Molecular cloning and expression of a fifth muscarinic acetylcholine receptor

A growing understanding of the role of muscarinic receptors in the molecular pathology and treatment of schizophrenia

Pre-clinical models, postmortem and neuroimaging studies all support a role for muscarinic receptors in the molecular pathology of schizophrenia. From these data it was proposed that activation of the muscarinic M1 and/or M4 receptor would reduce the severity of the symptoms of schizophrenia. This hypothesis is now supported by results from two clinical trials which indicate that activating central muscarinic M1 and M4 receptors can reduce the severity of positive, negative and cognitive symptoms of the disorder. This review will provide an update on a growing body of evidence that argues the muscarinic M1 and M4 receptors have critical roles in CNS functions that are dysregulated by the pathophysiology of schizophrenia. This realization has been made possible, in part, by the growing ability to visualize and quantify muscarinic M1 and M4 receptors in the human CNS using molecular neuroimaging. We will discuss how these advances have provided evidence to support the notion that there is a sub-group of patients within the syndrome of schizophrenia that have a unique molecular pathology driven by a marked loss of muscarinic M1 receptors. This review is timely, as drugs targeting muscarinic receptors approach clinical use for the treatment of schizophrenia and here we outline the background biology that supported development of such drugs to treat the disorder.

Discovery of a potent M5 antagonist with improved clearance profile. Part 2: Pyrrolidine amide-based antagonists

This Letter describes our ongoing effort to improve the clearance of selective M5 antagonists. Herein, we report the replacement of the previously disclosed piperidine amide (4, disclosed in Part 1) with a pyrrolidine amide core. Several compounds within this series provided good potency, subtype selectivity, and low to moderate clearance profiles. Interestingly, the left-hand side SAR for this series diverged from our earlier efforts.

Development of : A Potent, Highly Selective, and Systemically Active Orthosteric Antagonist of the M5 Muscarinic Acetylcholine Receptor for the Treatment of Opioid Use Disorder

The muscarinic acetylcholine receptor (mAChR) subtype 5 (M₅) represents a novel potential target for the treatment of multiple addictive disorders, including opioid use disorder. Through chemical optimization of several functional high-throughput screening hits, VU6019650 (27b) was identified as a novel M₅ orthosteric antagonist with high potency (human M₅ IC₅₀ = 36 nM), M₅ subtype selectivity (>100-fold selectivity against human M_1-4) and favorable physicochemical properties for systemic dosing in preclinical addiction models. In acute brain slice electrophysiology studies, 27b blocked the nonselective muscarinic agonist oxotremorine-M-induced increases in neuronal firing rates of midbrain dopamine neurons in the ventral tegmental area, a part of the mesolimbic dopaminergic reward circuitry. Moreover, 27b also inhibited oxycodone self-administration in male Sprague-Dawley rats within a dose range that did not impair general motor output.

The Muscarinic Acetylcholine Receptor M5: Therapeutic Implications and Allosteric Modulation

The muscarinic acetylcholine receptor (mAChR) subtype 5 (M₅) was the most recent mAChR to be cloned and has since emerged as a potential therapeutic target for a number of indications. Early studies with knockout animals have provided clues to the receptor's role in physiological processes related to Alzheimer's disease, schizophrenia, and addiction, and until recently, useful subtype-selective tools to further probe the pharmacology of M₅ have remained elusive. Small-molecule allosteric modulators have since gained traction as a means by which to selectively examine muscarinic pharmacology. This review highlights the discovery and optimization of M₅ positive allosteric modulators (PAMs) and negative allosteric modulators (NAMs).

Positive allosteric modulation of M1 and M4 muscarinic receptors as potential therapeutic treatments for schizophrenia

Current antipsychotic drugs provide symptomatic relief for positive symptoms of schizophrenia, but do not offer symptom management for negative and cognitive symptoms. In addition, many patients discontinue treatment due to adverse side effects. Therefore, there is a critical need to develop more effective and safe treatment options. Although the etiology of schizophrenia is unclear, considerable data from post-mortem, neuroimaging and neuropharmacology studies support a role of the muscarinic acetylcholine (mAChRs) in the pathophysiology of schizophrenia. Substantial evidence suggests that activation of mAChRs has the potential to treat all symptom domains of schizophrenia. Despite encouraging results in demonstrating efficacy, clinical trials of nonselective mAChR agonists were limited in their clinical utility due to dose-limiting peripheral side effects. Accordingly, efforts have been made to specifically target centrally located M1 and M4 mAChR subtypes devoid of adverse-effect liability. To circumvent this limitation, there have been tremendous advances in the discovery of ligands that bind at allosteric sites, binding sites distinct from the orthosteric site, which are structurally less conserved and thereby afford high levels of receptor subtype selectivity. The discovery of subtype-specific allosteric modulators has greatly advanced our understanding of the physiological role of various muscarinic receptor subtypes in schizophrenia and the potential utility of M1 and M4 mAChR subtypes as targets for the development of novel treatments for schizophrenia and related disorders. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Striatal cholinergic interneurons and Parkinson's disease

Giant, aspiny cholinergic interneurons (ChIs) have long been known to be key nodes in the striatal circuitry controlling goal-directed actions and habits. In recent years, new experimental approaches, like optogenetics and monosynaptic rabies virus mapping, have expanded our understanding of how ChIs contribute to the striatal activity underlying action selection and the interplay of dopaminergic and cholinergic signaling. These approaches also have begun to reveal how ChI function is distorted in disease states affecting the basal ganglia, like Parkinson's disease (PD). This review gives a brief overview of our current understanding of the functional role played by ChIs in striatal physiology and how this changes in PD. The translational implications of these discoveries, as well as the gaps that remain to be bridged, are discussed as well.

Drug Development in Alzheimer's Disease: The Contribution of PET and SPECT

Clinical trials aiming to develop disease-altering drugs for Alzheimer’s disease (AD), a neurodegenerative disorder with devastating consequences, are failing at an alarming rate. Poorly defined inclusion-and outcome criteria, due to a limited amount of objective biomarkers, is one of the major concerns. Non-invasive molecular imaging techniques, positron emission tomography and single photon emission (computed) tomography (PET and SPE(C)T), allow visualization and quantification of a wide variety of (patho)physiological processes and allow early (differential) diagnosis in many disorders. PET and SPECT have the ability to provide biomarkers that permit spatial assessment of pathophysiological molecular changes and therefore objectively evaluate and follow up therapeutic response, especially in the brain. A number of specific PET/SPECT biomarkers used in support of emerging clinical therapies in AD are discussed in this review.

From GTP and G proteins to TRPC channels: a personal account

By serendipity and good fortune, as a postdoctoral fellow in 1967, I landed at the right place at the right time, as I was allowed to investigate the mechanism by which hormones activate the enzyme adenylyl cyclase (then adenyl cyclase) in Martin Rodbell's Laboratory at the NIH in Bethesda, Maryland. The work uncovered first, the existence of receptors separate from the enzyme and then, the existence of transduction mechanisms requiring guanosine-5'-triphosphate (GTP) and Mg(2+). With my laboratory colleagues first and postdoctoral fellows after leaving NIH, I participated in the development of the field "signal transduction by G proteins," uncovered by molecular cloning several G-protein-coupled receptors (GPCRs) and became interested in both the molecular makeup of voltage-gated Ca channels and Ca2+ homeostasis downstream of activation of phospholipase C (PLC) by the Gq/11 signaling pathway. We were able to confirm the hypothesis that there would be mammalian homologues of the Drosophila "transient receptor potential" channel and discovered the existence of six of the seven mammalian genes, now called transient receptor potential canonical (TRPC) channels. In the present article, I summarize from a bird's eye view of what I feel were key findings along this path, not only from my laboratory but also from many others, that allowed for the present knowledge of cell signaling involving G proteins to evolve. Towards the end, I summarize roles of TRPC channels in health and disease.

