Heterogeneity of presynaptic muscarinic receptors mediating inhibition of sympathetic transmitter release: a study with M2- and M4-receptor-deficient mice

Neuronal cholinergic signaling constrains norepinephrine activity in the heart

It is well known that cholinergic hypofunction contributes to cardiac pathology; yet, the mechanisms involved remain unclear. Our previous publication has shown that genetically engineered model of cholinergic deficit, the vesicular acetylcholine transporter knockdown homozygous (VAChT KDHOM) mice exhibit pathological cardiac remodeling and a gradual increase in cardiac mass with aging. Given that an increase in cardiac mass is often caused by adrenergic hyperactivity, we hypothesized that VAChT KDHOM mice might have an increase in cardiac norepinephrine (NE) levels. We thus investigated the temporal changes in NE content in the heart from 3, 6 and 12 month-old VAChT mutants. Interestingly, mice with cholinergic hypofunction showed a gradual elevation in cardiac NE content, which was already increased at 6 months of age. Consistent with this finding, 6 month-old VAChT KDHOM mice showed enhanced sympathetic activity and a greater abundance of tyrosine hydroxylase positive sympathetic nerves in the heart. VAChT mutants exhibited an increase in peak calcium transient, and mitochondrial oxidative stress in cardiomyocytes along with enhanced GRK5 and NFAT staining in the heart. These are known targets of adrenergic signaling in the cell. Moreover, vagotomized-mice displayed an increase in cardiac NE content confirming the data obtained in VAChT KDHOM mice. Establishing a causal relationship between acetylcholine and NE, VAChT KDHOM mice treated with pyridostigmine, a cholinesterase inhibitor, showed reduced cardiac NE content, rescuing the phenotype. Our findings unveil a yet unrecognized role of cholinergic signaling as a modulator of cardiac NE, providing novel insights into the mechanisms that drive autonomic imbalance.

Acute autonomic effects of rose oxide on cardiovascular parameters of Wistar and spontaneously hypertensive rats

AIMS

Anti-inflammatory molecules, such as rose oxide (RO), are likely to exert therapeutic effects in systemic arterial hypertension (SAH), a disease associated with abnormal immune responses. We aimed to investigate acute autonomic effects of RO on hemodynamic parameters of Wistar and spontaneously hypertensive rats (SHR).

METHODS

Rats were anesthetized and femoral artery and veins were cannulated. Next day, blood pressure (BP) and heart rate (HR) were recorded. Acute effects of RO (1.25, 2.5, or 5.0 mg/kg; iv) on BP, HR, and variability of systolic arterial pressure (SAP) and pulse interval (PI) were assessed. The effects of RO were also investigated in SHR, which received atropine (2 mg/kg), propranolol (4 mg/kg), or hexamethonium (20 mg/kg) 15 min before receiving RO. Vasorelaxant effects of RO (10-10 to 10-4 M) on aortic rings of rats were also assessed.

KEY FINDINGS

In Wistar rats, none of the RO doses evoked significant changes in BP, HR, and variability of SAP and PI. On the other hand, in SHR, RO elicited reduction in mean arterial pressure (MAP), and prevented the increase in the low frequency power (LF) of the SAP spectra. Pretreatment with atropine or propranolol did not alter hypotension, but attenuated RO-induced bradycardia. Hexamethonium prevented RO-induced hypotension and bradycardia. RO exerted vasorelaxant effects on aortic rings with (Wistar and SHR) or without functional endothelium (SHR only).

SIGNIFICANCE

Rose oxide, a monoterpene with anti-inflammatory properties, acts as an antihypertensive molecule due to its ability to acutely promote hypotension and bradycardia in spontaneously hypertensive rats.

Resting cardiac sympathetic firing frequencies suppress terminal norepinephrine transporter uptake

The prejunctional norepinephrine transporter (NET) is responsible for the clearance of released norepinephrine (NE) back into the sympathetic nerve terminal. NET regulation must be tightly controlled as variations could have important implications for neurotransmission. Thus far, the effects of sympathetic neuronal activity on NET function have been unclear. Here, we optically monitor single-terminal cardiac NET activity ex vivo in response to a broad range of sympathetic postganglionic action potential (AP) firing frequencies. Isolated murine left atrial appendages were loaded with a fluorescent NET substrate [Neurotransmitter Transporter Uptake Assay (NTUA)] and imaged with confocal microscopy. Sympathetic APs were induced with electrical field stimulation at 0.2-10 Hz (0.1-0.2 ms pulse width). Exogenous NE was applied during the NTUA uptake- and washout phases to investigate substrate competition and displacement, respectively, on transport. Single-terminal NET reuptake rate was rapidly suppressed in a frequency-dependent manner with an inhibitory EF₅₀ of 0.9 Hz. At 2 Hz, the effect was reversed by the α₂-adrenoceptor antagonist yohimbine (1 μM) (p < 0.01) with no further effect imposed by the muscarinic receptor antagonist atropine (1 μM). Additionally, high exogenous NE concentrations abolished NET reuptake (1 μM NE; p < 0.0001) and displaced terminal specific NTUA during washout (1-100 μM NE; p < 0.0001). We have also identified α₂-adrenoceptor-induced suppression of NET reuptake rate during resting stimulation frequencies, which could oppose the effect of autoinhibition-mediated suppression of exocytosis and thus amplify the effects of sympathetic drive on cardiac function.

