A Rapid and Versatile Method to Label Receptor Ligands Using “Click” Chemistry: Validation with the Muscarinic M1 Antagonist Pirenzepine

Tagged biologically active molecules represent powerful pharmacological tools to study and characterize ligand-receptor interactions. However, the labeling of such molecules is not trivial, especially when poorly soluble tags have to be incorporated. The classical method of coupling usually necessitates a tedious final purification step to remove the excess of reagents and to isolate tagged molecules. To overcome this limitation, Cu(I)-catalyzed 1,3-dipolar cycloaddition, referred to as "click" chemistry, was evaluated as a tool to facilitate the access to labeled molecules. In order to validate the approach, we focused our attention on the incorporation of a fluorophore (Lissamine Rhodamine B), a nonfluorescent dye (Patent Blue VF), or biotin into a muscarinic antagonist scaffold derived from pirenzepine. The reaction performed in acetonitrile/water, in the presence of CuSO4 and Cu wire, allowed us to obtain three novel pirenzepine derivatives with high purity and in good yield. No coupling reagents were needed, and the quasi-stoichiometric conditions of the reaction enabled the straightforward isolation of the final product by simple precipitation and its use in bioassays. The affinity of the compounds for the human M1 muscarinic receptor fused to EGFP was checked under classical radioligand and FRET binding conditions. The three pirenzepine constructs display a nanomolar affinity for the M1 receptor. In addition, both dye-labeled derivatives behave as potent acceptors of energy from excited EGFP with a very high quenching efficiency.

Treatment of the overactive bladder syndrome with muscarinic receptor antagonists - a matter of metabolites?

Antagonists of muscarinic acetylcholine receptors, such as darifenacin, oxybutynin, propiverine, solifenacin, tolterodine, and trospium, are the mainstay of the treatment of the overactive bladder syndrome. Fesoterodine is a newer drug awaiting regulatory approval. We briefly review the pharmacological activity of their metabolites and discuss how active metabolites may contribute to their efficacy and tolerability in vivo. Except for trospium, and perhaps solifenacin, all of the above drugs form active metabolites, and their presence and activity need to be taken into consideration when elucidating relationships between pharmacokinetics and pharmacodynamics of these drugs. Moreover, the ratios between parent compounds and metabolites may differ depending on genotype of the metabolizing enzymes, concomitant medication, and/or drug formulation. Differential generation of active metabolites of darifenacin or tolterodine are unlikely to influence the overall clinical profile of these drugs in a major way because the active metabolites exhibit a similar pharmacological profile as the parent compound. In contrast, metabolites of oxybutynin and propiverine may behave quantitatively or even qualitatively differently from their parent compounds and this may have an impact on the overall clinical profile of these drugs. We conclude that more comprehensive studies of drug metabolites are required for an improved understanding of their clinical effects.

State-dependent regulation of sensory-motor transmission: role of muscarinic receptors in sensory-motor integration in the crayfish walking system

The aim of this study was to investigate a potential mechanism for state-dependent regulation of sensory-motor transmission from sensory afferents of a proprioceptor to motoneurons (MNs) in the walking system of the crayfish. This study was performed using an in vitro preparation of thoracic ganglia including motor nerves and the proprioceptor that codes movements of the second joint (coxo-basal chordotonal organ - CBCO) of the leg. Application of movements to the CBCO elicits resistance reflex responses intracellularly recorded from Dep MNs. This reflex response is enhanced when Dep MNs are depolarized either spontaneously or by current injection. This enhancement is abolished in the presence of scopolamine (an antagonist of muscarinic acetylcholine receptors). Using pharmacology, we demonstrate that the monosynaptic connection from CBCO sensory neurons to the Dep MNs includes both nicotinic and muscarinic components. In addition, the shape of monosynaptic excitatory postsynaptic potentials (EPSPs) depends on the membrane potential: at a subthreshold depolarizing membrane potential, the time constant of the falling phase of the EPSPs is significantly increased compared with its value at resting potential. This change is suppressed in the presence of scopolamine, indicating that the muscarinic component may contribute to the activation of the Dep MN pool by sensory activity. This state-dependent amplification of the sensory input may be important for increasing the strength of sensory feedback at times when central activation of the Dep MNs is very strong (e.g. during walking).