The muscarinic system, cognition and schizophrenia

An increasing body of evidence has implicated the central muscarinic system as contributing to a number of symptoms of schizophrenia and serving as a potential target for pharmaceutical interventions. A theoretical review is presented that focuses on the central muscarinic system's contribution to the cognitive symptoms of schizophrenia. The aim is to bridge the void between pertinent neuropsychological and neurobiological research to provide an explanatory account of the role that the central muscarinic system plays in the symptoms of schizophrenia. First, there will be a brief overview of the relevant neuropsychological schizophrenia literature, followed by a concise introduction to the central muscarinic system. Subsequently, we will draw from animal, neuropsychological and pharmacological literature, and discuss the findings in relation to cognition, schizophrenia and the muscarinic system. Whilst unifying the multiple domains of research into a concise review will act as a useful line of enquiry into the central muscarinic systems contribution to the symptoms of schizophrenia, it will be made apparent that more research is needed in this field.

A novel luminescence-based method for the detection of functionally active antibodies to muscarinic acetylcholine receptors of the M3 type (mAchR3) in patients' sera

In different bioassays, functional antibodies reacting with the human muscarinic acetylcholine receptor M3(mAchR3) have been detected in sera from patients with Sjögren's syndrome (SS), and there is strong evidence that those antibodies may have pathogenetic relevance. However, depending on the method of detection, their prevalence varied. Furthermore, those bioassays are difficult to standardize. We report on the development and optimization of a novel test system based on a luminometric method to determine downstream signalling of mAchR3 which produces specific and reproducible results. Chinese hamster ovarian (CHO) cells were transfected with plasmids encoding mAchR3 and a green fluorescence protein (GFP)/aequorin fusion protein. Incubation of cells with carbachol resulted in an increase in intracellular [Ca(2+)], which was detected by measuring light emission with a luminometer, and the effect of incubation with patients' immunoglobulins (Ig) was evaluated. Optimal cell density, Ig preparation and time of incubation with patients' sera were determined. Sera from patients with primary Sjögren's syndrome (pSS; n = 40), systemic sclerosis (SSc; n = 47), myasthenia gravis (MG; n = 133) and 50 blood donors were analysed. Optimal assay conditions were obtained with a cell density of 100 000 cells/ml, isolation of Ig by ammonium sulphate precipitation and short-term incubation. Based on this highly reliable assay, 50% of the pSS patients had antibodies which inhibited carbachol-induced activation of mAchR3; none of the SSc patients, 6% of the patients with MG and 12% of the blood donors had antibodies which reacted with the mAchR3. This method facilitates the determination of functional anti-mAchR3 antibodies in patients' sera, confirmed their high prevalence in pSS patients and may, therefore, help to analyse their pathogenetic and clinical relevance in more detail.

Vasomotor effects of acetylcholine, bradykinin, noradrenaline, 5-hydroxytryptamine, histamine and angiotensin II on the mouse basilar artery

We investigated the responsiveness of the mouse basilar artery to acetylcholine (ACh), bradykinin (BK), noradrenaline (NA), 5-hydroxytryptamine (5-HT), histamine (His) and angiotensin (Ang) II in order to characterize the related receptor subtypes in vitro. ACh and BK induced endothelium-dependent relaxation of precontracted arteries with U-46619 (a thromboxane A2 analogue). Atropine (a non-selective muscarinic receptor antagonist) and Nω-nitro-L-arginine (a NO synthase inhibitor, L-NNA) shifted the concentration-response curve for ACh to the right, whereas pirenzepine, methoctramine and pFHHSiD (muscarinic M1, M2 and M3 antagonists, respectively) had no significant effect. L-NNA and HOE140 (a B2 antagonist) shifted the concentration-response curve for BK to the right, whereas des-Arg(9)-[Leu(8)]-BK (a B1 antagonist) and indomethacin (a cyclooxygenase inhibitor) had no significant effect. NA failed to produce any vasomotor action. His and Ang II induced concentration-dependent contraction. Diphenhydramine (a H1 antagonist) shifted the concentration-response curve for His to the right, whereas cimetidine (a H2 antagonist) had no significant effect. Losartan (an AT1 antagonist) shifted the concentration-response curve for Ang II to the right, whereas PD123319 (an AT2 antagonist) had no significant effect. These results suggest that the H1 and AT1 receptor subtypes might play an important role in arterial contraction, whereas muscarinic receptor subtypes apart from M1, M2 and M3, and B2 receptors on the endothelium, might modify these contractions to relaxations.

Somatodendritic targeting of M5 muscarinic receptor in the rat ventral tegmental area: implications for mesolimbic dopamine transmission

Muscarinic modulation of mesolimbic dopaminergic neurons in the ventral tegmental area (VTA) plays an important role in reward, potentially mediated through the M5 muscarinic acetylcholine receptor (M5R). However, the key sites for M5R-mediated control of dopamine neurons within this region are still unknown. To address this question we examined the electron microscopic immunocytochemical localization of antipeptide antisera against M5R and the plasmalemmal dopamine transporter (DAT) in single sections through the rat VTA. M5R was located mainly to VTA somatodendritic profiles (71%; n = 627), at least one-third (33.2%; n = 208) of which also contained DAT. The M5R immunoreactivity was distributed along cytoplasmic tubulovesicular endomembrane systems in somata and large dendrites, but was more often located at plasmalemmal sites in small dendrites, the majority of which did not express DAT. The M5R-immunoreactive dendrites received a balanced input from unlabeled terminals forming either asymmetric or symmetric synapses. Compared with dendrites, M5R was less often seen in axon terminals, comprising only 10.8% (n = 102) of the total M5R-labeled profiles. These terminals were usually presynaptic to unlabeled dendrites, suggesting that M5R activation can indirectly modulate non-DAT-containing dendrites through presynaptic mechanisms. Our results provide the first ultrastructural evidence that in the VTA, M5R has a subcellular location conducive to major involvement in postsynaptic signaling in many dendrites, only some of which express DAT. These findings suggest that cognitive and rewarding effects ascribed to muscarinic activation in the VTA can primarily be credited to M5R activation at postsynaptic plasma membranes distinct from dopamine transport.

Muscarinic and nicotinic acetylcholine receptor agonists and allosteric modulators for the treatment of schizophrenia

Muscarinic and nicotinic acetylcholine (ACh) receptors (mAChRs and nAChRs) are emerging as important targets for the development of novel treatments for the symptoms associated with schizophrenia. Preclinical and early proof-of-concept clinical studies have provided strong evidence that activators of specific mAChR (M(1) and M(4)) and nAChR (α(7) and α(2)β(4)) subtypes are effective in animal models of antipsychotic-like activity and/or cognitive enhancement, and in the treatment of positive and cognitive symptoms in patients with schizophrenia. While early attempts to develop selective mAChR and nAChR agonists provided important preliminary findings, these compounds have ultimately failed in clinical development due to a lack of true subtype selectivity and subsequent dose-limiting adverse effects. In recent years, there have been major advances in the discovery of highly selective activators for the different mAChR and nAChR subtypes with suitable properties for optimization as potential candidates for clinical trials. One novel strategy has been to identify ligands that activate a specific receptor subtype through actions at sites that are distinct from the highly conserved ACh-binding site, termed allosteric sites. These allosteric activators, both allosteric agonists and positive allosteric modulators, of mAChR and nAChR subtypes demonstrate unique mechanisms of action and high selectivity in vivo, and may provide innovative treatment strategies for schizophrenia.

The M5 muscarinic receptor as possible target for treatment of drug abuse

Two reports published in the latter 1980s are generally given credit for being the first to announce the discovery of a new subtype of muscarinic acetylcholine receptor (mAChR), designated m5 or M5, and now officially M(5) (1). Both identifications were assigned using molecular biology techniques. Then - as now - no selective high-affinity ligands or toxins were available. In situ hybridization and reverse-transcriptase PCR have found M(5) AChR expression in brain to be distinct from that of the four other G protein-coupled mAChR subtypes and primarily localized to the substantia nigra, ventral tegmental area, hippocampus (CA1 and CA2 subfields), cerebral cortex (outermost layer) and striatum (caudate putamen). M(5) AChR brain region localization and involvement in the regulation of striatal dopamine release and in rewarding brain stimulation suggests a possible role for M(5) AChR as a target for novel therapy to treat excess hedonic drive, including drug abuse.

