Distribution and function of muscarinic receptor subtypes in the ovine submandibular gland

Treatment of Clozapine-Associated Sialorrhea: The Role of Benzamide Derivatives

PURPOSE

Hypersalivation is one of the most prevalent and distressing adverse effects associated with clozapine treatment. Currently, there is no standard therapeutic approach toward how to overcome it. Clinicians use various medications for managing this adverse effect. However, some of the agents are not effective enough, whereas others can induce other adverse effects. Recently, several reviews have been published on the treatment of clozapine-associated hypersalivation, in which the focus was on drugs from various pharmacological groups, and little attention was paid to drugs from the group of substituted benzamides. The intention of this brief narrative review is to draw the attention of clinicians to the use of the benzamide group for the treatment of this unpleasant adverse effect.

METHODS

A MEDLINE search was conducted to identify published treatment studies and case reports in the literature from 2000 to September 2021, concerning a treatment of clozapine-associated hypersalivation, mainly substituted benzamides.

RESULTS

Accumulating evidence during the last 2 decades indicates that agents derived from the benzamide group may be effective and safe agents for treatment of clozapine-associated hypersalivation. Whether with a psychotropic effect or without, medications from this group may produce a beneficial response.

CONCLUSIONS

Substitute benzamide derivatives have emerged as effective and well-tolerated agents for treatment clozapine-associated hypersalivation.

Anticholinergic medication: Related dry mouth and effects on the salivary glands

OBJECTIVE

Salivary glands are among the most sensitive target organs of medications with anticholinergic (AC) properties, interrupting the neural stimulation of saliva secretion and reducing saliva flow. Hyposalivation results in dry mouth, leading to dental caries, intraoral infection, orofacial pain, problems with speaking and swallowing, and diminished oral health--related quality of life. Current understanding of the pharmacokinetics of AC medications and their effect on muscarinic receptors in the salivary glands were reviewed to assist clinicians in predicting salivary damage in patients with AC medication-induced dry mouth.

STUDY DESIGN

We summarized the literature related to the mechanisms and properties of AC medications, anticholinergic adverse effects, and their effect on salivary function and management strategies to prevent oral health damage.

RESULTS

Although a large number of studies reported on the frequencies of medication-induced dry mouth, we found very limited data on predicting individual susceptibility to AC medication--caused hyposalivation and no prospective clinical studies addressing this issue.

CONCLUSION

Dry mouth is most frequently caused by medications with AC properties, which interrupt the neural stimulation of saliva secretion. Interdisciplinary care should guide pharmacotherapeutics and dental interventions should aim in preventing AC salivary adverse effects and reducing the oral health burden from AC medication-induced dry mouth.

Antimuscarinic use among older adults with dementia and overactive bladder: a Medicare beneficiaries study

OBJECTIVES

This study examined the incidence and predictors of antimuscarinic medication use including non-selective antimuscarinics among older adults with dementia and overactive bladder (OAB).

METHODS

The study used a new-user cohort design involving older adults (≥65 years) with dementia and OAB based on 2013-2015 Medicare data. Antimuscarinics included non-selective (oxybutynin, tolterodine, trospium, fesoterodine) and selective (solifenacin, darifenacin) medications. Descriptive statistics and multivariable logistic regression models were used to determine the incidence and predictors of new antimuscarinic use including non-selective antimuscarinics, respectively.

RESULTS

Of the 3.38 million Medicare beneficiaries with dementia, over one million (1.05) had OAB (31.03%). Of those, 287,612 (27.39%) were reported as prevalent antimuscarinics users. After applying continuous eligibility criteria, 21,848 (10.34%) incident antimuscarinic users were identified (77.6% non-selective; 22.4% selective). Most frequently reported antimuscarinics were oxybutynin (56.3%) and solifenacin (21.4%). Multivariable analysis revealed that patients ≥75 years, of black race, and those with schizophrenia, epilepsy, delirium, and Elixhauser's score were less likely to initiate antimuscarinics. Women, those with abnormal involuntary movements, bipolar disorder, gastroesophageal reflux disease, insomnia, irritable bowel syndrome, muscle spasm/low back pain, neuropathic pain, benign prostatic hyperplasia, falls/fractures, myasthenia gravis, narrow-angle glaucoma, Parkinson's disease, syncope, urinary tract infection and vulvovaginitis were more likely to initiate antimuscarinics. Further, patients with muscle spasms/low back pain, benign prostatic hyperplasia and those taking higher level anticholinergics had lower odds of receiving non-selective antimuscarinics, whereas white patients, black patients and those with schizophrenia and delirium were more likely to receive them.

