaS binding in rat cortical membranes with micromolar affinity. The response to carbachol was attenuated by himbacine and pirenzepine with pA2 of 8.2, 6.9 respectively consistent with the response being mediated, predominantly, via M2 and M4 receptors. Xanomeline-induced stimulation of [35S]-GTPgammaS binding was inhibited by himbacine with an apparent pKb of 6.3, was not attenuated by pirenzepine up to 3 microM and was inhibited by the selective 5-HT1A antagonist WAY100635 with an apparent pKb of 9.4. These data suggest the agonist effect of xanomeline in this tissue is, in part, via 5-HT1A receptors. Similar studies on human cloned receptors confirmed that xanomeline is an agonist at human cloned 5-HT1A and 5-HT1B receptors. In studies using the fluorescent cytoplasmic Ca2+ indicator FLUO-3AM, xanomeline induced an increase in cytoplasmic Ca2+ concentration in SH-SY5Y cells expressing recombinant human 5-HT2C receptors. Atropine antagonized this response, consistent with mediation via endogenously-expressed muscarinic receptors. In the presence of atropine, xanomeline antagonized 5-HT-induced cytoplasmic changes in Ca2+ concentration in cells expressing h5-HT2A, h5-HT2B and h5-HT2c receptors with potencies similar to its affinity at these receptors. These studies indicate that xanomeline is a potent agonist at 5-HT1A and 5-HT1B receptors and an antagonist at 5-HT2 receptor subtypes.

The profile of sabcomeline (SB-202026), a functionally selective M1 receptor partial agonist, in the marmoset

1. Sabcomeline (SB-202026, 0.03 mg kg(-1), p.o.), a potent and functionally selective M1 receptor partial agonist, caused a statistically significant improvement in the performance of a visual object discrimination task by marmosets. No such improvement was seen after RS86 (0.1 mg kg(-1), p.o.). 2. Initial learning, which only required an association of object with reward and an appropriate response to be made, was not significantly affected. Reversal learning, which required both the extinction of the previously learned response and the acquisition of a new response strategy, was significantly improved after administration of sabcomeline (0.03 mg kg(-1), p.o.). 3. Sabcomeline (0.03 and 0.1 mg kg(-1), p.o.) had no significant effect on mean blood pressure measured for 2 h after administration in the conscious marmoset. 4. Sabcomeline (0.03 mg kg(-1), p.o.) caused none of the overt effects such as emesis or behaviours often seen after the administration of muscarinic agonists, e.g. face rubbing and licking. 5. This is the first study to demonstrate cognitive enhancement by a functionally selective M1 receptor partial agonist in a normal (i.e. non-cognitively impaired) non-human primate and this effect was seen at a dose which did not cause side effects. 6. Perseverative behaviour and deficient acquisition of new information are seen in patients with Alzheimer's disease (AD). Therefore the data suggest that sabcomeline might be of therapeutic benefit in the treatment of AD.

Design of [R-(Z)]-(+)-alpha-(methoxyimino)-1-azabicyclo[2.2.2]octane-3-acetonitri le (SB 202026), a functionally selective azabicyclic muscarinic M1 agonist incorporating the N-methoxy imidoyl nitrile group as a novel ester bioisostere

Loss of cholinergic function is believed to be implicated in the cognitive decline associated with senile dementia of the Alzheimer type (SDAT). The disease is characterized by progressive loss of muscarinic receptors located on nerve terminals while postsynaptic muscarinic M1 receptors appear to remain largely intact. Muscarinic agonists acting directly on postsynaptic receptors offer the prospect of countering the cholinergic deficit in SDAT. This study describes a novel series of azabicyclic muscarinic agonists, which incorporate an oxime ether or modified oxime ether group as an ester bioisostere. Modification of the oxime ether function by the introduction of electron withdrawing groups led to the finding that the (Z)-N-methoxy imidoyl nitrile group serves as a stable methyl ester bioisostere. This culminated in the discovery of the quinuclidinyl N-methoxy imidoyl nitrile R-(+)-(Z)-5g which is a functionally selective muscarinic M1 partial agonist currently in phase III clinical trials for the treatment of SDAT. The selective profile of R-(+)-(Z)-5g can be rationalized in terms of the relative affinity of the compound at muscarinic receptor subtypes, the degree of agonist efficacy, and brain penetrancy.

Muscarinic receptor subtypes controlling the cationic current in guinea-pig ileal smooth muscle

1. The effects of muscarinic antagonists on cationic current evoked by activating muscarinic receptors with the stable agonist carbachol were studied by use of patch-clamp recording techniques in guinea-pig single ileal smooth muscle cells. 2. Ascending concentrations of carbachol (3-300 microM) activated the cationic conductance in a concentration-dependent manner with conductance at a maximally effective carbachol concentration (Gmax) of 27.4+/-1.4 nS and a mean -log EC50 of 5.12+/-0.03 (mean+/-s.e.mean) (n=114). 3. Muscarinic antagonists with higher affinity for the M2 receptor, methoctramine, himbacine and tripitramine, produced a parallel shift of the carbachol concentration-effect curve to the right in a concentration-dependent manner with pA2 values of 8.1, 8.0 and 9.1, respectively. 4. All M3 selective muscarinic antagonists tested, 4-DAMP, p-F-HHSiD and zamifenacin, reduced the maximal response in a concentration-dependent and non-competitive manner. This effect could be observed even at concentrations which did not produce any increase in the EC50 for carbachol. At higher concentrations M3 antagonists shifted the agonist curve to the right, increasing the EC50, and depressed the maximum conductance response. Atropine, a non-selective antagonist, produced both reduction in Gmax (M3 effect) and significant increase in the EC50 (M2 effect) in the same concentration range. 5. The depression of the conductance by 4-DAMP, zamifenacin and atropine could not be explained by channel block as cationic current evoked by adding GTPgammaS to the pipette (without application of carbachol) was unaffected. 6. The results support the hypothesis that carbachol activates M2 muscarinic receptors so initiating the opening of cationic channels which cause depolarization; this effect is potentiated by an unknown mechanism when carbachol activates M3 receptors. As an increasing fraction of M3 receptors are blocked by an antagonist, the effects on cationic current of an increasing proportion of activated M2 receptors are disabled.

