Therapeutic opportunities from muscarinic receptor research

Inhibition of transmitter release from rat sympathetic neurons via presynaptic M(1) muscarinic acetylcholine receptors

BACKGROUND AND PURPOSE

M(2), M(3) and/or M(4) muscarinic acetylcholine receptors have been reported to mediate presynaptic inhibition in sympathetic neurons. M(1) receptors mediate an inhibition of K(v)7, Ca(V)1 and Ca(V)2.2 channels. These effects cause increases and decreases in transmitter release, respectively, but presynaptic M(1) receptors are generally considered facilitatory. Here, we searched for inhibitory presynaptic M(1) receptors.

EXPERIMENTAL APPROACH

In primary cultures of rat superior cervical ganglion neurons, Ca(2+) currents were recorded via the perforated patch-clamp technique, and the release of [(3)H]-noradrenaline was determined.

KEY RESULTS

The muscarinic agonist oxotremorine M (OxoM) transiently enhanced (3)H outflow and reduced electrically evoked release, once the stimulant effect had faded. The stimulant effect was enhanced by pertussis toxin (PTX) and was abolished by blocking M(1) receptors, by opening K(v)7 channels and by preventing action potential propagation. The inhibitory effect was not altered by preventing action potentials or by opening K(v)7 channels, but was reduced by PTX and omega-conotoxin GVIA. The inhibition remaining after PTX treatment was abolished by blockage of M(1) receptors or inhibition of phospholipase C. When [(3)H]-noradrenaline release was triggered independently of voltage-activated Ca(2+) channels (VACCs), OxoM failed to cause any inhibition. The inhibition of Ca(2+) currents by OxoM was also reduced by omega-conotoxin and PTX and was abolished by M(1) antagonism in PTX-treated neurons.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that M(1), in addition to M(2), M(3) and M(4), receptors mediate presynaptic inhibition in sympathetic neurons using phospholipase C to close VACCs.

A novel derivative of xanomeline improved memory function in aged mice

OBJECTIVE

To characterize the function of a new xanomeline-derived M1 agonist, 3-[3-(3-florophenyl-2-propyn-1-ylthio)-1,2,5-thiadiazol-4-yl]-1,2,5,6- tetrahydro-1-methylpyridine Oxalate (EUK1001), the acute toxicity and the effects on synaptic plasticity and cognition of EUK1001 were evaluated.

METHODS

To examine the median lethal dose (LD50) of EUK1001, a wide dose range of EUK1001 was administered by p.o. and i.p. in aged mice. Furthermore, novel object recognition task and in vitro electrophysiological technique were utilized to investigate the effects of EUK1001 on recognition memory and hippocampal synaptic plasticity in aged mice.

RESULTS

EUK1001 exhibited lower toxicity than xanomeline, and improved the performance of aged mice in the novel object recognition test. In addition, bath application of 1 micromol/L EUK1001 directly induced long-term potentiation in the hippocampus slices.

CONCLUSION

We conclude that EUK1001 can improve the age-related cognitive deficits.

Synthesis and SAR of analogues of the M1 allosteric agonist TBPB. Part I: Exploration of alternative benzyl and privileged structure moieties

This Letter describes the first account of the synthesis and SAR, developed through an iterative analogue library approach, of analogues of the highly selective M1 allosteric agonist TBPB. With slight structural changes, mAChR selectivity was maintained, but the degree of partial M1 agonism varied considerably.

Synthesis and SAR of analogs of the M1 allosteric agonist TBPB. Part II: Amides, sulfonamides and ureas--the effect of capping the distal basic piperidine nitrogen

This letter describes the further synthesis and SAR, developed through an iterative analog library approach, of analogs of the highly selective M1 allosteric agonist TBPB by deletion of the distal basic piperidine nitrogen by the formation of amides, sulfonamides and ureas. Despite the large change in basicity and topology, M1 selectivity was maintained.

Novel selective allosteric activator of the M1 muscarinic acetylcholine receptor regulates amyloid processing and produces antipsychotic-like activity in rats

Recent studies suggest that subtype-selective activators of M(1)/M(4) muscarinic acetylcholine receptors (mAChRs) may offer a novel approach for the treatment of psychotic symptoms associated with schizophrenia and Alzheimer's disease. Previously developed muscarinic agonists have provided clinical data in support of this hypothesis, but failed in clinical development because of a lack of true subtype specificity and adverse effects associated with activation of other mAChR subtypes. We now report characterization of a novel highly selective agonist for the M(1) receptor with no agonist activity at any of the other mAChR subtypes, termed TBPB [1-(1'-2-methylbenzyl)-1,4'-bipiperidin-4-yl)-1H-benzo[d]imidazol-2(3H)-one]. Mutagenesis and molecular pharmacology studies revealed that TBPB activates M(1) through an allosteric site rather than the orthosteric acetylcholine binding site, which is likely critical for its unprecedented selectivity. Whole-cell patch-clamp recordings demonstrated that activation of M(1) by TBPB potentiates NMDA receptor currents in hippocampal pyramidal cells but does not alter excitatory or inhibitory synaptic transmission, responses thought to be mediated by M(2) and M(4). TBPB was efficacious in models predictive of antipsychotic-like activity in rats at doses that did not produce catalepsy or peripheral adverse effects of other mAChR agonists. Finally, TBPB had effects on the processing of the amyloid precursor protein toward the non-amyloidogenic pathway and decreased Abeta production in vitro. Together, these data suggest that selective activation of M(1) may provide a novel approach for the treatment of symptoms associated with schizophrenia and Alzheimer's disease.

Potential adverse effects of bronchodilators in the treatment of airways obstruction in older people: recommendations for prescribing

Asthma and chronic obstructive pulmonary disease (COPD) are common disorders that are associated with increasing morbidity and mortality in older people. Bronchodilators are used widely in patients with these conditions, but even when used in inhaled form can have systemic as well as local effects. Older people experience more adverse drug effects because of pharmacodynamic and pharmacokinetic changes and particularly drug-drug and drug-disease interactions. Cardiovascular disease is common in older people and beta-adrenoceptor agonists (beta-agonists) have inotropic and chronotropic effects that can increase arrhythmias and cardiomyopathy. They can also worsen or induce myocardial ischaemia and cause electrolyte disturbances that contribute to arrhythmias. Tremor is a well known distressing adverse effect of beta-agonist administration. Long-term beta-agonist use can be associated with tolerance, poor disease control, sudden life-threatening exacerbations and asthma-related deaths. Functional beta2-adrenoceptors are present in osteoblasts, and chronic use of beta-agonists has been implicated in osteoporosis. Inhaled anticholinergics are usually well tolerated but may cause dry mouth, which can be troublesome in older people. Pupillary dilatation, blurred vision and acute glaucoma can occur from escape of droplets from loosely fitting nebulizer masks. Although ECG changes have not been seen in randomized controlled trials of long-acting inhaled anticholinergics, supraventricular tachycardias have been observed in a 5-year randomized controlled trial of ipratropium bromide. Paradoxical bronchoconstriction can occur with inhaled anticholinergics as well as with beta-agonists, but tolerance has not been reported with anticholinergics. Anticholinergic drugs also cause central effects, most notably impairment of cognitive function, and these effects have been noted with inhaled agents. Use of theophylline is limited by its adverse effects, which range from commonly occurring gastrointestinal symptoms to palpitations, arrhythmias and reports of myocardial infarction. Seizures have been reported, but are rare. Theophylline is metabolized primarily by the liver, and commonly interacts with other medications. Its concentration in plasma should be monitored closely, especially in older people. Although many clinical trials have been conducted on bronchodilators in obstructive airways disease, the results of these clinical trials need to be interpreted with caution as older people are often under-represented and subjects with co-morbidities actively excluded from these trials.

