Overcoming Obstacles to Targeting Muscarinic Receptor Signaling in Colorectal Cancer

Despite great advances in our understanding of the pathobiology of colorectal cancer and the genetic and environmental factors that mitigate its onset and progression, a paucity of effective treatments persists. The five-year survival for advanced, stage IV disease remains substantially less than 20%. This review examines a relatively untapped reservoir of potential therapies to target muscarinic receptor expression, activation, and signaling in colorectal cancer. Most colorectal cancers overexpress M₃ muscarinic receptors (M₃R), and both in vitro and in vivo studies have shown that activating these receptors stimulates cellular programs that result in colon cancer growth, survival, and spread. In vivo studies using mouse models of intestinal neoplasia have shown that using either genetic or pharmacological approaches to block M₃R expression and activation, respectively, attenuates the development and progression of colon cancer. Moreover, both in vitro and in vivo studies have shown that blocking the activity of matrix metalloproteinases (MMPs) that are induced selectively by M₃R activation, i.e., MMP1 and MMP7, also impedes colon cancer growth and progression. Nonetheless, the widespread expression of muscarinic receptors and MMPs and their importance for many cellular functions raises important concerns about off-target effects and the safety of employing similar strategies in humans. As we highlight in this review, highly selective approaches can overcome these obstacles and permit clinicians to exploit the reliance of colon cancer cells on muscarinic receptors and their downstream signal transduction pathways for therapeutic purposes.

[Subtypes of muscarinic acetylcholine receptors involved in the persistent activity of layer V pyramidal neurons in the primary auditory cortex of young mice]

Cholinergic receptor activation and intracellular current injection lead to the persistent activity (PA), which may be involved in inducing neural plasticity. Our previous study showed that PA is closely related to the activation of muscarinic acetylcholine receptors (mAChRs) in pyramidal neurons of mouse primary auditory cortex (AI). However, the subtypes of mAChRs involved in PA remain unclear. Thus, using whole-cell patch-clamp recording and pharmacological methods, we investigated the role of different mAChR subtypes in inducing PA in AI layer V pyramidal neurons of young mice. The results showed that activation of mAChRs with intracellular depolarizing current induced PA in layer V pyramidal neurons. Blockade of M1, M2 or M3 subtypes prevented the PA, whereas M4 receptor antagonists did not affect the production of PA. The results suggest that the PA may be induced through a mechanism involving M1, M2 and M3 muscarinic receptors, but not M4 subtype.

Activation of muscarinic cholinergic receptors induces MCF-7 cells proliferation and angiogenesis by stimulating nitric oxide synthase activity

Muscarinic acetylcholine receptors (mAChR) are members of the G-protein coupled receptor family. These receptors play key physiological roles and changes in their expression and/or function are involved in several diseases. We had previously demonstrated that mAChR expression is up regulated in three different cell lines derived from distinct murine mammary adenocarcinomas that spontaneously arose in BALB/c female mice, in comparison with normal murine mammary cells. Stimulation of mAChR with the muscarinic agonist carbachol (CARB) potentiated different steps of tumor progression. We here evidence that similarly to previous results obtained in mice, human breast tumor homogenates over expressed mAChR in comparison with normal breast tissue. Thus, to test the muscarinic actions on human breast adenocarcinoma cells we investigate the effect of CARB on MCF-7 cells proliferation and neovascular response. Particularly we observe that: CARB stimulates tumor cells proliferation, being 10(-9) M the maximal effective dose for the muscarinic agonist. This action was due to M3 and M1 receptors activation being nitric oxide synthase (NOS) its effector enzyme via phospholipase C and protein kinase C signaling pathway. NOS1 and NOS3 isoforms are expressed in MCF-7 cells and its activation by CARB triggers nitric oxide synthesis and vascular endothelial growth factor expression increasing blood vessels formation induced by mammary tumor cells in vivo. We can conclude that nonneuronal cholinergic system activation stimulates MCF-7 tumor cells growth and neovascular response promoting tumor progression.

Effects of ovarian hormone treatment on the gene expression of muscarinic acetylcholine receptors in the ovariectomized rat myometrium

We investigate the effects of ovarian hormone on the gene expression of muscarinic acetylcholine receptors (M1-M5) in the myometrium using real-time PCR and evaluate the relationships between their expression and that of ovarian hormone receptors (ERα, ERβ, and PgR). Wistar rats were sham operated (SO) or ovariectomized (OVX) and treated with vehicle, estradiol (E2), progesterone (P4), or both E2 and P4 for 2 days beginning on postoperative day 33. M1 and M4 mRNA expressions were not detected in the myometrium. M2 mRNA expression did not change significantly in the OVX and OVX+P4 groups compared to the SO group, but increased significantly in the OVX+E2 group and was normalized in the OVX+E2P4 group. M3 mRNA expression increased significantly in the OVX and OVX+P4 groups compared to the SO group, but was normalized in the OVX+E2 and OVX+E2P4 groups. M5 mRNA expression did not change significantly in all experimental groups. ERα mRNA expression increased significantly in the OVX, OVX+E2, and OVX+P4 groups compared to the SO group, but was normalized in the OVX+E2P4 group. The changes in ERβ mRNA expression were similar to those of M3 mRNA expression in all experimental groups. In contrast, the changes in PgR mRNA expression did not correspond with that of M2, M3, or M5 mRNA expression in any of the experimental groups. Additionally, we evaluated the relationship between the expression of muscarinic acetylcholine receptors and ovarian hormone receptors in estrus cycle. M2 mRNA expression increased significantly in diestus and metaestrus compared in proestrus and estrus. M3 mRNA expression increased significantly in only diestrus compared in the other stages. In contrast, M5 mRNA expression did not change in estrus cycle. The changes in ERα mRNA expression appeared to be similar to those of M2 in estrus cycle, but no significant difference was found. The changes in ERβ mRNA expression were similar to those of M3 mRNA expression. The change in PgR mRNA expression increased significantly in diestrus compared in metaestrus, but did not correspond with that of M2, M3, or M5 mRNA expression in estrus cycle. When acetylcholine sensitivity in the myometrium was compared between diestrus and estrus, the sensitivity is significantly lower in estrus than in diestrus. These results suggest that ovarian hormones influence the expression of M2 and M3 in the myometrium by regulating the expression of hormone receptors. E2 may upregulate M2 via ERα, but P4 may downregulate M2 by inhibiting ERα via PgR. E2 may downregulate M3 by inhibiting ERβ, but P4 may not regulate the expression of M3 and ERβ. M5 may be a constitutive muscarinic receptor in the myometrium because neither E2 nor P4 influence the expression of M5. The combination of E2 and P4 may contribute the reproduction by quieting down the acetylcholine-induced myometrial contraction.

Quantitative Analysis of the Levels of Expression of Muscarinic Receptor Subtype RNA in the Detrusor Muscle of Patients with Overactive Bladder

INTRODUCTION

Antimuscarinic drugs have frequently been used for the treatment for patients with an overactive bladder (OAB) and there have been many studies on the distribution of muscarinic receptor subtypes in the bladder. However, the distribution of muscarinic receptor subtypes in OAB patients has not been well investigated. In this study we investigated the distribution of muscarinic receptor subtypes with mRNA and protein expressions in patients with and without OAB, and investigated both the dome and trigone area.

METHODS

Samples of bladder smooth muscle were obtained from 10 individuals, five patients with OAB and a non-OAB group consisting of five patients who received radical cystectomy.

RESULTS

The M2 receptor was predominant, but there was no significant difference in the level of M2 expression between the groups in the dome area. M5 expression in the dome area was significantly higher in the OAB group than in the non-OAB group. In the trigone area, the level of M2 mRNA expression was the highest in the non-OAB group, and was significantly lower in the OAB group. The levels of M1 and M5 mRNA expression were also observed in samples obtained from the trigone area.

CONCLUSION

The multiformity of the muscarinic receptor subtypes in human bladder smooth muscle was confirmed, and our results suggest that the efficacy of a given pharmacologic therapy differs from patient to patient.

