Mild gastritis alters voltage-sensitive sodium currents in gastric sensory neurons in rats

Heterologous expression of the human wild-type and variant NaV 1.8 (A1073V) in rat sensory neurons

BACKGROUND

Silent inflammatory bowel disease (IBD) is a condition in which individuals with the active disease experience minor to no pain. Voltage-gated Na+ (NaV ) channels expressed in sensory neurons play a major role in pain perception. Previously, we reported that a NaV 1.8 genetic polymorphism (A1073V, rs6795970) was more common in a cohort of silent IBD patients. The expression of this variant (1073V) in rat sympathetic neurons activated at more depolarized potentials when compared to the more common variant (1073A). In this study, we investigated whether expression of either NaV 1.8 variant in rat sensory neurons would exhibit different biophysical characteristics than previously observed in sympathetic neurons.

METHODS

Endogenous NaV 1.8 channels were first silenced in DRG neurons and then either 1073A or 1073V human NaV 1.8 cDNA constructs were transfected. NaV 1.8 currents were recorded with the whole-cell patch-clamp technique.

KEY RESULTS

The results indicate that 1073A and 1073V NaV 1.8 channels exhibited similar activation values. However, the slope factor (k) for activation determined for this same group of neurons decreased by 5 mV, suggesting an increase in voltage sensitivity. Comparison of inactivation parameters indicated that 1073V channels were shifted to more depolarized potentials than 1073A-expressing neurons, imparting a proexcitatory characteristic.

CONCLUSIONS AND INFERENCES

These findings differ from previous observations in other expression models and underscore the challenges with heterologous expression systems. Therefore, the use of human sensory neurons derived from induced pluripotent stem cells may help address these inconsistencies and better determine the effect of the polymorphism present in IBD patients.

Tetrodotoxin: A New Strategy to Treat Visceral Pain?

Visceral pain is one of the most common symptoms associated with functional gastrointestinal (GI) disorders. Although the origin of these symptoms has not been clearly defined, the implication of both the central and peripheral nervous systems in visceral hypersensitivity is well established. The role of several pathways in visceral nociception has been explored, as well as the influence of specific receptors on afferent neurons, such as voltage-gated sodium channels (VGSCs). VGSCs initiate action potentials and dysfunction of these channels has recently been associated with painful GI conditions. Current treatments for visceral pain generally involve opioid based drugs, which are associated with important side-effects and a loss of effectiveness or tolerance. Hence, efforts have been intensified to find new, more effective and longer-lasting therapies. The implication of VGSCs in visceral hypersensitivity has drawn attention to tetrodotoxin (TTX), a relatively selective sodium channel blocker, as a possible and promising molecule to treat visceral pain and related diseases. As such, here we will review the latest information regarding this toxin that is relevant to the treatment of visceral pain and the possible advantages that it may offer relative to other treatments, alone or in combination.

Upregulation of the TRPA1 Ion Channel in the Gastric Mucosa after Iodoacetamide-Induced Gastritis in Rats: A Potential New Therapeutic Target

Acute gastritis is often untreatable by acid secretion-inhibiting drugs. Understanding the protective mechanisms including the role of Transient Receptor Potential Ankyrin1 (TRPA1) and Vanilloid1 (TRPV1) channels localized on capsaicin-sensitive afferents and non-neuronal structures might identify novel therapeutic approaches. Therefore, we characterized a translational gastritis model using iodoacetamide (IAA) and investigated TRPA1/V1 expressions. Wistar rats and CD1, C57Bl/6J mice were exposed to IAA-containing (0.05, 0.1, 0.2, 0.3, 0.5%) drinking water for 7 or 14 days. Body weight and water consumption were recorded daily. Macroscopic lesions were scored, qualitative histopathologic investigation was performed, TRPA1/V1 immunopositivity and mRNA expressions were measured. IAA induced a concentration-dependent weight loss and reduced water intake in both species. Hyperemia, submucosal edema, inflammatory infiltration and hemorrhagic erosions developed after 7 days, while ulcers after 14 days in rats. Trpa1 mRNA/protein expressions were upregulated at both timepoints. Meanwhile, TRPV1 immunopositivity was upregulated in the gastric corpus after 0.05% IAA ingestion, but downregulated after 0.2%, whereas Trpv1 mRNA did not change. Interestingly, no macroscopic/microscopic changes were observed in mice. These are the first data for the concentration- and duration-dependent changes in the IAA-induced gastritis in rats accompanied by TRPA1 upregulation, therefore, its therapeutic potential in gastritis should further be investigated.

Is cervical region tightness related to vagal function and stomach symptoms?

The vagal nerve is a cranial nerve that carries mainly parasympathetic fibers (average 75%) with both sensory and motor functions. The vagal nerve contains a complex neuro-endocrine-immune network. The majority, at least 66%, of the gastric myenteric neurons receive direct cholinergic excitatory stimulation from the pre-enteric vagal nerve. Changes in vagal function may cause stomach problems, although the mechanisms that change the vagal function have not yet been fully illuminated. Considering the course of the vagal nerve in the cervical region, it is thought that conditions such as stiffness, tightness and decreased elasticity in this region may compress the vagal nerve andmay affect vagal function. According to this hypothesis, neuroinflammation and hyperalgesia may occur in the vagal nerve under mechanical pressure, resulting in increased complaints of pain and burning in the stomach increases. However, as the vagal nerve has various effects on the motility of the stomach and vagal dysfunction affects the motor function of the stomach, relaxation techniques applied to the soft tissues of the cervical region will provide mechanical relief in the nerve. Thus, the vagal nerve will be decompressed and be able to function optimally. According to our clinical observations, in patients whose soft tissues in the cervical region are relaxed, gastric symptoms are decreased. Based on research results and clinical experience, cervical region tightness can be considered to cause stomach problems through the vagal nerve, and soft tissue relaxation of the cervical region can be a promising treatment method for stomach symptoms.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

Acute pain signaling has a key protective role and is highly evolutionarily conserved. Chronic pain, however, is maladaptive, occurring as a consequence of injury and disease, and is associated with sensitization of the somatosensory nervous system. Primary sensory neurons are involved in both of these processes, and the recent advances in understanding sensory transduction and human genetics are the focus of this review. Voltage-gated sodium channels (VGSCs) are important determinants of sensory neuron excitability: they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and neurotransmitter release from sensory neuron terminals. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are all expressed by adult sensory neurons. The biophysical characteristics of these channels, as well as their unique expression patterns within subtypes of sensory neurons, define their functional role in pain signaling. Changes in the expression of VGSCs, as well as posttranslational modifications, contribute to the sensitization of sensory neurons in chronic pain states. Furthermore, gene variants in Nav1.7, Nav1.8, and Nav1.9 have now been linked to human Mendelian pain disorders and more recently to common pain disorders such as small-fiber neuropathy. Chronic pain affects one in five of the general population. Given the poor efficacy of current analgesics, the selective expression of particular VGSCs in sensory neurons makes these attractive targets for drug discovery. The increasing availability of gene sequencing, combined with structural modeling and electrophysiological analysis of gene variants, also provides the opportunity to better target existing therapies in a personalized manner.

The influence of voltage-gated sodium channels on human gastrointestinal nociception

BACKGROUND

Abdominal pain is a frequent and persistent problem in the most common gastrointestinal disorders, including irritable bowel syndrome and inflammatory bowel disease. Pain adversely impacts quality of life, incurs significant healthcare expenditures, and remains a challenging issue to manage with few safe therapeutic options currently available. It is imperative that new methods are developed for identifying and treating this symptom. A variety of peripherally active neuroendocrine signaling elements have the capability to influence gastrointestinal pain perception. A large and growing body of evidence suggests that voltage-gated sodium channels (VGSCs) play a critical role in the development and modulation of nociceptive signaling associated with the gut. Several VGSC isoforms demonstrate significant promise as potential targets for improved diagnosis and treatment of gut-based disorders associated with hyper- and hyposensitivity to abdominal pain.

