Muscarinic receptor subtypes controlling the cationic current in guinea-pig ileal smooth muscle

Direct modulation of TRPC ion channels by Gα proteins

GPCR-Gi protein pathways are involved in the regulation of vagus muscarinic pathway under physiological conditions and are closely associated with the regulation of internal visceral organs. The muscarinic receptor-operated cationic channel is important in GPCR-Gi protein signal transduction as it decreases heart rate and increases GI rhythm frequency. In the SA node of the heart, acetylcholine binds to the M2 receptor and the released Gβγ activates GIRK (I(K,ACh)) channel, inducing a negative chronotropic action. In gastric smooth muscle, there are two muscarinic acetylcholine receptor (mAChR) subtypes, M2 and M3. M2 receptor activates the muscarinic receptor-operated nonselective cationic current (mIcat, NSCC(ACh)) and induces positive chronotropic effect. Meanwhile, M3 receptor induces hydrolysis of PIP2 and releases DAG and IP3. This IP3 increases intracellular Ca2+ and then leads to contraction of GI smooth muscles. The activation of mIcat is inhibited by anti-Gi/o protein antibodies in GI smooth muscle, indicating the involvement of Gαi/o protein in the activation of mIcat. TRPC4 channel is a molecular candidate for mIcat and can be directly activated by constitutively active Gαi QL proteins. TRPC4 and TRPC5 belong to the same subfamily and both are activated by Gi/o proteins. Initial studies suggested that the binding sites for G protein exist at the rib helix or the CIRB domain of TRPC4/5 channels. However, recent cryo-EM structure showed that IYY58-60 amino acids at ARD of TRPC5 binds with Gi3 protein. Considering the expression of TRPC4/5 in the brain, the direct G protein activation on TRPC4/5 is important in terms of neurophysiology. TRPC4/5 channels are also suggested as a coincidence detector for Gi and Gq pathway as Gq pathway increases intracellular Ca2+ and the increased Ca2+ facilitates the activation of TRPC4/5 channels. More complicated situation would occur when GIRK, KCNQ2/3 (IM) and TRPC4/5 channels are co-activated by stimulation of muscarinic receptors at the acetylcholine-releasing nerve terminals. This review highlights the effects of GPCR-Gi protein pathway, including dopamine, μ-opioid, serotonin, glutamate, GABA, on various oragns, and it emphasizes the importance of considering TRPC4/5 channels as crucial players in the field of neuroscience.

Pharmacological effects of monoterpene carveol on the neuromuscular system of nematodes and mammals

The control of parasitic nematode infections relies mostly on anthelmintics. The potential pharmacotherapeutic application of phytochemicals, in order to overcome parasite resistance and enhance the effect of existing drugs, is becoming increasingly important. The antinematodal effects of carveol was tested on the free-living nematode Caenorhabditis elegans and the neuromuscular preparation of the parasitic nematode Ascaris suum. Carveol caused spastic paralysis in C. elegans. In A. suum carveol potentiated contractions induced by acetylcholine (ACh) and this effect was confirmed with two-electrode voltage-clamp electrophysiology on the A. suum nicotinic ACh receptor expressed in Xenopus oocytes. However, potentiating effect of carveol on ACh-induced contractions was partially sensitive to atropine, indicates a dominant nicotine effect but also the involvement of some muscarinic structures. The effects of carveol on the neuromuscular system of mammals are also specific. In micromolar concentrations, carveol acts as a non-competitive ACh antagonist on ileum contractions. Unlike atropine, it does not change the EC50 of ACh, but reduces the amplitude of contractions. Carveol caused an increase in Electrical Field Stimulation-evoked contractions of the isolated rat diaphragm, but at higher concentrations it caused an inhibition. Also, carveol neutralized the mecamylamine-induced tetanic fade, indicating a possibly different pre- and post-synaptic action at the neuromuscular junction.

Molecular mechanisms of cholinergic neurotransmission in visceral smooth muscles with a focus on receptor-operated TRPC4 channel and impairment of gastrointestinal motility by general anaesthetics and anxiolytics

Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles, wherein it binds to and activates two muscarinic receptors subtypes, M2 and M3, thus causing smooth muscle excitation and contraction. The first part of this review focuses on the types of cells involved in cholinergic neurotransmission and on the molecular mechanisms underlying acetylcholine-induced membrane depolarisation, which is the central event of excitation-contraction coupling causing Ca2+ entry via L-type Ca2+ channels and smooth muscle contraction. Studies of the muscarinic cation current in intestinal myocytes (mICAT) revealed its main molecular counterpart, receptor-operated TRPC4 channel, which is activated in synergy by both M2 and M3 receptors. M3 receptors activation is of permissive nature, while activation of M2 receptors via Gi/o proteins that are coupled to them plays a direct role in TRPC4 opening. Our understanding of signalling pathways underlying mICAT generation has vastly expanded in recent years through studies of TRPC4 gating in native cells and its regulation in heterologous cells. Recent studies using muscarinic receptor knockout have established that at low agonist concentration activation of both M2 receptor and the M2/M3 receptor complex elicits smooth muscle contraction, while at high agonist concentration M3 receptor function becomes dominant. Based on this knowledge, in the second part of this review we discuss the cellular and molecular mechanisms underlying the numerous anticholinergic effects on neuroactive drugs, in particular general anaesthetics and anxiolytics, which can significantly impair gastrointestinal motility. This article is part of the Special Issue on "Ukrainian Neuroscience".

Pico145 inhibits TRPC4-mediated mICAT and postprandial small intestinal motility

In intestinal smooth muscle cells, receptor-operated TRPC4 are responsible for the majority of muscarinic receptor cation current (mICAT), which initiates cholinergic excitation-contraction coupling. Our aim was to examine the effects of the TRPC4 inhibitor Pico145 on mICAT and Ca2+ signalling in mouse ileal myocytes, and on intestinal motility. Ileal myocytes freshly isolated from two month-old male BALB/c mice were used for patch-clamp recordings of whole-cell currents and for intracellular Ca2+ imaging using Fura-2. Functional assessment of Pico145's effects was carried out by standard in vitro tensiometry, ex vivo video recordings and in vivo postprandial intestinal transit measurements using carmine red. Carbachol (50 µM)-induced mICAT was strongly inhibited by Pico145 starting from 1 pM. The IC50 value for the inhibitory effect of Pico145 on this current evoked by intracellularly applied GTPγS (200 µM), and thus lacking desensitisation, was found to be 3.1 pM, while carbachol-induced intracellular Ca2+ rises were inhibited with IC50 of 2.7 pM. In contrast, the current activated by direct TRPC4 agonist (-)-englerin A was less sensitive to the action of Pico145 that caused only ∼43 % current inhibition at 100 pM. The inhibitory effect developed rather slowly and it was potentiated by membrane depolarisation. In functional assays, Pico145 produced concentration-dependent suppression of both spontaneous and carbachol-evoked intestinal smooth muscle contractions and delayed postprandial intestinal transit. Thus, Pico145 is a potent GI-active small-molecule which completely inhibits mICAT at picomolar concentrations and which is as effective as trpc4 gene deficiency in in vivo intestinal motility tests.

Antispasmodic Effect of Valeriana pilosa Root Essential Oil and Potential Mechanisms of Action: Ex Vivo and In Silico Studies

Infusions of Valeriana pilosa are commonly used in Peruvian folk medicine for treating gastrointestinal disorders. This study aimed to investigate the spasmolytic and antispasmodic effects of Valeriana pilosa essential oil (VPEO) on rat ileum. The basal tone of ileal sections decreased in response to accumulative concentrations of VPEO. Moreover, ileal sections precontracted with acetylcholine (ACh), potassium chloride (KCl), or barium chloride (BaCl2) were relaxed in response to VPEO by a mechanism that depended on atropine, hyoscine butylbromide, solifenacin, and verapamil, but not glibenclamide. The results showed that VPEO produced a relaxant effect by inhibiting muscarinic receptors and blocking calcium channels, with no apparent effect on the opening of potassium channels. In addition, molecular docking was employed to evaluate VPEO constituents that could inhibit intestinal contractile activity. The study showed that α-cubebene, β-patchoulene, β-bourbonene, β-caryophyllene, α-guaiene, γ-muurolene, valencene, eremophyllene, and δ-cadinene displayed the highest docking scores on muscarinic acetylcholine receptors and voltage-gated calcium channels, which may antagonize M2 and/or M3 muscarinic acetylcholine receptors and block voltage-gated calcium channels. In summary, VPEO has both spasmolytic and antispasmodic effects. It may block muscarinic receptors and calcium channels, thus providing a scientific basis for its traditional use for gastrointestinal disorders.

