M(3)-receptor knockout mice: muscarinic receptor function in atria, stomach fundus, urinary bladder, and trachea

Hybrid Allosteric Modulators of M1 Muscarinic Receptors Enhance Acetylcholine Efficacy and Decrease Locomotor Activity and Turning Behaviors in Zebrafish

Allosteric modulation of muscarinic acetylcholine receptors (mAChR) has been identified as a potential strategy for regulating cholinergic signaling in the treatment of various neurological disorders. Most positive allosteric modulators (PAMs) of mAChR enhance agonist affinity and potency, while very few PAMs selectively enhance G-protein coupling efficacy (e.g., amiodarone). The key structural features of amiodarone responsible for enhancement of mAChR efficacy were examined in CHO cells expressing M1 receptors. Subsequent incorporation of these structural features into previously identified allosteric modulators of potency (i.e., n-benzyl isatins) generated hybrid ligands that demonstrated similar or better enhancement of mAChR efficacy, lower in vivo toxicity, and higher allosteric binding affinity relative to amiodarone. Notable hybrid ligands include 8a and 8b which respectively demonstrated the strongest binding affinity and the most robust enhancement of mAChR efficacy as calculated from an allosteric operational model. Amiodarone derivatives and hybrid ligands were additionally screened in wildtype zebrafish (Danio rerio) to provide preliminary in vivo toxicity data as well as to observe effects on locomotor and turning behaviors relative to other mAChR PAMs. Several compounds, including 8a and 8c, reduced locomotor activity and increased measures of turning behaviors in zebrafish, suggesting that allosteric modulation of muscarinic receptor efficacy might be useful in the treatment of repetitive behaviors associated with autism spectrum disorder (ASD) and other neuropsychiatric disorders.

Diseases which cause generalized peripheral neuropathy: a systematic review

PURPOSE

Peripheral autonomic neuropathy, including enteric neuropathy, may be subtle and unrecognized for several years. Diagnosis of enteric neuropathy demands complicated examinations such as full-thickness bowel biopsy. We hypothesized that knowledge about simultaneous occurrence of different types of neuropathy would lead to faster recognition and diagnosis of autonomic/enteric neuropathy. The aim of the present systematic review was to increase the awareness of disease groups causing autonomic and enteric neuropathy along with sensorimotor neuropathy.

METHODS

A systematic search strategy was used in PubMed, Embase and Web of Science. First, 4978 articles were identified. Review of titles/abstracts rendered exclusion of animal studies, articles not written in English or full-length, case reports, conference abstracts and duplicates until 357 articles remained. The full-length evaluation resulted in 35 studies (27 non-systematic reviews) which described objectively verified peripheral autonomic, enteric and sensorimotor neuropathy within the same disease.

RESULTS

Diabetes is the most common disease in society rendering generalized peripheral neuropathy. Accumulation of tissue deposits in amyloidosis, Lewy body disorders and sarcoidosis lead to widespread peripheral neuropathy. Several autoimmune disorders such as systemic sclerosis and primary Sjögren's syndrome present themselves with neuropathy. Paraneoplastic neuropathy may appear prior to symptoms from the malignancy. Both the infection per se, as well as the autoimmune response to the infection, i.e., Guillain-Barré syndrome, may lead to widespread peripheral neuropathy. Hereditary disorders with disturbed metabolism lead to intermittent attacks of neuropathy.

CONCLUSIONS

The major causes of generalized peripheral neuropathy are diabetes, diseases with tissue deposits, autoimmunity, infections, malignancy and metabolic diseases.

Cardiac nicotinic receptors show β-subunit-dependent compensatory changes

Nicotinic receptors (NRs) play an important role in the cholinergic regulation of heart functions, and converging evidence suggests a diverse repertoire of NR subunits in the heart. A recent hypothesis about the plasticity of β NR subunits suggests that β2-subunits and β4-subunits may substitute for each other. In our study, we assessed the hypothetical β-subunit interchangeability in the heart at the level of mRNA. Using two mutant mice strains lacking β2 or β4 NR subunits, we examined the relative expression of NR subunits and other key cholinergic molecules. We investigated the physiology of isolated hearts perfused by Langendorff's method at basal conditions and after cholinergic and/or adrenergic stimulation. Lack of β2 NR subunit was accompanied with decreased relative expression of β4-subunits and α3-subunits. No other cholinergic changes were observed at the level of mRNA, except for increased M3 and decreased M4 muscarinic receptors. Isolated hearts lacking β2 NR subunit showed different dynamics in heart rate response to indirect cholinergic stimulation. In hearts lacking β4 NR subunit, increased levels of β2-subunits were observed together with decreased mRNA for acetylcholine-synthetizing enzyme and M1 and M4 muscarinic receptors. Changes in the expression levels in β4^-/- hearts were associated with increased basal heart rate and impaired response to a high dose of acetylcholine upon adrenergic stimulation. In support of the proposed plasticity of cardiac NRs, our results confirmed subunit-dependent compensatory changes to missing cardiac NRs subunits with consequences on isolated heart physiology.NEW & NOTEWORTHY In the present study, we observed an increase in mRNA levels of the β2 NR subunit in β4^-/- hearts but not vice versa, thus supporting the hypothesis of β NR subunit plasticity that depends on the specific type of missing β-subunit. This was accompanied with specific cholinergic adaptations. Nevertheless, isolated hearts of β4^-/- mice showed increased basal heart rate and a higher sensitivity to a high dose of acetylcholine upon adrenergic stimulation.

Functions of Muscarinic Receptor Subtypes in Gastrointestinal Smooth Muscle: A Review of Studies with Receptor-Knockout Mice

Parasympathetic signalling via muscarinic acetylcholine receptors (mAChRs) regulates gastrointestinal smooth muscle function. In most instances, the mAChR population in smooth muscle consists mainly of M2 and M3 subtypes in a roughly 80% to 20% mixture. Stimulation of these mAChRs triggers a complex array of biochemical and electrical events in the cell via associated G proteins, leading to smooth muscle contraction and facilitating gastrointestinal motility. Major signalling events induced by mAChRs include adenylyl cyclase inhibition, phosphoinositide hydrolysis, intracellular Ca2+ mobilisation, myofilament Ca2+ sensitisation, generation of non-selective cationic and chloride currents, K+ current modulation, inhibition or potentiation of voltage-dependent Ca2+ currents and membrane depolarisation. A lack of ligands with a high degree of receptor subtype selectivity and the frequent contribution of multiple receptor subtypes to responses in the same cell type have hampered studies on the signal transduction mechanisms and functions of individual mAChR subtypes. Therefore, novel strategies such as genetic manipulation are required to elucidate both the contributions of specific AChR subtypes to smooth muscle function and the underlying molecular mechanisms. In this article, we review recent studies on muscarinic function in gastrointestinal smooth muscle using mAChR subtype-knockout mice.

Novel M2 -selective, Gi -biased agonists of muscarinic acetylcholine receptors

BACKGROUND AND PURPOSE

More than 30% of currently marketed medications act via GPCRs. Thus, GPCRs represent one of the most important pharmacotherapeutic targets. In contrast to traditional agonists activating multiple signalling pathways, agonists activating a single signalling pathway represent a new generation of drugs with increased specificity and fewer adverse effects.

EXPERIMENTAL APPROACH

We have synthesized novel agonists of muscarinic ACh receptors and tested their binding and function (on levels of cAMP and inositol phosphates) in CHO cells expressing individual subtypes of muscarinic receptors, primary cultures of rat aortic smooth muscle cells and suspensions of digested native tissues from rats. Binding of the novel compounds to M₂ receptors was modelled in silico.

KEY RESULTS

Two of the tested new compounds (1-(thiophen-2-ylmethyl)-3,6-dihydro-2H-pyridinium and 1-methyl-1-(thiophen-2-ylmethyl)-3,6-dihydro-2H-pyridinium) only inhibited cAMP synthesis in CHO cells, primary cultures, and native tissues, with selectivity for M₂ muscarinic receptors and displaying bias towards the G_i signalling pathway at all subtypes of muscarinic receptors. Molecular modelling revealed interactions with the orthosteric binding site in a way specific for a given agonist followed by agonist-specific changes in the conformation of the receptor.

CONCLUSIONS AND IMPLICATIONS

The identified compounds may serve as lead structures in the search for novel non-steroidal and non-opioid analgesics acting via M₂ and M₄ muscarinic receptors with reduced side effects associated with activation of the phospholipase C signalling pathway.

