In vitro characterisation of the muscarinic receptor partial agonist, sabcomeline, in rat cortical and heart membranes.

We have investigated the pharmacology of the functionally selective muscarinic M1 receptor partial agonist, sabcomeline [SB-202026 (R-(Z)-(+)-alpha-(methoxyamino)-1-azabicyclo[2.2.2] octane-3-acetonitrile)], in rat cortex and heart using radioligand binding and functional studies. The Quinuclidinyl benzilate/Oxotremorine-M acetate ratio from radioligand binding studies suggested that sabcomeline and xanomeline [3(3-hexyloxy-1,25-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-met hylpyridine] are muscarinic receptor partial agonists in cortical and heart membranes. In [35S]GTPgammaS binding studies in rat cortex, carbachol stimulated binding via muscarinic M2/M4 receptors which could be blocked by sabcomeline with a pA2 of 7.2. In rat heart membranes, carbachol also stimulated [35S]GTPgammaS binding studies through muscarinic M2 receptors. Sabcomeline caused a small stimulation of basal [35S]GTPgammaS binding in both rat and heart tissues. Sabcomeline did not stimulate phosphoinositide hydrolysis in rat cortical slices, but did block the muscarinic M1 receptor-mediated response caused by carbachol with apparent pKb of 6.9. Xanomeline and milameline also had no effect on phosphoinositide hydrolysis up to 100 microM. In adenylyl cyclase studies in rat atria, sabcomeline inhibited forskolin-stimulated adenylyl cyclase activity to a similar extent to that of carbachol, xanomeline and milameline. The present study, using the techniques of radioligand binding, supports previous publications which have claimed that sabcomeline is a muscarinic receptor partial agonist. As expected, this study shows that the functional actions of this compound at muscarinic receptor subtypes and in different tissues will depend on receptor reserve.     

Differences in kinetics of xanomeline binding and selectivity of activation of G proteins at M(1) and M(2) muscarinic acetylcholine receptors

Xanomeline is a functionally selective M(1)/M(4) muscarinic acetylcholine receptor agonist that nevertheless binds with high affinity to all five subtypes of muscarinic receptors. A novel mode of interaction of this ligand with the muscarinic M(1) receptors characterized by persistent binding and receptor activation after extensive washout has been shown previously. In the present study, using human M(1) and M(2) receptors expressed in Chinese hamster ovary cells and [(3)H]N-methylscopolamine as a tracer, we show that persistent binding of xanomeline also occurs at the M(2) receptor with similar affinity as at the M(1) receptor (K(I) = 294 and 296 nM, respectively). However, kinetics of formation of xanomeline wash-resistant binding to M(2) receptors was markedly slower than to M(1) receptors. Xanomeline was a potent fast-acting full agonist in stimulating guanosine 5'-O-(3-[(35)S]thio)triphosphate binding at M(1) receptors, whereas at M(2) receptors it behaved as a potent partial agonist (40% of carbachol maximal response) only upon preincubation for 1 h. Development of xanomeline agonistic effects at the M(2) receptor was slower than its ability to attenuate carbachol responses. We also demonstrate that xanomeline discriminates better between G protein subtypes at M(1) than at M(2) receptors. Our data support the notion that xanomeline interacts with multiple sites on the muscarinic receptor, resulting in divergent conformations that exhibit differential effects on ligand binding and receptor activation. These conformations are both time- and concentration-dependent and vary between the M(1) and the M(2) receptor.     

Novel persistent activation of muscarinic M1 receptors by xanomeline

The muscarinic agonists, xanomeline and carbachol, displayed similar intrinsic activities in stimulating neuronal nitric oxide synthase at muscarinic M₁ receptors in Chinese hamster ovary (CHO) cells, with xanomeline being more potent. Pre-incubation (1 h) with 1 μM xanomeline, followed by extensive washing, resulted in a significantly elevated basal response, which was absent on co-incubation with atropine. This phenomenon was not observed with carbachol. This is the first report of a persistent, receptor-activating effect of a muscarinic agonist.     