General anaesthesia-related complications of gut motility with a focus on cholinergic mechanisms, TRP channels and visceral pain

General anesthesia produces multiple side effects. Notably, it temporarily impairs gastrointestinal motility following surgery and causes the so-called postoperative ileus (POI), a multifactorial and complex condition that develops secondary to neuromuscular failure and mainly affects the small intestine. There are currently limited medication options for POI, reflecting a lack of comprehensive understanding of the mechanisms involved in this complex condition. Notably, although acetylcholine is one of the major neurotransmitters initiating excitation-contraction coupling in the gut, cholinergic stimulation by prokinetic drugs is not very efficient in case of POI. Acetylcholine when released from excitatory motoneurones of the enteric nervous system binds to and activates M2 and M3 types of muscarinic receptors in smooth muscle myocytes. Downstream of these G protein-coupled receptors, muscarinic cation TRPC4 channels act as the major focal point of receptor-mediated signal integration, causing membrane depolarisation accompanied by action potential discharge and calcium influx via L-type Ca²⁺ channels for myocyte contraction. We have recently found that both inhalation (isoflurane) and intravenous (ketamine) anesthetics significantly inhibit this muscarinic cation current (termed mI _CAT) in ileal myocytes, even when G proteins are activated directly by intracellular GTPγS, i.e., bypassing muscarinic receptors. Here we aim to summarize Transient Receptor Potential channels and calcium signalling-related aspects of the cholinergic mechanisms in the gut and visceral pain, discuss exactly how these may be negatively impacted by general anaesthetics, while proposing the receptor-operated TRPC4 channel as a novel molecular target for the treatment of POI.

Propulsive colonic contractions are mediated by inhibition-driven poststimulus responses that originate in interstitial cells of Cajal

The peristaltic reflex is a fundamental behavior of the gastrointestinal (GI) tract in which mucosal stimulation activates propulsive contractions. The reflex occurs by stimulation of intrinsic primary afferent neurons with cell bodies in the myenteric plexus and projections to the lamina propria, distribution of information by interneurons, and activation of muscle motor neurons. The current concept is that excitatory cholinergic motor neurons are activated proximal to and inhibitory neurons are activated distal to the stimulus site. We found that atropine reduced, but did not block, colonic migrating motor complexes (CMMCs) in mouse, monkey, and human colons, suggesting a mechanism other than one activated by cholinergic neurons is involved in the generation/propagation of CMMCs. CMMCs were activated after a period of nerve stimulation in colons of each species, suggesting that the propulsive contractions of CMMCs may be due to the poststimulus excitation that follows inhibitory neural responses. Blocking nitrergic neurotransmission inhibited poststimulus excitation in muscle strips and blocked CMMCs in intact colons. Our data demonstrate that poststimulus excitation is due to increased Ca2+ transients in colonic interstitial cells of Cajal (ICC) following cessation of nitrergic, cyclic guanosine monophosphate (cGMP)-dependent inhibitory responses. The increase in Ca2+ transients after nitrergic responses activates a Ca2+-activated Cl− conductance, encoded by Ano1, in ICC. Antagonists of ANO1 channels inhibit poststimulus depolarizations in colonic muscles and CMMCs in intact colons. The poststimulus excitatory responses in ICC are linked to cGMP-inhibited cyclic adenosine monophosphate (cAMP) phosphodiesterase 3a and cAMP-dependent effects. These data suggest alternative mechanisms for generation and propagation of CMMCs in the colon.

Single-Walled Carbon Nanotubes Inhibit TRPC4-Mediated Muscarinic Cation Current in Mouse Ileal Myocytes

Single-walled carbon nanotubes (SWCNTs) are characterized by a combination of rather unique physical and chemical properties, which makes them interesting biocompatible nanostructured materials for various applications, including in the biomedical field. SWCNTs are not inert carriers of drug molecules, as they may interact with various biological macromolecules, including ion channels. To investigate the mechanisms of the inhibitory effects of SWCNTs on the muscarinic receptor cation current (mI_CAT), induced by intracellular GTPγs (200 μM), in isolated mouse ileal myocytes, we have used the patch-clamp method in the whole-cell configuration. Here, we use molecular docking/molecular dynamics simulations and direct patch-clamp recordings of whole-cell currents to show that SWCNTs, purified and functionalized by carboxylation in water suspension containing single SWCNTs with a diameter of 0.5–1.5 nm, can inhibit mI_CAT, which is mainly carried by TRPC4 cation channels in ileal smooth muscle cells, and is the main regulator of cholinergic excitation–contraction coupling in the small intestinal tract. This inhibition was voltage-independent and associated with a shortening of the mean open time of the channel. These results suggest that SWCNTs cause a direct blockage of the TRPC4 channel and may represent a novel class of TRPC4 modulators.

Isolation and Structure Elucidation of Additional Alkaloids from the Tropical Rainforest Tree Galbulimima baccata

The structures of five new natural products (GB 27-GB 31, 1-5), isolated as minor components from the bark of Galbulimima baccata, have been determined by 2D NMR spectroscopy in combination with DFT calculations. Among the alkaloids, GB 31 (5) belongs to Class I, GB 27 (1) and 28 (2) belong to Class II, and GB 30 (4) belongs to Class III GB alkaloids. GB 31 is the first non-nitrogen-containing GB "alkaloid", being a biosynthetic oxidation product of himbacine, himandravine, or himbeline. GB 29 (3) has an entirely new natural product scaffold but belongs to Class IV (miscellaneous alkaloids). The isolation of a new Galbulimima scaffold has revealed a new pathway in the biosynthesis of the GB alkaloids. The new molecules isolated have shed further light on the biogenetic relationship among these structurally unique and complex groups of alkaloids. We present, for the first time, a unified biogenesis for the GB alkaloids that were first isolated in the 1950s and now number over 40 examples. This work also brings full circle the story of Galbulimima alkaloids. A life-long project of Wal Taylor involving one of his first students (Lew Mander) and one of his last students (Peter Karuso), a story stretching over six decades, has come to a final conclusion.

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

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

Suppression of mICAT in Mouse Small Intestinal Myocytes by General Anaesthetic Ketamine and its Recovery by TRPC4 Agonist (-)-englerin A

A better understanding of the negative impact of general anesthetics on gastrointestinal motility requires thorough knowledge of their molecular targets. In this respect the muscarinic cationic current (mI_CAT carried mainly via TRPC4 channels) that initiates cholinergic excitation-contraction coupling in the gut is of special interest. Here we aimed to characterize the effects of one of the most commonly used “dissociative anesthetics”, ketamine, on mI_CAT. Patch-clamp and tensiometry techniques were used to investigate the mechanisms of the inhibitory effects of ketamine on mI_CAT in single mouse ileal myocytes, as well as on intestinal motility. Ketamine (100 µM) strongly inhibited both carbachol- and GTPγS-induced mI_CAT. The inhibition was slow (time constant of about 1 min) and practically irreversible. It was associated with altered voltage dependence and kinetics of mI_CAT. In functional tests, ketamine suppressed both spontaneous and carbachol-induced contractions of small intestine. Importantly, inhibited by ketamine mI_CAT could be restored by direct TRPC4 agonist (-)-englerin A. We identified mI_CAT as a novel target for ketamine. Signal transduction leading to TRPC4 channel opening is disrupted by ketamine mainly downstream of muscarinic receptor activation, but does not involve TRPC4 per se. Direct TRPC4 agonists may be used for the correction of gastrointestinal disorders provoked by general anesthesia.