Quantifying the association and dissociation rates of unlabelled antagonists at the muscarinic M3 receptor

Slow receptor dissociation kinetics has been implicated in the long clinical duration of action of the muscarinic receptor antagonist tiotropium. However, despite the potential benefits of new drugs with slow dissociation kinetics, the rate parameters of new compounds are seldom measured due to technical difficulties and financial implications associated with radiolabeling multiple ligands. Here we describe the development and optimisation of a medium throughput assay which is capable of measuring the kinetic parameters of novel, unlabelled compounds. Radioligand binding studies were performed with membranes derived from CHO cells recombinantly expressing the human M(3) muscarinic receptor.Initial characterisation of the radioligand [(3)H]-NMS yielded on and off rates of 4.1+/-0.2 x 10(8) M(-1) min(-1) and 0.015+/-0.0005 min(-1), respectively. The specific binding of [(3)H]-NMS was measured over time in the presence and absence of several concentrations of unlabelled competitor compounds. These data were analysed using a competition kinetic model to provide on and off rates for the unlabelled competitor. Comparison of the kinetically derived Kd (k(off)/k(on)) with K(i) values generated at equilibrium showed an excellent correlation (r(2)=0.99), providing good validation of the method. The on and off rates were also used in theoretical computer simulations to successfully predict the effect of incubation time on apparent IC(50) values. This study demonstrates that a medium-throughput competition kinetic binding assay can be used to determine accurate on and off rates of unlabelled compounds, providing the opportunity to optimise for kinetic parameters early in the drug discovery process.

Muscarinic allosteric modulators: atypical structure-activity-relationships in bispyridinium-type compounds

Allosteric modulators of receptor binding are known for a variety of membrane receptors. In case of muscarinic receptors, a considerable number of structurally divergent modulators have been described. For the M2 receptor subtype which has a high sensitivity to allosteric modulation most of the allosteric agents bind to the common allosteric binding site of the receptor protein. In this study, a series of DUO compounds characterized by a bispyridinium middle chain and lateral benzyloximeether moieties of a systematically varied substitution pattern has been evaluated with regard to their allosteric potency to affect M2 receptors, whose orthosteric site was blocked by [3H]N-methylscopolamine. The variations in potency were found to be surprisingly small and the structure-activity relationships of the DUO compounds diverged from those of correspondingly substituted hexamethonio-type allosteric modulators. One has to conclude that DUO compounds bind in an "atypical" manner which is in agreement with recently reported side-directed mutagenesis and molecular modeling studies.

Disparate cholinergic currents in rat principal trigeminal sensory nucleus neurons mediated by M1 and M2 receptors: a possible mechanism for selective gating of afferent sensory neurotransmission

Neurons situated in the principal sensory trigeminal nucleus (PSTN) convey orofacial sensory inputs to thalamic relay regions and higher brain centres, and the excitability of these ascending tract cells is modulated across sleep/wakefulness states and during pain conditions. Moreover, acetylcholine release changes profoundly across sleep/wakefulness states and ascending sensory neurotransmission is altered by cholinergic agonists. An intriguing possibility is, therefore, that cholinergic mechanisms mediate such state-dependent modulation of PSTN tract neurons. We tested the hypotheses that cholinergic agonists can modulate PSTN cell excitability and that such effects are mediated by muscarinic receptor subtypes, using patch-clamp methods in rat and mouse. In all examined cells, carbachol elicited an electrophysiological response that was independent of action potential generation as it persisted in the presence of tetrodotoxin. Responses were of three types: depolarization, hyperpolarization or a biphasic response consisting of hyperpolarization followed by depolarization. In voltage-clamp mode, carbachol evoked corresponding inward, outward or biphasic currents. Moreover, immunostaining for the vesicle-associated choline transporter showed cholinergic innervation of the PSTN. Using muscarinic receptor antagonists, we found that carbachol-elicited PSTN neuron hyperpolarization was mediated by M2 receptors and depolarization, in large part, by M1 receptors. These data suggest that acetylcholine acting on M1 and M2 receptors may contribute to selective excitability enhancement or depression in individual, rostrally projecting sensory neurons. Such selective gating effects via cholinergic input may play a functional role in modulation of ascending sensory transmission, including across behavioral states typified by distinct cholinergic tone, e.g. sleep/wakefulness arousal levels or neuropathic pain conditions.