Choline Attenuates Cardiac Fibrosis by Inhibiting p38MAPK Signaling Possibly by Acting on M3 Muscarinic Acetylcholine Receptor

Choline has been reported to produce a variety of cellular functions including cardioprotection via activating M₃ muscarinic acetylcholine receptor (M₃R) under various insults. However, whether choline offers similar beneficial effects via the same mechanism in cardiac fibrosis remained unexplored. The present study aimed to investigate the effects of choline on cardiac fibrosis and the underlying signaling mechanisms, particularly the possible involvement of M₃R. Transverse aortic constriction (TAC) mouse model was established to simulate the cardiac fibrosis. Transforming growth factor (TGF)-β1 treatment was employed to induce proliferation of cardiac fibroblasts in vitro. Choline chloride and M₃R antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) were used to unravel the potential role of M₃R. Cardiac function was assessed by echocardiography and interstitial fibrosis was quantified by Masson staining. Protein levels of collagens I and III were determined by Western blot analysis. The role of M₃R in the proliferation cardiac fibroblasts was validated by silencing M₃R with specific small interference RNA (siRNA). Furthermore, the mitogen-activated protein kinase (MAPK) signaling pathway including p38MAPK and ERK1/2 as well as the TGF-β1/Smad pathway were analyzed. M₃R protein was found abundantly in cardiac fibroblasts. M₃R protein level, as identified by Western blotting, was higher in mice with excessive cardiac fibrosis and in TGF-β1-induced cardiac fibrosis as well. Choline significantly inhibited interstitial fibrosis, and this beneficial action was reversed by 4-DAMP. Production of collagens I and III was reduced after choline treatment but restored by 4-DAMP. Expression silence of endogenous M₃R using siRNA increased the level of collagen I. Furthermore, the TGF-β1/Smad2/3 and the p38MAPK pathways were both suppressed by choline. In summary, choline produced an anti-fibrotic effect both in vivo and in vitro by regulating the TGF-β1/Smad2/3 and p38MAPK pathways. These findings unraveled a novel pharmacological property of choline linked to M₃R, suggesting that choline regulates cardiac fibrosis and the associated heart diseases possibly by acting on M₃R.

Distribution and function of the muscarinic receptor subtypes in the cardiovascular system

Muscarinic acetylcholine receptors belong to the G protein-coupled receptor superfamily and are widely known to mediate numerous functions within the central and peripheral nervous system. Thus, they have become attractive therapeutic targets for various disorders. It has long been known that the parasympathetic system, governed by acetylcholine, plays an essential role in regulating cardiovascular function. Unfortunately, due to the lack of pharmacologic selectivity for any one muscarinic receptor, there was a minimal understanding of their distribution and function within this region. However, in recent years, advancements in research have led to the generation of knockout animal models, better antibodies, and more selective ligands enabling a more thorough understanding of the unique role muscarinic receptors play in the cardiovascular system. These advances have shown muscarinic receptor 2 is no longer the only functional subtype found within the heart and muscarinic receptors 1 and 3 mediate both dilation and constriction in the vasculature. Although muscarinic receptors 4 and 5 are still not well characterized in the cardiovascular system, the recent generation of knockout animal models will hopefully generate a better understanding of their function. This mini review aims to summarize recent findings and advances of muscarinic involvement in the cardiovascular system.

Extracellular acidosis selectively inhibits pharmacomechanical coupling induced by carbachol in strips of rat gastric fundus

NEW FINDINGS

What is the central question of this study? Acute acidosis that results from short-term exercise is involved in delayed gastric emptying in rats and the lower responsiveness of gastric fundus strips to carbachol. Does extracellular acidosis decrease responsiveness to carbachol in tissues of sedentary rats? How? What is the main finding and its importance? Extracellular acidosis inhibits cholinergic signalling in the rat gastric fundus by selectively influencing the G_q/11 protein signalling pathway. Acute acidosis that results from short-term exercise delays gastric emptying in rats and decreases the responsiveness to carbachol in gastric fundus strips. The regulation of cytosolic Ca²⁺ concentrations appears to be a mechanism of action of acidosis. The present study investigated the way in which acidosis interferes with gastric smooth muscle contractions. Rat gastric fundus isolated strips at pH 6.0 presented a lower magnitude of carbachol-induced contractions compared with preparations at pH 7.4. This lower magnitude was absent in carbachol-stimulated duodenum and KCl-stimulated gastric fundus strips. In Ca²⁺ -free conditions, repeated contractions that were induced by carbachol progressively decreased, with no influence of extracellular pH. In fundus strips, CaCl₂ -induced contractions were lower at pH 6.0 than at pH 7.4 but only when stimulated in the combined presence of carbachol and verapamil. In contrast, verapamil-sensitive contractions that were induced by CaCl₂ in the presence of KCl did not change with pH acidification. In Ca²⁺ store-depleted preparations that were treated with thapsigargin, the contractions that were induced by extracellular Ca²⁺ restoration were smaller at pH 6.0 than at pH 7.4, but relaxation that was induced by SKF-96365 (an inhibitor of store-operated Ca²⁺ entry) was unaltered by extracellular acidification. At pH 6.0, the phospholipase C inhibitor U-73122 relaxed carbachol-induced contractions less than at pH 7.4, and this phenomenon was absent in tissue that was treated with the RhoA kinase blocker Y-27632. Thus, extracellular acidosis inhibited pharmacomechanical coupling in gastric fundus by selectively inhibiting the G_q/11 protein signalling pathway, whereas electromechanical coupling remained functionally preserved.

β1/2 or M2/3 Receptors Are Required for Different Gastrointestinal Motility Responses Induced by Acupuncture at Heterotopic or Homotopic Acupoints

Acupuncture at homotopic acupoints or heterotopic acupoints is known to either inhibit or facilitate gastrointestinal motility, depending on the acupoint location. However, little effort has been made to investigate the roles of specific receptors (such as adrenergic and muscarinic acetylcholine receptors) in mediating the effects of acupuncture at heterotopic and homotopic acupoints. Different adrenergic receptor subtypes or cholinergic receptor subtypes are predominantly expressed in various sections of the gut, resulting in variations between the effects of acupuncture at heterotopic or homotopic acupoints on gastrointestinal motility. Here, we investigated the role of β₁/β₂ receptors and M₂/M₃ receptors in gastrointestinal motility regulated by acupuncture at ST37, a heterotopic acupoint, and ST25, a homotopic acupoint, by simultaneously recording intraluminal pressures in the distal colon and stomach or jejunum and examining fecal phenol red excretion in β_1/2 receptor-knockout mice and M_2/3 receptor-knockout mice. We found that knockout of the M_2/3 receptor significantly inhibited ST37 acupuncture-induced enhancement of gastric motility, jejunal motility, and colonic motility. Additionally, knocking out of the β_1/2 receptor significantly diminished the ST25 acupuncture-induced inhibition of gastric motility and jejunal motility without significantly altering the enhancement of colonic motility induced by acupuncture at ST25. Acupuncture at ST37 significantly accelerated gastrointestinal transition in β_1/2 receptor-knockout mice and their wild-type littermates. However, this acceleration of gastrointestinal transition was markedly diminished in M_2/3 receptor-knockout mice relative to their wild-type littermates. Acupuncture at ST25 significantly increased gastrointestinal transition in β_1/2 receptor-knockout mice and significantly decreased gastrointestinal transition in M_2/3 receptor-knockout mice without altering gastrointestinal transition in wild-type littermates of either. Our study revealed that M_2/3 receptors are required for the gastrointestinal motility associated with whole gastrointestinal transition enhanced by acupuncture at heterotopic acupoints, whereas β_1/2 receptors are required for the same gastrointestinal motility processes inhibited by acupuncture at homotopic acupoints. Therefore, our findings reveal important biological mechanisms underlying acupuncture treatment of disorders involving gastrointestinal motility dysfunction.

Acupuncture at heterotopic acupoints facilitates distal colonic motility via activating M3 receptors and somatic afferent C-fibers in normal, constipated, or diarrhoeic rats

BACKGROUND

Previous studies have demonstrated the efficacy of somatic stimulation for patients with gastrointestinal motility disorders. However, little effort has been made to investigate the effects of acupuncture on colonic motility, particularly in pathological conditions. The precise mechanism employed in the regulation of acupuncture on colonic motility still remains unclear.