Differential Alterations of Receptor Densities of Three Muscarinic Acetylcholine Receptor Subtypes and Current Densities of the Corre-sponding K+ Channels in Canine Atria with Atrial Fibrillation Induced by Experimental Congestive Heart Failure

Parasympathetic tone and congestive heart failure (CHF) are two of promoting factors in initiation and perpetuation of atrial fibrillation (AF). Recent studies indicate co-existence of multiple muscarinic acetylcholine receptor subtypes (mAChRs) that mediate several distinct K+ currents in the heart; inward rectifier K+ current IKACh by the M2, and two delayed rectifier K+ currents IKM3 and IK4AP by the M3 and M4 receptors, respectively. We studied the alterations of atrial mAChRs and their coupled K+ channels in the setting of AF in dogs with ventricular tachypacing-induced CHF. Whole-patch-clamp recordings showed that the current densities of IKACh (induced by 1 mM acetylcholine) and IK4AP (induced by 1 mM 4-aminopyridine) were ñ45% and ñ55% lower, respectively, while that of IKM3 (induced by 10 mM choline) was ñ75% higher, at a plateau voltage of 0 mV in atrial myocytes from CHF than those from healthy hearts. In healthy hearts, IKACh comprised >60%, and IKM3 and IK4AP <30%, of the total outward K+ currents mediated by mAChRs at depolarized potentials (between -20 mV and +50 mV). In AF atria of CHF dogs, however, the contribution of IKM3 increased to approximately 50%, exceeding those of IKACh or IK4AP. Western blot analyses with atrial membrane protein samples indicated that receptor densities of the M2 and M4 subtypes decreased by approximately 33% and approximately 22%, respectively, whereas that of the M3 subtype increased by approximately 2.3 folds, in parallel to the alterations of the corresponding K+ currents. We conclude that differential alterations of mAChR subtypes underlie differential alterations of their coupled K+ channels in AF atria and these differential alterations may contribute to atrial remodeling in AF induced in the setting of CHF.

Contractile effect of acetylcholine on isolated isthmic segment of fallopian tubes

The Fallopian tubes are sparsely innervated with cholinergic nerve fibers. Acetylcholine is released from these nerves and contracts the smooth muscles of the tubes. The aim of our study was to investigate the effect of acetylcholine on the isthmic segment of the Fallopian tubes using selective antagonists in different hormonal settings. We investigated the effects of acetylcholine on the isolated isthmus of Fallopian tubes taken from 83 patients during abdominal hysterectomy with adnexectomy. Twenty-eight patients were in the follicular phase, 36 were in the luteal phase of the menstrual cycle, and 19 patients were in menopause. Selective and non-selective muscarinic and nicotinic receptor antagonists were used. Acetylcholine (1.8-658.6 micro M) produced concentration-dependent tonic contraction of isthmus taken from the patients in the follicular phase, the luteal phase and menopause. The nicotinic receptor antagonist mecamylamine (6.5 micro M) and local anesthetic lidocaine (230.8 micro M) did not alter the effect of acetylcholine. While M(1) and M(2)-selective muscarinic receptor antagonists pirenzepine (1.6 micro M) and methoctramine (0.9 micro M) did not show specific effect, atropine (0.01 micro M) and the selective M(3)-receptor antagonist p-fluoro-hexahydro-sila-difenidol (pFHHSiD; 0.2 micro M) effectively blocked contractions caused by acetylcholine (maximal pA(2) values 9.74 and 7.54, respectively). The affinity of pFHHSiD for muscarinic receptors was highest in the follicular phase. The results of our study suggest the existence of functional M(3) muscarinic receptors in the isthmus of the Fallopian tubes, located on the smooth muscle cells.

Gain of new exons and promoters by lineage-specific transposable elements-integration and conservation event on CHRM3 gene

The CHRM3 gene is a member of the muscarinic acetylcholine receptor family that plays important roles in the regulation of fundamental physiological functions. The evolutionary mechanism of exon-acquisition and alternative splicing of the CHRM3 gene in relation to transposable elements (TEs) were analyzed using experimental approaches and in silico analysis. Five different transcript variants (T1, T2, T3, T3-1, and T4) derived from three distinct promoter regions (T1: L1HS, T2, T4: original, T3, T3-1: THE1C) were identified. A placenta (T1) and testis (T3 and T3-1)-dominated expression pattern appeared to be controlled by different TEs (L1HS and THE1C) that were integrated into the common ancestor genome during primate evolution. Remarkably, the T1 transcript was formed by the integration event of the human specific L1HS element. Among the 12 different brain regions, the brain stem, olfactory region, and cerebellum showed decreased expression patterns. Evolutionary analysis of splicing sites and alternative splicing suggested that the exon-acquisition event was determined by a selection and conservation mechanism. Furthermore, continuous integration events of transposable elements could produce lineage specific alternative transcripts by providing novel promoters and splicing sites. Taken together, exon-acquisition and alternative splicing events of CHRM3 genes were shown to have occurred through the continuous integration of transposable elements following conservation.

Novel oxotremorine-related heterocyclic derivatives: Synthesis and in vitro pharmacology at the muscarinic receptor subtypes

A set of novel heterocyclic ligands (6-27) structurally related to Oxotremorine 2 was designed, synthesized and tested at muscarinic receptor subtypes (mAChRs). In the binding experiments at cloned human receptors (hm1-5), compounds 7 and 15 evidenced a remarkable affinity and selectivity for the hm2 subtype. The in vitro functional assays, performed on a selected group of derivatives at M(1), M(2), and M(3) tissue preparations, singled out the 3-butynyloxy-5-methylisoxazole trimethylammonium salt 7 as a potent unselective muscarinic agonist [pEC(50): 7.40 (M(1)), 8.18 (M(2)), and 8.14 (M(3))], whereas its 5-phenyl analogue 12 behaved as a muscarinic antagonist, slightly selective for the M(1) subtype [pK(B): 6.88 (M(1)), 5.95 (M(2)), 5.53 (M(3))]. Moreover, the functional data put in evidence that the presence of the piperidine ring may generate a functional selectivity, e.g., an M(1) antagonist/M(2) partial agonist/M(3) full agonist profile (compound 21), at variance with the corresponding quaternary ammonium salt (compound 22) which behaved as a muscarinic agonist at all M(1-3) receptors, with an appreciable selectivity for the cardiac M(2) receptors.

Phenotypic classification of mutants: a tool for understanding ligand binding and activation of muscarinic acetylcholine receptors

GPCRs (G-protein-coupled receptors) such as the M(1) muscarinic receptor have so far proved recalcitrant to direct structure determination. Nevertheless systematic mutagenesis, particularly alanine scanning, has advanced our understanding of their structure-function relationships. GPCRs exhibit multiple conformational states with different affinities for and abilities to activate their cognate G-proteins. Ligand binding alters these conformational equilibria, thus promoting or inhibiting signalling. Alanine-scanning mutagenesis probes the relative contributions of a particular amino acid side chain to the stability of the ground and activated states of the receptor and its complexes. These determine the phenotype of the mutant receptor. Classification of the phenotypes suggests functional roles for particular amino acid side chains, allowing us to group them accordingly. From a rhodopsin-based homology model of the M(1) mAChR, a coherent view emerges of how these clusters of residues function in ligand anchoring, transduction of binding energy, global structural stabilization and selective stabilization of the ground state or the activated state of the receptor. We can identify differences in ligand-binding modes, and suggest inter- and intra-molecular interactions that are weakened or broken, or formed or intensified during acetylcholine-induced activation. In due course, we may be able to extend these insights to activation by unconventional agonists.

Expression and function of muscarinic receptor subtypes on human cornea and conjunctiva

PURPOSE

To investigate the cellular distribution of the muscarinic receptor (MR) subtypes m1-m5 on the ocular surface and to determine their function in cell growth.

METHODS

Human limbal and conjunctival epithelial cells and conjunctival fibroblasts were isolated and cultured. RT-PCR, real-time PCR, immunostaining and Western blot analyses for m1-m5 were performed on cultured cells and tissues and a human conjunctival epithelial cell line (IOBA-NHC). Cell proliferation and p42/44 mitogen-activated protein (MAP) kinase (MAPK) activation in response to MR agonists and antagonists were analyzed by bromodeoxyuridine [BrdU] incorporation and Western blot analysis, respectively.

