Muscarinic receptors in human airways

Nicotinic acetylcholine receptor stimulation: A new approach for stimulating tear secretion in dry eye disease

Tear secretion is regulated by the lacrimal functional unit consisting of afferent and efferent nerve innervation. The afferent arm consists of trigeminal nociceptors on the ocular surface and nasal mucosa. When stimulated by agonists, nicotinic acetylcholine receptors on nerve endings in the nose initiate a reflex arc resulting in instantaneous tear secretion. Pharmacologic nasal neural stimulation to increase endogenous tear production is a novel approach to treating dry eye disease.

Cholinergic Synapse Pathway Gene Polymorphisms Associated With Late-Phase Responses in Allergic Rhinitis

Allergic rhinitis (AR) is characterized by an early-phase response (EPR), and in a subgroup of individuals, a late-phase response (LPR). We sought to investigate polymorphisms in cholinergic synapse pathway genes, previously associated with late-asthmatic responses, in the LPR. Twenty healthy participants and 74 participants with AR underwent allergen exposure using the Environmental Exposure Unit. Allergic participants were sub-phenotyped using self-reported nasal congestion scores; congestion is the predominant symptom experienced during the LPR. Acute congestion (AC, n = 36) participants developed only an EPR, while persistent congestion (PC, n = 38) participants developed both allergic responses. We interrogated blood samples collected before allergen exposure with genotyping and gene expression assays. Twenty-five SNPs located in ADCY3, AKT3, CACNA1S, CHRM3, CHRNB2, GNG4, and KCNQ4 had significantly different allele frequencies (P < 0.10) between PC and AC participants. PC participants had increased minor allele content (P = 0.009) in the 25 SNPs compared to AC participants. Two SNPs in AKT3 were associated with gene expression differences (FDR < 0.01) in PC participants. This study identified an association between the LPR and polymorphisms in the cholinergic synapse pathway genes, and developed a novel method to sub-phenotype AR using self-reported nasal congestion scores.

Regiospecific Introduction of Halogens on the 2-Aminobiphenyl Subunit Leading to Highly Potent and Selective M3 Muscarinic Acetylcholine Receptor Antagonists and Weak Inverse Agonists

Muscarinic M₃ receptor antagonists and inverse agonists displaying high affinity and subtype selectivity over the antitarget M₂ are valuable pharmacological tools and may enable improved treatment of chronic obstructive pulmonary disease (COPD), asthma, or urinary incontinence. On the basis of known M₃ antagonists comprising a piperidine or quinuclidine unit attached to a biphenyl carbamate, 5-fluoro substitution was responsible for M₃ subtype selectivity over M₂, while 3'-chloro substitution substantially increased affinity through a σ-hole interaction. Resultantly, two piperidinyl- and two quinuclidinium-substituted biphenyl carbamates OFH243 (13n), OFH244 (13m), OFH3911 (14n), and OFH3912 (14m) were discovered, which display two-digit picomolar affinities with K_i values from 0.069 to 0.084 nM, as well as high selectivity over the M₂ subtype (46- to 68-fold). While weak inverse agonistic properties were determined for the biphenyl carbamates 13m and 13n, neutral antagonism was observed for 14m and 14n and tiotropium under identical assay conditions.

Technological and practical challenges of dry powder inhalers and formulations

In the 50 years following the introduction of the first dry powder inhaler to the market, several developments have occurred. Multiple-unit dose and multi-dose devices have been introduced, but first generation capsule inhalers are still widely used for new formulations. Many new particle engineering techniques have been developed and considerable effort has been put in understanding the mechanisms that control particle interaction and powder dispersion during inhalation. Yet, several misconceptions about optimal inhaler performance manage to survive in modern literature. It is, for example still widely believed that a flow rate independent fine particle fraction contributes to an inhalation performance independent therapy, that dry powder inhalers perform best at 4 kPa (or 60 L/min) and that a high resistance device cannot be operated correctly by patients with reduced lung function. Nevertheless, there seems to be a great future for dry powder inhalation. Many new areas of interest for dry powder inhalation are explored and with the assistance of new techniques like computational fluid dynamics and emerging particle engineering technologies, this is likely to result in a new generation of inhaler devices and formulations, that will enable the introduction of new therapies based on inhaled medicines.