Muscarinic receptor heterogeneity in neonatal rat ventricular myocytes in culture

Carbachol increased ventricular automaticity in a concentration-dependent fashion from a control rate of 72 +/- 5 (mean +/- SEM) to 86 +/- 4 beats per minute at 10(-4) M carbachol. Pirenzepine, an M1-selective antagonist, and AFDX 116, an M2-selective antagonist, both at 10(-7) M, did not block the carbachol-induced positive chronotropic response. In contrast, 10(-7) M HHSiD, an M3-selective antagonist, completely blocked the positive chronotropic effect of carbachol. Carbachol stimulated the accumulation of IP1 in a concentration-dependent manner at concentrations > or = 3 x 10(-6) M. AFDX 116 had no effect on carbachol-induced IP1 accumulation. HHSiD significantly inhibited IP1 accumulation at concentrations > or = 3 x 10(-8) M, while pirenzepine inhibited IP1 accumulation only at concentrations > or = 10(-5) M. McN A343 and methacholine, two muscarinic receptor agonists with minimal M2 activities, and carbachol did not alter basal cAMP concentration, but all three agonists significantly attenuated the increase in cAMP accumulation in response to isoproterenol. Carbachol inhibited isoproterenol-mediated cAMP accumulation at concentrations > or = 10(-7) M. AFDX 116, HHSiD, and pirenzepine blocked the carbachol-induced inhibition of isoproterenol-stimulated cAMP accumulation. At equimolar concentrations, the inhibitory effects of HHSiD and AFDX-116 were similar, while that of pirenzepine was much less. Pretreatment with pertussis toxin for 24 h did not prevent the carbachol-mediated positive chronotropic response or accumulation of IP1 but completely abolished the inhibition of isoproterenol-stimulated cAMP accumulation. These results indicate that (a) neonatal ventricular myocytes in culture have a heterogeneous population of muscarinic (M2 and M3) receptors, (b) the M3 receptor is coupled to pertussis toxin-sensitive and pertussis toxin-insensitive G proteins, (c) M3 receptor stimulation activates phosphoinositide hydrolysis and increases automaticity via a pertussis toxin-insensitive G protein-dependent pathway, and (d) both M2 and M3 receptors couple to pertussis toxin-sensitive G protein(s) to mediate the inhibition of intracellular cAMP accumulation in response to isoproterenol stimulation.

Characterization of the interaction of zamifenacin at muscarinic receptors in vitro

The interaction of zamifenacin ((3R)-(+)-diphenylmethoxy-1-(3,4)-methylenedioxyphenethyl)pi peridine) at muscarinic receptor subtypes was studied using radioligand binding and functional techniques, in vitro. In radioligand binding studies, zamifenacin acted as a competitive antagonist, with the following pKi values; rat cerebral cortex (M1) 7.90 +/- 0.08, myocardium (M2) 7.93 +/- 0.13, submaxillary gland (M3) 8.52 +/- 0.04 and rabbit lung (M4) 7.78 +/- 0.04. In functional studies zamifenacin acted as a surmountable antagonist, exhibiting the following apparent affinity values; canine saphenous vein (putative M1) 7.93 +/- 0.09, guinea-pig left atria (M2) 6.60 +/- 0.04, guinea-pig ileum (M3) 9.31 +/- 0.06, guinea-pig oesophageal muscularis mucosae (M3) 8.84 +/- 0.04, guinea-pig trachea (M3) 8.16 +/- 0.04, and guinea-pig urinary bladder (M3) 7.57 +/- 0.15. Therefore, zamifenacin is selective for muscarinic M3 receptors in guinea-pig ileum, oesophageal muscularis mucosae, trachea and bladder over muscarinic M2 receptors in atria. The degree of muscarinic M3/M2 receptor selectivity depends upon the muscarinic M3 receptor preparation studied.

Characterization of muscarinic receptors mediating contractions of circular and longitudinal muscle of human isolated colon

1. The effects of seven muscarinic receptor antagonists were used to characterize the receptors which mediate carbachol-evoked contractions of intertaenial circular and taenial longitudinal muscle in human isolated colon. The effects of these antagonists were studied upon colon contractions induced by cumulatively added carbachol which had mean EC50 values of 11.7 +/- 2.3 microM (n = 8) and 12.6 +/- 2.3 microM (n = 8) respectively upon circular and longitudinal smooth muscle. 2. All antagonists displaced concentration-response curves to carbachol to the right in a parallel manner. The maximum concentration of each antagonist added (30 nM-10 microM) did not significantly suppress the maximum response. 3. In circular muscle, the M3 muscarinic receptor antagonists, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), hexahydrosiladiphenidol (HHSiD) and para-fluoro-hexahydrosiladiphenidol (p-F-HHSiD) inhibited responses with pA2 values of 9.41 +/- 0.23, 7.17 +/- 0.07, 6.94 +/- 0.18 respectively. The M2 muscarinic receptor antagonist, AF-DX 116, the M2/M4 muscarinic receptor antagonist, himbacine, and the M1 muscarinic receptor antagonist, pirenzepine, yielded pA2 values of 7.36 +/- 0.43, 7.47 +/- 0.14 and 7.23 +/- 0.48 respectively. The non-selective antagonist, atropine, had a pA2 of 8.72 +/- 0.28. 4. In longitudinal muscle 4-DAMP, HHSiD, p-F-HHSiD, AF-DX 116, himbacine and pirenzepine gave pA2 values of 9.09 +/- 0.16, 7.45 +/- 0.43, 7.44 +/- 0.21, 6.44 +/- 0.1, 7.54 +/- 0.40, 6.87 +/- 0.38 respectively. Atropine yielded a pA2 value of 8.60 +/- 0.08. 5. The pharmacological profile of antagonist affinities at the muscarinic receptor population responding to muscarinic agonist-evoked contraction is similar to that widely accepted as characterizing the activation of an M3 muscarinic receptor subtype, although pA2 values of some antagonists are lower than that seen in other investigations.

Retrotrapezoid nucleus muscarinic receptor subtypes localized by autoradiography

Microinjection of muscarinic receptor subtype antagonists into the region of the cat retrotrapezoid nucleus (RTN) decreases blood pressure (greatest efficacy; M2 subtype) and both baseline phrenic activity and CO2 sensitivity (greatest efficacy; M3/M1 subtype). Here we examine, in cat medullary sections at the level of the RTN, the effects of the same antagonists on binding of the high affinity muscarinic agonist quinuclidinyl benzilate (QNB). 3H-QNB binding was saturated and highly specific at 1 nM concentration and stable over 30 to 120 min (Kd, 0.49 nM; Bmax, 136 fm/mg protein). Studied biochemically, we found IC50 values for whole sections of 4.9 x 10(-6) M (M1 antagonist pirenzepine); 1.0 x 10(-6) M (M2 antagonist AFDX); and 0.64 x 10(-7) M (M3 antagonist DAMP; P < 0.03 vs PZ). Densitometric analysis of whole medullary cross section autoradiograms resulted in similar IC50 values as in the biochemical approach. Specific analysis of the RTN region demonstrated the presence of 3H-QNB binding and similar competition by the antagonists. Average IC50 values determined by densitometry were 14 x 10(-6) M (pirenzepine); 1.3 x 10(-6) M (AFDX; P < 0.01 vs PZ); and 0.53 x 10(-7) M (DAMP; P < 0.01 vs PZ). All three subtypes of muscarinic receptors identifiable via pharmacological antagonists appear to be present in the RTN region but we could not distinguish a subtype-specific pattern of receptor distribution.

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.

The involvement of muscarinic receptor subtypes in the mediation of hypothermia, tremor, and salivation in male mice

The potency of centrally administered non-selective (atropine and N-methyl scopolamine) and putatively selective muscarinic antagonists (pirenzepine, AF-DX 116 and 4-DAMP) in inhibition of oxotremorine-induced hypothermia, tremor and salivation in male mice has been compared with their potency in vitro in three functional systems, where muscarinic effects are mediated preferentially by M1 (i.e. superior cervical ganglion), M2 (i.e. atrium), and M3 (i.e. ileum) receptors. Atropine, N-methyl scopolamine and 4-DAMP potently abolished the effects of oxotremorine. Pirenzepine abolished tremor and salivation, whereas hypothermia was antagonized partially only. AF-DX 116 had but weak antagonistic effects. Atropine and N-methyl scopolamine were potent antagonists in all three in vitro test systems. High potency was also seen with 4-DAMP, in particular in the ileum preparation. Pirenzepine showed its highest potency in the ganglion preparation. AF-DX 116 was a weak and non-selective antagonist in all three in vitro preparations. Our studies indicate that the muscarinic induction of tremor and salivation may be preferentially mediated by M3 receptors whereas both M2 and M3 receptors may be involved in the mediation of hypothermia. However, the overall conclusion is that compounds with higher receptor subtype selectivity are needed.

