Expression of transient receptor potential channels and two-pore potassium channels in subtypes of vagal afferent neurons in rat

Lipids shape brain function through ion channel and receptor modulations: physiological mechanisms and clinical perspectives

Lipids represent the most abundant molecular type in the brain, with a fat content of ∼60% of the dry brain weight in humans. Despite this fact, little attention has been paid to circumscribe the dynamic role of lipids in brain function and disease. Membrane lipids such as cholesterol, phosphoinositide, sphingolipids, arachidonic acid, and endocannabinoids finely regulate both synaptic receptors and ion channels that ensure critical neural functions. After a brief introduction on brain lipids and their respective properties, we review here their role in regulating synaptic function and ion channel activity, action potential propagation, neuronal development, and functional plasticity and their contribution in the development of neurological and neuropsychiatric diseases. We also provide possible directions for future research on lipid function in brain plasticity and diseases.

Gaps in our understanding of how vagal afferents to the small intestinal mucosa detect luminal stimuli

Vagal afferents to the gastrointestinal tract are crucial for the regulation of food intake, signaling negative feedback that contributes to satiation and positive feedback that produces appetition and reward. Vagal afferents to the small intestinal mucosa contribute to this regulation by sensing luminal stimuli and reporting this information to the brain. These afferents respond to mechanical, chemical, thermal, pH, and osmolar stimuli, as well as to bacterial products and immunogens. Surprisingly, little is known about how these stimuli are transduced by vagal mucosal afferents or how their transduction is organized among these afferents' terminals. Furthermore, the effects of stimulus concentration ranges or physiological stimuli on vagal activity have not been examined for some of these stimuli. Also, detection of luminal stimuli has rarely been examined in rodents, which are most frequently used for studying small intestinal innervation. Here we review what is known about stimulus detection by vagal mucosal afferents and illustrate the complexity of this detection using nutrients as an exemplar. The accepted model proposes that nutrients bind to taste receptors on enteroendocrine cells (EECs), which excite them, causing the release of hormones that stimulate vagal mucosal afferents. However, evidence reviewed here suggests that although this model accounts for many aspects of vagal signaling about nutrients, it cannot account for all aspects. A major goal of this review is therefore to evaluate what is known about nutrient absorption and detection and, based on this evaluation, identify candidate mucosal cells and structures that could cooperate with EECs and vagal mucosal afferents in stimulus detection.

TRPV1 enhances cholecystokinin signaling in primary vagal afferent neurons and mediates the central effects on spontaneous glutamate release in the NTS

The gut peptide cholecystokinin (CCK) is released during feeding and promotes satiation by increasing excitation of vagal afferent neurons that innervate the upper gastrointestinal tract. Vagal afferent neurons express CCK1 receptors (CCK1Rs) in the periphery and at central terminals in the nucleus of the solitary tract (NTS). While the effects of CCK have been studied for decades, CCK receptor signaling and coupling to membrane ion channels are not entirely understood. Previous findings have implicated L-type voltage-gated calcium channels as well as transient receptor potential (TRP) channels in mediating the effects of CCK, but the lack of selective pharmacology has made determining the contributions of these putative mediators difficult. The nonselective ion channel transient receptor potential vanilloid subtype 1 (TRPV1) is expressed throughout vagal afferent neurons and controls many forms of signaling, including spontaneous glutamate release onto NTS neurons. Here we tested the hypothesis that CCK1Rs couple directly to TRPV1 to mediate vagal signaling using fluorescent calcium imaging and brainstem electrophysiology. We found that CCK signaling at high concentrations (low-affinity binding) was potentiated in TRPV1-containing afferents and that TRPV1 itself mediated the enhanced CCK1R signaling. While competitive antagonism of TRPV1 failed to alter CCK1R signaling, TRPV1 pore blockade or genetic deletion (TRPV1 KO) significantly reduced the CCK response in cultured vagal afferents and eliminated its ability to increase spontaneous glutamate release in the NTS. Together, these results establish that TRPV1 mediates the low-affinity effects of CCK on vagal afferent activation and control of synaptic transmission in the brainstem.NEW & NOTEWORTHY Cholecystokinin (CCK) signaling via the vagus nerve reduces food intake and produces satiation, yet the signaling cascades mediating these effects remain unknown. Here we report that the capsaicin receptor transient receptor potential vanilloid subtype 1 (TRPV1) potentiates CCK signaling in the vagus and mediates the ability of CCK to control excitatory synaptic transmission in the nucleus of the solitary tract. These results may prove useful in the future development of CCK/TRPV1-based therapeutic interventions.

Mechanical activation of vagal afferents involves opposing cation and TREK1 currents and NO regulation

Vagal afferents convey signals of mechanical stimulation in the gut to the brain, which is essential for the regulation of food intake. However, ion channels sensing mechanical stimuli are not fully understood. This study aimed to examine the ionic currents activated by mechanical stimulation and a possible neuro-modulatory role of nitric oxide on vagal afferents. Nodose neuronal currents and potentials, and intestinal afferent firing by mechanical stimulation were measured by whole-cell patch clamp, and in vitro afferent recording, respectively. Osmotically activated cation and two-pore domain K+ currents were identified in nodose neurons. The membrane potential displayed a biphasic change under hypotonic stimulation. Cation channel-mediated depolarization was followed by a hyperpolarization mediated by K+ channels. The latter was inhibited by l-methionine (TREK1 channel inhibitor) and l-NNA (nitric oxide synthase inhibitor). Correspondingly, mechanical stimulation activated opposing cation and TREK1 currents. NOS inhibition decreased TREK1 currents and potentiated jejunal afferent nerve firing induced by mechanical stimuli. This study suggested a novel activation mechanism of ion channels underlying adaptation under mechanical distension in vagal afferent neurons. The guts' ability to perceive mechanical stimuli is vital in determining how it responds to food intake. The mechanosensation through ion channels could initiate and control gut function.

Mechanism and potential treatments for gastrointestinal dysfunction in patients with COVID-19

The global coronavirus disease 2019 (COVID-19) has become one of the biggest threats to the world since 2019. The respiratory and gastrointestinal tracts are the main targets for severe acute respiratory syndrome coronavirus 2 infection for they highly express angiotensin-converting enzyme-2 and transmembrane protease serine 2. In patients suffering from COVID-19, gastrointestinal symptoms have ranged from 12% to 61%. Anorexia, nausea and/or vomiting, diarrhea, and abdominal pain are considered to be the main gastrointestinal symptoms of COVID-19. It has been reported that the direct damage of intestinal mucosal epithelial cells, malnutrition, and intestinal flora disorders are involved in COVID-19. However, the underlying mechanisms remain unclear. Thus, in this study, we reviewed and discussed the correlated mechanisms that cause gastrointestinal symptoms in order to help to develop the treatment strategy and build an appropriate guideline for medical workers.

Distribution and Assembly of TRP Ion Channels

In the last several decades, a large family of ion channels have been identified and studied intensively as cellular sensors for diverse physical and/or chemical stimuli. Named transient receptor potential (TRP) channels, they play critical roles in various aspects of cellular physiology. A large number of human hereditary diseases are found to be linked to TRP channel mutations, and their dysregulations lead to acute or chronical health problems. As TRP channels are named and categorized mostly based on sequence homology rather than functional similarities, they exhibit substantial functional diversity. Rapid advances in TRP channel study have been made in recent years and reported in a vast body of literature; a summary of the latest advancements becomes necessary. This chapter offers an overview of current understandings of TRP channel distribution and subunit assembly.

Neural signalling of gut mechanosensation in ingestive and digestive processes

Eating and drinking generate sequential mechanosensory signals along the digestive tract. These signals are communicated to the brain for the timely initiation and regulation of diverse ingestive and digestive processes - ranging from appetite control and tactile perception to gut motility, digestive fluid secretion and defecation - that are vital for the proper intake, breakdown and absorption of nutrients and water. Gut mechanosensation has been investigated for over a century as a common pillar of energy, fluid and gastrointestinal homeostasis, and recent discoveries of specific mechanoreceptors, contributing ion channels and the well-defined circuits underlying gut mechanosensation signalling and function have further expanded our understanding of ingestive and digestive processes at the molecular and cellular levels. In this Review, we discuss our current understanding of the generation of mechanosensory signals from the digestive periphery, the neural afferent pathways that relay these signals to the brain and the neural circuit mechanisms that control ingestive and digestive processes, focusing on the four major digestive tract parts: the oral and pharyngeal cavities, oesophagus, stomach and intestines. We also discuss the clinical implications of gut mechanosensation in ingestive and digestive disorders.