TRPC4 as a coincident detector of Gi/o and Gq/11 signaling: mechanisms and pathophysiological implications

TRPC channels are Ca²⁺-permeable nonselective cation channels activated downstream from phospholipase C (PLC). Although most TRPC channels can be activated by stimulating G_q/11-coupled receptors, TRPC4 requires simultaneous stimulation of G_i/o-coupled receptors, making it a perfect detector of coincident G_i/o and G_q/11 signaling. Evidence shows that activated Gα_i/o proteins work together with PLCδ1 to induce robust TRPC4 activation and the process is accelerated by stimulation of other PLC isozymes, such as PLCβ through G_q/11 proteins. Mechanistically, G_q/11-PLCβ activation produces triggering proton and calcium signals to initiate self-propagating PLCδ1 activity, crucial for G_i/o-mediated TRPC4 function. Thus, TRPC4-containing channels are activated under conditions not only when coincident G_i/o and G_q/11 stimulation occurs, but also when G_i/o stimulation coincides with proton and Ca²⁺ signals. The resulting cytosolic Ca²⁺ rise and membrane depolarization switch the inhibitory G_i/o response to excitation. The conditions and implications of G_i/o-mediated TRPC4 activation in physiology and pathophysiology warrant further investigation.

Intracellular acidification facilitates receptor-operated TRPC4 activation through PLCδ1 in a Ca2+ -dependent manner

KEY POINTS

Receptor-operated activation of TRPC4 cation channels requires G_i/o proteins and phospholipase-Cδ1 (PLCδ1) activation by intracellular Ca²⁺ . Concurrent stimulation of the G_q/11 pathway accelerates G_i/o activation of TRPC4, which is not mimicked by increasing cytosolic Ca²⁺ . The kinetic effect of G_q/11 was diminished by alkaline intracellular pH (pH_i ) and increased pH_i buffer capacity. Acidic pH_i (6.75-6.25) together with the cytosolic Ca²⁺ rise accelerated G_i/o -mediated TRPC4 activation. Protons exert their facilitation effect through Ca²⁺ -dependent activation of PLCδ1. The data suggest that the G_q/11 -PLCβ pathway facilitates G_i/o activation of TRPC4 through hydrolysing phosphatidylinositol 4,5-bisphosphate (PIP₂ ) to produce the initial proton signal that triggers a self-propagating PLCδ1 activity supported by regenerative H⁺ and Ca²⁺ . The findings provide novel mechanistic insights into receptor-operated TRPC4 activation by coincident G_q/11 and G_i/o pathways and shed light on how aberrant activation of TRPC4 may occur under pathological conditions to cause cell damage.

ABSTRACT

Transient Receptor Potential Canonical 4 (TRPC4) forms non-selective cation channels activated downstream from receptors that signal through G proteins. Our recent work suggests that TRPC4 channels are particularly coupled to pertussis toxin-sensitive G_i/o proteins, with a co-dependence on phospholipase-Cδ1 (PLCδ1). The G_i/o -mediated TRPC4 activation is dually dependent on and bimodally regulated by phosphatidylinositol 4,5-bisphosphate (PIP₂ ), the substrate hydrolysed by PLC, and intracellular Ca²⁺ . As a byproduct of PLC-mediated PIP₂ hydrolysis, protons have been shown to play an important role in the activation of Drosophila TRP channels. However, how intracellular pH affects mammalian TRPC channels remains obscure. Here, using patch-clamp recordings of HEK293 cells heterologously co-expressing mouse TRPC4β and the G_i/o -coupled μ opioid receptor, we investigated the role of intracellular protons on G_i/o -mediated TRPC4 activation. We found that acidic cytosolic pH greatly accelerated the rate of TRPC4 activation without altering the maximal current density and this effect was dependent on intracellular Ca²⁺ elevation. However, protons did not accelerate channel activation by directly acting upon TRPC4. We additionally demonstrated that protons exert their effect through sensitization of PLCδ1 to Ca²⁺ , which in turn promotes PLCδ1 activity and further potentiates TRPC4 via a positive feedback mechanism. The mechanism elucidated here helps explain how G_i/o and G_q/11 co-stimulation induces a faster activation of TRPC4 than G_i/o activation alone and highlights again the critical role of PLCδ1 in TRPC4 gating.

TRPC channels: Structure, function, regulation and recent advances in small molecular probes

Transient receptor potential canonical (TRPC) channels constitute a group of receptor-operated calcium-permeable nonselective cation channels of the TRP superfamily. The seven mammalian TRPC members, which can be further divided into four subgroups (TRPC1, TRPC2, TRPC4/5, and TRPC3/6/7) based on their amino acid sequences and functional similarities, contribute to a broad spectrum of cellular functions and physiological roles. Studies have revealed complexity of their regulation involving several components of the phospholipase C pathway, G_i and G_o proteins, and internal Ca²⁺ stores. Recent advances in cryogenic electron microscopy have provided several high-resolution structures of TRPC channels. Growing evidence demonstrates the involvement of TRPC channels in diseases, particularly the link between genetic mutations of TRPC6 and familial focal segmental glomerulosclerosis. Because TRPCs were discovered by the molecular identity first, their pharmacology had lagged behind. This is rapidly changing in recent years owning to great efforts from both academia and industry. A number of potent tool compounds from both synthetic and natural products that selective target different subtypes of TRPC channels have been discovered, including some preclinical drug candidates. This review will cover recent advancements in the understanding of TRPC channel regulation, structure, and discovery of novel TRPC small molecular probes over the past few years, with the goal of facilitating drug discovery for the study of TRPCs and therapeutic development.

Further characterization of the synergistic activation mechanism of cationic channels by M2 and M3 muscarinic receptors in mouse intestinal smooth muscle cells

In mouse ileal myocytes, muscarinic receptor-mediated cationic current (mI_cat) occurs mainly through synergism of M₂ and M₃ subtypes involving G_i/o-type GTP-binding proteins and phospholipase C (PLC). We have further studied the M₂/M₃ synergistic pathway. Carbachol-induced mI_cat was markedly depressed by YM-254890, a G_q/11 protein inhibitor. However, the mI_cat was unaffected by heparin, calphostin C, or chelerythrine, suggesting that mI_cat activation does not involve signaling molecules downstream of phosphatidylinositol 4,5-bisphosphate (PIP₂) breakdown. M₂-knockout (KO) mice displayed a reduced mI_cat (~10% of wild-type mI_cat) because of the lack of M₂-G_i/o signaling. The impaired mI_cat was insensitive to neuropeptide Y possessing a G_i/o-stimulating activity. M₃-KO mice also displayed a reduced mI_cat (~6% of wild-type mI_cat) because of the lack of M₃-G_q/11 signaling, and the mI_cat was insensitive to prostaglandin F_2α possessing a G_q/11-stimulating activity. These results suggest the importance of G_q/11/PLC-hydrolyzed PIP₂ breakdown itself in mI_cat activation and also support the idea that the M₂/M₃ synergistic pathway represents a signaling complex consisting of M₂-G_i/o and M₃-G_q/11-PLC systems in which both G proteins are special for this pathway but not general in receptor coupling.

TRPC1 as a negative regulator for TRPC4 and TRPC5 channels

Transient receptor potential canonical (TRPC) channels are calcium permeable, non-selective cation channels with wide tissue-specific distribution. Among 7 TRPC channels, TRPC 1/4/5 and TRPC3/6/7 are subdivided based on amino acid sequence homology. TRPC4 and TRPC5 channels exhibit cationic current with homotetrameric form, but they also form heterotetrameric channel such as TRPC1/4 or TRPC1/5 once TRPC1 is incorporated. The expression of TRPC1 is ubiquitous whereas the expressions of TRPC4 and TRPC5 are rather focused in nervous system. With the help of conditional knock-out of TPRC1, 4 and/or 5 genes, TRPC channels made of these constituents are reported to be involved in various pathophysiological functions such as seizure, anxiety-like behaviour, fear, Huntington's disease, Parkinson's disease and many others. In heterologous expression system, many issues such as activation mechanism, stoichiometry and relative cation permeabilites of homomeric or heteromeric channels have been addressed. In this review, we discussed the role of TRPC1 channel per se in plasma membrane, role of TRPC1 in heterotetrameric conformation (TRPC1/4 or TRPC1/5) and relationship between TRPC1/4/5 channels, calcium influx and voltage-gated calcium channels.

