Characterization of the peptidergic afferent innervation of the stomach in the rat, mouse and guinea-pig

Mapping the Organization and Morphology of Calcitonin Gene-Related Peptide (CGRP)-IR Axons in the Whole Mouse Stomach

Nociceptive afferent axons innervate the stomach and send signals to the brain and spinal cord. Peripheral nociceptive afferents can be detected with a variety of markers [e.g., substance P (SP) and calcitonin gene-related peptide (CGRP)]. We recently examined the topographical organization and morphology of SP-immunoreactive (SP-IR) axons in the whole mouse stomach muscular layer. However, the distribution and morphological structure of CGRP-IR axons remain unclear. We used immunohistochemistry labeling and applied a combination of imaging techniques, including confocal and Zeiss Imager M2 microscopy, Neurolucida 360 tracing, and integration of axon tracing data into a 3D stomach scaffold to characterize CGRP-IR axons and terminals in the whole mouse stomach muscular layers. We found that: 1) CGRP-IR axons formed extensive terminal networks in both ventral and dorsal stomachs. 2) CGRP-IR axons densely innervated the blood vessels. 3) CGRP-IR axons ran in parallel with the longitudinal and circular muscles. Some axons ran at angles through the muscular layers. 4) They also formed varicose terminal contacts with individual myenteric ganglion neurons. 5) CGRP-IR occurred in DiI-labeled gastric-projecting neurons in the dorsal root and vagal nodose ganglia, indicating CGRP-IR axons were visceral afferent axons. 6) CGRP-IR axons did not colocalize with tyrosine hydroxylase (TH) or vesicular acetylcholine transporter (VAChT) axons in the stomach, indicating CGRP-IR axons were not visceral efferent axons. 7) CGRP-IR axons were traced and integrated into a 3D stomach scaffold. For the first time, we provided a topographical distribution map of CGRP-IR axon innervation of the whole stomach muscular layers at the cellular/axonal/varicosity scale.

Astrocytes in the spinal cord contributed to acute stress-induced gastric damage via the gap junction protein CX43

Rat restraint water-immersion stress (RWIS) is a compound stress of high intensity and is widely used to study the pathological mechanisms of stress gastric ulcers. The spinal cord, as a part of the central nervous system, plays a dominant role in the gastrointestinal tract, but whether the spinal cord is involved in rat restraint water-immersion stress (RWIS)-induced gastric mucosal damage has not been reported. In this study, we examined the expression of spinal astrocytic glial fibrillary acidic protein (GFAP), neuronal c-Fos, connexin 43 (Cx43), and p-ERK1/2 during RWIS by immunohistochemistry and Western blotting. In addition, we intrathecally injected the astrocytic toxin L-a-aminoadipate (L-AA), gap junction blocker carbenoxolone (CBX), and ERK1/2 signaling pathway inhibitor PD98059 to explore the role of astrocytes in the spinal cord in RWIS-induced gastric mucosal damage and its possible mechanism in rats. The results showed that the expression of GFAP, c-Fos, Cx43, and p-ERK1/2 was significantly elevated in the spinal cord after RWIS. Intrathecal injection of both the astrocyte toxin L-AA and the gap junction blocker CBX significantly attenuated RWIS-induced gastric mucosal damage and decreased the activation of astrocytes and neurons induced in the spinal cord. Meanwhile, the ERK1/2 signaling pathway inhibitor PD98059 significantly inhibited gastric mucosal damage, gastric motility and RWIS-induced activation of spinal cord neurons and astrocytes. These results suggest that spinal astrocytes may regulate the RWIS-induced activation of neurons via CX43 gap junctions and play a critical role in RWIS-induced gastric mucosa damage through the ERK1/2 signaling pathway.

Topographical distribution and morphology of SP-IR axons in the antrum, pylorus, and duodenum of mice

Substance-P (SP) is a commonly used marker of nociceptive afferent axons, and it plays an important role in a variety of physiological functions including the regulation of motility, gut secretion, and vascular flow. Previously, we found that SP-immunoreactive (SP-IR) axons densely innervated the pyloric antrum of the flat-mount of the mouse whole stomach muscular layer. However, the regional distribution and morphology of SP-IR axons in the submucosa and mucosa were not well documented. In this study, the mouse antrum-pylorus-duodenum (APD) were transversely and longitudinally sectioned. A Zeiss M2 imager was used to scan the serial sections of each APD (each section montage consisted of 50-100 all-in-focus maximal projection images). To determine the detailed structures of SP-IR axons and terminals, we used the confocal microscope to scan the regions of interest. We found that 1) SP-IR axons innervated the muscular, submucosal, and mucosal layers. 2) In the muscular layer, SP-IR varicose axons densely innervated the muscles and formed varicose terminals which encircled myenteric neurons. 3) In the submucosa, SP-IR axons innervated blood vessels and submucosal ganglia and formed a network in Brunner's glands. 4) In the mucosa, SP-IR axons innervated the muscularis mucosae. Some SP-IR axons entered the lamina propria. 5) The muscular layer of the antrum and duodenum showed a higher SP-IR axon density than the pyloric sphincter. 6) SP-IR axons were from extrinsic and intrinsic origins. This work provided a comprehensive view of the distribution and morphology of SP-IR axons in the APD at single cell/axon/varicosity scale. This data will be used to create a 3D scaffold of the SP-IR axon innervation of the APD.

CGRP physiology, pharmacology, and therapeutic targets: migraine and beyond

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse physiological functions. Its two isoforms (α and β) are widely expressed throughout the body in sensory neurons as well as in other cell types, such as motor neurons and neuroendocrine cells. CGRP acts via at least two G protein-coupled receptors that form unusual complexes with receptor activity-modifying proteins. These are the CGRP receptor and the AMY1 receptor; in rodents, additional receptors come into play. Although CGRP is known to produce many effects, the precise molecular identity of the receptor(s) that mediates CGRP effects is seldom clear. Despite the many enigmas still in CGRP biology, therapeutics that target the CGRP axis to treat or prevent migraine are a bench-to-bedside success story. This review provides a contextual background on the regulation and sites of CGRP expression and CGRP receptor pharmacology. The physiological actions of CGRP in the nervous system are discussed, along with updates on CGRP actions in the cardiovascular, pulmonary, gastrointestinal, immune, hematopoietic, and reproductive systems and metabolic effects of CGRP in muscle and adipose tissues. We cover how CGRP in these systems is associated with disease states, most notably migraine. In this context, we discuss how CGRP actions in both the peripheral and central nervous systems provide a basis for therapeutic targeting of CGRP in migraine. Finally, we highlight potentially fertile ground for the development of additional therapeutics and combinatorial strategies that could be designed to modulate CGRP signaling for migraine and other diseases.

Quantification of CGRP-immunoreactive myenteric neurons in mouse colon

Quantitative data of biological systems provide valuable baseline information for understanding pathology, experimental perturbations, and computational modeling. In mouse colon, calcitonin gene-related peptide (CGRP) is expressed by myenteric neurons with multiaxonal (Dogiel type II) morphology, characteristic of intrinsic primary afferent neurons (IPANs). Analogous neurons in other species and gut regions represent 5-35% of myenteric neurons. We aimed to quantify proportions of CGRP-immunopositive (CGRP+) myenteric neurons. Colchicine-treated wholemount preparations of proximal, mid, and distal colon were labeled for HuC/D, CGRP, nitric oxide synthase (NOS), and peripherin (Per). The pan-neuronal markers (Hu+/Per+) co-labeled 94% of neurons. Hu+/Per- neurons comprised ∼6%, but Hu-/Per+ cells were rare. Thus, quantification was based on Hu+ myenteric neurons (8576 total; 1225 ± 239 per animal, n = 7). CGRP+ cell bodies were significantly larger than the average of all Hu+ neurons (329 ± 13 vs. 261 ± 12 μm² , p < .0001). CGRP+ neurons comprised 19% ± 3% of myenteric neurons without significant regional variation. NOS+ neurons comprised 42% ± 2% of myenteric neurons overall, representing a lower proportion in proximal colon, compared to mid and distal colon (38% ± 2%, 44% ± 2%, and 44% ± 3%, respectively). Peripherin immunolabeling revealed cell body and axonal morphology in some myenteric neurons. Whether all CGRP+ neurons were multiaxonal could not be addressed using peripherin immunolabeling. However, of 118 putatively multiaxonal neurons first identified based on peripherin immunoreactivity, all were CGRP+ (n = 4). In conclusion, CGRP+ myenteric neurons in mouse colon were comprehensively quantified, occurring within a range expected of a putative IPAN marker. All Per+ multiaxonal neurons, characteristic of Dogiel type II/IPAN morphology, were CGRP+.

