Tachykinin receptors are involved in the "local efferent" motor response to capsaicin in the guinea-pig small intestine and oesophagus

Activation of intestinal spinal afferent endings by changes in intra-mesenteric arterial pressure

KEY POINTS

A major class of mechano-nociceptors to the intestine have mechanotransduction sites on extramural and intramural arteries and arterioles ('vascular afferents'). These sensory neurons can be activated by compression or axial stretch of vessels. Using isolated preparations we showed that increasing intra-arterial pressure, within the physiological range, activated mechano-nociceptors on vessels in intact mesenteric arcades, but not in isolated arteries. This suggests that distortion of the branching vascular tree is the mechanical adequate stimulus for these sensory neurons, rather than simple distension. The same rises in pressure also activated intestinal peristalsis in a partially capsaicin-sensitive manner indicating that pressure-sensitive vascular afferents influence enteric circuits. The results identify the mechanical adequate stimulus for a major class of mechano-nociceptors with endings on blood vessels supplying the gut wall; these afferents have similar endings to ones supplying other viscera, striated muscle and dural vessels.

ABSTRACT

Spinal sensory neurons innervate many large blood vessels throughout the body. Their activation causes the hallmarks of neurogenic inflammation: vasodilatation through the release of the neuropeptide calcitonin gene-related peptide and plasma extravasation via tachykinins. The same vasodilator afferent neurons show mechanical sensitivity, responding to crushing, compression or axial stretch of blood vessels - responses which activate pain pathways and which can be modified by cell damage and inflammation. In the present study, we tested whether spinal afferent axons ending on branching mesenteric arteries ('vascular afferents') are sensitive to increased intravascular pressure. From a holding pressure of 5 mmHg, distension to 20, 40, 60 or 80 mmHg caused graded, slowly adapting increases in firing of vascular afferents. Many of the same afferent units showed responses to axial stretch, which summed with responses evoked by raised pressure. Many vascular afferents were also sensitive to raised temperature, capsaicin and/or local compression with von Frey hairs. However, responses to raised pressure in single, isolated vessels were negligible, suggesting that the adequate stimulus is distortion of the arterial arcade rather than distension per se. Increasing arterial pressure often triggered peristaltic contractions in the neighbouring segment of intestine, an effect that was mimicked by acute exposure to capsaicin (1 μm) and which was reduced after desensitisation to capsaicin. These results indicate that sensory fibres with perivascular endings are sensitive to pressure-induced distortion of branched arteries, in addition to compression and axial stretch, and that they contribute functional inputs to enteric motor circuits.

Effects of some natural carotenoids on TRPA1- and TRPV1-induced neurogenic inflammatory processes in vivo in the mouse skin

Mechanisms of the potent anti-inflammatory actions of carotenoids are unknown. Since carotenoids are incorporated into membranes, they might modulate transient receptor potential ankyrin 1 and vanilloid 1 (TRPA1 and TRPV1) activation predominantly on peptidergic sensory nerves. We therefore investigated the effects of three carotenoids (β-carotene, lutein and lycopene) on cutaneous neurogenic inflammation. Acute neurogenic edema and inflammatory cell recruitment were induced by smearing the TRPA1 agonist mustard oil (5%) or the TRPV1 activator capsaicin (2.5%) on the mouse ear. Ear thickness was then determined by micrometry, microcirculation by laser Doppler imaging and neutrophil accumulation by histopathology and spectrophotometric determination of myeloperoxidase activity. The effects of lutein on the stimulatory action of the TRPA1 agonist mustard oil were also tested on the guinea-pig small intestine, in isolated organ experiments. Mustard oil evoked 50-55% ear edema and granulocyte influx, as shown by histology and myeloperoxidase activity. Swelling was significantly reduced between 2 and 4 h after administration of lutein or β-carotene (100 mg/kg subcutane three times during 24 h). Lutein also decreased neutrophil accumulation induced by TRPA1 activation, but did not affect mustard oil-evoked intestinal contraction. Lycopene had no effect on any of these parameters. None of the three carotenoids altered capsaicin-evoked inflammation. It is proposed that the dihydroxycarotenoid lutein selectively inhibits TRPA1 activation and consequent neurogenic inflammation, possibly by modulating lipid rafts.

Localization of TRPV1 and contractile effect of capsaicin in mouse large intestine: high abundance and sensitivity in rectum and distal colon

We investigated immunohistochemical differences in the distribution of TRPV1 channels and the contractile effects of capsaicin on smooth muscle in the mouse rectum and distal, transverse, and proximal colon. In the immunohistochemical study, TRPV1 immunoreactivity was found in the mucosa, submucosal, and muscle layers and myenteric plexus. Large numbers of TRPV1-immunoreactive axons were observed in the rectum and distal colon. In contrast, TRPV1-positive axons were sparsely distributed in the transverse and proximal colon. The density of TRPV1-immunoreactive axons in the rectum and distal colon was much higher than those in the transverse and proximal colon. Axons double labeled with TRPV1 and protein gene product (PGP) 9.5 were detected in the myenteric plexus, but PGP 9.5-immunoreactive cell bodies did not colocalize with TRPV1. In motor function studies, capsaicin induced a fast transient contraction, followed by a large long-lasting contraction in the rectum and distal colon, whereas in the transverse and proximal colon only the transient contraction was observed. The capsaicin-induced transient contraction from the proximal colon to the rectum was moderately inhibited by an NK1 or NK2 receptor antagonist. The capsaicin-induced long-lasting contraction in the rectum and distal colon was markedly inhibited by an NK2 antagonist, but not by an NK1 antagonist. The present results suggest that TRPV1 channels located on the rectum and distal colon play a major role in the motor function in the large intestine.

Functional study on TRPV1-mediated signalling in the mouse small intestine: involvement of tachykinin receptors

Afferent nerves in the gut not only signal to the central nervous system but also provide a local efferent-like effect. This effect can modulate intestinal motility and secretion and is postulated to involve the transient receptor potential of the vanilloid type 1 (TRPV1). By using selective TRPV1 agonist and antagonists, we studied the efferent-like effect of afferent nerves in the isolated mouse jejunum. Mouse jejunal muscle strips were mounted in organ baths for isometric tension recordings. Jejunal strips contracted to the TRPV1 agonist capsaicin. Contractions to capsaicin showed rapid tachyphylaxis and were insensitive to tetrodotoxin, hexamethonium, atropine or L-nitroarginine. Capsaicin did not affect contractions to electrical stimulation of enteric motor nerves and carbachol. Tachykinin NK1, NK2 and NK3 receptor blockade by RP67580, nepadutant plus SR-142801 reduced contractions to capsaicin to a similar degree as contractions to substance P. The effect of the TRPV1 antagonists capsazepine, SB-366791, iodo-resiniferatoxin (iodo-RTX) and N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl)tetrahydropyrazine-1(2H)-carbox-amide (BCTC) was studied. Capsazepine inhibited contractions not only to capsaicin but also those to carbachol. SB-366791 reduced contractions both to capsaicin and carbachol. Iodo-RTX partially inhibited the contractions to capsaicin without affecting contractions to carbachol. BCTC concentration-dependently inhibited and at the highest concentration used, abolished the contractions to capsaicin without affecting those to carbachol. From these results, we conclude that activation of TRPV1 in the mouse intestine induces a contraction that is mediated by tachykinins most likely released from afferent nerves. The TRPV1-mediated contraction does not involve activation of intrinsic enteric motor nerves. Of the TRPV1 antagonists tested, BCTC combined strong TRPV1 antagonism with TRPV1 selectivity.