Sialogogic activities of SNI-2011 compared with those of pilocarpine and McN-A-343 in rat salivary glands: identification of a potential therapeutic agent for treatment of Sjörgen's syndrome

1. We examined the sialogogic activities in rat major salivary glands of SNI-2011, in comparison with those of pilocarpine and McN-A-343, and we characterized the subtypes of muscarine receptors that are involved in the sialogogic responses to SNI-2011 and McN-A-343. 2. SNI-2011 at doses ranging from 1 to 10 mg/kg (i.v.) increased the secretion of saliva in a dose-dependent manner. The dose-response curves for SNI-2011 were approximately parallel to curves for pilocarpine but the potency of SNI-2011 was about 25-fold lower than that of pilocarpine. 3. The total volume of saliva secreted in response to McN-A-343 was very much less than that secreted in response to SNI-2011. 4. The salivation induced by SNI-2011 and by McN-A-343 was inhibited by various antagonists with the following rank order of potency: 4-DAMP >> pirenzepine >> AF-DX 116. 5. Our results suggest that the sialogogic effects of SNI-2011 and McN-A-343 are mediated by direct stimulation of M3 receptors in salivary glands and that SNI-2011 may prove useful in the management of xerostomia in patients with Sjögren's syndrome.

Different muscarinic receptor subtypes mediating the phasic activity and basal tone of pig isolated intravesical ureter

1. We have studied the effects of muscarinic cholinoceptor agonists and specific antagonists on both phasic activity and basal tone of the isolated intravesical ureter of the pig by means of isometric techniques in vitro. 2. Acetylcholine in the presence and absence of physostigmine increased both phasic activity and basal tone of ureteral strips in a concentration-dependent manner. Moreover carbachol, methacholine and oxotremorine-M increased both contractile parameters while bethanechol and McN-A-343 evoked only increases in tone without affecting the frequency of the phasic contractions. 3. The nicotinic receptor blocker, hexamethonium (10(-6)-10(-4) M), failed to modify the contractions evoked by a single dose of carbachol (10(-5) M), whilst the muscarinic antagonist, atropine inhibited both phasic and tonic responses. 4. The muscarinic M1 (pirenzepine), M2 (AF-DX 116 and methoctramine), M3 (4-DAMP, HHSiD and p-F-HHSiD), and putative M4 receptor (tropicamide) antagonists significantly reversed increases in both frequency of phasic activity and baseline tone induced by a submaximal dose of carbachol (10(-5) M). The pIC50 values for inhibition of the induced phasic activity were: atropine (10.16) > 4-DAMP (9.12) > HHSiD (8.22) = methoctramine (7.98) = p-F-HHSiD (7.88 > tropicamide (7.62) = pirenzepine (7.53) = AF-DX 116 (7.45) and for inhibition of basal tone were: atropine (10.73) > 4-DAMP (9.32) > HHSiD (8.65) = pirenzepine (8.43) = p-F-HHSiD (8.38) > methoctramine (7.79) > tropicamide (7.53) > AF-DX 116 (7.04). 5. The antagonist profile indicates that an M1 receptor mediates the tonic response while the phasic activity could involve either both M2 and M3 or an M4 muscarinic receptor. These results suggest that different muscarinic receptor subtypes mediate the phasic and tonic contractile activity induced by a submaximal concentration of carbachol in the porcine intravesical ureter.

Muscarinic excitatory and inhibitory mechanisms involved in afferent fibre-evoked depolarization of motoneurones in the neonatal rat spinal cord

1. The involvement of acetylcholine and muscarinic receptors in spinal synaptic responses evoked by electrical and noxious sensory stimuli was investigated in the neonatal rat spinal cord in vitro. 2. Potentials were recorded extracellularly from a ventral root (L3-L5) of the isolated spinal cord, spinal cord-cutaneous nerve, and spinal cord-skin preparations of 1- to 4-day-old rats. Spinal reflexes were elicited by electrical stimulation of the ipsilateral dorsal root or the cutaneous saphenous nerve, or by noxious skin stimulation. 3. Single shock stimulation of supramaximum intensity of a dorsal root induced a mono-synaptic reflex in the corresponding ventral root. Bath-application of the muscarinic agonists, muscarine (0.3-30 microM) and (+)-cis-dioxolane (0.1-100 microM), produced an inhibition of the mono-synaptic reflex and a depolarization of motoneurones. Other muscarinic agonists, arecoline (10 nM-10 microM) and oxotremorine (10 nM-1 microM), inhibited the mono-synaptic reflex with little or no depolarization of motoneurones. Repetitive stimulation of the saphenous nerve at C-fibre strength induced a slow depolarizing response lasting about 30 s of the L3 ventral root. This slow ventral root potential (VRP) was also inhibited by arecoline (10 nM-10 microM) and oxotremorine (10 nM-1 microM). 4. In the spinal cord-saphenous nerve-skin preparation, a slow VRP was evoked by application of capsaicin (0.5 microM), bradykinin (3 microM), or noxious heat (47 degrees C) to skin. This slow VRP was depressed by the muscarinic agonists, arecoline (3 microM) and oxotremorine (1 microM). 5. Of the (+)-cis-dioxolane-induced inhibition of mono-synaptic reflex and motoneurone depolarization, the M2 antagonists, AF-DX 116 (0.1-1 microM) and methoctramine (100-300 nM), preferentially blocked the former response, whereas the M3 antagonists, 4-DAMP (3-10 nM) and p-F-HHSiD (0.3-3 microM), preferentially blocked the latter response. AF-DX 116 (0.1-1 microM) and methoctramine (100-300 nM) also effectively antagonized the arecoline- and oxotremorine-induced inhibition of the slow VRP. The pA2 values of AF-DX 116 and methoctramine against the arecoline-induced inhibition of the mono-synaptic reflex were both 6.79, and that of 4-DAMP against the (+)-cis-dioxolane-induced motoneurone depolarization was 8.16. 6. In the spinal cord-cutaneous nerve preparation, the saphenous nerve-evoked slow VRP was augmented by the anticholinesterase, edrophonium (5 microM). AF-DX 116 (1 microM) and methoctramine (100 nM) also potentiated the slow VRP, whereas 4-DAMP (10 nM) depressed the response. 4-DAMP(5-10 nM) depressed the capsaicin-induced slow VRP in the spinal cord-skin preparation.7. Oxotremorine (0.3 microM) and arecoline (1 AM) markedly depressed the depolarization of motoneurones evoked by application of capsaicin (3 9AM) to the spinal cord, whereas they depressed only slightly the depolarization induced by substance P (10 nM).8. The present study suggests that both excitatory (via M3-type receptors) and inhibitory (via M2-type receptors) muscarinic mechanisms are involved in afferent fibre-evoked nociceptive transmissions in the neonatal rat spinal cord.