Beneficial effects of estrogen in a mouse model of cerebrovascular insufficiency

BACKGROUND

The M(5) muscarinic acetylcholine receptor is known to play a crucial role in mediating acetylcholine dependent dilation of cerebral blood vessels. Previously, we reported that male M(5) muscarinic acetylcholine knockout mice (M5R(-/-) mice) suffer from a constitutive constriction of cerebral arteries, reduced cerebral blood flow, dendritic atrophy, and short-term memory loss, without necrosis and/or inflammation in the brain.

METHODOLOGY/PRINCIPAL FINDINGS

We employed the Magnetic Resonance Angiography to study the area of the basilar artery in male and female M5R(-/-) mice. Here we show that female M5R(-/-) mice did not show the reduction in vascular area observed in male M5R(-/-) mice. However, ovariectomized female M5R(-/-) mice displayed phenotypic changes similar to male M5R(-/-) mice, strongly suggesting that estrogen plays a key role in the observed gender differences. We found that 17beta-estradiol (E2) induced nitric oxide release and ERK activation in a conditional immortalized mouse brain cerebrovascular endothelial cell line. Agonists of ERalpha, ERbeta, and GPR30 promoted ERK activation in this cell line. Moreover, in vivo magnetic resonance imaging studies showed that the cross section of the basilar artery was restored to normal in male M5R(-/-) mice treated with E2. Treatment with E2 also improved the performance of male M5R(-/-) mice in a cognitive test and reduced the atrophy of neural dendrites in the cerebral cortex and hippocampus. M5R(-/-) mice also showed astrocyte swelling in cortex and hippocampus using the three-dimensional reconstruction of electron microscope images. This phenotype was reversed by E2 treatment, similar to the observed deficits in dendrite morphology and the number of synapses.

CONCLUSIONS/SIGNIFICANCE

Our findings indicate that M5R(-/-) mice represent an excellent novel model system to study the beneficial effects of estrogen on cerebrovascular function and cognition. E2 may offer new therapeutic perspectives for the treatment of cerebrovascular insufficiency related memory dysfunction.

Towards a muscarinic hypothesis of schizophrenia

Although the neurotransmitter dopamine plays a prominent role in the pathogenesis and treatment of schizophrenia, the dopamine hypothesis of schizophrenia fails to explain all aspects of this disorder. It is increasingly evident that the pathology of schizophrenia also involves other neurotransmitter systems. Data from many streams of research including pre-clinical and clinical pharmacology, treatment studies, post-mortem studies and neuroimaging suggest an important role for the muscarinic cholinergic system in the pathophysiology of schizophrenia. This review will focus on evidence that supports the hypothesis that the muscarinic system is involved in the pathogenesis of schizophrenia and that muscarinic receptors may represent promising novel targets for the treatment of this disorder.

Selective role for superoxide in InsP3 receptor-mediated mitochondrial dysfunction and endothelial apoptosis

Reactive oxygen species (ROS) play a divergent role in both cell survival and cell death during ischemia/reperfusion (I/R) injury and associated inflammation. In this study, ROS generation by activated macrophages evoked an intracellular Ca2+ ([Ca2+]i) transient in endothelial cells that was ablated by a combination of superoxide dismutase and an anion channel blocker. [Ca2+]i store depletion, but not extracellular Ca2+ chelation, prevented [Ca2+]i elevation in response to O2*- that was inositol 1,4,5-trisphosphate (InsP3) dependent, and cells lacking the three InsP3 receptor (InsP3R) isoforms failed to display the [Ca2+]i transient. Importantly, the O2*--triggered Ca2+ mobilization preceded a loss in mitochondrial membrane potential that was independent of other oxidants and mitochondrially derived ROS. Activation of apoptosis occurred selectively in response to O2*- and could be prevented by [Ca2+]i buffering. This study provides evidence that O2*- facilitates an InsP3R-linked apoptotic cascade and may serve a critical function in I/R injury and inflammation.

Decreased amphetamine-induced locomotion and improved latent inhibition in mice mutant for the M5 muscarinic receptor gene found in the human 15q schizophrenia region

M5 muscarinic receptors are coexpressed with D2 dopamine receptors in the ventral tegmentum and striatum, and are important for reward in rodents. Previously, we reported that disruption of the M5 receptor gene in mice reduced dopamine release in the nucleus accumbens. In this study, we established a polymerase chain reaction (PCR) genotyping method for M5 mutant mice, and, using RT-PCR, found that M5 mRNA expression was highest in the ventral tegmentum, striatum, and thalamus in wild-type mice. In the M5 mutant mice, D2 mRNA expression was increased in several brain structures, including the striatum. Genome mapping studies showed the M5 gene is localized to chromosome 2E4 in mice, and to 15q13 in humans in the region that has been linked to schizophrenia. Amphetamine-induced locomotion, but not baseline locomotion or motor functions, decreased in M5 mutant mice, consistent with lower accumbal dopamine release. Previous reports found latent inhibition improvement in rats following nucleus accumbens lesions, or blockade of dopamine D2 receptors with neuroleptic drugs. Here, latent inhibition was significantly increased in M5 mutant mice as compared with controls, consistent with reduced dopamine function in the nucleus accumbens. In summary, our results showed that M5 gene disruption in mice decreased amphetamine-induced locomotion and increased latent inhibition, suggesting that increased M5 mesolimbic function may be relevant to schizophrenia.

Designing human m1 muscarinic receptor-targeted hydrophobic eigenmode matched peptides as functional modulators

A new proprietary de novo peptide design technique generated ten 15-residue peptides targeting and containing the leading nontransmembrane hydrophobic autocorrelation wavelengths, "modes", of the human m(1) muscarinic cholinergic receptor, m(1)AChR. These modes were also shared by the m(4)AChR subtype (but not the m(2), m(3), or m(5) subtypes) and the three-finger snake toxins that pseudoirreversibly bind m(1)AChR. The linear decomposition of the hydrophobically transformed m(1)AChR amino acid sequence yielded ordered eigenvectors of orthogonal hydrophobic variational patterns. The weighted sum of two eigenvectors formed the peptide design template. Amino acids were iteratively assigned to template positions randomly, within hydrophobic groups. One peptide demonstrated significant functional indirect agonist activity, and five produced significant positive allosteric modulation of atropine-reversible, direct-agonist-induced cellular activation in stably m(1)AChR-transfected Chinese hamster ovary cells, reflected in integrated extracellular acidification responses. The peptide positive allosteric ligands produced left-shifts and peptide concentration-response augmentation in integrated extracellular acidification response asymptotic sigmoidal functions and concentration-response behavior in Hill number indices of positive cooperativity. Peptide mode specificity was suggested by negative crossover experiments with human m(2)ACh and D(2) dopamine receptors. Morlet wavelet transformation of the leading eigenvector-derived, m(1)AChR eigenfunctions locates seven hydrophobic transmembrane segments and suggests possible extracellular loop locations for the peptide-receptor mode-matched, modulatory hydrophobic aggregation sites.

Phospholipase C-gamma is required for agonist-induced Ca2+ entry

We report here that PLC-gamma isoforms are required for agonist-induced Ca2+ entry (ACE). Overexpressed wild-type PLC-gamma1 or a lipase-inactive mutant PLC-gamma1 each augmented ACE in PC12 cells, while a deletion mutant lacking the region containing the SH3 domain of PLC-gamma1 was ineffective. RNA interference to deplete either PLC-gamma1 or PLC-gamma2 in PC12 and A7r5 cells inhibited ACE. In DT40 B lymphocytes expressing only PLC-gamma2, overexpressed muscarinic M5 receptors (M5R) activated ACE. Using DT40 PLC-gamma2 knockout cells, M5R stimulation of ER Ca2+ store release was unaffected, but ACE was abolished. Normal ACE was restored by transient expression of PLC-gamma2 or a lipase-inactive PLC-gamma2 mutant. The results indicate a lipase-independent role of PLC-gamma in the physiological agonist-induced activation of Ca2+ entry.

