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

Insights on Platelet-Derived Growth Factor Receptor α-Positive Interstitial Cells in the Male Reproductive Tract

Male infertility is a significant factor in approximately half of all infertility cases and is marked by a decreased sperm count and motility. A decreased sperm count is caused by not only a decreased production of sperm but also decreased numbers successfully passing through the male reproductive tract. Smooth muscle movement may play an important role in sperm transport in the male reproductive tract; thus, understanding the mechanism of this movement is necessary to elucidate the cause of sperm transport disorder. Recent studies have highlighted the presence of platelet-derived growth factor receptor α (PDGFRα)-positive interstitial cells (PICs) in various smooth muscle organs. Although research is ongoing, PICs in the male reproductive tract may be involved in the regulation of smooth muscle movement, as they are in other smooth muscle organs. This review summarizes the findings to date on PICs in male reproductive organs. Further exploration of the structural, functional, and molecular characteristics of PICs could provide valuable insights into the pathogenesis of male infertility and potentially lead to new therapeutic approaches.

Function and Regulation of the Calcium-Activated Chloride Channel Anoctamin 1 (TMEM16A)

Various human tissues express the calcium-activated chloride channel Anoctamin 1 (ANO1), also known as TMEM16A. ANO1 allows the passive chloride flux that controls different physiological functions ranging from muscle contraction, fluid and hormone secretion, gastrointestinal motility, and electrical excitability. Overexpression of ANO1 is associated with pathological conditions such as hypertension and cancer. The molecular cloning of ANO1 has led to a surge in structural, functional, and physiological studies of the channel in several tissues. ANO1 is a homodimer channel harboring two pores - one in each monomer - that work independently. Each pore is activated by voltage-dependent binding of two intracellular calcium ions to a high-affinity-binding site. In addition, the binding of phosphatidylinositol 4,5-bisphosphate to sites scattered throughout the cytosolic side of the protein aids the calcium activation process. Furthermore, many pharmacological studies have established ANO1 as a target of promising compounds that could treat several illnesses. This chapter describes our current understanding of the physiological roles of ANO1 and its regulation under physiological conditions as well as new pharmacological compounds with potential therapeutic applications.

Ca2+ dynamics in interstitial cells: foundational mechanisms for the motor patterns in the gastrointestinal tract

The gastrointestinal (GI) tract displays multiple motor patterns that move nutrients and wastes through the body. Smooth muscle cells (SMCs) provide the forces necessary for GI motility, but interstitial cells, electrically coupled to SMCs, tune SMC excitability, transduce inputs from enteric motor neurons, and generate pacemaker activity that underlies major motor patterns, such as peristalsis and segmentation. The interstitial cells regulating SMCs are interstitial cells of Cajal (ICC) and PDGF receptor (PDGFR)α+ cells. Together these cells form the SIP syncytium. ICC and PDGFRα+ cells express signature Ca2+-dependent conductances: ICC express Ca2+-activated Cl- channels, encoded by Ano1, that generate inward current, and PDGFRα+ cells express Ca2+-activated K+ channels, encoded by Kcnn3, that generate outward current. The open probabilities of interstitial cell conductances are controlled by Ca2+ release from the endoplasmic reticulum. The resulting Ca2+ transients occur spontaneously in a stochastic manner. Ca2+ transients in ICC induce spontaneous transient inward currents and spontaneous transient depolarizations (STDs). Neurotransmission increases or decreases Ca2+ transients, and the resulting depolarizing or hyperpolarizing responses conduct to other cells in the SIP syncytium. In pacemaker ICC, STDs activate voltage-dependent Ca2+ influx, which initiates a cluster of Ca2+ transients and sustains activation of ANO1 channels and depolarization during slow waves. Regulation of GI motility has traditionally been described as neurogenic and myogenic. Recent advances in understanding Ca2+ handling mechanisms in interstitial cells and how these mechanisms influence motor patterns of the GI tract suggest that the term "myogenic" should be replaced by the term "SIPgenic," as this review discusses.

Altered functional responses by PAR1 agonist in murine dextran sodium sulphate-treated colon

Protease-activated receptor-1 (PAR1) is highly expressed in murine colonic smooth muscles. Responses to PAR1 activation are complex and result from responses in multiple cell types. We investigated whether PAR1 responses are altered in inflamed colon induced by dextran sodium sulfate (DSS)-treatment. Colitis was induced in C57BL/6 mice by administration of 3% DSS in drinking water for 7 days. Measurements of isometric force, transmembrane potentials from impaled smooth muscle cells, quantitative PCR and Western blots were performed. Thrombin, an activator of PAR1, caused transient hyperpolarization and relaxation of untreated colons, but these responses decreased in DSS-treated colons. Apamin caused depolarization and increased contractions of muscles from untreated mice. This response was decreased in DSS-treated colons. Expression of Kcnn3 and Pdgfra also decreased in DSS-treated muscles. A second phase of thrombin responses is depolarization and increased contractions in untreated muscles. However, thrombin did cause depolarization in DSS-treated colon, yet it increased colonic contractions. The latter effect was associated with enhanced expression of MYPT1 and CPI-17. The propagation velocity and frequency of colonic migrating motor complexes in DSS-treated colon was significantly higher compared to control colons. In summary, DSS treatment causes loss of transient relaxations due to downregulation of SK3 channels in PDGFRα⁺ cells and may increase contractile responses due to increased Ca²⁺ sensitization of smooth muscle cells via PAR1 activation.

Calcium transients in intramuscular interstitial cells of Cajal of the murine gastric fundus and their regulation by neuroeffector transmission

KEY POINTS

The cells responsible for mediating enteric neuroeffector transmission remain controversial. In the stomach intramuscular interstitial cells of Cajal (ICC-IM) were the first ICC reported to receive cholinergic and nitrergic neural inputs. Utilization of a cell specific calcium biosensor, GCaMP6f, the activity and neuroeffector responses of ICC-IM were examined. ICC-IM were highly active, generating stochastic intracellular Ca²⁺ -transients. Stimulation of enteric motor nerves abolished Ca²⁺ -transients in ICC-IM. This inhibitory response was preceded by a global rise in intracellular Ca²⁺ . Individual ICC-IM responded to nerve stimulation with a rise in Ca²⁺ followed by inhibition of Ca²⁺ -transients. Inhibition of Ca²⁺ -transients was blocked by the nitric oxide synthase antagonist, L-NNA. The global rise in Ca²⁺ was inhibited by the muscarinic antagonist, atropine. Simultaneous intracellular recordings with video imaging revealed that the global rise in intracellular Ca²⁺ and inhibition of Ca²⁺ -transients was temporally associated with rapid excitatory junction potentials followed by more sustained inhibitory junction potentials. The data presented support the premise of serial innervation of ICC-IM in excitatory and inhibitory neuroeffector transmission in the proximal stomach.

ABSTRACT

Enteric neurotransmission is critical for coordinating motility throughout the gastrointestinal (GI) tract. However, there is considerable controversy regarding the cells that are responsible for the transduction of these neural inputs. In the present study, utilization of a cell-specific calcium biosensor GCaMP6f, the spontaneous activity and neuroeffector responses of intramuscular ICC (ICC-IM) to motor neural inputs was examined. Simultaneous intracellular microelectrode recordings and high-speed video-imaging during nerve stimulation was used to reveal the temporal relationship between changes in intracellular Ca²⁺ and post-junctional electrical responses to neural stimulation. ICC-IM were highly active, generating intracellular Ca²⁺ -transients that occurred stochastically, from multiple independent sites in single ICC-IM. Ca²⁺ -transients were not entrained in single ICC-IM or between neighboring ICC-IM. Activation of enteric motor neurons produced a dominant inhibitory response that abolished Ca²⁺ -transients in ICC-IM. This inhibitory response was often preceded by a summation of Ca²⁺ -transients that led to a global rise in Ca²⁺ . Individual ICC-IM responded to nerve stimulation by a global rise in Ca²⁺ followed by inhibition of Ca²⁺ -transients. The inhibition of Ca²⁺ -transients was blocked by the nitric oxide synthase antagonist, L-NNA. The global rise in intracellular Ca²⁺ was inhibited by the muscarinic antagonist, atropine. Simultaneous intracellular microelectrode recordings with video-imaging revealed that the rise in Ca²⁺ was temporally associated with rapid excitatory junction potentials and the inhibition of Ca²⁺ -transients with inhibitory junction potentials. These data support the premise of serial innervation of ICC-IM in excitatory and inhibitory neuroeffector transmission in the proximal stomach. Abstract figure legend Intramuscular interstitial cells of Cajal (ICC-IM) of the gastric fundus receive nitrergic inhibitory and cholinergic excitatory neuroeffector motor inputs. Using a genetically encoded calcium sensor we demonstrate that ICC-IM are highly active cells generating stochastic intracellular Ca2 -transients. Stimulation of enteric motor nerves abolished Ca2 -transients in ICC-IM, produced an inhibitory junction potential (IJP) and muscle relaxation that was mediated by nitric oxide (left hand side of figure). This inhibitory response was often preceded by a global rise in intracellular Ca2 in ICC-IM, a rapid excitatory junction potential (EJP) and muscle contraction, that was mediated by acetylcholine (right hand side of figure). Individual ICC-IM could respond to both excitatory and inhibitory neural inputs. These data support the premise of serial innervation of ICC-IM in excitatory and inhibitory neuroeffector transmission in the proximal stomach. This article is protected by copyright. All rights reserved.

