Neurochemistry of visceral afferent neurones

Regulation of Carcinogenesis by Sensory Neurons and Neuromediators

Interactions between the immune system and the nervous system are crucial in maintaining homeostasis, and disturbances of these neuro-immune interactions may participate in carcinogenesis and metastasis. Nerve endings have been identified within solid tumors in humans and experimental animals. Although the involvement of the efferent sympathetic and parasympathetic innervation in carcinogenesis has been extensively investigated, the role of the afferent sensory neurons and the neuropeptides in tumor development, growth, and progression is recently appreciated. Similarly, current findings point to the significant role of Schwann cells as part of neuro-immune interactions. Hence, in this review, we mainly focus on local and systemic effects of sensory nerve activity as well as Schwann cells in carcinogenesis and metastasis. Specific denervation of vagal sensory nerve fibers, or vagotomy, in animal models, has been reported to markedly increase lung metastases of breast carcinoma as well as pancreatic and gastric tumor growth, with the formation of liver metastases demonstrating the protective role of vagal sensory fibers against cancer. Clinical studies have revealed that patients with gastric ulcers who have undergone a vagotomy have a greater risk of stomach, colorectal, biliary tract, and lung cancers. Protective effects of vagal activity have also been documented by epidemiological studies demonstrating that high vagal activity predicts longer survival rates in patients with colon, non-small cell lung, prostate, and breast cancers. However, several studies have reported that inhibition of sensory neuronal activity reduces the development of solid tumors, including prostate, gastric, pancreatic, head and neck, cervical, ovarian, and skin cancers. These contradictory findings are likely to be due to the post-nerve injury-induced activation of systemic sensory fibers, the level of aggressiveness of the tumor model used, and the local heterogeneity of sensory fibers. As the aggressiveness of the tumor model and the level of the inflammatory response increase, the protective role of sensory nerve fibers is apparent and might be mostly due to systemic alterations in the neuro-immune response. Hence, more insights into inductive and permissive mechanisms, such as systemic, cellular neuro-immunological mechanisms of carcinogenesis and metastasis formation, are needed to understand the role of sensory neurons in tumor growth and spread.

GHSR-1 agonist sensitizes rat colonic intrinsic and extrinsic neurons to exendin-4: A role in the manifestation of postprandial gastrointestinal symptoms in irritable bowel syndrome?

BACKGROUND

Patients with irritable bowel syndrome (IBS) may experience postprandial symptom exacerbation. Nutrients stimulate intestinal release of glucagon-like peptide 1 (GLP-1), an incretin hormone with known gastrointestinal effects. However, prior to the postprandial rise in GLP-1, levels of the hunger hormone, ghrelin, peak. The aims of this study were to determine if ghrelin sensitizes colonic intrinsic and extrinsic neurons to the stimulatory actions of a GLP-1 receptor agonist, and if this differs in a rat model of IBS.

METHODS

Calcium imaging of enteric neurons was compared between Sprague Dawley and Wistar Kyoto rats. Colonic contractile activity and vagal nerve recordings were also compared between strains.

KEY RESULTS

Circulating GLP-1 concentrations differ between IBS subtypes. Mechanistically, we have provided evidence that calcium responses evoked by exendin-4, a GLP-1 receptor agonist, are potentiated by a ghrelin receptor (GHSR-1) agonist, in both submucosal and myenteric neurons. Although basal patterns of colonic contractility varied between Sprague Dawley and Wister Kyoto rats, the capacity of exendin-4 to alter smooth muscle function was modified by a GHSR-1 agonist in both strains. Gut-brain signaling via GLP-1-mediated activation of vagal afferents was also potentiated by the GHSR-1 agonist.

CONCLUSIONS & INFERENCES

These findings support a temporal interaction between ghrelin and GLP-1, where the preprandial peak in ghrelin may temporarily sensitize colonic intrinsic and extrinsic neurons to the neurostimulatory actions of GLP-1. While the sensitizing effects of the GHSR-1 agonist were identified in both rat strains, in the rat model of IBS, underlying contractile activity was aberrant.

Distinct subclassification of DRG neurons innervating the distal colon and glans penis/distal urethra based on the electrophysiological current signature

Spinal sensory neurons innervating visceral and mucocutaneous tissues have unique microanatomic distribution, peripheral modality, and physiological, pharmacological, and biophysical characteristics compared with those neurons that innervate muscle and cutaneous tissues. In previous patch-clamp electrophysiological studies, we have demonstrated that small- and medium-diameter dorsal root ganglion (DRG) neurons can be subclassified on the basis of their patterns of voltage-activated currents (VAC). These VAC-based subclasses were highly consistent in their action potential characteristics, responses to algesic compounds, immunocytochemical expression patterns, and responses to thermal stimuli. For this study, we examined the VAC of neurons retrogradely traced from the distal colon and the glans penis/distal urethra in the adult male rat. The afferent population from the distal colon contained at least two previously characterized cell types observed in somatic tissues (types 5 and 8), as well as four novel cell types (types 15, 16, 17, and 18). In the glans penis/distal urethra, two previously described cell types (types 6 and 8) and three novel cell types (types 7, 14, and 15) were identified. Other characteristics, including action potential profiles, responses to algesic compounds (acetylcholine, capsaicin, ATP, and pH 5.0 solution), and neurochemistry (expression of substance P, CGRP, neurofilament, TRPV1, TRPV2, and isolectin B4 binding) were consistent for each VAC-defined subgroup. With identification of distinct DRG cell types that innervate the distal colon and glans penis/distal urethra, future in vitro studies related to the gastrointestinal and urogenital sensory function in normal as well as abnormal/pathological conditions may be benefitted.