RESULTS

RT-PCR revealed the presence of m1-m5 transcripts in cultured limbal and conjunctival epithelial cells and conjunctival fibroblasts. Relative quantitative real-time PCR showed that the m1 transcript level in conjunctival cells was higher than that in limbal cells; m2, m3, and m4 expression levels were higher in conjunctival fibroblasts than in epithelial cells. Absolute quantitative real-time PCR showed that the m5 mRNA level in the three cell types was higher than those of m1-m4. Immunohistochemistry and Western blot analysis confirmed the presence of m1-m5 proteins in the cultured cells and in tissues. Carbachol increased the incorporation of BrdU into conjunctival epithelial cells in a dose-dependent manner, which was totally inhibited by atropine, but only partially inhibited by pirenzepine, AF-DX116, and 4-DAMP. Carbachol also activated p42/44 MAPK in a time-dependent manner. Preincubation with U0126 abolished carbachol-induced p42/44 MAPK activation and cell proliferation.

CONCLUSIONS

All five MR subtypes were found on corneal and conjunctival cells. The MRs have a role in epithelial cell proliferation through the phosphorylation of p42/44 MAPK in a time-dependent fashion similar to EGF.

Acetylcholine synthesis, muscarinic receptor subtypes, neuropeptides and secretion of ferret salivary glands with special reference to the zygomatic gland

Studies on salivary secretion are usually focused on parotid and submandibular glands. However, the film of mucin, that protects the oral structures and is responsible for the feeling of oral comfort, is produced by the submucosal glands. The submucosal zygomatic and molar glands are particularly large in carnivores such as the ferret. Comparisons between the mucous sublingual, zygomatic and molar glands, serous parotid and sero-mucous submandibular glands showed the acetylcholine synthesis, in terms of concentration, to be three to four times higher in the mucous glands than in the parotid and submandibular glands. Bromoacetylcholine inhibited 95-99% of the synthesis of acetylcholine in the incubates of the five types of glands, showing the acetylcholine synthesis to depend on the activity of choline acetyltransferase. The high acetylcholine synthesis in the zygomatic gland was of nervous origin, since cutting the buccal nerve, aiming at parasympathetic denervation, and allowing time for nerve degeneration, reduced the acetylcholine synthesising capacity of the gland by 95%. A similar reduction (96%) in the parotid gland followed upon the avulsion of the parasympathetic auriculo-temporal nerve. Zygomatic saliva was very viscous. The salivary flow rate in response to electrical stimulation (20 Hz) of the buccal nerve (zygomatic gland), expressed per gland weight, was one-third of that to stimulation of the auriculo-temporal nerve (parotid gland) or the chorda-lingual nerve (submandibular gland). As previously shown for the parotid and submandibular gland, a certain fraction (25%) of the parasympathetic secretory response of the zygomatic gland depended on non-adrenergic, non-cholinergic transmission mechanisms, probably involving substance P and vasoactive intestinal peptide and possibly calcitonin gene-related peptide. Particularly, high concentrations of vasoactive intestinal peptide were found in the sublingual and molar glands, and of substance P in the submandibular, zygomatic and molar glands; notably, the concentration of calcitonin gene-related peptide of the sublingual gland was not detectable. All five muscarinic receptor subtypes were detected in the five glands. The receptor protein profile, as judged by immunoblotting and semi-quantitative estimations, was about the same in the glands: high level of M3, low level of M2 and levels roughly in the same range of M1, M4 and M5. Compared to the parotid and submandibular glands, the M5 receptor level was particularly low in the mucin-secreting glands. The present study points out both similarities and dissimilarities between the five types of glands investigated. The zygomatic gland, in particular, appears to be a suitable model for future studies aiming at causing relief of dry mouth by local pharmacological treatment.

Muscarinic acetylcholine receptor subtype expression in avian vestibular hair cells, nerve terminals and ganglion cells

Muscarinic acetylcholine receptors (mAChRs) are widely expressed in the CNS and peripheral nervous system and play an important role in modulating the cell activity and function. We have shown that the cholinergic agonist carbachol reduces the pigeon's inwardly rectifying potassium channel (pKir2.1) ionic currents in native vestibular hair cells. We have cloned and sequenced pigeon mAChR subtypes M2-M5 and we have studied the expression of all five mAChR subtypes (M1-M5) in the pigeon vestibular end organs (semicircular canal ampullary cristae and utricular maculae), vestibular nerve fibers and the vestibular (Scarpa's) ganglion using tissue immunohistochemistry (IH), dissociated single cell immunocytochemistry (IC) and Western blotting (WB). We found that vestibular hair cells, nerve fibers and ganglion cells each expressed all five (M1-M5) mAChR subtypes. Two of the three odd-numbered mAChRs (M1, M5) were present on the hair cell cilia, supporting cells and nerve terminals. And all three odd numbered mAChRs (M1, M3 and M5) were expressed on cuticular plates, myelin sheaths and Schwann cells. Even-numbered mAChRs were seen on the nerve terminals. M2 was also shown on the cuticular plates and supporting cells. Vestibular efferent fibers and terminals were not identified in our studies. Results from WB of the dissociated vestibular epithelia, nerve fibers and vestibular ganglia were consistent with the results from IH and IC. Our findings suggest that there is considerable co-expression of the subtypes on the neural elements of the labyrinth. Further electrophysiological and pharmacological studies should delineate the mechanisms of action of muscarinic acetylcholine receptors on structures in the labyrinth.

Structure–activity relationships of methoctramine-related polyamines as muscarinic antagonist: Effect of replacing the inner polymethylene chain with cyclic moieties

The aim of the present paper was to investigate the role of the octamethylene spacer of methoctramine (1) on the biological profile. Thus, this spacer was incorporated into a dianiline or dipiperidine moiety to determine whether flexibility and the basicity of the inner nitrogen atoms are important determinants of potency with respect to muscarinic receptors. The most potent compound was 4, which displayed, in the functional assays, a comparable potency at muscarinic M(2) receptors with respect to 1, and, in the binding assays, a loss of potency and selectivity toward muscarinic M(1) and M(3) receptor subtypes. Both compounds were endowed with antinociceptive activity. Furthermore, in microdialysis tests in rat parietal cortex, they enhanced acetylcholine release, most likely by antagonizing presynaptic muscarinic receptor subtypes.

Muscarinic receptor subtypes mediating positive and negative inotropy in the developing chick ventricle

The inotropic response to muscarinic receptor stimulation of isolated chick ventricular myocardium was examined at various developmental stages, and the receptor subtype involved was pharmacologically characterized. In embryonic chick ventricles, carbachol (CCh) produced positive inotropy at micromolar concentrations. In hatched chick ventricles, CCh produced negative inotropy at nanomolar concentrations. Neither positive nor negative inotropy was observed in the 19 - 21-day-old embryos. Both positive and negative inotropy were also observed with acetylcholine and oxotremoline-M. The CCh-induced positive inotropy in 7 - 9-day-old embryonic ventricles and the negative inotropy in 1 - 3-day-old hatched chick ventricles were antagonized by muscarinic receptor antagonists; pA(2) values for the positive and negative responses of pirenzepine were 7.5 and 7.2, those of AF-DX116 (11-[(2-[(diethylamino)methyl]-1-piperidinyl)acetyl]-5,11-dihydro-6H-pyrido[2,3-b][1,4] benzodiazepine-6-one) were 6.8 and 6.9, those of 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) were 9.0 and 8.5, and those of himbacine were 7.0 and 8.0, respectively. CCh had no effect on action potential configuration. In conclusion, the positive inotropy is most likely mediated by muscarinic M(1) receptors and the negative inotropy is mostly likely mediated by muscarinic M(4) receptors.

Relationship between muscarinic acetylcholine receptor subtypes

Muscarinic acetylcholine receptor belongs to the G-coupled receptor family, the cooperation between its five subtypes is crucially important in maintaining the normal physiological function. At present, many types of biological resources are increasing, which provides unprecedented opportunities to study the relationships between muscarinic acetylcholine receptor subtypes. In this study, we take advantage of different types of data, using bioinformatics tools and strategies, to analyze the relationship between muscarinic acetylcholine receptor subtypes from four aspects, including evolution, sequence similarity, expression correlation and protein-protein interaction network. From evolution and sequence similarity aspects, we found the five subtypes of muscarinic acetylcholine receptor can be classified into two subclasses. The first subclass includes M1, M3, and M5, and the second subclass includes M2 and M4. The evolutionary distance between two subtypes of the same subclass is relatively near, and the sequence similarity between them is relatively high. From expression correlation aspect, we found that the subtypes of the first subclass have a positive correlation with the subtypes of the second subclass, that is, there are potential co-expression relationships between them. From protein-protein interaction aspect, we found that the subtypes of the first subclass have indirect interactions with the subtypes of the second subclass, which indicates cooperation relationships.