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.

Inhibitory Effects of Jakyakgamcho-Tang (Glycyrrhiza uralensis and Paeonia lactiflora) on the Pacemaker Potential of the Interstitial Cells of Cajal in the Murine Small Intestine

Jakyakgamcho-tang (JYGCT) has been used to treat various diseases. The interstitial cells of Cajal (ICC) regulate gastrointestinal (GI) motility as pacemaker cells. Here, we examined the effects of JYGCT on the pacemaker potential of the ICC in the small intestine. We observed that JYGCT inhibited the pacemaker potential in a dose-dependent manner. Glibenclamide did not affect the pacemaker potential and on these conditions, JYGCT also had no effect on the pacemaker potential. Pretreatment with capsazepine or SB452533 blocked the JYGCT-induced effects. In the presence of SQ-22536, JYGCT did not inhibit the pacemaker potential. Additionally, JYGCT inhibited spontaneous [Ca2+]i oscillations and JYGCT-induced ITR increase was associated with TMEM16A, motilin and substance P activation. Moreover, JYGCT was effective in alleviating the symptoms of irritable bowel syndrome. Our results suggest that JYGCT inhibited the pacemaker potential of the ICC via KATP, the TRPV1 or the cyclic AMP pathway, and intracellular Ca2+ regulation, indicating that JYGCT can affect ICC and thus have the function of regulating GI motility. Therefore, JYGCT may be used as a GI motility disorder regulator or disease prevention agent.

Neuron-specific responses to acetylcholine within the spinal dorsal horn circuits of rodent and primate

Excitatory and inhibitory neurotransmission within the spinal dorsal horn is tightly controlled to regulate transmission of nociceptive signals to the brain. One aspect of this control is modulation of neuronal activity through cholinergic signaling. Nociceptive neurons in the dorsal horn express both nicotinic and muscarinic cholinergic receptors and activation of these receptors reduces pain in humans, while inhibition leads to nociceptive hypersensitivity. At a cellular level, acetylcholine (ACh) has diverse effects on excitability which is dependent on the receptor and neuronal subtypes involved. In the present study we sought to characterize the electrophysiological responses of specific subsets of lamina II interneurons from rat and marmoset spinal cord. Neurons were grouped by morphology and by action potential firing properties. Whole-cell voltage-clamp recordings from lamina II dorsal horn neurons of adult rats showed that bath applied acetylcholine increased, decreased or had no effect on spontaneous synaptic current activity in a cell-type specific manner. ACh modulated inhibitory synaptic activity in 80% of neurons, whereas excitatory synaptic activity was affected in less than 50% of neurons. In whole-cell current clamp recordings, brief somatic application of ACh induced cell-type specific responses in 79% of rat lamina II neurons, which included: depolarization and action potential firing, subthreshold membrane depolarization, biphasic responses characterized by transient depolarization followed by hyperpolarization and membrane hyperpolarization alone. Similar responses were seen in marmoset lamina II neurons and the properties of each neuron group were consistent across species. ACh-induced hyperpolarization was blocked by the muscarinic antagonist atropine and all forms of acetylcholine-induced depolarization were blocked by the nicotinic antagonist mecamylamine. The cholinergic system plays an important role in regulating nociception and this study contributes to our understanding of how circuit activity is controlled by ACh at a cellular level in primate and rodent spinal cord.

Synthesis and anticancer evaluation of 6-azacyclonol-2,4,6-trimethylpyridin-3-ol derivatives: M3 muscarinic acetylcholine receptor-mediated anticancer activity of a cyclohexyl derivative in androgen-refractory prostate cancer

We recently reported 2,4,5-trimethylpyridin-3-ol with C(6)-azacyclonol, whose code name is BJ-1207, showing a promising anticancer activity by inhibiting NOX-derived ROS in A549 human lung cancer cells. The present study was focused on structural modification of the azacyclonol moiety of BJ-1207 to find a compound with better anticancer activity. Ten new compounds (3A-3J) were prepared and evaluated their inhibitory actions against proliferation of eighteen cancer cell lines as a primary screening. Among the ten derivatives of BJ-1207, the effects of compounds 3A and 3J on DU145 and PC-3, androgen-refractory cancer cell lines (ARPC), were greater than the parent compound, and compound 3A showed better activity than 3J. Antitumor activity of compound 3A was also observed in DU145-xenografted chorioallantoic membrane (CAM) tumor model. In addition, the ligand-based target prediction and molecular docking study using DeepZema® server showed compound 3A was a ligand to M3 muscarinic acetylcholine receptor (M3R) which is overexpressed in ARPC. Carbachol, a muscarinic receptor agonist, concentration dependently increased proliferation of DU145 in the absence of serum, and it also activated NADPH oxidase (NOX). The carbachol-induced proliferation and NOX activity was significantly blocked by compounds 3A in a concentration-dependent manner. This finding might become a new milestone in the development of pyridinol-based anti-cancer agents against ARPC.

Role of Ano1 Ca2+-activated Cl- channels in generating urethral tone

Urinary continence is maintained in the lower urinary tract by the contracture of urethral sphincters, including smooth muscle of the internal urethral sphincter. These contractions occlude the urethral lumen, preventing urine leakage from the bladder to the exterior. Over the past 20 years, research on the ionic conductances that contribute to urethral smooth muscle contractility has greatly accelerated. A debate has emerged over the role of interstitial cell of Cajal (ICC)-like cells in the urethra and their expression of Ca²⁺-activated Cl^- channels encoded by anoctamin-1 [Ano1; transmembrane member 16 A (Tmem16a) gene]. It has been proposed that Ano1 channels expressed in urethral ICC serve as a source of depolarization for smooth muscle cells, increasing their excitability and contributing to tone. Although a clear role for Ano1 channels expressed in ICC is evident in other smooth muscle organs, such as the gastrointestinal tract, the role of these channels in the urethra is unclear, owing to differences in the species (rabbit, rat, guinea pig, sheep, and mouse) examined and experimental approaches by different groups. The importance of clarifying this situation is evident as effective targeting of Ano1 channels may lead to new treatments for urinary incontinence. In this review, we summarize the key findings from different species on the role of ICC and Ano1 channels in urethral contractility. Finally, we outline proposals for clarifying this controversial and important topic by addressing how cell-specific optogenetic and inducible cell-specific genetic deletion strategies coupled with advances in Ano1 channel pharmacology may clarify this area in future studies.NEW & NOTEWORTHY Studies from the rabbit have shown that anoctamin-1 (Ano1) channels expressed in urethral interstitial cells of Cajal (ICC) serve as a source of depolarization for smooth muscle cells, increasing excitability and tone. However, the role of urethral Ano1 channels is unclear, owing to differences in the species examined and experimental approaches. We summarize findings from different species on the role of urethral ICC and Ano1 channels in urethral contractility and outline proposals for clarifying this topic using cell-specific optogenetic approaches.

Ca2+ signaling driving pacemaker activity in submucosal interstitial cells of Cajal in the murine colon

Interstitial cells of Cajal (ICC) generate pacemaker activity responsible for phasic contractions in colonic segmentation and peristalsis. ICC along the submucosal border (ICC-SM) contribute to mixing and more complex patterns of colonic motility. We show the complex patterns of Ca²⁺ signaling in ICC-SM and the relationship between ICC-SM Ca²⁺ transients and activation of smooth muscle cells (SMCs) using optogenetic tools. ICC-SM displayed rhythmic firing of Ca²⁺transients ~ 15 cpm and paced adjacent SMCs. The majority of spontaneous activity occurred in regular Ca²⁺ transients clusters (CTCs) that propagated through the network. CTCs were organized and dependent upon Ca²⁺ entry through voltage-dependent Ca²⁺ conductances, L- and T-type Ca²⁺ channels. Removal of Ca²⁺ from the external solution abolished CTCs. Ca²⁺ release mechanisms reduced the duration and amplitude of Ca²⁺ transients but did not block CTCs. These data reveal how colonic pacemaker ICC-SM exhibit complex Ca^2+-firing patterns and drive smooth muscle activity and overall colonic contractions.

