Muscarinic acetylcholine receptor subtypes in the human iris

Management of use of urinary antimuscarinics and alpha blockers for benign prostatic hyperplasia in older adults at risk of falls: a clinical review

PURPOSE

We aimed to outline the existing information and the underlying mechanisms of risk of falls associated with the use of urinary antimuscarinics for overactive bladder (OAB) or alpha-blockers for benign prostatic hyperplasia (BPH) in older adults. In addition, we aimed to provide assistance to clinicians in decision-making about (de-)prescribing these drugs in older adults.

METHODOLOGY

Based on a literature search in PubMed and Google Scholar, we reviewed the literature, and identified additional relevant articles from reference lists, with an emphasis on the most commonly prescribed drugs in OAB and BPH in older patients. We discussed the use of bladder antimuscarinics and alpha-blockers, their potential side effects related to falls, and the deprescribing of these drugs in older adults.

RESULTS

Urinary urgency or incontinence and lower urinary tract symptoms due to untreated OAB and BPH contribute to fall risk. On the other hand, the use of bladder antimuscarinics and alpha-blockers is also related to fall risk. They contribute to (or cause) falling through dizziness, somnolence, visual impairment, and orthostatic hypotension while they differ in their side-effect profiles regarding these problems. Falls are common and can cause a remarkable amount of morbidity and mortality. Thus, preventive measures should be taken to lower the risk. If the clinical condition allows, withdrawal of bladder antimuscarinics and alpha-blockers is recommended in fall-prone older adults. There are practical resources and algorithms that guide and assist clinicians in deprescribing these drug groups.

CONCLUSIONS

The decision to prescribe or deprescribe these treatments in patients at high risk of falls should be individualized. In addition to explicit tools that are helpful for clinical decision-making in (de-)prescribing these drugs, STOPPFall (a recently developed expert-based decision aid specifically aiming to prevent falls) is present to assist prescribers in attaining decisions.

Tracking Eye Movements for Diagnosis in Myasthenia Gravis: A Comprehensive Review

BACKGROUND

Around 60%--75% of myasthenia gravis (MG) patients initially present with nonspecific ocular symptoms. Failed recognition of these symptoms may delay the diagnosis of MG up to 5 years or more, leading to a reduced likelihood of remission and increased morbidity. Current diagnostic tests are either poorly sensitive for patients presenting with ocular symptoms alone or are time consuming, invasive, require a high level of technical expertise, and generally are universally difficult to obtain. This review will explore quantitative eye and pupil tracking as a potential noninvasive, time-effective, and less technically demanding alternative to current diagnostic tests of MG.

EVIDENCE ACQUISITION

Comprehensive literature review.

RESULTS

Thirty-two publications using oculography for the diagnosis of MG and 6 studies using pupillometry were evaluated. In MG patients, extra ocular muscle fatigue was evident in reports of intersaccadic, intrasaccadic and postsaccadic abnormalities, changes in optokinetic nystagmus, slow eye movements, disconjugate saccades, and pupillary constrictor muscle weakness.

CONCLUSIONS

Our review identified several potentially useful variables that derive from oculography and pupillometry studies that could assist with a timely diagnosis of MG. Limitations of this review include heterogeneity in design, sample size, and quality of the studies evaluated. There is a need for larger, well-designed studies evaluating eye-tracking measures in the diagnosis of MG, especially for patients presenting with purely ocular symptoms.

The effects of tolterodine on anterior segment and choroidal thickness in patients with overactive bladder syndrome

BACKGROUND

To evaluate the effects of tolterodine on anterior segment and choroidal thickness by using the Pentacam system and optical coherence tomography (OCT) (Pentacam).

METHODS

A total of 122 eyes of 61 patients (34 female; 27 male) were included in the study. All patients underwent a regular ocular examination and intraocular pressure measurements before and after 3 months of antimuscarinic drug tolterodine treatment, in addition, pupil diameter, anterior chamber depth, anterior chamber volume, anterior chamber angle, and lens thickness were measured through Pentacam (Oculus Inc., Wetzlar, Germany) system. Measurements of choroidal thickness were performed by OCT (Spectralis^®, Heidelberg Engineering, Heidelberg, Germany).