METHODS

We assessed the effect of acupuncture at heterotopic acupoints on distal colonic motility using a warm-water-filled manometric balloon inserted 5-6 cm into the rectum of anesthetized normal rats or rats with diarrhea or constipation. Choline chloride, 4-DAMP, cobra venom and capsaicin were separately applied to investigate the role of M3 receptors in the regulation of distal colonic motility by acupuncture at heterotopic acupoints, and whether Aδ- and/or C-fibers are required for triggering distal colonic motility by acupuncture.

KEY RESULTS

Acupuncture at heterotopic acupoints increased distal colonic motility not only in normal rats but also in rats with constipation or diarrhea. M3 receptors play an important role in the facilitation of distal colonic motility triggered by acupuncture at heterotopic acupoints. Afferent nerve Aδ- and C-fibers mediate the transduction of the acupuncture signal and C-fibers are essential for enhancing the effect of acupuncture at the heterotopic acupoint on distal colonic motility.

CONCLUSIONS & INFERENCES

Our results reveal that acupuncture at heterotopic acupoints increases distal colonic motility regardless of normal or pathological conditions via predominately activating C-fibers of somatic afferent nerve and M3 receptors.

Role of various kinases in muscarinic M3 receptor-mediated contraction of longitudinal muscle of rat colon

The longitudinal muscle layer in gut is the functional opponent to the circular muscle layer during peristalsis. Differences in innervation of the layers allow for the contraction of one layer concurrently with the relaxation of the other, enabling the passage of gut contents in a controlled fashion. Differences in development have given the cells of the two layers differences in receptor populations, membrane lipid handling, and calcium handling profiles/behaviors. The contractile activity of the longitudinal muscle is largely mediated by cholinergic neural input from myenteric plexus. Activation of muscarinic receptors leads to rapid activation of several kinases including MLC kinase, ERK1/2, CaMKII and Rho kinase. Phosphorylation of myosin light chain (MLC₂₀) by MLC kinase (MLCK) is a prerequisite for contraction in both circular and longitudinal muscle cells. In rat colonic longitudinal muscle strips, we measured muscarinic receptor-mediated contraction following incubation with kinase inhibitors. Basal tension was differentially regulated by Rho kinase, ERK1/2, CaMKII and CaMKK. Selective inhibitors of Rho kinase, ERK1/2, CaMKK/AMPK, and CaMKII each reduced carbachol-induced contraction in the innervated muscle strips. These inhibitors had no direct effect on MLCK activity. Thus unlike previously reported for isolated muscle cells where CaMKII and ERK1/2 are not involved in contraction, we conclude that the regulation of carbachol-induced contraction in innervated longitudinal muscle strips involves the interplay of Rho kinase, ERK1/2, CaMKK/AMPK, and CAMKII.

Regulation of neuronal communication by G protein-coupled receptors

Neuronal communication plays an essential role in the propagation of information in the brain and requires a precisely orchestrated connectivity between neurons. Synaptic transmission is the mechanism through which neurons communicate with each other. It is a strictly regulated process which involves membrane depolarization, the cellular exocytosis machinery, neurotransmitter release from synaptic vesicles into the synaptic cleft, and the interaction between ion channels, G protein-coupled receptors (GPCRs), and downstream effector molecules. The focus of this review is to explore the role of GPCRs and G protein-signaling in neurotransmission, to highlight the function of GPCRs, which are localized in both presynaptic and postsynaptic membrane terminals, in regulation of intrasynaptic and intersynaptic communication, and to discuss the involvement of astrocytic GPCRs in the regulation of neuronal communication.

Assessing the agonist profiles of the prostacyclin analogues treprostinil and naxaprostene, particularly their DP₁ activity

In this study, the inhibitory profiles of the prostacyclin analogues treprostinil and naxaprostene on several isolated smooth muscle preparations have been investigated. Treprostinil was an agonist for prostanoid DP1, EP2 and IP receptors, but not EP4 receptors; its DP1 potency was only 3-4 times less than PGD2 itself. Naxaprostene was much more selective for IP receptors and tended towards partial agonism. Treprostinil is a 13,14-dihydro analogue and the role of conformation around C12-15 in controlling agonist specificity is debated; the synthesis of new analogues is proposed and possible clinical usage discussed. In terms of selective prostanoid antagonists employed, BW-A868C/MK-0524 (DP1), ACA-23 (EP2) and GW-627368 (EP4) were found fit for purpose. However, the IP antagonist RO-1138452 was compromised by α1 and α2-adrenoceptor-mediated contractile activity on rat tail artery and anti-muscarinic activity on mouse trachea. There is a need for IP receptor antagonists with better selectivity and higher affinity.

Acupuncture at heterotopic acupoints enhances jejunal motility in constipated and diarrheic rats

AIM: To investigate the effect and mechanism of acupuncture at heterotopic acupoints on jejunal motility, particularly in pathological conditions.

METHODS: Jejunal motility was assessed using a manometric balloon placed in the jejunum approximately 18-20 cm downstream from the pylorus and filled with approximately 0.1 mL warm water in anesthetized normal rats or rats with diarrhea or constipation. The heterotopic acupoints including LI11 (Quchi), ST37 (Shangjuxu), BL25 (Dachangshu), and the homotopic acupoint ST25 (Tianshu), were stimulated for 60 s by rotating acupuncture needles right and left at a frequency of 2 Hz. To determine the type of afferent fibers mediating the regulation of jejunal motility by manual acupuncture, the ipsilateral sciatic A or C fibers of ST37 were inactivated by local application of the A-fiber selective demyelination agent cobra venom or the C fiber blocker capsaicin. Methoctramine, a selective M₂ receptor antagonist, was injected intravenously to identify a specific role of M₂ receptors in mediating the effect of acupuncture on jejunal motility.

RESULTS: Acupuncture at heterotopic acupoints, such as LI11 and ST37, increased jejunal motility not only in normal rats, but also in rats with constipation or diarrhea. In normal rats, manual acupuncture at LI11 or ST37 enhanced jejunal pressure from 7.34 ± 0.19 cmH₂O to 7.93 ± 0.20 cmH₂O, an increase of 9.05% ± 0.82% (P < 0.05), and from 6.95 ± 0.14 cmH₂O to 8.97 ± 0.22 cmH₂O, a significant increase of 27.44% ± 1.96% (P < 0.01), respectively. In constipated rats, manual acupuncture at LI11 or ST37 increased intrajejunal pressure from 8.17 ± 0.31 cmH₂O to 9.86 ± 0.36 cmH₂O, an increase of 20.69% ± 2.10% (P < 0.05), and from 8.82 ± 0.28 cmH₂O to 10.83 ± 0.28 cmH₂O, an increase of 22.81% ± 1.46% (P < 0.05), respectively. In rats with diarrhea, MA at LI11 or ST37 increased intrajejunal pressure from 11.95 ± 0.35 cmH₂O to 13.96 ± 0.39 cmH₂O, an increase of 16.82% ± 2.35% (P < 0.05), and tended to increase intrajejunal pressure (from 12.42 ± 0.38 cmH₂O to 13.05 ± 0.38 cmH₂O, an increase of 5.07% ± 1.08%, P > 0.05), respectively. In contrast, acupuncture ST25, a homotopic acupoint, not only decreased intrajejunal pressure, but also significantly decreased frequency in normal rats and rats with constipation or diarrhea. Following demyelination of Aδ fibers, acupuncture at ST37 again augmented intrajejunal pressure to 121.48% ± 3.06% of baseline. Following capsaicin application for 24 h, acupuncture at ipsilateral ST37 increased intrajejunal pressure to 106.63% ± 1.26% of basal levels when compared to measurements prior to capsaicin treatment (P < 0.05). Acupuncture at LI11, ST37, or BL25 significantly rescued methoctramine-mediated inhibition of jejunal motility amplitude from 42.83% ± 1.65% to 53.43% ± 1.95% of baseline (P < 0.05), from 45.15% ± 2.22% to 70.51% ± 2.34% of baseline (P < 0.01), and from 38.03% ± 2.34% to 70.12% ± 2.22% of baseline (P < 0.01), respectively.

CONCLUSION: Acupuncture at heterotopic acupoints increases the amplitude of jejunal motility in rats. C fibers and M₂ receptors predominantly and (or) partially mediate the regulation of jejunal motility by acupuncture, respectively.