Are TREK Channels Temperature Sensors?

Internal human body normal temperature fluctuates between 36.5 and 37.5°C and it is generally measured in the oral cavity. Interestingly, most electrophysiological studies on the functioning of ion channels and their role in neuronal behavior are carried out at room temperature, which usually oscillates between 22 and 24°C, even when thermosensitive channels are studied. We very often forget that if the core of the body reached that temperature, the probability of death from cardiorespiratory arrest would be extremely high. Does this mean that we are studying ion channels in dying neurons? Thousands of electrophysiological experiments carried out at these low temperatures suggest that most neurons tolerate this aggression quite well, at least for the duration of the experiments. This also seems to happen with ion channels, although studies at different temperatures indicate large changes in both, neuron and channel behavior. It is known that many chemical, physical and therefore physiological processes, depend to a great extent on body temperature. Temperature clearly affects the kinetics of numerous events such as chemical reactions or conformational changes in proteins but, what if these proteins constitute ion channels and these channels are specifically designed to detect changes in temperature? In this review, we discuss the importance of the potassium channels of the TREK subfamily, belonging to the recently discovered family of two-pore domain channels, in the transduction of thermal sensitivity in different cell types.

Serotonin-induced vascular permeability is mediated by transient receptor potential vanilloid 4 in the airways and upper gastrointestinal tract of mice

Endothelial and epithelial cells form physical barriers that modulate the exchange of fluid and molecules. The integrity of these barriers can be influenced by signaling through G protein-coupled receptors (GPCRs) and ion channels. Serotonin (5-HT) is an important vasoactive mediator of tissue edema and inflammation. However, the mechanisms that drive 5-HT-induced plasma extravasation are poorly defined. The Transient Receptor Potential Vanilloid 4 (TRPV4) ion channel is an established enhancer of signaling by GPCRs that promote inflammation and endothelial barrier disruption. Here, we investigated the role of TRPV4 in 5-HT-induced plasma extravasation using pharmacological and genetic approaches. Activation of either TRPV4 or 5-HT receptors promoted significant plasma extravasation in the airway and upper gastrointestinal tract of mice. 5-HT-mediated extravasation was significantly reduced by pharmacological inhibition of the 5-HT2A receptor subtype, or with antagonism or deletion of TRPV4, consistent with functional interaction between 5-HT receptors and TRPV4. Inhibition of receptors for the neuropeptides substance P (SP) or calcitonin gene-related peptide (CGRP) diminished 5-HT-induced plasma extravasation. Supporting studies assessing treatment of HUVEC with 5-HT, CGRP, or SP was associated with ERK phosphorylation. Exposure to the TRPV4 activator GSK1016790A, but not 5-HT, increased intracellular Ca2+ in these cells. However, 5-HT pre-treatment enhanced GSK1016790A-mediated Ca2+ signaling, consistent with sensitization of TRPV4. The functional interaction was further characterized in HEK293 cells expressing 5-HT2A to reveal that TRPV4 enhances the duration of 5-HT-evoked Ca2+ signaling through a PLA2 and PKC-dependent mechanism. In summary, this study demonstrates that TRPV4 contributes to 5-HT2A-induced plasma extravasation in the airways and upper GI tract, with evidence supporting a mechanism of action involving SP and CGRP release.

TRP channels in COVID-19 disease: Potential targets for prevention and treatment

Coronavirus disease 2019 [COVID-19] is a global health threat caused by severe acute respiratory syndrome coronavirus 2 [SARS-CoV2] that requires two proteins for entry: angiotensin-converting enzyme 2 [ACE2] and -membrane protease serine 2 [TMPRSS2]. Many patients complain from pneumonia, cough, fever, and gastrointestinal (GI) problems. Notably, different TRP channels are expressed in various tissues infected by SARS-CoV-2. TRP channels are cation channels that show a common architecture with high permeability to calcium [Ca²⁺] in most sub-families. Literature review shed light on the possible role of TRP channels in COVID-19 disease. TRP channels may take part in inflammation, pain, fever, anosmia, ageusia, respiratory, cardiovascular, GI and neurological complications related to COVID-19. Also, TRP channels could be the targets for many active compounds that showed effectiveness against SARS-CoV-2. Desensitization or blocking TRP channels by antibodies, aptamers, small molecules or venoms can be an option for COVID-19 prevention and future treatment. This review provides insights into the involvement of TRP channels in different symptoms and mechanisms of SARS-CoV-2 , potential treatments targeting these channels and highlights missing gaps in literature.

Sensory Nociceptive Neurons Contribute to Host Protection During Enteric Infection With Citrobacter rodentium

BACKGROUND

Neurons are an integral component of the immune system that functions to coordinate responses to bacterial pathogens. Sensory nociceptive neurons that can detect bacterial pathogens are found throughout the body with dense innervation of the intestinal tract.

METHODS

In this study, we assessed the role of these nerves in the coordination of host defenses to Citrobacter rodentium. Selective ablation of nociceptive neurons significantly increased bacterial burden 10 days postinfection and delayed pathogen clearance.

RESULTS

Because the sensory neuropeptide CGRP (calcitonin gene-related peptide) regulates host responses during infection of the skin, lung, and small intestine, we assessed the role of CGRP receptor signaling during C rodentium infection. Although CGRP receptor blockade reduced certain proinflammatory gene expression, bacterial burden and Il-22 expression was unaffected.

CONCLUSIONS

Our data highlight that sensory nociceptive neurons exert a significant host protective role during C rodentium infection, independent of CGRP receptor signaling.

Hereditary gingival fibromatosis associated with the missense mutation of the KCNK4 gene

Hereditary gingival fibromatosis (HGF) is a rare oral condition that may appear as an isolated entity or as part of a genetic disease or syndrome. Molecular and biochemical mechanisms that trigger this pathologic process are not completely understood. In this article, we present a rare case of hereditary gingival fibromatosis in conjunction with a syndromic phenotype, associated with a rare missense mutation of the KCNK4 gene. This mutation induces a change in the structure of the TRAAK channel belonging to the 2-pore potassium channels. The gain of function promoted by the mutation could represent the pathogenetic basis of gingival fibromatosis.

Cannabidiol activation of vagal afferent neurons requires TRPA1

Vagal afferent neurons abundantly express excitatory transient receptor potential (TRP) channels, which strongly influence afferent signaling. Cannabinoids have been identified as direct agonists of TRP channels, including TRPA1 and TRPV1, suggesting that exogenous cannabinoids may influence vagal signaling via TRP channel activation. The diverse therapeutic effects of electrical vagus nerve stimulation also result from administration of the nonpsychotropic cannabinoid, cannabidiol (CBD); however, the direct effects of CBD on vagal afferent signaling remain unknown. We investigated actions of CBD on vagal afferent neurons, using calcium imaging and electrophysiology. CBD produced strong excitatory effects in neurons expressing TRPA1. CBD responses were prevented by removal of bath calcium, ruthenium red, and the TRPA1 antagonist A967079, but not the TRPV1 antagonist SB366791, suggesting an essential role for TRPA1. These pharmacological experiments were confirmed using genetic knockouts where TRPA1 KO mice lacked CBD responses, whereas TRPV1 knockout (KO) mice exhibited CBD-induced activation. We also characterized CBD-provoked inward currents at resting potentials in vagal afferents expressing TRPA1 that were absent in TRPA1 KO mice, but persisted in TRPV1 KO mice. CBD also inhibited voltage-activated sodium conductances in A-fiber, but not in C-fiber afferents. To simulate adaptation, resulting from chronic cannabis use, we administered cannabis extract vapor daily for 3 wk. Cannabis exposure reduced the magnitude of CBD responses, likely due to a loss of TRPA1 signaling. Together, these findings detail a novel excitatory action of CBD at vagal afferent neurons, which requires TRPA1 and may contribute to the vagal mimetic effects of CBD and adaptation following chronic cannabis use.NEW & NOTEWORTHY CBD usage has increased with its legalization. The clinical efficacy of CBD has been demonstrated for conditions including some forms of epilepsy, depression, and anxiety that are also treatable by vagus nerve stimulation. We found CBD exhibited direct excitatory effects on vagal afferent neurons that required TRPA1, were augmented by TRPV1, and attenuated following chronic cannabis vapor exposure. These effects may contribute to vagal mimetic effects of CBD and adaptation after chronic cannabis use.

