Nitric oxide as an endogenous peripheral modulator of visceral sensory neuronal function

Continuous electrical stimulation of superior laryngeal nerve inhibits initiation of swallowing in anesthetized rats

Pharyngeal electrical stimulation (PES) applies electrical stimulation to pharyngeal mucosa (PhM) and represents a useful approach to improve swallowing function in patients with dysphagia. To determine the optimal PES modality to treat dysphagia, the mechanism underlying the effects of PES on swallowing function must be elucidated. In this study, we evaluated how PES and electrical stimulation of the superior laryngeal nerve (SLN) modulate the initiation of swallowing in anesthetized rats. A swallow was evoked by electrical stimulation of the PhM, SLN, and nucleus of the solitary tract (nTS) and pharyngeal mechanical stimulation using a von Frey filament. A swallow was identified by electromyographic bursts in mylohyoid and thyrohyoid muscles. Bilateral SLN transection abolished the swallows evoked by PhM electrical stimulation. PhM and SLN electrical stimulation decreased swallowing frequency in a similar time-dependent manner. Intravenous administration of the GABAA receptor antagonist bicuculine did not affect the time-dependent change in swallowing frequency during SLN electrical stimulation. Continuous SLN electrical stimulation significantly inhibited pharyngeal mechanically and nTS-electrically evoked swallows compared with before and 5 min after stimulation. The present findings suggest that the SLN plays a primary role in PES-evoked swallows. Additionally, continuous SLN electrical stimulation inhibits the initiation of swallowing, and the modulation of central network associated with swallowing might be partially involved in this inhibition.

Self-Assembled Activatable Probes to Monitor Interactive Dynamics of Intracellular Nitric Oxide and Hydrogen Sulfide

The increasing understanding of the intricate relationship between two crucial gasotransmitters nitric oxide (NO) and hydrogen sulfide (H2S) in biological actions has generated significant interest. However, comprehensive monitoring of the dynamic fluctuations of endogenous NO and H2S remains a challenge. In this study, we have designed an innovative aggregation-induced reporter SAB-NH-SC with enhanced responsiveness to H2S for visualizing the fluctuations of intracellular NO and H2S. This probe leverages the hydrophilic properties of the pyridinium salt derivative, which can rapidly self-assemble into positively charged nanoparticles under physiological conditions, avoiding the introduction of organic solvents or tedious preparations. Notably, the reporter can repeatedly cycle S-nitrosation and SNO-transnitrosation reactions when successively treated with NO and H2S. Consequently, fluorescence alternation at 751 (H2S) and 639 nm (NO) facilitates the dynamic visualization of the alternating presence of H2S and NO within cells. This dynamic and reversible probe holds immense potential for unraveling the intricate interactions between NO and H2S in a complex network of biological applications.

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.

Cannabinoid 1 and mu-Opioid Receptor Agonists Synergistically Inhibit Abdominal Pain and Lack Side Effects in Mice

While effective in treating abdominal pain, opioids have significant side effects. Recent legalization of cannabis will likely promote use of cannabinoids as an adjunct or alternative to opioids, despite a lack of evidence. We aimed to investigate whether cannabinoids inhibit mouse colonic nociception, alone or in combination with opioids at low doses. Experiments were performed on C57BL/6 male and female mice. Visceral nociception was evaluated by measuring visceromotor responses (VMR), afferent nerve mechanosensitivity in flat-sheet colon preparations, and excitability of isolated DRG neurons. Blood oxygen saturation, locomotion, and defecation were measured to evaluate side effects. An agonist of cannabinoid 1 receptor (CB1R), arachidonyl-2'-chloroethylamide (ACEA), dose-dependently decreased VMR. ACEA and HU-210 (another CB1R agonist) also attenuated colonic afferent nerve mechanosensitivity. Additionally, HU-210 concentration-dependently decreased DRG neuron excitability, which was reversed by the CB1R antagonist AM-251. Conversely, cannabinoid 2 receptor (CB2R) agonists did not attenuate VMR, afferent nerve mechanosensitivity, or DRG neuron excitability. Combination of subanalgesic doses of CB1R and µ-opioid receptor agonists decreased VMR; importantly, this analgesic effect was preserved after 6 d of twice daily treatment. This combination also attenuated afferent nerve mechanosensitivity and DRG neuron excitability, which was inhibited by neuronal nitric oxide synthase and guanylate cyclase inhibitors. This combination avoided side effects (decreased oxygen saturation and colonic transit) caused by analgesic dose of morphine. Activation of CB1R, but not CB2R, decreased colonic nociception both alone and in synergy with µ-opioid receptor. Thus, CB1R agonists may enable opioid dose reduction and avoid opioid-related side effects.SIGNIFICANCE STATEMENT One of the most cited needs for patients with abdominal pain are safe and effective treatment options. The effectiveness of opioids in the management of abdominal pain is undermined by severe adverse side effects. Therefore, strategies to replace opioids or reduce the doses of opioids to suppress abdominal pain is needed. This study in mice demonstrates that cannabinoid 1 receptor (CB1R) agonists inhibit visceral sensation. Furthermore, a combination of subanalgesic doses of µ-opioid receptor agonist and CB1R agonist markedly reduce abdominal pain without causing the side effects of high-dose opioids. Thus, CB1R agonists, alone or in combination with low-dose opioids, may be a novel and safe treatment strategy for abdominal pain.