PURPOSE

In this article, we critically review key investigations that have evaluated the potential role that VGSCs play in visceral nociception and discuss recent advances related to this topic. Specifically, we discuss the following: (a) what is known about the structure and basic function of VGSCs, (b) the role that each VGSC plays in gut nociception, particularly as it relates to human physiology, and (c) potential diagnostic and therapeutic uses of VGSCs to manage disorders associated with chronic abdominal pain.

Homozygosity for the SCN10A Polymorphism rs6795970 Is Associated With Hypoalgesic Inflammatory Bowel Disease Phenotype

Background: Hypoalgesic inflammatory bowel disease (IBD), a condition in which patients with active disease do not perceive and/or report abdominal pain, is associated with serious complications and there is a lack of cost-effective, reliable diagnostic methods to identify “at-risk” patients. The voltage-gated sodium channels (VGSC's), Na_v1.7, Na_v1.8, and Na_v1.9, are preferentially expressed on nociceptive neurons, and have been implicated in visceral inflammatory pain. At least 29 VGSC single nucleotide polymorphisms (SNPs) have been implicated in chronic somatic pain syndromes, but little is known about their role in human visceral sensation. We hypothesized that disruptive VGSC polymorphisms result in anti-nociceptive behavior in IBD.

Methods and Findings: We performed targeted exome sequencing and/or TaqMan genotyping to evaluate the Na_v1.7, Na_v1.8, and Na_v1.9 genes (SCN9A, SCN10A and SCN11A) in 121 IBD patients (including 41 “hypoalgesic” IBD patients) and 86 healthy controls. Allelic and genotypic frequencies of polymorphisms were compared among study groups who had undergone characterization of intestinal inflammatory status and abdominal pain experience. Forty-nine total exonic SNPs were identified. The allelic frequency of only one non-synonymous SNP (rs6795970 [SCN10A]) approached significance in hypoalgesic IBD patients when compared to other IBD patients (p = 0.096, Fisher's exact test). Hypoalgesic IBD patients were more likely to be homozygous for this polymorphism (46 vs. 22%, p = 0.01, Fisher's exact test).

Conclusions: This is the first human study to demonstrate a link between a genetic variant of SCN10A and abdominal pain perception in IBD. These findings provide key insights into visceral nociceptive physiology and new diagnostic and therapeutic targets to consider in IBD and other gastrointestinal conditions associated with chronic abdominal pain. Further studies are required to elucidate the precise pathophysiological impact of the rs6795970 polymorphism on human gastrointestinal nociception.

Voltage-gated sodium channels: (NaV )igating the field to determine their contribution to visceral nociception

Chronic visceral pain, altered motility and bladder dysfunction are common, yet poorly managed symptoms of functional and inflammatory disorders of the gastrointestinal and urinary tracts. Recently, numerous human channelopathies of the voltage-gated sodium (NaV ) channel family have been identified, which induce either painful neuropathies, an insensitivity to pain, or alterations in smooth muscle function. The identification of these disorders, in addition to the recent utilisation of genetically modified NaV mice and specific NaV channel modulators, has shed new light on how NaV channels contribute to the function of neuronal and non-neuronal tissues within the gastrointestinal tract and bladder. Here we review the current pre-clinical and clinical evidence to reveal how the nine NaV channel family members (NaV 1.1-NaV 1.9) contribute to abdominal visceral function in normal and disease states.

Choosing an Animal Model for the Study of Functional Dyspepsia

Functional dyspepsia (FD) is a common functional gastrointestinal disorder with pain or discomfort in the upper abdomen as the main characteristic. The prevalence of FD worldwide varies between 5% and 11%. This condition adversely affects attendance and productivity in the workplace. Emerging evidence is beginning to unravel the pathophysiologies of FD, and new data on treatment are helping to guide evidence-based practice. In order to better understand the pathophysiologies of FD and explore better treatment options, various kinds of animal models of FD have been developed. However, it is unclear which of these models most closely mimic the human disease. This review provides a comprehensive overview of the currently available animal models of FD in relationship to the clinical features of the disease. The rationales, methods, merits, and disadvantages for modelling specific symptoms of FD are discussed in detail.

Inhibitory neurotransmission regulates vagal efferent activity and gastric motility

The gastrointestinal tract receives extrinsic innervation from both the sympathetic and parasympathetic nervous systems, which regulate and modulate the function of the intrinsic (enteric) nervous system. The stomach and upper gastrointestinal tract in particular are heavily influenced by the parasympathetic nervous system, supplied by the vagus nerve, and disruption of vagal sensory or motor functions results in disorganized motility patterns, disrupted receptive relaxation and accommodation, and delayed gastric emptying, amongst others. Studies from several laboratories have shown that the activity of vagal efferent motoneurons innervating the upper GI tract is inhibited tonically by GABAergic synaptic inputs from the adjacent nucleus tractus solitarius. Disruption of this influential central GABA input impacts vagal efferent output, hence gastric functions, significantly. The purpose of this review is to describe the development, physiology, and pathophysiology of this functionally dominant inhibitory synapse and its role in regulating vagally determined gastric functions.

Altered Ion Channel/Receptor Expression and Function in Extrinsic Sensory Neurons: The Cause of and Solution to Chronic Visceral Pain?

The gastrointestinal tract is unique in that it is innervated by several distinct populations of neurons, whose cell bodies are either intrinsic (enteric, viscerofugal) or extrinsic (sympathetic, sensory afferents) to the wall of the gut. We are usually completely unaware of the continuous, complicated orchestra of functions that these neurons conduct. However, for patients with Inflammatory Bowel Disease (IBD) or functional gastrointestinal disorders, such as Functional Dyspepsia (FD) and Irritable Bowel Syndrome (IBS) altered gastrointestinal motility, discomfort and pain are common, debilitating symptoms. Whilst bouts of inflammation underlie the symptoms associated with IBD, over the past few years there is increased pre-clinical and clinical evidence that infection and inflammation are key risk factors for the development of several functional gastrointestinal disorders, in particular IBS. There is a strong correlation between prior exposure to gut infection and symptom occurrence; with the duration and severity of the initial illness the strongest associated risk factors. This review discusses the current body of evidence for neuroplasticity during inflammation and how in many cases fails to reset back to normal, long after healing of the damaged tissues. Recent evidence suggests that the altered expression and function of key ion channels and receptors within extrinsic sensory neurons play fundamental roles in the aberrant pain sensation associated with these gastrointestinal diseases and disorders.

High fat diet attenuates glucose-dependent facilitation of 5-HT3 -mediated responses in rat gastric vagal afferents

KEY POINTS

Glucose regulates the density and function of 5-HT3 receptors on gastric vagal afferent neurones. Diet-induced obesity compromises the excitability and responsiveness of vagal afferents. In this study, we assessed whether exposure to a high fat diet (HFD) compromises the glucose-dependent modulation of 5-HT responses in gastric vagal afferents prior to the development of obesity. We show that HFD does not alter the response of gastric vagal afferent nerves and neurones to 5-HT but attenuates the ability of glucose to amplify 5-HT3-induced responses. These results suggest that glucose-dependent vagal afferent signalling is compromised by relatively short periods of exposure to HFD well in advance of the development of obesity or glycaemic dysregulation. Glucose regulates the density and function of 5-HT3 receptors on gastric vagal afferent neurones. Since diet-induced obesity attenuates the responsiveness of gastric vagal afferents to several neurohormones, the aim of the present study was to determine whether high fat diet (HFD) compromises the glucose-dependent modulation of 5-HT responses in gastric vagal afferents prior to the development of obesity. Rats were fed control or HFD (14% or 60% kilocalories from fat, respectively) for up to 8 weeks. Neurophysiological recordings assessed the ability of 5-HT to increase anterior gastric vagal afferent nerve (VAN) activity in vivo before and after acute hyperglycaemia, while electrophysiological recordings from gastric-projecting nodose neurones assessed the ability of glucose to modulate the 5-HT response in vitro. Immunocytochemical studies determined alterations in the neuronal distribution of 5-HT3 receptors. 5-HT and cholecystokinin (CCK) induced dose-dependent increases in VAN activity in all rats; HFD attenuated the response to CCK, but not 5-HT. The 5-HT-induced response was amplified by acute hyperglycaemia in control, but not HFD, rats. Similarly, although 5-HT induced an inward current in both control and HFD gastric nodose neurones in vitro, the 5-HT response and receptor distribution was amplified by acute hyperglycaemia only in control rats. These data suggest that, while HFD does not affect the response of gastric-projecting vagal afferents to 5-HT, it attenuates the ability of glucose to amplify 5-HT effects. This suggests that glucose-dependent vagal afferent signalling is compromised by short periods of exposure to HFD well in advance of obesity or glycaemic dysregulation.