Constipation Caused by Anti-calcitonin Gene-Related Peptide Migraine Therapeutics Explained by Antagonism of Calcitonin Gene-Related Peptide's Motor-Stimulating and Prosecretory Function in the Intestine

The development of small-molecule calcitonin gene-related peptide (CGRP) receptor antagonists (gepants) and of monoclonal antibodies targeting the CGRP system has been a major advance in the management of migraine. In the randomized controlled trials before regulatory approval, the safety of these anti-CGRP migraine therapeutics was considered favorable and to stay within the expected profile. Post-approval real-world surveys reveal, however, constipation to be a major adverse event which may affect more than 50% of patients treated with erenumab (an antibody targeting the CGRP receptor), fremanezumab or galcanezumab (antibodies targeting CGRP). In this review article we address the question whether constipation caused by inhibition of CGRP signaling can be mechanistically deduced from the known pharmacological actions and pathophysiological implications of CGRP in the digestive tract. CGRP in the gut is expressed by two distinct neuronal populations: extrinsic primary afferent nerve fibers and distinct neurons of the intrinsic enteric nervous system. In particular, CGRP is a major messenger of enteric sensory neurons which in response to mucosal stimulation activate both ascending excitatory and descending inhibitory neuronal pathways that enable propulsive (peristaltic) motor activity to take place. In addition, CGRP is able to stimulate ion and water secretion into the intestinal lumen. The motor-stimulating and prosecretory actions of CGRP combine in accelerating intestinal transit, an activity profile that has been confirmed by the ability of CGRP to induce diarrhea in mice, dogs and humans. We therefore conclude that the constipation elicited by antibodies targeting CGRP or its receptor results from interference with the physiological function of CGRP in the small and large intestine in which it contributes to the maintenance of peristaltic motor activity, ion and water secretion and intestinal transit.

Gut-brain communication by distinct sensory neurons differently controls feeding and glucose metabolism

Sensory neurons relay gut-derived signals to the brain, yet the molecular and functional organization of distinct populations remains unclear. Here, we employed intersectional genetic manipulations to probe the feeding and glucoregulatory function of distinct sensory neurons. We reconstruct the gut innervation patterns of numerous molecularly defined vagal and spinal afferents and identify their downstream brain targets. Bidirectional chemogenetic manipulations, coupled with behavioral and circuit mapping analysis, demonstrated that gut-innervating, glucagon-like peptide 1 receptor (GLP1R)-expressing vagal afferents relay anorexigenic signals to parabrachial nucleus neurons that control meal termination. Moreover, GLP1R vagal afferent activation improves glucose tolerance, and their inhibition elevates blood glucose levels independent of food intake. In contrast, gut-innervating, GPR65-expressing vagal afferent stimulation increases hepatic glucose production and activates parabrachial neurons that control normoglycemia, but they are dispensable for feeding regulation. Thus, distinct gut-innervating sensory neurons differentially control feeding and glucoregulatory neurocircuits and may provide specific targets for metabolic control.

Marine Toxins and Nociception: Potential Therapeutic Use in the Treatment of Visceral Pain Associated with Gastrointestinal Disorders

Visceral pain, of which the pathogenic basis is currently largely unknown, is a hallmark symptom of both functional disorders, such as irritable bowel syndrome, and inflammatory bowel disease. Intrinsic sensory neurons in the enteric nervous system and afferent sensory neurons of the dorsal root ganglia, connecting with the central nervous system, represent the primary neuronal pathways transducing gut visceral pain. Current pharmacological therapies have several limitations, owing to their partial efficacy and the generation of severe adverse effects. Numerous cellular targets of visceral nociception have been recognized, including, among others, channels (i.e., voltage-gated sodium channels, VGSCs, voltage-gated calcium channels, VGCCs, Transient Receptor Potential, TRP, and Acid-sensing ion channels, ASICs) and neurotransmitter pathways (i.e., GABAergic pathways), which represent attractive targets for the discovery of novel drugs. Natural biologically active compounds, such as marine toxins, able to bind with high affinity and selectivity to different visceral pain molecular mediators, may represent a useful tool (1) to improve our knowledge of the physiological and pathological relevance of each nociceptive target, and (2) to discover therapeutically valuable molecules. In this review we report the most recent literature describing the effects of marine toxin on gastrointestinal visceral pain pathways and the possible clinical implications in the treatment of chronic pain associated with gut diseases.

Distinct Expression of Phenotypic Markers in Placodes- and Neural Crest-Derived Afferent Neurons Innervating the Rat Stomach

BACKGROUND

Visceral pain is initiated by activation of primary afferent neurons among which the capsaicin-sensitive (TRPV1-positive) neurons play an important role. The stomach is a common source of visceral pain. Similar to other organs, the stomach receives dual spinal and vagal afferent innervation. Developmentally, spinal dorsal root ganglia (DRG) and vagal jugular neurons originate from embryonic neural crest and vagal nodose neurons originate from placodes. In thoracic organs the neural crest- and placodes-derived TRPV1-positive neurons have distinct phenotypes differing in activation profile, neurotrophic regulation and reflex responses. It is unknown to whether such distinction exists in the stomach.

AIMS

We hypothesized that gastric neural crest- and placodes-derived TRPV1-positive neurons express phenotypic markers indicative of placodes and neural crest phenotypes.

METHODS

Gastric DRG and vagal neurons were retrogradely traced by DiI injected into the rat stomach wall. Single-cell RT-PCR was performed on traced gastric neurons.

RESULTS

Retrograde tracing demonstrated that vagal gastric neurons locate exclusively into the nodose portion of the rat jugular/petrosal/nodose complex. Gastric DRG TRPV1-positive neurons preferentially expressed markers PPT-A, TrkA and GFRα₃ typical for neural crest-derived TRPV1-positive visceral neurons. In contrast, gastric nodose TRPV1-positive neurons preferentially expressed markers P2X₂ and TrkB typical for placodes-derived TRPV1-positive visceral neurons. Differential expression of neural crest and placodes markers was less pronounced in TRPV1-negative DRG and nodose populations.

CONCLUSIONS

There are phenotypic distinctions between the neural crest-derived DRG and placodes-derived vagal nodose TRPV1-positive neurons innervating the rat stomach that are similar to those described in thoracic organs.

The effect of spinal cord injury on the neurochemical properties of vagal sensory neurons

The vagus nerve is composed primarily of nonmyelinated sensory neurons whose cell bodies are located in the nodose ganglion (NG). The vagus has widespread projections that supply most visceral organs, including the bladder. Because of its nonspinal route, the vagus nerve itself is not directly damaged from spinal cord injury (SCI). Because most viscera, including bladder, are dually innervated by spinal and vagal sensory neurons, an impact of SCI on the sensory component of vagal circuitry may contribute to post-SCI visceral pathologies. To determine whether SCI, in male Wistar rats, might impact neurochemical characteristics of NG neurons, immunohistochemical assessments were performed for P2X3 receptor expression, isolectin B4 (IB4) binding, and substance P expression, three known injury-responsive markers in sensory neuronal subpopulations. In addition to examining the overall population of NG neurons, those innervating the urinary bladder also were assessed separately. All three of the molecular markers were represented in the NG from noninjured animals, with the majority of the neurons binding IB4. In the chronically injured rats, there was a significant increase in the number of NG neurons expressing P2X3 and a significant decrease in the number binding IB4 compared with noninjured animals, a finding that held true also for the bladder-innervating population. Overall, these results indicate that vagal afferents, including those innervating the bladder, display neurochemical plasticity post-SCI that may have implications for visceral homeostatic mechanisms and nociceptive signaling.