RESULTS

The mean age was 58.4 ± 7.3 years and 56.5 ± 11.1 years for female and male patients. The mean intraocular pressure was 15.10 ± 2.75 mmHg before treatment and 15.18 ± 2.65 mmHg after treatment. Pupil diameters were 3.09 ± 0.48 mm before treatment and 3.12 ± 0.43 mm after treatment. Anterior chamber depth, before and after treatment were 2.68 ± 0.65 mm and 2.70 ± 0.61 mm. The mean value for subfoveal choroidal thickness was 267.92 ± 81.35 μm before the administration of tolterodine, whereas the mean choroidal thickness was 271.83 ± 75.42 μm after the administration of tolterodine. The alterations in the subfoveal choroidal thickness were not statistically significant (p = 0.862).

CONCLUSIONS

After 3 months of therapy with tolterodine, there were no significant changes in anterior segment parameters and choroidal thickness.

Opposing effects of atropine and timolol on the color and luminance emmetropization mechanisms in chicks

This study analyzed the luminance and color emmetropization response in chicks treated with the nonselective parasympathetic antagonist atropine and the sympathetic β-receptor blocker timolol. Chicks were binocularly exposed (8h/day) for 4days to one of three illumination conditions: 2Hz sinusoidal luminance flicker, 2Hz sinusoidal blue/yellow color flicker, or steady light (mean 680lux). Atropine experiments involved monocular daily injections of either 20μl of atropine (18nmol) or 20μl of phosphate-buffered saline. Timolol experiments involved monocular daily applications of 2 drops of 0.5% timolol or 2 drops of distilled H2O. Changes in the experimental eye were compared with those in the fellow eye after correction for the effects of saline/water treatments. Atropine caused a reduction in axial length with both luminance flicker (-0.078±0.021mm) and color flicker (-0.054±0.017mm), and a reduction in vitreous chamber depth with luminance flicker (-0.095±0.023mm), evoking a hyperopic shift in refraction (3.40±1.77D). Timolol produced an increase in axial length with luminance flicker (0.045±0.030mm) and a myopic shift in refraction (-4.07±0.92D), while color flicker caused a significant decrease in axial length (-0.046±0.017mm) that was associated with choroidal thinning (-0.046±0.015mm). The opposing effects on growth and refraction seen with atropine and timolol suggest a balancing mechanism between the parasympathetic and β-receptor mediated sympathetic system through stimulation of the retina with luminance and color contrast.

Pharmaceutical intervention for myopia control

Myopia is the result of a mismatch between the optical power and the length of the eye, with the latter being too long. Driving the research in this field is the need to develop myopia treatments that can limit axial elongation. When axial elongation is excessive, as in high myopia, there is an increased risk of visual impairment and blindness due to ensuing pathologies such as retinal detachments. This article covers both clinical studies involving myopic children, and studies involving animal models for myopia. Atropine, a nonselective muscarinic antagonist, has been studied most extensively in both contexts. Because it remains the only drug used in a clinical setting, it is a major focus of the first part of this article, which also covers the many shortcomings of topical ophthalmic atropine. The second part of this article focuses on in vitro and animal-based drug studies, which encompass a range of drug targets including the retina, retinal pigment epithelium and sclera. While the latter studies have contributed to a better understanding of how eye growth is regulated, no new antimyopia drug treatments have reached the clinical setting. Less conservative approaches in research, and in particular, the exploration of new bioengineering approaches for drug delivery, are needed to advance this field.

A randomised, double-blind, parallel design, multi-institutional, non-inferiority phase IV trial of imidafenacin versus fesoterodine for overactive bladder

AIMS

Our objective was to compare the efficacy and safety of imidafenacin over fesoterodine in patients with overactive bladder (OAB).

METHODS

This study is a randomised, double-blind, parallel-group, fesoterodine-controlled study in patients with continuous OAB symptoms for ≥ 3 months, daily mean voiding frequency (DMVF) ≥ 8, and daily mean urgency or urgency incontinence frequency ≥ 2. A twice-daily 0.1 mg imidafenacin with placebo, or once-daily 4 mg fesoterodine with placebo were administered for 12 weeks. The primary efficacy end-point was the difference in DMVF at 12 weeks. The secondary efficacy end-points were differences in daily mean: (i) voiding frequency at 4 and 8 weeks; (ii) urgency frequency; (iii) urgency incontinence frequency; (iv) incontinence frequency; (v) nocturia frequency; and (vi) quality of life score. The variables for safety analysis were adverse events, vital signs, residual urine volume and clinical laboratory tests. An efficacy analysis was conducted in per-protocol patients and the safety analysis was conducted in all randomised patients.