Mutational disruption of a conserved disulfide bond in muscarinic acetylcholine receptors attenuates positive homotropic cooperativity between multiple allosteric sites and has subtype-dependent effects on the affinities of muscarinic allosteric ligands

The 2nd outer loop (o2) of muscarinic acetylcholine receptors (mAChRs) contains a highly conserved cysteine residue that is believed to participate in a disulfide bond and is flanked on either side by epitopes that are critical to the binding of many muscarinic allosteric modulators. We determined the allosteric binding parameters of the modulators gallamine, W84, and tetrahydroaminoacridine (THA) at M2 and M3 mAChRs in which these cysteine residues had been mutated to alanines. THA is known to bind to mAChRs with a strong positive homotropic cooperativity (a Hill slope of approximately 2) that implies that it must interact with multiple allosteric sites. The disulfide cysteine mutations in M2 receptors reduced the allosteric potencies of the tested modulators as if the critical adjacent residue (Tyr177) itself had been mutated. However, in M3 receptors, the disulfide cysteine mutations had no effect on the potencies of gallamine or W84 and even increased the potency of THA. It was most interesting that the strong, positive, homotropic interactions of THA at both M2 and M3 receptors were markedly reduced by the cysteine mutations. In addition, gallamine also displayed positive homotropic cooperativity in its interactions with M3 receptors (but not M2 receptors), and this cooperativity was not evident in the cysteine mutants. Thus, it seems that these cysteine residues play a role in linking cooperating allosteric sites, although it is not currently possible to say whether these multiple sites lie within one receptor or on two linked receptors of a dimer or higher order oligomer.

Up-Regulated PAR-2-Mediated Salivary Secretion in Mice Deficient in Muscarinic Acetylcholine Receptor Subtypes

Protease-activated receptor-2 (PAR-2) is expressed in the salivary glands and is expected to be a new target for the treatment of exocrine dysfunctions, such as dry mouth; however, the salivary secretory mechanism mediated by PAR-2 remains to be elucidated. Therefore, mechanism of the PAR-2-mediated salivary secretion was investigated in this study. We found that a PAR-2 agonist peptide, SLIGRL-OH, induced salivary flow in vivo and dose-dependent increase in [Ca(2+)](i) submandibular gland (SMG) acinar cells in wild-type (WT) mice and mice lacking M(3) or both M(1) and M(3) muscarinic acetylcholine receptors (mAChRs), whereas secretions in PAR-2 knockout (PAR-2KO) mice were completely abolished. The saliva composition secreted by SLIGRL-OH was similar to that secreted by mAChR stimulation. Ca(2+) imaging in WT acinar cells and beta-galactosidase staining in PAR-2KO mice, in which the beta-galactosidase gene (LacZ) was incorporated into the disrupted gene, revealed a nonubiquitous, sporadic distribution of PAR-2 in the SMG. Furthermore, compared with the secretion in WT mice, PAR-2-mediated salivary secretion and Ca(2+) response were enhanced in mice lacking M(3) or both M(1) and M(3) mAChRs, in which mAChR-stimulated secretion and Ca(2+) response in acinar cells were severely impaired. Although the mechanism underlying the enhanced PAR-2-mediated salivary secretion in M(3)-deficient mice is not clear, the result suggests the presence of some compensatory mechanism involving PAR-2 in the salivary glands deficient in cholinergic activation. These results indicate that PAR-2 present in the salivary glands mediates Ca(2+)-dependent fluid secretion, demonstrating potential usefulness of PAR-2 as a target for dry mouth treatment.

Rat Schwann cells express M1-M4 muscarinic receptor subtypes

The expression of different muscarinic receptor subtypes was analyzed in immature Schwann cells obtained from sciatic nerve of 2-day neonatal rats. By using RT-PCR analysis, we demonstrated the presence of M1, M2, M3, and M4 receptor subtypes in cultured Schwann cells, with M2 displaying the highest expression levels. Muscarinic subtypes were also quantified by immunoprecipitation and [3H]QNB binding. With this approach, we found the levels of receptor expression to be M2 > M3 > M1. M4 is expressed at very low levels, and M5 receptor was not detectable. Moreover, we also demonstrated that stimulation of the receptors by muscarinic agonists activates previously described signal transduction pathways, leading to a decrease of cAMP and an increase of IP3 levels not associated with an efficient intracellular Ca2+ release. The presence and activity of particular muscarinic receptors in immature Schwann cells suggest that ACh may play an important role in Schwann cell development.

On the physiological relevance of muscarinic acetylcholine receptors in Alzheimer's disease

The loss of cholinergic neurons, particularly in the forebrain, plays an important role in the pathophysiology of Alzheimer's disease (DAT). This concept has lead to the effective treatment of DAT by means of acetylcholine (Ach) esterase inhibitors. G-protein-coupled muscarinic acetylcholine receptors (mAchR) are classified in 5 subtypes, the M1 receptor stimulation and M2 inhibition being especially associated with cognitive skills. Modified cerebral muscarinic receptor profiles in patients with Alzheimer's disease in addition to loss of Ach releasing neurons help us to understand the pathophysiology of dementia and offer potential therapeutic approaches. Specific agonists and antagonists of muscarinic receptors are discussed as possible treatment options in DAT. Experimental results postulate a positive long lasting modulation of the pathological neuronal protein pattern in addition to their cholinomimetic effect.

Anticholinergic agents in asthma: chronic bronchodilator therapy, relief of acute severe asthma, reduction of chronic viral inflammation and prevention of airway remodeling

PURPOSE OF REVIEW

It is difficult to identify specific groups of asthmatic patients who may benefit from acute or chronic use of anticholinergic agents. Therefore, an important consideration is how anticholinergic agents can be used to achieve clinically effective treatment of asthma.

RECENT FINDINGS

A genotype-stratified study revealed that greater bronchoprotective effect of anticholinergic agents was observed in asthmatic patients with the Arg/Arg genotype of the beta2-adrenergic receptor. Anticholinergic agents could add to the bronchodilation obtained with beta2-agonists on acute severe asthma. CD8+ T lymphocytes induced by chronic hepatitis C viral infection causes chronic obstructive pulmonary disease-like inflammation in asthma. Virus-specific CD8+ T lymphocytes may induce cholinergic activation in asthma through M2 receptor dysfunction. Therefore, anticholinergic agents are highly effective for asthma associated with chronic viral infection. In contrast, asthma with chronic obstructive pulmonary disease-like inflammation appears to be poorly responsive to beta2-agonists and can lead to partially irreversible airflow limitation. Moreover, a recent report suggested that treatment with inhaled tiotropium bromide markedly inhibited the increase in airway smooth muscle mass, myosin expression, and contractility in asthma.

SUMMARY

Anticholinergic agents may benefit stable asthmatics, particularly those who have the Arg/Arg genotype. These agents have a demonstrated role in combination with beta2-agonists in the treatment of acute severe asthma, and may benefit asthmatics with chronic obstructive pulmonary disease-like inflammation. Moreover, these agents could be also beneficial in preventing airway remodeling in asthmatic airways.

Pharmacological characterization of muscarinic receptor subtypes mediating vasoconstriction of human umbilical vein

The present study attempted to pharmacologically characterize the muscarinic receptor subtypes mediating contraction of human umbilical vein (HUV).HUV rings were mounted in organ baths and concentration-response curves were constructed for acetylcholine (ACh) (pEC50: 6.16+/-0.04; maximum response 80.00+/-1.98% of the responses induced by serotonin 10 microM). The absence of endothelium did not modify the contractile responses of ACh in this tissue. The role of cholinesterases was evaluated: neither neostigmine (acetylcholinesterase inhibitor) nor iso-OMPA (butyrylcholinesterase inhibitor) modified ACh responses. When both enzymes were simultaneously inhibited, a significantly but little potentiation was observed (control: pEC50 6.33+/-0.03; double inhibition: pEC50 6.57+/-0.05). Atropine, nonselective muscarinic receptors antagonist, inhibited ACh-induced contraction (pKB 9.67). The muscarinic receptors antagonists pirenzepine (M1), methoctramine (M2) and pFHHSiD (M3) also antagonized responses to ACh. The affinity values estimated for these antagonists against responses evoked by ACh were 7.58, 6.78 and 7.94, respectively. On the other hand, PD 102807 (M4 selective muscarinic receptors antagonist) was ineffective against ACh-induced contraction.In presence of a blocking concentration of pirenzepine, pFHHSiFD produced an additional antagonism activity on ACh-induced responses. The M1 muscarinic receptors agonist McN-A-343 produced similar maximum but less potent responses than ACh in HUV. The calculated pA2 for pirenzepine against McN-A-343 induced responses was 8.54. In conclusion, the data obtained in this study demonstrate the role of M1 muscarinic receptor subtypes and suggest the involvement of M3 muscarinic receptor subtypes in ACh-induced vasoconstriction in HUV rings. In addition, the vasomotor activity evoked by ACh does not seem to be modulated by endothelial factors, and their enzymatic degradation appears to have little functional relevance in this tissue.