Excitatory cholinergic responses in mouse colon intramuscular interstitial cells of Cajal are due to enhanced Ca2+ release via M3 receptor activation

Colonic intramuscular interstitial cells of Cajal (ICC-IM) are associated with cholinergic varicosities, suggesting a role in mediating excitatory neurotransmission. Ca²⁺ release in ICC-IM activates Ano1, a Ca²⁺ -activated Cl^- conductance, causing tissue depolarization and increased smooth muscle excitability. We employed Ca²⁺ imaging of colonic ICC-IM in situ, using mice expressing GCaMP6f in ICC to evaluate ICC-IM responses to excitatory neurotransmission. Expression of muscarinic type 2, 3 (M₂ , M₃ ), and NK₁ receptors were enriched in ICC-IM. NK₁ receptor agonists had minimal effects on ICC-IM, whereas neostigmine and carbachol increased Ca²⁺ transients. These effects were reversed by DAU 5884 (M₃ receptor antagonist) but not AF-DX 116 (M₂ receptor antagonist). Electrical field stimulation (EFS) in the presence of L-NNA and MRS 2500 enhanced ICC-IM Ca²⁺ transients. Responses were blocked by atropine or DAU 5884, but not AF-DX 116. ICC-IM responses to EFS were ablated by inhibiting Ca²⁺ stores with cyclopiazonic acid and reduced by inhibiting Ca²⁺ influx via Orai channels. Contractions induced by EFS were reduced by an Ano1 channel antagonist, abolished by DAU 5884, and unaffected by AF-DX 116. Colonic ICC-IM receive excitatory inputs from cholinergic neurons via M₃ receptor activation. Enhancing ICC-IM Ca²⁺ release and Ano1 activation contributes to excitatory responses of colonic muscles.

Na+/Ca2 + Exchange and Pacemaker Activity of Interstitial Cells of Cajal

Interstitial cells of Cajal (ICC) are pacemaker cells that generate electrical slow waves in gastrointestinal (GI) smooth muscles. Slow waves organize basic motor patterns, such as peristalsis and segmentation in the GI tract. Slow waves depend upon activation of Ca²⁺-activated Cl^– channels (CaCC) encoded by Ano1. Slow waves consist of an upstroke depolarization and a sustained plateau potential that is the main factor leading to excitation-contraction coupling. The plateau phase can last for seconds in some regions of the GI tract. How elevated Ca²⁺ is maintained throughout the duration of slow waves, which is necessary for sustained activation of CaCC, is unknown. Modeling has suggested a role for Na⁺/Ca²⁺ exchanger (NCX) in regulating CaCC currents in ICC, so we tested this idea on murine intestinal ICC. ICC of small and large intestine express NCX isoforms. NCX3 is closely associated with ANO1 in ICC, as shown by immunoprecipitation and proximity ligation assays (PLA). KB-R7943, an inhibitor of NCX, increased CaCC current in ICC, suggesting that NCX, acting in Ca²⁺ exit mode, helps to regulate basal [Ca²⁺]_i in these cells. Shifting NCX into Ca²⁺ entry mode by replacing extracellular Na⁺ with Li⁺ increased spontaneous transient inward currents (STICs), due to activation of CaCC. Stepping ICC from −80 to −40 mV activated slow wave currents that were reduced in amplitude and duration by NCX inhibitors, KB-R7943 and SN-6, and enhanced by increasing the NCX driving force. SN-6 reduced the duration of clustered Ca²⁺ transients that underlie the activation of CaCC and the plateau phase of slow waves. Our results suggest that NCX participates in slow waves as modeling has predicted. Dynamic changes in membrane potential and ionic gradients during slow waves appear to flip the directionality of NCX, facilitating removal of Ca²⁺ during the inter-slow wave interval and providing Ca²⁺ for sustained activation of ANO1 during the slow wave plateau phase.

A Pilot Study of Non-invasive Sacral Nerve Stimulation in Treatment of Constipation in Childhood and Adolescence

Background/Aims: Constipation shows both, a high prevalence and a significant impact. However, it is often perceived as minor and treatment choices are limited. The neuromodulation approach is a valuable option to be considered. This study assesses the use of non-invasive sacral nerve stimulation to reduce constipation in children. Methods: Between February 2013 and May 2015, pediatric patients with chronic constipation were treated with this non-invasive neuromodulation procedure, adapted from classical sacral nerve stimulation. A stimulation device attached to adhesive electrodes on the lower abdomen and back generated an electrical field with a stable frequency of 15 Hz via variable stimulation intensity (1-10 V). The effect of therapy was evaluated in routine check-ups and by specialized questionnaires. Results: The study assessed non-invasive sacral nerve stimulation in 17 patients (9 boys, 8 girls, mean age 6.5 years). They underwent stimulation with 6-9 V for a mean of 11 h per day (range 0.5-24 h) over a mean of 12.7 weeks. Improvement of constipation was achieved in more than half of the patients (12/17) and sustained in almost half of these patients (5/12). Complications were minor (skin irritation, electrode dislocation). Conclusions: Non-invasive sacral nerve stimulation appears to be effective in achieving improvement in pediatric patients with chronic constipation. As an additional external neuromodulation concept, this stimulation may represent a relevant addition to currently available therapeutic options. Further studies are needed to confirm these results.

The Microbiota-Gut-Brain Axis

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson’s disease, and Alzheimer’s disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders.

Na-K-2Cl Cotransporter and Store-Operated Ca2+ Entry in Pacemaking by Interstitial Cells of Cajal

Pacemaker depolarization in interstitial cells of Cajal (ICCs) is believed to be induced by Ca²⁺ transients and activation of anoctamin-1 (Ano1) channels in the plasma membrane. However, block of store-operated calcium entry (SOCE) or the Na-K-2Cl cotransporter (NKCC1) terminates pacemaker activity in ICC, indicating these transporters are involved in the initiation or maintenance of pacemaker activity. We hypothesized that SOCE contributes to pacemaker depolarization by maintaining [Ca²⁺] in the endoplasmic reticulum, which is the underlying source of Ca²⁺ transients for activation of Ano1. NKCC1 maintains the Cl^- gradient supporting the driving force for inward current mediated by Ano1. Currently mechanisms sustaining release of Ca²⁺ and activation of Ano1 channels during the plateau phase of slow waves are unknown, but the reverse mode of the Na⁺/Ca²⁺ exchange may contribute. We generated a mathematical model of pacemaker activity based on current empirical observations from ICC of mouse small intestine that incorporates functions of SOCE and NKCC1. This model reproduces experimental findings, suggesting roles for SOCE and Ano1 channels: blocking of either NKCC1 or SOCE in our model terminates pacemaker activity. Direct contribution of NKCC1 to pacemaker activity in a beat-to-beat manner is not predicted by our model. Instead, NKCC1 plays a maintenance role supporting the driving force for Cl^- efflux. Incorporation of SOCE allows the model to drive pacemaker activity without a diastolic depolarization, as observed in cardiac pacemaking. Further biological experiments are necessary to validate and further refine the roles of NKCC1, Na⁺/Ca²⁺ exchange, and Ano1 in the pacemaker mechanism of ICC.

Tonic inhibition of murine proximal colon is due to nitrergic suppression of Ca2+ signaling in interstitial cells of Cajal

Spontaneous excitability and contractions of colonic smooth muscle cells (SMCs) are normally suppressed by inputs from inhibitory motor neurons, a behavior known as tonic inhibition. The post-junctional cell(s) mediating tonic inhibition have not been elucidated. We investigated the post-junctional cells mediating tonic inhibition in the proximal colon and whether tonic inhibition results from suppression of the activity of Ano1 channels, which are expressed exclusively in interstitial cells of Cajal (ICC). We found that tetrodotoxin (TTX), an inhibitor of nitric oxide (NO) synthesis, L-NNA, and an inhibitor of soluble guanylyl cyclase, ODQ, greatly enhanced colonic contractions. Ano1 antagonists, benzbromarone and Ani9 inhibited the effects of TTX, L-NNA and ODQ. Ano1 channels are activated by Ca²⁺ release from the endoplasmic reticulum (ER) in ICC, and blocking Ca²⁺ release with a SERCA inhibitor (thapsigargin) or a store-operated Ca²⁺ entry blocker (GSK 7975 A) reversed the effects of TTX, L-NNA and ODQ. Ca²⁺ imaging revealed that TTX, L-NNA and ODQ increased Ca²⁺ transient firing in colonic ICC. Our results suggest that tonic inhibition in the proximal colon occurs through suppression of Ca²⁺ release events in ICC. Suppression of Ca²⁺ release in ICC limits the open probability of Ano1 channels, reducing the excitability of electrically-coupled SMCs.