Role of central vagal 5-HT3 receptors in gastrointestinal physiology and pathophysiology

Vagal neurocircuits are vitally important in the co-ordination and modulation of GI reflexes and homeostatic functions. 5-hydroxytryptamine (5-HT; serotonin) is critically important in the regulation of several of these autonomic gastrointestinal (GI) functions including motility, secretion and visceral sensitivity. While several 5-HT receptors are involved in these physiological responses, the ligand-gated 5-HT3 receptor appears intimately involved in gut-brain signaling, particularly via the afferent (sensory) vagus nerve. 5-HT is released from enterochromaffin cells in response to mechanical or chemical stimulation of the GI tract which leads to activation of 5-HT3 receptors on the terminals of vagal afferents. 5-HT3 receptors are also present on the soma of vagal afferent neurons, including GI vagal afferent neurons, where they can be activated by circulating 5-HT. The central terminals of vagal afferents also exhibit 5-HT3 receptors that function to increase glutamatergic synaptic transmission to second order neurons of the nucleus tractus solitarius within the brainstem. While activation of central brainstem 5-HT3 receptors modulates visceral functions, it is still unclear whether central vagal neurons, i.e., nucleus of the tractus solitarius (NTS) and dorsal motor nucleus of the vagus (DMV) neurons themselves also display functional 5-HT3 receptors. Thus, activation of 5-HT3 receptors may modulate the excitability and activity of gastrointestinal vagal afferents at multiple sites and may be involved in several physiological and pathophysiological conditions, including distention- and chemical-evoked vagal reflexes, nausea, and vomiting, as well as visceral hypersensitivity.

Role of satellite glial cells in gastrointestinal pain

Gastrointestinal (GI) pain is a common clinical problem, for which effective therapy is quite limited. Sensations from the GI tract, including pain, are mediated largely by neurons in the dorsal root ganglia (DRG), and to a smaller extent by vagal afferents emerging from neurons in the nodose/jugular ganglia. Neurons in rodent DRG become hyperexcitable in models of GI pain (e.g., gastric or colonic inflammation), and can serve as a source for chronic pain. Glial cells are another element in the pain signaling pathways, and there is evidence that spinal glial cells (microglia and astrocytes) undergo activation (gliosis) in various pain models and contribute to pain. Recently it was found that satellite glial cells (SGCs), the main type of glial cells in sensory ganglia, might also contribute to chronic pain in rodent models. Most of that work focused on somatic pain, but in several studies GI pain was also investigated, and these are discussed in the present review. We have shown that colonic inflammation induced by dinitrobenzene sulfonic acid (DNBS) in mice leads to the activation of SGCs in DRG and increases gap junction-mediated coupling among these cells. This coupling appears to contribute to the hyperexcitability of DRG neurons that innervate the colon. Blocking gap junctions (GJ) in vitro reduced neuronal hyperexcitability induced by inflammation, suggesting that glial GJ participate in SGC-neuron interactions. Moreover, blocking GJ by carbenoxolone and other agents reduces pain behavior. Similar changes in SGCs were also found in the mouse nodose ganglia (NG), which provide sensory innervation to most of the GI tract. Following systemic inflammation, SGCs in these ganglia were activated, and displayed augmented coupling and greater sensitivity to the pain mediator ATP. The contribution of these changes to visceral pain remains to be determined. These results indicate that although visceral pain is unique, it shares basic mechanisms with somatic pain, suggesting that therapeutic approaches to both pain types may be similar. Future research in this field should include additional types of GI injury and also other types of visceral pain.

Exposure to a high fat diet during the perinatal period alters vagal motoneurone excitability, even in the absence of obesity

KEY POINTS

Obesity is recognized as being multifactorial in origin, involving both genetic and environmental factors. The perinatal period is known to be critically important in the development of neural circuits responsible for energy homeostasis and the integration of autonomic reflexes. Diet-induced obesity alters the biophysical, pharmacological and morphological properties of vagal neurocircuits regulating upper gastrointestinal tract functions, including satiety. Less information is available, however, regarding the effects of a high fat diet (HFD) itself on the properties of vagal neurocircuits. The present study was designed to test the hypothesis that exposure to a HFD during the perinatal period alters the electrophysiological, pharmacological and morphological properties of vagal efferent motoneurones innervating the stomach. Our data indicate that perinatal HFD decreases the excitability of gastric-projecting dorsal motor nucleus neurones and dysregulates neurotransmitter release from synaptic inputs and that these alterations occur prior to the development of obesity. These findings represent the first direct evidence that exposure to a HFD modulates the processing of central vagal neurocircuits even in the absence of obesity. The perinatal period is critically important to the development of autonomic neural circuits responsible for energy homeostasis. Vagal neurocircuits are vital to the regulation of upper gastrointestinal functions, including satiety. Diet-induced obesity modulates the excitability and responsiveness of both peripheral vagal afferents and central vagal efferents but less information is available regarding the effects of diet per se on vagal neurocircuit functions. The aims of this study were to investigate whether perinatal exposure to a high fat diet (HFD) dysregulated dorsal motor nucleus of the vagus (DMV) neurones, prior to the development of obesity. Whole cell patch clamp recordings were made from gastric-projecting DMV neurones in thin brainstem slices from rats that were exposed to either a control diet or HFD from pregnancy day 13. Our data demonstrate that following perinatal HFD: (i) DMV neurones had decreased excitability and input resistance with a reduced ability to fire action potentials; (ii) the proportion of DMV neurones excited by cholecystokinin (CCK) was unaltered but the proportion of neurones in which CCK increased excitatory glutamatergic synaptic inputs was reduced; (iii) the tonic activation of presynaptic group II metabotropic glutamate receptors on inhibitory nerve terminals was attenuated, allowing modulation of GABAergic synaptic transmission; and (iv) the size and dendritic arborization of gastric-projecting DMV neurones was increased. These results suggest that perinatal HFD exposure compromises the excitability and responsiveness of gastric-projecting DMV neurones, even in the absence of obesity, suggesting that attenuation of vago-vagal reflex signalling may precede the development of obesity.

Down-regulation of A-type potassium channel in gastric-specific DRG neurons in a rat model of functional dyspepsia

BACKGROUND

Although without evidence of organic structural abnormalities, pain or discomfort is a prominent symptom of functional dyspepsia and considered to reflect visceral hypersensitivity whose underlying mechanism is poorly understood. Here, we studied electrophysiological properties and expression of voltage-gated potassium channels in dorsal root ganglion (DRG) neurons in a rat model of functional dyspepsia induced by neonatal gastric irritation.

METHODS

Male Sprague-Dawley rat pups at 10-day old received 0.1% iodoacetamide (IA) or vehicle by oral gavage for 6 days and studied at adulthood. Retrograde tracer-labeled gastric-specific T8 -T12 DRG neurons were harvested for the patch-clamp study in voltage and current-clamp modes and protein expression of K(+) channel in T8 -T12 DRGs was examined by western blotting.

KEY RESULTS

(1) Gastric specific but not non-gastric DRG neurons showed an enhanced excitability in neonatal IA-treated rats compared to the control: depolarized resting membrane potentials, a lower current threshold for action potential (AP) activation, and an increase in the number of APs in response to current stimulation. (2) The current density of tetraethylammonium insensitive (transiently inactivating A-type current), but not the tetraethylammonium sensitive (slow-inactivating delayed rectifier K(+) currents), was significantly smaller in IA-treated rats (65.4 ± 6.9 pA/pF), compared to that of control (93.1 ± 8.3 pA/pF). (3) Protein expression of KV 4.3 was down-regulated in IA-treated rats.