Functional effects of KCNQ K(+) channels in airway smooth muscle

KCNQ (K_v7) channels underlie a voltage-gated K⁺ current best known for control of neuronal excitability, and its inhibition by G_q/11-coupled, muscarinic signaling. Studies have indicated expression of KCNQ channels in airway smooth muscle (ASM), a tissue that is predominantly regulated by muscarinic receptor signaling. Therefore, we investigated the function of KCNQ channels in rodent ASM and their interplay with G_q/11-coupled M₃ muscarinic receptors. Perforated-patch clamp of dissociated ASM cells detected a K⁺ current inhibited by the KCNQ antagonist, XE991, and augmented by the specific agonist, flupirtine. KCNQ channels begin to activate at voltages near resting potentials for ASM cells, and indeed XE991 depolarized resting membrane potentials. Muscarinic receptor activation inhibited KCNQ current weakly (~20%) at concentrations half-maximal for contractions. Thus, we were surprised to see that KCNQ had no affect on membrane voltage or muscle contractility following muscarinic activation. Further, M₃ receptor-specific antagonist J104129 fumarate alone did not reveal KCNQ effects on muscarinic evoked depolarization or contractility. However, a role for KCNQ channels was revealed when BK-K⁺ channel activities are reduced. While KCNQ channels do control resting potentials, they appear to play a redundant role with BK calcium-activated K⁺ channels during ASM muscarinic signaling. In contrast to effect of antagonist, we observe that KCNQ agonist flupirtine caused a significant hyperpolarization and reduced contraction in vitro irrespective of muscarinic activation. Using non-invasive whole animal plethysmography, the clinically approved KCNQ agonist retigabine caused a transient reduction in indexes of airway resistance in both wild type and BK β1 knockout (KO) mice treated with the muscarinic agonist. These findings indicate that KCNQ channels can be recruited via agonists to oppose muscarinic evoked contractions and may be of therapeutic value as bronchodilators.

Gastric body cholinergic contractile signal transduction in M2 and M3 receptor knockout mice

Although most smooth muscles express a greater density of M2 than M3 muscarinic receptors, based on the potency of subtype selective muscarinic receptor antagonists, the M3 subtype predominantly mediates contraction. The effect of inhibitors of putative contractile signal transduction pathway enzymes on carbachol-induced contractions was determined in wild-type (WT) mice and mice lacking either the M2 (M2KO) or the M3 (M3KO) receptor subtype. Contractile responses to KCl, then increasing carbachol concentrations in the presence and absence of enzyme inhibitors was determined. The KCl-induced contraction was not different between strains. The carbachol response was unaffected in the M2KO strain but decreased 42% in M3KO mice (p < 0.01). Darifenacin potency was high in both WT and M2KO strains, indicating M3-mediated contractions, and low in the M3KO strain, suggesting M2-mediated contractions. The phosphatidyl inositol-specific phospholipase C (Pi-PLC) inhibitor ET-18-OCH3 had no effect. Inhibition of phosphatidyl choline-specific phospholipase C (PC-PLC) and sphingomyelin synthase with D609 decreased maximal contraction in all strains. M3-mediated contractions in the M2KO strain were decreased 54% by the protein kinase C (PKC) inhibitor chelerythrine. M2-mediated contractions in the M3KO and WT strains were decreased by the Rho kinase (ROCK) inhibitor Y27632 as well as the ROCK, PKA and PKG inhibitor H89. The M3 subtype activates PKC and either PC-PLC or sphingomyelin synthase, while the M2 subtype activates ROCK and either PC-PLC or sphingomyelin synthase. These studies suggest that multiple parallel pathways mediate cholinergic contractions in stomach body smooth muscle.

Evidence for M2 and M3 muscarinic receptor involvement in cholinergic excitatory junction potentials through synergistic activation of cation channels in the longitudinal muscle of mouse ileum

Cholinergic nerve-mediated excitatory junction potentials (EJPs) in the longitudinal muscle of mouse ileum were characterized by using M2 or M3 muscarinic receptor-knockout (KO) mice and 1-[β-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SK&F 96365) and pertussis toxin (PTX). EJPs evoked by electrical field stimulation (EFS) in wild-type preparations, initially determined to be cholinergic in origin using tetrodotoxin, atropine, and eserine, were profoundly depressed after SK&F 96365 treatment known to block muscarinic receptor-operated cation channels. A similar depression of the EJPs was also observed by PTX treatment, which is predicted to disrupt M2-mediated pathways linked to cation channel activation. In M2-KO mouse preparations, cholinergic EJPs were evoked by EFS with their relative amplitude of 20%-30% to the wild-type EJP and strongly inhibited by SK&F 96365. No cholinergic EJP was seen in M3-KO as well as M2/M3 double-KO preparations. The results suggest that the wild-type cholinergic EJP is not a simple mixture of M2 and M3 responses, but due to synergistic activation of cation channels by both M2 and M3 receptors in the murine ileal longitudinal muscle.

Simo Decoction Stimulates Contractions of Antral Longitudinal Smooth Muscle via Multitudinous Mechanisms

The aim of the study was to investigate Simo decoction–induced contractions of antral smooth muscles of rats and its mechanisms. The contractile responses of longitudinal strips to consecutive concentrations of Simo decoction were characterized by atropine, gallamine, 4-diphenylacetoxy-N-methylpiperidine methiodide, and adrenaline, hexamethonium, L-arginine, and nifedipine and compared with Krebs solution (control) and acetylcholine-induced contractions. Simo decoction dose-dependently increased contractions of antral strips ( P = .000 vs control); its maximal effect was higher than acetylcholine (10−3 mol L−1; P < .05); Simo decoction–induced contractions were completely inhibited by atropine, 4-diphenylacetoxy-N-methylpiperidine methiodide, or 4-diphenylacetoxy-N-methylpiperidine methiodide + gallamine ( P = .000 for all) but were partly suppressed by gallamine, adrenaline, hexamethonium, L-arginine, and nifedipine ( P = .000 for all). Simo decoction promotes the contractions of antral strips mainly through activation of muscarinic M3 receptor, while partly through activation of M2 receptor, Ca2+ channel, nicotinic receptor, and inhibition of adrenergic receptor as well as release of nitric oxide.

Simo decoction promotes contraction of antral circular smooth muscle mainly via muscarinic M3 receptor

ETHNOPHARMACOLOGICAL RELEVANCE

Simo Decoction (SMD), a traditional Chinese medicine, included four elements, such as Fructus aurantii, Radix aucklandiae, Semen arecae and Radix linderae. It has been used to improve gastrointestinal dysmotility in clinical practice for a long history in China. However, the explicit mechanisms are unclear. The aim of this study was to investigate the effect of SMD on contractions of antral circular smooth muscle strips of Sprague-Dawley (SD) rats and its underlying mechanism.

MATERIALS AND METHODS

The antral circular strips were prepared in the organ bath under baseline or to be incubated with muscarinic receptor antagonist atropine (10(-6)M), muscarinic M3 receptor antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) (0.4×10(-6)M), muscarinic M2 receptor antagonist gallamine (10(-6)M), adrenergic receptor agonist adrenaline (10(-7)M), exogenous nitric oxide (NO) donor l-arginine (10(-4)M), nicotinic receptor antagonist hexamethonium chloride (10(-4)M) and Ca(2+) channel antagonist nifedipine (30nM), and consecutive concentrations of SMD were added to the bath to observe the strip responses. As a control, the responses of strips after administration with the same volume of Krebs solution as SMD were also noted. The strip responses to acetylcholine (10(-7)-10(-3)M) were also noted in organ bath to compare with SMD-induced contraction.

RESULTS

SMD dose-dependently evoked hypercontractility of antral circular strips, and the maximal contractile effect of circular smooth muscle induced by SMD was significantly higher than that induced by acetylcholine (10(-3)M). The responses of antral circular strips to SMD were completely antagonized by atropine, 4-DAMP or 4-DAMP+gallamine, but partly inhibited by gallamine and partly suppressed by adrenaline, l-arginine, hexamethonium chloride and nifedipine.

CONCLUSIONS

SMD promotes contractions of antral circular strips in rats mainly via activation of muscarinic M3 receptor, but partly via activation of muscarinic M2 receptor, Ca(2+) channel and nicotinic receptor, inhibition of adrenergic receptor and releasing of NO.

