Klaus Bielefeldt

Nitric Oxide as an Autocrine Regulator of Sodium Currents in Baroreceptor Neurons

Arterial baroreceptors are mechanosensitive nerve endings in the aortic arch and carotid sinus that play a critical role in acute regulation of arterial blood pressure. A previous study has shown that nitric oxide (NO) or NO-related species suppress action potential discharge of baroreceptors. In the present study, we investigated the effects of NO on Na+ currents of isolated baroreceptor neurons in culture. Exogenous NO donors inhibited both tetrodotoxin (TTX) -sensitive and -insensitive Na+ currents. The inhibition was not mediated by cGMP but by NO interaction with channel thiols. Acute inhibition of NO synthase increased the Na+ currents. NO scavengers (hemoglobin and ferrous diethyldithiocarbamate) increased Na+ currents before but not after inhibition of NO synthase. Furthermore, NO production in the neuronal cultures was detected by chemiluminescence and immunoreactivity to the neuronal isoform of NO synthase was identified in fluorescently identified baroreceptor neurons. These results indicate that NO/NO-related species function as autocrine regulators of Na+ currents in baroreceptor neurons. Modulation of Na+ channels may represent a novel response to NO.

TRPV1 Function in Mouse Colon Sensory Neurons Is Enhanced by Metabotropic 5-Hydroxytryptamine Receptor Activation

Using whole-cell patch-clamp methods, we examined the hypothesis that serotonin [5-hydroxytryptamine (5-HT)] receptor activation enhances TRPV1 function in mouse colon sensory neurons in lumbosacral dorsal root ganglia, which were identified by retrograde labeling with DiI (1,1′-dioctadecyl-3,3,3′,3-tetramethlindocarbocyanine methanesulfonate) injected into multiple sites in the wall of the descending colon. 5-HT increased membrane excitability at a temperature below body temperature in response to thermal ramp stimuli in colon sensory neurons from wild-type mice, but not from TRPV1 knock-out mice. 5-HT significantly enhanced capsaicin-, heat-, and proton-evoked currents with an EC50 value of 2.2 μm. 5-HT (1 μm) significantly increased capsaicin-evoked (100 nm) and proton-evoked (pH 5.5) currents 1.6- and 4.7-fold, respectively, and significantly decreased the threshold temperature for heat current activation from 42 to 38°C. The enhancement of TRPV1 by 5-HT was significantly attenuated by selective 5-HT2 and 5-HT4 receptor antagonists, but not by a 5-HT3 receptor antagonist. In support, 5-HT2 and 5-HT4 receptor agonists mimicked the facilitating effects of 5-HT on TRPV1 function. Downstream signaling required G-protein activation and phosphorylation as intracellularly administered GDP-β-S [guanosine 5′-O-(2-thiodiphosphate], protein kinase A inhibitors, and an A-kinase anchoring protein inhibitor significantly blocked serotonergic facilitation of TRPV1 function; 5-HT2 receptor-mediated facilitation was also inhibited by a PKC inhibitor. We conclude that the facilitation of TRPV1 by metabotropic 5-HT receptor activation may contribute to hypersensitivity of primary afferent neurons in irritable bowel syndrome patients.

Acid-Sensing Properties in Rat Gastric Sensory Neurons from Normal and Ulcerated Stomach

Gastric acid contributes to dyspeptic symptoms, including abdominal pain, in patients with disorders of the proximal gastrointestinal tract. To examine the molecular sensor(s) of gastric acid chemonociception, we characterized acid-elicited currents in dorsal root ganglion (DRG) and nodose ganglion (NG) neurons that innervate the stomach and examined their modulation after induction of gastric ulcers. A fluorescent dye (DiI) was injected into the stomach wall to retrogradely label gastric sensory neurons. After 1-2 weeks, gastric ulcers were induced by 45 s of luminal exposure of the stomach to 60% acetic acid injected into a clamped area of the distal stomach; control animals received saline. In whole-cell voltage-clamp recordings, all gastric DRG neurons and 55% of NG neurons exhibited transient, amiloride-sensitive, acid-sensing ion-channel (ASIC) currents. In the remaining 45% of NG neurons, protons activated a slow, sustained current that was attenuated by the transient receptor potential vanilloid subtype 1 antagonist, capsazepine. The kinetics and proton sensitivity of amiloride-sensitive ASIC currents differed between NG and DRG neurons. NG neurons had a lower proton sensitivity and faster kinetics, suggesting expression of specific subtypes of ASICs in the vagal and splanchnic innervation of the stomach. Effects of Zn2+ and N,N,N′,N′-tetrakis-(2-pyridylmethyl)-ethylenediamine on acid-elicited currents suggest contributions of ASIC1a and ASIC2a subunits. Gastric ulcers altered the properties of acid-elicited currents by increasing pH sensitivity and current density and changing current kinetics in gastric DRG neurons. The distinct properties of NG and DRG neurons and their modulation after injury suggest differential contributions of vagal and spinal afferent neurons to chemosensation and chemonociception.