Muscarinic M1 receptor agonist actions of muscarinic receptor agonists in rabbit vas deferens

In the electrically field-stimulated rabbit vas deferens, muscarinic receptor agonists increase twitch-height by actions at postjunctional M2 receptors and decrease twitch-height by actions at prejunctional M1 receptors. In the present studies, in contrast to previous reports, muscarinic receptor agonists primarily decreased twitch-height, produced minimal increases in twitch-height, and, produced identical responses in both epididymal and prostatic tissue segments, thus permitting a more detailed investigation of the M1 receptor component of action of muscarinic receptor agonists in the rabbit vas deferens. The nonselective muscarinic receptor agonist carbachol produced biphasic effects on twitch-height in the vas deferens: lower concentrations increased twitch-height to only approximately 25-30% over control, whereas higher concentrations inhibited the twitch. The selective M1 receptor antagonist pirenzepine blocked the inhibitory effects of carbachol, and unmasked carbachol-induced increases in twitch-height. Atropine, 4-DAMP (4-diphenylacetoxy-N-methylpiperidine methiodide) and AF-DX 116 (11-2[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro- 6H-pyrido[2,3-b][1,4]benzodiazepin-6-one) blocked both the inhibitory and stimulatory effects of carbachol, but atropine and 4-DAMP were more potent in blocking the inhibitory than the stimulatory effects of carbachol, whereas the reverse was true for AF-DX 116. McN-A-343 (4-hydroxy-2-butynyl)trimethylammonium chloride, m-chlorocarbanilate) and 12 other muscarinic receptor agonists from a variety of chemical classes also produced concentration-dependent decreases in twitch-height. The log IC50s of the muscarinic receptor agonists for decreasing twitch-height were highly correlated with their log Kis for inhibiting [3H]pirenzepine (r = 0.96) and [3H]oxotremorine-M (r = 0.85) binding in rat hippocampal membranes. The present results demonstrate that the muscarinic M1 receptor mediating inhibition of twitch-height in the rabbit vas deferens has pharmacologic properties similar to the muscarinic M1 receptor in rat hippocampus.

Interaction of selective compounds with muscarinic receptors at dispersed intestinal smooth muscle cells

1. The characterization of muscarinic receptors on single cells of the guinea-pig ileum longitudinal smooth muscle, devoid of neuronal elements, was functionally studied by estimating the affinities of muscarinic antagonists on acetylcholine-induced contractions. 2. Atropine (5 x 10(-11) to 5 x 10(-6) M), 4-diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP, 5 x 10(-8) to 5 x 10(-6) M), cyclohexyl(4-fluoro-phenyl) (3-piperidinopropyl) silanol (pFHHSiD, 5 x 10(-7) to 5 x 10(-5) M) as well as pirenzepine (5 x 10(-7) to 5 x 10(-5) M) competitively antagonized the acetylcholine-dependent contractions with different affinities (atropine > 4-DAMP > pFHHSiD > pirenzepine). 3. Methoctramine (5 x 10(-7) to 5 x 10(-5) M), and AF-DX 116 (5 x 10(-6) and 5 x 10(-5) M) also showed antagonist properties but these deviated from simple competition. These compounds, which discriminate between M2 and M3 receptors, showed a potency lower than that of pirenzepine, the rank order of potencies being pirenzepine > methoctramine > AF-DX 116. When concentrations of AF-DX 116, methoctramine and pirenzepine were increased an unspecific contractile effect occurred. 4. McN-A-343, a partial agonist on intact guinea-pig longitudinal smooth muscle strips, on this preparation induced a weak contraction (about 7% in comparison to control) that was not reversed by antimuscarinic agents. 5. These data indicate that M3 rather than M2 receptor sites are present on this tissue.

Ganglion-blocking activity of dequalinium in frog and rat sympathetic ganglia in vitro

The effect of dequalinium on ganglionic transmission and responses to exogenous acetylcholine receptor agonists were studied in frog and rat sympathetic ganglia. In frog ganglia, dequalinium reduced ganglionic transmission (measured using gross extracellular recording) with an EC50 of 2 microM. At 1 microM, dequalinium produced non-surmountable antagonism of the ganglion depolarization evoked by nicotinic receptor activation. Dequalinium reduced the amplitude of evoked and spontaneous excitatory postsynaptic potentials recorded intracellularly from frog neurones, but had no effect on the action potential elicited by injection of a depolarizing current pulse. Hyperpolarizing and depolarizing responses to muscarine recorded extracellularly from frog ganglia were antagonized by 3 microM dequalinium. In rat ganglia, synaptic transmission and depolarization by an exogenous nicotinic agonist were only slightly inhibited by dequalinium at a concentration of 100 microM. At 30 microM, dequalinium produced a 17% reduction in the depolarization of this tissue by muscarine (1 microM). These results add to the evidence of the diversity of nicotinic and muscarinic acetylcholine receptors, and indicate that dequalinium may be a valuable tool for the study of these receptors. However its usefulness as a probe for calcium-activated potassium channels may be limited by its actions at acetylcholine receptors.

Characterization of the interaction of the cervane alkaloid, imperialine, at muscarinic receptors in vitro

The action of the cervane alkaloid, imperialine, has been assessed at M1, M2 and M3 receptors in functional assays and at M1, M2, M3 and putative M4 sites in binding studies. In functional studies, imperialine acted as a selective surmountable antagonist at M2 receptors in guinea-pig isolated atria and uterus (-log KB = 7.7 and 7.4, respectively), in comparison to M1 receptors in canine isolated saphenous vein (-log KB = 6.9) or M3 receptors in a range of guinea-pig isolated smooth muscles including ileum, trachea, fundus, seminal vesicle or oesophagus (-log KB = 6.6-6.8). In rat aorta, the -log KB value at the M3 receptor (5.9) was slightly, but significantly, lower. In competition radioligand binding studies, imperialine was also selective toward to M2 sites in rat myocardium (-log Ki = 7.2) with respect to M1 and M3 sites (rat cerebral cortex, rat submaxillary gland; -log Ki = 6.1 and 5.7, respectively). However, it did not significantly discriminate between rat cardiac M2 sites and putative M4 sites in rabbit lung (-log Ki = 6.9). Imperialine resembles the alkaloid himbacine in terms of its pharmacological profile at muscarinic receptor subtypes in that it acts as an M2 selective antagonist with respect to M1 or M3 sites. It may also provide a second, commercially available, antagonist with which to discriminate between M1 and M4 receptors.

Muscarinic M2-selective ligands also recognize M4 receptors in the rat brain: evidence from combined in situ hybridization and receptor autoradiography

We have used autoradiographic techniques to examine the characteristics and distribution of the binding of reported selective M2 muscarinic ligands and compared them with the distribution of cells expressing mRNAs for the different subtypes of muscarinic receptors. Our results suggest that the M2 ligands used in the present study ([3H]OXO-M, ([3H]OXO-M,[3H]AF-DX384,AF-DX116, methoctramine) also recognize M4 receptors present in regions such as the striatum and olfactory tubercle. This is supported by 1) relative abundances of the different transcripts, with m2 mRNA being very scarce and m4 mRNA very abundant in these regions; 2) comparison of the pharmacological characteristics of M2-ligand binding sites in brain areas selected by their exclusive expression of M2 receptors versus areas enriched in M4 receptors. An important conclusion of these studies is that none of the muscarinic radioligands available at the present time appears to label specifically a single muscarinic receptor subtype population. Areas are suggested where autoradiographic techniques can be helpful in elucidating the subtype selectivity of existing and new ligands.