CONCLUSIONS

Nearly one-third of dementia patients had OAB and over one-fourth of them used antimuscarinics. Majority of the incident users were prescribed non-selective antimuscarinics with several demographic and clinical factors contributing to their use. Given the high prevalence of OAB among dementia patients, there is a need to optimize their antimuscarinic use, considering their vulnerability for anticholinergic adverse effects.

Effect of ischemic preconditioning on radiation damage to the submandibular gland in rats

To investigate the effects of radiation on rat submandibular glands and the possible protective effects of ischemic preconditioning, the submandibular glands of Wistar rats were subjected to in situ radiation after ischemic preconditioning. The glands were exposed to X-radiation at a single dose of 20 Gy. Ischemic preconditioning was achieved by three min of ischemia and three min of reperfusion, repeated three times before irradiation. Salivary secretion, histological changes, alterations in tight junctions, and the levels of oxidative stress, pro-inflammatory cytokines, and water secretion proteins mediated by the muscarinic acetylcholine M3 subtype receptor were determined at 1 and 12 weeks post-irradiation. In glands subjected to irradiation only, the secretion, superoxide dismutase activity, tight junction width, acinar cell number, and M3 receptor and aquaporin-5 levels were lower at 1 and 12 weeks than seen in the ischemically preconditioned irradiated glands. In contrast, tumor necrosis factor-α, malondialdehyde, myeloperoxidase activity, and the expression of the tight junction protein claudin-4 were significantly higher in the irradiated only glands. Our study revealed that radiation caused a series of injury-stress responses, especially damage to the water secretion pathway mediated by the M3 receptor that ultimately led to hyposecretion, which might play an important role in the dysfunction of the irradiated only glands. Ischemic preconditioning reduced the radiation-induced injury to submandibular glands and ameliorated salivary hyposecretion.

Heterogeneous localization of muscarinic cholinoceptor M1 in the salivary ducts of adult mice

We hypothesize variation in expression and localization, along the course of the glandular tubule, of muscarinic cholinergic receptor M₁ which plays as a distinct contribution, though minor in comparison with M₃ receptor, in saliva secretion. Localization of the M₁ receptor was examined using immunohistochemistry in three major salivary glands. Although all glandular cells were more or less M₁-immunoreactive, acinar cells were weakly immunoreactive, while ductal cells exhibited substantial M₁-immunoreactivity. Many ductal cells exhibited clear polarity with higher immunoreactivity in their apical/supra-nuclear domain. However, some exhibited indistinct polarity because of additional higher immunoreactivity in their basal/infra-nuclear domain. A small group of cells with intense immunoreactivity was found, mostly located in the intercalated ducts or in portions of the striated ducts close to the intercalated ducts. In immuno-electron microscopy, the immunoreactive materials were mainly in the cytoplasm including various vesicles and vacuoles. Unexpectedly, distinct immunoreactivity on apical and basal plasma membranes was infrequent in most ductal cells. The heterogeneous localization of M₁-immunoreactivity along the gland tubular system is discussed in view of possible modulatory roles of the M₁ receptor in saliva secretion.

Effects of cevimeline on excitability of parasympathetic preganglionic neurons in the superior salivatory nucleus of rats

The superior salivatory nucleus (SSN) contains parasympathetic preganglionic neurons innervating the submandibular and sublingual salivary glands. Cevimeline, a muscarinic acetylcholine receptor (mAChR) agonist, is a sialogogue that possibly stimulates SSN neurons in addition to the salivary glands themselves because it can cross the blood-brain barrier (BBB). In the present study, we examined immunoreactivities for mAChR subtypes in SSN neurons retrogradely labeled with a fluorescent tracer in neonatal rats. Additionally, we examined the effects of cevimeline in labeled SSN neurons of brainstem slices using a whole-cell patch-clamp technique. Mainly M1 and M3 receptors were detected by immunohistochemical staining, with low-level detection of M4 and M5 receptors and absence of M2 receptors. Most (110 of 129) SSN neurons exhibited excitatory responses to application of cevimeline. In responding neurons, voltage-clamp recordings showed that 84% (101/120) of the neurons exhibited inward currents. In the neurons displaying inward currents, the effects of the mAChR antagonists were examined. A mixture of M1 and M3 receptor antagonists most effectively reduced the peak amplitude of inward currents, suggesting that the excitatory effects of cevimeline on SSN neurons were mainly mediated by M1 and M3 receptors. Current-clamp recordings showed that application of cevimeline induced membrane depolarization (9/9 neurons). These results suggest that most SSN neurons are excited by cevimeline via M1 and M3 muscarinic receptors.