Using Network Pharmacology to Explore Potential Treatment Mechanism for Coronary Heart Disease Using Chuanxiong and Jiangxiang Essential Oils in Jingzhi Guanxin Prescriptions

Background

To predict the active components and potential targets of traditional Chinese medicine and to determine the mechanism behind the curative effect of traditional Chinese medicine, a multitargeted method was used. Jingzhi Guanxin prescriptions expressed a high efficacy for coronary heart disease (CHD) patients of which essential oils from Chuanxiong and Jiangxiang were confirmed to be the most important effective substance. However, the interaction between the active components and the targets for the treatment of CHD has not been clearly explained in previous studies.

Materials and Methods

Genes associated with the disease and the treatment strategy were searched from the electronic database and analyzed by Cytoscape (version 3.2.1). Protein-protein interaction network diagram of CHD with Jiangxiang and Chuanxiong essential oils was constructed by Cytoscape. Pathway functional enrichment analysis was executed by clusterProfiler package in R platform.

Results

121 ingredients of Chuanxiong and Jiangxiang essential oils were analyzed, and 393 target genes of the compositions and 912 CHD-related genes were retrieved. 15 coexpression genes were selected, including UGT1A1, DPP4, RXRA, ADH1A, RXRG, UGT1A3, PPARA, TRPC3, CYP1A1, ABCC2, AHR, and ADRA2A. The crucial pathways of occurrence and treatment molecular mechanism of CHD were analyzed, including retinoic acid metabolic process, flavonoid metabolic process, response to xenobiotic stimulus, cellular response to xenobiotic stimulus, cellular response to steroid hormone stimulus, retinoid binding, retinoic acid binding, and monocarboxylic acid binding. Finally, we elucidate the underlying role and mechanism behind these genes in the pathogenesis and treatment of CHD.

Conclusions

Generally speaking, the nodes in subnetwork affect the pathological process of CHD, thus indicating the mechanism of Jingzhi Guanxin prescriptions containing Chuanxiong and Jiangxiang essential oils in the treatment of CHD.

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.

Cholinergic activity as a new target in diseases of the heart

The autonomic nervous system is an important modulator of cardiac signaling in both health and disease. In fact, the significance of altered parasympathetic tone in cardiac disease has recently come to the forefront. Both neuronal and nonneuronal cholinergic signaling likely play a physiological role, since modulating acetylcholine (ACh) signaling from neurons or cardiomyocytes appears to have significant consequences in both health and disease. Notably, many of these effects are solely due to changes in cholinergic signaling, without altered sympathetic drive, which is known to have significant adverse effects in disease states. As such, it is likely that enhanced ACh-mediated signaling not only has direct positive effects on cardiomyocytes, but it also offsets the negative effects of hyperadrenergic tone. In this review, we discuss recent studies that implicate ACh as a major regulator of cardiac remodeling and provide support for the notion that enhancing cholinergic signaling in human patients with cardiac disease can reduce morbidity and mortality. These recent results support the idea of developing large clinical trials of strategies to increase cholinergic tone, either by stimulating the vagus or by increased availability of Ach, in heart failure.

M1 muscarinic acetylcholine receptor in Alzheimer's disease

The degeneration of cholinergic neurons and cholinergic hypofunction are pathologies associated with Alzheimer's disease (AD). Muscarinic acetylcholine receptors (mAChRs) mediate acetylcholine-induced neurotransmission and five mAChR subtypes (M1-M5) have been identified. Among them, M1 mAChR is widely expressed in the central nervous system and has been implicated in many physiological and pathological brain functions. In addition, M1 mAChR is postulated to be an important therapeutic target for AD and several other neurodegenerative diseases. In this article, we review recent progress in understanding the functional involvement of M1 mAChR in AD pathology and in developing M1 mAChR agonists for AD treatment.

Overview of muscarinic receptor subtypes

The physiological role of muscarinic receptors is highly complex and, although not completely understood, has become clearer over the last decade. Recent pharmacological evidence with novel compounds, together with data from transgenic mice, suggests that all five subtypes have defined functions in the nervous system as well as mediating the non neuronal, hormonal actions of acetylcholine. Numerous novel agonists, allosteric regulators, and antagonists have now been identified with authentic subtype specificity in vitro and in vivo. These compounds provide additional pharmacological opportunities for selective subtype modulation as well as a new generation of muscarinic receptor-based therapeutics.