Muscarinic Receptors: A Comparative Analysis of Structural Features and Binding Modes through Homology Modelling and Molecular Docking

Three-dimensional models of the five human muscarinic receptors were obtained from their known sequences. Homology modelling based on the crystallographic structure of bovine rhodopsin yielded models compatible with known results from site-directed mutagenesis studies. The only exceptions were the cytoplasmic loop 3 (CL3) in the five receptors, and the large C-terminal domain in M(1). Here, homology modelling with other closely related proteins allowed to solve these gaps. A detailed comparative discussion of the five models is given. The second part of the work involved docking experiments with the physiological ligand acetylcholine, again yielding results entirely compatible with results from mutagenesis experiments. The study revealed analogies and differences between the five receptors in the residues, and interactions leading to the recognition and binding of acetylcholine.

Differential expression of muscarinic acetylcholine receptors across excitatory and inhibitory cells in visual cortical areas V1 and V2 of the macaque monkey

Cholinergic neuromodulation, a candidate mechanism for aspects of attention, is complex and is not well understood. Because structure constrains function, quantitative anatomy is an invaluable tool for reducing such a challenging problem. Our goal was to determine the extent to which m1 and m2 muscarinic acetylcholine receptors (mAChRs) are expressed by inhibitory vs. excitatory neurons in the early visual cortex. To this end, V1 and V2 of macaque monkeys were immunofluorescently labelled for gamma-aminobutyric acid (GABA) and either m1 or m2 mAChRs. Among the GABA-immunoreactive (ir) neurons, 61% in V1 and 63% in V2 were m1 AChR-ir, whereas 28% in V1 and 43% in V2 were m2 AChR-ir. In V1, both mAChRs were expressed by fewer than 10% of excitatory neurons. However, in V2, the population of mAChR-ir excitatory neurons was at least double that observed in V1. We also examined m1 and m2 AChR immunoreactivity in layers 2 and 3 of area V1 under the electron microscope and found evidence that GABAergic neurons localize mAChRs to the soma, whereas glutamatergic neurons expressed mAChRs more strongly in dendrites. Axon and terminal labelling was generally weak. These data represent the first quantitative anatomical study of m1 and m2 AChR expression in the cortex of any species. In addition, the increased expression in excitatory neurons across the V1/V2 border may provide a neural basis for the observation that attentional effects gain strength up through the visual pathway from area V1 through V2 to V4 and beyond.

Molecular and pharmacological characterization of muscarinic receptors in retinal pigment epithelium: role in light-adaptive pigment movements

Muscarinic receptors are the predominant cholinergic receptors in the central and peripheral nervous systems. Recently, activation of muscarinic receptors was found to elicit pigment granule dispersion in retinal pigment epithelium isolated from bluegill fish. Pigment granule movement in retinal pigment epithelium is a light-adaptive mechanism in fish. In the present study, we used pharmacological and molecular approaches to identify the muscarinic receptor subtype and the intracellular signaling pathway involved in the pigment granule dispersion in retinal pigment epithelium. Of the muscarinic receptor subtype-specific antagonists used, only antagonists specific for M1 and M3 muscarinic receptors were found to block carbamyl choline (carbachol)-induced pigment granule dispersion. A phospholipase C inhibitor also blocked carbachol-induced pigment granule dispersion, and a similar result was obtained when retinal pigment epithelium was incubated with an inositol trisphosphate receptor inhibitor. We isolated M2 and M5 receptor genes from bluegill and studied their expression. Only M5 was found to be expressed in retinal pigment epithelium. Taken together, pharmacological and molecular evidence suggest that activation of an odd subtype of muscarinic receptor, possibly M5, on fish retinal pigment epithelium induces pigment granule dispersion.

Regulation of CD8+ cytolytic T lymphocyte differentiation by a cholinergic pathway

In this report, we provide evidence that muscarinic receptors play a role in the generation of CD8+ cytolytic T lymphocytes. Analysis of mice with targeted deletions of each of the known muscarinic receptors (M1-M5) showed that CD8+ T cells from M1 receptor-deficient mice had a defect in the ability to differentiate into cytolytic T lymphocytes. Additional pharmacological experiments support the role of muscarinic receptors in wild type mice and suggest that acetylcholine may be involved. Together, these findings suggest that the M1 muscarinic receptor is involved in CTL development, thus providing novel insights into CD8+ T cell biology and the potential role of cholinergic signaling in immune regulation.

Muscarinic receptor subtypes in human bladder detrusor and mucosa, studied by radioligand binding and quantitative competitive RT-PCR: changes in ageing

1. We investigated muscarinic receptors in the detrusor and mucosa of the human bladder body. Radioligand-binding studies with [(3)H]QNB were conducted using specimens collected from patients (36-77 years) with normal bladder function, undergoing surgery. For RT-PCR, biopsies of normal bladder were obtained from patients (30-88 years) undergoing check cystoscopy. 2. Binding of [(3)H]QNB in detrusor (n=20) was of high affinity (K(D) 77.1 (55.2-99.0) pM) and capacity (B(max) 181+/-7 fmol mg protein(-1)). Similar values were obtained in mucosa (n=6) (K(D) 100.5 (41.2-159.9) pM; B(max) 145+/-9 fmol mg protein(-1)). 3. Competition-binding experiments in detrusor membranes with muscarinic receptor antagonists including trospium, darifenacin, 4-DAMP, methoctramine, AQ-RA 741, AF-DX 116 and pirenzepine indicated a receptor population of 71% M(2), 22% M(3) and 7% M(1). In the mucosa, 75% of sites were M(2) receptors, with 25% M(3)/M(5). 4. Using RT-PCR, expression of M(1), M(2), M(3) and M(5) mRNA was demonstrated in both detrusor and mucosa. 5. The presence of a high density of mainly M(2) muscarinic receptors in the mucosa appears to be a novel finding and raises the question of their physiological significance and the source of their endogenous ligand. 6. There was a negative correlation of receptor number (B(max)) with age in detrusor muscle from male patients (P=0.02). Quantitative competitive RT-PCR demonstrated a selective age-related decrease in mRNA for muscarinic M(3) but not M(2) receptors, in both male (P<0.0001) and female (P=0.019) detrusor. These findings correspond with reports of decreased detrusor contractility with ageing.

A pentylenetetrazole-induced generalized seizure in early life enhances the efficacy of muscarinic receptor coupling to G-protein in hippocampus and neocortex of adult rat

We have previously shown that exposure to the anti-cholinesterase eserine provokes interictal-like discharges in the CA3 area of hippocampal slices from adult rats in which a generalized seizure has been induced by pentylenetetrazole (PTZ) when immature (at 20 days). Such increased responsiveness to acetylcholine (ACh) was not associated with any change in hippocampal acetylcholine or gamma-aminobutyric acid (GABA) content, GABAergic inhibition or density of ACh innervation, but was blocked by the muscarinic receptor antagonist atropine. We therefore turned to quantitative radioligand binding autoradiography, in situ hybridization and the [35S]GTPgammaS method to assess the properties of hippocampal and neocortical muscarinic receptors in adult rats having experienced a PTZ seizure at P20. The densities of M1 and M2 receptor binding sites, respectively labeled with [3H]pirenzepine and [3H]AFDX-384, as well as the amount of m1, m2 and m3 receptor mRNAs, did not differ from control in the hippocampus and neocortex of these rats. In contrast, in PTZ rats, both brain regions displayed a marked increase in [35S]GTPgammaS incorporation stimulated by ACh, bethanechol and particularly oxotremorine. This finding indicates that a generalized seizure in immature rat can entail a long-term and presumably permanent increase in the efficacy of G-protein coupling to muscarinic receptors in the hippocampus and neocortex of the adult. By analogy, such a mechanism could account for the susceptibility to epilepsy of human adults having suffered from prolonged convulsions in early life.