Targeting Chemosensory Ion Channels in Peripheral Swallowing-Related Regions for the Management of Oropharyngeal Dysphagia

Oropharyngeal dysphagia, or difficulty in swallowing, is a major health problem that can lead to serious complications, such as pulmonary aspiration, malnutrition, dehydration, and pneumonia. The current clinical management of oropharyngeal dysphagia mainly focuses on compensatory strategies and swallowing exercises/maneuvers; however, studies have suggested their limited effectiveness for recovering swallowing physiology and for promoting neuroplasticity in swallowing-related neuronal networks. Several new and innovative strategies based on neurostimulation in peripheral and cortical swallowing-related regions have been investigated, and appear promising for the management of oropharyngeal dysphagia. The peripheral chemical neurostimulation strategy is one of the innovative strategies, and targets chemosensory ion channels expressed in peripheral swallowing-related regions. A considerable number of animal and human studies, including randomized clinical trials in patients with oropharyngeal dysphagia, have reported improvements in the efficacy, safety, and physiology of swallowing using this strategy. There is also evidence that neuroplasticity is promoted in swallowing-related neuronal networks with this strategy. The targeting of chemosensory ion channels in peripheral swallowing-related regions may therefore be a promising pharmacological treatment strategy for the management of oropharyngeal dysphagia. In this review, we focus on this strategy, including its possible neurophysiological and molecular mechanisms.

Use of viruses for interrogating viscera-specific projections in central nervous system

Each internal organ may perform many different functions under central regulation, yet how these processes are coordinated is poorly understood. The last three decades have witnessed a renaissance in tract tracing with genetically engineered strains of viruses that rapidly interrogate viscera-specific projections in the CNS. The application of novel methods to study cell type-specific projections through trans-synaptically transmitted virus 'label' highlights projections exclusively originating from neurons expressing a very specific molecular phenotype. This has opened the door to neuroanatomical studies interrogating organ-specific projections in the CNS at an unprecedented scale. In this contribution to the Special Issue we present an overview of the present state and of future opportunities in charting viscera-brain specific connectivity and in linking brain circuits to internal organ function.

Chronic stress induces hypersensitivity of murine gastric vagal afferents

BACKGROUND

Stress exposure is known to trigger and exacerbate functional dyspepsia (FD) symptoms. Increased gastric sensitivity to food-related stimuli is widely observed in FD patients and is associated with stress and psychological disorders. The mechanisms underlying the hypersensitivity are not clear. Gastric vagal afferents (GVAs) play an important role in sensing meal-related mechanical stimulation to modulate gastrointestinal function and food intake. This study aimed to determine whether GVAs display hypersensitivity after chronic stress, and whether its interaction with leptin was altered by stress.

METHODS

Eight-week-old male C57BL/6 mice were exposed to unpredictable chronic mild stress or no stress (control) for 8 weeks. The metabolic rate, gastric emptying rate, and anxiety- and depression-like behaviors were determined. GVA mechanosensitivity, and its modulation by leptin, was determined using an in vitro single fiber recording technique. QRT-PCR was used to establish the levels of leptin and leptin receptor mRNA in the stomach and nodose ganglion, respectively.

KEY RESULTS

The stressed mice had lower body weight and food intake, and increased anxiety-like behavior compared to the control mice. The mechanosensitivity of mucosal and tension-sensitive GVAs was higher in the stressed mice. Leptin potentiated mucosal GVA mechanosensitivity in control but not stressed mice. The expression of leptin mRNA in the gastric mucosa was lower in the stressed mice.

CONCLUSIONS AND INFERENCES

In conclusion, chronic stress enhances GVA mechanosensitivity, which may contribute to the gastric hypersensitivity in FD. In addition, the modulatory effect of leptin on GVA signaling is lost after chronic stress exposure.

Gastric vagal afferent neuropathy following experimental spinal cord injury

Dramatic impairment of gastrointestinal (GI) function accompanies high-thoracic spinal cord injury (T3-SCI). The vagus nerve contains mechano- and chemosensory fibers as well as the motor fibers necessary for the central nervous system (CNS) control of GI reflexes. Cell bodies for the vagal afferent fibers are located within the nodose gangla (NG) and the majority of vagal afferent axons are unmyelinated C fibers that are sensitive to capsaicin through activation of transient receptor potential vanilloid-1 (TRPV1) channels. Vagal afferent fibers also express receptors for GI hormones, including cholecystokinin (CCK). Previously, T3-SCI provokes a transient GI inflammatory response as well as a reduction of both gastric emptying and centrally-mediated vagal responses to GI peptides, including CCK. TRPV1 channels and CCK-A receptors (CCKar) expressed in vagal afferents are upregulated in models of visceral inflammation. The present study investigated whether T3-SCI attenuates peripheral vagal afferent sensitivity through plasticity of TRPV1 and CCK receptors. Vagal afferent response to graded mechanical stimulation of the stomach was significantly attenuated by T3-SCI at 3-day and 3-week recovery. Immunocytochemical labeling for CCKar and TRPV1 demonstrated expression on dissociated gastric-projecting NG neurons. Quantitative assessment of mRNA expression by qRT-PCR revealed significant elevation of CCKar and TRPV1 in the whole NG following T3-SCI in 3-day recovery, but levels returned to normal after 3-weeks. Three days after injury, systemic administration of CCK-8 s showed a significantly diminished gastric vagal afferent response in T3-SCI rats compared to control rats while systemic capsaicin infusion revealed a significant elevation of vagal response in T3-SCI vs control rats. These findings demonstrate that T3-SCI provokes peripheral remodeling and prolonged alterations in the response of vagal afferent fibers to the physiological signals associated with digestion.

High-fat diet-induced vagal afferent dysfunction via upregulation of 2-pore domain potassium TRESK channel

Research shows that rats and humans on a high-fat diet (HFD) are less sensitive to satiety signals known to act via vagal afferent pathways. We hypothesize that HFD causes an upregulation of 2-pore domain potassium channels, resulting in hyperpolarization of nodose ganglia (NG) and decreased vagal response to satiety signals, which contribute to hyperphagia. We show that a 2-week HFD caused an upregulation of 2-pore domain TWIK-related spinal cord K+ (TRESK) and TWIK-related acid-sensitive K+ 1 (TASK1) channels by 330% ± 50% and 60% ± 20%, respectively, in NG. Patch-clamp studies of isolated NG neurons demonstrated a decrease in excitability. In vivo single-unit NG recordings showed that a 2-week HFD led to a 55% reduction in firing frequency in response to CCK-8 or leptin stimulation. NG electroporation with TRESK siRNA restored NG responsiveness to CCK-8 and leptin. Rats fed a 2-week HFD consumed ~40% more calories compared with controls. Silencing NG TRESK but not TASK1 channel expression in HFD-fed rats restored normal calorie consumption. In conclusion, HFD caused upregulation of TRESK channels, resulting in NG hyperpolarization and decreased vagal responsiveness to satiety signals. This finding provides a pharmacological target to prevent or treat HFD-induced hyperphagia.