Muscarinic agonists and antagonists: effects on the urinary bladder

Voiding of the bladder is the result of a parasympathetic muscarinic receptor activation of the detrusor smooth muscle. However, the maintenance of continence and a normal bladder micturition cycle involves a complex interaction of cholinergic, adrenergic, nitrergic and peptidergic systems that is currently little understood. The cholinergic component of bladder control involves two systems, acetylcholine (ACh) released from parasympathetic nerves and ACh from non-neuronal cells within the urothelium. The actions of ACh on the bladder depend on the presence of muscarinic receptors that are located on the detrusor smooth muscle, where they cause direct (M₃) and indirect (M₂) contraction; pre-junctional nerve terminals where they increase (M₁) or decrease (M₄) the release of ACh and noradrenaline (NA); sensory nerves where they influence afferent nerve activity; umbrella cells in the urothelium where they stimulate the release of ATP and NO; suburothelial interstitial cells with unknown function; and finally, other unidentified sites in the urothelium from where prostaglandins and inhibitory/relaxatory factors are released. Thus, the actions of muscarinic receptor agonists and antagonists on the bladder may be very complex even when considering only local muscarinic actions. Clinically, muscarinic antagonists remain the mainstay of treatment for the overactive bladder (OAB), while muscarinic agonists have been used to treat hypoactive bladder. The antagonists are effective in treating OAB, but their precise mechanisms and sites of action (detrusor, urothelium, and nerves) have yet to be established. Potentially more selective agents may be developed when the cholinergic systems within the bladder are more fully understood.

Muscarinic pain pharmacology: realizing the promise of novel analgesics by overcoming old challenges

The antinociceptive and analgesic effects of muscarinic receptor ligands in human and nonhuman species have been evident for more than half a century. In this review, we describe the current understanding of the roles of different muscarinic subtypes in pain modulation and their mechanism of action along the pain signaling pathway, including peripheral nociception, spinal cord pain processing, and supraspinal analgesia. Extensive preclinical and clinical validation of these mechanisms points to the development of selective muscarinic agonists as one of the most exciting and promising avenues toward novel pain medications.

Presynaptic muscarinic M(2) receptors modulate glutamatergic transmission in the bed nucleus of the stria terminalis

The anterolateral cell group of the bed nucleus of the stria terminalis (BNST(ALG)) serves as an important relay station in stress circuitry. Limbic inputs to the BNST(ALG) are primarily glutamatergic and activity-dependent changes in this input have been implicated in abnormal behaviors associated with chronic stress and addiction. Significantly, local infusion of acetylcholine (ACh) receptor agonists into the BNST trigger stress-like cardiovascular responses, however, little is known about the effects of these agents on glutamatergic transmission in the BNST(ALG). Here, we show that glutamate- and ACh-containing fibers are found in close association in the BNST(ALG). Moreover, in the presence of the acetylcholinesterase inhibitor, eserine, endogenous ACh release evoked a long-lasting reduction of the amplitude of stimulus-evoked EPSCs. This effect was mimicked by exogenous application of the ACh analog, carbachol, which caused a reversible, dose-dependent, reduction of the evoked EPSC amplitude, and an increase in both the paired-pulse ratio and coefficient of variation, suggesting a presynaptic site of action. Uncoupling of postsynaptic G-proteins with intracellular GDP-β-S, or application of the nicotinic receptor antagonist, tubocurarine, failed to block the carbachol effect. In contrast, the carbachol effect was blocked by prior application of atropine or M(2) receptor-preferring antagonists, and was absent in M(2)/M(4) receptor knockout mice, suggesting that presynaptic M(2) receptors mediate the effect of ACh. Immunoelectron microscopy studies further revealed the presence of M(2) receptors on axon terminals that formed asymmetric synapses with BNST neurons. Our findings suggest that presynaptic M(2) receptors might be an important modulator of the stress circuit and hence a novel target for drug development.

Sleep, activity, temperature and arousal responses of mice deficient for muscarinic receptor M2 or M4

AIMS

The type 2 muscarinic receptor (M2R) differs from the other G-protein-coupled muscarinic receptor (type 4, or M4R) in tissue distribution and physiologic effects. We studied the impact of these receptors on sleep and arousal by using M2R and M4R knock-out (KO) mice.

MAIN METHODS

M2R and M4R KO and genetically intact mice were compared in terms of normal patterns of sleep, responses to sleep loss, infectious challenge and acoustic startle, and acoustic prepulse inhibition of startle (PPI).