Fluorescent styryl dyes FM1-43 and FM2-10 are muscarinic receptor antagonists: intravital visualization of receptor occupancy

The fluorescent styryl dyes FM1-43 and FM2-10 have been used to visualize the endocytic and exocytic processes involved in neurotransmission in a variety of central and peripheral nerve preparations. Their utility is limited to some extent by a poorly understood vesicular-independent labelling of cells and tissues. We show here that one likely cause of this troublesome background labelling is that FM1-43 and FM2-10 are selective and competitive antagonists at both cloned and endogenously expressed muscarinic acetylcholine receptors. In radioligand binding studies, FM1-43 and FM2-10 bound with moderate affinity (23-220 nM) to membranes of Chinese hamster ovary (CHO) cells expressing cloned human muscarinic receptors (M1-M5). In functional studies in vitro, FM1-43 and FM2-10 inhibited electrical field stimulation (EFS) and acetylcholine-induced cholinergic contractions of guinea-pig tracheal strips (IC50: FM1-43, 0.4 +/- 0.1; FM2-10, 1.6 +/- 0.1 microM; concentration of antagonist producing a 2-fold leftward shift in the acetylcholine concentration-response curve (Kb): FM1-43, 0.3 +/- 0.1; FM2-10, 15.8 +/- 10.1 microM). Neither compound inhibited EFS-evoked, non-adrenergic non-cholinergic nerve-mediated relaxations or contractions of the airways, or contractions mediated by histamine H1 receptor or tachykinin NK2 receptor activation. Incubating freshly excised tracheal whole-mount preparations with 5 microM FM1-43 resulted in intense fluorescence labelling of the smooth muscle that was reduced by up to 90% in the presence of selective M2 and M3 receptor antagonists. The potency of the FM dyes as muscarinic receptor antagonists is within the concentration range used to study vesicular cycling at nerve terminals. Given that muscarinic receptors play a key role in the regulation of neurotransmitter release from a variety of neurones, the anticholinergic properties of FM dyes may have important implications when studying vesicular events in the nervous system. In addition, these dyes may provide a novel tool for visualizing muscarinic receptor occupancy in living tissue or cell preparations.

M1 muscarinic receptors contribute to, whereas M4 receptors inhibit, dopamine D1 receptor-induced [3H]-cyclic AMP accumulation in rat striatal slices

In rat striatal slices labelled with [(3)H]-adenine and in the presence of 1 mM 3-isobutyl-1-methylxantine (IBMX), cyclic [(3)H]-AMP ([(3)H]-cAMP) accumulation induced by the dopamine D(1) receptor agonist SKF-81297 (1 microM; 177 +/- 13% of basal) was inhibited by the general muscarinic agonist carbachol (maximum inhibition 72 +/- 3%, IC(50) 0.30 +/- 0.06 microM). The muscarinic toxin 7 (MT-7), a selective antagonist at muscarinic M(1) receptors, reduced the effect of SKF-81297 by 40+/-7% (IC(50) 251+/- 57 pM) and enhanced the inhibitory action of a submaximal (1 microM) concentration of carbachol (69 +/- 4% vs. 40 +/- 7% inhibition, IC(50) 386 +/- 105 pM). The toxin MT-1, agonist at M(1) receptors, stimulated [(3)H]-cAMP accumulation in a modest but significant manner (137 +/- 11% of basal at 400 nM), an action additive to that of D(1) receptor activation and blocked by MT-7 (10 nM). The effects of MT-7 on D(1) receptor-induced [(3)H]-cAMP accumulation and the carbachol inhibition were mimicked by the PKC inhibitors Ro-318220 (200 nM) and Gö-6976 (200 nM). Taken together our results indicate that in addition to the inhibitory role of M(4) receptors, in rat striatum acetylcholine stimulates cAMP formation through the activation of M(1 )receptors and PKC stimulation.