CONCLUSIONS & INFERENCES

A-type potassium channels are significantly down-regulated in the gastric-specific DRG neurons in adult rats with mild neonatal gastric irritation, which in part contribute to the enhanced DRG neuron excitabilities that leads to the development of gastric hypersensitivity.

Functional up-regulation of Nav1.8 sodium channel in Aβ afferent fibers subjected to chronic peripheral inflammation

Background

Functional alterations in the properties of Aβ afferent fibers may account for the increased pain sensitivity observed under peripheral chronic inflammation. Among the voltage-gated sodium channels involved in the pathophysiology of pain, Na_v1.8 has been shown to participate in the peripheral sensitization of nociceptors. However, to date, there is no evidence for a role of Na_v1.8 in controlling Aβ-fiber excitability following persistent inflammation.

Methods

Distribution and expression of Na_v1.8 in dorsal root ganglia and sciatic nerves were qualitatively or quantitatively assessed by immunohistochemical staining and by real time-polymerase chain reaction at different time points following complete Freund’s adjuvant (CFA) administration. Using a whole-cell patch-clamp configuration, we further determined both total I_Na and TTX-R Na_v1.8 currents in large-soma dorsal root ganglia (DRG) neurons isolated from sham or CFA-treated rats. Finally, we analyzed the effects of ambroxol, a Na_v1.8-preferring blocker on the electrophysiological properties of Na_v1.8 currents and on the mechanical sensitivity and inflammation of the hind paw in CFA-treated rats.

Results

Our findings revealed that Na_v1.8 is up-regulated in NF200-positive large sensory neurons and is subsequently anterogradely transported from the DRG cell bodies along the axons toward the periphery after CFA-induced inflammation. We also demonstrated that both total I_Na and Na_v1.8 peak current densities are enhanced in inflamed large myelinated Aβ-fiber neurons. Persistent inflammation leading to nociception also induced time-dependent changes in Aβ-fiber neuron excitability by shifting the voltage-dependent activation of Na_v1.8 in the hyperpolarizing direction, thus decreasing the current threshold for triggering action potentials. Finally, we found that ambroxol significantly reduces the potentiation of Na_v1.8 currents in Aβ-fiber neurons observed following intraplantar CFA injection and concomitantly blocks CFA-induced mechanical allodynia, suggesting that Na_v1.8 regulation in Aβ-fibers contributes to inflammatory pain.

Conclusions

Collectively, these findings support a key role for Na_v1.8 in controlling the excitability of Aβ-fibers and its potential contribution to the development of mechanical allodynia under persistent inflammation.

Gastric sensitivity and reflexes: basic mechanisms underlying clinical problems

Both reflex and sensory mechanisms control the function of the stomach, and disturbances in these mechanisms may explain the pathophysiology of disorders of gastric function. The objective of this report is to perform a literature-based critical analysis of new, relevant or conflicting information on gastric sensitivity and reflexes, with particular emphasis on the comprehensive integration of basic and clinical research data. The stomach exerts both phasic and tonic muscular (contractile and relaxatory) activity. Gastric tone determines the capacity of the stomach and mediates both gastric accommodation to a meal as well as gastric emptying, by partial relaxation or progressive recontraction, respectively. Perception and reflex afferent pathways from the stomach are activated independently by specific stimuli, suggesting that the terminal nerve endings operate as specialized receptors. Particularly, perception appears to be related to stimulation of tension receptors, while the existence of volume receptors in the stomach is uncertain. Reliable techniques have been developed to measure gastric perception and reflexes both in experimental and clinical conditions, and have facilitated the identification of abnormal responses in patients with gastric disorders. Gastroparesis is characterised by impaired gastric tone and contractility, whereas patients with functional dyspepsia have impaired accommodation, associated with antral distention and increased gastric sensitivity. An integrated view of fragmented knowledge allows the design of pathophysiological models in an attempt to explain disorders of gastric function, and may facilitate the development of mechanistically orientated treatments.

Effect of ECQ on Iodoacetamide-Induced Chronic Gastritis in Rats

This study investigated effect of extract containing quercetin-3-O-β-D-glucuronopyranoside from Rumex Aquaticus Herba (ECQ) against chronic gastritis in rats. To produce chronic gastritis, the animals received a daily intra-gastric administration of 0.1 ml of 0.15% iodoacetamide (IA) solution for 7 days. Daily exposure of the gastric mucosa to IA induced both gastric lesions and significant reductions of body weight and food and water intake. These reductions recovered with treatment with ECQ for 7 days. ECQ significantly inhibited the elevation of the malondialdehyde levels and myeloperoxidase activity, which were used as indices of lipid peroxidation and neutrophil infiltration. ECQ recovered the level of glutathione, activity of superoxide dismutase (SOD), and expression of SOD-2. The increased levels of total NO concentration and iNOS expression in the IA-induced chronic gastritis were significantly reduced by treatment with ECQ. These results suggest that the ECQ has a therapeutic effect on chronic gastritis in rats by inhibitory actions on neutrophil infiltration, lipid peroxidation and various steps of reactive oxygen species (ROS) generation.

Neural plasticity in the gastrointestinal tract: chronic inflammation, neurotrophic signals, and hypersensitivity

Neural plasticity is not only the adaptive response of the central nervous system to learning, structural damage or sensory deprivation, but also an increasingly recognized common feature of the gastrointestinal (GI) nervous system during pathological states. Indeed, nearly all chronic GI disorders exhibit a disease-stage-dependent, structural and functional neuroplasticity. At structural level, GI neuroplasticity usually comprises local tissue hyperinnervation (neural sprouting, neural, and ganglionic hypertrophy) next to hypoinnervated areas, a switch in the neurochemical (neurotransmitter/neuropeptide) code toward preferential expression of neuropeptides which are frequently present in nociceptive neurons (e.g., substance P/SP, calcitonin-gene-related-peptide/CGRP) and of ion channels (TRPV1, TRPA1, PAR2), and concomitant activation of peripheral neural glia. The functional counterpart of these structural alterations is altered neuronal electric activity, leading to organ dysfunction (e.g., impaired motility and secretion), together with reduced sensory thresholds, resulting in hypersensitivity and pain. The present review underlines that neural plasticity in all GI organs, starting from esophagus, stomach, small and large intestine to liver, gallbladder, and pancreas, actually exhibits common phenotypes and mechanisms. Careful appraisal of these GI neuroplastic alterations reveals that--no matter which etiology, i.e., inflammatory, infectious, neoplastic/malignant, or degenerative--neural plasticity in the GI tract primarily occurs in the presence of chronic tissue- and neuro-inflammation. It seems that studying the abundant trophic and activating signals which are generated during this neuro-immune-crosstalk represents the key to understand the remarkable neuroplasticity of the GI tract.

Roux-en-Y gastric bypass reverses the effects of diet-induced obesity to inhibit the responsiveness of central vagal motoneurones

Diet-induced obesity (DIO) has been shown to alter the biophysical properties and pharmacological responsiveness of vagal afferent neurones and fibres, although the effects of DIO on central vagal neurones or vagal efferent functions have never been investigated. The aims of this study were to investigate whether high-fat diet-induced DIO also affects the properties of vagal efferent motoneurones, and to investigate whether these effects were reversed following weight loss induced by Roux-en-Y gastric bypass (RYGB) surgery. Whole-cell patch-clamp recordings were made from rat dorsal motor nucleus of the vagus (DMV) neurones in thin brainstem slices. The DMV neurones from rats exposed to high-fat diet for 12-14 weeks were less excitable, with a decreased membrane input resistance and decreased ability to fire action potentials in response to direct current pulse injection. The DMV neurones were also less responsive to superfusion with the satiety neuropeptides cholecystokinin and glucagon-like peptide 1. Roux-en-Y gastric bypass reversed all of these DIO-induced effects. Diet-induced obesity also affected the morphological properties of DMV neurones, increasing their size and dendritic arborization; RYGB did not reverse these morphological alterations. Remarkably, independent of diet, RYGB also reversed age-related changes of membrane properties and occurrence of charybdotoxin-sensitive (BK) calcium-dependent potassium current. These results demonstrate that DIO also affects the properties of central autonomic neurones by decreasing the membrane excitability and pharmacological responsiveness of central vagal motoneurones and that these changes were reversed following RYGB. In contrast, DIO-induced changes in morphological properties of DMV neurones were not reversed following gastric bypass surgery, suggesting that they may be due to diet, rather than obesity. These findings represent the first direct evidence for the plausible effect of RYGB to improve vagal neuronal health in the brain by reversing some effects of chronic high-fat diet as well as ageing. Vagovagal neurocircuits appear to remain open to modulation and adaptation throughout life, and understanding of these mechanisms may help in development of novel interventions to alleviate environmental (e.g. dietary) ailments and also alter neuronal ageing.