KEY FINDINGS

Under basal conditions, M2R and M4R KO mice do not differ from the background strain or each other in the amount or diurnal pattern of sleep, locomotor activity, and body temperature. After enforced sleep loss, M2R KO mice, in contrast to the other two strains, show no rebound in slow-wave sleep (SWS) time, although their SWS is consolidated, and they show a greater rebound in time spent in REMS (rapid-eye-movement sleep) and REMS consolidation. During influenza infection, M2R KO mice, as compared with the other strains, show marked hypothermia and a less robust increase in SWS. During Candida albicans infection, M2R KO mice show a greater increase in SWS and a greater inflammatory response than do the other strains. M2R KO mice also show greater acoustic startle amplitude than does the background strain, although PPI was not different across the 3 strains over a range of stimulus intensities.

SIGNIFICANCE

Taken together, these findings support different roles for M2R and M4R in the modulation of sleep and arousal during homeostatic challenge.

High spatial resolution studies of muscarinic neuroeffector junctions in mouse isolated vas deferens

It is acknowledged that neurotransmission in the mouse vas deferens is predominantly mediated by ATP and noradrenaline (NA) released from sympathetic nerves while cholinergic transmission in the rodent vas deferens is often overlooked despite early literature. Recently we have characterized a cholinergic component of neurogenic contraction of mouse isolated vas deferens. In the present paper, by confocal imaging of Ca(2+) dynamics we detected acetylcholine (ACh) action at muscarinic cholinergic neuroeffector junctions at high-resolution. Experiments were carried out in the presence of prazosin (100 nM) and alpha,beta methylene ATP (alpha,beta-MeATP) (1 microM) to inhibit responses to NA and ATP respectively. Exogenous ACh (10 microM) elicited Ca(2+) transients, an effect blocked by the muscarinic receptor antagonist, cyclopentolate (1 microM). Ca(2+) transients were evoked by electrical stimulation of intrinsic nerves in the presence of the cholinesterase inhibitor neostigmine (10 microM). Stimulation produced a marked increase in the frequency and number of Ca(2+) transients. Cyclopentolate reduced the frequency of occurrence of spontaneous and evoked events to control levels. The alpha(2)-adrenoceptor antagonist yohimbine (300 nM) did not affect the spontaneous Ca(2+) transients, but increased the frequency of occurrence of evoked transients, an effect inhibited by cyclopentolate. The postjunctional effects of neuronally-released ACh are limited by the action of cholinesterase. Release of ACh appears to be tonically inhibited by NA released from sympathetic nerve terminals through action at prejunctional alpha(2)-adrenoceptors. Tetrodotoxin (TTX, 300 nM) abolished the nerve-evoked Ca(2+) events, with no effect on Ca(2+) transients elicited by exogenous ACh. In conclusion, the presence of spontaneous and evoked cholinergic Ca(2+) transients in smooth muscle cells of the mouse isolated vas deferens has been revealed. These events are mediated by ACh acting at M(3) muscarinic receptors. This action stands in marked contrast to the lack of effect of neuronally-released NA on smooth muscle Ca(2+) dynamics in this tissue.

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.

Synthesis, trafficking, and localization of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors are members of the G-protein coupled receptor superfamily that are expressed in and regulate the function of neurons, cardiac and smooth muscle, glands, and many other cell types and tissues. The correct trafficking of membrane proteins to the cell surface and their subsequent localization at appropriate sites in polarized cells are required for normal cellular signaling and physiological responses. This review will summarize work on the synthesis and trafficking of muscarinic receptors to the plasma membrane and their localization at the cell surface.

The detection of the non-M2 muscarinic receptor subtype in the rat heart atria and ventricles

Mammal heart tissue has long been assumed to be the exclusive domain of the M(2) subtype of muscarinic receptor, but data supporting the presence of other subtypes also exist. We have tested the hypothesis that muscarinic receptors other than the M(2) subtype are present in the heart as minor populations. We used several approaches: a set of competition binding experiments with pirenzepine, AFDX-116, 4-DAMP, PD 102807, p-F-HHSiD, AQ-RA 741, DAU 5884, methoctramine and tripinamide, blockage of M(1) muscarinic receptors using MT7 toxin, subtype-specific immunoprecipitation experiments and determination of phospholipase C activity. We also attempted to block M(1)-M(4) receptors using co-treatment with MT7 and AQ-RA 741. Our results show that only the M(2) subtype is present in the atria. In the ventricles, however, we were able to determine that 20% (on average) of the muscarinic receptors were subtypes other than M(2), with the majority of these belonging to the M(1) subtype. We were also able to detect a marginal fraction (6 +/- 2%) of receptors that, based on other findings, belong mainly to the M(5) muscarinic receptors. Co-treatment with MT7 and AQ-RA 741 was not a suitable tool for blocking of M(1)-M(4) receptors and can not therefore be used as a method for M(5) muscarinic receptor detection in substitution to crude venom. These results provide further evidence of the expression of the M(1) muscarinic receptor subtype in the rat heart and also show that the heart contains at least one other, albeit minor, muscarinic receptor population, which most likely belongs to the M(5) muscarinic receptors but not to that of the M(3) receptors.