RESULTS

The differences in DMVF at 12 weeks were -3.38 ± 3.63 and -2.45 ± 3.73 in the imidafenacin and fesoterodine groups, respectively, and the difference was not significant between the two groups. Imidafenacin was non-inferior to fesoterodine, and the lower limit of 95% two-sided confidence intervals was -0.53. The other six secondary end-points and variables for safety analysis showed no difference between the two groups.

CONCLUSIONS

Imidafenacin was non-inferior to fesoterodine in terms of efficacy, and showed no significant difference in terms of safety.

Ghrelin in ocular pathophysiology: from the anterior to the posterior segment

Ghrelin is a 28 amino acid acylated peptide produced in several organs that binds the growth hormone secretagogues receptor type 1a (GHSR-1a). It acts over a wide range of systems, e.g. the endocrine, cardiovascular, musculoskeletal and immune systems and the eye. The aim of this work is to review the physiologic and pathologic implications of the ghrelin-GHSR-1a in the eye. A systematic revision of studies published between 2000 and 2013 in English, Spanish or Portuguese in MEDLINE, EMBASE and Scopus was performed. Search words used included: ghrelin, GHSR-1a, ocular production, iris muscular kinetics, ciliary body, glaucoma, retinopathy and uvea. The production of ghrelin by the ocular tissue has been detected both in the anterior and posterior segments, as well as the presence of GHSR-1a. This peptide promotes the relaxation of the iris sphincter and dilator muscles, being this effect independent from GHSR-1a and dependent on prostaglandins release in the first case and dependent on GHSR-1a in the second. Regarding ocular pathology, ghrelin levels in the aqueous humor appear to be decreased in individuals with glaucoma. Moreover, ghrelin has been shown to decrease the intraocular pressure in animal models of ocular hypertension through GHSR-1a. In the posterior segment, the ghrelin-GHSR-1a system interferes with the development of oxygen-induced retinopathy, being protective in the vaso-obliterative phase and deleterious in the vaso-proliferative stage of the disease. Thus, the ghrelin-GHSR-1a system presents as a possible local regulatory mechanism in the eye, with pathophysiological implications, constituting a target for future clinical and therapeutic research and interventions.

Cardiovascular effects of antimuscarinic agents in overactive bladder

INTRODUCTION

The potential impact of antimuscarinics (AMs) on cardiac function is a major concern in the treatment of overactive bladder (OAB) patients, especially in older ones who are likely to present cardiovascular (CV) comorbidities and other risk factors that may predispose them to the adverse cardiac effects of this therapy.

AREAS COVERED

This article aims to review the literature on the impact on the CV system of AMs used in the treatment of OAB, giving a comprehensive explanation of the pathogenetic mechanisms of AMs' effects on CV system and the impact of each AM drug on cardiac function.

EXPERT OPINION

Although the CV safety of AM drugs seems to be good, evidence provided in this manuscript does not allow to exclude an increase in HR, QT prolongation or an increase in the CV risk due to drug-drug interactions in OAB patients who are usually elderly and have comorbidities. Clinical and electrocardiographic monitoring may be necessary throughout the administration period in selected populations such as patients aged > 80 years, those with coronary heart disease or congestive heart failure. Further studies are needed to understand whether the most recently developed AM drugs, such as imidafenacin, are safer than the old ones.

Pharmacokinetics and toxicity of antimuscarinic drugs for overactive bladder treatment in females

INTRODUCTION

Antimuscarinics (AMs) are the mainstay of pharmacological treatment of overactive bladder (OAB), a symptom complex defined by the presence of urinary urgency, usually associated with frequency and nocturia, with or without urgency urinary incontinence. The AMs used to treat OAB differ in their pharmacological profiles, which may affect their potential for causing adverse effects (AEs).

AREAS COVERED

The present article aims to review the literature about pharmacokinetics (PK) of the different AMs used in the treatment of OAB. Furthermore, the AEs related to the use of these drugs and their incidence are presented. This systematic review is based on material searched and obtained via Medline, Pubmed and EMBASE up to March 2012 using the search terms "adverse events, pharmacokinetics, tolerability" in combination with "darifenacin, fesoterodine, imidafenacin, oxybutynin, propiverine, solifenacin, tolterodine, and trospium."

EXPERT OPINION

Antimuscarinics are the first-line pharmacological treatment for OAB. Despite the development of new molecules that improve their efficacy/safety profile, there are some drugs that are pharmacokinetically more appropriate to be prescribed in specific populations such as patients with neurological disease or the elderly. Moreover, research should be encouraged in evaluating antimuscarinics in conjunction with other drugs such as estrogens or beta-agonists. The identification of prognostic criteria for pharmacological therapy would be helpful.