Muscarinic receptors on airway mesenchymal cells: novel findings for an ancient target

Since ancient times, anticholinergics have been used as a bronchodilator therapy for obstructive lung diseases. Targets of these drugs are G-protein-coupled muscarinic M(1), M(2) and M(3) receptors in the airways, which have long been recognized to regulate vagally-induced airway smooth muscle contraction and mucus secretion. However, recent studies have revealed that acetylcholine also exerts pro-inflammatory, pro-proliferative and pro-fibrotic actions in the airways, which may involve muscarinic receptor stimulation on mesenchymal, epithelial and inflammatory cells. Moreover, acetylcholine in the airways may not only be derived from vagal nerves, but also from non-neuronal cells, including epithelial and inflammatory cells. Airway smooth muscle cells seem to play a major role in the effects of acetylcholine on airway function. It has become apparent that these cells are multipotent cells that may reversibly adopt (hyper)contractile, proliferative and synthetic phenotypes, which are all under control of muscarinic receptors and differentially involved in bronchoconstriction, airway remodeling and inflammation. Cholinergic contractile tone is increased by airway inflammation associated with asthma and COPD, resulting from exaggerated acetylcholine release as well as increased expression of contraction related proteins in airway smooth muscle. Moreover, muscarinic receptor stimulation promotes proliferation of airway smooth muscle cells as well as fibroblasts, and regulates cytokine, chemokine and extracellular matrix production by these cells, which may contribute to airway smooth muscle growth, airway fibrosis and inflammation. In line, animal models of chronic allergic asthma and COPD have recently demonstrated that tiotropium may potently inhibit airway inflammation and remodeling. These observations indicate that muscarinic receptors have a much larger role in the pathophysiology of obstructive airway diseases than previously thought, which may have important therapeutic implications.

In vitro measurements of tracheal constriction using mice

Transgenic and knockout mice have been powerful tools for the investigation of the physiology and pathophysiology of airways(1,2). In vitro tensometry of isolated tracheal preparations has proven to be a useful assay of airway smooth muscle (ASM) contractile response in genetically modified mice. These in vitro tracheal preparations are relatively simple, provide a robust response, and retain both functional cholinergic nerve endings and muscle responses, even after long incubations. Tracheal tensometry also provides a functional assay to study a variety of second messenger signaling pathways that affect contraction of smooth muscle. Contraction in trachea is primarily mediated by parasympathetic, cholinergic nerves that release acetylcholine onto ASM (Figure 1). The major ASM acetylcholine receptors are muscarinic M2 and M3 which are G(i/o ;)and Gq coupled receptors, respectively(3,4,5). M3 receptors evoke contraction by coupling to Gq to activate phospholipase C, increase IP3 production and IP3-mediated calcium release from the sarcoplasmic reticulum(3,6,7). M2/G(i/o ;)signaling is believed to enhance contractions by inhibition of adenylate cyclase leading to a decrease in cAMP levels(5,8,9,10). These pathways constitute the so called "pharmaco-contraction coupling" of airway smooth muscle(11). In addition, cholinergic signaling through M2 receptors (and modulated by M3 signaling) involves pathways that depolarize the ASM which in turn activate L-type, voltage-dependent calcium channels (Figure 1) and calcium influx (so called "excitation-contraction coupling")(4,7). More detailed reviews on signaling pathways controlling airway constriction can be found(4,12). The above pathways appear to be conserved between mice and other species. However, mouse tracheas differ from other species in some signaling pathways. Most prominent is their lack of contractile response to histamine and adenosine(13,14), both well-known ASM modulators in humans and other species(5,15). Here we present protocols for the isolation of murine tracheal rings and the in vitro measurement of their contractile output. Included are descriptions of the equipment configuration, trachea ring isolation and contractile measurements. Examples are given for evoking contractions indirectly using high potassium stimulation of nerves and directly by depolarization of ASM muscle to activate voltage-dependent calcium influx (1. high K(+), Figure 1). In addition, methods are presented for stimulations of nerves alone using electric field stimulation (2. EFS, Figure 1), or for direct stimulation of ASM muscle using exogenous neurotransmitter applied to the bath (3. exogenous ACH, Figure 1). This flexibility and ease of preparation renders the isolated trachea ring model a robust and functional assay for a number of signaling cascades involved in airway smooth muscle contraction.

Muscarinic agonists and antagonists: effects on gastrointestinal function

Muscarinic agonists and antagonists are used to treat a handful of gastrointestinal (GI) conditions associated with impaired salivary secretion or altered motility of GI smooth muscle. With regard to exocrine secretion, the major muscarinic receptor expressed in salivary, gastric, and pancreatic glands is the M₃ with a small contribution of the M₁ receptor. In GI smooth muscle, the major muscarinic receptors expressed are the M₂ and M₃ with the M₂ outnumbering the M₃ by a ratio of at least four to one. The antagonism of both smooth muscle contraction and exocrine secretion is usually consistent with an M₃ receptor mechanism despite the major presence of the M₂ receptor in smooth muscle. These results are consistent with the conditional role of the M₂ receptor in smooth muscle. That is, the contractile role of the M₂ receptor depends on that of the M₃ so that antagonism of the M₃ receptor eliminates the response of the M₂. The physiological roles of muscarinic receptors in the GI tract are consistent with their known signaling mechanisms. Some so-called tissue-selective M₃ antagonists may owe their selectivity to a highly potent interaction with a nonmuscarinic receptor target.

Cardiac effects of muscarinic receptor antagonists used for voiding dysfunction

Antimuscarinic agents are the main drugs used to treat patients with the overactive bladder (OAB) syndrome, defined as urgency, with or without urgency incontinence, usually with increased daytime frequency and nocturia. Since the treatment is not curative and since OAB is a chronic disease, treatment may be life-long. Antimuscarinics are generally considered to be ‘safe’ drugs, but among the more serious concerns related to their use is the risk of cardiac adverse effects, particularly increases in heart rate (HR) and QT prolongation and induction of polymorphic ventricular tachycardia (torsade de pointes). An elevated resting HR has been linked to overall increased morbidity and mortality, particularly in patients with cardiovascular diseases. QT prolongation and its consequences are not related to blockade of muscarinic receptors, but rather linked to inhibition of the hERG potassium channel in the heart. However, experience with terodiline, an antimuscarinic drug causing torsade de pointes in patients, has placed the whole drug class under scrutiny. The potential of the different antimuscarinic agents to increase HR and/or prolong the QT time has not been extensively explored for all agents in clinical use. Differences between drugs cannot be excluded, but risk assessments based on available evidence are not possible.

Muscarinic mechanisms in psychotic disorders

Schizophrenia is a devastating disease with several broad symptom clusters and the current monoamine-based treatments do not adequately treat the disease, especially negative and cognitive symptoms. A proposed alternative approach for treating schizophrenia is through the use of compounds that activate certain muscarinic receptor subtypes, the so-called muscarinic cholinergic hypothesis theory. This theory has been revitalized with a number of recent and provocative findings including postmortem reports in schizophrenia patients showing decreased numbers of muscarinic M(1) and M(4) receptors in brain regions associated with schizophrenia as well as decreased muscarinic receptors in an in vivo imaging study. Studies with M(4) knockout mice have shown that there is a reciprocal relationship between M(4) and dopamine receptor function, and a number of muscarinic agonists have shown antidopaminergic activity in a variety of preclinical assays predictive of antipsychotic efficacy in the clinic. Furthermore, the M(1)/M(4) preferring partial agonist xanomeline has been shown to have antipsychotic-like and pro-cognitive activity in preclinical models and in clinical trials to decrease psychotic-like behaviors in Alzheimer's patients and positive, negative, and cognitive symptoms in patients with schizophrenia. Therefore, we propose that an agonist with M(1) and M(4) interactions would effectively treat core symptom clusters associated with schizophrenia. Currently, research is focused on developing subtype-selective muscarinic agonists and positive allosteric modulators that have reduced propensity for parasympathetic side-effects, but retain the therapeutic benefit observed with their less selective predecessors.