The neurochemical changes in the innervation of human colonic mesenteric and submucosal blood vessels in ulcerative colitis and Crohn's disease

BACKGROUND

Neurogenic inflammation involves vasodilation, oedema and sensory nerve hypersensitivity. Extrinsic sensory nerves to the intestinal wall mediate these effects and functional subsets of these extrinsic nerves can be characterized by immunohistochemical profiles. In this study such profiles were examined in samples from patients with inflammatory bowel disease (IBD), in particular ulcerative colitis (UC) and Crohn's disease (CD).

METHODS

Healthy margins from cancer patients were compared to specimens from IBD patients. All nerve fibres were labelled by PGP 9.5. Double and triple labelling with TH, NPY, SP, SOM, NOS, VIP, VAChT, CGRP, TRPv1 were performed. Perivascular nerve fibres in the mesentery, and submucosa, were examined. The percentage of all labelled nerve fibres was calculated with a transect method.

KEY RESULTS

Total number of varicosities on mesenteric vessels increased in IBD but decreased around submucosal vessels. The percentage of nerve fibres around submucosal arteries labelled by SP increased from 11% in controls to 20% (UC) and 24% (CD) and mesenteric artery nerve fibres were unchanged. Nerve fibres labelled by SOM were markedly reduced surrounding submucosal arteries, from 22% to 1% (UC) and 2% (CD), but not perivascular mesenteric nerve fibres. 87 to 93% of SP immunoreactive nerve fibres were also reactive for TRvP1. TRPv1 labelling without SP was 12%in controls and increased to 40% in CD submucosal specimens.

CONCLUSIONS & INFERENCES

There is an increase in SP and TRPv1, and a reduction in SOM immunoreactive nerve fibres in IBD. Changes in the perivascular functional nerve subclasses may underlie the hyperaemia, and ulceration, characteristic of IBD. Furthermore, pain may relate to underlying neural changes.

Autonomic control of gut motility: a comparative view

Gut motility is regulated to optimize food transport and processing. The autonomic innervation of the gut generally includes extrinsic cranial and spinal autonomic nerves. It also comprises the nerves contained entirely within the gut wall, i.e. the enteric nervous system. The extrinsic and enteric nervous control follows a similar pattern throughout the vertebrate groups. However, differences are common and may occur between groups and families as well as between closely related species. In this review, we give an overview of the distribution and effects of common neurotransmitters in the vertebrate gut. While the focus is on birds, reptiles, amphibians and fish, mammalian data are included to form the background for comparisons. While some transmitters, like acetylcholine and nitric oxide, show similar distribution patterns and effects in most species investigated, the role of others is more varying. The significance for these differences is not yet fully understood, emphasizing the need for continued comparative studies of autonomic control.

Gastrointestinal mechanisms of satiation for food

Satiation for food comprises the physiological processes that result in the termination of eating. Satiation is evoked by physical and chemical qualities of ingested food, which trigger afferent signals to the brain from multiple sites in the GI tract, including the stomach, the proximal small intestine, the distal small intestine and the colon. The physiological nature of each signal's contribution to satiation and overall control of food intake is likely to vary, depending on the level of the GI tract from which the signal arises. This article is a critical, though non-exhaustive, review of our current understanding of the mechanisms and adaptive value of satiation signals from the stomach and intestine.

Capsaicin-sensitive extrinsic afferents are involved in acid-induced activation of distinct myenteric neurons in the rat stomach

Challenge of the rat gastric mucosa with 0.5 mol L(-1) HCl activates nitrergic neurons in the myenteric plexus as visualized by c-Fos immunohistochemistry. In the present study, we characterized the activated neurons more extensively by their chemical coding and investigated whether a neural pathway that involves capsaicin-sensitive extrinsic afferents and/or cholinergic neurons transmitting via nicotinic receptors contributes to the activation of myenteric neurons. In multiple labelling experiments, c-Fos was examined for co-localization with nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), enkephalin (ENK), gastrin-releasing peptide (GRP), substance P (SP), calbindin D-28k (CALB) and neurofilament 145 (NF 145). All c-Fos-positive neurons were immunoreactive for NOS, VIP, NPY and NF 145, but not for SP, ENK, GRP and CALB. Nerve fibres co-expressing NOS, VIP and NPY were predominantly found in the external muscle layer and in the muscularis mucosae but rarely in the mucosa. Pre-treatment with capsaicin or hexamethonium or a combination of both pre-treatments reduced HCl-induced c-Fos expression by 54, 66 and 63%, respectively. Acid challenge of the stomach, therefore, leads to activation of presumably inhibitory motor neurons responsible for muscle relaxation. Activation of these neurons is partly mediated by capsaicin-sensitive afferents and involves ganglionic transmission via nicotinic receptors.

Consequences of intestinal inflammation on the enteric nervous system: neuronal activation induced by inflammatory mediators

The ENS is responsible for the regulation and control of all gastrointestinal functions. Because of this critical role, and probably as a consequence of its remarkable plasticity, the ENS is often relatively well preserved in conditions where the architecture of the intestine is seriously disrupted, such as in IBD. There are structural and functional changes in the enteric innervation in animal models of experimental intestinal inflammation and in IBD. These include both up and down regulation of transmitter expression and the induction of new genes in enteric neurons. Using Fos expression as a surrogate marker of neuronal activation it is now well established that enteric neurons (and also enteric glia) respond to inflammation. Whether this "activation" is limited to a short-term functional response, such as increased neuronal excitability, or reflects a long-term change in some aspect of the neuronal phenotype (or both) has yet to be firmly established, but it appears that enteric neurons are highly plastic in their response to inflammation.