Body thermal responses and the vagus nerve

While thermosensation from external environment has been extensively studied, physiological responses to temperature changes inside the body and the underlying regulatory mechanisms are less understood. As a critical link between body and brain that relays visceral organ information and regulates numerous physiological functions, the vagus nerve has been proposed to mediate diverse visceral thermal reflexes and indirectly regulate body temperature. However, the precise role of the vagus nerve in body thermal responses or visceral organ-related thermoregulation is still under debate due to extensive contradictory results. This data discrepancy is likely due to the high cell heterogeneity in the vagus nerve, as diverse vagal neuron types mediate numerous and sometimes opposite physiological functions. Here, we will review evidences that support and against the role of the vagus nerve in body thermosensation and thermoregulation and discuss potential future approaches for better understanding of this critical issue.

Increased TASK channel-mediated currents underlie high-fat diet induced vagal afferent dysfunction

We have previously demonstrated that satiety sensing vagal afferent neurons are less responsive to meal-related stimuli in obesity because of reduced electrical excitability. As leak K⁺ currents are key determinants of membrane excitability, we hypothesized that leak K⁺ currents are increased in vagal afferents during obesity. Diet-induced obesity was induced by feeding C57Bl/6J mice a high-fat diet (HFF) (60% energy from fat) for 8-10 wk. In vitro extracellular recordings were performed on jejunal afferent nerves. Whole cell patch-clamp recordings were performed on mouse nodose ganglion neurons. Leak K⁺ currents were isolated using ion substitution and pharmacological blockers. mRNA for TWIK-related acid-sensitive K⁺ (TASK) subunits was measured using quantitative real-time PCR. Intestinal afferent responses to nutrient (oleate) and non-nutrient (ATP) stimuli were significantly decreased in HFF mice. Voltage clamp experiments revealed the presence of a voltage-insensitive resting potassium conductance that was increased by external alkaline pH and halothane, known properties of TASK currents. In HFF neurons, leak K⁺ current was approximately doubled and was reduced by TASK1 and TASK3 inhibitors. The halothane sensitive current was similarly increased. Quantitative PCR revealed the presence of mRNA encoding TASK1 (KCNK3) and TASK3 (KCNK9) channels in nodose neurons. TASK3 transcript was significantly increased in HFF mice. The reduction in vagal afferent excitability in obesity is due in part to an increase of resting (leak) K⁺ conductance. TASK channels may account for the impairment of satiety signaling in diet-induced obesity and thus is a therapeutic target for obesity treatment. NEW & NOTEWORTHY This study characterized the electrophysiological properties and gene expression of the TWIK-related acid-sensitive K⁺ (TASK) channel in vagal afferent neurons. TASK conductance was increased and contributed to decreased excitability in diet-induced obesity. TASK channels may account for the impairment of satiety signaling in diet-induced obesity and thus is a promising therapeutic target.

Transient Receptor Potential Ankyrin 1 and Substance P Mediate the Development of Gastric Mucosal Lesions in a Water Immersion Restraint Stress Rat Model

BACKGROUND

Activation of substance P (SP) contributes to the development and maintenance of gastric lesions, but the mechanisms underlying the release of SP and SP-mediated damage to the gastric mucosa remain unknown. Transient receptor potential ankyrin 1 (TRPA1) is expressed in SP-positive neurons in the dorsal root ganglion (DRG) and stomach of rats. We hypothesized that water immersion restraint stress (WIRS) may activate and sensitize TRPA1 in DRG neurons, subsequently inducing the release of SP from DRG and stomach cells, causing the development of acute gastric mucosal lesions (AGML).

METHODS

Changes in TRPA1 and SP expression in T8-11 DRG sensory neurons and the stomach in an AGML rat model were determined by reverse transcription polymerase chain reaction, western blotting and immunohistochemistry. The SP levels of serum and gastric mucosa were measured by using an enzyme-linked immunosorbent assay (ELISA). Gastric lesions were evaluated by histopathological changes. The TRPA1 antagonist HC-030031 and TRPA1 agonists allyl isothiocyanate were used to verify effect of TRPA1 and SP on AGML.

RESULTS

SP and TRPA1 in the DRG and stomach were upregulated, and the serum and gastric mucosa levels of SP were increased after WIRS, which are closely associated with AGML. The release of SP was suppressed and AGML were alleviated following a selective TRPA1 antagonist HC-030031. TRPA1 agonists AITC increased release of SP and led to moderate gastric lesions. We confirmed that WIRS induced the release of SP in the DRG, stomach, serum and gastric mucosa, and in a TRPA1-dependent manner.

CONCLUSIONS

Upregulated SP and TRPA1 in the DRG and stomach and increased serum and gastric mucosa SP levels may contribute to stress-induced AGML. TRPA1 is a potential drug target to reduce stress-induced AGML development in patients with acute critical illnesses. This study may contribute to the discovery of drugs for AGML treatment.

Transient receptor potential (TRP) channels: a metabolic TR(i)P to obesity prevention and therapy

Cellular transport of ions, especially by ion channels, regulates physiological function. The transient receptor potential (TRP) channels, with 30 identified so far, are cation channels with high calcium permeability. These ion channels are present in metabolically active tissues including adipose tissue, liver, gastrointestinal tract, brain (hypothalamus), pancreas and skeletal muscle, which suggests a potential role in metabolic disorders including obesity. TRP channels have potentially important roles in adipogenesis, obesity development and its prevention and therapy because of their physiological properties including calcium permeability, thermosensation and taste perception, involvement in cell metabolic signalling and hormone release. This wide range of actions means that organ-specific actions are unlikely, thus increasing the possibility of adverse effects. Delineation of responses to TRP channels has been limited by the poor selectivity of available agonists and antagonists. Food constituents that can modulate TRP channels are of interest in controlling metabolic status. TRP vanilloid 1 channels modulated by capsaicin have been the most studied, suggesting that this may be the first target for effective pharmacological modulation in obesity. This review shows that most of the TRP channels are potential targets to reduce metabolic disorders through a range of mechanisms.

Gastric cooling and menthol cause an increase in cardiac parasympathetic efferent activity in healthy adult human volunteers

NEW FINDINGS

What is the central question of this study? How do gastric stretch and gastric cooling stimuli affect cardiac autonomic control? What is the main finding and its importance? Gastric stretch causes an increase in cardiac sympathetic activity. Stretch combined with cold stimulation result in an elimination of the sympathetic response to stretch and an increase in cardiac parasympathetic activity, in turn resulting in a reduction in heart rate. Gastric cold stimulation causes a shift in sympathovagal balance towards parasympathetic dominance. The cold-induced bradycardia has the potential to decrease cardiac workload, which might be significant in individuals with cardiovascular pathologies.

ABSTRACT

Gastric distension increases blood pressure and heart rate in young, healthy humans, but little is known about the effect of gastric stretch combined with cooling. We used a randomized crossover study to assess the cardiovascular responses to drinking 300 ml of ispaghula husk solution at either 6 or 37°C in nine healthy humans (age 24.08 ± 9.36 years) to establish the effect of gastric stretch with and without cooling. The effect of consuming peppermint oil capsules to activate cold thermoreceptors was also investigated. The ECG, respiratory movements and continuous blood pressure were recorded during a 5 min baseline period, followed by a 115 min post-drink period, during which 5 min epochs of data were recorded. Cardiac autonomic activity was assessed using time and frequency domain analyses of respiratory sinus arrhythmia to quantify parasympathetic autonomic activity, and corrected QT (QTc) interval analysis to quantify sympathetic autonomic activity. Gastric stretch only caused a significant reduction in QTc interval lasting up to 15 min, with a concomitant but non-significant increase in heart rate, indicating an increased sympathetic cardiac tone. The additional effect of gastric cold stimulation was significantly to reduce heart rate for up to 15 min, elevate indicators of cardiac parasympathetic tone and eliminate the reduction in QTc interval seen with gastric stretch only. Stimulation of gastric cold thermoreceptors with menthol also caused a significant reduction in heart rate and concomitant increase in the root mean square of successive differences. These findings indicate that gastric cold stimulation causes a shift in the sympathovagal balance of cardiac control towards a more parasympathetic dominant pattern.