Enhanced excitability of guinea pig inferior mesenteric ganglion neurons during and following recovery from chemical colitis

Postganglionic sympathetic neurons in the prevertebral ganglia (PVG) provide ongoing inhibitory tone to the gastrointestinal tract and receive innervation from mechanosensory intestinofugal afferent neurons primarily located in the colon and rectum. This study tests the hypothesis that colitis alters the excitability of PVG neurons. Intracellular recording techniques were used to evaluate changes in the electrical properties of inferior mesenteric ganglion (IMG) neurons in the trinitrobenzene sulfonic acid (TNBS) and acetic acid models of guinea pig colitis. Visceromotor IMG neurons were hyperexcitable 12 and 24 h, but not 6 h, post-TNBS during "acute" inflammation. Hyperexcitability persisted at 6 days post-TNBS during "chronic" inflammation, as well as at 56 days post-TNBS when colitis had resolved. In contrast, there was only a modest decrease in the current required to elicit an action potential at 24 h after acetic acid administration. Vasomotor neurons from inflamed preparations exhibited normal excitability. The excitatory effects of XE-991, a blocker of the channel that contributes to the M-type potassium current, and heteropodatoxin-2, a blocker of the channel that contributes to the A-type potassium current, were unchanged in TNBS-inflamed preparations, suggesting that these currents did not contribute to hyperexcitability. Riluzole, an inhibitor of persistent sodium currents, caused tonic visceromotor neurons to accommodate to sustained current pulses, regardless of the inflammatory state of the preparation, and restored a normal rheobase in neurons from TNBS-inflamed preparations but did not alter the rheobase of control preparations, suggesting that enhanced activity of voltage-gated sodium channels may contribute to colitis-induced hyperexcitability. Collectively, these data indicate that enhanced sympathetic drive as a result of hyperexcitable visceromotor neurons may contribute to small bowel dysfunction during colitis.

Colonic inflammation up-regulates voltage-gated sodium channels in bladder sensory neurons via activation of peripheral transient potential vanilloid 1 receptors

BACKGROUND

Primary sensory neurons express several types of ion channels including transient receptor potential vanilloid 1 (TRPV1) and voltage-gated Na(+) channels. Our previous studies showed an increased excitability of bladder primary sensory and spinal neurons triggered by inflammation in the distal colon as a result of pelvic organ cross-sensitization. The goal of this work was to determine the effects of TRPV1 receptor activation by potent agonists and/or colonic inflammation on voltage-gated Na(+) channels expressed in bladder sensory neurons.

METHODS

Sprague-Dawley rats were treated with intracolonic saline (control), resiniferatoxin (RTX, 10(-7 ) mol L(-1)), TNBS (colonic irritant) or double treatment (RTX followed by TNBS).

KEY RESULTS

TNBS-induced colitis increased the amplitude of total Na(+) current by two-fold and of tetrodotoxin resistant (TTX-R) Na(+) current by 78% (P ≤ 0.05 to control) in lumbosacral bladder neurons during acute phase (3 days post-TNBS). Instillation of RTX in the distal colon caused an enhancement in the amplitude of total Na(+) current at -20 mV from -112.1 ± 18.7 pA/pF (control) to -183.6 ± 27.8 pA/pF (3 days post-RTX, P ≤ 0.05) without changes in TTX resistant component. The amplitude of net Na(+) current was also increased by 119% at day 3 in the group with double treatment (RTX followed by TNBS, P ≤ 0.05 to control) which was significantly higher than in either group with a single treatment.

CONCLUSIONS & INFERENCES

These results provide evidence that colonic inflammation activates TRPV1 receptors at the peripheral sensory terminals leading to an up-regulation of voltage gated Na(+) channels on the cell soma of bladder sensory neurons. This mechanism may underlie the occurrence of peripheral cross-sensitization in the pelvis and functional chronic pelvic pain.

Involvement of subtypes γ and ε of protein kinase C in colon pain induced by formalin injection

Protein kinase C (PKC) has been widely reported to participate in somatic pain; however, its role in visceral pain remains largely unclear. Using a colon inflammatory pain model by intracolonic injection of formalin in rats, the present study was to examine the role of PKC in visceral pain and determine which subtypes may be involved. The colon pain behavior induced by formalin injection could be enhanced by intrathecal pretreatment with a PKC activator (PMA), and alleviated by a PKC inhibitor (H-7). Wide dynamic range (WDR) neurons in the L6-S1 spinal dorsal horn that were responsive to colorectal distension were recorded extracellularly. It was found that neuronal activity was greatly increased following formalin injection. Microdialysis of PMA near the recorded neuron in the spinal dorsal horn facilitated the enhanced responsive activity induced by formalin injection, while H-7 inhibited significantly the enhanced response induced by formalin injection. Western blot analysis revealed that membrane translocation of PKC-γ and PKC-∊, but not other subtypes, in the spinal cord was obviously increased following formalin injection. Therefore, our findings suggest that PKC is actively involved in the colon pain induced by intracolonic injection of formalin. PKC-γ and PKC-∊ subtypes seem to significantly contribute to this process.

Identifying the Ion Channels Responsible for Signaling Gastro-Intestinal Based Pain

We are normally unaware of the complex signalling events which continuously occur within our internal organs. Most of us only become cognisant when sensations of hunger, fullness, urgency or gas arise. However, for patients with organic and functional bowel disorders pain is an unpleasant and often debilitating reminder. Furthermore, chronic pain still represents a large unmet need for clinical treatment. Consequently, chronic pain has a considerable economic impact on health care systems and the afflicted individuals. In order to address this need we must understand how symptoms are generated within the gut, the molecular pathways responsible for generating these signals and how this process changes in disease states.

Increased TRPV1 gene expression in esophageal mucosa of patients with non-erosive and erosive reflux disease

BACKGROUND

Transient receptor potential channel vanilloid subfamily member-1 (TRPV1) may play a role in esophageal perception. TRPV1 mRNA and protein expression were examined in the esophageal mucosa of non-erosive reflux disease (NERD) and erosive esophagitis (EE) patients and correlated to esophageal acid exposure.

METHODS

Seventeen NERD patients, eight EE patients and 10 healthy subjects underwent endoscopy after a 3-week washout from proton pump inhibitors or H2 antagonists. Biopsies, obtained from the distal esophagus, were used for conventional histology, for Western blot analysis and/or quantitative real-time polymerase chain reaction (qPCR). Overall 13 NERD patients, four EE patients and five controls underwent ambulatory pH-testing.

KEY RESULTS

TRPV1 expression was increased in all NERD and EE patients, as measured by Western blot analysis (0.65 +/- 0.07 and 0.8 +/- 0.05 VS 0.34 +/- 0.04 in controls; P < 0.01) and by qPCR (1.98 +/- 0.21 and 2.52 +/- 0.46 VS 1.00 +/- 0.06; P < 0.01). Neutrophilic infiltration, in the mucosa, was detected only in EE patients.

CONCLUSIONS & INFERENCES

Non-erosive reflux disease and EE patients presented increased TRPV1 receptors mRNA and protein, although no correlation with acid exposure was demonstrated. Increased TRPV1 in the esophageal mucosa may contribute to symptoms both in NERD and EE patients and possibly account for peripheral mechanisms responsible for esophageal hypersensitivity in NERD patients.