BK channel β1 subunits regulate airway contraction secondary to M2 muscarinic acetylcholine receptor mediated depolarization

The large conductance calcium- and voltage-activated potassium channel (BK channel) and its smooth muscle-specific β1 subunit regulate excitation–contraction coupling in many types of smooth muscle cells. However, the relative contribution of BK channels to control of M2- or M3-muscarinic acetylcholine receptor mediated airway smooth muscle contraction is poorly understood. Previously, we showed that knockout of the BK channel β1 subunit enhances cholinergic-evoked trachea contractions. Here, we demonstrate that the enhanced contraction of the BK β1 knockout can be ascribed to a defect in BK channel opposition of M2 receptor-mediated contractions. Indeed, the enhanced contraction of β1 knockout is eliminated by specific M2 receptor antagonism. The role of BK β1 to oppose M2 signalling is evidenced by a greater than fourfold increase in the contribution of L-type voltage-dependent calcium channels to contraction that otherwise does not occur with M2 antagonist or with β1 containing BK channels. The mechanism through which BK channels oppose M2-mediated recruitment of calcium channels is through a negative shift in resting voltage that offsets, rather than directly opposes, M2-mediated depolarization. The negative shift in resting voltage is reduced to similar extents by BK β1 knockout or by paxilline block of BK channels. Normalization of β1 knockout baseline voltage with low external potassium eliminated the enhanced M2-receptor mediated contraction. In summary, these findings indicate that an important function of BK/β1 channels is to oppose cholinergic M2 receptor-mediated depolarization and activation of calcium channels by restricting excitation–contraction coupling to more negative voltage ranges.

The M2-muscarinic receptor inhibits the development of streptozotocin-induced neuropathy in mouse urinary bladder

We investigate the role of M(2)-muscarinic receptors in maintaining neurogenic bladder contraction during hyperglycemia. Mice were injected with a single dose of streptozotocin (125 mg/kg), and neurogenic contraction of urinary bladder from wild type and M(2)-muscarinic receptor knockout (M(2) KO) mice was measured at 8 to 24 weeks after treatment. In wild-type bladder lacking urothelium, the summation of the cholinergic (64%) and purinergic (56%) components of the electrical-field-stimulated response exceeded 100%, indicating a reserve capacity. Although the cholinergic component was slightly less in the M(2) KO mouse, the total electrical-field-stimulated contraction was the same as wild type. The cholinergic and purinergic components of contraction in wild-type bladder were minimally affected by streptozotocin treatment. In M(2) KO bladder, streptozotocin treatment reduced both the cholinergic (after 8-9 and 20-24 weeks) and purinergic (after 20-24 weeks only) components. The loss of function was approximately 50 to 70%. Similar results were observed in bladder with intact urothelium. M(2) KO bladder was more sensitive to the relaxant effect of isoproterenol compared with wild type, and this difference significantly increased at the early and late time points after streptozotocin treatment. In the presence of urothelium, however, this difference in isoproterenol sensitivity was smaller with streptozotocin treatment, but this trend reversed over time. Our results show that M(2) receptors oppose urinary bladder distension in wild-type bladder and inhibit streptozotocin-induced neuropathy.

Nerve-released acetylcholine contracts urinary bladder smooth muscle by inducing action potentials independently of IP3-mediated calcium release

Nerve-released ACh is the main stimulus for contraction of urinary bladder smooth muscle (UBSM). Here, the mechanisms by which ACh contracts UBSM are explored by determining Ca(2+) and electrical signals induced by nerve-released ACh. Photolysis of caged inositol 1,4,5-trisphosphate (IP(3)) evoked Ca(2+) release from the sarcoplasmic reticulum. Electrical field stimulation (20 Hz) induced Ca(2+) waves within the smooth muscle that were present only during stimulus application. Ca(2+) waves were blocked by inhibition of muscarinic ACh receptors (mAChRs) with atropine and depletion of sarcoplasmic reticulum Ca(2+) stores with cyclopiazonic acid (CPA), and therefore likely reflect activation of IP(3) receptors (IP(3)Rs). Electrical field stimulation also increased excitability to induce action potentials (APs) that were accompanied by Ca(2+) flashes, reflecting Ca(2+) entry through voltage-dependent Ca(2+) channels (VDCCs) during the action potential. The evoked Ca(2+) flashes and APs occurred as a burst with a lag time of approximately 1.5 s after onset of stimulation. They were not inhibited by blocking IP(3)-mediated Ca(2+) waves, but by blockers of mAChRs (atropine) and VDCCs (diltiazem). Nerve-evoked contractions of UBSM strips were greatly reduced by blocking VDCCs, but not by preventing IP(3)-mediated Ca(2+) signaling with cyclopiazonic acid or inhibition of PLC with U73122. These results indicate that ACh released from nerve varicosities induces IP(3)-mediated Ca(2+) waves during stimulation; but contrary to expectations, these signals do not appear to participate in contraction. In addition, our data provide compelling evidence that UBSM contractions evoked by nerve-released ACh depend on increased excitability and the resultant Ca(2+) entry through VDCCs during APs.

Impaired M3 and enhanced M2 muscarinic receptor contractile function in a streptozotocin model of mouse diabetic urinary bladder

We investigated the contractile roles of M2 and M3 muscarinic receptors in urinary bladder from streptozotocin-treated mice. Wild-type and M2 muscarinic receptor knockout (M2 KO) mice were given a single injection of vehicle or streptozotocin (125 mg kg(-1)) 2-24 weeks prior to bladder assays. The effect of forskolin on contractions elicited to the muscarinic agonist, oxotremorine-M, was measured in isolated urinary bladder (intact or denuded of urothelium). Denuded urinary bladder from vehicle-treated wild-type and M2 KO mice exhibited similar contractile responses to oxotremorine-M, when contraction was normalized relative to that elicited by KCl (50 mM). Eight to 9 weeks after streptozotocin treatment, the EC(50) value of oxotremorine-M increased 3.1-fold in urinary bladder from the M2 KO mouse (N = 5) compared to wild type (N = 6; P < 0.001). Analogous changes were observed in intact bladder. In denuded urinary bladder from vehicle-treated mice, forskolin (5 microM) caused a much greater inhibition of contraction in M2 KO bladder compared to wild type. Following streptozotocin treatment, this forskolin effect increased 1.6-fold (P = 0.032). At the 20- to 24-week time point, the forskolin effect increased 1.7-fold for denuded as well as intact bladders (P = 0.036, 0.01, respectively). Although streptozotocin treatment inhibits M3 receptor-mediated contraction in denuded urinary bladder, muscarinic contractile function is maintained in wild-type bladder by enhanced M2 contractile function. M2 receptor activation opposes forskolin-induced relaxation of the urinary bladder, and this M(2) function is enhanced following streptozotocin treatment.

Bile acid-induced arrhythmia is mediated by muscarinic M2 receptors in neonatal rat cardiomyocytes

Background

Intrahepatic cholestasis of pregnancy (ICP) is a common disease affecting up to 5% of pregnancies and which can cause fetal arrhythmia and sudden intrauterine death. We previously demonstrated that bile acid taurocholate (TC), which is raised in the bloodstream of ICP, can acutely alter the rate and rhythm of contraction and induce abnormal calcium destabilization in cultured neonatal rat cardiomyocytes (NRCM). Apart from their hepatic functions bile acids are ubiquitous signalling molecules with diverse systemic effects mediated by either the nuclear receptor FXR or by a recently discovered G-protein coupled receptor TGR5. We aim to investigate the mechanism of bile-acid induced arrhythmogenic effects in an in-vitro model of the fetal heart.

Methods and Results

Levels of bile acid transporters and nuclear receptor FXR were studied by quantitative real time PCR, western blot and immunostaining, which showed low levels of expression. We did not observe functional involvement of the canonical receptors FXR and TGR5. Instead, we found that TC binds to the muscarinic M₂ receptor in NRCM and serves as a partial agonist of this receptor in terms of inhibitory effect on intracellular cAMP and negative chronotropic response. Pharmacological inhibition and siRNA-knockdown of the M₂ receptor completely abolished the negative effect of TC on contraction, calcium transient amplitude and synchronisation in NRCM clusters.

Conclusion

We conclude that in NRCM the TC-induced arrhythmia is mediated by the partial agonism at the M₂ receptor. This mechanism might serve as a promising new therapeutic target for fetal arrhythmia.