Muscarinic receptor agonists and antagonists: effects on ocular function

Muscarinic agonists act mainly via muscarinic M₃ cholinoceptors to cause contraction of the iris sphincter, ciliary muscle and trabecular meshwork as well as increase outflow facility of aqueous humour. In the iris dilator, the effect of muscarinic agonists is species dependent but is predominantly relaxation via muscarinic M₃ receptors. In the conjunctiva, muscarinic agonists stimulate goblet cell secretion which contributes to the protective tear film. Muscarinic M₂ and M₃ receptors appear mainly involved. In the lens muscarinic agonists act via muscarinic M₁ receptors to produce depolarization and increase [Ca(2+)](i). All five subtypes of muscarinic receptor are present in the retina. In the developing retina, acetylcholine appears to limit purinergic stimulation of retinal development and decrease cell proliferation. In the adult retina acetylcholine and other muscarinic agonists may have complex effects, for example, enhancing light-evoked neuronal firing in transient ON retinal ganglion cells and inhibiting firing in OFF retinal ganglion cells. In the lacrimal gland, muscarinic agonists activate M₃ receptors on secretory globular acinar cells to stimulate tear secretion and also cause contraction of myoepithelial cells. In Sjögren's syndrome, antibodies to the muscarinic M₃ receptor disrupt normal gland function leading to xerophthalmia although the mechanism of action of the antibody is still not clear. Atropine and pirenzepine are useful in limiting the development of myopia in children probably by an action on muscarinic receptors in the sclera, although many other muscarinic receptor antagonists are not effective.

Stereoselectivity of satropane, a novel tropane analog, on iris muscarinic receptor activation and intraocular hypotension

AIM

To study the stereoselectivity of satropane (3-paramethylbenzene sulfonyloxy-6-acetoxy tropane), a novel tropane analog, on iris muscarinic receptor activation and intraocular hypotension.

METHODS

The assays for radioligand-receptor binding, the contractile responses of isolated iris muscle, the miosis response, and the intraocular hypotension of the enantiomers of satropane were investigated.

RESULTS

In the binding analysis, S(-)satropane (lesatropane) completely competed against the [3H]quinuclydinyl benzilate-labeled ligand at muscarinic receptors in the iris muscle, whereas R(+)satropane failed to completely compete. In an isolated iris contractile assay, R,S(+/-)satropane and S(-)satropane produced a concentration-dependent contractile response with similar efficacy and potency to that of carbachol. R(+)satropane did not induce any contractile response. In the pupil diameter measurement assay in vivo, S(-)satropane induced miosis much more effectively than pilocarpine, while R(+)satropane failed to produce any miosis. In the water loading-induced and methylcellulose-induced ocular hypertensive models, S(-)satropane, but not R(+)satropane, significantly suppressed intraocular pressure at a much lower concentration than pilocarpine.

CONCLUSION

The agonistic and hypotensive properties of satropane on rabbit eyes are stereoselective, with the S(-)isomer being its active form.

Calcium mobilization by activation of M(3)/M(5) muscarinic receptors in the human retinoblastoma

Activation of muscarinic acetylcholine receptors (mAChR) is one of the most important signal transduction pathways in the human body. In this study, we investigated the role of mAChR activation in relation to its subtypes in human retinoblastoma cell-lines (WERI-Rb-1) using Ca(2+) measurement, real-time PCR, and Western Blot techniques. Acetylcholine (ACh) produced prominent [Ca(2+)](i) transients in a repeated manner in WERI-Rb-1 cells. The maximal amplitude of the [Ca(2+)](i) transient was almost completely suppressed by 97.3 +/- 0.8% after atropine (1 microM) pretreatment. Similar suppressions were noted after pretreatments with thapsigargin (1 microM), an ER Ca(2+)-ATPase (SERCA) inhibitor, whereas the ACh-induced [Ca(2+)](i) transient was not affected even in the absence of extracellular calcium. U-73122 (1 microM), a PLC inhibitor, and xestospongin C (2 microM), an IP(3)-receptor antagonist, elicited 11.5 +/- 2.9% and 17.8 +/- 1.9% suppressions, respectively. The 50% inhibitory concentration of (IC(50)) values for blockade of a 100 microM ACh response by pirenzepine and 4-DAMP were 315.8 and 9.1 nM, respectively. Moreover, both M(3) and M(5) mAChRs were prominent in quantitative real-time-PCR. Taken together, the M(3)/M(5) subtypes appear to be the major contributor, leading to intracellular calcium mobilization from the internal store via an IP(3)-dependent pathway in the undifferentiated retinoblastoma cells.