Activation of TREK currents by riluzole in three subgroups of cultured mouse nodose ganglion neurons

Two-pore domain potassium channels (K2P) constitute major candidates for the regulation of background potassium currents in mammalian cells. Channels of the TREK subfamily are also well positioned to play an important role in sensory transduction due to their sensitivity to a large number of physiological and physical stimuli (pH, mechanical, temperature). Following our previous report describing the molecular expression of different K2P channels in the vagal sensory system, here we confirm that TREK channels are functionally expressed in neurons from the mouse nodose ganglion (mNG). Neurons were subdivided into three groups (A, Ah and C) based on their response to tetrodotoxin and capsaicin. Application of the TREK subfamily activator riluzole to isolated mNG neurons evoked a concentration-dependent outward current in the majority of cells from all the three subtypes studied. Riluzole increased membrane conductance and hyperpolarized the membrane potential by approximately 10 mV when applied to resting neurons. The resting potential was similar in all three groups, but C cells were clearly less excitable and showed smaller hyperpolarization-activated currents at -100 mV and smaller sustained currents at -30 mV. Our results indicate that the TREK subfamily of K2P channels might play an important role in the maintenance of the resting membrane potential in sensory neurons of the autonomic nervous system, suggesting its participation in the modulation of vagal reflexes.

Involvement of TRPV1 Channels in Energy Homeostasis

The ion channel TRPV1 is involved in a wide range of processes including nociception, thermosensation and, more recently discovered, energy homeostasis. Tightly controlling energy homeostasis is important to maintain a healthy body weight, or to aid in weight loss by expending more energy than energy intake. TRPV1 may be involved in energy homeostasis, both in the control of food intake and energy expenditure. In the periphery, it is possible that TRPV1 can impact on appetite through control of appetite hormone levels or via modulation of gastrointestinal vagal afferent signaling. Further, TRPV1 may increase energy expenditure via heat production. Dietary supplementation with TRPV1 agonists, such as capsaicin, has yielded conflicting results with some studies indicating a reduction in food intake and increase in energy expenditure, and other studies indicating the converse. Nonetheless, it is increasingly apparent that TRPV1 may be dysregulated in obesity and contributing to the development of this disease. The mechanisms behind this dysregulation are currently unknown but interactions with other systems, such as the endocannabinoid systems, could be altered and therefore play a role in this dysregulation. Further, TRPV1 channels appear to be involved in pancreatic insulin secretion. Therefore, given its plausible involvement in regulation of energy and glucose homeostasis and its dysregulation in obesity, TRPV1 may be a target for weight loss therapy and diabetes. However, further research is required too fully elucidate TRPV1s role in these processes. The review provides an overview of current knowledge in this field and potential areas for development.

Ethyl Vanillin Activates TRPA1

The nonselective cation channel transient receptor potential ankryn subtype family 1 (TRPA1) is expressed in neurons of dorsal root ganglia and trigeminal ganglia and also in vagal afferent neurons that innervate the lungs and gastrointestinal tract. Many TRPA1 agonists are reactive electrophilic compounds that form covalent adducts with TRPA1. Allyl isothiocyanate (AITC), the common agonist used to identify TRPA1, contains an electrophilic group that covalently binds with cysteine residues of TRPA1 and confers a structural change on the channel. There is scientific motivation to identify additional compounds that can activate TRPA1 with different mechanisms of channel gating. We provide evidence that ethyl vanillin (EVA) is a TRPA1 agonist. Using fluorescent calcium imaging and whole-cell patch-clamp electrophysiology on dissociated rat vagal afferent neurons and TRPA1-transfected COS-7 cells, we discovered that EVA activates cells also activated by AITC. Both agonists display similar current profiles and conductances. Pretreatment with A967079, a selective TRPA1 antagonist, blocks the EVA response as well as the AITC response. Furthermore, EVA does not activate vagal afferent neurons from TRPA1 knockout mice, showing selectivity for TRPA1 in this tissue. Interestingly, EVA appears to be pharmacologically different from AITC as a TRPA1 agonist. When AITC is applied before EVA, the EVA response is occluded. However, they both require intracellular oxidation to activate TRPA1. These findings suggest that EVA activates TRPA1 but via a distinct mechanism that may provide greater ease for study in native systems compared with AITC and may shed light on differential modes of TRPA1 gating by ligand types.

Vagus nerve is involved in the changes in body temperature induced by intragastric administration of 1,8-cineole via TRPM8 in mice

Transient Receptor Potential Melastatin 8 (TRPM8) is a cold receptor activated by mild cold temperature (<28°C). TRPM8 expressed in cutaneous sensory nerves is involved in cold sensation and thermoregulation. TRPM8 mRNA is detected in various tissues, including the gastrointestinal mucosa, and in the vagal afferent nerve. The relationship between vagal afferent nerve-specific expression of TRPM8 and thermoregulation remains unclear. In this study, we aimed to investigate whether TRPM8 expression in the vagal afferent nerve is involved in autonomic thermoregulation. We found that intragastric administration of 1,8-cineole, a TRPM8 agonist, increased intrascapular brown adipose tissue and colonic temperatures, and M8-B-treatment (TRPM8 antagonist) inhibited these responses. Intravenous administration of 1,8-cineole also showed similar effects. In vagotomized mice, the responses induced by intragastric administration of 1,8-cineole were attenuated. These results suggest that TRPM8 expressed in tissues apart from cutaneous sensory nerves are involved in autonomic thermoregulation response.

Increased Activation of the TRESK K+ Mediates Vago-Vagal Reflex Malfunction in Diabetic Rats

BACKGROUND & AIMS

Patients with diabetes have defects in the vagal afferent pathway that result in abnormal gastrointestinal function. We investigated whether selective increased activation of the 2-pore domain potassium channel TRESK (2-pore-domain weak inward-rectifying potassium channel-related spinal cord potassium channel) contributes to nodose ganglia (NG) malfunction, disrupting gastrointestinal function in diabetic rats.

METHODS

We conducted whole-cell current-clamp and single-unit recordings in NG neurons from diabetes-prone BioBreeding/Worcester rats and streptozotocin-induced diabetic (STZ-D) rats and compared them with control rats. NG neurons in rats or cultured NG neurons were exposed to pharmacologic antagonists and/or transfected with short hairpin or small interfering RNAs that reduced expression of TRESK. We then made electrophysiologic recordings and studied gastrointestinal functions.

RESULTS

We observed reduced input resistance, hyperpolarized membrane potential, and increased current threshold to elicit action potentiation in NG neurons of STZ-D rats compared with controls. NG neuron excitability was similarly altered in diabetes-prone rats. In vivo single-unit NG neuronal discharges in response to 30 and 60 pmol cholecystokinin octapeptide were significantly lower in STZ-D rats compared with controls. Reducing expression of the TRESK K+ channel restored NG excitability in vitro and in vivo, as well as cholecystokinin 8-stimulated secretion of pancreatic enzymes and secretin-induced gastrointestinal motility, which are mediated by vago-vagal reflexes. These abnormalities resulted from increased intracellular Ca2+ in the NG, activating calcineurin, which, in turn, bound to an nuclear factor of activated T cell-like docking site on the TRESK protein, resulting in neuronal membrane hyperpolarization.

CONCLUSIONS

In 2 rate models of diabetes, we found that activation of the TRESK K+ channel reduced NG excitability and disrupted gastrointestinal functions.

A Method to Target and Isolate Airway-innervating Sensory Neurons in Mice

Somatosensory nerves transduce thermal, mechanical, chemical, and noxious stimuli caused by both endogenous and environmental agents. The cell bodies of these afferent neurons are located within the sensory ganglia. Sensory ganglia innervate a specific organ or portion of the body. For instance, the dorsal root ganglia (DRG) are located in the vertebral column and extend processes throughout the body and limbs. The trigeminal ganglia are located in the skull and innervate the face, and upper airways. Vagal afferents of the nodose ganglia extend throughout the gut, heart, and lungs. The nodose neurons control a diverse array of functions such as: respiratory rate, airway irritation, and cough reflexes. Thus, to understand and manipulate their function, it is critical to identify and isolate airway specific neuronal sub-populations. In the mouse, the airways are exposed to a fluorescent tracer dye, Fast Blue, for retrograde tracing of airway-specific nodose neurons. The nodose ganglia are dissociated and fluorescence activated cell (FAC) sorting is used to collect dye positive cells. Next, high quality ribonucleic acid (RNA) is extracted from dye positive cells for next generation sequencing. Using this method airway specific neuronal gene expression is determined.