Distinct muscarinic acetylcholine receptor subtypes contribute to stability and growth, but not compensatory plasticity, of neuromuscular synapses

Muscarinic acetylcholine receptors (mAChRs) modulate synaptic function, but whether they influence synaptic structure remains unknown. At neuromuscular junctions (NMJs), mAChRs have been implicated in compensatory sprouting of axon terminals in paralyzed or denervated muscles. Here we used pharmacological and genetic inhibition and localization studies of mAChR subtypes at mouse NMJs to demonstrate their roles in synaptic stability and growth but not in compensatory sprouting. M(2) mAChRs were present solely in motor neurons, whereas M(1), M(3), and M(5) mAChRs were associated with Schwann cells and/or muscle fibers. Blockade of all five mAChR subtypes with atropine evoked pronounced effects, including terminal sprouting, terminal withdrawal, and muscle fiber atrophy. In contrast, methoctramine, an M(2/4)-preferring antagonist, induced terminal sprouting and terminal withdrawal, but no muscle fiber atrophy. Consistent with this observation, M(2)(-/-) but no other mAChR mutant mice exhibited spontaneous sprouting accompanied by extensive loss of parental terminal arbors. Terminal sprouting, however, seemed not to be the causative defect because partial loss of terminal branches was common even in the M(2)(-/-) NMJs without sprouting. Moreover, compensatory sprouting after paralysis or partial denervation was normal in mice deficient in M(2) or other mAChR subtypes. We also found that many NMJs of M(5)(-/-) mice were exceptionally small and reduced in proportion to the size of parental muscle fibers. These findings show that axon terminals are unstable without M(2) and that muscle fiber growth is defective without M(5). Subtype-specific muscarinic signaling provides a novel means for coordinating activity-dependent development and maintenance of the tripartite synapse.

Muscarinic receptor-mediated bronchoconstriction is coupled to caveolae in murine airways

Cholinergic bronchoconstriction is mediated by M(2) and M(3) muscarinic receptors (MR). In heart and urinary bladder, MR are linked to caveolin-1 or -3, the structural proteins of caveolae. Caveolae are cholesterol-rich, omega-shaped invaginations of the plasma membrane. They provide a scaffold for multiple G protein receptors and membrane-bound enzymes, thereby orchestrating signaling into the cell interior. Hence, we hypothesized that airway MR signaling pathways are coupled to caveolae as well. To address this issue, we determined the distribution of caveolin isoforms and MR subtype M2R in murine and human airways and investigated protein-protein associations by fluorescence resonance energy transfer (FRET)-confocal laser scanning microscopy (CLSM) analysis in immunolabeled murine tissue sections. Bronchoconstrictor responses of murine bronchi were recorded in lung-slice preparations before and after caveolae disruption by methyl-β-cyclodextrin, with efficiency of this treatment being validated by electron microscopy. KCl-induced bronchoconstriction was unaffected after treatment, demonstrating functional integrity of the smooth muscle. Caveolae disruption decreased muscarine-induced bronchoconstriction in wild-type and abolished it in M2R(-/-) and M3R(-/-) mice. Thus M2R and M3R signaling pathways require intact caveolae. Furthermore, we identified a presumed skeletal and cardiac myocyte-specific caveolin isoform, caveolin-3, in human and murine bronchial smooth muscle and found it to be associated with M2R in situ. In contrast, M2R was not associated with caveolin-1, despite an in situ association of caveolin-1 and caveolin-3 that was detected. Here, we demonstrated that M2R- and M3R-mediated bronchoconstriction is caveolae-dependent. Since caveolin-3 is directly associated with M2R, we suggest caveolin-3 as novel regulator of M2R-mediated signaling.

Loss of M2 muscarinic receptor function inhibits development of hypoxic bradycardia and alters cardiac beta-adrenergic sensitivity in larval zebrafish (Danio rerio)

Fish exposed to hypoxia develop decreased heart rate, or bradycardia, the physiological significance of which remains unknown. The general muscarinic receptor antagonist atropine abolishes the development of this hypoxic bradycardia, suggesting the involvement of muscarinic receptors. In this study, we tested the hypothesis that the hypoxic bradycardia is mediated specifically by stimulation of the M(2) muscarinic receptor, the most abundant subtype in the vertebrate heart. Zebrafish (Danio rerio) were reared at two levels of hypoxia (30 and 40 Torr PO(2)) from the point of fertilization. In hypoxic fish, the heart rate was significantly lower than in normoxic controls from 2 to 10 days postfertilization (dpf). At the more severe level of hypoxia (30 Torr PO(2)), there were significant increases in the relative mRNA expression of M(2) and the cardiac type beta-adrenergic receptors (beta1AR, beta2aAR, and beta2bAR) at 4 dpf. The hypoxic bradycardia was abolished (at 40 Torr PO(2)) or significantly attenuated (at 30 Torr PO(2)) in larvae experiencing M(2) receptor knockdown (using morpholino antisense oligonucleotides). Sham-injected larvae exhibited typical hypoxic bradycardia in both hypoxic regimens. The expression of beta1AR, beta2aAR, beta2bAR, and M(2) mRNA was altered at various stages between 1 and 4 dpf in larvae experiencing M(2) receptor knockdown. Interestingly, M(2) receptor knockdown revealed a cardioinhibitory role for the beta(2)-adrenergic receptor. This is the first study to demonstrate a specific role of the M(2) muscarinic receptor in the initiation of hypoxic bradycardia in fish.

Similarities and differences in the autonomic control of airway and urinary bladder smooth muscle

The airways and the urinary bladder are both hollow organs serving very different functions, i.e. air flow and urine storage, respectively. While the autonomic nervous system seems to play only a minor if any role in the physiological regulation of airway tone during normal breathing, it is important in the physiological regulation of bladder smooth muscle contraction and relaxation. While both tissues share a greater expression of M2 than of M3 muscarinic receptors, smooth muscle contraction in both is largely mediated by the smaller M3 population apparently involving phospholipase C activation to only a minor if any extent. While smooth muscle in both tissues can be relaxed by beta-adrenoceptor stimulation, this primarily involves beta2-adrenoceptors in human airways and beta3-adrenoceptors in human bladder. Despite activation of adenylyl cyclase by either subtype, cyclic adenosine monophosphate plays only a minor role in bladder relaxation by beta-agonists; an important but not exclusive function is known in airway relaxation. While airway beta2-adrenoceptors are sensitive to agonist-induced desensitization, beta3-adrenoceptors are generally considered to exhibit much less if any sensitivity to desensitization. Gene polymorphisms exist in the genes of both beta2- and beta3-adrenoceptors. Despite being not fully conclusive, the available data suggest some role of beta2-adrenoceptor polymorphisms in airway function and its treatment by receptor agonists, whereas the available data on beta3-adrenoceptor polymorphisms and bladder function are too limited to allow robust interpretation. We conclude that the distinct functions of airways and urinary bladder are reflected in a differential regulation by the autonomic nervous system. Studying these differences may be informative for a better understanding of each tissue.

M2 muscarinic receptors induce airway smooth muscle activation via a dual, Gbetagamma-mediated inhibition of large conductance Ca2+-activated K+ channel activity

Airway smooth muscle is richly endowed with muscarinic receptors of the M(2) and M(3) subtype. Stimulation of these receptors inhibits large conductance calcium-activated K(+) (BK) channels, a negative feed back regulator, in a pertussis toxin-sensitive manner and thus facilitates contraction. The underlying mechanism, however, is unknown. We therefore studied the activity of bovine trachea BK channels in HEK293 cells expressing the M(2) or M(3) receptor (M(2)R or M(3)R). In M(2)R- but not M(3)R-expressing cells, maximal effective concentrations of carbamoylcholine (CCh) inhibited whole cell BK currents by 53%. This M(2)R-induced inhibition was abolished by pertussis toxin treatment or overexpression of the Gbetagamma scavenger transducin-alpha. In inside-out patches, direct application of 300 nm purified Gbetagamma decreased channel open probability by 55%. The physical interaction of Gbetagamma with BK channels was confirmed by co-immunoprecipitation. Interestingly, inhibition of phospholipase C as well as protein kinase C activities also reversed the CCh effect but to a smaller (approximately 20%) extent. Mouse tracheal cells responded similarly to CCh, purified Gbetagamma and phospholipase C/protein kinase C inhibition as M(2)R-expressing HEK293 cells. Our results demonstrate that airway M(2)Rs inhibit BK channels by a dual, Gbetagamma-mediated mechanism, a direct membrane-delimited interaction, and the activation of the phospholipase C/protein kinase C pathway.

Treating patients with overactive bladder syndrome with antimuscarinics: heart rate considerations

In this excellent mini-review, the authors present an extensive and relevant paper on the effect of antimuscarinic agents on the heart. This is without doubt the most detailed and the most reader-friendly paper on this subject, and I am sure that it will help urologists to assist in further educating their patients when prescribing these compounds.