Presynaptic metabotropic receptors for acetylcholine and adrenaline/noradrenaline

Presynaptic metabotropic receptors for acetylcholine and adrenaline/noradrenaline were first described more than three decades ago. Molecular cloning has resulted in the identification of five G protein-coupled muscarinic receptors (M(1) - M(5)) which mediate the biological effects of acetylcholine. Nine adrenoceptors (alpha(1ABD),alpha(2ABC),beta(123)) transmit adrenaline/noradrenaline signals between cells. The lack of sufficiently subtype-selective ligands has prevented identification of the physiological role and therapeutic potential of these receptor subtypes for a long time. Recently, mouse lines with targeted deletions for all muscarinic and adrenoceptor genes have been generated. This review summarizes the results from these gene-targeting studies with particular emphasis on presynaptic auto- and heteroreceptor functions of muscarinic and adrenergic receptors. Specific knowledge about the function of receptor subtypes will enhance our understanding of the physiological role of the cholinergic and adrenergic nervous system and open new avenues for subtype-selective therapeutic strategies.

Cholinergic innervation of the guinea-pig isolated vas deferens

Recently, a cholinergic neurogenic component of contraction has been characterised in the aganglionic mouse vas deferens. In this paper, a cholinergic component of contraction in the guinea-pig vas deferens is characterised pharmacologically. A residual, tetrodotoxin-sensitive (TTX, 0.3 microM), neurogenic contraction was revealed after prolonged exposure (5 h) to the adrenergic neurone blocker bretylium (20 microM) or in the presence of prazosin (100 nM) and alpha,beta-methylene ATP (1 microM), a purinergic agonist which desensitizes P2X receptors. The bretylium-resistant component was potentiated by the acetylcholinesterase (AChE) inhibitor neostigmine (10 microM) and inhibited by the muscarinic-receptor (mAChR) antagonist cyclopentolate (1 microM). Nicotine (30 microM) enhanced the bretylium-resistant component. Neostigmine increased the second component of contraction in the presence of prazosin and alpha,beta-methylene ATP, whilst yohimbine (1 microM), an alpha(2) adrenergic receptor antagonist, enhanced both the first and second components of the electrically evoked contraction. These enhanced contractions were blocked by cyclopentolate in both cases. Nicotine enhanced the cholinergic component of contraction revealed by neostigmine but failed to have any detectable effects in the presence of cyclopentolate. Neostigmine alone increased the slow component of contraction which was reversed by cyclopentolate to control levels. The M(3) receptor-antagonist 4-DAMP (10 nM) markedly inhibited the cholinergic component of contraction to a level comparable with cyclopentolate. Laser microscopy has shown that neostigmine also increased the frequency of spontaneous Ca(2+) transients remaining in smooth muscle cells after perfusion with prazosin and alpha,beta-methylene ATP, an effect blocked by 4-DAMP. These experimental data show that there is a functional cholinergic innervation in the guinea-pig vas deferens whose action is limited by acetylcholinesterase, blocked by cyclopentolate and mediated through M3 receptors. Moreover, by blocking the cholinesterase, the increased amount of ACh generates spontaneous Ca(2+) transients in smooth muscle cells.

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.

M1 and M2 Muscarinic Acetylcholine Receptor Subtypes Mediate Ca2+ Channel Current Inhibition in Rat Sympathetic Stellate Ganglion Neurons

Muscarinic acetylcholine receptors (mAChRs) are known to mediate the acetylcholine inhibition of Ca2+ channels in central and peripheral neurons. Stellate ganglion (SG) neurons provide the main sympathetic input to the heart and contribute to the regulation of heart rate and myocardial contractility. Little information is available regarding mAChR regulation of Ca2+ channels in SG neurons. The purpose of this study was to identify the mAChR subtypes that modulate Ca2+ channel currents in rat SG neurons innervating heart muscle. Accordingly, the modulation of Ca2+ channel currents by the muscarinic cholinergic agonist, oxotremorine-methiodide (Oxo-M), and mAChR blockers was examined. Oxo-M–mediated mAChR stimulation led to inhibition of Ca2+ currents through voltage-dependent (VD) and voltage-independent (VI) pathways. Pre-exposure of SG neurons to the M1 receptor blocker, M1-toxin, resulted in VD inhibition of Ca2+ currents after Oxo-M application. On the other hand, VI modulation of Ca2+ currents was observed after pretreatment of cells with methoctramine (M2 mAChR blocker). The Oxo-M–mediated inhibition was nearly eliminated in the presence of both M1 and M2 mAChR blockers but was unaltered when SG neurons were exposed to the M4 mAChR toxin, M4-toxin. Finally, the results from single-cell RT-PCR and immunofluorescence assays indicated that M1 and M2 receptors are expressed and located on the surface of SG neurons. Overall, the results indicate that SG neurons that innervate cardiac muscle express M1 and M2 mAChR, and activation of these receptors leads to inhibition of Ca2+ channel currents through VI and VD pathways, respectively.