Effect of tolterodine on gastrointestinal transit and bowel habits in healthy subjects

Clinical trials and observations suggest that constipation is an uncommon side effect of treating overactive bladder with the muscarinic receptor antagonist tolterodine. Because muscarinic antagonism inhibits gastrointestinal motor activity, we evaluated the effects of tolterodine on bowel habits, gastrointestinal and colonic transit in healthy subjects. In this double-blind study, 36 healthy subjects were randomized to tolterodine extended release (ER, 4 mg daily) or placebo for 6 days. Gastric emptying (GE), small bowel and colonic transit were assessed on days 4-6 by scintigraphy. Bowel habits were recorded by diaries. Tolterodine did not significantly affect half-time for GE (GE t(half)) [116 +/- 6 min (mean +/- SEM) for placebo vs 126 +/- 7 min for tolterodine], small bowel transit measured by colonic filling at 6 h (45 +/- 6% for placebo vs 36 +/- 6% for tolterodine) or the geometric center of colonic transit at 24 h (2.9 +/- 0.2 for placebo vs 2.6 +/- 0.3 for tolterodine). Subjects who received tolterodine had slightly fewer bowel movements (i.e. 1.34 +/- 0.1 stools per day for placebo vs 1.0 +/- 0.1 for tolterodine; P = 0.02 for treatment effect). Tolterodine did not significantly affect stool consistency or ease of defecation. At the therapeutic dose used to treat overactive bladder, tolterodine did not significantly affect gastrointestinal or colonic transit and had minor effects on bowel habits in healthy subjects. Further studies are necessary to elucidate whether these observations are explained by tolterodine effects at muscarinic receptors which stimulate and inhibit gastrointestinal motility.

Contribution of the M3 muscarinic receptors to the vasodilator response to acetylcholine in the human forearm vascular bed

AIMS

Acetylcholine (ACh) is a muscarinic agonist that causes receptor-mediated, endothelium-dependent vasodilatation in the forearm vasculature. Previous indirect evidence suggests this effect may be mediated by muscarinic M(3) receptors. Darifenacin is a recently developed antimuscarinic drug with greater M(3) selectivity, and our main objective was to investigate whether darifenacin affects dose-dependent vasodilatation to ACh in the forearm circulation.

METHODS

Healthy subjects were enrolled in two studies designed to assess the effects of atropine and darifenacin on the forearm blood flow (FBF) response to ACh.

RESULTS

In both studies ACh caused similar dose-dependent vasodilation in the forearm vasculature. In study I (5 subjects), the FBF response to ACh was largely attenuated by pretreatment with the nonselective muscarinic antagonist atropine. In study II (10 subjects), oral administration of darifenacin 15 mg for 1 week significantly reduced the FBF dose-dependent response to ACh 20 microg min(-1) (mean difference from placebo 5.8 [95% confidence interval (CI) 3.1, 8.7] ml min(-1) per 100 ml of forearm volume, P < 0.001) and to ACh 60 microg min(-1)[mean difference from placebo 5.9 (95% CI 3.1, 8.7) ml min(-1) per 100 ml of forearm volume, P < 0.001]. After darifenacin, the AUC of change in FBF from baseline was reduced by almost 50% compared with placebo.

CONCLUSIONS

These results suggest that, in the forearm vasculature, muscarinic M(3) receptors play a major role in ACh-induced endothelium-mediated vasodilatation.

Affective analgesia following muscarinic activation of the ventral tegmental area in rats

Cholinergic stimulation of dopamine neurons in the ventral tegmental area (VTA) underlies activation of the brain reward circuitry. Activation of this circuit is proposed to preferentially suppress the affective reaction to noxious stimulation. Vocalization afterdischarges (VADs) are a validated model of the affective response of rats to noxious tail shock. The antinociceptive action of the acetylcholine agonist carbachol microinjected into the VTA on VAD threshold was compared with its effect on the thresholds of other tail shock-elicited responses (VDS, vocalizations during shock; SMR, spinal motor reflexes). Whereas VADs are organized within the forebrain, VDSs and SMRs are organized at medullary and spinal levels of the neuraxis, respectively. Carbachol (1 microg, 2 microg, and 4 microg) injected into VTA produced dose-dependent increases in VAD and VDS thresholds, although increases in VAD threshold were significantly greater than increases in VDS threshold. Administration of carbachol into VTA failed to elevate SMR threshold. Elevations in vocalization thresholds produced by intra-VTA carbachol were reversed in a dose-dependent manner by local administration of the muscarinic receptor antagonist atropine sulfate (30 microg and 60 microg). These results provide the first demonstration of the involvement of the VTA in muscarinic-induced suppression of pain affect.

PERSPECTIVE

Cholinergic activation of the brain reward circuit produced a preferential suppression of rats' affective reaction to noxious stimulation. The neurobiology that relates reinforcement to suppression of pain affect may provide insights into new treatments for pain and its associated affective disorders.

Sensitization of isolated rat vagal pulmonary sensory neurons by eosinophil-derived cationic proteins

It has been shown that airway exposure to eosinophil-derived cationic proteins stimulated vagal pulmonary C fibers and markedly potentiated their responses to lung inflation in anesthetized rats (Lee LY, Gu Q, Gleich GJ, J Appl Physiol 91: 1318–1326, 2001). However, whether the effects resulted from a direct action of these proteins on the sensory nerves was not known. The present study was therefore carried out to determine the effects of these proteins on isolated rat vagal pulmonary sensory neurons. Our results obtained from perforated whole cell patch-clamp recordings showed that pretreatment with eosinophil major basic protein (MBP; 2 μM, 60 s) significantly increased the capsaicin-evoked inward current in these neurons; this effect peaked ∼10 min after MBP and lasted for >60 min; in current-clamp mode, MBP substantially increased the number of action potentials evoked by both capsaicin and electrical stimulation. Pretreatment with MBP did not significantly alter the input resistance of these sensory neurons. In addition, the sensitizing effect of MBP was completely abolished when its cationic charge was neutralized by mixing with a polyanion, such as low-molecular-weight heparin or poly-l-glutamic or poly-l-aspartic acid, before its delivery to the neurons. Moreover, a similar sensitizing effect was also generated by other eosinophil granule-derived proteins (e.g., eosinophil peroxidase). These results demonstrate a direct, charge-dependent, and long-lasting sensitizing effect of cationic proteins on pulmonary sensory neurons, which may contribute to the airway hyperresponsiveness associated with airway infiltration of eosinophils under pathophysiological conditions.

Muscarinic acetylcholine receptors: mutant mice provide new insights for drug development

Muscarinic acetylcholine receptors (mAChRs), M(1)-M(5), regulate the activity of numerous fundamental central and peripheral functions. The lack of small-molecule ligands that can block or activate specific mAChR subtypes with high selectivity has remained a major obstacle in defining the roles of the individual receptor subtypes and in the development of novel muscarinic drugs. Recently, phenotypic analysis of mutant mouse strains deficient in each of the five mAChR subtypes has led to a wealth of new information regarding the physiological roles of the individual receptor subtypes. Importantly, these studies have identified specific mAChR-regulated pathways as potentially novel targets for the treatment of various important disorders including Alzheimer's disease, schizophrenia, pain, obesity and diabetes.

Human Idiopathic and Neurogenic Overactive Bladders and the Role of M2 Muscarinic Receptors in Contraction

OBJECTIVES

This study examines whether M(2) receptors contribute to direct contraction of the detrusor in human neurogenic and idiopathic overactive bladders.