The presence of m1 to m5 receptors in human sclera: evidence of the sclera as a potential site of action for muscarinic receptor antagonists

PURPOSE

The aim of this study was to identify the presence of muscarinic acetylcholine receptors (mAChRs) in human sclera in order to determine whether the sclera is a potential site of action for mAChR antagonists.

METHODS

Cell lines of human scleral fibroblasts were cultured in Dulbecco modified Ealge's medium. Reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analysis were used to detect mRNA expression of muscarinic acetylcholine receptors in the cell lines of the fibroblasts. Western blot analysis and immunocytochemistry were used to detect proteins of mAChRs in the cell lines. Immunohistochemical study was used to further detect the presence of mAChR proteins in the frozen scleral sections.

RESULTS

The cultured fibroblasts demonstrated mRNA expression of five mAChRs (m1 to m5) in RT-PCR and Northern blot analysis. The molecular size of mRNA expression was largest for the m3 receptor, followed by the m2, m4, m5, and m1 in both RT-PCR and Northern blot analysis. Proteins of the m1 to m5 receptors were present in cell line fibroblasts under Western blot analysis and immunocytochemistry with a range of molecular weight from 80 kDa (m5) to 60 kDa (m1) in Western blot analysis. The presence of these five receptors was also detected in scleral tissues with immunohistochemistry.

CONCLUSIONS

This study demonstrated the presence of mAChR subtypes (m1 to m5) in human scleral fibroblasts at both mRNA and protein levels. This finding indicates that the sclera is a potential site of action for the currently used mAChR antagonists in prevention of human myopia.

Muscarinic receptors: their distribution and function in body systems, and the implications for treating overactive bladder

1. The effectiveness of antimuscarinic agents in the treatment of the overactive bladder (OAB) syndrome is thought to arise through blockade of bladder muscarinic receptors located on detrusor smooth muscle cells, as well as on nondetrusor structures. 2. Muscarinic M3 receptors are primarily responsible for detrusor contraction. Limited evidence exists to suggest that M2 receptors may have a role in mediating indirect contractions and/or inhibition of detrusor relaxation. In addition, there is evidence that muscarinic receptors located in the urothelium/suburothelium and on afferent nerves may contribute to the pathophysiology of OAB. Blockade of these receptors may also contribute to the clinical efficacy of antimuscarinic agents. 3. Although the role of muscarinic receptors in the bladder, other than M3 receptors, remains unclear, their role in other body systems is becoming increasingly well established, with emerging evidence supporting a wide range of diverse functions. Blockade of these functions by muscarinic receptor antagonists can lead to similarly diverse adverse effects associated with antimuscarinic treatment, with the range of effects observed varying according to the different receptor subtypes affected. 4. This review explores the evolving understanding of muscarinic receptor functions throughout the body, with particular focus on the bladder, gastrointestinal tract, eye, heart, brain and salivary glands, and the implications for drugs used to treat OAB. The key factors that might determine the ideal antimuscarinic drug for treatment of OAB are also discussed. Further research is needed to show whether the M3 selective receptor antagonists have any advantage over less selective drugs, in leading to fewer adverse events.

Aceclidine and pilocarpine interact differently with muscarinic receptor in isolated rabbit iris muscle

The relationship between muscarinic receptor affinity states and the contractile response to the muscarinic agonists carbachol, aceclidine, and pilocarpine, has been examined in the isolated rabbit iris muscle. Contraction of the iris muscle by carbachol and aceclidine was more potent and/or more efficacious than the response to pilocarpine. Analysis of [3H]-Quinuclidinyl benzilate (QNB) binding showed that while both carbachol and aceclidine bound to high- and low-affinity forms of the muscarinic receptor, pilocarpine bound to one affinity state. The efficacy of carbachol and aceclidine to stimulate contraction of the iris muscle was consistent with receptor occupancy theory only when considering the low-affinity state of the muscarinic receptor, and activation of the low-affinity rather than high-affinity binding state of the receptor is likely to mediate the contraction of iris muscle. Therefore, the typical anti-glaucoma muscarinic agonists aceclidine and pilocarpine may interact differently with their target receptors in isolated rabbit iris muscle.