Biantennary Glycans as Well as Genetic Variants of α1-Acid Glycoprotein Control the Enantioselectivity and Binding Affinity of Oxybutynin

PURPOSE

The purpose of this study was to investigate the role of biantennary branching glycans of alpha1-acid glycoprotein (AGP) and its genetic variants in the enantioselective binding of oxybutynin (OXY).

METHOD

Human native AGP was separated using imminodiacetate-copper (II) affinity chromatography into two fractions, the A variant and a mixture of the F1 and S variants (F1-S). These fractionated AGPs were further separated by concanavalin A affinity chromatography into two fractions, with and without biantenarry glycans. An on-line high-performance liquid chromatography (HPLC) system consisting of a high-performance frontal analysis column, an extraction column, and an analytical HPLC column was developed to determine the binding affinities of OXY enantiomers for respective AGP species.

RESULTS

The total binding affinity as well as the enantiomeric selectivity of OXY in the F1-S mixed variant was significantly higher than that for the A variant, indicating that the chiral recognition ability of native AGP for the OXY enantiomers highly depends on the F1-S mixed variant. Furthermore, not only the genetic variants but also bianntenary glycans of AGP affect the binding affinity of OXY and are also responsible for the enantioselectivity.

CONCLUSIONS

Both genetic variants and glycan structures significantly contribute to the enantioselectivity and the binding affinity of OXY.

Role of the urothelium in urinary bladder dysfunction following spinal cord injury

A consequence of spinal cord injury is a change in bladder reflex pathways resulting in the emergence of detrusor hyperreflexia and increased activity of the urethral sphincter. A basis for some of these alterations could be changes in the environment of bladder sensory nerve endings at the target organ. Recent evidence suggests that the urothelium (the lining of the urinary bladder) plays a prominent role in modulating bladder sensory nerve ending excitability. It is conceivable that factors and processes affecting the plasticity of bladder neurons after spinal cord injury may be partly due to changes occurring in the urothelium. Although the urothelium has classically been thought of as a passive barrier to ions/solutes, a number of novel properties have been recently attributed to these cells. Our work and that of others clearly demonstrates that the urothelium exhibits both "sensor" (expression of sensor molecules or response to thermal, mechanical and chemical stimuli) as well as "transducer" (release of factors/transmitters) properties. Taken together, these and other findings discussed in this chapter suggest a sensory function for the urothelium and that alterations in urothelial properties may contribute to afferent abnormalities following spinal cord injury.

In vitro evidence and age-related changes for nicotinic but not muscarinic acetylcholine receptors in the central nervous system of Sepia officinalis