METHODS

Control detrusor muscle was obtained from patients undergoing cystectomy for bladder cancer, whilst overactive detrusor muscle was obtained from patients undergoing clam cystoplasty for idiopathic or neurogenic detrusor overactivity. The affinities of a range of subtype selective antagonists (DAMP, darifenacin, methoctramine R0-320-6206, and pirenzepine) were obtained in tissue bath experiments by using carbachol as the agonist. These affinity values were then compared with the known affinities for these antagonists at the muscarinic receptor subtypes.

RESULTS

An increased sensitivity to carbachol was observed in both the neurogenic and idiopathic overactive detrusors compared with the control human detrusor. The M(2)-selective antagonists (methoctramine, R0-320-6206) and M(1)-selective antagonist (pirenzepine) had low affinities, whilst the M(3)-selective antagonists (4-DAMP and darifenacin) had high affinities for the human detrusor muscarinic receptor in all three groups of tissues. The affinities (pK(B) values) for the five antagonists were consistent with antagonisms at the M(3) receptor in all three groups; Schild plot analysis indicated an action at this single receptor subtype.

CONCLUSIONS

Contraction mediated by muscarinic receptors is enhanced in idiopathic and neurogenic overactive detrusors compared with control detrusor. The direct contractile response to carbachol is mediated by the M(3) receptor in both human normal and overactive bladders, indicating no change in receptor subtype contribution to contraction in the disease state.

Structure-Function Studies of Allosteric Agonism at M2Muscarinic Acetylcholine Receptors

The M2 muscarinic acetylcholine receptor (mAChR) possesses at least one binding site for allosteric modulators that is dependent on the residues (172)EDGE(175), Tyr(177), and Thr(423). However, the contribution of these residues to actions of allosteric agonists, as opposed to modulators, is unknown. We created mutant M2 mAChRs in which the charge of the (172)EDGE(175) sequence had been neutralized and each Tyr(177) and Thr(423) was substituted with alanine. Radioligand binding experiments revealed that these mutations had a profound inhibitory effect on the prototypical modulators gallamine, alcuronium, and heptane-1,7-bis-[dimethyl-3'-phthalimidopropyl]-ammonium bromide (C7/3-phth) but minimal effects on the orthosteric antagonist [3H]N-methyl scopolamine. In contrast, the allosteric agonists 4-I-[3-chlorophenyl]carbamoyloxy)-2-butynyltrimethylammnonium chloride (McN-A-343), 4-n-butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl] piperidine hydrogen chloride (AC-42), and the novel AC-42 derivative 1-[3-(4-butyl-1-piperidinyl)propyl]-3,4-dihydro-2(1H)-quinolinone (77-LH-28-1) demonstrated an increased affinity or proportion of high-affinity sites at the combined EDGE-YT mutation, indicating a different mode of binding to the prototypical modulators. Subsequent functional assays of extracellular signal-regulated kinase (ERK)1/2 phosphorylation and guanosine 5'-(gamma-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding revealed minimal effects of the mutations on the orthosteric agonists acetylcholine (ACh) and pilocarpine but a significant increase in the efficacy of McN-A-343 and potency of 77-LH-28-1. Additional mutagenesis experiments found that these effects were predominantly mediated by Tyr(177) and Thr(423), rather than the (172)EDGE(175) sequence. The functional interaction between each of the allosteric agonists and ACh was characterized by high negative cooperativity but was consistent with an increased allosteric agonist affinity at the combined EDGE-YT mutant M2 mAChR. This study has thus revealed a differential role of critical allosteric site residues on the binding and function of allosteric agonists versus allosteric modulators of M2 mAChRs.

A novel multivalent ligand that bridges the allosteric and orthosteric binding sites of the M2 muscarinic receptor

THRX-160209 is a potent antagonist at the M(2) muscarinic acetylcholine (ACh) receptor subtype that was designed using a multivalent strategy, simultaneously targeting the orthosteric site and a nearby site known to bind allosteric ligands. In this report, we describe three characteristics of THRX-160209 binding that are consistent with a multivalent interaction: 1) an apparent affinity of the multivalent ligand for the M2 receptor subtype (apparent pK(I) = 9.51 +/- 0.22) that was several orders of magnitude greater than its two monovalent components (apparent pK(I) values < 6.0), 2) specificity of THRX-160209 for the M2 receptor subtype compared with the closely related M4 (apparent pK(I) = 8.78 +/- 0.24) and M1, M3, and M5 receptors (apparent pK(I) values <or= 8.0), and 3) acceleration (>10-fold) of the dissociation rate of tritium-labeled THRX-160209 from M2 receptors by competing monovalent ligands that are known to interact with either the orthosteric site (e.g., atropine) or a well characterized allosteric site (e.g., obidoxime) on the receptor. In complementary kinetic studies assessing allosteric modulation of the receptor, unlabeled THRX-160209 retarded dissociation of [3H]N-methyl scopolamine (NMS). The effects of THRX-160209 on retardation of [3H]NMS dissociation were competitively inhibited by obidoxime, suggesting that obidoxime and THRX-160209 bind to an overlapping region coincident with other typical muscarinic allosteric agents, such as 3-methyl-5-[7-[4-[(4S)-4-methyl-1,3-oxazolidin-2-yl]phenoxy]heptyl]-1,2-oxazole (W84) and gallamine. Taken together, these data are consistent with the hypothesis that THRX-160209 binds in a multivalent manner to the M2 receptor, simultaneously occupying the orthosteric site and a spatially distinct allosteric site.

Involvement of KCNQ2 subunits in [3H]dopamine release triggered by depolarization and pre-synaptic muscarinic receptor activation from rat striatal synaptosomes

KCNQ2 and KCNQ3 subunits encode for the muscarinic-regulated current (I(KM)), a sub-threshold voltage-dependent K+ current regulating neuronal excitability. In this study, we have investigated the involvement of I(KM) in dopamine (DA) release from rat striatal synaptosomes evoked by elevated extracellular K+ concentrations ([K+]e) and by muscarinic receptor activation. [3H]dopamine ([3H]DA) release triggered by 9 mmol/L [K+]e was inhibited by the I(KM) activator retigabine (0.01-30 micromol/L; Emax = 54.80 +/- 3.85%; IC50 = 0.50 +/- 0.36 micromol/L). The I(KM) blockers tetraethylammonium (0.1-3 mmol/L) and XE-991 (0.1-30 micromol/L) enhanced K+-evoked [3H]DA release and prevented retigabine-induced inhibition of depolarization-evoked [3H]DA release. Retigabine-induced inhibition of K+-evoked [3H]DA release was also abolished by synaptosomal entrapment of blocking anti-KCNQ2 polyclonal antibodies, an effect prevented by antibody pre-absorption with the KCNQ2 immunizing peptide. Furthermore, the cholinergic agonist oxotremorine (OXO) (1-300 micromol/L) potentiated 9 mmol/L [K+]e-evoked [3H]DA release (Emax = 155 +/- 9.50%; EC50 = 25 +/- 1.80 micromol/L). OXO (100 micromol/L)-induced [3H]DA release enhancement was competitively inhibited by pirenzepine (1-10 nmol/L) and abolished by the M3-preferring antagonist 4-diphenylacetoxy N-methylpiperidine methiodide (1 micromol/L), but was unaffected by the M1-selective antagonist MT-7 (10-100 nmol/L) or by Pertussis toxin (1.5-3 microg/mL), which uncouples M2- and M4-mediated responses. Finally, OXO-induced potentiation of depolarization-induced [3H]DA release was not additive to that produced by XE-991 (10 micromol/L), was unaffected by retigabine (10 micromol/L), and was abolished by synaptosomal entrapment of anti-KCNQ2 antibodies. Collectively, these findings indicate that, in rat striatal nerve endings, I(KM) channels containing KCNQ2 subunits regulate depolarization-induced DA release and that I(KM) suppression is involved in the reinforcement of depolarization-induced DA release triggered by the activation of pre-synaptic muscarinic heteroreceptors.