Ultrastructural evidence for communication between intramuscular vagal mechanoreceptors and interstitial cells of Cajal in the rat fundus

To assess whether afferent vagal intramuscular arrays (IMAs), putative gastrointestinal mechanoreceptors, form contacts with interstitial cells of Cajal of the intramuscular type (ICC-IM) and to describe any such contacts, electron microscopic analyses were performed on the external muscle layers of the fundus containing dextran-labelled diaminobenzidin (DAB)-stained IMAs. Special staining and embedding techniques were developed to preserve ultrastructural features. Within the muscle layers, IMA varicosities were observed in nerve bundles traversing major septa without contact with ICC-IM, contacting unlabelled neurites and glial cells. IMA varicosities were encountered in minor septa in contact with ICC-IM which were not necessarily in close contact with muscle cells. In addition, IMA varicosities were observed within muscle bundles in close contact with ICC-IM which were in gap junction contact with muscle cells. IMAs formed varicosities containing predominantly small agranular vesicles, occasionally large granular vesicles and prejunctional thickenings in apposition to ICC-IM processes, indicating communication between ICC and IMA via synapse-like contacts. Taken together, these different morphological features are consistent with a hypothesized mechanoreceptor role for IMA-ICC complexes. Intraganglionic laminar ending varicosities contacted neuronal somata and dendrites in the myenteric plexus of the fundus, but no contacts with ICC associated with Auerbach's plexus were encountered.

Survival dependency of intramuscular ICC on vagal afferent nerves in the cat esophagus

Interstitial cells of Cajal (ICC) have been proposed as stretch receptors for vagal afferent nerves in the stomach based on immunohistochemical studies. The aim of the present study was to use electron microscopy and the anterograde degeneration technique to investigate ultrastructural features and survival dependency of ICC associated with vagal afferent innervation of the cat esophagus. This is the first report on the ultrastructural characteristics of ICC in the cat esophagus. Intramuscular ICC (ICC-IM) were identified throughout the musculature, whereas ICC in the myenteric plexus were rare. ICC-IM were particularly numerous in septa aligned with smooth muscle bundles. They were in synapse-like contact with nerve varicosities and in gap junction contact with smooth muscle cells. Smooth muscle cells also made contact with ICC through peg and socket junctions. Precision damage through small-volume injection of saline in the center of the nodose ganglion from the lateral side, known to selectively affect sensory nerves, was followed within 24 h by degeneration of a subset of nerve varicosities associated with ICC-IM, as well as degeneration of the associated ICC-IM. Smooth muscle cells were not affected. Nerves of Auerbachs plexus and associated ICC were not affected. In summary, ICC-IM aligning the esophageal muscle bundles form specialized synapse-like contacts with vagal afferent nerves as well as gap junction and peg-and-socket contacts with smooth muscle cells. This is consistent with a role of ICC-IM as stretch receptors associated with vagal afferent nerves; the ICC-vagal nerve interaction appears essential for the survival of the ICC.

Contractile effect of TRPA1 receptor agonists in the isolated mouse intestine

TRPA1 is a member of the transient receptor potential (TRP) channel family expressed in sensory neurons. The present study focused on the effects of TRPA1 activation on contractile responses in isolated mouse intestine preparations. The jejunum, ileum, and proximal and distal colon were surgically isolated from male ddY mice. Intestinal motility was recorded as changes in isotonic tension. TRPA1, TRPM8, and TRPV1 expressions were examined by reverse transcription-polymerase chain reaction (RT-PCR). A TRPA1 agonist allyl isothiocyanate (AITC) dose-dependently induced contractions in the proximal and distal colon, whereas in the jejunum and ileum, even 100 muM AITC caused very little contraction. Likewise, a TRPA1 and TRPM8 agonist icilin, a TRPA1 agonist allicin, and a TRPV1 agonist capsaicin induced contractions in the colon. However, a TRPM8 agonist menthol induced long-lasting relaxation in the colon. Repeated exposure to AITC produced desensitization of its own contraction in the colon. Moreover, contractions induced by AITC generate cross-desensitization with icilin and capsaicin. Tetrodotoxin completely abolished AITC-induced contractions in the colon, whereas atropine significantly attenuated AITC-induced contractions in the distal colon, but not in the proximal colon. Menthol-induced relaxation in the colon was not inhibited by tetrodotoxin and atropine. RT-PCR analysis revealed the expression of TRPA1 and TRPV1, but not TRPM8, throughout the mouse intestine. These results suggest that TRPA1, but not TRPM8, are functionally expressed in the enteric nervous system throughout the mouse intestine on neurons that may also co-express TRPV1, yet the contractile responses to TRPA1 activation differ depending on their location along the intestine.

Multiple motor effects of ATP and their inhibition by P purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid in the small intestine of the guinea-pig

Adenosine 5'-triphosphate (ATP) may be an important neurotransmitter in the gastrointestinal tract. The present study examined the motor effects of exogenous ATP on longitudinally-oriented preparations of the guinea-pig isolated ileum and the influence of drugs on the ATP-induced responses. High micromolar concentrations of ATP caused two types of contraction, a phasic, cholinergic response and a tonic, tetrodotoxin-resistant contraction. The phasic contraction was reduced by hexamethonium (5x10(-5) M), but left uninfluenced by capsaicin tachyphylaxis or tachyphylaxis to alpha,beta-methylene ATP. The tonic response was resistant to atropine, hexamethonium, capsaicin, omega-conotoxin GVIA, or pretreatment with alpha,beta-methylene ATP. Both types of ATP-induced contraction were diminished or abolished by the P2 purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 3x10(-6) and 3x10(-5) M, respectively). In the precontracted, atropine-treated ileum ATP (10(-6)-10(-4) M) caused guanethidine-resistant relaxation. This response was not influenced by tetrodotoxin, omega-conotoxin GVIA, or NG-nitro-L-arginine, but was abolished by apamin (10(-7) M), and inhibited by PPADS (3x10(-5) M) or reactive blue 2 (10(-5) M), in a surmountable manner. A high degree of tachyphylaxis was observed with the relaxant effect of ATP (10(-5)-10(-4) M). A high concentration (3x10(-4) M) of PPADS failed to influence ileum contractions to exogenous acetylcholine or histamine. It is concluded that, in addition to its direct contractile action in the guinea-pig ileum, ATP can activate (partly preganglionic) cholinergic neurones, an effect whose mechanism is largely different from that of alpha,beta-methylene ATP. ATP also causes relaxation by a direct, probably P2Y-receptor-mediated effect on the smooth muscle. All motor effects of ATP are inhibited by the antagonist PPADS.