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

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

Fluorescent styryl dyes FM1-43 and FM2-10 are muscarinic receptor antagonists: intravital visualization of receptor occupancy

The fluorescent styryl dyes FM1-43 and FM2-10 have been used to visualize the endocytic and exocytic processes involved in neurotransmission in a variety of central and peripheral nerve preparations. Their utility is limited to some extent by a poorly understood vesicular-independent labelling of cells and tissues. We show here that one likely cause of this troublesome background labelling is that FM1-43 and FM2-10 are selective and competitive antagonists at both cloned and endogenously expressed muscarinic acetylcholine receptors. In radioligand binding studies, FM1-43 and FM2-10 bound with moderate affinity (23-220 nM) to membranes of Chinese hamster ovary (CHO) cells expressing cloned human muscarinic receptors (M1-M5). In functional studies in vitro, FM1-43 and FM2-10 inhibited electrical field stimulation (EFS) and acetylcholine-induced cholinergic contractions of guinea-pig tracheal strips (IC50: FM1-43, 0.4 +/- 0.1; FM2-10, 1.6 +/- 0.1 microM; concentration of antagonist producing a 2-fold leftward shift in the acetylcholine concentration-response curve (Kb): FM1-43, 0.3 +/- 0.1; FM2-10, 15.8 +/- 10.1 microM). Neither compound inhibited EFS-evoked, non-adrenergic non-cholinergic nerve-mediated relaxations or contractions of the airways, or contractions mediated by histamine H1 receptor or tachykinin NK2 receptor activation. Incubating freshly excised tracheal whole-mount preparations with 5 microM FM1-43 resulted in intense fluorescence labelling of the smooth muscle that was reduced by up to 90% in the presence of selective M2 and M3 receptor antagonists. The potency of the FM dyes as muscarinic receptor antagonists is within the concentration range used to study vesicular cycling at nerve terminals. Given that muscarinic receptors play a key role in the regulation of neurotransmitter release from a variety of neurones, the anticholinergic properties of FM dyes may have important implications when studying vesicular events in the nervous system. In addition, these dyes may provide a novel tool for visualizing muscarinic receptor occupancy in living tissue or cell preparations.

Involvement of M(2) muscarinic receptors in relaxant response of circular muscle of mouse gastric antrum

Our previous study showed that atropine significantly inhibited the sustained relaxation induced by electrical field stimulation (EFS) in the circular muscle strips prepared from the mouse antrum, and that pituitary adenylate cyclase activating peptide (PACAP) partially mediated the sustained relaxation. The muscarinic receptor subtype associated with the sustained relaxation was examined in the present study by using each muscarinic receptor subtype of knockout (KO) mice. EFS-induced sustained relaxation in the antrum prepared from M(2) receptor KO mice was significantly less than that of wild-type mice. Atropine failed to inhibit this relaxation. On the other hand, similar sustained relaxation and inhibitory effects of atropine to those of wild-type mice were observed in M(1), M(3) and M(4) receptor KO mice. Exogenously added PACAP-27 relaxed the antral strips of wild-type and M(2) receptor KO mice to a similar extent. Immunohistochemical study revealed that M(2) receptor immunoreactivity was localized with PACAP-immunoreactivity in enteric neurons within the antrum of wild-type mice. In contrast, atropine did not affect the EFS-induced sustained relaxation in the gastric fundus. These results suggest that M(2) receptors modulate the sustained relaxation, probably through the regulation of PACAP release, in the mouse antrum.

Distinct mixtures of muscarinic receptor subtypes mediate inhibition of noradrenaline release in different mouse peripheral tissues, as studied with receptor knockout mice

The muscarinic heteroreceptors modulating noradrenaline release in atria, urinary bladder and vas deferens were previously studied in mice in which the M(2) or the M(4) muscarinic receptor genes had been disrupted. These experiments showed that these tissues possessed both M(2) and non-M(2) heteroreceptors. The analysis was now extended to mice in which either the M(3), both the M(2) and the M(3), or both the M(2) and the M(4) genes had been disrupted (M(3)-knockout, M(2/3)-knockout and M(2/4)-knockout). Tissues were preincubated with (3)H-noradrenaline and then stimulated electrically (20 pulses per 50 Hz). In wild-type atria, carbachol (0.01-100 microM) decreased the electrically evoked tritium overflow by maximally 60-78%. The maximum inhibition of carbachol was reduced to 57% in M(3)-knockout and to 23% in M(2/4)-knockout atria. Strikingly, the effect of carbachol was abolished in M(2/3)-knockout atria. In wild-type bladder, carbachol (0.01-100 microM) reduced the evoked tritium overflow by maximally 57-71%. This effect remained unchanged in the M(3)-knockout, but was abolished in the M(2/4)-knockout bladder. In wild-type vas deferens, carbachol (0.01-100 microM) reduced the evoked tritium overflow by maximally 34-48%. The maximum inhibition of carbachol was reduced to 40% in the M(3)-knockout and to 18% in the M(2/4)-knockout vas deferens. We conclude that the postganglionic sympathetic axons of mouse atria possess M(2) and M(3), those of the urinary bladder M(2) and M(4), and those of the vas deferens M(2), M(3) and M(4) release-inhibiting muscarinic receptors.