The nitric oxide-cyclic guanosine monophosphate pathway inhibits the bladder ATP release in response to a physiological or pathological stimulus

The release of ATP from the epithelium of the urinary bladder (urothelium) in response to mechanical/chemical stimuli contributes to the visceral sensation in the micturition reflex. The nitric oxide (NO)-mediated induction of cyclic guanosine monophosphate (cGMP) has been detected in urothelial cells and may inhibit the micturition reflex. However, the function of the NO-cGMP pathway in the regulation of urothelial ATP release remains poorly understood in contrast to its effects on smooth muscles or primary afferent nerves. Therefore, we investigated the relevance of the NO-cGMP pathway to ATP release on the mucosal side in the present study. The administration of l-arginine (NO precursor) or NOC 12 (NO donor) significantly reduced ATP release to the mucosal side at a physiologically normal urine storage pressure (5 cmH2 O). L-NAME (NO synthase inhibitor) significantly increased the distention-induced release of ATP. The phosphodiesterase-5 inhibitor, sildenafil, which increases cGMP levels, inhibited distention-induced ATP release. Furthermore, sildenafil significantly reduced ATP release in response to the administration of lipopolysaccharide. These results suggest that the NO-cGMP pathway inhibited urothelial ATP release during the storage phase under both physiological and pathological conditions.

Distribution and morphology of P2X3-immunoreactive subserosal afferent nerve endings in the rat gastric antrum

The present study investigated the morphological characteristics of subserosal afferent nerve endings with immunoreactivity for the P2X3 purinoceptor (P2X3) in the rat stomach by immunohistochemistry of whole-mount preparations using confocal scanning laser microscopy. P2X3 immunoreactivity was observed in subserosal nerve endings proximal and lateral to the gastric sling muscles in the distal antrum of the lesser curvature. Parent axons ramified into several lamellar processes to form net-like complex structures that extended two-dimensionally in every direction on the surface of the longitudinal smooth muscle layer. The axon terminals in the periphery of P2X3-immunoreactive net-like structures were flat and looped or leaf-like in shape. Some net-like lamellar structures and their axon terminals with P2X3 immunoreactivity were also immunoreactive for P2X2. P2X3-immunoreactive nerve fibers forming net-like terminal structures were closely surrounded by S100B-immunoreactive terminal Schwann cells, whereas axon terminals twined around these cells and extended club-, knob-, or thread-like protrusions in the surrounding tissues. Furthermore, a retrograde tracing method using fast blue dye indicated that most of these nerve endings originated from the nodose ganglia of the vagus nerve. These results suggest that P2X3-immunoreactive subserosal nerve endings have morphological characteristics of mechanoreceptors and contribute to sensation of a mechanical deformation of the distal antral wall associated with antral peristalsis.

The role of the tripartite synapse in the heart: how glial cells may contribute to the physiology and pathophysiology of the intracardiac nervous system

One of the major roles of the intracardiac nervous system (ICNS) is to act as the final site of signal integration for efferent information destined for the myocardium to enable local control of heart rate and rhythm. Multiple subtypes of neurons exist in the ICNS where they are organized into clusters termed ganglionated plexi (GP). The majority of cells in the ICNS are actually glial cells; however, despite this, ICNS glial cells have received little attention to date. In the central nervous system, where glial cell function has been widely studied, glia are no longer viewed simply as supportive cells but rather have been shown to play an active role in modulating neuronal excitability and synaptic plasticity. Pioneering studies have demonstrated that in addition to glia within the brain stem, glial cells within multiple autonomic ganglia in the peripheral nervous system, including the ICNS, can also act to modulate cardiovascular function. Clinically, patients with atrial fibrillation (AF) undergoing catheter ablation show high plasma levels of S100B, a protein produced by cardiac glial cells, correlated with decreased AF recurrence. Interestingly, S100B also alters GP neuron excitability and neurite outgrowth in the ICNS. These studies highlight the importance of understanding how glial cells can affect the heart by modulating GP neuron activity or synaptic inputs. Here, we review studies investigating glia both in the central and peripheral nervous systems to discuss the potential role of glia in controlling cardiac function in health and disease, paying particular attention to the glial cells of the ICNS.

Novel Analgesics with Peripheral Targets

A limited number of peripheral targets generate pain. Inflammatory mediators can sensitize these. The review addresses targets acting exclusively or predominantly on sensory neurons, mediators involved in inflammation targeting sensory neurons, and mediators involved in a more general inflammatory process, of which an analgesic effect secondary to an anti-inflammatory effect can be expected. Different approaches to address these systems are discussed, including scavenging proinflammatory mediators, applying anti-inflammatory mediators, and inhibiting proinflammatory or facilitating anti-inflammatory receptors. New approaches are contrasted to established ones; the current stage of progress is mentioned, in particular considering whether there is data from a molecular and cellular level, from animals, or from human trials, including an early stage after a market release. An overview of publication activity is presented, considering a IuPhar/BPS-curated list of targets with restriction to pain-related publications, which was also used to identify topics.

Inducible nitric oxide synthase-derived nitric oxide reduces vagal satiety signalling in obese mice

KEY POINTS

Obesity is associated with disrupted satiety regulation. Mice with diet-induced obesity have reduced vagal afferent neuronal excitability and a decreased afferent response to satiety signals. A low grade inflammation occurs in obesity with increased expression of inducible nitric oxide synthase (iNOS). Inhibition of iNOS in diet-induced obese mice restored vagal afferent neuronal excitability, increased the afferent response to satiety mediators and distention of the gut, and reduced short-term energy intake. A prolonged inhibition of iNOS reduced energy intake and body weight gain during the first week, and reduced amounts of epididymal fat after 3 weeks. We identified a novel pathway underlying disrupted satiety regulation in obesity. Blocking of this pathway might be clinically useful for the management of obesity.