Acetylcholinesterase Inhibitors and Drugs Acting on Muscarinic Receptors- Potential Crosstalk of Cholinergic Mechanisms During Pharmacological Treatment

BACKGROUND

Pharmaceuticals with targets in the cholinergic transmission have been used for decades and are still fundamental treatments in many diseases and conditions today. Both the transmission and the effects of the somatomotoric and the parasympathetic nervous systems may be targeted by such treatments. Irrespective of the knowledge that the effects of neuronal signalling in the nervous systems may include a number of different receptor subtypes of both the nicotinic and the muscarinic receptors, this complexity is generally overlooked when assessing the mechanisms of action of pharmaceuticals.

METHODS

We have search of bibliographic databases for peer-reviewed research literature focused on the cholinergic system. Also, we have taken advantage of our expertise in this field to deduce the conclusions of this study.

RESULTS

Presently, the life cycle of acetylcholine, muscarinic receptors and their effects are reviewed in the major organ systems of the body. Neuronal and non-neuronal sources of acetylcholine are elucidated. Examples of pharmaceuticals, in particular cholinesterase inhibitors, affecting these systems are discussed. The review focuses on salivary glands, the respiratory tract and the lower urinary tract, since the complexity of the interplay of different muscarinic receptor subtypes is of significance for physiological, pharmacological and toxicological effects in these organs.

CONCLUSION

Most pharmaceuticals targeting muscarinic receptors are employed at such large doses that no selectivity can be expected. However, some differences in the adverse effect profile of muscarinic antagonists may still be explained by the variation of expression of muscarinic receptor subtypes in different organs. However, a complex pattern of interactions between muscarinic receptor subtypes occurs and needs to be considered when searching for selective pharmaceuticals. In the development of new entities for the treatment of for instance pesticide intoxication, the muscarinic receptor selectivity needs to be considered. Reactivators generally have a muscarinic M2 receptor acting profile. Such a blockade may engrave the situation since it may enlarge the effect of the muscarinic M3 receptor effect. This may explain why respiratory arrest is the major cause for deaths by esterase blocking.

Double-Blind, Randomized, Placebo-Controlled Trial of Metoclopramide for Hypersalivation Associated With Clozapine

Hypersalivation is a frequent, disturbing, and uncomfortable adverse effect of clozapine therapy that frequently leads to noncompliance. The aim of this study was to examine the efficacy of metoclopramide (dopamine D2 antagonist, antiemetic medication) as an option for management of hypersalivation associated with clozapine (HAC). A 3-week, double-blind, placebo-controlled trial was conducted in university-based research clinics from January 2012 to May 2014, on 58 inpatients treated with clozapine who were experiencing hypersalivation. The subjects were randomly divided into placebo and metoclopramide groups. The starting dose was 10 mg/d. Participants who did not respond were up-titrated 10 mg/d weekly to a total of 30 mg/d during the third week. The number of placebo capsules was increased accordingly up to 3 capsules per day. Primary outcome was the change from baseline to the end of study in the severity of hypersalivation as measured with the Nocturnal Hypersalivation Rating Scale and the Drooling Severity Scale. Secondary outcomes included Clinical Global Impression of Improvement scale and adverse effect scales. Significant improvement on the Nocturnal Hypersalivation Rating Scale was demonstrated in the metoclopramide group from the end of the second week (P < 0.004), and on the Drooling Severity Scale (P < 0.02) in the third week. Clinical Global Impression-Improvement scale scores revealed major improvement. Twenty subjects (66.7%) treated with metoclopramide reported significant decline or total disappearance of HAC in comparison to 8 patients (28.6%) who received placebo (P = 0.031). No adverse effects to metoclopramide were reported. Metoclopramide was found to be safe and effective for the treatment of HAC.