The clinical relevance of dry powder inhaler performance for drug delivery

BACKGROUND

Although understanding of the scientific basis of aerosol therapy with dry powder inhalers (DPIs) has increased, some misconceptions still persist. These include the beliefs that high resistance inhalers are unsuitable for some patients, that extra fine (<1.0 μm) particles improve peripheral lung deposition and that inhalers with flow rate-independent fine particle fractions (FPFs) produce a more consistent delivered dose to the lungs.

OBJECTIVES

This article aims to clarify the complex inter-relationships between inhaler design and resistance, inspiratory flow rate (IFR), FPF, lung deposition and clinical outcomes, as a better understanding may result in a better choice of DPI for individual patients.

METHODS

The various factors that determine the delivery of drug particles into the lungs are reviewed. These include aerodynamic particle size distribution, the inspiratory manoeuvre, airway geometry and the three basic principles that determine the site and extent of deposition: inertial impaction, sedimentation and diffusion. DPIs are classed as either dependent or independent of inspiratory flow rate and vary in their internal resistance to inspiration. The effects of these characteristics on drug deposition in the airways are described using data from studies directly comparing currently available inhaler devices.

RESULTS

Clinical experience shows that most patients can use a high resistance DPI effectively, even during exacerbations. Particles in the aerodynamic size range from 1.5-5 μm are shown to be optimal, as particles <1.0 μm are very likely to be exhaled again while those >5 μm may impact on the oropharynx. For DPIs with a constant FPF at all flow rates, less of the delivered dose reaches the central and peripheral lung when the flow rate increases, risking under-dosing of the required medication. In contrast, flow rate-dependent inhalers increase their FPF output at higher flow rates, which compensates for the greater impaction on the upper airways as flow rate increases.

CONCLUSIONS

The technical characteristics of different inhalers and the delivery and deposition of the fine particle dose to the lungs may be important additional considerations to help the physician to select the most appropriate device for the individual patient to optimise their treatment.

Role of parasympathetic nerves and muscarinic receptors in allergy and asthma

Parasympathetic nerves control the symptoms and inflammation of allergic diseases primarily by signaling through peripheral muscarinic receptors. Parasympathetic signaling targets classic effector tissues such as airway smooth muscle and secretory glands and mediates acute symptoms of allergic disease such as airway narrowing and increased mucus secretion. In addition, parasympathetic signaling modulates inflammatory cells and non-neuronal resident cell types such as fibroblasts and smooth muscle contributing to chronic allergic inflammation and tissue remodeling. Importantly, muscarinic antagonists are experiencing a rebirth for the treatment of asthma and may be useful for treating other allergic diseases.

Asthma

Asthma has been recognized as a disease since the earliest times. In the Corpus Hippocraticum, Hippocrates used the term “ασθμα” to indicate any form of breathing difficulty manifesting itself by panting. Aretaeus of Cappadocia, a well-known Greek physician (second century A.D.), is credited with providing the first detailed description of an asthma attack [13], and to Celsus it was a disease with wheezing and noisy, violent breathing. In the history of Rome, we find many members of the Julio-Claudian family affected with probable atopic respiratory disorders: Caesar Augustus suffered from bronchoconstriction, seasonal rhinitis as well as a highly pruritic skin disease. Claudius suffered from rhinoconjunctivitis and Britannicus was allergic to horse dander [529]. Maimonides (1136–1204) warned that to neglect treatment of asthma could prove fatal, whereas until the 19th century, European scholars defined it as “nervous asthma,” a term that was given to mean a defect of conductivity of the ninth pair of cranial nerves.