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

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

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.

Evaluation of [18F]fluoroxanomeline {5-{4-[(6-[18F]fluorohexyl)oxy]-1,2,5-thiadiazol-3-yl}-1-methyl-1,2,3,6-tetrahydropyridine} in muscarinic knockout mice

INTRODUCTION

We set out to develop a muscarinic M1-selective agonist (based on the structure of the functionally M1-selective xanomeline) that could be radiolabeled with fluorine-18 for use as an imaging agent for positron emission tomography.

METHODS

The radiochemical synthesis was achieved, employing the arts of organic and radiochemical syntheses. Binding selectivity studies employed biodistribution studies, using autoradiography and/or tissue dissection, in wild-type or muscarinic receptor knockout mice.

RESULTS

[(18)F]Fluoroxanomeline shows rather uniform uptake in all mouse brain regions and high specific binding, with a brain-to-blood ratio of 32 at 60 min postinjection. In addition, the specific binding is demonstrated by a 58% to 75% decrease in brain uptake upon coinjection with 5 nmol of unlabeled fluoroxanomeline or xanomeline. Brain uptake studies with [(3)H]xanomeline in muscarinic knockout mice show decreased uptake in M1 (17-34%) and M2 (2-20%) knockout mice compared with control. However, statistical significance was observed in only a few regions. Comparison of [(18)F]fluoroxanomeline in knockout mice showed no difference in M1 or M4 knockout mice but a general decrease in M2 (2-24%) knockout mice. The decrease of [(18)F]fluoroxanomeline uptake in M2 knockout mice reached statistical significance in brain stem, cerebellum, frontal cortex, hippocampus, inferior colliculus and superior colliculus.

CONCLUSION

Although xanomeline displays highly selective M1 agonist activity in functional assays, little selectivity for muscarinic subtype binding was observed for xanomeline or its fluorine-containing analogue, fluoroxanomeline. This emphasizes the lack of correlation between functional selectivity and binding selectivity.

Drugs, their targets and the nature and number of drug targets

What is a drug target? And how many such targets are there? Here, we consider the nature of drug targets, and by classifying known drug substances on the basis of the discussed principles we provide an estimation of the total number of current drug targets.

Dioxane and oxathiane nuclei: suitable substructures for muscarinic agonists

Muscarinic agonists, bearing 1,4-dioxane and 1,4-oxathiane nuclei, were synthesized and tested to evaluate their potency at M(1)-M(4) muscarinic receptor subtypes. The stereochemical relationship between the 2-side chain and the 6-methyl group plays an important role in drug-receptor interaction, since the cis isomers are more potent than the corresponding trans isomers. However, the latter are able to discriminate between the muscarinic receptor subtypes. Among them compound 5b proves particularly interesting, since it selectively activates the ileal M(3) receptor subtype and is devoid of agonist activity at the others.

The predicted 3D structures of the human M1 muscarinic acetylcholine receptor with agonist or antagonist bound

The muscarinic acetylcholine G-protein-coupled receptors are implicated in diseases ranging from cognitive dysfunctions to smooth-muscle disorders. To provide a structural basis for drug design, we used the MembStruk computational method to predict the 3D structure of the human M1 muscarinic receptor. We validated this structure by using the HierDock method to predict the binding sites for three agonists and four antagonists. The intermolecular ligand-receptor contacts at the predicted binding sites agree well with deductions from available mutagenesis experiments, and the calculated relative binding energies correlate with measured binding affinities. The predicted binding site of all four antagonists is located between transmembrane (TM) helices 3, 4, 5, 6, and 7, whereas the three agonists prefer a site involving residues from TM3, TM6, and TM7. We find that Trp 157(4) contributes directly to antagonist binding, whereas Pro 159(4) provides an indirect conformational switch to position Trp 157(4) in the binding site (the number in parentheses indicates the TM helix). This explains the large decrease in ligand binding affinity and signaling efficacy by mutations of Trp 157(4) and Pro 159(4) not previously explained by homology models. We also found that Asp 105(3) and aromatic residues Tyr 381(6), Tyr 404(7), and Tyr 408(7) are critical for binding the quaternary ammonium head group of the ligand through cation-pi interactions. For ligands with a charged tertiary amine head group, we suggest that proton transfer from the ligand to Asp 105(3) occurs upon binding. Furthermore, we found that an extensive aromatic network involving Tyr 106(3), Trp 157(4), Phe 197(5), Trp 378(6), and Tyr 381(6) is important in stabilizing antagonist binding. For antagonists with two terminal phenyl rings, this aromatic network extends to Trp 164(4), Tyr 179(extracellular loop 2), and Phe 390(6) located at the extracellular end of the TMs. We find that Asn 382(6) forms hydrogen bonds with selected antagonists. Tyr381(6) and Ser 109(3) form hydrogen bonds with the ester moiety of acetylcholine, which binds in the gauche conformation.

Effects of rat sinoaortic denervation on the vagal responsiveness and expression of muscarinic acetylcholine receptors

We studied heart rate (HR) responses to vagal electrical stimulation (VES) and the expression of muscarinic acetylcholine receptors (mAChRs) in the rat atria 1 day (SADa) and 20 days (SADc) after sinoaortic denervation (SAD). Arterial blood pressure (BP) was recorded in conscious, unrestrained rats and during vagal electrical stimulation of the vagus nerve. In the acute phase, SADa rats had hypertension, tachycardia, and increased blood pressure lability. In the chronic phase, heart rate and blood pressure in SADc rats returned to normal whereas blood pressure lability remained increased. VES produced a frequency-dependent bradycardia that was higher in SADa and SADc groups. Binding experiments with [H] N-methylscopolamine showed that in the chronic phase of SAD mAChRs density (SADc = 412.2 +/- 28.64, SADa = 273.38 +/- 48.37 and CTR = 241.5 +/- 25.35 fmol/mg of protein, P < 0.05) and affinity increased in SADc rats (reduced dissociation constant: SADc = 0.45 +/- 0.05, SADa = 1.01+/-0.26, and CTR = 0.98 +/- 0.12 mM, P < 0.05). Our study provides evidence that vagal hyperresponsiveness coexists with increased sympathetic activity in SADa rats without a concomitant increase in mAChRs density or affinity, suggesting that complex mechanisms might modulate the "accentuated antagonism" observed in the acute SAD phase. However, SADc rats had increased bradycardia to VES, increased affinity, and upregulation of mAChRs in the atria. Our results show that, 20 days after SAD in the rat, functional and cellular adaptations occur in the cardiac parasympathetic efferent pathway that may contribute to other regulatory mechanisms to compensate for cardiovascular changes provoked by baroreceptor arch disruption.

Muscarinic acetylcholine receptor subtypes in the rat seminal vesicle

The aim of the present study was to identify the muscarinic acetylcholine receptor (mAChR) mRNA subtypes in the rat seminal vesicle. Furthermore, the mAChR subtypes involved in the contraction of the seminal vesicle were also explored. Reverse transcriptase-polymerase chain reaction (PCR) was performed and five PCR products corresponding to M1-M5 mAChR mRNA subtypes were detected in this tissue. Functional pharmacological studies indicated that the rank order of mAChR antagonists in blocking the contractile effects of carbachol was p-fluoro-hexahydro-sila-difenidol (pF-HHSiD) >> tropicamide > methoctramine = pirenzepine. This antagonist profile indicates that M3 mAChR subtype is predominantly involved in the seminal vesicle contraction. Furthermore, immunohistochemical studies confirmed the presence of the M3 mAChR subtype in the smooth muscle layers. M2 mAChR subtype was also immunolocalized in smooth muscle cells and may be involved in the contraction of this tissue. The presence of M2 and M3 mAChR subtypes in the epithelial cells suggests that these receptors could be involved in the protein secretion. Taken together, the cholinergic neurotransmitter may be a factor controlling contractility and protein secretion in this tissue.