Failure of capsaicin-containing red pepper sauce suspension to induce esophageal motility response in patients with Barrett's esophagus

The physiologic importance of afferent sensory pathways in the esophageal motor functions has been recently recognised. Capsaicin-sensitive sensory afferents were shown to play a role in the maintenance of mucosal integrity of the GI tract, and regulation of human esophageal motility. The aim of this study was to investigate the effect of topical application of capsaicin-containing red pepper sauce (Tabasco, 25%v/v, pH:7.0) suspension on the phasic activity of the human esophagus of healthy volunteers and patients with Barrett's esophagus.

METHODS

The diagnosis of Barrett's esophagus was based on the findings of esophagoscopy and histology taken from the squamocolumnar junction of the esophagus. Esophageal motility was measured by perfusion manometry before and after application of red pepper sauce.

RESULTS

Capsaicin containing red pepper sauce increases the motility response (LES tone, contraction amplitude, propagation velocity) of the human esophagus in healthy volunteers. This response failed in patients with Barrett's esophagus.

CONCLUSION

Impaired esophageal sensory motor function may serve as one etiologic role in the development of Barrett's esophagus.

Potential of substance P antagonists as antiemetics

The introduction of serotonin 5-HT3 receptor antagonists into clinical practice allowed for a dramatic improvement in the management of nausea and vomiting. Despite this, postoperative and chemotherapy-induced emesis remains a significant, unresolved issue in many patients even when a combination of antiemetic drugs is used. Numerous neurotransmitters have been implicated in triggering emesis; however, the tachykinin substance P, by virtue of its localisation within both the gastrointestinal vagal afferent nerve fibres and brainstem emetic circuitry, and its ability to induce vomiting when administered intravenously, is thought to play a key role in emetic responses. Because substance P is the most likely endogenous ligand for the neurokinin-1 (NK1) receptor, the development of nonpeptide NK1 receptor antagonists led scientists to evaluate these compounds as antiemetics. The five NK1 receptor inhibitors that have been studied initially in humans are: vofopitant (GR-205171), CP-122721, ezlopitant (CJ-11974), MK-869 (L-754030) and its prodrug L-758298. Except for monotherapy in acute cisplatin-induced emesis, this new class of drugs has proven to be highly effective in the control of both chemotherapy-induced nausea and vomiting, and postoperative nausea and vomiting. No major adverse event was reported in the preliminary trials. Further investigation is mandatory in order to assess the optimal treatment regimen and to make sure the wide spectrum activity of the NK1 receptor inhibitors does not cause significant adverse effects in the context of the treatment of nausea and vomiting.

Effects of alverine citrate on cat intestinal mechanoreceptor responses to chemical and mechanical stimuli

BACKGROUND

Alverine citrate is commonly used in the treatment of painful affections of the colon.

AIM

To determine whether alverine citrate acts on the vagal sensory endings.

METHODS

Unitary recordings were performed at the level of the vagal fibres in the nodose ganglion of anaesthetized cats using extracellular glass microelectrodes, and the patterns of response to chemical and mechanical stimuli applied to identified vagal intestinal mechanoreceptors were studied.

RESULTS

The intestinal mechanoreceptors located at the endings of type C vagal fibres responded mainly to mechanical stimuli (distension and contraction), but also responded to chemical substances (cholecystokinin and substance P). The most conspicuous effect of alverine (2 mg/kg) was that it significantly inhibited the pattern of vagal activity produced in response to either cholecystokinin (5-10 microg/kg), substance P (5-10 microg/kg) or phenylbiguanide (5-10 microg/kg), a 5-HT3 receptor agonist. On the other hand, the unitary vagal response to the mechanical distension was slightly enhanced by alverine, as was any spontaneous activity present.

CONCLUSIONS

Based on the present data, alverine citrate can be said to decrease the sensitivity of the intestinal mechanoreceptors, which is consistent with its previously established anti-spasmodic effects.

Intraintestinal capsaicin transiently reduces CGRP-like immunoreactivity in rat submucosal plexus

Intraintestinal infusion of the sensory neurotoxin, capsaicin, transiently abolishes behavioral responses to chemical stimulation of the intestine. This desensitizing action of capsaicin may be due to an action on CGRP-containing nerve terminals, which are postulated to serve a sensory function in the enteric plexuses. To determine whether intraintestinal capsaicin treatment alters CGRP-like immunoreactivity (CGRP-li) in the enteric plexuses, we performed immunohistochemical analyses of the small intestinal submucosal and myenteric plexuses of rats at various times after intestinal infusion of capsaicin (5 mg) or its vehicle. Intestinal capsaicin treatment, but not vehicle treatment, reduced CGRP-li, but not substance-P-like immunoreactivity (SP-li), in nerve fibers of the submucosal plexus. CGRP-li was reduced in submucosal interganglionic connectives and in nerve fibers associated with submucosal blood vessels. CGRP-li of submucosal connectives was reduced by 1 h post-infusion. Reduction of CGRP-li in the submucosal fibers also was pronounced 24 h after intraintestinal capsaicin treatment. By 48 h after intestinal capsaicin infusion, CGRP-li was not distinguishable from vehicle-treated animals. There were no consistent immunohistochemical changes in CGRP-li or SP-li in the myenteric plexus at any time. These results indicate that intestinal capsaicin selectively induces transient reduction of CGRP-li in nerve fibers of the submucosal plexus. The chronology of depletion and reappearance of CGRP-li is congruent with previously reported, transient impairment of sensory function observed following intestinal capsaicin infusion.