Pain and inflammatory bowel disease

Abdominal pain is a common symptom of inflammatory bowel disease (IBD: Crohns disease, ulcerative colitis). Pain may arise from different mechanisms, which can include partial blockage and gut distention as well as severe intestinal inflammation. A majority of patients suffering from acute flares of IBD will experience pain, which will typically improve as disease activity decreases. However, a significant percentage of IBD patients continue experiencing symptoms of pain despite resolving inflammation and achieving what appears to be clinical remission. Current evidence suggests that sensory pathways sensitize during inflammation, leading to persistent changes in afferent neurons and central nervous system pain processing. Such persistent pain is not only a simple result of sensory input. Pain processing and even the activation of sensory pathways is modulated by arousal, emotion, and cognitive factors. Considering the high prevalence of iatrogenic as well as essential neuropsychiatric comorbidities including anxiety and depression in IBD patients, these central modulating factors may significantly contribute to the clinical manifestation of chronic pain. The improved understanding of peripheral and central pain mechanisms is leading to new treatment strategies that view pain as a biopsychosocial problem. Thus, improving the underlying inflammation, decreasing the excitability of sensitized afferent pathways, and altering emotional and/or cognitive functions may be required to more effectively address the difficult and disabling disease manifestations.

Gastrointestinal Manifestations of Systemic Sclerosis

Systemic sclerosis is a chronic disorder of connective tissue that affects the gastrointestinal tract in more than 80% of patients. Changes in neuromuscular function with progressive fibrosis of smooth muscle within the muscularis propria impair normal motor function, which may secondarily alter transit and nutrient absorption. Esophageal manifestations with gastroesophageal reflux and dysphagia are the most common visceral manifestation of the disease, often requiring more intense acid-suppressive medication. Gastric involvement may lead to gastroparesis, which can be found in up to 50% of patients. Severe small bowel disease can present as chronic intestinal pseudo-obstruction with distended loops of small intestine, bacterial overgrowth, impaired absorption and progressive development of nutritional deficiencies. While not studied as extensively, systemic sclerosis often also affects colorectal function resulting in constipation, diarrhea or fecal incontinence. Nutritional support and prokinetics have been used with some success to manage gastric and small or large bowel involvement in patients with systemic sclerosis. Despite advances in management, significant gastrointestinal manifestations of systemic sclerosis still carry a poor prognosis with a five-year mortality exceeding 50%.

Differential Responses of Bladder Lumbosacral and Thoracolumbar Dorsal Root Ganglion Neurons to Purinergic Agonists, Protons, and Capsaicin

The present study explored differences in sensitivity to purinergic agonists, protons, and capsaicin in lumbosacral (LS) and thoracolumbar (TL) sensory neurons that innervate the rat urinary bladder. The majority of LS neurons (93%) were sensitive to α,β-methyleneATP (α,β-metATP) compared with 50% of TL neurons. Based on inactivation kinetics, a slowly desensitizing current evoked by α,β-metATP predominated in LS neurons (86%) compared with mixed components that characterized TL neuron responses (58%). The density of the slowly desensitizing current was greater in LS than in TL neurons (LS, 34.4 ± 5.3 pA/pF; TL, 2.5 ± 0.8 pA/pF). Almost all neurons in both ganglia responded to protons and to capsaicin (LS, 100%; TL, 98%). Proton-activated currents in bladder sensory neurons exhibited distinct inactivation kinetics as fast, intermediate, slowly desensitizing, and sustained components. More than one component was expressed in every cell. Although there was no difference in the percentage of neurons expressing more than one component, the density of the sustained current was significantly greater in LS than in TL neurons (LS, 86.1 ± 16 pA/pF; TL, 30.3 ± 7 pA/pF). Similarly, the capsaicin-evoked current was greater in LS than in TL neurons (LS, 129.6 ± 17 pA/pF; TL, 86.9 ± 11 pA/pF). Finally, a greater percentage of TL neurons bound isolectin B4 than LS neurons (LS, 61%; TL, 85%). The greater degree of α,β-metATP, proton, and capsaicin responsiveness, in addition to differences in current type and current densities, in LS and TL neurons suggests that bladder pelvic and hypogastric/lumbar splanchnic afferents are functionally distinct and likely mediate different sensations arising from the urinary bladder.

Intestinal motility during hypoxia and reoxygenation in vitro.