Synthesis and pharmacological characterization of chiral pyrrolidinylfuran derivatives: the discovery of new functionally selective muscarinic agonists

Building on the previously and successfully applied hypothesis that stereochemical complication in the proximity of the critical cationic head of a cholinergic agonist would result in subtype selective compounds, we synthesized a series of chiral derivatives of furmethide and 5-methylfurmethide, with the aim of obtaining compounds that are useful for treating diseases derived from cholinergic receptor dysfunctions and/or useful for further characterizing subtypes of cholinergic receptors. Unlike their parent compounds, the new molecules lack nicotinic activity, being pure muscarinic ligands. While binding studies on the five cloned human muscarinic receptors showed no subtype selectivity, functional assays revealed that some of the molecules of the series are potent M 2 selective partial agonists with interesting pharmacological profiles.     

Changes in muscarinic receptors in the toad urothelial cell line TBM-54 following acrolein treatment

1. In cyclophosphamide-induced cystitis in the rat, cholinergic function of the bladder and muscarinic receptor expression are altered. In the present study, we investigated whether the toad urothelial cell line TBM-54 expresses functional muscarinic receptors and whether changes in muscarinic receptors can be induced in vitro by treating cells with acrolein, a metabolite of cyclophosphamide causing cystitis. 2. The occurrence of muscarinic receptors on cells was assessed by microphysiometry, a method analysing receptor function by measuring changes in the extracellular acidity rate (ECAR) in response to receptor stimulation. 3. Challenging untreated cells with the muscarinic receptor agonist carbachol gave rise to a concentration-dependent increase in changes in ECAR, with a maximal response at 1 mmol/L carbachol of 51 +/- 6%. Pre-incubating cells with different muscarinic receptor antagonists (i.e. pirenzepine (M(1) receptor selective), methoctramine (M(2)/M(4) receptor selective) and 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP; M(3)/M(1)/M(5) receptor selective)), gave rise to a concentration-dependent decrease in the effects of carbachol (0.5 mmol/L) on changes in ECAR. 4. Western blot analysis was used to determine the expression of all muscarinic receptor subtypes (M(1)-M(5)) by the cell line. Following acrolein treatment, cells were markedly less sensitive to carbachol and the expression of muscarinic M(2) receptors was decreased, whereas the expression of muscarinic M(3) receptors was increased. 5. In conclusion, the urothelial cell line TBM-54 expresses functional muscarinic receptors and exposure to acrolein leads to a modulation in the expression of muscarinic receptors. Consequently, acrolein may have direct effects on muscarinic receptor function and expression that contribute to the pathogenesis of cyclophosphamide-induced cystitis.     

L-689,660, a novel cholinomimetic with functional selectivity for M1 and M3 muscarinic receptors.

1. L-689,660, 1-azabicyclo[2.2.2]octane, 3-(6-chloropyrazinyl)maleate, a novel cholinomimetic, demonstrated high affinity binding (pKD (apparent) 7.42) at rat cerebral cortex muscarinic receptors. L-689,660 had a low ratio (34) of pKD (apparent) values for the displacement of binding of the antagonist ([3H]-N-methylscopolamine ([3H]-NMS) compared with the displacement of the agonist [3H]-oxotremorine-M ([3H]-Oxo-M), in rat cerebral cortex. Low NMS/Oxo-M ratios have been shown previously to be a characteristic of compounds that are low efficacy partial agonists with respect to stimulation of phosphatidyl inositol turnover in the cerebral cortex. 2. L-689,660 showed no muscarinic receptor subtype selectivity in radioligand binding assays but showed functional selectivity in pharmacological assays. At M1 muscarinic receptors in the rat superior cervical ganglion, L-689,660 was a potent (pEC50 7.3 +/- 0.2) full agonist in comparison with (+/-)-muscarine. At M3 receptors in the guinea-pig ileum myenteric plexus-longitudinal muscle or in trachea, L-689,660 was again a potent agonist (pEC50 7.5 +/- 0.2 and 7.7 +/- 0.3 respectively) but had a lower maximum response than carbachol. In contrast L-689,660 was an antagonist at M2 receptors in guinea-pig atria (pA2 7.2 (95% confidence limits 7, 7.4)) and at muscarinic autoreceptors in rat hippocampal slices. 3. The putative M1-selective muscarinic agonist, AF102B (cis-2-methylspiro-(1,3-oxathiolane 5,3')-quinuclidine hydrochloride) was found to have a profile similar to L-689,660 but had up to 100 times less affinity in binding and functional assays.(ABSTRACT TRUNCATED AT 250 WORDS)     