Sodium channels in normal and pathological pain

Nociception is essential for survival whereas pathological pain is maladaptive and often unresponsive to pharmacotherapy. Voltage-gated sodium channels, Na(v)1.1-Na(v)1.9, are essential for generation and conduction of electrical impulses in excitable cells. Human and animal studies have identified several channels as pivotal for signal transmission along the pain axis, including Na(v)1.3, Na(v)1.7, Na(v)1.8, and Na(v)1.9, with the latter three preferentially expressed in peripheral sensory neurons and Na(v)1.3 being upregulated along pain-signaling pathways after nervous system injuries. Na(v)1.7 is of special interest because it has been linked to a spectrum of inherited human pain disorders. Here we review the contribution of these sodium channel isoforms to pain.

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.

Trinitrobenzenesulphonic acid colitis alters Na 1.8 channel expression in mouse dorsal root ganglia neurons

Visceral inflammation evokes hyperexcitability in nociceptive dorsal root ganglia (DRG) neurons and these changes are associated with increased voltage-gated sodium channel (Na(v)) 1.8 current density, but the molecular determinants of these changes are unclear. This study used Western blotting to measure changes in Na(v) 1.7, 1.8 and 1.9 protein expression during trinitrobenzenesulphonic acid (TNBS) colitis and quantitative polymerase chain reaction (PCR) to examine corresponding changes in mRNA. Colonic neurons were labelled with the retrograde tracer Fast Blue injected into the wall of the distal colon and quantitative PCR performed on laser-captured labelled colonic neurons from ganglia at T9-13 or unlabelled DRG neurons from the upper spinal cord. Immunohistochemistry and western blots were performed on whole DRG from the same sites. Fast Blue-labelled neurons demonstrated Na(v) 1.7, 1.8 and 1.9 immunoreactivity. On day 7 of colitis, which correlated with electrophysiological studies, there was a threefold increase in Na(v) 1.8 protein in ganglia from T9 to 13, but Na(v) 1.7 and 1.9 levels were unchanged. There was no corresponding change in the Na(v) 1.8 alpha-subunit mRNA levels. However, on days 2 and 4, Na(v) 1.8 mRNA was decreased 10-fold. Na(v) 1.8 protein and mRNA levels were unchanged in neurons isolated from ganglia in the upper spinal cord, where colonic neurons are not found. These findings suggest that the TNBS evoked increase in Na(v) 1.8 currents is associated with increased numbers of channels. The absence of corresponding changes in transcript suggests a translational or post-translational mechanism, but the 10-fold recovery of transcript preceding this time point also demonstrates a complex transcriptional regulation.

Basic and clinical aspects of gastrointestinal pain

The gastrointestinal (GI) tract is a system of organs within multicellular animals which facilitates the ingestion, digestion, and absorption of food with subsequent defecation of waste. A complex arrangement of nerves and ancillary cells contributes to the sensorimotor apparatus required to subserve such essential functions that are with the exception of the extreme upper and lower ends of the GI tract normally subconscious. However, it also has the potential to provide conscious awareness of injury. Although this function can be protective, when dysregulated, particularly on a chronic basis, the same system can lead to considerable morbidity. The anatomical and molecular basis of gastrointestinal nociception, conditions associated with chronic unexplained visceral pain, and developments in treatment are presented in this review.

A rat model of chronic gastric sensorimotor dysfunction resulting from transient neonatal gastric irritation

BACKGROUND & AIMS

Although several pathophysiologic abnormalities have been noted in functional dyspepsia (FD), their pathogenesis is poorly understood. We hypothesized that chronic gastric hypersensitivity and gastric motor dysfunction seen in FD patients can be modeled in rats by transient gastric irritation during the neonatal period, a time of known neuronal vulnerability to long-term plasticity.

METHODS

Ten-day-old male rats received 0.2 mL 0.1% iodoacetamide (IA) in 2% sucrose daily by oral gavages for 6 days; controls received 2% sucrose. Rats in both groups were then followed to adulthood (8-10 weeks) at which point behavioral, visceromotor, and great splanchnic nerve responses to graded gastric balloon distention (GD; 20-80 mm Hg) and gastric motor function were tested.

RESULTS

IA-treated rats exhibited hypersensitivity to GD in a dose-dependent manner, as compared with the control group. The threshold of afferent nerve activation was lower and nerve responses to GD were significantly increased in IA-treated rats. Although IA-treated rats ingested food at a lower rate, gastric emptying was not significantly different between IA and control groups. However, gastric accommodation was significantly reduced in the IA group. No significant gastric pathology was seen in hypersensitive adult rats compared with controls.

CONCLUSIONS

These studies demonstrate that gastric irritation in the neonatal period can result in chronic gastric hypersensitivity and gastric motor dysfunction in adults even in the absence of significant detectable gastric pathology. Our results offer insight into the pathogenesis of chronic functional dyspepsia and provide a potential model for further study to this important clinical problem.

Neural mechanisms of pelvic organ cross-sensitization

Clinical observations of viscerovisceral referred pain in patients with gastrointestinal and genitourinary disorders suggest an overlap of neurohumoral mechanisms underlying both bowel and urinary bladder dysfunctions. Close proximity of visceral organs within the abdominal cavity complicates identification of the exact source of chronic pelvic pain, where it originates, and how it relocates with time. Cross-sensitization among pelvic structures may contribute to chronic pelvic pain of unknown etiology and involves convergent neural pathways of noxious stimulus transmission from two or more organs. Convergence of sensory information from discrete pelvic structures occurs at different levels of nervous system hierarchy including dorsal root ganglia, the spinal cord and the brain. The cell bodies of sensory neurons projecting to the colon, urinary bladder and male/female reproductive organs express a wide range of membrane receptors and synthesize many neurotransmitters and regulatory peptides. These substances are released from nerve terminals following enhanced neuronal excitability and may lead to the occurrence of neurogenic inflammation in the pelvis. Multiple factors including inflammation, nerve injury, ischemia, peripheral hyperalgesia, metabolic disorders and other pathological conditions dramatically alter the function of directly affected pelvic structures as well as organs located next to a damaged domain. Defining precise mechanisms of viscerovisceral cross-sensitization would have implications for the development of effective pharmacological therapies for the treatment of functional disorders with chronic pelvic pain such as irritable bowel syndrome and painful bladder syndrome. The complexity of overlapping neural pathways and possible mechanisms underlying pelvic organ crosstalk are analyzed in this review at both systemic and cellular levels.

Local inflammation in rat dorsal root ganglion alters excitability and ion currents in small-diameter sensory neurons

BACKGROUND

Chronic pain conditions may result from peripheral nerve injury, chronic peripheral inflammation, or sensory ganglia inflammation. However, inflammatory processes may also contribute to peripheral nerve injury responses. To isolate the contribution of local inflammation of sensory ganglia to chronic pain states, the authors previously developed a rat model in which long-lasting pain is induced by inflaming sensory ganglia without injuring the neurons. This results in prolonged mechanical pain, local increases in proinflammatory cytokines, increased neuronal hyperexcitability, and abnormal spontaneous activity.

METHODS

The authors used whole cell patch clamp in acutely isolated small-diameter neurons to determine how localized inflammation (3-5 days) of L4 and L5 ganglia altered voltage-gated K and Na currents.

RESULTS

Tetrodotoxin-sensitive Na currents increased twofold to threefold in neurons from inflamed ganglia. Tetrodotoxin-resistant Na currents increased more than twofold, but only in cells that bound isolectin B4. These increases occurred without shifts in voltage dependence of activation and inactivation. Similar results are seen in models of peripheral inflammation, except for the large magnitudes. Unlike most pain models, localized inflammation increased rather than decreased voltage-gated K currents, due to increased amplitudes of the sustained (delayed rectifier) and fast-inactivating transient components. The overall effect in current clamp experiments was an increase in excitability as indicated by decreased rheobase and lower action potential threshold.

CONCLUSIONS

Neuronal inflammation per se, in the absence of nerve injury, causes large increases in Na channel density and enhanced excitability. The unusual finding of increased K current may reflect regulation of excitability in the face of such large increases in Na current.