Genetic and pharmacological evidence for low-abundance TRPV3 expression in primary vagal afferent neurons

Primary vagal afferent neurons express a multitude of thermosensitive ion channels. Within this family of ion channels, the heat-sensitive capsaicin receptor (TRPV1) greatly influences vagal afferent signaling by determining the threshold for action-potential initiation at the peripheral endings, while controlling temperature-sensitive forms of glutamate release at central vagal terminals. Genetic deletion of TRPV1 does not completely eliminate these temperature-dependent effects, suggesting involvement of additional thermosensitive ion channels. The warm-sensitive, calcium-permeable, ion channel TRPV3 is commonly expressed with TRPV1; however, the extent to which TRPV3 is found in vagal afferent neurons is unknown. Here, we begin to characterize the genetic and functional expression of TRPV3 in vagal afferent neurons using molecular biology (RT-PCR and RT-quantitative PCR) in whole nodose and isolated neurons and fluorescent calcium imaging on primary cultures of nodose ganglia neurons. We confirmed low-level TRPV3 expression in vagal afferent neurons and observed direct activation with putative TRPV3 agonists eugenol, ethyl vanillin (EVA), and farnesyl pyrophosphate (FPP). Agonist activation stimulated neurons also containing TRPV1 and was blocked by ruthenium red. FPP sensitivity overlapped with EVA and eugenol but represented the smallest percentage of vagal afferent neurons, and it was the only agonist that did not stimulate neurons from TRPV3(-/-1) mice, suggesting FPP has the highest selectivity. Further, FPP was predictive of enhanced responses to capsaicin, EVA, and eugenol in rats. From our results, we conclude TRPV3 is expressed in a discrete subpopulation of vagal afferent neurons and may contribute to vagal afferent signaling either directly or in combination with TRPV1.

TRP channel functions in the gastrointestinal tract

Transient receptor potential (TRP) channels are predominantly distributed in both somatic and visceral sensory nervous systems and play a crucial role in sensory transduction. As the largest visceral organ system, the gastrointestinal (GI) tract frequently accommodates external inputs, which stimulate sensory nerves to initiate and coordinate sensory and motor functions in order to digest and absorb nutrients. Meanwhile, the sensory nerves in the GI tract are also able to detect potential tissue damage by responding to noxious irritants. This nocifensive function is mediated through specific ion channels and receptors expressed in a subpopulation of spinal and vagal afferent nerve called nociceptor. In the last 18 years, our understanding of TRP channel expression and function in GI sensory nervous system has been continuously improved. In this review, we focus on the expressions and functions of TRPV1, TRPA1, and TRPM8 in primary extrinsic afferent nerves innervated in the esophagus, stomach, intestine, and colon and briefly discuss their potential roles in relevant GI disorders.

TRPV1 Channels and Gastric Vagal Afferent Signalling in Lean and High Fat Diet Induced Obese Mice

Aim

Within the gastrointestinal tract vagal afferents play a role in control of food intake and satiety signalling. Activation of mechanosensitive gastric vagal afferents induces satiety. However, gastric vagal afferent responses to mechanical stretch are reduced in high fat diet mice. Transient receptor potential vanilloid 1 channels (TRPV1) are expressed in vagal afferents and knockout of TRPV1 reduces gastro-oesophageal vagal afferent responses to stretch. We aimed to determine the role of TRPV1 on gastric vagal afferent mechanosensitivity and food intake in lean and HFD-induced obese mice.

Methods

TRPV1+/+ and -/- mice were fed either a standard laboratory diet or high fat diet for 20wks. Gastric emptying of a solid meal and gastric vagal afferent mechanosensitivity was determined.

Results

Gastric emptying was delayed in high fat diet mice but there was no difference between TRPV1+/+ and -/- mice on either diet. TRPV1 mRNA expression in whole nodose ganglia of TRPV1+/+ mice was similar in both dietary groups. The TRPV1 agonist N-oleoyldopamine potentiated the response of tension receptors in standard laboratory diet but not high fat diet mice. Food intake was greater in the standard laboratory diet TRPV1-/- compared to TRPV1+/+ mice. This was associated with reduced response of tension receptors to stretch in standard laboratory diet TRPV1-/- mice. Tension receptor responses to stretch were decreased in high fat diet compared to standard laboratory diet TRPV1+/+ mice; an effect not observed in TRPV1-/- mice. Disruption of TRPV1 had no effect on the response of mucosal receptors to mucosal stroking in mice on either diet.

Conclusion

TRPV1 channels selectively modulate gastric vagal afferent tension receptor mechanosensitivity and may mediate the reduction in gastric vagal afferent mechanosensitivity in high fat diet-induced obesity.

TRPM8 function and expression in vagal sensory neurons and afferent nerves innervating guinea pig esophagus

Sensory transduction in esophageal afferents requires specific ion channels and receptors. TRPM8 is a new member of the transient receptor potential (TRP) channel family and participates in cold- and menthol-induced sensory transduction, but its role in visceral sensory transduction is still less clear. This study aims to determine TRPM8 function and expression in esophageal vagal afferent subtypes. TRPM8 agonist WS-12-induced responses were first determined in nodose and jugular neurons by calcium imaging and then investigated by whole cell patch-clamp recordings in Dil-labeled esophageal nodose and jugular neurons. Extracellular single-unit recordings were performed in nodose and jugular C fiber neurons using ex vivo esophageal-vagal preparations with intact nerve endings in the esophagus. TRPM8 mRNA expression was determined by single neuron RT-PCR in Dil-labeled esophageal nodose and jugular neurons. The TRPM8 agonist WS-12 elicited calcium influx in a subpopulation of jugular but not nodose neurons. WS-12 activated outwardly rectifying currents in esophageal Dil-labeled jugular but not nodose neurons in a dose-dependent manner, which could be inhibited by the TRPM8 inhibitor AMTB. WS-12 selectively evoked action potential discharges in esophageal jugular but not nodose C fibers. Consistently, TRPM8 transcripts were highly expressed in esophageal Dil-labeled TRPV1-positive jugular neurons. In summary, the present study demonstrated a preferential expression and function of TRPM8 in esophageal vagal jugular but not nodose neurons and C fiber subtypes. This provides a distinctive role of TRPM8 in esophageal sensory transduction and may lead to a better understanding of the mechanisms of esophageal sensation and nociception.

Regulation of Trek1 expression in nasal mucosa with allergic rhinitis by specific immunotherapy

Epithelial barrier dysfunction is involved in the pathogenesis of allergic disorders, such as nasal allergy. TWIK-related K(+) 1 (Trek1) potassium channels are required in the maintenance of the epithelial barrier function. This study aims to investigate the role of antigen-specific immunotherapy (SIT) in the regulation of Trek1 expression in the nasal mucosa. In this study, patients with nasal allergy were treated with SIT and/or Clostridium butyricum. The expression of Trek1 and histone demethylase 1 (HDAC1) in the nasal epithelia was assessed by real-time reverse transcription polymerase chain reaction and Western blotting. Serum cytokines were assessed by enzyme-linked immunosorbent assay. The results showed that Trek1 and HDAC1 were detected in the nasal epithelia. Trek1 was lower, whereas HDAC1 was higher in patients with allergic rhinitis as compared with healthy controls. Trek1-null RPMI2650 monolayers showed a markedly compromised epithelial barrier function. Treatment with SIT significantly increased the Trek1 levels in the nasal epithelia of allergic rhinitis patients that were further improved in conjunction of SIT and administration of probiotic C. butyricum. In conclusion, nasal epithelia express Trek1 that can be suppressed by allergic response. SIT can restore the expression of Trek1 in the nasal epithelia and can be further improved by conjunction with administration of C. butyricum.