Selectivity of partial agonists related to oxotremorine based on differences in muscarinic receptor reserve between the guinea pig ileum and urinary bladder

The muscarinic effects of analogs of oxotremorine were compared on strips of the guinea pig ileum and urinary bladder. In a series of eight analogs, full or nearly full contractile responses compared to carbachol were observed on the ileum. On the bladder, the analogs were full agonists, partial agonists, or competitive antagonists. Although EC50 values estimated on the bladder were 10- to 20-fold greater than those obtained on the ileum, the dissociation constant and relative efficacy of each agonist were similar in the two tissues, as were dissociation constants of competitive antagonists including pirenzepine. The ability to discriminate between responses in the ileum and bladder was related to intrinsic efficacy. Highly efficacious compounds such as carbachol and oxotremorine-M were full agonists in both tissues, although less potent on the bladder. Compounds having intermediate intrinsic efficacy, e.g., oxotremorine, were partial agonists on the bladder, whereas BM 5, having low intrinsic efficacy, was a competitive antagonist. These results suggest a mechanism, based on tissue differences in receptor reserve, by which selectivity may be achieved among muscarinic stimulants, even in the absence of distinct subtypes of muscarinic receptors.     

Competitive and non-competitive antagonism exhibited by 'selective' antagonists at atrial and ileal muscarinic receptor subtypes

The affinity of a number of 'selective' agonists and antagonists has been assessed at atrial or ileal muscarinic receptors by use of in vitro functional analysis. The most selective compound for ileal muscarinic receptors was silabenzhexol (approx. 50 fold), and to a lesser extent benzhexol (approx. 5 fold). Conversely, the most selective compound for the atrial muscarinic receptors was AF-DX 116 (approx. 6 fold). The novel M1-receptor antagonist, telenzepine and other antagonists such as propantheline and isopropamide did not distinguish between atrial and ileal receptors. Dicyclomine, adiphenine, hexahydroadiphenine and oxyphenonium exhibited competitive antagonism at atrial receptors but non-competitive antagonism at ileal receptors. No conclusions could, therefore, be drawn with regard to their selectivity. The agonists, arecaidine propargyl ester (APE), ethoxyethyltriethylammonium (EOE) and carbachol, exhibited some selectivity in potency but little difference in affinity. It is concluded that the study supports the existence of ileal and atrial muscarinic receptor subtypes. However, the use of dicyclomine and related compounds in receptor classification is limited.     

Muscarinic subtype affinity and functional activity profile of 1-methyl-2-(2-methyl-1,3-dioxolan-4-yl)pyrrolidine and 1-methyl-2-(2-methyl-1,3-oxathiolan-5-yl)pyrrolidine derivatives

Starting from two previously studied muscarinic full agonists, characterized by a 1,3-dioxolane ((+)-1) and a 1,3-oxathiolane ((+)-2) cycle, two new series of muscarinic ligands were designed, obtained by the steric complication of the parent compounds produced by freezing the aminoalkyl chain into a pyrrolidine ring. Both tertiary amines and the corresponding iodomethyl derivatives were synthesised and studied, and several compounds of the series which behaved as muscarinic agonists have been selected, on the basis of preliminary binding experiments on rat cortex homogenates, for the present work. Results are presented obtained from testing the affinity of the selected compounds against cloned human muscarinic receptors expressed in CHO cells, in order to evaluate subtype selectivity. Their functional activity on classical models of M1-M4 receptors, in guinea pig and rabbit tissues is also reported. With respect to parent compounds, the new molecules present some selectivity toward hm2 receptors; fair M2 selectivity is also evident in functional studies, where these compounds behave as partial agonists. Among the other compounds of the series (2S, 4'R, 2'S)-1,1-dimethyl-2-(2-methyl-1,3-dioxolan-4-yl)pyrrolidinium iodide (-)-3 and (2R, 5'S, 2'S)-1-methyl-2-(2-methyl-1,3-oxathiolan-5-yl)pyrrolidine (+)-5 present a promising pharmacological profile. Compound (-)-3 shows modest hm2 selectivity in binding experiments but a clearcut M2 selectivity in functional tests, where it behaves as a weak antagonist on M1 and M4 subtypes, as a weak full agonist on the M3 subtype and as a potent partial agonist on M2 subtype. Tertiary amine (+)-5 presents a quite similar profile but appears more interesting since, lacking a permanent charge on the nitrogen atom, it may represent an interesting tool to study CNS muscarinic receptors. Our results confirm that sterical complication of parent compounds (+)-1 and (+)-2 produces more selective muscarinic agonists.     