Role of specific muscarinic receptor subtypes in cholinergic parasympathomimetic responses, in vivo phosphoinositide hydrolysis, and pilocarpine-induced seizure activity

Muscarinic agonist-induced parasympathomimetic effects, in vivo phosphoinositide hydrolysis and seizures were evaluated in wild-type and muscarinic M1-M5 receptor knockout mice. The muscarinic agonist oxotremorine induced marked hypothermia in all the knockout mice, but the hypothermia was reduced in M2 and to a lesser extent in M3 knockout mice. Oxotremorine-induced tremor was abolished only in the M2 knockout mice. Muscarinic agonist-induced salivation was reduced to the greatest extent in M3 knockout mice, to a lesser degree in M1 and M4 knockout mice, and was not altered in M2 and M5 knockout mice. Pupil diameter under basal conditions was increased only in the M3 knockout mice. Pilocarpine-induced increases in in vivo phosphoinositide hydrolysis were completely absent in hippocampus and cortex of M1 knockout mice, but in vivo phosphoinositide hydrolysis was unaltered in the M2-M5 knockout mice. A high dose of pilocarpine (300 mg/kg) caused seizures and lethality in wild-type and M2-M5 knockout mice, but produced neither effect in the M1 knockout mice. These data demonstrate a major role for M2 and M3 muscarinic receptor subtypes in mediating parasympathomimetic effects. Muscarinic M1 receptors activate phosphoinositide hydrolysis in cortex and hippocampus of mice, consistent with the role of M1 receptors in cognition. Muscarinic M1 receptors appear to be the only muscarinic receptor subtype mediating seizures.

Use of M1-M5 muscarinic receptor knockout mice as novel tools to delineate the physiological roles of the muscarinic cholinergic system

In this review we report recent findings on the physiological role of the five known muscarinic acetylcholine receptors (mAChRs) as shown by gene targeting technology. Using knockout mice for each mAChRs subtype, the role of mAChRs subtypes in a number of physiological functions was confirmed and new activities were discovered. The M1 mAChRs modulate neurotransmitter signaling in cortex and hippocampus. The M3 mAChRs are involved in exocrine gland secretion, smooth muscle contractility, pupil dilation, food intake, and weight gain. The role of the M5 mAChRs involves modulation of central dopamine function and the tone of cerebral blood vessels. mAChRs of the M2 subtype mediate muscarinic agonist-induced bradycardia, tremor, hypothermia, and autoinhibition of release in several brain regions. M4 mAChRs modulate dopamine activity in motor tracts and act as inhibitory autoreceptors in striatum. Thus, as elucidated by gene targeting technology, mAChRs have widespread and manifold functions in the periphery and brain.

Mice lacking M2 and M3 muscarinic acetylcholine receptors are devoid of cholinergic smooth muscle contractions but still viable

Cholinergic agents elicit prominent smooth muscle contractions via stimulation of muscarinic receptors that comprise five distinct subtypes (M1-M5). Although such contractions are important for autonomic organs, the role of each subtype has not been characterized precisely because of the poor selectivity of the currently available muscarinic ligands. Here, we generated a mutant mouse line (M2-/-M3-/- mice) lacking M2 and M3 receptors that are implicated in such cholinergic contractions. The relative contributions of M2 and M3 receptors in vitro was approximately 5 and 95% for the detrusor muscle contraction and approximately 25 and 75% for the ileal longitudinal muscle contraction, respectively. Thus, M1, M4, or M5 receptors do not seem to play a role in such contractions. Despite the complete lack of cholinergic contractions in vitro, M2-/-M3-/- mice were viable, fertile, and free of apparent intestinal complications. The urinary bladder was distended only in males, which excludes a major contribution by cholinergic mechanisms to the urination in females. Thus, cholinergic mechanisms are dispensable in gastrointestinal motility and female urination. After 10 Hz electrical field stimulation, noncholinergic inputs were found to be increased in the ileum of M2-/-M3-/- females, which may account for the lack of apparent functional deficits. Interestingly, the M2-/-M3-/- mice had smaller ocular pupils than M3-deficient mice. The results suggest a novel role of M2 in the pupillary dilation, contrary to the well known cholinergic constriction. These results collectively suggest that an additional mechanism operates in the control of pupillary constriction-dilatation.