Unexpected Roles for the Second Brain: Enteric Nervous System as Master Regulator of Bowel Function

At the most fundamental level, the bowel facilitates absorption of small molecules, regulates fluid and electrolyte flux, and eliminates waste. To successfully coordinate this complex array of functions, the bowel relies on the enteric nervous system (ENS), an intricate network of more than 500 million neurons and supporting glia that are organized into distinct layers or plexi within the bowel wall. Neuron and glial diversity, as well as neurotransmitter and receptor expression in the ENS, resembles that of the central nervous system. The most carefully studied ENS functions include control of bowel motility, epithelial secretion, and blood flow, but the ENS also interacts with enteroendocrine cells, influences epithelial proliferation and repair, modulates the intestinal immune system, and mediates extrinsic nerve input. Here, we review the many different cell types that communicate with the ENS, integrating data about ENS function into a broader view of human health and disease. In particular, we focus on exciting new literature highlighting relationships between the ENS and its lesser-known interacting partners.

Applications of Spatio-temporal Mapping and Particle Analysis Techniques to Quantify Intracellular Ca2+ Signaling In Situ

Ca²⁺ imaging of isolated cells or specific types of cells within intact tissues often reveals complex patterns of Ca²⁺ signaling. This activity requires careful and in-depth analyses and quantification to capture as much information about the underlying events as possible. Spatial, temporal and intensity parameters intrinsic to Ca²⁺ signals such as frequency, duration, propagation, velocity and amplitude may provide some biological information required for intracellular signalling. High-resolution Ca²⁺ imaging typically results in the acquisition of large data files that are time consuming to process in terms of translating the imaging information into quantifiable data, and this process can be susceptible to human error and bias. Analysis of Ca²⁺ signals from cells in situ typically relies on simple intensity measurements from arbitrarily selected regions of interest (ROI) within a field of view (FOV). This approach ignores much of the important signaling information contained in the FOV. Thus, in order to maximize recovery of information from such high-resolution recordings obtained with Ca²⁺dyes or optogenetic Ca²⁺ imaging, appropriate spatial and temporal analysis of the Ca²⁺ signals is required. The protocols outlined in this paper will describe how a high volume of data can be obtained from Ca²⁺ imaging recordings to facilitate more complete analysis and quantification of Ca²⁺ signals recorded from cells using a combination of spatiotemporal map (STM)-based analysis and particle-based analysis. The protocols also describe how different patterns of Ca²⁺ signaling observed in different cell populations in situ can be analyzed appropriately. For illustration, the method will examine Ca²⁺ signaling in a specialized population of cells in the small intestine, interstitial cells of Cajal (ICC), using GECIs.

Nitric oxide and its role as a non-adrenergic, non-cholinergic inhibitory neurotransmitter in the gastrointestinal tract

NO is a neurotransmitter released from enteric inhibitory neurons and responsible for modulating gastrointestinal (GI) motor behaviour. Enteric neurons express nNOS (NOS1) that associates with membranes of nerve varicosities. NO released from neurons binds to soluble guanylate cyclase in post-junctional cells to generate cGMP. cGMP-dependent protein kinase type 1 (PKG1) is a major mediator but perhaps not the only pathway involved in cGMP-mediated effects in GI muscles based on gene deletion studies. NOS1+ neurons form close contacts with smooth muscle cells (SMCs), interstitial cells of Cajal (ICC) and PDGFRα+ cells, and these cells are electrically coupled (SIP syncytium). Cell-specific gene deletion studies have shown that nitrergic responses are due to mechanisms in SMCs and ICC. Controversy exists about the ion channels and other post-junctional mechanisms that mediate nitrergic responses in GI muscles. Reduced nNOS expression in enteric inhibitory motor neurons and/or reduced connectivity between nNOS+ neurons and the SIP syncytium appear to be responsible for motor defects that develop in diabetes. An overproduction of NO in some inflammatory conditions also impairs normal GI motor activity. This review summarizes recent findings regarding the role of NO as an enteric inhibitory neurotransmitter. LINKED ARTICLES: This article is part of a themed section on Nitric Oxide 20 Years from the 1998 Nobel Prize. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.2/issuetoc.

Spontaneous Electrical Activity and Rhythmicity in Gastrointestinal Smooth Muscles

The gastrointestinal (GI) tract has multifold tasks of ingesting, processing, and assimilating nutrients and disposing of wastes at appropriate times. These tasks are facilitated by several stereotypical motor patterns that build upon the intrinsic rhythmicity of the smooth muscles that generate phasic contractions in many regions of the gut. Phasic contractions result from a cyclical depolarization/repolarization cycle, known as electrical slow waves, which result from intrinsic pacemaker activity. Interstitial cells of Cajal (ICC) are electrically coupled to smooth muscle cells (SMCs) and generate and propagate pacemaker activity and slow waves. The mechanism of slow waves is dependent upon specialized conductances expressed by pacemaker ICC. The primary conductances responsible for slow waves in mice are Ano1, Ca2+-activated Cl- channels (CaCCs), and CaV3.2, T-type, voltage-dependent Ca2+ channels. Release of Ca2+ from intracellular stores in ICC appears to be the initiator of pacemaker depolarizations, activation of T-type current provides voltage-dependent Ca2+ entry into ICC, as slow waves propagate through ICC networks, and Ca2+-induced Ca2+ release and activation of Ano1 in ICC amplifies slow wave depolarizations. Slow waves conduct to coupled SMCs, and depolarization elicited by these events enhances the open-probability of L-type voltage-dependent Ca2+ channels, promotes Ca2+ entry, and initiates contraction. Phasic contractions timed by the occurrence of slow waves provide the basis for motility patterns such as gastric peristalsis and segmentation. This chapter discusses the properties of ICC and proposed mechanism of electrical rhythmicity in GI muscles.

Role of High-affinity Choline Transporter 1 in Colonic Hypermotility in a Rat Model of Irritable Bowel Syndrome

Background/Aims

Irritable bowel syndrome (IBS) is a common disease characterized by intestinal dysmotility, the mechanism of which remains elusive. We aim to determine whether the high-affinity choline transporter 1 (CHT1), a determinant of cholinergic signaling capacity, modulates intestinal motility associated with stress-induced IBS.

Methods

A rat IBS model was established using chronic water avoidance stress (WAS). Colonic pathological alterations were evaluated histologically and intestinal motility was assessed by intestinal transit time and fecal water content (FWC). Visceral sensitivity was determined by visceromotor response to colorectal distension. RT-PCR, western blotting, and immunostaining were performed to identify colonic CHT1 expression. Contractility of colonic muscle strips was measured using isometric transducers. enzyme-linked immunosorbent assay was used to measure acetylcholine (ACh). We examined the effects of MKC-231, a choline uptake enhancer, on colonic motility.

Results

After 10 days of WAS, intestinal transit time was decreased and fecal water content increased. Visceromotor response magnitude in WAS rats in response to colorectal distension was significantly enhanced. Protein and mRNA CHT1 levels in the colon were markedly elevated after WAS. The density of CHT1-positive intramuscular interstitial cells of Cajal and myenteric plexus neurons in WAS rats was higher than in controls. Ammonium pyrrolidine dithiocarbamate partly reversed CHT1 upregulation and alleviated colonic hypermotility in WAS rats. Pharmacological enhancement of CHT1 activity by MKC-231 enhanced colonic motility in control rats via upregulation of CHT1 and elevation of ACh production.

Conclusion

Upregulation of CHT1 in intramuscular interstitial cells of Cajal and myenteric plexus neurons is implicated in chronic stress-induced colonic hypermotility by modulation of ACh synthesis via nuclear factor-kappa B signaling.

The effect of pyriproxyfen on the motoric activity of rat intestine - In vitro study

The application of pyriproxyfen (PPF) to drinking water and constant exposure of the whole population to this insecticide is an unprecedented action on a world scale and presents a new and serious challenge for toxicology. The aim of the study was to evaluate the potential effect of PPF on the intestine muscle activity. The experiments were performed on isolated duodenum and jejunum strips of rat, in isometric conditions. Doses of PPF in the range of 0.032-100 μM were used in the experiments. The obtained results indicate that PPF affected significantly the spontaneous activity of duodenum and jejunum strips, PPF caused the muscle relaxation when used in the concentration of 0.8 μM and higher. The reaction to acetylcholine (ACh) when PPF preceded or followed ACh application was also reduced. It is demonstrated that the reduction of the contraction caused by ACh was stronger when duodenum strips were preincubated in the presence of PPF solution than in case of ACh-precontracted strips. The first significant reaction of duodenal strips appeared in the presence of PPF in a dose of 0.16 μM and 0.8 μM when the insecticide application preceded and followed ACh treatment, respectively. Besides, the duodenum turned out to be much more susceptible to the tested insecticide than jejunum. Taking into account PPF kinetic data obtained in animals, the observed disturbances were caused by the insecticide used in relatively high concentrations. However, the full risk estimation requires the kinetic data obtained in human, especially from monitoring studies on general population after long-term exposure to PPF.