Capsaicin-resistant vagal afferent fibers in the rat gastrointestinal tract: anatomical identification and functional integrity

The presence and distribution of vagal fibers and terminals throughout esophagus and gastrointestinal tract that could be anterogradely labeled by nodose ganglion tracer injections was quantitatively assessed in capsaicin- and vehicle-pretreated adult rats, in order to identify the capsaicin-resistant population. Up to 90% of the intraganglionic laminar endings (IGLEs), in the myenteric plexus of the esophagus, and 70-90% in the stomach, as well as 57% of the intramuscular endings or arrays (IMAs) in the fundic stomach survived the capsaicin treatment, while in the upper small intestine only few and in the lower small intestine, the cecum and colon, virtually no IGLEs survived capsaicin treatment. Intramucosal terminals were not assessed. Furthermore, gastric balloon distension-induced c-Fos expression in the dorsal vagal complex was not significantly decreased in capsaicin-treated rats. It is concluded that among primary vagal afferents there is a capsaicin-resistant population that primarily innervates the esophagus and upper gastrointestinal tract, and a capsaicin-sensitive population that innervates mainly the lower tract. At least vagal gastric tension-sensitive afferents also seems to be functionally intact in that they may be capable of synaptically activating second-order neurons in the brainstem.

Neurophysiology of micturition and continence in women

Micturition and continence involve the coordination of complex neural events between the central and peripheral nervous systems. An understanding of these events provides a foundation for the treatment of voiding disorders in women such as stress urinary incontinence, urge incontinence and interstitial cystitis. The purpose of this paper is to comprehensively review the neuroanatomy, neurophysiology and neuropharmacology of micturition and continence. However, a brief section discussing clinical correlations will follow each of these topics to help integrate the basic science with clinical observations.

Tachykinin NK1 receptor antagonists act centrally to inhibit emesis induced by the chemotherapeutic agent cisplatin in ferrets

These studies have compared the pharmacological profile of two non-peptide human type neurokinin1 (hNK1) receptor selective antagonists, L-741,671 and a quaternised compound L-743,310. In radioligand binding studies L-741,671 and L-743,310 had high affinity for ferret and cloned hNK1 receptors [Ki (nM) ferret 0.7 and 0.1; human 0.03 and 0.06, respectively] but low affinity for rodent NK1 receptors [Ki (nM) 64 and 17, respectively] suggesting that ferret receptors have hNK1-like binding pharmacology. Studies in vivo showed that L-741,671 and L-743,310 had equivalent functional activity in the periphery (ID50s of 1.6 and 2 micrograms/kg i.v., respectively) as measured by inhibition of plasma protein extravasation evoked in the oesophagus of guinea pigs by resiniferatoxin (7 nmol/kg i.v.). Using an in situ brain perfusion technique in anaesthetised rats, L-741,671 was shown to be much more brain penetrant than the quaternary compound L-743,310 which had an entry rate similar to the poorly brain penetrant plasma marker inulin. These compounds thus provided an opportunity to compare the anti-emetic effects of equi-active hNK1 receptor antagonists with and without brain penetration to central NK1 receptor sites. When tested against cisplatin-induced emesis in ferrets, L-741,671 (0.3, 1 and 3 mg/kg i.v.) produced marked dose-dependent inhibition of retching and vomiting but L-743,310 was inactive at 3 and 10 micrograms/kg i.v. In contrast, direct central injection of L-741,671 and L-743,310 (30 micrograms) into the vicinity of the nucleus tractus solitarius or L-743,310 (200 micrograms) intracisternally was shown to inhibit retching and vomiting induced by i.v. cisplatin. L-741,671 and L-743,310 had equivalent functional activity, at the same dose, against cisplatin-induced emesis when injected centrally. These observations indicated that had L-743,310 penetrated into the brain after systemic administration it would have been active in the cisplatin-induced emesis assay and so show that brain penetration is essential for the anti-emetic action of systemically administered NK1 receptor antagonists.

Calcitonin gene-related peptide innervation of the rat hepatobiliary system

The digestive system is densely innervated by calcitonin gene-related peptide (CGRP)-immunoreactive neurons. The present study investigated a) the distribution and origin of CGRP-immunoreactive fibers in the rat hepatobiliary tract, and b) their relation with substance P/tachykinin (SP/TK) immunoreactivity using immunohistochemical and radioimmunoassay techniques. CGRP-containing fibers form dense networks in the fibromuscular layer of the biliary tree and surrounding the portal vein. Thin, varicose fibers are present at the base of the mucosa of the ducts. In the liver, labeled fibers are restricted to the portal areas and the stromal compartment. Neonatal treatment with capsaicin, a neurotoxin for primary afferent neurons, or celiac/superior mesenteric ganglionectomy depletes CGRP-containing fibers in the biliary tract, and reduces those associated with the portal vein. In contrast, subdiaphragmatic vagotomy does not appreciably modify the density of these fibers. Radioimmunoassay studies show a reduction of CGRP-immunoreactive contents in the biliary tract and portal vein by 84% and 65%, respectively, following capsaicin treatment, and by 80% and 66%, respectively, following ganglionectomy. By contrast, CGRP concentrations in vagotomized animals are comparable to those of controls. Most CGRP-positive fibers appear to contain SP/TK immunoreactivity, as indicated by double-label studies. These results demonstrate that the rat hepatobiliary tract is prominently innervated by CGRP- and CGRP/SP/TK-immunoreactive fibers, which are likely to originate from spinal afferent neurons. The abundance of these fibers and their association with a variety of targets are in line with the involvement of these peptidergic visceral afferents in regulating hepatobiliary activities, including hemodynamic functions of the hepatic vasculature.