Store calcium mediates cholinergic effects on mIPSCs in the rat main olfactory bulb

The significance of endoplasmic reticulum (ER) store calcium in modulating transmitter release is slowly gaining recognition. One transmitter system that might play an important role in store calcium modulation of transmitter release in the CNS is acetylcholine (ACh). The main olfactory bulb (OB) receives rich cholinergic innervation from the horizontal limb of the diagonal band of Broca and blocking cholinergic signaling in the bulb inhibits the ability of animals to discriminate between closely related odors. Here we show that exposing OB slices to carbamylcholine (CCh), a hydrolysis-resistant analog of Ach, increases gamma-aminobutyric acid (GABA) release at dendrodendritic synapses onto the mitral cells. This increase in transmitter release is mediated by the activation of the M1 class of muscarinic receptors and requires the mobilization of calcium from the ER. The site of action of CCh for this effect is developmentally regulated. In animals younger than postnatal day 10, the major action of CCh appears to be on mitral cells, enhancing GABA release by reciprocal signaling resulting from increased glutamate release from mitral cells. In animals older than postnatal day 10, CCh appears to modulate transmitter release from dendrites of the interneurons themselves. Our results point to modulation of inhibition as an important role for cholinergic signaling in the OB. Our data also strengthen the emerging idea of a role for store calcium in modulating transmitter release at CNS synapses.

Changes in the cardiac beta-adrenergic system provoked by different T. cruzi strains

BACKGROUND

It has been demonstrated that the beta-adrenergic signal transduction system is altered somewhere along its pathway in Trypanosoma cruzi infected hearts and we think that these alterations would differ according to the infection phase and the parasite strain. Their study would be important for the understanding of the disease's pathophysiology.

METHODS

In the present work we studied important components of this system in mice hearts infected with T. cruzi, Tulahuen strain and with SGO-Z12 isolate, obtained from a patient of an endemic area, in the acute phase of the infection, determining: the plasma catecholamines levels, the beta-receptors density and affinity as well as their function, the cardiac concentration of cAMP and the cardiac contractility as the physiologic response to the initial stimulus.

RESULTS

Plasma catecholamines levels were diminished in both infected groups when compared to the uninfected one (P < 0.01). The receptor's affinity was also diminished (P < 0.05) while their density was augmented only in the SGO-Z12 infected one (P < 0.01). The cAMP levels were higher in both infected groups (P < 0.01), the basal contractile force however increased only in the Tulahuen infected one (P < 0.01) while the response to catecholamines remained unchanged. The hearts infected with the SGO Z12 isolate presented an inferior response to epinephrine (P < 0.05) than the ventricles infected with the Tulahuen strain.

CONCLUSIONS

This model represents an important approach to understand the biochemical, physiological and molecular changes in the cardiac beta-adrenergic signalling that clearly begin in the acute phase of Chagas' disease and reveal a clear differentiation in the alterations produced by different parasite strains.

Roles of M2 and M4 Muscarinic Receptors in Regulating Acetylcholine Release From Myenteric Neurons of Mouse Ileum

We investigated the subtype of presynaptic muscarinic receptors associated with inhibition of acetylcholine (ACh) release in the mouse small intestine. We measured endogenous ACh released from longitudinal muscle with myenteric plexus (LMMP) preparations obtained from M1–M5 receptor knockout (KO) mice. Electrical field stimulation (EFS) increased ACh release in all LMMP preparations obtained from M1–M5 receptor single KO mice. The amounts of ACh released in all preparations were equal to that in the wild-type mice. Atropine further increased EFS-induced ACh release in the wild-type mice. Unexpectedly, atropine also increased, to a similar extent, EFS-induced ACh release to the wild-type mice in all M1–M5 receptor single KO mice. In M2 and M4 receptor double KO mice, the amount of EFS-induced ACh release was equivalent to an atropine-evoked level in the wild-type mouse, and further addition of atropine had no effect. M2 receptor immunoreactivity was located in both smooth muscle cells and enteric neurons. M4 receptor immunoreactivity was located in the enteric neurons, being in co-localization with M2 receptor immunoreactivity. These results indicate that both M2 and M4 receptors mediate the muscarinic autoinhibition in ACh release in the LMMP preparation of the mouse ileum, and loss of one of these subtypes can be compensated functionally by a receptor that remained. M1, M3, and M5 receptors do not seem to be involved in this mechanism.