The enteric nervous system and gastrointestinal innervation: integrated local and central control

The digestive system is innervated through its connections with the central nervous system (CNS) and by the enteric nervous system (ENS) within the wall of the gastrointestinal tract. The ENS works in concert with CNS reflex and command centers and with neural pathways that pass through sympathetic ganglia to control digestive function. There is bidirectional information flow between the ENS and CNS and between the ENS and sympathetic prevertebral ganglia.The ENS in human contains 200-600 million neurons, distributed in many thousands of small ganglia, the great majority of which are found in two plexuses, the myenteric and submucosal plexuses. The myenteric plexus forms a continuous network that extends from the upper esophagus to the internal anal sphincter. Submucosal ganglia and connecting fiber bundles form plexuses in the small and large intestines, but not in the stomach and esophagus. The connections between the ENS and CNS are carried by the vagus and pelvic nerves and sympathetic pathways. Neurons also project from the ENS to prevertebral ganglia, the gallbladder, pancreas and trachea.The relative roles of the ENS and CNS differ considerably along the digestive tract. Movements of the striated muscle esophagus are determined by neural pattern generators in the CNS. Likewise the CNS has a major role in monitoring the state of the stomach and, in turn, controlling its contractile activity and acid secretion, through vago-vagal reflexes. In contrast, the ENS in the small intestine and colon contains full reflex circuits, including sensory neurons, interneurons and several classes of motor neuron, through which muscle activity, transmucosal fluid fluxes, local blood flow and other functions are controlled. The CNS has control of defecation, via the defecation centers in the lumbosacral spinal cord. The importance of the ENS is emphasized by the life-threatening effects of some ENS neuropathies. By contrast, removal of vagal or sympathetic connections with the gastrointestinal tract has minor effects on GI function. Voluntary control of defecation is exerted through pelvic connections, but cutting these connections is not life-threatening and other functions are little affected.

Deletion of leptin signaling in vagal afferent neurons results in hyperphagia and obesity

The vagal afferent pathway senses hormones released from the gut in response to nutritional cues and relays these signals to the brain. We tested the hypothesis that leptin resistance in vagal afferent neurons (VAN) is responsible for the onset of hyperphagia by developing a novel conditional knockout mouse to delete leptin receptor selectively in sensory neurons (Nav1.8/LepR (fl/fl) mice). Chow fed Nav1.8/LepR (fl/fl) mice weighed significantly more and had increased adiposity compared with wildtype mice. Cumulative food intake, meal size, and meal duration in the dark phase were increased in Nav1.8/LepR (fl/fl) mice; energy expenditure was unaltered. Reduced satiation in Nav1.8/LepR (fl/fl) mice is in part due to reduced sensitivity of VAN to CCK and the subsequent loss of VAN plasticity. Crucially Nav1.8/LepR (l/fl) mice did not gain further weight in response to a high fat diet. We conclude that disruption of leptin signaling in VAN is sufficient and necessary to promote hyperphagia and obesity.

Capsaicin receptor as target of calcitonin gene-related peptide in the gut

Calcitonin gene-related peptide (CGRP), a 37 aminoacid-residue peptide, is a marker of afferent fibers in the upper gastrointestinal tract, being almost completely depleted following treatment with the selective neurotoxin capsaicin that targets these fibers via transient receptor potential vanilloid type-1 (TRPV-1). It is widely distributed in the peripheral nervous system of mammals where it is present as alpha isoform, while intrinsic neurons of the enteric nervous systems express predominantly CGRP-beta. Many gastrointestinal functions involve CGRP-containing afferent fibers of the enteric nervous system such as defense against irritants, intestinal nociception, modulation of gastrointestinal motility and secretion, and healing of gastric ulcers. The main effects on stomach homeostasis rely on local vasodilator actions during increased acid-back diffusion. In humans, release of CGRP through the activation of TRPV-1 has been shown to protect from gastric damage induced by several stimuli and to be involved in gastritis. In both dyspepsia and irritable bowel syndrome the repeated stimulation of TRPV-1 induced an improvement in epigastric pain of these patients. The TRPV-1/CGRP pathway might be a novel target for therapeutics in gastric mucosal injury and visceral sensitivity.

Extrinsic primary afferent neurons projecting to the pylorus in the domestic pig--localization and neurochemical characteristics

The pig, as an omnivorous animal, seems to be especially valuable species in "gastrointestinal" experiments. The importance of the pylorus in the proper functioning of the digestive tract is widely accepted. Although it is commonly known that sensory innervation plays an important role in the regulation of gastric activity and gastrointestinal tissue resistance, there is complete lack of data on the extrinsic afferents projecting to the swine pylorus. The present experiment has been designed to discover the precise localization and neurochemical properties of the primary sensory neurons projecting to the porcine pylorus. Combined retrograde tracing technique and double immunocytochemistry were applied in five piglets. An additional RT-PCR reaction was used to confirm the presence of all investigated neurotransmitters in the studied ganglia. Traced neurons were localized within the bilateral nodose ganglia of the vagus and bilateral dorsal root ganglia spreading from Th4 to L1. Fast Blue-positive afferents expressed immunoreactivity to substance P, calcitonin gene-related peptide, neuronal isoform of nitric oxide synthase, and galanin. In the vagal and spinal ganglia, the percentages of traced neurons immunoreactive to these substances were 54.8, 10.7, 49.6, 7.4 % and 22.2, 75.5, 95.2 %, respectively, and the solitary perikarya were Gal immunoreactive. The presence of all substances studied in the ganglion tissue was confirmed by RT-PCR technique.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Roles of substance P and ATP in the subepithelial fibroblasts of rat intestinal villi

The ingestion of food and water induces chemical and mechanical signals that trigger peristaltic reflexes and also villous movement in the gut. In the intestinal villi, subepithelial fibroblasts under the epithelium form contractile cellular networks and closely contact to the varicosities of substance P and nonsubstance P afferent neurons. Subepithelial fibroblasts of the duodenal villi possess purinergic receptor P2Y1 and tachykinin receptor NK1. ATP and substance P induce increase in intracellular Ca(2+) and cell contraction in subepithelial fibroblasts. They are highly mechanosensitive and release ATP by mechanical stimuli. Released ATP spreads to form an ATP "cloud" with nearly 1μM concentration and activates the surroundings via P2Y1 and afferent neurons via P2X receptors. These findings suggest that villous subepithelial fibroblasts and afferent neurons interact via ATP and substance P. This mutual interaction may play important roles in the signal transduction of mechano reflex pathways including a coordinate villous movement and also in the maturation of the structure and function of the intestinal villi.

The neural crest- and placodes-derived afferent innervation of the mouse esophagus

BACKGROUND

The mouse is an invaluable model for mechanistic studies of esophageal nerves, but the afferent innervation of the mouse esophagus is incompletely understood. Vagal afferent neurons are derived from two embryonic sources: neural crest and epibranchial placodes. We hypothesized that both neural crest and placodes contribute to the TRPV1-positive (potentially nociceptive) vagal innervation of the mouse esophagus.

METHODS

Vagal jugular/nodose ganglion (JNG) and spinal dorsal root ganglia (DRG) neurons were retrogradely labeled from the cervical esophagus. Single cell RT-PCR was performed on the labeled neurons.

KEY RESULTS

In the Wnt1Cre/R26R mice expressing a reporter in the neural crest-derived cells we found that both the neural crest- and the placodes-derived vagal JNG neurons innervate the mouse esophagus. In the wild-type mouse the esophageal vagal JNG TRPV1-positive neurons segregated into two subsets: putative neural crest-derived purinergic receptor P2X(2) -negative/preprotachykinin-A (PPT-A)-positive subset and putative placodes-derived P2X(2) -positive/PPTA-negative subset. These subsets also segregated by the expression of TrkA and GFRα(3) in the putative neural crest-derived subset, and TrkB in the putative placodes-derived subset. The TRPV1-positive esophageal DRG neurons had the phenotype similar to the vagal putative neural crest-derived subset.