Xanomeline and the antipsychotic potential of muscarinic receptor subtype selective agonists

Binding studies initially suggested that the muscarinic agonist, xanomeline, was a subtype selective muscarinic M(1) receptor agonist, and a potential new treatment for Alzheimer's disease. However, later in vitro and in vivo functional studies suggest that this compound is probably better described as a subtype selective M(1)/M(4) muscarinic receptor agonist. This subtype selectivity profile has been claimed to explain the limited classical cholinomimetic side effects, particularly gastrointestinal, seen with xanomeline in animals. However, in both healthy volunteers and Alzheimer's patients many of these side effects have been reported for xanomeline and in the patient population this led to a >50% discontinuation rate. Clearly, the preclinical studies have not been able to predict this adverse profile of xanomeline, and this suggests that either xanomeline is not as subtype selective as predicted from preclinical research or that there are differences between humans and animals with regard to muscarinic receptors. Nevertheless, in Alzheimer's patients xanomeline dose-dependently improves aspects of behavioral disturbance and social behavior including a reduction in hallucinations, agitation, delusions, vocal outbursts and suspiciousness. The effects on cognition are not as robust and mainly seen at the highest doses tested. These effects in Alzheimer's patients have given impetus to the suggestion that muscarinic agonists have potential antipsychotic effects. The current review assesses the antipsychotic profile of xanomeline within the framework of the limited clinical studies with cholinergic agents in man, and the preclinical research on xanomeline using various models commonly used for the assessment of new antipsychotic drugs. In general, xanomeline has an antipsychotic-like profile in various dopamine models of psychosis and this agrees with the known interactions between the cholinergic and dopaminergic systems in the brain. Moreover, current data suggests that the actions of xanomeline at the M(4) muscarinic receptor subtype might mediate its antidopaminergic effects. Particularly intriguing are studies showing that xanomeline, even after acute administration, selectively inhibits the firing of mesolimbic dopamine cells relative to dopamine cell bodies projecting to the striatum. This data suggest that xanomeline would have a faster onset of action compared to current antipsychotics and would not induce extrapyramidal side effects. The preclinical data on the whole are promising for an antipsychotic-like profile. If in a new formulation (i.e., transdermal) xanomeline has less adverse effects, this drug may be valuable in the treatment of patients with psychosis.     

Evaluation of muscarinic agonist-induced analgesia in muscarinic acetylcholine receptor knockout mice

Centrally active muscarinic agonists display pronounced analgesic effects. Identification of the specific muscarinic acetylcholine receptor (mAChR) subtype(s) mediating this activity is of considerable therapeutic interest. To examine the roles of the M(2) and M(4) receptor subtypes, the two G(i)/G(o)-coupled mAChRs, in mediating agonist-dependent antinociception, we generated a mutant mouse line deficient in both M(2) and M(4) mAChRs [M(2)/M(4) double-knockout (KO) mice]. In wild-type mice, systemic, intrathecal, or intracerebroventricular administration of centrally active muscarinic agonists resulted in robust analgesic effects, indicating that muscarinic analgesia can be mediated by both spinal and supraspinal mechanisms. Strikingly, muscarinic agonist-induced antinociception was totally abolished in M(2)/M(4) double-KO mice, independent of the route of application. The nonselective muscarinic agonist oxotremorine showed reduced analgesic potency in M(2) receptor single-KO mice, but retained full analgesic activity in M(4) receptor single-KO mice. In contrast, two novel muscarinic agonists chemically derived from epibatidine, CMI-936 and CMI-1145, displayed reduced analgesic activity in both M(2) and M(4) receptor single-KO mice, independent of the route of application. Radioligand binding studies indicated that the two CMI compounds, in contrast to oxotremorine, showed >6-fold higher affinity for M(4) than for M(2) receptors, providing a molecular basis for the observed differences in agonist activity profiles. These data provide unambiguous evidence that muscarinic analgesia is exclusively mediated by a combination of M(2) and M(4) mAChRs at both spinal and supraspinal sites. These findings should be of considerable relevance for the development of receptor subtype-selective muscarinic agonists as novel analgesic drugs.     