ABSTRACT

Vagal afferents regulate feeding by transmitting satiety signals to the brain. Mice with diet-induced obesity have reduced vagal afferent sensitivity to satiety signals. We investigated whether inducible nitric oxide synthase (iNOS)-derived NO contributed to this reduction. C57BL/6J mice were fed a high- or low-fat diet for 6-8 weeks. Nodose ganglia and jejunum were analysed by immunoblotting for iNOS expression; NO production was measured using a fluorometric assay. Nodose neuron excitability and intestinal afferent sensitivity were evaluated by whole-cell patch clamp and in vitro afferent recording, respectively. Expression of iNOS and production of NO were increased in nodose ganglia and the small intestine in obese mice. Inhibition of iNOS in obese mice by pre-treatment with an iNOS inhibitor increased nodose neuron excitability via 2-pore-domain K⁺ channel leak currents, restored afferent sensitivity to satiety signals and reduced short-term energy intake. Obese mice given the iNOS inhibitor daily for 3 weeks had reduced energy intake and decreased body weight gain during the first week, compared to mice given saline, and lower amounts of epididymal fat at the end of 3 weeks. Inhibition of iNOS or blocking the action of iNOS-derived NO on vagal afferent pathways might comprise therapeutic strategies for hyperphagia and obesity.

Plasticity of vagal afferent signaling in the gut

Vagal sensory neurons mediate the vago-vagal reflex which, in turn, regulates a wide array of gastrointestinal functions including esophageal motility, gastric accommodation and pancreatic enzyme secretion. These neurons also transmit sensory information from the gut to the central nervous system, which then mediates the sensations of nausea, fullness and satiety. Recent research indicates that vagal afferent neurons process non-uniform properties and a significant degree of plasticity. These properties are important to ensure that vagally regulated gastrointestinal functions respond rapidly and appropriately to various intrinsic and extrinsic factors. Similar plastic changes in the vagus also occur in pathophysiological conditions, such as obesity and diabetes, resulting in abnormal gastrointestinal functions. A clear understanding of the mechanisms which mediate these events may provide novel therapeutic targets for the treatment of gastrointestinal disorders due to vago-vagal pathway malfunctions.

Zinc Transporter 3 (ZnT3) in the Enteric Nervous System of the Porcine Ileum in Physiological Conditions and during Experimental Inflammation

Zinc transporter 3 (ZnT3) is a member of the solute-linked carrier 30 (SLC 30) zinc transporter family. It is closely linked to the nervous system, where it takes part in the transport of zinc ions from the cytoplasm to the synaptic vesicles. ZnT3 has also been observed in the enteric nervous system (ENS), but its reactions in response to pathological factors remain unknown. This study, based on the triple immunofluorescence technique, describes changes in ZnT3-like immunoreactive (ZnT3-LI) enteric neurons in the porcine ileum, caused by chemically-induced inflammation. The inflammatory process led to a clear increase in the percentage of neurons immunoreactive to ZnT3 in all "kinds" of intramural enteric plexuses, i.e., myenteric (MP), outer submucous (OSP) and inner submucous (ISP) plexuses. Moreover, a wide range of other active substances was noted in ZnT3-LI neurons under physiological and pathological conditions, and changes in neurochemical characterisation of ZnT3⁺ cells in response to inflammation depended on the "kind" of enteric plexus. The obtained results show that ZnT3 is present in the ENS in a relatively numerous and diversified neuronal population, not only in physiological conditions, but also during inflammation. The reasons for the observed changes are not clear; they may be connected with the functions of zinc ions and their homeostasis disturbances in pathological processes. On the other hand, they may be due to adaptive and/or neuroprotective processes within the pathologically altered gastrointestinal tract.

Neuropeptide W modulation of gastric vagal afferent mechanosensitivity: Impact of age and sex

AIM

Gastric vagal afferents are activated in response to mechanical stimulation, an effect attenuated by neuropeptide W (NPW) in 20-week-old female mice. In this study we aimed to determine whether there were age and sex dependent effects of NPW on gastric vagal afferent mechanosensitivity.

METHODS

An in vitro gastro-oesophageal preparation was used to determine the effect of NPW on gastric vagal afferent mechanosensitivity from 8 and 20-week-old male and female C57BL/6 mice. Retrograde tracing and laser capture microdissection were used to selectively collect gastric vagal afferent cell bodies. Expression of NPW in the gastric mucosa and its receptor, GPR7, in gastric vagal afferent cell bodies was determined using quantitative RT-PCR.

RESULTS

NPW inhibited gastric tension sensitive vagal afferents from 20-week-old male and female mice, but not 8-week-old mice. In contrast, NPW inhibited the mechanosensitivity of gastric mucosal vagal afferents in 8-week-old male and female mice, but not 20-week-old mice. NPW mRNA expression in the gastric mucosa was higher in 20-week-old male mice compared to 8-week-old male mice. GPR7 mRNA expression in vagal afferent neurons innervating the gastric muscular layers was higher in 20-week-old mice compared to 8-week-old mice in both sexes.