Defensive and pathological functions of the gastrointestinal NK3 receptor

In general, normal gut functions are unaffected by selective NK(3) receptor antagonists such as talnetant (SB-223412), osanetant (SR 142901) or SB-235375. However, NK(3) receptors may mediate certain defensive or pathological intestinal processes. The precise mechanisms, by which this role is achieved, are not fully understood. In summary, intense stimulation of the intrinsic primary afferent neurones (IPANs) of the enteric nervous system is thought to release tachykinins from these neurones, to induce slow excitation (slow EPSPs) of connecting IPANs. This is hypothesised to cause hypersensitivity and disrupt intestinal motility, at least partly explaining why NK(3) receptor antagonism can reduce the level of disruption caused by supramaximal distension pressures in vitro. Tachykinin release from IPANs may also increase C-fibre sensitivity, directly or indirectly. Thus, NK(3) receptor antagonists can inhibit nociception associated with intestinal distension, in normal animals or after pre-sensitisation by restraint stress. Importantly, such inhibition has been found with SB-235375, a peripherally restricted antagonist. SB-235375 can also reduce a visceromotor response to brief colorectal distension without affecting similar responses to skin pinch, providing additional evidence for intestinal-specific activity. NK(3) receptor biology is, therefore, revealing a novel pathway by which disruptions in intestinal motility and nociception can be induced.

The prokinetic-like activity of ghrelin in rat isolated stomach is mediated via cholinergic and tachykininergic motor neurones

Ghrelin increases electrically evoked, neuronally mediated contractions of rat isolated forestomach, a prokinetic-like activity. Since the nerve type sensitive to ghrelin is unclear, we examined the activity of ghrelin in the presence of antagonists at receptors for the main gastric motor neurotransmitters. Electrical field stimulation (EFS; 5 Hz, 0.5 ms, +/-50 V, 30 s every 3 min) of circular muscle preparations evoked tetrodotoxin 1 microM-sensitive responses, consisting of a small initial contraction followed by a further contraction or more usually, by muscle relaxation. Termination of EFS evoked a large rapidly developing after-contraction. Atropine 1 microM prevented contractions during EFS, increased any relaxations and prolonged the after-contractions. Nomega-Nitro-L-arginine-methyl-ester-hydrochloride (L-NAME) 0.3 mM prevented relaxations during EFS, changing the triphasic response into a monophasic contraction. The tachykinin NK1 and tachykinin NK2 receptor antagonists N-acetyl-L-tryptophan-3,5-bistrifluoromethyl-benzyl-ester (L-732,138 1 microM) and Cyclo[Gln-Trp-Phe-Gly-Leu-CH2N(CH3)-Leu] (MDL-29,913 1 microM) each reduced EFS-evoked relaxations; the latter also reduced the after-contractions. The tachykinin NK3 receptor antagonist (-)-(S)-N-(alpha-ethylbenzyl)-3-(carboxymethoxy)-2-phenylquinoline-4-carboxamide (SB-235375, 0.1 microM) had no effects. The combination of tachykinin NK(1,2,3) receptor antagonists reduced the after-contractions and abolished relaxations during EFS, replacing this with a contraction. In control tissues, ghrelin 1 microM increased EFS-induced contractions and tended to reduce any relaxations. In the presence of atropine 1 microM, L-NAME 0.3 mM or the tachykinin receptor antagonists (as above), ghrelin 1 microM increased any EFS-induced contraction but in the presence of atropine had no effects on EFS-evoked relaxations. We conclude that EFS evokes responses mediated by acetylcholine, nitric oxide and tachykinins. Ghrelin facilitates both cholinergic and tachykininergic excitatory pathways, consistent with activity within the enteric nervous system and possibly the vagus nerve.

Innervation of the mammalian esophagus

Understanding the innervation of the esophagus is a prerequisite for successful treatment of a variety of disorders, e.g., dysphagia, achalasia, gastroesophageal reflux disease (GERD) and non-cardiac chest pain. Although, at first glance, functions of the esophagus are relatively simple, their neuronal control is considerably complex. Vagal motor neurons of the nucleus ambiguus and preganglionic neurons of the dorsal motor nucleus innervate striated and smooth muscle, respectively. Myenteric neurons represent the interface between the dorsal motor nucleus and smooth muscle but they are also involved in striated muscle innervation. Intraganglionic laminar endings (IGLEs) represent mechanosensory vagal afferent terminals. They also establish intricate connections with enteric neurons. Afferent information is implemented by the swallowing central pattern generator in the brainstem, which generates and coordinates deglutitive activity in both striated and smooth esophageal muscle and orchestrates esophageal sphincters as well as gastric adaptive relaxation. Disturbed excitation/inhibition balance in the lower esophageal sphincter results in motility disorders, e.g., achalasia and GERD. Loss of mechanosensory afferents disrupts adaptation of deglutitive motor programs to bolus variables, eventually leading to megaesophagus. Both spinal and vagal afferents appear to contribute to painful sensations, e.g., non-cardiac chest pain. Extrinsic and intrinsic neurons may be involved in intramural reflexes using acetylcholine, nitric oxide, substance P, CGRP and glutamate as main transmitters. In addition, other molecules, e.g., ATP, GABA and probably also inflammatory cytokines, may modulate these neuronal functions.

Tachykinins and tachykinin receptors in the gut, with special reference to NK2 receptors in human

Tachykinins (TKs), substance P (SP), neurokinin A (NKA) and B (NKB) are important peptide modulators of intestinal motility in animal species studied so far, including humans. Modulation of motility by TKs can occur at various levels, since these peptides are expressed in cholinergic excitatory motor neurons projecting to both circular and longitudinal muscle, interneurons, and intramural and extramural sensory neurons. The effects of SP, NKA and NKB are preferentially mediated through the stimulation of NK1, NK2 and NK3 receptors, respectively; however, the selectivity of natural TKs for their preferred receptors is relative. In addition, SP and NKA are expressed in similar quantities in the human intestine and adequate stimuli can release similar amount of these TKs from enteric nerves. Furthermore, a single anatomical substrate can express more than one TK receptor type, so that the blockade of a single receptor type may not reveal functional effects in integrated models of motility. In isolated human small intestine and colon circular muscle strips, both NK1 and NK2 receptors mediate contractile effects. Indeed, in the human small intestine, smooth muscle electrical and motor events induced by electrical field stimulation (EFS) can involve either or both NK1 and NK2 receptors or these latter receptors predominantly, depending on the experimental conditions. In contrast, in the human colonic smooth muscle, only the NK2 receptor-mediated component of the response to EFS is prominent and some evidence would suggest that this component is the main excitatory motor mechanism at this level. Furthermore, a NK2 receptor-mediated secretory component in the human colonic mucosa has been recently demonstrated. Thus, it could be speculated that the blockade of both NK1 and NK2 receptors will be necessary to antagonise motor effects induced by exogenous administration or endogenous release of TKs in the small intestine, whereas the blockade of the NK2 receptors would be sufficient to disrupt physiological motor and, possibly, secretory activity at the colonic level. Available evidence indicates that, in healthy volunteers, the infusion of NKA (25 pmol/kg/min i.v.) stimulated small intestine motility and precipitated a series of intestinal and non-intestinal adverse events. Nepadutant (8 mg i.v.), a selective NK2 receptor antagonist, antagonised small intestine motility induced by NKA and prevented associated intestinal adverse events. In another study, the same dose of nepadutant increased colo-rectal compliance during isobaric balloon distension in healthy volunteers pretreated with a glycerol enema, disclosing a NK2 receptor-mediated component in the regulation of colonic smooth muscle tone. However, the prolonged blockade of NK2 receptors by nepadutant (16 mg i.v. b.i.d. for 8 days) did not affect bowel habits, neither in term of movements nor of stool consistency. Altogether, these results indicate that, even when there is a significant redundance in the effects of TKs and in the role of their receptors, the selective blockade of tachykinin NK2 receptors can have functional consequences on human intestinal motility and perception, but this can occur without the disruption of the physiological functions.