Possible antagonistic effects of the TRPC4 channel blocker ML204 on M2 and M3 muscarinic receptors in mouse ileal and detrusor smooth muscles and atrial myocardium

ML204, a potent transient receptor potential canonical 4 (TRPC4) channel blocker, is often used to elucidate the involvement of TRPC4 channels in receptor-operated signaling processes in visceral smooth muscles. In the present study, we investigated the possible antagonistic actions of ML204 on M₂ and M₃ muscarinic receptors, which mediate contractions in mouse ileal and detrusor smooth muscles. In ileal and detrusor smooth muscle preparations, ML204 (3 or 10 µM) significantly inhibited electrical field stimulation (EFS)-evoked cholinergic contractions. However, it did not significantly inhibit high K⁺-induced and EFS-evoked non-cholinergic contractions in the ileal preparations. When the muscarinic agonist, carbachol was cumulatively applied, ML204 (1, 3 and 10 µM) caused a rightward parallel shift of the concentration-response curves of carbachol. Additionally, ML204 (1, 3 and 10 µM) inhibited carbachol-induced negative chronotropic response in atrial preparations, which is mediated by M₂ muscarinic receptors. Furthermore, ML204 significantly inhibited the contractions evoked by carbachol-induced intracellular Ca²⁺ release, which is mediated by M₃ muscarinic receptors. These results suggested that ML204 might exhibit antagonistic actions on M₂ and M₃ muscarinic receptors; in addition, the inhibitory effects of ML204 against EFS-induced cholinergic contractions might be attributed to this receptor antagonism rather than inhibition of TRPC4 channel activity. Therefore, these effects should be considered when ML204 is used as a TRPC4 channel blocker.

The cells and conductance mediating cholinergic neurotransmission in the murine proximal stomach

KEY POINTS

Enteric neurotransmission is essential for gastrointestinal (GI) motility, although the cells and conductances responsible for post-junctional responses are controversial. The calcium-activated chloride conductance (CaCC), anoctamin-1 (Ano1), was expressed by intramuscular interstitial cells of Cajal (ICC-IM) in proximal stomach and not resolved in smooth muscle cells (SMCs). Cholinergic nerve fibres were closely apposed to ICC-IM. Conductances activated by cholinergic stimulation in isolated ICC-IM and SMCs were determined. A CaCC was activated by carbachol in ICC-IM and a non-selective cation conductance in SMCs. Responses to cholinergic nerve stimulation were studied. Excitatory junction potentials (EJPs) and mechanical responses were evoked in wild-type mice but absent or greatly reduced with knockout/down of Ano1. Drugs that block Ano1 inhibited the conductance activated by carbachol in ICC-IM and EJPs and mechanical responses in tissues. The data of the present study suggest that electrical and mechanical responses to cholinergic nerve stimulation are mediated by Ano1 expressed in ICC-IM and not SMCs.

ABSTRACT

Enteric motor neurotransmission is essential for normal gastrointestinal (GI) motility. Controversy exists regarding the cells and ionic conductance(s) that mediate post-junctional neuroeffector responses to motor neurotransmitters. Isolated intramuscular ICC (ICC-IM) and smooth muscle cells (SMCs) from murine fundus muscles were used to determine the conductances activated by carbachol (CCh) in each cell type. The calcium-activated chloride conductance (CaCC), anoctamin-1 (Ano1) is expressed by ICC-IM but not resolved in SMCs, and CCh activated a Cl- conductance in ICC-IM and a non-selective cation conductance in SMCs. We also studied responses to nerve stimulation using electrical-field stimulation (EFS) of intact fundus muscles from wild-type and Ano1 knockout mice. EFS activated excitatory junction potentials (EJPs) in wild-type mice, although EJPs were absent in mice with congenital deactivation of Ano1 and greatly reduced in animals in which the CaCC-Ano1 was knocked down using Cre/loxP technology. Contractions to cholinergic nerve stimulation were also greatly reduced in Ano1 knockouts. SMCs cells also have receptors and ion channels activated by muscarinic agonists. Blocking acetylcholine esterase with neostigmine revealed a slow depolarization that developed after EJPs in wild-type mice. This depolarization was still apparent in mice with genetic deactivation of Ano1. Pharmacological blockers of Ano1 also inhibited EJPs and contractile responses to muscarinic stimulation in fundus muscles. The data of the present study are consistent with the hypothesis that ACh released from motor nerves binds muscarinic receptors on ICC-IM with preference and activates Ano1. If metabolism of acetylcholine is inhibited, ACh overflows and binds to extrajunctional receptors on SMCs, eliciting a slower depolarization response.

C60 fullerenes disrupt cellular signalling leading to TRPC4 and TRPC6 channels opening by the activation of muscarinic receptors and G-proteins in small intestinal smooth muscles

The effect of water-soluble pristine C₆₀ fullerene nanoparticles (C₆₀NPs) on receptor-operated cation channels formed by TRPC4/C6 proteins in ileal smooth muscle cells was investigated for the first time. Activation of these channels subsequent to acetylcholine binding to the expressed in these cells M₂ and M₃ muscarinic receptors represents the key event in the parasympathetic control of gastrointestinal smooth muscle motility and cholinergic excitation-contraction coupling. Experiments were performed on single collagenase-dispersed mouse ileal myocytes using patch-clamp techniques with symmetrical 125mM Cs⁺ solutions and [Ca²⁺]_i 'clamped' at 100nM in order to isolate the muscarinic cation current (mI_CAT). The current was induced by intracellular infusion of 200μM GTPγS, which activates G-proteins directly, i.e. bypassing the muscarinic receptors. C₆₀NPs applied at 10^-6M at peak response to activation of G-proteins caused mI_CAT inhibition by 47.0±3.5% (n=9). The inhibition developed rather slowly, with the time constant of 119±16s, was voltage-independent and irreversible. Thus, C₆₀NPs are unlikely to cause any direct block of TRPC4/C6 channels; rather, they may accumulate in the membrane and disrupt G-protein signalling leading to mI_CAT generation. C₆₀NPs may represent a novel class of biocompatible molecules for the treatment of disorders associated with enhanced gastrointestinal motility.

Inhalation anaesthetic isoflurane inhibits the muscarinic cation current and carbachol-induced gastrointestinal smooth muscle contractions

Gastrointestinal tract motility may be demoted significantly after surgery operations at least in part due to anaesthetic agents, but there is no comprehensive explanation of the molecular mechanism(s) of such adverse effects. Anesthetics are known to interact with various receptors and ion channels including several subtypes of transient receptor potential (TRP) channels. Two members of the canonical subfamily of TRP channels (TRPC), TRPC4 and TRPC6 are Ca²⁺-permeable cation channels involved in visceral smooth muscle contractility induced by acetylcholine, the primary excitatory neurotransmitter in the gut. In the present study, we aimed to study the effect of anesthetics on muscarinic receptor-mediated excitation and contraction of intestinal smooth muscle. Here we show that muscarinic cation current (mI_CAT) mediated by TRPC4 and TRPC6 channels in mouse ileal myocytes was strongly inhibited by isoflurane (0.5mM), one of the most commonly used inhalation anesthetics. Carbachol-activated mI_CAT was reduced by 63 ± 11% (n = 5), while GTPγS-induced (to bypass muscarinic receptors) current was inhibited by 44 ± 9% (n = 6). Furthermore, carbachol-induced ileum and colon contractions were inhibited by isoflurane by about 30%. We discuss the main sites of isoflurane action, which appear to be G-proteins and muscarinic receptors, rather than TRPC4/6 channels. These results contribute to our better understanding of the signalling pathways affected by inhalation anesthetics, which may cause ileus, and thus may be important for the development of novel treatment strategies during postoperative recovery.