Selective cholinergic immunolesioning affects synaptic plasticity in developing visual cortex

Cholinergic neurotransmission is known to affect activity-dependent plasticity in various areas, including the visual cortex. However, relatively little is known about the exact role of subcortical cholinergic inputs in the regulation of plastic events in this region during early postnatal development. In the present study, synaptic transmission and plasticity in the developing visual cortex were studied following selective immunotoxic removal of the basal forebrain cholinergic afferents in 4-day-old rat pups. The lesion produced dramatic cholinergic neuronal and terminal fibre loss associated with decreased mRNA levels for the M1 and M2 muscarinic receptors, as well as clear-cut impairments of long-term potentiation (LTP) in visual cortex slices. Indeed, after theta burst stimulation of layer IV a long-term depression (LTD) instead of an LTP was induced in immunolesioned slices. This functional change appears to be due to the lack of cholinergic input as exogenous application of acetylcholine prevented the shift from LTP to LTD. In addition, lesioned rats showed an increased sensitivity to acetylcholine (ACh). While application of 20 microm ACh produced a depression of the field potential in immunolesioned rat slices, in order to observe the same effect in control slices we had to increase ACh concentration to up to 200 microm. Taken together, our results indicate that deprivation of cholinergic input affects synaptic transmission and plasticity in developing visual cortex, suggesting that the cholinergic system could play an active role in the refinement of the cortical circuitry during maturation.

Genetic deletion of CB1 receptors improves non-associative learning

Habituation (a form of non-associative learning) was measured by assessing locomotion in novel activity monitors in CB1 receptor knockout mice and juxtaposed to habituation measured in muscarinic M2, M4, and double M2/M4 receptor knockout mice. M2 and M2/M4, but not M4, receptor knockout mice appeared to have an impaired ability to habituate, whereas CB1 receptor knockout mice showed enhanced habituation compared to wild-type animals. We conclude that CB1 receptor gene invalidation improves habituation tentatively through an increase in cholinergic neurotransmission.

Pilocarpine-induced status epilepticus: Monoamine level, muscarinic and dopaminergic receptors alterations in striatum of young rats

Behavioural changes, muscarinic and dopaminergic receptors density and levels of monoamines were measured in striatum of rats after pilocarpine-induced status epilepticus (SE). Wistar rats at the age of 21 days were treated with pilocarpine (400mg/kg; subcutaneously) whilst the control group was treated with 0.9% saline (s.c.). Both groups were sacrificed 1h following the treatment. SE induced a muscarinic receptor downregulation of 64% in pilocarpine group. This effect was also observed to be 57% in D(1) and 32% in D(2). In the dissociation constant (K(d)) values in muscarinic and D(1) receptor no alterations were verified. On the other hand, the K(d) value for D(2) was observed to increase 41%. High performance liquid chromatography determinations showed 63, 35, 77 and 64% decreases in dopamine, 3-methoxy-phenylacetic acid, serotonin and 5-hydroxyindoleacetic acid contents, respectively. The homovanilic acid level was verified to increase 119%. The noradrenaline content was unaltered. A direct evidence of monoamine levels alterations can be verified during seizure activity and receptor density changes appear to occur in an accentuated way in immature brain during the estabilishment of SE induced by pilocarpine.

Allosterism at Muscarinic Receptors: Ligands and Mechanisms

The evaluation of allosteric ligands at muscarinic receptors is discussed in terms of the ability of the experimental data to be interpreted by the allosteric ternary complex model. The compilation of useful SAR information of allosteric ligands is not simple, especially for muscarinic receptors, where there are multiple allosteric sites and complex interactions.

An interspecies comparison of mercury inhibition on muscarinic acetylcholine receptor binding in the cerebral cortex and cerebellum

Mercury (Hg) is a ubiquitous pollutant that can disrupt neurochemical signaling pathways in mammals. It is well documented that inorganic Hg (HgCl(2)) and methyl Hg (MeHg) can inhibit the binding of radioligands to the muscarinic acetylcholine (mACh) receptor in rat brains. However, little is known concerning this relationship in specific anatomical regions of the brain or in other species, including humans. The purpose of this study was to explore the inhibitory effects of HgCl(2) and MeHg on [(3)H]-quinuclidinyl benzilate ([(3)H]-QNB) binding to the mACh receptor in the cerebellum and cerebral cortex regions from human, rat, mouse, mink, and river otter brain tissues. Saturation binding curves were obtained from each sample to calculate receptor density (B(max)) and ligand affinity (K(d)). Subsequently, samples were exposed to HgCl(2) or MeHg to derive IC50 values and inhibition constants (K(i)). Results demonstrate that HgCl(2) is a more potent inhibitor of mACh receptor binding than MeHg, and the receptors in the cerebellum are more sensitive to Hg-mediated mACh receptor inhibition than those in the cerebral cortex. Species sensitivities, irrespective of Hg type and brain region, can be ranked from most to least sensitive: river otter > rat > mink > mouse > humans. In summary, our data demonstrate that Hg can inhibit the binding [(3)H]-QNB to the mACh receptor in a range of mammalian species. This comparative study provides data on interspecies differences and a framework for interpreting results from human, murine, and wildlife studies.

Cholinergic Control of Firing Pattern and Neurotransmission in Rat Neostriatal Projection Neurons: Role of CaV2.1 and CaV2.2 Ca2+ Channels

Besides a reduction of L-type Ca2+-currents (CaV1), muscarine and the peptidic M1-selective agonist, MT-1, reduced currents through CaV2.1 (P/Q) and CaV2.2 (N) Ca2+ channel types. This modulation was strongly blocked by the peptide MT-7, a specific muscarinic M1-type receptor antagonist but not significantly reduced by the peptide MT-3, a specific muscarinic M4-type receptor antagonist. Accordingly, MT-7, but not MT-3, blocked a muscarinic reduction of the afterhyperpolarizing potential (AHP) and decreased the GABAergic inhibitory postsynaptic currents (IPSCs) produced by axon collaterals that interconnect spiny neurons. Both these functions are known to be dependent on P/Q and N types Ca2+ channels. The action on the AHP had an important effect in increasing firing frequency. The action on the IPSCs was shown to be caused presynaptically as it coursed with an increase in the paired-pulse ratio. These results show: first, that muscarinic M1-type receptor activation is the main cholinergic mechanism that modulates Ca2+ entry through voltage-dependent Ca2+ channels in spiny neurons. Second, this muscarinic modulation produces a postsynaptic facilitation of discharge together with a presynaptic inhibition of the GABAergic control mediated by axon collaterals. Together, both effects would tend to recruit more spiny neurons for the same task.

The highly efficacious actions of N-desmethylclozapine at muscarinic receptors are unique and not a common property of either typical or atypical antipsychotic drugs: is M1 agonism a pre-requisite for mimicking clozapine's actions?

RATIONALE

Recent studies have suggested that the salutary actions of clozapine in schizophrenia may be due to selective activation of M(1) muscarinic receptors by clozapine and/or its major active metabolite N-desmethylclozapine.

OBJECTIVE

We systematically tested this hypothesis by screening a large number of psychoactive compounds, including many atypical antipsychotic drugs, for agonist activity at cloned, human M(1), M(3) and M(5) muscarinic receptors.

RESULTS

Only three of the 14 atypical antipsychotic drugs we tested were found to possess partial agonist actions at M(1) muscarinic receptors (fluperlapine, JL13, clozapine). A few additional miscellaneous compounds had a modest degree of M(1) agonist actions. Only carbachol and N-desmethylclozapine had appreciable M(3) muscarinic agonism at M(3) muscarinic receptors, although several were M(5) partial agonists including MK-212, N-desmethylclozapine and xanomeline.

CONCLUSION

Although M(1) muscarinic receptor-selective partial agonists have shown promise in some preclinical antipsychotic drug models, these studies indicate that it is unlikely that the salutary actions of clozapine and similar atypical antipsychotic drugs are mediated solely by M(1) muscarinic receptor activation. It is possible, however, that the M(1) agonism of N-desmethylclozapine contributes to the uniquely beneficial actions of clozapine. Thus, these results are consistent with the notion that a balanced degree of activity at multiple biogenic amine receptors, including M(1) muscarinic agonism, is responsible for the uniquely beneficial actions of clozapine.