Tachykinins are involved in local reflex modulation of vagally mediated striated muscle contractions in the rat esophagus via tachykinin NK1 receptors

The objective of the present study was to investigate the hypothesis of the presence of a local neural reflex modulating the vagally mediated contractions of striated muscle in the rat esophagus and to determine the possible involvement of tachykinins in such a local neural reflex. Electrical stimulation of the vagus nerve evoked twitch contractile responses that were abolished by d-tubocurarine (5 microM). Capsaicin (1-100 microM) inhibited the vagally mediated twitch contractions o f the normal rat esophageal preparations concentration-dependently but not those of the neonatally capsaicin-treated ones. NG-nitro-L-arginine methyl ester (100 microM), a nitric oxide synthase inhibitor, blocked the inhibitory effect of capsaicin and exogenous application of a nitric oxide donor (1 mM) inhibited the vagally mediated twitch contractions. Capsaicin suppressed acetylcholine release from the normal rat esophageal segments evoked by vagus nerve stimulation but not that from the neonatally capsaicin-treated ones. A selective tachykinin NK1 receptor antagonist (0.1 or 1 microM) attenuated the inhibitory effect of capsaicin. However, antagonists of tachykinin NK2, tachykinin NK3 and calcitonin gene-related peptide receptors (1 microM) did not have any effect. A tachykinin NK1 receptor agonist (1 or 5 microM) inhibited the vagally mediated twitch contractions, which was prevented by NG-nitro-L-arginine methyl ester (100 microM). These data suggest that the rat esophagus might have a local neural reflex inhibiting the vagally mediated striated muscle motility, which consists of capsaicin-sensitive sensory neurons and myenteric nitrergic neurons, and that tachykinins might be involved in the neural reflex through tachykinin NK1 receptors.

Facilitation and inhibition by capsaicin of cholinergic neurotransmission in the guinea-pig small intestine

The effects of capsaicin on [3H]acetylcholine release and muscle contraction were studied on the myenteric plexus-longitudinal muscle preparation of the guinea-pig ileum preincubated with [3H]choline. Capsaicin concentration-dependently increased both basal [3H]acetylcholine release (pEC50 7.0) and muscle tone (pEC50 6.1). The facilitatory effects of capsaicin were antagonized by 1 microM capsazepine (pK (B) 7.0 and 7.6), and by the combined blockade of NK1 and NK3 tachykinin receptors with the antagonists CP99994 plus SR142801 (each 0.1 microM). This suggests that stimulation by capsaicin of TRPV1 receptors on primary afferent fibres causes a release of tachykinins which, in turn, mediate via NK1 and NK3 receptors an increase in acetylcholine release. The capsaicin-induced acetylcholine release was significantly enhanced by the NO synthase inhibitor L-NG-nitroarginine (100 microM). This indicates that tachykinins released from sensory neurons also stimulate nitrergic neurons and thus lead, via NO release, to inhibition of acetylcholine release. Capsaicin concentration-dependently reduced the electrically-evoked [3H]acetylcholine release (pEC50 6.4) and twitch contractions (pEC50 5.9). The inhibitory effects were not affected by either capsazepine, NK1 and NK3 receptor antagonists, the cannabinoid CB1 antagonist SR141716A or by L-NG-nitroarginine. Desensitization of TRPV1 receptors by a short exposure to 3 microM capsaicin abolished the facilitatory responses to a subsequent administration, but did not modify the inhibitory effects. In summary, capsaicin has a dual effect on cholinergic neurotransmission. The facilitatory effect is indirect and involves tachykinin release and excitation of NK1 and NK3 receptors on cholinergic neurons. The inhibition of acetylcholine release may be due to a decrease of Ca2+ influx into cholinergic neurons.

Effects of capsaicin on visceral smooth muscle: a valuable tool for sensory neurotransmitter identification

Studying the visceral effects of the sensory stimulant capsaicin is a useful and relatively simple tool of neurotransmitter identification and has been used for this purpose for approximately 25 years in the authors' and other laboratories. We believe that conclusions drawn from experiments on visceral preparations may have an impact on studies dealing with the central endings of primary afferent neurons, i.e. research on nociception at the spinal level. The present review concentrates on the effects of capsaicin--through the transient receptor potential vanilloid receptor type 1 (TRPV1) receptor--on innervated gastrointestinal, respiratory and genitourinary smooth muscle preparations. Tachykinins and calcitonin gene-related peptide (CGRP) are the most widely accepted transmitters to mediate "local efferent" effects of capsaicin-sensitive nerves in tissues taken from animals. Studies more and more frequently indicate a supra-additive interaction of various types of tachykinin receptors (tachykinin NK(1), NK(2), NK(3) receptors) in the excitatory effects of capsaicin. There is also evidence for a mediating role of ATP, acting on P(2) purinoceptors. Non-specific inhibitory actions of capsaicin-like drugs have to be taken into consideration while designing experiments with these drugs. Results obtained on human tissues may be sharply different from those of animal preparations. Capsaicin potently inhibits tone and movements of human intestinal preparations, an effect mediated by nitric oxide (NO) and/or vasoactive intestinal polypeptide.

Inhibition of nitric oxide synthesis blocks the inhibitory response to capsaicin in intestinal circular muscle preparations from different species

Moderate concentrations of the sensory stimulant drug capsaicin caused relaxation in human and animal intestinal circular muscle preparations (guinea-pig proximal, mouse distal colon, human small intestine and appendix) in vitro. With the exception of the guinea-pig colon, the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine (L-NOARG; 10(-4) M) strongly inhibited the relaxant effect of capsaicin. Tetrodotoxin, an inhibitor of voltage-sensitive Na+ channels failed to significantly reduce the inhibitory effect of capsaicin in the guinea-pig colon, human ileum and appendix; it caused an approximately 50% reduction in the mouse colon. The relaxant effect of capsaicin was strongly reduced in colonic preparations from transient receptor potential vanilloid type (TRPV1) receptor knockout mice as compared to their wildtype controls. It is concluded that nitric oxide, possibly of sensory origin, is involved in the relaxant action of capsaicin in the circular muscle of the mouse and human intestine.