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)     

Xanomeline: A selective muscarinic agonist for the treatment of Alzheimer's disease

Xanomeline is a novel muscarinic receptor agonist relatively devoid of parasympathomimetic side effects. Xanomeline had high affinity for muscarinic receptors and much lower affinity for a variety of other neuronal receptors in radioligand binding assays. Functional studies in cell lines transfected with the muscarinic receptor subtypes demonstrated that xanomeline had higher potency and efficacy for m1 and m4 receptors than m2, m3, and m5 receptor subtypes. Similarly, in isolated tissue studies, xanomeline had higher potency and efficacy for M₁ receptors in rabbit vas deferens than at M₂ receptors in guinea pig atria or M₃ receptors in guinea pig bladder. Secretion of soluble amyloid precursor protein from m1 cell lines was potently stimulated by xanomeline. In vivo, xanomeline robustly stimulated phosphoinositide hydrolysis in brain, consistent with m1 agonism. Xanomeline produced modest increases in brain acetylcholine levels and did not produce bradycardia, suggesting little, if any, m2 agonist activity in vivo. Additionally, xanomeline did not induce nonselective cholinergic agonist side effects such as tremor, hypothermia and salivation. In animal behavior studies, xanomeline reduced locomotion and blocked memory deficits that were induced by a muscarinic antagonist in a passive avoidance paradigm. Xanomeline was found to be safe and reasonably well tolerated in safety studies in humans. In a placebo controlled double blind clinical trial of 6 months duration, xanomeline halted cognitive decline in patients with Alzheimer's disease. Furthermore, behavioral symptoms associated with Alzheimer's disease such as hallucinations, delusions and vocal outbursts were significantly decreased by xanomeline treatment. Additional clinical trials are under way to assess the novel therapeutic effects of xanomeline. Drug Dev. Res. 40:158–170, 1997. © 1997 Wiley-Liss, Inc.     

M1胆碱受体激动剂治疗阿尔茨海默病的研究进展

阿尔茨海默病(Alzheimer disease，AD)是一种以胆碱能神经元进行性退变、老年斑和神经元缠结为病理特征的神经退行性疾病。尽管AD发病机制尚未阐明，但β淀粉样肽沉积和tau蛋白磷酸化与胆碱能神经退变的恶性循环(vicious cycle)无疑是造成AD的重要病理机制之一。大量研究表明胆碱能神经突触后膜的M1受体的数目在整个病程中变化不大，M1受体选择性激动剂不但可以直接补偿胆碱能的功能，而且可以调节β淀粉样前体蛋白代谢和降低tau蛋白的过度磷酸化，有助于打破这一恶性循环，改善AD的学习记忆功能并延缓病情的发展。因此M1胆碱受体激动剂被认为是最有前途的治疗AD药物之一。目前Xanomeline、Sabcomeline等具有相对选择性M1受体激动剂业已进入新药临床试验阶段。     

Heterocyclic muscarinic agonists. Synthesis and biological activity of some bicyclic sulfonium arecoline bioisosteres

A number of S-methylsulfonium analogues of the conformationally restricted muscarinic agonists of the 3-alk-oxy-4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine (O-alkyl-THPO) type have been synthesized. The effects on muscarinic receptors of these 3-alkoxy-5-methyl-6,7-dihydro-4H-thiopyrano[3,4-d]isoxazol-5 -ium (O-alkyl-S-methyl-DHTO) analogues (7a-d) were assessed in receptor-binding experiments with tritiated oxotremorine M, pirenzepine, and quinuclidinyl benzilate as ligands and were supported by studies on the isolated guinea pig ileum. The degree of muscarinic agonist activity of the compounds (M-agonist index) and their selectivity for M-1 or M-2 muscarinic receptor subtypes (M-2/M-1 index) were estimated on the basis of receptor-binding studies. The in vitro pharmacological profiles of the compounds were compared with those of arecoline and its sulfonium and 3-methoxyisoxazole isosteres, sulfoarecoline and O,5-dimethyl-THPO, respectively. While O-methyl-DHTO (5a) and N-methyl-DHTO (6a) were inactive, all of the sulfonium analogues 7a-d were muscarinic agonists with the exception of O-ethyl-S-methyl-DHTO (7b), which showed a muscarinic antagonist profile.     