The spinal muscarinic receptor subtypes contribute to the morphine-induced antinociceptive effects in thermal stimulation in mice

The present study was undertaken to clarify how spinal muscarinic receptors can be involved in the antinociceptive effects induced by morphine in thermal stimulation. The morphine-induced antinociceptive effects (26.6 micromol/kg, s.c.) was inhibited by an intrathecal (i.t.) injection of the muscarinic antagonist (M) atropine and the M(1)/M(4) antagonist pirenzepine in a dose-dependent manner. In contrast, the M(2) antagonist methoctramine and the M(3) antagonist 4-DAMP did not inhibit the morphine-induced antinociceptive effects. Injection (i.t.) of the putative M(1) agonist McN-A-343 resulted in dose-dependent antinociceptive effects in thermal stimuli. In addition, antinociceptive effects induced by the i.t. injection of morphine were not inhibited by the M(1)/M(4) antagonist pirenzepine, although pirenzepine did inhibit the intracerebroventricular (i.c.v.) injection of morphine-induced antinociceptive effects. These results suggest that the morphine-induced antinociceptive effects in thermal stimuli are regulated by the M(1) or M(4) receptor in the spinal cord.

Quantitative analysis of binding parameters of [3H]N-methylscopolamine in central nervous system of muscarinic acetylcholine receptor knockout mice

We have studied binding parameters (Kd, Bmax) of [3H]N-methylscopolamine ([3H]NMS) in various brain regions and spinal cord of wild-type (WT) and muscarinic acetylcholine receptor (mAChR) subtype (M1-M5) knockout (KO) mice. In the M1-M4 KO mice, the number of [3H]NMS binding sites (Bmax) was decreased throughout the central nervous system (CNS) with significant regional differences. Our results collectively suggest that M1 receptor was present in a relatively high density in the cerebral cortex and hippocampus, and the densities of M1 and M4 subtypes were highest in the corpus striatum. M2 receptor appeared to be the major subtype in the thalamus, hypothalamus, midbrain, pons-medulla, cerebellum and spinal cord. These findings may contribute significantly not only to the further understanding of the physiological roles of mAChR subtypes in the central cholinergic functions, but also to the development of selective therapeutic agents targeting specific subtype.

Lung function improvements with once-daily tiotropium in chronic obstructive pulmonary disease

The defining feature of chronic obstructive pulmonary disease (COPD) is progressive deterioration in lung function. Measures of lung function are used to confirm the diagnosis, assess the severity of disease, and evaluate the efficacy of interventions. Forced expiratory volume in 1 second (FEV1), determined by spirometry, is the best known of these measures; however, it does not correlate well with dyspnea or exercise capacity, which are important targets for improvement in COPD management. Airflow obstruction in COPD often causes lung hyperinflation, which further inhibits the patient's ability to breathe. The degree of hyperinflation has been shown to correlate well with dyspnea and exercise capacity, but it is less convenient to measure than FEV1. This article briefly reviews the key lung function measurements used in monitoring patients with COPD. To illustrate how these measurements can be used to demonstrate the improvements in lung function elicited by effective bronchodilator therapy, the changes associated with the once-daily, long-acting bronchodilator tiotropium are presented.

Habituation to a test apparatus during associative learning is sufficient to enhance muscarinic acetylcholine receptor-immunoreactivity in rat suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is engaged in modulation of memory retention after (fear) conditioning, but it is unknown which pathways and neurotransmitter system(s) play a role in this action. Here we examine immunocytochemically whether muscarinic acetylcholine receptors (mAChRs), mediating cholinergic signal transduction in the SCN, are involved. For this purpose, mAChR immunoreactivity (mAChR-ir) was studied in the SCN after various stages of passive shock avoidance (PSA) and active shock avoidance (ASA) training and, for ASA, at various posttraining time points. mAChR-ir was significantly enhanced in SCN neurons as a result of the training procedure, and the number of mAChR-positive glial cells in the SCN increased significantly. The increase in mAChR-ir as a result of PSA and ASA training was not due to fear conditioning or the number of correct avoidances (in case of ASA training) but rather to behavioral arousal as a consequence of (brief) exposure to a novel environment (the test apparatus). This finding was confirmed by a cage-change experiment in which the rats were allowed to stay in a novel cage for 15 min or 24 hr. Only the brief exposure to the fresh cage triggered alterations for SCN mAChRs 24 hr later. These results shed new light on a possible function of the cholinergic system in the SCN mediated by mAChRs in relation to modulation of memory processes and demonstrate that behavioral arousal during (the habituation stage of) a learning task is sufficient to alter the mAChR system in the SCN.

Functional analysis of muscarinic acetylcholine receptors using knockout mice

Because of the low selectivity of available ligands, pharmacological approaches to elucidate the functional difference among muscarinic acetylcholine receptor (mAChR) subtypes have been problematic. As an alternative approach, we have established a series of mutant mouse lines deficient in each mAChR subtype (mAChR KO mice). The systematic analyses of these mice have been useful in revealing the functional difference among mAChR subtypes. Here, we review our prior research on these mutant mice and also some notable findings reported by other research groups.

Tetrodotoxin-sensitive Na+ channels and muscarinic and purinergic receptors identified in human erythroid progenitor cells and red blood cell ghosts

This article concerns the identification of different types of voltage-gated Na(+) channels and of muscarinic and purinergic receptors that are expressed in human erythroid precursor cells and red cell ghosts. We analyzed, by RT-PCR, RNA that was extracted from purified and synchronously growing human erythroid progenitor cells, differentiating from erythroblasts to reticulocytes in 7 to 14 days. These extracts were free of white cell and platelet contamination. Two types of voltage-gated, tetrodotoxin-sensitive Na(+) channels were found. These were Na(v)1.4 and Na(v)1.7, the former known to be present in skeletal muscle and the latter in peripheral nerve. By using a pan Na(+) channel antibody and Western blotting, an immunoreactive channel was detected in ghosts of human red blood cells, consistent with the expression of these two channels. The transcripts for four of the five known subtypes of muscarinic receptors were also identified, including subtypes M2, M3, M4, and M5, whereas subtype M1 was not found. Expression was also detected for the purinergic type receptors P2X(1), P2X(4), P2X(7), and P2Y(1) whereas types P2Y(2), P2Y(4), and P2Y(6) were not found. We also searched for but did not find transcripts for hBNP-1, a type 1b human brain sodium phosphate cotransporter, and cystic fibrosis transmembrane conductance regulator (CFTR). Implications regarding the presence of these different types of channels and receptors in human red blood cells and their functional significance are discussed.

Homo- or hetero-dimerization of muscarinic receptor subtypes is not mediated by direct protein-protein interaction through intracellular and extracellular regions

The oligomerization of G-proteincoupled receptors (GPCRs) has been shown to occur by various mechanisms, such as via disulfide covalent linkages, noncovalent (ionic, hydrophobic) interactions of the N-terminal, and/or transmembrane and/or intracellular domains. Interactions between GPCRs could involve an association between identical proteins (homomers) or non-identical proteins (heteromers), or between two monomers (to form dimers) or multiple monomers (to form oligomers). It is believed that muscarinic receptors may also be arranged into dimeric or oigomeric complexes, but no systematic experimental evidence exists concerning the direct physical interaction between receptor proteins as its mechanism. We undertook this study to determine whether muscarinic receptors form homomers or a heteromers by direct protein-protein interaction within the same or within different subtypes using a yeast two-hybrid system. Intracellular loops (i1, i2 and i3) and the C-terminal cytoplasmic tails (C) of human muscarinic (Hm) receptor subtypes, Hm1, Hm2 and Hm3, were cloned into the vectors (pB42AD and pLexA) of a two-hybrid system and examined for heteromeric or homodimeric interactions between the cytoplasmic domains. No physical interaction was observed between the intracellular domains of any of the Hm/Hm receptor sets tested. The results of our study suggest that the Hm1, Hm2 and Hm3 receptors do not form dimers or oligomers by interacting directly through either the hydrophilic intracellular domains or the C-terminal tail domains. To further investigate extracellular domain interactions, the N-terminus (N) and extracellular loops (o1 and o2) were also cloned into the two-hybrid vectors. Interactions of Hm2N with Hm2N, Hm2o1, Hm2o2, Hm3N, Hm3o1 or Hm3o2 were examined. The N-terminal domain of Hm2 was found to have no direct interaction with any extracellular domain. From our results, we excluded the possibility of a direct interaction between the muscarinic receptor subtypes (Hm1, Hm2 and Hm3) as a mechanism for homo- or hetero-meric dimerization/oligomerization. On the other hand, it remains a possibility that interaction may occur indirectly or require proper conformation or subunit formation or hydrophobic region involvement.