M1 Receptors Play a Central Role in Modulating AD-like Pathology in Transgenic Mice

We investigated the therapeutic efficacy of the selective M1 muscarinic agonist AF267B in the 3xTg-AD model of Alzheimer disease. AF267B administration rescued the cognitive deficits in a spatial task but not contextual fear conditioning. The effect of AF267B on cognition predicted the neuropathological outcome, as both the Abeta and tau pathologies were reduced in the hippocampus and cortex, but not in the amygdala. The mechanism underlying the effect on the Abeta pathology was caused by the selective activation of ADAM17, thereby shifting APP processing toward the nonamyloidogenic pathway, whereas the reduction in tau pathology is mediated by decreased GSK3beta activity. We further demonstrate that administration of dicyclomine, an M1 antagonist, exacerbates the Abeta and tau pathologies. In conclusion, AF267B represents a peripherally administered low molecular weight compound to attenuate the major hallmarks of AD and to reverse deficits in cognition. Therefore, selective M1 agonists may be efficacious for the treatment of AD.

Highly Chiral Muscarinic Ligands: The Discovery of (2S,2‘R,3‘S,5‘R)-1-Methyl-2-(2-methyl-1,3-oxathiolan-5-yl)pyrrolidine 3-sulfoxide Methyl Iodide, a Potent, Functionally Selective, M2 Partial Agonist

By further steric complication of previously studied highly chiral muscarinic agonists, we have obtained a small chiral library of enantiomeric and diasteromeric 1-methyl-2-(2-methyl-1,3-oxathiolan-5-yl)pyrrolidine 3-sulfoxides. Binding studies on cloned human muscarinic receptors expressed in CHO cells show that the introduction of a fourth stereogenic center gives undetectable affinity for hm1, hm3, hm4 and hm5 subtypes while leaving a quite modest affinity only for hm2 subtypes. However, functional studies on model M1-M4 muscarinic tissues have shown that three compounds of the series [(-)-5, (-)-7, (+)-8] are endowed with functional activity and behave as M2 selective partial agonists. Among them, compound (2S,2'R,3'S,5'R)-1-methyl-2-(2-methyl-1,3-oxathiolan-5-yl)pyrrolidine 3-sulfoxide methyl iodide [(+)-8] is particularly interesting, as it is a potent partial agonist on guinea pig atrium (force) (M2; pD2=7.65, alpha=0.41) while being a poor antagonist on M1, M3, and M4 model tissues (pKb<5).

Potent anti-muscarinic activity in a novel series of quinuclidine derivatives

The synthesis and biological evaluation of a novel family of M(3) muscarinic antagonists are described. A systematic modification of the substituents to a novel alkyne-quinuclidine scaffold yielded original compounds displaying potent in vitro anticholinergic properties.

In vitro effects of bethanechol on equine gastrointestinal contractility and functional characterization of involved muscarinic receptor subtypes

The goal of this study was to investigate the effect of bethanechol (BeCh) on contractility patterns of smooth muscle preparations of equine duodenum descendens, jejunum, caecum and pelvic flexure in vitro. Concentration-response relationships were developed for BeCh using in vitro assays with and without preincubation of muscarinic (M) receptor antagonists for M2 and M3 receptors. BeCh induced a significant, concentration-dependent increase in contractile response in equine intestine in specimens with circular orientation. The maximal effect was largest for jejunal specimens with no difference in EC50 within the different locations investigated. The M2 antagonist, AF-DX 116, caused a rightward shift of the concentration-response curve and the M3 antagonist, 4-DAMP (1,1-dimethyl-4-diphenylacetoxypiperidinium iodide), almost completely inhibited the effect of BeCh over the entire concentration-response curve. These data provide evidence that, although the effect of BeCh is predominantly mediated by M3 receptors, M2 muscarinic receptors also play a role in BeCh-induced contraction in specimens of equine intestine. The involvement of other muscarinic receptor subtypes cannot be excluded. Further studies are necessary to understand the effect of BeCh in vivo including diseased animals.

Dopamine-glutamate reciprocal modulation of release and motor responses in the rat caudate-putamen and nucleus accumbens of "intact" animals

Functional interactions between dopaminergic neurotransmission and glutamatergic neurotransmission are well known to play a crucial integrative role in the striatum, the major input structure of the basal ganglia now widely recognized to contribute to the control of motor activity and movements but also to the processing of cognitive and limbic functions. However, the nature of these interactions is still a matter of debate and controversy. This review (1) summarizes anatomical data on the distribution of dopaminergic and glutamatergic receptors in the striatum-accumbens complex, (2) focuses on the dopamine-glutamate interactions in the modulation of each other's release in the striatum-accumbens complex, and (3) examines the dopamine-glutamate interactions in the entire striatum involved in the control of locomotor activity. The effects of dopaminergic and glutamatergic receptor selective agonists and antagonists on dopamine and glutamate release as well on motor responses are analyzed in the entire striatum, by reviewing both in vitro and in vivo data. Regarding in vivo data, only findings from focal injections studies in the nucleus accumbens or the caudate-putamen of "intact" animals are reviewed. Altogether, the available data demonstrate that dopamine and glutamate do not uniformly interact to modulate each others' release and postsynaptic modulation of striatal output neurons. Depending on the receptor subtypes involved, interactions between dopaminergic and glutamatergic transmission vary as a multiple and complex combination of tonic, phasic, facilitatory, and inhibitory properties.

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.

Expression and localization of muscarinic acetylcholine receptor subtypes in rat efferent ductules and epididymis

The expression of muscarinic acetylcholine receptor (mAChR) subtypes (M(1)-M(5)) was studied in the rat efferent ductules and epididymis at the mRNA and protein levels. The relative abundance of each mAChR transcript subtype differed depending on the tissue and the epididymal region analyzed. The M(1) mAChR mRNA level was more abundant in the efferent ductules than in the caput and cauda of the epididymis. The M(2) mAChR mRNA level was similar between the efferent ductules and caput of the epididymis and higher in the cauda region. The M(3) mAChR mRNA level was low in the efferent ductules and caput of the epididymis, but high levels were detected in the cauda region. mRNAs for M(4) and M(5) mAChRs were not detected in these tissues. Our studies indicated a variable degree of immunostaining for each mAChR subtype in a cell-type and tissue-specific pattern. M(1) mAChR was detected over the efferent ductule epithelium. M(2) and M(3) mAChRs were observed in the apical region of the ciliated cells. Apical and narrow cells of the initial segment showed distinct staining by M(1) antibody, whereas a supranuclear reaction was noted in the principal cells of the caput of the epididymis. In addition, staining for M(1) and M(2) mAChRs was visible in the apical membrane of some epithelial cells of the cauda region. M(3) mAChR was detected in the peritubular smooth muscle of the efferent ductules and epididymis. Functional studies suggested the involvement of this subtype in epididymal tubule contraction. Thus, the cell-specific expression of the various mAChR subtypes in the efferent ductules and epididymis suggests that these receptors play a role in the modulation of luminal fluid composition and smooth muscle contraction.

Modulation of memory and visuospatial processes by biperiden and rivastigmine in elderly healthy subjects

RATIONALE

The central cholinergic system is implicated in cognitive functioning. The dysfunction of this system is expressed in many diseases like Alzheimer's disease, dementia of Lewy body, Parkinson's disease and vascular dementia. In recent animal studies, it was found that selective cholinergic modulation affects visuospatial processes even more than memory function.