Phenotypic changes of morphologically identified guinea-pig myenteric neurons following intestinal inflammation

We investigated the responses of morphologically identified myenteric neurons of the guinea-pig ileum to inflammation that was induced by the intraluminal injection of trinitrobenzene sulphonate, 6 or 7 days previously. Electrophysiological properties were examined with intracellular microelectrodes using in vitro preparations from the inflamed or control ileum. The neurons were injected with marker dyes during recording and later they were recovered for morphological examination. A proportion of neurons with Dogiel type I morphology, 45% (32/71), from the inflamed ileum had a changed phenotype. These neurons exhibited an action potential with a tetrodotoxin-resistant component, and a prolonged after-hyperpolarizing potential followed the action potential. Of the other 39 Dogiel type I neurons, no changes were observed in 36 and 3 had increased excitability. The afterhyperpolarizing potential (AHP) in Dogiel type I neurons was blocked by the intermediate conductance, Ca(2+)-dependent K(+) channel blocker TRAM-34. Neurons which showed these phenotypic changes had anally directed axonal projections. Neither a tetrodotoxin-resistant action potential nor an AHP was seen in Dogiel type I neurons from control preparations. Dogiel type II neurons retained their distinguishing AH phenotype, including an inflection on the falling phase of the action potential, an AHP and, in over 90% of neurons, an absence of fast excitatory transmission. However, they became hyperexcitable and exhibited anodal break action potentials, which, unlike control Dogiel type II neurons, were not all blocked by the h current (I(h)) antagonist Cs(+). It is concluded that inflammation selectively affects different classes of myenteric neurons and causes specific changes in their electrophysiological properties.

In vivo and transgenic animal models used to study visceral hypersensitivity

Measurement of visceral sensitivity in animals is mainly based on 'pseudoaffective' responses, which are brain stem reflexes. For example, in female, but not male rats, acute partial restraint stress induces hypersensitivity to colorectal distension. Mucosal mast cell density increases in rats after nematode infection or maternal deprivation, and both also induce colon hypersensitivity. Significantly, the proximity between nerves and mast cells has been found to be increased in adult rats submitted to maternal deprivation. Protease activation of the proteinase-activated receptor-2 also increases visceral nociception in rats, suggesting that an increase in paracellular permeability may be the primum movens in several animal models of visceral hypersensitivity. Accumulating evidence suggests that sensitization of visceral afferents is not restricted to the presumed nociceptor population, suggesting that most of the mechanosensitive afferent population can contribute to visceral discomfort and pain. Other inflammation-produced changes (e.g. subunit composition of purine-gated P2X channels) in visceral sensory neurones may also contribute to visceral hypersensitivity. This article discusses use of in vivo strategies (and transgenic mouse models) to reveal putative roles in mechanosensitivity and sensitization for molecules not previously considered to have mechanosensory functions.

Phenotypic characterization of gastric sensory neurons in mice

Recent studies suggest that the capsaicin receptor [transient receptor potential vanilloid (TRPV)1] may play a role in visceral mechanosensation. To address the potential role of TRPV1 in vagal sensory neurons, we developed a new in vitro technique allowing us to determine TRPV1 expression directly in physiologically characterized gastric sensory neurons. Stomach, esophagus, and intact vagus nerve up to the central terminations were carefully dissected and placed in a perfusion chamber. Intracellular recordings were made from the soma of nodose neurons during mechanical stimulation of the stomach. Physiologically characterized neurons were labeled iontophoretically with neurobiotin and processed for immunohistochemical experiments. As shown by action potential responses triggered by stimulation of the upper thoracic vagus with a suction electrode, essentially all abdominal vagal afferents in mice conduct in the C-fiber range. Mechanosensitive gastric afferents encode stimulus intensities over a wide range without apparent saturation when punctate stimuli are used. Nine of 37 mechanosensitive vagal afferents expressed TRPV1 immunoreactivity, with 8 of the TRPV1-positive cells responding to stretch. A small number of mechanosensitive gastric vagal afferents express neurofilament heavy chains and did not respond to stretch. By maintaining the structural and functional integrity of vagal afferents up to the nodose ganglion, physiological and immunohistochemical properties of mechanosensory gastric sensory neurons can be studied in vitro. Using this novel technique, we identified TRPV1 immunoreactivity in only one-fourth of gastric mechanosensitive neurons, arguing against a major role of this ion channel in sensation of mechanical stimuli under physiological conditions.

Analysis of the variation in use-dependent inactivation of high-threshold tetrodotoxin-resistant sodium currents recorded from rat sensory neurons

This study addressed variation in the use-dependent inactivation (UDI) of high-threshold tetrodotoxin-resistant Na+ currents (TTX-R currents) and action potential firing behavior among acutely isolated rat dorsal root ganglion (DRG) cells. UDI was quantified as the percent decrease in current amplitude caused by increasing the current activation rate from 0.1-1.0 Hz for 20 s. TTX-R current UDI varied from 6% to 66% among 122 DRG cells examined, suggesting the existence of two or more levels of UDI. The voltage-dependency of the TTX-R currents was consistent with Na(V)1.8, regardless of UDI. However, TTX-R currents with more UDI had a more negative voltage-dependency of inactivation, a greater tendency to enter slow inactivation, and a slower recovery rate from slow inactivation, compared with those with less UDI. TTX-R currents with more UDI ran down faster than those with less UDI. However, UDI itself changed little over time, regardless of the initial UDI level observed in a particular DRG cell. Together, these two observations suggest that individual DRG cells did not express mixtures of TTX-R channels that varied regarding UDI. TTX-R current UDI was correlated with expression of a low-threshold A-current and whole-cell capacitance, suggesting that it varied among different nociceptor types. Whole-cell inward currents (WCI-currents), recorded without channel blockers, also exhibited UDI. WCI-current UDI varied similarly to TTX-R current UDI in magnitude, and relative to whole-cell capacitance and A-current expression, suggesting that the WCI-currents were carried predominantly by TTX-R channels. DRG cells with more WCI-current UDI exhibited a greater decrease in action potential amplitude and number, and a greater increase in action potential threshold over seven ramp depolarizations, compared with DRG cells with less WCI-current UDI. Variation in UDI of Na(V)1.8 channels expressed by different nociceptor types could contribute to shaping their individual firing patterns in response to noxious stimuli.

Hyperexcitability of convergent colon and bladder dorsal root ganglion neurons after colonic inflammation: mechanism for pelvic organ cross-talk

Clinical studies reveal concomitant occurrence of several gastrointestinal and urologic disorders, including irritable bowel syndrome and interstitial cystitis. The purpose of this study was to determine the mechanisms underlying cross-organ sensitization at the level of dorsal root ganglion (DRG) after acute and subsided gastrointestinal inflammation. DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) and Fast Blue were injected into the distal colon and urinary bladder of male rats, respectively. Convergent DRG neurons were found in L1-L3 and L6-S2 ganglia with an average distribution of 14% +/- 2%. The resting membrane potential (RMP) of cells isolated from upper lumbar (UL) ganglia was -59.8 +/- 2.7 mV, whereas lumbosacral (LS) neurons were more depolarized (RMP = -49.4 +/- 2.1 mV, P < or = 0.05) under control conditions. Acute trinitrobenzene sulfonic acid (TNBS) colitis (3 days) decreased voltage and current thresholds for action potential firing in LS but not UL convergent capsaicin-sensitive neurons. This effect persisted for 30 days in the absence of overt colonic inflammation. The current threshold for action potential (AP) firing in UL cells was also decreased from 165.0 +/- 24.5 pA (control) to 85.0 +/- 19.1 pA at 30 days (P < or = 0.05), indicating increased excitability. The presence of a subpopulation of colon-bladder convergent DRG neurons and their persistent hyperexcitability after colonic inflammation provides a basis for pelvic organ cross-sensitization.

Inflammatory hyperalgesia: a role for the C-fiber sensory neuron cell body?