Long-acting muscarinic receptor antagonists for the treatment of respiratory disease

The use of muscarinic receptor antagonists in the treatment of chronic obstructive pulmonary disease (COPD) is well established. More recently, the potential for long-acting muscarinic receptor antagonists (LAMAs) in the treatment of asthma has also been investigated. While LAMAs offer advantages over short-acting muscarinic receptor antagonists, in terms of a reduced dosing frequency, there remains a need for therapies that improve symptom control throughout both the day and night, provide better management of exacerbations and deliver improved health-related quality of life. Furthermore, the potential for unwanted anticholinergic side effects, particularly cardiovascular effects, remains a concern for this class of compounds. Novel LAMAs in clinical development for the treatment of respiratory disease include: aclidinium bromide, NVA237 (glycopyrronium bromide), GP-MDI, EP-101, CHF-5259, umeclidinium bromide, CHF-5407, TD-4208, AZD8683 and V-0162. These compounds offer potential advantages in terms of onset of action, symptom control and safety. In addition, a number of LAMAs are also being developed as combination treatments with long-acting β2-agonists (LABAs) or inhaled glucocorticosteroids, potentially important treatment options for patients who require combination therapy to achieve an optimal therapeutic response as their disease progresses. More recently, compounds such as GSK961081 and THRX-198321 have been identified that combine LAMA and LABA activity in the same molecule, and have the potential to offer the benefits of combination therapy in a single compound. Here, we review novel LAMAs and dual action compounds in clinical development, with a particular focus on how they may address the current unmet clinical needs in the treatment of respiratory disease, particularly COPD.

Antimuscarinics for the treatment of overactive bladder: understanding the role of muscarinic subtype selectivity

INTRODUCTION AND HYPOTHESIS

Antimuscarinic agents appear to exert their therapeutic activity in overactive bladder (OAB) via blockade of the M(3) muscarinic receptor subtype. Antimuscarinics are broadly similar in efficacy, but their safety and tolerability profiles vary, which may reflect differences in muscarinic receptor selectivity profiles.

METHODS

This review of available literature aims to determine whether antimuscarinic agents with greater M(3) selectivity have clinical advantages over less selective drugs.

RESULTS

Antimuscarinic agents differ widely in their propensity to cause cognitive and cardiovascular (CV) effects, which appear mainly to be related to differences in their relative selectivity for binding to non-M(3) receptors, including M(1) receptors in the brain and cardiac M(2) receptors.

CONCLUSIONS

Cognitive and CV effects are especially pertinent for the OAB patient who tends to be older with various comorbidities and is often taking multiple medications. Hence, it is important to consider the risk/benefit balance of antimuscarinic agents when selecting OAB treatment.

Intestinal epithelial stem/progenitor cells are controlled by mucosal afferent nerves

Background

The maintenance of the intestinal epithelium is of great importance for the survival of the organism. A possible nervous control of epithelial cell renewal was studied in rats and mice.

Methods

Mucosal afferent nerves were stimulated by exposing the intestinal mucosa to capsaicin (1.6 mM), which stimulates intestinal external axons. Epithelial cell renewal was investigated in the jejunum by measuring intestinal thymidine kinase (TK) activity, intestinal ³H-thymidine incorporation into DNA, and the number of crypt cells labeled with BrdU. The influence of the external gut innervation was minimized by severing the periarterial nerves.

Principal Findings

Luminal capsaicin increased all the studied variables, an effect nervously mediated to judge from inhibitory effects on TK activity or ³H-thymidine incorporation into DNA by exposing the mucosa to lidocaine (a local anesthetic) or by giving four different neurotransmitter receptor antagonists i.v. (muscarinic, nicotinic, neurokinin1 (NK1) or calcitonin gene related peptide (CGRP) receptors). After degeneration of the intestinal external nerves capsaicin did not increase TK activity, suggesting the involvement of an axon reflex. Intra-arterial infusion of Substance P (SP) or CGRP increased intestinal TK activity, a response abolished by muscarinic receptor blockade. Immunohistochemistry suggested presence of M3 and M5 muscarinic receptors on the intestinal stem/progenitor cells. We propose that the stem/progenitor cells are controlled by cholinergic nerves, which, in turn, are influenced by mucosal afferent neuron(s) releasing acetylcholine and/or SP and/or CGRP. In mice lacking the capsaicin receptor, thymidine incorporation into DNA and number of crypt cells labeled with BrdU was lower than in wild type animals suggesting that nerves are important also in the absence of luminal capsaicin, a conclusion also supported by the observation that atropine lowered thymidine incorporation into DNA by 60% in control rat segments.