Structural requirements of transmembrane domain 3 for activation by the M1 muscarinic receptor agonists AC-42, AC-260584, clozapine, and N-desmethylclozapine: evidence for three distinct modes of receptor activation

Transmembrane domain 3 (TM3) plays a crucial role mediating muscarinic acetylcholine receptor activation by acetylcholine, carbachol, and other muscarinic agonists. We compared the effects of point mutations throughout TM3 on the interactions of carbachol, 4-n-butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl] piperidine hydrogen chloride (AC-42), a potent structural analog of AC-42 called 4-[3-(4-butylpiperidin-1-yl)-propyl]-7-fluoro-4H-benzo[1,4]oxazin-3-one (AC-260584), N-desmethylclozapine, and clozapine with the M(1) muscarinic receptor. The binding and activation profiles of these ligands fell into three distinct patterns; one exemplified by orthosteric compounds like carbachol, another by structural analogs of AC-42, and a third by structural analogs of N-desmethylclozapine. All mutations tested severely reduced carbachol binding and activation of M(1). In contrast, the agonist actions of AC-42 and AC-260584 were greatly potentiated by the W101A mutation, slightly reduced by Y106A, and slightly increased by S109A. Clozapine and N-desmethylclozapine displayed substantially increased maximum responses at the Y106A and W101A mutants, slightly lower activity at S109A, but no substantial changes in potency. At L102A and N110A, agonist responses to AC-42, AC-260584, clozapine, and N-desmethylclozapine were all substantially reduced, but usually less than carbachol. D105A showed no functional responses to all ligands. Displacement and dissociation rate experiments demonstrated clear allosteric properties of AC-42 and AC-260584 but not for N-desmethylclozapine and clozapine, indicating that they may contact different residues than carbachol to activate M(1) but occupy substantially overlapping spaces, in contrast to AC-42 and AC-260584, which occupy separable spaces. These results show that M(1) receptors can be activated in at least three distinct ways and that there is no requirement for potent muscarinic agonists to mimic acetylcholine interactions with TM3.     

Effects of muscarinic toxins MT2 and MT7, from green mamba venom, on m1, m3 and m5 muscarinic receptors expressed in Chinese Hamster Ovary cells.

Several small proteins called muscarinic toxins (MTs) have been isolated from venom of green mamba (Dendroaspis angusticeps). They have previously been shown in radioligand binding studies to have high selectivity and affinity for individual muscarinic receptor subtypes, but less is known of their functional effects. This study has examined the actions of two of these MTs, MT2 and MT7, using changes in cytosolic Ca(2+) ([Ca(2+)](i)) measured using the fluorescent indicator fura-2 in Chinese Hamster Ovary (CHO) cells stably transfected with individual muscarinic receptor subtypes, m1, m3 and m5. MT2 activated the m1 receptor: at concentrations above 100 nM it caused significant and concentration-dependent increases in [Ca(2+)](i). From 25 to 800 nM MT2 also produced increases in [Ca(2+)](i) by activating m3 receptors, although these increases in [Ca(2+)](i) were not strictly concentration-dependent with only intermittent responses being recorded (i.e. it was not always possible to obtain a response to the agonist with each application of the compound). MT2 (800-1600 nM) also caused significant increases in [Ca(2+)](i) in CHO cells expressing the m5 muscarinic receptor subtype. MT7 (1 microM) displayed no agonist activity at any of the muscarinic receptors but was a potent non-competitive antagonist (at 20 nM) at the m1 muscarinic receptor subtype. It had no antagonist activity at the m3 or m5 subtypes. These results indicate that MT7 is a highly specific antagonist at the m1 muscarinic receptor subtype as suggested by results from radioligand binding studies. However, MT2 is less selective for the m1 muscarinic receptor than previously described as it also exhibits agonist activity at the m3 and m5 muscarinic receptors, which was not detected in radioligand binding studies.     