OBJECTIVE

In the current study, we tried to replicate those findings. In order to investigate the acute effects of cholinergic drugs on memory and visuospatial functions, a selective anticholinergic drug, biperiden, was compared to a selective acetylcholinesterase-inhibiting drug, rivastigmine, in healthy elderly subjects.

METHODS

A double-blind, placebo-controlled, randomised, cross-over study was performed in 16 healthy, elderly volunteers (eight men, eight women; mean age 66.1, SD 4.46 years). All subjects received biperiden (2 mg), rivastigmine (3 mg) and placebo with an interval of 7 days between them. Testing took place 1 h after drug intake (which was around Tmax for both drugs). Subjects were presented with tests for episodic memory (wordlist and picture memory), working memory tasks (N-back, symbol recall) and motor learning (maze task, pursuit rotor). Visuospatial abilities were assessed by tests with high visual scanning components (tangled lines and Symbol Digit Substitution Test).

RESULTS

Episodic memory was impaired by biperiden. Rivastigmine impaired recognition parts of the episodic memory performance. Working memory was non-significantly impaired by biperiden and not affected by rivastigmine. Motor learning as well as visuospatial processes were impaired by biperiden and improved by rivastigmine.

CONCLUSIONS

These results implicate acetylcholine as a modulator not only of memory but also of visuospatial abilities.

Evaluation of levetiracetam effects on pilocarpine-induced seizures: Cholinergic muscarinic system involvement

Levetiracetam (LEV) is a new antiepileptic drug effective as adjunctive therapy for partial seizures. It displays a unique pharmacological profile against experimental models of seizures, including pilocarpine-induced seizures in rodents. Aiming to clarify if anticonvulsant activity of LEV occurs due to cholinergic alterations, adult male mice received LEV injections before cholinergic agonists' administration. Pretreatment with LEV (30-200 mg/kg, i.p.) increased the latencies of seizures, but decreased status epilepticus and death on the seizure model induced by pilocarpine, 400 mg/kg, s.c. (P400). LEV (LEV200, 200 mg/kg, i.p.) pretreatment also reduced the intensity of tremors induced by oxotremorine (0.5 mg/kg, i.p). [3H]-N-methylscopolamine-binding assays in mice hippocampus showed that LEV200 pretreatment reverts the downregulation on muscarinic acetylcholine receptors (mAChR), induced by P400 administration, bringing back these density values to control ones (0.9% NaCl, i.p.). However, subtype-specific-binding assays revealed that P400- and LEV-alone treatments result in M1 and M2 subtypes decrease, respectively. The agonist-like behavior of LEV on the inhibitory M2 mAChR subtype, observed in this work, could contribute to explain the reduction on oxotremorine-induced tremors and the delay on pilocarpine-induced seizures, by an increase in the attenuation of neuronal activity mediated by the M1 receptors.

Effects of testosterone on muscarinic acetylcholine receptors in the rat epididymis

The effect of testosterone on the expression of muscarinic acetylcholine receptor (mAChR) subtypes was studied in the rat epididymis, at mRNA and protein level. The rat androgen status was monitored by measuring plasma testosterone level and caput and cauda epididymis wet weight. Ribonuclease protection assay (RPA) and [3H]quinuclidinyl benzilate ([3H]QNB) binding assay were performed in the caput and cauda epididymis from control (50-day old), castrated, castrated and treated with testosterone and sexually immature (30-day old) rats. The expression of each mAChR transcript subtype differed depending on the epididymal region analyzed and rat testosterone and/or testicular factors status. In control rats, RPA showed the presence of mRNA for M1, M2 and M3 mAChR in the caput and cauda epididymis. The abundance of m2 and m3 transcripts in the cauda was higher than that in the caput epididymis. Low amount of m1 transcript was observed in both regions. Orchidectomy increased m1 mRNA amount in the caput and cauda epididymis when compared to control rats, an effect slightly modified by testosterone replacement. Although orchidectomy down-regulated the level of m2 transcript in both epididymal regions, castration significantly increased m3 mRNA amount in the caput region. These effects on m2 and m3 transcripts were prevented by testosterone replacement to castrated rats. Similar abundance of m3 transcript, however, was detected in the cauda epididymis of all experimental group tested. [3H]QNB binding studies revealed that orchidectomy down-regulated the number of mAChR detected in both epididymal regions, an effect also prevented by testosterone replacement. Thus, testosterone and/or testicular factors may play a role in the regulation of mAChR expression in the rat epididymis.

Medikament�se Therapie der weiblichen Harninkontinenz

Drug treatment for female urinary incontinence requires a thorough knowledge of the differential diagnosis and pathophysiology of incontinence as well as of the pharmacological agents employed. Pharmacotherapy has to be tailored to suit the incontinence subtype and should be carefully balanced according to efficacy and side effects of the drug. Women with urge incontinence require treatment that relaxes or desensitizes the bladder (antimuscarinics, estrogens, alpha-blockers, beta-mimetics, botulinum toxin A, resiniferatoxin, vinpocetine), whereas patients with stress incontinence need stimulation and strengthening of the pelvic floor and external sphincter (alpha-mimetics, estrogens, duloxetine). Females with overflow incontinence need reduction of outflow resistance (baclofen, alpha-blockers, intrasphincteric botulinum toxin A) and/or improvement of bladder contractility (parasympathomimetics). If nocturia or nocturnal incontinence are the major complaints, control of diuresis is obtained by administration of the ADH analogue desmopressin. Future developments will help to further optimize the pharmacological therapy for female urinary incontinence.

Involvement of the intralaminar parafascicular nucleus in muscarinic-induced antinociception in rats

The thalamic contribution to cholinergic-induced antinociception was examined by microinjecting the acetylcholine (ACh) agonist carbachol into the intralaminar nucleus parafascicularis (nPf) of rats. Pain behaviors organized at spinal (spinal motor reflexes), medullary (vocalizations during shock), and forebrain (vocalization afterdischarges, VADs) levels of the neuraxis were elicited by noxious tailshock. Carbachol (0.5, 1, and 2 microg/side) administered into nPf produced dose-dependent elevations of vocalization thresholds, but failed to elevate spinal motor reflex threshold. Injections of carbachol into adjacent sites dorsal or ventral to nPf failed to alter vocalization thresholds. Elevations in vocalization thresholds produced by intra-nPf carbachol were reversed in a dose-dependent manner by local administration of the muscarinic receptor antagonist atropine (30 and 60 microg/side). These results provide the first direct evidence supporting the involvement of the intralaminar thalamus in muscarinic-induced antinociception. Results are discussed in terms of the contribution of nPf to the processing of the affective dimension of pain.

M1 muscarinic receptor activation protects neurons from β-amyloid toxicity. A role for Wnt signaling pathway

Amyloid-beta-peptide (Abeta) deposits are one of the hallmark features of Alzheimer's disease. Signal transduction alterations are implicate in the neuronal responses to Abeta, which include neurotransmitter systems and pathways involved in the maintenance of the nervous system. In this context, we have recently found that Abeta-neurotoxicity triggers a loss of Wnt signaling. We report here that M1-acetylcholine-muscarinic-receptor (mAChR) activation protects neurons from Abeta-toxicity. Concomitant with this effect, a modulation of the Wnt signaling was observed. M1 mAChR activation inhibits glycogen-synthase-kinase-3beta (GSK-3beta) activity, stabilizes cytoplasmic and nuclear beta-catenin, and induces the expression of the Wnt target genes engrailed and cyclin-D1, reverting the switch off of the Wnt pathway caused by Abeta-toxicity. Neurons from mice that overexpress GSK-3beta allow us to establish that M1 mAChR stimulation leads to GSK-3beta inactivation. We conclude that the cross-talk between the muscarinic signaling and Wnt components underlie the neuroprotective effect of the M1 mAChR activation.