SOCE mediated by STIM and Orai is essential for pacemaker activity in the interstitial cells of Cajal in the gastrointestinal tract

Electrical pacemaker activity generates phasic contractions and motility patterns such as segmentation and peristalsis in the gastrointestinal tract. Pacemaker currents are generated in interstitial cells of Cajal (ICC), which release Ca2+ from intracellular stores that stimulates Ca2+-activated Cl- channels (CaCCs) in the plasma membrane. Thus, Ca2+ stores must be maintained to sustain pacemaker activity. Store-operated Ca2+ entry (SOCE) facilitates the refilling of Ca2+ stores by a mechanism dependent upon interactions between STIM and Orai proteins. We investigated the role of SOCE in ICC pacemaker activity. Reintroduction of extracellular Ca2+ in store-depleted ICC resulted in CaCC activation. Blocking CaCCs revealed an inwardly rectifying current with properties of a Ca2+ release-activated current (ICRAC). An inhibitory peptide that interfered with the STIM-Orai interaction blocked ICRAC in HEK 293 cells expressing STIM1 and Orai1 and blocked spontaneous transient inward currents (STICs) and slow wave currents in ICC. STICs, which are fundamental pacemaker events in ICC, were blocked by an Orai antagonist. Imaging of Ca2+ transients linked to pacemaker activity in ICC in intact muscles showed that the Orai antagonist blocked Ca2+ transients in ICC. These data suggest that Ca2+ recovery through STIM-Orai interactions is necessary to maintain ICC pacemaker activity.

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.

Inhibitory Neural Regulation of the Ca 2+ Transients in Intramuscular Interstitial Cells of Cajal in the Small Intestine

Gastrointestinal motility is coordinated by enteric neurons. Both inhibitory and excitatory motor neurons innervate the syncytium consisting of smooth muscle cells (SMCs) interstitial cells of Cajal (ICC) and PDGFRα+ cells (SIP syncytium). Confocal imaging of mouse small intestines from animals expressing GCaMP3 in ICC were used to investigate inhibitory neural regulation of ICC in the deep muscular plexus (ICC-DMP). We hypothesized that Ca2+ signaling in ICC-DMP can be modulated by inhibitory enteric neural input. ICC-DMP lie in close proximity to the varicosities of motor neurons and generate ongoing Ca2+ transients that underlie activation of Ca2+-dependent Cl- channels and regulate the excitability of SMCs in the SIP syncytium. Electrical field stimulation (EFS) caused inhibition of Ca2+ for the first 2-3 s of stimulation, and then Ca2+ transients escaped from inhibition. The NO donor (DEA-NONOate) inhibited Ca2+ transients and Nω-Nitro-L-arginine (L-NNA) or a guanylate cyclase inhibitor (ODQ) blocked inhibition induced by EFS. Purinergic neurotransmission did not affect Ca2+ transients in ICC-DMP. Purinergic neurotransmission elicits hyperpolarization of the SIP syncytium by activation of K+ channels in PDGFRα+ cells. Generalized hyperpolarization of SIP cells by pinacidil (KATP agonist) or MRS2365 (P2Y1 agonist) also had no effect on Ca2+ transients in ICC-DMP. Peptidergic transmitter receptors (VIP and PACAP) are expressed in ICC and can modulate ICC-DMP Ca2+ transients. In summary Ca2+ transients in ICC-DMP are blocked by enteric inhibitory neurotransmission. ICC-DMP lack a voltage-dependent mechanism for regulating Ca2+ release, and this protects Ca2+ handling in ICC-DMP from membrane potential changes in other SIP cells.

Excitatory Neuronal Responses of Ca2+ Transients in Interstitial Cells of Cajal in the Small Intestine

Interstitial cells of Cajal (ICC) regulate smooth muscle excitability and motility in the gastrointestinal (GI) tract. ICC in the deep muscular plexus (ICC-DMP) of the small intestine are aligned closely with varicosities of enteric motor neurons and thought to transduce neural responses. ICC-DMP generate Ca²⁺ transients that activate Ca²⁺ activated Cl^- channels and generate electrophysiological responses. We tested the hypothesis that excitatory neurotransmitters regulate Ca²⁺ transients in ICC-DMP as a means of regulating intestinal muscles. High-resolution confocal microscopy was used to image Ca²⁺ transients in ICC-DMP within murine small intestinal muscles with cell-specific expression of GCaMP3. Intrinsic nerves were stimulated by electrical field stimulation (EFS). ICC-DMP exhibited ongoing Ca²⁺ transients before stimuli were applied. EFS caused initial suppression of Ca²⁺ transients, followed by escape during sustained stimulation, and large increases in Ca²⁺ transients after cessation of stimulation. Basal Ca²⁺ activity and the excitatory phases of Ca²⁺ responses to EFS were inhibited by atropine and neurokinin 1 receptor (NK1) antagonists, but not by NK2 receptor antagonists. Exogenous ACh and substance P (SP) increased Ca²⁺ transients, atropine and NK1 antagonists decreased Ca²⁺ transients. Neurokinins appear to be released spontaneously (tonic excitation) in small intestinal muscles and are the dominant excitatory neurotransmitters. Subcellular regulation of Ca²⁺ release events in ICC-DMP may be a means by which excitatory neurotransmission organizes intestinal motility patterns.

The effects of mitochondrial inhibitors on Ca2+ signalling and electrical conductances required for pacemaking in interstitial cells of Cajal in the mouse small intestine

Interstitial cells of Cajal (ICC-MY) are pacemakers that generate and propagate electrical slow waves in gastrointestinal (GI) muscles. Slow waves appear to be generated by the release of Ca²⁺ from intracellular stores and activation of Ca²⁺-activated Cl^- channels (Ano1). Conduction of slow waves to smooth muscle cells coordinates rhythmic contractions. Mitochondrial Ca²⁺ handling is currently thought to be critical for ICC pacemaking. Protonophores, inhibitors of the electron transport chain (FCCP, CCCP or antimycin) or mitochondrial Na⁺/Ca²⁺ exchange blockers inhibited slow waves in several GI muscles. Here we utilized Ca²⁺ imaging of ICC in small intestinal muscles in situ to determine the effects of mitochondrial drugs on Ca²⁺ transients in ICC. Muscles were obtained from mice expressing a genetically encoded Ca²⁺ indicator (GCaMP3) in ICC. FCCP, CCCP, antimycin, a uniporter blocker, Ru360, and a mitochondrial Na⁺/Ca²⁺ exchange inhibitor, CGP-37157 inhibited Ca²⁺ transients in ICC-MY. Effects were not due to depletion of ATP, as oligomycin did not affect Ca²⁺ transients. Patch-clamp experiments were performed to test the effects of the mitochondrial drugs on key pacemaker conductances, Ano1 and T-type Ca²⁺ (Ca_V3.2), in HEK293 cells. Antimycin blocked Ano1 and reduced Ca_V3.2 currents. CCCP blocked Ca_V3.2 current but did not affect Ano1 current. Ano1 and Cav3.2 currents were inhibited by CGP-37157. Inhibitory effects of mitochondrial drugs on slow waves and Ca²⁺ signalling in ICC can be explained by direct antagonism of key pacemaker conductances in ICC that generate and propagate slow waves. A direct obligatory role for mitochondria in pacemaker activity is therefore questionable.

Colonic Hypermotility in a Rat Model of Irritable Bowel Syndrome Is Associated with Upregulation of TMEM16A in Myenteric Plexus

BACKGROUND

Irritable bowel syndrome (IBS) is a common disease with intestinal dysmotility, whose mechanism remains elusive. TMEM16A is a calcium-activated chloride channel (CaCC) involved in intestinal slow-wave generation.

AIMS

To investigate whether TMEM16A is involved in colonic dysmotility in IBS.

METHODS

A rat model of IBS was established by chronic water avoidance stress (WAS). Colonic pathological alterations were evaluated histologically, and intestinal motility was assessed by intestinal transit time (ITT) and fecal water content (FWC). Visceral sensitivity was determined by visceromotor response (VMR) to colorectal distension (CRD). TMEM16A expression was evaluated by RT-PCR, Western blot, and immunofluorescence. Colonic muscle strip contractility was measured by isometric transducers, and the effect of niflumic acid (NFA), a CaCC antagonist, on colonic motility was examined.

RESULTS

After 10 days of WAS exposure, ITT was decreased and FWC was elevated. Furthermore, VMR magnitude of WAS rats in response to CRD was significantly enhanced. Protein and mRNA levels of TMEM16A in colon were considerably increased after WAS. The percentage of TMEM16A-positive neurons in myenteric plexus (MP) of WAS rats was significantly higher than controls. Pharmacological blockade of TMEM16A activity by NFA considerably enhanced ITT, with concentration-dependent declines in FWC and VMR magnitude in NFA-treated rats. Further, spontaneous contraction of colonic strips of NFA-treated rats was significantly ameliorated in a concentration-dependent manner in vitro.