Binding putative muscarinic ([3H]-NMS and [3H]-QNB) or nicotinic ([3H]-cytisine) acetylcholine receptors was quantitatively studied through the use of in vitro binding experiments on either membrane preparations or brain sections of juvenile (3 months), mature (15 months) or senescent (23 months) cuttlefish. No specific binding could be detected with muscarinic receptor ligands under any of the experimental conditions employed (ligand concentrations, buffers, ionic charges, types of tissue, i.e., brain sections or membrane preparations). On the other hand, [3H]-cytisine demonstrated a specific and saturable binding with a single class of high affinity binding sites (Kd of 2.6-34.6 nM; Bmax of 128-1682 fmol/mg tissue equivalent, depending on the central structure). This binding was found to be heterogeneous throughout the central regions (optic lobe>pedal lobe; superior frontal lobe>...precommissural lobe; vertical lobe>...anterior basal lobe; subvertical lobe; inferior frontal lobe; median basal lobe). These results question the existence of muscarinic-like receptors in the cuttlefish brain, or at least of a pharmacological dissimilarity from vertebrate muscarinic receptors. In contrast, nicotinic-like receptors are widely present; interestingly, their density was found to be significantly reduced in most nervous central lobes of senescent cuttlefish when compared with mature animals. The most significant decrease (-71%) was found in the anterior part of the superior frontal lobe, which is involved in visual learning; this might be related to the changes, previously demonstrated, in cholinergic neurons in this lobe in the course of aging.

Inhibition of ligand binding to G protein-coupled receptors by arachidonic acid

Arachidonic acid (AA), released in response to muscarinic acetylcholine receptor (mAChR) stimulation, previously has been reported to function as a reversible feedback inhibitor of the mAChR. To determine if the effects of AA on binding to the mAChR are subtype specific and whether AA inhibits ligand binding to other G protein-coupled receptors (GPCRs), the effects of AA on ligand binding to the mAChR subtypes (M1, M2, M3, M4, and M5) and to the micro-opioid receptor, beta2-adrenergic receptor (beta2-AR), 5-hydroxytryptamine receptor (5-HTR), and nicotinic receptors were examined. AA was found to inhibit ligand binding to all mAChR subtypes, to the beta2-AR, the 5-HTR, and to the micro-opioid receptor. However, AA does not inhibit ligand binding to the nicotinic receptor, even at high concentrations of AA. Thus, AA inhibits several types of GPCRs, with 50% inhibition occurring at 3-25 MuM, whereas the nicotinic receptor, a non-GPCR, remains unaffected. Further research is needed to determine the mechanism by which AA inhibits GPCR function.

Decreased density of muscarinic receptors in the superior temporal gyrusin schizophrenia

Recent studies have indicated that muscarinic receptors are involved in the pathophysiology in schizophrenia, particularly in cognitive deficits. The superior temporal gyrus (STG) is an area that has also been strongly implicated in the pathophysiology of schizophrenia. Therefore, in this study, we investigated the binding density of two muscarinic antagonists, [(3)H]pirenzepine and [(3)H]AF-DX 384, in the STG of schizophrenia patients compared with controls. A significant decrease (44% in the superficial layers and 48% in the deep layers, P<0.01) in binding density of [(3)H]pirenzepine was observed in schizophrenia patients, which suggested a reduction of muscarinic M1 and M4 receptor densities in the STG of schizophrenia patients. A tendency toward decreased [(3)H]AF-DX 384 binding density (34%, P=0.09) was also observed in schizophrenia patients compared with controls. Because of the positive correlation between [(3)H]pirenzepine and [(3)H]AF-DX 384 binding, and, insofar as both ligands have high affinities for the M4 receptor, the involvement of M4 receptor alteration is also suggested in the STG in schizophrenia. These results suggest that changes of the muscarinic receptors M1 and M4 might contribute to the STG pathology in schizophrenia.

In vivo co-ordinated interactions between inhibitory systems to control glutamate-mediated hippocampal excitability

We present an overview of the long-term adaptation of hippocampal neurotransmission to cholinergic and GABAergic deafferentation caused by excitotoxic lesion of the medial septum. Two months after septal microinjection of 2.7 nmol alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), a 220% increase of GABA(A) receptor labelling in the hippocampal CA3 and the hilus was shown, and also changes in hippocampal neurotransmission characterised by in vivo microdialysis and HPLC. Basal amino acid and purine extracellular levels were studied in control and lesioned rats. In vivo effects of 100 mm KCl perfusion and adenosine A(1) receptor blockade with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) on their release were also investigated. In lesioned animals GABA, glutamate and glutamine basal levels were decreased and taurine, adenosine and uric acid levels increased. A similar response to KCl infusion occurred in both groups except for GABA and glutamate, which release decreased in lesioned rats. Only in lesioned rats, DPCPX increased GABA basal level and KCl-induced glutamate release, and decreased glutamate turnover. Our results evidence that an excitotoxic septal lesion leads to increased hippocampal GABA(A) receptors and decreased glutamate neurotransmission. In this situation, a co-ordinated response of hippocampal retaliatory systems takes place to control neuron excitability.