CONCLUSION

The inhibitory effect of NPW on gastric tension sensitive and mucosal vagal afferents is age but not sex-dependent. These findings suggest that the physiological role of NPW varies depending on the age of the mice.

Diet-dependent modulation of gastro-oesphageal vagal afferent mechanosensitivity by endogenous nitric oxide

Neuronal nitric oxide (NO) plays an important role in gastric motor activity and modulates the mechanosensitivity of gastro-oesophageal vagal afferents. Effects of NO on food intake are dependent on feeding status. We sought to determine the effect of NO on gastro-oesophageal vagal afferent activity in the normally fed and food-restricted states and the second messenger pathways mediating these effects. Eight week old female C56BL/6 mice were fed ad libitum or food restricted for 14 h. An in vitro preparation was used to determine the functional effects of NO and the second messenger pathways involved. Expression of NO signal transduction molecules in vagal afferents was determined by reverse-transcription polymerase chain reaction (RT-PCR). Endogenous NO and the NO donor S-nitroso-N-acetylpenicillamine (SNAP) inhibited vagal mucosal afferent responses to tactile stimuli in mice fed ad libitum. After a 14 h fast endogenous NO and SNAP potentiated tension and mucosal afferent responses to mechanical stimulation. The excitatory effect of NO was blocked by the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor apocynin. After a 14 h fast expression of NADPH oxidase 2 (NOX2) mRNA in whole nodose ganglia was significantly reduced and the excitatory effect of NO on gastro-oesophageal vagal afferents was lost. Under fasting conditions the inhibitory effect of NO was blocked with the hyperpolarisation-activated cyclic nucleotide-gated (HCN) channel blocker ivabradine and mRNA expression of HCN3 in the nodose ganglia was elevated. In conclusion, the role of NO in the peripheral modulation of gastro-oesophageal vagal afferents is dynamic and dependent on feeding status.

Functional roles of capsaicin-sensitive intrinsic neural circuit in the regulation of esophageal peristalsis in rats: in vivo studies using a novel method

A well-developed myenteric plexus exists in the esophagus composed of striated muscle layers, but its functional role in controlling peristaltic movements remains to be clarified. The purpose of this study was to clarify the role of a local neural reflex consisting of capsaicin-sensitive primary afferent neurons and intrinsic neurons in esophageal peristalsis. We firstly devised a method to measure peristaltic movement of esophagus in vivo in rats. Rats were anesthetized with urethane, and esophageal intraluminal pressure and propelled intraluminal liquid volume were recorded. In the experimental system, an intraluminal pressure stimulus evoked periodic changes in intraluminal pressure of the esophagus, which were consistently accompanied by intraluminal liquid propulsion. Bilateral vagotomy abolished changes in intraluminal pressure as well as liquid propulsion. These results indicate that the novel method is appropriate for inducing peristalsis in the esophagus composed of striated muscles. Then, by using the method, we examined functional roles of the local reflex in esophageal peristalsis. For that purpose, we used rats in which capsaicin-sensitive neurons had been destroyed. The esophagus of capsaicin-treated rats showed a multiphasic rise in intraluminal pressure, which may due to noncoordinated contractions of esophageal muscles, whereas a monophasic response was observed in the intact rat esophagus. In addition, destruction of capsaicin-sensitive neurons increased the propelled liquid volume and lowered the pressure threshold for initiating peristalsis. These results suggest that the local neural reflex consisting of capsaicin-sensitive neurons and intrinsic neurons contributes to coordination of peristalsis and suppresses mechanosensory function of vagal afferents in the esophagus.

Characterization of the motor inhibitory role of colonic mucosa under chemical stimulation in mice

The main roles of the colonic mucosa are the absorption of water and electrolytes and the barrier function that preserves the integrity of the colonic wall. The mediators and mechanisms to accomplish these functions are under continuous investigation, but little attention has been paid to a possible control of colonic motility by the mucosa that would fine tune the relationship between absorption and motility. The purpose of this study was to establish the role of the mucosa in the control of induced colonic contractility. Young ICR-CD1 mice (3-5 mo old) were studied. Isometric tension transducers were used to record contractility in full-thickness (FT) and mucosa-free (MF) strips from proximal colon. Proximal FT strips showed lower KCl- and bethanechol-induced responses than MF strips. The difference was not due to mechanical artefacts since the contractile response of FT strips to electrical field stimulation was around 50% lower than in MF. The inhibitory effects of the mucosa on FT strips were mimicked by immersion of separate strips of mucosa in the organ bath but not by addition of mucosal extract, suggesting gaseous molecules as mediators of this effect. Incubation of MF strips with synthase inhibitors of nitric oxide, carbon monoxide, and hydrogen sulfide abolished the inhibition caused by addition of the mucosal strip, indicating that mucosal gasotransmitters are the mediators of these effects. This suggests that the control of colonic motility exerted by the mucosa could fine tune the balance between transit and absorption.