Identification and characterization of muscarinic acetylcholine receptor subtypes expressed in human skin melanocytes

The present study was designed to identify and characterize muscarinic acetylcholine receptors in normal human melanocytes. We used subtype-specific oligonucleotide primers to localize the five genetically defined mAChR mRNAs (ml through m5) by reverse transcription-polymerase chain reaction. These experiments showed that all five mAChR subtype mRNAs are expressed in melanocytes. The PCR products were verified by restriction analysis and Southern blotting. Receptors were visualized in cultures of normal human melanocytes and specimens of normal human skin by subtype-specific rabbit anti-receptor polyclonal antibodies. Radioligand binding assays with the lipophilic drug [3H]quinuclidinyl benzilate demonstrated approximately 9,000 high affinity binding sites/cell. Micromolar concentrations of muscarine or carbachol transiently increased intracellular Ca2+, which could be attenuated by atropine, demonstrating coupling of the receptors to mobilization of intracellular free Ca2+. Lower concentrations of muscarine induced spontaneous repetitive spike-like increases of intracellular Ca2+ which is characteristic for the activation of muscarinic receptors. These results indicate that normal human skin melanocytes express the ml, m2, m3, m4, and m5 subtypes of classic muscarinic acetylcholine receptors on their cell membrane and that these receptors regulate the concentration of intracellular free Ca2+, which may play an important physiologic role in melanocyte behavior and skin pigmentation.

Novel fragments of the Sjögren's syndrome autoantigens alpha-fodrin and type 3 muscarinic acetylcholine receptor generated during cytotoxic lymphocyte granule-induced cell death

OBJECTIVE

To determine whether the Sjögren's syndrome autoantigens alpha-fodrin and the type 3 muscarinic acetylcholine receptor (M3R) are cleaved during cytotoxic lymphocyte granule-induced death, to yield novel fragments.

METHODS

Primary salivary gland epithelial cells, human salivary gland cells, and HeLa cells were incubated with granule contents. The susceptibility to cleavage and the generation of novel fragments of Sjögren's syndrome autoantigens in this form of apoptosis was assessed by immunoblotting. Cleavage of M3R was further characterized by assays performed on the M3R molecule generated by in vitro translation.

RESULTS

This study demonstrated that alpha-fodrin was uniquely cleaved during cytotoxic lymphocyte granule-induced cell death, generating a 155-kd fragment distinct from those generated by caspase 3 in other forms of apoptosis. The study also demonstrates that M3R (which is restricted in expression to the peripheral autonomic organs) was efficiently cleaved by granzyme B (but not by caspases) at several sites, both in vitro and in intact cells. This is the first description of cleavage of a transmembrane autoantigen by granzyme B.

CONCLUSION

The observation that both ubiquitously expressed autoantigens (e.g., alpha-fodrin, La, and nuclear mitotic apparatus protein) and tissue-restricted autoantigens (e.g., M3R) targeted in Sjögren's syndrome are specifically cleaved by granzyme B, generating unique fragments, strongly suggests that a common biochemical event (novel autoantigen cleavage during granule-induced epithelial cell death) is responsible for selecting this apparently unconnected group of molecules for a high-titer autoantibody response. The data focus attention on the role of cytotoxic lymphocytes in the initiation and propagation of Sjögren's syndrome.

Multiple functional defects in peripheral autonomic organs in mice lacking muscarinic acetylcholine receptor gene for the M3 subtype

Muscarinic acetylcholine receptors consist of five distinct subtypes and have been important targets for drug development. In the periphery, muscarinic acetylcholine receptors mediate cholinergic signals to autonomic organs, but specific physiological functions of each subtype remain poorly elucidated. Here, we have constructed and analyzed mutant mice lacking the M(3) receptor and have demonstrated that this subtype plays key roles in salivary secretion, pupillary constriction, and bladder detrusor contractions. However, M(3)-mediated signals in digestive and reproductive organs are dispensable, likely because of redundant mechanisms through other muscarinic acetylcholine receptor subtypes or other mediators. In addition, we have found prominent urinary retention only in the male, which indicates a considerable sex difference in the micturition mechanism. Accordingly, this mutant mouse should provide a useful animal model for investigation of human diseases that are affected in the peripheral cholinergic functions.

Inhibitory effects of muscarinic receptor autoantibodies on parasympathetic neurotransmission in Sjögren's syndrome

OBJECTIVE

Sjögren's syndrome (SS) is an autoimmune disorder characterized by dry eyes and mouth (sicca syndrome) and lymphocytic infiltration of the lacrimal and salivary glands. Abnormalities of parasympathetic neurotransmission may contribute to the glandular dysfunction. In this study, we used a functional assay to investigate autoantibody-mediated effects on parasympathetic neurotransmission and smooth muscle contraction.