Acute desensitization of muscarinic receptors in the isolated guinea-pig ileal longitudinal muscle

1. The effects of acute desensitization of muscarinic receptors mediating contractile responses of the guinea-pig ileal longitudinal muscle were studied in vitro, using similar conditions for both functional and radioligand binding studies. 2. The pA2 values for a number of muscarinic antagonists (pirenzepine, methoctramine, (+/-)para-fluoro-hexahydro-siladifenidol and 4-diphenylacetoxy-N-methyl piperidine-methiodide) indicated that the contractile response to carbachol was mediated through an M3 muscarinic receptor. In binding experiments the muscarinic receptor subtype population in ileal longitudinal muscle was found to be heterogeneous, consisting of approximately 77% M2 and 23% M3 receptors. 3. Pre-exposure of ileal longitudinal muscle to 10 microM carbachol for 30 min produced a reduction (28 +/- % of control maximum) in the maximum contractile response and a dextral shift in the concentration-effect curve to carbachol. Prior equilibration (60 min) with (+/-)p-F-HHSiD (1 microM), but not with methoctramine (1 microM) or pirenzepine (0.3 microM), prevented the desensitization. Desensitization under these conditions did not alter either the apparent affinity, the total number of binding sites or the relative, proportions of M2 and M3 muscarinic receptors, as determined in radioligand binding studies. Desensitization did not cause any meaningful change in either the apparent affinity of carbachol or the proportion of the high and low affinity binding sites. 4. It is concluded that desensitization of the contractile responses of the guinea-pig ileal longitudinal muscle is a result of M3 but not M2 muscarinic receptor desensitization. Acute desensitization, therefore, is not accompanied by meaningful changes in the total number of both M2 and M3 receptors or by alterations in the affinity of the receptor to ligands.

The action of (+/-)L-660,863 [(+/-)3-(3-amino-1,2,4-oxadiazole-5-yl)-quinuclidine] at muscarinic receptor subtypes in vitro

1. The muscarinic pharmacology of a novel oxadiazole muscarinic agonist, (+/-) L-660,863, [+/-3-(3-amino-1,2,4-oxadiazole-5-yl)-quinuclidine] has been studied using pharmacological, radioligand binding and biochemical techniques, in vitro. 2. In isolated tissue experiments, (+/-)L-660,863 was a more potent agonist than carbachol in all preparations studied, being most potent at muscarinic receptors mediating negative chronotropy in guinea-pig right, spontaneously beating atria and least potent at receptors mediating contractions in canine saphenous vein and endothelial denuded rabbit aorta (-log EC50 values were 8.8, 6.6 and 6.3, respectively. The apparent affinities (-log KA) of (+/-)L-660,863) estimated by receptor inactivation, showed some selectivity toward the atrial M2 muscarinic receptor (-log KA = 7.6) in comparison to the M1 or M3 muscarinic receptors (-log KA = 5.4 and 6.2) respectively. This degree of selectivity was also observed in competition radioligand binding studies. 3. At M3 muscarinic receptors mediating inositol phosphates (IPs) accumulation in longitudinal muscle of guinea-pig ileum, the potency of (+/-)L-660,863 (-log EC50 value = 6.2) was similar to the apparent affinity calculated at M3 muscarinic receptors in the functional studies (see above). In contrast, at muscarinic receptors mediating IPs accumulation in guinea-pig atria and ventricles, the potency for (+/-)L-660,863 (-log EC50 = 6.2 and 6.4, respectively) was lower than the apparent affinity calculated at M2 muscarinic receptors from inotropic and binding studies in cardiac tissue (see above).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of muscarinic receptor subtypes in canine left ventricular membranes

The pharmacological characteristics of muscarinic receptor (mAChR) subtypes in canine left ventricular membranes (LVM) were determined using [3H] quinuclidinyl benzilate ([3H]QNB) and [3H]N-methyl scopolamine ([3H]NMS) as ligands. Binding of [3H]QNB and [3H]NMS was saturable with respect to the radioligand concentrations. Analysis of binding isotherms by Scatchard plot showed that [3H]QNB and [3H]NMS bound to an apparently homogeneous population of mAChRs in LVM, with KD values of 390 +/- 100 and 285 +/- 34 pM and Bmax values of 240 +/- 20 and 133 +/- 9 fmol/mg protein, (n = 6), respectively. The Hill coefficients for [3H]QNB and [3H]NMS binding were 0.95 +/- 0.02 and 0.99 +/- 0.01, respectively. Based on the competitive inhibition of [3H]ligand binding, atropine and NMS as well as the selective M1 antagonist PZ revealed no selectivity for these mAChRs. PZ competed with [3H]QNB or [3H]NMS for a single binding site with a Ki value of 0.23 +/- 0.03 microM and 0.62 +/- 0.10 microM, (n = 6), respectively, which is close to the values of M2 or M3 receptors. The data indicate that the M1 receptor subtype did not exist in canine LVM. Competition of [3H]ligand binding with selective M2 antagonists, AF-DX 116 and methoctramine and the selective M3 antagonists, 4-DAMP and hexahydrosiladifenidol, gave a best fit for a two-binding site model. The inhibition of carbachol-mediated phosphoinositide hydrolysis by PZ, AF-DX 116 and 4-DAMP, generated an affinity profile for this response also dissimilar to that described for the classical cardiac M2 response. Although no other muscarinic receptor mRNA has been detected in this tissue, these data suggest the presence of a second population of muscarinic sites, which may signify an M2 receptor diversity.

Characterization of muscarinic receptors in dog tracheal smooth muscle cells

1. The tritiated muscarinic antagonist N-methyl scopolamine, [3H]-NMS, was used to characterize the muscarinic receptors associated with the intact dog tracheal smooth muscle cells. Based on receptor binding assays, the intact tracheal smooth muscle cells had specific, saturable, high-affinity binding sites for [3H]-NMS. 2. Specific binding was cell concentration- and time-dependent. The specific binding of [3H]-NMS was increased linearly with increasing cell concentrations. The equilibrium for association of [3H]-NMS with the muscarinic receptors was attained within 30 min at 37 degrees C. 3. Binding was saturable with respect to [3H]-NMS concentrations. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 320 +/- 20 pM and a maximum receptor density (Bmax) of 13.7 +/- 1.4 fmole per 5 x 10(4) cells. The Hill coefficient for [3H]-NMS binding was 1.00 +/- 0.01. The association (K1) and dissociation (K-1) rate constants were determined to be (1.19 +/- 0.23) x 10(8) M-1 min-1 and 0.034 +/- 0.09 min-1, respectively. KD, calculated from the ratio of K-1 and K1, was 286 +/- 65 pM; this value is close to the value of KD calculated from Scatchard plots of binding isotherms. 4. The non-selective muscarinic antagonist atropine and M1 selective antagonist pirenzepine did not reveal any selectivity of these muscarinic receptors. Pirenzepine competed with [3H]-NMS for a single binding site with a Ki value of (6.02 +/- 0.69) x 10(-7) M which is close to the value of M2 or M3 receptors, indicating that the M1 receptor subtype did not exist in the intact tracheal smooth muscle cells. 5. Competition with cardioselective antagonist (M2), methoctramine; smooth muscle selective antagonists (M3), hexahydrodifenidol and hexahydrosiladifenidol; as well as carbachol, were best fit by a two-binding site model. The results suggest that both M2 and M3 receptor subtypes exist at the cell surface of tracheal smooth muscle cells.