Zebrafish M2 muscarinic acetylcholine receptor: cloning, pharmacological characterization, expression patterns and roles in embryonic bradycardia

1. A zebrafish M2 muscarinic acetylcholine receptor (mAChR) gene was cloned. It encodes 495 amino acids in a single exon. The derived amino acid sequence is 73.5% identical to its human homologue. 2. Competitive binding studies of the zebrafish M2 receptor and [(3)H]-NMS gave negative log dissociation constants (pK(i)) for each antagonist as follows: atropine (9.16)>himbacine (8.05)>/=4-DAMP (7.83)>AF-DX 116 (7.26)>/=pirenzepine (7.18)>/=tropicamide (6.97)>/=methoctramine (6.82)>/=p-F-HHSiD (6.67)>carbachol (5.20). The antagonist affinity profile correlated with the profile of the human M2 receptor, except for pirenzepine. 3. Reverse transcription polymerase chain reaction and Southern blotting analysis demonstrated that the M2 mAChR mRNA levels increased during the segmentation period (12 h post-fertilization; h.p.f.) in zebrafish. By whole-mount in situ hybridization, the M2 mAChR was first detectable in the heart, vagus motor ganglion, and vagus sensory ganglion at 30, 48 and 60 h.p.f., respectively. 4. The muscarinic receptor that mediates carbachol (CCh)-induced bradycardia was functionally mature at 72 h.p.f. The effect of CCh-induced bradycardia was antagonized by several muscarinic receptor antagonists with the order of potency (pIC(50) values): atropine (6.76)>methoctramine (6.47)>himbacine (6.10)>4-DAMP (5.72)>AF-DX 116 (4.77), however, not by pirenzepine, p-F-HHSiD, or tropicamide (<10 micro M). 5. The effect of CCh-induced bradycardia was abolished completely before 56 h.p.f. by M2 RNA interference, and the bradycardia effect gradually recovered after 72 h.p.f. The basal heart rate was increased in embryos injected with M2 mAChR morpholino antisense oligonucleotide (M2 MO) and the effect of CCh-induced bradycardia was abolished by M2 MO in a dose-dependent manner. In conclusion, the results suggest that the M2 mAChR inhibit basal heart rate in zebrafish embryo and the M2 mAChR mediates the CCh-induced bradycardia.

Deletion of the M5 muscarinic acetylcholine receptor attenuates morphine reinforcement and withdrawal but not morphine analgesia

Little is known about the physiological roles of the M5 muscarinic receptor, the last member of the muscarinic receptor family (M1-M5) to be cloned. In the brain, the M5 receptor subtype is preferentially expressed by dopaminergic neurons of the substantia nigra and the ventral tegmental area. Dopaminergic neurons located in the ventral tegmental area are known to play important roles in mediating both the rewarding effects of opiates and other drugs of abuse and the manifestations of opiate/drug withdrawal symptoms. We therefore speculated that acetylcholine-dependent activation of M5 receptors might modulate the manifestations of opiate reward and withdrawal. This hypothesis was tested in a series of behavioral, biochemical, and neurochemical studies using M5 receptor-deficient mice (M5-/- mice) as novel experimental tools. We found that the rewarding effects of morphine, as measured in the conditioned place preference paradigm, were substantially reduced in M5-/- mice. Furthermore, both the somatic and affective components of naloxone-induced morphine withdrawal symptoms were significantly attenuated in M5-/- mice. In contrast, the analgesic efficacy of morphine and the development of tolerance to the analgesic effects of morphine remained unaltered by the lack of M5 receptors. The finding that M5 receptor activity modulates both morphine reward and withdrawal processes suggests that M5 receptors may represent a novel target for the treatment of opiate addiction.

Quantitation of mRNAs for M(1) to M(5) subtypes of muscarinic receptors in rat heart and brain cortex

It has been generally accepted that, of the five subtypes of muscarinic receptors (M(1)-M(5)), only the M(2) subtype is expressed in mammalian heart. This notion has recently been challenged by a series of reports indicating that mRNAs for some or all non-M(2) subtypes are also present in mammalian heart, in parallel with the M(2) mRNA. However, the quantities of relevant mRNAs reported to be present in the heart are not known, which makes it difficult to evaluate their likely significance. We measured the concentrations of the five muscarinic mRNAs by competitive reverse transcription-polymerase chain reaction and discovered that the M(2) mRNA represents more than 90% of total muscarinic mRNAs in rat atria and in either ventricle. The concentrations of total muscarinic mRNAs and of the M(2) mRNA were more than twice as high in the atria than in the ventricles. mRNAs for all non-M(2) muscarinic receptor subtypes were also detected but represented less than 1% (M(1) and M(4)), less than 3% (M(3)), and less than 5% (M(5)) of total muscarinic RNAs in the atria and ventricles. The findings support the concept of the prevalent role of the M(2) muscarinic receptors in the cholinergic control of the heart. When the same method of quantitation was applied to rat cerebral cortex, mRNAs for individual subtypes were found to represent 36% (M(1)), 21% (M(2)), 25% (M(3)), 11% (M(4)), and 7% (M(5)) of total muscarinic mRNAs.

Increased drinking in mutant mice with truncated M5 muscarinic receptor genes

The rarest and least understood of the muscarinic receptors is the M5 subtype. Recombinant methods were used to create mutant mice with a deletion in the third intracellular loop of the M5 receptor gene. Salivation induced by the nonselective muscarinic agonist pilocarpine (1 mg/kg s.c.) was reduced in homozygous mutants from 15 to 60 min after injection as compared with wild-type mice. After 18-h food and water deprivation, drinking was increased in homozygous mutants, but feeding was not increased. The mutant and wild-type mice had similar responses in tests of open-field exploration, seizures induced by pilocarpine (300 mg/kg) or hypothermia induced by pilocarpine (1-3 mg/kg). These results indicate that M5 muscarinic receptors are important for fluid intake and suggest that M5 receptors are involved in slow secretory processes.

Homologous mutations near the junction of the sixth transmembrane domain and the third extracellular loop lead to constitutive activity and enhanced agonist affinity at all muscarinic receptor subtypes

Previous studies have found that a mutation near the junction of the sixth transmembrane domain (TM6) and the third extracellular loop of the M5 muscarinic receptor leads to constitutive activation and enhanced agonist affinity for the mutated receptor. These results were consistent with the extended ternary complex model, which predicts a correlation between agonist affinity and constitutive activity. We have introduced the homologous mutation into all five subtypes of the highly conserved muscarinic receptor family; SerThr-->TyrPro was introduced into M1 and M5, and AsnThr-->TyrPro was introduced into M2, M3, and M4. In binding assays, these mutations produced increases in affinities toward acetylcholine and carbachol that ranged from 5-fold at the M2 receptor to 15- to 20-fold at M1, M3, and M4, to 40-fold at M5. In functional assays, all five mutant receptors exhibited constitutive activity, at levels ranging between 30 and 80% of the maximal response elicited by carbachol. In every case, the muscarinic antagonist atropine inhibited this constitutive activity with high affinity. Thus, despite differences in effector coupling and in wild-type sequence at the mutation site, all five subtypes were activated by this mutation at the top of TM6. Previous studies of the M5 subtype have indicated that TM6 is a ligand-dependent switch that sets the activation state of the receptor. Based on the results of the present study, it is possible that TM6 represents a general switch for the activation of the muscarinic receptor family.

Cholinergic dilation of cerebral blood vessels is abolished in M(5) muscarinic acetylcholine receptor knockout mice

The M(5) muscarinic receptor is the most recent member of the muscarinic acetylcholine receptor family (M(1)-M(5)) to be cloned. At present, the physiological relevance of this receptor subtype remains unknown, primarily because of its low expression levels and the lack of M(5) receptor-selective ligands. To circumvent these difficulties, we used gene targeting technology to generate M(5) receptor-deficient mice (M5R(-/-) mice). M5R(-/-) mice did not differ from their wild-type littermates in various behavioral and pharmacologic tests. However, in vitro neurotransmitter release experiments showed that M(5) receptors play a role in facilitating muscarinic agonist-induced dopamine release in the striatum. Because M(5) receptor mRNA has been detected in several blood vessels, we also investigated whether the lack of M(5) receptors led to changes in vascular tone by using several in vivo and in vitro vascular preparations. Strikingly, acetylcholine, a powerful dilator of most vascular beds, virtually lost the ability to dilate cerebral arteries and arterioles in M5R(-/-) mice. This effect was specific for cerebral blood vessels, because acetylcholine-mediated dilation of extra-cerebral arteries remained fully intact in M5R(-/-) mice. Our findings provide direct evidence that M(5) muscarinic receptors are physiologically relevant. Because it has been suggested that impaired cholinergic dilation of cerebral blood vessels may play a role in the pathophysiology of Alzheimer's disease and focal cerebral ischemia, cerebrovascular M(5) receptors may represent an attractive therapeutic target.