Ischemia-reperfusion injury leads to profoundfunctional and structural alterations of thegastrointestinal tract. We developed an in vitro modelof reperfusion injury to study the changes in intestinal motility during hypoxia followed byreoxygenation. We recorded the spontaneous motoractivity of intestinal rings from the proximal mousejejunum, using force displacement transducers. Inaddition to the rhythmic contractions, we studied thecontractile response to transmural stimulation ofintrinsic nerves. During hypoxia, the frequency of thespontaneous contractions and the resting tensiondecreased. While 29% of the tissues still responded toneural stimulation after 15 min of hypoxia, electricalfield stimulation did not evoke any response after 60min of hypoxia. Reoxygenation resulted in a transient increase in the baseline tension and an initialnormalization of the spontaneous rhythmic contractions,which subsequently became irregular. The percentage oftissues that recovered their ability to respond to electrical field stimulation 10 min afterreoxygenation decreased from 100% after 15 min ofhypoxia to 47% after 60 min of hypoxia. Theadministration of the antioxidant glutathione preventedthe functional abnormalities seen 10 min after reoxygenation.The pharmacological inhibition of Cu,Zn superoxidedismutase exacerbated the functional reoxygenationdamage. Conversely, the overexpression of thisradical-scavenging enzyme in transgenic mice increased thelikelihood of functional recovery. Reoxygenation in acalciumfree solution also prevented prolonged functionaldamage of the muscle rings. We conclude thathypoxia-reoxygenation significantly alters intestinal motility. Thegeneration of reactive oxygen species and disruptions inthe calcium homeostasis play an important role in thepathogenesis of reoxygenation damage. Interventions that alter the intracellular redox state oraffect the secondary changes in the intracellularcalcium concentration can prevent or blunt the effectsof reoxygenation injury on intestinalmotility.

Development, plasticity and modulation of visceral afferents

Visceral pain is the most common reason for doctor visits in the US. Like somatic pain, virtually all visceral pain sensations begin with the activation of primary sensory neurons innervating the viscera and/or the blood vessels associated with these structures. Visceral afferents also play a central role in tissue homeostasis. Recent studies show that in addition to monitoring the state of the viscera, they perform efferent functions through the release of small molecules (e.g. peptides like CGRP) that can drive inflammation, thereby contributing to the development of visceral pathologies (e.g. diabetes Razavi, R., Chan, Y., Afifiyan, F.N., Liu, X.J., Wan, X., Yantha, J., Tsui, H., Tang, L., Tsai, S., Santamaria, P., Driver, J.P., Serreze, D., Salter, M.W., Dosch, H.M., 2006. TRPV1+ sensory neurons control beta cell stress and islet inflammation in autoimmune diabetes, Cell 127 1123-1135). Visceral afferents are heterogeneous with respect to their anatomy, neurochemistry and function. They are also highly plastic in that their cellular environment continuously influences their response properties. This plasticity makes them susceptible to long-term changes that may contribute significantly to the development of persistent pain states such as those associated with irritable bowel syndrome, pancreatitis, and visceral cancers. This review examines recent insights into visceral afferent anatomy and neurochemistry and how neonatal insults can affect the function of these neurons in the adult. New approaches to the treatment of visceral pain, which focus on primary afferents, will also be discussed.

Basic and clinical aspects of visceral sensation: transmission in the CNS

Abstract  Pain and discomfort are the leading cause for consultative visits to gastroenterologists. Acute pain should be considered a symptom of an underlying disease, thereby serving a physiologically important function. However, many patients experience chronic pain in the absence of potentially harmful stimuli or disorders, turning pain into the primary problem rather than a symptom. Vagal and spinal afferents both contribute to the sensory component of the gut–brain axis. Current evidence suggests that they convey different elements of the complex sensory experience. Spinal afferents play a key role in the discriminatory dimension, while vagal input primarily affects the strong emotional and autonomic reactions to noxious visceral stimuli. Drugs, surgical and non‐pharmacological treatments can target these pathways and provide therapeutic options for patients with chronic visceral pain syndromes.

Neonatal Colon Insult Alters Growth Factor Expression and TRPA1 Responses in Adult Mice

&NA; Inflammation or pain during neonatal development can result in long‐term structural and functional alterations of nociceptive pathways, ultimately altering pain perception in adulthood. We have developed a mouse model of neonatal colon irritation (NCI) to investigate the plasticity of pain processing within the viscerosensory system. Mouse pups received an intracolonic administration of 2% mustard oil (MO) on postnatal days 8 and 10. Distal colons were processed at subsequent timepoints for myeloperoxidase (MPO) activity and growth factor expression. Adult mice were assessed for visceral hypersensitivity by measuring the visceromotor response during colorectal distension. Dorsal root ganglion (DRG) neurons from adult mice were retrogradely labeled from the distal colon and calcium imaging was used to measure transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1) responses to acute application of capsaicin and MO, respectively. Despite the absence of inflammation (as indicated by MPO activity), neonatal exposure to intracolonic MO transiently maintained a higher expression level of growth factor messenger RNA (mRNA). Adult NCI mice displayed significant visceral hypersensitivity, as well as increased sensitivity to mechanical stimulation of the hindpaw, compared to control mice. The percentage of TRPA1‐expressing colon afferents was significantly increased in NCI mice, however they displayed no increase in the percentage of TRPV1‐immunopositive or capsaicin‐sensitive colon DRG neurons. These results suggest that early neonatal colon injury results in a long‐lasting visceral hypersensitivity, possibly driven by an early increase in growth factor expression and maintained by permanent changes in TRPA1 function.