Interaction of p-F-HHSiD (p-Fluoro-hexahydrosila-difenidol) at muscarinic receptors in guinea-pig trachea

1. para-Fluoro-hexahydrosila-difenidol (p-F-HHSiD) has been proposed as an M3 selective antagonist. However, the M3 selectivity is variable in that it exhibits a high pA2 value for M3 muscarinic receptors in guinea-pig ileum but a low value at muscarinic receptors in guinea-pig trachea. 2. The pA2 value in the trachea was found to be agonist independent since similar pA2 values were found when acetylcholine, carbachol, (+)-cis-dioxolane or OXA-22 were used (7.13, 7.03, 6.85 and 6.97, respectively). The pA2 value was not meaningfully increased when the equilibrium period was increased from 60 to 180 min. The pA2 value was unaffected by blockade of M1 or M2 receptors, using 0.1 microM pirenzepine or methoctramine (7.03 and 7.14, respectively). p-F-HHSiD and atropine appeared to act at the same site, as adjudged by combination concentration-ratio studies. 3. The pA2 values for p-F-HHSiD vary by 10 fold between ileal (8.0) and tracheal M3 receptors (7.0). The precise reason for this is unknown, but appears to be unrelated to conditions of disequilibrium that could be detected. The antagonist should therefore only be employed to distinguish M3 or M1 from M2 receptors. In this respect, although the M1/M3 vs M2 discrimination is relatively large (68 fold), p-F-HHSiD exhibits similar properties to other putative M3 selective antagonists such as 4-diphenylacetoxy-N-methyl piperidine methiodide (4-DAMP) or the parent compound, hexahydrosiladifenidol (HHSiD).

Heterogeneity of vascular muscarinic receptors

Muscarinic receptors mediate diverse effects on the vasculature. Recently, a consensus has been arrived at with regard to muscarinic receptor classification (Levine & Birdsall, 1989). As a result, it may now be possible to clarify the role of each subtype in the responses of vascular tissues to muscarinic agonists. It is apparent that vascular muscarinic receptors form a heterogeneous population. M1 receptors contract canine venous tissue, whilst M3 receptors contract porcine and bovine coronary arteries. M3 receptors also mediate EDRF-dependent relaxant responses in the majority of tissues studied to date. M2 receptors elicit relaxations by a decrease in sympathetic outflow in canine femoral vein, rabbit ear artery and rat portal vein. These conclusions are primarily derived from functional estimations of equilibrium dissociation constants, since comparable radioligand binding data are both scarce and contradictory. It is concluded that all three major subtypes of receptors are present in the vasculature. However, the limited selectivity of the available antagonists, the lack of extensive use of such compounds and the unavailability of selective agonists clearly indicate the need for more definitive studies to be undertaken.

Effects of two new pirenzepine analogs on the contractile response of the guinea-pig oesophageal muscularis mucosae to acetylcholine, bethanechol, histamine and high potassium

The guinea-pig oesophageal muscularis mucosae was used to determine the affinity for muscarinic receptors of two new tricyclic compounds, DF 545 and DF 594, which are structurally related to pirenzepine. Both acetylcholine and bethanechol induced a concentration-dependent contraction of the muscularis mucosae. This contraction was competitively antagonized by DF 545 and DF 594 over the dose range 10(-7)-10(-5) M, while at higher concentrations both antagonists caused a depression of the maximal response to the cholinomimetics. The potency of DF 545 and DF 594 appeared to be comparable to that of pirenzepine and approximately 50 times lower than that of atropine. By comparing the affinities of DF 545 and DF 594 with those of selective antagonists (methoctramine and 4-DAMP) which discriminate between M2/M3 muscarinic receptor subtypes, it emerged that pirenzepine as well as DF 545 and DF 594 might act on M3 receptors, which seem to be predominant in the guinea-pig oesophageal muscularis mucosae. McN-A-343 exhibited no agonist activity while it acted as a competitive antagonist against acetylcholine and bethanechol. None of the compounds exhibited calcium antagonist properties. DF 545 inhibited the contractile responses to histamine, but DF 594 and pirenzepine did not.

The interaction of parafluorohexahydrosiladiphenidol at muscarinic receptors in vitro

1. The antagonistic actions of parafluorohexahydrosiladiphenidol (pFHHSiD) at muscarinic receptors has been studied in cardiac muscle, smooth muscle and cell culture preparations. In this paper, the classification scheme of Doods et al. (1987) is employed. This scheme is based upon differential affinities of muscarinic antagonists. pFHHSiD exhibited high pA2 values at M3 receptors mediating contractions of guinea-pig ileum and oesophageal muscularis mucosae (7.8 and 8.2 respectively) whereas low values were determined at M2 receptors mediating negative inotropic responses in guinea-pig atria (6.0). Intermediate pA2 values were determined at M1 receptors mediating contractions of the canine femoral and saphenous veins. 2. The pA2 values of pFHHSiD at receptors mediating endothelial-dependent relaxation of rat aortic rings, rabbit jugular vein and canine femoral artery (7.6-7.9) were similar to those determined on the ileum. However, the pA2 values of pFHHSiD at receptors mediating contractions of the guinea-pig trachea (7.1), which has been previously shown to possess M3 receptors, were different from those determined in the ileum. 3. The similarity in pA2 values of pFHHSiD between the M3 receptors in guinea-pig ileum and the receptors mediating endothelial-dependent relaxations provide further evidence for the role of M3 receptors in this vascular response. Taken together, pA2 values for pFHHSiD range from 7.1 to 8.2, depending upon the M3 preparation used. The selectivity of the compound therefore for the M3 versus the M2 muscarinic receptor ranged from 13 to 163 fold. 4. At muscarinic receptors mediating stimulation of phosphatidylinositol hydrolysis, pFHHSiD paradoxically displayed a high affinity for the M1 receptor in the SH-SY5Y cell line (pA2 = 7.9) as well as for the M3 receptor in the human astrocytoma (1321 NI cell line (pA2 = 7.6). The value at the M1 receptor in SH-SY5Y cells was greater than was observed at M1 receptors mediating contractions of both the canine saphenous and femoral veins (7.1). 5. pFHHSiD, therefore, clearly delineated M3 from M2 muscarinic receptors, whilst the separation between M1 and M3 receptors was variable. The reason for the anomalous affinity estimates in some functional studies remains unclear. These data indicate that the pA2 values for pFHHSiD appear to be tissue-dependent since the M3 selectivity varies according to the preparations studied. As a result the utility of pFHHSiD in muscarinic receptor classification is limited.