Functional expression of M(1), M(3) and M(5) muscarinic acetylcholine receptors in yeast

The goal of this study was to functionally express the three G(q)-coupled muscarinic receptor subtypes, M(1), M(3) and M(5), in yeast (Saccharomyces cerevisiae). Transformation of yeast with expression constructs coding for the full-length receptors resulted in very low numbers of detectable muscarinic binding sites (B(max) < 5 fmol/mg). Strikingly, deletion of the central portion of the third intracellular loops of the M(1), M(3) and M(5) muscarinic receptors resulted in dramatic increases in B(max) values (53-214 fmol/mg). To monitor productive receptor/G-protein coupling, we used specifically engineered yeast strains that required agonist-stimulated receptor/G-protein coupling for cell growth. These studies showed that the shortened versions of the M(1), M(3) and M(5) receptors were unable to productively interact with the endogenous yeast G protein alpha-subunit, Gpa1p, or a Gpa1 mutant subunit that contained C-terminal mammalian Galpha(s) sequence. In contrast, all three receptors gained the ability to efficiently couple to a Gpa1/Galpha(q) hybrid subunit containing C-terminal mammalian Galpha(q) sequence, indicating that the M(1), M(3) and M(5) muscarinic receptors retained proper G-protein coupling selectivity in yeast. This is the first study to report the expression of muscarinic receptors in a coupling-competent form in yeast. The strategy described here, which involves structural modification of both receptors and co-expressed G proteins, should facilitate the functional expression of other classes of G protein-coupled receptors in yeast.

Neuromodulation of Na+ channels: an unexpected form of cellular plasticity

Voltage-gated Na+ channels set the threshold for action potential generation and are therefore good candidates to mediate forms of plasticity that affect the entire neuronal output. Although early studies led to the idea that Na+ channels were not subject to modulation, we now know that Na+ channel function is affected by phosphorylation. Furthermore, Na+ channel modulation is implicated in the control of input-output relationships in several types of neuron and seems to be involved in phenomena as varied as cocaine withdrawal, hyperalgesia and light adaptation. Here we review the available evidence for the regulation of Na+ channels by phosphorylation, its molecular mechanism, and the possible ways in which it affects neuronal function.

Pharmacological and functional characterization of muscarinic receptor subtypes in developing oligodendrocytes

This study focused on the molecular and pharmacological characterization of muscarinic acetylcholine receptors expressed by progenitors and differentiated oligodendrocytes. We also analyzed the role of muscarinic receptors in regulating downstream signal transduction pathways and the functional significance of receptor expression in oligodendrocytes. RT-PCR analysis revealed the expression of transcripts for M3, and to a lesser extent M4, followed by M1, M2 and M5 receptor subtypes in both progenitors and differentiated oligodendrocytes. Competition binding experiments using [(3)H]N-methylscopolamine and several antagonists, as well as inhibition of carbachol-mediated phosphoinositide hydrolysis, showed that M3 is the main subtype expressed in these cells. In progenitors the activation of p42/44-mitogen-activated protein kinase (MAPK) and cAMP-response element binding protein (CREB) as well as c-fos mRNA expression were blocked by the M3 relatively selective antagonist, 4-DAMP, and its irreversible analogue, 4-DAMP-mustard. Carbachol increased proliferation of progenitors, an effect prevented by atropine and 4-DAMP, as well as by the MAPK kinase inhibitor PD98059. These results indicate that carbachol modulates oligodendrocyte progenitor proliferation through M3 receptors, involving activation of a MAPK signaling pathway. Receptor density and phosphoinositide hydrolysis are down-regulated during oligodendrocyte differentiation. Functional consequences of these events are a reduction in carbachol-stimulated p42/44(MAPK) and CREB phosphorylation, as well as induction of c-fos.

Acetylcholine increases the free intracellular calcium concentration in podocytes in intact rat glomeruli via muscarinic M(5) receptors

The effects of acetylcholine (ACh) on the free intracellular calcium concentration ([Ca2+](i)) of microdissected glomeruli were investigated using fura-2 fluorescence digital imaging and two-photon confocal microscopy. ACh caused a concentration-dependent [Ca2+](i) increases with an initial peak followed by a sustained plateau, which was suppressed by reduced extracellular Ca2+ concentrations. The [Ca2+](i) plateau was not affected by the L-type Ca2+ channel blocker nicardipine, whereas gadolinium and lanthanum (both at 1 microM) blocked the plateau. Diphenylacetoxy-N:-methylpiperidine methiodide (100 nM), an M(3)/M(5) receptor antagonist, and pirenzepine (1 microM), an M(1) receptor antagonist, completely inhibited the effect of ACh. [Ca2+](i) measurements using two-photon excitation of fluo-3 and staining of the cells with calcein/acetoxymethyl ester, for observation of the capillary network together with the glomerular cells, showed that [Ca2+](i) was increased in single podocytes. Immunohistochemical studies did not demonstrate M(3) receptor expression in glomerular cells. M(1) receptors could be detected only in the parietal sheet of Bowman's capsule, whereas M(5) receptors were found only in podocytes. The data show that ACh increases [Ca2+](i) in podocytes of intact glomeruli, most likely via muscarinic M(5) receptors.

Steroidal sigma receptor ligands affect signaling pathways in human spermatozoa

In human spermatozoa, Ca(2+) entry is stimulated by progesterone or prostaglandin E(1) (PGE(1)). The regulation of cation currents by progestins involves sigma receptors, and sigma binding sites are abundant in testis. We examined the effects of sigma ligands on human spermatozoa. Ca(2+) entry induced by progesterone or PGE(1) was not altered by the sigma ligands haloperidol and ditolylguanidine. However, the steroidal sigma ligands RU 3117 and RU 1968 had distinct effects. Stimulation by RU 3117 resulted in activation and homologous desensitization of the sperm progesterone receptor but not of the PGE(1) receptor. Because haloperidol and ditolylguanidine did not affect RU 3117 and progesterone actions in spermatozoa, we conclude that sigma receptors are not involved. However, RU 1968 potently inhibited both the progesterone- and PGE(1)-induced Ca(2+) entry and acrosome reaction. At higher concentrations, RU 1968 also inhibited hormonal Ca(2+) signaling in fibroblasts. Despite suppression of Ca(2+) mobilization, inhibition of phospholipase C by RU 1968 was not observed. Furthermore, RU 1968 did not impair the binding of inositol-1,4,5-trisphosphate to its endoplasmic reticulum receptor. Because RU 1968 preferentially inhibits signaling pathways in spermatozoa, the future development of more selective drugs structurally related to RU 1968 may be a novel approach for pharmacological contraception.

Isolation and functional characterization of the chick M5 muscarinic acetylcholine receptor gene

The chick is a widely used system for study of the actions of muscarinic acetylcholine receptors in the cardiovascular, visual, and nervous systems. We report the isolation and functional analysis of the gene encoding the chick M5 muscarinic receptor. RT-PCR analysis indicates that the M5 receptor is expressed at low levels in embryonic chick brain and heart. When expressed in stably transfected Chinese hamster ovary cells, the M5 receptor exhibits high-affinity binding to muscarinic antagonists and mediates robust activation of phospholipase C activity.

Asymmetric distribution of muscarinic acetylcholine receptors in Madin-Darby canine kidney cells

We have characterized the muscarinic ACh receptors (mAChRs) expressed in Madin- Darby canine kidney (MDCK) strain II epithelial cells. Binding studies with the membrane-impermeable antagonist N-[(3)H]methylscopolamine demonstrated that mAChRs are approximately 2.5 times more abundant on the basolateral than on the apical surface. Apical, but not basolateral, mAChRs inhibited forskolin-stimulated adenylyl cyclase activity in response to the agonist carbachol. Neither apical nor basolateral mAChRs exhibited detectable carbachol-stimulated phospholipase C activity. Carbachol application to the apical or the basolateral membrane resulted in a threefold increase in intracellular Ca(2+) concentration, which was completely inhibited by pertussis toxin on the apical side and partially inhibited on the basolateral side. RT-PCR analysis showed that MDCK cells express the M(4) and M(5) receptor mRNAs. These data suggest that M(4) receptors reside on the apical and basolateral membranes of polarized MDCK strain II cells and that the M(5) receptor may reside in the basolateral membrane of a subset of cells.