Sensory nerves, neurogenic inflammation and pain: missing components of alternative irritation strategies? A review and a potential strategy

The eyes and skin are highly innervated by sensory nerves; stimulation of these nerves by irritants may give rise to neurogenic inflammation, leading to sensory irritation and pain. Few in vitro models of neurogenic inflammation have been described in conjunction with alternative skin and eye irritation methods, despite the fact that the sensory innervation of these organs is well-documented. To date, alternative approaches to the Draize skin and eye irritation tests have proved largely successful at classifying severe irritants, but are generally poor at discriminating between agents with mild to moderate irritant potential. We propose that the development of in vitro models for the prediction of sensory stimulation will assist in the re-classification of the irritant potential of agents that are under-predicted by current in vitro strategies. This review describes the range of xenobiotics known to cause inflammation and pain through the stimulation of sensory nerves, as well as the endogenous mediators and receptor types that are involved. In particular, it focuses on the vanilloid receptor, its activators and its regulation, as these receptors function as integrators of responses to numerous noxious stimuli. Cell culture models and ex vivo preparations that have the potential to serve as predictors of sensory irritation are also described. In addition, as readily available sensory neuron cell line models are few in number, stem cell lines (with the capacity to differentiate into sensory neurons) are explored. Finally, a preliminary strategy to enable assessment of whether incorporation of a sensory component will enhance the predictive power of current in vitro eye and skin testing strategies is proposed.

Extending the mutual information measure to rank inferred literature relationships

Background

Within the peer-reviewed literature, associations between two things are not always recognized until commonalities between them become apparent. These commonalities can provide justification for the inference of a new relationship where none was previously known, and are the basis of most observation-based hypothesis formation. It has been shown that the crux of the problem is not finding inferable associations, which are extraordinarily abundant given the scale-free networks that arise from literature-based associations, but determining which ones are informative. The Mutual Information Measure (MIM) is a well-established method to measure how informative an association is, but is limited to direct (i.e. observable) associations.

Results

Herein, we attempt to extend the calculation of mutual information to indirect (i.e. inferable) associations by using the MIM of shared associations. Objects of general research interest (e.g. genes, diseases, phenotypes, drugs, ontology categories) found within MEDLINE are used to create a network of associations for evaluation.

Conclusions

Mutual information calculations can be effectively extended into implied relationships and a significance cutoff estimated from analysis of random word networks. Of the models tested, the shared minimum MIM (MMIM) model is found to correlate best with the observed strength and frequency of known associations. Using three test cases, the MMIM method tends to rank more specific relationships higher than counting the number of shared relationships within a network.

Neurokinin NK1 and NK3 receptors as targets for drugs to treat gastrointestinal motility disorders and pain

NK1 and NK3 receptors do not appear to play significant roles in normal GI functions, but both may be involved in defensive or pathological processes. NK1 receptor antagonists are antiemetic, operating via vagal sensory and motor systems, so there is a need to study their effects on other gastro-vagal functions thought to play roles in functional bowel disorders. Interactions between NK1 receptors and enteric nonadrenergic, noncholinergic motorneurones suggest a need to explore the role of this receptor in disrupted colonic motility. NK1 receptor antagonism does not exert consistent analgesic activity in humans, but similar studies have not been carried out against pain of GI origin, where NK1 receptors may have additional influences on mucosal inflammatory or "irritant" processes. NK3 receptors mediate certain disruptions of intestinal motility. The activity may be driven by tachykinins released from intrinsic primary afferent neurones (IPANs), which induce slow EPSP activity in connecting IPANs and hence, a degree of hypersensitivity within the enteric nervous system. The same process is also proposed to increase C-fibre sensitivity, either indirectly or directly. Thus, NK3 receptor antagonists inhibit intestinal nociception via a "peripheral" mechanism that may be intestine-specific. Studies with talnetant and other selective NK3 receptor antagonists are, therefore, revealing an exciting and novel pathway by which pathological changes in intestinal motility and nociception can be induced, suggesting a role for NK3 receptor antagonism in irritable bowel syndrome.

Characterization of capsaicin-induced, capsazepine-insensitive relaxation of ileal smooth muscle of rats

The mechanisms underlying the capsaicin-induced relaxation of the acetylcholine- as well as KCl-contraction were studied by measuring isometric force and phosphorylation of 20-kDa regulatory light chain subunit of myosin (MLC(20)) in ileal longitudinal smooth muscles of rats. Capsaicin relaxed acetylcholine- and KCl-stimulated preparations in a concentration-dependent manner; the former was less sensitive to capsaicin than the latter and maximum responses to capsaicin (a percentage of papaverine-induced relaxation) were 70.6+/-7.5%, n=10 and 97.1+/-0.9%, n=13, P<0.05, respectively. The response showed no desensitization. Like nifedipine, capsaicin relaxed the tissue precontracted with an agonist of L-type Ca(2+) channels as well. The relaxant effect of capsaicin was not inhibited by capsazepine (a selective antagonist of vanilloid VR1 receptors), nitro-l-arginine, indomethacin, guanethidine, nor by inhibitors of soluble guanylate cyclase. Capsaicin inhibited acetylcholine-induced transient contraction in a Ca(2+)-free, EGTA solution. Phosphorylation of MLC(20) (a percentage of phosphorylated to total MLC(20)) was increased 1 min after application of 10 microM acetylcholine (7.8+/-2.0%, n=6 vs. 22.6+/-3.2%, n=6) and of 65.9 mM KCl (2.2+/-0.3%, n=8 vs. 10.7+/-1.7%, n=12). Capsaicin reduced the KCl-induced increase more markedly than acetylcholine-induced increase in MLC(20) phosphorylation. When the tissue was contracted for 20 min with acetylcholine, MLC(20) phosphorylation was increased, and capsaicin reduced markedly the contraction and abolished MLC(20) phosphorylation both elicited by acetylcholine. It is suggested that capsaicin relaxes the rat ileum via its direct action on smooth muscle, and that capsaicin inhibits contractile mechanisms involving extracellular Ca(2+) influx via non-L-type Ca(2+) channels, possibly via store-operated Ca(2+) channels and Ca(2+) release from intracellular storage sites. The effects of capsaicin on acetylcholine- and KCl-induced contraction could be explained by a decrease in MLC(20) phosphorylation.

Localization of neurokinin 1 receptor (NK1R) immunoreactivity in rat esophagus

The aim of the present immunohistochemical study was to investigate the localization of neurokinin 1 receptor (NK1R) in rat esophagus and examine the relationship between NK1Rs and intrinsic cholinergic, nitrergic, or substance P (SP) neurons. NK1R immunoreactivity (IR) was observed on the nerve cell bodies in the myenteric ganglia throughout the esophagus, but not on striated muscles and smooth muscle cells of the muscularis mucosae. The frequency of occurrence of NK1R neurons was highest in the cervical esophagus and lowest in the lower thoracic esophagus. Considerable immunoreactivity was seen on the nerve cell surfaces and was also present in the cytoplasm of cell somas and in the initial part of the axons, but not in any other nerve fibers or terminals. Dogiel type I-like morphology was observed in some of the NK1R neurons; however, the majority exhibited polymorphic morphology. Double immunolabeling indicated that a majority (77%) of the NK1R neurons were immunoreactive for choline acetyltransferase (ChAT), while a minority (23%) were immunoreactive for nitric oxide synthase (NOS)-IR. Most of the NK1R neurons (92%) were innervated by the SP nerve fibers. Triple immunolabeling indicated that 70% of the NK1R neurons were associated with intrinsic SP nerve fibers (without CGRP-IR), 59% were associated with extrinsic SP nerve fibers (with CGRP-IR), and 35% were associated with both intrinsic and extrinsic SP nerve fibers. These results suggest that SP/tachykinin released from the SP nerve fibers of intrinsic and/or extrinsic origin activates the predominantly intrinsic cholinergic neurons via NK1Rs to influence neuronal transmission or motility in rat esophagus.