CONCLUSIONS & INFERENCES

The TRPV1-positive (potentially nociceptive) vagal afferent neurons innervating the mouse esophagus originate from both neural crest and placodes. The expression profile of the receptors for neurotrophic factors is similar between the neural crest-derived vagal and spinal nociceptors, but distinct from the vagal placodes-derived nociceptors.

TRPA1: A gatekeeper for inflammation

Tissue damage evokes an inflammatory response that promotes the removal of harmful stimuli, tissue repair, and protective behaviors to prevent further damage and encourage healing. However, inflammation may outlive its usefulness and become chronic. Chronic inflammation can lead to a host of diseases, including asthma, itch, rheumatoid arthritis, and colitis. Primary afferent sensory neurons that innervate target organs release inflammatory neuropeptides in the local area of tissue damage to promote vascular leakage, the recruitment of immune cells, and hypersensitivity to mechanical and thermal stimuli. TRPA1 channels are required for neuronal excitation, the release of inflammatory neuropeptides, and subsequent pain hypersensitivity. TRPA1 is also activated by the release of inflammatory agents from nonneuronal cells in the area of tissue injury or disease. This dual function of TRPA1 as a detector and instigator of inflammatory agents makes TRPA1 a gatekeeper of chronic inflammatory disorders of the skin, airways, and gastrointestinal tract.

Solid gastric emptying mediated by the serotonin (5-HT)3 receptor in mice is a simple marker to predict emesis

Nausea and emesis are often observed as side effects with many medicines and may lead to poor treatment compliance. In the present study, we aimed to establish simple methods for predicting nausea and/or emesis in mice, which do not vomit, using drugs and chemicals known to evoke nausea and/or emesis. The gastrointestinal transit test, the liquid gastric emptying by phenol red solution (Phenol red method) and the solid gastric emptying by resin beads (Beads method) were used and the effects of antispasmogenics (atropine, 0.1-3 mg/kg i.p.; salmon calcitonin, 1-30 units/kg i.m.), nauseants (copper sulfate, 1-30 mg/kg p.o.; apomorphine, 0.01-0.3 mg/kg s.c.) and chemotherapeutics (cisplatin, 0.3-10 mg/kg i.v.; doxorubicin, 0.3-10 mg/kg i.v.) were evaluated. In addition, the effects of ondansetron, a serotonin (5-HT)(3) receptor antagonist, on the inhibition of solid gastric emptying induced by salmon calcitonin, copper sulfate, cisplatin and doxorubicin were also assessed. Only the solid gastric emptying method could detect changes of gastric emptying by all drugs and chemicals. We also found that the inhibition of solid gastric emptying induced by cisplatin and doxorubicin was dose-dependently antagonized by ondansetron. However, ondansetron failed to antagonize the salmon calcitonin-induced delay, but exerted only very weak effects with copper sulfate. Solid gastric emptying may be more suitable than gastrointestinal intestinal transit or liquid gastric emptying in mice to predict nausea and/or emesis. Our results also suggest that chemotherapeutic-induced delay of solid gastric emptying mediated via 5-HT(3) receptors in mice could also be useful for prediction purposes.

The influence of ileitis on the neurochemistry of the caudal mesenteric ganglion in the pig

BACKGROUND

Some literature data suggest that there is a regulatory neuronal circuit between the small and the large bowel. To verify this hypothesis the present study investigated: (i) the distribution, chemical coding and routing of caudal mesenteric ganglion (CaMG) neurons participating in an intestinointestinal reflex pathway involving ileal descending neurons and viscerofugal colonic neurons and (ii) possible changes in the neuroarchitecture of this pathway evoked by chemically induced ileitis in juvenile pigs (n=16).

METHODS

Combined retrograde tract tracing and transections of the intermesenteric or caudal colonic nerves were applied. In addition, double immunostainings was used to investigate the chemical coding of retrogradely labeled CaMG neurons and intraganglionic nerve terminals apposed to them, under normal and inflammatory conditions.

KEY RESULTS

The majority of the ileum-projecting neurons were found in the caudal part of CaMG. Disruption of particular nerve pathways resulted in diminished number of retrogradely labeled neurons, ipsilateral to the side of manipulation. In normal pigs, ileum-projecting CaMG neurons stained for tyrosine hydroxylase, dopamine-β-hydroxylase, neuropeptide Y (NPY), somatostatin and galanin (GAL). The number and chemical coding of the neurons in the inflamed animals were similar to those observed in the normal pigs. However, in the inflamed pigs, the number of NPY-, GAL- or substance P-positive nerve terminals supplying retrogradely labeled neurons was increased.

CONCLUSIONS & INFERENCES

The present results suggest that inflammatory processes of the porcine ileum are able to induce changes in the intraganglionic architecture of a sympathetic ganglion located at discrete distance from the affected bowel segment.

The analgesic effects of the GABAB receptor agonist, baclofen, in a rodent model of functional dyspepsia

BACKGROUND

The amino acid γ-aminobutyric acid (GABA) is an important modulator of pain but its role in visceral pain syndromes is just beginning to be studied. Our aims were to investigate the effect and mechanism of action of the GABA(B) receptor agonist, baclofen, on gastric hypersensitivity in a validated rat model of functional dyspepsia (FD).

METHODS

10-day-old male rats received 0.2 mL of 0.1% iodoacetamide in 2% sucrose daily by oral gavages for 6 days. Control group received 2% sucrose. At 8-10 weeks rats treated with baclofen (0.3, 1, and 3 mg kg(-1) bw) or saline were tested for behavioral and electromyographic (EMG) visceromotor responses; gastric spinal afferent nerve activity to graded gastric distention and Fos protein expression in dorsal horn of spinal cord segments T8-T10 to noxious gastric distention.

KEY RESULTS

Baclofen administration was associated with a significant attenuation of the behavioral and EMG responses (at 1 and 3 mg kg(-1)) and expression of Fos in T8 and T9 segments in neonatal iodoacetamide sensitized rats. However, baclofen administration did not significantly affect splanchnic nerve activity to gastric distention. Baclofen (3 mg kg(-1)) also significantly reduced the expression of spinal Fos in response to gastric distention in control rats to a lesser extent than sensitized rats.

CONCLUSIONS & INFERENCES

Baclofen is effective in attenuating pain associated responses in an experimental model of FD and appears to act by central mechanisms. These results provide a basis for clinical trials of this drug in FD patients.

Transient receptor potential (TRP) channels as drug targets for diseases of the digestive system

Approximately 20 of the 30 mammalian transient receptor potential (TRP) channel subunits are expressed by specific neurons and cells within the alimentary canal. They subserve important roles in taste, chemesthesis, mechanosensation, pain and hyperalgesia and contribute to the regulation of gastrointestinal motility, absorptive and secretory processes, blood flow, and mucosal homeostasis. In a cellular perspective, TRP channels operate either as primary detectors of chemical and physical stimuli, as secondary transducers of ionotropic or metabotropic receptors, or as ion transport channels. The polymodal sensory function of TRPA1, TRPM5, TRPM8, TRPP2, TRPV1, TRPV3 and TRPV4 enables the digestive system to survey its physical and chemical environment, which is relevant to all processes of digestion. TRPV5 and TRPV6 as well as TRPM6 and TRPM7 contribute to the absorption of Ca²⁺ and Mg²⁺, respectively. TRPM7 participates in intestinal pacemaker activity, and TRPC4 transduces muscarinic acetylcholine receptor activation to smooth muscle contraction. Changes in TRP channel expression or function are associated with a variety of diseases/disorders of the digestive system, notably gastro-esophageal reflux disease, inflammatory bowel disease, pain and hyperalgesia in heartburn, functional dyspepsia and irritable bowel syndrome, cholera, hypomagnesemia with secondary hypocalcemia, infantile hypertrophic pyloric stenosis, esophageal, gastrointestinal and pancreatic cancer, and polycystic liver disease. These implications identify TRP channels as promising drug targets for the management of a number of gastrointestinal pathologies. As a result, major efforts are put into the development of selective TRP channel agonists and antagonists and the assessment of their therapeutic potential.