Putative roles of neuropeptides in vagal afferent signaling

The vagus nerve is a major pathway by which information is communicated between the brain and peripheral organs. Sensory neurons of the vagus are located in the nodose ganglia. These vagal afferent neurons innervate the heart, the lung and the gastrointestinal tract, and convey information about peripheral signals to the brain important in the control of cardiovascular tone, respiratory tone, and satiation, respectively. Glutamate is thought to be the primary neurotransmitter involved in conveying all of this information to the brain. It remains unclear how a single neurotransmitter can regulate such an extensive list of physiological functions from a wide range of visceral sites. Many neurotransmitters have been identified in vagal afferent neurons and have been suggested to modulate the physiological functions of glutamate. Specifically, the anorectic peptide transmitters, cocaine and amphetamine regulated transcript (CART) and the orexigenic peptide transmitters, melanin concentrating hormone (MCH) are differentially regulated in vagal afferent neurons and have opposing effects on food intake. Using these two peptides as a model, this review will discuss the potential role of peptide transmitters in providing a more precise and refined modulatory control of the broad physiological functions of glutamate, especially in relation to the control of feeding.

Modulation of murine gastric vagal afferent mechanosensitivity by neuropeptide W

AIM

Neuropeptide W (NPW) is an endogenous ligand for the receptors GPR7 and GPR8 and is involved in central regulation of energy homeostasis. NPW in the periphery is found in gastric gastrin (G) cells. In the stomach, energy intake is influenced by vagal afferent signals, so we aimed to determine the effect of NPW on mechanosensitive gastric vagal afferents under different feeding conditions.

METHODS

Female C57BL/6 mice (N > 10 per group) were fed a standard laboratory diet (SLD), high-fat diet (HFD) or were food restricted. The relationship between NPW immunopositive cells and gastric vagal afferent endings was determined by anterograde tracing and NPW immunohistochemistry. An in vitro gastro-oesophageal preparation was used to determine the functional effects of NPW on gastric vagal afferents. Expression of NPW in the gastric mucosa and GPR7 in whole nodose ganglia was determined by quantitative RT-PCR (QRT-PCR). The expression of GPR7 in gastric vagal afferent neurones was determined by retrograde tracing and QRT-PCR.

RESULTS

Neuropeptide W immunoreactive cells were found in close proximity to traced vagal afferents. NPW selectively inhibited responses of gastric vagal tension receptors to stretch in SLD but not HFD or fasted mice. In the nodose ganglia, GPR7 mRNA was specifically expressed in gastric vagal afferent neurones. In fasted mice gastric mucosal NPW and nodose GPR7, mRNA was reduced compared with SLD. A HFD had no effect on gastric NPW mRNA, but down-regulated nodose GPR7 expression.

CONCLUSION

Neuropeptide W modulates gastric vagal afferent activity, but the effect is dynamic and related to feeding status.

Influence of nitric oxide synthase inhibition on the motility and sensitivity of distal colon in man

BACKGROUND

Alterations of nitrergic innervation have been implicated in the pathophysiology of motor-sensory abnormalities of post infectious functional dyspepsia and could be involved in the pathophysiology of post infectious irritable bowel syndrome. The role of nitrergic neurons in the control of distal colonic sensorimotor function in man is not known. The aim of this study is to evaluate the motility and sensitivity of distal colon in healthy subjects before and after a nitric oxide synthase inhibitor (L-NMMA).

METHODS

A 700-mL balloon connected with a barostat-manometry assembly was placed in the descending colon of 10 healthy subjects and distension (4 mmHg/2 min) was performed. Intra-balloon pressure was then set at minimal distending pressure + 2 mmHg for 30 min, placebo or L-NMMA (8 mg kg(-1) h(-1)) was administered i.v. in double-blind, randomized, cross-over design and distensions were repeated.

KEY RESULTS

Placebo and L-NMMA did not influence colonic compliance, motility index, and tone. Placebo did not affect thresholds for first perception and discomfort and the areas under the pressure-perception curve. L-NMMA did not alter thresholds for first perception, but significantly decreased the pressure thresholds for discomfort (P = 0.008) and increased the areas under the pressure-perception score (P = 0.01).

CONCLUSIONS & INFERENCES

In man, inhibition of nitric oxide synthase sensitizes the distal colon to distension. Impaired nitrergic innervation is a mechanism that may be involved in the pathogenesis of hypersensitivity to colonic distension.

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.

Evidence of nNOS and ChAT positive phenotypes in nervous ganglia of the retrostyloid space

The cholinergic and nitrergic phenotypes in human fetal ganglia (inferior) of the glossopharyngeal and vagus nerves were overlooked in basic research. Lack of a positive neuronal NO synthase (nNOS) phenotype in the inferior vagal fetal ganglion was recently suggested to be an individually variable phenotype. Choline acetyltransferase (ChAT) was not evaluated previously in ontogenesis. We aimed to evaluate these phenotypes in human midterm fetuses. Samples from five specimens with gestational ages varying from 4 to 6 months were used. Immunohistochemistry for nNOS, ChAT, neurofilaments, and S100 protein was performed. Neuronal somata were positively stained for nNOS, ChAT and neurofilaments in the inferior glossopharyngeal and vagal ganglia. S100 protein distinctively labelled the satellite glial cells ensheating the respective neurons. In human midterm fetuses vagal and glossopharyngeal inferior ganglia are nitrergic and cholinergic. To evaluate a functional role of these phenotypes in ontogenesis, the specific anatomic circuits should be further checked. Differences in immune labelling should be evaluated by use of similar antibodies from different manufacturers.