3H-noradrenaline release from mouse iris?ciliary body: role of presynaptic muscarinic heteroreceptors

Sympathetic neurotransmitter release and its modulation by presynaptic muscarinic heteroreceptors were studied in mouse iris-ciliary bodies. Tissue preparations were preincubated with (3)H-noradrenaline and then superfused and stimulated electrically. Firstly, experimental conditions were defined, allowing study of presynaptic sympathetic inhibition in mouse iris-ciliary body. If tissue was stimulated four times with 36 pulses/3 Hz, tritium overflow peaks were reliably and reproducibly measured. As expected, these stimulation conditions led to marked alpha(2)-autoinhibition as indicated by the release-enhancing effect of the alpha(2)-antagonists phentolamine and rauwolscine. To ensure autoinhibition-free (3)H-noradrenaline release, which is optimal for studying presynaptic sympathetic inhibition, alpha(2)-receptors were blocked in all subsequent experiments. Under these conditions, evoked tritium overflow was almost completely abolished in the presence of the sodium channel blocker tetrodotoxin, indicating a neuronal origin of (3)H-noradrenaline release. Secondly, muscarinic inhibition of (3)H-noradrenaline release was characterized using the conditions described above (36 pulses/3 Hz; phentolamine 1 muM and rauwolscine 1 muM throughout). The muscarinic receptor agonist oxotremorine M decreased evoked tritium overflow in a concentration-dependent manner with an IC(50) of 0.33 muM and maximal inhibition of 51%. The concentration-response curve of oxotremorine M was shifted to the right by the muscarinic antagonists ipratropium and methoctramine, whereas pirenzepine was ineffective. The observed rank order of antagonist potencies, ipratropium > methoctramine > pirenzepine, which is typical for the M(2) subtype, indicates that presynaptic muscarinic receptors on sympathetic axons of mouse iris-ciliary bodies are predominantly M(2). Finally, inhibition of (3)H-noradrenaline release by endogenously secreted acetylcholine was investigated. Longer pulse trains, 120 pulses/3 Hz and 600 pulses/5 Hz, were used and the cholinesterase inhibitor physostigmine was added to the superfusion medium to increase synaptic levels of endogenous acetylcholine. Under these conditions, ipratropium approximately doubled the evoked overflow of tritium, indicating that endogenously released acetylcholine can activate presynaptic muscarinic heteroreceptors. In conclusion, the present experiments establish measurement of the electrically induced release of (3)H-noradrenaline from mouse iris-ciliary bodies. As in other species, noradrenaline release in this preparation was subject to presynaptic muscarinic inhibition. Our results also indicate that the presynaptic muscarinic receptors on sympathetic axons in mouse iris-ciliary body are predominantly M(2). Moreover, these receptors can be activated by both exogenous agonists and endogenously released acetylcholine and, hence, may operate physiologically in the interplay between the parasympathetic and sympathetic nervous system.

Muscarinic acetylcholine receptor knockout mice: novel phenotypes and clinical implications

Muscarinic acetylcholine receptors (mAChRs; M1-M5) play key roles in regulating the activity of many important functions of the central and peripheral nervous system. Because of the lack of ligands endowed with a high degree of receptor subtype selectivity and the fact that most tissues or cell types express two or more mAChR subtypes, identification of the physiological and pathophysiological roles of the individual mAChR subtypes has proven a difficult task. To circumvent these difficulties, several laboratories recently employed gene-targeting techniques to generate mutant mouse strains deficient in each of the five mAChR subtypes. Phenotyping studies showed that each mutant mouse line displayed characteristic physiological, pharmacological, behavioral, biochemical, or neurochemical deficits. The novel insights gained from these studies should prove instrumental for the development of novel classes of muscarinic drugs.

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

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

Novel insights into muscarinic acetylcholine receptor function using gene targeting technology

Muscarinic acetylcholine receptors (mAChRs) modulate the activity of an extraordinarily large number of physiological functions. Individual members of the mAChR family (M(1)-M(5)) are expressed in a complex, overlapping fashion in most tissues and cell types. However, the identification of the precise physiological roles of individual mAChR subtypes remains a challenging task because, with the exception of a few snake toxins, mAChR ligands that can activate or inhibit specific mAChR subtypes with a high degree of selectivity are not yet available. Knowledge of the specific roles of mAChR subtypes is of considerable interest for the development of novel, clinically useful mAChR ligands. In this article, recent studies of mutant mouse strains developed, using gene targeting techniques, to be deficient in one of the three G(q)-coupled mAChR subtypes (M(1), M(3) and M(5)) are discussed. These investigations have led to many important new insights into the physiological roles of these receptor subtypes.