Muscarinic receptor binding, plasma concentration and inhibition of salivation after oral administration of a novel antimuscarinic agent, solifenacin succinate in mice

1 A novel muscarinic receptor antagonist, solifenacin succinate, inhibited specific binding of [N-methyl-(3)H]-scopolamine ([(3)H]-NMS) in the mouse bladder, submaxillary gland and heart in a concentration-dependent manner. This inhibitory effect was greatest in the submaxillary gland, followed by the bladder and heart. 2 After oral administration of oxybutynin (76.1 micromol kg(-1)) or solifenacin (62.4, 208 micromol kg(-1)), a significant dose- and time-dependent increase in K(D) values for specific [(3)H]-NMS binding was seen in the bladder, prostate, submaxillary gland, heart, colon and lung, compared with control values. The increase in K(D) induced by oxybutynin in each tissue reached a maximum 0.5 h after oral administration and then rapidly declined, while that induced by solifenacin was greatest 2 h after administration and was maintained for at least 6 or 12 h, depending on the dose. The muscarinic receptor binding of oral solifenacin was slower in onset and of a longer duration than that of oxybutynin. 3 Plasma concentrations of oxybutynin and its active metabolite (N-desethyl-oxybutynin, DEOB) were maximum 0.5 h after its oral administration and then declined rapidly. Oral solifenacin persisted in the blood for longer than oxybutynin. 4 Pilocarpine-induced salivary secretion in mice was significantly reduced by oral administration of solifenacin and was completely abolished 0.5 h after oral oxybutynin. Although the suppression induced by solifenacin was more persistent than that due to oxybutynin, the antagonistic effect of solifenacin on the dose-response curves to pilocarpine was significantly weaker than that of oxybutynin. It is concluded that oral solifenacin persistently binds to muscarinic receptors in tissues expressing the M(3) subtype, such as the bladder.

Blockade of β-Adrenoceptors and Muscarinic Cholinergic Receptors Modulates Effect on Nitric Oxide on Heart Rate in Rats

Nitroglycerine in doses of 0.4-1.0 mg/kg decreased the heart rate in rats, which was associated with inhibition of adrenergic influences realized via beta-adrenoceptors. The negative chronotropic effect of sodium nitroprusside in a dose of 1 mg/kg was more significant compared to that of nitroglycerine (by 2-3 times). It was associated with inhibition of adrenergic and stimulation of cholinergic influences mediated via beta-adrenoceptors and muscarinic cholinergic receptors, respectively. During blockade of beta-adrenoceptors and muscarinic cholinergic receptors, sodium nitroprusside increased the time of atrioventricular conduction. These data indicate that function of myocytes in the heart conduction system of rats depends on the PQ interval.

Darifenacin in the treatment of overactive bladder

Antimuscarinic drug therapy has been shown to be effective in the management of patients with symptoms of the overactive bladder syndrome (OAB), but the bothersome antimuscarinic adverse effects of dry mouth, constipation, somnolence and blurred vision often affect compliance with medication. The development of bladder selective M3 specific antagonists offers the possibility of increasing efficacy whilst minimising adverse effects. The M3 specific antagonist solifenacin has recently been marketed, and darifenacin will soon be available. The purpose of this article is to review the pharmacology and clinical trial data available for darifenacin, in addition to examining its role in the treatment of the OBS.