Channeling satiation: a primer on the role of TRP channels in the control of glutamate release from vagal afferent neurons

Obesity results from the chronic imbalance between food intake and energy expenditure. To maintain homeostasis, the brainstem nucleus of the solitary tract (NTS) integrates peripheral information from visceral organs and initiates reflex pathways that control food intake and other autonomic functions. This peripheral-to-central neural communication occurs through activation of vagal afferent neurons which converge to form the solitary tract (ST) and synapse with strong glutamatergic contacts onto NTS neurons. Vagal afferents release glutamate containing vesicles via three distinct pathways (synchronous, asynchronous, and spontaneous) providing multiple levels of control through fast synaptic neurotransmission at ST-NTS synapses. While temperature at the NTS is relatively constant, vagal afferent neurons express an array of thermosensitive ion channels named transient receptor potential (TRP) channels. Here we review the evidence that TRP channels pre-synaptically control quantal glutamate release and examine the potential roles of TRP channels in vagally mediated satiety signaling. We summarize the current literature that TRP channels contribute to asynchronous and spontaneous release of glutamate which can distinctly influence the transfer of information across the ST-NTS synapse. In other words, multiple glutamate vesicle release pathways, guided by afferent TRP channels, provide for robust while adaptive neurotransmission and expand our understanding of vagal afferent signaling.

External QX-314 inhibits evoked cranial primary afferent synaptic transmission independent of TRPV1

The cell-impermeant lidocaine derivative QX-314 blocks sodium channels via intracellular mechanisms. In somatosensory nociceptive neurons, open transient receptor potential vanilloid type 1 (TRPV1) receptors provide a transmembrane passageway for QX-314 to produce long-lasting analgesia. Many cranial primary afferents express TRPV1 at synapses on neurons in the nucleus of the solitary tract and caudal trigeminal nucleus (Vc). Here, we investigated whether QX-314 interrupts neurotransmission from primary afferents in rat brain-stem slices. Shocks to the solitary tract (ST) activated highly synchronous evoked excitatory postsynaptic currents (ST-EPSCs). Application of 300 μM QX-314 increased the ST-EPSC latency from TRPV1+ ST afferents, but, surprisingly, it had similar actions at TRPV1- ST afferents. Continued exposure to QX-314 blocked evoked ST-EPSCs at both afferent types. Neither the time to onset of latency changes nor the time to ST-EPSC failure differed between responses for TRPV1+ and TRPV1- inputs. Likewise, the TRPV1 antagonist capsazepine failed to prevent the actions of QX-314. Whereas QX-314 blocked ST-evoked release, the frequency and amplitude of spontaneous EPSCs remained unaltered. In neurons exposed to QX-314, intracellular current injection evoked action potentials suggesting a presynaptic site of action. QX-314 acted similarly at Vc neurons to increase latency and block EPSCs evoked from trigeminal tract afferents. Our results demonstrate that QX-314 blocked nerve conduction in cranial primary afferents without interrupting the glutamate release mechanism or generation of postsynaptic action potentials. The TRPV1 independence suggests that QX-314 either acted extracellularly or more likely entered these axons through an undetermined pathway common to all cranial primary afferents.

Effects of acid on vagal nociceptive afferent subtypes in guinea pig esophagus

Acid reflux-induced heartburn and noncardiac chest pain are processed peripherally by sensory nerve endings in the wall of the esophagus, but the underlying mechanism is still unclear. This study aims to determine the effects of acid on esophageal vagal nociceptive afferent subtypes. Extracellular single-unit recordings were performed in guinea pig vagal nodose or jugular C fiber neurons by using ex vivo esophageal-vagal preparations with intact nerve endings in the esophagus. We recorded action potentials (AP) of esophageal nodose or jugular C fibers evoked by acid perfusion and compared esophageal distension-evoked AP before and after acid perfusion. Acid perfusion for 30 min (pH range 7.4 to 5.8) did not evoke AP in nodose C fibers but significantly decreased their responses to esophageal distension, which could be recovered after washing out acid for 90 min. In jugular C fibers, acid perfusion not only evoked AP but also inhibited their responses to esophageal distension, which were not recovered after washing out acid for 120 min. Lower concentration of capsaicin perfusion mimicked acid-induced effects in nodose and jugular C fibers. Pretreatment with TRPV1 antagonist AMG9810, but not acid-sensing ion channel (ASIC) inhibitor amiloride, significantly inhibited acid-induced effects in nodose and jugular C fiber. These results demonstrate that esophageal vagal nociceptive afferent nerve subtypes display distinctive responses to acid. Acid activates jugular, but not nodose, C fibers and inhibits both of their responses to esophageal distension. These effects are mediated mainly through TRPV1. This inhibitory effect is a novel finding and may contribute to esophageal sensory/motor dysfunction in acid reflux diseases.

The role of trigeminal nasal TRPM8-expressing afferent neurons in the antitussive effects of menthol

The cold-sensitive cation channel TRPM8 is a target for menthol, which is used routinely as a cough suppressant and as an additive to tobacco and food products. Given that cold temperatures and menthol activate neurons through gating of TRPM8, it is unclear how menthol actively suppresses cough. In this study we describe the antitussive effects of (-)-menthol in conscious and anesthetized guinea pigs. In anesthetized guinea pigs, cough evoked by citric acid applied topically to the tracheal mucosa was suppressed by menthol only when it was selectively administered as vapors to the upper airways. Menthol applied topically to the tracheal mucosa prior to and during citric acid application or administered continuously as vapors or as an aerosol to the lower airways was without effect on cough. These actions of upper airway menthol treatment were mimicked by cold air delivered to the upper airways but not by (+)-menthol, the inactive isomer of menthol, or by the TRPM8/TRPA1 agonist icilin administered directly to the trachea. Subsequent molecular analyses confirmed the expression of TRPM8 in a subset of nasal trigeminal afferent neurons that do not coincidently express TRPA1 or TRPV1. We conclude that menthol suppresses cough evoked in the lower airways primarily through a reflex initiated from the nose.

Changes in gastrointestinal tract function and structure in functional dyspepsia

Functional dyspepsia is an extremely common disorder of gastrointestinal function. The disorder is thought to be heterogeneous, with different pathophysiological mechanisms underlying varied symptom patterns. A diversity of changes in gastrointestinal tract function and structure has been described in functional dyspepsia. These involve alterations in the stomach, such as impaired accommodation, delayed gastric emptying and hypersensitivity, and alterations in the duodenum, such as increased sensitivity to duodenal acid and/or lipids and low-grade inflammation. In this Review, we summarize all these abnormalities in an attempt to provide an integrated overview of the pathophysiological mechanisms in functional dyspepsia.

Gastric vagal afferent modulation by leptin is influenced by food intake status

Energy intake is strongly influenced by vagal afferent signals from the stomach, and is also modulated by leptin. Leptin may be secreted from gastric epithelial cells, so we aimed to determine the direct effect of leptin on gastric vagal afferents under different feeding conditions. Female C57BL/6 mice were fed standard laboratory diet, high-fat diet or were food restricted. The expression of leptin receptor (Lep-R) and its signal transduction molecules in vagal afferents was determined by retrograde tracing and reverse-transcription polymerase chain reaction, and the relationship between leptin-immunopositive cells and gastric vagal afferent endings determined by anterograde tracing and leptin immunohistochemistry. An in vitro preparation was used to determine the functional effects of leptin on gastric vagal afferents and the second messenger pathways involved. Leptin potentiated vagal mucosal afferent responses to tactile stimuli, and epithelial cells expressing leptin were found close to vagal mucosal endings. After fasting or diet-induced obesity, potentiation of mucosal afferents by leptin was lost and Lep-R expression reduced in the cell bodies of gastric mucosal afferents. These effects in diet-induced obese mice were accompanied by a reduction in anatomical vagal innervation of the gastric mucosa. In striking contrast, after fasting or diet-induced obesity, leptin actually inhibited responses to distension in tension receptors. The inhibitory effect on gastric tension receptors was mediated through phosphatidylinositol 3-kinase-dependent activation of large-conductance calcium-activated potassium channels. The excitatory effect of leptin on gastric mucosal vagal afferents was mediated by phospholipase C-dependent activation of canonical transient receptor potential channels. These data suggest the effect of leptin on gastric vagal afferent excitability is dynamic and related to the feeding state. Paradoxically, in obesity, leptin may reduce responses to gastric distension following food intake.