In vitro characterisation of the muscarinic receptor partial agonist, sabcomeline, in rat cortical and heart membranes

We have investigated the pharmacology of the functionally selective muscarinic M1 receptor partial agonist, sabcomeline [SB-202026 (R-(Z)-(+)-alpha-(methoxyamino)-1-azabicyclo[2.2.2] octane-3-acetonitrile)], in rat cortex and heart using radioligand binding and functional studies. The Quinuclidinyl benzilate/Oxotremorine-M acetate ratio from radioligand binding studies suggested that sabcomeline and xanomeline [3(3-hexyloxy-1,25-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-met hylpyridine] are muscarinic receptor partial agonists in cortical and heart membranes. In [35S]GTPgammaS binding studies in rat cortex, carbachol stimulated binding via muscarinic M2/M4 receptors which could be blocked by sabcomeline with a pA2 of 7.2. In rat heart membranes, carbachol also stimulated [35S]GTPgammaS binding studies through muscarinic M2 receptors. Sabcomeline caused a small stimulation of basal [35S]GTPgammaS binding in both rat and heart tissues. Sabcomeline did not stimulate phosphoinositide hydrolysis in rat cortical slices, but did block the muscarinic M1 receptor-mediated response caused by carbachol with apparent pKb of 6.9. Xanomeline and milameline also had no effect on phosphoinositide hydrolysis up to 100 microM. In adenylyl cyclase studies in rat atria, sabcomeline inhibited forskolin-stimulated adenylyl cyclase activity to a similar extent to that of carbachol, xanomeline and milameline. The present study, using the techniques of radioligand binding, supports previous publications which have claimed that sabcomeline is a muscarinic receptor partial agonist. As expected, this study shows that the functional actions of this compound at muscarinic receptor subtypes and in different tissues will depend on receptor reserve.

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

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

Functional effects of the muscarinic receptor agonist, xanomeline, at 5-HT1 and 5-HT2 receptors

Xanomeline [3(3-hexyloxy-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-me thylpyridine] has been reported to act as a functionally selective muscarinic partial agonist with potential use in the treatment of Alzheimer's disease. This study examined the functional activity of xanomeline at 5-HT1 and 5-HT2 receptors in native tissue and/or human cloned receptors. Xanomeline had affinity for muscarinic receptors in rat cortical membranes where the ratio of the displacement affinity of [3H]-Quinuclidinyl benzilate vs that of [3H]-Oxotremorine-M was 16, indicative of partial agonist activity. Radioligand binding studies on human cloned receptors confirmed that xanomeline had substantial affinity for M1, M2, M3, M4, M5 receptors and also for 5-HT1 and 5-HT2 receptor subtypes. Carbachol and xanomeline stimulated basal [35S]-GTPgammaS binding in rat cortical membranes with micromolar affinity. The response to carbachol was attenuated by himbacine and pirenzepine with pA2 of 8.2, 6.9 respectively consistent with the response being mediated, predominantly, via M2 and M4 receptors. Xanomeline-induced stimulation of [35S]-GTPgammaS binding was inhibited by himbacine with an apparent pKb of 6.3, was not attenuated by pirenzepine up to 3 microM and was inhibited by the selective 5-HT1A antagonist WAY100635 with an apparent pKb of 9.4. These data suggest the agonist effect of xanomeline in this tissue is, in part, via 5-HT1A receptors. Similar studies on human cloned receptors confirmed that xanomeline is an agonist at human cloned 5-HT1A and 5-HT1B receptors. In studies using the fluorescent cytoplasmic Ca2+ indicator FLUO-3AM, xanomeline induced an increase in cytoplasmic Ca2+ concentration in SH-SY5Y cells expressing recombinant human 5-HT2C receptors. Atropine antagonized this response, consistent with mediation via endogenously-expressed muscarinic receptors. In the presence of atropine, xanomeline antagonized 5-HT-induced cytoplasmic changes in Ca2+ concentration in cells expressing h5-HT2A, h5-HT2B and h5-HT2c receptors with potencies similar to its affinity at these receptors. These studies indicate that xanomeline is a potent agonist at 5-HT1A and 5-HT1B receptors and an antagonist at 5-HT2 receptor subtypes.

Signaling mechanisms underlying muscarinic receptor-mediated increase in contraction rate in cultured heart cells

We have investigated the mechanisms by which stimulation of cardiac muscarinic receptors result in paradoxical stimulatory effects on cardiac function, using cultured neonatal rat ventricular myocytes as a model system. Application of low concentrations of carbachol (CCh) (EC50 = 35 nM) produced an atropine-sensitive decrease in spontaneous contraction rate, while, in cells pretreated with pertussis toxin, higher concentrations of CCh (EC50 = 26 microM) elicited an atropine-sensitive increase in contraction rate. Oxotremorine, an m2 muscarinic acetylcholine receptor (mAChR) agonist, mimicked the negative but not the positive chronotropic response to CCh. Reverse transcription followed by polymerase chain reaction carried out on mRNA obtained from single cells indicated that ventricular myocytes express mRNA for the m1, m2, and, possibly, m4 mAChRs. The presence of m1 and m2 mAChR protein on the surface membranes of the cultured ventricular myocytes was confirmed by immunofluorescence. The CCh-induced positive chronotropic response was significantly inhibited by fluorescein-tagged antisense oligonucleotides directed against the m1, but not the m2 and m4, mAChR subtypes. The response was also inhibited by antisense oligonucleotides against Gqalpha protein. Finally, inhibition of CCh-induced phosphoinositide hydrolysis with 500 microM neomycin or 5 microM U73122 completely abolished the CCh-induced positive chronotropic response. These results are consistent with the stimulatory effects of mAChR activation on the rate of contractions in cultured ventricular myocytes being mediated through the m1 mAChR coupled through Gq to phospholipase C-induced phosphoinositide hydrolysis.

Receptor-activated Ca2+ influx via human Trp3 stably expressed in human embryonic kidney (HEK)293 cells. Evidence for a non-capacitative Ca2+ entry

Ca2+ release from its internal stores as a result of activation of phospholipase C is accompanied by Ca2+ influx from the extracellular space. Ca2+ influx channels may be formed of proteins homologous to Drosophila Trp. At least six non-allelic Trp genes are present in the mouse genome. Full-length human, bovine, mouse, and rat cDNAs for Trp1, 3, 4, 6 have been cloned. Expression of these genes in various mammalian cells has provided evidence that Trp proteins form plasma membrane Ca2+-permeant channels that can be activated by an agonist that activates phospholipase C, by inositol 1,4, 5-trisphosphate, and/or store depletion. We have stably expressed human Trp3 (hTrp3) in human embryonic kidney (HEK)293 cells. Measurement of intracellular Ca2+ concentrations in Fura2-loaded cells showed that cell lines expressing hTrp3 have significantly higher basal and agonist-stimulated influxes of Ca2+, Mn2+, Ba2+, and Sr2+ than control cells. The increase in Ca2+ entry attributable to the expression of hTrp3 obtained upon store depletion by thapsigargin was much lower than that obtained by stimulation with agonists acting via a Gq-coupled receptor. Addition of agonists to thapsigargin-treated Trp3 cells resulted in a further increase in the entry of divalent cations. The increased cation entry in Trp3 cells was blocked by high concentrations of SKF 96365, verapamil, La3+, Ni2+, and Gd3+. The Trp3-mediated Ca2+ influx activated by agonists was inhibited by a phospholipase C inhibitor, U73122. We propose that expression of hTrp3 in these cells forms a non-selective cation channel that opens after the activation of phospholipase C but not after store depletion. In addition, a subpopulation of the expressed hTrp3 may form heteromultimeric channels with endogenous proteins that are sensitive to store depletion.