Carbamoylcholine homologs: synthesis and pharmacology at nicotinic acetylcholine receptors

In a recent study, racemic 3-(N,N-dimethylamino)butyl-N,N-dimethylcarbamate (1) was shown to be a potent agonist at neuronal nicotinic acetylcholine receptors with a high selectivity for nicotinic over muscarinic acetylcholine receptors [Mol. Pharmacol. 64 (2003) 865-875]. Here we present the synthesis and pharmacological characterization of a series of analogs of, where the methyl group at C-3 has been replaced by different alkyl substituents. Ring systems have been incorporated into the carbon backbone of some of the molecules, or the amino group has been build into ring systems. Furthermore, the (+)- and (-)-enantiomers of have been separated, and X-ray crystallography has revealed that (-)-1 possesses (S)-configuration. The compounds have been characterized pharmacologically at recombinant nicotinic receptor subtypes. The structure-activity relationship study has provided valuable insight into the mode of interactions of and its analogs with neuronal nicotinic acetylcholine receptors.

Effects of carbachol on rat Sertoli cell proliferation and muscarinic acetylcholine receptors regulation: an in vitro study

The aim of the present work was to study the effect of muscarinic agonist on cell proliferation and muscarinic acetylcholine receptors (mAChRs) regulation in rat Sertoli cells. Primary cultures of Sertoli cells were obtained from 8-day and 15-day old male Wistar rats. In proliferation assays, [methyl-3H]thymidine incorporation in Sertoli cells from 8-day and 15-day old rats reached a plateau after 60 min of carbachol incubation and decreased after 120 min of agonist incubation. Binding studies with [N-Methyl-3H]scopolamine ([3H]NMS) indicated a rapid loss of cell surface mAChRs when Sertoli cells from 15-day old rats were incubated with carbachol at 35 degrees C for 2 min. This effect was temperature-dependent. When the incubation of the cells was prolonged at 35 degrees C or at 4 degrees C, after the agonist had been washed away, 94% of mAChRs were present in the cell surface after 120 min incubation at 35 degrees C. At 4 degrees C, however, a low percentage of mAChRs was detected in the cell surface. In the presence of cycloheximide, the recycling of mAChRs to the cell surface was not changed, suggesting that the appearance of mAChRs on cell surface was not dependent on de novo receptor synthesis. In conclusion, our studies indicate that the activation of mAChRs may play a role in rat Sertoli cell proliferation. These receptors may be under regulation (internalization and recycling) when cells are exposed to muscarinic cholinergic agonist.

Direct and indirect mechanisms regulating secretion of glucagon-like peptide-1 and glucagon-like peptide-2

The proglucagon-derived peptide family consists of three highly related peptides, glucagon and the glucagon-like peptides GLP-1 and GLP-2. Although the biological activity of glucagon as a counter-regulatory hormone has been known for almost a century, studies conducted over the past decade have now also elucidated important roles for GLP-1 as an antidiabetic hormone, and for GLP-2 as a stimulator of intestinal growth. In contrast to pancreatic glucagon, the GLPs are synthesized in the intestinal epithelial L cells, where they are subject to the influences of luminal nutrients, as well as to a variety of neuroendocrine inputs. In this review, we will focus on the complex integrative mechanisms that regulate the secretion of these peptides from L cells, including both direct and indirect regulation by ingested nutrients.

Muscarinic (M) Receptors in Coronary Circulation

OBJECTIVE

Determining the role of specific muscarinic (M) receptor subtypes mediating responses to acetylcholine (ACh) has been limited by the specificity of pharmacological agents. Deletion of the gene for M5 receptors abolished response to ACh in cerebral blood vessels but did not affect dilation of coronary arteries. The goal of this study was to determine the M receptors mediating responses to ACh in coronary circulation using mice deficient in M2 or M3 receptors (M2-/-, M3-/-, respectively).

METHODS AND RESULTS

Coronary arteries from respective wild-type, M2-/-, or M3-/- mice were isolated, cannulated, and pressurized. Diameter was measured with video microscopy. After preconstriction with U46619, ACh produced dose-dependent dilation of coronary arteries that was similar in wild-type and M2-/- mice. In contrast, dilation of coronary arteries from M3-/- mice to ACh was reduced by approximately 80% compared with wild type. The residual response to ACh was atropine insensitive. Relaxation of coronary arteries to other stimuli was similar in M2-/- and M3-/- mice. Similar results were obtained in aorta rings.

CONCLUSIONS

These findings provide the first direct evidence that relaxation to ACh in coronary circulation is mediated predominantly by activation of M3 receptors.

Necrotizing enterotyphlocolitis in dogs treated with a potent antimuscarinic

To assess the safety of a potent muscarinic receptor antagonist (antimuscarinic) for human clinical testing, repeat-dose oral toxicity studies were conducted in the Beagle dog. Treatment resulted in unexpected gastrointestinal system effects that were likely mediated by the exaggerated pharmacologic effects of the antimuscarinic on intestinal motility. Dogs developed profound anorexia followed by severe bloody diarrhea, and at necropsy, the intestinal tract was distended, filled with red-tinged fluid, and the mucosa was reddened. Histologically, intestinal lesions consisted of severe epithelial necrosis and mucosal atrophy localized predominantly to the ileum and large intestine. Feces from dogs with diarrhea were culture-positive for Clostridium perfringens (CP), suggesting that CP might be the etiologic agent; however, the intestinal lesions were not consistent with histologic findings reported in dogs with hemorrhagic canine enteritis.

Emerging Pharmacologic Approaches for the Treatment of Lower Urinary Tract Disorders

Lower urinary tract disorders include disorders affecting continence (stress urinary incontinence, urge urinary incontinence, and benign prostatic hyperplasia) and male erectile dysfunction. Although none of these conditions are fatal, they affect overall quality of life. Throughout modern medicine the treatment of these conditions was limited to psychological counseling or surgical intervention. In recent years, research defining the physiological mechanisms of continence and male sexual function has aided in the pharmacologic design of approaches to these conditions. These agents can act both centrally or on the peripheral genitourinary smooth muscle to alleviate disease symptoms. Incontinence is primarily treated with agents that act directly on the bladder smooth muscle such as muscarinic antagonists. However, afferent blockade to attenuate the spinalbulbospinal reflex pathway including mixed norepinephrine/serotonin reuptake inhibitors may provide a key breakthrough. Erectile dysfunction treatment has been revolutionized via the discovery of the nitric oxide pathway and phosphodiesterase 5 inhibitors. New peripheral targets as well as centrally acting agents represent potential emerging therapies. In this review, the pharmacologic basis of treatment of these disorders is discussed with special emphasis on emerging new therapeutics.

Differential agonist-induced regulation of human M2 and M3 muscarinic receptors

We have compared the regulation of M(2) and M(3) muscarinic receptors heterologously expressed in HEK-293 cells upon long-term exposure towards the agonist carbachol. Carbachol time- and concentration-dependently reduced M(2) receptor density with a maximum reduction of about 60%. Treatment with 1mM carbachol for 24hr was accompanied by desensitisation of carbachol-induced Ca(2+) elevations (maximum response reduced by 70%) but not by alterations in the expression of various G-protein alpha-subunits. Consistently, heterologous desensitisation of Ca(2+) elevations by the purinergic receptor agonist ATP or by sphingosine-1-phosphate was not detected. In contrast, carbachol time- and concentration-dependently up-regulated M(3) receptors with maximum increases to about 350% of control values. The up-regulation was fully blocked by cycloheximide indicating that it was dependent on protein synthesis. Concomitant with the up-regulation of the M(3) receptor was a reduction in the expression of the alpha-subunit of G(q/11). The net effect of these two opposite regulatory mechanisms was a lack of alteration of carbachol-stimulated Ca(2+) elevation. However, the reduction of G(q/11) was accompanied by a heterologous desensitisation of Ca(2+) elevations by ATP and sphingosine-1-phosphate. Levels of M(2) and M(3) receptor mRNA as assessed by real-time PCR were not significantly altered by carbachol exposure for either receptor, suggesting that alterations of mRNA stability did not contribute to the observed changes in receptor number. We conclude that M(2) and M(3) receptor expression within the same cell undergoes differential agonist-induced regulation being accompanied by distinct regulation of G-protein expression leading to differential effects on signal transduction by other receptor systems.