CONCLUSIONS

Upregulation of TMEM16A in MP neurons may play an important role in chronic stress-induced colonic hypermotility, making CaCC-blocking drugs a putatively effective treatment method for colonic hypermotility in IBS.

Regulation of neural ion channels by muscarinic receptors

The excitable behaviour of neurons is determined by the activity of their endogenous membrane ion channels. Since muscarinic receptors are not themselves ion channels, the acute effects of muscarinic receptor stimulation on neuronal function are governed by the effects of the receptors on these endogenous neuronal ion channels. This review considers some principles and factors determining the interaction between subtypes and classes of muscarinic receptors with neuronal ion channels, and summarizes the effects of muscarinic receptor stimulation on a number of different channels, the mechanisms of receptor - channel transduction and their direct consequences for neuronal activity. Ion channels considered include potassium channels (voltage-gated, inward rectifier and calcium activated), voltage-gated calcium channels, cation channels and chloride channels. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Effects of new-generation TMEM16A inhibitors on calcium-activated chloride currents in rabbit urethral interstitial cells of Cajal

Interstitial cells of Cajal (ICC) isolated from the rabbit urethra exhibit Ca²⁺-activated Cl^- currents (I _ClCa) that are important for the development of urethral tone. Here, we examined if TMEM16A (ANO1) contributed to this activity by examining the effect of "new-generation" TMEM16A inhibitors, CACC_inh-A01 and T16A_inh-A01, on I _ClCa recorded from freshly isolated rabbit urethral ICC (RUICC) and on contractions of intact strips of rabbit urethra smooth muscle. Real-time quantitative PCR experiments demonstrated that TMEM16A was highly expressed in rabbit urethra smooth muscle, in comparison to TMEM16B and TMEM16F. Single-cell RT-PCR experiments revealed that only TMEM16A was expressed in freshly isolated RUICC. Depolarization-evoked I _ClCa in isolated RUICC, recorded using voltage clamp, were inhibited by CACC_inh-A01 and T16A_inh-A01 with IC₅₀ values of 1.2 and 3.4 μM, respectively. Similarly, spontaneous transient inward currents (STICs) recorded from RUICC voltage clamped at -60 mV and spontaneous transient depolarizations (STDs), recorded in current clamp, were also inhibited by CACC_inh-A01 and T16A_inh-A01. In contrast, spontaneous Ca²⁺ waves in isolated RUICC were only partially reduced by CACC_inh-A01 and T16A_inh-A01. Finally, neurogenic contractions of strips of rabbit urethra smooth muscle (RUSM), evoked by electric field stimulation (EFS), were also significantly reduced by CACC_inh-A01 and T16A_inh-A01. These data are consistent with the idea that TMEM16A is involved with CACCs in RUICC and in contraction of rabbit urethral smooth muscle.

Clustering of Ca2+ transients in interstitial cells of Cajal defines slow wave duration

Electrical slow waves in the small intestine are generated by pacemaker cells called interstitial cells of Cajal. Drumm et al. record clusters of Ca²⁺ transients in these cells that are entrained by voltage-dependent Ca²⁺ entry and which define the duration of the electrical slow waves.

Interstitial cells of Cajal (ICC) in the myenteric plexus region (ICC-MY) of the small intestine are pacemakers that generate rhythmic depolarizations known as slow waves. Slow waves depend on activation of Ca²⁺-activated Cl⁻ channels (ANO1) in ICC, propagate actively within networks of ICC-MY, and conduct to smooth muscle cells where they generate action potentials and phasic contractions. Thus, mechanisms of Ca²⁺ regulation in ICC are fundamental to the motor patterns of the bowel. Here, we characterize the nature of Ca²⁺ transients in ICC-MY within intact muscles, using mice expressing a genetically encoded Ca²⁺ sensor, GCaMP3, in ICC. Ca²⁺ transients in ICC-MY display a complex firing pattern caused by localized Ca²⁺ release events arising from multiple sites in cell somata and processes. Ca²⁺ transients are clustered within the time course of slow waves but fire asynchronously during these clusters. The durations of Ca²⁺ transient clusters (CTCs) correspond to slow wave durations (plateau phase). Simultaneous imaging and intracellular electrical recordings revealed that the upstroke depolarization of slow waves precedes clusters of Ca²⁺ transients. Summation of CTCs results in relatively uniform Ca²⁺ responses from one slow wave to another. These Ca²⁺ transients are caused by Ca²⁺ release from intracellular stores and depend on ryanodine receptors as well as amplification from IP₃ receptors. Reduced extracellular Ca²⁺ concentrations and T-type Ca²⁺ channel blockers decreased the number of firing sites and firing probability of Ca²⁺ transients. In summary, the fundamental electrical events of small intestinal muscles generated by ICC-MY depend on asynchronous firing of Ca²⁺ transients from multiple intracellular release sites. These events are organized into clusters by Ca²⁺ influx through T-type Ca²⁺ channels to sustain activation of ANO1 channels and generate the plateau phase of slow waves.

Transcriptome of interstitial cells of Cajal reveals unique and selective gene signatures

Transcriptome-scale data can reveal essential clues into understanding the underlying molecular mechanisms behind specific cellular functions and biological processes. Transcriptomics is a continually growing field of research utilized in biomarker discovery. The transcriptomic profile of interstitial cells of Cajal (ICC), which serve as slow-wave electrical pacemakers for gastrointestinal (GI) smooth muscle, has yet to be uncovered. Using copGFP-labeled ICC mice and flow cytometry, we isolated ICC populations from the murine small intestine and colon and obtained their transcriptomes. In analyzing the transcriptome, we identified a unique set of ICC-restricted markers including transcription factors, epigenetic enzymes/regulators, growth factors, receptors, protein kinases/phosphatases, and ion channels/transporters. This analysis provides new and unique insights into the cellular and biological functions of ICC in GI physiology. Additionally, we constructed an interactive ICC genome browser (http://med.unr.edu/physio/transcriptome) based on the UCSC genome database. To our knowledge, this is the first online resource that provides a comprehensive library of all known genetic transcripts expressed in primary ICC. Our genome browser offers a new perspective into the alternative expression of genes in ICC and provides a valuable reference for future functional studies.

Na+-K+-Cl- cotransporter (NKCC) maintains the chloride gradient to sustain pacemaker activity in interstitial cells of Cajal

Interstitial cells of Cajal (ICC) generate electrical slow waves by coordinated openings of ANO1 channels, a Ca²⁺-activated Cl^- (CaCC) conductance. Efflux of Cl^- during slow waves must be significant, as there is high current density during slow-wave currents and slow waves are of sufficient magnitude to depolarize the syncytium of smooth muscle cells and PDGFRα⁺ cells to which they are electrically coupled. We investigated how the driving force for Cl^- current is maintained in ICC. We found robust expression of Slc12a2 (which encodes an Na⁺-K⁺-Cl^- cotransporter, NKCC1) and immunohistochemical confirmation that NKCC1 is expressed in ICC. With the use of the gramicidin permeabilized-patch technique, which is reported to not disturb [Cl^-]_i, the reversal potential for spontaneous transient inward currents (E_STICs) was -10.5 mV. This value corresponds to the peak of slow waves when they are recorded directly from ICC in situ. Inhibition of NKCC1 with bumetanide shifted E_STICs to more negative potentials within a few minutes and reduced pacemaker activity. Bumetanide had no direct effects on ANO1 or Ca_V3.2 channels expressed in HEK293 cells or L-type Ca²⁺ currents. Reducing extracellular Cl^- to 10 mM shifted E_STICs to positive potentials as predicted by the Nernst equation. The relatively rapid shift in E_STICs when NKCC1 was blocked suggests that significant changes in the transmembrane Cl^- gradient occur during the slow-wave cycle, possibly within microdomains formed between endoplasmic reticulum and the plasma membrane in ICC. Recovery of Cl^- via NKCC1 might have additional consequences on shaping the waveforms of slow waves via Na⁺ entry into microdomains.

Influence of intracellular Ca2+ and alternative splicing on the pharmacological profile of ANO1 channels

Anoctamin-1 (ANO1) is a Ca(2+)-activated Cl(-) channel expressed in many types of cells. Splice variants of ANO1 have been shown to influence the biophysical properties of conductance. It has been suggested that several new antagonists of ANO1 with relatively high affinity and selectivity might be useful for experimental and, potentially, therapeutic purposes. We investigated the effects of intracellular Ca(2+) concentration ([Ca(2+)]i) at 100-1,000 nM, a concentration range that might be achieved in cells during physiological activation of ANO1 channels, on blockade of ANO1 channels expressed in HEK-293 cells. Whole cell and excised patch configurations of the patch-clamp technique were used to perform tests on a variety of naturally occurring splice variants of ANO1. Blockade of ANO1 currents with aminophenylthiazole (T16Ainh-A01) was highly dependent on [Ca(2+)]i Increasing [Ca(2+)]i reduced the potency of this blocker. Similar Ca(2+)-dependent effects were also observed with benzbromarone. Experiments on excised, inside-out patches showed that the diminished potency of the blockers caused by intracellular Ca(2+) might involve a competitive interaction for a common binding site or repulsion of the blocking drugs by electrostatic forces at the cytoplasmic surface of the channels. The degree of interaction between the channel blockers and [Ca(2+)]i depends on the splice variant expressed. These experiments demonstrate that the efficacy of ANO1 antagonists depends on [Ca(2+)]i, suggesting a need for caution when ANO1 blockers are used to determine the role of ANO1 in physiological functions and in their use as therapeutic agents.