Cloning and expression of a novel mammalian homolog of Drosophila transient receptor potential (Trp) involved in calcium entry secondary to activation of receptors coupled by the Gq class of G protein

Hormonal stimulation of Gq-protein coupled receptors triggers Ca2+ mobilization from internal stores. This is followed by a Ca2+ entry through the plasma membrane. Drosophila Trp and Trpl proteins have been implicated in Ca2+ entry and three mammalian homologues of Drosophila Trp/Trpl, hTrp1, hTrp3 and bTrp4 (also bCCE) have been cloned and expressed. Using mouse brain RNA as template, we report here the polymerase chain reaction-based cloning and functional expression of a novel Trp, mTrp6. The cDNA encodes a protein of 930 amino acids, the sequence of which is 36.8, 36.3, 43.1, 38.6, and 74. 1% identical to Drosophila Trp and Trpl, bovine Trp4, and human Trp1 and Trp3, respectively. Transient expression of mTrp6 in COS.M6 cells by transfection of the full-length mTrp6 cDNA increases Ca2+ entry induced by stimulation of co-transfected M5 muscarinic acetylcholine receptor with carbachol (CCh), as seen by dual wavelength fura 2 fluorescence ratio measurements. The mTrp6-mediated increase in Ca2+ entry activity was blocked by SKF-96365 and La3+. Ca2+ entry activity induced by thapsigargin was similar in COS cells transfected with or without the mTrp6 cDNA. The thapsigargin-stimulated Ca2+ entry could not be further stimulated by CCh in control cells but was markedly increased in mTrp6-transfected cells. Records of whole cell transmembrane currents developed in response to voltage ramps from -80 to +40 mV in control HEK cells and HEK cells stably expressing mTrp6 revealed the presence of a muscarinic receptor responsive non-selective cation conductance in Trp6 cells that was absent in control cells. Our data support the hypothesis that mTrp6 encodes an ion channel subunit that mediates Ca2+ entry stimulated by a G-protein coupled receptor, but not Ca2+ entry stimulated by intracellular Ca2+ store depletion.

Subtypes of the muscarinic receptor in smooth muscle

Muscarinic receptors are expressed in smooth muscle throughout the body. In most instances, the muscarinic receptor population in smooth muscle is composed of mainly the M2 and M3 subtypes in an 80% to 20% mixture. The M3 subtype mediates phosphoinositide hydrolysis and calcium mobilization, whereas the M2 subtype mediates an inhibition of cAMP accumulation. In addition, a variety of ionic conductances are elicited by muscarinic receptors. Muscarinic agonists stimulate a nonselective cation conductance that is pertussis toxin-sensitive and dependent on calcium. The pertussis toxin-sensitivity of this response suggests that it is mediated by M2 receptors. Following agonist induced depolarization of smooth muscle, voltage dependent calcium channels are activated to enable an influx of calcium. In some instances, muscarinic agonists enhance this conductance through a mechanism involving protein kinase C, whereas in other instances, muscarinic agonists suppress this calcium conductance. Smooth muscle often contains calcium activated potassium channels that tend to repolarize the membrane following calcium influx. Activation of muscarinic receptors suppresses this potassium conductance in some smooth muscles. Under standard conditions, muscarinic agonists elicit pertussis toxin-insensitive contractions through activation of the M3 receptor. When most of the M3 receptors are inactivated, it is possible to measure a pertussis toxin-sensitive contractile response to muscarinic agonists that is most likely mediated through M2 receptors. M2 receptors also cause an indirect contraction by inhibiting the relaxant effects of agents that increase cAMP (e.g., forskolin and isoproterenol).

The M5 (m5) receptor subtype: fact or fiction?

By comparison to the other subtypes of muscarinic receptors, very little is known about the binding properties, locations, mechanisms and physiological functions of the M5 (m5)* receptor subtype. Studies of the m5 receptor have been hampered by the lack of m5-selective ligands or antibodies and a source that endogenously expresses predominantly the m5 receptor subtype. We have developed a pharmacological labeling strategy using the non-selective muscarinic antagonist [3H]NMS, in the presence of muscarinic antagonists and toxins in green mamba venom to occlude the m1-m4 receptor subtypes, to selectively label the m5 receptor subtype. This m5-selective labeling approach, along with those developed for the other four receptor subtypes, has permitted for the first time a comparison of the relative expression levels and anatomical localizations of the five muscarinic receptor subtypes in the brain. The distribution profile of the m5 receptor is distinct from the other four subtypes and is enriched in the outer layers of the cortex, specific subfields of the hippocampus, caudate putamen, olfactory tubercle and nucleus accumbens. These studies have also demonstrated that the levels of m5 receptor protein expression are apparently higher and more widespread than anticipated from previous in situ hybridization and immunoprecipitation studies. Taken together, the results suggest a unique and potentially physiologically important role for the m5 receptor subtype in modulating the actions of acetylcholine in the brain.

Studies on the characterization of the subtype(s) of muscarinic receptor involved in prostacyclin synthesis in rabbit cardiomyocytes

The present study was conducted to localize and characterize the subtype(s) of muscarinic receptor involved in prostacyclin (PGI2) production elicited by the cholinergic transmitter acetylcholine (ACh) in various cell types in the rabbit heart. ACh increased PGI2 synthesis measured as 6-keto-PGF1 alpha, in cultured coronary endothelial cells and freshly dissociated ventricular myocytes in a dose dependent manner but not in cultured coronary smooth muscle cells of rabbit heart. McN-A-343, a partially selective M1 muscarinic ACh receptor (mAChR) agonist, did not alter 6-keto-PGF1 alpha synthesis in these cell types. ACh induced 6-keto-PGF1 alpha synthesis in coronary endothelial cells and ventricular myocytes was not altered by a low concentration (10(-8) M) of pirenzipine, an M1 mAChR antagonist but was reduced by a higher concentration (10(-6) M). In coronary endothelial cells ACh induced 6-keto-PGF1 alpha production was reduced by hexahydro-sila-difendial (HHSiD), an M3 mAChR antagonist, and in ventricular myocytes by both 11-¿2-[(di-ethylamino) methyl]-1-piperidinyl]acetyl-5,11-dihydro-6-H-pyrido-[2,3-b]-benzodiazep ine-6 one¿ (AF-DX 116), an M2 receptor antagonist, and HHSiD. The decrease by ACh of isoporterenol stimulated cAMP accumulation was minimized by AF-DX 116 but not by HHSiD or pirenzipine. Pertussis toxin treatment minimized ACh induced decrease in isoproterenol stimulated rise in cAMP and ATP release, but not ACh induced 6-keto-PGF1 alpha synthesis. These data suggest that ACh stimulates prostacyclin production in coronary endothelial cells via M3 mAChR and in ventricular myocytes M2 and M3 mAChR. Moreover, ACh induced decrease in cAMP, but not the increase in 6-keto-PGF1 alpha production, is mediated by pertussis toxin sensitive G alpha i proteins in these cells.

Involvement of Gs and Gi proteins in dual coupling of the luteinizing hormone receptor to adenylyl cyclase and phospholipase C

Binding of lutropin/choriogonadotropin to its cognate receptor results in the activation of adenylyl cyclase and phospholipase C. The mechanism underlying the generation of this bifurcating signal is presently not known. To analyze the coupling mechanism of the LH receptor, activated G proteins were labeled with [alpha-32P]GTP azidoanilide and identified by selective immunoprecipitation. In membranes of bovine corpora lutea and of L cells stably expressing the murine LH receptor (LHR cells), human chorionic gonadotropin (hCG) led to incorporation of the label into alphas and alphai2. Stimulation of LHR cells or of L cells expressing the M5 muscarinic receptor (LM5 cells) with the respective agonist resulted in activation of phospholipase C in both cell lines. However, alphaq and alpha11 were only labeled upon stimulation of the M5 muscarinic receptor. Agonist-induced Ca2+ mobilization and inositol phosphate accumulation were partially sensitive to pertussis toxin, and the expression of the betagamma-stimulable phospholipase C isoforms beta2 and beta3 could be demonstrated in LHR cells. Overexpression of phospholipase C-beta2 led to increased hCG-stimulated inositol phosphate accumulation, and expression of a beta-ARK1 C-terminal polypeptide effectively suppressed hCG-mediated phosphatidylinositol hydrolysis. Thus, the LH receptor couples to both Gs and Gi, and betagamma-subunits released from either G protein contribute to the stimulation of phospholipase C-beta isoforms.