Human muscarinic receptors expressed in A9L and CHO cells: activation by full and partial agonists.

1. A comparative study of receptor activation by ten full and partial muscarinic agonists was undertaken on the five subtypes of human muscarinic receptors expressed at similar receptor densities in Chinese hamster ovary (CHO-K1) cells. In addition, m1, m2 and m3 receptors were expressed in mouse fibroblast A9L cells in order to compare the influences of cell type on agonist activation of these receptors. 2. Receptor-effector coupling efficiencies were greater in CHO than A9L cells and agonists displayed greater potencies and similar or greater intrinsic activities at CHOm1 and CHOm3 than A9Lm1 and A9Lm3 receptors. Although m2 receptor density was 6 fold higher in A9L than CHO cells, carbachol elicited significantly greater inhibition of adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation in CHOm2 cells. These data suggest that not only receptor density but receptor-effector coupling and/or coupling efficiencies play significant roles in agonist-induced responses. 3. In CHO cells, receptor-effector coupling efficiencies were m3 = m1 > m5. Although CHOm5 receptors were the least efficiently coupled, some partial agonists displayed higher intrinsic efficacies at m5 than m3 receptors suggesting that, in CHO cells, m5 and m3 receptors may activate different G proteins and/or effectors to stimulate inositol monophosphate (IP1) formation. 4. McN-A-343 was a functionally selective m4 agonist. It had little or no agonist activity at m3 receptors expressed in either A9L or CHO cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Study of the muscarinic receptor subtypes in N1E 115 mouse neuroblastoma cells.

Muscarinic receptors in N1E 115 mouse neuroblastoma cells were characterized by competition binding experiments using three agonists and five antagonists, including 4-DAMP and AF-DX 116, and by studying the effect of agonist stimulation on the cellular cAMP and cGMP content. The results of the binding studies with the antagonists suggest that only one single homogeneous binding site of the M1 muscarinic receptor subtype is present. For the binding with the agonists, two binding sites were detected, one with high affinity for the ligand (between 53 and 77% of the total binding sites depending on the agonist) and one with low affinity. In contrast to the results obtained with the binding experiments using antagonists, the study of the cellular cyclic nucleotide response upon carbachol stimulation suggested the presence of both the M1 and M2 subtypes as there was an increase in cyclic GMP concentration while at the same time, the prostaglandin-stimulated synthesis of cyclic AMP was inhibited. Considering both binding and functional data we suggest that in N1E 115 cells a majority of M1 and a minority of M2 muscarinic receptors are present; there is no evidence for the presence of M3 muscarinic receptors.     

Functional characterization of rat submaxillary gland muscarinic receptors using microphysiometry

1. Muscarinic cholinoceptors (MChR) in freshly dispersed rat salivary gland (RSG) cells were characterized using microphysiometry to measure changes in acidification rates. Several non-selective and selective muscarinic antagonists were used to elucidate the nature of the subtypes mediating the response to carbachol. 2. The effects of carbachol (pEC(50) = 5.74 +/- 0.02 s.e.mean; n = 53) were highly reproducible and most antagonists acted in a surmountable, reversible fashion. The following antagonist rank order, with apparent affinity constants in parentheses, was noted: 4-DAMP (8.9)= atropine (8.9) > tolterodine (8.5) > oxybutynin (7.9) > S-secoverine (7.2) > pirenzepine (6.9) > himbacine (6.8) > AQ-RA 741 (6.6) > methoctramine (5.9). 3. These studies validate the use of primary isolated RSG cells in microphysiometry for pharmacological analysis. These data are consistent with, and extend, previous studies using alternative functional methods, which reported a lack of differential receptor pharmacology between bladder and salivary gland tissue. 4. The antagonist affinity profile significantly correlated with the profile at human recombinant muscarinic M(3) and M(5) receptors. Given a lack of antagonists that discriminate between M(3) and M(5), definitive conclusion of which subtype(s) is present within RSG cells cannot be determined.