Functional adaptation in female rats: the role of estrogen signaling

Background

Sex steroids have direct effects on the skeleton. Estrogen acts on the skeleton via the classical genomic estrogen receptors alpha and beta (ERα and ERβ), a membrane ER, and the non-genomic G-protein coupled estrogen receptor (GPER). GPER is distributed throughout the nervous system, but little is known about its effects on bone. In male rats, adaptation to loading is neuronally regulated, but this has not been studied in females.

Methodology/Principal Findings

We used the rat ulna end-loading model to induce an adaptive modeling response in ovariectomized (OVX) female Sprague-Dawley rats. Rats were treated with a placebo, estrogen (17β-estradiol), or G-1, a GPER-specific agonist. Fourteen days after OVX, rats underwent unilateral cyclic loading of the right ulna; half of the rats in each group had brachial plexus anesthesia (BPA) of the loaded limb before loading. Ten days after loading, serum estrogen concentrations, dorsal root ganglion (DRG) gene expression of ERα, ERβ, GPER, CGRPα, TRPV1, TRPV4 and TRPA1, and load-induced skeletal responses were quantified. We hypothesized that estrogen and G-1 treatment would influence skeletal responses to cyclic loading through a neuronal mechanism. We found that estrogen suppresses periosteal bone formation in female rats. This physiological effect is not GPER-mediated. We also found that absolute mechanosensitivity in female rats was decreased, when compared with male rats. Blocking of adaptive bone formation by BPA in Placebo OVX females was reduced.

Conclusions

Estrogen acts to decrease periosteal bone formation in female rats in vivo. This effect is not GPER-mediated. Gender differences in absolute bone mechanosensitivity exist in young Sprague-Dawley rats with reduced mechanosensitivity in females, although underlying bone formation rate associated with growth likely influences this observation. In contrast to female and male rats, central neuronal signals had a diminished effect on adaptive bone formation in estrogen-deficient female rats.

Upper gastrointestinal dysmotility after spinal cord injury: is diminished vagal sensory processing one culprit?

Despite the widely recognized prevalence of gastric, colonic, and anorectal dysfunction after spinal cord injury (SCI), significant knowledge gaps persist regarding the mechanisms leading to post-SCI gastrointestinal (GI) impairments. Briefly, the regulation of GI function is governed by a mix of parasympathetic, sympathetic, and enteric neurocircuitry. Unlike the intestines, the stomach is dominated by parasympathetic (vagal) control whereby gastric sensory information is transmitted via the afferent vagus nerve to neurons of the nucleus tractus solitarius (NTS). The NTS integrates this sensory information with signals from throughout the central nervous system. Glutamatergic and GABAergic NTS neurons project to other nuclei, including the preganglionic parasympathetic neurons of the dorsal motor nucleus of the vagus (DMV). Finally, axons from the DMV project to gastric myenteric neurons, again, through the efferent vagus nerve. SCI interrupts descending input to the lumbosacral spinal cord neurons that modulate colonic motility and evacuation reflexes. In contrast, vagal neurocircuitry remains anatomically intact after injury. This review presents evidence that unlike the post-SCI loss of supraspinal control which leads to colonic and anorectal dysfunction, gastric dysmotility occurs as an indirect or secondary pathology following SCI. Specifically, emerging data points toward diminished sensitivity of vagal afferents to GI neuroactive peptides, neurotransmitters and, possibly, macronutrients. The neurophysiological properties of rat vagal afferent neurons are highly plastic and can be altered by injury or energy balance. A reduction of vagal afferent signaling to NTS neurons may ultimately bias NTS output toward unregulated GABAergic transmission onto gastric-projecting DMV neurons. The resulting gastroinhibitory signal may be one mechanism leading to upper GI dysmotility following SCI.

Expression of K2P channels in sensory and motor neurons of the autonomic nervous system

Several types of neurons within the central and peripheral somatic nervous system express two-pore-domain potassium (K2P) channels, providing them with resting potassium conductances. We demonstrate that these channels are also expressed in the autonomic nervous system where they might be important modulators of neuronal excitability. We observed strong mRNA expression of members of the TRESK and TREK subfamilies in both the mouse superior cervical ganglion (mSCG) and the mouse nodose ganglion (mNG). Motor mSCG neurons strongly expressed mRNA transcripts for TRESK and TREK-2 subunits, whereas TASK-1 and TASK-2 subunits were only moderately expressed, with only few or very few transcripts for TREK-1 and TRAAK (TRESK ≈ TREK-2 > TASK-2 ≈ TASK-1 > TREK-1 > TRAAK). Similarly, the TRESK and TREK-1 subunits were the most strongly expressed in sensorial mNG neurons, while TASK-1 and TASK-2 mRNAs were moderately expressed, and fewer TREK-2 and TRAAK transcripts were detected (TRESK ≈ TREK-1 > TASK-1 ≈ TASK-2 > TREK-2 > TRAAK). Moreover, cell-attached single-channel recordings showed a major contribution of TRESK and TREK-1 channels in mNG. As the level of TRESK mRNA expression was not statistically different between the ganglia analysed, the distinct expression of TREK-1 and TREK-2 subunits was the main difference observed between these structures. Our results strongly suggest that TRESK and TREK channels are important modulators of the sensorial and motor information flowing through the autonomic nervous system, probably exerting a strong influence on vagal reflexes.

Comparative pharmacology of cholecystokinin induced activation of cultured vagal afferent neurons from rats and mice

Cholecystokinin (CCK) facilitates the process of satiation via activation of vagal afferent neurons innervating the upper gastrointestinal tract. Recent findings indicate CCK acts on these neurons via a ruthenium red (RuR) sensitive pathway that involves members of the vanilloid (V) subfamily of transient receptor potential (TRP) channels. To further test this mechanism, the mouse provides an ideal model in which genetic tools could be applied. However, whether CCK acts by similar mechanism(s) in mice has not been determined. In the present study we explored the actions of CCK on nodose neurons isolated from Sprague Dawley (SD) rat and two strains of mice; C57BL/6 and BalbC using fluorescence-based calcium imaging. With minor exceptions nodose neurons isolated from all species/strains behaved similarly. They all respond to brief depolarization with a large calcium transient. A significant subset of neurons responded to capsaicin (CAP), a TRPV1 agonist, although neurons from C57BL/6 were 10-fold more sensitive to CAP than SD rats or BalbC mice, and a significantly smaller fraction of neurons from BalbC mice responded to CAP. CCK-8 dose-dependently activated a subpopulation of neurons with similar dose dependency, percent responders, and overlap between CCK and CAP responsiveness. In all species/strains CCK-8 induced activation was significantly attenuated (but not completely blocked) by pretreatment with the TRPV channel blocker RuR. Surprisingly, the CCK analogue JMV-180, which is reported to have pure antagonistic properties in rat but mixed agonist/antagonist properties in mice, behaved as a pure antagonist to CCK in both rat and mouse neurons. The pure antagonistic action of JMV-180 in this in vitro preparation suggests that prior reported differential effects of JMV-180 on satiation in rats versus mouse must be mediated by a site other than vagal afferent activation.

Recent progress in histochemistry and cell biology

Studies published in Histochemistry and Cell Biology in the year 2011 represent once more a manifest of established and newly sophisticated techniques being exploited to put tissue- and cell type-specific molecules into a functional context. The review is therefore the Histochemistry and Cell Biology's yearly intention to provide interested readers appropriate summaries of investigations touching the areas of tissue biology, developmental biology, the biology of the immune system, stem cell research, the biology of subcellular compartments, in order to put the message of such studies into natural scientific-/human- and also pathological-relevant correlations.