PAR-2 agonists induce contraction of murine small intestine through neurokinin receptors

Protease-activated receptor-2 (PAR-2) is a G protein-coupled receptor and is expressed throughout the gut. It is well known that PAR-2 participates in the regulation of gastrointestinal motility; however, the results are inconsistent. The present study investigated the effect and mechanism of PAR-2 activation on murine small intestinal smooth muscle function in vitro. Both trypsin and PAR-2-activating peptide SLIGRL induced a small relaxation followed by a concentration-dependent contraction. The sensitivity to trypsin was greater than that to SLIGRL (EC50 = 0.03 vs. 40 microM), but maximal responses were similar (12.3 +/- 1.6 vs. 13.7 +/- 1.3 N/cm2). Trypsin-evoked contraction (1 microM) exhibited a rapid desensitization, whereas the desensitization of response to SLIGRL was less even at high concentration (50 microM). Atropine had no effect on PAR-2 agonist-induced contractions. In contrast, TTX and capsaicin significantly attenuated those contractions, implicating a neurogenic mechanism that may involve capsaicin-sensitive sensory nerves. Furthermore, contractions induced by trypsin and SLIGRL were reduced by neurokinin receptor NK1 antagonist SR-140333 or NK2 antagonist SR-48968 alone or were further reduced by combined application of SR-140333 and SR-48968, indicating the involvement of neurokinin receptors. In addition, desensitizing neurokinin receptors with substance P and/or neurokinin A decreased the PAR-2 agonist-evoked contraction. We concluded that PAR-2 agonists induced a contraction of murine intestinal smooth muscle that was mediated by nerves. The excitatory effect is also dependent on sensory neural pathways and requires both NK1 and NK2 receptors.

Actions of endothelin and corticotropin releasing factor in the guinea-pig ileum: no evidence for an interaction with capsaicin-sensitive neurons

Both endothelins and corticotropin releasing factor (CRF) appear in capsaicin-sensitive neurons. We have investigated the effects of human endothelin-1 (ET-1) and CRF in the guinea-pig ileum longitudinal and circular preparations and sought for ways of specific antagonism. With the aid of tachyphylaxis to capsaicin (i.e., rendering capsaicin-sensitive neurons functionally impaired) it was tested if these neurons played a mediating role in the effects of ET-1 or CRF. We also tried to find out whether endogenous endothelin or CRF plays a role in the excitatory and inhibitory effects of capsaicin in the ileum. In preparations at basal tone, both exogenous ET-1 (1-100 nM) and CRF (3-100 nM) caused contraction. These responses were not influenced by capsaicin tachyphylaxis. The contractile effect of ET-1 was not affected by tetrodotoxin (1 microM), atropine (1 microM), methysergide (100 nM), chloropyramine (100 nM) or SR140333 (100 nM) but was significantly inhibited or even abolished by the receptor antagonist BQ123 (3 microM) or BQ788 (3 microM). CRF caused contraction that was fully sensitive to tetrodotoxin (1 microM), tachyphylaxis to CRF or to atropine (1 microM) plus the tachykinin NK1 receptor antagonist SR140333 (200 nM). Atropine alone had a weak inhibitory effect on the contractile action of CRF. Neither the antagonist BQ123 (3 microM) nor CRF tachyphylaxis inhibited the contractile action of capsaicin (2 microM), even in the presence of a mixture of GR82334 (3 microM) and SR142801 (100 nM), for blocking tachykinin NK1 and NK3 receptors, respectively--a treatment that by itself significantly reduced the effect of capsaicin. Exogenous ET-1 (0.3-5 nM), but not CRF (30-100 nM), caused relaxation of the atropine-treated, histamine-precontracted ileum. This effect of ET-1 was significantly inhibited or abolished by BQ123 (10 microM), or BQ788 (3 microM), but was not influenced by capsaicin tachyphylaxis. Likewise, relaxation of the atropine-treated, histamine-precontracted ileum in response to capsaicin was not influenced by the endothelin receptor antagonist BQ788 (3 microM) or BQ788 (3 microM) plus BQ123 (3 microM). Apamin (300 nM) was also without effect on the capsaicin-induced relaxation. In circular muscle strips ET-1 inhibited the indomethacin-induced spontaneous activity. This effect was abolished by BQ123 (3 microM) or BQ788 (3 microM). CRF caused a stimulation of the circular muscle. This stimulatory effect was not influenced by atropine (1 microM) alone, but was inhibited by atropine plus tachykinin NK1 and NK2 receptor antagonists (SR140333 (200 nM) and SR48968 (200 nM)) and also by tetrodotoxin (1 microM). It is concluded that capsaicin-sensitive neurons do not play a role in the effects of exogenous ET-1 or CRF in the guinea-pig ileum. ET-1 can both contract and relax the ileal longitudinal smooth muscle directly, probably via both ETA and ETB receptors. CRF acts by specifically stimulating excitatory (but not inhibitory) neurons of the myenteric plexus. Neither endogenous ET-1 nor CRF seems to play a role in the excitatory or inhibitory effects of capsaicin.

Modifications of capsaicin-sensitive neurons in isolated guinea pig ileum by [6]-gingerol and lafutidine

A segment of guinea pig ileum was used to confirm the hypothesis that [6]-gingerol and lafutidine interact with capsaicin-sensitive neurons. Addition of 30 and 100 microM [6]-gingerol (a pungent constituent of ginger) induced contraction of the ileum immediately. Like capsaicin, [6]-gingerol-induced contraction was inhibited by antagonists of the vanilloid receptor (capsazepine and ruthenium red), tetrodotoxin, and atropine. Treatment with [6]-gingerol up to 0.3 microM, which alone had no effect, enhanced 3 microM capsaicin-induced contraction, but greater than 3 microM [6]-gingerol significantly inhibited capsaicin-induced contraction. Treatment with lafutidine (a new type of antagonist of the histamine H(2) receptor), which was suggested to interact with capsaicin-sensitive neurons in vivo, also showed both stimulatory and inhibitory effects on capsaicin-induced contraction depending on the concentrations. Lafutidine alone had no effect. The enhanced contraction induced by capsaicin in the [6]-gingerol- or lafutidine-treated ileum was also inhibited by antagonists of the vanilloid receptor, tetrodotoxin, and atropine. Capsaicin and [6]-gingerol, but not lafutidine, at 30 microM stimulated [(3)H]choline release from the prelabeled slices of the ileum. These findings suggest that [6]-gingerol and lafutidine act on capsaicin-sensitive cholinergic neurons and modulate the contraction in isolated guinea pig ileum.