Antimuscarinics for treatment of overactive bladder

For many years, antimuscarinic drugs have been the first-line pharmacological treatment for urgency, frequency, and urge incontinence, all symptoms of the disorder termed overactive bladder. Antimuscarinic treatment is not always effective and is associated with side-effects that limit its clinical use. The clinical significance of the effects of antimuscarinic drugs has been questioned lately. In this review, the rationale for the use of these drugs in the management of overactive bladder is re-examined and the results of treatment are discussed. I conclude that these drugs are the only treatment with undisputed effectiveness in the treatment of overactive bladder. They may not be the perfect treatment for all patients with this disorder, but their value for individual patients should not be underestimated. Further clinical trials with improvement in quality of life as the primary endpoint are needed and may give a fair reflection of the clinical value of antimuscarinic drugs.

M2 muscarinic receptors in pontine reticular formation of C57BL/6J mouse contribute to rapid eye movement sleep generation

Microinjecting the acetylcholinesterase inhibitor neostigmine into the pontine reticular formation of C57BL/6J (B6) mouse causes a rapid eye movement (REM) sleep-like state. This finding is consistent with similar studies in cat and both sets of data indicate that the REM sleep-like state is caused by increasing levels of endogenous acetylcholine (ACh). Muscarinic cholinergic receptors have been localized to the pontine reticular formation of B6 mouse but no previous studies have examined which of the five muscarinic receptor subtypes participate in cholinergic REM sleep enhancement. This study examined the hypothesis that M2 receptors in pontine reticular formation of B6 mouse contribute to the REM sleep-like state caused by pontine reticular formation administration of neostigmine. B6 mice (n=13) were implanted with electrodes for recording states of sleep and wakefulness and with microinjection cannulae aimed for the pontine reticular formation. States of sleep and wakefulness were recorded for 4 h following pontine reticular formation injection of saline (control) or neostigmine. Experiments designed to gain insight into the muscarinic receptor subtypes mediating REM sleep enhancement involved pontine reticular formation administration of neostigmine after pertussis toxin, neostigmine after methoctramine, and neostigmine after pirenzepine. Pertussis toxin was used to block effects mediated by M2 and M4 receptors. Methoctramine was used to block M2 and M4 receptors, and pirenzepine was used to block M1 and M4 receptors. Pertussis toxin and methoctramine significantly decreased the neostigmine-induced REM sleep-like state. In contrast, pretreatment with pirenzepine did not significantly decrease the REM sleep-like state caused by neostigmine. These results support the interpretation that M2 receptors in the pontine reticular formation of B6 mouse contribute to the generation of REM sleep.

Carbamoylcholine homologs: novel and potent agonists at neuronal nicotinic acetylcholine receptors

The classic muscarinic acetylcholine receptor (mAChR) agonist carbamoylcholine (carbachol) does not seem to be the most obvious lead for the development of selective ligands at nicotinic acetylcholine receptors (nAChRs). In the past, however, N-methylations of carbachol have provided N-methylcarbamoylcholine and N,N-dimethylcarbamoylcholine (DMCC), which predominantly display nicotinic activity. In this study, 12 homologous analogs of DMCC and its corresponding tertiary amine, N,N-dimethylcarbamoyl-N,N-dimethylaminoethanol, were synthesized and their binding affinities to native mAChR and nAChR sites estimated. One of the compounds in the series, 3-N,N-dimethylaminobutyl-N,N-dimethylcarbamate (7), displayed low nanomolar binding affinity to nAChRs and a 400-fold selectivity for nAChRs over mAChRs. Hence, a new series of compounds was synthesized in which alkyl and aryl groups and different ring systems were introduced in the carbamate moiety of 7. In a [3H]epibatidine binding assay, the Ki values of 7 and its analogs at rat alpha2beta2, alpha4beta2, alpha2beta4, alpha3beta4, and alpha4beta4 nAChRs, stably expressed in mammalian cell lines, ranged from low nanomolar to midmicromolar concentrations, whereas all of the compounds displayed weak binding to an alpha7/5-HT3 chimera and to native mAChRs. Compound 7 and its analogs were determined to be agonists at the alpha3beta4 nAChR subtype. This series includes the most potent and selective nicotinic agonists structurally derived from ACh to date. Furthermore, the compounds are tertiary amines, implying some advantages in terms of bioavailability pertinent to future in vivo pharmacological studies. Finally, observations made in the study hold promising perspectives for future development of ligands selective for specific nAChR subtypes.

Activation of M1 muscarinic receptors triggers transmitter release from rat sympathetic neurons through an inhibition of M-type K+ channels

Acetylcholine has long been known to excite sympathetic neurons via M1 muscarinic receptors through an inhibition of M-currents. Nevertheless, it remained controversial whether activation of muscarinic receptors is also sufficient to trigger noradrenaline release from sympathetic neurons. In primary cultures of rat superior cervical ganglia, the muscarinic agonist oxotremorine M inhibited M-currents with half-maximal effects at 1 microM and induced the release of previously incorporated [3H]noradrenaline with half-maximal effects at 10 microM. This latter action was not affected by the nicotinic antagonist mecamylamine which, however, abolished currents through nicotinic receptors elicited by high oxotremorine M concentrations. Ablation of the signalling cascades linked to inhibitory G proteins by pertussis toxin potentiated the release stimulating effect of oxotremorine M, and the half-maximal concentration required to stimulate noradrenaline release was decreased to 3 microM. Pirenzepine antagonized the inhibition of M-currents and the induction of release by oxotremorine M with identical apparent affinity, and both effects were abolished by the muscarinic toxin 7. These results indicate that one muscarinic receptor subtype, namely M1, mediates these two effects. Retigabine, which enhances M-currents, abolished the release induced by oxotremorine M, but left electrically induced release unaltered. Moreover, retigabine shifted the voltage-dependent activation of M-currents by about 20 mV to more negative potentials and caused 20 mV hyperpolarisations of the membrane potential. In the absence of retigabine, oxotremorine M depolarised the neurons and elicited action potential discharges in 8 of 23 neurons; in its presence, oxotremorine M still caused equal depolarisations, but always failed to trigger action potentials. Action potential waveforms caused by current injection were not affected by retigabine. These results indicate that the inhibition of M-currents is the basis for the stimulation of transmitter release from sympathetic neurons via M1 muscarinic receptors.

N-Homolupinanoyl and N-(ω-lupinylthio)alkanoyl derivatives of some tricyclic systems as ligands for muscarinic M1 and M2 receptor subtypes

A set of N-homolupinanoyl- and N-(omega-lupinylthio)alkanoyl derivatives of tricyclic systems (as phenothiazine, iminodibenzyl and dihydropyridobenzodiazepinone) has been prepared and tested for affinity for rat muscarinic M(1) and M(2) receptor subtypes labeled with [3H]pirenzepine and [3H]AF-DX 384. Good affinity for both M(1) and M(2) subtypes was displayed by most compounds, often with nanomolar K(i) values, which for lupinylthiopropionyl- and lupinylthiobutyryl-phenothiazines (13-16) were comparable to those of pirenzepine and methoctramine, respectively. However, only moderate selectivity for one or the other subtype was seen.