Muscarinic receptor subtypes mediating negative chrono- and inotropic responses in isolated, blood-perfused dog right atria

1. Negative chrono- and inotropic responses to both carbachol (CCh) into the sinus node artery and electrical stimulation of vagal nerve fibres (ES) were studied in the isolated, blood-perfused canine right atrium, using four muscarinic receptor antagonists, atropine, 4-DAMP, AF-DX 116 and pirenzepine. 2. ES and CCh evoked negative chrono- and inotropic responses in a frequency-dependent manner and in a dose-related manner, respectively. 3. Each antagonist inhibited these negative chrono- and inotropic responses in a dose-dependent manner. The ranking order of blocking potency (ID50) was atropine greater than or equal to 4-DAMP greater than AF-DX 116 greater than pirenzepine. 4. The ID50 of atropine, 4-DAMP or AF-DX 116 against sinus rate decreases induced by CCh or ES was not significantly different from that against the atrial tension decreases. In contrast, the ID50 of pirenzepine against sinus rate decreases evoked by CCh (17 nmol) or ES (20 nmol) was significantly smaller than that for the atrial tension decrease (CCh, 200 and ES, 53 nmol, respectively). 5. These results suggest that, in the isolated dog atrium, M2-receptor-mediated-responses are predominant. However, M1-receptor activation may also be involved in sinus rate response.

The interaction of hexamethonium with muscarinic receptor subtypes in vitro

1. The action of hexamethonium has been studied at a range of muscarinic receptors in vitro by use of both functional and radioligand binding studies. 2. In functional studies, hexamethonium exhibited little or no significant (P less than 0.05) antagonism of contractile responses to carbachol at muscarinic receptors in the guinea-pig ileum, oesophageal muscularis mucosae, urinary bladder and trachea. However, antagonism was observed at muscarinic receptors in the guinea-pig left atria mediating negative inotropic responses and the calculated pKB value was 3.80. Hexamethonium also antagonized contractile responses to carbachol in the canine saphenous vein. The pKB value at these receptors was 3.75. 3. In the presence of 3.2 mM hexamethonium, the pA2 value for methoctramine at atrial muscarinic receptors was reduced by approximately 10 fold (control pA2 value was 7.81 +/- 0.05; pA2 value in hexamethonium was 6.73 +/- 0.04). In contrast at tracheal muscarinic receptors, the pA2 values for methoctramine were unaffected in the presence of 3.2 mM hexamethonium (control pA2 = 5.58 +/- 0.07; pA2 value in hexamethonium was 5.63 +/- 0.12). All values quoted are mean +/- s.e. mean, n = 8. 4. In competition radioligand binding studies, hexamethonium exhibited a higher affinity for cardiac M2 receptors (pKi = 3.68) than for cerebrocortical M1 receptors (pKi = 3.28) or for submaxillary gland M3 receptors (pKi = 2.61). At M2 receptors hexamethonium at concentrations of 0.1-10 mM, increased the half life of the dissociation rate of [3H]-N-methylscopolamine 1.6-4.3 fold. This was observed at M3 receptors only at 10 mM, when the half life was increased 1.7 fold. 5. We conclude that hexamethonium, in addition to its well characterized nicotinic antagonist properties, can act as a weak muscarinic antagonist and differentiates between cardiac M2 receptors and glandular/smooth muscle M3 receptors. However, hexamethonium differentiates less clearly between M1 and M2 receptors. The selectivity between M2 and M3 receptors observed in the present study with hexamethonium is comparable to other M2 selective antagonists such as AF-DX 116 and himbacine. 6. Caution should be exercised with regard to the inclusion of hexamethonium in functionsal studies of M2 muscarinic receptor subtypes at concentrations of 0.1 mm and above.

Direct labeling of rat M3-muscarinic receptors by [3H]4DAMP

The muscarinic receptors of rat submaxillary gland, rat heart and rat cortex were directly labeled using the ligand [3H]4-diphenylacetoxy-N-methyl-piperidine methiodide [( 3H]4DAMP). In the rat submaxillary gland, [3H]4DAMP predominantly bound with high affinity (Kd = 0.2 nM) to a population of binding sites that displayed the pharmacology of the M3 muscarinic receptor subtype. In rat heart, [3H]4DAMP labeled the M2 muscarinic receptor with low affinity (Kd = 4 nM). In rat cortex [3H]4DAMP predominantly bound to a population of sites with high affinity (Kd = 0.2 nM). The pharmacology of these sites was consistent with [3H]4DAMP labeling both M1 and M3 muscarinic receptors present in rat cortex with high affinity. These data indicate that [3H]4DAMP represents a useful ligand for selectively labeling the M1 and M3 muscarinic receptor subtypes.

The pharmacological properties of the peptide, endothelin

1. The effect of endothelin (ET-1) has been studied on isolated vascular and non-vascular preparations, using both functional and competition radioligand binding techniques. The effects of endothelin on blood pressure were studied in both anaesthetised, chemically denervated normotensive and spontaneously hypertensive rats (SHR). 2. Endothelin elicited contractile responses in the rat thoracic aorta, perfused mesenteric bed, rabbit mesenteric artery and portal vein. The maximal responses in the rat aorta were enhanced by removal of the endothelium, and were reduced in the presence of either a cyclo-oxygenase inhibitor (indomethacin) or a thromboxane receptor antagonist (SQ 29,548). In terms of potency, the most sensitive preparation was the rat endothelium-denuded aorta and rat perfused mesenteric bed (-log EC50 values = 8.2 +/- 0.07 and 8.2 +/- 0.12, mean +/- s.e.mean, n = 4, respectively). In the perfused mesenteric bed of the rat the maximum response to endothelin (219 +/- 12 mmHg, n = 4) was greater than that to either phenylephrine (maximal response = 67 +/- 9 mmHg; n = 4) or KCl (maximal response = 110 +/- 6 mmHg, n = 4). 3. Endothelin elicited contractile responses of the guinea-pig isolated ileum, oesophageal muscularis mucosae and uterus. Responses were also observed in the rat fundic strip and paced left atria. The guinea-pig urinary bladder, trachea, rat vas deferens and anococcygeus exhibited little or no response to endothelin at the concentrations studied (1 x 10(-12)-3.2 x 10(-8) M). Of the above preparations, the ileum and oesophageal muscularis mucosae were the most sensitive to endothelin (-log EC50 = 8.5 +/- 0.11 and 8.4 +/- 0.06, n = 6, respectively), exhibiting potencies similar to those observed in the endothelium-denuded aorta of the rat. 4. In competition-radioligand binding studies, endothelin did not displace either [3H]-PN 210-100 or [125I]-(-)-omega-conotoxin GVIA from binding sites in membranes from rat cerebral cortex and, skeletal muscle or from guinea-pig cerebral cortex and hippocampus, respectively. This indicates a lack of direct interaction of endothelin at the dihydropyridine binding site and the N-type calcium channel, respectively. However, in functional studies, contractile responses to endothelin (1 x 10(-8) M) in the endothelium-denuded aorta of the rat were potently reversed by nifedipine, verapamil, and prenylamine (-log IC50 values = 8.0 +/- 0.13, 7.2 +/- 0.09 and 6.6 +/- 0.08, n = 4-8, respectively). In addition, the responses to endothelin were virtually abolished in the presence of Krebs physiological salt solution containing no calcium but with 1 x 10-M EDTA added. Preequilibration with either (-)-w-conotoxin (1 x 10-6M) or tetrodotoxin (1 x 10-6M) did not affect responses to endothelin. 5. In chemically denervated rats, endothelin (1pmolkg-'-10nmolkg- , i.v.) exhibited pressor responses, which were unaffected by a 3 h pretreatment with indomethacin. In the SHR, the effects on blood pressure were not significantly different from those observed in normotensive animals at any of the doses studied. A transient (duration < 30 s) depressor response was also observed in all groups studied at a dose of 0.1-1 nmol kg-1 i.v. 6. In conclusion, endothelin is a potent contractile agonist in both vascular and non-vascular muscle. It appears to elicit responses partly via the entry of extracellular calcium (by a mechanism distinct from that of other calcium facilitators) and partly by release of endoperoxides.