Control of pancreatic exocrine secretion via muscarinic receptors: which subtype(s) are involved? A review

The present report gives an overview of the experimental, pharmacological and molecular investigations that have been undertaken during the past two decades to characterize and identify the muscarinic receptor subtype(s) involved in the cholinergic control of pancreatic exocrine secretion in humans and different animal species. The results published in the literature clearly indicate that both M1 and M3 receptors contribute to the regulation of pancreatic enzyme secretion, although contradictory conclusions have been drawn from secretory studies using specific M1 and M3 receptor antagonists in vivo and in vitro. Binding studies using specific M1 and M3 receptor antagonists have supported the existence of both M1 and M3 receptors on pancreatic acinar cells, which was confirmed by the demonstration of specific mRNA for both receptor subtypes in rat pancreatic acinar cells. In addition, experimental evidence exists that nonacinar (possibly presynaptic) M1 receptors also contribute to the control of pancreatic enzyme secretion. The role of the different muscarinic receptor subtypes in the control of pancreatic fluid and bicarbonate output, however, still needs to be clarified. Future research should cover the evaluation of the relative contribution of the different receptor subtypes to the regulation of pancreatic exocrine function, the localization of the receptors involved as well as possible species differences.

Effect of M4-Cholinoceptor Blockade on Haloperidol-Produced Catatonic Syndrome in Rats

We studied the relationship between the efficiency of muscarinic receptor antagonists in preventing haloperidol-induced catatonia and their activity in tests for the interaction of ligands with various subtypes of muscarinic receptors (M1-M4) in rats. Mathematical modeling showed that affinity of the ligand for M4 receptors positively affects its ability to correct extrapyramidal disorders (catatonic syndrome) produced by haloperidol, while affinity for M2 receptors had a negative effect on this characteristic.

Effects of Cholinoblockers on Acetylcholine Content in Rat Striatum in Neuroleptic-Induced Parkinsonism

Correction of neuroleptic-induced parkinsonism in rats with two central cholinoblockers atropine and pentifine (acetylene aminoalcohol synthesized at Institute of Toxicology) were studied by measuring the content of acetylcholine in the striatum. The content of the transmitter secretion was estimated from the content of bound acetylcholine fraction in homogenates of the above-mentioned compartment of the brain. The results indicate that atropine and pentifine in doses equally effectively preventing catalepsy in rats had different effects on acetylcholine secretion in the striatum. Hence, cholinolytics with different pharmacological selective effects differently interact with central muscarine receptor subtypes.

Role of muscarinic receptor subtypes in the constriction of peripheral airways: studies on receptor-deficient mice

In the airways, increases in cholinergic nerve activity and cholinergic hypersensitivity are associated with chronic obstructive pulmonary disease and asthma. However, the contribution of individual muscarinic acetylcholine receptor subtypes to the constriction of smaller intrapulmonary airways that are primarily responsible for airway resistance has not been analyzed. To address this issue, we used videomicroscopy and digital imaging of precision-cut lung slices derived from wild-type mice and mice deficient in either the M1 (mAChR1-/- mice), M2 (mAChR2-/- mice), or M3 receptor subtype (mAChR3-/- mice) or lacking both the M2 and M3 receptor subtypes (mAChR2/3-/- double-knockout mice). In peripheral airways from wild-type mice (mAChR+/+ mice), muscarine induced a triphasic concentration-dependent response, characterized by an initial constriction, a transient relaxation, and a sustained constriction. The bronchoconstriction was diminished by up to 60% in mAChR3-/- lungs and was completely abolished in mAChR2/3-/- lungs. The sustained bronchoconstriction was reduced in mAChR2-/- bronchi, and, interestingly, the transient relaxation was absent; the bronchoconstriction in response to 10-8 M muscarine was increased by 158% in mAChR1-/- mice. Quantitative reverse transcriptase-polymerase chain reaction analysis revealed that the disruption of specific mAChR genes had no significant effect on the expression levels of the remaining mAChR subtypes. These results demonstrate that cholinergic constriction of murine peripheral airways is mediated by the concerted action of the M2 and M3 receptor subtypes and suggest the existence of pulmonary M1 receptor activation, which counteracts cholinergic bronchoconstriction. Given the important role of muscarinic cholinergic mechanisms in pulmonary disease, these findings should be of considerable therapeutic relevance.

Acetylcholine release in the pontine reticular formation of C57BL/6J mouse is modulated by non-M1 muscarinic receptors

Pontine acetylcholine (ACh) contributes to the regulation of electroencephalographic and behavioral arousal in all mammals so far investigated. The mouse is recognized as a powerful model for pharmacogenomics but the synaptic mechanisms regulating ACh release in mouse pontine reticular formation have not been characterized. Drug delivery by microdialysis was used in isoflurane-anesthetized C57BL/6J (B6) mice (n=33) to test the hypothesis that muscarinic autoreceptors modulate ACh release in the pontine reticular nucleus, oral part (PnO). Dialysis delivery of tetrodotoxin to the PnO significantly decreased ACh by 58% below control levels, confirming that measured ACh reflected neurotransmitter release. The muscarinic antagonist scopolamine increased ACh release in the PnO by 21% (3 nM), 48% (10 nM), 56% (30 nM), and 104% (100 nM). The muscarinic agonist bethanechol dialyzed into the PnO significantly decreased ACh release by 60% compared with control. Dialysis delivery of relatively subtype selective muscarinic antagonists to the PnO revealed the following order of potency for increasing ACh release: scopolamine (3 nM)>AF-DX 116 (100 nM)=pirenzepine (100 nM). These data support the conclusion that ACh release in PnO of B6 mouse is modulated by non-M1 muscarinic receptors.

Antimuscarinics for the treatment of overactive bladder: current options and emerging therapies

Antimuscarinic drugs have been the mainstay in the treatment of overactive bladder (OAB) for over two decades. An ideal antimuscarinic medicine is one that can normalize bladder function without interfering with parasympathetic regulation of other organs. Currently, extended-release formulations of tolterodine (tolterodine-ER) and oxybutynin (oxybutynin-ER and oxybutynin-TDS) serve as the cornerstone in the pharmacotherapy of OAB. Although these products represent a significant improvement over older agents, especially with respect to convenience of dosing schedule, their tolerability concerns and modest efficacy make them less than ideal therapies. Advances in our understanding of muscarinic receptor pharmacology have raised optimism in our ability to widen the therapeutic index and increase the efficacy of antimuscarinics by selectively targeting one or more of the five muscarinic subtypes. A structurally diverse group of molecules, having varying receptor-selectivity profiles (non-selective, M3 selective, M2 selective, M2 sparing and M5 sparing), are in development for OAB. Results of clinical trials with these drugs must be awaited before their therapeutic value can be accurately judged.

Muscarinic receptor subtypes in the human colon: lack of evidence for atypical subtypes

Characteristics of muscarinic receptors were investigated in circular muscle from normal human colon. In saturation studies (n=18), binding of [3H]quinuclidinyl benzylate (QNB) was of high affinity (K(d) 87.3 pM) and capacity (B(max) 362+/-27 fmol/mg protein), with no differences between ascending and sigmoid colon. Kinetic studies gave a K(d) of 55 pM. Methoctramine and darifenacin displayed biphasic binding profiles, the high affinity components being compatible with a population of approximately 80+/-5% M(2) and 13+/-2% M(3) muscarinic receptors, respectively. Pirenzepine, mamba toxin 1 and mamba toxin 3 were very weak competitors, indicating negligible expression of muscarinic M(1) and M(4) receptors. Six other subtype-preferring antagonists exhibited K(i) values typical of those reported at cloned human muscarinic M(2) receptors. In the presence of methoctramine, pre-treatment with alkylating agent 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride (4-DAMP mustard) inhibited [3H]quinuclidinyl benzylate binding to 26% of sites. Following alkylation of muscarinic M(3) receptors, darifenacin bound to a single low affinity site, indicating binding to muscarinic M(2) receptors.