Molecular and functional identification of m1 muscarinic acetylcholine receptors in rat ventricular myocytes

The expression of muscarinic acetylcholine receptor (mAChR) subtypes in freshly isolated adult rat ventricular myocytes was investigated by reverse transcription of cellular mRNA followed by amplification of cDNA using the polymerase chain reaction (PCR). After reverse-transcriptase PCR, bands were obtained corresponding to the expected sizes for the m1 and m2 but not for the m3 to m5 mAChRs. The identity of the m1 and m2 bands was confirmed by single-cell PCR, restriction digest mapping, and Southern blot analysis. The presence of m1 and m2, but not m3, mAChR protein in these cells was shown by indirect immunofluorescence studies using subtype-specific antibodies. It was further investigated whether the identified m1 mAChR was responsible for the stimulatory effects on Ca2+ transients by high concentrations of carbachol ( > 10 mumol/L) known to occur in these cells. In pertussis toxin-treated ventricular myocytes electrically stimulated at 1 Hz, carbachol (300 mumol/L) increased the basal Ca2+ level from 96 +/- 7 to 118 +/- 8 nmol/L and the peak Ca2+ transient level from 519 +/- 32 to 640 +/- 36 nmol/L (mean +/- SEM P < .05 for both, n = 8). These effects of carbachol on Ca2+ transients were antagonized by 10 nmol/L pirenzepine, an m1 mAChR-selective antagonist. In contrast, the m2 mAChR-selective antagonist methoctramine (up to 100 nmol/L) did not inhibit the response. These results are the first to use single-cell PCR to probe cardiomyocyte-specific gene expression and indicate that m1 mAChRs are expressed on adult rat ventricular myocytes in addition to m2 mAChRs. The results further suggest that m1 mAChRs mediate the stimulatory responses on Ca2+ transients to high concentrations of cholinergic agonists seen in these cells.

trp, a novel mammalian gene family essential for agonist-activated capacitative Ca2+ entry

SUMMARY

Capacitative calcium entry (CCE) describes CA2+ influx into cells that replenishes CA2+ stores emptied through the action of IP3 and other agents. It is an essential component of cellular responses to many hormones and growth factors. The molecular basis of this form of Ca2+ entry is complex and may involve more than one type of channel. Studies on visual signal transduction in Drosophila led to the hypothesis that a protein encoded in trp may be a component of CCE channels. We reported the existence of six trp-related genes in the mouse genome. Expression in L cells of small portions of these genes in antisense orientation suppressed CCE. Expression in COS cells of two full-length cDNAs encoding human trp homologs, Htrp1 and Htrp3, increased CCE. This identifies mammalian gene products that participate in CCE. We propose that trp homologs are subunits of CCE channels, not unlike those of classical voltage-gated ion channels.

Muscarinic modulation of sodium current by activation of protein kinase C in rat hippocampal neurons

Phosphorylation of brain Na+ channels by protein kinase C (PKC) decreases peak Na+ current and slows macroscopic inactivation, but receptor-activated modulation of Na+ currents via the PKC pathway has not been demonstrated. We have examined modulation of Na+ channels by activation of muscarinic receptors in acutely-isolated hippocampal neurons using whole-cell voltage-clamp recording. Application of the muscarinic agonist carbachol reduced peak Na+ current and slowed macroscopic inactivation at all potentials, without changing the voltage-dependent properties of the channel. These effects were mediated by PKC, since they were eliminated when the specific PKC inhibitor (PKCI19-36) was included in the pipette solution and mimicked by the extracellular application of the PKC activator, OAG. Thus, activation of endogenous muscarinic receptors on hippocampal neurons strongly modulates Na+ channel activity by activation of PKC. Cholinergic input from basal forebrain neurons may have this effect in the hippocampus in vivo.

Autoradiographic localization of functional muscarinic receptors in the rat superior cervical sympathetic ganglion reveals an extensive distribution over non-synaptic surfaces of neuronal somata, dendrites and nerve endings

Fast synaptic transmission in sympathetic ganglia is mediated by acetylcholine, acting on nicotinic receptors, yet muscarinic receptors are also present and are involved in the production of slow postsynaptic potentials. In order further to elucidate the role of muscarinic receptors in ganglionic transmission their distribution in the rat superior cervical sympathetic ganglion was investigated autoradiographically by use of the tritiated irreversible muscarinic ligand propylbenzilylcholine mustard. It was observed that this agent blocked the carbachol-evoked hydrolysis of inositol phospholipids in the ganglion and that this response to carbachol is itself inhibitable by selective muscarinic antagonists with a potency sequence which indicates involvement primarily of M1 receptors. Light microscope autoradiography showed that labelling inhibitable by atropine and by the M1-selective muscarinic antagonist pirenzepine was essentially confined to the margins of neuronal somata and regions of dendritic arborization, which include synaptic contacts. Quantitative electron microscope autoradiography showed that binding of the radioligand, of which approximately 70% was inhibitable by atropine and 68% by pirenzepine, was associated predominantly with surface membranes of neuronal somata, dendrites, other neurites (including axons and uncharacterized dendrites) and nerve terminal profiles, in the approximate ratios 95:85:52:45. Of the inhibitable binding over neuronal membranes in the ganglion little more than 3% was found to be synaptically located, and this involved para- or peri-synaptic regions of nerve terminal contacts rather than the specialized synaptic zone. About 5% of the inhibitable binding over neuronal membranes involved non-synaptic surfaces of nerve terminals and preterminal axon segments; almost 70% was distributed over non-synaptic surfaces of neuronal somata and dendrites, and about 21% upon other neurites. Binding sites were found not to be more highly concentrated at or adjacent to synapses than over other regions of neuronal surface membranes. About 50%, possibly more, of the binding on non-synaptic surfaces of nerve endings, and about 7% of binding upon dendritic membranes, was of non-M1, possibly M2 type, inhibitable by atropine but not by pirenzepine. Non-synaptic neuro-neuronal appositions, which involve dendrites and somata and often lie adjacent to synapses, showed rather more than twice the binding expected for each membrane individually; and neuronal membrane exposed to basal lamina lining ganglionic tissue spaces showed high levels of binding. Little inhibitable binding was seen over membranes of satellite and Schwann cells, or over cytoplasmic territories or ganglionic interstitial tissue. A model was constructed of the distribution of label, which showed that the observed results for total binding could be approximately matched by assuming the following relative densities of ligand binding sites: interstitial tissue space and supporting cells 1, soma cytoplasm 3, cytoplasm of dendrites, neurites and nerve terminals 4.5, surfaces of mesodermal elements 15, surfaces of neurites and nerve endings including sites of synapse 45, surfaces of dendrites 90, surfaces of neuronal somata 120, non-synaptic neuro-neuronal appositions 180. It is concluded that functional muscarinic receptors in this sympathetic ganglion, predominantly of the M1 type linked with slow depolarizations, but including some non-M1 receptors, are widely distributed over non-synaptic surfaces of the neuronal somata and dendrites and are not concentrated at synapses. Presynaptic autoreceptors are also present, of which half or more are of non-M1, possibly M2, type which might be inhibitory. The presence of M4 receptors is not excluded...

Promoter region of the rat m4 muscarinic acetylcholine receptor gene contains a cell type-specific silencer element

We describe here the characterization of the rat m4 muscarinic acetylcholine receptor gene and the identification of its regulatory region. Two 5'-noncoding exons are located approximately 5 kilobases upstream from the coding exon, and at least two alternatively spliced variants of m4 mRNA are expressed in the neuronal cell line PC12D. There are two transcription initiation sites. The promoter region is GC-rich, contains no TATA-box, but has two potential CAAT boxes and several putative binding sites for transcription factors Sp1 and AP-2. We assessed the m4 promoter activity functionally in transient expression assays using luciferase as a reporter. The proximal 435-base pair (bp) sequence of the 5'-flanking region produced luciferase activity in both m4-expressing neuronal cell lines (PC12D and NG108-15) and non-neuronal cell lines (L6 and 3Y1B). A longer fragment containing an additional 638-bp sequence produced luciferase activity only in m4-expressing neuronal cell lines. These data suggest that the proximal 435-bp sequence contains a constitutive promoter and that a 638-bp sequence farther upstream contains a cell type-specific silencer element. A consensus sequence for the neural-restrictive silencer element is found within this 638-bp segment.