Plasticity in vagal afferent neurones during feeding and fasting: mechanisms and significance

The ingestion of food activates mechanisms leading to inhibition of food intake and gastric emptying mediated by the release of regulatory peptides, for example cholecystokinin (CCK), and lipid amides, e.g. oleylethanolamide from the gut. In addition, there are both peptides (e.g. ghrelin) and lipid amides (e.g. anandamide) that appear to signal the absence of food in the gut and that are associated with the stimulation of food intake. Vagal afferent neurones are a common target for both types of signal. Remarkably, the neurochemical phenotype of these neurones itself depends on nutritional status. CCK acting at CCK1 receptors on vagal afferent neurones stimulates expression in these neurones of Y2-receptors and the neuropeptide CART, both of which are associated with the inhibition of food intake. Conversely, in fasted rats when plasma CCK is low, these neurones express cannabinoid (CB)-1 and melanin concentrating hormone (MCH)-1 receptors, and MCH, and this is inhibited by exogenous CCK or endogenous CCK released by refeeding. The stimulation of CART expression by CCK is mediated by the activation of CREB and EGR1; ghrelin inhibits the action of CCK by promoting nuclear exclusion of CREB and leptin potentiates the action of CCK by the stimulation of EGR1 expression. Vagal afferent neurones therefore constitute a level of integration outside the CNS for nutrient-derived signals that control energy intake and that are capable of encoding recent nutrient ingestion.

Role of capsaicin-sensitive primary afferent neurons and non-protein sulphydryl groups on gastroprotective effect of amifostine against ethanol-induced gastric damage in rats

BACKGROUND

Amifostine has been widely tested as a cytoprotective agent against a number of aggressors in different organs. Recently, a gastroprotective effect was observed for this drug in a model of indomethacin-induced gastric injury. Our objective was to investigate the effect of amifostine on ethanol-induced gastric injury and the role played in this mechanism by afferent sensory neurons, non-protein sulfhydryl groups, nitric oxide, ATP-sensitive potassium channels, and cyclooxygenase-2.

METHODS

Rats were treated with amifostine (22.5, 45, 90, or 180 mg/kg, PO or SC). After 30 min, the rats received absolute ethanol (5 ml kg(-1), PO). One hour later, gastric damage was quantified with a planimeter. Samples from the stomach were also taken for histopathological assessment and for assays of non-protein sulfhydryl groups. The other groups were pretreated with L-NAME (10 mg kg(-1), IP), glibenclamide (10 mg kg(-1), PO), or celecoxib (10 mg kg(-1), PO). After 30 min, the animals were given amifostine (90 mg kg(-1), PO or SC), followed 30 min later by gavage with absolute ethanol (5 ml kg(-1)). Other rats were desensitized with capsaicin (125 mg kg(-1), SC) 8 days prior to amifostine treatment.

RESULTS

Amifostine administration PO and SC significantly and dose-dependently reduced ethanol-induced macroscopic and microscopic gastric damage by restoring glutathione levels in the stomach mucosa. Amifostine-promoted gastroprotection against ethanol-induced stomach injury was reversed by pretreatment with neurotoxic doses of capsaicin, but not by L-NAME, glibenclamide, or celecoxib.

CONCLUSIONS

Amifostine protects against ethanol-induced gastric injury by increasing glutathione levels and stimulating the afferent sensory neurons in the stomach.

TRPV1: a target for next generation analgesics

Transient Receptor Potential Vanilloid 1 (TRPV1) is a Ca²⁺ permeant non-selective cation channel expressed in a subpopulation of primary afferent neurons. TRPV1 is activated by physical and chemical stimuli. It is critical for the detection of nociceptive and thermal inflammatory pain as revealed by the deletion of the TRPV1 gene. TRPV1 is distributed in the peripheral and central terminals of the sensory neurons and plays a role in initiating action potentials at the nerve terminals and modulating neurotransmitter release at the first sensory synapse, respectively. Distribution of TRPV1 in the nerve terminals innervating blood vessels and in parts of the CNS that are not subjected to temperature range that is required to activate TRPV1 suggests a role beyond a noxious thermal sensor. Presently, TRPV1 is being considered as a target for analgesics through evaluation of different antagonists. Here, we will discuss the distribution and the functions of TRPV1, potential use of its agonists and antagonists as analgesics and highlight the functions that are not related to nociceptive transmission that might lead to adverse effects.

Acid-induced CGRP release from the stomach does not depend on TRPV1 or ASIC3

BACKGROUND

Acid-sensing and regulating reactions are vitally important in the upper gastrointestinal tract and disturbances are common. Sensory neurons in the mucosa detect the intrusion of hydrogen ions and, by their release of vasoactive neuropeptides, seem to play a predominantly protective role in these tissues.

METHODS

The model to investigate sensory transduction of proton stimuli in the isolated everted mouse stomach was to measure the induced calcitonin gene-related peptide (CGRP) release as an index of neuronal activation.

KEY RESULTS

Proton concentrations in the range of pH 2.5-0.5 stimulated the release of CGRP and substance P and profoundly decreased the prostaglandin E2 formation in outbred CD mice. A similar linearly pH-dependent CGRP release was observed in inbred C57BL/6 mice, fully dependent on extracellular calcium at pH 2, partially at pH 1. Both transient receptor potential vanilloid type 1 (TRPV1) and acid-sensing ion channel type 3 (ASIC3) are expressed in the sensory neurons innervating the stomach walls and are responsible for the transduction of acidic stimuli in other visceral organs. However, the proton-induced gastric CGRP release in mice lacking the TRPV1 or the ASIC3 receptor-channels was the same as in corresponding wild-type mice. Nonetheless, the pharmacological blockers N-(4-tertiarybutylphenyl)-4-(3-chlorophyridin-2-yl)tetrahydropyrazine-1(2H)carboxamide and amiloride, respectively, inhibited the acid-stimulated CGRP release, although to the same extend in wild types as TRPV1 and ASIC3 knockout mice.

CONCLUSIONS & INFERENCES

Adequate proton concentrations inhibit prostaglandin and stimulate CGRP release from the stomach wall, however, the transduction mechanism in the gastric sensory neurons remains unclear.

Melanocortin-4 receptor expression in a vago-vagal circuitry involved in postprandial functions

Vagal afferents regulate energy balance by providing a link between the brain and postprandial signals originating from the gut. In the current study, we investigated melanocortin-4 receptor (MC4R) expression in the nodose ganglion, where the cell bodies of vagal sensory afferents reside. By using a line of mice expressing green fluorescent protein (GFP) under the control of the MC4R promoter, we found GFP expression in approximately one-third of nodose ganglion neurons. By using immunohistochemistry combined with in situ hybridization, we also demonstrated that approximately 20% of GFP-positive neurons coexpressed cholecystokinin receptor A. In addition, we found that the GFP is transported to peripheral tissues by both vagal sensory afferents and motor efferents, which allowed us to assess the sites innervated by MC4R-GFP neurons. GFP-positive efferents that co-expressed choline acetyltransferase specifically terminated in the hepatic artery and the myenteric plexus of the stomach and duodenum. In contrast, GFP-positive afferents that did not express cholinergic or sympathetic markers terminated in the submucosal plexus and mucosa of the duodenum. Retrograde tracing experiments confirmed the innervation of the duodenum by GFP-positive neurons located in the nodose ganglion. Our findings support the hypothesis that MC4R signaling in vagal afferents may modulate the activity of fibers sensitive to satiety signals such as cholecystokinin, and that MC4R signaling in vagal efferents may contribute to the control of the liver and gastrointestinal tract.