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

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

Amikacin: A Novel Modulator of Vesical and Prostate Efferences. An in vitro Experimental Study

The autonomic efferent neurotransmission to the bladder and prostate smooth muscle is a potential target for drug therapy of specific low urinary tract disfunction (LUTD). Since amikacin and other amynoglicosides were reported to affect neurotransmission by a pre-junctional mechanism, we investigated the effect of amikacin on isolated rat and human detrusor smooth muscle contraction and on isolated rat and human prostate contraction, to further evaluate its potential relaxant properties.

Materials and Methods

Samples of detrusor smooth muscle and prostate tissue, obtained from 97 rats and 16 patients undergoing surgery, were studied through the measurement of isometric contraction induced by electrical field stimulation (EFS) and other pharmacological stimuli in the presence or absence of 1mM amikacin in a low-Ca medium.

Results

Amikacin 1 mM significantly reduced contraction of isolated rat and human detrusor muscle and prostate, achieved with pre-junctional stimulation, while no significant effect was observed on contraction induced by pharmacological post-junctional stimulators. EFS contraction inhibited by amikacin was restored after addition of calcium chloride. The amikacin effect was comparable to the effect of magnesium ions, which are known to exert a pre-junctional inhibition of neurotransmitter release.

Conclusions

Amikacin significantly inhibited rat and human detrusor and prostate contraction evoked by pre-junctional stimulation in vitro, suggesting a depressant effect on autonomic efferent neurotransmission. Further pharmacokinetics studies and researches on related compounds may hold potential for future development in the treatment of specific low urinary tract disfunction (LUTD).

Methacholine-induced pulmonary gas trapping in a mouse model of allergic asthma: effect of inhaled budesonide and ciglitazone

Previously, we found pulmonary gas trapping to be a rapid, simple and objective measure of methacholine-induced airway obstruction in naïve mice. In this study we extended that finding by using methacholine-induced pulmonary gas trapping to differentiate airway responses of ovalbumin-sensitized, ovalbumin-exposed (Positive Control) and ovalbumin-sensitized, sodium chloride-exposed (Negative Control) mice. Additionally, pulmonary gas trapping and enhanced pause were compared following methacholine exposure in sensitized and nonsensitized mice. Finally, we examined by nose-only inhalation the ability of the glucocorticosteroid budesonide and the peroxisome proliferator-activated receptor-gamma agonist ciglitazone to modify methacholine-induced airway responses in ovalbumin-sensitized mice. Positive Controls exhibited a 7.8-fold increase in sensitivity and a 2.4-fold enhancement in the maximal airway obstruction to methacholine versus Negative Controls. Following methacholine, individual Positive and Negative Control mouse enhanced pause values overlapped in 9 of 9 studies, whereas individual Positive and Negative Control mouse excised lung gas volume values overlapped in only 1 of 9 studies, and log[excised lung gas volume] correlated (P=0.023) with in vivo log[enhanced pause] in nonsensitized mice. Finally, budesonide (100.0 or 1000.0 microg/kg) reduced methacholine-mediated airway responses and eosinophils and neutrophils, whereas ciglitazone (1000.0 microg/kg) had no effect on methacholine-induced pulmonary gas trapping, but reduced eosinophils. In conclusion, pulmonary gas trapping is a more reproducible measure of methacholine-mediated airway responses in ovalbumin-sensitized mice than enhanced pause. Also, excised lung gas volume changes can be used to monitor drug interventions like budesonide. Finally, this study highlights the importance of running a positive comparator when examining novel treatments like ciglitazone.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Delayed goblet cell hyperplasia, acetylcholine receptor expression, and worm expulsion in SMC-specific IL-4Ralpha-deficient mice

Interleukin 4 receptor alpha (IL-4Ralpha) is essential for effective clearance of gastrointestinal nematode infections. Smooth muscle cells are considered to play a role in the type 2 immune response-driven expulsion of gastrointestinal nematodes. Previous studies have shown in vitro that signal transducer and activator of transcription 6 signaling in response to parasitic nematode infection significantly increases smooth muscle cell contractility. Inhibition of the IL-4Ralpha pathway inhibits this response. How this response manifests itself in vivo is unknown. In this study, smooth muscle cell IL-4Ralpha-deficient mice (SM-MHC(Cre)IL-4Ralpha(-/lox)) were generated and characterized to uncover any role for IL-4/IL-13 in this non-immune cell type in response to Nippostrongylus brasiliensis infection. IL-4Ralpha was absent from alpha-actin-positive smooth muscle cells, while other cell types showed normal IL-4Ralpha expression, thus demonstrating efficient cell-type-specific deletion of the IL-4Ralpha gene. N. brasiliensis-infected SM-MHC(Cre)IL-4Ralpha(-/lox) mice showed delayed ability to resolve infection with significantly prolonged fecal egg recovery and delayed worm expulsion. The delayed expulsion was related to a delayed intestinal goblet cell hyperplasia, reduced T helper 2 cytokine production in the mesenteric lymph node, and reduced M3 muscarinic receptor expression during infection. Together, these results demonstrate that in vivo IL-4Ralpha-responsive smooth muscle cells are beneficial for N. brasiliensis expulsion by coordinating T helper 2 cytokine responses, goblet hyperplasia, and acetylcholine responsiveness, which drive smooth muscle cell contractions.

Age differences in cholinergic airway responsiveness in relation with muscarinic receptor subtypes

Children seem more susceptible to increased airway reactivity than adults. Such an age-dependent discrepancy in airway reactivity may involve different airway smooth muscle functions. Therefore, we compared the in vivo and in vitro responsiveness of airway smooth muscles between two age groups of animals. Rats of 6 and 21 weeks old were challenged in vivo with acetylcholine (ACh) infused intravenously and airway resistance (R(aw)) was measured. Tracheal muscle was also isolated and the isometric force developed to ACh or KCl was measured. Furthermore, the level of genes encoding muscarinic receptor subtypes (M(1-3)) and acetylcholinesterase (AChE) expressed in the tracheal muscle was determined by RT-PCR. In results, the basal R(aw) was similar in the two age groups. The R(aw) at each ACh dose was significantly greater in young rats than older rats (p<0.05, n=22-27). Tracheal muscles from young rats were more sensitive to ACh than older rats (p<0.05, n=20-21), while receptor-independent muscle contraction to KCl was greater in older rats (p<0.05, n=10-19). Genes encoding AChE, M(2) and M(3) muscarinic receptors were more highly expressed in the tracheal muscles from young than older rats (p<0.05, n=4-6). In conclusion, airway smooth muscle in young rat is more sensitive to cholinergic stimulation in vivo and in vitro compared to older rats, which may be due to a higher expression of M(2) and M(3) muscarinic receptors in airway smooth muscle.

In vitro effects of bethanechol on specimens of intestinal smooth muscle obtained from the duodenum and jejunum of healthy dairy cows

OBJECTIVE

To describe the in vitro effects of bethanechol on contractility of smooth muscle preparations from the small intestines of healthy cows and define the muscarinic receptor subtypes involved in mediating contraction.

SAMPLE POPULATION

Tissue samples from the duodenum and jejunum collected immediately after slaughter of 40 healthy cows.

PROCEDURES

Cumulative concentration-response curves were determined for the muscarinic receptor agonist bethanechol with or without prior incubation with subtype-specific receptor antagonists in an organ bath. Effects of bethanechol and antagonists and the influence of intestinal location on basal tone, maximal amplitude (A(max)), and area under the curve (AUC) were evaluated.

RESULTS

Bethanechol induced a significant, concentration-dependent increase in all preparations and variables. The effect of bethanechol was more pronounced in jejunal than in duodenal samples and in circular than in longitudinal preparations. Significant inhibition of the effects of bethanechol was observed after prior incubation with muscarinic receptor subtype M(3) antagonists (more commonly for basal tone than for A(max) and AUC). The M(2) receptor antagonists partly inhibited the response to bethanechol, especially for basal tone. The M(3) receptor antagonists were generally more potent than the M(2) receptor antagonists. In a protection experiment, an M(3) receptor antagonist was less potent than when used in combination with an M(2) receptor antagonist. Receptor antagonists for M(1) and M(4) did not affect contractility variables.

CONCLUSIONS AND CLINICAL RELEVANCE

Bethanechol acting on muscarinic receptor sub-types M(2) and M(3) may be of clinical use as a prokinetic drug for motility disorders of the duodenum and jejunum in dairy cows.