A high n-6 polyunsaturated fatty acid diet reduces muscarinic M2/M4 receptor binding in the rat brain

The aim of this study was to examine the influence of different fat diets on muscarinic acetylcholine receptor binding. Nineteen male Sprague-Dawley rats were divided into four groups and fed a diet of either high saturated fat, n-6 polyunsaturated fatty acid (PUFA), n-3 PUFA or low fat (control) for 8 weeks. Using quantitative autoradiography, [(3)H]pirenzepine binding to muscarinic M1/M4 receptors and [(3)H]AF-DX384 binding to M2/M4 receptors were measured throughout the brain in all four groups. The main findings were that compared to the low fat control group, M2/M4 receptor binding was significantly reduced in the dorsolateral, dorsomedial and ventromedial parts of the caudate putamen (61-64%, p < 0.05), anterior cingulate cortex (59%, p < 0.01), dentate gyrus and CA1-3 fields of the hippocampus (32-43%, p < 0.01) of rats on a high n-6 PUFA diet; however, no differences in M1/M4 receptor binding densities between the four groups were observed. These results suggest that a diet high in n-6 PUFA, but not of n-3 PUFAs or saturated fat, may selectively alter M2/M4 receptor-mediated signal transduction in the rat brain.

In vivo demonstration of muscarinic receptor binding activity of N-desethyl-oxybutynin, active metabolite of oxybutynin

The present study was undertaken to characterize in vivo muscarinic receptor binding of N-desethyl-oxybutynin (DEOB), active metabolite of oxybutynin (anticholinergic agent), in the bladder, submaxillary gland, heart and colon of rats, in relation to the plasma concentrations and inhibition of salivation. In the in vitro experiment, DEOB, as well as oxybutynin, inhibited the concentration-dependently specific [3H]N-methylscopolamine (NMS) binding in rat tissues and the affinity of DEOB in the rat bladder, submaxillary gland and colon was significantly (about 2 times) greater than that of oxybutynin. Following i.v. injection of DEOB (2.73-27.3 micromol/kg), there were dose- and time-dependent increases in the apparent dissociation constant (Kd) for specific [3H]NMS binding in the bladder, submaxillary gland, heart and colon of rats, compared with control values, and the effect was similar to that by i.v. injection of oxybutynin (2.54-25.4 micromol/kg). Plasma concentrations of DEOB and oxybutynin in these rats showed dose- and time-dependent increases. The pilocarpine-induced salivary secretion in rats was equipotently reduced by the i.v. injection of DEOB and oxybutynin. In conclusion, it has been shown that intravenously injected DEOB, as well as oxybutynin, binds significantly to muscarinic receptors in rat tissues including the bladder and salivary gland and the receptor binding activity of DEOB is roughly similar to that of oxybutynin.

Antagonist binding in the rat muscarinic receptor A study by docking and X-ray crystallography

A series of agonists to the rat muscarinic receptor have been docked computationally to the active site of a homology model of rat M1 muscarinic receptor. The agonists were modelled on the X-ray crystal structure of atropine, which is reported here and the docking studies are shown to reproduce correctly the order of experimental binding affinities for the agonists as well as indicate where there appear to be inconsistencies in the experimental data. The crystal and molecular structure of atropine (tropine tropate; alpha-[hydroxymethyl]benzeneacetic acid 8-methyl[3.2.1]oct-3-yl ester C17H23NO3) has been determined by X-ray crystallography using an automated Patterson search method, and refined by full-matrix least-squares to a final R of 0.0452 for 2701 independent observed reflections and 192 parameters using Mo Kalpha radiation, lambda=0.71073A at 150K. The compound crystallises in space group Fdd2 with Z=16 molecules per unit cell.