Cellular sites of expression of the neurokinin-1 receptor in the rat gastrointestinal tract

In the digestive system, substance P is an excitatory transmitter to muscle, a putative excitatory neuro-neuronal transmitter, a vasodilator, and a mediator in inflammatory processes. Many of the biological effects of substance P are mediated by a high-affinity interaction with the tachykinin receptor neurokinin-1. The aim of the present study was to identify the sites of expression of this receptor in the rat stomach and intestine by immunohistochemistry with a polyclonal antiserum raised to the intracellular C-terminal portion of the rat neurokinin-1 receptor. Neurokinin-1 receptor immunoreactivity is present in a large population of enteric neurons. The relative density of these neurons along the gut is colon > ileum >> stomach. In the intestine, stained neurons have a smooth cell body with processes that can be followed within and between plexuses, and make close approaches to other neuronal cells, but do not appear to project outside the plexuses, suggesting that they are interneurons. In the stomach, neurokinin-1 receptor-immunoreactive neurons are infrequent and have a poorly defined and irregular shape. Neurokinin-1 receptor immunoreactivity is also localized to numerous non-neuronal cells in the inner portion of the circular muscle layer of the small intestine, which have the appearance of small dark smooth muscle cells or interstitial cells of Cajal. These cells are postulated to form a "stretch-sensitive" system with the deep muscular plexus and thus constitute an important site of regulation of muscle activity. Double labeling immunofluorescence was used to simultaneously localize neurokinin-1 receptor and substance P/tachykinin immunoreactivities. These experiments demonstrate that in the enteric plexuses, substance P/tachykinin-immunoreactive varicose fibers encircle the cell bodies of most neurokinin-1 receptor-containing neurons, and in the inner portion of the circular muscle layer of the small intestine they lie close to neurokinin-1 receptor-immunoreactive non-neuronal cells. In addition, some enteric neurons express both neurokinin-1 receptor and substance P/tachykinin immunoreactivities. The present study provides strong evidence that the neurokinin-1 receptor is the tachykinin receptor mediating the actions of substance P on enteric neurons and smooth muscle.

The anti-emetic effects of CP-99,994 in the ferret and the dog: role of the NK1 receptor

1. The selective NK1 receptor antagonist, CP-99,994, produced dose-related (0.1-1.0 mg kg-1, s.c.) inhibition of vomiting and retching in ferrets challenged with central (loperamide and apomorphine), peripheral (CuSO4) and mixed central and peripheral (ipecac, cisplatin) emetic stimuli. 2. Parallel studies with the enantiomer, CP-100,263 (1 mg kg-1, s.c.), which is > 1,000 fold less potent as a NK1 antagonist, indicated that it was without significant effect against CuSO4, loperamide, cisplatin and apomorphine-induced emesis. Against ipecac, it inhibited both retching and vomiting, expressing approximately 1/10th the potency of CP-99,994. 3. The 5-HT3 receptor antagonist, tropisetron (1 mg kg-1, s.c.) inhibited retching and vomiting to cisplatin and ipecac, but not CuSO4 or loperamide. 4. CP-99,994 (1 mg kg-1, i.v.) blocked retching induced by electrical stimulation of the ventral abdominal vagus without affecting the cardiovascular response, the apnoeic response to central vagal stimulation or the guarding and hypertensive response to stimulation of the greater splanchnic nerves. CP-99,994 (1 mg kg-1, i.v.) did not alter baseline cardiovascular and respiratory parameters and it failed to block the characteristic heart rate, blood pressure and respiratory rate/depth changes in response to i.v. 2-methyl-5-HT challenge (von Bezold-Jarisch reflex). 5. Using in vitro autoradiography, [3H]-substance P was shown to bind to several regions of the ferret brainstem with the density of binding in the nucleus tractus solitarius being much greater than in the area postrema. This binding was displaced by CP-99,994 in a concentration-related manner. 6. In dogs, CP-99,994 (40 micrograms kg-1 bolus and 300 micrograms kg-1 h-1, i.v.) produced statistically significant reductions in vomiting to CuSO4 and apomorphine as well as retching to CuSO4. 7. Together, these studies support the hypothesis that the NK1 receptor antagonist properties of CP-99,994 are responsible for its broad spectrum anti-emetic effects. They also suggest that CP-99,994 acts within the brainstem, most probably within the nucleus tractus solitarius although the involvement of the area postrema could not be excluded.

Visceral pain in infants

Visceral pain in infants represents a complexity of interacting neural, developmental, psychosocial, and environmental factors, which must be separately and conjointly evaluated. Inhibitory mechanisms are not fully developed in infants and thus nociception is not readily dampened. Heightened behavioral responses to pain (e.g., crying) are likewise not easily inhibited. Esophageal pain and behaviors perceived by the caregiver to represent pain (e.g., crying and retching) can potentially affect normal growth and development. The response of the infant to pain and other visceral sensory stimuli and the ability to cope with these sensations (painful and nonpainful) are shaped by the relationship of the infant with the primary caregiver, usually the mother. Neural mechanisms of pain transmission and inhibition are reviewed, as well as biopsychosocial and environmental characteristics that can shape or contribute to infant pain syndromes. Proposed multifaceted clinical treatment strategies are aimed at decreasing efforts to dampen excitatory neural sensory signaling and improving the mother/infant relationship and maternal behavioral response to the crying infant.