Amino acids in the rat intestinal lumen regulate their own absorption from a distant intestinal site

Intestinal nutrient transport is altered in response to changes in dietary conditions and luminal substrate level. It is not clear, however, whether an amino acid in the intestinal lumen can acutely affect its own absorption from a distant site. Our aim is to study the effect of an amino acid present in rat small intestinal segment on its own absorption from a proximal or distal site and elucidate the underlying mechanisms. The effect of instillation of alanine (Ala) in either jejunum or ileum on its own absorption at ileal or jejunal level was examined in vivo. The modulation of this intestinal regulatory loop by the following interventions was studied: tetrodotoxin (TTX) added to Ala, subdiaphragmatic vagotomy, chemical ablation of capsaicin-sensitive primary afferent (CSPA) fibers, and IV administration of calcitonin gene-related peptide (CGRP) antagonist. In addition, the kinetics of jejunal Ala absorption and the importance of Na+-dependent transport were studied in vitro after instilling Ala in the ileum. Basal jejunal Ala absorption [0.198 +/- 0.018 micromol x cm(-1) x 20 min(-1) (means +/- SD)] was significantly decreased with the instillation of 20 mM Ala in the ileum or in an adjacent distal jejunal segment (0.12 +/- 0.015; P < 0.0001 and 0.138 +/- 0.014; P < 0.002, respectively). Comparable inhibition was observed in the presence of proline in the ileum. Moreover, basal Ala absorption from the ileum (0.169 +/- 0.025) was significantly decreased by the presence of 20 mM Ala in the jejunum (0.103 +/- 0.027; P < 0.01). The inhibitory effect on jejunal Ala absorption was abolished by TTX, subdiaphragmatic vagotomy, neonatal capsaicin treatment, and CGRP antagonism. In vitro studies showed that Ala in the ileum affects Na+-mediated transport and increases K(m) without affecting Vmax. Intraluminal amino acids control their own absorption from a distant part of the intestine, by affecting the affinity of the Na+-mediated Ala transporter, through a neuronal mechanism that involves CSPA and CGRP.

Inside information: the unique features of visceral sensation

Most of what is written and believed about pain and nociceptors originates from studies of the "somatic" (non-visceral) sensory system. As a result, the unique features of visceral pain are often overlooked. In the clinic, the management of visceral pain is typically poor, and drugs that are used with some efficacy to treat somatic pain often present unwanted effects on the viscera. For these reasons, a better understanding of visceral sensory neurons-particularly visceral nociceptors-is required. This review provides evidence of functional, morphological, and biochemical differences between visceral and non-visceral afferents, with a focus on potential nociceptive roles, and also considers some of the potential mechanisms of visceral mechanosensation.

The pharmacological challenge to tame the transient receptor potential vanilloid-1 (TRPV1) nocisensor

The transient receptor potential vanilloid-1 (TRPV1) cation channel is a receptor that is activated by heat (>42 degrees C), acidosis (pH<6) and a variety of chemicals among which capsaicin is the best known. With these properties, TRPV1 has emerged as a polymodal nocisensor of nociceptive afferent neurones, although some non-neuronal cells and neurones in the brain also express TRPV1. The activity of TRPV1 is controlled by a multitude of regulatory mechanisms that either cause sensitization or desensitization of the channel. As many proalgesic pathways converge on TRPV1 and this nocisensor is upregulated and sensitized by inflammation and injury, TRPV1 is thought to be a central transducer of hyperalgesia and a prime target for the pharmacological control of pain. As a consequence, TRPV1 agonists causing defunctionalization of sensory neurones and a large number of TRPV1 blockers have been developed, some of which are in clinical trials. A major drawback of many TRPV1 antagonists is their potential to cause hyperthermia, and their long-term use may carry further risks because TRPV1 has important physiological functions in the peripheral and central nervous system. The challenge, therefore, is to pharmacologically differentiate between the physiological and pathological implications of TRPV1. There are several possibilities to focus therapy specifically on those TRPV1 channels that contribute to disease processes. These approaches include (i) site-specific TRPV1 antagonists, (ii) modality-specific TRPV1 antagonists, (iii) uncompetitive TRPV1 (open channel) blockers, (iv) drugs interfering with TRPV1 sensitization, (v) drugs interfering with intracellular trafficking of TRPV1 and (vi) TRPV1 agonists for local administration.

Control of gastric acid secretion in health and disease

Recent milestones in the understanding of gastric acid secretion and treatment of acid-peptic disorders include the (1) discovery of histamine H(2)-receptors and development of histamine H(2)-receptor antagonists, (2) identification of H(+)K(+)-ATPase as the parietal cell proton pump and development of proton pump inhibitors, and (3) identification of Helicobacter pylori as the major cause of duodenal ulcer and development of effective eradication regimens. This review emphasizes the importance and relevance of gastric acid secretion and its regulation in health and disease. We review the physiology and pathophysiology of acid secretion as well as evidence regarding its inhibition in the management of acid-related clinical conditions.

Ablation of primary afferent neurons by neonatal capsaicin treatment reduces the susceptibility of the portal hypertensive gastric mucosa to ethanol-induced injury in cirrhotic rats

Primary sensory afferent neurons modulate the hyperdynamic circulation in cirrhotic rats with portal hypertension. The stomach of cirrhotic rats is prone to damage induced by ethanol, a phenomenon associated with reduced gastric hyperemic response to acid-back diffusion. The aim of this study was to examine the impact of ablation of capsaicin-sensitive neurons and the tachykinin NK(1) receptor antagonist A5330 on the susceptibility of the portal hypertensive gastric mucosa to ethanol-induced injury and its effects on gastric cyclooxygenase (COX) and nitric oxide synthase (NOS) mRNA expression. Capsaicin was administered to neonatal, male, Wistar rats and the animals were allowed to grow. Cirrhosis was then induced by bile duct ligation in adult rats while controls had sham operation. Ethanol-induced gastric damage was assessed using ex vivo gastric chamber experiments. Gastric blood flow was measured as well as COX/NOS mRNA expression. Topical application of ethanol produced significant gastric damage in cirrhotic rats compared to controls, which was reversed in capsaicin- and A5330-treated animals. Mean arterial and portal pressure was normalized in capsaicin-treated cirrhotic rats. Capsaicin and A5330 administration restored gastric blood flow responses to topical application of ethanol followed by acid in cirrhotic rats. Differential COX and NOS mRNA expression was noted in bile duct ligated rats relative to controls. Capsaicin treatment significantly modified gastric eNOS/iNOS/COX-2 mRNA expression in cirrhotic rats. Capsaicin-sensitive neurons modulate the susceptibility of the portal hypertensive gastric mucosa to injury induced by ethanol via tachykinin NK(1) receptors and signalling of prostaglandin and NO production/release.

Prolonged elevation of galanin and tachykinin expression in mucosal and myenteric enteric nerves in trinitrobenzene sulphonic acid colitis

Diverticulitis causes recurrent abdominal pain associated with increased mucosal expression of mucosal galanin and substance P (SP). We studied changes in mucosal and myenteric plexus neuropeptides in adult rats using a model of colonic inflammation, trinitrobenzenesulphonic acid colitis. We assessed the effects on the pan-neuronal markers protein gene product 9.5 (PGP9.5) and neurofilament protein, as well as specific neuropeptides at 1, 2, 3, 4, 6, 8, 10 and 14 weeks. Following the acute injury there was macroscopic resolution of inflammation but minor microscopic abnormalities persisted. Percent area stained of mucosal PGP9.5 fell initially but average levels on days 21 and 28 levels were significantly elevated (P < 0.001), returning to normal by day 42. Percent area staining of PGP9.5 in the muscle rose immediately and remained significantly elevated at 70 days (P < 0.001). SP, neuropeptide K and galanin followed a similar overall pattern. SP to PGP9.5 ratio was significantly increased in the muscle both acutely (days 1-28) and in the long term (days 70 and 98), whereas the galanin to PGP9.5 ratio was significantly increased in the mucosa throughout the study. Low-grade chronic inflammation after an acute initial insult causes a persistent increase in the expression of galanin in the mucosa and SP in muscle layer.

Dichotomizing axons in spinal and vagal afferents of the mouse stomach

Visceral sensory input is typically poorly localized. We hypothesized that gastric sensory neurons frequently dichotomize, innervating more than one anatomically distinct region and contributing to the poor spatial discrimination.