Nitric oxide modulates bladder afferent nerve activity in the in vitro urinary bladder-pelvic nerve preparation from rats with cyclophosphamide induced cystitis

Effects of a nitric oxide (NO) donor (SNAP), NO substrate (l-arginine), and NO synthase inhibitor (l-NAME) on bladder afferent nerve (BAN) activity were studied in an in vitro bladder-pelvic nerve preparation from untreated or cyclophosphamide (CYP) treated rats. Distension of the bladder induced phasic bladder contractions (PBC) that were accompanied by multiunit afferent firing. Intravesical administration of SNAP (2mM) which did not change the amplitude of PBC significantly decreased peak afferent firing from 79 ± 15 spikes/s to 44 ± 8 spikes/s in CYP pretreated but not untreated preparations. In CYP treated preparations SNAP also decreased by 33-55% BAN firing induced by isotonic distension of the bladder at 10-40 cmH(2)O pressures. Electrical stimulation on the surface of the bladder elicited action potentials (AP) in BAN. SNAP significantly increased the voltage threshold by 75% (p<0.05) and decreased by 45% (p<0.05) the area of the AP evoked at submaximal stimulus intensity. Bath application of SNAP (2mM) or l-arginine (50mM) elicited similar inhibitory effects on the distension evoked BAN firing. The effects of l-arginine were blocked by bath application of l-NAME (20mM). l-NAME alone did not alter BAN firing. In preparations from normal rats SNAP or l-arginine did not alter BAN activity. These results suggest that exogenous as well as endogenously generated NO depresses the excitability of sensitized but not normal BAN and that NO may have an antinociceptive function and modulate bladder hyperactivity induced by pathological conditions.

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.

Peripheral neural targets in obesity

Interest in pharmacological treatments for obesity that act in the brain to reduce appetite has increased exponentially over recent years, but failures of clinical trials and withdrawals due to adverse effects have so far precluded any success. Treatments that do not act within the brain are, in contrast, a neglected area of research and development. This is despite the fact that a vast wealth of molecular mechanisms exists within the gut epithelium and vagal afferent system that could be manipulated to increase satiety. Here we discuss mechano- and chemosensory pathways from the gut involved in appetite suppression, and distinguish between gastric and intestinal vagal afferent pathways in terms of their basic physiology and activation by enteroendocrine factors. Gastric bypass surgery makes use of this system by exposing areas of the intestine to greater nutrient loads resulting in greater satiety hormone release and reduced food intake. A non-surgical approach to this system is preferable for many reasons. This review details where the opportunities may lie for such approaches by describing nutrient-sensing mechanisms throughout the gastrointestinal tract.

Genetic tracing of Nav1.8-expressing vagal afferents in the mouse

Nav1.8 is a tetrodotoxin-resistant sodium channel present in large subsets of peripheral sensory neurons, including both spinal and vagal afferents. In spinal afferents, Nav1.8 plays a key role in signaling different types of pain. Little is known, however, about the exact identity and role of Nav1.8-expressing vagal neurons. Here we generated mice with restricted expression of tdTomato fluorescent protein in all Nav1.8-expressing afferent neurons. As a result, intense fluorescence was visible in the cell bodies, central relays, and sensory endings of these neurons, revealing the full extent of their innervation sites in thoracic and abdominal viscera. For instance, vagal and spinal Nav1.8-expressing endings were seen clearly within the gastrointestinal mucosa and myenteric plexus, respectively. In the gastrointestinal muscle wall, labeled endings included a small subset of vagal tension receptors but not any stretch receptors. We also examined the detailed innervation of key metabolic tissues such as liver and pancreas and evaluated the anatomical relationship of Nav1.8-expressing vagal afferents with select enteroendocrine cells (i.e., ghrelin, glucagon, GLP-1). Specifically, our data revealed the presence of Nav1.8-expressing vagal afferents in several metabolic tissues and varying degrees of proximity between Nav1.8-expressing mucosal afferents and enteroendocrine cells, including apparent neuroendocrine apposition. In summary, this study demonstrates the power and versatility of the Cre-LoxP technology to trace identified visceral afferents, and our data suggest a previously unrecognized role for Nav1.8-expressing vagal neurons in gastrointestinal functions.

Impact of l-NAME on the cardiopulmonary reflex in cardiac hypertrophy

There is evidence that in cardiac failure, there is defective baroreceptor reflex control of sympathetic nerve activity. Often, cardiac failure is preceded by a state of cardiac hypertrophy in which there may be enhanced performance of the heart. This study investigated whether in two different models of cardiac hypertrophy, there was an increased contribution of nitric oxide (NO) to the low-pressure baroreceptor regulation of renal sympathetic nerve activity (RSNA) and nerve-dependent excretory function. Administration of a volume load, 0.25* body wt/min saline for 30 min, in normal rats decreased RSNA by 40* and increased urine flow by some 9-fold. Following nitro-l-arginine methyl ester (l-NAME) administration, 10 μg·kg−1·min−1 for 60 min, which had no effect on blood pressure, heart rate, or RSNA, the volume load-induced renal sympathoinhibitory and excretory responses were markedly enhanced. In cardiac hypertrophy states induced by 2 wk of isoprenaline/caffeine or 1 wk thyroxine administration, the volume challenge failed to suppress RSNA, and there were blunted increases in urine flow in the innervated kidneys, but following l-NAME infusion, the volume load decreased RSNA by 30–40* and increased urine flow by some 20-fold in the innervated kidneys, roughly to the same extent as observed in normal rats. These findings suggest that the blunted renal sympathoinhibition and nerve-dependent diuresis to the volume load in cardiac hypertrophy are related to a heightened production or activity of NO within either the afferent or central arms of the reflex.