Conclusion

Enteric nerves are of importance in maintaining the intestinal epithelial barrier.

Expression of neural receptors in mouse meibomian gland

PURPOSE

To investigate gene and protein expression profiles of neural receptors found in the mouse meibomian gland. RNA and protein levels were determined for neuropeptide Y (NPY) receptor, vasoactive intestinal peptide (VIP) receptor, substance P (SP) receptor, and muscarinic cholinergic receptor (mAChR) subtypes M1-M5 in the mouse meibomian gland.

METHODS

Frozen sections of Balb/c mouse eyelids were subjected to laser capture microdissection to isolate pure samples of meibomian gland ductal and acinar cells. Real-time polymerase chain reaction, immunolabeling, and Western blot analysis for SP receptor, VIP receptor, NPY receptor, and mAChR subtypes M1-M5 were performed on meibomian gland ductal and acinar cells.

RESULTS

Expression of NPY1 receptor, VIP receptor 1, SP receptor, and all 5 mAChR subtypes was found in all meibomian gland ductal and acinar cells analyzed by real-time polymerase chain reaction. Immunolabeling and Western blot analysis confirmed the presence of NPY1 receptor, VIP receptor 1, SP receptor, and all 5 mAChR subtypes in the meibomian gland. The levels were variable with the duct showing greater levels of NPY1 receptor, SP receptor, and mAChRs 1, 2, 4, and 5 than with the gland.

CONCLUSIONS

VIP receptor 1, SP receptor, NPY1 receptor, and mAChR subtypes may be involved in the regulation of meibomian gland secretion. Laser capture microdissection in conjunction with gene expression analysis provides an excellent approach for studying meibomian gland cells about which relatively little is known at the molecular level.

Activation of transient receptor potential vanilloid subtype 1 increases secretion of the hypofunctional, transplanted submandibular gland

Hyposecretion occurs in most patients early after submandibular gland autotransplantation for severe keratoconjunctivitis sicca. Endogenous transient receptor potential vanilloid subtype 1 (TRPV1) has been recently demonstrated in rabbit submandibular glands, and activation of TRPV1 by capsaicin increases secretion in isolated glands, but the TRPV1-mediated secretory mechanism remains to be elucidated. The purpose of this study was to verify whether activation of TRPV1 by capsaicin could improve the secretion of transplanted gland and its underlying mechanism. The salivary flow of the transplanted glands was significantly decreased, and the mRNA and protein levels of TRPV1 and aquaporin 5 (AQP5) were downregulated in the transplanted glands. Topical capsaicin cream increased secretion and upregulated levels of TRPV1 and AQP5 in transplanted glands. Moreover, in cultured submandibular gland cells, capsaicin increased the mRNA expression of AQP5 and led to redistribution of AQP5 from the cytoplasm to the plasma membrane via TRPV1 activation. Capsaicin enhanced the phosphorylation of extracellular signal-regulated kinase (ERK). Preincubation of cells with PD98059, an inhibitor of ERK kinase, suppressed the capsaicin-induced mRNA expression of AQP5. In summary, the capsaicin-induced secretory mechanism involved activation of TRPV1 and upregulation of AQP5 in an ERK-dependent manner and promoted the redistribution of AQP5 in submandibular gland cells. Activation of TRPV1 may provide a new therapeutic strategy to improve submandibular gland hypofunction.

Characterization of Prejunctional Muscarinic Receptors: Effects on the Release of VIP and Functional Responses and Receptor Expression in the Ovine Submandibular Gland

In the in vivo experiments on anaesthetized sheep, it was presently examined whether muscarinic receptor antagonists with diverse selectivity affect the release of VIP in response to electrical stimulation of the parasympathetic chorda tympanic nerve differently, and if the changes in the release could be associated to altered secretory and vasodilator responses. The location of the muscarinic receptor subtypes was examined also. In the experiments, blood was collected out of the submandibular venous drainage before and during electrical stimulation of chorda tympani nerve in the absence and presence either of pirenzepine or methoctramine. While metchoctramine increased the output of protein, pirenzepine inhibited flow of saliva and increased protein output, vasodilatation, and VIP output. In morphological examinations, the inhibitory muscarinic M4 receptor occurred interacinarily in the gland. It is concluded that prejunctional muscarinic receptors, most likely of the M4 subtype, exert inhibitory modulation of the parasympathetic release of VIP in the ovine submandibular gland.