METHODS

Serum and purified IgG were obtained from patients with primary and secondary SS and from control subjects. Contraction of isolated bladder strips in response to stimulation of M3-muscarinic receptors by a muscarinic receptor agonist, carbachol, or by endogenous acetylcholine released from postganglionic parasympathetic nerves was measured before and after the addition of patient serum or IgG.

RESULTS

Sera from 5 of 9 patients with primary SS and from 6 of 6 patients with secondary SS inhibited carbachol-evoked bladder contraction by approximately 50%. Sera from these patients also inhibited the action of neuronally released acetylcholine at M3-muscarinic receptors. Sera from 7 of 8 healthy individuals, from patients with rheumatoid arthritis without sicca symptoms, and from patients with systemic lupus erythematosus had no effect. The anti-muscarinic receptor activity was localized in the IgG fraction, since purified IgG from patients with SS also inhibited agonist- and nerve-evoked contractions. In this preliminary study, the autoantibodies seemed to be associated with the presence of bladder symptoms and other autonomic features.

CONCLUSION

Autoantibodies that act as antagonists at M3-muscarinic receptors on smooth muscle occur in a subset of patients with primary and secondary SS. Their presence in secondary SS was unexpected and provides new evidence for a common pathogenetic link between primary and secondary SS. These autoantibodies appear to contribute to sicca symptoms and may explain associated features of autonomic dysfunction in some patients.

Anticholinergic effects of desloratadine, the major metabolite of loratadine, in rabbit and guinea-pig iris smooth muscle

Allergic conjunctivitis is the most common ocular allergic disease. Although very symptomatic it does not endanger vision, and topical antihistamines or chromones are the first choice treatment in clinical practice. Recently, equivalent nanomolar affinities for histamine H and muscarinic M 1 and M3 cloned human receptors have been reported for desloratadine, the active metabolite of loratadine, a widely prescribed antihistamine. This property might enhance its utility in the treatment of asthma, but could induce adverse anticholinergic effects after topical administration. In the present study, we compare the anticholinergic activity of desloratadine with other known muscarinic antagonists and antihistamines on rabbit and guinea-pig iris smooth muscle. Desloratadine was found to be a competitive antagonist (pA2 = 6.67+/-0.09) of carbachol-induced contractions in isolated rabbit iris smooth muscle. Atropine (pA2 = 9.44+/-0.02) and NPC-14695 (pA2 = 9.18+/-0.03) also behaved as competitive antagonists, whereas tiotropium bromide (pD'2 = 9.06+/-0.02) exhibited a non-competitive behaviour in this tissue. Carebastine (pA2 = 5.64+/-0.04) and fexofenadine (pA2 < 4.0) were also studied. After topical administration on the guinea-pig eye conjunctiva, desloratadine produced a potent (ED50 = 2.3 mg/ml) and long lasting mydriasis (> 120 min at the ED50) in conscious animals. Fexofenadine and carebastine were inactive even at the highest concentration tested (10 mg/ml). Atropine (ED50 = 30 microg/ml) and tiotropium bromide (ED50 = 10 microg/ml) were much more potent than desloratadine or pirenzepine (ED50 = 3 mg/ml) in this model. The competitive muscarinic antagonism of desloratadine in vitro, and its potency and duration of action in vivo, suggest that topical treatment of allergic conjunctivitis and rhinitis with desloratadine could produce undesirable peripheral anticholinergic side effects such as mydriasis and xerostomia.

Mapping of five subtype genes for muscarinic acetylcholine receptor to mouse chromosomes

Muscarinic acetylcholine receptors in mammals consist of five subtypes (M1-M5) encoded by distinct genes. They are widely expressed throughout the body and play a variety of roles in the peripheral and central nervous systems. Although their pharmacological properties have been studied extensively in vitro, colocalization of the multiple subtypes in each tissue and lack of subtype-specific ligands have hampered characterization of the respective subtypes in vivo. We have mapped mouse genomic loci for all five genes (Chrm1-5) by restriction fragment length variant (RFLV) analyses in interspecific backcross mice. Chrm1, Chrm2, and Chrm3 were mapped to chromosome (Chr) 19, 6, and 13, respectively. Both Chrm4 and Chrm5 were mapped to Chr 2. Although a comparison of their map positions with other mutations in their vicinities suggested a possibility that the El2 (epilepsy 2) allele might be a mutation in Chrm5, sequencing analyses of the Chrm5 gene in the El2 mutant mice did not support such a hypothesis.