Central distribution of substance P, calcitonin gene-related peptide and 5-hydroxytryptamine in vagal sensory afferents in the rat dorsal medulla

The central distribution of vagal afferents in the medulla containing either substance P, calcitonin gene-related peptide or 5-hydroxytryptamine was examined using a double-labelling technique and laser scanning confocal microscopy. Areas of the nucleus tractus solitarii, dorsal motonucleus of the vagus nerve and area postrema were scanned for double-labelled axon profiles. Analysis of this material revealed that all three neurochemicals were contained within the central terminals of vagal nerve sensory neurons. However, the distribution of vagal nerve afferents containing each of these putative transmitters differed. Afferents containing 5-hydroxytryptamine were detected mainly in the areas postrema and the adjacent nucleus tractus solitarii, with a smaller number in the ventral subnuclei of the solitary tract. In contrast afferents containing calcitonin gene-related peptide were found primarily in the medial and commissural regions of the nucleus tractus solitarii. Afferents containing substance P-immunoreactivity were surprisingly few in number and did not appear to be associated with any particular region. These results establish the presence of 5-hydroxytryptamine, substance P and calcitonin gene-related peptide in the central axons of vagal sensory afferents. Furthermore, the differential distribution of afferents immunoreactive for these neurochemicals seen in this study, together with previous demonstrations of the viscerotopic organization of vagal sensory afferents suggests a possible "chemical coding" for individual end organs.

Sympathetic activation of cat spinal neurons responsive to noxious stimulation of deep tissues in the low back

Prior findings from diverse studies have indicated that activity in axons located in the lumbar sympathetic chains contributes to the activation of spinal pain pathways and to low back pain; these studies have utilized sympathetic blocks in patients, electrical stimulation of the chain in conscious humans, and neuroanatomical mapping of afferent fiber projections. In the present study, dorsal horn neurons receiving nociceptor input from lumbar paraspinal tissues were tested for activation by electrical stimulation of the lumbar sympathetic chain in anesthetized cats. Of 83 neurons tested, 70% were responsive to sympathetic trunk stimulation. Excitatory responses, observed in both nociceptive specific and wide-dynamic-range neurons, were differentiable into two classes: non-entrained and entrained responses. Non-entrained responses were attenuated or blocked by systemic administration of the alpha-adrenergic antagonist phentolamine and are thought to result from sympathetic efferent activation of primary afferents in the units' receptive fields. Entrained responses were unaffected by phentolamine and are thought to result from electrical activation of somatic and/or visceral afferent fibers ascending through the sympathetic trunk into the dorsal horn. These findings from nocireceptive neurons serving lumbar paraspinal tissues suggest that low back pain may be exacerbated by activity in both efferent and afferent fibers located in the lumbar sympathetic chain, the efferent actions being mediated indirectly through sympathetic-sensory interactions in somatic and/or visceral tissues.

Silent ischemia: a hypothetical mechanism

Ischemia of visceral organs, especially the heart, is often a painful and potentially life-threatening condition. However, in at least 75% of all cases myocardial ischemia may be "silent" (i.e., without pain or sensation). Yet, the mechanisms responsible for silent ischemia are not well understood. As such, many different theories have been advanced to explain silent ischemia; however, none have been able to adequately explain all of the experimental and clinical findings. This paper proposes a hypothetical mechanism that may help to understand mechanisms of silent ischemia.

Transient expression of somatostatin peptide is a widespread feature of developing sensory and sympathetic neurons in the embryonic rat

Previous studies from this and other laboratories demonstrated that many embryonic sensory ganglion cells in the rat transiently express the catecholamine synthesizing enzyme tyrosine hydroxylase (TH), a trait not expressed by most mature sensory neurons. We, therefore, sought to determine whether transient expression was uniquely associated with catecholaminergic traits, or, alternatively, whether embryonic ganglion cells transiently expressed peptidergic properties as well. Of the four peptides examined (somatostatin [somatotropin release inhibiting factor] (SRIF), galanin (Gal), calcitonin gene-related peptide (CGRP), and substance P (SP)), only SRIF was found to be transiently expressed during early stages of sensory gangliogenesis. Surprisingly, SRIF immunoreactivity was observed in virtually all cranial and spinal sensory ganglion cells on embryonic day (E) 12.5. In addition to perikaryal labeling, intense SRIF immunoreactivity was also observed in the central and peripheral processes of E12.5 sensory neurons, suggesting the peptide may be released from nerve endings. The time course of SRIF appearance in cranial ganglion cells paralleled that previously described for TH, and double-labeling studies revealed extensive co-localization of these two phenotypes. By E16.5, however, the number of neurons expressing SRIF had diminished markedly, indicating that SRIF is only transiently expressed by most sensory neurons during early stages of ganglion development. An unexpected finding was that transient expression of SRIF is also a prominent feature of sympathetic ganglion cells; however, the temporal pattern of staining in the sympathetic and sensory ganglia differed substantially. Whereas virtually no SRIF staining was observed in E12.5 sympathetics, the vast majority of cells in the E16.5 superior cervical ganglion (SCG) were labeled. This contrasted sharply with the adult SCG, in which only low levels of SRIF expression were found. These findings demonstrate that SRIF peptide is transiently expressed at high levels in peripheral sensory and sympathetic neurons during embryogenesis. The time course and widespread distribution of SRIF expression indicates that the peptide may play a role in early stages of ganglion cell growth and development. Moreover, these data, in conjunction with previous studies demonstrating SRIF immunoreactivity in developing central neurons, suggest that transient expression of this peptide is a common property of diverse neuronal cell types.