The activation of Rac1 by M3 muscarinic acetylcholine receptors involves the translocation of Rac1 and IQGAP1 to cell junctions and changes in the composition of protein complexes containing Rac1, IQGAP1, and actin

The abilities of the M(3) muscarinic acetylcholine receptor (mAChR) and Rac1 to regulate similar cellular responses, including cadherin-mediated adhesion, prompted us to investigate Rac1 regulation by M(3) mAChR. We characterized changes in Rac1 induced by stimulating transfected M(3) mAChR in Chinese hamster ovary cells stably expressing hemagglutinin (HA)-tagged wild-type or mutant Rac1. mAChR activation converts endogenous Rac1 to the GTP-bound form in cells expressing HA-Rac1 but not in cells expressing dominant negative HA-Rac1(Asn-17) or constitutively active HA-Rac1(Val-12). The competitive binding of endogenous IQGAP1 by HA-Rac1(Val-12) may diminish the mAChR-mediated activation of endogenous Rac1. HA-Rac1 and HA-Rac1(Val-12), but not HA-Rac1(Asn-17), accumulate with IQGAP1 at cell junctions during mAChR-induced cell-cell compaction. Co-localization studies suggest that Rac1 can accumulate at junctions without IQGAP1, but IQGAP1 cannot accumulate at junctions without Rac1. mAChR activation also induces GTP-independent changes in Rac1 because mAChR activation redistributes HA-Rac1(Asn-17), which does not bind GTP. Actin associates with complexes containing HA-Rac1 or HA-Rac1(Val-12) after prolonged mAChR activation. We also demonstrate that Rac1 participates in mAChR-induced cell-cell compaction and c-Jun phosphorylation. These results indicate that M(3) mAChR activation converts Rac1 to the GTP-bound form, alters interactions between Rac1, IQGAP1, and actin, and causes the junctional accumulation of Rac1 and IQGAP1.

Excitatory non-adrenergic-non-cholinergic neuropeptides: key players in asthma

Professor David de Wied first introduced the term 'neuropeptides' at the end of 1971. Later peptide hormones and their fragments, endogenous opioid (morphine-like) peptides and a large number of other biogenic peptides became classified as neuropeptides. All of these peptides are united by a number of common features including their origin (nervous system and peptide-secreting cells found in various organs such as skin, gut, lungs), biosynthesis, secretion, metabolism, and enormous effectiveness. Neuropeptides are biologically active at extremely low concentrations. The past decade, neuropeptide research has revealed that neuropeptides also participate strongly in immune reactions. The neuro-immune concept has opened up a whole new research area. In the last 20 years, significant advances have been made in investigations of the interaction between immune and nervous systems in chronic inflammatory diseases such as asthma. The goal of this review is to bring together the functional relevance of excitatory non-adrenergic-non-cholinergic (NANC) nerves and the interaction with the immune system in asthma.

Effects of inhalation exposures to an M1-receptor agonist on ventilation in rhesus monkeys

Information was needed on effects of possible occupational inhalation exposure to an M1-receptor agonist (xanomeline) such as might occur during the manufacturing process. Both acute and repeated inhalation exposures to xanomeline were carried out in six male rhesus monkeys using a head-dome exposure system. Exposure concentrations ranged from 0.3 to 10 mg/m3. The exposure durations were up to 2 weeks. Decreases in tidal volume and increases in respiratory frequency were both time and concentration related during acute exposures. These effects were blocked with atropine pre-treatment. Correlation with pulmonary resistance measurements in two monkeys suggested that these were bronchoconstrictive changes that increased with severity with time at a given concentration and with concentration when measured after a constant exposure time. The dose-response was relatively steep with 10 mg/m3 becoming intolerable to the monkeys after approximately 15 minutes, but no measurable effects were observed at 0.3 mg/m3 after up to 4 hours of exposure. To investigate the effects of repeated exposures, monkeys were exposed for 4 hr/day, 5 days/wk for 2 weeks to 0.0 (air only), 0.3, and 1.2 mg xanomeline/m3 of air. When compared to the air-only exposure, 0.3 mg/m3 caused no significant changes in tidal volume. In contrast, 1.2 mg/m3 caused a rapid and significant decrease in tidal volume that was sustained throughout the 4-hr exposure. A slower rise in breathing frequency also occurred. Repeated exposures did not alter the effects seen after a single exposure. It is concluded that xanomeline, a M1-receptor agonist, can acutely alter normal ventilation in non-human primates at airborne concentrations > or = 0.6 mg/m3 and should be carefully controlled in a manufacturing environment. The no-observed-effect concentration was 0.3 mg/m3.