Overview of Antagonists Used for Determining the Mechanisms of Action Employed by Potential Vasodilators with Their Suggested Signaling Pathways

This paper is a review on the types of antagonists and the signaling mechanism pathways that have been used to determine the mechanisms of action employed for vasodilation by test compounds. Thus, we exhaustively reviewed and analyzed reports related to this topic published in PubMed between the years of 2010 till 2015. The aim of this paperis to suggest the most appropriate type of antagonists that correspond to receptors that would be involved during the mechanistic studies, as well as the latest signaling pathways trends that are being studied in order to determine the route(s) that atest compound employs for inducing vasodilation. The methods to perform the mechanism studies were included. Fundamentally, the affinity, specificity and selectivity of the antagonists to their receptors or enzymes were clearly elaborated as well as the solubility and reversibility. All the signaling pathways on the mechanisms of action involved in the vascular tone regulation have been well described in previous review articles. However, the most appropriate antagonists that should be utilized have never been suggested and elaborated before, hence the reason for this review.

Regulator of G-protein signalling and GoLoco proteins suppress TRPC4 channel function via acting at Gαi/o

Transient receptor potential canonical 4 (TRPC4) forms non-selective cation channels implicated in the regulation of diverse physiological functions. Previously, TRPC4 was shown to be activated by the Gi/o subgroup of heterotrimeric G-proteins involving Gαi/o, rather than Gβγ, subunits. Because the lifetime and availability of Gα-GTP are regulated by regulators of G-protein signalling (RGS) and Gαi/o-Loco (GoLoco) domain-containing proteins via their GTPase-activating protein (GAP) and guanine-nucleotide-dissociation inhibitor (GDI) functions respectively, we tested how RGS and GoLoco domain proteins affect TRPC4 currents activated via Gi/o-coupled receptors. Using whole-cell patch-clamp recordings, we show that both RGS and GoLoco proteins [RGS4, RGS6, RGS12, RGS14, LGN or activator of G-protein signalling 3 (AGS3)] suppress receptor-mediated TRPC4 activation without causing detectable basal current or altering surface expression of the channel protein. The inhibitory effects are dependent on the GAP and GoLoco domains and facilitated by enhancing membrane targeting of the GoLoco protein AGS3. In addition, RGS, but not GoLoco, proteins accelerate desensitization of receptor-activation evoked TRPC4 currents. The inhibitory effects of RGS and GoLoco domains are additive and are most prominent with RGS12 and RGS14, which contain both RGS and GoLoco domains. Our data support the notion that the Gα, but not Gβγ, arm of the Gi/o signalling is involved in TRPC4 activation and unveil new roles for RGS and GoLoco domain proteins in fine-tuning TRPC4 activities. The versatile and diverse functions of RGS and GoLoco proteins in regulating G-protein signalling may underlie the complexity of receptor-operated TRPC4 activation in various cell types under different conditions.

Identification of muscarinic receptor subtypes involved in catecholamine secretion in adrenal medullary chromaffin cells by genetic deletion

BACKGROUND AND PURPOSE

Activation of muscarinic receptors results in catecholamine secretion in adrenal chromaffin cells in many mammals, and muscarinic receptors partly mediate synaptic transmission from the splanchnic nerve, at least in guinea pigs. To elucidate the physiological functions of muscarinic receptors in chromaffin cells, it is necessary to identify the muscarinic receptor subtypes involved in excitation.

EXPERIMENTAL APPROACH

To identify muscarinic receptors, pharmacological tools and strains of mice where one or several muscarinic receptor subtypes were genetically deleted were used. Cellular responses to muscarinic stimulation in isolated chromaffin cells were studied with the patch clamp technique and amperometry.

KEY RESULTS

Muscarinic M₁, M₄ and M₅ receptors were immunologically detected in mouse chromaffin cells, and these receptors disappeared after the appropriate gene deletion. Mouse cells secreted catecholamines in response to muscarinic agonists, angiotensin II and a decrease in external pH. Genetic deletion of M₁, but not M₃, M₄ or M₅, receptors in mice abolished secretion in response to muscarine, but not to other stimuli. The muscarine-induced secretion was suppressed by MT7, a snake peptide toxin specific for M₁ receptors. Similarly, muscarine failed to induce an inward current in the presence of MT7 in mouse and rat chromaffin cells. The binding affinity of VU0255035 for the inhibition of muscarine-induced currents agreed with that for the M₁ receptor.

CONCLUSIONS AND IMPLICATIONS

Based upon the effects of genetic deletion of muscarinic receptors and MT7, it is concluded that the M₁ receptor alone is responsible for muscarine-induced catecholamine secretion.

Canonical transient receptor potential 4 and its small molecule modulators

Canonical transient receptor potential 4 (TRPC4) forms non-selective cation channels that contribute to phospholipase C-dependent Ca(2+) entry into cells following stimulation of G protein coupled receptors and receptor tyrosine kinases. Moreover, the channels are regulated by pertussis toxin-sensitive Gi/o proteins, lipids, and various other signaling mechanisms. TRPC4-containing channels participate in the regulation of a variety of physiological functions, including excitability of both gastrointestinal smooth muscles and brain neurons. This review is to present recent advances in the understanding of physiology and development of small molecular modulators of TRPC4 channels.

Functional and pharmacological characterization of volume-regulated anion channels in human normal and cystic fibrosis bronchial and nasal epithelial cells

Volume-regulated anion channels (VRACs) are widely present in various cell types and have important functions ranging from regulatory volume decrease to control of cell proliferation and apoptosis. Here we aimed to compare the biophysical features and pharmacological profiles of VRAC currents in healthy and cystic fibrosis (CF) respiratory epithelial cells in order to characterize these currents both functionally and pharmacologically. Whole-cell electrophysiology was used to characterize the VRAC current in normal (16HBE14o-; HBE) and CF cell lines (CFBE14o-; CFBE), as well as in native human nasal epithelial cells. Application of hypotonic solution produced current responses of similar sizes in both HBE and CFBE cells. Biophysical properties of VRACs, such as instantaneous activation and deactivation upon voltage step, some inactivation at potentials positive to 40 mV and outwardly-rectifying I-V curves, were indistinguishable in both cell types. Extensive pharmacological analysis of the currents revealed a similar pharmacological profile in response to three blockers--NPPB, DCPIB and DIDS. Native primary human nasal epithelial cells from both healthy and CF volunteers also showed typical VRAC responses of comparable sizes. VRACs in these cells were more sensitive to external solution hypotonicity compared to HBE and CFBE cells. In all cell types studied robust VRAC currents could be induced at constant cell volume by G-protein activation with GTPγS infusion. This study provides the first extensive comparative functional and pharmacological analysis of VRAC currents in normal and CF airway epithelial cells and shows that VRACs are unimpaired molecularly or functionally in CF.