Peripheral nerve injury increases the chemosensitivity and excitability of injured afferents, resulting in ectopic activity arising from within dorsal root ganglia. Studies of dissociated sensory ganglion neurons in vitro suggest afferent somata might be sensitized by persistent inflammation. However, it is unknown whether this inflammation-induced sensitization is manifest in somata within the intact ganglia. To explore this possibility, intracellular electrophysiologic recording was used with a sciatic nerve-L4-dorsal root ganglia preparation to compare excitability and chemosensitivity of cutaneous C-fiber somata from control and inflamed rats. Cutaneous afferents were identified with the retrograde dye DiI. Excitability was assessed before and after the application of inflammatory soup (IS) containing bradykinin, serotonin, and prostaglandin E2 all at a pH of 7.0. Persistent inflammation decreased the excitability of cutaneous afferents in intact ganglia and had no significant influence on the magnitude of IS-induced increase in excitability. Opposite to the effects observed in intact ganglia, excitability was greater in dissociated cutaneous nociceptors obtained from inflamed rats, although the magnitude of the IS-induced increase in excitability was not significantly affected by inflammation. These results suggest that the cell bodies of putative cutaneous nociceptors in the intact ganglia contribute minimally to pain and hyperalgesia associated with persistent inflammation.

PERSPECTIVE

Results of the present study suggest that inflammation-induced changes in afferent somata are minimal. However, they also suggest that inflammatory mediator-induced increase in the excitability of sensory neuron somata might contribute to global changes in nociception observed under high systemic inflammatory mediator loads.

Capsaicin receptor (TRPV1) and non-erosive reflux disease

BACKGROUND/AIM

Non-erosive reflux disease (NERD) is a common and heterogeneous disorder. We hypothesized that changes in peripheral innervation may lead to hyperalgesia and contribute to the development of the disorder.

METHODS

Patients referred for evaluation of reflux symptoms with wireless pH monitoring were asked to provide demographic and clinical data and complete a survey related to severity of reflux symptoms. Endoscopies were performed to rule out macroscopic abnormalities of the esophageal mucosa. Biopsies obtained 2 cm above the gastroesophageal junction were stained for protein gene product 9.5 (PGP 9.5; general neuronal marker) and TRPV1 (capsaicin receptor) immunoreactivity. The density of immunoreactive fibers in the esophageal mucosa was determined morphometrically.

RESULTS

A total of 39 patients without evidence of Barrett's metaplasia, erosive or ulcerative esophagitis were enrolled. Most patients had daily symptoms. The total esophageal acid exposure time was 5.6+/-0.6%, with 16 patients (41%) having increased acid reflux. Immunoreactivity for PGP 9.5 or TRPV1 was detected in papillary structures as well as within the epithelium (free intra-epithelial endings). Total acid-exposure time, but not symptom score or duration correlated significantly with density of PGP 9.5 immunoreactivity and TRPV1 positive fibers.

CONCLUSION

Even in the absence of macroscopic injury, esophageal acid exposure is associated with changes in mucosal innervation of the esophagus, thus potentially further enhancing symptoms in patients with gastroesophageal reflux.

Gastrointestinal pain in functional bowel disorders: sensory neurons as novel drug targets

Functional bowel disorders (FBDs) are defined by symptoms of gastrointestinal (GI) dysfunction, discomfort and pain in the absence of a demonstrable organic cause. Since the prevalence of FBDs, particularly functional dyspepsia and irritable bowel syndrome, can be as high as 20%, FBDs represent a significant burden in terms of direct healthcare and productivity costs. There is emerging evidence that the discomfort and pain experienced by many FBD patients is due to persistent hypersensitivity of primary afferent neurons, which may develop in response to infection, inflammation or other insults. This concept identifies vagal and spinal sensory neurons as important targets for novel therapies of GI hyperalgesia. Sensory neuron-specific targets can be grouped into three categories: receptors and sensors at the peripheral nerve terminals, ion channels relevant to nerve excitability and conduction and transmitter receptors. Particular therapeutic potential is attributed to targets that are selectively expressed by afferent neurons, such as the transient receptor potential channel TRPV1, acid-sensing ion channels and tetrodotoxin-resistant Na + channels.

Inflammation increases the excitability of masseter muscle afferents

Temporomandibular disorder is a major health problem associated with chronic orofacial pain in the masticatory muscles and/or temporomandibular joint. Evidence suggests that changes in primary afferents innervating the muscles of mastication may contribute to temporomandibular disorder. However, there has been little systematic study of the mechanisms controlling the excitability of these muscle afferents, nor their response to inflammation. In the present study, we tested the hypotheses that inflammation increases the excitability of sensory neurons innervating the masseter muscle of the rat and that the ionic mechanisms underlying these changes are unique to these neurons. We examined inflammation-induced changes in the excitability of trigeminal ganglia muscle neurons following intramuscular injections of complete Freund's adjuvant. Three days after complete Freund's adjuvant injection acutely dissociated, retrogradely labeled trigeminal ganglia neurons were studied using whole cell patch clamp techniques. Complete Freund's adjuvant-induced inflammation was associated with an increase in neuronal excitability marked by a significant decrease in rheobase and increase in the slope of the stimulus response function assessed with depolarizing current injection. The increase in excitability was associated with significant decreases in the rate of action potential fall and the duration of the action potential afterhyperpolarization. These changes in excitability and action potential waveform were associated with significant shifts in the voltage-dependence of activation and steady-state availability of voltage-gated K(+) current as well as significant decreases in the density of voltage-gated K(+) current subject to steady-state inactivation. These data suggest that K(+) channel subtypes may provide novel targets for the treatment of pain arising from inflamed muscle. These results also support the hypothesis that the underlying mechanisms of pain arising from specific regions of the body are unique suggesting that it may be possible, if not necessary to treat pain originating from different parts of the body with specific therapeutic interventions.

Morphological and electrophysiological changes in mouse dorsal root ganglia after partial colonic obstruction

There is evidence that sensitization of neurons in dorsal root ganglia (DRG) may contribute to pain induced by intestinal injury. We hypothesized that obstruction-induced pain is related to changes in DRG neurons and satellite glial cells (SGCs). In this study, partial colonic obstruction was induced by ligation. The neurons projecting to the colon were traced by an injection of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate into the colon wall. The electrophysiological properties of DRG neurons were determined using intracellular electrodes. Dye coupling was examined with an intracellular injection of Lucifer yellow (LY). Morphological changes in the colon and DRG were examined. Pain was assessed with von Frey hairs. Partial colonic obstruction caused the following changes. First, coupling between SGCs enveloping different neurons increased 18-fold when LY was injected into SGCs near neurons projecting to the colon. Second, neurons were not coupled to other neurons or SGCs. Third, the firing threshold of neurons projecting to the colon decreased by more than 40% (P < 0.01), and the resting potential was more positive by 4-6 mV (P < 0.05). Finally, the number of neurons displaying spontaneous spikes increased eightfold, and the number of neurons with subthreshold voltage oscillations increased over threefold. These changes are consistent with augmented neuronal excitability. The pain threshold to abdominal stimulation decreased by 70.2%. Inflammatory responses were found in the colon wall. We conclude that obstruction increased neuronal excitability, which is likely to be a major factor in the pain behavior observed. The augmented dye coupling between glial cells may contribute to the neuronal hyperexcitability.

Inflammation alters sodium currents and excitability of temporomandibular joint afferents

Inflammation-induced changes in voltage-gated sodium currents (I(Na)) in primary afferent neurons may contribute to hyperexcitability and pain. The present study was designed to test the hypothesis that persistent inflammation of the temporomandibular joint (TMJ) increases I(Na) in TMJ afferents. Acutely dissociated retrogradely labeled TMJ afferents were studied using whole-cell patch clamp techniques three days following Complete Freund's Adjuvant-induced inflammation of the TMJ. Inflammation was associated with a decrease in tetrodotoxin (TTX)-sensitive Na+ conductance and no significant change in slowly inactivating TTX-resistant Na+ conductance. However, inflammation increased the excitability of TMJ afferents. These results suggest that changes in ion channels other than those underlying TTX-sensitive and the slowly inactivating TTX-resistant Na+ conductance are likely to account for the inflammation-induced increase in the excitability of TMJ afferents.