Genetically transmitted cholinergic hyperresponsiveness predisposes to experimental asthma

The excitatory innervation of the airway smooth muscle is primarily cholinergic in nature. However, the potential neural mechanism(s) underlying airway hyperresponsiveness, one of the hallmarks of asthma, is not fully understood. In this study, cholinergic hyperresponsive Flinders sensitive line (FSL) rats and their control counterparts, Flinders resistant line (FRL) rats, were repeatedly challenged with different doses of nebulized methacholine (0, 4, 16, 64, and 256 mg/ml) for 5 min. Airway responsiveness was assessed in spontaneously breathing, unrestrained animals by means of whole body plethysmography. Increased airway responsiveness of FSL rats was evidenced as a more pronounced increase in Penh value (enhanced pause, an index of bronchoconstriction) across different concentrations of methacholine. In subsequent experiments, FSL and FRL rats were sensitized to ovalbumin and challenged with nebulized antigen. Our results indicate that the genetically transmitted cholinergic hyperresponsiveness of the FSL rat is paralleled by an increased susceptibility to allergen-induced bronchoconstriction and inflammation of the airways. This study provides further evidence that neural factors can play an important role in determining airway responsiveness and thus may be relevant for the expression of asthma. In addition, the FSL rat may be a useful model for studies of airway hyperresponsiveness.

Diagnosis and management of rhinitis: complete guidelines of the Joint Task Force on Practice Parameters in Allergy, Asthma and Immunology. American Academy of Allergy, Asthma, and Immunology

This document contains complete guidelines for diagnosis and management of rhinitis developed by the Joint Task Force on Practice Parameters in Allergy, Asthma and Immunology, representing the American Academy of Allergy, Asthma and Immunology, the American College of Allergy, Asthma and Immunology and the Joint Council on Allergy, Asthma and Immunology. The guidelines are comprehensive and begin with statements on clinical characteristics and diagnosis of different forms of rhinitis (allergic, non-allergic, occupational rhinitis, hormonal rhinitis [pregnancy and hypothyroidism], drug-induced rhinitis, rhinitis from food ingestion), and other conditions that may be confused with rhinitis. Recommendations on patient evaluation discuss appropriate use of history, physical examination, and diagnostic testing, as well as unproven or inappropriate techniques that should not be used. Parameters on management include use of environmental control measures, pharmacologic therapy including recently introduced therapies and allergen immunotherapy. Because of the risks to patients and society from sedation and performance impairment caused by first generation antihistamines, second generation antihistamines that reduce or eliminate these side effects should usually be considered before first generation antihistamines for the treatment of allergic rhinitis. The document emphasizes the importance of rhinitis management for comorbid conditions (asthma, sinusitis, otitis media). Guidelines are also presented on special considerations in patients subsets (children, the elderly, pregnancy, athletes and patients with rhinitis medicamentosa); and when consultation with an allergist-immunologist should be considered.

Superiority of ipratropium plus albuterol over albuterol alone in the emergency department management of adult asthma: a randomized clinical trial

STUDY OBJECTIVE

The use of nebulized ipratropium in combination with beta-agonists for the treatment of acute asthma in adults is controversial. We wished to test the hypothesis that combined aerosol treatment results in a greater rate of airflow improvement and a lower proportion of hospital admission in adults with acute asthma.