TRPC4- and TRPC4-containing channels

TRPC4 proteins comprise six transmembrane domains, a putative pore-forming region, and an intracellularly located amino- and carboxy-terminus. Among eleven splice variants identified so far, TRPC4α and TRPC4β are the most abundantly expressed and functionally characterized. TRPC4 is expressed in various organs and cell types including the soma and dendrites of numerous types of neurons; the cardiovascular system including endothelial, smooth muscle, and cardiac cells; myometrial and skeletal muscle cells; kidney; and immune cells such as mast cells. Both recombinant and native TRPC4-containing channels differ tremendously in their permeability and other biophysical properties, pharmacological modulation, and mode of activation depending on the cellular environment. They vary from inwardly rectifying store-operated channels with a high Ca(2+) selectivity to non-store-operated channels predominantly carrying Na(+) and activated by Gαq- and/or Gαi-coupled receptors with a complex U-shaped current-voltage relationship. Thus, individual TRPC4-containing channels contribute to agonist-induced Ca(2+) entry directly or indirectly via depolarization and activation of voltage-gated Ca(2+) channels. The differences in channel properties may arise from variations in the composition of the channel complexes, in the specific regulatory pathways in the corresponding cell system, and/or in the expression pattern of interaction partners which comprise other TRPC proteins to form heteromultimeric channels. Additional interaction partners of TRPC4 that can mediate the activity of TRPC4-containing channels include (1) scaffolding proteins (e.g., NHERF) that may mediate interactions with signaling molecules in or in close vicinity to the plasma membrane such as Gα proteins or phospholipase C and with the cytoskeleton, (2) proteins in specific membrane microdomains (e.g., caveolin-1), or (3) proteins on cellular organelles (e.g., Stim1). The diversity of TRPC4-containing channels hampers the development of specific agonists or antagonists, but recently, ML204 was identified as a blocker of both recombinant and endogenous TRPC4-containing channels with an IC50 in the lower micromolar range that lacks activity on most voltage-gated channels and other TRPs except TRPC5 and TRPC3. Lanthanides are specific activators of heterologously expressed TRPC4- and TRPC5-containing channels but can block individual native TRPC4-containing channels. The biological relevance of TRPC4-containing channels was demonstrated by knockdown of TRPC4 expression in numerous native systems including gene expression, cell differentiation and proliferation, formation of myotubes, and axonal regeneration. Studies of TRPC4 single and TRPC compound knockout mice uncovered their role for the regulation of vascular tone, endothelial permeability, gastrointestinal contractility and motility, neurotransmitter release, and social exploratory behavior as well as for excitotoxicity and epileptogenesis. Recently, a single-nucleotide polymorphism (SNP) in the Trpc4 gene was associated with a reduced risk for experience of myocardial infarction.

Purinergic and muscarinic modulation of ATP release from the urothelium and its paracrine actions

The urothelium is a newly recognized sensory structure that detects bladder fullness. Pivotal to this sensory role is the release of ATP from the urothelium. However, the routes for urothelial ATP release, its modulation by receptor-mediated pathways, and the autocrine/paracrine role of ATP are poorly understood, especially in native tissue. We examined the action of key neurotransmitters: purinergic and muscarinic agonists on ATP release and its paracrine effect. Guinea pig and human urothelial mucosa were mounted in a perfusion trough; superfusate ATP was measured using a luciferin-luciferase assay, and tissue contractions were recorded with a tension transducer. Intracellular Ca²⁺ was measured in isolated urothelial cells with fura-2. The P2Y agonist UTP but not the P2X agonist α,β-methylene-ATP generated ATP release. The muscarinic agonist carbachol and the M₂-preferential agonist oxotremorine also generated ATP release, which was antagonized by the M₂-specific agent methoctramine. Agonist-evoked ATP release was accompanied by mucosal contractions. Urothelial ATP release was differentially mediated by intracellular Ca²⁺ release, cAMP, exocytosis, or connexins. Urothelium-attached smooth muscle exhibited spontaneous contractions that were augmented by subthreshold concentrations of carbachol, which had little direct effect on smooth muscle. This activity was attenuated by desensitizing P2X receptors on smooth muscle. Urothelial ATP release was increased in aging bladders. Purinergic and muscarinic agents produced similar effects in human urothelial tissue. This is the first demonstration of specific modulation of urothelial ATP release in native tissue by purinergic and muscarinic neurotransmitters via distinct mechanisms. Released ATP produces paracrine effects on underlying tissues. This process is altered during aging and has relevance to human bladder pathologies.

The roles of G proteins in the activation of TRPC4 and TRPC5 transient receptor potential channels

TRPC4 and TRPC5 channels are important regulators of electrical excitability in both gastrointestinal myocytes and neurons. Much is known regarding the assembly and function of these channels including TRPC1 as a homotetramer or a heteromultimer and the roles that their interacting proteins play in controlling these events. Further, they are one of the best-studied targets of G protein-coupled receptors and growth factors in general and Gαq protein coupled receptor or epidermal growth factor in particular. However, our understanding of the roles of Gαi/o proteins on TRPC4/5 channels is still rudimentary. We discuss potential roles for Gαi/o proteins in channel activation in addition to their known role in cellular signaling.

Muscarinic agonists and antagonists: effects on gastrointestinal function

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

Selective Gαi subunits as novel direct activators of transient receptor potential canonical (TRPC)4 and TRPC5 channels

The ubiquitous transient receptor potential canonical (TRPC) channels function as non-selective, Ca(2+)-permeable channels and mediate numerous cellular functions. It is commonly assumed that TRPC channels are activated by stimulation of Gα(q)-PLC-coupled receptors. However, whether the Gα(q)-PLC pathway is the main regulator of TRPC4/5 channels and how other Gα proteins may regulate these channels are poorly understood. We previously reported that TRPC4/TRPC5 can be activated by Gα(i). In the current work, we found that Gα(i) subunits, rather than Gα(q), are the primary and direct activators of TRPC4 and TRPC5. We report a novel molecular mechanism in which TRPC4 is activated by several Gα(i) subunits, most prominently by Gα(i2), and TRPC5 is activated primarily by Gα(i3). Activation of Gα(i) by the muscarinic M2 receptors or expression of the constitutively active Gα(i) mutants equally and fully activates the channels. Moreover, both TRPC4 and TRPC5 are activated by direct interaction of their conserved C-terminal SESTD (SEC14-like and spectrin-type domains) with the Gα(i) subunits. Two amino acids (lysine 715 and arginine 716) of the TRPC4 C terminus were identified by structural modeling as mediating the interaction with Gα(i2). These findings indicate an essential role of Gα(i) proteins as novel activators for TRPC4/5 and reveal the molecular mechanism by which G-proteins activate the channels.

Identification of ML204, a novel potent antagonist that selectively modulates native TRPC4/C5 ion channels

Transient receptor potential canonical (TRPC) channels are Ca(2+)-permeable nonselective cation channels implicated in diverse physiological functions, including smooth muscle contractility and synaptic transmission. However, lack of potent selective pharmacological inhibitors for TRPC channels has limited delineation of the roles of these channels in physiological systems. Here we report the identification and characterization of ML204 as a novel, potent, and selective TRPC4 channel inhibitor. A high throughput fluorescent screen of 305,000 compounds of the Molecular Libraries Small Molecule Repository was performed for inhibitors that blocked intracellular Ca(2+) rise in response to stimulation of mouse TRPC4β by μ-opioid receptors. ML204 inhibited TRPC4β-mediated intracellular Ca(2+) rise with an IC(50) value of 0.96 μm and exhibited 19-fold selectivity against muscarinic receptor-coupled TRPC6 channel activation. In whole-cell patch clamp recordings, ML204 blocked TRPC4β currents activated through either μ-opioid receptor stimulation or intracellular dialysis of guanosine 5'-3-O-(thio)triphosphate (GTPγS), suggesting a direct interaction of ML204 with TRPC4 channels rather than any interference with the signal transduction pathways. Selectivity studies showed no appreciable block by 10-20 μm ML204 of TRPV1, TRPV3, TRPA1, and TRPM8, as well as KCNQ2 and native voltage-gated sodium, potassium, and calcium channels in mouse dorsal root ganglion neurons. In isolated guinea pig ileal myocytes, ML204 blocked muscarinic cation currents activated by bath application of carbachol or intracellular infusion of GTPγS, demonstrating its effectiveness on native TRPC4 currents. Therefore, ML204 represents an excellent novel tool for investigation of TRPC4 channel function and may facilitate the development of therapeutics targeted to TRPC4.