Central muscarinic receptor subtypes involved in pilocarpine-induced salivation, hypertension and water intake

BACKGROUND AND PURPOSE

Recent evidence has suggested that pilocarpine (ACh receptor agonist) injected peripherally may act centrally producing salivation and hypertension. In this study, we investigated the effects of specific M(1) (pirenzepine), M(2)/M(4) (methoctramine), M(1)/M(3) (4-DAMP) and M(4) (tropicamide) muscarinic receptor subtype antagonists injected into the lateral cerebral ventricle (LV) on salivation, water intake and pressor responses to peripheral pilocarpine.

EXPERIMENTAL APPROACH

Male Holtzman rats with stainless steel cannulae implanted in the LV were used. Salivation was measured in rats anaesthetized with ketamine (100 mg per kg body weight) and arterial pressure was recorded in unanaesthetized rats.

KEY RESULTS

Salivation induced by i.p. pilocarpine (4 micromol per kg body weight) was reduced only by 4-DAMP (25-250 nmol) injected into the LV, not by pirenzepine, methoctramine or tropicamide at the dose of 500 nmol. Pirenzepine (0.1 and 1 nmol) and 4-DAMP (5 and 10 nmol) injected into the LV reduced i.p. pilocarpine-induced water intake, whereas metoctramine (50 nmol) produced nonspecific effects on ingestive behaviours. Injection of pirenzepine (100 nmol) or 4-DAMP (25 and 50 nmol) into the LV reduced i.v. pilocarpine-induced pressor responses. Tropicamide (500 nmol) injected into the LV had no effect on pilocarpine-induced salivation, pressor responses or water intake.

CONCLUSIONS AND IMPLICATIONS

The results suggest that central M(3) receptors are involved in peripheral pilocarpine-induced salivation and M(1) receptors in water intake and pressor responses. The involvement of M(3) receptors in water intake and pressor responses is not clear because 4-DAMP blocks both M(1) and M(3) receptors.

Lack of specificity of commercially available antisera: better specifications needed

The ideal antiserum for immunohistochemical (IHC) applications contains monospecific high-affinity antibodies with little nonspecific adherence to sections. Many commercially available antibodies are "affinity" purified, but it is unknown if they meet "hard" specificity criteria, such as absence of staining in tissues genetically deficient for the antigen or a staining pattern that is identical to that of an antibody raised against a different epitope on the same protein. Reviewers, therefore, often require additional characterization. Although the affinity-purified antibodies used in our study on the distribution of muscarinic receptors produced selective staining patterns on sections, few passed the preabsorption test, and none produced bands of the anticipated size on Western blots. More importantly, none showed a difference in staining pattern on sections or Western blots between wild-type and knockout mice. Because these antibodies were used in most studies published thus far, our findings cast doubts on the validity of the extant body of morphological knowledge of the whole family of muscarinic receptors. We formulate requirements that antibody-specification data sheets should meet and propose that journals for which IHC is a core technique facilitate consumer rating of antibodies. "Certified" antibodies could avoid fruitless and costly validation assays and should become the standard of commercial suppliers.

Distinct pathways of ERK activation by the muscarinic agonists pilocarpine and carbachol in a human salivary cell line