Subclassification of atrial and intestinal muscarinic receptors of the rat--direct binding studies with agonists and antagonists

1. Although extensively investigated, the extent of differences between receptors mediating negative inotropic and chronotropic responses is still unclear. In the present study atrial and intestinal muscarinic receptors were identified by [3H]-N-methyl-scopolamine ([3H]-NMS) binding and the affinities of some presumably inotropy- or chronotropy-selective agonists and several antagonists determined. 2. All the agonists tested showed similar affinity for right and left atrial receptors. Accepting an affinity difference of 0.4 log units as experimental error, none of the agonists tested was selective for either atrium. 3. Affinity differences of the cardioselective antagonists himbacine, AF-DX 116 and methoctramine and the M1-selective antagonist dicyclomine for right and left atrial muscarinic receptors were also minimal (less than 2 fold selective). When compared to intestinal receptors, AF-DX 116 was 3 to 4 fold, methoctramine 10 to 13 fold selective and himbacine and dicyclomine non-selective. 4. These data provide evidence for differences between atrial and intestinal but not between right and left atrial muscarinic receptors.

M2 muscarinic receptors on the iris sphincter muscle differ from those on iris noradrenergic nerves

The pre- and postjunctional affinity constants of a series of muscarinic antagonists were determined in guinea pig and rabbit irises. Field stimulation-evoked [3H]noradrenaline release from superfused isolated irises was concentration dependently inhibited by (+/-)-methacholine, confirming the presence on the iris noradrenergic nerves of prejunctional inhibitory muscarinic receptors. The affinity constants of the antagonists at the pre- and postjunctional receptors are compatible with the coexistence in the iris of two different M2 receptors: the cardiac (M2 alpha) subtype on the noradrenergic nerves and the smooth muscle (M2 beta) subtype on the iris sphincter muscle. The rank order of potency of the antagonists studied at the prejunctional site was: atropine greater than himbacine greater than AF-DX 116 greater than pirenzepine greater than hexahydrosiladifenidol. The order of potency at the postjunctional receptors mediating the methacholine-induced isotonic contraction of the isolated rabbit iris sphincter was: atropine greater than hexahydrosiladifenidol greater than pirenzepine greater than himbacine greater than AF-DX 116.

Comparison of the muscarinic receptors in the coronary artery, cerebral artery and atrium of the pig

The affinity of various muscarinic antagonists for the muscarinic receptors mediating contraction (induced by acetyl-beta-methylcholine) of the isolated pig coronary and basilar artery was determined in order to compare the muscarinic receptor subtype involved in the contractile response of these arteries. In order to identify the muscarinic receptor subtype(s) involved, the affinity of the antagonists for the M2 receptor present in the pig atria was also investigated. The following muscarinic antagonists were used: atropine, pirenzepine, AF-DX 116 (11-2[[2-[(diethylamino)methyl]-1- piperidinyl]acetyl]-5,11-dihydro-6H- pyrido[2,3-b][1,4]benzodiazepin-6-one),4-DAMP(4-diphenylacetoxy-N- methylpiperidine methiodide), HHSiD (hexahydrosiladifenidol), methoctramine (N,N'-bis[6-[(2- methoxybenzyl)amino]hexyl]-1,8-octane-diamine tetrahydrochloride) and ipratropium. The order of affinity of the antagonists with respect to the muscarinic receptor in the coronary artery was clearly different from that for the muscarinic receptor in the basilar artery. The order of affinity established on the basilar artery closely resembled that for the M2 receptor in the atria. It is concluded that the muscarinic receptors on smooth muscle of the coronary and basilar arteries are not identical. The muscarinic receptor involved in the contraction of the basilar artery adheres to the M2 receptor subtype. A comparison of the selectivity of the antagonists suggests that the muscarinic receptor involved in the contraction of the coronary artery belongs to the M3 (like in exocrine glands) or M4 (as found in ileal smooth muscle) receptor subtype.

Affinity of muscarinic receptor antagonists for three putative muscarinic receptor binding sites

1. A range of muscarinic receptor antagonists were examined for affinity at the M1 muscarinic binding site, present in rat cerebrocortical membranes and the M2 muscarinic binding sites of rat cardiac and submaxillary gland membranes. 2. The results obtained were consistent with the presence of three classes of muscarinic binding site. 3. Both the M1 binding site, labelled by [3H]-pirenzepine ([3H]-Pir) in rat cerebrocortical membranes, and the M2 gland binding site, labelled by [3H]-N-methyl scopolamine ([3H]-NMS) in rat submaxillary gland membranes, displayed higher affinity for pirenzepine, dicyclomine, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and cyclohexylphenyl (2-piperidinoethyl) silanol (CPPS) than did the M2 binding sites of cardiac membranes labelled by [3H]-NMS. 4. The M2 cardiac sites displayed higher affinity for methoctramine, himbacine and AF-DX 116 than did either the M1 binding site of cerebrocortical membranes or the M2 gland binding site present in rat submaxillary gland membranes. 5. The M1 and M2 gland binding sites could only be distinguished by considering the absolute affinity of compounds for these two sites. Thus, all compounds, with the exception of 4-DAMP, displayed between a 2 and 8 fold higher affinity for the M1 than for the M2 gland binding site. There were no antagonists with higher M2 gland than M1 affinity.

Bretylium tosylate binds preferentially to muscarinic receptors labelled with [3H]oxotremorine M (SH or 'high affinity' receptors) in rat heart and brain cortex

Bretylium tosylate is an antiarrhythmic agent. In guinea pig atria it showed the properties of a competitive muscarinic (cholinergic) antagonist and could distinguish between two muscarinic receptor classes or states in cardiac membranes. We decided to further investigate its binding properties at muscarinic cholinergic receptors of the rat heart and brain (cortex), keeping in mind the recently discovered heterogeneity of muscarinic receptor protein. Bretylium tosylate recognized two receptor classes or states in the heart with Ki values of 0.9 and 11 microM. All cardiac membrane receptors showed a homogeneous (11 microM) Ki value for the drug in the presence of GTP in the incubation medium, or after in vivo pretreatment with islet activating protein (IAP). Bretylium tosylate was able (but only at a high concentration, 1 mM) to slow the dissociation kinetics of the tracer, which suggests that it also bound to an allosteric site on the muscarinic receptor, or that it affected the receptor environment. In the brain cortex, as in the heart, bretylium tosylate displayed a high affinity for receptors labelled with the agonist [3H]oxotremorine M (Ki value: 0.8 microM for the SH-or cardiac-type high-affinity receptors), and a 8- to 10-fold lower affinity for cortex M and L receptors. These data suggest that the antagonist bretylium tosylate had binding properties in rat cardiac membranes analogous to those of the partial agonist pilocarpine and that it interacted with a single type of receptor.