Vagal afferent innervation of the rat fundic stomach: morphological characterization of the gastric tension receptor

Although the gastric tension receptor has been characterized behaviorally and electrophysiologically quite well, its location and structure remains elusive. Therefore, the vagal afferents to the rat fundus (forestomach or nonglandular stomach) were anterogradely labeled in vivo with injections of the carbocyanine dye Dil into the nodose ganglia, and the nerves and ganglia of the enteric nervous system were labeled in toto with intraperitoneal Fluorogold injection. Dissected layers and cryostat cross sections of the fundic wall were mounted in glycerin and analyzed by means of conventional and laser scanning confocal microscopy. Particularly in the longitudinal, and to a lesser extent in the circular, smooth muscle layers, Dil-labeled fibers and terminals were abundant. These processes, which originated from fibers coursing through the myenteric ganglia and connectives, entered either muscle coat and then ran parallel to the respective muscle fibers, often for several millimeters. They ran in close association with the Fluorogold-labeled network of interstitial cells of Cajal, upon which they appeared to form multiple spiny appositions or varicosities. In the myenteric plexus, two different types of afferent vagal structures were observed. Up to 300 highly arborizing endings forming dense accumulations of small puncta similar to the esophageal intraganglionic laminar endings (Rodrigo et al., '75 Acta Anat. 92:79-100) were found in the fundic wall ipsilateral to the injected nodose ganglion. They often covered small clusters of myenteric neurons or even single isolated ganglion cells (mean = 5.8 neurons) and tended to extend throughout the neuropil of the ganglia. In a second pattern, fine varicose fibers with less profuse arborizations innervated mainly the central regions of myenteric ganglia. Camera lucida analyses established that single vagal afferent fibers had separate collaterals in both a smooth muscle layer and the myenteric ganglia. Finally, Dil-labeled afferent vagal fibers were also found in the submucosa and mucosa. Control experiments in rats with supranodose vagotomy as well as rats with Dil injections directly in the distal cervical vagus ruled out the possibility of colabeling of afferent fibers of passage. In triple labeling experiments, in conjunction with Dil labeling of afferents and Fluorogold labeling of enteric neurons, the carbocyanine dye DiA was injected into the dorsal motor nucleus of the vagus to anterogradely label the efferent vagal fibers and terminals. The different distributions and morphological characteristics of the vagal afferents and efferents could be simultaneously compared. In some instances the same myenteric ganglion was apparently innervated by an afferent laminar ending and an efferent terminal.(ABSTRACT TRUNCATED AT 400 WORDS)

Capsaicin-sensitive vagal stimulation-induced gastric acid secretion in the rat: evidence for cholinergic vagal afferents

1. The effects of electrical vagal stimulation on frequency-dependent gastric acid secretion were investigated in urethane-anaesthetized rats in vivo. 2. Stimulation at 4, 16 or 32 Hz was performed in rats treated with atropine (1 mg kg-1, i.v.), hexamethonium (10 mg kg-1, i.v. bolus and 1 mg kg-1 min-1, i.v. infusion) or atropine and hexamethonium (doses as above); in some experiments pentagastrin (1.2 micrograms kg-1 h-1, i.v. infusion) was infused prior to stimulation. 3. Maximal acid secretion occurred at 16 Hz. This was significantly reduced but not abolished by atropine or hexamethonium and completely abolished after atropine and hexamethonium. In the presence of pentagastrin, the acid secretory response to 16 Hz stimulation was augmented, atropine or hexamethonium reduced stimulated secretion by about 70%, whereas atropine and hexamethonium completely abolished stimulated secretion. 4. In rats in which the vagus nerve was pretreated with capsaicin 10-14 days before experimentation there was a significant reduction (by about 40%) in stimulated acid secretion at 16 Hz, which was virtually abolished by atropine treatment. After acute treatment of the vagus nerve with capsaicin (at the time of experimentation) maximally stimulated acid secretion was significantly reduced by about 50%. 5. Taken together, these results indicate that capsaicin-sensitive afferent fibres contribute to the acid secretory response induced by electrical vagal stimulation in the rat. Based on pharmacological evidence, the capsaicin-sensitive afferent fibres may be cholinergic, since atropine and hexamethonium totally abolish vagal stimulation-induced acid secretion.

New insights into peptidergic abnormalities in Hirschsprung's disease by wholemount immunohistochemistry

In a pilot study previously reported, we showed that individual nerves could be traced in the different layers of the gut in Hirschsprung's disease (HD) using wholemount immunohistochemistry (WI). Little is known about the course of the important nonadrenergic, noncholinergic nerves containing neuropeptides in HD. Therefore, we studied the distribution of neuropeptides in 9 HD patients and 5 controls using WI. The new findings include the following: (1) there were two populations of substance P (SP) nerves--in aganglionic gut, SP-efferent nerves were decreased but SP-afferent fibres innervating blood vessels and mucosa remained unchanged; (2) met-enkephin was present only in efferent nerves to muscle and was decreased in aganglionic gut; and (3) peptidergic nerves have a disorganised pattern in HD affecting not only aganglionic gut but also "normal" gut at the colostomy site. These peptidergic abnormalities may play an important role in the pathophysiology of HD. In particular, the imbalance of afferent and efferent innervation, a finding not previously described in HD, warrants special attention in future studies.

Localization of substance P immunoreactivity in phrenic primary afferent neurons

Retrograde tracing with a fluorescent dye (Fast Blue) combined with immunohistochemistry was used to localize the putative neurotransmitter, substance P, in phrenic primary afferent neurons. Fast Blue was injected into the diaphragm and was found to label phrenic primary afferent neurons in sections from the fifth and sixth cervical dorsal root ganglia. The same sections were then treated with antiserum to substance P. A total of 11.4% of labelled phrenic primary afferent neurons contained substance P immunoreactivity. The diameters of the neurons ranged between 17 to 45 microns with a mean size of 29.7 +/- 0.7 microns (N = 81). The results suggest that substance P could be involved in mediating the transmission of sensory information from the diaphragm to the CNS.