Genomic variants in the coding region of neuronal nitric oxide synthase (NOS1) in infantile hypertrophic pyloric stenosis

BACKGROUND

Infantile hypertrophic pyloric stenosis (IHPS) is a common childhood pathology affecting 1 to 5:1000 newborns, with a genetic background suggested by familial occurrence. Neuronal nitric oxide synthase (NOS1) is a candidate gene owing to its role in the relaxation of smooth musculature and the association of the -84g>a variant of NOS1 with IHPS.

METHODS

We investigated NOS1 through sequencing of the complete NOS1 coding region in DNA from 43 patients with IHPS compared the genotype frequencies to 47 controls using the Cochran-Armitage trend or Fisher exact tests.

RESULTS

We found 19 polymorphisms in the coding region of NOS1. The variants c.3827-42_3827-43 del_insTA and c.+276 c>t were more frequent in IHPS with statistically significant exact P values (P = .010 and P = .039, respectively) yet failed to show significance after Bonferroni adjustment for multiple testing. We have also found a marginally significant occurrence of the variants c.-460A (P = .065) and c.2823+15G (P = .076). There was a significant correlation between the variants c.2706C>T ⬄ c.2823+15A>G, (r(2) = 1.00) and c.3258 C>T ⬄ c.3235+31A>G (r(2) = 1.00).

CONCLUSIONS

We conclude that NOS1 variants are present in patients with IHPS yet show no significant statistical association with the IHPS phenotype, suggesting at best an adjuvant role for NOS1 in IHPS.

Overactive and underactive bladder dysfunction is reflected by alterations in urothelial ATP and NO release

ATP and NO are released from the urothelium in the bladder. Detrusor overactivity (DO) following spinal cord injury results in higher ATP and lower NO release from the bladder urothelium. Our aim was to study the relationship between ATP and NO release in (1) early diabetic bladders, an overactive bladder model; and (2) "diuretic" bladders, an underactive bladder model. To induce diabetes mellitus female rats received 65mg/kg streptozocin (i.v.). To induce chronic diuresis rats were fed with 5% sucrose. At 28 days, in vivo open cystometry was performed. Bladder wash was collected to analyze the amount of ATP and NO released into the bladder lumen. For in vitro analysis of ATP and NO release, a Ussing chamber was utilized and hypoosmotic Krebs was perfused on the urothelial side of the chamber. ATP was analyzed with luminometry or HPLC-fluorometry while NO was measured with a Sievers NO-analyzer. In vivo ATP release was increased in diabetic bladders and unchanged in diuretic bladders. In vitro release from the urothelium followed the same pattern. NO release was unchanged both in vitro and in vivo in overactive bladders whereas it was enhanced in underactive bladders. We found that the ratio of ATP/NO, representing sensory transmission in the bladder, was high in overactive and low in underactive bladder dysfunction. In summary, ATP release has a positive correlation while NO release has a negative correlation with the bladder contraction frequency. The urinary ATP/NO ratio may be a clinically relevant biomarker to characterize the extent of bladder dysfunction.

Nitric oxide/cyclic guanosine monophosphate signalling mediates an inhibitory action on sensory pathways of the micturition reflex in the rat

BACKGROUND

Overactive bladder can be associated with a hyperexcitability of bladder afferent C-fibres. Several studies have suggested that nitric oxide (NO) or its downstream signalling could modulate the micturition reflex (MR) by reducing the excitability of bladder afferents.

OBJECTIVES

To evaluate the role of the NO/cyclic guanosine monophosphate (cGMP) signalling pathway on the MR in a model of bladder hyperactivity (BHA) associated with C-fibre activation in the rat.

DESIGN, SETTING, AND PARTICIPANTS

Adult female Sprague Dawley rats were used.

MEASUREMENTS

Cystometry was performed in anaesthetised rats. The effects of 0.1 mg/kg of sodium nitroprusside (SNP), an NO donor; 10 mg/kg of 8Br-cGMP, a cGMP analogue; 3 mg/kg of sildenafil and 1 mg/kg of vardenafil, two phosphodiesterase type 5 inhibitors (PDE5-I); 10 mg/ml of L-N(G)-nitroarginine methyl ester (L-NAME), an NO synthase inhibitor; and 1 mg/kg of LY-83583, a guanylate cyclase inhibitor, were investigated on BHA during intravesical capsaicin (30 micromol/l) instillation. All drugs were delivered intravenously except for L-NAME, which was intravesically administered.

RESULTS AND LIMITATIONS

SNP, 8Br-cGMP, and PDE5-I increased the intercontraction interval (ICI), while SNP and PDE5-I increased the micturition pressure threshold (MPT). L-NAME and LY-83583 decreased MPT, and L-NAME decreased ICI. 8Br-cGMP decreased the maximum intravesical pressure (MP), contrary to L-NAME and LY-83583. SNP and PDE5-I had no effect on MP. SNP increased the voided volume (VV). PDE5-I and 8Br-cGMP also increased VV, although not significantly. In contrast, L-NAME tended to decrease VV. Although 8Br-cGMP decreased the baseline intravesical pressure, LY-83583 increased it. Neither SNP nor PDE5-I nor L-NAME had any effect on baseline pressure.