Muscarinic activation of Ca2+-activated Cl- current in interstitial cells of Cajal

Interstitial cells of Cajal (ICC) provide pacemaker activity and functional bridges between enteric motor nerve terminals and gastrointestinal smooth muscle cells. The ionic conductance(s) in ICC that are activated by excitatory neural inputs are unknown. Transgenic mice (Kit(copGFP/+)) with constitutive expression of a bright green fluorescent protein were used to investigate cellular responses of ICC to cholinergic stimulation. ICC displayed spontaneous transient inward currents (STICs) under voltage clamp that corresponded to spontaneous transient depolarizations (STDs) under current clamp. STICs reversed at 0 mV when E(Cl) = 0 mV and at -40 mV when E(Cl) was -40 mV, suggesting the STICs were due to a chloride conductance. Carbachol (CCh, 100 nm and 1 μm) induced a sustained inward current (depolarization in current clamp) and increased the amplitude and frequency of STICs and STDs. CCh responses were blocked by atropine (10 μm) or 4-DAMP (100 nm), an M(3) receptor antagonist. STDs were blocked by niflumic acid and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (both 100 μm), and CCh had no effect in the presence of these drugs. The responses of intact circular muscles to CCh and stimulation of intrinsic excitatory nerves by electrical field stimulation (EFS) were also compared. CCh (1 μm) caused atropine-sensitive depolarization and increased the maximum depolarization of slow waves. Similar atropine-sensitive responses were elicited by stimulation of intrinsic excitatory neurons. Niflumic acid (100 μm) blocked responses to EFS but had minor effect on responses to exogenous CCh. These data suggest that different ionic conductances are responsible for electrical responses elicited by bath-applied CCh and cholinergic nerve stimulation.

Assessment of gastrointestinal motility using three different assays in vitro

The protocols detailed in this unit are designed to assess the motor activity of different gastric and intestinal muscle preparations in vitro and the effects of drugs that modulate gastrointestinal motility. The preparations described are characterized by different contractile behaviors, consisting of spontaneous (duodenum), neurogenic (ileum), and drug-stimulated (fundus, ileum) motility; these reproduce motility patterns occurring in the gut wall in vivo. These protocols document the variety of factors that can influence the responses of isolated tissues and describe how such tissues can be used for testing substances that affect gut movements. These preparations allow evaluation of direct interactions with the processes that control contractile machinery, as well as indirect effects resulting from the modification of neurotransmitter release from myenteric neurons. These models can be exploited to assay novel compounds undergoing preclinical development or to evaluate the functional toxicity exerted by environmental or alimentary pollutants, like xenobiotics and naturally occurring toxins, as well as the mechanisms underlying these effects.

Characterization of bencycloquidium bromide, a novel muscarinic M(3) receptor antagonist in guinea pig airways

In this study we have investigated the antagonist affinity, efficacy and duration of action of bencycloquidium bromide (BCQB), a selective muscarinic M(3) receptor antagonist, as a possible clinical bronchodilator for the treatment of chronic obstructive pulmonary disease (COPD) and asthma. In competition studies, BCQB showed high affinity toward the M(3) receptor in Chinese hamster ovary (CHO) cells (M(3) pKi=8.21, M(2) pKi=7.21, and M(1) pKi=7.86); pA(2)=8.85, 8.71 and 8.57 in methacholine-induced contraction of trachea, ileum and urinary bladder, 8.19 in methacholine-induced bradycardia of right atrium in vitro, respectively. In function studies, duration of inhibition of carbachol-induced tonic contraction, BCQB and ipratropium had a very similar onset and offset of action, but onset faster and offset slower than that of tiotropium. After treatment with intratracheally instilled or the inhalation route, BCQB protects against methacholine or antigen-induced bronchoconstriction in a dose-dependent manner in the normal and sensitized guinea pigs in vivo. BCQB and ipratropium-induced inhibitory activity was short lasting, as it declined quickly when compared to tiotropium. These results suggest that BCQB bind muscarinic M(3) receptors with high affinity. On this basis we speculate that a putative BCQB-based therapy for COPD might require more than once-a-day administration to be as effective as the currently employed once-daily therapy with tiotropium. Nevertheless, Inhalable M(3)-selective compounds may spare M(2)-cardiac receptors and reduce the risks of cardiovascular events associated with the long-term treatment of these agents.

Expression and physiological roles of TRP channels in smooth muscle cells

Smooth muscles are widely distributed in mammal body through various systems such as circulatory, respiratory, gastro-intestinal and urogenital systems. The smooth muscle cell (SMC) is not only a contractile cell but is able to perform other important functions such as migration, proliferation, production of cytokines, chemokines, extracellular matrix proteins, growth factors and cell surface adhesion molecules. Thus, SMC appears today as a fascinating cell with remarkable plasticity that contributes to its roles in physiology and disease. Most of the SMC functions are dependent on a key event: the increase in intracellular calcium concentration ([Ca(2+)](i)). Calcium entry from the extracellular space is a major step in the elevation of [Ca(2+)](i) in SMC and involves a variety of plasmalemmal calcium channels, among them is the superfamily of transient receptor potential (TRP) proteins. TRPC (canonical), TRPM (melastatin), TRPV (vanilloid) and TRPP (polycystin), are widely expressed in both visceral (airways, gastrointestinal tract, uterus) and vascular (systemic and pulmonary circulation) smooth muscles. Mainly, TRPC, TRPV and TRPM are implicated in a variety of physiological and pathophysiological processes such as: SMC contraction, relaxation, growth, migration and proliferation; control of blood pressure, arterial myogenic tone, pulmonary hypertension, intestinal motility, gastric acidity, uterine activity during parturition and labor. Thus it is becoming evident that TRP are major element of SMC calcium homeostasis and, thus, appear as novel drug targets for a better management of diseases originating from SMC dysfunction.

Arecoline excites the contraction of distal colonic smooth muscle strips in rats via the M3 receptor – extracellular Ca2+ influx – Ca2+ store release pathway

Areca is a Chinese herbal medicine that is widely used for constipation. However the mechanisms of its action are not clear. We investigated the effects of arecoline, the most active component of areca, on the motility of rat distal colonic smooth muscle strips. In longitudinal muscle of distal colon (LMDC) and circular muscle of distal colon (CMDC), arecoline increased the contraction in a dose-dependent manner. Tetrodotoxin (TTX) did not inhibit the effects of arecoline. The contractile response to arecoline was completely antagonized by atropine. 4-Diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) strongly depressed the response to arecoline, but gallamine and methoctramine did not. Nifedipine, 2-aminoethoxydiphenyl borate (2-APB), and Ca2+-free Krebs solution with EGTA partly inhibited the effects of arecoline. The sum of Ca2+-free Krebs solution, EGTA, and 2-APB completely inhibited the effects of arecoline. The results show that arecoline stimulates distal colonic contraction in rats via the muscarinic (M3) receptor – extracellular Ca2+ influx – Ca2+ store release pathway. It is likely that the action of areca in relieving constipation is due to its stimulation of muscle contraction.

Deletion of TRPC4 and TRPC6 in mice impairs smooth muscle contraction and intestinal motility in vivo

BACKGROUND & AIMS

Downstream effects of muscarinic receptor stimulation in intestinal smooth muscle include contraction and intestinal transit. We thought to determine whether classic transient receptor potential (TRPC) channels integrate the intracellular signaling cascades evoked by the stimulated receptors and thereby contribute to the control of the membrane potential, Ca-influx, and cell responses.

METHODS

We created trpc4-, trpc6-, and trpc4/trpc6-gene-deficient mice and analyzed them for intestinal smooth muscle function in vitro and in vivo.

RESULTS

In intestinal smooth muscle cells TRPC4 forms a 55 pS cation channel and underlies more than 80% of the muscarinic receptor-induced cation current (mI(CAT)). The residual mI(CAT) depends on the expression of TRPC6, indicating that TRPC6 and TRPC4 determine mI(CAT) channel activity independent of other channel subunits. In TRPC4-deficient ileal myocytes the carbachol-induced membrane depolarizations are diminished greatly and the atropine-sensitive contraction elicited by acetylcholine release from excitatory motor neurons is reduced greatly. Additional deletion of TRPC6 aggravates these effects. Intestinal transit is slowed down in mice lacking TRPC4 and TRPC6.

CONCLUSIONS

In intestinal smooth muscle cells TRPC4 and TRPC6 channels are gated by muscarinic receptors and are responsible for mI(CAT). They couple muscarinic receptors to depolarization of intestinal smooth muscle cells and voltage-activated Ca(2+)-influx and contraction, and thereby accelerate small intestinal motility in vivo.