METHODS

In a randomized, double-blind, placebo-controlled trial, 55 adult asthmatic patients with peak expiratory flow rate (PEFR) less than 200 L/min were randomly assigned to nebulization treatment with albuterol alone (2.5 mg initial dose followed by 2 more doses at 20-min intervals.), or the same albuterol regimen plus ipratropium (.5 mg combined with the initial dose of albuterol only). Patients were recruited from an emergency department at an urban academic medical center. The primary endpoints were changes in PEFR and in percent predicted PEFR over time (ie, treatment by time effect). PEFRs were assessed at baseline and at 20-minute intervals for a 1-hour period. The proportion of admissions in the two groups were examined as secondary endpoints.

RESULTS

The increases in PEFR and percent predicted PEFR over time were both significantly greater in the combined ipratropium plus albuterol treatment group (P < or = .001). In addition, the proportion admitted patients was less in this group (3/27) than the proportion in the albuterol-only group (10/28). The 95% confidence interval for the absolute difference of 25% in the proportion admitted was 3% to 46%, P = .03. Most of the baseline clinical and historical features in the two groups were similar.

CONCLUSION

These data suggest that ipratropium should be combined with initial albuterol nebulization in the ED treatment of acute asthma in adults, especially those with PEFRs less than 200 L/min.

M2 receptor activation of nonselective cation channels in smooth muscle cells: calcium and Gi/G(o) requirements

Muscarinic stimulation of fura 2-loaded smooth muscle cells evoked a rapidly inactivating Ca(2+)-activated Cl- current [ICl(Ca)] and a sustained nonselective cation current (Icat) as well as a transient (delta Ca(tran)) and a sustained (delta Ca(sus)) elevation of cytosolic Ca2+ concentration ([Ca2+]i). Caffeine and inositol 1,4,5-trisphosphate induced delta Ca(tran) and ICl(Ca) but not Icat or delta Ca(sus). M2 receptor antagonism blocked muscarinic activation of Icat and delta Ca(sus) but not ICl(Ca) and delta Ca(tran). M3 antagonism blocked activation of ICl(Ca) and Icat and a rise in [Ca2+]i, but application of caffeine with methacholine restored Icat and delta Ca(sus). After depletion of intracellular Ca2+ stores, methacholine failed to induce Icat or a [Ca2+]i increase and, in pertussis toxin-treated cells, ICl(Ca) and delta Ca(tran) but not Icat or delta Ca(sus) were evoked. Anti-G alpha i-1/G alpha i-2 antibodies and anti-G alpha i-3/ G(o) alpha antibodies blocked Icat but did not affect ICl(Ca). Anti-Gq alpha/ G alpha 11 antibodies greatly inhibited ICl(Ca) but did not affect Icat. Activation of M2 receptors leads to the opening of nonselective cation channels through Gi/G(o) proteins in smooth muscle cells, resulting in a sustained rise in [Ca2+]i. Arise in [Ca2+]i is necessary but not sufficient for activation of nonselective cation channels.

Selective muscarinic receptor agonists and antagonists

Muscarinic receptors are composed of a family of four subtypes each of which can be distinguished pharmacologically and structurally. The physiological role of each subtype in the central and peripheral nervous systems remains to be clarified, due, in part, to a lack of agonists and antagonists with adequate subtype selectivity. Nonetheless, several agonists with functional selectivity for M1 receptors are now in advanced clinical evaluation for Alzheimer's disease, while selective M1/M3 antagonists may prove useful in the treatment of disorders of smooth muscle function. These novel compounds thus provide an advance over earlier therapeutics with which the clinical efficacy was compromised by the side effect profile. This review attempts to assess novel, selective agonists and antagonists, both in terms of their use in defining muscarinic receptor subtypes and their potential clinical utility.