Spontaneous Ca(2+) transients in interstitial cells of Cajal located within the deep muscular plexus of the murine small intestine

KEY POINTS

Interstitial cells of Cajal at the level of the deep muscular plexus (ICC-DMP) in the small intestine generate spontaneous Ca(2+) transients that consist of localized Ca(2+) events and limited propagating Ca(2+) waves. Ca(2+) transients in ICC-DMP display variable characteristics: from discrete, highly localized Ca(2+) transients to regionalized Ca(2+) waves with variable rates of occurrence, amplitude, duration and spatial spread. Ca(2+) transients fired stochastically, with no cellular or multicellular rhythmic activity being observed. No correlation was found between the firing sites in adjacent cells. Ca(2+) transients in ICC-DMP are suppressed by the ongoing release of inhibitory neurotransmitter(s). Functional intracellular Ca(2+) stores are essential for spontaneous Ca(2+) transients, and the sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) pump is necessary for maintenance of spontaneity. Ca(2+) release mechanisms involve both ryanodine receptors (RyRs) and inositol triphosphate receptors (InsP3 Rs). Release from these channels is interdependent. ICC express transcripts of multiple RyRs and InsP3 Rs, with Itpr1 and Ryr2 subtypes displaying the highest expression.

ABSTRACT

Interstitial cells of Cajal in the deep muscular plexus of the small intestine (ICC-DMP) are closely associated with varicosities of enteric motor neurons and generate responses contributing to neural regulation of intestinal motility. Responses of ICC-DMP are mediated by activation of Ca(2+) -activated Cl(-) channels; thus, Ca(2+) signalling is central to the behaviours of these cells. Confocal imaging was used to characterize the nature and mechanisms of Ca(2+) transients in ICC-DMP within intact jejunal muscles expressing a genetically encoded Ca(2+) indicator (GCaMP3) selectively in ICC. ICC-DMP displayed spontaneous Ca(2+) transients that ranged from discrete, localized events to waves that propagated over variable distances. The occurrence of Ca(2+) transients was highly variable, and it was determined that firing was stochastic in nature. Ca(2+) transients were tabulated in multiple cells within fields of view, and no correlation was found between the events in adjacent cells. TTX (1 μm) significantly increased the occurrence of Ca(2+) transients, suggesting that ICC-DMP contributes to the tonic inhibition conveyed by ongoing activity of inhibitory motor neurons. Ca(2+) transients were minimally affected after 12 min in Ca(2+) free solution, indicating these events do not depend immediately upon Ca(2+) influx. However, inhibitors of sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) pump and blockers of inositol triphosphate receptor (InsP3 R) and ryanodine receptor (RyR) channels blocked ICC Ca(2+) transients. These data suggest an interdependence between RyR and InsP3 R in the generation of Ca(2+) transients. Itpr1 and Ryr2 were the dominant transcripts expressed by ICC. These findings provide the first high-resolution recording of the subcellular Ca(2+) dynamics that control the behaviour of ICC-DMP in situ.

Airway acidification initiates host defense abnormalities in cystic fibrosis mice

Cystic fibrosis (CF) is caused by mutations in the gene that encodes the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel. In humans and pigs, the loss of CFTR impairs respiratory host defenses, causing airway infection. But CF mice are spared. We found that in all three species, CFTR secreted bicarbonate into airway surface liquid. In humans and pigs lacking CFTR, unchecked H(+) secretion by the nongastric H(+)/K(+) adenosine triphosphatase (ATP12A) acidified airway surface liquid, which impaired airway host defenses. In contrast, mouse airways expressed little ATP12A and secreted minimal H(+); consequently, airway surface liquid in CF and non-CF mice had similar pH. Inhibiting ATP12A reversed host defense abnormalities in human and pig airways. Conversely, expressing ATP12A in CF mouse airways acidified airway surface liquid, impaired defenses, and increased airway bacteria. These findings help explain why CF mice are protected from infection and nominate ATP12A as a potential therapeutic target for CF.

Myocardial Telocytes: A New Player in Electric Circuitry of the Heart

The heart is an electrically conducting organ with networked bioelectric currents that transverse a large segment of interstitial space interspersed with the muscular parenchyma. Non-excitable connective cells in the interstitial space contributed importantly to many structural, biochemical, and physiological activities of cardiac homeostasis. However, contribution of interstitial cells in the cardiac niche has long been neglected. Telocyte is recently recognized as a distinct class of interstitial cell that resides in a wide array of tissues including in the epicardium, myocardium, and endocardium of the heart. They are increasingly described to conduct ionic currents that may have significant implications in bioelectric signaling. In this review, we highlight the significance of telocytes in such connectivity and conductivity within the interstitial bioelectric network in tissue homeostasis.

A novel exon in the human Ca2+-activated Cl- channel Ano1 imparts greater sensitivity to intracellular Ca2

Anoctamin 1 (Ano1; TMEM16A) is a Ca(2+)-activated Cl(-) channel (CACC) expressed in interstitial cells of Cajal. The mechanisms by which Ca(2+) regulates Ano1 are incompletely understood. In the gastrointestinal tract, Ano1 is required for normal slow wave activity and is involved in regulating cell proliferation. Splice variants of Ano1 have varying electrophysiological properties and altered expression in disease states. Recently, we identified a transcript for human Ano1 containing a novel exon-"exon 0" upstream of and in frame with exon 1. The electrophysiological properties of this longer Ano1 isoform are unknown. Our aim was to determine the functional contribution of the newly identified exon to the Ca(2+) sensitivity and electrophysiological properties of Ano1. Constructs with [Ano1(+0)] or without [Ano1(-0)] the newly identified exon were transfected into human embryonic kidney-293 cells. Voltage-clamp electrophysiology was used to determine voltage- and time-dependent parameters of whole cell Cl(-) currents between isoforms with varying concentrations of intracellular Ca(2+), extracellular anions, or Cl(-) channel inhibitors. We found that exon 0 did not change voltage sensitivity and had no impact on the relative permeability of Ano1 to most anions. Ano1(+0) exhibited greater changes in current density but lesser changes in kinetics than Ano1(-0) in response to varying intracellular Ca(2+). The CACC inhibitor niflumic acid inhibited current with greater efficacy and higher potency against Ano1(+0) compared with Ano1(-0). Likewise, the Ano1 inhibitor T16Ainh-A01 reduced Ano1(+0) more than Ano1(-0). In conclusion, human Ano1 containing exon 0 imparts its Cl(-) current with greater sensitivity to intracellular Ca(2+) and CACC inhibitors.

Uterine Contractility in the Nonpregnant Mouse: Changes During the Estrous Cycle and Effects of Chloride Channel Blockade

Mechanisms involved in the generation of spontaneous uterine contractions are not fully understood. Kit-expressing interstitial cells of Cajal are pacemakers of contractile rhythm in other visceral organs, and recent studies describe a role for Ca(2+)-activated Cl(-) currents as the initiating conductance in these cells. The existence and role of similar specialized pacemaker cells in the nonpregnant uterus remains undetermined. Spontaneous contractility patterns were characterized throughout the estrous cycle in isolated, nonpregnant mouse uteri using spatiotemporal mapping and tension recordings. During proestrus, estrus, and diestrus, contraction origin predominated in the oviduct end of the uterus, suggesting the existence of a dominant pacemaker site. Propagation speed of contractions during estrus and diestrus were significantly slower than in proestrus and metestrus. Five major patterns of activity were predominantly exhibited in particular stages: quiescent (diestrus), high-frequency phasic (proestrus), low-frequency phasic (estrus), multivariant (metestrus), and complex. Kit-immunopositive cells reminiscent of pacemaking ICCs were not consistently observed within the uterus. Niflumic acid (10 μM), anthracene-9-carboxylic acid (0.1-1 mM), and 5-nitro-2-(3-phenylpropylamino)benzoic acid (10 μM) each reduced the frequency of spontaneous contractions, suggesting involvement of Cl(-) channels in generating spontaneous uterine motor activity. It is unlikely that this conductance is generated by the Ca(2+)-activated Cl(-) channels, anoctamin-1 and CLCA4, as immunohistochemical labeling did not reveal protein expression within muscle or pacemaker cell networks. In summary, these results suggest that spontaneous uterine contractions may be generated by a Kit-negative pacemaker cell type or uterine myocytes, likely involving the activity of a yet-unidentified Cl(-) channel.