Cholinergic-muscarinic receptor agonists are used to alleviate mouth dryness, although the cellular signals mediating the actions of these agents on salivary glands have not been identified. We examined the activation of ERK1/2 by two muscarinic agonists, pilocarpine and carbachol, in a human salivary cell line (HSY). Immunoblot analysis revealed that both agonists induced transient activation of ERK1/2. Whereas pilocarpine induced phosphorylation of the epidermal growth factor (EGF) receptor, carbachol did not. Moreover, ERK activation by pilocarpine, but not carbachol, was abolished by the EGF receptor inhibitor AG-1478. Downregulation of PKC by prolonged treatment of cells with the phorbol ester PMA diminished carbachol-induced ERK phosphorylation but had no effect on pilocarpine responsiveness. Depletion of intracellular Ca2+ ([Ca2+]i by EGTA did not affect ERK activation by either agent. In contrast to carbachol, pilocarpine did not elicit [Ca2+]i mobilization in HSY cells. Treatment of cells with the muscarinic receptor subtype 3 (M3) antagonist N-(3-chloropropyl)-4-piperidnyl diphenylacetate decreased ERK responsiveness to both agents, whereas the subtype 1 (M1) antagonist pirenzepine reduced only the carbachol response. Stimulation of ERKs by pilocarpine was also decreased by M3, but not M1, receptor small interfering RNA. The Src inhibitor PP2 blocked pilocarpine-induced ERK activation and EGF receptor phosphorylation, without affecting ERK activation by carbachol. Our results demonstrate that the actions of pilocarpine and carbachol in salivary cells are mediated through two distinct signaling mechanisms-pilocarpine acting via M3 receptors and Src-dependent transactivation of EGF receptors, and carbachol via M1/M3 receptors and PKC-converging on the ERK pathway.

Expression of muscarinic receptor subtypes in salivary glands of rats, sheep and man

In rat parotid, submandibular and sublingual glands and in ovine parotid and in human labial glands, the expression of muscarinic receptor subtypes was examined by immunoblotting and immunohistochemistry. Functional correlates were searched for in rat salivary glands. In the rat submandibular and sublingual glandular tissues clear signals of muscarinic M1 and M5 receptors could be detected in the immunoblotting and vague bands for muscarinic M3 and, in particular for, M4 receptors. The rat parotid gland differed. In this gland, the signal was less obvious for the muscarinic M1 receptor, and further, muscarinic M4 receptors appeared more strongly marked than in the submandibular glands. The results from the immunohistochemistry could be interpreted as the muscarinic M4 receptors are located on nerve fibres, since the outer layer of lobuli were densely stained. Intraglandular vessels in the rat submandibular and parotid glands showed expression of M3 receptors. In contrast to the parotid gland, the submandibular vessels also expressed M1 and M2 receptors. Occasionally M5 receptors appeared in the arteries and veins also. The functional studies in the rat confirmed muscarinic M1 receptor mediated secretion in the submandibular gland. Since the M1 receptor blockade did not affect submandibular blood flow, indirect vascular effects could not in total explain the secretory inhibition. Also in the human labial glands, muscarinic M1, M3 and M5 receptors occurred. No or low amounts of muscarinic M2 and M4 receptors could be detected. In patients with Sjögren-like symptoms an up-regulation of M3, M4 and M5 receptors was apparent in the labial glands. In ovine parotid glands all receptors could be detected, but constantly with vague bands for muscarinic M2 receptors. In conclusion, muscarinic M1 receptors seem to be expressed in seromucous/mucous glands. A secretory effect by muscarinic M5 receptors is not to be excluded, since they were expressed in all the glands examined. However, other functions, such as promotion of inflammation, cell growth and proliferation are possible as well.

Autoantibodies against M1 muscarinic acetylcholine receptor in myasthenic disorders

The Lambert-Eaton myasthenic syndrome (LEMS), often associated with small-cell lung carcinoma (SCLC), is a disorder of acetylcholine (ACh) release from motor nerve terminals. In most patients, it is caused by autoantibodies against the P/Q-type voltage-gated calcium channels (VGCC) that trigger ACh release. However, these antibodies are not detected in approximately 15% of clinically and electrophysiologically typical cases. The M1-type pre-synaptic muscarinic ACh receptor (M1 mAChR) modulates cholinergic neuromuscular transmission by linking to P/Q-type VGCC, and may partially compensate for the reduced calcium entry. Immunoblotting against solubilized human M1 mAChR, we detected autoantibodies in: (a) 14 of 20 (70%) anti-VGCC-positive LEMS patients; (b) all five anti-VGCC-negative LEMS patients, one of whose serum had previously passively transferred LEMS-type electrophysiological defects to mice; (c) all five LEMS patients with autonomic symptoms; (d) seven of 25 (28%) myasthenia gravis (MG) patients in whom increased ACh release partially compensates for post-synaptic defects; (e) none of 10 SCLC patients without LEMS. Although not proving primary pathogenicity of anti-M1 mAChR antibodies, the present results highlight their potential to affect synaptic compensatory mechanisms, more in LEMS than MG.