Cellular distribution of vanilloid VR1 receptor immunoreactivity in the guinea-pig myenteric plexus

Recent investigations suggest that vanilloid receptor-1 (VR1) immunoreactivity occurs in the intestine. We have determined and quantified this immunoreactivity in the myenteric plexus with respect to cholinergic and neurofilament protein-positive neurones. Guinea-pig and rat preparations were dual-labelled with specific antibodies raised in rabbit or goat against vanilloid receptor-1 and against other neurochemical markers. In the rat ileum, both vanilloid receptor antibodies were co-distributed, whereas in the guinea-pig ileum and colon, tertiary fibres were also detected with the goat antibody. In the guinea-pig, all vanilloid receptor-1-immunoreactive cell bodies were choline acetyltransferase-immunopositive (100%) and showed some immunoreactivity to neurofilament proteins (NFP-200 kDa (79%) or triplet (10.8%)) or calretinin. Immunoreactive fibres in the secondary plexus co-localised with calcitonin gene-related peptide (CGRP) and with substance P, calretinin and synapsin I in the tertiary plexus. Subpopulations of cholinergic neurones including sensory, interneuronal and secretory neurones express vanilloid receptor-1. Co-localisation with substance P and calretinin in fibres suggests that vanilloid receptor-1 may be expressed by excitatory motor neurones. The association of vanilloid receptors with calcitonin gene-related peptide and synaptic protein in fibres implies a role for vanilloid receptors in neurotransmitter/neuropeptide release. Although it is likely that at least some of the vanilloid receptor-bearing fibres originate in immunopositive myenteric soma, the origin of all these fibres cannot be identified in the present study.

The guinea-pig oesophagus is a versatile in vitro preparation for pharmacological studies

1. The isolated oesophagus of the guinea-pig is a useful preparation that can be used as an alternative to the phrenic nerve-diaphragm preparations that have been used traditionally in the determination of the actions of drugs and toxins at the neuromuscular junction. The guinea-pig isolated oesophagus can also provide information about effects at ganglionic nicotinic receptors, which are not present in the diaphragm preparations. 2. The muscularis externa of the body of the oesophagus consists exclusively of striated muscle fibres. The myenteric plexus is found between the outer longitudinally arranged layer and the inner circular layer. The muscularis mucosae contains smooth muscle fibres arranged longitudinally. 3. The vagal nerves are comprised of special vagal efferents that innervate the striated muscle fibres directly and 'parasympathetic' vagal fibres that synapse in the myenteric ganglia and, subsequently, affect the smooth muscle of the muscularis mucosae. Thus, both striated and smooth muscle responses to vagal nerve stimulation can be studied. In addition, afferent neurons run in the vagus. 4. Studies using various isolated oesophageal preparations have been reviewed. These consist of the whole oesophagus (including striated muscle, myenteric plexus and smooth muscle), the isolated mucosal layer (striated muscle and the myenteric plexus both absent) and the whole oesophagus with the vagus nerve attached. Comparative studies of the effects of drugs acting directly on the muscularis mucosae and/or indirectly via the intramural plexuses can be performed using the whole oesophagus and the isolated mucosal layer. 5. The use of the guinea-pig isolated vagus nerve-oesophagus preparation allows potency determinations for both neuromuscular and ganglion blockade of various non-depolarizing muscle relaxants to be performed simultaneously under identical conditions. Furthermore, the phenomenon of fade, a waning of tetanic tension during the stimulation period, can be studied. 6. A potential application of this preparation is the simultaneous screening of the constituents of snake venoms for activity at the neuromuscular junction and/or the ganglion. It is also suggested that new calcium channel blockers could be screened in this preparation because different voltage-sensitive calcium channels are involved in neurotransmitter release from nerve terminals at the neuromuscular junction and autonomic cholinergic neuroeffector sites.

Localization of neurokinin B receptor in mouse gastrointestinal tract

AIM: To observe the location of neurokinin receptor (NK3r) in the mouse gastrointestinal tract.

METHODS: The abdomens of 8 male Kunming mice were opened under anaesthesia with sodium pentobarbital. The exposed gut organs were kept moisture and temperature at the same time. Then the esophagus, jejulum, ileum, colon, etc were respectively cut and the segments from the stomach to the distal colon were opened along the mesenteric border. A circular 4 mm-6 mm enteric part(pieces of 1 cm² were to be prepared) and mucosa and submucosa were removed, then the longitudinal muscle layer was pulled off from the circular muscle layer under microphotograph. They were rinsed in 50 nmol·L^-1 potassium phosphate-buffered saline(PBS). Immunohistochemistry and immunoreactive fluorescence were used in the staining procedures.

RESULTS: There was not NK3r-Like(-Li) positive material on the smooth muscle cells of the esophagus, stomach, and intestines and other regions. The nerve cell bodies with immunoreactivity for NK3r were mainly distributed in the submucousal nerve plexus or myenteric nerve plexus of the gastrointestinal tract except for the esophagus, stomach and rectum. The reaction product was located on the surface of the nerve cell plasma. It was occasionally observed in the cell plasma endosomes, but was very weakly stained. Among the NK3-like positive neurons in the plexus,the morphological type in many neurons appeared like Dogiel II type cells. Some neuron cell bodies were big, having many profiles, some were long ones or having grading structure. Cell body diameter was about 10 μm-46 μm and 8 μm-42 μm in myenteric plexus and submucous plexus.

CONCLUSION: This study not only described the distribution of neurokinin B receptor in the mouse gut in detail, but also provided a morphological basis for deducing the functional identity of the NK3r-LI immunoreactivity neurons, suggesting the possibility that these neurons were closely related to gastrointestinal tract contraction and relaxing activity.

Tachykinin receptors in the gut: physiological and pathological implications

The tachykinins substance P and neurokinin A participate in the regulation of gastrointestinal motility, secretion, vascular permeability and pain sensitivity. Advances made during the past two years corroborate a causal involvement of tachykinins in inflammation-induced disturbances of gut function, such as dysmotility, secretory diarrhoea, oedema and hyperalgesia. It would therefore appear that tachykinin receptors, which in the digestive system are expressed in a cell-specific manner, represent attractive targets for novel therapeutics in gastroenterology.

Differential effects of anandamide on acetylcholine release in the guinea-pig ileum mediated via vanilloid and non-CB1 cannabinoid receptors

1. The effects of anandamide on [3H]-acetylcholine release and muscle contraction were studied on the myenteric plexus-longitudinal muscle preparation of the guinea-pig ileum preincubated with [3H]-choline. 2. Anandamide increased both basal [3H]-acetylcholine release (pEC(50) 6.3) and muscle tone (pEC(50) 6.3). The concentration-response curves for anandamide were shifted to the right by 1 microM capsazepine (pK(B) 7.5 and 7.6), and by the combined blockade of NK1 and NK3 tachykinin receptors with the antagonists CP99994 plus SR142801 (each 0.1 microM). The CB1 and CB2 receptor antagonists, SR141716A (1 microM) and SR144528 (30 nM), did not modify the facilitatory effects of anandamide. 3. Anandamide inhibited the electrically-evoked release of [3H]-acetylcholine (pEC(50) 5.8) and contractions (pEC(50) 5.2). The contractile response to the muscarinic agonist methacholine was not significantly affected by 10 microM anandamide. 4. The inhibitory effects of anandamide were not changed by either capsazepine (1 microM), SR144528 (30 nM) or CP99994 plus SR142801 (each 0.1 microM). SR141716A (1 microM) produced rightward shifts in the inhibitory concentration-response curves for anandamide yielding pK(B) values of 6.6 and 6.2. 5. CP55940 inhibited the evoked [3H]-acetylcholine release and contractions, and SR141716A (0.1 microM) shifted the concentration-response curves of CP55940 to the right with pK(B) values of 8.4 and 8.9. 6. The experiments confirm the existence of release-inhibitory CB1 receptors on cholinergic myenteric neurones. We conclude that anandamide inhibits the evoked acetylcholine release via stimulation of a receptor that is different from the CB1 and CB2 receptor. Furthermore, anandamide increases basal acetylcholine release via stimulation of vanilloid receptors located at primary afferent fibres.