Muscarinic Allosteric Enhancers of Ligand Binding: Pivotal Pharmacophoric Elements in Hexamethonio-Type Agents

Bisphthalimidopropyl-substituted hexamethonio compounds have been established as allosteric modulators of ligand binding to muscarinic acetylcholine receptors. Enhancers of ligand binding are of special interest. This study aimed to unravel the structural elements inducing positive cooperativity with the binding of an antagonist. [(3)H]-N-methylscopolamine binding to muscarinic M(2) receptors was measured in porcine heart homogenates. Dimethylation, but not monomethylation, of the lateral propyl chain in combination with an affinity increasing aromatic imide moiety, such as a 5-methylphthalimide and naphthalimide, on the same side of the molecule shifts the cooperativity toward positive values, resulting in enhancers of antagonist binding. Thus, lateral side chain dimethylation is a pivotal pharmacophoric element for positive cooperativity in hexamethonio-type muscarinic allosteric agents.

C(8)-substituted 1-azabicyclo[3.3.1]non-3-enes: a novel scaffold for muscarinic receptor ligands

The [3.3.1]-bicyclic amine, exo-8-benzyloxymethyl-3-ethoxycarbonyl-4-hydroxy-1-azabicyclo[3.3.1]non-3-ene (1), has been shown to be a potent competitive antagonist against the hM(1)-hM(5) muscarinic receptors. This heterocyclic system has not been extensively evaluated despite the notable activities reported for other bicyclic amines. Synthetic strategies permitted the selective alteration of five structural sites in 1. Pharmacological evaluation demonstrated that modification of either the C(3) alkoxycarbonyl or the C(4) enol units in 1 gave compounds with high affinity for the hM(1)-hM(5) muscarinic receptors with selectivity for the hM(2) receptor.

Molecular modulation of muscarinic antagonists. Synthesis and affinity profile of 2,2-diphenyl-2-ethylthio-acetic acid esters designed to probe the binding site cavity

The synthesis and preliminary pharmacological profile of a new series of muscarinic antagonists, derived from previously studied 2,2-diphenyl-2-ethylthio-acetic acid esters, are reported. The parent molecules were decorated with linkers of different length, carrying an amino group to catch a putative anionic function outside the recognition site of the receptor. It was hoped that the interception of this function would give molecules with higher potency and selectivity. The attempt has not been successful, but a new series of compounds with a peculiar pharmacological profile has been identified.

Changes in acetylcholinesterase activity and muscarinic receptor bindings in mu-opioid receptor knockout mice

Anatomical evidence indicates that cholinergic and opioidergic systems are co-localized and acting on the same neurons. However, the regulatory mechanisms between cholinergic and opioidergic system have not been well characterized. In the present study, we investigated whether there are compensatory changes of acetylcholinesterase activity and cholinergic receptors in mice lacking mu-opioid receptor gene. The acetylcholinesterase activity was determined by histochemistry assay. The cholinergic receptor binding was carried out by quantitative autoradiography using [3H]-quinuclidinyl benzilate (nonselective muscarinic receptors), N-[3H]-methylscopolamine (nonselective muscarinic receptors), [3H]-pirenzepine (M1 subtype muscarinic receptors) and [3H]-AF-DX384 (M2 subtype muscarinic receptors) in brain slices of wild-type and mu-opioid receptor knockout mice. The acetylcholinesterase activity of mu-opioid receptor knockout mice was higher than that of the wild-type in the striatal caudate putamen and nucleus accumbens, but not in the cortex and hippocampus areas. In addition, the bindings in N-[3H]-methylscopolamine and [3H]-AF-DX384 of mu-opioid receptor knockout mice were significantly lower when compared with that of the wild-type controls in the striatal caudate putamen and nucleus accumbens. However, there were no significant differences in bindings of [3H]-quinuclidinyl benzilate and [3H]-pirenzepine between mu-opioid receptor knockout and wild-type mice in the cortex, striatum and hippocampus. These data indicate that there are up-regulation of acetylcholinesterase activity and compensatory down-regulation of M2 muscarinic receptors in the striatal caudate putamen and nucleus accumbens of mu-opioid receptor knockout mice.