Protease-activated receptors modulate excitability of murine colonic smooth muscles by differential effects on interstitial cells

Protease-activated receptors (PARs) are G protein-coupled receptors activated by proteolytic cleavage at their amino termini by serine proteases. PAR activation contributes to the inflammatory response in the gastrointestinal (GI) tract and alters GI motility, but little is known about the specific cells within the tunica muscularis that express PARs and the mechanisms leading to contractile responses. Using real time PCR, we found PARs to be expressed in smooth muscle cells (SMCs), interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor α positive (PDGFRα(+)) cells. The latter cell-type showed dominant expression of F2r (encodes PAR1) and F2rl1 (encodes PAR2). Contractile and intracellular electrical activities were measured to characterize the integrated responses to PAR activation in whole muscles. Cells were isolated and ICC and PDGFRα(+) cells were identified by constitutive expression of fluorescent reporters. Thrombin (PAR1 agonist) and trypsin (PAR2 agonist) caused biphasic responses in colonic muscles: transient hyperpolarization and relaxation followed by repolarization and excitation. The inhibitory phase was blocked by apamin, revealing a distinct excitatory component. Patch clamp studies showed that the inhibitory response was mediated by activation of small conductance calcium-activated K(+) channels in PDGFRα(+) cells, and the excitatory response was mediated by activation of a Cl(-) conductance in ICC. SMCs contributed little to PAR responses in colonic muscles. In summary, PARs regulate the excitability of colonic muscles; different conductances are activated in each cell type of the SMC-ICC-PDGFRα(+) cell (SIP) syncytium. Motor responses to PAR agonists are integrated responses of the SIP syncytium.

Intracellular Ca(2+) release from endoplasmic reticulum regulates slow wave currents and pacemaker activity of interstitial cells of Cajal

Interstitial cells of Cajal (ICC) provide pacemaker activity in gastrointestinal muscles that underlies segmental and peristaltic contractions. ICC generate electrical slow waves that are due to large-amplitude inward currents resulting from anoctamin 1 (ANO1) channels, which are Ca(2+)-activated Cl(-) channels. We investigated the hypothesis that the Ca(2+) responsible for the stochastic activation of ANO1 channels during spontaneous transient inward currents (STICs) and synchronized activation of ANO1 channels during slow wave currents comes from intracellular Ca(2+) stores. ICC, obtained from the small intestine of Kit(+/copGFP) mice, were studied under voltage and current clamp to determine the effects of blocking Ca(2+) uptake into stores and release of Ca(2+) via inositol 1,4,5-trisphosphate (IP3)-dependent and ryanodine-sensitive channels. Cyclocpiazonic acid, thapsigargin, 2-APB, and xestospongin C inhibited STICs and slow wave currents. Ryanodine and tetracaine also inhibited STICs and slow wave currents. Store-active compounds had no direct effects on ANO1 channels expressed in human embryonic kidney-293 cells. Under current clamp, store-active drugs caused significant depolarization of ICC and reduced spontaneous transient depolarizations (STDs). After block of ryanodine receptors with ryanodine and tetracaine, repolarization did not restore STDs. ANO1 expressed in ICC has limited access to cytoplasmic Ca(2+) concentration, suggesting that pacemaker activity depends on Ca(2+) dynamics in restricted microdomains. Our data from studies of isolated ICC differ somewhat from studies on intact muscles and suggest that release of Ca(2+) from both IP3 and ryanodine receptors is important in generating pacemaker activity in ICC.

Dominant role of interstitial cells of Cajal in nitrergic relaxation of murine lower oesophageal sphincter

Oesophageal achalasia is a disease known to result from reduced relaxation of the lower oesophageal sphincter (LES). Nitric oxide (NO) is one of the main inhibitory transmitters. NO-sensitive guanylyl cyclase (NO-GC) acts as the key target of NO and, by the generation of cGMP, mediates nitrergic relaxation in the LES. To date, the exact mechanism of nitrergic LES relaxation is still insufficiently elucidated. To clarify the role of NO-GC in LES relaxation, we used cell-specific knockout (KO) mouse lines for NO-GC. These include mice lacking NO-GC in smooth muscle cells (SMC-GCKO), in interstitial cells of Cajal (ICC-GCKO) and in both SMC/ICC (SMC/ICC-GCKO). We applied oesophageal manometry to study the functionality of LES in vivo. Isometric force studies were performed to monitor LES responsiveness to exogenous NO and electric field stimulation of intrinsic nerves in vitro. Cell-specific expression/deletion of NO-GC was monitored by immunohistochemistry. Swallowing-induced LES relaxation is strongly reduced by deletion of NO-GC in ICC. Basal LES tone is affected by NO-GC deletion in either SMC or ICC. Lack of NO-GC in both cells leads to a complete interruption of NO-induced relaxation and, therefore, to an achalasia-like phenotype similar to that seen in global GCKO mice. Our data indicate that regulation of basal LES tone is based on a dual mechanism mediated by NO-GC in SMC and ICC whereas swallow-induced LES relaxation is mainly regulated by nitrergic mechanisms in ICC.

Heme oxygenase-1 has antitumoral effects in colorectal cancer: involvement of p53

The expression of heme oxygenase-1 (HO-1) has been shown to be up-regulated in colorectal cancer (CRC), but the role it plays in this cancer type has not yet been addressed. The aims of this study have been to analyze HO-1 expression in human invasive CRC, evaluate its correlation with clinical and histo-pathological parameters and to investigate the mechanisms through which the enzyme influences tumor progression. We confirmed that HO-1 was over-expressed in human invasive CRC and found that the expression of the enzyme was associated with a longer overall survival time. In addition, we observed in a chemically-induced CRC animal model that total and nuclear HO-1 expression increases with tumor progression. Our investigation of the mechanisms involved in HO-1 action in CRC demonstrates that the protein reduces cell viability through induction of cell cycle arrest and apoptosis and, importantly, that a functional p53 tumor suppressor protein is required for these effects. This reduction in cell viability is accompanied by modulation of the levels of p21, p27, and cyclin D1 and by modulation of Akt and PKC pathways. Altogether, our results demonstrate an antitumoral role of HO-1 and points to the importance of p53 status in this antitumor activity.

Interstitial cells: regulators of smooth muscle function

Smooth muscles are complex tissues containing a variety of cells in addition to muscle cells. Interstitial cells of mesenchymal origin interact with and form electrical connectivity with smooth muscle cells in many organs, and these cells provide important regulatory functions. For example, in the gastrointestinal tract, interstitial cells of Cajal (ICC) and PDGFRα(+) cells have been described, in detail, and represent distinct classes of cells with unique ultrastructure, molecular phenotypes, and functions. Smooth muscle cells are electrically coupled to ICC and PDGFRα(+) cells, forming an integrated unit called the SIP syncytium. SIP cells express a variety of receptors and ion channels, and conductance changes in any type of SIP cell affect the excitability and responses of the syncytium. SIP cells are known to provide pacemaker activity, propagation pathways for slow waves, transduction of inputs from motor neurons, and mechanosensitivity. Loss of interstitial cells has been associated with motor disorders of the gut. Interstitial cells are also found in a variety of other smooth muscles; however, in most cases, the physiological and pathophysiological roles for these cells have not been clearly defined. This review describes structural, functional, and molecular features of interstitial cells and discusses their contributions in determining the behaviors of smooth muscle tissues.

The significance of interstitial cells in neurogastroenterology

Smooth muscle layers of the gastrointestinal tract consist of a heterogeneous population of cells that include enteric neurons, several classes of interstitial cells of mesenchymal origin, a variety of immune cells and smooth muscle cells (SMCs). Over the last number of years the complexity of the interactions between these cell types has begun to emerge. For example, interstitial cells, consisting of both interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive (PDGFRα(+)) cells generate pacemaker activity throughout the gastrointestinal (GI) tract and also transduce enteric motor nerve signals and mechanosensitivity to adjacent SMCs. ICC and PDGFRα(+) cells are electrically coupled to SMCs possibly via gap junctions forming a multicellular functional syncytium termed the SIP syncytium. Cells that make up the SIP syncytium are highly specialized containing unique receptors, ion channels and intracellular signaling pathways that regulate the excitability of GI muscles. The unique role of these cells in coordinating GI motility is evident by the altered motility patterns in animal models where interstitial cell networks are disrupted. Although considerable advances have been made in recent years on our understanding of the roles of these cells within the SIP syncytium, the full physiological functions of these cells and the consequences of their disruption in GI muscles have not been clearly defined. This review gives a synopsis of the history of interstitial cell discovery and highlights recent advances in structural, molecular expression and functional roles of these cells in the GI tract.