Opioid, cannabinoid and vanilloid receptor localization on porcine cultured myenteric neurons

Opioids and cannabinoids have profound inhibitory actions on intestinal motility which are mediated in part by their cognate receptors in the enteric nervous system. In the present study, we examined the expression of immunoreactivity for delta- and kappa-opioid receptors, CB(1)-cannabinoid receptors and type 1 vanilloid receptors by immunocytochemistry and confocal laser scanning microscopy on ileal myenteric neurons, isolated from juvenile pigs, that were <70 microm diameter in either axis and maintained for 1-2 weeks in primary culture. Immunoreactivities for delta-opioid and cannabinoid receptors were present in neurons immunoreactive for the cholinergic marker, choline acetyltransferase. Some neurons with delta-opioid receptor-like immunoreactivity were also immunoreactive for kappa-opioid, cannabinoid or vanilloid receptors. These observations indicate that receptors for cannabinoids or vanilloids are co-localized in opioid receptor-expressing myenteric neurons which modulate intestinal sensorimotor function.

Tachykinins mediate slow excitatory postsynaptic transmission in guinea pig sphincter of Oddi ganglia

Intracellular recording techniques were used to test whether tachykinins could be mediators of slow excitatory postsynaptic potentials (EPSPs) in guinea pig sphincter of Oddi (SO) ganglia. Application of the tachykinin substance P (SP) onto SO neurons caused a prolonged membrane depolarization that was reminiscent of the slow EPSP in these cells. Pressure ejection of the neurokinin 3 (NK3) receptor-specific agonist senktide caused a similar depolarization; however, no responses were detected on application of NK1 or NK2 receptor agonists. The NK3 receptor antagonist SR-142801 (100 nM) significantly inhibited both SP-induced depolarization and the stimulation-evoked slow EPSP, as did NK3 receptor desensitization with senktide. Capsaicin, which causes the release of SP from small-diameter afferent fibers, induced a depolarization that was similar to the evoked slow EPSP in both amplitude and duration. The capsaicin-induced depolarization was significantly attenuated in the presence of SR-142801. These data indicate that tachykinins, released from extrinsic afferent fibers, act via NK3 receptors to provide slow excitatory synaptic input to SO neurons.

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.

Neurokinin receptor antagonists

Studies on tachykinin peptides and the corresponding neurokinin receptors (NKr) have increased dramatically recently due to the discovery of selective, orally-active, metabolically stable and sometimes CNS penetrating NKr antagonists. After demonstrating the potential use for NKr antagonists in animal models, some compounds have recently progressed into clinical trials and a few results have been published. NKr antagonists have demonstrated efficacy for the treatment of emesis and depression, while results in other areas have been disappointing. Nonetheless, this area is coming to the exciting time of proof of concept in humans. Demonstration of the involvement of tachykinin peptides in biological functions continues to grow, as do the potential indications for NKr antagonists. More drug candidates are undergoing clinical trials for various conditions and these results could widen the potential use for NKr antagonists.

Evidence for the involvement of ATP, but not of VIP/PACAP or nitric oxide, in the excitatory effect of capsaicin in the small intestine

The contractile effect of capsaicin in the guinea-pig small intestine involves an activation of enteric cholinergic neurons. Our present data show that the P(2) purinoceptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 30 microM) significantly reduces the contractile response to capsaicin (2 microM) in the presence, but not in the absence, of the tachykinin receptor antagonists [O-Pro(9), (Spiro-gamma-lactam)Leu(10), Trp(11)]physalaemin (1-11) (GR 82334; 3 microM) and (S)-(N)-(1-(3-(1-benzoyl-3-(3, 4-dichlorophenyl)piperidin-3-yl)propyl)-4-phenylpiperidine-4-yl)-N -methylacetamide (SR 142804: 100 nM) (for blocking tachykinin NK1 and NK3 receptors, respectively). PPADS (30 microM) fails to influence submaximal cholinergic contractions evoked by cholecystokinin octapeptide (CCK-8; 2-3 nM) or senktide (1 nM), or the direct smooth muscle-contracting effect of histamine (100-200 nM). A higher concentration (300 microM) of PPADS is also without effect against the stimulatory action of cholecystokinin octapeptide. This means that PPADS can probably be safely used as a purinoceptor antagonist in intestinal preparations. The putative pituitary adenylate cyclase activating peptide (PACAP) receptor antagonist PACAP-(6-38) (3 microM) significantly reduces the contractile effect of PACAP-(1-38) (10 nM) and abolishes that of vasoactive intestinal polypeptide (VIP; 10 nM). PACAP-(6-38) (3 microM) fails to influence the effect of capsaicin (2 microM) both in the absence and in the presence of tachykinin receptor antagonists. The nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine (L-NOARG; 100 microM) also fails to inhibit the capsaicin-induced motor response. We conclude that an endogenous ligand of PPADS-sensitive P(2) purinoceptors (possibly ATP), but not a VIP/PACAP-like peptide or NO, is involved in the nontachykininergic activation of cholinergic neurons in the course of the capsaicin-induced contraction.

Connections between P2 purinoceptors and capsaicin-sensitive afferents in the intestine and other tissues

Relations between P2 purinoceptors and capsaicin-sensitive sensory neurons include an excitatory action of P2 purinoceptor agonists on spinal afferent neurons, as well as release of ATP from afferents at their central and peripheral endings, and a possible participation of ATP in nociception and/or in 'local efferent' responses mediated by sensory nerves at the periphery. The present paper briefly summarizes available evidence on these interrelations. Ample evidence shows that ATP and other P2 purinoceptor agonists can activate primary afferent neurons, through P2X3 receptors and probably other purinoceptors as well, but evidence for an involvement of P2 purinoceptors in nociception or in 'local efferent' responses due to activation of primary afferents is, at best, circumstantial. The possibility is also dealt with that P2 purinoceptor activation may cause small intestinal contraction with the mediation of capsaicin-sensitive sensory neurons and that the motor response to capsaicin in this tissue may involve the release of a P2 purinoceptor stimulant from sensory nerves. Our data show that cholinergic contractions of the guinea-pig ileum in response to the P2 purinoceptor agonist alpha,beta-methylene ATP (alpha,beta-meATP) are blocked by atropine, but not by in vitro capsaicin pretreatment (which completely blocks the contractile action of capsaicin). Cholinergic ileum contractions due to capsaicin (2 microM) are insensitive to suramin (a P2 purinoceptor antagonist; 100 microM). In the presence of antagonists acting at tachykinin NK1 and NK2 receptors, however, suramin (100 microM) causes a significant inhibition of the capsaicin-evoked contraction. These data indicate that capsaicin-sensitive nerves are not involved in the excitatory effect of alpha,beta-methylene ATP on myenteric neurons. On the other hand, ATP is probably involved in the 'non-tachykininergic' component of the capsaicin-induced excitatory response of the small intestine. ATP may originate from sensory neurons and probably acts as activator of myenteric nerves.