CONCLUSIONS

Compounds activating the NO/cGMP pathway inhibited BHA, whereas compounds inhibiting the NO/cGMP pathway increased it. These results indicate that the NO/cGMP signalling pathway is involved in the regulation of the MR, with an action that seems more predominant on the sensory rather on the motor component of the MR in a rat model of BHA associated with C-fibre afferent activation.

Kisspeptin signaling is required for peripheral but not central stimulation of gonadotropin-releasing hormone neurons by NMDA

NMDA and kisspeptins can stimulate gonadotropin-releasing hormone (GnRH) release after peripheral or central administration in mice. To determine whether these agonists act independently or through a common pathway, we have examined their ability to stimulate GnRH/luteinizing hormone (LH) release after peripheral or central administration in Kiss1- or Gpr54 (Kiss1r)-null mutant mice. Peripheral injection of NMDA failed to stimulate GnRH/LH release in prepubertal or gonadally intact mutant male mice. Dual-labeling experiments indicated a direct activation of Kiss1-expressing neurons in the arcuate nucleus. In contrast, central injection of NMDA into the lateral ventricle increased plasma LH levels in both Kiss1 and Gpr54 mutant male mice similar to the responses in wild-type mice. Central injection of NMDA stimulated c-Fos expression throughout the hypothalamus but not in GnRH neurons, suggesting an action at the nerve terminals only. In contrast, kisspeptin-10 stimulated LH release after both central and peripheral injection but induced c-Fos expression in GnRH neurons only after central administration. Finally, central injection of NMDA induces c-Fos expression in catecholamine- and nitric oxide-producing neurons in the hypothalamus of mutant mice, indicating a possible kisspeptin-independent GnRH/LH release by NMDA through activation of these neurons. Thus, NMDA may act at both GnRH cell bodies (kisspeptin-independent) and nerve terminals (kisspeptin-dependent) in a dual way to participate in the GnRH/LH secretion in the male mouse.

Nitric oxide regulates BDNF release from nodose ganglion neurons in a pattern-dependent and cGMP-independent manner

Activity of arterial baroreceptors is modulated by neurohumoral factors, including nitric oxide (NO), released from endothelial cells. Baroreceptor reflex responses can also be modulated by NO signaling in the brainstem nucleus tractus solitarius (NTS), the primary central target of cardiovascular afferents. Our recent studies indicate that brain-derived neurotrophic factor (BDNF) is abundantly expressed by developing and adult baroreceptor afferents in vivo, and released from cultured nodose ganglion (NG) neurons by patterns of baroreceptor activity. Using electrical field stimulation and ELISA in situ, we show that exogenous NO nearly abolishes BDNF release from newborn rat NG neurons in vitro stimulated with single pulses delivered at 6 Hz, but not 2-pulse bursts delivered at the same 6-Hz frequency, that corresponds to a rat heart rate. Application of L-NAME, a specific inhibitor of endogenous NO synthases, does not have any significant effect on activity-dependent BDNF release, but leads to upregulation of BDNF expression in an activity-dependent manner. The latter effect suggests a novel mechanism of homeostatic regulation of activity-dependent BDNF expression with endogenous NO as a key player. The exogenous NO-mediated effect does not involve the cGMP-protein kinase G (PKG) pathway, but is largely inhibited by N-ethylmaleimide and TEMPOL that are known to prevent S-nitrosylation. Together, our current data identify previously unknown mechanisms regulating BDNF availability, and point to NO as a likely regulator of BDNF at baroafferent synapses in the NTS.

Visceral afferents - determinants and modulation of excitability

An essential property of visceral sensory afferents is to be able to alter their firing properties in response to changes in the microenvironment at the level of the sensory ending. Significant progress has been made in recent years in understanding the ionic mechanisms of the regulation of afferent neuronal excitability, and in identifying the mechanisms by which this can be altered. This article will review some of the recent developments in the state of knowledge regarding mechanisms of increased excitability after inflammation, and pharmacological modulation of excitability, concentrating on afferent nerves innervating the GI tract and urinary bladder.

Post-inflammatory modification of colonic afferent mechanosensitivity

1. The present review discusses interactions between the immune and nervous systems in post-infectious irritable bowel syndrome (PI-IBS). 2. Visceral pain is the single symptom that most affects the quality of life of patients with irritable bowel syndrome (IBS), yet it is the least successfully managed. An underlying hypersensitivity of colonic afferents to mechanical stimuli has long been implicated in visceral pain in IBS, but little more is known of the physiological aetiology. 3. The PI-IBS patients are a cohort of IBS patients who attribute their symptoms to a preceding gastrointestinal infection by pathogens such as Campylobacter or Salmonella. Current evidence suggests that the immune system remains activated in these patients and contributes to their visceral hypersensitivity. This is characterized by a shift in the phenotype of circulating immune cells towards a Type 1 (Th1 predominating) state. Products from these immune cells sensitize colonic afferents to mechanical stimuli. 4. Rectal instillation of trinitrobenzene sulphonic acid induces a Th1-mediated inflammatory response, consistent with clinical observations in PI-IBS. The visceral hypersensitivity observed in this model is biphasic, with an initial onset characterized by visceral hypersensitivity correlating with histological damage followed by a delayed phase that occurs after histological recovery. Interestingly, this chronic visceral hypersensitivity is mediated by afferents in closest apposition to blood vessels, but furthest from the initial site of damage. 5. Both clinical and experimental evidence indicates that chronic dysregulation of the immune system induces visceral afferent hypersensitivity and, therefore, may be the central mechanism underlying PI-IBS.