Calcitonin gene-related peptide in visceral afferent nerve fibres: quantification by radioimmunoassay and determination of axonal transport rates

An antibody specific for the C-terminus of rat alpha calcitonin gene-related peptide has been used in radioimmunoassay to measure concentrations of immunoreactive peptide in the upper gastrointestinal tract of capsaicin-treated and coeliac ganglionectomized rats, and to measure axonal transport velocities in the vagus and splanchnic nerves. In adult rats that had been treated soon after birth with capsaicin, immunoreactive calcitonin gene-related peptide in the stomach and duodenum was undetectable (less than 0.1 pmol/g) compared with 4-10 pmol/g in control rats. Removal of the coeliac ganglion also reduced concentrations of immunoreactive calcitonin gene-related peptide by 5-fold, but Leu-enkephalin and Met-enkephalin Arg6Gly7Leu8-immunoreactivities (which are thought to occur in intrinsic gut neurons) were unchanged by coeliac ganglionectomy. Concentrations of calcitonin gene-related peptide immunoreactivity in coeliac ganglia were depressed by 90% in capsaicin-treated rats but concentrations of opioid peptide immunoreactivity were similar to control. The results suggest calcitonin gene-related peptide-immunoreactivity in the upper gastrointestinal tract in the rat is predominantly of extrinsic afferent origin. Chromatographic separation on Sephadex G50, or high-performance liquid chromatography revealed that the major immunoreactive form in stomach extracts corresponded to intact calcitonin gene-related peptide, although there was evidence of smaller, less hydrophobic C-terminal fragments. Direct evidence of transport of calcitonin gene-related peptide toward the gut was obtained by ligation of the cervical vagus and greater splanchnic nerves. There was accumulation on the central side of ligatures, which suggested axonal transport velocities in the vagus of about 1.5 mm/h and 0.7 mm/h in splanchnic nerves.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptide immunocytochemistry in afferent neurons from lower gut in rats

Several peptides were detected in primary sensory neurons located in nodose and dorsal root ganglia and projecting from rat cecum and rectosigmoid, through a combination of retrograde staining by the fluorescent tracer DY-2HCl and of the immunofluorescent procedure of Coons. The three larger cell populations thus identified stored immunoreactive components respectively similar to calcitonin gene-related peptide (CGRP), substance P (SP), and a peptide related to peptide histidine methionine (PHM). The later immunoreactivity consisted of a single molecular form with an apparent molecular weight smaller than PHM itself. Fewer cells contained components immunologically similar to somatostatin 14 (ST14), to the 1-14 N-terminal sequence of somatostatin 28 (1-14 S28), and to neuropeptide Y (NPY). Neonatal treatment with capsaicin resulted in a drastic reduction of immunoreactivity for SP, PHM, ST14, 1-14 S28, and in a partial reduction of CGRP-like positive perikarya. These results demonstrate that several peptides are potentially involved in the sensory innervation of the lower gut in rat.

Speculations on the structure/function relationship for vagal and splanchnic afferent endings supplying the gastrointestinal tract

This paper discusses some of the unsettled issues in the study of the afferent innervation of the gastrointestinal (GI) tract. Afferent fibres in the vagus and splanchnic nerves have been studied electrophysiologically and much has been learnt from single fibre recordings. Splanchnic afferent fibres generally terminate in multiple mechanosensitive endings in the mesentery and serosa where they are in a position to monitor tension on the mesenteric attachments. Other mechanoreceptors following a mainly vagal pathway behave as if they are functionally in-series with the muscle elements of the gut wall and signal muscle tension generated passively by distension and actively during contraction. A third group of afferent endings supply the GI mucosa where they are in a position to signal information on the physical and chemical environment of the gut lumen. A complex picture of mucosal sensitivity has emerged with subpopulations of receptors with polymodal sensitivity and quality-specific mechanoreceptors, thermoreceptors and chemoreceptors. Unfortunately, there is little concensus amongst the different research groups because of different experimental paradigms. One group describes specific chemoreceptors, other groups fail to find them. In this minireview I have speculated on the cause of the often conflicting data on GI afferents and the implications this has for the interpretation of visceral receptor mechanisms.

The sensory-efferent function of capsaicin-sensitive sensory neurons

Capsaicin-sensitive sensory neurons convey to the central nervous system signals (chemical and physical) arising from viscera and the skin which activate a variety of visceromotor and neuroendocrine reflexes integrated at various levels (intramurally in peripheral organs, at level of prevertebral ganglia, spinal and supraspinal level). Much evidence is now available that peripheral terminals of certain sensory neurons, widely distributed in skin and viscera have the ability to release, upon adequate stimulation, their transmitter content. In addition to the well-known "axon reflex" arrangement, the capsaicin-sensitive sensory neurons have the ability to release the stored transmitter also from the same terminal which is excited by the environmental stimulus. The efferent function of these sensory neurons is realized through the direct and indirect (i.e. mediated by activation of other cells) effects of released mediators. The action of released transmitters on postjunctional elements covers a wide range of effects which may have a physiological or pathological relevance. Development of drugs capable of controlling the sensory-efferent functions of the capsaicin-sensitive sensory neurons represent a new and very promising area of research for pharmacological treatment of various human diseases.

Calcitonin gene-related peptide and substance P in afferents to the upper gastrointestinal tract in the rat

Combined immunohistochemistry and retrograde tracing was used to investigate the origin of the sensory calcitonin gene-related peptide (CGRP) innervation of the rat stomach. Up to 85% of spinal gastric afferents (T6-L1) contained CGRP immunoreactivity compared with less than 6% of vagal gastric afferents. By comparison substance P immunoreactivity occurred in about 50% of spinal gastric afferents and less than 2% of vagal afferents. The vagal afferents to the oesophagus were 14 and 26% substance P- and CGRP-immunoreactive respectively. The results suggest an important role for CGRP in gastric spinal afferents.