METHODS

The neurochemical phenotype and projections of gastro-duodenal sensory neurons were determined in adult mice. Choleratoxin B (CTB) coupled to different fluorophors was injected into fundus, corpus, antrum, and/or distal duodenum. Immunoreactivity for TRPV1, neurofilament (N52), calcitonin gene-related peptide (CGRP), presence of isolectin B4 (IB4) and labeling for retrograde labels was determined.

RESULTS

Depending on the distance between injection sites, staining for two retrograde tracers was seen in 6-48% of neurons. Most dorsal root ganglion (DRG) neurons showed immunoreactivity for TRPV1 and CGRP. In contrast, about half of the gastric nodose ganglion (NG) neurons had TRPV1 immunoreactivity or showed IB4 labeling with only 10% CGRP-positive neurons. N52 immunoreactivity was present in one-fourth of gastroduodenal DRG and NG neurons.

CONCLUSION

Visceral sensory neurons have neurochemical properties and may project to more than one anatomically distinct area. Neurons with such dichotomizing axons may contribute to the poor ability to localize or discriminate visceral stimuli.

Tachyphylaxis of the ECL-cell response to PACAP: receptor desensitization and/or depletion of secretory products

BACKGROUND AND PURPOSE

Rat stomach ECL cells secrete histamine and pancreastatin in response to gastrin and pituitary adenylate cyclase-activating peptide-27 (PACAP). This study applies microdialysis to explore how ECL cells in situ respond to PACAP and gastrin.

EXPERIMENTAL APPROACH

Both peptides were administered by microinfusion into the gastric submucosa. The microdialysate was analysed for histamine and pancreastatin (ECL-cell markers) and for somatostatin (D-cell marker).

KEY RESULTS

Microinfusion of PACAP (0.01-0.3 nmol microl(-1)) raised microdialysate histamine and pancreastatin dose-dependently. The response was powerful but short-lived. The response to gastrin was sustained at all doses tested. It is unlikely that the transient nature of the histamine response to PACAP reflects inadequate histamine synthesis, since the pancreastatin response to PACAP was short-lived too, and both gastrin and PACAP activated ECL-cell histidine decarboxylase. Unlike gastrin, PACAP mobilized somatostatin. Co-infusion of somatostatin abolished the histamine-mobilizing effect of PACAP. However, pretreatment with the somatostatin receptor type-2 antagonist (PRL-2903) did not prolong the histamine response to PACAP, suggesting that mobilization of somatostatin does not explain the transient nature of the response. Repeated administration of 0.1 nmol microl(-1) of PACAP (1 h infusions, 1 h intervals) failed to induce a second histamine response. Pretreatment with a low dose of PACAP (0.03 nmol microl(-1)) abolished the response to a subsequent near-maximal PACAP challenge (0.3 nmol microl(-1)).

CONCLUSION

The transient nature of the histamine response to PACAP reflects desensitization of the PACAP receptor and/or exhaustion of a specific storage compartment that responds to PACAP but not to gastrin.

Role of calcitonin receptor-like receptor in colonic motility and inflammation

Calcitonin gene-related peptide (CGRP) mediates neurogenic inflammation and modulates intestinal motility. The CGRP receptor is a heterodimer of calcitonin receptor-like receptor (CLR) and receptor-associated modifying protein 1. We used RNA interference to elucidate the specific role of CLR in colonic motility and inflammation. Intramural injection of double-stranded RNA (dsRNA) against CLR (dsCLR) into the colonic wall at two sites caused the spatial and temporal downregulation of CLR in the colon within 1 day of dsRNA injection. Knockdown of CLR persisted for 7-9 days, and the effect of knockdown spread to approximately 2 cm proximal and distal to the injection sites, whereas control dsRNA injection did not affect CLR expression. Measurement of isometric contractions of isolated colonic muscle segments revealed that in control dsRNA-injected rats, CGRP abrogated contractions entirely and decreased resting muscular tone, whereas in dsCLR-injected rats, CGRP decreased muscle tone but slow-wave contractions of varying amplitude persisted. In trinitrobenzene sulfonic acid-induced colitis, rats with knockdown of CLR displayed a significantly greater degree of edema and necrosis than saline- or control dsRNA-injected rats. Levels of the proinflammatory cytokines TNF-alpha and IL-6 were markedly upregulated by trinitrobenzene sulfonic acid treatment. TNF-alpha mRNA levels were further increased in CLR knockdown rats, whereas levels of IL-6 were unaltered. Thus this study demonstrates that CLR is a functional receptor for CGRP.

Innervation of the gastrointestinal tract: patterns of aging

The gastrointestinal (GI) tract is innervated by intrinsic enteric neurons and by extrinsic projections, including sympathetic and parasympathetic efferents as well as visceral afferents, all of which are compromised by age to different degrees. In the present review, we summarize and illustrate key structural changes in the aging innervation of the gut, and suggest a provisional list of the general patterns of aging of the GI innervation. For example, age-related neuronal losses occur in both the myenteric plexus and submucosal plexus of the intestines. These losses start in adulthood, increase over the rest of the life span, and are specific to cholinergic neurons. Parallel losses of enteric glia also occur. The extent of neuronal and glial loss varies along an oral-to-anal gradient, with the more distal GI tract being more severely affected. Additionally, with aging, dystrophic axonal swellings and markedly dilated varicosities progressively accumulate in the sympathetic, vagal, dorsal root, and enteric nitrergic innervation of the gut. These dramatic and consistent patterns of neuropathy that characterize the aging autonomic nervous system of the GI tract are candidate mechanisms for some of the age-related declines in function evidenced in the elderly.

Gastrocardiac afferent convergence in upper thoracic spinal neurons: a central mechanism of postprandial angina pectoris

The aim of this study was to examine whether gastric afferent information converged onto upper thoracic spinal neurons that received noxious cardiac input. Extracellular potentials of single upper thoracic (T3) spinal neurons were recorded in pentobarbital-anesthetized, paralyzed, ventilated male rats. Gastric distension (GD) (20, 40, 60 mm Hg, 20 seconds) was produced by air inflation of a latex balloon surgically placed in the stomach. A catheter was placed in the pericardial sac to administer bradykinin solution (10 microg/mL, 0.2 mL, 1 minute) as a noxious cardiac stimulus. Noxious GD (> or =40 mm Hg) altered activity of 26 of 31 (84%) spinal neurons receiving cardiac input. Twenty-two (85%) gastrocardiac convergent neurons were excited, and 1 neuron was inhibited by both intrapericardial bradykinin and GD; the remainder exhibited biphasic response patterns. Twenty-three of 26 (88%) gastrocardiac neurons also received convergent somatic input from the chest, triceps, and upper back areas. Bilateral cervical vagotomy did not significantly affect excitatory responses to GD in 5 of 5 neurons tested. Spinal transection at the C1 segment after vagotomy did not affect excitatory responses to GD in 3 of 4 neurons but abolished the GD response in 1 neuron. These data showed that a gastric stimulus excited T3 spinal neurons with noxious cardiac input primarily by way of intraspinal ascending pathways.

PERSPECTIVE

Convergence of gastric afferent input onto upper thoracic spinal neurons receiving noxious cardiac input that was observed in the present study may provide a spinal mechanism that explains stomach-heart cross-organ communication, such as postprandial triggering and worsening of angina pectoris in patients with coronary artery disease.