Isoform-specific inhibition of TRPC4 channel by phosphatidylinositol 4,5-bisphosphate

Full-length transient receptor potential (TRP) cation channel TRPC4alpha and shorter TRPC4beta lacking 84 amino acids in the cytosolic C terminus are expressed in smooth muscle and endothelial cells where they regulate membrane potential and Ca(2+) influx. In common with other "classical" TRPCs, TRPC4 is activated by G(q)/phospholipase C-coupled receptors, but the underlying mechanism remains elusive. Little is also known about any isoform-specific channel regulation. Here we show that TRPC4alpha but not TRPC4beta was strongly inhibited by intracellularly applied phosphatidylinositol 4,5-bisphosphate (PIP(2)). In contrast, several other phosphoinositides (PI), including PI(3,4)P(2), PI(3,5)P(2), and PI(3,4,5)P(3), had no effect or even potentiated TRPC4alpha indicating that PIP(2) inhibits TRPC4alpha in a highly selective manner. We show that PIP(2) binds to the C terminus of TRPC4alpha but not that of TRPC4beta in vitro. Its inhibitory action was dependent on the association of TRPC4alpha with actin cytoskeleton as it was prevented by cytochalasin D treatment or by the deletion of the C-terminal PDZ-binding motif (Thr-Thr-Arg-Leu) that links TRPC4 to F-actin through the sodium-hydrogen exchanger regulatory factor and ezrin. PIP(2) breakdown appears to be a required step in TRPC4alpha channel activation as PIP(2) depletion alone was insufficient for channel opening, which additionally required Ca(2+) and pertussis toxin-sensitive G(i/o) proteins. Thus, TRPC4 channels integrate a variety of G-protein-dependent stimuli, including a PIP(2)/cytoskeleton dependence reminiscent of the TRPC4-like muscarinic agonist-activated cation channels in ileal myocytes.

Sub-plasmalemmal [Ca2+]i upstroke in myocytes of the guinea-pig small intestine evoked by muscarinic stimulation: IP3R-mediated Ca2+ release induced by voltage-gated Ca2+ entry

Membrane depolarization triggers Ca²⁺ release from the sarcoplasmic reticulum (SR) in skeletal muscles via direct interaction between the voltage-gated L-type Ca²⁺ channels (the dihydropyridine receptors; VGCCs) and ryanodine receptors (RyRs), while in cardiac muscles Ca²⁺ entry through VGCCs triggers RyR-mediated Ca²⁺ release via a Ca²⁺-induced Ca²⁺ release (CICR) mechanism. Here we demonstrate that in phasic smooth muscle of the guinea-pig small intestine, excitation evoked by muscarinic receptor activation triggers an abrupt Ca²⁺ release from sub-plasmalemmal (sub-PM) SR elements enriched with inositol 1,4,5-trisphosphate receptors (IP₃Rs) and poor in RyRs. This was followed by a lesser rise, or oscillations in [Ca²⁺]_i. The initial abrupt sub-PM [Ca²⁺]_i upstroke was all but abolished by block of VGCCs (by 5 μM nicardipine), depletion of intracellular Ca²⁺ stores (with 10 μM cyclopiazonic acid) or inhibition of IP₃Rs (by 2 μM xestospongin C or 30 μM 2-APB), but was not affected by block of RyRs (by 50–100 μM tetracaine or 100 μM ryanodine). Inhibition of either IP₃Rs or RyRs attenuated phasic muscarinic contraction by 73%. Thus, in contrast to cardiac muscles, excitation–contraction coupling in this phasic visceral smooth muscle occurs by Ca²⁺ entry through VGCCs which evokes an initial IP₃R-mediated Ca²⁺ release activated via a CICR mechanism.

Three distinct muscarinic signalling pathways for cationic channel activation in mouse gut smooth muscle cells

Using mutant mice genetically lacking certain subtypes of muscarinic receptor, we have studied muscarinic signal pathways mediating cationic channel activation in intestinal smooth muscle cells. In cells from M2 subtype-knockout (M2-KO) or M3-KO mice, carbachol (100 microM) evoked a muscarinic cationic current (mI(Cat)) as small as approximately 10% of mI(Cat) in wild-type (WT) cells. No appreciable current was evoked in M2/M3 double-KO cells. All mutant type cells preserved normal G-protein-cationic channel coupling. The M3-KO and WT mI(Cat) each showed a U-shaped current-voltage (I-V) relationship, whereas the M2-KO mI(Cat) displayed a linear I-V relationship. Channel analysis in outside-out patches recognized 70-pS and 120-pS channels as the major muscarinic cationic channels. Active patches of M2-KO cells exhibited both 70-pS and 120-pS channel activity usually together, either of which consisted of brief openings (the respective mean open times O(tau) = 0.55 and 0.23 ms). In contrast, active M3-KO patches showed only 70-pS channel activity, which had three open states (O(tau) = 0.55, 3.1 and 17.4 ms). In WT patches, besides the M2-KO and M3-KO types, another type of channel activity was also observed that consisted of 70-pS channel openings with four open states (O(tau) = 0.62, 2.7, 16.9 and 121.1 ms), and patch current of this channel activity showed a U-shaped I-V curve similar to the WT mI(Cat). The present results demonstrate that intestinal myocytes are endowed with three distinct muscarinic pathways mediating cationic channel activation and that the M2/M3 pathway targeting 70-pS channels, serves as the major contributor to mI(Cat) generation. The delineation of this pathway is consistent with the formation of a functional unit by the M2-Go protein and the M3-PLC systems predicted to control cationic channels.

Functional roles of muscarinic M2 and M3 receptors in mouse stomach motility: Studies with muscarinic receptor knockout mice

Functional roles of muscarinic acetylcholine receptors in the regulation of mouse stomach motility were examined using mice genetically lacking muscarinic M(2) receptor and/or M(3) receptor and their corresponding wild-type (WT) mice. Single application of carbachol (1 nM-30 microM) produced concentration-dependent contraction in antral and fundus strips from muscarinic M(2) receptor knockout (M(2)R-KO) and M(3) receptor knockout (M(3)R-KO) mice but not in those from M(2) and M(3) receptors double knockout (M(2)/M(3)R-KO) mice. A comparison of the concentration-response curves with those for WT mice showed a significant decrease in the negative logarithm of EC(50) (pEC(50)) value (M(2)R-KO) or amplitude of maximum contraction (M(3)R-KO) in the muscarinic receptor-deficient mice. The tonic phase of carbachol-induced contraction was decreased in gastric strips from M(3)R-KO mice. Antagonistic affinity for 4-diphenylacetoxy-N-methyl-piperidine (4-DAMP) or 11-([2-[(diethylamino)methyl]-1-piperdinyl]acetyl)-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepine-6-one (AF-DX116) indicated that the contractile responses in M(2)R-KO and M(3)R-KO mice were mediated by muscarinic M(3) and M(2) receptors, respectively. Electrical field stimulation (EFS, 0.5-32 Hz) elicited frequency-dependent contraction in physostigmine- and N(omega)-nitro-L-arginine methylester (l-NAME)-treated fundic and antral strips from M(2)R-KO and M(3)R-KO mice, but the cholinergic contractile components decreased significantly compared with those in WT mice. In gastric strips from M(2)/M(3)R-KO mice, cholinergic contractions elicited by EFS were not observed but atropine-resistant contractions were more conspicuous than those in gastric strips from WT mice. Gastric emptying in WT mice and that in M(2)/M(3)R-KO mice were comparable, suggesting that motor function of the stomach in the KO mice did not differ from that in the WT mice. The results indicate that both muscarinic M(2) and M(3) receptors but not other subtypes mediate carbachol- or EFS-induced contraction in the mouse stomach but that the contribution of each receptor to concentration-response relationships is distinguishable. Although there was impairment of nerve-mediated cholinergic responses in the stomach of KO mice, gastric emptying in KO mice was the same as that in WT mice probably due to the compensatory enhancement of the non-cholinergic contraction pathway.