Characterization of T9-T10 spinal neurons with duodenal input and modulation by gastric electrical stimulation in rats

Gastric electrical stimulation (GES) has been suggested as a therapy for patients with gastric motility disorders or morbid obesity. However, it is unclear whether GES also affects intestinal sensory and motor functions. Furthermore, little is known about intraspinal visceroreceptive transmission and processing for duodenal afferent information. The aims of this study were to characterize responses of thoracic spinal neurons to duodenal distension, to determine the afferent pathway and to examine the effects of GES on activity of these neurons. Extracellular potentials of single T9-T10 spinal neurons were recorded in pentobarbital anesthetized, paralyzed, ventilated male rats (n=19). Graded duodenal distension (DD, 0.2-0.6 ml, 20 s) was produced by water inflation of a latex balloon surgically placed into the duodenum. One pair of platinum electrodes (1.0-1.5 cm apart) was sutured onto the serosal surface of the lesser curvature of the stomach. GES with four sets of parameters was applied for one minute: GES-A (6 mA, 0.3 ms, 40 Hz, 2 s on, 3 s off), GES-B (6 mA, 0.3 ms, 14 Hz, 0.1 s on, 5 s off), GES-C (6 mA, 3 ms, 40 Hz, 2 s on, 3 s off) and GES-D (6 mA, 200 ms, 12 pulses/min). Results showed that 33/117 (28%) spinal neurons responded to noxious DD (0.4 ml, 20 s). Of these, 7 (6%) neurons had low-threshold responses to DD (<or=0.2 ml) and 26 (22%) had high-threshold responses to DD (>or=0.4 ml). DD-responsive spinal neurons were encountered more frequently in deeper (depth: 0.3-1.2 mm) than in superficial laminae (depth: <0.3 mm) of the dorsal horn (24/67 vs. 9/50, P<0.05). DD excited all 9 superficial neurons. In contrast, 20 deeper neurons were excited and 4 neurons were inhibited by DD. Activity of DD-responsive neurons was affected more frequently with GES-C (13/15, 87%) than GES-A (6/16, 38%), -B (3/15, 20%) and -D (5/14, 36%) (P<0.01). Bilateral cervical vagotomy did not significantly alter the effects of DD and GES on 5/5 neurons. Resiniferatoxin (2.0 microg/kg, i.v.), an ultrapotent agonist of transient receptor potential vanilloid receptor-1 (TRPV1), abolished DD responses and GES effects on all neurons examined in vagotomized rats. Additionally, 29/33 (88%) DD-responsive neurons received inputs from somatic receptive fields on the back, flank and medial/lateral abdominal areas. It was concluded that GES mainly exerted an excitatory effect on T9-T10 spinal neurons with duodenal input transmitted by sympathetic afferent fibers expressing TRPV1; spinal neuronal responses to GES were strengthened with an increased pulse width and/or frequency of stimulation; T9-T10 spinal neurons processed input from the duodenum and might mediate effects of GES on duodenal sensation and motility.

Modulatory effects and afferent pathways of gastric electrical stimulation on rat thoracic spinal neurons receiving input from the stomach

Gastric electrical stimulation (GES) has been suggested as a potential therapy for patients with obesity or gastric motility disorders. The aim of this study was to investigate the spinal mechanism of GES effects on gastric functions. Extracellular potentials of single spinal (T9-T10) neurons were recorded in pentobarbital anesthetized, paralyzed, ventilated male rats (n=19). Gastric distension (GD) was produced by air inflation of a balloon. One pair of platinum electrodes (1.0-1.5cm apart) was sutured onto the serosal surface of the lesser curvature of the stomach. GES with four sets of parameters was applied for 1min: GES-A (6mA, 0.3ms, 40Hz, 2s on, 3s off), GES-B (6mA, 0.3ms, 14Hz, 0.1s on, 5s off), GES-C (6mA, 3ms, 40Hz, 2s on, 3s off), GES-D (6mA, 200ms, 12pulses/min). 62/158 (39%) spinal neurons responded to GD (20, 40, 60mmHg, 20s. Most GD-responsive neurons (n=43) had excitatory responses; the remainder had inhibitory (n=12) or biphasic responses (n=7). GES-A, -B, -C and -D affected activity of 12/33 (36%), 4/31 (13%), 22/29 (76%) and 13/30 (43%) GD-responsive neurons, respectively. Bilateral cervical vagotomy did not significantly alter mean excitatory neuronal responses to GD (n=5) or GES (n=6). Resiniferatoxin (2.0microg/kg, i.v.), an ultrapotent agonist of vanilloid receptor-1, abolished excitatory responses to GD and GES in 4/4 neurons recorded in vagotomized rats. The results suggested that GES mainly had an excitatory effect on T9-T10 spinal neurons with gastric inputs; neuronal responses to GES were strengthened with stimulation at an increased pulse width and/or number of pulses. The modulatory effect of GES involved thoracic spinal (sympathetic) afferent fibers containing vanilloid receptor-1.

Adenosine 5'-triphosphate and its relationship with other mediators that activate pelvic nerve afferent neurons in the rat colorectum

Evidence of a role for purinergic signalling in visceral afferents involving P2X₂, P2X₃ and P2Y₁ receptors exists, which appears to be important during inflammation. This study aimed to evaluate the degree of interaction between adenosine 5′-triphosphate (ATP) and other mediators that activate sensory nerves in the colorectum. Recordings from pelvic nerve afferents were made during application of agents to the in-vitro colorectal preparation. Analysis allowed calculation of single unit activity. When applied individually, bradykinin (78%) and 5-hydoxytryptamine (77%) activated the greatest number of neurons, followed by substance P, protons, ATP and capsaicin. Prostaglandin E₂ stimulated the least number (54%) and had a longer latency. Seventy-seven percent of all units studied either responded to both ATP and capsaicin or to neither, giving the greatest degree of activity correlation. Five percent of units were activated by all seven agents and no units were activated by a single agent alone. 5-hydroxytryptamine, capsaicin and protons, when co-applied with ATP, increased pelvic nerve activity to a greater degree than the sum of the individual responses. It is concluded that ATP activates pelvic nerve afferents and acts synergistically with protons, capsaicin and 5-hydroxytryptamine. The pattern of neuronal activation suggests that visceral afferents are polymodal but the receptor expression on their terminals is variable.

Nitric oxide synthase activity in rat gastric mucosa contributes to mucin synthesis elicited by calcitonin gene-related peptide

The majority of research for the calcitonin gene-related peptide (CGRP) in the stomach has been devoted to the submucosal blood flow, and only slight attention has been paid to its involvement in the gastric epithelial function. In this study, we examined the age-related change in the CGRP-containing nerves and its effects on the mucus metabolism. We compared the immunoreactivity for CGRP in the gastric mucosa of 7-week-old rats (young) to that of 52-week-old animals (middle-aged). The effects of CGRP on the mucin biosynthesis were compared using the stomachs from both young and middle-aged rats. The nitric oxide synthase (NOS) activity was measured in the surface and deep mucosa of the gastric corpus. The density of the CGRP nerve fibers was reduced in both the lamina propria and submucosa of the middle-aged rats compared to the young rats. CGRP stimulated the mucin biosynthesis in the cultured corpus mucosa from the 7-week-old rats, but not from the 52-week-old rats. The total NOS activity of the surface layer in the corpus mucosa was markedly reduced in the middle-aged rats compared to the young rats. These findings demonstrate the age-dependent reduction in the CGRP-induced mucin biosynthesis, as well as in the density of the CGRP fibers in the rat stomach. The decreased NOS activity in the surface layer of the oxyntic mucosa in the aged rats may also be a principal cause for the lack of regulation of the mucin biosynthesis by CGRP.

Co-localization of TRPV1-expressing nerve fibers with calcitonin-gene-related peptide and substance P in fundus of rat stomach

The localization of vanilloid receptor TRPV1 was studied in rat gastric fundus by an immunohistochemical technique. Numerous TRPV1-immunoreactive nerve fibers were found around arterioles in the submucosal layer. A large number of the nerve fibers were also seen in the smooth muscle layer and in the myenteric nerve plexus, but the cell bodies could not be found. TRPV1 nerve fibers within the circular muscle layers were running parallel to the muscle fibers. Virtually all TRPV1 axons were immunoreactive for calcitonin-gene-related peptide (CGRP), with particularly extensive double labeling seen in axons of the submucosa around blood vessels. TRPV1 nerve fibers containing substance P were found running in longitudinal muscle and circular muscle. The TRPV1 axons seem to be predominantly extrinsic and contain CGRP and substance P in gastric fundus. TRPV1 neurons are thought to be sensory afferent neurons that operate to maintain gastric motility and blood flow.